Single-Crystal Structures of Fully and Partially Dehydrated Zeolite Y (FAU, Si/Al = 1.56) Ni²⁺ Exchanged at a Low pH, 4.9

The crystal structures of zeolite Y Ni²⁺ exchanged at pH 4.9, fully dehydrated (|Ni_29.7(Ni₄AlO₄)_1.1H_12.3|[Si₁₁₇Al₇₅O₃₈₄]-FAU) and partially dehydrated (|Ni_14.1(NiOH₂)_16.4(Ni₄AlO₄)_1.1(H₃O)_9.6|[Si₁₁₇Al₇₅O₃₈₄]-FAU), were determined by single-crystal synchrotron X-ray diffraction techniques. They were refined to the final error indices <i>R</i>₁/<i>wR</i>₂ = 0.044/0.115 and 0.052/0.153, respectively, in the cubic space group <i>Fd</i>3̅<i>m</i>. Some dealumination of the zeolite framework occurred during Ni²⁺ exchange. In the fully dehydrated structure, Ni²⁺ ions primarily occupy sites I and II, with additional Ni²⁺ ions at site I′, site II′, and a second site II. In the partially dehydrated structure, Ni²⁺ occupies six different sites: I, I′, a second site I′, II, a second site II, and III′. Because of the low pH of the Ni²⁺ exchange solution, some H₃O⁺ ions are observed in partially dehydrated Ni²⁺–Y; correspondingly, some H⁺ ions are required for charge balance in fully dehydrated Ni²⁺–Y. The nonframework oxygen atoms in water and orthoaluminate coordinate to some of the Ni²⁺ ions. Upon full dehydration, a substantial Ni²⁺ migration from I′ sites to site I is seen. Unlike the situation at pH 6.2, no hydrolyzed Ni²⁺ ions (NiOH⁺, Ni₂OH³⁺, Ni₄(OH)₄⁴⁺) appear to have exchanged into the zeolite