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

Industrial Chemistry & Materials Pub Date: 2023-01-06 DOI: 10.1039/d2im00041e

Abstract
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.
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