ROLE OF THE ALKALI-METAL CO-CATION IN THE ION-EXCHANGE OF Y-ZEOLITES .1. ALKALI-METAL NICKEL ION-EXCHANGE EQUILIBRIA

Ion-exchange isotherms describing the uni-univalent ion exchange of NaY and KY with Li+, Na+ (or K+), Rb+ and Cs+ and the uni-divalent exchange of LiY, NaY, KY, RbNaY and CsNaY with Ni2+ at a total normality of 0.1 mol equiv.-1 and at 298, 330 and 373 K are presented. In the case of the uni-univalent system the maximum level of exchange is temperature-independent, the extent of hydronium exchange is negligible and exchange is reversible without hysteresis. The upper loading limit in the uni-divalent system, on the other hand, varies with temperature and exchange reversibility depends on the thermal history of the sample. The lithium, sodium and potassium ions readily exchange with the small-cage indigenous ions whereas the in-going rubidium and cesium ions are restricted to exchange with the accessible supercage parent ions. While diffusion of hydrated nickel ions into the hexagonal prism is sterically hindered, a partial occupancy of the small cages was observed even at the lowest exchange temperature. A solution phase activity correction factor was taken into account in the measurement of the selectivity coefficients, and based on recently available experimentally measured activity coefficients the correction factor for alkali metal and nickel ion exchanges varies from 0.8 to 1.2 and 0.5 to 3.1, respectively, depending on composition and temperature. The relationship between in-going ion loading and selectivity was fitted to third-order polynomial equations, and the polynomial coefficients and the error of the fits are presented. Such considerations as exchange rate, ion location, site heterogeneity, selectivity reversal and intracrystalline diffusion are discussed. Values of the equilibrium constant, standard energy, enthalpy and entropy changes were measured for the reversible systems, and the thermodynamic affinity sequence of Li+ <Na+ <K+ <Rb+ <Cs+ <Ni2+ is established. The out-going cation has a considerable influence on the progress of exchange. The data presented point to the presence of a stable hydrate species in the small cages of LiY, NaY, RbNaY and CsNaY which is not as prevalent in the KY framework. The overall preference of the zeolite phase for the entering Ni2+ ions decreases in the order LiY > NaY > KY > RbNaY > CsNaY.