Polymeric ionic liquids containing copper(I) and copper(II) ions as gas chromatographic stationary phases for olefin separations

Abstract

Copper(I) ions (Cu+) are used in olefin separations due to their olefin complexing ability and low cost, but their instability in the presence of water and gases limits their widespread use. Ionic liquids (ILs) have emerged as stabilizers of Cu+ ions and prevent their degradation, providing high olefin separation efficiency. There is limited understanding into the role that polymeric ionic liquids (PILs), which possess similar structural characteristics to ILs, have on Cu+ ion-olefin interactions. Moreover, copper ions with diverse oxidation states, including Cu+ and Cu2+ ions, have been rarely employed for olefin separations. In this study, gas chromatography (GC) is used to investigate the interaction strength of olefins to stationary phases composed of the 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C6MIM+][NTf2−]) IL and the poly(1-hexyl-3-vinylimidazolium [NTf2−]) (poly([C6VIM+][NTf2−])) PIL containing monovalent and divalent copper salts (i.e., [Cu+][NTf2−] and [Cu2+]2[NTf2−]). The chromatographic retention of alkenes, alkynes, dienes, and aromatic compounds was examined. Incorporation of the [Cu2+]2[NTf2−] salt into a stationary phase comprised of poly(dimethylsiloxane) resulted in strong retention of olefins, while its addition to the [C6MIM+][NTf2−] IL and poly([C6VIM+][NTf2−]) PIL allowed for the interaction strength to be modulated. Olefins exhibited greater affinities toward IL and PIL stationary phases containing the [Cu2+]2[NTf2−] salt compared to those with the [Cu+][NTf2−] salt. Elimination of water from both copper salts was observed to be an important factor in promoting olefin interactions, as evidenced by increased olefin retention upon exposure of the stationary phases to high temperatures. To evaluate the long-term thermal stability of the stationary phase, chromatographic retention of probes was measured on the [Cu2+]2[NTf2−]/[C6MIM+][NTf2−] IL stationary phase after its exposure to helium at a temperature of 110 °C.