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 (CO 2 ) 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 CO 2 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, CO 2 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.