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Fabrication of Polyimide Membrane Incorporated with Functional Graphene Oxide for CO2 Separation: The Effects of GO Surface Modification on Membrane Performance Ting Wang, Cheng Cheng, Li-guang Wu, Jiangnan Shen, Bart Van der Bruggen, Qian Chen, Di Chen, and Chunying Dong Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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Environmental Science & Technology

4

Fabrication of Polyimide Membrane Incorporated with Functional Graphene Oxide for CO2 Separation: The Effects of GO Surface Modification on Membrane Performance

5

Ting Wanga, Cheng Chenga,Li-guang Wua, *,Jiang-nan Shenb,*,Bart Van der Bruggenc, Qian

6

Chena,Di Chena, Chun-ying Donga

7

a

8

b

9

Hangzhou 310014, China

1 2 3

School of Environ. Sci. & Eng., Zhejiang Gongshang University, Hangzhou, 310012, China Center for Membrane and Water Science, Ocean College, Zhejiang University of Technology,

10

c

11

KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

12

Corresponding Author: Li-guang Wu, e-mail: [email protected].

Department of Chemical Engineering, Process Engineering for Sustainable Systems (ProcESS),

13

ACS Paragon Plus Environment


14

ABSTRACT. Two kinds of isocyanate were used to modify graphene oxide (GO) samples. Then,

15

polyimide (PI) hybrid membranes containing GO and modified GO were prepared by in situ

16

polymerization. The permeation of CO2 and N2 was studied using these novel membranes. The

17

morphology experiments showed that the isocyanate groups were successfully grafted on the

18

surface of GO by replacement of the oxygen-containing functional groups. After modification,

19

the surface polarity of the GO increased, and more defect structures were introduced into the GO

20

surface. This resulted in a good distribution of more modified GO samples in the PI polymer

21

matrix. Thus, the PI hybrid membranes incorporated by modified GO samples showed a high gas

22

permeability and ideal selectivity of membranes. In addition, enhancement of the selectivity due

23

to the solubility of CO2 played a major role in the increase in the separation performance of the

24

hybrid membranes for CO2, although the diffusion coefficients for CO2 also increased. Both the

25

higher condensability and the strong affinity between CO2 molecules and GO in the polymer

26

matrix caused an enhancement of the solubility selectivity higher than the diffusion selectivity

27

after GO surface modification.

28

TOC Art

29


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30 31


INTRODUCTION The use of fossil fuels increases atmospheric CO2 levels and is a leading contributor to global 1, 2

32

warming

33

emission of CO2 and other greenhouse gases

34

technologies are needed to solve the current global warming problem caused by greenhouse

35

gases. One common industrial technology to sequester CO2 is to employ amine solvents in the

36

absorption process

37

used in the absorption process also have a high environmental effect in production. Polymeric

38

membranes are an attractive technology because membrane filtration is an energy-saving and

39

pollution-free method compared to amine absorption

40

studied as a potential membrane material because of its high thermal stability, desirable gas

41

permeation selectivity and good mechanical properties 11, 12. However, the low CO2 permeability

42

of the PI membrane limits its application in the separation of CO2 11, 12.

43

. Low carbon emission and the development of new energy sources can reduce the

5-7

3, 4

. However, more separation and recycling

. Except low efficiency and high secondary pollution effect, the solvents

8-10

. Polyimide (PI) has been extensively

Currently, the development of organic–inorganic hybrid membranes provides a new direction 13-15

44

to solve this issue

. The addition of nanomaterials into the polymer matrix to fabricate

45

organic–inorganic hybrid membranes can effectively enhance the gas permeability of the

46

membrane

47

membranes containing zero-dimensional, one-dimensional and two-dimensional nanomaterials

48

such as TiO2 and SiO2 nanoparticles 16, 17, single-walled and multi-walled carbon nanotubes 18, 19,

49

graphene and graphene oxide

50

because of their excellent CO2 adsorption

51

that nanoporous graphene membranes with appropriate pore sizes and geometry are expected to

13-15

. Researchers have designed and fabricated many novel polymeric hybrid

20, 21

. Carbon nanotubes and graphene are particularly potential 22-25

. Sun et al.26 used molecular simulations to show



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52

have a high permeability and selectivity for separating CH4/CO2, CH4/H2S and CH4/N2 mixtures.

