Ionic-Liquid-Based CO2 Capture Systems: Structure, Interaction and

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Ionic-Liquid-Based CO2 Capture Systems: Structure, Interaction and Process Shaojuan Zeng,†,# Xiangping Zhang,†,‡,# Lu Bai,† Xiaochun Zhang,† Hui Wang,† Jianji Wang,§ Di Bao,†,‡ Mengdie Li,†,‡ Xinyan Liu,†,‡ and Suojiang Zhang*,† †

Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ College of Chemical and Engineering, University of Chinese Academy of Sciences, Beijing 100049, China § School of Chemistry and Environmental Science, Henan Normal University, Xinxiang, Henan 453007, China ABSTRACT: The inherent structure tunability, good affinity with CO2, and nonvolatility of ionic liquids (ILs) drive their exploration and exploitation in CO2 separation field, and has attracted remarkable interest from both industries and academia. The aim of this Review is to give a detailed overview on the recent advances on IL-based materials, including pure ILs, IL-based solvents, and IL-based membranes for CO2 capture and separation from the viewpoint of molecule to engineering. The effects of anions, cations and functional groups on CO2 solubility and selectivity of ILs, as well as the studies on degradability of ILs are reviewed, and the recent developments on functionalized ILs, IL-based solvents, and IL-based membranes are also discussed. CO2 separation mechanism with IL-based solvents and IL-based membranes are explained by combining molecular simulation and experimental characterization. Taking into consideration of the applications and industrialization, the recent achievements and developments on the transport properties of IL fluids and the process design of IL-based processes are highlighted. Finally, the future research challenges and perspectives of the commercialization of CO2 capture and separation with IL-based materials are posed.

CONTENTS 1. Introduction 2. Structure Effects 2.1. Pure Ionic Liquids 2.1.1. CO2 Solubility in Ionic Liquids 2.1.2. CO2 Selectivity in Ionic Liquids 2.2. Ionic Liquid Blending Solvents 2.2.1. Ionic Liquid-Water Blends 2.2.2. Ionic Liquid−Organic Solvents 2.2.3. Ionic Liquid−Ionic Liquid Systems 2.3. Ionic Liquid-Based Membranes 2.3.1. Supported Ionic Liquid Membranes 2.3.2. Poly(Ionic Liquid) Membranes 2.3.3. Ionic Liquid-Based Composite Membranes 3. Interaction Mechanism of CO2 Separation 3.1. Ionic Liquid-Based Solvents 3.1.1. Lewis Acid and Base Interaction 3.1.2. Rearrangements 3.1.3. Free Volume 3.1.4. Weak Interaction 3.1.5. Amine−CO2 Chemical Interaction 3.1.6. Non-Amine−CO2 Chemical Interaction 3.2. Ionic-Liquid-Based Membranes 3.2.1. Solution-Diffusion Mechanisms 3.2.2. Facilitated Transport Mechanisms 4. Degradation of Ionic Liquids

© 2017 American Chemical Society

4.1. Degradation in the Presence of Microorganism 4.2. Thermal Degradation 5. Scale-up and Process Design for CO2 Separation 5.1. Transport Properties of Ionic Liquid Systems 5.1.1. Bubble Behaviors in Ionic Liquids 5.1.2. Mass Transfer Properties in Ionic Liquids 5.1.3. CFD on Transport Phenomena in Ionic Liquid Systems 5.2. Process Design of Ionic Liquid Systems 5.3. Scale-up of Ionic Liquid-Based Membranes 6. Conclusions and Outlook Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments References

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Special Issue: Carbon Capture and Separation Received: January 30, 2017 Published: July 7, 2017 9625

DOI: 10.1021/acs.chemrev.7b00072 Chem. Rev. 2017, 117, 9625−9673

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ine ([aemmim][Tau]) can reach 0.90 mol CO2·mol IL−1.65 However, the corresponding gravimetric capacities of these ILs are usually lower than 0.10 g CO2·g IL−1 because of the high molecular weight of ILs, whereas the gravimetric capacity is much more interested by industries.47 Therefore, since then, researchers have made great efforts for the improvement of CO2 gravimetric capacities in ILs. For instance, our research group developed a new dual amino-functionalized IL, and its CO2 gravimetric capacities can reach 18.5%, which corresponds to 1.05 mol CO2·mol IL−1.68 In addition, higher CO2 selectivity toward other gases in the practical separation process is also considered. The ether-functionalized ILs were designed for CO2 selective separation from natural gas and biogas, and the CO2/CH4 selectivity was improved by an average of 50% compared with nonfunctionalized analogues.67 Besides the CO2 absorption performances, the viscosity of ILs is also a key property considering the application. Generally, the viscosities of functionalized ILs are relatively high and increase after CO2 absorption due to the chemical reaction. For example, the viscosity of 3-aminopropyl tributylphosphonium amino acid ([aP4443][AA]) increases nearly 3-fold after absorption of CO2,66 and the viscosity of trihexyl(tetradecyl)phosphonium isoleucinate ([P66614][Iso]) increases over 200fold when exposed to 0.10 MPa of CO2.69 Therefore, the development of new ILs with low viscosity is extremely desirable. It was reported that the viscosity of trihexyl(tetradecyl)phosphonium imidazole ([P66614][Im]) decreases from 810.4 to 648.7 mPa·s upon uptake of CO270 because of the combination of chemisorption and physisorption. Our group also found an efficient way to decrease the viscosities of ILs by introducing ether groups into the cations. The viscosities of ether-functionalized ILs are lower and obviously decrease with the increasing number of ether oxygen atoms, and the decrease degree of the viscosity is up to 43% compared with nonfunctionalized analogues.67 Another useful way to solve the inherently high viscosities of ILs is adding some molecular solvents into ILs to form IL-based solvents. For instance, when ILs instead of some water are mixed with organic amine solvents, not only the viscosity of IL-based absorbent significantly decreases but also CO2 absorption capacities have no obvious change, which is regarded as one of the promising technologies.60 In addition, combining the ILs with polymer membranes to prepare IL-based membranes is another good option to avoid the high viscosity of ILs, and more importantly, gas separation performances are significantly improved. It was found that the addition of free ILs can increase the permeability of CO2, N2, and CH4 by 300−600% for the poly(IL)-IL composite membranes compared with the analogous neat poly(IL) membranes.71 For the mixed matrix membranes (MMM) containing poly(IL) or polymers, free ILs and porous particles, the presence of ILs can improve the interfacial adhesion between particles and organic polymers, resulting in the increase of CO2 permeability and selectivity.72,73 Hence, both IL-based solvents and membranes show great potentials for industrial applications of CO2 capture. Up to date, several review papers related on CO2 capture and separation with IL-based solvents and membranes have been published. For example, Ramdin et al. provided an overview of the experimental data of CO2 solubility, selectivity, and diffusivity in different ILs. Then the state of art on task-specific ILs and supported IL membranes was discussed.74 Bara et al. mainly focused on the CO2 separation performance of imidazolium-based ILs, including conventional ILs, function-

