| Autor: |
Zhu, L., Seff, K., Olson, D. H., Cohen, B. J., Dreele, R. B. Von |
| Zdroj: |
The Journal of Physical Chemistry - Part B; November 25, 1999, Vol. 103 Issue: 47 p10365-10372, 8p |
| Abstrakt: |
The crystal chemistry of hydronium ions in zeolite X has been investigated by single-crystal X-ray and powder neutron diffraction. By titration of Na96Si96Al96O384 with HCl, a break in the pH was observed at 6.35, at about 33 H3O+ ions per unit cell. The structure of zeolite X in equilibrium with 0.1 M aqueous Na+ at pH 6.35 was determined by single-crystal X-ray diffraction techniques in the cubic space group Fd&thremacr; at 20 °C after partial dehydration (Na60(H3O)32(H2O)24Si100Al92O384, a = 25.128(4) Å) and in Fd&thremacr;m after complete dehydration (Na60H32Si100Al92O384, a = 25.101(2) Å). In the partially dehydrated crystal, Na+ ions are found at three different crystallographic sites and H3O+ ions are found at two. Two different site I positions are occupied by 16 Na+ and 16 H3O+ ions, respectively, per unit cell; a water molecule bridges between two of each in each sodalite cage to give eight near tetrahedral (H3O+)2H2O(Na+)2 clusters. Two different site III positions in the supercage are occupied by 16 Na+ and 16 H3O+ ions, respectively; a water molecule bridges between one of each to give 16 (H3O+)(H2O)(Na+) clusters. Thus a structural basis for the great selectivity of zeolite Na−X for H3O+ at near neutral pHs is demonstrated. The remaining 28 Na+ ions are at site II. In the fully dehydrated single crystal, the 60 Na+ ions are found at four crystallographic sites. Approximately 6 and 20 Na+ ions per unit cell at sites I and I, respectively, effectively fill the double six-rings. Thirty-two Na+ ions fill site II and the remaining two are at site III. To avoid Na+ ions, at least 20 of the 32 H+ ions per unit cell should be in the supercage. Finally, neutron diffraction of Na54(D3O)42Si96Al96O384• ~ 80D2O at 10 K revealed the structures of the 16 D3O+ ions in the sodalite units: D−O = 1.13(1) Å and D−O−D = 88(1)°. Each deuteron hydrogen bonds to a framework oxygen atom at 1.53(1) Å. The O···O hydrogen-bonding distance is 2.63(1) Å and the O−D···O angle is 161°. |
| Databáze: |
Supplemental Index |
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