Geoscience Reference
In-Depth Information
The mechanisms of zeolite formation are very complex due to the plethora of
chemical reactions, equilibria, and solubility variations that occur through the
heterogeneous synthesis mixture during the crystallization process.
The process of zeolitization 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. The variables in the synthesis of zeolites are temperature,
alkalinity (pH), and chemical composition of the reactant mixtures. These variables
do not necessarily determine the products obtained in hydrothermal reactions
because nucleation appears to be kinetically rather than thermodynamically deter-
mined and controlled. The kinetic variables include the treatment of reactants prior
to crystallization, their chemical, and physical nature
[10,64]
.
With reference to the source materials, one would hope that a mole of SiO
2
would be equivalent to another SiO
2
, from whatever source, when compounded
into a gel. Experience teaches us that this is not the case; SiO
2
and Al
2
O
3
are to
some degree polymeric before gelation. Any reagent contains some unsuspected
impurities, which can be active on the synthesis. To duplicate the work, one has to
repeat with the same source material. The starting material cannot be just silica,
but precipitated silica, fumed silica,orsilica sol, from a designated supplier. For
very high silica phases, the alumina content of the silica source may be very impor-
tant. Besides, one has to carefully prepare the gel and the step-by-step procedure
has to be followed as noted. These steps include the order of mixing, the device
used to mix, the aging time and temperature, nucleation, and batch composition.
For example, the ingredients are not added sequentially but combined into two (or
three) subgroups that are blended to make the final gel. After a careful preparation
of the precursor, a suitable reaction vessel has to be selected to provide the desired
temperature, pressure, agitation, and the facility to withdraw an occasional sample.
Several companies from the United States of America, Japan, and Germany manu-
facture autoclaves with all such facilities for the synthesis of zeolites. Figure 3.27
shows the Parr autoclave commonly used in zeolite synthesis. For the commercial
production of zeolites, large-size autoclaves or batch reactors are usually employed.
The rate of heating is very important. If possible, a periodic/continuous monitoring
facility is necessary to obtain the values of pH, percent of crystallinity, and the rate
of crystallization. After carrying out the experimental runs, the filtration, washing,
drying, and yield estimation are to be done carefully. For a better reproducibility of
the results, the above aspects are to be kept in mind.
Several techniques are used to characterize the zeolite phases obtained. The most
commonly used techniques are X-ray powder diffraction, Rietvield technique of
structure refinement, neutron scattering, NMR, IR, thermal, SEM, laser Raman,
measurement of sorptive capacity, particle size and pore size distribution, and so on.
Davis and Lobo
[10]
have proposed two extremes of the mechanisms of zeolite
synthesis: (i) the solution-mediated transport mechanism and (ii) the solid-phase
transformation mechanism.
Figure 6.6
shows the schematic illustrations of the
solution-mediated transport and solid-hydrogen transformation crystallization
mechanisms. The first one involves the diffusion of aluminate, silicate, and/or
aluminosilicate species from the liquid phase to the nucleation site for crystal
