Role of cation-water disorder during cation exchange in small-pore zeolite sodium natrolite

By combining X-ray diffraction with oxygen K-edge absorption spectroscopy we track changes occurring during the K⁺-Na⁺ cation exchange of Na-natrolite (Na-NAT) as tightly bonded Na⁺ cations and H₂O molecules convert into a disordered K⁺-H₂O substructure and the unit cell expands by ca. 10% after 50% cation exchange. The coordination of the confined H₂O and nonframework cations change from a tetrahedral configuration, similar in ice I_h, with Na ⁺ near the middle of the channels in Na-NAT to two-bonded configuration, similar in bulk water, and K⁺ located near the walls of the framework in K-NAT. This is related to the enhanced ion-exchange properties of K-NAT, which, in marked contrast to Na-NAT, permits the exchange of K⁺ by a variety of uni-, di-, and trivalent cations.