Dehydroxylation of silica surface

Scheme 2 Different siloxane bridge structures formed upon dehydroxylation of silica surface. The increasing dimension of silicon rings and, consequently, of the Si - O - Si angle reflects a decreasing of the strain of these structures...

Figure 3. Thermogravimetric weight loss for 37.5FRM-PVAc with 3 regions highlighted A- adsorbed, physically trapped species B- PVAc decomposition, removal C dehydroxylation of silica surface.

The same surface species is obtained at ambient temperature by the reaction of Bu3SnH and the silanol groups, suggesting that the Sn-H bond is more reactive in this case than the Sn-C bond. The surface reaction depends upon the degree of dehydroxylation of the surface of silica. On silica dehydroxylated at 500°C the reaction leads to one well-defined surface complex. On the other hand, on silica dehydroxylated at 200°C, the evolution of alkane is continuous. The difference in the latter case is related to the presence of neighboring OH groups, because the number of the surface vicinal OH groups capable of... 

When a silica gel is heated to progressively higher temperatures water is lost first from any mesopores and supermicropores then from any ultramicropores, and finally by the decomposition of surface hydroxyls. The dehydroxylation of the surface... 

Experiments with CPGs subjected to progressive thermal treatment leading to partial dehydroxylation of the surface and its enrichment in boron atoms showed specific adsorption properties of these materials differentiating them from silica gels [30,31,73]. It was found that depending on the type of adsorbate two trends in the adsorption behaviour of heated CPG can be observed [30,31]. The increase of thermal treatment time leads to the increase of CPG adsorption properties in relation to water, chloroform or hexane [30] (e.g. see Fig. 13). 

Tagieva and V. Kiselev (322) showed that when quartz and silica were exposed to 7-radiation (60Co source), a strong dehydroxylation of the surface occurred. The adsorption of water vapor by Si02 following such an exposure decreased sharply. 

The comparison of the results obtained by the author [3-5] with those reported in the literature covers the following topics (i) limiting temperature for removing physically adsorbed water from the hydroxylated surface (ii) completely hydroxylated state of the surface (iii) dehydroxylation of the surface (iv) rehydroxylation of the surface (v) stmcturally bound water inside silica particles. Owing to the lack of space in this abstract, references are made only to those results which closely... 

Reactions A-E which follow have been proposed by Brinker and Haaland [96] as possible schemes for surface nitridation via ammonolysis. Lewis acid adsorption (A) is a possible scheme for electrophilic metals capable of formally increasing their coordination numbers, e.g., trigonally coordinated boron or tetrahedrally coordinated aluminum. Lewis acid adsorption may be followed by dissociative chemisorption as in B. This scheme depends on the Lewis acidity of the metal site but does not necessarily involve a stable Intermediate with a formal Increase in coordination number. As discussed in the previous section, dehydroxylation of the silica surface at temperatures above 250°C progressively creates strained surface silicon species with enhanced Lewis acidity. The Importance of scheme B for silica is therefore expected to increase with the extent of suface dehydroxylation. Reaction C was proposed by Mulfinger [97] to account for dehydroxylation of silica... 

Though silica supports are amorphous, the surface may exhibit some local order, such as that of the mineral /3-crystoballite (Fig. 5.23). The surfaces of silica support contain OH groups at densities of between 4 and 5.5 OH per nm that of cristobal-lite is 4.55 OH per nm. Silica surfaces contain only terminal OH groups, i.e. bound to a single Si atom. Heating leads to dehydroxylation, and at high temperatures only the isolated OH groups remain. 

Alumina is known to have more ionic character and its surface has a more complex structure than that of silica. Reaction of Bu3SnH with the surface of partially dehydroxylated aluminas was followed and it was found that the extreme sensitivity of tin chemical shifts to the molecular environment constitutes a method whereby surface organometallic complexes of tin can be used as molecular probes for determining surface structures of oxides.248... 

The second approach (Equation(3)) has a number of advantages over the first one (Equation(2)). The alkyl complexes are more reactive than the related alkoxides, the latter being for group 4 elements generally associated into dimers or trimers 48 also, reaction (2) liberates an alcohol which may further react with the surface of silica, whereas the alkane ( Equation(3)) is inert. It was demonstrated by various spectroscopic techniques and elemental analysis that with a silica dehydroxylated at 500 °C under vacuum, the stoichiometry of reaction (3) corresponds to n = 1.45,46 Moreover, a better control of the surface reaction was achieved with the procedure represented in Equation(3). 

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