Enhanced Performance of Polyurethane Hybrid Membranes for CO2

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Enhanced Performance of Polyurethane Hybrid Membranes for CO2 Separation by Incorporating Graphene Oxide: The Relationship between the Membrane Performance and the Morphology of Graphene Oxide Ting Wang, Li Zhao, Jiang-nan Shen, Liguang Wu, and Bart Van der Bruggen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00138 • Publication Date (Web): 29 May 2015 Downloaded from http://pubs.acs.org on June 6, 2015

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Enhanced Performance of Polyurethane Hybrid Membranes for CO2 Separation by Incorporating Graphene Oxide: The Relationship between the Membrane Performance and the Morphology of Graphene Oxide Ting Wang a, Li Zhao a, Jiang-nan Shen b, Li-guang Wu a*, Bart Van der Bruggen c a

School of Environ. Sci. & Eng., Zhejiang Gongshang University, Hangzhou, 310012, China

b

Center for Membrane and Water Science, Ocean College, Zhejiang University of Technology,

Hangzhou 310014, China c

Department of Chemical Engineering, Process Engineering for Sustainable Systems

(ProcESS), KU Leuven, W. de Croylaan 46, B-3001 Leuven, Belgium

ABSTRACT: Polyurethane hybrid membranes containing graphene oxide (GO) with different morphologies were prepared by in situ polymerization. The separation of CO2/N2 gas mixtures was studied using these novel membranes. The results from the morphology characterization of GO samples indicated that the oxidation process in the improved Hummers method introduced oxygenated functional groups into graphite, making graphite powder exfoliate into GO nanosheets. The surface defects on the GO sheets increased when oxidation increased due to the introduction of more oxygenated functional groups. Both the increase in oxygenated functional groups on the GO surface and the decrease in the number of GO layers leads to a better


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distribution of GO in the polymer matrix, increasing thermal stability and gas separation performance of membranes. Addition of excess oxidant destroys the structure of GO sheets to form structural defects, which depressed the separation performance of membranes. The hybrid membranes containing well-distributed GO showed higher permeability and permeability selectivity for the CO2. The formation of GO aggregates in the hybrid membranes depressed the membrane performance at a high content of GO.

INTRODUCTION The atmospheric concentration of CO2 has increased rapidly in recent decades, and its main environmental effect is global warming 1, 2. Separation of CO2 from the flue gas is an important step towards reducing the global emissions of CO2. The efficient and economical separation of CO2 from flue gas and low-grade natural gas is a key technology towards reducing the global emissions of CO2 3. A common industrial technology to separate CO2 is to employ amine solvents in the absorption process 4. Compared to this traditional system, polymeric membranes are an attractive alternative, because membrane filtration is not energy-intensive and requires no phase transfer in the process 5, 6. However, the selectivity of CO2 to N2 at room temperature that can be obtained with typical membranes 5, 6 is not high enough. Therefore, development of novel and energy efficient membranes for gas separation is attracting immense research interest. Over the last decades, organic–inorganic hybrid membranes have been developed as attractive materials because of their desirable organic and inorganic properties

7, 8

. Several studies have

indicated that such hybrid membranes substantially increased the permeability without deteriorating the selectivity

7, 8

. However, fabrication of organic-inorganic hybrid membranes



with high performance for CO2 separation can be very difficult. One of the challenges of developing hybrid membranes is the search for a nanomaterial that has a high performance for separating CO2 and that can be well distributed in the polymer matrix 9. Since its discovery in 2004, graphene has attracted a wide interest due to a large variety of potential applications, owing to its extremely high surface area as well as exceptional electrical, optical, thermal, and mechanical properties

10

. Graphene oxide (GO) is an atomic sheet of

graphite decorated by several oxygenated functional groups on its surface, resulting in a hybrid structure comprising a mixture of sp2 and sp3 hybridized carbon atoms 11. Currently, more and more researches report the application of graphene and graphene oxide for gas adsorption, gas storage and gas separation 12, 13. Schrier et al. employed molecular simulations to investigate the effect of nanoporous structuring and fluorination of graphene surfaces on the adsorption of CH4, CO2, N2, O2, H2S, SO2, and H2O 12. Results showed that graphene was very suitable for CO2 capture and separation. Lee et al. found that a graphene membrane could efficiently separate CO2 from CO2/O2, CO2/N2 and CO2/CH4 mixtures, because its single-atom thick length provided higher selectivity and faster transport of gas molecules compared to a carbon nanotube 14

. Based on this, we used GO to fabricate the hybrid membranes to separate CO2/N2 gas

mixtures. From the results in the literature,

13-15

the surface oxidation of GO is the key factor

that affects its morphology and surface properties, thus it will affect its gas sorption properties 15

. In addition, the dispersion of GO in the polymer matrix is also determined by its morphology

and surface properties and is crucial in improving the properties of the resulting hybrid membranes, when they are employed as addition agents. In this work, the amount of oxidizing agents during the preparation process was first


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regulated to generate GO samples with different surface properties and morphologies via the improved Hummers method, according to several published papers.

