Zeolite exchangers. Thermodynamic treatment when not all ions are exchangeable

Abstract

Zeolites normally provide a number of distinct intracrystalline environments for cations. These cations then have different exchange properties for each such environment. The standard free energy, enthalpy, entropy and equilibrium constant for exchange in the whole crystal have been related to those of cations in the several environments. Three possible limiting cases of partial ion exchange can be visualised: (1) a pseudo-equilibrium is rapidly attained but a further slow exchange takes place over still longer times; (2) true equilibrium is reached rapidly, but exchange for certain entering ions never proceeds beyond a temperature independent limit; (3) true equilibrium is rapidly attained but exchange for certain entering ions never proceeds beyond a limit which in this case is reversibly dependent on temperature. A thermodynamic treatment of case 1 is not possible. For cases 2 and 3 a simple extension of the thremodynamic treatment for complete exchange is fully adequate.