Activation-Controlled Structural Integrity in A520 MOF Membranes for Efficient CO₂/N₂ and CO₂/CH₄ Separation

Aluminum fumarate (A520) is a promising metal-organic framework (MOF) for gas adsorption and separation. However, the fabrication of dense, pristine polycrystalline A520 membranes has not yet been reported. Herein, we present a robust seeded-growth protocol to fabricate continuous A520 polycrystalline membranes with a thickness of approximately 3 μm on α-alumina substrates. Our results demonstrate that the post-synthetic activation process plays a critical role in determining membrane integrity. In particular, a methanol exchange (ME) pretreatment prior to thermal activation effectively suppresses lattice distortion induced by capillary stress during solvent evaporation, as evidenced by X-ray diffraction and gas permeation tests. The resulting A520 (ME) membranes exhibit excellent molecular sieving performance, with ideal CO₂/N₂ and CO₂/CH₄ selectivities of 71 and 112, respectively. Mixed-gas permeation measurements reveal pronounced composition-dependent separation behavior, where the CO₂/N₂ separation factor reaches 153 at a 20 mol % CO₂ feed, exceeding the 2019 Robeson upper bound, but decreases at higher CO₂ concentrations. This behavior is associated with composition-dependent competitive adsorption and diffusion within the confined A520 channels. Grand canonical Monte Carlo (GCMC) simulations reveal that increasing N₂ partial pressure enhances N₂ uptake but significantly reduces its diffusional mobility, suggesting that additional N₂ molecules occupy less favorable transport environments and experience increased diffusion resistance. In contrast, CO₂/CH₄ separation remains primarily governed by the intrinsic molecular sieving capability of the A520 framework, where the larger kinetic diameter of CH₄ results in stronger diffusion limitation. These results highlight the importance of controlled activation strategies and provide insights into the interplay between competitive adsorption and confined diffusion in A520 membranes for carbon capture applications.