Two anhydrous zeolite X crystal structures, Mn28Cs36-X and Mn21.5Rb49-X

The crystal structures of fully dehydrated Mn2+- and Cs+-exchanged zeolite X, Mn28Cs36-X (X = Si100Al92O384; a = 24.732(1) Angstrom). and Mn2+- and Rb +-exchanged zeolite X, Mn21.5Rb49-X (a = 24.742(1) Angstrom), have been det ermined by single-crystal X-ray diffraction methods in the cubic space grou ps Fd (3) over barm and Fd (3) over bar at 21(1) degreesC, respectively. Tw o crystal structures were refined to the final error indices R-1 = 0.069 an d R-2 = 0.054, with 130 reflections, and R-1 = 0.087 and R-2 = 0.071, with 203 reflections, respectively, for which I > 3 sigma (I). In Mn28CS36-X, 28 Mn2+ ions per unit cell were found at three crystallographic sites: 14 at site I, 4 at site I', and the remaining 10 at site II. The Mn2+ ion at site II was recessed 0.47(1) Angstrom into the supercage, where it was coordina ted to three framework oxygens at 2.16(1) Angstrom. Thirty-six Cs+ ions wer e found at three sites: 7 at site II', 7 at site II, and 22 at site III. Th e Cs+ ion at site II was recessed 2.16(1) Angstrom into the supercage, wher e it was coordinated to three framework oxygens at 2.93(1) Angstrom. Most o f the Mn2+ ions at site I were not exchanged with Cs+ ions but some Mn2+ io ns at site II were disturbed and exchanged with Cs+ ions. In Mn21.5Rb49-X, 21.5 Mn2+ ions were found at three sites: 11.5 at site I, 6 at site II', an d 4 at site I'. Forty-nine Rb+ ions were found at four sites: 5 at site I', 18 at site II, 6 at site II', and 20 at site III. The Rb+ ion at site II w as recessed 1.876(4) A into the supercage, where it was coordinated to thre e framework oxygens at 2.71(2) A. The Mn 21 ions at site I and II were dist urbed and exchanged with Rb+ ions. Rb+ ions, which are smaller and more dif fusible than Cs+ ions, occupied site I'. The maximum Cs+ and Rb+ ion exchan ges were 39% and 53%, respectively. Because these cations were too large to enter the small cavities and their charge distributions in the zeolite str ucture may have been unfavorable, cation-sieve effects were shown.