Ionic liquids in gas separation processing

Ionic liquids in gas separation processing

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Ionic liquids in gas separation processing Dawei Shanga,b, Xinyan Liua, Lu Baia, Shaojuan Zenga, Qiuxia Xua,c, Hongshuai Gaoa and Xiangping Zhanga Ionic liquids (ILs) are recognized as novel solvents for both acid and base gases separations in industrial processes to solve the possible problems caused by the traditional organic solvent methods, such as high energy consumption and second pollutions. In this paper, the absorption performance and absorption mechanism of several typical gases, including CO2, SO2, H2S and NH3, in conventional and functionalized ILs are briefly summarized and discussed. Besides, the method of screening ILs with COSMOS-RS, and the process simulation, assessment and design for new IL-based gas separation processes are also reviewed. Addresses a Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex System, Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China b College of Chemical and Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China c School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China Corresponding author: Zhang, Xiangping ([email protected])

Current Opinion in Green and Sustainable Chemistry 2017, 5:74–81 This review comes from a themed issue on Green Solvents 2017 Edited by Charlotta Turner and Jianji Wang

separation technologies is an enduring R&D topic, and the key is to design new solvents or materials and then the corresponding novel processes. In recent years, ionic liquids (ILs) have emerged as a promising option for gas separations [6], which are entirely composed of cations and anions. Compared with traditional solvents, their desirable properties, such as negligible vapor pressures, high thermal stability and tunable structures, endow ILs the advantages of lower energy consumption, less loss of absorbents, no waste water discharge and so on [7]. The visual comparison of CO2 absorption by monoethanolamine (MEA) aqueous solution and 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) is as shown in Figure 1. The undesired industrial gases considered in this paper are mainly divided into acid gases and base gases, i.e., CO2, SO2, H2S and NH3. Research on such gases absorption by both conventional and functionalized ILs and the absorption mechanism, absorbents screening method with COSMO-RS, and process simulation for gas separation are also reviewed.

Absorption performance and mechanism of gases in ionic liquids CO2 solubility in ionic liquids and the absorption mechanism Physical absorption

The rapid development of human civilization during recent centuries is based on the energy and environment and at present, environmental degradation has become one of the main impediments for further progress of the society. For example, a large sum of undesired gases emitted from industries cause global warming [1,2], acid rain [3], fog and haze [4,5], which have severely threatened the living environment of human beings. Gas separation is an important operation unit to treat the industrial tail gases or process gases and there are already many technologies for gas separations, like physical or chemical solvents scrubbing, pressure swing absorption (PSA) and gas membrane separation. However, due to the complexity of the gas components and diverse conditions, most technologies still suffer from high energy consumption, high cost and secondary pollutions. Thus, developing new gas

The most common way in industries to capture CO2 is the absorption by organic amine solutions, such as MEA [8e10] and methyldiethanolamine/piperazine (MDEA/ PZ) aqueous solutions [11,12]. However, the drawbacks including pipeline corrosion, absorbent degradation and high energy consumption limit the applications of these methods. In order to solve the problem, many new absorbents have been developed [13] and among them, ILs are a promising option. In 1999, Blanchard et al. [14] first absorption by 1-butyl-3reported CO2 methylimidazolium hexafluorophosphate ([Bmim] [PF6]) and found the high solubility of CO2 in IL but insolubility of IL in CO2. Since then research on conventional and functionalized ILs for CO2 capture received great attentions. For traditional ILs, the interactions between CO2 and ILs are mainly composed of electrostatic, Van der Waals and hydrogen-bonding forces [15e19]. Therefore, the solubility of CO2 in such ILs is related to the structures of cations and anions. Aki et al. [20] measured CO2 solubility in ILs with different length of the cations’ alkyl chain and demonstrated that the longer the length of alkyl chain, the higher the CO2

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Ionic liquids in gas separation processing Shang et al.


