Acidity of silanol groups

The acidity of silanol groups on the surface of silica or polysilicic acid has been examined by Schindler and Kambcr (40), who calculated the intrinsic acidity constant, K f, from results of titrating silica gel at 25°C in 0.1 M NaCIO, solution ... 

Hydrolysis is most rapid and complete when catalysts are employed [47]. Although mineral acids or ammonia are most generally used in sol-gel processing, other known catalysts are acetic acid, KOH, amines, KF, HF, titanium alkoxides, and vanadium alkoxides and oxides [47]. Many authors report that mineral acids are more effective catalysts than equivalent concentrations of base. However, neither the increasing acidity of silanol groups with the extent of hydrolysis and condensation (acidic silanols may neutralize basic catalysts [48]) nor the generation of unhydrolyzed monomers via base-catalyzed alcoholic or hydrolytic depolymerization processes have generally been taken into acount. 

Condensation occurs most readily at a pH value equal to the piC of the participating silanol group. This representation becomes less vaUd at pH values above 10, where the rate constant of the depolymerization reaction k 2 ) becomes significant and at very low pH values where acids exert a catalytic influence on polymerization. The piC of monosilicic acid is 9.91 0.04 (51). The piC value of Si—OH decreases to 6.5 in higher order sihcate polymers (52), which is consistent with piC values of 6.8 0.2 reported for the surface silanol groups of sihca gel (53). Thus, the acidity of silanol functionahties increases as the degree of polymerization of the anion increases. However, the exact relationship between the connectivity of the silanol sihcon and SiOH acidity is not known. 

The condensation catalyzed by a strong base is first order with respect to substrate and catalyst (74,75). Because of the high acidity of silanol, all the alkah metal base (MtOH) is usually transformed into the silanolate anion. In the rate-determining step, the sdanolate anion attacks the siHcon atom in the silanol end group (eq. 12 and 13). 

It is well know that the zeolite materials synthesized in alkaline systems usually have a high number of silanol groups (=SiOH) named defect groups [10] which possess a moderated Bronsted acidity [11]. Oppositely, Silicalite-1 synthesized in fluorine media are relatively defect-free [12] and the fluorine ions remain in the small cages of the MFI structure even after the calcination process [12]. The 29Si-NMR analyses carried out on samples Na-Silicalite-1 and F-Silicalite-1 confirm the presence of silanol groups only on the SI support surface (results not showed). Delaminated zeolites (ITQ-6) are obtained by exfoliation of as-synthesized lamellar precursor zeolites [13]. After this process, the final structure of the delaminated zeolite results in a completely hydroxylated and well-ordered external surface [13]. 

Very few direct measurements of the reaction of surface silanol groups on quartz have been reported. This is apparently caused by the small effects due to the limited surface areas available. Adsorption of sodium ions on quartz was measured by radioactive tracer techniques by Gaudin et al. (293). Saturation was achieved at high pH (>10) and sodium ion concentrations above 0.07 Jlf. The calculated packing density of silanol groups was 4.25/100 A. Goates and Anderson (294) titrated quartz with aqueous sodium hydroxide and alcoholic sodium ethylate. The occurrence of two types of acidic groups was reported. 

The above interpretation can also explain the IR spectra of the HYUS sanples. Indeed, the HYUS-8 sample shows two bands at 3555 and 3635 cm, a considerable amount of silanol groups, and hydroxyl vibrating at 3675 and--3695 cm (Fijg. 4a). After pyridine is adsorbed the bands at 3555 and 3635 cm disappear, while the other two bands remain and a new band at 3615 cm is now clearly visible (Fig. 4b. If the difference spectrum is obtained a band at 3600-3610 cm (Fig. 4c) again indicates the presence of strongly acid hydroxyl groups. ... 

Silica is an amorphous product with several types of silanol groups on its surface [4,5]. In Figure 2.1, the different types are shown. From the IR data, the presence of isolated silanol groups with a sharp absorption band at 3746 cm is evident. These isolated silanol groups are the more acidic ones (pK 5) and can act as the cation exchanger. Their acidity depends on the surface impurities... 

Technological development has not been equally intensive over the entire GC field. An area in which surprisingly little has happened during recent years is the further development of the separation columns where there are still some problems. A major problem concerns the properties of modern columns. Because of the presence of silanol groups, the surface of the fused-silica tubing is acidic. Such surface silanols have to be more or less entirely removed to be able to elute basic compounds from the column. In a recent comparison of commercially available columns, it was shown that these are not entirely satisfactory concerning... 

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