Oxidation Sites in Extra-framework Positions

Redox-type zeolite catalysts can also be prepared in which the active sites occupy extra-framework positions and are obtained by different post-synthesis procedures, including anchoring the transition metal complexes (TMC) [92-94], CVD deposition, trapping the TMC within the pores during zeolite synthesis [95,96], or encapsulation of the TMC by the ship-in-a-bottle teehnique. 

The ship-in-a-bottle technique is perhaps the most common method for encapsulation of transition metal complexes. In this way the tetradentate Schiff base ligand SALEN (bis-salicylidene) ethylenediamine can diffuse through the 12 MR windows of faujasite. Then, when complexed with a previously exchanged metal ion, nearly square planar coordination geometry is formed inside the a-cages [97-100], Mn complexes with a chiral ligand, prepared by the ship-in-a-bottle technique inside Y and EMT zeolites, have enantioselectively carried at the epoxidation of olefins [101,102]. 

Other complexes including phthalocyanines and metal clusters have also been encapsulated in several zeolites such as Y, EMT, VPI-5 and mesoporous materials [103-110], 

Zeolites can be used as microreaetors in which the size of the reactor, i. e. the size of pores and cavities can be varied. The adsorption properties of zeolites can be modified and this, with their molecular sieve properties either for reactants, products, or transition states can be very useful for directing the reaction towards the desired products. 

Acidic, basic, and redox active sites can be introduced either in framework or in extra-framework positions. A good understanding of the nature of the active sites and their interaction with the substrates is being achieved by combining theoretical calculations, characterization, and reactivity studies. It seems that the interactions between active sites and reactants are more complex than previously supposed and that the reactant adsorbed and the transition state will interact not only with the particular acidic or basic site, but also with neighboring framework 

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