under hydrothermal conditions. For example, synthetic analogues of the mineral
faujasite (FAU) can be made from reaction between sodium silicate and sodium
aluminate under basic conditions at 60–150
C. Variations in the temperature,
charge-compensating cation, time of reaction, and the pH, amongst other variables, yield a variety of low-silica, aluminosilicate zeolites, though all such reactions are carried out under basic conditions. Post-synthesis modifications, such as
steaming or treatment with silicon tetrachloride, have been developed to increase
the Si/Al ratios of these materials. A major breakthrough in respect of the direct
synthesis of high-silica materials took place in the 1970s with the development of
synthetic routes involving the hydroxides of quaternary organic amines in place of
the normal inorganic base. These organic amines, which normally play the role
of template molecules or structure-directing agents, are incorporated into the final
product where they replace the usual charge-compensating cations. They play two
important roles: (i) the size and shape of the quaternary amine can lead to the
creation of new pore structures which cannot be made in other ways; (ii) the large
sizes of these cations, when compared with traditional metallic cations, means that
they can only compensate for a relatively low framework charge, thus leading to
the formation of high Si/Al materials. The organic cation can normally be removed
from the zeolite by calcination, leaving behind the acid form of the zeolite (see
above). A classical example is zeolite ZSM-5, which is made in the presence of tetramethylammonium cations, TMA [28]. Curiously, over 20 years after its synthesis
in the laboratory, a natural analogue of ZSM-5, known as mutinaite, has recently
been discovered [29]; its Si/Al ratio, equal to 7.6, is the highest found in any natural zeolite.
In addition to the aluminosilicates, some obvious synthetic analogues, such as
borosilicates, gallosilicates and ferrisilicates, have also been explored. The borosilicates are more difficult to prepare, probably because the much smaller boron,
with its tendency towards planar, three-fold coordination, is not an obvious substitute for the role of aluminum. Gallium and iron(III) materials, though, are well
known and usually have properties similar to those of the aluminosilicates. There
is evidence, however, for some unique behavior with gallosilicates, as in their ability to catalyze cyclization reactions and activate methane [30]. Other silicate-based
systems include the titanosilicates, mentioned earlier, and some recently discovered vanadium silicates that show excellent thermal stability and can potentially be
activated for catalysis [31].
Another interesting development has been the direct synthesis of pure silica
materials with no exchangeable cations. The synthesis of the silica polymorphs,
which include silicalite (MFI) [32] and siliceous ferrierite (FER) [33], may still require template molecules or structure-directing agents, and there is an interesting
question as to how the charge of the quaternary amine is compensated in the absence of framework aluminum. Since they are completely ordered (once the organic has been removed), they provide excellent vehicles for detailed characterization by
17 O and
29
Si NMR studies [34], as well as for computer simulation studies
[35]. They are also very interesting candidates in the context of low-dielectric materials for integrated circuit manufacting [36] since they can be made defect free
18 Nanoporous Materials
594
faujasite (FAU) can be made from reaction between sodium silicate and sodium
aluminate under basic conditions at 60–150
C. Variations in the temperature,
charge-compensating cation, time of reaction, and the pH, amongst other variables, yield a variety of low-silica, aluminosilicate zeolites, though all such reactions are carried out under basic conditions. Post-synthesis modifications, such as
steaming or treatment with silicon tetrachloride, have been developed to increase
the Si/Al ratios of these materials. A major breakthrough in respect of the direct
synthesis of high-silica materials took place in the 1970s with the development of
synthetic routes involving the hydroxides of quaternary organic amines in place of
the normal inorganic base. These organic amines, which normally play the role
of template molecules or structure-directing agents, are incorporated into the final
product where they replace the usual charge-compensating cations. They play two
important roles: (i) the size and shape of the quaternary amine can lead to the
creation of new pore structures which cannot be made in other ways; (ii) the large
sizes of these cations, when compared with traditional metallic cations, means that
they can only compensate for a relatively low framework charge, thus leading to
the formation of high Si/Al materials. The organic cation can normally be removed
from the zeolite by calcination, leaving behind the acid form of the zeolite (see
above). A classical example is zeolite ZSM-5, which is made in the presence of tetramethylammonium cations, TMA [28]. Curiously, over 20 years after its synthesis
in the laboratory, a natural analogue of ZSM-5, known as mutinaite, has recently
been discovered [29]; its Si/Al ratio, equal to 7.6, is the highest found in any natural zeolite.
In addition to the aluminosilicates, some obvious synthetic analogues, such as
borosilicates, gallosilicates and ferrisilicates, have also been explored. The borosilicates are more difficult to prepare, probably because the much smaller boron,
with its tendency towards planar, three-fold coordination, is not an obvious substitute for the role of aluminum. Gallium and iron(III) materials, though, are well
known and usually have properties similar to those of the aluminosilicates. There
is evidence, however, for some unique behavior with gallosilicates, as in their ability to catalyze cyclization reactions and activate methane [30]. Other silicate-based
systems include the titanosilicates, mentioned earlier, and some recently discovered vanadium silicates that show excellent thermal stability and can potentially be
activated for catalysis [31].
Another interesting development has been the direct synthesis of pure silica
materials with no exchangeable cations. The synthesis of the silica polymorphs,
which include silicalite (MFI) [32] and siliceous ferrierite (FER) [33], may still require template molecules or structure-directing agents, and there is an interesting
question as to how the charge of the quaternary amine is compensated in the absence of framework aluminum. Since they are completely ordered (once the organic has been removed), they provide excellent vehicles for detailed characterization by
17 O and
29
Si NMR studies [34], as well as for computer simulation studies
[35]. They are also very interesting candidates in the context of low-dielectric materials for integrated circuit manufacting [36] since they can be made defect free
18 Nanoporous Materials
594