Biomass to Furanics: Renewable Routes to Chemicals and Fuels

Perspective pubs.acs.org/journal/ascecg

Biomass to Furanics: Renewable Routes to Chemicals and Fuels Benjamin R. Caes,†,‡,⊥ Rodrigo E. Teixeira,‡,§ Kurtis G. Knapp,§ and Ronald T. Raines*,†,‡,∥

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Department of Chemistry, University of Wisconsin−Madison, 1101 University Avenue, Madison, Wisconsin 53706-1322, United States ‡ DOE Great Lakes Bioenergy Research Center, Wisconsin Energy Institute, 1552 University Avenue, Madison, Wisconsin 53726, United States § Hyrax Energy, Inc., 3475 Edison Way−Suite N, Menlo Park, California 94025, United States ∥ Department of Biochemistry, University of Wisconsin−Madison, 433 Babcock Drive, Madison, Wisconsin 55706-1544, United States ABSTRACT: The quest to achieve a sustainable supply of both energy and chemicals is one of the great challenges of this century. 5-(Hydroxymethyl)furfural (HMF), the long-known dehydration product of hexose carbohydrates, has become an important nexus for access to both liquid fuels and chemicals. One such biofuel is 2,5-dimethylfuran (DMF), which is a product of HMF hydrogenolysis and contains an energy density 40% greater than that of ethanol. In recent years, much work has been done to effect the chemical conversion of fructose, glucose, cellulose, and even lignocellulosic biomass into HMF in high yield. Here, we provide an overview of methods to access HMF from carbohydrates with the highest potential to reach an industrial scale, along with a discussion of unmet technological needs necessary for commercialization. KEYWORDS: Cellulose, Chemurgy, 2,5-Dimethylfuran, 5-(Hydroxymethyl)furfural, Ionic liquid, Lignocellulose

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environments.5 Yet, the demand for petroleum continues to grow. Meanwhile, the Earth can generate enough biomass in 10 years to match the energy content of all existing petroleum reserves.6 Hence, technologies that convert biomass into fuels and chemicals on massive scales could play a leading role in the next chapter of human civilization. The cellulose in biomass is the most abundant organic molecule on Earth. Versatile derivatives of furan, such as 5(hydroxymethyl)furfural (HMF), can be accessed from cellulose without breaking a single C−C bond (Figure 2). More specifically, HMF can be accessed from the furanose form of hexose sugars by three dehydration reactions (Figure 3),7,8 and forms spontaneously when sugars are heated, especially under acidic conditions. HMF has been detected in foods such as coffee and dried fruits,9 and is prevalent in baked goods;10 its daily intake is estimated to be 30−150 mg per person.11 Although HMF can be toxic to humans when ingested at high concentrations, multigram doses of HMF as “Aes-103” (Baxter International, Inc.) are being administered to sickle-cell disease patients in a Phase 2 clinical trial. Here, we focus on the potential of HMF to become a platform for industrial fuels and chemicals. HMF can be transformed into a large number of useful compounds through simple chemical reactions (Figure 4).12−16

INTRODUCTION In the early 20th century, William Jay Hale was a member of the faculty at the University of Michigan and later a scientist at Dow Chemical (Figure 1). During most of his professional life, Hale advocated the use of agricultural carbohydrates as industrial feedstocks for hundreds of commercial products. In the 1930s, he coined the term “chemurgy” from Greek expressions meaning “chemistry at work”. According to Hale,1 “Chemurgy brings out in relief the correct interpretation of agriculture. No longer a pursuit to supply man with food and raiment, but a pursuit that shall bring into existence a vast array of chemical compounds to fit a myriad of needs...Both the lignin and cellulose, out of wood, admirably fit into this picture.” Hale believed that ethanol would eventually become the greatest agricultural product.2 These prescient thoughts preceded by nearly a century alcohol reaching cost-parity with gasoline in Brazil. The use of chemistry to transform the farm gained early traction, but fell out of favor in the 1950s due to the emergence of inexpensive petroleumderived products.3,4 The term “chemurgy” was never adopted broadly. Since the Neolithic Revolution, humans have been rudimentary chemurgists. We relied on animal power, agriculturally derived chemicals and materials, and biomass burning for heat. More recently, the Industrial Revolution was unleashed by tapping into vast deposits of coal and then petroleum. Now, two centuries later, the carbon cycle has been altered on a sufficiently short time scale to pose an imminent threat to micro- and macro© 2015 American Chemical Society

Received: May 28, 2015 Revised: August 18, 2015 Published: September 20, 2015 2591

DOI: 10.1021/acssuschemeng.5b00473 ACS Sustainable Chem. Eng. 2015, 3, 2591−2605

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ACS Sustainable Chemistry & Engineering

Rehydration of HMF causes it to decompose into levulinic and formic acids,17−21 both valuable commodity chemicals. Levulinic acid is a precursor to the liquid fuel γ-valerolactone,22−24 which can also solubilize biomass.25−28 Oxidation of the hydroxymethyl group leads to 2,5-diformylfuran and 2,5-furandicarboxylic acid. The latter, which has been accessed by electrochemical oxidation of HMF,29 is of particular interest for its use in the manufacturing of polyamides, polyesters, and polyurethanes as a substitute for terephthalic and isophthalic acid polymers.30 Reduction of the formyl moiety accesses 2,5-bis(hydroxymethyl)furan, whereas hydrogenation forms 2,5-bis(hydroxymethyl)tetrahydrofuran and 2,5-diformyltetrahydrofuran. These products can undergo condensation to generate polymers that can ultimately become liquid alkanes.31−33 Finally, hydrogenolysis yields 2,5-dimethylfuran (DMF), 2,5-dimethyltetrahydrofuran, and 2-methyltetrahydrofuran. With its high energy density, low volatility, and immiscibility with water, DMF has evoked substantial interest as a fuel.34−36 HMF and its derivatives hold vast potential and could eventually match the array of compounds derived from petroleum, allowing biomass to become a viable source of energy and materials. To reach this goal, conversion technologies must reach high yields without cost-prohibitive catalysts or processes that are too complex. Many investigators have manifested the conversion of mono- and polysaccharides into HMF in high yields, but no single route has prevailed to date. Diverse aspects of HMF production and utilization have been reviewed elsewhere.15,16,36−45 Herein, we focus on chemical strategies for accessing HMF that we believe hold special promise. In addition, we highlight routes to the candidate biofuel: DMF.

Figure 1. Photograph of William Jay Hale (1876−1955) in 1917, when he was an Associate Professor at the University of Michigan. Hale was a visionary advocate of the use of farm carbohydrates as industrial feedstocks for plastics, paints, gasohol, and other commercial products.1,2 Photograph is Courtesy of the Chemical Heritage Foundation Collections.

