Efficient adsorption removal of NO2 by covalent triazine frameworks with fine-tuned binding sites

One promising method for mitigating low-temperature and low-concentration NO2 pollution is selective adsorption using porous solid materials. However, the corrosive nature of NO2 renders the development of an adsorbent with high and cyclable NO2 uptake a formidable challenge. Herein, we attempted to address this challenge by developing a microporous covalent triazine framework (CTF) as effective adsorbents for NO2 capture, achieving unprecedented high cyclable capacity enabled by simultaneous high adsorbent stability and moderate adsorption affinity via tuning specific binding sites. Using dynamic column breakthrough experiments, we demonstrated that the pristine CTF exhibited a high NO2 adsorption capacity (dry: 3.83 mmol/g; wet: 5.26 mmol/g) overtaking most reported solid adsorbents, thanks to abundant active sites (e.g., nitrogen/carbon/triazine rings). Post-synthesis converted the surface nitrogen to protonated nitrogen site and further to transition metal site affording more effective binding sites that allows for moderately strong adsorption, resulting in elevated NO2 adsorption capacity (dry: 6.11 mmol/g; wet: 8.97 mmol/g) and a highest reversible NO2 adsorption (retaining 91 % and 64 % at dry and wet conditions after five cycles, respectively), as illustrated by our mechanistic study using in situ DRIFTS and DFT calculations. This work affords not only promising adsorbents for real-world applications in mitigating NO2 pollution, but also a strategy of developing robust adsorbents with desired affinity generally applicable to adsorptive capture of corrosive and toxic gases.