Theoretical insights into NH3 absorption mechanisms with imidazolium-based protic ionic liquids†

Ionic liquids (ILs) provide a promising way for efficient absorption and separation of ammonia (NH₃) due to their extremely low vapor pressures and adjustable structures. However, the understanding of absorption mechanisms especially in terms of theoretical insights is still not very clear, which is crucial for designing targeted ILs. In this work, a universal method that integrates density functional theory and molecular dynamic simulations was proposed to study the mechanisms of NH₃ absorption by protic ionic liquids (PILs). The results showed that the NH₃ absorption performance of the imidazolium-based PILs ([BIm][X], X= Tf₂N, SCN and NO₃) is determined by not only the hydrogen bonding between the N atom in NH₃ and the protic site (H–N³) on the cation but also the cation–anion interaction. With the increase in NH₃ absorption capacity, the hydrogen bonding between [BIm][Tf₂N] and NH₃ changed from orbital dominated to electrostatic dominated, so 3.0 mol NH₃ per mol IL at 313.15 K and 0.10 MPa was further proved as a threshold for NH₃ capacity of [BIm][Tf₂N] by the Gibbs free energy results, which agrees well with the experimental results. Furthermore, the anions of [BIm][X] could also compete with NH₃ for interaction with H-N³ of the cation, which weakens the interaction between the cation and NH₃ and then decreases the NH₃ absorption ability of PILs. This study provides further understanding on NH₃ absorption mechanisms with ILs, which will guide the design of novel functionalized ILs for NH₃ separation and recovery.

Keywords: Protic ionic liquids; NH₃ absorption; Interaction mechanisms; Simulation calculations.