ionic liquid modified graphene oxide mixed matrix membranes for enhanced CO2 capture

Author’s Accepted Manuscript Pebax/Ionic liquid modified graphene oxide mixed matrix membranes for enhanced CO2 capture Guoji Huang, Ali Pournaghshband Isfahani, Ansori Muchtar, Kento Sakurai, Binod Babu Shrestha, Detao Qin, Daisuke Yamaguchi, Easan Sivaniah, Behnam Ghalei www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(18)31183-9 https://doi.org/10.1016/j.memsci.2018.08.026 MEMSCI16400

To appear in: Journal of Membrane Science Received date: 30 April 2018 Revised date: 30 July 2018 Accepted date: 20 August 2018 Cite this article as: Guoji Huang, Ali Pournaghshband Isfahani, Ansori Muchtar, Kento Sakurai, Binod Babu Shrestha, Detao Qin, Daisuke Yamaguchi, Easan Sivaniah and Behnam Ghalei, Pebax/Ionic liquid modified graphene oxide mixed matrix membranes for enhanced CO2 capture, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.08.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Pebax/Ionic liquid modified graphene oxide mixed matrix membranes for enhanced CO2 capture Guoji Huanga,b, Ali Pournaghshband Isfahania, Ansori Muchtara,b, Kento Sakuraia, Binod Babu Shresthaa, Detao Qina, Daisuke Yamaguchia,b, Easan Sivaniah a,b*, Behnam Ghalei a,b* a

Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, 606-8501

Kyoto, Japan. b

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,

Kyoto, Japan. [email protected] [email protected] *Corresponding author. Tel.: +8175 7539865; Fax: +8175 7539820.

Abstract The development of mixed matrix membranes (MMMs) is mostly challenged by the filler dispersion and the fabrication of defect-free membranes with an ultra-thin selective layer. Graphene oxide based MMMs are promising materials for gas separation application. The low filler content and preference of extended GO lamella to align perpendicular to a membrane surface and hence the gas flow direction permits the development of thin film composite membrane (TFC). Here, facilitated transport MMMs were fabricated by incorporating ionic liquid functionalized graphene oxide (GO-IL) into poly(ether-block-amide) (Pebax 1657). The 1-(3-aminopropyl)-3-methylimidazolium bromide ionic liquid was reacted to graphene oxide sheets, enhancing the CO2 solubility and CO2/gas selectivity of the MMMs. Moreover, hydrogen bonding interactions between the ionic liquid and amide moieties in Pebax provide a homogeneous dispersion of GO-IL. The pure (H2, CO2, O2, N2, CH4) and mixed (CO2/H2, CO2/N2) gas permeability of the membranes were performed at 25°C and 4 bar. The gas permeability measurements indicate an improvement of over 90% in CO2/N2 selectivity and 50% in CO2 permeability for the GO-IL MMMs compared to the pure Pebax membrane. The resulting TFC membranes showed high CO2 permeance up to 900 GPU (10-6 cm3 (STP) cm-2s-1 cmHg-1) and the CO2/N2 and CO2/H2 selectivities of about 45 and 5.8, respectively. Our finding underlines the importance of GO-IL nanosheet to design high selective thin film membranes, providing a direction to fulfil the concept of mixed matrix membranes for practical applications. Keywords: Gas separation membrane; graphene oxide; thin film composite; ionic liquid; CO2 capture

1. Introduction Reduction of CO2 emissions from biogas/natural gas sweetening and flue gas treatment is one of 1

the most pressing environmental issues. Among various separation techniques, membrane-based separation processes have received attention for CO2 capture due to its high energy efficiency, simplicity and low capital cost [1, 2]. Design and development of new materials with selective transport pathways and high gas permselectivity is a practical approach to improve the gas separation performance of membranes [3, 4]. Up to now, polymer membranes, zeolite molecular sieves, and mixed matrix membranes (MMMs) have been widely studied, and some of them have been commercialized. Various types of polymer membranes have been synthesized and scaled up for industrial applications. However, polymer membranes usually suffer from a trade-off between permeability and selectivity, the so-called Robeson’s upper bound [5, 6]. Inorganic membranes (e.g., CMS) overcome this limitation with the size exclusion mechanism; they often possess non-tortuous pores that can serve as fast transport channels with a size-sieving ability for gas molecules but have poor processability [7, 8]. Therefore, current efforts are being undertaken to meet this challenge through the development of MMMs to combine the processability of polymers with a good gas separation performance of inorganic materials [9-12]. A broad range of inorganic additives with different specific surface chemistry and functionality like silica, metal oxides, zeolites, metal organic frameworks (MOFs), activated carbons and graphene oxide have been utilized to fabricate MMMs [13-17]. The results have shown that the introduction of these inorganic additives enhances the overall gas transport properties of the polymer matrix. However, poor filler-polymer compatibilities and filler aggregation may have a detrimental impact on the mechanical and separation performances of the final membranes. Also, the presence of non-ideal defects such as interfacial microvoids, polymer chain rigidification, and particle pore blockage is considered as undesirable factors for the development of high-performance MMMs [17, 18]. Therefore, several attempts have been devoted to fabricating defect-free MMMs by constructing an ideal interface between the inorganic fillers and polymer matrix without sacrificing the porous structure of fillers and mechanical strength of the polymer membranes [19]. For instance, surface modification of inorganic additives is employed to improve the compatibility between inorganic fillers and the polymer matrix, thus minimizing the formation of non-selective voids between two phases [20, 21]. However, it is still challenging to obtain a desired interfacial morphology and good dispersion of fillers through the polymer matrix. Two-dimensional nanosheet graphene oxide (GO) with a high aspect ratio (>1000), tuneable surface functionality, facile synthesis and high mechanical and thermal properties have been used as a promising filler to develop MMMs [22]. The high aspect ratio of GO nanosheets provides longer and more tortuous paths for larger gas molecules to pass through the polymer membranes. This property efficiently decreases the permeability of larger gas molecules while enhances the overall gas selectivity. Also, the intrinsic nanochannels with tuneable functionality

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between the layers of stacked GO nanosheets offer selective transport pathways for gas separation [16]. Furthermore, the presence of polar groups on the surface of GO (e.g., hydroxyl, epoxide) enhances the compatibility of the filler with the polymer matrix which results in non-defective MMMs [23, 24]. For example, it was shown that the CO2/N2 selectivity of Pebax MMMs was greatly increased with the addition of 0.1 wt% GO [16]. Also, incorporation of 0.065 wt% GO into poly(ethylene oxide)–poly(butylene terephthalate) led to an enhancement of about 40% in CO2/N2 selectivity compared to the pristine polymer membrane [23]. That such low additions of GO can induce large material changes is considered a testament to the extended nature of the sheet-like additives. MOF nanosheets embedded into a polyimide (PI) matrix shows 20-80% higher CO2/CH4 selectivity than the pure PI membrane as reported by Rodenas et al. [25]. The influence of filler structure on the gas separation performance of polybenzimidazole was also evaluated by employing nanocubes and nanosheets MOF as the fillers [26]. The best performance was obtained in membranes with MOF nanosheets due to the high sieving ability which is favorable for highly selective gas separation membranes. Graphene oxide can be modified with different functional moieties to increase the transport for polar gases like CO2 [27-29]. For instance, GO was successfully functionalized with amino acids, polyethylene glycol monomethyl ether and polyethyleneimine to enhance the CO2 solubility and selectivity due to the presence of abundant carboxylic acid, ethylene oxide, and primary amine groups [27, 28]. Imidazolium-based ionic liquids (IL), with basic nature, have gained attentions because of better CO2-philic properties [30, 31]. IL has been extensively used to functionalize fillers, such as MOFs and silver nanopowder. The modified fillers are incorporated into the polymer matrix to increase the CO2 solubility and the compatibility of the fillers with polymer chains which result in an enhanced gas separation performance [32-34].

