While catalysis of lignin conversion in ILs has not received the same attention as cellulose and monosaccharides, there is growing interest in the application of ILs to the catalysis of lignin conversion. The main goal of much of the research relating to lignin in ILs have been for the purpose of pretreatment [76, 80, 85, 133]. Sun et al. demonstrated the ability to fractionate wood into cellulose rich and lignin rich samples using EMIMAc and acetone/water as an antisolvent for cellulose followed by evaporation of acetone to precipitate the lignin [74]. Other groups then used this discovery as a stepping stone to pretreatment of biomass. While some use ILs as a path towards delignification for pretreatment [76], others have shown that a loss of cellulose crystallinity is also a source of IL pretreatment efficacy [14, 77, 79]. The other work in catalysis of lignin conversion has covered thermal and chemical depolymerization and hydrodeoxygenation. As lignin is a complex, amorphous polymer, most studies on the catalysis of lignin conversion work with lignin model compounds as a way to test a process while keeping analytical complications to a minimum.

One of the simplest treatments of lignin in ILs is to simply dissolve lignin and heat the IL/lignin mixture for a period of time. Kubo et al. performed a series of experiments with the lignin model compound guaiacylglycerol-p-guaiacyl ether (GG) mixed with BMIMCl, EMIMAc, or AMIMCl at 120 °C. This model compound simulates the p-O-4 ether linkage, which is the most common struc­ture in lignin. The main product of this reaction was 3-(4-hydroxy- 3methoxyphenyl)-2-(2-methoxyphenoxy)2-propenol, which is an enol ether (EE). EE is the dehydration product of GG, and an analogous process has been implicated as an intermediate in the depolymerization of lignin under both acidic and alkaline conditions [134]. This intermediate has been detected in other studies involving lignin model compounds.

The cleavage of the P-O-4 ether linkage is a possible pathway both to general delignification of biomass and to the utilization of lignin as a feedstock for fuel and chemical production [135, 136]. A number of methods have been explored to sever this bond. In one study, N-bases were used in 1-butyl-2,3-dimethylimidazolium chloride (BDMIMCl). It was demonstrated that the base 1,3,5-triazabicyclo[4.4.0] dec-5-ene (TBD) was effective at cleaving the p-O-4 ether linkage in GG with a yield of up to 23 %. As in the study by Kubo et al., EE was observed as an intermediate. Other N-bases, including 7-methyl-1,3,5-triazabicyclo[4.4.0]dec-5- ene (MTBD), did not show the same activity as TBD, indicating a unique func­tionality for TBD. It was suggested that TBD could act as a combination base and nucleophile to break down lignin using the same mechanism as kraft pulping, although the process was not shown to be catalytic [137]. The trialkylimidazolium IL was utilized in this study as opposed to the more common dialkylimidazolium ILs because the hydrogen at the 2 position on the dialkylimidazolium ring can be extracted under basic conditions to form a reactive carbene [49]. Another method, using metal chlorides as catalysts, was demonstrated to be effective at the hydro­lysis of both phenolic and non-phenolic lignin model compounds. In this study, GG and veratrylglycerol-p-guaiacyl ether (VG) were used to model the P-O-4 ether linkage in lignin. FeCl3, CuCl2, and AlCl3 were shown to be effective at catalyzing the hydrolysis of the ether linkage with AlCl3 showing the highest yield of 80 % for GG and 75 % for VG. The metal chlorides most likely acted as acid catalysts to break the bond through the same mechanism of other acid promoted systems [138].

The task-specific acidic IL HMIMCl has been shown to catalytically hydrolyze the ether bonds in both GG and VG up to a 71.5 % yield. The mechanism for this hydrolysis starts with dehydration into an enol ether structure, which is then susceptible to acidic attack of the P-O-4 ether linkages. This process, as shown in Fig. 8.8, occurs both in the individual model compounds and dimers of the model compound that form under reaction conditions [139]. This method was extended to a number of other acidic ILs. ILs composed of 1-H-methylimidazolium cations and chloride, bromide, hydrogensulfate, and tetrafluoroborate anions, along with BMIMHSO4 were used to hydrolyze the P-O-4 ether linkage in GG and VG. HMIMCl was found to be the most effective of these ILs. The Hammett acidity of each of these ILs was measured using UV-vis measurements to determine protonation of 3-nitroanaline added to the ILs, but the acidity of the IL did not correlate with the yield of hydrolysis products. The efficacy of acid catalyzed hydrolysis in these ILs was determined by the ability of the anion to hydrogen bond with the hydroxyl groups on the lignin model compounds [88]. Further study demonstrated the ability of HMIMCl to depolymerize lignin through acid catalyzed hydrolysis of the P-O-4 ether linkage. The lignin used was extracted from oak wood using EMIMAc. Treated lignin was demonstrated to be reduced in size from the untreated lignin and the disappearance of the ether structures was observed through NMR and IR spectroscopy [136].

