Selective hydrogenation of benzyl cyanide to 2-phenylethylamine over a Pd/Al2O3 catalyst promoted by synergistic effects of CO2 and water

The selective hydrogenation of benzyl cyanide (BC) to the primary amine of 2-phenylethylamine (PEA) was investigated with a Pd/Al2O3 catalyst in different multiphase reaction media including n-hexane, water, and/or CO2. In neat n-hexane, the hydrogenation of BC occurs at a large rate but no PEA was formed and the secondary amine of N,N-bis(2-phenylethyl)amine (BPEA) was produced as the main product. The pressurization of n-hexane with CO2 decreased the rate of hydrogenation and the main product was BPEA. In this n-hexane–CO2 medium, the catalyst lost its activity during the reaction. When a similar volume of water was added to n-hexane, the rate of hydrogenation did not change so much but a small amount of the primary amine was detected to form in addition to the main product of BPEA. In contrast to these reaction media, a high PEA selectivity of >90% was obtained and no catalyst deactivation occurred in an n-hexane–water–CO2 medium. The rate of hydrogenation was smaller than that in neat n-hexane but comparable to those in n-hexane–water and n-hexane–CO2 media. Those results show that synergistic effects appear on the selective hydrogenation of BC to PEA when both water and CO2 are included in the reaction medium. Possible roles and actions of water and CO2 have been discussed on the basis of the results of hydrogenation runs under different conditions and in situ ambient and high pressure Fourier transform infrared spectroscopy measurements that give insights into molecular interactions of CO2 with reacting species.