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SOLVENTS Carbohydrate polymers can form inter- and intrastrand hydrogen bonds, making them more recalcitrant to dissolution, and hence inaccessible for chemical reactions. A select few solvents are able to accomplish dissolution.46−49 Of these privileged solvents, ionic liquids are especially noteworthy. Ionic liquids are organic salts that melt near room temperature, typically below 100 °C.50−52 As salts, they have a negligible vapor pressure. The binary combination, resulting from the pairing of a cation (e.g., ammonium, imidazolium, phosphonium, or pyridinium) with an anion (e.g., acetate, bromide, chloride, or hexafluorophosphate), allows access to a staggering number of species.51 This variety allows for the customization of properties for specific tasks. One such task is the dissolution of polysaccharides.53−56 For a solvent to dissolve cellulose, it must out-compete the hydrogen-bond network. Ionic liquids do so by manifesting a high concentration of anionic species that form more stable hydrogen bonds with the hydroxyl groups of cellulose.53 Dialkylimidazolium and dialkylpyridinium chloride ionic liquids are especially well-known for their capacity to dissolve cellulose at significant concentrations.46,54,57 The reuse of an ionic liquid after a bioconversion process mitigates its cost,58,59 and the addition of a fluorous tag to an ionic liquid can facilitate separations.60 Moreover, ionic liquids themselves can catalyze the transformation of hexose sugars into HMF,61 but yields are enhanced by exogenous catalysts, as described below. In addition to serving as the solvent for biomass conversions, ionic liquids also dissolve algal cell walls to extract intracellular contents, including lipids,62 and serve as the basis for pretreatment strategies.63−65

Figure 2. Route for the conversion of cellulose to HMF highlighting the retained connectivity of all carbon atoms.

Figure 3. Putative mechanism for the dehydration of fructose to form HMF.7,8

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Figure 4. Synthetic routes from HMF to fuels and chemicals by (i) rehydration, (ii) oxidation, (iii) reduction, (iv) hydrogenolysis, and (v) hydrogenation.13,41,45 BHF, 2,5-bis(hydroxymethyl)furan; BHM-THF, 2,5-bis(hydroxymethyl)tetrahydrofuran; DFF, 2,5-diformylfuran; DF-THF, 2,5-diformyltetrahydrofuran; DMF, 2,5-dimethylfuran; FDCA, 2,5-furandicarboxylic acid; HMF, 5-(hydroxymethyl)furfural; M-HMF, 5-methyl-2hydroxymethylfuran; MHM-THF, 5-methyl-2-hydroxymethyltetrahydrofuran.

Table 1. Representative Conversions of Carbohydrates to HMF Using Mineral Acid Catalysts carbohydrate

catalyst

solvent

T (°C)

time

HMF molar yield (%)

ref

fructose fructose fructose fructose glucose sucrose inulin starch cellobiose fructose fructose glucose sucrose inulin fructose fructose fructose fructose fructose

H2SO4 H2SO4 HCl HCl HCl HCl HCl HCl HCl H3PO4 H2SO4 H2SO4 H2SO4 H2SO4 HCl HCl HCl HCl HBr

subcritical H2O ethylene glycol dimethyl ether 7:3 (8:2 H2O/DMSO)/PVP/7:3 MIBK/2-butanol 5:5 H2O/DMSO/7:3 MIBK/2-butanol 4:6 H2O/DMSO/7:3 MIBK/2-butanol 4:6 H2O/DMSO/7:3 MIBK/2-butanol 5:5 H2O/DMSO/7:3 MIBK/2-butanol 4:6 H2O/DMSO/7:3 MIBK/2-butanol 4:6 H2O/DMSO/7:3 MIBK/2-butanol subcritical H2O 9:1 supercritical acetone/H2O 9:1 supercritical acetone/H2O 9:1 supercritical acetone/H2O 9:1 supercritical acetone/H2O H2O H2O H2O 1:5 H2O/DMSO/MIBK/2-butanol sulfolane

250 200 200 170 170 170 170 170 170 240 180 (20 MPa) 180 (20 MPa) 180 (20 MPa) 180 (20 MPa) 200 200 185 (17 bar) 185 (20 bar) 100

32 s 3.3 h 3 min 4 min 10 min 5 min 5 min 11 min 10 min 2 min 2 min 2 min 2 min 2 min 1s 1 min 1 min 1 min 1h

53 70 75 85 23 50 75 26 27 65 77 48 56 78 33 53 54 83 93

7 75 76 77 77 77 77 77 77 79 81 81 81 81 82 82 83 83 84

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CATALYSTS

In 1895, HMF was synthesized with intent from inulin by Düll66 and from sugar cane by Kiermayer67 using oxalic acid as a catalyst. These initial reports led to more extensive work by Fenton and co-workers.68−70 In 1919, Middendorp reported a detailed study on the synthesis, physical characteristics, and chemical behavior of HMF.71 In subsequent years, Reichstein72 and Haworth and Jones73 made immense contributions by putting forth a detailed process and chemical mechanism for the acid-catalyzed formation of HMF from fructose. In recent years, numerous advancements have been made in the use of mineral acids to transform carbohydrates (Table 1). Using H2SO4, Antal and co-workers reported the dehydration of fructose in subcritical water at 250 °C to access HMF in 53% yield.7 Moreover, they were able to conclude that fructose dehydrates to HMF through closed-ring intermediates due to the ease of HMF formation from fructose and the fructosyl moiety of sucrose, as well as the facile conversion of 2,5-anhydro-D-

Mineral Acids. The use of common mineral acids (e.g., hydrochloric, sulfuric, and phosphoric acids) to catalyze the conversion of sugars to furanics has a long history. This approach had its origins in the so-called “furfuraldehyde tests” for carbohydrates that were developed in the late 19th century. The first, Molisch’s test, is based on pentoses and hexoses being dehydrated by sulfuric acid to form furfural and HMF, respectively, which condense with α-naphthol to form a purple dye. Seliwanoff’s test for ketoses relies on ketoses (like fructose) undergoing hydrochloric acid-catalyzed dehydration to yield furfural derivatives more rapidly than do aldoses (like glucose). These derivatives react with resorcinol to form a xanthine dye with a deep red color. In Bial’s test, orcinal instead of resorcinol and added FeCl3 is used to distinguish between pentoses and hexoses. 2593


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DMSO. Raines and co-workers showed that a versatile industrial solvent, sulfolane, supports the HBr-catalyzed conversion of fructose to HMF in 93% yield in 1 h.84 Acid catalysis, while serving as a common method to access HMF from carbohydrates, also hydrolyzes cellulosic polysaccharides to monomeric sugars. The Amarasekara group used the acidic ionic liquids 1-(1-propylsulfonic)-3-methylimidazolium chloride and 1-(1-butylsulfonic)-3-methylimidazolium chloride to dissolve and hydrolyze cellulose to glucose and other reducing sugars in the presence of water.85 The highest yields for reducing sugars (62%) and, specifically, glucose (14%) were obtained with 1 h of preheating at 70 °C and continued heating for 30 min after water addition. Seddon and co-workers used H2SO4 and methanesulfonic acid to study the kinetics of cellobiose hydrolysis, and observed a linear dependence on acid concentration.86 The maximal amount of glucose obtained from cellobiose was 68%, though degradation of glucose occurred in the presence of the strong acids. Hydrolysis of lignocellulosic biomass (miscanthus grass) gave only 5% glucose; though if lignin were extracted first, 30% yields were obtainable, as were 25% yields of xylose. In another study, Binder and Raines used HCl in the ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) to accomplish the hydrolysis of corn stover, another lignocellulosic biomass source.87 When untreated biomass was hydrolyzed with 20 wt % HCl with gradual water addition to enhance glucose stability, the yields of glucose and xylose were 42% and 71%, respectively. A second stage hydrolysis of the precipitated, unhydrolyzed residues allowed more sugar to be accessed, with final yields of 70% for glucose and 79% for xylose. Those yields were increased later by the same group to 92% for glucose and 95% for xylose after reoptimization of HCl concentration, gradual water addition in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), and separation of the products by simulated moving bed chromatography.88 The Dumesic group achieved 44−48% and 57−59% yields of glucose and xylose, respectively, in biomass-derived γ-valerolactone containing water and 0.05 wt % H2SO4.28 Although HMF has great potential to serve as a platform chemical, a related compound, 5-(chloromethyl)furfural (CMF), is also valuable for its ability to form furanic ethers, which are used as diesel additives. The Mascal group discovered a method for its conversion and isolation using a two-phase reaction medium.89 By heating microcrystalline cellulose with 5 wt % LiCl in concentrated HCl at 65 °C, CMF was obtained as the major product. Continuous extraction in 1,2-dichloroethane gave a total recovered CMF yield of 71%. Amino Acids and Enzymes. Two recent contributions borrowed from biochemistry. Su, Li, Cheng, and their co-workers showed that the conversion of sucrose to HMF in ionic liquids can be catalyzed by amino acids, with tyrosine being the best.90 A maximum yield of 76% HMF was obtained in [EMIM]Br after 4 h at 160 °C. The authors suggested that tyrosine lends a proton to catalyze the hydrolysis of sucrose, while the unprotonated amino group helps to isomerize glucose to fructose. The Afonso group converted glucose to HMF by first isomerizing to fructose in water using sweetzyme, an industrial enzyme used in the production of high-fructose corn syrup. Subsequent dehydration to HMF was investigated in the presence of a number of Brønsted acid catalysts. By using H2SO4 and HNO3 catalysts, yields reached 80% and 87%, respectively.91 Boric and Boronic Acids. Recently, researchers utilized boron-containing compounds to facilitate the transformation of carbohydrates to HMF. Boric acid, which is known to dehydrate

