Abstract
A series of fluorinated NiMo/Al 2O 3 catalysts containing different amounts of fluorine were prepared and their activity with respect to the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was compared with that of fluorinated CoMo/Al 2O 3 catalysts. Fluorine modifies two properties of NiMo/Al 2O 3 catalysts: metal dispersion and catalyst acidity. In the HDS of DBT, the catalytic activity is enhanced up to 0.5 wt.% added fluorine due to increased dispersion of the metal, and lowered by further fluorine addition because the catalysts lose a significant amount of initial surface area. The hydrogenation of the DBT aromatic ring is enhanced to a greater extent than the hydrogenolysis of the CS bond due to the fluorine addition. In the HDS of 4,6-DMDBT, however, the catalytic activity is enhanced in proportion to the fluorine content up to 5.0 wt.% added fluorine. The hydrogenolysis of the CS bond is enhanced to a greater extent than the hydrogenation of aromatic rings, in contrast to the trend observed in the HDS of DBT. A facilitated migration of methyl groups in the aromatic ring of 4,6-DMDBT due to an increase in the amounts of acidic sites of the catalysts is responsible for the enhanced hydrogenolysis of the CS bond. The optimum fluorine content to yield the maximum amounts of either the direct desulfurization (DDS) or ring-hydrogenated (HYD) products is different depending on the reactants, DBT or 4,6-DMDBT, as well as the catalysts, NiMo/Al 2O 3 or CoMo/Al 2O 3. The characteristic reaction results obtained in this study can be explained by considering the relative contributions of the rates of ring hydrogenation and methyl-group migration to product distribution for different cases of reactants and fluorinated catalysts.
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