Figure 1

CO2 absorption by MEA aqueous solution (a) and [Bmim][BF4] (b).

solubility. Anthony et al. [21] designed a few cations paired with different anions and found that the effect of anions on the CO2 solubility is much larger than the cations, which was also confirmed by other researchers [22,23]. Chemical absorption

Op]) could absorb 1.58 mol CO2/mol IL under ambient conditions and the probable absorption mechanism was as shown in Figure 2. SO2 solubility in ionic liquids and the absorption mechanism Physical absorption

In order to achieve higher CO2 absorption capacity to compete with conventional amine solutions, functionalized ILs by introducing amino or other groups onto cation or anion are developed and the higher CO2 absorption capacity is attributed to the chemical interactions between ILs and CO2 [24e36]. The first aminofunctionalized IL 1-butyl-3-propylamineimidazolium tetrafluoroborate ([NH2p-bim][BF4]) using for CO2 capture was reported by Bates et al. [24] and the molar ratio of CO2/IL was about 0.5 at 295 K, 100 kPa. The possible absorption mechanism was the reaction between CO2 and the amino group and then carbamate was formed. In addition to append amino on the cation, researchers also designed ILs with amino-functionalized anions and the ILs could even achieve equimolar absorption of CO2 [26,27]. Zhang et al. [37] synthesized a kind of dual amino-functionalized cation-tethered IL and the CO2 gravimetric capacity of the IL reached up to 18.5 wt %. However, not all NH2-functionalzied ILs can achieve such high CO2 absorption capacity for the reason that the HOMO and LUMO energies for CO2 and various NH2-functionalzied ILs were different [25,38]. For the sake of overcoming the high viscosities and complex synthesis of these amino-functionalized ILs, Wang et al. [30,32,33,35] developed a series of nonamino ILs to capture CO2, such as super base-derived protic ILs and diverse phenolic ILs with multi-active sites on anions. The results showed that trihexyl (tetradecyl)phosphonium 2- hydroxypyridinate ([P66614][2-

Another typical acid gas is SO2, often existing in flue gases emitted from power plants. In the past decades, design and synthesis of various ILs as efficient absorbents for SO2 absorption and separation offer a new opportunity for developing novel separation processes that are capable of reversibly capturing SO2 with a high capacity and low absorption enthalpy [3,39e49]. The relationship between the structures and the absorption performance of the conventional ILs had been clarified by many researchers [3,50]. Zeng et al. [51] systematically investigated the effect of the alkyl chain length of cations and various anions on SO2 solubility in ILs. The results revealed that the length of the alkyl chain had a minor influence on SO2 absorption performance, while anions played a dominant role in SO2 absorption. Moreover, the surprisingly high absorption capacity of SO2 in N-butylpyridinium thiocyanate ([C4Py][SCN]) was mostly attributed to the stronger electrostatic interaction between the anion and SO2. Ether and nitrile-functionalized ILs also displayed excellent capacity for SO2 capture and the enhanced absorption is due to the stronger physical interaction between ILs and SO2 in comparison with non-functionalized analogs [C4Py][SCN] [52,53].

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Chemical absorption

Wu et al. [54] first reported TMG-based IL (1,1,3,3tetramethylguanidinium lactate, [TMG][L]) for SO2 absorption through physical and chemical interactions and

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Figure 2

The plausible mechanism of CO2 absorption by [P66614][2-Op] through multiple-site cooperative interactions [35] (Reproduced with permission from Ref. [35]. Copyright 2014WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

the IL can efficiently absorb SO2 with high capacity under both ambient and low partial pressures. Further explanation about the interactions between SO2 and TMG-based ILs combining experiments, molecular dynamic simulation and ab-initio calculation were conducted by other researchers [50,55,56]. On the basis of this, Wu et al. [56] proposed a new method to distinguish the functionalized ILs and non-functionalized ILs according to the pKa values of the acids forming the ILs. Subsequently, azole and phenolate-based ILs were developed to further improve SO2 capacity through multiple-site interactions and the SO2 solubility was up to 5.7 mol SO2/mol IL with tunable absorption enthalpy [42,46,47,57,58]. Interestingly, Zeng et al. [59] revealed the unusual viscosity changes of 1-(2-diethylaminoethyl)pyridinium thiocyanate ([NEt2C2Py][SCN]) during SO2 absorption owing to different absorption stages of chemical and physical interaction respectively, as shown in Figure 3.

