Hydronium ions in zeolites. 1. Structures of partially and fully dehydrated Na, H{sub 3}O-X by X-ray and neutron diffraction
The crystal chemistry of hydronium ions in zeolite X has been investigated by single-crystal X-ray and powder neutron diffraction. By titration of Na{sub 96}Si{sub 96}Al{sub 96}O{sub 384} with HCl, a break in the pH was observed at 6.35, at about 33 H{sub 3}O{sup +} ions per unit cell. The structure of zeolite X in equilibrium with 0.1 M aqueous Na{sup +} at pH 6.35 was determined by single-crystal X-ray diffraction techniques in the cubic space group Fd{bar 3} at 20 C after partial dehydration (Na{sub 60}(H{sub 3}O){sub 32}(H{sub 2}O){sub 24}Si{sub 100}Al{sub 92}O{sub 384}, a = 25.128(4) {angstrom}) and in Fd{bar 3}m after complete dehydration (Na{sub 60}H{sub 32}Si{sub 100}Al{sub 92}O{sub 384}, a = 25.101(2) {angstrom}). in the partially dehydrated crystal, Na{sup +} ions are found at three different crystallographic sites and H{sub 3}O{sup +} ions are found at two. Two different site I{prime} positions are occupied by 16 Na{sup +} and 16 H{sub 3}O{sup +} ions, respectively, per unit cell; a water molecule bridges between two of each in each sodalite cage to give eight near tetrahedral (H{sub 3}O{sup +}){sub 2}H{sub 2}O(Na{sup +}){sub 2} clusters. Two different site III{prime} positions in the supercage are occupied by 16 Na{sup +} and 16 H{sub 3}O{sup +} ions, respectively; a water molecule bridges between one of each to give 16 (H{sub 3}O{sup +})(H{sub 2}O)(Na{sup +}) clusters. Thus a structural basis for the great selectivity of zeolite Na-X for H{sub 3}O{sup +} at near neutral pHs is demonstrated. The remaining 28 Na{sup +} ions are at site II. In the fully dehydrated single crystal, the 60 Na{sup +} ions are found at four crystallographic sites. Approximately 6 and 20 Na{sup +} ions per unit cell at sites I and I{prime}, respectively, effectively fill the double six-rings. Thirty-two Na{sup +} ions fill site II and the remaining two are at site III{prime}. To avoid Na{sup +} ions, at least 20 of the 32 H{sup +} ions per unit cell should be in the supercage. Finally, neutron diffraction of Na{sub 54}(D{sub 3}O){sub 42}Si{sub 96}Al{sub 96}O{sub 384}{sm{underscore}bullet}{approximately}80D{sub 2}O at 10 K revealed the structures of the 16 D{sub 3}O{sup +} ions in the sodalite units: D-O = 1.13(1) {angstrom} and D-O-D = 88(1){degree}. Each deuteron hydrogen bonds to a framework oxygen atom at 1.53(1) {angstrom}. The O{hor{underscore}ellipsis}O hydrogen-bonding distance is 2.63(1) {angstrom} and the O-D{hor{underscore}ellipsis}O angle is 161{degree}.
- Research Organization:
- Univ. of Hawaii at Manoa, Honolulu, HI (US); Univ. of Pennsylvania, Philadelphia, PA (US); Los Alamos National Lab., Los Alamos, NM (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-36
- OSTI ID:
- 20013641
- Journal Information:
- Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Vol. 103, Issue 47; Other Information: PBD: 25 Nov 1999; ISSN 1089-5647
- Country of Publication:
- United States
- Language:
- English
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