Accelerated Oxidative Degradation of Phosphonium-Type Ionic Liquid with l-Prolinate Anion: Degradation Mechanism and CO2 Separation Performance

Amino acid ionic liquids (AAILs) are regarded as greenalternativesto existing CO2-sorptive materials because amino acidsare readily available from renewable sources in large quantities.For widespread applications of AAILs, including direct air capture,the relationship between the stability of AAILs, especially towardO(2), and the CO2 separation performance is ofparticular importance. In the present study, the accelerated oxidativedegradation of tetra-n-butylphosphonium l-prolinate ([P-4444]-[Pro]), a model AAIL that has beenwidely investigated as a CO2-chemsorptive IL, is performedusing a flow-type reactor system. Upon heating at 120-150 degrees Cand O-2 gas bubbling to [P-4444]-[Pro], both thecationic and anionic parts undergo oxidative degradation. The kineticevaluation of the oxidative degradation of [P-4444]-[Pro]is performed by tracing the decrease in the [Pro](-) concentration. Supported IL membranes composed of degraded [P-4444]-[Pro] are fabricated, and the membranes retain CO2 permeability and CO2/N-2 selectivityvalues in spite of the partial degradation of [P-4444]-[Pro].

Automated summary

Amino acid ionic liquids (AAILs) are considered as environmentally friendly alternatives to current CO2-sorptive materials due to the availability of amino acids from renewable sources. The stability of AAILs, especially towards O(2), is crucial for their wide applications including direct air capture. This study investigates the accelerated oxidative degradation of tetra-n-butylphosphonium l-prolinate ([P-4444]-[Pro]), a commonly studied AAIL, and demonstrates the fabrication of supported IL membranes composed of degraded [P-4444]-[Pro] that retain CO2 permeability and CO2/N-2 selectivity values.