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A molecular sieve is a material with pores of uniform size comparable to that of individual
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s, linking the interior of the solid to its exterior. These materials embody the molecular sieve effect, in which molecules larger than the pores are preferentially sieved, allowing for the selective
adsorption 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 ...
of specific compounds based on their molecular size. Many kinds of materials exhibit some molecular sieves, but
zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a meta ...
s dominate the field. Zeolites are almost always
aluminosilicate Aluminosilicate refers to materials containing anionic Si-O-Al linkages. Commonly, the associate cations are sodium (Na+), potassium (K+) and protons (H+). Such materials occur as minerals, coal combustion products and as synthetic materials, of ...
s, or variants where some or all of the Si or Al centers are replaced by similarly charged elements.


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
aluminosilicate Aluminosilicate refers to materials containing anionic Si-O-Al linkages. Commonly, the associate cations are sodium (Na+), potassium (K+) and protons (H+). Such materials occur as minerals, coal combustion products and as synthetic materials, of ...
s (
zeolites Zeolites are a group of several Microporous material, microporous, crystalline aluminosilicate minerals commonly used as commercial adsorption, adsorbents and Catalysis, catalysts. They mainly consist of silicon, aluminium, oxygen, and have the ge ...
) with Si/Al molar ratio less than 2, but there are also examples of
activated carbon Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that greatly increase the surface ar ...
and
silica gel Silica gel is an amorphous and porosity, porous form of silicon dioxide (silica), consisting of an irregular three-dimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain wate ...
. The pore diameter of a molecular sieve is measured in ångströms (Å) or
nanometres 330px, Different lengths as in respect to the Molecule">molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm), or nanometer (American spelling), is a unit of length ...
(nm). According to
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
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). The sieving properties of molecular sieves are classified as *microporous (3-10 Å pores) including zeolite A, LTA, and FAU. Some clays, active carbon, and porous glass meet this criterion. *mesoporous materials (<2 nm pores) *macroporous materials (2–50 nm pores), e.g., in the form of
Silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...
(used to make silica gel): 24 Å (2.4 nm)


Applications

Some molecular sieves are used in
size-exclusion chromatography Size-exclusion chromatography, also known as molecular sieve chromatography, is a chromatography, chromatographic method in which molecules in Solution (chemistry), solution are separated by their Chemical structure, shape, and in some cases molec ...
, a separation technique that sorts molecules based on their size. Another important use is as a
desiccant 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. Desiccant ...
. They are often utilized in the
petrochemical Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable s ...
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 Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large a ...
.


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.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. Under the term zeolites, molecular sieves are used for a wide range of catalytic applications. They catalyze isomerisation,
alkylation Alkylation is a chemical reaction that entails transfer of an alkyl group. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting al ...
, 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 Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that greatly increase the surface ar ...
, finally being used to charge breathing air tanks. 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. 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 Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
dehydration), or heating under high vacuum. Regeneration temperatures range from to depending on molecular sieve type. In contrast, silica gel can be regenerated by heating it in a regular oven to 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.


Adsorption capabilities


3A

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


Production

3A molecular sieves are produced by cation exchange of
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
for
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
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 Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
, air,
refrigerant A refrigerant is a working fluid used in the cooling, heating, or reverse cooling/heating cycles of air conditioning systems and heat pumps, where they undergo a repeated phase transition from a liquid to a gas and back again. Refrigerants are ...
s,
natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...
and unsaturated hydrocarbons. The latter include cracking gas,
acetylene Acetylene (Chemical nomenclature, systematic name: ethyne) is a chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is u ...
,
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...
,
propylene Propylene, also known as propene, is an unsaturated organic compound with the chemical formula . It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petroleum-like o ...
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

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


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 4A sieves serve as the precursor to 3A and 5A sieves through cation exchange of
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
for
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
(for 3A) or
calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
(for 5A)Intraglobal
/ref>


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 Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water requires less soap for the same cleaning effort, as soap is not wasted bonding with calcium ions. Soft water also extend ...
. 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 through
calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
ion exchange, remove and prevent the deposition of dirt. They are widely used to replace
phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
. 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 Soap is a salt (chemistry), salt of a fatty acid (sometimes other carboxylic acids) used for cleaning and lubricating products as well as other applications. In a domestic setting, soaps, specifically "toilet soaps", are surfactants usually u ...
forming agent and in
toothpaste Toothpaste is a paste or gel dentifrice that is used with a toothbrush to clean and maintain the aesthetics of Human tooth, teeth. Toothpaste is used to promote oral hygiene: it is an abrasive that aids in removing dental plaque and food from th ...
.


Other purposes

# The
metallurgical industry Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...
: separating agent, separation, extraction of brine potassium,
rubidium Rubidium is a chemical element; it has Symbol (chemistry), symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have ...
,
caesium Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only f ...
, etc. # Petrochemical industry,
catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
,
desiccant 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. Desiccant ...
, adsorbent # Agriculture:
soil conditioner A soil conditioner is a product which is added to soil to improve the soil’s physical qualities, usually its fertility (ability to provide nutrition for plants) and sometimes its mechanics. In general usage, the term "soil conditioner" is often ...
# Medicine: load silver
zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a meta ...
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

*
Lime (mineral) Lime is an inorganic material composed primarily of calcium oxides and hydroxides. It is also the name for calcium oxide which is used as an industrial mineral and is made by heating calcium carbonate in a kiln. Calcium oxide can occur as a ...
*
Zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a meta ...


Notes


References


External links


Sieves Put A Lid On Greenhouse Gas




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