1. INTRODUCTION A large quantity of greenhouse gases in atmosphere, such as CO2 emitted from industries, has caused serious global warming problems.1−3 On the other hand, in the field of energy gas resources, such as natural gas, shale gas, biogas, syngas and so on, contains a certain amount of CO2 as impurity that decreases the heating values and the qualities of the gases, demands high energy consumption for conversion and transport, and corrodes pipelines and equipment.4−7 Therefore, CO2 separation has become one of the important solutions to green-house gas control and gas upgrading and purification. Up to date, there are many technologies for CO2 separation, for example, pressure swing absorption (PSA),8−11 physical or chemical solvent scrubbing,12−16 and gas membrane separation.17−22 However, because of the complexity of the gas components and diverse conditions, most technologies still suffer from higher energy consumption, higher cost, and serious secondary pollutions.23−26 Thus, the development of new gas separation technologies is an enduring R&D topic, and the key is to design new solvents/materials and corresponding novel processes.27−44 In the past 15 years, ionic liquids (ILs) have emerged as potential candidates for CO2 capture and separation owing to their unique molecular structures consisting of cations and anions, and special functional groups, as well as correspondingly outstanding properties, such as nonvolatility, designability, higher CO2 solubility, and selectivity.45−53 Especially, the nonvolatility of ILs makes the captured CO2 be easily released from the saturated solvents, resulting in decreasing the energy consumption and the environmental concerns in CO2 capture process than the traditional amine scrubbing method to a great extent.23 For example, it was evaluated by process simulation that compared with the well-known methyldiethanolamine (MDEA) process, the single-stage and multistage IL-based CO2 capture process can reduce the total energy including electricity and thermal energy by 42.8% and 66.04%, respectively.54 It was also found that the process of IL-amine hybrid solvents 1butylpyridinium tetrafluoroborate ([Bpy][BF4]) monoethanolamine (MEA) for CO 2 capture can save about 15% regeneration heat duty in the reboiler compared with the MEA method.55 Moreover, the substitution of the conventional volatile solvents by ILs can also avoid the loss of solvents into the purge gas so as to prevent atmospheric pollutions caused by organics.56 Additionally, the structure tunability of ILs provides an extra degree of freedom for designing solvents with certain specific characteristics.57,58 Therefore, ILs as designable solvents are potential options for energy and cost efficient capture of CO2. Since Blanchard et al.59 first reported that CO2 can be efficiently dissolved in ILs at 25 °C and pressure up to 40 MPa, but the ILs are insoluble in CO2, the extensive scientific research on CO2 absorption with ILs, including conventional ILs with physisorption and functionalized ILs with chemisorption, have been carried out in succession.47,60−62 For conventional ILs, the anions generally play a dominant role in CO2 absorption, and the effect of the cations is relatively minor. However, CO2 absorption capacity and selectivity of these physisorption ILs cannot compete with the traditional amine solvents. In order to further improve CO2 absorption performances, functionalized ILs were designed and developed subsequently.63−67 For example, the solubility in the aminefunctionalized IL 1-aminoethyl-2,3-dimethylimidazolium taur9626

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IL mixtures, absorption mechanisms with molecular simulation, transport property, and process design and assessment.47 By analyzing the relevant review papers for CO2 capture and separation, it is clearly found that the research interests were mainly focused on the following aspects: (1) the design and synthesis of new ILs, especially task-specific ILs for improving CO2 absorption performances by the introduction of functional groups into cations and anions, and the study on the mechanism of CO2 capture; (2) combining ILs with membrane materials and other molecular solvents, such as water and amines, to form new IL-based membranes and solvents, such as SILMs, poly(IL) membranes, IL-based composite membranes, IL-amine mixtures, and so on. The aim of the modification is to improve CO2 capacity and selectivity and avoid the high viscosity of ILs and accelerate gas mass transfer in solvents according to industrial applications; (3) the measurement and prediction of physiochemical properties of the multicomponent systems containing ILs, CO2, and other components, and the establishment of suitable thermodynamic models; and (4) the investigation of transport properties, absorption kinetics, and IL-based CO2 capture process design and assessments considering the energy consumption and economic performances from the perspective of applications. Although a great deal of progresses have been made in the four aspects mentioned above, the comprehensive review covering structures effects, microscopic interaction, transport behaviors, and scale up and process design for IL-based CO2 separation from the viewpoint of industrialization, especially, the latest progresses of new IL-based materials for CO2 capture, has not been reported. Therefore, the aim of this paper is to give a systematic review on the advances of IL-based CO2 capture systems from molecule to engineering, and mainly focuses on the structure−property relationship of pure ILs and IL-based solvents and membranes, CO2 separation mechanism with molecular simulation and experimental characterization, transport properties of IL fluid and scale up, as well as the process design and assessment of IL-based CO2 separation processes. Moreover, the future directions and prospects for CO2 capture and separation with IL-based materials are also presented.

alized ILs, IL-amine solutions, and supported IL membranes. The group contribution theory for interpretation of CO2 solubility and selectivity in imidazolium-based ILs was conducted.75 Subsequently, he presented the research progress and perspective of room-temperature ionic liquids (RTILs), ILamine mixtures, and IL-based composite materials for CO2 capture, focusing on supported ionic-liquid membranes (SILMs), poly(RTIL) membranes, and poly(RTIL)-RTIL composites membranes.76 MacDowell et al. overviewed three large-scale CO2 capture technologies, i.e., solvent-based chemisorption technique, carbonate looping technology, and oxyfuel process. As novel and designable solvents, the feasibility of using ILs to capture CO2 was discussed, mainly focusing on the structure effects on CO2 absorption and IL-amine mixtures.77 Larachi et al. reviewed a variety of IL-based techniques involving RTILs, task-specific ILs, SILMs and polymerized ILs for CO2 capture, extending from low to high temperature applications, and discussed advantages and drawbacks of the corresponding IL-based methods.78 Karadas et al. summarized and compared CO2 solubilities in several ILs, and analyzed the CO2−IL interaction theoretically. Also, the absorption abilities of other gases using ILs, such as H2S and hydrocarbons were reviewed, and IL-based CO2 absorption process design was involved.79 Torralba-Callejan et al. summarized 112 solubility data of CO2 in totally 65 ILs including imidazolium, pyrrolidinium, pyridinium, ammonium, and phosphonium-based ILs under different temperatures and pressures, and simply discussed the effect cations and anions on CO2 absorption.80 Kumar et al. reviewed the state-of-the-art of comprehensive applications of aqueous amines, ILs, IL-amine blends, and binding organic liquids for natural gas sweetening. Physicochemical properties of ILs (such as density, viscosity, melt point, surface tension, and thermal stability), CO2 solubility in ILs and CO 2−IL interaction mechanism, thermodynamic models, as well as process simulation investigations were introduced.81 Cui et al. focused on the recent progresses in CO 2 chemisorption through site interaction by IL systems, such as amino-based ILs, aminofree ILs and IL-amine blends. Furthermore, CO2 capture mechanisms of these site-containing liquid absorbents, and novel strategies to activate the existent reactive sites and develop novel reactive sites were presented.82 Tomé et al. demonstrated a perspective on different strategies that use ILbased materials as a unique tunable platform to design taskspecific advanced materials for CO2 separation membranes. Based on the analysis of the data hitherto reported, different membranes including supported IL membranes, polymer/IL composite membranes, gelled IL membranes and poly(IL)based membranes were evaluated in detail in terms of CO2 separation efficiency. Finally, an integrated perspective of ILbased membranes on technology, economy and sustainability was provided.83 Hu et al. discussed the key factors influencing the solubility of gases in ILs, including sample purity, experimental methodology, molecular characteristics of ILs, temperature, and pressure. The experimental, computational, and theoretical developments in conformational equilibria of ions, in nanosegregated polar and nonpolar domains in ILs, and in the mechanisms for dissolution of gases in ILs were discussed and the new microscopic mechanism for dissolving the gases in ILs were proposed.32 Zhang et al. reviewed the progress of CO2 capture with ILs from the viewpoint of multiscale, including CO2 absorption capacity in pure ILs and

2. STRUCTURE EFFECTS The tunability of structures as one of the most important features of ILs makes them possess unique properties compared with other organic solvents.84 Millions of possible cations and anions, as well as functional groups, can tailor not only specific physiochemical properties of ILs, but also their CO2 separation performances.85−87 Therefore, in this section, we will concentrate on the effect of anions, cations, and functional groups on CO2 solubility and selectivity of pure ILs including physisorption ILs and chemisorption ILs, and the recent progresses on IL-based solvents and IL-based membranes. 2.1. Pure Ionic Liquids

