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              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
                                                                                                ◦
              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-
                 ◦
              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),
                                          ◦
              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
                           ◦
                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.
                   ◦
                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
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