Sustainable Productions of Organic Acids and Their Derivatives from

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Sustainable Productions of Organic Acids and Their Derivatives from Biomass via Selective Oxidative Cleavage of C-C bond Min Wang, Jiping Ma, Huifang Liu, Nengchao Luo, Zhitong Zhao, and Feng Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03790 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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ACS Catalysis

Sustainable Productions of Organic Acids and Their Derivatives from Biomass via Selective Oxidative Cleavage of C–C Bond Min Wang, Jiping Ma, Huifang Liu, Nengchao Luo, Zhitong Zhao, Feng Wang* State Key Laboratory of Catalysis (SKLC), Dalian National Laboratory for Clean Energy (DNL), Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences, Dalian 116023 (China), E-mail: [email protected]

Abstract

Biomass is a renewable and the most abundant carbon resource which shows great potential for sustainable production of chemicals in the future. Concerning the diminishing of the limited fossil reserves, biomass conversion has aroused global attention. The use of biomass as a resource has developed rapidly in recent years, and various kinds of chemicals could be produced from biomass. Although biomass is annually renewable and much abundant, it is important to process it in the most efficient way. Before rushing into biomass conversion, it should be thinking that what chemicals are reasonable and economical produced from biomass. In this review, we first analyzed the products from biomass based on the structural properties and economics. Taking into account of the oxygen-rich character of the feedstock, it is a reasonable route to convert the biomass into valuable oxygen-containing fine chemicals, among which organic acids are one class of important and widely used fine chemicals. Then, we provided insights into the recent 1

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progress in the oxidative cleavage of biomass into organic acids and their derivatives, such as esters and anhydrides. The biomass resources cover the lignocellulose biomass, sugars, chitin, platform molecules and fats. As biomass resources are generally polymers and the C–C bond is the backbone, the oxidative cleavage of C–C bond can break up the biomass to small molecules and introduce acid functionality at the same time. This review particularly focuses on the generation of acids via C–C bond oxidative cleavage process. Various methods, catalytic systems and C–C bond cleavage mechanisms are summarized. Finally, we conclude the challenges in the oxidative conversion of biomass and the possible research direction in this area. Keywords: Biomass, lignin, 5-hydroxylmethyl furfural, fatty acid, oxidation, formic acid, C-C bond cleavage

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1. Introduction

Modern society depends heavily on fossil resources, which provide various energy, chemicals and materials for our daily life. Although fossil reserves are forecast to grow significantly over the next two decades, especially the shale oil and gas, the economics of extraction of these resources are still unidentified. More importantly, to the best of our knowledge, the fossil resource is unrenewable and its reserve is limited. With increasing energy demand of human society and the progressive reduction of fossil resources, the energy crisis has become a major problem for the human society in the future. Sustainable development has become an irresistible trend worldwide for the academia, industry and governments. Many alternative renewable energies, such as wind, sun, nuclear, and water, can meet the future energy demands. However, these sources cannot provide carbon resources to meet the needs of society. The most promising approach is to use biomass as a renewable carbon resource. Biomass generally refers to the plants, such as grass, tree, flower, agriculture crop, algae. Organic materials in biomass are originally formed via biological photosynthesis from readily available atmospheric carbon dioxide (CO2), water, and sunlight. Biomass also includes fat in animals and chitin in shrimp. Therefore, biomass is a renewable, abundant and green carbon resource. There is no doubt that biomass is the main choice of feedstock for chemicals and materials in the future. 3


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Nature provides large amounts of biomass each year. Globally, 170 billion metric tonnes of biomass sources are produced.1 The main components of plants are organic molecules, such as carbohydrates, lignin and fats. Carbohydrates, including cellulose and hemicellulose, is the most abundant organic compounds. Carbohydrates are tightly bounded to the lignin, and these integrates are the main component of plant cell walls. Materials primarily composed of carbohydrates-lignin are commonly referred to as lignocellulosic biomass, which is abundant in plant, such as wood, grass, bamboo, corn stalks and sugarcane. Lignocellulose is the most abundant biomass resource and its composition varies with different kinds of biomass resources. In general, lignocellulose contains 20–35% of hemi-cellulose, 35–50% of cellulose and 10–25% of lignin. Cellulose is a linear polymer with the formula (C6H10O5)n and consists of D-glucose units connected with 1,4-glycosidic bond. The chain length of cellulose heavily affect its properties. The chain lengths of wood pulp cellulose is between 300 and 1700 units, while the cellulose from cotton has chain lengths between 800 to 10,000 units.2 Cellulose is a linear polymer which is rich in hydrogen-bonds. The hydrogen-bonds make the cellulose rigid, crystalline and insoluble in water. Consequently, cellulose is not easy to be hydrolyzed in mild conditions.3 Hemicellulose is a polysaccharide composed not only of glucose but also of xylose, mannose, galactose, rhamnose and arabinose. Xylose is the dominant unit in the hemicellulose. Hemicellulose occupies about 20% of most plant biomass, which serves as the linkage 4


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ACS Catalysis

between lignin and cellulose. Different from the crystalline cellulose, hemicellulose is an amorphous and branched polymer, and contains shorter chains.4 These properties make hemicellulose easily be depolymerized to monomers via hydrolysis. Lignin is a cross-linked aromatic biopolymer and abundant in wood. Lignin contributes to about one third of the non-fossil organic carbon and twenty to thirty five percent of the dry mass of wood.5-7 Unlike the cellulose, lignin lacks a defined primary structure. Although the structure of lignin has been studied for many years, the exact structure of lignin remains unclear. But, generally, there are three basic units: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol.8 The structure of lignin also varies from species to species. The content of the three monomers varies depending on the biomass source. Even if the same plants grow at different places or at different ages, the lignin structure is also different. The variability of the lignin makes its fractionation and depolymerization more complicate. The oil and fats are another kind of renewable feedstock which is abundant in nature and utilized widely in our daily life. Notably, large amount of plant oil turns into wasted gutter oil after consumption. Reasonable use of gutter oil resource, can not only prevent the return of waste cooking oil, but also reduce environmental pollution and other social problems. In China, about 5 million tonnes of gutter oil is produced per year. In consideration of the limited fossil resources, the direct transformation of the gutter oil to high-value chemicals is a promising route. Natural animal fats and vegetable oils are 5


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mainly present as triglycerides, which contain a large number of unsaturated fatty acids (UFAs). UFAs could be converted to valuable monocarboxylic- and dicarboxylic acids. Chitin is a major constituent in crustacean shells.9-10 About 6 - 8 million tonnes of waste crab, shrimp and lobster shells are produced per year. Chitin accounts for 15–40 wt% of a crab’s mass. N-acetyl-D-glucosamine monosaccharide is the major unit of chitin, which is linked together by β-glycosidic bonds in a pyranose ring conformation. Different from the cellulose, an acetyl amide group is on the C2 position instead of a hydroxyl group in chitin. The transformation of biomass to fuels and chemicals has aroused worldwide research interests and has already achieved some successes. Various chemicals and fuels have been directly or indirectly produced from biomass. One issue needs to be addressed is that what products are more rational and economical to be produced from biomass. One option is the conversion of biomass to liquid fuels via removing excess oxygen atoms to increase their energy density. In addition to dehydration, hydrogenation is mostly used to remove oxygen atoms. Presently, hydrogen is industrially produced through the fossil route, mainly through water gas shift reaction and steam reforming of methane. Moreover, the overall energy balance must be considered. Although the raw-biomass resources such as corn stalks and sugarcanes are abundant and inexpensive, their energy densities are very low 6


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ACS Catalysis

and the distribution is scattered over a large area. The collection and accumulation of biomass resources and the conversion of biomass to fuels require additional energy input. The net energy is substantially reduced after subtracting the consumed energy during the production process. In addition, there are other renewable and clean energy resources, such as wind, sun, nuclear and water, which may meet the future energy demand. These energies can be stored and transformed to electrical energy. In recent years, the electric vehicle technology has been rapidly developed. Electric cars and buses are now appearing in our daily life and the amount is increasing dramatically. Therefore, the current conversion of biomass to transport fuels is uneconomical and also competes with other new energy technologies. However, in some cases, the conversion of biomass to high energy density fuels, such as jet fuels, is significant. Compared to the fuels for the cars, the consumption amount of jet fuels is smaller. Moreover, at present, there is no other alternative power for the airplanes.

Figure 1. Acids consumption in China. 7


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Another option is to convert biomass to high-value fine chemicals. Considering the small consumption and high-cost accounts, it is a better way to produce fine chemicals from biomass for the valorization of biomass. A successful example is the production of vanillin from lignin sulfate.11-15 Furthermore, some fine chemicals are easier to be produced from biomass but are difficult to be produced from petroleum. Oxygen-containing products are of great significance in our daily life. Organic acids and their derivatives, such as esters and anhydrides, are value-added fine chemicals. They are widely used as solvents, cosmetics, polymer monomers and food additives. The consumption of typical acids is shown in Figure 1Figure 1. Acids consumption in China. Presently, organic acids are mainly produced from fossil resources. For example, formic acid is industrially produced from fossil feedstocks, such as methanol carbonylation/methyl formate hydrolysis and partial oxidation of naphtha.16 Acetic acid is produced through methanol carbonylation,17 while methanol is produced from syngas which is derived from coal gasification or methane steam reforming.18 The maleic anhydride or maleic acid is produced by the oxidation of butane or benzene.19 The hydrocarbons in fossil resources are oxygen-deficient feedstocks. The production of organic acids from fossil resources requires the destruction of the inert C–H or C–C bonds of the hydrocarbons and the formation of C–O bonds via insertion of oxygen atom. In contrast, biomass is oxygen-enriched with abundant C–O bonds. Retaining oxygen atoms to produce organic acids is economically feasible. Moreover, the 8


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ACS Catalysis

C–C or C–H bonds of biomass are activated by oxygen atoms and are more susceptible to oxidation than hydrocarbons. Figure 2 shows the O/C and H/C ratio of the acids, biomass resources and fossil resources. The O/C and H/C ratio of acids is more similar to that of biomass resources. Thus, the direct conversion of biomass to the valuable organic acids is a very promising avenue and may be competitive to the present petroleum routes.

Figure 2. The O/C and H/C ratio of acids, biomass resource and fossil resource.

For example, 38 kt of bio-based succinic acid was globally produced in 2013 and the market value is about $108 million, while, about 40 kt of fossil-based succinic acid are produced with a $100 million market value.20 The current market price of bio-based succinic acid is about 2,860 $/t, which is relatively larger than that of fossil-based equivalent (around 2,500 $/t).20 At larger scale and with increasing of the oil price, the price of bio-based succinic acid is supposed to be lower than fossil-derived succinic acid. 9


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Biomass resources, such as (hemi)cellulose and lignin, are generally polymers. To utilize these resources, one should cut them down to small molecules. The C–C bond oxidative cleavage is an effective way for breaking down the polymer and reconstruction of the molecules. More importantly, it also introduces acid functionality at the same time, making it a powerful method for biomass valorization. This review systematically summarized the advances in production of organic acids from biomass via C–C bond oxidative cleavage (Figure 3). Various methods, catalytic systems and C–C bond cleavage mechanisms were summarized. The biomass resources include carbohydrates, lignin, chitin and platform molecules and unsaturated fatty acids. Platform molecules include 5-hydroxylmethyl furfural (HMF), furfural, levulinic acid (LA) and malic acid (MCA). The C–C bond cleavage via a retro-aldol reaction or decarboxylation under oxygen-free conditions is excluded from this review. Conversion of the alcohols or aldehydes to acids is not included herein, for example, oxidation of glucose and HMF to the corresponding acids. Some excellent reviews have covered these topics.3,21-22

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ACS Catalysis

Figure 3. Scheme of the oxidative cleavage of biomass resources to the acids and their derivatives.

2. Oxidation of carbohydrates

The major challenge for oxidative cleavage of carbohydrates to aliphatic acid is to selectively break up the C–C bond. Due to the different hydroxyl or carbonyl functional groups, the reactivity of the C–C bond in carbohydrates and consequent intermediates is varied. Highly hydroxyl-functionalized and hydrogen-bonding character make it hard to be broken up. Harsh condition easily leads to the formation of CO2. For cellulose, the intra- and inter-molecular hydrogen-bonding results in the poor solubility in common solvents. The insolubility makes it hard to be contacted by the catalyst and thus shows low activity. Recent works have developed a series of catalysts 11


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and strategies to overcome these issues and have achieved great progress.

2.1. Oxidation of carbohydrates to formic acid

Formic acid (FA) is an important and widely used organic chemical. In addition, FA is also used as a hydrogen donor for many hydrogenation reactions, and even for some important biomass hydrogenation, instead of hydrogen.23-24 More importantly, FA is believed to be a hydrogen-storage carrier in the future because it can be readily decomposed into hydrogen under mild conditions.25-29 Considering the energy distribution and storage, hydrogen has the potential to be one of the ultimate energy sources for mobile applications.28 The industrial production of FA is via hydrolysis of methyl formate formed by methanol carboxylation.16 Also, FA can be obtained through partial oxidation of butanes or naphtha.16 Recently, the hydrogenation of CO2 is a promising alternative route to FA, attracting a great deal of interest, but so far it has not been industrialized.27 As the limited reserve and unrenewable nature of the fossil feedstocks, a lot of researches have been devoted to making FA from renewable and abundant biomass resources. The results of recent oxidative cleavage of biomass resources to FA were shown in Table 1.

2.1.1. Hydrothermal oxidation without catalyst

Carbohydrates biomass with an aliphatic polyhydroxyl structure is a potential resource for the FA production via complete C–C bond oxidative cleavage. For

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ACS Catalysis

example, glucose, a model compound of carbohydrates, has six carbon atoms linked by five C–C bonds. Theoretically, one glucose molecule can provide six molecules of FA by oxidative cleavage of five C–C bonds. Even without the addition of a catalyst, FA could be formed via thermal oxidation of glucose at high temperature.30-33 In 2008, Jin reported a selective synthesis of FA from biomass via hydrothermal oxidation.31 Glucose was hydrothermally oxidized in alkali hydroxide solution at 250 °C, and hydrogen peroxide (H2O2) was used as the oxidant. FA with 75% yield was obtained. But, a high alkali concentration (1.2 M, 5 equiv) was required to prevent the decomposition of FA. The maximum FA yield was decreased to 24% without the addition of alkali salts. Many complex biomass can be hydrothermally oxidized through C–C bonds oxidative cleavage, leading to various organic acids, while the product distribution of organic acids was not selective, and the yield of FA is generally low.34-36 Cellulose was hydrothermally oxidized to a mixture of FA, acetic acid, glycolic acid and lactic acid with a highest total yield of 43%.37 The yield of FA is around 8%. A two-step method, that is first degradation of cellulose to small molecular intermediates at 110 °C under atmospheric pressure, and then oxidation with H2O2 at 50 °C for 4 h gave 32.8% yield of FA.38

2.1.2. Catalytic oxidation with POM as catalyst

To avoid harsh reaction conditions, catalytic oxidative cleavage of biomass has received much attention. The most commonly used catalyst for biomass 13


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oxidative cleavage is polyoxometallate (POM). POM is a kind of anionic metal–oxygen

clusters

and

is

widely

used

in

catalysis.39-43

Acidic

phosphovanadomolybdates, H3+nPVnMo12−nO40 (HPA-n), with n = 0–5, are widely used as catalyst in oxidation reactions.44-56 In 2010, Wasserscheid and co-workers reported a H5PV2Mo10O40 (HPA-2) catalyst for the conversion of glucose in O2 atmosphere.44 A 50% yield of FA was achieved at 353 K in 3 MPa oxygen for 26 h. In 2012, Fu’ group also reported that a 52% yield of FA was obtained from glucose over HPA-2.45 Detailed optimization of reaction conditions was conducted. The longer reaction time and higher reaction temperature would lead to FA decomposition. A high oxygen pressure (> 2 MPa) was needed to re-oxidize the HPA-2 after being reduced by the glucose. However, for more complex water-insoluble biomass such as cellulose, low efficiency was shown due to the poor solubility. To overcome this problem, the combination of acid catalyst and HPA was employed for cellulose and biogenuin biomass conversion to FA. Cellulose was firstly hydrolyzed to glucose over acid catalyst, and then the oxidative cleavage of glucose occurred over HPA catalyst. Wasserscheid’ group found that 22% yield of FA was obtained from oxidative cleavage of cellulose over HPA-2 with the assistance of p-toluenesulfonic acid (TSA) at 363 K in 3 MPa of O2 for 66 h.46 TSA promoted the hydrolysis of cellulose into soluble glucose, thereby enhancing the oxidative cleavage of the cellulose to FA. Some biogenuine 14


