DEEP DESULFURIZATION OF LIGHT OIL .2. HYDRODESULFURIZATION OF DIBENZOTHIOPHENE, 4-METHYLDIBENZOTHIOPHENE AND 4,6-DIMETHYLDIBENZOTHIOPHENE

The hydrodesulfurization of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was investigated under deep desulfurization conditions (sulfur concentration < 0.05 wt.-%) using Co Mo/Al2O3. The conversion of DBTs into cyclohexylbenzenes (CHBs) was nearly the same while that of DBTs into biphenyls (BPs) decreased in order DBT > 4 MDBT > 4,6-DMDBT. This result indicates that, when an aromatic ring in DBTs is hydrogenated, the extent of steric hinderance of the methyl group decreases and differences between the HDS rates of DBTs disappear. Data for DBTs could be arranged by the Langmuir-Hinshelwood rate equation. Activation energies of DBT, 4-MDBT and 4,6-DMDBT were 24, 31 and 40 kcal/mol, respectively. Heats of adsorption for DBT, 4-MDBT and 4,6-DMDBT were 12,20 and 21 kcal/mol, respectively. These results show that 4-MDBT or 4,6-DMDBT can be adsorbed on the catalyst through a pi-electron in the aromatic rings more strongly than DBT, and that the C-S bond cleavage of adsorbed DBTs is disturbed by steric hinderance of the methyl group.