thumb|Typical molecular sieves are of the LTA type. They feature sodium aluminosilicates cages (sodium not shown) that have high affinity for water.
thumb|Vials of [[mesoporous silica]]
A molecular sieve is a material with pores of uniform size comparable to that of individual molecules, linking the interior of the solid to its exterior. These materials embody the molecular sieve effect, in which molecules larger than the pores are preferentially sieved, allowing for the selective adsorption of specific compounds based on their molecular size. Many kinds of materials exhibit some molecular sieves, but zeolites dominate the field. Zeolites are almost always aluminosilicates, or variants where some or all of the Si or Al centers are replaced by similarly charged elements.
The pore diameter of a molecular sieve is measured in ångströms (Å) or nanometres (nm). 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).
The sieving properties of molecular sieves are classified as
- microporous (3-10 Å pores) including zeolite A, LTA, and FAU. Some clays, active carbon, and porous glass meet this criterion.
- mesoporous materials (< 2 nm pores)
- macroporous materials (2–50 nm pores), e.g., in the form of Silicon dioxide (used to make silica gel): 24 Å (2.4 nm)
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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.-->
Applications
Some molecular sieves are used in size-exclusion chromatography, a separation technique that sorts molecules based on their size.
Another important use is as a desiccant. They are often used in the petrochemical industry for drying gas streams. For example, in the liquid natural gas (LNG) industry, the water content of the gas needs to be reduced to less than 1 ppmv to prevent blockages caused by ice or methane clathrate.
Laboratory use
In the laboratory, molecular sieves are used to dry solvents. Molecular sieves have proven to be superior to traditional drying techniques, which often employ aggressive desiccants.
Under the term zeolites, molecular sieves are used for a wide range of catalytic applications. They catalyze isomerisation, alkylation, and epoxidation, and are used in large scale industrial processes, including hydrocracking and fluid catalytic cracking.
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, depending on the application, is filled with molecular sieve 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.
FDA approval
The US FDA has as of April 1, 2012, approved sodium aluminosilicate for direct contact with consumable items under 21 CFR 182.2727. Prior to this approval the European Union 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.
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 depending on molecular sieve type. In contrast, silica gel can be regenerated by heating it in a regular oven to 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.
Adsorption capabilities
{| class="wikitable"
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! Name !! Alias !! Pore diameter (Ångström) !! Bulk density (g/mL) !! Adsorbed water (% w/w) !! Attrition or abrasion, W (% w/w) !! Usage
|-
| 3A || A-3, K-A || 3 || 0.60–0.68 ||19–20 || 0.3–0.6 || Desiccation of petroleum cracking gas and alkenes, selective adsorption of H<sub>2</sub>O in insulated glass (IG) and polyurethane, drying of ethanol fuel for blending with gasoline.
|-
| 4A || A-4, Na-A || 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 SO<sub>2</sub>, CO<sub>2</sub>, H<sub>2</sub>S, C<sub>2</sub>H<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, and C<sub>3</sub>H<sub>6</sub>. Generally considered a universal drying agent in polar and nonpolar media; 4A sieves serve as the precursor to 3A and 5A sieves through cation exchange of sodium for potassium (for 3A) or calcium (for 5A)
Uses
The main use of zeolitic molecular sieves is in laundry detergents. In 2001, an estimated 1.2 million tons of zeolite A were produced for this purpose, which entails water softening.