Surface Groups on Silica

Only with silica was the nature of the surface groups studied as extensively as with carbon. Silica, like carbon, has several polymorphs. Apart from the amorphous state, it is known to exist in numerous crystalline modifications. The most important forms are quartz, tridymite, and cristobalite. Each of these can occur in a low-temperature form and in a high-temperature form of somewhat higher symmetry. Tridymite is only stable if small amounts of alkali ions are present in the lattice 159). Ar. Weiss and Al. Weiss 160) discovered an unstable fibrous modification with the SiSj structure. Recently, a few high-pressure modifications have been synthesized keatite 161), coesite 162), and stishovite 16S). The high-pressure forms have been found in nature in impact craters of meteorites, e.g., in the Arizona crater or in the Ries near Nbrdlingen (Bavaria). 

Stishovite is very interesting because it has the rutile structure with octahedral coordination of silicon. In all other forms of silica, each silicon atom is surrounded tetrahedrally by four oxygen atoms. 

The bonding is intermediate in type between purely covalent and ionic 164). There is some d-n—pn bonding between silicon and oxygen. 

The bond angle is near 140° (165). A review on the structural peculiarities of silica and silicates was recently given by Noll 166), 

Amorphous silica is similar to the crystalline modifications in the close ordering of the atoms. However, the three-dimensional array of the Si04 tetrahedra is not as regular. 

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Figure 8 (a) Schematic diagram showing distribution of fillers in different parts of anionic elastomer [27]. (b) Proposed structural model showing interaction of silanol groups on silica surface with carboxylale groups [27]. 

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

Most work on the surface of silica has been done with amorphous silica of colloidal dimensions. This is due to its large surface area and to its technical importance. We shall therefore discuss first the identification of surface groups on amorphous silica. 

In principle, there is no difference in the surface groups on quartz and on amorphous silica. The most important question discussed in the literature is whether the structure of crystalline quartz is represented in its surface, too. Many investigators (282-287) reported that there is a disturbed layer of amorphous character present on the quartz surface. It is more readily dissolved by water or by hydrofluoric acid. Holt and King (288) claimed that only a monomolecular layer of silicic acid was adsorbed on quartz surfaces. 

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. 

In a volume otherwise heavily concerned with mechanistic questions, the editors have included a contribution (H. P. Boehm) from another sphere of investigative interest which struck us as having the quality of the imported spice to stimulate new interests within the accustomed sphere of catalytic gastronomy. It concerns the chemical nature of the surface groups on a series of materials (e.g. silica, alumina) which happen to be so frequently and universally used by the catalytic researcher. 

The SULPHOS-containing rhodium and ruthenium complexes retained their catalytic activity in heteroarene hydrogenation when supported on styrene-divinylbenzene polymer [180] or on silica [181], and showed even higher activity than in homogeneous solution. This effect is attributed to the diminished possibility of dimerization of the active catalytic species to an inactive dimer on the surface of the support relative to the solution phase. The strong hydrogen bonds between the surface OH-groups on silica and the -SO3 substituent in 31 withheld the catalyst in the solid phase despite the rather drastic conditions (100 °C, 30 bar H2). 

Silica-alumina is the most popular mixed oxide support, combining characteristic features of silica and alumina, including (i) high surface area, (ii) persistent OH population at high temperature and (iii) strong Lewis acidic sites. The predominant surface hydroxyl groups on silica-alumina are [=Si-OH], while [Als-OH] surface species have not been observed [79, 89, 90]. Note that the silica-alumina bulk is mainly composed of [=Si-0-Si=] along with [=Si-0-Als] moieties (Lewis... 

Figure 9. Schematic representation of active groups on silica surfaces (A) isolated silanols (B) siloxane bonds (C) geminal silanols (D) hydrogen-bonded silanols ( ) hydrogen-bonded water. (Reproduced with permission from L. Johnston in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 359.)...

The Bronsted or proton acidity of the surface hydroxyl groups on silica and alumina is weak (51, 52, 53). Evidence for this comes from IR studies of pyridine adsorption, no surface pyridinium species (py H+)... 

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. 

Activation energies for the coverage of surface hydroxyl groups on silica gel... 

A remarkable feature is the perturbation of the 2270 cm 1 Si-H stretching vibration. Upon ammoniation, this band splits into at least 3 distinct bands. Actually, this band contains all the information on the concentration and the nature of the surface groups on ammoniated, trichlorosilylated silica gel. This means that knowledge of the intensity and peak position of the different shoulders in the Si-H vibration band results in a prediction of the concentration of the N and Cl containing species on the surface and in a distinction of primary and secondary species. These fascinating characteristics of the silane vibration will be fully covered further in this chapter. 

M.L. Hair, Hydroxyl Groups on Silica Surface , J. Non-Cryst. Solids, 19 299-309 (1975). 

We have shown that grafted surface silicon hydride groups can influence the structure of filled polymers and polymerization of unsaturated monomers owing to formation of polymer-filler covalent Si-C bonds.3,4 The presence of both methylsilyl and chemically active silicon hydride groups on silica may provide improved compatibility, to obtain a more uniform filler distribution of the filled composite. 

Ammonia interacting with a surface, as sketched in Fig. 4.57, will form NH4 ions coordinated to the bridging oxygen atom. Most oxide surfaces behave very similarly to the MgO surfaces. An exception is the surface generated on silica. In Si02 the Si cations are tetrahedrally coordinated. This coordination can be restored on the surface when the Si02 surface becomes exposed to H2O. Silanol groups develop, as sketched in Fig. 4.58. 

Peri and Hensley [22] proposed the existence of these groups on silica surface, although their existence on the surface has not been confirmed. These groups have only been experimentally observed on monomeric organosilicon compounds in solution. 

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