Catalytic Transformation of Lignocellulose into Chemicals and Fuel

Review pubs.acs.org/CR

Catalytic Transformation of Lignocellulose into Chemicals and Fuel Products in Ionic Liquids Zhanrong Zhang, Jinliang Song,* and Buxing Han* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ABSTRACT: Innovative valorization of naturally abundant and renewable lignocellulosic biomass is of great importance in the pursuit of a sustainable future and biobased economy. Ionic liquids (ILs) as an important kind of green solvents and functional fluids have attracted significant attention for the catalytic transformation of lignocellulosic feedstocks into a diverse range of products. Taking advantage of some unique properties of ILs with different functions, the catalytic transformation processes can be carried out more efficiently and potentially with lower environmental impacts. Also, a new product portfolio may be derived from catalytic systems with ILs as media. This review focuses on the catalytic chemical conversion of lignocellulose and its primary ingredients (i.e., cellulose, hemicellulose, and lignin) into value-added chemicals and fuel products using ILs as the reaction media. An outlook is provided at the end of this review to highlight the challenges and opportunities associated with this interesting and important area.

CONTENTS 1. Introduction 1.1. General Overview 1.2. Scope of This Review 2. Conversion of Cellulose in ILs 2.1. Hydrolysis of Cellulose to Reducing Sugars in ILs 2.1.1. Hydrolysis of Cellulose over Mineral Acids in ILs 2.1.2. Hydrolysis of Cellulose over Soluble Lewis Acids in ILs 2.1.3. Soluble Quaternary Ammonium Perrhenates for Hydrolysis of Cellulose in ILs 2.1.4. Heterogeneous Hydrolysis of Cellulose in ILs 2.1.5. Hydrolysis of Cellulose over ILs-Based Catalyst in ILs 2.2. Dehydration of Glucose and Cellulose to 5Hydroxymethylfurfural (HMF) in ILs 2.2.1. Dehydration of Glucose to HMF in ILs 2.2.2. Dehydration of Cellulose to HMF in ILs 2.3. Hydrogenation of Cellulose to Sugar Alcohols in ILs 2.4. Conversion of Glucose and Cellulose to Levulinic Acid in ILs 2.4.1. Conversion of Glucose to LA in ILs 2.4.2. Conversion of Cellulose to LA in ILs 2.5. Conversion of Glucose and Cellulose to Lactic Acid in ILs 2.6. Conversion of Cellulose-Derived HMF in ILs 3. Conversion of Hemicellulose in ILs © XXXX American Chemical Society

3.1. Hydrolysis of Hemicellulose to Sugars in ILs 3.2. Conversion of Xylose and Hemicellulose to Furfural 3.2.1. Metal Salts for Conversion of Xylose and Hemicellulose to Furfural in ILs 3.2.2. Homogeneous Brønsted Acids for Conversion of Xylose and Hemicellulose to Furfural in ILs 3.2.3. Solid Acids for Conversion of Xylose and Hemicellulose to Furfural in ILs 3.2.4. Acidic ILs for Conversion of Xylose and Hemicellulose to Furfural 4. Catalytic Transformation of Lignin in ILs 4.1. Acid/Base-Catalyzed Depolymerization of Lignin in ILs 4.1.1. Acid-Catalyzed Hydrolysis of Lignin and Its Model Compounds 4.1.2. Base-Catalyzed Hydrolysis of Lignin and Its Model Compounds in ILs 4.2. Catalytic Hydroprocessing of Lignin and Its Model Compound in ILs 4.2.1. Catalytic Hydroprocessing of Lignin and Its Model Compounds in Homogeneous Catalytic Systems 4.2.2. Catalytic Hydroprocessing of Lignin and Its Model Compounds in Heterogeneous Catalytic Systems

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Special Issue: Ionic Liquids Received: July 15, 2016

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DOI: 10.1021/acs.chemrev.6b00457 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 4.3. Catalytic Oxidation of Lignin and Its Model Compounds in ILs 4.3.1. Homogeneous Catalytic Oxidation of Lignin and Its Model Compounds in ILs 4.3.2. Heterogeneous Catalytic Oxidation of Lignin and Its Model Compounds in ILs 4.3.3. Metal-Free Catalytic Oxidation of Lignin and Its Model Compounds in ILs 4.3.4. Electrocatalytic Oxidation of Lignin and Its Model Compounds in ILs 5. Catalytic Transformation of Lignocellulosic Biomass in ILs 6. Concluding Remarks and Outlook Author Information Corresponding Authors Notes Biographies Acknowledgments References

Review

of the dry weight of both woody and herbaceous plant materials and represents the most abundant form of biomass.16 With an annual production of around 170 billion metric tons, it has for a long time been recognized as a promising renewable candidate to substitute fossil resources for the production of chemicals and fuels. However, until now no more than 5% of the produced lignocellulose has been used by humans for diverse purposes.17 Lignocellulose is essentially a composite material constructed primarily from three oxygen-containing high-molecular-weight organic polymers, namely, cellulose, hemicellulose, and lignin (Figure 1), together with minor amounts of extraneous materials (e.g., terpenes, oils, and inorganic minerals). The weight percentage of each component varies and depends on the wood/plant species, but in general, typical wood biomass contains 30−50% cellulose (a polymer of glucose), 20−35% hemicellulose (a heteropolymer containing primarily xylose), and 15−30% lignin (an amorphous aromatic macromolecule).18 In the concept of developing biorefineries, significant efforts have been and are still being devoted in the field of transformation of lignocellulose for the production of high-value products. In this context, many catalytic strategies have been developed to produce value-added chemicals and fuel products, aiming to exploit the full chemical potential of this currently underutilized but valuable resource.19−23 Different value-added chemicals and high-quality fuel products have been generated from lignocellulose, such as organic acids (e.g., gluconic acid,24 formic acid,25 lactic acid,26 levulinic acid27) and alcohols (sugar alcohols,28 ethylene glycol, and propylene glycol29) from cellulose, furfuralbased compounds (e.g., 5-hydroxymethylfurfural,30 furfural31) from cellulose and hemicellulose, and various aromatic chemicals from lignin.32 Moreover, some of these lignocellulose-derived chemicals can be considered as platform chemicals which can be further converted to many kinds of value-added compounds through various reaction routes. This aspect is sometimes recognized as an expanded field of lignocellulose conversion.33−38 With the rapid development of green chemistry and in terms of developing biorefineries, the transformation of feedstocks in an environmentally benign way represents another important issue to be taken into account. In this aspect, utilization of green solvents (e.g., water, supercritical fluids, polyethylene glycols, ILs, etc.) instead of volatile organic solvents (VOCs) has become a hot topic, as solvents play important roles in most of the chemical and chemical engineering processes.40−42 As a promising class of green and functional solvents, ILs have attracted enormous attention in recent years, owing to their unusual properties such as near-zero vapor pressure, wide electrochemical window, high thermal stability, nonflammability, excellent solvent power for both organic and inorganic substances, etc.43−45 More importantly, the properties of ILs related with hydrophobicity, polarity, and solvent power can be tuned by appropriate combination or modification of the cations and anions.46,47 Due to these unique properties, ILs have been applied as efficient solvents in many fields, such as material synthesis, separation, electrochemistry, and catalysis.48−53 With the finding that ILs can dissolve lignocellulose,54−56 many works have been conducted in the field of using ILs for dissolution, fractionation, and pretreatment of biomass, with the major aim to achieve delignification of biomass. In recent years, application of ILs in conversion of lignocellulose to chemicals and fuel products has gained much research interest and been studied extensively. Many efficient routes in ILs have been developed for converting lignocellulose to fuels or value-added

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1. INTRODUCTION 1.1. General Overview

Nowadays, most of the essential fuels and chemicals used by mankind are predominantly produced from nonrenewable fossil resources mainly including coal, petroleum, and natural gas. However, the reserves of these fossil resources are diminishing, while the demands for energy and chemicals are continuously increasing. Meanwhile, consumption of the nonrenewable fossil resources leads to the generation of large amounts of greenhouse gas (GHG). The GHG emission causes global climate change, which is one of the most challenging issues for humankind. Therefore, the search for sustainable alternatives to depleting fossil resources for chemicals and energy supply represents an urgent task and a worldwide grand research challenge. Making innovative and sufficient use of naturally abundant and renewable resources is of great importance in the pursuit of a sustainable future. Biomass, the only renewable organic carbon resource in nature,1−3 has attracted significant attention as the renewable carbon resource to produce a variety of value-added chemicals, functional materials, and fuel products,4−8 in addition to its role as the fourth largest energy source in the world for the generation of heat and power following oil, coal, and natural gas.9 Conversion of biomass into valuable chemicals and fuels can help us to alleviate our heavy dependence on fossil resources and also has been recognized as an effective way to reduce the net emission of CO2 by combination of photosynthesis and chemical methods.10 Moreover, the utilization of naturally abundant renewable biomass as a feedstock for value-added products also imbues the principles of green chemistry within the context of sustainability. Therefore, with the globally increasing awareness and concerns over the depletion of nonrenewable fossil carbon reserves and climate change, interest in biomass transformation has shown a continually rapid increase during the past several decades and this field has drawn significant attention from both academia and industry. Not surprisingly, the production of valueadded chemicals, fuel products, and functional materials from biomass has become an intensively active research area.11−15 Among different types of biomass feedstocks (e.g., triglycerides, lignocellulose, chitin, starch), lignocellulosic biomass (wood and other plant biomass) primarily comprises the bulk B


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Figure 1. Schematic illustration of lingocellulose. Reproduced from ref 39 with permission. Copyright 2013 Royal Society of Chemistry.

desired products from lignocellulose. Additionally, in some cases, functionalized ILs serve simultaneously as the reaction media and catalyst for the reactions. Herein, we review the state-of-the-art of catalytic conversion of lignocellulosic biomass, focusing on cellulose, hemicellulose, and lignin, to fine chemicals and fuel products using ILs as the reaction media.

chemicals, which has become an interesting research topic in the field of lignocellulose transformation. The application of ILs as the reaction media can promote the reaction efficiency by dissolving the lignocellulosic macromolecules and thus improving the accessibility of targeted bonds for catalysts. As a result, the reaction conditions required are typically less harsh than those required in other solvents. Meanwhile, ILs can protect and/or stabilize the formed active intermediates (e.g., radicals, carbocations) or suppress the occurrence of side reactions (e.g., repolymerization, self-condensation) due to their special properties,57−59 which is beneficial toward high selectivities of

1.2. Scope of This Review

Through searching the publications we found that there have been many excellent works related to both lignocellulose and ILs in the past decade. In this review, we attempt to provide a holistic overview of the catalytic transformation of lignocellulose using C


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adding water gradually into the HCl/[EMIM]Cl catalytic system, a highest glucose yield of around 90% could be achieved from cellulose at 105 °C. In addition, they studied the hydrolysis of cellulose to form glucose in various ILs, including 1-ethyl-3methylimidazolium acetate ([EMIM]OAc, 0%), 1-ethyl-3methylimidazolium nitrate ([EMIM]NO3, 0%), 1,3-dimethylimidazolium dimethylphosphate (0%), 1-ethyl-3-methylimidazolium bromide ([EMIM]Br, 4%), 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl, 90%), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM[BF4], 0%), 1-ethyl-3-methylimidazolium triflate ([EMIM]OTf, 5%), 1-butyl-3-methylimidazolium chloride ([BMIM]Cl, 66%), 1-butyl-4-methylpyridinium chloride (73%), 1-ethylpyridinium chloride (69%), and 1-ethyl-2,3dimethylimidazolium chloride (46%). The results indicated that the ILs with Cl− anion could promote the hydrolysis of cellulose to glucose with moderate to high yields (46%−90%), while other ILs showed no or very low ( Hβ> HZSM-5 > SAPO-34, which was consistent with their pore sizes. X-ray diffraction (XRD) characterization revealed that the framework structure of HY was particularly stable in [BMIM]Cl, and the cell parameter of HY was enlarged due to the dilatation effect of [BMIM]Cl. During the catalytic hydrolysis reaction, the F


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Figure 4. Possible ion exchange between SPS-DVB and [BMIM]Br. Adapted from ref 99 with permission. Copyright 2013 Elsevier B.V.

Figure 5. Plausible mechanism for the hydrolysis of microcrystalline cellulose (MCC) over CMC−SO3H in [BMIM]Cl. Adapted from ref 103 with permission. Copyright 2016 Elsevier B.V.

hydrolysis of cellulose in 1-butyl-3-methylimidazolium bromide ([BMIM]Br).99 The yield of TRS could reach 60.7% with a glucose yield of 48.1% under the optimal conditions (temperature 120 °C; time 10 h). After the reaction, the acidity of the catalyst decreased due to ion exchange between the acidic sites of sulfonated catalyst and [BMIM]Br (Figure 4), leading to decreased catalytic activity during catalyst recycling experiments when the catalysts were used directly without regeneration. In order to facilitate the separation of catalysts from the resulting hydrolysis residues, magnetic solid acid catalysts have been designed for hydrolysis of cellulose in ILs. For example, Guo et al. prepared a superparamagnetic carbonaceous solid acid catalyst by incomplete hydrothermal carbonization of cellulose followed by Fe3O4 grafting and −SO3H group functionalization.100 This superparamagnetic carbon catalyst containing −SO3H, −COOH, and phenolic −OH groups exhibited good performance for the hydrolysis of cellulose in [BMIM]Cl. A TRS yield of 68.9% was obtained in [BMIM]Cl at 130 °C. The activity of the recycled catalyst decreased with recycling times, attributed to unavoidable losses of the catalyst and adsorption of humin (a common byproduct resulting from thermal treatment of cellulosic matter and also from hemicellulose and lignin condensation and polymerization) on the active sites of the catalyst. Xiong et al. prepared a core−shell Fe3O4@SiO2-SO3H solid acid catalyst by the immobilization of sulfonic acid groups on the surface of silica-encapsulated Fe3O4 nanoparticles.101

Cellulose could be hydrolyzed to reducing sugars over this synthesized magnetic catalyst in [BMIM]Cl, and the highest yield of reducing sugars of about 73.2% could be achieved at 130 °C. Liu et al. prepared a magnetic acid catalyst (Fe3O4@CSO3H) by sulfonation of the core−shell structured [email protected] Cellulose could be hydrolyzed in the Fe3O4@C-SO3H/[BMIM] Cl catalytic system effectively to afford reducing sugars (yield 72.1%) with a glucose selectivity of 82.5%. Very recently, Hu et al. prepared a magnetic carbonaceous solid acid (CMC-SO3H) containing chlorine (−Cl) groups and −SO3H groups for hydrolysis of cellulose in [BMIM]Cl.103 The results indicated that −Cl groups with stronger electron negativity could not only improve its adsorption to cellulose but also enhance the acidity of SO3H groups (Figure 5), leading to a TRS yield of 78.5% from hydrolysis of cellulose. Some other types of solid acids have also been developed for hydrolysis of cellulose in ILs. For example, Zhang et al. synthesized CaFe2O4-based solid acid catalyst by calcination of the hydroxides coprecipitated from aqueous Ca(NO3)2·6H2O and Fe(NO3)3·9H2O solutions in the presence of urea.104 The synthesized CaFe2O4 could catalyze the hydrolysis of cellulose in [AMIM]Cl, and a maximum hydrolysis yield of 49.8% and glucose selectivity of 74.1% could be reached at 150 °C. In the catalytic process, the authors found that solid CaFe2O4 rather than Fe2O3 and the trace leached Ca2+ catalyzed the hydrolysis of cellulose as control experiments indicated that Fe2O3 and Ca2+ G


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Figure 6. Reaction scheme for grafting poly(styrenesulfonated acid) (PSSA) and poly(vinylimidazolium chloride) IL from a glass or ceramic membrane substrate. Adapted from ref 105 with permission. Copyright 2013 Royal Society of Chemistry.

chloride ([C3SO3HMIM]Cl) showed a higher hydrolytic activity than 1-(1-butylsulfonic)-3-methylimidazolium chloride ([C4SO3HMIM][Cl]). The authors suggested that the shorter alkyl chain between the imidazole nucleus and the SO3H group could improve the catalytic activity of IL toward hydrolytic conversion of cellulose. Maximum TRS yield of 62% with a glucose yield of 14% was obtained in [C3SO3HMIM]Cl at 70 °C. Subsequently, Liu et al. synthesized six kinds of SO3Hfunctionalized acidic ILs (Figure 8) for hydrolysis of cellulose in [BMIM]Cl.111 Among the tested ILs, triethyl-(3-sulfo-propyl)ammonium hydrogen sulfate showed the best performance with a maximum TRS yield up to 99% at 100 °C. In addition, the

could not promote the reaction. Additionally, the recovered CaFe2O4 showed similar properties with the fresh one, retaining high activity and selectivity for cellulose hydrolysis. In another work, Qian et al. synthesized novel solid polymeric catalysts comprising poly(styrene sulfonic acid) (PSSA) polymer chains and poly(vinyl imidazolium chloride) ionic liquid (PIL) polymer chains (Figure 6).105 The neighboring PIL chains were helpful to solubilize lignocellulosic biomass and enhance the catalytic activity of the PSSA chains. These novel polymeric solid acid catalysts demonstrated excellent activity (yield of TRS over 97%) toward the hydrolysis of cellulose in [EMIM]Cl at 140 °C. Recently, Xu et al. synthesized the mesocellular silicon foam supported poly(chloromethylstyrene-co-divinylbenzene) (MCFcopolymer).106 After sulfonation with concentrated sulfuric acid, MCF-copolymer-supported sulfonic acid catalyst was obtained. The solid acid catalyst could catalyze the hydrolysis of cellulose efficiently in [BMIM]Cl with a high yield of TRS (88.42%) at 110 °C. The high catalytic activity resulted from the large pore sizes of the synthesized catalyst, which favored the diffusion of molecules especially for bulky molecules. 2.1.5. Hydrolysis of Cellulose over ILs-Based Catalyst in ILs. The utilization of acidic ILs for hydrolysis of cellulose to generate reducing sugars is considered to be an advanced strategy, which potentially offers a more sustainable, greener, and less costly route in comparison with traditional methods.107 Several kinds of acidic ILs have been developed for hydrolysis of cellulose by using them either simultaneously as the catalyst and solvent or only as catalysts in other ILs. SO3H-functionalized acidic ILs represent the most extensively studied ILs for hydrolysis of cellulose.108 Amarasekara and Owereh designed several kinds of acidic ILs with SO3H groups, including methylimidazolium-, pyridinium-, and triethanolammonium-based ILs (Figure 7). The functionalized ILs were used for the dissolution and hydrolysis of cellulose.109 Cellulose was initially dissolved in these ILs and subsequently hydrolyzed with the addition of water. Among the examined ILs, methylimidazolium-based ILs resulted in higher yields of TRS and glucose than pyridinium- and ammonium-based ones. This effect was primarily due to the higher solubility of cellulose in the imidazolium-based ILs.110 Within the two synthesized imidazolium-based ILs, 1-(1-propylsulfonic)-3-methylimidazolium

Figure 8. Acidic ILs used in Liu’s study. Adapted from ref 111 with permission. Copyright 2012 Elsevier Ltd.

authors pointed out that controlling the amount of water to a relatively low level was crucial in these catalytic systems. The presence of too much water in [BMIM]Cl reduced the solubility of cellulose in the IL solvent, thus imposing a negative impact on the subsequent cellulose hydrolysis step. Another six acidic ILs with SO3H groups based on 2-phenyl-2-imidazoline were synthesized (Figure 9) for hydrolysis of cellulose in [BMIM]Cl by Zhuo et al.112 It was demonstrated that the acidic ILs with HSO4− and Cl− anions showed better performance for cellulose hydrolysis than those with H2PO4− due to the stronger acidity of

Figure 9. Chemical structures of the ILs used in Zhuo’s study. Adapted from ref 112 with permission. Copyright 2014 Elsevier Ltd.

