Significant Promotion of Surface Oxygen Vacancies on Bimetallic CoNi Nanocatalysts for Hydrodeoxygenation of Biomass-derived Vanillin to Produce Methylcyclohexanol

Constructing surface defective structures (e.g., oxygen vacancies) on metal catalysts may alter their surface electronic properties, thus controlling the absorption and activation of reactant molecules and resultantly governing their catalytic activity. Herein, a series of bimetallic CoNi nanocatalysts were fabricated to be employed in the hydrodeoxygenation (HDO) of lignin-derived vanillin to produce methylcyclohexanol (MCYL). It was demonstrated that surface CoOx-decorated CoNi nanoparticles (NPs) could be generated from Co–Ni–Al-layered double hydroxide precursors. The as-fabricated bimetallic CoNi nanocatalyst with a Co/Ni atomic ratio of 2:1 exhibited an unprecedented catalytic HDO performance with nearly 100% yield of MCYL and an ultrahigh turnover frequency of 1303 h–1 under mild reaction conditions (200 °C and 1.0 MPa hydrogen pressure). XPS spectra and in situ FT-IR absorption results demonstrated that the introduction of Co into bimetallic CoNi NPs was beneficial to the formation of favorable electron-rich Co0 species and abundant surface-defective CoOx species. Combining with density functional theory calculations and experimental results, it was revealed that surface oxygen vacancies stemming from CoOx species significantly promoted the adsorption and activation of reactants, especially vanillin and the 2-methoxy-4-methylphenol intermediate, and meanwhile, surface electron-rich Co0 species on CoNi NPs could favor the activation of oxygen-containing groups. Correspondingly, HDO could proceed rapidly via a direct deoxygenation process of the carbonyl group or methoxy group, with the assistance of double active hydrogen species originating from molecular hydrogen and isopropanol solvent, greatly accelerating the multipath tandem reactions. The present findings provide an advanced approach for designing high-performance non-noble-metal catalysts applied in the catalytic HDO transformation of various biomass derivatives.