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Hydrodesulfurization (HDS) reactivity of a refractory sulfur species, 4,6-dimethyldibenzothiophene (4,6-DMDBT), was examined over a Y-zeolite containing CoMo/Al 2 O 3 (CoMo/Al 2 O 3zeolite), conventional CoMo/Al 2 O 3 , and NiMo/Al 2 O 3 . Isomerization and considerable transalkylation of 4,6-DMDBT into 3,6-DMDBT and into tri-or tetramethyldibenzothiophenes, respectively, were observed characteristically over CoMo/Al 2 O 3 -zeolite catalyst. Migration of methyl groups enhances the HDS reactivity of the refractory sulfur species by diminishing the steric hindrance. Among the catalysts examined, CoMo/Al 2 O 3 -zeolite exhibited the best activity for HDS of the gas oil through the effective desulfurization of refractory alkyldibenzothiophenes.

A gas oil (bp 227-377 °C) was fractionated by the distillation at intervals of 20 °C into five fractions. Each fraction was desulfurized over the NiMo and CoMo catalysts, respectively, in an autoclave reactor at 360 °C under a total pressure of 2.9 MPa to examine its hydrodesulfurization (HDS) reactivities. The fractions exhibited the different reactivities which were expressed by the pseudo-second-order kinetics, giving the decreasing rate constants of 0.636, 0.270, 0.118, 0.039, and 0.023 [min-wt %]_1 with an increase of their boiling point ranges. Composition analyses of the fractions indicated that alkyldibenzothiophenes, polyaromatic hydrocarbons, and nitrogen compounds in the heavier fractions restricted their reactivities. Steric hindrance of methyl groups as well as the competitive retardation of latter compounds are suggested. The NiMo catalyst showed higher activity for HDS of the heavier fractions than the CoMo through its higher hydrogenation activity, while the latter was slightly superior to the former in HDS of the lighter fractions. The polyaromatic species existing dominantly in the heaviest fraction (>340 °C) are responsible for the fluorescent color in the oils. On the basis of such understanding, a deep HDS process which combined a single-stage HDS for lighter fractions and a multistage HDS for the heavier fractions was proposed and examined. Such a process achieved the sulfur level below 0.05 wt % with the color removal and its superiority over the conventional HDS process was discussed.