Acid-Modulated Synthesis of High Surface Area Amine-Functionalized MIL-101(Cr) Nanoparticles for CO₂ Separations

Metal–organic frameworks (MOFs) have been gaining significant interest for separations involving CO₂. In this study, amine-functionalized MIL-101­(Cr) (i.e., MIL-101­(Cr)-NH₂) MOFs with sub-20 nm particle size and an ultrahigh Brunauer–Emmett–Teller surface area of 3700 ± 200 m²/g were directly synthesized from 2-aminoterephthalic acid in water with the aid of acidic modulators. Acidic modulators are shown to yield significant changes in nuclei formation and crystal growth during MOF particle synthesis by enhancing the solubility of the 2-aminoterephthalic acid ligand in aqueous media through protonation of the amine groups while slightly suppressing the deprotonation of carboxylic acids. Owing to the ultrahigh surface area and Lewis basic amine chemistry, superior CO₂ uptake of 5.4 mmol/g was obtained by the synthesized MIL-101­(Cr)-NH₂ at 1 bar and 278 K, which surpasses other MIL-101­(Cr) MOFs reported to date. In addition, MIL-101­(Cr)-NH₂ exhibits significantly higher CO₂/N₂ selectivities than the parent MIL-101­(Cr) benchmark for an idealized flue gas mixture composed of 0.15 bar CO₂ and 0.85 bar N₂, as determined using ideal adsorbed solution theory at 323 K. The high CO₂ uptake and CO₂/N₂ selectivity are attributed to the high isosteric heat of CO₂ adsorption, calculated to be −59.5 kJ/mol at zero coverage. The successful preparation of high surface area amine-functionalized MIL-101­(Cr) nanoparticles provides a great platform for further exploring the functionalities and applications of MIL-101­(Cr)-based adsorbents and membranes for CO₂ separations.