Thermochemistry and thermodynamics of ion exchange in a near-faujasite

Abstract

The thermochemistry and thermodynamics of the exchange of Li +, K +, Rb +, Cs +, Ca 2+, Sr 2+ and Ba 2+ with the Na + ions in Linde Sieve X have been investigated. At 25°C complete replacement of Na + by Rb +, Cs + and Ba 2+ could not be achieved; 35 per cent of the Na + was not exchanged by the monovalent ions and 26 per cent of the Na + was not replaced by Ba 2+. These limits to the exchange are helpful in elucidating the cation site positions in the zeolite. From the determination of the standard free energy of exchange, ΔG°, the following affinity sequence was established: Li + < Na + < Ca 2+, K + < Sr 2+, Cs + < Ba 2+ < Rb +. In this affinity series ΔG° is derived from the value of the equilibrium constant formulated with maximum obtainable exchanges taken as 100 per cent exchange. The standard heats of ion exchange ΔH° were found to be negative whenever a larger monovalent ion replaced a smaller one and also when Ba 2+ replaced Na + but when Ca 2+ and Sr 2+ replaced the reaction was endothermic. In the exchanges involving the divalent ions, equilibrium was not attained in the calorimeter and ΔS° for these exchanges could not be derived. With the monovalent ions, however, ΔS° was found to be negative when a larger monovalent ion replaced a smaller one.