Influences of sodium and potassium cations on the hydrodesulfurization performances of Pd and Pt catalysts supported on siliceous MCM-41

Abstract

Na+ and K+ were introduced to siliceous MCM-41 (Si-MCM-41) by ion-exchange of Si-MCM-41 with sodium oxalate or potassium oxalate, respectively. The supported Pd and Pt catalysts were prepared by an incipient wetness impregnation method. Their hydrodesulfurization (HDS) performances were evaluated using dibenzothiophene as the model molecule. The introduction of Na+ or K+ led to a decrease in the metal dispersions but an increase in the electron densities of metal sites. It also strongly inhibited the adsorption, dissociation, and probably the spillover of H2 on the catalysts. Independent of the type of metal, a push-pull effect of Na+ and K+ on the HDS performances of the Pd and Pt catalysts was observed. They on the one hand enhanced the direct desulfurization pathway (DDS), but on the other hand hindered the hydrogenation route. The DDS activity of Pt increased almost linearly with its electron density. These are discussed by considering both the negative effects of Na+ and K+ on the formation of active hydrogen species and their positive role on the electron densities of the metal sites. The Pt catalysts deactivated faster than the Pd catalysts. The kinetic study indicates that Na+ and K+ did not affect the deactivation of the catalysts.