15, 16

Secondly,

polyurethane (PU) was used as a substrate for the preparation of the hybrid membranes with different GO samples by in situ polymerization. This study aimed to investigate the effect of the oxidation levels of GO on the morphology and surface, thus on the morphology and gas separation performance of the resulting hybrid membranes.

EXPERIMENTAL Materials Graphite power (G, 8000 mesh) was purchased from Reagent Chemical Manufacturing (Shanghai, China). Sulfuric acid (H2SO4), potassium permanganate (KMnO4) and sodium nitrate (NaNO3) were purchased from Shanghai Reagent Factory (Shanghai, China). N,N-dimethylformamide (DMF), phenyl isocyanate (MDI), 1,4-butanediol (BDO), and dibutyltin dilaurate (DBTDL) were purchased from Reagent Chemical Manufacturing (Shanghai, China) and used as received without further purification. The polysulfone film used had a molecular weight cut-off of 20,000 (MWCO = 20,000) and was purchased from Hangzhou Development Center of Water Treatment Technology (Hangzhou, China). Preparation of graphene oxide Graphene oxide (GO) was synthesized by the oxidation of graphite powder using an improved Hummers method, followed by ultrasonication. Concentrated H2SO4 (130 mL) was added to a mixture of graphite powder (5.0 g) and NaNO3 (2.5 g), and the mixture was cooled to 0 °C. Then,



different amounts of KMnO4 (shown in Figure 1) were added slowly in portions to keep the reaction temperature below 5 °C for 30 min. The reaction was heated to 40 °C and stirred for 1.5 h, at which time water (230 mL) was added slowly, producing a large exotherm to 98 °C. External heating was introduced to maintain the reaction temperature at 98 °C for 20 min, then the heat was removed and the reaction was cooled using a water bath for 10 min. Additional water (700 mL) and 30% H2O2 (50 mL) were added to the suspension. Under ultrasonication, the GO suspension was washed by repeated filtration with 5% HCl aqueous solution and water until the pH of the solution became neutral with no SO42-. The GO nanosheets were obtained by sifting, filtration, multiple washings, centrifugations and decanting, vacuum drying, as described in the order of GO-1, GO-2, GO-3, GO-4 and GO-5. Preparation of GO/polyurethane hybrid membrane Different GO samples were dispersed in 15 mL DMF using an ultrasonic probe (VC750, 150W, 20Hz) for 20 min and then stirred in a water bath at 45 °C for 10 min. MDI (15.75 g) was dissolved in 37.5 mL DMF using an ultrasonic bath (70 W, 42 Hz) and then added to the DMF containing the GO under stirring at 45 °C. After 30 min of constant stirring, 4.7 mL BDO and 50 µL DBTDL were sequentially added to the mixture while stirring vigorously at 45 °C to initiate the polymerization reaction. When the viscosity of the reaction system reached about 300 mPa.s, the reaction mixture was used to coat a polysulfone film (molecular weight cut-off, MWCO = 20,000). The hybrid membranes with different GO samples on the polysulfone film were incubated and obtained by continuing the polymerization reaction in a vacuum oven at 45 °C for another 10 h to allow the evaporation of the solvent. The thickness of the top polymer membrane


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was measured at about 25 µm by scanning electron microscopy (SEM). Characterization The morphologies of different graphene oxides (GO) and GO-containing membranes were characterized using a JEM-1230 transmission electron microscope (TEM: Jeol Co., Ltd.). The structure of the different GO samples and the GO/PU hybrid membranes were characterized by Fourier transform infrared (FTIR) spectroscopy (Nexus-670, Nicolet Co.) and Raman spectroscopy (LabRAM HR UV, USA). The chemical composition and the state of the elements present in the outermost parts of the GO nanosheets were investigated by X-ray photoelectron spectroscopy (XPS) measurements using an ESCA-2000, VG Microtech Ltd. The thermal stability of the hybrid membrane was determined by differential scanning calorimetry (DSC: Perkin-Elmer DSC7), ranging from 50 °C to 650 °C under N2 at a heating rate of 10 °C/min. The zeta potential measurements of the hybrid membranes were conducted using a solid surface potentiometer (SurPASS, Anton Paar Co., Ltd.) and the static water contact angles of the hybrid membranes were determined by using a contact angle goniometer (SL200B, Kono Co., Ltd.).

