Geoscience Reference
In-Depth Information
solution into microporous crystalline zeolites. The complex chemical processes
involved in this transformation can be denoted as zeolitization. The process of zeoliti-
zation is thermally activated and usually takes place at elevated temperatures in order
to achieve a high yield of crystals in an acceptable period of time.
In spite of the success of the last 60 years in the synthesis of zeolites, the factors
controlling the synthesis of porous crystalline aluminosilicates or aluminopho-
sphates are not well defined. There are contrasting opinions on the mechanism of
their formation and the role of organics in the hydrothermal system. Several labora-
tories around the world have synthesized zeolites with large and small pores by
using a combination of inorganic and organic materials to guide the structures as
they are formed. Meanwhile, several other researchers especially from Germany
and France discovered that, by carefully controlling the processing parameters dur-
ing the growth, they could form a large-pore zeolite without the need for expensive
organic compounds typically used to guide their architecture as they grow. This
motivates for the cheaper catalysts
[62]
. Tsapatsis and colleagues
[63]
at the
University of Minnesota have discovered a breakthrough method for chemical
separations by using ultrathin zeolite membranes, which is highly useful for a wide
variety of chemically selective filters. In this connection, the IZA Synthesis
Commission has given an outline of a thorough report of a molecular sieve synthe-
sis and the following product characterization
[61]
. The priority designation is
a consensus opinion of the committee as to the importance that we attach to the
item. The simplest experiments will lack details in some of these categories but not
in all cases. The author considered whether the particular procedure is unique in
any way and might be significant in the overall result. Negative answers to the
implied question can be helpful.
Similarly, the IZA Synthesis Commission gives two more outlines dealing with
the background material and synthesis report of a hydrothermal zeolite. The back-
ground material covers aspects like source materials, gel preparation, crystalliza-
tion, product recovery, and characterization. These aspects have been discussed in
great detail by several authors from time to time.
Synthetic zeolites are generally made by mixing solutions of aluminates and
silicates, often with the formation of a gel, and by maintaining the mixture at
temperatures of 100
C or more for selected periods
[40,61]
.
The synthesis of zeolites involves several steps, as shown below:
Reactant
k
Reactant mixtures
k
Nucleation
k
Crystal growth
