250997 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 (WIL/IRMOF-1), the selectivity of CO2/N2 at infinite dilution is enhanced. At a given WIL/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 WIL/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.