Other transformations of lignin have been demonstrated in ILs as well. Binder et al. performed work with many catalysts in EMIMCl and 1-ethyl-3-methylimi — dazolium triflate (EMIMOTf). While a number of catalysts were able to dealkylate the lignin model compound eugenol, these catalysts failed to produce monomeric products from organosolv lignin [140]. Further use of metals for catalysis of lignin in ILs was demonstrated by Jiang and Ragauskas. This study

Hibberts Ketones +Unknowns

Hibbert’s Ketones

Fig. 8.8 Proposed acid-catalyzed mechanism for hydrolysis of P-O-4 bonds lignin model com­pounds (Adapted with permission from [138]. Copyright 2010 John Wiley and Sons) dealt with the use of vanadyl acetylacetonate in BMIMPF6 along with Cu(II) or Cu(I) co-catalysts to selectively oxidize aromatic alcohols into carbonyl or carboxylic acid groups. While most of this work focused on a wide variety of alcohols, 3,4-dimethoxybenzyl alcohol and 1-(3,4-dimethoxyphenyl)ethanol were specifically noted as being model compounds for lignin [141]. Other work has been performed with metal catalysts in ILs for the purpose of deoxygenation of lignin model compounds. In a study by Yan and coworkers, cyclohexanol was dehydrated into cyclohexene with Br0nsted acidic ILs. Then, by combining the acidic ILs with Ru, Rh, or Pt nanoparticles, phenolic lignin model compounds were hydrogenated and deoxygenated to non-aromatic hexane species [142]. Other work on hydrodeoxygenation of lignin in ILs have been limited because the temperature at which traditional hydrodeoxygenation catalysts function exceeds the stability limit of IL, especially of those that have the ability to solubilize biomass.

Soiene Guiheneuf, Alfredo Santiago Rodriguez Castillo, Ludovic Paquin, Pierre-Francois Biard, Annabelle Couvert, and Abdeltif Amrane

Abstract The coupling of absorption in a gas-liquid contactor and biodegradation in a two-phase partitioning bioreactor (TPPB) has been shown to be a promising technology for the removal of hydrophobic volatile organic compounds. The choice of the organic phase is crucial, and consequently only two families of compounds comply with the requested criteria, silicone oils and ionic liquids. These latter solvents appear especially promising owing to their absorption capacity towards hydrophobic compounds and their low volatility, as well as the possibility of IL tailoring, allowing a fine-tuning of their physicochemical properties, leading to a wide range of products with various characteristics. Some results on common ionic liquids are highlighted in this chapter: biodegradation rates reported by some authors show that phenol biodegradation in the presence of ILs is up to 40 % higher than those obtained in other multiphase reactors; there is a strong affinity of toluene and DMDS for imidazolium salts, [C4Mim][PF6] or [C4Mim][NTf2]. Performance improvements may be expected from the tailoring of ionic liquid structure, espe­cially towards toxicity reduction. Positive results recorded after cell acclimation to target compounds let expect an important gain from more complex acclimation strategies, including microbial acclimation to both ionic liquids and pollutants.

Keywords Absorption • Activated sludge • Biodegradation • Ionic liquids • Two — phase partitioning bioreactors • Toxicity • Volatile organic compounds • Separation

S. Guiheneuf • L. Paquin

Universite de Rennes 1, Sciences Chimiques de Rennes, UMR CNRS 6226, Groupe Ingenierie Chimique & Molecules pour le Vivant (ICMV), Bat. 10A, Campus de Beaulieu, Avenue du General Leclerc, CS 74205, 35042 Rennes Cedex, France

Universite europeenne de Bretagne, Rennes Cedex, France

A. S.R. Castillo • P.-F. Biard • A. Couvert • A. Amrane (*)

Ecole Nationale Superieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du General Leclerc, CS 50837, 35708 Rennes Cedex 7, France

Universite europeenne de Bretagne, Rennes Cedex, France e-mail: abdeltif. amrane@univ-rennes1.fr

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_12,

© Springer Science+Business Media Dordrecht 2014

12.1 Introduction

When absorption is used to remove pollutants present in the atmosphere, an aqueous phase is generally employed, either water or water containing reagents (acid, basis, oxidant, etc.). More rarely, some organic phases (solvents) are implemented, but their cost implicates their recycling, and then, their regener­ation. The use of this type of liquid phase becomes primordial when gaseous pollutants belong to hydrophobic compounds family (i. e. toluene, benzene, xylene, etc.) since their affinity for the liquid phase in which they have to be absorbed must be important. This means that there is an important issue in finding new absorbents, displaying high absorption potentialities facing many volatile organic compounds (VOC), and able to be regenerated. For this purpose, absorbent regeneration could be considered after VOC biodegradation; the latter low cost process appears therefore promising. This implies that the solvent will have to fulfil several conditions, especially the absence of biodegradability.

Among the solvents available on the market, a wide number appears biode­gradable, even if an acclimation time is often needed. Indeed, various com­pounds having very low degradation rates or totally refractory towards microorganisms are described as bio-recalcitrant. However, a total absence of biodegradation, even after an acclimation time is required for the proposed process. Therefore and based on biodegradability and biocompatibility criteria, among the available solvents two classes can be selected: silicone oils and ionic liquids [1]. These latter appear especially promising owing to their solvent capacity and their low volatility (saturated vapour pressure close to zero), as well as the possibility of IL tailoring to fit the characteristics required for specific applications [2]. Only few reports are available dealing with the use of ILs for pollutants removal; phenol biodegradation has been investigated in the presence of an IL [3]; while the previous works of Quijano et al. [4, 5] are the only reports dealing with the affinity of ILs for the absorption of hydropho­bic odorous compounds and their biodegradation in a multiphase bioreactor. These works led to promising results for toluene after cell acclimation to the VOC, while for DMDS more complex strategies, including acclimation to ILs, should be subsequently considered.

The potential of this class of compounds for the absorption of hydrophobic VOCs and the subsequent biodegradation of these compounds in a multiphase bioreactor involving ILs (containing the absorbed VOC) as an organic phase and an aqueous phase containing microorganisms are discussed thereafter. The process considered to implement such solvents in a whole operation is schematically described in Fig. 12.1.

