Hydrogenation of aniline to cyclohexylamine in supercritical carbon dioxide: Significance of phase behaviour

Hydrogenation of aniline to cyclohexylamine was carried out in supercritical carbon dioxide using a variety of noble metal (Pt, Pd and Rh) catalysts. At 80 degrees C and 8 MPa of CO2 pressure, >95% of aniline conversion with 93% selectivity to cyclohexylamine was achieved on 5% Rh/Al2O3. A strong influence of phase behaviour related to the CO2 pressure was found on the conversion and selectivity. Optimization of reaction parameters resulted in a higher overall activity in the biphase (liquid substrate + gaseous H-2 and CO2) than in the single phase (liquid substrate-CO2-H-2) condition. It has been found that the interaction of CO2 with amine leads to the formation of solid carbamic acid, which enhanced the selectivity of cyclohexylamine, but reduced the conversion significantly. Furthermore, reaction temperature played a crucial role in preventing the formation of carbamic acid and also maintained a reasonably high reaction performance in terms of conversion and selectivity. (C) 2011 Elsevier B.V. All rights reserved.