Two Crystal Structures of Fully Dehydrated, Fully Ag~+-Exchanged Zeolite X. Dehydration in Oxygen Prevents Ag~+ Reduction. Without Oxygen, Ag8~(n+) (Td) and cyclo-Ag_4~(m+) (near S_4) Form

摘要

Two crystal structures of fully dehydrated, fully Ag~+-exchanged zeolite X have been determined by single-crystal 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 ℃ for 2 days, followed by cooling to 21 ℃, 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 Fd3m (a = 25.177(4)A) 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)A) was similarly dehydrated without oxygen. Its structure was determined in Fd3 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)A) and 24 Ag~+ ions occupy the nearby site I' positions (Ag~+-O = 2.220(10)A), avoiding I-I' contacts. Thirty-two Ag~+ ions fill site II (Ag~+-O = 2.360(10)A), and the remaining 32 Ag~+ ions occupy a III' site (Ag~+-O = 2.368(12) and 2.586(15)A). No reduction of Ag~+ has occurred. In the second crystal, 16 Ag~+ ions fill site I (Ag~+-O = 2.573(10)A), 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)A), and 16 Ag~+ ions half fill the 32-fold site II' inside the sodalite cavity (Ag~+-O = 2.461(11)A), 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.