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Black plate (253,1) described in the biological literature), such as HO, HO2, RO, RO2, or heterolytic and involve oxygen-atom transfer processes, or indeed a mixture of them, depending mainly on the type of metal. When the metal in its highest oxidation state is both a Lewis Acid and a weak oxidant, e.g. early transition metal ions with a d0 configuration, such as Mo(VI), W(VI) and Ti(IV), heterolytic peroxometal (M-OOR) routes are favoured. The metal ion functions as a Lewis Acid and so its oxidation level remains constant throughout the catalytic cycle. Strong single-electron oxidants, such as Cr(VI), Mn(III), Co(III), Fe(III), encourage oxometal path- ways andyor homolytic decomposition of RO2H. Vanadium(V) is something of an all-rounder, since its mechanistic role appears to involve all three types of activity, on account that it is a strong Lewis Acid in addition to being a fairly strong single-electron oxidant82. When hydrocarbons are oxidised using O2, hydroper- oxides are formed according to the classical mechanism of auto- xidation with RO2H species as intermediates78. These species may undergo subsequent heterolytic oxygen-atom transfers, and in oxidations with RO2H or H2O2 as reagents, homolytic reactions may result in the formation of free O2 as a result of bimolecular terminations of RO2 or HO2 radicals (Russell Mechanism) e.g. RO þ RO ?ROOR þ O : 222 The O2 can feed-back into an autoxidation type mechanism by reacting further with free radicals present in the system. It is not always an easy matter to distinguish between homolytic and heterolytic pathways, because the same products can be produced from both routes. There are simple means for inquiring systems about free radicals, for example adding radical scavengers which necessarily inhibit a genuine homolytic chain-mechanism, and if the product concentration is reduced on deoxygenation, e.g. with argon, there is the strong likelihood that O2 is involved in the reaction. The relative rates at which secondary and tertiary C2H bonds are oxidised may also be used to indicate the nature of the attacking species, and is is in the range 2–3 for HO reacting with R3CHyR2CH296, bearing in mind that the relative rates depend on the choice of solvent and on other factors. Among peroxometal mediated pathways are the epoxidation, sulfoxidation and oxidation of alkenes, while typical oxometal-mediated or autoxidation reac- tions include the oxidation of allylic and benzylic compounds and the oxidation of alkanes, where the two mechanisms are often hard to distinguish. Alcohol oxidation may involve both peroxometal or oxometal intermediates, and titanium-substituted molecular sieves www.scienceprogress.co.uk Properties and applications of zeolites 253PDF Image | Properties and applications of zeolites
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