mannose to HMF. Grin and co-workers observed a kinetic isotope effect using HCl and D2O for the dehydration of fructose to HMF, indicating that a proton was transferred in the ratelimiting step of the reaction.74 Kuster and co-workers used H2SO4 to access HMF from the fructose acetonide 1,2:4,5-di-oisopropylidene-β-D-fructopyranose (prepared via the condensation of fructose with acetone).75 As fructose has limited solubility in alcohols, transformation to the fructose acetonide enabled the volatile ethylene glycol dimethyl ether to be used as a solvent, providing a facile means for HMF recovery and solvent recycling. Dumesic and co-workers employed biphasic reaction systems, both to form HMF from fructose and to extract the HMF with an organic solvent. Using HCl as a catalyst, fructose was dehydrated at 85% selectivity in an aqueous phase containing dimethyl sulfoxide (DMSO) and poly(1-vinyl-2-pyrrolidinone) (PVP), which were added to suppress unwanted side reactions.76 HMF was extracted continuously into an organic phase of methylisobutylketone (MIBK), which was phase-modified with 2butanol to improve the partitioning of the HMF from the aqueous phase. These workers were also able to adjust the biphasic system conditions for monomeric sugars and thereby achieve HMF selectivities of 89% and 53% for fructose and glucose, respectively, and 91% selectivity for furfural from xylose.77 Furthermore, these optimal conditions could be applied toward disaccharides sucrose (a glucose−fructose dimer) and cellobiose (a glucose dimer), along with the polysaccharides inulin (a polyfructan), starch (a polyglucan), and xylan (a polyxylose). HMF selectivities were 77% from sucrose, 52% from cellobiose, 77% from inulin, and 43% from starch, with a furfural selectivity of 66% from xylan. Finally, the use of inorganic salts was shown to increase the partitioning of HMF in biphasic systems (NaCl being the most beneficial), and tetrahydrofuran (THF) was shown to have a superior extraction ability for HMF, with an attained selectivity of 83%.78 The Yoshida group investigated the use of a variety acids in subcritical water to dehydrate fructose.79 They found that an HMF yield of 65% was attained using H3PO4, although HMF was also formed even in the absence of acid catalysts at increased temperatures. They also used HCl to study the dehydration of fructose to HMF followed by a rehydration to form levulinic and formic acids in subcritical water, as well as the formation of decomposition products.80 They determined that soluble polymer byproducts were formed not only from fructose, but also from HMF, and no soluble polymers were formed from levulinic and formic acids. Vogel and co-workers used H2SO4 to catalyze the dehydration of fructose in sub- and supercritical acetone−water mixtures,81 accessing HMF in 77% yield. They also used their system to access HMF from glucose (48%), sucrose (56%), and inulin (78%). Riisager and co-workers used HCl to catalyze a microwaveassisted dehydration of concentrated aqueous fructose solutions to access HMF.82 Water, which normally results in low HMF yields when used as the reaction medium, performed substantially better with microwave irradiation to give HMF rapidly with selectivity of 63% and fructose conversion of 52%. A reaction time of 60 s gave 95% fructose conversion, but resulted in a decreased HMF selectivity of 55%. Loebbecke and coworkers also used HCl to dehydrate fructose in an aqueous solution at increased temperatures and pressures (185 °C and 17 bar) in a microreactor.83 Whereas an HMF yield of 54% was obtained in 1 min, addition of DMSO as a cosolvent and extracting with a MIBK/2-butanol mixture increased the HMF yield to 83%. Unfortunately, HMF is difficult to separate from 2594


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ACS Sustainable Chemistry & Engineering Table 2. Representative Conversions of Carbohydrates to HMF Using Metal Ion Catalysts carbohydrate

catalyst

solvent

T (°C)

time

HMF molar yield (%)

ref

glucose maltose cellobiose starch cellulose pine wood fructose cellulose fructose glucose sucrose inulin cellobiose starch fructose glucose fructose glucose glucose cellulose corn stover mannose galactose lactose tagatose cellulose cellulose reed sucrose glucose fructose fructose glucose fructose fructose fructose fructose fructose glucose glucose

Al3·6H2O Al3·6H2O Al3·6H2O Al3·6H2O Al3·6H2O Al3·6H2O WCl6 MnCl2 SnCl4 SnCl4 SnCl4 SnCl4 SnCl4 SnCl4 CrCl3 CrCl2 NaBr CrCl2 CrCl3 CrCl3/HCl CrCl3/HCl CrCl2 CrBr3 CrBr3 CrCl2 CrCl2/CuCl2 CrCl2/RuCl3 CrCl2/RuCl3 CrCl3/NH4Br CrCl3/NH4Br CrCl3/NH4Br NHC/CrCl2 NHC/CrCl2 LaCl3 NdCl3 EuCl3 DyCl3 LuCl3 YbCl3 Yb(OTf)3

3:1 THF/H2O 3:1 THF/H2O 3:1 THF/H2O 3:1 THF/H2O 3:1 THF/H2O 3:1 THF/H2O [BMIM]Cl/THF [SA-BMIM]HSO4 [EMIM]BF4 [EMIM]BF4 [EMIM]BF4 [EMIM]BF4 [EMIM]BF4 [EMIM]BF4 [EMIM]Cl [EMIM]Cl DMA DMA−LiCl/[EMIM]Cl DMA−LiCl/[EMIM]Cl DMA−LiCl/[EMIM]Cl DMA−LiCl/[EMIM]Cl DMA−LiBr DMA DMA DMSO [EMIM]Cl [EMIM]Cl [EMIM]Cl DMA DMA DMA [BMIM]Cl [BMIM]Cl DMSO DMSO DMSO DMSO DMSO [OMIM] [BMIM]Cl