3.2.1 Dissolution of Cellulose in Non-derivatizing Media

Cellulose is a highly valuable biopolymer of glucose from which chemical platforms, intermediates, ethanol and fuel additives can be then produced [4, 5]. All these processes initially imply the catalytic deconstruction of cellulose to glucose. The high crystallinity of cellulose is a serious bottleneck which is at the origin of the low accessibility of cellulose to (bio)catalysts. Hence, in many cases, harsh conditions of pressure and temperature are required for the deconstruction of cellulose making the control of the reaction selectivity very difficult. To overcome this issue, cellulose is generally subjected to a pre-treatment process prior to catalytic deconstruction. This pre-treatment aims at favoring a better accessibility of the cellulosic backbone to catalyst by reducing its crystallinity or particle size or degree of polymerization for instance. In this context, much effort has been recently devoted to the search of innovative media capable of dissolving and thus disrupting the supramolecular organization of cellulose. Dissolution of cellulose in a non-derivatizing solvent is an interesting approach that allows a change of the cellulose structure from a highly crystalline to a low crystalline structure, a key parameter in the subsequent catalytic hydrolysis of cellulose to glucose. After the dissolution process, cellulose is generally recovered, by precipitation, upon addition of an anti solvent such as ethanol or water. Historically, mixtures of DMSO/LiCl or DMA/LiCl (among other combinations) and more recently N-methylmorpholine-N-oxide (Lyocell process) have been used for the dissolution/decrystallization of cellulose [6]. Although these systems ensure a drastic decrease in the crystallinity index of cellulose, their recycling is difficult and rather expensive. Recently, ionic liquids (ILs) have received considerable attentions because of their ability to dissolve and thus to decrease the crystallinity index of cellulose (Scheme 3.2).[1]

Dissolution of cellulose in ILs has been firstly demonstrated at the beginning of the twentieth century in particular using ethyl ammonium nitrate [8, 9]. Nowadays, the use of ionic liquids for the dissolution/decrystallization of cellulose is now witnessing a sort of renaissance with the recent emergence of room temperature

ILs recycling

I—————————————————— 1

Scheme 3.2 Decrystallization of the cellulosic fraction of lignocellulosic biomass by dissolution in ILs

ILs. To date, plenty of works have been recently reported in this field of chemistry and this topic is too large to be summarized here. Complementary information to this section can be found in excellent recent reviews.[2]

Analysis of the specialized literature reveals that two parameters mainly govern the dissolution of cellulose in ILs (1) the ability of ILs to disrupt the extensive hydrogen-bond network of cellulose and (2) hydrophobic interactions. In ILs, dissolution rate of cellulose closely depends on the temperature, time of heating and molecular weight of ILs.[3]

Anion of the ILs plays an important role in the dissolution process by inducing polar-interaction with the hydroxyl groups of cellulose thus weakening the hydro­gen bond network of cellulose. To date, chloride is one of the most efficient anion but its exact role is still subject to controversy in the current literature. Other anions such as acetate, formate or phosphate have been also proven to be effective. More generally, anion with a basic character seems to be more favorable for the disso­lution of cellulose. The cation composing the ILs plays also a major role in the dissolution process. Due to intra — and intermolecular hydrogen bonding, cellulose is composed of flat ribbons with sides that differ markedly in their polarity. Hence, amphiphilic cations are generally required in order to ensure an efficient dissolution of cellulose. The size of the cation is also a parameter that is taken into account in few literatures and an optimal size should be found in order to favor the diffusion of ILs within the cellulose microfibrils. In this context, the imidazolium moiety closely meets all these requirements.

Although elucidation of the exact mechanism governing the dissolution of cellulose in ILs is still not really clear, combination of an amphiphilic cation with a basic anion seems to be a good compromise. It is more or less accepted that the

cation has the role to slide open the cellulose fibrils and to transport the anion within the cellulose backbone where it interacts with the hydrogen bond network. To date, 1-butyl-3-methyl-imidazolium chloride and 1-ethyl-3-methyl-imidazolium acetate are considered as the best ILs for the dissolution of cellulose. Despite the remark­able ability of these ILs to dissolve cellulose, their industrial emergence is unfor­tunately hampered by their high cost, reactivity, toxicity and high viscosity. Additionally, RTILs are highly hygroscopic and presence of water, even in a trace amount, has a detrimental effect on the dissolution of cellulose. Hence, room temperature ILs are nowadays only regarded as excellent models to under­stand the mechanism governing the dissolution/decrystallization of cellulose.

For fully soluble finely pulverized materials, the dissolution is relatively rapid for all components. Dissolution is much faster compared to coarse materials and wood converts to a completely solvated state. Solvation was not complete with the coarse

materials. The following dissolution and fractionation mechanism seemed to differ greatly from the pulverized materials, based on compositional analyses of isolated fractions (Table 6.2) and XRD-analysis of the sawdust fraction 1 and the original sawdust (Fig. 6.3).

For the incomplete dissolution of the coarser materials (sawdust and TMP), fraction 1 was mostly composed of the materials that remained solid (but seemingly swollen) during the whole dissolution/extraction period. This was determined to be mainly lignin and polysaccharide. Fraction 2 was determined to be mainly cellu­lose, based on Klason lignin and ATR-IR analyses (Fig. 6.4). As fraction 2 was 33 % of the original sawdust fraction, meaning that most of the cellulose is extracted from the partially soluble wood sample leaving an insoluble matrix of lignin and hemicellulose. Further evidence of this is found after XRD analysis of the regenerated fraction 2, in comparison to the starting sawdust. After extraction of cellulose, the amorphous LCC network was remaining. One should ask the ques­tions, ‘Why is there an insoluble fraction when the purified polymers are all soluble in the IL?’ and ‘Why can lignin not be separated efficiently from the polysaccha­ride, even when the finely pulverized samples are completely soluble?’.

