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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.31 No.1 pp.1-15

Recent Advances on Ionic Liquid based Mixed Matrix Membrane for CO2 Separation

Chaerim Wang*, Rajkumar Patel**
*Nano Science and Engineering, Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, Songdogwahak-ro, Yeonsu-gu, Incheon 21983, South Korea
**Energy and Environmental Science and Engineering (EESE), Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, Songdogwahak-ro, Yeonsu-gu, Incheon 21983, South Korea
Corresponding author(e-mail:,
December 10, 2020 ; December 23, 2020 ; January 6, 2021


The membrane-based CO2 capture is a fast-growing branch in gas separating field. Ionic liquid assisted mixed matrix membrane (MMM), which consists of organic fillers with dispersed ionic liquid, shows high potentiality as a candidate for CO2 separation medium. In MMM, various kinds of ionic liquid and inorganic filler are incorporated into polymer to enhance gas separating performance. Especially, the strong interaction between ionic liquid and organic filler gives huge influence on enhancing the separating performance by increasing affinity, selectivity and adsorption of CO2 into the framework. Also the mechanical properties of metal organic framework are positively tuned by input of ionic liquid to improve CO2 permeability and selectivity. In this review, study of various combinations of ionic liquid and metal organic framework (MOF) in the polymeric membrane for carbon dioxide separation is discussed.

CO2 분리를 위한 이온성 액체 기반 혼합 매트릭스 멤브레인의 최근 발전

황 채 림*, 라즈쿠마 파텔**
*연세대학교 언더우드 국제대학 나노과학 공학과(NSE) 융합과학공학과(ISED)
**연세대학교 언더우드 국제대학 에너지 환경 공학부(EESE) 융합과학공학과(ISED)


이온성 액체 기반 혼합 멤브레인(MMM)은 이산화탄소 분리 매체로써 다양한 종류의 이온성 액체와 무기성 필러 폴리머에 의해 만들어진다. 이온성 액체와 무기성 필러의 강한 상호작용은 이산화탄소의 금속 유기체의 프레임워크 내 친화 력, 선택성, 흡착성을 높여 분리성능을 향상시킨다. 또, 이온성 액체에 의해 혼합 매트릭스 멤브레인의 구조적 특성이 조절되 어 이산화탄소 투과성과 선택성을 향상시킨다. 이 리뷰에서는 이산화탄소 분리를 위한 고분자 막에서의 이온성 액체(IL)와 금속 유기체 프레임워크(MOF)의 다양한 조합에 대한 연구가 논의될 것이다.

    1. Introduction

    As the use of fossil fuels such as petroleum and coal has increased, the amount of carbon dioxide released into atmosphere increased dramatically. The residual carbon dioxide in the atmosphere is the main cause of global warming among greenhouse gases by destroying ecosystem and humanity as a whole. In order to address this issue, CO2 capture technology has been developed. Among the separating technologies, membrane-based separation, especially gas separation by polymeric membrane, exhibits great potentiality owing to its efficiency, energy saving and environmentally friendliness. Ionic liquid based mixed matrix membrane composed of polymer or inorganic fillers’ crystals in the matrix shows significantly enhanced properties of gas separation with high selectivity and permeability of CO2[1-17].

    The inorganic filler is a material that is incorporated into the polymer membrane in order to produce the composite with better gas separating properties. Inorganic filler makes huge impact on determining the properties of composite depending on its types, loading amount or state of distribution into the polymer. The metal organic framework (MOF), consisting of positively charged metal ions and organic ligands, is one of the promising crystalline porous filler due to the favorable characteristics of gas separation. MOF exhibits ultrahigh surface area, distinctive structure, great tunability, and excellent adsorption performance. By utilizing different types of metal ions and organic linkers, various structures of MOF with different properties can be fabricated. One example is zeolitic imdazolate frameworks (ZIFs), made of transitional metal linked by imdazolate. Many studies regarding ZIF have been increased in the field of separation due to its high regenerability with great chemical and thermal stability. Especially, ZIF-8 becomes the most promising material for the CO2 adsorbent with high affinity toward the gas molecules. Another example of MOF is copper benzene-1,3,5-tricarboxylate (HKUST-1) which is metals ions linked with benzene-1,3,5-tricarboxylate. MOF can now become a substitute for the conventional nonporous adsorbents, which have been known to be expensive and inefficient.

    In the fabrication of the ionic liquid based MMM, the choice of ionic liquids (ILs) is as important as the choice of the inorganic filler. Ionic liquid is form of salt in the liquid state. Ionic liquid has many suitable properties as the medium for the gas separation such as minor volatility, inflammable state, high stability and great ionic conductivity; above all, its tenability can maximize the CO2 separation performance by modifying the properties of polymer resulting from the tuning the nano-cage of composite with cations and anions as monomers. This shows that the insertion of ILs into MOFs has resulted in a new MMM with superior gas separating properties. In this review, various ionic liquids are used to analyze the various combinations of composite. One will compare the thermal and chemical properties of various types of MOF/IL composite through nano-characterization methods and compare the CO2 separation performance by measuring CO2 selectivity and permeability. Schematic of the separation process is presented in Fig. 1 and summarized in Table 1.

