Green synthesis of MIL-101(Cr) with enhanced direct air capture of CO2 through synergistic effects of polyethyleneimine and additives

To address the continuously rising levels of CO2 in the atmosphere and mitigate the greenhouse effect, CO2 direct air capture (DAC) has become an increasingly important technology. Amines-functionalized metal-organic frameworks (MOFs) are promising to serve as adsorbents for CO2 adsorption in DAC applications. This study presents a green synthesis approach for MIL-101(Cr) (MIL = Materials of Institut Lavoisier) using NaOH as a modulator, achieving a high specific surface area and uniform morphology while ensuring an eco-friendly process free from toxic chemicals and organic solvents at relatively low reaction temperature. Then, we explored the impact of co-impregnation of polyethyleneimine (PEI) and various additives (including hydroxylcontaining surfactants, alkanolamine and amine small molecules) into MIL-101(Cr) on the improvement of low concentration CO2 adsorption at room temperature. The hydroxyl-containing surfactants not only provided hydroxyl groups to synergistically interact with amine sites for the DAC of CO2, but also enhanced the dispersion of PEI within MIL-101(Cr) for the improved accessibility of the adsorption sites. As a result, the combination of PEI and polyethylene glycol (PEG) achieved the highest CO2 adsorption capacity up to 1.65 mmol/g and amine efficiency of approximately 0.159 under 400 ppm CO2 level. It also showed excellent adsorption kinetic performance and cyclic capacity. Breakthrough adsorption tests further validated the rapid and sustained removal of CO2 with high selectivity under simulated dry and humid air environments, contributing to the development of more efficient adsorbents for DAC technology.