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Review Cite This: Chem. Rev. 2018, 118, 505−613

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Catalytic Conversion of Carbohydrates to Initial Platform Chemicals: Chemistry and Sustainability László T. Mika,*,† Edit Cséfalvay,‡ and Á ron Németh§ †

Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, Mű egyetem rkp. 3., Budapest 1111, Hungary ‡ Department of Energy Engineering, Budapest University of Technology and Economics, Budapest 1111, Hungary § Department of Applied Biotechnology and Food Science, Budapest University of Technology and Economics, Budapest 1111, Hungary ABSTRACT: The replacement of fossil resources that currently provide more than 90% of our energy needs and feedstocks of the chemical industry in combination with reduced emission of carbon dioxide is one of the most pressing challenges of mankind. Biomass as a globally available resource has been proposed as an alternative feedstock for production of basic building blocks, which could partially or even fully replace the currently utilized fossil-based ones in well-established chemical processes. The destruction of lignocellulosic feed followed by oxygen removal from its cellulose and hemicellulose content by catalytic processes results in the formation of initial platform chemicals (IPCs). However, their sustainable production strongly depends on the availability of resources, their efficient or even industrially viable conversion processes, and replenishment time of feedstocks. Herein, we overview recent advances and developments in catalytic transformations of the carbohydrate content of lignocellulosic biomass to IPCs (i.e., ethanol, 3-hydroxypropionic acid, isoprene, succinic and levulinic acids, furfural, and 5-hydroxymethylfurfural). The mechanistic aspects, development of new catalysts, different efficiency indicators (yield and selectivity), and conversion conditions of their production are presented and compared. The potential biochemical production routes utilizing recently engineered microorganisms are reviewed, as well. The sustainability metrics that could be applied to the chemical industry (individual set of sustainability indicators, composite indices methods, material and energy flow analysis-based metrics, and ethanol equivalents) are also overviewed as well as an outlook is provided to highlight challenges and opportunities associated with this huge research area.

CONTENTS 1. Introduction 1.1. Scope of the Review 2. Synthesis of Platform Chemicals 2.1. Ethanol as a C2 Basic Chemical 2.1.1. Production of Ethanol 2.1.2. Ethanol Separation and Purification 2.1.3. Energy Demands of Ethanol Production 2.1.4. Conclusion 2.2. C3-Basic Chemicals 2.2.1. 3-Hydroxypropionic Acid (3-HP) 2.2.2. Propionic Acid (PA) 2.3. Succinic Acid as a C4-Basic Chemical 2.3.1. Biochemical Pathways for Succinic Acid Production 2.3.2. Biotechnological Production of Succinic Acid (SUA) 2.3.3. Processing of Succinic Acid (downstream) 2.3.4. Conclusions 2.4. C5 Basic Chemicals 2.4.1. Furfural 2.4.2. Levulinic Acid (LA) © 2017 American Chemical Society

2.4.3. Isoprene (IP) 2.5. 5-Hydroxymethylfurfural (5-HMF) as a C6Basic Chemical 2.5.1. Mechanistic Aspects of Formation of 5HMF 2.5.2. Process Chemistry for Conversion of C6Monomers to 5-HMF 3. Sustainability Metrics Relating to the Chemical Industry 3.1. Introduction to Sustainability Metrics 3.2. Individual/Set of Sustainability Indicators 3.3. Composite Indices Methods 3.4. Material and Energy Flow Analysis-Based Metrics 3.5. Ethanol Equivalents of Initial Platform Chemicals 3.6. Conclusions on Sustainability Measurements 4. Concluding Remarks and Outlook

506 509 509 509 510 514 514 514 515 515 520 524 524 524 527 527 528 528 534

543 544 545 555 581 581 582 582 583 589 590 590

Special Issue: Sustainable Chemistry Received: June 30, 2017 Published: November 20, 2017 505

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

Chemical Reviews Author Information Corresponding Author ORCID Notes Biographies Acknowledgments Abbreviations References Note Added after ASAP Publication

Review

Biomass, of which ca. 75% are carbohydrates with an empirical formula of C6H12O6 is produced by Nature utilizing sunlight (Scheme 1) to convert carbon dioxide and water via photosynthesis.

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Scheme 1. Generalized Equation of Biomass Production

The simplest carbohydrates naturally occur in the forms of five- and six-carbon monosaccharides such as xylose, fructose, and glucose, representing carbohydrate units of complex lignocellulosic biomass as a nonedible resource. Lignocellulosic biomass comprises a robust composite material constructed primarily from different oxygen-containing high-molecularweight polymers as follows: (i) cellulose as a polymer of C6glucose units linked by β(1−4) glycosidic bonds, (ii) hemicellulose as a heteropolymer primarily composed of C5 xylose units and to a lesser extent of mannose, galactose, rhamnose, and arabinose, and (iii) lignin as an amorphous polymer containing three phenylpropanolic monomers (monolignols) linked by carbon−carbon and ether bonds.13 A schematic illustration of lignocellulose is depicted in Figure 1. Cellulose and hemicellulose are encapsulated in a lignin shelter and fixed by hydrogen and covalent bonds. The percentage of each component depends on the plant species and varies between 30−50 wt % cellulose, 20−40 wt % hemicellulose, and 10−20 wt % lignin. The compositions of selected lignocellulosic resources are presented in Table 1. It should be noted that freely accessible carbohydrates could also be present in the structure. Other components are tannins, resins, fatty acids, and inorganic salts as microcomponents as well as special substances that can also be isolated from biomass, such as vitamins, dyes, flavors, aromatic essences, certain oils, and proteins. Although the annual production of biomass is ca. 170 billion metric tons7 shared 50:50 between oceans and land,17 no more than 5% of the produced lignocellulose has been efficiently utilized. Concerning elemental composition, the fundamental difference between fossil and biobased resources is demonstrated by the latter’s significantly higher oxygen content. It could be up to 40 wt %. For comparison, the elemental composition of coals of different origin, petroleum, and selected examples of renewable raw materials are summarized in Table 2. The detailed chemical composition of biomass was overviewed by Vassilev and coworkers.32 The active debate regarding “fuel versus food” highlighted the importance of identifying waste biomass streams, including food waste and manure, as feedstocks for production of fuels and chemicals.16,45 Properties of these resources were presented in a recent review.46 Because of the complex structure of biomass and biomassbased waste materials, its efficient chemical or biological conversion to platform chemicals requires a pretreatment step(s). To obtain monomers or even C5- and C6-platform chemicals, the corresponding fractions of lignocellulose have to be released by an appropriate pretreatment. In most cases, at least two pretreatment steps are needed. An effective technology should meet the following requirements: (i) break the complex structure of lignocellulose, (ii) reduce the crystallinity of cellulose, (iii) preserve pentoses from hemicelluloses, (iv) limit the formation of degradation products that inhibit subsequent conversion steps, such as hydrolysis and/or

1. INTRODUCTION Our modern society utilizes an enormous amount of energy and consumer products of the chemical industry, including transportation fuels, fertilizers, polymers and composites, pharmaceuticals, detergents, food additives, electronics, sports equipment, clothes, dyes, and agrochemicals. The utilization of fossil carbon resources received only occasional applications until the industrial revolution. However, since the middle of the 18th century, the fossil carbon resources such as coal, and later on, crude oil and natural gas, have become the primary sources of energy and carbon-based chemicals,1 establishing the improved living standards. Today, refinery processing of crude oil continuously provides bulk chemicals for the chemical industry that produces value-added consumer products in million-ton scales. In addition, the growing population of Earth has increased the world’s overall energy demand, which is expected to double by 2035 that of 2000. It should be noted that increased energy consumption correlates with increasing carbon dioxide emission, which reached 33.4 GT in 2016.2,3 Increasing energy and resource demands, the global efforts to reduce CO2 emission according to the Kyoto Protocol, and possible production of carbon-neutral end products, as well as diminishing reserves, have directed researchers’ attention to identify and develop innovative solutions for replacement of fossil-based resources by renewable resources. The contribution of hydropower, wind, and photovoltaic energy4,5 to overall energy demand and utilization of carbon dioxide6 and biomass as a carbon resource for the chemical industry have emerged.7−16 Thus, biomass has a crucial role and a strategic position in sustainable carbon-atom management and therefore sustainable production of carbon-based consumer end products. From the sustainability point of view, four strategic objectives have been identified as follows: (i) pollution caused by chemical industry is to be reduced, (ii) risks arising from the use or production of hazardous chemicals are to be diminished, (iii) the volume of raw materials has to be reduced by more efficient conversion technologies and reaction routes, and (iv) use of abundant carbon-neutral biomass as raw material and renewable energy source has to be preferred. Development of processes enables a shift toward the use of renewable resources as raw materials, and Green Chemistry plays a major role in its realization. Biomass as a globally available, renewable, and natural carbon resource that has been used as feedstock for production of chemicals since ancient times could also be an ideal alternative supply for production of carbon-based chemicals in the future. Accordingly, the idea of “biorefinery concept”, including integration of conversion processes to produce fuels, power, heat, and value-added chemicals by targeting the utilization of all carbon atoms of the resource, was born. However, it has only been partially realized by transformation of the carbohydrate content of renewable resources. 506

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Figure 1. Schematic illustration of lignocellulose. Modified with permission from ref 282. Copyright 2013 Royal Society of Chemistry.

supercritical CO2,49 ionic-liquid-assisted deconstruction,50,51 or utilization of deep eutectic solvents (DES) for these goals.52 The corresponding fraction of lignocellulosic biomass can be subsequently converted either by chemo- or biocatalytic processes to platform chemicals that could fully or partially replace the currently utilized fossil-based building blocks of well-established chemical processes. A series of potential candidates obtained from carbohydrates, such as 5-hydroxymethylfurfural (5-HMF) from cellulose, furfural (FAL) from hemicellulose as well as levulinic acid (LA), 3-hydroxypropionic acid (3-HP), furanedicarboxylic, and fumaric acids, etc. was collected by Werpy and Petersen in 2004,53 and updated by Bozell in 2010.54 Of these basic chemicals initially formed from C5- or C6-carbohydrate fractions, the “Initial Platform Chemicals” (IPCs) have to be distinguished. Chemocatalytic conversion of C5-units and C6-units leads to the formation of FAL and 5-HMF, respectively. Biochemical conversion of cellulosic feedstocks can be applied for the synthesis of ethanol, 3-hydroxypropionic, propionic and succinic acids, and isoprene (Figure 2), from which the production of ethanol and succinic acid has been industrialized. To reach similar elemental composition of biomass-based platform molecules to that of crude oil, the primary claim of conversion processes is selective oxygen removal, which can be carried out by dehydration reactions. Catalysis plays a distinguished role for these purposes.13 Mineral acid catalysts have been practically used since the early 20th century. Most common plants use diluted or concentrated H2SO4 operating either on an industrial scale (furfural production)55 or pilot scale such as the Biofine process.56 Because the final reaction mixtures contain dissolved catalyst(s) (mineral or Lewis acids), the significance of some well-known drawbacks (i.e., product isolation from the reaction mixture and catalyst recovery) have to be emphasized. In addition, the generally low products’ concentrations could result in a large amount of generated waste.57,58 Heterogeneous catalytic conversions have good tolerance to impurities and a longer lifetime than homogeneous or biocatalytic systems. Therefore, they could be the basis of potential industrially viable processes. Moreover, robust heterogeneous systems can transform a wide range of

Table 1. Composition of Selected Lignocellulosic Resources in Wt % cellulose energy crops average rape straw miscanthus miscanthus straw switchgrass agricultural residues average wheat straw corn stover rape straw corn cob sunflower husk forestry residues average hardwood stem softwood stem poplar sawdust food waste average shells (hazelnut, walnut) sugar cane bagasse rice hull orange peel banana peel

hemicellulose

lignin

ref

30−50

20−40

10−20

18

45.0 44−45 44.7 45.0 35.0−55.0

19.0 18−30 29.6 31.4 25.0−35.0

18.0 21−22 21.0 12.0 15.0−30

19, 20 21, 22 21, 22 23 18

39.2

25.6

22.9

28−51 36−45 41−52 48.4 40.0−50.0

28−31 20−27 32−36 34.6 25.0−35.0

11−14 20 6−15 17.0 20.0−30.0

22, 24, 26 25, 26 21, 27 21, 26 25 18

40.0−55.0 45.0−50.0 57.6

24.0−40.0 25.0−35.0 14.1

18.0−25.0 25.0−35.0 18.0

23 23 24

25.6−26.8

22.1−30.4

42.9−52.3

25

34−36 30.9−33.9 12.0 11.5

29−43 16.5−16.8 14.5 25.5

19−21 21.4−35.9 2.2 9.8

28, 29 26, 30 31 31

fermentation, (v) minimize energy inputs and use of extraneous chemicals, (vi) be simple to implement, (vii) generate a high value lignin coproduct, (viii) minimize the production of toxic and hazardous wastes, if any, and (ix) generate the minimum amount of wastewater.47 Accordingly, several methods were developed for this purpose. The conventional processes are acid or base hydrolysis and steam explosion.48 To improve the viability of biomass conversion, some alternative pretreatment processes were also developed, including the application of 507

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Table 2. Elemental Compositions of Fossil (Entries 1−5) and Selected Biomass Resources (Entries 6−21) entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

anthracite (Pennsylvania coal) bituminous coal (Pennsylvania coal) petroleum black coal peat miscanthus fresh switchgrass rape straw sorghum stalk sugar cane bagasse wheat straw straw (average) sawdust bagasse corn cob corn stover bark forest residues wood without bark sunflower husk shells (hazelnut, walnut)

carbon (wt %)

hydrogen (wt %)

oxygen (wt %)

nitrogen (wt %)

sulfur (wt %)

91.0−94.0 83.0−89.0 83.0−87.0 76.0−87.0 52.0−56.0 47.3−47.7 43.5−47.5 58.5 40.0−46.1 48.6 42.9−47.6 45.0−47.0 50.0 48.0 49.0 42.6 48.0−52.0 48.0−52.0 48.0−52.0 47.4 51.6−53.5

2.0−4.0 4.0−6.0 10.0−14.0 3.5−5.0 5.0−6.5 5.8−6.0 5.8−6.2 8.6 5.2−5.8 5.9 5.1−5.9 5.8−6.0 6.3 6.0 5.4 5.1 5.7−6.8 6.0−6.2 6.2−6.4 5.8 6.2−6.6

2.0−5.0 3.0−8.0 0.05−1.5 2.8−11.3 30.0−40.0 42.1−43.5 37.6−44.8 23.5 40.6−40.7 42.9 40.4−42.4 40.0−46.0 42.5 42.0 44.6 36.5 24.3−40.2 40.0−44.0 38.0−42.0 41.4 35.5−40.2

0.6−1.2 1.4−1.6 0.1−2.0 0.8−1.5 1.0−3.0 0.33−0.45 0.36−0.77 3.67 0.39−1.40 0.16 0.43−0.73 0.4−0.6 0.8

0.6−1.2 1.4−1.7 0.05−6.0 0.5−3.1 0.05−0.3 0.05−0.08 0.04−0.19 n.a. 0.20−0.27 0.04 0.09−0.29 0.05−0.2 0.03 − − 0.09 55 g L−1 with a productivity of 2.23 g L−1 h−1 was obtained in fed-batch HCD fermentation at pH 6.5. By a three-stage simulated fed-batch process in serum bottles, 49.2 g L−1 propionic acid was produced with a yield of 0.53 g g−1 and productivity of 0.66 g L−1 h−1. These productivities, yields, and PA titers were among the highest ever obtained in free-cell PA fermentation on glucose.198 To make PA fermentation more feasible, useful byproducts formed simultaneously during the fermentation should also be extracted. Therefore, an efficient fermentation-strengthening approach was developed to improve the anaerobic production of PA and vitamin B12 at the same time with Propionibacterium freudenreichii. While glucose-based vitamin B12 production resulted in a relatively high productivity (0.35 mg L−1 h−1) and a propionic acid yield of 0.55 g g−1, glycerol gave higher PA yield (0.63 g g−1) and low productivity (0.16 g L−1 h−1). Cofermentation of glycerol and glucose with a gradual addition 522

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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three columns. The fermentation broth with free cells was circulated through the expanded-bed-adsorption column to separate the propionic acid in the expanded bed every 12−24 h when 10 g L−1 propionic acid was produced. To optimize the fermentation performance, a strategy of gradually adding feedstock was employed, in which glycerol was introduced at set times (12, 24, 48, and 72 h) with a constant mass ratio of 2.5 to glucose. When glycerol was added after 48 h, the highest product concentration (propionic acid, 42.7 g L−1), the highest yield (propionic acid, 0.71 g g−1), and highest productivity (propionic acid, 0.36 g L−1 h−1) of this experiment were obtained.199 The highest PA productivity was achieved by sequential batch (hereinafter SB) fermentation with cell recycling.198 Propionibacterium acidipropionici ACT-1 was used in the study. Cells in the late exponential phase (cultured in 3 L of medium with 50 g L−1 glucose at pH 6.5) were collected by centrifugation and used to seed the batch fermentation in a 1 L bioreactor containing 500 mL of medium agitated at 100 rpm. A total of four sequential batches were performed at pH 6.5 for a total duration of 65 h. In the SB fermentation with high initial cell density, the PA yield was stable at 0.45 g g−1 and the volumetric productivity increased from 2.06 g L−1 h−1 for the first batch to 2.74 g L−1 h−1 for the fourth batch, which was also proportional to the average cell density in these SB fermentations. It should be noted that a significantly higher yield of 0.48 g g−1 and productivity of ca. 3 g L−1 h−1 were obtained in the third batch. At this stage, the highest cell density of ca. 49 g L−1 was achieved in terms of the dry weight of the cell (corresponding to OD 244.8). The highest propionic acid titer of 42.7 g L−1 obtained in the second batch also confirmed that ACT-1 had a relatively high tolerance to propionate at pH 6.5.198 2.2.2.3. Processing of PA (Downstream). It is a real challenge to compose a comprehensive review of downstream operations because their efficiency is strongly determined by the different feedstocks (yielding the matrix), the producer strain (which determines accompanying byproducts), and the required final product quality and formula. In addition, existing recovery reports focus either on recovery after synthetic production or recovery from model solutions, but almost none of them address recovery from real fermentation broths. However, we compared the available processes on the basis of their key operations, and the overall recovery yields obtained. The importance of propionic acid production by fermentation makes it necessary to develop new, efficient methods for PA separation from fermentation broths and requires substantial improvement in existing recovery technologies. Around half of the total production costs are associated with downstream processing,204 therefore, these improvements have a similar impact to developments on fermentation processes generally. As a result, this overview introduces the possible processes regarding model solutions. Because the purity of the product is often required to be higher than 99.5%, a great extent of recovery is needed in the downstream process. Low energy consumption, as well as low consumption of chemicals, is necessary during product recovery. Therefore, efficient levels of mass and heat transfer must be exhibited during recovery. To meet all of these, the downstream process has to fulfill the following key requirements: (i) removal of large particles during clarification, (ii) removal of the product from the bulk aqueous solution containing major impurities for primary recovery, (iii) counterion removal by replacing the cation of a

strategy gave high yields at the same time (propionic acid: 0.71 g g−1; vitamin B12:0.72 mg g−1) and productivities (propionic acid: 0.36 g L−1 h−1; vitamin B12:0.36 mg L−1 h−1). Thus, the integrated feedstock and fermentation system strengthening strategy seemed to be an efficient method for the economic production of biobased propionic acid and vitamin B12.199 Propionic acid production by Propionibacterium freudenreichii from molasses and hydrolyzed waste Propionibacterium cells as nitrogen source was studied in a plant fibrous-bed bioreactor (PFB). With nontreated molasses as a carbon source, 12.69 ± 0.40 g L−1 of propionic acid was attained in free-cell fermentation, whereas the PFB fermentation yielded 41.22 ± 2.06 g L−1 for 120 h with faster cell growth. To optimize the fermentation outcomes, the fed-batch process was performed with hydrolyzed molasses in PFB, giving 91.89 ± 4.59 g L−1 of propionic acid in 254 h. Further experiments were carried out using hydrolyzed waste propionibacterium cells as a substitute nitrogen source resulting in a propionic acid concentration of 79.81 ± 3.99 g L−1 for ca. 300 h. This study suggested that the low-cost molasses and waste Propionibacterium cells can be utilized for the green and economical production of propionic acid by P. freudenreichii.200 As can be concluded from Table 10, the highest propionic acid titer is 106 g L−1, which was reported by Zhang and coworkers.203 In this experiment, researchers used a mutant strain of Propionibacterium acidipropionici ATCC 4875 with acetatekinase (AcK) gene knockout. An immobilized-cell-containing FBB was applied and connected to a 5 L fermenter to control the pH and temperature through a recirculation loop. The FBB itself had an operation volume of ca. 600 mL, and the complete system contained ca. 2 L of the medium. After inoculation, the FBB was operated under a repeated batch mode for several batches with glucose as the substrate to increase the cell density in the reactor system. After the cell density had reached the desired value on glucose, the glycerol feed was started. The fermentation lasted for four months, which equates to 3000 h. The glycerol fed-batch fermentation was conducted five times. The long fermentation time resulted in low propionic acid productivity (0.04 g L−1 h−1), in contrast to the highest propionic acid titer (106 g L−1).203 The highest carbohydrate-based PA production was achieved in a very similar PFB system with P. freundenriechii CCTCC M2017015 in a fed-batch mode on molasses resulting in almost 92 g L−1 PA, according to Table 10. Instead of cotton cloth, chopped bagasse was applied for cell immobilization; otherwise it was a very similar system to the already presented FBB. In Table 10, the highest propionic acid yields are 0.70 and 0.71 g g−1. While these were reached by utilization of glycerol, the highest PA yield on carbohydrate substrate was 0.66 g g−1 utilizing lactose, but considering sustainability, the use of renewable carbohydrate is more important. Thus, the highest PA yield was achieved on glucose with 0.6 g g−1. A high PA yield could be obtained by cofermenting glucose and glycerol using P. freudenreichii resulting in a propionic acid yield of 0.71 g g−1.199 In this report, batch fermentations were studied in three parallel 200 mL expanded-bed-adsorption bioreactors (EBAB) connected with a recirculation loop to a 5 L stirred-tank fermenter to control the temperature and pH. The EBAB was made of a glass column packed with Duolite A30 resin, and both ends of the column were equipped with a stainless steel wire mesh. In this process, propionic acid was adsorbed semicontinuously from the broth, and replenishment with fresh resin was possible when needed by alternating the 523

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12 Value-Added Chemicals proposed by Petersen.53 Its first isolation was completed from amber by Georgicus Agricola in 1546;214 however, SUA’s industrial potential was recognized in 1980.215 The SUA market has been steadily growing. Recently, its worldwide production volume has an amount around 58500 tons annually.216 Its market size of SUA was 157.2 million USD in 2015 and is projected to reach 75000 million USD in 2021. SUA was manufactured for a long time via fermentation, but later, the petrochemical route became more feasible.218 2.3.1. Biochemical Pathways for Succinic Acid Production. The central metabolism in most microorganisms has a glycolysis part breaking down C6 units into C3 from glucose through glyceraldehyde-phosphate until pyruvate, resulting in a net two moles of ATP per mol of glucose. The next part of the metabolic map is the TCA cycle, involving succinate at the middle of the cycle. Therefore, overproduction of SUA is possible through both normal direction of TCA (oxidative pathway with CO2 release) from pyruvate via isocitrate to SUA or reverse direction from pyruvate via maleate to SUA (reductive pathway CO2 fixation) (Scheme 14). Jansen and co-worker217 reported the same metabolism in eukaryotic cells like Saccharomyces cerevisiae, in which TCA is located in mitochondria, and additionally, they also indicated beneficial pathway modifications via gene knockout of glycerol- and ethanol-producing pathways in cytosol. 2.3.2. Biotechnological Production of Succinic Acid (SUA). While SUA is currently manufactured on a large scale economically via chemical synthesis, such as reduction of maleic, fumaric acids, or maleic anhydride, due to the favored utilization of different agricultural and other biomass wastes, its biotechnological production has gained a lot of interest in the past two decades.218 According to Choi et al.219 the biotechnological production SUA has been commercialized by four companies: Myriant, BioAmber (a joint venture of DNP and ARD), Succinity (a joint venture of BASF and Purac), and Reverdia (a joint venture of DSM and Roquette). BioAmber produces SUA with the help of recombinant E. coli in Pomacle (France), around 3000 t/yr capacity. The pH is kept constant adjusted with ammonium or sodium hydroxide resulting in the corresponding succinate. Clarification is carried out via crossflow membranes, followed by electrodialysis to remove

propionate with H+ to produce propionic acid (if required), and (iv) concentration/purification: removal of the bulk solvent or capture of the propionic acid, achieving the concentration as well as removal of remaining impurities.205 To summarize, different techniques can be found in the literature for PA recovery: adsorption,206,207 electrodialysis,208 extraction,209,210 and reactive extraction.211,212 The highest recovery of 91% is reported via the reactive extraction method. The detailed discussion of these processes is beyond our recent scope. 2.2.2.4. Conclusions. In conclusion, for efficient biotechnological PA production, three different strategies are used to overcome product inhibition: (i) decrease the toxic effect by pH controlling resulting salts instead of nondissociated acids, (ii) increase the PA tolerance by mutagenesis or adaptation methods, and (iii) in situ product removal, for example, by extraction. The most effective way seemed to be the application of FBBs resulting in culture adaptation to high PA and allowing substrate feed and achieving one of the highest concentrations (71.8 g L−1) and yields (0.5 g g−1). Thus, a relatively high product titer and yield can be obtained, but in terms of productivity further developments are needed. 2.3. Succinic Acid as a C4-Basic Chemical

Succinic acid (SUA) [CAS: 110-15-6, Scheme 13] as a fourcarbon dicarboxylic acid has been described as a versatile C4 Scheme 13. Succinic Acid

basic chemical with the potential of replacing the maleic anhydride platform,213 representing the same concept as that based on fumaric or maleic acid. SUA can be used as a precursor of many industrially important chemicals (i.e., dimethylsuccinate, 1,4-butanediol, tetrahydrofuran, N-methylpyrrolidone, 2-pyrrolidone, and γ-butyrolactone); as well, it can be used as an additive, corrosion inhibitor, etc. (Figure 10).214 Accordingly, it is a biomass-based candidate on the list of Top-

Figure 10. Chemicals from succinic acid. 524

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Scheme 14. Central Metabolism with Succinic Acid as Intermediate of Tricarboxylic Acid (TCA) Cyclea

a

SUA can be reached from both oxidative and reductive directions (CoA, coenzyme-A).

yields of SUA, but with a proper gene modification, they can be fine-tuned and turned from low to excellent high producing strains. Therefore, every microorganism that already has been reported to be able to produce SUA has the potential to give some benefits during technology investigation. These special behaviors are briefly described below. Aspergilli has industrial application (e.g., in citric acid production), which is an antagonist of SUA because both products form in the same metabolism. Therefore, any investigation on citric acid reduces the production of SUA and vice versa.236 The metabolism is under pH control: when the pH drops below 3, then citric acid is the major product, but in buffered neutral pH, SUA is formed in a higher amount. Filamentous fungi have versatile tools depending on their morphology (like a single cell, filamentous, pellet, etc.). They can be especially useful in the case of solid-state fermentation (SSF), in which biomass or different wastes can be converted into a high value-added product like SUA directly, without submerged fermentation. SSF fermentation is often used for hydrolytic enzyme production by fungi, which can be applied even in SUA production.235 Wild-type A. saccharolyticus has really small potential because the SUA titer is 4 g L −1 , but through recombinant supplementation with fumarate-reductase enzyme (from Trypanosoma brucei), its glucose-based SUA production can be increased to four times higher value (i.e., 17 g L−1). In earlier reviews,214,224 some fungi (B. nivea, L. degener, and P. varioti) were indicated as SUA producers, but their epoxy-SUA production was studied because it accompanied SUA as a byproduct.

hydroxides. Finally, purification is carried out by ion-exchange chromatography, and nanofiltration is used for final concentration. Myriant uses lactic acid recovery technology of ThyssenKrupp Uhde having a capacity of 13.5 kilotons SUA per year in Lake Providence, Louisiana. Production is planned to be extended to an annual 63.0 kilotons.220 In this process, a recombinant E. coli fermentation is used with neutralization by ammonium carbonate. Solids are removed by centrifugation and ultrafiltration to avoid the cell mass getting mixed with a filter aid. The final purification involves active carbon treatment and nanofiltration to remove colors, sulfate ions, and 50% of remaining sugars. Succinity, a joint venture of BASF and Corbion Purac operates a pilot plant for succinic fermentation at neutral pH using Basfia succininiproducens, followed by lactic acid recovery technology of Purac. A 25.0 kilotons per year plant is planned to be built in Spain.221 DSM and Roquette (i.e., Reverdia in Cassano Spinola, Italy)222 have developed a recombinant Saccharomyces cerevisiae strain to produce SUA by low-pH fermentation, thus minimizing inorganic acid and base consumption. Their recovery scheme resembles the itaconic acid recovery scheme but involves recrystallization after decolorization and ion exchange.223 Because SUA is an intermediate of the TCA cycle belonging to the central catabolism, it can be found in almost every microbial, plant, and animal cell.214 In contrast to the industrial application, which uses only some fungi, Gram-negative and Gram-positive bacteria can also be used. Our aim was to collect almost every microorganism that has already been described to be able to produce SUA. There are wild natural strains having low efficiency and producing low 525

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Table 11. Succinic Acid Fungi Producers, with Main Featuresa titer (g L−1)

productivity (g L−1 h−1)

yield (g g−1)

no.

strain

fermentation technique/scale/substrate

1 2

Aspergillus niger Aspergillus fumigatus

n.a./n.a/xylose since Song and Lee have reported that these fungi were PA producer, these strains could only be found in studies focusing on raw material pretreatment with fungal enzymes for SUA fermentation with A. succinogenes

0.025

∼0

∼0

234 235

wt: batch/0.25L/glucose

4

0.027

0.05

236

rec( f rd+):/0.25L/glucose almost every review mentions these strains as SA producer, but in fact, their epoxy-succinic acid production was studied with SA as byproduct; their potential is really low, and no technoeconomic data are available

17

0.118

0.21

batch/0.01L/n-alkanes

23.6

0.123

0.67

238

aerob batch/10L/ethanol

5.2

0.076

0.33

239

aerob batch/10L/ethanol

9.4

0.38

0.39

239

wt control: batch/0.2L/glucose rec(PMCFfg): aerob,batch/3L/glucose

0.11

0.001

0.002

240

13

0.108 0.06b

0.13

n.a.

n.a.

Aspergillus awamori Aspergillus saccharolyticus

3

Byssochlamys nivea

4

Lentinus degener Paecilomyces varioti Penicillium viniferum Candida brumptii Candida catenulate Candida zeylanoides Saccharomyces cerevisiae

5 6 7 8

9 10 11

Penicillium simplicissimum Rhizopus sp. recombinant Yarrowia lipolytica

237

nongrowing hifa bioconversion/1.7L/glucose

n.a./n.a./glucose/glycerol

ref

45.5

241

242

n.a., not available; wt, wild type; rec ( f rd+), recombinant with the code of heterologous gene. bProduction was given in g CDW−1 h−1, where CDW is the cell dry weight. a

Table 12. Succinic Acid Gram + Bacteria Producers, with Main Features strain

fermentation technique/scale/substrate

titer (g L−1)

productivity (g L−1 h−1)

yield (g g−1)

ref

Clostridium thermosuccinogenes Corynebacterium glutamicum recombinant C. glutamicum Enterococcus faecalis Enterococcus faecalis RKY1 Ruminococcus flavefaciens

batch/2L/inulin aerob batch/1L/D-glucose fedbatch/n.a./glucose n.a./n.a. /fumarate n.a./n.a. /fumarate and rice bran batch/0.5L/pulp-and-paper

3 7.08 146 82.5 4.75 2

0.125 1.778 3.17 3.5 2.2 0.05

0.31 0.47 0.92 0.825 0.95 0.2

243 244 245 246 247 248

cultivate rumen bacteria because some of them are hardly culturable.225 These bacteria act as enzyme or gene sources and have less industrial relevance as strains. One of the most promising bacteria is Actinobacillus succinogenes because 55 g L−1 SUA was reported on sulfuric acid-pretreated molasses in a fed-batch anaerobic fermentation for 48 h, resulting in 1.15 g L−1 h−1 productivity. Similar results were obtained in a recent study on different and differently pretreated lignocellulosic feedstocks providing 42.8 g L−1 SUA concentration, 0.75 g g −1 yield, and 1.27 g L −1 h −1 productivity.226 An interesting remark in this study was that this bacterium is able to detoxify furfural to furfuryl alcohol; therefore, acetate and furans have stronger inhibition. A. succinogenes was used in an internal FBB, with the fibrous bed mounted on the stirrer shaft, in contrast to FBBs in the propionic acid section, and reached 98.7 g L−1 SUA together with 0.89 g g−1 yield and 2.77 g L−1 h−1 productivity because of the high cell concentration and defensive environment.227

Different yeasts also have the potential to produce SUA, but the maximum concentrations are generally low. However, the workhorse Saccharomyces cerevisiae was also tested240 because it has good acid tolerance. The wild type produced hardly detectable 0.11 g L−1 SUA, but through several gene deletions, an engineered strain was obtained that was able to produce more than 6 g L−1 SUA. After deletion of the key enzyme in the glycerol-forming pathway (namely glycerol-phosphate-dehydrogenase, GDP 1p), the reached titer further increased up to 8 g L−1 and up to 9.98 g L−1 with the regulation of biotin and urea concentration. Finally, after optimization of CO2 supply, the highest concentration obtained was 12.97 g L−1, which is a hundred times higher than that obtained for the starting wildtype organism. Among bacteria, several strains have better results than yeasts and fungi. Most of them were isolated from the rumen, which is an optimal life-space for SUA formation because there are anaerobic conditions with elevated CO2 concentration. However, in several cases there were great challenges to 526

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Table 13. Succinic Acid Gram − Bacteria Producers, with Main Features strain Actinobacillus succinogenes

Anaerobiospirillum succiniciproducens Bacteroides fragilis Bafisa succiniproducens recombinant Escherichia coli Fibrobacter succinogenes Klebsiella pnaumoniae Mannheimia succiniciproducens Succiniclasticum ruminis* Selenomonas ruminatium*

fermentation technique/scale/substrate

titer (g L−1)

productivity (g L−1 h−1)

yield (g g−1)

ref

batch/0.1L/molass batch/5L/molass fedbatch/5L/molass fed-batch/3L intern.FBB/glucose batch/10L/glucose batch/10L/glucose continuous/n.a./glycerol aerob fed-batch/1L/glucose batch/0.3L/pulp-and-paper anaerob batch/0.065/glucose fed-batch/6.6L/glucose + glycerol

50.6 46.4 55.2 98.7 1.18 12 5.29 58.3 2.8 2.1 90.68 homofermentative

0.833 0.966 1.15 2.77 0.05 n.a. 0.094 0.988 0.28 0.21 3.49

79.5 n.a. n.a. 0.89 0.65 0.33 1.02 0.557 0.07 0.088 0.75

249

fed-batch/14L/L-lactic acid continuous/14L/L-lactic acid

5 3.3

0.083 0.182

0.8

227 250 251 252 253 248 254 255 256

Veillonella parvula*

2.3.3. Processing of Succinic Acid (downstream). Similarly to any other organic acid processing, SUA can be recovered from fermentation broth by three classical methods: (i) formation of calcium succinate (i.e., precipitation), followed by filtration and acidification with sulfuric acid resulting gypsum as byproduct, and finally filtration of the byproduct and ion exchange of the main product followed by concentration and crystallization; (ii) recovery of SUA from succinate salts (obtained during fermentation via compensation of pH decrease) by electrodialysis followed by an ion exchange for the final purification; and (iii) reactive extraction of SUA with a tertiary amine.214 The application of carbonate salts has a special benefit in SUA fermentation because the produced acid will result in CO2 release, and the higher available CO2 amount results in a higher SUA concentration due its incorporation by pyruvate-decarboxylase enzyme.257 There is a recent trend to develop fermentation technologies running at low pH, avoiding the salt formation by pH control because the carboxylate salt should be then reacidified resulting in free carboxylic acid and inorganic salt as byproducts having almost no market nor utilization, thus going to waste.258 Because at low pH the weak carboxylic acids are mainly in undissociated form, fermentation running at low pH requires much less base, resulting in less inorganic salt as waste or byproduct. Unfortunately, the low pH caused by the product carboxylic acid is very unfavorable to the producing strain because the undissociated form of carboxylic acid can enter the cytoplasm with simple passive diffusion, and it will dissociate intracellularly, decreasing the cytosol pH to a toxic level and triggering the proton gradient of ATP production. 2.3.4. Conclusions. In conclusion, biotechnological SUA production is a reality with isolated C6-fractions of renewable feedstocks. Expansion of industrial-scale production of SUA from glucose is due to the successful adaptation of a recovery method from lactic acid process, thus resulting in an economical production. It has to be empahized that a recent trend has arisen to develop second and third generations of biosuccinic acid from lignocellulosic feedstock. In recent studies, different lignocellulosic fractions are converted into SUA with the listed natural SUA producer strains having a final titer range of 15−70 g L−1, yield range of 0.05−1.24 g g−1, and productivity range of 0.02−3.3 g L−1 h−1.259 Although hemicellulose-based production of SUA was successfully demonstrated on a laboratory scale, the bottleneck of the

Continuous cultivation was not so successful for this strain.228,229 Among recombinant strains, E. coli is the basic one, which was reengineered to be capable of SUA production as well. Because E. coli is the best-known host organism, there are many reports on improvements of its SUA production displaying great steps forward in the final SUA titer but far from the economic range (21 g L−1,230 22.4 g L−1,231 and 28.6 g L−1,232 etc.) Two attempts have significantly higher SUA concentration: Lin and co-workers reached 58.3 g L−1253 and Jiang and co-workers obtained 68.12 g L−1.233 In Table 11, we summarize the known SUA producers’ good to excellent results, and in Tables 12 and 13, we summarize the SUA gram + and − bacterial producers, respectively, with their main features. The highest SUA yield was achieved among yeasts by recombinant Yarrowia lipolytica242 providing a titer of 45.5 g L−1. The benefit for applying yeast is to ferment at low pH, requiring less base for neutralization, thus resulting in less byproduct. The highest efficiency of all (i.e., final SUA concentration) was reached by Okino and co-workers,245 as can be determined on the basis of data in Table 11. Because they could both overexpress the pyc gene (reductive way) encoding pyruvatecarboxylase in engineered C. glutamicum, furthermore they disrupted the ldhA gene encoding L-lactate dehydrogenase and obtained a new strain able to produce 146 g L−1 of SUA in 46 h with a fed-batch fermentation technique on glucose under “oxygen-free” conditions ( THF > 2-butanol > guaiacol > isopropanol > methyl acetate > acetone >1-butanol >1-propanol > diethyl ether > methanol > cyclopentanone > hexadecane > furan > water > ethylene glycol > hydroxyl acetone > 2,5-dimethoxytetrahydrofuran. Although DMSO gave the highest yield of FAL (>80%, 160 °C) from the point of view of both production and separation of FAL, methyl formate was proposed as the best solvent, due to its high selectivity and efficiency (yield ca. 70% for 20 min) 532

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Table 14. Production of Furfural from Various Resources no.

substrate

subst conc (wt %)

catalyst

catalyst conc

T (°C)

− 50 mM 3.5 wt % 0.05 wt % 2 wt % − 15 wt %

200 200

conv (%)

select (%)

yield (%)

yield (wt %)

ref

1h 7 min

89 100

55 76

49 76

31 49

282 285

150 150 200 120

6 3 1 3

53 78 na 99

42 35 na 72

22 27 16 71

14 17 10 46

306 307 282 278

t

1 2

water water

xylose xylose

1 na

3 4 5 6

water water water water:toluene = 2:3 (v/v) water:toluene = 1:4.4 (v/v) water:toluene = 1:1 (v/v) water:toluene = 1:1 (v/v) water:toluene = 1:1 (v/v) water:onitrotoluene= 3:1 (v/v) water:MeTHF = 1:2 (v/v) DMF

xylose xylose xylan xylose

1 10 1 11

− HCl NaCl NbOH Sn-MMT − Nb2O5

xylose

2

[BMIM][HSO4]

5 wt %

140

6h

100

71

71

46

317

xylose

2

1 wt %

155

2h

95

98

93

59

309

xylose

0.53

MPrSO3HMCM-41 WO3/SiO2 (SG)

0.13 wt %

170

8h

na

na

61

36

311

xylose

0.53

Ga2O3/SiO2 (SG)

0.13 wt %

170

8h

na

na

54

32

311

xylose

na

formic acid

1.75 wt %

190

75 min

86

86

74

47

289

xylose

2

Glu-TsOH-Ti

1 wt %

180

30 min

na

na

51

33

313

xylose

3

8h

na

na

46

29

300

xylose xylose xylose xylose hemicellulose

2.4 10 1.8 1.8 0.53

6 wt % 3 wt % 0.6 wt % 2 wt % 0.4 wt % 0.4 wt % 0.13 wt %

100

GVL SBp/NaCl-DMSO DMSO DMSO water:toluene = 1:1 (v/v) water:toluene = 1:1 (v/v) water:GVL = 1:9 (w/w) water:GVL= 9:1 (w/w) water:GVL= 9:1 (w/w) water water seawater

