Highly selective sorption and unique packing geometries of unsaturated hydrocarbons and CO2 in a fluorine-substituted organic–inorganic ionic crystal

Abstract

An ionic crystal [Cr₃O(OOCCF₃)₆(H₂O)₃]₃[α-PW₁₂O₄₀]·8CH₃COCH₃·8H₂O·CHCl₃ [Ia] was synthesized by the complexation of a fluorine-substituted macrocation with a phosphododecatungstate. Compound Ia possessed a layered structure with an interlayer distance of ca. 3.5 Å, and the inner surface of the layer was decorated with CF₃ groups of the macrocations. The solvent molecules (acetone and chloroform) spontaneously desorbed and exchanged with water under an ambient atmosphere and a hydrated phase [Cr₃O(OOCCF₃)₆(H₂O)₃]₃[α-PW₁₂O₄₀]·25H₂O [Ib] was formed. The water molecules were partially desorbed by the treatment of Ibin vacuo or under a dry N₂ or He flow at 298–303 K, and [Cr₃O(OOCCF₃)₆(H₂O)₃]₃[α-PW₁₂O₄₀]·15H₂O [Ic] was formed. The powder XRD pattern of Ic well agreed with that calculated for Ia, showing that the structure was maintained after the exchange and partial desorption of guests. Compound Ic sorbed CO₂ and unsaturated hydrocarbons, while saturated hydrocarbons such as ethane and methane were almost excluded despite the similar kinetic diameters. Acetylene/methane, CO₂/methane, and ethylene/ethane sorption ratios were 13, 15, and 4.9, respectively, at 198 K and 100 kPa. Typical Monte Carlo-based optimized geometries of acetylene and CO₂ showed different alignments in the structure of Ic despite the similar amounts of sorption and molecular sizes. Acetylene and CO₂ were aligned mainly parallel and perpendicular, respectively, to the layer, which is probably due to the differences in the electron density distributions of the HOMO orbitals.