53

The selectivity for permeating gases (i.e., CO2, H2S and N2) was much higher than that of the

54

non-permeating gases (i.e., CH4).

55

We previously prepared polyurethane (PU) hybrid membranes incorporating multi-walled 27, 28

56

carbon nanotubes (MWCNTs) and graphene oxide (GO) to separate CO2 gas

57

of MWCNTs and GO significantly improved the separation of hybrid membranes for CO2, due

58

to the effective capture and adsorption for CO2 by MWCNTs or GO in polymer matrix. In

59

addition, the use of MWCNTs and GO introduces interface gaps in the polymer, which also

60

enhances the diffusion of gas molecules. However, it is difficult for GO nanosheets to distribute

61

homogeneously in the polymer matrix. In prior work 28, only a small amount of GO nanosheets

62

was uniformly dispersed in the PU polymer to fabricate a hybrid polymer with high-performance

63

(1.0 wt% of GO). It was difficult to maintain a good distribution of GO nanosheets in the PU

64

polymer when the amount of GO nanosheets exceeded 1.0 wt%. This significantly decreased the

65

separation performance and limits the application of graphene oxide and other carbon

66

nanomaterials as gas separation tools.

67

. The addition

Surface modification of carbon nanotubes or GO can improve their distribution in polymer 29, 30

. Kim et al.31 functionalized graphene nanosheets with polyethylene. After

68

matrices

69

modification, the dispersion of GO nanosheets in the solvent was greatly improved. This was

70

confirmed with transmission electron microscopy. Here, based on the literature

71

prior work

72

polyimide (PI) hybrid membranes containing modified-GO samples were fabricated via insitu

73

polymerization. The surface properties of GO were first changed by modification to enhance the

25, 30, 31

and our

28

, two isocyanates were employed to modify the GO surface. Subsequently,


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74

distribution of GO in the polyimide polymer. On the other hand, GO nanosheets may have a

75

good distribution during polymerization via insitu polymerization. Gas permeation experiments

76

are carried out to evaluate the changes in separation performance of the different hybrid

77

membranes. Finally, the impact of GO surface modification on the diffusion coefficients and gas

78

permeability was measured.

79

EXPERIMENTAL

80

Materials

81

Graphite power (8000 mesh) was purchased from Reagent Chemical Manufacturing (Shanghai,

82

China). Sulfuric acid (H2SO4), potassium permanganate (KMnO4) and sodium nitrate (NaNO3)

83

were

84

4,4'-(Hexafluoroisopropylidene) diphthalic anhydride

85

(ODA), and N,N-dimethylacetamide (DMAC), and N,N-dimethyl formamide (DMF) were

86

purchased from Reagent Chemical Manufacturing (Shanghai, China). 2,4-tolylene diisocyanate

87

(TDI) and dicyclohexylmethylmethane-4,4′-diisocyanate (HMDI) were purchased from Tokyo

88

Chemical industry. The structure of TDI and HMDI are shown in Scheme S1(Supporting

89

information-SI). High-purity (>99.99%) CO2 and N2 gases were purchased from Hangzhou

90

Jingong Gas Co. Ltd.

91

Synthesis of GO

92

purchased

from

Shanghai

Reagent

Factory

(Shanghai,

China).

(6FDA), 4,4’-diaminodiphenylether

GO was synthesized by oxidizing graphite powder with the improved Hummers method 28, 32

93

according to previous reports

. Our previous results showed that the GO nanosheets showed

94

the best morphology and distribution in polymer matrix when 5.0 g of graphite powder, 2.5g of



95

NaNO3, and 20 g of KMnO4 were added during GO preparation. Therefore, this study also

96

maintained these conditions in synthesizing the GO nanosheets.

97

Modification of GO by TDI or HMDI

98

Briefly, 0.5g of the as-prepared GO and 50 mL of anhydrous DMF were added in a triflask.

