Metal–organic framework supported ionic liquid membranes for CO2 capture: anion effects

Abstract

IRMOF-1 supported ionic liquid (IL) membranes are investigated for CO₂ capture by atomistic simulation. The ILs consist of identical cation 1-n-butyl-3-methylimidazolium [BMIM]⁺, but four different anions, namely hexafluorophosphate [PF₆]⁻, tetrafluoroborate [BF₄]⁻, bis(trifluoromethylsulfonyl)imide [Tf₂N]⁻, and thiocyanate [SCN]⁻. As compared with the cation, the anion has a stronger interaction with IRMOF-1 and a more ordered structure in IRMOF-1. The small anions [PF₆]⁻, [BF₄]⁻, and [SCN]⁻ prefer to locate near to the metal-cluster, particularly the quasi-spherical [PF₆]⁻ and [BF₄]⁻. In contrast, the bulky and chain-like [BMIM]⁺ and [Tf₂N]⁻ reside near the phenyl ring. Among the four anions, [Tf₂N]⁻ has the weakest interaction with IRMOF-1 and thus the strongest interaction with [BMIM]⁺. 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]⁻ < [PF₆]⁻ < [BF₄]⁻ < [SCN]⁻. This hierarchy is predicted by the COSMO-RS method, and largely follows the order of binding energy between CO₂ and anion estimated by ab initio calculation. In the [BMIM][SCN]/IRMOF-1 membrane with W_IL/IRMOF-1 = 1, [SCN]⁻ is identified to be the most favorable site for CO₂ adsorption. [BMIM][SCN]/IRMOF-1 outperforms polymer membranes and polymer-supported ILs in CO₂ permeability, and its performance surpasses Robeson's upper bound. This simulation study reveals that the anion has strong effects on the microscopic properties of ILs and suggests that MOF-supported ILs are potentially intriguing for CO₂ capture.