The nature of active sites on zeolites: VI. The influence of calcination temperature on the structural hydroxyl groups and acidity of stabilized hydrogen Y zeolite

Abstract

The infrared spectra of magnesium ammonium Y zeolite have been observed after calcination at temperatures up to 800 °C. Thermogravimetric, differential thermal, and X-ray diffraction analyses were also made. The observations are similar to those previously reported for ammonium Y zeolite except no indications of structural collapse were observed. The spectra of chemisorbed pyridine were observed for the zeolite after calcination. The concentration of pyridinium ion parallels that of the hydroxyl groups represented by the 3643-cm −1 absorption band and is a measure of the Brönsted acid site concentration. The concentration of Lewis acid sites increases with increasing calcination temperatures and decreasing hydroxyl content. The concentration of Brönsted acid sites plus twice the concentration of Lewis acid sites remains constant over a wide range of temperatures. As for ammonium Y zeolite, the acidic hydroxyl groups are considered to be the catalytically active centers. No hydroxyl groups attached to magnesium cations are detected.