99

GO was first dispersed in DMF after stirring for 30 min under nitrogen. Then, organic isocyanate

100

(0.3228 g of HMDI, and 0.2143g of TDI) dissolved in DMF were added to the system under

101

stirring in a nitrogen atmosphere for 24 h. Upon completion of the reaction, the reaction mixture

102

was poured into 500 mL of methylene chloride; the products quickly precipitated. The products

103

were obtained by several cycles of centrifugation–redispersion–washing and then dried in the

104

vacuum drying oven at room temperature. The modification mechanism of GO by two

105

isocyantes is listed in Scheme S2 (SI). The modified GO samples were denoted as GO–HMDI

106

and GO-TDI, respectively.

107

Preparation of IGO/PI hybrid membrane

108

Different GO samples were dispersed in 12 mL of DMAC with an ultrasonic probe

109

(KQ-300TDE; 300 W, 80 KHz) for 40 min. Subsequently, 0.8970 g of ODA was added to the

110

GO suspension under stirring. After 10 min of constant stirring, 2.000 g of 6FDA was

111

sequentially added to the mixture under vigorous stirring at a temperature below 10°C to initiate

112

polymerization. The molar ratio of 6FDA to ODA is 1.005 to 1. When the viscosity of the

113

reaction system reached approximately 300 mPa·s, the reaction mixture was cast onto a glass

114

pane and dried at room temperature for 30 min. Hybrid membranes with different GO contents

115

were incubated and subjected to continuous polymerization in a vacuum oven at a heating rate of


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116

2°C/min at 80°C for 2 h and at 150 °C, 240 °C, and 300 °C for 1 h. (The fabricated mechanism

117

of hybrid membranes incorporated by GO with or without modification are listed in Schemes S3

118

and S4, SI) The thickness of the top polymer membrane was about 25 µm by SEM analysis.

119

Characterization

120

The morphologies of different graphene oxides (GO) were characterized using a JEM-1230

121

transmission electron microscope (TEM: Jeol Co., Ltd.). The structures of the different GO

122

samples were characterized with Fourier transform infrared (FTIR) spectroscopy (Nexus-670,

123

Nicolet Co.) and Raman spectroscopy (LabRAM HR UV, USA). The chemical composition and

124

the state of the elements present on the GO nanosheet surface were investigated with X-ray

125

photoelectron spectroscopy (XPS) measurements using an ESCA-2000, VG Microtech Ltd.

126

Hybrid membranes containing different GO samples were analyzed using S-4700 SEM (Hitachi

127

Co., Ltd., Japan).

128

Gas permeance measurement

129

The

pure

gas

permeability

values

were

determined

using

the

33, 34

130

constant-volume/variable-pressure method

131

permeation test apparatus on both sides under high vacuum at 30 °C. The increase in permeation

132

pressure with time was measured with a pressure transducer. The permeabilities of all gases were

133

measured at 30 °C at a constant pressure of 10 bar. The gas permeability was determined from

134

. The membranes were degassed in the

ܲ = ‫ = ܵ × ܦ‬10ଵ଴ × ௣

௏௅

ೠ೛

× ஺ோ்

ௗ௣(௧) ௗ௧

,

(1)

135

where P is the gas permeability (Barrer) [1Barrer=10-10 cm3(STP)cm cm-2 s-1cmHg-1], pup is the

136

upstream pressure(cmHg), dp/dt is the steady-state permeate-side pressure increase (cmHgs-1), V



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137

is the calibrated permeate volume(cm3), L is the membrane thickness(cm), A is the effective

138

membrane area(cm2), T is the operating temperature(K), and R is the gas constant [0.278cm3

139

cmHg cm-3(STP)K-1].

