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.