(544f) Thermodynamic Studies/Analysis of NH3 Absorption in Pyridinium-Based Protic Ionic Liquids

Ammonia (NH₃) is undoubtedly another major threat to environment and human health since it is considered as a main precursor of PM2.5 (Particulate matter ≤ 2.5 μm)^[1]. Meanwhile, NH₃ is an important chemical raw material and widely applied in some fields, such as fertilizers, nitric acid, freezing medium and so on. Therefore, the recovery of ammonia from tail gas is of great significance for pollution control and resource utilization. Although the water scrubbing has been widely used in industry, it possesses some inherent drawbacks, such as high energy consumption, huge water consumption. Hence, it is extremely essential to develop new absorbents with high efficiency, reversibility and low energy consumption for NH₃ recovery.

In our previous research, we developed the hydroxyl-functionalized ionic liquids^[2] and protic ionic liquids^[3] for absorption NH₃, respectively. They all displayed effective absorption performance for NH₃ through the hydrogen bonds. In this work, a novel type of pyridinium-based ionic liquids (ILs) with acidic protons and hydroxyl groups were designed and synthesized for NH₃ absorption. The NH₃ solubility in five pyridinium-based protic ILs at temperatures from 303.15 to 343.15 K and pressures up to 600 kPa were measured using gas-liquid equilibrium method, and the thermodynamic properties including reaction enthalpy, entropy, Gibbs free energy were further obtained by a Reaction Equilibrium Thermodynamic Model (RETM)^[4] to better understand the NH₃ absorption process. The results indicated that the PIL [4-MeOHPy][NTf₂] exhibits the maximum NH₃ solubility of 3.43 mol NH₃/mol IL at 313.15 K and atmospheric pressure, surpassing any nonmetallic ILs previously reported. Furthermore, the characteristics of isotherms under low pressures behaved an obvious chemical reaction between ILs and NH₃. This work provides an important reference for the thermodynamic properties of NH₃ absorption process in practical application.