Metal-organic framework supported ionic liquid membranes for CO 2 capture: Anion effects

Abstract

IRMOF-1 supported ionic liquid (IL) membranes are investigated for CO 2 capture by atomistic simulation. The ILs consist of identical cation 1-n-butyl-3-methylimidazolium [BMIM] +, but four different anions, namely hexafluorophosphate [PF 6] -, tetrafluoroborate [BF 4] -, bis(trifluoromethylsulfonyl) imide [Tf 2N] -, 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 [PF 6] -, [BF 4] -, and [SCN] - prefer to locate near to the metal-cluster, particularly the quasi-spherical [PF 6] - and [BF 4] -. In contrast, the bulky and chain-like [BMIM] + and [Tf 2N] - reside near the phenyl ring. Among the four anions, [Tf 2N] - 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 CO 2/N 2 at infinite dilution is enhanced. At a given W IL/IRMOF-1, the selectivity increases as [Tf 2N] - < [PF 6] - < [BF 4] - < [SCN] -. This hierarchy is predicted by the COSMO-RS method, and largely follows the order of binding energy between CO 2 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 CO 2 adsorption. [BMIM][SCN]/IRMOF-1 outperforms polymer membranes and polymer-supported ILs in CO 2 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 CO 2 capture. © the Owner Societies 2012.