Hydroxyl Groups in Silica

Many studies have been devoted to the multiplicity of the surface hydroxyl groups of aluminas. After the work of Peri [116], and of Tsyganenko and Filimonov [111], 

3 Surface Hydroxyl Croups of Other Sesquioxides and of Spinel-Type Mixed Oxides 

4 Surface Hydroxyl Croups of Rock-Salt-Type Metal Oxides 

5 Surface Hydroxyl Croups of the Oxides of Tetravalent Metals 

6 The Surface Hydroxyl Croups of Sulfated and Tungstated Oxides 

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As mentioned above, HOSi(OA)3 may be taken as the simplest cluster model of the terminal hydroxyl group in silicas. Indeed, even with this cluster CNDO/BW provided a quite satisfactory description of the lower part of the curve representing potential energy as a function of the OH stretching vibration coordinate ROH (Fig. 2) (48,49). The respective experimental curve was plotted by Kazansky et al. (49) based on the analysis of the fundamental frequency vOH and the first overtone of the characteristic OH stretching vibration in terms of the Morse potential function. The frequencies of the second and third overtones were also determined in that work, and it was shown that the Morse potential reproduced well the potential curve within a rather wide range of ROH. 

In this section we shall discuss the results of quantum-chemical calculations of the chemisorption interaction of the surface terminal hydroxyl groups in silicas with water and ammonia molecules. Two types of one-center coordination of H20 molecules by a hydroxyl group of Si02 are presented in Fig. 3. CNDO/BW calculations using these clusters gave adsorption energies... 

Deuterium exchange with DjO was used by Shuravlev and Kiselev (199) in the determination of surface hydroxyl groups of silica gel. Adsorption isotherms of HgO and DjO were determined gravi-metrically they agreed with each other within the limits of experimental error. 

Summarizing, it can be said that the existence of surface hydroxyl groups on silica-alumina is beyond doubt. However, in chemical reactions all of the hydroxyl groups behave just like silanol groups on silica. No conclusive evidence for the existence of hydroxyl groups bonded to aluminum ions was ever obtained. The most that can be said is that surface silanol groups are much more stable than A1—OH groups. 

Alumina will also bind Cr03 and stabilize it to 900°C, and it can polymerize ethylene when reduced to Cr(II). High surface area y alumina can be made having the porosity necesssary for good activity. Besides the electronic differences between Si—O—Cr and A1—O—Cr bonds, such alumina catalysts typically have 50-100% more hydroxyl groups than silica at normal calcining temperatures. This is clear in Fig. 21, which shows the hydroxyl populations of three different supports. The hydroxyl concentration was measured by reaction with methylmagnesium iodide. 

There is just a handful of papers in which hydroxyl groups of silica gel are studied directly by high-resolution H NMR techniques (385-387). In particular, Hunger et al. (387) were able to observe two spectral lines in the H MAS NMR spectrum of amorphous silica-alumina gels of different composition The line at 2 ppm from TMS was attributed to nonacidic hydroxyls, since it also occurs in silica and alumina the line at 7 ppm, the... 

The 4 -monophosphate was also prepared from the same compound as above (Figure 5). Although the 4 -hydroxyl group in 12 has rather low reactivity, its phosphorylation could be performed with phenyl phosphate and dicyclohexylcarbodiimide (DCC) in pyridine. The reaction product was converted into the benzyl phenyl ester ( ) to facilitate purification. In contrast to the above dibenzyl ester of the glycosyl phosphate, the benzyl phenyl ester of the 4 -phosphate was stable and could be purified by silica gel column chromatography after removal of the propenyl group without decomposition. Hydrogenolytic deprotection of (first with Pd-black then with PtC>2) afforded the 4 -monophosphate 17. 

After had been treated with one equivalent of butyllithium and then with dibenzyl phosphorochloridate as above, the mixture was hydrogenolyzed immediately. Of the two free hydroxyl groups in 3, the more acidic one on C-l was phosphorylated selectively. The desired 1-a-monophosphate was isolated after purification with a silica gel column (CHCl3-Me0H-H20 40 10 1) as its sodium salt (65% from ). 

Since there were variations in the experimental conditions (such as the nature of the aldehyde or the number of performed reaction cycles) it remains possible that the event or the extent of symmetry breaking depend on these. Even if the first reaction cycle gives rise to only a small ee, further ones will certainly amplify this small bias and push it with its proper enantiomeric direction to the edges. Further influence could come from achiral additives. Kawasaki et al. assume in the case of the addition of achiral silica gel that this additive may provide an improved reaction platform by coordination of the aldehyde and involvement of a zinc atom from the reaction of diisopropylzinc with the acidic hydroxyl group of silica gel [40]. Further systematic experimental studies, which can also shed more light on the basic reaction mechanism, are required to better understand the differences in the results. 

Since Kiselev1 discovered the surface hydroxyl groups on silica in 1936, many studies on the quantification of the silanol number (a0H number of hydroxyl groups per nm2) and on the characterization of the different hydroxyl types have been published. These studies can be divided into theoretical calculations, physical methods and chemical methods. 

Inspired by these developments, Osinski and coworkers employed polyglycerol 1 as the soluble polymer instead of a silica or polystyrene support in order to obtain similarly recyclable, but homogeneous, hydroformylation catalysts [42], In this context the hydroxyl groups in polyglycerol have been... 

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