Supported alkene oxidation

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

An unanticipated catalytic reaction of olefinic hydrocarbons was described in 1964 by Banks and Bailey.1 2 They discovered that C3-C8 alkenes disproportionate to homologs of higher and lower molecular weight in the presence of alumina-supported molybdenum oxide [Eq. (12.1)], cobalt oxide-molybdenum oxide, molybdenum hexacarbonyl, or tungsten hexacarbonyl at 100-200°C, under about 30 atm pressure ... 

Studies in this field are just beginning, and the number of publications hardly exceeds a dozen. The most interesting results were obtained by the research groups of Yamada [160-162], Neumann [163,164] and Kozhevnikov [165, 166], Using various type catalysts (Ru porphyrene complexes, polyoxometalates, supported metals), the authors conducted selective oxidations of various types. These include epoxidation of alkenes, oxidation of alcohols, oxidation of alkylaromatics, oxidation and aromatiza-tion of dihydroanthracenes, and some other reactions. The experiments were typically conducted at 373—423 K under 1.0 MPa pressure of nitrous oxide. 

For commercial processes, formed supports are more useful. Compared with other supports, fumed oxide supports showed new catalytic effects [41]. Some intensively investigated applications for these supports are abstracted in the following. SiC>2 pellets have been successfully introduced in a new generation of precious metal supports in vinylacetate monomer production [42]. This resulted in better selcctivities and an up to 50% higher space-time yield compared with supports based on natural alumo-silicates. In alkene hydration fumed silica pellets serve as a support for phosphoric acid. In this case, an increased catalyst lifetime and a higher space-time yield were observed [43]. Pyrogenic TiC>2 powder can be used as a starting material for the manufacture of monolithic catalysts [44] for the selective reduction of NOv with ammonia. 

Applications of HT-type catalysts, prepared by the above methods, have been reported in recent years for basic catalysis (polymerization of alkene oxides, aldol condensation), steam reforming of methane or naphtha, CO hydrogenation as in methanol and higher-alcohol synthesis, conversion of syngas to alkanes and alkenes, hydrogenation of nitrobenzene, oxidation reactions, and as a support for Ziegler-Natta catalysts (Table 2). 

The reactivity of O with simple alkanes and alkenes has been studied with both MgO and supported metal oxides as substrates. 

The development of solid-supported chiral oxidants is a challenging area that has yielded interesting results in the development of a chiral supported dioxiran precursor. The preparation of non-racemic epoxides has been extensively studied in recent years since they are important building blocks in stereoselective synthesis. A supported dioxirane precursor based on a-fluorotropinones was shown to promote the epoxidation of alkenes [28, 29]. The reactant was anchored on meso-porous MCM-41 and amorphous silicas. It has shown comparable activity to its homogenous counterpart and good stability on recycling. The enantiomerically enriched version efficiently promotes the enanhoselective epoxidation of alkenes, with ee values up to 80% (Scheme 4.4). 

The triblock poly(alkene oxide)-supported catalysts 88 and 90 can be simply prepared using the same chemistry used earlier to prepare poly(ethylene oxide)-immobilized hydrogenation catalysts (Eqs. 29 and 30) [124]. Once prepared, these ligands and their Rh(I) complexes separate as oil-in-water emulsions from water at temperatures in the 0-50 °C range. The actual LCST depends on the substrate concentration and on the ratio of hydrophilic/hy-... 

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