Fixed-bed hydrogenation of organic compounds in supercritical carbon dioxide

The Pd/C hydrogenation of cyclohexene to cyclohexane was performed in a continuous fixed-bed reactor employing CO, to solubilize the reaction mixture in a single supercritical (sc) phase surrounding the solid catalyst. Employing an equimolar feed of reactants (cyclohexene and hydrogen) in 90% CO2 and an olefin space velocity of 20 h(-1), excellent temperature control and stable catalyst activity were demonstrated at 343 K and 13.6 MPa, which represent dense supercritical operating conditions. Total cyclohexane selectivity was observed with its productivity being approximately 16 kg/kg cat/h. Gradual catalyst deactivation (2% loss in cyclohexene conversion per hour) occurred with the as-received cyclohexene feed that typically contained 180 ppm organic peroxides. When these peroxides were mitigated to 6 ppm or less by pretreating the cyclohexene feed through an alumina trap, the catalyst activity was stable with no measurable losses in either surface area or pore volume. Based on prior studies in our laboratory, it has been well established that organic peroxides can catalyze the formation of olefinic oligomers that can adsorb strongly on the catalyst surface and cause deactivation by fouling. No CO was detected in the reactor effluent and no H-2 was observed during the post-run depressurization step, indicating that neither the reverse water-gas shift activity between CO2 and H-2 nor the formation of Pd formate complexes is significant enough at our operating conditions to deactivate the Pd sites. This conclusion is also supported by the fact that no measurable loss of hydrogenation activity was observed after prolonged ( similar to 35 h) catalyst exposure to CO2 + H-2 at reaction conditions. These insights on how to operate an exothermic reaction in scCO(2) with tight temperature control and stable catalyst activity pave the way for systematic fundamental investigations of fixed-bed hydrogenations of functional groups on supported catalysts. Clearly, such investigations are essential for rational design and scaleup of scCO(2)-based hydrogenations. (C) 2001 Elsevier Science Ltd. All rights reserved.