LITMIR.BIZ

Scheme 2.8 Proposed acid‐catalyzed cleavage of lignin models (specific bond types are highlighted in bold). Reaction conditions: 1,4‐dioxane (solvent), ethylene glycol (4 eq., based on the substrate), Fe(OTf)₃ (10 mol%, based on the substrate), 140 °C, 15 minutes. Source: Based on Deuss et al. [113].

In distinct contrast to cellulose, lignin is a structurally branched biopolymer that is soluble in many organic solvents and can be cleaved under relatively mild conditions. The principal problem in the processing of lignin is that depolymerization products tend to rapidly repolymerize in the process forming thermodynamically stable C–C linkages [8,33,109]. For example, reactions of various model substances in 1,4‐dioxane in the presence of 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

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