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Lignocellulosic biomass, an alternative and sustainable energy source for the future, can be used as a feedstock for bioconversion into fermentable sugars or other value-added chemicals. The ability of ionic liquids to dissolve lignocellulosic materials provides new opportunities for the utilization of this renewable resource. In this chapter, the catalytic conversion of cellulose, lignin, hemicellulose, and raw lignocellulosic biomass in the presence of ionic liquids to sugars or other value-added chemicals, such as furfural and 5-hydroxymethylfurfural, is reviewed. Mineral acids, solid acids, metal salts, and acidic ionic liquids are efficient catalysts for the conversion of lignocelluloses in ionic liquids. This conversion process can be carried out under mild conditions, which enables the energy-efficient and cost-effective conversion of biomass to biofuels and platform chemicals. However, several challenges in this area need to be addressed and some of these are considered at the end of this chapter.

The fossil fuel-based economy is facing several problems and challenges, which involve the increasing emissions of CO2, decreasing reserves, and increasing energy prices.1  These challenges have driven the search for new transportation fuels and bioproducts to substitute the fossil carbon-based materials. Biomass is defined as organic matter available on a renewable basis, and it includes forest and mill residues, agricultural crops and wastes, wood and wood wastes, animal wastes, livestock operation residues, aquatic plants, and municipal and industrial wastes.2  Biomass is deemed a sustainable and green feedstock for the production of fuels and fine chemicals, although perhaps not always in the way they are proposed to be used.

A major source of biomass is lignocellulosic biomass, which is particularly well suited for energy applications because of its large-scale availability, low cost, and environmentally benign production. Lignocelluloses are composed of cellulose, hemicellulose, lignin, extractives, and several inorganic materials, of which the first three biopolymers are the main components. The cellulose microfibrils that are present in the hemicellulose–lignin matrix are often associated in the form of bundles or macrofibrils.3  The structure of these naturally occurring cellulose fibrils is mostly crystalline in nature and highly resistant to attack by enzymes. In addition, the presence of lignin also impedes enzymatic hydrolysis, as enzymes bind onto the surface of lignin and hence do not act on the cellulose chains.4 

Usually, conversion of lignocellulosic biomass is carried out in the presence of catalysts, such as strong liquid and solid acids. Various types of lignocellulosic biomass, such as wood chips, sawdust, corncobs, and walnut shells, have been tentatively processed by liquid acid-catalyzed hydrolysis5–8  with H2SO4, HCl, H3PO4, etc. Despite the relatively high catalytic activity of these liquid acids in the hydrolysis of cellulosic materials, by and large their uses are still uneconomical because the process suffers from severe corrosion, a requirement for special reactors, and costly separation and neutralization of waste acids.9 

Recently, attention has been paid to the use of solid catalysts in the depolymerization of lignocellulosic biomass. Several types of solid acids, such as Nafion, Amberlyst, −SO3H functionalized amorphous carbon or mesoporous silica, H-form zeolites like HZSM-5, heteropolyacids, and even metal oxides (e.g., γ-Al2O3) have been explored for their catalytic performance in the hydrolysis of lignocellulosic biomass.10–13  It has been shown that solid Brønsted acids are efficient catalysts for the hydrolysis of lignocellulosic biomass.14,15 

The ability of ionic liquids (ILs, now defined as salts with melting points below 100 °C16 ) to dissolve biomass provides new opportunities for the pretreatment and conversion of lignocellulosic biomass. In 2002, we reported that certain ILs, such as 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) can dissolve cellulose by as much as 25 wt% without any pretreatment.17  Since then, increasing numbers of scientific papers, patents, and conference abstracts in this area have been published, and ILs have become one of the “hot-topics” in polysaccharide research. Up to now, ILs have been shown to be able to dissolve a number of pure biopolymers, including cellulose,17–22  hemicellulose,23  lignin,24,25  chitin,26  starch,27  silk,28  wool,29  as well as a variety of raw biomass, such as wood,30,31  bagasse,32,33  corn stover,34  wheat straw35  and shrimp shell.26  Not only is the dissolution of biomass in ILs widely studied, but also its conversion into value-enhanced products has drawn the attention of scientists.

In this chapter, the catalytic dissolution and degradation of pure cellulose, lignin (including lignin model compounds), hemicellulose, and raw lignocellulosic biomass materials in the presence of ILs will be reviewed. Several challenges in this area will also be addressed.

Lignocellulosic biomass presents a greater challenge for dissolution because of the tight, covalent, hydrogen bonded matrix of carbohydrate polymers (cellulose and hemicellulose) and phenolic polymers (lignin),36  resulting in insolubility in common solvents. Various pretreatment methods for lignocelluloses have been developed to open the compact structure and make the conversion easier, and these methods include those that are physical (irradiation), chemical (alkali, acid, organosolv, ammonia explosion), physicochemical (steam explosion, CO2 explosion), or some combination of these.37  Extreme conditions involving strong acids or bases, high temperatures, and high pressures are typically used at the expense of fragmentation of the components.

