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Gelled graphene oxide-ionic liquid composite membranes with enriched ionic liquid surfaces for improved CO separation 2

Winny Fam, Jaleh Mansouri, Hongyu Li, Jingwei Hou, and Vicki Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Applied Materials & Interfaces

Gelled graphene oxide-ionic liquid composite membranes with enriched ionic liquid surfaces for improved CO2 separation Winny Fam,a Jaleh Mansouri,a,b Hongyu Li,a Jingwei Hou,a,c and Vicki Chena* a

UNESCO Centre for Membrane Science and Technology, School of Chemical engineering, University of New South Wales, Sydney, New South Wales 2052, Australia. b

c

Cooporative Research Centre for Polymers, Notting Hill, Victoria 3168, Australia.

Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB3 0FS, UK.

KEYWORDS CO2 separation, ionic liquid, graphene oxide, mixed matrix membrane, composite membrane.

ABSTRACT Blends containing ionic liquid (IL) 1-ethyl-3-methyimidazolium tetrafluoroborate [emim][BF4] gelled with Pebax®1657 block copolymers were modified by adding graphene oxide (GO) and fabricated in the form of thin film composite hollow fiber membranes. Their carbon dioxide (CO2) separation performance was evaluated using CO2 and N2 gas permeation and low-pressure adsorption measurements, and the morphology of films was characterized

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using scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Upon small addition of GO into the IL-dominated environment, the interaction between IL and GO facilitated the migration of IL to the surface while supressing the interaction between IL and Pebax, which was confirmed using Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Amplified migration of IL to the surface and better dispersion of GO stacks were further achieved under alkaline condition. With the enriched IL on the surface, the gas permeations through the films at 0.5wt% GO and approximately 80wt% IL loading reached 1000 GPU for CO2 with their CO2/N2 selectivity (up to 44) approaching that of pure IL.

1. Introduction Ionic liquids are molten salts made of organic cation and inorganic anion, which have high thermal stability, low vapor pressure and tunable properties by combining different cations and anions. Since the discovery that ionic liquids (ILs) are capable to solubilize CO2 by Blanchard et al,1 they have been extensively investigated for laboratory-scale carbon capture. It was later found that effect of anion type on CO2 solubility is particularly important attributed to the Lewis acid-base-like interaction between them.2 Off-the-shelf and tailored ionic liquids have been utilized in the form of supported ionic liquid membranes (SILMs), polymerized ionic liquid membranes (PILMs), or simply gelled using organic gellators or polymers.3 Recent research on IL-based membranes focuses on the use of ILs as a minor component in mixed matrix membranes (MMMs) by direct blending,4–6 or impregnating IL into 3-dimensional nanoparticles to tailor the separation properties.7,8 Direct blending of IL often significantly enhances the permeability as IL reduces the crystallinity of polymer, and it may at the same time slightly improve the gas pair selectivity attributed to better interface compatibility between the


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organic polymer matrix and the inorganic dispersed particles. For example, Li et al. reported Pebax/ZIF-8/[bmim][Tf2N] to have CO2 permeability of 225 Barrer, four times higher than that of neat Pebax membranes with preserved CO2/N2 selectivity.9 On the other hand, impregnation of nanoparticles with IL results in higher improvement in gas pair selectivity due to decrease in the particle aperture size that improves their molecular sieving properties. Ban et al. incorporated [bmim][Tf2N] into ZIF-8 particles via in-situ ionothermal synthesis with the corresponding MMMs showing remarkable CO2/CH4 selectivity of 66 at 20 bar, about three times higher than that of neat polymer.7 There was a minor improvement in CO2 permeability, arguably attributed to higher intrinsic permeability of the gas through bulk IL although it remains unclear how such improvement could rise from entrapping a small amount of IL within the nanoparticles. In addition to the heavily explored 3-D nanoparticles, graphene oxide (GO), a 2-dimensional nanosheets which is mostly known for its gas barrier properties10 has drawn significant interest in membrane applications. GO is synthesized from oxidized graphite that can be easily dispersed in water and exfoliated into a single or few sheets. When the interlayer spacing between the sheets is carefully adjusted, ultrathin GO membranes show interesting gas separation properties. Kim et al. demonstrated that H2 selective GO membranes can be fabricated by repeated dipping and spin-coating, whereas CO2 selective membranes with more highly interlocked layer structure can be fabricated by layer-by-layer drop spin coating method.11 When used in MMMs, the random orientation of GO stacks can inhibit the gas transport as the diffusion pathways become more tortuous. However, when the GO stacks were assembled more perpendicularly to the membrane surface, they can provide a faster pathway, or exhibit molecular sieving effect for gases smaller than the interlayer spacing. Shen et al. accomplished engineering of the GO sheets assembly


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through the formation of hydrogen bonding between GO and polymer matrix,12 whereas Zhang et al. utilized external shearing from dip coating withdrawal speed to orient the GO sheets.13 In addition to having the capacity to form hydrogen bonding, GO contains functional groups (epoxide, hydroxyl, carbonyl and hydroxyl) that allow chemical functionalization and formation of weak – stacking interactions between the GO aromatic rings and other aromatic species. As the result, covalently modified GO such as imidazole-functionalized GO MMMs14 or simple blending IL/GO thin composite membranes with remarkable CO2/N2 separation properties have been produced.15 In our previous work, the gas separation performance of Pebax/IL thin film composite (TFC) hollow fiber membranes was studied.16 In this work, we further explore the performance of Pebax/IL membranes by adding GO sheets at high IL loading. GO was selected in anticipation of improving the separation properties of the Pebax/IL blends because GO exhibits affinity for CO2 while its sheet alignment may impede other gases due to the increase of tortuosity of gas diffusion pathway. 1-ethyl-3-methylimidizalium tetrafluoroborate [emim][BF4] was selected as the IL because it can be easily obtained and less costly than a number of alternative ILs. After blending the IL with GO at different loading composition, the mixture was stabilized by gelling them using Pebax® 1657 that is easily dissolved in ethanol:water mixture. Their CO2 separation properties and the morphology of the films were characterized using single and mixed-gas permeation tests and microscopy techniques. The effect of GO loading and change of pH during the fabrication on the CO2 separation properties and interactions among the components were also investigated to understand the underlying gas transport mechanism in these ternary mixed matrix membranes.

