NATURE OF ACTIVE SITES ON ZEOLITES .X. ACIDITY AND CATALYTIC ACTIVITY OF X ZEOLITES

The infrared spectra of pyridine chemisorbed on several alkali, alkaline earth, transition metal, and rare earth cation forms of X zeolites have been studied. Pyridinium ion, characterized by an absorption band at 1545 cm-1, is a measure of the Bronsted acidity and coordinately bound pyridine, with an absorption band near 1451 cm-1, is a measure of Lewis acidity. All of the zeolites studied, apart from the alkali cation and copper forms, are Bronsted acids. After dehydration at 480 °C, only the rare earth forms showed detectable Lewis acidity. The acidity of the X zeolites is much less than that of the Y zeolites. Reasons for this difference are suggested. For the alkaline earth forms, the Bronsted acid site concentration increases with decreasing cation radius and increasing electrostatic field. No simple relationship between the acidity of the transition metal forms and physical parameters was found. The catalytic activity for several reactions is, in general, related to the Bronsted acidity. However, although the relationship is good for the alkaline earth and rare earth forms, it is poor for the transition metal forms. Differences in the activation energy for the different groups of cations suggest that different mechanisms or sites might be involved. © 1969.