Reaction kinetics and mechanism of ketonization of aliphatic carboxylic acids with different carbon chain lengths over Ru/TiO2 catalyst

A kinetics study of the ketonization of carboxylic acids with varying alkyl chain lengths (acetic, propionic, and butyric) has been conducted on a pre-reduced Ru/TiO2 catalyst. A thorough analysis built upon a Langmuir-Hinshelwood (LH) model and transition state theory (TST) shows that the reaction follows a second-order expression with respect to the surface coverage of carboxylic acids. The heats of adsorption are very similar for the three different acids and independent of the carbon chain length. Moreover, they are significantly higher than those of the reaction products, that is, ketone, water, and CO2. At the same time, the change in adsorption entropy of the acids (in absolute value) with respect to the gas phase was found to decrease with increasing alkyl chain length. These results are consistent with a strongly adsorbed bidentate configuration, in which the main interaction with the surface is via the carboxylic group while the alkyl group moves rather freely. Application of the LH model in the fitting of the reaction data at varying temperatures allowed us to calculate the true activation energy and the activation entropy of the reaction. Both were found to increase with increasing carbon chain length of the acids. This compensation effect can be interpreted in terms of the nature of the transition state. It is concluded that ketonization proceeds through a beta-ketoacid intermediate with an early transition state, in which the formation of the C-C bond is the rate-limiting step. (c) 2014 Elsevier Inc. All rights reserved.