Both questions can be addressed by the explanation of precipitation based on molecular weight. However, most solvents will preferentially precipitate one com­ponent over another and this is simply not the complete picture. Both questions can be answered if you take in to consideration that wood is suggested to contain LCCs and it is actually the LCC network that is preventing dissolution. Only material that is not bound into the LCC network is extractable, under mild dissolution conditions.

This is entirely consistent with a report by Lawoko et al. [69] showing that almost all isolatable LCCs from Norway spruce consist of lignin, which is chemically bonded with hemicelluloses. Whereas, only a minor portion of spruce LCCs have been found to contain lignin-cellulose type LCCs. With this literature confirmation it is no wonder that we can extract cellulose from an insoluble LCC matrix.

As anticipated, it was nearly impossible to derivatize and dissolve any further material from fraction 1 for SEC, due to its insolubility. In case of highly pulverized wood, physical degradation of all polymeric components seem to take place and overall polydispersity decreases. Fractions from ball milled TMP pulp further gave more evidence about the close association with lignin and carbohydrates and how these structures could control the total dissolution of wood. The FT-IR analysis offered some details about the carbohydrate compositions in isolated fractions. Neutral sugar analysis is traditionally used to characterize the carbohydrate moie­ties in lignocellulose, but in this work FT-IR was used instead as a fast, non-destructive, and semi-quantitative analytical method. IR spectra of the isolated fractions from two spruce materials, ball-milled TMP and sawdust, are presented in Fig. 6.4. When spectra of fraction 1 from the two materials are compared, the significant presence of hemicelluloses can be seen for sawdust, whereas in milled

TMP the carbohydrates seem to be mostly cellulose. The hemicelluloses are more present in the lower molecular weight fraction 2 for the case of milled TMP. Other significant differences can be found in composition and yield of the water-soluble fraction 4. For sawdust it seems that the majority of the hemicelluloses have remained totally water insoluble in fraction 1, for some yet unknown reason. For milled wood around half of the original hemicelluloses were converted to be water soluble and dissolved together with relatively large portions of lignin. No lignin was observed to be water soluble from sawdust crude fractions.

Once again, this observation could be explained by the covalent attachment of hemicelluloses that are released during the milling via the fragmentation of the supporting lignin polymers, that otherwise would prevent them from being extracted during the water washing.

Sawdust and TMP pulp preparations represent structurally quite unaltered wood. Our results suggest that swelling and dissolution of native or relatively intact fibers start from the amorphous and crystalline domains of cellulose. The solvated cellulose polymers then diffuse to the bulk solution (fraction 2) leaving behind the lignin-hemicellulose matrix that remains in a rather swollen form and is restricted from complete dissolution (fraction 1). Only minor fractions of lignin (fraction 3) or hemicelluloses (fraction 4) seem to be unbound and transfer to bulk solution. Molecular weight analysis showed that the isolated cellulose-rich fraction 2 had a significantly lower molecular weight than reported for e. g. softwood pulps.

From the dissolution treatment, it is hard to estimate if there is significant depolymerization during the 48 h dissolution period. For the sawdust fractionation, some depolymerization of the carbohydrate components during the dissolution seems evident. Molecular weight of the isolated fraction 2 is low, considering the fact that this fraction was composed mostly of cellulose, as our analysis revealed (see Table 6.2 and Fig. 6.4). In light of a recent study by Gazit and Katz [42], the depolymerization of cellulose during long dissolution periods, even in purified IL, is not surprising. Their results indicate that trace level formation of acidic by-products can take place during dissolution, even below the temperatures that were used in our work. The unfortunate fact is that technical pulps have very defined specifications, in terms of molecular weight distributions, and not only their lignin and hemicellulose contents. This means that controlling acidic and oxidative impurities during an IL-mediated fractionation will be critical in the future to obtain technically useful pulps that fit existing value-chains. In many cases even the present commercial ILs contain these impurities. More must be done to quantify and understand the effects of these impurities.

The single experiment that was performed with Eucalyptus resulted in a lignin poor fraction that was the first to precipitate from the IL-solution (fraction 1). As mentioned previously, precipitation was dependent on molecular weight for Euca­lyptus, as well as milled spruce. It remains as a topic for further studies how differences in covalent structures between lignin and polysaccharide will affect the selectivity of separation. Other ILs like [emim][OAc] have been reported to fully dissolve hardwood [8]. This may be due to chemical degradation of LCC-matrix during the treatment, as discussed earlier. It remains possible that other ILs could facilitate higher yields of cellulose-enriched materials by gradual precipitation, only if the LCC cleavage and carbohydrate depolymerization remains at a low level during the dissolution.

6.3 Conclusions

Due to the rapidly expanding field of IL mediated wood processing, our knowledge in this area has increased to a new level. Many new technical advances are apparent, including more refined ILs, electrolytes, pre-treatments and processing techniques. However, the application of fundamental knowledge related to the connectivity of wood biopolymers, wood morphology, wood ultrastructure and even the solubility of wood in ILs seems to have been largely neglected. Increasing awareness related to IL reactivity has brought both challenges and possibilities to wood fractionation. Depolymerization during fractionation can result in undesired products. This is most relevant when molecular weight distributions should be maintained, e. g. for the production of cellulosic pulps. However, in some cases degradation may be beneficial, e. g. for dissolving the LCC network or reducing the recalcitrance of wood for biofuel production.

