Organometallic Models

Figure 4.23. Infrared spectra of NO probe molecules on sulfided Mo, Co, and Co-Mo hydrodesulfurization catalysts. The peak assignments are supported by the IR spectra of organometallic model compounds. These spectra allow for a quantitative titration of Co and Mo sites in the Co-Mo catalyst.

Particularly the step of CC-bond formation is of interest, and has been investigated thoroughly by use of organometallic model compounds a survey of this subject has been given by IV. A. Herrmann [6], In the following, some more recent results are discussed briefly. 

P.R. Raithby (eds), vol 2, Wiley-VCH, Weinheim, 1999, 741 (b) R.A. Sanchez-Delgado, Organometallic Modeling of the Hydrodesulfurization and Hydrodenitrogenation Reactions, Kluwar Academic Publishers, Dordrecht, 2002. 

R.A. Sanchez-Delgado, Organometallic Modeling of the Hydrodesuljurization and Hydrodenitrogenation Reactions. Kluwer, Dordrecht, 2002. 

This simple mechanistic picture cannot be applied to thiophene derivatives, whose HDS mechanism is still under study. Research on organometallic models... 

Mechanistic studies have mostly focussed on the 1-alkene formation by polymerization of surface CFI2 (methylenes). The formation of the CH2 species by a deoxygenation/hydrogenation sequence of adsorbed CO is not well understood as there are few convincing organometallic models, but it is usually depicted as shown in Figure 16. The path involves stepwise de-oxygenation to a surface carbide followed by sequential H transfer to make various intermediate CiHx(ad) (x = 0 - 4) species and finally methane which is liberated. 

Since HDN occurs in the same process as HDS, H2S is always present. In fact, H2S is necessary to maximize the HDN activity of the C0-M0/AI2O3 and Ni-Mo/A Os catalysts. One role of the H2S is to maintain the catalyst in the sulfided form, but there is evidence to suggest that it is also more directly involved in the HDN mechanism. Less is known about possible mechanisms for HDN than HDS based on organometallic models, but recent studies of amine, pyrrole, pyridine, and quinoline binding and reactions at metal centers offer some possibilities. 

The allylic oxidation of propene typifies the so-called bimetallic heterogeneous catalysis [4], a terminus technicus to emphasize cooperative effects in catalytic conversions (for multicomponent homogeneous catalysis, see Section 3.1.5). Nevertheless, the SOHIO-type oxidation is included in this book because one can imagine a number of mechanistic implications on a molecular platform, too. Studies on organometallic model compounds and reactions are available in ref. [2]. 

This trend is interesting in that it is consistent with the one observed for the relation between adsorption constants and HDS activities of thiophenes on Co-MO/AI2O3 catalysts nevertheless, a similar trend was also observed in the case of the T -S bonded complexes [CpfCOljRufri S-Th)] also studied by Angelici [44] (see Section 2.2) and therefore no clear conclusions can be safely drawn from these organometallic model compounds as to apreferred mode of surface adsorption (r S or Tj ) being related to HDS activity. 

The results described in this Chapter demonstrate that transition metal complexes can be considered reasonable models for mimicking some of the steps known or believed to occur in heterogeneous HDN. The most relevant points in this organometallic modeling may be summarized as follows ... 

The fact that Nb complexes are capable of oxidatively adding the C-N bond of anilines takes the organometallic modeling a step closer to the more conventional HDN mechanisms (of e.g. quinoline). Some interesting mechanistic considerations have been put forward in this case, but more detailed studies are required in order to better understand this reaction. When other non-aromatic amines have been used as model substrates, electron-rich late metals (Ru, Ni) are better suited for cleaving C-N bonds,... 

Some early proposals for the modes of adsorption of thiophenes on metal sulfides have been probed by comparisons with the structures of well-characterized metal complexes this has allowed the identification of the most reasonable alternatives and of new possibilities not previously considered. Tlieoretical studies on such complexes at increasing levels of sophistication have also contributed in an Important manner to provide a clear and consistent picture of the different possible bonding modes of thiophenes to metal centers. When these theoretical and experimental results from molecular chemistry are combined with the information available from surface techniques and heterogeneous catalysis, the chemisorption of this type of organosulfur compounds on metal sulfides arises as a very well understood phenomenon. This is no doubt one of the most important achievements of the organometallic modeling approach to HDS chemistry. 

Similar considerations have been made in the case of HDN-related substrates like pyrroles and pyridines. Nevertheless, the advances in this direction are much more modest and less clear-cut than those made for HDS models, and therefore, an exciting opportunity is open for new research in the organometallic modeling of the adsorption and reactivity of organonitrogen compounds. 

In this Epilogue, it is attempted to do that in a systematic and also in a somewhat provocative manner. This will hopefully stimulate new thoughts and further discussions, not only on the general subjects of HDS and HDN, but also on the validity and of the usefulness of the organometallic modeling approach as an additional tool to tackle problems in heterogeneous catalysis. Of course this modeling is not meant to represent a panacea, as we are well aware of the many limitations it contains, which will also be pointed out ... 

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