ILs with HSO4− and Cl− than those with H2PO4−. The authors found that a suitable amount of water was beneficial for TRS production due to the decrease in the dehydration rate of TRS. Using 1-propyl sulfonic acid-2-phenyl imidazoline hydrogensulfate as the acidic catalyst in [BMIM]Cl, the TRS yield of 85.1% could be achieved at 100 °C. Except for the SO3H-functionalized acidic ILs, Zhao et al. synthesized a series of Brønsted−Lewis-acidic ILs based on N-

Figure 7. Brønsted-acidic ILs used in Amarasekara’s study. Adapted from ref 109 with permission. Copyright 2009 American Chemical Society. H


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Figure 10. Synthesis of sulfonic acid-functionalized acidic ionic liquid modified silica catalyst. Adapted from ref 115 with permission. Copyright 2010 Elsevier B.V.

2.2. Dehydration of Glucose and Cellulose to 5-Hydroxymethylfurfural (HMF) in ILs

methyl-2-pyrrolidonium (NMP) and metal chlorides ([HNMP]Cl/MClx, where M = Fe, Zn, Al, or Cu) through the reaction of metal chlorides and [HNMP]Cl (formed by the reaction of Nmethyl-2-pyrrolidinone and hydrochloric acid) with a molar ratio of 1:1.113 Then the obtained ILs [HNMP]Cl/MClx were used to catalyze the hydrolysis of cellulose. The results revealed that IL [HNMP]Cl/FeCl3 presented the best performance, and a 98.8% TRS yield from microcrystalline cellulose was achieved at 100 °C in [BMIM]Cl, which was better than or similar to other −SO3Hcontaining acidic ILs. Density functional theory calculations indicated that interactions between the metal chlorides and the cellulose including charge-transfer interactions were important for the hydrolysis of cellulose, and FeCl3 showed stronger interaction with cellulose than other used metal chlorides. Therefore, IL [HNMP]Cl/FeCl3 showed the best catalytic performance for cellulose hydrolysis. Very recently, Amarasekara et al. developed a metal salt−Brønsted-acidic IL system comprising ZnCl2·1.74H2O-1-(1-propylsulfonic)-3-methylimidazolium chloride (BAIL) for hydrolysis of cellulose near room temperature.114 The authors suggested that ZnCl2 could interact with carbohydrate and probably change the conformation of the polysaccharides, thus facilitating the BAIL-catalyzed hydrolytic reaction. In this system, untreated cellulose could be hydrolyzed to TRS (yield 78%) with a glucose yield of 19% at 37 °C for 4 days. Acidic ILs could also be immobilized on supporting materials to prepare heterogeneous acidic IL catalysts for hydrolysis of cellulose with other ILs as the reaction media. This can be considered as an expanding application of ILs, although the intrinsic property of the supported ILs may have been changed. Amarasekara et al. prepared a silica catalyst modified by sulfonic acid-functionalized acidic IL.115 The simple two-step preparation process involves nucleophilic substitution of chlorine with imidazole followed by condensation with 1,3-propane sultone and acidification using HCl (Figure 10). This silica-supported acidic IL catalyst was effective for the hydrolysis of cellulose in [BMIM]Cl at 70 °C, generating glucose and TRS with yields up to 26% and 67%, respectively. Recently, Liu et al. synthesized an acidic IL-functionalized polymer (PDVB-SO3H-[C 3vim][SO3CF3]) with abundant nanoporous structures and high acid strength for hydrolysis of cellulose to sugars in [BMIM]Cl.116 The synthesized acidic IL-functionalized polymer showed much better performance than mineral acids, homogeneous acidic ILs, and acidic resins such as Amberlyst 15. The authors suggested that the enhanced catalytic activity offered by the polymer resulted from synergistic effects between the strong acidic groups and the ILs grafted onto the polymer which by itself could break down the crystalline structures of cellulose. Under optimal conditions, the total yield of TRS and mono- and disaccharides could reach almost 100% with a glucose yield of 77.0%.

As a versatile platform molecule, 5-hydroxymethylfurfural (HMF) has been recognized as one of the most useful building block platform chemicals generated from biomass117 and can be transformed into many kinds of valuable chemicals through diverse selective reactions.118−123 In recent years, production of HMF from biomass has attracted much attention, and a lot of works for conversion of biomass into HMF have been reported. Herein, we only highlight the dehydration of glucose and cellulose into HMF using ILs as the reaction media (Table 1). 2.2.1. Dehydration of Glucose to HMF in ILs. Imidazolium-based ILs represent the most extensively studied ones for dehydration of glucose into HMF. Zhao et al. reported that metal halides in 1-alkyl-3-methylimidazolium chlorides could be used as effective catalysts for dehydration of glucose into HMF. A catalytic system comprising CrCl2 in [EMIM]Cl was the most effective one for this transformation, in which HMF was obtained from dehydration of glucose with a yield around 70% at 100 °C for 3 h.124 The authors studied the reaction mechanism. They proposed that the CrCl3− anion formed from CrCl2 and [EMIM]Cl played an important role to facilitate mutarotation of α-anomer of glucose to the β-anomer, which was the key step for the dehydration (Figure 11). 1H NMR analysis revealed that the critical role of CrCl3− was to affect a formal hydride transfer by the formation of hydrogen bonds with the hydroxyl groups of glucose, leading to isomerization of glucose to fructose which could be easily dehydrated to HMF. Yong et al. used N-heterocyclic carbenes (NHCs)-modified CrCl2 or CrCl3 to catalyze the conversion of glucose to HMF in [BMIM]Cl.125 Different from Zhao et al.’s work, CrCl2 and CrCl3 showed similar catalytic activities for the reaction. The activity of the catalysts was significantly affected by the stereochemical properties of the NHC ligands (Figure 12). Highest HMF yields around 81% could be obtained from catalytic systems comprising catalysts with the most bulky NHC ligands such as 1,3-bis(2,6-diisopropylphenyl)imidazolylidene and 1,3-bis(2,6-diisopropyl)phenylimidazolinylidene. It was speculated that sterically crowded complexes could protect the Cr center from reacting with [BMIM]Cl and therefore provided the highest catalytic efficiency. Furthermore, Li et al. found that microwave irradiation could promote the dehydration of glucose significantly, and a high HMF yield of 91% could be obtained in the CrCl3/[BMIM]Cl catalytic system with a remarkable reduced reaction time (from hours to minutes).126 In order to avoid the use of toxic chromium salts, several research groups have developed more environmentally benign catalytic systems for dehydration of glucose into HMF (Table 1). For instance, Hu et al. found that SnCl4 in 1-ethyl-3methylimidazolium tetrafluoroborate ([EMIM]BF4) could efficiently catalyze the dehydration of glucose to HMF, and the yield of HMF could reach about 62%.127 On the basis of 1HNMR analysis and control experiments with different alcohols (i.e., I


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Table 1. Dehydration of Glucose and Cellulose into HMF in Various ILs substrate (amount) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.05 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g)

IL (amount)

catalyst (amount)

T (°C)

a

b

t

HMF yield (%)

ref

[EMIM]Cl (0.5 g)

CrCl2 (6 mol %)

100

3h

∼68

124

[EMIM]Cl (0.5 g)

CrCl3 (6 mol %)

100

3h

∼45

124

[BMIM]Cl (0.5 g)

1c/CrCl2 (9 mol %)

100

6h

66

125

[BMIM]Cl (0.5 g)

2c/CrCl2 (9 mol %)

100

6h

65

125

[BMIM]Cl (0.5 g)

3c/CrCl2 (9 mol %)

100

6h

62

125

[BMIM]Cl (0.5 g)

4c/CrCl2 (9 mol %)

100

6h

80

125

[BMIM]Cl (0.5 g)

5c/CrCl2 (9 mol %)

100

6h

50

125

[BMIM]Cl (0.5 g)

6c/CrCl2 (9 mol %)

100

6h

81

125

[BMIM]Cl (0.5 g)

7c/CrCl2 (9 mol %)

100

6h

70

125

[BMIM]Cl (0.5 g)

8c/(CrCl2)2 (9 mol %)

100

6h

81

125

[BMIM]Cl (0.5 g)

8c/CrCl2 (9 mol %)

100

6h

14

125

c

[BMIM]Cl (0.5 g)

4 /CrCl3 (9 mol %)

100

6h

78

125

[BMIM]Cl (0.5 g)

5c/CrCl3 (9 mol %)

100

6h

72

125

[BMIM]Cl (0.5 g)

6c/CrCl3 (9 mol %)

100

6h

78

125

[BMIM]Cl (0.5 g)

7c/CrCl3 (9 mol %)

100

6h

81

125

[BMIM]Cl (1 g)

CrCl3·6H2O (0.006 g)

MI

1 min

91

126

[BMIM]Cl (1 g)

CrCl3·6H2O (0.006 g)

100

1h

17

126

[EMIM]BF4 (1 g)

SnCl4·5H2O (19.5 mg)

100

3h

62

127

[EMIM]Cl (1 g)

CeCl3 (10 mol %)

140

6h

3

128

[EMIM]Cl (1 g)

PrCl3 (10 mol %)

140

6h

13

128

[EMIM]Cl (1 g)

NdCl3 (10 mol %)

140

6h

12

128

[EMIM]Cl (1 g)

DyCl3 (10 mol %)

140

6h

10

128

[EMIM]Cl (1 g)

YbCl3 (10 mol %)

140

6h

5

128

[EMIM]Cl (1 g)

Yb(OTf)3 (10 mol %)

140

6h

10

128

[BMIM]Cl (1 g)

CeCl3 (10 mol %)

140

6h

3

128

[BMIM]Cl (1 g)

PrCl3 (10 mol %)

140

6h

7

128

[BMIM]Cl (1 g)

NdCl3 (10 mol %)

140

6h

8

128

[BMIM]Cl (1 g)

DyCl3 (10 mol %)

140

6h

10

128

[BMIM]Cl (1 g)

YbCl3 (10 mol %)

140

6h

12

128

[BMIM]Cl (1 g)

Yb(OTf)3 (10 mol %)

140

6h

24

128

[EMIM]Cl (1 g)

H3BO3 (27.5 mg)

120

3h

41

129

[BMIM]Cl (1 g)

H3BO3 (27.5 mg)

120

3h

14

129

[HeMIM]Cl (1 g)

H3BO3 (27.5 mg)

120

3h

32

129

[OMIM]Cl (1 g)

H3BO3 (27.5 mg)

120

3h

26

129

J

d


Chemical Reviews

Review

Table 1. continued substrate (amount) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.04 g) glucose (0.04 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (10 wt %) glucose (10 wt %) glucose (10 wt %) glucose (10 wt %) glucose (0.117 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (5 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g)

t

HMF yield (%)

[BMIM]Cl (2 g)

GeCl4 (10 mol %)

100

75 min

38.4

130

[DMIM]Cl (2 g)e

GeCl4 (10 mol %)

100

75 min

23.3

130

[BMIM]Cl (2 g)

GeCl4 (10 mol %)f

100

2h

48.4

130

[EMIM]Cl (0.2 g)

AlCl3 (10 mol %)

120

6h

1.6

131

[EMIM]Cl (0.2 g)

AlEt3 (10 mol %)

120

6h

51

131

[BMIM]Cl (2 g)

ScCl3 (10 mol %)

110

2h

32.3

132

[BMIM]Cl (2 g)

ScCl3 (10 mol %)

MId

2.5 min

73.4

132

[BMIM]Cl (2 g)

HfCl4 (10 mol %)

100

4h

34.5

133

[BMIM]Cl (1 g)

12-TPA/H3BO3 (20 wt %/10 wt %)g

140

40 min

51.9

134

[BMIM]Cl (1 g)

CrCl3/H3BO3 (10 mol %/20 mol %)

120

30 min

78.8

135

[BMIM]Cl (2 g)

Cr-HAP (100 mg)h

MId

2.5 min

40

136

[BMIM]Cl (3 mL)

Dowex 50W-1x4 (10 wt %)

100

6h

3

137

[BMIM]Cl (3 mL)

Dowex 50Wx8-100 (10 wt %)

100

6h

34

137

[BMIM]Cl (3 mL)

Dowex 50Wx8-200 (10 wt %)

100

6h

44

137

[BMIM]Cl (3 mL)

Dowex 50Wx8-200 (10 wt %)

100

3h

53

137

120

6h

50

138

IL (amount)

catalyst (amount)

i

T (°C)

a

b

ref

[EMIM]Cl (0.5 g)

Cr0-NPs (10 mol %)

[BMIM]Cl (1 g)

Hβ-zeolite (Si/Al = 25) (60 mg)

150

50 min

50.3

139

[BMIM]Cl (1 g)

HY-zeolite (Si/Al = 5) (40 mg)

140

30 min

11.8

139

[BMIM]Cl (1 g)

H-mordenite (Si/Al = 15) (40 mg)

140

30 min

13.1

139

[BMIM]Cl (1 g)


140

30 min

18.4

139

[BMIM]Cl (1 g)


140

30 min

23.7

139

[BMIM]Cl (1 g)

HZSM (Si/Al = 50) (40 mg)

140

30 min

20.5

139

[BMIM]Cl (1 g)

HZSM (Si/Al = 100) (40 mg)

140

30 min

16.4

139

[BMIM]Cl (1 g)

HZSM (Si/Al = 300) (40 mg)

140

30 min

8.3

139

[BMIM]Cl (1 g)

HZSM (Si/Al = 25) (40 mg)

140

30 min

14.2

139

[BMIM]Cl (1 g)

Sn-MCM-41 (0.1 g)j

110

4h

70

140

[BMIM]Cl (1.5 g)

Cr3+-modified ion-exchange resins (0.1 g)

110

30 min

61.3

141

[BMIM]Br (16 g)

modified mordenite (Si/Al = 11.2) (0.5 g)k

100

6h

64

142

TEACl (1 g)

CrCl3 (10 mol %)

130

10 min

71.3

143

Bu-DBUCl (1 g)m

CrCl3 (10 wt %)

100

3h

64

144

m

CrCl3 (10 wt %)

100

3h

63

144

HEOE-DBUCl (1 g)m

CrCl3 (10 wt %)

100

3h

62

144

Et-DBUBS (1 g)n

CrCl3 (10 wt %)

110

2

83.4

145

Al-DBUBS (1 g)n

CrCl3 (10 wt %)

110

2

78.5

145

Oc-DBUCl (1 g)

K


Chemical Reviews

Review

Table 1. continued substrate (amount) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (0.1 g) glucose (68 mg) glucose (36 mg) cellulose (0.1 g) cellulose (0.1 g) cellulose (0.1 g) cellulose (0.1 g) cellulose (50 mg) cellulose (50 mg) cellulose (0.35 g) cellulose (0.35 g) cellulose (0.15 g) cellulose (0.15 g) cellulose (0.15 g) cellulose (0.15 g) cellulose (0.1 g) cellulose (0.1 g) cellulose (0.1 g) cellulose (0.1 g) cellulose (0.1 g) cellulose (0.1 g) cellulose (0.1 g) cellulose (0.1 g) cellulose (21.6 mg)

