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

Abstract

Trabajo de Máster Universitario en Simulación Molecular (2023/24). Director: Dr. Felipe Jiménez Blas. Climate emergency has led to the investigation of CO2 valorisation routes capable of converting this greenhouse gas into a valuable resource. A competitive process included in this circular economy transition framework is the catalytic CO2 cycloaddition to epoxides, to produce cyclic carbonates. Halide-based Ionic liquids have been postulated to be a competitive choice to catalyse this reaction by their favourable physico-chemical properties, which can be enhanced towards specific objectives by cation and anion selection and functionalization. In this context of improvement, experimental and DFT theoretical studies have been carried out, leading to inconsistencies and contradictions regarding the cations and anions effects, resulting impossible until recently to set generalizations that permit the construction of an effective route for molecule design towards the optimization of the catalytic activity in terms of the carbonate yield. Latest studies have found that cation-anion dissociation constant is a key descriptor of the catalytic activity and is intimately linked to the cation-anion interaction strength. It has been found that cation-anion interaction is predominant, confirmed experimentally by low conductivities of the epoxide medium with dissolved Ionic Liquid. This result has led to question the most extended paradigm that considers ionic liquids ions as dissociated free interacting species, which is the fundament of the most extended reaction mechanism. In this work, the problem was addressed by a different simulation technique, classical molecular dynamics. To replicate experimental conditions, propylene epoxide and carbonate force fields found in the literature were evaluated in terms of their vapor-liquid phase equilibria. With the epoxide and carbonate ready and with CL&P force field model for ionic liquids, solubilities were tested for [N4444] and [N2222] cations combined independently with the halide anions [I], [Br] and [Cl]. Results showed that [N2222] cation-based ionic liquids crystalized and were therefore insoluble in the epoxide/carbonate medium, whereas [N4444] cation-based ionic liquids demonstrated slow diffusion. Both results were endorsed experimentally. Simulation visualizations showed ion pairs rather than dissociation species. Lastly, reaction medium interactions were studied between key atoms in terms of the reaction mechanism. It was found that cation-anion interaction follows the catalytic activity trend, being [I] the halide anion less associated with the three studied cations ([N4444], [bmim] and [emim]), and at the same time, being those ionic liquids the ones that displayed a better interaction between the cation and the epoxide Oxygen, phenomena that can be liked to epoxide activation and reaction intermediates stabilization. Therefore, the path towards the understanding of the mechanisms underlying the catalytic activity has been widen, with new simulation techniques that have enabled a better understanding of the complex interactions interplay present in this catalytic system.