MOLECULAR SIEVES

Definition:

Molecular sieves are a microporous material which can selectively adsorb gases and liquids.  They are synthetic zeolites (crystalline metal aluminosilicates) which upon dehydration can efficiently and selectively remove water or other solvents.  The selectivity of a molecular sieve is determined by its pore size, molecules with a critical diameter which is less than the pore size will be efficiently adsorbed while larger molecules will be excluded.  See Table 2 for a list of common molecules and their critical diameters.  Standard sieves come in 3 Å, 4Å, 5Å, and 10Å (13X) pore sizes.
Literature Reference:  Breck, D.W.  J. Chem. Ed. 1964, 41, 678.

Regeneration of Molecular Sieves:

To regenerate the adsorptive power of the material, it is usually heated and purged with carrier gas (large scale applications).  For smaller scale applications, they can be dried in a drying oven followed by slow cooling in a closed environment (a dessicator).

 

Table I: Adapted from SigmaAldrich.com (see reference below)
Type: 3A
Composition: 0.6 K2O: 0.40 Na2O : 1 Al2O3 : 2.0 ± 0.1 SiO2 : x H2O
Description: The 3A form is made by substituting potassium cations for the inherent sodium ions of the 4A structure, reducing the effective pore size to ~3Å, excluding diameter >3Å, e.g., ethane.
Major Applications: Commercial dehydration of unsaturated hydrocarbon streams, including cracked gas, propylene, butadiene, acetylene; drying polar liquids such as methanol and ethanol. Useful for adsorption of molecules such as NH3 and H2O from a N2/H2 flow. Considered a general-purpose drying agent in polar and non-polar media.
Regeneration Temperature: 175-260 ° C
Type: 4A
Composition: 1 Na2O: 1 Al2O3: 2.0 ± 0.1 SiO2 : x H2O
Description: This sodium form represents the type A family of molecular sieves. Effective pore opening is 4Å, thus excluding molecules of effective diameter >4Å, e.g., propane.
Major Applications: 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.
Regeneration Temperature: 200-315 °C
Type: 5A
Composition: 0.80 CaO : 0.20 Na2O : 1 Al2O3: 2.0 ± 0.1 SiO2: x H2O
Description: Divalent calcium ions in place of sodium cations give apertures of ~5Å which exclude molecules of effective diameter >5Å, e.g., all 4-carbon rings, and iso-compounds.
Major Applications: Separation of normal paraffins from branched-chain and cyclic hydrocarbons; removal of H2S, CO2 and mercaptans from natural gas. Molecules adsorbed include nC4H10, nC4H9OH, C3H8 to C22H46, and dichlorodifluoro-methane (Freon 12®).
Regeneration Temperature: 200-315 °C
Type: 13X
Composition: 1 Na2O: 1 Al2O3 : 2.8 ± 0.2 SiO2 : x H2O
Description: The sodium form represents the basic structure of the type X family, with an effective pore opening in the 9-10 Å range. Will not adsorb (C4F9)3N, for example.
Major Applications: Commercial gas drying, air plant feed purification (simultaneous H2O and CO2 removal) and liquid hydrocarbon/natural gas sweetening (H2S and mercaptan removal).
Regneration Temperature: 200-315 °C

 

Table 2: Critical Diameters of Common Molecules (adapted from Sigma Aldrich.com)
 
Molecule
Critical Diameter (A)
  
Molecule
Critical Diameter (A)
Helium
2.0
   Ammonia
3.6
Propylene
5.0
   Cyclohexane
6.1
Hydrogen
2.4
   Hydrogen Sulfide
3.6
Ethyl Mercaptan
5.1
   Benzene
6.7
Acetylene
2.4
   Argon
3.8
1-Butene
5.1
   Toluene
6.7
Oxygen
2.8
   Methane
4.0
Trans-2-Butene
5.1
   p-Xylene
6.7
Carbon Monoxide
2.8
   Ethylene
4.2
1,3-Butadiene
5.2
   Carbon Tetrachloride
6.9
Carbon Dioxide
2.8
   Ethylene Oxide
4.2
Chloro-Difluoro-Methane
5.3
   Chloroform
6.9
Nitrogen
3.0
   m-Xylene
7.1
Thiophene
5.3
   Methyl Mercaptan
4.5
Water
3.2
   o-Xylene
7.4
Ethane
4.4
   Triethyl-Amine
8.4
Propane
4.9
   n-Butane to n-Docosane
4.9
Isobutane to Isodocosane
5.6
    
 

Source: Sigma Aldrich Technical Information Bulletin: Mineral Adsorbents, Filter Agents, and Drying Agents. http://www.sigmaaldrich.com/Brands/Aldrich/Tech_Bulletins/AL_143/Molecular_Sieves.html Accessed 7/23/08.