β-O-4 Bond Cleavage Mechanism for Lignin Model Compounds over Pd Catalysts Identified by Combination of First-Principles Calculations and Experiments

Lignin in lignocellulosic biomass is the only renewable source for aromatic compounds, and effective valorization of lignin remains a significant challenge in biomass conversion processes. We have performed density functional theory calculations and experiments to investigate the cleavage mechanism of the C-O ether bond in the lignin model compound 2-phenoxy-1-phenylethanol with a beta-O-4 linkage over a Pd(111) catalyst surface model. We propose the favorable reaction pathway to proceed as follows: the dilignol reactant gets dehydrogenated first on the a-carbon and then on the -OH group to generate its corresponding ketone 2-phenoxy-1-phenylethanone; the ketone continues to get dehydrogenated on the beta-carbon by first a equilibrated keto-enol tautomerization to its enol form and then -OH dehydrogenation; the C-O ether bond cleavage happens afterward, leading to one-aromatic-ring surface intermediates followed by hydrogenation to yield acetophenone and phenol.