Role of Ni/Al Molar Ratio in Ni─Al Mixed Oxide Catalysts for Upgraded Vanillin Hydrodeoxygenation

Abstract Ni–Al mixed oxide catalysts were synthesized from layered double hydroxide (LDH) precursors with Ni/Al molar ratios of 2, 3, and 4, using a low‐supersaturation coprecipitation method. The materials were calcined and reduced to evaluate their catalytic performance in the hydrodeoxygenation (HDO) of vanillin, a representative lignin‐derived compound. Catalytic tests performed in a batch reactor at 533 K and 3.92 MPa H₂ demonstrated that the Ni/Al = 3 catalyst exhibited superior activity, achieving full vanillin conversion and 79% selectivity toward fully deoxygenated products, namely methylcyclohexane and cyclohexane. Characterization by XRD, TPR, and XPS revealed that the coexistence of metallic Ni⁰ particles and oxygen vacancies was critical in enhancing both hydrogenation and deoxygenation reactions. Notably, the presence of anionic vacancies allowed for heterolytic H₂ dissociation and hydride formation, contributing to high catalytic efficiency even in the absence of surface‐exposed Ni⁰. The Ni–Al interaction modulated the electronic environment and reducibility of the active sites, influencing product selectivity and catalytic stability. Recycling experiments confirmed the catalyst’s structural robustness over three consecutive cycles. This study demonstrates that tuning the Ni/Al ratio in LDH‐derived mixed oxides enables the design of stable and bifunctional catalysts with synergistic active sites, offering a promising strategy for efficient bio‐oil upgrading via selective HDO pathways.