The ability of ILs to dissolve pure cellulose, lignin, and hemicellulose prompted us to study if ILs could also dissolve raw lignocellulosic biomass. In 2007, Professor Moyna, along with our group, reported that [C4mim]Cl can dissolve different sources of wood with varying hardness.31  Later, we showed that 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) is a better solvent for wood than [C4mim]Cl under the same reaction conditions.30  But even using [C2mim][OAc], it took 46 h to completely dissolve 0.5 g Southern yellow pine in 10 g IL at 110 °C, and the lignin content in the regenerated cellulose-rich material was 23.5%.30  It was clear that a better separation of the lignin from the cellulose was needed.

At the time, we hypothesized that part of the difficulty in dissolving lignocellulosic biomass arose from the covalent linkages which hold lignin and the carbohydrate together via ether, ester, or glycoside bonds.38  We thus sought a catalyst which would selectively cleave these bonds and facilitate the dissolution and separation of lignin and the carbohydrate. Polyoxometalates (POMs), together with O2, had been shown to be promising systems for pulp delignification39  and we found that when POMs ([PV2Mo10O40]5−) were present in the IL system, the dissolution of wood was greatly enhanced (e.g., 0.5 g of Southern yellow pine could be dissolved in 10 g [C2mim][OAc] in 15 h vs. 46 h without POM).38  The lignin content in the regenerated pulp was drastically reduced, and the lowest lignin content observed was 5.4% (vs. 23.5% without POM).

The form of POM (acidic POM vs. [C2mim][POM]), POM concentration, and reaction time all affected delignification efficiency. Using [C2mim][POM], longer heating time, and higher POM loadings led to better delignification and higher lignin losses. This research indicated that the presence of an appropriate catalyst could indeed facilitate the cleavage of lignin from carbohydrate.

Of the biopolymers in lignocellulosic biomass, cellulose is the most abundant and indeed is the most abundant renewable biodegradable biopolymer. It is a linear polysaccharide chain consisting of d-anhydroglucopyranose linked together through β-glycosidic bonds. An extensive network of inter- and intra-molecular hydrogen bonds and van der Waals forces results in a complex crystalline supramolecular structure.40  Decrystallization and hydrolytic cleavage of cellulose polymers to other products has been a bottleneck in the path toward energy-efficient and economical utilization of cellulose and many efforts have been devoted to the depolymerization of cellulose. These include acidic hydrolysis,41  enzymatic hydrolysis,42  and hydrolysis in supercritical water.43  However, progress has been limited partly due to the lack of solubility of cellulose in water and contamination of enzyme by the presence of other components. The dissolution of cellulose in ILs improves the reactivity of cellulose44  and thus recently, more attention has been paid to the hydrolysis of cellulose in ILs.

In 2007, Li and Zhao reported the hydrolysis of cellulose in [C4mim]Cl in the presence of mineral acids, such as H2SO4, HCl, HNO3, H3PO4.45  The catalytic activity of H2SO4, HCl, and HNO3 were similar, while H3PO4 showed lower catalytic activity, indicating that the acidity played an important role in the hydrolysis of cellulose in [C4mim]Cl. Catalytic amounts of acid were sufficient to drive the hydrolysis reaction. For example, when the acid/cellulose mass ratio was 0.46, yields of total reducing sugar (TRS) and glucose were 64% and 36%, respectively, after 42 min at 100 °C.46  However, when excess amounts of acid were loaded to the IL system, sugar yields decreased because side reactions tended to occur which consumed the hydrolysis products.

Hydrolysis of cellulose dissolved in 1-ethyl-3-methylimidazolium chloride ([C2mim]Cl) and [C4mim]Cl catalyzed by mineral acids can lead to the formation of glucose, cellobiose, and 5-hydroxymethylfurfural (5-HMF) as the main products.47,48  The initial rate of glucose formation was determined to be of first order in the concentrations of dissolved glucan and acid concentration, and of zero order in the concentration of water. The independence on water concentration suggested that cleavage of the β-1,4-glycosidic bonds near chain ends is irreversible. The absence of oligosaccharides longer than cellobiose indicated that cleavage of interior glycosidic bonds is reversible due to the slow diffusional separation of cleaved chains in the highly viscous glucan–IL solution. Gradual addition of water during the glucan hydrolysis inhibited the rate of glucose dehydration to 5-HMF and the formation of humins. It was proposed that the inhibition was attributed to the stronger interactions of protons with water than the 2-OH atom of the pyranose ring of glucose, the critical step in the formation of 5-HMF. The reduction in humin formation associated with water addition was ascribed to the lowered concentration of 5-HMF, since the humins were formed through the condensation polymerization of 5-HMF with glucose.

