Co-Mo-S catalysts

The mechanism proposed involves hydrogenation of the C2 C3 double bond, formation of 2-vinylthiophenol by an E2 elimination, and hydrocarbon elimination by homolysis of the S—Caryi bond. This pathway rationalizes the primary formation of (104) observed in some HDS reactions of (102) over Co/Mo/S catalysts, as well as the kinetic evidence that the rate-determining step on real catalysts is the removal of surface sulfur.158-160... 

Another novel catalyst modification has been suggested in which the active Co-Mo-S catalyst is used in combination with an acidic catalyst such as a zeolite. This combination has the potential of opening another reaction pathway by isomerization of the alkyl groups on molecules such as 4,6-DMDBT to positions that do not sterically interfere with adsorption or oxidative addition. This is illustrated in Fig. 33. Gates and co-workers reported many years ago that the 2,8- and 3,7-dimethyldibenzothiophenes are much more easily desulfurized than 4,6-DMDBT (see Table XII) (26). Therefore, a combination of an isomerization catalyst and a desulfurization catalyst could be synergistic for removing dialkylbenzothiophenes. 

Curtis has provided some homogeneous work that represents without doubt the closest model available for real Co-Mo-S catalysts. Heterobimetallic clusters, which contain the right combination of metals (Co-Mo) linked through sulfur bridges, were used, thereby beautifully mimicking a Co-Mo-S site [68, 69]. Even though this complex could be considered as just another example of a heterobimetallic cluster, its unique relation to real HDS catalysts seemed to justify its description in some detail in a separate section of its own. 

Hi) To heterogeneous catalysis sulfides of group S(a) often show catalytic activity which may be strongly enhanced in the presence of metal centers of group B elements (c/. the important hydrodesulfurization reactions utilizing Co—Mo—S catalysts). The catalytic activity may be discussed in terms of the different electron densities at the metal centers and the sulfur atoms attached to them. 

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