Beta-keto acids are important intermediates in food metabolism and for the industrial preparation of ketones and biofuels from carboxylic acids by the catalytic decarboxylative ketonization on metal oxides. Debates on the details of the decarboxylation mechanism in solution continued for almost a century, but the key important step is the required activation of the carbonyl group by Lewis acids. In aqueous solution, protons take the role of Lewis acid, while on surface of metal oxides it is a metal center. The preferred mode of adsorption of beta-keto acids on ZrO2, and TiO2 found by DFT studies by at least five research groups is through binding by both the carbonyl and the carboxyl groups. Thus, the carbonyl group is predictably activated for the decarboxylation. In the current study, we present kinetics of four beta-keto acids decarboxylation in solution and on surface of KOH-doped and undoped ZrO2, and KOH-doped anatase TiO2. The four acids selected for the study represent intermediates proposed for the cross-ketonization of acetic and isobutyric acids and studied by DFT computations in our previous work.[1] The reaction follows first-order kinetics in solution and on surface. It is faster on surface confirming the catalytic effect of metal oxides. The rates of decarboxylation measured at 18-53 °C temperature are extrapolated to the typical range of ketones formation, 300-450 °C. The rate of the decarboxylation step in the reaction mechanism is several orders of magnitude faster compared to the overall rate of the ketones formation from carboxylic acids. The results serve as the experimental evidence that the decarboxylation is not the rate-limiting step of the ketones production. A strong effect of the molecules geometry on the decarboxylation activity is found. Surprisingly, KOH doping of anatase TiO2 catalyst promotes formation of cross-ketones vs. symmetrical ketones for both the decarboxylation step in the current study and the overall catalytic formation of ketones from carboxylic acids reported previously.[2] In contrast, decarboxylation of beta-keto acids to symmetrical ketones and their overall formation from carboxylic acids are faster with zirconia catalysts, while for the decarboxylation in solution there is almost no preference.

Kinetics of beta-keto acids decarboxylation in solution and on surface of metal oxides

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.