PHASE-TRANSITION UPON K-EXCHANGE INTO NA LOW SILICA X - COMBINED NMR AND SYNCHROTRON X-RAY-POWDER DIFFRACTION STUDY( ION)

The mechanism by which K+ ions exchange into zeolite Na-low silica X ( LSX) (Na96Al96Si96O384. nH(2)O) has been determined by studying struct ures of the Na-LSX and K-LSX end members in the Na-K LSX solid solutio n series as well as samples exchanged at the 20%, 42% and 80% K+ level s. A preliminary investigation using Si-29 MAS NMR spectroscopy reveal ed a two-phase region in the solid solution near 80% K+ exchange. Riet veld analysis of the powder diffraction data collected from hydrated s amples showed that, up to 42% of K+ exchange, K+ ions were located pre ferentially at site I', just outside the double 6-ring (D6R) in the so dalite cage, and at site II, above the single 6-ring (S6R) in the supe rcage. Introduction of K+ ions into site I' repositioned Na+ ions into site I, at the center of the D6R. An abrupt change in the cubic latti ce parameter from 25.0389(5) to 25.2086(5) Angstrom marked the formati on of a second phase at the 80% K+-exchange level as K+ ions began to occupy site I. No coexistence of phases was observed for the fully K+- exchanged sample (a = 25.2486(2) Angstrom), where sites I and II were fully occupied by K+ ions.