The low-temperature NO<inf>2</inf> removal by tailoring metal node in porphyrin-based metal-organic frameworks

Abstract

Nitrogen dioxide (NO2) is the most toxic and prevalent form of nitrogen oxides (NOx) pollutant and its removal from ambient air is a pressing challenge. The state-of-the-art deNOx technologies such as selective catalytic reduction (SCR) can only work at elevated temperatures (>250–300 °C), but ineffective for the NOx 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.