Zeolite Structure
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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.
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| Molecular Sieve Type A |
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Molecular Sieve Type X
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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.
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The pore opening of the sodium form of zeolite X (13X) is approximately 8 Ångstrom.
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