Metal chloride anion-based ionic liquids for efficient separation of NH3

A large number of NH3 emissions from industries, such as ammonia plant purge gas, tail gases of urea plants and ammonium molybdate plants, inevitably cause serious air pollution problems, such as fog and haze, which has raised extensive attentions worldwide. However, the commercial methods, like water scrubbing and acid scrubbing, substantially transfer the NH3 pollute from gas phase to liquid or solid phase, which does not mean total elimination. How to realize efficiently separate and recover NH3 form NH3-containing gas mixtures is still a great challenge. In order to develop efficient absorbents for NH3 removal, a series of metal (copper (Cu), stannum (Sn), nickel (Ni), manganese (Mn), cadmium (Cd), zinc (Zn), ferrum (Fe)) ionic liquids (MILs) were synthesized, and their physicochemical properties and NH3 absorption performance were systematically investigated. The results implied that the incorporation of metal center into ILs has a great effect on NH3 absorption. Among the investigated MILs, bis(1-butyl-3methyl imidazolium) copper tetrachloride salt ([Bmim](2)[CuCl4]) and bis(1-butyl-3-methyl imidazolium) stannum tetrachloride salt ([Bmim](2)[SnCl4]) show excellent NH3 absorption capacities up to 0.172 gNH(3)center dot gIL(-1) and 0.106 gNH(3)center dot gIL(-1), respectively, which are far more than most of the conventional ILs. Moreover, the high selectivity for NH3/N-2, NH3/CO2 and NH3/O-2 of [Bmim](2)[CuCl4] and [Bmim](2)[SnCl4] indicated that these MILs can selectively separate NH3 from NH3-containing gases. The absorption mechanism was further studied using Fourier transform infrared (FT-IR) spectroscopy and quantitative chemical calculations. It was demonstrated that the chemical complexation between [Bmim](2)[CuCl4] and NH3 plays a dominant role in NH3 absorption, but the stronger hydrogen bonding between [Bmim](2)[SnCl4] and NH3 enhances the absorption performance. In addition, [Bmim](2)[SnCl4] can keep good stability after five absorption and desorption cycles, implying this MIL shows great potentials as an efficient and reversible absorbent for NH3 capture applications. (C) 2018 Elsevier Ltd. All rights reserved.