Ordered mesoporous solids, synthesi formation

Titanium has also been incorporated into ordered mesoporous silicas of the SBA-15 type (126). The influence of synthesis parameters on the properties of titanium-substituted SB A-15 silicas prepared by a direct one-step synthesis (cocondensation) was systematically investigated (127) through characterizing the products by N2 physisorption, XRD, diffuse reflectance UV-vis spectroscopy, and elemental analysis. The results showed that when a low titanium precursor concentration (i.e., less than 0.05 mol/1) was used in the initial synthesis gel, incorporation of titanium into the solid was not completed. Under these conditions, titanium ions were well dispersed in the silica framework and present mainly in tetrahedral coordination in the product. When the isolated titanium species present in the gel reached a critical concentration, an increase in titanium incorporation into the sofid was observed as a consequence of the formation of anatase clusters on the material s surface. The titanium loading at which anatase formation was observed was found to he strongly influenced by the synthesis conditions. Moreover, when the formation of anatase takes place, the amorphous titanium species... 

Ordered mesoporous silica containing triazine moieties (Figure 3), based on the SBA-15 structure, has also been reported as an efficient and a reusable catalyst for the synthesis of chromenes 20 xmder solvent-free reaction conditions (Scheme 12) [75]. The catalyst was prepared from 3-mercaptopropyl-grafted SBA-15 by thiol-ene click reaction with 2,4,6-triallyloxy-l,3,5-triazine. The characterization data of the solid demonstrated the total disappearance of C=C bonds in triazine moiety. This feature strongly suggested the formation of bridges between the triazine and the mesopore walls. 

Microporous nanoparticles with ordered zeolitic structure such as Ti-Beta are used for incorporation into walls or deposition into pores of mesoporous materials to form the micro/mesoporous composite materials [1-3], Microporous particles need to be small enough to be successfully incorporated in the composite structure. This means that the zeolite synthesis has to be stopped as soon as the particles exhibit ordered zeolitic structure. To study the growth of Ti-Beta particles we used 29Si solid-state and liquid-state NMR spectroscopy combined with x-ray powder diffraction (XRPD) and high-resolution transmission electron microscopy (HRTEM). With these techniques we monitored zeolite formation from the initial precursor gel to the final Ti-Beta product. 

The synthesis of mesoporous silicas was performed from an isotropic reaction mixture using cationic surfactant as a structure directing agent. The decrease in pH, which causes the formation of solid particles, was achieved by hydrolysis of methyl acetate. The procedure enabled to obtain not only siliceous MCM-41 but also a less well-ordered hexagonal silica with extraordinary large surface area and silica with bimodal mesoporous structure containing the MCM-41 mesopore system and a system of mesopores with a mean diameter of 14 nm. 

---