Al2O3-Ni-Al amberlyst-15 FeCl3·6H2O Sn-MMT P-C-SO3H C-SO3H SAPO-44

170 180 150 150 170

10 min 30 min 2h 3h 8h

na 93 92 45 93

na 82 99 98 68

87 77 91 44 63

56 49 58 28 37

294 307 307 307 310

hemicellulose

0.53

HUSY

0.13 wt %

170

8h

na

na

56

33

310

xylan/cellulose

0.3/ 0.3

Al-beta zeolite

1.2 wt %

175

100 min

100

37

37

24

302

arabinose

2

H2SO4

0.05 M

170

15 min

na

na

87

56

301

arabinose

2

H-beta

3.75 wt %

160

40 min

na

na

73

47

301

corncob corncob corncob

33 11 63

90 min 5 min na

na na 98

na na 74

61 37 73

39 24 47

287 288 296

corncob

0.6

0.5 M 2 v/v% 60 mM, 2 wt % 2.4 wt %

170 180a 190

water:GVL = 1:9 (w/w) water

H2SO4 HCl FeCl3 acetic acid steam Al-beta zeolite

185

85 min

na

na

na

na

302

empty fruit bunch sugar cane bagasse

20

H2SO4

1 v/v%

198

11 min

na

na

30

19

283

0.6

Al-beta zeolite

2.4 wt %

185

85 min

na

na

na

na

302

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 a

solvent

water:GVL = 1:9 (w/w)

h h h h

Microwave irradiation.

properties of the catalyst but also the recycle experiments, too. They showed that the SAPO-44 catalyst’s activity could be maintained for a remarkable eight runs in 8-h-long reactions at 170 °C.310 In a subsequent work of Dhepe, the preparation of WO3/SiO2 and Ga2O3/SiO2 catalyst by sol−gel and impregnation methods was shown. These catalysts showed stability under similar reaction conditions as found in the previous work, and similar effectivity in xylose conversion (highest 61% FAL yield by WO3/SiO2).311 Poly(ethylene glycol)-bound sulfonic acid (PEG-OSO3H) was prepared and used for furfural production from pentoses. The furfural yield was as high as 75% from xylose in the presence of MnCl2 as a promoter for isomerization of xylose to xylulose at 120 °C. The catalyst was mild, nonvolatile, and noncorrosive and can be recycled multiple times (>10) without an intermediate regeneration

catalytic activity was in accordance with that of their total acid sites at the same catalysts’ weight. However, the selectivity of furfural from H3PW12O40 was lower than that of Amberlyst-15 and NKC-9 owing to the difference in the Brønsted to Lewis acid ratios of the three catalysts.308 Methylpropylsulfonic acid catalyst (MPrSO3H-MCM-41), which was prepared by a co-condensation method using dodecyltrimethylammonium bromide (DTAB) surfactant, produced outstanding catalytic performance, yielding FAL up to 93% at 155 °C for 2 h. However, the catalyst’s stability needed more development because the furfural selectivity rapidly decreased through the recycles.309 Softwood-derived hemicellulose as raw material gave 63% FAL yield using SAPO44 catalyst (silicoaluminophosphate) in a one-pot reaction. Dhepe and Bhamuik examined not only the physicochemical 533

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step, and no significant leaching of −OSO3H groups was observed. Noteworthy, this yield was higher than that obtained by sulfuric acid-catalyzed reactions.312 Carbonaceous solid acids could represent an important kind of heterogeneous catalyst for xylose conversion. Sulfonated TiO2 nanoparticles were prepared and characterized by AbuOmar et al. for the synthesis of FAL from xylose. The GluTsOH-Ti catalyst had Brønsted (-HSO3) and Lewis (TiO2) acid sites (density ratio: 1.2) for the isomerization and dehydration of xylose. The reactions were performed in water: MeTHF solvent mixtures providing 51% yield of FAL after 30 min at 180 °C. The catalyst was also reusable four times without significant loss of activity.313 Prominent FAL selectivity and conversion rates (highest: 100% and 98%, respectively; 150 °C for 3 h) of the xylose-to-furfural transformation by sulfonated polymer impregnated carbon composite (P−C−SO3H) was presented by Jain et al. The catalyst was recovered and reused many times without notable loss of activity.314 Ionic liquids could play a double role (catalyst and solvent) for FAL synthesis. Bogel-Łukasik applied [BMIM][HSO4] acidic ionic liquid for conversion of xylose. Reactions were carried out under homogeneous conditions and biphasic conditions using typical organic extraction solvents [toluene, methyl isobutyl ketone (MIBK), dioxane]. The best FAL yield (82%) with almost complete conversion was achieved in a biphasic system formed from dioxane and [BMIM][HSO4] at 140 °C.315 When the same solvent system was used as the reaction medium of the last step in the production of FAL from Eucalyptus globulus wood by Parajó, only moderate, 59% of the potential substrates (pentoses, oligo-, and polysaccharides) could be converted to FAL at the optimized conditions (160 °C for 4 h).316 In a water−toluene mixture, 71% product yield was detected by the use of [BMIM][HSO4] as catalyst for the same feedstock (140 °C, 6 h).317 Table 14 gives a comparison of catalytic conversions of different feedstocks including xylose as model substrate and “real” biomass resources to FAL. In water, the average yield of conversion of xylose is 27 wt %, which is much lower than water-containing biphasic system where higher than ca. 42 wt % could be achieved. In organic solvents, even under biphasic conditions, ca. two-times higher selectivity was detected at the same conversion rates. Because various homogeneous and heterogeneous catalysts operated under different reaction conditions, no clear conclusion could be stated as to which type represented better performance. In general, the applied temperature range is 120−200 °C. It is clear that generally rather low substrate concentrations were used, resulting in significant solvent need for the reaction. Although several active and selective catalysts were developed, these drawbacks dramatically increased the environmental factor (see section 3.4) of the technology. However, it should be emphasized that technology using FeCl3/acetic acid, steam, and seawater as the reaction medium, 63% feedstock load gave very promising yields of FAL and overcame some separation difficulties (entry 25). Accordingly, the product concentrations were also low, leading to serious separation issues, which have high energy needs (i.e., for distillation lowering sustainability). 2.4.1.3. Conclusions. Although the production of FAL from renewable resources is an industrial-scale process, intensive research activities were devoted to improve both catalysts’ activity and selectivity and process efficiency using well-know catalysts. From the sustainability point of view, instead of pure

subsrates used on laboratory scale generally, the application of complex lignocellulosic materials (i.e., hemicelluloserich waste streams) have to be preferred. It can be concluded that singlephase aqueous systems using either homogeneous or heterogeneous catalysts gave rather low (50%) efficiency due to in situ product extraction. Ionic liquids could also give high product yields. While the large scale applicability of biphasic systems is determined by the price of the cosolvent, the application of ionic liquids is strictly limited by their availability and their price. Another elegant approach is the application of combined solid acid catalysts having Lewis and Brønsted acidity that can accelerate one-pot pentose isomerization and conversion. Depending on structure, they could represent a robust system(s) and be a subject of scale-up. 2.4.2. Levulinic Acid (LA). Levulinic acid (LA) [CAS: 12376-2, Scheme 22], 5-ketovaleric acid, 4-oxopentenoic acid, is Scheme 22. Levulinic Acid

the simplest γ-oxocarboxylic acid, which was first observed as a product of sucrose degradation in the presence of mineral acids by Mulder in 1840.318 The substance was described and named levulinic acid by Grote in 1875319 and selected as a highpotential C5-building block for synthesis of value-added chemicals in the 1950’s.320 In 2004, Werpy and Petersen highlighted LA in the list of “TOP 12 platform chemicals.”53 Its increasing application, especially in pesticides, solvents, pharmaceuticals, food additive, and cosmetics industries is anticipated to boost growth in the coming years. Global LA market demand was 2606.2 tons in 2013 and is estimated to be 3820 tons by 2020. LA market volume share by its application was estimated as follows in 2013: pharmaceuticals 23.3%, agriculture 42.8%, cosmetics 13%, and food additive 21%.321 Its price was estimated at 5000 USD/ton in 2008; however, due to the increased capacity, the price has varied between 1000−3000 USD/ton since 2010. Because LA is a rehydrated product of 5-hydroxymethylfurfural (5-HMF, see section 2.5), it is not exclusively an IPC. However, there is no doubt concerning its importance in biorefinery concept indicated by a wide range of chemicals as LA derivatives (Figure 14)53,322,323 and increasing number of scientific papers focusing on the chemistry of LA (Figure 15). Furthermore, numerous synthesis routes are devoted to produce LA directly from biomass without isolation of 5HMF. In this section, the mechanistic aspect of formation of LA from 5-HMF and recent developments in its process chemistry are overviewed. 2.4.2.1. Mechanistic Aspects of Formation of Levulinic Acid. The acid-catalyzed conversion of carbohydrates, preferably C6-units to LA starts with the dehydration of carbohydrates to 5-HMF as an initial stable C6-species. However, depending on the reaction conditions, 5-HMF readily undergoes a rehydration reaction to form LA and formic acid under applied conditions, considering the presence 534

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Figure 14. Examples: chemicals derived from levulinic acid.

The first proposed mechanism by Horvat and Šunjić for conversion of 5-HMF to LA dates back to 1995.324,325 In accordance with their NMR studies, the reaction sequence of hydration of 5-HMF (Scheme 23) initiated by the addition of a water molecule to the C-2, C-3 olefinic bond (4,5-addition) of the furan ring in the presence of H+ to form an unstable tricarbonyl intermediate 2,5-dioxohex-3-enal, which readily decomposes to LA by elimination of a molecule of formic acid (hereinafter FA). Thus, LA and FA form in equimolar amounts in the reaction. Although intermediates drawn in brackets on Scheme 23 were not characterized, the presence of a tricarbonyl intermediate was indicated by its 13C NMR spectrum as follows (δ, ppm): C-1:26.8, C-2:207.5, C-3:128.8, C-4:138.7, C-5:209.8, and C-6:184.6. It should be noted that the 2,3-addition of a water to 5-HMF (Scheme 24) resulted in the formation of a 5-hydroxy-5-(hydroxymethyl)-4,5-dihydrofuran-2-carbaldehyde, which can turn to 2,5-dioxo-6-hydroxyhexanal having a reactive aldehyde group. The latter that was also evidenced by its 13C NMR spectrum could either be polymerized after isomerization or react further with 5-HMF. These aldol addition/condensation reactions (Scheme 25) were investigated in detail by Lund, concluding that humins formed from these reactions retain the furan ring and hydroxymethyl group of HMF but that the carbonyl group of HMF is not present.326 Weitz also probed the pathway of the HMF rehydration to LA by applying 13C-labeled 5-HMF produced

Figure 15. Number of publications on levulinic acid annually from January 2000 to December 2016. Source: Web of Science (keyword: levulinic acid).

of water and any kind of acid as an active catalyst of this transformation. The mechanistic aspects of formation of 5HMF from hexoses is discussed in section 2.5.1. This section focuses on the mechanisms for conversion of 5-HMF to LA, which has significantly less interest than that of 5-HMF formation from hexoses.

Scheme 23. Proposed Mechanism for the Conversion of 5-HMF to Levulinic Acid by Šunjića

a

Adapted and modified with permission from ref 324. Copyright 1985 Elsevier. 535

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Scheme 24. 2,3-Addition of Water to 5-HMF Proposed by Šunjić

Scheme 25. Aldol Addition and Condensation Reactions of 5-HMF and 2,5-Dioxo-5-hydroxy-hexanal

Scheme 26. Proposed Mechanism for Conversion of 5-HMF to Levulinic Acid by Pidko. Modified with permission from ref 327. Copyright 2012 Elsevier.

from 13C-labeled D-fructose in a mixture of DMSO-d6 and H2O (1:1) and for different catalysts such as Amberlyst-70 and H2SO4. It was demonstrated that the C-1 and C-6 carbons of 5HMF are mapped onto the carbon of FA and C-5 carbon of LA, respectively.432 Focusing on molecular mapping of conversion of 5-HMF to LA, Pidko’s laboratory performed a detailed DFT calculation involving numerous pathways to determine the most probable sequence.327 Of the evaluated 22 potential pathways, only three did not lead to the formation of LA and FA. The most favorable reaction route is depicted in Scheme 26. On the basis of this reaction pathway, the maximum theoretical yield of LA is 64.4 wt % from hexoses, 67.8 wt % from sucrose, and 71.6 wt % from cellulose. However, recently Leahy and co-workers revealed that FA was produced over the theoretical yield,328 which is consistent with previous results published for LA synthesis in various solvents.329−331 They found that FA contents compared with LA in the final reaction mixture obtained from different substrates were as follows: Dfructose, D-glucose, D-mannose, and D-galactose were shown to be 1.08 ± 0.05, 1.15 ± 0.08, 1.20 ± 0.10, and 1.19 ± 0.08, respectively. They suggested that there could be at least four potential pathways depending on reaction conditions which could be responsible for the excess FA, through furfuryl alcohol and furfural formation and through the transformation of Derythrose and pyruvaldehyde. To conclude, there are only a few studies focusing on the spectroscopic investigation of the conversion of 5-HMF to LA. Although several pathways and structure were calculated by a DFT study, there is no spectroscopic evidence for their presence under real reaction conditions. Concerning the humin formation in LA formation, two possible reasons have to be

considered as follows: (i) humins are formed during the acidcatalyzed dehydration of hexoses to 5-HMF (see section 2.5.1) and (ii) aldol addition and condensation reactions of 5-HMF via 2,5-dioxo-6-hydroxy-hexanal. 2.4.2.2. Process Chemistry of Levulinic Acid Production. LA can be obtained using different process chemistries (Scheme 27). The first one is a five-step petrochemical route.332,333 In this process, maleic acid is converted to its diethyl ester followed by an acylation of CC bond. The addition proceeds via a free radical mechanism and could yield up to 81%. Although good overall yields for LA (up to 80%) can be achieved, this process operated by DSM, Linz, Austria, at low (3 tons/day) scale. Moreover, it could not be sustainable on a large scale. Fermentation of glucose also results in the formation of LA via pyruvate and acetaldehyde as key intermediates.334 It should be noted that no overall yield from glucose was provided. The conversion furfural to furfuryl alcohol265,335 and subsequently to LA336 integrating the C5 (xylose) and C6 (glucose/fructose) reaction pathways is of importance, as well.337 It could associate xylose with the chemistry of cellulose-based compounds. Because of the relatively high prices of furfural (500−1500 USD/ton) and furfuryl alcohol (>1500 USD/ton), this approach could not be economically favorable.338 Obviously, the most important route for production of LA is an acid-catalyzed multistep conversion of polysaccharide content of biomass. It includes hydrolysis of polysaccharides to monosaccharides, dehydration of C6-units to 5-HMF, and its rehydration to LA and FA. While the synthesis of 5-HMF is preferably performed in organic solvents, ionic liquids, and other designer reaction media (see Section 2.5.2), water is the best solvent for LA synthesis. Because 5-HMF is an intermediate in the reaction 536

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Scheme 27. Production Routes of Levulinic Acid

grounded (size distribution 0.5−1 cm) raw materials are mixed with recycled diluted acid, (ii) acid hydrolysis, (iii) product concentration, where the water content of reaction mixture is adjusted as well as FA and furfural is recovered, (iv) LA and recycle acid separation, and finally (v) LA recovery, where the product is either further purified or converted to derivative product(s). The process is based on a continuous two-stage technology, in which the reactor system consists of a plug-flow and a stirred-tank reactor. To form a relatively high amount of 5-HMF, short residence time (ca. 12 s) and high temperature (210−220 °C) are used in the first stage. The second reactor is operated at slightly lower temperature (190−200 °C) with contact time ca. 20 min. On the basis of the hexose content of the carbohydrate-containing feedstock, the LA yield is >60%.339 Recently, Italian chemical firms have developed the production of LA from cellulosic feedstocks and estimates scale-up to 8000 tons by 2017.340 A detailed input−output analysis of various processes (i.e., Biofine process56 process developed by Shen and Wyman341 and Dumesic342) was performed by Huber.343 Comprehensive overviews on LA production can be found in reviews published by Rackemann,344 Dumont,345 Pandey,346 Yan,347 and Galletti.323 2.4.2.2.1. Conversion of Monosaccharides. To optimize reaction conditions further and maximize yields for LA production, preferably, hexoses (fructose and glucose) have been used as model substrates. Horváth’s group investigated the sulfuric acid catalyzed conversion of fructose and glucose and its 1:1 mixture to LA in GVL at 130 °C for 4 h. The yields of LA and FA from glucose were 51% and 56%, respectively.

sequence, the overall yields for LA are lower than those reported for 5-HMF. Furthermore, the process chemistry of LA production is operated at higher acid concentration, which results in the extensive formation of polymeric byproducts, socalled humins. In general, mineral acids such as HCl or H2SO4 give the highest efficiency. However, the application of aqueous HCl could result in serious environmental concerns. Accordingly, the manufacture of LA from lignocellulosic biomass, the “Biofine process” using sulfuric acid as a catalyst was developed in the 1980’s and commercialized in the 1990’s.56 The Biofine process leading to a commercial grade LA involves five steps (Figure 16): (i) feedstock preparation and mixing, in which the

Figure 16. Simplified flowchart of biofine process. Adapted with permission from ref 338. Copyright 2016 John Wiley and Sons. 537

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Table 15. Conversion of Fructose to Levulinic Acid Under Aqueous Conditions no. 1 3 4 2 5 6 7 8 a

solvent water water water water water water water:GVL water:[BMIM-SO3H] [HSO4] = 1:3

fructose conc (wt %)

catalyst

catalyst loading

T (°C)

t

conv (%)

select (%)

LA yield (%)

LA yield (wt %)

ref

5 5 5 9 6 4 3 2.5

H2SO4 H2SO4 HCl HCl 20%Nb/Al amberlyst-15 H2SO4 −

2 wt % 2M 2M 2M 10 wt % 6 wt % 7.3% −

150 170a 170a 95 190 120 130 95

2h 30 min 30 min 3h 10 min 24 h 4h 1h

100 na na 100 na 93 na 100

71 na na 63 na 56 na 70

71 42 49 63 74 52 68 70

46 28 32 41 48 33 44 45

328 350 350 389 356 361 329 370

Microwave irradiation.

with 1:1, 1:2, and 2:1 weight ratios of CrCl3 and HY zeolite were characterized, prepared by using a wetness impregnation method, and compared in the glucose-to-LA conversion. The experiments showed that hybrid 1:1 catalyst gave full conversion with outstanding LA selectivity (63%) at 160 °C after 180 min.354 Over homogeneous catalytic systems, the heterogeneouscatalyzed transformations were also developed to provide a possible recyclable catalyst. Wang and co-workers performed studies on the catalytic conversion of glucose applying a lysine functional heteropolyacid nanospheres (Ly0.5H2.5PW). Reactions were carried out in ChCl for 30 min at 130 °C providing 53% yield of LA by 75% conversion of glucose. The catalyst performed well under high substrate concentrations (67 wt %) and maintained its performance for six runs after washing.355 Nb/Al catalysts were synthesized by a coprecipitation method. Because of its large surface area and high acidity, 20%Nb/Al provided a high 74% yield of LA from fructose at 190 °C after 10 min and 48% yield from glucose at 200 °C after 15 min in water. The catalyst was reusable five times keeping almost the same activity.356 Yang et al. observed the conversion of glucose into LA utilizing Nb-containing catalysts. The catalysts were prepared by a hydrothermal synthesis method. Fe-doped niobium phosphate (Fe-NbP) showed the highest activity toward LA due to its high acid density and appropriate Brønsted/Lewis ratio, providing 64% yield at 180 °C after 3 h in water. The reaction conditions were optimized, and as a consequence, the catalyst maintained its good performance after four consecutive runs.357 An extended work of Rackemann et al. showed a comparison of methanesulfonic acid (MSA) and sulfuric acid-catalyzed conversions of glucose and xylose mixtures. Under rough reaction conditions, the MSA provided higher furfural yields than sulfuric acid. It was shown that fast heating rates led to maximal yields (>60%) of LA and furfural. They also examined the effects of the presence of both glucose and xylose in the mixture: LA yields were slightly higher when both monosaccharides were employed.358 Application of heterogeneous graphene oxide bearing the −SO3H group (GO−SO3H) catalytic system was investigated by Hwang. A high yield of LA (78%) was reached in aqueous solution after 2 h at 200 °C. The good performance of GO−SO3H is attributed to its layered morphology and sulfonate groups. The catalyst could be easily recycled.359 p-Toluenesulfonic acid (p-TSA) as catalyst was investigated by Xu for conversion of glucose and sulfonating the glucose. Higher water/p-TSA ratio provided more LA than sulfonated carbon. The highest LA yield was 53% at 170 °C after 7 h at 100% conversion of glucose.360

These yields were ca. 20% lower than obtained from fructose. Maximum yields from sucrose as a model disaccharide reached 52% LA and 57% FA, which were similar to those of a 1:1 mixture of fructose and glucose (53% LA and 58% FA). The latter results show that the acid-catalyzed hydrolysis of the glycosidic bond in sucrose is fast and “sucrose” behaves as the 1:1 mixture of fructose and glucose.329 Several combined catalyst systems containing Lewis acid, preferably metal(III) chlorides and mineral acid, were investigated for conversion of glucose to LA. A combination of homogeneous Lewis acids MCl3 (M = Cr, Cu, Fe, Al) and Brønsted acids (H3PO4, HCl, H2SO4) was studied by Shubin. Expectedly, the best LA selectivity (55.2%) was observed with 100% glucose conversion by coupling CrCl3 and H3PO4 as a mixed catalyst at 170 °C for 6 h. It was demonstrated that single Brønsted and Lewis-acid catalysts proved to be less efficient. A detailed kinetic model based on pseudo first-order kinetics was also developed, exhibiting good correlation between the calculations and the experimental data.348 Kinetic and catalytic investigations of Lewis and Brønsted-acid catalysts for the formation of LA from glucose were performed by Sandler and Vlachos. It was shown that Cr(III) as an active Lewis acid was responsible for isomerization of glucose to fructose as well as the combined Lewis and Brønsted-acid (HCl) catalysts performed both isomerization and dehydration/rehydration reactions. A Cr(III) speciation model in conjunction with kinetics results indicated that the hydrolyzed chromium complex [Cr(H2O)5OH]2+ played a key role in glucose isomerization. In water, 46% yield of LA was achieved under optimized conditions (140 °C, 350 min).349 It should be noted that comparable 49% and 48% yields were obtained in the absence of Lewis acid from fructose and glucose by the use of microwave-irradiation technique under optimized conditions (2 M H2SO4, 170 °C, 30 min), respectively.350 HMF and LA production from glucose by combining maleic acid (MA) with AlCl3 was presented by Mosier et al. With this combination, the reaction was 1.7-times faster than using the HCl−AlCl3 system; furthermore, the selectivity was also higher (3.3-times) by using the MA−AlCl3 combination.351 Amin developed a new hybrid catalyst comprising of zeolite and CrCl3. The novel catalyst had good catalytic effect on the conversion of glucose. After the optimization, 55.2% LA yield was achieved at 145.2 °C, 146.7 min by 12% catalyst loading.352 The same group performed the optimization of glucose conversion using an acidic ionic liquid, 1-sulfonic acid-3-methylimidazolium tetrachloroferrate(III) ([SMIM][FeCl4]). Reaction temperature, time, feedstock, and catalyst loading as variables were optimized observing 69% LA yield from glucose.353 In further developments, hybrid catalyst 538

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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Table 16. Conversion of Glucose to Levulinic Acid under Aqueous Conditions no.

a

solvent

glucose loading

1 2 3 4 5 6

water water water water water water

5 5 0.1 M 0.0555 M 0.0555 M 10 wt %

7 8 9 10 11 12 14 15 16

water water water water water water water:ChClb water:GVLc water:[BMIM][HSO4]

1 wt % 21 wt % 13 wt % 6 wt % 1 wt % 0.1 M 40 wt % 3 wt % 2 wt %

catalyst

catalyst conc

H2SO4 HCl H2SO4 H3PO4/CrCl3 H2SO4/FeCl3 HCl CrCl3 Fe-NbP pTSA GO-SO3H 20%Nb/Al SA-SO3H MSA Ly0.5H2.5PW H2SO4 [BMIM][HSO4]

2M 2M 0.25 M 0.02 M, 0.02 M 0.02 M, 0.02 M 0.1 M 3 wt % 0.5 wt % 21 wt % 0.2 wt % 10 wt % 1 wt % 0.25 M 0.03 M 7.3% 0.9 M

T (°C)

t

conv (%)

sel (%)

yield (%)

yield (wt %)

ref

170a 170a 200 170 170 140

30 min 30 min 30 min 240 min 240 min 350 min

na na na 100 75 96

na na na 54 48 48

40 48 45 54 36 46

26 31 29 35 23 30

350 350 358 348 348 349

180 170 200 190 180 200 130 130 145

3h 7h 2h 10 min 12 h 30 min 30 min 4h 104 min

99 100 89 na na na 75 na na

65 53 88 na na na 71 na na

64 53 78 48 61 51 53 51 71

41 34 50 31 39 33 34 34 46

357 360 359 356 376 358 355 329 378

Microwave irradiation. bChCl, choline chloride. cGVL, γ-valerolactone.

fractions of lignocellulosic biomass in water:GVL solvent mixture was demonstrated. Corn stover was applied as substrate. An outstanding 66% yield of LA was reached after 19 h in 20:80 v/v % water:GVL. It should be noted that high furfural yield (96%) could be observed from alkaline hydrogen peroxide pretreated corn stover.362 The same group extended this work for cellulose conversion by the use of amberlyst-70 catalyst. The reactions were carried out in GVL:water (9:1 wt/ wt) at 160 °C for 16 h, providing an excellent 69% yield of LA. Under the same conditions but in aqueous solution, low (20% yield) efficiency was obtained. The catalyst could be reactivated by washing with H2O2 solution. It is noteworthy that LA was upgraded to GVL without neutralization or purification.363 The production of LA in the presence of HCl in GVL under biphasic conditions was investigated as well. The aqueous solutions contained HCl and NaCl, converting cellulose to LA and FA. Partition coefficients for LA and FA varying between 1.5 and 4 for LA and FA depending on NaCl content were determined. Other solutes such as LiCl and NaNO3 also led to the formation of a biphasic system. However, low partition coefficients were observed for those. As a result, higher than 75% of the LA was extracted into the organic phase, and a maximum LA yield of 72% was reached using 1.25 M HCl after the first bolus of cellulose (2 wt %) at 155 °C for 1.5 h. The produced LA was converted into 4-hydroxyvaleric acid over Ru−Sn catalyst.364 The effects of alkali metal halides on direct production of LA from cellulose were investigated by Zhang et al. They found that high ion concentration played the main role in the microwave-assisted one-pot catalytic process rather than the ion type. High ionic strength and microwave irradiation jointly accelerated the reaction; the reaction time was reduced to 60 min, reaching 67.3% LA yield in H3PO4 (1.5 M) solution at 170 °C using NaCl.365 The possible role of solid heteropolyacid-type catalyst was investigated for cellulose degradation and synthesis of LA. Wang synthesized a series of new temperature-responsive heteropolyacids ([(CH3)3NCH2CH2OH]nH5−nAlW12O40 (n = 0−5) by a precipitation/ion exchange method for cellulose transformation. The most effective catalyst was ChH4AlW12O40 in water:MIBK solvent mixture. The choline and Al groups of

Tables 15 and 16 summarize the conversion of fructose and glucose to LA. When comparing these results with those obtained for 5-HMF, it is clear that yields of LA are generally lower. From a series of mineral acids, both HCl and H2SO4 were used. It should be noted that heterogeneous catalysts developed for cellulose conversion are preliminarily tested on model substrates, generally on glucose. 2.4.2.2.2. Conversion of Polysaccharides. Cellulose has been widely applied as a model polysaccharide for LA production. Because of the presence of hemicellulose fraction in the feedstock, formation of furfural has to be also taken into account. Because water is the preferred solvent for this reaction, numerous studies were focused on its application. Huber investigated a two-step cellulose-based process mimicking the Biofine process for LA production. In the first step, cellulose was converted into glucose and 5-HMF, and then in the second step, these compounds were converted into LA. The efficiencies of HCl, zirconium phosphate, and amberlyst-70 as catalysts were compared. Amberlyst-70 solid acid catalyst was applied for the reaction under aqueous conditions. The effects of substrate loading, reaction time, and reaction temperature were observed. Under optimum conditions, 34% yield of LA could be achieved at 160 °C after 6 h. Water-soluble organic compounds (i.e., fructose, glucose, cellobiose, levoglucosan, 5HMF, and furfural) were detected in the reaction mixture. Their distribution during the reaction could be significantly affected by reaction temperature. While lower temperature (190 °C) favored monosaccharide formation, high LA content was detected at 270 °C. The catalyst significantly lost its activity after further runs. Representative mass flow rates were also provided in the study.341 Synthesis of LA from polysaccharides using microwave (MW) heating was optimized. Cellulose, chitin, and chitosan were treated under conditions as follows, 2 M H2SO4, 170 °C, 50 min. The highest yield of LA achieved was 34% from cellulose. Less eco-friendly HCl is also an applicable catalyst, providing even higher yields. Glucosaminebased glycans were also tested for the formation of LA.350 Dumesic’s group extensively studied the conversion of cellulosic biomass to LA and its subsequent conversion to GVL under mono- and biphasic conditions. The sulfuric acidcatalyzed one-pot conversion of cellulose and hemicellulose 539

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Chemical Reviews

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the catalyst provide appropriate Lewis and Brønsted acidity. This catalytic system provided an excellent 75% yield of LA at 120 °C after 10 h. The catalyst was easily recoverable. The accelerating effect of microwave irradiation was also demonstrated.366 Similarly, 73% product yield was obtained under optimal conditions in water:sulfolane = 1:9 w/w mixture in the presence of H2SO4 at 180 °C for 1 h. It was shown that a lower amount of water increases the yield of furfural. The catalyst was reusable four times without significant loss of activity.367 A series of heteropolyacid ionic liquids [C4H6N2(CH2)3SO3H]3‑nHnPW12O40 ([MIMPSH]nH32PW12O40, n = 1, 2 3) were synthesized and used for the one-pot depolymerization of cellulose. The hydrolysis process was demonstrated to be efficient, and the conversion of cellulose and the yield of glucose reached 55.1% and 36.0%, respectively, at 140 °C for 5 h in a water:MIBK biphasic system. The [MIMPSH]2PW catalyst was able to produce a high amount of LA (63%) in the biphasic system at 140 °C for 12 h. The products and the catalysts were readily separated, and the recycled catalyst was reused in six runs for hydrolysis of cellulose without its activity loss.368 Liu and co-workers presented the selective cellulose conversion to LA using microwave irradiation in SO3H-functionalized ionic liquids (SFILs). Because of the acidity of the anions, HSO4− anion proved to have the best catalytic activity, and CH3SO3− and H2PO4− were less effective. 35−45% LA yield was observed in water at 160−170 °C for 30 min using [C3SO3HMIM][HSO4] as a catalyst.369 Saccharified biomass hydrolysate from corn stover was converted to LA through glucose, fructose, and HMF by Alipour. Glucose from the feed was isomerized into fructose (by GXI enzyme), and then it was transformed into LA in [BMIM-SO3H][HSO4] acidic ionic liquid and water without any additional acid catalyst. The highest yield of LA obtained was 70% at 95 °C after 1 h. The overall yield of the process was 58%. The catalyst was reusable for three consecutive runs providing the same yield.370 Wang et al. observed the conversion of cellulose into LA by several niobium-based solid-acid catalysts. The carbon-containing product distributions were monitored as a function of reaction temperature, revealing the optimum for LA at 180 °C.371 It is in good agreement with sulfuric acid catalyzed conversion of cellulose under MW irradiation.350 An outstanding 53% yield of LA was achieved by Al-NbOPO4 at 180 °C after 24 h in water. It was revealed that appropriate Lewis/ Brønsted acid ratio and acid strength had an important role in the process. Additional (TfO)3La, a strong Lewis acid, increased the selectivity toward LA. The catalyst was reused six times; however, after the fifth cycle, it had to be regenerated. Hydrothermal treatment of cellulose using ZrO2 catalyst was demonstrated by Joshi. Total cellulose conversion was observed at 180 °C after 3 h by applying 2 wt % catalyst and the same amount of cellulose. The catalyst’s selectivity and catalytic activity was maintained after recycling it five times. Formation of humins during cellulose hydrolysis and dehydration was observed and confirmed by NMR and FTIR analysis.372 Various polymers bearing acidic −SO3H functional groups as proposed separable acidic catalysts were synthesized and applied for cellulose conversion. The design method and the synthesis of substituted sulfonated hyperbranched poly(arylene oxindole)s (SHPAOs) was published by Sels. Among others, 5Cl-SHOPAOs was compared with SO3H-5-Cl-Isatin and SO3H-B3 under the same reaction conditions (165 °C, 5 h). The 5-Cl-SHOPAOs (Scheme 28) catalyst showed the highest

Scheme 28. Schematic Representation of Sulfonated Hyperbranched Poly(arylene Oxindole)s373

LA yield of 48%, but 5-F-SHPAOs, 5-Br-SHPAOs, and methylated 5-Cl-SHPAOs catalysts also reached excellent yields in the range of 38−47%. Using other biomass-derived carbohydrates such as 5-HMF, inulin, and starch as substrates were also investigated, and excellent results were obtained by applying the 5-Cl-SHPAOs.373 The same group reported a novel one-pot reaction pathway to the production of LA from microcrystalline cellulose. Initially, cellulose was converted to ethyl glucoside then ethyl levulinate in EtOH at 160 °C over 5-Cl-SHPAO. The second reaction step focused on the removal of EtOH followed by addition of water to hydrolyze the ethyl levulinate at 140 °C. Excellent 50−60% LA yields can be achieved in this two-step approach by the full conversion of cellulose. The catalyst’s recovery was carried out by ultrafiltration, and the recovered catalyst was reusable without significant activity loss.374 Sulfonated chloromethyl polystyrolene (CP) resin (CPSO3H-1.69) catalyst was also evaluated for direct conversion of microcrystalline cellulose to LA in an aqueous system. Excellent 65% yield was achieved by employing water:GVL = 9:1 wt/wt at 170 °C. The recycling experiments revealed that the catalyst’s activity was slightly decreased during cellobiose hydrolysis.375 Qi revealed an eco-friendly method for cellulose and glucose conversion using mesoporous cellulase-mimetic solid-acid catalyst. The results of the experiments showed that high 51% LA yield can be obtained in the pure water/solid-acid catalytic system from cellulose and remarkable yield of 61% LA, when glucose was used as the starting material. The reusability of SA-SO3H catalyst was also investigated. H2O2 solution was used after five runs and the regenerated catalyst had 95% of the initial catalytic activity.376 Over microcrystalline cellulose and high-cellulose-containing feedstock such as wheat straw, several lignocellulosic biomass and biomass wastes were utilized for LA production. Conversion of tomato plant waste to LA was tested by Cravotto in the presence of HCl. The reactions were performed for 2 min under MW irradiation and 40 bar N2 pressure in aqueous solution. The catalyst concentration (12−0.5 M) had insignificant effect on the yield. The highest yield of LA achieved was 63% by 1 M catalyst at 225 °C. Cellulose and chitosan were also used as substrates for the reaction.377 Oil palm fronds (OPF) conversion to LA using an acidic ionic liquid, 1-sulfonic acid-3-methylimidazolium tetrachloroferrate(III) ([SMIM]FeCl4]), was performed by Ramli et al. Reaction temperature, time, feedstock, and catalyst loading as variables were optimized observing 69% and 25% LA yields from glucose and OPF, respectively.353 Glucose and fructose content of kiwifruit waste residue could also be converted to LA by Nb/Al catalysts resulting in excellent (99%) and good (76.2%) conversion of glucose and fructose at 200 °C for 15 min, respectively. However, the final concentration of LA was rather low (8.5 mg L−1).356 Amin developed a new hybrid catalyst comprising of zeolite and CrCl3. The novel catalyst had good 540

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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Table 17. Synthesis of Levulinic Acid from Various Biomass Resources no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

water water water water water water water water water water water water water water water water water

18

22 23 24 25

water:MIBK = 10:1 (v/v) water:MIBK = 10:1 (v/v) water: sulfolane = 1:9 water:GVL = 9:1 GVL water water water

26

water

27 28

water water

29 30

water water

31

water

32 33 34 35

water water water water

36

water:GVL = 2:8 water:GVL = 1:9 water:GVL = 1:9 water water water

19 20 21

37 38 39 41 42 a

substrate conc (wt %)

catalyst

catalyst conc

T (°C)

t

conv (%)

select (%)

5-HMF sucrose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose ball-milled cellulose cellulose

5 4 4 29 8 2 5 2 4 2 2 3.5 8 6 2 1 1

H2SO4 CP-SO3H-1.69 H2SO4 amberlyst-70 [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] Al-NbOPO4 ZrO2 HCl 5-Cl-SHIPAOs methylated 5-Cl-SHIPAOs HCl H3PO4 (1.5 M) + NaCl H3PO4 (1.5M) + KCl amberlyst-70 SA-SO3H SA-SO3H

2 wt % 13 wt % 18 wt % 0.1 M 31 wt % 33 wt % 4 wt % 2 wt % 18 wt % 2 wt % 2 wt % 0.927 M 30 wt % 35 wt % 6 wt % 1 wt % 1 wt %

150 170 170a 160 160 MW 160 MW 180 180 170a 165 165 180 170 MW 170 MW 160 180 180

2h 1h 50 min 6h 30 min 30 min 24 h 3h 50 min 5h 5h 20 min 1h 1h 16 h 12 h 12 h

100 100 na 91 na na 95 100 na 100 100 100 na na na na na

84 49 na 37 na na 56 54 na 48 41 61 na na na na na

2

[MIMPSH]H2OW

4 wt %

140

12 h

na

cellulose

2

ChH4AlW12O40

4 wt %

120

7h

cellulose

7

H2SO4

0.7 wt %

180

cellulose

4

CP-SO3H-1.69

13 wt %

cellulose inulin starch sugar cane bagasse sugar cane bagasse giant reed tobacco chops paper sludge sweet sorghum juice waxy corn starch rice husks giant reed giant reed tomato plant waste corn stover

2 2 2 10

HCl (35 wt % NaCl) 5-Cl-SHIPAOs 5-Cl-SHIPAOs H2SO4

10

AHP corn stover cellulose

solvent

substrate

chitosan potato peel wheat straw mix

yield (wt %)

ref

84 49 34 34 45 55 53 54 46 48 41 61 67 61 19 46 52

77 17 24 24 32 39 38 39 33 34 29 44 48 44 14 33 37

328 375 350 343 369 369 371 372 350 373 373 392 365 365 363 376 376

na

63

45

368

99

76

75

54

366

1h

na

na

73

52

367

170

10 h

100

66

66

47

375

1.25 M 2 wt % 2 wt % 0.55 M

155 165 165 150

1.5 h 5h 7h 6h

na na na na

na na na na

72 na na 63

52 63 50 45

364 373 373 386

H2SO4

0.11 M

200

1h

na

na

50

36

386

7 7

cc HCl cc HCl

0.4 M 2 wt %

190 200

1h 30 min

na na

na na

62 59

44 42

379 382

7 −

cc HCl H2SO4

2 wt % 2M

200a 160a

15 min 30 min

na na

na na

56 na

40 31

382 383

1

HCl

2M

180

15 min

na

na

na

55

390

9 6 6 10

HCl cc. HCl HCl HCl

6 2 2 1

170 190 180 225a

60 min 1h 20 min 2 min

na na na 78

na na na 81

na na na 63

59 23 21 45

385 381 380 377

7

H2SO4

0.2 M

na

19 h

na

na

66

47

362

5

H2SO4

0.025 M

na

30 min

na

na

10

7

362

2

amberlyst-70

6 wt %

160

16 h

na

na

69

49

363

2 10 10

SnCl4·H2O H2SO4 H2SO4

2 wt % 2M 2M

200a 170a 170a

30 min 0.5 h 0.5 h

na na na

na na na

na 21 15.5

24 15 11

391 384 384

wt % wt % wt % M

yield (%)

Microwave irradiation.

