Silica based materials

The basic chemical principle behind sol-gel processing of silica-based materials is the transformation of Si—OR- and Si—OH-containing species to siloxane compounds by condensation reactions. From a structural point of view, this corresponds to connecting Si04 tetrahedra (or RSiOs tetrahedra in hybrid materials) by corner sharing. To obtain a stable gel, the number of siloxane bonds (Si—O—Si) has to be maximized and consequently the number of silanol (Si—OH) and alkoxo (Si—OR) groups has to be minimized. 

The chemical reactions during sol-gel processing can be formally described by three equations (Eq. (1.2)). In alkoxide-based systems, hydrolysis reactions of Si—OR groups must precede condensation to generate the Si—OH groups, which are necessary for condensation. The fact that the reactive groups must be created in the first place is an important difference to typical organic polymerization reactions. Condensation (i.e., formation of Si—O—Si units) takes place by either alcohol or (more often) water elimination. 

The most important differences between the two precursor types are the following  

The reaction mechanisms are the same for both silicate and alkoxysilane precursors. However, from a mechanistic point of view, reactions under acidic or basic conditions have to be considered separately. Under acidic conditions, that is, at a pH below the PZC, the oxygen atom of a =Si—0 , =Si—OH, or =Si—OR group is protonated in a rapid first step (Eq. (1.3)). A good leaving group (water or alcohol) is thus created. In addition, electron density is withdrawn from the central silicon atom, rendering it more electrophilic and thus more susceptible to attack by water (in hydrolysis reactions) or silanol groups (in condensation reactions). 

Under basic conditions, the reaction proceeds by nucleophilic attack of either an OH (in hydrolysis reactions) or a =Si—0 ion (in condensation reactions) to the silicon atom with an SN2-type mechanism (Eq. (1.4)). The entering OH or =SiO group is formed by deprotonation of water or a =Si—OH group. Under strongly alkaline conditions, the Si—O—Si bonds can be cleaved again by OH . 

Tine pore stxuctxire is diaracterized by such estinates as the specific 2 

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Guth J-L and Kessler H 1999 Synthesis of aluminosilicate zeolites and related silica-based materials Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L Puppe (Berlin Springer) pp 1-52... 

Eor the selective pre-concentration of deactivated phenols a new silica-based material with the grafted 2,3,5-triphenyltetrazole was proposed. This method is based on the formation of molecular chai ge-transfer comlexes of 2,3,5-triphenyltetrazole (7t-acceptor) with picric acid (7t-donor) in the phase of the sorbent. Proposed SPE is suitable for HPEC analysis of nitrophenols after their desorption by acetonitrile. Test-system for visual monitoring of polynitrophenols under their maximum concentration limits was developed using the proposed adsorbent. 

DIRECT ATOMIC ABSORPTION ANALYSIS OF SILICA BASED MATERIALS AND SOME OF ITS PECULIARITIES... 

Recently, new approaches of sorbent construction for reversed-phase chromatography have been developed. Silicas modified with hydrocarbon chains have been investigated the most and broadly utilized for these aims. Silica-based materials possess sufficient stability only in the pH 2-8 range. Polymeric HPLC sorbents remove these limitations. Tweeten et al. [108] demonstrated the application of stroongly crosslinked styrene-divinylbenzene resins for reversed-phase chromatography of peptides. 

The analysis demonstrates the elegant use of a very specific type of column packing. As a result, there is no sample preparation, so after the serum has been filtered or centrifuged, which is a precautionary measure to protect the apparatus, 10 p.1 of serum is injected directly on to the column. The separation obtained is shown in figure 13. The stationary phase, as described by Supelco, was a silica based material with a polymeric surface containing dispersive areas surrounded by a polar network. Small molecules can penetrate the polar network and interact with the dispersive areas and be retained, whereas the larger molecules, such as proteins, cannot reach the interactive surface and are thus rapidly eluted from the column. The chemical nature of the material is not clear, but it can be assumed that the dispersive surface where interaction with the small molecules can take place probably contains hydrocarbon chains like a reversed phase. 

Q. Huo, D. I. Margolese, G. D. Stucky, Surfactant control of phases in the synthesis of mesoporous silica-based materials , Chem Mater. 1996, 8,1147-1160. 

Silica-Based Materials for Advanced Chemical Applications By Mario Pagliaro Mario Pagliaro 2009... 

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. 

Counter to intuition that would exclude non-conductive glassy materials from the field of electrochemistry, organically modified silica-based materials have a rich and varied electrochemistry39 made possible by the accessible inner porosity. This allows oxidant and reducing reactant molecules to diffuse through the material and eventually to the surface either of a conducting electrode or of a conductive material (Figure 1.18). 

Catalysis by sol gel doped silica-based materials has become in the last 20 years a prominent tool to synthesize a vast number of useful molecules both in the laboratory and in industrial plants.12 The underlying basic concept of all sol-gel applications is unique one or more host molecules are entrapped by a sol-gel process within the cages of an amorphous metal oxide where they are accessible to diffusible reactants through the inner pore network, which leads to chemical interactions and reactions (Figure 5.3). 

Hydrofluoric acid HF Commercially available concentrated acid is 50% (26 M) HF must be stored in plastic containers, since it attacks glass very damaging to skin Dissolving silica-based materials and stainless steel... 

Molecular imprinting is not limited to organic polymer matrices, but can also be applied to silica-based materials and even proteins. Proteins freeze-dried in the presence of a transition state analogue as template have been used successfully as catalysts, e.g., for the dehydrofluorination of a fluorobutanone. For instance, lyophilized 3-lactoglobulin imprinted in this manner with N-isopropyl-N-ni-trobenzyl-amine could accelerate the dehydrofluorination by a factor of 3.27 compared to the non-imprinted protein see Table 5 [62]. In a similar procedure, BSA was imprinted with N-methyl-N-(4-nitrobenzyl)-S-aminovaleric acid and showed an enhancement of the catalytic effect by a factor of 3.3 compared to the control protein for the same reaction see Table 5 [113]. 

A disadvantage of the hybrid materials is that they give somewhat lower efficiency (even for neutral compounds) than purely silica based materials [5], This factor is hardly surprising, considering they have some polymeric character, and that polymeric materials are known to give lower efficiency (see later sections). 

Unfortunately, even this modified equation does not describe the true practical situation in LC, as it is complicated by the fact that all silica-based materials exhibit exclusion properties. The pore diameter of silica-based stationary phases can range from, perhaps, 2-3 Angstrom to as much as 1000-2000 Angstrom. Consequently, some, otherwise open pores, are accessible to the solute while others are not, depending on the size of the molecule. Therefore, only those pores that have a diameter equal to, or greater than, that of the solute molecules are accessible and only the stationary phase within those pores can effect retention. In addition, the static interstitial volume between the particles can also exhibit exclusion properties and some of the static interstitial volume may also be inaccessible to the larger solutes. As a consequence, equation (12) must be further modified to give,... 

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