An Amine-Functionalized Ultramicroporous Metal-Organic Framework for Postcombustion CO₂ Capture

Among the most promising methods by which to capture CO₂ from flue gas, the emission of which has accelerated global warming, is energy-efficient physisorption using metal-organic framework (MOF) adsorbents. Here, we present a novel cuprous-based ultramicroporous MOF, Cu(adci)-2 (adci^- = 2-amino-4,5-dicyanoimidazolate), which was rationally synthesized by combining two strategies to design MOF physisorbents for enhanced CO₂ capturing, i.e., aromatic amine functionalization and the introduction of ultramicroporosity (pore size <7 Å). Synchrotron powder X-ray diffraction and a Rietveld analysis reveal that the Cu(adci)-2 structure has one-dimensional square-shaped channels, in each of which all affiliated ligands, specifically NH₂ groups at the 2-position of the imidazolate ring, have the same orientation, with a pair of NH₂ groups therefore facing each other on opposite sides of the channel walls. While Cu(adci)-2 exhibits a high CO₂ adsorption capacity (2.01 mmol g^-1 at 298 K and 15 kPa) but a low zero-coverage isosteric heat of adsorption (27.5 kJ mol^-1), breakthrough experiments under dry and 60% relative humidity conditions show that its CO₂ capture ability is retained even in the presence of high amounts of moisture. In a Monte Carlo simulation and a radial distribution analysis, the preferential CO₂ binding site of Cu(adci)-2 was predicted to be between two ligands, forming a sandwich-like structure and implying that its CO₂ adsorption properties originate from the enhancement of Lewis base-acid and London dispersion interactions due to the amino groups and ultramicroporosity, respectively.