[C4MIM][HSO4] ionic liquid as catalyst. 71% LA yield was

catalytic effect on the conversion of empty fruit bunch and kenaf. After the optimization, 55.2% LA yield was achieved at 145.2 °C and 146.7 min by 12% catalyst loading.352 Bamboo shoot shell (BSS) was also introduced into LA synthesis using

achieved by the optimized reaction conditions: 145 °C, 104 min, 0.9 M [C4MIM][HSO4], and 2 wt % of BSS.378 541

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hot water and enzymes. The chosen temperature range was 160−200 °C, while 3−90 min residence time was investigated. The results of both thermal pretreatment and sugar conversion (67% LA yield) offer promising opportunities for further utilization of sugar cane bagasse.388 LA was synthesized from corn stover by Alipour and Omidvarborna. Saccharified biomass hydrolysate was used as feed, and reactions were catalyzed by HCl. Glucose content of the biomass was converted to fructose first, and then fructose was further converted into LA. Under optimal conditions (95 °C, 3 h, water as solvent) 63% yield of LA was obtained.389 Three different cornstarches and the effects of different heatings were studied for the synthesis of LA by Dumont and Mukherjee. The substrates were normal, high-amylose and waxy cornstarches that were different in amylase/amylopectin ratios. The reactions were carried out in aqueous solution by HCl catalyst at various reaction times and temperatures. Microwave heating and oil bath produced almost the same yields, but the oil bath required a shorter reaction time. LA yield of 55 wt % was obtained after 15 min at 180 °C from waxy cornstarch. All three substrates gave similar yields.390 The hydrolysis of chitosan as an animal origin model substrate was studied in the arresting work of Kerton et al. Using SnCl4·H2O 24 wt % LA yield was achieved in water solvent under microwave irradiation at 200 °C for 30 min. Other substrates such as glucosamine hydrochloride and chitin were treated similarly; 13 wt % and approximately 32 wt % LA were produced, respectively.391 Table 17 provides an overview of the synthesis of LA from various biomass resources. It can be concluded from multiple publications that high-cellulose-containing biomass sources are the most widely studied substrates yielding LA up to ca. 50% with high conversion (>90%) rates depending on reaction temperature, which was optimized by several studies as 150− 190 °C for water. However, the average yields were ca. 38 wt %. It is in excellent agreement with that obtained for fructose (39 wt %, Table 15) and those of glucose (35 wt %, Table 16) taking into account versatility of catalysts and reaction conditions. Although in designer reaction media, the efficiency could be increased and the complex solvent mixtures could result in difficulties for isolation of LA. For the conversions of lignocellulosic biomass residues, comparable productivities were reported. It is evidenced by the presence of other convertible sugars (i.e., glucose content of feedstock). No significant differences can be distinguished between homogeneous mineral acid and well-designed heterogeneous catalysts. However, for the latter, the recyclability can easily be carried out without any loss of activity. The low substrate loading and significant amount of byproduct formation even in the case of full conversion with moderate selectivity resulted in notable waste generation and accordingly serious separation issues. The latter represents moderate sustainability assessed by their environmental factor (see section 3.4). 2.4.2.3. Conclusions. High-cellulose-containing biomass sources are proven to be excellent feedstocks of LA production. Although high conversion rates (>90%) were achieved in a temperature range of 150−190 °C using water as the reaction medium, average yields were ca. 38 wt % that is in correspondence with that obtained for pure model substrates (fructose, 39 wt %; glucose, 35 wt %). Several publications provide GC-yields only ignoring the quantity of auxiliary substances, missing mass balance, and making their results more impressive. Isolated yields, however, would better reflect

Galletti’s group extensively investigated the conversion of various high-cellulose-containing raw materials to LA. A nonfood-type feedstock, giant reed (Arundo donax L.) was successfully converted using HCl as a catalyst. After 1 h at 190 °C as an optimized temperature in water and N2 at 30 atm, 62% yield of LA could be reached, calculated on the cellulose content of the biomass. LA was hydrogenated toward GVL over a heterogeneous Ru catalyst.379 Concerning the harvest time of giant reed, no effect on the microwave-assisted conversion performed in the presence of diluted HCl was detected.380 The characterization of the solid residue after hydrothermal conversion of Arundo donax L. was completed. It was revealed that the solid contained aromatic hydroxyl groups, and carbonyl moieties deriving from lignin and carbohydrates degradation reactions, while the aliphatic hydroxyl and methoxyl groups were almost completely removed.381 A wide range of biomass (poplar sawdust, paper mill sludge, tobacco chops, wheat straw, and olive tree pruning) as substrate was tested for LA formation. Various reaction parameters were optimized, and microwave irradiation was applied. It should be noted that the feedstock was characterized by its cellulose, hemicellulose, lignin, and ash contents. The highest LA yield obtained was 59% from tobacco chops (25.0 wt % cellulose content) at 200 °C after 30 min in aqueous solution by Cl catalyst. Other substrates could produce comparable yields as well. They first utilized niobium phosphate catalyst for transformation of inulin and wheat straw.382 A study on the conversion of sweet sorghum (Sorghum bicolor) into LA by sulfuric acid catalyst was reported. Centrifugal separation was performed for removing insoluble materials. The influences of reaction conditions such as microwave irradiation time, catalyst dosage, reaction temperature, and pretreatment time were screened. The highest yield of LA was 31 wt % after 20 min pretreatment and 30 min MW irradiation at 160 °C in 2 M H2SO4. 5-Hydroxymethylfurfural could also be produced by this method.383 Microwave-assisted valorization of lignocellulosic wastes such as shells, sunflower husk, peels of vegetables, and fruits to LA was demonstrated. It was revealed that no significant difference was found between applying regular conventional and MW dielectric heating methods, but the reaction time could be significantly reduced to 30 min from the 8 h for the latter. The reached LA yields were in the range of 10−25%, depending on the waste material. The work also revealed that the influence of the water content had no effect on the LA yields, so the energy necessities of the drying process can be eliminated.384 When pressurized acid hydrolysis of rice husks into LA by HCl as catalyst was performed, 59% yield of LA was reached at 170 °C at 56 bar after 1 h in aqueous solution.385 Sugar cane bagasse was also evaluated as a feedstock for LA synthesis. Hayes investigated its conversion in water by H2SO4. The effects of reaction temperature (150−200 °C) and acid concentration (0.11−0.55 M) were tested. By 0.55 M sulfuric acid at 150 °C, LA yield of 63% was obtained, which is equal to 194 kg LA from 1 ton of dry sugar cane bagasse. A kinetic model for the reaction was also developed.386 Ackermann’s study revealed the effect of pretreatment on methanesulfonic acid-catalyzed hydrolysis. The pretreatment did not influence significantly the LA yields (70−80%); however, it affected the furfural’s production.387 Schmidt et al. also investigated the LA synthesis from fractionated sugar cane bagasse, focusing on the LA production’s integration into a biorefinery. The material’s fractionation was actually a thermal-enzymatic treatment using 542

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the efficiency of conversion processes. The low substrate loading and significant amount of byproduct formation even in the case of full conversion with moderate selectivity result in notable waste generation and accordingly serious separation issues. Concomitant humin formation (byproduct) cannot be avoided, so the utilization of humins has to be solved to increase viability of the conversion process. Efficiency could be increased by using designer reaction media and controversially could result in separation difficulties as well. No significant differences can be distinguished between homogeneous mineral acid and well-designed heterogeneous catalysts. Industrialization is a hot topic in current research trends: an economically viable process would use cheap mineral acid or robust heterogeneous catalyst operating in a cheap solvent. 2.4.3. Isoprene (IP). Isoprene (IP) [CAS: 78−79−5, Scheme 29], 2-methyl-1,3-butadiene, is the building block of Scheme 29. Isoprene

Figure 17. Utilization of isoprene.

Pentose-Phosphate Pathways (PPP) via glycerol-aldehyde-3phosphate (G-3-P) into pyruvate. In the case of the MEP module, these two metabolites are condensed to form 1-deoxyxylulose-5-phosphate (DXP), which is then reduced to form MEP. In the next step, MEP is converted into diphosphocytidyl-methylerythritol (CDP-ME), which is then reacted with CDP-ME to form CDP-MEP. The latter is converted into methylerythritol−cyclodiphosphate (MECPP), which is reduced in the next reaction to hydroxyl-methylbutenylpyrophosphate (HMBPP). In the last reduction, the precursor of dimethylallyl-diphosphate (DMAPP) is formed. In the last (third) isoprene-forming module, DMAPP is converted into isoprene, by different isoprene-synthase enzymes. If the middle module is changed to the MVA module then aceto-acetyl-CoA is arriving from the Feeding module and will be converted to hydroxy-methyl-glutaryl-CoA (HMG-CoA). This is then reduced to mevalonate. After three phosphorylation steps, DMAPP is obtained. This metabolism is strictly regulated by MECPP as the key metabolite.400 Finally, according to Scheme 30, the last module is the same in both pathways, meaning to have a common bottleneck of isoprene synthase (ispS) enzyme. Thus, for recombinant strain improvement, ispS (isolated from plants) is a major target. This is an enzyme with temperature optimum of 40−50 °C and pH optimum for 7.0−8.5, located in chloroplasts.401 Its Km values are quite high, which is considered as an evolutionary advantage resulting in a carbon flux mainly to essential physiology and not to isoprene synthesis.402 2.4.3.2. Biotechnological Production of Isoprene. While among the bioproducers bacteria, yeasts, and even plants can be found in nature, and the latter produce more than 600 million tons IP annually in their chloroplasts (probably for thylakoid membrane stabilization against thermal and oxidative stresses),398 currently isoprene is completely produced from crude oil with naphtha crackers in the C5 fraction. The global synthetic isoprene production was below 1 million ton per year in 2010.403 It should be noted that the huge isoprene amount released by plants are carbon-neutral in contrast to the petrolbased synthetic one. The highest yield natural producer is the common soil bacterium called Bacillus subtilis,404 producing IP

several natural products such as terpenes and biologically active compounds, for example vitamin-A and the steroid sex hormones.393 It was first synthesized in 1860 by Williams by the pyrolysis of natural rubber (NR).394 While its commercial importance was negligible until the Second Word War, the role of IP as a monomer of synthetic rubber changed by development of novel methods for obtaining isoprene from crude oil. Isoprene has been described as a C4 basic chemical with the potential of replacing the maleic anhydride platform.395 According to recent estimations, the annual world production of NR is ca. 9 million tons of which 70% is utilized for tires. NR production based on the rubber tree (Hevea brasiliensis) has several limitations because this plant has special living area and conditions.396 Most of the isoprene is utilized on a large scale in the polymer industry for manufacture of poly(cis-1,4-isoprene) so-called isoprene rubber as well as the second in the production of styrene−isoprene−styrene (SIS) block copolymers (Figure 17).393 Although the production of IP from renewables is not competitive with fossil-based IP and not commercially viable, several major companies, such as DuPont, Goodyear Tire, and Rubber Company and Bridgestone Corporation, just to name a few, profiled for global biobased isoprene. It was valued at 1.93 billion USD in 2015 and is projected to reach 2.96 billion USD by 2021. The price of IP was ca. 1500 USD/ton in 2015.397 2.4.3.1. Biochemical Pathways for IP production. In natural IP producers, two pathways can be distinguished for isoprene production as follows, mevalonate pathway (MVA) and methylerythritol-phosphate (MEP) (Scheme 30).398 While the MVA pathway is only used in eukaryotes, archaea, and cytosol of plants, the MEP pathway is widely applied in bacteria, cyanobacteria, microalgae, and plant plastis.399 Fortunately, at the same time, the MEP pathway is theoretically more efficient, resulting in higher IP yield on glucose, and it is energetically balanced in comparison with the MVA pathway, as well. Both of them are divided into the following parts. The feeding module converts the carbon sources via Embden− Meyerhof−Parnas (EMP) or Embden−Doudoroff (EDP) or 543

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discolor), and some bacteria (Bacillus subtilis, Escherichia coli, and Synechocystis) could act as sources of ispS (EC 4.2.3.27). All the bacterial hosts and additionally Saccharomyces cerevisiae (baker’s yeast) were improved via genetic modifications. Because the reached isoprene titer was in every case less than 0.01 g/L except for E. coli, according to a recent review of Yu and his co-workers,398 herein we focus on the latter. In the case of E. coli, the feeding module presented on Scheme 30 produces an imbalanced ratio between G-3-P and pyruvate, manufacturing much more pyruvate, suggesting G-3-P as a limiting factor.408 Therefore, one strategy is to enhance G3-P production via balancing the EMP feeding module or redirecting the metabolism to an alternative feeding pathway such as EDP.409 Equimolar production of G-3-P and pyruvate resulted in higher efficiency in carbon flux toward lycopene and isoprene, providing three times higher titer and six times higher yield for the latter (from 71.4 to 219.4 mg L−1). To improve the conversion of G-3-P and pyruvate to IP, the general regulatory role of DXP synthase (dxS) should be considered. The overexpression of dxS resulted in almost double IP concentration, or the heterologous expression of B. subtilis dxS increased IP concentration much more.410 Another strategy can be to supplement the metabolism of host E. coli having MEP pathway with an exogenous MVA pathway, which can help host cells overcome flux limitations.411 According to our knowledge, the best results were achieved by combination of bacterial enzymes from E. faecalis (converting acetyl-CoA to MVA) and yeast-originated enzymes (converting MVA to DMAPP) to obtain a hybrid MVA and MEP combined path supplemented with ispS of Populus alba (i.e., white poplar). After optimizing fermentation, 60 g L−1 isoprene was reached in a continuous cultivation with a productivity of 2 g L−1 h−1 and yield of 11% from glucose. Although it is far away from sustainable production, the dramatic influence of gene engineering is clearly demonstrated. This is the basis of the patent of Danisco and Goodyear Tire and Rubber Company.412 Isoprene has a high vapor pressure at the temperature of the fermentation, therefore, unlike other fermentative products, IP will leave the fermenter in the off-gas. The contaminants are the rest of the oxygen, nitrogen passing inertly through the fermentation, furthermore the formed CO2, and finally the humidity. Bioisoprene can be separated by two methods:403 either by adsorption or by solvent extraction. Adsorption by activated carbon can remove IP until the carbon load reaches 10−15 wt % and will result in a purity of 99.5%. By fortune, catalyst poisons of the polymerization (other C5 molecules like cyclopentadiene etc.) are not among the residual impurities. IP desorption can be carried out by either steam or nitrogen, followed by condensation. 2.4.3.3. Conclusions. In conclusion, while several microorganisms are able to ferment and release isoprene, the most effective biotechnological isoprene producers are plants. Thus, for biorefinery purposes, plant genes must be implemented into any microbial hosts resulting in gene-modified host organism. There are several strategies to overcome biochemical and metabolic bottlenecks, and Danisco and Goodyear is near to industrialization having 60 g L−1 final concentration and more than 2 g L−1 h−1 productivity.

Scheme 30. Isoprene Producing Natural MEP and MVA Pathwaysa

a Adapted with permission from ref 398. Copyright 2016 Elsevier. Enzymes: dxR, DXP reductoisomerase; dxS, DXP synthase; ispD, DXP-ME synthase; ispE, CDP-ME kinase; ispF, MECPP synthase; ispG, HMBPP synthase; ispH, HMBPP reductase; idi, IPP isomerase; ERG10, acetyl-coA acetyl transferase; HMGS, HMG-CoA synthase; HMGR, HMG-CoA reductase; MK, mevalonate kinase; PMK, mevalonate-5-phosphate kinase; MVD, mevalonate-5-diphosphate decarboxylase; IDI1, IPP isomerase; ispS, isoprene synthase. Pathways: EMP, Embden-Meyerhof pathway; EDP, Entner-Doudoroff pathway; PPP, pentose phosphate pathway; MEP pathway, 2-C-methyl-Derythritol 4-phosphate pathway; MVA pathway, mevalonate pathway. Pathway intermediates: G-3-P: glyceraldehyde-3-phosphate, DXP, 1deoxy-D-xylulose 5-phosphate; MEP, 2-C-methyl-D-erythritol 4phosphate; CDP-ME, 4-diphosphocytidyl-2C-methyl-D-erythritol; CDP-MEP, 4-diphosphocytidyl-2-C-methyl-D-erythritol 2-phosphate; MECPP, 2-C-methyl-D-erythritol 2,4-cyclopyrophosphate; HMBPP, 1hydroxy-2-methyl-2-(E)-butenyl 4-pyrophosphate; IPP, isopentenyl pyrophosphate; DMAPP, dimethylallyl pyrophosphate; Ac-CoA, acetyl-CoA; AcAc-CoA, acetoacetyl-CoA; HMG-CoA, 3-hydroxy-3methylglutaryl coenzyme-A; PMev, mevalonate 5-phosphate; PPMev, mevalonate pyrophosphate.

via the MEP pathway.405 Its ispS has a pH optimum of 6.2 without requiring a high amount of divalent cations.406 On the basis of Brenda Enzyme Database,407 several plants (Arabidopsis, Campylopus introflexus, Casuarina, Eucalyptus, Ficus septica, Ipomoea batatas, Mucuna pruriens, Nicotiana tabacum, Populus sp, Pueraria monatana, Puertus petraea, and Salix

2.5. 5-Hydroxymethylfurfural (5-HMF) as a C6-Basic Chemical

5-Hydroxymethylfurfural (5-HMF) [CAS: 67−47−0], 5(hydroxymethyl)-2-furaldehyde, is a natural substance that 544

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Figure 18. 5-HMF as a platform chemical.

2016.424 The current price of 5-HMF is estimated between 500 and 1500 USD/kg, which is 3 orders of magnitude higher than that of currently utilized fossil-based bulk chemicals. A price around 1 USD/kg would be economically acceptable. The most effective protocol to obtain 5-HMF is the multistep acid-catalyzed dehydration of hexoses, preferably fructose, glucose, as well as cellulose, representing the C6fraction of biomass. In this section, the mechanistic aspects of formation of 5HMF from fructose and glucose as well as recent achievements in its process chemistry are reviewed. The mechanistic studies on hexose formation from polysaccharides is not our subject herein. 2.5.1. Mechanistic Aspects of Formation of 5-HMF. 2.5.1.1. Conversion of Fructose. While numerous studies were devoted to convert monosaccharides, preferably D-fructose or D-glucose as C6-units of biomass, to 5-HMF, no clear conclusion on the mechanistic investigations have been proposed yet. However, the molecular mapping of transformation of hexoses is fundamentally important to understand the formation of byproducts such as humins. Moreover, the molecular level understanding of the reactions is crucial to increase the efficiency and could help to design more efficient catalytic systems that may be applied on industrial scales. This paragraph deals with the mechanistic aspects of dehydration of hexoses to 5-HMF focusing on background and recent results. In general, three possible mechanisms have been proposed for transformation of hexoses, preferably D-fructose and Dglucose. The first and most widely discussed transformation involves the consecutive removal of three water molecules from corresponding hexoses to form 5-HMF. The second one is possible via the Maillard reaction of hexoses in the presence of amino acids and amines. The third and most widely discussed route is direct dehydration of hexoses to 5-HMF, which could take place via either acyclic intermediates416,425−428 or cyclic intermediates.429 The proposed acyclic conversion pathway involves the isomerization of D-fructose, D-glucose, and D-mannose via 1,2enediol, a key species of the mechanism, which can undergo three consecutive dehydration steps to form 5-HMF. The reaction sequence involves the formation of 3-deoxyglucos-2-

occurs in foods such as honey, vegetables, coffee, and other beverages in small amounts and is one of the constituents of aroma in liqueurs.413 The first publication regarding 5-HMF can be dated back to 1895.414 It was identified as a highpotential initial C6-platform chemical serving biomass-based alternatives for polymers, pharmaceuticals, agrochemicals, flavors and fragrances, macro- and heterocycles, and natural products as well as it could be a precursor for fuel components.415 The most important 5-HMF-based chemicals are summarized on Figure 18. The significant interest in using 5-HMF is clearly evidenced by increasing numbers of publications on its production and utilization (Figure 19) in

Figure 19. Number of publications on 5-HMF annually from January 2000 to December 2016. Source: Web of Science (keyword: 5hydroxymethylfurfural).

the last 16 years. Its importance can also be indicated by the number of reviews that were published on its role in biomassbased production of chemicals and fuels (e.g., by Kuster,416 Heeres and de Vries,417 Palkovits,418 Gallezot,15 Afonso,419 Zhang,420 Shanks,421 Zhang,422 and Raghavan.423) The global market for 5-HMF is expected to reach about 123279 million USD by 2022 from 116750 million USD in 545

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Scheme 31. Acyclic Reaction Pathway of Dehydration of Hexoses to 5-HMF as Revisited by Moreaua

a

Modified with permission from ref 427. Copyright 1996 Elsevier.

deoxyglucosene completing the final dehydration step toward 5-HMF could result in the formation of furfural.416,427 The cyclic conversion pathway (Scheme 33) is proposed to be generally initiated by a dehydration of D-fructofuranose at the C-2 position to form a tertiary carbenium cation followed by subsequent β-dehydration resulting in (2R,3S,4S)-2(hydroxymethyl)-5-(hydroxyl-methylene)tetrahydrofuran-3,4diol (3,4-DIOL). 3,4-DIOL turns via a subsequent dehydration to (4S,5R)-4-hydroxy-5-(hydroxymethyl)-4,5-dihydrofuran-2carbaldehyde (carbaldehyde-5), which can readily be dehydrated to 5-HMF.425,428,429 It is important to emphasize that most of the earlier proposed mechanisms were referred to catalytic systems operating in aqueous condition and the reaction media could change the mechanism. Because a recent comprehensive review discussed mechanistic aspects in detail,417 only a few studies have been reported on deeper insights of the cyclic pathway of fructose dehydration. Horváth and co-workers investigated the conversion mechanism using the 13C isotope labeling technique in DMSO. Identification and characterization of the intermediates of the cyclic pathway by assigning the corresponding 13C NMR peaks (Scheme 34) were reported.431 It was first established that all five isomers of D-fructose were detected in the solution, and the five-membered ring fructosyl oxocarbenium ion that formed via protonation and dehydration of D-fructofuranose could undergo deprotonation to form either 3,4-DIOL or 2,6anhydro-β-D-fructofuranose. While carbaldehyde-5 can easily be

ene (Scheme 31), which is present in equilibrium with its tautomer form 3-deoxy-D-glucosone (3-DG). This conversion route is the so-called “ene-diol mechanism.” Recently, Bols and co-workers investigated and compared the formation of 5-HMF from D-fructose, D-glucose, and 3-DG. When 3-DG was used as a substrate, significantly higher reaction rate was observed.430 Consequently, it is hard to exclude 3-DG from the conversion pathway. It should be noted that depending on the type of acid, 3-DG was also found to be an excellent starting material for preparation of chloromethylfurfural and bromomethylfurfural (Scheme 32). For these halogenated furans, 1- and 2-orders of magnitude reaction rates were detected compared with rate of formation of 5-HMF. In addition, the isomerization of 1,2enediol to 2,3-enediol opens a way to form 2-hydroxyacetylfuran by triple dehydration, as well as the decarbonylation of 3,4Scheme 32. Conversion of 3-Deoxy-D-Glucosone to Corresponding Substituted Furfural

546

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Scheme 33. Generally Proposed Cyclic Pathway of Dehydration of Fructose to 5-HMF

Scheme 34. Proposed Intermediates of Acid-Catalyzed Dehydration of D-Fructose to 5-HMF via Cyclic Pathwaya

a

Adapted with permission from ref 431. Copyright 2012 Royal Society of Chemistry.

HMF, the second one leads to the formation of oligomers and humins. Concerning the cyclic pathway, Hensen et al. performed detailed DFT calculations for Brønsted-acid catalyzed dehydration of D-fructose considering more than 100 potential reaction paths. They concluded that the hydroxyl group at the anomeric carbon in α-D-glucopyranose and β-Dfructofuranose (O1−H and O2−H, respectively) shows the highest affinity toward protons. Water removal from these sites is facile and resulted in the formation of activated carbocationic sugar intermediates.327 Because the starting species of the cyclic reaction pathway is D-fructose, it can be supposed that isomerization of other hexoses such as D-glucose to form Dfructose proceeds prior to the dehydration sequence. In situ NMR studies by Weitz using 13C-1-D-fructose and 13 C-6-D-fructose verified that fructose C-1 forms the formyl group and C-6 forms hydroxymethyl carbon of 5-HMF (Scheme 35), which fit both acyclic and cyclic mechanisms.

dehydrated to 5-HMF, the 2,6-anhydro-β-D-fructofuranose as a key species in the reaction mixture could also form a sixmembered ring fructopyranosil oxocarbenium ion as a starting species of difructose dianhydrides (DFAs) and (3S,4R,5R)-2(hydroxymethylene)-tetrahydro-2H-pyran-3,4,5-triol (3,4,5TRIOL), which then turns to (3R,4S)-3,4-dihydroxy-3,4dihydro-2H-pyran-6-carbaldehyde (carbaldehyde-6) via dehydration. It is important to note that 3,4,5-TRIOL can also be derived from the D-fructopyranose form. The utilization of D2O as deuterium source in the reaction mixture showed that every step after the initial dehydration was irreversible, making the acyclic pathway highly unlikely. Consequently, the equilibrium reactions between the fructose isomers, five- and six-membered oxocarbenium cations, and the presence of 2,6-anhydro-β-Dfructofuranose divide the conversion pathway into two parallel directions. While the first one, which is in excellent agreement with the proposed cyclic pathway, leads to the formation of 5547

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Scheme 35. Conversion of Fructose to 5-HMF and Levulinic and Formic Acidsa

a

Adapted from ref 432. Copyright 2012 American Chemical Society.

Scheme 36. Noncatalytic Conversion of D-Fructose to 5-HMF in DMSOa

a

Adapted from ref 433. Copyright 2013 American Chemical Society.

These labeled carbon atoms were subsequently assigned in hydrated products of 5-HMF as well. Both water and DMSO were applied as a reaction medium in the presence of mineral (H2SO4) or solid (amberlyst-70 and PO43−/niobic acid) acids as catalysts. When PO43−/niobic acid was used in water, furfural was detected in the final reaction mixture. However, it was established that furfural is not a reaction product from 5HMF.432 In DMSO, no furfural formation was detected. It is important that the population of fructose’s isomers in the selected solvent at different temperatures could be crucial for the final yield of 5-HMF. When determination of the effect of temperature on fructose isomers’ distribution, the effect of different solvents (DMSO, MeOH, and H2O) were compared by Matubayasi,433 the following tendencies were observed: (i) The D-fructopyranose form decreased dramatically; (ii) the fivemembered D-fructofuranose was the major product even at higher temperatures, which fits the results reported by Amarasekara;434 and (iii) the concentration of D-fructofuranose is significantly higher in DMSO than in H2O and MeOH. This is in accordance with the observation of solvent selection for enhanced synthesis of 5-HMF from fructose.496 The effect of solvent on acid-free dehydration of D-fructose to 5-HMF in DMSO (Scheme 36), water, and methanol was also investigated. In DMSO, 3,4-dihydroxy-2-dihydroxymethyl-5hydroxymethyltetrahydrofuran having no double bond was identified as a new intermediate prior to carbaldehyde-5, and the outstanding (95%) yield of 5-HMF was obtained at 90 °C for 5 h. Although neither 3,4-dihydroxy-2-dihydroxymethyl-5hydroxymethyltetrahydrofuran nor carbaldehyde-5 can be detected in water, their presence also cannot be excluded. In accordance with their structure, they were not considered stable enough in the protic reaction environment on the NMR time scale.433 Expectedly, LA and FA were also detected in water. In methanol, three anhydrosugars (Scheme 37) were identified as products representing completely different product distribution compared with DMSO and water. Promising results by Amarasekara also proved that DMSO acts both as a catalyst and as a solvent and carbaldehyde-5 was identified by 1H- and 13C NMR.434 These results are fully

Scheme 37. Identified Anhydrosugars from D-Fructose in Methanol

consistent with the generally proposed cyclic mechanism depicted in Scheme 33, where water presented as a solvent. Weitz et al. also reported a combined theoretical and experimental study of the acid-catalyzed dehydration of Dfructose in DMSO applying different catalysts.435 The 13C, 1H, and 17O isotope-labeling technique established that DMSO played a key role in the overall mechanism through the formation of key intermediate (2-(hydroxydimethylsulfinyloxy)-β-D-fructofuranose) (Scheme 38). By this interaction, the formation of humins can be reduced compared with the catalytic system operating in water under identical conditions. Although the experimental methods could not distinguish 3,4DIOL and 4-dihydroxy-5-hydroxymethyltetrahydrofuran-2-carbaldehyde, the results of theoretical calculations indicated that both forms were at local minima on the potential energy surface as well as the keto form having a lower free energy than that of enol. Besides the application of conventional organic solvents, a significant interest toward the use of alternative solvents such as ionic liquids436 or DES systems in biomass conversion processes has been seen. Consequently, detailed mechanistic studies were performed on fructose and glucose either in the presence or absence of a catalyst to explore the possible role of these solvents in the transformation. The influence of halides X− (X preferably Cl) components of ionic liquids, which are usually used as catalysts or reaction media for fructose conversion, was investigated by Raines.437 To account for the significant influence of halides on conversion and selectivity, two possible reactions of a fivemembered ring fructosyl oxocarbenium ion were proposed as 548

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Scheme 38. Proposed Role of DMSO in 5-HMF Formationa

a

Adapted with permission from ref 435. Copyright 2016 Elsevier.

Scheme 39. Putative Nucleophilic and Basic Mechanisms for Halide Participation for Transformation of Fructose to 5-HMFa

a

Adapted from ref 437. Copyright 2009 American Chemical Society.

Scheme 40. Proposed Tautomeric Equilibrium of Fructose in ZnCl2 Molten Salta

a

Adapted from ref 438. Copyright 2014 American Chemical Society.

follows, first X− reacts with oxocarbenium ion leading to the formation of a 2-deoxy-2-halo intermediate, which is less prone to side reactions and subsequently turns to 3,4-DIOL (Scheme 39, nucleophile path). On the other hand, X− could form 3,4DIOL simply via deprotonation of C-1 (Scheme 39, base path). The conversion of fructose in ZnCl2 molten salt medium and its synergistic effect with Sn(IV) as Lewis acid were investigated by Hou.438 The tautomeric distribution of D-fructose was significantly affected by the concentration of ZnCl2 according to the 13C NMR spectra as well as the changes in chemical shifts suggesting the formation of a β-pyranose-Zn(II) adduct (Scheme 40). It was also revealed that concentrations of furanose forms were significantly decreased with increased salt concentration; meanwhile, the pyranose tautomers were increased significantly corresponding to the humin formation. Huang and co-workers studied the N-methyl-2-pyrrolidinium chloride [NMP][Cl] catalyzed conversion of D-fructose to 5HMF in DES systems formed from [EMIM][Cl] and C1−C4 aliphatic alcohols.439 It was shown that the polarity of an

alcohol and its stereostructure were major factors influencing the yield of 5-HMF, which was 74% in the presence of isopropanol as the optimal solvent. The 1H NMR studies established the presence of two key intermediates: 4-dihydroxy5-hydroxymethyltetrahydrofuran-2-carbaldehyde and carbaldehyde-5. It was proposed that alcohol molecules could form a hydrogen bond between [HNMP]+ and D-fructose [Scheme 41 (a)] and between [EMIM]+, [HNMP]+, and 4-dihydroxy-5hydroxymethyltetrahydrofuran-2-carbaldehyde (Scheme 41b). Otherwise, the sequence of intermediates corresponded to the general cyclic mechanism (Scheme 33). Noteworthy, the presence of 4-dihydroxy-5-hydroxymethyltetrahydrofuran-2carbaldehyde was verified by a peak at 5.35 ppm and carbaldehyde-5 by a peak at 6.27 ppm. The concentrations of these intermediates were found to be strongly dependent on the type of alcohol as well as the presence of [EMIM][Cl]. It indicated that the alcohols played a key role in the formation and transformation of intermediates during the D-fructose dehydration. It was also verified by the final yield of 5-HMF 549

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glucose anion or its degradation by nucleophile attack of OH− has to be considered. Applying deuterated D-glucose at the C-2 position (2-[D]-glucose) showed kobs = 4.9 × 10−4 s−1 that corresponded to kinetic isotope effect kH/kD = 3.8. Accordingly, C-2 hydrogen plays a substantial role in the conversion of the opened form of glucose. Moreover, isotope labeling revealed competitive reactions of the open form of glucose, which could lead to the formation of byproducts. It was shown that significantly higher conversion, fructose yield, and selectivity (47%, 30%, and 64%, respectively) were obtained for 2-[H]glucose than with 2-[D ]-glucose (36%, 7%, and 20%, respectively). The proposed mechanism based on isotope labeling and kinetic measurements is depicted in Scheme 43. The mechanism involves the following steps: (i) ionization of cyclic glucose to form acyclic glucose anion, (ii) removal of a hydrogen atom to form enediol intermediate, (iii) formation of fructose anion in opened form, and (iv) ring closing and proton removal from water to establish the cyclic fructose. Two possible mechanisms could be considered to explain the formation of the enediol intermediate. The first could be a bimolecular deprotonation with [OH]− followed by reprotonation from water according to Scheme 42. The second one is

Scheme 41. Proposed Hydrogen Bonds Between Alcohol, [HNMP]+, and Intermediates of D-Fructose Conversion

that the effect of solvents decreases in the following order: isopropanol > 2-butanol > propanol > ethanol > methanol. 2.5.1.2. Conversion of Glucose. Beyond fructose, other aldohexoses (D-glucose, D-mannose, and D-galactose) and aldopentoses (D-xylose, L-arabinose, and D-ribose) as readily available components of biomass and biomass wastes could also be used as feedstocks for production of 5-HMF. However, isomerization reactions are proposed to be a prior step for their transformation to 5-HMF. The reactions can be catalyzed by both Brønsted bases, Lewis acids, as well as in the presence of immobilized enzymes. The equilibrium limited transformation (Keq ≈ 1, at 25 °C) catalyzed by an immobilized enzyme (xylose isomerase) is slightly endothermic (ΔHr = 3 kJ mol−1)440 and utilized on an industrial scale (ca. 8.5 × 106 tons in 2015 in the USA)441 for the production of highfructose-containing syrup. The equilibrium-based, maximum thermodynamic yield of fructose is 57.4% at 100 °C.442 It is important to note that typical yields of 5-HMF from fructose are superior to those obtained from glucose or other hexoses under the same reaction conditions. Palkovits,443 Bai,444 and Davis445 reviewed a broad range of both homogeneous and heterogeneous chemocatalytic isomerization of glucose, including the application of inorganic hydroxides, organic bases, zeolites, hydrotalcites, anion exchange resins, and Lewis acids. Herein, we focus on the main aspects and recent achievements in mechanistic studies of isomerization of glucose, as a much cheaper feedstock for fructose production. Bases were first utilized as catalysts for the isomerization of carbohydrates, including glucose. The reaction is called the Lobry de Bruyn−Alberda van Ekenstein transformation discovered in 1895,447 in which the isomerization of an aldose species could lead to the formation of a ketose as well as an epimeric form of starting species (Scheme 46). The formation of an ene−diol intermediate has been considered as a key step of the reaction, which has directed research attention toward developing a more active catalyst system for a long while. The recent developments of the base-catalyzed carbohydrate isomerization were summarized by Palkovits.443 Although inorganic bases [e.g., NaOH or Ca(OH)2] are cheap and available in large quantities, due to the low isomerization rate, their efficiency is rather low. Water is primarily the solvent of carbohydrates. Therefore, detailed mechanistic investigations were devoted to understanding the isomerization mechanism under aqueous conditions. An extensive mechanistic and kinetic study on the organic Brønsted base-catalyzed isomerization in the temperature range of 80−120 °C and pH 9.5−11.5 was reported by Tessonnier.446 A detailed reaction pathway was mapped, including an isomerization sequence and possible degradation routes. For the latter, the thermal decomposition of acyclic

Scheme 42. Base-Catalyzed Isomerization of D-Glucose

based on an intramolecular hydrogen shift from C-2 atom to O5 atom (Scheme 43). Tessonnier’s study concluded that none of them could be excluded during the conversion sequence. DFT studies performed by Faungnawakij showed the effect of catalysts on the reaction mechanism and energy profile of glucose isomerization in hot compressed water. The ring opening of β-glucose was predicted as the rate-limiting step in the noncatalytic system, while the tautomerization or the Hshift was proposed as the rate-limiting step in both basic and acidic conditions.448 For the latter, the reactivity of the basic catalyst was shown to be lower than that of acidic systems. The 550

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Scheme 43. Proposed Mechanism for Amine-Catalyzed Isomerization of Glucose to Fructose Under Aqueous Conditionsa

a

Adapted from ref 446. Copyright 2015 American Chemical Society.

Scheme 44. Sn-beta-Catalyzed Isomerization of 2-[D]-Glucose to Fructosea

a

Adapted with permission from ref 451. Copyright 2010 Wiley-VCH.

2 positions, initiating a 1,2-hydride shift. The in situ27 Al-NMR verified the presence of coordinated Al species throughout the reaction. An obvious kinetic isotope effect (kH/kD = 2.7 ± 0.3) was shown by the use of 2-[D]-glucose, confirming the intramolecular hydride shift from C-2 to C-1, which was proposed as the rate-limiting step of isomerization. By comparing the activity of Cr(III) and Sn(IV), the following order was observed: CrCl3 > AlCl3 > SnCl4. The activation energy of the Al-catalyzed reaction was estimated to be 110 ± 2 kJ mol−1 at 100−130 °C.452 Vlachos and co-workers investigated the isomerization of glucose to fructose in the presence of a combined catalyst containing a Lewis acid (CrCl 3 ) and a Brønsted acid (HCl) under aqueous conditions.349,453 The kinetic results indicated that the aqua complex, [Cr(H2O)5OH]2+, was the most active species in the isomerization. They additionally showed a complex interaction between the two types of catalyst as follows: (i) due to the decreased concentration of [Cr(H2O)5OH]2+ in the presence of HCl, Brønsted acidity retards the isomerization, (ii) Lewis acidity accelerated the side reaction, which was indicated by increased fructose consumption, (iii) in the absence of HCl, the hydrolysis of Cr(III) decreased the pH of the reaction mixture; consequently, this intrinsic Brønsted acidity readily catalyzed both dehydration of fructose to 5-HMF and its rehydration to LA. Mushrif utilized Car−Parrinello molecular dynamics to obtain deeper insights into the mechanism. In accordance with experimental results, it was verified that the Cr(III)-catalyzed isomerization took place via a 1,2-hydride shift, and the first step of isomerization was [Cr(C6H12O6)(H2O)3OH]2+-assisted deprotonation of glucose.454 The kinetic isotope effects (kH/

activation energies of the rate-limiting steps for noncatalytic, acidic, and basic systems were found to be 184.09, 157.82, and 53.59 kJ mol−1, respectively. Heeres reported interconversion of different ketoses (fructose, sorbose, and tagatose) and aldoses (glucose, mannose, and galactose), involving kinetic and mechanistic investigations via aqueous dehydration. They also concluded that an isomerization of ketose would be required prior to dehydration.449 Davis and co-workers first demonstrated the conversion of glucose to fructose over Sn silicate with β-zeolites topology, socalled Sn-beta, exhibiting Lewis acidity.450 Mechanistic studies were also performed on Sn-beta catalyzed glucose isomerization under aqueous conditions using 2-[D]-glucose. The 1H- and 13 C NMR spectra showed that the deuterium atom of 2-[D]glucose transferred to the C-1 position of fructose. Therefore, it could be assumed that the glucose isomerization reaction with a solid Lewis-acid catalyst in pure water proceeded by an intramolecular 1,2-hydride shift. In addition, the conversion of 2-[D]-glucose compared with unlabeled substrate revealed a 2fold decrease in the initial reaction rate (kH/kD = 1.98), showing a considerable kinetic isotopic effect.451 The proposed mechanism (Scheme 44) is similar to that of metal-salt catalyzed reaction. Concerning water-soluble Lewis acids, a proposed mechanism for AlCl3-catalyzed isomerization was reported by Tang et al. The ESI−MS/MS measurements established that the [Al(OH)2(aq)]+ complex contributed to the isomerization as well as it accelerated the ring-opening process of glucose. The latter observation was based on ATR-IR. The binding of acyclic glucose with [Al(OH)2(aq)]+ species occurs at the O-1 and O551

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Scheme 45. Proposed Mechanism of Isomerization of Glucose Into Fructose in the Presence of Water-Soluble Lewis acids. M = Cr, Al. Modified from Ref 443

Scheme 46. Proposed Mechanism for Chromium(II)-Catalyzed Isomerization of Glucose to Fructose in an Ionic Liquida

a

Adapted with permission from ref 458. Copyright 2010 Wiley-VCH.

preferably in ionic liquids.457−460 Hensen and co-workers performed detailed DFT and EXAFS studies. They demonstrated that the distorted tetrahedral [CrCl4]2− anion was the reactive species in the chromium-catalyzed glucose dehydration in [EMIM][Cl].458 It was revealed that the ring opening of glucose was catalyzed by a mononuclear Cr complex as well as the isomerization to fructose proceeds via a binuclear Cr complex (Scheme 46).459,460 The formation of the latter was proposed to be crucial for the hydrogen transfer (H-shift) for the isomerization of glucose to fructose, and therefore, it is generally referred to as the “1,2-hydride-shift mechanism.” Raines reported mechanistic insights on the conversion of 2-

kD ) were estimated for Cr(III)- and Al(III)-catalyzed conversion of 2-[D]-glucose as 1.77 ± 0.1 and 1.71 ± 0.1, respectively.455 These are comparable with that determined by Labinger for Sn-beta.451 Consequently, a metal-salt-catalyzed reaction path operating under aqueous conditions was proposed (Scheme 45). Since the seminal paper published by Zhang and coworkers,456 in which the uniquely efficient chromium(II) chloride assisted conversion of glucose yielding 70% of 5HMF with 95% conversion was demonstrated, several studies have focused on the mechanistic investigation of the Lewisacid-catalyzed isomerization in alternative reaction media, 552

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proved that the addition of ethylene glycol, which could form a five-membered chelate ring with Sn, resulted in an inhibition effect. In the presence of ethanol and 1,3-propanediol, no influence on the conversion was detected.463 Zhao et al. performed similar mechanistic studies on germanium(IV)assisted conversion of glucose to 5-HMF applying D-glucose2-13C revealing a similar reaction sequence to that reported by Han.464 Hafnium(IV) chloride-assisted conversion of both fructose and glucose to 5-HMF was reported by Zhang and co-workers using [BMIM][Cl] as the solvent.465 Expectedly, high 5-HMF yields were obtained in the Cl−-containing ILs [i.e., [RMIM][Cl] (R = butyl, hexyl, octyl)]. It should be noted that the replacement of Cl − by OAc − resulted in comparable conversion; however, a dramatic decrease in 5-HMF yield from fructose could be observed. When [BMIM][Cl] and [BMIM][Br] were used, 76.7% and 75% yields were obtained, respectively. This influence could be explained by the possible side reactions between the imidazolium ring and fructose induced by a base OAc−.466 The study also concluded that [HfCln](n−4)− (n = 5, 6), which formed from HfCl4 and [BMIM][Cl], acted as a catalytically active species for both glucose isomerization, including formation of β-D-glucose from α-D-glucose to fructose via a 1,2-hydride-shift step and subsequently fructose dehydration to 5-HMF (Scheme 49). A similar sequence for glucose isomerization was proposed by Ma in the presence of Lewis acids FeCl3, CrCl3, and AlCl3 in [BMIM][Cl].467 As opposed to 1,2-hydride shift-containing mechanisms for conversion of glucose to fructose, a new approach was reported by Riisager and Fristrup using boric acid as a promoter in an ionic liquid.468 DFT studies showed that the glucose to fructose isomerization in the absence of any catalyst was energetically unfavored by 10.4 kJ mol−1. When transformation was assisted by a monocoordinated boric acid, the overall isomerization was energetically favored by 14 kJ mol−1. Beyond theoretical calculations, the conversion of 2-[D]-glucose was also performed. If the reaction took place via a 1,2-hydride mechanism, the deuterium labeling would completely appear at the C-2 position of fructose, consequently, 50−50% at C-1 position of 5-HMF (Scheme 50) according to Weitz’s conclusion.432 Unexpectedly, less than 5% of deuterium was incorporated into the formyl group (C-1 position) of 5-HMF. This observation was compatible with the “ene-diol mecha-

[D]-glucose to 5-HMF. The isotope labeling established that ca. 33% of deuterium incorporated into 5-HMF by a chromiumcatalyzed conversion. It is consistent with a mechanism involving a 1,2-hydride shift (Scheme 47).461 Scheme 47. Proposed 1,2-Hydrogen Shift of the Isomerization of Glucose to Fructosea

a

Adapted with permission from ref 458. Copyright 2010 Wiley-VCH.

