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

Ionic crystals [Cr3O­(OOCCH2X)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 −CH2X 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 (−OOCCH2X) 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.

离子晶体[Cr₃O(OOCCH₂X)₆(H₂O)₃]₄[α-SiW₁₂O₄₀]·14H₂O（X=Cl [Ia]、Br [IIa]）通过卤素取代大阳离子与硅十二钨酸根（silicododecatungstate）的配位反应合成。化合物Ia与IIa具有相似的一维蜿蜒通道，通道壁由硅十二钨酸根的氧原子与大阳离子的−CH₂X基团构成。Ia（IIa）中的结晶水可通过在298~303 K下真空处理或于干燥N₂、He气流中脱除，得到相应的无客体相Ib（IIb）。在298 K、100 kPa条件下，Ib对丙烯/丙烷、乙烯/乙烷的吸附比分别为6.1和3.6；IIb在相同条件下对乙烯/乙烷的吸附比为2.8。上述数值远高于传统吸附剂的对应水平。原位13C魔角旋转核磁共振（13C MAS NMR）光谱研究表明，吸附于Ib中的乙烯与宿主的相互作用强于乙烷。通过密度泛函理论（DFT）计算与蒙特卡洛（Monte Carlo）模拟，阐明了该高乙烯/乙烷吸附比的本质：高电负性卤素的引入使桥连羧基（−OOCCH₂X）发生极化。乙烯以σ_h平面朝向通道壁的方式沿通道壁排布，而乙烷则在Ib与IIb的通道中无规分布。上述结果表明，烯烃的吸附可通过烯烃π电子与高极化的卤素取代大阳离子及/或硅十二钨酸根之间的静电相互作用得到增强。