    2. Metal Organic Framework (MOF)

    2.1. Zeolitic imidazolate framework (ZIF-8)

    The effect of ionic liquid on the microenvironment of the MOF nanocage was reported by Ban et al. When the room temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (RTIL[bmim] [Tf2N]) is incorporated into the ZIF-8, the pore sizes of the microporous material is adjusted[17]. Then, the molecular sieving properties of ZIF-8 is modified, thereafter the gas capturing performance is enhanced. In this process, the RTIL[bmim][Tf2N] acts as both cavity occupant and the solvent. The IL confinement ratio in the nanocage is 1.4 per the cage. The confinement of IL into the nanocage is characterized by X-ray diffraction (XRD) and the attenuation total reflectance-Fourier transform infrared (ATR-FTIR). According to the XRD, the change of the XRD intensity is caused by the change of packing density inside the nanocage after the penetration of anions in IL. FT-IR shows the confinement of IL due to the shift of the SO2 stretching band. Shifting of peaks is the evidence for the presence of a strong interaction between IL and the ZIF-8. As ILs occupies the volume of pore on the ZIF-8, Brunauer Emmett Teller (BET) shows the decrease in the surface area and pore volume at each 374 m2/g and 0.18 mL/g respectively. Comparing the IL@ZIF-8 MMMs and the pristine polysulfone (PSF) membrane structure, IL@ZIF-8 MMMs shows the best performance in capturing CO2 with high selectivity. This is because IL@ZIF-8 MMM increases the CO2 permeability by about 50% while maintains N2 and CH4 permeability. So that, the selectivity of MMM exceeds the upper bound of the pristine membrane structure. Also, when the permeability is compared between IL@ZIF-8 MMM and MOF based MMM, IL@ZIF-8 MMM significantly shows about 50% enhanced CO2 permeability whereas reduced N2 and CH4 permeability. Interestingly, IL@ZIF-8 MMMs shows opposite trend with PSF membrane depending on the pressure. The pristine PSF membrane shows the decrease of the separation factor in the increasing pres-sure due to the plasticization by CO2. On the contrary, IL@ZIF-8 exhibits high value of separating factor of CO2/N2 as 152 at 10 bar. Such distinct gas separating property at different pressure is due to the intrinsic property of the filler and is suitable to be used in various field of separating. Also, membrane-based CO2 separation used for gas sweetening, biogas upgrading, and post-combustion CO2 capture can be effectively achieved through IL@ZIF-8 MMMs.

    IL-1657 MMM filled with well dispersed ZIF-8 particles was synthesized by two steps of processes. First, 3-di-n-butyl-2-mthylimidazolim chloride (DnBMCL) ionic liquid is incorporated into 1657[18]. Then, ZIF-8s particles were incorporated into the IL modified 1657 to fabricate the mixed matrix membrane (MMM) through the coating method. ZIF-8 particles were synthesized by different molar ratios of precursor to investigate the CO2 separation performance from CH4, N2 and H2. The ZIF-8 particles with different molar ratios of precursors are analyzed through SEM and XRD characterization. It was found that the molar ratio of precursor of 2-methyl imidazole (MeIM)/Zn2+ effects on the size of the ZIF-8 particle because as the as the molar ratio of precursor increases, the ZIF-8 particle size decreases. According to the N2 adsorption analysis, among different ratio of MeIM/Zn2+ with 32/1, 8/1 and 2/1, sample with precursor ratio of 32/1 shows higher adsorption tendency with high microporosity and pore volume. Along with the result of the N2 adsorption, BET result shows that the higher molar ratio of MeIM/Zn2+ exhibits high surface area along with smallest mean crystal size. Then the FT-IR and 13C NMR were used to examine the intrinsic interaction in IL-1657 MMM. Thermogravimetric analysis (TGA) showed that the IL-modified membrane has a higher temperature for degradation, which signifies higher thermal stability than other unmodified MMM. Permeability of CO2 depends on the solubility of gas, therefore, it shows the high permeability in modified IL-1657 based MMM. IL only acts as the compatibilizing agent between polymer and metal organic framework (MOF) and the permeability is more affected by the concentration of the precursor loading amount rather than the IL itself. In the comparison of selectivity between modified and unmodified IL-1657 based MMM, the modified IL-1657 based MMM shows higher selectivity in CO2/CH4, CO2/N2 and CO2/H2 due to the newly formed carbon-carbon bonds in the ZIF-8 nanoparticles.

    However, despite these advantages, it has not emerged as a promising method owing to the two reasons. First, this method is not feasible because CO2 selectivity in the mixed gas test exhibits inferior gas separating performance compared to that of the pure gas test. Second, increasing pressure facilitates the plasticization of polyethylene oxide (PEO) in block in in membrane, causing MMM to loss the CO2/N2, CO2/CH4 and CO2/H2 selectivity. Particularly, the DnBMCl-modified 1657/ZIF-8 MMM are affected significantly by the changes in pressure. This is because rise of pressure causes the improvement in overall permeability of gas molecules, therefore, the selectivity of gas molecules in DnBMClmodified 1657/ZIF-8 MMM is highly reduced.