140

The diffusivity (D) was determined from ଶ

141

‫ ܮ = ܦ‬ൗ6ߠ ,

142

where θ is the time lag when a steady dp/dt rate is obtained on the downstream side in the

143

permeation tests 35. The solubility (S) was determined from

144

ܵ = ܲൗ‫ ܦ‬,

145

and the ideal selectivity (α) was determined from

146

ߙ=

ܲ஺ ‫ܦ‬ ܵ ൗܲ = ஺ൗ‫ ∙ ܦ‬஺ൗܵ = ߙ஽ ∙ ߙௌ . ஻ ஻ ஻

(2)

(3)

(4)

147

Here, PA and PB are the permeabilities of pure gases CO2 and N2, respectively. Terms αD and αS

148

are the respective solubility selectivity and diffusivity selectivity. The solution–diffusion

149

transport model

150

membranes containing different GO samples, and the selectivities of membranes for gas CO2

151

were expressed relative to gas N2 using Eq. (4).

152

RESULTS AND DISCUSSION

153

Morphology of different GO samples

36

was used for discussing the gas transport properties of dense PI hybrid


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C-N

Transmittance /%

=CH2 CHN

-C=O

N-H

GO GO-HMDI GO-TDI 4000

3500

2500

2000

1500

1000

500

Wavenumber/cm-1

154 155

3000

Figure 1 Fourier transform infrared spectra for different GO samples

156

FTIR analysis. Figure 1 shows that the most characteristic peaks in the FTIR spectra of the

157

two modified GO samples are similar to the pristine GO. The absorption bands correspond to the

158

C=O carbonyl stretching at 1720 cm-1, C–C stretching at 1605 cm−1, C-OH stretching at 1392

159

cm−1, and C-O stretching at 1069 cm−1 37. The peak at 3383 cm−1 is a resonance peak assigned to

160

the absorbed hydroxyl groups in the samples

161

stretching at 1720 cm-1 became very weak in modified GO versus pristine GO. New absorption

162

bands corresponding to the C-N stretching vibration (at 1518 cm-1) and the N-H bending

163

vibrations (at 1577 cm-1) were found in the FTIR spectra of GO-HMDI samples. TDI

164

modification caused a very weak absorption band corresponding to the C=O carbonyl stretching

165

and a new absorption band at 1577 cm-1 (the N-H bending vibrations). This shows that the

166

isocyanate groups of HMDI and TDI were successfully grafted on the GO surface by reaction

167

between the isocyanate groups and the oxygen-containing functional groups. Combining with the

168

conclusions in literatures

38

. The absorption band of the C=O carbonyl

39, 40

, we think the hydroxyl (-OH) and carboxyl(-COOH) groups on



169

GO surface would react with isocyanate, as shown in Scheme S2(SI). The little change in the OH

170

peak in Figure 1 may be due to the small amount of hydroxyl groups reacted with isocyanates.

171

TEM analysis. TEM images of GO (as listed in Figure S1a, SI) and modified GO samples (as

172

shown in Figure S2, SI) show that surface modification only slightly changes the morphology of

173

the GO nanosheets. The images showed that all nanosheets in three GO samples became highly

174

transparent indicating that the morphology consisted of a monolayer or just a few layers of

175

nanosheets. The selected area electron diffraction (SAED) pattern of the GO sample also

176

suggests that the GO samples with high oxidation levels have a more regular carbon framework

177

36

178

HDMI or TDI could bring some impurities into the GO hexagonal lattice. This was also seen in

179

the SAED patterns as a vague ring-like pattern. This meant that surface modification of GO

180

samples would lead to destroy or distort some hexagonal lattice of GO. The HRTEM images of

181

three GO samples shown in Figure S3 (SI) also confirmed this conclusion. The HRTEM image

182

of GO shows many obvious lattice structures on the GO surface (circles in Figure S3a). After

183

modification by HMDI or TDI, the grafted groups on the GO surface would destroy the lattice

184

structures of GO. Thus, the lattice structures on GO-HMDI or GO-TDI samples become

185

indistinct with some disordered surface defects or lattice distortion. This caused impurities in the

186

two SAED patterns of modified-GO samples.

. However, the SAED patterns of modified GO samples show that the surface modification by


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a

b

187 188

Figure 2 The SAED patterns of different modified GO samples

189

a. GO-HMDI; b. GO-TDI

190

Raman spectrum. The Raman spectra in Figure 3 show two obvious characteristic peaks in

191

the spectra of GO or two modified GO samples: the first peak at approximately 1350 cm-1

192

represents the disordered carbon band (D-band) and typically corresponds to the surface defects

193

of the GO sheets; the second peak at approximately 1580 cm-1 represents the graphitized band

194

(G-band) and corresponds to the formation of sp2-bonded crystalline carbon in GO samples 41-43.

