href="#ulink_7b25bdb3-cce1-5fd1-b2fe-5e6f64a29d60">[50]. Mixtures of DMA and LiCl (present in differing ratios depending on the targeted application) are ionic media that have already been extensively employed in cellulose refining technologies [58]. This solvent system enabled the direct processing of biomass into HMF in good yields (up to 54 mol%, based on the cellulose content, Table 2.1) in the presence of the combined acid catalyst chromium(II) or chromium(III) chloride and hydrochloric acid [50]. With lignocellulosic substrate (corn stover), FF was also obtained (in addition to HMF), owing to acid‐catalyzed reactions of xylans (yield up to 37 mol%, based on the xylan content, Table 2.1). The yields of HMF could be improved during the direct processing of MCC in [C2mim]Cl in the presence of Lewis acid‐assisted Lewis acid catalyst CuCl2/CrCl2 (58%) [51], or in the cosolvent system [C2mim]OAc (1‐ethyl‐3‐methylimidazolium acetate)/1‐(4‐sulfobutyl)‐3‐methylimidazolium methanesulfonate ([C4SO3Hmim]CH3SO3) in the presence of CuCl2 (70 wt%) [52], but these methods have not been applied to the valorization of native biomass. Combined treatment of lignocellulosic biomass (wood chips and rice straw) in aqueous sodium hydroxide solution (3%), followed by the CrCl3‐catalyzed transformation in [C4mim]Cl, enabled inspiring yields of HMF (up to 79 mol%) under relatively mild processing conditions (120 °C, two hours, Table 2.1) [53]. Apparently, treatment in aqueous basic solution removes a substantial portion of lignin and hemicellulose from the biomass, facilitating to the rapid dissolution and hydrolytic processing of the substrate in the ionic solvent. This method may become industrially viable, once the efficient recovery of the reaction system and the targeted furaldehyde have been engineered.
Scheme 2.5 Proposed formation of catalytic species in [Cnmim]Cl. R, alkyl; n, integer.
Although ILs are excellent media for the valorization of carbohydrates, these systems suffer some drawbacks, mostly related to the high cost of common ionic solvents, and sometimes to intricacies relating to their recycling [4]. These downsides pose a barrier to their widespread industrial acceptance. In this regard, many researchers are currently investigating less‐expensive ionic solvents for the valorization of biomass [4]. Zinc chloride hydrate solvents, with the conventional formula ZnCl2·nH2O (these systems are true ILs with the molecular formula [Zn(OH2)6][ZnCl4] in the case of n = 3), have proved to be suitable for some biorefinery applications [35,54,82–86]. Such ionic systems have been historically employed as solvents in cellulose refining technologies and have been found useful in the production of cellulose aerogels, low‐molecular‐weight saccharides, and their derivative sugar alcohols [82–85]. Our recent systematic studies [35,54,86] demonstrate that ZnCl2·nH2O possesses intrinsic catalytic activity, which promotes the conversion of polysaccharides into value‐added molecules (Scheme 2.2 and Scheme 2.6). Moreover, it is possible to adjust the activity of ZnCl2·nH2O by manipulating the hydration number n [35,54,86]. Less‐hydrated media, ZnCl2·2.5–3.0H2O, 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.