2.1.1. CO2 Solubility in Ionic Liquids. 2.1.1.1. Physisorption Ionic Liquids. Since the first report on efficient absorption of CO2 by the conventional IL 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), 0.75 mol CO2·mol IL−1 at 25 °C and 8.30 MPa,59 ILs have received extensive attention as promising candidates for CO2 capture in recent years. The ILs can absorb CO2 through the physical interaction between cations and anions of ILs and CO2. Therefore, CO2 solubility is generally determined by the types of cations and anions. 9627

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fluorine-substituted IL results in the greater free volume to accommodate more CO2 gas and the stronger interaction between CO2 and the fluorinated alkyl chains, which is favorable for physical dissolution of CO2 in ILs.108 However, some functional groups, such as the hydrogen of the C2 position on the imidazolium ring substituted with methy groups, ether groups, or nitrile groups on cations, have a slightly negative impact on CO2 solubility.109−112 For example, Brenneck et al.109 studied the effect of the hydrogen on the C2 position on CO2 solubility in three pairs of ILs (Figure 2):

Imidazolium-based ILs are most widely investigated for CO2 capture, and a large number of CO2 solubility data in these ILs has been reported in the literature.88−91 Comparing with cations of ILs, anions are considered to play a primary role in CO2 absorption. Anthony et al.89 systematically investigated the effect of cations (such as imidazolium, ammonium, pyrrolidinium, and phosphonium), and anions (Tf2N, PF6 and BF4) on CO2 solubility. They found that when the anion is the same, the types of cation have a slight influence on CO2 solubility. In contrast, the effect of anions is more obvious. Hou et al.92 and Yunus et al.93 also found that the imidazolium and pyridniumbased ILs with the same anions, which have the same alkyl chain and anion but different heterocyclic aromatic rings, have the similar CO2 solubility. Brennecke et al. measured CO2 solubilities in the ILs with the same cation ([Bmim]) and several different anions. The solubility of CO2 in [Bmim]-based ILs increased in the following order: [NO3] < [dca] < [BF4] < [PF6] < [TfO] < [Tf2N] < [methide] (tris(trifluoromethylsulfonyl)methide) at 25 °C,90,91 which was confirmed by the COSMO-RS simulation by Maiti and Sistla’s work.94,95 In addition, the ILs containing fluor groups on the anion have better affinity for CO2 than the ILs with nonfluorinated anions, and CO2 solubility in ILs increases with the increasing number of fluor groups on the anions.96−99 For example, the trend of CO2 solubility followed the order: [BF4] < [PF6] < [Tf2N] < [methide] < [FAP] (tris(pentafluoroethyl)trifluorophosphate).90,91 Zhang et al. using COSMO-RS method further proved that the longer fluoroalkyl chain in the anion (e.g., [FAP] anion), the higher CO2 solubility.97 Although the contribution of cations to CO2 solubility is relatively minor, the cation effects are not inconsequential. For the ILs with the same anion, the presence of long alkyl chains, the fluorination of cations, ester groups on cations are favorable for improving marginally CO2 solubility.91,100−106 For example, CO2 solubility increased with the alkyl chain length on the imidazolium ring in the following cation order: [Omim] > [Hmim] > [Bmim].90 The similar results were obtained from the Yunus’s work.93 Almantariotis et al.107 found that the solubility of CO 2 in the fluorine-substituted IL 1(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8H4F13mim][Tf2N]) is higher than 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Omim][Tf2N]) as shown in Figure 1. The main reason is that the increase of the side chain length and the

Figure 2. Effect of hydrogen at the 2-position on the imidazodium ring: [Emim][Tf2N] ■; [Emmim][Tf2N] □; [Bmim][PF6] ●; [Bmmim][PF6] ○; [Bmim][BF4] ▲; [Bmmim][BF4] Δ.109 Reproduced with permission from ref 109. Copyright 2004 American Chemical Society.

[Bmim][PF6] and 1-butyl-2,3-dimethylimidazolium hexafluorophosphate ([Bmmim][PF6]); 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([Bmmim][BF4]); and 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][Tf2N]) and 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([Emmim][Tf2N]). There is slightly decreased solubility for the methyl-substituted IL compared to the hydrogen-substituted IL in each pair, implying the weak influence of the cations on CO2 solubility. Kazarian et al.113 thought that higher CO2 solubility in the hydrogensubstituted ILs was mostly attributed to the hydrogen bonding between the acidic hydrogen on the imidazolium ring and CO2. Bara et al.110 found that the Henry’s constants for the etherbased ILs [Pnmim][Tf2N] (P = C2H4O, n = 1, 2, 3) are slightly higher than those for nonfunctionalized counterparts [Cmmim][Tf2N] (m = 4, 7, 10), indicating that CO2 solubility is mildly affected by the presence of short oligo(ethylene glycol) units on cations. 2.1.1.2. Chemisorption Ionic Liquids. Although the emerging of ILs provides a new and exciting option for CO2 capture, physisorption ILs cannot still compete with current commercially available solvents because of the lower CO2 absorption capacity, especially for postcombustion flue gas with low CO2 concentration. Therefore, the design and synthesis of functionalized ILs with chemisorption or chemiphysisorption for highly efficient capture of CO2 becomes the hot topic. The functionalized ILs, including amino-based ILs and non-amino ILs, can absorb CO2 through chemical interaction between basic groups and CO2. 2.1.1.3. Amino-Based Ionic Liquids. Taking a clue from the chemistry of organic amines reacting with CO2, Bates et al. first introduced a primary amine into the imidazolium cation and synthesized the amino-functionalized IL, that is, 1-butyl-2-

Figure 1. Effect of fluorination groups on CO2 solubility.107 Reproduced with permission from ref 107. Copyright 2010 American Chemical Society. 9628

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dual amino-functionalized ILs were developed by tethering amino groups on both cations and anions, respectively, such as [aP4443][AA] and [aemmim][Tau]. CO2 absorption capacities of these ILs are found to approach 1.00 mol CO2·mol IL−1, rather than the theoretical absorption capacity of 1.50 mol CO2·mol IL−1 because of the 1:2 mechanism of the aminetethered cations and 1:1 mechanism of the amine-tethered anions.64,65 One explanation for this phenomenon was that the complex formed by the amine-tethered anion reacting with CO2 according to 1:1 stoichiometry was unstable, and could further react with another amine (1:2 mechanism).65 Lv et al.119 and Zhou et al.120 developed two highly efficient absorbents of amine-based amino acid-functionalized ILs 1aminopropyl-3-methylimidazolium glycinate ([APmim][Gly]) and 1-aminopropyl-3-methylimidazolium lysine ([APmim][Lys]), respectively. The CO2 capacities of [APmim][Gly] and [APmim][Lys] aqueous solutions were found to be 1.23 and 1.80 mol CO2·mol IL−1, respectively, which were much higher than that of the most existing dual functionalized ILs because of their low molecular weight. In addition to dual amino-functionalized ILs, Bhattacharyya et al.121 designed the ether functionalized choline tethered amino acid ILs by combing functionalization of choline cation with amino acid anion. They found that the etherification can reduce the viscosity of ether-functionlized ILs by more than 3-fold as compared to conventional choline ILs, and the high CO2 capacity of up to 1.62 mol CO2·mol IL−1 (19.02 wt % CO2) can be achieved. In addition, several methods for enhancing CO2 absorption rate or CO2 capacity through introducing a hydrogen acceptor or an electron-withdrawing site into amino-functionalized anions were also reported. For example, Luo et al.122 introduced a hydrogen acceptor such as N or O atom into amino-functionalized anions to decrease the viscosity of aminofunctionalized ILs after CO2 capture by forming intramolecular hydrogen bond. Compared with the corresponding pure IL, the viscosity of trihexyl(tetradecyl)phosphonium sarcosine ([P66614][Me-Gly]) after CO2 capture increased more than 130-fold, but that of trihexyl(tetradecyl)phosphonium acetylglycine ([P66614][Ac-Gly]) after CO2 capture reduced about 18%. Chen et al.123 introduced an electron-withdrawing site such as activate carboxylate groups into amino acid anions, and superior CO2 capacity of up to 1.69 mol CO2·mol IL−1 in the aminopolycarboxylate-based IL was achieved through multiplesite interactions between amino and carboxylate groups in amino acid anions and CO2. The related reaction mechanism of CO2 with functionalized ILs will be discussed in section 3. 2.1.1.4. Non-amino Ionic Liquids. In spite of relatively great enhancement in CO2 absorption by amino-functionalized species, the introduction of amine groups generally results in higher viscosity of ILs than conventional ILs or commercially available absorbents, and even more, the viscosity of ILs after reacting with CO2 dramatically increase by forming highly viscous glassy or gel-like materials.124,125 For example, the viscosities of amino acid ILs, such as trihexyl(tetradecyl)phosphonium glycinate ([P66614][Gly]), trihexyl(tetradecyl)phosphonium Alanate ([P66614][Ala]), trihexyl(tetradecyl)phosphonium sarcosinate ([P 66614 ][Sar]), and trihexyl(tetradecyl)phosphonium isoleucinate ([P66614][Ile]), upon the addition of CO2 reach 17 000−100 000 mPa·s and increase 48−240 fold as that of the pure ILs at 25 °C, which severely impacts the utilizations of ILs in a normal absorber-stripper configuration for CO2 capture.126 The great increase in

