BRIDGING HYDROXYL-GROUPS IN ZEOLITIC CATALYSTS - A COMPUTER-SIMULATION OF THEIR STRUCTURE, VIBRATIONAL PROPERTIES AND ACIDITY IN PROTONATED FAUJASITES (H-Y ZEOLITES)

The structure and properties of the four possible bridging hydroxyl groups in silicon-rich H-faujasite (zeolite Y) are studied by lattice-energy minimization carried out within the classical shell model. The most likely proton-accepting sites are the O1 and O3 oxygens. Their hydroxyl stretching frequencies are shown to be responsible for the characteristic high-frequency (HF) and low-frequency (LF) bands, respectively, in the infrared spectra of H-faujasites. The variation of the OH stretching frequencies for the four isolated sites is correlated with changes of local geometry such as the 0-H bond length and the Si-O(H)-Al and the Si-O-H angles. However, there appears to be no correlation with differences in the acid strength as evidenced by the values of the calculated deprotonation energy of the O(1)H and O(3)H hydroxyl groups for which only a minor difference is predicted.