Molecular sieve
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A '''molecular sieve''' is a material containing tiny pores of a precise and uniform size that is used as an [[adsorption |adsorbent]] for [[gas]]es and [[liquid]]s.
[[Molecule]]s small enough to pass through the pores are [[adsorption |adsorbed]] while larger molecules are not. It is different from a common filter in that it operates on a molecular level. For instance, a water molecule may be small enough to pass through while larger molecules are not. Because of this, they often function as a [[desiccant]]. A molecular sieve can adsorb water up to 22% of its own weight.<ref name="molecularsieve.org">[http://www.molecularsieve.org/ Molecular sieve,and molecular sieves study<!-- Bot generated title -->]</ref>
Often they consist of [[aluminosilicate]] [[mineral]]s, [[clay]]s, porous [[glass]]es, microporous [[charcoal]]s, [[zeolite]]s, active [[carbon]]s, 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 (element)|mercury]] content of natural gas is extremely harmful to the aluminum 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 fuel|ethanol]] dehydration), or heating under high vacuum.
==Adsorption capabilities<ref name="Fieser and Fieser">Feiser and Feiser, Reagents for Organic Synthesis, Vol. 1, Wiley, New York, 1967, p. 703</ref><ref name="Breck">Breck, D.W. J. Chem. Ed., 41, 678 (1964)</ref>==
* 3A (pore size 3 Å): Adsorbs NH<sub>3</sub>, H<sub>2</sub>O, (not C<sub>2</sub>H<sub>6</sub>), good for drying polar liquids.
* 4A (pore size 4 Å): Adsorbs H<sub>2</sub>O, CO<sub>2</sub>, SO<sub>2</sub>, H<sub>2</sub>S, C<sub>2</sub>H<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, C<sub>3</sub>H<sub>6</sub>, EtOH. Will not adsorb C<sub>3</sub>H<sub>8</sub> and higher hydrocarbons. Good for drying nonpolar liquids and gases.
* 5A (pore size 5 Å): Adsorbs normal (linear) hydrocarbons to n-C<sub>4</sub>H<sub>10</sub>, alcohols to C<sub>4</sub>H<sub>9</sub>OH, mercaptans to C<sub>4</sub>H<sub>9</sub>SH. Will not adsorb isocompounds or rings greater than C<sub>4</sub>.
* 10X (pore size 8 Å): Adsorbs branched hydrocarbons and aromatics. Useful for drying gases.
* 13X (pore size 10 Å): Adsorbs di-n-butylamine (not tri-n-butylamine). Useful for drying [[hexamethylphosphoramide|HMPA]].
==See also==
Some of the many types of molecular sieves are:
*[[Activated carbon]]
*[[Desiccant]]
*[[Lime (mineral)]]
*[[Silica gel]]
*[[Zeolite]]
== References ==
{{Reflist}}
== External links ==
* [http://www.physorg.com/news65284632.html Sieves put a lid on greenhouse gas]
* [http://www.patentinformationsearch.com/molecular_sieve/index.php molecular sieve patents]
* [http://www.molecularsieve.org/ Molecular sieve faq.]
* [http://www.zettapac.com/molecular-sieve-details.html#_opencycle An Open Cycle Molecular Sieve Dehydration System.]
* [http://www.sigmaaldrich.com/Brands/Aldrich/Tech_Bulletins/AL_143/Molecular_Sieves Sigma-Aldrich]
[[Category:Filters]]
[[Category:Desiccants]]
[[Category:Vacuum]]
[[Category:Chemical engineering]]
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