Development of a Magnetically Recyclable (Ni/Co)MoS2 catalysts supported on magnetic greigite: high performance and stability in hydrodesulfurization

Magnetic MoS2/Fe3S4, CoMoS/Fe3S4, NiMoS/Fe3S4 composites that consist of greigite as the core and (Co or Ni) molybdenum sulfide catalyst as the top layer were were synthesized and compared to MoS2/Fe3O4 (synthesized and studied in our previous work [1]) for hydrodesulfurization (HDS). The composites prepared in this work containing greigite core were found to have higher hydrogenation (HYD) selectivity in HDS of dibenzothiophene (DBT) compared to the one with a magnetite core and showed higher stability during different cycles of DBT HDS test. When previous techniques [2] failed to utilize promoted sulfide catalysts in slurry reactors, the novel technique used in this study showed the possibility of promoting MoS2 and increase the activity of the catalyst. The promoting atoms (Ni and Co) were found to be uniformly dispersed over the catalysts and no separate Ni or Co sulfide phase was detected. NiMoS/Fe3S4 was found to have the highest activity in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and DMDBT followed by CoMoS/Fe3S4 and MoS2/Fe3S4. For the promoted catalysts, the direct desulfurization (DDS) pathway was found to be more selectively enhanced than the HYD route in HDS of DBT. HYD pathway was found to be the dominant route in HDS of dimethyldibenzothiophene (DMDBT). MoS2/Fe3S4 was found to be more acidic than the promoted catalysts in the HDS of DBT. This was assigned to the difference in the acidic nature of –SH groups formed through the dissociation of H2S over the coordinatively unsaturated sites (CUS) for the promoted and unpromoted catalysts. In the presence of a weak metal-sulfur bond as in promoted catalysts, the –SH groups were considered to work as nucleophilic centers while over MoS2/Fe3S4 with a strong metal-sulfur bond, they act as Bronsted acid acites. High activity toward the refractory sulfur-containing compounds, high stability, and the magnetic properties associated with these nano-composites as shown in Figure 1, makes them an excellent choice for hydroprocessing of heavy petroleum oils in slurry reactors as they can be easily separated and reused.