As described in the previous section, chloride type ILs have a strong ability to dissolve cellulose, and it is predominantly attributed to the anion to form hydrogen bonding with the hydroxyl groups of cellulose. However, most chloride salts have both a high melting point and high viscosity. These properties are not suitable for the improvement of cellulose solubilization. Various attempts have been made to reduce the melting point of chloride-based ILs, as discussed above. Despite the prepared chloride salts being in their liquid state at room temperature, heating is necessary to dissolve cellulose. Since the necessity of the continuous heating requires an excessive amount of energy consumption, this leads the increase in the total cost of the cellulose treatment process. It is therefore strongly desired to develop novel ILs to dissolve cellulose with low energy cost. Design of anion structure is required because there is a limitation to overcome the problem by only optimization of cations of chloride-based salts.

To design novel CDILs, we should have an analytical method to evaluate the hydrogen bonding basicity of ILs, because chloride type ILs dissolve cellulose through making favorable hydrogen bonds with hydroxyl groups of cellulose. The analysis of physicochemical properties of ILs is essential for the design of CDILs. There are many ways to investigate or predict the proton accepting ability, in other words, hydrogen bond basicity. For example, Hansen solubility parameters [14], COSMO-RS [15], and the Kamlet-Taft parameters [16, 17] are known as useful empirical or semi-empirical polarity scales. Especially, it is known that Kamlet — Taft parameters are very useful, which requires three solvatochromic dyes

Diethyl-4-nitroaniline

Scheme 2.2 Structure of prove dyes for Kamlet-Taft parameter measurements [16] (Scheme 2.2). From the shift of the absorption maximum wavelength of the individual dye molecules shown in Scheme 2.2, three Kamlet-Taft parameters such as а, в, and n values are calculated. These three parameters, а, в, and n values represent hydrogen bond acidity, hydrogen bond basicity, and polarizability, respectively [16,17]. Since ILs are conductive materials, it is not easy to determine the polarity with conventional electrochemical methods. Considering this, Kamlet — Taft parameters are quite useful to evaluate the polarity of ILs.

Brandt and co-workers compiled the correlation between cellulose dissolving ability and the Kamlet-Taft в value of several ILs (Fig. 2.1) [18]. Although the plotted data were measured at different conditions (e. g. different temperature, dissolution time, degree of polymerization (DP) of cellulose, moisture content, purity of ILs, etc.), there is a certain correlation between cellulose solubility and the в value of the ILs. ILs with в value of less than 0.6 have no power to dissolve cellulose under any condition. The ILs having a в value of more than 0.6 start to dissolve cellulose and solubility increases with an increase of their в value. Here, it should be noted that the в value is not only the factor to govern the cellulose solubility. There are still many ILs that cannot solubilize cellulose in spite of their larger в value [19]. Other factors such as a value and ion structure should also be considered for the design of cellulose solvents. Although the в value does not entirely determine the cellulose dissolving ability, it is a useful design parameter for CDILs.

According to the data compiled by Ohno and co-workers, a series of carboxylate salts (Scheme 2.3) were confirmed to have strong hydrogen bond basicity (Table 2.3) [20]. Since there are a wide variety of carboxylic acid derivatives, carboxylate anions have been selected as good anions to construct CDILs [21].

From the structures listed in a patent by Swatloski and co-workers, BASF reported that imidazolium ILs bearing acetate anions are effective for the dissolu­tion of cellulose [22]. Since 1-ethyl-3-methyl-imidazolium acetate ([C2mim]OAc) is less toxic, and less viscous, this IL is a favorable solvent for cellulose. Fukaya and

[C4mim][RCOO] [Amim][HCOO]

R = H : [HCOO], CnH2n+i (n = 1~3) : [CnCOO], C(CH3)3 : [t-C4COO]

Scheme 2.3 Structure of carboxylate type salts (Reprinted with permission from Ohno and Fukaya [20], Copyright (2009) The Chemical Society of Japan)

co-workers also reported that a series of carboxylate-type ILs for cellulose disso­lution [23]. They suggested that 1-allyl-3-methylimidazolium formate ([Amim] formate, IL3 in Scheme 2.4) is a good solvent to dissolve cellulose. This ionic liquid shows no melting temperature but low glass transition temperature (—76 °C) and low viscosity (66 cP at 25 °C) (Table 2.4). The hydrogen bond basicity of IL3

Scheme 2.4 Structure of formate salts with imidazolium cations which have different length of alkyl chains (Reprinted with permission from Fukaya et al. [23], Copyright (2006) American Chemical Society)

was higher than that of chloride salts. The IL3 was confirmed to have a good ability to dissolve cellulose under mild condition. It solubilized 10 wt% cellulose at 60 °C though [Amim]Cl required 100 ° C to dissolve the same concentration of cellulose (Fig. 2.2).

After the appearance of these carboxylate type CDILs, many studies were reported about the cellulose dissolving mechanism by carboxylate salts. Remsing and co-workers analyzed the solvation mechanisms of acetate and chloride type salts, such as [C4mim]Cl, [Amim]Cl, and 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) using 35/37Cl and 13C NMR relaxation [24]. The 35/37Cl and 13C relaxation rates of anions showed a strong dependency on the carbohydrate con­centration in the ILs having acetate or chloride anions. Especially, in the case of [C2mim][OAc], with the increase of carbohydrate concentration, the reorientation rate of the anion decreased faster than that of cations. They suggested that the interactions between the cations and carbohydrates are nonspecific, and concluded that the solvation mechanism was almost the same regardless of the structure of the anions.

Zhang and co-workers also analyzed the interaction between [C2mim][OAc] and cellobiose, a repeating unit of cellulose (Scheme 2.5), using 1H-NMR spec­troscopy [25]. The acetate anion made hydrogen bonds with hydroxyl groups of cellobiose, and the imidazolium cation also interacted with the oxygen atom of

hydroxyl group of cellobiose, especially via the most acidic proton in the C-2 position (Fig. 2.3).

Liu and co-workers carried out molecular dynamics simulations to clarify the interaction of cellulose and ILs [26]. They suggested that the interaction energy between a series of (1-4) linked в-D-glucose oligomers and [C2mim][OAc] was stronger than that with water or methanol. The estimated energy for hydrogen bonding between the hydroxide group on glucose unit and water or ethanol was estimated to be around 5 kcal mol_1, whereas that in [C2mim][OAc] was estimated to be 14 kcal mol_1. Furthermore, some of these cations interacted with these polysaccharides through hydrophobic interactions. Xu and co-workers reported that the cellulose solubility of [C4mim][OAc] was certainly improved by addition of lithium salts [27]. They have suggested that lithium cation interacts with an oxygen atom of C3-hydroxyl group of cellulose, and it causes cleavage of the O(6)H-O(3) inter-molecular hydrogen bonding. This result means that cations also make a certain contribution to dissolve cellulose depending on their structure.

