Silicone mesoporous materials

For application of protein-immobilized porous materials to sensor fields, use of an electroactive substance as the framework material is important. DeLouise and Miller demonstrated the immobilization of glutathione-S-transferase in electrochemically etched porous silicon films [134], which are attractive materials for the construction of biosensors and may also have utility for the production of immobilized enzyme bioreactors. Not limited to this case, practical applications of nanohybrids from biomolecules and mesoporous materials have been paid much attention. Examples of the application of such hybrids are summarized in a later section of this chapter. 

The smallest pores that can be formed electrochemically in silicon have radii of < 1 nm and are therefore truly microporous. However, confinement effects proposed to be responsible for micropore formation extend well into the lower mesoporous regime and in addition are largely determined by skeleton size, not by pore size. Therefore the IUPAC convention of pore size will not be applied strictly and all PS properties that are dominated by quantum size effects, for example the optical properties, will be discussed in Chapter 7, independently of actual pore size. Furthermore, it is useful in some cases to compare the properties of different pore size regimes. Meso PS, for example, has roughly the same internal surface area as micro PS but shows only negligible confinement effects. It is therefore perfectly standard to decide whether observations at micro PS samples are surface-related or QC-related. As a result, a few properties of microporous silicon will be discussed in the section about mesoporous materials, and vice versa. Properties of PS common to all size regimes, e.g. growth rate, porosity or dissolution valence, will be discussed in this chapter. 

The acidity of thermally stable mesoporous aluminophosphates (AlPO) and sili-coaluminophosphates (SAPO) has also been stndied by microcalorimetry [245]. By contrast with microporous crystalline alnminophosphate molecnlar sieves, mesoporous compounds are amorphous and characterized by Al/P ratios greater than 1. These particularities are responsible for a strong Lewis acidity, making these mesoporous materials more acidic than the microporons analognes, with an amonnt of strong acid sites that increases with the silicon content. 

Polyacrylic acid Hybrid organic/inorganic network polymers were formed via the reaction of polyacrylic acid with tin(IV). titanium(IV), and silicon (IV) alkoxides and subsequent hydrolysis to form mesoporous materials. Treatment by nitric acid removed the polyacrylate template and produced microporous inorganic hydrous metal oxides Surface areas characterized by BET measurements 130... 

Substitution of silicon by other atoms like Ti or Al was reported to improve the thermal and hydrothermal stability to some extent [6]. It was also reported that improved hydrothermal stability could be achieved by adjusting the gel pH several times during crystallization process [7], Post-synthesis silylation technique has also been reported to enhance the hydrothermal stability of mesoporous materials by increasing the hydrophobicity of the samples [8,9]. However, it is most desirable to develop a method for preparing hydrother-mally stable mesoporous material by direct synthesis route. 

Chemical Thermodynamics Mesoporous Materials, Synthesis and Properties Micelles Silicone (Siloxane) Surfactants... 

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... 

Mesoporous phosphomolybdic acid has been hydrotheimally synthesized with the surfaetant template cetyltrimethylammonium bromide, phosphomolybdic acid and tetraethyl orthosilicate as the silicon source. The parent materials are extracted in a solvent to remove the oi anic structuredirecting agent. The materials have been characterized by chemical analysis. X-ray diffraction, TEM, Infrared spectra and nitrogen BET surface measurement. After extraction of the silica-modified salts with HCI/EtOH, the surface area of the mesoporous material is up to 140 m /g, and the pore diameter is 30 40 nm. 

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... 

The possible compositions for the walls of the ordered mesoporous materials go beyond the field of inorganic chemistry, and materials with hybrid organosilica walls have been prepared [81-84], Some mesoporous benzene-silica hybrids are stable at a temperature higher than 500 °C [84], Mesoporous materials prepared from polysilazanes and nonionic surfactants can be activated to form silicon carbonilride ceramics, which retain an ordered mesoporosity up to 1500 °C [85],... 

We report here preliminary results of the physicochemical characterization of a composite material obtained by combining the cethyltrimethylammonium cations clay insertion procedure with the room temperature synthesis of mesoporous materials inside of clay layers. The Romanian bentonite, containing 64% montmorillonite was used. The organic cations are incorporated within the interlayer region of the clay, serving to prop of>en the layers and to allow incorporation of the silicon source for MCM-4I synthesis. The obtained materials display a high thermal stability and molecular sieve properties. 

Mesoporous materials such as ordered mesoporous silicas (OMS), initially developed by the Mobil labs in the early 1990s have very different properties (8-11). While primarily comprised of silica (SiOa), the pores are larger (4 to 10 nm) and the materials are amorphous (i.e. noncrystaUine). However, the pores of most OMS materials possess long-range ordering. While it is possible in these materials to substitute some silicon atoms in the amorphous matrix as in the case of zeolites, the resulting materials do not typically possess the thermal stability of zeolites, particularly in the presence of water vapor/steam. These materials are currently under intense investigation and their commercial use has been much more limited. 

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