Heating-induced phase transformation in zeolite brewsterite: new 4- and 5-coordinated (Si,Al) sites

The dehydration-rehydration processes and phase transformations of brewsterite (space group P2(1)/ m, ideal formula (Sr,Ba)Al2Si6O16 . 5H(2)O) were studied by single crystal X-ray diffraction on two samples heated in an evacuated capillary for 24 h at 280 degrees C and 330 degrees C, respectively, and then cooled to room temperature. After the data collection, the capillary of the crystal heated at 280 degrees C was broken, the sample was kept at room conditions for one month, and diffraction data were recollected. Brewsterite heated in vacuum at 280 degrees C shows a strong decrease of the unit cell volume (about 10%) and the statistical breaking of the T1-O7-T2 bridge of the 4-ring of the 4(2)5(4) PBU. T1 cation, in 31% of cases, migrates to a new tetrahedral site, which shares three vertices with the previously occupied one; the fourth vertex is on the mirror plane, and joins two adjacent layers. T2 cation, in 20% of cases, migrates to a new site, which is characterized by a 5-fold coordination. This coordination polyhedron shares three vertices with the previously occupied tetrahedron; the fourth vertex is on the mirror plane, and joins two adjacent layers; the fifth one is unshared. The formation of these new T-O-T bridges prevents the re-expansion and rehydration of the heated brewsterite; as a consequence, the crystal brought back to room conditions does not show any noticeable structural difference with respect to that heated at 280 degrees C. The exchangeable cations, which occupy only one extraframework site in the untreated brewsterite, spread over several sites. The sample heated at 330 degrees C shows a further decrease in the unit cell volume of about 2% with respect to the sample heated at 280 degrees C, and an increase in the migration percentage of T1 and T2 cations to the new sites.