The low-temperature NO₂ removal by tailoring metal node in porphyrin-based metal-organic frameworks

Nitrogen dioxide (NO₂) is the most toxic and prevalent form of nitrogen oxides (NO_x) pollutant and its removal from ambient air is a pressing challenge. The state-of-the-art deNO_x technologies such as selective catalytic reduction (SCR) can only work at elevated temperatures (>250-300 °C), but ineffective for the NO_x removal under ambient conditions. The adsorptive removal of NO₂ is an alternative approach to SCR, whose success depends on the design of stable adsorbents capable of selectively capturing NO₂ with a highly reversible capacity. Here we synthesized and developed five porphyrin-based metal-organic frameworks (PMOFs) as robust ambient NO₂ adsorbents, including three aluminum-based (Al-PMOF) isostructures, and two zirconium-based (Zr-PMOFs) isostructures. Of them, Al-PMOF stands out to be the most promising candidate by showing the highest NO₂ adsorption capacity (1.85 mmol/g), high stability, and good regenerability (retaining 87% capacity after five cycles of adsorption) at dry conditions. The NO₂ adsorption capacity of Al-PMOF was approximately doubled (3.61 mmol/g) at wet conditions. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed the NO₂ adsorption mechanism - the hydrogen bonding occurs between bridging hydroxyl (-OH) (attached to the metal node) and NO₂ molecules. Our work demonstrates that PMOFs are promising NO₂ adsorbents and will provide guidance for designing robust and reusable adsorbents for efficiently removing NO₂ at ambient temperature.