2. Experimental


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2.1 Materials For graphene oxide synthesis, graphite flakes and potassium permanganate were purchased from Sigma Aldrich, sulfuric acid (98%) was purchased from Merck, phosphoric acid was purchased from Ajax Finechem, and hydrogen peroxide (30%) and hydrochloric acid (32%) were purchased from Chem Supply. Pebax® 1657 pellets were acquired from Arkema, whereas the ionic liquid [emim][BF4] (≥ 98%) was purchased from Sigma-Aldrich. The hollow fibers polyvinylidene fluoride (PVDF) with the average pore size of 0.05 µm (OD = 1.1 mm, ID = 0.5 mm) were kindly provided by OriginWater Pure Tech Co. (China). Poly(1-trimethylsilyl-1-propyne) (PTMSP) was purchased from Gelest. Sodium hydroxide pellets and all other reagents including ethanol and hexane were purchased from Chem Supply. All pure gases (N2 and CO2) and mixed-gases of CO2/N2 (20:80) with 0.96% NOx and CO2/CH4 (20:80) used in the permeation tests were purchased from Coregas. All materials were used without any further purification.

2.2 Membrane preparation Graphene oxide was synthesized according to improved Hummers’ method.17 For the selective layer solution, Pebax® 1657 pellets were weighed and dissolved in ethanol: water mixture (70:30 w/w) under magnetic stirring for 2 h at 70°C to make 3wt% solution. IL and GO were weighed and mixed in 15 mL of ethanol:water mixture under sonication for 15-20 minutes. The mixture was added into Pebax solution that had been cooled down to room temperature. To change the pH of the solution, 0.1 M NaOH was added until desired pH was reached, as indicated by universal pH indicator strips.

The final solution was sonicated for another 15

minutes to ensure uniform dispersion of GO before coating. The weight composition of GO is


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expressed based on the total weight of solutes (Pebax + IL + GO), and the total weight of IL and GO always made up to 80 wt%. Table S1 summarizes the compositions of 100 g of selective layer coating solutions. The thin film composite membranes were prepared by dip-coating the PVDF hollow fiber with 2wt% PTMSP gutter layer solution in hexane. The step was repeated for four times before the selective layer coating. Finally, a coating with PTMSP solution was deposited to protect the selective layer and to seal off any defects.16 Figure S1 illustrates the laboratory-scale dip coating process and the composite structure of a coated hollow fiber.

2.3 Membrane characterization FTIR spectra of the GO, ionic liquid and dense membranes were obtained using attenuated total reflection (ATR) method at room temperature using Bruker Alpha spectrometer with wavelength range of 4000-400 cm-1. The morphology and the thickness of the hollow fibers were imaged using Nova NanoSEM 450 by applying an accelerating voltage at 5kV. The cross section of the fibers was prepared by cryogenic fracturing in liquid nitrogen, and all samples were mounted on aluminum stubs, and sputter-coated with 10nm of platinum before imaging. The synthesized graphene oxide was characterized using X-ray diffraction (PANalytical Xpert Multipurpose XRD) with Cu-Kα radiation for 2θ from 5° to 50°with 0.026° step size. The surface morphology of the TFC membranes was analyzed using Bruker Dimension ICON SPM (BRUKER, Germany) operated in ScanAsyst mode with ScanAsyst-air probe. The samples were scanned at the rate of 0.5 Hz with 1 µm2 scan area and 512 samples/ line, and the images were processed using free Gwyddion 2.40 software. The TEM images of the selective layer were acquired using Philips CM200 with an accelerating voltage of 200kV. After the embedding preparation, the sample blocks were sectioned using Leica Ultracut UC 6 at room temperature


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with a diamond knife, and the resulting thin slices were placed on Formvar/carbon coated copper grids. The surface elemental compositions of the TFC selective layers were determined using XPS data collected from Thermo ESCALAB250Xi XPS (Thermo Scientific, UK) equipped with a mono-chromated Al K-alpha (1486.68 eV) source.

2.4 Gas permeation measurement

The hollow fibers were potted in a stainless-steel module with outside-in configuration and effective membrane area of 4.84 cm2. The single and mixed-gas permeation test setups followed the previous work.16 Gas permeation test was carried out in a constant pressure-variable volume system at 35°C and 3 bar gauge pressure unless stated otherwise, with the permeate flow rate was measured using a digital flowmeter (Agilent ADM). The schematic of the gas permeation setup is shown in Figure S2. The pure gas permeability (P) which is a measure of membrane’s productivity can be calculated using the following formula: .

= ∆

(1)

where is the volumetric flow rate of the permeate gas (cm3/s), is the membrane surface area (cm2), ∆ is the pressure difference across the membrane (cmHg), is the thickness of the membrane (cm). The unit of permeability is often expressed in Barrer where 1 Barrer = 10-10 cm3(STP).cm/(cm2.s.cmHg). In this work, permeance (P/l) will be used instead of permeability to characterize the performance of the TFC membranes. The unit of permeance is reported in gas permeation unit (GPU) where 1 GPU = 10-6 cm3(STP)/(cm2.s.cmHg). The ideal selectivity () of a membrane, which is a measure of membrane’s efficiency to separate a gas pair was calculated as the ratio the permeance of fast gas A to slow gas B:


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⁄ )

⁄ = ⁄)

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(2)

The mixed-gas permeance was calculated using the formula as follows:

where

=

×

×

(3)

and ! are the feed and permeate gas concentrations, and and are the feed and

permeate pressures, respectively. Since the mixed-gas permeation did not utilize any sweep gas or vacuum, the gas separation was limited by the feed-to-permeate ratio. Hence, the mixed-gas selectivity was calculated using ideal selectivity (Eq. 2) to obtain a better insight into the membrane’s separation capacity.