bromopropylamine imidazolium tetrafluoroborate ([NH2pbim][BF4]). The IL exhibits a high capacity of nearly 0.50 mol CO2·mol IL−1 (a stoichiometry of one CO2 to two amines, so-called 1:2 stoichiometry), and it reacts with CO2 in a manner similar to aqueous amines by forming a carbamate salt. Furthermore, the IL shows excellent reversibility and keeps stable absorption performances even after five consecutive absorption and desorption cycles.114 Sharma et al.115 and Sistla et al.116 studied CO2 absorption in a series of primary aminefunctionalized ILs with different anions. It was found that amine-functionalized imidazolium cation increases CO2 absorption capacity by 300% compared to nonfunctionalized ILs, and these ILs also follow the above absorption mechanism. However, Comparing with ILs with amino-functionalized cations, more favorable stoichiometry than one CO2 to two amines is achieved by tethering the amine to anions instead of cations. Gurkan et al. reported two phosphonium-based amino acid ILs, trihexyl(tetradecyl)phosphonium prolinate ([P66614][Pro]) and trihexyl(tetradecyl)phosphonium methioninate ([P66614][Met]), and CO2 absorption capacity of both ILs reached 0.90 mol CO2·mol IL−1, which nearly approaches the 1:1 stoichiometry (Figure 3). Higher absorption capacities than

Figure 3. CO2 absorption by amino-acid-based ILs [P66614][Pro] and [P66614][Met] at 22 °C.117 Reproduced with permission from ref 117. Copyright 2010 American Chemical Society.

those of cation-functionalized ILs and aqueous amines are ascribed to the different reaction mechanisms. Other than the cation-tethered ILs, tethering the amine to the anion favors the carbamic acid rather than the carbamate due to the instability of the produced dianion, therefore resulting in an equimolar CO2 absorption.117 Sistla et al.118 studied CO2 absorption in nine amino-acid-based ILs with [Bmim] cation. The results indicated that these ILs show higher CO2 capacity than amine-functionalized cation ILs and nonfunctionalized ILs, and the IL with arginate anion ([Arg]) has the highest CO2 absorption capacity of among all the ILs in terms of CO2 molar uptake due to the availability of more accessible amine groups. To further enhance the CO2 absorption capacity of ILs, Zhang et al. synthesized a dual amino-functionalized cationtethered IL, 1,3-di(2-aminoethyl)- 2-methylimidazolium bromide (DAIL). CO2 absorption capacity of the aqueous solution of DAIL (10%) could be up to 1.05 mol CO2·mol IL−1 at 30 °C and 0.10 MPa, which is close to the theoretical capacity on the basis of 1:2 stoichiometry. The presence of H2O may contribute to CO2 uptake in the excess of 1.00 mol CO2·mol IL−1 for aqueous solution of DAIL.68 In addition, a series of 9629

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addition, Wang et al.129 also synthesized several superbasederived protic ILs by neutralization of superbases with weak proton donors, such as fluorinate alcohols, imidazoles, pyridines. Among the investigated ILs, 1,3,4,6,7,8-hexahydro1-methyl-2H-pyrimido[1,2-a]pyrimidine trifluoroethanol ([MTBDH][TFE]) and imidazole ([MTBDH][Im]) show higher absorption capacity of 1.13 and 1.03 mol CO2·mol IL−1 at 23 °C and 0.10 MPa, respectively. Similarly, the absorption kinetic is rapid and reach equilibration within 5 min for [MTBDH][TFE], which is related to its low viscosity of 8.63 mPa·s at room temperature. Besides, higher capacities of CO2 absorption for chemisorption ILs are mostly along with stronger interaction than physisorption ILs, therefore resulting in difficult desorption and high energy demand for regeneration. The absorption enthalpy is an important parameter to estimate the interaction between gas and liquid solvent. The enthalpy of CO2 physisorption by ILs is about −20 kJ·mol−1, while the enthalpy of CO2 absorption in amino-functionalized ILs is nearly −80 kJ· mol−1, which means that higher CO2 capacity is achieved at the expense of increasing the enthalpy and energy consumption. Several non-amino functionalized ILs were also developed for improving CO2 capture with both high capacity and low enthalpy.69,130,131 Wang et al.130 designed and synthesized a series of phenolic ILs to achieve highly efficient and energysaving absorption of CO2 through tuning the substituent of the IL anions. For example, CO2 absorption capacity of trihexyl(tetradecyl)phosphonium p-methylphenolate ([P66614][4-MePhO]), trihexyl(tetradecyl)phosphonium p-methoxyphenolate ([P66614][4-MeO-PhO]), trihexyl(tetradecyl)phosphonium phenolate ([P 66614 ][4-H-PhO]), trihexyl(tetradecyl)phosphonium p-chlorophenolate ([P 66614 ][4-Cl-PhO]), trihexyl(tetradecyl)phosphonium p-trifluorophenolate ([P66614][4-CF3−PhO]), and trihexyl(tetradecyl)phosphonium p-nitrophenolate ([P66614][4-NO2-PhO]) is 0.91, 0.92, 0.85, 0.82, 0.61, and 0.30 mol CO2·mol IL−1, respectively. The results indicated that the stronger electron-withdrawing group (Cl, CF 3 , and NO 2 ), the lower CO 2 absorption capacity. Conversely, the introduction of an electron-donating group (MeO and Me) on anions increased CO2 capacity. The changes in the enthalpy of CO2 absorption agree well with the results of CO2 capacity. The reason is related to a quantitative relationship between CO2 capacity, absorption enthalpy and the basicity of phenolic ILs. CO2 capacity decreased with the decreasing pKa value of the anion (Figure 6), and the absorption enthalpy of the phenolic ILs increased from −17.1 to −49.2 kJ·mol−1 with the increasing of Mulliken charge on the oxygen atom, which offers a promising method for CO2

viscosity was attributed to the formation of strong and pervasive hydrogen-bond networks between the zwitterions and dication species during CO2 absorption by aminofunctionalized ILs.127,128 To overcome this problem, an efficient strategy for improving CO2 absorption without forming hydrogen-bond networks was proposed by nonamino functionalized ILs.70,129 Some typical anion structures of ILs were listed in Figure 4.

Figure 4. Typical structures of anions of non-amine ILs for CO2 capture: [Im], imidazole; [Tetz], tetrazole; [2-Op], 2-hydroxypyridine; [3-Op], 3-hydroxypyridine; [4-Op], 4-hydroxypyridine; [3-OCH3-2Op], 3-methoxy-2-hydroxypyridine; [2-Ph-Im], 2-phenylimidazole; [4Br-C6H4COO], 4-bromo-benzoic acid; [C6H4COO], benzoic acid; [4Br-C6H4O], 4-bromophenol; [Benim], benzimidazole; [BenTriz], benzotriazole.

