Metal-containing Mesoporous Silica-based Materials

3 Metal-containing Mesoporous Silica-based Materials 

The introduction of heteroatoms into the mesostructured silica wall will change the chemical properties of mesoporous silica materials. Many examples have been reported. Here, only a few of examples are introduced. 

We choose Ti-containing materials as an example to discuss heteratom-containing silica material and nonsilica mesoporous materials, because Ti-containing materials are useful catalysts and were studied extensively. 

Microporous titanium silicate (e.g., TS-1, Ti-(3, Ti-ZSM-12, Ti-mordenite) is an effective molecular-sieve catalyst for the selective oxidation of alkanes, the hydroxyla-tion of phenol, and the epoxidation of alkenes with aqueous H202. The range of organic compounds that can be oxidized is greatly limited, however, by the relatively small pore size (about 0.6 nm) of the host framework. 

Since the first synthesis of Ti-MCM-41, several Ti-containing mesoporous molecular sieves, such as Ti-HMS, Ti-MCM-48, Ti-MSU-1, and Ti-SBA-15 have been synthesized and characterized. In most instances the syntheses were accomplished using organic precursors, namely, TEOS and titanium tetraethoxide (TEOT) or titanium tetrabutoxide (TBOT) as the Si and Ti sources, respectively. It is easy to form a homogeneous gel using organic precursors. 

---

The oxide catalysts are microporous or mesoporous materials or materials containing both types of pores. In the latter case, the applicability is larger in terms of the molecular size of the reactants. Acid-base properties of these materials depend on the covalent/ionic character of the metal-oxygen bonds. These sites are involved in several steps of the catalytic oxidation reactions. The acid sites participate with the cation redox properties in determining the selective/unselective catalyst behavior [30,31]. Thus, many studies agree that partial oxidation of organic compounds almost exclusively involves redox cycles and acid-base properties of transition metal oxides and some authors have attempted to relate these properties with activity or selectivity in oxidation reactions [31,42]. The presence of both Bronsted and Lewis acid sites was evidenced, for example, in the case of the metal-modified mesoporous sihcas [30,39,43]. For the bimetallic (V-Ti, Nb-Ti) ions-modified MCM-41 mesoporous silica, the incorporation of the second metal led to the increase of the Lewis sites population [44]. This increased concentration of the acid sites was well correlated with the increased conversion in oxidation of unsaturated molecules such as cyclohexene or styrene [26,44] and functionalized compounds such as alcohols [31,42] or phenols [45]. 

---