Scheme 1. Schematic diagram of the gas transport through the GO-IL based membrane

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Recently, IL functionalized GO composites with enhanced specific area received tremendous attention for application in energy storage, ultrasensitive determination, adsorptions and gas separation [35-38]. For instance, Chen et al. reported that incorporation of IL into Pebax membrane could increase the CO2 permeability. The CO2/N2 selectivity of the membrane further improved by adding GO nanosheets and adjustment of pH [39, 40]. However, the reported membrane fabrication process is relatively complicated and required a significant amount of IL. Therefore, developing a straightforward and practical approach for combining GO and IL to improve the separation performance of polymer membranes is still a challenge that needs to be addressed. Herein, we report a novel alternative strategy for fabricating Pebax1657 MMMs with 1-(3-aminopropyl)-3-methylimidazolium bromide ionic liquid (IL-NH2) covalent functionalized GO (GO-IL). An important aspect of this research is synthesizing of GO-IL with good dispersibility in the polymer matrix that can be easily applied to the development of thin film membranes. The high-aspect ratio of GO-IL, as well as its high affinity with CO2, enhances the selectivity of CO2 over other light gases (Scheme 1). The modification of GO fillers also improves the filler-polymer interface compatibility. 2. Experimental 2.1 Materials Natural graphite powders (325 mesh) were purchased from Alfa Aesar and Pebax1657 (Pebax) was supplied by Arkema as pellets. 3-Bromopropylamine hydrobromide (98%), 1-methylimidazole (98%), ethanol and ethyl acetate (99.8%) were obtained from Sigma-Aldrich. Concentrated sulfuric acid (H2SO4, 98wt%), hydrochloric acid (HCl), and hydrogen peroxide (H2O2, 30wt%), potassium permanganate (KMnO4) and potassium hydroxide were purchased from Wako Pure Chemical Industries and used as received. 2.2 Synthesis of GO and GO-IL nanosheets Graphene oxide (GO) was prepared by the Improved Hummers’ method [41, 42]. Graphite powder (1 g) was added to 23 mL of sulfuric acid at 0 °C, and the mixture was stirred in an ice bath for 30 min. 3 g of KMnO4 was gradually added to the reaction, and the mixture was stirred at 35 °C for 2 h. Distilled water (46 mL) was slowly added while the temperature was maintained at 100°C for 15 min. Finally, the mixture was poured slowly into 130 mL of 30% H2O2 solution. The product was filtered and washed with 10% aqueous HCl (750 mL) and thoroughly diluted with distilled water. The brown-yellow powder GO was dried for further modification. IL-NH2 was synthesized according to the reported procedure [43]. 3-Bromopropylamine hydrobromide (2.200 g, 10 mmol) and 1-methylimidazole (0.790 mL, 10

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mmol) were added to 25 mL of ethanol and refluxed under nitrogen for 24 h. The solid powder was obtained in ethyl acetate from ethanol and dried under vacuum at 60 °C overnight. GO-IL fillers were synthesized by an epoxide ring-opening reaction between GO and IL-NH2 [44]. In the typical procedure, 10 mg of IL-NH2 was added into 10 mL of GO solution (0.5 mg/mL) and stirred for 30 min, followed by addition of 10 mg KOH. After 30 min of sonication, the homogeneous solution was vigorously stirred at 80 °C for 24 h. The GO-IL was subsequently centrifuged, washed with ethanol and water, and dried under vacuum. 2.3 Membrane preparation The membranes were prepared by the solution-casting method. A certain amount of fillers (0.05-0.5 wt%) was dispersed and sonicated in Pebax1657 solution (ethanol/ water 70:30 wt%) for 12 h. The solution was cast into a Teflon petri dish and slowly evaporated at room temperature for 48 h. The membranes were peeled off from and further dried under vacuum at 60 °C for 24 h. The pristine Pebax 1657 membrane was fabricated via the same method without adding particles. The thickness of the pure and composite membranes varied between 40 to 50 μm, depending on the loading of fillers as measured by a micrometer. Thin film composite membranes (TFC) were fabricated from 2 wt% polymer dope solution coated on PVDF support (MWCo. 50 kDa, AMI Co. US) using roller blade coater (RK print coater instrument, UK). Before coating the polymer, PTMSP (2 wt% in cyclohexane) was first coated as the gutter layer for smoothing the surface and preventing the pore penetration of Pebax solution. PTMSP with ultrahigh gas permeability and low CO2 selectivity is a wise choice for the gutter layer as it almost doesn’t effect on the separation performance [45, 46]. The thickness of the selective layer was found to be around 100 nm by using cross-sectional SEM images. 2.4 Membrane characterization The as-prepared GO, GO-IL and membranes were characterized by ATR-FTIR spectra (Shimadzu IRTracer-100 spectrometer, Japan) in the range of 4000-600 cm-1 with a resolution of 2 cm-1 and 64 scans. The crystalline structure of fillers and membranes were investigated by wide-angle X-ray diffraction (WAXD) (Rigaku RINT XRD, Japan) in the range of 5° to 40° at the scan rate of 10°/min using Cu K-alpha radiation under a voltage of 40 kV and a current of 200 mA. The glass transition temperature (Tg) of membranes was measured using differential scanning calorimeter (DSC) (Bruker DSC 3100SA, Germany) under a nitrogen atmosphere in the range of -100 °C to 250 °C with a 10 °C/min scanning rate. Thermo-gravimetric analysis (TGA, Rigaku Thermo plus EVO2, Japan) was utilized under a nitrogen atmosphere at a 10 °C/min ramp rate to evaluate membrane thermal stability of the membranes. The morphology of membranes was observed by FESEM (Hitachi S-4800) instrument. The samples were freeze-fractured in liquid nitrogen and sputtered with osmium to prevent charging. The surface morphology of the membranes also characterized using atomic force microscope (NanoWizard

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III, JPK Instruments, Japan) in tapping mode (AFM). High resolution TEM (transmission electron microscope) images were taken using a JEOL JEM-2200FS under an operating voltage of 200 kV equipped with a field emission gun. An ethanol dispersion of the GO and GO-IL fillers after sonication were dropped onto a thin carbon film with a plastic microgrid on a copper mesh and dried by standing at room temperature. Mechanical properties of the Pebax and representative MMMs were characterized by using a universal testing machine (84-76 Series Tension Tester, TMI Co.) in accordance with ASTM D882 test method. The gauge length and the crosshead speed were 100 mm and 50 mm/min, respectively. 2.5 Gas permeation measurements Pure gas permeabilities of the membranes were measured using the variable pressure–constant volume method at 4 bar and 25 °C. Gas permeability data for H2, O2, N2, CH4, and CO2 were recorded separately. Gas permeability was expressed in term of barrer (1 barrer = 1×10−10 cm3 (STP) cm cm−2 s−1 cm Hg−1). The reported gas permeation values are the average of four membranes samples. The gas permeability (P) and ideal selectivity (α) were calculated using the following equation: P  Ji

l 273.15 V dp l  1010 ( ) p 76 AT dt p

(1)

