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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.

This work investigates the effect of the type of support (SiO2, Al2O3, TiO2, ZrO2, CeO2, and CeZrO2) on the performance of Pd-based catalysts for the hydrodeoxygenation of phenol at 573 K using a fixed-bed reactor. Product distribution is significantly affected by the type of support. Benzene was the major product over Pd/TiO2 and Pd/ZrO2; on the other hand, cyclohexanone was the main compound over Pd/SiO2, Pd/Al2O3, Pd/CeO2, and Pd/CeZrO2. A reaction mechanism based on the tautomerization of phenol was proposed on the basis of DRIFTS experiments and catalytic tests with the intermediate products. The high selectivity to benzene over Pd/TiO2 and Pd/ZrO2 catalysts is likely due to the oxophilic sites of this support represented by incompletely coordinated Ti4+ and Zr4+ cations in close proximity to the periphery of metal particles. The greater interaction between oxygen in the keto-tautomer intermediate with oxophilic sites promotes the selective hydrogenation of CO bond. Pd/SiO2, Pd/Al2O3, Pd/TiO2, and Pd/ZrO2 catalysts significantly deactivated during TOS. However, Pd/CeO2 and Pd/CeZrO2 were more stable, and only slight losses in activity were observed. Carbon deposits were not detected by Raman spectroscopy after reaction. DRIFTS experiments under reaction conditions revealed a buildup of phenoxy and intermediate species during reaction. These species remained adsorbed on the Lewis acid sites, blocking those sites and inhibiting further reactant adsorption. The growth of Pd particle size and the reduction in acid site density during HDO of phenol were the primary routes of catalyst deactivation. The higher stability of Pd/CeO2 and Pd/CeZrO2 catalysts is likely due to the higher amount of oxygen vacancies of these supports.

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