    Guo et al., devises the two-step adsorption/infiltration strategy to incorporate ionic liquid into the MOF nanocage so that the mixed matrix membranes can be fabricated for the better gas capturing performance[19]. At first, ZIF-8 incorporates the small size of the [BMIM] [PF6] into the outer space of ZIF-8. Then through the heat treatment, the large size of [BMIM][PF6] can be transported into the dilated pores. When the conducting environment returns to the room temperature, the pore size contracts to 3.4 Ångström, which results in the trap of [BMIM][PF6] in the MOF nano-cage. Finally, the DMF washing removes the extra IL on the outer space of the composite and the synthesized IL@ZIF-8/ MMM is able to achieve the better CO2/N2 selectivity by tailoring the cage size according to the amount of IL loadings. SEM and EDS were used to study the incorporation of [BMIM][PF6] into the ZIF-8. The ZIF-8 is coated with the [BMIM][PF6] layers and the distinct particles are observed in the boundary of the ZIF-8. Through power X ray diffraction (PXRD), it was found that the XRD analysis before and after incorporating [BMIM][PF6] is the same, which suggests that the input of IL does not affect in change of the morphology of the composite structure. Then, the FTIR spectrum shows the presence of the interaction of [BMIM][PF6] through the red shift of the absorption band and based on the direct interaction, the composite structure shows the low decomposition temperature. As the incorporated [BMIM][PF6] can adjust in the nano cage, the pore size on the ZIF-8 can be tailored to achieve the size exclusion of the adsorbing gas molecules. Therefore, [BMIM] [PF6]@ZIF-8 shows the decreasing N2 adsorption amount compared to the pristine ZIF-8 after the cage size of ZIF-8 is effectively adjusted to a size with diameter of CO2 over N2. [BMIM][PF6]@ZIF-8/PEBAX® MMM shows increasing in the selectivity as the IL loading amount increases to 25 wt%. However, one of the concerns about the [BMIM][PF6]@ZIF-8/MMM is that it shows the slight low permeability of CO2. This is because as the pore narrows down its size, the amount of CO2 to be transported is decreasing resulting from the unsuccessful cage screening effect. [BMIM][PF6]@ZIF-8/ MMM shows the CO2 permeability as 117 barrer and CO2/N2 selectivity as 84.5 the best performance, and this is higher than the existing reported 2008 Robberson upper bound of other MMM structures.

    Upon incorporation of 1-butyl-3-methlimidazolim hexafluorophosphate [BMIM][PF6] to ZIF-8 results in strong interaction between them and it affects on the performance of gas uptake[20] (Figs. 2~4) The strong direct interaction between ZIF-8 and ionic liquid is confirmed by various characterization methods. First, through the SEM and XRD, it was found that MOF structure was well maintained after IL incorporation in a rigid structure, so ionic liquid was well preserved in the framework. Also, using the BET equation, one can figure out the input of the [BMIM][PF6] into ZIF-8 was successful from the decreasing of pore volume and surface area of ZIF-8. Then, according to the TGA, [BMIM] [PF6]/ZIF-8 shows the decrease of the onset temperature and weight loss (remaining mass of 33 wt% of initial mass), which results in the change of the decomposition mechanism. The significant of change in decomposition mechanism is that it guides one to figure out the direct interaction between IL and MOF, which behaves differently depending on degradation mechanism. TGA and IR spectroscopy provides the information regarding the interaction between the frameworks and ionic liquid. As the [BMIM][PF6] is incorporated into the ZIF-8, there is a red shift at the stretching frequencies of the [PF6] anion at 83, 837, 792, 557 and 536. This indicates that the incorporated [BMIM][PF6] weakens the P-F bond, so the P-F bond shares electrons between the anion and ZIF-8. This is clear evidence of the presence of interaction between the imidazole ring in the framework ZIF-8 and ionic liquid [BMIM][PF6]. Along with the characterization of interaction between MOF and ionic liquid, one figures out the adsorption isotherms for CO2, CH4, and N2 gases after incorporating [BMIM] [PF6]/ZIF-8 to see the effect of the strong interaction. According to the grand canonical Monte Carlo (GCMC) simulation, the gas adsorption is enhanced overall and this enhancement is mostly at low pressure with the help of newly formed adsorption sites for CO2. For instance, the selectivity of CO2/N2 and CO2/CH4 by ZIF-8 enhances almost doubled amount at low pressure. At the same time by the introduction of IL into ZIF-8, ideal and mixture selectivity increased for CO2/N2 increased from 7.832 and 7.85 to 24.21 and 28.02 at the 0.1 bar. From GCMC calculation it was observed that the improvement in selectivity is due to the isosteric heats of adsorption.