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ACS Catalysis

biomass, such as poplar, pine, waste paper, etc., were also converted to FA.47 Furthermore, FA could be simply extracted from the aqueous reaction mixture. A 30 kg of FA was obtained from 100 kg of cellulose over HPA-2 for 24 h. The pH value showed great effect on HPA-catalyzed cellulose oxidation to FA. Wu and coworkers48 studied the effect of pH value on the HPA catalytic performance. The combination of HPA-2 and H2SO4 achieved a 61% yield of FA from cellulose with molecular oxygen in aqueous solution. Decreasing the pH value increased the oxidation potential and electron affinity because of the formation of VO2+ species and protonated HPA-2, which is beneficial for the catalyst reduction and the substrate oxidation. The hydrolysis of cellulose to glucose was accelerated in the low pH value, favoring the formation of FA but also inducing side reactions, such as further hydrolysis of glucose to levulinic acid. The incorporation of transition metal into Mo-V-P heteropolyacid could enhances its catalytic performance in the oxidation of cellulose to FA.49 FA with 66% yield was obtained from mechanically activated microcrystalline cellulose over Co-HPA-2 catalyst at pH = 1.5. Liu and coworkers prepared heteropolyanion-based ionic liquids for the conversion of cellulose to FA in an O2 atmosphere.50 This functionalized ionic liquids contains SO3H groups and PMo11VO404− anions, and served as a bifunctional catalysts in which SO3H groups catalyzed hydrolysis of cellulose to glucose and PMo11VO404− anions catalyzed oxidative cleavage of glucose to FA. FA in 50% yield was achieved from cellulose oxidation. 15


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Another issue for oxidative cleavage of biomass into FA is to prohibit the further decomposition of FA to CO2. Han and coworkers observed that decreasing vanadium content of the HPA-n catalysts led to the increase of FA selectivity.51

Compared

to

multi-V-substituted

phosphomolybdic

acid,

mono-V-substituted phosphomolybdic acid (HPA-1) possesses lower oxidation capacity and prevents FA from being over-oxidized to CO2, resulting in a higher selectivity towards FA. A 68% yield of FA was obtained by the oxidation of cellulose over HPA-1 catalyst under optimized conditions. HPA-1 was also effective for conversion of raw cellulosic biomass bagasse and hay to FA in about 60% yield. Another effective method to increase the FA selectivity is to in-situ extract the FA from the reaction solution. Albert developed a water – organic biphasic system to prohibit FA decomposition.52 Compared with the monophasic aqueous media, the biphasic system was more selective. The oxidation reaction mainly occurred in the water phase over HPA-5 catalyst. The in-situ extraction of FA to organic phase with long-chain primary alcohols can avoid the deep degradation of FA to CO2 by the catalyst in the water phase. Furthermore, the formation of FA will lead to the decrease of the pH value of the aqueous phase. When the pH value is below 1.5, HPA-5 is inclined to produce a large amount of CO2. The in-situ extraction of FA to organic phase limits the decrease of pH value and increases the selectivity of FA. A maximum 85% yield of FA was obtained from glucose. Employing the biphasic reaction 16


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ACS Catalysis

system, even beech wood was converted into FA in 61% yield. The biphasic protocol shows promising to produce pure FA from wood in a robust, integrated, and low-temperature process. Although HPA catalyst is effective for the oxidation of carbohydrates to FA, high oxygen pressures (typically > 2 MPa) is needed to re-oxidize the reduced catalyst. It is a rate-determining step for the biomass conversion to FA with HPA-2. Wasserscheid’s group found that an increase of V-substitution degree could facilitate re-oxidation of the reduced catalyst, which showed better performance.53 Different V substituted H3+nPVnMo12−nO40 (HPA-n, n = 0-6) catalysts were used as catalyst for the oxidation of glucose to FA with molecular oxygen and TSA. The higher V-substituted HPA (n = 2–6) exhibited better performance than the lower V-substituted HPA (n = 0–1). HPA-5 showed the best performance with a total 60% and 28%FA yield for glucose and cellulose, respectively. Generally, oxidation catalyzed by HPA has been shown to be carried out through

the

Mars−van

Krevelen

Mechanism

involving

the

electron

transfer-oxygen transfer type process.54 Oxygen atoms are transferred from the

polyoxometallate

to

the

substrate,

simultaneously

reducing

the

polyoxometallate. And then, the reduced polyoxometallate is re-oxidized by molecular oxygen. HPA break the C–C bond of the alcohol through an oxygen insertion process into the C–C bond.54 Since glucose possesses polyol groups, its cleavage is analogous to that of alcohols. Oxidation of glucose with 6 17


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equivalents of polyoxometallate under anaerobic conditions yields 5 equivalents of FA, and 1 equivalent of HCHO by cleavage of five C–C bonds. The oxidation reaction is probably via several intermediates, such as glycolaldehyde, glyoxylaldehyde and glyceraldehyde (Figure 4). These compounds are susceptible to be oxidized to FA under the reaction conditions. Labelling experiments showed that CO2 was mainly derived from C5-carbon. This indicates that CO2 not only comes from the decomposition of FA but also directly forms during the cleavage of C–C bond. Therefore, it is hard to completely inhibit the formation of CO2. To minimize the generation of CO2, the catalyst should selectively cleave the C–C bonds to FA and have poor activity for the decomposition of FA.

Figure 4. The reaction mechanism for the oxidation of cellulose to FA over HPA catalyst.

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2.1.3. Catalytic oxidation with vanadium salts as catalyst

Wang and coworkers found that simple VOSO4 was also active in biomass oxidation to FA, but a relative high reaction temperature was needed (> 413 K).57 The oxidative cleavage of C–C bond is achieved through the redox conversion of VO2+/VO2+ under oxygen conditions. The addition of methanol or ethanol prohibited the formation of CO2 and significantly increased the FA yield to 70–75% at 413 K with 2 MPa of oxygen in 3 h. With the addition of ethanol, FA with 70% yield was obtained from oxidation of ball-milled cellulose at 433 K and 2 MPa of oxygen within 5 h. The reaction mechanism is shown in Figure 5. Glucose was first isomerized to fructose catalyzed by VO2+ species, and then fructose

was

subsequently

converted

to

glyceraldehyde

and

1,3-dihydroxyacetone via retro-aldol reaction. FA was formed via the cleavage of the C3 and C2 intermediates under an O2 atmosphere. Oxidative decarboxylation of glyoxylic acid to FA may be the main pathway.

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OH OH HO

OH O

VIV H2O

O

isomerization

OH OH glucose

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OH

HO OH OH fructose

VIV H2O

retro-aldo

OH HO

OH

HO OH

O H

O

O

HO OH

H

OH

HO

O

O

O

O

O

HO H

CO2

O

O

HO

H

OH

O

OH

OH

O

H

CO2

OH

Figure 5. The reaction mechanism for the VOSO4 catalyzed oxidation of glucose to FA.57 NaVO3 is also effective for the oxidative cleavage of biomass to FA. Wu and coworkers studied the conversion of wheat straw to FA in aqueous solution with molecular oxygen using NaVO3-H2SO4 catalyst.58-59 The products majorly derived from polysaccharides in wheat straw via hydrolysis of polysaccharides

to

monosaccharides

and

oxidative

cleavage

of

monosaccharides to FA. H2SO4 was used for the hydrolysis of polysaccharides to

monosaccharides

and

NaVO3

catalyzed

the

degradation

of

monosaccharides to FA. Deep hydrolysis to byproducts (levulinic acid) also occurred in the presence of H2SO4 and competed with the oxidation to FA. The reaction temperature, oxygen pressure and H2SO4 concentration showed remarkable effects on this catalytic system. High temperatures enhanced the deep hydrolysis reaction. High O2 pressure favored the catalytic oxidation and suppressed the deep hydrolysis. Moreover, the increase of H2SO4 20


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ACS Catalysis

concentration promoted deep hydrolysis and inhibited the catalytic oxidation. To obtain a high yield of FA, the H2SO4 concentration, oxygen pressure and the reaction temperature should be carefully optimized. A 47% yield of FA was obtained using 2 wt% H2SO4 at 160 °C under 3 MPa of O2 for 5 minutes.

2.1.4. Other catalyst

Calcined Mg-Al hydrotalcite was used to produce FA from monosaccharides such as glucose, galactose, xylose, arabinose and lyxose with H2O2 as an oxidant in ethanol solvent.60 For the glucose oxidation, the FA yield and H2O2 utilization efficiency reached 78% and 100% at 343 K for 5 h, respectively. The used hydrotalcite catalyst were separated and reused twice. Jung and coworkers discovered that NHC-amidate Pd(II) complex could oxidize saccharides using stoichiometric H2O2 at 313 K.61 The catalyst could be separated via extracting with methylene chloride and reused in a sequential reaction. Excess bases were needed. However, the high price and complexity of Pd nobel metal limited its use in practice. Copper catalyst was also effective for oxidation of biomass to FA. About 23% yield of FA was obtained at 463 K over CuO catalyst with O2 in the alkaline solution. A 65% FA yield was obtained from glucose with a magnesia-supported copper catalyst with H2O2 as oxidant at 393 K.62 Sulfonated iron(III) porphyrin in ppm concentration efficiently catalyzed the degradation of cellulose to FA in an aqueous alkaline medium in 64% yield.63 21


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Page 22 of 147

Iron(III)-peroxo was the active species for the oxidative cleavage of gluconic acid to FA. The electrochemical conversion of carbohydrates into FA in an aqueous medium was investigated by Parpot.64 At the NiOOH anode, D-xylose provided 42% selectivity towards FA in alkaline medium. The selectivity of D-glucose transformation into FA was 38% when RuO2/Ti was used as the working electrode in 0.5 M NaOH medium. The byproducts were mainly oxalate anion and

carbonates.

Although

it

showed

low

efficiency

presently,

the

electrochemical method is a potential method for the conversion of biomass to FA, because it can be carried out at low temperatures, which may inhibit the decomposition of FA. Photocatalysis is also very promising for biomass degradation since very active oxygen species, such as O2•- and •OH, formed under photo-irradiation conditions. These oxygen species are very active in oxidative degradation reactions. Using a nano TiO2 catalyst, 35% yield of FA was obtained from glucose in 0.03 M NaOH aqueous solution.65

2.2. Oxidation of carbohydrates to acetic acid

Acetic acid (ACA), is an important organic acids and the global production amount in 2014 is 12.9 million tonnes.66 It is industrially produced by methanol carbonylation.17 Recently, exploring new routes to produce ACA from alternative resources has received much attention.67-68 22


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ACS Catalysis

The oxidative cleavage of biomass is a promising way to produce ACA. Minor amount of ACA was observed in the oxidation of biomass and organic waste to FA.51,69-72 Hydrothermal oxidation of biomass is widely employed for producing ACA.73-76 Wet oxidation of lignin model compound guaiacol at 573 K afforded a 10% yield of ACA.77 Fujie's group

78

studied the ACA production

from some organic waste and compounds with H2O2 as oxidant via hydrothermal oxidation. A mixture of carboxylic acids was formed with ACA as the main product. ACA was obtained from dry-waste fish entrails at about 26 mg g−1 at 623 K. About 29 mg g−1 of ACA was produced from glucose. The yield of ACA is generally low in normal hydrothermal oxidations. Efforts have been made to increase the yield of ACA by the addition of bases, acids and metal oxides. The addition of base into the hydrothermal solution increased the ACA yield. About 17% yield of ACA was produced from glucose at 250 °C in the presence of H2O2

and NaOH.79 Jin and coworkers80

proposed a two-step method to improve the ACA yield by controlling the reaction pathways (Figure 6a). The two-step reaction process comprises anaerobic

and

hydrothermolysis

subsequent was

oxidation

conducted

under

reaction.

In

oxygen-free

the

first

conditions,

step, and

carbohydrates were mainly converted to lactic acid, HMF and 2-furaldehyde. In the second step, oxygen was charged. Compared with the carbohydrates polymers, the reaction products in the first step were easier to be oxidized into ACA. ACA with a 16% yield was obtained from cellulose by the two-step 23


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process. The contribution of furans and lactic acid to ACA formation was 85-90%.

Figure 6. Two step method for the hydrothermal oxidation of carbohydrates to ACA.

Later on, Jin and coworkers81-82 further developed an alkali two-step process (Figure 6b). The introduction of base in the first step promoted the conversion of carbohydrates to lactic acid, and in the second step, lactic acid was then oxidized to ACA by H2O2 or air. By adding Ca(OH)2 or bentonite, a maximum 27% ACA yield was obtained from glucose. The use of CuO as oxidant instead of H2O2 or air further increased the ACA yield. A 32% yield of ACA from glucose and 5% from cellulose were achieved with CuO oxidant under alkaline hydrothermal conditions.37,83 In the alkaline two-step method, using CuO as oxidant in the second step significantly improved the ACA. Glucose, cellulose and starch were converted to ACA in yields of 26%, 22% and 23%, respectively.84 With the addition of acid catalyst in the first step, an acidic two-step 24


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ACS Catalysis

method was employed to improve ACA yield under hydrothermal conditions (Figure 6c).85-86 The first step, the acid catalyst catalyzes the hydrolysis of carbohydrates to levulinic acid. In the second step, levulinic acid was oxidatively cleaved to ACA with newly supplied H2O2. The ACA yield reached 23% and 26% from glucose and fructose using the acidic two-step method, respectively.

2.3. Oxidation of chitin to acetic acid Chitin is a major constituent in crustacean shells.9-10 N-acetyl-D-glucosamine monosaccharide is the major unit of chitin, which is linked together by β-glycosidic bonds (Figure 7). Chitin is a suitable resource for ACA production because 25% of the acetyl groups are naturally present in the chitin. The acetyl amide could be readily hydrolyzed to ACA. In addition, chitin pyranose ring in the framework can also be oxidatively cleaved to ACA, similar to cellulose. Therefore, theoretically, more than 25% yield can be obtained from chitin. Yan and coworkers pioneered the conversion of chitin to ACA.87 CuO was used as catalyst with molecular oxygen as oxidant in NaOH solution at 573 K. ACA with a 38% yield was obtained from chitin. Furthermore, about 48% yield of ACA was produced from crude shrimp shells. Acetyl amide was initially hydrolyzed to ACA and the remaining compounds were subsequent oxidized to ACA.

25


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Figure 7. The oxidation of chitin to ACA. 2.4. Oxidation of carbohydrates to oxalic acid

Oxalic acid is a useful chemical and widely used in the production of celluloid and rayon, leather manufacture and dressing, extraction of rare earth from monazite, etc. Oxalic acid has been produced from cellulose by nitric acid oxidation processes.88 However, this process requires high concentrations of nitric acid (ca. 40 wt %), which is very costly and corrosive and requires special reaction apparatus in terms of scaling up for industrial production. Traditionally, oxalic acid was also produced by fusion of cellulose in alkaline solutions at high temperatures (473–493 K).89-90 Chitin was oxidized to oxalic acid in 5% yield,87 and cellulose could be efficiently converted into oxalic acid using CuO as a catalyst and O2 as an oxidant.91 A 42% yield of oxalic acid was obtained from microcrystalline cellulose at 473 K and 0.3 MPa of oxygen for 2 h. Glycolic acid and lactic acid are possible intermediates. As the oxidation of glycolic acid to oxalic acid is faster than that of lactic acid, the reaction may be predominantly through the glycolic acid intermediate. A plausible reaction mechanism for cellulose conversion to oxalic acid was proposed (Figure 8). Cellulose was firstly hydrolyzed to glucose under base conditions. Then, glucose may coordinate 26


Page 26 of 147

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ACS Catalysis

on the surface Cu site37,92 and was further transformed into glyceraldehyde via C3–C4 cleavage of glucose (pathway 1) and glycolaldehyde via C2–C3 scission (pathway 2). Besides, glyceraldehyde could be converted to formaldehyde and glycol aldehyde through retro-aldol condensation. In pathway 2, glycolic acid was derived from glycol aldehyde via base-catalyzed oxidation in NaOH solution. Glycolic acid is easily oxidized to oxalic acid via oxidation of the hydroxyl groups in base conditions.93-94 In pathway 1, glyceraldehyde could produce lactic acid, which was then oxidized to pyruvic acid and 2-hydroxyacrylic acid. 2-Hydroxyacrylic acid is an isomerization product of pyruvic acid. Glyceric acid could be formed from 2-hydroxyacrylic acid,95 or from the direct oxidation of glyceraldehyde,96-101 and then underwent base-catalyzed oxidation to form 2-hydroxy-3-oxopropanoic acid which was further converted into glycoxylic acid via retro-aldol condensation. Glycoxylic acid was oxidized to oxalic acid as in pathway 2.

