Exploiting the Pi-Complexation Capability of Silver(I) and Copper(I) Ions in Ionic Liquids for Olefin and Paraffin Separations

Abstract

Separating olefins from structurally-similar paraffins is difficult as they generally possess indistinguishable chemical properties. To achieve their separation, the selective and reversible π-complexation capability of silver(I) and copper(I) ions has been employed in separation systems. However, the utilization of traditional solvents (e.g., water and organic solvents) for dissolving such metal ions is limited as they are often plagued with solvent evaporation and poor facilitation of π-complexation, resulting in a diminished olefin/paraffin selectivity. Ionic liquids (ILs) are nonmolecular solvents possessing low vapor pressure, high thermal stability, a melting point lower than 100 °C, and structural tunability that can alternate traditional solvents in olefin/paraffin separation systems involving silver(I) and copper(I) ions. However, limited information on the π-complexation capability of such metal ions in IL media prevents their broader applications. In this study, experimental factors that may affect the interaction between olefins and silver(I) ion/IL media, such as silver salt type, IL structure, temperature, and exposure gas type, are systematically examined by gas chromatographic (GC) methods. When silver bis[(trifluoromethyl)sulfonyl]imide ([Ag+][NTf2-]) was dissolved in imidazolium-based ILs with [NTf2-] anions, stable silver(I) ion-olefin complexation could be achieved even under harsh conditions, including elevated temperatures and hydrogen atmosphere. Moreover, longer alkyl substituents on the imidazolium cation of the IL provided a stronger interaction between the silver(I) ion/IL stationary phase and olefins. Based on the optimized conditions, a theoretical equilibria model was developed to describe olefin partitioning between inert carrier gas (mobile phase), 1-decyl-3-imidazolium ([C10MIM+]) [NTf2-] IL solvent, and [Ag+][NTf2-] that was treated as a pseudophase. Analyte partition coefficients and thermodynamic parameters of solvation were determined using the model. Moreover, changes in olefin partition coefficients upon silver(I) ion reduction promoted by applying hydrogen-involving gas mixtures and elevated temperatures to the [Ag+][NTf2-]/[C10MIM+][NTf2-] IL stationary phases were monitored. As reduction proceeded within the silver(I) ion/IL stationary phase, chromatographic selectivity for olefins over paraffins tended to decrease gradually. Olefin/paraffin separation performance of other systems utilizing π-complexation capability of metal ions was also examined. By applying the poly(1-decyl-3-vinylimidazolium [NTf2-]) polymeric ionic liquid (PIL) impregnated with silver(I) ion as a GC stationary phase, silver(I) ion-olefin complexation was facilitated more effectively than the IL with structural similarities ([C10MIM+][NTf2-]). Partitioning of olefins and paraffins in a GC system involving copper(I) ion/IL stationary phases could also be studied using the chromatographic equilibria model. When [Cu+][NTf2-] was incorporated into the [C10MIM+][NTf2-] IL, copper(I) ion acted as a pseudophase similar to the case of silver(I) ion-containing IL. Interestingly, its π-complexation strength can be enhanced by heating to elevated temperatures while water introduction results in an opposite effect.