**1. Introduction**

The ordered mesoporous silicas (OMSs) containing transition metals are versatile catalytic materials for oxidation of a wide range of organic compounds [1–3]. Mesoporous materials with various structures such as MCM-41 (2d hexagonal p6mm) [4–6], MCM-48 (3d cubic Ia3d) [7, 8], SBA-15 (large-pore 2d hexagonal p6mm) [2, 9–12], SBA-16 (large-pore 3d cubic Im3m) [13], KIT-5 (well-ordered cage-type mesoporous with cubic Fm3m) [2], and KIT-6 (large-pore cubic with interpenetrated cylindrical mesopores Ia3d) [2, 14–18] have been used as supports for transition metals. These supports with controlled morphology and accessible metallic center for the reactant molecules also offered the opportunity to

immobilize the transition metal complexes and their heterogenization [19, 20]. These catalysts have attracted much interest due to the desirable characteristics of the silica supports such as narrow pore size, high surface area and large pore volume, tunable mesoporous channels with well-defined pore-size distribution, controllable wall composition, and modifiable surface properties. Pore diameter of mesoporous silicas (2–50 nm) and porous structure are usually tailored by the choice of the template surfactant or the incorporation of swelling agents to expand the surfactant micelles during synthesis [21, 22]. In condition of typical synthesis environment of the mesoporous molecular supports, the incorporation of metal methods varies with properties of their precursors.

In order to obtain active catalysts, different active redox metal sites have been introduced into specific locations (mesoporous channels and framework) of the OMSs supports by direct synthesis methods (framework substitution) or postsynthetic methods. In any case, Me*n*<sup>+</sup> can be simultaneously present in different coordination geometries and positions (surface, lattice) [16, 23]. In a directsynthesis preparation, the condensations of silicon and metal species around the organic micelles occur simultaneously, and it is likely that some of the metal species are trapped in the silica walls during the formation of OMSs supports. This may influence the unit cell parameters, the wall thickness, and the long-range ordering of the material. By contrast, metal species introduced by a postsynthesis treatment (template ion exchange, impregnation, grafting, chemical vapor deposition methods) are mostly located at the surface of the mesopores and do not modify the internal composition of the silica walls, mainly when the samples are prepared in alcohol. The synthesis method offers the advantage that the dispersion and location of metal species are easily controlled. This may be a great advantage with respect to conventional synthesis methods to prepare materials with specific applications in catalysis. The activity of the obtained materials was demonstrated in various reactions, mostly oxidation reactions of the organic compounds in the liquid or gaseous phase. All the reported results show that the localization of the metal ion, morphology, particle and pore channel sizes and their interaction with the support, and other metal (bimetallic catalysts) influence the oxidation state of the catalytic sites, respectively their redox properties. Therefore, the redox properties of these materials are the result of the support and metal cation synergistic effect.
