Geoscience Reference
In-Depth Information
In zeolite synthesis, the term templating is used in a loose sense to refer to the
filling of the void spaces of a zeolite structure by the bulky organic/inorganic
cations frequently used as structure-directing or structure-stabilizing agents. They
contribute to the formation of the zeolite lattice during the zeolitization by influ-
encing the gelation and/or nucleation processes and by lowering the chemical
potential of the lattice formed by the inclusion of the templates during zeolite syn-
thesis. Templating contributes to the stability through new interacts (hydrogen
bonds and electrostatic and London dispersion interactions) and further controls the
formation of a particular topology through its geometry (form and size)
[96]
. Thus,
the action of a template appears to have both electronic and steric components. It is
evident that changes in the cation density provoked by geometrical or physical
properties of the template will be reflected in the chemical composition (Si/Al
ratio) of a given topology. In the early 1960s, the TMA (Thermal-Mechanical
Analysis) cation was introduced as the first organic cation to be used in zeolite syn-
thesis
[47,97]
.
Thus far, it has not been possible to predict the actual role of templates and which
template is reported to obtain a particular structure and composition. Therefore, in
selecting possible templates, one has to bear in mind some general criteria regarding
the templating potential in zeolitization, such as solubility in the solution, stability
under synthesis conditions, steric compatibility, and possible framework stabilization,
and the most important practical issue is to remove the template without destroying the
framework. The synthesis of omega zeolite is strongly favored by the presence of
TMA
1
ions; thus, templates can be structure-making or structure-breaking agents
[98,84]
. Also, it is well known that neutral molecules, as well as cations or ion pairs,
are able to fulfill structure- and composition-directing functions. Each one of these can
be organic or inorganic in nature. Recently, Feng et al.
[99]
have carried out the
amine-directed synthesis for some of the zeolites. It is clear from the literature data that
multiple small organic cations cooperatively template the extra-large micropores, in
contrast, highly siliceous zeolites template as one bulk large molecule. On the basis of
structural analysis of the extra-large micropore compounds, Li et al.
[58]
proposed the
following formation mechanism (
Figure 6.15
): In the initial reaction, the inorganic and
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