Chemistry Reference
In-Depth Information
4.4 Monoliths
Rigid monoliths are of great significance to optical devices. The molding of the
powders into monoliths is also necessary with several advantages including
mechanical stability, ease of handling and recovery, and greater structural uniform-
ity to meet the broad needs of industry and household. They can be directly used
across the areas of catalysis, sorption, separation, and water treatment.
Periodic mesoporous titanium phosphonate (PMTP-2) monoliths were synthe-
sized by combining autoclaving process and EISA strategy. After experiencing
low-temperature hydrothermal aging of the reaction mixture of HEDP and TiCl
4
in the presence of oligomer surfactant Brij 56, the complete condensation and
coordination of titanium and phosphonic acid could result in the generation of a
transparent liquid with great viscosity. The solvent was subsequently evaporated
at 50 °C, similar to the EISA method, resulting in titanium phosphonate mono-
liths (Fig.
4.6
) [
21
,
22
]. This might be caused by the polymerization of the organic
bridged groups in the network with the inorganic species, making them more like
some kinds of macromolecular polymer with mechanical strength and ductility. A
mild ethanol exaction process could efficiently remove the involved soft template
molecules, leaving highly ordered hexagonal mesostructures [
21
]. The as-synthe-
sized samples could be molded into various macroscopic morphologies, and valu-
ably, the monolithic shape could be well preserved even after surfactant removal,
which may potentially fulfill the qualifications for some industrial devices. The
resultant surface area and pore volume were determined to be 1,034 m
2
g
−
1
and
0.51 cm
3
g
−
1
, respectively. The organophosphonate groups were homogeneously
incorporated in the framework of the periodic mesoporous hybrid solids, present-
ing a thermal stability up to approximately 450 °C.
A major challenge for the practical use of MOFs is to deliver them in a suitable
shape. However, coordination polymers are mostly obtained and characterized in a
powdered crystalline state, and thus typically compounding with binders and pel-
letizing is required causing a reduction of the inner surface area and pore blocking.
On the contrary, coordination polymer gels should provide a versatile access to
the synthesis of porous solid bodies of any desired shape at relatively low costs.
Aerogels combine both high surface area and good surface accessibility due to
their bimodal micro- and macroporous structure. Therefore, they can be utilized
as catalyst supports or as catalysts themselves. Kaskel and coworkers reported the
synthesis of iron-BTC aerogels with high permanent porosity and total pore vol-
umes of up to 5.6 cm
3
g
−
1
by a solgel approach [
23
]. The resulting purified gel is
placed in an autoclave and subsequently the adsorbed ethanol is exchanged with
CO
2
for a time of about 24 h to produce monolithic aerogels. Depending on the
initial concentration of the solutions, aerogels with densities of 14.5, 32.4, 62.1,
and 110.5 mg cm
−
3
are obtained from 0.05, 0.1, 0.2, and 0.4 M trimesic acid solu-
tions, respectively. Figure
4.7
showed an aerogel and the corresponding xerogel
that was dried at 40 °C for 48 h. The picture highlights the shrinkage of air-dried
gels due to the syneresis.
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