Absorption characteristics of ammonia by ImFSA-based low melting mixtures: Experimental study and computational analysis

Ammonia (NH₃) is a versatile raw material. The efficient capture and separation of NH₃ is vital for green chemistry improvement. In this study, imidazolium bis(fluorosulfonyl)imide salt (ImFSA) with properties favorable for NH₃ absorption was designed as hydrogen bond acceptor (HBA) and further mixed with various hydrogen bond donors (HBDs) to form low melting mixtures (LMMs) for the effective capture of NH₃. The key physical properties of the LMMs were measured, and the influences of HBD types, molar compositions, and temperature on NH₃ absorption performance were experimentally evaluated. Results showed that ImFSA primarily contributed to NH₃ capture, while HBDs provided good fluidity and operational stability for the LMMs. ImFSA-Triz (1:1) demonstrated impressive capacity of 0.2187 g NH₃/g LMM at 303.15 K and 0.1 MPa, with less than 2 % capacity loss after five consecutive cycles. Spectroscopic analysis and theoretical calculations collectively revealed the molecular insights on NH₃ absorption mechanism, involving four steps: robust NH₃ capture by Im⁺, proton transfer, further binding of NH₄⁺ to NH₃, and VDW interactions stabilizing system.