Non-ideal behavior of ionic liquid mixtures to enhance CO2 capture

Abstract

In this work, the non-ideal behavior of ionic liquid (IL) mixtures is analyzed to explore potential synergistic effects on CO2 capture through physical absorption. For this purpose, a preliminary thermodynamic analysis was performed based on the quantum-chemical COSMO-RS method. The Henry's constants of CO2 in more than 400 (reciprocal or not) IL binary mixtures were computed at different compositions, and the predictions were compared with available experimental data of neat ILs and their binary mixtures. Henry's constant deviation (ΔKHCO2) was defined to expedite the analysis of the non-ideal mixing effects on the solubility of CO2 in IL binary mixtures. After, the effect of both the cation and anion constituents of the IL mixtures on the ΔKHCO2 values was systematically analyzed. The Kirkwood-Buff-based formalism was successfully applied to describe the Henry's constant behavior in the CO2 - IL mixture system. Thus, the activity coefficients of the IL components in the mixture, predicted by COSMO-RS, were demonstrated useful thermodynamic parameters to understand the CO2 solubility behavior in IL binary mixtures. Based on this analysis, a representative sample of 8 new imidazolium-based binary and reciprocal binary IL mixtures were experimentally evaluated by gravimetric measurements using a high pressure sorption analyzer with magnetic suspension balance. Their ability to absorb CO2 in the pressure range up to ca. 16 bar at 298.2 K was determined at equimolar composition of the IL mixture. Henry's constants were estimated from the CO2 absorption isotherms for pure ILs and IL mixtures. Overall results demonstrate significant synergistic effects of IL mixtures with positive deviations from ideality, which can be used to enhance the physical absorption for CO2 capture.