Molecular sieve

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Typical molecular sieves are of the LTA type. They feature sodium aluminosilicates cages (sodium not shown) that have high affinity for water. LTA (cropped).png
Typical molecular sieves are of the LTA type. They feature sodium aluminosilicates cages (sodium not shown) that have high affinity for water.
Vials of mesoporous silica MSN vials.JPG
Vials of mesoporous silica

A molecular sieve is a material with pores (voids or holes), having uniform size comparable to that of individual molecules, linking the interior of the solid to its exterior. These materials embody the molecular sieve effect, the preferential sieving of molecules larger than the pores. [a] 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. [2]

Contents

Sieving process

The diameters of the pores that comprise molecular sieves are similar in size to small molecules. 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. Most of molecular sieves are aluminosilicates (zeolites) with Si/Al molar ratio less than 2, but there are also examples of activated charcoal and silica gel. [2] [3] [4]

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). [5]


The sieving properties of molecular sieves are classified as

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 utilized 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 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 catalyze isomerisation, alkylation, and epoxidation, and are used in large scale industrial processes, including hydrocracking and fluid catalytic cracking. [8]

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. [9] Such filtration can remove particulates and compressor exhaust products from the breathing air supply.

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. [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]

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. [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]

Adsorption capabilities

NameAliasPore diameter (Ångström)Bulk density (g/mL)Adsorbed water (% w/w) Attrition or abrasion, W (% w/w)Usage [14]
3AA-3, K-A30.60–0.6819–200.3–0.6 Desiccation of petroleum cracking gas and alkenes, selective adsorption of H2O in insulated glass (IG) and polyurethane, drying of ethanol fuel for blending with gasoline.
4AA-4, Na-A40.60–0.6520–210.3–0.6Adsorption 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; [12] separation of natural gas and alkenes, adsorption of water in non-nitrogen sensitive polyurethane
5A-DW50.45–0.5021–220.3–0.6Degreasing and pour point depression of aviation kerosene and diesel, and alkenes separation
5A small oxygen-enriched50.4–0.8≥23Specially designed for medical or healthy oxygen generator[ citation needed ]
5AA-5, Ca-A50.60–0.6520–210.3–0.5Desiccation and purification of air; dehydration and desulfurization of natural gas and liquid petroleum gas; oxygen and hydrogen production by pressure swing adsorption process
10XF-9, Ca-X80.50–0.6023–240.3–0.6High-efficient sorption, used in desiccation, decarburization, desulfurization of gas and liquids and separation of aromatic hydrocarbon
13XF-9, Na-X100.55–0.6523–240.3–0.5Desiccation, desulfurization and purification of petroleum gas and natural gas
13X-AS100.55–0.6523–240.3–0.5 Decarburization and desiccation in the air separation industry, separation of nitrogen from oxygen in oxygen concentrators
Cu-13XCu-X100.50–0.6023–240.3–0.5 Sweetening (removal of thiols) of aviation fuel and corresponding liquid hydrocarbons

3A

Production

3A molecular sieves are produced by cation exchange of potassium for sodium in 4A molecular sieves (See below)

Usage

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 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 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.

4A

Production

For the production of 4A sieve, typically aqueous solutions of sodium silicate and sodium 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 through cation exchange of sodium for potassium (for 3A) or calcium (for 5A) [16] [17]

Uses

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 entails water softening. [2]

4A molecular sieves are widely used to dry laboratory solvents. [7] 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.

Bottle of 4A molecular sieves 4A sieves.JPG
Bottle of 4A molecular sieves

Some 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.

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.

Morphology of molecular sieves

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.

See also

Notes

  1. ↑ "With respect to porous solids, the surface associated with pores communicating with the outside space may be called the internal surface. Because the accessibility of pores may depend on the size of the fluid molecules, the extent of the internal surface may depend on the size of the molecules comprising the fluid, and may be different for the various components of a fluid mixture." [1]

Related Research Articles

<span class="mw-page-title-main">Zeolite</span> Microporous, aluminosilicate mineral

Zeolite is a family of several microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula Mn+
1/n(AlO
2)−
(SiO
2)
x･yH
2O where Mn+
1/n is either a metal ion or H+.

<span class="mw-page-title-main">Adsorption</span> Phenomenon of surface adhesion

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid is dissolved by or permeates a liquid or solid. While adsorption does often precede absorption, which involves the transfer of the absorbate into the volume of the absorbent material, alternatively, adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface and into the bulk of the adsorbent. The term sorption encompasses both adsorption and absorption, and desorption is the reverse of sorption.

<span class="mw-page-title-main">Silica gel</span> Chemical compound

Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain water or some other liquids, or may be filled by gas or vacuum. In the last case, the material is properly called silica xerogel.

