Can the tricyanomethanide anion improve CO₂ absorption by acetate-based ionic liquids?

Area of Science:

• Materials Science
• Chemical Engineering
• Physical Chemistry

Background:

• Ionic liquids (ILs) are promising absorbents for carbon dioxide (CO2) capture.
• Understanding the absorption behavior of IL mixtures is crucial for optimizing CO2 capture processes.
• The influence of different anion compositions on CO2 solubility and mass transfer in ILs requires detailed investigation.

Purpose of the Study:

• To experimentally determine the CO2 absorption capacity of mixtures of 1-butyl-3-methylimidazolium acetate ([C4C1Im][OAc]) and 1-butyl-3-methylimidazolium tricyanomethanide ([C4C1Im][C(CN)3]).
• To investigate the effect of temperature, pressure, and anion composition on CO2 absorption.
• To elucidate the mechanisms of physical and chemical CO2 absorption and their contributions in IL mixtures.

Main Methods:

• Experimental determination of CO2 absorption in IL mixtures at varying temperatures (303-343 K) and pressures (up to 10 bar).
• Thermodynamic analysis to calculate equilibrium (Keq) and Henry's law (KH) constants.
• Identification of dissolved species to understand the stoichiometry of CO2-IL interactions.

Main Results:

• CO2 absorption decreased with increasing [C4C1Im][C(CN)3] concentration and temperature.
• Maximum CO2 absorption (0.4 mole fraction) observed in pure [C4C1Im][OAc] at 303 K.
• The tricyanomethanide anion reduced Henry's law constants, indicating increased physical absorption, and enhanced mass transfer by increasing mixture fluidity.

Conclusions:

• Chemical CO2 sorption occurs with a 1:2 stoichiometry between CO2 and the acetate-based IL.
• The [C4C1Im][C(CN)3] anion does not significantly alter the chemical reaction equilibrium but enhances physical absorption and mass transfer.
• These findings suggest that IL mixtures containing tricyanomethanide anions are promising for efficient CO2 capture due to improved absorption and mass transfer properties.