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Catalysis in Ionic Liquids
From Catalyst Synthesis to Application
By Chris Hardacre, Vasile Parvulescu
The Royal Society of ChemistryCopyright © 2014 The Royal Society of Chemistry
All rights reserved.
Catalytic Conversion of Biomass in Ionic Liquids
HUI WANG, LEAH E. BLOCK AND ROBIN D. ROGERS
The fossil fuel-based economy is facing several problems and challenges, which involve the increasing emissions of CO2, decreasing reserves, and increasing energy prices. 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. 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. 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.
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 hydrolysis 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.
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. It has been shown that solid Brønsted acids are efficient catalysts for the hydrolysis of lignocellulosic biomass.
The ability of ionic liquids (ILs, now defined as salts with melting points below 100 °C) 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. 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, hemicellulose, lignin, chitin, starch, silk, wool, as well as a variety of raw biomass, such as wood, bagasse, corn stover, wheat straw and shrimp shell. 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.
1.2 Catalytic Dissolution of Lignocellulosic Biomass
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), 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. 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. 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. 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%. 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. 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 delignification and we found that when POMs ([PV2Mo10 O40]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). 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.
1.3 Catalytic Degradation of Biomass
1.3.1 Catalytic Degradation of Cellulose
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 ß-glycosidic bonds. An extensive network of inter- and intra-molecular hydrogen bonds and van der Waals forces results in a complex crystalline supramolecular structure. 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, enzymatic hydrolysis, and hydrolysis in supercritical water. 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 cellulose and thus recently, more attention has been paid to the hydrolysis of cellulose in ILs.
220.127.116.11 Degradation Catalyzed by Mineral Acids
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. 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. 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. 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 ß-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. 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, 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, 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.
18.104.22.168 Degradation Catalyzed by Solid Acids
Rinaldi et al. first reported that solid acids are powerful catalysts for the hydrolysis of cellulose dissolved in ILs. The factors responsible for the control of depolymerization of cellulose in [C4mim]Cl using Amberlyst 15DRY as the catalyst were determined. 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. 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.
22.214.171.124 Degradation Catalyzed by Metal Salts
Zhao et al. found that CrCl2 and CrCl3 were efficient catalysts for the hydrolysis of cellulose in [C2mim]Cl to HMF. 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. 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). The yield compared well with results of HMF synthesis from cellulose in the patents using aqueous acid or ILs. 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. These paired metal chlorides showed high catalytic activity for the hydrolytic cleavage of ß-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). 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.
Excerpted from Catalysis in Ionic Liquids by Chris Hardacre, Vasile Parvulescu. Copyright © 2014 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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