195

The ratio of the D-band to the G-band (D/G ratio) in the two modified GO samples was larger

196

than that of GO samples due to the conversion of some sp2-hybridized carbons in the GO sheets

197

to sp3 hybridization. This also means that the surface defects on the GO sheets increased after

198

surface modification. This again confirms the results of the SAED patterns and HRTEM images.



D

ID / IG

Intensity /a.u.

GO-TDI

Page 12 of 31

G

1.68

GO-HMDI

1.53

GO

1.44

GO-TDI

GO-HMDI

GO

500

750

1000

1250

1500

1750

199

Raman shift /cm-1

200

Figure 3 Raman spectra of different GO samples

201

(excitation at λ = 632.8 nm)

2000

202

With respect to the Raman spectra of two modified GO samples in Figure 3, it can be seen that

203

modification of TDI would cause a larger D/G ratio than the HMDI modification. This indicates

204

that more surface defects were formed on the GO-TDI surface. TDI is smaller and causes less

205

steric hindrance during surface grafting. Thus, more TDI molecules could be grafted to the GO

206

surface and introduce more disordered surface defects. In addition, a π-π bond could be formed

207

between the benzene ring in the molecule of TDI and the GO surface in the GO-TDI sample.

208

This would further damage the lattice structure of the GO surface and introduce more surface

209

defects.

210

The effect of GO surface modification on the lattice structure is also reflected in the XRD

211

patterns of three GO samples (Figure S4). The XRD patterns of three GO samples all had a

212

characteristic (002) peak at about 10.8o 44. Three patterns show that the characteristic peak of the

213

modified GO samples is weaker and wider than the pristine GO sample. This decrease in

214

intensity of the 002 characteristic peak is due to the introduction of surface defects onto modified


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GO samples. After TDI modification, more surface defects were formed on the GO surface. Thus,

216

the characteristic peak in the XRD pattern of the GO-TDI sample was the weakest and widest

217

among the three XRD patterns in Figure S4.

218

XPS analysis. The XPS spectra of GO and modified GO samples as well as their elemental

219

composition are shown in Figure S5 and Table S1, respectively (SI). The results confirm that the

220

two isocyanates are successfully grafted onto the GO surface. To further investigate the effect of

221

surface modification on the GO surface, XPS spectra of C1s are shown in Figure 4.

GO-TDI

GO-HMDI

GO

222 223 224

Figure 4 XPS spectra for the C1s region of GO and different modified GO samples. The GO samples have main XPS peaks (Figure 4). The peak at 284.5 eV is assigned to

225

sp2-hybridized carbon atoms

28, 45

226

modification GO samples with sp3-hybridized orbits. After deconvolution of high-resolution

227

XPS profiles, the C1s XPS spectrum of GO shows a considerable degree of oxidation

228

corresponding to carbon atoms consisting of four components in different functional groups: the

. The other peak originates from C-O on the surface of GO or



229

C=C/C-C in aromatic rings, C-O (epoxy and alkoxy), the carbonyl carbon, and the carboxylate

230

carbon (O-C=O); these were at 284.8, 286.6, 287.1, and 288.9 eV, respectively. Table S2 (SI)

231

provides oxygen/carbon intensity ratios, ratio of sp2- to sp3-bonded carbon atoms, and intensity

232

ratio of hydroxyl, carboxyl, epoxy groups and carbon-nitrogen with respect to the C-C peak.

233

Figure 4 and Table S2 show that both the sp2/sp3 ratio and the carbon/oxygen intensity ratio

234

increase after modification with the two isocyanates

235

modified GO exhibits the same oxygen functionalities as GO, the intensities of the peaks at

236

286.6 eV and 287.1 eV decrease. This shows that most of the oxygen-containing functional

237

groups in graphene are removed by the two isocyanates, especially the epoxy and alkoxy groups.