Gas permeance measurement The gas permeability properties of the hybrid membranes were determined using the variable-pressure constant-volume method with a pre-calibrated permeation cell as described in previous work 17. A mixture of CO2/N2 (volume ratio, 1:9) was used as test gas

17

. The gas

permeability and permeability selectivity for CO2/N2 were calculated by the following equations 17-19:

PA =

N Al p2 − p1

(1)

where NA is the steady state gas flux through the film, l is the film thickness, and p2 and p1 are the upstream and downstream partial pressures of gas A, respectively. Gas permeability selectivity of the membranes is defined as the ratio of two gas



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permeability:

α CO

2

N2

=

PCO2 PN 2

(2)

RESULTS AND DISCUSSION Morphology of the GO samples with different oxidation levels

Figure 1 Preparation of different GO samples According to the literature,

20, 21

an increase in the amount of oxidizing agents (KMnO4)

during the Hummers method or the improved Hummers method introduces more oxygenated functional groups into the samples to form GO samples with a high oxidation level, which will change the properties of the GO samples, including gas adsorption, surface polarity and optical properties. In this work, it can be observed from Figure 1 that there was a change in color, from black into blackish brown and finally into a brownish yellow color, when the GO samples with different oxidation levels were dispersed in water solution. This phenomenon was due to the fact that the higher oxidation level GO had a more obvious brownish yellow color, which has been observed before 15, 20.


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Besides the optical properties, the morphologies of the GO samples with different oxidation levels were also different, as shown in the TEM images (Figure S1, SI). All samples showed a large flake like morphology with different transparencies, which was caused by the number of layers present in the stacked structure of the GO samples with different oxidation levels. A

B

Figure 2 TEM images of different GO samples. The inset in each image shows the SAED pattern of the corresponding sample A. GO-3; B. GO-5 From the TEM images (GO-1, GO-2 and GO-5 are listed in Figure S2, supporting information - SI), GO-1 and GO-2 show the multi-layer sheet like morphology comprised of partially oxidized graphite oxide. With an increase in the oxidation level, GO samples (GO-3, GO-4, GO-5) became highly transparent, which indicates that the morphology consisted of a monolayer or just a few layers of GO. The increase in the oxidation level increased the amount of oxygenated functional groups in the GO, which makes them suitable for exfoliation into a monolayer or just a few layers of GO after ultrasonication. AFM images for the different GO samples (Figure S3, SI) were also in agreement with the TEM results. The average lateral



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dimension of the GO samples with high oxidation level (GO-4 and GO-5) was 0.898 nm and 0.776 nm, respectively, this is in accordance with the thickness of single-layer graphene (～1 nm) proved by previous work

20, 21

. An overall analysis of TEM and AFM both indicated that

the level of oxidation and the exfoliation strategy determined the sheets morphology, dimension and the transparency. The SAED patterns of GO-1 and GO-2 indicated a typical ring-like pattern arising from the merging of the diffraction spots due to the greater number of layers in the samples 20. The other three SAED patterns, showed that there were clear diffraction spots with a six-fold pattern that was consistent with the hexagonal lattice 22. These observations indicated that the graphitic AB stacking order was preserved in the lattice even after higher oxidation levels. This is in accordance with the previous studies of Jeong et al.

22

With an increase in the oxidation level,

the SAED patterns became clearer suggesting that GO samples with high oxidation levels have a more regular carbon framework. The structure analysis of different GO samples Figure 3 show that the FT-IR spectrum of graphite does not exhibit any significant absorption band in the studied region. However, the presence of several bands was detected in the other GO samples spectra. The characteristic FTIR spectrum of GO shows the absorption bands corresponding to the C＝O carbonyl stretching at 1720 cm 1, the C-OH stretching at 1403 cm 1, -

-

-

and the C-O stretching at 1050 cm 1 23. When the oxidation level increased, these corresponding peaks became stronger. This confirms that more oxygenated functional groups were introduced -

to the GO surface during the oxidation process. The peak found at 1620 cm 1 is a resonance


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peak that can be assigned to the C–C stretching and absorbed hydroxyl groups in the GO According to our previous work and the literature,

17, 25

24

.

these oxygenated functional groups

introduced onto the surface of the GO sheets provide the possibility of in situ polymerization between the polyimide and the GO.

