Enhanced oxidative depolymerization of lignin in cooperative imidazolium-based ionic liquid binary mixtures

Abstract

The aerobic oxidation of lignin model 2-phenoxyacetophenone (2-PAP) in cooperative ionic liquid mixtures (CoILs) with 1-ethyl-3-methylimidazolium acetate ([C2C1im]OAc) and 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BZC1im]NTf2) was investigated. Complete degradation of 2-PAP was achieved with [C2C1im]OAc/[BZC1im]NTf2 molar ratio (RIL) of 1/1 and 1/2 at 100 °C for 2 h. The conversion and product yields from CoILs were higher than those in pure ILs, indicating the cooperative effects of [C2C1im]OAc/[BZC1im]NTf2 on cleaving aryl–ether bonds. [C2C1im]OAc promoted the catalytic cleavage of aryl–ether bonds and solvation, and [BZC1im]NTf2 induced the formation of alkyl radicals and enhanced the product selectivity. Accordingly, the highest conversion of alkali lignin (79.8%) was obtained with RIL of 5/1 at 100 °C for 2 h, and phenol monomers (306 mg/g) were selectively produced. The CoILs exhibited good catalytic capacities for oxidative depolymerization of lignin, which strongly depends on the changes in intermolecular interactions and structural organization with varying RIL.