Location of Na + and Cs + cations in CsNaY zeolites studied by 23Na and 133Cs magic-angle spinning nuclear magnetic resonance spectroscopy combined with X-ray structure analysis by Rietveld refinement

Abstract

Solid-state 23Na and 133Cs nuclear magnetic resonance (NMR) spectroscopy was used in conjunction with X-ray powder diffraction (XRD) to study the location of Na + and Cs + cations in dehydrated CsNaY zeolites. Rietveld refinement of the structure of a 72% cesium-exchanged CsNaY zeolite dehydrated in vacuo at 623 K revealed sodium cations at SI and SII′ sites while cesium cations were found at SI, SI′ SII, and SIII sites. The results of the Rietveld refinement agree with the cation sitings derived from the 23Na and 133Cs magic-angle spinning (MAS) NMR spectra of this sample. Four lines identified in the 133Cs MAS NMR spectra of CsNaY zeolites were attributed to the distinct cation sites. In addition, the potential of 133Cs MAS NMR for the quantitative determination of site populations is demonstrated. 23Na and 133Cs MAS NMR is used to locate sodium and cesium cations in a series of CsNaY zeolites with different degrees of cesium exchange and dehydrated at different temperatures. With increasing Cs + content, the Na + cations at SI′/SII sites are exchanged first for Cs +. In addition, Na + cations at SI sites are replaced by Cs + cations at high cesium exchange levels. Upon increasing the calcination temperature of the 72% cesium-exchanged sample from 473 K to 773 K, further migration of Cs + and Na + to SI sites is observed. Calcination and rehydration of a 72% cesium-exchanged CsNaY zeolite leads to a partial migration of sodium cations from the sodalite cages into the supercages making them accessible to further ion exchange. Cesium exchange levels up to 90% were achieved by a three-fold exchange-calcination cycle of the 72% cesium-exchanged sample without damage to the zeolite structure.