Effect of H2S partial pressure on the HDS of dibenzothiophene and 4,6-dimethyldibenzothiophene over sulfided NiMoP/Al2O3 and CoMoP/Al2O3 catalysts

The effect of hydrogen sulfide (H2S) partial pressure on the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (46DMDBT) was studied over commercial NiMoP/Al2O3 and CoMoP/Al2O3 catalysts under conditions close to those commonly used in the hydrotreating of diesel fuels (280 or 340 degrees C) total pressure between 2.5 and 5.5 MPa). The sulfur compounds diluted in n-heptane were in proportions representative of those found in diesel fuels (0.0014 MPa of DBT or 46DMDBT, 0.0058-0.1 MPa of H2S generated from dimethyldisulfide). A significant negative effect of H2S on the HDS of the dibenzothiophenes (DBT and 46DMDBT) was observed whatever the total pressure (2.5-5.5 MPa) and the catalyst used. However, the reactivity of DBT was more reduced than that of 46DMDBT. Moreover, the "direct desulfurization pathway" (DDS) of both reactants was more inhibited by H2S than the so-called "hydrogenation pathway" (HYD) over the two catalysts. It was also found that the NiMoP/Al2O3 catalyst was more sensitive to H2S than its CoMoP/Al2O3 counterpart whatever the experimental conditions. Several hypotheses were considered and discussed to explain these findings: specific centres for each of the two reactions (DDS and HYD) or centres being similar in nature (sulfur vacancies associated to sulfur anions) but which would differ in their acid-base properties. It was also suggested that the centres could be identical but that DDS and HYD could have different rate-limiting steps which would not involve the same part of the dual site so that their sensitivity to H2S would be different. (c) 2006 Elsevier B.V. All rights reserved.