Aqueous choline acetate as reaction medium for the oxidation of kraft lignin with hydrogen peroxide

Abstract

Recently, choline-based ionic liquids (ILs), a class of biocompatible ILs, have been successfully employed in the pretreatment of lignocellulosic biomass. Nevertheless, their potential as solvents alternative to the more popular yet unfriendly imidazolium ILs in the oxidative depolymerization of lignin appears only marginally explored. In this study, aqueous cholinium acetate, [Ch][AcO], a readily available IL, was used as the reaction solvent for the oxidative depolymerization of Kraft lignin (KL) with H₂O₂/MoO₃. Experimental optimization design was helpful to estimate optimal reaction parameters. Under reasonably mild conditions (H₂O₂ to KL weight ratio 0.33, 9.2% MoO₃, 8% NaOH, 77 °C, 5 h), KL could be converted to 37% of depolymerization oil (KL_Oil), containing nearly 2.5% of aromatic monomers (ArMo), of which vanillin is the most abundant (up to 67%, yield 1.64%), and to 55% of oxidized lignin (OKL) solid fraction. When increasing H₂O₂/KL up to 1, ArMo yield decreased in favor of aliphatic acids (mainly, malonic acid), originating from ArMo over-oxidation, while OKL fraction enriched with carbonyl functional groups. Interestingly, in the absence of [Ch][AcO], lower ArMo yields (ArMo 1.68%) were observed, with vanillin selectivity dropping to 36% (yield 0.60%), hinting a possible stabilizing effect of the IL on reactive depolymerization intermediates and products. [Ch][AcO] could be regenerated, with <3% mass loss and unaltered chemical structure, and recycled without significant changes in product yields.