About Zeolites - Molecular Sieves - Adsorbents
Active coals and clays, non-organic gels (such as silica gel or active aluminium oxide), and crystal aluminosilicates (which are zeolites, or molecular sieves) are all applicable as industrial adsorbents, due to their well-developed system of pores for the separation of gases and vapours
Active coals, active aluminium oxide, and silica gel do not have a regular crystalline structure, and are therefore characterized by heterogeneous porosity. The distribution of pores in relation to diameters of adsorbent particles can be narrow (from 20–50 A) or wide (from 20 to a few thousand angstroms). Zeolites have homogenous pores (from 3–10 A), the size of which is clearly defined by the construction of the elementary structure of the crystal. All the pores are unable to adsorb molecules, the size of which exceeds the diameter of the pores.
The term 'molecular sieve' was first introduced by J. McBen to designate adsorbents capable of acting as a sieve in the separation of different types of molecules. Now other synonyms are used such as 'synthetic zeolite', 'sorbent', and 'adsorbent'.
Currently it is known that the most powerful molecular sieve effects are attained by dehydrated zeolite crystals. Due to their system of channels or cavities, these adsorbents possess a well-developed internal surface accessible to adsorbent molecules. In addition, the proportion of external surface of zeolite (molecular sieve) comprises only a very small part of the total surface.
Zeolites (molecular sieves) demonstrate a degree of selectivity with regard to different types of molecules and the molecular sieve (zeolite) activity can be expressed at varying degrees. In the case of a complete molecular sieve (zeolite) activity effect, certain molecules diffuse within the crystal, while the diffusion of other molecules is excluded.
Zeolites (molecular sieves) now are used not only in adsorption, but in other very diverse areas of chemistry, for example in catalysis and ion exchange. As before, they are still referred to as 'molecular sieves', although this term suggests many other areas of use of zeolites, and does not reflect, in particular, the well-known property of zeolites to display sieve effects in catalytic and ion exchange reactions.