Basic OH group

Gilman found that the a, w-dihydroxy-decaphenylpentasilane formed by a normal cleavage reaction with bromine followed by hydrolysis reacts to form a trisilane when the solution is run through a column of aluminium oxide8 The terminal Si atoms are apparently cleaved by the basic OH groups of the A1203 ... 

The number of these sites, as measured by Fink (246, 262), vary between 1.2 and 1.8 X 1013/cm2. This value nicely coincides with the number of sites that convert pyridine to the pyridone species (see Section IV.D.l). Thus, the X-sites certainly contain reactive and strongly basic OH groups and they may be identical with the sites responsible for the pyridone formation and the hydrolysis of ketones and nitriles. The highest-frequency OH group vanishes preferentially in all these surface reactions. Because the X-sites are Al-OH pair sites created by the formation of oxide vacancies in the immediate vicinity of the reactive OH groups, the above result lends some support to the interpretation of Dunken and Fink (116) that the reactive OH groups (3800 cm-1) are surrounded by four oxide vacancies rather than Peri s (120) assumption that they are surrounded by four O2- ions (see Section IV.A.l). Rosynek (267a) thinks that a free carbonate ion also exists on the surface and contributes to the band at 1480 cm-1. 

The Al-OH groups involved in this reaction are likely to be the basic ones, since the adsorption reaction is probably to be thought of as analogous to the hydrolysis of the PtClk2 ) species that occurs in aqueous solution at higher pH values. The fact that the platinum adsorption capacity of a typical y-alumina is of the order of 1.5 pmol/m, while the amount of basic OH groups is typically 3 to 3.5 pmol/m2, as determined by titration with Mo02(acetylacetonate)2, fits in nicely with this idea. 

The interaction between AHM and an Y-AI2O3 surface has been extensively studied. Two adsorption reactions take place. In the first instance, HM ions react with the basic OH groups according to... 

As Fig. 10.15A shows, the OH stretch region of a spectrum of Ti02 contains at least three peaks. The one at about 3730 cm-1 disappears when the surface is exchanged with F ions and corresponds to basic OH groups. The other peaks around 3670 and 3630 cm-1 correspond to neutral and acidic hydroxyl groups, respectively [34]. Similar correlations exist for the O-H stretch frequencies of OH groups on alumina supports [32]. 

The specific adsorption of anions at titanium dioxide (discussed in the following part of this chapter) seems to occur via the replacement of the basic OH groups. It appears difficult, at the first sight, to find a common point among the anions known to give rise to the super-equivalent adsorption on the surface of Ti02. 

In strongly alkaline solutions also the singly co-ordinated (basic) OH groups may presumably react in a similar way, i.e.. 

Fig. 11.15. Infrared spectra of the OH stretch region of Ti02, (A) before and (B) after exchange with F confirm that the peak aroimd 3720 cm" is due to basic OH " groups (from van Veen et al. [34]).

Basic salt A salt containing a basic OH group. 

Titania is crystalline TiO2 with basic OH groups on its surface (this is in contrast to silica) therefore it is stable at high pH. It can be used in both the normal-phase and the reversed-phase mode. 

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