Dehydroxylation of silica

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...

Chlorine could be even used for efficient dehydroxylation of silica gels [79] at 800 °C due to the reaction ... 

SCHEME 32 Dehydroxylation of silica is enhanced by (A) CO through the water gas shift reaction, or (B) by CS2 through an unknown reaction, perhaps as shown here, or (C) by replacement by bromide or iodide followed by oxidative burn-off. 

FIGURE 121 Extent of dehydroxylation of silicas treated in various gases at high temperatures. Measurements were made by reaction with CHjMgl and referenced to the OH group population measured after calcination in N2 at 200 °C. 

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.

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. 

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... 

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). 

To follow the mode of grafting of an organometallic species onto Si02, the tris (neopentyl)neopentylidene tantalum complex has been used to show the reactivity of its groups with silanol groups of silica (dehydroxylated at 300 to 700 °C)... 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

As the loading of STA on the catalyst support is decreased, incomplete anhydride conversion is observed and significant hydrolysis of the anhydride to form iso-butyric acid is observed (Table 2). Use of silica supported phosphoric acid results in lower ketone yields and significant hydrolysis of the iso-butyric anhydride. Blank reactions (catalyst and anhydride, 90°C, 30 min) indicates that hydrolysis of anhydride is observed in the presence of these catalysts and may result from either dehydroxylation of the silica support or residual water in the catalyst, ffowever this reaction is slow (42%STA/silica, 44% conversion and 70%P[3PO4/silica, 86% conversion respectively). 

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