Based on our work, with sawdust and highly pulverized spruce wood, we have demonstrated that wood is not completely soluble in [amim]Cl in its native state. This is confusing as isolated lignin, cellulose and hemcellulose preparations have been dissolved efficiently in several publications. One possible reason for this is the presence of an extended LCC matrix in wood that is simply of too high molecular weight and is too interconnected to dissolve. This property can be utilized to extract cellulose, as it is not covalently bound to the insoluble LCC matrix. Cellulose is extracted and by careful control of non-solvent addition, the insoluble lignin — hemicellulose rich fraction can be first isolated, followed by regeneration of relatively pure cellulose. This cellulose extraction procedure is not yet at a stage that would yield a technically useful pulp, due to apparent depolymerization, in comparison to technical pulps and holo-cellulose. However, the more we learn about the stability of wood and lignocellulose, in technical and pure ILs, the better are our chances of yielding close-to native polymers.

The implications for biofuels production are more straight-forward, in regard to pre-treatment mechanisms. Certain ionic liquids are excellent media for cellulose dissolution and regeneration to a state, which is easier to process. Presence of impurities or intentionally added catalysts, that may depolymerize the biopolymers during this process, are beneficial, provided the IL is somewhere between 99 and 100 % recoverable. This is a function of the high cost of ILs, at present. The method of biopolymer regeneration, to enhance separate lignin from polysaccharide, is therefore quite important. If degradation is significant enough breakage of covalent linkages between lignin and polysaccharide should facilitate this. IL recyclability is a major challenge here due to the buildup of monomers, dimers, oligomers, silicates and other inorganics. Therefore improving IL recyclability will greatly enhance the chances of success.

6.4 Experimental

6.5.1 Materials

Synthesis of the [amim]Cl was performed according to a method adapted from Wu et al. [74]. Allyl chloride (200 mL, 2.51 mol) and 1-methylimidazole (160 mL,

2.1 mol) were added to a flask under nitrogen atmosphere. The mixture was refluxed at 50 °C with stirring under positive pressure of nitrogen for 18 h. The reaction was determined to be complete by 1H NMR. The mixture was transferred under nitrogen atmosphere to a rotary evaporator, attached to a high vacuum pump. The excess of allyl chloride was removed at 50 °C. The cloudy crude product was further purified by heating, at 80 °C for 18 h, with activated charcoal (3.0 g) and water (200 mL). The mixture was then filtered through Celite in agrade-3 sinter. Water was removed at 65 °C by rotary evaporation over 18 h, under high vacuum, to yield [amim]Cl as a pale yellow viscous oil. 1H NMR (300 MHz, CDCl3) 5 3.97 (3H, s, NCH3), 4.86 (2H, d, J = 6.4 Hz, NCH2), 5.33-5.26 (2H, m, C=CH-C), 5.86 (1H, ddt, J = 16.9, 10.3,

6.5 Hz, C=CH2), 7.40 (1H, s, C=CH), 7.65 (1H, s, C=CH), 10.39(1H, s, NCHN).

Unbleached Norway spruce (Picea abies) thermomechanical pulp (TMP) was

donated from a Swedish mill. Norway spruce sawdust (particle size <0.2 mm by sieving) was prepared with a belt grinder (grade 60), in-house. Eucalyptus grandis was supplied by Novozymes, NC, USA. Prior to ball-milling treatments, Norway spruce TMP was first milled in a Wiley mill with a 20 mesh (0.84 mm) sieving screen. After Wiley milling the 20 mesh powder was extracted in a Soxhlet extractor for 48 h with acetone. A portion of this fibrous material was further sieved to pass a coarse 40 mesh (0.40 mm) sieve. The remaining extracted 20 mesh Norway spruce powder was rotary ball milled in a ceramic plated 5.5 L steel jar with 470 ceramic balls (diameter 0.9 cm) and a rotation speed 60 rpm, for 28 day period. After milling, the fine powder was dried in vacuum oven. The average particle size was determined to be less than 200 mesh (75 pm).

Eucalyptus chips were Soxhlet extracted with acetone for 48 h. Remaining tannins were removed by refluxing in 0.075 M NaOH solution (1:50 w/v ratio) for 1 h prior to milling of the dried sample. Milling was performed in a Fritsch Pulverette planetary ball-mill, with a 20 mL tungsten carbide grinding bowl and steel balls, at a rotation speed of 420 rpm for 48 h in total. The total milling time was made up of a repetitive milling cycle of 30 min milling time and 20 min brake, to avoid burning of the sample. All the wood materials were dried in vacuum oven over night at 40 °C prior to their use.

For practical applications, the concentration of feedstock in the reaction mixture should be as concentrated as possible. However, 5-HMF yield and selectivity

normally decrease with increasing initial carbohydrate concentration since the formed 5-HMF can combine with monosaccharides, and cross-polymerize to form humins [25]. In aqueous systems including aqueous mixture systems, losses due to humin formation can be as high as 35 % for 18 wt% fructose solution, although this value decreases to 20 % for 4.5 wt% fructose solutions [25]. It was reported that in aqueous mixtures, the formation of humins for fructose conversions in ionic liquids can be largely inhibited [20, 45, 65]. Qi et al. [20] examined the effect of initial fructose concentration on fructose conversion and 5-HMF selectiv­ity in [BMIM][Cl]. When the fructose/[BMIM][Cl] weight ratio was increased from

0. 01 to 0.05, the 5-HMF selectivity changed slightly from 86 to 85.2 %. As the fructose/[BMIM][Cl] weight ratio was increased from 0.05 to 0.1, the 5-HMF selectivity decreased by about 5 %. Additional increases in the fructose/[BMIM] [Cl] weight ratio did not lead to significant lowering of the 5-HMF selectivity, which is similar to the trends noted by Yong et al. for the conversion of fructose in [BMIM][Cl] with NHC/CrCl2 (NHC=N-heterocyclic carbene) complexes as cata­lysts [45]. Xie et al. [65] studied the dehydration of fructose in [BMIM] [Cl] catalyzed by lignosulfonic acid, and the 5-HMF yields of 92.7, 88.0, 84.1, and 73 % were obtained when 5, 10, 25, and 50 wt% of fructose were used, respectively. Increasing amounts of humins (0.007, 0.019, and 0.046 g) was observed for the increasing fructose concentrations, supporting the hypothesis that high concentrations of sugars and 5-HMF in ILs promote degradation and polymerization of 5-HMF.

