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Zeolite Molecular Sieve
Zeolite molecular sieves are characterized by the following properties:
Zeolite - Structure and Properties
Zeolite molecular sieves are crystalline, highly porous materials, which belong to the class of aluminosilicates. These crystals are characterised by a three-dimensional pore system, with pores of precisely defined diameter. The corresponding crystallographic structure is formed by tetrahedras of (AlO4) and (SiO4). These tetrahedras are the basic building blocks for various zeolite structures, such as zeolites A and X, the most common commercial adsorbents.
Due to the presence of alumina, zeolites exhibit a negatively charged framework, which is counter-balanced by positive cations resulting in a strong electrostatic field on the internal surface. These cations can be exchanged to fine-tune the pore size or the adsorption characteristics. For instance, the sodium form of zeolite A has a pore opening of approximately 4 Ångstrom (4 x 10–10 m), called 4A molecular sieve. If the sodium ion is exchanged with the larger potassium ion, the pore opening is reduced to approximately 3 Ångstrom (3A molecular sieve). On ion exchange with calcium, one calcium ion replaces two sodium ions. Thus, the pore opening increases to approximately 5 Ångstrom (5A molecular sieve). Ion exchange with other cations is sometimes used for particular separation purposes.
The pore opening of the sodium form of zeolite X (13X) is approximately 8 Ångstrom.
The ability to adjust the pores to precisely
determined uniform openings allows for molecules smaller than its
pore diameter to be adsorbed whilst excluding larger molecules,
hence the name “molecular sieve”. The different pore
sizes of synthetic zeolites open up a wide range of possibilities
in terms of "sieving" molecules of different size or shape
from gases and liquids.
Selective Adsorption of Water and other Polar Substances
The up-take of water or other species in zeolites is called adsorption
and functions on the basis of physisorption. The main driving force
for adsorption is the highly polar surface within the pores. This
unique characteristic distinguishes zeolites from other commercially
available adsorbents, enabling an extremely high adsorption capacity
for water and other polar components even at very low concentrations.
In addition, the pore size plays a significant role, allowing or prohibiting the entrance of molecules to the pore system.
The adsorption on molecular sieves is therefore dependent on the following physical molecular properties:
Of note is the high capacity of Zeolite Molecular Sieves even at low water concentration, allowing to dry to very low water contents. The Zeolite molecular sieve can retain its high capacity at high temperature, which makes it the optimal material if drying needs to be carried out at comparatively high temperatures.
The adsorption process is fully reversible and of purely physical nature. The structure of the zeolite stays intact during the adsorption process (and its later regeneration), and dissolution effects like with other drying agents like calcium compounds can not happen.
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