NH₃ absorption performance and reversible absorption mechanisms of protic ionic liquids with six-membered N-heterocyclic cations

Ammonia (NH₃) is one of the main reasons for the formation of aerosols. NH₃ pollution mainly comes from a large amount of emissions in industrial production. Ionic liquids (ILs), as a green solvent, can efficiently separate and recover NH₃ from industrial NH₃-containing tail gas. In this work, novel protic ionic liquids (PILs) with hydrogen bond donating ability were designed and synthesized by incorporating six-membered N-heterocyclic compounds including pyridinium and piperidinium as cations. Three pyridinium-based PILs with single protic H showed higher NH₃ capacity with the maximum of 3.078 mol NH₃/mol IL at 313.1 K and 0.10 MPa than the piperidinium-based PILs with double interaction sites. However, there was obvious reduction in NH₃ capacity for pyridinium-based PILs during absorption and absorption-desorption cycles. The reason is that the acidic protic H of cations of pyridinium-based PILs is easily stripped in supersaturated alkaline environment due to the smaller pK_a of pyridine derivatives than that of NH₃, forming the NH₄⁺ and volatile neutral pyridine derivatives. This work will provide useful guidance to the design of novel ILs used for efficient and reversible absorption of NH₃.