Framework Modifications and Dehydration Path of a Ag+-Modified Zeolite with STI Framework Type

摘要

The effect of Ag+ incorporation into stellerite, a natural zeolite with STI framework type, was investigated by means of single-crystal X-ray diffraction (SC-XRD), molecular dynamics (MD) simulations, and X-ray absorption fine structure spectroscopy (XAFS). At room temperature (RT), the complete exchange of the original extraframework ions with Ag+ provoked a distortion of the framework accompanied by symmetry reduction from orthorhombic Fmmm to monoclinic F2/m space group. Ag+ ions were strongly disordered, with occupancies ranging from 0.02 to 0.24, at partially occupied sites within zeolitic cages. The combination of ab initio MD simulations and XAFS spectroscopy suggested that Ag+ is coordinated by three H2O oxygens at 2.37 angstrom and two framework oxygen atoms between 2.55 and 3.00 angstrom. The thermal stability was monitored in situ by SC-XRD (from 25 to 400 degrees C) and XAFS (from 25 to 650 degrees C). Upon heating, the structure transformed to three different topologies: B phase, D' phase, observed here for the first time, and D phase. The unit cell volume contracted from 4392.85(14) at RT to 3644.4(4) angstrom(3) measured at 400 degrees C. Possible Ag+ to Ag-0 reduction could be excluded, although the formation of Ag+-Ag+ clusters could not be unambiguously ruled out.