The formation of ketones from carboxylic acids has been carried out on industrial scale more than a century ago by pyrolysis of their salts. More recently, the reaction is performed in the presence of catalysts at elevated temperature. Therewith, it fulfils the principles of Green Chemistry as a high atom efficiency is achieved and only innocuous by-products such as water and carbon dioxide produced.<br/>The mechanism has been under debate for a long time, but in recent years the so-called “beta-keto acid mechanism” becomes more and more predominant as the result of theoretical, spectroscopic and experimental studies. Herein, evidences for this mechanism on the monoclinic zirconium oxide surface are discussed and compared to other metal oxides.<br/>Monoclinic zirconium oxide is an active and selective catalyst for the ketonic decarboxylation at 400 °C, with good stability with time on stream. The well-defined surface allowed a theoretical elucidation of the mechanism and DFT studies favored the “beta-keto acid” mechanism over the “concerted” model. Thereby, one molecule is deprotonated in alfa-position to the carboxyl group and this carbon atom attacks nucleophilically the carbonyl group of a second molecule. This sequence puts the focus on the substitution pattern in alfa-position and therefore, the behavior of branched carboxylic acids was studied in detail.<br/>When comparing the reaction of linear and mono-substituted carboxylic acids, the same apparent activation energy was experimentally determined, while the reaction rates differed by a factor of 30. A reason for this might be the change in entropy, which was measured to differ by nearly 50 kJ/mol. It was supposed that the origin for this was that molecules have to align in the right way on the surface, hindered by the substituent in the case of the branched molecules.<br/>The “beta-keto acid” mechanism predicts that tertiary carboxylic acids, such as pivalic acid, cannot be transformed into ketones. Experimentally, this was confirmed with different materials as e.g. zirconium oxide, barium oxide and magnesium oxide, all of them suitable catalysts for less substituted molecules. When forcing the reaction conditions (&gt;450 °C) a retro-Koch reaction was observed, providing an olefin, carbon monoxide and water, and other pyrolysis products, but no symmetrical ketone or ketone derived products. All these observations on branched and tertiary carboxylic acids are clearly in line with the favored “beta-keto acid” mechanism.

Mechanism of the ketonic decarboxylation of carboxylic acids on zirconium oxide and other oxides

2019 Spring National Meeting

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