EFFECTS OF STRUCTURAL CHARACTERISTICS OF ZEOLITES ON THE PROPERTIES OF THEIR BRIDGING AND TERMINAL HYDROXYL-GROUPS

Hydrogen forms of zeolites are active catalysts in numerous acid-catal yzed reactions. Hydroxyl groups are responsible for the Bronsted acidi ty of amorphous silica-aluminas and of zeolites. Generally, the acid s trength of OH groups in H forms of zeolite depends on the Si/Al ratio, the degree of cation exchange and the degree of dehydroxylation. The present aim of our investigation is to rationalize an understanding of the role of hydroxyl groups both bridging and terminal on the acidity of zeolites with the help of local density functional (LDF) vibration al frequency calculation. The effects of geometry (Si-O and Al-O bond lengths and Si-O-H as well as Si-O-Al angles) of zeolites on the vibra tional frequencies of their OH groups were modeled by H3SiOHAlH3; wher eas to study the influence of chemical composition models with formula H3SiOSi(OH)(3) and H2AlOSi(OH)(3) have been studied. It is observed t hat structural characteristic influences the vibrational frequency mor e in comparison to chemical composition. A case study with H-Y zeolite has been performed with the models having formula (HO)(3)SiOHAl(OH)(3 ) and [(HO)(3)SiOAl(OH)(3)](-). It is observed that between O-1 and O- 4 type oxygens present in H-Y zeolite, the O-1-H group lies at a highe r frequency than the O-4-H group. All the results were compared with e xperimental IR frequencies. (C) 1998 Elsevier Science B.V. All rights reserved.