Time-Dependent Ni²⁺-Ion Exchange in Zeolites Y (FAU, Si/Al = 1.56) and Their Single-Crystal Structures

Seven single crystals of fully dehydrated Ni²⁺-exchanged zeolite Y (Si/Al = 1.56) were prepared via cation exchange of Na₇₅–Y (|Na₇₅|[Si₁₁₇Al₇₅O₃₈₄]-FAU) by flowing 0.05 M aqueous solutions of Ni­(NO₃)₂·6H₂O (pH 4.9 and 294 K) with various ion-exchange times. Their crystal structures were completely determined by single-crystal synchrotron X-ray diffraction techniques in cubic space group <i>Fd</i>3̅<i>m</i> at 100(1) K. In all seven structures, Ni²⁺ ions occupy sites I, I′, and II, and sometimes site II′ or a second site II, or both, preferring site I; residual Na⁺ ions in crystals 1 and 2 are found at site III and/or a second site III. The level of Ni²⁺ exchange monotonically increased from 75.2 to 89.6% [from 28.2(2) to 33.6(8) Ni²⁺ ions per unit cell] with increasing exchange time until 18 h. The dealumination of the zeolite frameworks was observed in the center of sodalite cavities after 24 h, suggesting that this process occurs during ion exchange, or subsequent dehydration. H⁺ ions are present in all seven crystals for a charge balance. Both the unit cell constants and Na⁺ contents decreased with increasing levels of Ni²⁺ exchange and ion-exchange time