Hydrolysis-dominated catalytic system: Hydrogen-free hydrogenolysis of lignin from Pd-MoO_x/TiO₂

Lignin is continuously investigated by various techniques for valorization due to its high content of oxygen-containing functional groups. Catalytic systems employing hydrolysis‑hydrogenolysis, leveraging the synergistic effect of redox metal sites and acid sites, exhibit efficient degradation of lignin. The predominance of either hydrolysis or hydrogenolysis reactions hinges upon the relative activity of acid and metal sites, as well as the intensity of the reductive atmosphere. In this study, the Pd-MoO_x/TiO₂ catalyst was found to primarily catalyze hydrolysis in the lignin depolymerization process, attributed to the abundance of moderate acidic sites on Pd and the redox-assisted catalysis of MoO_x under inert conditions. After subjecting the reaction to 240 °C for 30 h, a yield of 48.22 wt% of total phenolic monomers, with 5.90 wt% consisting of diphenols, was achieved. Investigation into the conversion of 4-propylguaiacol (4-PG), a major depolymerized monomer of corncob lignin, revealed the production of ketone intermediates, a phenomenon closely linked to the unique properties of MoO_x. Dehydrogenation of the propyl is a key step in initiating the reaction, and 4-PG could be almost completely transformed, accompanied by an over 97 % of 4-propylcatechol selectivity. This distinctive system lays a new theoretical groundwork for the eco-friendly valorization of lignin.