Binder and Raines also reported the degradation of cellulose in [C2mim]Cl with H2SO4 or HCl.49  HMF was the main product, with moderate yields of glucose. The production of HMF at the expense of glucose suggested that glucose was dehydrated to HMF. To test this hypothesis, glucose was hydrolyzed in [C2mim]Cl containing varying amounts of water. In the absence of both acid and water, glucose was recovered intact. Adding H2SO4 to glucose–IL solution, with little or no water, led to the rapid decay of glucose into HMF and other products. Interestingly, it was found that increasing the water content to 33 wt% in the same acidic solution enabled nearly 90% of the glucose to remain intact after 1 h, which was in accordance with Le Chatelier's principle,50  showing that water disfavors dehydrative reactions, including glucose oligomerization and conversion into HMF. Additionally, the authors proposed that the highly nucleophilic chloride anions of the IL coordinate strongly to the carbohydrates,51  accelerating acid-catalyzed dehydration reactions. High concentrations of water solvated chloride and thus prevented it from interacting with carbohydrates. Therefore, when water was in a large amount, the hydrolysis reaction was inhibited.

Rinaldi et al. first reported that solid acids are powerful catalysts for the hydrolysis of cellulose dissolved in ILs.52,53  The factors responsible for the control of depolymerization of cellulose in [C4mim]Cl using Amberlyst 15DRY as the catalyst were determined.54  It was found that the acidic resin released H+ into the solution, controlling the initial rate of depolymerization. The initial size of the cellulose chains was crucial in the control of initial product distribution. Long chains were preferably cleaved into shorter ones instead of producing glucose, accounting for the induction period observed for the release of glucose or total reducing sugars. Activation of cellulose towards hydrolysis requires a strong acid, which prohibits the utilization of ILs composed of a weakly basic anion, such as acetate or phosphonate. These anions could capture the available H+ species and prevent the activation of the glycosidic bonds. Additionally, the presence of N-methylimidazole, an impurity in [C4mim]Cl, decreased the catalytic performance of this system. Amberlyst 15DRY could be recycled, and after being washed with sulfuric acid, this catalyst showed the same catalytic activity as the fresh resin.

Solid acid-catalyzed hydrolysis of cellulose in IL can be substantially improved by microwave heating. H-form zeolites with a lower Si/Al molar ratio and a larger surface area showed high catalytic activity.55  These solid catalysts exhibited better performance than styrene-based sulfonic acid resin. Compared with conventional oil bath heating, microwave irradiation at an appropriate power significantly reduced the reaction time (e.g., <10 min at 240 W) and increased the yields of reducing sugars. A typical hydrolysis reaction with Avicel cellulose produced glucose in ∼37% yield within 8 min, in comparison with 7.1% by using oil bath heating at 100 °C for 10 h. Cellulose hydrolysis catalyzed by solid acids was more environmentally friendly, as it could simplify the downstream processes and circumvent waste acids and water disposal.

Zhao et al. found that CrCl2 and CrCl3 were efficient catalysts for the hydrolysis of cellulose in [C2mim]Cl to HMF.56  Later, an efficient strategy for CrCl3-mediated production of HMF in ca. 60% and 90% isolated yields from cellulose and glucose, respectively, in [C4mim]Cl under microwave irradiation was reported by the same group.57  When water was used as the solvent, glucose dehydration was essentially restrained, indicating that [C4mim]Cl was a solvent superior to water. If H2SO4 was used in lieu of CrCl3, the dehydration reaction afforded HMF in only 49% yield, and formation of insoluble humins was observed.

Dissolution of purified cellulose in a mixture of N,N-dimethylacetamide (DMAc)–LiCl and [C2mim]Cl with the addition of CrCl2 or CrCl3 produced HMF in yields up to 54% within 2 h at 140 °C (with 60 wt% [C2mim]Cl).58  The yield compared well with results of HMF synthesis from cellulose in the patents using aqueous acid59  or ILs.56  Neither lithium iodide nor lithium bromide alone produced high yields of HMF because these salts in DMAc do not dissolve cellulose. However, using lithium bromide along with DMAc–LiCl did enable modest improvements in HMF yield. Likewise, using HCl as a cocatalyst also enhanced the HMF yields.

A novel catalytic system involving CuCl2 (an example of a primary metal chloride catalyst) paired with a second metal chloride, such as CrCl2, PdCl2, CrCl3, or FeCl3 in [C2mim]Cl, was found to substantially accelerate the rate of cellulose depolymerization under mild conditions.60,61  These paired metal chlorides showed high catalytic activity for the hydrolytic cleavage of β-1,4-glycosidic bonds when compared with the rates of H2SO4-catalyzed hydrolysis. In contrast, single metal chlorides with the same total molar loading showed much lower activity under the same reaction conditions. Possible mechanisms involved in the paired CuCl2–PdCl2 catalytic system were studied experimentally in combination with theoretical calculations. Results indicated that Cu(ii) was reduced during the reaction to Cu(i) only in the presence of a second metal chloride and a carbohydrate source such as cellulose in the IL system. Cu(ii) generated protons by hydrolysis of water to catalyze the depolymerization step, and served to regenerate Pd(ii) from Pd(0) (the added PdCl2 was reduced to Pd(0) by side reactions). Pd(ii) was suggested to facilitate the depolymerization step by coordinating the catalytic protons and also promoting the formation of HMF.

