HETEROGENEITY OF HYDROXYL-GROUPS IN ZEOLITES

Bridging Si-OH-Al groups in NaHZSM-5 and NaHY zeolites are heterogeneous; few kinds of hydroxyls of various acid strength exist. On the other hand, in the case of NaHX zeolite Si-OH-Al groups were found to be homogeneous (only one kind of hydroxyl). These results agree with the hypothesis that homogeneous Si-OH-Al groups are expected in zeolites in which all hydroxyls have the same Si-O and Al-O bond distances, the same Si-OH-Al bond angle, and also the same number of Al in close vicinity, and that heterogeneous Si-OH-Al groups would exist in zeolites in which Si-O and Al-O distances, Si-OH-Al bond angle, or the number of Al close to Si-OH-Al differs.