A kind of non-amino phosphonium ILs with different azole anions including [P 66 6 1 4 ][Im], trihexyl(tetradecyl)phosphonium pyrazole ([P66614][Pyr]), trihexyl(tetradecyl)phosphonium trizole ([P66614][Triz]), trihexyl(tetradecyl)phosphonium tetrazole ([P66614][Tetz]), trihexyl(tetradecyl)phosphonium oxazolidinone ([P66614][Oxa]), trihexyl(tetradecyl)phosphonium pyrazole ([P66614][Pyr]), and trihexyl(tetradecyl)phosphonium phenol ([P66614][Pho]), not only reach equimolar absorption of CO2 because of the tunable anion basicity but also it only takes about 10 min to complete the absorption, substantially faster than amine-functionalized ILs (Figure 5). The rapid absorption rate mainly ascribes to a little change of the viscosities of the non-amino ILs during CO2 absorption because of the absence of hydrogen-bond networks. For example, the viscosity of [P66614][Pyr] increases from 245.4 to 555.1 mPa·s at 23 °C, whereas the viscosity of [P66614][Im] decreases from 810.4 to 648.7 mPa·s upon uptake of CO2.70 In

Figure 5. Rapid CO2 absorption by non-amino functionalized ILs: ◊ [P66614][Pyr], □ [P66614][Im], ▽[P66614][Triz], ○ [P66614][Oxa], ◁ [P66614][Tetz], Δ [P66614][PhO].70 Reproduced with permission from ref 70. Copyright 2011 John Wiley & Sons.

Figure 6. Relationship between CO2 absorption capacity and pKa values of anions.130 Reproduced with permission from ref 130. Copyright 2012 John Wiley & Sons. 9630

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capture with both high capacity and excellent reversibility. Zhang et al.132 introduced the fluorine atom into the phenolic anion to synthesize low viscous fluorine-substituted phenolic ILs, including tetrabutylphosphonium 2-fluorophenolate ([P4444][2-F-PhO]), tetrabutylphosphonium 3-fluorophenolate ([P4444][3-F-PhO]), and tetrabutylphosphonium 4-fluorophenolate ([P4444][4-F-PhO]). The viscosities of the three fluorinesubstituted phenolic ILs are found to be less than 64.8 mPa·s at 30 °C, which results in the very fast rate of CO2 absorption with the absorption equilibrium time within 230 s at 40 °C. CO2 absorption capacities of [P4444][2-F-PhO], [P 4444][3-FPhO], [P4444][4-F-PhO] at 0.10 MPa and 40 °C are as high as 0.673, 0.741, 0.842 mol CO2·mol IL−1, respectively. Vafaeezadeh et al.133 reported that a novel phenolic-based IL 1-(2hydroxyethyl)-2,3-dimethy lim id azolium pheno xide ([EOHmmim][PhO]) has very high CO2 capacity of 1.50 mol CO2·mol IL−1 because of the cooperative interactions between phenoxide and hydroxyl groups and CO2. Subsequently, Wang et al.131 also proposed another effective way for efficient and reversible CO2 absorption by new nonamino ILs making use of multiple site cooperative interactions. In their study, pyridine-containing functionalized ILs were synthesized by introducing a nitrogen-based interacting site on the phenolate anion, and an extremely high capacity of up to 1.60 mol CO2·mol IL−1 was achieved for trihexyl(tetradecyl)phosphonium 3-methoxy-2-hydroxypyridine ([P66614][3-OMe32-Op]) through two site interactions between electronegative oxygen and nitrogen atoms in the anion and CO2, respectively, which is significantly greater than 1:1 stoichimetry. Therefore, the use of cooperative interactions in gas separation also provides a general strategy for enhancing the capacity of other acid gases. 2.1.2. CO2 Selectivity in Ionic Liquids. On the basis of the above review, a large number of CO2 solubility data in various ILs are available from the literature.91,129,134−138 However, in practice, gas separation and purification processes usually involve gas mixtures, such as flue gas, natural gas, and biogas containing impurities (like SO2, H2S, CH4, CO, N2, O2, and H2) besides CO2. Thus, only CO2 solubility data is not enough to judge the separation performances of an absorbent, so the study on selectivity for different gas components is also necessary. Unfortunately, the selectivity data of CO2 in ILs and related systematical research are relatively scarce. Anderson et al.139 measured different gas solubilities in 1-hexyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([Hmpy][Tf2N]) following the order: N2 < O2 < CH4 < C2H6 < C2H4 < CO2 < SO2 (Figure 7), but the gas selectivity was not definitely given. In general, the ideal solubility selectivity was used to evaluate the selectivity of CO2 toward other gas in ILs, which can be obtained by the ratio of Henry’s law constant of other gas to that of CO2 in ILs under the same temperature.5 Therefore, lower N2 and O2 solubility than hydrocarbon solubility implies the higher CO2/N2 and CO2/O2 selectivity for [Hmpy][Tf2N] than CO2/hydrocarbon selectivity. Similar results are also found for 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][Tf 2 N]) 89 and [Bmim][PF6].88 2.1.2.1. CO 2 /Hydrocarbon Selectivity. Noble et al.110,111,140,141 and Zhang et al.142 reported CO2 and CH4 solubility data in some conventional imidazolium-based ILs, such as [Bmim][BF4], [Bmim][Tf2N], 1-butyl-3-methylimidazolium nitrate ([Bmim][NO3]), and 1-butyl-3-methylimidazolium dicyanamide ([Bmim][dca]). The results showed that the

Figure 7. Gas solubility in [Hmpy][Tf2N] at 25 °C: ● SO2; ○CO2; ■ C2H4; □ C2H6; ▲ CH4; ΔO2; ▼ N2.139 Reproduced with permission from ref 139. Copyright 2007 American Chemical Society.

solubility of CH4 is much lower than that of CO2 in these ILs, but the CO2/CH4 selectivity in ILs is lower than 12, which is still unable to compete with the traditional physical absorbents, such as Rectisol and sulfolate. Subsequently, Ramdin et al.143 studied CO2 and CH4 solubility, as well as CO2/CH4 selectivity in ten ILs such as piperidinium, pyrrolidinium, ammonium, and phosphonium-based ones. Among the investigated ILs, 1-allyl3-methylimidazolium dicyanamide ([Amim][dca]) and 1-butyl1-methylpyrrolidinium dicyanamide ([Bmpyrr][dca]) showed the higher CO2/CH4 selectivity of 21.8 and 16.7 due to the lower CH4 solubility, respectively. However, compared with the imidazolium-based ILs mentioned above, CO2/CH4 selectivity in other kinds of ILs is not significantly improved due to the simultaneous increase of CO2 and CH4 solubility. For example, Henry’s law constants of CO2 and CH4 in [Bmim][Tf2N] was 41 and 420 bar at 40 °C, respectively,111 whereas Henry’s law constants of CO2 and CH4 in 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide ([Bmpip][Tf2N]) decreased drastically to 5.01 and 43.93 bar, respectively.143 It was thought that CO2/CH4 selectivity decreased with the increasing IL molecular weight, implying that high CO2/CH4 selectivity can be obtained using the ILs with low molecular weight. As we know, CO2 solubility usually increases with the increase of IL molecular weight and free volume,74 while CH4 solubility is mainly determined by the CH4−IL interaction but not directly related to the IL molecular weight. Therefore, there is a tradeoff between CO2 solubility and CO2/CH4 selectivity. Carvalho et al.144 studied the effect of IL polarity on CH4 solubility and CO2/CH4 selectivity. CH4 was more soluble in ILs with longer nonpolar alkyl chains, and CO2/CH4 selectivity was correlated well with Kamlet−Taft β parameter, which proposes a key to the design of ILs with enhanced selectivities. Pereira et al.145 investigated the phase equilibrium of CO2, CH4 and N2 in [Bmim][dca], and compared Henry’s constants for the studied gases and CO2 selectivity with other ILs and commercial solvents, such as trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([THTDP][Tf2N]), 1-ethyl-3methylimidazolium methylphosphonate ([C 2 C 1 im][CH3OHPO2]), 1-butyl-3-methylimidazolium thiocyanate ([C4C1im][SCN]), N-methyl-2-hydroxyethylammonium propionate (2mHEAPr), glycol monoethyl ether (TEGMME), and aqueous solutions of MEA (40 wt %) in Figure 8. The results 9631