27


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Page 28 of 147

Figure 8. The proposed pathways for conversion of MCC to oxalic acid.91

Table 1. The oxidative cleavage of carbohydrates to FA Substrate

Catalyst

Reaction conditions

Yield

Ref.

75%

31

Hydrothermal, 250 °C, 1.25 M KOH, H2O2, 1 min

glucose

HPA-2

70 °C, 30 bar O2, 7 h

50%

102

HPA-2

70 °C, 2 MPa O2, 3 h

55%

45

17%

64

21%

64

Constant current intensity Ni electrode of 0.25 A. 0.2 M NaOH. Ru/TiO2

Constant current intensity 28


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ACS Catalysis

electrode

of 0.5 A. 0.2 M NaOH.

HPA-5

90 °C, 30 bar O2, 8 h,

60%

53

75%

57

180 °C, 1 MPa O2, 1 h

55%

50

180 °C, 1 MPa O2, 3 h

55%

103

44%

58

70 °C, H2O2, 5 h

78%

60

CuCTAB/MgO

120 °C, H2O2, 12 h

65%

62

Iron(III)

0.5 mol L-1 NaOH, 2.0 MPa 25%

63

55%

103

44%

58

49%

60

140 °C, 2 MPa O2, 3 h, VOSO4 H2O-Methanol solvent [MIMPS]3HPM o11VO40 HPA-1

H2SO4, 160 °C, 3 MPa O2, NaVO3 1 min Calcined Mg-Al-HT

porphyrin

O2, 150 °C, 3 h

HPA-1

180 °C, 1 MPa O2, 3 h H2SO4, 160 °C, 3 MPa O2,

NaVO3 1 min

fructose Calcined °

70 C, H2O2, 5 h Mg-Al-HT

29%

glucuronic HPA-1

150 °C, 1 MPa O2, 3 h

35%

103

HPA-2

70 °C, 30 bar O2, 7 h

56%

102

acid sorbitol

29


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Page 30 of 147

H4PVMo10O40

180 °C, 1 MPa O2, 3 h

44%

103

HPA-2

80 °C, 30 bar O2, 26 h

54%

102

36%

64

180 °C, 1 MPa O2, 3 h

33%

103

70 °C, H2O2, 5 h

80%

60

70 °C, H2O2, 5 h

80%

60

70 °C, H2O2, 5 h

80%

60

7%

64

70 °C, H2O2, 5 h

74%

60

Constant current intensity Ni electrode of 0.25 A. 0.2 M NaOH. xylose HPA-1 Calcined Mg-Al-HT Calcined arabinose Mg-Al-HT Calcined xylose Mg-Al-HT Constant potential of 2V

PbO2 electrode

vs. SCE; 0.2 M NaOH.

D-galactose Calcined Mg-Al-HT sucrose

HPA-2

80 °C, 30 bar O2, 26 h

48%

102

cellobiose

HPA-2

80 °C, 30 bar O2, 26 h

47%

102

HPA-2

80 °C, 30 bar O2, 26 h

33%

102

H5PV2Mo10O40 TSA, 90 °C, 30 bar O2, 66 h

63%

46

TSA, 90 °C, 30 bar O2, 24 h

58%

53

Constant current intensity

3%

64

xylan

HPA-5

trehalose

Ni electrode of 0.25 A. 0.2 M NaOH. 30


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ACS Catalysis

Hydrothermal, 328 °C, -

8%

37

33%

38

NaOH, CuO, 1 min Two step, hydrothermal, H 2 O2 HPA-2

80 °C, 30 bar O2, 26 h

1%

102

HPA-2

TSA, 90 °C, 30 bar O2, 66 h

22%

46

HPA-2

HCl, 90 °C, 30 bar O2, 66 h

34%

45

61%

48

66%

49

H2SO4, 180 °C, 3 MPa O2, HPA-2 5 min pH = 1.5, 160 °C, 20 bar Co-HPA-2 O2 , 5 h

cellulose HPA-5

TSA, 90 °C, 30 bar O2, 24 h

28%

53

VOSO4

160 °C, 2 MPa O2, 5 h

70%

57

180 °C, 1 MPa O2, 1 h

55%

50

180 °C, 0.6 MPa O2, 3 h

68%

103

58%

58

23%

91

64%

63

[MIMPS]3HPM o11VO40 HPA-1

H2SO4, 160 °C, 3 MPa O2, NaVO3 10 min CuO

190 °C, 0.5 MPa O2, 2 h

Iron(III)

0.5 mol L-1 NaOH, 2.0 MPa

porphyrin

O2, 150 °C, 6 h

31


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Page 32 of 147

inulin

VOSO4

140 °C, 2 MPa O2, 1.5 h

39%

57

starch

VOSO4

140 °C, 2 MPa O2, 1.5 h

46%

57

HPA-2

80 °C, 30 bar O2, 26 h

14%

102

HPA-2

TSA, 90 °C, 30 bar O2, 66 h

32%

46

HPA-5

TSA, 90 °C, 30 bar O2, 24 h

32%

53

HPA-2

80 °C, 30 bar O2, 26 h

19%

102

pomace

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

55%

47

cane trash

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

49%

47

fruit pulp

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

45%

47

spruce chips

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

35%

47

poplar splint

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

31%

47

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

30%

47

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

22%

47

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

22%

47

lignin

poplar wood sawdust

grass clippings chondrus crispus chlorella

32


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ACS Catalysis

oak bark

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

21%

47

willow bark

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

16%

47

cyanobacter

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

16 %

47

spirulina

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

16%

47

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

16%

47

straw

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

12%

47

nettle leaf

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

11%

47

ulva lactuca

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

8%

47

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

16%

47

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

11%

47

HPA-2

TSA, 90 °C, 3 bar O2, 24 h

1%

47

H2SO4, 160 °C, 3 MPa O2,

47%

59

ascophyllum nodosum

effluent sludge railway sleeper beech condensate

wheat straw

NaVO3 5 min

33


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Page 34 of 147

3. Oxidation of lignin

Lignin is an aromatic biopolymer, and is formed form three primary structure of phenylpropanols: coumaryl alcohol (4-hydroxycinnamyl), coniferyl alcohol (3-methoxy

4-hydroxycinnamyl)l

and

sinapyl

alcohol

(3,5-dimethoxy

4-hydroxycinnam-yl) (Figure 9). The three monolignols are named as p-hydroxyphenyl (H) , guaiacyl (G) and syringyl (S) units. The richness of lignin in nature and its potential to provide low-molecular weight aromatics arouse a great research interest in the depolymerization of lignin.

Figure 9. The structure of monolignols and linkages in lignin. The lignin units are majorly connected by C−O and C−C bonds. The major linkages between two monolignols can be categorized into several types, as shown in Figure 10. The major linkages between the monolignols are β-O-4, β−β, β-5, β-1, α-O-4, 4-O-5 and 5−5, where the β-O-4 is the dominant linkage and constitutes about half of the lignin linkages.104 These basic link units are 34


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ACS Catalysis

often used as models for the lignin cleavage study. Some lignin linkage derivatives were also used as lignin models. For example, the β-1 and β-O-4 ketone are derived from the oxidation of Cα-OH of β-1 and β-O-4. The oxidation of the lignin linkages to the corresponding ketones have been successfully achieved for both lignin models and organosolv lignin.105-110 The key to lignin depolymerization is to effectively break up the linkages. Acid-base hydrolysis,111-116 pyrolysis,117-120 hydrogenolysis121-132, oxidation133 and their combinations105,134-135 are used for the depolymerization of the lignin linkages. Lignin is degraded by the enzymes in nature. The oxidative depolymerization of lignin tends to form aromatic compounds with oxygen-containing functionality, which is a value-added process. Many of these functionalized aromatic compounds are either served as target chemicals or as platform chemicals used for subsequent transformation. Molecular oxygen, hydrogen peroxide, nitrobenzene and metal oxides are the commonly used oxidants. The oxidative cracking reaction involves cleavage of C–O and C–C bonds, or the deconstruction of aromatic rings. Aromatic acids are widely used chemicals and are produced industrially from petroleum via hydrocarbon oxidation. It is significant to prepare aromatic acids from lignin. About one third of the linkages in lignin are C–C bonds, and their oxidative cleavage can not only lead to the lignin depolymerization, but also introduces oxygen into the products. Aromatic acids can be obtained from lignin via the C–C bonds oxidative cleavage. The key issue is to avoid the 35


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Page 36 of 147

overoxidation or repolymerizaiton of the products, especially in the presence of phenolic groups. Historically, in paper industry, lignin is oxidatively degraded into waste in the delignification process.136-145 It needs to develop more selective catalysts to convert lignin into valuable chemicals. Recent work has focused on the lignin models, as a “bottom-to-up” strategy due to the structure complexity of lignin. In this section, we will summarize the recent work on the C–C bond oxidative cleavage of lignin models to aromatic acids and their derivatives or aromatic aldehydes, and some real lignin to acids. The catalytic systems can be classified into following types: vanadium-based catalysts, copper-based catalysts, Co/Mn/Br, Re, POM, and metal-free catalysts. In addition, photocatalytic oxidation of lignin is also included in this section.

HO

O

R

O

4

O

O

O

-5 4-12% O

OH

1 5 O O

4 1

4

2 3

1

HO

O

3 2

4-O-5 4-7%

O O

O

OH

O

O

5-5 4-25%

-O-4 43-65%

4

HO

O

O

O

5

3 4O 2 1 5 1 5 O 4 32

O

O O

O

-1 3-7%

Figure 10. The typical lignin models and their abundance. 36


O

2-7%

O

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ACS Catalysis

3.1. Thermal catalytic reaction

3.1.1. Vanadium based catalyst β-O-4 is the predominant bonding mode in lignin with 43-65% percentage of the all lignin linkages. Enormous works have been done on the oxidative cleavage of β-O-4 linkages. Vanadium-based homogenous catalyst is effective for the oxidative cleavage of C−C bonds in vicinal diols,146 and has been widely used in the oxidative cleavage of lignin models. Toste, Baker and Hanson prepared a series of vanadium-based complexes (Figure 11).147-154 2-Phenoxy-1-phenylethanol and 1, 2-diphenyl-2- methoxyethanol were first used

as

substrates

with

(dipic)V(O)OiPr

complex

as

catalyst.154

2-Phenoxy-1-phenylethanol underwent oxidation to benzoic acid with a 81% yield in DMSO (Figure 12). Solvent has a great effect on the product distribution (Figure 13). For 1, 2-diphenyl-2-methoxyethanol, benzaldehyde (73% yield) was the major product in DMSO, while in pyridine solvent, benzoic acid and methyl benzoate with 85% and 84% yields, respectively, were formed as the major products.

Figure 11. The structure of (dipic)V(O)OiPr and (HQ)2V(O)(OiPr).

37


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Figure

12.

Oxidative

cleavage

of

2-phenoxy-1-phenylethanol

Page 38 of 147

over

(dipic)V(O)OiPr complex.154

Figure 13. Oxidative cleavage of 1,2-diphenyl-2-methoxyethanol over (dipic)V(O)OiPr complex.154

The vanadium catalyst for the oxidation reaction is initiated by the C−H bonds cleavage of the hydroxyl group, forming the ketone or aldehyde intermediate. Further oxidation of the ketone or aldehyde intermediate resulted in C−C bonds cleavage. In the oxidation of 1,2-diphenyl-2-methoxyethanol, ketone benzoin methyl ether intermediate was initially the major product and was then further converted to benzoic acid and methyl benzoate. The C–H bond oxidation is the key step for the vanadium catalyzed C–C bond cleavages. The acceleration for reaction with addition of the base is ascribed to the abstraction of the C−H bond on the alcohol carbon. Susannah 38


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ACS Catalysis

L. Scott and coworkers studied the mechanism of C−H bond deprotonation using probe molecules, such as isopropanol and benzyl alcohols.155 In the case of (dipic)V(O)OiPr, pyridine coordinates to the vanadium center and then, another pyridine molecule attacks the C−H bond of the isopropoxy group (Figure 14). It is also possible that pyridine directly attack the C−H bond of the isopropoxy ligand of (dipic)V(O)OiPr without prior pyridine coordination. (HQ)2V(O)(OiPr) shows a similar mechanism to (dipic)V(O)OiPr. Abstraction of the C−H bond of the benzyl alcoholate ligand by the base (NEt3) is in concerted with 2e-reduction of the vanadium center. The base shuttles a proton from the benzylic C−H to the vanadium-oxo, generating the vanadium(III) hydroxide intermediate [(HQ)2VIII(OH)(OCHPh)].

39


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Figure 14. Base-assisted dehydrogenation for (dipic)V(O)OiPr (a) and (HQ)2V(O)(OiPr) (b).155

The ligand shows great effect on the product distribution (Figure 15). In the oxidation of phenolic β-O-4 models, (HQ)2V(O)(OiPr) initially deprotonates O–H in phenolic group to generate a phenoxy radical intermediate, leading to the breaking up the C(alkyl)–C(phenyl) bond and affording products 2,6-dimethoxybenzoquinone and acrolein derivative.153 The vanadium Schiff base complex (SB)V(O)(OMe) initially cleaves the benzylic C–H bond, 40


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ACS Catalysis

resulting in breaking up the C–O bond and affording ketones and phenols.147,152

Figure 15. Oxidation of the phenolic β-O-4 models by (SB)V(O)(OMe) and (HQ)2V(O)(OiPr).153

Simple vanadium salts can also catalyze the oxidative cleavage of β-O-4 lignin

models

with

the

1-phenyl-2-phenoxyethanol,

assistance Xu

and

of

acids.

In

coworkers156-157

the

oxidation

found

that

of the

combination of VO(acac)2 with aliphatic acid greatly enhanced C–C bond cleavage. In propionic acid or butyric acid solvent, 83% C–C bond cleavage selectivity were obtained at 94% conversion. Bolm and coworkers158 reported the copper-vanadium double-layered hydrotalcites for the oxidative cleavage of lignin models (Figure 16). A 31% yield of acid was obtained in pyridine solvent. However, the catalyst was unstable, as copper and vanadium leached during the reaction. 41


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Page 42 of 147

Figure 16. Oxidative cleavage of lignin model with copper-vanadium double-layered hydrotalcites.158

3.1.2. Copper catalyst Copper-based catalysts are widely used for the oxidation of alcohols.159 Recent studies also show that copper is an efficient catalyst for the C–C bond oxidative cleavage.160 Copper catalyst has also been used for the oxidative cleavage

of

lignin.

The

CuCl/TEMPO

[(2,2,6,6-tetramethylpiperidin-1-yl)oxyl)]/pyridine mixture was efficient for the oxidation

of

simple

lignin

models.

In

the

oxidation

1,2-diphenyl-2-methoxyethanol with O2 as oxidant, benzaldehyde

of

and methyl

benzoate was obtained using CuCl and TEMPO in pyridine at 100 °C (Figure 17).161 However, the drawback is the instability of the active copper species. To achieve 92% conversion, of CuCl in two additions (10 mol% each) and TEMPO in three additions of (10 mol % each) were required during the reaction. 42


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ACS Catalysis

Figure

17.

