A Cobalt-Based RHO-Type Metal-Azolate Framework for Selective CO₂/N₂ and CO₂/CH₄ Separation

The efficient separation of CO₂ from flue gas and natural gas is critical for mitigating climate change and reducing industrial emissions. To address this challenge, we synthesized a cobalt-based metal-azolate framework (MAF) with rho topology named MAF-stu-112, featuring 0.6 nm channels and 2 nm 26-faced polyhedral cages, decorated with high-density aldehyde and methyl groups on the channel and cage surface. These functional groups enable exceptional CO₂ selectivity over N₂ and CH₄, as demonstrated by single-component adsorption capacities (47.92/4.09/23.54 cm³ g^-1, for CO₂, N₂ and CH₄, respectively) and IAST selectivities of 44.4 (CO₂/N₂, 15:85) and 4.7 (CO₂/CH₄, 50:50) at 298 K and 1 bar. Dynamic breakthrough experiments further confirmed its selective CO₂ capture performance, with elution time gaps of 32 min g^-1 (CO₂/N₂, 15:85) and 20 min g^-1 (CO₂/CH₄, 50:50). Theoretical calculations and simulations revealed that CO₂ adsorption is driven by strong C-H···O interactions between CO₂ molecules and the framework's imidazole hydrogen and methyl groups, as well as C···O interactions with the framework's aldehyde groups. These results demonstrated that MAF-stu-112 can be a promising adsorbent for industrial CO₂ capture from flue gas and natural gas under ambient conditions.