Selective Sorption of Olefins by Halogen-Substituted Macrocation-Polyoxometalate Porous Ionic Crystals

Ionic crystals [Cr3O(OOCCH2X)(6)(H2O)(3)](4)[alpha-SiW12O40]center dot 14H(2)O (X = Cl [Ia], Br [IIa]) are synthesized by complexation of halogen-substituted macrocations with a silicododecatungstate. Compounds Ia and IIa possess analogous one-dimensional winding channels, and the channel walls are composed of oxygen atoms of silicododecatungstates and -CH2X groups of macrocations. Water of crystallization in Ia (IIa) is desorbed by the treatment in vacuo or under a dry N-2 or He flow at 298-303 K, and the corresponding guest free phases Ib (IIb) are obtained. Propylene/propane and ethylene/ethane sorption ratios of Ib at 298 K and 100 kPa are 6.1 and 3.6, respectively. The ethylene/ethane sorption ratio of IIb at 298 K and 100 kPa is 2.8. These values are much larger than those of conventional sorbents. In situ C-13 MAS NMR spectra of ethylene or ethane sorbed in Ib suggest that host-guest interaction for ethylene is larger than that for ethane. DFT calculation and Monte Carlo simulation are carried out to elucidate the nature of the high ethylene/ethane sorption ratios: The bridging carboxylates (-OOCCH2X) are polarized by incorporation of highly electronegative halogens. Ethylene resides along the channel wall with the sigma(h) plane facing the channel wall, while ethane resides randomly in the channels of Ib and IIb. These results suggest that the olefin sorption is enhanced by electrostatic interaction between the it-electron of olefins and highly polarized halogen-substituted macrocations and/or silicododecatungstates.