Parker, Qamreen; (2018) Molecular simulations of ionic liquids for CO₂ capture. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Ionic liquids (ILs) are molten salts at temperatures below 100 °C or at room temperature, achieving this by possessing ions that pack weakly, preventing the formation of a stable crystal lattice. The very low or non-existent volatility of the liquids is one of the most important reasons for why ILs are explored for carbon dioxide (CO2) capture, along with interesting properties such as thermal stability, nonflammability and tunability for high CO2 solubility. It is therefore important to understand, at a molecular level, the structure and properties of ILs. The work presented in this thesis employs classical molecular dynamics (MD) simulations to investigate ILs at varying temperature and varying loadings of CO2. Initially, the interatomic potentials or force field (FF) that can be used to simulate ILs are researched, before choosing two: The Generalised Amber FF (GAFF) and Canongia Lopez and Padua FF (CL&PFF). After a comparison of densities and structures resultant, further validation on the CL&PFF is reported. Subsequently, a phosphonium based IL, [P66614][NTf2], is investigated in a pure state at varying temperatures, to compare with experimental data. We study the density, structure and diffusion of the system, in terms of cation, anion and ion pair. We reinforce the results of our initial FF comparison, as the density is calculated to a high degree of accuracy compared with experiment. We continue to describe the influence of temperature on the structure and dynamics, comparing with experimental data where available. Finally, we considered the IL’s reported CO2 solubility and explored different loadings of CO2 in the IL system. We observe significant changes in IL structure and diffusion with even small loadings of CO2, along with interesting CO2 interactions and diffusion. Following on from this, we detail the initial modelling of a phosphonium based superbase ionic liquid, with a combination of FFs and scaling of atomic charges to better detail the diffusivity of the IL. Thus, in this thesis, we present ILs as a media that offers significant performance benefits when compared to traditional organic solvents for carbon capture. Additionally, we confirm the suitability of MD simulations for the accurate description and elucidation of structural and diffusive properties of ILs.

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