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:
- 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.
- 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.
- Sometimes the synthetic zeolites (molecular sieves) are denoted according to the name of corresponding natural mineral, for example «zeolite Analcime- type", "zeolite Mordenite- type".
- 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 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.
(NaA, CaA, KA)
|Zeolite||Adsorbate||t, C||Vp, сm³/g||V'p,А³/unit cell|
|Zeolite Na12(A) •24H2O||H2O||25||0,289||842|
|Zeolite Ca6(A) •30H2O||H2O||25||0,305||885|
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).
Free volume in zeolite NaX (Si/Al = 1,25).
|Adsorbate||t,C||Vp, сm³/g||Number of moleculars per unit cell|
Free volume in zeolite NaX (Si/Al = 1,33).
|T, K||Vp, сm³/g||Number of moleculars per unit cell|