2.5 Gas adsorption measurement

N2 and CO2 adsorption isotherm measurements were carried out using Micromeritics Tristar II Plus surface area and pore size analyzer (USA). Each sample weighing 0.14-0.15 g was dried at 50°C under vacuum overnight to remove residual moisture. Prior the measurement process, the samples were degassed at 70°C for 12 h to remove any guest molecules. The nitrogen sorption measurement was performed at 77 K using liquid nitrogen, whereas the CO2 sorption measurement was performed at 273 K by immersing the sample tubes in ice water bath.

3. Results and discussion 3.1 GO characterization and suspension stability in solution mixture

The synthesized GO sheets were characterized using AFM, XRD, and FTIR (Fig. S3). The single layer thickness is ~1.1 nm with the lateral size < 0.5µm. The oxidization process of graphite into GO was demonstrated by the disappearance of the characteristic (002) peak of graphite at


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2θ=26.6° (d-spacing = 0.34 nm) and the emergence of the characteristic (001) peak of GO at 2θ =10.1 (d-spacing = 0.88 nm) on the XRD spectrum of GO. The increase in d-spacing indicates the formation of oxygen-containing functional groups which weaken the van der Waals interaction between the sheets. The presence of the oxygen-containing functional groups on GO was confirmed with FTIR with the spectra which show the characteristic broad peak of O-H group from alcohol or carboxylic acid from 3400-3200 cm-1, and other stretching peaks at 1726, 1632, 1218, and 1053 cm-1, which represent C=O, C=C, C-OH, and C-O groups, respectively. High oxidation degree, which can be found in GO synthesized using Hummer’s method results in more abundant functional groups, which make the GO more hydrophilic and stable when exfoliated in aqueous solution.18 In the solvent mixture of ethanol:water (70:30 w/w), stable suspension of 0.1 mg/mL GO was observed for at least two weeks although GO suspension in ethanol has been reported to exhibit stability up to a few days only.19 Alkaline environment has been reported to improve the dispersion of GO in IL due to the electrostatic attraction between deprotonated carboxyl groups and the cations of IL. In addition, the aromatic rings on GO layers may interact with imidazolium rings through weak π-π and/or cation-π interactions20 as illustrated in Figure 1a. For the coating solution used in this work, GO was dispersed in Pebax/IL mixture in ethanol:water with the pH of the resulting mixture was approximately 6. After the ultrasonication process, GO appeared well suspended although the stability lasted for several minutes and GO completely settled after 24 hours (Fig. 1b). When the pH of the solution was raised to 12 by adding 0.1 M NaOH solution in the freshly made Pebax/IL/GO mixture, GO suspension remained stable even after 24 hours due to the ionic complexation between GO and IL cations. It was also noted that the mixture at higher pH slightly gelled over time with the viscosity increased by four times from 3.9 cP to 18 cP (Table


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S2). Although the gelation mechanism was unknown, it helped trapping the already-dispersed GO particles in the solution. Interestingly, stable suspension was not obtained when NaOH was added into Pebax/IL/GO mixture that had been prepared a week before. In this case, GO eventually settled at the bottom of the container as for the case of pH = 6 despite the high alkaline environment.

Figure 1. (a) Interaction between GO sheets and IL in the forms of ionic complexation (2-D model) at high pH21 and π-π or cation-π interaction between GO basal plane and cation of IL (ball and stick model), (b) stable dispersion of Pebax/IL/GO-0.5 solution at pH =12.

3.2 Film morphology characterization

The morphology of the coating layer on TFC membranes was characterized using SEM as shown in Figure 2. The surface of the PTMSP gutter layer is not completely smooth, but shows the formation of micron-scale hills and valleys which may be caused by water vapor condensation on the film during the membrane formation process.22 After the subsequent coating of Pebax/IL/GO, the total thickness of the film increases and the surface becomes smoother. At 2 wt% GO loading, the total film thickness increases by up to 25% compared to 0.5 wt% loading with less distinguishable pit feature on the surface. Fibers that were coated with solution at


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higher pH in (Fig. 2g-h) reveal interesting morphology with more gel-like appearance and significantly higher film thickness due to the increase in solution viscosity. During the SEM sample preparation, sputtered platinum coatings on the fibers fabricated at higher pH showed poor adherence, suggesting they have a different surface chemistry from the other fibers.

