As the name might suggest these are molecules that can sieve out other molecules, acting like a filter but on the molecular scale. They are usually composed of a highly porous mineral or organometallic compound, the tiny pores of which are usually of uniform size and shape.
Clays, porous glasses, microporous charcoals, active carbons, aluminosilicate minerals, zeolites, and various synthetic compounds which we’ve discussed in Elemental Discoveries in the past allow much smaller molecules to enter and either pass through or be adsorbed on to the inside surface of the molecular sieve. As such. molecular sieves can be used to absorb liquids and gases. For instance, water molecules are often small enough to enter such porous materials while larger molecules are not, making molecular sieves useful as drying agents, or desiccants. A molecular sieve can absorb water up to 22% of its own weight. The petroleum industry makes wide use of molecular sieves for purifying gas streams. You can find out more here.