Considering the toxicity and environmental concerns of Cr-based catalysts, Tao and coworkers explored the catalytic activity of non-toxic and inexpensive FeCl2 and CoSO4 in the depolymerization of cellulose. It was found that functional acidic ILs, with the addition of FeCl2 or CoSO4, were an effective system for the hydrolysis of microcrystalline cellulose (MCC).62,63  The IL, 1-(4-sulfonic acid)-butyl-3-methylimidazolium hydrogen sulfate, in combination with the metal catalyst, was found to be the most efficient system for the hydrolysis of cellulose at 150 °C, and conversion of MCC reached 84% in 300 min. The yields of HMF and furfural were up to 34% and 19%, respectively, for the FeCl2 system and were shown to be 24% and 17%, respectively, for the system containing CoSO4. Additionally, small amounts of levulinic acid and reducing sugars (8% and 4%, respectively) were detected. Dimers of furan compounds were the main by-products as detected by HPLC-MS, and the components of gas products, analyzed by MS, were shown to contain methane, ethane, CO, CO2, and H2. The IL and catalyst could be recycled by removing the solvents and reused in the hydrolysis of cellulose with favorable catalytic activity over five repeated runs.

Enzymatic hydrolysis is another important method for the conversion of cellulose. However, one of the disadvantages of ILs is their strong tendency to denature enzymes. Turner et al.64  studied the hydrolysis of cellulose by Trichoderma reesei cellulase in [C4mim]Cl that contained 5% cellulose in 50 mM citrate buffer at 50 °C. The hydrolytic rate in this IL was poor, at least 10-fold less than that performed in aqueous buffer. Low activity in [C4mim]Cl was attributed to the high concentration of the Cl ion which led to unfolding and inactivation of the enzymes, and this inactivation was shown to be irreversible. Hence, in order to preserve the activity of cellulases in the hydrolysis of cellulose, cellulose needs to be regenerated from the IL solutions and all traces of chloride-containing IL should be completely removed. This undoubtedly introduces a regeneration–separation step into the process, which would increase the overall cost and preclude the development of a single stage continuous process for conversion of lignocellulosic materials, unless more IL-tolerant enzymes can be used.

With the aim of eliminating the need to regenerate and separate cellulose, Kamiya et al. investigated in situ enzyme saccharification of cellulose in enzyme compatible IL 1-ethyl-3-methylimidazolium diethylphosphate.65  Cellulase was directly added to the aqueous–IL mixture containing cellulose at 40 °C. The ratio of IL to water (citrate buffer with pH=5.0) greatly affected the cellulase activity. When the volume of IL to water was greater than 3:2, low cellulase activity was observed. However, decreasing the volume ratio to 1:4 enhanced cellulase activity and over 70% of the added cellulose was converted to glucose and cellobiose.65  A similar study by Engel et al.66  demonstrated cellulase activity of up to 30% on α-cellulose in the presence of 10% (v/v) 1,3-dimethylimidazolium dimethylphosphate. Increasing viscosity and ionic strength led to decrease in enzyme activity.

A 100% conversion of cellulose to industrially useful chemicals was achieved in a one-step reaction by the use of cooperative IL pairs for combined dissolution and catalytic degradation.67  One IL was selected to dissolve cellulose, while the second one was used for the catalytic conversion of the dissolved cellulose to products with low molecular weight. During the dissolution and catalytic reaction, an immiscible organic solvent, e.g., hexane, was placed on top of the IL pair system to extract any small soluble organic products (MW<300 g mol−1). The first IL could be [C2mim][OAc], [C4mim]Cl, or [C4mim][OAc], and the second one was an acidic IL, such as [C4mim][HSO4], [C1mim][HSO4], or [C4H8SO3Hmim][HSO4]. The acidity of the second IL was directly related to the catalytic degradation of cellulose. The strongest IL acid tested, [C4H8SO3Hmim][HSO4], showed the highest catalytic performance and yielded the lowest average molecular weight products. Using the [C4H8SO3Hmim][HSO4]–[C4mim]Cl pair at 200 °C, 45.8 wt% of the cellulose was selectively converted to 2-(diethoxymethyl)furan.

Lignin is a three-dimensional cross-linked amorphous phenolic polymer.68  The composition, molecular weight, and amount of lignin differ from plant to plant, with lignin abundance generally decreasing in the order of softwoods > hardwoods > grasses.69  Lignin fills the spaces between cellulose and hemicellulose, and acts as a resin to confer strength and rigidity to the plant.70  Lignin is mainly composed of phenylpropane monomers that link together primarily through the C–O linkage of α- and β-ether bonds, and the β-O-4 linkage is found to be dominant.71,72  Schematic representations of the softwood and hardwood lignin structures showing common linkages are depicted in Figure 1.1.73  (The structures are merely pictorial and do not imply a particular sequence.) The chemical structure of lignin suggests that this polyphenolic material could potentially serve as a renewable chemical feedstock if suitable conversion chemistry is developed.

Figure 1.1

Schematic representations of softwood lignin (top) and hardwood lignin (bottom), redrawn from Ref. 73.

