Catalytic hydrodeoxygenation of benzoic acid as a bio-oil model compound: reaction and kinetics using nickel-supported catalysts

The development of technologies for the bio-oil upgrading process is a crucial step towards achieving sustainable energy production. This study investigates the effects of support properties during the hydrodeoxygenation (HDO) of benzoic acid as a bio-oil model compound with the aim to produce a catalyst of superior activity and selectivity. Three Ni-based catalysts were prepared: microporous m-Ni/ZSM-5, mesoporous h-Ni/ZSM-5, and Ni/SiO₂. The h-Ni/ZSM-5 exhibited the highest concentration of acid sites, strongest metal-support interaction and best metal dispersion. The highest conversion of benzoic acid was recorded over the h-Ni/ZSM-5 catalyst (97%). Ni/SiO₂ catalysts produced toluene, while others produced benzene and cyclohexane in addition. This was linked to a synergy between support acidity and metal sites. The catalyst from the nearly neutral support, Ni/SiO₂, showed higher activity (91% conversion) compared to m-Ni/ZSM-5 (84%), which was attributed to the mesoporous nature of Ni/SiO₂, allowing more access to active sites for bulk benzoic acid molecules. A kinetic model was developed using the Langmuir–Hinshelwood–Hougen–Watson (LHHW) approach. A mechanism assuming dual-site adsorption of dissociatively adsorbed hydrogen was shown to be the most accurate representation of the three-phase benzoic acid HDO. The observed activation energy from the model was 137.2 kJ mol⁻¹.