Biomass Valorization. Группа авторов

[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 (MCl_n, M = metal, n = integer) [4,61]. The solvent–catalyst interaction presumably leads to the formation of acidic catalytic complexes, [C_nmim]⁺[MCl_n+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 mixture (LiCl, 10 wt%) [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 [C_nmim]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].