Insights into the oxygen vacancies in titanium-aluminum composite oxides for Ru-catalyzed bio-oil upgrading using guaiacol as a model compound

The high quality liquid products obtained from upgrading of bio-oil through hydrogenation can serve as viable alternatives to fossil fuels, thereby alleviating the strain on energy supply. In this study, a series of titanium-aluminum composite oxides (Ti x Al y O) was prepared by a sol-gel method and used as supports to fabricate ruthenium (Ru)-based catalysts for the hydrogenolysis upgrading of bio-oil. Various techniques, including in situ pyridine adsorption-Fourier-transform infrared spectroscopy (in situ Py-FTIR), in situ CO diffuse-reflectance Fourier-transform infrared spectroscopy (in situ CO DRIFTS), and CO 2 temperature-programmed desorption (CO 2 -TPD), were employed to characterize the physicochemical and electronic structures of the catalysts. Using guaiacol as a model compound for bio-oil, the compositions and structures of the Ti x Al y O supports, as well as the effect of calcination treatment of the supports in an N 2 atmosphere, on the hydrogenolysis reactivity, were investigated in detail. The results indicated that the hydrogenolysis rate of C aromatic −O bonds showed a significant linear correlation with the oxygen vacancy/Lewis acid content. Furthermore, the presence of oxygen vacancies facilitated the formation of small Ru nanoparticles. Importantly, calcination of the support in an N 2 atmosphere facilitated the formation of metal/oxide interface sites in the Ru/Ti 5 Al 5 O-N catalyst, further augmenting its Lewis acidity and reducing the activation energy for the hydrogenolysis of the C aromatic −O bonds. The hydrogenolysis rate of the C aromatic −O bond in guaiacol over the oxygen vacancy-rich catalyst Ru/Ti 5 Al 5 O-N was increased by a factor of 2–4 compared to those over Ru-based catalysts with a single oxide support. This study exemplified a novel strategy of oxygen vacancy-promoted hydrogenolysis upgrading of bio-oil.