160 160 160 160 180 180 50 150 100 100 100 100 100 100 80 100 100 100 100 140 140 100 120 120 120 120 120 120 100 100 100 100 100 100 100 100 100 100 160 140

30 min 10 min 10 min 10 min 30 min 30 min 4h 5h 3h 3h 3h 3h 3h 24 h 3h 3h 2h 6h 6h 2h 2h 2h 3h 3h 2h 8h 3h 2h 1h 1h 1h 6h 6h 4h 12 h 12 h 12 h 12 h 1h 6h

65 57 28 50 37 35 72 37 62 61 65 40 57 47 70 68 93 62 67 54 48 69 33 41 27 59 60 41 87 74 92 96 81 95 91 92 93 95 23 24

98 98 98 98 98 98 99 100 101 101 101 101 101 101 102 102 103 103 103 103 103 104 104 104 104 109 110 110 111 111 111 112 112 115 115 115 115 115 117 117

alcohols,92,93 was shown in the Riisager laboratory to convert glucose into HMF in a 41% yield in [EMIM]Cl.94 Computational modeling suggested the formation of a glucose−borate complex to isomerize glucose into fructose through an enediol mechanism prior to dehydration to HMF. The Raines group developed a boron-containing organocatalyst for HMF production.95 Their 2-alkoxycarbonylphenyl boronic acids mediate the transformation of fructose, glucose, and cellulose (including cotton, paper towels, and newspaper) into HMF. Although catalyst loadings for rapid (∼2 h) conversions are high, this liability is overcome by the facility of recovering the catalyst or, potentially, by its immobilization. Metal Ions. Metal ion catalysts are also used for the transformation of carbohydrates into HMF (Table 2). A study by the Chohan group used a variety of metal chlorides to test the conversion of glucose to HMF.96 All metal ions tested resulted in an enhancement in the rate of HMF production relative to acid catalysis, which became even more pronounced at increased catalyst concentrations and increased temperatures. Another

study done by Tyrlik and co-workers used aluminum salts in various oxygenated solvents to serve as ligands to the metal center for the conversion of glucose to HMF.97 The ligands were seen to influence both the yield and the selectivity of HMF formation, with ethanol proving the most efficient. The Abu-Omar group used AlCl3·6H2O in a biphasic system of water and THF to transform glucose into HMF in 65% yield.98 They also obtained good yields from other sugar sources such as maltose, cellobiose, starch, and cellulose. Nevertheless, their best HMF yields were only 35% from pine wood biomass materials, although furfural was obtained in yields greater than 60%. Yugen Zhang and his group used another biphasic system of THF and [BMIM]Cl to transform fructose into HMF.99 Using tungsten salts, notably WCl6, they were able to achieve conversion to HMF with 72% yield at 50 °C; even at room temperature, they achieved a 60% yield of HMF. Further, they demonstrated the recyclability of the catalyst. Chou and co-workers used MnCl2 in the acidic ionic liquid 1(1-butylsulfonic)-3-methylimidazolium hydrogen sulfate ([SA2595


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ACS Sustainable Chemistry & Engineering BMIM]HSO4) to transform cellulose into HMF.100 They reported HMF yields of 37%, although some levulinic acid was produced from the rehydration of the HMF product. Han and co-workers utilized SnCl4 to transform carbohydrate materials into HMF in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4).101 Good HMF yields were obtained from fructose (62%) and glucose (61%). They further accessed HMF by utilizing their system on polysaccharides sucrose (65%), inulin (40%), cellobiose (57%), and starch (47%). They postulated that the formation of a five-membered ring chelate complex of the Sn with C1 and C2 hydroxyls of αglucose facilitated an enolization of glucose to fructose before dehydration to HMF. Conrad Zhang and his group screened many metal chloride catalysts, and CrCl2 and CrCl3 emerged as best for the conversion of monosaccharides to HMF.102 In [EMIM]Cl, glucose and fructose were transformed in yields near 70%. This pioneering work established Cr(II) and Cr(III) as privileged catalysts for the conversion of carbohydrates to HMF. Binder and Raines used CrCl2 and CrCl3 in a solvent mixture of N,N-dimethylacetamide (DMA)−LiCl and [EMIM]Cl.103 Importantly, they discovered that their system was not only amenable to cellulose dissolution but also enabled an HMF yield of 48% from untreated corn stover, along with a 37% yield of furfural. This work was the first “one-pot” conversion of crude biomass to a furanic. Further work by the same group demonstrated that the catalytic system could be applied to convert other sugars such as mannose, galactose, lactose, and tagatose into HMF,104 and to transform xylose and xylan into furfural.105 Fructose is a ketose and prefers to exist in a furanose form,106 which is a ready precursor to HMF. In contrast, glucose is an aldose and prefers a pyranose form. The Zhang group proposed that chromium ions catalyzed this key aldose-to-ketose transformation through a chromium enolate intermediate (Figure 5A).102 Later work by Hensen and co-workers demonstrated that a chromium ion could form a complex with sugar hydroxyl groups.107 Another plausible mechanism enlists chromium ions to expedite a 1,2-hydride shift (Figure 5B). Interestingly, known enzymic catalysts of hexose isomerization use each pathway.108 The enolization mechanism is used by phosphoglucose isomerase, which does not employ a metal cofactor, whereas the hydride-shift mechanism is used by xylose isomerase, the manganese-dependent enzyme used for the commercial isomerization of glucose to fructose. Deuterium-labeling studies by the Raines group indicated that chromium ion mediated a 1,2hydride shift (Figure 5B).104,105 The success of chromium chloride catalysts to convert carbohydrates to HMF inspired attempts to enhance its transformative properties by pairings with other metal chlorides. Further work by the Zhang group used CrCl2 and CuCl2 in [EMIM]Cl to convert cellulose into HMF with yields consistently near 57%.109 HMF was extracted and the system maintained catalytic performance while being recycled for repeated use. Furthermore, they demonstrated that the bicatalyst system was able to depolymerize cellulose more rapidly than typical acid hydrolysis procedures. Cho and co-workers paired CrCl2 with RuCl3 in [EMIM]Cl to access HMF from cellulose in yields of 60%, and from reed biomass in a 41% yield with a 26% furfural yield.110 A pairing of CrCl3 with ammonium halides, notably NH4Br, in DMA by Zhang and co-workers was used to transform sucrose, glucose, and fructose into HMF at 87%, 74%, and 92% yields.111 They observed a halide effect by varying

Figure 5. Putative mechanisms for the metal-mediated conversion of an aldose (e.g., D-glucose) to HMF via a ketose (e.g., D-fructose). Replacement of deuterium for hydrogen in the aldose (red) or water (blue) can be used to differentiate between a mechanism based on (A) enolization or (B) hydride shift.104,105

chloride, bromide, and iodide as the anion of the ammonium salt for glucose conversion to HMF, with bromide being the best. This halide effect is consistent to that observed previously in the Raines laboratory.103 The Zhang and Ying groups used N-heterocyclic carbenes (NHC) as ligands to complex with Cr(II) and Cr(III) in an effort to enhance their catalytic activity.112 They were able to convert fructose and glucose into HMF at 96% and 81% yields using 1,3bis(2,6-diisopropylphenyl)imidazolylidene as the NHC ligand. Both the catalysts and the ionic liquid in this system could be recycled for continued use. More recently, Dumesic, Rinaldi, and co-workers combined AlCl3 catalyst with an aqueous−organic biphasic reactor and microwave heating. Interestingly, they started from beechwood, sugar cane bagasse, or cellulose, and impregnated the biomass with HCl before comminution. This procedure created oligosaccharides that were hydrolyzed quickly (in 6 min) into monosaccharides and converted into HMF. Yields of 60−69% for HMF and 74−84% for furfural were obtained depending on the biomass.113 Lanthanide metals can serve as Lewis acids in their ionic forms to catalyze the conversion of saccharide materials to HMF. Ishida and Seri found that glucose could be dehydrated by lanthanide(III) ions, potentially due to the high affinity of lanthanide ions for oxygen atoms.114 Still, the nascent HMF also suffered decomposition. In further work, they demonstrated that high yields of HMF (>90%) could be obtained from fructose using the chloride salts of lanthanides in DMSO.115 Similarly, all the lanthanide(III) ions were shown to dehydrate hexoses in water 2596


