THE STRUCTURE-CONTROLLING ROLE OF ORGANIC TEMPLATES FOR THE SYNTHESIS OF POROSILS IN THE SYSTEM SIO2/TEMPLATE/H2O

Hydrothermal syntheses of porosils (porous tectosilicates with framework composition SiO2) in the simplified system SiO2/template/H2O revealed the structure-controlling role of (mostly) organic molecules added as templates. Sixty-one molecules different in size, shape, and chemical character were used to synthesize 13 different silica host frameworks. The cage- or channel-like voids of the silica host frameworks were found to contain the templating molecules as guests. The templates show a clear preference for a certain type of void and, therefore, stabilize a certain porosil structure type. Size and shape of the template determine the size and shape of the void, demonstrating a remarkably good geometrical fit between the guest molecules and the host framework. The host-guest arrangement is optimized for building up van der Waals contacts as the most important interactions. The chemical character of the guest species, however, had no influence on the type of porosil structure formed. Among the experimental parameters of temperature, pressure, and concentration, the synthesis temperature has the most pronounced influence on the reaction product. From our results, one should be able to predict in the system SiO2/template/H2O the porosil structure type that is most likely to be formed for a given guest molecule.