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 S_I 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.