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Black plate (254,1) act via peroxometal pathways and effect the epoxidation of alkenes. They would not, however, work well in allylic or benzylic oxidations, and using titanium or vanadium-substituted molecular sieves to promote the oxidation of alkenes with H2O2 would be unlikely to involve heterolytic oxygen-atom transfer. More reason- ably, hydroxyl and hydroperoxyl radicals are formed in these systems, by the metal ion induced homolysis of H2O2, as is the case in homogeneous vanadium catalysts97. 7. Photooxidation in zeolite cages The oxidation of small alkanes, alkenes and aromatic compounds (derived from crude oil and natural gas) is pivotal to much of the chemical industry. Molecular oxygen is an ideal oxidant in principle because it is so widely and readily available, but it tends to lack selectivity, and almost invariably results in the production of unwanted by-products which require separation using processes that are demanding in terms of their energy costs. This lack of selectivity is due to the fact that oxidation reactions in gas or liquid phases tend to proceed via free radical intermediates, and are notorious for resulting in over-oxidation and other products resulting from radical coupling and disproportionation. A relative exothermicity is also a feature of reactions with O2, and unrestricted mobility of free radicals encourages indiscriminate attack on initial hydrocarbon chains and their primary oxidation products. It is an unlucky dispensation of nature that the oxidised product molecules tend to be more reactive toward free radicals than their parent hydrocarbons are, and hence the kinetic competition favours the formation of poly-substitution or over-oxidation products. As the reaction proceeds and its products accumulate, the effect becomes more pronounced and in most practical situations conversion to the desired products is limited to just a few percent78. Various approaches have been explored by which to optimise the outcome of oxidation reactions more viably. An O2 molecule and a hydro- carbon molecule may be together activated to induce an electron transfer from the latter to the former, resulting in the formation of the hydrocarbon radical cation along with the superoxide radical anion (O). The resulting alkane and alkene radical cations are 2 highly acidic and so can transfer a proton to the superoxide moiety, yielding an alkyl or allyl radical and the hydroperoxyl radical (HOO). Although these are the same radicals as are produced in normal gas-phase or liquid-phase oxidation reactions, at ambient temperature and in a restricted environment, their reactivity might 254 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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