Effect of Framework Charge Density on the Ion Exchange Properties of Zeolites

Abstract

Members of a group of zeolites such as X and Y differ from each other only in the degree to which aluminium has isomorphously replaced silicon within the framework. As far as ion exchange properties are concerned, the effects of isomorphously increasing the number of aluminiums per unit cell are fourfold: (a) the unit cell volume increases; (b) the framework charge density increases; (c) there is a decrease in the ratio of the number of available crystallographic cationic sites to the number of framework charges on the lattice; (d) the zeolite becomes more hydrophilic. The effects of changing isomorphously the framework charge on the degree of hydrolysis, stability, and selectivity of the zeolite are discussed. The problems of attempting to correlate changes in selectivity with bulk properties of zeolites is first considered and then experimental data on hydronium exchange, the sodium/ammonium exchange in faujasites and lead exchanges in faujasites are presented. With respect to the latter, the limitations of dielectric theory are emphasized.