Two anhydrous zeolite X crystal structures, Pd18Tl56Si100Al92O384 and Pd21Tl50Si100Al92O384

The crystal structures of fully dehydrated Pd2+- and Tl+-exchanged zeolite X, Pd18Tl56Si100Al92O384 (Pd18Tl56X, a = 24.935(4) Angstrom) and Pd21Tl50Si 100Al92O384 (Pd21Tl50-X, a = 24.914(4) Angstrom), have been determined by s ingle-crys tal X-ray diffraction methods in the cubic space group Fd (3) ov er bar at 21(1) degreesC. The crystals were prepared using an exchange solu tion that had a Pd(NH3)(4)Cl-2 :TINO3 mole ratio of 50 : 1 and 200 : 1, res pectively, with a total concentration of 0.05 M for 4 days. After dehydrati on at 360 degreesC and 2 x 10(-6) Torr in flowing oxygen for 2 days, the cr ystals were evacuated at 21(1)OC for 2 hours. They were refined to the fina l error indices R-1 = 0.045 and R-2 = 0.038 with 344 reflections for Pd18Tl 56-X, and R-1 = 0.043 and R-2 = 0.045 with 280 reflections for Pd21Tl50- X; I >3 sigma (I). In the structure of dehydrated Pd18Tl56-X, eighteen Pd2+ i ons and fourteen Tl+ ions are located at site I'. About twenty-seven Tl+ io ns occupy site II recessed 1.74 Angstrom 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 3, 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 concen tration of exchange was much higher than that of Tl+ ions. Pd2+ ions tend t o 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 approxim ately two and one-half I' sites per sodalite cage may be occupied by Pd2+ i ons. The remaining I' sites an occupied by Tl+ inns with Tl-O bond distance that is shorter than the sum of their ionic radii. The electrostatic repul sion between two large Tl+ ions and between Tl+ and Pd2+ ions in the same b eta -cage pushes each other to the charged six-ring planes. It causes the T l-O bond to have some covalent character. However, TIC ions at site II form ionic bonds with three oxygens because the supercage has the available spa ce to obtain the reliable ionic bonds.