Modified metal–organic framework by a novel coordinatively unsaturated amine grafting mechanism for direct air capture of CO₂

This study aimed to improve the carbon dioxide (CO₂) capture and kinetics performance for direct air capture (DAC) while demonstrating the amine grafting reaction principles and its constraints using N-(2-aminoethyl)ethanolamine (AEEA) to functionalize coordinatively unsaturated metal sites in MIL-100(Fe), UiO-66(Zr), and MIL-100(Cr). Grafting experiments indicated that the grafting process is the reaction between metal sites and secondary amines of AEEA, while MOFs with high surface area and low acidity can effectively promote amine grafting without destroying the active site. Moreover, the adsorbed CO₂ amounts of MF-Cr-AEEA at 400 ppm were 1.91 mmol/g of −25 °C and 2.42 mmol/g of 0 °C, and the structural decomposition temperature exceeded 400 °C, demonstrating excellent thermal and cold stability. As a result of the better crystal and surface structure, the internal heat and mass transfer processes were accelerated, resulting in low semi-adsorption times below 21 min. Moreover, CO₂ capture cycles were established at 25 °C for adsorption and 80 °C for desorption. The results show that the adsorption capacity of MF-Cr-AEEA remained 1.86 mmol/g after seven cycles, demonstrating low renewable energy consumption and high stability.