Recently, a novel type of CDIL has been developed that has high added-value as well as cellulose dissolving ability. Ito and co-workers reported that some amino acid type ILs dissolved cellulose [19]. Especially, N, N-diethyl-N- (2-methoxyethyl)-N-methylammonium alanine dissolved cellulose well at 100 °C. These amino acid-based ILs are halogen-free and polar ILs [34, 35]. Since amino acids are biomolecules, cheap products, and environmentally-friendly materials.

25 30 35 40 45 50 55 60 65 70
Temperature / °С

ILs composed of amino acids are expected to generate more interest as potential solvents in the near future [36—38]. More recently, they reported that the alanine containing salt dissolved 23 wt% cellulose at room temperature with the aid of DMSO [39]. Mixtures of ILs and molecular liquids are gaining attention of IL researchers as new liquids containing the advantages of both IL and molecular liquids. The properties of these mixtures are of course a function of the mixing ratio. In other words, it is not difficult to control the fluid properties by adjusting the mixing ratio when adequate IL and molecular liquids are chosen.

For most processes, highly pure ILs are needed to maintain the efficiency of the cellulose dissolution due to keep their unique properties. ILs have a characteristic properties about vapor pressure, namely ILs are non — or very low-volatile liquids. In other words, it is quite difficult to purify ILs by distillation. Polar and distillable ILs are expected to improve some processes. One of solutions is the use of ILs prepared by the neutralization [40]. King and co-workers reported the distillable acid-base conjugate ILs which has cellulose dissolving ability [41]. They found that the neutralized salt of 1,1,3,3-tetramethylguanidine (TMG) with propionic acid ([TMGH][CO2Et]) has been shown to be technically distillable, and it dissolved 5 wt% cellulose within 10 min at 100 °C. This dissolution capability and distillable property are dependent upon the relative basicity of the competing base, and the equilibrium is temperature dependent.

Phosphonate type salts require low energy cost to dissolve cellulose, and they would be potential solvents for cellulose technology. However, most CDILs have a

Table 2.5 IL structure and cellulose solubility

Table 2.5 (continued)

critical drawback for biomass treatment process, especially for an energy conver­sion system. Addition of a small amount of water to the ILs certainly decreases the cellulose dissolving ability. These ILs cannot dissolve cellulose in the presence of a certain amount of water. Mazza and co-workers reported that the influence of water on the precipitation of cellulose in ILs [42]. Addition of a small amount of water was reported to greatly decrease the cellulose dissolving ability of CDILs. Gericke and co-workers analyzed cellulose precipitation from CDILs by addition of several anti-solvents including water [43]. According to the paper, once dissolved cellulose was easily precipitated from CDILs by the addition of 20 wt% water. This precip­itation was found in all ILs used in the study, namely [C4mim]Cl, [Amim]Cl, and [C2mim][OAc]. Hauru and co-worker also reported the cellulose precipitation from CDILs [44]. The cellulose solutions became turbid by the addition of 2-3 equiva­lents of water, which is equivalent to 20-25 wt% water content. ILs easily absorb water from air [45], and especially CDILs have a high water absorption rate because they are very polar. Generally polar materials are hydrophilic. Troshenkova and co-workers reported on the water absorbability of a CDIL, [C2mim][OAc] [46]. This IL adsorbed up to 27 wt% of water from air at 25 ° C. [C2mim][OAc] was hydrated by the water exothermically (11 kJ mol_1), such values being

comparable to the thermal effect of chemical reactions. This means that CDILs should be sufficiently dried before cellulose treatment, and this might require a considerable amount of energy.

Quite recently, a novel IL derivative was reported as a cellulose solvent which dissolves cellulose without heating even in the presence of water. Abe and co-workers reported that tetra-n-butylphosphonium hydroxide (TBPH) containing 30-50 wt% water dissolved cellulose (15-20 wt% at final concentration) without heating at 25 °C (Table 2.6) [47]. Since this solution contained water, we do not need to dry the cellulose materials before dissolution process. TBPH/water mixture is expected as a potential solvent for cellulose regardless of water content.

New types of ILs specifically designed to wood fractionation applications have been introduced to the field, and some of the new generations of ILs have also being designed to be more suitable for processing steps, such as recycling of the IL after the treatment. Promising alternatives have been found to the most commonly used dialkylimidazolium acetates and chlorides.

