IR study of heterogeneity of OH groups in zeolite HY-splitting of OH and OD bands

Abstract

We have been studying the problem of heterogeneity of OH groups in zeolites HY for a long time. The heterogeneity was suggested by the shift of the IR band of OH groups restoring upon ammonia desorption and also by the fact that the band of OH groups forming hydrogen bonds was relatively broad (broader than for homogeneous acidic OH). In the present study we present another important argument for heterogeneity: the splitting of the IR band of free OH and OD groups in a zeolite of Si/Al=8.3 dealuminated by (NH 4) 2SiF 6 treatment. Such a splitting is the best seen in low temperature spectra of OD groups. We found less acidic 3640 cm −1 (AlO)(SiO) 2Si O 1 HAl(OSi) 3 and more acidic 3625 cm −1 (SiO) 3Si O 1 HAl(OSi) 3 groups. The presence of these two kinds of hydroxyls corresponds to the presence of Si(2Al) and Si(1Al), respectively, detected in 29Si MAS NMR spectra. We also found a small amount of strongly acidic 3599 cm −1 hydroxyls interacting with extraframework Al species.