Ultra-thin nickel oxide overcoating of noble metal catalysts for directing selective hydrogenation of nitriles to secondary amines

Abstract

Hydrogenation of nitriles represents an efficient and environmentally benign route to synthesis of high-value amines. However, the amine selectivity is commonly low, and the mixtures of amines, imines, and even low-value hydrogenolysis byproducts are generally obtained. Herein, we report that overcoating of Pd and Pt catalysts with an ultra-thin NiOx layer via atomic layer deposition enables to close up the undesired hydrogenolysis pathway completely and to prompt a yield of secondary amines drastically to 96% under mild conditions in the hydrogenation of nitriles. The NiOx-coated catalysts also exhibit a much higher recyclability than the uncoated ones. Through detailed structural characterization and control experiments of catalytic performance tests, we show that the NiOx overcoat plays at least three important roles in the promotion of catalytic performance: (i) Enhancement of the adsorption of both imine intermediates and primary amines, to stimulate the subsequent condensation of these two molecules to form secondary amines exclusively with a high activity; (ii) Isolation of large Pd and Pt ensembles to inhibit the hydrogenolysis byproduct; and (iii) Acting as a protective layer to stabilize metal nanoparticles against leaching, and to prevent the formation of undesired metal hydride phase, thus leading to a significant improvement in catalytic stability. These findings provide a robust methodology for directing selective hydrogenation of nitriles to secondary amines exclusively, and the insights here would facilitate the rational design of oxide overcoating layers for advanced catalysis.