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In-Depth Information
kept in mind is that template removal has to be possible without structural col-
lapse. The most common method to remove the surfactant templates is mild
extraction or ion exchange, and sometimes, high-temperature calcination is also
used that may cause partial collapse of mesoporous networks and inevitably intro-
duce some inorganic impurities. Extraction processes are best suited for nonionic
surfactants. If ionic surfactants shall be extracted, the extraction typically has to be
combined with an ion exchange because the surfactant also compensates frame-
work charges, which may bring new functionalities such as high H
+
exchange
capability.
Finally, if mesoporous materials shall be used under harsh conditions such
as high-temperature reactions, problems can arise during reaction process.
Crystallization of the hybrid framework represents an alternative way. However,
the presence of the mesostructure and crystallinity of the walls are typically not
compatible because crystalline materials can in most cases not accommodate the
type of curvatures present in the mesostructures. Thus, as soon as crystallization
of the walls to bulk occurs, the mesostructures deteriorate or collapse, as has been
observed in many cases. Thus, further exploration including new synthesis strate-
gies involving novel emerging techniques is worthy of tremendous endeavors.
3.2.3 Mesostructure Design
The adjustment of mesostructures is still a challenging task in the preparation of
periodic mesoporous hybrid materials, while it is a key step in regulating their phys-
icochemical properties. A lot of factors could cause the phase transformation of the
mesopores, including interactions between the organic and inorganic species [
60
],
the reaction temperature and time [
61
], the addition of some inorganic additives
[
62
], the nature of surfactant that could be clarified using the molecular surfactant
packing parameter [
63
], the molar ratios of the surfactant and inorganic precursor
[
64
], and so forth. Herein, the molar ratios of the reactants were found to be critical
for determining the final mesostructures of the hybrid materials, and different meso-
phases could be obtained by adjusting the amount of added reagents and surfactant.
A synthesis condition map of periodic mesoporous titanium phosphonates with vari-
ous phases has been explored (Fig.
3.12
) [
65
]. The molar ratios of Ti/P should be
fixed as 3:4 and 1:4 for hexagonal and cubic mesostructures, respectively, under the
experimental conditions, while mixed phases with poor pore periodicity also existed
at these two Ti/P ratios. It was easy to understand the Ti/P ratio in hexagonal meso-
phases because one Ti atom was coordinated to four P atoms, whereas one P was
coordinated to three Ti atoms and was also connected to one C atom. The P spe-
cies was superfluous in the cubic mesophases, which was probably due to the exist-
ence of some P atoms with a low-coordination state. By varying the molar ratios
of C
16
TABr/Ti, a general range for the synthesis of different mesophases was con-
firmed, namely in the region of 0.1 < C
16
TABr/Ti < 0.4 (Ti/P
=
3:4) for a hexagonal
phase, at 0.4 < C
16
TABr/Ti < 1.9 (Ti/P
=
3:4, 1:4) for a mixed phase with poor pore
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