(771f) 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.