Insights into the catalytic performance of Ni/Nb2O5 catalysts for vanillin hydrodeoxygenation in aqueous phase: The role of Nb2O5 crystal structures

Hydrodeoxygenation (HDO) in aqueous phase is green and efficient approaches to convert the biomass and its derivative to high value-added chemicals and renewable fuels. However, it remains a great challenge for synthesizing catalysts with the excellent hydrodeoxygenation performance. Three kinds of Ni/Nb2O5 catalyst with different crystal structures (Ni/TT-Nb2O5, Ni/T-Nb2O5, Ni/H-Nb2O5) were prepared for vanillin hydrodeoxygenation. Compared with the Ni/TT-Nb2O5 and Ni/H-Nb2O5 catalyst, the Ni/T-Nb2O5 catalyst had the most concentration of oxygen vacancies (Ov), which resulted in the excellent hydrodeoxygenation performance. At 180 ℃, 1 MPa H2, and the water/ethanol ratio of 8/12, 96.1% conversion of vanillin and 79.2% selectivity of 2-methoxy-4-methylphenol (MMP) were obtained. The abundant Ov of Ni/T-Nb2O5 adsorbed and activated the C-OH bonds of vanillyl alcohol (VA), while water formed a stronger transition state with VA to further reduce the disassociation energy of C-OH bonds. The synergistic effect between Ov and water promoted the hydrodeoxygenation of vanillin in aqueous phase, achieving the high yield of MMP. This work provides a new idea for the preparation of catalysts with high catalytic performance in aqueous phase.