Site selectivity of cations and adsorption properties in zeolite

Abstract

The site selectivity of alkali metal ions in the dehydrated and hydrated zeolite A is discussed by using the semiempirical potential energy functions. The net charges of framework atoms are calculated by the electronegativity equalization method. In the dehydrated zeolite A, the calculated solvation heats of gaseous cations by dehydrated and hydrated zeolite A frameworks agree semiquantitatively with the experimental data. The symmetry of cation positions and size of cations explain the site selectivity. When the dehydrated zeolite A is hydrated, the first four water molecules are bound to the cations on sites II and III, with the binding energy of -25.9 and -28.5 kcal/mole respectively. The site selectivity of sodium ions in the hydrated zeolite A is shown to be site(I) site(II) site(III). Although $Mg^{+2}$ and $Ca^{+2}$ ions show site I selectivity, $Ba^{+2}$ ion is distributed over two sites. To study zero-coordinated cations, X-ray data were used for $Ca_6-A$, $Sr_6-A$ and K-A. The greater tendency of $K^+$ ion to occupy eight-ring sites is discussed in therms of site selectivities of the divalent cations. The collapse of dehydrated $Ba_6-A$ is due to that the site II preference of $Ba_6$ ions in dehydrated $Ba_6-A$ destabilizes the framework of zeolite A