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.