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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.33 No.5 pp.240-247

Ionic Liquid Consisted of Composite Membrane for Carbon Dioxide Separation: A Review

Young Simon Shi Young, Rajkumar Patel†
Energy and Environmental Science and Engineering, Integrated Science and Engineering Division, Underwood International College, Yonsei University, Incheon 21983, Korea
Corresponding author(e-mail:;
August 19, 2023 ; August 28, 2023 ; September 18, 2023


Even among gas separation methods, CO2 capture and separation via membranes is an ever-growing field, with many different membrane compositions continually being developed. Ionic liquid (IL) based composite membranes show excellent performance values in separating CO2. Similarly, various copolymer/IL composite membranes also display improved performance. The addition of fillers such as graphene oxide to these copolymer/IL composite membranes shows a further enhanced version of these fillers, most likely due to the strong interactions that occur between ILs and organic fillers, which consequently improves factors such as the affinity, selectivity, and adsorption of CO2. Copolymer/IL composite membranes utilizing a metal-organic framework (MOF) showed improved CO2 permeability. This review discusses the study of various combinations of ionic liquid and copolymer composite membranes for carbon dioxide separation.

이산화탄소 분리를 위한 이온성 액체 기반 복합 멤브레인: 총설

영 사이먼 시 영, 라즈쿠마 파텔†
연세대학교 언더우드학부 융합과학공학부 에너지환경융합전공


가스 분리 방법 중에서도, 멤브레인을 이용한 CO2 포집 및 분리는 지속적으로 개발되고 있는 꾸준히 성장하는 분 야이다. 이온성 액체(IL) 기반 복합 막은 CO2를 분리하는 데 있어 우수한 성능값을 보여준다. 유사하게, 다양한 공중합체/IL 복합막 또한 향상된 성능을 보여준다. 이러한 공중합체/IL 복합만에 산화그래핀과 같은 필러를 첨가하면 IL과 유기 필러 사 이에서 발생하는 강한 상호작용으로 인해 필러의 효과가 더욱 향상되며, 이는 결과적으로 CO2의 친화도, 선택도 및 흡착과 같은 요소를 향상시킨다. 금속-유기 구조체(MOF)를 사용하는 공중합체/IL 복합 막은 향상된 CO2 투과도를 보여주었다. 이 총설에서는 이온성 액체와 공중합체복합막의 다양한 조합에 따른 이산화탄소분리성능에 대한 상관관계를 논의한다.

    1. Introduction

    The usage of membranes in the carbon dioxide separation process has been a constant source of interest[1-3]. This is mainly due to membranes requiring comparatively low energy consumption as opposed to the energy consumption levels in more common separation processes such as distillation, drying, extraction, adsorption, and so on. Using membranes requires low capital, and operating costs only increase the incentive for researchers to investigate the implementation and extension of membranes further to facilitate the separation process within industrial settings. As it relates to gas separation processes, permeability is defined as a measure of productivity, while selectivity can be defined as a measure of separation efficiency[4-7]. The separation of gases in a membrane separation process occurs due to differing levels of diffusivity and solubility between membrane and gas.

    Gas separation via membranes has been increasingly considered in gas separation applications as they are more attractive and practical than other methods. Three distinct types of membranes are currently being investigated for practical use: composite membranes, graphene oxide filler membranes, and metal-organic framework membranes. Composite membranes are combinations of ionic liquids (IL) and polymers[8-11]. Graphene oxide filler membranes utilize graphene oxide’s gas barrier properties alongside IL/polymer composite membranes. Metal-Organic Framework Membranes are those which use MOFs partially organic nature in hopes of higher compatibility of a polymer chain within IL/polymer composite membranes[12-14]. This work will compare the performances of various types of membranes with a specific focus on CO2 selectivity and permeability.

    2. Ionic Liquid and Polymer Composite Membrane

    Hayashi et al. prepared a composite membrane consisting of sulfonated polyimide (SPI) and cheap protic ionic liquid[15]. Composite membrane was prepared by solution casting method.

