Theoretical investigation on the reaction mechanisms of O2-initiated gas-phase oxidation of lignin model compounds

Transforming renewable lignin into high value-added chemicals is a forward-looking strategy to address the resource waste caused by insufficient utilization of biomass resources. On this basis, studying the efficient conversion of lignin to aldehydes/acids and their reaction mechanisms has become an attractive topic. A systematic investigation of the gas-phase oxidation reaction mechanisms of the three model compounds initiated by O₂ was carried out at the atomic and molecular levels by using density functional theory (DFT). Further revealing of oxidation behavior on two reaction sites of phenolic hydroxyl group and hydroxymethyl group were accomplished in detail. The potential energy surface information of 21 possible reaction channels of two pathways were obtained at B3LYP/6-311+G(d,p) level. The influence of substituent effects on the reaction energy barrier was estimated. The calculation results showed that the reactivity of phenolic hydroxyl group is stronger than that of hydroxymethyl group, because the reaction Gibbs potential barriers are lower by about 4.9–8.7 kcal/mol. The reaction energy barriers on phenolic hydroxyl group site and hydroxymethyl group site decrease with the increase of the number of methoxy groups. Revealing the oxidation processes of lignin model compounds will provide a deeper understanding on the reaction mechanism and provide theoretical support for further experimental research on the conversion of lignin into high value-added chemicals.