Decarboxylative ketonization is used for the industrial preparation of ketones from carboxylic acids and for biofuels as part of the biomass deoxygenation process.The reaction mechanism proceeds through a reversible alpha proton abstraction from surface carboxylates, i.e., enolization, thus, generating a reactive nucleophile for the condensation with another carboxylate on catalyst surface. The condensation step forms beta keto acid intermediate which decomposes into ketone and CO2. Condensation is the limiting step of the mechanism, the rate of which depends on the concentration of the enolized carboxylates. For kinetic and mechanistic studies of the decarboxylative ketonization reaction it is important to know the rate of the enolization, the activation energy and the equilibrium constant. In the current work we present data on the catalytic H/D isotopic exchange of alpha protons in carboxylic acids with ZrO2 catalyst at the temperatures 180-230 °C, which is below the typical range of the decarboxylative ketonization reaction, but at which the enolization can still take place. The isotopic exchange in this case proceeds via two elementary steps, alpha proton abstraction generating enolized carboxylate, followed by addition of deuterium via the reverse reaction. In kinetics, H/D exchange is a classic case of pseudo-first-order reaction approaching the equilibrium. The McKay equation is used to determine the actual rate constant of the reaction. It is found to follow the first order on the acid concentration. The rate of H/D exchange found for the mixture of isobutyric acid, (CH3)2CHCO2H with D2O was about 4 times faster compared to the rate of D/H exchange between alpha-deuterated isobutyric acid (CH3)2CDCO2D with H2O at 190 °C. From the temperature dependence of the rate constant, the activation energies, 17.0 ± 1.6 kcal/mol and 21.1 ± 0.9 kcal/mol have been found for the H and D abstraction, respectively. A conclusion is made that the limiting step of the isotopic exchange is the abstraction of alpha-hydrogen of the carboxylic acid.

Kinetic isotopic effect in the catalytic enolization of carboxylic acids on ZrO2 surface

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

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