Q.What are zeolites? Describe their general structure and explain how their negatively charged aluminosilicate framework is charge-balanced.

[!TLDR] Zeolites are 3-D aluminosilicate frameworks where some Si is replaced by Al, leaving a negative charge balanced by loosely-held exchangeable cations like Na+/Ca2+. [!ANSWER] Zeolites are crystalline, 3-D framework aluminosilicates (some SiO4 tetrahedra replaced by AlO4); each Al substitution leaves one unit …

Zeolites are a special class of three-dimensional framework silicates, in which the fully corner-shared $\text{SiO}_4$ tetrahedral network (which, on its own, as in quartz, carries no net charge since every oxygen is shared equally between two silicons) has a fraction of its silicon atoms replaced by aluminium atoms, isomorphously substituted into the same tetrahedral framework positions. Because aluminium contributes only three valence electrons to bonding compared with silicon's four, each such $\text{Si}\to\text{Al}$ substitution leaves the surrounding framework with one extra unit of negative charge. This negative charge is balanced not by further covalent bonding within the rigid framework, but by loosely-associated exchangeable cations — commonly $\text{Na}^+$, $\text{K}^+$ or $\text{Ca}^{2+}$ — which sit within the regular cavities and channels that run through the crystal structure, rather than being fixed covalently in place. A general formula is $M_{x/n}[(\text{AlO}_2)_x(\text{SiO}_2)_y]\cdot z\text{H}_2\text{O}$, where $M^{n+}$ is the compensating cation. Because these cations are held only loosely (electrostatic …

Describe zeolites as framework silicates with isomorphous Si-to-Al substitution, explain why each substitution creates a negative charge, an …

Forgetting that the parent framework silicate (all-silicon) is neutral, so the charge specifically arises from the Al substitution; describing the balancing cations as co …