Hydrodesulphurization mechanism of thiophene and tetrahydrothiophene on a cobalt molybdenum catalyst

The hydrodesulphurization mechanism of thiophene and tetrahydrothiophene has been developed at high pressures and over a broad range of temperature and contact time on a commercial CoMo—Al2O3 catalyst. The influence of the pretreatment on the catalyst activity and stability was also studied. The pretreatment with a mixture of H2 and H2S was found to be the most convenient. It was found that the sulphur uptake of the fresh catalyst increases with temperature and that an excess of sulphur in the catalyst leads to an initial higher activity.The thiophene reaction seems to occur simultaneously by two pathways: one consists of ring opening, and the second is yielding tetrahydrothiophene. The latter is the slowest step. The tetrahydrothiophene reacts faster than the thiophene and its reaction mechanism involves mainly the rupture of the C—S bond. However, thiophene was detected in small concentrations, showing the contribution of a second route for the tetrahydrothiophene hydrodesulphurization. Experiments carried out with benzene seem to indicate the existence of three different kinds of active sites in the catalyst: desulphurization, aromatics hydrogenation, and olefin saturation sites.On a élaboré le mécanisme d'hydrodésulfurisation du thiophène et du tétrahydrothiophène à haute pression et sur une large échelle de température et de temps de contact avec une catalyseur commercial mixte: CoMo—Al2O3. On a également étudié la stabilité ainsi que l'influence d'un traitement préalable sur l'activité du catalyseur. On a trouvé que le traitement préalable, par un mélange de H2 et de H2S, est celui qui convient le mieux. On a constaté que la fixation de soufre sur le catalyseur frais augmente avec la température et que l'excès de soufre sur le catalyseur conduit à une activité initiale plus forte.