1-Ethyl-3-methylimidazolium acetate ionic liquid as simple and efficient catalytic system for the oxidative depolymerization of alkali lignin

The oxidative depolymerization of alkali lignin (AL) in 1-ethyl-3-methylimidazolium acetate ([C 2 C 1 im]OAc) system without additional catalyst was investigated under mild conditions (initial O 2 pressure of 1.5 MPa, 80 °C–100 °C). Compared with other ionic liquids (ILs), the cooperation of imidazolium cation and acetate anion successfully enhanced AL conversion. Among the investigated imidazolium acetate ILs with ethyl- to octyl-side chains, [C 2 C 1 im]OAc presented the best catalytic capacity for AL oxidative depolymerization. Adding an appropriate amount of water to [C 2 C 1 im]OAc can further improve the reaction efficiency. In the [C 2 C 1 im]OAc system with the addition of 0.10–0.25 mL of water, approximately 77 wt% AL was depolymerized into small molecule soluble products at 100 °C for 2 h. The extracted oil was composed mainly of phenolic derived compounds. With the use of the [C 2 C 1 im]OAc-based system, the specific inter-unit linkages of lignin were broken down, and residual lignin with low molecular weight and narrow polydispersity index (1.88–1.96) was obtained. Compared with that in AL conversion with fresh [C 2 C 1 im]OAc, only a minimal decrease (~3.2%) was observed with the recovered IL until the fifth cycle. These findings revealed that [C 2 C 1 im]OAc–based system is a simple and efficient catalytic system for lignin oxidative depolymerization.