graphite GO-2 GO-3

Transmittance /%

GO-4

GO-5

1050 cm−1 C-O

1250 cm−1 C-O-C

1403 cm−1 C-OH 1620 cm−11720 cm−1 C=O

1000

1200

1400

1600

1800

2000

Wavenumber /cm-1

Figure 3 Fourier transform infrared spectra for different GO samples From the XPS spectrum of graphite (as shown in Figure S4, SI), it was observed that there was only a peak at 284.5 eV assigned to C–C bonds

20, 26

and this C1s peak was caused by

sp2-hybridized carbon in the graphite. It was also found in Figure S4 that the additional peaks that appear in the XPS spectra of the GO samples after oxidation and the sp2-hybridized C1s peak intensity was significantly reduced with the oxidation level. For further comparison, the oxygen/carbon intensity ratios, ratio of sp2- to sp3-bonded carbon atoms, and the intensity ratio of the hydroxyl, carboxyl and epoxy groups with respect to the C-C peak are provided in Table 1.



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Table 1 XPS data of different GO samples

Sample

Ic/Ioa

C-C

C-OH

C-OO

C-O-C

percentage /%

percentage /%

percentage /%

percentage /%

sp2/sp3 b

GO-1

4.05

1.82

50.84

42.12

7.04

0.00

GO-2

2.72

1.12

40.45

46.39

4.59

8.57

GO-3

2.60

0.93

33.83

50.06

5.52

10.59

GO-4

2.58

1.04

29.76

51.79

4.91

13.54

GO-5

2.12

0.7

27.97

50.94

5.37

15.72

Note: a. Oxygen/carbon intensity ratios of GO samples; b. Ratio of sp2- to sp3-bonded carbon atoms in the GO samples. It can be observed that both sp2/sp3 ratio and oxygen/carbon intensity ratio decreased with an increase in oxidation level, which corresponds well with the literature

27

. The hydroxyl and

carboxyl groups, on further oxidation, lead to the formation of the epoxide groups, which resulted in the increase in interlayer spacing and exfoliation of graphitic oxide into GO. Increase in the oxidation level resulted in the decrease of the intensity ratio of the hydroxyl, carboxyl and epoxy groups with respect to the C-C domains. These results also confirmed that more oxygenated functional groups were introduced to the oxidized GO surface. The hydroxyl and carboxyl groups were formed at lower oxidation levels (GO-1) and were converted into epoxy groups when the oxidation level increased. The dimanganese heptoxide produced during preparation process caused the epoxidation of unsaturated oxygenated groups and the formation of O–C–O groups in the GO samples 28, 29.


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The surface morphology of different GO samples Raman spectroscopy is usually employed in many studies to characterize the surface of graphene and GO

30, 31

. This technique is used because the characteristic peaks that appear in

the spectrum can be attributed to the surface defects which were caused by surface modification or introduction of groups on the graphene and GO surface. The Raman spectra in Figure 4 show two obvious characteristic peaks in the spectra of GO samples with different oxidation levels: one peak at approximately 1350 cm-1, which represents the disordered carbon band (D-band) and typically corresponds to the surface defects of the GO sheets; and a second peak at approximately 1580 cm-1, which represents the graphitized band (G-band) and corresponds to the formation of sp2-bonded crystalline carbon in the graphite or GO samples. 30 The strength of these two bands increased with the oxidation level, which suggests that more surface defects on the GO sheets were generated due to the oxidation during the preparation with the Hummers method. The ratio of D-band to G-band intensity (D/G ratio) increased with the oxidation levels of the GO samples, as a result of the conversion of some sp2-hybridized carbons in the GO sheets to sp3 hybridization. That also means that the surface defects on the GO sheets increased when the oxidation level increased.



D

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ID / IG G-5 1.65 G-4 1.44 G-3 1.41 G-2 1.39 G-1 1.38 graphite 0.25

Intensity (a.u)

G

1000

1200

1400

1600

1800

Raman Shift /cm-1

2000

2200

2400

Figure 4 Raman spectra of different GO samples (excitation at λ = 632.8 nm) Comparing the Raman spectra of GO-1 ~ GO-4, the D/G ratio increased slowly with the oxidation level. However, the D/G ratio of GO-5 increased significantly to 1.65, which was higher than for the other four GO samples. This means that more defects formed in the GO-5 sheets. These defects structure were confirmed by HRSEM and HRTEM (as shown in Figures S5 and 6). In Figure S5 it can be observed that the surface of GO-3 showed a yarn-like sheet structure and showed a flat surface in the HRSEM image. With an increase in the oxidation level, the surface of the GO sheets becomes rougher. This can be observed in the SEM and HRSEM images. This was also caused by the introduction of more oxygenated functional groups and the formation of more defects with oxidation level. A few structural defects appeared on the surface of the GO-5 sheet shown in Figure S6, which led to the high ratio of D-band to G-band intensity obtained from Raman analysis. This might be caused by the damage of excess oxidant


