The nature of active sites on zeolites: I. The decationated Y zeolite

Abstract

The infrared spectra of ammonium Y zeolite have been observed after calcination at a series of temperatures up to 750 °C. At least three different types of hydroxyl groups exist on the surface after the ammonium ions have been decomposed. The hydroxyl group concentration reaches a maximum at 350 °C, remains constant to 550 °C, and decreases with increasing temperature. The spectra of chemisorbed pyridine were observed on the zeolite after calcination. The concentration of pyridinium ion parallels that of the hydroxyl groups and is a measure of the concentration of Brönsted acid sites. The concentration of coordinately bound pyridine (Lewis acidity) is low below 475 °C but increases rapidly with increasing calcination temperature. The hydroxyl groups are the Brönsted acid centers and the Lewis acid centers are tricoordinated aluminum atoms formed by dehydroxylation of the zeolite. Comparison of the hydroxyl group and Brönsted acid site concentrations with catalytic activity shows that the active centers are the acidic hydroxyl groups.