Using a Chromatographic Pseudophase Model To Elucidate the Mechanism of Olefin Separation by Silver(I) Ions in Ionic Liquids

Silver­(I) ions undergo selective olefin complexation and have been utilized in various olefin/paraffin separation techniques such as argentation chromatography and facilitated transport membranes. Ionic liquids (ILs) are solvents known for their low vapor pressure, high thermal stability, low melting points, and ability to promote a favorable solvation environment for silver­(I) ion–olefin interactions. To develop highly selective separation systems, a fundamental understanding of analyte partitioning to the stationary phase and the thermodynamic driving forces behind solvation is required. In this study, a chromatographic model treating silver­(I) ions as a pseudophase is constructed and employed for the first time to investigate the olefin separation mechanism in silver­(I) salt/IL mixtures. Stationary phases containing varying amounts of noncoordinated silver­(I) salt ([Ag⁺]­[NTf₂^–]) dissolved in the 1-decyl-3-methylimidazolium bis­[(trifluoromethyl)­sulfonyl]­imide ([C₁₀MIM⁺]­[NTf₂^–]) IL are utilized to determine the partition coefficients of various analytes including alkanes, alkenes, alkynes, aromatics, aldehyde, esters, and ketones. As ligand coordination to silver­(I) ions is known to lower its olefin complexation capability, this study also examines two different types of coordinated silver­(I) ion pseudophases, namely, monocoordinated silver­(I) salt ([Ag⁺(1-decyl-2-methylimidazole, DMIM)]­[NTf₂^–]) and dicoordinated silver­(I) salt ([Ag⁺(1-methylimidazole, MIM)­(DMIM)]­[NTf₂^–]). The extent of olefin partitioning to the coordinated silver­(I) ion pseudophases over the carrier gas and IL decreased by up to two orders of magnitude. Values for enthalpy, entropy, and free energy of solvation were determined for the three silver­(I) ion-containing systems. Olefin retention was observed to be enthalpically dominated, while ligand coordination to the silver­(I) ion pseudophase resulted in variations for both enthalpic and entropic contributions to the free energy of solvation. The developed model can be used to study chemical changes that occur in silver­(I) ions over time as well as identify optimal silver­(I) salt/IL mixtures that yield high olefin selectivity.