Molecular sieve

A sealed canister, commonly filled with silica gel and other molecular sieves used as desiccant in drug containers to keep contents dry. The US Quarter in the photo is 26.5 millimetres (1.04 in) in diameter.

Vials of mesoporous silica

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.^[1]

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).^[2]

Materials

Molecular sieves can be microporous, mesoporous, or macroporous material.

Microporous material (<2 nm)

• Zeolites (aluminosilicate minerals, not to be confused with aluminium silicate)
  • Zeolite LTA: 3–4 Å^[3]
• 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.^[4]

Mesoporous material (2–50 nm)

• Silicon dioxide (used to make silica gel): 24 Å (2.4 nm)^[5]

Macroporous material (>50 nm)

• Mesoporous silica, 200–1000 Å (20–100 nm)^[6]

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.

FDA approval

The FDA has as of April 1, 2012 approved sodium aluminosilicate for direct contact with consumable items under 21 CFR 182.2727.^[9] 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.^[10]

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.^[11] 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.^[12]

Adsorption capabilities

3Å

• 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, 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.^[14]

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 alkali's.

4Å

• Chemical formula: Na₂O•Al₂O₃•2SiO₂•9/2H₂O
• Silica-alumina ratio: SiO₂/ Al₂O₃≈2

4Å molecular sieves are widely used to dry laboratory solvents.^[15] 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).

Bottle of 4A molecular sieves.

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.

Other purposes

1. The metallurgical industry - separating agent, separation, extraction of brine potassium, rubidium, cesium, etc.
2. Petrochemical industry, catalyst, desiccant, adsorbent
3. Agriculture - soil conditioner;
4. Medicine - load silver zeolite antibacterial agent.

5Å

• 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 contains tiny pores of a precise and uniform size, and it 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.

See also

• Desiccant
• Activated carbon
• Lime (mineral)
• Silica gel
• Zeolite

References

External links

• 'MOLE SIEVE DEHYDRATION DESIGN AND TROUBLESHOOTING'
• Sieves Put A Lid On Greenhouse Gas
• Molecular Sieve Safety