Liu et al. [38] presented a promising choline chloride/CO2 (ChCl/CO2) system that could tolerate high fructose concentration with stable 5-HMF yield. The research group showed that as high as 100 wt% of fructose could be dehydrated into 5-HMF in ChCl/CO2 system without substantial loss of yield (66 %) (Fig. 9.7).

This process provides a practical and eco-efficient synthesis method for 5-HMF. The authors ascribed the surprising tolerance of the ChCl/CO2 system to a high content of fructose to a change of the physico-chemical properties of the ChCl/ fructose mixture. Even in the presence of water, ChCl is capable of forming various deep eutectic solvents (DESs) with many hydrogen-bond donors such as polyols, urea, or carboxylic acid. After fructose is dehydrated into 5-HMF, ChCl and 5-HMF tend to form a new DES, which stabilizes the 5-HMF allowing its reactivity to be drastically reduced thus inhibiting its decomposition.

Preface

Interest in biofuels and value-added chemicals that can be produced from biomass is increasing daily as societies look to sustainable sources of energy. Ionic liquids are used for the pretreatment and chemical transformation of biomass due to their unique ability for dissolving lignocellulosic materials. Although there are many books on the topic of either biomass conversion or ionic liquids, the unique feature of this book is that it links biomass conversion with ionic liquids and chemicals such that processing, chemistry, biofuel production, enzyme compatibility and environmental treatment are covered for conceptual design of a biorefinery. This book is the first book of the series entitled Biofuels and Biorefineries.

This book consists of 12 chapters contributed by leading world-experts on biomass conversion with ionic liquids. Each chapter was subjected to peer-review and carefully revised by the authors and editors so that the quality of the material could be improved. The chapters are arranged in five parts:

Part I: Synthesis and Fundamentals of Ionic Liquids for Biomass Conversion (Chaps. 1, 2, and 3).

Part II: Dissolution and Derivation of Cellulose and Fractionation of Lignocellu — losic Materials with Ionic Liquids (Chaps. 4, 5, and 6).

Part III: Production of Biofuels and Chemicals in Ionic Liquids (Chaps. 7, 8, and 9). Part IV: Compatibility of Ionic Liquids with Enzymes in Biomass Treatment (Chaps. 10 and 11).

Part V: Ionic Liquids for Absorption and Biodegradation of Organic Pollutants in Multiphase Systems (Chap. 12).

Chapter 1 introduces the fundamentals of ionic liquids related to biomass treatment. Chapter 2 gives an outline of design and synthesis of ionic liquids for cellulose dissolution and plant biomass treatment. Chapter 3 overviews the recent advances made with choline-chloride (ChCl) not only for the activation of biomass but also for its conversion to value-added chemicals. Chapter 4 summarizes approaches to design of ionic liquids that have good capability for dissolving cellulose and discusses factors for realizing efficient room-temperature dissolution of cellulose dissolution and subsequent enzymatic hydrolysis. Chapter 5 provides a

comprehensive overview about the use of ionic liquids for the chemical derivati — zation of cellulose. Chapter 6 reviews the current state of knowledge and process development in the area of ionic liquid fractionation of wood, and reports findings on factors that control the solubility of wood in ionic liquids. Chapter 7 provides an overview on the biodiesel production in ionic liquids, ionic liquids-catalyzed biodiesel production, ionic liquids-modified enzymes for biodiesel production, purification of bio-alcohols with ionic liquids, and prospects. Chapter 8 focuses on catalytic transformations of biomass into fuels and chemicals in ionic liquids. Chapter 9 describes the efficient methods for producing the platform chemical, 5-hydroxymethylfurfural with ionic liquids. Chapter 10 covers the biocompatibility issues of ionic liquids, for example, biocatalysts in ionic liquids media, the effect of ionic liquids properties on the activity and stability of enzymes, approaches to enhance the activity and stability of enzymes in the ionic liquids containing medium, and rational design of ionic liquids for use with enzymatic reactions. Chapter 11 focuses on the application of enzyme technology in ionic liquids. Chapter 12 introduces the potential of ionic liquids for hydrophobic organic pollutants absorption and biodegradation in multiphase systems.

This book reviews many aspects of the ionic liquids techniques necessary for efficient development of biomass resources. The text should be of interest to students, researchers, academicians and industrialists in the area of ionic liquids and biomass conversion.

Beijing, People’s Republic of China Sendai, Japan

Tianjin, People’s Republic of China

Acknowledgements

First and foremost, we would like to thank all the contributing authors for their many efforts to insure the reliability of the information given in the chapters. Contributing authors have really made this project realizable.

