Crystallographic studies on the site selectivity of Ca²⁺, K⁺, and Rb⁺ ions within zeolite Y (Si/Al = 1.56)

Abstract

The single-crystals of Ca²⁺, K⁺-exchanged zeolite Y, and Ca²⁺, Rb⁺-exchanged zeolite Y were prepared by using flow method with mixed ion-exchange solution, whose Ca(NO₃)₂:KNO₃ mole ratios were 1:1 (crystal 1) and 1:100 (crystal 2), and Ca(NO₃)₂:RbNO₃ mole ratios were 1:1 (crystal 3) and 1:100 (crystal 4), respectively, with a total concentration of 0.05 M. They were fully dehydrated by vacuum dehydration at 723 K and 1 × 10⁻⁶ Torr for 2 days. Their crystals were determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group $Fd \overline{3}$ m, respectively, and were refined to the final error indices R ₁/wR ₂ = 0.057/0.196, 0.073/0.223, 0.055/0.188, and 0.049/0.175 for crystals 1, 2, 3, and 4, respectively. In the structure of crystal 1 (|Ca₂₃K₂₉|[Si₁₁₇Al₇₅O₃₈₄]-FAU), 23 Ca²⁺ ions per unit cell occupy sites I, II′, and II; 29 K⁺ ions per unit cell are at sites II′, II, and III′. In the structure of crystal 2 (|Ca_18.5K₃₈|[Si₁₁₇Al₇₅O₃₈₄]-FAU), 18.5 Ca²⁺ ions per unit cell occupy sites I, I′, and II; 38 K⁺ ions are at sites I′, II, and III′. In the structure of crystal 3 (|Ca₂₇Rb₂₁|[Si₁₁₇Al₇₅O₃₈₄]-FAU), 27 Ca²⁺ ions per unit cell occupy sites I, II′, and II; 21 Rb⁺ ions per unit cell are at sites II′, II, and III. In the structure of crystal 4 (|Ca₁₈Rb₃₉|[Si₁₁₇Al₇₅O₃₈₄]-FAU), 18 Ca²⁺ ions per unit cell occupy sites I and II; 39 Rb⁺ ions per unit cell are at sites I′, II′, II, III, and III′. In the four crystals, the Ca²⁺ ion which has much smaller size and higher charge than other cations such as K⁺ and Rb⁺ energetically preferred at site I and so the first to be filled on it. Unlike Ca²⁺ ion, no K⁺ and Rb⁺ ions are found at site I, which are clearly less favorable for K⁺ and Rb⁺ ions.