Abstract
Zeolites show remarkable properties that can be tuned through cation exchange of their original extraframework content. In this respect, the response of the modified zeolite to the heating stimuli, in terms of structural modifications and thermal stability, can drastically change and is, therefore, an important factor to consider. In this study, the dehydration mechanism of a natural levyne previously exchanged with Cd2+ has been monitored in situ by single crystal X-ray diffraction. The initial dehydration trend between 50 and 175 °C is similar to that observed for the pristine material, levyne-Ca. The water loss is accompanied by extraframework cation migration within the zeolitic cavities and the unit-cell volume slightly contracts from 3503.8(1) to 3467.8(6) Å3. From 200 to 250 °C, a pronounced drop of the unit-cell volume (− 7%) is observed. The dehydrated structure at 250 °C corresponds to levyne B topology of natural levyne, characterized by the statistical rupture of the T–O–T bonds of the double six-ring membered cage. However, in contrast to levyne-Ca, the fraction of broken connections reached 50% instead of 37%, and no additional structural modifications were detected up to 350 °C. At 400 °C, diffraction data pointed to the onset of the structural collapse. At this temperature, the measured unit-cell volume was 8% smaller compared to that of the RT structure. The corresponding contracted structure did not rehydrate after exposure to humid conditions for 21 days.
Highlights
The mineral levyne belongs to the group of zeolites
In the frame of a research aimed at investigating the role of the EF cations on the thermal stability of natural zeolites, we recently reported on the crystal structure of levyne after Cd-exchange in aqueous solution (Cametti et al 2021)
Ca atoms are found at C3 site and, in addition, at C4 site. This tiny rearrangement of the Cd and Ca atoms lead to a shifted position of the H 2O at the W3 site, whereas no significant differences are found for W1, W2 and W4. Their atomic coordinates are in agreement with those previously reported for Cd-exchanged levyne (Cd-LEV) (Cametti et al 2021)
Summary
The mineral levyne belongs to the group of zeolites. At room temperature (RT), levyne is rhombohedral, space group R-3 m, ahex = 13.35 chex = 22.90 Å, V = 3534 Å3 (Gottardi and Galli 1985). The structure consists of interconnected tetrahedra TO4 (Ti = Si+4, Al3+), which form single and double-membered rings stacked along the c-axis according to the AABCBBCAA sequence. The LEV framework type (Baerlocher et al 2007) is built up by the so-called lev cavity [496583], which alternates with double six-rings (D6R) polyhedra [ 4662] along [001]. Two 8-memebered ring channels run perpendicular to this direction. The dominant extraframework (EF) cations are Ca and Na (Coombs et al 1997), distributed along the threefold axis at different crystallographic positions that can vary depending on the sample
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