(591d) Elucidating the Effect of Ionic Liquid Structure on the Separation of Hydrofluorocarbon Mixtures: A Molecular Modeling Study

Global warming concerns have led world leaders to call for the phaseout of high global warming potential (GWP) hydrofluorocarbon (HFC) refrigerants. Ionic liquids (ILs) are being considered as potential solvents which can be used to separate azeotropic HFC mixtures and recycle low GWP HFCs.

Molecular dynamics (MD) simulations were used to study mixtures consisting of CH₂F₂ (HFC-32) and CHF₂CF₃ (HFC-125) along with four ILs including 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]), 1-n-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]), and 1-n-hexyl-3-methylimidazolium chloride ([HMIM][Cl]). The mixtures consisted of pure HFC-32 and HFC-125 at different concentrations as well as a 50:50 wt% mixture of the two HFCs (known as HFC-410A). Extensive analyses were carried out to understand the effect of IL structure on various properties of the HFC/IL mixtures. Density, diffusivity, and viscosity of the pure ILs were calculated and compared with experimental values. The good agreement between computed and experimental results suggests that the applied force fields are reliable. The calculated center of mass radial distribution functions (RDFs), partial RDFs, spatial distribution functions and coordination numbers provide a sense of how the distribution of HFC changes with IL structure. Detailed analysis reveals that the cation and anion represent opposite selectivity towards HFC-32 and HFC-125 among the studied ILs and work synergistically.

Next, we report an accurate and efficient alchemical free energy calculation workflow for calculating HFC solubility isotherms of the HFC/IL binary mixtures mentioned above. The workflow includes thermodynamic cycle construction, system minimization and equilibration, Hamiltonian replica exchange molecular dynamics production runs in the isothermal-isobaric ensemble, free energy difference estimation, phase equilibrium calculation, and isotherm interpolation. Convergence checks ensured equilibration of the production runs, which is important given the sluggish dynamics of ILs. Cross-comparison between multiple free energy estimators, including thermodynamic integration, free energy perturbation, and multistate Bennett acceptance ratio method, ensured that sufficient sampling and reasonable spacing of the alchemical intermediate states were used. The Peng-Robinson equation of state was shown to be an accurate and efficient way to calculate the excess chemical potential of HFCs in the vapor phase. Our simulations can perfectly reproduce experimental HFC-125 results, and slightly overestimate experimental HFC-32 solubility. Overall, simulated Henry's law constants, isotherm curvature, and solubility trends match experimental results very well.

The molecular insight provided in the current work and the solubility calculations help elucidate the effect of IL structure on HFC separation performance and set the stage for future screening studies to search for ILs optimized to separate azeotropic HFC mixtures.