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

Abstract

The performance of Pd catalysts supported on SiO 2, Al 2 O 3 and ZrO 2 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 SiO 2 and Al 2 O 3 exhibits high selectivity to cyclohexanone, when supported on an oxophilic support such as ZrO 2, 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/SiO 2 and Pd/Al 2 O 3 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/ZrO 2 catalyst. These results demonstrate that the selectivity for HDO of phenol can be controlled by using supports of varying oxophilicity.