Reversible absorption of nitrogen dioxide by choline chloride-based deep eutectic solvents and their aqueous mixtures

The adoption of cost-effective and reusable materials is critical for developing absorption liquids for industrial applications such as scrubbing technology. Here, we demonstrate that the use of deep eutectic solvents (DESs) and their aqueous mixtures is a new strategy for capturing nitrogen dioxide (NO2) because of their high absorption capacity and good reusability. The DESs are composed of a hydrogen bond acceptor (HBA) (i.e., choline chloride (ChCl)) and a hydrogen bond donor (HBD) (i.e., glycerol (Gly) or ethylene glycol (EG)). The results indicate that ChCl-based DESs exhibit excellent NO2 absorption capability, which increases with increasing HBD content, and the highest value reaches 55.1 wt% (NO2/DES) for the ChCl-EG DES at 298 K with a NO2 partial pressure of 101.3 kPa. Moreover, with the introduction of water, the DES aqueous mixtures retain high NO2 solubility while exhibiting a lower viscosity. This improvement in the transport properties largely advances the applicability of the proposed DESs as industrial scrubbing solutions for NO2 elimination. The compiled results provide insights into the factors determining the interaction of NO2 with DESs, where the NO2 solubility is directly affected by the HBD, and inform future material design for improving the NO2 absorption performance.