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CO2/C3H8, CO2/C2H6 and CO2/CH4 were 9.69, 2.90, 1.33, and 0.62 at 40 °C, respectively. 2.1.2.2. CO2/Diatomic Gas Selectivity. For diatomic gases, for example, N2, O2, CO, and H2, the solubilities in ILs were found to increase in the following trend: H2 < CO < N2 < O2 < CH4 < CO2, indicating higher selectivity for CO2 to these diatomic gases than CO2/CH4 selectivity. Jacquemin et al.152 measured the solubilities of CO2, CH4, N2, O2, CO, and H2 in [Bmim][PF6] under different conditions, and found some discrepancies in the solubility data in comparison with other work reported by different authors, especially for N2, O2, CO, and H2. For example, Anthony et al.89 could not detect CO solubility in [Bmim][PF6], while the Henry’s constant of CO at 20 °C was reported to be 3270 bar by Ohlin et al.153 and 1236 bar by Jacquemin et al.,152 respectively. In spite of large discrepancies in these gases with relatively low solubilities, these gas solubilities in a chosen IL still obey the similar trend mentioned above. Moreover, these gases were more soluble in [Bmim][PF6] than those in [Bmim][BF4] (except similar solubility of H2 in these two ILs) because of the free volume contribution of [PF6] anion, leading to higher selectivities of CO2/N2, CO2/O2, and CO2/CO in [Bmim][PF6].154 Finotello et al.140 focused on CO2, CH4, H2, and N2 solubility and CO2 selectivity in [Emim][Tf2N], [Emim][BF4], 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Hmim][Tf 2 N]), and 1,3-dimethylimidazolium methyl sulfate ([mmim][MeSO4]) at different temperatures. They found that higher ideal selectivity of gas pairs (CO2/N2, CO2/H2, and CO2/CH4) is related to the ILs with the smaller molar volumes, for example, [mmim][MeSO4] and [Emim][BF4] at all temperatures, which agrees well with previous studies on 1ethyl-3-methylimidazolium dicyanamide ([Emim][dca]) and 1ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim][CF3SO3]).155,156 Shi et al.157,158 further confirmed that H2 solubility is highly dependent upon the molar volume of ILs by both molecular simulations and experimental studies. They proposed to use ILs, for example, 1-ethyl-3-methylimidazolium acetate ([Emim][Ac]) with small molar volume (leading to low H2 solubility) and strong interaction with CO2 to achieve high CO2 solubility and high CO2/H2 selectivity. Simulation results indicated that H2 solubility in [Emim][Ac] are about 6 times smaller than that for [Hmim][Tf2N]. The low H2 solubility in [Emim][Ac] is due to the small molar volume of IL (about half of that for [Hmim][Tf2N]), and the weak interactions between H2 and the IL. Ohlin et al.153 determined CO solubility in 37 ILs using high-pressure 13C NMR spectroscopy, and studied the effect of IL structure on CO solubility. It was found that CO solubility is similar to, or higher than, that of H2 for the ILs,159 but CO solubility is more dependent upon the nature of ILs than H2 presumably due to the presence of a dipole moment and higher polarizability. For [Bmim]-based ILs, CO solubility increases according to the series of anions: [BF4] < [PF6] < [SbF6] (hexafluoroantimonate) < [CF3CO2] (trifluoroacetic acid) < [Tf2N], which coincides with an increase in the anion size and a decrease in π* interactions. For [Tf2N]-based ILs, CO solubility increases with the chain length of the alkyl substituent on the cations, and decreases when the alkyl group is replaced by a benzyl group because of an increase in π* interactions. 2.1.2.3. CO2/H2S and CO2/SO2 Selectivity. Compared with that of the hydrocarbon and diatomic gases, the selectivity data of CO2/H2S and CO2/SO2 in ILs are relatively scarce. In general, acid and polar gases, for example, H2S and SO2

Figure 8. CO2/CH4 selectivity of ILs and some organic solvents at 40 °C.145 Reproduced with permission from ref 145. Copyright 2016 Elsevier.

indicated that high CO2/CH4 selectivity can be achieved in most ILs compared to traditional solvents. Although CH4 solubility in [Bmim][dca] is similar to that of other sizeequivalent ILs, its CO2 solubility is lower, leading to one of the lowest selectivities in these reported ILs. A similar behavior is reported for [C2C1im][CH3OHPO2] and is an indication of the unfavorable interactions of IL-CH4, CO2, N2O, and N2 present on highly polar ILs.146 The observed results highlighted that a delicate balance between the solvent polarity and its molar volume should be considered for screening a highly selective solvent for CO2/CH4 separation, which is also applied to CO2/ N2 separation. In addition, introducing nonreactive and polar functional groups into cations of ILs, such as ethers and nitriles, is regarded as an effective way to improve CO2/CH4 and CO2/N2 selectivity.67,75,110,111,147,148 Compared with that of the nonfunctionalized analogues, the presence of ether chains and nitrile groups has no substantial effect on CO2 solubility; however, ideal selectivities for CO2/CH4 and CO2/N2 are enhanced by 33−75% because of an appreciable reduction in CH4 and N2 solubility. For instance, the decrease in CH4 solubility for nitrile-based ILs [NCCnmim][Tf2N] relative to [H3CCnmim][Tf2N] was 47%, 45%, and 35% for n = 1, 3, and 5, respectively. Likewise, the solubility reduction for N2 was found to be 38%, 40%, and 33% for n = 1, 3. and 5, respectively. However, the decrease in CO2 solubility was 17% for 1ethanenitrile-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([NCC1mim][Tf2N]), and no observable difference for 1-hexanenitrile-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([NCC5mim][Tf2N]) was found. In other words, the improvement of the CO2/CH4 and CO2/N2 selectivity in functionalized ILs with polar units is by decreasing CH4 and N2 solubility rather than increasing CO2 solubility. Besides CH4, Althuluth et al.149 also measured the solubilities of other small hydrocarbons, for example, ethane (C2H6), propane (C3H8), and butane (C4H10) in [Emim][FAP] under similar conditions, which decreased in the following order: C4H10 > CO2 > C3H8 > C2H6 > CH4. With the longer hydrocarbon chain, the solubility of hydrocarbons in [Emim][FAP] increases because of the stronger dispersive forces between the alkyl chain of ILs and the longer chain of hydrocarbons.150 Similar results are found for the solubility of the hydrocarbons in [Bmim][Tf2N].151 Conversely, CO2 selectivity decreases as the hydrocarbon chain became longer, and followed the order: CO2/C4H10 < CO2/C3H8 < CO2/C2H6 < CO2/CH4. For example, the selectivities of CO2/C4H10, 9632

