Kinetics of Adiponitrile Hydrogenation Over Rhodium-Alumina Catalysts

Abstract

The catalytic hydrogenation of adiponitrile is generally carried out industrially at high temperature and pressure over Ni-Raney catalysts, which have the disadvantages of possessing low mechanical resistance and of being pyrophoric. In this work adiponitrile was hydrogenated in a three phase stirred slurry reactor using rhodium over alumina powder as a catalyst instead. Catalysts were prepared with a ion exchange technique by contacting 100 micron alumina powder with a solution of hydrate rhodium cloride for 36 h. The reaction runs were conducted in a 100 cc vessel at a pressure of 3 MPa and temperatures of 72 – 92 °C varying the initial concentration of adiponitrile. A variety of series-parallel reactions took place when adiponitrile was contacted with hydrogen in our reactor. Attention was concentrated on hexamethylenediamine, the final product, and on aminocapronitrile, a very interesting intermediate which could be used subsequently for caprolactam production (and from this the direct synthesis of nylon). Samples were withdrawn from the reactor at fixed interval of time and analysed with a gas-chromatograph so that time dependency of reactant conversion and product selectivity could be determined. The results showed the expected qualitative behaviour: adiponitrile conversion increased with time, as did the hexamethylenediamine selectivity, whereas aminocapronitrile selectivity showed a clear maximum and then decreased to negligible values. The “Chemical reaction engineering” approach was used and proved sufficient to describe quantitatively the observed kinetic behaviour of the reactor.