Elucidating reaction mechanisms in the oxidative depolymerization of sodium lignosulfonate for enhancing vanillin production: A Density Functional Theory study

Abstract

Lignin is a vital organic constituent of plant biomass and it can be transformed into value-added chemicals through chemical processes. This study employs the Density Functional Theory (DFT) to investigate the mechanistic effects of various oxidants on the depolymerization of sodium lignosulfonate model compounds into vanillin. Under alkaline conditions, OH- ions participation in the hydrolysis of β-O-4 bonds has been predicted. The dehydration of dihydroconiferyl alcohol to form eugenol, with an energy barrier of 69.0kcal/mol was identified as the major rate-determining step in the oxidation reaction. In the absence of an oxidant, the energy barrier for the oxidation of eugenol to vanillin was 77.8kcal/mol. However, when using nitrobenzene or H2O2 as oxidants, the energy barrier has significantly decreased to 12.6kcal/mol and 21.4kcal/mol, respectively. The effects of temperature and pressure on the rate-determining step of the reaction were analyzed using Statistical Product and Service Solutions (SPSS) software. These results have demonstrated that the energy barrier for the hydrolysis reaction's rate-determining step is positively correlated with temperature but negatively correlated with pressure. These theoretical findings would potentially help in designing the experiments and aid in enhancing the selectivity and yield of vanillin.