Oxidation

of

1,

2-diphenyl-2-methoxyethanol

by

CuCl/TEMPO/pyridine.161

The lignin models were also broken down to aldehydes and phenols with stoichiometric CuCl/TEMPO (Figure 18).150 After 40 h, 89% conversion was achieved, affording a mixture of 3,5-dimethoxybenzaldehyde, 3,5-dimethoxybenzoic acid, 2-methoxyphenol, ketones, dehydrated ketone, and formic acid. The reaction was not via ketone intermediates. The instability was a major drawback of the CuCl/TEMPO catalytic system, and a high catalyst loading was required.

Figure 18. Oxidation of β-O-4 lignin models by CuCl/TEMPO/pyridine.150 Subsequently, Baker et al.161 screened a number of different nitrogen ligands in association with various copper salts in the aerobic oxidation of β-O-4

models.

Copper(I) 43


trifluoromethanesulfonate

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Page 44 of 147

[Cu(OTf)]/2,6-lutidine/TEMPO was found to be the best catalyst, and a 20 mol % of Cu(OTf)/TEMPO was needed. Oxidation of the non-phenolic β-O-4 models proceeded with high Cα–Cβ bond cleavage selectivity with aldehyde as the dominant product (Figure 19). Whereas, the oxidation of the phenolic models prefers to cleave Cα–Caryl bond instead of Cα–Cβ bond to form 2,6-dimethoxybenzoquinone.

However,

a

stoichiometric

amount

of

Cu(OTf)/2,6-lutidine/TEMPO was still needed to dominantly cleave the Cα–Caryl bond (Figure 20). When the catalyst amount was reduced to 10 mol %, β-O-4 ketones was the major product. Using polybenzoxazine as support and ligand for the CuCl2, β-O-4 lignin model were readily underwent Cα–Cβ bond cleavage to acid and phenol product under room temperature with H2O2 as oxidant.162

Figure

Oxidation

19.

of

nonphenolic

β-O-4

models

over

Cu(OTf)/TEMPO/2,6-lutidine.161 OCH 3

OCH 3 OH H 3CO HO

O

OCH 3 O OH

OCH 3

O

O +

+ M eO

Cu(OTf)/TEMPO

21%

46%

2,6-lutidine (10 equiv.), O 2, toluene 100 oC, 18 h

O H 3CO

O O

OCH3

H 3CO

O +

conversion:100% HO

OH OCH 3 5%

44


OCH3 O

HO OCH 3 9%

Page 45 of 147 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure

20. Oxidation

of

phenolic

β-O-4

models

by

stoichiometric

Cu(OTf)/TEMPO/2,6-lutidine.161

Studies in recent years have found that copper-based catalysts is efficient in the oxidative cleavage of ketones,163-174 and are also active in the oxidative cleavage of β-O-4 ketone. A two-step method, that is first oxidation of β-O-4 models to the ketones and subsequent oxidative cleavage of β-O-4 ketone over a copper catalyst, was employed for β-O-4 and β-1 cleavage. Recent researches on the successful oxidation of β-O-4 linkages to β-O-4 ketone promoted the investigation of β-O-4 ketone.105-108 Liu and coworkers175 reported

the

oxidation

of

β-O-4

ketone

in

methanol

solvent

over

CuCl2/BF3OEt2/pyridine catalytic system. The selectivity of C–C bond cleavage is low and a mixture of methyl benzoate, methyloxy acetophenones and

phenols

were

formed.

Wang

and

coworkers176

reported

that

Cu(OAc)2/BF3OEt2 catalyst is efficient for the cleavage of β-O-4 and β-1 ketone in alcohols solvent, yielding esters and phenols (Error! Reference source not found.and Error! Reference source not found.). The key step for the C–C bond cleavage is the Cβ−H bond cleavage. BF3OEt2 coordinates the oxygen atom in carbonyl group and decreases Cβ−H bond dissociation energy. Without BF3OEt2, Cu(NO3)2 alone was active in the oxidation of β-O-4 ketone to acid in acetonitrile solvent.177 The acetonitrile solvent is crucial for the reaction, which increased the reduction potential of copper and favored the activation of substrate via electron transfer. With Copper−N-Heterocyclic 45


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Carbene as catalyst, β-1 lignin models majorly converted into aldehyde via a retro-aldol reaction.178 Table 2. Oxidation of β-O-4 ketone over CuCl2/BF3OEt2/Pyridine.176

Table 3. Oxidation of β-1 ketone over CuCl2/BF3OEt2/Pyridine models. 176

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In the presence of amines, amides could be generated from oxidation of lignin ketones over copper catalyst. Loh and coworkers179 reported CuI catalyzed oxidative cleavage of lignin ketones to a mixture of β-ketoamides, amide and phenols (Table 4). Only secondary amines were used as substrates. Primary amines were not used possibly because of the self-coupling of primary amines to imines under the oxidative conditions. Table 4. Oxidation of β-O-4 ketone to amides over CuI catalyst.179

Recently, Wang and coworkers reported a two-step strategy for oxidation of lignin to aromatic acids and phenols (Figure 21).180 Firstly, β-O-4 alcohol 48


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ACS Catalysis

was oxidized to β-O-4 ketone by VOSO4/TEMPO. In the next step, the Cα–Cβ bond was selectively cleaved over a Cu(OAc)2/1,10-phenanthroline catalyst, leading to the formation of acids and phenols. A wide range of methoxyl-substituted β-O-4 ketones were converted to aromatic acids with 80-95% yields (Table 5). However, the phenols are not stable and easy deep oxidized and polymerized under the reaction conditions and have low yields, especially for phenols with methoxyl substituents.

Figure 21. Two-step strategy for the oxidative cleavage of lignin.180

49


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Table

Oxidation

5.

of

β-O-4

Page 50 of 147

ketone

models

catalyzed

by

Cu(OAc)2/1,10-phenanthroline catalyst.180

The C–C bond cleavage may proceed as follows (Figure 22). Firstly, the Cα–OH

was

oxidized

Copper-oxo-bridged

to

a

dimer

ketone, was

activating

formed

the

via

Cβ–H

the

bond.181

reaction

of

Cu(OAc)2/1,10-phenanthroline complex with oxygen. This active species abstracts the H from Cβ–H bond by its one oxygen, and the remaining part is bound to the other oxygen atom, forming C–O bond. The Cα–Cβ bond energy in the such intermediate decreases from 307.7 to 205.5 kJ mol-1, which makes the Cα–Cβ bond to be easily broken up,182-185 leading to the formation of benzoic

acid

and

phenol

formate.

By

the

combination

of

Cu(OAc)2/1,10-phenanthroline and KOH base, one-step oxidative cleavage of 50


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ACS Catalysis

C–C bond in β-O-4 was realized via β-O-4 ketone intermediate.186

Figure 22. Proposed reaction mechanism for the β-O-4 oxidative cleavage by Cu(OAc)2/1,10-phenanthroline.180

3.1.3. Fe catalyst

Figure 23. Oxidative cleavage of β-O-4 model by Fe(TAML)Li as catalyst.187

51


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Table 6. Oxidation of β-O-4 models over Fe-DABCO catalyst.188

Andrioletti and co-workers187 reported the cleavage of β-O-4 models using Fe(TAML)Li catalyst and PhI(OAc)2 oxidant (Figure 23). The corresponding ketone was the main product in 31% yield, while the products from C–C bond cleavage, such as veratraldehyde and veratric acid, were formed in 15% and 3% yields, respectively. Bolm and coworker188 reported that FeCl3-derived iron 52


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ACS Catalysis

catalysts catalyzed the oxidation of β-O-4 models using H2O2 in DMSO solvent. DMSO was not just used as solvent, and it could generate methyl radicals which were active species for β-O-4 cleavage. Various methoxy-substituted lignin models were oxidized to methoxyphenols and benzaldehydes (Table 6).

3.1.4. Methyltrioxo rhenium Methyltrioxo rhenium (MTO) has been reported to be effective in activating the oxidizing agent as a highly active species and was widely used in oxidation reactions. MTO is also used in the oxidation of lignin or lignin models due to the high reactivity.189-194 Saladino and coworkers immobilized the MTO on polymers (Figure 24) which is used lignin model oxidation. With H2O2 as oxidant, phenolic β-O-4 models underwent C–C bond oxidative cleavage to provide 10-25% yield of 4-hydroxyl-3-methoxyl benzoic acid (Table 7). The lignin from sugar cane and red spruce were oxidized with an acid content of 0.81-1.76 g mmol-1.

Figure 24. Schematic structure of the immobilized MTO catalyst.191 53


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Page 54 of 147

Table 7. Oxidation of phenolic lignin model with H2O2 and catalysts I-IV. 191

3.1.5. Co/Mn/Br Co/Mn/Br was industrially used as catalyst in the oxidation of hydrocarbons to acids. Partenheimer195 reported the oxidation of lignin using Co/Mn/Zr/Br catalyst. Five kind of lignin samples were oxidized using air over the Co/Mn/Zr/Br catalyst in acetic acid solvent. The products were very complex and up to 18 products were identified. Up to 11 wt % of aromatic products, including

4-hydroxybenzaldehyde,

4-hydroxy-3-methoxy-benzaldehyde,

4-hydroxybenzoic

4-hydroxy-3-methoxybenzoic

4-hydroxy-3,5-dimethoxybenzalde-hyde

and

acid, acid,

4-hydroxy-3,5-

dimethoxy-benzoic acid, were obtained from organosolv lignin under optimized conditions. The poor solubility of lignin makes it difficult to contact with the catalyst and therefore it is inert to decompose. Increasing the solubility of lignin can enhance its transformation. Ionic liquids is a good solvent for macromolecules 54


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ACS Catalysis

such as lignin and cellulose and thus represents a new opportunity for depolymerization of lignin to value-added chemicals.196 Wasserscheid reported the oxidative cleavage of beech lignin using Mn(NO3)2 catalsyt in 1-ethyl-3-methylimidazolium

trifluoromethanesulfonate

[EMIM]

[CF3SO3],

giving a 63% conversion at 100 °C for 24 h. The catalyst loading has great effect on the selectivity. Reducing the catalyst loading to 2 wt %, the product was shifted from syringaldehyde to 2,6-dimethoxy-1,4-benzoquinone.

3.1.6. Pd catalyst Wang et al. reported that Pd/CeO2 catalyzed the oxidative cleavage of 2-phenoxy-1-phenylethanol in methanol with O2 as the oxidant, affording phenol, acetophenone and methyl benzoate.173 This reaction is proceeded via ketone intermediate. Pd nanoparticles catalyzed the oxidation of the Cα-OH into the Cα=O group. The ketone intermediate is more active to undergo the Cα–Cβ bond cleavage over CeO2 to produce benzoic acid and methyl benzoate. In addition, 8.47 wt % yields of vanillin, guaiacol and 4-hydroxybenzaldehyde was obtained from organosolv lignin over Pd/CeO2. Pd/Al2O3 catalysts were used as catalyst for the oxidation of alkaline lignin to vanillin and syringaldehyde in low yield.197-199

3.1.7. Metal free process Direct oxidative cleavage of lignin models has been reported to occur with strong oxidizing agents, such as Na2S2O8, affording aromatic aldehydes as the 55


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Page 56 of 147

major product.200 Recent studies have shown that C(CO)–C bonds in ketones are susceptible to be oxidative cleaved. The oxidation of Cα-OH in β-O-4 to ketones has been well studied.107-108 Treatment of β-O-4 ketone model with H2O2 in the presence of base provided veratric acid and guaiacol in 88% and 42% yields, respectively. (Figure 25).

Figure 25. Oxidation of β-O-4 ketone model with NaOH and H2O2.90

Graphene oxide was reported to catalyze the oxidative cleavage of β-O-4 models.201 The metal impurities in the graphene oxide played a marginal role in the catalytic activity. Bolm and coworkers reported a one-pot two-step cleavage of β-O-4 models using organo catalyst.202 First oxidation of the primary

over

the

secondary

hydroxyl

groups

with

a TEMPO/DAIB

[(diacetoxy)iodobenzene] system and then the Cα–Cβ bond are cleaved by proline-catalysed via retro-aldol reactions, producing aromatics aldehyde and phenol in high yields. Cary–Calky bond of β-O-4 ketone can be broken up via Baeyer-Villiger oxidation.203-205 Meier and coworkers reported a two-step method for the oxidative cleavage of Cary–Calky bond.203 In the first step, β-O-4 ketone was formed via oxidation of benzyl alcohol. Then, in the presence of formic acid and H2O2, the Cary–Calky bond was broken up via Baeyer-Villiger 56


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ACS Catalysis

oxidation to esters. Further hydrolysis of the esters led to a mixture of aromatic acids and phenols. β-O-4 Ketone can be oxidatively degraded in metal-free conditions promoted by ionic liquid. 2-Phenoxyacetophenone was completely oxidized to benzoic

acid

and

phenol

in

1-benzyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)-imide ([BnMIm][NTf2]) solvent with a small amount of H3PO4 (Figure 26).206 Benzoic acid (89% yield) and phenol (84% yield) were obtained at 403 K in 3 h under 1 MPa of O2. It was proposed that [BnMIm][NTf2] promoted the formation of a •OOH free radical which is the active species for the C–C bond cleavage.

Figure 26. Oxidation of 2-phenoxyacetophenone in ionic liquid solvent with H3PO4.206

3.2. Photocatalytic oxidation

Lignin is naturally formed by photosynthesis through a single electron process. It is promising to depolymerize lignin by photocatalysis.207-210 In the paper industry, the degradation of lignin polymer via photo irradiation has long been studied in the delignification process. Tanaka and coworkers have studied the complete photo-degradation of lignin over TiO2.211 Chang and coworkers studied the treatment of waste water containing lignin employing TiO2 assisted 57


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by UV light.212 The degradation of calcium salt of lignosulfonates to CO2 and H2O has been reported by using gamma ray irradiation.213 However, in all of the photocatalysis processes described above, the carbon of lignin is dominantly converted to CO2.214-218 It is desirable that the lignin is selectively cleaved into value-added chemicals.219-221 Argyropoulo and Sun222 investigated the solid-state photodegradation of lignin models and real lignin in oxygen atmosphere. The β-O-4 ketone models were converted to ketones, aldehydes and acids via C–O and C–C cleavage (Figure 27). Benzoic acid was the major product during the C–C bond cleavage, which amounted to 41%. The β-O-4 alcohol model was more reluctant to degradation under photo-irradiation, and only about 50% of the substrate was converted after irradiation for 4 h (Figure 28). The yields of C–C bond oxidative cleavage products, such as 3-methoxy-4-hydroxybenzaldehyde and 3-methoxy-4-hydroxy benzoic acid, were 17% and 9%, respectively. About 0.5 mmol of carboxylic acid was formed upon irradiation of milled wood lignin.

58


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ACS Catalysis

Figure 27. Photodegradation of β-O-4 ketone in oxygen atmosphere. 222

Figure

28. Photodegradation

of

phenolic

β-O-4

model

in

oxygen

atmosphere.222

The C–C bond of β-O-4 models can be photocatalytically broken up with the combination of 1,4-hydroquinone and a copper nanoparticle as catalyst.223 With only DDQ, the oxidation of the benzyl alcohol to a ketone occurred and no C–C bond cleavage occurred.224 The combination of 1,4-hydroquinone and a copper nanoparticle showed activity in the C–C bond oxidative cleavage of lignin models under visible light irradiation, but the yields of aromatic aldehydes and phenols were very low (< 50%). A more efficient vanadium complex photocatalyst for lignin C–C bond oxidative cleavage was developed by Soo and coworkers (Figure 29).225 59


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Page 60 of 147

Vanadium complex effectively realized the chemoselective cleavage of C–C bond of β-O-4 models under visible light (> 420 nm) irradiation. Aromatic aldehyde (70% yield), aromatic acid (24% yield) and FA (32% yield) was obtained with 10 mol % of vanadium complex in CH3CN solution for 24 h.