Figure 1.1

Schematic representations of softwood lignin (top) and hardwood lignin (bottom), redrawn from Ref. 73.

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Lignin represents a vastly underutilized resource. Despite being one of the most abundant polymers on earth, ∼98% of lignin is burned as a source of energy,74,75  primarily in the pulp and paper industry, at very low efficiency. With the rapidly growing interest in conversion of lignocellulosic biomass to fuels, a large amount of lignin will be available. This will undoubtedly spur new concepts for using lignin as a resource for value-added products.

Degradation of lignin or lignin model compounds in the presence of ILs has gained more and more attention in recent years. Beech lignin was oxidatively cleaved in four ILs, including 1-ethyl-3-methylimidazolium methylsulfonate, ([C2mim][MeSO3]), 1-ethyl-3-methylimidazolium ethylsulfonate ([C2mim][EtSO3]), 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([C2mim][CF3SO3]), and 1-methyl-3-methylimidazolium methylsulfate ([C1mim][MeSO4]), to give phenols, unsaturated propylaromatics, and aromatic aldehydes.76  Mn(NO3)2, used as the catalyst, in combination with [C2mim][CF3SO3] proved to be the most effective reaction system. By adjusting the reaction conditions, the selectivity of the process could be shifted from syringaldehyde as the main product to 2,6-dimethoxy-1,4-benzoquinone (DMBQ). DMBQ could be isolated as a pure substance by a simple extraction–crystallization process,76  and this compound is reported to be of relevance in the context of anti-tumor drugs.

Alcell and soda lignin were dissolved in the IL 1-ethyl-3-methylimidazolium diethylphosphate ([C2min][DEP]) and subsequently oxidized by O2 in the presence of several transition metal catalysts.77  CoCl2·6H2O in [C2min][DEP] proved particularly effective for the oxidation. Both the Alcell lignin and soda lignin readily dissolved in [C2min][DEP] to yield dark brown solutions. Samples were taken each hour, and a small portion of each sample was used for spectroscopic analysis. Analysis of the reaction solution was conducted by first performing ethyl acetate extraction in an attempt to isolate any possible low molecular weight, monomeric products formed during the reaction; however, no monomeric products were detected by GC-MS. Attenuated total reflection infrared spectroscopy (ATR-IR) analysis of the lignin reaction samples indicated oxidation of lignin. IR peaks corresponding to alcohol and aldehyde stretches were observed, which suggested that the dissolved lignin was selectively oxidized. The absence of monomeric products, however, indicated either that the linkages in the lignin remained intact or were insufficiently disrupted to yield aromatic products of sufficiently low molecular weight for GC-MS detection.

Analysis of depolymerization of various lignin model compounds in the same system indicated that the catalyst rapidly oxidized benzyl and other alcohol functionalities in lignin, but left the phenolic functionalities and 5–5′, β-O-4, and phenylcoumaran linkages intact.77  This system represents a potential method in a biorefinery platform to increase the oxygen functionality of lignin prior to depolymerization or functionalization of the already depolymerized lignin.

ILs based on the 1-methylimidazolium ([Hmim]+) cation with chloride, bromide, hydrogen sulphate [HSO4], and tetrafluoroborate [BF4] counter ions along with [C4mim][HSO4] were employed to degrade two lignin model compounds, guaiacylglycerol-β-guaiacyl ether (GG) and veratrylglycerol-β-guaiacyl ether (VG).78  All the tested acidic ILs were successful in breaking down the lignin model compounds by hydrolyzing the β-O-4 ether linkages. While the acidic environment of the ILs catalyzed the hydrolysis reaction, the anions in the ILs had a significant effect on the guaiacol yield. At 150 °C, the relative guaiacol yield produced by each IL decreased in the order: [Hmim]Cl > [C4mim][HSO4] > [Hmim]Br > [Hmim][HSO4] > [Hmim][BF4]. Thus, it was assumed that the ability of the anion to hydrogen bond with the model compound was a major contributor to the ability of an acidic IL to effectively catalyze the hydrolysis of the β-O-4 ether linkage with stronger coordination leading to a chemical environment more conducive to ether bond hydrolysis.

In other work by the same group, the β-O-4 bonds of phenolic and non-phenolic lignin model compounds, GG and VG, underwent catalytic hydrolysis to guaiacol as the primary product in acidic [Hmim]Cl.79  More than 70% of the β-O-4 bonds of both model compounds reacted with water to produce guaiacol at 150 °C. The IL could be recycled and reused without extra treatment or appreciable loss of activity.

Similarly, Binder et al. reported that ILs were suitable media for the Brønsted acid-catalyzed dealkylation of lignin model compounds, such as eugenol, 2-phenylethyl phenyl ether, and 4-ethylguaiacol.80  A wide range of strong acidic catalysts enabled the formation of guaiacol in [C2mim]Cl or 1-ethyl-3-methylimidazolium triflate ([C2mim][OTf]), with product yields as high as 11.6%.

