Kinetics of Two Pathways for 4,6-Dimethyldibenzothiophene Hydrodesulfurization over NiMo, CoMo Sulfide, and Nickel Phosphide Catalysts

Abstract

The kinetics for the initial stage of the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) and dibenzothiophene (DBT) were comparatively examined over NiMo and CoMo sulfide catalysts and newly developed nickel phosphide catalysts. The HDS can proceed through an indirect hydrogenation (HYD) pathway and a direct desulfurization (DDS, or hydrogenolysis) pathway. The rate constants for the HYD and DDS pathways (k 1 and k 2, respectively) were estimated using a method that involved extrapolation to zero conversion for the initial selectivity ratio of primary products. The overall rate constants (k 1 + k 2) for 4,6-DMDBT on a catalyst weight basis (in units of 10-5 s-1 g·cat-1) at 573 K under a pressure of 20.4 atm increased in the order of CoMo sulfide (34.1) < Ni2P/USY (51.5) < Ni2P/SiO2 (66.4) < NiMo sulfide (83.2); however, the values based on active sites (in units of s-1 active site-1) ranked in a different order (CoMo sulfide (4.0) < NiMo sulfide (8.8) < Ni2P/USY (15.2) < Ni2P/SiO2 (23.7)). The rate constants for the HYD pathway of 4,6-DMDBT HDS at 573 K based on active sites were strongly dependent on the type of catalyst used; however, those for the DDS pathway were less sensitive to the type of catalyst, as can be observed from the corresponding values (k 1; k 2) for CoMo sulfide (2.2; 1.8), NiMo sulfide (6.7; 2.1), Ni2P/USY (12.7; 2.5), and Ni2P/SiO2 (21.6; 2.1). The NiMo sulfide catalyst, which favored the HYD pathway, was more active than the CoMo sulfide catalyst for 4,6-DMDBT. The nickel phosphide catalysts showed higher activity in 4,6-DMDBT HDS than the sulfide catalysts, based on rate constants normalized to active sites; they operated through the HYD pathway. A comparison with DBT HDS at 573 K under a hydrogen (H2) pressure of 20.4 atm showed that the presence of the methyl groups at the 4- and 6-positions dramatically inhibited the DDS pathway, because of steric hindrance around the S atom, and thus made the HYD pathway more important. Higher H2 pressure further enhanced the HYD pathway, whereas increased temperature increased the contribution of the DDS pathway for 4,6-DMDBT HDS.

Development of Highly Active Co(Ni)Mo Catalysts for the Hydrodesulfurization of Dibenzothiophene Compounds

“…Compared with Co(i)Mo(i) or Ni(i)Mo(i), both Co(c)Mo(i) and Ni(c)Mo(i) promoted the DDS pathway, producing BP or DMBP, to a greater extent than the HYD pathway in the HDS of either DBT or 4,6-DMDBT. These results agreed with the XPS results that the interactions between Co or Ni and Mo were increased when the promoter was added by CVD because this promoted the formation of the Co-Mo-S or Ni-Mo-S species, both of which were more active for the DDS than for the HYD route [41,42].…”

“…The above results indirectly support the notion that the promoter, either Co or Ni, was selectively deposited onto, and interacted efficiently with, the Mo sulfides when added by CVD. The addition of promoters to MoS 2 preferentially enhanced the DDS rate over the HYD rate in the HDS of DBT compounds [41][42][43]. As shown in Tables 3 and 4, the conversions were higher when either Co or Ni was added to Mo(i) using CVD instead of impregnation.…”

Nature of active sites in Ni2P hydrotreating catalysts as probed by iron substitution

“…The table lists (DMBCH). The DMBP has been considered as the product from direct desulfurization (DDS) pathway, whereas the MCHT and DMBCH from hydrogenation (HYD) pathway [72,73]. A previous study using similar condition also pointed out that DMBP cannot be produced by a secondary dehydrogenation pathway as the parallel dehydrogenation of tetralin to naphthalene did not occur [42].…”

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

“…Pseudo-first-order rate constants were determined using the method reported in our recent kinetic study [22]. The rate constants of simultaneous HDS of DBT and of 4,6-DMDBT over the unsupported MoS 2 , NiMoS 2 and CoMoS 2 catalysts and commercial NiMo and CoMo alumina supported catalysts are shown in Fig.…”

“…These Mo sulfide catalysts can be synthesized directly via a thermal decomposition method of ammonium tetrathiomolybdate (ATTM) and do not need further sulfidation. The Mo sulfide generated from decomposition of ATTM is more active than that from sulfidation of molybdenum oxide or www.elsevier.com/locate/cattod Catalysis Today 130 (2008) [14][15][16][17][18][19][20][21][22][23] the reagent MoS 2 , and thus ATTM has been used as catalyst precursor for generation of active MoS 2 catalyst. Recently, the new method for preparing highly dispersed unsupported Mo sulfide was reported by Song et al [14][15][16].…”