Figure 2. SEM surface (left) and cross-section (right) images of the PVDF hollow fibers coated with (a-b) PTMSP and subsequent coating of (c-d) Pebax/IL/GO-0.5, or (e-f) Pebax/IL/GO-2.0, or (g-h) Pebax/IL/GO-0.5(pH=12). Figure S4 shows the surface topology and phase images of 1 µm2 scans of TFC hollow fibers characterized using AFM. Based on the previous work,16 the microphase separation between the PEO and PA segments of Pebax was expected to be less distinguishable at high IL loading


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because IL interacts with both segments and reduces the polymer crystallinity significantly. With the addition of GO at 0.5 wt% GO loading, the microphase separation is surprisingly still observable where the hard, more crystalline segments appear darker and soft segments give lighter contrast on the inphase channel. At GO loading of 2.0 wt%, the contrast between both phases becomes more distinctive and the surface becomes rougher as indicated by larger Ra and Rms values in Table S3. Rougher surface normally suggests a higher three-dimensional surface area. The more distinctive microphase separation with higher GO loading may be due to the increased interaction between IL and GO, which subsequently reduces hydrogen bonding interaction between IL and the polymer host. The reduced IL-polymer interaction is also confirmed by the shifting of FTIR signal of methyl-N peak of the IL cation in Pebax/IL/GO approaching the signal of pure IL with increasing GO (Fig S5). When the pH of the solution was increased to 12, the resulting morphology of Pebax/IL/GO0.5 is drastically different from that of original pH condition. The membrane surface appears smoother with the formation of island-like features with the acquired surface area being 20% higher than the projected two-dimensional scan area of 1µm2 despite having similar Ra and Rms values as the membranes fabricated at pH = 6 (Fig. S4, Table S3). Since the image surface area is obtained from the sum of triangle areas formed by three adjacent points, the existence of island-like features with long horizontal sides may significantly contribute to the increase in surface area. Moreover, there is less contrast between the hard and soft phases of the polymers, which suggests the surface is more uniform with one phase dominating over the other. To investigate how the GO was dispersed in Pebax/IL matrix, TEM images of the film cross sections at the original pH and pH = 12 were acquired. Since GO sheets have crystalline nature, Fast Fourier Transform (FFT) was used to detect the presence of GO in the matrix, which are


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shown as diffraction spots in reciprocal space. It was observed GO orientations were more random at the pH = 6, whereas GO stacks were aligned in the same orientation at higher pH as shown in Figure 3. It was expected that the d-spacing of GO sheets in Pebax/IL matrix would be close to that of as-synthesized GO sheets of 0.88 nm (Fig. S3b). However, Figure 3b shows that the d-spacing of GO sheets in Pebax/IL matrix at pH = 6 is only 0.35 nm, similar to the d-spacing of graphene. The small decrease in d-spacing of GO sheets in Pebax has been reported elsewhere, owing to the confinement effects of the polymer chains and interaction between GO and the polymer matrix via hydrogen bonding.12 Nevertheless, the fact of the d-spacing of GO sheets coincides that of graphene suggests that GO sheets are possibly reduced into graphene. Karunakaran et. al. reported the sign of GO reduction in IL/GO by the color change of GO from brownish to dark black after 24 hours.15 Even in its solid form at room temperature, GO has been reported to gradually reduce upon prolonged storage.23 In this work, there was no noticeable color change of Pebax/IL/GO observed after 24 hours. However, FTIR analysis of GO collected from the Pebax/IL/GO solutions in Figure S6 reveals the diminished signal intensity of OH and C=O peaks for GO collected at pH = 6. Moreover, the OH broad peak completely disappeared for GO collected from the basic Pebax/IL/GO solution, consistent with the reported accelerated GO reduction under alkaline condition.24 Since GO sheets lost their –COOH functional groups in IL-rich environment during the course of the membrane fabrication, the ionic complexation between GO and cations in the formed membrane can be ruled out. Hence, IL-GO interaction between the cation and the outer basal plane of GO stacks occurs via π-π and/or cation-π interactions only.


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Figure 3. TEM cross section image of Pebax/IL/GO-0.5wt% and the d-spacing of GO stacks fabricated at pH= 6 (a-b) and pH = 12 (c-d). Figure 3d shows that d-spacing of GO sheets at pH = 12 shrinks to 0.26 nm. The decrease is in agreement with results reported by Huang et al, where the electrical double screening effect predominates and lowers the zeta potential of GO sheets at pH = 9-12, which shrinks the


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interlayer distance25 although it remains unclear how the d-spacing is smaller than that of graphene. Nevertheless, these results show that despite the better dispersion and more aligned GO sheets obtained at higher pH, reduced GO acts more like impermeable fillers, substantially reducing the possibility of molecular sieving characteristics through the interlayer space of stacked GO sheets. Therefore, the gas transport through the Pebax/IL/GO membranes with high IL loading is expected to be dominated by solution-diffusion mechanism.

3.3 Film surface chemistry characterization

To investigate the changes in film surface chemistry with GO loading and pH, XPS and surface energy calculation on the fiber surface were conducted. The summary of the elemental composition for the fibers without GO and with 0.5 wt% GO loading at different pH is given in Figure 4. The results were obtained from the average of at least two measurements with insignificant standard deviation. Boron and fluorine are present in IL only, nitrogen is present in both Pebax and IL, oxygen is present in both Pebax and GO, and carbon is present in all three components. Sodium was found to be present in the fiber at pH = 12 due to the addition of NaOH. The survey scan shows that the incorporation of 0.5 wt% GO enhances % oxygen on the surface from 11% to 18%, and slightly decreases % fluorine because of the presence of GO which may replace some IL on the surface. More detailed analysis on the N1s spectra (Fig S7 shows that there are more IL cations migrating to the surface compared to their composition on Pebax/IL membrane surface. These cations are probably associated with GO sheets by π-π and/or cation-π interactions. As the result, the anions on the surface of Pebax/IL/GO-0.5 are more available to interact with CO2 gas.


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Figure 4. XPS survey scan and elemental composition of Pebax/IL and Pebax/IL/GO-0.5 TFC hollow fibers fabricated at pH = 6 and pH = 12. The addition of GO at high pH further promotes the migration of IL to the surface as indicated by substantial increase in anion composition ( %B and F) in Figure 4 and higher cation composition represented by larger N1s peak at 401 eV in Figure S7. Study on the effect of pH on the adsorption of IL at oil-water interface by Asadabadi and Saien shows that OH- has the tendency at adsorb at the interface, which lowers the repulsion between the IL cations, and thus bolsters IL adsorption and lowers the interfacial tension of the surface.26 We postulate that similar phenomenon is applicable to our Pebax/IL/GO solution at high pH. However, instead of surface tension, the effect of pH on surface energy of the film was investigated.


