Molecular sieve

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Molecular sieve (or short molecular sieve ) is the functional description of natural and synthetic zeolites or other substances that have a high adsorption capacity for gases have, vapors and dissolved substances with specific molecular sizes. With a suitable choice of molecular sieve it is possible to separate molecules of different sizes. In addition to zeolites, there are also carbon molecular sieves or molecular sieving carbon. On the one hand, these are somewhat more expensive due to the process, on the other hand, they can be specialized for a certain separation during pyrolysis .

description

The molecular sieves have a large internal surface area (600–700 m 2 / g) and have uniform pore diameters which are of the order of magnitude of the diameter of molecules.

In technology, the pore diameters are often given in angstroms . The molecular sieve zeolite A has a pore size of 3 Å = 0.3 nm. Only molecules with a smaller (effective) diameter than 0.3 nm can get into the pores. Molecular sieves can be used to dynamically dry gases and solvents , separate mixtures of straight-chain and branched alkanes , or soften water.

Zeolite molecular sieves that absorb nitrogen work in medical oxygen concentrators via pressure swing adsorption ( PSA ). On the other hand, to obtain nitrogen, CMS ( English Carbo Molecular Sieves ) can be used.

Molecular sieves in powdered, rod or pearl form are commercially available. The most common pore sizes are 3 Å, 4 Å, 5 Å and 10 Å. Rod and pearl-shaped molecular sieves are also available with a moisture indicator to indicate that the capacity limit has been reached.

A 4 Å molecular sieve is suitable for drying most common solvents in laboratory chemistry. For solvents with relatively small molecules (for example methanol , ethanol , dichloromethane or acetonitrile ) the pore size 4 Å is unsuitable, as the solvent molecules themselves can penetrate the pores and force out the water in them. Molecular sieves with a pore size of 3 Å are used for such substances.

Selectivities

The selectivity of the material varies depending on the pore size :

  • 3A (3 Å pore size): Adsorbs NH 3 , H 2 O, (but not C 2 H 6 ), suitable for drying polar solvents.
  • 4A (4 Å pore size): Adsorbs H 2 O, CO 2 , SO 2 , H 2 S, C 2 H 4 , C 2 H 6 , C 3 H 6 , EtOH. Does not adsorb C 3 H 8 and higher hydrocarbons. Suitable for drying non-polar solvents and gases.
  • 5A (5 Å pore size): Adsorbs normal (linear) hydrocarbons up to nC 4 H 10 , alcohols up to C 4 H 9 OH, mercaptans up to C 4 H 9 SH. Does not adsorb iso compounds and rings larger than C 4 .
  • 10X (8 Å pore size): Adsorbs branched hydrocarbons and aromatics. Suitable for drying gases.
  • 13X (10 Å pore size): Adsorbs di-n-butylamine (but not tri-n-butylamine). Suitable for drying HMPT .

regeneration

One advantage of molecular sieves is that they can be regenerated almost as often as required by heating to 350–400 ° C. Care must be taken to warm up slowly, otherwise the molecular sieve will age too quickly or be completely destroyed. Depending on how strongly the substance is bound to the molecular sieve, the highest temperature must be adjusted during the drying process.

Molecular sieves can also be regenerated in other ways. For example, if it is in equilibrium with the external substance concentration and if this is lowered, for example by reducing the pressure, the adsorbed molecules evaporate again until a new equilibrium has been established.

Individual evidence

  1. ↑ Airtexx gas systems & equipment: PSA / N2 generator , accessed on April 27, 2013.
  2. ^ Sigma-Aldrich: Carbon Molecular Sieve (CMS) Adsorbents , accessed April 27, 2013.
  3. ↑ Japan EnviroChemicals: PSA: Gas separation by Pressure Swing Adsorption
  4. ↑ Louis F. Fieser , Mary Fieser : Reagents for organic synthesis. Volume 1. Wiley, New York et al. 1967, ISBN 0-471-25875-X , p. 703.
  5. ^ DW Breck: Crystalline Molecular Sieves. In: Journal of Chemical Education. Vol. 41, No. 12, 1964, p. 678, doi: 10.1021 / ed041p678 .