Solubility and dissociation of ionic liquids in epoxides and cyclic carbonate by molecular dynamics simulation

Abstract

Climate emergency has led to the investigation of CO₂ valorization routes. A competitive process included in this framework is the catalytic CO₂ cycloaddition to epoxides, to produce cyclic carbonates. Halide-based Ionic liquids (ILs) have been postulated to be a competitive choice. Nevertheless, the structure-performance relation for different ILs is still a topic of debate, being the cation-anion dissociation constant a key descriptor. In this work, the ions effect is tackled by Molecular Dynamics (MD) simulations. Propylene oxide and carbonate force fields were tested and used for 1,2-epoxyhexane and hexylene carbonate force field construction, while ILs were modelled by the CL&P force field. Solubilities in an epoxide-carbonate medium were tested for ILs composed of [$N{4444}^{+}$] or [$N{2222}^{+}$] cations combined with the halide anions: Iodide [$I^{-}$], Bromide [$B{r}^{-}$] and Chloride [$C{l}^{-}$]. Results showed that [$N{2222}^{+}$] cation-based ionic liquids were insoluble in the epoxide/carbonate medium, whereas [$N{4444}^{+}$] cation-based ionic liquids demonstrated diffusion. Reaction medium interactions were studied between key atoms for experimentally soluble ILs. It was found that cation-anion interaction follows the catalytic activity trend, being [$I^{-}$] the halide anion less associated with ([$N{4444}^{+}$], [$\mathrm{bmi}{m}^{+}$] and [$\mathrm{emi}{m}^{+}$]) cations. A correlation between the first peak integration of the radial distribution functions and the experimental yields (including [$N{1111}^{+}$][$B{r}^{-}$]) could be established with a regression coefficient of 0.86. Additionally, [$I^{-}$] based ILs displayed a better interaction between the cation and the epoxide oxygen, phenomena linked to epoxide activation and intermediates stabilization. Therefore, the path towards the understanding of this catalytic system has been widened.