Infrared and temperature-programmed desorption study of the acidic properties of ZSM-5-type zeolites

Infrared spectroscopy and temperature-programmed desorption have been used to investigate the acidic properties of ZSM-5-type zeolites. Active acidic sites, in concentration related to the aluminum content and most probably located at the channel intersections, are characterized by an ir band at 3600 cm−1 and a desorption peak near 773 K (γ state). Weaker acidic sites are characterized by an ir band at 3720–3740 cm−1 and a desorption peak at about 500 K (β state). They probably correspond to terminal silanol groups on the external surface of the zeolite or possibly nonzeolitic impurities. Lewis acidic sites are also characterized by the γ-state desorption peak, while Na adsorption centers for ammonia give rise to a β-state peak. Pyridine desorption helps dehydroxylation of the zeolite.