Surface hydroxyl groups on silica

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

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. 

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... 

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... 

C.P. Tripp and J.R. Combes, The interaction of supercritical C02 with the adsorbed water and surface hydroxyl groups on silica, Langmuir, 14, p.7348 (1998). 

B3. Basila, M. R., Hydrogen bonding interaction between adsorbate molecules and surface hydroxyl groups on silica. J. Chem. Phys. 36, 1151-1158 (1961). 

Figure 9.17 Surface hydroxyl groups on silica (A) isolated groups (B) hydrogen bonded groups (C) geminated hydroxyl groups (from the same Si atom). (After Hindryckx et al., 1997.)...

The surface properties of amorphous silicas are largely influenced by the nature of the surface silanol (SiOH) groups [1-3]. Lewis acid—base sites are absent unless the silica has been activated at very high temperatures, Brpnsted acidity at the gas—solid interface is low or nonexistent, and the sUoxane bridges are relatively unreactive toward most molecules. This chapter discusses some methods that employ chemical modiflcation and H-D exchange to probe the nature of the surface hydroxyl groups on silica. Infrared spectroscopy is the main technique used, and one objective has been to compare the silanol groups on a fumed silica with those on a precipitated silica. 

The interaction of titanium tetraisopropoxide with CrO was investigated in refluxing carbon tetrachloride, which gives the Cr(III) species Cr[(R0)j-Ti-0] j. The presence of such a compound indicates that the titanium tetraisopropoxide need not react with the surface hydroxyl group on silica in a separate step, but may react with the CrO first, and this intermediate material would react with the surface hydroxyl groups to lead to the type of structure shown in Figure 3.8, Structure (II). 

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