Deep Desulfurization of Light Oil. (Part 4). Hydrodesulfurization of Methyl-substituted Dibenzothiophenes Catalyzed by Various Co-Mo/Al2O3 and Ni-Mo/Al2O3 Catalysts.

Abstract

Hydrodesulfurization (HDS) of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) were investigated in deep desulfurization conditions (sulfur concentration 4-MDBT>4,6-DMDBT. These results indicate that, when an aromatic ring in DBTs is hydrogenated, the extent of steric hindrance of a methyl group decreases and the difference among HDS rates of DBTs disappear. HDS rates of DBTs were arranged by Langmuir-Hinshelwood rate equation. Activation energies of DBT, 4-MDBT and 4,6-DMDBT were 22±2, 29±1 and 33±2 kcal/ mol, respectively, while heats of adsorption for each item were 11±1, 19±1 and 21±1 kcal/mol, respectively, for all Co-Mo/Al 2 O 3 catalysts. These results showed that 4-MDBT or 4,6-DMDBT could be adsorbed on the catalyst through π-electrons in aromatic rings more strongly than DBT, and that the C-S bond cleavage of adsorbed DBTs could be affected by steric hindrance of the methyl group. In the case of Ni-Mo/Al 2 O 3 catalyst, hydrogenation of an aromatic ring of methyl-substituted dibenzothiophenes proceeded, and the conversions of DBTs to CHBs reached a range of about 60%, at 340°C. Compared with performances of Co-Mo/Al 2 O 3 catalysts, all the conversions of DBT, 4-MDBT and 4,6-DMDBT were enhanced, and the routes of formation of CHBs from BPs in the HDS reactions became important in the mechanism of the Ni-Mo/Al 2 O 3 catalyst.