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 3Ì 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 à . 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.