Multiple cation sites in dehydrated brewsterite. An in-situ X-ray synchrotron powder diffraction study

Abstract

The dehydration process of the zeolite brewsterite, Sr 0.67Ba 0.33Al 2Si 6O 16·5H 2O, has been studied in situ by X-ray synchrotron radiation powder diffraction. Sixty-one consecutive powder patterns were collected between 310 and 684 K, in steps of 6 K, at 5 min intervals, and the crystal structures were refined using the Rietveld method. At 310 K, the original cation and four water sites are fully occupied. As water is expelled, three alternative cation sites successively appear. These cation sites generally have more framework oxygens in their coordination spheres, and/or show an increasing ability to share water molecules between themselves. Most aspects of the brewsterite dehydration process can be related to the variations in the cation site occupancies. The total water content during the dehydration can be fully modelled, based on the occupancies of two of the cation sites. Also, the maxima in the dehydration rate and the reduction in unit cell volume are strongly correlated to cation site occupancies. The connection between the tetrahedral sheets building the brewsterite framework, the T4O9T4 angle, is reduced by 25° between 550 and 610 K. The strain at this ‘weak link’ is believed to be the direct cause of the destruction of crystallinity, whereas the clustering of cations inherent to an effective water sharing is considered as the cause of this framework strain.