Ion exchange in zeolites: some recent developments in theory and practice

Zeolites find relatively few direct applications as ion exchangers and it is perhaps for this reason that the study of their ion exchange behaviour is somewhat neglected in comparison to their other properties. Nevertheless, ion exchange is an essential part of preparative procedures for the manufacture of zeolitic sorbents and catalysts, and there is now increasing recognition of the importance of using strictly controlled conditions during ion exchange in order to avoid hydrolysis and crystal damage. Some practical points concerning ion exchange are therefore considered, and in addition two aspects of the theory of ion exchange are reviewed. Firstly, some recent discussions regarding the propriety of using the Gaines and Thomas thermodynamic formulation to evaluate activity coefficients within the exchanger phase are considered. Secondly, recent progress in the prediction of multicomponent exchange equilibria is discussed. Finally, some suggestions are made regarding possible fruitful future areas of research.