IL (amount)

catalyst (amount)

T (°C)

a

b

t

HMF yield (%)

ref

Bu-DBUBS (1 g)n

CrCl3 (10 wt %)

110

2

70.1

145

Bn-DBUBS (1 g)n

CrCl3 (10 wt %)

110

2

72.4

145

[Bn-MMP]Cl (1 g)

SnCl4·5H2O (10 wt %)

100

3

14.4

146

[Bn-MMP]BF4 (1 g)


100

3

18.7

146

[Bn-MMP]BS (1 g)


100

3

33.1

146

[Al-MMP]Br (1 g)


100

3

40.1

146

[Al-MMP]BF4 (1 g)


100

3

42.5

146

[Al-MMP]BS (1 g)


100

3

63.4

146

[Et-MMP]Br (1 g)


100

3

54.0

146

[Et-MMP]BF4 (1 g)


100

3

37.5

146


100

3

67.6

146

CrCl2 (2.1 mol %)

110

2

47

147

[Et-MMP]BS (1 g) [C10(EPy)2]2Br− (2 g)

o

choline dihydrogen citrate (738 mg) + H3BO3 (124 mg) glycolic acid (190 mg) [BMIM]Cl (2 g) CrCl3·6H2O (10 mg)

140

2

60

148

MId

2 min

62

126

CrCl3·6H2O (10 mg)

130

2h

41

144

CrCl3·6H2O (10 mg)

130

2h

42

144

HEOE-DBUClm (1 g)

CrCl3·6H2O (10 mg)

130

2h

24

144

[EMIM]Cl (0.5 g)

CuCl2 + CrCl2 (37 μmol/g [EMIM]Cl at XCuCl2 = 0.17)

120

8h

60

149

[EMIM]Cl (0.5 g)

CrCl2 + RuCl3 (10 mol % at CrCl2/RuCl3 = 4/1)

120

2h

60

150

[EMIM]OAc (10 mL)

CuCl2 (0.1 mol/L) + [C4SO3Hmim]CH3SO3 (1.5 mL)p

160

3.5 h

69.7

151

[EMIM]OAc (10 mL)

CuCl2 (0.1 mol/L) + [C4SO3Hmim]HSO4 (1.5 mL)p

160

3.5 h

64.9

151

[BMIM]Cl (3 mL)

MnCl2 (3 mol %)+IL-1 (9 mol %)q

120

1h

62.63

152

[BMIM]Cl (3 mL)

MnCl2 (3 mol %) + IL-2 (9 mol %)q

120

1h

66.5

152

[BMIM]Cl (3 mL)

MnCl2 (3 mol %) + IL-3 (9 mol %)q

120

1h

52.46

152

[BMIM]Cl (3 mL)

MnCl2 (3 mol %) + IL-4 (9 mol %)q

120

1h

54.51

152

[EMIM]Cl (2 g)

ATP-SO3H-Cr(III) (0.1 g)r

120

2h

31.2

156

120

2h

41.22

156

Bu-DBUClm (1 g) Oc-DBUCl

m

(1 g)

r

[EMIM]Cl (2 g)

HNTs-SO3H-Cr(III) (0.1 g)

[EMIM]Cl (2 g)

HNTs-PSt-PDVB-SO3H(I) (0.1 g)r

120

2h

28.22

157

[EMIM]Cl (2 g)

HNTs-PSt-PDVB-SO3H(II) (0.05 g)r

120

2h

32.86

157

[EMIM]Cl (2 g)

catalyst-110° (30 mg)s

120

30 min

34.6

158

120

30 min

37.1

158

s

[EMIM]Cl (2 g)

catalyst-10° (30 mg)

[EMIM]Cl (1 g)

H-form zeolite (20 wt %) + LiCl (6 mol %)

160

30 min

70.3

159

[EMIM]Cl (1 g)

H-form zeolite (20 wt %) + NaCl−KCl (3 mol % + 3 mol %)

120

2h

58.2

159

[EMIM]Cl (493.4 mg)

H2SO4 (0.88 wt %) + MgCl2·6H2O (300 mol %) + 2-methoxycarbonylphenyl boronic acid (120 mol %)

105

1h

41

160

L


Chemical Reviews

Review

Table 1. continued substrate (amount) cellulose (0.1 g)

IL (amount) [BMIM]Cl (2 g)

catalyst (amount) Cr([PSMIM]HSO4)3 (0.05 g)

T (°C) 120

a

b

t 5h

HMF yield (%)

ref

53

161

a T = Temperature. bt = Time. cStructure of ligands in Figure 12. dMI = Microwave irradiation at 400 W. e[DMIM]Cl = 1-Decyl-3methylimidazolium chloride. fWith the addition of 0.5 g of 5 Å molecular sieves. g12-TPA = 12-Tungstophosphoric acid. hCr-HAP = Hydroxyapatite-supported chromium chloride. iCr0-NPs = Chromium(0) nanoparticles. jSn-MCM-41 = A tin-containing silica molecular sieve. kThe mordenite was treated with 1 M NH4Cl. lTEAC = Tetraethylammonium chloride. mStructure of the ILs in Figure 17a. nStructure of the ILs in Figure 17b. o[C10(EPy)2]2Br− = 1,1′-Decane-1,10-diylbis(3-ethylpyridinium) dibromide. pStructure of the ILs in Figure 20. qStructure of the ILs in Figure 21. rATP = attapulgite, HNTs = halloysite nanotubes, PSt = polystyrene, and DVB = polydivinylbenzene. sCatalyst-110° = Brønsted-acidic polymer nanotubes with hydrophobic surface wettability, and catalyst-10° = Brønsted-acidic polymer nanotubes with hydrophilic surface wettability.

Figure 11. Proposed metal halide interaction with glucose over CrCl2 in [EMIM]Cl. Adapted from ref 124 with permission. Copyright 2007 American Association for the Advancement of Science.

Figure 12. Structures of NHC ligands used in Yong et al.’s work. Adapted from ref 125 with permission. Copyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

opposite to the trend observed in chromium-based catalytic systems. The reason for this phenomenon could probably be that ytterbium was less prone to form complexes with the imidazolium chlorides. Furthermore, Ståhlberg et al. also developed a metal-free process for converting glucose into HMF using boric acid as a promoter in various imidazoliumbased ILs.129 The boric acid/[EMIM]Cl was the most effective catalytic system for this transformation, and the yield of HMF was up to 41%. Through density functional theory (DFT) calculations they confirmed that the formation of 1:1 glucose− borate complexes facilitated the conversion pathway from glucose to fructose (Figure 14). Zhang et al. found that germanium(IV) chloride in ILs could catalyze the direct conversion of glucose into HMF, and HMF yield of 48.4%

ethanol, ethylene glycol, and 1,3-dipropanediol), they suggested that the five-membered-ring chelate complex of the Sn atom and two neighboring hydroxyl groups in glucose may play a key role for the formation of the enol intermediate, which represents a crucial step in the dehydration of glucose to HMF (Figure 13). Ståhlberg et al. used various lanthanide salts as catalysts in several imidazolium-based ILs for the direct transformation of glucose.128 Ytterbium chloride or triflate offered the best catalytic activity in alkylimidazolium chlorides including [EMIM]Cl, [BMIM]Cl, 1-hexyl-3-methylimidazolium chloride ([HeMIM]Cl), and 1-octyl-3-methylimidazolium chloride ([OMIM]Cl). However, the highest yield of HMF was only around 24% in these catalytic systems. The HMF yield increased with the increasing hydrophobicity of the imidazolium ring, which was M


Chemical Reviews

Review

(HPA)/boric acid (B(OH)3) could effectively catalyze the conversion of glucose into HMF with a good yield of 51.9% in [BMIM]Cl.134 In this catalytic system, boric acid may promote the formation of 1,2-enediol, which can not only be directly dehydrated to HMF catalyzed by 12-tungstophosphoric acid but also be indirectly dehydrated to HMF through fructose promoted by 12-tungstophosphoric acid and boric acid (Figure 16). Furthermore, they developed an efficient process for dehydration of glucose into HMF using CrCl3 and boric acid as double catalyst in [BMIM]Cl, and a high HMF yield of 78.8% was achieved.135 The fact that boric acid was favorable to this transformation may be attributed to its promoting effect for the glucose to fructose isomerization as fructose is more readily to be dehydrated to yield HMF. To address the issues associated with catalyst separation, heterogeneous catalysts for dehydration of glucose to HMF have attracted much interest in the past few years. In this aspect, Zhang et al. developed hydroxyapatite-supported chromium chloride (Cr-HAP) for dehydration of glucose into HMF in [BMIM]Cl.136 They found that a HMF yield of 40% could be obtained in 2.5 min under microwave irradiation, and the CrHAP/[BMIM]Cl catalytic system could be reused at least for five cycles. The transformation from glucose to HMF could also be catalyzed by acidic ion-exchange resins in [BMIM]Cl.137 The yield of HMF could reach 53% when Dowex 50W ion-exchange resins were used as the catalyst. Furthermore, chromium(0) nanoparticles (Cr0-NPs) generated in situ from Cr(CO)6 could be considered as a strong Lewis acid for dehydration of glucose in [EMIM]Cl.138 The reaction mechanism for the transformation catalyzed by Cr0-NPs was similar to those over other Lewis acids such as CrClx. Hu et al. found that a variety of zeolites could dehydrate glucose to HMF heterogeneously in [BMIM]Cl.139 The results indicated that Hβ-zeolite with a unique BEA structure and a moderate Si/Al ratio of 25 resulted in the highest HMF yield of 50.3% with 80.6% glucose conversion. This was achieved through the synergistic catalytic effect between Lewis acid sites and Brønsted acid sites of Hβ-zeolite. Moreover, a tin-

Figure 13. Proposed processes of glucose conversion to produce HMF catalyzed by SnCl4 in [EMIM]BF4. Adapted from ref 127 with permission. Copyright 2009 Royal Society of Chemistry.

could be obtained in GeCl4/[BMIM]Cl catalytic system by the addition of 5 Å molecular sieves to remove water from the system.130 13C NMR characterization indicated that there was a dedicated interaction between glucose and GeCl4 and that fructose was formed during the reaction (Figure 15). Recently, cheap aluminum alkyl or alkoxy compounds were found to be good Lewis acid catalysts for dehydration of glucose into HMF in [EMIM]Cl. The yield of HMF up to 51% could be obtained in the AlEt3/[EMIM]Cl catalytic system, which was much higher than that obtained in AlCl3/[EMIM]Cl (1.6%).131 Additionally, ScCl3 in [BMIM]Cl132 and HfCl4 in [BMIM]Cl133 have also been reported as effective catalytic systems for dehydration of glucose into HMF (Table 1). The combination of two catalytic active components is an efficient strategy for dehydration of glucose in ILs. Hu et al. found that the paired synergetic catalysts of 12-tungstophosphoric acid

Figure 14. Putative mechanism for the dehydration of glucose to HMF in imidazolium chlorides with boric acid as promoter. Adapted from ref 129 with permission. Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. N


Chemical Reviews

Review

Figure 15. Proposed mechanism of HMF formation from glucose catalyzed by GeCl4. Adapted from ref 130 with permission. Copyright 2011 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 16. Putative reaction mechanism for the conversion of glucose into HMF by HPAs and B(OH)3 in [BMIM]Cl. Adapted from ref 134 with permission. Copyright 2012 Elsevier Ltd.

Figure 17. Structures of DBU-based ILs used for dehydration of glucose to HMF. (a) Adapted from ref 144 with permission. Copyright 2013 Royal Society of Chemistry. (b) Adapted from ref 145 with permission. Copyright 2014 Royal Society of Chemistry.

al. used modified mordenite (treated with 1 M NH4Cl) as a heterogeneous catalyst to catalyze the dehydration of glucose in [BMIM]Br.142 The modified mordenite with a moderate Si/Al ratio of 11.2 and a high content of strong Brønsted acid sites (TPDNH3 acidity of 1.39 mmol g−1) showed a good catalytic activity. HMF yield was 64% with 97% glucose conversion in the modified mordenite/[EMIM]Br catalytic system. The authors suggested that the quick adsorption and strong interaction of the

containing silica molecular sieve (Sn-MCM-41) was designed as the heterogeneous catalyst for conversion of glucose to HMF with a yield of 70% obtained at 110 °C in [EMIM]Br.140 The SnMCM-41 could be easily recovered and reused without a significant loss in activity. Liu et al. found that glucose could be dehydrated to HMF over Cr3+-modified ion-exchange resins in [BMIM]Cl.141 A HMF yield of 61.3% could be achieved for the first catalytic cycle at 110 °C. However, the catalytic activity significantly decreased after six recycles. Very recently, Mamo et O


Chemical Reviews

Review

IL with the zeolite, which hindered the diffusion of the products, enhanced the role of zeolite mesoporosity in the catalytic activity. Apart from the mostly employed imidazolium-based ILs, several types of novel functionalized ILs have been developed as the solvent for converting glucose into HMF. For example, Hu et al. reported that the inexpensive tetraethylammonium chloride (TEAC) could be used as an effective solvent for glucose dehydration with CrCl3 as the catalyst.143 A HMF yield of 71.3% could be achieved at 130 °C in this CrCl3/TEAC catalytic system, which was tolerant to high water content and high glucose concentration. Our research group designed a series of DBU-based (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) ILs as efficient solvents for dehydration of glucose (Figure 17).144,145 Ethyl-DBU benzenesulfonate (Et-DBUBS) was the most effective solvent for the reaction when CrCl3 was used as the catalyst, and a high yield of HMF (83.4%) could be achieved in the CrCl3/Et-DBUBS catalytic system. The slow rate of generation of byproducts directly from glucose in the CrCl3/ Et-DBUBS catalytic system contributes most to the high HMF yield. In addition, we also synthesized several morpholine-based (MMP) ILs with benzenesulfonate (BS) anion and found that SnCl4/[Et-MMP]BS was the most efficient catalytic system with HMF yield up to 67.6%.146 Chinnappan et al. reported the conversion of glucose to HMF using 1,1′-decane-1,10-diylbis(3ethylpyridinium) dibromide ([C10(EPy)2]2Br−) as the reaction medium.147 A HMF yield of 47% could be generated from glucose in the CrCl2/[C10(EPy)2]2Br− catalytic system (Figure 18). With the aim of developing an alternative to Cr/IL catalytic systems, Matsumiya et al. used deep eutectic solvents (DES) composed of choline salts and carboxylic acids as the reaction

Figure 19. Pathway for direct transformation of cellulose into HMF. Adapted from ref 150 with permission. Copyright 2011 Royal Society of Chemistry.

further converted into HMF in a similar manner to the conversion reaction catalyzed by CrCl2 in [EMIM]Cl discussed previously.124 Song et al. designed three DBU-based ILs (i.e., Nbutyl-DBU chloride (Bu-DBUCl), N-octyl-DBU chloride (OcDBUCl), and N-2-(2-hydroxyethoxy)ethyl-DBU chloride (HEOE-DBUCl), Figure 17a) and used them as solvents for the conversion of cellulose to generate HMF over CrCl3. A highest HMF yield of 41% could be obtained in the CrCl3/BuDBUCl catalytic system.144 During the catalytic dehydration reactions of cellulose, the yield of HMF could be improved by combining two metal salts in ILs. The direct conversion of cellulose into HMF was investigated in the CuCl2/CrCl2/[EMIM]Cl catalytic system by Su et al.149 The activation effect of CrCl2 on the CuCl2-rich catalyst was observed for the hydrolytic cleavage of both α- and β-1,4-glycosidic bonds. A balanced metal chlorides composition (i.e., CuCl2 and CrCl2 at XCuCl2 = 0.17) offered the highest yield of HMF (60%), which was much higher than the results obtained over single metal chlorides. Kim et al. found that the combination of CrCl2 and RuCl3 was very effective for direct transformation of cellulose into HMF in [EMIM]Cl, and the yield of HMF up to 60% could be achieved when the molar ratio of CrCl2 and RuCl3 was 4:1.150 The presence of RuCl3 in the catalytic system could enhance the hydrolytic efficiency of cellulose, thus facilitating the generation of HMF. It is noteworthy that the CrCl2/RuCl3/ [EMIM]Cl catalytic system could be applied for gram scales for the synthesis of HMF from cellulose. Furthermore, the transformation of cellulose into HMF could be effectively catalyzed by combined metal Lewis acids and acidic ILs. For instance, Ding et al. studied the transformation of cellulose into HMF using a pair of acidic ILs and CuCl2 as catalysts in [EMIM]OAc.151 Among the various acidic ILs tested (Figure 20), 1-(4-sulfonic acid) butyl-3-methylimidazolium methyl sulfate ([C4SO3HMIM]CH3SO3) showed the best performance. HMF was obtained with a yield of 69.7% in the CuCl2/[C4SO3HMIM]CH3SO3/[EMIM]OAc catalytic system. It was supposed that combination of the Brønsted acid (acidic ILs) and Lewis acid (CuCl2) could weaken the glycosidic bonds through binding with a glycosidic oxygen atom, leading to the dehydration of polysaccharides to generate monosaccharides. Meanwhile, [CuCl2(CH3SO3)n]n− complexes formed from CuCl2 and the acidic ILs could facilitate the conversion of αglucose to β-glucose and isomerization of β-glucose to fructose, which was beneficial for the generation of HMF (Figure 21). Shi et al. synthesized four dual-core sulfonic acid ILs (Figure 22) and used them in combination with MnCl2 as catalysts for direct

Figure 18. Conversion of glucose into HMF in [C10(Epy)2]2Br−. Adapted from ref 147 with permission. Copyright 2015 Elsevier B.V.

media for HMF production from glucose with boric acid as the promoter.148 They found that the DES comprising choline dihydrogen citrate and glycolic acid was the best choice, affording an HMF yield of about 60% at 140 °C by adding a small volume of water as a cosolvent. 2.2.2. Dehydration of Cellulose to HMF in ILs. Direct transformation of cellulose into HMF is more attractive but challenging compared with dehydration of glucose. In general, three steps are involved to convert cellulose into HMF: hydrolysis of cellulose into monosaccharides, isomerization of aldose-type sugars to their ketose-type counterparts, and subsequent dehydration of ketose-type sugars to HMF (Figure 19). Single metal salts could be used as efficient catalysts for direct dehydration of cellulose into HMF in ILs. Li et al. reported that CrCl3 could catalyze dehydration of cellulose effectively with an isolated HMF yield of 60% under microwave irradiation in [BMIM]Cl.126 It was suggested that CrCl3 in [BMIM]Cl could form [BMIM]n[CrCl3+n] (n = 1−3) complexes, and [CrCl3+n]n− could partially weaken the 1,4-glycosidic bonds. Hence, the water molecules have more liability to attack the cellulose macromolecule to liberate glucose and oligomers, which could be P


Chemical Reviews

Review

Figure 20. Structures of ILs prepared and used in Ding’s work. Adapted from ref 151 with permission. Copyright 2012 Elsevier Ltd.