A subsequent study from Hensen demonstrated that Cr3+ showed higher activity and selectivity toward 5-HMF than that obtained for Cr2+. This was explained by higher Lewis acidity of Cr3+, which could result in a more stable negative intermediate formed during the isomerization of glucose to fructose.460 The same group extended the detailed mechanistic study to copperand iron-containing systems as well.462 While neither fructose nor glucose were converted by the FeCl3-containing system, the copper exhibited similar activity compared with chromiumbased systems. However, the selectivity of the CuCl2 catalyst was negligible. The interaction of O1 oxygen of the glucose initializing its ring opening via mononuclear complexes is necessary for both selective and nonselective transformations in the presence of Cr2+ and Cu2+ chlorides, respectively. For Cr2+, the reactive metal chloride species are able to coordinate directly to the substrate at the O-1 site. However, calculations proved that coordination of Cu2+ and the substrate to form the copper(II) chloride complex was unfavorable. The nonselective conversion of glucose is catalyzed by a mobile Cl− ligand of [CuCl4]2− complex that could deprotonate a glucose at the O-1 site and initialize its nonselective conversion to form byproducts (Scheme 48). Han and co-workers introduced the common Lewis acid SnCl4 in 5-HMF production in ionic liquid [EMIM][BF4]. They suggested that the formation of a five-membered chelate ring containing the Sn atom with the two neighboring hydroxyl groups of glucose could play an important role in the isomerization via the 1,2-hydride-shift mechanism. It was

Scheme 48. Proposed Reaction Routes for Metal(II) Activation of Glucosea

a

Adapted with permission from ref 462. Copyright 2012 Wiley-VCH. 553

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Scheme 49. Proposed Reaction Routes for Hf(II)-Catalyzed Isomerization of Glucosea

a

Adapted with permission from ref 465. Copyright 2012 Wiley-VCH.

Scheme 50. Deuterium Incorporation During Fructose Isomerization and Its Subsequent Dehydration to 5-HMF

Scheme 51. Boron Acid-Catalyzed Isomerization of Glucose to Fructosea

a

Adapted with permission from ref 468. Copyright 2011 Wiley-VCH.

42% from glucose and as much as 66% from sucrose was obtained in [EMIM][Cl] at 120 °C for 3 h. The isomerization of glucose to fructose prior to dehydration was treated as an essential step, but this theory first changed by Bols et al.430 Their experiments established the formation of 5HMF from 3-DG, which could be derived from dehydration of

nism,” in which all deuterium from C-2 was discarded. It was concluded that borate ionic-liquid-mediated isomerization of fructose to glucose proceeded via different routes than that reported for either enzyme469 or generally accepted Lewis-acid catalyzed reaction. The proposed isomerization mechanism is depicted in Scheme 51. By applying B(OH)3, a notable yield of 554

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Scheme 52. Proposed Mechanism for Conversion of Glucose to 5-HMF by Bols et al.430

It also has to be considered that the solvent could play an important role in the reaction mechanism. For example, DMSO and ionic liquids are not only present as solvents but also play active roles as catalysts. The isomerization of glucose to fructose as an initial step in the reaction sequence of the transformation is currently accepted. However, it represents a great challenge because the isomerization is a base-catalyzed reaction and dehydration proceeds in the presence of acid. To solve this challenge, significant work has been devoted to developing bifunctional catalyst systems. 2.5.2. Process Chemistry for Conversion of C6Monomers to 5-HMF. The process chemistry of production of 5-HMF contains multistep dehydration of fructose as discussed by points of different pathways in section 2.5.1. In a short summary, the synthesis of 5-HMF before the 1980’s almost exclusively focused on the homogeneous acid-catalyzed transformation of C6-monomers in water as a commonly and frequently utilized solvent for carbohydrate chemistry. However, in the past three decades, both common organic (DMSO, DMF, DMA, alcohols, etc.) and alternative solvents such as ionic liquids or their proper combination have become the focus of interest, which usually operate under biphasic conditions and could provide an excellent opportunity to immobilize the catalyst species as well as prevent the product from subsequent reaction (i.e., rehydration to LA).417 In general, recently more than 90% of studies as basic research activities were focused on the conversion of D-fructose and Dglucose as model substrates for development of efficient catalytic systems, including solvents screening, catalyst preparation and characterization, optimization of reaction conditions, product separation, and investigation of possible catalyst recycling. Accordingly, the recent developments representing good to excellent yields of 5-HMF are overviewed in the sections as follows: dehydration reactions in water (section 2.5.2.1), in organic solvents (section 2.5.2.2), in organic/aqueous mixtures involving biphasic systems (section 2.5.2.3), and ionic liquids (section 2.5.2.4). The process chemistry of 5-HMF production has clearly become the most widely studied area of carbohydrate conversion, as indicated by the large number of publications annually. 2.5.2.1. Fructose and Glucose Dehydration in Water. Mineral acid, preferably HCl, catalyzed conversion of D-fructose to 5-HMF can be dated back to 1977. Since the pioneer work of Kuster and co-workers showed that the transformation resulted in moderate yield of 5-HMF (ca. 20%), many efforts

glucose. When the relative rates of the 5-HMF formation from glucose, fructose, and 3-DG were compared, the following order was determined: glucose:fructose:3-DG = 1:22:61. The proposed mechanism is depicted in Scheme 52. It is important to emphasize that their conclusions were also verified by theoretical calculations. The DFT studies by Faungnawakij showed the effect of catalysts on the reaction mechanism and energy profile of glucose isomerization. The ring opening of β-glucose was predicted as the rate-limiting step in the noncatalytic system, while the tautomerization or the H-shift was proposed as the rate-limiting step in both basic and acidic conditions, respectively.448 Horváth investigated the Brønsted-acid (H2SO4) mechanism of conversion of glucose to 5-HMF using isotope labeling technique in GVL. When 2-[13C]-glucose was converted, three intermediates: 1,6-anhydro-β-D-glucofuranose, 1,6-anhydro-βD-glucopyranose, and levoglucosenone (Scheme 53) were detected in the reaction mixture.329 Scheme 53. Identified Intermediates of Glucose Conversion in γ-Valerolactone

2.5.1.3. Conclusion. In spite of several theoretical calculations and experimental investigations by the use of isotope 2 H-, 13C-, 17O-labeling techniques, there are several uncertainties regarding the hexose dehydration sequence. In general, only limited evidence has been provided for both (cyclic and acyclic) reaction networks. It should be emphasized that due to the intrinsic time scales of analytical techniques, the detection of most exciting/unrevealed intermediates can strongly be limited and has still remained a great challenge. On the other hand, the first dehydration step could be proposed as rate limiting, resulting in further complications in the detection/ characterization of all subsequent steps, which proceed much faster. Despite these difficulties, several important intermediates were successfully characterized, revealing that a cyclic mechanism could be more likely. 555

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Scheme 54. Proposed Formic Acid Assisted Conversion of Fructose472

surface properties in terms of Brønsted sites and Lewis-acid sites.476 When niobium phosphates (NbPO) bearing a lot of Lewis and Brønsted-acid sites were used, 45% product yield was obtained from fructose with NbPO-pH2-type species at 130 °C for 30 min.477 The selectivities of SiO2, Al2O3, ZrPO4, Nb2O5, MOR zeolite, and amberlyst-15 were tested in water, showing that the activity in the transformation of fructose correlates with the strength of the Lewis-acid sites of the catalysts as follows, Nb2O5 > ZrPO4 > Al2O3 > SiO2−Al2O3. The most effective catalysts were MOR and amberlyst-15, representing selectivity of ca. 60%.478 Furthermore, it was revealed that Brønsted acidity increased the selectivity toward 5-HMF; however, Lewis acidity had the opposite effect as indicated by intensive humin formation. A similar observation was reported for mesoporous AlSiO catalyst, which was efficient for conversion of other substrates (e.g., sucrose, cellobiose, and cellulose). AlSiO-20 with a Si/Al ratio of 18 produced a moderate 63% yield of HMF at 160 °C for 90 min.479 By applying Nb2O5/CeO2 mixed oxides in which the Nb was the most active component, the highest conversion of fructose was 82%, while the selectivity was just 48%. The number of strong acid sites per surface area was increased with the increasing amount of Nb2O5.480 The stability of the Nb2O5· nH2O catalyst was improved by applying Ba2+ and Nd3+ as doping ions, and a detailed kinetic study was reported by Cartini. The kinetic rate constants of deactivation were determined and associated with a faster (0.004 < kdeact,1/h−1 < 0.058) and a slower (0.001 < kdeact,2/h−1 < 0.005) deactivation phenomenon.481 The layered HNb3O8 could also convert fructose with a yield of 56% in water at 155 °C and high substrate/catalyst ratio. This system worked with other carbohydrates, such as glucose, inulin, and sucrose, as well.482 A cetyltrimethylammonium salt of Cr(III)-substituted polyoxometalate (C16H3PW11CrO39) was tested by Wang et al. for the dehydration of fructose and glucose. The highest yields in water were 41% from fructose and 35% from glucose after 2 h at 130 °C. The catalytic system maintained its activity after six consecutive runs.483 Another PCP(Cr)-SO3H·Cr(III) bearing both Lewis and Brønsted-acid sites solid catalysts were used for conversion of glucose providing 81% product yield under the aqueous phase at 180 °C for 4 h. After the reaction, THF was added as an extracting agent to establish the catalyst recycling. After regeneration, the loss of Cr was insignificant and the catalyst was reusable five times.484 Similar activity was reported for bifunctional solid Brønsted acid based Cr(III)PSFSI-MSMA15/SiO2 and Cr(III)-PDVB-0.3-SSFBI catalyst. The latter gave ca. 95% conversion of fructose with 60% selectivity for 7 h, and due to the water-tolerant PhSO2NHSO2C4F9 groups, it showed excellent recyclability (12 cycles without significant activity loss).485 Auroux et al. investigated the transformation of fructose into 5-HMF by tungstated zirconia oxides. The reactions were

were attempted to increase the efficiency that were comprehensively summarized up to 2013 by de Vries and Heeres.417 Heeres et al. presented an excellent detailed kinetic study for the dehydration of fructose into 5-HMF and LA by H2SO4 catalyst in water.470 According to their model, the maximum attainable HMF yield in the experimental window was 56 mol % (Cfructose = 0.1 M; Csulfuric acid = 0.005 M, 166 °C), which is close to the highest experimental value within the range (53 mol %) and considerably higher than that reported for Dglucose. Afonso et al. developed a special way of producing 5-HMF. First, part of the glucose was isomerized into fructose in an enzyme-catalyzed reaction in wet [TEA][Br]. Then HNO3 was used as s catalyst yielding 91% of 5-HMF at 80 °C for 15 min.471 Hanefeld proposed a sustainable autocatalytic production of 5-HMF from fructose using FA as a species that is formed from the rehydration reaction of 5-HMF to LA and could act as a catalyst under aqueous conditions (Scheme 54). The optimized system gave 47% of 5-HMF at 200 °C for 5 min. Expectedly, the addition of FA at the beginning of the transformation dramatically increased the humin formation.472 Essayem performed a systematic study on dehydration of fructose and glucose in neat hot water (T = 25−250 °C) as well as in the presence of a series of mineral and solid acid or basic catalysts at 150 °C.473 The yield showed a maximum of 28 mol % at 220 °C. As a function of pH, the conversions were much higher in the range of 2.1−1, and the maximum of 5-HMF yield was determined for pH = 1.5. It is in excellent accordance with Horváth’s observation.431 It was shown that some inorganic anions (Cl−, Br−, I−, and NO3−) hardly accelerated the rate of fructose decomposition, but the organic acid anions (acetate, oxalate, formate, and benzoate) did. However, they lowered the selectivity toward 5-HMF to 25%.474 Heterogeneous catalysts having Lewis and Brønsted-acidic characteristics have been widely used for these purposes. Faungnawakij investigated four phosphate-based catalysts, which showed good activity in fructose and glucose dehydration giving HMF yields of 20−21% and 34−39%, respectively. The yields of glucose and HMF from hydrolysis/ dehydration of cellulose were highest at 34% over αSr(PO3)2.475 When H3PO3 and NaOH were used, comparable conversions were achieved; however, the yields for the latter were slightly lower. 448 The optimal temperature was determined as 220 °C. The same group developed, characterized, and utilized nanostructured copper hydrogen phosphate monohydrate and copper pyrophosphate for catalyzing dehydration of fructose in hot compressed water at 200 °C. The Cu2P2O7 catalysts with weak acid strength (+3.3 ≤ H0 ≤ + 4.8) were highly active and selective toward 5-HMF with a yield of 36%, while no metal leaching was detected after the conversion. Expectedly, their catalytic activity was related to 556

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Table 18. Dehydration of Fructose to 5-HMF in Watera

a

no.

fructose conc (wt %)

catalyst

catalyst loading

T (°C)

time

conv (%)

select (%)

yield (%)

yield (wt %)

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

9 30 30 9 10 9 6.5 1.8 1 1 1 4.8 9 9 9 5.6 5.6 30 6 6 6 7 7 10 10 10 9 9 7 9 1 23 10 6 23

− − − H3PO4 H3PO4:NaH2PO4 NaOH H2SO4 H2SO4 lactic acid acetic acid formic acid InCl3 CaP2O6 α-Sr(PO3)2 α-Cu2P2O7 HSO3-fiber H2PO3-fiber levulinic acid amberlyst-15 MOR zeolite Al2O3 NbPO-pH2 NbPO-pH7 HNb3O8+MI HNb3O8 HNb3O8+MI AgSTA AgSTA Ct2 CuCl2·2H2O 16.8-WO3/ZrO2 C16H3CrPW11 5M41S550 H-MOR (C16)H4PW11Ti

− − − 0.1 M 0.5 M:0.5 M 0.1 M 0.1 M 1M 50% 50% 20% 0.15 wt % 1 wt % 1 wt % 1 wt % 8.6% 8.6% 50 n% 1 wt % 1 wt % 1 wt % 7 wt % 7 wt % 0.2 wt % 0.2 wt % 2 wt % 10 wt % 10 wt % 0.1 wt % 10 wt % 1.3 wt % 0.1 mmol 0.3 wt % 1.3 wt % 0.9 wt %

200 200 190 200 150 200 120 166 150 150 150 180 200 200 200 120 120 190 135 135 135 130 130 155 155 155 120 140 170 120 130 130 170 165 130

5 min 50 min 40 min 5 min 30 min 5 min 60 min 200 min 2h 2h 2h 15 min 5 min 5 min 5 min 6h 6h 40 min na na na 30 min 30 min 18 min 12 min 18 min 120 min 120 min 4.5 h 120 min 4h 90 min 300 min 5h 90 min

94 94 70 93 92 81 70 na 96 84 99+ 100 82 88 82 72 79 93 19 5 25 58 68 96 92 88 98 100 100 82 na 90 59 85 100

22 50 61 30 68 24 50 na 45 58 53 79 40 44 44 47 34 53 60 47 15 78 50 58 51 49 87 69 60 23 na 45 96 40 76

21 47 43 28 63 20 35 56 63 47 53 79 34 39 36 34 27 49 na na na 45 34 56 47 43 86 69 60 19 12 40 57 34 76

14 33 43 20 44 14 25 39 44 33 37 55 24 27 25 24 19 34 na na na 32 24 39 33 30 60 48 42 13 8 28 40 24 53

475 472 472 475 493 448 449 470 473 473 473 494 475 475 476 489 489 472 478 478 478 477 477 482 482 482 488 488 486 488 487 483 495 594 605

Yield (%) refers to mol %.

performed with various WO3 contents representing different acidic sites. It was also confirmed that the ratio of acidic to basic sites of the catalyst was the key parameter affecting selectivity, while the conversion of fructose was mainly related to the presence of acidic sites of a given strength 150 > Qdiff > 100 kJ mol−1 NH3 h−1. Similar results were reported for different recyclable carbon catalysts (Ct2) bearing oxygen- and sulfurcontaining groups.486 At optimum, 12% yield of 5-HMF was obtained by 16.8-WO3/ZrO2 at 130 °C for 4 h in water.487 The Ag-exchanged modified acid sites containing silicotungstic acid (AgSTA) operating in superheated water was also applied for conversion of the C6 model substrates. The highest yield of 5HMF was 86% from fructose and 63% from sucrose. The catalytic system kept its good performance after eight runs.488 Fructose decomposition to 5-HMF in the presence of HSO3/ H2PO3-grafted polyethylene fiber was performed by Oyola. Because of the presence of sulfonic and phosphonic groups on the catalyst, it had a strong interaction with the water. The reactions were carried out at 120 °C and could achieve a moderate 5-HMF yield. The method reduces environmental risk as it does not need any toxic organic solvents, and the catalysts could be recycled.489

Zhao et al. revealed a one-pot transformation method for glucose using an immobilized thermophilic glucose isomerase enzyme with a base (−NH2) functionalized mesoporous silica (aminopropyl-FMS) for isomerization and combined it with a solid-acid (−SO3H) functionalized mesoporous silica (propylsulfonic acid-FMS) for dehydration. By this system, 61% fructose and 30% 5-HMF yield could be achieved.490 Microwave-assisted conversion of glucose by Cu-containing aluminosilicates was demonstrated by Romero. Although the selectivity of 5-HMF was moderate after short reaction times (2−5 min), remarkable selectivities of reduced product of 5HMF could be reached (85%) after 10−30 min. The study also proved that even a small amount of Zn (0.2 wt %) increased the selectivity significantly.491 Qiao and Hou et al. presented the results of the direct conversion of chitin biomass to 5-HMF in concentrated ZnCl2 aqueous solution. 99% of the D-glucosamine (GlcNH2) was converted successfully to 5-HMF, and the reaction gave 21.9% 5-HMF yield at 120 °C for 90 min in 67 wt % ZnCl2 solution. Other cocatalysts, such as CdCl2, CuCl2, and NH4Cl had no effect on the 5-HMF yields; however, positive effects were observed when AlCl3 and B(OH)3 were employed.492 557

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Table 19. Conversion of Glucose to 5-HMF in Watera

a

no.

glucose conc (wt %)

catalyst

catalyst loading

T (°C)

t

conv (%)

select (%)

yield (%)

yield (wt %)

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14

9 1 1 9 9 10 9 10 10 10 9 12 10 1

− − HCl H3PO4 NaOH H3PO4:NaH2PO4 α-Sr(PO3)2 FeCl3 6H2O AlCl3 CrCl3 6H2O CaP2O6 C16H3CrPW11 20 wt % Sn-VPO PCP(Cr)-SO3H· Cr(III)

− − pH = 1 0.1 M 0.1 M 0.5 M: 0.5 M 1 wt % 1 wt % 1 wt % 1 wt % 1 wt % 0.1 mmol 0.5 wt % 0.7 wt %

200 220 180 200 200 150 200 120 120 120 200 130 100 180

5 min 2h 2h 5 min 5 min 90 5 min 6h 5h 2h 5 min 2h 6h 4h

41 68 47 68 41 50 61 na na na 71 84 52 na

15 38 9 13 12 20 34 na na na 28 42 38 na

6 26 4 9 5 10 21 2 11 13 20 35 20 50

4 18 3 6 4 7 15 1.4 7.7 9.1 14 25 14 35

475 473 473 475 448 493 475 467 467 467 475 483 510 484

Yield (%) refers to mol %.

outstanding conversions were achieved with yield of 19−42% using TiO2 nanoparticles.499 Noteworthy, when protic alcohols were used, the catalyst system can be fine-tuned for production of 5-HMF-derived ethers and LA esters. The TiO2 catalyst was successfully recycled for five consecutive runs. Dumesic performed excellent biomass-derived solvent screening to identify the best reaction medium for production of 5-HMF. THF, MeTHF, or its mixture, as well as γ-lactones were examined in the presence of amberlyst-70 heterogeneous catalyst. From fructose, >71% product yields were obtained for GVL, GHL, and THF at 130 °C for 10 min. The best catalytic performance for glucose was obtained in the presence of combined amberlyst-70 and Sn-β as solid-acid catalysts were yields of 55, 59, 60, and 63% using GHL, GVL, THF:MTHF = 1:1, and THF as solvents, respectively.500 When THF was used, the product separation could be achieved by distillation. In contrast to GVL and GHL having relatively high boiling points, the product could be extracted by cyclopentane. Cheap and readily available Lewis-acid catalysts (i.e., FeCl3· 6H2O, CrCl3·6H2O, and AlCl3) were evaluated for dehydration of glucose in water and DMSO, as well as kinetic studies were performed by Ma.467 The effects of solvent/catalyst on the performance of the reaction were investigated at 100 and 130 °C. The 5-HMF yield in the different solvents followed a decreasing order as DMSO > > H2O and for catalysts by CrCl3· 6H2O > AlCl3 > FeCl3·6H2O. It was found that the initial glucose concentration has a significant effect on product formation, showing an optimum at 10 wt % for the chromiumbased system. The optimal temperature and reaction time were determined between 120 and 130 °C and 30 and 480 min, respectively, resulting in product yield of 54.4% in DMSO with CrCl3·6H2O at 130 °C for 480 min and 52% with AlCl3 at 120 °C for 240 min. By the use of SnCl4 in the presence of [TBA][Br], good 5-HMF yield of 69% was also obtained from glucose under similar conditions. This catalytic system is also efficient with other substrates, such as fructose, sucrose, inulin, and starch.501 Huang et al. utilized heteropolyacid salt catalysts in DMSO.502 When H3PW12O40 was applied, 94.7% yield was obtained at 120 °C for 2 h. The Cs3PW12O40-catalyzed reaction operating in DMSO resulted in a slightly lower (73%) yield. By introduction of 1-(3-sulfonic acid)propyl-3-methylimidazolium phosphotungstate ([MIMPS]3PW12O40) catalyst, which was recycled for six consecutive runs without decrease in its activity,

Tables 18 and 19 summarize the catalytic dehydration of fructose and glucose in water focusing on the best system reported in the cited references. For better comparison of the reported results, for same cases, different conditions were indicated to compare the effect of the corresponding parameter and/or modification of the catalyst on the efficiency. It can be stated that the average yield was as high as ca. 45% (ca. 32 wt %) from fructose, which represents ca. 70% generated waste at high (>90%) conversion rates. From glucose, these values were slightly lower. In addition, the generally applied initial substrate concentrations were rather low (1−30 wt %), which result in separation issues and significant solvent usage. It should be noted that large differences could be observed in the reported yields under very similar conditions. 2.5.2.2. Fructose and Glucose Dehydration in Organic Solvents. Besides water, several organic solvents, preferably DMSO, DMF, DMA, THF, and alcohols, such as ethanol, isopropanol, as well as in some cases their combination, were tested as reaction media for dehydration of hexoses in the presence of either homogeneous or a wide range of heterogeneous catalysts, including heteropolyacid salts, Lewis acids, mesoporous material, and metal−organic frameworks (MOF). Horváth and co-workers performed a systematic screening of conventional organic solvents and GVL for dehydration of fructose in the presence of HCl. Although DMSO and THF were the best media, GVL was comparable to MeCN and acetone, with ca. 70% selectivity of 5-HMF. In the case of THF, in addition to the well-known issues of formation of peroxides during storage under air, the formation of side product 4chlorobutan-1-ol was also observed.496 Estrine et al. demonstrated the conversion of carbohydrates into 5-HMF without applying any catalysts in DMSO with different modes of heating (microwave irradiation, thermal heating) and different atmospheres (vacuum, air, and nitrogen). A remarkable 92% yield of 5-HMF was obtained under 900 W microwave irradiation after only 4 min from fructose. The in situ formation of strong acids could provide the efficiency of this system.497 Pidko used sulfonic acid-modified mesoporous SBA-15 silica, and hybrid organosilica catalysts tested for conversion of fructose in DMSO and water; however, the product formation in the aqueous phase was insignificant compared with DMSO (88%, 120 °C, 3 h, SBA-C2Ph-cocatalyst).498 When fructose was treated in DMSO, THF, MeCN, DMF, and DMA, 558

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outstanding catalytic performance as >99% conversion with >98% selectivity was achieved in sec-butanol at 120 °C for 2 h. Interestingly, the structure of the alcohol significantly influenced the 5-HMF yield as follows: sec-butanol > n-butanol > isobutanol. When the [MIMPS]3PW12O40 catalyst was tested on glucose conversion in DMSO, one-sixth of the actual activity was observed.503 Cellulose sulfuric acid as a proposed biosupported catalyst was tested for 5-HMF production in various solvents as follows: DMSO, DMA, NMP, and DMF. All of them could result in around 90% of 5-HMF yield with almost complete conversion at 100 °C for 45 min.504 Yang immobilized Cr3+ with SO3H-functionalized solid polymeric ionic liquids to obtain solid polymeric ionic liquids (FPILs). The 5-HMF yields were ca. 90% with 98% conversion in the dehydration of fructose at 120 °C for 60 min in DMSO. The synthesized FPILs showed good thermal stability, and the kinetics study revealed that the catalyst reduced the activation energy of the glucose conversion. Expectedly, it was effectively separated from the reaction mixture, and it could be easily reused without significant activity loss.505 Yan et al. observed the effects of high internal phase emulsions (HIPEs) for the transformation of carbohydrates. The catalyst had Brønstedacidic −SO3H sites for the conversion into 5-HMF and Lewis acidic CrIII for the isomerization to fructose from glucose. After optimizing the reaction conditions, 81% yield of 5-HMF was obtained from fructose at 120 °C in DMSO. The catalyst was effectively reusable four times.506 Metal (M: Cr, Yb, Ge, Sn, and Al) halides and triflates were screened to produce 5-HMF. It was shown that the most efficient catalyst was Al(OTf)3, giving 60% yield under optimal conditions (140 °C, 15 min) in DMSO.507 The transformation of fructose by Nb2O5 catalyst was tested by Dong et al. The catalysts were prepared from niobic acid by calcinations at 300−700 °C. The highest yield of 5-HMF obtained 86% by Nb2O5 produced at 400 °C.508 Yang et al. presented the synthesis of bifunctional partially hydroxylated AlF3. The mesoporous AlF3 exhibited high catalytic performance not only for the isomerization of glucose to fructose but also for the fructose dehydration as well. Lewis and Brønsted-acid sites containing AlF3-150 gave a yield of 57.3% with 95.5% conversion in DMSO at 140 °C for 10 h.509 Isomerization and subsequent dehydration of glucose by the tin-promoted vanadium phosphate (Sn-VPO) catalyst was also demonstrated by Parida. The optimal conditions were 20 wt % Sn (compared with no-promoted VPO) at 110 °C for 6 h in DMSO, resulting in 74% yield of 5-HMF. The catalytic system was reusable four times without significant loss of activity.510 Cr-incorporated zirconium phosphate (Zr−P−Cr) was prepared for the transformation of fructose providing an excellent 95% yield as well as being reusable six times with only slight loss of activity.511 Chou et al. prepared mesoporous zirconium oxophosphate with different P/Zr mole ratios for the fructose dehydration under mild conditions. With almost 91% fructose conversion, M-ZrPO-0.75 gave the best 5-HMF yield (69%), but M-ZrPO-0.25 showed the highest 80.5% selectivity toward 5-HMF in DMSO.512 Polypropylene fiber supported ionic liquid-type catalysts (Scheme 55a) were developed to increase recyclability. With the use of DMSO, 72−86% product yields were detected at 100 °C and [PPFPy][HSO4]-type catalyst was recycled for 10 runs without loss of activity. In protic solvents (i.e., iPrOH and 2butanol), lower activity was achieved.513 Bifunctionalized mesoporous silica nanoparticles (MSN) bearing a sulfonic

Scheme 55. Polypropylene Fiber Supported IL Catalysts

acid (HSO3) and an ionic liquid part (Scheme 55b) activated by CrCl2 were also utilized for this purpose giving the yield of 5-HMF as 73% after 3 h at 90 °C in DMSO. The MSN could be used four times with the same activity.514 A well-ordered KIT-6 mesoporous silica operating at 165 °C in DMSO showed higher activity represented by a shorter reaction time (30 min).515 Slightly higher (89%) yield with excellent (95%) selectivity was achieved by applying a recyclable (five times after ion exchange) Nafion-modified mesocellular silica foam (MCF) at 90 °C for 2 h.516 When β-cyclodextrin-SO3H carbonaceous catalyst was used, similar results were achieved for fructose.517 Comparable yield of 5-HMF was produced by poly(p-styrenesulfonic acid)-grafted carbon nanotubes (CNTPSSA) (Scheme 55c) at 130 °C. The catalyst could be reused three times. It should be noted that other substrates (i.e., inulin or sucrose) provided high yields as well.518 An efficient system by using magnetic lignin-derived solid acid carbonaceous catalyst was reported by Hu. The catalyst showed a yield of 81.1% (130 °C, 40 min, DMSO) and could be readily separated from the reaction mixture using an external magnet.519 Catalyst prepared by acid functionalization on mesoporous carbon/silica material via infiltration, carbonization, and sulfonation sequence gave slightly lower yield (70%).520 It should be noted that magnetically separable acid catalysts (i.e., [email protected]/Ph-SO3H),521 chromium-exchanged hydroxyapatite γ-Fe2O3 (γ[email protected]),522 and [email protected] C−SO3H523 also exhibited remarkable (82−89%) yield at 110− 120 °C for 3−4 h in DMSO. Poly(benzylammonium chloride) resin (PBnNH3Cl) catalyst was applied for the production of 5-HMF from various carbohydrates by Zhang. The catalytic system operated both in DMSO and under biphasic conditions; however, the addition of water decreased the conversion of glucose. The highest yield of 5-HMF from glucose was 58% at 120 °C after 10 h. Other carbohydrates such as sucrose, starch, and cellulose were also observed, providing comparable results.524 Hu et al. synthesized 5-HMF from fructose by applying formyl-modified polyaniline (FS-PAN) with excellent 5-HMF yield (90%) at 140 °C for 4 h in DMSO. It was also demonstrated that the amide site of the catalyst was the active phase. The catalyst maintained its good performance after four consecutive runs.525 559

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sites. The latter was reusable four times with no significant loss of activity and found to be effective with other substrates, such as inulin, glucose, and sucrose.531 Sulfonated carbon-based solid acid catalyst (CS-2)532 and sulfonated graphene oxides (SGOs)533 were characterized as efficient heterogeneous catalysts for conversion of hexose in DMSO with a yield of 5-HMF 90% (160 °C, 1.5 h) and 85% (SGO-3, 120 °C, 1 h), respectively. The latter was also applicable with high fructose dosage up to 20 wt %. Both catalysts were reused five times without significant loss in their activity. Over widespread applications of DMSO as a pure reaction medium for carbohydrate conversion, it has usually been applied as a component of solvents mixture. Some studies were focused on investigation of conversion of fructose in THF:DMSO mixtures in the presence of a catalyst such as bamboo-derived carbon (SBC) bearing SO3H, COOH, and phenolic OH groups,534 sulfonated carbonaceous material (GTS),535 P-VI-0 base, and P-SO3H-154 acid mixture,536 and Sn-Mont537 with yields of 50−95%. Ionic liquids were successfully applied as catalysts for carbohydrate conversion in organic solvents. The structures of ionic liquids are presented in section 2.5.2.4 in Scheme 59. An extended work by Huang revealed that 5-HMF formation could be significantly affected by varying the structure of ionic liquids as applied catalysts. While [EMIM][OH], [BMIM][OH], [BMIM]2[CO3], and [BMIM][PhCOO] were active for fructose dehydration in DMSO at 160 °C, the introduction of 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate(III) [C2OHMIM][BF4] could convert fructose to 5-HMF with outstanding yield (>92%) and glucose with moderate (30%) yield under identical conditions.503 The optimization of temperature in the range of 140−180 °C, catalyst concentration between catalyst and substrate ratio of 0.1−1, the structure of ionic-liquid type, and solvent screening verified that [C2OHMIM][BF4] showed the best performance, and the yield of 5-HMF reached as high as 67% from glucose at 180 °C for 1 h in DMSO. The catalyst was efficiently separated from the reaction mixture by distillation and reused six times without any decrease in its activity. A kinetic analysis was also performed showing first order for glucose, and the values of the activation energy and the pre-exponential factor for the reaction were 55.77 kJ mol−1 and 1.6 × 104 min−1, respectively. When an acidic ionic liquid [CMIM][Cl]538 or [BMIM][OH]539 was applied as catalyst for the conversion of fructose under comparable conditions, higher yields (>95%) were reported. An excellent 5-HMF yield of 91.2% was obtained using ionic liquid [HO2CMMIm]Cl in iPrOH at 110 °C for 30 min. The thermoregulated system’s recyclability was also investigated, and the catalyst’s activity showed no loss for five consecutive runs.540 Besides amorphous Cr2O3, SnO2, and SrO, graphene oxide−ferric oxide (GO−Fe2O3) showed excellent performance in 5-HMF production from glucose according to the work of Cheng et al. The reactions were carried out in [EMIM][Br] and DMSO, the conditions were 140 °C for 4 h. It was also shown that the calcinated metal oxides totally lost their catalytic activities.541 Mixtures of DMSO and [BMIM][Cl] including a less toxic GeCl4 as catalyst were found to be an active reaction medium for hexose conversion at low temperature (25 °C). It was shown that the accompanying anion of [BMIM] had a significant effect on activity. The highest yield was provided by Cl− (42%), compared with OAc− and [BF6]. The study also

Metal−organic frameworks (MOF) named NUS-6(Zr) and NUS-6(Hf) were used for the conversion of fructose. Both of them showed high stability and strong Brønsted acidity and due to the latter and more suitable pore size, NUS-6(Hf) was the more active catalyst, providing an outstanding 98% yield of 5HMF after 1 h at 100 °C in DMSO.526 Chen et al. demonstrated that both conversion and selectivity increases with higher density of −SO3H sites of MOF-type MOF-SO3H. An outstanding 90% yield of 5-HMF was reached by MIL101(Cr)-SO3H in DMSO at 120 °C after 1 h. Other metals in the catalyst (i.e., Zr or Al) also provided high yields.527 Nanoporous sulfonated polytriphenylamine (SPPTPA-1, Scheme 56) was synthesized and applied as a solid-acid catalyst Scheme 56. Structure of SPPTPA-1 Catalyst

for the direct conversion of sugar to HMF in DMSO. They reported that the catalyst had extremely high surface area (1437 m2 g−1) and surface acidity, and it was successfully used in microwave-assisted reactions. Extreme 5-HMF yield of 94.6% was achieved at 140 °C for 20 min, with a turnover number of 6.11.528 A sulfated ZrO2 hollow nanostructure (Scheme 57) was developed by Yin et al., which was able to dehydrate fructose selectively to 5-HMF with a yield of 64% in DMSO.529 A slightly higher performance (99% conversion and 97% selectivity) for fructose was observed by the use of a sulfonated two-dimensional crystalline covalent organic framework termed TFP-DABA530 as well as the same product yield being reported by utilizing mesoscopically assembled sulfated zirconium nanostructures (MASZN) having Lewis and Brønsted-acid Scheme 57. Sulfonated Zn-Based Hollow Shell Type Catalysta

a

Adapted from ref 529. Copyright 2013 Wiley-VCH. 560

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showed that addition of LiCl increased the yield of 5-HMF significantly.542 In the same environment, cornstarch was converted to 5-HMF by the AlCl3·6H2O catalyst in a microwave-mediated reaction. Under optimal conditions (150 °C, 20 min) 86% yield of HMF was obtained. The catalyst maintained its good performance for four cycles.543 Pyridinium-based dicationic ionic-liquid catalysts were investigated by Kim et al. on the conversion of fructose into 5-HMF. The two catalytic systems were 1,1′-hexane-1,6diylbis(3-methylpyridinium) tetrachloronickelate(II) ([C6(Mpy)2][NiCl4]) in DMSO and 1,1′-decane-1,10-diylbis(3-ethylpyridinium) dibromide ([C10(Epy)2]Br2) without DMSO. The reaction parameters were optimized. Both catalysts provided excellent yields of 5-HMF: 96% by [C6(Mpy)2][NiCl4] after 1 h at 110 °C and 87% by [C10(Epy)2]Br2 after 90 min at 100 °C.544 Ionic-liquid polyoxometalate salts (IL-POM) were tested as catalysts for the conversion of fructose into 5-HMF and ethyl levulinate by Chen et al. The most efficient salt was phosphotungstic acid-derived IL-POM ([3·2H]3[PW12O40]2), resulting in a 92% yield of 5-HMF. The optimum conditions were 100 °C for 1 h in DMSO. The catalyst was easily separable from the mixture, and it showed high activity toward the dehydration of other carbohydrates (e.g., inulin, sucrose, and cellobiose).545 Soni investigated the effect of two symmetrical ([MMIM][HSO4] and [MMBIM][HSO4]) and two unsymmetrical ([PSMBIM][HSO4] and [HMBIM] [HSO4]) acidic ionic liquids on the transformation of fructose (Scheme 58). The

Wang et al. characterized novel catalysts by the immobilization of chromium(III) Schiff base complexes [i.e., Cr(Salen), Cr(Salen-Br), Cu(Salen), and acidic ionic liquids] onto the surface of MCM-41. Using Cr(Salen)-IM-HSO4-MCM-41 catalyst in DMSO at 140 °C for 4 h provided a moderate 43.5% 5-HMF yield.549 Tables 20 and 21 give an overview of efficient catalyst systems developed for glucose and fructose conversion in DMSO and DMSO-containing mixtures. It can be concluded that average yields (ca. 75%) for glucose are higher even in lower temperatures than that in water, indicating the solvent effect on fructose and glucose transformation. Concerning glucose, ca. 50% average yield was observed. However, the high boiling point of DMSO (189 °C) could result in separation issues. Although DMSO and DMSO-containing mixtures were proved to be better reaction media for hexose dehydration, several studies investigated the transformation in other common organic solvents. N,N-dimethylacetamide (DMAC) was utilized as a polar aprotic reaction environment by Xu et al. It was found that the metal halides (MCl3, where M: Al, Ga, In, Fe, La and AlX3, where X: Br, I) were efficient catalysts for the glucose conversion. When AlI3 was used, 5-HMF was obtained with a yield of 52% at 120 °C for 15 min. It was shown by 13C NMR spectroscopy and HPLC analysis that the AlI3 could promote three consecutive reactions: mutarotation of αglucopyranose to ß-glucopyranose, isomerization of glucose to fructose, and finally the dehydration of fructose to 5-HMF.559 When Sc, La, and Y chlorides were utilized at 145 °C, a yield as high as 68% was obtained.560 Focusing on the application of alcohols, cross-linked 4vinylpyridinium polymers were synthesized for the dehydration of fructose. Poly(N-alkylvinylpyridinium bromides) showed the highest activity, providing 77% yield at 180 °C after 30 min in EtOH. It was shown that the length of the alkyl chain had a significant effect on the yields.561 Comparable yields were obtained by applying poly(benzylicammonium chloride) (PBnNH3Cl) resin with an optimal ratio of BnNH3Cl (21%) and BnCl (79%). A small amount of water (3 v%) in iPrOH could improve the 5-HMF yield to an excellent 77% at 140 °C for 3 h.562 Slightly higher activity (yield: 85%, at 130 °C) was demonstrated for recyclable (six runs) poly(vinyl alcohol) (PVA) functionalized mesoporous DICAT-1 catalyst bearing −SO3H sites for the dehydration of fructose in iPrOH.563 Chloride salts of Li, Cu, Ni, Sn, Fe, and Cr in the absence of mineral acids were also tested in iPrOH;564 however, the activities of these systems were lower. Vigier et al. discussed the effect of a ChCl/CO2 catalytic system on the hydrolysis of fructose. At optimal conditions (120 °C, 90 min, pCO2 = 4 MPa), the 72% yield of 5-HMF was obtained in a mixture of ChCl and MIBK. It was also proven that without MIBK, the yield was much lower.565 The effect of acid-based heteropolyacids (HPAs) with lysine catalysts (Ly3−xHxPW) for the synthesis of 5-HMF from fructose was investigated. The acidity and basicity of the catalysts were influenced by different amounts of HPA and lysine. The reactions were carried out in ChCl at 110 °C for 1 min. The highest yield of 5-HMF was 92% by Ly2HPW. The catalytic system worked with highly concentrated feedstock and could be reused several times after regeneration by washing.566 The catalytic activity of choline chloride-p-TSA, a DES, on the transformation of fructose was also investigated. Under optimal conditions (2.5 wt % feed, 1 h, 80 °C, DES mole ratio

