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<0.05wt%) using three commercially available Co-Mo/Al2O3 catalysts and one commercially available Ni-Mo/Al2O3 catalyst. For every Co-Mo/Al2O3 catalyst, the conversions of DBTs to cyclohexylbenzenes (CHBs) were nearly the same, while that of DBTs to biphenyls (BPs) decreased in the order DBT>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±2kcal/mol, respectively, while heats of adsorption for each item were 11±1, 19±1 and 21±1kcal/mol, respectively, for all Co-Mo/Al2O3 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/Al2O3 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/Al2O3 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/Al2O3 catalyst.
