Kinetics of ion exchange in mordenite

Abstract

The rates of exchange of the ion pairs Na⁺/K⁺, Na⁺/Rb⁺ and Na⁺/Cs⁺ in mordenite have been measured. These rates have been compared with those predicted by the Helfferich and Plesset theory in the first two exchange systems. The agreement between theory and experiment is poor and an attempt has been made with the Na/K system to establish the reasons for the inadequacies of the theory. The theory assumes an ideal exchange reaction but analysis of the Na/K isotherm establishes that this system is not ideal. Secondly, the theory assumes that D^*_Na and D^*_K, the respective self-diffusion coefficients of Na⁺ and K⁺ in mordenite, are independent of concentration and this has been shown to be untrue. When allowances are made for these factors an improvement between theory and experiment is obtained. Rates of exchange have also been measured starting from exchangers which contained various ratios of Na⁺ to K⁺ initially. These rate measurements and the variable self-diffusion coefficients indicate that site heterogeneity exists and has a pronounced effect on the rates of exchange.