Co-Mo-S clusters

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. 

The starting material is the butterfly shaped tetranuclear cluster [Cp 2Mo2Co2S3(CO)4] (Cp = MeCp ), which reacts with thiophene in solution under hydrogen (150 C, 15 atm) to produce the free C4 hydrocarbons corresponding to HDS (together with small amounts of cracked products) and a new cubane cluster [Cp 2M02C02S4(C0)2] in which the extruded sulfur has been incorporated (Eq. 4.22). 

This cubane cluster can be converted back to the original one by reaction with CO (30 atm) to form COS, thus completing a cyclic reaction. Although this pair of reactions indicates the possibility of constructing a catalytic cycle for the HDS of thiophene, the presence of free CO unfortunately inhibits the forward desulfurization reaction and therefore the process cannot be rendered catalytic [68]. The exact mechanism of thiophene HDS by use of these clusters in not clearly understood, mainly because the reaction is fast and no intermediates have been isolated or identified. Nevertheless, Curtis has reported extensive related mechanistic studies on the desulfurization of a variety of thiols and sulfides using the same metal cluster Cp 2Mo2Co2S3(CO)4 and as a result of that, the main reaction pathways for desulfurization of these simpler substrates are now well understood [69], 

Fig 4 The mechanism of HDS of thiols by Cp jMo COjSjfCOA her ligands ommited for clarity). 

A further point of great interest that emerges from the work of Curtis and from the mechanism depicted in Fig. 4.14 is the concept of latent vacancies and sulfur mobility . This can be related to some recent important considerations on the existence of real anionic vacancies on Co-Mo-S surfaces, a frequently encountered key feature of HDS mechanisms. Tlie Co atom, which is known to be the primary site of attack of thiols in this case, is electronically saturated, but the empty site required for 

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The overall degradation of (103) assisted by the cluster [(Cp )2 M o2Co2S3(CO)4] (Cp = CH3C5H4) is the model reaction that best resembles the heterogeneous counterparts, particularly those classified as Co/Mo/S phase,158 in terms of both structural motif and HDS activity.229 Morever, the Co/Mo/S cluster has successfully been employed to show that the C—S bond scission in the desulfurization of aromatic and aliphatic thiols occurs in homolytic fashion at 35 °C and that thiolate and sulfido groups can move over the face of the cluster as they are supposed to do over the surface of heterogeneous catalysts.230... 

In relation to HDS catalysis, C-S bond cleavage of thiols has also been extensively investigated. Co/Mo/S cluster shown in Scheme 3.79 reacts with benzenethiol to give a new /r-sulhdo cluster by releasing benzene. Such reactions are also regarded as one of the model reactions of heterogeneous HDS catalyst... 

Scheme 11 Desulfurization of thiols on Co-Mo-S clusters. Reproduced from Organometallic modeling of the hydrodesulfurization and hydrodenitrogenation, by R. A. Sanchez-Delgado in Catalysis by Metal Complexes] B. R. James and P. W. N. M. van Leeuwen, Eds. Kluwer Academic Publishers Dordrecht, Vol. 24, p. 126 (2002). With kind permission of Springer Science and Business Media.

Thus, theoretical calculations can be a very useful tool in mechanistic understanding. As it is now known that the most active Co-Mo-S or SBMS catalysts consist of very small clusters of Mo atoms (as few as seven), it is within the realm of practical computational procedures to completely model the catalyst/S-molecule interactions. Assumptions made about steric crowding around the catalytic site may be quite different for such small clusters as the catalytic site is not an extended planar surface, as discussed in the previous section. Future work in this area should provide new insight into the true limitations in HDS of dialkyldibenzothiophenes. 

Nowadays, it is generally accepted that the HDS activity is associated with the presence of the Co-Mo-S structures that consist of small M0S2 or WS2 clusters with promoter atoms located at the edges of M0S2 slabs (25). 

The reaction proceeds with co-evolution of hydrogen. Nitrogenase has been studied extensively and appears to contain three different kinds of Fe/Mo/S clusters distributed over two proteins. The actual nitrogen reduction takes place at the FeyMoSs cluster shown in Fig. 7.15. 

It was shown by EXAFS that the structure and dispersion of the Mo sulfide species in MoSx/NaY were unaltered by a prolonged sulfidation of 20-h at 673 Kina stream of H,S/Hj. This fact indicates that highly dispersed Mo sulfide species are thermally stable. On the other hand, with MoSx/AljOj prepared by using Mo(CO) [12], a considerable agglomeration of highly dispersed Mo sulfide species was observed at a shorter treatment time. It is considered that highly dispersed Mo sulfide clusters are thermally stabilized in zeolite cavities. 

Figure 4 Mo-Fe-S clusters containing two cubane-type MoFe3S4 cores, which are the extensively studied, synthetic models of FeMo-co...

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