Nanoporous Ti₃C₂O₂ monolayer towards high permeance and selectivity for CO₂/N₂ separation: CO₂-MXene interaction tuning

MXene, as a new gas separation membrane, has good stability and suitable pore topological structures to mitigate the greenhouse effect caused by excessive CO₂ emission. However, the large quadrupole and polarizability of CO₂ make it a strong interaction with MXene, thus leading to a strong solubility and weak diffusivity of CO₂. Here, we design nanoporous Ti₃C₂O₂ monolayers (Y_mX_n) with exposed C atoms that could tune CO₂-Ti₃C₂O₂ interaction so as to improve gas separation performance by density functional theory and molecular dynamics simulation. The scope of “Cut-off” size is obtained by analyzing the gas separation state from un-permeation to permeation. Results reveal that nanoporous Ti₃C₂O₂ membranes achieve both outstanding CO₂ selectivity and permeance. The best-performance Y₃X₃-Ti possesses CO₂ permeance of 1.01 × 10^-4 and 2.56 × 10^-5 mol s⁻¹ m⁻² Pa⁻¹ for equimolar gas mixtures and flue gases, respectively, and both the CO₂/N₂ selectivity reach 100 %. The gas separation mechanism is dominated by diffusion selectivity rather than solution selectivity in nanoporous Ti₃C₂O₂. The dynamic processes of gas permeation, including the center of mass, mean square displacement, and radial distribution function of the gases, are precisely investigated. The electrostatic repulsion between the C atoms in the channel of the membrane and CO₂ follows the sequence Y₃X₃-C > Y₂X₄ > Y₃X₃-Ti, which is the key to regulating gas diffusion. Results of this work highlight that tuning gas-membrane interaction and designing “Cut-off” size are effective strategies to achieve excellent CO₂ separation performance, and shed new light on the intrinsic mechanism in high-performance gas separation membranes. © 2024 Elsevier B.V.