As smartly recently stated by M. Francisco et al. [14] finding an eco-efficient solvent for the dissolution of cellulose and more largely lignocellulosic biomass is becoming the Achille’s heel of renewable chemicals and biofuels processing.

In this context, few groups have attempted the design of bio-inspired ILs from choline chloride (ChCl) with the aim of dissolving (and thus decreasing the crystallinity index of cellulose) in a more sustainable way. ChCl is a very cheap (<2€/kg), biodegradable and non-toxic quaternary ammonium salt which can be either extracted from biomass or readily synthesized from fossil reserves (million metric tons per year) through a very high atom economy process. Its ionic structure makes of this organic salt a suitable candidate for the design of safer solvents that are particularly promising for biomass processing. In this context, two strategies are employed to produce media from ChCl (1) a chloride metathesis to produce the so-called bio-inspired ILs or (2) its combination with safe hydrogen bond donors such as urea, renewable carboxylic acids (e. g. oxalic, citric, succinic or aminoacids) or renewable polyols (e. g. glycerol, carbohydrates) to produce a deep eutectic solvent (DES). More information regarding DES is provided later in the chapter. To date the number of examples involving bio-renewable ChCl-derived solvents for the dissolution of cellulose and more largely lignocellulosic biomass is rather scarce mostly due to the novelty of these systems.

ChCl being a solid with a high melting point, one of the main strategies reported in the current literature consists in properly exchanging the chloride anion by acetate or an amino acid affording ILs that are liquid at a temperature below than 70 °C. All these so-called bio-inspired ILs were conveniently prepared by neutral­ization of the commercially available choline hydroxide derivative with the corresponding acid (yield >95 %). Main advantages of these ILs stem from their convenient synthesis, biodegradability, low toxicity and low price. To date, these bio-inspired ILs were essentially used in various catalytic reactions such as aldol [15] and Knoevenagel [16] reactions for instance but their use for biomass processing remained scarce.

In 2010, C. S. Pereira and his co-workers reported the efficient use of cholinium ethanoate and lactate for the dissolution of refined cork, an insoluble residue from the cork manufactures, composed of ~20 wt% of polysaccharides, ~30 wt% of poly (phenolics) (“lignin-like”) and ~50 wt% suberine [17]. Ability of choline-derived ILs to dissolve refined cork has been investigated at 100 °C. The residue (not soluble) was then analyzed by ATR-FTIR to identify which polymer of refined cork has been extracted. Choline ethanoate was able to dissolve a larger quantity of refined cork than the reference 1,3-dialkylimidazolium ionic liquid, especially of the aromatic suberin component (Table 3.1).

Other anions such as butanoate, hexanoate, methylpropanoate and lactate were also tested. Among them, lactate was found the less efficient (extraction efficiency of 20.7 and 39.7 % for lactate and ethanoate, respectively). An increase of the chain length of the anion led to an improvement of the extraction efficiency and best results were obtained with the hexanoate anion for which the extraction efficiency reached 64.9 %. The ability of choline-derived ILs to dissolve refined cork follow the pKa value of the conjugate acid of the anion i. e. an increase of the basicity led to an increase of the extraction efficiency. ATR-FTIR analyses revealed a drastic reduction in the aliphatic and aromatic bands of refined cork after extraction suggesting that tested choline-derived ILs mostly extract suberin.

In 2012, Zhang et al. investigated the dissolution of microcrystalline cellulose (PH AVICEL 106) in choline acetate [18]. Although choline acetate ([Ch]OAc) was not capable of dissolving microcrystalline cellulose in a large extent (solubility <0.2 and ~0.5 wt% after 5 min and 12 h, respectively), it is noteworthy that microcrystal­line cellulose started to swell after immersion in [Ch]OAc for 12 h at 110 °C. After filtration, cellulose looked like a flocky precipitate rather than a powder, suggesting that this media does affect the supramolecular organization of cellulose. To ensure a complete dissolution of cellulose, effect of additives was investigated. Among various tested additives, it was shown that addition of 5-15 wt% of tributylmethyl ammonium chloride ([TBMA]Cl) in [Ch]OAc dramatically enhanced the dissolution of microcrystalline cellulose. In particular, in the presence of 15 wt% of ([TBMA] Cl), 6 wt% of cellulose was dissolved within only 10 min at 110 °C. For comparison, when the dissolution of microcrystalline cellulose was performed in neat 1-butyl-3- methylimidazolium chloride (commonly used for the dissolution of cellulose), only 4 wt% of MCC were dissolved after 8 h of reaction, further demonstrating the effectiveness of the [Ch]OAc/[TBMA]Cl mixture.

