Deammoniation and dehydroxylation of calcium ammonium chabazites

Abstract

The deammoniation and dehydroxylation of calcium ammonium chabazites has been studied by thermal analysis and infrared spectroscopy. At low ammonium-exchange levels dehydroxylation starts only at extremely high temperatures (ca. 1200 K) owing to the occupancy of the crystallographic SI cation sites by Ca and hence to the existence of isolated hydroxyls at 3630 cm–1 vibrating in the plane of the eight-membered rings. At higher exchange levels a second band at 3550 cm–1 and a dehydroxylation step at much lower temperatures (890–990 K) develop, pointing to the existence of hydroxyls in close proximity which vibrate in the hexagonal prisms or in the six-membered rings. Steaming and calcination results in the appearance of new i.r. bands at 3670 and 3750 cm–1 which are assigned to hydroxyls vibrating in the large cavities and to lattice-terminating OH groups, respectively. Steaming of highly ammonium-exchanged chabazite shifts the frequencies of the acidic hydroxyls to higher values but does not result (as in the case of other zeolites) in a substantial release of aluminium from the framework. The structure of the highly exchanged but steamed H-chabazites is in contrast to the non-steamed samples, which are thermostable after rehydration, indicating that during such steam treatment less acidic hydroxyl groups are formed.