A theoretical study of ion selectivity of zeolites—application to chabazite

Abstract

As a contribution towards a better theoretical understanding of zeolite properties, the mechanism of ion migration through zeolite lattices has been studied with special reference to ion selectivity in chabazite. We have started from the Cartesian coordinates obtained by combining experimental data and the values for a model having standard bond lengths and angles. Then we have applied the Del Re method (with appropriate parameters for Si and Al −) to determine the net charges of the framework atoms. Then we have computed the potential energy profiles for one or more cations crossing the larger cavity of the chabazite unit cell ( ca 200 atoms). The interaction energy contains electrostatic terms associated with point-charge interaction and polarization effects. Two different pictures have been analyzed: a totally idealized one, where ion motion takes place in the fully dehydrated zeolite, and a more realistic one, where the ion is seen as a moving aquo complex. In the latter case, the presence of the solvent has been taken into account by introducing a dielectric constant depending on ion size and channel diameter. Whereas no significant differences between ions having the same formal charges have been found in the dehydrated case, polarization effects being too small to bring about a distinct selectivity, the model taking solvation into account does predict the expected selectivity, and agrees well with available experimental results. Equilibrium sites, the corresponding free energies, and energy barriers between them have been used for the discussion. The value of the two models in attempts to understand elementary physico-chemical processes in zeolites is emphasized, with extensive reference to existing literature.