A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecules migrate through the stationary bed of porous, semi-solid substance referred to as a sieve (or matrix), the components of highest molecular weight (which are unable to pass into the molecular pores) leave the bed first, followed by successively smaller molecules. Some molecular sieves are used in size-exclusion chromatography, a separation technique that sorts molecules based on their size. Other molecular sieves are used as desiccants (some examples include activated charcoal and silica gel). The pore diameter of a molecular sieve is measured in ångströms (Å) or nanometres (nm). According to IUPAC notation, microporous materials have pore diameters of less than 2 nm (20 Å) and macroporous materials have pore diameters of greater than 50 nm (500 Å); the mesoporous category thus lies in the middle with pore diameters between 2 and 50 nm (20–500 Å). Materials Molecular sieves can be microporous, mesoporous, or macroporous material. Microporous material (<2 nm) Zeolites (aluminosilicate minerals, not to be confused with aluminium silicate) Zeolite LTA: 3–4 Å Porous glass: 10 Å (1 nm), and up Active carbon: 0–20 Å (0–2 nm), and up Clays Montmorillonite intermixes Halloysite (endellite): Two common forms are found, when hydrated the clay exhibits a 1 nm spacing of the layers and when dehydrated (meta-halloysite) the spacing is 0.7 nm. Halloysite naturally occurs as small cylinders which average 30 nm in diameter with lengths between 0.5 and 10 micrometres. Mesoporous material (2–50 nm) Silicon dioxide (used to make silica gel): 24 Å (2.4 nm) Macroporous material (>50 nm) Mesoporous silica, 200–1000 Å (20–100 nm) Applications Molecular sieves are often utilized in the petroleum industry, especially for drying gas streams. For example, in the liquid natural gas (LNG) industry, the water content of the gas needs to be reduced to less than 1 ppmv to prevent blockages caused by ice or methane clathrate. In the laboratory, molecular sieves are used to dry solvent. "Sieves" have proven to be superior to traditional drying techniques, which often employ aggressive desiccants. Under the term zeolites, molecular sieves are used for a wide range of catalytic applications. They catalyze isomerisation, alkylation, and epoxidation, and are used in large scale industrial processes, including hydrocracking