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on the GO sheets. After the generation of the monolayer GO sheet (GO-4), excess oxidant added in the preparation process would destroy the surface of the GO to form a few structural defects. These structural defects are very small and few, and cannot be detected by TEM or SAED analysis. Morphology of different PU hybrid membranes All results show that the morphology and surface properties of GO both changed when the oxidation level increases, which must affect the morphology of GO/PU hybrid membranes. First, TEM analysis (as shown in Figure 5) on the PU hybrid membranes with different GO sheets showed that most of the GOs dispersed well in the PU polymer matrix, which is hard to achieve by mixing, thus the GO should be directly bonded to the PU. The GO samples with different oxidation levels showed different distribution in the PU polymer matrix. The 1.0 wt% loading of GO was selected for this research because the PU hybrid membrane containing 1.0 wt% of MWCNTs showed the best performance in previous work 17. A

B

C

Figure 5 TEM micrographs of PU membranes containing different GO samples (1.0wt% loading): A. GO-3/PU; B. GO-4/PU; C. GO-5/PU From TEM micrographs of hybrid membranes, the dispersion of the GO samples in the



hybrid PU membranes improved with the oxidation level (TEM micrograph of GO-1/PU membrane and TEM images with large magnification of different hybrids membranes are shown in Figures S7 and S8, SI). The increase in the oxidation level enhanced the surface wettability of the GO, thus improving the affinity between the GO sheets and the polymer materials. The introduction of oxygenated functional groups to the surfaces of the GO sheets by the oxidation also increased the binding between the GO and polymers. In addition, the fewer the number of layers of GO, the more homogenous its dispersion in the polymer matrix was. Therefore, the dispersion of GO-4 and GO-5 was significantly better than that of the other GO samples in the membranes when the same quantity of GO was added. The difference in the distribution of GO samples with different oxidation levels in the PU membranes could also be confirmed by FTIR, TGA and DSC analysis. From the FTIR spectra (Figure S9), it can be observed that all the absorption peaks in the spectra corresponded to the PU polymer and there were no absorption peaks that corresponded to the GO sheets. This might also be due to the low GO content and good distribution of the GO in the PU polymer matrix. The TGA curves in Figure S10 show that the addition of GO samples increased the decomposition temperature of the hybrid membranes. In the DSC thermograms, the endothermic peak for the PU membrane observed near 333 °C suggested the decomposition of the membrane. When GO was added the endothermic decomposition peaks in the DSC curves shifted to a higher temperature. Thus, the hybrid membrane with GO samples had a higher thermal stability than the membrane without GO samples. Comparing the two DSC curves for the hybrid membranes, GO-4 showed a more homogeneous distribution in the PU polymer than


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that of GO-2. Therefore, the hybrid membrane containing GO-4 had a higher thermal stability than the GO-2/PU membranes. PU GO-2/PU

Heat Flow /W·g-1

GO-4/PU

50

150

250

350

450

550

650

Temperature / °C

Figure 6 DSC thermographs for different PU membranes containing 1.0 wt% of GO These differences in the distribution of GO in the PU membranes were also observed in the zeta potential values and water contact angles measurements of different GO/PU hybrid membranes and (as shown in Figure 7) it was observed 7 that the zeta potentials for the hybrid membrane with GO-4 was the highest among all hybrid membranes in Figure 7. That is to say, the GO-4/PU membrane surface showed the highest electrical property, which caused GO-4/PU membrane to exhibit the best surface hydrophilicity as shown in Figure 7 (the static water contact angles of GO-1/PU ~ GO-5/PU were 99.8º, 84.7º, 61.8º, 48.6º, 53.2º, respectively). Combining the results by FTIR and XPS analysis, both the distribution in the polymer matrix of the GO and the introduction of more oxygenated functional groups during the oxidation process caused the changes in the zeta potential and the surface hydrophilicity of the different hybrid



membranes. GO-4 had a monolayer morphology and there were many oxygenated functional groups on its surface, it also showed better distribution in the PU membrane. Therefore, the GO-4/PU membrane had the highest electrical property and the best surface hydrophilicity of the GO-4/PU membrane.

