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

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 are also analyzed and are the first to account for vanillin
production under these conditions. Major β-O-4 bond cleavage
and dimer decomposition paths have been identified. In neutral water,
the dominant path is quinone methide formation followed by homolysis.
In basic water, two paths dominate: one leads to a stable vinyl ether,
and the other leads to a highly unstable homovanillin.