alt="Chemical reaaction depicts the unusual acid-catalyzed transformations of cellulose in zinc chloride hydrate solvents into furan-type molecules."/>
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 monomers at modest reaction temperature and into FF at elevated temperature and extended reaction time (Table 2.1). In contrast, the direct processing of softwood chips with high cellulose content converts only hemicellulose into FF (yield 55 mol%, Table 2.1) and leaves a fine cellulosic powder as an unreacted portion of the biomass [57]. The cellulose so formed was shown to be highly amenable for the subsequent acid‐catalyzed transformation into low‐molecular‐weight saccharides and platform chemicals (furans and alkyl levulinate), likely associated with reduced size of the particulate substrate [57]. Such combined technologies are valuable with a view to a multistage sustainable biorefinery.
The chemistry covered to this stage shows that representative transformations of cellulosic biomass lead mostly to the formation of low‐molecular‐weight saccharides and furan derivatives. Evidently, the processing into these products requires somewhat similar reaction conditions (Table 2.1), albeit that there is a requirement for the presence of Lewis acid activity for efficient conversions into furanoids (Scheme 2.2). The production of functionalized organic acids or their derivatives via the rehydration of furans, or via retro‐aldol reaction of monosaccharides, is often favored by more forcing processing parameters (temperature 160–280 °C) [4,7]. Such cellulose‐derived products (e.g. LevA, formic acid (ForA), or LacA) are another group of platform chemicals with a broad scope for commercial applications [3,4]. Even though ILs are shown to be efficient for certain hydrolytic transformations, the need for more forcing conditions for the production of the named acids potentially causes issues with the ionic media: some of them may decompose at elevated temperatures, while others may increase the rates of side reactions, reducing the selectivity of the targeted product(s) [86,93,94]. Instead, water and alcohols become suitable media for the preparation of cellulose‐derived acids and esters, respectively. For example, the production of LevA and ForA, rehydration products derived from HMF, is commonly conducted in aqueous media. These platform chemicals are produced commercially from cellulosic materials by the Biofine process [95–97]. The technology involves the two‐step hydrolytic processing of low‐value cellulosic materials catalyzed by sulfuric acid (Figure
