Insights into co-solvent roles in oxidative depolymerization of lignin to vanillin and lactic acid

Abstract

Efficient low-boiling-point co-solvent systems were developed for the oxidative depolymerization of lignin to high-value platform chemicals, mainly vanillin and lactic acid, using molybdenum (Mo)-based solid oxide catalysts and hydrogen peroxide (H2O2) oxidant. The co-solvent, designed with the matching Hansen solubility parameter (HSP) values to lignin, facilitated efficient solvation of lignin. Various factors, including co-solvents, catalyst supports, H2O2 concentrations, and active metals, were detailly investigated. Optimal yields of total liquid products (EA oils: 19.8 %) were obtained in water-tetrahydrofuran (H2O-THF) co-solvent systems, increased by 61.0 % compared to that of the sole aqueous solution, employing a MoO3/Al2O3 catalyst. Additionally, the oxidized lignin (O-lignin) or EA oils was analyzed using UV–vis, CHN, GPC, and FT-IR techniques. Results indicated that the co-solvent system remarkably inhibited the repolymerization of lignin fragments, and enhanced the oxidative cleavage of lignin side chains. Lastly, a hypothetical mechanism explained the reaction pathway of lignin depolymerization in co-solvent systems.