Figure 7 Zeta potential values of different PU membranes and the shape of water droplets on the different PU membranes (1.0wt% loading): a. GO-1/PU; b. GO-2/PU; c. GO-3/PU; d. GO-4/PU; e. GO-5/PU Separation performance of different PU hybrid membranes The separation of the CO2/N2 gas mixture was employed to evaluate the effects of the addition of GO with different oxidation levels on the performance of the hybrid membranes. Figures S11 and S12 show that the permeability for CO2 and N2 of different GO/PU hybrid membranes was higher than that of PU without GO, which is due to the strong adsorption properties of GO for gases. The maximum permeability for CO2 of GO/PU hybrid membranes


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was about 8 times more than that of PU without GO. The maximum permeability for N2 of the hybrid membranes was twice as much as that of PU. Comparing these two figures, it can be observed that the improvement in CO2 permeability was much stronger than that of the N2 permeability. This was caused by the preferential adsorption of CO2 by GO nanomaterials. According to the literature, 12, 13 GO has a 2D structure with conjugated π bonds, which shows affinity towards CO2. The effective π–π stacking interaction between GO and CO2 generated in the hybrid membranes, helped to capture and absorb CO2 molecules. In addition, the difference in electronegativity between C and O in the CO2 molecule leads to polar bonds,

32

which also helps the CO2 to be absorbed by the GO and

improves the diffusion of the CO2 in the hybrid membrane containing GO. The addition of the GO samples increased the separation performance of the PU hybrid membranes for CO2/N2 (up to 1 wt% for all hybrid membranes containing GO with different oxidation levels), as shown in Figure 8. This phenomenon is similar to the observed separation performance of the PU membranes with multi-walled carbon nanotubes (MWCNTs), 17 which have a 1D structure with conjugated π bonds. Due to the larger specific surface area of GO compared to MWCNTs, the maximum permeability selectivity of the GO/PU membranes was about 3 times more than that of the MWCNTs/ PU membranes, and it was even higher than that of functionalized MWCNTs/PU membranes in our previous work 17. This maximum permeability selectivity for CO2/N2 of GO/PU membranes in the present work was also significantly higher than the permeability selectivity found in the literature. 14, 33



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80

Permeability Selectivity for CO2/N2

70 60 50

40 30 20

GO-1 GO-2 GO-3 GO-4 GO-5

10 0 0

1

2

3

4

Mass percentage of graphene oxide (wt%)

5

Figure 8 Effect of the addition of different GO samples on the selectivity of CO2/N2 gas mixture of the hybrid membranes. Besides the specific surface area, the hybrid membrane with GO-4 showed the highest permeability selectivity, due to the fact that GO-4 had a monolayer morphology and there were many oxygenated functional groups on its surface, it also showed a better distribution in the PU membrane. The decrease in the permeability selectivity of the hybrid membrane with GO-5 was caused by the structural defects. In addition, the curves in permeation experiments indicated that the permeability performance and permeability selectivity of the membranes with all GO samples initially increased but eventually decreased with the addition of GO. The decrease in the separation performance of hybrid membranes was also due to the aggregation of GO in the membranes. The addition of GO in excess into the PU membranes also caused aggregation (as shown in Figure S13), which evidently hindered the membrane performance. The aggregation of GO in the membranes was


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also confirmed by DSC analysis (Figure 9) and the zeta potential values of the membranes (as shown in Figure S14). From Figure 9, it is obvious that the endothermic peak corresponding to the membrane decomposition moved to lower temperatures due to the aggregation of GO-4 in PU when the content of GO-4 increased from 1.0 wt% to 5.0 wt%. This means that the hybrid membrane with 5.0 wt% of GO-4 showed the lowest thermal stability. In Figure S14, the aggregation of GO in PU membranes also resulted in a decrease in the zeta potential values of the hybrid membranes.

Heat Flow /W·g-1

1.0 wt% 3.0 wt% 5.0 wt%

50

150

250

350

450

550

650

Temperature / °C

Figure 9 DSC thermographs for PU membranes containing GO-4 in different quantities

AUTHOR INFORMATION Corresponding Author Phone: +86 571 28008204; Fax: +86 571 28008215 e-mail: [email protected] . Notes The authors declare no competing financial interest.



ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China Grants (Contracts 21376218 and 21076190) and the Natural Science Foundation of Zhejiang Province (Contracts LY14B060001) is gratefully acknowledged is gratefully acknowledged.

SUPPORTING INFORMATION AVAILABLE TEM images with wide range of GO samples, TEM images of GO-1, GO-2 and GO-5 with the SAED pattern, Atomic force microscopic image, the line profile of the GO nanosheets, XPS spectra for the C1s region of GO with different degrees of oxidation, SEM and HRSEM images of different GO samples, HRTEM images of different GO samples, TEM micrograph of GO-2 incorporated PU membrane (1.0wt% loading), TEM images with large magnification of GO/PU mixed matrix membranes (1.0wt% loading of GO), FTIR spectra of PU membranes containing different GO sheets (1.0wt% loading of GO), TGA thermograms of different PU membranes, Dependence of GO content on the CO2 permeability of the membranes, Dependence of GO content on the N2 permeability of the membranes, TEM micrographs of GO-3 and GO-4 incorporated PU membranes, and Zeta potential values of different PU membranes are shown in Figure S1~S14, respectively. This information is available free of charge via the Internet at http://pubs.acs.org.

