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

Abstract

Climate emergency has led to the investigation of CO$_{2}$ valorization routes. A competitive process included in this framework is the catalytic CO$_{2}$ 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 [$\mathrm{N4444^+}$] or [$\mathrm{N2222^+}$] cations combined with the halide anions: Iodide [$\mathrm{I^-}$], Bromide [$\mathrm{Br^-}$] and Chloride [$\mathrm{Cl^-}$]. Results showed that [$\mathrm{N2222^+}$] cation-based ionic liquids were insoluble in the epoxide/carbonate medium, whereas [$\mathrm{N4444^+}$] 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 [$\mathrm{I^-}$] the halide anion less associated with ([$\mathrm{N4444^+}$], [$\mathrm{bmim^+}$] and [$\mathrm{emim^+}$]) cations. A correlation between the first peak integration of the radial distribution functions and the experimental yields (including [$\mathrm{N1111^+}$][$\mathrm{Br^-}$]) could be established with a regression coefficient of 0.86. Additionally, [$\mathrm{I^-}$] based ILs displayed a better interaction between the cation and the epoxide oxygen, phenomena linked to epoxide activation and intermediates stabilization.