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Black plate (259,1) photo-reaction. Since this is between 1 and 3 orders of magnitude greater than the concentration of any defect sites (Lewis acid or free radical sites) that might be present in the zeolite, the result confirms that the sole relevant physical property of the zeolite cage in the photo-reaction is the high electrostatic field in the vicinity of the Naþ cations, which is responsible for inducing the reaction, as has been previously assumed116. Frei has published a note in Science which emphasises the importance of the rate of diffusion of the products out of the zeolite in any practical preparative application of these photooxidations and that this poses a major obstacle to commercial scale-up117. The importance of the relative orientation of the photo-reactants in the zeolite cage to accessing the CT complex is also discussed. Interestingly, a paper by another group reports Density Functional Theory (DFT) calculations on the initial CT step for alkene photo-oxidation in zeolite Y. The model system used represents a part of the Y-zeolite supercage containing two SII sites with alkaline-earth cations and 2,3-dimethylbutene-2 (DMB) and O2 co-adsorbed on them. It is predicted that the electrostatic field of the zeolite cavity plays only a minor role in the stabilization of a charge-transfer state, while the relative orientation and the distance between the DMB and O2 molecules are the critical factors. On the basis of these results, the photo-oxidation is considered as being due to a confinement effect in which the reagents are oriented in a suitable ‘‘pre-transition state’’ configuration118. The same group have reported a further DFT study of the conformations of akanes adsorbed on zeolite cations119. 8. Novel zeolites Post-synthetic modification120,121 is a method that is employed to reduce the effective channel dimensions of a zeolite, to reduce the number of framework acid sites or to introduce some desired new reactive functionality. Haw and his coworkers recently functionalised the large-pore zeolite H-beta by reaction with trimethylphenylsilane at 648K, which was found to convert some of the acid sites to their trimethylsilyl derivatives122. This work has been extended to a two-step post-synthetic sequence in which HZSM-5 is functionalised with a high selectivity at ambient temperatures. The framework acid sites of the zeolite are initially reacted with phenylsilane to replace the acidic proton with a hydrosilyl (–SiH3) group covalently linked to the frame- work (step 1). This group readily couples to acetone in a step 2 to form a framework-bound hydrosilyl isopropyl ether. While this www.scienceprogress.co.uk Properties and applications of zeolites 259PDF Image | Properties and applications of zeolites
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