Molecular sieve

Molecular sieve
A molecular sieve inside an antibiotics container (contents removed). See below for size comparison image.
One common type of a molecular sieve used in medicinal containers to keep contents dry; shown here next to a U.S. quarter for size comparison.

A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.

Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a molecular level and traps the adsorbed substance. For instance, a water molecule may be small enough to pass through the pores while larger molecules are not, so water is forced into the pores which act as a trap for the penetrating water molecules, which are retained within the pores. Because of this, they often function as a desiccant. A molecular sieve can adsorb water up to 22% of its own weight.[1] The principle of adsorption to molecular sieve particles is somewhat similar to that of size exclusion chromatography, except that without a changing solution composition, the adsorbed product remains trapped because in the absence of other molecules able to penetrate the pore and fill the space, a vacuum would be created by desorption.

Often they consist of aluminosilicate minerals, clays, porous glasses, microporous charcoals, zeolites, active carbons, or synthetic compounds that have open structures through which small molecules, such as nitrogen and water can diffuse.

Molecular sieves are often utilized in the petroleum industry, especially for the purification of gas streams and in the chemistry laboratory for separating compounds and drying reaction starting materials. The mercury content of natural gas is extremely harmful to the aluminium piping and other parts of the liquefaction apparatus—silica gel is used in this case.

Methods for regeneration of molecular sieves include pressure change (as in oxygen concentrators), heating and purging with a carrier gas (as when used in ethanol dehydration), or heating under high vacuum. Temperatures typically used to regenerate water-adsorbed molecular sieves range from 130 °C to 250 °C.[2]

Contents

Adsorption capabilities

  • 3A (pore size 3 Å): Adsorbs NH3, H2O, (not C2H6), good for drying polar liquids.
  • 4A (pore size 4 Å): Adsorbs H2O, CO2, SO2, H2S, C2H4, C2H6, C3H6, ethanol. Will not adsorb C3H8 and higher hydrocarbons. Good for drying nonpolar liquids and gases.
  • 5A (pore size 5 Å): Adsorbs normal (linear) hydrocarbons to n-C4H10, alcohols to C4H9OH, mercaptans to C4H9SH. Will not adsorb isocompounds or ring structures with more than four carbon atoms.
  • 10x (pore size 8 Å): Adsorbs branched hydrocarbons and aromatics. Used for drying gases.
  • 13x (pore size 10 Å): Adsorbs di-n-butylamine (not tri-n-butylamine). Used for drying HMPA.[3][4]

FDA approval

The United States FDA has not yet approved molecular sieve for direct contact with consumable items, although in Europe it is used with pharmaceuticals. Lack of government approval for the use of molecular sieves in food and drug packaging has limited its more widespread use. Independent testing suggests that molecular sieves meet all government requirements but the industry has been unwilling to fund the expensive testing required for government approval.[5]

Difference between molecular sieves and zeolite

Molecular sieves Zeolites
Able to distinguish materials on the basis of their size Special class of molecular sieves with aluminosilicates as skeletal composition
May be crystalline, non-crystalline, para-crystalline or pillared clays Highly crystalline materials
Variable framework charge with porous structure Anionic framework with microporous and crystalline structure

See also

References

  1. ^ Molecular Sieves Study
  2. ^ Sudhir Joshi, James R. Fair (November 1988). "Adsorptive drying of toluene". Ind. Eng. Chem. Res. 27 (11): 2078–2085. 
  3. ^ Fieser; Fieser (1967). Reagents for Organic Synthesis. 1. New York: Wiley. p. 703. 
  4. ^ Breck, D. W. (1964). "Crystalline molecular sieves". J. Chem. Ed. 41 (12): 678. Bibcode 1964JChEd..41..678B. doi:10.1021/ed041p678. 
  5. ^ Molecular Sieves' Use

External links


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Look at other dictionaries:

  • molecular sieve — n. any of a class of zeolites or similar materials, natural or synthetic, having small, precisely uniform, pores in their crystal lattices that can absorb molecules small enough to pass through the pores: used as for separating or drying gases… …   English World dictionary

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  • molecular-sieve chromatography — molekulinių sietų chromatografija statusas T sritis chemija apibrėžtis Chromatografija, pagrįsta makromolekulių išskirstymu pagal jų matmenis ir pavidalą kolonėlėje su molekuliniais sietais. atitikmenys: angl. molecular sieve chromatography; size …   Chemijos terminų aiškinamasis žodynas

  • molecular sieve — Chem. a compound with molecule size pores, as some sodium aluminum silicates, that chemically locks molecules in them: used in purification and separation processes. [1925 30] * * * ▪ chemistry       a porous solid, usually a synthetic or a… …   Universalium

  • molecular sieve — molec′ular sieve′ n. chem. a compound with molecule size pores, as some sodium aluminum silicates, that chemically locks molecules in them: used in purification and separation processes • Etymology: 1925–30 …   From formal English to slang

  • molecular sieve — noun Date: 1869 a crystalline substance (as a zeolite) characterized by uniformly sized pores of molecular dimension that can adsorb small molecules and is used especially in separations …   New Collegiate Dictionary

  • molecular sieve — a crystalline substance having uniform pores of molecular size that adsorbs smaller but not larger molecules; used in chemical separation …   Medical dictionary

  • molecular sieve — noun : a crystalline substance (as a zeolite) that is characterized by pores of molecular dimensions and uniform size formed by heating to drive off the water of hydration and that by its ability to adsorb small molecules but not large ones can… …   Useful english dictionary

  • molecular sieve — noun a material, usually a zeolite silicate mineral, having pores of a precise and uniform size, that can be used to separate molecules of different shape and size …   Wiktionary

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