H2S solubility in ionic liquids and the absorption mechanism

Natural gas and biogas often contain the poisonous acid gas H2S, which should be removed before utilization. The traditional amine aqueous solutions for natural gas sweetening suffer from many impediments [60]. Therefore, ILs are introduced to remove H2S from these gases. Pomelli et al. [61] studied the solubility of H2S in a series of imidazolium-based cation ILs with different anions and as expected, the cation has a moderate effect on the H2S solubility and strong hydrogen bonds formed between H2S and the anions, which is confirmed by Aparicio et al. [62] and Sakhaeinia et al. [63]. Dual Lewis base functionalized ILs were reported by Huang et al. for the selective absorption of H2S and the solubility reached 0.84 mol H2S/mol IL at 333 K, 100 kPa, attributing to the formation of the N/H/S hydrogen bond [64]. Ma et al. [65] studied the chemical reaction between 1.6

Figure 3

The viscosity of pyridinium-based ILs with SO2 absorption capacity [59] (Reproduced with permission from Ref. [59]. Copyright 2015 American Chemical Society). Current Opinion in Green and Sustainable Chemistry 2017, 5:74–81

Ionic liquids in gas separation processing Shang et al.

[Et3NHCl]$[FeCl3] (triethylamine hydrochloride blending with ferric chloride, molar rate 1.6:1) and H2S and the solubility was up to 6.36 g/L in the Fe-based ILs. Huang et al. [66,67] further researched the selective absorption of H2S over CO2 with hydrophobic protic ionic liquids and substituted benzoate-based ILs, and the H2S/ CO2 selectivity was up to 37.2 at 298 K, 100 kPa. Also, Huang et al. [68] used ILs to serve as the efficient media for the Claus reaction of SO2 and H2S and 1-hexyl-3methylimidazolium chloride ([Hmim][Cl]) enabled the transformation of H2S to S8 with a conversion ratio as high as > 96% within 3 min. NH3 solubility in ionic liquids and the absorption mechanism

Direct emission of NH3 from chemical industries heavily pollutes the environment and the water scrubbing method is one of the main methods to treat the exhausted gases. Compared with water or acid scrubbing methods, ILs serving as NH3 absorbents have showed great advantages, such as no waste water discharge and recovery of pure NH3 product. Yokozeki et al. [69,70] first reported 8 kinds of ILs for NH3 absorption at ranges of temperatures and pressures and they concluded intermolecular complex interactions might form between NH3 and ILs. Shi et al. [71] declared that NH3 interacted more strongly with cations than the anions, which was in contrast to the situation of CO2 absorption by ILs. The further work about the influence of the length of the cation’s alkyl chain on the NH3 solubility was studied by Li et al. [72]. Systematically, Palomar et al. [73,74] screened 272 kinds of ILs and ultimately hydroxyl-functionalized ILs, 1e2 (-Hydroxyethyl)-3-methylimidazolium tetrafluoroborate and cholinium bis(tri([EtOHmim][BF4]) fluoromethylsulfonyl)imide ([choline][NTf2]), were selected to absorb NH3 and achieved high solubility. Li et al. [75] found the strong hydrogen bond interaction between NH3 and the H atom of the hydroxyl group, which resulted in the high NH3 absorption. Our group proposed protic ionic liquid 1-butyl imidazolium bis(trifluoromethylsulfonyl)imide ([Bim][NTf2]) with strong hydrogen bond donating ability for NH3 absorption and


solubility is up to 2.69 mol NH3/mol IL at 313 K, 100 kPa and the possible absorption mechanism was as shown in Figure 4 [76].

