Ionic liquids and deep eutectic solvents for NH₃ absorption and separation: a review

Ammonia (NH3) provides an essential nitrogen source for the entire ecosystem. It is also a critical raw material for industrial production and human survival. NH3 emissions during various utilization processes will result in serious environmental problems and waste of valuable sources. Green solvents like ionic liquids (ILs) and deep eutectic solvents (DESs) appear as potential materials for gas capture due to their tunable properties. This review systematically summarizes their utilization of NH3 absorption and separation. The main contents include ILs (conventional ILs, hydroxyl-functionalized ILs, protic ILs, metallic ILs, multiple ILs, polyionic ILs, and IL-supported materials) and DES materials (common DESs, hydroxyl-functionalized DESs, protic DESs, multiple-sites DESs, metallic DESs, and DES-supported materials). The influence of components in the green solvents on their physical property, absorption mechanism, and absorption and separation performance are systematically discussed. The future investigation direction and prospects are also discussed on the basis of analysis of present green solvents. This review is helpful for researchers to gain an overall understanding of ILs and DESs for NH3 capture.