Catalytic Hydrogenolysis of Lignin into Propenyl-monophenol over Ru Single Atoms Supported on CeO2 with Rich Oxygen Vacancies

Lignin is the most abundant aromatic source of natural products, but developing efficient catalysts to depolymerize it into valuable monophenol with high yield and unique selectivity remains a challenge. Herein, we report a Ru single-atom catalyst (SAC) supported on rod CeO2 with oxygen vacancies (Ov) for the depolymerization of birch dioxane acidolysis lignin (DAL). A near-theoretical maximum monophenol yield (14.8 wt %) with good selectivity to 4-n-propenyl guaiacol (51.4%), as well as high catalyst stability, was achieved. The calculated turnover (TON) was 387 molaromatics/molRu, which is 55× higher than that of the Ru/C catalyst. The possible reaction for this catalyst was proposed by studying a series of lignin model compounds and in situ DRIFT measurements. The mechanism involves the cleavage of Cα–OH and Cβ–O bonds to produce coniferyl alcohol, followed by the removal of γ-OH to generate 4-n-propenyl guaiacol. The effects of some key parameters like solvent, Ru content, temperature, reaction time, and H2 pressure were also investigated in terms of monophenol yields and average molecular weight. This work provides an economically feasible method for the depolymerization of lignin into highly valuable monophenols.