Unraveling the Role of Metal in M/NiAl₂O₄ (M = Pt, Pd, Ru) Catalyst for the Self-Reforming-Driven Hydrogenolysis of Lignin

Previously, we found that, by self-reforming-driven hydrogenolysis of lignin or lignin oils, 4-alkylphenols can be produced directly under hydrogen-free conditions over Pt//NiAl2O4 catalyst (ACS Catal. 2020, 10, 15197-15206). That work mainly focused on the exploration of a new strategy and the reaction pathways of the hydrogen contained in lignin during its valorization. In this work, we use M/NiAl2O4 (M = Pt, Pd, Ru) catalysts to investigate the rate-determined step, structure-activity relationships, the adsorbed species, and the essence of different metals and finally screen a good catalyst with excellent performance. A temporal profile of reaction using a model compound and kinetic measurements disclosed that demethoxylation is the rate-determined step, in which Ru/NiAl2O4 has superior ability probably due to the faster removal of products that enables the rapid recovery of active sites for demethoxylation when compared to Pt/NiAl2O4. In contrast, side reactions and a low rate of demethoxylation were observed over Pd/NiAl2O4. In the self-reforming-driven hydrogenolysis of birch lignin and lignin oil, Ru/NiAl2O4 exhibited again its excellence in gaining 4-alkylphenols with yields of 40.7 and 92.8 mol % (5.7 and 44.5 wt %), from lignin and lignin oil, respectively. This comprehensive study would help to design catalysts with higher activity for the self-reforming process of biomass resources.