Molecular Sieve is crystalline metal alumina silicates having a three dimensional interconnecting network of silica and alumina tetrahedral. Natural water of hydration is removed from this network by heating to produce uniform cavities which selectively adsorb molecules of a specific size.

A 4 to 8-mesh sieve is normally used in gas phase applications, while the 8 to 12-mesh type is common in liquid phase applications. The powder forms of the 3A, 4A, 5A and 13X sieves are suitable for specialized applications.

Long known for their drying capacity (even to 90°C), molecular sieves have recently demonstrated utility in synthetic organic procedures, frequently allowing isolation of desired products from condensation reaction that are governed by generally unfavourable equilibria. These synthetic zeolites have been shown to removed water, alcohols (including methanol and ethanol), and HCI form such system as ketamine and enamine synthetics, ester condensation, and the conversation of unsaturated aldehydes to polyenals.