Enhanced Carbon Dioxide Capture from Diluted Streams with Functionalized Metal–Organic Frameworks

Capturing carbon dioxide from diluted streams, such as flue gas originating from natural gas combustion, can be achieved using recyclable, humidity-resistant porous materials. Three such materials were synthesized by chemically modifying the pores of metal-organic frameworks (MOFs) with Lewis basic functional groups. These materials included aluminum 1,2,4,5-tetrakis(4-carboxylatophenyl) benzene (Al-TCPB) and two novel MOFs: Al-TCPB(OH), and Al-TCPB(NH₂), both isostructural to Al-TCPB, and chemically and thermally stable. Single-component adsorption isotherms revealed significantly increased CO₂ uptakes upon pore functionalization. Breakthrough experiments using a 4/96 CO₂/N₂ gas mixture humidified up to 75% RH at 25 °C showed that Al-TCPB(OH) displayed the highest CO₂ dynamic breakthrough capacity (0.52 mmol/g) followed by that of Al-TCPB(NH₂) (0.47 mmol/g) and Al-TCPB (0.26 mmol/g). All three materials demonstrated excellent recyclability over eight humid breakthrough-regeneration cycles. Solid-state nuclear magnetic resonance spectra revealed that upon CO₂/H₂O loading, H₂O molecules do not interfere with CO₂ physisorption and are localized near the Al-O(H) chain and the -NH₂ functional group, whereas CO₂ molecules are spatially confined in Al-TCPB(OH) and relatively mobile in Al-TCPB(NH₂). Density functional theory calculations confirmed the impact of the adsorbaphore site between of two parallel ligand-forming benzene rings for CO₂ capture. Our study elucidates how pore functionalization influences the fundamental adsorption properties of MOFs, underscoring their practical potential as porous sorbent materials.