Addition of manganese to alumina and its influence on the formation of supported NiMo catalysts for dibenzothiophene hydrodesulfurization application

The influence of adding manganese to NiMo/Al2O3 hydrodesulfurization (HDS) catalysts was studied by impregnation of manganese acetate at the surface of an Al2O3 support obtained by a sol?gel method. The as-obtained Mn-containing alumina supports were then impregnated with nickel nitrate, Ni(NO3)26H2O, and ammonium heptamolybdate, (NH4)6Mo7O244H2O, dried at 120 C for 12 h, and finally calcined in air at 400 C for 4 h. The different solids were then sulfided using 10 mol.% H2S in H2 at 400 C for 4 h and evaluated in the hydrodesulfurization of dibenzothiophene (DBT) at T = 300 C and P = 30 bars H2. Catalysts were characterized at each step of the preparation: after drying, after calcination, and finally in the sulfided state. Both the pH of the impregnation solution of Ni and Mo precursors and the Mn content (1 or 5 mol.% as MnO) strongly influence the nature of the Ni and Mo species formed. In this respect, the pH of the impregnation solution plays a critical role at low Mn content (1 mol.%). While impregnation at pH 7 results in well-dispersed molybdate species easily sulfided into a highly Ni-promoted MoS2 phase, increasing the pH to 9 favors the formation of more polymerized Mo species and the loss of some nickel into a NiMnO spinel, leading after sulfidation to a lower promotion degree and a lower dispersion. At higher Mn content (5 mol.%), whatever the pH of impregnation, the formation of MoO3 clusters and the loss of Ni into a NiMnO spinel phase leads to deleted HDS activity after sulfidation. Finally, our results also demonstrate a strong positive influence of manganese on the intrinsic activity of the NiMoS phase if conditions of preparation are optimized.