Figure 29. Photocatalytic degredation of β-O-4 model over vanadium complex.225

Li and coworkers226 studied the photocatalytic degradation of pine kraft lignin polymer with C-60-modified Bi2TiO4F2 as photocatalyst under visible light irradiation. A mixture of phenols, aldehydes and acids were detected. The Bi2TiO4F2 photocatalyst was unstable and easily deactivated possibly caused by lignin adsorption on the catalysts surface and the change of the structure under the reaction conditions. The modification of C60 improved the stability of Bi2TiO4F2. OMe

OH

CuOX/CeO2/TiO2-nanotube MeO

OH

MeCN, O2, 455 nm, 24 h

O

OMe

Figure

30.

MeO

OMe Yield:

Photocatalytic

degradation

99%

96%

of

60


O

+

MeO

β-1

model

over

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ACS Catalysis

CuOx/CeO2/TiO2-nanotube.227 Very recently, Wang and coworkers reported a hybrid CuOx/ceria/anatase nanotube catalyst for the cleavage of β-1 models under visible light irradiation.227 Nearly quantitative yields of aromatic aldehyde were obtained (Figure 30). Mechanism investigation indicates Cβ–H abstraction by photogenerated holes is a vital step for the cleavage of C–C bond. CuOx clusters present on the ceria domains increase the concentration of oxygen vacancies and accordingly improved the photocatalytic activity; the CuOx clusters

decorated

on

anatase

suppressed

the

side

reaction

(oxydehydrogenation without C-C bond cleavage) by lower the valence band oxidation potential of anatase. The two roles of CuOx account for the high selectivity of the photocatalzyed C–C bond cleavage.

4. Oxidation of biomass-derived platform molecules

In some cases, the direct conversion of cellulose and hemicellulose to targeted chemicals is difficult due to the high functionality and large molecular weight. First transformation of cellulose and hemicellulose via acid catalyzed process to small platform molecules, such as furfural, 5-hydroxymethylfurfural (HMF), lactic acid and levulinic acid (LA), and then conversion of the platform molecules to targeted compounds is an alternative route for the biomass conversion. Moreover, some products can be prepared from platform molecules but can not be derived from cellulose. These indirect methods using 61


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various platform molecules as starting materials provide more possibilities for the biomass utilization.

4.1. Oxidation of Furfural to C4 acid chemicals The production of furfural from agricultural waste has been industrialized.228 It is thus believed furfural is a good feedstock to produce maleic anhydride (MA) from biomass. Oxidative cleavage of furfural can provide MA, maleic acid (MAc), furanone (FU) and succinic acid (SA) (Figure 31).

Figure 31. Oxidative cleavage of furfural. 4.1.1. Oxidation of furfural to maleic anhydride

In 1928, Sessions reported the synthesis of MA from furfural via vapor-phase oxidation over V2O5 catalyst at 473–573 K.229 In 1949, Nielsen reported the oxidation of furfural to MA over iron molybdates in gas phase and the yield of

62


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MA was above 75%.230 Later on, the mechanism and kinetics of furfural oxidation to MA were studied with Sn(VO3)4 and V-Mo-P as solid catalysts.231-234 Ojeda and coworkers reported a route to convert furfural to MA by gas-phase oxidation and 73% yield was achieved at 593 K with 5.7 kPa of O2 using VOx/Al2O3 as a solid catalyst (Figure 32).235-236 The Al2O3-supported polyvanadates were intrinsically more active than monovanadates (VO4) and V2O5 crystals. Furfural was first converted to furan via C–C bond oxidative cleavage, releasing CO2.237-240 Further oxidation of furan gave 2-furanone as the reaction intermediate, which was rapidly transformed into MA via C–H oxidation.241-242

Figure 32. The proposed mechanism for the oxidation of FA to MA. 235 Since gas phase oxidation operates at high temperatures and has high energy consumption, liquid-phase oxidation under lower temperature is more attractive. Recent studies were focused on the liquid-phase oxidation of furfural. In 2014, Yin and coworkers demonstrated a liquid phase oxidation of furfural to MA using HPA-5 and Cu(CF3SO3)2 catalysts.243 In MeCN solvent, MA was formed as the major product. About 30% yield of MA was obtained over HPA-5, and the addition of Cu(CF3SO3)2 increased the yield to 54%. This mechanism is different from vapor phase oxidation through furan intermediates 63


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Page 64 of 147

since furan can hardly be converted to MA. The author proposed a mechanism (Figure 33). First, hydrogen abstraction generates the furfural radical (1), and further electron transfer leads to the formation of furfural cation (2). Then, H2O attacks the furfural cation to form 3 which is converted to 4 via 1,4 rearrangement. Compound 4 is converted to 5 via oxidative decarboxylation. H2O attacks 5 to form 6 which is transformed to MA by oxidative dehydrogenation of hydroxyl group. However, only intermediate 6 was detected.

Another

possible

route

is

via

furan

intermediate.

Hydrogen-abstraction of the aldehyde functional group generates the intermediate 7 which further undergoes decarbonylation and electron transfer to generate 8. Hydration of 13 forms the 9 which further undergoes 1,4-rearrangement to produce 10. After hydrogen abstraction from 10 and electron transfer, intermediate 5 is generated and further converted to MA.

64


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Figure 33. The proposed mechanism for the synthesis of MA from furfural oxidation with HPA-5 catalyst.243

Zhang and coworkers have investigated the oxidation of furfural to MA using Mo–V-O as catalysts.244 The phase composition of the catalyst and the solvent have a large effect on the reaction. The Mo–V binary metal oxides showed better performance than the simple V2O5 and MoO3. Since MA can be further hydrolyzed to MAc, a total yield of 65% of the MA and MAc mixture was achieved over the Mo4VO14 catalyst in acetic acid. The yield of MA was 47%. The author suggested a free radical mechanism as demonstrated by following reason. MA could still be produced when H2O2 (free radical initiator) was 65


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added without catalyst, while, upon the addition of 4-tert-butylphenol (free radical inhibitor), the reaction was significantly prohibited in the presence of the Mo4VO14 catalyst. This is different from the mechanism proposed by Yin.243 It is still in debate whether the furfural oxidation to MA is a free radical reaction. The Mo4VO14 catalyst was unstable with leaching of Mo and V, also accompanied by the change of morphology. After the fourth recycle, the yield of MA decreased significantly.

4.1.2. Oxidation of furfural to maleic acid

Currently, MAc is produced by the hydration of MA, which has been widely used to produce fumaric acid, succinic acid and malic acid. Significantly, MAc is widely used in the area of plasticizers, agricultural chemicals, lubricant additives, resins, surface coatings and polymers.19 The preparation of MAc from furfural is an attractive route. Many works have been reported for oxidizing furfural with various oxidants, but the major product is furoic acid.

66


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Figure 34. Proposed mechanism for furfural oxidation to maleic acid.245 In 2011, Yin and coworkers introduced a new route to synthesize MAc from the renewable furfural in an aqueous phase using dioxygen as oxidant.245 Furfural is unstable under oxygen atmosphere and easy polymerize to resin. MAc in 49% yield was achieved over Cu(NO3)2/H3PMo12O40 catalytic system. A proposed mechanism was shown in Figure 34. First, hydrogen abstraction generates the furfural radical (1) which undergoes electron transfer to phosphomolybdic acid to generate the furfural cation (2). Then, H2O attacks furfural cation to form compound 3 which is converted to compound 4 via 1,4 rearrangement. Hydrolysis of compound 4 generates compound 5 which is converted to MAc via oxidative decarboxylation. Furfural radical (compound 1) is very active and can initialize polymerization to resins. The major challenge for this route is to avoid the furfural polymerization under oxidative conditions.

Yin and coworkers further developed an aqueous/organic biphase system to increase the selectivity of MAc.246 The furfural and was mainly distributed in the organic phase. Because the phosphomolybdic acid catalyst is soluble in water, the oxidation reaction happened in the aqueous phase. As the oxidation reaction proceeded , the furfural in organic phase was gradually transferred from to aqueous phase, making the furfural concentration in the aqueous phase always at a low level. Furfural polymerization was reduced at low concentration and thus improved the selectivity of MAc. Tetrachloroethane was the best solvent among the tested solvents, including nitrobenzene, 67


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tetrachloromethane, toluene, nitromethane, p-xylene, cyclohexane and tetradecane. The reaction could achieve a 50% conversion of furfural and a 69% selectivity of MAc. With H2O2 as oxidant, proton acids, such as HCl, H2SO4, ACA and FA, was active for the oxidation of furfural to MAc. A 95% yield of MAc was achieved in formic acid solvent.247 Vigier and cowokers reported a betaine hydrochloride catalyst and a 92% total yield of MAc (61%) and fumaric acid (31%) was achieved.248 Betaine hydrochloride is a kind of quaternary ammonium salt, and can be recycled via evaporation of the water solvent. Betaine hydrochloride was reused for at least 4 cycles with comparable yields. Compared to homogeneous catalyst, heterogeneous catalyst is easier recyclable, and some studies have been done to develop heterogeneous catalyst for the oxidation of furfural to MAc. Granados and coworkers reported a titanium silicalite (TS-1) catalyst for the furfural oxidation with H2O2 as an oxidant.249-250 Under the optimization reactions, 78% yield of MAc was obtained. To increase the utilization efficiency of H2O2, a two-step sequential reaction was conducted with TS-1 and Amberlyst 70 catalysts which were consecutively added in the first and second steps, respectively. A H2O2/furfural molar ratio of 4.4 was required. Using the two-step method, the yield of MAc reached 92% under 4.6 wt% of furfural concentration over 52 h at 323 K. However, TS-1 was not stable under this reaction conditions and Ti leaching was observed. Although leaching occurred, TS-1 was reused for six times 68


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without obvious deactivation. The possible reaction mechanism was shown in Figure 35. 249-252 In the first step, furfural-epoxide was formed by epoxidation of the less sterically hindered double bond, and subsequent converted to the (Z)-4-oxopent-2-enedial through a rapid rearrangement The Baeyer-Villiger oxidation of the aldehyde functional group further occurred at 1-position of the Z-4-oxopent-2-enedial, resulting in the cleavage of C–C bond connecting the aldehyde and ketone groups. The as-formed ester was rapidly hydrolyzed to β-formylacrylic acid, which was in equilibrium with the hydroxyfuranone. Finally, hydroxyfuranone was directly oxidized to MA.

OH HO

O O

O

O

epoxidation

H2O2

HO

O

O

O

O 5-hydroxy-furan-2(5)-one

O

H2O HCOOH O O

O

H2O2

O O

O O

Baeyer-Villiger (Z)-oxopent-2-enedial

O O OH

hydrolysis β -formylacrylic acid

Figure 35. The possible reaction mechanism for the oxidation of furfural to MAc using TS-1 and H2O2.249-250 Compared to H2O2, molecular oxygen is relatively inert. The oxidation of furfural to MAc with molecular oxygen is usually inactive. Metalloporphyrin, the 69


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core of the enzymes, is known to be effective for the molecular oxygen. Maleic acid could be achieved in 44% yield using Fe porphyrin as catalyst under optimal conditions.253 The CaCu-phosphate catalyst was reported as an active heterogeneous catalyst with molecular oxygen as oxidant and 37% yield was achieved.254

4.1.3. Oxidation of furfural to 2(5H)-furanone

2(5H)-Furanone, as an important chemical, exists in many natural products and shows diverse biological activities.255-258 2(5H)-Furanone can also be used to synthesize surfactants235 and as intermediates for the synthesis of lactones and diols, which are wieldy used in the fields of medicines, chemicals and polymers.259-261 Furanone can be produced via 2-methoxyfuran hydrolysis, conversion of hydroxybutyrolactones, deoxygenation of butanoic acids and cyclocarbonylation of terminal alkynols.257-258,262-264 Currently, 2(5H)-furanone is dominantly synthesized via the oxidation of furfural. H2O2 is the commonly used oxidant. Baeyer–Villiger oxidation first occurred to form formic ester (Figure 36). Further hydrolysis of the formic ester would afford 2(3H)-furanone and 2(5H)-furanone. 2(3H)-furanone could be isomerized to 2(5H)-furanone. The selective formation of 2(5H)-furanone is a great challenge, because many side reactions may occur, such as deep oxidation, polymerization, isomerization and hydrolysis.265-272 Furanone can be formed by oxidizing furfural with H2O2 without a catalyst. 70


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In 1996, Liu reported the oxidation of furfural with H2O2 under reflux.273 The oxidation product of 2(3H)-furanone and 2(5H)-furanone were separated by distillation. The addition of Et3N increased the yield 2(5H)-furanone to 67% via isomerization of 2(3H)-furanone. The addition of sodium molybdate and potassium bichromate into the reaction solution did not increase the yield of 2(5H)-furanone.274 Hoffmann reported the production of 2(5H)-furanone via oxidation of furfural with H2O2.275 N, N-dimethylethanolamine was used for 2[3H]-furanone isomerization to 2(5H)-furanone. Overall, 2(5H)-furanone was obtained in 45% yield. The pH value showed great effect on the formation of 2(5H)-furanone.276 At pH = 1, 50% yield of 2(5H)-furanone was obtained. No 2(5H)-furanone was formed at pH > 5.

Figure 36. Oxidation of furfural with H2O2 under reflux.

Recent works have studied the catalytic oxidation of furfural to furanones. In the oxidation of furfural to MA using HPA-5 and Cu(CF3SO3)2 catalysts with molecular oxygen as oxidant, 5-acetoxyl-2(5H)-furanone was detected in minor

amount

(8%

yield).243

Grunskaya

reported

a

32%

yield

of

2(5H)-furanone from the furfural oxidation in the presence of Na2MoO4 and 30 71


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wt% H2O2 at 60 °C.277 Poskonin reported a niobium(V) acetate tetrahydrate catalyst for this reaction. The reaction was very slow. More than 80 h was required to achive a 60% yield.278 Formic acid can be used as catalyst when H2O2 was used as the oxidant. Performic acid was formed via reaction of H2O2 and formic acid, which was very active for the Baeyer–Villiger oxidation reaction. A 43% yield was obtained in 9 h.260 The yield can be further increased using a bi-phase system. Furfural and 2(5H)-furanone prefers to stay in the organic phase, which inhibits their deep oxidation with H2O2 and hydrolysis in the aqueous phase. In the homogeneous system, the deep oxidation to acid and polymerization were the major reactions, and low yield of 2(5H)-furanone was obtained. With bi-phase system, 60–62% yield was achieved.259,279 The yield can be further increased 90% in combination of Pt/TiO2–ZrO2 catalyst and formic acid.

4.1.4. Oxidation of furfural to succinic acid

Succinic acid (SA) is used as feedstock in the area of pharmaceutical, food, polymer,280 fine chemicals and agrochemical industries.281-283 Hydrogenation and

dehydration

of

succinic

acid

can

produce

the

1,4-butanediol,

γ-butyrolactone and tetrahydrofuran, which were applied as solvents and intermediates to synthesize pharmaceuticals, agrochemicals and polymers.284 SA was among the top 12 value-added chemicals derived from biomass. SA is industrially obtained via the hydrogenation of MA or MAc or fermentation 72


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of sugars.285 SA can also be produced by electrolytic reduction of MAc.286 It is a promising route to direct synthesis of SA from furfural. Sodium molybdate catalyst gave a 25% yield of SA with H2O2 oxidant.274 Tachibana and coworkers reported a two-step method for SA produciton.280,287 In the first step, the oxidation of furfural to fumaric acid take place in water in the presence of sodium chlorate and vanadium pentoxide catalyst, and then Pd/C was added to reduce fumaric acid, affording 47% overall yield of SA. Bronsted acid is effective for this reaction.288 Ebitani and coworkers reported the catalytic oxidation of furfural to produce SA in the presence of acid catalysts and H2O2 oxidant.289 Among the tested acids, including Brönsted acid and Lewis acid, Amberlyst-15 and TSA showed the best performance with 74% and 72% yields for SA, respectively, in aqueous solution. Amberlyst-15 could be reused three runs with 68-74% yield. The reaction mechanism is shown in Figure 37. First, furfural was subjected to Baeyer-Villiger reaction under acidic conditions, and then further converted to 2(3H)-furanone. 2(3H)-furanone was converted to SA via hydrolysis and oxidation.

