Tunable product selectivity in the hydrodeoxygenation of guaiacol over Nix-Fey/Al2O3 catalysts under mild conditions

The selective hydrodeoxygenation (HDO) of lignin-derived phenolics such as guaiacol into high-value chemicals under mild conditions presents a persistent challenge in biomass valorization. In this work, a series of Ni x -Fe y /Al 2 O 3 catalysts were synthesized by a sol-gel method and evaluated for the HDO of guaiacol. Characterization (XRD, XPS, H 2 -TPR, NH 3 -TPD, etc.) revealed that the Ni x -Fe y /Al 2 O 3 catalysts possess enhanced textural properties, improved metal dispersion, and tailored surface acidity, collectively contributing to their high activity and selectivity. The introduction of Fe promotes the formation of a Ni 3 Fe alloy and defect-rich FeO x species. Under mild conditions (180 °C, 2 MPa H 2 ), Ni 5 /Al 2 O 3 achieved full guaiacol conversion with > 85% selectivity to 1,2-cyclohexanediol. By introducing Fe and increasing the temperature to 260 °C (2 MPa H 2 ), the Ni 5 -Fe 1 /Al 2 O 3 catalyst shifted the reaction pathway toward deeper deoxygenation, yielding cyclohexane with over 78% selectivity. This tunable selectivity originates from a synergistic mechanism in which metallic Ni/Ni 3 Fe sites facilitate hydrogenation and hydrogenolysis, while acid sites and FeO x -derived oxygen vacancies promote the dehydration of diol intermediates, especially at higher temperatures. This study provides a design-oriented bimetallic strategy for the targeted valorization of lignin into distinct high-value chemicals.