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Surface energy of a solid material "# is the result of cohesive interactions between the atoms and molecules. According to van Oss-Good theory, the interactions can be divided into dispersive "#$ ) and polar "# ) components.27 The polar component can be further differentiated into an acid component and a base component. The dispersive component characterizes van der Waals type of interactions, whereas the polar component characterizes the tendency of the surface to have specific interactions, such as dipole-dipole, induced dipole-dipole and hydrogen bonding to another surface. The total surface energy is defined to be the additive sum of each component. The detail of the surface energy calculation using van Oss theory is available in the supporting information. The resulting surface energy calculation of the TFC membranes is given in Table 1. The surface energy of Pebax/IL/GO-0.5 fabricated at pH = 12 is lower than that at pH = 6, affirming that basic condition enhances IL adsorption on the surface, and thus increases the gas permeance. In addition, the polar component contribution for pH = 12 is three times higher than that of pH = 6, which indicates its higher tendency to have specific interactions with gases with large quadruple moments, such as CO2.

Table 1. Surface energy of Pebax/IL/GO-06 TFC hollow fibers fabricated at pH = 6 and pH = 12. Pebax/IL/GO-0.5

"#$ (mN/m)

"# (mN/m) "# (mN/m) Polar component contribution

pH = 6

48.3

3.9

52.2

7.4%

pH=12

37.8

9.8

47.6

21%


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3.4 Gas separation performance

3.4.1

Effect of GO loading and withdrawal speed on single gas permeation

To investigate the effect of GO addition to Pebax/IL blend, the composition of GO was varied from 0 to 2.0 wt% with respect to the weight of the total solutes. Figure 5a shows the single gas permeation results of the Pebax/IL/GO membranes measured at feed gauge pressure of 3 bar and 25°C. The incorporation of GO clearly affects the CO2 permeance with a significant increase from 573 GPU at 0 wt% loading to 773 GPU at 0.5wt% loading, but the gas permeance later decreases with higher loading. The CO2/N2 selectivity remains constant for all GO compositions at approximately 30, with a slight decrease at 2wt% GO loading. It should be noted that the decrease in gas permeance after 0.5 wt% GO loading may result from the combination of the decrease in intrinsic gas permeability and increase in the film thickness. To determine the intrinsic gas permeability, the thickness of the film needs to be measured accurately. Since the SEM images in Figure 2 show that the interface between the intermediate gutter layer and selective layer was indistinguishable,28 the thickness of the selective layer can be estimated by subtracting the thickness of the PTMSP layer (~4µm) from the total coating thickness. For Pebax/IL TFC membranes, employing this method yields the estimated selective layer thickness of 1.6 µm and the corresponding CO2 permeability of 938 Barrer. This estimated result is notably higher than the permeability measurement for cast dense Pebax/IL membranes in the preceding work (PCO2 = 270 Barrer).16 Thus, solely relying on the SEM images to obtain the film thickness and the subsequent gas permeabilities is inadequate to isolate the cause to the decrease in gas permeance.


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The effect of withdrawal speed was also investigated on Pebax/IL/GO-0.5 membranes (Fig. 5b). Increasing the withdrawal speed from 0.2 cm/s to 0.5 cm/s decreases CO2 permeance by 22% due to the thickness increase in liquid film and the subsequent dry film thickness. The withdrawal speed also affects the gas pair selectivity. At withdrawal speed of 0.2 cm/s, lower CO2/N2 selectivity may be caused by incomplete coverage of the selective layer over the gutter layer as well as misalignments of GO sheets that result in defects. On the other hand, thicker films produced at withdrawal speed > 0.3 cm/s exhibit slight decrease in gas pair selectivities. This may be due to longer drying time required by thicker liquid film, which allows GO sheets to realign and produce more heterogeneous structures.29 As the result, the optimum withdrawal speed for Pebax/IL/GO-0.5 wt% is 0.3 cm/s, which will be used for further study. Similar withdrawal speed effect was obtained for previous Pebax/GO study,13 confirming that the performance of the membranes is not only affected by the concentration of the two-dimensional fillers, but also by the uniformity of their dispersion in the polymer matrix.

Figure 5. (a) Single gas permeation of Pebax/IL/GO TFC membranes at 3 bar feed gauge pressure and 25°C by varying GO concentration by up to 2wt% with total wt% of IL + GO


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always adds up to 80wt%. (b) Effect of withdrawal speed in CO2 separation properties of Pebax/IL/GO-0.5 TFC membranes at 3 bar feed gauge pressure and 25°C. 3.4.2

Effect of pH on gas separation performance

The performances of hollow fibers coated with Pebax/IL/GO-0.5 wt% fabricated at the original pH = 6 and pH = 12 were evaluated using mixed gas permeation tests. The summary of their performance is given in Table 2 for CO2:N2 feed mixture containing 0.96% NOx and CO2:CH4 feed mixture. Fibers that were fabricated at pH = 12 exhibit higher CO2 permeance and selectivity simultaneously for both feed gas mixtures by at least 50% compared to fibers fabricated at the original condition. Table 2. Mixed-gas performance of Pebax/IL/GO-0.5wt% fabricated at pH = 6 and pH=12 at 35°C and 3 bar gauge pressure. CO2:N2 (20:80) with 0.96%NOx

CO2:CH4 (20:80)

N2 (GPU)

CO2 (GPU)

