Molecular Sieves

Molecular sieves are synthetic and naturally occurring zeolites with well-defined structures that have found extensive use for the separation of permanent (or fixed) gases. Molecular sieves have a pore size that is defined by the particular ion used in the preparation

Table 1 General application fields of adsorbents

Absorption material

Applications

Molecular sieves

Permanent gases, hydrogen isotopes,

CO, N2O

Porous polymers

Volatile polar and nonpolar compounds;

samples containing water; CFCs;

solvents

Alumina

Hydrocarbon impurities in C1-C5

hydrocarbons

Carbon

CO and CO2 in air; impurities in ethylene

Silica

C1-C3 hydrocarbons, sulfur gases;

hydrocarbon and semipolar impurities;

samples containing water

CFC, chlorofluorocarbon.

CFC, chlorofluorocarbon.

of the material - calcium aluminium silicate gives a pore size of 0.5 nm, whereas sodium aluminium silicate gives a pore size of approximately 1 nm. These are the two commonest molecular sieves; other pore sizes are available but are less widely used. The separation on a molecular sieve is based on more than one retention mechanism. The first selection depends on size - molecules that are smaller than the pore size will diffuse inside the pores. Once inside the cavities, the small molecules can interact with a large surface area, which means that they will have relatively long retention times. Large molecules such as branched alkanes or sulfur hexafluoride (SF6) are too big to enter the pores and these compounds will elute earlier. Compounds that are too large to enter the pores will only be retained by relatively weak adsorption on active sites on the outside of the particles, and thus give shorter retention times.

The retention of components with dipole interaction and hydrogen bond formation, like water and carbon dioxide, is very high. Carrier gas and samples should be as dry as possible. Water is absorbed by the molecular sieve and will cause a reduction of retention times. The water can be removed by heating for a few hours at 300°C. A molecular sieve of pore size 0.5 nm is an ideal adsorbent for the separation of permanent gases; this is also the main use of the column. Normally molecular sieves are not used for separations above C2 except for hydrocarbon type analysis. Higher boiling compounds are strongly adsorbed and can only be eluted by using undesirably high temperatures (molecular sieves are good catalysts!), vicinal exchange coupled with backflush techniques or even destruction of the sieve with hydrogen fluoride. A typical separation of a permanent gas mixture is shown in Figure 1. The 0.53 mm fused silica column is usually coated with a 50 |im layer of 0.5 nm molecular sieve to generate sufficient retention to make a high resolution separation possible at temperatures above ambient. The separation of argon and oxygen is baseline. The 50 | m layer provides a relatively high retention for carbon dioxide. If the separation of argon-oxygen is not important, a 15 |im film can be used. For a comparison of the separations with 50 and 15 | m layers, see Figures 1 and 2. Several other applications have been reviewed. Molecular sieves are also successfully coated onto Ultimetal capillary columns of 0.53 mm i.d. Applications of these columns are especially of interest for analyser systems where reliability is a major issue (see metal PLOT columns below).

Molecular sieves of the 13X type are also used. These materials have a lower absolute retention due to the larger pores. They are used in the petroleum

Figure 1 Permanent gases on a 50 |im Molsieve 0.5 nm PLOT column. Experimental details: column, 50 m x 0.53 mm fused silica; oven, 30°C; carrier gas, hydrogen; injection, split; detection, TCD. Peaks: 1, helium; 2, neon; 3, argon; 4, oxygen ; 5, nitrogen; 6, methane.

industry for the type separation of naphthenes from paraffins, olefins, naphthenes and aromatics (PONA analysis), mainly in packed column configurations.

Figure 2 Fast separation of permanent gases on a 15 |im Molsieve 0.5 nm PLOT column. Experimental details: column, 30 m x 0.53 mm fused silica, Molsieve 0.5 nm df = 15 |im; oven, 50°C; carrier gas, hydrogen; injection, split; detection, TCD. Peaks: 1, helium # neon; 2, argon # oxygen; 3, nitrogen; 4, methane; 5, carbon monoxide.

Figure 2 Fast separation of permanent gases on a 15 |im Molsieve 0.5 nm PLOT column. Experimental details: column, 30 m x 0.53 mm fused silica, Molsieve 0.5 nm df = 15 |im; oven, 50°C; carrier gas, hydrogen; injection, split; detection, TCD. Peaks: 1, helium # neon; 2, argon # oxygen; 3, nitrogen; 4, methane; 5, carbon monoxide.

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Solar Panel Basics

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