Role of Keto Intermediates in the Hydrodeoxygenation of Phenol over Pd on Oxophilic Supports

The performance of Pd catalysts supported on SiO2, Al2O3 and ZrO2 for the hydrodeoxygenation (HDO) of phenol has been compared in the gas phase, at 300 °C and 1 atm using a fixed bed reactor. While Pd supported on SiO2 and Al2O3 exhibits high selectivity to cyclohexanone, when supported on an oxophilic support such as ZrO2, it favors the selectivity toward benzene, reducing the formation of ring-hydrogenated products, cyclohexanone and cyclohexanol. Diffuse reflectance infrared Fourier transform spectroscopy experiments support the participation of a keto-tautomer intermediate (2,4-cyclohexadienone) in the reaction. This intermediate can be hydrogenated in two different pathways. If the ring is hydrogenated, cyclohexanone and cyclohexanol are dominant products, as in the case of Pd/SiO2 and Pd/Al2O3 catalysts. By contrast, if the carbonyl group of the keto-intermediate tautomer is hydrogenated, benzene is directly formed via rapid dehydration of the unstable cyclohexadienol intermediate. This is observed in the case of Pd/ZrO2 catalyst. These results demonstrate that the selectivity for HDO of phenol can be controlled by using supports of varying oxophilicity.