REFERENCE (1) Stern, N. Stern Review on the Economics of Climate Change; Cambridge University Press: Cambridge, U.K., 2006.


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(2) Gebald, C.; Wurzbacher, J.A.; Borgschulte, A.; Zimmermann, T.; Steinfeld, A. Single-component and Binary CO2 and H2O Adsorption of Amine-functionalized Cellulose.

Environ. Sci. Technol. 2014, 48, 2497−2504. (3) Vieira, R.B.; Pastore, H.O. Polyethylenimine-magadiite Layered Silicate Sorbent for CO2 Capture. Environ. Sci. Technol. 2014, 48, 2472−2480. (4) Webley, P.A. Adsorption Technology for CO2 Separation and Capture: A perspective.

Adsorption 2014, 20, 225−231. (5) Shao, L.; Low, B.T.; Chung, T.S.; Greenberg, A.R. Polymeric Membranes for the Hydrogen Economy: Contemporary approaches and prospects for the future. J. Membr. Sci. 2009, 327, 18–31. (6) Bastani, D.; Esmaeili, N.; Asadollahi, M. Polymeric Mixed Matrix Membranes Containing Zeolites as a Filler for Gas Separation Applications: A review. J. Ind. Eng. Chem. 2013, 19, 375–393. (7) Xiao, Y.; Low, B.T.; Hosseini, S.S.; Chung, T.S.; Paul, D.R. The Strategies of Molecular Architecture and Modification of Polyimide-based Membranes for CO2 Removal from Natural Gas—A review. Prog. Polym. Sci. 2009, 34, 561–580. (8) Boroglu, M.S.; Gurkaynak, M.A. The Preparation of Novel Silica Modified Polyimide Membranes: Synthesis, characterization, and gas separation properties. Polym. Adv. Technol. 2011, 22, 545–553. (9) Goh, P.S.; Ismail, A.F.; Sanip, S.M.; Ng, B.C. Aziz, M. Recent Advances of Inorganic Fillers in Mixed Matrix Membrane for Gas Separation. Sep. Purif. Technol. 2011, 81, 243–264. (10) Liu, Z.K.; Li, J.H.; Sun, Z.H.; Tai, G.A.; Lau, S.P.; Yan, F. The Application of Highly



Doped Single-layer Graphene as the Top Electrodes of Semitransparent Organic Solar Cells.

ACS Nano 2012, 6, 810–818. (11) Dikin, D.A.; Stankovich, S.; Zimney, E.J.; Piner, R.D.; Dommett, G.H.B.; Evmenenko, G. Preparation and Characterization of Graphene Oxide Paper. Nature 2007, 448, 457–460. (12) Schrier, J. Fluorinated and Nanoporous Graphene Materials as Sorbents for Gas Separations. Appl. Mater. Interf. 2011, 3, 4451–4458. (13) Burress, J.W.; Gadipelli, S.; Ford, J.; Simmons, J.M. Zhou, W.; Yildirim, T. Graphene Oxide Framework Materials: Theoretical predictions and experimental results. Angew. Chem.

Int. Ed. 2010, 49, 8902–8904. (14) Lee, J.; Aluru, N.R. Water-solubility-driven Separation of Gases Using Graphene Membrane. J. Membr. Sci. 2012, 428, 546–553. (15) He, Y.Q.; Zhang, N.N.; Wu, F.; Xu, F.Q.; Liu, Y.; Gao, J.P. Graphene Oxide Foams and Their Excellent Adsorption Ability for Acetone gas. Mater. Res. Bull. 2013, 48, 3553–3558. (16) Marcano, D.C.; Kosynkin, D.V.; Berlin, J.M.; Sinitskii, A.; Sun, Z.Z.; Slesarev, A.; Alemany, L.B.; Lu, W.; Tour, J.M. Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4, 4806–4814. (17) Wang, T.; Shen, J.N.; Wu, L.G.; Bruggen, B.V. Improvement in the Permeation Performance of Hybrid Membranes by the Incorporation of Gunctional Multi-walled Carbon Nanotubes. J.