Timo Leskinen, Alistair W. T. King, and Dimitris S. Argyropoulos

Abstract Ionic liquids (ILs) have been recognized as a promising way to fractionate lignocellulosic biomass. During recent years, a number of publications have intro­duced a variety of technical developments and solvent systems based on several types of ILs to fractionate lignocellulose into individual polymeric components, after full or partial dissolution. In this chapter we briefly review the latest developments and knowledge in this field of study and introduce an alternative fractionation method based on the controlled regeneration of components from 1-allyl-3-methyl — imidazolium chloride ([amim]Cl). Norway spruce (Picea abies) and Eucalyptus grandis woods were dissolved in their fibrous state or by utilizing ball milling to improve solubility. The resulting wood solutions were precipitated gradually into fractions by addition of non-solvents, such as acetonitrile and water. Further water extraction of the crude fractions resulted in better separations. By analyzing molec­ular weight distributions of the fractions, together with their chemical composition, we have obtained fundamental information concerning the mechanisms of wood fractionation with ILs. Fractionation efficiency is found to be highly dependent on the modification of the wood cell wall ultrastructure and the degree of reduction of the molecular weights of the main components, arising from mechanical degradation.

T. Leskinen

Departments of Forest Biomaterials and Chemistry, North Carolina State University, Raleigh, NC 27695-8005, USA

A. W.T. King

Department of Chemistry, University of Helsinki, Helsinki 00014, Finland D. S. Argyropoulos (*)

Departments of Forest Biomaterials and Chemistry, North Carolina State University, Raleigh, NC 27695-8005, USA

Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia e-mail: dsargyro@ncsu. edu

Z. Fang et al. (eds.), Production of Biofuels and Chemicals with Ionic Liquids, Biofuels and Biorefineries 1, DOI 10.1007/978-94-007-7711-8_6,

© Springer Science+Business Media Dordrecht 2014

Isolation of cellulose enriched fractions was archived with Spruce sawdust and ball milled Eucalyptus, evidently following from distinct dissolution mechanisms.

Keywords Wood • Cellulose • Lignin • LCC • Ionic liquids • 1-allyl-3-methylimi — dazolium chloride • Fractionation • Extraction • Separation • Molecular weight • Pulp • Biofuels

6.1 Introduction

In a relatively short time, the research area of ionic liquid-mediated fractionation and pretreatment of wood has emerged from the interest of a small group of scientists into a noteworthy and diverse field of study. Global interest in lignocellulosic biomass is experiencing a renaissance, not only because of the growing financial potential in lignocellulose-based liquid biofuels, but also because it represents a source of bio-based materials and chemicals. Ionic liquids (ILs) have been recognized to have potential in many applications that can be categorized under the advanced utilization of grassy and woody biomass. The ability to dissolve various biopolymers and a general status as a green alternative to organic solvents makes IL platform technologies attractive to industry. This is mostly in areas pertaining to the manu­facture of novel polymeric materials, by derivatization or blending of cellulose. Alternatively, in biomass pretreatments, including structural or compositional alter­ation of plant cell walls and acid catalysed hydrolysis of plant polysaccharides, for the purposes of biofuel production [1-3]. Aside from the use of ILs as a media for modification, fractionation of lignocellulosic biomass can be integrated into a variety of applications as it can also be used as a method to obtain purified or specified polymeric raw materials, for further use [4]. IL-mediated fractionation is suitable for the general concept of a biorefinery, serving the demand of component separation for subsequent multiple product streams. Ideally this method should be tunable. How­ever, the selectivity of fractionation of native woods using ILs is still poorly devel­oped or understood, from a mechanistic point of view.

Understanding the fundamentals of the separation of polymeric cell wall com­ponents has improved after initial publications concerning cellulose and whole wood dissolution into ILs [5—7]. Ideally, there are two ways to fractionate ligno- cellulose: (1) complete pre-dissolution of biomass followed by selective precipita­tion of the sought components as purified fractions, by addition of a non-solvent, or

(2) selective extraction of components from the biomass. The first efforts to isolate purified fractions using ILs can be roughly categorized under either of the afore­mentioned approaches [6, 8—10]. However, the complex recalcitrant structure of wood greatly hinders a complete dissolution and efficient fractionation. During the last few years, a variety of new methods have emerged resulting in enhanced fractionation. In addition to introducing our work on wood fractionation, in this effort we will also present a brief overview of the latest technical advances in the IL-based fractionation systems and of our findings related to the mechanisms controlling the dissolution and fractionation of the complex materials of the wood cell wall.

Our work concerning wood fractionation has focused on dissolution of Norway spruce (softwood) and Eucalyptus grandis (hardwood) woods, as completely as possible under mild conditions, followed by a stepwise regeneration of wood com­ponents, with the addition of non-solvents. From initial screening, non-solvents were chosen in an attempt to enhance the selectivity of component precipitation. In this selection we considered the optical brightness of the precipitated samples, the ease of recovery (defined precipitate vs. emulsions) and the ability to fractionally precipitate the dissolved material. In our overall work we have selected the IL 1-allyl-3- methylimidazolium chloride ([amim]Cl) for the fractionation experiments, which has been demonstrated to have a good dissolution capability for cellulose and wood materials [7,11-13]. The starting materials were either coarse TMP softwood pulp or sawdust and fine ball-milled powders from soft — or hardwood. In contrast to the usual approach, in component regeneration from IL solutions, we did not use excess of non-solvent causing rapid precipitation, but gradually increased the amount of a single polar non-solvent to control the amount of precipitated material. By this method, only a fraction of the dissolved material was precipitated, while the rest of the material remained in solution. Careful gel-permeation chromatographic analyses of the fractions offered a visualization of the molecular weight and the distribution of species within the dissolved components. Ball-milled wood dissolved completely in the IL, but the coarse sawdust or TMP pulp preparations were not fully soluble on a microscopic level. It has been noticed earlier that the solubility of softwood in [amim] Cl and subsequent phosphitylation of all hydroxyl functionalities is greatly dependent on the preliminary mechanical treatment [14]. Surprisingly, this partial insolubility of sawdust has enabled a more efficient component separation, by selective extraction of components, compared to the soluble fine powder preparations, which separate according to molecular weight distributions. From the coarse material, a cellulose — rich fraction was extracted and the rest of the lignin-hemicellulose matrix could be isolated as an insoluble material. In the case of increased pulverization, the observed better solubility was rationalized by the fragmentation of the matrix formed by lignin — carbohydrate complexes (LCC). An increase in the amount of water extractable lignin from crude fractions of milled wood, after the IL treatment, points to the presence of soluble fragments, originating from an LCC matrix.