As commonly performed in the case of imidazolium-based ILs, cellulose was then regenerated by addition of ethanol and recovered. XRD analyses performed on regenerated cellulose revealed that the cellulose structure was successfully changed from a high to a low crystalline structure indicating that the [Ch]OAc/[TBMA]Cl mixture was capable of disrupting the hydrogen bond network of cellulose. During the dissolution process, the glucose units were not damaged (checked by infra-red) while viscosimetry analyses revealed that the degree of polymerization of cellulose remained unchanged before and after the dissolution process. After washing of regenerated cellulose, no nitrogen was detected by elemental analysis suggesting that the [Ch]OAc/[TBMA]Cl mixture can be conveniently separated from cellulose.

After filtration of regenerated cellulose and removal of ethanol, the [Ch]OAc/ [TBMA]Cl mixture was recycled three times without appreciable decrease of its dissolution abilities. After the third run, the ability of the [Ch]OAc/[TBMA]Cl

Scheme 3.3 Dissolution/regeneration of microcrystalline cellulose in Choline acetate

mixture to dissolve cellulose however started to drop mainly due to the accumula­tion of impurities in the solution that hampered the long term recycling of this system. A similar trend is observed with imidazolium-based ILs. The whole dissolution/regeneration process is summarized in Scheme 3.3.

In the same year, Zong and his co-workers reported the design of ILs by combining cholinium as a cation and amino acids as anions affording the so-called [Ch][AA] with AA = glycine, alanine, serine, proline, among many others. Liu et al. [19] Ability of these bio-inspired ILs to dissolve biopolymers such as lignin, xylan (a model of hemicellulose) and cellulose was investigated. All prepared [Ch][AA] were fully characterized in term of viscosity, stability (TGA analysis), melting point (DSC analysis), alkalinity and optical rotation in order to rationalize the efficiency of [Ch][AA] in the dissolution of biopolymers. In a first approximation, authors observed that [Ch][AA] with a high alkalinity and low viscosity are more favorable for the dissolution of lignin. In particular, [Ch][gly — cine] was found to be the best ILs with a dissolution of up to 220 mg of lignin per gram of ILs at 90 °C. Although alkalinity and viscosity of [Ch][AA] also exerted an influence on the dissolution of xylan, this biopolymer was found much less soluble than lignin. Cellulose, a recalcitrant biopolymer, was however insoluble.

Despite the low solubility of xylan and cellulose in [Ch][glycine], it can be used for the pre-treatment of rice straw. In particular, a pre-treatment of rice straw in [Ch][glycine] at 90 °C for 24 h prior to enzymatic hydrolysis, led to an enhance­ment of the glucose production. For instance, after pre-treatment of rice straw in [Ch][glycine], the concentration of glucose obtained after enzymatic hydrolysis was improved from 0.31 to 2.05 g L-1 which was attributed to the ability of the [Ch]

[glycine] to partly dissolve lignin, thus making more accessible the hemicellulosic and cellulosic fraction to enzymes. Optimization has been recently reported by same authors [20] (Table 3.2).

In 2012, Itoh and co-workers reported the use of what they have called “ionic liquids inspired by Nature” for the dissolution of cellulose [21]. Although this work does not deal with the use of choline, it opens key data for improving the ability of ChCl-derived ILs to dissolve cellulose. After examination of the protein sequences of several cellulase (enzymes responsible for the hydrolysis of cellulose), authors hypothesized that amino-acids might be suitable anions to disrupt the hydrogen bond network of cellulose and thus enable its dissolution. As a cation, authors have highlighted the superior performances of N, N-diethyl-N-(2-methoxyethyl)-N — methylammonium, a cation with a structure close to that of an etherified cholinium. In particular, after combination of this cation with amino-acids such as tryptophan,

Scheme 3.4 Dissolution of microcrystalline cellulose in amino acid derived ILs

alanine, cysteine, lysine, among many others, cellulose was efficiently dissolved at 100 °C. Among tested amino-acids, alanine was found to be the best achieving up to 6 wt% of dissolution of cellulose at 60 °C. All other tested anions such as Cl~, AcO~, HO~ were found inefficient further supporting the pivotal role played by amino acid in the dissolution of cellulose. More importantly, these amino acid — derived ILs were tolerant to the presence of up to 2 wt% of water which is an important point considering that biomass always contained water. At higher tem­perature (150 °C), the dissolution rate of cellulose as well as the solubility of cellulose was greatly improved but a degradation of ILs was noticed at such temperature and authors recommended to not proceeding dissolution experiments at a temperature higher than 120 °C. Once dissolved, cellulose can be regenerated after addition of water as an anti-solvent. Precipitated cellulose was of the Type II supporting that such ILs are able to induce a change of the crystalline structure of cellulose. Impact of this change of crystallinity on the recalcitrance of cellulose to deconstruction was then evaluated in the enzymatic hydrolysis of cellulose. After regeneration, 88 % of cellulose was converted after 10 h of reaction at 50 °C and pH5 which has to be compared to the 40 % obtained without pre-treatment in ILs. In our views, this work is of prime importance within the scope of this chapter since it indirectly suggests that combination of amino acid with an etherified cholinium cation should provide competitive bio-inspired ILs for the dissolution of cellulose. Such aspect is the topic of current investigations in our group (Scheme 3.4).