Molecular sieve

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A sealed canister, commonly filled with silica gel and other molecular sieves used as desiccant in drug containers to keep contents dry; shown here next to a U.S. quarter for size comparison.
Vials of mesoporous silica

A molecular sieve is a material with very small holes of precise and uniform size. These holes are small enough to block large molecules while allowing small molecules to pass. Many molecular sieves are used as desiccants. Some examples include Activated charcoal and silica gel.[1]

The diameter of a molecular sieve is measured in Angstroms (Å) or nanometers (nm). The unit conversion for 1 Å is 0.1 nm.

According to IUPAC notation,[2] 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)
  • 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.[3]

Mesoporous material (2-50 nm)

Macroporous material (>50 nm)

Properties

Molecular sieves are used as 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.[7] 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.

Applications

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. For example, in the liquid natural gas (LNG) industry, the water content of the gas needs to be reduced to very low values (less than 1 ppmv) to prevent it from freezing (and causing blockages) in the cold section of LNG plants.

They are also used in the filtration of air supplies for breathing apparatus, for example those used by scuba divers and firefighters. In such applications, air is supplied by an air compressor and is passed through a cartridge filter which, dependent on the application, is filled with molecular sieve and/or activated carbon, finally being used to charge breathing air tanks.[8] Such filtration can remove particulates and compressor exhaust products from the breathing air supply.

Regeneration

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. Regeneration temperatures range from 175 °C to 315 °C depending on molecular sieve type.[9] In contrast, silica gel can be regenerated by heating it in a regular oven to 120 °C (250 °F) for two hours. However, some types of silica gel will "pop" when exposed to enough water. This is caused by breakage of the silica spheres when contacting the water.[10]

Adsorption capabilities

Model Pore diameter (Ångström) Bulk density (g/ml) Adsorbed water (% w/w) Attrition abrasion W (%) Usage[11]
3A 3 0.60~0.68 19~20 0.3~0.6 Desiccation of petroleum cracking gas and alkenes, selective adsorption of H2O in insulated glass (IG) and polyurethane
4A 4 0.60~0.65 20~21 0.3~0.6 Adsorption of water in sodium aluminosilicate which is FDA approved (see below) used as molecular sieve in medical containers to keep contents dry and as food additive having E-number E-554 (anti-caking agent); Preferred for static dehydration in closed liquid or gas systems, e.g., in packaging of drugs, electric components and perishable chemicals; water scavenging in printing and plastics systems and drying saturated hydrocarbon streams. Adsorbed species include SO2, CO2, H2S, C2H4, C2H6, and C3H6. Generally considered a universal drying agent in polar and nonpolar media;[9] separation of natural gas and alkenes, adsorption of water in non-nitrogen sensitive polyurethane
5A-DW 5 0.45~0.50 21~22 0.3~0.6 Degreasing and pour point depression of aviation kerosene and diesel, and alkenes separation
5A small oxygen-enriched 5 0.4-0.8 ≥23 ? Specially designed for medical or healthy oxygen generator[citation needed]
5A 5 0.60~0.65 20~21 0.3~0.5 Desiccation and purification of air; dehydration and desulphurization of natural gas; desulphurization of petroleum gas; oxygen and hydrogen production by pressure swing adsorption process
10X 8 0.50~0.60 23~24 0.3~0.6 High-efficient sorption, be used in desiccation, decarburization, desulphurization of gas and liquids and separation of aromatic hydrocarbon
13X 10 0.55~0.65 23~24 0.3~0.5 Desiccation, desulphurization and purification of petroleum gas and natural gas
13X-AS 10 0.55~0.65 23~24 0.3~0.5 Decarburization and desiccation in the air separation industry, separation of nitrogen from oxygen in oxygen concentrators
Cu-13X 10 0.50~0.60 23~24 0.3~0.5 Sweetening (removal of thiols) of aviation fuel and corresponding liquid hydrocarbons

FDA approval

The FDA has as of April 1, 2012 approved sodium aluminosilicate (sodium silicoaluminate) for direct contact with consumable items under 21 CFR 182.2727.[12] Prior to this approval Europe had used molecular sieves with pharmaceuticals and independent testing suggested that molecular sieves meet all government requirements but the industry had been unwilling to fund the expensive testing required for government approval.[13]

Distinction from 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. ↑ http://chemistry.about.com/od/chemistryglossary/g/Molecular-Sieve-Definition.htm
  2. ↑ J. Rouquerol et al. (1994). "Recommendations for the characterization of porous solids (Technical Report)" (free download pdf). Pure & Appl. Chem 66 (8): 1739–1758. doi:10.1351/pac199466081739. 
  3. ↑ Brindley, George W. (1952). "Structural mineralogy of clays". Clays and Clay Minerals 1: 33–43. Bibcode:1952CCM.....1...33B. doi:10.1346/CCMN.1952.0010105. 
  4. ↑ http://www.sorbentsystems.com/desiccants_types.html
  5. ↑ Mann, B. F.; Mann, A. K. P.; Skrabalak, S. E.; Novotny, M. V. (2013). "Sub 2-μm Macroporous Silica Particles Derivatized for Enhanced Lectin Affinity Enrichment of Glycoproteins". Analytical Chemistry 85 (3): 1905–1912. doi:10.1021/ac303274w. PMC 3586544. PMID 23278114. 
  6. ↑ http://www.faqs.org/patents/app/20100068474
  7. ↑ Molecular Sieves Study
  8. ↑ http://www.lawrence-factor.com/direc_filters.html
  9. ↑ 9.0 9.1 http://www.sigmaaldrich.com/chemistry/chemical-synthesis/learning-center/technical-bulletins/al-1430/molecular-sieves.html
  10. ↑ Spence Konde, "Preparation of High-Silica Zeolite Beads From Silica Gel," retrieved 2011-09-26
  11. ↑ Specification and Function
  12. ↑ "Sec. 182.2727 Sodium aluminosilicate.". U.S. Food and Drug Administration. 1 April 2012. Retrieved 10 December 2012. 
  13. ↑ Molecular Sieves' Use

External links

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