Where Ji is the flux of gas "i"; p is the pressure difference across the membrane; l is the 𝑑𝑝 𝑑𝑡

membrane thickness; ( ) is the steady-state slope of the permeate pressure versus t curve; V, A, and T are the permeate volume, the membrane area, and the temperature, respectively. The ideal selectivity (αi/j) is defined as the permeability of gas “i” relative to that of gas “j”, and is expressed by: (αi/j)=Pi/Pj

(2)

The diffusion coefficient (D) for a specific gas was derived from the thickness of the membrane and the time lag (θ) (eq. (3)): D= l2/6θ

(3)

The solubility (S) was calculated from eq. (4): S=P/D

(4)

The ideal selectivity of gas pairs is defined as: αA/B=PA/PB=( DA/DB)×(SA/SB)

(5)

where DA/DB is the diffusivity selectivity and SA/SB is the solubility selectivity.

3. Results and discussion 3.1 FTIR characterization The FTIR spectra of GO, GO-IL, and fabricated membranes are shown in Fig. 1. In general, the

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hydroxyl and carbonyl groups of GO appear at 3400 cm-1 and 1726 cm-1, respectively. The peaks observed at 1405 and 1052 cm-1 are attributed to the O–H bending vibration and the C-O of epoxy groups, respectively [47]. The reaction of GO and amine-terminated ionic liquid is illustrated in Scheme 2. After modification, the peak appeared at 1164 cm-1 (black arrow) is related to the ring in-plane asymmetric stretching of the imidazolium ring, indicating the successful attachment of amine-terminated IL-NH2 to GO nanosheets [44].

Scheme 2. Illustration of the preparation of GO-IL.

FTIR spectra were further used to study the hydrogen bonding interactions between fillers and polymer chains. The peaks appear at 3300, and 1094 cm-1 are related to the –NH– and C-O-C groups, respectively [48]. The significant changes in the FTIR spectra of the prepared samples could be observed in the region of –NH– and the carbonyl stretching zone at 1600-1800 cm-1. It is reported that the –NH– groups in the hard segments (PA units) can make hydrogen bonding with (i) carbonyl of amide groups and (ii) C-O-C of the soft segments (PEO segments) [49, 50]. Therefore, the peak of carbonyl groups splits into the bands at 1730 and 1640 cm-1 which are associated with the bonded and free carbonyl groups, respectively. The peak at higher frequency corresponds to the free carbonyl groups while the peak appearing at lower frequency relates to the hydrogen bonded carbonyl groups. As shown in Fig. 1, the peaks of the –NH– and the bonded carbonyl groups of the pure Pebax shift to a lower frequency with the incorporation of GO-IL. However, the peaks of the GO Pebax samples are almost unchanged. This result infers that the GO-IL makes hydrogen bonding with the PA hard segment of Pebax. In contrast, the C-O-C band of PEO segment has remained unchanged by incorporation of GO-IL fillers which infers a relatively weak interaction between GO-IL and the soft regions.

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Fig. 1. (a) FTIR spectra of GO, GO-IL, and as-prepared membranes; (b) and (c) wave number from 3000-3500 cm-1 (–NH–) and 1550-1800 cm-1 (–HN–C=O–).

3.2 X-ray diffractions The XRD patterns were carried out to analyze the crystalline properties of pristine GO, GO-IL, and Pebax based MMMs (Fig. 2a). The diffraction patterns of the Pebax nanocomposites display a broad halo peak at 2θ=15 to 23°. The sharp peak appeared at 2θ=23° is associated with the crystalline amide blocks of Pebax. The X-ray diffraction analysis of GO and GO-IL fillers exhibits <001> peak at 11.1° (interlayer space is ca. 0.79 nm) and 10.0° (d-spacing is 0.88 nm), respectively. The increase in the interlayer spacing of modified GO-IL is ascribed to the covalent attachment of IL-NH2 to graphene plane and ring opening of epoxy groups [44]. The X-ray diffraction of GO is not observed at the relatively low content of GO. However, at higher loading of GO, i.e., 0.5 wt%, the new peak appeared at 11.4° corresponding to the reflection of GO fillers. Interestingly, the X-ray diffraction of GO-IL is not present even at high filler contents. Also, the intensity of XRD peak corresponding to the crystalline regions in the hard segments (amide units) is found to increase with the GO-IL content. This observation can be explained by the nanometric dispersion and the nucleation of GO-IL in the PA units, as discussed in the FTIR section. 3.3 Thermal and mechanical analysis DSC measurement was applied to explore the effect of GO and GO-IL fillers on the glass transition temperature (Tg) of the pristine Pebax (Fig. 2b). The effect of T g on the chain flexibility and the ultimate gas separation performance of the membranes has been reported [51]. The PEO soft domains possess a low Tg at -52 °C and a melting transition at 30 °C. The PA part of the Pebax shows a broad melting temperature that starts at 160°C while the Tg of the hard segment cannot be detected. The small changes in Tg of MMMs elucidate that both GO, and

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GO-IL fillers could not provide strong interactions with PEO segments. However, the slope of Tg increases with the incorporation of GO-IL, which indicates that PEO chains undergo relatively few interferences with particles and PA segments [52]. In addition, at higher GO loadings, the agglomeration of the GO sheets and the tendency of the particles to disperse in PEO segments rather than the PA domains caused that the T g shifts to higher temperatures. This result is in agreement with FTIR and XRD observation. Fig. S1 shows TGA and DTGA curves of the pure Pebax and composite films containing GO and GO-IL fillers. Pebax showed one‐step decomposition related to the random chain scission mechanism of the main polymer chain. The composite films showed one‐step decomposition, similar to the pure Pebax, indicating that the presence of the GO and GO-IL fillers does not significantly influence the thermal degradation pattern in the MMMs.

Fig. 2. (a) XRD patterns and (b) DSC thermograms of pure Pebax and Pebax MMMs.

The mechanical properties of the GO Pebax and GO-IL Pebax MMMs were analyzed by a tensile test (Table S1). By adding both GO and GO-IL particles, the tensile strength reduced, while the elasticity modulus increased. For example, for GO Pebax MMM with 0.2 wt% filler loading, the tensile strength decreased from 22.6 MPa (Pure Pebax) to 14.1 MPa, and the

elasticity modulus increased to 231 MPa. The results indicated that the MMMs had more rigid and brittle structure than the pristine Pebax membrane. The introduction of GO sheets into the Pebax matrix limited the molecular rearrangement of polymer chains during the deformation process. The MMM comprising GO-IL sheets showed less change in mechanical properties compared with GO based MMMs that could be due to the higher miscibility and stronger interaction between GO-IL fillers and Pebax matrix. 3.4 Morphology The morphologies of GO and GO-IL sheets were analyzed by AFM and the results are shown in 9

Fig. 3. AFM images of (a) GO and (b) GO-IL sheets (scan area: 2 µm×2 µm); AFM images of the surfaces of (c) Pure Pebax membrane, (d) 0.2 wt% GO Pebax MMMs, and (e) 0.2 wt% GO-IL Pebax MMMs, (scan area: 10 µm×10 µm).

