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The sorption properties of zeolites are controlled by several factors, i.e. Si/Al ratio of the aluminosilicate framework [AlSiO 4 ] - , the type and position of the extraframework (EF) cations, and the applied temperature. Here we investigate the flexibility of CHA framework as a function of EF cation-content and temperature (20-350 °C). Two CHA forms (Na- and Cu-CHA) with Si/Al = 2 were analysed. The main objectives were: (i) to shed light on the HT behaviour of Na-CHA, for which contrasting results exist in literature; (ii) define the role of temperature and EF cation-type in the response to the heating stimuli. We show that at 75 °C, Na-CHA undergoes a severe contraction of the unit-cell volume (-12%) accompanied by a symmetry lowering (R-3m to I2/m). The transformation is reversible, if the dehydrated Na-CHA is exposed to ambient conditions. In contrast, Cu-CHA experiences a significant different dehydration path, which involves minor changes of the CHA framework, and a net positive thermal-expansion after dehydration. The implications of the observed transformations for gas separation processes are finally discussed., (© 2024. The Author(s).)

For the first time, the structural investigation of a Pb-exchanged zeolite (Pb 13.4 (OH) 10 Al 17.4 Si 54.6 O 144 ∙38H 2 O) with STI framework type, revealed a highly unusual and intriguing sudden volume increase under continuous heating. Understanding the fundamental mechanisms leading to such an unusual behaviour is essential for technological applications and interpretation of chemical bonding in zeolites. The dehydration was tracked in situ from 25 to 450 °C by single crystal X-ray diffraction, infrared and X-ray absorption spectroscopy. Further interpretation of the experimental observations was supported by ab initio molecular dynamics simulations. Initially, Pb-STI unit-cell volume contracts (ΔV = - 3.5%) from 25 to 100 °C. This agrees with the trend observed in STI zeolites. Surprisingly, at 125 °C, the framework expanded (ΔV =  + 2%), adopting a configuration, which resembles that of the room temperature structure. Upon heating, the structure loses H 2 O but no de-hydroxylation occurred. The key mechanism leading to the sudden volume increase was found to be the formation of Pb x (OH) y clusters, which prevent the shrinking of the channels, rupture of the tetrahedral bonds and occlusion of the pores. This zeolite has therefore an increased thermal stability with respect to other STI metal-exchanged zeolites, with important consequences on its applications., (© 2022. The Author(s).)

The crystal structure of mimetite Pb 5 (AsO 4 ) 3 Cl, a phosphate with apatite structure-type has been investigated in situ at 123, 173, 273, 288, 353 and 393 K. A careful inspection of the diffraction pattern and subsequent structure refinements indicated that mimetite transforms from the monoclinic to the hexagonal polymorph with increasing temperature. At 123 K, a monoclinic superstructure, mimetite-2M, with cell parameters a = 20.4487 (9),  b = 7.4362 (2), c = 20.4513 (9) Å, β = 119.953 (6)°, V = 2694.5 (2) Å 3 and space group P2 1 was observed. From 173 to 353 K, the reflections of the supercell were evident only along one direction of the corresponding hexagonal apatite-cell and the structure transforms to the polymorph mimetite-M with space group P2 1 /b and unit-cell parameters a = 10.2378 (3), b = 20.4573 (7), c = 7.4457 (2) Å, β = 120.039 (5)°, V = 1349.96 (9) Å 3 . Only at higher temperature, i.e. 393 K, does mimetite adopt the hexagonal space group P6 3 /m characteristic of apatite structure-types. The role of the electron lone pairs of Pb atoms in the phase transition was investigated through the analysis of the electron localization function (ELF) calculated based on the DFT-geometry optimized structures of the three polymorphs. The changes in spatial distribution of the 6s 2 electron density during the phase transitions were explored by means of the Wannier Function Centres (WFCs) derived from ab initio molecular dynamics trajectories. In the high-temperature hexagonal structure the 6s 2 electrons are spherically symmetric relative to the position of Pb atoms. At low temperature the maximum of 6s 2 electron density is displaced relative to the position of Pb atom contributing to the polar interaction in the monoclinic polymorphs., (open access.)

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 Cd 2+ 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) Å 3 . 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., Supplementary Information: The online version contains supplementary material available at 10.1007/s00269-021-01146-6., Competing Interests: Conflict of interestsThe author has non-financial interest to disclose., (© The Author(s) 2021.)

Ag-exchanged zeolites are known to have improved sorption and catalytic properties compared to the raw natural material. The application range of Ag-exchanged zeolites is linked to the stability of the exchanged form and its structural evolution at high temperatures. In this study, we investigated the thermal stability of a Ag-exchanged zeolite with an LEV framework type. The dehydration path was monitored in situ by single-crystal X-ray diffraction (SC-XRD) and X-ray absorption fine structure spectroscopy (XAFS). The experimental data were compared with those extrapolated from molecular dynamics (MD) trajectories. Our results showed that Ag-exchanged levyne (Ag-LEV) follows a different dehydration path compared to that of the natural levyne (Ca-LEV). Between 25 and 350 °C, the unit cell volume contraction was -4% with respect to that measured at room temperature. Upon dehydration, Ag-LEV transformed to the LEV B topology: such transformation is accompanied by the change from R 3̅ m to R 3 m space group and by the onset of the rupture of one T-O-T connection at 250 °C. Differently from Ca-LEV, no additional change to LEV B' configuration was detected. XAFS analysis indicated that each Ag is approximately surrounded by four oxygen atoms between 2.15 and 2.40 Å. This local environment was maintained up to 650 °C. Weak Ag +1 -Ag +1 interactions, detected in the whole investigated temperature range, are mainly ascribed to the presence of Ag-erionite (Ag-ERI) intergrown with Ag-LEV. No reduction to elemental Ag 0 occurred upon heating., Competing Interests: The authors declare no competing financial interest., (© 2020 American Chemical Society.)

The exposure of humans to erionite fibres of appropriate morphology and dimension has been unambiguously linked to the occurrence of Malignant Mesothelioma. For this reason, a detailed morpho-structural investigation through Electron Microscopy techniques has been performed on erionite samples collected at two different localities, Durkee (ED) and Rome (ER), Oregon, USA. The sample from Rome has been also investigated after a prolonged leaching with Gamble's solution (ER4G) in order to evaluate the possible occurrence of morpho-structural modifications induced by this Simulated-Lung-Fluid (SLF). Here we report how the micrometric erionite fibres evolve in irregular ribbon- or rod-like bundles as a function of different nano-structural features. The reasons for the observed morphological variability have been explained by considering the structural defects located at ED surface fibrils (bi-dimensional ribbons) and the presence of nontronite, an iron-bearing clay mineral embedding the ER fibrils (mono-dimensional rods). ER4G shows a decrease in width of the rod-like fibres due to their partial digestion by SLF leaching, which synchronously dissolves nontronite. The reported results represent a valuable background toward the full comprehension of the morphological mechanisms responsible for potentially damage of lung tissue through the potential relocation of fibers to extrapulmonary sites, increasing the carcinogenic risk to humans.