Carbon-carbon coupling and hydrodeoxygenation during beechwood hydropyrolysis gas upgrading on TiO2: Oxygen vacancies, lewis acidity and basicity

Anatase TiO ₂ (TiO ₂-A) has been utilized for biomass upgrading processes such as hydrodeoxygenation (HDO) and Carbon-Carbon (C-C) coupling reactions (ketonisation and aldol condensation), where Ti-O Lewis acid-base pairs (LABPs) serve as active sites. Altering the metal oxide's reduction state can modify its acid-base properties, yet the effects of oxygen vacancy coverage on TiO ₂ during biomass vapor upgrading remain unclear. This study investigates the dynamics between C-C coupling and HDO reactions in the ex-situ upgrading of beechwood pyrolysis vapors at 600 °C and 1 atm. LABPs properties were tuned by varying degrees of oxygen vacancy on TiO ₂, and the catalyst was characterized by BET, XRD, NH ₃-TPD, CO ₂-TPD, H ₂-TPR, Raman, UV–vis, SEM-EDX, and FTIR. Our study demonstrated that decreasing the O/Ti ratio (i.e., increasing oxygen vacancies) promotes C–C coupling and HDO reactions. The highest C-C coupling and moderate HDO observed on an O/Ti ratio of 1.7 produced the highest jet-fuel fraction (56.5%) compared to other TiO ₂ variants. The C2+ selectivity shifted from 85.2% of hydropyrolysis oil to 99.2 wt%, while the O/C and H/C ratios changed from 0.45 and 1.55 of hydropyrolysis oil to 0.06 and 1.39, respectively, on TiO ₂ with an O/Ti ratio of 1.7. The adsorption behavior of the acetone, furan, and guaiacol on LABPs was evaluated on the (1 0 1) plane of A-TiO ₂ by DFT, which corroborated the experimental findings. This is the first time a deep correlation has been provided on the influence of oxygen vacancies on the vapor phase upgrading of real biomass feedstock.