Group 8 VIII synthesis

In addition to the processes mentioned above, there are also ongoing efforts to synthesize formamide direcdy from carbon dioxide [124-38-9J, hydrogen [1333-74-0] and ammonia [7664-41-7] (29—32). Catalysts that have been proposed are Group VIII transition-metal coordination compounds. Under moderate reaction conditions, ie, 100—180°C, 1—10 MPa (10—100 bar), turnovers of up to 1000 mole formamide per mole catalyst have been achieved. However, since expensive noble metal catalysts are needed, further work is required prior to the technical realization of an industrial process for formamide synthesis based on carbon dioxide. 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

A new method to synthesis nanoparticles of group VIII-X elements has been developed by Choukroun et al. [178] by reduction of metalUc precursors with CP2V. Mono- and bimetallic colloids of different metals (stabilized by polymers) have been prepared in this way (Fe, Pd, Rh, Rh/Pd). These colloids are then used as catalysts in various reactions such as hydrogenation of CC, CO, NO or CN multiple bonds, hydroformylation, carbonylation, etc. 

This preparation is an illustration of the hydroformylation of olefins (oxo synthesis). The reaction occurs in the presence of soluble catalytic complexes containing metals of Group VIII of the periodic system. Although the metal originally used by Roelen and still largely used in the industry for the production of aliphatic aldehydes and alcohols is cobalt, the most active and selective catalysts are rhodium-containing compounds. The catalytic activity of the other Group VIII metals is in... 

Fischer-Tropsch Catalysts. - It is well known that all Group VIII transition metals are active for F-T synthesis. However, the only F-T catalysts, which have sufficient CO hydrogenation activity for commercial application, are composed of Ni, Co, Fe or Ru as the active metal phase. These metals are orders-of-magnitude more active than the other Group VIII metals and some characteristics of Ni-, Fe-, Co- and Ru-based F-T catalysts are summarized in Table 2. 

Group VIII sulfides are obtained by the low-temperature precipitation described above. However, a preferred synthesis involves the direct sulfidation with hydrogen sulfide of ammonium hexachlorometallates. A good example is found in the synthesis of RuS2 and Rh2S3 (28). However, synthetic techniques and the solid-state chemistry of the Group VIII sulfides are not as well developed as those for the Group VII sulfides, and much research needs... 

The reaction between a low-valent Group VIII metal complex and an alkyl halide belongs to the class known as oxidative addition and has attracted much study and controversy as to the mechanism. Recent evidence suggests free radicals as intermediates in many cases. The oxidative-addition reaction is of widespread occurrence and importance in transition metal chemistry, due in part to its use in synthesis and to its implication in many catalytic systems. In one of its forms it is described by... 

Krocher, O., Koppel, R.A., Froba, M. and Baiker, A. (1998) Silica hybrid gel catalysts containing group(VIII) transition metal complexes preparation, structural, and catalytic properties in the synthesis of N, N-dimethylformamide and methyl formate from supercritical carbon dioxide. Journal of Catalysis, 178, 284-298. 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

Finally, methyl acetate could be made directly from methanol via carbonyla-tion [33]. Preferentially, the catalyst system is based on rhodium, but other metals of Group VIII or Zr, Hf have also been patented. The addition of iodine seems necessary for good selectivities and conversions. Intermediates similar to those discussed for the acetic acid synthesis have been proposed [34] and Scheme 10 shows some species. 

Vannice, M.A., The catalytic synthesis of hydrocarbons from Ha/CO mixtures over the group VIII metals (I. The specific activities and product disthbutions of supported metals), J. Catal. 37 (1975) 449-461. 

The models are extended from Pt/TiOg to some other systems for which data are available. The Inhibition of Cu/Zn0/Al203 methanol synthesis catalysts by low levels of Group VIII metals, especially cobalt, Is shown to fit the same pattern of SMSI the active metal... 

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