Thermal behavior of stilbite and stellerite revisited and dehydration of their Na-exchanged forms: Considerations on the memory effect of the STI framework type

• Published in: Microporous and mesoporous materials Vol. 253; pp. 239 - 250
• Main Authors: Cametti, Georgia, Fisch, Martin, Armbruster, Thomas
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 15.11.2017
• Subjects: Na-stellerite; Na-stilbite; Stellerite; STI-framework type; Stilbite; Stellerite; Na-stellerite; Stilbite; STI-framework type; Na-stilbite
• ISSN: 1387-1811; 1873-3093
• DOI: 10.1016/j.micromeso.2017.06.051

The thermal behavior of Na-exchanged stellerite and stilbite was investigated by in-situ single crystal X-ray diffraction. For comparison with the exchanged forms new data were collected on natural stellerite and stilbite under the same experimental conditions. With the increase of temperature, strong disorder at T and O sites of the tetrahedra of the four-membered ring developed in natural forms. Such disorder was associated with the rupture of T-O-T connections and transition from the A to the B phase. Differently from previous studies, stellerite B at T > 300 °C was found to be monoclinic (space group A2/m). In addition, at 400 °C, a new T-O-T connection occurred, analogous to that in the B phase of barrerite. Na-stellerite and Na-stilbite were at RT monoclinic, space group F2/m. Upon heating, they also displayed the same structural modifications as observed in natural barrerite and Na-barrerite and adopted space group A2/m. Compared to natural stellerite and stilbite different T-O-T connections ruptured leading to a different topology of the B phase. The total volume contraction was −16% at 350 °C compared to −8% of pristine materials. The highly-condensed D phase, which does not form in natural stellerite and stilbite, was obtained by heating a Na-stellerite crystal ex-situ at 525 °C. The structure corresponded to the D phase of natural barrerite and Na-barrerite. All investigated STI members, after being exchanged with Na, have identical symmetry and demonstrate corresponding behavior upon heating and associated dehydration. Thus, a previously assumed memory effect of the symmetry of the natural parent structure, is not confirmed. [Display omitted] •B phase of natural stellerite changed from Amma to A2/m at 300 °C.•B phase of natural stilbite remained monoclinic (s.g. A2/m).•Strong disorder developed with the increase of T in both B stellerite and B stilbite.•Na-stilbite and Na-stellerite structure at RT was the same as Na-barrerite.•All Na-forms of STI group behave identically upon heating.