Fig. 3. Both GO, and GO-IL sheets were exfoliated to a single layer and dispersed homogenously in the ethanol/water mixed solvent. The prepared GO and GO-IL sheets have the lateral sizes of 1-2 µm with a thickness of ca. 1–1.5 nm (Fig. 3c and d) which is in good agreement with TEM images (Fig. S2) and previous reported data [53, 54]. The filler dispersion in MMMs was further studied by AFM images. As shown in Fig. 3, the Pebax film consists of randomly distributed soft and hard domains which inferred the incomplete phase-separated morphology. Pebax is a segmented copolymer with soft rubbery and hard glassy phases, which can lead to microphase separation. Some large flake-like bulges appeared after the GO sheets were embedded into the polymer chains (Fig. 3). In particular, the GO aggregation can be easily investigated in comparison to GO-IL based membrane with the same loading of 0.2 wt%. FESEM analysis was also employed to investigate the dispersion and interfacial adhesion of GO and GO-IL within Pebax matrix. The cross-sectional SEM images of prepared membranes were observed by FESEM in Fig. 4. Pebax copolymer membrane had a smooth and dense cross-sectional morphology (Fig. 4a and b). However, by adding GO sheets, rough morphologies appeared on the surface of membranes. The GO laminates formed in the MMMs

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were clearly observed and are shown in Fig. 4c and d. The agglomeration of GO was observed at high filler loading (0.2 wt%) because of the strong hydrogen bonding between GO sheets (Fig. 4c and d). Compared with GO MMMs, the GO-IL MMMs exhibit homogeneous morphology (also seen in the AFM image of Fig. 3) indicating the uniform dispersion of GO-IL in the polymer matrix without any aggregation and voids at the GO-IL/polymer interface even at relatively high GO-IL contents (i.e., 0.2 wt%). These results justify that the surface modification of GO fillers with IL-NH2 improved the interface compatibility by strong hydrogen bonding between GO-IL and Pebax matrix, which is required for efficient gas separation applications. 3.5 Gas separation performance Gas permeation tests were carried out to investigate the effect of GO nanosheet concentrations on the membrane separation performances. Pure gas permeabilities for several GO and GO-IL containing membranes were measured at 4 bar and 25 °C (Fig. 5 and Table 1). The CO2 permeability and CO2/N2 selectivity of the pristine Pebax membrane is 92 barrer and 42, respectively, which is comparable to the previously reported data [49, 50]. The order of gas permeability of GO Pebax MMMs is CO2＞H2＞ CH4＞N2. This data follows the same trend as the results reported by Shen et al. for GO Pebax MMMs [16].

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Fig. 4. Cross-section SEM images of (a), (b) pure Pebax; (c), (d) 0.2 wt% GO Pebax MMM; (e), (f) 0.2 wt% GO-IL Pebax MMM.

Fig. 5 CO2 permeability and ideal selectivity of (a) CO2/N2 and (b) CO2/H2 for Pebax, GO Pebax and

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GO-IL Pebax MMMs.

Table 1. Pure gas separation properties of the prepared membranes at 4 bar and 25°C. Permeability (barrer) Membrane

CO2

N2

Pure Pebax

92.4±4.5

2.2±0.2

0.01 wt% GO Pebax

104±5.1

0.05 wt% GO Pebax

Selectivity H2

CH4

O2

CO2/N2

CO2/H2

9.9±0.8

4.9±0.4

5±0.4

42.0±4.3

9.3±0.9

2±0.2

9.1±0.8

5.3±0.4

5.2±0.4

52.0±5.8

11.4±1.2

113+5.4

1.56±0.1

9.3±0.8

5.1±0.4

4.7±0.4

72.4±5.7

12.2±1.3

0.1 wt% GO Pebax

106.2±5.0

1.83±0.1

10.8±0.9

6.5±0.5

5.2±0.4

58.0±4.2

9.8±0.9

0.2 wt% GO Pebax

88±3.8

2±0.1

11±0.9

6.8±0.5

5.7±0.5

44.0±2.9

8.0±0.7

0.5 wt% GO Pebax

74.5±3.1

2.13±0.2

12.5±1.0

7.1±0.6

6.1±0.5

35.0±3.6

6±0.5

0.01 wt% GO-IL Pebax

101.3±4.8

1.81±0.1

10±0.9

4.77±0.3

4.2±0.3

56.0±4.7

10.1±1.0

0.05 wt% GO-IL Pebax

127.6±5.6

1.98±0.1

10.2±0.9

5.3±0.4

4.9±0.4

64.4±6.9

12.5±1.2

0.1 wt% GO-IL Pebax

132.7±5.9

1.95±0.1

10.5±1.0

5.7±0.4

4.5±0.4

68.1±6.6

12.6±1.3

0.2 wt% GO-IL Pebax

143±6.2

1.8±0.1

10.4±0.9

6.25±0.5

5.3±0.4

79.4±5.5

13.8±1.3

0.5 wt% GO-IL Pebax

114.4±5.3

1.5±0.1

10.3±0.9

6.42±0.5

6.67±0.5

76.3±3.9

11.1±1.1

As the GO loading increases up to 0.05 wt%, the permeability of CO2 and its selectivities increased to that of pure polymer. For example, the 0.05 wt% GO Pebax membrane showed the highest CO2 permeability of 113 barrer and CO2/N2 and CO2/H2 selectivity of 72 and 12.2, respectively. Compared with Pebax membrane, the GO Pebax MMMs containing 0.05 wt% GO exhibits improved CO2 permeability (23%), CO2/N2 selectivity (71%) and CO2/H2 selectivity (31%). At higher loadings, i.e. up to 0.5 wt%, both permeability and selectivity of typical gas pairs decreased which is due to aggregation of GO nanosheets inside the polymer matrix. In higher particle concentrations, heterogeneous membranes were formed which cracked upon casting. The enhancement of the CO2 permeability and the selectivity can be explained by the higher CO2 solubility in the GO Pebax MMMs, whilst H2 and N2 solubilities are not changed. The interaction of the GO with PA chains (see FTIR and DSC results) reduces the interferences of the PEO groups with PA segments, providing more sorption sites for CO2 molecules to interact with polar PEO groups. This interaction is stronger in the membranes comprising GO-IL filler due to its higher CO2 sorption capability. However, the agglomeration and the presence of GO nanosheets in the PEO domains, the most permeable part of the Pebax, at higher loadings (0.2 and 0.5 wt%) cause a significant reduction in both permeability and selectivity of all gases. This behaviour is related to the lower CO2 solubility and gas diffusivity through the membranes. In the GO Pebax MMMs, the gas can pass through both the polymer and the interlayer spacing of GO nanosheets. The stacking of GO nanosheets in the polymer matrix plays a significant role

