Hydrogenation of Phenol in Supercritical Carbon Dioxide Catalyzed by Palladium Supported on Al‐MCM‐41: A Facile Route for One‐Pot Cyclohexanone Formation

Abstract

AbstractThe hydrogenation of phenol has been carried out in supercritical carbon dioxide (scCO2) under very mild reaction conditions at the temperature of 50 °C over palladium supported Al‐MCM‐41 (metal loading ∼1%). This palladium catalyst is shown to be highly active and promotes the selective formation of cyclohexanone (∼98%), an industrially important compound, in a “one‐pot” way. The effects of different variables like carbon dioxide and hydrogen pressure, reaction time and also silica/alumina ratio of the MCM‐41 support along with palladium dispersion are presented and discussed. The pressure effect of carbon dioxide is significantly prominent in terms of conversion and cyclohexanone selectivity. Moreover, the silica/alumina ratio was also found to be an important parameter to enhance the effectiveness of the catalyst as it exhibits a remarkable increase in phenol conversion from 20.6% to 98.4% as the support changes from only silica MCM‐41 to Al‐MCM‐41. A plausible mechanism for the hydrogenation of phenol to cyclohexanone over the palladium catalyst has been proposed. The proposition is validated by transition state calculations using density functional theory (DFT), which reveal that cyclohexanone is a favorable product and stabilized by <19 kcal mol−1 over cyclohexanol in scCO2 medium. Under similar reaction conditions, phenol hydrogenation was also carried out with rhodium, supported on Al‐MCM‐41. In contrast to the palladium catalyst, a mixture of cyclohexanone (57.8%) and cyclohexanol (42.2%) was formed. Detailed characterization by X‐ray diffraction and transmission electron microscopy confirmed the presence of metal nanoparticles (palladium and rhodium) between 10–20 nm. Both the catalysts exhibit strikingly different product distributions in solventless conditions compared to scCO2. This method can also be successfully applied to the other hydroxylated aromatic compounds.