Hydrogenation of Benzonitrile over Supported Pd Catalysts: Kinetic and Mechanistic Insight

The liquid phase hydrogenation of benzonitrile over a 5 wt % Pd/C catalyst using a stirred autoclave is investigated. The reaction conforms to a consecutive reaction sequence: first benzonitrile is hydrogenated to produce benzylamine, which subsequently undergoes a hydrogenolysis step to form toluene. Benzonitrile hydrogenation obeys first-order kinetics with an activation energy of 27.6 kJ mol(-1). In contrast, the benzylamine hydrogenolysis stage obeys zero-order kinetics and exhibits an activation energy of 80.1 kJ mol(-1). A 1 wt % Pd/Al2O3 catalyst is additionally examined, which is also seen to support hydrogenolysis activity alongside the hydrogenation pathway. Gas phase transmission infrared spectroscopic measurements of the hydrogenation of benzonitrile and benzylamine over the 1 wt % Pd/Al2O3 catalyst utilizing hydrogen and deuterium are undertaken, which enable reaction schemes incorporating adsorption geometries of intermediate adsorption complexes to be proposed.