Effect of the framework charge density on zeolite ion exchange selectivities

The ion exchange selectivity of zeolites has been reassessed by calculations with dielectric theory. For uni–univalent Na+/K+ and Na+/Rb+ exchanges, the observed selectivities, measured as thermodynamic equilibrium constants, decreased with increasing zeolite aluminium content and charge density. For uni–divalent Na+/Ca2+ and Na+/Sr2+ exchanges, the observed trend was the opposite, the selectivities increased with increasing zeolite charge density. Dielectric theory predicts these trends. Moreover, the quantitative agreement with the theory and observed thermodynamic equilibrium constants is good or fair at low charge densities. This agreement is better than what could be anticipated from previous studies of zeolite selectivities. Dielectric theory clearly fails, however, at higher zeolite charge densities.