Ketonization of Carboxylic Acids: Mechanisms, Catalysts, and Implications for Biomass Conversion

Ketonization is a reaction in which two carboxylic acids convert into a ketone, carbon dioxide, and water. While this reaction once found its industrial application for acetone production, it is regaining interest for its value in the upgrading of biomass-derived oxygenates, for example, bio-oils obtained from the fast pyrolysis of biomass. Namely, ketonization is crucial to reduce the detrimental effects of carboxylic acids in bio-oil. This review addresses reaction mechanisms, families of materials that catalyze the reaction (metal oxides and zeolites), and current applications of ketonization in the upgrading of biomass-derived oxygenates. A variety of mechanisms have been proposed to explain the ketonization reaction, and these proposals are critically discussed. The role of the α-hydrogen has been proven as a critical requirement for ketonization over catalysts that are active for surface ketonization and serves as the initial basis for the discussion. The role of crucial reaction intermediates such as ketene, beta-keto-acids, carboxylates, and acyl carbonium ions is critically evaluated. Finally, the importance of amphoteric properties of metal oxides on the ketonization reaction is explained. This analysis also explains the positive influence of additives, as well as prereduction treatments, on catalytic performance.