Recently, P. Abbott and co-workers have introduced the concept of Deep Eutectic Solvents (DES). A DES is a fluid generally composed of two or three cheap and safe components which are capable of associating each others, often through hydrogen bond interactions, to form a eutectic mixture. The resulting DES is characterized by a melting point lower than that of each individual component. Generally, DESs are characterized by a very large depression of freezing point and are liquid at temper­atures lower than 150 °C [22]. Note that most of them are liquid between room temperature and 70 °C. In most cases, a DES is obtained by mixing a quaternary

t і

I Safe hydrogen bond-donors

I (renewable carboxylic acids, urea, polyols

etc…)

Scheme 3.5 Deep Eutectic Solvents ammonium salt with metal salts or a hydrogen bond donor (HBD) that has the ability to complex the halide anion of the quaternary ammonium salt.

Owing to its low cost, biodegradabity and low toxicity, choline chloride (ChCl) and more recently glycine betaine (zwitterionic or protonic form) have been recently proposed as an organic salt to produce eutectic mixtures generally in combination with cheap and safe HBDs such as urea, renewable carboxylic acids (e. g. oxalic, citric, succinic or amino acids) or renewable polyols (e. g. glycerol, carbohydrates). As compared to the traditional ionic liquids (ILs), DESs derived from ChCl gather many advantages such as (1) low price, (2) 100 % atom-economy synthesis (no purification is required), and (3) most of them are biodegradable [23], biocompatible [24] and non-toxic [25] reinforcing the sustainability of these media. Physicochemical properties of DESs (density, viscosity, refractive index, conduc­tivity, surface tension, etc.) are very close to those of common ILs. Thereby, they have the potential to advantageously replace ILs in many applications such as metal and oxides dissolution, catalysis, electrochemistry and material preparations. Addi­tionally, through hydrogen bond interaction, DESs have the unique ability to stabilize and thus to lower the reactivity of water, opening the route to chemical transformations that are normally not feasible in hygroscopic solvents [26] (Scheme 3.5).

In 2007, Abbott and co-workers defined DESs using the general formula R1R2R3R4N+,X^Y^ [27]:

Type I DES Y = MClx, M = Zn, Sn, Fe, Al, Ga Type II DES Y = MClx. yH2O, M = Cr, Co, Cu, Ni, Fe Type III DES Y = R5 Z with Z = — CONH2, — COOH, — OH

Note that the same group also defined a fourth type of DES which is composed of metal chlorides (e. g. ZnCl2) mixed with different HBDs such as urea, ethylene glycol, acetamide or hexanediol (type IV DES).

Similar to the case of ChCl-derived bio-inspired ILs, the use of DES for dissolution of cellulose has been scarcely reported mostly due to the novelty of these systems. Although based on the state of the art, protic groups such as — OH or -COOH are clearly not favorable for the dissolution of cellulose, their involvement in the formation of a DES drastically reduces their protic nature (the — OH group being involved in hydrogen bond interaction) thus offering a better chance to achieve the dissolution of cellulose.

In 2012, Georgia Tech Research Corporation has patented the dissolution of microcrystalline cellulose (AVICEL) in various DESs made of ChCl and betaine monohydrate [28]. Neat DES made of ChCl and urea (or malonic acid or formamide) were not able to dissolve microcrystalline cellulose. When DESs were diluted with a basic solution (NaOH or NaOAc) together with a prolonged incubation time, a swelling of cellulose was however observed. By means of XRD analyses, a decrease of the crystallinity index of cellulose of 15-20 % was noticed suggesting that these systems can partly interact with the hydrogen bond network of cellulose. In agree­ment with previous results, regenerated cellulose was less recalcitrant to hydrolysis after pretreatment in basified ChCl-derived DES. Although neat DESs do not dissolve cellulose, one should however mention that their combination with a basic solution allowed avoiding the large amount of base traditionally required for the dissolution of cellulose. Next, authors highlighted the possible formation of DES from betaine monohydrate and urea. As compared to ChCl, betaine monohydrate is more attractive due to its lower cost and its direct availability from biomass (co-product of the sugar beet industry). DESs are made with more difficulty from betaine than from ChCl and only urea was found eligible as a hydrogen bond donor in such case. Such betaine derived DESs is however highly viscous. Following the same strategy than that used from ChCl authors found that a pretreatment of cellulose in the betaine/urea DES led to a decrease of 15 % in the crystallinity index of cellulose. Although these systems have allowed the crystallinity index of cellulose to be slightly decreased, DESs are however not capable of dissolving cellulose presumably because DES components are already involved in hydrogen bond interaction making difficult their interaction with the hydrogen bond network of cellulose. Additionally, removal of DESs from regenerated cellulose is not an easy task and extensive washing are required.

In the same year, M. Francisco and co-workers investigated a series of 26 dif­ferent DESs in the dissolution of cellulose, lignin and starch [14]. Since selected mixtures exhibited no melting point by differential scanning calorimetry (only glass transition), such mixtures were more considered as low transition temperature mixing (LTTM) rather than real DESs. All solubility measurements were deter­mined by using the cloud point method within a range of temperature of 60 and 100 °C. This method consists in the progressive addition of a biopolymer in LTTMs up to the observation of a turbid solution. Among them, LTTMS made of lactic acid-ChCl were found particularly efficient for lignin dissolution. A clear solubility enhancement of lignin was even observed with an increase of the lactic acid content. Reversely, LTTMs made of malic acid were found more efficient for the dissolution of starch than for the dissolution of lignin. Among tested melts, malic acid-proline melt efficiently dissolved starch and dissolution ability can be

improved by increasing of the proline ratio. Note that no clear rationalization was proposed and the search of LTTMs for the dissolution of lignin or starch still remains empirical.

In agreement with the above-described work patented by Georgia Tech Research Corporation, no significant dissolution of cellulose was observed in all tested LTTMs. Nevertheless, in the typical case of LTTMs derived from proline, turbid solution was observed with cellulose and no evidence of solid particle was further detected further supporting the superior ability of aminoacid for interacting with cellulose as described above.

Having all these results in hand, authors next checked the ability of LTTMs for the delignification of wheat straw. Using a Lactic acid-ChCl melt (2/1), 2 wt% of lignin was extracted after incubation overnight at 60 °C. Although no solubility data was provided, authors claimed that the solubility of wheat straw can be improved using a malic acid/proline melt (1/3) which is consistent with results presented in Table 3.3.