    The properties of the composite membrane prepared by mixing of PIL is compared with another composite membrane when the same polymer was composited with aprotic ionic liquid (AIL) named as [N1123] [NTf2]. Although the PIL composite membrane possess lower permeability but the selectivity of the membrane is higher than AIL composite membrane. Kim et al. reported prepares a composite membrane of 1-Aminopyridinium iodide and cadmium oxide for carbon dioxide separation[16]. The fabrication schematic of the membrane is presented in Fig. 2.

    Composite membranes incorporate various combinations of polymers and ILs which results in unique performance values.

    Hayashi et al. prepared composite membrane of SPI and ionic liquid to find out the role of cation in the sepation of CO2 and their structure is represented in Fig. 3[17]. Phase separation in the composite membrane play an important role in the gas separation process. Gas separation behavior of PIL and AIL is presented in Fig. 3.

    Cheng et al. examines the use of cellulose (CA) in synthesizing cellulose/IL composite membranes[18]. First, two completely new kinds of cellulose esters utilizing imadzaolium cations were synthesized, namely CA 1-butyl-3-methylimidazolium chloride (CA-BmimCl) and CA 1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (CA-BmimTf2N). The resulting cationized cellulose esters were shown to effectively lock in various ILs such as C10mimTf2N due to electrostatic interactions. As a result of strong attraction interactions, the resultant cellulose ester/ILs composite membranes were found to be uniform, smooth, and highly transparent. Of the two cellulose ester/IL composite membrane combinations, the CA-BmimTf2N/C10mimTf2N membrane combination exhibited the highest CO2 permeability, 38 times higher than CA membrane and 17 times higher than CA-BmimTf2N membrane with 91.1 Barrer, as well as showing a tensile strength of 10~55 MPa and an elongation break at 10~30% as well aspermselectivity of CO2/N2 levels at 24.3.

    Dai et al. research investigates hybrid membranes of Nafion and the IL 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) for applications in carbon [19]. CO2 permeability was found to be greatly enhanced upon usage of the composite membranewhilst surrounded by water in the gas stream, most likely as a result of IL nanochannels forming within the Nafion matrix. [Bmim][BF4] in the Nafion/IL hybrid membranes functions as a “first promoter” to improve gas transport, whilst water vapor acts as a “second promoter”. [Bmim][BF4]/Nafion hybrid membrane exhibited CO2/N2 selectivity of about 30 at 100% RH, 3 times more than same membrane in dry state and 200 times more than dry nafion. [Bmim][BF4]/Nafion hybrid membrane also exhibited mixed gas CO2 permeability of 390 Barrer. These performance statistics strongly suggest a synergetic relationship between IL and water vapor in regards to the gas transportation properties of Nafion.

    Estahbanati et al. research investigates the incorporation of the superior characteristics of Pebax 1657 which includes strong mechanical resistance and outstanding gas permeability especially in regards to polar gases, with the IL 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4])’s affinity to CO2[20]. Synthesizing the polymer/IL composite membrane resulted in membranes that were more amorphous and less crystalline which was shown to be beneficial factors in increasing permeability. In addition, both CO2 permeability and selectivity was shown to increase simultaneously as IL content was incrementally increased, most likely due to the high affinity of CO2 of both components of the composite membrane synthesized in the research. In a 50% wt hybrid membrane, CO2 permeability was found to be 190 Barrer while selectivity of CO2/N2 was found to be 105.6 as well as a tensile strength of 153.8 MPa. which results in increasing the permselectivity of the membranes for CO2/light gases separation.

    Lu et al. research explored the design of a blending method of the various ILs including DEMSTFSI and polysulfone (PSf)[21]. The method devised by the researchers involves the ILs being immobilised, withapplications in high-pressure gas separation in mind. The method results inmembranes which are tunable and have a porous surface, The size and distribution of which were found to be dependent on both the type and the amount of IL blended with PSf. The best performance statstics in regards to gas separation was observed in a hybrid membrane using DEMSTFSI, with permeability of 580 Barrer and selectivity of 30. Based on these researches on IL/Polymer composite membranes it is clear that the polysulfone/IL composite membrane has the best performance values.

