Silanol group esterification

First investigations revealed a comparison of batch-scale performance with micro-channel processing [91]. Further investigations of the role of surface effects catalyzing the esterification concerned the deliberate enhancement in the number of silanol groups on the glass surface of such micro-channel reactors. 

Many other methods have been used to prepare bonded phases these include esterification of the surface silanol groups with alco-Tiols, or conversion of the silanol groups to Si—Cl using thionyl chloride, followed by reaction with an organometallic compound. If you are interested, there are details in the textbooks by Knox or by Hamilton and Sewell. 

A marked decrease in the chlorine uptake was observed with increasing temperature of pretreatment of the samples. Very little chlorine was bound when the surface silanol groups were protected by esterification (see Section III,A,l,/and Table XVII). 

Esterification. Another reaction which can be used in the determination of surface silanol groups is their esterification with alcohols. This reaction has also found industrial application. The formation of surface esters by the action of alcohols on silica is covered in a patent to Her (226). The reaction products, called estersils, are hydrophobic. 

Esterification of Silanol Groups on Aerosil Surfaces [Calculated from Measurements by... 

After esterification After SOCl 2 reaction silanol groups ... 

As appears from the examination of the equations (giving the best fit to the rate data) in Table 21, no relation between the form of the kinetic equation and the type of catalyst can be found. It seems likely that the equations are really semi-empirical expressions and it is risky to draw any conclusion about the actual reaction mechanism from the kinetic model. In spite of the formalism of the reported studies, two observations should be mentioned. Maatman et al. [410] calculated from the rate coefficients for the esterification of acetic acid with 1-propanol on silica gel, the site density of the catalyst using a method reported previously [418]. They found a relatively high site density, which justifies the identification of active sites of silica gel with the surface silanol groups made by Fricke and Alpeter [411]. The same authors [411] also estimated the values of the standard enthalpy and entropy changes on adsorption of propanol from kinetic data from the relatively low values they presume that propanol is weakly adsorbed on the surface, retaining much of the character of the liquid alcohol. 

In Section 3, Cr(VI) attachment to silica was described as an esterification with pairs of silanol groups on a fully hydrated surface. Such a reaction cannot occur, however, if the silica has first been calcined and if the impregnation of chromium is done from an anhydrous, aprotic solvent. Nevertheless, such a catalyst can be made and Cr(VI) does attach through reaction with siloxanes. Probably, the most reactive siloxanes are strained or distorted. Such catalysts have been found to provide some unique and... 

Various methods, either chemical (4-8) or physical (9-15), can be used for the determination of these surface groups, and their number and type can be easily modified by chemical (e.g., esterification upon reaction with alcohols) or heat treatment. However, for heat treatment, as shown by Fripiat (16), the modification of the surface chemical properties is much more complex than would be expected when only considering the curves relating weight loss to temperature. Thus it should be of interest to relate the evolution of surface silanol groups to the surface free energy of silica samples. 

The hydrolysis reaction (eq 4) replaces alkoxide groups with hydroxyl groups. Subsequent condensation reactions involving the silanol groups produce siloxane bonds plus the by-products alcohol (eq 5) or water (eq 6). The reverse of hydrolysis is esterification, in which hydroxyl groups are replaced with alkoxides. The reverse of condensation is siloxane bond alcoholysis (eq 5) or hydrolysis (eq 6). 

Porous silica beads have been used in GSC for separating mixtures of gases and volatile organic hydrocarbons (VOCs). Retention characteristics arise from adsorption by the micropores, 50-500 nm in diameter, and polar properties of the surface silanol groups. Retention of VOCs decreases as the pore size decreases and surface area increases. Hydrothermal treatment with steam at 850°C for up to 24 h is used to increase pore size to over lOOOnm and chemical modification by silanisation of the silanol groups reduces polar character. Esterification of the silanol groups with methanol and ethanol has also been used. 

The synthetic methods used for the preparation of bonded phase materials are illustrated in Figure 6.35. One of the first reported bonded phases, the alkoxy silanes (1) also referred to as silicate esters, was prepared by the direct esterification of silanol groups with alcohols. The major disadvantage of this packing material was its hmited hydrolytic stability, as it is readily hydrolysed by aqueous alcohol eluants. 

Vogt and coworkers [86] discovered that the reactivity of the corner situated silanol group in oligosilsesquioxanes for esterification with a chlorophos-phite can be enhanced by conversion into the corresponding thallium salt (Scheme 2.97). 

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