Manufacturing Acidities of Hydrogen-Bond Donors in Deep Eutectic Solvents for Effective and Reversible NH3 Capture

Solvents with appropriate acidity are required to simultaneously realize efficient and reversible absorption of NH3. To achieve this goal, a series of 1-ethyl-3-methylimidazolium ([emim]Cl)-based DESs with different weak acids as the hydrogen-bond donors (HBDs) were prepared and characterized. The acidities of these weak acids cover a wide range, with the pK(a) values in DMSO ranging from 8.2 to 18.6. The NH3 absorption and desorption performance of DESs were investigated systematically and correlated with the acidities of HBDs. The different behavior of NH3 absorption was analyzed by spectroscopic characterizations and thermodynamic calculations. By manufacturing the acidities of HBDs in DESs, solvents having not only high capacities but also good reversibility for NH3 absorption were identified. The optimal DESs were then selected as representatives to investigate the effects of HBA to HBD molar ratios and temperatures on NH3 absorption, as well as the selective absorption of NH3 from other gases.