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in passages of gas molecules. The GO nanosheets can be oriented either perpendicular or parallel to the surface of the membrane. The gas molecules pass through the interlayer spacing of perpendicular GO nanosheets while the gas diffusion may be delayed by other GO nanosheets which are located parallel to the membrane surface. This hindrance by the parallel oriented GO nanosheets decreases the gas permeation at high filler loadings, more than 0.05 wt%. The dispersion of GO-IL in the polymer matrix is uniform even at high filler loading, i.e. 0.2 wt.%. The CO2 permeability and CO2/N2 selectivities of GO-IL MMMs were increased up to 0.2 wt.% filler concentration. For example, the CO2 permeability of 92 barrer for the pure Pebax increased to 127 and 143 barrer in 0.05 wt% and 0.2 wt% GO-IL Pebax MMMs, respectively (Fig. 5a). Moreover, the CO2/H2 selectivity of 0.2 wt% GO Pebax increased from 8 to 13.8 for 0.2 wt% GO-IL Pebax MMMs (Fig. 5b). The enhancement in CO2 permeability and CO2/N2 and CO2/H2 selectivities can be due to the presence of ILs as CO2-philic moieties in the interlayer spaces of GO nanosheets. The presence of IL functional groups on the surface of GO nanosheets also leads to strong hydrogen bonding between the hard segments of the polymer matrix and the fillers (as confirmed by FTIR). The formation of these hydrogen bonds enhances the interfacial compatibility between polymer chains and the GO-IL fillers, which are required for fast and selective gas separation process. At higher GO-IL loadings, i.e. 0.5 wt%, the CO2 permeability decreased, but notably, the selectivity of the gases remains at a higher plateau value than that of the pure polymer matrix. Furthermore, the ideal selectivity of GO-IL based Pebax is enhanced more compared to GO-based Pebax MMMs. The ideal selectivity of the CO2/N2 gas pair is 64 for 0.05 wt% GO-IL Pebax, 80 for 0.2 wt% GO-IL Pebax, and 76 for 0.5 wt% GO-IL Pebax, respectively. MMMs with 0.05 and 0.2 wt.% GO, and GO-IL loadings were chosen to investigate further the effect of feed pressure, which was varied from 2 to 12 bar. The comparative result with the neat Pebax membrane is shown in Fig. S3. The permeability of CO2 increased with pressure for all the membranes due to increase in solubility and the driving force required for mass transfer. For neat Pebax, CO2 permeability risen from 89.3 to 101.2 Barrer while the CO2/N2 selectivity was almost constant. Compared with the pristine Pebax membrane, the increase in CO2 permeability and CO2/N2 selectivity became more significant with increasing GO content. The higher CO2 permeability and CO2/N2 selectivity may result from increasing CO2 solubility, due to its strong

14

affinity with GO and GO-IL functional groups, while the hydrostatic compressive effect of gas molecules decreased. Indeed, the increased crystallinity and more rigid nature of the GO Pebax MMMs, contributed to this phenomenon. The structural stability of the membranes is an important factor from the view of practical application. Therefore, the structural stability of the GO-IL Pebax MMMs (0.2wt% filler content) was investigated. Continuous CO2 and N2 gas permeation tests were conducted at 4 bar and 25 ˚C. The membrane performance remains stable within the test period (Fig. S4), indicating the stable structure of GO-IL based MMMs under CO2 and N2 gas feeds.

Table 2. Gas diffusivity and solubility coefficients of the membranes at 4 bar and 25°C. Membrane

DCO2a

DN2a

SCO2b

SN2b

DCO2/ DN2

SCO2/SN2

Pure Pebax

3.70±0.29

3.51±0.21

2.5±0.23

0.63±0.04

1.1±0.1

39.7±4.4

0.05 wt% GO Pebax

4.19±0.38

2.58±0.15

2.7±0.25

0.603±0.02

1.6±0.2

44.8±4.7

0.05 wt% GO-IL Pebax

4.12±0.32

2.94±0.16

3.09±0.24

0.67±0.03

1.4±0.1

45.9±4.4

0.2 wt% GO Pebax

2.83±0.24

2.31±0.14

3.11±0.22

0.87±0.04

1.2±0.1

35.7±3.0

0.2 wt% GO-IL Pebax

4.58±0.41

2.96±0.17

3.12±0.23

0.61±0.038

1.5±0.2

51.3±4.5

a

Diffusivity coefficient [cm2 s-1] ×107. b Solubility coefficient [cm3(STP)/cm3 cmHg] ×103.

Table 3. Pure gas transport properties of Pebax and GO-IL Pebax TFC membranes at 4 bar and 25°C. Permeance (GPU)

Selectivity

Membrane CO2

N2

H2

CO2/N2

CO2/H2

Pure Pebax

722.6±60.5

23.7±2.2

172.0±6.8

30.5±3.8

4.2±0.4

0.05 wt% GO-IL Pebax

905.4±48.0

20.2±1.5

156.1±7.5

44.8±4.1

5.8±0.4

0.2 wt% GO-IL Pebax

783.3±31.6

18.2±0.8

141.3±5.9

43.0±2.6

5.5±0.3

Detailed analysis of gas diffusivity and solubility coefficients of GO and GO-IL based Pebax MMMs are summarized in Table 2. It can be seen that the increase in CO2 permeability of 0.05 wt% GO Pebax membrane can be attributed to the enhancement of both solubility and diffusivity. However, the decreased diffusivity of N2 compared with pure Pebax, resulted in increased CO2/N2 selectivity. This is mainly due to the high aspect ratio of GO sheets which rendered more tortuous gas transport channels. This is favorable for hindering the penetration of N2 with larger molecular size [16]. The higher loadings of GO, i.e., 0.2 wt%, led to a lower diffusion coefficient due to aggregation of GO aggregation inside the polymer matrix. For GO-IL based Pebax, as the loading of GO-IL increases, both the diffusion coefficient and the

15

solubility of CO2 increased. GO-IL based Pebax showed significantly larger CO2 solubility coefficient values accompanied by a moderately enhanced selectivity of CO2/N2 than GO-based Pebax.

Fig. 6. (a) Comparing the performance of GO-IL Pebax thin membranes (red filled circles) with some developed commercial membranes (The target performance of membrane for CO2 capture from flue gas is reproduced from ref. [55]): () are data points for poly(amidoamine) dendrimer composite membrane [56, 57], () Supported ionic liquid membrane of aqueous diethanolamine [58], () Polyvinylamine thin film membrane[59], () Polyallylamine blend thin membrane[60], () GKSS membrane [61], () and ()Hendricks at al. [62], () Crosslinked PU [50], () PolarisTM [55]; (b) Cross-sectional SEM image of the thin film composite membrane of GO-IL Pebax on α-alumina porous substrate; Comparison of (c) CO2/N2 and (d) CO2/H2 separation performance of GO and GO-IL MMMs with the Robeson's upper bounds [6]. : Ref [16], : Ref [23], : Ref [34] are reported literature data of GO-based MMMs.

