Ionic Liquid Membranes Supported by Hydrophobic and Hydrophilic Metal-Organic Frameworks for CO2 Capture

Atomistic simulation is performed to investigate CO2 capture in ionic liquid (IL) membranes supported on metal-organic frameworks (MOFs). The IL considered is 1-n-butyl-3-methylimidazolium thiocyanate [BMIM][SCN], while hydrophobic ZIF-71 and hydrophilic Na-rho-ZMOF with the same topology and similar pore size are used as supports. The [SCN](-) anion prefers to locate near the metal cluster of ZIF-71 and the Na+ ion of Na-rho-ZMOF, in contrast to the bulky and chainlike [BMIM](+) cation that resides in the open cage. In both membranes, the [SCN](-) interacts more strongly than the [BMIM](+) with the MOF supports. With regard to CO2 capture from CO2/N-2 mixture, CO2 adsorption is greater than N-2, while CO2 diffusion is slower in both membranes, particularly in [BMIM][SCN]/ZMOF because the Na+ ions in Na-rho-ZMOF act as strong binding sites for CO2. The permselectivity of CO2 over N-2 is governed by adsorption selectivity, as diffusion selectivity remains a constant over the pressure range examined. Compared to many polymer membranes and polymer-supported ILs, [BMIM][SCN]/ZMOF exhibits higher permeability and permselectivity, and also surpasses the Robeson's upper bound. On the basis of the two MOF-supported [BMIM][SCN] membranes examined for CO2 capture, the simulation study suggests that hydrophilic support is superior to the hydrophobic counterpart.