One successful effort that has been made towards more sustainable systems is the development of ‘distillable’ ILs capable of dissolving cellulose, by King et al. [15] This group of ILs is based on the acid-base conjugates of 1,1,3,3- tetramethylguanidine (TMG) and common carboxylic acids, such as acetic or propionic acid. The distillation ability arises from the fact that the acid-base equilibrium can be shifted to a sufficient extent, at high temperature, to produce volatile neutral species. Yet, the applicability to wood fractionation has not yet been examined. Anugwom et al. have achieved the selective extraction of hemicelluloses using ‘switchable’ ILs (SIL), that are not capable of dissolving cellulose or lignin from the wood matrix [16, 17]. SILs can be formed by reacting CO2, and 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) with alcohols. They can be converted back to neutral solvents by removal of CO2, under reduced pressure or bubbling with nitrogen. Reversing the IL equilibrium back to volatile components is an interesting potential method for solvent recycling.

In other areas of the IL field, work has been done with the aim of tuning the hydrogen-bonding properties of traditional ILs, to be more selective towards specific wood components. An example of this, by Froschauer et al. [18] demon­strates modified properties of dialkyl phosphate ILs by using sulfur or selenium to replace one of the oxygen atoms in the anion structure. As a result, the hydrogen — bond accepting ability, as described by the Kamlet-Taft parameter p, was reduced. The new type of anions showed a selective dissolution of hemicelluloses out of hemicellulose-rich pulp. In this regard, Kamlet-Taft parameterization of a range of ILs and co-solvents is starting to allow for a better understanding of solvent and wood biopolymer interactions. In work by Hauru et al., the fundamentals of cellulose dissolution and recovery were presented. They have proposed a different approach to interpret the solvent properties of ILs. Rather than simple evaluation of hydrogen bonding accepting properties ф), the donor ability (a) should be consid­ered as well, resulting in an effective or net basicity value ф—a) that better describes the cellulose dissolution capability of ILs [19]. Undoubtedly an in-depth understanding of the hydrogen-bonding properties has a key role in sophisticated design of ILs that can selectively extract various main wood compo­nents. For lignin isolation, Pinkert et al. have applied dialkylimidazolium sulfamates [20], which do not possess any ability to dissolve cellulose and thus can be used to extract lignin out of biomass. Of note, for this class of IL’s, is an earlier application of acesulfamate as a food additive, which makes them promising from an ecotoxicity point-of-view.

Inedible lignocellulosic biomass is a prime candidate as a starting material for 5-HMF production because it presumably would not compete directly with food sources. The development of efficient routes for converting lignocellulose biomass into 5-HMF is essential for achieving sustainable production of 5-HMF. Many research works focus on the transformation of cellulose, since it is the constituent of biomass that can be used to make 5-HMF. However, cellulose is insoluble in many conventional solvents [54—56]. The main advantage of using ionic liquids as reaction media for biomass conversion is the possibility of ionic liquids to dissolve carbohydrate polymers and subsequently form products in one-pot reactions. The conversion of cellulose to 5-HMF can be thought to involve three chemical processes: hydrolysis, isomerisation and dehydration. Although hydrolysis of cel­lulose in ionic liquids in the presence of mineral acids has been studied in detail [57, 58], the efficient conversion of cellulose into 5-HMF with high yield has not been realized until CrCl2 was found to be active for dehydration of glucose into 5-HMF [41]. Su et al. [59] presented a single-step process for cellulose conversion into 5-HMF by using an ionic liquid solvent system with a pair metal chlorides (CuCl2-CrCl2) catalyst, and obtained a 5-HMF yield of 55 % under relatively mild conditions of 120 °C in 8 h reaction time. After the product 5-HMF was separated from the solvent, the catalytic performance of recovered [EMIM] [Cl] and the catalysts were used in repeated experiments. Under these conditions, cellulose depolymerizes at a rate that is about one order of magnitude higher than when using a homogeneous acid catalysis. Single metal chlorides at the same total loading showed considerably less activity under similar conditions [59]. Binder et al. [44] studied the conversion of cellulose, corn stover, and pine dust in DMA-LiCl — EMIM][Cl] mixture using chromium(II) or chromium(III) chlorides as catalysts and hydrochloric acid as a co-catalyst. The mixed system converted cellulose into 5-HMF with a yield of 54 %. When [EMIM][Cl] was used as solvent and CrCl2-HCl as the catalytic system, results were about the same. These examples show the presence of a catalyst is essential for the reaction system and a 5-HMF yield of only 4 % could be obtained in the absence of the catalyst. For lignocellu — losic feedstocks, 5-HMF yields varied from 16 to 47 %. A 5-HMF yield of 16 % was obtained for corn stover substrate regardless of whether it was subjected to pretreatment of ammonia fiber expansion. Lignin and proteins did not seem to affect the results.

