Highly active MoS₂, CoMoS₂ and NiMoS₂ unsupported catalysts prepared by hydrothermal synthesis for hydrodesulfurization of 4,6-dimethyldibenzothiophene

This paper reports on a comparative study of unsupported MoS₂ and Me/MoS₂ (Me = Co, Ni) catalysts prepared by hydrothermal synthesis in our laboratory. Hydrothermal synthesis using water and organic solvent under hydrogen was found to produce highly active Mo based sulfide nano-size particles. MoS₂ had high surface area (320 m²/g) and large pore volume (0.72 m³/g). Addition of Co or Ni promoter decreased surface area and pore volume. Compared to MoS₂, downward shifts of the reduction temperatures were observed in TPR for Co(Ni)-Mo-S sulfides which suggested a decrease in metal sulfur bond strength. HRTEM and XRD results showed that MoS₂ formed large crystallized particles but the particle growth was inhibited when promoters were incorporated. For HDS, 4,6-dimethyldibenzothiophene (4,6-DMDBT) was slightly more reactive than dibenzothiophene (DBT) over the MoS₂. The liquid-phase adsorption showed that 4,6-DMDBT had higher capacity and stronger interaction with the adsorption site on MoS₂ than DBT. In distinct contrast to MoS₂, the promoted NiMo or CoMo sulfide showed higher liquid-phase adsorption selectivity for DBT than for 4,6-DMDBT. The promoter increased activity of MoS₂ and changed the contribution of the direct-desulfurization and of hydrogenation pathways. The adsorption and HDS results indicated that promoter affects both the number and the activity of active sites of the Mo sulfides. HDS activity of the unsupported Mo based sulfides was much higher than that of the sulfided commercial Co(Ni)Mo/Al₂O₃ catalysts.