Accelerated Oxidative Degradation of Phosphonium-Type Ionic Liquid with l-Prolinate Anion: Degradation Mechanism and CO(2) Separation Performance

[Image: see text] Amino acid ionic liquids (AAILs) are regarded as green alternatives to existing CO(2)-sorptive materials because amino acids are 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 toward O(2), and the CO(2) separation performance is of particular importance. In the present study, the accelerated oxidative degradation of tetra-n-butylphosphonium l-prolinate ([P(4444)][Pro]), a model AAIL that has been widely investigated as a CO(2)-chemsorptive IL, is performed using a flow-type reactor system. Upon heating at 120–150 °C and O(2) gas bubbling to [P(4444)][Pro], both the cationic and anionic parts undergo oxidative degradation. The kinetic evaluation 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 CO(2) permeability and CO(2)/N(2) selectivity values in spite of the partial degradation of [P(4444)][Pro].