CO2/N2

CH4

CO2

CO2/CH4

Dry feed

22.3±4.0

642±50

29±6

71.0±9.3

464±65

6.5±1.3

Wet feed (5060% RH)

29.2±1.8

737±125

25±4

88.2±8.3

521±83

5.9±1.1

Dry feed

22.4±4.4

981±118

44±10

63.4±5.6

683±120

10.9±1.0

Wet feed (5060% RH)

27.6±2.0

1083±197

39±7

98.4±6.3

874±117

8.9±1.3

Pebax/IL/GO-0.5 TFC membranes

pH = 6

pH = 12

When the feed was humidified at 50-60%RH, the gas permeances of both types of fibers increase despite the lower gas pair selectivity compared to dry feed mixture performance (Table 2). Since all components of the membranes are hydrophilic, they may interact and bond with water strongly. When exposed to humid condition, pure GO membranes, for instance, have been reported to exhibit high water vapor permeability due to swollen GO sheets with enlarged


20

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interlayer spacing.30 In addition, the majority of membrane was made up by Pebax/IL blends, and thus swelling is most likely to occur within the matrix, which leads to enhancement in gas permeance by increasing gas diffusivity through the membranes. In this case, the increase in N2 permeance is higher than CO2, which consequently decreases the gas pair selectivity at humid condition. It is also worth mentioning that the stability of the membranes was maintained even after the exposure to trace composition of NOx in the CO2:N2 feed mixture. Based on our previous works for single gas permeations, the ideal CO2/CH4 selectivity for neat Pebax®1657 is 19,31 whereas the reported CO2/CH4 selectivity for [emim][BF4] SILM is 22.32 Although the single-gas permeation results for CO2/CH4 separation of Pebax/IL/GO are not presented here, it is reasonable to assume that the results will be lying in between the properties of the pure materials. However, the mixed-gas permeations with CO2:CH4 in Table 2 clearly exhibit lower gas pair selectivities than the expected properties for the blend materials. Similar decrease in CO2/CH4 selectivity for mixed-gas feed has been observed in the previous work with Pebax/80% IL membranes,16 which affirms that competitive sorption between CO2 and CH4 continues to persist in Pebax/IL/GO membranes. The effect of operating temperature on Pebax/IL/GO-0.5 TFC membranes fabricated at different pH conditions was also investigated for 25-55 °C. Figure 6 shows the CO2/N2 separation performance with the feed gauge pressure was maintained at 3 bar. Since permeation is an activated process, its temperature dependence can be described using Arrhenius equation as follows: = % & '( /*+

(4)

where the , is the apparent activation energy of permeation. The increase in both CO2 and N2 permeances with temperature can be clearly observed, but the increase of CO2 permeance is


21


Page 22 of 45

lower than that of N2, possibly due to significant decrease in CO2 solubility with temperature.33 Consequently, the gas pair selectivity decreases drastically with temperature by 62% to 12 for Pebax/IL/GO( pH = 6 ) and 57% to 17 for Pebax/IL/GO (pH = 12) at the operating temperature of 55°C. Despite the considerable decrease in selectivity, the permeation results for CO2 separation is still attractive, particularly for fibers fabricated at alkaline condition where CO2 permeance exceeds 1000 GPU.

Figure 6. Effect of temperature on the single-gas performance of Pebax/IL/GO-0.5 TFC membranes fabricated at pH = 6 (solid symbols) and pH = 12 (open symbols). (a) N2 and CO2 permeances, and (b) CO2/N2 selectivity. Feed gauge pressure was maintained at 3 bar. 3.4.3

Gas adsorption capacity

The change in intrinsic gas permeability can be caused by change in gas solubility and/or gas diffusivity. To elucidate the change in gas solubility due to the addition of GO, gas adsorption experiments

were

performed

for

Pebax/IL,

Pebax/IL/GO-0.5,

Pebax/IL/GO-2.0,

and

Pebax/IL/GO-0.5 (pH = 12) TFC membranes. The quantity of each gas adsorbed was plotted against the relative pressure (p/po), where po is the saturation pressure of the gas at the measurement temperature. The adsorption isotherm of light gases on the selective layer is


22

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expected to be linear with pressure due to the rubbery nature of the host polymer.34 Figure 7a shows the N2 isotherm obtained for the relative pressure ranging from 0-1 at 77K. The shape of the isotherms and the hysteresis between the adsorption and desorption curves are typically observed for glassy microporous polymer, such as PTMSP.35 This denotes that N2 was mostly adsorbed through the lumen sides of the fibers on the PVDF support and PTMSP gutter layers, and the adsorption on the selective layer was negligible. The CO2 isotherm in Figure 7b shows a linear relationship with the relative pressure up to 0.03, consistent with the reported isotherm of rubbery Pebax polymer.36 The quantity of CO2 adsorbed for a fixed absolute pressure appears much less than the quantity of N2 adsorbed because the CO2 measurement was carried out at a higher temperature of 273K. The addition of GO at the original pH condition increases the amount of CO2 adsorbed although the difference is marginal for the pressure measured. The increase may arise from reduced interaction between Pebax and IL due to the presence of GO as described in Section 3.2, which allows more IL to interact with CO2. In addition, the polar groups attached to GO surface is also known to exhibit preferential adsorption with CO2 through dipole-quadrupole interaction37 although this effect may be minor considering the amount of GO used. Among the membranes fabricated at pH =6, Pebax/IL/GO-0.5 shows the highest adsorption capacity for both CO2 and N2, followed by Pebax/IL/GO-2.0 which has CO2 adsorption capacity slightly higher than that of Pebax/IL. Since the change in GO loading has minor effect on the sorption capacity, the change in the intrinsic permeability will be highly dependent on the change in gas diffusivity. The random orientations of GO stacks (Fig. 3a) increase the tendency to form aggregates at higher GO loading, which will lower the gas permeability as they behave as gas barriers by increasing the gas diffusion


23


Page 24 of 45

pathway.38 These aggregates may also cause the formation of voids, which decrease gas pair selectivity observed at 2.0 wt% loading.