Figure 37. The reaction mechanism of oxidation of furfural to SA by acid 73


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catalyst.289

4.2. Oxidation of 5-Hydroxymethylfurfural

Figure 38. Oxidative cleavage of HMF to MA, MAc and SA.

5-Hydroxymethylfurfural (HMF) can be obtained via the hydrolysis and dehydration of cellulosic biomass resources. Many value-added products, such as dimethylfuran, LA, 1,6-hexanediol, 2,2-diformylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA), can be derived from HMF. The synthesis of DFF and FDCA via the oxidation of HMF have well summarized by some reviews22,290. MA, MAc and SA can be obtained through oxidative cleavage of HMF. MA, MAc and SA can be transformed into each other under certain reaction conditions (Figure 38).

4.2.1. Oxidation of 5-Hydroxymethylfurfural to maleic anhydride

Oxidative cleavage of HMF provides a new route to synthesize MA. The 74


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structure of MA and HMF both contain furyl rings, and it is possible to produce MA via C–C bond oxidative cleavage of HMF. Vanadium is the mostly used element for this reaction. Xu and coworkers investigated the oxidation of HMF over VO(acac)2 catalyst in acetonitrile (containing 1% acetic acid).291 The oxygen pressure played a key role in the reaction pathways. When the reaction was performed under 0.1 MPa of oxygen, the major product was DFF. Increasing oxygen pressure led to the formation of MA. When the pressure of oxygen was increased from 0.1 MPa to 1.0 MPa, the conversion of HMF and the yield of MA were greatly increased. About 52% total yield of MA and MAc was achieved at 362 K in 4 h. With vanadium-substituted heteropolyacid catalyst, the total yield of MA and MAc reached 64%.292 The reaction was not a free radical reaction as the addition of 2,6-di-tert-butyl-p-cresol (a typical free radical inhibitor) did not affect the reaction. DFF or FDCA could not be converted to MA under the same condition, indicating they were not the reaction intermediates. The author proposed a reaction mechanism (Figure 39). The cleavage of C–C bond and the oxidation of hydroxymethyl group occurs simultaneously. When the reaction was conducted under 0.1 MPa of O2, the oxidation of hydroxymethyl group to afford DFF is the major route (route b). In higher oxygen pressure (1 MPa of O2), the C–C bond connecting the hydroxymethyl group and furan ring preferred to be broken up and a ketonic group form via further resonance. MA was formed via further cleavage of the 75


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aldehyde group (route a). DFF was also present in the reaction solution but could not be converted to MA.

Figure 39. Oxidation of HMF over vanadium-based catalyst in different oxygen pressure.291

Immobilization of the vanadium-oxo on graphene oxide significantly improved the efficiency, achieving 95% yield of MA.293 The enrichment of HMF around the vanadium-oxo sites via adsorption on the oxygen-containing groups may be responsible for the high efficiency. Zhang and coworkers reported V2O5 and V2O5/SiO2 heterogeneous catalyst for the oxidation of HMF to MA.294 A maximum 79% total yield of MA and MAc was obtained at 373 K under 5 bar O2 for 4 h in acetonitrile solvent. Both the V2O5 and V2O5/SiO2 catalyst showed good recyclability. Interestingly, the author further investigated the direct production of MA from fructose in one-pot two-step procedures (Figure 40). In the first step, fructose was converted to HMF with HCl catalyst in 2-propanol. Then the evaporation of 2-propanol solvent give67% yield of crude HMF product. In the second step, the vanadium catalysts as well as 76


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acetic acid solvent were added to the reactor for the oxidation of HMF to MA. A 50% overall yield of MA was achieved using V2O5 catalyst.

Figure 40. Production of MA from fructose.294

4.2.2. Oxidation of 5-Hydroxymethylfurfural to succinic acid

K. Ebitani and coworkers reported the oxidation of HMF with Amberlyst-15 catalyst and H2O2 oxidant (Figure 41).295 A 19% yield of SA was obtained at 353 K for 24 h. 2-Oxoglutaric acid and SA were the major products. HMF was first transformed to 2-oxoglutaric acid, and further oxidative decarboxylation of 2-Oxoglutaric acid led to SA. Recently, direct oxidative cleavage of glucose to SA was achieved over Ru Catalyst and N-Doped graphene with 87.5% and 60% yields, respectively.296-297 In this case, HMF was not the intermediate. The reaction was initiated by the direct oxidative cleavage of the C–C bond in vicinal diols.

Figure 41. Oxidation of HMF to SA in the presence of Amberlyst-15.295 77


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4.3. Oxidation of levulinic acid

LA can be produced from cellulose and hemi-cellulose by acidic processing of biomass with high yields,298-300 and can also be derived from the hydrolysis of the biomass-derived HMF or 5-(chloromethyl)furfural.301 Upon scaled up, the price of LA may be decreased to less than $1 kg-1.298 Therefore, the conversion of LA into value-added chemicals is promising in terms of sustainability and economics.302-303

4.3.1. Oxidation of levulinic acid to succinic acid and succinates

Succinates are important plasticizers, lubricants and chemical intermediates. As early as in 1879, Tollens first reported the oxidaiton of LA to SA via C–C bond oxidative cleavage using nitric acid as the oxidant. However, the product is a mixture of organic acids and the yield of SA was very low.304 In 1934, Ponsford used hydrogen peroxide as oxidant and a cupric salt as catalyst, and only trace amount of SA was found.305

78


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Figure 42. Oxidation of methyl levulinate catalyzed by Mn(OAc)3.306

Recent processes have been emphasized on the green chemical oxidation of LA to SA. In 2013, Xu and coworkers306 reported a Mn(OAc)3 catalyzed oxidation of methyl levulinate to dimethyl succinate in 52% at 363 K under 5 bar of O2 in a 20 h reaction. With levulinic acid as starting material, a mixture of C4

product,

including

succinic

anhydride,

MA

and

2-methyl-5-oxotetrahydro-2-furanyl acetate, were produced.307 The author demonstrated the reaction pathway (Figure 42). Oxidative cleavage of C–C bond at the α1-position gave CO2 and a C4 compound. Breaking up C–C bond at the α2 position afforded acetic acid and malonate which was easily decomposed into acetic acid and CO2 under the reaction conditions. Moreover, further oxidation of the methylene group generated oxomalonate which was further converted to C2 products such as oxalate. Mn(OAc)3 favored the cleavage of C–C at the α1-position. Besides, other aliphatic methyl ketones 79


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Page 80 of 147

could be also oxidatively cleaved the same way. Podoleanet and coworkers308 used Ru(III) immobilized magnetic catalyst in the conversion of LA to SA under O2 atmosphere in water. The catalyst could be magnetically separated and reused for three times. Caretto

and

Perosa309

reported

the

heating

LA

in

a

dimethylcarbonate–base mixture at 473 K generated a mixture of methyl levulinate, dimethyl succinate and methylated product. Up to 20% yield of dimethyl succinate was achieved. In 2015, Roman-Leshkov and coworkers310 reported the production of succinate from oxidation of methyl levulinate with peroxides oxidant and Brönsted and Lewis acid catalysts (Figure 43). The C–C cleavage was via the Baeyer–Villiger (BV) mechanism. Strong solvent effect on the product distribution was observed. Hydrogen-bonding solvent, such as methanol, favored the methyl group migration with the formation of succinate (route b). Non-hydrogen-bonding solvent, such as heptane, favored the branched alkyl group migration with the formation of acetate (route a). A 56% conversion of methyl levulinate and 60% selectivity to succinate were obtained over PSA catalyst at 353 K for 6 h with H2O2 as oxidant in methanol solvent. When the solvent was changed to heptane, the succinate/acetate ratio was decreased from 1.6 to 0.3. Note that, theoretically, B-V reaction route is 100% atom economy. No CO2 was formed in this process. Mascal and coworkers311 further found trifluoroacetic acid (TFA) was a more efficient acid catalyst for the 80


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oxidation of LA to SA. Moreover, TFA can be fully recycled. SA could be isolated through distillation of the TFA solvent and byproducts, and 60% isolated yield was obtained under 363 K for 6 h.

Figure 43. The oxidative cleavage of methyl levulinate into dimethyl succinate.310 4.3.2. Oxidation of levulinic acid to maleic anhydride In 2015, Chatzidimitriou and Bond312 presented a catalytic pathway for the synthesis of MA from LA via oxidative cleavage of the methyl carbon over supported vanadates (Figure 44). The reaction was operated in a continuous flow, packed bed reactor. The single-pass MA yield as high as 71% was 81


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obtained at 573 K. SA was firstly formed as intermediate which subsequently underwent dehydration and oxidative dehydrogenation consequently to form MA.

Figure 44. Oxidation of LA to MA over supported vanadates.312

4.3.3. Oxidation of levulinic acid to 3-hydroxypropanoic acid

3-Hydroxypropanoic acid (HPA) is recognized as a new building blocks for the synthesis of a variety of high value-added compounds. It is widely used to synthesize acrylic acid and its derivatives. HPA can also be oxidized to malonic acid, hydrogenated to 1,3-propaneiol, or used as feedstock to produce polyesters and oligomers.313 The global production amount of acrylic acid in 2011 was 5000 kMT with a market size of 11.5 billion dollars.314 Traditionally, HPA was produced via oxidation of allylic alcohol, hydration of acrylic acid and fermentation of glucose or glycerol.315 In the oxidation of LA to SA, HPA is a byproduct formed via methyl 82


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migration.189,311 In 2015, Mark Mascal316 reported the production of HPA by the oxidative cleavage of LA with H2O2 as oxidant. Previous study showed that C–C bond in α1-position was preferentially broken up via B-V reaction in the presence of acid (Figure 43), while in base conditions, C–C bond in α2-position was more inclined to be cleaved, leading to HPA and acetic acid. About 47% yield was obtained by dropwise addition of H2O2 and KOH into the LA aqueous solution at 388 K for 90 min. Performing the reaction at a temperate between 273 K and room temperature for six hours led to the formation of 3-(hydroperoxy)propanoic acid (HPPA) in 82% yield, which was quantitatively hydrogenated to HPA over Pd/C catalyst. LA can also be converted to ketones via oxidative decarboxylation, such as methyl vinyl ketone and butanones.317-319 These ketones are high-value chemicals. Lin and coworkers318-319 reported the vapor phase oxidative decarbonylation of LA to ketones in KH2PO4 solution (pH = 3.2). When CuO was used as catalyst, 60% yield of methyl ethyl ketone (MEK) was obtained at 300 °C for 2 h. While, CuO/CeO2 could oxidize LA at a lower temperature, but the major product was changed to methyl vinyl ketone with the yield of about 20% at 448 K. Other minor products were acetaldehyde, acetone, methyl vinyl ketone and 2,3-butanedione.

4.4. Oxidation of malic acid to dimethyl malonate

Malonic acid and its ester are important feedstocks used to synthesize drugs, 83


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flavors and vitamins. For example, malonic acid is used for the synthesis of barbiturate which is a kind of ataractics. Industrially, it is synthesized via hydrogen cyanide process using highly toxic sodium cyanide and chloro-acetic acid. It is very promising to produce malonic acid and esters from biomass in an environmentally benign process.

Figure 45. Oxidation of malic acid to dimethyl malonate.320 Malic acid is produced via biomass carbohydrates fermentation at about 100 kt per year. In 2012, Xu and coworkers320 first reported the production of dimethyl malonate from oxidation of malic acid (Figure 45). Heteropolyacids were used as the catalyst and HPA-2 was the best catalyst. A 68% yield of dimethyl malonate was obtained at 100 °C under 1.0 MPa of O2 for 10 h in methanol solvent. HPA-2 has two roles that are catalyzing the oxidative decarboxylation and esterification. The authors suggested a mechanism by which malic acid was first esterified to dimethyl malate catalyzed by protons, and then the C–C bond oxidative cleavage led to hemiacetal intermediate. The 84


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dehydration of hemiacetal intermediate gave methyl 3,3-dimethoxypropionate as a by-product. And dimethyl malonate was formed via oxidation of the hydroxyl group in hemiacetal intermediate.

5. Oxidation of fatty acids As one kind of important and abundant raw materials, oils and fats are widely used for food and daily chemical industry.321-326 It has been showing steady increase on the global production and consumption of oils and fats in the last decades,327 especially for the production of biofuels and chemicals, meeting the need of world energy changes and the exhaust of fossil fuels. Recently, there has been growing concern about the oxidation of unsaturated fatty acids, to mono- and dibasic acids which are widely used for the production of fibers, pharmaceuticals,

cosmetics,

plastics

and

adhesives.328-330

Several

reviews331-335 have been conducted on the utilization of oil and fats, including the chemical and biotechnological utilization of oleochemicals, and some special issues on the oxidation of oil derivatives have been reported.336-339 This part in our review is focused on the production of organic acid products from unsaturated fatty acids, regarding the development of various catalyst systems with oxidants. Fatty acids, as one of the major oleochemical base materials, can be generated from the hydrolysis of triglycerides widely present in natural oil and fats. Several typical unsaturated fatty acids (UFAs) derived from vegetable oils are summarized in Table 8. Generally, UFAs obtained from vegetable oils have 16-18 carbon atoms and 1-3 double bonds, while oil and fats from animal lipids can also give unsaturated fatty acids in longer carbon chain. 85


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Table 8. Common unsaturated fatty acids derived from typical oil and fats.340 Common name

Systematic name

Structure

Palmitoleic

cis-9-Hexadecenoic

cis-Vaccenic

cis-11-Octadecenoic

Oleic

cis-9-Octadecenoic

Petroselinic

cis-6-Octadecenoic

Linoleic

9,12-Octadecadienoic

α-Linolenic

9,12,15-Octadecatrienoic

γ-Linolenic

6,9,12-Octadecatrienoic

C OOH

Figure 46. The routes of oxidative cleavage of oleic acid.

Oleic acid (OA) is one of the most abundant unsaturated fatty acid in nature, having numerous applications such as food diets, and ingredients for soaps and detergents in chemical industry. The oxidation network of oleic acid contains three main transformations, i.e. epoxidation, dihydroxylation and the direct C=C bond cleavage (Figure 46). These three steps are interrelated and each step has been studied. The corresponding mono carboxylic acid (pelargonic acid) or diacid (azelaic acid) product, could be generated from the 86


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direct cleavage process, as well as the tandem multi-step procedures with the epoxide or 1,2-diols as intermediates.339,341-342 Dibasic acids from the crude plant oils can be used to prepare industrially important polymers like polyesters and polyamides.324 For example, azelaic acid or brassylic acid with 9 or 13 carbon atoms respectively, are monomers for the synthesis of advanced Nylon-69 and Nylon-1313.343 These polymers can be synthesized from biomass feedstock other than petroleum route,344-345 which can reduce the dependence on fossil resources. Our interest in this section covers the advances in the oxidation of unsaturated fatty acids to carboxylic acids, especially for the transformation of oleic acid to perlargonic and azelaic acids in detail. Processes to obtain epoxy intermediates, diols, and aldehydes from cleavage, are not specifically described here. The key to oxidative conversion of unsaturated fatty acids or derivatives is the oxidative cleavage of C=C double bonds. This section are categorized into several groups according to the oxidants used, from traditional strong oxidants to greener H2O2 or O2. The results were summarized in Table 9.