    1-n-butyl-3-methlimidazolim tetrafluoroborate [BMIN] [BF4] is an ionic liquid that shows the great separation performance by increasing and modifying the absorption pores on the surface on ZIF-8[21]. In this work it is reported that the interaction between MOF and ionic liquid leads to the gas uptake enhancement after incorporating ionic liquid. According to the SEM and XRD, it was found that the addition of ionic liquid into ZIF-8 up to 28 wt% loading was successful without changing the structure of the framework. According to EDX, the distribution of ionic liquid in ZIF-8 is found uniform throughout the surface. From TGA analysis and FTIR spectroscopy, the effect of interaction between [BMIN][BF4] and ZIF-8 was analyzed. According to the TGA, the interaction between MOF and ionic liquid effects on the change of the thermal decomposition mechanism resulting in decreasing stability, and according to the FTIR, the deconvoluted peaks and the shifted band position resulting from the change in electron densities in ZIF-8 and [BMIN][BF4] are observed. The significance of the presence of the direct interaction between the framework and the ionic liquid is that it modifies the gas separation and adsorption performance. After incorporating the 20 wt% of [BMIN] [BF4] into the ZIF-8 at 0.1 bar, CO2 uptake increase by 9% and CH4 and N2 uptake decrease in overall pressure. After incorporating the 28 wt% of [BMIN][BF4] into ZIF-8 at 0.1 bar, the CO2/CH4, CO2/N2 and CH4/N2 selectivity increases from 2.2, 6.5 and 3 to 4, 13.3 and 3.4 respectively. The increase of the selectivity at low pressure is explained by the isosteric heat increase after the input of [BMIN][BF4] similar to the previous report by the same author. Another reason for enhanced selectivity at low pressure is that the pore volume decreases at the high pressure, resulting in the hindering of the gas separation.

    The same research group investigated the separating performance of CO2/CH4 and CO2/N2 after incorporating 1-n-butyl-3-methylimidazolium thiocyanate [BMIM] [SCN] into the framework ZIF-8[22]. It was characterized by XRD and SEM and found that the input of [BMIM][SCN] into the framework ZIF-8 does not damage the integrity of the framework itself. There are no defects in the SEM image and the XRD feature of ZIF-8 and [BMIM][SCN]/ZIF-8 are corresponding before and after the IL input without any structural modification. Also, it was observed in BET that the surface area and pore volume decreased, which signifies the successful IL incorporation process. Then, the TGA revealed that the decomposition mechanism changes after the IL incorporating process, and this reveals the presence of the interaction between IL and the framework which affects in the adsorption performance of the [BMIM][SCN]/ZIF-8. The gas uptake was compared using volumetric gas absorption of CO2, CH4 and N2. The gas uptake is generally decreasing as the pressure goes up, while at the low pressure of 1 mbar, each gas molecules shows different uptake level. For example, uptake of CO2, CH4 and N2 decreased by 66, 25, and 16% from the amount of absorption generated by pristine ZIF-8 at the low pressure of 1 mbar. The significance difference makes [BMIM][SCN]/ZIF-8 to be better separating device and at 1 bar [BMIM][SCN]/ ZIF-8 shows the CO2/CH4 and CO2/N2 selectivity to be enhanced 2.6 and 4 times compared to the that of pristine ZIF-8. The ionic liquid at the low pressure plays dominant role in determining the gas uptake performance by modifying the existing absorption pores and forming new absorption pores.

    2.2. HKUST (acronyms for Hong Kong University of Science and Technology)

    Nozari et al. reported that the interionic interaction energy of IL is a key factor that determines the IL/MOF composite gas separating performance[23]. Composite of 1-n-butyl-3-methylimidazolim tetrafluoroborate [BMIM] [BF4] and CuBTC was prepared and characterized by XRD, FTIR SEM analysis. It showed that IL successfully incorporated into the MOF structure because the morphology of CuBTC was intact without any defect after IL addition. Especially, FTIR figures out the existence of the interaction between CuBTC and [BMIM] [BF4] because the weakening of the bond leads to the slight red shift in the wavelength by sharing the electrons between the copper atoms in the MOF and [BMIM][BF4] composite. According to FTIR analysis, it can also be inferred that the interaction energy between ions in IL will affect the electronic environment because the interaction energy between ions will increase its energy as the Cu-O bond becomes weaker. The interionic interaction is important factor that regulates the electronic environment at the adsorption site of the unsaturated metal gas in the MOF framework. Not only that, as the interionic interaction increase, the stability limit is decreasing. As the stability limit is decreasing, the thermal decomposition mechanism is changed, and this suggests that the interionic interaction between IL and MOF takes major role in changing the characteristics of the composite. The gas adsorption performance is affected by the interionic interaction. As the anion of the IL has strong interaction with the metal sites in the MOF, therefore, it hinders the gas adsorption. When the correlations between CH4 and the CO2 adsorption amount is compared, the R2 of the CO2 is always higher than that of the. This suggests that there is a difference between the uptaking amount between CH4 and CO2, so that is helps to enhance selectivity of CO2 over CH4. These properties of the interionic interaction energy help to tailor the chemical and physical characteristics of the composite, thereby, IL and MOF composite will be a promising material for the future gas adsorbing and separating industry.

