Efficient Capture and Low Energy Release of NH3 by Azophenol Decorated Photoresponsive Covalent Organic Frameworks

Abstract

In NH₃ 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 CO₂ 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 NH₃ uptake capacity of 7.7 mmol g⁻¹ and a NH₃/N₂ selectivity of 158. In the adsorbed NH₃, about 29.0 % could be removed by vis-light irradiated cis-trans isomerization at 25 °C, and the remaining NH₃ 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 NH₃ 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 NH₃ and the smart COF, as well as the increased polarity and number of hydrogen bond sites after the trans-cis isomerization.