Unsupported NiMo Sulfide Catalysts Obtained from Nickel/Ammonium and Nickel/Tetraalkylammonium Thiomolybdates: Synthesis and Application in the Hydrodesulfurization of Dibenzothiophene

Abstract

Ammonium and tetraalkylammonium tetrathiomolybdates impregnated with nickel nitrate were used as precursors of unsupported NiMo sulfide catalysts. The precursors were decomposed either in situ during the course of a dibenzothiophene (DBT) hydrodesulfurization (HDS) test or ex situ through sulfidation by H2S/H2 (15% v/v H2S). The catalysts were characterized by thermogravimetric analysis, N2 adsorption, scanning electron microscopy (SEM), and X-ray diffraction. Textural and catalytic properties of these NiMo catalysts were strongly influenced both by the nature of the precursor and the activation procedure. For ex-situ activated NiMo catalysts, the use of carbon-containing tetraalkylammonium thiosalts as precursors did not lead to a significant improvement in HDS activity. For in situ activated NiMo catalysts, the role of carbon is more complex. The use of tetramethyl- or tetrapropylammonium tetrathiomolybdate salts led to a poor final HDS activity while using tetrabutylammonium tetrathiomolybdate, a net increase in HDS activity was observed compared to the use of the non-carbon containing ammonium tetrathiomolybdate. This was related to the development of a mesoporous structure and to a high increase in surface area. This result is in agreement with those found previously for CoMo catalysts and confirms that tetraalkylammonium tetrathiomolybdate salts with long alkyl chains lead to Co- or Ni-promoted MoS2-based catalysts with enhanced HDS activity if in situ activated.