Geoscience Reference
In-Depth Information
counterparts in natural zeolites. New synthetic zeolites' phases will continue to
grow in numbers faster than the natural zeolites.
6.5 Synthesis of Zeolites
Efforts to synthesize zeolites can be traced as far back as 1848 when W¨hler
[38]
first recrystallized apophyllite by heating it in water solutions at 180
190
C under
10
12 atm. Followed by this, several more attempts were made during the 1860s
when Claireville
[39]
synthesized zeolites by the hydrothermal method in 1862.
Today, over 3 million tons of zeolites are produced annually for use in laundry
detergents, cat litter, filters, molecular sieves, selective catalysis, medical, and so
on with China alone producing more than 2 million tons followed by countries
such as South Korea, Japan, Jordan, Slovakia, Turkey, and United States of
America. The hydrothermal reaction merges into a low-temperature reaction and
indeed, in zeolite synthesis, the trend has been inter alia for greater experimental
convenience, to grow zeolites at or below 100
C provided the reaction rates are
adequate. It was only during the 1940s that great attention was devoted to zeolite
synthesis as a result of the pioneering work by Barrer
[40]
and Milton
[41]
. Even
now, the area of zeolite synthesis is still expanding and hence is resulting in the
discovery of synthetic zeolites with new topologies and new catalytic sorption and
separation properties. Generally, three processes are used to produce zeolites and
are as follows:
1. Preparation of molecular sieve zeolites as high-purity crystalline powders.
2. The conversion of clay minerals into zeolites.
3. Processes based on the use of other naturally occurring raw materials.
It is not surprising that much effort is made to unravel the mechanisms responsi-
ble for the formation of zeolites from its precursors. Using this knowledge,
concepts may be derived for tailor-made zeolite synthesis, making the art of creat-
ing zeolite synthetics a real scientific issue. Hydrothermal synthesis of aluminosili-
cate zeolites involves a few elementary steps by which a mixture of Si and Al
species, metal cations, organic molecules, and water is converted via an alkaline
supersaturated solution into a microporous crystalline aluminosilicate. The complex
chemical processes involved in this transformation can be denoted as zeolitization.
Since the 1940s, systematic studies on synthesis have been disclosed. During 1948,
Barrer
[40]
synthesized the first zeolite that did not have a natural counterpart.
However, zeolite technology was initiated during the late 1940s on a large scale,
essentially by the groups of Barrer and Milton. They developed hydrothermal
synthesis using reactive alkali-metal aluminosilicate gels at
low temperatures
100
C) and pressure (autogeneous). By 1958, under the leadership of Milton,
the Linde Division of Union Carbide had successfully synthesized nearly all the
commercially important zeolites
[41
(
B
45]
. The next advance in zeolite synthesis
was again due to Barrer
[40]
. In 1961, Barrer and Denny
[46]
reported zeolites'
