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Types and structure of synthetic zeolites (molecular sieves)

Nowadays it is known about 34 natural and more than 100 types of synthetic zeolites  (molecular sieves - sorbents), but only some of them are used in practice. The point is that some zeolites after dehydration has a system of narrow channels, diffusion in them is practically impossible because of structural damages. Other zeolites – as a result of dehydration have such changes that their structure partially damaged and dehydration process become irreversible. But as a molecular sieve is possible to use such a zeolite which structure remains without changes after full dehydration process.

 Synthetic zeolites (molecular sieves - sorbents) are more suitable for researching and meet the requirements of industrial standards because of high homogeneity and purity. It is especially important where a high level of repeatability of measurements is needed (for example, separation processes) and where small quantity of impurities can cause unwanted effects (for example, influence of iron on heterogeneous catalysis).

 Parameters and classification of Aluminum Silicates such as zeolites           (molecular sieves) is difficult as there is no specified system of chemical classification. If to use IUPAC standards, then as a base we should take the composition of unit cell. Classification of such type is complicated and it is needed to know the unit cell composition (for example, zeolite natural Analcite Na16 (AlO2)16 (SiO2)32 • 16H2O should be called 16-Aqua 16-Alumina-32- Sodium silicate).


Nowadays the following notations for zeolites are used:

  1. Synthetic zeolite (molecular sieve) is denoted by the letters which were introduced by the first researcher: for example, zeolite A,   zeolite K-G, zeolite ZK-5 and so on.
  2. These symbols are used for the zeolites which are received at synthesis. The letter ‘ A ‘ means that here is used a synthetic zeolite  Na16 (AlO2)16 (SiO2)32 • 16H2O (molecular sieve  4А), got from  Na2O, Al2O3, SiO2, H2O. It is a typical zeolite composition. Also the following symbols are used «zeolite A -type", "zeolite X-type" and so on.
  3. Sometimes the synthetic zeolites (molecular sieves) are denoted according to the name of corresponding natural mineral, for example «zeolite Analcime- type", "zeolite  Mordenite- type".
  4. Different cationic forms got by the method of ion exchange can also have special notations. For example, calcic form of zeolite A (molecular sieve 5А) briefly is denoted as zeolite CaA (or molecular sieve 5A).But the hyphen between Ca and A, i.e. zeolite Ca-A, means other one.
Zeolite A

Zeolite A has 2 types of cavities : small ones
 (located in alpha-spaces and available only for small water molecules ) and big ones (located in beta-spacesand available for gases like argon,oxygen and nitrogen). Free volume on zeolite unit cell is  926А3.  
 A-type zeolite
(NaA, CaA, KA)

 

ZeoliteAdsorbatet, CVp, сm³/gV'p,А³/unit cell
Zeolite Na12(A) •24H2O H2O 25 0,289 842
  C2O -75 0,252 725
  O2 -183 0,213 612
Zeolite Ca6(A) •30H2O H2O 25 0,305 885
  O2 -183 0,242 700
  Ar -183 0,261 738
  N2 -196 0,297 857
  n-Butane 25 0,226 655

 

Zeolite X

For this type of zeolite the volume of the pores determined on water is  7908 А³ on unit cell. Most adsorbates except water fill only large cavities of the zeolite (molecular sieve). Total pore volume determined for large cavities on adsorption of Argon or Oxygen is 6700 А³ on unit cell. About 1200 А³ of unit cell volume (150 А³ on each of eight beta-cavities) is available only for water molecule. The volume of each large 26-face cavity in the zeolite X is 822 А³. Free volume on a unit cell of zeolite is 7832 А³ (for water is 7908 А³).    
X-typeZeolite
(NaX, CaX)



Free volume in zeolite NaX (Si/Al = 1,25).

Adsorbatet,CVp, сm³/gNumber of moleculars per unit cell
H2O 25 0,36 265
C2O -78 0,33 120
Ar -183 0,30 140
Kr -183 0,27 116
Xe -78 0,25 74
O2 -183 0,31 149
N2 -196 0,35 134
n-Pentane 25 0,30 34
Neopentane 25 0,26 29
2,2,4-Trimethylpentane 25 0,27 22
Benzole 25 0,30 45
(C4H9)3N 25 0,29 16,4



Free volume in zeolite NaX (Si/Al = 1,33).


Hydrocarbon		T, KVp, сm³/gNumber of moleculars per unit cell
n-Pentane 298 0,311 35,8
Isopentane 298 0,309 34,8
n-Hexane 298 0,309 31,3
n-Heptane 298 0,311 28,3
n-Octane 313 0,308 24,4
Isooctane 298 0,282 22,7
Benzole 303 0,295 43,3
Toluene 313 0,301 36,8
Cyclopentane 303 0,334 45,1
Cyclohexane 313 0,268 33,0

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