Atomistic Simulation to Design MOF-Supported Ionic Liquid Membranes for CO2 Capture

Carbon capture and sequestration (CCS) is now not only a scientific interest, but a societal issue for environmental protection. There has been increasing interest to use neat as well as supported ionic liquids (ILs) for CO₂ capture. Nevertheless, most studies to date have used polymeric and inorganic supports to prepare supported ILs. In the last decade, metal-organic frameworks (MOFs) have emerged as a new class of porous materials. The enormous choices of organic linkers and metal oxides allow the structures and functions of MOFs to be tuned in a rational manner, thus we envision that MOFs are versatile supports to fabricate supported IL for CO₂ separation from flue gas.

In the current work, atomistic simulations have been performed to investigate CO₂ separation in IRMOF-1 supported IL membranes. In addition, the microscopic properties of ILs in IRMOF-1 are also investigated. The ILs consist of identical cation 1-n-butyl-3-methylimidazolium [BMIM]⁺, but four different anions, namely hexafluorophosphate [PF₆]ˉ, tetrafluoroborate [BF₄]ˉ, bis(trifluoromethylsulfonyl)imide [Tf₂N]ˉ, and thiocyanate [SCN]ˉ. The cations and anions in IRMOF-1 are more packed compared with bulk phase due to the confinement effect. The simulation results also suggest that anion has a stronger interaction with IRMOF-1 than cation. The small anions [PF₆]ˉ, [BF₄]ˉ, and [SCN]ˉ prefer to locate near metal-cluster, particularly the quasi-spherical [PF₆]ˉ and [BF₄]ˉ. In contrast, the bulky and chain-like [BMIM]⁺ and [Tf₂N]ˉ reside near phenyl ring. Among the four anions, [Tf₂N]ˉ has the weakest interaction with IRMOF-1 and thus the strongest interaction with [BMIM]⁺.

Different IL loadings into IRMOF-1 are also considered to elucidate the performance of membranes towards CO₂ capture. With increasing the weight ratio of IL to IRMOF-1 (W_IL/IRMOF-1), the selectivity of CO₂/N₂ at infinite dilution is enhanced. At a given W_IL/IRMOF-1, the selectivity increases as [Tf₂N]ˉ < [PF₆]ˉ < [BF₄]ˉ < [SCN]ˉ. This is in accordance with the prediction from COSMO-RS method and largely similar to the order of binding energy between CO₂ and anion. In [BMIM][SCN]/IRMOF-1 membrane with W_IL/IRMOF-1 = 1, [SCN]ˉ is identified to be the most favorable site for CO₂ adsorption. [BMIM][SCN]/IRMOF-1 outperforms polymer-supported ILs in CO₂ permeability and its performance surpasses the Robeson's upper bound. This simulation study reveals that anion has strong effects on the microscopic properties of ILs and suggests that MOF-supported ILs are potentially intriguing for CO₂ capture.