SOLID-STATE CESIUM-133 NMR-STUDIES OF CATIONS IN CS/LI/NA ZEOLITE-A - AN EXAMPLE OF CATION DYNAMICS INVOLVING 3 SITES

Cs-133 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+, Na+, and Cs+ 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+ 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+ 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+ ions. The exchange between the S6R and S8R sites is considered to be the elementay step of the Cs+ diffusion process.