A Comparative Study of the Thermal Transformations of Ba- Exchanged Zeolites A, X and LSX

In this investigation samples of exhaustively Ba-exchanged zeolites Ba–A, Ba–X, and Ba–LSX were subjected to thermal treatments in the temperature range 200–1550 C for times up to 40 h. Their thermal transformations were comparatively studied by room temperature X-ray diffraction. Upon heating, the three zeolites undergo the following sequence of transformations: zeolite! amorphous phase!hexacelsian!monoclinic celsian in strongly different ways. In particular it was found that: (1) The crystallisation of nuclei of monoclinic celsian dispersed in the amorphous phase arising from the thermal collapse of the structure of zeolite Ba–A, recorded above 500 C in a previous work, did not occur for zeolites Ba–X and Ba–LSX. (2) Zeolite Ba–A exhibited a far greater reactivity than zeolites Ba–X and Ba–LSX; actually, the conversion of hexacelsian into monoclinic celsian attained completion at 1300 C, starting from zeolite Ba–A, whereas did not attain completion in the most severe experimental conditions experienced (1550 C, 24 h), starting from zeolites Ba–X and Ba–LSX. Such different thermal behaviours could be reasonably ascribed to the different middle-range order occurring in the amorphous phases arising from the thermal collapse of the microporous structures of zeolites Ba–A, Ba–X, and Ba–LSX.