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-catalyzed reactions. Hydroxyl groups are responsible for the Bronsted acidity of amorphous silica-aluminas and of zeolites. Generally, the acid strength 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) vibrational 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 vibrational frequencies of their OH groups were modeled by H₃SiOHAlH₃; whereas to study the influence of chemical composition models with formula H₃SiOSi(OH)₃ and H₂ AlOSi(OH)₃ have been studied. It is observed that structural characteristic influences the vibrational frequency more in comparison to chemical composition. A case study with H-Y zeolite has been performed with the models having formula (HO)₃SiOHAl(OH)₃ and [(HO)₃SiOAl(OH)₃]^-. It is observed that between O₁ and O₄ type oxygens present in H-Y zeolite, the O₁-H group lies at a higher frequency than the O₄-H group. All the results were compared with experimental IR frequencies.