Base-mediated cleavage of β-O-4 bonds in a dimeric phenolic lignin model compound GG in 1-butyl-2,3-dimethylimidazolium chloride ([C4C1mim]Cl) has also been reported.81  Nitrogen bases of varying basicity and structures were tested at temperatures up to 150 °C. An enolether, 3-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxy-phenoxy)-2-propenol, was found to be the primary product in all cases. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene was shown to be the most active catalyst, leading to more than 40% of the β-O-4 bond cleaved, and its high activity was assumed to associate with the exposed nature of the nitrogen atoms, which made this catalyst function as both a base and a nucleophile.

The hydrolytic cleavage of GG and VG was also studied in [C4mim]Cl with metal chlorides and water.82  FeCl3, CuCl2, and AlCl3 were found to be effective catalysts in cleaving the β-O-4 bond of GG, and it was observed that an increase in available water could lead to more β-O-4 bond cleavage of GG. After 120 min at 150 °C in the presence of FeCl3 and CuCl2, GG conversion reached 100%, and about 70% of the β-O-4 bonds were hydrolyzed, liberating guaiacol; while using AlCl3 as the catalyst, about 80% of the β-O-4 bonds of GG were hydrolyzed with 100% GG conversion. AlCl3 functioned more effectively than FeCl3 and CuCl2 in cleaving the β-O-4 bond of VG. About 75% of the β-O-4 bonds of VG could be hydrolyzed after 240 min at 150 °C in the presence of AlCl3. The authors proposed that the catalytic activity was associated with HCl, working as the acidic catalyst, formed in situ by the hydrolysis of the metal chlorides.

Reichert and coworkers presented an approach of electro-oxidative cleavage of 5 wt% alkaline lignin solutions in a special protic IL, triethylammonium methanesulfonate ([HNEt3][MeSO3]), using electrodes coated with ruthenium–vanadium–titanium mixed oxide.83  This protic IL provided a suitable medium for dissolution of lignin, ensured electrolysis at higher potentials, and promoted an oxidative lignin cleavage. The mixed oxide coating exhibited great oxidative stability combined with a remarkable catalytic activity for the oxidation of lignin. A wide range of aromatic fragments, such as aldehydes and ketones (benzaldehyde, 3-furaldehyde, m-tolualdehyde, vanillin, acetovanillone) were identified as the cleavage products as detected by GC-MS, and HPLC experiments confirmed an additional oxidative step, namely the conversion of vanillin to vanillic acid.

Hemicellulose, a mixture of polysaccharides containing xylose, arabinose, glucose, galactose, mannose, and other sugars,84  is another major component of lignocellulosic biomass material. Studies on the degradation of hemicellulose in ILs are relatively few. Dee and Bell studied the hydrolysis of xylan, a material representing hemicellulose, in [C2mim]Cl catalyzed by H2SO4.48  They found that the rate of hemicellulose hydrolysis was approximately 1.4 times faster than that of cellulose hydrolysis. Experiments with gradual water addition produced primarily saccharide products with very low yields of dehydration products. After 30 min, the yields of xylose and furfural from xylan were 62% and 4%, respectively. The increase in furfural and xylose production from xylan prior to increasing the water concentration to 43 wt% at 60 min, resulted in the production of a black precipitate of humins. The concentration of humins continued to increase as the xylose yield decreased from 73% at 60 min to 63% at 120 min, while furfural yield only increased from 5% to 7% over the same period of time.

Using the IL [C2mim]Cl as solvent and Brønsted acids as catalysts, Enslow and Bell investigated the kinetics of hydrolysis of xylan and subsequent dehydration/degradation reactions.85  Xylobiose and xylose were detected as the primary products. Furfural was produced from the dehydration of xylose, and glucose as well as arabinose were also identified. Also, humins began to form and continued to accumulate throughout the duration of the experiment, which are suggested to be formed from the coupling loss reaction between xylose and furfural.86 

The hydrolysis of xylan was compared with that of cellulose.85  The observed initial rate constant of xylan hydrolysis was ca. 8 times higher on average than that of cellulose under similar reaction conditions over the temperature range from 80 to 100 °C. The lower initial rate of cellulose hydrolysis was likely due to the presence of the additional hydroxymethyl groups within cellulose. Unlike the secondary alcohols present in glucose and xylose, the primary alcohol in the methoxy group of glucose has the ability to abstract catalytic protons from solution through an equilibrium protonation–deprotonation reaction,87  lowering the concentration of catalyst available for hydrolyzing the β-1,4-glycosidic bonds in cellulose. Thus, at the same initial catalyst loadings, fewer catalytic protons were available for cellulose hydrolysis, resulting in a lower initial rate when compared with hydrolysis of hemicellulose. This chemical process presents a viable pathway for producing sugars capable of being chemically (via dehydration/hydrogenation) or biologically (via fermentation) upgraded to potential fuel molecules.