    3. Ionic Liquid and Graphene Oxide Composite Membrane

    IL/Polymer composite membranes that utilize graphene oxide as a filler are membranes that utilize GOs unique gas barrier properties to further increase membrane performance. Fam et al. first prepared blend of Pebax 1657 which is a block copolymer of polyamide and polyethylene glycol with ionic liquid[22]. By introduction of small amount of modified graphene oxide as shown in Fig. 6 into the blended system, tremendously increase the movement of ionic liquid to the membrane surface while reducing the interaction of BCP with IL in the bulk of the system.

    Migration of IL to the membrane surface is observed by SEM which is presented in Fig. 5.

    This membrane has excellent selectivity for CO2/N2 separation with CO2 permeability of 1000 GPU.

    Huang et al. looks into the synthesis of an alternate Pebax/IL composite membrane with GO fillers, this time using the IL 1-(3-aminopropyl)-3-methylimidazolium bromide[23]. A lower wt% of GO was used in this research, but displayed better performance values then the previous research. Best performance was observed in 0.05 %wt GO-IL Pebax membrane with permeability of 900 GPU and selectivity of 45 and a tensile strength of 17.3 MPa. Karunakran et al. investigated the design of a three component material consisting of the ionic liquids emimAc and emimBF4, graphene oxide and a highly permeable low selective polymeric coating derived from polyacrylonitryle[24]. The resulting membrane displayed excellent performance statistics as well as stability spanning over 120 hours. As with all previous GO filler membranes, during membrane formation the GO films stacks create sieving nanochannels which result in improved performance levels. The unselective but highly permeable sealing layer and the GO incorporation are the key factors for the improved CO2 selectivity. Best performance was observed in GO-IL hybrid membrane of 0.5 %wt GO and emimAc which showed a permeability flux of 37 GPU and selectivity of 130.

    Wu et al. research investigated the synthesis of a GO supported ionic liquid membrane (GO-SILM) through the use of a strategy known as reduced pressure- assisted vapor penetration[25]. Researchers used the IL ([EMIM] [BF4]) and GO, with Al2O3 for support. The vapors from the IL is shown to be capable of sealing any defects or pores in the GO membrane, as well as penetrate the 2D nanochannels. As seen across all GO/IL composite membranes, due to the interactions between GO and IL, the IL can be confined within the nanochannels, which results in fast and selective CO2 transport. In addition, the transport distance is significantly shortened due to the ultrathin layers. All these factors help display high permeance of 56.6 GPU and selectivity of 63. Upon examining all the research, it can be concluded that the GO-filler membrane with the best performance is the one that was synthesized using Pebax1656 and 1-(3-aminopropyl)-3- methylimidazolium bromide.

    4. Ionic Liquid and Metal Organic Framework Composite Membrane

    Composite membranes which utilize MOFs refer to those that make use of the part-organic nature of MOFs that leads to increased compatibility with the organic chains of the polymers in the composite membranes. Casado-Coterilloet al research details the synthesis of mixed matrix membranes (MMMs) using the IL [emim][Ac] at 5 wt%, the biopolymer chitosan (CS), and nanometre-sized MOF ZIF-8 or HKUST-1 particles to improve the selectivity of the MMM[26]. The use of MOFs is shown to vastly improve the thermal stability of the resulting MMMs, which would al- low for applications in high temperature separation. The best CO2 permeability and CO2/N2 selectivity performance is obtained for 10 wt% ZIF-8 and 5 wt% HKUST-1 at 5413 Barrer and 11.5 and 4754 Barrer and 19.3 respectively. The better performance of the ZIF-8 MOF MMM is attributed to a better adhesion and smaller particle size of ZIF-8 than HKUST-1 nanoparticles with respect to the IL–CS continuous matrix.

