The role of defect sites and oxophilicity of the support on the phenol hydrodeoxygenation reaction

This work studies the effect of support defect sites on the performance of Pd/CexZr1-xO2 (x = 0.00; 0.25; 0.50; 0.75; 0.90) catalysts for the hydrodeoxygenation of phenol in the gas phase at 573 K. The activity and selectivity for hydrodeoxygenation of phenol depends significantly on the support used. Increasing the Zr content from x = 0.0 to 0.5, the reaction rate for hydrodeoxygenation and the selectivity to benzene remains very low. However, upon increasing the Zr content above x = 0.5 a sudden jump in reaction rate and selectivity to benzene is observed. Interestingly, this activity and selectivity boost has no direct correlation with the density of acid sites or the concentration of defects on the support. Rather, the selectivity to deoxygenated products is found to depend on the oxophilicity of the support. Increasing the Zr content enhances the strength of the interaction between the 0 of the carbonyl group and the oxophilic site. It is proposed that the oxophilicity of these catalysts is related to the structure of the CexZr1-xO2 solid solution formed. In addition, it is observed that the degree of deactivation during the reaction also depends on the Ce/Zr molar ratio of the support. Pd/ZrO2, Pd/Ce0.10Zr0.90O2 and Pd/Ce0.25Zr0.75O2 catalysts readily deactivate during reaction, whereas the phenol conversion only slightly decreases for Pd/CeO2, Pd/Ce0.75Zr0.25O2 and Pd/Ce0.50Zr0.50O2 catalysts. The results reveal that the density of Zr species on the surface is responsible for catalyst deactivation. The stronger adsorption between the oxygen from the phenol with Zr cations resulted in an accumulation of O-containing byproducts and catalyst deactivation.