A crystallographic study of Sr²⁺ and K⁺ ion-exchanged zeolite Y (FAU, Si/Al = 1.56) from binary solution with different mole ratio of Sr²⁺ and K⁺

To study the Sr²⁺-ion selectivity of zeolite Y (Si/Al = 1.56) in binary solution with different Sr²⁺ and K⁺ concentration during exchange, two single-crystals of fully dehydrated, Sr²⁺- and K⁺-exchanged zeolites Y were prepared by the flow method using a mixed ion-exchange solution whose Sr(NO₃₎₂:KNO₃ mol ratios were 1:1 (crystal 1) and 1:100 (crystal 2), respectively, with a total concentration of 0.05 M, followed by vacuum dehydration at 723 K. Their crystal structures were determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fd3 ¯ m, respectively, and were refined to the final error indices R₁/wR₂ = 0.0429/0.1437 and 0.0356/0.1239 for crystals 1 and 2, respectively. In the structure of |Sr₂₈K₁₉|[Si₁₁₇Al₇₅O₃₈₄]-FAU (crystal 1), 28 Sr²⁺ ions per unit cell occupy four different crystallographically distinct sites; 15, 2, 2, and 9 are at sites I, I′, II′, and II, respectively, whereas, the K⁺ ions occupy only one site: 19 are at site II. In the structure of |Sr₁₇K₄₁|[Si₁₁₇Al₇₅O₃₈₄]-FAU (crystal 2), 17 Sr²⁺ ions per unit cell occupy three equipoints; 10.5, 2, and 4.5 are at sites I, I′, and II, respectively. The residual 41 K⁺ ions per unit cell are found at four different sites; 8, 24, 3, and 6 are at sites I′, II, III′a, and III′b, respectively. The degrees of ion exchange are 74.7 and 45.3% for crystals 1 and 2, respectively. This result shows that the degree of Sr²⁺ exchange decreased sharply by decreasing the initial Sr²⁺ concentration and increasing the initial K⁺ concentration in the given ion-exchange solution.