Theoretical Study of the Mechanism of Propionic Acid Deoxygenation on the Palladium Surface

Abstract

Using the DFT–PBE method in the scalar relativistic approximation, the mechanisms of the two main pathways of propionic acid deoxygenation on the rough and flat (111) palladium surfaces have been studied. According to the calculations, in the decarboxylation mechanism on rough and flat surfaces, the formation of the following intermediates is preferable: C₂H₅COO, C₂H₄COO, and C₂H₄. For the second deoxygenation pathway via decarbonylation reactions, the mechanisms on different surfaces of palladium differ. Thus, on a rough surface, the most likely steps are C₂H₅COOH → C₂H₅CO → C₂H₅ → C₂H₄, and on the Pd(111) surface the most likely steps are C₂H₅COOH → C₂H₄COOH → C₂H₄CO → C₂H₄. The coordination unsaturation of palladium atoms contributes to a decrease in the activation barriers of the reaction by 8–13 kcal/mol. Thus, the flat surface of palladium particles is less active in the deoxygenation of carboxylic acids. The type of palladium surface insignificantly affects the selectivity of deoxygenation. On a rough surface, the decarbonylation rate is slightly higher than the rate of decarboxylation. On the Pd(111) surface, the rate of decarboxylation is higher. The difference in the activation barriers of these pathways of deoxygenation is small (0.7 kcal/mol).