An investigation of the surface acidity of mesoporous Al-containing MCM-41and of the external surface of ferrierite through pivalonitrile adsorption

The surface's acidity of amorphous and mesoporous MCM-41-type silica and si lica-aluminas has been investigated by using IR spectroscopy of the surface OHs and of adsorbed pivalonitrile, and catalytic isomerization of 1-butene . IR experiments allowed also characterizing the external sites of ferrieri te. In contradiction to part of the recent literature, the conclusion is th at the Bronsted acidic OHs associated to Al insertion into the silica frame work are terminal on the surfaces of both amorphous and mesoporous samples and also on the external surface of ferrierite. These sites participate to the generation of the O-H stretching band near 3745 cm(-1). These surfaces also carry Lewis acid sites arising from Al cations in a tetrahedral-like u nsaturated coordination sphere. It is concluded that bridging Si-OH-Al ions , giving rise to strong Bronsted acidity of protonic zeolites, are only for ced to be formed into their internal cavities, but cannot occur on "flat" o r sufficiently "open" surfaces. (C) 1999 Elsevier Science B.V. All rights r eserved.