Task-Specific Ionic Liquids Functionalized by Cobalt(II) Salen for Room Temperature Biomimetic Dioxygen Binding Qinghe Zheng, Samuel John Thompson, Shaojun Zhou, Marty Lail,* Kelly Amato, Aravind V. Rayer, Jeff Mecham, Paul Mobley, Jianping Shen, and Brenda Fletcher Technology Advancement & Commercialization Division, RTI International, Durham, North Carolina 27709, United States * S Supporting Information 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 [P C1C2C3C4 ] + (C1 = C2 = C3 = 2, 4, or 6; C4 = 5, 8, or 16) and anions including N -methylglycinate ([NmGly] - ) or/and bis- (trifluoromethanesulfonyl)amide ([Tf 2 N] - ) 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 O 2 over N 2 . Among the TSILs, [P 2225 ] 2 [NmGly][Tf 2 N][Co(salen)] showed the highest O 2 sorption capacity, with maximum sorption of 4368 μLO 2 /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 O 2 diffusivity could be enhanced by lowering the dynamic viscosity of the TSILs, which resulted in a further increase in O 2 sorption capacity. 1. INTRODUCTION In biological systems, respiratory pigments (e.g., hemoglobins, myoglobins, hemerythrins, and hemocyanins) are able to reversibly bind dioxygen from the atmosphere without appreciable loss of activity. It is common knowledge that transition metals with certain organic coordination environ- ments play important roles in the oxygen binding in these natural oxygen carriers. Synthetic access to biomimetic substances containing a transition metal center that reversibly binds oxygen has attracted considerable interest. 1,2 By far the greatest number of synthetic oxygen complexes are based on cobalt, and some of the most thoroughly investigated systems are based on the Schiff base complex N,N′-bis(salicylidene)- ethylenediaminocobalt(II), i.e., (Co(II)salen) which is also labeled as “Salcomine”, and its ring-substituted derivatives. 3 The salen molecule is a tetradentate ligand coordinating to the central metal cation through hydroxyl and amine groups. 4 The cobalt salen complex is planar, which crystallizes in a layer of lattice, and has shown to uptake oxygen at a ratio of Co:O 2 = 2:1 (binuclear) in solid state and of 2:1 or 1:1 (mononuclear) in various solvation environments. 1 Unprocessed solid-phase Co(II)salen does not bind oxygen efficiently, which has prompted intensive research endeavors to improve the oxygenation rate of Co(II)salen, for example, by nano- particulate refining, by axial-adduct formation with mono- dentate Lewis bases promoting oxygen binding to the metal, and by stabilization of the Co-O 2 superoxo complexes via associated cations or tethered hydrogen bonding function- alities. 5-8 Room-temperature ionic liquids (RTILs) have become popular as nonvolatile organic solvent media for separation processes due to their inherent features such as tunable structure, negligible vapor pressure, nonflammability, high thermal stability, environmentally nontoxicity, and recycla- bility. 9-12 Over the past decade, developments in the design of organic salts have made it possible to create task-specific ionic liquids (TSILs), in which a functional group is covalently attached to the cation or the anion or both. This has enabled a supply of various functionalities to conventional IL media for different separation applications. 13 Kohno et al. first developed a thermoresponsive cobalt(II)- based TSIL that reversibly binds O 2 over N 2 with high selectivity. 14 This TSIL is a neat, liquid-state material that not only maintains the desired material properties of an IL but also can be impregnated onto porous materials or membranes for Received: August 30, 2018 Revised: November 21, 2018 Accepted: December 10, 2018 Published: December 10, 2018 Article pubs.acs.org/IECR Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.iecr.8b04224 Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by YORK UNIV on 12/21/18. For personal use only.