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

Ammonia (NH 3 ) is one of the main reasons for the formation of aerosols. NH 3 pollution mainly comes from a large amount of emissions in industrial production. Ionic liquids (ILs), as a green solvent, can efficiently separate and recover NH 3 from industrial NH 3 -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 NH 3 capacity with the maximum of 3.078 mol NH 3 /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 NH 3 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 p K a of pyridine derivatives than that of NH 3 , forming the NH 4 + and volatile neutral pyridine derivatives. This work will provide useful guidance to the design of novel ILs used for efficient and reversible absorption of NH 3 .