Crystallographic study on the selectivity, occupancy, and distribution of Sr²⁺ ions within zeolite Y in the presence of competing Na⁺ ions in aqueous exchange solution

Abstract

Three single-crystals of fully dehydrated and partially Sr²⁺-exchanged zeolites Y (Si/Al = 1.56) were prepared by the flow method using mixed ion-exchange solutions. The Sr(NO₃)₂:NaNO₃ molar ratios of the ion exchange solution were 1:100 (crystal 1), 1:250 (crystal 2), and 1:500 (crystal 3) with a total concentration of 0.05 M. The single-crystals were then vacuum dehydrated at 723 K and 1 × 10⁻⁴ Pa for 2 days. The structures of the crystals were determined by single-crystal synchrotron X-ray diffraction technique in the cubic space group $Fd\bar {3}m$, at 100(1) K, and were then refined to the final error indices of R₁/wR₂ = 0.044/0.167, 0.051/0.175, and 0.069/0.219 for crystals 1, 2, and 3, respectively. The unit-cell formulas of crystals 1, 2, and 3 were |Sr₁₇Na₄₁|[Si₁₁₇Al₇₅O₃₈₄]-FAU, |Sr₁₃Na₄₉|[Si₁₁₇Al₇₅O₃₈₄]-FAU, and |Sr₉Na₅₇|[Si₁₁₇Al₇₅O₃₈₄]-FAU, respectively. In all three crystals, the Sr²⁺ ions preferred to occupy site I in the D6Rs, with the remainder occupying site II. The Sr²⁺ ions are also located at site I′ in crystal 1. On the other hand, the Na⁺ ions are located at sites I′, II, and III′ in all crystals. With increasing the molar concentration of Na⁺ in the given exchange solution, the occupancies of the Sr²⁺ ions at sites I, I′, and II decreased, whereas the occupancies of the Na⁺ ions at sites I′, II, and III′ increased.