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

Our plain-English summary of 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, by G. Cametti, A.C. Scheinost, S.V. Churakov (ACS Omega, 2020).

The study

• Authors: G. Cametti, A.C. Scheinost, S.V. Churakov
• Journal: ACS Omega
• Published: 2020;5(49):31774-31783
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

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 3m 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 Ag0 occurred upon heating.

© 2020 American Chemical Society.

In plain English

This laboratory study examined how levyne, a zeolite mineral whose natural calcium had been exchanged for silver (Ag-LEV), changed when heated. It did not involve asbestos, people or disease. The authors heated a single crystal from 50 to 350 °C and followed it by X-ray diffraction; at 350 °C only the unit-cell dimensions could be measured. They also heated a powder, which contained the zeolite erionite grown together with the levyne, from 25 to 650 °C to study the silver by X-ray absorption spectroscopy, and compared the results with computer simulations. As water was lost, the crystal's unit cell shrank by about 4%. From 250 °C some framework links began to break. At 300 °C about 20% of one type of framework site was involved, giving a mix of the original structure and a form called levyne B. Unlike calcium levyne, no further change to levyne B prime was seen. Silver kept a similar oxygen surrounding up to 650 °C and no metallic silver formed. Close silver-silver distances were attributed mainly to the erionite. The authors noted that the erionite prevented an unequivocal interpretation of some spectroscopy results.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory materials-science experiment: in situ heating with single-crystal X-ray diffraction and X-ray absorption spectroscopy, compared with ab initio molecular dynamics simulations
• Who was studied: A silver-exchanged levyne (zeolite) sample from the authors' previous study, intergrown with erionite. A single levyne crystal fragment was used for X-ray diffraction, and a powdered sample containing both levyne and erionite (ratio 54:46) was used for X-ray absorption spectroscopy.
• Exposure: Heating, not asbestos. The single crystal was heated from 50 to 300 °C in 25 °C steps and from 300 to 350 °C in 50 °C steps. The powder was measured from 25 to 650 °C.
• Main results:
  • Across the temperature range studied, the Ag-LEV crystal's unit cell volume contracted by approximately 4% compared with room temperature.
  • In Ag-LEV, breaking of framework (T11-O22-T11) linkages began at 250 °C.
  • At 300 °C, framework rupture involved approximately 20% of the T11 tetrahedral sites, so the original LEV and new LEV B topologies coexisted.
  • From 200 °C, the Ag-LEV unit cell volume stayed almost steady up to 300 °C, when it was 2% larger than that measured for Ca-LEV.
  • At 25 °C, the powdered sample showed approximately one oxygen at 2.15 Å and 2.2 oxygen atoms at 2.40 Å around silver, plus one more oxygen at 2.94 Å.
  • On heating, silver moved within the structure but kept a coordination very similar to that at room temperature.
  • Silver-silver distances of about 3.3 Å were not seen in the heated Ag-LEV crystal structure, and the authors ascribed them to silver in erionite.
  • No elemental (metallic) silver was found.
• The authors' conclusion: The authors concluded that the main difference between silver-exchanged and calcium levyne on dehydration is that Ag-LEV did not transform to the levyne B prime phase. They explained the missing second dehydration step by the lower water content of Ag-LEV at room temperature.
• Limitations the authors state:
  • At 350 °C, only the unit-cell dimensions could be obtained, not the full structure.
  • The 200 °C diffraction dataset could not be refined.
  • The structural refinements did not locate all the silver predicted by the chemical composition.
  • Water release could not be unambiguously monitored.
  • Because the powder contained erionite, the spectroscopy results could not be interpreted unequivocally, and the role of Ag-erionite in the temperature-related change remains unclear.

Topics

Medical terms in this study, explained in our Lexicon: Zeolite, X-Ray Diffraction (XRD), Thermal Stability, erionite.

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