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

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.