Phase Transition upon K⁺ Ion Exchange into Na-Low Silica X: Combined NMR and Synchrotron X-ray Powder Diffraction Study

The mechanism by which K⁺ ions exchange into zeolite Na-low silica X (LSX) (Na₉₆Al₉₆-Si₉₆O₃₈₄·nH ₂O) has been determined by studying structures of the Na-LSX and K-LSX end members in the Na-K LSX solid solution series as well as samples exchanged at the 20%, 42% and 80% K⁺ levels. A preliminary investigation using ²⁹Si MAS NMR spectroscopy revealed a two-phase region in the solid solution near 80% K⁺ exchange. Rietveld analysis of the powder diffraction data collected from hydrated samples showed that, up to 42% of K⁺ exchange, K⁺ ions were located preferentially at site I′, just outside the double 6-ring (D6R) in the sodalite cage, and at site II, above the single 6-ring (S6R) in the supercage. 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 lattice parameter from 25.0389(5) to 25.2086(5) Å marked the formation 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) Å), where sites I and II were fully occupied by K⁺ ions.