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ZEOLITES
What are zeolites?
Zeolites are a class of naturally
occurring crystalline pourous alumino-silcates. They
have three-dimensional structures arising from a framework of [SiO4]4-
and [AlO4]5- coordination polyhedra (figure 1) linked
by all their corners. Collective polyhedra (figure
2) form frameworks that are generally open and contain channels and cavities
in which are located metal cations and water molecules.
Many of the naturally occurring zeolites can be created synthetically. One major advantage of zeolites is that since they are naturally occurring, they are often very cheap. Additionally, since they are composed largely of silicon, a major component of the earth’s crust, they find many uses in a more environmentally aware society.
Characteristics
The assemblages of tetrahedral atoms
create their porous structure with regular arrays of openings, or pores. These pores are of such a size as to be able to selectively
take up some molecules into their porous structure while rejecting others
on the basis of their larger effective molecular dimensions. This is the property of ‘molecular sieving,’ largely unique
to zeolites. Zeolites’ resistance to water degradation
account for their well-known use as a desiccant.
High-purity zeolites exhibit uniform
pore size that can be further tailored. Synthetic
zeolite characteristics are tailored by post treatment modification – including
pore size tailoring, surface treatments, acidity tuning, and changes in silica-alumuna
ratios.
Finally, zeolites exhibit high resistance
to changes in temperature, pressure, acidity, and moisture.
Structures
There are about 40 known naturally
occurring zeolites species in addition to about 150 synthetic species, with
possibly more based upon structural definitions (8).
As stated earlier, the tetrahedral
arrangements of [SiO4]4- and [AlO4]5-
coordination polyhedra create numerous lattices where the oxygen atoms are
shared with another unit cell. The net negative charge
is then balanced by cations (e.g. K+ or NH4+).
Small recurring unit can be defined for zeolites named, ‘secondary building units.’ Some of these building units are shown in figure 3 (3,19).
These unit cell cavities vary with
the building units and the number of cations present within a zeolite structure
and is determined by the number of [AlO4]5- tetrahedral
included in the framework. This arises from the isomorphous
substitution of Al3+ and Si4+ into the component polyhedra.
Types
The first zeolites recognized where
classified from volcanic rocks. Often collected by
museums because of there spectacular appearance, it is now recognized that
they are one of the most abundant natural occurring species. Natural zeolites are found in various settings such as
alkaline lakebeds, soils and land surfaces, marine deposits, and geothermal
deposits.
Initial efforts to synthesize zeolites
were carried out under high pressure/temperature conditions in order to
simulate those natural zeolites first discovered from volcanic deposits. Significant progress was made when synthesis was started
under normal atmospheric conditions (<100oC and atmospheric
pressure). Synthesis was also focused on recreating
natural zeolites, however, it was soon realized that many new structures
could easily be created. Computer predictions have
say that there are about six million conceivable zeolites (2).
General synthesis starts from crystallization
from an inhomogeneous gel, created from a silica source and an alumina source
combined with water. Some of the parameters that
control the type of zeolite formed are pH of the solution, temperature,
pressure, and crystallization time.
Synthetic zeolites are very small
compared to natural zeolites. This is due to the very
long crystallization time of natural zeolites in the earth.
Some of the zeolite properties that
are determined during synthesis include (2):
-Structure
-SiO2:Al2O3 ratio
-Pore size
-Density
Numerous practical applications of
zeolites exist. Some of the general principles are:
1.
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.
AS MOLECULAR SIEVES: CLEANING THE AIR WITH ZEOLITES
2.
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 function
as acid catalysts and can be used as supports for active metals or reagents.
They have also been used as oxidation catalysts. The main industrial application
areas are: petroleum refining, synfuels production, and petrochemical production.
Synthetic zeolites are the most important catalysts in petrochemical refineries.
3. Ion Exchange – The presence of the counterbalancing cations in the zeolite framework present the possibility for ion exchange if these cations are mobile. This ion exchange ability accounts for the greatest volume use of zeolites today (8). For example, zeolite A, a synthetic zeolite with sodium as a cation has widely replaced environmentally harsh phosphates as detergent water softeners. They do this by exchanging the sodium in the zeolite for the calcium and magnesium present in ‘hard’ water.
AS ION EXCHANGERS: REMOVING METALS FROM NATURE
4.
Table of
Other Environmental Applications of Zeolites (8)
