Effect of the framework charge density on zeolite ion exchange selectivities

The ion exchange selectivity of zeolites has been reassessed by calculation s with dielectric theory. For uni-univalent Na+/K+ and Na+/Rb+ exchanges, t he observed selectivities, measured as thermodynamic equilibrium constants, decreased with increasing zeolite aluminium content and charge density. Fo r uni-divalent Na+/Ca2+ and Na+/Sr2+ exchanges, the observed trend was the opposite, the selectivities increased with increasing zeolite charge densit y. Dielectric theory predicts these trends. Moreover, the quantitative agre ement with the theory and observed thermodynamic equilibrium constants is g ood or fair at low charge densities. This agreement is better than what cou ld be anticipated from previous studies of zeolite selectivities. Dielectri c theory clearly fails, however, at higher zeolite charge densities.