Beta-keto acids are critical intermediates in the mechanism of the catalytic decarboxylative ketonization on metal oxides. The preferred mode for the decarboxylation of beta-keto acids on tetragonal and monoclinic ZrO2, anatase and rutile form of TiO2 assumed in several DFT studies is through binding by both the carbonyl and the carboxyl groups.[1] Such binding of the carbonyl group to the Lewis acid center makes beta-keto acids unstable and immediately activates them for the decarboxylation. In the current study we have synthesized four beta-keto acids proposed as intermediates for the cross-ketonization of acetic and isobutyric acids and deposited them on surface of metal oxides: KOH-doped and undoped monoclinic ZrO2 and KOH-doped anatase TiO2. Infrared spectroscopy revealed the preferred mode of adsorption on all catalysts without binding carbonyl group. First-order rates of decarboxylation have been measured at 18-53 °C temperature range, and extrapolated to the typical range of ketones formation, 300-450 °C. The rate of the decarboxylation step is faster compared to the overall rate of the decarboxylative ketonization of carboxylic acids, which serves as the experimental evidence that the decarboxylation is not the rate-limiting step of the reaction mechanism. Negative entropy of activation was found and associated with the binding of the carbonyl group to the surface as the step required for the decarboxylation. Additionally, a minor direction for the decomposition of beta-keto acids by a retro-condensation reaction to carboxylic acids was confirmed by mass-spectrometry analysis. This result is consistent with the reversibility of the decarboxylative ketonization mechanism. Surprisingly, the effect of KOH doping of anatase TiO2 catalyst promotes formation of cross-ketones vs. symmetrical ketones for both the decarboxylation step found in the current study and for the overall catalytic formation of ketones from carboxylic acids reported previously.[2] In contrast, decarboxylation of beta-keto acids leading to the symmetrical ketones and their overall formation from carboxylic acids is faster with zirconia catalysts. The symmetry related entropy change along the mechanistic path was used to explain the difference in the cross-selectivity.

Decarboxylation of beta-keto acids on surface of metal oxides: selectivity driven by entropy

ACS Spring Meeting 2021

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