238

These results were consistent with that in FTIR analysis. Nevertheless, the intensity of the peak

239

at 289.0 eV slightly increased, showing that the carboxylate carbon increased, which is

240

consistent with the group features of modified GO samples. Moreover, an extra peak at 285.9 eV

241

shows C-N indicating that TDI and HMDI were grafted on the GO surface.

46

. Although the C1s XPS spectrum of

242

Figure S6 (SI) shows the differences in the distribution of GO or modified GO samples in

243

dimethyl acetamide(DMAC) solvent after one week (the GO content is 10 g·L−1). The modified

244

GO samples show a better distribution in DMAC, a typical polar solvent. The improved

245

dispersion of GO in DMAC is due to the increase in the surface polarity of GO. Both the grafting

246

of isocyanate groups and the introduction of surface defects can increase the surface polarity of

247

GO. This enhances the dispersion of GO in DMAC. The surface of the GO-TDI has more

248

isocyanate groups and more surface defects. Moreover, the TDI molecule is less symmetrical

249

than the HMDI molecule. These two factors can both increase the surface polarity of the

250

GO-TDI, and the GO-TDI has a better distribution in DMAC than GO-HMDI.


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251


Morphology of different membranes containing GO and modified GO samples

252

FTIR analysis is first employed to characterize PI and different hybrid membranes and the

253

results are listed in Figure S7 (SI). From Figure, it can be observed that the characteristic bands

254

of PI polymer in our work are detected at 1780 and 1370 cm–1 due to C=O and N-C vibrations of

255

the imide ring, which is consistent with the result in literature 47. All the absorption peaks in the

256

spectra of different hybrid membranes corresponded to the PI polymer, which can confirm the PI

257

formation in different hybrid membranes. And it is also found from these FTIR spectra that there

258

are no absorption peaks that corresponded to the GO sheets, due to the low GO content in the

259

membranes.

260

The surface modification can also affect the distribution of GO in the PI polymer matrix. The

261

photographs of the PI polymer and different hybrid membranes are given in Figure S8, SI; these

262

show some obvious black points when the samples have a high GO content (circles, Figure S8b).

263

This suggests the formation of GO aggregates in the polymer matrix. As more modified GO is

264

added, both GO-HMDI and GO-TDI can distribute homogeneously in the polymer matrix. No

265

GO aggregation is observed with the modified GO samples. The difference in the morphology of

266

hybrid membranes with GO and modified GO samples can be further confirmed by SEM

267

analysis.



b

a

268 269

Figure 5 SEM images of surface of different PI hybrid membranes. The inset in each image

270

shows the corresponding morphologies of cross-section of membranes

271

(4.0 wt% loading): a. GO–HMDI/PI; b. GO–TDI/PI.

272

SEM images of different hybrid membrane. The SEM images in Figure S9 (SI) shows that

273

pristine GO without modification aggregates significantly in the hybrid membrane when added

274

at 4.0 wt%. All SEM images of hybrid membranes containing modified GO (4.0 wt% loading) in

275

Figure 5 show that both GO-HMDI and GO-TDI are distributed homogeneously in the PI

276

polymer matrix due to the grafting of isocyanate groups and the introduction of surface defects

277

on the GO surface. The different distribution of three GO samples in PI polymer matrix can be

278

confirmed by TEM analysis (Figure S10, SI). As shown in all TEM images, GO and modified

279

GO sheets remain stable in PI polymer matrix after treatment at 300 °C, due to the protection of

280

polymer matrix on GO nanosheets. It is also found that two modified GO samples show better

281

distribution in PI polymer matrix than that of GO without modification, when the GO content in

282

all hybrid membranes is 4.0 wt%.

283

These differences in the distribution of GO or modified GO in the PI polymer matrix are also

284

observed in the zeta potential values and water contact angles of different hybrid membranes (as


Page 16 of 31

Page 17 of 31


285

shown in Figures S11 and S12, SI). The GO and modified GO samples all have polar surface.

