Theoretical insights into NH 3 absorption mechanisms with imidazolium-based protic ionic liquids

Abstract

NH 3 absorption in [BIm][Tf 2 N] is governed by hydrogen bonds between NH 3 and the protic site on the cation and considerable effects of anions due to their interaction with the protic site. Ionic liquids (ILs) provide a promising way for efficient absorption and separation of ammonia (NH 3 ) 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 NH 3 absorption by protic ionic liquids (PILs). The results showed that the NH 3 absorption performance of the imidazolium-based PILs ([BIm][X], X= Tf 2 N, SCN and NO 3 ) is determined by not only the hydrogen bonding between the N atom in NH 3 and the protic site (H–N 3 ) on the cation but also the cation–anion interaction. With the increase in NH 3 absorption capacity, the hydrogen bonding between [BIm][Tf 2 N] and NH 3 changed from orbital dominated to electrostatic dominated, so 3.0 mol NH 3 per mol IL at 313.15 K and 0.10 MPa was further proved as a threshold for NH 3 capacity of [BIm][Tf 2 N] by the Gibbs free energy results, which agrees well with the experimental results. Furthermore, the anions of [BIm][X] could also compete with NH 3 for interaction with H-N 3 of the cation, which weakens the interaction between the cation and NH 3 and then decreases the NH 3 absorption ability of PILs. This study provides further understanding on NH 3 absorption mechanisms with ILs, which will guide the design of novel functionalized ILs for NH 3 separation and recovery.