Crucial role of oxygen vacancies for the efficient hydrodeoxygenation of lignin-based phenolic model compounds with Ni/Ti1−xZrxO2

Abstract

Lignin holds enormous potential for biofuel production through thermal and thermochemical processes. However, its high oxygen content in the form of phenolics leads to a low heating value, low stability, and high viscosity. Catalytic hydrodeoxygenation (HDO) with metal-containing catalysts with appropriate oxygen vacancies and active sites represents an efficient route for removing oxygen and producing high-quality liquid hydrocarbons. In this context, our present work focuses on synthesising Ni-doped mixed oxide (Ti_1−xZr_xO₂) supports for the HDO of lignin model compounds (anisole, phenol, cresol). Notably, 3 wt% Ni doped Ti_0.50Zr_0.50 (Ni/Ti_0.50Zr_0.50O₂) exhibits superior activity and selectivity, achieving a 94% yield of cyclohexane as the primary deoxygenated product, with complete conversion of anisole at 230 °C. In contrast, the mono metal oxide-based support catalysts Ni/ZrO₂ and Ni/TiO₂ yield 29% and 49% of cyclohexane, respectively. The higher activity and selectivity towards deoxygenated products can be attributed to high oxygen vacancies, Lewis-acid strength and strong metal–support interaction compared to Ni/TiO₂ and Ni/ZrO₂, as confirmed by the O₂-temperature programmed desorption, X-ray photoelectron spectroscopy, NH₃-diffuse reflectance infrared Fourier transform spectroscopy and H₂-temperature programmed reduction studies. The use of a low percentage (3 wt%) of non-precious metal doped mixed oxide (Ti_1−xZr_xO₂) supports with enhanced oxygen vacancies opens a new window for exploring effective heterogeneous catalysts for the transformation of lignin-derived bio-oil into fuel-grade hydrocarbons.