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.