Solid-state cesium-133 NMR studies of cations in cesium/lithium/sodium zeolite A: an example of cation dynamics involving three sites

Abstract

{sup 133}Cs NMR spectra show marked changes in the line shape as a function of temperature for a fully hydrated zeolite. A sample containing a mixture of Li{sup +}, Na{sup +}, and Cs{sup +} ions. A single Lorentzian peak is observed at temperature above 285 K, and three resolvable peaks are observed at temperatures below 268 K. These three peaks arise from the Cs{sup +} ion at the sites of the single eight-membered ring (S8R), the single six-membered ring (S6R), and the {alpha}-cage sites near the four-membered rings. The experimental line shapes are satisfactorily simulated by the modified Bloch equations that include the chemical exchange kinetics between these three sites. The analysis yields a lifetime, {tau}, of 0.12 ms for the Cs{sup +} at S8R site at 297 K. The Arrhenius plot yields an activation energy of 44 kJ/mol for the site-exchange process of the charge-balancing Cs{sup +} ions. The exchange between the S6R and S8R sites is considered to be the elementary step of the Cs{sup +} diffusion process.