Reactions of carboxylic acids on oxides: 2. bimolecular reaction of aliphatic acids to ketones

The reaction of aliphatic carboxylic acids over oxidic catalysts
has been studied. Ketones are the main product in this reaction. Up
to now there has been no agreement in the literature concerning the
mechanism of this ketonization reaction. In the case of acetic acid,
it appears that the ketone can be formed via two different routes.
On oxides with a low lattice energy, bulk acetates are formed, decomposition of which leads to acetone. On oxides with a high lattice energy, the reaction to acetone takes place on the surface and leaves the bulk structure of the catalyst unaltered. The surface reaction to ketones probably proceeds via an intermediate that is oriented parallel to the surface and that has chemical interactions with the catalyst via both the carboxyl group and the α-carbon of the alkyl group. For the latter interaction abstraction of an α-hydrogen atom is required. The alkyl group of this intermediate can react with a neighboring carboxylate to give the ketone. The remaining carboxyl group forms CO2. The intermediate is very likely to be in pseudoequilibrium with the corresponding ketene.