Hydrothermal Synthesis and Growth of Zeolites - Hydrothermal Technology - page 302

K 1 , and Rb 1 , which
also interact with the water molecules, break the original hydrogen bonds, and,
therefore, are known to break the water structure [84] . However, there are some
large cations, like tetra alkylammonium, which seem to organize the water struc-
ture; the other templating neutral molecules used are amines, ethers, alcohols, di,
and triols, and so on [96,100] .
The presence of salts such as NaCl, KCl, KBr, CaF 2 , BaCl 2 , and BaBr 2 during
zeolite synthesis can be an important factor as these ion pairs may be occluded
(possibly next to water molecules) into the pore system, thereby stabilizing the
pore structure. These nonvolatile species increase the silica, which is soluble with
an increase in temperature, and open several new pathways for zeolites synthesis
and also for new products besides improving crystallinity and yield.
As mentioned earlier, the use of organics in the zeolite synthesis has opened up
a debate whether their presence is required at all or the zeolite synthesis can be
carried out even without these organics in the system. Application of templating
methods to produce zeolite materials with hierarchical bi- or trimodal pore size is
a popular area of research, although several nontemplated approaches also exist. In
general, mesopores can be templates in zeolite materials by the use of solid tem-
plating, supramolecular templating, or indirect templating [101] . The researchers
are contemplating the development of room temperature synthesis of mesoporous
zeolites through an appropriate and highly efficient template selection [102] .
Recently, Na et al. [103] have come out with a breakthrough in zeolite synthesis,
which leads to the direction of zeolite structures into hierarchically nanoporous
architectures. The organics also influence the size and morphology of the zeolites
in addition to the structure-building activity. The surface characteristics are also
altered significantly thus making the surface either hydrophobic or hydrophilic for
specific applications.
The reader can go through the excellent reviews given in Refs [101]

The structure-breaking cations are large cations, like NH 4 ;

[105] on
the role of organic substances in the synthesis of zeolites. Table 6.16 indicates that
organic bases facilitate the formation of several novel structures, especially those
with extremely silica-rich zeolites, such as the pentasils, ZSM-5 and ZSM-11, and
also zeolite Nu-1 and Fu-1 [40] . Table 6.17 shows the organic zeolite structure
relationships [89] . In recent years, several zeolites, like ZSM-5, mordenite, and
other related zeolite structures, could be synthesized in the absence of any organic
compound; here the hydrated Na 1 ions are able to function as templates for the
formation of SBUs.
In recent years, templating has found several new applications [106] . Among the
inorganic materials, zeolites have been widely used as hosts for a variety of materi-
als, including conjugated polymers, selenium, chains, and so on. Zeolites, as well
as glass or alumina nanoporous membranes, have been used to fabricate organic
and inorganic tubules or fibrils [107] . Carbon nanotubes can serve as removable
templates for other inorganic nanorods/nanowires, e.g., GaN, Cu, or V 2 O 5 [108] .
The mesoporous materials can be fabricated with a variety of shapes and sizes,
from nanostructures to almost millimeter-size objects [109] .