Scheme 58. Cations of Brønsted-Acidic Ionic Liquida

a

Prepared by Soni, ref 546.

reactions were performed in DMSO solvent, while the reaction times, temperatures, and catalyst amounts were optimized. An outstanding 73% yield of HMF was obtained by [PSMBIM]HSO4 at 80 °C after 1 h. It was proven that unsymmetrical ionic liquids were slightly more effective than symmetrical ones. The catalysts maintained their good performance after five runs.546 A DMSO and iPrOH mixture was used as solvent to optimize the dehydration of fructose by graphene oxide catalyst, which gave 90% yield at 120 °C for 6 h. It was also proven that sulfonic groups and abundance of oxygencontaining groups have significant roles in the high activity of the catalytic system.547 Low boiling 1,4-dioxane in the presence of a small amount of DMSO was used as the solvent for an amberlyst-15-catalyzed fructose conversion at 110 °C for 3 min giving 92% yield under continuous conditions, which is 75 times higher than the batch case. The long-term stability test showed that the activity of the amberlyst-15 catalyst remained the same after 96 h.548 561

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Table 20. Conversion of Fructose to 5-HMF in DMSO and DMSO-Containing Solvent Mixtures no.

solvent

fructose conc (wt %)

1 2 3 4 5 6 7 8 9 10 11 12

DMSO DMSOa DMSOb DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSOc

11 10 15 10 11 4 8 9.3 4.7 2 2 14

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO

5 1 0.5 0.5 4.3 4.3 5.1 0.3 10.5 8 4.3 4.3 9.3 9.3 1.75 2 5 3 3 5

33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO

8 5 5 2 2 3 3 2 2.2 2.2 5 5 5 5 5 5 8 8 8 na 3 4 3 3 1.2 0.9

catalyst − − − − H2SO4 H2SO4 H2SO4 CrCl3·6H2O HfCl4 Nb2O5 niobic acid Nb2O5 γ-AL2O3 H-ZSM-5 TiO2 nanop. H3PW12O40 Cs3PW12O40 TFP-DABA TFP-DABA cellulose sulfuric acid HSO3-MSN amberlyst-15 amberlyst-15 TFP-DABA TFP-DABA FPIL H3PW12O40 [email protected]/Ph-SO3H MLC-SO3H MLC-SO3H Cs2Cr3SiW12 PZS-2.0 SnCl4:(NH4)2HPO4 = 1:2 CNT-PSSA GO SGO-3 nafion NR50 nafion NR50 nafion(15)/MCF PZODA-3.5 coal-H-SO3H ([3·2H]3[PW12O40]2 ([3·2H]3[SiW12O40]2 HCCP CNC HCl SBA-C2Ph-coc SBA-C2Ph-c-HT MASZN-3 MIL-101(Cr)-SO3H UIO-66(Zr)-SO3H poly(VMPS)-PW bifunctionalized MSN [email protected] Zr-P-Cr [email protected] γ[email protected] SO4/TiO2-SiO2 β-cyclodextrin-SO3H

T (°C)

t

conv (%)

select (%)

yield (%)

− − − − 0.5 M 1 wt % 5 wt % 2.8 wt % 10 mol % 0.2 wt % 0.2 wt % 0.55 wt % 0.83 wt % 2.5 wt % 0.5 wt % 0.25 wt % 0.25 wt % 10 mol % 5 mol % 1.4 wt % 0.3 wt % 2.6 wt % 0.8 wt % 10 mol % 5 mol % 2.8 wt % 2.8 wt % 1.75 wt % 50 wt % 50 wt % 0.2 wt % 1.6 wt % 30 mol %

120 − − 150 120 125 130 120 100 120 120 150

2h 4 min 7 min 300 min 2h 30 min 60 min 1h 2h 2h 2h 5h

na 100 99 100 na 85 94 96 na 100 100 na

na 92 69 90 na 79 56 92 na 86 80 na

20 92 68 90 80 67 84 88 67 86 80 78

14 64 48 63 56 47 37 62 47 60 56 55

431 497 497 497 431 532 554 505 465 508 508 558

110 150 120 120 100 100 100 60 120 120 100 100 120 120 110 130 130 130 90 135

12 h 3h 2h 2h 1h 1h 45 min 15 h 1h 30 min 1h 1h 1h 1h 3h 40 min 40 min 0.5 h 0.5 h 1h

− 99+ 98 96 >99 67 100 66.3 na. 89 >99 67 98.3 85 >99 100 100 76.6 na na

− 30 95 70 97 97 94 70 na. 79 97 97 90 53 84 84 83 58.9 na na

65 30 93 73 97 65 94 46.1 83 70 97 65 88.3 45 83 83.7 83.4 45.1 97.2 14

21 65 51 68 45 66 32 58 49 68 45 62 32 58 59 58 32 68 10

641 499 502 502 530 530 504 689 550 518 530 530 505 505 521 519 519 573 551 580

0.8 wt % 0.5 wt % 0.5 wt % 0.9 wt % 0.9 wt % 1.5 wt % 1.6 wt % 0.2 wt % 0.02 mol % 0.02 mol % 1 mol % 1 mol % 1 mol % 2.5 wt % 2.5 wt % 0.3 wt % 5 wt % 5 wt % 5 wt % na 0.5 wt % 2 wt % 0.2 wt % 2 wt % 0.89 wt % 0.9 wt %

120 120 120 80 80 90 90 140 100 100 90 90 90 120 120 110 120 120 130 90 100 120 100 120 110 140

30 min 1h 1h 2h 5h 2h 0.5 h 140 min 1h 1h 2h 2h 2h 3h 3h 2h 1h 1h 90 min 3h 10 h 2h 10 h 3h 2h 2h

100 70 90 76 95 94 na na 99 99 99 96 98 99 99 99 99 99 99 97 97 99 80 96 92 na

89 96 94 71 83 95 na na 93 91 92 88 90 89 86 93 91 86 85 75 82 87 50 93 80 na

89 67 85 57 79 89 82.2 85 92 90 91 84 88 88 85 92 90 85 84 73 80 86 40 89 74 96

62 47 59 38 55 62 57.5 60 64 63 64 59 62 62 60 64 63 60 59 51 56 60 28 62 52 67

518 533 533 557 557 516 551 552 545 545 553 553 553 498 498 531 527 527 554 514 523 511 523 522 555 517

catalyst loading

562

yield (wt %)

ref

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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Table 20. continued no.

fructose conc (wt %)

3 3 4 4 4 7 na na 5 1 1.5 3 2 1.5 1.6 0.5 1.6 8 8 8 8 5 5 45 7 5 7 7 5.4

SBC SBC amberlyst-15

4 4

[email protected] graphene oxide

1.4 wt % 1.4 wt % 1.2 × 10−6 mol/ mL min 0.7 wt % 0.4 wt %

5

graphene oxide

5

95 96

select (%)

yield (%)

yield (wt %)

ref

na

81

57

506

165 165 140 140 140 130 100 100 130 110 160 120 80 160 160 120 160 80 80 80 80 110 110 100 110 160

30 min 30 min 4h 4h 1h 1h 1h 1h 2 min 4h 2h 2h 2h 8h 2h 2h 2h 1h 1h 1h 1h 1h 1h 30 min 1h 1h

na 100 100 100 99 98 99+ 99+ 90 na 58.9 100 70 94 na 99 na 94 90 75 88 95 96 na 96 99.0

na 100 45 90 80 90 84 98 80 na 92.9 95 44 96.8 na 75 na 78 79 76 80 96 100 na 86 99.0

59 84 45 90 79 88 84 98 72 65 54.7 95.4 31 91.6 58 76 95 73 71 57 70 91 96 86 83 98.0

41 59 62 63 55 62 59 69 50 46 38 67 22 64 40 53 67 51 50 40 49 64 67 60 58 69

515 515 525 525 532 534 526 526 563 556 539 538 557 539 503 502 503 546 546 546 546 544 544 513 544 535

130 130 110

1h 1h 3 min

98 98 98

90 91 94

88 89 92

62 60 64

534 534 548

80 120

10 h 6h

98 100

31 90

30 90

21 63

523 547

0.5 wt %

120

6h

98

89

87

61

547

SGO-3

0.5 wt %

120

1h

100

94

94

66

533

20

SGO-3

0.5 wt %

120

1h

na

na

75

53

533

3

GeCl4

10 mol %

25

12 h

na

na

70

49

542

3 5

KIT-6-Pr-SO3H GTS

1.3 wt % 2.5 wt %

165 160

30 min 1h

94 99

61 99

57 98

40 69

515 535

DMSO:ethanol = 1:3 DMSO:isopropanol= 5.6:1 DMSO:isopropanol= 1:6 DMSO:[AMIM]Cl = 1:1 DMSO:[AMIM]Cl = 1:1 DMSO:[BMIM][Cl] = 1:5 DMSO:DMF DMSO:THF = 30:70 v/v

94

conv (%) na

DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO:THF = 30:70 v/v DMSO:THF = 11:1 DMSO:THF = 3:1 DMSO:1,4-dioxane

93

t 3h

60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85

92

T (°C) 120

1.7

91

catalyst loading 0.5 wt % 0.2 wt % 1.3 wt % 1.3 wt % 3 wt % 3 wt % 1 wt % 1.4 wt % 3.5 mol % 3.5 mol % 9 wt % na 0.75 wt % 6.3 wt % 0.2 wt % 0.75 wt % 0.8 wt % 0.25 wt % 0.8 wt % 0.8 wt % 0.8 wt % 0.8 wt % 0.8 wt % 15 wt % 15 wt % na 20 wt % 2.5 wt %

DMSO

89 90

catalyst PEs-SO3H PEs-CrIII Al-MCM-41 KIT-6-Pr-SO3H S-PAN FS-PAN CS-2 SBC NUS-6(Zr) NUS-6(Hf) DICAT-1 PC-SO3H [BMIM][Cl] [CMIm][Cl] [BMIM][Cl] [BMIM][OH] [BMIM][OH] [MIMPS]3[PW12O40] [C2OHMIM][BF4] [PSMBIM][HSO4] [MMBIM][HSO4] [MMIM][HSO4] [HMBIM][HSO4] [C6(Mpy)2]2Br− [C6(Mpy)2][NiCl4] [PPFPy] [HSO4] [C10(Epy)2][Br] GTS

59

86 87 88

a

solvent

900 W microwave irradiation. b500 W microwave irradiation. cMicrowave irradiation.

selectivity toward 5-HMF. The catalyst could be reused five times.569 Tables 22 and 23 summarize the transformation of fructose and glucose to 5-HMF in common organic solvents. Approximately 63% and 50% as average yields were obtained for fructose and glucose, respectively. These values are comparable with those that resulted from DMSO-containing systems; however, some reaction media (e.g., alcohols) have lower boiling points that could significantly reduce the energy need of evaporation, for example. 2.5.2.3. Fructose and Glucose Dehydration in Organic/ Aqueous Mixtures. This section describes research performed on the conversion of D-fructose and D-glucose to 5-HMF in a

1:1), 91% of 5-HMF yield was obtained. The study also showed that p-TSA was both the hydrogen-bond donor and the catalyst.567 When a molten mixture containing choline dihydrogencitrate and glycolic acid was used for 5-HMF production from both glucose and fructose by using B(OH)3 as a promoter, the glucose conversion showed around 60% of 5HMF yield at 140 °C for 4 h.568 Various SO3H-functionalized acidic ILs were applied as catalysts for the conversion of glucose into 5-HMF in MIBK. An outstanding 79% yield of HMF was obtained under optimal conditions (140 °C, 6 h) by IL-5. The study showed that stronger acidity increases the yield, while longer reaction time, higher temperature, and additional water decreases the 563

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

Chemical Reviews

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Table 21. Conversion of Glucose to 5-HMF in DMSO and DMSO-Containing Solvent Mixtures no.

solvent

glucose conc (wt %)

t

conv (%)

select (%)

yield (%)

yield (wt %)

ref

1 wt % 1 wt % 1 wt % 0.8 wt % 0.7 wt % 0.4 wt % 0.4 wt % 0.4 wt % 0.05 M 1.9 wt % 1.9 wt % 1.2 wt % 1.5 wt % 1.2 wt % 0.9 wt % 0.5 wt % 0.2 wt % 0.01 wt % 1.4 wt % 2.1 wt % 6.1 wt %, 2.9 wt % 3.2 wt % 1 wt %

120 120 120 180 180 110 110 110 140 140 140 100

360 min 300 min 120 min 1h 4h 6h 6h 6h 15 min 10 h 10 h 2h

na na na na na 100 94 78 96 95.5 81 na

na na na na na 74 57 38 63 60 53 na

13 53 54 67 9 74 54 30 60 57.3 43 69

9.1 37 38 47 6 52 38 21 42 40 30 48

467 467 467 503 484 510 510 510 507 509 509 501

100 140 120

2h 2h 3h

na na na

na na na

20 47 58

14 33 41

501 517 506

150

4h

na

na

56

39

558

140 120

4h 7h

99 na

44 na

43.5 50.6

30 35

549 538

160 100

3h 10 h

98.4 na

54 na

53.5 95

37 67

537 536

1 wt %

100

10 h

na

na

94

66

536

1 wt %

100

10 h

na

na

76

53

536

1 wt %

200

10 min

92

16

15

14

618

0.2 wt %

120

3h

na

na

55

39

576

catalyst conc

FeCl3·6H2O AlCl3 CrCl3·6H2O [C2OHMIM][BF4] PCP(Cr)-SO3H·Cr(III) 20 wt % Sn-VPO 10 wt % Sn-VPO VPO Al(OTF)3 AlF3-150 AlF3-250 + HCl (0.01 wt % HCl) SnCl4 TBAB SnCl4 β-cyclodextrin-SO3H PEs-SO3H PEs-CrIII Nb2O5 γ-AL2O3 Cr(Salen)-IM-HSO4-MCM-41 [CMIM][Cl], ZrOCl2·6 8H2O

1 2 3 4 5 6 7 8 9 10 11 12

DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO

10 10 10 1.6 0.9 9 9 9 16 4.7 4.7 8

13 14 15

DMSO DMSO DMSO

8 0.9 1.7

16

DMSO

14

17 18

DMSO DMSO

4.26 2.9

19 20

DMSO:THF = 3:7 DMSO:THF= 1.5:1

7 2

21

THF:DMSO= 1:1.5

2

22

THF:DMSO = 1:1.5

2

23

DMSO:[HexylMIM] [Cl] = 1:1 w/w DMSO:P[BVIM][Cl] = 5:1

2.5

Sn-Mont P-VI-0 P-SO3H-154 P-VI-108 P-SO3H-154 P-VI-150 P-SO3H-154 ZrO2

3

CrCl2

24

T (°C)

catalyst

resulting in excellent 86% yield from fructose by addition of LiCl as cocatalyst. Reasonable yields were obtained for other substrates, such as sucrose, maltose, starch, and cellulose.581 Sulfanilic acid was tested for the conversion of carbohydrates into 5-HMF by Karimi and Mirzaei. The process was effective for various substrates (even from untreated lignocellulosic biomass) and totally metal-free, which makes the system environmentally benign. A good 78% yield of 5-HMF was performed by C6H4(SO3H)(NH2) catalyst from glucose at 150 °C after 1 h in a DMSO:water:2-butanol:MIBK solvent mixture. The catalyst could be reused five times with insignificant loss of activity.582 The study by Zhang et al. discussed the influence of polymerbound sulfonic acids on the transformation of glucose. The two catalysts were poly(ethylene glycol)-bound sulfonic acid (PEGOSO3H) and polystyrene-PEG-OSO3H that contained both Brønsted and Lewis-acid sites. The reaction resulted in an excellent yield of 86% by PS-PEG-OSO3H with LiCl cocatalyst in aqueous DMSO at 120 °C for 1 h. Reasonable yields were obtained for other substrates such as sucrose, maltose, starch, and cellulose.581 The group of Dumesic performed extensive research on 5HMF production in different solvent mixtures involving combination of THF with water. Their inorganic SBA-48type silica (E0) and two organic ethane-bridged SBA-15-type silicas (E45 and E90) as well as a commercial silica (p-SO3H−

solvent mixture containing water and organic solvent(s) that usually form a biphasic system under applied conditions. Experiments concerning the solvent selection for biphasic systems solvent optimization for the extraction of 5-HMF from catalysts containing aqueous phase were performed by Palkovits and co-workers.577 Factors including temperature, concentration, and fructose addition were used for experimental validation of the predictive power of COSMO-RS solvent selection software. The advances in 5-HMF production from biomass under biphasic conditions were reviewed by de Vries417 and Abu-Omar.578 Because DMSO was proven to be an excellent solvent for the dehydration reaction, its combination with water to improve efficiency was studied. Qi et al. performed synthesis of 5-HMF by the use of functionalized silica nanoparticles SiNP-SO3HC16. Because of the higher hydrophobicity by strongly acidic SO3H sites, an excellent yield of 87% was obtained at 120 °C in water:DMSO medium with a catalyst loading of 4 wt %.579 Similar reaction media were used for dehydration of sugars by in situ generated tin phosphate catalyst (SnCl4/(NH4)2HPO4), exhibiting a yield up to 71% from fructose at 135 °C. It was found that tin valence number, phosphate-type, and Sn/PO4 ratio had a significant influence on the yield.580 Zhang introduced a combined catalyst containing poly(ethylene glycol)-bound sulfonic acid (PEG-OSO3H) and polystyrenePEG-OSO3H bearing both Brønsted and Lewis-acid sites, 564

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

Chemical Reviews

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Table 22. Dehydration of Fructose to 5-HMF in Conventional Organic Solvents no.

fructose conc (wt %)

catalyst

catalyst loading

T (°C)

t

conv (%)

select (%)

yield (%)

yield (wt %) 38 25 41 27 48

ref

0.5 wt % 0.5 wt % 10 mol % 2.2 wt % 0.7 wt % 2.5 wt % 0.5 wt % 3 wt % 1.4 wt % 49.6 wt % 10 wt % 0.4:1:1 1:2.7:4 0.6:1:1 10 mol % 10 mol % 1.3 wt % 0.25 wt % 0.2 wt % 0.25 wt % 1.5 wt % 10 mol % 10 mol % 10 mol % 0.03 wt % 0.1 mmol 0.25 wt % 0.25 wt % 0.25 wt % 1.7 wt %

150 150 100 100 145 110 150 120 100 100 130 120 120 120 100 100 165 120 130 120 100 180 180 180 100 130 120 120 120 140

3h 3h 2h 10 min 40 min 12 h 3h 1h 4 5 min 45 min 2 min 20 min 20 min 20 min 2h 2h 30 min 2h 0.5 h 2h 12 h 2h 2h 2h 4h 90 2h 2h 2h 3h

99+ 99+ na 73 88 − 99+ na. 98 99 99 na na na n.a. n.a. 99 >99 89.2 99 99 99 99 84 na 100 99 99 99 na

54 35 na 52 77 − 31 na. 87 86 77 na na na n.a. n.a. 44 17 50.3 75 47 73 71 48 na 73 74 99 56 na

54 35 58 38 68 5 31 64 85.5 84.9 76 68 93 70 80 82 44 17 44.9 74 47 72 70 40 16 73 75 99 57 77

21 45 60 59 53 48 65 49 56 57 31 12 31 52 33 50 49 28 11 51 53 70 40 54

499 499 465 655 560 641 499 550 504 570 563 571 571 571 572 572 515 502 573 502 564 561 561 561 643 483 502 502 502 562

2 4 6.5 6.5 10 10

TiO2 nanop. TiO2 nanop. HfCl4 lignosulfonic acid YCl3·6H2O H-ZSM-5 TiO2 nanop. amberlyst-15 cellulose sulfuric acid [AMIM][Cl] DICAT-1 AlCl3:H2SO4:H3PO4 AlCl3:H2SO4:H3PO4 AlCl3:H2SO4:H3PO4 FeCl3 + NBS SnCl4 + NBS KIT-6-Pr-SO3H H3PW12O40 H4SiW12O40 [MIMPS]3PW12O40 NH4Cl C5H11Br C7H15Br C3H7I FeCl3 C16H3CrPW11 [MIMPS]3PW12O40 [MIMPS]3PW12O40 [MIMPS]3PW12O40 BnNH3Cl/BnCl = 21 :79 [HO2CMMIm]Cl [HO2CMMIm]Cl DICAT-1 DICAT-1 NH4Cl −

4 wt % 4 wt % 11 wt % 11 wt % 1.5 wt % −

110 110 130 (MI) 130 100 160

30 min 30 min 2 min 2.5 h 12 h −

100 93 95 na 100 >99

92 90 89 na 68 81

91.2 83.4 85 50 68 84

64 58 60 35 48 56

540 540 563 563 564 615

10





160



96

82

79

55

615

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

THF DMAC DMAC DMAC DMAC DMAC DMF DMF DMF DMF DMFa DMF DMF DMF NMP NMP NMP MIBK MIBK MIBK ethanol ethanol ethanol ethanol ethanol n-butanol n-butanol i-butanol 2-butanol iPrOH

1 1 4.7 9 41 5 1 12 5.1 3.6 6 5 5 5 6.5 6.5 3 0.5 3 0.5 10 6 6 6 0.4 23 0.5 0.5 0.5 4.3

31 32 33 34 35 36

iPrOH iPrOH iPrOH iPrOH iPrOH (HCl-KBr)aq: GVL = 1:2a (HCl-NaCl)aq: GVL = 1:2a

37 a

solvent

Microwave irradiation.

Table 23. Dehydration of Glucose to 5-HMF in Conventional Organic Solvents no.

solvent

glucose conc (wt %)

catalyst

1

THF

10

CrCl3, HCl

2 3 4 5

DMAC DMAC DMAC DMAC

na 4.5 4.5 9

6 7

DMAC DMAC

5 4

8 9 10

DMAC DMAC:[EMIM][Cl] = 5:1 EtOH:[HexylMIM][Cl] = 1:1 w/w

41 3 2.5

All3 AlBr3 All3 TEAB CrCl3 AlCl3 AlCl3 NaI YCl3·6H2O CrCl2 ZrO2

catalyst loading 18.6 mM, 0.1 M 20 wt % 0.7 wt % 2 wt % 7.5 wt % 1.0 wt % 0.3 wt % 0.3 wt % 4 wt % 2.2 wt % 0.2 wt % 1 wt %

565

T (°C)

t

conv (%)

select (%)

yield (%)

yield (wt %)

ref

140

180 min

91

65

59

41

559

120 130 120 120

15 15 15 90

min min min min

99 na 99 na

53 na 53 na

52 44 52 73

36 31 36 51

559 559 559 574

130 130

15 min 15 min

70 86

51 72

36 62

25 43

575 575

145 120 200

2h 3h 10 min

100 na 92

33 na 23

33 58 21

23 41 14

560 576 618

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

Chemical Reviews

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Sc) were tested in a tubular reactor operated at 130 °C with THF:water = 4:1 mixed solvent. It was shown that the selectivity of 5-HMF with ordered structures was much higher (60−75%) compared with the nonordered structure (max 20%).583 Intercalation of polyvinylpyrrolidone (PVP), a polar aprotic polymer, into mesopores of acid-functionalized silicas by incipient wetness impregnation was also developed. The PVP-modified silica-based materials (PVP-p-SO3H-SBA-15 and PVP-p-SO3H-MCM-41) achieved much higher HMF selectivities (>80%), than the unmodified catalysts (p-SO3H-SBA-15 and p-SO3H-MCM-41) under the same conditions. The experiments on the recycling methods showed that the solids could be readily recovered and the final products could be easily separated from the reaction mixture.584 The effect of a combination of Lewis and Brønsted acids on the dehydration of glucose in 2-sec-butylphenol (SBP):water solvent at 170 °C was also discussed. Several Lewis acids with HCl were tested, but AlCl3 was the most efficient, yielding 97% of 5-HMF in the organic phase. After it was extracted and purified, the isolated yield was 62%. However, this method could be useful because it avoids expensive solvents and toxic catalysts.585 This work was extended for lanthanide-based Lewis acids revealing the efficiency of both Yb and La. Although moderate yield was obtained (42%) under near-neutral conditions (pH = 5.5), most of the 5-HMF resolved in SBP, and the aqueous phase could be reused. The system showed activity toward other carbohydrates, such as starch and cellobiose.586 The same group reported the dehydration of fructose in THF:water:2,5(dihydroxymethyl)tetrahydrofuran mixture by the amberlyst-70 catalyst. It was proven that higher DHM:THF:water ratio provides significantly increased conversion, selectivity, and yield after short reaction times. Lower amounts of substrate resulted in higher yields, as expected.587 Abu-Omar investigated the combination of layered zirconosilicate (Na2ZrSi4O11) and solid acid amberlyst-15 for fructose conversion; however, a low product yield (39%) was achieved in a THF:water mixture.588 Yield could be slightly increased by a titanium hydrogen phosphate (TiHP) resulting in 55% 5HMF from fructose at 140 °C for 3 h.589 By replacement of THF by EtOH, in the presence of AlCl3, higher furan yields could be achieved. However, the formation of 5-HMF and 5ethoxymethylfurfural (5-EMF) was competitive to the formation of LA and ethyl levulinate, as expected. Other substrates also provided sufficient yields, such as sucrose, maltose, cellobiose, and starch.590 A biphasic system formed from THF, water, and AlCl3 was also used for mineral acid-catalyzed microwave-assisted conversion of glucose, resulting in comparable (60%) yield at 160 °C. The effect of the pH on the selectivity and conversion was investigated, and the maximum 5-HMF selectivity was observed over the pH range of 0.5−1.591 When boric acid was added to the system, no significant improvement was achieved.592 MIBK forming a biphasic system with water was also widely studied as an extracting agent for carbohydrate conversion. Nijhuis et al. performed the dehydration of glucose with four metal-phosphate (PO) catalysts (i.e., AlPO, TiPO, ZrPO, and NbPO), from which the latter gave the highest conversion (>80%) of glucose and selectivity (55%) toward 5-HMF at 135 °C. It was also proven that higher Brønsted to Lewis-acid site ratio increased the selectivity significantly.593 Continuing the catalyst development, outstanding yields utilizing heterogeneous zeolites (MOR, ZSM-5, and BEA) and amorphous aluminosilicates catalyst were also reported.594 Although

zeolites and Al2O3-SiO2 catalysts exhibited fructose conversion above 60% in water for 3.3 h, the selectivity of 5-HMF reached a maximum at 48% only in the case of alumina at 50% conversion. However, it was found that the selectivity could be enhanced by the addition of MIBK as a cosolvent to form a biphasic system. At low conversion rates, the ratio of water to MIBK had a significant effect on selectivity. Over 40% conversion, this effect became negligible. Compared with an aqueous system, ca. 20% higher selectivities were achieved. The accelerating effect of MIBK decreased in the order of MOR > ZSM-5 > BEA > amorphous aluminosilicate, which corresponds to the order of the decrease in the strength of the acid sites. In an extended work by the same group, zirconium phosphate (ZrPO) foam-based catalysts were evaluated for hexoses to 5HMF dehydration. The reaction gave 35% yield of 5-HMF at 70% glucose conversion over the silylated ZrP-f/Al(Ic)-10. The treatment of catalyst with tetraethyl orthosilicate increased the selectivity up to 60%. The activity of the foam did not decrease significantly for the second cycle, indicating a stable structure.595 Higher product yields (ca. 77%) were obtained by applying hydrophilic silicoaluminophosphate (SAPO-44)596 and large-pore mesoporous tin phosphate (LPSnP-1)597 catalysts operating in a MIBK:water solvent saturated with NaCl at 175 and 150 °C, respectively. This catalytic system also provided reasonable yields from maltose, cellobiose, and starch. While both catalysts could be reused five times without regeneration, the SAPO-44 also provided reasonable yields from maltose, cellobiose, and starch. Comparable results were achieved via an autocatalytic dehydration of fructose in MIBK:water. Although a good 74% yield of 5-HMF was reached after 2 h at 160 °C, expectedly side products (LA and FA) were formed as well.598 Maireles-Torres et al. performed the dehydration of glucose by using aluminum-doped mesoporous MCM-41 silica catalysts in MIBK:water at 195 °C for 150 min. They also verified that the addition of 20 wt % aqueous NaCl solution increased the yield to 63%. Because of its high selectivity, no LA formation was detected.599 The HSO3-MCM41 mesoporous silica showed higher activity (83% yield) at 190 °C after 80 min in the same biphasic system.600 The phosphoric acid-doped polybenzimidazole (PA−PBI) catalyst was also found to be active in the MIBK:water mixture. The reactions were carried out at high fructose dosage (50 wt %) and showed high selectivity (94%) toward 5-HMF. The highest yield reached was 71% after 2 h at 180 °C. The catalyst was reusable four times without significant loss in catalytic activity. 601 Shimanouchi et al. optimized the fructose conversion in a slug flow reactor resulting in the highest yield of HMF as 89% at 180 °C and 3.1 mm/s velocity with MIBK:water ratio of 1:1 with HCl as catalyst.602 Jerome and co-workers stated that ChCl can be considered as a safe additive capable of assisting the catalytic conversion of glucose. Under optimized conditions, 5-HMF was produced with 70% yield (150 °C), which is competitive with those obtained in traditional imidazolium-based ionic liquids in the presence of hazardous chromium salts. By screening various Lewis acids, it was revealed that applied metal chlorides can be classified by their activity as follows: AlCl3, CrCl2, CrCl3 > FeCl3, CuCl2 > MgCl2, FeCl2 > MnCl2, BiCl3 > ZnCl2.603 Alcohols, preferably butanols, which could easily be obtained from biomass, were also applied as extracting compounds in biphasic systems. For these systems, the heterogeneously catalyzed reaction has been preferred. Beltramini prepared and characterized TiO2 catalyst for glucose conversion, which 566

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

Chemical Reviews

Review

Table 24. Dehydration of Fructose to 5-HMF in Aqueous Systems Including Biphasic Systems no.

solvent

fructose conc (wt %)

17 4.5 9 15 8 15 0.5 0.6

H3PO4 Si semiconductor H3PO4 Si-OH semiconductor SiNP-SO3H SiNP-SO3H-C3 SiNP-SO3H-C16 SnCl4:(NH4)2HPO4 = 1:2 SnCl4:K2HPO4 = 1:2 SnCl2:(NH4)2HPO4 = 2:1 SnCl4:K2HPO4 = 1:2 SnCl2:(NH4)2HPO4 = 2:1 BHC BHC BHC amberlyst-15 amberlyst-15 amberlyst-15 FePO4 beta(OF)-Cal450

0.6 15 42 45 1.7 10 10 2 10 10 30 10 50 3.5 3 26 2

1

water:DMSO= 1:1

0.3

2

water:DMSO= 1:1

0.3

3 4 5 6

water:DMSO water:DMSO water:DMSO water:DMSO

1:4 1:4 1:4 35:65

4 4 4 5

7 8

water:DMSO = 35:65 water:DMSO = 35:65

5 5

9 10

water:DMSO = 35:65 water:DMSO = 35:65

5 5

11 12 13 14 15 16 17 18

aqueous glycerol water:MIBK water (ChCl):MIBK water + 10 wt %TPAB water + 10 wt %TEAB water + 10 wt % TEAB water (NaCl):THF = 1:3 v/v DMSO:THF:H2O= 2.5:75:22.5 v/v DMSO:THF:H2O = 2.5:75:22.5 v/v water:MIBK = 1:3 water:MIBK= 1:3 water:MIBK = 2:8 water:MIBK= 1:1.6 water:MIBK = 1:2 water:MIBK = 1:2 water:MIBK= 1:4 water:MIBK = 1:1.25 water:MIBK = 1:1 water:MIBK = 1:2 water:MIBK = 1:2 water:MIBK = 1:2 (v/v) water-MIBK (1:1 v/v%) water:2-butanol = 2:3(v/v) water:n-butanol = 1:3 water:butanol= 1:2.4

5.4 40 20 20 1

41 42 43 44 45

acetone:water = 3:1 ChCl ChCl:MIBK = 1.5:1 ChCl:MIBK = 1.5:1 water:MIBK = 1:5 [BMIM] [Cl] = 2 wt % acetonitrile:water = 8:1 THF:water = 4:1 THF:water = 4:1 THF:water = 4:1 THF:water = 3:1

2.1 na na na 10

46

THF:water = 3:1

10

47 48 49 50 51

THF-water = 5:1 THF:water = 9:1 GHL:water = 9:1 GVL:water = 9:1 water- toluene

4 2 2 2 5

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

= = = =

catalyst loading

catalyst

T (°C)

t

conv (%)

select (%)

yield (%)

yield (wt %)

ref

31 wt %, 0.7 wt % 31 wt %

80

5h

na

na

42

29

621

80

5h

na

na

97

68

621

2 wt % 2 wt % 2 wt % 30 mol %

120 120 120 135

3 3 3 1

100 100 100 na

60 69 87 na

60 69 87 71

42 48 61 50

579 579 579 580

30 mol % 30 mol %

135 135

1h 1h

na na

na na

70 25

49 18

580 580

30 mol % 30 mol %

135 135

1h 1h

na na

na na

70 25

49 18

580 580

42 wt % 11 wt % 9 wt % 10 wt % 10 wt % 10 wt % 0.12 wt % 0.5 wt %

110 100 110 100 100 100 140 180

25 min 2h 1h 15 15 15 15 min 0.5 h

na. na. na. na. na. na. 99 81

na. na. na. na. na. na. 63 76

57 57 84 91 97 97 62.6 62

40 40 56 64 68 68 44 43

619 619 619 617 617 617 622 623

beta-HBL

2.6 wt %

180

0.5 h

97

85

82

58

623

− − [PPFPy]HSO4 LPSnP-1 H2SO4 5M41S550 SAPO-44 HCl HCl HSO3-MCM41 HSO3-MCM41 PA-PBI H-MOR zeolite WO3 (0.5%)-Ta2O5 (C16)H4PW11Ti HCl 1000 rpm stirring HCl Ly3PW CO2 CO2 AlCl3·6H2O

− − na 0.4 wt % 0.3 wt % 0.3 wt % 0.56 wt % 0.025 M 0.025 M 0.3 wt % 0.3 wt % 3 wt % 0.7 wt % 0.2 wt % 1 wt % 0.3 M

160 160 100 150 170 170 175 180 180 170 190 180 165 180 130 170

2h 2h 45 min 20 min 40 min 16 min 1h 9.1 mm/s 3.1 mm/s 80 min 80 min 2h 5h na 90 min 25 min

98 95 na na 80 65 89 94 97 98 94 86 60 na 98 94

78 47 na na 87 91 88 90 92 77 88 83 60 na 49 88

76 45 85 77 70 59 78 85 89 75 83 71 36 62 48 83

53 32 60 54 49 41 55 60 62 53 58 50 25 43 34 58

598 598 513 597 495 495 596 602 602 600 600 601 594 606 605 607

3.3 mM 0.25 mol % p = 4 MPa p = 2 MPa na

180 110 120 20 140

130 min 1 min 90 min 90 min 20 min

98 81 na na na

79 98 na na na

77 79 72 66 70

54 55 50 46 49

613 566 565 565 709

[MBClM]SO3Cl E45 pSO3H-Sc E0 HCl NaCl Sn-beta-F, HCl NaCl TiHP amberlyst-70 amberlyst-70 amberlyst-70 H-ZSM-5

1.4 wt % na na na na

80 130 130 130 160

4h 26 h 30 h 27 h 1h

na 68 14 78 99

na 69 7 72 77

89 na na na 76

62 na na na 53

612 583 583 583 624

na

160

1h

99

70

69

48

624

2 wt % 3.3 wt % 3.3 wt % 3.3 wt % 2.5 wt %

140 130 130 130 90

3h 10 min 10 min 9 min 12 h

na 91 91 89 na

na 85 81 80 na

55 77 74 71 50

39 50 52 50 35

589 500 500 500 641

567

h h h h

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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Table 24. continued no. 52 53 54 55 55 57

solvent DMC: water = 3:1 v/v DEC water = 3:1 v/v 1,2-PC: water = 3:1 v/v DHMTHF:water = 9:1 nitromethane:water = 10:1 nitromethane:water = 10:1

fructose conc (wt %) 4.7 4.7 4.7 0.2 3 3

catalyst loading

T (°C)

1.7 wt % 1.7 wt % 1.7 wt % 0.34 wt % 2 mol % 2 mol %

150 150 150 130 140 140

catalyst CeP3 CeP3 CeP3 amberlyst-70 SBA-15-PrSO3H SBA-15-EtPhSO3H

gave an outstanding 81% yield of 5-HMF at 175 °C for 3 h in nBuOH:water = 2:1 biphasic solvent. The catalyst maintained its good performance for six consecutive runs.604 Huo et al. reported heteropolyacid salts (C16)H4PW11Ti having Brønsted and Lewis-acidic properties as catalysts for the conversion of various carbohydrates. The system utilizing n-BuOH:water = 3:1 resulted in 77% yield of HMF from fructose at 130 °C for 2 h. The catalyst maintained its good performance for six runs after regeneration, and it was tolerant to high substrate concentrations.605 WO3−Ta2O5 catalyst, whose acidity can be fine-tuned by different WO3 to Ta2O5 ratios, gave slightly lower product yield (62%) at 180 °C in 2-BuOH:water mixture.606 Qi et al. investigated the effect of stirring on the dehydration of fructose. The reactions were carried out in aqueous n-BuOH by 0.3 M HCl at 170 °C for 30 min and stirring at 1000 rpm, resulting in 82% yield of 5-HMF. It was proven that higher stirring speed provides higher yield and mechanical stirring is more effective than magnetic stirring. The amounts of side products such as LA and glucose were insignificant.607 Lignin-based extractive solvents (LBS) containing alkylsubstituted phenolics [i.e., sec-butylphenol (SBP) or propyl guaiacol (PG)] and water mixtures was also introduced into transformation of C6 sugars. Li et al. monitored several Lewis acids for glucose-to-5-HMF conversion in a SBP:water mixture. It was unequivocally visible that 5-HMF yields were decreased according to the following order: Al(OTf)3 (45%) > Sc(OTf)3 = La(OTf) 3 (35%) > Yb(OTf) 3 (20%). It was also demonstrated that the higher valence state of the Lewis acid salt (Hf(OTf)4) could manage higher glucose conversion rates.608 D-Glucose was converted into 5-HMF by three Nb/C catalysts with different hydrophilicity in the same solvent mixture. The different catalysts were located in the organic phase, in the aqueous phase, and on the interface. Although the most effective was Nb/CB-2-DP, it provided only a low yield of 20% at 170 °C after 2 h.609 Work on 5-HMF production in SBP or PG was also performed at 170 °C. The dehydration of glucose was carried out starting from 5 wt % glucose in the presence of AlCl3 in an aqueous 0.1 M hydrochloric acid solution saturated with NaCl. Using PG:water = 2:1 mass ratio, 58% selectivity toward HMF was obtained at 85% conversion, and an excellent amount (82%) of the total 5-HMF was retained in the organic phase. While LBS exhibited a similar behavior for selectivity, it was able to extract up to 88% of 5HMF. These are comparable to glucose conversion (91%) and 5-HMF selectivity (67%) obtained using SBP as the extractive organic phase.610 Several groups performed solvent screening focusing on a combination of common organic solvents and/or ionic liquids to form biphasic systems for hexose transformation. Karimi demonstrated the efficiency of mesoporous sulfonic acid catalysts for fructose conversion. Reactions were carried out in CH3NO2:water at 140 °C for 30 min. The most effective

t 6h 6h 6h 30 min 30 min 30 min

conv (%)

select (%)

yield (%)

