Efficient Capture and Low Energy Release of NH 3 by Azophenol Decorated Photoresponsive Covalent Organic Frameworks.

In NH ₃ capture technologies, the desorption process is usually driven by high temperature and low pressure (such as 150-200 °C under vacuum), which accounts for intensive energy consumption and CO ₂ emission. Developing light responsive adsorbent is promising in this regard but remains a great challenge. Here, we for the first time designed and synthesized a light responsive azophenol-containing covalent organic framework (COF), COF-HNU38, to address this challenge. We found that at 25 °C and 1.0 bar, the cis -COF exhibited a NH ₃ uptake capacity of 7.7 mmol g ^-1 and a NH ₃ /N ₂ selectivity of 158. In the adsorbed NH ₃ , about 29.0 % could be removed by vis-light irradiated cis-trans isomerization at 25 °C, and the remaining NH ₃ might be released at 25 °C under vacuum. Almost no decrease in adsorption capacity was observed after eight adsorption-desorption cycles. As such, an efficient NH ₃ capture and low energy release strategy was established thanks to the multiple hydrogen bond interactions (which are strong in total but weak in individuals) between NH ₃ and the smart COF, as well as the increased polarity and number of hydrogen bond sites after the trans-cis isomerization.
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