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

•Mn addition to Al2O3 and its influence on the preparation of NiMo catalysts analyzed.•The pH of the impregnation solution and the Mn content influence Ni and Mo species.•The presence of NiMnO spinel limits the formation of the promoted NiMoS phase.•If properly optimized, strong beneficial role of Mn toward HDS activity.

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)2·6H2O, and ammonium heptamolybdate, (NH4)6Mo7O24·4H2O, 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.

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