Li et al. [47] studied the conversion of cellulose and raw lignocellulosic biomass with microwave irradiation using CrCl3 as a catalyst in [BMIM][Cl] solvent. Initially the investigation was conducted on transformation of cellulose to 5-HMF using cellulose of various degrees of polymerization (Avicel, Spruce, Sigamcell, R-cellulose). The 5-HMF yields varied from 53 to 62 % for all substrates. The authors proposed that the coordination of cellulose with [CrCl3 + n]n- species is responsible for the partial weakening of the 1,4-glucosidic bonds, thus making cellulose more susceptible to attack by water in the hydrolysis step. The authors observed that the use of microwave heating has significant effect in reducing the reaction time, this was also observed by Qi et al., who examined the conversion of cellulose into 5-HMF in [BMIM][Cl] under microwave irradiation, and a 5-HMF yield of 54 % in 10 min reaction time at 150 °C was obtained [19].

A reaction system that could convert microcrystalline cellulose into 5-HMF under mild conditions was reported by Zhang et al. [60] They designed a green process for the conversion of cellulose into water-soluble reducing carbohydrates with a total yield as high as 97 % in the absence of added acid catalysts with the main point of originality of the process being added water to the ionic liquid — cellulose system. The formed carbohydrates could be transformed into 5-HMF in 89 % yield when CrCl2 was added. Such a high 5-HMF yield of nearly 90 % means that not only glucose, but also other water-soluble reducing sugars were converted to product. The total reducing sugars and 5-HMF yields depend not only on reaction temperature and time but also on the amount of added water. By using ab initio calculations, the authors demonstrated that the favorable results obtained in a catalyst system was probably due to the dissociation constant (Kw) of water that increased with the addition of purified ILs. The increased Kw of water by ILs makes the IL-water mixture exhibits higher concentrations of both [H+] and [OH-] than pure water, thus enabling acid — or base-catalyzed reactions to occur. For example, water in the presence of 15 mol% of the [EMIM][Cl] at 120 °C exhibits Kw values up to three orders of magnitude higher than those of pure water under ambient conditions [60]. This intrinsic property of the IL-water mixtures can not only be used in biomass processing and conversion, but also in organic catalysis, electro­chemistry, or other relative research fields.

Due to the inherent environmental risk of chromium, finding of chromium-free processes for conversion of cellulose into 5-HMF is of great significance for biomass utilization. The transformation of cellulose into 5-HMF involves three chemical processes, that is, hydrolysis of cellulose to glucose catalyzed by acid catalyst, isomerization of glucose into fructose by base catalyst, and dehydration of fructose into 5-HMF [22, 24]. Peng and co-workers synthesized acid-base bifunctionalized mesoporous silica nanoparticles with large pore sizes of around 30 nm (LPMSNs) for cooperative catalysis of one-pot cellulose to 5-HMF conver­sion [61]. They used a grafting method to functionalize LPMSN with an acid (SO3H) or/and a base (NH2) group in order to prepare SO3H, NH2, and both SO3H and NH2 functionalized LPMSN (denoted as LPMSN-SO3H, LPMSN — NH2, and LPMSN-both, respectively). Results show that LPMSN-SO3H and LPMSN-NH2 could increase the yield of 5-HMF converted from the reactions that need acid and base catalysts, respectively. The LPMSN-SO3H catalysts were found to promote for one-pot conversion of cellulose to 5-HMF and gave enhanced 5-HMF yields.

