On the Mechanism of Catalytic Decarboxylation of Carboxylic Acids on Carbon-Supported Palladium Hydride

The high chemical stability of aliphatic carboxylic acid makes catalytic decarboxylation at low temperatures challenging. We show that arylaliphatic acids (Ar-C_nH_2n-COOH, n ≥ 1) decarboxylate on carbon-supported Pd nanoparticles (Pd/C) at 90 °C with 100% selectivity. In situ XANES during decarboxylation of preadsorbed substrates indicates that the active phase is α-phase palladium hydride (α-PdH_x). The reaction rate is enhanced by one order of magnitude when hydrogen is preadsorbed. Tracing deuterium labeling positions, it is concluded that carboxylic acid (Ar-C_nH_2n-COOH) undergoes an α-C–H bond dissociation on the Pd surface to the Ar-(CH₂)_n−1-CH*-COO* intermediate in the first step, followed by the C–COO scission, and finally, Ar-(CH₂)_n−1-CH* reacts with two sorbed H to produce Ar-(CH₂)_n−1-CH₃. The high rates are related to the concentration of hydride present on the catalyst particles to complete the catalytic cycle in a Mars–van Krevelen-type mechanism and the rate of H/D exchange at the α-C–H position.