Vicinal silanols condensation

It will be shown in chapter 5 that in the temperature range 473-673 K mainly vicinal hydroxyls condense, whereas in the region above 673 K mainly isolated hydroxyls condense with increasing difficulty. Temperatures > 1473 K are required to remove all silanols.21... 

However, it is hard to see from the common models of the silica surface just how the hydroxyls could come close enough to chromium to coordinate. For example in the model of Peri and Hensley,with the chromium occupying vicinal pairs,the nearest hydroxyl neighbor still seems to be 3 or 4A away. Nonetheless, the same problem is often encountered in explaining how neighboring silanols condense, so it is clear that our understanding of the silica surface is incomplete. Some type of distant coordination might even be possible between chromium and hydroxyls which does not completely block the active center but still influences its behavior. 

The increase in relative intensity of the D2 band at intermediate temperatures is correlated with a reduction in relative intensity of the ca. 3740 cm" band assigned to surface SiO-H stretching vibrations [147]. (See Fig. 45.) This suggests that the species responsible for the D2 band forms by condensation reactions involving isolated vicinal silanols on the silica gel surface as confirmed by 0 isotopic enrichment studies [160,161]. 

The NMR-Raman data indicate that in gels three-membered rings are absent at low temperatures. They form at intermediate temperatures primarily on the silica gel surface by condensation reactions involving isolated vicinal silanol groups located on unstrained precursors [32] ... 

The rate constant data indicates that the rate, thus ease, of pyridine removal from surface silanol sites increases with increasing pretreatment temperature for both silicas. If diminished acidity of silanol sites results in a weaker hydrogen bond to pyridine, then the data indicates that increasing treatment temperature results in not only fewer silanols, but also silanols with reduced acidic strength. An important correlation is that thermal treatment of silica in the 200-400°C range is known to result in the condensation of silanols that experience mutual hydrogen bonds. Most commonly these are vicinal sites... 

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