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

Abstract

Ionic crystals [Cr3O(OOCCH2 X)6(H2O)3]4[α-SiW12O40]·14H2O (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 −CH2 X groups of macrocations. Water of crystallization in Ia (IIa) is desorbed by the treatment in vacuo or under a dry N2 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 13C 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 (−OOCCH2 X) are polarized by incorporation of highly electronegative halogens. Ethylene resides along the channel wall with the σh 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 π-electron of olefins and highly polarized halogen-substituted macrocations and/or silicododecatungstates.

Microporous metal–organic frameworks for storage and separation of small hydrocarbons

“…The unsaturated hydrocarbons acetylene, ethylene, and propylene display the side- ) was reported by Mizuno and co-workers. 105 They are built up from halogen-substituted macrocations and polyoxometalates (POMs). These two compounds possess analogous one-dimensional winding channels, and the channel 30 walls are composed of oxygen atoms of silicododecatungstates and CH 2 X groups of macrocations.…”

Microporous cesium salts of tetravalent Keggin-type polyoxotungstates Cs4[SiW12O40], Cs4[PW11O39(Sn-n-C4H9)], and Cs4[PW11O39(Sn-OH)] and their adsorption properties

“…Ionic crystals, which are important families of crystalline materials, have been attracting considerable interest because of their unique chemical and physical properties, which depend on their crystalline structures and constituent ions [1][2][3][4][5][6][7][8][9]. Normally, cations and anions are densely packed in the crystal lattice, with the coordination numbers determined by the relative sizes and charges of the constituent ions since the Coulombic forces are strong and directionless.…”

Alternatives to Cryogenic Distillation: Advanced Porous Materials in Adsorptive Light Olefin/Paraffin Separations

“…Mizuno et al reported that olefins such as ethylene and propylene are preferentially adsorbed in the ionic crystals [Cr 3 O(OOCCH 2 X ) 6 (H 2 O) 3 ] 4 [α‐SiW 12 O 40 ]·14H 2 O ( X = Cl or Br) . Different from MOFs where metal moieties are interconnected via coordination bonds, these porous ionic crystals are composed with 0‐D coordination complexes and polyoxometalate anions ( Figure a,b).…”

“…d) Local structure of the 1D channel. e) C 2 H 4 (circle) and C 2 H 6 (square) sorption isotherms of [Cr 3 O(OOCCH 2 Cl) 6 (H 2 O) 3 ] 4 [α‐SiW 12 O 40 ] and [Cr 3 O(OOCCH 2 Br) 6 (H 2 O) 3 ] 4 [α‐SiW 12 O 40 ] at 298 K. Modified and reproduced with permission . Copyright 2012, American Chemical Society.…”