Surface cristobalite model

The fact that silsesquioxane molecules like 2-7 contain covalently bonded reactive functionalities make them promising monomers for polymerization reactions or for grafting these monomers to polymer chains. In recent years this has been the basis for the development of novel hybrid materials, which offer a variety of useful properties. This area of applied silsesquioxane chemistry has been largely developed by Lichtenhan et al With respect to catalysis research, the chemistry of metallasilsesquioxanes also receives considerable current interest. As mentioned above, incompletely condensed silsesquioxanes of the type R7Si70g(0H)3 (2-7, Scheme 4) share astonishing structural similarities with p-tridymite and p-cristobalite and are thus quite realistic models for the silanol sites on silica surfaces. Metal... 

Others have examined the geometry of the chromate and dichromate ions and compared then to models of the silica surface (12). Again, the uncertainty is high because we have very little idea what the silica surface really looks like. Hydroxyl spacings derived from a particular face of cristobalite or tridymite may have little relevance to amorphous, high surface area silica gel. 

The early theoretical study of Peri and Hensley,2 performing a Monte Carlo analysis on the idealized 111 and 100 faces of cristobalite concluded that 85 % to 95 % of all surface silanols are paired (geminal), whereas the earlier study of De Boer and Vleeskens10 modelled the 0001 face of /3-cristobalite to conclude that no geminal hydroxyls are present at all. 

In the low temperature region, the surface is almost completely covered with vicinal (bridged) silanols. This is consistent with the theoretical model of Peri and Hensley,9 who performed Monte-Carlo simulations on the (100) plane of /3-cristobalite. They concluded that the surface of a hydroxylated silica is completely covered with bridged silanols. The hypothesis that a fully hydroxylated silica is only covered with bridged hydroxyls is probably not entirely correct, as will be evidenced later in this section. 

Until recently, when Peri 155) reported on a model of the silica-alumina surface, there were no detailed models for the surfaces of mixed oxides available. Beside the presence of Br nsted and Lewis acid sites, Peri 156) had proposed the existence of a sites on the Si02—A1203 surface, which he described as acid-base pair sites rather than simple Lewis acid sites. Various molecules, such as acetylene, butene, and HC1, are adsorbed very selectively on these a sites, whereas NH3 and H20 are also held by many other sites 157). To rationalize the formation of these sites, Peri 155) developed a semiquantitative surface model for certain silica-aluminas, which were prepared by reaction of A1C13 with the surface silanol groups of silica and subsequent hydrolysis and dehydration. The model is entirely based on a surface model of silica, which suggests an external surface resembling a (100) face of the cristobalite structure 158). It should be mentioned in this connection that Peri s surface model of silica may... 

D iogenically precipitated silica is a metastable silica polymorph which must eventually alter to quartz under the earth s surface conditions. Present observations of deep-sea sediments suggest that this transformation may occur directly or through an intermediate, alpha cristobalite. Several models have been proposed to ascertain the rate at which these processes occur. This series of papers tests these models and offers simple but powerful methods for detecting changes in crystal form as a function of geologic age. 

Surface Models. A surface corresponding to the 111 faces of / -cristobalite would bear single silanols on alternate Si atoms (36). The 0-0 distance of two fn silanols is about 0.51 nm in an idealized structure of / -cristobalite with the orientation Fd3m (38). The corresponding surface... 

Sm is a single silanol, somewhere on the middle of a surface 111 (Figure 6a). The idealized 0-0 distance with a fn silanol, which is 0.51 nm, only applies to one-fourth of the silanols in a more realistic model of / -cristobalite in the orientation P2i3 (33). The three other silanols, which are... 

All these H-altered H- Si CP-MAS experiments, together with a spin-exchange experiment in which H- Si cross-polarization follows a sequence analogous to the predetection portion of the H CRAMPS spin-exchange experiment described in Fig. 18 [60], lead to a model of the silica surface that corresponds to the 100 face of p-cristobalite for Si(OH)2 silanols and to the 111 face of p-cristobalite for SiOH silanols. These faces are depicted in Fig. 29, in which one can see that the distance between OH groups on adjacent Si(OH)2 groups is much smaller than between adjacent SiOH groups, or even between the... 

MAS) is an invaluable approach for observing local silicon environments on the silica surface. H NMR approaches distinguish clustered (hydrogen-bonded) and isolated surface silanols. Correlations between Si and H NMR behaviors in silicas have led to detailed structural models of the silica surface based on intersections of OH-terminated 100 and 111 faces of -cristobalite. Other nuclides (e.g., H, C,... 

The speculated presence of dicarbonyl and monocarbonyl species on the silica surface was further confirmed by ONIOM calculations. The model cut from the (100) face of p-cristobalite was applied to mimic the local structures of the silica surface. Two different molecular models with replaceable and irreplaceable silox-ane ligand were built for the dicarbonyl and monocarbonyl species, respectively, as shown in Fig. 18. The calculated relative shifting for the symmetric and asymmetric CO stretching was 11 cm, very close to the experimental value of 12 cm, which revealed information on the local coordination environment of the Cr(II) site (see structures 5e and fie in Scheme 11). 

These spectra, and analogous results obtained by a variety of NMR techniques, show that the Q2 silanols are more extensively hydrogen bonded than the Q3 silanols. From results of this type in samples prepared with various water contents, it has been possible to develop a model of the silica gel (and probably nanosilica) surface in terms of surface segments that resemble specific surface faces of p-cristobalite (Her 1979, Fegrand 1998, Brown et al. 1999, Chuiko 2001, 2003). 

Silica with its constituents Si, O, and H may be regarded as one of the simplest supports. The huge number of varieties however shows that the architecture of the material is manifold and sensitively dependent on the preparation. Attempts to construct silica or just an amorphous Si02 by a computer program gave unsatisfactory results [9]. Since the basic work of Peri and Hensley [10], a simplified model of cristobalite planes is used to explain the properties of the silica surface, neglecting the aspect that the fractal dimension of the latter may reach almost the value three. The thermal behaviour of geminal, vicinal, or isolated OH... 

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