Apart from the efforts of authors, we would also like to acknowledge the individuals listed below for carefully reading the book chapters and giving con­structive comments that significantly improved the quality of many aspects of the text:

Dr. Leigh Aldous, the University of New South Wales, Australia;

Dr. Agnieszka Brandt, Imperial College London, UK;

Prof. Johann Gdrgens, Stellenbosch University, South Africa;

Prof. Yanlong Gu, Huazhong University of Science and Technology, China;

Prof. Mohd Ali Hashim, University of Malaya, Malaysia;

Prof. Noriho Kamiya, Kyushu University, Japan;

Dr. Takao Kishimoto, Toyama Prefectural University, Japan;

Dr. Jong Min Lee, Nanyang Technological University, Singapore;

Dr. Sang Hyun Lee, Konkuk University, South Korea;

Prof. Jean-Marc Leveque, Universite de Savoie, France,

Dr. Ruigang Liu, Chinese Academy of Sciences, China;

Prof. Rafael Luque, Universidad de Cordoba, Spain;

Dr. Patrick Navard, Ecole des Mines de Paris/CNRS, France;

Dr. Vladimir Raus, Academy of Sciences of the Czech Republic;

Prof. Robin D. Rogers, The University of Alabama, USA.;

Prof. Roger A. Sheldon, Delft University of Technology, The Netherlands;

Prof. Run-Cang Sun, Beijing Forestry University;

Dr. Yugen Zhang, Institute of Bioengineering and Nanotechnology, Singapore;

We are also grateful to Ms. Becky Zhao (senior editor) and Ms. Abbey Huang (editorial assistant) for their encouragement and assistant with the guidelines during preparation of the book.

Finally, we would like to express our deepest gratitude towards our family for their kind cooperation and encouragement, which helped us in completion of this project.

Zhen FANG, August 1, 2013 in Kunming Richard L. Smith, Jr., August 1, 2013 in Sendai Xinhua Qi, August 1, 2013 in Tianjin

Thomas Heinze and Martin Gericke

Abstract The chapter provides a comprehensive overview of the chemical deriv­atization of cellulose in ionic liquids (ILs). Different types of chemical reactions, including esterification, etherification, and grafting reactions, that have been performed in these novel type of polysaccharide solvents are discussed separately regarding efficiencies and unique characteristics. With respect to the use of ILs in technical scale, specific limitations and open questions are discussed such as the chemical reactivity of certain ILs, their high viscosity and hydrophilicity, and the need to develop efficient recycling strategies. Finally, an outlook on the develop­ment of task-specific ILs and IL/co-solvent systems as reaction media for cellulose is presented.

Keywords Cellulose • Ionic liquids • Homogeneous synthesis • Polysaccharide derivatives • Side reactions • Co-solvents • Task-specific solvents

5.1 Introduction

In recent years, ILs have received enormous interest in different areas of polysac­charide research. They are intensively studied in different areas for processing of cellulose and cellulosic biomass:

1. Cellulose is the most abundant bioresource worldwide and ILs can find use in the extraction of cellulose from lignocellulosic biomass and/or the selective sepa­ration from other plant components, such as hemicelluloses and lignin [1—3].

T. Heinze (*) • M. Gericke

Institute of Organic Chemistry and Macromolecular Chemistry,

Centre of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, HumboldtstraBe 10, D-07743 Jena, Germany e-mail: thomas. heinze@uni-jenna. de

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_5,

© Springer Science+Business Media Dordrecht 2014

2. ILs can also be used for the conversion of biomass into monosaccharides or platform chemicals. They may act either as reaction medium for cellulose hydrolysis or as efficient pretreatment agents to improve saccharification of the polysaccharide [2—4].

3. With respect to the environmental and safety concerns of the viscose and NMMO process, ILs are studied as alternative solvents for shaping of cellulose into fibers, sponges, beads, and other cellulosic objects [5—7].

4. Several ILs could be exploited as efficient homogeneous reaction media for the chemical modification of cellulose [8].

Regarding the complexity of all these topics, the present chapter is only devoted to the latter issue. It should be noted at this point that the ability to dissolve cellulose is not an inherent property of ILs but merely limited to a small fraction of this group with specific structural features. If not explicitly stated otherwise, however, the term ‘IL’ used in the chapter refers to ones that act as cellulose solvents.

Bio-oil is a renewable liquid fuel, having negligible contents of sulfur, nitrogen, and ash, and is widely recognized as one of the most promising renewable fuels that may one day replace fossil fuels. Fast pyrolysis of biomass technologies for the production of bio-oil have been developed extensively in recent years, which is usually carried out by the rapid (in a few seconds) raising of temperature to around 450-550 °C under atmospheric pressure and anaerobic conditions. The bio-oils are of high oxygen content of high viscosity, thermal instability, corrosiveness, and chemical complexity. These characteristics limit the applications of bio-oils, pre­cluding it from being used directly as a liquid fuel [100, 101]. Therefore, bio-oils need to be upgraded to improve its fuel properties. In order to develop a mild pyrolysis process with higher selectivity to favored compounds, the ILs-based technology was also introduced into this area. Several reports have demonstrated that ILs could be used as solvents or catalysts for this purpose. For example, Sheldrake et al. reported that dicationic molten salts were used as solvents for the controlled pyrolysis of cellulose to anhydrosugars [102]. It was demonstrated that the use of serials of dicationic ionic liquids for the pyrolysis of cellulose gave levoglucosenone as the dominant anhydrosugar product at 180 °C. An acidic dicationic IL were prepared and used as the catalyst to upgrade bio-oil through the esterification reaction of organic acids and ethanol at room temperature [103]. It was found that no coke and deactivation of the catalyst were observed. The yield of upgraded oil reached 49 %, and its properties were significantly improved with higher heating value of 24.6 MJ/kg, an increase of pH value to 5.1, and a decrease of moisture content to 8.2 wt%. The data showed that organic acids could be success­fully converted into esters and that the dicationic IL can facilitate the esterification to upgrade bio-oil. It is also found that microware irradiation could promote the pyrolysis of rice straw and sawdust with 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium tetrafluoroborate ILs as catalysts, and the bio-oil yield from rice straw reached 38 % and that from sawdust reached 34 % [104]. However, due to the high cost of ILs, and thermo stability during the pyrolysis, the use of ILs for the pyrolysis of biomass will not be the right direction.

