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

Silver(I)\nions (Ag<sup>+</sup>) undergo selective π-complexation\nwith olefins and have been employed as separation media for the isolation\nof olefins from structurally similar paraffins. Ionic liquids (ILs)\npossess minimal vapor pressures, exceptional thermal stabilities,\nlow melting points, as well as provide a favorable environment for\nπ-complexation between Ag<sup>+</sup> ions and olefins. The\ndevelopment of molecular drivers capable of highly selective olefin/paraffin\nseparation systems with Ag<sup>+</sup>-containing ILs necessitates\na comprehensive understanding of all factors that affect olefin solubility\nand selectivity. This study examines how coordinating different ligand\nspecies to Ag<sup>+</sup> ions produces separation media with varying\ninteraction strengths to olefins. Four coordination compounds, (4,4′-dimethyl-2,2′-bipyridine)silver(I)\nbis[(trifluoromethyl)sulfonyl]imide ([Ag<sup>+</sup>(DMBP)][NTf<sub>2</sub><sup>–</sup>]), bis(pyridine)silver(I) [NTf<sub>2</sub><sup>–</sup>] ([Ag<sup>+</sup>(Py)<sub>2</sub>][NTf<sub>2</sub><sup>–</sup>]), bis(2,6-lutidine)silver(I) [NTf<sub>2</sub><sup>–</sup>] ([Ag<sup>+</sup>(Lut)<sub>2</sub>][NTf<sub>2</sub><sup>–</sup>]), and (triphenylphosphine)silver(I) [NTf<sub>2</sub><sup>–</sup>] ([Ag<sup>+</sup>(PPh<sub>3</sub>)][NTf<sub>2</sub><sup>–</sup>]) were dissolved in the 1-decyl-3-methylimidazolium\n[NTf<sub>2</sub><sup>–</sup>] ([DMIM<sup>+</sup>][NTf<sub>2</sub><sup>–</sup>]) IL and employed as stationary phases for inverse\ngas chromatography. Ligand coordination to the Ag<sup>+</sup> ion\nwas observed to modulate interactions of unsaturated hydrocarbons.\nThe [Ag<sup>+</sup>(Py)<sub>2</sub>][NTf<sub>2</sub><sup>–</sup>] complex offered the greatest olefin retention among the coordination\ncomplexes reaching 54% of the 1-octene retention factor of the uncoordinated\n[Ag<sup>+</sup>][NTf<sub>2</sub><sup>–</sup>]. Hydrogen (H<sub>2</sub>) exposure studies showed ligand-dependent rates of reduction\nfrom Ag<sup>+</sup> ion to elemental silver (Ag<sup>0</sup>). The\n[Ag<sup>+</sup>(PPh<sub>3</sub>)][NTf<sub>2</sub><sup>–</sup>] complex exhibited superior stability, compared to the neat [Ag<sup>+</sup>][NTf<sub>2</sub><sup>–</sup>] salt, reducing the retention\nfactor of 1-octene by 15.3% and 19.4%, respectively, after 200 h of\nH<sub>2</sub> exposure at 70 °C. The results from this study\nshow that coordination complexes with Ag<sup>+</sup> ions are useful\nin highly selective and efficient petroleum processing systems.