Chemisorption sites on porous silica glass and on mixed-oxide catalysts

Abstract

It has been shown previously that two types of adsorption sites exist on the surface of porous silica glass (Corning code no. 7930) for the adsorption of polar molecules, such as ammonia. One site is the surface hydroxyl group which forms a hydrogen bond to the adsorbate molecule. The second type of site is provided by traces of foreign oxides on the glass surface.In the present study, the infrared spectrum of ammonia adsorbed on the silica glass has been compared with that of ammonia adsorbed on samples prepared by adding Bz03, Zr02, and A1203 to a pure silica powder. This comparison suggests that surface boron a t o~n s (a Lewis acid) are the adsorption sites for ammonia on the porous silica glass. An additional type of site (a Bronsted acid or a different type of Lewis acid site, such as aluminium atoms) may exist in small concentrations on the glass and be responsible for isomerization and polymerization reactions with olefins.

Broensted acid sites on porous glass from membrane potentials

“…Their method would not apply to Br0nsted acid sites produced by tetrahedrally coordinated aluminum or boron.21 •22 Other infrared studies have examined the nature of active sites on porous glass. [8][9][10][11][12][13][14][15][16] There is general agreement that in addition to silanol groups there are sur-face B-OH groups12-14 and surface sites demonstrating Lewis acidity which have been assigned to trigonally coordinated boron, The previous emf studies of porous glass membranes have shown the applicability of the Teorell-Meyer model but have ignored the weak electrolyte character of the surface acid sites.18 •19 The present study includes the dissociation constant and the hydrogen ion activity in the bulk and interfacial region to gain additional information about the nature of these acid sites.…”