The possibility of using DES to extract glycerol was successfully demonstrated on palm oil-derived biodiesel using KOH as a basic catalyst. Authors have used a DES composed of ChCl and glycerine [42] as a solvent to extract residual glycerine contained in biodiesel. They have shown that the best DES/biodiesel ratio is inversely proportional with the percentage of extracted glycerine. The DES com­position is also of prime importance. DESs with low content of glycerol are generally preferred to extract residual glycerine from biodiesel. Generally, the composition of starting ChCl/glycerol eutectic mixture is adjusted in order to have, after extraction of the residual glycerol from biodiesel, an ChCl/glycerine close to the ideal composition of the DES. Best separation was achieved using a DES:biodiesel and a ChCl/glycerine DES molar composition of 1:1. More impor­tantly, at the end of the reaction, ChCl can be recovered by precipitation and reused in combination with glycerol. Shabaz et al. [43] have studied the removal of glycerol from palm oil-derived biodiesel using phosphonium-based salt with dif­ferent hydrogen bond donors (HBD). Novel DESs based on methyltriphenylpho — sphonium bromide as salts and glycerin, ethylene glycol, and triethyleneglycol as hydrogen bond donor were prepared. Glycerol-based DESs were not highly effi­cient to remove residual glycerine contained in biodiesel. Only DESs composed of a 1:2 ChCl/glycerol molar ratio while respecting a DES:biodiesel molar ratio of 2:1, 2.5:1, and 3:1 were found to be efficient. However, DESs made of ethylene glycol or triethylene glycol were found to be more efficient in removing residual glycerol from biodiesel. The optimum DES/biodiesel molar ratios using ethylene glycol or triethylene glycol were 0.75:1.

The authors have also demonstrated that the residual catalyst KOH used in the transesterification of oils can be removed from the reaction media using DES based on choline chloride or methyltriphenylphosphonium bromide (MTPB) salts [44]. In such case, glycerol, ethylene glycol 2,2,2-trifluoroacetamide and triethylene glycol were used as hydrogen bond donors. An increase of the DES/biodiesel and ChCl/ HBD led to a higher KOH extraction efficiency. For instance, the ChCl/glycerol and MTPB/glycerol DESs allowed removal of 98.5 and 94.6 % respectively of KOH from palm oil-based biodiesel.

Very recently, Pablo Dominguez de Maria and coworkers have studied the (trans)esterification of HMF with different acyl donors (ethyl actetate, ethyl hexanoate, dimethyl carbonate, soybean oil, propionic acid, hexanoic acid, lauric acid) in the presence of a biocatalyst [45]. Under solvent free conditions, the yields of HMF esters were higher than 80 % after 24 h of reaction at 40 °C. Although no solvent was used during the (trans) esterification reaction, the selective separation of the unreacted HMF from HMF esters is necessary since the reaction was not complete. In this context, ChCl-based DESs were used for the selective extraction of HMF from HMF esters. Following this approach, more than 90 % of HMF esters, along with a purity higher than 99 %, were recovered after selective extraction of residual HMF by the DES. Investigated DESs were composed of ChCl and either glycerol or xylitol or urea. Regardless of the nature of the DES, the separation was always very selective and the optimal DES/reaction mixture volume ratio was found to be 1. It is noteworthy that this study confirms the previous work of the same authors where alcohol-esters mixtures were efficiently separated using DES [46] (Scheme 3.11).

3.2

Conclusion

From 2000, new generation of solvents so-called bio-inspired ILs and DES derived from ChCl and glycine betaine have emerged as promising candidates for biomass processing. More than a sustainable alternative to the traditional imidazolium — derived ILs (low price, low ecological footprint), these neoteric solvents have clearly processing advantages that no other solvent can provide in the field of biomass. In particular, their tunable viscosity, their ability to dissolve carbohydrates and related biopolymers, their ability to chemically stabilize polar molecules and their immiscibility with commonly used low boiling point solvents has open the route to the design of eco-efficient processes.

In the field of biopolymer dissolution, ILs derived from ChCl are quite efficient especially when combined with a basic anion derived from amino-acid. Although these systems can dissolve various biopolymers such as lignin, starch or suberine, their ability to dissolve cellulose is unfortunately more problematic mainly due to the presence of — OH group on the cholinium cation which is clearly not favorable for the dissolution of cellulose. Addition of additives drastically improves the ability of ChCl-derived ILs to dissolve cellulose but at the expense of the sustainability of the process. One should comment that recent reported works on cations exhibiting close structure to ChCl has shown the way how to design more efficient system from ChCl. In particular, the etherification of the — OH group of choline should be an attractive way. The direct etherification of cholinium cation with short chain alcohols is however quite difficult to be performed under compet­itive route and innovation in this direction is required in order to widen the scope and use of these systems.

Like ChCl-derived ILs, DESs are capable of dissolving various biopolymers except cellulose presumably because their formation results in the auto-association of two components through hydrogen bond interaction, thus preventing an efficient interaction of these systems with the hydrogen bond network of cellulose. DESs are however much more efficient in the conversion of monomeric carbohydrates such

as fructose or glucose. In particular, the ability of ChCl to produce DES with monomeric carbohydrates or low molecular weight biopolymers such as inulin have allowed the production of HMF in a more competitive way than using conventional solvents. Additionally, the ability of ChCl to stabilize hydrogen bond donor such as HMF provides catalytic processes that are tolerant to high loading of fructose, a main drawback encountered with other solvents.