Dehydration of hemicellulosic material, xylose, into furfural was investigated in several ILs88  and [C2mim]Cl as well as [C4mim]Cl were found to be the most efficient ones. A CrCl2 catalyst loading of 20 mol% with respect to xylose gave optimum conversion and yield amongst the different catalyst loadings used. For a catalyst combination of CrCl2 and CuCl2, the optimum mole fraction of the mixture was 0.6:0.4 (CrCl2:CuCl2), resulting in a maximum conversion of 90% and furfural yield of 49%. The addition of mineral acids had an adverse effect on furfural yield and it resulted in lower conversions when compared with reactions without the acid catalysts.

Lignocellulosic biomass presents a more significant challenge for hydrolysis than does cellulose or hemicellulose. In addition to intractable crystalline cellulose, lignocellulosic biomass includes protective hemicellulose and lignin, heterogeneous components that are major obstacles to many biomass hydrolysis processes.89  The dissolution of lignocellulose using ILs as the solvents offers several attractive features, and catalytic conversion of lignocelluloses in ILs has been widely studied.

Acids in ILs were demonstrated as efficient systems for hydrolysis of lignocellulosic materials with desirable TRS yields under mild conditions.90  TRS yields could reach 66%, 74%, 81%, and 68% for hydrolysis of corn stalk, rice straw, pinewood, and bagasse, respectively, in [C4mim]Cl in the presence of 7 wt% HCl at 100 °C under atmospheric pressure within 60 min. Analysis of hydrolyzates indicated the formation of monosaccharaides, such as glucose, xylose, arabinose, and galactose. The reactivity of other mineral acids in [C4mim]Cl against corn stalks was also explored and it was found that hydrolysis rates decreased in the order: HCl>HNO3>H2SO4>HO2CCHCHCO2H>H3PO4. A novel outcome of this work was the discovery that acidic ILs 1-(4-sulfobutyl)-3-methylimidazolium bisulfate ([Sbmim][HSO4]) and [C4mim][HSO4] could act as both catalyst and solvent for hydrolysis of corn stalk. It was found that this biomass material dissolved quickly in the acidic ILs, forming a solution with lower viscosity, implying that polysaccharide depolymerization readily occurred; however, low TRS yields (15–23%) were obtained. It was thus concluded that the strong acidic ILs might not only promote the depolymerization reaction, but also speed up the rate of sugar degradation. Corn stalk was subsequently hydrolyzed in [C4mim]Cl, using the acidic [C4mim][HSO4] or [Sbmim][HSO4] as the catalyst, and TRS yield increased to 68% and 71%, respectively.

Recently Binder and Raines49  reported an effective process for the hydrolysis of untreated corn stover, in which water was gradually added to a catalytic system containing [C2mim]Cl and acid catalyst (H2SO4 or HCl). Corn stover was hydrolyzed with [C2mim]Cl in the presence of 10 wt% HCl at 105 °C, producing a 71% yield of xylose and 42% yield of glucose (yields were based on the xylan and cellulose content of the stover). Dilution of the reaction mixture to 70% water caused precipitation of unhydrolyzed polysaccharides and lignin. These residues were then dissolved in [C2mim]Cl and subjected to a second-stage hydrolysis, which released additional xylose and glucose, leaving behind lignin-containing solids. After the two-stage HCl-catalyzed hydrolysis process, glucose and xylose yields reached 70% and 79%, respectively. Ion-exclusion chromatography allowed recovery of the IL and delivered sugar feedstocks that supported the vigorous growth of ethanologenic microbes. In this technique, passing the solution through a charged resin separated a mixture containing electrolyte and nonelectrolyte solutes. The charged IL was excluded from the resin, while nonelectrolytes, e.g., sugars and furfural, were retained. The nonpolar HMF and furfural were adsorbed more strongly than sugars, and eluted later. Passing the corn stover hydrolyzate through a column of [C2mim]-exchanged Dowex® 50 resin allowed laboratory-scale separation of the IL from the sugars, and recovery of the IL was higher than 95%, and those of glucose and xylose were 94% and 88%, respectively. Additionally, this chromatographic step removed inhibitory compounds such as HMF and furfural.

Miscanthus, which can be grown with lower water and soil nutrient requirements compared to other biofuel feedstocks91  has been identified as one of several lignocellulosic feedstocks for biofuel production. The hydrolysis of raw Miscanthus dissolved in [C2mim]Cl using H2SO4 as the catalyst was reported by Dee and Bell.48  The kinetics of the hydrolysis of the cellulosic and hemicellulosic portions of Miscanthus was first order in acid concentration and zero order in water concentration. When compared with the hydrolysis of pure cellulose and xylan, it was found that rates for the hydrolysis of the cellulosic and hemicellulosic portions of Miscanthus were much lower, attributed to the inhibiting effects of lignin in raw Miscanthus.

Cleavage of the lignin–hemicellulose linkages by ethylene diamine pretreatment of Miscanthus increased the rate of hydrolysis of both the cellulosic and hemicellulosic materials. The conversion of Miscanthus to sugar products was improved by gradual addition of water to the reaction mixture, which limited the dehydration of the saccharides to furfurals and the formation of humins. Increasing the concentration of the acid catalyst increased the conversion of the cellulosic portion of Miscanthus to glucose but decreased the conversion of hemicellulose to xylose due to dehydration of this product to furfural and its subsequent condensation with glucose and xylose to form humins. High yields of saccharides were achieved with initial Miscanthus loadings of up to 9 wt%, but further increasing the initial loading, e.g., 18 wt%, lowered the conversion to soluble products, most significantly for the cellulosic component.

