Solvent-like properties, zeolites

The intracrystalline channel cavity-pore-cage system in zeolites is surrounded by the lattice and therefore is submitted to the zeolite crystal field. This results in solvent-like and even electrolyte-type properties. One has seen above how cations could be easily exchangeable. It may also exist an interaction between any occluded ionic compound and the zeolitic framework. Salts, especially salts of univalent anions, have been shown to penetrate the zeolite structure and fill the available space even if the openings of the cavities (as the 0 -ring of 0.24 nm in size in sodal te cage of Y zeolite) is smaller than the size of the anion (CIO, NO for instance). The interesting feature is then the enhanced thermal stability of the occluded salt. 

The zeolitic framework involving channels, cavities, cages induces very peculiar properties. Due to local crystal and electrical field particular ionization properties are observed with a solvent-like nature. 

Zeolites, crystalline solids with regular, molecular-scale pores that incorporate catalytic groups, exert unique catalytic influence because of the molecular-sieving properties of their pore structures and the solvent-like character of their narrow pores. 

We would like to emphasize that the global properties of molecular sieves, often described in terms of hydrophobicity and hydrophilicity [240] or fiamework polarity [241] will markedly influence the chemical preference for the sorption of molecules. This may lead to quite different relative concentrations of reactants in the zeolite pores (well demonstrated and discussed for the case of TS-1 in this review [163]) and in the intra ciystalline void space. The consequence of which is unexpected solvent-type effects. While these possibilities are hardly utilized up to now, one could think of systematically applying this approach for organic synthesis steps in which, for example, a high concentration of reactants is needed around the active site, but for which one wishes to dilute the products in the intra crystalline void to prevent further reaction. In such a case a solvent of different polarity than the reacting substrate should be chosen and the choice of the zeolite should be made in accordance to the polarity of the reactant(s). 

Note that many MOFs exhibit permanent porosity like zeolites, but it is not retained after the removal of the solvent for a number of systems (eg, clays, unless they are pillared by specially introduced agents). The preservation of the porosity after the removal of the solvent upon evacuation plays a decisive role for gas-phase catalytic reactions, as well as for the separation and storage of gases. However, for catalysis in condensed phases, this property is not significant. 

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