We are fully convinced that ChCl or glycine betaine-derived ILs and DES do have the potential to open new horizons in the field of catalysis applied to biomass. Although promising results have been reported, this approach is not mature yet for use on a large scale and few issues need to be overcome such as the relative instability of ChCl at temperature higher than 120 °C, in some applications their viscosity and, as mentioned above, the necessity to find sustainable routes for the chemical functionalization of the cholinium cation with the aim of widen their use.

The lipids extraction from various resources is essential for production of biodiesel. Usually, the lipids extraction process uses large amounts of organic solvents, such as hexane, CHCl3, etc. which also results in significant losses and energy consump­tion during the solvent recycling process. Hexanes and related hydrocarbon extractants are also becoming an environmental and health concerns. Therefore, exploration of new extraction technologies has received much attention [21].

A new class of “switchable solvents” has been proposed [22, 23], which are based on an exothermic transformation from an organic base, an alcohol and an acid gas (e. g. CO2). These solvents are capable of changing composition reversibly under mild conditions to shift between molecular liquid and ionic liquid, in associated with switching properties, such as polarity and viscosity [24—27]. Thus, ‘switchable solvents’ have been tested to extract lipids from soybean flakes [28]. It was found that the combination of an amidine and excess water gave superior solvent/oil separation, adequate oil extraction. The contamination levels of residual amidine in the soy oil are very low. This method takes advantage of the fact that amidines can be made to switch their hydrophilicity by application or removal of CO2 in the system. However, the extraction efficiency is lower than those of traditional hexane, and ethanol.

Microalgae are one of the most important emerging resources for lipids. In 2010, Samori, et al. extended the switchable solvent to the lipid extraction from water- suspended and dried microalgae Botryococcus braunii. It was found that DBU/octanol exhibited the highest yields of extracted hydrocarbons from both freeze-dried and liquid algal samples (16 and 8.2 % respectively against 7.8 and 5.6 % with n-hexane) [29]. Their follow-up research demonstrated that a new switchable system based on the reversible reaction of N, N-dimethylcyclohexylamine (DMCHA) with water showed better performance in lipid extraction of wet algal samples or cultures (Fig. 7.2) [30].

The lipid extraction efficiency of the system applied to both of wet and dry biomass, was higher than that obtained through a typical extraction procedure with CHCl3-MeOH (Tables 7.1, 7.2). The FAMEs yield was very good for all of the tested algae, independent of the biomass/DMCHA ratio. The higher extraction content may be because DMCHA had access to structural lipids which are resistant to extraction with CHCl3-MeOH [30].

In 2010, Young et al. reported the ability of a co-solvent system composed of a hydrophilic IL 1-ethyl-3-methyl imidazolium methyl sulfate and methanol at a mass ratio of 1.2:1 to extract and auto partition lipids from various biomass. The extraction yields of lipids were summarized in Table 7.3. The results suggest that the ILs-methanol co-solvent is successful in complete extraction of the lipids from these biomass sources. The proposed IL-methanol co-solvent system differs from traditional organic co-solvent systems which both dissolve and solubilize the extracted lipid and thus suffer extraction efficiencies limited by the solvent’s carrying capacity [31].

Considering the main components (e. g. polysaccharide, protein, lipids) of algae, Teixeira investigated the use of traditional ILs to extract lipids and produced chemical feedstocks from algae without acids, bases or other catalysts, which is based on the fact that a dissolution and hydrolysis of wet alga biomass in ILs. Deconstruction reached completion in <50 min regardless of algae species, at 100-140 °C and atmospheric pressure. The fast rate of hydrolysis without acids or bases suggests the ionic liquid itself is acting as both a solvent and a catalyst. Depolymerization of algae cell wall polysaccharides could result in the deconstruc­tion of the cell wall, including the phospholipid membrane, and creation of a cell — free mixture that can be separated into constituent fractions, and result in a full separation of lipids from algae. This work presented a strategy of full utilization of algae biomass [32].