    In new IL-assisted method GO-IL/MOF composite is fabricated. By mixing the GO-IL solution with precursor solution of MOF, GO-IL/MOF composite is prepared[ 24]. The GO-IL/MOF composite has better CO2 adsorption performance. In this work, three different ionic liquids, triethylene tetramine acetate (TETA-Ac), triethylene tetramine tetrafluoroborate (TETA-BF4) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIN] [BF4]) are used to check the effect of IL on the framework. Among various ionic liquid, TETA-Ac seems the most efficient IL to generate most high gas separating performance MOF layer because four amine groups on cation of TETA-Ac and counter acetate anion can prevent the GO sheet to be distorted, and thereby more adsorption sites for Cu2+ is produced. Therefore, Cu3(BTC)2 layer with many Cu2+ binding sites and Go-TAc/MOF shortens the CO2 transfer speed. When different concentration of IL is incorporated in the composite, the highest CO2 uptake is for GO-TAc/MOF-60 is 5.62 mmol/g. Different characterizations technique were used to characterize GO-IL/MOF. According to the XRD, XRD pattern of GO-TAc/MOF and Cu3(BTC)2 show high similarity and this signifies that the fabricated Cu3(BTC)2 layers in GO-TAc/MOF is intact when the GO sheet was grown. SEM image shows the GO-TAc/ MOF-60 is a superimposed structure with the GO sheets and loosely arranged MOF nanocrystals. Also, by analyzing the porous properties of the composite, GO-TAc/ MOF-60 showed the surface area of 2043 m2/g and pore volume of 0.196 cm3/g and this is highest values in the IL mixed MOF structures. According to the flame atomic absorption spectrometry (FAAS), the copper contents which can be used for the synthesis of the IL is highest in the GO-TAc/MOF-60 composite. The significance of this research is that the cations and anions in IL affect the growth of the MOF crystal seed layer by changing the absorption of the copper content of the GO sheet. Now, when the CO2 adsorption of the isotherms is compared among different combination of GO-IL/MOF composite, GO-TAc/MOF-60 showed the highest absorbed amounts. Specifically, GO-TAc/MOF-60 shows a high CO2 adsorption capacity of 5.62 mmol/g at 25°C and 100 kPa. On the contrary to the high CO2 adsorption capacity, GO-TAc/MOF-60 shows low N2 adsorption capacity as 0.27 mmol/g at 25 and 100 kPa, resulting in the high selectivity of CO2/N2 as 20.81. Therefore, the generated GO-IL/MOF based on GO sheet can be used as the material for the CO2 adsorption with its high selectivity and capacity. This will play a great role in the future separation technology field by integrating with various fields such as catalysts, electronic or drug carriers.

    Lin et al. suggests the way to reduce the incompatibility between the filler and polymer by using the ionic liquid[25] (Figs. 5~8). To show such reduction, the Emim[Tf2N] covered HKUST-1 is incorporated into the 6FDA-Durene [4,4’-(hexafluoroisopropylidene0-diphthalic anhydriede-2,3,5,6-tetramethyl-1,3-phenyldiamine) a polyimide polymer by ionic decoration method to fabricate the mixed membrane matrix (MMM). Fabricated mixed matrix membrane gets rid of about above 90 vol% of nonselective voids and improves gas separation performance by having IL as a MOFs/polymer interfacial binder. Then, through the following impregnation process and washing using chloroform, the per meability of CO2 is enhanced by reducing the number of excessive free-IL on the external surface. Whether IL is well absorbed onto surface of HKUST-1 is observed through XRD, SEM, and BET. According to the XRD, the HKUST-1 and HKUST-IL diffraction pattern shows high similarity because the input of IL does not effect on the change of the morphology. SEM shows that IL fills the gap between the HKUST-1 and the BET shows the decrease in the HKUST-IL surface due to the blockage of the pores. Also, by showing the shift of (C-F) and (C-H) HKUST-IL spectrum compared to that of the pristine IL, the FTIR exhibits the presence of the interaction between IL and the HKUST-1.Since the gas permeability and the selectivity of MMM is highly relied on the interfacial morphology of the membrane, it become important to see the interfacial void on the surface of the MMM. SEM reveals that the HKUST-IL MMM has reduced number of visible HKUST-1 fillers due to the less coverage of polymer on HKUST-1. This suggests that the addition of IL prevent formation of the interfacial void. Next, explained by FE-SEM, small volume fraction of interfacial void in HKUST-IL MMM was observed as 0.0023%. This is very low value compared to the interfacial void in HKUST-1 MMM as 0.028%. The volume fraction reduces almost 91.8%. One explains the reduction of the interfacial void is due to the strong interaction between HKUST-1 and IL because it attributes to improve adhesion of IL to polymer. Now, one compares the permeability and selectivity of each molecules in MMM. Comparing between HKUST-1 MMM, un-HKUST-IL MMM and HKUST-IL MMM, it was found that HKUST-IL MMM shows high CO2 permeability and low permeability of all other gases. To be specific, HKUST-IL MMM shows 1101.6 barrer for the CO2 permeability. This is because by having the HKUST-1 particles in the MMM, it increases the number of free volumes for the CO2 diffusion. On the contrary, when IL was confined into the un-HKUST-IL MMM, the free volume of un-HKUST was blocked, which causes the low rate of transfer of other gases. Looking at the selectivity values, HKUST-IL MMM showed the highest CO2/N2 and CO2/CH4 selectivity at 27.1 and 29.3, respectively. This means that IL acts as interfacial binder for efficient membrane separation and contributes to improving the CO2 adsorption ability above existing 2008 Roberson tradeoff for CO2/CH4. However, excessive IL can reduce the free volume of the polymer, so that it can reduce CO2. Therefore, producing MOF-IL MMM through IL decoration will lead to the great efficiency in the field of natural gas and biogas separation industries.

