Synthesis and characterisation of aluminosilicate and framework modified sodalites

Abstract

The synthesis and characterisation of a wide range of sodalites has been achieved via ionic sustitution of both framework and non-framework species. These materials conform to the general formula M₈[ABO₄]₆.X₂, where M = Na, Li, K, Ag; A = Al and Ga; B = Si and Ge; and X includes Cl, Br, I, ClO₄, MnO₄ and SCN. Aluminosilicates were generally prepared by structure conversion of zeolite 4A at 800^oC, or from condensation reactions in basic solution at 120^oC. Gallium and germanium substituted sodalites have been synthesised using hydrothermal reaction between 90 and 180^oC, for which the type of framework precursor has been shown to significantly affect the product purity and crystallinity. Experiment has shown that gallogermanates are the most difficult of the framework substituted sodalites to prepare, with only the halides producing phase pure products. Post synthetic exchange of sodium by lithium and potassium was achieved via nitrate melt reactions in the temperature range 270-370^oC for 3-16 hours; silver was introduced by reflux with concentrated silver nitrate solution at 100^oC for 16 hours.

All of the sodalites adopt the space group P43n indicating a perfectly ordered alternating array of framework tetrahedra, thus showing that Loewenstein's rule of aluminium avoidance within zeolites can be extended to include framework linkages such as Ga-O-Ga. Rietveld profile refinement of powder x-ray and neutron diffraction data has been undertaken and yields structural parameters which have been used to investigate structure-property relationships revealing how the introduction of gallium and germanium produces subtle changes in bond lengths and angles. For a particular entrapped anion, the larger frameworks undergo greater relative cell collapse which is principally accommodated by co-operative rotations of the TO₄ tetrahedra manifested by variation in tetrahedral tilt angles and framework T-O-T bond angles.

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