After cellullose dissolution in ILs and its regeneration with an anti-solvent, the IL is usually filtered (or centrifuged) and washed with ethanol, acetone, ethyl ether, or water several times to remove by-products of wood degradation. Due to the IL low vapor pressure [60], the excess can be removed with a rotary evaporator, possibly at high temperatures, before the IL reuse [25, 31, 46, 55, 89]. It can also be separated with ethyl ether, then dissolved in acetonitrile/ethyl acetate and frozen for 24 h. The IL is then placed in a vacuum over at 90°C for 8 h before reuse [16].

Using these recycling procedures, the reuse of ILs for the pretreatment of native biomass through multiple cycles was reported. [BAIM][Cl] and [MAIM][Cl] could dissolve Metasequoia glyptostroboides wood sawdust without any efficiency loss after five cycles [16]. After five cycles, [EMIM][OAc] only lost 10% of its effi­ciency to dissolve maple wood flour [25]. The dissolution of rice straw in [EMIM][OAc] was repeated for 20 cycles with no reported efficiency loss. The cellulose recovery even increased over time due to the accumulation of dissolved cellulose residues that can be recovered in later cycles [46]. ILs, such as [BMIM][PF6] and [BMIM][BF4], were successfully recycled through multiple reaction cycles. Their recyclability was attributed mainly to their low solubility in some organic solvents or water. They can thus be extracted with an organic solvent or washed with water [60].

The IL recyclability is limited by the formation and accumulation of by­products or impurities. The degradation of cellulose was reported in reactions conducted at high temperatures [22, 36] or with acid catalysts [6, 32, 53]. The dehydration of free monosaccharides could lead to the formation of 5-hydrox — ymethylfurfural and furfural [53]. After the IL use for the dissolution of native biomass, 31P NMR spectra revealed signatures from 5-hydroxymethylfurfural, acetol, 2-methoxy-4-methylphenol, catechol, and acetic acid [32]. Even if these by-products can be avoided, the lignin extracted from the biomass accumulated in the recycled ILs with the increasing number of cycles [25, 32]. There was also accumulation of hemicelluloses, which are polar and have good affinity with polar ILs, such as 1-allyl-3-methylimidazolium chloride [32].

Wood naturally contains acid groups that can become free by hydrolysis and generate acids in the IL solution, such as acetic acid (pKa = 4.76) and glucuronic acid (pKa = 3.18) [31]. The generation of strong acids can protonate the acetate anion in [EMIM][OAc], for example, reduce the IL dissolution efficiency and complicate its recovery for reuse [31, 36]. Recycling is further limited by the high viscosity of ILs, which complicates handling and purification steps [36]. There­fore, efficient methods to separate the different dissolved products are necessary for the recycling of ILs [53].

Recycling efficiency depends on the anti-solvent used for the regeneration of wood after dissolution. Using water as the anti-solvent resulted in a higher yield of regenerated wood than using methanol [31]. The glucose yield after enzymatic hydrolysis after four cycles was also higher using water as an anti-solvent. In the case of E. grandis, the lower yield with methanol was explained by the larger amount of extractives dissolved in the methanol/IL mixture. However, after four recycling cycles, the recycled IL yield was 96% with methanol as the anti-solvent and 91% with water. At an industrial scale, water is preferable to methanol, since it is cheaper and more benign environmentally [31].

Another possibility is to replace the anti-solvent by an aqueous solution of phosphate, carbonate, or sulfate. The addition of a K3PO4 solution to the biomass solution led to the precipitation of the dissolved biomass and the appearance of a biphasic system with an IL-rich phase and a salt-rich phase. The extracted IL can then be dried and reused [43]. Phenylboronic acid and naphthalene-2-boronic acid were used to extract more glucose, xylose, cello- biose from IL/corn stover solutions after enzymatic hydrolysis in order to improve recyclability [45].