Facile Ionization of the Nanochannels of Lamellar Membranes for Stable Ionic Liquid Immobilization and Efficient CO2 Separation

Two-dimensional (2D) lamellar membranes, with highly ordered nanochannels between the adjacent layers, have revealed potential application prospects in various fields. To separate gases with similar kinetic diameters, intercalation of a functional liquid, especially an ionic liquid (IL), into 2D lamellar membranes is proved to be an efficient method due to the capacity of imparting solubility-based separation and sealing undesired defects. Stable immobilization of a high content of liquid is challenging but extremely required to achieve and maintain high separation performance. Herein, we describe the intercalation of a typical IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), into the ionized nanochannels of sulfonated MXene lamellar membranes, where the sulfonate groups are anchored onto MXene nanosheets through a facile method based on metal-catechol chelating chemistry. Thanks to the intrinsic benefits of MXene as building blocks and the decorated sulfonate groups, the optimal membrane possesses adequate interlayer spacing (∼1.8 nm) and high IL uptake (∼47 wt %) and therefore presents a CO2 permeance of 519 GPU and a CO2/N2 selectivity of 210, outperforming the previously reported liquid-immobilized lamellar membranes. Moreover, the IL loss rate of the membrane within 7 days at elevated pressure (5 bar) is measured to be significantly decreased (from 43.2 to 9.0 wt %) after growing sulfonate groups on the nanochannel walls, demonstrating the excellent IL storage stability.