7.3 Conclusion and Prospects

Biodiesel and bioalcohols are major biofuels that will offer many advantages over traditional fossil fuels and chemicals. In consideration of the unique properties of ILs and the key issues of biodiesel production from lipids, and bioalcohols separa­tion from fermentation process, ILs including functionalized acidic and basic ILs, switchable ILs and deep eutectic solvents have been used for more efficient production of biodiesel and bioalcohols. Although satisfactory results have been achieved in terms of lipids extraction, catalytic conversion of lipids and fatty acids, biodiesel purification, and bioalcohols separation, major challenges remain in this area in terms of lowering the costs, improving recyclability and environmental compatibility of ILs. In the future, the effect of possible residual ILs on the quality of biofuel products and downstream application need to be addressed. Bearing all of this in mind, new switchable ionic liquid systems may have great potential in application because of their unique properties, such as easy preparation and good recyclability. It is expected that ILs will be applied in a wider and more integrated way for biofuel production from various raw materials.

In this chapter, a summary of issues regarding enzymes in ionic liquids has been provided. In general, most of enzymes can be used in adequate ionic liquids. In most cases, it is found that ionic liquids with hydrophobic nature, less viscosity, kosmotropic anion and chaotropic cation usually enhance the activity and stability of enzymes. The activity and stability of enzyme in ionic liquids can be improved by immobilization, modification with activated stabilizing agents, pretreatment with polar organic solvents or by reaction media engineering such as mircroemulsion, using co-solvent, and design of biocompatible ionic liquids, etc. Furthermore, the information regarding structure and conformation dynamics of protein in ionic liquids could be helpful for engineering and scientific communities
to understand how ionic liquids enhance the stability and activity of enzymes, and thereafter be useful for choosing or designing ionic liquids for specific enzymatic reactions with the help of quantum chemistry.

ILs have many intriguing properties, such as low vapor pressure, high chemical and thermal stability, wide electrochemical window, non-flammability, wide liquid range and recognition ability of biomaterials. They are applied in a variety of fields, including extraction, organic synthesis, catalysis/biocatalysts, materials science, electrochemistry and separation technology. Furthermore, because ILs have ionic nature, they may interact with charged groups in the enzyme, either in the active site or at its periphery, causing changes in the enzyme’s structure.

ILs are promising solvents, for reaction and separation, offers tremendous possibilities for the development of sustainable industry, advanced materials and chemicals. Up to date, cellulose dissolution with ILs has been well developed. And the problems of ILs like high cost, which is obstacle to the industrial scale of ILs application, and the future researches trends and orient of ILs in cellulose/biomass applications, such as the binary systems including complex ILs or solvents, addi­tives and catalysts, will be described more clearly.

The recycle of ILs in an efficient way is also should be well developed. The process of dissolving cellulose or biomass applications in ILs should be optimized to reduce the loss of cellulose or biomass. Binary and ternary system of ILs perhaps will be more efficient for the dissolution of cellulose/biomass, meanwhile, the large scale of synthesis and functional design of ILs with high stabilities and low viscosities will be developed. Considering of the environmental effect and special purposes, bio-degradable ILs and chiral ILs, acid or base enhanced ILs with lower

l, 8-Diazabicyclo[5.4.0]undec-7-enium

saccharinate

1, 8-diazabicyclo [5.4.0] undec-7-enium bis(trifluoromethanesulfonyl)imide 8-Methyl-l,8-diazabicyclo[5.4.0]undec- 7-enium hydrogensulfate 8-Methyl-l,8-diazabicyclo[5.4.0]undec- 7-enium chloride

8-Octyl-1,8-diazabicyclo[5.4.0]undec- 7-enium chloride 1 — Butyl-4-methylpyridinium hexafluorophosphate 1 -Butyl-4-methylpyridinium chloride

38 AMMOENG 110 formate

39 AMMOENG 110 acetate

40 Trihexyltetradecyl phosphonium

dicyanamide

41 AMMOENG 110 dicyanamide

42 Tetrabutylammonium formate

43 N, N,Ntriethyl-3,6,9-trioxadecy-

lammonium acetate

44 Benzyldimethyl(tetradecyl)ammonium

chloride

45 l-(2-Hydroxyethyl)-3-methylimi-

dazolium chloride

46 l-Allyl-2,3-dimethylimidazolium [AdMIM]Br

chloride

47 l-(3,6,9-Trioxadecyl)-3- [Me(OEt)3-Et-

ethylimidazolium Im] [OAc]

aat 298.15K; bat 303.15K; cat 296.65K; dat 293.15K.

Cellulose [123]

Cellulose [123]

Cellulose [123]

Cellulose [22]

Cellulose [22]

Insoluble [115]

Avicel, spruce sulfite, pulp, Cotton linters, cellulose

[22]

Avicel [63]

Avicel [63]

898a [111] Avicel [63]

Avicel [63]

Avicel [63]

Avicel [63, 117]

Spruce sulfite, avicel, pulp, cotton linters [124, 125]

Not mentioned [126]

Avicel, spruce sulfite, pulp, cotton linters[63, 127]

Avicel [63]

r->

viscosity, higher thermal stability would be designed and synthesized by calculat­ing the structure-function relationship or predicting the properties with group contribution method or semi-rational formula.

Acknowledgements This research was supported financially by the Projects of International Cooperation and Exchanges NSFC (No. 21210006), Natural Science Foundation of Beijing of China (No.2131005, No.2132055) and National High Technology Research and Development Program of China (863 Program) (No. 2012AA063001).