    2.3. Others

    Chen et al. investigated the gas separating performance of the chromium metal-organic framework when the bifunctional ionic liquid is confined[26]. The used bifunctional ionic liquid is diethylenetriamine-based ionic liquid with multi-amine-tethered cation acetate anion (DETA-AC) and the framework is chromium metal-organic framework MIL-101(Cr). DETA-Cl based ionic liquid was synthesized by one-step acid-base neutralization of DETA with HCl. DETA-Ac@MIL-101(Cr) was prepared by mixing activated MOF and DETA-Cl followed by ion exchange chloride group with acetate group.

    XRD and SEM results showed that the added ionic liquid does not effect on the integrity of the MIL-101 (Cr). XRD peak does not shift after incorporating DETA-AC and the SEM image shows the intact structure of DETA-Cl@MIL-101(Cr). Then FTIR analysis suggests that compared with the pristine DETA-Cl and MIL-101(Cr) spectrum, the DETA-Cl@MIL-101(Cr) makes the shift in the 2800~3000 peak to high wavelength due to the CH2 aliphatic stretching. When the CO2 and N2 adsorption at 25°C in 1.0 bar was measured, the CO2 adsorption amount in the DETA-Cl@ MIL-101(Cr) improves about 92.6% compared to the pristine MIL-101(Cr). The CO2 adsorption at 25°C and 1 bar in MIL-101(Cr) is 1.22 mmol/g and rose to 2.35 mmol/g for DETA-Cl@MIL-101(Cr) and for DETA-Ac @MIL-101(Cr) is 2.46 mmol/g. As the more acetate anion is incorporated into the framework, the higher solubility is introduced and the CO2 adsorption is improved, resulting in the 101.6% enhancement. Therefore, this phenomenon proves that IL regulates the adsorption sites in the composite material and enhances the affinity of CO2 to the composite material. In addition to that, the well dispersion of the IL promotes CO2 adsorption at low pressure because it creates the more ad- zsorption sites by giving more availability to open metal sites on the surface. Therefore, DETA-Ac-Imp- MIL-101(Cr) shows high affinity for the composite. However, too high pressure restrains the transport of the CO2 due to the highly packed IL in the pores; the grafting of DETA-based ILs onto MIL-101(Cr) at low pressure makes superior quantity of CO2 adsorbing. This high affinity of CO2 is also explained through the increase of adsorption values of isosteric heat. Using ideal adsorbed solution theory (IAST) combined Dual-Site Langmuir-Freundlich (DSLF) model, CO2/N2 (0.15/0.85) selectivity is measured and the CO2 absorbing quantity in DETA-Ac-Imp-MIL-101(Cr) is much higher compared to that of the N2. Compared to the pristine MOF, the DETA-Ac-Imp-MIL-101(Cr) exhibits 39 times enhanced CO2/N2 selectivity, which makes DETA-Ac-Imp-MIL- 101(Cr) as promising material in CO2 capture industry. One investigates the stability and regenerating ability of the composite through the six cyclic tests and this demonstrates that the stability and regenerating ability of composites can be sufficiently maintained after six cyclic tests. The DETA-Ac@MIL-101(Cr) can be used in the variable separating environment. Also, the gas affinity of the composite is fully modified via adjusting the adsorption sites through input of IL, and this will make synergistic effects in gas selectivity and adsorbing. MMM’s CO2 gas separating performance will stand out in the natural gas purification industry with many variables.

    Xia et al. explains the reason for the reduced CO2 adsorption by MOF/IL composite by simulation and experiment[27] (Figs. 9~11). The chosen two MOFs are UiO-66 and NU-1000 and selected amino acids ionic liquids (AAIL) are [Emim][Gly] and [Emim][Phe]. The selected MOF and IL show high thermal stability, and according to the results of PXRD and SEM, no MOF morphology modification after IL incorporating was confirmed. In addition, BET showed reduced surface area and pore volume, and this demonstrated successful IL incorporation along with SEM and PXRD results. Unlike the simulation result, the experimental results show decrease of the CO2 adsorption as the IL concentration increases. In order to figure out the reason, the pore size distribution of IL/MOF was measured, and it was found that the pore size is identical while the pore size volume changes. One assumes that this is because the adsorption channels are blocked by the IL rather having the uniform distribution with IL. To see the validity of the assumption, one makes simulation with random model in which IL is randomly distributed in NU-1000 and the blocked model in which the ILs are artificially accumulated in the channel of NU-1000 to see the difference mechanism of CO2 adsorption. The blocked model exhibits similar CO2 absorption tendency with the experiment, and this suggests that the blocked channel is the reason for the CO2 transport hindrance. For the further analysis of the blocked model, different loading amount of IL is incorporated into the MOF; 5, 10, and 30 vol%. And it was observed it hinders the CO2 adsorption more as the higher concentration of IL is observed. Therefore, it becomes significant to find out the way to distribute IL uniformly in the composite for the better CO2 adsorption.

