Oxidative depolymerization of lignin to vanillin and lactic acid in an aqueous solution

A heterogeneous molybdenum (Mo)-based oxide catalyst system in combination with hydrogen peroxide as an oxidant was developed for oxidative depolymerization of lignin to yield high-value platform chemicals in a nearly neutral aqueous solution. The impact of various reaction conditions, including the ratios of H2O2 and/or MoO3/Al2O3 catalyst to lignin, reaction temperature and duration, and lignin concentration, were investigated in a batch system or a continuous flow fixed-bed system. The structural changes between dealkaline lignin and the recovered lignin (O-lignin and M -lignin) were analyzed using UV-vis, FT-IR, and GPC techniques. The com-bination of MoO3 and H2O2 resulted in a superior peroxo complex, promoting the formation of vanillin and lactic acid. The medium and weak acid feature of MoO3/Al2O3 catalyst induced the highest vanillin yield (5.6%) and the selectivity (86.9%) of main platform products. However, the highest yield of lactic acid (3.4%) was observed on MoO3/SiO2 with more weak acid sites. Hypothetical formation mechanisms for vanillin and lactic acid were proposed based on the depolymerization of lignin model compounds.

Automated summary

A heterogeneous Mo-based oxide catalyst system with hydrogen peroxide as an oxidant was developed for the oxidative depolymerization of lignin to produce high-value platform chemicals in a nearly neutral aqueous environment. The effects of various reaction conditions, including reactant ratios, temperature, duration, and lignin concentration, were investigated. The combination of MoO3 and H2O2 resulted in a superior peroxo complex, leading to the formation of vanillin and lactic acid. The highest yield and selectivity of vanillin were achieved with a MoO3/Al2O3 catalyst, while the highest yield of lactic acid was obtained with MoO3/SiO2.