Two Anhydrous Zeolite X Crystal Structures, Cd46Si100Al92O384 and Cd24.5Tl43Si100Al92O384

The structures of fully dehydrated, fully Cd 2+ -exchanged zeolite X, Cd 46 Si 100 Al 92 O 384 (Cd 46 -X; a = 24.935(8) Å), and that of fully dehydrated Cd 2+ - and Tl + -exchanged zeolite X, Cd 24.5 Tl 43 Si 100 Al 92 O 384 (Cd 24.5 Tl 43 -X; a = 24.858(9) Å), have been determined by single-crystal X-ray diffraction methods in the cubic space group Fd 3̄ at 21(1) °C. Cd 46 -X was prepared by ion exchange in a flowing stream of 0.05 M aqueous Cd(NO 3 ) 2 for 2 days. Cd 24.5 Tl 43 -X was prepared similarly using a solution 0.025 M each in Cd(NO 3 ) 2 and TlNO 3 . Each crystal was then dehydrated at 450 °C and 2 × 10 -6 Torr for 2 days. Their structures were refined to the final error indices R 1 = 0.055 and R 2 = 0.077 with 544 reflections for Cd 46 -X, and R 1 = 0.054 and R 2 = 0.051 with 272 reflections for Cd 24.5 Tl 43 -X; I > 3σ( I ). In the structure of dehydrated Cd 46 -X, Cd 2+ ions are located at two different crystallographic sites. Sixteen Cd 2+ ions fill site I, at the centers of the double six-rings; each Cd 2+ ion is octahedrally coordinated by framework oxygens, all at 2.35(1) Å. The remaining 30 Cd 2+ ions nearly fill the 32-fold site II in the single six-rings; each is three-coordinate planar to framework oxygens at 2.16(1) Å. The fractional occupancies in dehydrated Cd 24.5 Tl 43 -X are most easily explained with two types of unit cell: half have 14 Cd 2+ ions at site I and four Tl + ions at site I‘; the remaining half have 15 Cd 2+ ions at site I and two Tl + ions at site I‘. The remaining ten Cd 2+ ions occupy site II; 22 Tl + ions extend 1.52 Å into the supercage from their three oxygen planes to complete the filling of site II. The remaining 18 Tl + ions are statistically distributed over site III, a 48-fold equipoint in the supercages on twofold axes; Tl−O = 2.79(2) Å. It appears that Cd 2+ ions prefer sites I and II in that order, and that Tl + ions occupy the remaining sites, except that they are too large to be stable at site I.