Figure 21. (a) Putative mechanism of CuCl2 and [C4SO3HMIM]CH3SO3 promote conversion of cellulose into β-glucose. (b) Putative mechanism of CuCl2 and [C4SO3HMIM]CH3SO3 promote conversion of β-glucose into HMF. Adapted from ref 151 with permission. Copyright 2012 Elsevier Ltd.

dehydration of cellulose in [BMIM]Cl.152 The highest yield of HMF in the MnCl2/[bi-C3SO3HMIM][CH3SO3]/[BMIM]Cl catalytic system was 66.5%. It was found that the

[MnCl2(HSO4)n]n− complexes formed from MnCl2, and the acidic ILs play an important role for improving the yield of HMF, which was consistent with Ding’s work.151 Q


Chemical Reviews

Review

Figure 22. Four dual-core sulfonic acid ILs used in Shi’s work. Adapted from ref 152 with permission. Copyright 2013 Royal Society of Chemistry.

Figure 23. Proposed scheme for the conversion of cellulose to furan derivatives in the presence of ILs, H-form zeolite, and alkali metal chlorides. Adapted from ref 159 with permission. Copyright 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V.

Figure 24. Catalytic direct conversion of cellulose into sugar alcohols. Adapted from ref 162 with permission. Copyright 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

cellulose conversion. Furthermore, by cationic polymerization and subsequent sulfonation, they developed two Brønsted-acidic polymer nanotubes with hydrophobic and hydrophilic surface wettability (i.e., catalyst-110° and catalyst-10°).158 The catalyst110° showed comparable catalytic activity to catalyst-10° in [EMIM]Cl for the conversion of cellulose to HMF, although the acid strength of catalyst-10° was higher than that of catalyst-110°. This was attributed to the hydrophobicity of catalyst-110°, which was beneficial for isolation of HMF from water molecules. Thus, the side reactions of HMF were reduced. These results indicated that not only the acid strength of the catalysts affects their catalytic activity but also the surface wettability of catalysts plays an important role. Cellulose could also be converted into HMF using H-form zeolite combined with alkali metal chlorides as catalysts in [EMIM]Cl, as reported by Abou-Yousef et al.159 It was the gaseous HCl generated from cation-exchange reactions between [EMIM]+, M+, and protons on the surface of H-form zeolite that promotes the transformation. Indeed, the liberated gaseous HCl could catalyze dehydration of cellulose to HMF in the absence of water in [EMIM]Cl. Meanwhile, the resulted limited amount of water with gaseous HCl may initiate the hydrolysis of cellulose chain to afford glucose (Figure 23).

Another attractive route for the transformation of cellulose into HMF is using heterogeneous acidic catalysts in ILs.153−155 In this aspect, Zhang et al. synthesized two acid-chromic chloride bifunctionalized catalysts (i.e., ATP-SO3H-Cr(III) and HNTsSO3H-Cr(III)) by grafting −SO3H and Cr(III) onto the surface of treated attapulgite (ATP) and halloysite nanotubes (HNTs).156 These two bifunctional catalysts could catalyze the one-pot conversion of cellulose to HMF in [EMIM]Cl, and under optimized conditions, the yield of HMF up to 31.20% and 41.22% was obtained over ATP-SO3H-Cr(III) and HNTsSO3H−Cr(III), respectively. In a following study, through precipitation polymerization and Pickering emulsion polymerization, they prepared two catalyst precursors based on halloysite nanotubes (HNTs), namely, HNTs-polystyrene(PSt)polydivinylbenzene(DVB)(I) and HNTs-PSt-PDVB(II).157 After sulfonation by 98% H2SO4, two polymeric solid acid catalysts (i.e., HNTs-PSt-PDVB-SO3H(I) and HNTs-PStPDVB-SO3H(II)) were obtained for direct conversion of cellulose to HMF in [EMIM]Cl. HNTs-PSt-PDVB-SO3H(II) showed better catalytic performance than HNTs-PSt-PDVBSO3H(I) owing to the presence of more strong acidic sites in the former. This effect highlights the key role of strong acidic sites in R


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Figure 25. Process for synthesis of the boronic acid binding agent in Zhu et al.’s work. Adapted from ref 168 with permission. Copyright 2010 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 26. Catalytic conversion of cellulose to hexitols by reversible interaction over a boronic acid binding agent and an IL-stabilized Ru nanparticle catalyst. Adapted from ref 168 with permission. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Brønsted acid (originated from the HSO4−) and the binding of Cr3+ and SO42− with the glycosidic oxygen atom, which was beneficial for the decomposition of cellulose to glucose and the subsequent dehydration of glucose to HMF.

Among the catalytic systems studied, zeolite/LiCl/[EMIM]Cl showed the best performance with a HMF yield of 70.3% at 160 °C. In addition, Caes et al. developed a low-temperature, one-pot route using o-carboxyl-substituted phenylboronic acids as organocatalysts in combination with MgCl2·6H2O and mineral acids to convert cellulose to HMF in [EMIM]Cl.160 The yields of HMF obtained in this catalytic system were comparable to those catalyzed by toxic heavy metal catalysts. The highest HMF yield of 41% could be achieved in 2-methoxycarbonylphenyl boronic acid/MgCl2·6H2O/H2SO4/[EMIM]Cl catalytic system at 105 °C. Isotopic labeling studies indicated that the key aldose-toketose transformation occurred via an enediol intermediate. Zhou et al. designed two bifunctional metallic ILs (i.e., Cr([PSMIM]HSO4)3 and CuCr([PSMIM]SO4)5) through the reaction between 1-(3-sulfonic acid) propane-3-methylimidazole hydrosulfate ([PSMIM]HSO4) and CrCl3 or CrCl3−CuCl2.161 Cr([PSMIM]HSO4)3 showed higher catalytic activity than CuCr([PSMIM]SO4)5 and [PSMIM]HSO4 for the conversion of cellulose to HMF in [BMIM]Cl due to its bifunctionality and higher Brønsted acidity. The maximum yield of HMF was 53% over Cr([PSMIM]HSO4)3 at 120 °C. The mechanism study indicated that the glycosidic bonds in cellulose were weakened by

2.3. Hydrogenation of Cellulose to Sugar Alcohols in ILs

Sugar alcohols including sorbitol and mannitol could be derived from cellulose via hydrogenation (Figure 24).162−166 These alcohol compounds are widely used as food additives and are also important precursors to synthesize fuels and value-added chemicals. The hydrogenation of cellulose to produce sugar alcohols in ILs has also been reported by different research groups. Yan et al. reported that cellulose could be converted to sugar alcohols in [BMIM]Cl catalyzed by Ru nanocluster.167 Later, Zhu et al. developed a highly efficient and selective approach to produce hexitols from cellulose via hydrogenation in ILs.168 They synthesized a conjugate from an ionic liquid moiety and a boronic acid binding agent (Figure 25), which was very effective for catalyzing the transformation of cellulose to hexitols in combination with IL-stabilized Ru nanoparticles. This catalytic system showed a high selectivity toward sorbitol, and the highest yield of sorbitol around 94% was achieved using [BMIM]Cl as S


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Figure 27. Scheme for the transformation of cellulose to levulinic acid (LA).

reduce the activation energy, thus accelerating the glucose conversion. 2.4.2. Conversion of Cellulose to LA in ILs. Conversion of cellulose into LA requires acidic environment and generally involves three major steps: (i) hydrolysis of cellulose to glucose, (ii) conversion of glucose to HMF, and (iii) hydration of HMF to generate LA (Figure 27). Using SO3H-functionalized ILs, highly selective transformation of cellulose to LA could be achieved under microwave irradiation.176 For the 1-methyl-3-(3-sulfopropyl)imidazoliumbased ILs, their catalytic activity depends on the anions and follows the order HSO4− > CH3SO3− > H2PO4−, which is in good consistency with their acidity order. The effect of different kinds of SO3H-functionalized cations (i.e., imidazolium, pyridinium, and ammonium) on the catalytic activity was negligible. Furthermore, no obvious variation on the formation of LA could be observed by changing the chain length of the alkyl chain near the sulfonic group (e.g., from propyl to butyl). The maximum yield of LA of about 55% was obtained at 160 °C under microwave irradiation in 1-methyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate ([C3SO3HMIM]HSO4). Similar results were also reported by Shen et al.177 for the SO3Hfunctionalized 1-(4-sulfonic acid) butyl-3-methylimidazolium ([BSMIM])-based ILs; their catalytic activity depends on the anions and follows the same trend in accordance with their acidity (CF3SO3− > HSO4− > OAc−). The highest yield of LA was around 45.1% at 120 °C in the H2O/[BSMIM]CF3SO3 catalytic system. Moreover, it was found that InCl3 had a negative impact on the formation of LA in comparison with the catalytic systems without InCl3. For the catalytic systems with InCl3, the side reactions became prominent and byproducts including lactic acid and acetic acid were generated.

the solvent and sodium formate as the hydrogen source. The yield of sorbitol was around 89% when hydrogen was used instead of sodium formate. The authors suggested that the synthesized boronic acid-based receptor could conjugate with cellulose through the 1,2-diols present along the cellulose chains (Figure 26), which could promote the hydrolysis of cellulose to glucose and subsequent hydrogenation of glucose to sorbitol. Reductive splitting of cellulose to sugar alcohols was reported by Ignatyev et al.169 It was found that the combination of a heterogeneous metal catalyst (Pt/C or Rh/C) and a homogeneous ruthenium catalyst (HRuCl(CO)(PPh3)3) could efficiently catalyze the hydrogenation reaction of cellulose to sugar alcohols, with a very small amount of KOH in [BMIM]Cl. Sorbitol was assigned to be the dominant product with yields ranging from 51% to 74%. The results indicated that the ruthenium species could facilitate depolymerization of cellulose by complexation to alcohol or diol moieties. More importantly, the homogeneous catalyst (HRuCl(CO)(PPh3)3) acted as a hydrogen transport agent in the IL via the formation of hydride compounds, thus improving the efficiency of cellulose hydrogenation to afford sugar alcohols. 2.4. Conversion of Glucose and Cellulose to Levulinic Acid in ILs

Through acid-catalyzed hydrolysis processes, glucose and cellulose can be transformed into levulinic acid (LA),170−174 which can be used to produce various value-added chemicals, polymers, flavor substances, and fuel additives. In terms of converting glucose and cellulose into LA, several kinds of acidfunctionalized ILs as alternatives for mineral acids such as HCl and H2SO4 have been developed. In addition to its role as catalysts, these acid-functionalized ILs also served as the solvent for the transformation. 2.4.1. Conversion of Glucose to LA in ILs. For the conversion of glucose into LA, Ramli et al. prepared three acidic functionalized ILs including 1-butyl-3-methylimidazolium tetrachloroferrate ([BMIM]FeCl4], 1-sulfonic acid-3-methylimidazolium chloride ([SMIM]Cl), and 1-sulfonicacid-3-methylimidazolium tetrachloroferrate ([SMIM]FeCl4).175 Combined with a suitable amount of water (weight ratio of water and ILs 1.5:1), glucose could be effectively converted into LA using these three ILs. Among them, [SMIM]FeCl4 was the most effective catalyst for this conversion with a LA yield of 68% obtained from glucose. The high catalytic activity of [SMIM]FeCl4 was due to its stronger acidity and the presence of both Brønsted and Lewis acid sites in its structure. Additionally, the kinetic analysis implied that introducing functional groups into [SMIM][FeCl4] could

2.5. Conversion of Glucose and Cellulose to Lactic Acid in ILs

Lactic acid has been recognized as a commodity chemical, which is widely used in food and pharmaceutical industries and to fabricate biodegradable plastics and fine chemicals. Nowadays, lactic acid is primarily produced through the fermentation of starch in industry. Direct chemical transformation of glucose and cellulose to produce lactic acid provides a more attractive strategy,178−182 especially using ILs as the reaction media. In this context, Huang et al. investigated the transformation of glucose to lactic acid in hydrophobic imidazolium-based ILs using basic catalysts including Ca(OH)2, Sr(OH)2, Ba(OH)2, MgO, CaO, SrO, LiOH, and NaOH.183 The ILs offered a unique solvent effect for glucose and bases and promoted the conversion of glucose into lactic acid. The highest yield of lactic acid around 97% was achieved in the Ca(OH)2/[OMIM]Cl catalytic system T


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under the optimal conditions (temperature 90 °C; time 1 h). The generated calcium lactate could be readily separated from the catalytic system with the addition of water. Following this work, the same authors reported that lactic acid could be catalytically generated from cellulose using SnCl4 in 1,3-dimethylimidazolium methylsulfate IL.184 The yield of lactic acid was 15.1% at 140 °C in this catalytic system. A mechanism study revealed that the catalytic systems containing SnCl4 could promote the hydrolysis of cellulose to glucose, which was subsequently isomerized to fructose and transformed to lactic acid. In addition, this catalytic system was also capable of converting glucose to lactic acid with a yield of about 65%.

Hydrogenation of HMF has drawn significant attention in the past few years, from which a series of valuable chemicals such as 2,5-dimethylfuran, 2,5-dihydroxymethylfuran, and 2,5-dimethyltetrahydrofuran could be derived. Chidambaram et al. investigated the catalytic effects of carbon-supported Pd, Pt, Ru, and Rh for the hydrogenation of HMF in [EMIM]Cl.185 As illustrated in Figure 29, the six principal products were assigned to 5-methylfurfural (MF), 2,5-dihydroxymethylfuran (DHMF), 5-methylfurfuryl alcohol (MFA), 2,5-dimethylfuran (DMF), 5methyltetrahydrofurfuryl alcohol (MTHFA), and 2,5-hexadione (HD). In the most effective catalytic system (Pd/C/[EMIM]Cl), the conversion of HMF was 19% and the selectivity of the desired product (DMF) was only 13% at 120 °C for 1 h. By adding acetonitrile into [EMIM]Cl, the conversion of HMF could be increased to 47% with a higher selectivity toward DMF (32%). The low conversion of HMF and selectivity of the desired product in these catalytic systems was mainly attributed to the relatively low reaction temperature, short reaction time, as well as the low solubility of hydrogen in ILs. In addition, the authors pointed out that the low yield of DMF might partially result from the inhibiting effect of generated DHMF. As shown in Figure 30, HMF could be transformed to 2,5dihydroxymethylfuran (DHMF) and 5-hydroxymethylfuranoic

2.6. Conversion of Cellulose-Derived HMF in ILs

As mentioned previously, HMF represents an important cellulose-derived platform molecule, which can be transformed into many kinds of value-added chemicals through diverse selective reactions (Figure 28). Conversion of HMF in ILs is an interesting field, and transformation of HMF is generally considered as an important aspect in terms of cellulose valorization.

Figure 30. Cannizzaro reactions of HMF to DHMF and HMFA. Adapted from ref 186 with permission. Copyright 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V.

acid (HMFA) via Cannizzaro reaction (base-induced disproportionation reaction of an aldehyde lacking a hydrogen atom at an α-position to the carbonyl group).186 Six ILs including 1butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM]TFSI), 1-ethyl-3-meth ylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]TFSI), 1-butyl-3methylimidazolium tetrafluoroborate ([BMIM]BF4), 1-butyl-3methylimidazolium hexafluorophosphate ([BMIM]PF6), 1-

Figure 28. Various value-added chemicals derived from HMF. Adapted from ref 118 with permission. Copyright 2013 American Chemical Society.

