Molecular sieve

A molecular sieve is a material with pores of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, 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. 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. Most of molecular sieves are aluminosilicate zeolites with Si/Al molar ratio less than 2, but there are also examples of activated charcoal and silica gel.

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).

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
  • Zeolite FAU: 7-10 Å
• 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.

Mesoporous material (2–50 nm)

• Silicon dioxide (used to make silica gel): 24 Å (2.4 nm)

Macroporous material (>50 nm)

• Macroporous silica, 200–1000 Å (20–100 nm)

Applications

Molecular sieves are often utilized in the petroleum industry, especially 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.

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.

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 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.

Morphology of molecular sieves

Molecular sieves are available in diverse shape and sizes. Molecular sieve can be used directly in its powered form in many applications, such as in insulated glass or coatings. However, to make handling the material more manageable, especially for regenerative processes, molecular sieve powder is commonly mixed with a clay binder material and formed into an extrudate or a bead before being activated.

Originally, the molecular sieve zeolite and clay binder mixture was extruded into a pellet form. While pellets are still commonly used, manufacturers eventually began forming the mixture into a beaded form, which provides more favorable adsorption characteristics. Depending on the process conditions and the size of the process chamber, both extrudates and beads are available in a range of nominal sizes. For example, industrial ethanol dehydration processes use a larger 4x8 mesh bead because the process chambers are large, up to 40 feet tall or more. On the other end, a portable oxygen concentrator uses a smaller bead size, approximately 20x40 mesh, which allows higher output in a smaller vessel size by improving the adsorption/desorption rate of the molecular sieve.

Spherical beads have advantages over other shapes as they offer lower pressure drop, are attrition resistant as they do not have any sharp edges, and have good strength, i.e. crush force required per unit area is higher. Certain beaded molecular sieves offer lower heat capacity thus lower energy requirements during regeneration. The other advantage of using beaded molecular sieves is that bulk density is usually higher than other shapes, thus for a given adsorption requirement, the molecular sieve volume required is less. While de-bottlenecking, one may use beaded molecular sieves to load more adsorbent in the same volume while avoiding vessel modifications.

FDA approval

The U.S. 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 175 °C (350 °F) to 315 °C (600 °F) depending on molecular sieve type.

Adsorption capabilities

Zeolite

Zeolites are captivating minerals that can either occur naturally or be created synthetically to meet specific industrial needs. The natural formation of zeolite entails a fascinating combination of volcanic activity and groundwater. It originates from volcanic ash interacting with alkaline groundwater over thousands of years, resulting in the creation of porous crystalline structures, endowing this material with unique properties that make it valuable in various industrial processes. The volcanic ash, rich in silica and alumina, acts as the building blocks for the porous structures of molecular sieves. As water percolates through these ash deposits, it gradually dissolves some of the minerals and carries them along. Over time, these dissolved minerals recombine under specific conditions to form the crystalline framework that defines zeolite structures. This natural process gives rise to the remarkable adsorption capabilities that zeolite is renowned for. Owing to its natural structure, zeolite has a high affinity for water molecules, making it particularly useful in applications where water removal is essential, such as gas drying processes.

3A

• Approximate chemical formula: ((K₂O)_2⁄3 (Na₂O)_1⁄3) • Al₂O₃• 2 SiO₂ • 9/2 H₂O
• Silica-alumina ratio: SiO₂/ Al₂O₃≈2

Production

3A molecular sieves are produced by cation exchange of potassium for sodium in 4A molecular sieves.

Usage

3A 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. 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 as ethanol, air, refrigerants, natural gas and unsaturated hydrocarbons. The latter include cracking gas, acetylene, ethylene, propylene and butadiene.