72.6 49.4 46 86 93 96

93.2 97.8 85.3 70 75 69

68 48 39 60 70 66

yield (wt %)

ref

47 34 27 42 49 46

616 616 616 587 611 611

catalyst was SBA-15-PrSO3H with the 5-HMF yield of 70%. The study showed that a hydrophilic surface increases the activity of the catalyst.611 Fructose dehydration in aqueous acetonitrile in the presence of ionic liquids was investigated by Qiao. 1-Methyl-3-(butyl-4-chlorosulfonyl)imidazolium chlorosulfate ([MBClM][SO3Cl]) ionic liquid was utilized as a catalyst. Under optimal conditions, high 5-HMF yield of 89% was obtained at 80 °C after 4 h in the acetonitrile−water mixture.612 High fructose syrup was converted to 5-HMF by Woodley et al. applying HCl as catalyst. The reactions were carried out in water:acetone mixture with additional NaCl in a tubular flow reactor. The study proved that acetone−water ratio had an insignificant effect on the selectivity. The yield of 5-HMF obtained was 77% after 130 min at 180 °C with a 3:1 acetone:water ratio.613 It was shown by Repo et al. that bromide salts (instead of chlorides) dissolved in the aqueous phase lead to higher 5HMF yields for Cr(III)-catalyzed glucose dehydration reactions. By use of KBr, 47% 5-HMF yield could be reached, and by KCl only 36% could be achieved in salt(aq)−(toluene/ acetone) media at 140 °C for 2 h.614 An extended work investigated the microwave-enhanced aqueous biphasic dehydration reactions in the presence of alkali metal salts. Organic solvents (MeCN and GVL) in the presence of HCl were successfully utilized as extractors resulting in a conversion of 99% of fructose with the excellent 84% yield in GVL (and KBr, saturated solution) at 160 °C for 1 min, under microwave conditions.615 Dibenedetto used [(Ce(PO4)1.5(H2O)H3O)0.5(H2O)0.5)] catalyst with high selectivity (up to 93%) and around 70% 5HMF yields were reached by the conversion of fructose at 423 K after 6 h in the biphasic system of dimethyl carbonate (DMC) and water. DMC proved to be the best with its 73% conversion ratio; however, the examined diethyl carbonate (DEC), 1,2-probene carbonate (1,2-PC), and diallyl carbonate (DAIC) gave high selectivity for 5-HMF, as well.616 Afonso et al. published work on amberlyst-15 (10 wt %)-catalyzed fructose degradation in tetrapropylammonium bromide (TPAB) and tetraethylammonium bromide (TEAB). They reported outstanding results for both reaction media, the highest 5-HMF yield of 91% was observed in TPAB at 100 °C for 15 min and 91% at 100 °C for 15 min when TEAB was used. The experiments showed that adding small amounts of water (10−15 wt %) was needed to achieve a clean transformation.617 Smith developed a ZrO2-catalyzed transformation of glucose in ionic liquid/water mixture. A synergistic effect on glucose conversion to 5-HMF was demonstrated that the presence of water in the reaction mixture accelerated the glucose to fructose isomerization and promoted the dehydration of fructose to 5HMF, which has a maximum yield of ca. 53% with 93% glucose conversion at 200 °C for 10 min. When other protic solvents, 568

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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Table 25. Conversion of Glucose to 5-HMF in Aqueous Systems Including Biphasic Systems catalyst loading

T (°C)

t

conv (%)

select (%)

yield (%)

PS−PEG-OSO3H PEG-OSO3H PBnNH3Cl IL-5 IL-5 Al-MCM-41 Al-MCM-41 Al-MCM-41 20% NaCl AlCl3·6H2O

9 wt % 3 wt % 5 wt % 2.4 wt % 2.5 wt % 1 wt % 1 wt % 1 wt % na

120 120 120 140 120 185 195 195 140

1h 1.5 h 10 h 6h 6h 150 min 150 min 30 min 20 min

98 99 88 99 100 88 86 96 na

88 79 66 79 78 28 42 66 na

26 2 5 10 1.8

(C16)H4PW11Ti 15P-TiO2 15P-TiO2 15P-TiO2 C6H4(SO3H)(NH2)

1 wt % 1 wt % 1 wt % 1 wt % 12 mol %

130 175 175 175 150

2 3 3 3 1

h h h h h

92 97 97 97 92

1.8

C6H4(SO3H)(NH2)

24 mol %

150

1h

1.8 2.5

C6H4(SO3H)(NH2) amberlyst-15 -ZrSil

150 180

4.8 1.3 1.3 1.3 2 2 5 10 10 3 3 3 0.1 0.1 0.1 2 2

InCl3 WO3-MoO3 WO3 MoO3 TiO2 P-TiO2 V-TiO2 Sn-beta-F, HCl Sn-beta-F, HCl NaCl AlSiO-20 AlSiO-10 AlSiO-20 Nb0.2WO3 Nb0.03WO3 WO3 TiO2-ZrO2 TiO2−ZrO2: amberlyst = 70:1 AlCl3·6H2O B(OH)3 AlCl3·6H2O B(OH)3 PCP(Cr)SO3H· Cr(III) PTA−PCP(Cr)SO3H· Cr(III) Al(OTf)3 Hf(OTf)3 AlCl3·6H2O AlCl3·6H2O ZrPO/Si CrPO4 PBnNH3Cl LaCl3 YbCl3 AlCl3·6H2O AlCl3·6H2O AlCl3·6H2O Nb/CB-2-DP Sn-Mont beta-Cal500

24 mol % 2.5−10 wt % 0.15 wt % 0.24 wt % 0.60 wt % 0.37 wt % 0.5 wt % 0.5 wt % 1.25 wt % na na 3 wt % 3 wt % 0.7 wt % 1 wt % 1 wt % 1 wt % 1 wt % 1 wt % 0.5 wt % 8 wt % 0.5 wt % 8 wt % 0.2 wt %

glucose conc (wt %)

no.

solvent

1 2 3 4 5 6 7 8 9

5 5 7 8 4 3 3 3 1

16 17

DMSO:water = 2:1 DMSO:water = 2:1 DMSO:water = 10:1 MIBK:water = 6:1 MIBK:water = 6:1 MIBK:water = 4:1 MIBK:water = 4:1 MIBK:water = 4:1 MIBK:water = 5:1 [BMIM][Cl] = 2 wt % water:n-butanol = 1:3 water:n-butanol = 1:2 water:n-butanol = 1:2 water:n-butanol = 1:2 water:DMSO:2-butanol:MIBK = 1:2:2:5 water:DMSO:2-butanol:MIBK = 1:2:2:5 water:MIBK = 1:2 THF/H2O (10:1)

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

THF-water THF:water THF:water THF:water THF:water THF:water THF:water THF:water THF:water THF:water THF:water THF:water THF:water THF:water THF:water THF:water THF:water

35

THF:water = 2.5:1 + NaCl

1.3

36

THF:water = 3.5:1 + NaCl

1.3

37

THF:water:NaCl = 5:3:1

0.3

38

THF:water:NaCl = 5:3:1

0.3

39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

water/sec-butyl phenol water/sec-butyl phenol H2O (NaCl)/THF H2O/THF water:MIBK = 1: 3 H2O (NaCl)/THF DMSO:MIBK:water = 3:11:1 SBP:water = 2:1 SBP:water = 2:1 ethanol:water = 9:1 ethanol:water = 3:7 ethanol: water SBP:water = 2:1 THF-DMSO = 7:3 DMSO:THF:H2O = 2.5:75:22.5 v/v

2.2 2.2 10.6 10.6 1.6 2 2 5 5 4 4 4 5 7 0.6

10 11 12 13 14 15

(3:1 v/v) = 5:2 = 5:2 = 5:2 = 4:1 = 4:1 = 4:1 = 3:1 = 3:1 = 3:1 (v/v) = 3:1 (v/v) = 3:1 (v/v) = 8:1 = 8:1 = 8:1 = 3.5:1 = 3.5:1

catalyst

569

yield (wt %)

ref

86 78 58 79 78 24 36 63 68

60 55 41 55 55 17 25 44 48

581 581 524 569 569 599 599 599 709

84 84 62 46 74

77 81 60 45 68

54 57 42 32 48

605 604 604 604 582

98

80

78

55

582

1h 1.5 h

80 87

20 45

16 39

11 27

582 588

180 170 170 170 175 175 175 160 160 160 160 160 120 120 120 175 175

10 min 40 min 40 min 40 min 105 min 105 min 105 min 1h 1h 90 min 90 min 90 min 3h 3h 3h 3h 3h

90 99 98 97 90 94 100 77 78 92 99 97 100 100 100 95 100

67 62 58 29 81 88 36 17 33 69 49 69 59 57 59 76 86

60 61 57 28 73 83 36 13 26 63 48 67 59 57 59 72 86

42 43 40 20 51 58 25 9 18 44 34 47 41 40 41 50 60

625 626 626 626 627 627 627 624 624 479 479 479 628 628 628 704 704

170

40 min

na

na

49

34

592

170

40 min

na

na

52

36

592

180

4h

na

na

81

57

484

0.2 wt %

180

4h

na

na

45

32

484

25 mM 25 mM 5.7 wt % 5.7 wt % 0.3 wt % 0.3 wt % 2 wt % 25 mM 25 mM 2.3 wt % 2.3 wt % 2.3 wt % na 3.2 wt % 0.5 wt %

160 160 160 160 165 140 140 170 170 160 160 160 170 160 180

25 min 25 min 10 min 10 min 6.6 h 30 min 10 h 80 min 80 min 15 min 15 min 15 min 2h 3h 3h

75 58 99 99 40 99 80 81 90 na na na 78 98.4 57

83 79 62 53 52 64 69 44 45 na na na 26 54 36

62 46 61 52 21 63 53 36 42 57 32 44 20 53.5 21

43 32 43 36 15 44 37 25 29 40 22 31 14 37 14

608 608 686 686 595 629 524 586 586 590 590 590 609 537 630

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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Table 25. continued no.

solvent

glucose conc (wt %)

54 55 56 57 58

DMSO:THF:H2O = 2.5:75:22.5 v/v THF:water = 9:1 GHL:water = 9:1 GVL:water = 9:1 PG:water = 2:1

0.6 2 2 2 5

59

LBS:water = 2:1

5

60

water-MeTHF/NMP (3:7 v/v)

9

beta-Cal750 amberlyst-70 amberlyst-70 amberlyst-70 AlCl3 HCl AlCl3 HCl ACl3 ·6H2O + HCl

61 62 63 64 65 66 67 68 69 70

water-MeTHF/NMP (3:7 v/v) water-DMOE (1:4 v/v) KCl(aq) KBr(aq) ChCl-water:MIBK = 2−2:2 TEAC [BMIM][Cl]:water = 1:1 w/w [HexylMIM][Cl]:water = 1:1 w/w TBAC TBAC

9 3.5 10 10 10 5 2.5 2.5 20 40

phosphated TiO2 AlCl3 CrCl3·6H2O CrCl3·6H2O AlCl3 CrCl3·6H2O ZrO2 ZrO2 CrCl3·6H2O CrCl3

catalyst

catalyst loading

T (°C)

t

0.5 wt % 6.6 wt % 6.6 wt % 6.6 wt % 0.05 M 0.1 M 0.05 M 0.1 M 10 mM+ 0.1 M 1.1 wt % 0.15 wt % 10 mol % 10 mol % 3 mol % 1 wt % 1 wt % 1 wt % 2.5 wt % 3.4 wt %

180 130 130 130 170

conv (%)

select (%)

yield (%)

yield (wt %)

3h 50 min 30 min 30 min 35 min

78 90 85 92 85

55 25 30 29 58

ref

43 23 26 32 49

30 16 18 21 35

630 500 500 500 610

170

35 min

82

59

48

34

610

175

45 min

99.9

87

87

61

631

175 150 140 140 150 130 200 200 110 110

80 min 45 min 2h 2h 15 min 10 min 10 min 10 min 4h 4h

97.9 98 na na 90 na 86 92 na na

93 58.5 na na 78 na 37 37 na na

90.7 57 36 47 70 72 32 58 49 54

63 40 25 33 48 50 22 41 34 37

631 632 614 614 603 658 618 618 641 641

Scheme 59. Cations ([MMIM]: 1,3-Dimethylimidazolium; [EMIM]: 1-Ethyl-3-methylimidazolium; [BMIM]: 1-Butyl-3methylimidazolium; [HexMIM]: 1-Hexyl-3-methylimidazolium; [OctMIM]: 1-Octyl-3-methylimidazolium; [C16MIM]: 1Hexadecyl-3-methylimidazolium; [C2OHMIM]: 1-(2-Hydroxyethyl)-3-methylimidazolium; [AMIM]: 1-Allyl-3methylimidazolium; [Me3N]: Tetramethylammonium; [Et3N]: Tetraethylammonium) and Anions ([Tf2N]: Bis((trifluoromethyl)sulfonyl)imide; [MeS]: Methanesulfonate)

such as DMSO, DMF, acetone, water, and C1−C2 alcohols were applied, a similar effect was detected; however, the influences were lower than that of water.618 Vigier and co-workers demonstrated that the application of betaine hydrochloride (BHC)-based media could be very promising in the dehydration processes of carbohydrates (i.e., fructose and inulin). The solid BHC was mixed with glycerol, water, and ChCl to make it into the liquid phase. First, BHC/ glycerol media was used for the reactions, when the conversion of lowest fructose content (10 wt %) substrate resulted in a 5HMF yield of 57% at 110 °C from BHC/glycerol (50:50) mixture, while the [BHC/water (80:20)]/[MIBK] medium

produced 5-HMF with the same 57% yield at 100 °C for 2 h. When the [ChCl/BHC]/MIBK system was used, excellent 84% yield at 110 °C for 1 h was observed.619 Aqueous GVL was also utilized for conversion of fructose and glucose in the presence of a strong solid −SO3H functionalized acid catalyst, exhibiting high performance. From fructose, 78% yield of 5-HMF was detected at 130 °C for 30 min.620 Tables 24 and 25 give an overview of the conversion of fructose and glucose to 5-HMF in the presence of various catalysts in water, including biphasic systems. 570

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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2.5.2.4. Conversion of Fructose and Glucose in Ionic Liquids. Since the seminal paper published by Moreau,633 tremendous research works have been done to perform conversion of biomass compound(s) in ionic liquids, preferably in imidazolium-based ones. Bogel-Łukasik published elaborate reviews on ionic-liquid-mediated synthesis of 5-HMF,436 solubilities of numerous carbohydrates in a wide range of ionic liquids,634 and a summary of a sustainable approach to cellulose and lignocellulosic biomass conversion without an additional catalyst in ionic liquids.635 Furthermore, reviews by Riisager636 and Valnete637 were also focused on this topic. Very recently, Han and co-workers reviewed the role of ionic liquids, which could act as reaction media as well as catalyst species for conversion of carbohydrates to value-added chemicals.50 Since then, several results were reported on the development of 5HMF production in ionic liquids. The most widely used cations and anions are summarized in Scheme 59. The special, unconventional ionic liquids are presented in the corresponding paragraph. Two roles have to be distinguished when planning sugar conversion in ionic liquids. Ionic liquids bearing necessary functionality [e.g., acidic functional group(s) such as SO3H] could act as a catalyst or preferably common ionic liquids without any functionality and can be utilized as the solvent. In this section, we focus on the latter; however, the special ionicliquid catalysts synthesized are presented. The transformations were mainly performed in the presence of either Al, Fe, Cr, Hf, Cu-based Lewis acids or heterogeneous nanoparticles. Stark et al. pointed out that product separation is crucial; they compared the efficiency of several extracting agents through thermodynamic measurements and extraction experiments and proposed the following common organic solvents as reaction media: THF, MIBK, and BuOAc.638 From the series of 1,3-dialkylimidazolium-based ionic liquids, the most extensively utilized medium is a commercially available but relatively expensive [BMIM][Cl]. It also should be noted that the synthesis trees of [BMIM]-type ionic liquids involve >20 chemical transformation steps.639 Several groups performed catalyst screening and optimization of reaction conditions (i.e., temperature, catalyst and substrate concentration, and reaction time using fructose and glucose as model substrates). In addition, possible effects of any cocatalyst as well as auxiliary substances on activity were also monitored. Zhang et al. investigated the effect of both anion and cation of ionic liquids on 5-HMF formation in a CF3SO3H-catalyzed conversion of fructose. It was demonstrated that the anions in [BMIM]-based ionic liquids have a strong influence on yield. The [HSO4] and [Cl] gave higher yields. By varying the length of the alkyl chain on the imidazolium cation, another effect was detected: a decrease in the alkyl chain length in the cation of the ionic liquid caused an increased 5-HMF yield.640 Bols et al. investigated the conversion of both glucose and fructose in [BMIM][Cl] compared with organic solvents by applying the H-ZSM-5 catalyst. In conventional organic solvents (DMA, DMSO) 5−65% 5-HMF yields were obtained from D-fructose at 90−110 °C; however, for D-glucose, these conditions could not be applied successfully. By replacing the solvent with [BMIM][Cl], 22−45% yield of 5-HMF were obtained from glucose with significantly shorter reaction times.641 Although the relatively expensive imidazolium-based ionic liquid could be replaced by [TBA][Cl], their activities were comparable (yields: 50−55%), the necessity of CrCl2 catalyst could result in serious environmental issues.

Hafnium(IV) chloride as a Lewis-acid mediated conversion of fructose, glucose, as well as oligo- and polysaccharides using chloride-containing ionic liquids was reported by Zhang.465 The study proved that fructose could be efficiently converted yielding 77.5% 5-HMF in [BMIM][Cl] at 100 °C. It was also shown that the length of the alkyl chain on the cation of the IL has a slight effect on product formation; simultaneously, the same effect was confirmed for glucose conversion in the presence of aluminoxanes.642 The trialkoxy and alkylaryloxy aluminum [Al(OiPr)3, Al(OtBu)3, MeAl(BHT)2]-catalyzed reactions also gave 5-HMF yield in the range of 45−50%.642 The replacement of HfCl4 by FeCl3 resulted in an outstanding 5-HMF yield of 91% from fructose at 110 °C.643 With a combination of CrCl3 and B(OH)3 (ratio 1:2), excellent (78.8%) yield was obtained from glucose at 120 °C. The effect of water content on efficiency was also tested showing that a small amount of water had just a little effect on the final yield.644 When B(OH)3 was added as a cocatalyst to 12tungstophosphoric acid (12-TPA) during the conversion of glucose, under optimal conditions only moderate productivity (maximum yield 52%, 140 °C) was detected. The negligible effect of water in [BMIM][Cl] was also proven.645 While the same activity was shown for Hβ-zeolite with Si:Al = 25,646 a higher yield (71% at 110 °C) was detected for Cr(III)-modified cation-exchange resin (D001-cc) for glucose conversion.647 The activity of ScCl3 was demonstrated by applying a 2 min microwave irradiation at 400 W, yielding 94.7% from fructose as a model substrate.648 Dehydration of fructose by various organic catalysts was investigated by Han. Hexachlorotriphosphazene (N3P3Cl6), trichloromelamine (C3N6H3Cl3), and N-bromosuccinimide (NBS) were utilized at 80 °C for 20 min in [BMIM][Cl]. All of the catalysts provided an excellent yield over 80%, but N3P3Cl6 was the most efficient with 93% yield. The effect of catalyst amount, temperature, and reaction time were also tested, but no significant influence on 5-HMF formation was proven. The system also worked with other substrates, such as inulin, sucrose, and glucose.649 Various metal chloride catalysts were compared for the conversion of fructose to 5-HMF in [BMIM][HSO4] by Hallett, verifying the excellent activity of CrCl3, which gave an almost quantitative conversion of fructose with a yield of 96%. Optimal conditions were 100 °C for 3 h; however, the addition of water moderately decreased the efficiency.650 The silica-supported boric acid (SSBA) catalyst was also regarded as an efficient catalyst for fructose conversion in [BMIM][HSO4] at 120 °C. The catalyst maintained its good performance after four consecutive runs.651 A cellulose-derived carbonaceous catalyst (CCC) bearing −COOH, −SO3H, and phenolic −OH groups was prepared by Hu. Its activity was shown for hydrolysis of carbohydrates, isomerization, and dehydration reactions. At 160 °C for 15 min, the moderate 46.4% 5-HMF yield was obtained from glucose. The regeneration of CCC could be completed by carbonization and sulfonation processes without significantly losing its activity.652 Qi et al. prepared a special amorphous cellulose via sulfonation with H2SO4 (CSS) or by both sulfonation and chemical activation with KOH (a-CSS). The CSS material had a lower surface area but higher −SO3H content, while the activated form had reversed properties. The highest yield of 5HMF was 83%, obtained from fructose at 80 °C after 10 min with 4.3 wt % of CSS catalyst. The a-CSS had lower activity in the reaction than the nonactivated one.653 H3PW12O40 and H4SiW12O40 were also tested for fructose conversion. Both 571

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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product 5-HMF was continuously extracted into the organic solvent phase (CO2 + acetone). At the mass ratio of 1:3:1 (5HMF in the organic solvent phase to IL phase) in the biphasic system, the excellent 84% 5-HMF yield was achieved from fructose at 393 K, 8 MPa, 4 h with TfOH catalyst, in [Omim][Cl]. The recyclability of the CO2−acetone mixture was also demonstrated.664 Over conventional and readily available ionic liquids, some work focused on the development of novel ionic liquid structures. A series of designer ionic liquids was evaluated in the dehydration of glucose, cellobiose, and maltotetraose by Dyson and Laurenczy, revealing the best performance of the Cr-catalyzed system in [C2OHMIM][Cl] for glucose (yield: 65%, 100 °C 3 h, from glucose). By replacement of the methyl group with CH 2 CH 2 Ph giving a bifunctionalized [C2OC2PhIM][Cl] and its combination with [C2OHMIM][Cl], the efficiency could be increased (yield 81%) under identical conditions. The addition of cosolvent such as toluene, DMA, etc. did not increase the product yields.665 Ionic liquids containing bis(N-methylimidazolium) cations bearing short oligo ethylene glycol linkers (Scheme 61) were

catalytic systems provided excellent HMF yields of 99% at 80 °C after 5 min, and both of them were reusable for 10 times with a slight loss of activity.654 Lignosulfonic acid (LS), a waste from the paper industry, was also characterized as a highly efficient catalyst for the conversion of fructose, giving the highest yield of 94% in [BMIM][Cl] for 10 min, with 0.5 wt % catalyst and 5 wt % substrate at 100 °C.655 Functionalized mesoporous SBA-15-SO3H-X (X = 5−20) catalysts were used for fructose conversion in [BMIM][Cl] (Scheme 60). By applying SBA-15-SO3H-10-type catalyst, Scheme 60. Formation of Mesoporous Materials SBA-15SO3Ha

a

Ref 656.

excellent conversion (99%) and 5-HMF yield (81%) were observed. These results were comparable to H2SO4-catalyzed system that resulted in full conversion and 82% yield.656 While the initial fructose concentration in the range of 10−60 wt % had no significant effect on conversion, at higher substrate contents (>35 wt %), lower product yields were obtained at 120 °C for 1 h. Me(II) chloride (Me = Cr, Cu, and Fe)-catalyzed conversion of fructose and glucose was reported by Hensen using [EMIM][Cl]-mediated Lewis-acid catalysts. From fructose, excellent conversion (90−100%) and reasonable yields (up to 87%) were achieved for Cu- and Cr-containing systems at 100 °C for 3 h. When glucose was used as the substrate, comparable results were obtained for Cr-catalyzed reactions; however, copper was not found to be selective similarly to FeCl2modified systems because they gave negligible product yields in both cases.462 Similar observation was found by Hassan for the series of FeCl3, CrCl2, CrCl3, and CuCl2 for conversion of cellulose in [EMIM][Cl].657 With the replacement of solvent by relatively cheap [TEA][Cl], no significant changes were observed for the Cr(III)-catalyzed glucose conversion (yield = 72% at 130 °C for only 10 min) in [TEA][Cl].658 Lower yields were reported for hexoses in the presence of CrCl3(THF)3,659 Cr(CO)6,660 Cr0 nanoparticles, mesoporous titania, and zirconia nanoparticles (HT-MTN and HTMZrN),661 operating in the temperature range of 80−120 °C. Porous tin(IV) phosphonate (SnBPMA) and zirconium phosphonate (ZrBPMA)-based catalyst systems operating in [EMIM][Br] were optimized for dehydration of hexoses resulting in an excellent yield of 83% of 5-HMF at 100 °C for 1 h by SnBPMA.662 Li and Ma performed mechanistic and kinetic studies on the conversion of glucose to 5-HMF catalyzed by Cr(III) in alkylimidazolium ionic liquids (i.e., [BMIM][Cl], [EMIM][Cl], and [AMIM][Cl]). It was shown that the reaction showed second-order kinetics in glucose with an activation energy of 134.9 kJ mol−1. The catalytic studies revealed that the highest yield of 5-HMF was 69% after 30 min at 100 °C.663 Zhang reported the application of a switchable biphasic reaction/separation coupling system with sub/supercritical CO2 as a “phase separation switch.” The active catalyst phase was an ionic liquid (containing dissolved CO2), and the

Scheme 61. Bis(N-methylimidazolium)-Based Cations

employed for fructose dehydration reactions. In the absence of any additive, 92.3% of 5-HMF yield was achieved with one equivalent of [TetraEG(mim)2][MeS]2 at 120 °C for 40 min. By applying [TetraEG(mim)2][OMs]2 or [DiEG(mim)2][OMs]2 bearing shorter glycol chains, the catalytic activity was lower. The study discussed the effect of reaction time, mole ratio, temperature, and cocatalyst, such as metal halides (FeCl3, CuCl2·6H2O, CoCl2·6H2O, NiCl2·6H2O) on product formation resulting in full substrate conversion with moderate selectivity for copper and excellent for nickel-based cocatalysts. Because of a proposed interaction between the metal halides and glycol segment in the ionic liquid, the catalyst system could operate at a lower temperature (100 °C).666 Unsymmetrical dicationic ionic liquids (Scheme 62) were synthesized and utilized for fructose dehydration. Imidazolium, Scheme 62. Cations of Dicationic Ionic Liquids

pyridinium, and triethylammonium were investigated as cation components accompanied by [HSO4], [MeS], and [Br]. Excellent 5-HMF yield and selectivity rates of 92% and 95%, respectively, were achieved with 97% fructose conversion under mild conditions (at 70 °C, for 40 min) by ([TetraEG(mim)(triethylamino)][HSO4]2) IL.667 Song et al. presented the synthesis of several DBU (1,8diazabicyclo[5.4.0]undec-7-ene)-based ionic liquids (Scheme 572

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Water was utilized as a reaction medium for conversion of microcrystalline cellulose in the presence of bimodal-HZ-5 zeolite catalyst to glucose and 5-HMF by Bokade. In spite of solubility limitations (67%), cellulose could be converted and 46% 5-HMF yield was achieved at 190 °C for 4 h. The catalyst could be recycled for four cycles keeping its activity.681 The Agexchanged modified acid sites containing silicotungstic acid (AgSTA) reported by Jadhav et al., operating in superheated water was also applied for conversion of saccharose resulting in moderate 63.9% 5-HMF yield at 140 °C.488 Sun et al. reported that 52.2% yield of 5-HMF was obtained from bamboo fiber degradation using NH2SO3H catalyst and 50 Hz microwave heating under aqueous conditions. The system showed good performance on the conversion of other raw lignocellulosic materials (i.e., cotton, hardwood, bamboo, and softwood). However, for the latter two, lower product yields (10−30%) were achieved.682 Zhao et al. showed that 5-HMF can be produced from ultrasonic pretreated rice husk by its thermochemical conversion in hot compressed water. 6.87% relative content was detected in the chemical composition of the heavy oil prepared at 300 °C with short contact time in hot compressed water.683 Organic solvents have also been proven to be better reaction media for di- and polysaccharides. Conversion of sucrose was performed by Zhang in the presence of CrCl3 catalyst with various alkali metal and ammonium halide promoters, from which the most efficient promoter was NH4Br, yielding 87% of 5-HMF in DMAC at 100 °C. The investigation of the effect of reaction time showed that the yield slightly decreased with longer reaction times than 1 h. The study proved that this catalytic system works with glucose and fructose as well.684 Yang et al. optimized the InCl3−ionic liquid-catalyzed process for the transformation of cellulose, revealing that a moderate yield of 45% was obtained in DMSO at 160 °C. The most efficient ionic liquid was [C3SO3HMIM][HSO4]. The catalytic system maintained its good activity after five successive runs.685 Under biphasic conditions, AlCl3·6H2O was used as a catalyst for the conversion of maltose, cellobiose, starch, and cellulose. NaCl was added to the reaction mixtures to decrease the lactic acid formation, but it had no significant effect on the 5-HMF yields. The reactions were carried out in THF:water at 160 °C for 10 min. With 99% glucose conversion, it succeeded in having 61% 5-HMF yield. Cellobiose as the starting material gave 58% yield of 5-HMF when the reaction was catalyzed by AlCl3·6H2O. For comparison, HCl as catalyst could produce only 10% 5-HMF.686 The TiO2/mordenite((K2.8Na2Ca2)· (Si39.2Al8.8O96)·34H2O) solid-acid catalyst was used for converting inulin to 5-HMF under biphasic conditions in 2BuOH:water. The one-pot heterogeneous catalytic reaction could give 61% yield of 5-HMF at 160 °C for 60 min under N2 atmosphere.687 A slightly lower yield was obtained from cellulose using FePO4 catalyst by Xia et al. The catalyst is insoluble at room temperature; however, it worked as a homogeneous catalyst at higher temperatures when it dissolved partially. Because of this property, FePO4 was easily separable from the reaction mixture by cooling. The 5-HMF yield of 50% could be achieved at 160 °C after 1 h in THF:water. Although the structure of the catalyst transformed from amorphous to crystalline form, it maintained its performance for five runs.688 Wu and co-workers reported an integrated enzyme cascadechemocatalytic conversion of cellulose to 5-HMF. The hydrolysis and isomerization reactions were carried out in aqueous (enzyme) system, while the dehydration process was

63) with benzenesulfonate (BS) anion. These solvents were successfully used for the dehydration of glucose to 5-HMF in Scheme 63. Designed Ionic Liquids by Han

the presence of CrCl3. Al-DBUBS, Bu-DBUBS, and Bn-DBUBS showed good performance, but the highest yield (83.4%) was achieved with Et-DBUBS solvent at 110 °C for 2 h.668 Dai et al. developed a new three-phase catalytic system for conversion of fructose. The system included an aqueous phase, a hydrophobic IL phase, and solid-acid catalyst of nanostructured vanadium phosphate (VPO) and mesostructured cellular foam silica (MCF) (solid phase). With the combination of two solvents (H2O and [BMIM][Tf2N]), 89% of the fructose could be converted to 5-HMF with the selectivity of 91%, but the reactions needed 20 h at 120 °C.669 In a detailed study by Zhang et al. [BMIM][Cl]-glycol dimethyl ether (GDE)-H2O ternary biphasic system was prepared and used for 5-HMF production from high concentration glucose solution (80 wt % with respect to the ionic liquid). Not only mentioning the 64.5% 5-HMF yield (108 °C, 60 min), 56% 5-HMF extraction efficiency was reached due to the use of GDE, which served multiple functions, including the extraction, saturation, and it mediated the viscosity reduction of the [BMIM][Cl].670 Although the ionic liquids could be excellent media for carbohydrate conversion, only a few studies were devoted to the investigation of separation of 5-HMF from ionic liquids, which is crucial for its large scale and viable use. Wang and coworkers demonstrated that interactions between the 5-HMF and ionic liquids were mainly ascribed to the strong hydrogen bonds of 5-HMF with anions of the corresponding ionic liquid as well as the formation abilities of hydrogen bonds of the anions with its OH group were found to decrease in the following order: CH 3COO −, C2 H5 COO − > HSO4 − > CF3COO− > N(CN)2− > NO3− > CH3OSO3− > BF4−. The study combined FT-IR, 1H NMR, and quantum chemistry calculations.671 Tables 26 and 27 summarize the conversion of fructose and glucose to 5-HMF in ionic liquids. High yields having an average of ca. 75% were reported with an applied reaction temperature between 80−140 °C, which is comparable to that utilized for conventional organic solvents. Several ionic liquids bearing Brønsted acidity could act both as a catalyst and a solvent. In addition, the stabilizing effect of the ionic liquids that prevents the subsequent rehydration of 5-HMF to LA has to be highlighted. It is important to note that several studies reported yields by the direct analysis of the reaction mixture and no product isolation was performed. 2.5.2.5. Conversion of Di- and Polysaccharides. The catalytic systems developed for conversion of model substrates to 5-HMF presented in sections 2.5.2.1−2.5.2.4 are generally tested on conversion of di- or polysaccharides such as saccharose, cellobiose, maltose, inulin, cellulose, and starch in various solvents. 573

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Table 26. Dehydration of Fructose to 5-HMF in Ionic liquids

no.

solvent

fructose conc (wt %)

catalyst

1 2 3 4 5 6 7

[EMIM][Cl] [EMIM][Cl] [EMIM][Cl] [EMIM][Cl] [EMIM][Cl] [BMIM][Cl] [BMIM][Cl]

15 15 8 8 11 10 10

8 9 10 11 12 13 14 15 16

[EMIM][Cl] [EMIM][Cl] [EMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl]

10 10 10 4.7 8.3 6 9 9 5

TRGO GO LPMSN-SO3H LPMSN-NH2 NbCl5 SBA-15 SBA-15SO3H-10 CrCl2 CuCl2 FeCl2 HfCl4 CrCl3-Al2O3 NbCl5 CSS a-CSS N3P3Cl6

17

[BMIM][Cl]

5

C3N6H3Cl3

18

[BMIM][Cl]

5

NBS

19

[BMIM][Cl]

9

20

[BMIM][Cl]

13

21

[BMIM][Cl]

0.4

lignosulfonic acid amberlyst-15 (ground) FeCl3

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

[BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM] [Cl]:[BMIM] [BF4] [BMIM][Br] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [hexMIM][Cl] [octMIM][Cl] [C16MI][Cl] [tetraEG(mim)2][OMs]2 [TriEG(mim)2][OMs]2 [DiEG(mim)2][OMs]2 [tetraEG(mim)2][OMs]2

3 na na na 28 28 28 69 8.3 9 4.5 4.7 na na na 4.7 4.7 9 90 90 90 90

44

[tetraEG(mim)2][OMs]2

90

45 46 47 48 49 50

[tetraEG(mim) (triethylamo)][HSO4]2 [tetraEG(mpyri) (triethylamo)][HSO4]2 [BMIM][Cl]:MIBK (1:1 v/v) DMA:NaBr = 10:2 wt/wt DMA:KBr = 10:2 wt/wt [BMIM][Cl]:ethanol = 1:1

90 90 5 4.3 4.3 0.4

Zr-P-Cr CrCl3·6H2O − CrCl2 H3PW12O40 H4SiW12O40 H3PW12O40 H3PW12O40 Cr(III)-Al2O3 − amberlyst-15 HfCl4 CrCl3·H2O CrCl3·H2O CrCl2 HfCl4 HfCl4 − − − − Co-cat: CoCl2 H2O Co-cat: NiCl2 H2O − − Zr(O)Cl2 Nb-NTMPA Nb-NTMPA FeCl3

574

catalyst loading

T (°C)

0.5 wt % 0.5 wt % 2 wt % 2 wt % 3 wt % 1% 1%

140 140 120 120 80 120 120

0.4 wt % 0.4 wt % 0.4 wt % 10 mol % 8.3 wt % 2 wt % 4.3 wt % 4.3 wt % 0.24 wt % 0.24 wt % 0.24 wt % 2.2 wt %

conv (%)

select (%)

yield (wt %)

4h 4h 3h 3h 60 1h 1h

na na na na na 89.2 99

na na na na na 47 82

ref

69 85 70 66 70 42 81

48 59 49 46 49 29 57

672 672 699 699 673 656 656

80 80 80 100 90 80 80 80 80

3h 3h 3h 30 min 40 min 50 10 min 10 min 20 min

100 90 18 na na na na na na

67 93 17 na na na na na na

67 84 3 77 67.6 79 83 52 93

47 59 2 54 47 55 58 36 65

462 462 462 465 675 673 653 653 649

80

20 min

na

na

90

63

649

80

20 min

na

na

82

57

649

100

10 min

98

96

94

66

655

6 wt %

80

10 min

99

83

82

57

674

0.03 wt % 2 wt % 7 mol % − 7 mol % 16 wt % 16 wt % 16 wt % 16 wt % 8.3 wt % − 4.5 wt % 10 mol % 7 mol % 7 mol % 7 mol % 10 mol % 10 mol % − 50 mol % 50 mol % 50 mol % 33 mol %

100

4h

na

na

91

64

643

120 100 100 100 80 80 60 80 90 80 25 100 100 120 100 100 100 120 120 120 120 100

1h 3h 3h 3h 5 min 5 min 10 min 40 min 40 min 8 min 3h 30 min 3h 3h 3h 30 min 30 min 12 min 150 min 150 min 150 min 40 min

100 96 69 87 99 99 83 99 na 100 na na na na na na na 100 >99 >99 >99 >99

95 na na na 99 99 96 99 na 91 na na na na na na na 88 93 77 70 74

95 na na na 99 99 80 98 84.3 91 56 75 96 89 87 72 71 88 93 77 70 74

67 na na na 69 69 56 67 59 64 39 53 67 62 61 51 50 62 65 49 54 65

511 650 650 650 654 654 654 654 675 676 677 465 650 650 650 465 465 676 666 666 666 666

33 mol %

100

40 min

>99

93

93

65

666

10 wt % 10 wt % 10 mol % 2.1 wt % 2.1 wt % 0.03 wt %

70 70 120 100 100 100

40 min 40 min 5 min 1.5 h 1.5 h 4h

97 92 na 100 98.6 na

95 90 na 86 82 na

92 83 84 85.6 80.5 80

64 58 59 60 56 56

667 667 678 679 679 643

t

yield (%)

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Table 27. Dehydration of Glucose to 5-HMF in Ionic liquids no.

solvent

glucose conc (wt %)

catalyst

T (°C)

t

conv (%)

select (%)

yield (%)

yield (wt %)

ref

100 100 100 100 120 100 100 140 100 120 120 100 120

3h 3h 3h 30 min 6h 3h 3h 4h 3h 6h 24 h 3h 30 min

100 82 4 na na na na na 99 na na na na

68 6 0 na na na na na 57 na na na na

68 5 0 35 51 71 69 77 57 49 41 71 78.8

48 4 0 25 35.7 50 48 54 40 34 29 49.7 55.2

462 462 462 465 642 659 659 672 680 660 660 659 644

140

40 min

na

na

52

36

645

90 80

180 min 2.5 min

84 93

68 91

57 85

40 60

674 674

140

30 min

38

47

18

13

646

3.5 wt %

140

30 min

48

50

24

17

646

3.5 wt %

150

50 min

81

62

50

35

646

9 wt %

150

50 min

83

54

45

32

646

110 110 110 110 160 120

30 30 30 30 15 30

na na na na 77.8 na

na na na na 60 na

53 66 71 61 46.4 78.8

37 46 50 43 32.5 55.2

647 647 647 647 652 644

110 120 120 120 100 100 120 110 110 120 120

8 30 min 30 min 60 min 1h 1h 90 min 2h 2h 12 min 5 min

na na na na 90 93 na 99 97 75 na

na na na na 78 89 na 83.7 80.4 45 na

45 11 28 32 70 83 88 83.4 78.5 34 66

32 7.7 19.6 22.4 49 58 62 58 55 24 46

641 467 467 467 662 662 651 668 668 676 678

catalyst loading 0.4 wt % 0.4 wt % 0.4 wt % 10 mol % 1 wt % na na 0.5 wt % 2.5 10 mol % 10 mol %

1 2 3 4 5 6 7 8 9 10 11 12 13

[EMIM][Cl] [EMIM][Cl] [EMIM][Cl] [BMIM][Cl] [EMIM][Cl] [EMIM][Cl] [EMIM][Cl] [EMIM][Cl] [EMIM][Cl] [EMIM]Cl [EMIM]Cl [EMIM][Cl] [BMIM][Cl]

10 10 10 4.7 16.5 10 10 15 16 na na 10 10

CrCl2 CuCl2 FeCl2 HfCl4 Et3Al CrCl3(THF)3 CrCl3(n-BuOH)3 SGO CrCl2-IMes (1:0.5) Cr(CO)6 Cr(CO)6 CrCl3(THF)3 CrCl3·6H2O, B(OH)3

14

[BMIM][Cl]

9

15 16

[BMIM][Cl] [BMIM][Cl]

13 13

17

[BMIM][Cl]

9

18

[BMIM][Cl]

9

19

[BMIM][Cl]

9

20

[BMIM][Cl]

15

21 22 23 24 25 26

[BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl]

6 6 3 6 8.8 9

12-TPA B(OH)3 CrCl3 CrCl3 MI Hβ-zeolite (Si/Al = 15) Hβ-zeolite (Si/Al = 25) Hβ-zeolite (Si/Al = 55) Hβ-zeolite (Si/Al = 15) Cr3+-D001-cc Cr3+-D001-cc Cr3+-D001-cc Cr3+-D001-cc CCC CrCl3·6H2O, B(OH)3

27 28 29 30 31 32 33 34 35 36 37

[BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [BMIM][Cl] [EMIM][Br] [EMIM][Br] [BMIM][HSO4] Et-DBUBS Al-DBUBS [C16MIM][Cl] [BMIM][Cl] - MIBK (1:1 v/v) [EMIM]Cl:DMAC = 1:5 P[BVIM]Cl:DMSO = 1:5 P[BVIM]Cl:DMF = 1:5 [BMIM]HSO4 ChCl:p-TSA= 1:1 (mol) ChCl:p-TSA= 1:1 (mol) ChCl:p-TSA= 1:1 (mol) ChCl:p-TSA= 1.5:1 (mol) [TEA][Cl] [TBA][Cl] [TBA][Cl]

5 10 10 10 9 9 9 9 9 9 5

H-ZSM-5 FeCl3·6H2O AlCl3 CrCl3·6H2O ZrBPMA SnBPMA SSBA CrCl3 CrCl3 − Zr(O)Cl2

3 wt % 11 wt % 6 wt % 6 wt % 3.5 wt % 1.3 wt %, 0.3 wt % 10 wt % 1 wt % 1 wt % 1 wt % 2.2 wt % 2.2 wt % 1.3 wt % 0.8 wt % 0.8 wt % − 10 mol %

