Hydration/Dehydration and Cation Migration Processes at High Temperature in Zeolite Chabazite

Abstract

High temperature structural behavior of a natural chabazite of composition (Ca 1.1 Na 0.4 K 0.7 )-[Si 8.6 Al 3.4 O 24 ] • 14.4H 2 O has been characterized by means of in situ HT single-crystal X-ray diffraction (SC-XRD) and thermogravimetric analysis. Lattice dimensions have been measured in the 25-700 °C range and crystal structure refined from XRD data collected at T ) 25, 100, 125, 175, 250, 300, 425, and 600 °C. Variations of unit-cell parameters as a function of temperature reveal two discontinuities at 100 and 200 °C, to which no symmetry changes are associated, and an overall volume reduction of 2.8%. Between 200 and 250 °C, a steep contraction of cell volume is associated with a significant broadening of diffraction profiles, which turn sharp and narrow at higher temperatures. As the dehydration process proceeds with increasing T, cationic sites partly coordinated by extraframework water molecules become unstable and cations migrate toward new positions. Cations occupying the C2, C3, and C4 sites at room temperature move first toward C2 while T is raised to 200 °C, and for T > 200 °C they start migrating toward smaller cavities where coordination is assured by oxygen atoms of the framework only, the sixmembered double ring (C1 site), and a peripheral position within the eight-membered ring. The latter position has been labeled as C5. Sites C1 and C5 are stable up to 700 °C. Reversal experiments demonstrated that the whole process is reversible under the conditions of this study; by decreasing temperature, water enters the structure again and cations migrate back to their original positions.

Understanding the AC conductivity and permittivity of trapdoor chabazites for future development of next-generation gas sensors

“…It can be observed that the mass of all three materials decreased with increased temperature, which is attributed to the progressive removal of water molecules from different crystallographic sites. As shown in the works on natural chabazites by Fialips and Zema, different energetic types of water exist in these materials [36,37]. In all samples, dehydration began as soon as the temperature was raised, however the thermal stability and mass variations are different for each material.…”

Structural intergrowth of merlinoite/phillipsite and its temperature-dependent dehydration behaviour: a single-crystal X-ray study

“…Without knowing the chemical composition of the Monte Somma chabazite, and considering the weak intensity of X-ray reflections of the subordinate chabazite phase, the interpretations of extra-framework occupants have only a semiquantitative character. At 50ºC the major cation site C2 (Zema et al, 2008) crowns on the sixmembered ring of tetrahedra, while the opposite side is shielded by H 2 O. Note that this site was split into K1, K2 and Na2 subsites by Yakubovich et al (2005).…”

“…Note that this site was split into K1, K2 and Na2 subsites by Yakubovich et al (2005). The centres of eight-membered rings are plugged by the low-occupancy C5 site (labelled according to Zema et al, 2008). At 125ºC, the main part of H 2 O coordinating C2 is released and C2 moves closer to the centre of a six-membered ring; the occupancy of C5 FIG.…”

A metamorphic inorganic framework that can be switched between eight single-crystalline states

“…Although there are some inorganic oxides that exhibit small volume changes 18 19 20 21 22 . Chabazite, one of the most widespread natural zeolites and one of the first zeolites to be studied for a wide range of industrial and technological applications, showed a volume change of 0.1% after high-temperature treatment (873 K) 22 . The key building blocks of the crystal shown here are the doughnut-shaped inorganic molecular metal oxide rings, [P 8 W 48 O 184 ] 40− ({P 8 W 48 }), connected to each other by dicationic cobalt linkers to form a lattice 23 .…”