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Amolecular sieve is a material withpores of uniform size comparable to that of individualmolecules, linking the interior of the solid to its exterior. These materials embody themolecular sieve effect, in which molecules larger than the pores are preferentiallysieved, allowing for the selectiveadsorption of specific compounds based on their molecular size.[a] Many kinds of materials exhibit some molecular sieves, butzeolites dominate the field. Zeolites are almost alwaysaluminosilicates, or variants where some or all of the Si or Al centers are replaced by similarly charged elements.[2]
The diameters of the pores that comprise molecular sieves are similar in size to small molecules. Large molecules cannot enter or beadsorbed, while smaller molecules can. As a mixture of molecules migrates through the stationary bed of porous, semi-solid substance referred to as a sieve (or matrix), the components of the highest molecular weight (which are unable to pass into the molecular pores) leave the bed first, followed by successively smaller molecules. Most of molecular sieves arealuminosilicates (zeolites) with Si/Al molar ratio less than 2, but there are also examples ofactivated carbon andsilica gel.[2][3][4]
The pore diameter of a molecular sieve is measured inångströms (Å) ornanometres (nm). According toIUPAC notation,microporous materials have pore diameters of less than 2 nm (20 Å) andmacroporous materials have pore diameters of greater than 50 nm (500 Å); themesoporous category thus lies in the middle with pore diameters between 2 and 50 nm (20–500 Å).[5]
The sieving properties of molecular sieves are classified as
Some molecular sieves are used insize-exclusion chromatography, a separation technique that sorts molecules based on their size.
Another important use is as adesiccant. They are often utilized in thepetrochemical 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 1ppmv to prevent blockages caused by ice ormethane clathrate.
In the laboratory, molecular sieves are used to dry solvent. "Sieves" have proven to be superior to traditional drying techniques, which often employ aggressive desiccants.[7]
Under the term zeolites, molecular sieves are used for a wide range of catalytic applications. They catalyzeisomerisation,alkylation, andepoxidation, and are used in large scale industrial processes, includinghydrocracking and fluid catalyticcracking.[8]
They are also used in the filtration of air supplies for breathing apparatus, for example those used byscuba divers andfirefighters. In such applications, air is supplied by anair compressor and is passed through a cartridge filter which, depending on the application, is filled with molecular sieve and/oractivated carbon, finally being used to charge breathing air tanks.[9] Such filtration can remove particulates and compressor exhaust products from the breathing air supply.
The U.S. FDA has as of April 1, 2012, approvedsodium aluminosilicate for direct contact with consumable items under 21 CFR 182.2727.[10] 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.[11]
Methods for regeneration of molecular sieves include pressure change (as in oxygen concentrators), heating and purging with a carrier gas (as when used inethanol dehydration), or heating under high vacuum. Regeneration temperatures range from 175 °C (350 °F) to 315 °C (600 °F) depending on molecular sieve type.[12] 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.[13]
| Name | Alias | Pore diameter (Ångström) | Bulk density (g/mL) | Adsorbed water (% w/w) | Attrition or abrasion, W (% w/w) | Usage[14] |
|---|---|---|---|---|---|---|
| 3A | A-3, K-A | 3 | 0.60–0.68 | 19–20 | 0.3–0.6 | Desiccation ofpetroleum cracking gas and alkenes, selective adsorption of H2O ininsulated glass (IG) and polyurethane, drying ofethanol fuel for blending with gasoline. |
| 4A | A-4, Na-A | 4 | 0.60–0.65 | 20–21 | 0.3–0.6 | Adsorption of water insodium aluminosilicate which is FDA approved (seebelow) used as molecular sieve in medical containers to keep contents dry and asfood additive havingE-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;[12] separation ofnatural gas andalkenes, adsorption of water in non-nitrogen sensitivepolyurethane |
| 5A-DW | 5 | 0.45–0.50 | 21–22 | 0.3–0.6 | Degreasing and pour point depression ofaviationkerosene anddiesel, and alkenes separation | |
| 5A small oxygen-enriched | 5 | 0.4–0.8 | ≥23 | Specially designed for medical or healthy oxygen generator[citation needed] | ||
| 5A | A-5, Ca-A | 5 | 0.60–0.65 | 20–21 | 0.3–0.5 | Desiccation and purification of air;dehydration anddesulfurization of natural gas andliquid petroleum gas;oxygen andhydrogen production bypressure swing adsorption process |
| 10X | F-9, Ca-X | 8 | 0.50–0.60 | 23–24 | 0.3–0.6 | High-efficient sorption, used in desiccation, decarburization, desulfurization of gas and liquids and separation ofaromatic hydrocarbon |
| 13X | F-9, Na-X | 10 | 0.55–0.65 | 23–24 | 0.3–0.5 | Desiccation, desulfurization 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 | Cu-X | 10 | 0.50–0.60 | 23–24 | 0.3–0.5 | Sweetening (removal ofthiols) ofaviation fuel and correspondingliquid hydrocarbons |
3A molecular sieves are produced by cation exchange ofpotassium forsodium in 4A molecular sieves (See below)
3A molecular sieves do not adsorb molecules with diameters are larger than 3 Å. The characteristics of these molecular sieves include fast adsorption speed, frequent regeneration ability, good crushing resistance andpollution resistance. These features can improve both the efficiency and lifetime of the sieve. 3A molecular sieves are the necessary desiccant in petroleum and chemical industries for refining oil, polymerization, and chemical gas-liquid depth drying.
3A molecular sieves are used to dry a range of materials, such asethanol, air,refrigerants,natural gas andunsaturated hydrocarbons. The latter include cracking gas,acetylene,ethylene,propylene andbutadiene.3A molecular sieves are stored at room temperature, with a relative humidity not more than 90%. They are sealed under reduced pressure, being kept away from water, acids and alkalis.
For the production of 4A sieve, typically aqueous solutions ofsodium silicate andsodium aluminate are combined at 80 °C. The product is "activated" by "heating" at 400 °C[15] 4A sieves serve as the precursor to 3A and 5A sieves throughcation exchange ofsodium forpotassium (for 3A) orcalcium (for 5A)[16][17]
The main use of zeolitic molecular sieves is in laundry detergents. In 2001, an estimated 1200 kilotons of zeolite A were produced for this purpose, which entailswater softening.[2]
4A molecular sieves are widely used to dry laboratory solvents. They can absorb water and other species with a critical diameter less than 4 Å such as NH3, H2S, SO2, CO2, C2H5OH, C2H6, and C2H4.
Some molecular sieves are used to assist detergents as they can produce demineralized water throughcalcium ion exchange, remove and prevent the deposition of dirt. They are widely used to replacephosphorus. The 4A molecular sieve plays a major role to replace sodium tripolyphosphate as detergent auxiliary in order to mitigate the environmental impact of the detergent. It also can be used as asoap forming agent and intoothpaste.
Molecular sieves are available in diverse shape and sizes. Spherical beads have advantage over other shapes as they offer lower pressure drop and are mechanically robust.
