Abstract
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 °C and 8 MPa of CO 2 pressure, >95% of aniline conversion with 93% selectivity to cyclohexylamine was achieved on 5% Rh/Al 2O 3. A strong influence of phase behaviour related to the CO 2 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 CO 2) than in the single phase (liquid substrate–CO 2–H 2) condition. It has been found that the interaction of CO 2 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.
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