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 R3̅m 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
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.