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

Nitrogen dioxide (NO 2 ) 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 NO 2 is an alternative approach to SCR, whose success depends on the design of stable adsorbents capable of selectively capturing NO 2 with a highly reversible capacity. Here we synthesized and developed five porphyrin-based metal-organic frameworks (PMOFs) as robust ambient NO 2 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 NO 2 adsorption capacity (1.85 mmol/g), high stability, and good regenerability (retaining 87% capacity after five cycles of adsorption) at dry conditions. The NO 2 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 NO 2 adsorption mechanism – the hydrogen bonding occurs between bridging hydroxyl (-OH) (attached to the metal node) and NO 2 molecules. Our work demonstrates that PMOFs are promising NO 2 adsorbents and will provide guidance for designing robust and reusable adsorbents for efficiently removing NO 2 at ambient temperature.