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Scheme 2.3 Proposed structure and mechanism of catalytic action of the cellulase mimetic catalyst. Source: Based on Shuai and Pan [44].

The sustainability of solid acids is somewhat undermined by the insolubility of both the cellulose and the catalyst, the latter by design, in aqueous reaction mixtures [4,40]. The heterogeneity of the system limits the number of effective substrate–catalyst interactions during the processing, leading to several technical and economical hurdles. Firstly, there is a need for high loadings of the catalyst [4,44–46]. This may lead to the production of solid deactivated catalyst waste, if it is not fully recyclable. Another downside relates to the use of pretreated cellulose, instead of largely available lignocellulosic biomass. Pretreatment helps to ameliorate issues pertaining to mass transfer in heterogeneous systems, owing to the reduced physical size and sometimes lower molecular weight of pretreated polysaccharides. Examples of pretreated substrates are MCC, a medium value product obtained by the treatment of wood pulp in diluted aqueous acids, and ball‐milled cellulose [16,58]. Ball milling is arguably the most commonly used method for the pretreatment of cellulosic biomass. Although this approach is efficient at the bench scale and quite effectively depolymerizes cellulose to some extent, it becomes highly energy demanding at the larger scale and remains mostly unemployed in the industry [4]. Some studies, however, claim that some of the significant energy implications associated with this method of pretreatment may be avoided for acid‐assisted mechanochemical depolymerization [47,59]. For instance, mechanochemical depolymerization of beechwood and poplar wood may be realized at kilogram scale by ball milling of biomass in the presence of sulfuric acid [47]. Nevertheless, these processes require subsequent dilution of the pretreated acidified systems with water and hydrolytic treatment under forcing reaction conditions, which does not significantly differ from other processes in aqueous solvents (Table 2.1). Considering the energy costs accompanied by a need for corrosion‐resistant equipment, such means of depolymerization cannot be accepted as sustainable, unless the energy demands of ball milling can somehow be reduced.

Table 2.1 Conditions and results of the acid‐catalyzed processing of cellulosic biomass into carbohydrates and furansa.