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ACS Sustainable Chemistry & Engineering Table 3. Representative Conversions of Carbohydrates to HMF Using Heterogeneous Catalysts carbohydrate

catalyst

solvent

T (°C)

time

HMF molar yield (%)

ref

fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose fructose glucose cellulose corn stover husk fructose fructose fructose fructose fructose glucose glucose fructose glucose glucose fructose glucose glucose starch cellulose fructose fructose glucose glucose fructose

DOWEX 50WX8 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 Amberlyst-15 [MIMPS]3PW12O40 Ag3PW12O40 Ag3PW12O40 Cr[(OSO3C12H25) H2PW12O40]3 Cr[(OSO3C12H25) H2PW12O40]3 Cr[(OSO3C12H25) H2PW12O40]3 SO42−−ZrO2 SO42−−ZrO2 SO42−−ZrO2 CSZ CSZA CSZ CSZA Anatase TiO2 Anatase TiO2 ZrO2 Anatase TiO2 Anatase TiO2 ZrO2 Anatase TiO2 Anatase TiO2 TiO2 nanoparticles TiO2 nanoparticles TiO2 nanoparticles TiO2 nanoparticles DAHM

70:30 acetone/DMSO [BMIM]Cl/acetone [BMIM]Cl/DMSO [BMIM]Cl/methanol [BMIM]Cl/ethanol [BMIM]Cl/ethyl acetate [BMIM]Cl/scCO2 [BMIM]BF4/DMSO [BMIM]PF6/DMSO [BMIM]Cl [BMIM]Cl sec-butanol 1:2.25 H2O/MIBK 1:2.25 H2O/MIBK H2O H2O H2O [BMIM]Cl H2O acetone/DMSO DMSO DMSO DMSO DMSO hot-compressed H2O hot-compressed H2O hot-compressed H2O 3:1 H2O/n-butanol 1:10 H2O/MIBK 1:10 H2O/MIBK 1:10 H2O/MIBK 1:5 H2O/MIBK H2O DMSO H2O DMSO 1:5 H2O/MIBK

150 25 25 25 25 25 35 (15 MPa) 80 80 80 120 120 120 130 150 150 150 100 200 180 130 130 130 130 200 200 200 200 180 180 180 270 120 140 120 140 165

10 min 6h 6h 6h 6h 6h 6h 32 h 24 h 10 min 1 min 2h 1h 4h 2h 2h 2h 30 min 5 min 20 min 4h 4h 4h 4h 5 min 5 min 5 min 2 min 2 min 2 min 2 min 2 min 5 min 5 min 2 min 5 min 2h

82 78 78 82 80 81 79 87 78 83 82 99 78 76 53 31 36 88 36 73 68 56 18 48 24 23 18 18 29 21 15 35 34 54 22 37 69

119 120 120 120 120 120 120 121 121 122 122 123 124 124 125 125 125 126 127 127 128 128 128 128 129 129 129 130 130 130 130 130 131 131 131 131 132

without rehydration to levulinic and formic acids.116 A kinetic analysis indicated that the rate-determining step in the dehydration is the reaction of the hexose−catalyst complex rather than the association of the two to form the complex. The activity of lanthanide catalysts has also been investigated in ionic liquids. The Riisager group demonstrated the conversion of glucose to HMF in ionic liquids with ytterbium chloride (YbCl3) and ytterbium triflate (Yb(OTf)3).117 They achieved their best yield for YbCl3 (23%) in 1-octyl-3-methylimidazolium chloride ([OMIM]Cl) and for Yb(OTf)3 (24%) in [BMIM]Cl. Although these yields are modest in comparison to chromium catalysts, the ytterbium catalysts tended to give higher HMF yields in less hydrophilic ionic liquids, whereas chromium preferred more hydrophilic ionic liquids such as [EMIM]Cl. A study in the Okuda laboratory compared lanthanum chloride (LaCl3) to the rare earth salts ytterium chloride (YCl3) and scandium chloride (ScCl3) on conversion of glucose and cellobiose to HMF in the organic solvent DMA.118 They concluded that the conversion and selectivity depend on the ionic radii of the metal center, with the smaller radius giving greater activity.

Heterogeneous Catalysts. Heterogeneous catalysts are also used to convert carbohydrates into HMF (Table 3). Their use in conversion reactions can be advantageous for several reasons: facile product separation, catalyst recyclability, high temperature-tolerance, and modulation of surface properties (acidity, basicity, and pore morphology) to achieve maximal HMF selectivities and yields. Here, we discuss ion-exchange resins, heteropolyacids, zirconia compounds, and H-form zeolites. Smith and co-workers investigated the dehydration of fructose using DOWEX 50WX8 ion-exchange resin with microwave heating.119 Whereas initial work had used pure DMSO as the solvent, they were interested in using acetone as a cosolvent to serve as a promotor for higher HMF selectivities. In only 10 min, HMF yields could be obtained at 82% with this system, and the catalytic activity of the resin was maintained over multiple runs. In another study by the same group, Amberlyst-15 ion-exchange resin was used to dehydrate fructose in [BMIM]Cl with the addition of various cosolvents.120 All of the cosolvents tested (acetone, DMSO, methanol, ethanol, ethyl acetate, and supercritical CO2) accessed HMF yields of 78−82%. Dumesic and coworkers achieved somewhat lower yields by using biomass2597


