New topology of levyne B under quasi-equilibrium conditions: A temperature-dependent in situ single crystal X-ray diffraction study

Cametti, Georgia (2018). New topology of levyne B under quasi-equilibrium conditions: A temperature-dependent in situ single crystal X-ray diffraction study. Microporous and mesoporous materials, 265, pp. 162-171. Elsevier 10.1016/j.micromeso.2018.02.002

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The dehydration behavior of natural levyne-Ca was investigated from 25 to 400 °C by in-situ single-crystal X-ray diffraction under dry conditions. The dehydration started at 50 °C. From 125 to 250 °C the structure changed to levyne B topology, characterized by statistical breaking of T1-O-T1 bonds in the original D6R units. At 275 °C, additional rupture of T2-O-T2 bonds in 6mR units started. This second rupture associated with the release of remaining water was not reported before. At 300 °C the structure is assumed to be anhydrous. The new levyne B topology, referred to levyne B′, is characterized by 37% T1B and 15% T2B that are new tetrahedral sites arising as a consequence of the T-O-T breaking process. In contrast to the T1B site, T2B is at general position, originating a system of T2B face sharing tetrahedra, mutually exclusive. Complete migration of T sites to the new positions would lead to different LEV topologies with different kind of cages and, as a consequence, of stacking sequences depending on the position occupied by the T2B tetrahedra. The different results obtained in this study compared to previous ones, confirmed that the applied experimental set-up is fundamental in tracking the transformations upon dehydration in zeolitic materials. Understanding these differences is crucial for subsequent applications of these microporous compounds.

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