Zeolites

Zeolites are a special, industrially and environmentally important class of three-dimensional framework silicates (see the previous section) in which a fraction of the framework's silicon atoms have been isomorphously replaced by aluminium atoms. Because aluminium contributes only three valence electrons to bonding compared with silicon's four, each such substitution leaves the overall aluminosilicate framework with one unit of negative charge for every aluminium atom introduced. This negative framework charge is balanced by "exchangeable" cations — typically $\text{Na}^+$, $\text{K}^+$ or $\text{Ca}^{2+}$ — that sit loosely within the cavities and channels of the structure rather than being covalently bonded into the rigid $\text{Si/Al}$–O framework itself. A general formula for a zeolite can be written as $M_{x/n}[(\text{AlO}_2)_x(\text{SiO}_2)_y]\cdot z\text{H}_2\text{O}$, where $M^{n+}$ is the balancing cation.

The defining structural feature of zeolites is that this rigid aluminosilicate framework encloses a regular, crystallographically well-defined network of cavities and channels of essentially molecular dimensions — different zeolite structures have different, precisely fixed pore sizes. This gives zeolites their characteristic "molecular sieve" behaviour: molecules smaller than a given zeolite's pore opening can diffuse freely into (and react within, or be adsorbed by) the internal cavity structure, while larger molecules are simply excluded, unable to enter at all. …