In particular, the major contribution to the increase in selectivity of CO2/N2 comes from the solubility coefficient while the CO2/N2 diffusivity selectivity is slightly decreased in comparison with GO-based Pebax. After modification of GO sheets with IL-NH2, the interlayer space between GO-IL sheets was increased slightly (as confirmed by XRD in Fig. 2a), which is beneficial to develop membranes with higher loadings without aggregation due to the weaken hydrogen bonding between GO-IL fillers. Also, the IL-NH2 moieties on the surface of GO

16

sheets could enhance the facilitated transport of CO2 through a reversible reaction, leading to a higher CO2/gas selectivity with improved filler–polymer interface compatibility. Development of thin film composite membranes (TFC) on porous support is required for industrial application of gas separation membranes. The selective layer with a few hundred nanometres thickness provides high gas flux and productivity while the porous support is responsible for mechanical stability. The gas separation properties of pure Pebax and GO-IL Pebax MMM in TFC configuration were immediately measured after preparation and drying, as shown in Table 3. The thickness of the membranes was found to be around 300 nm from the cross-sectional SEM images (Fig 6b). The trend of relevant TFC gas separation properties is similar to the performance obtained from the bulk membranes with the thickness of 40-50 µm. The CO2/N2 and CO2/H2 selectivities of Pebax thin film membranes enhanced from 30 and 4.2 to 45 and 5.8 for 0.05 wt% GO-IL Pebax, respectively. A higher degree of non-equilibrium free volume during the fast film forming process makes the gas selectivity of TFC membranes lower than the bulk separation properties [49]. The separation performance of the prepared membranes is compared with some developed polymer membranes reported in literature for CO2 capture (Fig 5). The GO-IL Pebax membranes exhibit both high CO2 permeance (900 GPU) and CO2/N2 selectivity of 45, which are more preferable than other reported materials with either high CO2 permeance or CO2/N2 selectivity (Table S2). Practically, both high CO2 flux and CO2/N2 selectivity are essential to overcome the issues of pressure-ratio limit and low CO2 feed concentration of carbon capture process. Robeson upper bound plots of the GO and GO-IL based MMMs for gas pairs of CO2/N2 and CO2/H2 are presented in Fig. 6. The pure Pebax possesses high permeability but relatively low selectivity for CO2/N2 and CO2/H2 gas pairs. Incorporation of GO fillers into Pebax membranes will enhance the gas selectivity without significant change in CO2 permeability. Modification of GO with IL could further improve the permeability and selectivity of the MMMs compared to GO-based Pebax membranes, even at relatively low loading. For instance, the separation performance of 0.2 wt% GO-IL Pebax membrane is at a more elevated position than GO Pebax with the same filler concentration. The overall gas separation performance of 0.2 wt% GO-IL Pebax far exceeds 2008 upper bound, making it possible to be used in CO2/N2 and CO2/H2 separation processes. The mixed gas separation performance of pure Pebax and 0.2 wt% GO-IL Pebax membranes were measured using CO2/N2 (30/70 vol/vol%) and CO2/H2 (30/70 vol/vol%) mixed gas at 25 °C and 4 bar, as presented in Table 4. The CO2 mixed gas permeability and CO2/N2 and CO2/H2 selectivities are lower than the values of the pure gas measurement. This observation could be attributed to the competitive interaction of two gases and non-ideal effects in mixed gas experiments [63-65]. This is the general trend for rubbery and most of the glassy polymers

17

as reported elsewhere [66]. It is important to note that, typical flue gas and syngas feeds are partially dehydrated which can significantly influence on the membrane performance, durability and functionality of IL moieties. It was reported that a trace of water content can enhance the selectivity, whilst the competition between the water and other gas species in the streams with high water content reduces the separation performance [67, 68]. Table 4. Mixed gas separation performance of Pebax and 0.2wt% GO-IL Pebax membranes Membrane

α (CO2/N2)

α (CO2/H2)

P(CO2)/barrer

P(N2)/barrer

P(H2)/barrer

Pure Pebax

37±4.0

8.1±0.5

73.9±3.3

2.0±0.2

9.1±0.4

0.2wt% GO-IL Pebax

71±5.3

9.5±0.6

118.6±5.4

1.67±0.1

12.4±0.5

4. Conclusion In summary, a series of facilitated MMMs were fabricated by incorporating IL-modified GO into the Pebax matrix. The introduction of GO with high-aspect-ratio sheet structure leads to longer and more tortuous paths for larger molecules, enhancing diffusivity selectivity. The incorporation of IL-NH2 improved the interfacial interaction by stronger hydrogen bonding as well as the filler–polymer interface compatibility between GO nanosheets and the polymer. Additionally, existing IL-NH2 on the interspaces of GO nanosheets facilitate the transport of CO2 through reversible reaction. The as-prepared GO-IL based membrane featured excellent preferential CO2 permeation, with an extraordinary high CO2/N2 and CO2/H2 separation performance, which surpassed the 2008 Robeson's upper bound line. The membrane formation by this method has a distinct advantage regarding facile membrane fabrication for practical CO2 capture applications. We believe that our strategy may provide an effective strategy to improve MMMs performance with both high permeability and selectivity further. The approach presented in this study could be extended as an efficient method to fabricate other kinds of advanced membrane materials.

Acknowledgements The authors gratefully acknowledge JST-Mirai project funding and JST-A-Step funding. iCeMS is supported by the World Premier International Research Initiative (WPI), MEXT, Japan.

References [1] R.W. Baker, Future Directions of Membrane Gas Separation Technology, Ind. Eng. Chem. Res., 41 (2002) 1393-1411. [2] T.C. Merkel, M. Zhou, R.W. Baker, Carbon dioxide capture with membranes at an IGCC power plant, J. Membr. Sci., 389 (2012) 441-450. [3] B. Ghalei, Y. Kinoshita, K. Wakimoto, K. Sakurai, S. Mathew, Y. Yue, H. Kusuda, H. Imahori, E.

18

Sivaniah, Surface functionalization of high free-volume polymers as a route to efficient hydrogen separation membranes, J. Mater. Chem. A, 5 (2017) 4686-4694. [4] A. Pournaghshband Isfahani, M. Sadeghi, K. Wakimoto, B.B. Shrestha, R. Bagheri, E. Sivaniah, B. Ghalei, Pentiptycene-based polyurethane with enhanced mechanical properties and CO2-plasticization resistance for thin film gas separation membranes, ACS Appl. Mater. Interfaces, 10 (2018) 17366–17374. [5] N. Du, H.B. Park, M.M. Dal-Cin, M.D. Guiver, Advances in high permeability polymeric membrane materials for CO 2 separations, Energy Environ. Sci., 5 (2012) 7306-7322. [6] L.M. Robeson, The upper bound revisited, J. Membr. Sci., 320 (2008) 390-400. [7] O. Shekhah, J. Liu, R. Fischer, C. Wöll, MOF thin films: existing and future applications, Chem. Soc. Rev., 40 (2011) 1081-1106. [8] H. Furukawa, K.E. Cordova, M. O’Keeffe, O.M. Yaghi, The chemistry and applications of metal-organic frameworks, Science, 341 (2013) 1230444. [9] B. Ghalei, K. Sakurai, Y. Kinoshita, K. Wakimoto, A.P. Isfahani, Q. Song, K. Doitomi, S. Furukawa, H. Hirao, H. Kusuda, Enhanced selectivity in mixed matrix membranes for CO2 capture through efficient dispersion of amine-functionalized MOF nanoparticles, Nature Energy, 2 (2017) nenergy201786. [10] B. Ghalei, A. Pournaghshband Isfahani, M. Sadeghi, E. Vakili, A. Jalili, Polyurethane-mesoporous silica gas separation membranes, Polym. Adv. Technol., 29 (2018) 874-883. [11] A.P. Isfahani, M. Sadeghi, A.H.S. Dehaghani, M.A. Aravand, Enhancement of the gas separation properties of polyurethane membrane by epoxy nanoparticles, J. Ind. Eng. Chem., 44 (2016) 67-72. [12] J. Dechnik, J. Gascon, C.J. Doonan, C. Janiak, C.J. Sumby, Mixed‐Matrix Membranes, Angew. Chem. Int. Ed., 56 (2017) 9292-9310. [13] T.-S. Chung, L.Y. Jiang, Y. Li, S. Kulprathipanja, Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci., 32 (2007) 483-507. [14] G. Dong, H. Li, V. Chen, Challenges and opportunities for mixed-matrix membranes for gas separation, J. Mater. Chem. A, 1 (2013) 4610-4630. [15] D. Bastani, N. Esmaeili, M. Asadollahi, Polymeric mixed matrix membranes containing zeolites as a filler for gas separation applications: A review, J. Ind. Eng. Chem., 19 (2013) 375-393. [16] J. Shen, G. Liu, K. Huang, W. Jin, K.R. Lee, N. Xu, Membranes with fast and selective gas‐transport channels of laminar graphene oxide for efficient CO2 capture, Angew. Chem., 127 (2015) 588-592. [17] E. Ameri, M. Sadeghi, N. Zarei, A. Pournaghshband, Enhancement of the gas separation properties of polyurethane membranes by alumina nanoparticles, J. Membr. Sci., 479 (2015) 11-19. [18] T.T. Moore, W.J. Koros, Non-ideal effects in organic–inorganic materials for gas separation membranes, J. Mol. Struct., 739 (2005) 87-98. [19] H. Vinh-Thang, S. Kaliaguine, Predictive models for mixed-matrix membrane performance: a review, Chem. Rev., 113 (2013) 4980-5028.

