Abstract: The acidic properties of four REUSY zeolites were studied by NH_3-TPD and IR spectra. The results show that the spectra of hydroxyl groups of REUSY are very complicated. It can be distinguished as three kinds of hydroxyl groups i.e. silicon hydroxyl groups, high frequency hydroxyl groups and low frequency hydroxyl groups. The acidic or nonacidic hydroxyl groups can also be seperated by the IR differential spectra. When the acidic properties were related to the distribution of framework silicon and aluminum and extraframework aluminun measured by ~(29)Si and ~(27)Al MAS NMR, and also to the secondary mesopores formed in the process of dealumination, it was found that when (Si/Al)_F is about 3 to 4, the acid sites corresponding to the Si(3Al) or Si(2Al) are mainly removed from the α-cages. As the (Si/Al) decreases with the Si(2Al) are removed from the β-cages and hexagonal cages. The results also show that a portion of framework acidic high frequency hydroxyl groups can not be accessed by pyridine molecules, but the low frequency hydroxyl groups can be accessed by Py molecules due to the formation of secondary mesopores. On dealumination, the Bronsted, Lewis and the total acid sites all decrease, and the strength of strong acid sites and Bronsted acid sites increase.

Key words: REUSY, Dealumination, Hydroxyl groups, Acidic property