Critical Role in Structural Optimization and Activation of CAU‐23 Membranes for CO ₂ Separation

Abstract Metal–organic framework (MOF) membranes have emerged as promising candidates for energy‐efficient CO ₂ separations due to their tunable pore structures, high surface areas, and molecular‐level selectivity. However, their performance is highly dependent on both their structural integrity and effective activation to remove residual pore‐blocking species. In this study, the structural optimization and post‐synthetic activation of CAU‐23 membranes for CO ₂ separation are investigated. Membranes with two distinct morphologies are fabricated on porous alumina substrates via a secondary growth method. The effects of thermal treatment and methanol solvent exchange are systematically compared, revealing that methanol activation is significantly more effective in restoring pore accessibility and enhancing gas permeation. Single‐ and mixed‐gas permeation tests demonstrated that methanol‐activated fine‐grain CAU‐23 membranes exhibit outstanding separation performance, achieving CO ₂ /N ₂ and CO ₂ /CH ₄ separation factors as high as 95.3 and 318, respectively. These findings highlight the critical role of morphology engineering and activation strategy in unlocking the full potential of MOF membranes and position CAU‐23 as a competitive material for CO ₂ capture from flue gas and natural gas streams.