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A deep hydrodesulfurization (HDS) catalyst, which was considerably active to desulfurize dibenzothiophene (DBT), 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 γ-Al2O3-supported catalysts. This is attributed to the higher dispersion of the active species and more Co–Mo pairs generated on the surface of MCM-41. In the liquid product obtained at temperatures over 280°C, 70–80% BP was generated and its selectivity changed a little with increasing HDS reaction temperature. The selectivity of cyclohexylbenzene (CHB) decreased whereas the selectivity of benzene and cyclohexane increased when increasing the reaction temperature. It is shown that HDS of DBT over MCM-41-supported Co–Mo sulfides includes mainly three reaction pathways: hydrogenolysis, prehydrogenation followed by desulfurization, and hydrocracking of CHB. The hydrogenolysis of DBT, i.e., direct extraction of sulfur atom from DBT 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 mechanism for HDS is proposed, which is in accordance with the isotope tracer experiment results, the well-established rim–edge model, and the DFT calculations. |
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