•
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.
• Macroporous material (>50 nm) Mesoporous silica 、Zeolite LTA
Pore ​size
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.
Molecular sieves can be microporous, mesoporous, or macroporous material. • Microporous material (<2 nm) Zeolites 、Porous glass 、Active carbon 、Clays
• Mesoporous material (2-50 nm) Silicon dioxide
THE END
• Desiccant
பைடு நூலகம்egeneration
• 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.
Molecular sieve
李瑞琦 应化 01
Principle
• A molecular sieve is a material with very small holes of precise and uniform size. These holes are small enough to block large molecules while allowing small molecules to pass.
• Because of this, they often function as a desiccant. A molecular sieve can adsorb water up to 22% of its own weight.
Differents from the Size-exclusion
• 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.
• Regeneration temperatures range from 175 °C to 315 °C depending on molecular sieve type.[9] 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.
chromatography
• 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. • Size-exclusion chromatography (SEC) is a chromatographic method in which molecules in solution are separated by their size, and in some cases molecular weight.
Applications
• breathing apparatus
• 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 activated carbon, finally being used to charge breathing air tanks. Such filtration can remove particulates and compressor exhaust products from the breathing air supply.
Application
• Many molecular sieves are used as desiccants. Some examples include Activated charcoal and silica gel.
• Activated charcoal
• Silica gel
Materials
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.