New insight into the thermal transformation of barium exchanged zeolite A to celsian

The thermal transformation of a sample of Ba-exchanged zeolite A, containing a residual Na content of 0.20 meq/g, into monoclinic celsian is thoroughly investigated. The samples were thermally treated at temperatures ranging from 200 to 1500 degreesC for times up to 40 h and subsequently were characterized by room-temperature X-ray diffraction. The following are the main results of this investigation: (1) Above 500 degreesC, the presence of small crystalline aggregates of monoclinic celsian dispersed in the amorphous phase arising from the thermal collapse of Ba-exchanged zeolite A is recorded.(2) Above 900 degreesC, crystallization of hexacelsian occurs.(3) Above 1100 degreesC, hexacelsian converts into monoclinic celsian. The conversion is complete with thermal treatments at 1300, 1400, and 1500 degreesC within 22, 15, and 22 h, respectively. The presence of monoclinic celsian nuclei above 500 degreesC appears to favor the conversion of hexacelsian into monoclinic celsian at temperatures higher than 1100 degreesC. The residual Na content of Ba-exchanged zeolite A was found to strongly affect the temperatures and times at which the transformations of amorphous phase --> hexacelsian and hexacelsian --> monoclinic celsian occur. Actually a higher Na residual content results in lower temperatures and times, ceteris paribus.