Types and structure of synthetic zeolites (molecular sieves)

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At this time, 34 natural and over 100 synthetic zeolite types are known, but for now only a few of these have practical meaning. This is due to the fact that some zeolites after dehydration develop a system of very narrow unconnected channels, which make diffusion structurally difficult. Other zeolites, as a result of dehydration, undergo such changes in the frame and nature of cation locations that their structure becomes disrupted partially and the process of dehydration becomes irreversible. The structure of a molecular sieve zeolite must remain unchanged after dehydration.

Synthetic zeolite molecular sieves – sorbents are more suitable for research and are much better for industry requirements due to high uniformity and purity. These requirements are especially important where high degree of result repeatability is required (e.g. in industrial separation processes) and where even insignificant amounts of impurities may cause undesirable effects (such as the influence of iron usually contained in minerals on heterogeneous catalysis).

The characteristics and classification of complex aluminum silicates, such as zeolites (molecular sieves), is complicated by the absence of a definitive chemical nomenclature system. Under IUPAC, this should be based on the size of the elementary cell. This kind of nomenclature is cumbersome and requires knowledge of elementary cell composition, which is not always possible (e.g. natural zeolite analcime Na16(AlO2)16(SiO2)32 ∙ 16H2O, should be called 17-water 16-alumo-32-sodium silicate).

Therefore, at present, the following zeolite designation methods are used:

  1. Syntheric zeolite (molecular sieve) is designated by the letters introduced by the first researcher (by right of priority), e.g.: zeolite A, zeolite K-G, zeolite ZK-5 etc.

  2. These symbols are used for designation of the synthesized modification of the zeolite. The letter A, for example, means a synthetic Na16 zeolite (AlO2)16(SiO2)32 ∙ 16H2O (type 4A molecular sieve), obtained from the system of Na2O, Al2O3, SiO2 and H2O. The formula shows typical zeolite composition. Other designations include “Type A zeolite”, “Type X zeolite” etc.

  3. Sometimes, synthetic zeolites (molecular sieves) are designated by the name of the corresponding natural material, e.g. “analcime type zeolite” or “mordenite type zeolite”.

  4. Various cationic forms, obtained from ion exchange, may also have special designations. For example, the calcium form for A zeolite is designated as CaA (or 5A type molecular sieve). However, a hyphen between Ca and A, i.e. Ca-A, means an entirely different zeolite.

Zeolite A.

Zeolite A has two types of cavities: small (located in alpha cavities and are only accessible for small water molecules) and large (located in beta-cavities and are accessible to molecules of gases such as argon, oxygen and nitrogen). The free volume per elementary cell of zeolite is 926A3.

Zeolite

Adsorbed
substance

t, C

Vp, cm3/g

V’p, A3/el.cell

Zeolite
Na12(A) ∙ 24H2O

H2O

C2O

O2

  25

-75

-183

0.289

0.252

0.213

842

725

612

Zeolite
Ca6(A) ∙ 30H2O

H2O

O2

Ar

N2

n-Butane

  25

-183

-183

-196

  25

0.305

0.242

0.261

0.297

0.226

885

700

738

857

655

Zeolite X

The pore volume of this zeolite defined with water is 7908 A3 per elementary cell. Most adsorbed substances, apart form water, fill the large cavities of the zeolite (molecular sieve). Total pore volume, determined for large cavities when adsorbing argon or oxygen is 6700 A3 per elementary cell. Approximately 1200 A3 of the volume (150 A3 per each of the eight beta0cavities) of the elementary cell is only available for water molecule. The volume of each 26-hedral cavity in zeolite X is 822 A3. Free volume per elementary cell of the zeolite is 7832 A3 (7908 A3 for water).

Free volume in zeolite NaX (Si/Al = 1.25)

Adsorbed substance

t, C

Vp, cm3/g          

Number of molecules per elementary 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

Benzol

  25

0.30

45

(C4H9)3N

  25

0.29

16.4

Free volume in zeolite NaX (Si/Al = 1.33)

Hydrocarbon    

T, K

Vp, cm3/g

Number of molecules per elementary 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

Benzol

303

0.295

43.3

Toluene

313

0.301

36.8

Cyclopentane

303

0.334

45.1

Cyclohexane

313

0.268

33.0

Free volume in zeolite NaX (Si/Al = 1.33)

Hydrocarbon     T, KVp, cm3/gNumber of molecules per elementary cell
n-Pentane2980.31135.8
Isopentane2980.30934.8
n-Hexane2980.30931.3
n-Heptane2980.31128.3
n-Octane3130.30824.4
Isooctane2980.28222.7
Benzol3030.29543.3
Toluene3130.30136.8
Cyclopentane3030.33445.1
Cyclohexane3130.26833.0

Free volume in zeolite NaX (Si/Al = 1.33)                        

Hydrocarbon

T, K

Vp, cm3/g

Number of molecules per elementary 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

Benzol

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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обновлено: June 2, 2016 автором: globecore