Ionic liquid-pillared MXene membranes with tunable sub-angstrom channels for CO₂/N₂ separation

Two-dimensional (2D) lamellar membranes with sub-nanometer channels hold promise for precise gas separation, but tuning interlayer spacing at the sub-angstrom scale while synergizing solution-diffusion and molecular sieving remains challenging. Here we report the confinement of an ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF 4 ]), within the galleries of few-layer Al@Ti 3 C 2 T x nanosheets to create ionic liquid-confined lamellar MXene membranes (Al@Ti 3 C 2 T x -ILLMMs). The residual Al 3+ species in the nanosheets stabilize the interlayer architecture through electrostatic interactions, while the abundant surface terminations (-F, -OH) render the nanosheets negatively charged, promoting uniform IL distribution. By precisely controlling the IL loading via an equilibration–spin-coating approach, the interlayer spacing is continuously tunable from 0.309 to 0.375 nm. The optimal channel height of 0.342 nm lies within the gap between the kinetic diameters of CO 2 (0.33 nm) and N 2 (0.36 nm), closer to that of CO 2 . This precision engineering shifts the gas transport mechanism from Knudsen diffusion to a coupled solution-diffusion and size-sieving regime. The resulting membrane achieves a CO 2 permeance of 150 GPU and a CO 2 /N 2 selectivity of 128.5, representing 4.5-fold and 109-fold improvements over the pristine MXene membrane and offering a favorable balance compared to reported IL-supported membranes. The demonstrated quantitative loading-spacing correlation provides a designable platform for sub-angstrom channel engineering in 2D material membranes.