Hydrodesulfurization of DBT, 4-MDBT, and 4,6-DMDBT on fluorinated CoMoS/Al2O3 catalysts

CoMoS/Al2O3 catalysts containing different amounts of fluorine have been te sted for the hydrodesulfurization (HDS) of dibenzothiophene (DBT), 4-methyl dibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT), an d the results have been analyzed based on three fundamental reactions invol ved in the HDS mechanism: hydrogenation of the aromatic ring, hydrogenolysi s of the C-S bond, and migration of methyl groups in the ring structure. Fl uorine addition to the catalyst promotes all of these three reactions due t o the enhancement of two factors: the metal dispersion and the catalyst aci dity. The extents that the HDS rates are improved by fluorine addition incr ease in the order of DBT<4-MDBT<4,6-DMDBT. Product distributions change in characteristic trends with fluorine addition depending on the individual re actants. That is, in DBT HDS, CHB obtained by the ring saturation is enhanc ed more than BP produced by the direct desulfurization, while the opposite trend is observed in 4-MDBT HDS. 4,6-DMDBT HDS shows an intermediate trend: products of both types are promoted to similar extents on fluorinated cata lysts. The migration of methyl groups in the reactant ring structure due to the catalyst acidity, which reduces the steric hindrance to the C-S bond, is responsible for the characteristic trends in the product distribution ob served with the individual reactants. (C) 2000 Elsevier Science B.V. All ri ghts reserved.