5.1. Ozonolysis Ozone, an allotrope of oxygen, can be inserted rapidly into the C=C double bonds, generating aldehydes or carboxylic acids from alkenes without using any metal catalyst. The traditional technology industrially used for oxidative cleavage of fatty acid is ozonolysis, and it has been used for the production of azelaic acid from oleic acid, as the sole commercialized way.346 Besides, stoichiometric pelargonic acid (PA) is formed. This method in Goebel’s 87


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patent347 at an early stage could achieve 78% yield of azelaic acid. During ozonolysis, in the first step, OA reacted with ozone via 1,3 cyclo-addition generates 1,2,3-trioxolane which is then converted to a 1,2,4-trioxolane. In the second step, the 1,2,4-trioxolane is transformed to carboxylic acids (Figure 47).346 Tremendous efforts have been done to modify the ozonolysis process to improve the azelaic acid yield.348-357

Figure 47. Ozonolysis of oleic acid.346

Other dibasic acids or acid mixtures could also been produced through ozonolysis from some natural raw materials. For example, brassylic acid (72-82% yield) of high purity, was reported by means of a two-stage cleavage of erucic acid with ozone and oxygen, with pelargonic acid as a co-product, and this method was also translated to a small pilot-scale operation.358 Ozonolysis is a relatively well-developed technical process with apparent advantages like good selectivity and simple reprocessing. However, hazardous and environmental problems associated with ozone utilization hampered it to be widely used. Alternatives which are more economical and 88


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ACS Catalysis

environment-friendly to replace ozonolysis are needed for the production of dibasic acids.

5.2. Oxidation with strong oxidants Several other strong oxidants, such as K2Cr2O7, HNO3, KMnO4, oxone, and NalO4, were used as stoichiometric oxidants for the oxidation of double bond.359 As using these strong oxidants are not economic and cause environmental problems, it has been abandoned for a long time and is briefly summarized here.

5.2.1. Oxidation with KMnO4 and (or) periodate Permanganate oxidation of olefins could obtain products like diol, ketone and dialdehyde,360-361

and

even

the

carboxylic

acid

products

in

acidic

solutions.362-363 Aldehyde products could be achieved in high selectivity through the emulsion method, like oil in water emulsion364 or using biphasic solvents365, to quench the manganese(III) or manganese(IV) intermediates. The emulsion technology was also used in permanganate oxidation of oleic acid to pelargonic and azelaic acids at neutral pH, along with other kinds of byproducts.366 Periodate (NaIO4) was widely used for oxidative cleavage of vicinal diols. Based on this point, a multistep process involving epoxidation and hydrolysis of C=C bond to diols, followed by diols cleavage in the presence of NaIO4 is proposed for the unsaturated fatty acid cleavage. R. U. Lemieux and E. Vonrudloff found that C=C double bonds were easily oxidized by periodate in the presence of catalytic amount of permanganate.367 89


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The main oxidation courses of C=C bond involved first oxidation to hydroxyketones by permanganate, then the rapid cleavage by periodate to aldehyes, which were subsequently oxidized by the permanganate to carboxylic acid.

5.2.2. Oxidation with oxone and periodate Another effective oxidizing agent is oxone, which is widely used in peroxidation, hydrolysis and cleavage of C=C double bond. Oxone is potassium peroxymonosulfate, i.e., the potassium salt of peroxymonosulfuric acid (2KHSO5·KHSO4·K2SO4).

Figure 48. Cascade oxidative cleavage of double bond to acid products with oxone and periodate.368 The combination of oxone and periodate is effective for the cleavage of C=C bond to carboxylic acids via sequential reactions with epoxide, diol and aldehyde as the intermediates. With oxone and periodate as oxidants, even without catalyst, the oxidation of methyl oleate, oleic acid, elaidic acid and erucic acid methyl ester all nearly quantitatively gave corresponding acid products via C=C bond cleavage.368 The oxidation is achieved via a cascade reaction including C=C bond epoxidation by oxone, ring-opening of the epoxide to a diol, the diol cleavage to aldehydes by periodate, and aldehydes further oxidation to acids by oxone in one pot (Figure 48).

90


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ACS Catalysis

5.2.3. Catalytic oxidation with Ru(Os)O4 and periodate (oxone) Catalysts such as Os or Ru are mostly used with oxone and periodate oxidant for the cleavage of C=C bond.369-377 Os and Ru tetroxide themselves can be used as powerful oxidants for the C=C bond cleavage.378-384 NaIO4 or oxone could not only be used for oxidizing the diol intermediates to aldehydes, but also for the oxidation of reduced Os and Ru to tetroxides (Figure 49).371,385-387

Figure 49. Catalytic oxidative cleavage of C=C bonds by RuO4/NaIO4 or oxone.387 Ruthenium trichloride was used as a catalyst in the developed oxidation protocols to cleave olefins to carbonyl compounds, including aldehydes and carboxylic acids. RuCl3-Oxone-NaHCO3 and RuCl3-NaIO4 systems were effective to cleave different kinds of olefins into aldehydes.385 In these systems, RuO4 could be formed in situ through the reaction between RuCl3 (RuO2) and NaIO4.388 The Sharpless system389 uses periodate as the oxidant and RuCl3 as catalyst in solvent mixtures of CCl4:MeCN:H2O (2:2:3, v/v), and this 91


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Page 92 of 147

Ru-catalyzed system has been improved by changing the solvent mixtures.374 The solvent showed a great effect on the oxidation reactions. In H2O/MeCN/AcOEt cosolvnt, azelaic and pelargonic acid in 73% and 65% yield, respectively, were obtained from OA oxidation.390 The Oget group

390-392

reported some improvements on this catalytic system. The use of the surfactant

Aliquat®

336

(methyl-trioctylammonium

chloride)

and

ultrasonification greatly increased the reaction rate. Azelaic and pelargonic acid in 81% and 96% yield, respectively, were obtained under optimized conditions.391 9-Decenoic acid and 10-undecenoic acid were converted to azelaic acid and sebacic acid with 96% and 85% yield, respectively. With OsO4 as a catalyst and the oxone as oxidant, methyl oleate was converted to nonanoic acid and nonanedioic acid monomethyl ester in 80% isolated yield in the presence of OsO4/oxone.372,376 As ruthenium shows effective activity in catalyzing cis-dihydroxylation and oxidative cleavage of alkenes, and much interests have been put in the alternative metal catalysts for alkene dihydroxylations. Che et al.393 reported a hydroxyapatite (nano-RuHAP) supported ruthenium NPs for the oxidative cleavage of C=C bond cleavage with NaIO4 as the oxidant. Solvent showed a great effect on the reaction route in “nano-RuHAP + NaIO4” protocol. Dihydroxylation of alkenes took place in EtOAc/MeCN/H2O solvent along with 20

mol%

of

H2SO4,

while

alkenes

cleavage

occurred

in

1,2-dichloroethane/H2O solvent. However, the catalytic system showed lower activity for oxidation of UFAs, only 16% of methyl oleate was oxidized to aldehyde.

92


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5.2.4. Catalytic oxidation with H2O2 and periodate To replace oxone, Gebbink and coworkers394 introduced a Fe-based coordinative metal complexes for the oleic acid cleavage with H2O2/NaIO4 as the oxidant. The iron complexes have shown activity for the olefin epoxidation and cleavage.395 Iron complex [Fe(OTf)2(mix-BPBP)] was used as the catalyst for oleic acid oxidation (Figure 50). H2O2 was used to epoxidize the C=C bond catalyzed by iron complex.396 After epoxidation, acid and water were added for the hydrolysis of epoxide to diols. The diols were cleaved in the presence of NaIO4 (Figure 51). The methyl oleate and oleic acid were converted into Nnonanal in yields of 96% and 90%, respectively.

Figure 50. Iron complexes isomers with OTf and BPBP ligands.394

Figure 51. The reaction route of [Fe(OTf)2(mix-BPBP)] catalyzed oxidative cleavage of C=C bond with H2O2 and NaIO4.394 In order to obtain acid products, [Fe(OTf)2(6-Me-PyTACN)] was used as catalyst (Figure 52).397 The reaction includes the direct cis-dihydroxylation of 93


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C=C bond to diols, cleavage of the diols to aldehydes and subsequent deep-oxidation of the aldehdyes to the carboxylic acids (Figure 53). Methyl oleate was converted into 50-55% yields of both nonanoic and azelaic acid in a one-pot procedure, as well as epoxide and aldehyde intermediates. The addition of sulfuric acid promoted the hydrolysis of epoxide to diols, and thus could further improve the yields of nonanoic and azelaic acid to 80-85%. However, the oxidizing ability of iron is low and long reaction time is needed.

Figure 52. [Fe(OTf)2(6-Me-PyTACN)] catalyzed oxidative cleavage of oleic acid or its derivatives to acid products.397

Figure 53. Mechanism of [Fe(OTf)2(6-Me-PyTACN)] catalyzed oxidative cleavage of oleic acid or derivatives with H2O2 and NaIO4.397 Because of the low selectivity, various side reactions, and waste pollution, these non eco-friendly methods using strong stoichiometric oxidants have not been adopted for a long time. The use of safe and environmentally benign oxidants, like H2O2 and O2, is more desirable. 5.3. Catalytic oxidation with H2O2 Recently, alternative methods have been reported for the cleavage of UFAs 94


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using transition metal catalysts combined with environmentally benign oxidants. Herein, related studies are presented in different transition metals catalysts with H2O2 as the oxidant, especially the mostly used Ru and W catalysts. Compared to the oxone, O3 or periodate, H2O2 is a greener oxidant, and is also widely used for C=C bond epoxidation. Using H2O2 as oxidant is mostly studied for the unsaturated fatty acid cleavage, either via sequential oxidation reactions involving epoxide and diol intermediates, or just as terminal oxidant for regenerating the substrate -reduced catalyst. 5.3.1. Ru catalyst Behr and co-workers tried to avoid the use of NaIO4 with Ru-based metal complexes.398-399 However, the reaction terminated at epoxide stage in the presence of Ru(acac)3/2,6-dipicolinic (acac : acetylacetone) acid under room temperature. Under optimized reaction conditions, the epoxidized methyl oleate was obtained with 90% yield.398 Using an excess amount of 2,6-dipicolinic acid and elevating the reaction temperature made the C=C bond cleavage possible with only H2O2.399 Azelaic and pelargonic acid in 86% and 81% yields, respectively, were obtained from methyl oleate. While, 66% and 59% yields of azelaic and pelargonic acid were derived from oleic acid, respectively. 9-Decanoic acid methyl ester was converted to azelaic acid with 53% yield. 5.3.2. Mo and W catalysts As several metal elements like Mo, W and V show variable valance states, they are ready to form peroxo species and be used in many oxidation reactions, and widely applied in the oxidation of double bond. 95


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Page 96 of 147

Turnwald400-401 reported a molybdenum-2,6-dipicolinate complex formed via the reaction of tetroxide with pyridine-2,6-dicarboxylic acid for the oxidation of oleic acid into pelargonic and azelaic acid with excess H2O2. Azelaic acid in 82% yield was obtained. However, the large amount of H2O2 used in this system makes it unsuitable for large-scale applications. Tungsten is the mostly studied catalyst in combination with H2O2 for oxidative cleavage of unsaturated fatty acid because tungsten-containing materials have unique interactions with H2O2. Compared with Ru and Os, W is cheaper and less toxic.402 With H2WO4 as the catalyst,

403

oleic acid was

oxidized to azelaic acid and pelargonic acid in 91% and 69% yield, respectively. Recently, W-based POMs were widely studied as the catalyst for the oxidative cleavage of UFAs. Generally, a phase transfer agent (PTA), such as quaternary ammonium salts (Q), is used to enhance the contact of substrates with oxidants in the biphasic system. Tungstophosphoric acid (TPA, H3PW 12O40) was used as the W precursor. A peroxo–tungsten complex Q3{PO4[WO(O2)2]4} is formed in situ via the addition of H2O2 to PTA and TPA solution. The mostly used quaternary ammonium salts are cetylpyridinium chloride

(CPC),

tetrabutylammonium

chloride,

methyltrioctylammonium

chloride (Aliquat®336) and tetraoctylammonium chloride. Antonelliet

et

al.404

reported

a

methyltrioctylammonium

tetrakis

(oxodiperoxotungsto)phosphate catalyst for the oxidation of oleic acids with H2O2 catalyzed in two-phase conditions, giving 79% and 82% yield of azelaic acid

and

pelargonic

acid,

respectively

at

80

°C

for

5

h.

Peroxo-tris(cetylpyridinium)12-tungsto phosphate (PCWP) catalyst gave 64% 96


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ACS Catalysis

yield of azelaic acid at 90 °C for 10 h.401 Increasing the catalyst amount, a higher yield (86% for azelaic acid) was achieved.405 But the catalyst was unstable and easily decomposed. Replacing the cetylpyridinium with Cs+ increased the thermal stability of the complex, but resulting in lower yield. Under the same reaction conditions, 84% yield of 3-hydroxynonanoic was obtained from oxidation of ricinoleic acid. The replace of Aliquat®336 with cetylpyridinium

chloride (CPC) showed

little effect

on the catalytic

performance.406

Figure 54. Phase-transfer catalysts (PTCs).407 The phase transfer agent (PTA) also showed obvious effect on the catalytic performance. Among the investigated PTAs, such as CPC, Aliquat®336, tetrabutyl and tetraoctyl ammonium chloride, CPC showed the best results in the oxidation of oleic acid.408 Under optimized reaction conditions, 81% and 86% yields of azelaic and pelargonic acid were achieved in an organic solvent-free system, respectively. The use of polyethyleneimine led to the formation of aldehydes from methyl oleate. Nonanal in 97% yield was obtained under 70 °C for 24 h.409 Kadyrov and Hackenberger407 studied the role anions of PTA in Na2WO4/H3PO4/PTA system. For different unsaturated fatty acids, the best PTA for the oxidation of oleic acid (54% yield of azelaic acid), linoleic acid (54% yield of azelaic acid) and erucic acid (82% yield of tridecanedioic acid) are [n-C16H33N(C2H4OH)(CH3)2]H2PO4 (PTC-1), 97


ACS Catalysis 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[PhCH2(C12-C16)N(CH3)2]Cl (PTC-2) and [(C13-C17-CONHC2H4)N(C2H4OH)CH3] [SO3(OCH3)] (PTC-3) (Figure 54), respectively. Noureddini and coworkers410 compared the catalytic performance of various transition metal catalyst, such as tungsten, tantalum, molybdenum, zirconium, and niobium, in the oxidation of oleic acid with H2O2 as oxidant,. The results turned out that tungsten and tungstic oxide showed the highest activity and selectivity. Metal oxide is the active phase for catalyzing the oxidation of unsaturated fatty acid. The pure metals were firstly oxidized to active metal oxides. Loading tungstic oxide on the carriers enhanced the catalytic performance probably due to the dispersion of tungstic oxide. Silica supported tungstic oxide showed the best performance for the oxidation of oleic acid with 85% conversion of oleic acid in 1 h. CTAB-capped molybdenum oxide afforded 83 and 68% yields of azelaic and pelargonic acids, respectively411.

5.4. Catalytic oxidation with molecular oxygen Molecular oxygen (O2) is abundant in nature and 21% percentage of air is oxygen. Molecular oxygen is an ideal oxidant for oxidation reactions considering the availability and price. Molecular oxygen is widely used in the oxidation reactions, such as hydrocarbon oxygenation, alcohols oxidation and amine oxidation.412-414 However, molecular oxygen is naturally inert due to the triplet state. Therefore, only a few works have been reported for the oxidative cleavage with O2. Utilization of oxygen as terminal oxidant currently showed low selectivity due to the decarbonylation side reaction, and led to a mixture of 98


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ACS Catalysis

acids with different carbon chains (Figure 55).415 In most cases, a small amount of other oxidant or additive is needed and the efficiency is generally low.

