Improvements in the CO2 permeation selectivities of amino acid ionic liquid-based facilitated transport membranes by controlling their gas absorption properties

•Amino acid ionic liquids (AAILs) with different cation size were synthesized.•A design guideline of AAILs for high CO2 selective membrane was proposed.•The N2 solubility of the AAILs decreased with decreasing cation size.•The N2 permeability of AAIL based membranes decreased with decreasing AAIL size.•High CO2 selective membrane was fabricated by using the AAIL with small cation.

A series of amino acid ionic liquids (AAILs) composed of different cations of different sizes, including trihexyl(tetradecyl)phosphonium glycinate, tetrabutylphosphonium glycinate and triethyl(pentyl)phosphonium glycinate have been synthesized for the carrier of CO2 facilitated transport membrane. Their physical properties, including their viscosity, density, molar volume and N2 absorption amount were investigated. The amount of N2 absorption decreased as the size of the cation of the AAIL decreased. Facilitated transport membranes containing these AAILs were prepared, and their CO2 and N2 permeabilities were measured. The N2 permeabilities were systematically investigated both experimentally and theoretically with the aim of improving the selectivity of the CO2 permeation of the AAIL-based facilitated transport membrane. As expected, the CO2 permselectivity was improved using triethyl(pentyl)phosphonium glycinate, which contained the smallest cation of the AAILs investigated in this study. We have proposed a methodology for improving the CO2 selectivity of AAIL-based facilitated transport membranes based on reducing their N2 permeabilities.