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ACS Sustainable Chemistry & Engineering derived solvents (tetrahydrofuran, γ-valerolactone, γ-hexalactone, or methyltetrahydrofuran) to access HMF from glucose with a combination of Lewis (Sn-β) and Brønsted (Amberlyst70)25 or H-Beta zeolite solid catalysts.27 Lansalot-Matras and Moreau compared the effect of two ionic liquids, the hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) and the hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6), on the dehydration of fructose using Amberlyst-15.121 Using DMSO as a cosolvent to assist with sugar solubility, they observed HMF yields near 80% in both systems. But without the DMSO cosolvent, HMF yields dropped to 52% in [BMIM]BF4, and fructose was not soluble in pure [BMIM]PF6. The Smith group was able to access HMF in yields as high as 83% using the same catalyst in [BMIM]Cl with microwave heating at 80 °C in only 10 min.122 When the temperature was increased to 120 °C, a similar yield could be accessed in only 1 min. Furthermore, they demonstrated that 5 wt % water content or lower in the ionic liquid had no adverse effects. As HMF can be difficult to separate from ionic liquids, Huang and co-workers used the heteropolyacid salt of an ionic liquid cation as a catalyst to convert fructose in sec-butanol.123 Using 1(1-propylsulfonic)-3-methylimidazolium phosphotungstate ([MIMPS]3PW12O40), they achieved an HMF yield as high as 99% in 2 h. Furthermore, simple precipitation of the catalyst enabled its recycling in further reactions with no loss of activity. Xiaochong Wang and co-workers used another heteropolyacid salt, Ag3PW12O40, to transform both fructose and glucose into HMF.124 Using a biphasic system of water and MIBK to extract the HMF product, HMF yields of 78% and 76% were obtained from fructose and glucose, respectively. The catalyst was recycled easily, demonstrated no loss of activity over multiple reactions, and was tolerant to high feedstock concentrations with minimal byproduct formation. Further, Wang and co-workers used the Brønsted−Lewis−surfactant-combined heteropolyacid Cr[(OSO3C12H25)H2PW12O40]3 to achieve both the depolymerization of cellulose and its conversion to HMF.125 A 53% yield of HMF was obtained in 2 h at 150 °C from pure cellulose, and yields of 31% and 36% were obtained from corn stover and Xanthoceras sorbifolia Bunge husk (which is native to Northern China), respectively. The catalyst existed as an emulsion during extraction of HMF using MIBK, allowing facile recovery for reuse. The group attributed the high activity of the catalyst to its dual Brønsted and Lewis acidities. Heterogeneous zirconia (ZrO2) compounds can facilitate dehydration of carbohydrates. Qi and co-workers impregnated ZrO2 with H2SO4 to serve as a catalyst for conversion of fructose to HMF in [BMIM]Cl.126 An 88% yield was obtained in only 30 min at 100 °C. The solid catalyst and ionic liquid were able to be recycled with constant activity for multiple runs. Another study by Qi, Smith, and co-workers coupled the sulfated zirconia with microwave heating to dehydrate fructose in an acetone−DMSO mixture.127 At 180 °C, an HMF yield of 73% was attained in 20 min. When the reaction was carried out in water, however, the HMF yield was only 36%, demonstrating that ZrO2 has little activity in aqueous systems. Hu and co-workers used an ethylene dichloride solution of chlorosulfonic acid to impregnate Zr(OH)4 (CSZ) and Zr(OH)4−Al(OH)3 (CSZA) to serve as catalysts for fructose and glucose conversion to HMF with maximal yields of 68% and 48%, respectively.128 The zirconium contained both Brønsted and Lewis acid sites, and the aluminum contained basic sites. They found that increased basicity promoted isomerization of glucose to fructose, and an ideal

mole ratio of 1:1 Zr:Al resulted in the highest yield of HMF. The catalyst was also recoverable with minimal loss of activity. Inomata and co-workers used both TiO2 (anatase and rutile) and ZrO2 to convert glucose and fructose into HMF in hotcompressed water.129 They found that rutile TiO2 was inactive in glucose conversion, but both anatase TiO2 and ZrO2 were able to isomerize glucose into fructose. Anatase TiO2 also dehydrated fructose, suggesting that ZrO2 is a base catalyst and anatase TiO2 has both basic and acidic sites. McNeff and co-workers again used anatase TiO2 and ZrO2 in a fixed bed reactor to access HMF from a variety of carbohydrates (i.e., fructose, glucose, starch, and cellulose).130 An HMF yield of 35% was obtained from cellulose after extraction with MIBK, and the catalysts were regenerated once their activity diminished by heating to 450 °C for 5 h. Mesoporous TiO2 nanoparticles were used by Bhaumik, Saha, and co-workers to convert carbohydrate materials with microwave heating in aqueous and organic solvents.131 The maximal HMF yields from fructose were 34% and 54% in water and DMSO, respectively, and 37% from glucose in DMSO. The nanoparticles were shown to retain their catalytic activity over 4 cycles. The Moreau group employed dealuminated H-form mordenites (DAHM) in a biphasic system of water and MIBK to transform fructose into HMF.132 They achieved an HMF yield of 69% with the maximal reaction rate attained using H-mordenites with an 11:1 Si:Al ratio. A correlation was observed between HMF selectivity and the bidimensional structure of the Hmordenites, particularly the absence of cavities within the structure that prevented secondary product formation. Continuous extraction of HMF using MIBK for countercurrent circulation in a continuous heterogeneous pulsed column reactor resulted in an increase in HMF selectivity. Heterogeneous catalysts can accomplish the depolymerization and hydrolysis of cellulose. Schüth and co-workers used Amberlyst-15DRY in [BMIM]Cl to produce reducing sugars and cellooligomers from microcrystalline cellulose and wood biomass after an induction period of 1 h.133 When they used ptoluenesulfonic acid, however, no induction period was apparent. The Shimizu group employed the heteropolyacids H3PW12O40 and H4SiW12O40 to access reducing sugars from cellulose in an aqueous phase.134 They found that a stronger Brønsted acidity resulted in more active reactions. Catalysis by sulfonated silica/ carbon nanocomposites were investigated by Jacobs, Sels, and co-workers to access high yields of glucose from cellulose.135 They attributed the high yields to the hybrid surface facilitating the adsorption of the β-1,4-glucan. Hara and co-workers used amorphous carbon with sulfuric acid, hydroxyl, and carboxylic acid groups to hydrolyze solid cellulose in water directly.136 They too attributed the activity of the catalysts on their ability to adsorb the cellulose glucan onto the catalytic surface. 2,5-Dimethylfuran as a Product. In an effort to supplement petroleum-derived fuels with a renewable resource, gasoline is blended with ethanol. Some chemical properties of ethanol, such as miscibility with water and low enthalpy of vaporization, are undesirable.137 Even with substantial progress being made on accessing ethanol from cellulosic materials, the question of its ultimate viability and sustainability as a biofuel remains unanswered.138 Among the alternatives, DMF, which is formed by the catalytic hydrogenolysis of HMF, stands out. It has an increased energy density compared to ethanol (31.5 MJ/L versus 23 MJ/L), lower volatility (bp 92−94 °C versus 78 °C), and immiscibility with water, making it a potential drop-in replacement to gasoline and diesel.34 2598


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served to dehydrate the fructose, which upon addition of a Pd/C catalyst in tetrahydrofuran, underwent hydrogenolysis using the formic acid as a hydrogen source to access 2,5-bis(hydroxymethyl)furan before deoxygenation to DMF. The Chidambaram group likewise used a Pd/C catalyst to access DMF from glucose in an ionic liquid/acetonitrile mixture in a two-step process.145 Glucose was converted into HMF using the heterocatalyst 12-molybdophosphoric acid, after which the catalyst could be replaced with the Pd/C without isolation of HMF to access DMF in a 16% yield. The lower yield was attributed to decreased reaction temperature, duration, and solubility of H2 in ionic liquids. Catalyst combinations were used to access DMF from HMF at high yields. Yanqin Wang and co-workers used a Ru/Co3O4 catalyst to convert DMF at 93.4% yield at 130 °C and 0.7 MPa.146 Further, they showed that their catalyst can be reused up to 5 times without loss of activity. Saha, Bohn, and Abu-Omar demonstrated the synergy of ZnCl2 with Pd/C to achieve an 85% yield of DMF at 150 °C and 0.8 mPa.147 Their catalyst was recycled up to 4 times without loss of activity. The Chatterjee and Kawanami groups used supercritical CO2 and water to effect the hydrogenation of HMF in the presence of various metal catalysts.148 They reported 100% yields of DMF in 2 h with a Pd/C catalyst and relatively mild reaction conditions: 80 °C, pCO2 = 10 MPa and pH2 = 1 MPa. Selectivity toward other products was tuned by varying pCO2 alone. When pCO2 was 10 MPa, 2,5-dimethyltetrahydrofuran was the major product. The catalyst could be recycled without significant loss of activity.

