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A molecular sieve is a material with pores (voids or holes), having uniform size comparable to that of individual molecules, linking the interior of the solid to its exterior. These materials embody the molecular sieve effect, the preferential sieving of molecules larger than the pores.
Size-exclusion chromatography, also known as molecular sieve chromatography, [1] is a chromatographic method in which molecules in solution are separated by their shape, and in some cases size. [2] It is usually applied to large molecules or macromolecular complexes such as proteins and industrial polymers . [ 3 ]
The typical molecular sieve used is a synthetic zeolite with a pore diameter around 0.4 nanometer ( Type 4A ) and a surface area of about 500 m 2 /g. The sorption pump contains between 300 g and 1.2 kg of molecular sieve. A 15-liter system will be pumped down to about 10 −2 mbar by 300 g molecular sieve. [1]
For example, the major molecular sieves, 3A, 4A and 5A, are all LTA (Linde Type A). Most commercially available natural zeolites are of the MOR, HEU or ANA-types. An example of the notation of the ring structure of zeolite and other silicate materials is shown in the upper right figure.
Canisters are commonly filled with silica gel and other molecular sieves as desiccants in drug containers to keep contents dry Silica gel in a sachet or porous packet. A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness (desiccation) in its vicinity; it is the opposite of a humectant.
A nitrogen generator Bottle of 4Å molecular sieves. Pressure swing adsorption provides separation of oxygen or nitrogen from air without liquefaction. The process operates around ambient temperature; a zeolite (molecular sponge) is exposed to high pressure air, then the air is released and an adsorbed film of the desired gas is released.
Classic oxygen concentrators use two-bed molecular sieves; newer concentrators use multi-bed molecular sieves. The advantage of the multi-bed technology is the increased availability and redundancy, as the 10 L/min molecular sieves are staggered and multiplied on several platforms. With this, over 960 L/min can be produced.
For low boiling azeotropes distillation may not allow the components to be fully separated, and must make use of separation methods that does not rely on distillation. A common approach involves the use of molecular sieves. The sieves can be subsequently regenerated by dehydration using a vacuum oven.