Hydrogenation of nitrile in supercritical carbon dioxide: a tunable approach to amine selectivity

Abstract

The use of supercritical carbon dioxide (scCO₂) on the hydrogenation of benzonitrile was investigated over Pd and other metal catalysts. Without any additive, benzonitrile was hydrogenated to benzylamine with high conversion (90.2%) and selectivity (90.9%) using the Pd/MCM-41 catalyst. A strong influence of CO₂ pressure on the conversion and selectivity were observed. As the CO₂ pressure increases, the conversion was increased, and after reaching the maximum at around 8–10 MPa, it decreased. Moreover, simply by tuning the CO₂ pressure, it is possible to obtain benzylamine or dibenzylamine. For instance, at lower pressure CO₂ acts as a protecting agent, leading to the formation of the primary amine, but at higher pressure, the yield of primary amine as well as the solubility of the imine intermediate in CO₂ increases, which results high selectivity for dibenzylamine. A plausible mechanism has been proposed to show the role of CO₂ on the selectivity toward primary and secondary amines. The results confirm that the presence of CO₂ is mandatory for the formation of benzylamine with high selectivity. Furthermore, the other reaction parameters, such as reaction time, H₂ pressure, temperature etc., also affect the conversion as well as selectivity of benzylamine. This process has been extended to the hydrogenation of a series of different nitrile compounds.