Synthesis and Crystal Structure of Ag₄Br₄ Nanoclusters in the Sodalite Cavities of Fully K+-Exchanged Zeolite A (LTA)

摘要

Ag₄Br₄ nanoclusters have been synthesized in about 75% of the sodalite cavities of fully K+-exchanged zeolite A (LTA). An additional KBr molecule is retained in each large cavity as part of a near square-planar K₄Br3+ cation. A single crystal of Ag₁₂-A, prepared by the dynamic ion-exchange of Na₁₂-A with aqueous 0.05 M AgNO₃ and washed with CH₃OH, was placed in a stream of flowing 0.05 M KBr in CH₃OH for two days. The crystal structure of the product K₉(K₄Br)Si₁₂Al₁₂O₄₈·0.75Ag₄Br₄, a = 12.186(1) A) 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 = 0.080 for the 99 reflections for which F_o > 4ヲ (F_o). The thirteen K+ ions per unit cell are found at three crystallographically distinct positions: eight K+ ions in the large cavity fill the six-ring site, three K+ ions fill the eight-rings, and two K+ ions are opposite four-rings in the large cavity. One bromide ion per unit cell lies opposite a four-ring in the large cavity, held there by two eight-ring and two six-ring K+ ions (K₄Br3+). Three Ag+ and three Br- ions per unit cell are found on 3-fold axes in the sodalite unit, indicating the formation of nano-sized Ag₄Br₄ clusters (interpenetrating tetrahedra; symmetry T_d; diameter ca. 7.9 A) in 75% of the sodalite units. Each cluster (Ag-Br = 2.93(3) A) is held in place by the coordination of its four Ag+ ions to the zeolite framework (each Ag+ cation is 2.52(3) A from three six-ring oxygens) and by the coordination of its four Br- ions to K+ ions through six-rings (Br-K = 3.00(4) A).