19

[20] Y. Kinoshita, K. Wakimoto, A.H. Gibbons, A.P. Isfahani, H. Kusuda, E. Sivaniah, B. Ghalei, Enhanced PIM-1 membrane gas separation selectivity through efficient dispersion of functionalized POSS fillers, J. Membr. Sci., 539 (2017) 178-186. [21] R. Mahajan, W.J. Koros, Mixed matrix membrane materials with glassy polymers. Part 1, Polymer Engineering & Science, 42 (2002) 1420-1431. [22] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev., 39 (2010) 228-240. [23] M. Karunakaran, R. Shevate, M. Kumar, K.-V. Peinemann, CO 2-selective PEO–PBT (PolyActive™)/graphene oxide composite membranes, Chem. Commun., 51 (2015) 14187-14190. [24] G. Dong, J. Hou, J. Wang, Y. Zhang, V. Chen, J. Liu, Enhanced CO2/N2 separation by porous reduced graphene oxide/Pebax mixed matrix membranes, J. Membr. Sci., 520 (2016) 860-868. [25] T. Rodenas, I. Luz, G. Prieto, B. Seoane, H. Miro, A. Corma, F. Kapteijn, F.X.L. i Xamena, J. Gascon, Metal–organic framework nanosheets in polymer composite materials for gas separation, Nat. Mater., 14 (2015) 48. [26] Z. Kang, Y. Peng, Z. Hu, Y. Qian, C. Chi, L.Y. Yeo, L. Tee, D. Zhao, Mixed matrix membranes composed of two-dimensional metal–organic framework nanosheets for pre-combustion CO 2 capture: a relationship study of filler morphology versus membrane performance, J. Mater. Chem. A, 3 (2015) 20801-20810. [27] Q. Xin, Z. Li, C. Li, S. Wang, Z. Jiang, H. Wu, Y. Zhang, J. Yang, X. Cao, Enhancing the CO 2 separation performance of composite membranes by the incorporation of amino acid-functionalized graphene oxide, J. Mater. Chem. A, 3 (2015) 6629-6641. [28] X. Li, Y. Cheng, H. Zhang, S. Wang, Z. Jiang, R. Guo, H. Wu, Efficient CO2 capture by functionalized graphene oxide nanosheets as fillers to fabricate multi-permselective mixed matrix membranes, ACS Appl. Mater. Interfaces, 7 (2015) 5528-5537. [29] H. Matsuyama, A. Terada, T. Nakagawara, Y. Kitamura, M. Teramoto, Facilitated transport of CO2 through polyethylenimine/poly (vinyl alcohol) blend membrane, J. Membr. Sci., 163 (1999) 221-227. [30] R.D. Noble, D.L. Gin, Perspective on ionic liquids and ionic liquid membranes, J. Membr. Sci., 369 (2011) 1-4. [31] Z. Dai, R.D. Noble, D.L. Gin, X. Zhang, L. Deng, Combination of ionic liquids with membrane technology: A new approach for CO2 separation, J. Membr. Sci., 497 (2016) 1-20. [32] M.A. Aroon, A.F. Ismail, T. Matsuura, M.M. Montazer-Rahmati, Performance studies of mixed matrix membranes for gas separation: A review, Sep. Purif. Technol., 75 (2010) 229-242. [33] Y. Ban, Z. Li, Y. Li, Y. Peng, H. Jin, W. Jiao, A. Guo, P. Wang, Q. Yang, C. Zhong, Confinement of Ionic Liquids in Nanocages: Tailoring the Molecular Sieving Properties of ZIF‐8 for Membrane‐Based CO2 Capture, Angew. Chem., 127 (2015) 15703-15707. [34] H. Li, L. Tuo, K. Yang, H.-K. Jeong, Y. Dai, G. He, W. Zhao, Simultaneous enhancement of

20

mechanical properties and CO 2 selectivity of ZIF-8 mixed matrix membranes: Interfacial toughening effect of ionic liquid, J. Membr. Sci., 511 (2016) 130-142. [35] Z. She, D. Ghosh, M.A. Pope, Decorating Graphene Oxide with Ionic Liquid Nanodroplets: An Approach Leading to Energy-Dense, High-Voltage Supercapacitors, ACS Nano, 11 (2017) 10077-10087. [36] R. Sitko, P. Janik, B. Feist, E. Talik, A. Gagor, Suspended Aminosilanized Graphene Oxide Nanosheets for Selective Preconcentration of Lead Ions and Ultrasensitive Determination by Electrothermal Atomic Absorption Spectrometry, ACS Appl. Mater. Interfaces, 6 (2014) 20144-20153. [37] X. Zhou, Y. Zhang, Z. Huang, D. Lu, A. Zhu, G. Shi, Ionic liquids modified graphene oxide composites: a high efficient adsorbent for phthalates from aqueous solution, Scientific Reports, 6 (2016) 38417. [38] M. Karunakaran, L.F. Villalobos, M. Kumar, R. Shevate, F.H. Akhtar, K.V. Peinemann, Graphene oxide doped ionic liquid ultrathin composite membranes for efficient CO2 capture, J. Mater. Chem. A, 5 (2017) 649-656. [39] W. Fam, J. Mansouri, H. Li, V. Chen, Improving CO2 separation performance of thin film composite hollow fiber with Pebax® 1657/ionic liquid gel membranes, J. Membr. Sci., 537 (2017) 54-68. [40] W. Fam, J. Mansouri, H. Li, J. Hou, V. Chen, Gelled Graphene Oxide–Ionic Liquid Composite Membranes with Enriched Ionic Liquid Surfaces for Improved CO2 Separation, ACS Appl. Mater. Interfaces, 10 (2018) 7389-7400. [41] G. Huang, C. Hou, Y. Shao, B. Zhu, B. Jia, H. Wang, Q. Zhang, Y. Li, High-performance all-solid-state yarn supercapacitors based on porous graphene ribbons, Nano Energy, 12 (2015) 26-32. [42] W.S. Hummers Jr, R.E. Offeman, Preparation of graphitic oxide, JACS, 80 (1958) 1339-1339. [43] Y. Zhang, Y. Shen, J. Yuan, D. Han, Z. Wang, Q. Zhang, L. Niu, Design and synthesis of multifunctional materials based on an ionic‐liquid backbone, Angew. Chem., 118 (2006) 5999-6002. [44] H. Yang, C. Shan, F. Li, D. Han, Q. Zhang, L. Niu, Covalent functionalization of polydisperse chemically-converted graphene sheets with amine-terminated ionic liquid, Chem. Commun., (2009) 3880-3882. [45] J. Benito, J. Sánchez-Laínez, B. Zornoza, S. Martín, M. Carta, R. Malpass-Evans, C. Téllez, N.B. McKeown, J. Coronas, I. Gascón, Ultrathin Composite Polymeric Membranes for CO2/N2 Separation with Minimum Thickness and High CO2 Permeance, ChemSusChem,

