Reversibility of the catalytic ketonization of carboxylic acids and of beta-keto acids decarboxylation

Decarboxylation of beta-keto acids in enzymatic and heterogeneous catalysis has been considered in the literature as an irreversible reaction due to a large positive entropy change. We report here experimental evidence for its reversibility in heterogeneous catalysis by solid metal oxide(s) surfaces. Ketones and carboxylic acids having C-13-labeled carbonyl group undergo C-13/C-12 exchange when heated in an autoclave in the presence of (CO2)-C-12 and ZrO2 catalyst. In the case of ketones, the carbonyl group exchange with CO2 serves as evidence for the reversibility of all steps of the catalytic mechanism of carboxylic acids ketonic decarboxylation, i.e. enolization, condensation, dehydration and decarboxylation.