Dehydration dynamics of barrerite: an in situ synchrotron XRPD study

The thermally induced structural modifications of the natural zeolite barrerite [[Na.sub.16] [Al.sub.16][Si.sub.56][O.sub.144] x 52[H.sub.2]O, a = 13.6239(4) [Angstrom], b = 18.2033(5) [Angstrom], c = 17.8317(7) [Angstrom], V = 4422.3(3) [[Angstrom].sup.3], space group Amma, framework type STI] were studied in a temperature-resolved X-ray powder diffraction experiment, using synchrotron radiation, in the temperature range 339-973 K. In the initial stage of heating, up to 508 K, barrerite Phase A (space group Amma) is stable, the unit-cell volume decreases by about 4% and a water release of about 66% is observed. Between 521 and 598 K, a phase transition to the collapsed so-caned barrerite Phase B (space group Amma) is observed. During the transition, the rotation of the [4.sup.2][5.sup.4] secondary building units causes a large decrease in cell volume and deformation of the channel system. Phase B, at 611 K, shows the statistical breaking of T-O-T bridges in the 4-rings and the migration of the involved tetrahedral atoms to new 'face-sharing' tetrahedra, with a consequent reduction of the free volume of the channels parallel to [100]. The new structure is stable up to 741 K and the total volume decrease is about 16%. A new phase appears from 754 K with cell parameters similar to those reported for the highly deformed barrerite Phase D and is stable up to 910 K, which is the temperature at which the total volume decrease is 22.5%. The material does not undergo amorphization up to the highest temperature investigated. Keywords: Zeolite, barrerite, dehydration, X-ray powder diffraction, synchrotron radiation, crystal structure