Figure 55. Degradation of intermediates during the oxidation of diol.415 A two-step method was generally employed and O2 was used for the oxidative cleavage of diol intermediates. In the first step, the C=C bond was converted to diols by other oxidant, mostly with H2O2. Then the diols underwent C–C bond oxidative cleavage to acids with O2. The successful epoxidation of UFAs enables it easy to obtain dihydroxy derivativess.398,416-419 Therefore, the oxidative cleavage of the dihydroxyl derivatives of UFAs aroused much attention. Woodward420

developed

a

H2WO4/Co(acac)3/N-hydroxy-phthalimide

(NHPI) catalytic system for the oxidation of UFAs with H2O2/O2 as oxidant. Oleic acid, methyl oleate and methyl erucate were dihydroxylated by H2O2 in the presence of H2WO4. Co(acac)3/NHPI was responsible for the oxidative cleavage of the diols intermediate to acids. This system used a limited amount of H2O2, but the yield was very low. 99


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Page 100 of 147

Similarly, Santacesaria and coworkers341,421-422 reported a cobalt polyoxo metalate for the oxidative cleavage of the diols. The oxidative cleavage of UFAs was a two-step process. Firstly, the C=C bond was oxidized to diols with H2O2. Secondly, the oxidative cleavage of the diols with O2 over in situ formed H6CoW 12O40 catalyst. However, using this catalytic system, the yield of azelaic acid production from 9,10-dihydroxystearic acid was not high (53%, 70 °C, 270 min). To improve the efficiency of diols cleavage and addressed the recycle of the

catalyst,

heterogeneous

catalysts

have been

explored

for

this

transformation. Kulik and coworkers423-424 reported the oxidative cleavage of diols from oleic acid, methyl oleate, and erucic acid using a supported gold catalyst with O2 as the oxidant. Among the investigated supports of Al2O3, CeO2, TiO2 and ZrO2, Au/Al2O3 showed the best result. Azelaic acid and pelargonic

acid

in

86%

and

99%

yields

were

obtained

from

9,10-dihydroxystearic acid, respectively. However, catalytic activity significantly decreased after reaction. After the first recycle, the conversion was declined from 94% to 77%, as well as decreasing the yields of azelaic and pelargonic acids by approximately 30%. Also, a large amount of NaOH was needed. Mechanism study indicated that the hydroxyketone and diketone might be the active intermediates. The diols were firstly oxidatively dehydrogenated to hydroxyketone or diketone which were then cleaved to a mixture of acid products.424 In order to improve the efficiency with O2 as the oxidant, aldehyde is added into the catalytic system. Aldehyde is easy to be oxidized to peracid even in the absence of catalyst,425-427 which is more active than O2 and widely used for the 100


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ACS Catalysis

epoxidation

of

olefins.417,428

Köckritz

and

coworkers429

used

O2/isobutyraldehyde as the oxidation system in the oxidation of methyl oleate and oleic acid. Monomethyl azelate and pelargonic acid with 50–70% yields were obtained from methyl oleate with OsO4 catalyst. The oxidation of aldehyde by O2 generated peracid which was the active oxidant for the cleavage of C=C bond. 2,2-Azobisisobutyronitrile (AIBN) was used as the radical initiator to promote the oxidation of isobutyraldehyde to peracid. Fujitani and coworkers430 employed Co–Mn–Br catalytic system in the oxidation of UFAs. Co–Mn–Br is widely used in the hydrocarbon oxidation to acid. It is also effective in the preparation of dicarboxylic acids from UFAs. For example, industrial oleic acid, containing oleic acid, palmitoleic acid, linoleic acid, and linolenic acid, were converted into azelaic acid and suberic acid . Soybean acid and tall acid majorly consisting of linoleic acid and linolenic acid were also oxidized to the related dicarboxylic acids. Ikushima and coworkers431 investigated mesoporous silicas (Cr-MCM-41, Mn-MCM-41, Co-MCM-41) and microporous zeolites (Cr-APO-5, Co-MFI, Mn-MFI) supported catalysts in the oxidation of oleic acid with O2. Chromium-containing catalysts showed the best performance with 95% conversion. The catalyst were recycled without obvious decrease of catalytic activity. Although this catalyst was highly active, the selectivity was low (32% for azelaic acid, and 32% for pelargonic acid) due to the degradation to C6–C10 acids. Table 9. Different catalytic systems with various oxidants for the oxidative cleavage of UFAs into carboxylic acids.

101


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Page 102 of 147

Reaction Reactants

Catalyst/oxidant

Yield

Ref.

conditions Oxone/periodate in Oleic acid

Reflux, 6 h

PA: 99%

368

MeCN:H2O RuCl3/NaIO4 in

AA: 73% 2 h, RT

Oleic acid co-solvents

390

PA: 65% 0.75 h, RT,

Oleic acid

RuCl3/NaIO4

ultrasonic

AA: 81%

391

radiation 8 h, RT, ultrasonic Oleic acid

radiation,

RuCl3/NaIO4

AA: 62%

392

organic solvent-free [MoO(O2)2,6Oleic acid

5 h, 90 °C

AA: 82%

401

dipicolinate)](H2O)/H2O2 [Fe(OTf)2(6-Me-PyTACN)

H2SO4, 48 h, PA: 85%

Oleic acid ]/NaIO4

RT

H6CoW12O40/H2O2–O2

4.5 h, 70 °C

AA: Oleic acid

397

422

52.5% 399

[Ru(2,6-dipicolinate)2]/H2 Oleic acid

24 h, 80 °C

PA : 59%

O2 H2WO4 and AA: 15% Oleic acid

Co(acac)3/H2O2 and

5 h, 70–75 °C PA: 15%

NHPI in O2

102


420

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ACS Catalysis

5 h, 90 °C Oleic acid

PCWP/H2O2

organic

AA: 57%

401

solvent-free AA: 86% Oleic acid

PCWP/H2O2

4 h, 80 °C

405

PA: 82% A peroxo–tungsten AA: 79% Oleic acid

complex with Aliquat®

5 h, 80 °C

404

PA: 82% 336 as PTA/H2O2 5 h, 85 °C, AA: 81% Oleic acid

PCWP/H2O2

organic

408

PA: 86% solvent-free Tungsten oxide

AA: 25%

Oleic acid

1 h, 130 °C supported on silica/H2O2

PA: 28% AA: 16%

Oleic acid

Tungsten oxide/H2O2

410

1 h, 130 °C

410

PA: 17%

CTAB-molybdenum Oleic acid

AA: 83% 3.5 h, 85 °C PA: 68%

oxide /H2O2 Chromium supported on

8 h, 80 °C

AA: 31%

MCM-41/O2

in scCO2

PA: 30%

OsO4/O2/aldehyde/AIBN

5 h, 90 °C

Oleic acid AA: 43% Oleic acid

411

431

429

PA: 36% DEE Mixture of

Conv.: Co(OAc)2-Mn(OAc)2-HBr/

88% 8 h, 100 °C

Industrial

430

DA9+8:

O2

Oleic Acid

84%

Methyl OsO4/oxone in DMF

3 h, RT

oleate 103


PA: 93%

372

ACS Catalysis 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Methyl

Oxone/periodate in

oleate

MeCN:H2O

Methyl

[Fe(OTf)2(6-Me-PyTACN)

oleate

Page 104 of 147

Reflux, 6 h

PA: 70%

36 h, 0 °C

PA: 55%

24 h, 80 °C

PA: 81%

4 h, 85 °C

MEA:

Organic

83%

solvent-free

PA: 84%

368

397

]/H2O2 and NaIO4 A ruthenium-based POM:

Methyl

[Ru(2,6-dipicolinate)2]/H2

oleate

O2

398

A peroxo–tungsten Methyl oleate

complex with Aliquat®

406

336 as PTA/H2O2 H2WO4 and

MEA:

Methyl Co(acac)3/H2O2 and

5 h, 70–75 °C

19%

420

oleate NHPI in O2

PA: 20%

9,10–dihydr oxystearic

260 min,

AA: 86%

80 °C

PA: 99%

Au supported on Al2O3/O2

423

acid 9,10–dihydr Co(OAc)2-Mn(OAc)2-HBr/ oxystearic

AA: 89% 8 h, 100 °C

O2

430

PA: 85%

acid Methyl

H2WO4 and Co(acac)3

MB: 41% 5 h, 70–75 °C

erucate

/H2O2 and NHPI in O2

420

PA: 54%

A peroxo–tungsten

4 h, 85 °C,

complex with

organic

Aliquat®336 as PTA/H2O2

solvent-free

MA: 85%

Methyl

PA 84%

ricinoleate

104


406

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ACS Catalysis

RT: room temperature, PA: pelargonic acid, AA: azelaic acid, NL: nonanal, OA: oleic acid, MO: methyl oleate, MA: methyl azelate, and MB: methyl brassylate. SAA and SPA mean the selectivity of AA and PA, respectively.

6. Outlook

With the diminishing of the oil resources and increasing demand of fuels and organic materials by the modern society, the investigation of renewable alternative carbon sources to meet the human society has aroused worldwide research interests. Biomass is abundant in nature and renewable. However, most of the biomass resource is not well utilized. Biomass, such as lignocellulose, has plenty of oxygen atoms. Compared to the conversion of biomass to fuels via removing oxygen atoms, it is more desirable to convert the biomass into oxygen-containing fine chemicals in the view of element economics. On the other hand, although biomass conversion has been widely studied in recent years, there is few commercialized biorefinery process. Currently, biomass derived from inedible plant is considered to be wastes and are majorly burned to provide heat. Better utilization of these wastes aroused an increasing attention in the future. For the oxidative cleavage of biomass into acid, one risk is the overoxidation to CO2. Particularly in the oxidation of biomass into FA, strong oxidant and highly active catalyst are used for completely breaking up all C–C bond. A lot of CO2 is usually produced. The CO2 is not just from the 105


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decomposition of FA, but also forms during the C–C bond cleavage. Therefore, a more active and selective catalyst is needed to prohibit the formation of CO2. This needs a profound understanding of the C–C bond cleavage mechanism and utilization of the recently fast developed nanotechnology. The knowledge of the C–C bond cleavage are still needed to be devoted. Direct oxidative cleavage of original biomass to acid product in one pot is more desirable, but it is a more challenging task. The original biomass, such as cellulose, is a polymer and hard to be dissolved, which makes the contact of the catalyst with substrate difficult. Using cascade reaction involving several steps, for example the combination of hydrolysis and oxidation reactions, is supposed to be an effective way for one-pot conversion of original biomass to chemicals. Each step should simultaneously work well under the same reaction conditions, and this needs a multifunctional catalyst or to combine several catalytic components together. Further works should be focused on the delicate synthesis of new catalyst with finely distributed multi-active sites in the nanoscale. Recently, the nanotechnology has been fast developed. Taking advantage of the recent achievements in the synthesis of nanomaterials for the biomass conversion will attract much attention in the future. Also, to develop new solvent system for dissolving the original biomass is important for biomass conversion. Ionic liquid is recently shown effective for dissolving original biomass, such as cellulose and lignin. Designing functional ionic liquids with catalytic sites, such as acidic or redox sites, is a promising strategy for the 106


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oxidative cleavage of original biomass. The use of ionic liquid is also convenient for the product isolation via extraction. The oxidative cleavage of UFAs involving of C=C double bond cleavage, which is different from the C-C single bond cleavage. Compared with the C-C bond, direct cleavage of C=C double bond is more difficult due to the bigger bond dissociating energy. Strong oxidant, such as O3, are generally needed to direct break up the C=C double bond. One promising mild route is to first convert the C=C double bond to the functionalized C–C single bond, such as diols, which subsequently undergoes cleavage under mild conditions. The oxidative cleavage of platform molecules to the C4 acid is very complex because it usually consists of several kinds of reaction, such as redox and acid-base catalyzed reactions. Moreover, the furfural and HMF are unstable and easily polymerized under the reaction conditions. Therefore, to effective cleave the platform molecules to the targeted acid, a profound understanding of the mechanism of the each reaction and sided reactions should be further investigated. Based on the obtained knowledge, multi-functionalized catalyst with geometrically and electronically distributed redox and acid-base sites should be prepared. The active sites should be delicately adjusted to facilitate the cooperative effect between each sites. The combination of the catalytic process and new reactors may be helpful for improving the efficiency. For example, using membrane reactor to isolate the desired products in situ can avoid their further conversion to byproducts, 107


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thus improving the selectivity. The usage of microreactor can allow a well mixing of reagents and catalysts, thus enhancing the diffusion step. Moreover, the microreactor can control thermal or concentration of gradients within the microreactor

which

will

provide

chance

for

the

efficient

chemical

transformations. In the future, using new equipment to enhance the reaction process and combine multiple reaction steps will show promising in the biomass conversion. Selective cleavage of C–C bond and controlled functional group transformation to synthesize various acid products from biomass is still needed to be explored. Beside the design and preparation of new catalyst, the use of new techniques will show promise in the synthesis of tailor-made products from biomass. For example, photocatalysis and electrocatalysis are two new techniques. Photo irradiation can effectively activate the molecular oxygen to produce active oxygen species. Photo reactions are different from the thermal reaction. Thermally unfavorable oxidation reactions may be achieved by photocatalyzed reactions. Also, the product selectivity may be varied, and therefore, photocatalyzed oxidation reactions may provide chance to synthesize different acid products from biomass. Actually, photocatalytic oxidation of biomass, known as biomass reforming to H2,432 has been already used in the biomass oxidation for a long time, but usually the biomass was completely converted into CO2. Further studies should be devoted to tailoring the redox properties of the photocatalyst according to the properties of 108


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biomass resources. Electrocatalytic reaction is another promising method for the biomass conversion. It has already been widely used in the electrocatalytic hydrogenation and oxidation of platform molecules.433-438 The character of electrocatalytic reaction is that the selectivity can be controlled via adjusting the over-potentials. Presently, electrocatalytic oxidative cleavage of biomass is little

explored.439

Moreover,

the

combination

of

photocatalysis

and

electrocatalysis together would show unexpected results in the biomass oxidation. Another point is that the process for the biomass oxidation should be in accordance with the “green chemistry”. Using environmentally benign oxidants, such as molecular oxygen and H2O2, and green solvents, prohibiting the greenhouse gas emission and lowering energy consumption can reduce the negative environmental impact of the chemical industry. Before rushing into the commercialization of the biomass conversion process, a life cycle assessment (LCA) should be considered. LCA, defined as the "compilation and evaluation of the inputs, outputs and potential environmental impacts of a product system throughout its life cycle",440 is an effective tool for evaluating environmental benefits of a reaction. It evaluates the environmental impacts of products at all stages in their life cycle so that some cases can be avoided, such as solving an environmental problem by shifting it to another stage in goods production.441 All greenhouse gases emitted during a product’s lifetime are quantified and transformed into CO2 depending on their abilities to contribute 109


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to global warming.442 Also, the techno-economic analysis (TEA) should be made, and it is an essential part ensuring that market-driven prices can be achieved. It evaluates the economic feasibility of a specific project and compare economic quality of different technology routes providing the same service.443

Abbreviations

AA

azelaic acid

MB

methyl brassylate

ACA

acetic acid

MCC

microcrystalline cellulose

AIBN

2, 2-azobisisobutyronitrile

MO

methyl oleate

BV

Baeyer–Villiger mechanism

NHPI

N-hydroxy-phthalimide

CMF

5-(chloromethyl)furfural

NL

nonanal

CPC

cetylpyridinium chloride

OA

oleic acid

CTAB

cetyltrimethyl

ammonium PA

pelargonic acid

bromide DFF

2,2-diformylfuran

PCWP

peroxo-tris(cetylpyridinium) 12-tungsto phosphate

FA

fomic acid

POM

polyoxometallate

FDCA

2,5-furandicarboxylic acid

PTA

phase transfer agent

GBL

γ-butyrolactone

PTC

phase-transfer catalysts

HMF

5-hydroxylmethyl furfural

Q

quaternary ammonium salts

110


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HPA

3-hydroxypropanoic acid

RT

room temperature

HPA-n

H3+nPVnMo10-nO40

TEMP

[(2,2,6,6-tetramethylpiperidi

O

n-1-yl)oxyl)]

TFA

trifluoroacetic acid

TPA

tungstophosphoric

HPPA

3-(hydroperoxy)propanoic acid

HT

hydrotalcite

acid

H3PW 12O40 LA

levulinic acid

MEA

methyl azelate

MA

maleic anhydride

TS-1

titanium silicalite

MAc

maleic acid

TSA

p-toluenesulfonic acid

MCA

malic acid

UFAs

unsaturated fatty acids

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21603219), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17020300), department of Science and Technology of Liaoning province under contract of 2015020086-101 and DICP (DICP ZZBS201613).

111


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Tachibana,

Y.;

Masuda,

T.;

Funabashi,

M.;

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