Activation of alumina-supported hydrotreating catalysts by organosulfides:: comparison with H2S and effect of different solvents

The sulfidation of nonpromoted and Co(Ni)-promoted MoS2/Al2O3 catalysts with different organosulfide compounds (dimethylsulfide, dimethyldisulfide, and ethanethiol) has been investigated in this study. Comparison with H2S sulfidation was performed using similar procedures of activation except the origin of sulfur (particularly the H2S partial pressure generated inside the reactor during the activation procedure). The aim of this study was to better evaluate the different possibilities proposed in the literature to explain the well-known advantage of using organosulfides instead of H2S during the activation of hydrotreating catalysts. Using organosulfide agents with alkane solvents to perform sulfidation of NiMo/Al2O3 leads to a very limited gain in activity for the HDS of dibenzothiophene or 4,6-dimethyidibenzothiophene compared to that of H2S sulfidation. Even if the formation of thiolate species and their interaction with the catalyst surface is demonstrated, their role does not seem of prime importance to reach highly active catalytic systems. Moreover, in our experimental conditions, the catalytic results rule out a better balance between reduction and sulfidation when using organosulfides instead of H2S. If the sulfiding agents are used under rigorously identical conditions, there is no significant difference in HDS performances of the obtained catalysts at the laboratory scale. Therefore, it is concluded that the actual beneficial effect of organosulfides as activating agents observed in industrial practice is not mainly chemical in nature. A thermal well effect of the organosulfides diluting the effect of the exothermicity of the oxide-sulfide transformation should be rather envisaged to explain the better efficiency of organosulfides. Finally, suppressing H2S added to the reactant feed during the HDS reaction clearly evidences the higher activity of NiMo catalysts compared to CoMo catalysts at low H2S concentration. Increasing the H2S partial pressure results in a more severe inhibition of NiMo catalysts than CoMo catalysts, leveling off HDS activity differences between these two catalysts. (C) 2004 Elsevier Inc. All rights reserved.