Glucose is an isomer of fructose and since it occurs as the monomeric unit in cellulose, it can be considered to be the most abundant monosaccharide in nature. Therefore, glucose is more appropriate than fructose as starting material for 5-HMF. Efficient routes for converting glucose to 5-HMF are an active research topic. However, glucose has been shown to be difficult to convert to 5-HMF (yields <30 %) with solvents such as water [22], organics [39] and organic-water mixtures

[40] . The reason for this is apparently because glucose tends to form a stable six-membered pyranoside structure that has a low enolization rate [25]. Since enolisation rate is the rate-determining step for 5-HMF formation, glucose will react much slower than fructose. Thus, glucose is more likely to undergo cross­polymerization with reactive intermediates and 5-HMF, since it can form true oligosaccharides that contain reactive reducing groups [25]. Because of this limi­tation in the fundamental chemistry, there were no efficient processes for the selective dehydration of glucose into 5-HMF, until a major breakthrough came in 2007 when Zhao et al. [41] published a method for transforming glucose into 5-HMF with an ionic liquid solvent (1-ethyl-3-methylimidazolium chloride, [EMIM] [Cl]) and CrCl2 catalyst. In that work, a 5-HMF yield of 68 % was obtained at a temperature of 100 °C for a reaction time of 3 h. In this reaction, CrCl3~ anion is thought to not only play a key role in proton transfer that facilitates mutarotation of glucose in [EMIM][Cl], but also to play a critical role in the isomerization of glucose to fructose by a formal hydride transfer. Once fructose is formed, it is rapidly dehydrated to 5-HMF in the ionic liquid in the presence of the catalyst. Inspired by this work, a series of papers were reported for the conversion of glucose to 5-HMF using chromium chloride as catalyst [19, 42—45].

Yong et al. [45] studied the production of 5-HMF from fructose and glucose in 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) using CrCl2 as catalyst, and 5-HMF yields of 96 % and 81 %, respectively for reaction conditions at 100 °C for 6 h reaction time achieved. Those authors considered that NHC/CrClx (NHC=N — heterocyclic carbene) complexes played the key role in glucose dehydration in [BMIM][Cl]. Additionally, in the CrCl2/EMIM system, a NHC/Cr complex could be formed under the reaction conditions and therefore serves as a catalyst [45]. Remarkably, 5-HMF yields were approximately 14 % higher for the reaction carried out in air than that conducted in argon. Zhang et al. [46] studied the production of 5-HMF from glucose catalyzed by hydroxyapatite supported chro­mium chloride in an ionic liquid (1-butyl-3-methylimidazolium chloride), and a maximum 5-HMF yield of 40 % was obtained. In the work of Zhao et al., glucose conversions and 5-HMF yields were lower when CrCl3 was used instead of CrCl2

[41] . However, subsequent studies suggested that there are only minor differences in the catalytic activity of bivalent and trivalent chromium salts [19, 47]. Compared with the strongly reductive Cr (II), the trivalent form, Cr (III), possesses higher stability in the environment, and Cr (III) is essential for mammals in removing glucose from the bloodstream [48]. Binder and Raines [44] made an extensive investigation on glucose conversion in dimethylacetamide (DMA) with the addition of halide salts. Addition of 10 wt% LiCl or LiBr along with CrCl2, CrCl3, or CrBr3 resulted in 5-HMF yields up to 80 %.

A zero-valent Cr(CO)6-based catalyst system was found to be effective for the conversion of glucose to 5-HMF in ionic liquid [EMIM][Cl], even at low catalyst loadings [49]. Through in-situ, ex-situ, and quantitative poisoning experiments, it was demonstrated that small, uniform Cr0-nanoparticles, either preformed via microwave irradiation (3.6-0.7 nm) or generated in-situ via thermolysis (2.3-0.4 nm) during the reaction, are active species responsible for the observed catalysis when using Cr(CO)6 as the precatalyst [49]. In view of some of the advantages of the Cr(CO)6-derived nanoparticles catalyst system, including the relatively low cost and air-stability of the precatalyst as well as its ability to maintain high efficiency at low catalyst loadings, the results should provide a new method to develop more effective metal-nanoparticles catalysts for glucose or related biomass conversion processes.

Two analogous chromium catalysts other than chromium chloride have been reported to be effective for conversion of glucose to 5-HMF [32]. Han and co-workers [50] used 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM] [BF4]) with SnCl4 as catalyst, and obtained a 5-HMF yield of ca. 60 % at 100 °C for 3 h. They screened metal chlorides and ILs, for which only chromium(III), aluminum(III), and tin(IV) chlorides exhibited activity, and tin(IV) was concluded to be the most active catalyst. Out of the eight ILs examined, [EMIM][BF4] was found to be most favorable. Those authors proposed that the formation of a five — membered-ring chelate complex consisting of Sn and two neighboring hydroxyl groups in glucose was a probable intermediate in the formation of 5-HMF (Fig. 9.3). Their 1H-NMR measurements showed that chloride in SnCl4 was transferred and interacted with hydrogen atoms, and Sn atoms interacted with oxygens to promote the formation of a straight-chain glucose required for transfor­mation to the enol intermediate and formation of 5-HMF [50]. Stanlberg et al. [51] examined ionic liquids with lanthanide catalyst, and obtained a 5-HMF yield of 24 %. Lanthanide(III) salts have also been tried for the dehydration of glucose to 5-HMF [51]. The use of the strongest Lewis acid Yt(OTf)3 resulted in 24 % of 5-HMF yield, and the catalytic effect increased with increasing atom number in the lanthanide series. Furthermore, the 5-HMF yield was observed to increase with increasing the chain length of the alkyl groups on the imidazolium cation, that is, 1-octyl-3-imidazolium chloride ([OMIM][Cl]) had a significantly higher yield than [EMIM][Cl]. This phenomenon has not been observed with other catalyst systems where [EMIM][Cl] has been superior or equivalent to other methylimidazolium chlorides [51].

Although the catalysts such as CrCl2, CrCl3, Cr(CO)6 and SnCl4 are effective for the dehydration of glucose into 5-HMF, they are poisonous, difficult to recycle and have high environmental risk. The use of inherently nonhazardous catalysts and solvent systems are needed for application in today’s society. Considering that the dehydration of glucose to 5-HMF involves two steps, namely, isomerization of glucose into fructose through base catalysis that is followed by dehydration of

Fructose

furanose

Fig. 9.3 Proposed mechanism for glucose dehydration to 5-HMF catalyzed by SnCl4 in [EMIM] [BF4] (Reproduced with permission from reference [50]. Copyright © 2009 Royal Society of Chemistry) fructose by acid catalysis to give 5-HMF [22], Qi et al. developed a method for glucose conversion into 5-HMF with ionic liquid-water mixture without using chromium-analogous catalysts [52]. They found that the addition of an appropriate amount of water into the ionic liquid has a synergistic effect on the glucose conversion to 5-HMF, and promoted the formation of 5-HMF from glucose com­pared with that in either pure water or in the pure ionic liquid solvent. In the proposed reaction system, a 5-HMF yield of 53 % could be obtained in 50:50 w/w% 1-hexyl-3-methyl imidazolium chloride-water mixture in 10 min reaction time at 200 °C in the presence of ZrO2. It was confirmed that 1,3-dialkylimidazolium ILs with Cl_ and HSO4~ anion were effective for 5-HMF formation from glucose in IL-H2O mixture. The addition of the other protic solvents such as methanol and ethanol into the ionic liquid had a similar synergistic effect as water and promoted fructose and 5-HMF formation [52].

