Nitrogen dioxide (NO2), the most prevalent form of nitrogen oxides (NOx), is a major air pollutant that brings huge damage to the environment and people’s health. The immoderate atmospheric emission of NO2 contributes to the formation of photochemical smog, fine particulate matter (PM2.5), acid rain, as well as many human diseases, such as asthma and emphysema. The development of techniques to reduce NO2 emissions is therefore significant. Although the mature technology of selective catalytic reduction (SCR) applicable for NO2 conversion at high temperatures (250-600 oC) has been widely used in industry, the control and abatement of ambient NO2 emission is still quite a complex and challengeable issue. Gas adsorption by solid adsorbents, which is based on the host-guest interactions, is a promising approach for NO2 capture. Some porous solid adsorbents, such as silica, carbon materials, zeolites, and metal-organic frameworks (MOFs) and its composites, have been attempted for NO2 capture suitable for ambient conditions. However, as NO2 is a highly reactive, oxidizing, and corrosive gas, it’s challenging for people to develop the adsorbents simultaneously showing the high NO2 capacity, satisfactory selectivity, and good regenerability in real cases. In this work, we successfully developed one type of porphyrin-based metal-organic framework (Al-PMOF) as low-temperature NO2 adsorbent. The NO2 capture ability was tuned by changing the atom-isolated transition metals (TMs) in the porphyrin ring. The NO2 breakthrough experiments indicated the Al-PMOF with inserted nickel (Al-PMOF(Ni)) showed the highest NO2 adsorption capacity of 2.30 mmol/g (500ppm NO2/helium/air, detection limit: 20 ppm NO2). In-situ diffuse reflectance infrared Fourier transform spectroscopy (in situ-DRIFTS) and synchrotron X-ray diffraction experiments were conducted to determine the adsorption mechanism. NO2 would partially transform into N2O4 in the framework of adsorbents, and both of the two molecules could interact with Al-node by hydrogen bonding and inserted TMs via π-backbonding.

Porphyrin-based metal-organic frameworks as low-temperature NO2 adsorbent

ACS Spring Meeting 2021

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