COSMO-RS for ionic liquids screening Apart from the synthesis of ILs, measurements of properties and solubility as well as understanding the absorption mechanism by experimental characteristics are also important. Thus another aspect in this area is to screen and design new ILs according to the thermodynamic model combining with process simulation method. UNIFAC [77], UNIQUAC and EOS models [78] are the traditional methods for predicting the thermodynamic properties of vapor-liquid systems and new methods have been developed. Compared with COSMO, COSMO-RS goes far beyond simple continuum solvation models (CSMs) because of the integrated concepts from quantum chemistry, dielectric continuum models, electrostatic surface interactions and statistical thermodynamics [79,80]. Since COSMO-RS is independent of experimental data and an alternative to structure interpolating group contribution methods, it has been widely used for a quick solvent screening or property predictions. Zhang et al. [81] used the COSMO-RS model to screen ILs for CO2 capture and obtained the Henry’s constants of CO2 in 408 ILs. They found that the ILs with anion [FEP] showed relative higher CO2 absorption capability than other ILs, while Maiti et al. [82] reported that the ILs with all guanidinium-functionalized cations and [BF4] anion possessed a higher CO2 solubility. Marco et al. [83] developed a simple model to predict the solubility of CO2 by combining COSMO-RS for unsymmetrical activity coefficient with PR EOS for calculation of CO2 fugacity coefficient. They found that the molecular mass of ILs has a great effect on Henry’s constant of CO2. Palomar et al. [73,74] performed quantumchemical COSMO-RS analysis to analyze NH3-solvent interactions and then a rational screening of Henry’s coefficient of NH3 over 272 ILs was done to select potential high solvents for NH3 capture. Based on the COSMO-RS model, Liu et al. [84] established a more

Figure 4

Possible NH3 absorption mechanism by [Bim][NTf2] [76] (Reproduced with permission from Ref. [76]. Copyright 2016, Royal Society of Chemistry).

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comprehensive and systematic method for the screening of ILs. Not only CO2 solubility and CO2/CH4 selectivity, but also the properties of viscosity and toxicity were taken into consideration. Further, a new CO2 capture process with the screened IL was developed and the energy consumption of the process was evaluated. The results showed that the IL-based process can realize energy-saving and environmental friendly target for carbon capture. Similarly, COSMO-RS models are also useful for other gases separation processes [85,86].

Simulation and assessment of ionic liquidsbased gas separation process The screening of appropriate ILs including experimental and COSMO-RS methods is the fundamental work for gas separations, but process simulation and assessment are also an indispensable step for industrial design. However, the thermodynamic properties of ILs are absent and these data are very necessary not only in process simulations but also in the assessment of a new technology. Huang et al. [87] developed a new fragment contribution-corresponding states (FC-CS) method to predict critical properties of ILs and the predicted results agreed well with the experimental data. Based on this reliable thermodynamic model, three ILs blending with MEA for CO2 capture processes were simulated and the CO2 capture cost of the IL-based process was evaluated [88]. Compared with the MEA process, the [C4Py] [BF4]-MEA process was regarded as energy-saving and cost-efficient carbon capture process and other researchers have also obtained the similar conclusions [89e 91]. Xu et al. [92] established process simulation for biogas upgrading and they found the energy consumption and green degree of the IL scrubbing process were respectively the lowest and highest among the three technologies studied. On the basis of COSMO-based process simulations, Ruiz et al. [93] evaluated NH3 absorption refrigeration cycles with ILs as absorbents and the ILs method provided the best cycle’s performance.

adsorption materials. These hybrid materials can keep the excellent features of both ILs and porous materials and meanwhile avoid the possible drawbacks of the pure ILs and traditional porous materials. For example immobilization of ILs in membranes has been extensively investigated to combine the advantages of membranes and ILs [94,95]. Metal organic frameworks (MOF), as an important kind of adsorption materials, have great potential as new porous materials for gas separations, which could be functionalized by ILs to create more adsorption sites for gas molecules and increase the gas selectivity of materials [96,97].

Acknowledgement This work was financially supported by the National Natural Science Fund for Distinguished Young Scholars (21425625), the National Natural Science Foundation of China (51574215, 21606233).

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Conclusions The absorption of various undesired gases with ILs has displayed potential industrial applications, but there are still many challenges, such as how to design new ILs for better gas absorption performance and overcome the high price and reduce the viscosities of ILs. As mentioned above, it is suggested to screen ILs from the point of molecular simulation and then clarify the interactions of cationeanion, cation-gas and anion-gas and that would be very helpful and efficient. At present stage, blending ILs with organic solvents is a compromising but reasonable method to be applied into the gas separation process in order to reduce the viscosity and cost and however, the drawbacks of organic solvents cannot be totally avoided. In addition, new materials introducing ILs have become promising options for gas separations yet, including the IL-based membranes and Current Opinion in Green and Sustainable Chemistry 2017, 5:74–81

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