Activation of alumina-supported hydrotreating catalysts by organosulfides or H2S: Effect of the H2S partial pressure used during the activation process
Résumé
The influence on the hydrodesulfurization (HDS) of dibenzothiophene (DBT) of different activation procedures using H2S or dimethyldisulfide (DMDS) has been investigated on phosphorus-free and phosphorus-doped CoMo and NiMo industrial catalysts supported on alumina. The comparison of the two sulfiding agents was performed using strictly similar procedures of sulfidation to det. the origin of the well-known beneficial effect of organosulfides for the activation of hydrotreating catalysts. Partial pressures in H2 and/or in sulfiding agents have been modified to observe activity variations among the different sulfiding agents used. Comparison between H2S and DMDS has also been performed on phosphorus-doped CoMoP/Al2O3 and NiMoP/Al2O3 catalysts. Results emphasized the importance of supplying H2S to the catalyst at low temps. of activation (around 423 K) even in a low proportion ( P H 2 S as low as 3.3 kPa) to sulfide correctly NiMo and CoMo catalysts. Indeed, through a low consumption of H2S, O-S exchange and intramol. redox reactions can be initiated at low temps. of activation leading to a level of sulfidation sufficiently advanced before reaching temps. (above 573 K) at which a strong competition between redn. and sulfidation starts. DMDS appears less efficient than H2S for activating NiMo and CoMo catalyst due to its inability to provide H2S to the catalyst at these low temps. of activation. At higher temps. (T > 543 K), a good activation cannot be obtained if a high PH2/PH2S ratio is used whatever the sulfiding agent. Finally, the difficulty of activating the P-doped CoMoP/Al2O3 and NiMoP/Al2O3 catalysts was confirmed. This study confirms that the beneficial role of organosulfide compds. as activating agents is not related to a pure "chem." phenomenon but more probably to a "thermal well" effect limiting the exothermic character of the oxide-sulfide transformation.