Figure 29. Reaction pathway for the hydrogenation of HMF in [EMIM]Cl and acetonitrile solvent using Pd/C as catalyst. Adapted from ref 185 with permission. Copyright 2010 Royal Society of Chemistry. U


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authors proposed the carbene catalytic cycle for this upgrading transformation of HMF (Figure 32).

methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([PMPyrr]TFSI), and N,N-diethyl-N-methyl-N-(2methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (N122,201-TFSI) were prepared and used as solvents for this transformation (Figure 31). The highest yields of DHMF

3. CONVERSION OF HEMICELLULOSE IN ILS Hemicellulose represents the second largest fraction (20−35%) of lignocellulosic biomass and surrounds/interacts with cellulose through hydrogen bonding and acts as a linkage between cellulose and lignin.188,189 Unlike cellulose, which only contains glucose in its structure, hemicellulose has a heteropolysaccharide makeup and is a branched polymer consisting of various polymerized monosaccharides (Figure 1). Major constituent monosaccharides include C5 sugars (pentoses) such as xylose, arabinose, and C6 sugars (hexoses) such as mannose, glucose, and galactose, as well as some uronic acids (e.g., glucuronic acid).190 The ratio of these constituent monomers depends on the origins of biomass, while in general xylose represents the most abundant one. Hemicellulose has lower molecular weights than cellulose with only around 150 repeating monosaccharide units compared to that of repeating glucose units in cellulose (5000−10 000).191 Also different from cellulose, hemicellulose is mainly amorphous due to its random polymerized nature and the presence of side chains, which prevents the formation of ordered rigid crystalline structures and renders it much more susceptible/ reactive and less stable than cellulose.192 Similar to cellulose, hemicellulose could also be converted into various chemicals and fuels through a series of reactions. Herein, we only focus on the transformation of hemicellulose using ILs as the reaction media.

Figure 31. Structures of the six ILs used in Cannizzaro reactions of HMF. Adapted from ref 186 with permission. Copyright 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V.

(100%) and HMFA (84%) were obtained in [EMIM]TFSI with 5 equiv of NaOH. Imidazolium-based ILs showed better performance than ammonium-based ILs as both the polarizability and the mobility of the latter was lower. It is noteworthy that in the study the TFSI-based ILs could be recovered almost quantitatively. A rapid, highly selective manner for the conversion of HMF into DMHF (a promising C12 kerosene/jet fuel intermediate) with high yields was reported by Liu et al. using N-heterocyclic carbenes in [EMIM]OAc.187 The yield of DMHF was up to 98% (determined by HPLC or NMR) or 87% (unoptimized, isolated yield) under industrially favorable conditions (i.e., ambient atmosphere and 60−80 °C). Through mechanistic studies, the

3.1. Hydrolysis of Hemicellulose to Sugars in ILs

Hydrolysis is an essential step for converting hemicellulose into value-added chemicals and fuel products.193−196 In the context of transforming hemicellulose into value-added products, hydrolysis of hemicellulose into sugars using ILs as the reaction media has also attracted attention. For example, Enslow and Bell reported that hemicellulose (xylan) could be hydrolyzed to yield xylose (yield 90%) with dehydration products (5 wt %) and humins (4 wt %) at 80 °C in the H2SO4/[EMIM]Cl catalytic

Figure 32. Proposed catalytic cycle for umpolung self-condensation of HMF to DHMF by a catalytic IL [EMIM]OAc. Adapted from ref 187 with permission. Copyright 2012 Royal Society of Chemistry. V


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Figure 33. Possible reaction mechanism for xylose isomerization into xylulose using MnCl2/PEG-OSO3H as a catalyst in [BMIM]PF6. Adapted from ref 208 with permission. Copyright 2014 Royal Society of Chemistry.

furfural. In the AlCl3·6H2O/ChCl/oxalic acid catalytic system, the yield of furfural was 32.4% and the conversion of xylose was 74.3% at 100 °C. The difference between the catalytic activity of metal chlorides was attributed to the ionization potential of the metal cations. In this catalytic system, ChCl/oxalic acid acted as both a Brønsted acid catalyst and the reaction medium. Meanwhile, metal chlorides promoted the enolization of xylose and induced xylose dehydration to form furfural via the xylose isomerization to xylulose. 3.2.2. Homogeneous Brønsted Acids for Conversion of Xylose and Hemicellulose to Furfural in ILs. Sievers et al. reported that H2SO4 could catalyze the dehydration of xylose to furfural in [BMIM]Cl.207 However, the highest furfural yield was only 13% due to the formation of solid degradation products. Furthermore, polyethylene glycol (PEG)-bound sulfonic acid (PEG-OSO3H) could catalyze the conversion of pentoses to furfural in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6).208 With the addition of MnCl2 as a cocatalyst, a furfural yield of 75% could be achieved from xylose over PEGOSO3H. In the MnCl2/PEG-OSO3H/[BMIM]PF6 catalytic system, MnCl2 promoted the isomerization of xylose to xylulose, which was also catalyzed by PEG-OSO3H and represented the key step for dehydration of xylose to furfural (Figure 33). Hence, a catalytic system that contains both Brønsted acid and Lewis acid which could combine the isomerization process with the dehydration step could efficiently promote the conversion of xylose into furfural. 3.2.3. Solid Acids for Conversion of Xylose and Hemicellulose to Furfural in ILs. H3PW12O40, Amberlyst-5, and NKC-9 (macroporous styrene-based sulfonic acid resin) were used as solid catalysts by Zhang et al. for the generation of furfural from xylose and xylan in [BMIM]Cl under microwave irradiation.209 H3PW12O40 showed the highest catalytic activity with a furfural yield of 82.7% in comparison with Amberlyst-5 and NKC-9. This trend of catalytic activity of the solid acids was in accordance with that of their total acid sites when the weight of catalysts was equivalent. However, the selectivity of furfural from H3PW12O40 was lower than that from Amberlyst-15 and NKC-9 owing to the difference in the Brønsted to Lewis acid ratios of the three catalysts. In general, catalysts with a higher number of Lewis acid sites were more effective for the formation of furfural, but its selectivity depended on the Brønsted acid sites.210 Therefore, the highest yield of furfural was obtained over H3PW12O40, which has a higher number of both Lewis acid sites

system when water was added stepwise.197 For the hemicellulose hydrolysis reaction, kinetic studies revealed that the initial rate dependence is zero order with respect to water and first order with respect to the concentrations of both free protons and β-1,4glycosidic linkages. The activation energy of hemicellulose hydrolysis was lower than that of cellulose hydrolysis. A similar trend was also observed for the degradation of xylose and glucose (xylose was more susceptible to degrade than glucose). As mentioned above, this was attributed to the inherent structural differences between hemicellulose and cellulose. 3.2. Conversion of Xylose and Hemicellulose to Furfural

Furfural is a highly versatile and key derivative from hemicellulose with diverse applications. It could be used as the precursor for the production of various chemicals and materials, a solvent, and a selective extraction agent, etc.35,198−202 In comparison with aqueous-based processes for the production of furfural from hemicellulose, direct catalytic transformation in ILs provides an effective alternative process. 3.2.1. Metal Salts for Conversion of Xylose and Hemicellulose to Furfural in ILs. Zhang et al. used various mineral acids (H2SO4, HCl, and H3PO4) and metal chlorides (i.e., CrCl3, CuCl2·2H2O, CrCl3/LiCl, FeCl3·6H2O, LiCl, CuCl, and AlCl3) as catalysts for conversion of xylan into furfural under microwave irradiation in [BMIM]Cl.203 Among these catalysts, AlCl3 showed the best performance for furfural production with yields of furfural derived from xylan and xylose of 84.8% and 82.2%, respectively. A mechanism study indicated that the [AlCln](n−3)− complexes formed between AlCl3 and [BMIM]Cl, in a similar manner to that of LnCl3,204 could weaken the glycosidic bonds by bonding with a glycosidic oxygen atom and promote the hydrolysis of xylan to xylose. Subsequently, αxylopyranose anomers were converted into β-xylopyranose anomers promoted by the complexes through hydrogen bonding between the chloride anions and the hydroxyl groups in xylose. Then the cyclic aldoses reversed to acyclic form, and xylose was isomerized to form an enolate structure led by [AlCln](n−3)−, followed by dehydration to produce furfural. In another study, the catalytic conversion of xylose and xylan to furfural in a deep eutectic solvent formed by combing biorenewable choline chloride (ChCl) and oxalic acid was investigated in the presence of trivalent metal chlorides (i.e., CrCl3·6H2O, FeCl3·6H2O, AlCl3·6H2O, CeCl3·7H2O, and LaCl3·7H2O).205,206 Among the tested metal chlorides, AlCl3· 6H2O was the most efficient one for the conversion of xylose to W


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monolignols further generate corresponding phenyl paranoids, p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) lignin subunits, in which forms they are incorporated into lignin macromolecules.214 During the biosynthesis of lignin, various cross-linkages (e.g., β-O-4, α-O-4, 4-O-5) and C−C interunit linkages (e.g., β-1, β-5, β-β, 5-5) are formed. Among them, the βO-4 linkage represents the most abundant one, constituting approximately 50−65% of the structure (Figures 36 and 37).213,215 Lignin, as the only abundant and renewable aromatic resource in nature, represents a valuable feedstock for the generation of chemicals and fuel products.5,216 To this end, considerable efforts have been devoted to investigate the viability of recovering aromatics and hydrocarbons from lignin through depolymerization and selective conversion. Due to the complex chemical composition of lignin, many studies associated with catalytic lignin conversion employ lignin model compounds that possess predominant bonds and structures present in the lignin macromolecular matrix as feedstocks to elucidate possible reaction mechanisms and explore optimum reaction conditions. The major strategies currently applied for chemical depolymerization and transformation of lignin into lower molecular weight products mainly includes acid- and/or base-catalyzed depolymerization,32,217−219 hydroprocessing (including hydrogenation,220,221 hydrodeoxygenation,222−224 and hydrogenolysis225−229), oxidation,230−232 thermochemical treatments (e.g., pyrolysis and gasification),22,233 and biocatalytis.234 There are several comprehensive review articles available from the literature with regard to valorization of lignin for chemical and fuel products.21,235−237 Herein, we review the catalytic chemical transformation of lignin and its model compounds in ILs in the following order: acid/base-catalyzed depolymerization, hyroprocessing, and oxidation.

and total acid sites, and the Amberlyst-15 which contains more Brønsted acid sites offered the highest selectivity toward furfural. Later, Wu et al. studied the conversion of xylose to furfural using lignosulfonic acid (LS, Figure 34) as a homogeneous

Figure 34. Representative structure of lignosulfonic acid (LS). Adapted from ref 211 with permission. Copyright 2014 American Chemical Society.

Brønsted acid catalyst in [BMIM]Cl.211 In the LS/[BMIM]Cl catalytic system, the yield of furfural was 21% at 100 °C for 1.5 h. This combination of biomass-based feedstock (xylose) and biomass-derived catalyst (LS) as well as a green solvent (IL) represents a potential pathway to realize the green conversion of xylose to furfural. 3.2.4. Acidic ILs for Conversion of Xylose and Hemicellulose to Furfural. Furfural could be derived from xylose in an acidic IL ([BMIM]HSO4) which simultaneously serves as the catalyst and solvent, as reported by Peleteiro et al.212 More than 95% of xylose was converted at 120 or 140 °C, while only 36.7% of them was transformed to furfural. In order to improve the yields of furfural, biphasic reaction systems (using toluene, methyl-isobutyl ketone, or dioxane as extraction solvents) were employed for this transformation. At 140 °C, the yields of furfural increased to 73.8%, 80.3%, and 82.2% using the abovementioned three extraction solvents, respectively.

4.1. Acid/Base-Catalyzed Depolymerization of Lignin in ILs

Acid- and/or base-catalyzed depolymerization has been for a long time studied and applied for depolymerization and generation of simple aromatic monomers from lignin under relatively mild conditions. To date, most reported work in this field focused predominantly on the hydrolytic cleavage of the aryl−alkyl ether bonds (e.g., β-O-4 bonds), since these bonds are the weakest linkages in lignin structure in comparison with others such as aryl−aryl ether bonds, phenolic C−O bonds, and C−C bonds between aromatic moieties (Figure 37). Also, cleavage of aryl−alkyl ether bonds of lignin represents a dominant reaction in acidic and/or alkaline delignification of wood and other lignocellulosic biomass.238 4.1.1. Acid-Catalyzed Hydrolysis of Lignin and Its Model Compounds. In this regard, acid catalysts such as Lewis acids in ILs and functional ILs which serve both as the solvent and catalysts, have been tested for the hydrolytic cleavage of lignin and its model compounds, predominantly focused on the cleavage of β-O-4 linkages. Lewis acids such as metal chlorides could catalyze the depolymerization of lignin and related model compounds in water or low molecular alcohol to generate phenols with relatively lower yields. It has been found that the catalytic activity of these Lewis catalysts are more efficient when ILs are used as the reaction media for the transformation of lignin and related model compounds.235,239 For example, two β-O-4-type lignin model compounds (phenol-based guaiacylglycerol-β-guaiacyl ether (GG) and anisole-based veratrylglycerol-β-guaiacyl ether (VG)) could be hydrolyzed in [BMIM]Cl in the presence of

4. CATALYTIC TRANSFORMATION OF LIGNIN IN ILS Lignin is a renewable aromatic polymer which accounts for about 15−30% of biomass by weight. Specifically, lignin is a highly complicated cross-linked three-dimensional amorphous resin that surrounds the outer layer of polysaccharide fibers, protecting the inside carbohydrates and proteins from microbial attack and water damage while providing structural integrity and rigidity.18,213 At the molecular level, lignin represents a complex, highly branched, and recalcitrant polyphenolic macromolecule substance consisting of a wide irregular variety of “hydroxyl-” and “methoxy-”substituted phenylpropane-type units. In general, the composition and structure of lignin depends upon the type of biomass, and even different parts of plants could possess different lignin structures. Typically, three monolignol (monomer units of lignin) building blocks are present in lignin, namely, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 35). These

Figure 35. Chemical structures of the three primary building blocks of lignin (monolignols). X


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Figure 36. Representative structure models of lignin. Reproduced from ref 235 with permission. Copyright 2015 American Chemical Society.

Figure 37. Six common linkages found in lignin: A = β-O-4, B = β-5, C = β-β′, D = 5-5′, E = 4-O-5, F = β-1′. Adapted from ref 18 with permission. Copyright 2016 American Chemical Society.

Scheme 1. β-O-4 Bond Cleavage of Lignin Model Compoundsa

a

Adapted from ref 240 with permission. Copyright 2011 American Chemical Society.

water and metal chlorides (Scheme 1).240 The Lewis acid/H2O/ [BMIM]Cl catalytic systems comprising FeCl3, AlCl3, and CuCl2 are very effective for cleavage of β-O-4 bonds in GG to afford guaiacol (yield 69−80%) with 100% conversion at 150 °C for 2 h reaction (GG 10 mg; IL 100 mg; metal catalyst 5 mol % to GG; H2O 32:1 to GG by mol). For the nonphenolic lignin model compound VG, it is more resistant to degrade than GG under identical conditions. Only around 75% of the β-O-4 bonds in VG

were reacted with water to liberate guaiacol in AlCl3/H2O/ [BMIM]Cl catalytic system at 150 °C for 4 h (AlCl3 10 mol % to VG). Hibbert’s ketones were generated in both cases due to the repolymerization and recondensation of unstable intermediates. The hydrochloric acid formed from in situ hydrolysis of the metal chlorides catalyzed the hydrolysis reaction, and no reaction was observed without addition of water. The fact that GG is more susceptible toward hydrolysis is partially attributed to its Y


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Scheme 2. Proposed Acid-Catalyzed Mechanism for Hydrolysis of the β-O-4 Bonds of GG and VGa

a

Adapted from ref 241 with permission. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

treat either GG or VG from 8 wt % to 32 wt % without obvious loss of activity and also demonstrates excellent reusability. It is noteworthy that prior to this study, Kubo et al. also observed and studied in detail the formation of EE as a major dehydration product from GG in dialkylimidazolium-based chloride and acetate ILs including [BMIM]Cl, [AMIM]Cl, and [EMIM]OAc. In this study, guaiacol was not obtained as these nonacidic ILs were not capable of cleaving the β-O-4 bond of GG without additional catalysts.243 Oak wood-derived lignin (extracted by [EMIM]OAc from oak wood) was also subjected to [HMIM]Cl for depolymerization. An increase in the processing temperature from 110 to 150 °C leads to accelerated reaction rate but does not obviously reduce the sizes of remaining lignin fragments as characterized by gel permeation chromatography (GPC). NMR and IR analysis demonstrates that the lignin was depolymerized via hydrolysis of the alkyl−aryl ether linkages, which is inconsistent with the above-mentioned model compound study using GG and VG as substrates.244 Furthermore, by exploring the reactivity of GG and VG in a variety of 1-methylimidazolium-based acidic ILs with chloride, bromide, and hydrogen sulfate as the anions ([HMIM]Cl, [HMIM]Br, and [HMIM]HSO4) as well as a dialkylated IL ([BMIM]HSO4), it was revealed that not only the acidic environment of the ILs affects the yield of the major product guaiacol but also the nature of the anions and their interactions with the model substrates significantly influence the reaction efficiency and mechanism.245 Given the fact that the ability of the above-mentioned ILs for the cleavage of the β-O-4 bonds to afford guaiacol ([HMIM]Cl > [BMIM]HSO4 > [HMIM]Br > [HMIM]HSO4 > [HMIM]BF4) does not necessarily correlate to

phenolic nature as it possesses a phenolic hydroxide group which could serve as an additional proton donor and interact with metal chlorides to generate more hydrochloric acid, thus further increasing the acidity of the catalytic system. The same research group also explored the acidic hydrolysis of GG and VG using a Brønsted-acidic IL 1-H-3-methylimidazolium chloride ([HMIM]Cl) as both an acid catalyst and solvent.241 Both GG and VG underwent catalytic hydrolysis to afford guaiacol as the primary product with yields more than 70% at 150 °C. During hydrolysis of GG, an enol ether, 3-(4-hydroxy3-methoxyphenyl)-2-(2-methoxy-phenoxy)-2-propenol (EE), was formed as an intermediate, and in the case when VG was used as the substrate, an EE analogue (VEE) was produced (Scheme 2). From the experimental results, it was speculated that both monomeric and dimeric intermediates were involved before guaiacol was liberated. More specifically, guaiacol was formed directly either from GG and VG monomers or from EE (or VEE) and GG (or VG) dimers during the reaction. In the latter case, Hibbert’s ketones were generated as a side product. The mechanism and kinetics for the formation of Hibbert’s ketones in acidic environments has been intensively investigated.242 When lignin model compounds containing β-aryl ether bonds are used as substrates in acidic environments, these compounds initially undergo an acid-catalyzed dehydration reaction which is also the rate-determining step for hydrolysis of these compounds to afford EE as an intermediate product. Subsequently, EE could be hydrolyzed to yield guaiacol and α-ketocarbinol. Under the reaction conditions, the α-ketocarbinol could be gradually transformed to a mixture containing four compounds entitled “Hibbert’s ketones” through allylic rearrangement and intermolecular redox reactions.242 The acidic IL [HMIM]Cl could Z


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Scheme 3. Pathways of GG and VG Degradation in Acidic ILs; Analogous Chemistry Occurs with GG and VG Dimersa

a

Adapted from ref 245 with permission. Copyright 2011 Elsevier Ltd.

