Ionic liquid–based green processes for ammonia separation and recovery

doi:10.1016/j.cogsc.2020.100354

Current Opinion in Green and Sustainable Chemistry

Volume 25, October 2020, 100354

https://doi.org/10.1016/j.cogsc.2020.100354 Get rights and content

The emergence of ionic liquids (ILs) has provided a highly effective and energy-saving way for separation and recovery of NH₃ owing to their remarkable advantages, such as very low volatility and structure designability. This review presents a comprehensive summary on the latest progresses of pure ILs, IL hybrid solvents, and IL composite materials for NH₃ separation from the perspective of material design to process simulation, involving the role of anions, cations, and functional sites of ILs in NH₃ separation performance of IL-based systems, the interaction mechanisms between ILs and NH₃, as well as the simulation of IL-based NH₃ separation process. Finally, the research challenges and perspectives of IL-based NH₃ separation and recovery in future are discussed.

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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Review articleIonic liquid–based green processes for ammonia separation and recovery

Introduction

Section snippets

Conventional ILs

Ionic liquid hybrid solvents for NH3 absorption

Ionic liquid composite materials for NH3 separation

Interaction mechanisms between ILs and NH3

Simulation of ionic liquid–based NH3 separation process

Conclusions

Declaration of competing interest

Acknowledgements

Environ Pollut

Environ Pollut

Appl Energy

Chem Eng J

Energy Environ Sci

AIChE J

Separ Purif Technol

RSC Adv

J Chem Technol Biotechnol

J Chem Thermodyn

ACS Sustainable Chem Eng

Chem Eng J

Chem Commun

Membranes

Ind Eng Chem Res

Increased atmospheric ammonia over the world's major agricultural areas detected from space

Geophys Res Lett

Highly selective capture of CO2 by ether-functionalized pyridinium ionic liquids with low viscosity

Energy Fuel

Ionic-liquid-based CO2 capture systems: structure, interaction and process

Chem Rev

Improving SO2 capture by tuning functional groups on the cation of pyridinium- based ionic liquids

RSC Adv

Selective separation of hydrogen sulfide with pyridinium-based ionic liquids

Ind Eng Chem Res

Efficient and reversible absorption of ammonia by cobalt ionic liquids through Lewis acid-base and cooperative hydrogen bond interactions

Green Chem

Efficient adsorption of ammonia by incorporation of metal ionic liquids into silica gels as mesoporous composites

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Simulation and optimization of ammonia recovery with ionic liquid from purge gas in ammonia synthesis plant

Chin J Process Eng

Highly efficient CO2 capture by tunable alkanolamine-based ionic liquids with multidentate cation coordination

Chem Commun

Protic ionic liquids for the selective absorption of H2S from CO2: thermodynamic analysis

AIChE J

Review article
Ionic liquid–based green processes for ammonia separation and recovery

Ionic liquid hybrid solvents for NH₃ absorption

Ionic liquid composite materials for NH₃ separation

Interaction mechanisms between ILs and NH₃

Simulation of ionic liquid–based NH₃ separation process

Highly selective capture of CO₂ by ether-functionalized pyridinium ionic liquids with low viscosity

Ionic-liquid-based CO₂ capture systems: structure, interaction and process

Improving SO₂ capture by tuning functional groups on the cation of pyridinium- based ionic liquids

Highly efficient CO₂ capture by tunable alkanolamine-based ionic liquids with multidentate cation coordination

Protic ionic liquids for the selective absorption of H₂S from CO₂: thermodynamic analysis