Understanding the Molecular Features Controlling the Solubility Differences of R-134a, R-1234ze(E), and R-1234yf in 1-Alkyl-3-methylimidazolium Tricyanomethanide Ionic Liquids

Abstract

This work reports a study of the absorption of fluorinated refrigerant gases in ionic liquids (ILs) from a combined experimental and computational approach to gain insights into the solvation of these greenhouse gases. The results allow the selection of solvents for fluorinated gas recovery, based on molecular characteristics, and improve the recyclability of these high-value-added compounds. The solubility of R-134a, R-1234yf, and R-1234ze(E) was measured experimentally in 1-alkyl-3-methylimidazolium tricyanomethanide ILs at 283.15-323.15 K and up to 5 bar to investigate the influence of the alkyl side chain length of the cations in the ILs. Molecular dynamics was used to study solvation environments around refrigerant molecules in terms of radial distribution functions, coordination numbers, and spatial distribution functions. Simulations were carried out with the recent CL&Pol polarizable force field for ILs, which includes polarization effects explicitly. The force field was validated against experimental solubility through free-energy perturbation calculations. The main effects influencing the solubility of fluorinated refrigerants in ILs result from interactions with the anions and with the cation head group alkyl chain, setting a basis for a rational selection and design of ILs for processes to recover and recycle refrigerants in a circular economy model.