Unraveling synergistic CO2 binding in anion-pillared materials and amine-modified MOFs for direct air capture (DAC) via combined DFT-D3 calculation and PALS experiment

To elucidate the direct air capture (DAC) performance and CO 2 selective sieving mechanisms of distinct porous materials under trace CO 2 conditions, three types of materials, anion-pillared materials, open site MOFs, and amine-modified materials, were synthesized, with their DAC performance systematically characterized. DFT-D3 calculations revealed that in NbOFFiVE-1-Ni, the C···F distances (3.04 Å) between CO 2 and the pillared NbOF₅ 2− anions, as well as the O···H distances (2.89 Å) between CO 2 and pyrazine linker hydrogens, are shorter than those in SIFSIX-3-Ni and GEFSIX-3-Ni. These synergistic C···F van der Waals (vdW) and O···H hydrogen-bonding interactions afford NbOFFiVE-1-Ni the highest CO 2 binding energy (62.74 kJ/mol), exceeding that of SIFSIX-3-Ni (57.79 kJ/mol), GEFSIX-3-Ni (60.47 kJ/mol), and MIL-101(Cr) (22.86 kJ/mol). Consistently, NbOFFiVE-1-Ni exhibits the highest atmospheric CO 2 uptake (1.15 mmol/g) among these materials. Given the low atmospheric CO 2 uptake of pristine MIL-101(Cr), two amines (PEI and TEPA) were used to modify it, leveraging its abundant unsaturated metal sites to enhance CO 2 adsorption capacity. DAC performance of amine-modified MIL-101(Cr) first increased then decreased with amine loading, with optimal values of 2.07 mmol/g (50 wt% TEPA) and 1.52 mmol/g (70 wt% PEI). DFT-D3 calculations demonstrated that TEPA molecules and PEI chains bind directly to the unsaturated metal sites of MIL-101(Cr), forming tadpole-like amine-modified MOF structures with binding energies of 210.01 kJ/mol and 178.30 kJ/mol, respectively. Positron annihilation lifetime spectroscopy (PALS) confirmed that TEPA incorporation generates new cavities, microporous structures, and abundant active amine sites in MIL-101(Cr) crystals, synergistically enhancing its DAC performance.