Functional silicon alkoxides

Functional Silicon Alkoxides Precursors of Flybrid Materials... 

Silica-based materials obtained by the sol-gel process are perhaps the most promising class of functional materials capable to meet such a grand objective. In the sol-gel process liquid precursors such as silicon alkoxides are mixed and transformed into silica via hydrolytic polycondensation at room temperature. Called soft chemitry or chimie douce, this approach to the synthesis of glasses at room temperature and pressure and in biocompatible conditions (water, neutral pH) has been pioneered by Livage and Rouxel in the 1970s, and further developed by Sanchez, Avnir, Brinker and Ozin. 

Small molecule imprinting in sol-gel matrices has received considerable interest in recent years, undoubtedly due to the flexibility offered by the sol-gel process.5 Two different approaches have been utilized covalent assembly and noncovalent self-assembly.9 In the covalent assembly approach, the polymerizable functional group (i.e., the silicon alkoxide group) is covalently attached to the imprint molecule. The functionalized imprint molecule is then mixed with appropriate monomers (i.e., TMOS) to form the imprinted materials. After polymerization, the covalent bonds are cleaved to release the template and leave the molecular recognition pocket. Figure 20.4 shows a diagram of this process. 

Coupling of functional groups onto the walls of mesoporous silica materials can also be achieved by surfactant-templated assembly of an organosilicon alkoxide and silicon alkoxide reagents. Recent reports that use this synthetic strategy include hybrids of methacrylate-mesoporous silica [104], phenyl-amino (thiol or allyl)-mesoporous silica [105], and organic chromo-phore-mesoporous silica [106]. 

The present paper provides an overview of the physical properties of silica mesostructures with representative wormhole and lamellar framework structures assembled through S°P pathways. The wormhole framework silica, denoted HMS silica, was assembled using an alkylamine surfactant as the structure director and a silicon alkoxide as the inorganic precursor. The lamellar framework silica with a vesicular hierarchical structure, denoted MSU-G silica, was obtained from tetraethylorthosilicate (TEOS) as the silica precursor and a bi-functional gemini amine surfactant of the type RNH(CH2)2NH2. We then provide examples of the catalytic activity of these disordered mesostructures in comparison to more ordered framework mesostructures such as hexagonal MCM-41. 

In the evolution of solids from solution, a wide spectrum of structures can be formed. In Fig. 4, a simple schematic representation of the structural boundary condition for gel formation is presented. At one extreme of the conditions, linear or nearly linear polymeric networks are formed. For these systems, the functionality of polymerization /, is nearly 2. This means there is little branching or cross-linking. The degree of cross-linking is nearly 0. In silica, gels of this type can be readily formed by catalysis with HCl or HNO3 under conditions of low water content (less than 4 mol water to 1 mol silicon alkoxide). The ideal fractal dimension for such a linear chain structure is 1. The phe-... 

Silicon alkoxide functionalized fullerenes can be profitably used in the preparation of HPLC stationary phases [152]. The silicon alkoxide group, in fact, guarantees the chemical grafting to silica (Scheme 9). Standard chromatographic tests with simple aromatic compounds revealed that the new phase works with high efficiency both in organic and water-rich media. Specific inter-... 

Monolithic silica is the most recently introduced class of stationary phases for electrochromatography on microchips. They benefit from their very high surface area, adjustable pore size, and controllable surface chemistry. Functionalities required for the separation in reversed-phase mode are typically incorporated onto the silica monolith by including an appropriate silicon alkoxide in the precursor mixture or by silanization of the surface after the monolith has been formed. Monolithic silica-based columns are generally known to exhibit superior performance in HPLC separations of small molecules, which is atftibuted to the presence of mesopores entailing large surface areas. 

Following preparation (via condensation of titanium and silicon-alkoxide precursors around a tetrapropylammonium hydroxide template), the material is hydrothermally treated to produce a highly crystalline solid with a distinct X-ray diffraction profile analogous to that of ZSM-5. It contains isolated titanium sites in a silica matrix. These function significantly differently from bulk titanium dioxide, which is inactive in selective oxidation reactions using hydrogen peroxide, as the latter promotes the thermal decomposition of hydrogen peroxide. ... 

This problem can be solved by addition of the less-reactive precursor to the solution where the second, more-reactive precursor is partially prehydrolyzed, or equahzation of the reactivity by selection of precursors with different molecular structure, or by using a complexing agent to control the hydrolysis rate. For preparation of aluminum silicates with acidic functionality created by entrapment of tetracoordinated aluminum ions in the silica matrix, the prehydrolysis of the less-reactive silicon alkoxide before addition of aluminum alkoxide suppresses the early formation of Al—OH—A1 bridges and subsequent... 

Metal alkoxides are soluble in their corresponding alcohols. In practice, dissolution of solid alkoxides or dilution of liquid alkoxides is normally performed in the corresponding alcohol. As outlined below for the case of silicon alkoxides, the alcohol may also serve an additional function as a mutual solvent for the alkoxide and water when they are immiscible. Metal alkoxides also have the ability to exchange alkoxide groups with alcohols, as in Eq. (5.13), for example. 

Poly(vinyl-acetate), the polymer chosen for this study, is functionally incapable of covalently bonding with the polymerizing silicon alkoxide, TEOS. Therefore, the Si02/PVAc composites may be considered class I hybrid materials. Reasons for interest in mixing PVAc with silica include its excellent optical transparency, a refractive index close to that of silica, and its solubility in ethanol-water mixtures. The solubility of PVAc in ethanol-water mixtures allows the polymer to be added directly to a typical alkoxide sol-gel formulation of alkoxide, alcohol, water, and catalyst, followed by tiie polymerization of the inorganic network around the organic polymer domains. 

For several decades, carbon black and fumed silica have been used in tires, tubings, sealants and so on.[162,163] More recently, functionalized silica nanoparticles were reported to copolymerize with MMA to produce transparent films with good surface finish and controllable refractive index. [162] The sol-gel process is another research topic to prepare the polymer nanocomposites. The principal sol-gel process includes two steps the formation of a colloidal suspension, i.e. sol, and the gelation of the sol to formulate a network in a continuous liquid phase, i.e. gel. For example, the hydrolysis and poly condensation of silicon alkoxides in the presence of MMA will produce a polymer nanocomposite with superior optical and mechanical properties.[163]... 

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