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 deNO_x 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. © 2023 Elsevier B.V.