metal triflates demonstrated that β‐O‐4 lignin models can be fully depolymerized into derivative aromatic aldehyde with a carbonyl group at the β‐position and guaiacol (140 °C, 15 minutes, Scheme 2.8) [113]. The reaction may also occur toward a derivative ketone, analogous to Hibbert ketones (products of the acidolysis of lignin) [114]; this pathway has not been confirmed (Scheme 2.8) [113]. In contrast, a β‐5 model undergoes ring‐opening reactions into the corresponding trans‐stilbene derivatives (Scheme 2.8), while a β‐β model undergoes only epimerization in the acidic media (Scheme 2.8 does not show this transformation). Fe(OTf)3 was found to possesses optimal activity for the cleavage of β‐O‐4 linkages. It has been hypothesized that the activity of metal triflates relates to the generation of Brønsted acidic triflic acid in the reaction media [113]. This is nonetheless unlikely, as metal triflates are known to generate Brønsted acidity via an assisted acidity mechanism (Scheme 2.1), not by the hydrolysis into triflic acid [7,21,103]. Additionally, targeted studies have demonstrated that triflic acid is not causative [115,116]. Another important observation in the study [113] is that repolymerization can be effectively suppressed by the addition of ethylene glycol to form stable acetals from reactive products. The subsequent acid‐catalyzed depolymerization of varied substrates under defined optimal conditions (solvent dioxane, catalyst Fe(OTf)3, 140 °C, 15 minutes [113,117]) showed the relationship between the amount of produced monomers and the β‐aryl ether content of lignin. The conversion of lignins obtained by organosolv processes (extraction of the product with low molecular weight alcohols) using substrates with high β‐aryl ether content led to improved yields of phenolic acetals (up to 36 wt% of monomers), while the processing of industrial kraft lignins (lignins obtained by treatment of cellulose with aqueous Na2S and NaOH) with their high content of C–C linkages provided lower yields of product (around 10 wt%) [117]. This apparently relates to the repolymerization of labile C—O bonds into stable C—C linkages during the kraft pulping process. Although there are isolated instances where the ratio between C–O and C–C linkages is not so clearly linked to reaction outcomes [109], it is clear that the composition of the substrate influences the overall progress of the reaction. The selection of lignin types is therefore essential for the development of specific catalytic methods.
There has been some effort to promote the valorization of lignin in ILs. The transformations of varied β‐O‐4 model compounds in common imidazolium salts in the presence of Brønsted or Lewis acidic catalysts demonstrate good activity of such systems for the hydrolytic processing of the substrates [118–121]. The presence of some form of Brønsted acidity is requisite for the cleavage of β‐O‐4 bonds, whether this is achieved via the addition of protic acids, hydrolysis of metal salts, or by Lewis acid‐assisted Brønsted acidity. Hydrolysis and reduction of reactive intermediates into stable alcohol derivatives may be accomplished by the simultaneous hydrolysis/reduction of 2‐(2‐methoxyphenoxy)‐1‐phenylethanone into guaiacol and 2‐phenylethanol (yields nearly 60% each [122]) in the mixed ionic system comprising 1‐butyl‐2,3‐dimethylimidazolium bis(trifluoromethanesulfonyl)imide, the Brønsted acidic IL 1‐(4‐sulfobutyl)‐3‐methylimidazolium triflate, and IL‐stabilized ruthenium nanoparticles (a catalyst for the hydrogenation step) [122]. These studies have improved fundamental understanding of the processes involved [118–122]; translation of such model reactions to the conversion of macromolecular lignin is yet to be done.
Lastly, zinc chloride hydrate solvents under hydrogen pressure have been successfully applied to the conversion of unrefined pinewood lignin into value‐added alkyl phenols or cycloalkanes [123]. Alkyl phenols and cycloalkanes hold potential for the production of biosurfactants and biofuel, respectively [110,111,123]. It has been identified that the processing of lignin in 63 wt% ZnCl2 aqueous solution (corresponding to ZnCl2·4.5H2O; reaction conditions: H2 pressure 4 MPa, 200 °C, and six hours) in the presence of HCl cocatalyst yields a range of alkyl phenol products (47 wt%, based on lignin input) [123]. These results suggest that the acidic reaction media is capable of catalyzing the cleavage of all types of linkages of lignins, despite the relatively low selectivity at present. Importantly, the addition of Ru/C catalyst (instead of hydrochloric acid; C = activated carbon support) promotes subsequent hydrogenation of alkyl phenol derivatives into cycloalkanes, mostly containing two ring structures (yield 54 wt%, based on lignin input) [123]. It is worth noting that the study utilized lignin obtained after sulfuric acid‐assisted fractionation of pinewood [123], i.e. unrefined lignin. This represents a step forward in lignin processing toward an industrially implementable process.
2.4 Conclusions and Perspectives
Ongoing research toward the catalytic valorization of biomass is creating a foundation for sustainable chemical development. In numerous bench‐scale processes, acid catalysis has been shown to be a universal tool to source value‐added products from reactions of low value or waste cellulosic materials. The abundance of residual biomass derived from existing manufactories, such as in agriculture, algal aquaculture, forestry, and the food processing industry, can readily provide a sizeable amount of renewable substrates for the production of chemicals. Importantly, many products derived from such sources under acid‐catalyzed conditions, products such as low‐molecular‐weight carbohydrates, furaldehydes, functional acids and esters, phenols, and alkanes can be immediately employed in extant commercial settings. For example, cellulose‐derived alkyl (poly)glucosides can form a part of detergent formulas, alkyl levulinates and lignin‐based cycloalkanes can be used in the production of automotive fuels, and hydroxy acids can be polymerized into commodity polyesters. Nevertheless, most laboratory methods need substantial improvements before they may fully translate into commercial production. On the one hand, this translation requires further scientific research to render targeted catalytic reactions as efficient as possible. These studies will involve improved reaction conditions, separation processes,
