Crystallographic Verification that Copper(II) Coordinates to Four of the Oxygen Atoms of Zeolite 6-Rings. Two Single-Crystal Structures of Fully Dehydrated, Largely Cu²⁺-Exchanged Zeolite Y (FAU, Si/Al = 1.56)

Two single crystals of fully dehydrated, mostly Cu²⁺-exchanged zeolite Y were prepared by the exchange of Na₇₅–Y (|Na₇₅|[Si₁₁₇Al₇₅O₃₈₄]-FAU, Si/Al = 1.56) with an aqueous stream 0.05 M in Cu(NO₃)₂, pH = 4.2, at 294 K for 18 and 24 h, respectively, followed by vacuum dehydration at 673 K and 1 × 10^–6 Torr. Their crystal structures were determined by synchrotron X-ray diffraction techniques in the cubic space group Fd3̅m at 100(1) K and were refined to the final error indices R₁/wR₂ = 0.050/0.165 and 0.050/0.163, respectively, for |Cu_32.6Na_5.3H_4.5|[Si₁₁₇Al₇₅O₃₈₄]-FAU and |Cu_33.0Na_3.9H_5.1|[Si₁₁₇Al₇₅O₃₈₄]-FAU, respectively. Cu²⁺ ions occupy the 6-ring sites I′ and II, preferring I′; Na⁺ ions also occupy sites I′ and II; neither site is filled. All Cu²⁺ ions in these structures coordinate not only to three trigonally arranged oxygen atoms of their 6-rings, but also to a fourth oxygen atom of those 6-rings to achieve planar, severely distorted square, 4-coordination. This result is in agreement with DFT calculations by Pierloot et al. and Berthomieu et al. With increasing Cu²⁺-exchange time, both the Cu²⁺ and H⁺ contents of the zeolite increase.