Synthesis and Crystal Structure of Zinc Iodide in the Sodalite Cavities of Zeolite A (LTA)

Abstract

The crystal structure of ZnI2 molecule synthesized in zeolite A (LTA) has been studied by single-crystal X-ray diffraction techniques. A single crystal of |Zn6|[Si12Al12O48]-LTA, synthesized by the dynamic ion-exchange of |Na12|[Si12Al12O48]-LTA with aqueous 0.05 M Zn(NO3)2 and washed with deionized water, was placed in a stream of flowing 0.05 M KI in CH3OH at 294 K for four days. The resulting crystal structure of the product (|K6Zn3(KI)3(ZnI2)0.5|[Si12Al12O48]-LTA, a = 12.1690(10) Å) was determined at 294 K by single-crystal X-ray diffraction in the space group Pm m. It was refined with all measured reflections to the final error index R1 = 0.078 for 431 reflections which Fo > 4σ (Fo). At four crystallographically distinct positions, 3.5 Zn 2+ and nine K + ions per unit cell are found: three Zn 2+ and five K + ions lie on the 3-fold axes opposite 6-rings in the large cavity, two K + ions are off the plane of the 8-rings, two K + ions are recessed deeply off the plane of the 8-rings, and the remaining a half Zn 2+ ion lie on the 3-fold axes opposite 6-rings in the sodalite cavity. A half Zn 2+ ion and an I − ion per unit cell are found in the sodalite units, indicating the formation of a ZnI2 molecule in 50% of the sodalite cavities. Each ZnI2 (Zn-I = 3.35(5) Å) is held in place by the coordination of its one Zn 2+ ion to the zeolite framework oxygens and by the coordination of its two I − ions to K + ions through 6-rings (I-K = 3.33(8) Å). Three additional I − ions per unit cell are found opposite a 4-ring in the large cavity and form a K3I 2+ and two K2ZnI 3+ ionic clusters, respectively.