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Encyclopedia of Physical Science and Technology EN009I-420 July 10, 2001 15:8
Mesoporous Materials, Synthesis and Properties 375
D. Typical Synthesis Procedures temperature and the as-synthesized mesoporous silica
obtained via filtration. The as-synthesized mesoporous
Three general steps are involved in forming mesoporous silica is then washed with plenty of distilled water,
molecular sieves: synthesis, drying, and template removal. air-dried, and finally calcined in air at 550 C for 10 hours
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The four main components (reagents) of the synthesis gel to remove the template resulting in the mesoporous silica.
(mixture) are the structure director (template), a source of
the inorganic species, a solvent, and a catalyst (an acid or a
base). The synthesis is usually performed under room tem- III. COMPOSITIONS
perature or mild hydrothermal conditions (typically below
150 C). The choice of inorganic species is crucial in deter- The key property required of the inorganic species is abil-
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mining the preparation procedure, as illustrated in the fol- ity to build up (polymerize) around the template molecules
lowing example for mesoporous silica. When nonmolecu- into a stable framework. As is already evident in this ar-
lar silica sources (such as fumed silica or water glass) are ticle, the most commonly used inorganic species are sili-
used, a gel that contains all of the reagents is formed from cate ions, which yield a silica framework. The silica can be
a nonhomogeneous solution. The gel is then treated hy- dopedwithawidevarietyofotherelements(heteroatoms),
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drothermally at between 50 and 150 C for several hours to which are able to occupy positions within the framework.
several days. For this procedure, surfactant concentrations For example, addition of an aluminium source to the syn-
in the range 10–30 wt% can be used. For molecular sil- thesis gel provides aluminosilicate ions and ultimately an
ica sources, e.g., alkoxysilanes (such as tetraethyorthosil- aluminosilicate mesoporous molecular sieve. Other non-
icate), the aqueous surfactant solution and catalyst are silica metal oxides can also be used to construct stable
first combined to form a homogeneous micellar solution mesoporous materials. These include alumina, zirconia,
to which the molecular alkoxide (silica source) is added. and titania. Metal oxide mesophases, of varying stabil-
The inorganic/surfactant mesophase forms almost imme- ity, have also been obtained from metals such as anti-
diately and surfactant concentrations as low as 0.5 wt% mony (Sb), iron (Fe), zinc (Zn), lead (Pb), tungsten (W),
can be used. molybdenum (M), niobium (Nb), tantalum (Ta), and man-
Once the inorganic species and surfactant are assem- ganese(Mn).Thethermalstability,aftertemplateremoval,
bled into a surfactant/inorganic mesophase, the surfactant and structural ordering of these mesostructured metal ox-
(template) is removed leaving behind a well-ordered solid ides, is far lower, however, than that of mesoporous silica.
inorganic framework. The diameter of pores in the result- Other compositions that are possible include mesostruc-
ing material is determined primarily by the size of the tured metal sulfides (though these are unstable to template
template surfactant molecules. The pores can be tailored removal) and mesoporous metals (e.g., platinum, Pt).
in the 2.0–300 nm range. The materials exhibit specific
2
surface areas typically in the range 500–1500 m /g and A. Compositional Modification
3
pore volumes greater than 0.5 cm /g associated with a
Puresilicamesoporousmolecularsievesarethemostcom-
uniform or narrow distribution of pore sizes. The method
mon mesoporous materials. In the following discussion,
and conditions used to remove the template can affect the
M41S type (i.e., MCM-41 or MCM-48) mesoporous silica
final pore volume fraction, porosity, and the pore size of
is used to illustrate the compositional flexibility of meso-
the materials, but (except under extreme conditions) not
porous materials. The chemical composition of meso-
the connectivity or arrangement (geometry) of the pores.
porous silica can be easily modified to generate new ma-
Various methods are commonly used to remove the tem-
terials with altered properties. It is however worth noting
plate. This include solvent extraction, calcination, oxygen
that compositional modification of mesoporous silica has
plasma treatment, and supercritical extraction.
A typical synthesis procedure for mesoporous silica is structural implications; in most cases the introduction of a
“guest” heteroatom reduces the long-range structural or-
given here to illustrate the key preparation steps.
dering. In some cases the pore size is also altered.
40.4 g of water (solvent) and 6.2 g of cetyltrimethy- The compositional modification of silica frameworks
lammonium bromide (template) are mixed and stirred by other elements (often metals) is achieved via a number
at room temperature until all the template is dissolved.
of methods. The most frequently used method is the direct
Then, to the surfactant solution, 10 g of a 20% TEAOH
addition of the guest elements’ precursors to the synthesis
solution (catalyst) is added. Finally, 4.1 g of Fumed
solution (gel). Hence, the modifying elements are incor-
silica (silica/silicate ions source) is added. The mixture
porated into the solid structure along with silica during
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is stirred at 70 C for 2 hours and then aged at room
mesophase formation. This method enables a uniform dis-
temperature for a further 24 hours. The mixture is then
transferred into a Teflon-lined autoclave and heated at tribution of the guest element in the resulting framework.
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150 C for 48 h under autogenous pressure. After the A second modification route is to introduce the guest ele-
hydrothermal treatment the autoclave is cooled to room ment after formation of the silica mesostructure. This can