Molecular dynamics simulations of CO2 permeation through ionic liquids confined in γ-alumina nanopores

Abstract

CO2 permeation through imidazolium-based ionic liquids (ILs, [BMIM][Ac], [EMIM][Ac], [OMIM][Ac], [BMIM][BF4], and [BMIM][PF6]) confined in 1.0, 2.0, and 3.5 nm γ-alumina pores was investigated using molecular dynamics simulation. It was found that the nanopore confinement effect influenced the structure of confined ILs greatly, resulting in a layered structure and anisotropic orientation of ILs. In the center of 2.0-nm pore, the long alkyl chain of [BMIM]+ tended to be parallel to the wall, providing a straight diffusion path benefiting the CO2 permeation. The CO2 diffusion coefficients in confined [EMIM][Ac], [BMIM][Ac], and [OMIM][Ac] were 2.3–4.1, 2.4–6.4, and 14.4–21.7 × 10−10 m2 s−1, respectively. This order was opposite to that in the bulk ILs, because the longer alkyl chain led to a more ordered structure, facilitating CO2 diffusion. In addition, the CO2 solubilities were 445–722 mol m−3 MPa−1 for the five ILs confined in 1.0 nm pore, which were larger than those in 2.0 and 3.5 nm pores (196–335 mol·m−3 MPa−1), due to the larger free volume. Both parallel orientation of alkyl chain and large free volume could increase the CO2 permeability in confined ILs.