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not enough to enhance O2 solubility by a significant amount. The results also agree well with subsequent reinvestigation.168 In other words, if one or both gases in a binary mixture are sparingly soluble, one can expect that the mixture will be close to ideal. While for a mixture containing two solutes in which both have relatively high solubilities, such as CO2/SO2, competition will exist and ideal selectivities should not be expected because of the strong interaction between CO2 or SO2 and ILs. Yolozeki et al.169,170 developed a reliable equation of state (EOS) model for predicting CO2/SO2 selectivity of the CO2/SO2 binary mixture in 1-butyl-3-methylimidazolium methyl sulfate ([Bmim][MeSO4]) and [Hmim][Tf 2 N], respectively. CO2/SO2 selectivity in these two ILs is mainly dependent on CO2/SO2 feed ratios and IL amount. As shown in Figure 9, for large and equimolar CO2/SO2 mole ratios,

generally show higher solubilities in ILs than CO2. Ramdin et al.160 obtained the solubility of precombustion gases CO2, CH4, CO, H2, N2, and H2S in [Bmim][Tf2N] by Monte Carlo simulations. The results showed that gas solubility trend obeys the following order: H2S > CO2 > CH4 > CO > N2 > H2, and H2S solubility in [Bmim][Tf2N] is over three times higher than CO2 solubility (CO2/H2S ideal selectivity of 0.33), implying that it is difficult to selectively capture CO2 from H2S by [Bmim][Tf2N] at precombustion conditions. Therefore, the syngas, natural gas and flue gas is often desulfurized prior to CO2 removal. Huang et al. successively synthesized different kinds of functionlizaed ILs, such as carboxylic acid-based ILs,5 amino acid-based ILs,6 tertiarty amine-based ILs,7 and benzoate-based ILs,161 for selective absorption of H2S from CO2. For example, the order of H2S/CO2 selectivity in four protic ILs including methyldiethano lammonium acetate ([MDEAH][Ac]), methyldiethanolammonium formate ([MDEAH][For]), dimethylethanolammonium acetate ([DEAH][Ac]), and dimethylethanolammonium formate ([DMEAH][For]) follows the sequences of [DMEAH][For] > [DMEAH] [Ac] > [MDEAH][For]> [MDEAH][Ac], and the highest value is 19.5 for [DMEAH][For] at 30 °C.5 The selectivity of H2S/CO2 in the tertiary amine-based IL bis(2dimethylaminoethyl)ether bis(trifluoromethylsufonyl)imde ([BDMAEE][Tf2N]) can reach 37.2 (H2S solubility at 0.10 MPa vs CO2 solubility at 0.10 MPa) and 15.4 (H2S solubility at 0.01 MPa vs CO2 solubility at 0.10 MPa) at 25 °C.7 Similarly, selective absorption of SO2 from CO2 by azole-based ILs162,163 and pyridium-based ILs164,165 were also investigated. For instance, our work demonstrated that SO2 capacity in four pyridium-based ILs, including 1-(2-diethylaminoethyl)pyridinium thiocyanate ([NEt2C2Py][SCN]), 1-ethyoxylethylpyridinium thiocyanate ([C4OPy][SCN]), and 1-valeronitrilepyridinium thiocyanate ([C4CNPy][SCN]) and [BPy][SCN], is much higher than CO2 capacity because of the stronger interaction between ILs and SO2, therefore leading to SO2/ CO2 selectivity in the range from 56 to 79.165 2.1.2.4. Real Selectivity of CO2/Mixed-Gas. The CO2 selectivity presented above is regarded as “ideal selectivity”, which is determined from pure gas solubility with the assumption of ideal mixing. However, real gases usually involve many other compounds besides CO2, and interaction and competition between different solutes may influence the dissolution process, which means that the presence of CO2 and any other component can affect gas solubilities and change CO2 selectivity. Consequently, the measurement of the solubility of gas mixtures in ILs is of great importance. However, given the difficulties associated with performing mixed-gas measurements, the limited investigations for the simultaneous solubility of gas mixtures in ILs have been carried out so far. Hert et al.166 measured CO2/O2 and CO2/CH4 mixture solubilitites in [Hmim][Tf2N]. They thought that comparing with the pure gas, the presence of CO2 increases O2 and CH4 solubilities due to favorable dispersion interactions between dissolved CO2 and either O2 or CH4, while CO2 solubility decreases because of a reduction in free volume caused by O2 and CH4 absorption. In contrast, Shi et al.167 predicted the CO2 and O2 solubilities for CO2/O2 mixture by molecular simulation, which are nearly to the solubility of single gas, with little enhancement or competition between the two gases. The reason is that the interaction energy between CO2 and O2 is extremely small, suggesting that it is unlikely that CO2 interacts with O2 to any appreciable extent, and certainly

Figure 9. (a) Plots of calculated selectivity vs [Hmim][Tf2N] amount with different CO2/SO2 feed ratios at 25 °C and 0.10 MPa. (b) Selectivity plots without [Hmim][Tf2N] as a function of total pressure. Lines: dotted line = 1/9 CO2/SO2 feed ratio; broken line = 1/1 CO2/ SO2 feed ratio; solid line = 9/1 CO2/SO2 feed ratio.169 Reproduced with permission from ref 169. Copyright 2009 American Chemical Society.

CO2/SO2 selectivity was nearly independent of the amount of IL addition. However, for small CO2/SO2 mole ratios the IL addition significantly increased CO2/SO2 selectivity. The feed ratio of 9/1 (CO2/SO2) with [Bmim][MeSO4] and [Hmim][Tf2N] had a selectivity range of 29−31 and 226−348, respectively, whereas the corresponding case without ILs only showed a selectivity range of 3−9 (Figure 9). A similar method is applied for CO2/H2S binary mixture in [Bmim][MeSO4],171 [Bmim][PF6],172 and 1-octyl-3-methylimidazolium hexafluorophosphate ([Omim][PF6]).173 For 9633

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Table 1. CO2 Absorption Capacities of IL Blending Solventsa IL mixtures

CO2 absorption capacity (mol CO2·mol IL−1)

conditions (MPa/°C)

ref

[Bmim][NO3] + H2O (Rmo = 98.01:1.99) [Hopmim][NO3] + H2O (Rmo = 95.89:4.11) [iBu3MeP][TOS] + H2O (Rmo = 4:96) [N2224][Ac] + H2O (Rmo = 1:2) [P66614][2-CNPyr] + H2O (Rma = 95.5:4.5) [Hmim][Tf2N] + MEA (Rmo = 1:1) [N1111][Gly] + MDEA (Rma = 1:1) [N2222][Gly] + MDEA (Rma = 1:1) [N1111][Lys] + MDEA (Rma = 1:1) [N2222][Lys] + MDEA (Rma = 1:1) [POHmim][Cl] + MEA (Rmo = 1:2) [EOHmim][dca] + MEA (Rma = 1:3) [Bmim][dca] + MEA (Rma = 1:3) [choline][Pro] + PEG-200 (Rma = 1:1) [Emim][Ac] + [Emim][TFA] (Rmo= 49.98:50.02) [Emim][BF4]+ [Omim][Tf2N] (Rma= 1:1) [Bmim][BF4] + [Omim][Tf2N] (Rma= 1:1) [Emim][EtSO4] + [Emim][Ac] (Rmo= 1:1)

∼0.1 ∼0.099 ∼0.030 0.197 0.9 0.5 0.56 0.64 0.69 0.74 0.396 0.638 0.652 0.520 0.124 0.32 0.34 0.19

0.153/50 0.233/45 0.45/16 0.01/25 0.01/22 0.01/40 0.0097/25 0.0097/25 0.0097/25 0.0097/25 0.01/35 0.0185/40 0.0233/40 0.0107/65 0.01/50 0.218/40 0.222/40 0.393/25

175 175 176 177 178 179 180 180 180 180 181 182 182 183 184 138 138 185

a Rma = mass ratio; Rmo = mole ratio; [iBu3MeP][TOS] = tri-iso-butyl(methyl)phosphonium tosylate; [N2224][Ac] = triethylbutylammonium acetate; [P66614][2-CNPyr] = trihexyl(tetradecyl)phosphonium 2-cyano-pyrrolide; [N1111][Gly] = tetramethylammonium glycinate; [N2222][Gly] = tetraethylammonium glycinate; [N1111][Lys] = tetramethylammonium lysinate; [N2222][Lys] = tetraethylammonium lysinate; [POHmim][Cl] = 1hydropropyl-3-methylimidazolium chloride; [EOHmim][dca] = 1-(2-hydroxyethyl)-3-methyl-imidazolium dicyanamide; [Choline][Pro] = (2hydroxyethyl)trimethylammonium-2-pyrrolidine-carboxylic acid salt; [Emim][TFA] = 1-ethyl-3-methylimidazolium trifluoroacetate; [Emim][EtSO4] = 1-ethyl-3-methylimidazolium ethylsulfate.