Figure 7. Adsorption isotherms for Pebax/IL, Pebax/IL/GO-0.5, Pebax/IL/GO-2.0, and Pebax/IL/GO-0.5 fabricated at pH = 12 with (a) N2 at 77K with po =101 kPa and (b) CO2 at 273 K with po =3479.72 kPa. po is the saturation pressure of the gas at the measurement temperature.

Pebax/IL/GO-0.5 TFC membrane fabricated at pH = 12 shows more superior CO2 adsorption capacity compared to other fibers fabricated at the original pH condition (Fig.7b). The excellent CO2 sorption capacity is supported by the morphology and surface characterization results, where the addition of base changes the film morphology with enriched IL on the surface (Fig. 4) and higher surface area (Table S3). Coupled with absence of GO aggregates due to more aligned GO stacks (Fig. 3c), the membrane show enhancement in CO2 separation with increased CO2 permeance and CO2/N2 selectivity from the mixed-gas permeation approaching the ideal CO2/N2 selectivity of free [emim][BF4] at 44.39

3.5 Membrane stability


24

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One of the main problems faced by ionic liquid-based membranes is their performance stability over time. Using the fibers that have been aged for six months, the gas separation of the Pebax/IL/GO-0.5 (pH = 12) TFC membranes were tested for 28 h and measured at various time intervals as shown in Figure 8. Stable CO2 gas permeance at 568 GPU and CO2/N2 selectivity at 40 were observed for the measured period. However, compared to the mixed-gas performance of the freshly-made fibers tested 1-2 weeks after their fabrication (Table 2), the CO2 permeance of the aged fibers have decreased by 42%, whereas the CO2/N2 selectivity is preserved. Since there is no significant change in the gas pair selectivity, it can be concluded that IL remains stable within the membrane matrix, and the membranes are chemically robust even after long exposure to trace amount of NOx. The decrease in gas permeance is most likely caused by the accelerated aging of PTMSP when fabricated in the form of thin films. Dorkenoo and Pfromm reported that the 1- and 3-μm-thick PTMSP membranes lost 60% of their gas permeability within 400 h.40 Consequently, the gas transport resistance due to the aging of PTMSP gutter and protective layers would increase over time and decrease the overall gas permeability. Nonetheless, the Pebax/IL/GO membranes have demonstrated potential for real application in terms of their stability and excellent separation performance although more rigorous testing will be required, and alternative materials that are more resistant to physical aging such as polydimethylsiloxane (PDMS) should be considered to maintain the gas separation efficiency over time.


25


Page 26 of 45

Figure 8. Gas separation performance of Pebax/IL/GO-0.5(pH = 12) TFC membranes with time with CO2:N2 (20:80) mixed-feed gas containing 0.96% NOx at feed gauge pressure of 3 bar and 35°C. The fibers used for the stability measurement were six months old.

3.6 Comparison with other membranes

The performance Pebax/IL/GO TFC membranes are compared with other GO-based or ILbased membranes as shown in Table 3. The results of the TFC membranes are presented in GPU because the thickness of the selective layer could not be determined accurately. As previously mentioned, GO membrane itself is highly impermeable to gases with CO2 permeability of 0.76 Barrer. However, when the membrane is made thin enough with properly stacked structures, it can behave as molecular sieving membranes with CO2/N2 selectivity of 20. Incorporation of GO or modified GO into polymer in general greatly increases the CO2/N2 selectivity performance. For instance, Pebax/imidazole-functionalized-GO (ImGO) with filler loading of 0.8 wt% exhibits improvement in CO2/N2 gas selectivity by 50% due the rigidified interface between the polymer and the interaction between CO2 and imidazole groups. The performance of Pebax/PEG-PEI-GO


26

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is even more impressive with simultaneous two-fold improvement in permeability and selectivity at filler loading of 10 wt% due to the presence of additional PEO groups and amine carriers which facilitates CO2 transport, particularly at low pressure and humid condition. Table 3. Comparison with other membranes that contain Pebax®1657, GO, or IL. Membrane

CO2 permeability

CO2/N2

CO2/CH4

CO2/H2

Test condition

Ref

110 GPU (0.76 Barrer)

20

10

6

35°C, 1 bar

11

GO

37 GPU Barrer)

130

39

16

25°C, 1 bar

15

PAN/[emim][Ac]GO/PTMSP

103 GPU (86.4 Barrer)

59

18

-

35°C, 3 bar

16

Pebax Pebax/GO

108 Barrer

49

17

9

35°C, 7 bar

41

Pebax/GO

100 Barrer

91

-

-

25°C, 3 bar

12

Pebax/porous reduced GO

119 Barrer

104

-

-

30°C, 2 bar

42

PolyActiveTM/GO

143 Barrer

73

21

12

25°C, 0.5 bar

43

Pebax/ImGO

76 Barrer

106

-

25°C, 4 bar

14

Pebax/PEG-PEI-GO

142 Barrer

63

24

-

30°C, 2 bar, dry feed

44

-

Pebax/[emim][BF4]

270 Barrer

42

27

-

35°C, 3 bar

16

Pebax/ZIF-8/[bmim][TF2N]

120 Barrer

42

17

-

23°C, 1 bar

6

642 GPU

34

-

-

35°C, 3 bar

This work

981 GPU

44

-

-

35°C, 3 bar

This work

Pebax/[emim][BF4]/GO Pebax/[emim][BF4]/GO (pH = 12)

