Crystal structure of full La {sup 3+}-exchanged zeolite X: An intrazeolitic La{sub 2}O{sub 3} continuum, hexagonal planar and trigonally monocapped trigonal prismatic coordination
Three single crystals of sodium zeolite X were fully La{sup 3+}-exchanged at 90 C from aqueous solution (pH = 7.45, 7.45, and 6.89 at 22 C, respectively). One was vacuum dehydrated at 400 C (crystal 1), another at 22 C (crystal 2), and the third remained fully hydrated (crystal 3). Their structures at 22 C were determined by single-crystal X-ray crystallography in the cubic space groups Fd{bar 3}m (a = 24.999(4) and 25.059(4){angstrom}) and Fd{bar 3} (a - 25.154(3){angstrom}), respectively. They were refined to the final error indices R{sub 1} = 0.076, 0.091, and 0.078 with 471, 374, and 725 reflections, respectively, for which F{sub 0} > 4{sigma}(F{sub 0}). The number of La{sup 3+}, H{sub 3}O{sup +}, and OH{sup {minus}} ions found in the three structures are about 38, 0, 18; 38, 28, 50; and 33, 16, 24, respectively, per unit cell. Crystal 1 also has 48 nonframework oxide ions and 92 H{sup +} ions per unit cell. In all cases overexchange occurred. In crystal 1, 32 La{sup 3+} ions per unit cell fill site I{prime}, with the remaining six at 12-ring centers (site V). Each of these six coordinates to three OH{sup {minus}} ions and three water molecules in a near hexagonal planar manner. A tetrahedron of four La{sup 3+} ions at site I{prime} is found in each sodalite unit; each La{sup 3+} coordinates to three framework oxygens. This tetrahedron is interpenetrated by a tetrahedron of four oxide ions to give a distorted La{sub 4}O{sub 4}{sup 4+} cube. These cubes are linked by oxide ions at site 1 to form a neutral La{sub 2}O{sub 3} continuum; a binary intrazeolitic continuum had not been previously reported. In crystal 2, 29 La{sup 3+} ions are at site I{prime}, eight at site V, and two at supercage centers (site IV). Four H{sub 3}O{sup +} ions are at site I{prime}, 12 at site III{prime}, and 12 at site II. In all, 64 water molecules per unit cell coordinate to La{sup 3+} ions or bridge between hydrated La{sup 3+} ions. Half of the sodalite units contain complete La{sub 4}(OH){sub 4}{sup 8+} cubes, and the other half contain La{sub 3}(OH)(OH{sub 2}){sub 3}{sup 8+} units. Two of the eight supercages per unit cell are filled with large clusters. Each consists of a La{sup 3+} ion at its center (site IV), four La{sup 3+} ions near site V, six H{sub 3}O{sup +} ions, 19 water molecules, and 15 OH{sup {minus}} ions. In crystal 3, eight La{sup 3+} ions are at site I{prime}, 24 at site II, and one at site V. Each sodalite unit contains a La{sup 3+} ion at site I{prime}; this ion coordinates to three framework oxygens and to three OH{sup {minus}} ions. Each OH{sup {minus}}ion bridges to a La{sup 3+} ion at site II, and each triplet of OH{sup {minus}} ions is capped by a H{sub 3}O{sup +} ion. Each supercage contains three La{sup 3+} ions at site II and 19 water molecules. For the first time, the coordination geometries hexagonal planar (crystals 1 and 2) and trigonally monocapped trigonal-prismatic (crystal 3) are seen.
- Research Organization:
- Univ. of Hawaii, Honolulu, HI (US)
- OSTI ID:
- 20026914
- Journal Information:
- Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Vol. 104, Issue 10; Other Information: PBD: 16 Mar 2000; ISSN 1089-5647
- Country of Publication:
- United States
- Language:
- English
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