Enhanced Selectivity of Phenol Hydrogenation in Low-Pressure CO2 over Supported Pd Catalysts

Selective hydrogenation of phenol to cyclohexanone is an important process in both chemical industry and renewable feedstock processing. However, direct hydrogenation of phenol to cyclohexanone under mild conditions over catalysts with high reactivity, selectivity, and facile preparation is still a challenge. In the present study, we report that 99% conversion and 99% selectivity can be achieved over as-prepared Pd/gamma-Al2O3 catalyst under the medium of low-pressure CO2 (0.05-0.2 MPa) and H2O at 373 K. According to experiment results, ab initio calculations and in situ high-pressure FTIR measurements indicated enhanced selectivity of cyclohexanone in low-pressure CO2; this result originated from the molecular interaction between cyclohexanone and CO2 and can prevent the further hydrogenation of cyclohexanone. Notably, enhancement of selectivity to cyclohexanone in low-pressure CO2 was also achieved using commercial Pd/gamma-Al2O3 and Pd/C catalysts.