Crystal structure of Ag-exchanged levyne intergrown with erionite: Single-crystal X-ray diffraction and Molecular Dynamics simulations

Abstract

The modification of natural zeolites via ion exchange is an efficient technique used to improve their performances and tune their properties for specific applications. In this study, a natural levyne-Ca intergrown with erionite was fully exchanged by Ag+ and its structure [with idealized chemical composition Ag6(Si,Al)18O36·18H2O] was investigated by combining a theoretical and experimental approach. Single-crystal X-ray diffraction data demonstrated that Ag-levyne maintained the R3m space group, characteristic of the natural levyne. Ag ions distribute over partially occupied sites along the threefold axis and, differently from the pristine material, at the wall of the 8-membered ring window of the lev cavity. The lack of ~30% of Ag ions that could not be located by the structural refinement is ascribed to the strong disorder of the extraframework occupants. The structural results obtained by Molecular Dynamics simulations are in overall agreement with the experimental data and showed that, on average, Ag+ is surrounded by ~2 H2O and 1 framework oxygen at distances between 2.43 and 2.6 Å. Molecular Dynamics trajectories indicate that the occurrence of silver inside the D6R cage depends on the water content: silver occupancy of D6R cages is estimated to be 83, 30, and 0% when the structure contains 3, 2.5, and 2 H2O per Ag ion, respectively. The cation-exchange process, as demonstrated by scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS) spectrometry, affects the intergrown erionite as well. A structural characterization of the Ag-erionite phase (with dimension <100 μm) was possible by means of a CuKα micro-focus source: structure solution pointed to P63/mmc space group, indicating no change with respect to natural erionite. In agreement with previous studies, K ions in the cancrinite cage could not be exchanged, whereas Ag+ is found in the eri cavity.

Structural Evolution of Ag-LEV Zeolite upon Heating: An In Situ Single-Crystal X-ray Diffraction (SC-XRD) and X-ray Absorption Spectroscopy (XAS) Study

“…18 This specimen consisted of levyne intergrown with erionite. 18 For the single-crystal X-ray experiments, a single fragment of Aglevyne (chemical composition Ag 6.1 Al 6.3 Si 11.8 O 36 •18.0H 2 O 18 ) could be selected, and, consequently, the erionite had no influence on the obtained results. In contrast, XAFS measurements were performed on the powdered sample (see Section 2.3) that included erionite fragments as well.…”

“…The single-crystal used for X-ray diffraction experiments was the same used and characterized at room temperature (RT) in our previous study. 18 Diffraction data were collected using a Bruker Apex II diffractometer equipped with Mo Kα radiation (0.71073 Å) and a CCD detector. The dehydration process was investigated from 50 to 300 °C in steps of 25 °C and from 300 to 350 °C in steps of 50 °C, using the same experimental apparatus described in ref 17.…”

“…Atom labels and coordinates of framework atoms were those used by Cametti and Churakov, 2020. 18 Extraframework species were located by difference Fourier maps. Atomic sites related by a pseudocenter of symmetry in R3m were labeled by doubling the number of digits of the corresponding site in R3̅ m. For instance, in R3̅ m, two independent tetrahedral sites T1 and T2 constitute the framework.…”

“…The scattering paths were calculated by FEFF 8.2 25,26 based on the structure of Ag-levyne. 18 The amplitude reduction factor (S 0 2 ) was fixed to 0.9. For all scattering paths, a global phase shift ΔE 0 was fitted and the Debye−Waller factor was constrained from 0.001 to 0.01 Å 2 .…”

“…The dehydration behavior of levyne-Ca was also investigated by in situ single-crystal X-ray diffraction (SC-XRD). 17 Recently, we reported the room-temperature structure of Ag-exchanged levyne (Ag-LEV) in space group R3̅ m. 18 Silver ions were found to be strongly disordered at partially occupied sites (occupancy lower than 0.32) distributed along the threefold axis and, differently from levyne-Ca (Ca-LEV), at the wall of the eight-membered ring window of the lev cage. In the same study, we could perform a chemical and structural characterization of an erionite fragment (another zeolite belonging to the ABC 6 family 15 ) intergrown with levyne.…”