Catalytic Transfer Hydrogenolysis of Lignin Models and Native Lignin over Oxygen Vacancy–Rich Ni–Ru/HO–TiO x Using Isopropanol as a Hydrogen Donor

Abstract

Developing sustainable and hydrogen-efficient catalytic strategies for lignin valorization remains a significant challenge for renewable energy and environmental applications. Herein, we report a defect-engineered, oxygen vacancy–rich Ni–Ru/HO–TiOx catalyst for selective catalytic transfer hydrogenolysis (CTH) of β-O-4 lignin model compounds and native hardwood lignin into value-added chemicals and fuels. Surface hydroxyl enrichment and oxygen vacancies are proposed to enhance metal–support interactions, hydrogen spillover, and C–O bond activation over the optimized 3Ni1Ru/HO–TiOx catalyst. Using isopropanol as both solvent and hydrogen donor, the catalyst achieved >99% conversion of 2-phenoxy-1-phenylethanol at 160 °C, with tunable selectivity toward aromatic and cyclic hydrocarbons in the absence of externally supplied molecular hydrogen. Mechanistic and kinetic studies suggest that in situ alcohol dehydrogenation, defect-rich interfaces, and modulated surface acidity facilitate hydrogen transfer, selective ether bond cleavage, and controlled hydrogenation. The catalyst showed good stability, recyclability, and minimal metal leaching. Furthermore, the catalytic system upgraded organosolv hardwood lignin into C6–C9 cyclic hydrocarbons. CHEM21 green metrics analysis indicated improved material efficiency and reduced environmental impact under optimized conditions. These findings demonstrate the potential of defect-engineered catalysts for sustainable lignin upgrading using catalytic transfer hydrogenolysis pathways.