Crystal structure of an acetylene sorption complex of dehydrated fully Cd2+-exchanged zeolite X

Abstract

The crystal structure of an acetylene sorption complex of dehydrated fully Cd²⁺-exchanged zeolite X, Cd₄₆Si₁₀₀Al₉₂O₃₈₄ · 28C₂H₂ [a = 24.874(5) Å], has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd$\text{3}$ at 21(1)°C. Cd₄₆−X was dehydrated at 450°C and 2 × 10⁻⁶ Torr for 2 d, followed by exposure to ca. 250 Torr of acetylene gas at 21(1)°C. The structure was determined in this atmosphere and was refined to the final error indices R₁ = 0.058 and R₂ = 0.067 with 661 reflections for which I > 3σ(I). In this structure, Cd²⁺ ions are found at three crystallographic sites: 13 Cd²⁺ ions are at site l, at the centers of the double six-oxygen rings; five Cd²⁺ ions lie at site I′, in the sodalite cavity opposite the double six-oxygen rings; and the remaining 28 Cd²⁺ ions are at site II, in the supercage near single six-oxygen rings. Each of these 28 is recessed 0.46 Å into the supercage where it coordinates laterally to an acetylene molecule (CC = 1.22(7) Å). These Cd²⁺ ions have a distorted tetrahedral environment, 2.192(8) Å from three framework oxygens and 2.79(5)Å from each carbon atom of an acetylene molecule (here counted as a monodentate). As compared to dehydrated Cd₄₆−X, five Cd²⁺ ions have relocated from sites I and II to I′, as they did in Cd₄₆−X · 28CO, primarily for electrostatic reasons.