Figure 38. Chemical structures of [BDMIM]Cl, TBD, MTBD, TMG, and DBU.

Figure 39. Possible reaction pathways on the conversion of GG into the primary products.

hydrolysis mechanism (Scheme 3A). In the case where ILs containing less coordinating anions such as BF4− were used, significantly higher amounts of vinyl ether (VE) were formed instead of EE. VE and formaldehyde were produced through deprotonation of the γ-hydroxyl group followed by the cleavage of the γ-carbon (Scheme 3B). As the stability of VE is higher than that of EE, ILs that could promote the formation of EE (i.e., those with stronger coordinating abilities) tend to afford higher yields of guaiacol. 4.1.2. Base-Catalyzed Hydrolysis of Lignin and Its Model Compounds in ILs. Maybe attributed to the instability of most common dialkyl-substituted imidazolium-based ILs under basic conditions,246 much less work has been reported for the base-mediated hydrolysis of lignin and related model

the Hammett acidity of the ILs ([HMIM]Cl > [HMIM]BF4 > [HMIM] HSO4 ≈ [HMIM]Br ≈ [BMIM]HSO4), it was inferred that the ability of the anions to coordinate with the alcohol groups on the model compounds through hydrogen bonding has significant impacts on the reaction efficiency and pathway. This coordination effect could stabilize the generated intermediates and facilitate the cleavage of the ether bonds and liberation of guaiacol. As illustrated in Scheme 3, when ILs containing strong coordinating anions such as chloride, bromide, and hydrogen sulfate were used as reaction media, the hydroxyl group on the model compound was stabilized and deprotonation was prevented through coordination. In this situation, the enol ether (EE) was assigned to be the predominant intermediate. The reaction proceeds through a typical Brønsted acid-catalyzed AA


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Table 2. Catalytic Hydroprocessing of Lignin and Its Model Compounds in ILs

AB


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Table 2. continued

a

T = temperature, t = time, P = pressure. bConv. = Conversion. cThe tested metal NPs include Pd, Pt, Rh, and Ru. The conversion and yield data reported herein are the best results. For detailed information please refer to ref 265. dSelectivity (%). eYield (%). f[HTMA]MeSO3 = hexadecyltrimethylammonium methanesulfonate. g[TBA]MeSO3 = tetrabutylammonium methanesulfonate.

smaller molecular weights such as lignin oligomers, phenols, and other high-value products. It is also a commonly applied protocol for the upgrading of biomass-derived small chemical compounds such as bio-oils resulted from pyrolysis of biomass to hydrocarbon fuel products. With the aim to preserve and make full use of the aromatic nature of lignin, the catalytic reduction of lignin and/or its model compounds into aromatic products is receiving increasing attention.236 In the context of hydroprocessing of lignin, a major aspect that must be taken into consideration is that the reduction reactions caused by hydrogen are in many cases not controllable and cannot be avoided. Hence, side hydrogenation reactions of desired products may become the prominent reaction, especially in the presence of metal in the catalytic system which could act as a catalyst for the reduction.228 It is crucial to address this effect in terms of product quality control and hydrogen economy, which is of particular importance for the development of future biorefineries.236 In recent years, significant research advances for the hydroprocessing of lignin and/or model compounds have been reported. Common routes for valorization of lignin or its downstream processing products through reductive methods include hydrogenation, hydrogenolysis, hydrodeoxygenation, hydroalkylation, and sometimes combined/integrated ones of the above-mentioned reactions. In addition, owing to the complex chemical structure of lignin, depolymerization of lignin in one single step generally results in a complicated mixture of products which are difficult to separate. The incorporation of a hydroprocessing step into the conversion process (e.g., depolymerization followed by hydrodeoxygenation) could afford less mixed products.248 According to the published work, these reactions are generally conducted in water, alcohols, and other organic solvents using various metal catalysts. Relatively fewer works associated with hydroprocessing of lignin and/or its model compounds in ILs have been reported. Herein, we review the state-of-the-art of research status of depolymerization of lignin and its model compounds through hydroprocessing using ILs as reaction media, and some results are summarized in Table 2. This section is organized in the following order according to the

compounds in comparison with acid-catalyzed hydrolysis reactions using ILs as the reaction media. Nonaqueous organic N-bases of varying basicity and structure including 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD), 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene (MTBD), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,1,3,3-tetramethylguanidine (TMG), and 1,4-diazabicyclo[2.2.2]octane (DABCO) were tested for their activity for the hydrolytic cleavage of β-O-4 type lignin model compound GG in 1-butyl-2,3-dimethylimidazolium chloride ([BDMIM]Cl).247 Figure 38 shows the chemical structures of the employed IL and organic bases. The TBD/[BDMIM]Cl catalytic system was the most effective one for the conversion of GG. Under the given conditions (GG 0.025 mmol; IL 100 mg; TBD 0.025 mmol; temperature 150 °C; time 2 h), more than 90% of GG reacted in the TBD/[BDMIM]Cl catalytic system and more than 40% of the β-O-4 bond cleavage product was obtained. However, since additional studies with lower TBD to GG ratios and longer reaction times cannot afford guaiacol with yields exceeding the amount of TBD added, the cleavage reaction was found to be noncatalytic over TBD. As illustrated in Figure 39, similar to other base-promoted conversion of β-aryl as well as α-aryl ethers, the formation of quinone methide was the key intermediate step, leading to the conversion of GG.238 According to the experimental results, it was speculated that TBD probably acts as a dibasic nucleophile due to the presence of two exposed nitrogen atoms (N1 and N2, red color), attacking both the α- and the β-carbon of quinone methide and thereby assisting the cleavage reaction of GG. Hence, in addition to its role as a base in forming quinone methide, TBD may also act as a nucleophile for the reaction. Similar to acid-catalyzed depolymerization reactions of β-aryl ethers, the enol ether (EE) was also found to be a primary decomposition product in all tested reaction systems. 4.2. Catalytic Hydroprocessing of Lignin and Its Model Compound in ILs

Selective catalytic hydrotreatment of lignin in a reducing environment (e.g., in the presence of hydrogen) represents another effective and widely applied strategy for the deconstruction of lignin macromolecule into products with AC


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Pt, and Rh) were dispersed and stabilized in ILs using a capping agent (poly(1-vinyl-3-butylimidazolium chloride-co-N-vinyl-2pyrrolidone). Various lignin derivatives including phenol, anisole, p-cresol, and 4-ethylphenol could be effectively transformed into their respective alkanes with high conversion and selectivity in either [BMIM]BF4 or [BMIM]Tf2N under mild conditions (substrate 1 mmol; solvent 1 mL; metal/substrate 300/1; acidic IL catalyst 1 in Figure 40 0.2 M in solvent; H2 40 atm; temperature 130 °C; time 4 h). The reaction proceeds through metal-catalyzed hydrogenation followed by acidcatalyzed dehydration. It was found that both the Hammett acidity of the Brønsted-acidic IL catalysts and the nature of metal nanoparticles influence the final product distribution. Furthermore, it is noteworthy that a nearly quantitative yield of ethylcyclohexane could be generated from 4-ethylphenol when Ru and Rh nanoparticles are used together under the abovementioned reaction conditions. This finding opens new opportunities to explore the synergistic and cooperating effects between different metal NPs and acidic catalysts for this kind of reaction. During acid-catalyzed cleavage of lignin and its model compounds either by Brønsted-acidic ILs or by Lewis acids, the formed intermediates are unstable and tend to repolymerize and/or recondense to afford undesirable secondary products such as Hibbert’s ketones and other higher molecular weight products, thus resulting in relatively low yields of targeted monomeric products. To address this problem, the stabilization of these reactive intermediates is recognized as a crucial step in order to maximize the yields of monomeric products.257−259 In this context, Scott et al. reported the stabilization of the aldehyde intermediate which was originated from Brønsted-acidic ILcatalyzed depolymerization of a dimeric β-O-4 type lignin model compound by in situ hydrogenation using Ru nanoparticles dispersed in IL media (Figure 41).260 The strategy here was to

nature of catalytic system: homogeneous and heterogeneous catalysis. 4.2.1. Catalytic Hydroprocessing of Lignin and Its Model Compounds in Homogeneous Catalytic Systems. Catalytic systems comprising soluble metal nanoparticles have received increasing attention due to their high catalytic efficiency.249 Unlike heterogeneous catalysts in which the metal nanoparticles are immobilized on solid surfaces, soluble nanoparticles are potentially more active with their rotational freedom and spherically symmetrical geometry.250 ILs as a kind of green solvent are promising media for soluble nanoparticle catalysts and provide an opportunity to combine the advantages of homogeneous and heterogeneous processes in a single system.251 The rotationally free catalytic centers of nanoparticles are preserved by immobilization of metal nanoparticles in the IL phase instead of a solid surface. Metal nanoparticles are stable with high activity in ILs attributed to the structural directionality (IL effects), self-organization, and an electrostatic stabilization effect.252,253 Also, owning to these unique effects, ILs represent an ideal media for the preparation of small nanoparticles and for controlling extended ordering of nanoscale structures.254 In particular, imidazolium-based ILs can act as good stabilizers for transition-metal nanoparticles. Catalytic systems involving soluble metal nanoparticles in ILs are sometimes referred to as “pseudo-homogeneous catalysis in ILs”. The potential application of these catalytic systems for hydroprocessing of lignin and its model compounds, especially upgrading of lignin-derived products, has been demonstrated by several research groups. It is noteworthy that in many cases the hydroprocessing of lignin and/or lignin derivatives is combined with other transformation protocols (e.g., acid-catalyzed depolymerization followed by hydroprocessing for further upgrading). On the basis of their previous work about hydrogenation of benzene and other arenes using rhodium nanoparticles stabilized by an IL-like copolymer (poly[(N-vinyl-2-pyrrolidone)-co-(1vinyl-3-butylimidazolium chloride)]) in [BMIM]BF4,255 Yan et al. reported a one-pot process for the conversion of ligninderived phenols into their respective alkanes employing bifunctional catalysts (metal nanoparticles combined with functionalized Brønsted-acidic ILs) and nonfunctionalized ILs including [BMIM]BF4 and [BMIM]Tf2N as solvents.256 A range of Brønsted-acidic ILs with SO3H group covalently linked to the imidazolium ring through an alkyl chain were prepared (Figure 40) and tested as acidic IL catalysts. Metal nanoparticles (Ru, Pd,

Figure 41. Structures of IL medium and Brønsted-acidic ILs (top) as well as the approach to cleavage and conversion of reactive intermediates (bottom). Adapted from ref 260 with permission. Copyright 2015 Royal Society of Chemistry.

employ an IL 1-butyl-2,3-dimethylimidazolium N,N-bis(trifluoromethylsulfonyl)imide ([BM2IM]NTf2) as solvent due to its known ability to stabilize Ru nanoparticles.261 Also, since this IL does not possess an acidic imidazolium proton, it is expected to offer controlled acidity in the reaction system. Two Brønsted-acidic ILs were employed as acidic catalysts for the deconstruction of the β-O-4 lignin model substrate. As illustrated in Figure 41, the dimeric lignin model compound was degraded

Figure 40. Brønsted-acidic ILs used in Yan et al.’s study. Adapted from ref 256 with permission. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. AD


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neous catalysis in ILs, the work reported in this field is relatively less, and this is probably attributed to mass transfer concerns especially when lignin and relatively large lignin model compounds are used as substrates. Binder et al. reported the dealkylation of a simple lignin model compound eugenol (2methoxy-4-(2-propenyl)phenol) to afford guaiacol (yield up to 11.6%) in 1-ethyl-3-methylimidazolium triflate ([EMIM]OTf) at moderate temperatures below 200 °C catalyzed by a wide variety of homogeneous or heterogeneous acid catalysts in the presence of hydrogen.266 These catalytic systems could also be used to cleave the ether bonds of 2-phenylethyl phenyl ether to generate phenol. However, they are not able to efficiently convert 4-ethylguaiacol, and no monomeric products could be obtained from organosolv lignin. Monophenolic compounds including phenol and catechol could be selectively generated through hydrogenolysis of Kraft lignin in catalytic systems consisting of choline-derived ILs and commercial Pd/C.267 The choline (Ch)-derived ionic acid acts simultaneously as a solvent and an acidic catalyst, while the Pd/C represents a metal catalyst, forming a dual acid/redox catalytic system. For the Pd/C/[Ch]MeSO3 catalytic system, under the optimum conditions (H2 2 MPa; time 5 h; temperature 200 °C; mass ratio of [Ch]MeSO3 to Pd/C 1; Pd/C loading 3.5 wt %), 20.3% of Kraft lignin was converted and selectivities to phenol and catechol were 18.4% and 18.1%, respectively. Further study using a β-O-4-type model compound (guaiacylglycerol-βguaiacyl ether) in the above-mentioned catalytic system reveals that the lignin was initially fragmented which was catalyzed by both the acid (IL) and the metal catalyst (Pd/C). Then the various C−O and C−C bonds contained in the fragmented intermediates were cleaved through acid-catalyzed reactions to afford phenol and catechol.

into guaiacol and an unstable aldehyde intermediate. The unstable intermediate was simultaneously converted to more stable monomeric products (4 and 5 in Figure 41) together with trace amounts of products resulting from hydrogenation of the benzene ring. Moreover, it was found that the aldehyde intermediate could also be effectively in situ stabilized and transformed in the presence of ethylene glycol to yield acetals such as 2-benzyl-1,3-dioxolane (Figure 42).

Figure 42. Cleavage of β-O-4 model compound 1 with IL-H+1 in IL1 in the presence of ethylene glycol (EG) to give 2-benzyl-1,3-dioxolane 8. Adapted from ref 260 with permission. Copyright 2015, Royal Society of Chemistry.

