Influence of Compressed Carbon Dioxide on Hydrogenation Reactions in Cyclohexane with a Pd/C Catalyst

Abstract

The potential of CO2-expanded liquid media for chemical reactions has been examined in this work, using cyclohexane as a solvent and Pd/C as a heterogeneous catalyst for hydrogenation of styrene, citral, and nitrobenzene with H2. The rate of hydrogenation reactions is increased, and the product selectivity is altered in the CO2-expanded cyclohexane phase. In the hydrogenation of citral, the selectivity to citronellal decreases with CO2 pressure, which changes from ∼80% in the neat cyclohexane to ∼65% at 16 MPa. The CO2 dissolved in the cyclohexane phase may accelerate the rate of further hydrogenation of citronellal. In the hydrogenation of nitrobenzene, the selectivity to aniline is large >90% in the absence of dense phase CO2 but it increases up to >95% on CO2 pressurization of the liquid reaction phase.