Mesoporous materials, polymerization

UV-vis DRS spectra of the calcined titanium mesoporous materials are shown in Figure 2. An absorption band centered at ca. 220 nm is observed in all the samples, and this band is usually assigned to a low-energy charge-transfer transition between tetrahedral oxygen ligands and central Ti4 ions [9, 10]. The shoulder at 270 nm probably corresponds to partially polymerized hexa-coordinated Ti species [10], and some polymeric species are suspected to co-exist with the isolated Ti sites in all the mesoporous samples prepared. However, anatase-like TiO, phase at 330 nm was absent in these samples. 

Functionalization of the matrix allows incorporation of a variety of catalytic activities into the material. Recently, procedures were developed to add functional groups that are electrostatically or hydrophobically attractive to the ammonium surfactant head groups and are able to compete with silicate anions during self-assembly. This has led to a class of mesoporous materials that are functionalized only on the inside of the pores. Highly selective polymerization and cooperative catalytic systems have been developed from these materials.3 Furthermore, by incorporating caps onto the pores, chemical reagents can be stored in the channels,... 

It has also been demonstrated that mesoporous materials are viable candidates for optical devices [90]. Silicon nanoclusters were formed inside optically transparent, free-standing, oriented mesoporous silica film by chemical vapor deposition (CVD) of disilane within the spatial confines of the channels. The resulting silicon-silica nanocomposite displayed bright visible photoluminescence and nanosecond lifetimes (Fig. 2.12). The presence of partially polymerized silica channel walls and the retention of the surfactant template within the channels afforded very mild 100-140°C CVD conditions for the formation of... 

In addition to creating semiconductor or metal replicas of the channels of mesoporous materials, which are expected to display electron and hole quantum confinement effects, forming fibers of polymers could lead to materials with novel electrical, magnetic, optical or mechanical properties. To this end, oxidative polymerization of aniline within the channels of mesoporous silica has been reported [91]. Convincing spectroscopic evidence for intrachannel polymerization of aniline to poly(aniline) was provided. Extracted polymer had a molecular weight considerably smaller than that of the bulk material under similar conditions indicative of a diffusion constraint imposed upon the polymerization and growth of monomer inside the channel space of mesoporous silica. 

The hydrothermal stability is mainly dependent on the wall thickness and the degree of polymerization. KIT-1 and SBA-15 are highly resistant to hydrothermal treatment due to their highly polymerized and thicker walls. Under mild steaming conditions, the pore structure might lead to different hydrothermal stability for materials with comparable wall thickness. Cubic MCM-48 exhibits less structural degradation than the hexagonal mesoporous materials. A complete hydrothermal stability order reported by Cassiers etal. is as follows KIT-1 > SBA-15 > MCM-48 (fumed silica and TEOS), PCH > FSM-16, MCM-41 (fumed silica and TEOS), HMS. 

In the acidic route (with pH < 2), both kinetic and thermodynamic controlling factors need to be considered. First, the acid catalysis speeds up the hydrolysis of silicon alkoxides. Second, the silica species in solution are positively charged as =SiOH2 (denoted as I+). Finally, the siloxane bond condensation rate is kinetically promoted near the micelle surface. The surfactant (S+)-silica interaction in S+X 11 is mediated by the counterion X-. The micelle-counterion interaction is in thermodynamic equilibrium. Thus the factors involved in determining the total rate of nanostructure formation are the counterion adsorption equilibrium of X on the micellar surface, surface-enhanced concentration of I+, and proton-catalysed silica condensation near the micellar surface. From consideration of the surfactant, the surfactants first form micelles as a combination of the S+X assemblies, which then form a liquid crystal with molecular silicate species, and finally the mesoporous material is formed through inorganic polymerization and condensation of the silicate species. In the S+X I+ model, the surfactant-to-counteranion... 

From a view point of reaction time, the typical preparation of mesoporous material can be divided three main steps (1) interaction between surfactant and silica (or other inorganic) species in solution and the formation of ordered mesostmcture (2) the further reaction (polymerization or condensation for silica) at a certain temperature for a time period. A possible phase transformation may occur (3) recovery of solid product by filtration, washing, and drying. The phase transformation may also occur in this step (4) removal of template from the solid product by calcination or extraction with solvent. The phase transformation is also possible even in this step. 

Other interesting solids for the catalytic degradation of polymeric wastes are the various silica-based mesophases which have recently been discovered.4,5 These materials are characterized by the presence of ordered and regular pore systems and high surface areas, typically over 1000 m2 g l. The most common member of this family is MCM-41, which has a hexagonal array of uniform pores with diameters that can be tailored in the range 1.5-10 nm by varying the synthesis conditions. These mesoporous materials can be prepared with a wide... 

The catalytic properties of mesoporous materials with embedded nanoparticles are mainly determined by the type of the inclusion (particle). All catalytic reactions, which are normally known for the particular metals or alloys, can be carried out with mesoporous soHds containing nanoparticles. The important advantage of mesoporous oxides is their stability at high temperatures. Due to this feature, mesoporous oxides with nanoparticles can be successfully used as catalysts in such reactions where nanoparticles embedded in polymeric systems cannot be employed. Another probable advantage of mesoporous catalysts is an appropriate use of pores as nanoreactors of certain size. This can be applicable to large molecules or to cyclization reaction where pore size and shape will influence the reactive path [90]. However, for mesoporous solids with nanoparticles such applications are not reported so far. 

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