A Hydrophobic Fe-Based Metal-Organic Framework with Hierarchical Micropores for Adsorption of Indoor Trace VOCs

Indoor volatile organic compounds (VOCs) with kinetic diameters (2.5 ∼ 8.0 Å) commonly coexist at trace concentrations but pose serious threat to human health. Adsorption using porous adsorbents is a promising method for VOC removal especially at the adsorbent pore size matching the VOC molecule size. However, conventional adsorbents such as activated carbons are common with broad pore size distributions showing most pores smaller or much larger than VOC molecules, while most novel adsorbents with uniform narrow micropores cannot match the varied VOC sizes. Moreover, adsorption of VOCs is largely limited at high humidity by the competition of water molecules. Herein, a novel hydrophobic Fe-based metal-organic framework (MOF), ZJU-700-P, having a cross-helicoidal layer structure to form hierarchical micropores of 3.58 ± 0.30, 4.58 ± 0.45, 5.01 ± 0.45, 5.73 ± 0.59, and 8.20 ± 1.02 Å, was synthesized to match the coexisting indoor VOC molecules for adsorption at high humidity. ZJU-700-P has a specific surface area of 1056 m²/g and a water contact angle of 134.9°, showing excellent adsorption for formaldehyde (28.6 mg/g), acetone (29.8 mg/g), benzene (60.3 mg/g), toluene (37.1 mg/g), and m-xylene (10.1 mg/g) at 1 ppm and 298 K. Notably, adsorption of VOCs by ZJU-700-P retains 84-93% even at relative humidity up to 90%. Density functional theory simulation indicates that the binding sites of ZJU-700-P for VOCs are located at the hierarchical micropores in the cross-helicoidal layer structure due to the pore size matching effects. Therefore, ZJU-700-P is a promising physisorbent for trace indoor VOC capture.