Xu and co-workers compared DMF to both gasoline and ethanol as a fuel. One such study was to compare the combustion performance of DMF in a gasoline direct-injection (GDI) engine to gasoline and ethanol.139 They found that using pure DMF did not adversely affect the performance of the research engine. Different loads of DMF gave different initial combustion durations (through gasoline had a longer duration) and engine knock was induced at 7.1 bar indicated mean effective pressure. Furthermore, emissions of CO, hydrocarbons, NOx, and particulate matter were all similar to gasoline emissions (although ethanol emissions were lower). Similar conclusions were reached using a dual-injection of DMF or ethanol with gasoline in a spark-ignition engine.140 High performance gains were attained with increased direct injection fractions of both DMF and ethanol, and emissions were mostly reduced using the dual-injection strategy (although CO2 and NOx emissions increased for DMF blends with gasoline). They concluded that the dual-injection strategy was most effective at lower fractions of the biofuels for port fuel injection. In another study by Xu and co-workers, the effect of spark timing and load of DMF was analyzed on a direct-injection sparkignition engine.141 As compared to gasoline, they found that DMF was more resistant to engine knock, had a lower initial and total combustion duration (as did ethanol, indicating the rapid combustion rate of oxygenated fuels), and had a similar volumetric consumption rate, allowing for a similar driving range. Additionally, DMF had a greater loss of thermal energy due to higher combustion temperatures and similar engine-out emissions to gasoline. Ethanol combustion was not limited by engine knock, but it has a lower volumetric consumption rate than both gasoline and DMF. Unsurprisingly, they also found that ethanol had the highest laminar burning velocity, followed by gasoline and then DMF.142 Thus, a laminar flame propagates most quickly in ethanol, though the velocities of gasoline and DMF were similar. As the advantages of DMF acquired a more solid foundation, researchers have sought to access it from biomass materials. The Dumesic group developed a two-step process to transform fructose into DMF.34 Acid-catalyzed dehydration of fructose in a biphasic reactor led to the formation of HMF, which was then extracted into 1-butanol. The HMF was subjected to hydrogenolysis using a 3:1 atomic ratio copper−ruthenium (Cu:Ru/ C) catalyst to access DMF in a 71% yield. The Cu:Ru/C catalyst was more resistant to poisoning by chloride anions than were copper catalysts, and could be regenerated by flowing hydrogen at the reaction temperature of 220 °C. The Raines group used the same catalyst to access DMF from corn stover biomass in a twostep process.103 Using CrCl3 in an [EMIM]Cl/DMA−LiCl mixture, they converted corn stover into HMF, which was purified with ion-exclusion chromatography to remove chloride anions and prevent poisoning of the Cu:Ru/C catalyst. The recovered HMF was subjected to hydrogenolysis in 1-butanol to give DMF in 49% yield, with an overall 9% yield from corn stover. Luijkx, Maat, van Bekkum, and co-workers used a palladium on carbon (Pd/C) catalyst to access DMF from HMF in 1propanol.143 By GC−MS, they observed formation of 5hydroxymethyl-2-(propyloxymethyl)furan during the initial stages of hydrogenolysis. The alcohol bond was the first to undergo hydrogenolysis, followed by the ether bond to access DMF. Similar reactivity was observed in 2-propanol, but 2,5bis(hydroxymethyl)furan became the major product in 1,4dioxane. The Rauchfuss group developed a one-pot process to convert fructose into DMF using formic acid.144 The formic acid

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CONCLUSIONS The Quaker Oats process developed in the 1920s produces mainly furfural and furfuryl alcohol from agricultural residues such as corn cobs and cotton hulls.149 The yields are less than 50%, generate large waste streams, and require high operating costs. Today, almost all furfural plants are located in China. To the best of our knowledge, HMF has not yet been produced at commercially viable quantities and cost from any feedstock. Still, HMF and furfural are versatile renewable platform chemicals that could feed the manufacture of molecules in a multitude of markets (Figure 6). Research into routes to substitute petroleum have enjoyed an environmental imperative

Figure 6. Notional cycle for the renewable production of fuels and chemicals via HMF. 2599


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furandicarboxylic acid could replace terephthalic acid in the production of polyethylene terephthalate-like polymers. Still, access to distribution channels, safety to consumers, and other factors must be resolved prior to broad commercial adoption. In any scenario, a shift in focus toward research that more closely mimics a holistic process will be a prerequisite to displace petroleum and yield a green, sustainable economy like that envisioned long ago by “Billy” Hale.

in many institutions and countries. Unfortunately, the vast majority of research is focused solely on increasing rates and yields. Little regard is given to other important parts of an industrial process, such as product and solvent recoveries, or to a realistic perspective of economic feasibility. Both technical and economic aspects of the entire process, and not merely parts of the process, must be considered with great care before production can attain a meaningful scale. To date, attempts to reach beyond the pilot scale have been limited to only furfural and aqueous solvents. HMF yields remain limited by rehydration into levulinic and formic acids, a side-reaction that has not been prevented effectively. Furthermore, humin formation has several detrimental consequences, including the coating of reactor walls and deactivation of heterogeneous catalysts. The reactive-extraction approach taken by Dumesic and coworkers addresses several key technical and economic aspects necessary for industrial practice.76 This work showed that a systematic design of “phase modifiers” (cosolvents) can both suppress side reactions and extract the HMF target into an organic phase, thus preventing its degradation (e.g., rehydration and condensation) reactions as well. As a result, specificity reached a high of 85% as HMF was extracted continuously into a solvent that could be distilled from the product. A technoeconomic study concluded that this approach would reach an HMF minimum cost of $1.80/kg from $0.55/kg fructose and a plant capacity of 300 ton/day.150 Other reactive-extraction approaches that address a process as a whole would further realistic progress. Various technical and economic challenges regarding ionic liquid solvents also remain. Processes in ionic liquids are unique in providing access to furanics directly from lignocellulosic biomass and doing so with respectable yields (vide supra). Nevertheless, few studies report on the recovery and recycling of ionic liquids from a biomass conversion process.151−153 Indeed, there has been a persistent, but unsubstantiated, belief that ionic liquid recycling is problematic. In 2011, a compressed CO2 phase was used to extract furanics from ionic liquids such as dialkylimidazolium halides.154 Recently, this observation was reproduced by Mu and co-workers in a pressure reactor containing [BMIM]Cl and CO2 to effect the simultaneous conversion glucose to HMF and extraction of HMF in compressed CO2, achieving both enhanced yield and product isolation in a single step.155 Another prevalent belief is that ionic liquids are generally expensive and therefore cannot be used in a cost-effective process, even if recycled efficiently. In reality, the cost of ionic liquids spans a wide range, from roughly $1/kg to over $100/kg when manufactured in bulk. Fortunately, common and simple ionic liquids such as [EMIM]Cl and [BMIM]Cl, which have been used to access HMF from both glucose and lignocellulose with good yields, cost around $7/kg if sourced in multiton quantities. (This price is derived from 2015 prices for delivered 1methylimidazole ($8.0/kg) and chlorobutane ($3.5/kg), and application of the techno-economic model by Hallett and coworkers.156) Moreover, 1-methylimidazole can be manufactured from the simple chemicals ammonia, methylamine, formaldehyde, and glyoxal. Hence, its cost could decrease if production is scaled up to meet future demands. Many obstacles have yet to be overcome for HMF to become an industrial platform chemical. Not all of these obstacles are technical. Even if HMF were readily accessible, its importance relies on the market-penetration of its derivatives (Figure 4). For example, DMF could substitute for gasoline, and 2,5-

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ENVOI

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AUTHOR INFORMATION

“Sunshine and its conversion into chemical energy and material through the pathways of carbon reduction and quantum conversion in the green plant is our ultimate energy source; it is relatively free of political control, universally available on an annually renewal basis, and environmentally clean.”Melvin Calvin (1911−1997), 1978 Priestley Medal Address to the American Chemical Society.

