LOCATION OF NA-23 AND CS-133 MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY COMBINED WITH X-RAY STRUCTURE-ANALYSIS BY RIETVELD REFINEMENT( AND CS+ CATIONS IN CSNAY ZEOLITES STUDIED BY NA)

Solid-state Na-23 and Cs-133 nuclear magnetic resonance (NMR) spectros copy was used in conjunction with X-ray powder diffraction (XRD) to st udy the location of Na+ and Cs+ cations in dehydrated CsNaY zeolites. Rietveld refinement of the structure of a 72% cesium-exchanged CsNaY z eolite 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 si tes. The results of the Rietveld refinement agree with the cation siti ngs derived from the Na-23 and Cs-133 magic-angle spinning (MAS) NMR s pectra of this sample. Four lines identified in the Cs-133 MAS NMR spe ctra of CsNaY zeolites were attributed to the distinct cation sites. I n addition, the potential of Cs-133 MAS NMR for the quantitative deter mination of site populations is demonstrated. Na-23 and Cs-133 MAS NMR is used to locate sodium and cesium cations in a series of CsNaY zeol ites with different degrees of cesium exchange and dehydrated at diffe rent 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. Upo n increasing the calcination temperature of the 72% cesium-exchanged s ample from 473 K to 773 K, further migration of Cs+ and Na+ to SI site s is observed. Calcination and rehydration of a 72% cesium-exchanged C sNaY zeolite leads to a partial migration of sodium cations from the s odalite cages into the supercages making them accessible to further io n exchange. Cesium exchange levels up to 90% were achieved by a three- fold exchange-calcination cycle of the 72% cesium-exchanged sample wit hout damage to the zeolite structure.