Three single-crystal structures of Cd2+-exchanged zeolite Y (FAU). Cd(OH)2 imbibition increases with increasing ion exchange temperature

Abstract

Three single crystals of excessively Cd2+-exchanged zeolite Y were prepared by the exchange of Na-Y (|Na75|[Si117Al75O384]-FAU, Si/Al=1.56) with an aqueous stream 0.25M each in Cd(NO3)2 and Cd(C2H3O2)2 at 294K (crystals 1 and 2) and 353K (crystal 3), followed by vacuum dehydration at 1×10−6Torr and 423K (crystal 1) and 723K (crystals 2 and 3). Their crystal structures were determined by diffraction experiment using synchrotron X-radiation in the cubic space group Fd3¯m. They were refined to the final error indices R1/wR2=0.0502/0.1445, 0.0702/0.1960, and 0.0643/0.1747 for crystals 1, 2, and 3, respectively. The number of Cd2+, H2O, OH−, and O2− ions or molecules in the three crystal structures are about 43.5, 23, 12, 0; 43.5, 0, 0, 6; and 47.5, 0, 0, 10 per unit cell for crystals 1, 2, and 3, respectively. In all cases, over-exchange due to uptake of OH− occurred. Crystals 1 and 2 had imbibed about 6 Cd(OH)2 molecules per unit cell, perhaps as CdOH+ ions, crystal 3 ten, indicating this increase in imbibition as temperature increased from 294K to 353K. These decomposed upon vacuum dehydration at 723K to give, with additional Cd2+ ions, Cd8O48+ clusters (symmetry 4¯3m(Td)) in the sodalite cavities of crystals 2 and 3. The crystal compositions are |Cd8.5(CdOH)12(CdOH2)23|[Si117Al75O384]-FAU (crystal 1), |Cd31.5(Cd8O4)1.5|[Si117Al75O384]-FAU (crystal 2), and |Cd27.5(Cd8O4)2.5|[Si117Al75O384]-FAU (crystal 3), respectively.