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 titaniumaluminum composite oxides (TixAlyO) 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 CO2 temperature-programmed desorption (CO2-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 TixAlyO supports, as well as the effect of calcination treatment of the supports in an N2 atmosphere, on the hydrogenolysis reactivity, were investigated in detail. The results indicated that the hydrogenolysis rate of Caromatic- 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 N2 atmosphere facilitated the formation of metal/oxide interface sites in the Ru/Ti5Al5O-N catalyst, further augmenting its Lewis acidity and reducing the activation energy for the hydrogenolysis of the Caromatic- O bonds. The hydrogenolysis rate of the Caromatic- O bond in guaiacol over the oxygen vacancy-rich catalyst Ru/Ti5Al5O-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.