Weak Ag +–Ag + bonding in zeolite X. Crystal structures of Ag 92Si 100Al 92O 384 hydrated and fully dehydrated in flowing oxygen

Abstract

Two crystal structures of fully Ag +-exchanged zeolite X, one hydrated ( a=24.996(4) Å) and the other fully dehydrated ( a=25.200(4) Å), have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd 3 ̄ at 21(1)°C. Each initial Na 92–X crystal was ion exchanged in a flowing stream of 0.05 M aqueous AgNO 3. The second crystal was dehydrated at 360°C for two days in a flowing stream of oxygen gas (790 Torr) followed by evacuation at 400°C and 2×10 −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 σ( I). Both structures show weakly attractive 3.0–3.3 Å 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) Å) to those at site I, 32 fill site II in the supercages, and 28 occupy four different III ′ sites. Some H 2O 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) Å). Three linear Ag 3 + clusters per unit cell with atoms at sites I ′, I, and I ′, respectively, lie along 3-fold axes, and two bent 168(2)° Ag 3 2+ clusters per unit cell are in the sodalite cavities. It remains possible, considering Ag–Ag and Ag–O 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.