IR Spectroscopic Studies of Dealuminated and Realuminated Zeolite HY

Dealumination of zeolite NaHY with water vapor at 820 K results in the disappearance of the 3550 and 3640 cm -1 infrared bands from hydroxyl groups and the appearance of new bands at 3600 cm -1 (assigned by some authors to strongly acidic hydroxyls interacting with extraframework Al) and at 3690 cm -1 (from the Al−OH groups). Sorption of benzene reveals that two kinds of Al−OH groups coexist in dealuminated zeolites: one accessible and another inaccessible to benzene molecules, corresponding to IR bands at 3681 and 3697 cm -1, respectively. NMR has shown that realumination using an aqueous solution of KOH at 350 K results in the reinsertion of most of Al into the framework and the reconstruction of the hydroxyls IR bands at 3550 and 3640 cm -1 . These hydroxyl groups react with ammonia and pyridine in the same way as in the nondealuminated zeolite. Weak bands at 3600 and 3690 cm -1, characteristic of OH groups in dealuminated zeolite Y, are still present after realumination, which indicates that some extraframework Al remains in the sample. Pyridine sorption shows that in both the realuminated and the nondealuminated samples the low-frequency hydroxyls situated inside the hexagonal prisms are inaccessible to the bulky molecule, indicating that realumination with water vapor at 820 K does not create vacancies which could facilitate access to these hydroxyls, as is the case with dealumination by H 4 EDTA. Benzene sorption measurements show that the acid strength of the 3640 cm -1 hydroxyls in the realuminated zeolite is higher than prior to dealumination. We attribute this to the changed relative populations of the Si( n Al) groupings in the course of the dealumination−realumination process. The contribution of the Si(1Al) sites, responsible for the creation of most of the acidic hydroxyls, increases upon realumination.