The synthesis of HMF is nowadays one of the most investigated reactions from biomass. HMF is indeed considered as a chemical platforms from which new generations of biofuels (ex: dimethylfurane) and a wide range of intermediates, monomers and many other fine chemicals can be then produced [29, 30]. This old reaction is now witnessing a sort of renaissance due to the scarcity of oils. HMF is produced through a triple acid-catalyzed dehydration of hexoses. In this reaction the nature of the solvent plays a pivotal role by ensuring (i) the dissolution of carbohy­drates, including biopolymers (ii) the dilution of released water, thus limiting side reactions such as the rehydration of HMF to levulinic and formic acids (Scheme 3.6) and (iii) determine the choice of the work-up procedure. Obviously, the solvent

should be also inert. In the current literature, several solvents have been proposed. Among them, dimethylsulfoxide (DMSO), water, mixtures of water and organic compounds, and ionic liquids have been particularly investigated.

In DMSO, yields of HMF greater than 85 % were obtained but the extraction of HMF from DMSO still remains rather complex and thus expensive. Additionally, under acidic conditions, DMSO may be decomposed leading to the formation of toxic products decreasing the sustainability/attractiveness of the process. In water, yields of HMF are rather low due to the side acid-catalyzed rehydration of HMF to levulinic and formic acids (Scheme 3.6). For this reason, water is often used in combination with organic solvents. This strategy affords higher yields but one should notice that reported yields are still lower than in the presence of DMSO.

In recent years, ILs have received considerable attention for this reaction. Yields of HMF obtained in ILs are comparable to those obtained in DMSO while HMF can be conveniently recovered by liquid-liquid phase extraction using for instance methyliso — butylketone (MIBK), tetrahydrofurane or butanol. Note that the extraction of HMF from ILs can be carried out in a continuous mode, thus allowing side reactions involving HMF to be limited at the same time. It should be noted that same strategy was employed in water (biphasic system) in order to limit the rehydration of HMF. In such case, sodium chloride is generally used in order to facilitate the extraction of HMF (salt-out effect). Unfortunately, the price and toxicity of ILs together with problems linked to their long term recycling are currently hampering their utilization at an industrial scale. Clearly, the industrial emergence of HMF requires chemists to urgently develop innovative processes to produce HMF in a more sustainable way from biomass. In this context, the above-described DESs have received more and more attention for the conversion of hexoses to HMF. In the next section, we present the most recent innovative works reported in this field of chemistry. Although this topic has emerged very recently, we wish to demonstrate here that DESs have the potential to produce HMF in a more rational way than in conventional solvents (Scheme 3.7).

In 2008, B. Han and co-workers have reported that HMF can be produced in acidic ChCl-derived DESs [31]. In particular, in a melt composed of ChCl and citric acid (a cheap and renewable carboxylic acid), authors have shown that fructose can be converted to HMF with more than 76 % yield (at 80 °C, 1 h, ratio DES/fructose = 5). Other DESs made of renewably sourced carboxylic acids such as oxalic and malonic acids have been also successfully used. Owing to the low solubility of HMF in the ChCl/citric acid DES, the process can be performed in a biphasic system using ethyl acetate as an extraction solvent. Like in the case of ILs, continuous extraction of HMF not only facilitated the isolation of HMF but also allowed the selectivity to HMF to be increased. Indeed, under such conditions, HMF was obtained with a yield as high as 91 %. After removal of the ethyl acetate phase containing HMF, the ChCl/citric acid DES was recycled. A slight decrease of the HMF yield was observed upon recycling experiments mainly due to the accumulation of water in the reaction media that affects the selectivity of the process (presumable acid catalyzed rehydra­tion of HMF to levulinic and formic acid). After drying of the used ChCl/citric acid eutectic mixture, the initial yield of HMF was recovered further demonstrating (i) the negative effect of water on the HMF selectivity and (ii) the stability of the ChCl/citric acid DES under reported conditions.

Next, the same group has transposed this work to the tandem hydrolysis/dehy — dration of inulin, a biopolymer of fructose, to HMF in the presence of acidic DESs such as ChCl/citric acid monohydrate or ChCl/oxalic acid dehydrate [35]. They have shown that, at 70 °C, the solubility of inulin was 150 and 28 mg. g-1 in ChCl/ oxalic acid and ChCl/citric acid, respectively. The tandem hydrolysis/dehydration of inulin (81.0 mg, 0.5 mmol fructose units) to HMF was performed at a temper­ature within a range of 50 and 90 °C. At 80 °C, the maximum yield of HMF was 55 % in both DESs within 2 h. Note that the conversion rate of fructose to HMF is higher in the ChCl/oxalic acid DES than in the ChCl/citric acid one due to a difference of acid strength. The authors have also demonstrated that addition of a suitable amount of water as soon as the beginning of the reaction has a positive effect on the formation of HMF. In particular, authors have shown that in ChCl/ oxalic acid and ChCl/citric acid DESs water does not affect the selectivity of the reaction as soon as the water/fructose units molar ratio remained lower than 31. When the water content was increased, secondary reactions such as rehydration of HMF became the dominant reactions. The reaction temperature is of prime impor­tance in this process and closely governs the selectivity of each elementary step. The hydrolysis of inulin to fructose was optimal at 50 °C while 80 °C was found to be necessary to dehydrate fructose to HMF. In this context, the one pot process was carried out in two steps involving (i) hydrolysis of inulin at 50 °C for 2 h and (ii) heating of the solution to 80 °C for another 2 h in order to dehydrate in-situ produced fructose to HMF. Using this procedure, the selectivity to HMF (65 %) was found to be higher. For the same reasons to those described above, the HMF production rate was found to be higher in the ChCl/oxalic acid DES than in the ChCl/citric acid one. To further increase the yield of HMF, authors have attempted the reaction in a biphasic system using acetyl acetate as an extraction solvent. In agreement with previous reports, the yield of HMF was increased from 57 to 64 %

and the extraction was found to be fully selective to HMF. After elimination of the ethyl acetate phase containing HMF, the possible recycling of the DES was investigated. Although water was unavoidably accumulated in the DES phase, at least six runs were performed without appreciable decrease of the HMF yield.

