Exchange of ammonium and sodium ions in synthetic faujasites

Abstract

Ion-exchange isotherms for the Na ⇌ NH₄ equilibrium in zeolites X and Y were constructed at 298 K and a total solution normality of 0.1 g-equiv. dm^–3. Values for the standard free energy of exchange were calculated and shown to be consistent with the affinity sequence predicted using dielectric theory. Comparisons are made between previously reported studies on the Na ⇌ NH₄ exchange in X and Y and this current work, and the differences in experimental data are discussed. Statistical thermodynamic considerations provide a greater understanding of the Na ⇌ NH₄ exchange equilibrium in X and Y, and collectively the evidence suggests that high framework charge and ion-sieving factors in X are the dominant properties determining exchange selectivity and maximum exchange levels in this zeolite. In Y, the ion-sieve effect and repulsion between the entering ammonium ions are the dominant factors.