Location of cations in metal ion-exchanged zeolites. Part 2. Crystal structures of a fully silver-exchanged heulandite

Abstract

The crystal structure of a fully silver-exchanged heulandite is reported. The freshly prepared monoclinic crystals, space group C2/m, have cell parameters a= 17.736(9), b= 18.104(9), c= 7.447(6)Å, β= 116.2(1)°, Z= 1, and formula Ag_7.3Al_7.2Si_28.8O₇₂·18H₂O (from electron microprobe analysis). The final R index was 0.068 using 1 602 independent reflections in the least-squares refinement. X-Ray analysis results suggest that Ag⁺ has exchanged two sites CS(1) and CS(2) already occupied by Ca²⁺ and Na⁺ in the natural heulandite. Furthermore, a new site, CS(3), with a low occupancy has been ascribed to Ag²⁺. However, the total amount of silver ions (4.1 per unit cell) detected from X-ray refinement is significantly smaller than that derived from chemical analysis. This suggests that the remainder of the Ag⁺ ions are spread out in the zeolite pores. Since crystals after long exposure in air darken, a second crystal structure analysis was carried out using a crystal of (Ag)heu exchanged under different conditions and after exposure to air for two months. No significant difference was found with the above results.