Zeolites

Zeolites are microporous crystalline aluminosilicates composed of a three-dimensional framework of corner-sharing SiO4 and AlO4 tetrahedra. The tetrahedra create channels and cages that can host water and other molecules. Substitution of silicon by aluminum introduces negative charges on the framework, which are balanced by extra-framework cations such as Na+, K+, or Ca2+. These cations and the hydrated pores confer zeolites with ion-exchange capacity, hydrophilicity, and a strong affinity for polar molecules.

Zeolite pores are rigid and highly specific in size and shape, giving rise to selective adsorption and catalytic properties. Frameworks are divided by pore size into microporous materials, typically less than 2 nanometers. Zeolites occur naturally (for example clinoptilolite, mordenite, chabazite) and are also synthesized in laboratories. Well-known synthetic zeolites include FAU-type faujasite (X, Y), LTA-type zeolite A, MFI-type ZSM-5, and BEA-type beta.

Industrial uses cover ion exchange for water softening, catalytic processes in petroleum refinement (acid catalysis in ZSM-5 and related materials), and separation and purification of gases and liquids. Zeolites also play a role in environmental applications such as CO2 capture and removal of volatile organic compounds. Their stability under heat and acid conditions and the ability to tailor pore size and acidity by composition make them versatile catalysts and adsorbents.

Synthesis commonly involves hydrothermal crystallization of gels containing silica and alumina sources under alkaline conditions, often with structure-directing agents to control the framework type. Post-synthesis modifications, such as dealumination or ion exchange, further tune acidity, hydrophilicity, and pore characteristics. The International Zeolite Association classifies zeolites by framework type and topology, highlighting the wide diversity of available structures.