Activation of alumina-supported hydrotreating catalysts by organosulfides or H_2S:Effect of the H_2S partial pressure used during the activation process

摘要

The influence on the hydrodesulfurization (HDS) of dibenzothiophene (DBT) of different activation procedures using H_2S 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 determine the origin of the well-known beneficial effect of organosulfides for the activation of hydrotreating catalysts.Partial pressures in H_2 and/or in sulfiding agents have been modified to observe activity variations among the different sulfiding agents used.Comparison between H_2S and DMDS has also been performed on phosphorus-doped CoMoP/Al_2O_3 and NiMoP/Al_2O_3 catalysts.Results emphasized the importance of supplying H_2S to the catalyst at low temperatures of activation (around 423 K) even in a low proportion (P_(H_2S) as low as 3.3 kPa) to sulfide correctly NiMo and CoMo catalysts.Indeed,through a low consumption of H_2S,O-S exchange and intramolecular redox reactions can be initiated at low temperatures of activation leading to a level of sulfidation sufficiently advanced before reaching temperatures (above 573 K) at which a strong competition between reduction and sulfidation starts.DMDS appears less efficient than H_2S for activating NiMo and CoMo catalyst due to its inability to provide H)2S to the catalyst at these low temperatures of activation.At higher temperatures (T > 543 K),a good activation cannot be obtained if a high P_(H_2) /P_(H2s) ratio is used whatever the sulfiding agent.Finally,the difficulty of activating the P-doped CoMoP/Al_2O_3 and NiMoP/Al_2O_3 catalysts was confirmed. This study confirms that the beneficial role of organosulfide compounds as activating agents is not related to a pure "chemical" phenomenon but more probably to a "thermal well" effect limiting the exothermic character of the oxide-sulfide transformation.