3A molecular sieve is utilized to remove water from ethanol, which can later be used directly as a biofuel or indirectly to produce various products such as chemicals, foods, pharmaceuticals, and more. Due to the formation of an azeotrope at approximately 95.6 percent concentration by weight, normal distillation alone cannot remove all the water (an undesirable byproduct from ethanol production) from ethanol process streams. To break the azeotrope, molecular sieve is used to separate ethanol and water on a molecular level by adsorbing water into the 3A molecular sieve crystal while allowing the ethanol to pass freely. Once the molecular sieve is saturated with water, temperature or pressure can be manipulated to allow water to release from the molecular sieve in a process called regeneration.

3A molecular sieves should be 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.

4A

• Chemical formula: Na₂O•Al₂O₃•2SiO₂•9/2H₂O
• Silicon-aluminium ratio: 1:1 (SiO₂/ Al₂O₃≈2)

Production

Production of 4A sieve is relatively straightforward as it requires neither high pressures nor particularly high temperatures. Typically, aqueous solutions of sodium silicate and sodium aluminate are combined at 80°C. The solvent-impregnated product is "activated" by "baking" at 400°C.

Relation to 3A and 5A

4A sieves serve as the precursor to 3A and 5A sieves through cation exchange of sodium for potassium (for 3A) or calcium (for 5A). In effect, this ion exchange allows the selectivity of molecular sieve to differentiate where 3A molecular sieve is more selective than 4A molecular sieve while 5A molecular sieve is less selective than 4A molecular sieve and can adsorb larger molecules.

Usage

Drying solvents

4A 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. They are 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 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 a soap forming agent and in toothpaste.

Harmful waste treatment

4A 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. 4A 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, caesium, etc.
2. Petrochemical industry, catalyst, desiccant, adsorbent
3. Agriculture: soil conditioner
4. Medicine: load silver zeolite antibacterial agent.

5A

• Chemical formula: 0.7CaO•0.30Na₂O•Al₂O₃•2.0SiO₂ •4.5H₂O
• Silica-alumina ratio: SiO₂/ Al₂O₃≈2

Production

5A molecular sieves are produced by cation exchange of calcium for sodium in 4A molecular sieves.

Usage

Five-ångström (5A) 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.

Five-ångström 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.

Five-ångström 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.

Type 13X

• Chemical formula: [(AlO₂)_a(SiO₂)_b]•_cH₂O

Production

Specialized synthesis methods have been developed to tailor the properties of zeolite for diverse industrial applications. By carefully controlling the chemical composition and reaction conditions, manufacturers can modify the size, shape, and surface properties of zeolite to create synthetic 13X molecular sieve, allowing it to be optimized for specific functions. For air separation processes, carefully synthesized 13X molecular sieve can be engineered to selectively adsorb certain gas molecules while allowing others to pass through, making it invaluable in separating and purifying gases on an industrial scale.

Relation to Type A Molecular Sieve

Type X crystals are shaped differently from Type A crystals are tend to offer much larger pore sizes, about 9 Angstroms in diameter.

Usage

In the natural gas and biogas industries, the presence of impurities such as carbon dioxide and hydrogen sulfide can significantly impact the quality and usability of these valuable resources. 13X zeolite is a key component in removing these impurities. Leveraging its high selectivity for carbon dioxide and hydrogen sulfide, 13X effectively purifies natural gas and biogas, ensuring that these resources meet stringent purity standards for safe utilization. Moreover, in air separation applications, the selective adsorption capabilities of 13X come into play by effectively removing impurities from the air. This is particularly crucial in industries where high-purity gases are essential for various processes. The ability of 13X zeolite to selectively capture water molecules and other impurities contributes to achieving the desired purity levels required for industrial applications. Beyond gas separation and purification, 13X zeolite also serves as an invaluable asset in the drying of various gases and liquids. Its exceptional water adsorption capacity makes it an indispensable tool for removing moisture from industrial processes. Whether it’s the drying of natural gas, biogas, or liquid streams, 13X zeolite plays a critical role in maintaining the purity and integrity of these essential substances.

See also

• Lime (mineral)
• Zeolite

References

External links

• Sieves Put A Lid On Greenhouse Gas
• Molecular Sieve Safety
• About Molecular Sieves
• 3A, 4A, 5A, and 13X, What's the Difference