Hydrothermal Decomposition of a Lignin Dimer under Neutral and Basic Conditions: A Mechanism Study

Abstract

Efficient cleavage of the β-O-4 linkages in lignin is the key to obtaining phenolic chemicals from lignin. The hydrothermal treatment of lignin in neutral and alkaline water is highly utilized but remains poorly understood, preventing the development of strategic depolymerization methods. Herein, the hydrothermal decomposition of a guaiacol-based β-O-4 lignin dimer is systematically explored at varying hydroxide and carbonate base concentrations at 175 °C and supplemented by density functional theory calculations to elucidate the mechanisms of monomeric phenolic product formation. Weakly basic conditions were found to efficiently cleave the β-O-4 linkage yielding guaiacol as the primary product and vanillin and acetovanillone as minor products. Intramolecular substitution, bimolecular elimination, and intermolecular substitution mechanisms are proposed for β-O-4 bond cleavage, and their theoretical rates are compared by transition state theory calculations. The downstream product pathways of this reaction...