Heterogeneity of hydroxyl groups in zeolites studied by IR spectroscopy

Abstract

Abstract A hypothesis has been proposed that homogeneous hydroxyl groups would be present in zeolites in which all the hydroxyls have the same number of Al atoms in the close vicinity and also have the same bridge geometry (the same SiO and AlO bond distances and the same SiOHAl bridge angle). However, heterogeneous hydroxyl groups would appear in zeolites in which there are hydroxyls with various Al numbers or with various geometries. Heterogeneity of hydroxyl groups was expected in NaHA and NaHX zeolites (the same bridge geometry and the same number of Al atoms). However, heterogeneity was expected in NaHY zeolite (the same geometry but various number of Al atoms), in NaHZSM-5 and NaH-ferrisilicate (the same Al or Fe number but various geometries) and also in NaH-mordenites (various Al numbers and various geometries). The heterogeneity of hydroxyl groups was studied by analysis of the IR band of hydroxyls hydrogen-bonded to aromatic hydrocarbons, their derivatives and to ethene. Principal component analysis evidenced that the splitting of the IR band of hydrogen-bonded hydroxyl groups is due to the heterogeneity of the hydroxyls and not to Fermi resonance. The results of our IR studies concerning the properties of hydroxyl groups were compared with the results of quantum-chemical calculations (MNDO method) and also the results of 29 Si MAS NMR studies of properties of framework Si atoms.