Dehydration dynamics of levyne: a combined synchrotron XRPD and single crystal diffraction study

Zeolite dehydration has been widely studied because of the potential use of these porous phases as molecular sieves, sorbents, and catalysts. Knowledge of the structural modifications induced by temperature and of the stability field of these materials is of prime importance to assess their persistence and effectiveness in technological applications. The reversibility of dehydration process, which is considered one of the most remarkable properties of zeolites, is in general - but not always - completely fulfilled, at least in the short term. Some porous materials, when completely dehydrated, undergo irreversible changes in the framework topology. In general, these changes are due to the strain induced by migration of cations - initially located in the channels and surrounded by water molecules - to achieve a better coordination with framework anions after the water release. Notwithstanding the geological and technological interest for porous materials with levyne topology (LEV), its thermal behavior and stability has not been studied in detail up to now. In this work, the thermal induced structural modifications of a natural levyne (Ca7:8 Na2:2K1:1Al19:8Si31:2O10851H2O, a= 13.3619(4) Å, c= 22.8820(1) Å, V = 3538.0(3) Å3, s.g. R?3m) were studied by both temperature-resolved synchrotron X-ray powder diffraction experiments (SR-XRPD), and by conventional-source high-temperature single crystal X-ray diffraction (SC-XRD). LEV framework consists of stacking 6-rings perpendicular to [0001] (6mRs?[0001]) in the sequence AABCCABBC.. . (Merlino et al. 1975). The repeat distance of this long sequence is about 22.90 Å. Between pairs of 6mRs?[0001] (d6R building units) are two levyne cages (LEV), which contain the non-framework cations and water molecules. One d6R and two LEV cages built up channels along [0001]. Moreover three equivalent channels, perpendicular to the threefold axis, confined by eight-membered rings are present in LEV-type framework. The SR-XRPD patterns were collected in continuum from room-T to 800°C, while single crystal experiments were performed at room-T, 30, 100 and 250°C. Below 100°C a lattice parameter increases, while c decreases, accounting for a very low volume decrease and only few water molecules are lost. Between 100 and 200 °C the release of more than 23 water molecules induces the cations migration toward the framework oxygen atoms, so to achieve a better coordination. Above 230°C, a further water loss induces the migration of Ca cations - previously hosted in the channels- inside the d6R building units, which are empty before the thermal treatment. This induces an abrupt cell volume reduction accompanied by symmetry reduction. No complete amorphization is observed up to the highest T studied. The X-ray structure refinement performed on the pattern collected upon reverse to room-T after transformation revealed that structural modifications are not reversible and the room-T cell parameters are not regained. Some aspects of levyne dehydration dynamics are similar to those found by Zema et al. (2008) for zeolite chabazite. Merlino, S., Galli, E. and Alberti, A. (1975) The crystal structure of levyne. Tschermaks. Mineral. Petrogr. Mitt. 22, 117-129. Zema, M., Tarantino, S.C., and Montagna, G. (2008) Hydration/dehydration and cation migration processes at high temperature in zeolite chabazite. Chem. Mater., 20 (18), 5876–5887.