(0.56

In this work, IL dominates the overall performance of Pebax/IL/GO because it makes up the majority of the membrane by up to 80 wt%. By adding a small amount of GO and increasing the pH of the solution, the gas permeance increases significantly while maintaining moderate CO2/N2 selectivity. The large improvement in permeance and the capability of making the


27


Page 28 of 45

membranes thin are more attractive for commercial application because the enrichment in the permeate is ultimately limited by the feed-to-permeate pressure ratio, not by the intrinsic separation properties of the membranes.45

4. Conclusions In this study, the addition of GO into Pebax1657/IL at high IL loading was investigated in the form TFC hollow fiber membranes. Incorporating GO by up to 0.5wt% GO relative to the total solute weight increases the gas permeance while maintaining CO2/N2 selectivity compared to Pebax/IL membranes. However, further addition of GO is shown to lower the permeance due to thicker selective layer and possible formation of GO aggregates. XPS results reveal the migration of IL to the surface due to the presence of GO on the interface, which brings IL along via π-π and/or cation-π interactions. FTIR spectra of the solution-cast dense films of Pebax/IL/GO also show diminished interactions between IL and Pebax, consistent with AFM surface mapping that indicates the resurgence of microphase separation between the hard and soft phases of block copolymers upon the addition of GO. Moreover, alkaline condition enhances the gas permeance and selectivity simultaneously by facilitating further migration of IL to the surface. Consequently, the film surface becomes more polar and has higher tendency to interact with CO2 molecules that have significant quadrupole moments than with non-polar N2 molecules. In ILrich environment, GO sheets were reduced to some degrees, with basic condition accelerating the process and further shrinking the interlayer distance between the stacks, thus likely reducing the potential for molecular sieving of GO stacks to contribute to the transport and affirming the solution-diffusion mechanism through the polymer/IL matrix. In summary, the fibers fabricated at high pH exhibit excellent performance in terms of gas permeance and stability with feed


28

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containing traces of water vapor and NOx. The fabrication process is also easily scalable, and the thin film structure allows relatively low production cost, which makes them attractive for commercial carbon capture applications. ASSOCIATED CONTENT Supporting Information. (1) Composition of selective layer coating solution, (2) Schematic of laboratory-scale dip coating technique and Pebax/IL/GO TFC membrane structure, (3) Schematic of gas permeation setup, (4) AFM, XRD and FTIR characterization of synthesized GO, (5) viscosity measurement of Pebax/IL/GO solutions, (6) AFM topological images of Pebax/IL/GO membranes, (7) Surface roughness and surface area of Pebax/IL/GO TFC membranes, (8) FTIR spectra of neat Pebax, Pebax/IL/GO blends, and pure IL, (9) FTIR spectra of GO collected from the Pebax/IL/GO solutions, (10) XPS analysis for N1s region for Pebax/IL/GO blends, and (11) surface energy calculation. The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Funding Sources


29


Page 30 of 45

This research was supported by the Australian Government Research Training Program Scholarship.

ACKNOWLEDGMENT Authors would like to thank Dr. Jason Scott from UNSW Particle and Catalysis Research Group for N2 and CO2 adsorption measurement and analysis, Sigrid Fraser for TEM sample preparation, and Dr. Qiang Zhu for the technical assistance on TEM at the Electron Microscope Unit at UNSW.

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Figure 1. (a) Interaction between GO sheets and IL in the forms of ionic complexation (2-D model) at high pH21 and π-π or cation-π interaction between GO basal plane and cation of IL (ball and stick model), (b) stable dispersion of Pebax/IL/GO-0.5 solution at pH =12. 261x105mm (150 x 150 DPI)


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Figure 2. SEM surface (left) and cross-section (right) images of the PVDF hollow fibers coated with (a-b) PTMSP and subsequent coating of (c-d) Pebax/IL/GO-0.5, or (e-f) Pebax/IL/GO-2.0, or (g-h) Pebax/IL/GO0.5(pH=12). 147x211mm (150 x 150 DPI)



Figure 3. TEM cross section image of Pebax/IL/GO-0.5wt% and the d-spacing of GO stacks fabricated at pH= 6 (a-b) and pH = 12 (c-d). 204x209mm (150 x 150 DPI)


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Figure 4. XPS survey scan and elemental composition of Pebax/IL and Pebax/IL/GO-0.5 TFC hollow fibers fabricated at pH = 6 and pH = 12. 289x414mm (300 x 300 DPI)



Figure 5. (a) Single gas permeation of Pebax/IL/GO TFC membranes at 3 bar feed gauge pressure and 25°C by varying GO concentration by up to 2wt% with total wt% of IL + GO. 299x114mm (150 x 150 DPI)


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Figure 6. Effect of temperature on the single-gas performance of Pebax/IL/GO-0.5 TFC membranes fabricated at pH = 6 (solid symbols) and pH = 12 (open symbols). (a) N2 and CO2 permeances (b) CO2/N2 selectivity. Feed gauge pressure was maintained at 3 bar. 127x60mm (300 x 300 DPI)



Figure 7. Adsorption isotherms for Pebax/IL, Pebax/IL/GO-0.5, Pebax/IL/GO-2.0, and Pebax/IL/GO-0.5 fabricated at pH = 12 with (a) N2 at 77K with po =101 kPa and (b) CO2 at 273 K with po =3479.72 kPa. po is the saturation pressure of the gas at the measurement temperature. 92x32mm (300 x 300 DPI)


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Figure 8. Gas separation performance of Pebax/IL/GO-0.5(pH = 12) TFC membranes with time with CO2:N2 (20:80) mixed-feed gas containing 0.96% NOx at feed gauge pressure of 3 bar and 35°C. The fibers used for the stability measurement were six months old. 186x159mm (300 x 300 DPI)


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