Silica surface hydroxyl groups

M L. Hair and W. Herd, Reactivity of Boria-Silica Surface Hydroxyl Groups , J. Phys. Chem., 77 [16] 1965-9(1973). 

Figure 2. Dependencies of shift of valence vibration band of silica surface hydroxyl groups at adsorption of small organic bases (a) and organosilicon compounds (b) on the maximum negative charge of the bases (a) and on the ionization potential of organosilicon compounds (b).

Surface hydroxyl groups on Ti02 and Si02 are typically less acidic than those on alumina.On silica surfaces, hydroxyl groups are associated with tetrahedrally coordinated silicon atoms. Functionalization of the surface silanols is readily achieved by reaction with chloro- or alkoxysUane reagents. Reactions such as those in equations (2) and (3) may be used to add phosphine, cyclopentadienyl, and other functionalities to the surface. 

Initially the 3-isocyanatopropyltriethoxylsilane is reacted with the silica surface hydroxyl groups in the following manner... 

Various hydrated aluminum hydroxides serve as starting material for TLC alumina. By a series of nonuniform thermal dehydration processes, a variety of aluminas are obtained, and the ones most suitable for TLC are the crystalline modifications of x-ALOs and y-ALO (Rossler, 1969 Snyder, 1975). The physicochemical properties or the exact nature of adsorption sites of alumina are not well understood. Snyder (1975) has suggested that exposed A1 atoms, strained Al-O bonds, and perhaps other cationic sites serve as adsorption sites, whereas, unlike silica, surface hydroxyl groups are probably not important. Acids are probably retained by interaction with basic sites such as surface oxide ions. Gasparic and Churacek (1978) report that every A1 atom is surrounded by six atoms of... 

Late metals can also be bound to silica. For example, fMeRhL2(CO)] (L = PMes) reacts with silica surface hydroxyl groups to give a surface-bound Rh(I) species, 932, with release of methane. ... 

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 ...

The possibility exists that a surface hydroxyl group on silica can itself function as a proton donor (12),... 

Scheme 3 Anchoring reaction of chromate on a silica support. Adjacent surface hydroxyl groups are consumed and chromiimi attaches to the siuface by oxygen linkages, either in mono-, di- or polychromate forms...

In this case some of the Sll silica was freeze-dried and degassed at 10-3 Torr and 150°C and then reacted with trichloromethylsilane in the vapour phase this replaced the surface hydroxyl groups by chloro groups. The particles were then redispersed (with the aid of ultrasonics) in a 4 1 volume ratio toluene THF solvent mixture. "Living" polystyrene (PS19), in a similar solvent mixture was then introduced and the grafting reaction allowed to proceed for 24 hr at room temperature. 

In an earlier study in this laboratory (8) it had been shown that PS does not adsorb onto hydrophobised (methylated) silica from DMF. It is clear from the present results, however, (sample S12/PS 19, table III) that PS does adsorb onto silica particles which retain their surface hydroxyl groups, despite the strong hydrogen bonding between DMF and these groups referred to above. However, this adsorption is weak and reversible, since the PS... 

A determination of the surface hydroxyl groups was made by Noll et al. (183). The total water content was determined by heating to 1100°. The content ofmolecular water was titrated by the Karl Fischer method. Silanol groups react with this reagent only very slowly. Good agreement was observed with silanol contents determined independently by other methods. With silica gel, 5.2 silanol groups were found per 100 A. ... 

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. 

The rhenium perhydrocarbyl complex Re(=CBu )(= CHBu )(CH2Bu )2 reacts with the surface hydroxyl groups of a silica(7oo) (Scheme 2.29) to form a well-defined surface species, monografted on silica and containing one alkyl, one alkylidene and one alkylidyne ligands according to mass balance analysis IR, NMR, EXAFS [79-82] and calculations [83, 84]. 

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