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.

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The selective hydrodesulfurization (HDS) reaction of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in decane containing a dominant amount of naphthalene was examined over blends of CoMo/Al2O3 and Ru/Al2O3, and their activity was compared to those of CoMo, NiMo, and Ru/Al2O3 in their single use. The blend exhibited an excellent activity for HDS of 4,6-DMDBT through its selective hydrogenation without excess hydrogenation of the aromatic partner. NiMo/Al2O3 and CoMo/Al2O3 examined in the present study were inferior to the blend for the HDS of 4,6-DMDBT, because the hydrogenation activity of the first catalyst is toward the aromatic partner while the second catalyst is inferior in the hydrogenation activity for 4,6-DMDBT.

The catalytic activities of sulfided Ru−CoMo/Al2O3 were examined for the desulfurization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in decane and decane with naphthalene to find selective catalysts which desulfurize 4,6-DMDBT in the presence of naphthalene through preferential hydrogenation of its phenyl ring. Addition of Ru to CoMo catalyst exhibited an excellent activity for HDS of 4,6-DMDBT and suffered less inhibition by coexisting naphthalene, whereas significant retardation was observed over NiMo/Al2O3 and Ru−NiMo/Al2O3. The ternary combination required much less amount of Ru to maximize the selective HDS reaction, compared to the blend of Ru/Al2O3 and CoMo/Al2O3. Several structural characteristics of the ternary catalyst were revealed according to XPS and HREM:  (1) Mo and Ru existed separately on the same alumina support in forms of respective sulfides, (2) crystal size of MoS2 increased when Ru was present on the same support, and (3) MoS2 was easily reduced in the presence of Ru. RuS2 is suggested as the site for the selective hydrogenation of 4,6-DMDBT, the hydrogenated product of which is transferred immediately to the Co−Mo−S active site to be desulfurized.