What molecules does molecular sieve adsorbed and excluded?


Zeolite adsorbents are synthetically produced molecular sieves that are microporous, crystalline, metal aluminosilicates. The uniform crystalline structure of molecular sieve adsorbents provides very predictable and reliable adsorptive properties. Metal cations contained in the crystalline structure of molecular sieve adsorbents balance the negative charge of the framework. These metal cations create an electrical field, hence their strong affinity for polar molecules.

Depending on the type of crystalline structure and the occupying cation of the molecular sieve adsorbent,molecules may be readily adsorbed or completely excluded according to their relative molecular size. For example, a 3A molecular sieve is particularly useful for the dehydration of olefins. It will adsorb water, but will exclude an olefin molecule.  Zeolite molecular sieve adsorbents have a strong affinity for polar and polarizable molecules and even exhibit a selective preference for various polar molecules. Based on their degree of polarity. Water is the most polar molecule known, and is therefore the most preferred and strongly adsorbed onto zeolite molecular sieves.

The following is a table list of molecules different types of molecular sieve adsorb and exclude:

Molecule Sieves: Molecules Adsorbed and Excluded

Type Molecules Adsorbed**  Molecules Excluded
3A Molecules with an effective diameter <3 angstroms including H2O and NH3 Molecules with an effective diameter >3 angstroms (ethane)
4A Molecules with an effective diameter <4 angstroms including ethanol, H2S, CO2, SO2, C2H4, C2H6 and C3H6 Molecules with an effective diameter
>4 angstroms (propane)
5A Molecules with an effectivediameter <5 angstroms including nC4H9OH, n-C4H10, C3H8 to C22H46 and R-12 Molecules with an effective diameter >5 angstroms
(propane) (iso compounds and all 4-carbon rings)
13X Molecules with an effective diameter <8 angstroms including benzene and toluene Molecules with an effective diameter >8 angstroms (C4F9)3N

* Chart depicts basic molecular sieve types only. In all applications, these basic forms are customized for specific use.
** Each type adsorbs listed molecules plus those of preceding type.