Recyclability of Encapsulated Ionic Liquids for Post-Combustion CO₂ Capture

Abstract

Ionic liquids (ILs) with aprotic heterocyclic anions (AHAs) are promising candidates for post-combustion carbon capture technologies since they react with CO2 stoichiometrically and reversibly. CO2 solubilities in two AHA-ILs, triethyl(octyl)phosphonium 2-cyanopyrrolide ([P2228][2CNPyr]) and triethyl(octyl)phosphonium benzimidazolide ([P2228][BnIm]), are reported for multiple temperatures and a third, triethyl(octyl)phosphonium 6-bromobenzimidazolide ([P2228][6-BrBnIm]), at one temperature. Ionic liquid [P2228][2CNPyr] and phase-change ionic liquid (PCIL) tetraethylphosphonium benzimidazolide ([P2222][BnIm]) were encapsulated in a chemically compatible and CO2-permeable polydimethylsiloxane (PDMS) polymer shell in order to enhance absorption and desorption kinetics. Both the free and encapsulated [P2228][2CNPyr] and [P2222][BnIm] were subjected to thermodynamic testing. The CO2 solubilities in the encapsulated IL and PCIL were in good agreement with the free IL and PCIL, meaning that the encapsulation of IL and PCIL greatly enhanced the kinetics of CO2 absorption while maintaining the high CO2 capture capacity. Recyclability testing was also performed on both the free and encapsulated [P2228][2CNPyr] and [P2222][BnIm]. The IL and PCIL materials, as well as the capsules, were stable upon cycling, with the CO2 capacities for each cycle remaining unchanged. The IL and the PCIL showed no sign of degradation after cycling, which demonstrated excellent performance.