Time-Resolved Diffraction Studies of Ion Exchange: K+ and Na+ Exchange into (Al, Ge) Gismondine (GIS) Na24Al24Ge24O96·40H2O and K8Al8Ge8O32·8H2O

Time-resolved in situ synchrotron X-ray powder diffraction (XRPD) was used to study ion-exchange mechanisms and pathways in K+ and Na+ forms of an aluminogermanate with the zeolite gismondine topology (AlGe−GIS). The Na+ and K+ forms differ in their distribution of extraframework cations with the Na+ ions ordered in the C-centered monoclinic Na−AlGe−GIS of unit-cell dimensions a = 14.490(3) Å, b = 9.3840(2) Å, c = 23.530(5) Å, and β = 105.90(3)° and K+ ions disordered in the I-centered monoclinic K−AlGe−GIS of unit-cell dimensions a = 10.311(2) Å, b = 9.749(1) Å, c = 10.238(1) Å, and β = 90.000(2)°. Rietveld structure refinements indicate K+ first occupies sites K2, K4, and K6 in the [1̄01] channel of the Na−AlGe−GIS structure. After 10% (±2%) K+ exchange into the [1̄01] channel, the I2/a unit cell forms and subsequent replacement of Na+ is consistent with site independent exchange along the [1̄01] and [201] channels. Ion exchange proceeded to approximately 90% (±1%) substitution of K+ into Na−AlGe−GIS within the course of the experiment. However, in the reverse exchange of Na+ into the K−AlGe−GIS, an abrupt growth of Na−AlGe−GIS occurred at 4.5% (±3.5%) Na+ exchange and ended at 10% (±2.5%) Na+ exchange. Bond valence calculations demonstrate that K+ has a stronger interaction with framework O2- (0.423 v.u.) than Na+ (0.242 v.u.) and the valence matching principle shows that Na+ has a stronger affinity to interstitial H2O. These results imply that the AlGe−GIS structure has a preference for K+ as the charge-balancing extraframework cation.