On the mechanism of catalytic hydrogenation of nitriles to amines over supported metal catalysts

Hydrogenation of nitriles over NaY-supported transition metal catalysts has been studied in the liquid phase in an autoclave and in the gas phase in a microflow reactor. The results show that all reaction steps converting nitriles to primary, secondary and tertiary amines and unsaturated compounds take place on the catalysts surface. At low temperature the selectivity to a particular amine is controlled by the nature of metal: tertiary amines are preferentially formed over Pt/NaY, secondary amines over Pd/NaY, and primary amines over Ru/NaY. With butyronitrile, the selectivity to the primary amine over a variety of metals shows the same selectivity pattern for gas phase and liquid phase hydrogenation, it decreases in the order of Ru, Ph, Ni, Pd, Pt. Clearly, no liquid phase is required for the formation of the higher amines. In the liquid phase hydrogenation of butyronitrile over PdNi/NaY in the presence of pentylamine, small amounts of the enamines: N-butenyl-dibutylamine, N-butenyl-N-butyl-pentylamine, and N-butenyl-dipentylamine have been identified by CC-MS. Higher concentrations of an enamine are found in the liquid phase hydrogenation of benzylcyanide in the presence of diethylamine over Co/Al2O3 catalyst. The concentration of the enamine, N-(2-phenylvinyl)-diethylamine, passes through a maximum with reaction time. Amine addition to the reaction mixture lowers the hydrogenation rate of either nitrile and promotes primary amine formation. (C) 1999 Elsevier Science B.V. All rights reserved.