favor the transformation of cellulose into furans, namely, HMF, furyl hydroxymethyl ketone (FHK), and FF (Figure 2.1). FHK and FF are unusual major dehydration products, and their formation is accordingly seldom recorded for the reactions of cellulose. FHK is considered to originate by the dehydration of the intermediate ketohexose (isomerization product derived from fructose), while FF is thought to form from fructose via an intermediate pentose (e.g. arabinose), as shown in Scheme 2.6. These two furanoids are used as specialty solvents, pharmaceutical intermediates, and in the production of performance resins. Interestingly, the selectivity to these unusual furanoids may be significantly improved when performing reactions in the biphasic system ZnCl2·3.0H2O/anisole [54,86]. This biphasic system is especially useful for the production of FF from native biomass because of the simultaneous conversion of both cellulose and hemicellulose into the targeted furaldehyde (yield up to 42 wt% based on cellulose and hemicellulose content in biomass, Table 2.1) [54]. In distinct contrast to less‐hydrated solvents, highly hydrated molten salts ZnCl2·4.0–4.5H2O transform cellulose predominantly into HMF (yield up to 21 mol%) and low‐molecular‐weight saccharides (total yield up to 48 wt%, Figure 2.1) [35]. The correlation between selectivity of the products and hydration levels of ILs is presumed to be related to the acidity of the reaction media, which diminishes with rising n, as was shown by pH readings and NMR spectroscopy [35]; however, the exact nature of the catalytic action of ZnCl2·nH2O remains to be established. After optimizing the process, high yields of HMF (up to 35 mol%), FF (up to 29 wt%), and sugars (up to 61 wt%) are achievable by performing the conversion of native lignocellulose (corncob and softwood) and algal biomass (macroalga Ulva lactuca or microalga Porphyridium cruentum) in ZnCl2·4.25H2O under relatively mild conditions (Table 2.1) [35]. In addition, the transformation of lignocellulose in zinc chloride hydrate solvents enabled the recovery of a lignin‐containing residue [35,87]. However, not all types of biomass were found to transform efficiently in the inorganic solvent. For example, native softwood is less amenable for the catalytic conversion (Table 2.1). Additionally, economical methods to recover products and solvents demand further investigations.
Scheme 2.6 Unusual acid‐catalyzed transformations of cellulose in zinc chloride hydrate solvents into furan‐type molecules. n, integer. Source: Bodachivskyi et al. [86].
Figure 2.1 Acid‐catalyzed transformation of cellulose into low‐molecular‐weight molecules in ZnCl2·nH2O. The figure specifies combined yields of mono‐, di‐, tri‐, and tetrasaccharides in wt% and yields of furans in mol%. Reaction conditions: MCC (50 mg), solvent–catalyst (5.000 g), 80 °C, 2.5 hours, then 120 °C, 1 hour [35]. ■: saccharides; □: 5‐(hydroxymethyl)furfural;
DESs are another class of alternative ionic media that exhibit some advantages relatively to imidazolium‐based salts. These relate to the ease of the preparation of DESs, which often require a one‐step combination of less toxic and naturally renewable precursors at moderate temperatures [88]. DESs are useful for the pretreatment and fractionation of cellulosic biomass and for the transformation of bulk cellulose into micro‐ or nanocrystalline cellulosic materials [89–91]. Some DESs are able to dissolve biomacromolecules and enable their subsequent catalytic conversion [55–57,92]. For example, acidic eutectic systems composed of ChCl and organic acids (usually oxalic acid or citric acid) are suited to this task and are recoverable media for the acid‐catalyzed transformation of inulin, a β(1 → 2) linearly linked fructose polymer with occasional chain‐terminating glucose units, and of xylans, yielding HMF (64%, Table 2.1 [55]) and FF (69 mol%, Table 2.1 [56]), respectively. However, in most applications, such DESs fail to convert cellulose into low‐molecular‐weight derivative products, with the exception of the process in the cosolvent system [C4mim]Cl/ChCl/oxalic acid, as discussed above (Table 2.1) [49]. A study of the reactivity of several polycarbohydrates in the neat ChCl/oxalic acid solvent showed that starch, hemicellulose, and inulin are all soluble and convertible into monosaccharides and furans in this DES, while the linear polymer cellulose was almost insoluble and unreactive under similar conditions (reaction temperature 60–100 °C, time one hour) [57]. Arguably, there is an apparent correlation between solubility and reactivity of carbohydrates in the acidic DES. The subsequent processing of lignocellulose (corn husk, corncobs, and softwood chips) and algal biomass (U. lactuca, P. cruentum, and Chlorella vulgaris) provided conversions of native starch, xylans, and fructans into monosaccharides (glucose yield up to 68 wt%, xylose yield up to 73 wt%, based on respective polysaccharide content in biomass), HMF (up to 13 mol%), and FF (up to 72 mol%) in the neat DES or in the biphasic system ChCl/oxalic acid/methyl isobutyl ketone (MIBK) [57]. Some of these instances are given in Table 2.1, demonstrating that the formation of specific product(s) is favorable under specific reaction conditions and for specified substrate types [57]. The polysaccharide component of the microalgae P. cruentum, comprising predominantly structurally branched glucans and xylans, may be transformed into the respective