On the mechanism of Pd-catalyzed low pressure gas-solid hydrogenation

A palladium-catalyzed, low pressure hydrogenation of phenylalkynyl derivatives in a gas-solid reaction in the absence of a solvent was studied. The distinctive initiation step was found to be related to the catalyst activation stage and was directly affected by the treatment of the metallic catalyst: deoxygenating of the catalyst helped to diminish the initiation step, while partial poisoning of the catalyst resulted in extending the slower initial step. In the following propagation step, triple bonds are being hydrogenated to give initially a phenylalkenyl and phenylalkanyl products mixture. The chemoselectivity of the hydrogenation depends both on hydrogen pressure and on the mode of preparation of the reacting phenylalkynyl/catalyst powder. A mechanism, which is based on hydrogen mobility in the phenylalkynyl lattice, is suggested and a direct measurement of hydrogen mobility in the lattice was demonstrated.