acids. We proposed that the catalytic activity of the DES is generated by Lewis acid‐assisted Brønsted acidity, owing to the complexation between Lewis acidic ChCl and Brønsted acidic oxalic acid [49]. It is worth mentioning that reactions in the cosolvent system required the addition of water after dissolution of the substrate to promote the hydrolysis into monomer sugars, similar to the previous instances [34].
Scheme 2.4 Acid‐catalyzed hydrolysis of cellulose into low‐molecular‐weight carbohydrates in ILs via oligosaccharides. n, integer; m, 0, 1, and 2 for cellobiose, cellotriose, and cellotetraose, respectively.
Acid‐catalyzed depolymerization of cellulose in ILs opens numerous avenues to generate significantly value‐added chemicals from low‐molecular‐weight sugars. Pertinent examples include the fermentative production of ethanol for biofuel applications, or transition metal‐catalyzed synthesis of hexitols for food and medical uses [34,69,70]. Another interesting application is found in their transformation into alkyl glycosides, a class of biodegradable surfactants with widespread employment in differentiated products such as cosmetics, body care, and cleaning formulations [71]. The Corma research group from the Polytechnic University of Valencia published a series of papers dedicated to the synthesis of alkyl glycosides through the hydrolysis of cellulose into low‐molecular‐weight carbohydrates in an ionic solvent, followed by glycosidation into alkyl glycosides and alkyl polyglycosides [72–74]. This was accomplished by the addition of long‐chain alcohols, such as 1‐octanol, 1‐decanol, or 1‐dodecanol, after depolymerization of cellulose in [C4mim]Cl in the presence of acidic resin Amberlyst® 15. The method permitted the production of surfactants in high yield (up to 82 mol%), under mild processing conditions (temperatures around 100 °C) [72]. It is worth noting that alkyl glucosides are commercially synthesized from low‐molecular‐weight carbohydrates or structural polysaccharides in two steps, namely, glycosidation of the substrate with low‐molecular‐weight alcohols, followed by transacetalization with long‐chain alcohols [71]. The production of alkyl glycosides from cellulosic biomass in ILs is therefore a promising alternative pathway to generate bio‐based surfactants. The ongoing challenge relates to the separation of glycosides from ILs and is currently based on conventional chromatographic methods, which are difficult to sustain at larger scale, especially with mid‐priced performance chemicals [74]. Nevertheless, the problem may be potentially solved by the use of simulated moving bed chromatography that has already been applied to the simultaneous recovery of monosaccharides and ionic solvents [C4mim]Cl at the multigram scale [70]. Simulated moving bed chromatography is largely employed in the fractionation of carbohydrates, and this method demonstrates significant commercial potential [70].
Among the various biorefinery processes, there is a particular focus on the transformation of cellulosic saccharides into furan derivatives [4,75]. These are useful targets because HMF (a cellulose‐derived product) and FF (a hemicellulose‐derived product) are raw materials for the production of biofuels, bioplastics, food additives, and pharmaceuticals [75,76]. The synthesis of furans is well investigated in aqueous media, mostly based on the acid‐catalyzed transformation of low‐molecular‐weight sugars such as fructose, sucrose, and xylose [75,76]. The direct conversion of undervalued polysaccharides into furans in aqueous solvents is difficult, mostly as a consequence of the chemical reactivity of aldehydes in the acidic reaction media [4]. As a case in point, furaldehydes are convertible into LevA and its derivatives (Scheme 2.2) and also to high‐molecular‐weight by‐products such as humins (condensation products of saccharides and aldehydes), which have a limited scope of applications [75,77,78]. Additional complexities arise in the planning and execution of these reactions because the conversion of polysaccharides into furans involves aldose–ketose isomerization promoted by Lewis acids (Scheme 2.2), whose activity is often compromised in aqueous reaction media [4,7].
The abovementioned issues have been largely alleviated by the employment of ILs, often because of the stabilization of the reactive furanoids and the catalyst in the ionic media [4,61,79]. A commonly applied strategy is the processing of polysaccharides in imidazolium‐based solvents in the presence of metal chloride catalysts (MCln, M = metal, n = integer) [4,61]. The solvent–catalyst interaction presumably leads to the formation of acidic catalytic complexes, [Cnmim]+[MCln+1]– in the case of 1‐alkyl‐3‐methylimidazolium chloride solvents (Scheme 2.5), and these complexes tend to promote the requisite Lewis acid‐catalyzed aldose–ketose isomerization [79]. Brønsted acidity is likely achievable by the hydrolysis of some of these species in the presence of water with concomitant formation of metal aquo complexes and (hydrated) hydrogen cations (Scheme 2.5), as is commonly observed in aqueous systems [80]. Decomposition of imidazolium salts into N‐heterocyclic carbenes and HCl may also be a source of Brønsted acid activity [81]. However, in many instances, the processing requires the addition of protic acids to the reaction media [4,61]. The direct conversion of native cellulose and lignocellulose has been conducted in a cosolvent system comprising [C2mim]Cl (20–80 wt%, based on the reaction system) and dimethylacetamide (DMA)/LiCl