Noble metal nanoparticles are known for their high catalytic activity for both hydrogenation and C−O bond activation reactions.262−264 Recently, Chen et al. reported a pseudohomogeneous catalytic system comprising uniformly dispersed and stabilized noble metal nanoparticles in ILs in which they investigated the hydrodeoxygenation of monomeric lignin model compounds and cleavage of C−O bonds between aromatic units in dimeric lignin model compounds.265 By in situ reducing metal salt precursor using methanol in various ILs, metal nanoparticles (Pd, Pt, Rh, and Ru) were synthesized and well dispersed without aggregation in various ILs as characterized by TEM, SEM, XPS, and XRD, forming various metal nanoparticles/IL catalytic systems. The Pt/[BMIM]PF6 catalytic system was found to be the most effective for the hydrodeoxygenation of monomeric lignin model compounds (phenol and guaiacol) and C−O bond cleavage of dimeric lignin model compounds (diphenyl ether and benzyl phenyl ether) with the addition of small amounts of H3PO4 under relatively mild conditions (temperature 130 °C; time 10 h; pressure 5 MPa). All model compounds could be effectively transformed with nearly complete conversion and high selectivity (maximum around 97%) in the Pt/[BMIM]PF6 catalytic system. From the experimental results the authors proposed possible reaction mechanisms for each model compound, and the selectivity of the metal nanoparticles toward the reductive cleavage of C−O bonds and HDO (Pt > Rh ≈ Ru ≫ Pd) was attributed to the sizes of metal nanoparticles (Pd ≫ Pt > Rh ≈ Ru). Similar to many previous works using both lignin and its model compounds as substrates, the Pt/[BMIM]PF6 catalytic system is much less efficient for the depolymerization and HDO of authentic lignin samples such as dealkalized lignin and organosolv lignin under identical conditions (temperature 130 °C; time 10 h; pressure 5 MPa). This is because the C−O bonds contained in the lignin macroaromatic network are much less approachable and active than those contained in model compounds. Despite the conversions for these two lignin macromolecules being very low (less than 5%), the main product (cyclohexane) accounts for more than 50% of degraded products. 4.2.2. Catalytic Hydroprocessing of Lignin and Its Model Compounds in Heterogeneous Catalytic Systems. Hydrotreatment of lignin, lignin model compounds, and other lignin derivatives into value-added products using heterogeneous catalysts in ILs has also been reported by different researchers. Despite there being many advantages associated with heteroge-

4.3. Catalytic Oxidation of Lignin and Its Model Compounds in ILs

Oxidative cracking is another promising route for valorization of lignin and tends to generate aromatic products such as aromatic alcohols, aldehydes, and acids. In contrast to hydrolysis and catalytic reduction reactions which tend to disrupt and eliminate the chemical functionalities of lignin and its model compounds and to produce chemicals with simpler structures (e.g., phenols), depolymerization of lignin in oxidative environments affords polyfunctional aromatic compounds.21,235 In general, the oxidative products of lignin have a variety of additional functionalities. Some of these products are themselves targeted fine chemicals, and others could be further converted into useful products. The employment of ILs as reaction media for the catalytic oxidative transformation of lignin is a promising strategy due to its high solubility of both lignin and oxygen; thus, the oxidation reactions could be conducted under relatively mild conditions.268,269 In recent years, the catalytic oxidative conversion of lignin and its model compounds using ILs as the reaction media has been demonstrated by many researchers, and some of these results are summarized in Table 3. 4.3.1. Homogeneous Catalytic Oxidation of Lignin and Its Model Compounds in ILs. Lignin and its model compounds could be oxidized to afford various value-added products in homogeneous catalytic systems with ILs as the reaction media under oxidative conditions. As an example, benzylic and allylic alcohols could be oxidized to their corresponding aldehydes or ketones with excellent conversions and yields using a two-component catalyst VO(acac)2/DABCO (1,4-diazabicyclo-[2.2.2]octane) and molecular oxygen as the AE


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Table 3. Catalytic Oxidation of Lignin and Its Model Compounds in ILs

AF


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Table 3. continued

a T = temperature, t = time, P = pressure. bConv. = Conversion. cDABCO = 1,4-diazabicyclo-[2.2.2]octane. dYield (%). eThe reactions were conducted in compressed air and the number given is air pressure. fThe selectivity to these products could be tuned by adjusting the catalyst loading. g The reactions were conducted under microwave irradiation.

oxidizing agent in [BMIM]PF6 under relatively mild conditions. For instance, 4-methoxybenzyl alcohol could be oxidized to 4methoxybenzaldehyde in the presence of 5 mol % VO(acac)2 and 10 mol % DABCO at 80 °C for 6 h reaction in an oxygen atmosphere (0.1 MPa) in [BMIM]PF6 (Figure 43). Under this

Zakzeski et al. studied the catalytic oxidation of Alcell and soda lignin in catalytic systems consisting of simple cobalt(II) salts (e.g., CoCl2·6H2O) dissolved in 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM]DEP) in the presence of NaOH using molecular oxygen as the oxidant.272 The catalytic system (Co/[EMIM]DEP/OH−) is capable of oxidizing the benzyl and other alcohol functionalities to aldehyde and/or acid groups while leaving phenolic functionality, 5-5′, β-O-4, and phenylcoumaran linkages intact. A proposed plausible change of lignin structure is illustrated in Figure 45. These results were verified by ATR-IR spectroscopy and a detailed model compound study involving various aromatic compounds. Lignin model compounds including veratryl alcohol, cinnamyl alcohol, and vannilyl alcohol could be oxidized into their corresponding aldehydes and/or acids. Veratryl alcohol could be selectively oxidized to veratraldehyde in this catalytic system with a maximum turnover frequency of 1440 h−1. Cinnamyl alcohol was oxidized to cinnamaldehyde or cinnamic acid, together with products generated through disruption of the carbon−carbon double bond on the propyl chain including benzoic acid and epoxide. In contrast, the phenolic functionalities contained in phenolic lignin model compounds such as guaiacol, syringol, and vanillyl alcohol could not be converted in this catalytic system, although the benzyl groups in vannillyl alcohol were oxidized to afford vanillin as the sole product. The authors also found that the incorporation of strongly binding ligands such as tetradentate porphryin or salen ligands to the metal catalyst reduces their catalytic activity, as these strongly binding ligands might inhibit the transition of metal to the active form. Despite no monomeric aromatic compounds being detected by GC-MS during oxidation of the authentic lignin samples (i.e., Alcell and soda lignin), this approach represents a potential pretreatment route to increase the oxygen functionality in lignin or to add additional functionalities to lignin-depolymerized products. To gain deeper insight into the reaction mechanisms of aerobic oxidation of lignin and its model compounds in the Co/OH−/[EMIM]DEP catalytic system, a combined in situ spectroscopic characterization study using ATR-IR, Raman, and UV−vis spectroscopy of

Figure 43. Scheme for the conversion of 4-methoxybenzyl alcohol to 4methoxybenzaldehyde in [BMIM]PF6. Adapted from ref 270 with permission. Copyright 2006 Elsevier Ltd.

condition, the conversion and isolated yield of the desired product are 98% and 91%, respectively.270 By incorporating copper(II) 2-ethylhexanoate as a cocatalyst to the abovementioned catalytic system, 4-methoxybenzoic acid could be obtained as an overoxidized product of 4-methoxybenzyl alcohol. The yield of 4-methoxybenzoic acid could reach 77% when the reaction was conducted in 1-hexyl-3-methylimidazolium trifluoromethansulfonate ([HeMIM]OTf) in oxygen (0.1 MPa) at 95 °C for 15 h reaction (Figure 44). These reaction systems are also capable of transforming a variety of other primary alcohols to their corresponding aldehydes and acids. It is noteworthy that the catalysts and ILs could be recovered and reused at least three times without obvious loss of catalytic activity.271

Figure 44. Scheme for the conversion of 4-methoxybenzyl alcohol to 4methoxybenzoic acid in [HeMIM]OTf. Adapted from ref 271 with permission. Copyright 2007 American Chemical Society. AG


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Figure 45. Possible structural changes in lignin samples as a result of oxidation. Adapted from ref 272 with permission. Copyright 2010 Royal Society of Chemistry.

Table 4. C−O Bond Cleavage of Various 2-Aryloxy-1-aryethanols276a

conv. (%)b

yield of a (%)b

yield of b (%)b

subs.

R1

R2

R3

Cc

Md

Cc

Md

Cc

Md

1 2 3 4 5 6

H H H OCH3 OCH3 OCH3

OCH3 H OCH3 H OCH3 OCH3

H H OCH3 H H OCH3

98 96 92 94 95 94

98 97 98 98 98 97

60 46 48 21 19 25

64 65 61 41 37 29

67 52 47 30 28 28

69 66 57 50 46 41

a

Reprinted with permission from ref 276. Copyright 2015 Royal Society of Chemistry. bReaction conditions: substrate (0.2 mmol), MTO: 5 mol %, [BMIM]NTf2: 1 g, temperature: 180 °C, time: 14 h; cC = conventional heating; dM = microwave heating.

the complexes involved during the process was conducted.273 It was revealed that the alcohol-containing substrates were coordinated to the cobalt followed by the formation of Co− superoxo species and hydroxide. This coordination step was crucial for the oxidation reaction to take place. Moreover, careful choices of anions and cations of ILs are of great importance as the ILs not only stabilized the reactive intermediates formed during oxidation but also favored the coordination of substrates to cobalt over direct oxidation of the cobalt. Catalytic oxidation of organosolv beech lignin was achieved in the IL [EMIM]CF3SO3 with Mn(NO3)2 as the catalyst in compressed air.274 Under optimum conditions (84 × 105 Pa compressed air as oxidant, 100 °C, 24 h), lignin conversion reached 63% (2 wt % of Mn(NO3)2 relative to lignin) to a maximum of 66.3% (20 wt % of Mn(NO3)2). The product distribution could be tuned by adjusting the reaction conditions and catalyst loading. For instance, at low catalyst loading (e.g., 2 wt %), GC-MS analysis indicates that syringaldehyde was the major product together with vanillin, 2,6-dimethoxy-1,4benzoquinone (DMBQ), and small amounts of syringol. At high catalyst loading (e.g., 20 wt %), the selectivity of DMBQ increased significantly and was assigned to the predominant product which could be isolated from the reaction mixture as a pure substance (>95%) by precipitation in ethanol. DMBQ

represents a potential antitumor agent and could be used as a synthon in organic chemistry.275 This product was proved to be derived from overoxidation of syringaldehyde by a model compound study in which syringaldehyde was oxidized to yield DMBQ as the only product in the above-mentioned catalytic system. With the assistance of microwave heating, the oxidative depolymerization of a series of β-O-4-type dimer lignin model compounds (2-aryloxy-1-arylethanols) and birth wood-derived organosolv lignin into monomeric aromatic chemicals was investigated by Zhang et al.276 The reactions were conducted over methyltrioxorhenium (MTO) in a serious of imidazoliumbased ILs (i.e., [BMIM]Cl, [BMIM]BF 4 , [BMIM]PF 6 , [BDMIM]Cl, [BMIM]NTf2) without any other oxidants under microwave heating. It was found that the activity and selectivity of catalyst strongly depends on the employed IL. Also, the yields of products obtained in the MTO/[BMIM]NTf2 catalytic system are much higher than those obtained in other catalytic systems under both conventional and microwave heating. The β-O-4type dimer lignin model compounds could be efficiently transformed into their corresponding phenols which represent the primary products with high yields up to 69%. Microwave irradiation could facilitate and promote the cleavage of aryl ether bonds and significantly reduces the reaction time from 18 h to 2 AH


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Figure 46. Degraded organosolv lignin products under microwave irradiation within 2 min, as determined by GC-MS. Adapted from ref 276 with permission. Copyright 2015 Royal Society of Chemistry.

under a constant oxygen pressure (2.5 MPa). The structures of the employed ILs are illustrated in Figure 48. By extracting the

min. Also, as illustrated in Table 4, both the conversion and the yields were higher when the reactions were conducted under microwave irradiation. This effect is attributed to the ionic nature of ILs and their dielectric properties which promote the conversion of irradiation energy into heat. Infrared spectroscopy analysis revealed that the MTO was reduced to methyldioxorhenium (MDO) during the reaction. A series of monomeric syringyl- and guaiacyl-type phenolic compounds (Figure 46) could be produced from birth wood-derived organosolv lignin under microwave heating at constant power (240 W) at 180 °C for 2 min with a total yield 34.2 wt %. Among the various products derived from catalytic oxidation of lignin, aromatic aldehydes (e.g., vanillin, syringaldehyde, and p-hydroxybenzalde) are a group of products that are of significant interest. These aromatic aldehydes are widely used by a variety of industrial sectors for the production of flavors, pharmaceuticals, and agricultural products.277 However, in some oxidation processes, the yields of these aldehydes are not satisfactory as these compounds are susceptible to further oxidation in oxidative catalytic systems, yielding a variety of acid derivatives such as vanillic acid, syringic acid, and/or other byproducts.278 Recently, as an attempt to address this problem, an integrated approach which couples the oxidation of organosolv lignin and simultaneous extraction of the aldehyde products using methylisobutylketone (MIBK) was developed (Figure 47).279 The oxidation reactions were catalyzed by CuSO4, and a variety of ILs including [MMIM]Me2PO4, [MPy]Me2PO4, [MTEtN]Me2PO4, and [MMO]Me2PO4 were tested as reaction media

Figure 48. Structures of employed ILs in Liu et al.’s study. Adapted from ref 279 with permission. Copyright 2013 Royal Society of Chemistry.

aldehyde products into the MIBK fraction immediately when they were generated, the highest yield of these aldehydes (around 30%) was reached using the CuSO4/[MMIM]Me2PO4 catalytic system coupled with MIBK as the extracting agent. Similar to other oxidation reactions of lignin, it was supposed that the IL could stabilize and promote the catalytic activity of the transition metal salt catalyst, and the reaction proceeds via a free radical route. Moreover, the CuSO4/[MMIM]Me2PO4 catalytic system could be easily reused at least six times without obvious loss of catalytic abilities. Kumar et al. reported that veratryl alcohol could be oxidized over iron(III) porphyrins and horseradish peroxidase (HRP) using hydrogen peroxide as the oxidant in [BMIM]PF6. Veratraldehyde and 2-hydroxymethyl-5-methoxy-2,5-cyclohexadiene-1,4-dione were obtained as two major products at room temperature (Scheme 4). The catalytic activity of the iron(III) porphyrin or HRP was enhanced by immobilizing them in ILs.280 In the presence of a monomeric phenol (4-tert-butyl-2,6Scheme 4. Scheme for the Oxidative Conversion of Veratryl Alcohola

Figure 47. Batch process for the production of aromatic aldehydes from lignin with simulated countercurrent extraction. Adapted from ref 279 with permission. Copyright 2013 Royal Society of Chemistry.

a

Adapted from ref 280 with permission. Copyright 2007 Rights Managed by Georg Thieme Verlag KG Stuttgart New York.

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the conversion and yield were much lower when these two catalysts were used separately. A maximum yield of veratryl aldehyde (94.9%) was reached under optimal conditions (veratryl alcohol 0.1 g; IL 1.0 g; CuO 0.02 g; 5 wt % Ru@ZIF8 7 mg; distilled water 1.8 g; O2 0.5 MPa; temperature 130 °C; time 55 min). The yield of this product dropped after the optimum reaction time (55 min) due to side reactions such as further oxidation of veratryl aldehyde and disproportionation reactions. Taking advantage of the basic nature of the employed IL, sequential aldol condensation of veratryl aldehyde with acetone could be achieved in one pot simply by adding acetone into the reaction mixture after the oxidation reaction and release of the remaining oxygen. Catalyzed by the BIL, the maximum yield of 3,4-dimethoxybenzylideneacetone (86.4%) was reached at 85 °C for 3 h. In this study, the BIL not only acts as the reaction media for the oxidation reaction but also promotes the subsequent aldol condensation reaction to allow adjustments of carbon numbers of the final product. 4.3.3. Metal-Free Catalytic Oxidation of Lignin and Its Model Compounds in ILs. Despite the fact that most transition-metal catalysts could effectively transform lignin and its model compounds into value-added products, the use of metal generates harmful metal wastes which require additional energy and cost for their post-treatment. Also, taking into consideration that the reserves of metal (especially noble metal) are limited in earth, there is a growing interest to develop effective metal-free catalytic systems, preferably using molecular oxygen or hydrogen peroxide as the oxidant.283,284 In this context, our group recently found that the β-O-4-type lignin model compound 2phenoxyacetophenone could be converted into benzoic acid and phenol without metal catalysts in 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide ([BnMIM]NTf2) with a small amount of water and catalytic amount of H3PO4 in oxygen.285 Under optimum conditions (substrate 1 mmol; IL 1 g; 85% H3PO4 16 μL; H2O 46 μL; O2 1 MPa; 130 °C; 3 h), the model compound can be completely converted and the yields for benzoic acid and phenol are 89% and 84%, respectively. In this

dimethylphenol), organosolv lignin and Klason lignin could be extensively depolymerized into much lower molecular weight lignin oligomers in 1-ethyl-3-methylimidazolium xylenesulfonate ([EMIM]ABS) and [BMIM]MeSO4 under oxidative conditions catalyzed by Cu/EDTA (ethylenediamine-N,N,N′,N′-tetraacetic acid) through a redistribution mechanism. Despite no monomeric products being obtained directly, this approach represents a promising way to break down the lignin macromolecules into much smaller intermediates (lignin oligomers), which could be further transformed into value-added products.281 4.3.2. Heterogeneous Catalytic Oxidation of Lignin and Its Model Compounds in ILs. The catalytic oxidation of veratryl alcohol to veratryl aldehyde and subsequent conversion of the aldehyde to 3,4-dimethoxybenzylideneacetone through aldol condensation reaction were studied in a basic IL (BIL) 1butyl-3-methylimidazolium 5-nitrobenzimidazolide, which simultaneously acts as the solvent and provides a basic environment required to perform the reactions (Figure 49).282

Figure 49. Sequential oxidation and aldol-condensation reactions of veratryl alcohol: (I) veratryl alchol; (II) veratryl aldehyde; (III) 3,4dimethoxy-benzylideneacetone. Adapted from ref 282 with permission. Copyright 2013 Royal Society of Chemistry.

