Structure Determination and Time-Resolved Raman Spectroscopy of Yttrium Ion Exchange into Microporous Titanosilicate...

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, multistep ion exchange process. The ion exchange process was followed using in situ time-resolved Raman spectroscopy, and the crystal structures 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 stepwise 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, we constructed molecular models to explain the changes in the Raman spectrum during ion exchange. The multistep 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 O^2-. Crystal structures 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.