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Structure determination and time-resolved Raman spectroscopy of yttrium ion exchange into microporous titanosilicate ETS-4
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  • Aaron J. Celestian,
  • J. Caleb Chappell,
  • Melinda J. Rucks,
  • Pauline Norris
Aaron J. Celestian
Natural History Museum of Los Angeles County (NHM)
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J. Caleb Chappell
Miami University
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Melinda J. Rucks
State University of New York at Stony Brook
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Pauline Norris
Western Kentucky University (WKU)
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The ion exchange of yttrium, one of the five most critical rare earth elements as outlined by the U.S. Department of Energy, into ETS-4 is a dynamic, multi-step ion exchange process. The ion exchange process was followed using in situ time-resolved Raman spectroscopy, and the crystal structure of the pre-exchange and post-exchange forms were determined by single crystal X-ray diffraction. In situ Raman spectroscopy is an ideal tool for this type of study as it measures the spectral changes that are a result of molecular geometry changes at fast time intervals, even where symmetry and unit volume changes are minimally detected by X-ray diffraction. By tracking the step-wise changes in the peak positions and intensities in the spectra, where we focused primarily on the strong spectral features corresponding to titania quantum wires and three membered-ring bending and breathing modes, molecular models were constructed to explain the changes in the Raman spectrum during ion exchange. The multi-step ion exchange process started with rapid absorption of Y into the Na2 site causing titania quantum wires to kink. After this initial uptake, the exchange process slowed, likely caused by hydration coordination changes within the channels. Next, Y exchange accelerated again during which time the Y site moved closer to the framework \(\mathrm{O^{2-}}\). Crystal structure of the maximal Y exchanged ETS-4 material were determined, and confirmed the splitting of the Y site. Inductively coupled plasma optical emission spectroscopy was also used to quantify the extent of Y exchange, and to measure if there were indications of titania leaching from the framework.