<span class="mw-page-title-main">Desiccant</span> Substance used to induce or sustain dryness

A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness (desiccation) in its vicinity; it is the opposite of a humectant. Commonly encountered pre-packaged desiccants are solids that absorb water. Desiccants for specialized purposes may be in forms other than solid, and may work through other principles, such as chemical bonding of water molecules. They are commonly encountered in foods to retain crispness. Industrially, desiccants are widely used to control the level of water in gas streams.

<span class="mw-page-title-main">Membrane gas separation</span> Technology for splitting specific gases out of mixtures

Gas mixtures can be effectively separated by synthetic membranes made from polymers such as polyamide or cellulose acetate, or from ceramic materials.

<span class="mw-page-title-main">Mesoporous material</span> Material with pores between 2 and 50 nm

A mesoporous material is a nanoporous material containing pores with diameters between 2 and 50 nm, according to IUPAC nomenclature. For comparison, IUPAC defines microporous material as a material having pores smaller than 2 nm in diameter and macroporous material as a material having pores larger than 50 nm in diameter.

<span class="mw-page-title-main">Activated alumina</span>

Activated alumina is manufactured from aluminium hydroxide by dehydroxylating it in a way that produces a highly porous material; this material can have a surface area significantly over 200 m2/g. The compound is used as a desiccant (to keep things dry by adsorbing water from the air) and as a filter of fluoride, arsenic and selenium in drinking water. It is made of aluminium oxide (alumina; Al2O3). It has a very high surface-area-to-weight ratio, due to the many "tunnel like" pores that it has. Activated alumina in its phase composition can be represented only by metastable forms (gamma-Al2O3 etc.). Corundum (alpha-Al2O3), the only stable form of aluminum oxide, does not have such a chemically active surface and is not used as a sorbent.

Sodium aluminosilicate refers to compounds which contain sodium, aluminium, silicon and oxygen, and which may also contain water. These include synthetic amorphous sodium aluminosilicate, a few naturally occurring minerals and synthetic zeolites. Synthetic amorphous sodium aluminosilicate is widely used as a food additive, E 554.

<span class="mw-page-title-main">Cetrimonium bromide</span> Quaternary ammonium surfactant and antiseptic agent

Cetrimonium bromide, also known with the abbreviation CTAB, is a quaternary ammonium surfactant with a condensed structural formula [(C16H33)N(CH3)3]Br.

<span class="mw-page-title-main">Moisture sorption isotherm</span>

The relationship between water content and equilibrium relative humidity of a material can be displayed graphically by a curve, the so-called moisture sorption isotherm. For each humidity value, a sorption isotherm indicates the corresponding water content value at a given temperature. If the composition or quality of the material changes, then its sorption behaviour also changes. Because of the complexity of sorption process the isotherms cannot be determined explicitly by calculation, but must be recorded experimentally for each product.

<span class="mw-page-title-main">Nanoporous materials</span>

Nanoporous materials consist of a regular organic or inorganic bulk phase in which a porous structure is present. Nanoporous materials exhibit pore diameters that are most appropriately quantified using units of nanometers. The diameter of pores in nanoporous materials is thus typically 100 nanometers or smaller. Pores may be open or closed, and pore connectivity and void fraction vary considerably, as with other porous materials. Open pores are pores that connect to the surface of the material whereas closed pores are pockets of void space within a bulk material. Open pores are useful for molecular separation techniques, adsorption, and catalysis studies. Closed pores are mainly used in thermal insulators and for structural applications.

Mesoporous silicates are silicates with a special morphology.

<span class="mw-page-title-main">Mesoporous silica</span> Nano-scale porous silica compound

Mesoporous silica is a form of silica that is characterised by its mesoporous structure, that is, having pores that range from 2 nm to 50 nm in diameter. According to IUPAC's terminology, mesoporosity sits between microporous (<2 nm) and macroporous (>50 nm). Mesoporous silica is a relatively recent development in nanotechnology. The most common types of mesoporous nanoparticles are MCM-41 and SBA-15. Research continues on the particles, which have applications in catalysis, drug delivery and imaging. Mesoporous ordered silica films have been also obtained with different pore topologies.

<span class="mw-page-title-main">ZSM-5</span> Zeolite used as a catalyst in oil refining

ZSM-5, Zeolite Socony Mobil–5 (framework type MFI from ZSM-5 (five)), is an aluminosilicate zeolite belonging to the pentasil family of zeolites. Its chemical formula is NanAlnSi96–nO192·16H2O (0<n<27). Patented by Mobil Oil Company in 1975, it is widely used in the petroleum industry as a heterogeneous catalyst for hydrocarbon isomerization reactions.