Table 7.2 Lipids fractionation of oils obtained through extraction of dried algae samples (CHCl3- MeOH) and wet algal biomass (DMCHA), expressed on algal dry weight basis (Reprinted with permission from [30]. Copyright © 2012 The Royal Society of Chemistry)

Many enzymatic reactions in ionic liquids have been reported over the last decades. The performance of biocatalyst in ionic liquids reveals that ionic liquids are not only environmentally friendly alternatives to organic solvents but also good solvents for many enzymes and whole cell catalysts. The first example of biotransformation ionic liquids was reported by Lye et al. in 2000 [17]. It involved a whole cell biotransformation of 1,3-dicyanobenzene to 3-cyanobenzamide, with a Rhodococcus sp. in a biphasic [Bmim][PF6]/H2O medium. In this example, ionic liquids essentially act as a reservoir for the substrate and product, thereby decreasing the substrate and product inhibition observed in water. In principle, an organic solvent could be used for the same purpose but it was found that ionic liquids caused less damage to the microbial cell than organic solvents, for example, toluene [18].

The first use of isolated enzymes in ionic liquids was commonly recognized by the report of Erbeldinger et al. in 2000 [19] although Magnuson et al. earlier demonstrated the activity and stability of alkaline phosphatase in aqueous mixtures of [EtNH3][NO3] in 1984. However, the finding of Magnuson et al. did not attract much attention due to the lack of knowledge of ionic liquids at that time. In the work of Erbeldinger et al., thermolysin-catalyzed synthesis of Z-aspartame in [Bmim][PF6]/H2O (95/5, v/v) showed comparable reaction rate to those observed

in ethylacetate/H2O. In addition, the enzyme exhibited a higher stability in the ionic liquids/water medium although the small amount (3.2 mg • mL-1) of enzyme that dissolved in ionic liquids was catalytically inactive. At the same time, Sheldon et al. [20] showed that Candida antarctica lipase B(CALB), either free enzyme (SP525) or an immobilized form (Novozym 435) is able to catalyze a variety of biotransformation in [Bmim][BF4] or [Bmim][PF6]. Since then, a wide number of enzymes have been examined in ionic liquids such as lipase, protease, oxidoreduc — tase, peroxidase, cellulase, whole cells, and so forth (Table 10.1). Among them, lipase is the major enzyme reported so far in ionic liquids since it is considered as “work horses” of biocatalyst in various potential applications from fine chemical, chiral compounds, biopharmaceuticals to bulk chemicals such as surfactants, and biodiesel.

The uses of ionic liquids in biocatalysis can be classified into anhydrous ionic liquid system, aqueous/ionic liquids system using ionic liquids as co-solvent or additives, aqueous/ionic liquid two phase system, organic solvent/ionic liquids system, and supercritical CO2/ionic liquid systems. A large number of enzymes (e. g. lipases, proteases, peroxidases, dehydrogenases, glycosidases) and reactions (e. g. esterification, kinetic resolution, reductions, oxidations hydrolysis, etc.) have been tested in monophasic system based on ionic liquid [14, 15, 33]. While most water-miscible ionic liquids have been shown to act as enzyme — deactivating agents at low water content, water-immiscible ionic liquids (e. g. [Bmim][Tf2N], [Bmim] [PF6], etc.) act as suitable reaction media for enzymatic catalysis in the same conditions. The hygroscopic character of water-immiscible ionic liquids could be regarded as an additional advantage of these solvents, because enzymes require a certain degree of hydration to become active [34]. In addition, biphasic systems based on ionic liquid-water or ionic liquid-organic solvent have been assayed for biocatalytic processes [35, 36]. For example, the lipase catalyzed kinetic resolution of rac-ibuprofen in a water — ionic liquids biphasic medium [37], lipase catalyzed production of isoamyl acetate in an ionic liquid-alcohol biphasic system [38], and lipase catalyzed kinetic resolution of rac-1-phenylethanol in both ionic liquid/ hexane [39]. In these biphasic reaction systems, the products can be easily recov­ered by liquid-liquid extraction. However, in the case of hydrophobic compounds, liquid-liquid extraction with organic solvents might breakdown the greenness of the process. However, product recovery from ionic liquid-enzyme reaction media by another non-aqueous green solvent, such as supercritical CO2, is nowadays considered the most interesting strategy for developing integral clean chemical processes [34].

Many works and excellent reviews have been published during the last decade regarding the use of enzymes in ionic liquids [14—16, 40—47]. The results showed that enzymes in ionic liquids have many advantages compared to those in organic solvents such as high activity, high selectivity including substrate, region- and enantioselectivity, better enzyme stability as well as better recoverability and recyclability. In addition, ionic liquids can be used for carrying out biotransforma­tion with polar or hydrophilic substrates (e. g. amino acid, carbohydrates) which are not soluble or sparingly soluble in most organic solvents.

