A multiphase reaction medium including pressurized carbon dioxide and water for selective hydrogenation of benzonitrile with a Pd/Al2O3 catalyst

• Water and CO2 are useful for selective hydrogenation of nitrile to primary amine.
• Water suppresses the reaction between primary amine and imine intermediate.
• Water prevents the catalyst from being deactivated by carbamate accumulation.
• CO2 molecules interact with cyano group of benzonitrile decreasing its reactivity.

The hydrogenation of benzonitrile was studied with a Pd/Al2O3 catalyst in the presence of pressurized CO2 and water. This multiphase reaction system was effective for the selective production of the desired primary amine of benzylamine with no deactivation of the catalyst. The enhanced selectivity to the primary amine resulted from its transfer into water phase, which prevented the primary amine from reacting with an intermediate of imine into the secondary amine in the organic phase (benzonitrile). In addition, interactions of CO2 molecules with the primary amine could also increase its selectivity. In the absence of water, the catalyst was deactivated by the accumulation of a carbamate salt on the surface of catalyst. But this deactivation was avoided by the addition of water because the salt was soluble in water and it did not deposit on the catalyst existing at interfacial layer between the water and organic phases.

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