The effect of the sulfation of zirconia catalysts on their structure, acidity/basicity, and catalytic activity/selectivity toward the ketonization of organic acids is investigated by a combined experimental and computational method. Here, we show that, upon sulfation, zirconia catalysts exhibit a significant increase in their Brønsted and Lewis acid strength, whereas their Lewis basicity is significantly reduced. Such changes in the interplay between acid–base sites result in an improvement of the selectivity toward the ketonization process, although the measured conversion rates show a significant drop. We report a detailed DFT investigation of the putative surface species on sulfated zirconia, including the possible formation of dimeric pyrosulfate (S 2 O 7 2– ) species. Our results show that the formation of such a dimeric system is an endothermic process, with energy barriers ranging between 60.0 and 70.0 kcal mol –1 , and which is likely to occur only at high SO 4 2– coverages (4 S/nm 2 ), high temperatures, and dehydrating conditions. Conversely, the formation of monomeric species is expected at lower SO 4 2– coverages, mild temperatures, and in the presence of water, which are the usual conditions experienced during the chemical upgrading of biofuels.

We report a detailed survey of the calculated bulk properties of zirconia using GGA and meta-GGA DFT functionals, Grimme's dispersion, and Hubbard correction in order to identify the best DFT approach for in silico description of zirconia polymorphs.

No abstract