3 3

CrCl2 CrCl2

0.2 wt % 0.2 wt %

120 120

3h 3h

na na

na na

58 55

41 39

576 576

3 9 2.5 5 10 2.5

CrCl2 SSBA − − − −

0.2 wt % 1.3 wt % − − − −

120 120 80 80 80 80

3 3 1 1 1 1

na na na na na na

na na na na na na

66 81 91 69 52 59

46 57 64 48 36 41

576 651 567 567 567 567

5 20 40

CrCl3·6H2O CrCl3·6H2O CrCl3

1 wt % 2.5 wt % 3.4 wt %

130 110 110

10 min 4 4

na na na

na na na

72 49 54

50 34 37

658 641 641

38 39 40 41 42 43 44 45 46 47 48

10 mol %, 20 mol % 2 wt % 1 wt % 1.3 wt % 1.3 wt % 5W 3.5 wt %

executed in an organic solvent system. [email protected] catalyst was used at 60 °C for 24 h, at pH 4.8, for cellulose-to-

min min min min min min

h h h h h h

glucose conversion. HSO3-MSN catalyst was employed for the fructose-to-5-HMF conversion in phosphate buffer/DMSO 575

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purity (HP, 99.5%), medium purity (MP, 98%) and low purity (LP, 93%) media for cellulose conversion at 180 °C for 5 min in a precedent study. It was shown that the MP ionic liquid had the highest activity toward 5-HMF, while HP had no activity at all. The effect of different metal halides in reaction media on efficiency was investigated in [EMIM][Cl]-HP. Expectedly, Cr was the most effective of them, Cu provided moderate 5-HMF yield, and Fe was an inhibitor of the reaction at 160 °C for different reaction times.695 Zhang et al. investigated the hydrolysis of cellulose to cellobiose, glucose, and 5-HMF by various zeolite catalysts. The yield of all three products unexpectedly could reach 97%; however, the yield of 5-HMF was rather low (max 24%) in [BMIM][Cl].696 Biomass hydrochar-derived sulfonated catalysts were tested, as well. The optimal conditions were 100 °C, 1 h in [AMIM][Cl]. The highest yield of 5-HMF obtained was 64% by the L225-SO3H catalyst; however, significantly lower yield (37%) was extracted with ethyl acetate.697 Dehydration of cellulose utilizing dualionic-liquid catalyst was investigated by Wu et al. Various reaction parameters were tested. Under optimal conditions, a moderate 5-HMF yield of 53% was obtained. This yield of 5HMF was provided by metallic ionic liquid Cr([PSMIM][HSO4])3 in [BMIM][Cl] after 5 h at 120 °C. The catalytic system could be reused with cyclic utilization (Scheme 65).698

mixed solution for 15 h, and the 5-HMF yield could reach 46.1%; however, the conversion was only 66.3%.689 In a one-pot reaction, Abu-Omar et al. investigated the conversion of starch. A series of SO42−/ZrO2-Al2O3 (CSZA) bifunctional catalysts was tested at 150 °C in DMSO:water solvent with different acidity and basicity of the catalyst and different reaction times. The study demonstrated that the highest yield was obtained with high acidity and moderate basicity by using CSZA-3 catalyst after 6 h. CSZA-1 had the highest and CSZA-5 had the least amount of acid sites. Addition of water increased the yield of 5-HMF by promoting the hydrolysis of starch to glucose.690 By the combination of acid−base bifunctionalized mesoporous silica nanoparticles (MSNs-SO3H-NH2) and microporous polymer foam poly(HIPE) PHs, Pan prepared an efficient catalyst for the synthesis of 5-HMF from cellulose. The acidic and basic features of the prepared catalysts were also investigated; the [email protected] had the highest total acidity of 3.1 mmol g−1 and basicity of 0.45 mmol g−1. After the optimization of the reaction conditions, 5-HMF yield was 44.5% from cellulose in an ILbased solvent and 88.0% in the DMSO:H2O system from glucose, respectively.691 The application of ionic liquids as reaction media for conversion of carbohydrates to generate reducing sugars (i.e., fructose and glucose and subsequently 5-HMF have been considered as an advanced strategy for the valorization of the carbohydrate content of biomass or even biomass-based wastes). The role of ionic liquids for polysaccharides’ conversion was reviewed very recently by Han and coauthors.50 The structures of common ionic liquids mentioned in this section are presented in Scheme 59. Ionic liquids in the presence of Lewis-acid catalysts were also tested for 5-HMF production from polysaccharide content of biomass or biomass-based wastes (i.e., rice husk, wheat straw, etc). Kim et al. observed the Cr-catalyzed dehydration of sucrose to 5-HMF in [C10(EPy)2][Br]2 ionic liquid (Scheme 64) as a reaction media. The highest yield of 5-HMF obtained

Scheme 65. [PSMIM][HSO4]: 1-Methyl-3-(3sulfopropyl)imidazolium Hydrogensulfate

Wu et al. reported an efficient one-pot method for converting cellulose to 5-HMF in [EMIM][Cl]. Large-pored mesoporous silica nanoparticles (LPMSNs) were synthesized and functionalized with SO3H (acid), NH2 (base) functional groups, and an acid−base bifunctionalized LPMSN nanomaterial was also characterized. The results showed that there was only a small difference in the 5-HMF yields between the variously functionalized LPMSNs in the fructose-to-5-HMF conversion (yields were in the range of 66−70%). Both LPMSN-NH2 and LPMSN catalysts provided around 13% 5-HMF yield from the glucose-to-5-HMF conversion, but it was clearly seen that the LPMSN-SO3H catalyst exhibited the highest yields of 5-HMF (18.9%) when the cellulose-to-HMF conversion was carried out. All reactions were executed at 120 °C for 3 h.699 Saha et al. used Brønsted-acidic [NMP][MeS] and [DMA][MeS] catalysts for conversion of weeds (grasses). By the use of [DMA][MeS] catalyst and DMA-LiCl solvent, a 58% yield of 5HMF was obtained from the straw part of foxtail at 120 °C for 2 min. What makes this production more important, the catalyst and the solvent system were efficiently recycled for four cycles.700 Wang et al. reported on the development of an environmentally friendly method by the elimination of toxic chromium salts. The effects of different metal salts, solvents, reaction times and temperatures, and various amounts of water were tested with cellulose. It was found that by MnCl 2 in [biC3SO3HMIM][MeS] (Scheme 66) and [BMIM][Cl] is the most efficient system. After 1 h at 120 °C, a remarkable yield of

Scheme 64. [C10(EPy)2][Br]2 Dicationic Ionic Liquids Developed by Kim

was 66% at 110 °C after 1 h. The influence of reaction time, temperature, catalyst, and substrate loading was also tested. The catalyst maintained its performance for four cycles.692 Different metal chlorides MClx (M: Cr, Cu, Sn, W, x = 2, 3) were investigated as catalysts for the conversion of microcrystalline cellulose. Reaction parameters were optimized indicating, expectedly, that CrCl3 was the most effective catalyst, providing 63% yield of 5-HMF in [BMIM][Cl] at 120 °C after 3 h. Filter paper and cotton were also investigated as feedstock, giving 40% and 12% yields, respectively. The catalytic system could be reused seven times without loss of activity.693 Detailed studies were performed on the microwaveassisted dehydration of cellulose with a metal chloride catalyst. It was proven that low toxicity ZrCl4 was the most active at 220 °C for 3.5 min in [BMIM][Cl]. By MW irradiation at 400 W, 51% yield of 5-HMF was reached. The catalytic system was reusable six times.694 The role of organic impurities in ionic liquids on efficiency was tested for [EMIM][Cl] with high 576

DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613

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for glucose conversion, lower product yield was achieved than that for fructose under identical conditions. Although 5-HMF could react further to form levulinic and formic acids via rehydration reactions, some studies claimed very high selectivities with high conversion rates. It also should be noted that the applied substrate concentrations were rather low, generally 1−10 wt % resulting in a low concentration of 5-HMF in the final reaction mixture. It could result in very high environmental factors and separation issues. Although in a few cases, higher initial fructose content (ca. 30%) was used, no significant improvement in yields was achieved. Both polar/aprotic (THF, DMSO, DMF, DMAC, MIBK, and NMP) and polar/protic (preferably C2−C4 alcohols) common organic solvents were widely tested for dehydration of hexoses representing high conversion rates (>80%) at lower temperatures (100−150 °C). There is no doubt that the yields are significantly higher in organic solvents even in the presence of a bifunctional (isomerization and dehydration) catalysts (Tables 20−23). From the series of proposed solvents, DMSO was found to be the best medium for conversion of both fructose and glucose. Under optimized conditions, yields of >70% were reported. Because of the proposed stabilizing effect of DMSO via its coordination to 5-HMF, very low or even no formation of levulinic and formic acids were reported. In alcohols, comparable yields were reported. Similar to the aqueous system, rather low substrate concentration was used to achieve high 5-HMF yields. Furthermore, the efficiency of homogeneous or heterogeneous catalysis could not be distinguished for these systems, as well. However, the latter could be recycled several times without any loss in their activity. By introducing organic/aqueous biphasic conditions to establish in situ product extraction, high yields were achieved from fructose under optimized conditions (Tables 24, 25). Even though high conversion rates were achieved from glucose, the yields of 5-HMF remained low. The application of ionic liquids as reaction medium for conversion of carbohydrates to generate reducing sugars (i.e., fructose and glucose and subsequently 5-HMF) have been considered as an advanced strategy for the valorization of the carbohydrate content of biomass or even biomass-based wastes. From the series of ionic liquids having Brønsted-acidic functionality, the imidazolium-based types were the most widely utilized media. In general, high yields were reported (Tables 26 and 27). Compared with the organic solvents, slightly higher substrate concentrations up 15 wt % were used. However, some groups used extremely high substrate loading, up to 90 wt %. It was proposed that the main advantage of ionic liquids is the stabilizing effect preventing the subsequent rehydration reactions of 5-HMF. However, it also results in separation and scale-up difficulties. Concerning the applied catalyst, a huge variety of them have been developed, characterized, and tested. While the application of mineral acids has had less importance, the use of Lewis acids as well as novel heterogeneous catalysts have come into focus of interest. It can be concluded from several reports that Cr-based catalysts play a key role in obtaining high 5-HMF yield from fructose via its ability to isomerize glucose to fructose. Both Cr(II) and Cr(III) were active, yielding similar amounts of 5-HMF. For heterogeneous catalysts bearing acidic and basic sites, it was established that the ratio of acidic to basic sites was the key parameter affecting selectivity toward 5-HMF, while the conversion of fructose was mainly related to the presence of acidic sites. The Brønsted acidity of the catalysts

Scheme 66

67% was reached. The solvent and the catalyst were both reusable four times. Lignocellulosic substrates like straw or reed also provided moderate yields.701 Designer ionic liquids (Scheme 67) were prepared and tested for the conversion of cellulose to 5-HMF by Jessop and Dyson. Scheme 67. Designer Ionic Liquids Developed by Dyson and Jessop

The one-step reactions were carried out in [C2OHMIM][Cl]/ IL-X solvent mixture for 4 h at 140 °C with CrCl2 as the catalyst. The highest yield of 5-HMF obtained was 62% in [(C1Ph)2IM][Cl] and acidic [MC4SIM][CF3SO3]. 5-HMF was extracted by MIBK or 1,2-dimethoxyethane.702 Table 28 summarizes the conversion of di- and polysaccharides into 5-HMF in different solvents. At relatively low temperatures (80−130 °C), high product yields were reported. It should be noted that some ionic liquids are good solvents of cellulose, as was demonstrated by treatment of lignocellulosic biomass in [BMIM][Cl] (Table 29). 2.5.2.6. Conclusions on the Process Chemistry of 5-HMF Production. The conversion of carbohydrates to 5-HMF is one of the most widely studied catalytic transformation of biomass utilization. Accordingly, several catalytic systems were investigated in a wide range of solvents or solvent mixtures. For aqueous conversion (Tables 18 and 19), some trends could be concluded, such as a decrease in selectivity with increasing conversion rates as well as in the absence of a catalyst rather low efficiencies were detected for fructose and glucose in the temperature range of 120−200 °C. Because of the great variation in presented yields, no clear conclusion can be drawn concerning the application of homogeneous or heterogeneous catalysts. Expectedly, when a specified catalytic system was used 577

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sucrose sucrose sucrose sucrose sucrose sucrose sucrose sucrose sucrose

sucrose sucrose

sucrose sucrose sucrose

sucrose sucrose sucrose sucrose sucrose sucrose sucrose sucrose sucrose sucrose cellobiose cellobiose

cellobiose cellobiose cellobiose cellobiose maltose maltose inulin

inulin inulin inulin inulin

10 11

12 13 14

15 16 17 18 19 20 21 22 23 24 25 26

27 28 29 30 31 32 33

34 35 36 37

substrate

1 2 3 4 5 6 7 8 9

no.

578

DMSO [AMIM]Cl [AMIM]Cl [AMIM]Cl

DMA water:MIBK= 4:1 Bu-DBUCl THF:water = 3.5:1 DMSO:water = 2:1 [AMIM]Cl DMAC

DMSO:MIBK: water = 3:11:1 water:MIBK = 1:1.6 THF:water = 3.5:1 [BMIM]HSO4 [AMIM]Cl [AMIM]Cl [BMIM][Cl] [C10(EPy)2]2Br− [BMIM]Br [AMIM]Cl water:MIBK = 1:1.6 DMAC

water:n-butanol = 1:3 DMSO:water= 2:1 DMAC

DMAC DMAC

water water water DMSO DMSO DMSO DMSO DMAC DMAC

solvent

6 4 4 4

39 2 9 2 8 6

2 1.7 2 9 5 6 6 3 6 6 1.7 4

26 8 10

9 4

9 9 9 4 0.8 0.8 0.9 10 9

substr conc AgSTA AgSTA CuCl2·2H2O FS-PAN [BMIM]OH [AEMIM]BF4 β-cyclodextrin-SO3H CrCl3 CrCl3 ·6H2O NH4Br CrCl3 ·6H2O AlCl3 NaI (C16)H4PW11Ti PS-PEG-OSO3H CrCl3 NH4Br PBnNH3Cl LPSnP-1 TiO2-ZrO2: Amberlyst15 = 70:1 SSBA MASZN-3 [NPM][HSO4] [NPM][HSO4] CrCl2 [NPM][HSO4] [C3SO3HMIM] [HSO4] LPSnP-1 AlCl3 NaI YCl3·6H2O SAPO-44 CrCl3·6H2O TiO2-ZrO2: amberlyst15 = 70:1 PS-PEG-OSO3H [C3SO3HMIM] [HSO4] AlCl3 NaI CNT-PSSA L225-SO3H D225-SO3H C265-SO3H

catalyst

Table 28. Dehydration of Di- and Polysaccharides to 5-HMF in Different Reaction Media

10 wt % 10 wt % 10 wt % 3 wt % 0.4 wt % 0.4 wt % 0.9 wt % 9.5 mol % 9.5 mol % 0.16 M 9.5 mol % 0.3 wt % 4 wt % 1 wt % 9 wt % 9.5 mol % 0.16 M 2 wt % 0.4 wt % 1 wt % 1.3 wt % 0.3 wt % 9 mol % 9 mol % 2.3 mol % 9 mol % 9 mol % 0.4 wt % 0.3 wt % 4 wt % 2.3 wt % 0.56 wt % 0.9 wt % 1 wt % 9 wt % 9 mol % 0.3 wt % 4 wt % 0.8 wt % 4 wt % 4 wt % 4 wt %

catalyst conc

120 100 100 100

145 175 100 180 120 120 130

140 150 180 120 120 120 120 120 120 120 150 130

80 120 100

100 130

120 140 120 140 160 160 140 100 100

T (°C)

90 min 1h 1h 1h

2h 6h 3h 3h 1h 1h 15 min

11 h 20 min 3h 3h 3h 1h 1h 2h 1h 1h 20 min 15 min

5h 1h 1h

1h 15 min

160 min 160 min 160 min 4h 3h 8h 2h 1h 1h

t

100 na na na

100 na na 99 98 98 99

96 96 99 na 99 98 96 81 95 99 94 99

100 97 na

na 99

92 100 78 100 na na na na na

conv (%)

87 na na na

30 na na 82 69 57 49

58 53 88 na 44 89 84 72 81 84 41 47

66 78 na

na 64

68 64 14 43 na na na na na

select (%)

87 64 60 49

30 56 39 81 68 56 49

56 51 87 80 44 87 81 58 77 83 39 47

66 76 87

66 63

63 64 11 43 54 69 85 66 87

yield (%)

61 45 42 34

21 39 27 57 48 39 34

39 36 61 56 31 61 57 41 54 58 27 33

46 53 61

46 44

44 45 8 30 38 48 60 46 61

yield (wt %)

ref

518 697 697 697

560 596 706 704 581 705

524 597 704 651 531 705 705 692 705 705 597

605 581 684

684

488 488 488 525 703 703 517 684 684

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579

cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose cellulose

53 54 55 56 57 58 59 60 61 62 63 64 65

cellulose cellulose cellulose cellulose cellulose

cellulose cellulose

51 52

67 68 69 70 71

cellulose

50

cellulose

cellulose

49

66

inulin inulin inulin inulin inulin inulin inulin cellulose cellulose cellulose cellulose

substrate

38 39 40 41 42 43 44 45 46 47 48

no.

Table 28. continued

water:n-butanol = 1:3 [BMIM][Cl]:water = 10:1 [EMIM][Cl] [EMIM][Cl] [BMIM][Cl]

[BMIM][Cl]

[EMIM][Cl]-MP [EMIM]Cl-HP [EMIM]Cl-HP [BMIM]HSO4 [BMIM][Cl] [BMIM][Cl]:water = 3:1 [BMIM][Cl] water:n-BuOH:DMSO = 1:5:11 [BMIM][Cl] water:n-BuOH = 1:5 [EMIM][Cl] [EMIM] [Cl] [BMIM][Cl]

Bu-DBUCl [C2OHmim][Cl]

[C2OHmim][Cl]

[C2OHmim][Cl]

[AMIM]Cl [AMIM]Cl DMSO Bu-DBUCl DMSO water:DMSO = 4:1 [BMIM]HSO4 water:THF:NaCl = 3:8:1 water:THF:NaCl = 3:8:1 water:THF:NaCl = 3:8:1 [C2OHmim][C l]

solvent

26 4 9 9 3.4

4

9 9 9 13 13 13 3 0.26 5 0.35 8 8 4

9 9

9

9

4 9 0.9 4 9 1.2 2.4 4.7 9

4

substr conc W265-SO3H MASZN-3 FS-PAN CrCl3·6H2O β-cyclodextrin -SO3H SiNP-SO3H-C3 SSBA FePO4 FePO4 FePO4 CrCl2 IL-1 CrCl2 IL-4 CrCl2 IL-1 IL-9 CrCl3·6H2O CrCl2 IL-4 IL-9 − CrCl2 CuCl2 CrCl3·6H2O CrCl3·6H2O CrCl3·6H2O CrCl3·6H2O ZnCl2 CrCl3 ZnCl2 HT-MTN HT-MZrN ZrCl4 MI ZrCl4 MI (C16)H4PW11Ti HY zeolite CrCl3 CrCl3/CuCl2 MnCl2 [bi-C3SO3HMIM]

catalyst 4 wt % 0.3 wt % 3 wt % 0.9 wt % 0.9 wt % 2 wt % 1.3 wt % 1.2 wt % 1.2 wt % 1.2 wt % 1 wt % 9 wt % 1 wt % 9 wt % 1 wt % 9 wt % 1 wt % 0.9 wt % 1 wt % 9 wt % 1 wt % − 6 mol % 6 mol % 7 mol % 7 mol % 7 mol % 7 mol % 11 wt % 2 wt % 15 wt % 2.2 wt % 2.2 wt % 8 wt % 400 W 8 wt % 640 W 1 wt % 11.1 mol % 0.27 wt % 0.54 wt % 3 mol %

catalyst conc

160 130 140 140 120

100−220

180 160 160 120 120 120 150 190 120 190 120 120 100−220

100 140

140

140

100 110 140 100 140 120 120 160 160 160 140

T (°C) h h h h h h h h h h h

8h 2h 10 min 10 min 1h

3.5 min

5 min 30 min 8 min 3h 3h 1h 1h 100 min 3h 100 min 3h 3h 3.5 min

2h 4h

4h

4h

1 2 4 3 2 3 5 1 1 1 4

t

91 na 85 80 na

na

na na na na na na na na na na na na na

na na

na

na

na 95 100 na na na na na na na na

conv (%)

45 na 42 50 na

na

na na na na na na na na na na na na na

na na

na

na

na 58 52 na na na na na na na na

select (%)

41 24 36 40 67

27

29 24 6 5 54 51 58 80 63 38 18 29 51

41 62

48

44

63 55 52 46 92 65 88 50 48 37 41

yield (%)

29 17 25 28 47

19

20 17 4 4 38 36 41 56 44 27 13 20 36

29 43

34

31

44 39 36 32 64 46 62 35 34 26 29

yield (wt %)

ref

605 696 657 657 701

694

695 695 695 707 707 707 707 708 693 708 661 661 694

706 702

702

702

697 531 525 706 517 579 651 688 688 688 702

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cellulose

cellulose cellulose

microcrystalline cellulose

starch starch starch starch starch starch starch starch normal corn starch high amylose corn starch waxy corn starch

73 74

75

76 77 78 79 80 81 82 83 84 85 86

substrate

72

no.

Table 28. continued

water:MIBK= 4:1 DMSO:MIBK: water= 3:11:1 DMSO:water = 5:1 DMSO:water = 5:1 water:MIBK = 1:5 [BMIM][Cl] = 2 wt % DMSO:water = 5:1 DMSO SBP:water = 2:1 DMSO/[BMIM][Cl] (1 wt %) DMSO/[BMIM][Cl] (1 wt %) DMSO/[BMIM][Cl] (1 wt %)

DMSO

[BMIM][Cl] [BMIM][Cl]

[BMIM][Cl]

solvent

2 2 0.33 0.33 1 0.33 0.9 5 na na na

0.75

5 4

3.4

substr conc [CH3SO3] MnCl2 [bi-C3SO3HMIM] [HSO4] Cr([PSMIM] HSO4)3 ZrCl4 MI InCl3 [C3SO3HMIM] [HSO4] SAPO-44 PBnNH3Cl CSZA-1 CSZA-3 AlCl3·6H2O CSZA-5 β-cyclodextrin -SO3H YbCl3 AlCl3·6H2O AlCl3·6H2O AlCl3·6H2O

catalyst 9 mol % 3 mol % 9 mol % 2.5 wt % 8 wt % 240 W 0.68 wt % 23 wt % 0.56 wt % 2 wt % 0.18 wt % 0.18 wt % na 0.18 wt % 0.9 wt % 25 mM na na na

catalyst conc

175 140 150 150 140 150 140 170 150 150 150

160

120 100−220

120

T (°C)

6h 12 h 6h 6h 20 min 6h 5h 80 min 20 min 20 min 20 min

5h

5h 3.5 min

1h

t

na 96 na na na na na na na na na

85

95 na

na

conv (%)

na 43 na na na na na na na na na

53

56 na

na

select (%)

68 41 38 55 64 45 10 42 86 73 93

45

53 17

63

yield (%)

48 29 27 39 45 32 7 29 60 51 65

32

37 12

44

yield (wt %)

596 524 690 690 709 690 517 586 543 543 543

685

698 694

701

ref

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Table 29. Dehydration of Lignocellulosic Feedstocks to 5-HMF in [BMIM][Cl] no.

substrate

substr conc (wt %)

catalyst

1 2 3 4 5 6

cotton wood chops wood chops rice straw rice straw straw

5 5 5 5 5 3.4

CrCl3 CrCl3·6H2O CrCl3·6H2O CrCl3·6H2O CrCl3·6H2O MnCl2 [bi-C3SO3HMIM] [HSO4]

catalyst conc 2 4 4 4 4 3 9

wt % wt % wt % wt % wt % mol % mol %

T (°C) 120 120 60 120 60 120

t 3 2 2 2 2 1

h h h h h h

conv (%)

select (%)

yield (%)

yield (wt %)

ref

na na na na na 85

na na na na na 74

12 79 63 76 67 63

8 55 44 53 47 44

693 710 710 710 710 701

units on economic, environmental, and social conditions separately, but they cannot fulfill all the requirements of policymakers. The United Nations conference on environment and development called on individual countries to identify indicators to improve the information basis for decision-makers. As a result, more than 500 sustainable indicators have been developed up to now. Policymakers, however, want an aggregate index that can be interpreted by and communicated to the general public. According to the European Union, sustainable development can be defined simply as “the pursuit of a better quality of life for both present and future generations. It is a vision of progress that links economic development, protection of the environment and social justice, and therefore concerns all citizens of the EU, and indeed of the whole world.”712 The European Commission, in cooperation with the Member States, European Free Trade Association, and the candidate countries developed a set of sustainable development indicators (SDIs), which are formulated around 10 sustainable development strategy objectives which all have their own indicator. Strategy objectives−indicators are as follows: socioeconomic development GDP/inhabitant; sustainable consumption and production resource productivity index; social inclusion EU-25 at-risk-of-poverty rate after social transfers; demographic changes employment rate of workers aged 55−64 years; public health EU-15 healthy life years and life expectancy at birth; climate change and energy, which has two indicators: (i) total EU-15 greenhouse-gas emissions and (ii) share of renewables in gross inland energy consumption; sustainable transport energy consumption of transport expressed as an index with the base year set at 2000; natural resources of the common bird index with the base year set at 2000; global partnership of the EU-15 official development assistance; and good governance (no defined headline indicator for this objective.)712 The United Nations created a new sustainable development agenda in 2015 and countries adopted 17 goals, which have been followed-up and reviewed using a set of global indicators. Global indicators are complemented by indicators at regional and national level. The Open Working Group on Sustainable Development Goals (SDG) published a report on an indicator framework that supports these targets. This report proposes 100 global monitoring indicators accompanied by suggestions for complementary national indicators that can monitor the sustainable development targets in an integrated, clear, and effective manner. Because this paragraph is dedicated to sustainability metrics on biomass-based production of commodity chemicals, only the relevant indicators are listed here. Primary energy by type (indicator no. 7.1), fossil fuel subsidies (indicator no. 7.2), and share of energy from renewables (indicator no. 7.3) are prior to sustainable energy for all and

generally increased the selectivity toward 5-HMF; however, Lewis acidity had the opposite effect indicated by intense humin formation. One of the key challenges of large-scale production of 5HMF is an increase in substrate/feed concentration to reduce solvent need of the transformation and energy/materials needs of the separation. While several transformations were successful on a small laboratory case in special (i.e., bifunctional or immobilized catalyst) or designed reaction media, the industrial realization requires a robust system, which has higher tolerance toward impurities and changes in feed composition. By overcoming these difficulties, the price of 5-HMF could be significantly reduced. If the price could reach ca. 1 USD/kg, the bulk-scale production would be realized.

3. SUSTAINABILITY METRICS RELATING TO THE CHEMICAL INDUSTRY 3.1. Introduction to Sustainability Metrics

The need for evaluation methods of production routes of biomass-based chemicals from the view of sustainability has emerged in the past decade. The general aim was to create a concise set of sustainability metrics with which the fossil- and biobased production routes could be compared. Because several sustainability metrics were developed for the estimation of sustainability of biomass conversion, the result highly depends on the method applied. For example, atom economy based exclusively on the stoichiometry of the reaction does not consider any auxiliary substances, which is on the other hand involved in the calculation of the E-factor. Therefore, in spite of a calculated atom economy below 100%, the subsequent utilization of byproduct(s) or recycling of any auxiliary substances (e.g., solvent), decreases the E-factor resulting in an increased sustainability of the process. Because of the numerous assessment methods and sustainability metrics, an overview of their evolution and possible application is highly desired. In this section, we overview the definitions of sustainability as well as sustainability metrics, which have relevance to the biomass utilization and conversion. Finally, the application of ethanol equivalent for IPCs is demonstrated. The definition of sustainability can be dated back to 1987, composed by the Brundtland Committee. The report “Our Common Future” states that sustainable development should meet the requirements of the present generation without compromising the needs of future generations to meet their own needs.711 The concept comprises three pillars of sustainability: people, environment, and economy; however, integrated economic welfare, environmental quality, and social coherence seemed to be not clearly measurable in a single metric. In addition, the identification of operational indicators has been realized, and these indicators may provide manageable 581

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belong to complementary national indicators for goal 7. Biomass is mentioned as biomass energy in indicator no. 7.1, and biomass as raw material has not been discussed or evaluated in the detailed description of SDG and their monitoring.713 In the last 25 years, different methods have been developed to facilitate the sustainability analysis besides the existing individual/set of indicators, such as composite indices, socially responsible investment indices, material and energy flow analysis, and life-cycle analysis. Angelakoglou and Gaidajis have recently reviewed the methods suitable for the assessment of environmental sustainability of industrial systems.714 Their comprehensive review offers a good overview of 48 methods that were identified to be capable to fulfill five criteria: ability to promote actions of improvement, ability to help decisionmaking, potential for benchmarking, applicability and ease of use, and integration of wider spatial and temporal characteristics. Focusing on biomass-based chemicals and their production, the numerous methods are narrowed to those that may be suitable for their description. The suitable methods reviewed here are classified to three groups as (i) individual/set of indicators, (ii) composite indices, and (iii) material and energy flow analysis.

effluents, and waste. Indicators are normalized preferably per added value.717 Meanwhile, the indicators of sustainable development for industry (ISDI) created in modular domains was developed, and it makes it possible to assess systems by a set of indicators belonging either to environmental, economic, or social modules. ISDI follows a life-cycle approach, taking into consideration the complete life cycle of materials and energy utilized. Indicators included in this method are applicable to most types of industries.718 However, biomass-based chemical production has not been evaluated with this method yet. Indicators of sustainable production (ISP) method is based on 22 core and other supplemental indicators for measuring the progress toward sustainable production systems. The method is applied by following an eight-step approach in which indicators are classified into three categories: energy and material use, natural environment, and products.719 The classification of indicators and their aggregation into one index helped to widen the palette of methods to create composite indices. 3.3. Composite Indices Methods

Composite indices involve clusters of approaches, which relate to each other. Atkisson720 introduced a new aggregation, scaling, and presentation methodology, called “The Compass Index of Sustainability,” based on the clusters proposed by Daly. “Daly’s Pyramid” (Figure 20) had been improved further

3.2. Individual/Set of Sustainability Indicators

The Wuppertal Sustainability Indicators (WSI) method proposes a set of indicators and takes into account the relationships among environmental, economic, and social indicators. The developed method follows a proactive approach aiming to identify foreseeable problems. Resource intensity and resource productivity that are crucial for biomass conversion were included715 and meet the conditions set by de Swaan Arons and co-workers,716 namely, the consumption rate of resources in technological applications should not be higher than the rate of resource production in the ecosphere, otherwise the needs of mankind may be endangered. From this point of view, resource productivity seems a key issue in sustainability. de Swaan Arons et al. emphasized another crucial condition of sustainability: the emissions of the technosphere must not endanger the ecological systems. In other words, the production of harmless products (wastes) may not exceed the assimilative uptake rate of these products in the ecosphere. Consequently, a chemical process is classified as nonsustaining if it takes raw materials from the ecosphere at a rate faster than the raw material is being generated or produces products (typically waste) that can damage the ecological mechanism and hence resource production. Their approach forecasts the need of integration of renewable resources in the production technology, therefore meeting both basic conditions. The same group also introduced a renewability factor that reflects the fraction of renewable exergy in total exergy consumption. This factor somehow points beyond the above-stated conditions, and expresses the valuable work potential (exergy) instead of energy. This approach binds directly to the third group of material and energy flow analysis. The Institution of Chemical Engineers (IChemE) developed IChemE sustainable development progress metrics, consisting of a set of indicators that can be used to measure the sustainability performance of industrial facilities in different scales. Economic, social, and environmental aspects of sustainability can also be assessed. Twenty-four environmental indicators are used to measure potential impacts of emissions,

Figure 20. Herman Daly’s triangle, adapted from ref 723.

by Meadows,721 which was then applied in Atkisson’s compass index of sustainability.722 Indicators and assessment scores were clustered into four quadrants based on the metaphor of compass NESW, namely nature, economy, society, and wellbeing. In application, the Compass turns a complex indicator set into a series of four performance indices, one for each Compass Point, on a scale of 0−100. Normative decisions based on both scientific and social values determine the conversion formula for each indicator. The four indices can be superaggregated to produce an “overall sustainability index.” Of the group of composite indices, the BASF method has to be highlighted as a pathfinder because it assesses environmental behavior and impacts on human and ecosystems health following the cradle-to-grave approach. Impacts are assessed according to five aspects: consumption of raw materials, consumption of energy, emissions, toxicity potential, and abuse and risk potential. Their life-cycle related costs are also included in the assessment. After normalization and weighting, a composite index, the so-called environmental fingerprint could be produced.724 582

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Their ratio IEcP/IEvP is defined as the Index of Sustainable Performance.730 Emergy and emergy analysis are bound up with energy analysis and its extensions, discussed among the material and energy flow analysis section 3.4.

Composite sustainability performance index (CSPI) method works similarly to that of BASF’s method, assessing five categories and worked out for the sustainability assessment of the steel industry. Weighting is executed by the analytical hierarchy process, aggregation according to Z-score and Liberatone methods. After estimating a composite index per category, a final estimation of CSPI final index would represent a complex sustainability. Although this method focused on the sustainability assessment of the steel industry, the authors provide its further application to other industries. It is noteworthy that this method has not been applied to biomass-based chemical industry yet.725 Krajnc and Glavič designed a model for obtaining a composite sustainable development index (ICSD) to track integrated information on environmental, economic, and social performance of a company. These three subindices were composed into an overall indicator by using the concept of an analytic hierarchy process. In their case study, a set of SDIs were classified using the widely accepted approach of the Global Reporting Initiative. Their work demonstrated that ICSD provided by the model could be applied to deliver composite indicators of sustainability performance of the Henkel company.726 By widening the number of evaluation categories, newer indices can be created. AIChE Sustainability Index (AIChE SI) evaluates sustainability through seven sustainability-related factors/categories: strategic commitment, environmental performance, safety performance, product stewardship, social responsibility, sustainability innovation, value-chain management.727,728 Each category is rated on a scale of 1−7, by comparing the performance of the assessed company/industry to best available practices. The environmental performance category takes into account the energy (including renewable energy), material (including renewable materials), and water use on the input side and calculates the greenhouse gas and other emissions on the output side. Environmental performance index (EPI) focuses on the environmental dimension of sustainability and has two overarching environmental objectives: (i) reducing environmental stresses to human health and (ii) promoting ecosystem vitality and sound natural resource management. Within the framework, six categories of environmental health, air pollution (effects on ecosystems), water pollution (effects on the ecosystem), biodiversity and habitat, productive natural resources, and climate change are assessed. All variables are normalized on a scale from 0 to 100. After data aggregation and weighting, EPI can be calculated.729 Lou and co-workers introduced a set of new sustainability indices and proposed to assess environmental and economic performances of industrial systems in a uniform structure. They introduced a common unit: emergy as the value of nonmoneyed and moneyed resources, services, and commodities.730 By definition: “The ecological cost of nature’s products and services may be estimated as the amount of energy used directly or indirectly in its manufacture, which is named emergy.”731 The same work team assessed both economic (IEcP) and environmental sustainability performance (IEvP), in which the emergy of nonrenewable and renewable resources, waste generated, and recycling flows were included. The novelty of these emergy-based sustainability indices compared with the ones used for agricultural or natural ecological systems, is that they are addressing the unique features of industrial systems (i.e., waste treatment, recovery, reuse, and recycle).

3.4. Material and Energy Flow Analysis-Based Metrics

A very popular aggregate sustainable development index referred to as material and energy flow analysis is the ecological footprint, which is based on the quantitative land and water requirements to sustain a living standard and can be expressed in square meters. By definition: “theoretical area (in global hectares) required to produce the resources consumed and to assimilate wastes generated by the system under examination.”732 The environmental impact potential of the useful product in terms of its material and energy input can be described by the Ecological Rucksack (ER) that is based on a material flow. ER refers to the material imput minus the mass of the product itself. The methodology was developed by the European Environment Agency. The concept of material input per unit of service (MIPS) was to transform the use of environmental space to a measurable value with a standardized methodology. MIPS is an interlinkage of indicators based on the material intensity analysis (MAIA or MIA) developed at Wuppertal Institute. It measures material inputs at all levels (product, company, national economy, and region).733,734 Material input is calculated by multiplying the inputs of the system with their respective conversion factors.735 Via material and resource efficiency, MIPS and ER are capable of assessing environmental sustainability. One of the first developed indices was the Sustainable Process Index (SPI), which estimates the impact of a process by calculating the total area required for the complete assimilation of a process into the ecosphere.736 Aggregating the area needed to provide raw materials, energy, and infrastructure and on the other hand the area requirements of assimilation of emissions results in the SPI. The fewer the total requirements in area, the less is the impact on the environment.737 One of the conclusions is that in a future sustainable economy, solar energy has to cover our energy needs. Embodied energy (EE) reflects the sum of the direct and indirect energy required to produce a product.738,739 EE estimates the quantity of nonrenewable energy per unit of weight of a product, as well it can provide valuable information about the efficiency of energy use. Energy seems a crucial factor in plenty of sustainable assessment methods, and its different transformations act as valuable components, too. An improvement of energy flow analysis resulted in the introduction of emergy, first used in agriculture. According to Brown and Ugiati,740 emergy is defined as “the energy of one kind that is used up in transformations directly and indirectly to obtain a product or service,” and the method which applies emergy is called emergy analysis (EA). EA is based on the evaluation of the energy used for making products or services and this “used” energy is called emergy.731 Current approaches use emergy expressed as the amount of solar energy consumed during the process. Certainly solar transformation factors are necessary to perform the estimations. Emergy is not used independently but simultaneously with composite indicators, such as environmental loading ratio, emergy yield ratio, or emergy investment ratio.741 583

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impact decreases. The limiting value of exergy efficiency is 100%, when the environmental impact approaches zero and sustainability approaches infinity because the process approaches reversibility (no loss is observed during conversion). As exergy efficiency approaches zero, sustainability approaches zero because nothing is accomplished and environmental impact approaches infinity because of the increasing quantity of resources generating an increasing amount of exergy-containing waste emitted to the environment. These concepts were demonstrated via electrical generating station consisting of four main sections, steam generators, turbine generators and transformers, condensers, and preheating heat exchangers and pumps, and has not been applied for chemical conversions yet. By the EXA method, the extra and the lacking amounts of exergy-containing flows can be identified and the efficiency of energy and resource use can be increased.746 Further improvements have been introduced in quantitatively describing the sustainability of resource utilization in terms of depletion rates of resources. Quantification is based on the principles of thermodynamics and shows the role of exergy. de Swaan Arons and co-workers kept the original idea of renewability parameter and overall efficiency (defined previously) and “relates the exergy value of the useful products to the exergy required to produce the products and to abate harmful emissions that occur either during production or during destruction stage of products.” The advantage of the improvement is that it defines the resource depletion time and transforms it to an abundance factor that shows the ratio of depletion time to the sum of depletion time and a reference time. The reference time is the time needed to reach an abundance factor of 0.5. Nonlinearity of the abundance factor makes it more sensitive than linear functions and brings a common sense to the quantification method by averaging the abundance factors of individual resources while using their exergy flows to the process as averaging weights.747 The different forms of exergy-based indicators, for example exergy depletion index, cycling ratio of material exergy, or exergy renewability seems to be applicable for the evaluation of the efficiency of resource utilization and the environmental potential effect. It visualizes its applicability in industrial ecology and the sustainability assessment of industrial processes.748 Life-cycle analysis (LCA) is an efficient tool and a widely used assessment method for the global evaluation of production technologies. The LCA technique provides a framework to incorporate ideas through the use of appropriate quantitative metrics. Although a branch of LCA methods is available and widely used for the assessment of sustainability, herein we only name a few that are able to assess chemical industrial systems without giving a comprehensive review of them. All of them named here use environmental approaches in the evaluation: bridges to sustainability framework, carbon footprint, ecosystem damage potential, life-cycle sustainability dashboard, uniform system for the evaluation of substance, CML 2001, ecoindicator 99, EDIP 97 and its upgraded version EDIP 2003, and environmental priority strategies in product development 2000. Area-specific LCA methods are the LIME method used in Japan and TRACI method used in North America. As many different assessment methods exist, their variable combinations can lead to different combined methods, which are usually viable in assessment software. Globally used combined methods are Impact 2002+ (including Impact 2002, Eco-indicator 99, CML, and IPCC methods) and ReCiPe (Eco-indicator 99 and CML 2001 methods).

