A molecular sieve is a material with very small holes of precise and uniform size. These holes are small enough to block large molecules and allow small molecules to pass. Many molecular sieves are used as desiccants. Examples: Activated charcoal and silica gels are molecular sieves.
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).
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
- 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.
- Montmorillonite intermixes
Mesoporous material (2-50 nm)
Macroporous material (>50 nm)
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. 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.
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. Such filtration can remove particulates and compressor exhaust products from the breathing air supply.
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. 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.
|Model||Pore diameter (Ångström)||Bulk density (g/ml)||Water absorbing||Attrition abrasion W (%)||Used in|
|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; 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|
|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 absorption 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 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 of aviation and corresponding liquid hydrocarbons|
The FDA has as of April 1, 2012 approved sodium aluminosilicate (sodium silicoaluminate) for direct contact with consumable items under 21 CFR 182.2727. 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.
Distinction from zeolite
|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|
- 3A Molecular sieve
- 4A Molecular Sieve
- 5A Molecular Sieve
- Activated carbon
- Lime (mineral)
- Silica gel
- 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.
- Brindley, George W. (1952). "Structural mineralogy of clays". Clays and Clay Minerals 1: 33–43. doi:10.1346/CCMN.1952.0010105.
- Molecular Sieves Study
- Spence Konde, "Preparation of High-Silica Zeolite Beads From Silica Gel," retrieved 2011-09-26
- Specification and Function
- "Sec. 182.2727 Sodium aluminosilicate.". U.S. Food and Drug Administration. 1. Retrieved 10 December 2012.
- Molecular Sieves' Use