Three single-crystal structures of Cd²⁺-exchanged zeolite Y (FAU). Cd(OH)₂ imbibition increases with increasing ion exchange temperature
2013
Seo, Sung Man | Lim, Woo Taik
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, Cd₈O₄ ⁸⁺ 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.
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