The heterogeneous catalytic hydrogenation of nitriles is an important industrial process for the production of diverse amines.
The reaction scheme based on the von Braun and Greenfield proposals has been widely accepted for many years and it remains the most
frequently cited reaction sequence for the formation of secondary and tertiary amines via the hydrogenation of nitriles. Over the past decade,
there has been a stream of published papers that, using modern scientific techniques, have intensively investigated the detailed
mechanism underlying the surface reactions of heterogeneously catalyzed nitrile hydrogenation and the surface intermediates. On the one
hand, the results of these studies bring some light to this issue; on the other hand, entirely new, unanswered questions arise from new
knowledge. The studies suggest that the mechanism of nitrile hydrogenation on solid catalysts is much more complicated than expected
based on the reaction formalism of von Braun and Greenfield. Unfortunately, there are as yet no physicochemical studies available that
could confirm in a straightforward manner the intimate details of the reaction mechanism of surface reactions. In this review, the various
speculative mechanisms proposed for heterogeneous nitrile hydrogenation are discussed. It seems very likely from current knowledge
that aminocarbene complexes and aldimines, coordinated to a metal via the π-system of a C=N double bond and/or via the nitrogen lone
pair, will prevail among the surface intermediates suggested for nitrile hydrogenation on palladium or platinum, whereas hydrogenation
on a cobalt or nickel surface will likely be associated with the formation of nitrene intermediates. This duality provides a satisfactory explanation
for the selectivity differences between metal catalysts in nitrile hydrogenation to primary amines.
Keywords: Adsorption, Benzonitrile, Carbene, Enamine, Heterogeneous catalysis, Hydrogenation, Imine, N-methylalkylamines disproportionation,
Nitrene, Nitrile, Surface species, Transamination.
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