Active sites in zeolites: III. Selective poisoning of Bronsted sites on synthetic Y zeolites

Abstract

2,6-Dimethylpyridine and pyridine adsorption on CaY, HY and NaY zeolites is followed by quantitative infrared spectroscopy. 2,6-Dimethylpyridine reacts selectively with the hydroxyl groups. After adsorption of the base at temperatures from 25 up to 400 °C the OH groups are titrated first and only thereafter the Lewis sites interact with the base. The selectivity of pyridine towards Brønsted and Lewis sites is much less pronounced. Infrared identification of the ion or of the Lewis bound base was made through the frequency shift of the ν 8a and ν 8b vibrations. Poisoning with 2,6-dimethylpyridine of the cumene cracking activity of the HY samples amounts to an upper limit of active sites of 1.8 × 10 20 per gram. This number is much lower than the amount titrated with pyridine.