Nanosized unsupported metal sulfide catalysts by hydrothermal synthesis for simultaneous hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene

This lecture discusses the synthesis and catalytic properties of unsupported MoS2 and Me/MoS2 (Me=Co, Ni) catalysts by a hydrothermal synthesis method. Hydrothermal synthesis using water and organic solvent under hydrogen was found to produce nano-size particles of MoS2 with high surface area (320 m2/g) and large pore volume (0.72 m3/g). Co or Ni addition decreased surface area and pore volume. Compared to MoS2, downward shifts of the reduction temperatures were observed in TPR for Co(Ni)MoS sulfides, suggesting a decreased metal-sulfur bond strength. HRTEM and XRD showed that MoS2 formed nano-sized, longer slabs; the addition of promoters led to the increase in curvature of MoS2 slabs and the decrease in slab length. For simultaneous HDS of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), the activity of the unsupported Co(Ni)Mo sulfides was much higher than that of the sulfided commercial Co(Ni)Mo/Al2O3 catalysts tested. In contrast to supported catalysts, 4,6-DMDBT is not much less reactive than DBT over all of the unsupported catalysts (approximately 0.8 times compared with 2-6 times as reported in the literature). Surprisingly, 4,6-DMDBT is even slightly more reactive than DBT over the unsupported MoS2 catalyst. The liquid-phase adsorption of DBT and DMDBT was carried out and the results revealed that 4,6-DMDBT showed higher adsorption capacity and stronger interaction with the adsorption site on MoS2 than DBT. In 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 promoter affects both the number and the activity of active sites of the Mo sulfides. The turn over frequency (TOF) values were also determined for the promoted and unpromoted MoS2 catalysts by using the adsorption capacities as a measurement of the number of active sites.