ILs are highly tunable systems, and their physical and chemical characteristics can be modulated for specific tasks, providing high levels of flexibility for acid‐catalyzed processing [65–67]. It is possible to design acidic ILs, which can simultaneously act as a solvent and catalyst, enabling the dissolution of substrates (solvent effect) and their subsequent conversions (catalyst effect) [49,61,63,66,67]. Such approaches potentially eliminate the need for highly corrosive mineral acid catalysts, such as hydrochloric or sulfuric acid, potentially avoiding the associated technological downsides accompanying their use. In an interesting study involving mixed solvent systems [63], the use of ammonium salts functionalized with Brønsted acidic sulfonic acid groups and hydrogen sulfate anion was investigated. N,N,N‐Triethyl‐N‐(3‐sulfopropyl)ammonium hydrogen sulfate showed exceptional activity as a cosolvent in [C4mim]Cl. The hydrogen sulfate system provided the Brønsted acid catalyst for the hydrolytic processing of MCC, affording very high yields of low‐molecular‐weight carbohydrates (yields of total reducing sugars 99%). The yields were determined using a colorimetric method based on the interaction of reducing carbohydrates with dinitrosalicylic acid [68], which may be subject to errors because of the reaction of dinitrosalicylic acid with other cellulose‐derived reducing substances. Nonetheless, the reported yields are impressive. More recently, our group has probed the use of a mixed ionic solvent system formed by [C4mim]Cl and biorenewable acidic deep eutectic solvents (DESs; DESs are eutectic mixtures of Brønsted and Lewis acids and bases, often forming ILs) formed from choline chloride (ChCl) and oxalic acid dihydrate [49]. This mixed ionic system afforded high yields of low‐molecular‐weight saccharides (glucose yield up to 55 wt% and xylose yield up to 40 wt%, based on the substrate; yields were determined by liquid chromatography–mass spectrometry analysis, providing unambiguous detection of targeted products [49]) after processing of non‐pretreated cellulose (eucalyptus and Pinus) and cellulosic materials of terrestrial (corncobs) and marine origin (micro‐ and macroalgae, Table 2.1), avoiding the use of corrosive 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