Enhancing the anisole hydrodeoxygenation activity over Ni/Nb2O5−x by tuning the oxophilicity of the support

Abstract

The present work focuses on designing a robust non-noble metal-based niobia-supported catalyst with a high number of oxygen vacancies for an efficient HDO activity of anisole. Various synthesis procedures, such as hydrothermal (Nb₂O₅-HT), precipitation (Nb₂O₅-P) and hydrolysis (Nb₂O₅-H), are adopted for the preparation of the Nb₂O₅ support, followed by 3 wt% Ni impregnation. As a benchmark, Ni has also been impregnated on commercial Nb₂O₅ (Nb₂O₅-C). The catalysts display a large discrepancy in surface areas, the crystallite size of Nb₂O₅ and NiO_x, strong metal–support interaction (SMSI), active interfacial Ni–NbO_x species and oxygen vacancies. In particular, Ni impregnated on Nb₂O₅-H (Ni/Nb₂O₅-H) possesses the highest amount of oxygen vacancies (202 μmol g⁻¹) (O₂-temperature programmed desorption), which is 1.1 and 3.5 fold higher than that for Ni/Nb₂O₅-HT and Ni/Nb₂O₅-P, respectively, indicating the importance of the synthesis procedure of the support. Interestingly Ni/Nb₂O₅-H has 200-fold higher oxygen vacancies than Ni/Nb₂O₅-C. Likewise, Ni/Nb₂O₅-H possesses a high Ni³⁺/Ni²⁺ and Nb⁴⁺/Nb⁵⁺ ratio, based on X-ray photoelectron spectroscopy, suggesting high dispersion of small NiO_x particles on the NbO_x surface, facilitating the redistribution of the electrons at the interface of NiO and Nb₂O₅ due to SMSI. This leads to the formation of active interfacial Ni–NbO_x species and oxygen vacancies (confirmed by H₂-TPD and XPS), contributing to the activation of hydrogen and anisole adsorption and thereby improving HDO activity substantially. At similar conversions of anisole, Ni/Nb₂O₅-H exhibits the highest rate of HDO of 0.63 mmol min⁻¹ g⁻¹, which is 1.4 to 3.2 fold higher than those of the other catalysts, including Nb₂O₅-C. Under optimised reaction conditions, Ni/Nb₂O₅-H shows a remarkable activity towards HDO of anisole with an excellent selectivity to cyclohexane (96.5%) at the near-quantitative conversion of anisole (>98%) at 240 °C, 20 bar H₂, 2 h. Ni/Nb₂O₅-H also displays good to excellent activity towards various lignin-based model compounds, affording 50–96% cyclohexane yield.