Membr. Sci. 2014, 466, 338–347. (18) Matteucci, S.; Kusuma, V. A.; Swinnea, S.; Freeman, B. D. Gas Permeability, Solubility and Diffusivity in 1,2-Polybutadiene Containing Brookite Nanoparticles. Polymer, 2008, 49, 757–773. (19) Sanders, D. F.; Guo, R. L.; Smith Z. P.; Stevens, K. A.; Liu Q.; McGrath J. E.; Paul, D. R.;


Page 24 of 26

Page 25 of 26


Freeman, B. D. Influence of Polyimide Precursor Synthesis Route and Ortho-position Functional Group on Thermally Rearranged (TR) Polymerproperties: Pure Gas Permeability and Selectivity.

J. Membr. Sci. 2014, 463, 73–81. (20) Krishnamoorthy, K.; Veerapandian, M.; Yun, K.; Kim, S.-J. The Chemical and Structural Analysis of Graphene Oxide with Different Degrees of Oxidation. Carbon 2013, 53, 38–49. (21) Wilson, N.R.; Pandey, P.A.; Beanland, R.; Young, R.J.; Kinloch, I.A.; Gong, L.; Liu, Z.; Suenaga, K.; Rourke, J.P.; York, S.J.; Sloan, J. Graphene Oxide: structural analysis and application as a highly transparent support for electron microscopy. ACS Nano 2009, 3, 2547–2556. (22) Jeong, H.-K.; Lee, Y.P.; Lahaye, R.J.W.E.; Park, M.-H.; An, K.H.; Kim, I.J.; Yang, C.-W.; Park, C.Y.; Ruoff, R.S.; Lee, Y.H. Evidence of Graphitic AB Stacking Order of Graphite Oxides.

J. Am. Chem. Soc. 2008, 130, 1362–1366. (23) Gunasekaran, V.; Krishnamoorthy, K.; Mohan, R.; Kim, S.-J. An Investigation of the Electrical Transport Properties of Graphene-oxide Thin Films. Mater. Chem. Phys. 2012, 132, 29–33. (24) Stankovich, S.; Piner, R.D.; Nguyen, S.T.; Ruoff, R.S. Synthesis and Exfoliation of Isocyanate-treated Graphene Oxide Nanoplatelets. Carbon 2006, 44, 3342–3347. (25) Lee, Y. R.; Raghu, A. V.; Jeong, H. M.; Kim, B. K. Properties of Waterborne Polyurethane/Functionalized Graphene Sheet Nanocomposites Prepared by An In Situ Method.

Macromol. Chem. Phys. 2009, 210, 1247–1254. (25) Zhao, G.; Shao, D.; Chen, C.; Wang, X. Synthesis of Few-layered Graphene by H2O2 Plasma Etching of Graphite. Appl. Phys. Lett. 2011, 98, 1–3.



(26) Lee, D. W.; Seo, J. W. Sp2/sp3 Carbon Ratio in Graphite Oxide with Different Preparation Times. J. Phys. Chem. C 2011, 115, 2705–2708. (27) Dash, S.; Patel, S.; Mishra, B.K. Oxidation by Permanganate: Synthetic and mechanistic aspects. Tetrahedron 2009, 65, 707–739. (28) Tromel, M.; Russ, M. Dimanganese Heptoxide for the Selective Oxidation of Organic Substrates. Angew. Chem. Int. Ed. Eng. 1987, 26, 1007–1009. (29) Krishnamoorthy, K.; Veerapandian, M.; Mohan, R.; Kim, S.-J. Investigation of Raman and Photoluminescence Studies of Reduced Graphene Oxide Sheets. Appl. Phys. A 2012, 106, 501–506. (30) Ferrari, A. C.; Robertson. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. J. Phys. Rev. B, 2000, 61, 14095–14107. (31) Appel, A. M.; Bercaw, J. E.; Bocarsly, A. B.; Dobbek, H.; DuBois, D. L.; Dupuis, M.; Ferry, J. G.; Fujita, E.; Hille, R.; Kenis, P. J. A.; Kerfeld, C. A.; Morris, R. H.; Peden, C. H. F.; Portis, A. R.; Ragsdale, S. W.; Rauchfuss, T. B.; Reek, J. N. H.; Seefeldt, L. C.; Thauer, R. K.; Waldrop, G. L. Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation. Chem. Rev. 2013, 113, 6621–6658. (32) Shan, M. X.; Xue, Q. Z; Jing, N. N.; Ling, C C.; Zhang, T.; Yan, Z. F.; Zheng, J. T. Influence of Chemical Functionalization on the CO2/N2 Separation Performance of Porous Graphene Membranes. Nanoscale 2012, 4, 5477–5482.


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