Enhanced CO₂ Uptake in Cobalt-Based MOFs via Textural Optimization and Water-Induced Flexibility

Developing metal-organic frameworks (MOFs) that combine high CO₂ uptake, ultramicroporosity, and stability under humid conditions remains a challenge for practical carbon capture. Here, a rational and optimized synthetic strategy allows to eliminate Co(OH)₂ impurities, unlocking previously inaccessible ultramicroporosity in a robust cobalt-based MOF, constructed from Co(II) nodes and the multitopic 3,6 N-ditriazolyl-2,5-dihydroxy-1,4-benzoquinone (trz₂An) linker. This protocol increases the accessible surface area of 50% while preserving a narrow pore size distribution centered at 3.6 Å, critical for CO₂:N₂ selectivity. Static CO₂ adsorption measurements reveal uptake values of 5 and 4 mmol g^-1 at 0 and 30 °C, respectively. Dynamic breakthrough experiments with 5-10% CO₂:N₂ mixtures demonstrate excellent separation performance and stability over 15 cycles, with mild regeneration in N₂ at room temperature. Notably, the optimized material exhibits a 24% and 46% increase in CO₂ uptake at 10 and 25 °C, respectively, compared to the nonoptimized analogue. In addition, and notably in contrast to most CO₂ adsorbents, CO₂ uptake further increases under humid conditions, reaching a 65% enhancement, highlighting a beneficial role of water in the adsorption process. This increase is reasonably attributed to transient pore opening due to guest-induced linker flexibility, allowing ultramicropore accessibility without compromising structural integrity. These findings demonstrate how targeted synthetic control can activate latent porosity in rigid ultramicroporous MOFs, offering a viable pathway toward CO₂ capture under realistic operating conditions.