Two Crystal Structures of Fully Dehydrated, Fully Ag⁺-Exchanged Zeolite X. Dehydration in Oxygen Prevents Ag⁺ Reduction. Without Oxygen, Ag₈ⁿ⁺ (T_d) and cyclo-Ag₄^m+ (near S₄) Form

Abstract

Two crystal structures of fully dehydrated, fully Ag + -exchanged zeolite X have been determined by singlecrystal X-ray diffraction techniques. Ion exchange was accomplished by allowing 0.05 M aqueous AgNO 3 to flow past each crystal for 3 days to give hydrated Ag 92 Si 100 Al 92 O 384 . One crystal was dehydrated at 400 °C for 2 days, followed by cooling to 21 °C, all in a flowing stream of dry oxygen gas (790 Torr), followed by evacuation at 2 × 10 -6 Torr for 20 min. The structure was determined in the cubic space group Fd3 hm (a ) 25.177(4) Å) and was refined to the final error indices R 1 ) 0.048 and R 2 ) 0.078 with 291 reflections for which I > 3σ(I). The second crystal (a ) 24.871(4) Å) was similarly dehydrated without oxygen. Its structure was determined in Fd3 h and refined to R 1 ) 0.069 and R 2 ) 0.072 with 320 reflections as above. In the first crystal, 4 Ag + ions were found at the octahedral site I (Ag + -O ) 2.689(7) Å) and 24 Ag + ions occupy the nearby site I′ positions (Ag + -O ) 2.220(10) Å), avoiding I-I′ contacts. Thirty-two Ag + ions fill site II (Ag + -O ) 2.360(10) Å), and the remaining 32 Ag + ions occupy a III′ site (Ag + -O ) 2.368(12) and 2.586(15) Å). No reduction of Ag + has occurred. In the second crystal, 16 Ag + ions fill site I (Ag + -O ) 2.573(10) Å), 32 partially reduced Ag + ions half fill two different site I′ positions that are unusually deep in the sodalite cavities (Ag-O ) 2.628(10) and 2.708(14) Å), and 16 Ag + ions half fill the 32-fold site II′ inside the sodalite cavity (Ag + -O ) 2.461(11) Å). Sixteen Ag + ions half fill the 32-fold site II (Ag + -O ) 2.231(11) Å), and eight Ag + ions lie at two different III′ sites. In total, only about 86 silver atoms or ions were found per unit cell of crystal 2. The remaining six have migrated out of the zeolite framework to form small silver crystallites on the surface of the zeolite single crystal. In this structure, each sodalite cavity hosts a cationic silver cluster. In half, eight silver atoms form interpenetrating tetrahedra, Ag 8 n+ , with T d symmetry. The other half host cyclo-Ag 4 m+ cations with near S 4 symmetry. Likely values of n and m are 4 and 2, respectively.

Formation of Ag clusters in zeolite X studied by in situ EXAFS and infrared spectroscopy

“…57 Å and that of Ag-Ag as 2.705 Å for Ag 86 -X[7]. On the other hand, in the present EXAFS results for Ag 86 -X, the distance of Ag-O(1) is $2.34 Å and that for Ag-Ag is $2.83 Å and such…”

“…We could estimate the Ag 8 cluster for the small cluster (cluster I) because the evaluated N Ag-Ag was the same as EXAFS result (N Ag-Ag = 3.0). Kim et al [7] proposed that 4 Ag + were bound by zeolite framework and the charge of such a cluster is 4+(Ag 8 4+ , cluster I). On the other hand, we could speculate the cluster model Ag 12 as the large cluster II because the average coordination number for this type of cluster was 4.7, which coincided with the EXAFS result (N Ag-Ag = 4.7).…”

“…X-ray diffraction (XRD) method has long been applied to the structural determination of several types of Ag-zeolites, and it gives successful structural results for Ag-zeolite X recently [7].…”

Synthesis and Characterization of the Large Single Crystal of Fully K+-exchanged Zeolite X (FAU), |K80|[Si112Al80O384]-FAU (Si/Al=1.41)

“…Further description is available. 7,9,[18][19][20] Space Group Considerations. If the Si/Al = 1.00, then Si and Al atoms would alternate in the FAU framework in obeyance of Lowenstein's rule (-Al-O-Al-sequences are forbidden).…”

Local structure change of luminescent Ag zeolite-A and -X studied by in situ XAFS and IR spectroscopy

“…Zeolites show unique properties owing to the cavities and cages existing in the crystalline aluminosilicates. Several types of cations can be easily exchanged with Na + ions in the Natype zeolites and their unique physical and chemical properties will depend on the substituted cations (Kim et al, 2003).…”