Advantages of using enzymes in ionic liquids are summarized as follows:

— Ionic liquids can be designed for particular bioprocesses

— Enzymes show excellent operational and thermal stability in ionic liquids. Therefore, bioprocesses can be conducted at high temperature due to high thermal stability of enzymes in ionic liquids

— Enzymes can be easily recovered and recycled simply by filtration or centrifugation

— Products and substrates can be recovered by evaporation, extraction with organic solvents or with supercritical CO2

ILs used in cellulose and biomass pretreatment has attracted much interest due to their excellent properties, such as the low melting point, high thermal stability and solvation capacity, especially the hydrogen-bonds among the cations and anions [43]. Moreover, the promising diversity of ILs suggests that appropriate ILs will be non-volatile polar solvents for carbohydrate dissolution and biomass pretreatment.

1.5.1 Melting Point

The melting point of a compound represents the lower limit of the liquid range and together with thermal stability defines the temperature window in which it can be used as a solvent [44].

The melting point is defined as the temperature of equilibrium between solid and liquid state in thermodynamic. Since the change of the Gibbs free energy equals zero at the equilibrium, the melting point (Tm) is defined as follows [45]:

Recently, many researchers study the effect of structural features on melting point because the melting point is an important factor for employment of ILs as a reaction media. Factors influencing in ILs melting point are charge, size and the distribution of charge on the constituent ions. For the same class of ILs, small changes in the shape of the uncharged, covalent regions of the ions need to be considered [46]. For example, the melting point of imidazolium based ILs is influenced by four factors: electron delocalization, H-bonding ability, the symmetry of the ions, and van der Waals interactions. Both alkyl substitutions on the cation and ion asymmetry have been shown to interfere with the packing efficiency of ions in the crystalline lattice [47]. Researchers are always try to find that how the chemical structure affects the melting point by many different measuring methods. The following trends can be concluded from open literature:

1. The sizes and shapes of cations of ILs are important factors influencing the melting points of ILs. In general, as the size of the cation increases, the melting point of the salts decreases.

2. With increasing the size of the anion, the melting point of the salts decreases, which reflects the weaker columbic interactions in the crystal lattice. For instance, from Cl — to [BF4]- to [PF6]- to [AlCl4]-, the melting points of the sodium salts decrease from 801 to 185 °C with increasing thermochemical radius of the anion [47].

3. With the same anion, symmetry of the alkyl substitution also affects the melting point of ILs. Generally, highly asymmetric alkyl substitution has been identified as important for obtaining high melting point. For example, the melting point of [Mmim]Cl is 124.5 °C while the melting point of [Bmim]Cl is 65 °C.

4. The alkyl chain length of cations also affect melting point of ILs. For example, as for 1-alkyl-3-methylimidazolium tetrafluoroborate, with an increase in the alkyl chain length (up to n = 8), the melting point decrease, where n is the number of carbon atoms. But the melting point of ILs increases gradually with increasing chain length when n > 8. The same condition occurs in 1-alkyl-3- methylimidazoliumbis (trifyl) imide [46].

5. For ILs in which the only difference is the degree of branching within the alkyl chain at the imidazolium ring, higher degree of branching within the alkyl chain, higher melting point of ILs.

Owing to their unique properties, ILs are widely used as a kind of versatile solvents in biomass separation/conversion. Most ILs used for biomass pretreatment have low melting points, so that they are liquid at room temperature, the melting points of the ILs listed in Fig. 1.2 are all lower than 350 K [26]. That’s because

lower melting points of ILs make them easy to handle and are known as a technical advantage on the recyclability of the solvents. In recent years, some new types of ILs that have lower melting points and a sufficient polarity to further process carbohydrates have been claimed to replace chloride-based ILs [48]. These ILs are formate, acetate or phosphate salts with imidazolium based cations. They have been proven to be potential solvents to dissolve cellulose under mild condition.

Phenyl carbamates of celluloses from different sources, including bacterial cellu­lose, have been prepared in BMIMCl as solvent by homogeneous conversion with the corresponding isocyanate [14, 15]. No byproducts are formed during this reaction and the DS could be tuned over the whole range, up to a maximum of 3.0, by adjusting reaction time and the amount of derivatization reagent. ‘Carbanilation’ proceeds with only minor chain degradation and yields organosoluble derivatives. Consequently it is an important tool for determination of the molecular weight of cellulose by size exclusion chromatography [74]. More­over, cellulose carbamates can be utilized as chiral stationary phase for the chro­matographic separation of enantiomers [59].

