Complex Olefin Separations by Coordinated Silver(I) Containing Ionic Liquid Stationary Phases Using Comprehensive Two-Dimensional Gas Chromatography

Olefins containing single or multiple carbon-carbon double bonds serve as vital intermediates in the chemical and materials industries. The development of highly selective separation media for olefin separations has become a growing need due to their increased use as platform chemicals. Silver(I) ([Ag⁺]) containing ionic liquids (ILs) have emerged as favorable separation media as they can be employed across numerous separation technologies, given that they undergo selective, reversible π-complexation with olefins. This study evaluates six coordinated [Ag⁺]/IL mixtures based on (triphenylphosphine)silver(I) bis[(trifluoromethyl)sulfonyl]imide ([Ag⁺(PPh₃)][NTf₂⁻]) and bis(pyridine)silver(I) [NTf₂⁻] ([Ag⁺(Py)₂][NTf₂⁻]) complexes dissolved in the 1-butly-3-methylimidazolium [NTf₂⁻] ([BMIM⁺][NTf₂⁻]) and 1-(2-(2-methoxyethoxy)ethyl)-3-methylimidazolium [NTf₂⁻] ([PEG₂MIM⁺][NTf₂⁻]) ILs as stationary phases in comprehensive two-dimensional gas chromatography (GC × GC) and examines their chromatographic selectivity in separating structurally similar olefins, diolefins, and paraffins. Coordinated [Ag⁺]/IL mixtures capable of strong π-complexation provided better selectivity and resolution among the tested stationary phases. Specifically, the [Ag⁺(Py)₂][NTf₂⁻]/[BMIM⁺][NTf₂⁻] mixture achieved a resolution of 3.16 between n-octane and trans-2-octene when used as a second dimension stationary phase. When separating diolefins, 1,5-hexadiene exhibited a more favorable free energy of solvation (ΔG) compared to 1,6-heptadiene and 1,7-octadiene for all stationary phases tested. The ΔG values at 55 °C using the [Ag⁺(PPh₃)][NTf₂⁻]/[PEG₂MIM⁺][NTf₂⁻] stationary phase for 1,5-hexadiene (-16,627 ± 6 J mol⁻¹) and 1,7-octadiene (-15,505 ± 7 J mol⁻¹) suggest that chelation and olefin molecular structure outweigh olefin carbon chain length in governing [Ag⁺]-olefin interactions. Results from this work demonstrate olefin selectivity can be fine-tuned by methodically altering the [Ag⁺] complex and IL, highlighting their utility in complex hydrocarbon separations.