In 2009, Konig and co-workers investigated the production of HMF from a melt composed of D-fructose (40 wt%) and N, N’-dimethyl urea (DMU) heated at 110 °C for 2 h in the presence of CrCl2 and CrCl3 (10 mol% each). Unfortunately, even in biphasic conditions, (using ethyl acetate as an extraction solvent) low yields of HMF were obtained (6 and 2 % with CrCl2 and CrCl3, respectively). Other catalysts such as FeCl3 and AlCl3 gave similar results. Only Amberlyst 15, a sulfonated ion-exchange resin, provided a 27 % yield of HMF in this system. Authors have next investigated the production of HMF using different urea derivatives (urea, DMU and N, N’-tetramethyl urea (TMU)). Reactions were performed in the pres­ence of FeCl3 (10 mol%) and heated at 100 °C for 1 h. Conversely to urea and DMU, TMU-based melt gave HMF with an excellent yield of 89 %. These results tend to show that the presence of — NH — groups on urea and DMU are detrimental for the selectivity of the reaction. Although high yields have been obtained with TMU, the toxicity and problem of separation arising from the use of TMU represent two serious limitations. It is noteworthy that these results are in accordance with a previous work of B. Han and co-workers who have reported that basic DESs (ChCl/ urea) in combination with Lewis acid (ZnCl2, CrCl3) or ChCl/metal chlorides-based DES were poorly efficient in the dehydration of fructose to HMF [32]. Konig et al. have next developed novel carbohydrate-derived melts (based on choline chloride as a hydrogen bond acceptor) with low melting point, low viscosity, low toxicity and high sugar content. The pH values of the different melts are presented in Scheme 3.8. Production of HMF in these melts was tested using a content of fructose of 40 wt% and Amberlyst 15 or FeCl3 as catalysts. Reactions were carried out at 100 °C for 1 h. Among all tested melts, only ChCl/fructose DES has allowed the production of 25 and 40 % yield of HMF in the presence of A15 and FeCl3, respectively. When no ChCl was employed, levulinic acid (20 %) was detected instead of HMF further supporting the key role played by ChCl.

Based on these results, authors have screened the activity/selectivity of several homogeneous, heterogeneous, Bronsted and Lewis acid catalysts in ChCl/carbohy — drate melts. Results are reported in Table 3.4. Except in the case of ZnCl2, it was found that, for all other tested catalysts (Table 3.4), HMF can be obtained with 40-60 % yields from melts composed of ChCl and fructose or inulin. Tested solid catalysts such as Montmorillonite and Amberlyst 15 were also capable of promot­ing the dehydration of fructose to HMF (40-49 % yield). Conversely to the case of fructose, Montmorillonite was found however poorly active from inulin which was ascribed to the low efficiency of Montmorillonite in the catalytic hydrolysis of inulin to fructose, a pre-requisite step prior formation of HMF.

Conversion of glucose to HMF is more challenging and requires first an isom­erization step to fructose before dehydration to HMF. In accordance with a previous work of Zhao et al. performed in imidazolium-based ILs [33], chromium-based catalysts were found the most efficient catalysts in tested DES affording HMF with 31-45 % yield and 43-62 % yield from glucose and sucrose, respectively.

In 2012, K. De Oliveira Vigier et al. reported that betaine hydrochloride (BHC), a co-product of the sugar beet industry, can be used as a renewably sourced Bronsted acid in combination with ChCl and water for the production of HMF from fructose and inulin [34] In a ternary mixture ChCl/BHC/water (10/0.5/2), HMF was produced with 63 % yield (at 130 °C from 40 wt% of fructose). As observed by B. Han and co-workers, when the reaction was performed in a biphasic system using methylisobutylketone (MIBK) as an extraction solvent, HMF was recovered with a purity higher than 95 % (isolated yield of 84 % from 10 wt% of fructose) further demonstrating that these systems similarly behave to the

traditional imidazolium-derived ILs (Scheme 3.9). After recovery of the MIBK phase containing HMF, the ChCl/BHC/water system was successfully recycled seven times. In same work, authors have shown that the dehydration of fructose to HMF can also conveniently take place in a BHC/glycerol (1:1) mixture, a DES exclusively made of renewably-sourced chemicals. Although 51 % yield of HMF was successfully obtained at 110 °C (from 10 wt% of fructose), extraction of HMF from the BHC/glycerol medium remained very difficult due to the very high solubility of HMF in this mixture.

Later, F. Liu et al. have shown that, under exposure to CO2, fructose and inulin can be converted to HMF in a ChCl/fructose DES [35]. The pKa of carbonic acid is low enough to catalyze the dehydration of fructose to HMF as previously shown by B. Han [36]. In this work, CO2 initially reacted with water contained in fructose and in the DES resulting in the formation of carbonic acid in a sufficient amount to initiate the dehydration of fructose to HMF. After 90 min of reaction at 120 ° C under 4 MPa of CO2, a yield of 74 % of HMF was obtained from 20 wt% of fructose. Due to the high selectivity of the reaction and the insolubility of ChCl in MIBK, it was also possible to selectively extract HMF which was recovered with a purity of 98 %. Remarkably, the present system was tolerant to very high loading of fructose whereas most of reported solvents suffer from a low selectivity to HMF

when the fructose loading was higher than 20 wt%. For instance, fructose with a loading of 100 wt% was successfully dehydrated to HMF in a ChCl/CO2 system without appreciable decrease of the HMF yield (66 %). The authors have ascribed the tolerance of such system to high loading of fructose to strong interaction between produced HMF and ChCl resulting in the stabilization of HMF in the reaction media. As shown in Scheme 3.10, when neat HMF and ChCl were mixed together a melt was readily obtained at a fructose content higher than 60 wt%. Such melt might be responsible for the surprising stability of HMF in such system when using high loading of fructose. It is indeed known that when a chemical is engaged in the formation of a DES its reactivity is drastically reduced.

Haibo Xie and Zongbao Kent Zhao

Abstract In consideration of unique properties of ionic liquids, the research into using ionic liquids as solvents and catalysts for lipids extraction, biodiesel produc­tion and purification, as well as bioalcohol extraction from fermentation booth have been investigated to develop clean and cost-competitive new technologies. This review summarizes up-to-date progress in these areas and analyzes examples with the aim to provide an in-depth understanding of how to integrate ionic liquids-based technologies into traditional biofuel production processes.

Keywords Ionic liquids • Lipids • Extraction • Transesterification • Biodiesel • Bioethanol • Biobutanol • Catalysis

7.1 Introduction

The search for alternative resources for transportation fuel production is driven by the increasing concerns on global warming and fossil resources depletion. Biomass refers to all organic matters derived from the process of photosynthesis. It is produced in large quantity, with an estimated 140 billion metric tons per year. Thus, sugar, starch, vegetable oils, agricultural wastes, forest residues, and dedi­cated industrial materials from plants, all belong to biomass by definition. Ligno — cellulose, is mainly consisted of cellulose, hemicellulose and lignin, is by far the most abundant form of biomass. Biomass has been considered as the most impor­tant alternative feedstock for the production of fuels, chemicals and materials [1].

