Clathrates, template synthesis

The zeolites are aluminosilicate framework minerals of general formula M", [AI4Sil0lr+>JJ -zH20.y They are characterized by open structures that permit exchange of catioas and water molecules (Fig. 16.2). In the synthetic zeolites the aperture and channel sizes may sometimes be controlled by a sort of template synthesis—the zeolite is synthesized around a particular organoammonium canon. This yields channels of the desired size. The zeolite framework thus behaves in some ways like a clathrate cage about a guest molecule (Chapter 8). The synthesis of zeolites also involves several other factors such as the Al/Si ratio, the pH. the temperature and pressure, and the presence or absence of seed crystals - ... 

At the opposite, there are microporous solids with covaloit bonded framewoilcs (e. g., silicon fnunewoite with clathrate-type stractures of 4.1.2).For the time bdng, however, they are few in number. Synthesis difficulties (reactivity, templating possibility, directivity of the bonds) are probably more responsible of that situadtm than stability problems. 

A series of silica end-members of zeolites (zeosils) as well as clathrate compounds with framework composition Si02 (clathrasils) have been synthesized in the recent past [1, 2). It has been concluded that the organic guest species alone act as templates for the formation of the open framework and that these guest molecules have a structure directing role. In a detailed study it has been shown that the size and dimensionality of the pore system in porous tectosilicates is dependent on the properties of the guest species used for synthesis [3]. 

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