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Hydrodesulfurization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in decane has been investigated over a Co−Mo/C catalyst, using a magnetically stirred autoclave reactor. The hydrodesulfurization reaction was found to be pseudo-first order with respect to 4,6- DMDBT. Direct desulfurization was detected to be Arrhenius-dependent with an activation energy of 35.7 kcal/mol, while hydrogenation pathway showed a maxima at 340 °C. The activation energy of hydrogenation in the Arrhenius-dependent region was calculated to be 17.9 kcal/mol, as hydrogenation is thermodynamically limited at high temperature. The reaction of hydrodesulfurization appears to proceed via two different partially dependent routes, hydrogenation and desulfurization, over the present catalyst. Hydrogenation of 3,3‘-dimethylbiphenyl at different temperatures was also measured to identify the rates of the step involved in the hydrodesulfurization network of 4,6-DMDBT. Attention was particularly paid to the effects of naphthalene and H2S on the two different reaction pathways to propose the mechanism of their inhibition. Two different catalytic active sites, one for hydrogenation and the other for both hydrogenation and direct desulfurization, are discussed to be present.

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Several series of Co−Mo-based carbon catalysts series were synthesized using three methods, successive impregnation, successive impregnation with sulfidation in between, and equilibrium adsorption methods. Co/C and Mo/C were also prepared by impregnation. The activities of sulfided catalysts for hydrodesulfurization of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) as models of the most refractory sulfur-containing compounds found in petroleum oil were studied using an autoclave reactor operating at 2.9 MPa hydrogen pressure. The activity results were found to be highly dependent on the method of preparation. The series that was prepared by impregnation but with the sulfidation step before the cobalt addition exhibited superior activities for hydrodesulfurization (HDS) of DBT and 4,6-DMDBT. The activity results were compared with that of the conventional commercial Co−Mo/Al2O3 catalyst. Co−Mo/C catalysts had about twice the activity of the commercial catalyst. It was found that the activity of HDS depends on the metal loading as well as the order of metal addition during preparation. The highest synergetic effect between Co and Mo for HDS activity was observed at a Co/Mo atomic ratio of 0.325.