Ionic liquids (ILs) are specifically referred to salts that melt below 100 °C. ILs are usually organic salts comprised of cations and anions. Some ILs exist as liquids

H. Xie (*) • Z. K. Zhao (*)

Division of Biotechnology, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS), 457 Zhongshan Road,

Dalian 116023, People’s Republic of China e-mail: hbxie@dicp. ac. cn; zhaozb@dicp. ac. cn

Z. Fang et al. (eds.), Production of Biofuels and Chemicals with Ionic Liquids, Biofuels and Biorefineries 1, DOI 10.1007/978-94-007-7711-8_7,

© Springer Science+Business Media Dordrecht 2014 at relatively low temperatures, even below room temperature, so they are usually called as room temperature ILs. Due to their unique structural properties, ILs may have some of the following properties, such as non-detectable volatile under atmosphere pressure, good solubility to most of inorganic and organic compounds, high thermal stability, high ionic conductivity, tunable miscibility with traditional solvents, etc. Notably, the chemical and physical properties of ILs can be tuned by combining different cation and anion [2]. These unique properties suggest their important roles in green chemical processes, especially in the topic of biomass processing and conversion towards a foundation of sustainable biorefinery process [3, 4]. Up to date, the use of ILs for biomass processing and conversion mainly focuses on the following subjects:

• Dissolution, derivation, and regeneration of biopolymers

• Catalytic conversion of biopolymers and their monomers into platform

chemicals

• Biomass pretreatment, components fractionation and structural elucidation

• Separation, production of biofuels

Currently, biodiesel, bio-alcohols (e. g. ethanol, butanol) and bio-oil from pyrol­ysis are among the most extensively studied biofuels [5]. Biodiesel is produced mainly from vegetable oils and animal fats via transesterification process. These kinds of oils (lipids) are regarded as important energetic biorenewable resources and chemical raw materials [6, 7]. Bio-alcohols are normally made by microbial fermentation of sugars produced from carbohydrates or starch crops such as sugar­cane or corn [8,9]. With the significant progress in the conversion and utilization of biomass, it is well-recognized that clean and efficient technologies are in high demand to develop a cost-effective bioenergy production system, such as extraction of lipids and bio-alcohols, biodiesel preparation and purification [10].

The applications of ILs as a solvent and/or catalyst in conversion of biomass in a wide perspective have been covered in other chapters of this book or being reviewed elsewhere [11—13]. Especially, with ILs-based pretreatment technology, lignocellulosic biomass is also being developed as a feedstock for the production of bio-alcohols and biodiesel [14, 15]. This chapter focuses on those areas, in which ILs are used for the production of biofuel molecules in a more direct way. Specif­ically, the chapter will provide an up-to-date overview on how to apply ILs in the following areas: lipids separation, biodiesel production, bio-alcohols extraction, and bio-oil production through pyrolysis and upgrading.

Growing concerns on global warming and the depletion of traditional resources have driven us to look for green and sustainable energy sources. As a biomass- derived platform chemical, the production of 5-HMF has been one of the hottest topics in biomass utilization in these few years, and a large number of publications have appeared. Many works focus on improving 5-HMF yield by applying different substrates, different catalysts, different solvents, different reaction conditions and extraction methods. Application of ionic liquids has been shown to be efficient for the preparation of 5-HMF, and the rapid growing interest in 5-HMF production with ionic liquids from biomass holds great promise for the future. However, this field is still in its infancy and some issues concerning the chemistry and process engineer­ing should be addressed as follows for practical large-scale processes:

1. The high viscosity of ionic liquids leads to some disadvantages in the process, and most of the present used ionic liquids are prepared with petroleum-derived chemicals as starting material. The synthesis and use of ionic liquids in which cation and/or anion are originated from natural resources should be developed in the 5-HMF production. Choline based ionic liquids are good examples that have been reported [38, 63]. The other possible alternatives include glycerol, diethylethanolamine, or amino acid based ionic liquids. The use of these biorenewable ionic liquids permits the processes to be even more green and sustainable. Chapters 2 and 4 provide some exciting new avenues for ionic liquids that dissolve cellulose and that have low viscosity.

2. In the production of 5-HMF from carbohydrates in various solvents, an impor­tant side reaction is the formation of soluble or insoluble polymers known as humins that greatly affect 5-HMF yield. These humins are formed from different intermediates in the reaction and their formation increases with increasing substrate concentration. The formation of humins remains a troublesome prob­lem for 5-HMF synthesis even in ionic liquids. More investigations are required to make clear the detailed formation mechanism of humins and restrain its formation. Interesting results have been reported for the addition of acetonitrile as co-solvent to the glucose — [BMIM][Cl] or [EMIM][Cl] reaction system. Addition of acetonitrile to the reaction system inhibited the formation of humins and enhanced the glucose conversion up to 99 %, with a 98 % 5-HMF selectivity. Moderate amounts of humins of up to 20 % were formed in the absence of acetonitrile as a co-solvent [103]. These results could help to reveal the forma­tion mechanism of humins and find a way to inhibit their formation and to provide a practical method to produce 5-HMF from carbohydrates.

3. To date, efficient production of 5-HMF is mainly from fructose, and the only proposed efficient catalytic system for production of 5-HMF from glucose and cellulose is chromium or tin chlorides that are hazardous to environment. Therefore, more efforts are needed for more environmentally-friendly catalysts that are both efficient and renewable. Functionalized carbonaceous materials possibly could provide new avenues for research in this area [104—106].

4. The starting materials used in most of studies on 5-HMF production are model chemicals such as sugars (glucose, fructose and sucrose) and microcrystalline cellulose, but the use of crude lignocellulosic biomass is relatively lack. How­ever, the composition complexity of the crude biomass will require that the process of 5-HMF production in ionic liquids be greatly different from that when pure chemicals are used. Consequently, more work is required for practical applications in the treatment of actual biomass for 5-HMF production.

5. More works are needed for the separation of 5-HMF product from ionic liquid systems, especially for continuous operation. The extraction of 5-HMF from ionic liquids by supercritical carbon dioxide with co-solvent addition may be a possibility for 5-HMF separation and the creative use of CO2 in forming biphasic systems for separations could provide both an environmental and efficient separation method.

6. Since 5-HMF is unstable, it might be further reacted to produce other chemicals such as furan and esters, and these derivatives would probably be more soluble in CO2 and thus it might be possible to extract them with supercritical CO2.

The synthetic routs are greatly related to the structures and composition of ILs, such as metathesis, protic synthesis, halogen free synthesis and other special methods, in this review, the synthesis procedures are summarized into one step method, two step method, enhanced methods and others especial methods, such as the synthetic method of chiral ILs.

1. One step method

The ILs synthesized in one step method are mainly produced by the nucleophilic solvents reacting with the alkyl halide or esters, and the tertiary amine neutral­ized with acid. For example, the alkylimidazole halide, quaternary ammonium halide, alkyl sulfate, alkyl phosphate and neutralization reaction [27—31]. The alkylimidazole based ILs are synthesized according to Eq. 1.1.

2. Two step method

In the two step method, the alkylimidazole halide was first synthesized, then the halide was changed to targeted anions by complex reaction, metathesis reaction, ions exchange and electrolytic method. Many common used ILs are produced with this method, for examples, the [Bmim][FeCl3], [Bmim][PF6], [Bmim]2[SO4] and [bpy][NO3] [32—36]. The synthetic routs are shown in the Eq. 1.2.

MY/HY

R

Lewis Acid N ® N. [MXy+l]

3. Enhanced methods

In the synthesis process, microwave or ultrasonic are used to enhance the reaction to increase the reaction and conversion rate. For example, the [Bmim] [BF4] is synthesized in the route as shown in Eq. 1.3 [37].

