Monolithic silica precursors

The ethylene glycol-containing silica precursor has been combined, as mentioned above, with most commercially important polysaccharides and two proteins listed in Table 3.1. In spite of the wide variety of their nature, structure and properties, the jellification processes on addition of THEOS to solutions of all of these biopolymers (Scheme 3.2) had a common feature, that is the formation of monolithic nanocomposite materials, proceeding without phase separation and precipitation. The syner-esis mentioned in a number of cases in Table 3.1 was not more than 10 vol.%. It is worthwhile to compare it with common sol-gel processes. For example, the volume shrinkage of gels fabricated with the help of TEOS and diglyceryl silane was 70 and 53 %, respectively [138,141]. 

FIGURE 1.4 Morphology and porosity of a typical monolithic rod, prepared by copolymerization of silane precursors, (a) SEM micrograph of the fractured surface of a monolithic silica gel rod. (b) Pore size distribution of a representative monolithic silica rod. (Reprinted from Guiochon, G., J. Chromatogr. A, 1168, 101, 2007. Copyright 2007, with permission from Elsevier.)... 

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. 

In order to run photocatalytic oxidative remediation of airborne VOC on titania, Cao et al. (21) made low-density silica-titania aerogels by first preparing a silica aerogel and then impregnating the monolithic silica by a solution containing the Ti precursor. Finally, the impregnated silica aerogels were one more time supercritically... 

Even if supercritical drying permits to obtain perfectly monolithic and transparent sihca-based thermal insulators, the process to obtain such crack-free large plates remains stUl too far from industrial large-scale commercialization. This was the initial reason why different processes have been studied to develop subcritical routes to access rapid massive commercialization. Among the various studies, aging of the gels in silica precursor containing solution has permitted to reach room temperature thermal conductivities as low as... 

Until now, a considerable variety of functionalized and non-fimctionalized monolithic materials based on either organic or inorganic polymers are available. While inorganic monoliths are usually prepared from silica precursors, for example, Si(OR)4, via sol-gel techniques, organic continuous beds have... 

Brook, M.A., Chen, Y., Guo, K., Zhang, Z. and Brennan, J.D. (2004) Sugar-modified silanes precursors for silica monoliths. Journal of Materials Science, 14, 1469-1479. 

In a tetralkoxysilane/alkyltrialkoxysilane system, the hydrolysis and condensation reactions proceed not only between similar precursor molecules but also between the two different precursor species. Figure 8.3 illustrates a simplified scheme of hydrolysis and condensafion reactions in a hybrid system containing TMOS and an alkyltrimethoxysilane. Notice the difference from the scheme in Figure 8.1 the alkyl group, represented by R, does not participate in the hydrolysis and condensation reactions that lead to the silica monolith. The sol-gel process kinetics in a hybrid sol-gel... 

Various Silane Precursors Used to Synthesize Organo-Silica Hybrid Monolithic Columns... 

In principle, many organosilanes can be used as a second precursor to form the organo-silica monolith. Various coprecursors reported for the fabrication of hybrid organo-silica monolithic columns for EC or CEC are listed in Table 8.1. The structure of the organo-silica material is affected by the ratio of the tetralkoxysilane to the alkyltrialkoxysilane in the sol-gel reactions, the type of precursor used, and the processing parameters. This can be seen, for example, in the phase separation behavior of... 

FIGURE 8.6 SEM of an organo-silica hybrid monolith prepared with the precursor A-octadecyldimethyl[3-(trimethoxysilyl)propyl] ammonium chloride to provide reversed phase stationary phase and anodic EOF. (a) Cross-sectional view (magnification 1800 times) and (b) longitudinal view (magnification 7000 times). (Reprinted from J. D. Hayes, A. Malik, Anal. Chem., 72 4090 (2000). With permission. Copyright American Chemical Society 2000.)... 

Silica-based monolithic columns (Figure 9) are generally prepared using sol-gel technology. This involves the preparation of a sol solution and the gelation of the sol to form a network in a continuous liquid phase within the capillary. The precursors for the synthesis of these monoliths are normally metal alkoxides that react readily with water. The most widely used are alkoxysilanes such as tetramethoxysilane (TMOS) and TEOS. 

Although inorganic, monolithic columns attracted considerable attention in the last 10 years, the preparation of silica-based monoliths does not yet offer the broad chemical variety of precursors and porogens for specific adjustment of separation compared with their organic counterpart. The preparation of silica monoliths uses the classical sol-gel process of hydrolysis and polycondensation of organosilicium compounds. 

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

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