Zeolites - Frequently Asked Questions
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Zeolites are three-dimensional, micro porous, crystalline solids with
well-defined structures that contain aluminum, silicon, and oxygen in
their regular framework; cations and water are located in the pores. The
silicon and aluminum atoms are tetrahedrally coordinated with each other
through shared oxygen atoms. Zeolites are natural minerals that are mined
in many parts of the world. Zeolites have void space (cavities or channels)
that can host cations, water, or other molecules. Because of their known
structure, they behave in a predictable fashion.
In 1755, the Swedish mineralogist Axel Fredrick Cronstedt discovered that
stilbite, a natural mineral, visibly lost water when heated, and he named
the class of minerals zeolites from the classical Greek words meaning
'boiling stones.' Zeolites were considered an obscure group of minerals
with unique properties for almost 200 years, and Cronstedt was remembered
primarily for discovering the element nickel.
ADSORPTION - Zeolites are used to adsorb a variety of materials.
This includes applications in drying, purification, and separation. They
can remove water to very low partial pressures and are very effective
desiccants, with a capacity of up to more than 25% of their weight in
water. They can remove volatile organic chemicals from air streams,
separate isomers and mixtures of gases.
CATALYSIS - Zeolites can be shape-selective catalysts either by
transition state selectivity or by exclusion of competing reactants on
the basis of molecular diameter. Zeolites can also be acid catalysts and
can be used as supports for active metals or reagents. They have also
been used as oxidation catalysts.
ION EXCHANGE - The largest volume use for zeolites is in detergent
formulations where they have replaced phosphates as water-softening
agents. They do this by exchanging the sodium in the zeolite for the
calcium and magnesium present in the water.
Many zeolites are thermally stable to over 500 degrees Celsius. Some are
stable in an alkaline environment, and some are stable in acidic media.
They are also stable to ionizing radiation and can be used to adsorb
radioactive cations.
Zeolites can separate molecules based on size, shape, polarity, and degree
of saturation, among others.
Zeolites should they become saturated with adsorbed water due to continual
levels above equilibrium can be regenerated using relatively easy methods,
such as heating or placing in sunlight to remove adsorbed water.
The combination of many properties, among them: the micro porous character
of the uniform pore dimensions, the ion exchange properties, the ability to
develop internal acidity, the high thermal stability, the high internal
surface area. These make zeolite unique among inorganic oxides.
