Mesoporous silicates silica source

Mesoporous silica was prepared using sodium silicate as silica source and cetyltrimethylammoniumbromide (CTMABr) as template as elsewhere [7]. The aluminosilicates were prepared with a similar procedure by using sodium aluminate as aluminum source, which was dissolved in the surfactant solution as described before [3]. Si/Al molar ratios were of 6, 10, 30 and 80, the impregnated iron amount was 6 wt. 

To study the influence of the silica source on the mesoporous materials formation, we have performed synthesis by adding different source of silica neutral TMOS and ionic sodium silicate to the micellar solution. Syntheses were made either under acidic (pH = 2) or basic (pH = 10) conditions for TMOS. Table 1 contains essential information of the obtained products. 

Polyoxyethylene alkyl ether and in particular decaoxyethylene oleyl ether CigHj5 (CH2CH20)io can be used as templating agent for silica mesoporous materials formation. The synthesis can be performed under both acidic or basic conditions with tetramethoxysilane (TMOS) as silica source while no mesoporous compound was obtained with sodium silicate. 

The preparation of mesoporous surfactant-templated silicates is in principle straightforward, requiring a silica source, a catalyst for silica polymerisation, the template and a solvent. The templating reaction can be done under acidic, alkaline or neutral conditions. The solvent for these syntheses is usually water, although some work in solvent mixtures has been reported, as well as a few syntheses in nonaqueous sol-... 

The interactions taking place between silicates and surfactants are the key factor for the formation of organic/inorganic mesostructured composite systems. Depending on the synthesis conditions, the silica source, or the type of surfactant used (Table 18.1), many mesoporous materials can be synthesized. Table 18.2 summarizes the different interactions that may occur between the inorganic components and the head groups of the surfactants. 

Figure 1. Scheme for the liquid crystalline templating mechanism proposed by Kresge et al 1 for synthesis of mesoporous silica MCM-41. Formation of a hexagonal array of cylindrical micelles possibly mediated by silicate anions followed by condensation of the silicate anions from the silicate source (tetraethylorthosilicate) leads to templated framework structure. Calcination or extraction of the template produces hexagonally ordered mesoporous silica. 

There are two main techniques for the incorporation of atomically dispersed titanium into a mesoporous silica framework. In the cocondensation method a titanium source is added during the preparation of the silica material framework and formation proceeds via simultaneous condensation of both titanium and silicon precursors, resulting in titanium incorporation throughout the material. Conversely, postmodification involves the grafting of a titanium-alkoxide precursor to the pore surface by condensation with surface hydroxyls of a preformed silica material. This forms a surface-modified titanium silicate. 

The key property required of the inorganic species is ability to build up (polymerize) around the template molecules into a stable framework. As is already evident in this article, the most commonly used inorganic species are silicate ions, which yield a silica framework. The silica can be doped with a wide variety of other elements (heteroatoms), which are able to occupy positions within the framework. For example, addition of an aluminium source to the synthesis gel provides aluminosilicate ions and ultimately an aluminosilicate mesoporous molecular sieve. Other nonsilica metal oxides can also be used to construct stable mesoporous materials. These include alumina, zirconia, and titania. Metal oxide mesophases, of varying stability, have also been obtained from metals such as antimony (Sb), iron (Fe), zinc (Zn), lead (Pb), tungsten (W), molybdenum (M), niobium (Nb), tantalum (Ta), and manganese (Mn). The thermal stability, after template removal, and structural ordering of these mesostructured metal oxides, is far lower, however, than that of mesoporous silica. Other compositions that are possible include mesostructured metal sulfides (though these are unstable to template removal) and mesoporous metals (e.g., platinum, Pt). 

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