Hydrodesulfurization of Dibenzothiophene over Siliceous MCM-41-Supportd Catalysts

摘要

A deep hydrodesulfurization (HDS) catalyst, which was consid- erably active to desulfurize dibenzothiophene (DHT), was prepared by depositing Co-Mo species over siliceous MCM -41. The extremely high surface area of MCM-41 favors the dispersion of the active species, resulting in very high HDS activity. The optimal Co/Mo atomic ratio for this series of catalysts is 0.75, higher than the conventional y-AlzO3-supported catalysts. This is attributed to the higher dispersion of the active species and more Co-Mo pairs gen- erated on the surface ofMCM-41. In the liquid product obtained at temperatures over 280°C, 70-80% HP was generated and its selec- tivity changed a little with increasing HDS reaction temperature. The selectivity of cyclohexylbenzene (CHH) decreased whereas the selectivity of benzene and cyclohexane increased when increasing the reaction temperature. It is shown that HDS ofDHT over MCM- 41-supported Co-Mo sulfides includes mainly three reaction path- ways: hydrogenolysis, prehydrogenation followed by desulfuriza- tion, and hydrocracking of CHH. The hydrogenolysis of DHT, i.e., direct extraction of sulfur atom from DHT molecule, predominately progresses in the HDS reactions, while benzene mainly results from hydrocracking of CHB. 35S isotope tracer investigation revealed that sulfur atoms retained on the surface could be released only by the introduction of a sulfur-containing compound, indicating that sulfur atom exchange between sulfur-containing compounds and the active sites is involved in the HDS reaction. A reaction mecha- nism for HDS is proposed, which is in accordance with the isotope tracer experiment results, the well-established rim-edge model, and the DFf calculations.