    Hao et al. investigated into the effects of ZIF-8 MOF in applications in natural gas sweetening and capture of CO2 post combustion when used with either polymerizable room temperature ionic liquids (poly(RTIL)) or free room temperature ionic liquids (RTIL)[27]. The research shows that ZIF-8 MOF in MMMs significantly improves permeability levels. The P[vbim] [NTf2]/[emim][B(CN)4]/ZIF-8 system displayed an interesting phenomenon where the performance statistics improved as ZIF-8 wt% increased. Best performance was observed with P[vbim][NTf2]/[emim][B(CN)4]/ZIF-8 system with 25.8 %wt ZIF-8 with a permeability of 1062.4 Barrer and selectivity of 24.2.

    Li et al. designed a toughened ZIF-8-pebaxMMMthrough the confinement ofthe IL [bmim][Tf2N] at room temperature into ZIF-8 cages[28]. The diffusion of the IL out of the cages was restrained by water- washing. The resulting membrane’s mechanical properties and gas separation performance were shown to have significantly improved. The molecular sieving properties were also shown to have improved in comparison to ZIF-8/Pebax membranes, as a result of the stiffer interphase between ZIF-9 and pebax as well as a reduction ofZIF-8’s effective aperture size. The best performance of the resulting membranes showed permeability of 104.9 Barrer and selectivity of 83.9 as well as a tensile strength of 12.2 MPa. From exploring the various research into membranes that utilize MOF-fillers, it can be concluded that the membrane with the best performance is ZIF-8 filler membrane that was synthesized with the biopolymer chitosan and the IL emimAc.

    5. Conclusions

    Each composite membrane explored has shown potential to be used in various applications such as gas separation in water or gas separation in high heat environments. Ionic liquids are excellent materials used for adsorption of CO2. Composite membrane prepared by compositing by various kind of polymer are applied as CO2 separation membrane. Anion present in the ionic liquid be easily tuned to enhance the gas separation efficiency. MOF is another interesting type of material that can be incorporated into the polymer/IL binary system to make the ternary composite membrane more selective CO2 separation membrane. Different type of functional group can be introduced into the inorganic- organic framework to enhance CO2 dissolution ability along with enhancing the mechanical strength and thermal behavior like glass transition temperature. These are the issues discussed in this review.



    Coordination behavior in 1-butyl-3-methylimidazolium tetrafluoroborate/CdO/1-aminopyridinium iodide composite (Reproduced with permission from Kim et al.[16], Copyright 2019, Springer Nature).


    Chemical structures of protic ionic liquid (PIL), aprotic ionic liquid (AIL), and sulfonated polyimide (SPI) used in this study (Reproduced with permission from Hayashi et al.[17], Copyright 2019, MDPI).


    (a) Volume expansion of IL as a function of mole fraction of dissolved CO2 and (b) molarities of dissolved CO2 as a function of the pressure of CO2 for PIL [C3imH][NTf2] and AIL [C2mim][NTf2] at 30°C (Reproduced with permission from Hayashi et al.[17], Copyright 2019, MDPI).


    (a) Interaction between GO sheets and IL in the form of ionic complexation (2-D model) at a high pH (21) and ππ or cation−π interaction between the GO basal plane and cation of IL (ball and stick model) and (b) stable dispersion of the Pebax/IL/GO-0.5 solution at pH = 12 (Reproduced with permission from Fam et al.[22], Copyright 2018, American Chemical Society).


    SEM surface (left) and cross-sectional (right) images of the PVDF hollow fibers coated with (a, b) PTMSP and subsequent coating of (c, d) Pebax/IL/GO-0.5, (e, f) Pebax/IL/GO-2.0, or (g, h) Pebax/IL/GO-0.5 (pH = 12) (Reproduced with permission from Fam et al.[22], Copyright 2018, American Chemical Society).



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