Ion exchange in mordenite

Abstract

Ion-exchange isotherms have been measured for synthetic mordenite involving the cation pairs Na⁺⇌Cs⁺, NH⁺₄⇌K⁺, NH⁺₄⇌Na⁺, NH⁺₄⇌Li⁺, 2NH⁺₄⇌Ca²⁺, 2NH⁺₄⇌Sr²⁺ and 2NH⁺₄⇌Ba²⁺. In all cases except those involving Ca²⁺ and Sr²⁺ reversible exchange was effected. The values of the thermodynamic equilibrium constant, K_a, and the standard free energy of exchange, ΔG^⊖, were calculated for the reversible pairs. For additional combinations of the reversible ionic pairs, K_a and ΔG^⊖ were obtained using the triangle rule. The thermodynamic affinity sequence is Cs⁺ > K⁺ > NH⁺₄ > Na⁺ > Ba²⁺ > Li⁺. The thermodynamic affinity sequence among monovalent alkali metal ions established in this work for mordenite was found to be repeated for other zeolites, provided exchanges which do not go to completion are normalised by considering the equilibrium only in terms of the exchangeable fraction of the ions.