Weak Ag+-Ag+ bonding in zeolite X.: Crystal structures of Ag92Si100Al92O384 hydrated and fully dehydrated in flowing oxygen

Two crystal structures of fully Ag+-exchanged zeolite X, one hydrated (a = 24.996(4) Angstrom) and the other fully dehydrated (a = 25.200(4) Angstrom), have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd (3) over bar at 21(1)degreesC. Each initial Na-92-X crystal was ion exchanged in a flowing stream of 0.05 M aqueous AgNO3. The second crystal was dehydrated at 360 degreesC for two days in a flowing stream of oxygen gas (790 Torr) followed by evacuation at 400 degrees \C and 2 x 10(-6) Torr for 2 h. Their structures were refined to the final error indexes R-1/R-2 = 0.088/0.104 with 216 reflections, and R-1/R-2 = 0.047/0.041 with 312 reflections, respectively, for which I > 3 sigma (I). Both structures show weakly attractive 3.0-3.3 Angstrom Ag+-Ag+ interactions. In the hydrated crystal, 92 Ag+ ions were found at seven crystallographic sites: 16 fill site I at the centers of the double six-rings, 16 at site I' in the sodalite cavities opposite double six-rings bond weakly (3.045(3) Angstrom) to those at site I, 32 fill site II in the supercages, and 28 occupy four different III' sites. Some H2O molecules were found at two different 3-fold axis sites: 16 coordinate to site I' Ag+ ions in the sodalite cavities, and 32 coordinate to site II Ag+ ions in the supercage. In the dehydrated crystal, Ag ions or atoms were found at eight crystallographic sites: three Ag+ ions are at site I, 26 Ag+ ions and six Ag-0 atoms are at two I' sites in the sodalite cavities filling site I', 32 Ag+ ions fill site II as in crystal 1, two Ag-0 atoms are on 2-fold axes in the sodalite cavities, and 23 Ag+ ions occupy three different III' sites. The 26 Ag+ ions at site I' bond weakly in pairs (3.224(3) Angstrom). Three linear Ag: clusters per unit cell with atoms at sites I/, I, and I', respectively, lie along 3-fold axes, and two bent 168(2)degrees Ag-3(2+) clusters per unit cell are in the sodalite cavities. It remains possible, considering Ag-Ag and Ag-0 distances, that no Ag-0 atoms have formed, that the product is (Ag+)(92)-X, and that the bonding in the clusters, both of which would then be Ag-3(3+), is due to additional Ag+-Ag+ interactions. (C) 2000 Elsevier Science B.V. All rights reserved.