Search

For the first time, the structural investigation of a Pb-exchanged zeolite (Pb13.4(OH)10Al17.4Si54.6O144 ∙38H2O) 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 H2O but no de-hydroxylation occurred. The key mechanism leading to the sudden volume increase was found to be the formation of Pbx(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.

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. Supplementary Information The online version contains supplementary material available at 10.1007/s00269-021-01146-6.

Small-pore zeolites are successfully employed as catalysts, sorbents and molecular sieves. Their physiochemical properties can be tuned by modifying their extraframework cation (EF) composition via ion exchange. In this study, we investigate the crystal structure of a Cd-exchanged levyne (LEV) intergrown with erionite (ERI) by combining Single Crystal X-ray Diffraction (SCXRD), Molecular Dynamic simulations (MD) and Extended X-ray Absorption Fine-Structure spectroscopy (EXAFS). Data obtained from the different techniques consistently indicate that Cd2+ in LEV is arranged in a nearly ordered fashion. In contrast, strong disorder of the EF species (Cd2+ and H2O) is observed in the ERI cavities. Here, Cd2+ forms aqueous complexes that are more mobile in comparison to LEV, where Cd2+ bonds to both H2O and framework-oxygen atoms. The formation of Cd-clusters is excluded based on EXAFS analysis. Finally, to discriminate between thermal and static disorder, we propose a new approach based on combined MD and geometry optimization.

The possibility to develop a process for lead removal from wastewater, based on calcium carbonate minerals, depends on the overall efficiency of the uptake process. Aqueous Pb tends to form cerussite (CER) via a dissolution–precipitation reaction when interacting with aragonite (ARG) and calcite (CAL). From a thermodynamic perspective, the two processes have a similar driving force, because the solubility of CAL and ARG is approximatively the same (Ks ≈ 1 × 10–8). Experimentally, the macroscopic yield of reaction was found to be very different. Using ex situ electron microscopy, diffraction, and in situ atomic force microscopy, we demonstrate that the Pb uptake mechanism by the two most abundant CaCO3 polymorphs is controlled by the kinetics of processes at the solid–solid and solid–liquid interfaces. Aragonite is isostructural with the product phase (CER) that easily precipitates taking advantage of the template effect offered by the surfaces of the substrate. The reaction proceeds through an interface-coupled dissolution–precipitation that leads to a mineral replacement. Because of a crystallographic mismatch, the reaction between CAL and CER mainly occurs as a simple solvent-mediated transformation. Our study unveiled the mechanistic reasons behind the different reaction yields shown by CAL and ARG toward Pb uptake. Similar conclusions can be extended to other contaminant(aq)-CaCO3(s) systems, thus increasing the predictability of limestone efficiency toward the uptake of heavy metals.

The effect of Ag+ incorporation into stellerite, a natural zeolite with STI framework type, was investigated by means of Single Crystal X-ray Diffraction (SC-XRD), Molecular Dynamics (MD) simulations and X-ray Absorption Fine Structure Spectroscopy (XAFS). At room temperature the complete exchange of the original extraframework ions with Ag+ provoked a distortion of the framework accompanied by symmetry reduction from orthorhombic Fmmm to monoclinic F2/m space group. Ag+ ions were strongly disordered, with occupancies ranging from 0.02 to 0.24, at partially-occupied sites within zeolitic cages. The combination of ab initio molecular dynamic simulations and XAFS spectroscopy suggested that Ag+ is coordinated by three H2O oxygen at 2.37 Å and by two framework oxygen atoms between 2.55 and 3.00 Å. The thermal stability was monitored in situ by SC-XRD (from 25 to 400°C) and by XAFS (from 25 to 650°C). Upon heating the structure transformed to three different topologies: B phase, D’ phase, observed here for the first time, and D phase. The unit-cell volume contracted from 4392.85(14) at room temperature to 3644.4(4) Å3, measured at 400°C. Possible Ag+ to Ag0 reduction could be excluded although the formation of Ag+-Ag+ clusters could not be unambiguously ruled out.

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 temperature. In this study, we investigated the thermal stability of an Ag-exchanged zeolite with 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-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-3m to R3m 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 4 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 Ag0 occurred upon heating.