286

Addition of GO samples into PI matrix will improve the surface electrical property and surface

287

polarity of mixed matrix membrane, thus increasing zeta potential values and decreasing water

288

contact angles of hybrid membranes. More GO nanosheets with polar surface disperse

289

homogenously in the mixed matrix membranes, which leads to a larger zeta potential value and

290

smaller water contact angle of the mixed matrix membrane. After modification of GO with two

291

isocyanates, the distribution of GO in the polymer matrix can be significantly improved.

292

Therefore, the hybrid membrane with 4.0 wt% modified GO samples has the largest zeta

293

potential value and the lowest water contact angles. This means that 4.0 wt% of the modified GO

294

samples can be distributed homogeneously in the polymer matrix. Under the same preparation

295

conditions, the hybrid membrane containing GO-TDI shows the largest zeta potential value and

296

the lowest water contact angles because it has the highest surface polarity and best distribution.

297

The differences in the hybrid membranes with 4.0 wt% of all three GO samples can be

298

confirmed by TGA analysis (Figure S13, SI).

299

Figures S14 (SI) and Figure 6 show the permeability and ideal selectivity for pure gas CO2

300

and N2 of hybrid membranes incorporated by different GO samples. And the diffusivity,

301

solubility and permeability parameters for the base polymer, GO added, modified GO added

302

materials are listed in Tables S3 and S4. The data show that CO2 permeability was much larger

303

than N2 permeability for all hybrid membranes. According to the literatures 28, 48, 49, GO has a 2D

304

structure with conjugated π bonds, giving affinity towards CO2 molecules. The effective π–π

305

stacking interactions between GO and CO2 molecules capture and absorb CO2 molecules.



Page 18 of 31

306

Secondly, the difference in electronegativity between C and O in the CO2 molecule leads to polar

307

bonds,

308

CO2 in the hybrid membrane containing GO. In addition, since more modified GO samples with

309

the surface defects are added, the increase in interface gaps between the polymer and modified

310

GO sheets also benefits the diffusion of gas molecules. The increase in interface gaps of PI

311

hybrid membranes can be confirm by XRD analysis, as shown in Figure S15 (SI). More

312

modified GO nanosheets can distribute homogenously in the PI polymer matrix due to the

313

enhancement on the surface polarity and GO surface defects after modification. Therefore, the

314

optimal content of modified GO in PI polymer matrix can reach 4.0 wt%, after modification with

315

these two isocyanates. However, the optimal content of GO without modification in the PI

316

polymer matrix is only 1.0 wt%.

39

which also helps the CO2 to be absorbed by the GO and improves the diffusion of the

50 GO-TDI-PI GO-HMDI-PI

45

GO-PI 40

CO2/N2 ideal selectivity

35 30 25 20 15 10 5 0 0

317 318 319 320

1

2

3

4

5

GO cotent in membranes /wt%

6

7

Figure 6 The effect of the addition of different GO samples on the ideal selectivity of CO2/N2. Modifying the GO samples can also increases the CO2 and N2 diffusivity coefficients, as


Page 19 of 31


321

shown in Figure S16 (SI). The modification causes surface defects on the GO samples and

322

introduces interface gaps between the polymer and nanomaterials. This enhances the diffusion

323

coefficients for the two gases. The diffusion coefficients for both gases through the PI hybrid

324

membranes increase upon addition of GO. When GO is aggregated in the PI polymer matrix, the

325

GO aggregates inhibit gas diffusion. Thus, the gas diffusion coefficients through the PI hybrid

326

membranes decreased. CO2 gas is a polar gas and improving the GO surface polarity could

327

enhance the diffusion coefficients of CO2 through the PI hybrid membranes. The GO-TDI

328

samples have the most surface defects and the highest surface polarity. This causes the highest

329

diffusion coefficients for the two gases through the PI hybrid membranes with the same GO

330

addition content.

