Hydrodeoxygenation of Oxygenates Derived from Biomass Pyrolysis Using Titanium Dioxide-Supported Cobalt Catalysts

Bio-oil upgrading to produce biofuels and chemicals has become an attractive topic over the past decade. However, the design of cost- and performance-effective catalysts for commercial-scale production remains a challenge. Herein, commercial titania (TiO₂) was used as the support of cobalt (Co)-based catalysts (Co/TiO₂) due to its low cost, high availability, and practicability for commercialization in the future. The Co/TiO₂ catalysts were made with two different forms of TiO₂ (anatase [TiO₂-A] and rutile [TiO₂-R]) and comparatively evaluated in the hydrodeoxygenation (HDO) of 4-propylguaicol (4PG), a lignin-derived model compound. Both Co/TiO₂ catalysts promoted the HDO of 4PG following a similar pathway, but the Co/TiO₂-R catalyst exhibited a higher activity in the early stages of the reaction due to the formation of abundant Ti³⁺ species, as detected by X-ray photoelectron spectroscopy (XPS) and hydrogen-temperature programed reduction (H₂-TPR) analyses. On the other hand, the Co/TiO₂-A catalyst possessed a higher acidity that enhanced propylcyclohexane production at prolonged reaction times. In terms of reusability, the Co/TiO₂-A catalyst showed a higher stability (less Co leaching) and reusability compared to Co/TiO₂-R, as confirmed by transmission electron microscopy (TEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses. The HDO of the real bio-oil derived from pyrolysis of Leucaena leucocephala revealed that the Co/TiO₂-A catalyst could convert high oxygenated aromatics (methoxyphenols, dimethoxyphenols, and benzenediols) to phenols and enhanced the phenols content, hinting at its potential to produce green chemicals from bio-feedstock.