Task-Specific Ionic Liquids Functionalized by Cobalt(II) Salen for Room Temperature Biomimetic Dioxygen Binding

Abstract

Development of task-specific ionic liquids (TSILs) with delicately designed chemical functionalities allow selective chemisorption of specific gas components for separation applications based on membrane or sorbent systems. The present study explored syntheses of various types of TSILs for biomimetic dioxygen binding for room-temperature air separation. IL ligands composed of different combinations of phosphonium-type cations [PC1C2C3C4]+ (C1 = C2 = C3 = 2, 4, or 6; C4 = 5, 8, or 16) and anions including N-methylglycinate ([NmGly]−) or/and bis(trifluoromethanesulfonyl)amide ([Tf2N]−) were synthesized and complexed with Co(II)salen to yield 11 TSIL samples. Material characterizations (NMR, CHN, FT-IR, and UV–vis) confirmed the formations of the expected material structures and the coordination bonds between IL ligands and Co(II) metal centers. The dioxygen sorption capacity and reversibility of the as-synthesized TSILs were investigated via both gravimetric and volumetric approaches under various gas sorption conditions. Under ambient conditions, the studied TSILs were all found to selectively absorb O2 over N2. Among the TSILs, [P2225]2[NmGly][Tf2N][Co(salen)] showed the highest O2 sorption capacity, with maximum sorption of 4368 μL O2/g sample and nearly half of the capacity reached within the initial 30 min of sorption time on stream. By increasing sorption temperature from 25 to 50 °C, the O2 diffusivity could be enhanced by lowering the dynamic viscosity of the TSILs, which resulted in a further increase in O2 sorption capacity.