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 °C 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 °C, 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 °C, crystallization of hexacelsian occurs.(3) Above 1100 °C, hexacelsian converts into monoclinic celsian. The conversion is complete with thermal treatments at 1300, 1400, and 1500 °C within 22, 15, and 22 h, respectively.The presence of monoclinic celsian nuclei above 500 °C appears to favor the conversion of hexacelsian into monoclinic celsian at temperatures higher than 1100 °C. 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.