4. The synthetic method of chiral ILs [38, 39]

Chiral ILs have more unique properties than that of common ILs, which combining the advantages of chiral material and ILs, and the chiral materials or asymmetric synthesis are both used in the procedure. For example, the chiral

compound (1S, 2R)-(+)-N, N-dimethylephdrinium cation(Fig. 1.1) can be used to synthesized ephedrinium based chiral ILs [39], and the nicotine based chiral ILs are synthesized according to the Eq. 1.4 [40, 41].

Now ILs are applied in a variety of fields, so many companies that produce and sale ILs are occurred all over the world. The common companies that produce ILs are listed in Table 1.3.

Cellulose sulfates (CSs), the sulfuric acid half ester of cellulose in their sodium form, are useful compounds for certain biomedical applications and for the formation of polyelectrolyte complex superstructures [66]. However, the homoge­neous synthesis of these derivatives in ILs proofed to be more challenging com­pared to the preparation of organic cellulose esters [33]. The same holds true for toluene sulfonic acid esters of cellulose, usually referred to as tosyl celluloses (TOSCs), that are versatile intermediates for the preparation of polysaccharide derivatives via nucleophilic displacement reaction [35]. Both cellulose derivatives are usually prepared at or below 25 °C to prevent side reactions. Sulfation at elevated temperature often induces severe polymer degradation whereas tosylation might yield chlorinated or cross-linked products. At these low temperatures, the exceptional high viscosity of cellulose/IL solutions (see Sect. 5.3.1) prevents efficient mixing during the reactions, which results in non-uniform product mix­tures. Interestingly, EMIMAc, a cellulose dissolving IL with rather low viscosity, cannot be applied as reaction medium. The acetate anion, present in high concen­trations, forms mixed anhydrides with the tosylation and sulfation reagents and acetatylation instead of formation of the desired cellulose derivatives occurs (see Sect. 5.3.2) [67]. Certain co-solvents, such as DMSO, DMF, pyridine, or N, N — ‘-dimethylimidazolidinone (DMI) can be added to cellulose/IL solutions to dimin­ish viscosity (see Sect. 5.3.1) [33, 68, 69]. Thus, completely homogeneous sulfation and tosylation of cellulose could be achieved at 25 °C with little polymer degrada­tion using BMIMCl/co-solvent mixtures as reaction media [33, 35]. CSs prepared by this procedure exhibit good water solubility even at low DS values of 0.2-0.3. The derivatization reaction shows higher regioselectivity compared to other het­erogeneous or quasi-homogeneous methods for sulfation of cellulose; at DS < 1 the vast majority of sulfate groups are located at position 6. Moreover, the DS and consequently the product properties can be tailored by adjusting the molar ratio AGU to sulfation reagent (Fig. 5.3). In a comparable approach, TOSC with DStosyi < 1.14 have been prepared by homogeneous derivatization of cellulose in BMIMCl/pyridine and BMIMCl/DMI mixtures [35]. The amount of deoxy-chloro moieties introduced as well as the extent of polymer degradation can be controlled by adjusting the reaction time from 4 to 8 h.

8.2.1 Pretreatment of Biomass

The production of ethanol through fermentation is already a common process for the utilization of biomass resources [39]. While simple sugars and starches can be easily used in this process, feed stocks of cellulose and hemicellulose would provide a source of sugars that would not compete with food and could be produced in otherwise unused land area. The conversion of these polysaccharides into simple sugars adds a significant challenge compared to the use of corn or sugarcane feed stocks [20]. In order for cellulosic biomass to be used in ethanol production, the feedstock must be pretreated and saccharified to provide a substrate suitable to the ethanol producing yeast. Pretreatment is a key step in a number of catalytic biomass processes [9], so while the pretreatment step itself may not be catalytic, it is important to an understanding of processing of biomass in ILs.

Because the structure of biomass, especially the presence of lignin, inhibits the saccharification of structural carbohydrates, pretreatment is needed to open up the structure of plant matter (Fig. 8.5). While a number of methods, such as steam explosion, dilute acid treatment, ammonia explosion, and milling have been explored, the unique solvent properties of ILs have garnered significant attention as a pretreatment option [9, 73]. What makes ILs promising for the catalytic treatment of biomass, namely their ability to make homogeneous solutions of lignocellulose, is also what make ILs a good medium for pretreatment. Because much of the recalcitrance of biomass to saccharification comes from the structure of the cell wall and the presence of lignin, when the structure is disrupted through dissolution in ILs, the carbohydrates are made available for enzymatic attack [70].

The general procedure for IL pretreatment of biomass is to dissolve or swell the biomass with an IL solvent. After treatment at a given temperature for a given time, an antisolvent, such as water, ethanol, or an acetone/water mixture, is added

Pretreatment

Decreased Cellulose Crystallinity

Fig. 8.5 Schematic of the role of pretreatment in the conversion of biomass to fuel (Adapted with permission from [73]. Copyright 2009 American Chemical Society) to precipitate the biomass and wash away the IL (see Fig. 8.6). The biomass is then dried and saccharified through enzymatic or chemical methods. This process builds off of the work by Rogers in which lignocellulose was fractionated into a cellulose and a lignin rich phase through dissolution in 1-ethyl-3-methylimi — dazolium acetate (EMIMAc). In this work, an acetone/water solution was added to precipitate the cellulose while keeping the lignin in solution. Evaporation of the acetone precipitates the lignin after the cellulose had been filtered from the solution [74].

This method was applied to pretreatment using a variety of ILs by a number of different researchers. Lynam conducted a study to measure the effect of a few different ILs on the composition and structure of lignocellulose in which ground rice hulls pretreated with 1-ethyl-3-methylimidazolium acetate, 1-allyl-3- methylimidazolium chloride (AMIMCl), or 1-hexyl-3-methylimidazolium chloride (HMIMCl) and either ethanol or water as antisolvents for cellulose precipitation. In this study, the EMIMAc was able to completely remove the lignin and significant amounts of hemicellulose from the rice hulls, while the other ILs removed less lignin but more hemicellulose [75]. Lee et al. worked with EMIMAc and other ILs to treat wood flour for enzymatic saccharification. By removing the lignin and reducing the cellulose crystallinity, the IL treated wood flour was 95 % digestible to cellulase enzymes from Trichoderma viride [76]. It has been demonstrated that IL pretreatment works even at a biomass loading well above the solubility limit of the biomass in the IL [77]. Other ILs, such as 1-ethyl-3-methylimidazolium diethyl phosphate, alkyloxyalkyl substituted imidazolium acetate, or alkyloxyalkyl substituted ammonium acetate have also been investigated [78, 79]. The use of

ILs as a pretreatment strategy has been compared to a more common method, namely pretreatment with dilute acid. In this study, it was found that IL pretreated samples produced a higher yield of monosaccharides and in a shorter time than samples pretreated with dilute acid [14]. Some studies have employed variations on dissolution and washing, such as the addition of an ammonia treatment step [80] or combining the pretreatment and saccharification into one step with aqueous ILs and enzymes [78, 81, 82].

