Effect of surface proton exchange on hydrodesulfurization performance of MCM-41-supported catalysts

Ni-Mo. Co-Mo. Ni-W and Pt were supported over siliceous MCM-41 (Si-MCM-41) or the proton-exchanged Si-MCM-41 (H+-MCM-41) in order to investigate the effect of support surface modification on the performance of the catalysts. The prepared catalysts were evaluated by the hydrodesulfurization (HDS) of dibenzothiophene (DBT). It is indicated that the supported Ni-Mo sulfides exhibited the highest HDS activity regardless of the support, and that Ni-W sulfides showed the highest hydrogenation activity. The H+-MCM-41-supported Ni-Mo, Ni-W or Pt catalyst performed much better in HDS of DBT than the Si-MCM-41-supported counterpart. From the relative selectivity of cyclohexylbenzene (CHB) to biphenyl (BP) (S-CHB/S-BP) it could be concluded that the higher HDS performance of H+-MCM-41-supported catalysts may be attributed to the enhanced hydrogenation activity. Nevertheless, a little difference in HDS activity was observed for Co-Mo sulfides supported on Si-MCM-41 or H+-MCM-41. Since the cleavage of sulfur atoms mainly takes the route of hydrogenolysis in HDS of DBT catalyzed by Co-Mo sulfides, the improvement in hydrogenation activity was not significant for Co-Mo sulfides when Si-MCM-41 was ion-exchanged with HNO3. TPR study showed that the profiles of Ni-Mo/Si-MCM-41 and Ni-W/Si-MCM-41 changed markedly after the proton exchange of Si-MCM-41 while the profile of Co-Mo/Si-MCM-41 changed a little. It may be concluded that the removal of sulfur from DBT and hydrogenation take place on separate active sites. and that the spillover of the dissociated hydrogen species over the surface is essential to the hydrogenation pathway. Hydroxyl groups on H+-MCM-41 favors the spillover of the hydrogen species, enhancing the hydrogenation activity of its supported catalysts. On the other hand, Na+ cations on Si-MCM-41 may "trap" the spillover hydrogen species, lowering the hydrogenation activity of its supported catalysts. (C) 2003 Elsevier B.V. All rights reserved.