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

• Published in: Applied catalysis. A, General Vol. 182; no. 2; pp. 225 - 235
• Main Authors: Trombetta, Marcella, Busca, Guido, Lenarda, Maurizio, Storaro, Loretta, Pavan, Massimo
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 21.06.1999; Elsevier
• Subjects: Brønsted acid; Catalysis; Catalysts: preparations and properties; Chemistry; Exact sciences and technology; Ferrierite zeolite; General and physical chemistry; Lewis acid; Mesoporous solids; Pivalonitrile adsorption; Silica–alumina; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Silica–alumina; Lewis acid; Mesoporous solids; Brønsted acid; Pivalonitrile adsorption; Ferrierite zeolite; Characterization; Adsorption; Acidity; Zeolite; Alumina; Silica; Mesoporosity; Molecular sieve
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/S0926-860X(99)00005-8

The surface's acidity of amorphous and mesoporous MCM-41-type silica and silica–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 ferrierite. In contradiction to part of the recent literature, the conclusion is that the Brønsted acidic OHs associated to Al insertion into the silica framework 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 unsaturated coordination sphere. It is concluded that bridging Si–OH–Al ions, giving rise to strong Brønsted acidity of protonic zeolites, are only forced to be formed into their internal cavities, but cannot occur on “flat” or sufficiently “open” surfaces.