Influences Of The Modification Of Support On Catalytic Performances In Hydrodesulfurization Over Palladium And Platinum Catalysts

Catalyst support has key effect to the catalytic performances of noble metal catalysts.In the hydrodesulfurization(HDS)reaction,the properties of the support can influence the electronic state,the dispersion,the metal particles' distribution as well as the adsorption and dissociation of the hydrogen species.In the present work,a series of Pd and Pt supported catalysts were prepared with MCM-41 and activated carbon as supports.Several methods(such as introduction of alkaline cations,acid and alkaline treatment and silylation)were used to modify the supports.The hydrodesulfurization(HDS)performances of the noble metal catalysts were evaluated using dibenzothiophene as the model molecule.Na+ and K+ were introduced to MCM-41 by ion-exchange of MCM-41 with sodium oxalate or potassium oxalate,respectively.An opposite effect of Na+ and K+ on the HDS performances of the MCM-41 supported Pd and Pt catalysts was observed,which on the one hand enhanced the direct desulfurization pathway(DDS)activity,but on the other hand inhibited the hydrogenation pathway(HYD)activity,The DDS pathway selectivity of the Pd and Pt catalysts was enhanced due to the introduction of Na+ or K+,with the selectivity of 80%and 95%respectively.This may due to 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.Both the HNO3 treatment of activated carbon(A-C)and the alkaline treatment(NaOH and KOH)of H-C has enhanced the dispersion of the Pd particles on the catalyst supports as well as the HDS activity.The HDS activity of the catalysts decreased in the order:Pd/K-C>Pd/Na-C>Pd/H-C>Pd/A-C.The HNO3 treatment of A-C favors the HYD pathway more,whereas the alkaline treatment of H-C as well as the introduction of Na+ and K+ enhanced the DDS pathway activity but inhibited the HYD pathway activity,the same pattern was observed on the MCM-41 supported noble metal catalysts.The DDS pathway selectivity of Pd/Na-C and Pd/K-C both reached 95%.Two supported Pd catalysts were prepared by impregnation of MCM-41 with a solution of PdCl2 in diluted HCl and a pre-silylated MCM-41(Si-MCM-41)with a toluene solution of Pd(OAc)2,these two catalysts showed distinctive difference in metal particle distributions in nanoscale range,the catalytic performances also behaved differently.The Pd particles were almost all confined in the mesopores of MCM-41,whereas spherical Pd nanoparticles were exclusively stay on the surface of Si-MCM-41.The electron density of Pd clusters in Pd/Si-MCM-41 was lower than that in Pd/MCM-41.In the HDS reaction,Pd/Si-MCM-41 possessed higher HYD pathway selectivity(83%).Compared with Pd/MCM-41,Pd/Si-MCM-41 exhibited much better selective hydrogenation performance but a lower hydrogenolysis activity.The differences in catalytic performances between the two catalysts can be attributed to their different metal particle distribution profiles and average electron density differences of Pd.The catalytic performance of noble metal catalysts is tunable by simply controlling the metal particle distribution in the pores or the surface of the support.