Positions of cations and molecules in zeolites with the chabazite framework III. Dehydrated Na-exchanged chabazite

Abstract

Chabazite from Bozen, Tyrol was ion-exchanged with NaCℓ solution to Na 15.2Aℓ 15.2Si 32.8O 96.nH 2O. After vacuum dehydration at 320°C, the crystal structure was determined at room temperature. Reduction of the ideal rhombohedral symmetry to monoclinic (C2/m or lower; a 19.319(3) b 13.833(2) c 11.849(2)Å β 11.348(3)°) is interpreted to result from near-elliptical distortion of most 8-rings to accommodate 57% of the Na + cations. All these cations have 4 framework oxygens as nearest neighbors, but positional disorder complicates interpretation of the distances which range from 2.34 to 2.92Å. Most other Na + cations (38%) are displaced into the large cavity from 6-rings with 3 framework oxygens as nearest neighbors at 2.32 to 2.49Å. Diameters of three types of 8-rings range from 4.5 to 9.4Å, but the fourth type with very low occupancy of Na + is less distorted with diameters ranging from 6.0 to 7.0Å. Correlation between T-O distances and T-O-T angles is semi-quantitatively similar to that in mordenite.