Kinetics of two pathways for 4,6-dimethyldibenzothiophene hydrodesulfurization over NiMo, CoMo sulfide, and nickel phosphide catalysts

The kinetics for the initial stage of the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) and dibenzothiophene (DBT) were comparatively examined over NiMo and CoMo sulfide catalysts and newly developed nickel phosphide catalysts. The HDS can proceed through an indirect hydrogenation (HYD) pathway and a direct desulfurization (DDS, or hydrogenolysis) pathway. The rate constants for the HYD and DDS pathways (k₁ and k₂, respectively) were estimated using a method that involved extrapolation to zero conversion for the initial selectivity ratio of primary products. The overall rate constants (k₁ + k₂) for 4,6-DMDBT on a catalyst weight basis (in units of 10^-5 s^-1 g.cat^-1) at 573 K under a pressure of 20.4 atm increased in the order of CoMo sulfide (34.1) < Ni₂P/USY (51.5) < Ni₂P/SiO₂ (66.4) < NiMo sulfide (83.2); however, the values based on active sites (in units of s^-1 active site ^-1) ranked in a different order (CoMo sulfide (4.0) < NiMo sulfide (8.8) < Ni₂P/USY (15.2) < Ni₂P/SiO₂ (23.7)). The rate constants for the HYD pathway of 4,6-DMDBT HDS at 573 K based on active sites were strongly dependent on the type of catalyst used; however, those for the DDS pathway were less sensitive to the type of catalyst, as can be observed from the corresponding values (k1; k2) for CoMo sulfide (2.2; 1.8), NiMo sulfide (6.7; 2.1), Ni₂P/USY (12.7; 2.5), and Ni₂P/SiO₂ (21.6; 2.1). The NiMo sulfide catalyst, which favored the HYD pathway, was more active than the CoMo sulfide catalyst for 4,6-DMDBT. The nickel phosphide catalysts showed higher activity in 4,6-DMDBT HDS than the sulfide catalysts, based on rate constants normalized to active sites; they operated through the HYD pathway. A comparison with DBT HDS at 573 K under a hydrogen (H₂) pressure of 20.4 atm showed that the presence of the methyl groups at the 4- and 6-positions dramatically inhibited the DDS pathway, because of steric hindrance around the S atom, and thus made the HYD pathway more important. Higher H₂ pressure further enhanced the HYD pathway, whereas increased temperature increased the contribution of the DDS pathway for 4,6-DMDBT HDS.