Two Anhydrous Zeolite X Crystal Structures, Pd₁₈Tl₅₆Si₁₀₀Al₉₂O₃₈₄ and Pd₂₁Tl₅₀Si₁₀₀Al₉₂O₃₈₄

摘要

The crystal structures of fully dehydrated Pd2+ - and Tl+ -exchanged zeolite X, Pd₁₈Tl₅₆Si₁₀₀Al₉₂O₃₈₄ (Pd₁₈Tl_56-X, a = 24.935(4)∈ and Pd₂₁Tl₅₀Si₁₀₀Al₉₂O₃₈₄ (Pd₂₁Tl_50-X, a = 24.914(4)∈), have been determined by single-crystal X-ray diffraction methods in the cubic space group Fd3 at 21(1)∩. The crystals were prepared using an exchange solution that had a Pd(NH₃)₄Cl₂ :TlNO₃ mole ratio of 50 : 1 and 200 : 1, respectively, with a total concentration of 0.05M for 4 days. After dehydration at 360∩ and 2 】10-6 Torr in flowing oxygen for 2 days, the crystals were evacuated at 21(1)∩ for 2 hours. They were refined to the final error indices R₁ = 0.045 and R₂ = 0.038 with 344 reflections for Pd₁₈Tl_56-X, and R₁ = 0.043 and R₂ = 0.045 with 280 reflections for Pd₂₁Tl_50-X; I > 3ヲ(I). In the structure of dehydrated Pd₁₈Tl_56-X, eighteen Pd2+ ions and fourteen Tl+ ions are located at site I`. About twenty-seven Tl+ ions occupy site II recessed 1.74∈ into a supercage from the plane of three oxygens. The remaining fifteen Tl+ ions are distributed over two non-equivalent III` sites, with occupancies of 11 and 4, respectively. In the structure of Pd₂₁Tl_50-X, twenty Pd2+ and ten Tl+ ions occupy site I`, and one Pd2+ ion is at site I. About twenty-three Tl+ ions occupy site II, and the remaining seventeen Tl+ ions are distributed over two different III` sites. Pd2+ ions show a limit of exchange (ca. 39% and 46%), though their concentration of exchange was much higher than that of Tl+ ions. Pd2+ ions tend to occupy site I`, where they fit the double six-ring plane as nearly ideal trigonal planar. Tl+ ions fill the remaining I` sites, then occupy site II and two different III` sites. The two crystal structures show that approximately two and one-half I` sites per sodalite cage may be occupied by Pd2+ ions. The remaining I` sites are occupied by Tl+ ions with Tl-O bond distance that is shorter than the sum of their ionic radii. The electrostatic repulsion between two large Tl+ ions and between Tl+ and Pd2+ ions in the same モ-cage pushes each other to the charged six-ring planes. It causes the Tl-O bond to have some covalent character. However, Tl+ ions at site II form ionic bonds with three oxygens because the super-cage has the available space to obtain the reliable ionic bonds.