Examining the Influence of Silver(I) Ion Coordination Environment in Ionic Liquids on Olefin–Paraffin Separations using Inverse Gas Chromatography

Silver(I) ions (Ag⁺) undergo selective π-complexation with olefins and have been employed as separation media for the isolation of olefins from structurally similar paraffins. Ionic liquids (ILs) possess minimal vapor pressures, exceptional thermal stabilities, low melting points, as well as provide a favorable environment for π-complexation between Ag⁺ ions and olefins. The development of molecular drivers capable of highly selective olefin/paraffin separation systems with Ag⁺-containing ILs necessitates a comprehensive understanding of all factors that affect olefin solubility and selectivity. This study examines how coordinating different ligand species to Ag⁺ ions produces separation media with varying interaction strengths to olefins. Four coordination compounds, (4,4'-dimethyl-2,2'-bipyridine)silver(I) bis[(trifluoromethyl)sulfonyl]imide ([Ag⁺(DMBP)][NTf₂^-]), bis(pyridine)silver(I) [NTf₂^-] ([Ag⁺(Py)₂][NTf₂^-]), bis(2,6-lutidine)silver(I) [NTf₂^-] ([Ag⁺(Lut)₂][NTf₂^-]), and (triphenylphosphine)silver(I) [NTf₂^-] ([Ag⁺(PPh₃)][NTf₂^-]) were dissolved in the 1-decyl-3-methylimidazolium [NTf₂^-] ([DMIM⁺][NTf₂^-]) IL and employed as stationary phases for inverse gas chromatography. Ligand coordination to the Ag⁺ ion was observed to modulate interactions of unsaturated hydrocarbons. The [Ag⁺(Py)₂][NTf₂^-] complex offered the greatest olefin retention among the coordination complexes reaching 54% of the 1-octene retention factor of the uncoordinated [Ag⁺][NTf₂^-]. Hydrogen (H₂) exposure studies showed ligand-dependent rates of reduction from Ag⁺ ion to elemental silver (Ag⁰). The [Ag⁺(PPh₃)][NTf₂^-] complex exhibited superior stability, compared to the neat [Ag⁺][NTf₂^-] salt, reducing the retention factor of 1-octene by 15.3% and 19.4%, respectively, after 200 h of H₂ exposure at 70 °C. The results from this study show that coordination complexes with Ag⁺ ions are useful in highly selective and efficient petroleum processing systems.