Zeolite A (LTA): structure, molecular sieves and uses

Zeolite A is one of the zeolites, crystalline aluminosilicates whose SiO₄ and AlO₄ tetrahedra share corners to enclose cages and channels holding water and loosely bound cations that balance the charge of the AlO₄ units (Liguori et al. 2019). Its framework type is LTA, for Linde Type A.

The framework is built from double four-rings (D4R), its main secondary building unit (Wang et al. 2021). Spectroscopy during synthesis shows a gel rich in D4R units from which both kinds of cage appear as the crystals form (Ren et al. 2011): small β (sodalite) cages and large α (lta) cages, arranged on a body-centred cubic pattern (Chen et al. 2025; Ooe et al. 2023). Zeolite A shares its polyhedral building units with zeolite X of the faujasite family (Liguori et al. 2019).

The α cages connect through eight-membered rings 4.1 × 4.1 Å across, the only way into the large cavities (Chen et al. 2025). With Si/Al = 1, the maximum aluminium content (Ooe et al. 2023), the framework carries a high negative charge and many cations. In the sodium form there are 12 Na⁺ per unit cell (Chen et al. 2025), spread over three sites: at the six-rings (site I), in the eight-rings (site II) and at the four-rings (site III). Na-LTA is cubic with a = 24.555 Å (Ooe et al. 2023).

Because every eight-ring window has a cation sitting in it, exchanging the cation changes what can pass. The three commercial molecular sieves are forms of the same LTA framework:

• 4A is the sodium form, NaA. Its Na⁺ ions sit in the plane of the eight-rings and act as door-keepers: a gas molecule gets in only when it pushes the cation aside, and NaA admits small krypton more readily than larger xenon (Chen et al. 2025).
• 3A is a potassium-containing (K/Na) form with Si/Al ≈ 1 (Wu et al. 2019). Its micropores are about 3 Å, small enough to keep out dichloromethane (kinetic diameter 3.3 Å) while taking up water (Jović et al. 2017).
• 5A is calcium-exchanged LTA (Chen et al. 2025). Divalent Ca²⁺ means fewer cations are needed to balance the framework, leaving cation sites vacant, and Ca²⁺ prefers the six-ring sites over the eight-rings. In silver–calcium LTA, exchanging half the cation sites for Ca²⁺ unblocked the windows enough for nitrogen to enter at 77 K (Chen et al. 2025).

The same door-keeper idea can be tuned further: replacing Na⁺ by Ag⁺ reverses the selectivity so that larger xenon is admitted while krypton is kept out, with an IAST Xe/Kr selectivity over 1600 (Chen et al. 2025).

• Detergent builder. Synthetic zeolites have been replacing phosphates as builders in laundry detergents; they are hygroscopic ion exchangers, and the intact zeolite is not bioavailable by mouth, skin or inhalation (Fruijtier-Pölloth 2009). Zeolite 4A removes calcium ions from the wash water during the washing cycle (Hui and Chao 2006), and a 4A made from coal fly ash reached a cation exchange capacity of 4.5 meq Ca²⁺ g⁻¹ (Cardoso et al. 2015).
• Drying solvents and gases. Molecular sieve 3A dries dichloromethane to ppm levels in a packed bed: the solvent is excluded from the micropores, so water adsorbs as it would from pure water vapour (Jović et al. 2017). A good zeolite A takes up about 26–28% of its weight in water at 16 torr and 298 K (Aprea et al. 2011).
• Gas separation and CO₂ capture. The 4.1 Å opening admits CO₂ (3.3 Å); LTA made from kaolin clay adsorbed 5.2 mmol g⁻¹ CO₂ at 273 K and kept its capacity over 50 adsorption–desorption cycles (Li et al. 2026). Cation-exchanged LTA separates xenon from krypton (Chen et al. 2025).

For a paper, slide or poster, show three levels: a single double four-ring (a cube of eight tetrahedral atoms), a sodalite (β) cage, and the cubic cell with the large α cage in the middle and its eight-ring windows facing each neighbour. Draw the framework as T–T sticks or as SiO₄/AlO₄ tetrahedra, and add the cations in the windows when the molecular-sieve action matters. In the LabCanvas figure editor the Library has a zeolite object and an LTA crystal file that draws the framework as a high-resolution SVG; the LabCanvas 3D Studio has an LTA model you can turn, cut and export as an image.