The nature of active sites on zeolites: III. The alkali and alkaline earth ion-exchanged forms

Abstract

The infrared spectra of pyridine adsorbed on the alkali and alkaline earth cation forms of Y zeolite have been studied. Pyridinium ion, characterized by an infrared band at 1545 cm −1, is a measure of the Brönsted acidity and coordinately bound pyridine, with an absorption band at 1451 cm −1, is a measure of Lewis acidity. None of the zeolites studied exhibits Lewis acidity if the calcination temperature is less than 500 °C. At 650 °C calcination temperature, Lewis acidity is observed. The alkaline earth forms are Brönsted acids whereas the alkali cation forms are not. The population of Brönsted acid sites increases with decreasing cation radius and increasing electrostatic potential and field. The divalent cation forms also have structural hydroxyl groups. The electrostatic potential or field due to the divalent cation dissociates adsorbed water, producing MOH + and structural hydroxyl groups. The hydroxyl groups are the Brönsted acid sites. Pyridine is also coordinately bonded directly to the cation; the strength of the bonding increases with cation field for both the alkali and alkaline earth cation forms. The variation of catalytic activity with cation was also examined. Catalytic activity is greatest for the small divalent cations for both cumene and hexane conversion. The Brönsted acidity, hydroxyl group concentration, and catalytic activity vary in a parallel manner with cation size. The acidic hydroxyl groups are similar to those of hydrogen Y zeolite and are considered to be the active sites.