Study on Shape Selectivity in Hydrocarbon Oxidation catalyzed by Zedites

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We have found remarkable shape selectivity in the epoxidation of alkenes and oxygenation of alkanes. Here we have investigated the factors determining the rates of oxidation of various substrates which have different steric and electronic characters over titanium silicalites TS-1 and TS-2.The catalytic reaction system is rather complicated. The liquid phase is separated into an organic phase and an aqucous phase. Since the solid catalyst exists almost exclusively in the aqueous phase, the substrates must transfer to the aqueous phase, where the catalyst and H_2O_2 are present. This accounts for the promoting effect of organic cosolvents in the oxidation of hydrophobic hydrocarbons.The next step is diffusion into zeolite pores. We have found that cyclic alkanes and alkenes are much more difficult to oxidize than linear ones. Thus the diffusion of molecules which have similar dimensions to the zeolites is affected by the coexisting molecules including water. The polarity of substrates an … More d hydrophobicity/- philicity of zeolites might also be taken into account.Even if the substrate is in the zeolite pores, the group to be oxidized must approach the active site. In the oxidation of unsaturated alcohols the influence on the chemoselectivity between terminal and internal double bonds was found quite different and interpreted in terms of their coordination ability to the Ti active site. The terminal CH_2=CH- double bonds, being less sterically demanding, appear to be able to interact strongly with the Ti active site, preventing the alcoholic group from approaching the active site.It has been revealed that the hydroxylation activity is proportional to the intensity of IR Ti=O stretching intensity. Therefore titanyl species might consist of active site. On the other hand, there is no relationship between the epoxidation activity and the intensity of the Ti=O stretching band.However, access to the active site is not the sufficient condition for the reaction. For the oxidation of unsaturated alcohols reactivity toward epoxidation was opposite to the coordination ability; higher turnover of epoxidation of the internal double bond than that of the terminal double bond would result from an electrophilic nature of the attack of active oxygen species against C=C double bonds.The final stage is the diffusion of products into the outside of the pores. We have found this process some- times limits the rate of the oxidation; the diffusion of reactants into the pore filled with products must be retard- ed. Thus it has been revealed that there are several prerequisite conditions to the completion of the H_2O_2 oxidation of organic substrates on titanium silicalites. Less