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.