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

An ionic crystal [Cr3O(OOCCF3)(6)(H2O)(3)](3)[alpha-PW12O40]center dot 8CH(3)COCH(3)center dot 8H(2)O center dot 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 angstrom, 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)[alpha-PW12O40]center dot 25H(2)O [Ib] was formed. The water molecules were partially desorbed by the treatment of Ib in vacuo or under a dry N-2 or He flow at 298-303 K, and [Cr3O(OOCCF3)(6)(H2O)(3)](3)[alpha-PW12O40]center dot 15H(2)O [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.