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Imidazolium-based ionic liquids (ILs) bearing an alkylphosphite anion, were highly efficient for the selective removal of acetylenes in olefins. Comparison of solubility data at 313 K and at atmospheric pressure shows that the solubilities of acetylene and propyne in 1,3-dimethylimidazolium methylphosphite ([DMIM][MeHPO(3)]) are about 45 and 20 times higher than those of ethylene and propylene, respectively. Computational and (1)H NMR results clearly demonstrate that there are substantial interactions between the acidic hydrogen atom or atoms of acetylenes and the phosphite anion.

Room temperature ionic liquids (RTILs) are proposed as the alternative solvents for the acetylene separation in ethylene generated from the naphtha cracking process. The solubility behavior of acetylene in RTILs was examined using a linear solvation energy relationship based on Kamlet-Taft solvent parameters including the hydrogen-bond acidity or donor ability (α), the hydrogen-bond basicity or acceptor ability (β), and the polarity/polarizability (π*). It is found that the solubility of acetylene linearly correlates with β value and is almost independent of α or π*. The solubility of acetylene in RTILs increases with increasing hydrogen-bond acceptor (HBA) ability of the anion, but is little affected by the nature of the cation. Quantum mechanical calculations demonstrate that the acidic proton of acetylene specifically forms hydrogen bond with a basic oxygen atom on the anion of a RTIL. On the other hand, although C-H···π interaction is plausible, all optimized structures indicate that the acidic protons on the cation do not specifically associate with the π cloud of acetylene. Thermodynamic analysis agrees well with the proposed correlation: the higher the β value of a RTIL is, the more negative the enthalpy of acetylene absorption in the RTIL is.