ILs have been exploited successfully for the homogeneous preparation of several cellulose-gra/f-copolymers. Grafting of poly-L-lactide and poly(e-caprolactone) chains onto the cellulose backbone has been achieved by ring opening polymeri­zation with DMAP or stannous octanoate as catalyst [43—45, 47]. Depending on the amount of monomer used, different DS values in the range of 0.1-2.7 could be achieved; the average number of repeating units was rather low («1-5). Thermo­plastic mixed derivatives containing acetyl moieties and grafted poly (L-lactide) chains have been obtained as well in a combined ‘one-pot’ reaction [46]. Cellulose — graff-poly(acrylamide) and — poly(accrylate) have also been prepared in ILs, by radical polymerization, induced by y-ray irradiation or persulfate initiation [48, 49]. In the latter case, the reaction was performed under microwave heating and a certain amount of bivalent cross linker was present, i. e., a heterogeneous polymer gel was obtained. Also the grafting of copolymers onto cellulose via ATRP has

been studied in ILs but no advantages over common molecular solvents in terms of the conversion rate could be observed [26]. Thus, it is important to decide if ILs can be beneficial in this process. At DS values >0.6, the macro initiators required in the first step dissolves, e. g., in DMF, dioxane, or butanone, i. e., ILs or other cellulose solvents are not required [23]. However, if low grafting densities are desired, ILs might prove to be useful as reaction media for homogeneous grafting of polymers onto cellulose via ATRP.

Recently, Su et al. demonstrated the ability to use paired metal chlorides for the depolymerization of cellulose into monosaccharides and other products. In this work, CuCl2, CrCl2, CrCl3, PdCl2, and FeCl3 were initially tested as catalysts, but none were effective on their own. It was then discovered that by pairing CuCl2 and PdCl2, high yields of monosaccharides could be obtained. The total yield of products (including glucose, cellobiose, 5-hydroxymethylfurfural, and other) from this reaction system was found to be as high as 70 % with glucose yields up to roughly 45 %. This yield is much higher and occurs much faster than when using sulfuric acid under the same conditions [99].

11.3.1.1 IL Polarity

One of the key parameters in the biocompatibility of ILs with enzymes is their polar character, which is not to be confused with the non-water miscibility of some of them (Fig. 11.5).

The subject of ionic liquid polarity has been addressed using a variety of methodologies, including measuring the absorption maximum of a solvatochromic dye, eg. Nile Red or Reichardt’s dye, using a fluorescent probe, measuring the keto — enol equilibria (which are known to depend on the polarity of the medium), microwave dielectric spectroscopy measurements, etc. [79—82]. As a rule, the polar character of anions decreases as a function of the size/delocalization of the negative charge (e. g. [Cl] > [NO2] > [NO3] > [BF4] > [NTf2] > [PF6]), while, in the case of cations, polarity is mainly determined by the length of the alkyl chain groups. Simple chemical reasoning predicts that a polar medium will dissolve polar compounds, such as carbohydrates. By this measure, the ionic liquid [BMIM][BF4], which is hydrophilic, and [BMIM][PF6], which is hydrophobic, fail the polarity test, because both dissolve less than 0.5 g/L of glucose at room temperature [68]. The concept of solvent polarity with ILs is too elusive to serve as a basis to predict, for example, solubility or reaction rates. There are, moreover, indications that solvent-solute interactions of ionic liquids obey a dual interaction model (i. e. ionic liquids behave as non-polar solvents with non-polar solutes but display a polar character with polar solutes), even to the extent that ionic liquids should be regarded as nanostructured materials [8]. However, the IL polarity-enzyme activity correlation has not been established for many enzymatic reactions performed in ILs [15, 19, 83].

Yang and Pan [9] suggested that enzyme activity may be related more to the viscosity and less to the polarity of ionic liquids. Reaction rates have usually been compared in different ionic liquids when the same amount of water is present in the reaction system (e. g. 2 % v/v of water). Therefore, the higher reaction rates in more polar ionic liquids [25, 84] can be explained by the effect of viscosity. Under such conditions, the solvent of higher polarity would leave less water associated with the enzyme and more water remaining in the solvent: the former would result in a lower reaction rate, while the latter would lead to a reduction in the viscosity of the ionic liquid and, in turn, to an improvement in the mobility of the protein molecule.