Crystal facet dependence of the ketonization of propionic acid on anatase TiO2†

Abstract

Exploring reactions on well-defined surfaces may contribute to a better understanding of structure–activity relationships for metal oxide-mediated reactions. Herein, anatase TiO₂ with predominantly exposed (101), (100) and (001) facets were synthesized and tested for vapor-phase ketonization of propionic acid. The intrinsic ketonization rates based on both mass and surface area at 350 °C increase following the order of TiO₂(100) < TiO₂(101) < TiO₂(001), with the corresponding turnover frequency based on the density of the acid–base pair being 51.8, 71.3, and 185.2 h⁻¹, respectively. The ketonization rate cannot be correlated with either acid/base property or concentration of oxygen vacancies, but an integral band intensity ratio of monodentate/bidentate propionate, suggesting that the monodentate configuration is likely the more reactive intermediate toward C–C coupling. Density functional theory calculation of propionic acid adsorption indicates that the high activity of ketonization results from the longer shortest Ti_5c–Ti_5c distance, and the square arrangement of surface Ti_5c centers in the nearly flat surface of the (001) facet. These results indicate that the surface geometrical structure of the metal oxide plays a crucial role in the ketonization of carboxylic acids, and the minority (001) facet on the surface may predominantly contribute to the overall activity in ketonization on anatase TiO₂. Our results also suggest that facet engineering may greatly enhance the C–C coupling reactions mediated on the acid–base pair of the metal oxide.