<span class="mw-page-title-main">SSZ-13</span>

SSZ-13 (framework type code CHA) is a high-silica aluminosilicate zeolite possessing 0.38 × 0.38 nm micropores. It belongs to the ABC-6 family of zeolites as well as offretite, cancrinite, erionite and other related small-pore zeolites. The framework topology is the same as that of chabazite but SSZ-13 has a high silica composition with Si/Al > 5, which leads to low cation exchange capacity. The typical chemical formula of the unit cell can be described as QxNayAl2.4Si33.6O72•zH2O (1.4 < x <27)(0.7 < y < 4.3)(1 < z <7), where Q is N,N,N-1-trimethyladamantammonium. The material was patented by Chevron research Company in 1985, and could potentially be used as a solid catalyst for the methanol-to-olefins (MTO) process and the selective catalytic reduction (SCR) of NOx.

Mesoporous organosilica are a type of silica containing organic groups that give rise to mesoporosity. They exhibit pore size ranging from 2 nm - 50 nm, depending on the organic substituents. In contrast, zeolites exhibit pore sizes less than a nanometer. PMOs have potential applications as catalysts, adsorbents, trapping agents, drug delivery agents, stationary phases in chromatography and chemical sensors.

<span class="mw-page-title-main">MCM-41</span> Catalyst or catalyst support material

MCM-41 is a mesoporous material with a hierarchical structure from a family of silicate and alumosilicate solids that were first developed by researchers at Mobil Oil Corporation and that can be used as catalysts or catalyst supports.

<span class="mw-page-title-main">Aerogel</span> Synthetic ultralight solid material

Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low density and extremely low thermal conductivity. Aerogels can be made from a variety of chemical compounds. Silica aerogels feel like fragile styrofoam to the touch, while some polymer-based aerogels feel like rigid foams.

A zeolite membrane is a synthetic membrane made of crystalline aluminosilicate materials, typically aluminum, silicon, and oxygen with positive counterions such as Na+ and Ca2+ within the structure. Zeolite membranes serve as a low energy separation method. They have recently drawn interest due to their high chemical and thermal stability, and their high selectivity. Currently zeolites have seen applications in gas separation, membrane reactors, water desalination, and solid state batteries. Currently zeolite membranes have yet to be widely implemented commercially due to key issues including low flux, high cost of production, and defects in the crystal structure.

References

  1. ↑ "Molecular Sieve Effect". IUPAC GoldBook.
  2. 1 2 3 Kresge, Charles T.; Dhingra, Sandeep S. (2004). "Molecular Sieves". Kirk-Othmer Encyclopedia of Chemical Technology. doi:10.1002/0471238961.1315120511210812.a01.pub2. ISBN   978-0-471-48494-3.
  3. ↑ Mazur, Michal; Přech, Jan; Čejka, Jiří (2019). "Zeolites and Other Micro- and Mesoporous Molecular Sieves". Kirk-Othmer Encyclopedia of Chemical Technology. pp. 1–36. doi:10.1002/0471238961.zeolcejk.a01.pub2. ISBN   978-0-471-48494-3.
  4. ↑ Cohen, Alan P. (2003). "Desiccants". Kirk-Othmer Encyclopedia of Chemical Technology. doi:10.1002/0471238961.0405190903150805.a01.pub2. ISBN   978-0-471-48494-3.
  5. ↑ J. Rouquerol; et al. (1994). "Recommendations for the characterization of porous solids (Technical Report)" (free download pdf). Pure Appl. Chem. 66 (8): 1739–1758. doi: 10.1351/pac199466081739 . S2CID   18789898.
  6. ↑ "Desiccant Types". SorbentSystems.com. Retrieved 2014-02-26.
  7. 1 2 Williams, D. B. G., Lawton, M., "Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants", The Journal of Organic Chemistry 2010, vol. 75, 8351. doi : 10.1021/jo101589h
  8. ↑ Pujadó, P. R.; Rabó, J. A.; Antos, G. J.; Gembicki, S. A. (1992-03-11). "Industrial catalytic applications of molecular sieves". Catalysis Today. 13 (1): 113–141. doi:10.1016/0920-5861(92)80191-O.
  9. ↑ Archived April 16, 2012, at the Wayback Machine
  10. ↑ "Sec. 182.2727 Sodium aluminosilicate". U.S. Food and Drug Administration. 1 April 2012. Retrieved 10 December 2012.
  11. ↑ "Molecular Sieve Desiccant". DesiccantPacks.net. Retrieved 2014-02-26.
  12. 1 2 "Molecular Sieves". Sigma-Aldrich . Retrieved 2014-02-26.
  13. ↑ Spence Konde, "Preparation of High-Silica Zeolite Beads From Silica Gel," retrieved 2011-09-26
  14. ↑ "Molecular Sieve, Yiyuan Molecular Sieves". Chemicalpackingcorp.com. Retrieved 2014-02-26.
  15. ↑ US 3433588,Max Michel&Denis Papee,"Method for the preparation of 4 angstrom unit zeolites",published 1969-03-18,issued 1969-03-18
  16. ↑ Zeochem
  17. ↑ Intraglobal
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