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ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 22, Number 10
BKCSDE 22(10)
October 20, 2001 |
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Title |
Two Anhydrous Zeolite X Crystal Structures, Mn28Cs36-X and Mn21.5Rb49-X |
Author |
Myung Nam Bae, Mee Kyung Song, Yang Kim |
Keywords |
Structure, Zeolite X, Manganese, Cesium, Rubidium.
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Abstract |
The crystal structures of fully dehydrated Mn2+ - and Cs+ -exchanged zeolite X, Mn28Cs36-X (X=Si100Al92O384; a = 24.732(1) Å), and Mn2+ - and Rb+ -exchanged zeolite X, Mn21.5Rb49-X (a = 24.742(1) Å), have been determined by single-crystal X-ray diffraction methods in the cubic space groups Fd3m and Fd3 at 21(1)℃, respectively. Two crystal structures were refined to the final error indices R1 = 0.069 and R2 = 0.054, with 130 reflections, and R1 = 0.087 and R2 = 0.071, with 203 reflections, respectively, for which I > 3σ (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)Å into the supercage, where it was coordinated to three framework oxygens at 2.16(1)Å. Thirty-six Cs+ ions were found at three sites: 7 at site Ⅱ', 7 at site Ⅱ, and 22 at site Ⅲ. The Cs+ ion at site Ⅱ was recessed 2.16(1) Å into the supercage, where it was coordinated to three framework oxygens at 2.93(1)Å. Most of the Mn2+ ions at site I were not exchanged with Cs+ ions but some Mn2+ ions 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', and 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 was recessed 1.876(4)Å into the supercage, where it was coordinated to three framework oxygens at 2.71(2) Å. The Mn2+ ions at site I and Ⅱ were disturbed and exchanged with Rb+ ions. Rb+ ions, which are smaller and more diffusible than Cs+ ions, occupied site I'. The maximum Cs+ and Rb+ ion exchanges were 39% and 53%, respectively. Because these cations were too large to enter the small cavities and their charge distributions in the zeolite structure may have been unfavorable, cation-sieve effects were shown. |
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1081 - 1088 |
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