Review articleIonic liquid–based green processes for ammonia separation and recovery
Introduction
As a critical precursor of particle matter 2.5 (PM 2.5), basic ammonia (NH3) gas that can react with nitric acid and sulfuric acid in atmosphere to form ammonium salts is a primary factor for the formation of fog and haze [1,2]. Meanwhile, NH3 is also an important and fundamental material for chemical industries and plays as dominant working medium in absorption refrigeration [3]. The NH3-containing gases have widespread sources from industries, such as the production processes of ammonia, melamine, urea, and inorganic chemicals (e.g. ammonium molybdate and molybdenum oxide), as well as exhaust gases of nitric acid plants and so on. The direct emission of NH3 from industries into atmosphere not only results in the aggravated environmental issues but also is accompanied by serious loss of NH3 resources. Therefore, NH3 separation and recovery is of great significance for environmental protection and effective utilization of NH3 resources. Water scrubbing and acid scrubbing are two commercial technologies in industries. However, the two methods cannot radically solve NH3 pollution and recovery in a green and sustainable way. For example, the water method consumes a lot of energy for NH3 regeneration and recycle because of the high volatility of water, along with a great amount of NH3-containing wastewater. The acid solution shows strong reactivity easily with NH3 and generates irreversible and low-valued salts [4]. Hence, the development of novel efficient and energy-saving technology for NH3 separation and recovery is an important trend for green processes.
As an novel media, ionic liquids (ILs) have been widely applied for different gases (e.g. CO2 [5,6], SO2 [7,8], H2S [9], and NH3 [10,11]) separation as novel absorbents because of their special ion structures and nonvolatility, such as the sodium chloride. The extremely low vapor pressure of ILs can effectively eliminate solvent entrainment loss, as well as simplify NH3 recovery and the absorbents recycling, which can significantly reduce energy consumption for NH3 desorption [12,13]. Based on preliminary estimation, the IL-based method can save more than 17% energy consumption compared with the traditional water washing for NH3 purification and recovery from NH3 purge gas [14]. More importantly, ILs can be functionalized through tuning the combination of cations and anions or incorporating specific groups to improve gas capacity and selectivity [15,16]. Therefore, the emergence of ILs provides an efficient pathway for separating and recovering NH3 from industrial NH3-containing gases.
Up to date, a number of conventional and functionalized ILs (e.g. hydroxyl ILs, metal ILs [MILs], and protic ILs [PILs]), IL hybrid solvents (e.g. deep eutectic solvents [DESs]) as well as IL composite materials (e.g. IL-supported materials and encapsulated IL materials) have been developed for NH3 absorption and adsorption and shows excellent performances and great potentials for NH3 separation applications. In this paper, a systematic review on the latest progresses of pure ILs, IL hybrid solvents, and IL composite materials for NH3 separation including the effect of IL structure on NH3 capacity, and selectivity, the interaction between ILs and NH3 as well as the simulation of IL-based NH3 separation process were summarized (Figure 1). Furthermore, the future research and prospects of IL-based systems for NH3 separation and recovery are discussed.
Section snippets
Conventional ILs
The reported works indicated that the conventional ILs generally show good NH3 solubility [17, 18, 19, 20, 21, 22]. In 2007, Yokozeki et al. [4,17] firstly tested the solubility of NH3 in seven conventional ILs by vapor–liquid equilibria and obtained the thermodynamic properties of NH3 in these ILs. It was demonstrated that the highest NH3 solubility in these ILs is 32 mg NH3∙(g IL)−1 at 298.15 K and 0.174 MPa, and the anions have no significant effect on NH3 solubility when the cations are the
Ionic liquid hybrid solvents for NH3 absorption
Pure ILs have made great achievements on NH3 absorption by means of their outstanding properties, but the higher viscosity and complex synthesis steps of several functionalized ILs than common absorbents hinder further applications. Therefore, to overcome such problems and enhance NH3 absorption performance, ILs have been applied through combining with other molecular solvents to form IL hybrid solvents, especially IL-based deep eutectic solvents (DESs) [30]. As a class of IL analogs, DESs
Ionic liquid composite materials for NH3 separation
In addition, the combination of ILs and porous solids to prepare IL composite adsorbents or membranes is also an effective method for solving the application of highly viscous or solid ILs in NH3 separation.
For low concentration of NH3 removal from air, Ruckart et al. [41] synthesized a series of adsorbents by supporting 1-methyl-3-(propylsulfonic acid)-imidazolium triflate ([C3mimSO3H][TfO]) with the content from 3 to 46 wt% onto SBA-15. Compared with the parent SBA-15, the functional group of
Interaction mechanisms between ILs and NH3
Based on the aforementioned discussion, it was clear that NH3 separation performance of IL-based systems is greatly affected by the structures of ILs; therefore, understanding the interaction mechanisms of IL–NH3 systems is vital to rationally design novel functionalized IL-based materials. Based on the literature, the interaction between IL-based systems involving conventional ILs and functionalized ILs and NH3 was mainly classified as hydrogen bonding, chemical complexation, as well as size
Simulation of ionic liquid–based NH3 separation process
From the view molecular to the macroscopic engineering aspect, the performance of IL-based NH3 absorption together with desorption in a large-scale process is the other key aspect to be considered because more and more new ILs are discovered with high NH3 capacity and selectivity. However, it always takes intensive time and huge work to conduct the lab-scale experiments in each potential IL. To evaluate the separation performance and have a better understanding of the IL application, process
Conclusions
In this review, the latest progresses on NH3 separation performance with pure ILs, IL hybrid solvents, and IL composite materials, and the interaction mechanisms of conventional ILs and functionalized ILs as well as IL-based process simulation have been summarized in detail. Although ILs show great potentials in NH3 separation applications through the absorption–desorption technology, there are still many challenges for their industrial applications to be faced.
First, in order to obtain the ILs
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financially supported by the National Key R&D Program of China (2017YFB0603401-03), the National Natural Science Foundation of China (21890764), the Beijing Municipal Natural Science Foundation (2182071), the Hebei Natural Science Foundation (B2019103011), and the Zhengzhou High Level Talent (20180200029).
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