Crystal Phase Effects on the Gas-Phase Ketonization of Small Carboxylic Acids over TiO₂ Catalysts

The choice of TiO₂ crystal phase (i. e., anatase, rutile, or brookite) greatly influences catalyst performance in the gas-phase ketonization of small volatile fatty acids, such as acetic acid and propionic acid. Rutile TiO₂ was found to perform best, combining superior activity, as exemplified by an exceptional reaction rate of 141.8 mmol h^-1  g_cat ^-1 (at 425 °C and 24 h^-1 ) with excellent ketone selectivity when propionic acid was used. Brookite, to the best of our knowledge never reported before as a viable ketonization catalyst, was found to outperform the well-studied anatase phase, but not rutile. Operando Fourier-transform IR spectroscopy measurements combined with on-line mass spectrometry showed that bidentate carboxylates were the most abundant surface species on the rutile and brookite surfaces, while on anatase both monodentate and bidentate carboxylates co-existed. The bidendate carboxylates were thought to be precursors to the active ketonization species, likely monodentate intermediates more prone to C-C coupling. Ketonization activity did not directly correlate with acidity; the observed, strong crystal phase effect did suggest that ketonization activity is influenced strongly by geometrical factors that determine the ease of formation of the relevant surface intermediates.