A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are of the dimensions of small molecules, thus large molecules cannot be absorbed, while smaller molecules can. Many molecular sieves are used as desiccants. Some examples include activated charcoal and silica gel.
The 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).
- 1 Materials
- 2 Properties
- 3 Applications
- 4 Regeneration
- 5 Adsorption capabilities
- 6 3Å
- 7 4Å
- 8 5Å
- 9 See also
- 10 References
- 11 External links
Microporous material (<2 nm)
- Zeolites (aluminosilicate minerals, not to be confused with aluminium silicate)
- Zeolite LTA: 3–4 Å
- 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.
|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|
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.
The 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 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.
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)||Adsorbed water (% w/w)||Attrition or abrasion, W (% w/w)||Usage|
|3Å||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|
|4Å||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|
|5Å-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|
|5Å small oxygen-enriched||5||0.4–0.8||≥23||Specially designed for medical or healthy oxygen generator|
|5Å||5||0.60–0.65||20–21||0.3–0.5||Desiccation and purification of air; dehydration and desulphurization of natural gas and liquid 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|
- Approximate chemical formula: 2/3K2O•1/3Na2O•Al2O3• 2 SiO2 • 9/2 H2O
- Silica-alumina ratio: SiO2/ Al2O3≈2
3Å molecular sieves do not adsorb molecules whose diameters are larger than 3 Å. The characteristics of these molecular sieves include fast adsorption speed, frequent regeneration ability, good crushing resistance and pollution resistance. These features can improve both the efficiency and lifetime of the sieve. 3Å molecular sieves are the necessary desiccant in petroleum and chemical industries for refining oil, polymerization, and chemical gas-liquid depth drying.
3Å molecular sieves are used to dry a range of materials, such as ethanol, air, refrigerants, natural gas and unsaturated hydrocarbons. The latter include cracking gas, acetylene, ethylene, propylene and butadiene.
3Å molecular sieve is utilized to remove water from ethanol, which can later be used directly as a bio-fuel or indirectly to produce various products such as chemicals, foods, pharmaceuticals, and more. Since normal distillation cannot remove all the water (an undesirable byproduct from ethanol production) from ethanol process streams due to the formation of an azeotrope at around 95 percent concentration, molecular sieve beads are used to separate ethanol and water on a molecular level by adsorbing the water into the beads and allowing the ethanol to pass freely. Once the beads are full of water, temperature or pressure can be manipulated, allowing the water to be released from the molecular sieve beads.
3Å 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.
- Chemical formula: Na2O•Al2O3•2SiO2•9/2H2O
- Silica-alumina ratio: SiO2/ Al2O3≈2
4Å molecular sieves are widely used to dry laboratory solvents. They can absorb water and other molecules with a critical diameter less than 4 Å such as NH3, H2S, SO2, CO2, C2H5OH, C2H6, and C2H4. It is widely used in the drying, refining and purification of liquids and gases (such as the preparation of argon).
Polyester agent additives
These molecular sieves are used to assist detergents as they can produce demineralized water through calcium ion exchange, remove and prevent the deposition of dirt. They are widely used to replace phosphorus. The 4Å 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 a soap forming agent and in toothpaste.
Harmful waste treatment
4Å molecular sieves can purify sewage of cationic species such as ammonium ions, Pb2+, Cu2+, Zn2+ and Cd2+. Due to the high selectivity for NH4+ they have been successfully applied in the field to combat eutrophication and other effects in waterways due to excessive ammonium ions. 4Å molecular sieves have also been used to remove heavy metal ions present in water due to industrial activities.
- The metallurgical industry - separating agent, separation, extraction of brine potassium, rubidium, cesium, etc.
- Petrochemical industry, catalyst, desiccant, adsorbent
- Agriculture - soil conditioner;
- Medicine - load silver zeolite antibacterial agent.
- Chemical formula: 0.7CaO•0.30Na2O•Al2O3•2.0SiO2 •4.5H2O
- Silica-alumina ratio: SiO2/ Al2O3≈2
5Å 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. They contain tiny pores of a precise and uniform size, and are mainly used as an adsorbent for gases and liquids.
5Å molecular sieves are used to dry natural gas, along with performing desulfurization and decarbonation of the gas. They can also be used to separate mixtures of oxygen, nitrogen and hydrogen, and oil-wax n-hydrocarbons from branched and polycyclic hydrocarbons.
5Å molecular sieves are stored at room temperature, with a relative humidity less than 90% in cardboard barrels or carton packaging. The molecular sieves should not be directly exposed to the air and water, acids and alkalis should be avoided.
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