Hydroxyl on silica

Table 7.13. The frequencies of straight vibrations of the terminal and geminal hydroxyls on silica surface and their isotopic shifts [80, 89]...

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)...

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

The number of leaving alkyl groups seems to depend on the degree of hydroxylation of silica. When the silica is heated, water is driven off and the surface concentration of OH groups decreases. At 450°C siloxane bonds are also formed (Scheme 7.8). 

M. Iwamoto, K. Matsukami, and S. Kagawa, Catalytic oxidation by oxide radical ions. 1. One-step hydroxylation of benzene to phenol over group 5 or 6 oxides supported on silica gel, J. Phys. Chem. 87,903 (1983). 

The trans elimination can take place if the basic sites of the alumina attack the hydrogen from one side of the plane and the hydroxyl group is removed from the opposite side of the plane by the acidic sites of the alumina. This may be possible if the reaction occurs within the pores of molecular dimensions (46) or within the crevices of the aluminas. Crevice sites on silica-alumina catalyst have been proposed by Burwell and co-workers (57) on the basis of racemization and exchange reactions of hydrocarbons. 

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. 

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