Crystal Structure of Dehydrated Partially Ag+-Exchanged Zeolite A treated with Cesium Vapor at 250?é

摘要

The crystal structure of partially Ag+-exchanged zeolite A, Ag3.2Na8.8-A, vacuum dehydrated at 360∩ and then exposed to 0.1 torr of cesium vapor for 12 hours at 250∩ has been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3m (a = 12.262(2)∈) at 21(1)∩. The structure was refined to the final error indexes R1 = 0.068 and R2 = 0.072 by using 338 reflections for which Io > 3ヲ(Io) and the composition of unit cell is Ag3.2Cs8.8-A. 3 Cs+ ions lie on the centers of the 8-rings at sites of D4h symmetry. Two crystallographycally different 6-ring Cs+ ions were found: 1.5 Cs+ ions at Cs(2) are located inside of sodalite cavity and 4.3 Cs+ ions at Cs(3) are located in the large cavity. The fractional occupancies observed at Cs(2) and Cs(3) indicate that the existence of at least three types of unit cells with regard to the 6-ring Cs+ ions. For example, 50% of unit cells may have two Cs+ ions at Cs(2) and 4 Cs+ ions at Cs(3). 30% of unit cells may have one Cs+ ion at Cs(2) and 5 Cs+ ions at Cs(3). The remaining 20% would have one Cs+ ion at Cs(2) and 4 Cs+ ions at Cs(3). On threefold axes of the unit cell two non-equivalent Ag atom positions are found in the large cavity, each containing 0.64 and 1.92 Ag atoms, respectively. A crystallographic analysis may be interpreted to indicate that 0.64 (Ag5)+ clusters are present in each large cavity. This cluster may be viewed as a tetrasilver molecule (Ag4)0(bond length, 2.84∈) stabilized by the coordination of one Ag+ ion.