    Metal organic framework (MOF) and the covalent organic framework (COF) are promising materials in the CO2 capture industry because it produces the synergistic effect with the ionic liquid[28]. With the help of the IL, MOF and COF are able to adjust the existing adsorption sites to be fit in the CO2 capture and increase the adsorption sites on the surface. Xue et al. explain the dispersion effect of IL in MOF and COF using five MOFs and eight COFs by incorporating 1-n-butyl-3-methy limidazolium thiocyanate [BMIM] [SCN]. Since the dispersion of IL has great influences on the CO2 separation, we need to look at the feature of distributed IL in MOF and COF to see the effects of the composite. They compared the coordination number that represents the degree of IL aggregation in MOF and COF and it was observed that the coordination number in MOF was expressed much lower than that of COF. This indicates that IL is better dispersed in MOF. This dispersion of IL is related to Coulombic interaction, and in fact, MOFs have strong Coulombic interaction due to the metal ions. Also, the 2D material COF and MOF with 1D pore structures have better structure in aggregating IL compared to the 3D structures. To determine how IL dispersion affects gas separation, isosteric heats of adsorption of CO2, N2, and CH4 were measured. IL/MOF showed a significant increase in isosteric heats of adsorption of CO2 due to strong interaction with CO2. This result demonstrates that when IL dispersion is well performed, the adsorption sites of IL are increasing their numbers for the selective adsorption of CO2. Based on this, adsorption selectivity of CO2/N2 and CO2/CH4 were investigated. Both IL/MOF and IL/COF showed a large selective percentage improvement, especially MOF such as UiO- 66, CYCU-3, ZIF-8, and IRMOF-1 showed a great enhancement. In addition, the structural difference between MOF and COF affects gas separation differently. MOF is better to be used as an inorganic metal ion subunit linked by the organic ligand and using a twoor one-dimension pore structure in framework is better to prevent an aggregation of IL molecules. However, MOF-74 showed relatively low selectivity enhancement than others, which means that using a nanoporous material with too many adsorption sites is not very helpful in adsorption.

    Liu et al. explains the incorporation of UiO-66-PEI @[bmim][Tf2N] particles into the mixed matrix membrane structure to enhance the CO2 separation performance. MMM was prepared by mixing UiO-66-PEI@ [bmim][Tf2N] particles in post-synthesize the UiO-66 framework and added to polyethyleneimine (PEI) in a N2 atmosphere. The resulting UiO-66-PEI@[bmim][Tf2N] particles are impregnated into 6FDA-ODA matrix, a polyimide polyer via solution casting and solvent evaporation method to create a Uio-66-PEI@[bmim][Tf2N]/ 6FO MMM. The UiO-66-PEI@[bmim][Tf2N] particles in mixed membrane matrix improve the gas separation performance by improving the morphology of the interface. Different characterization methods are used to study the influence of IL on MMM. According to the SEM image and BET results, the surface area and pore volume was observed decreased after the IL addition. XRD also showed successful IL incorporation by forming three different characteristic peaks after adding IL. FTIR found that IL does not affect in generating the strong interaction between the particles and the matrix, but hydrogen bonds are formed between the UiO-66-PEI particles and the 6FDA-ODA matrix. This formation is explained by the presence of the blue shift of N-H bond and this hydrogen bond helps to increase the compatibility between the filler and MMM. Then thermogravimetric analysis (TGA) was used to investigate the decomposition temperature after IL addition into the MOF/ 6FO-10 and the stability of UiO-66-PEI@[bmim][Tf2N]/ 6FO was enhanced with IL. At the same time, the glass transition of UiO-66-PEI@[bmim][Tf2N] MMM is reduced because the IL coating can prevent the crystallization process from proceeding. Now the gas permeation effect of 6FO membrane, MOF/6FO-15 and MOF@IL/6FO-15 membrane were evaluated, and the permeability of CO2 was enhanced after incorporating UiO-66-PEI particle because the high affinity to CO2 increases the gas permeation through the membrane. The addition of IL also reduces the permeability of CH4 by improving the interaction between UiO-66-PEI@ [bmim][Tf2N] and the matrix. With the help of the IL and the UiO-66-PEI particles, highest selectivity of UiO-66-PEI@[bmim][Tf2N] can increase up to 59.99 with 15 wt% IL loading. Compared with the pristine 6FO-10 and MOF/6FO-10, this greatly improves the selectivity, while UiO-66-PEI@[bmim][Tf2N]/6FO MMM shows higher permeability than the 2008 Roberson upper limit of the pristine 6RO membrane. Since too much IL hinders the selectivity, the optimum amount of IL should be dealt in the real-life application. Also, the pressure is another factor for changing the permeability of CO2 and the conducting environment should be in the low pressure due to the plasticization of CO2 at high pressure.