Production of HMF and furfural from lignocellulosic biomass was also studied in [C4mim]Cl and [C4mim]Br in the presence of CrCl3 under microwave irradiation.92  Corn stalk, rice straw, and pine wood treated under typical reaction conditions produced HMF and furfural in yields of 45–52% and 23–31%, respectively, within 3 min. It was postulated that CrCl3 in [C4mim]Cl formed complexes [CrCl3+n]n in a similar manner to LnCl3.93  These complexes would promote rapid conversion of α-anomers of glucose or xylose to β-anomers through hydrogen bonding between the Cl anions and the hydroxyl groups, followed by cyclic aldoses reverting to the acyclic form which combines with the chromium complex to form an enolate structure. Enolate formation would enable conversion of the aldoses to ketoses, followed by dehydration to produce HMF and furfural (Scheme 1.1). Thus, in the presence of IL, the β-1,4-glycosidic bonds were weakened at the cellulose hydrolysis step under microwave irradiation because of coordination with [CrCl3+n]n. As a result, the β-1,4-glycosidic bonds were easily attacked by water to form glucose and oligomers. This method could facilitate energy-efficient and cost-effective conversion of biomass to biofuels and platform chemicals.

Scheme 1.1

Putative mechanism of CrCl3-promoted conversion of glucose and D-xylose into HMF and furfural.92 

Scheme 1.1

Putative mechanism of CrCl3-promoted conversion of glucose and D-xylose into HMF and furfural.92 

Close modal

HMF could also be readily produced from untreated lignocellulosic biomass, such as corn stover and pine sawdust, under mild conditions in the mixture of DMAc–LiCl and [C2mim]Cl with CrCl2 or CrCl3.58  Yields of HMF from untreated corn stover were nearly identical to those for stover subjected to ammonia fiber expansion pretreatment. Other biomass components, such as lignin, did not interfere substantially in the process, as yields of HMF based on the cellulose content of the biomass were comparable to those from purified cellulose.

Researchers in the US Department of Energy's Joint BioEnergy Institute (JBEI) have engineered the first strains of Escherichia coli bacteria that can digest switchgrass biomass pretreated by ILs.94  Both cellulolytic and hemicellulolytic strains were further engineered with three biofuel synthesis pathways to demonstrate the production of fuel substitutes or precursors suitable for gasoline, diesel, and jet engines directly from IL-treated switchgrass. This work might enable reduction in fuel production costs by consolidating two steps— depolymerizing cellulose and hemicelluloses into sugars, and fermenting the sugars into fuels—into a one-step operation, thus providing an economical route to produce advanced biofuels.

Ionic liquids hold the key to unlocking the bottleneck to the production of biofuels from lignocellulosic materials. By dissolving biomass materials in ILs, the hydrogen bonds that link the biopolymers together can be disrupted, which will benefit the further conversion of the biopolymers to sugars or other value-added products. Moreover, conversion of lignocellulosic biomass in the presence of ILs can be carried out under milder conditions, which can save energy; another important sustainability requirement of modern society.

However, several shortcomings of using ILs in biofuel production still exist, especially in process development and optimization. First, the choice of ILs can be a compromise between solubilizing power and catalyst compatibility. For instance, the Cl anion is a superior anion for biomass dissolution, but also causes many enzymes to denature. For an effective integrated process that would use enzymes in situ with ILs, more research on enzyme compatible ILs is required. ILs containing acetate anion are more efficient at dissolving biomass materials than the Cl-containing ILs,23  but the acidic catalyst for the hydrolysis of biopolymers cannot coexist with basic anions, e.g., acetate, in the solution.

Second, effective and energy-saving IL recycling methods should be developed for each intended process. The current method to recycle ILs in biomass processing mainly centers on evaporation of the antisolvent, which would consume a lot of energy, especially in the case of IL–water solutions. Thus, further efforts to develop effective methods to facilitate the IL recycling are needed.

Third, the price of ILs is still very high, which will make the conversion process uneconomical. This concern can be addressed by the development of new manufacturing methods, efficient scale-up technologies, and by developing better ILs. For example, better sources (e.g., renewable) of raw materials, or new and effective synthetic routes could be developed.

The particular ability of some ILs to dissolve biopolymers, accompanied by a series of advantages, certainly might facilitate the conversion of raw biomass materials to value-added chemicals. However, this field is still in its infancy and much more research is needed in this area, such as optimizing large-scale pretreatment conditions, performing post-pretreatment steps in ILs, recycling and reusing ILs with reduced energy consumption and enhancing process efficiency. Moreover, the nature of lignin suggests that it might be used in the manufacture of high value chemicals, traditionally obtained from petroleum. Much of this work is now underway around the world and we look forward to the fascinating results yet to come.

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