One of the more difficult challenges facing pretreatment of biomass with ILs or saccharification in ILs is the separation of the ILs and carbohydrates after the treatment is completed. Ideally, the products will be insoluble in the IL or be precipitated with an antisolvent. These options may not be sufficient, such as when monosaccharides must be extracted from the IL. Brennan and coworkers developed a liquid-liquid extraction procedure for the removal of sugars from an IL phase using organic soluble boronic acids that have an affinity for sugars [83]. Another method relies on the use of kosmotropic, or water-structuring, salts to induce a biphasic system with water and an IL [84]. This effect has been used to separate and reuse ILs after pretreatment of biomass [85].

Bioethanol can be produced from lignocellulosic biomass feedstocks (wood, grasses, agricultural residues, and waste materials). The general steps for producing ethanol include pretreatment of substrates, saccharification to release the ferment­able sugars from polysaccharides, fermentation of the released sugars and, finally, a distillation step to separate the ethanol (Fig. 11.3).

Hydrolysis is usually catalyzed by cellulase enzymes and the fermentation is carried out by yeast or bacteria. Pretreatment of lignocellulosic materials is a prerequisite to facilitate the separation of cellulose, hemicellulose and lignin, so that complex carbohydrate molecules constituting the cellulose and hemicellulose can be broken down by enzyme-catalysed hydrolysis into their constituent simple sugars. Lignin consists of phenols and, for practical purposes, is not fermentable, while hemicellulose consists of 5-carbon sugars, and, although they are easily broken down into their constituent sugars such as xylose and pentose, the fermen­tation process is much more difficult, and requires efficient microorganisms that are able to ferment 5-carbon sugars to ethanol [2, 7, 48, 49]. Besides, in their natural state, cellulose fibers are highly crystalline and tightly packed, so pretreatment is necessary to increase the porosity and the accessible surface area for hydrolytic enzymes. Various biomass pretreatment methods have been used for the production of bioethanol, either simple such as steam explosion alone, or combined treatments such as steam explosion/ammonia, explosion/acid or alkali, fiber explosion/CO2, chemical hydrolysis/enzymatic processes [50—53]. The advantages of biological pretreatment include low energy requirement and mild environmental conditions, but the hydrolysis rate is very low.

In a pioneering study by Swatloski et al. [41], several ILs, in particular [BMIM] [Cl], were found to be capable of dissolving up to 25 % cellulose (by weight), forming highly viscous solutions. This prompted several other groups to test a variety of other ILs for their ability to dissolve cellulose [34, 43, 44, 54—61]. To optimize the use of lignocellulosic materials any pretreatment method should extract lignin and decrease the cellulose crystal structure. Lignocellulosic biomass can also be dissolved in ILs such as [AMIM][Cl], [BMIM][Cl] and [EMIM] [Ac] [62—64] and, for ionic liquids containing the same cation [BMIM], the ability to dissolve the residual lignin was dependent on the anion, as follows (in order): [MSO4] > [Cl] >> [Br] >>> [PF6] [65]. However, not all ILs have the capacity to dissolve cellulose. For example, it has been reported that, due to the presence of cationic hydroxy and allyl groups, alkanolammonium ILs cannot dissolve the crystal structure of cellulose [66]. In this context, ILs possessing coordinating anions (e. g., [Cl], [NO3], [Ac], [(MeO)2PO2]) which are strong hydrogen bond acceptors, have been found to be capable of dissolving cellulose in mild conditions by forming strong hydrogen-bonds with cellulose and other carbohydrates at high temperatures [22]. A good compromise between the solubility of lignin and cellu­lose is achieved with [EMIM][Ac] [67].

Liu et al. [68] provided an extensive review on the mechanism of cellulose dissolution in ionic liquids, demonstrating that the key parameters in the capacity of ILs, for cellulose dissolution are the cation and anion size and the ability to form hydrogen bonds with cellulose. Besides, the presence of water is disadvantageous to the solubility of carbohydrates, but is necessary for cellulose hydrolysis. However, high concentrations of water solvate the ions of the ionic liquid and thus prevent it from interacting with carbohydrates. A compromise must be reached between the water content and the cellulose hydrolysis rate in imidazolium ionic liquids [69].

Froschauer et al. [70] reported that a mixture of the cellulose dissolving IL EMIM OAc with 15-20 wt% of water is able to selectively extract hemicelluloses when mixed with a paper-grade kraft pulp for 3 h at 60 °C. This fractionation

method suggests the use of a cellulose solvent, which can serve repeatedly for complete dissolution of the purified cellulose fraction when applied undiluted.

It seems that although hydrophilic ILs are effective for the dissolution of cellulose, the activity of cellulases decreases significantly in their presence, which is consistent with what has been found for other enzymatic reactions. To overcome the negative effect of ILs on the enzymes, many research groups have regenerated cellulose from ILs prior to enzymatic saccharification, observing the faster hydrolysis of IL-regenerated cellulose compared to untreated cellulose [61, 67, 71]. Ionic liquid-treated cellulose was found to be essentially amorphous and more porous than native cellulose, both of which are effective parameters for enhancing the enzymatic action [72]. Figure 11.4 shows a scheme for regenerating cellulose from ILs prior to enzymatic saccharification for bioethanol synthesis.

After the pretreatment of cellulose with ionic liquids, the ILs must be removed; for this, methanol, ethanol and deionized water can be used as anti-solvents to regenerate cellulose from the cellulose/IL solutions. Enzymatic hydrolysis is affected by the anti-solvents used, the pretreatment temperature and the residual amount of ILs [73]. However, it has been shown that microwave irradiation (or sonication to a lesser degree) enhances the efficiency of dissolution compared with thermal heating, and, when used along with ionic liquid pretreatment, it increases the conversion of cellulose during the enzymatic reaction by making the external and internal surface area of cellulose more accessible. Similarly, Kamiya et al. [74] reported enzymatic in situ saccharification of cellulose in aqueous-ILs by adjusting the ratio of [EMIM] [DEP] to water, and Yang et al. [75] presented a new approach for enzymatic saccharification of cellulose in ionic liquids ([MMIM][DMP)-aqueous media, in which ultrasonic pretreatment is used to enhance the conversion of cellulose. Another strategy proposed to improve the stability of the cellulose in [BMIM][Cl] was to coat the immobilized enzyme particles with hydrophobic ILs. In this way, the stability of cellulase in hydrophobic IL/[BMIM][Cl] mixtures being greatly improved with respect to [BMIM] [Cl] alone [76].

Besides, it must be taken into account that cellulase activity is inhibited by cellobiose and, to a lesser extent, by glucose. Several methods have been developed to reduce such inhibition, including the use of high enzyme concentrations,
supplementation with в-glucosidases during hydrolysis, and removing sugars dur­ing hydrolysis. At this point, the importance of the purity should be mentioned, since water, halides, unreacted organic salts and organics easily accumulate in ionic liquids, thus influencing the solvent properties of the IL, and/or interfering with the biocatalyst.