    Hanioka et al. explains the supported liquid membrane (SLM) based on a task specific ionic liquid[30]. The used task specific ionic liquid is three different types; amine terminated ionic liquid (task-specific ionic liquid) such as N-aminopropyl-3-methylimidazolium trifluoromethanesulfonate [C3NH2mim][CF3SO3], N-aminopropyl- 3-methylimidazolim bis(trifluoromethylsulfonyl) imide [C3NH2mim][Tf2N], and 1-butyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide [C4mim][Tf2N]. The synthesized SLM based IL can improve the gas separating performance with better selectivity. Due to the amine moiety from the ionic liquid, the permeability of CO2 at low CO2 partial pressure is high and especially, the [C3NH2 mim][CF3SO3] and [C3NH2 mim][Tf2N] shows much better CO2 permeability at low pressure. Contrast to that, the permeability of CH4 is constant in overall pressures, and this contributes to the increase of the CO2/CH4 selectivity at low pressure. The CO2/CH4 selectivity is changed with the increase of the temperature. As the temperature increases, the permeability of CO2 and CH4 increases, and thereby, the CO2/CH4 selectivity decreases. It is better to keep the low temperature to produce the high selectivity. Also, along with the IL characteristics of high stability and negligible volatility in overall temperature, SLM with polymerized amine shows a stable tendency in the long term. Hence, the CO2/CH4 selectivity can be retained high for a long period of time. The proposed material fused with task specific ionic liquid is effective in CO2 separation with high selectivity and stability.

    3. Conclusion

    In this review, the potentiality of the membrane-based CO2 capture with ionic liquid has been discussed. The ionic liquid based mixed matrix membrane can have the superior gas separating performance due to the interionic interaction energy between the ionic liquid and the polymer. As the IL is confined into the polymer framework, it modifies the morphology of the framework to have enhancing CO2 adsorption and perme ability. The ionic liquid increases the number of the adsorption sites on the framework and adjust the pore size of the framework to be fit in the CO2 capture. Contrast to that, the modification of the framework is inferior to the other gas such as N2 and CH4. With this different behavior of the framework, high CO2 selectivity is achieved. In this review, many different ionic liquids were discussed and there are two similar tendencies observed. First, the CO2 adsorption and selectivity are improved mostly at the low pressure. Second, excessive free ionic liquid can hinder the permeability of the CO2 by decreasing the free volume of the polymer. The mixed matrix membrane with ionic liquid/polymer composite will be utilized in the bio-separating industry in the near future.



    Schematic diagram of mixed matrix membrane.


    Morphology of (a) ZIF-8 and (b) [BMIM][PF6]/ ZIF-8 samples obtained from SEM analysis (Reproduced with permission from Kinik et al., 20, Copyright 2016, American Chemical Society).


    Lowest energy equilibrium conformer of [BMIM] [PF6]-incorporated ZIF-8 optimized at B3LYP/6-31G(d). For clear representation, ZIF-8 and [BMIM][PF6] are demonstrated in wireframe and ball-and-bond-type representation, respectively (Reproduced with permission from Kinik et al., 20, Copyright 2016, American Chemical Society).


    Single-component excess adsorption isotherms of CO2, CH4, and N2 in (a) ZIF-8 and (b) [BMIM][PF6]/ZIF-8 measured at room temperature (Reproduced with permission from Kinik et al., 20, Copyright 2016, American Chemical Society).


    Schematic diagram of MMM containing IL-decorated HKUST-1 (Cu, green; C, gray; and O, red) (Reproduced with permission from Lin et al., 25, Copyright 2016, American Chemical Society).


    Multistaged of 6FDA-durene polyimide synthesis. (Reproduced with permission from Lin et al., 25, Copyright 2016, American Chemical Society).


    SEM images of (a,b) HKUST-1, (c,d) un-HKUSTIL, and (e,f) HKUST-IL (Reproduced with permission from Lin et al., 25, Copyright 2016, American Chemical Society).


    Cross-section SEM images of (a,b) HKUST-1 MMM and (c,d) HKUST-IL MMM (arrows point to the MOF crystals embedded in polymer matrix) (Reproduced with permission from Lin et al., 25, Copyright 2016, American Chemical Society).


    PXRD of (a) [Emim][Gly]/UiO-66 and (b) [Emim] [Gly]/NU-1000 composites. SEM images of (c) UiO-66, (d) [Emim][Gly]/UiO-66-30%, (e) NU-1000, and (f) [Emim][Gly]/ NU-1000-30% (Reproduced with permission from Xia et al., 27, Copyright 2019, American Chemical Society).


    CO2 adsorption isotherms of (a) [Emim][Gly]/UiO- 66, (b) [Emim][Phe]/UiO-66, (c) [Emim][Gly]/NU-1000, and (d) [Emim][Phe]/NU-1000 (Reproduced with permission from Xia et al., 27, Copyright 2019, American Chemical Society).


    Schematic illustration of various possible blockage resulting from AAILs inside MOFs assuming the same amount of AAILs is loaded: (a,d,g) a single channel that was completely blocked by ILs with varying distributions in supercell of MOFs; (b,e,h) three blocked channels that were partially filled by ILs with varying distributions in supercell of MOFs; (c,f,i) five blocked channels that were partially filled by ILs with varying distributions in supercell of MOFs (Reproduced with permission from Xia et al., 27, Copyright 2019, American Chemical Society).


    Summary of Mixed Matrix Membrane


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