CO2 solubility of in two hygroscopic ILs [Bmim][NO3] and hydroxypropylmethylimidazolium nitrate ([Hopmim][NO3]) and found that when the water concentration is low, CO2 solubility in [Bmim][NO3] water solution is slightly higher than that of in pure [Bmim][NO3]. However, when the water concentration becomes higher, it will lead to lower CO2 solubility because of the positive excess molar volumes of the aqueous solutions under the working conditions. Similar investigations on other IL−water systems for CO2 separation also showed that the presence of water in ILs can result in different reduction in the viscosity and CO2 absorption capacity.176,177,186 Wang et al.177 found that [N2224][Ac] has a little higher absorption capacity than that of its aqueous solution due to the different reaction mechanisms. Pure [N2224][Ac] can only react with CO2 through Lewis acid− base interaction, whereas [N2224][Ac]−H2O system easily reaches CO2 saturation due to the formation of a stable acetic acid-H2O compound. Goodrich et al.186 found that CO2 capacity in [P66614][Pro] with 14 wt % water decreases by approximately 0.20 mol CO2·mol IL−1 at 0.025 MPa and by 0.10 mol CO2·mol IL−1 at 0.10 MPa, respectively. However, for different ILs, adding a certain amount of water into the ILs slightly increase CO2 solubility in IL−water systems. Romanos et al.187 reported that the addition of water into 1-alkyl-3methylimidazolium tricyanomethanide ([CnC1im][TCM]) enhances CO2 capture in IL−H2O system compared to the pure ILs. A molecular exchange mechansim between CO2 in the gas phase and H2O in the liquid phase was used to explain the enhanced CO2 absorption in the hybrid solvents. When the concentration of water reaches a critical value, the interaction between IL and H2O molecules will be broken and H2O will be replaced by CO2. A series of [P66614][2-CNPyr]−H2O mixtures below the saturation limit was studied for CO2 absorption, and also showed that an increase of H2O content leads to a slightly increased CO2 solubility and the shape of the isotherm is

CO2/H2S [Bmim][MeSO4] system, both CO2/SO2 feed ratios and IL amount have the same effect on CO2/H2S selectivity. However, CO2/H2S selectivity in [Bmim][PF6] with the feed ratio of 9/1 (CO2/H2S) and [Omim][PF6] with the feed ratio of 4/1 (CO2/H2S) is nearly independent of IL amount and ranged from 3.0 to 4.0, which is much lower than that of [Bmim][MeSO4] with the feed ratio of 9/1 (CO2/H2S) (7.4− 13). Unlike CO2/SO2 system, CO2/H2S selectivity in [Bmim][MeSO4] with large (9/1) and intermediate (1/1) CO2/H2S feed ratios is high (10 to 13, compared with MDEA + PZ + [Bmim][Cl] + H2O ≈ MDEA + PZ + H2O > MDEA + PZ + [Bmim][NO3] + H2O, indicating that the addition of [Bmim][BF4] can decrease the sensible heat.224 Therefore, the energy consumption of IL−alkanolamine mixtures is lower than that of aqueous alkanolamine solution due to a certain amount of water replacing by ILs, indicating great potentials of IL−alkanolamine mixtures for CO2 separation applications. As mentioned before, the high water content of traditional water-based amine systems for CO2 capture requires the high energy of regeneration due to the high specific heat and vaporization heat of water.225,226 Therefore, nonaqueous solvent systems were investigated to reduce the energy penalty associated with boiling and condensing water. CO2 binding organic liquids (CO2BOL) as a unique class of nonaqueous switchable ILs, originally developed by Jessop et al.,227 were further modified and proposed by Heldebrant et al.228 It was found that CO2BOLs act like aqueous amines, but use alcohols in place of water and a non-nucleophilic base in place of nucleophilic primary and secondary amines. As absorbing CO2, they can form ILs or zwitterionic liquids.29,229−233 For example, The diazabicyclo[5.4.0]-undec-7-ene (DBU)/1-hexanol CO2BOL can absorb 1.30 mol CO2·mol DBU−1 due to the combination of chemisorption and physisorption, which is much higher than that for 30% MEA aqueous solution. Futhermore, although CO2BOLs can chemically bind CO2 weakly as alkylcarbonate salts, the binding energies for CO2 is no more than 10 kJ·mol−1, which is lower than CO2 release from bicarbonate salts observed in aqueous systems.228 Privalova et al.234 studied the CO2 capacity in the equimolar mixtures of DBU and different (amino) alcohols, and DBU-4amino-1-butanol showed the highest capacity of CO2, which increases by 21% in CO2 uptake compared with aqueous 4amino-1-butanol solution. Similar to the CO2BOLs, aminosilicones also have been shown to react with CO2 forming the switchable ILs, which can be reversed by heating at high temperatures.235−240 GE’s gamma amino propyl (GAP) aminosilicones are one of these solvent systems. To further facilitate the mass transfer of CO2, a mixture of GAP/ triethylene glycol (TEG) was developed by adding TEG as a cosolvent into GAP aminosilicones for CO2 capture. The mixture shows a higher CO2 capacity (8.20 wt %), lower heat capacity (∼2.00 J·g °C1−), and lower vapor pressure than 30% aqueous MEA solution.241,242 On the basis of the above strategy, Wang et al.243 developed the integrated sorption systems consisting of 1:1 mixtures of an alcohol-functionalized IL and a superbase instead of the DBU−alcohol system. The DBU-1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Im21OH][Tf2N]) system has the high CO2 absorption capacity of 1.04 mol CO2·mol absorbent−1 at 20 °C and 0.10 MPa. Subsequently, considering the weak acidity of C-2 proton of imidazolium-based ILs, Wang et al.244 investigated the CO2 absorption performances in the 9636

DOI: 10.1021/acs.chemrev.7b00072 Chem. Rev. 2017, 117, 9625−9673

Chemical Reviews

Review

their gas separation performance. The results showed that CO2 permeability is from 350 barrer (for [Cl]−) to 1000 barrer (for [Tf2N]−), and CO2/N2 ideal selectivities is from 15 (for [Cl]−) to 61 (for [dca]−), which is obviously above the Robeson upper-bound of representative polymers.271 Moreover, the order for CO2 permeabilities of SILMs ([Tf2N]− > [CF3SO3]− > [dca]− > [Cl]−) is similar to that for CO2 solubility in ILs ([Tf2N]− > [dca]− ≈ [CF3SO3]− > [Cl]−). Therefore, CO2 solubility of ILs is an important and decisive factor in the performance of SILMs. Their ideal selectivity versus permeability ratios are above the Robeson upper-bound for polymers,271 and the SILM with [Emim][dca] is the best among the investigated membranes with CO2 permeability of 610 Barrers and ideal selectivities of 61 for CO2/N2 and 20 for CO2/CH4. Further, the mixed-gas permeabilites and selectivities of SILMs with imidazolium-based ILs were investigated by Scovazzo et al.272 and Neves et al.273 It was found that these membranes are more stable than the traditional polymeric membranes, possibly due to a better chemical affinity between RTILs and the supported membrane. Comparing with dry gas stream, the presence of water in a gas stream increases the SILMs gas permeability but decreased CO2/N2 and CO2/CH4 selectivity significantly. The selectivities of CO2/N2 and CO2/ CH4 are 20−32 and 98−200, respectively. Zhao et al.274 investigated the effect of wate content in [Bmim][BF4]poly(ether sulfone) (PES) SILMs on the separation performance, which showed that adding a small amount of water (