Zeolite is allumosilicate with sceletal structure containing holes
occupied by large ions and water molecules which have substantial
freedom of movement. This leads to possibilities of ion-exchange
and reversible dehydratation.
The zeolite minerals form the most
numerous group of the silicate minerals - more than 40 mineral
species. All they differ each from another by their structure
and chemical composition. The zeolite frameworks are constructed
by Si and Al oxide thetrahedrons, forming regular system of holes.
The Si/Al ratio varies(1:1 - 6:1 - N:1), but is never less than
1. There are several isostructural zeolite minerals, having one
and the same framework, but they have different zeolite chemistry
(framework chemistry - Si/Al ratio and cationic chemistry - the
cation content).
Some pictures of zeolite frameworks must be here
Overall chemical composition of the zeolites can be represented
with the following oxide formula:
Mx/nO.Al2O3.xSiO2.yH2O
or more precizely as
Mx/n[AlxSiy)2(x+y)].pH2O
where :
- M are monovalent (Na,K,Li) and/or bivalent
(Ca, Mg,Ba etc.) cations;
- n is the charge of the cations;
- 1 < y/x < 6;
- 1 < p/x < 4;
- Water sorption/desoprtion
The water molecules in the zeolite channels are bonded by the
forces of dipole interactions to the cations, forming their
close surrounding. The water from the zeolite crystals can be
removed during thermal treatment, often called "thermal activation"
or "dehydration". The dehydration process is equilibric and
is depending on the temperature and the partial water pressure.
The curve of the dehydration (water loss vs. temperature at
a constant pressure) is informatible for the water state in
the zeolite - the "wide porous" zeolites show wide temperature
range of the dehydration starting at room temperature, when
"narrow porous" zeolites show narrow and more distinguishable
curves with the beginning of the dehydration process at higher
temperatures. The overall water content in saturated state and
the character of the dehydration depend not only to the zeolite
framework type, but also to their cationic content.
During the dehydration process the frameworks of some zeolites
are changed irreverively. After these changes are observed,
the ability of reverse water saturation ("rehydration") is lost
permanently. That's why some zeolite types are consider as "thermostable"
and the temperature of the framework destruction is cited as
their thermostability.
The dehydrated stable zeolites are very good water sorbents,
especially at low partial pressure of water vapours. That's
why they are used as dessicants in several industrial processes.
- Adsorption, molecular
sieve effects
- Ion exchange
- Catalytical properties
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