Thermodynamics and thermochemistry of cation exchange in zeolite Y

Abstract

A thermodynamic and thermochemical study has been made of the exchanges Na + → Li +, Na + → K +, Na + → Rb +, Na + → Cs +, Na + → Cg +, Na + → TV, 2Na + → Ca 2, 2Na 2+, 2Na + → Sr 2+ and 2Na + → Ba + in the near-faujasite Linde Sieve Y. Of these ions only Li +, K + and Ag + were found to replace all Na + at 25°C; the remainder replaced approximately 70 per cent of the Na +. An attempt has been made to correlate this limit with known cation positions of the Na + ions. From the exchange equilibria thermodynamic affinity sequences have been obtained for the ions that give complete or partial exchange with Na +. For the former group Ag + > K + > Na + > Li +; and for the latter Tl + > Cs + > Rb + > Ba 2+ > Na + > Sr 2+ > Ca 2+. The thermodynamic equilibrium constants were calculated from exchange capacities corresponding to the maximum observed extents of exchange. From exchange heats obtained by direct calorimetry standard heats of partial and complete exchange have been evaluated. Exchanges of Na + by Li +, Ca 2+ and Sr 2+ were endothermic, the remainder exothermic. Exchange of Na + by K + in methanolic solutions showed that the solvent had little effect upon this equilibrium but greatly influenced the rate of exchange. Standard entropy changes for the replacement of Na + have been evaluated where possible. These were negative for uni-univalent exchanges except for Na + → Ag + and Na + → Tl +. Positive entropy changes were observed for exchanges of Na + by divalent ions. The thermodynamic functions for the formation of the mixed exchangers showed that the end members formed non-ideal solid solutions with each other. This behaviour was also reflected in the activity coefficients of the exchange ions in the mixed exchangers.