Ketonization of Carboxylic Acids in Biomass Conversion over TiO2 and ZrO2 Surfaces: A DFT Perspective

Carboxylic acids play a fundamental role in the transformation of biomass into liquid fuels and other useful chemicals. In order to reduce the O/C content of biofuels, carboxylic acids need to be decomposed by decarboxylation, dehydroxylation, or decarbonylation unimolecular reactions, or they need to be converted into ketones via complex bimolecular reaction mechanisms. Ketonization, that is, the transformation of carboxylic acids into ketones, carbon dioxide, and water, is promoted by heterogeneous catalysts based on oxide materials. Among the most active catalysts are titania and zirconia surfaces. In recent years, a large body of experimental data has been complemented by specific investigations performed with first-principles electronic structure calculations based on density functional theory (DFT). In this review, I discuss the present level of understanding of the bonding modes of carboxylic acids (acetic acid in particular) on the TiO2 and ZrO2 surfaces as obtained from DFT calculations. Enolization and ketonization reaction mechanisms determined at the DFT level on TiO2 and ZrO2 surfaces are also discussed, and the results are analyzed in view of the experimental evidence. Finally, the role of supported metal particles, of the redox properties of the oxide catalyst, and the nature of the active sites on the surface of titania and zirconia are discussed.