Contain Chromium, Molybdenum and Tungsten

Indirect Reduction with Mediators Containing Chromium, Molybdenum, and Tungsten... 

The mononuclear metal carbonyls contain only one metal atom, and they have comparatively simple structures. For example, nickel tetracarbonyl is tetrahedral. The pentacarbonyls of iron, ruthenium, and osmium are trigonal bipyramidal, whereas the hexacarbonyls of vanadium, chromium, molybdenum, and tungsten are octahedral. These structures are shown in Figure 21.1. 

Epoxidation with hydroperoxides is the basis for the large-scale indirect production of propylene oxide by a process that has been called the Oxirane or Halcon processes. Early work was reported by Smith in a patent issued in 1956 [457], which described soluble heteropoly acids containing transition metals such as chromium, molybdenum, and tungsten that could be employed as homogeneous catalysts for the reaction of olefins with organic hydroperoxides and hydrogen peroxide. 

Studies on the behavior of chromium, molybdenum and tungsten oxide-doped monolithic carbon aerogels in the isomerization of 1-butene have been carried out by the group of Moreno-Castilla [38]. Samples were prepared by polymerization of a resorcinol-formaldehyde mixture which contained metallic salts of the elements and then the polymer was carbonized at both 500 and 1000 °C. The catalyhc performance of the samples was studied at temperatures ranging from 50 to 425 °C. 

Some commercial cobalt-aUoy compositions are listed in Table 24.1. They typically contain chromium as well as molybdenum and/or tungsten, making them relatively resistant to both reducing and oxidizing conditions. High resistance to abrasion results from the precipitation of carbides, which can be carbides of chromium, molybdenum, and tungsten [3]. 

In the case of metal-containing PAN the spectral picture is qualitatively the same (as compared with the initial PAN) however, ftie nature of tiie metal carbonyl introduced into the polymer somewhat influences the character of file transformations. By FTIR, ESCA and ESR, it was found that under thermal treatment in air at 220 C, first metal carbonyl complexes decompose (by FTIR spectroscopy tungsten and molybdenum carbonyl complexes lose carbon monoxide at 150-160 C, chromium carbonyl ones at 110°C) and chromium, molybdenum and tungsten oxide particles form. 

Chromium Molybdenum, and Tungsten. Triple metal-metal bonded complexes of the types [MaLe] (M = Mo or W, L = NRj, OR, or R) and [ M(CO)aCp a] have been reviewed. Although MO calculations suggest the structures of the former should be eclipsed, the known examples, which contain bulky terminal ligands, have ethane-like, staggered conformations. Attempts to determine the mechanism by which [MaLg] complexes are formed have, as yet, been unsuccessful although it is clear that metal-metal metathesis reactions between [MOaLe] and [WaLe] occur far less readily than do those between [ Mo(CO)aCp a] and [ W(CO)aCp a]. ... 

There are few examples of complexes of chromium with molecular nitrogen, although molybdenum and tungsten form many such complexes.1 Unstable species containing Cr—N2 bonds produced by matrix isolation techniques have not been included.56... 

Ethylene is not sufficient for the reduction of molybdenum and tungsten catalysts containing metals in high oxidation states. Hydrogen, Na or NaH must be used. We assume that these centres and their reactions are similar to centres with chromium [201]. 

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