Crystallographic configurations of water molecules and exchangeable cations in a hydrated natural CHA-zeolite (chabazite)

Abstract

Crystallographic configurations of water molecules and exchangeable cations in a hydrated natural CHA-zeolite (chabazite) with a composition of (Ca 1.57Na 0.49)(Al 3.39Si 8.55)O 24 · 12.47H 2O have been examined from the structure analyses based on single-crystal X-ray diffraction data. The structure analyses have revealed the presence of five partially occupied water sites (OW1–OW5) and four partially occupied exchangeable-cation sites (Ca1–Ca4), and the final structure refinement converged to R = 0.0283 and R w = 0.0276 for 1255 reflections. Of these sites, the Ca4 and OW2 sites are located at the center of the [4 66 2]-ring and the center of the 8-ring window in the [4 126 28 6]-cavity, respectively, and the remaining sites are inside the [4 126 28 6]-cavity. The Ca1 atoms can be coordinated only by water molecules, and fully coordinated Ca1 atoms can adopt an octahedral coordination or a triangular-prismatic coordination depending on configurations of water molecules. The Ca2 and Ca3 atoms can be coordinated by both framework oxygen atoms and water molecules, and fully coordinated Ca2 and Ca3 atoms can adopt a sevenfold coordination and an octahedral coordination, respectively. The Ca4 atoms are coordinated octahedrally by six framework oxygen atoms. Judging from OW⋯O and OW⋯OW distances, the hydrogen bonds formed between water molecules and framework oxygen atoms are expected to be very weak, whereas strong hydrogen bonds can exist between water molecules.