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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 22, Number 1
BKCSDE 22(1)
January 20, 2001 

 
Title
Two Anhydrous Zeolite X Crystal Structures, Pd18Tl56Si100Al92O384 and Pd21Tl50Si100Al92O384
Author
Bo Young Yoon, Mee Kyung Song, Seok Hee Lee, Yang Kim
Keywords
Structure, Zeolite X, Thallium, Palladium, Ion-exchange.
Abstract
The crystal structures of fully dehydrated Pd2+ - and Tl+ -exchanged zeolite X, Pd18Tl56Si100Al92O384 (Pd18Tl56-X, a = 24.935(4)Å and Pd21Tl50Si100Al92O384 (Pd21Tl50-X, a = 24.914(4)Å), 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 Pd18Tl56-X, and R1 = 0.043 and R2 = 0.045 with 280 reflections for Pd21Tl50-X; I > 3σ(I). In the structure of dehydrated Pd18Tl56-X, eighteen Pd2+ ions and fourteen Tl+ ions are located at site I'. About twenty-seven Tl+ ions occupy site II recessed 1.74Å 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 Pd21Tl50-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 β-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.
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30 - 36
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