Functional Solution Composed of Cu(I) Salt and Ionic Liquids to Separate Propylene from Propane

Polymeric ionic liquids containing copper(I) and copper(II) ions as gas chromatographic stationary phases for olefin separations

Copper(I) ions (Cu+) are used in olefin separations due to their olefin complexing ability and low cost, but their instability in the presence of water and gases limits their widespread use. Ionic liquids (ILs) have emerged as stabilizers of Cu+ ions and prevent their degradation, providing high olefin separation efficiency. There is limited understanding into the role that polymeric ionic liquids (PILs), which possess similar structural characteristics to ILs, have on Cu+ ion-olefin interactions. Moreover, copper ions with diverse oxidation states, including Cu+ and Cu2+ ions, have been rarely employed for olefin separations. In this study, gas chromatography (GC) is used to investigate the interaction strength of olefins to stationary phases composed of the 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C6MIM+][NTf2-]) IL and the poly(1-hexyl-3-vinylimidazolium [NTf2-]) (poly([C6VIM+][NTf2-])) PIL containing monovalent and divalent copper salts (i.e., [Cu+][NTf2-] and [Cu2+]2[NTf2-]). The chromatographic retention of alkenes, alkynes, dienes, and aromatic compounds was examined. Incorporation of the [Cu2+]2[NTf2-] salt into a stationary phase comprised of poly(dimethylsiloxane) resulted in strong retention of olefins, while its addition to the [C6MIM+][NTf2-] IL and poly([C6VIM+][NTf2-]) PIL allowed for the interaction strength to be modulated. Olefins exhibited greater affinities toward IL and PIL stationary phases containing the [Cu2+]2[NTf2-] salt compared to those with the [Cu+][NTf2-] salt. Elimination of water from both copper salts was observed to be an important factor in promoting olefin interactions, as evidenced by increased olefin retention upon exposure of the stationary phases to high temperatures. To evaluate the long-term thermal stability of the stationary phase, chromatographic retention of probes was measured on the [Cu2+]2[NTf2-]/[C6MIM+][NTf2-] IL stationary phase after its exposure to helium at a temperature of 110 °C.

Elucidating the role of temperature and water on the π-complexation strength of copper(I) ion-containing ionic liquids using inverse gas chromatography

BACKGROUND

The π-complexation capability of copper(I) ion has been exploited in olefin/paraffin separations, but its propensity of undergoing disproportionation to copper(II) ion and copper metal has limited its use. Imidazolium-based ionic liquids (ILs) can serve as solvents for copper(I) ions as they facilitate copper(I) ion-olefin complexation and can enhance its stability. To precisely monitor how copper(I) ions complex with olefins in ILs and evaluate the effects of environmental factors, it is necessary to construct an experimental platform capable of quantitatively measuring their molecular-level interactions.

RESULTS

This study employs an innovative inverse chromatography platform to measure changes in molecular-level interactions between copper(I) ions and olefins when the temperature and water content in the system are carefully controlled. Gas chromatographic stationary phases comprised of the 1-decyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C10MIM+][NTf2-]) IL containing 0.5 M [Cu+][NTf2-] were pre-heated to 140 °C maximizing copper(I) ion's π-complexation capability. The chromatographic retention of alkenes, dienes, and alkynes on the copper(I) ion/IL stationary phase was observed to be predominantly influenced by their partitioning between the carrier gas and copper(I) ion as well as between the IL and copper(I) ion. Upon introducing water to the system, the Gibbs free energy of solvation for olefins showed less favorable solvation into the stationary phase. In contrast, their solvation was significantly enhanced when the [Cu+][NTf2-]/[C10MIM+][NTf2-] stationary phase was heated to an elevated temperature, indicating that the π-complexation capability of copper(I) ion can be regenerated as needed.

SIGNIFICANCE

This study demonstrates that the stability of copper(I) ions can be improved by dissolving them into appropriate IL solvents. Moreover, the olefin separation performance of the copper(I) ion/IL stationary phase was found to be adjustable by the application of different column treatment conditions (i.e., heating and water introduction), opening the possibility of devising more stable, reliable, and efficient olefin separation systems based on copper(I) ion and IL solvents.

Alternatives to Cryogenic Distillation: Advanced Porous Materials in Adsorptive Light Olefin/Paraffin Separations

As primary feedstocks in the petrochemical industry, light olefins such as ethylene and propylene are mainly obtained from steam cracking of naphtha and short chain alkanes (ethane and propane). Due to their similar physical properties, the separations of olefins and paraffins-pivotal processes to meet the olefin purity requirement of downstream processing-are typically performed by highly energy-intensive cryogenic distillation at low temperatures and high pressures. To reduce the energy input and save costs, adsorptive olefin/paraffin separations have been proposed as promising techniques to complement or even replace cryogenic distillation, and growing efforts have been devoted to developing advanced adsorbents to fulfill this challenging task. In this Review, a holistic view of olefin/paraffin separations is first provided by summarizing how different processes have been established to leverage the differences between olefins and paraffins for effective separations. Subsequently, recent advances in the development of porous materials for adsorptive olefin/paraffin separations are highlighted with an emphasis on different separation mechanisms. Last, a perspective on possible directions to push the limit of the research in this field is presented.