The oxidation reactions were conducted under pressurized oxygen and catalyzed by Ru@ZIF-8 (zeolite imidazolate framework-8) and CuO. It was the synergistic effect between Ru@ZIF-8 and CuO that promoted the oxidation reactions, as

Figure 50. Possible mechanism of oxidation of lignin model compound 2-phenoxyacetophenone. Adapted from ref 285 with permission. Copyright 2015 Royal Society of Chemistry. AJ


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Figure 51. Structures of the anions and cations of ILs, lignin model compounds, and lignin in Chen et al.’s study. Adapted from ref 289 with permission. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

reaction conditions. These new findings are crucial given the importance of product purification and recycling of ILs. 4.3.4. Electrocatalytic Oxidation of Lignin and Its Model Compounds in ILs. Electrochemical techniques offer an alternative route for the transformation (including hydroprocessing and oxidation) of lignin as well as related model compounds, allowing the reactions to be conducted under relatively milder conditions and potentially with higher selectivity.21,287 This is particularly attractive when ILs are used as electrolytes due to their ionic nature and many unique properties such as inherent conductivity, wide electrochemical windows, and tailored electrochemical responses.237,288 In this context, Chen et al. initially demonstrated the potential application of electrochemical techniques on lignin and related model compounds.289 In their study, they investigated the abrasive stripping voltammetry of lignin and four related model compounds in four ILs including [C4MIM]NTf2, [N6,2,2,2]NTf2, [C4MIM]OTf, and [N6,2,2,2]OTf on a gold macrodisk, aiming to fingerprint the functional groups within the lignin molecule (Figure 51). By mechanical abrasion of lignin and/or its model compounds onto the surface of the electrode and supporting the electrode in the ILs (electrolytes), voltammetry could be measured when the adhered lignin and/or related compounds were electrochemically stripped off. Multiple peaks were observed at different potentials for lignin model compounds with different structures. Hence, this work demonstrates the feasibility of using the abrasive stripping voltammetry approach for characterization of the functional groups and fingerprint lignin samples. Moreover, this work highlights the potential application of the electrochemical approach for oxidation of lignin and its model compounds. By changing potential, the conversion and selectivity of the reaction could be controlled.237 Reichert et al. reported the electro-oxidative cleavage of alkaline lignin in a protic IL triethylammonium methanesulfonate ([Et3NH]CF3SO3) using electrodes coated with ruthenium−vanadium−titanium mixed oxide.290 By dissolving lignin in the protic IL, electrolysis of lignin could be conducted at

reaction, the [NTf2] anion was crucial for this oxidation reaction to occur, while there was a synergistic and/or cooperating effect between the H3PO4 and the [NTf2] anion. As depicted in Figure 50, the reaction proceeds through a free radical conversion mechanism revealed by UV−vis spectroscopy and electron paramagnetic resonance (EPR) analysis. Initially, the model compound 2-phenoxyacetophenone interacts with oxygen to form a contact charge-transfer complex (CCTC) via van der Waals force, which is not stable. This CCTC generates OOH free radicals and then were oxidized into ROOH, which was further converted to afford benzoic acid, phenol, and formic acid. The anion of the IL ([NTf2]) is crucial to promote the generation of free radicals due to the strong electronegativity of the heteroatoms contained. Furthermore, another lignin model compound (2-phenyloxy-1-phenylethanol) as well as organosolv lignin could also be catalytically transformed in the H3PO4/ H2O/[BnMIM]NTf2 catalytic system. Very recently, Prado et al. reported that the lignin in the black liquor could be directly depolymerized into monomer aromatic compounds in butylimidazolium hydrogen sulfate [HC4IM]HSO4 and triethylammonium hydrogen sulfate [Et3NH]HSO4 employing H2O2 as the oxidant.286 The black liquor was obtained through a delignification process of Miscanthus giganteus using mixtures of water and the above-mentioned ILs as extracting solvents. It was found that the [HC4IM]HSO4-derived lignin was more susceptible toward depolymerization, and vanillic acid was identified as the major oxidation product when 10% H2O2 (related to the total weight of IL and lignin) was used. Other major products include aromatic acids such as benzoic acid and 1,2-benzenedicarboxylic acid. The best phenolic yield was obtained using 5% H2O2 from [Et3NH]HSO4-derived lignin, and the major product was assigned to guaiacol. Despite [Et3NH]HSO4 being less efficient than [HC4IM]HSO4 during pretreatment, the aromatic products generated using [Et3NH]HSO4 were free from contaminants resulting from IL oxidation and [Et3NH]HSO4 were not affected by H2O2 under the given AK


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could also be obtained from combinations of various mineral acids (sulfuric acid, nitric acid, phosphoric acid, and maleic acid) and other imidazolium-based ILs ([HeMIM]Cl, [BMIM]Br, [AMIM]Cl, [BMIM]HSO4, and 1-(4-sulfobutyl)-3-methylimidazolium bisulfate ([SBMIM]HSO4)). However, in comparison with the HCl/[BMIM]Cl catalytic system, to obtain similar yields of TRS requires longer reaction time for other catalytic systems. Despite no very obvious differences being observed, the catalytic activity of acids on hydrolysis of raw lignocellulosic biomass generally follows the trend hydrochloric acid > nitric acid > sulfuric acid > maleic acid > phosphoric acid. FT-IR and elemental analysis also revealed that chemical modification (e.g., esterification, sulfonation) of lignin also occurred in the case when sulfuric acid was used as the catalyst. It was found that the hydrolysis reactions proceed through a consecutive first-order reaction sequence, and the determined rate constants for TRS formation (k1) and TRS degradation (k2) were 0.068 and 0.007 min−1, respectively. Sievers et al. studied the acid-catalyzed hydrolysis of loblolly pine wood in [BMIM]Cl using trifluoroacetic acid as the catalyst in the presence of water.294 Without any pretreatment, almost all of the carbohydrate content of wood was converted into watersoluble products including monosaccharides, oligosaccharides, furfural, and HMF at 120 °C for 2 h, while the lignin content remained as a solid residue which was modified slightly. Complete separation of carbohydrates and lignin was achieved in this system, but the selectivity of the carbohydrate-degraded products needs to be further improved. In another study, Li et al. reported that not only the sugar content of three wood species (i.e., Eucalyptus grandis, Southern pine, and Norway spruce thermomechanical pulp) was degraded in [AMIM]Cl with diluted HCl but also lignin was degraded into smaller aromatic molecules such as catechol, methylcatechol, methylguaiacol, etc.295 The degradation of lignin was not observed in their previous study using the HCl/[BMIM]Cl catalytic system. In comparison, for the samples treated in water at equivalent acid concentrations, only hemicellulose was hydrolyzed, leaving the cellulose and lignin content intact. Corn stalk could be selectively hydrolyzed in HCl/[BMIM]Cl through a two-stage process to produce the hydrolysates of hemicellulose and cellulose separately and consecutively by adjusting the pH of the catalytic system.296 In the first stage (pH 4.5), the lignin−hemicellulose matrix of corn stalk was disrupted and the hemicellulose was hydrolyzed to afford water-soluble monosaccharides such as xylose (yield 23.1%) at 90 °C for 7 h. Subsequently, the pH of the catalytic system was adjusted to 2−3 in the second stage, and the cellulose-rich residues were completely degraded into glucose at 90 °C after another 7 h. Using microwave irradiation as an alternative heating source, biomass could be effectively converted under much milder conditions within a short period of time (in general several minutes). For example, corn stalk, rice straw, and pine wood could be selectively hydrolyzed in imidazolium-based ILs (e.g., [BMIM]Cl and [BMIM]Br) within 3 min in the presence of water and catalytic amounts of CrCl3 to afford HMF and furfural with yields up to 45−52% and 23−31%, respectively.297 Herein, the reported yields of HMF and furfural are based on the respective hexose and pentose content of the raw biomass. CrCl3/LiCl could significantly promote the microwave-assisted hydrolysis of untreated wheat straw to produce HMF in [BMIM] Cl in the presence of water.298 The yield of HMF is almost comparable with that obtained from pure cellulose (62.3%) in

higher potentials. A wide range of aromatic lignin cleavage products including vanillin, guaiacol, and syringol were detected by GC-MS and/or HPLC. The product distribution was significantly affected by the applied potential. With higher potentials, more molecules with smaller molecular weights could be derived. This is in good consistency with the work reported by Chen et al.289

5. CATALYTIC TRANSFORMATION OF LIGNOCELLULOSIC BIOMASS IN ILS In the concept of green chemistry and developing future biorefineries, making innovative and effective use of low-value and currently underutilized raw lignocellulosic biomass is of crucial importance. They have also been recognized as a valuable and renewable resource for the generation of value-added chemical products. In the above sections, the catalytic conversion of the three major components of lignocellulosic biomass (i.e., cellulose, hemicellulose, and lignin) and related model compounds using ILs as reaction media has been described. In comparison, fewer works are reported on direct catalytic transformation of authentic raw lignocellulosic biomass using ILs as reaction media. This is probably due to the notoriously complex and recalcitrant nature of raw lignocellulosic biomass feedstocks and their inherent heterogeneity. Thus far, most work associated with processing of raw biomass in ILs was about using ILs for the dissolution, pretreatment, and fractionation of raw biomass, taking advantage of their super solubility for macromolecules. Since the pretreatment of raw biomass is out of the scope of this review, herein, we provide the readers an update of direct catalytic transformation of raw biomass in ILs with regard to the generation of valuable chemicals. In this aspect, almost all works are focused on catalytic hydrolysis of the carbohydrate fraction of raw lignocellulosic biomass (i.e., cellulose and hemicellulose) to afford reducing sugars including monosaccharides and oligosaccharides as well as carbohydrate derivatives (e.g., HMF and furfural), while the lignin fraction generally remains intact or slightly modified. Hence, in most related studies, the ILs offer a unique environment for the catalytic hydrolysis reactions of polysaccharides, and these reactions proceed in a similar manner to those for the hydrolysis reactions of cellulose and hemicellulose, which has been descried in the above sections. By dissolving raw lignocellulosic biomass in ILs, the accessibility of glycosidic bonds in carbohydrates could be significantly increased. Thus, in comparison with the acid- or base-catalyzed hydrolysis reactions conducted in aqueous media, the efficiency of the hydrolysis of polysaccharides is much improved when the reactions are conducted in ILs. Also, it is noteworthy that the conversion of raw biomass in ILs could be achieved under relatively milder conditions than those required to hydrolyze the crystalline cellulose in aqueous media (in general between 140 and 220 °C).291,292 Since some ILs are also capable of dissolving the raw biomass samples as a whole, the presence of lignin matrix and other ingredients such as protein in the reaction system does not hinder or abate the hydrolysis process. In this context, Li et al. reported the direct hydrolysis of raw lignocellulosic biomass using acids in various imidazolium-based ILs.293 The polysaccharides of raw biomass could be effectively reduced to monosaccharides. The yields of total reducing sugars (TRS) were up to 66%, 74%, 81%, and 68% for corn stalk, rice straw, pine wood, and bagasse in [BMIM]Cl in the presence of HCl (7 wt %) at 100 °C for 1 h. Similar product distributions AL


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the CrCl3/LiCl/[BMIM]Cl catalytic system at 160 °C for 10 min irradiation. The potential application of functional acidic ILs as both the solvent and the catalyst for hydrolysis of lignocellulosic biomass has also been demonstrated. Soybean straw and corn straw were hydrolyzed to afford reducing sugars in [HMIM]Cl with the assistance of ultrasound irradiation.299 Under optimum reaction conditions with the assistance of ultrasound (weight ratio of water/sample 5; weight ratio of IL/sample 25; temperature 70 °C; time 2 h), reducing sugars (around 50 mg) could be derived from soybean straw and corn straw (0.2 g). The ultrasound could probably reduce the viscosity of the reaction mixture and enhance the movement rate of chloride ions, thus accelerating the dissolution of biomass and improving the overall reaction efficiency.

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6. CONCLUDING REMARKS AND OUTLOOK Efficient transformation of abundant and renewable lignocellulosic biomass into value-added chemicals and fuel products using environmentally benign routes is of great importance, especially for sustainable development of our society. This represents not only a long-time grand research task and challenge but also a huge opportunity. In the context of green chemistry/green chemical engineering as well as developing biorefineries for a future biobased economy, it is crucial that these chemical valorization processes are conducted effectively using greener solvents or media. Taking advantage of the unusual properties of ILs, we can not only make the transformation processes greener and more efficient but also solve many challenging problems and develop various new technologies. In recent years, significant advances have been achieved in this interesting field. This review provides a holistic overview of the developed chemical processes and technologies that use ILs as the reaction media for converting lignocellulosic matters into chemicals and fuel products. According to the reported works in this field, there is no doubt that using ILs as the reaction media for the chemical valorization of lignocellulosic biomass is very promising and encouraging. It also reflects that this field is still in its infancy, and there are many challenges that need to be addressed. Herein, we would like to highlight the following points. (1) The existing routes for the transformation of lignocellulose should be optimized. More importantly, innovative and new strategies should be developed to make full use of the advantages of ILs. In some cases, even new products should be derived. This involves the design of highly efficient and green catalysts and new types of functional ILs. Attention should be paid to the development of robust and effective catalysts that are durable in a catalytic system involving ILs and optimization of various catalyst characteristics such as activity, selectivity, stability, and reusability, etc. New functional solvents including new types of (functionalized) ILs and combination of different green and functional solvents (e.g., IL/IL, water/IL, supercritical CO2/IL) should be developed and investigated. In this context, it is crucial to take into consideration of the costs of (functionalized) ILs, especially for large-scale applications in order to make the biorefineries be feasible economically. (2) As in many cases, the performance of catalysts generally depends upon the solvents and other components in the catalytic systems. Hence, in terms of optimizing and developing new types of catalysts and solvents, the

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coupling design of catalysts and solvent systems is another key issue to be taken into account. For the transformation of the lignocellulose macromolecules in catalytic systems involving ILs as solvents, the solubility of the feedstock also significantly affects the overall reaction efficiency. Hence, it is important to make careful choices of combinations of catalyst and suitable solvents. Getting insight into the reaction mechanisms in the catalytic reactions of lignocellulose materials in the developed catalytic systems (catalyst plus solvents) is a very important aspect from both fundamental and practical points of view. Efforts should be devoted to understand the molecular interactions between the feedstock, the catalyst, and the solvents. This will contribute to a more fundamental understanding and also potentially provide more opportunities for the development of new catalytic systems. Process intensification is another important aspect in terms of developing future biorefineries. Due to the super solubility of lignocellulosic materials in some ILs, they are extensively studied for pretreatment of biomass, although this is not included in the current contribution. Hence, for processes using ILs, it is important to integrate the pretreatment, conversion, and postprocessing in order to design technically and economically feasible transformation strategies. A complicated mixture of products is generally obtained after the reaction using lignocellulosic biomass and/or its primary components. Hence, the development of technologies and methods for highly efficient separation and fractionation of products from solvents/catalytic systems and further purification is very important. Especially for catalytic systems involving ILs as solvents, ineffective separation might results in decreased reusability and recyclability of the catalytic systems. Among the three major components of lignocellulosic biomass (i.e., cellulose, hemicellulose, and lignin), limited success has been achieved in deconstruction of lignin for its valorization, even on a pilot plant scale. This is primarily due to its structural variability and recalcitrant amorphous nature. Given the fact the lignin is actually the only abundant renewable aromatic resource on earth, more attention should be paid to conversion of this currently underutilized but valuable material into high-value products. In this context, exploring new conversion strategies that can preserve its aromatic character represents a promising research field. In addition to converting lignocellulosic biomass into small molecules, larger molecules with various functionalities through selective cleavage of chemical bonds in lignocellulosic biomass or its primary components should be derived. Also, innovative use of these larger functionalized molecules should be explored. Despite being out of the scope of this review, chemical modification of lignocellulosic biomass and its primary components for the production of functional (polymeric) materials is another very interesting and promising area. This strategy can retain most structures of feedstock materials, and therefore, the processes can potentially be more energetically efficient. ILs as a super solvent and reaction media offer more options for these strategies. Large-scale application of the lignocellulosic biomass feedstock is important, but this is also very challenging due DOI: 10.1021/acs.chemrev.6b00457 Chem. Rev. XXXX, XXX, XXX−XXX

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to variabilities in their supply, composition, and properties, etc. Many current technologies are technically viable but economically prohibitive. The successful development of energetically, environmentally, and economically feasible processes requires both multinational and multidisciplinary approaches, including chemistry, material science, process engineering, etc.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Zhanrong Zhang was born in 1988 and obtained his Bachelor’s degree from Chongqing University (China) in 2010. He continued his graduate study at the Green Chemistry Centre of Excellence, University of York (UK), where he received his Master’s and Ph.D. degrees in 2011 and 2015, respectively. In the same year, he joined Prof. Buxing Han’s group in the Institute of Chemistry, Chinese Academy of Sciences (CAS), as an assistant professor. His research interest covers catalytic conversion of biomass in green solvents and the synthesis and design of novel catalysts and catalytic systems. Jinliang Song was born in 1981 and received his Ph.D. degree in 2009 from the Institute of Chemistry, Chinese Academy of Sciences (CAS), under the supervision of Prof. Buxing Han. After graduation, he started working in Prof. Han’s group as an assistant professor and then he was promoted to be an associate professor in 2012. His current research interest covers utilization and conversion of biomass and carbon dioxide, applications of green solvents, and design and synthesis of novel catalysts and catalytic systems. Buxing Han received his Ph.D. degree at the Institute of Chemistry, Chinese Academy of Sciences (CAS), in 1988 and did postdoctoral research from 1989 to 1991 at the University of Saskatchewan, Canada. He has been a professor at the Institute of Chemistry, Chinese Academy of Sciences, since 1993. He is an Academician of CAS and Fellow of the Royal Society of Chemistry and was a titular member of Organic and Biomolecular Chemistry Division, IUPAC (2012−2015) and the Chairman of the IUPAC Subcommittee on Green Chemistry (2008− 2012). His research interests include properties of green solvent systems and applications of green solvents in chemical reactions and material science.

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21133009, 21603236, 21673249), Chinese Academy of Sciences (QYZDY-SSW-SLH013), and Ministry of Science and Technology of China. REFERENCES (1) Klass, D. L. Biomass for Renewable Energy, Fuels and Chemicals; Academic Press: San Diego, 1998. (2) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering. Chem. Rev. 2006, 106, 4044−4098. (3) Lin, Y.-C.; Huber, G. W. The critical role of heterogeneous catalysis in lignocellulosic biomass conversion. Energy Environ. Sci. 2009, 2, 68− 80. (4) Gallezot, P. Conversion of biomass to selected chemical products. Chem. Soc. Rev. 2012, 41, 1538−1558. AN


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