2. A zeolite mineral is a crystalline substance with a structure
characterized by a framework of linked tetrahedra, each consisting of four O
atoms surrounding a cation. This framework contains open cavities in the form
of channels and cages. These are usually occupied by H2O molecules and
extra-framework cations that are commonly exchangeable. The channels are
large enough to allow the passage of guest species. In the hydrated phases,
dehydration occurs at temperatures mostly below about 400 °C and is largely
reversible. The framework may be interrupted by (OH, F) groups; these
occupy a tetrahedron apex that is not shared with adjacent tetrahedra
Inorganic, crystalline, aluminosilicate materials – 3D network-cavities and
cages
Consist of interconnected SiO2 and AlO2 tetrahedra , Cations and H2O
Chemically and thermally stable
Tetrahedra form 3D frameworks with linked channels down to molecular level
Usually microporous
Recommended Nomenclature for Zeolite Minerals: Report of The Subcommittee on
Zeolites of the International Mineralogical Association, Commission on New Minerals and
Mineral Names. The Canadian Mineralogist 35 (1997) 1571 – 1606.
4. Zeolite Building blocks
Schematically, oxygen are shown as
straight lines (bonds) and Si at the
corners
All corners are connected → 4 lines from
each Si
O
Si
Si
O
Si
Si
O
O
Si
Zeolite - Silicalite
5. APPLICATION:
- cation exchange
- adsorption and related molecular sieving
- dehydration and rehydration
- biological activity
- catalytic
.
*** the ability to exchange inherent cations for
other cations on a basis of ion selectivity- used to
reomve the heavy metals from effluents
zeolites have different dimensions of pores and channels responsible for
their physical and chemical properties
6. Product Shape Selectivity;
benzene + methanol = xylene
Para-xylene is far more valuable than ortho or meta xylene -
used in polyester manufacture
8. Adsorption and related molecular sieving
The adsorption on molecular sieves is therefore dependent on the
following physical molecular properties:
- Size and Shape; molecules or ions larger than the pore opening of
the zeolite can not be adsorbed, smaller molecules or ions can.
-Molecular Polarity; due the charge on outer and inner zeolitic
surface particle, molecules with large polarity or polarisability can
be adsorbed preferentially under identical conditions.
The adsorption process is fully reversible and of purely physical
nature. The structure of the zeolite is unchanged during the
adsorption process and its later regeneration.
9. difference between natural zeolite minerals is manifested in content of hydrated
cations which occupied pores and channels, ratio of Si/Al in structure and number
of water molecules
• Trap selectively gas molecules of the right size
• Trap selectively gas molecules of the right shape
• Trap selectively gas molecules of the right polarity
• Highly polar materials, ion conductors and ion exchangers
• Al+3 acidic site: catalyze a number of chemical reactions
• Room temperature operation
• High specific surface
Use of Zeolites for sensing
