Ambient temperature NO2 removal by adsorption on robust DMOFs: Regulating water stability, acid stability, and NO2 capacity by methyl functionalization

Nitrogen dioxide (NO2) is a dominant contributor to air pollution and urgently needs to be abated. The catalysis-based deNOx technologies can efficiently remove NO2 at elevated temperatures but fail to effectively deal with ambient NO2 pollution. Selective adsorption by solid porous adsorbents, especially metal-organic frameworks (MOFs), emerges as a promising strategy for ambient NO2 removal. To overcome the biggest challenge in developing low-temperature NO2 adsorbents today – poor stability and thus the inferior cyclable capacity, especially in the presence of water vapor, here we investigated the functionalization of a series of DMOFs with nonpolar methyl (–CH3) groups to regulate their water stability and NO2 adsorption capacity. Among these isoreticular DMOFs, the tetramethyl (TM) functionalized one (Ni-TM DMOF) exhibited a two-fold and 60-times increase in NO2 adsorption capacity under dry (2.2 mmol/g) and wet (1.8 mmol/g) conditions, respectively, over its counterpart without the TM groups (Ni-BDC DMOF). Careful structural characterizations attributed the elevated NO2 adsorption performance of DMOFs to the improved hydrophobicity induced by methyl group decoration under the wet condition. The combination of column breakthrough experiment, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study, and density functional theory (DFT) calculations revealed the NO2 adsorption mechanism under both dry and wet conditions. This work affords not only promising adsorbents for ambient NO2 removal but also new knowledge to guide the design of porous MOFs as robust adsorbents for acid gases capture.