Corresponding Author

*R. T. Raines. E-mail: [email protected]. Present Address ⊥

Siegwerk USA Co., 3535 SW 56th Street, Des Moines, IA 50321

Notes

The authors declare the following competing financial interest(s): Hyrax Energy, Inc. is the exclusive licensee of patents relating to ionic liquid bioprocessing that were developed at the University of Wisconsin−Madison and are owned by the Wisconsin Alumni Research Foundation (WARF). R.E.T., K.G.K., and R.T.R. are shareholders in Hyrax Energy, Inc. Biographies

Dr. Benjamin R. Caes received his Ph.D. degree in chemistry from the University of Wisconsin−Madison in 2012 under the direction of Prof. Ronald T. Raines, working at the Great Lakes Bioenergy Research Center. He then joined Siegwerk, a global printing ink supplier, as a chemist in its Global Innovation Network (GIN). Today, he leads the U.S.-based GIN team in its development of next-generation printing ink technologies. 2600


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furanics, and lignin from lignocellulosic biomass, and he is a Co-Founder of Hyrax Energy, Inc.

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ACKNOWLEDGMENTS Work on biomass conversion at the University of Wisconsin− Madison is supported by the DOE Great Lakes Bioenergy Research Center (DOE Basic Energy Research Office of Science DE-FC02-07ER64494). Biorefinery process development at Hyrax Energy is supported by the DOE Office of Energy Efficiency and Renewable Energy (DE-SC0010126) and by the NSF Industrial Innovation and Partnerships (IIP-1314699).

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Dr. Rodrigo E. Teixeira obtained his M.S. and Ph.D. degrees in Chemical Engineering from Stanford University, working under the supervision of Profs. Eric Shaqfeh and Steven Chu (Physics). Today, he is a Scientist at the Great Lakes Bioenergy Research Center, and Chief Technology Officer and Co-Founder of Hyrax Energy, Inc., a Silicon Valley startup developing an industrial process for the conversion of biomass into clean, low-cost fermentable sugars for the production of renewable materials, chemicals and fuels.

REFERENCES

(1) Hale, W. J. In The Farm Chemurgic; The Stratford Company: Boston, MA, 1934; pp 142−143. (2) Hale, W. J. Prosperity Beckons: Dawn of the Alcohol Era; The Stratford Company: Boston, MA, 1936. (3) Yergin, D. The Prize: The Epic Quest for Oil, Money and Power; Simon & Schuster: New York, NY, 1991. (4) Yergin, D. The Quest: Energy, Security and the Remaking of the Modern World; Penguin Books: New York, NY, 2011. (5) Field, C. B., Barros, V., Stocker, T. F., and Dahe, Q. Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation: Special Report of the Intergovernmental Panel on Climate Change; Cambridge University Press: New York, NY, 2012. (6) Lewis, N.; Nocera, D. Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729− 15735. (7) Antal, M. J., Jr.; Mok, W. S. L.; Richards, G. N. Mechanism of formation of 5-(hydroxymethyl)-2-furaldehyde from D-fructose and sucrose. Carbohydr. Res. 1990, 199, 91−109. (8) Amarasekara, A. S.; Williams, L. D.; Ebede, C. C. Mechanism of the dehydration of D-fructose to 5-hydroxymethylfurfural in dimethyl sulfoxide at 150 °C: An NMR study. Carbohydr. Res. 2008, 343, 3021−3024. (9) Murkovic, M.; Pichler, N. Analysis of 5-hydroxymethylfurfual in coffee, dried fruits and urine. Mol. Nutr. Food Res. 2006, 50, 842−846. (10) Husøy, T.; Haugen, M.; Murkovic, M.; Jöbstl, D.; Stølen, H.; Bjellaas, T.; Rønninborg, C.; Glatt, H.; Alexander, J. Dietary exposure to 5-hydroxymethylfurfural from Norwegian food and correlations with urine metabolites of short-term exposure. Food Chem. Toxicol. 2008, 46, 3697−3702. (11) Janzowski, C.; Glaab, V.; Samimi, E.; Schlatter, J.; Eisenbrand, G. 5-Hydroxymethylfurfural: Assessment of mutagenicity, DNA-damaging potential and reactivity towards cellular glutathione. Food Chem. Toxicol. 2000, 38, 801−809. (12) Lewkowski, J. Synthesis, chemistry and applications of 5hydroxymethylfurfural and its derivatives. ARKIVOC 2001, 17−54. (13) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Liquid-phase catalytic processing of biomass-derived oxygenated hydrocarbons to fuels and chemicals. Angew. Chem., Int. Ed. 2007, 46, 7164−7183. (14) Verevkin, S. P.; Emel’yanenko, V. N.; Stepurko, E. N.; Ralys, R. V.; Zaitsau, D. H. Biomass-derived platform chemicals: Thermodynamic studies on the conversion of 5-hydroxymethylfurfural into bulk intermediates. Ind. Eng. Chem. Res. 2009, 48, 10087−10093. (15) Tong, X.; Ma, Y.; Li, Y. Biomass into chemicals: Conversion of sugars to furan derivatives by catalytic processes. Appl. Catal., A 2010, 385, 1−13. (16) Rosatella, A. A.; Simeonov, S. P.; Frade, R. F. M.; Afonso, C. A. M. 5-Hydroxymethylfurfural (HMF) as a building block platform: Biological properties, synthesis and synthetic applications. Green Chem. 2011, 13, 754−793. (17) Horvat, J.; Klaić, B.; Metelko, B.; Šunjić, V. Mechanism of levulinic acid formation. Tetrahedron Lett. 1985, 26, 2111−2114. (18) Horvat, J.; Klaić, B.; Metelko, B.; Šunjić, V. Mechanism of levulinic acid formation in acid catalysed hydrolysis of 2-hydroxymethylfurane

Dr. Kurtis G. Knapp obtained his M.S. and Ph.D. degrees in Chemical Engineering from Stanford University, working under the supervision of Prof. James Swartz. Today, he is the President and Co-Founder of Hyrax Energy, Inc., a Silicon Valley startup developing an industrial process for the conversion of biomass into clean, low-cost fermentable sugars for the production of renewable materials, chemicals, and fuels.

Dr. Ronald T. Raines is the Henry Lardy Professor of Biochemistry, the Linus Pauling Professor of Chemical Biology, and a Professor of Chemistry at the University of Wisconsin−Madison. He leads a project at the Great Lakes Bioenergy Research Center on obtaining sugars, 2601


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