Thermochemical properties of ammonium and hydrogen exchanged mordenite

Abstract

The thermochemical properties of ammonium and hydrogen exchanged synthetic mordenite have been examined by i.r. spectroscopy, thermal analysis, and high temperature X-ray diffractometry. Differential thermal analysis (d.t.a.) of ammonium exchanged mordenite in air shows an intense ammonia oxidation exotherm at 525°C and crystal collapse at 1030°C. Anomalous results are obtained if an instrument in which the purge gas passes over the zeolite bed is used. Thermal gravimetric analysis (t.g.a.) indicates that dehydration, deammoniation and dehydroxylation occur as discrete steps. I.r. analyses show that water can be removed by evacuation at room temperature and that the position of the zeolitic hydroxy group band shifts from 3637 to 3612 cm^–1 with increasing temperature. Several bands in the framework region increase in frequency at higher temperatures. High temperature X-ray diffractometry indicates that variations in the unit cell dimensions are very complex and depend upon temperature, time, bed geometry, and calcination atmosphere. The diffraction pattern is different for samples fired in a thin bed and thick bed configuration; the latter environment results in a more stable crystal structure and intense low angle peaks. I.r. spectra of hydrogen mordenite show that the zeolitic hydroxy group band occurs at 3612 cm^–1 and is invariant with temperature. However, shifts can be noted in the framework region. It is not possible to remove water by room temperature evacuation. D.t.a. and t.g.a. show that dehydroxylation occurs near 750°C. X-ray diffraction patterns are very similar to ammonium exchanged mordenite and also show an effect of bed configuration during firing. In view of the enhanced stability of NH₄–Y, ammonium exchanged mordenite, and hydrogen mordenite caused by firing in a deep bed configuration, it is very likely this can be considered a general phenomenon of siliceous zeolites.