n/a-n/a.

[46] T. Li, Y. Pan, K.-V. Peinemann, Z. Lai, Carbon dioxide selective mixed matrix composite membrane containing ZIF-7 nano-fillers, J. Membr. Sci., 425 (2013) 235-242. [47] G. Huang, C. Hou, Y. Shao, H. Wang, Q. Zhang, Y. Li, M. Zhu, Highly strong and elastic graphene fibres prepared from universal graphene oxide precursors, Scientific reports, 4 (2014) 4248. [48] J.H. Kim, Y.M. Lee, Gas permeation properties of poly (amide-6-b-ethylene oxide)–silica hybrid membranes, J. Membr. Sci., 193 (2001) 209-225. [49] A.P. Isfahani, B. Ghalei, R. Bagheri, Y. Kinoshita, H. Kitagawa, E. Sivaniah, M. Sadeghi,

21

Polyurethane gas separation membranes with ethereal bonds in the hard segments, J. Membr. Sci., 513 (2016) 58-66. [50] A.P. Isfahani, B. Ghalei, K. Wakimoto, R. Bagheri, E. Sivaniah, M. Sadeghi, Plasticization resistant crosslinked polyurethane gas separation membranes, J. Mater. Chem. A, 4 (2016) 17431-17439. [51] V. Bondar, B. Freeman, I. Pinnau, Gas transport properties of poly (ether‐b‐amide) segmented block copolymers, J. Polym. Sci., Part B: Polym. Phys., 38 (2000) 2051-2062. [52] A.P. Isfahani, M. Sadeghi, K. Wakimoto, A.H. Gibbons, R. Bagheri, E. Sivaniah, B. Ghalei, Enhancement of CO2 capture by polyethylene glycol-based polyurethane membranes, J. Membr. Sci., 542 (2017) 143-149. [53] S. Song, Y. Ma, H. Shen, M. Zhang, Z. Zhang, Removal and recycling of ppm levels of methylene blue from an aqueous solution with graphene oxide, RSC Adv., 5 (2015) 27922-27932. [54] N. Mohanty, V. Berry, Graphene-based single-bacterium resolution biodevice and DNA transistor: interfacing graphene derivatives with nanoscale and microscale biocomponents, Nano Lett., 8 (2008) 4469-4476. [55] T.C. Merkel, H. Lin, X. Wei, R. Baker, Power plant post-combustion carbon dioxide capture: an opportunity for membranes, J. Membr. Sci., 359 (2010) 126-139. [56] T. Kai, T. Kouketsu, S. Duan, S. Kazama, K. Yamada, Development of commercial-sized dendrimer composite membrane modules for CO2 removal from flue gas, Sep. Purif. Technol., 63 (2008) 524-530. [57] S. Duan, F.A. Chowdhury, T. Kai, S. Kazama, Y. Fujioka, PAMAM dendrimer composite membrane for CO2 separation: addition of hyaluronic acid in gutter layer and application of novel hydroxyl PAMAM dendrimer, Desalination, 234 (2008) 278-285. [58] L. Bao, M.C. Trachtenberg, Facilitated transport of CO2 across a liquid membrane: Comparing enzyme, amine, and alkaline, J. Membr. Sci., 280 (2006) 330-334. [59] T.-J. Kim, B. Li, M.-B. Hägg, Novel fixed-site–carrier polyvinylamine membrane for carbon dioxide capture, J. Polym. Sci., Part B: Polym. Phys., 42 (2004) 4326-4336. [60] Y. Cai, Z. Wang, C. Yi, Y. Bai, J. Wang, S. Wang, Gas transport property of polyallylamine– poly(vinyl alcohol)/polysulfone composite membranes, J. Membr. Sci., 310 (2008) 184-196. [61] L. Zhao, R. Menzer, E. Riensche, L. Blum, D. Stolten, Concepts and investment cost analyses of multi-stage membrane systems used in post-combustion processes, Energy Procedia, 1 (2009) 269-278. [62] J.P. Van Der Sluijs, C.A. Hendriks, K. Blok, Feasibility of polymer membranes for carbon dioxide recovery from flue gases, Energy Convers. Manage., 33 (1992) 429-436. [63] W. Koros, G. Fleming, S. Jordan, T. Kim, H. Hoehn, Polymeric membrane materials for solution-diffusion based permeation separations, Prog. Polym. Sci., 13 (1988) 339-401. [64] H.B. Park, C.H. Jung, Y.M. Lee, A.J. Hill, S.J. Pas, S.T. Mudie, E. Van Wagner, B.D. Freeman, D.J. Cookson, Polymers with cavities tuned for fast selective transport of small molecules and ions, science, 318 (2007) 254-258.

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[65] L. Shao, T.-S. Chung, In situ fabrication of cross-linked PEO/silica reverse-selective membranes for hydrogen purification, Int. J. Hydrogen Energy, 34 (2009) 6492-6504. [66] J. Schultz, K.-V. Peinemann, Membranes for separation of higher hydrocarbons from methane, J. Membr. Sci., 110 (1996) 37-45. [67] P. Bakonyi, N. Nemestóthy, K. Bélafi-Bakó, Biohydrogen purification by membranes: An overview on the operational conditions affecting the performance of non-porous, polymeric and ionic liquid based gas separation membranes, Int. J. Hydrogen Energy, 38 (2013) 9673-9687. [68] W. Zhao, G. He, L. Zhang, J. Ju, H. Dou, F. Nie, C. Li, H. Liu, Effect of water in ionic liquid on the separation performance of supported ionic liquid membrane for CO2/N2, J. Membr. Sci., 350 (2010) 279-285.

Highlights 

Mixed matrix membranes were fabricated using ionic liquid functionalized graphene oxide and Pebax.



GO-IL enhanced the membranes solubility and diffusivity selectivity.



The IL-NH2 could improve filler–polymer interface compatibility.



Membrane performance enhanced by 90% in CO2/N2 selectivity and 50% in CO2 permeability.

Graphical abstract

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