Introduction of exergy can be the answer for the need for quantitative figures dealing with natural resources-consuming processes. It reflects the thermodynamic concept of sustainability (i.e., during the conversion processes, the natural resources either in the form of material or energy are converted to consumer materials and heat during the loss of given extent in work potential). By definition, exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir.742 Szargut et al. proposed a seminal theory in the late 1980’s by introducing the exergy analysis of thermal, chemical, and metallurgical processes.743 Their work was published almost at the same time as the Report of the Brundtland Committee, so at that time exergy analysis was not classified as a sustainability assessment method. Hinderink, van der Kooi, and de Swaan Arons were the first who used exergy analysis as a sustainability assessment method and calculated exergies of material streams in 1996; moreover they implemented their calculations into a flow-sheeting simulator.744 They focused on input and output streams of chemical conversion processes because this governs the overall thermodynamic efficiency. They concluded that development of chemical processes should focus on exergyneutral reactions, and the best source of exergy is from renewable resources. They pointed out that the chemical industry should focus on the development of exergy-neutral reactions, and an intuitive approach toward efficiency and sustainability using renewable resources (energy and material) is highly desired.745 The improvement of exergy analysis theory enlightens the need to fulfill the requirements of two basic conditions: (i) the consumption rate of resources cannot be faster than their natural reproduction and (ii) the waste produced and emitted into the technosphere should not endanger the resource production capacity. A renewability parameter (α) was defined as the fraction of renewable exergy consumption with respect to the total exergy consumption. Exergy analysis was implemented into the development of a sustainability indicator via two efficiency parameters: (a) the environmental parameter (η1) and (b) the process efficiency (η2). The overall efficiency (η = η1η2) is then averaged with the renewability parameter forming the sustainability coefficient (S) as follows: S = α × η/2.716 Simultaneously, Rosen and Dincer introduced exergy analysis (EXA) in the field of industrial ecology and illustrated the relation between exergy, sustainability, and associated environmental impact.746 As can be seen in Figure 21, as exergy efficiency of a process increases, sustainability increases, and conversely environmental

Figure 21. Relation between environmental impact and sustainability and its exergy efficiency, adapted from ref 746. 584

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indicators, however, provide information about all these three dimensions of sustainability. The hierarchical framework for sustainability metrics is illustrated in Figure 23. Initial step (step 1) in the process evaluation is the proper system definition. One needs to define the system boundary as well as the surroundings and needs to identify the inputs and outputs through the system boundary. Inputs are usually materials, water, and energy, while outputs are the main products, coproducts, and byproducts (emission and energy). In the second step (step 2), the identification of a number of three-dimensional metrics is required as an inventory analysis. Having enough numbers of metrics, economic analyses can be performed (step 3). If needed, two-dimensional and one-dimensional indicators are to be identified and calculated, which provide specific information about the system (step 4). Finally, improvement of the process can be achieved as results of the metrics calculations and cost estimation (step 5). An essential step is to collect the necessary data, and from a practical point of view, choose a small set of metrics. Martins and co-workers proposed four three-dimensional metrics, namely material intensity, energy intensity, potential chemical risk, and potential environmental impacts, which are applicable to a wide range of processes or systems and reflect all three dimensions of sustainability. Material intensity and energy intensity can be classified into the group of material and energy flow analyses, while chemical risk and environmental impact offer other complex evaluations and approaches. By definition, they are as follows. (i) Energy intensity measures the energy demands of a process, primarily focuses on nonrenewable energy, and calculated per unit mass of products. This is arguably the most important metric that can be used for the assessment of a manufacturing process. (ii) Material intensity measures the amount of nonrenewable resources required to obtain a unit mass of products. Applying this to chemical processes, it includes raw materials, solvents, and other ingredients. (iii) Potential chemical risk measures the potential risk to human health associated with manipulation, storage, and use of hazardous chemical compounds in the process relating to unit mass of products. (iv) Potential environmental impact measures the potential impact to the environment that is due to the emissions and the discharge of hazardous chemicals to the environment relating to unit mass of products.750 Nonrenewable energy and material use affect the environment negatively because waste is generated during their production; contrarily, they are positive for the economy because via their use value-added products are produced. Similarly, to the dual effect on the environment, the use of nonrenewable energy and material can have positive and negative effects on the living standards of the society, too. The intensive use of energy represents high living standards, but the depletion of fossil resources and the slow development of efficient renewable resource forces restricted living standards, therefore, representing a kind of negative effect on the society. On the basis of this approach, energy intensity and material intensity belong to 3−D metrics. Potential chemical risk expresses the potential to human health causing illnesses (Figure 24). Potential environmental impact clearly represents its possible effect on the environment. Both potential chemical risk and potential environmental impact can have indirect economic aspect such as financial loss due to reduced labor supply because of illness or environmental fines. During the assessments, potential chemical risk and potential environ-

Eco-indicator 99 and CML 2001 methods extended with the principal component analysis (PCA) was used by Di Paola et al.749 to assess environmental footprints of biofuels (including first- and second-generation bioethanol, biodiesel from rapeseed oil, and biogas from waste). Although they pointed out that the existence of mutual relations among different indicators has to be handled, and statistical methods are to be used to highlight the correlations, by PCA they took into account three principal components (human health, ecosystem quality, and resources) with mixing combinations according to a mixing triangle. Full LCA methods are especially designed to measure sustainability via plenty of impact categories, but they are laborious and time-consuming. Noteworthy, the assessment of a single order of sustainability depends on the existence of a single leading principal component; otherwise, the different components correspond to competing sustainability metrics. Thus, arbitrary selection of metrics, impact categories, and weighting methods leads uncertainty into the assessment. It can be claimed that scientists and engineers prefer metrics, which rely on thermodynamics and suffice for an evaluation. Although we disregard the evaluation of social-based sustainability indices, it has to be emphasized that the selection of sustainable development variables can be quite country specific, therefore in general, strongly influenced by the social development state of a given country. On the basis of these considerations, we can state that choosing variables, normalization methods, and weighting are associated with subjective judgments and reflect the nature of the sustainability.67 We can conclude that works on indicators elaborated here show they either do not truly reflect all three aspects of sustainability or they are too many and therefore difficult to apply. For a critical evaluation production technology, however, this election of adequate set of metrics is very important, especially for the comparative analyses of versions of that process. The metrics must be a coherent set of quantifiable variables; moreover, they must be clear, simple, and unambiguous. Martins and co-workers750 applied a new framework for sustainability metrics to industrial processes, including chemical processes. Correlating to the three pillars of sustainability (i.e., the three dimensions), they identified three different hierarchical groups depicted in Figure 22. One-dimensional (1−D) indicators provide information in only one dimension of sustainability: economic, ecological, or societal. Two-dimensional (2−D) indicators provide information about two dimensions such as socioecological, socioeconomic, or economic-ecological dimension. Three-dimensional (3−D)

Figure 22. Three dimensions of sustainability. Adapted from ref 750. Copyright 2007 American Chemical Society. 585

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Figure 23. Representation of sustainability metrics framework. Adapted from ref 750. Copyright 2007 American Chemical Society.

Figure 24. Derivation of potential chemical risk. Adapted from ref 750. Copyright 2007 American Chemical Society.

Figure 25. Potential environmental impact. Adapted from ref 750. Copyrignt 2007 American Chemical Society.

mental impact are to be compared, prioritized, and the one having higher impact has to be involved in the decision-making. Inventory analyses have to be performed at first including the code and name of the chemicals used in the production process, their quantities and frequency of use, and information about their hazard characteristics. Potential exposure class is determined by both quantity and frequency classes. Quantity class is divided into five levels, based on the relative value related to the sum of chemicals used in the process. Similarly to the quantity class, the frequency class is divided into five groups, as well. Depending on the R phrases provided in the Material Safety Data Sheet, chemicals are classified into one of the five hazard classes. When a compound has more than one hazard class, the largest value is selected to evaluate its potential chemical risk. After determination of hazard class and potential exposure class, potential chemical risk can be identified. Both quantity and frequency classes were worked out by Vincent et al.751 Potential chemical risk is then determined by an arbitrary weighting by the multiplication of 1 or 3 by 10 to the power of 1−6. Although determination of potential chemical risk relies on the quantity, time, and hazard domain, the arbitrary weighting due to the potential exposure makes the continuous processes unfavorable. It should be noted that the probability of unordinary operation of continuous production technologies is lower than that of batch processes because of the less frequent start-up and shutdown. Therefore, we can conclude

that this approach does not involve the overall safety of production technologies and material integration or recycling in closed loops, which would significantly lower the overall value of potential chemical risk. The evaluation method of potential environmental impact is similar to that of potential chemical risk and based on relative quantity, hazard characteristic, physical state, and the environmental impact of the chemicals on the receiving medium (air, water, and soil) (Figure 25). One of the weaknesses of this sustainable metric is the arbitrary-determined quantity threshold value. The potential environmental impact greatly depends on the transfer medium which the chemical contacts (or is emitted into), and it is determined by the physicochemical properties of the chemical and the transfer coefficients. However, the values of transfer coefficients seem arbitrarily determined (varying from 0.001 to 0.95) and could be redefined in view of more specified (moreover measured) data on transfer coefficients. Interestingly, the overall potential environmental impact is determined first, and then by knowing the transfer coefficients of each chemicals in each receiving medium, the potential impact on air, water, and soil can be determined. Martins and co-workers successfully applied their proposed frameworks, and the selected 3−D sustainability metrics for the assessment of chlorine production alternatives with mercury cells, diaphragm cells, and membrane cells; however, it can be a challenge to adapt this method to biomass-based chemicals.750 586

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The European Union COST Action CM0903 “Utilization of biomass for sustainable fuels and chemicals, UBIOCHEM” was launched in 2009 and emphasized the utilization of lignocellulosic biomass, algae, and nonedible crops. Their aim was to obtain sustainable transport fuels and commodity chemicals from biomass, including valorization of byproducts. The main objective of UBIOCHEM was to inspire scientists with different expertise to work together, utilizing biomass to produce sustainable transport fuels and basic chemicals. The secondary objective was to create or shape metrics for comparing different biomass conversion processes to sustainable fuels and basic chemicals.752 The journal Catalysis Today (Elsevier) published a special issue devoted to sustainability metrics of chemicals from renewable biomass.753 This special issue enlightened the definition of sustainability from the critical points of view and defined two fundamental conditions for sustainable replacements and sustainable waste treatment. To translate this to a technological level, the following two objective conditions were addressed: (1) “The natural resources should not be used at rates that result in their depletion.” and (2) “Waste should not be generated at rates that preclude its assimilation by the natural environment.” These were originally defined by Cséfalvay and Horváth, and reformulated by Sheldon.69 These conditions are in good agreement with the conditions set in previous studies, such as resource intensity and resource productivity included in the WSI715 and conditions proposed by de Swaan Arons and co-workers.716 Although the latter study constricts waste to harmless products, both expressions have much in common. In a transferred sense, these conditions meet the requirements of all three pillars of sustainability. Going into detail, reliability and availability as well as measurability of the data for quantification over a longer time horizon are highly desired. From the point of view that all renewable-based chemical production, technology-centered indices shall be discussed. The difficult question “How to measure sustainability of biomass-based products?” has emerged in several workshops. Researchers agree that technological and environmental impacts of processes are of utmost importance and the formulation of sustainable metrics has to rely on the principles of thermodynamics. In the late 1980’s, Sheldon introduced the environmental factor (E-factor)68 as a metric applicable for the assessment of the performance of a production process, which is now generally accepted and used by the chemical industry. Efactor is defined as the mass ratio of waste to desired product. Another single metric was the atom economy or atom utilization that is defined as the molecular weight of the desired products divided by the sum of the molecular weights of all products produced in the stoichiometric equation proposed by Trost.70 Atom Economy highlights the use of transition metal catalysts to increase selectivity of chemical reaction, thus decreasing the formation of byproducts and wastes. Both E-factor and atom economy are strongly related to the greenness of the chemical industry and represent widely accepted measures of the environmental impact of chemical processes. A concise set of meaningful metrics could provide scientists and policymakers with a simple and quick tool by which the sustainability of biomass-based routes for the manufacturing basic chemicals can be assessed. Moreover, via four basic sustainability metrics, the sustainability of biomass-based and petrochemicals routes of production of a given chemical can be

easily compared. Material efficiency, energy efficiency, land use, and costs are suggested as sustainability metrics that can supply sufficient comparison. It must be emphasized that these metrics do not include the product’s characteristics, such as biodegradability, recyclability or toxicity; they rather focus on the production process. Material efficiency involves the E-factor and is defined as follows: the total weight of useful products divided by the total weight of useful products + waste.754 The usefulness of byproducts is often a key factor in material efficiency, therefore Horváth suggested to treat the useful byproducts or side products as a secondary resource.77 Sheldon and co-workers emphasize the importance of the use of renewable energy beside fossil energy and formulated the total energy efficiency. Total energy efficiency is defined as the caloric value of the end product + caloric value of all the useful side products divided by the sum of all the fossil and renewable energy inputs, including all agricultural inputs for fertilizers, tractor, transport, and the chemical processes and downstream processing.754 Land use gives the amount of soil (given in hectares) required to produce 1 ton of products. Basically sugar, starch, protein, oil, and lignocellulosic material yields from sugar beet, corn, or rapeseed have to be considered.754 The land use of basic chemicals derived from corn−ethanol conversion processes can be explicitly calculated by knowing the ethanol yield of the cornfield.69 In an economical manner, costs represent the crucial component in the assessment. Capital costs and raw material costs are expressed in EUR per product.754 Petrochemicalbased and biomass-based production of chemicals can greatly differ in correspondence with the prices of crude oil and biomass that are greatly influenced by political decisions and stock changes and may lead to erroneous consequences. In addition, biomass-based production of chemicals is realized at a laboratory scale, and there is deficient information on upscaling and the costs of industrial production. Lastly, the cost of raw material always depends on its purpose of use and can be significantly decreased when utilizing biomass wastes rather than biomass grown directly as raw material. Horváth, Sheldon, and Poliakoff highlighted the opportunities for valorization existing residual biomass (the byproducts of present agricultural and food-processing stream), therefore revealing possibilities of cheaper and widely available resources/raw materials of commodity chemical production.45 Sheldon and Sanders evaluated the possible production of 1butanol, 1,2-propane diol, acrylonitrile, isoprene, lactic acid, succinic acid, and methionine from biomass-derived carbohydrate building blocks, although adverted the importance of origin and composition of biomass.754 The same set of sustainability metrics (i.e., material efficiency, energy efficiency, land use, and costs regarded as green metrics was applied by Uyttebroek and co-workers and the 1-butanol production by petrochemical and biobased ways were compared). When acetone, ethanol, fiber, and protein were also treated as products of ABE fermentation, the E-factor was decreased and the overall material efficiency was increased to 42%, which was less than half of that of oxo-synthesis. Their assessment revealed that the biobased production at the current stage cannot compete with the very efficient oxo-synthesis improved during one hundred years of petrochemistry. The significance of biobased production of chemicals will increase if only the crude oil reserves are endangered or the prices of biomass raw materials decrease.755 Juodeikiene et al. used material efficiency, including the E-factor, total energy 587

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Figure 26. Assessment methodology of sustainability index ratio, adapted with permission from ref 762. Copyright Elsevier 2015.

calculated costs of bioisoprene were slightly higher than the actual market price of its fossil counterpart, the energy efficiency and material efficiency values were similar, thus revealing that the bacteria-based isoprene production is a good alterative to the petrochemical process.760 Two different biobased routes and a chemical route were compared according to the above-mentioned green sustainability metrics for the production of methionine. The concise evaluation revealed that both biobased processes were attractive, showing higher material efficiency and lower energy input than the chemical process.761 Patel’s group proposed the sustainability index ratio (SIR) as a tool for measuring sustainability. They defined SIR as “the ratio of the total score of the bio-based process to that of the fossil-based process.” Accordingly a SIR < 1 indicates that the biobased route is favorable. As a demonstration, the sustainability assessment of production of higher alcohols from ethanol was performed by this method. Their work represents a kind of interface between sustainability assessment with indicators and LCA analysis. On the basis of their own laboratory work (Guerbet reaction was applied for the synthesis of higher alcohols), reaction yields, concentrations, and reaction temperature were included in the evaluation. Their methodology compares the performance of a novel chemical process using ethanol as raw material with a conventional process. Economics constraint, process costs, and environmental impact, and environmental impact of raw materials were used as indicators, and environmental impacts and economic aspects were used as impact categories. The categories and respective weighting factors are represented in Figure 26. Because the indicators used are impacts, the lowest possible total score was aimed for. The analysis showed that the biobased process was less favorable at the current state than the fossil-based process. However, they pointed out that the outcome depends on the feedstock parameters, system choices, and market scenarios, too.762 Horváth and co-workers proposed the use of ethanol equivalent (EE), which is defined as “the mass of ethanol (expressed in kilogram, tonnes or million tonnes) needed to

efficiency, land use, and costs for the comparison of lactic acid production by chemical and fermentative methods. They pointed out that the fermentative utilization of renewable carbohydrate-containing biomass offers a low-temperature, lowenergy consuming but lower-yield process representing lower total energy efficiency. The significant advantage of the biotechnological route over chemical production is the use of cheap renewable carbohydrate biomass; conversely, the advantage of chemical production is the zero land use.756 These green metrics were applied by Pinazo et al. for the assessment of succinic acid production by two different biomass-based and petrochemical routes. The evaluation of green metrics showed that energy efficiency for biomass-based succinic acid production is slightly higher, while material efficiency is rather lower than with the petrochemical route. Remarkably, calculated costs of biobased production were lower than the actual prices for petrochemical production. In this manner, biobased succinic acid production appeared to be competitive with the petrochemical route.757 Comparison of different routes for the synthesis of acrylonitrile by GuerreroPérez and Banares revealed that the catalytic conversion of glycerol is an interesting green chemistry alternative of the petrochemical propane ammoxidation. Green metrics indicate that the petrochemical process was more advantageous in terms of land use, raw materials costs, and energy efficiency. Considering the materials efficiency, the glycerol route is more advantageous.758 In the case of 1,2-propanediol, the largescale available renewable resources (glycerol) have allowed the development of biomass-based production route for commercial production of 1,2-propanediol. The evaluation of the green metrics pointed out that the biomass-based route can provide a viable alternative to the petrochemical route. Higher material efficiency and lower E-factor values were calculated, simultaneously more efficient energy use was revealed during the glycerol-based production.759 Green metrics were used as a tool to validate and compare the petrochemical and biological processes of isoprene production published by Bogel-Łukasik et al. The Sumitomo process and modified Escherichia coli bacterial process were selected for comparison. Although the 588

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Table 30. Ethanol Equivalent of IPCs, Based on Carbon-Atom Equivalency and ERoE = 2.3 no.

IPCs

formula

annual production in the USA

carbon-atom eq

EE [Mt]

EE2.3 [Mt]

field [km2]a

field [km2]b

1 2 3 4 5 6 7 8

ethanol 3-HP PA SUA FAL IP LA 5-HMF

C2H6O C3H6O3 C3H6O2 C4H6O4 C5H4O2 C5H8 C5H8O3 C6H6O3

364.979 million barrel

1 1.5 1.5 2 2.5 2.5 2.5 3

45.787

65.694

188207.093

0.373 0.039

0.535 0.056

1531.447 160.117

1.688

2.422

6939.324

411.049 314.858 382.862 320.235 491.942 693.932 407.120 449.804

0.4 Mt 0.05 Mt 1 Mt

a

Area of cornfield required to replace the current volume of chemicals by corn-ethanol. bArea of cornfield required to replace 0.1 Mt of assumed quantity of chemicals by corn-ethanol.

Table 31. Field Requirements and Ethanol Equivalent of Chemicals Based on Carbon-Atom Equivalency and ERoE = 2.3, Idealized Diameter of an Ethanol-Based Biorefinery cover the current volume of chemicals

cover an assumed 100 kt volume of chemicals

no.

chemicals

EE2.3 [Mt]

field (km )

area-times Easter Island

1 2 3 4 5 6 7 8

ethanol 3-HP PA SUA FAL IP LA 5-HMF

65.694

188207.093

1150

38

0.535 0.056

1531.447 160.117

9 1

3 1

2.422

6939.324

42

7

2 a

a

diameter (km)

a

field (km2)b

area-times Easter Islandb

diameter (km)b

411.049 314.858 382.862 320.235 491.942 693.932 407.120 449.804

3 2 2 2 3 4 2 3

2.0 1.6 1.6 1.6 2 2.3 1.6 2

a

Area of cornfield required to replace the current volume of chemicals by ethanol. bArea of cornfield required to replace 0.1 Mt of assumed quantity of chemicals by ethanol.

on the initial chemical dehydration of corn-ethanol to ethylene followed by its conversion by existing chemical processes. Indices were calculated for ethylene, propylene, toluene, pxylene, styrene, and ethylene oxide, accordingly.77

deliver the equivalent amount of energy from a given feedstock or produce the equivalent amount of mass of a carbon-based chemical using thermodynamic equivalency.” Their calculations were based on the first-generation corn-based bioethanol technology as commercially practiced in the US in 2008 providing ethanol, which then was applied as a biobased raw material. EE was used as a translational tool between fossil- and biomass-based feedstocks. By knowing the corn-ethanol yields, the required mass of biomass feedstock, the size of land, and even the volume of water for the corn plant growth, irrigation, and ethanol production could be calculated.69 Definition and the formulation of ethanol equivalent was indeed in good correlation with the two conditions set in the preface of the special issue dedicated to sustainability,753 although the approach on the no. 1 condition was the reproduction rate of the resources instead of the depletion rate of resources. Horváth and co-workers formulated two sustainability conditions as follows: “resources including energy should be used at a rate at which they can be replaced naturally and the generation of waste cannot be faster than the rate of their remediation.”69,77 By accepting these approaches, sustainability can be regarded as an intrinsic property of a material, an energy source, a reaction, a process, and seems independent of social and economic issues. Poor understanding of sustainability issues or the vested interests of the stakeholders/policymakers could lead to suitable developments with minimal or not even identifiable sustainable components. On the basis of the principles of thermodynamics, carbon-atom-equivalency, energy-equivalency, three sustainability metrics, the sustainability value of resource replacement (SVrep), the sustainability value of the fate of waste (SVwaste), and the sustainability indicator (SUSind) were defined for biomass-based carbon chemicals by using the EE as a common currency. Calculations were based

3.5. Ethanol Equivalents of Initial Platform Chemicals

In spite of the numerous sustainability assessment methods, ethanol equivalent calculated on a carbon-atom equivalency basis offers a quick and rough estimate for the sustainability of molecules. Assuming that all carbon atoms of the named chemicals (ethanol, 3-hydroxypropionic and propionic acids, succinic acid, isoprene, levulinic acid, and 5-hydroxymethylfurfural) are derived from carbon atoms of ethanol, and ignoring the real conversion and selectivity values (both treated as 100%), the calculation offers an absolutely positive estimation on the required ethanol quantity. Because the production of ethanol from biomass requires a significant amount of energy which in a sustainable world would be produced from ethanol as well, the industrial-scale energy balance of ethanol production has to be accounted for. According to the latest USDA report, 1 unit of fossil fuel used in fertilizers, powering farm machinery, transportation, processing and purification, returns 2.1 units of energy in the form of ethanol in a plant that uses the dry milling process and sells dry distillers’ grains.145,146 The energy balance ratio for the same plant modeled by engineering software is slightly higher than the real value and stayed the same as found in 2008 at 2.3.145,146 When biomass power (energy content of corn stover) is integrated into the energy input to replace external power by 50% or 100%, an even higher energy ratio of 4.1 and 58 was estimated, respectively.145 The effectiveness of the ethanol production technology characterized by its ethanol return of ethanol or ERoE was integrated into the calculations: ERoE = 2.3 indicating that out of the total of 3.3 units of bioethanol, 1 589

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unit is required for the production of 2.3 units of bioethanol. As an example, 364.979 million barrels of ethanol were produced in the US in 2015, which represents 45.787 million metric tons (Mt) (see Table 30, entry 2). Taking into consideration ERoE, EE must contain the energy input, therefore generating EE2.3 [calculated as follows: EE2.3 = EE × (1 + 1/2.3)]. The current corn-to-ethanol productivity at industrial-scale production (473 gallons ethanol/acre = 4424 L ethanol/ hectare)72 determines the cornfield requirements: ca. 188 thousand km2 land would be required for the corn and cornethanol production, or in comparison, to produce 1 Mt isoprene on an ethanol basis, only one-third of this area would be needed. The current available produced volumes were indicated (where available) and used for the calculation. When annual production volumes were not accessible, an assumed volume of 100 kt (i.e., 0.1 Mt) was applied, and the corresponding required cornfields are shown in the last column of Table 30. For better visualization of the required areas, calculated cornfields were compared with the size of Easter Island. From these values, it can be concluded that the corn-ethanol-based production of the selected chemicals, either in the case of real or assumed annual volumes, would require at least the area of Easter Island, or even several times greater fields (Table 31). It indicates the diameter of an idealized cornfield around an ethanol-producing plant of both real (indicated with diameter [km]*) and assumed volumes of chemicals (indicated with diameter [km]**). The assumption of 100 kt as a typical volume of intermediary chemical production leads to realizable ethanol and cornfield requirements. It should be noted, however, involving the conversion and selectivity of the reaction chain into the estimation, even higher ethanol volumes and land would be required. When currently produced volume of ethanol (ca. 365 million barrel, ca. 46 Mt) would be produced exclusively from cornstarch, the total EE2.3 requirements would be ca. 65 Mt of ethanol, which includes the total energy requirement as an ERoE of 2.3. All together, 188 thousand km2 land would be needed for corn production, which is 1150 times the area of Easter Island. If we assume a round-shaped cornfield and a centrally located ethanol plant, the diameter of the cornfield would be 38 km, which is dramatically several times greater than the size of world-leading industrial companies. If we assume a 100 kt annual production for ethanol, the ethanol equivalent would be much less and only a 411 km2 cornfield would be required for covering the ethanol, which is only threetimes greater than Easter Island. With the assumption of an identical location of the ethanol plant, a 2-km-sized diameter would be required.

In accordance with our knowledge, the perfect sustainability index does not exist, but several research groups are working to improve current metrics or to develop new metrics aiming to meet the requirements as follows: (i) the clearly thermodynamic-based indices seem to fulfill the recently formulated conditions of sustainability: “Resources including energy should be used at a rate at which they can be replaced naturally.” (ii) “The generation of waste cannot be faster than the rate of their remediation.” In the view of biomass-based production of chemicals, both the E-factor and atom economy represent widely accepted measures of the environmental impact of chemical processes. As summarized in a recent thematic issue of Catalysis Today, Material efficiency, energy efficiency, land use, and costs are suggested as sustainability metrics that can permit sufficient comparison. Extending basic sustainability metrics, ethanol equivalent can be used as a translational tool between fossiland biomass-based feedstocks. It must be emphasized that these metrics do not include the product’s characteristics such as biodegradability, recyclability, or toxicity; they rather focus on the production process. The sustainability index provided by Horváth and co-workers incorporates the biodegradability of wastes and byproducts formed during the chemical conversions.77 Taking all into consideration, we conclude that resource availability is the principal factor determining the biomassbased production, and then material and energy efficiency play the second crucial role. Furthermore, an efficient biomass-based chemical production can be attained when conversion and selectivity are high, the reaction does not require enormous volumes of solvents, takes places at low temperature in the presence of properly designed catalyst, and generates small amounts of wastes; moreover, the byproducts can be utilized either as it is or for energy production.

4. CONCLUDING REMARKS AND OUTLOOK An efficient and even industrially viable transformation of renewable lignocellulosic carbon resources to consumer end products is one of the most important challenges of mankind and a key issue of sustainability, which has been generally introduced as theory addressing interactions between the growing population, food consumption, industrial production, the use of natural resources, and environmental damages. Thus, the sustainability of mankind depends on whether we can continuously supply our dramatically increasing population with food, drinking water, and carbon-based chemicals independently of fossil-based feedstocks. In accordance with the theory of green chemistry, the utilization of biomass as a globally available, renewable, and natural carbon resource that has been used as raw material for the production of chemicals since ancient times could be an ideal alternative solution. The carbon conservation from biomass waste to value-added chemicals has also become a key issue for the development of fossil-independent chemical technology lowering the carbon footprint of manufactured transportation fuels, fuel additives, and carbon-based end products. The intensive research activities on the transformation of carbohydrate-containing biomass have led to the identification of basic chemicals, socalled platform chemicals that could fully and/or partially replace the currently used fossil-based chemicals in welldeveloped chemical processes. Among these platform chemicals, the initial species that are formed from the corresponding fraction of lignocellulose have to be distinguished, and

3.6. Conclusions on Sustainability Measurements

It can be concluded that in spite of the numerous sustainability indicators, individual or set of metrics, composite indices or material and energy flow analysis-based metrics, neither of them reflects truly all three aspects (environmental, economic, and social) of sustainability. For a critical evaluation production technology, however, the selection of an adequate set of metrics is very important and must rely on thermodynamic principles and must be coherent and quantifiable; moreover, they must be clear, simple, and unambiguous, and finally free of vested interests of the stakeholders. 590

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tons in the USA in 2016. The production of lignocellulosicbased second-generation ethanol has been already realized on a demonstration scale. Moreover, taking into consideration the land requirements of corresponding platform chemicals, which could be estimated by their ethanol equivalent, the coverage and therefore the sustainability of many chemical consumer products became crucial. Although commercial processes to produce monomeric sugars are developed, these streams are very diluted and impure, therefore, their application for either ethanol production or synthesis of fine chemicals is strongly limited. By the application of proper pretreatment technology, second-generation ethanol could be competitive with firstgeneration corn-based ethanol, resulting in comparable ethanol prices. Although the further transformation of ethanol to basic chemicals has already been demonstrated as secondary biorefinery, viability is still economically crucial. (8) The biochemical conversion of C5- and C6-carbohydrate units of biomass or biomass-based waste streams could offer an ideal, fossil-independent alternative production route to manufacture industrially important C3 (propionic and 3-hydroxypropionic acids), C4 (succinic acid), and for example C5 (isoprene) building blocks. However, the low productivity, low substrate concentration, and even the diluted and large volume of fermentation broth could result in serious energy needs for product isolation and concentration in the downstream process. It should be emphasized that engineered enzymes and microorganisms could provide an attractive solution to overcome the current limitation of fermentation techniques derived from the nature of microorganisms and/or enzymes (i.e., product inhibition, low titer, and productivity). Succinic acid as a C4-platform chemical is the only biobased one listed above that is manufactured on kTonne scale from C6carbohydrate units of biomass. (9) The chemocatalytic conversion of hemicellulose fraction yielding furfural as a C5unit is a widely studied and commercialized process with an average furfural yield of ca. 50%. The results of intensive academic research on furfural production could contribute to the improvement of productivity on an industrial scale. (10) The transformation of C5-building units of lignocellulose to 5hydroxymethylfurfural or levulinic acid is a very hot topic in academic research, as indicated by multitudinous published papers. Although in some cases excellent catalyst activity and selectivity were demonstrated, the scale-up and economically feasible production involving catalyst and solvent recycling and even product purification still have been a great challenge for both chemists and engineers. (11) In general, prices of biomassderived platform chemicals are currently much higher than that of fossil-based bulk chemicals. By further developments and optimization of process chemistry, the prices could be lowered, and economically viable production is envisioned in the near future. Furthermore, the fluctuation of crude oil price influences the price of biomass-based chemicals and the latter cannot be competitive with fossil-origin at the current state. (12) From a sustainability point of view, the selection of an adequate set of metrics is important when a proper critical evaluation of a chemical production technology is targeted. In spite of the numerous indicators, there are only a few that are able to characterize the biomass-based production of chemicals. Proper indicators are E-factor, atom economy, material flow, energy (or exergy) flow, land use, ethanol equivalent, and sustainability index. Sustainability metrics must be independent of political, social, and economic interests, and able to describe the two conditions of sustainability: (i) “The natural resources should

consequently, their efficient productions are crucial for sustainability. According to the recent research work on the catalytic valorization of biomass, herein the following issues would be highlighted. (1) The sustainability of a target platform molecule is primarily determined by the availability and distribution of corresponding raw materials that can easily be expressed by their land requirement and replenish time. Accordingly, to improve sustainable production of these key building blocks, the valorization of biomass-based waste streams having very similar compositions to raw biomass has to be preferred. (2) The sustainable production of IPCs from carbohydrate-containing biomass is additionally determined by the selectivity of catalytic systems. Therefore, the molecular level understanding of the operation of a catalytic system, the identification and characterization of key intermediates of the catalytic cycle is fundamentally important and could help to improve the performance of existing procedures. The in situ spectroscopy combined with isotope labeling technique could provide deeper insights and help to design better catalysts. (3) The performance of catalytic systems is generally affected by the solvent(s) and other auxiliary materials. Recently, several outstanding catalysts were prepared and tested in designer reaction media; however, their large-scale utilization is crucial. Hence, catalyst developments must be focused on robust catalysts and reduced solvent needs. Moreover, the solvents have to be cheap and readily available to achieve technically and economically feasible processes. A possibility of efficient solvent recycling in the technology or its subsequent exploitation in an integrated biorefinery concept could significantly reduce the generated waste and environmental factor, as well. (4) In most of the existing conversion techniques, generally very low substrate concentrations (1−10 wt %) have been applied, which rigorously result in large-volume solvents handling and therefore crucial separation issues. Although in some studies very high substrate feeds were reported, those systems operated in a special designer media making them technically viable but economically noncompetitive. It should be noted that several papers gave only chromatographic yields of the products, and only few data were reported on demonstration of product isolation and pilot or industrial-scale applications. For better evaluations, isolated yields have to be provided in the studies. The product isolation and subsequent purification has to be a part of the development of a viable catalytic system. The latter could be critical when scaling-up. (5) The alternative utilization of byproduct(s) formed has/have to be accelerated, thus increasing the carbon conversion of feedstocks. Moreover, decreasing the environmental factor makes the process more sustainable. (6) Process identification involving energy and material integration is another important issue of an efficient biomass conversion. Any improvements in pretreatment processes could dramatically affect the productivity of the integrated processes. (7) Ethanol is the largest volume produced platform chemical, of which production has been accelerated by dramatically increasing demands of biofuel and biofuels blending compounds. Its production via biotechnological routes has a key role in today’s chemical industry, and it is predicted to keep its role in the future. Because it can be dehydrated to ethylene, which then can be converted to propylene, butene, benzene, etc. by existing petrochemical technology, biobased ethanol could replace the fossil-based raw materials of the chemical industry. Currently, most of the ethanol is produced via fermentation of carbohydrates, such as sucrose and starch, reaching an annual amount of 46 million 591

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Á ron Németh was born in 1979 in Budapest, Hungary. He studied bioengineering (Budapest University of Technology and Economics, BME) and received a Ph.D. in Chemical Engineering from BME in 2008, under the supervision of Prof. Béla Sevella. He has worked as an assistant professor since 2008. After the untimely death of Prof. Béla Sevella, he became the head of the Fermentation Pilot Plant Laboratory. His current research activity focuses on applied microbiology and biotechnology with fermentation techniques in the laboratory, thus both upstream and downstream processes, including agricultural, food, pharmaceutical, and white biotechnology. He was the Secretary of the Bioengineering Working Committee of the Hungarian Academy of Science between 2008−2011, and in 2011, he was elected the Chairman of the same organization. Since 2012, he has been the Secretary of the Scientific Board of the Faculty of Chemical Technology and Biotechnology.

not be used at rates that result in their depletion” and (ii) “Waste should not be generated at rates that preclude its assimilation by the natural environment.” In general, no additional resources and energy needs are considered for these metrics, resulting in as positive approaches as they could be. Accordingly, a very optimistic calculation was done. If incorporating auxiliary substances, electricity needs, energy used for transportation, and storage, etc. were considered, less sustainable cases would be depicted. The use of secondary resources, however, could increase sustainability as well as the replacement of fossil energy with renewable energy covering the technology’s energy needs. Over chemistry and chemical engineering, to achieve a sustainable biomass conversion requires strict multidisciplinary approach involving material scientists, energy, process and bioengineers, biologists, and economists.

ACKNOWLEDGMENTS The authors thank Mr. Dániel Fodor and Mr. Á dám Hajnal, undergraduate students, and Assistant Prof. Jozsef M. Tukacs at Budapest University of Technology and Economics for their valuable help in graphical and technical artworks. L.T.M. is grateful to the support József Varga Foundation, Budapest University of Technology and Economics and National Research, Development and Innovation Office−NKFIH (PD 116559). L.T.M. and E.C. are grateful for the support of the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] Tel: +36 1 463 1263. Fax: +36 1 463 3197. ORCID

László T. Mika: 0000-0002-8520-0065 Notes

The authors declare no competing financial interest. Biographies

ABBREVIATIONS 3-DG 3-deoxy-D-glucosone 3-HP 3-hydroxypropionic acid 3-HPA 3-hydroxypropionaldehyde 3,4,5-TRIOL (3S,4R,5R)-2-(hydroxymethylene)-tetrahydro2H-pyran-3,4,5-triol 5-HMF 5-hydroxymethylfurfural AcK acetatekinase AFEX ammonia fiber explosion AIChE SI AIChE sustainability index BHC betaine hydrochloride BNICE biochemical network integrated computational explorer carbaldehyde-6 (3R,4S)-3,4-dihydroxy-3,4-dihydro-2H-pyran6-carbaldehyde CDP-ME diphosphocytidyl-methylerythritol CDW cell dry weight CE cellulosic-ethanol ChCl choline-chloride CSPI composite sustainability performance index DAC diallyl carbonate DAP dilute acid pretreatment DEC diethyl carbonate DES deep eutectic solvents DFT density functional theory DHM 2,5-(dihydroxymethyl)tetrahydrofuran DMAC N,N-dimethylacetamide DMAPP dimethylallyl-diphosphate DMF dimethyl-formamide DMR mechanical refining DMSO dimethyl-sulfoxide DryPB dry acid pretreatment and biodetoxification DTAB dodecyltrimethylammonium bromide DXP 1-deoxy-xylulose-5-phosphate

László T. Mika was born in 1976 in Budapest, Hungary. He studied chemical engineering (University of Veszprém, Veszprém, Hungary) and chemistry (Eötvös University, Budapest, Hungary) and received a Ph.D. degree in organic and organometallic chemistry from Eötvös University in 2010, under the supervision of Prof. István T. Horváth. He worked as an assistant professor at the Institute of Chemistry, Eötvös University, until 2012, when he joined the Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics (BME), as an associate professor. Since 2016, he has been the head of Department and Laboratory of Catalysis. His current research addresses catalytic conversion of biomass, applications of green reaction media for catalysis, thermodynamic modeling of vapor−liquid equilibria, and design of novel catalytic systems. He received the Bolyai János Scholarship of the Hungarian Academy of Sciences (2014−2017) and József Varga Foundation (2017−2018). Edit Cséfalvay was born in 1981 in Baja, Hungary. She studied environmental engineering at Budapest University of Technology and Economics, Budapest, Hungary. She received her Ph.D. degree in the discipline of Bio, Environmental, and Chemical Engineering in the field of application of green technologies in process water treatment in 2009 at the same university. She worked as an assistant professor from 2008−2009 at Eötvös University, and then she returned to her Alma Mater. She worked as an assistant professor from 2009−2015 at the Department of Chemical and Environmental Process Engineering, Faculty of Chemical Technology and Biotechnology, Budapest University of Technology and Economics (BME). Since 2015, she has been working at the Department of Energy Engineering, Faculty of Mechanical Engineering, and became an associate professor in 2017. She received the János Bolyai Research Scholarship of the Hungarian Academy of Sciences (2017−2020). Her current research activities are sustainable metrics on a thermodynamic basis, and the ignition and emissions of biobased lighter fluids. 592

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SHCF

dxR dxS E-factor EA EBAB EDP EE EMP EPI ER ERoE EXA FA FAL FBB G-3-P Glu GVL HCD HFMC HMBPP HMG-CoA IChemE ICSD IEcP IEvP IP IPCs ISDI

DXP reductoisomerase DXP synthase environmental factor emergy analysis expanded-bed-adsorption bioreactors Entner-Doudoroff pathway ethanol equivalent Embden-Meyerhof pathway environmental performance index ecological rucksack ethanol return of ethanol exergy analysis formic acid furfural fibrous-bed bioreactor glycerol-aldehyde-3-phosphate glutamate γ-valerolactone high-cell-density hollow-fiber membrane contactor hydroxyl-methylbutenyl-pyrophosphate hydroxy-methyl-glutaryl-CoA Institution of Chemical Engineers composite sustainable development index economic sustainability performance environmental sustainability performance isoprene initial platform chemicals indicators of sustainable development for industry ISP indicators of sustainable production ispS isoprene-synthase LA levulinic acid LBS lignin-based extractive solvents LCA life-cycle analysis MAIA or MIA material intensity analysis MAVS membrane-assisted vapor stripping MCR malonyl-CoA reductase MECPP methylerythritol−cyclodiphosphate MEP methylerythritol-phosphate MIBK methyl isobutyl ketone MIPS material input per unit of service MSA methanesulfonic acid MVA mevalonate NESW nature, economy, society, and well-being OAA oxaloacetate p-TSA p-toluenesulfonic acid PA propionic acid PCA principal component analysis PCC phosphoenolpyruvate carboxylase PEP phosphoenolpyruvate PFB plant fibrous-bed bioreactor PG propyl guaiacol PPP pentose-phosphate pathways PTA phosphotransacetylase RON research octane number S sustainability coefficient SB sequential batch SBP sec-butylphenol SDG sustainable development goals SDIs sustainable development indicators SE steam explosion

SHF SIR SIS SPI SSF SUA SUSind SVrep SVwaste TCA THF TOA TPAB WSI α-KG

simultaneous saccharification and cofermentation separate hydrolysis and fermentation sustainability index ratio styrene−isoprene−styrene sustainable process index saccharification and fermentation succinic acid sustainability indicator sustainability value of resource replacement sustainability value of the fate of waste tricarboxylic acid tetrahydrofuran Trin-octylamine tetrapropylammonium bromide Wuppertal sustainability indicators α-ketoglutarate

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published to the Web on November 20, 2017, with errors in Scheme 27. These were corrected in the version published to the Web on December 14, 2017.

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DOI: 10.1021/acs.chemrev.7b00395 Chem. Rev. 2018, 118, 505−613