Effect of Metal Catalysts on Bond Cleavage Reactions of Lignin Model Compounds in Supercritical Water

Lignin depolymerization into its aromatic monomers is necessary for the valorization of lignocellulosic biomass, but cleavage of the C–O ether bonds and C–C bonds between the monomers is challenging. In this study, we investigated bond cleavage reactions of benzyl phenyl ether, diphenyl ether, and biphenyl, which served as models for lignin compounds with α–O–4, 4–O–5, and 5–5 linkages, respectively, by using various metal catalysts in supercritical water at 673 K and a water density of 0.5 g cm −3 without added hydrogen gas. Benzyl phenyl ether was decomposed without catalyst. We found that palladium, platinum, and rhodium catalysts supported on carbon showed activity for cleavage of the C–O ether bonds in benzyl phenyl ether and diphenyl ether but not for the bonds between the aromatic units of biphenyl. Using anisole as a model compound, we also found that methoxy groups, which are present in lignin containing coniferyl and sinapyl alcohol monomers, were converted into phenol groups or were decomposed. The reaction conditions described herein have great potential utility for the conversion of lignin, which can be decomposed and dissolved in supercritical water, into valuable aromatic compounds.