The palladium catalysed hydrogenation of multi-functional aromatic nitriles

A series of model compounds and a commercial Pd/C catalyst were used to study the issues relevant to the hydrogenation of aromatic nitrile molecules that are associated with an industrial agrichemicals process, where a primary amine is the target product. Benzonitrile hydrogenation was found to be converted to high value benzylamine before an unexpected hydrogenolysis reaction led to the loss of ammonia to ultimately yield toluene as the final product. Indeed, gas phase infrared studies unambiguously showed the formation of ammonia for the first time. On closer investigation, the reaction was found to be a consecutive process where the order of reaction changed from first order for hydrogenation to zero order for hydrogenolysis. Co-adsorption studies proved that the two reactions occurred independently on two distinct Pd sites. The choice of catalyst and the use of an acid additive were shown to improve selectivity to benzylamine. A dramatic change was noted when the aliphatic chain was extended. For benzyl cyanide hydrogenation, conversion was observed but, by way of a “spillover” process, the amine product was retained by the catalyst. Extending the chain further resulted in a complete loss in reactivity showing that electronic and structural factors had a major effect on activity and product distribution. Mandelonitrile hydrogenation required an acid additive to facilitate conversion since a series of co-adsorption studies showed that under neutral conditions an intermediate hydroxyamine acted as a poison. Recycling of the catalyst showed that a cumulative poisoning effect was evident, but manipulation of Pd particle shape and size resulted in an extended lifetime and superior selectivity. Introducing additional functionality to the aromatic ring meant that stabilised imine species were observed in the liquid phase. The nature of the substituent also affected product distribution and catalyst lifetime. MeO-, Me- and Cl-substituents all showed signs of reduced catalyst performance, but an OH-substituent exhibited greater durability, albeit with reduced selectivity to the primary amine. These systems also indicated the presence of a high energy site on the catalyst, which was responsible for the formation of secondary and tertiary amines