Controlled Secondary Growth of CAU-1-NH₂ Membranes with Improved CO₂ Separation Performance

Metal-organic framework (MOF) membranes have attracted increasing interest for energy-efficient gas separation due to their tunable pore structures and selective adsorption properties. In this study, CAU-1-NH₂ membranes were fabricated on porous α-alumina substrates via a seeded growth method. The effects of precursor concentration and ligand-to-metal ratio in the secondary growth solution on membrane morphology and gas separation performance were systematically investigated. Among the five membrane variants synthesized, the optimized CAU-1-NH₂(B) membrane exhibited a continuous and low-defect structure, as confirmed by scanning electron microscopy and confocal fluorescence microscopy. Single-gas permeation measurements on the CAU-1-NH₂(B) membrane at 35 °C and 3 bar showed H₂, CO₂, N₂, and CH₄ permeances of 207.1, 110.2, 6.0, and 6.6 GPU, respectively, corresponding to ideal CO₂/N₂ and CO₂/CH₄ selectivities of 19.4 and 17.6. Mixed-gas permeation tests revealed significantly enhanced separation performance, with CO₂/N₂ separation factors ranging from 59.3 to 89.2 under various feed compositions. Grand canonical Monte Carlo (GCMC) simulations further indicate that strong competitive adsorption of CO₂ within the CAU-1-NH₂ framework plays a dominant role in governing the observed mixed-gas separation behavior. These findings demonstrate the potential of CAU-1-NH₂ membranes for efficient CO₂ separation.