Crystal Structures of Fully La3+-Exchanged Zeolite X: an Intrazeolitic La2O3 Continuum, Hexagonal Planar and Trigonally Monocapped Trigonal Prismatic Coordination

Three single crystals of sodium zeolite X were fully La 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 3̄ m ( a = 24.999(4) and 25.059(4) Å) and Fd 3̄ ( a = 25.154 (3) Å), respectively. They were refined to the final error indices R 1 = 0.076, 0.091, and 0.078 with 471, 374, and 725 reflections, respectively, for which F o > 4σ( F o ). The number of La 3+, H 3 O +, and OH - 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 + ions per unit cell. In all cases overexchange occurred. In crystal 1, 32 La 3+ ions per unit cell fill site I‘, with the remaining six at 12-ring centers (site V). Each of these six coordinates to three OH - ions and three water molecules in a near hexagonal planar manner. A tetrahedron of four La 3+ ions at site I‘ is found in each sodalite unit; each La 3+ coordinates to three framework oxygens. This tetrahedron is interpenetrated by a tetrahedron of four oxide ions to give a distorted La 4 O 4 4+ cube. These cubes are linked by oxide ions at site I to form a neutral La 2 O 3 continuum; a binary intrazeolitic continuum had not been previously reported. In crystal 2, 28 La 3+ ions are at site I‘, eight at site V, and two at supercage centers (site IV). Four H 3 O + ions are at site I‘, 12 at site III‘, and 12 at site II. In all, 64 water molecules per unit cell coordinate to La 3+ ions or bridge between hydrated La 3+ ions. Half of the sodalite units contain complete La 4 (OH) 4 8+ “cubes”, and the other half contain La 3 (OH)(OH 2 ) 3 8+ units. Two of the eight supercages per unit cell are filled with large clusters. Each consists of a La 3+ ion at its center (site IV), four La 3+ ions near site V, six H 3 O + ions, 19 water molecules, and 15 OH - ions. In crystal 3, eight La 3+ ions are at site I‘, 24 at site II, and one at site V. Each sodalite unit contains a La 3+ ion at site I‘; this ion coordinates to three framework oxygens and to three OH - ions. Each OH - ion bridges to a La 3+ ion at site II, and each triplet of OH - ions is capped by a H 3 O + ion. Each supercage contains three La 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.