Silica hydroxyl groups

The number of silica hydroxyl groups decreases after heat treatments. Hence, the study of the influence of such treatment on the 5bet(CH2C12) values and on the corresponding AEDF is of interest. 

Stereospecificity. The adsorption of isotactic, stereotactic and atactic PMMA to silica from CDCI3 was studied by IH liquid spectra, where the methyl peaks spectrally separate into isotactic, heterotactic and syndiotactic sequences. The intensity ratios of the liquid IH signals of polymers adsorbed to silica were compared to solution spectra. In Fig. 17 the comparison of traces A and D shows that the isotactic component (mm) has significantly decreased in intensity relative to the other two components in spectrum D, and a selective adsorption of i-PMMA was concluded. The stereospecificity decreased with adsorbed amount, and was also reduced for adsorption above the conformational transition temperature. Comparison to adsorption to a hydrophobised surface, which did not show stereospecific immobilisation, proved that the silica hydroxyl groups were essential to attract the isotactic component specifically [59]. 

Nonetheless, Karol et al. [25] and Lunsford et al. [27]postulate that the chromium species in Figure 3.17 is the predominant structure on silica calcined at 800°C, because silica calcined at 800 C is believed to contain only isolated silica-hydroxyl groups, therefore the deposition reaction shown in Figure 3.18 is not possible. 

Examination of Figure 3.20 shows two interesting features from the chromocene deposition e >eriments on silica that were dehydrated at 100°C and 600°C. For siUca dehydrated at 100 C, the solvent effect is not observed. Most likely, this is due to the removal of both of the cydopen-tadienyl rings in chromocene, by reacting with the high concentration of silica hydroxyl groups on this type of silica. The structure shown in Figure 3.19 is not present after the deposition process. The chemically attached, disubstituted chromimn species prepared on siUca dried at 100°C... 

Species A in Figure 3.41 requires adjacent silica hydroxyl groups and would most likely be formed on silica dehydrated at 200°C, while species B requires isolated silica hydroxyl groups that are the predominate species on silica dehydrated at 400 and 800°C. Bade et al. found infrared evidence for the reaction illustrated in Figure 3.41C and suggested that this type of reaction takes place on silica dehydrated at 800°C where strained siloxane linkages may be present. The catalyst prepared on 800°C silica was unique in that it was the only catalyst that exhibited infrared bands associated with a q -bonded allyl group attached to the chromium center. 

The catalyst described above was supported on silica by first dissolving the bis(n-butylcyclopentadienyl) zirconium dichloride compound into the methylalumoxane before this solution was added to the calcined silica. In this method, the MAO is chemically attached to the silica by reacting the silica hydroxyl group (Si-OH) with the AI-CH3 group present in the MAO as shown below ... 

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

Fig. 5.16 Surface concentration of hydroxyl groups of silica, as a function of the temperature of dehydration. Data are +, from Fripiat and Uytterhoeven A, from Kiselev and Zhuralev O, from Taylor (cf.

The relationship between the BET monolayer capacity of physically adsorbed water and the hydroxyl content of the surface of silica has been examined by Naono and his co-workers in a systematic study, following the earlier work by Morimoto. Samples of the starting material—a silica gel—were heated for 4 hours in vacuum at a succession of temperatures ranging from 25 to 1000°C, and the surface concentration of hydroxyl groups of each sample was obtained from the further loss on ignition at 1100°C combined with the BET-nitrogen area. Two complete water isotherms were determined at 20°C on each sample, and to ensure complete... 

In Table 5.3, is compared with the total hydroxyl concentration (Ni, + N ) of the corresponding fully hydroxylated, sample. The results clearly demonstrate that the physical adsorption is determined by the total hydroxyl content of the surface, showing the adsorption to be localized. It is useful to note that the BET monolayer capacity n JH2O) (= N ) of the water calculated from the water isotherm by the BET procedure corresponds to approximately 1 molecule of water per hydroxyl group, and so provides a convenient means of estimating the hydroxyl concentration on the surface. Since the adsorption is localized, n.(H20) does not, of course, denote a close-packed layer of water molecules. Indeed, the area occupied per molecule of water is determined by the structure of the silica, and is uJH2O) 20A ... 

Representative results are given in Table 5.4. From column 7, it is seen that the ratio iV,/ Afj - - N/,) is in the region of 1 2 (in contrast to the 1 1 found with silica) suggesting that each molecule of water in the physisorbed monolayer is bonded to two surface hydroxyl groups. 

Table 2. Absorption Peaks of Gel-Silica Monolith Pore Water and Surface Hydroxyl Groups ...

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

It should be noted that, due to the strong polarity of the hydroxyl groups on the silica, the initial adsorption of the ethyl acetate on the silica surface is extremely rapid. The individual isotherms for the two adsorbed layers of ethyl acetate are shown in Figure 8. The two curves, although similar in form, are quite different in magnitude. The first layer, which is very strongly held, is complete when the concentration of ethyl acetate is only about l%w/w. At concentrations in excess of l%w/w, the second layer is only just being formed. The formation of the second layer is much slower and the interactions between the solvent molecules with those already adsorbed on the surface are much weaker. 

Nowadays, almost all commercially available HPLC stationary phases are also applicable to planar chromatography. In addition to the polar hydroxyl groups present on the surface of native silica, other polar functional groups attached to the silica skeleton can also enter into adsorptive interactions with suitable sample molecules (34). Silica with hydrophilic polar ligands, such as amino, cyano, and diol functions, attached to the silica skeleton by alkyl chains, all of which have been well proven in HPLC, have also been developed for TLC (34). 

Nevertheless, silica gel is the material of choice for the production of the vast majority of LC stationary phases. Due to the reactive character of the hydroxyl groups on the surface of silica gel, various organic groups can be bonded to the surface using standard silicon chemistry. Consequently, the silica gel surface can be modified to encompass the complete range of interactive properties necessary for LC ranging from the highly polar to almost completely dispersive. 

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