Ordered distribution of Al atoms in the framework of analcimes

Abstract

Ordered distributions of Al atoms in the framework of analcime have been deduced under the restriction of Loewenstein's Al–O–Al avoidance rule. Distributions of extra-framework cations, Na⁺ or Ca²⁺, were also determined for each Al configuration. From these distributions, one can determine which Al atoms are easily replaceable by an Si atom. The existence of such Al atoms leads to deviation of the composition from the symmetrically ideal, as is frequently observed with zeolites. The models obtained explain well the literature experimental results for structural data, allowed Al content, ion-exchange properties and ²⁹Si MAS NMR spectra. Furthermore, it was predicted that a highly siliceous (Si/Al=3) analcime of R3 symmetry will be very stable.

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