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Capturing CO₂ at trace concentration remains a critical challenge in sustainable carbon management via adsorption, as conventional adsorbents suffer from low CO₂ selectivity, poor moisture tolerance, and energy-intensive regeneration requirements. Here, we report a hierarchical Ba²⁺-exchanged silicoaluminophosphate (Ba²⁺-CSAPO-34) composite synthesized via confined-space crystallization within an activated carbon matrix. Comprehensive characterization revealed a confined nucleation mechanism and the successful incorporation of Ba²⁺ active sites within the SAPO-34 framework, achieved via a two-step liquid ion-exchange protocol. The core-shell architecture combines the selective CO₂ binding of Ba²⁺-functionalized SAPO-34 with the hydrophobic protection of the carbon shell. Fixed-bed adsorption tests demonstrated strong CO₂ bindingmore » (at 500-2500 ppm), no roll-up, and effective suppression of water affinity, while maintaining high selectivity even at 90% relative humidity. A phenomenological adsorption model, validated against dynamic breakthrough data, accurately predicted dynamic adsorption behavior under real-world operating conditions, enabling rational process design for direct air capture (DAC) and closed-loop life support systems. Furthermore, these results establish Ba²⁺-CSAPO-34 as a scalable, moisture-resistant adsorbent that addresses key limitations in trace CO₂ capture, advancing practical implementation of carbon removal technologies.« less

Here, a Co₂(pzdc)₂(bpy)(H₂O)_m (pzdc: pyrazine-2,3dicarboxylate; bpy: 4,4'-bipyridine) porous coordination polymer (PCP) was studied for CO₂ uptake and concomitant structural changes at ambient temperature. Extended structural characterization included evaluation of lattice parameter changes upon CO₂ adsorption and in situ synchrotron X-ray powder diffraction data. The PCP effective pore size increased by ~2% with gas uptake over the pressure range of 1-50 atm, allowing the adsorption capacity to double. Furthermore, the hysteretic behaviors seen during CO₂ adsorption at moderate pressures are commensurate with the structural changes from synchrotron data. The adsorption and hysteresis occur with rotation of the linking carboxylate groups, andmore » yet only minor changes in unit cell volume (ΔV ≈ 6 ų) are observed. This contrasts the findings for [Cu₂(pzdc)₂(bpy)]_n, where a combination of pillar bpy rotations and significant lattice expansion (ΔV ≈ 68 Å(3)) takes place upon hysteretic adsorption of CO₂. In situ high-temperature X-ray diffraction revealed that the Co(II)-based material has good thermal stability up to ca. 200° C. Finally, the CO₂ uptake also appears to be at a physisorption level, with adsorbent-adsorbate interactions that are ca. 30% stronger than what has been reported for CO₂ adsorption onto [Cu₂(pzdc)₂(bpy)]_n.« less

Flexible metal–organic frameworks (MOFs) can show exceptional selectivity and capacity for adsorption of CO₂. The incorporation of CO₂ into flexible MOFs that have Cu²⁺ coordination centers and organic pillar ligands is accompanied by a distortion of the framework lattice arising from chemical interactions between these components and CO₂ molecules. CO₂ adsorption yields a reproducible lattice expansion that is enabled by the rotation of the pillar ligands. The structures of Cu₂(pzdc)₂(bpy) and Cu₂(pzdc)₂(bpe), CPL-2 and CPL-5, were evaluated using in situ synchrotron X-ray powder diffraction at room temperature at CO₂ gas pressures up to 50 atm. The structural parameters exhibit hysteresismore » between pressurization and depressurization. The pore volume within CPL-2 and CPL-5 increases at elevated CO₂ pressure due to a combination of the pillar ligand rotation and the overall expansion of the lattice. Volumetric CO₂ adsorption measurements up to 50 atm reveal adsorption behavior consistent with the structural results, including a rapid uptake of CO₂ at low pressure, saturation above 20 atm, and hysteresis evident as a retention of CO₂ during depressurization. Finally, a significantly greater CO₂ uptake is observed in CPL-5 in comparison with predictions based on CO₂ pressure-induced expansion of the pore volume available for adsorption, indicating that the flexibility of the CPL structures is a key factor in enhancing adsorption capacity.« less