331

Figure 7 obviously shows that the GO surface modification leads to a more significant

332

enhancement on solubility selectivity than diffusion selectivity, although the diffusion

333

coefficients for CO2 do increase. The literature suggests that the diffusion selectivity is mainly

334

determined by the size difference between the penetrant molecules and the size-sieving ability of

335

the polymer matrix; the solubility selectivity is controlled by the relative condensability (i.e.,

336

critical temperature) of the penetrants and the relative affinity between the penetrants and the

337

polymer matrix. The sizes (critical volume) of CO2 and N2 molecules are 93.9 and 89.8

338

cm3/mole, respectively. The similar size of the two gas molecules causes small differences in

339

diffusion coefficients for CO2 and N2. Even after modification with these two isocyanates, the

340

differences in diffusion coefficients for the two gases are minimal. Therefore, the largest

341

diffusion selectivity for the PI hybrid membranes in Figure 7 is only ~ 4.



Page 20 of 31

14 S D of GO-PI S D of GO-HMDI-PI S D of GO-TDI-PI S S of GO-PI S S of GO-HMDI-PI S S of GO-TDI-PI

12

Selectivity

10

8

6

4

2

0 0

342 343 344

1

2

3

4

GO cotent in membranes /wt%

5

6

7

Figure 7 The solubility selectivity and diffusivity selectivity of different PI hybrid membranes.

345

For the solubility selectivity, pure CO2 has higher solubility coefficients based on their

346

condensability; the condensability of CO2 and N2 molecules is 304.21 and 126.2 K, respectively.

347

In addition, the π–π stacking interactions between GO and CO2 molecules result in a strong

348

affinity between CO2 molecules and the polymer matrix. The surface polarity of GO also

349

increases the affinity between CO2 and GO in the hybrid membranes. The strong affinity

350

between CO2 and GO can be confirmed via the TPD-CO2 results (Figure S17, SI). The GO

351

samples and the modified GO samples all strongly absorb CO2. Therefore, the increased

352

solubility selectivity for CO2 upon GO addition increases the CO2 separation performance of the

353

hybrid membranes. After modification, more GO can be distributed homogeneously in the

354

polymer matrix. This generates a strong affinity between the CO2 molecules and the polymer

355

matrix. Thus, the PI hybrid membranes incorporating modified GO samples have a significant

356

higher solubility selectivity than the GO-PI membrane. The solubility selectivity of the hybrid


Page 21 of 31


357

membranes with GO-TDI samples is the highest of all hybrid membranes with the same GO

358

content. This is because TDI has the highest surface polarity and a benzene ring structure.

359

The significant increase in solubility selectivity for CO2 gas of membranes can be confirmed

360

by the CO2 and N2 sorption isotherms of different membranes, as shown in Figure S18 (SI). The

361

sorption isotherms also show that all membranes had an adsorption capacity for CO2 gas than N2

362

gas, due to the effective π–π stacking interactions between CO2 molecules and GO in polymer

363

matrix. Because of the better distribution in PI polymer matrix, the hybrid membranes containing

364

modified GO have much higher adsorption capacity for CO2 gas than that of hybrid membrane

365

incorporated by unmodified GO samples.

366

The effect of TDI addition content on the performance of hybrid membranes

367

To investigate further the effect of surface modification, TDI with different addition content

368

were first employed to modify GO nanosheets (TDI concentrations were shown in Table S5, SI).

369

XPS analysis for different GO-TDI samples and the corresponding explanation were listed in

370

Figure S19 (SI). From the XPS results, it was shown that the amount of nitrogen in GO-TDI

371

samples initially increased but eventually decreased with addition content of TDI. This means

372

that the amount of TDI grafted on the GO surface is limited. When excess TDI is added, the TDI

373

molecules cannot be grafted on the GO surface. They are distributed in the solvent, which results

374

in a binding force between the molecules and hampers grafting. Therefore, the amount of C-N

375

bonds decreases slightly, with more than 70 wt% of TDI addition (as shown in Tables S6 and

376

S7).

377

After modification, the different GO-TDI/PI hybrid membranes were fabricated. Figure 8



Page 22 of 31

378

shows the diffusion coefficients and gas permeability for two gases of the hybrid membranes

379

incorporating different GO-TDI samples. As the addition of TDI increases in modification, more

380

defects are introduced on the GO surface. The diffusion coefficients and gas permeability for the

381

two gases of hybrid membranes also increase with TDI addition. The change in diffusion

382

coefficients and gas permeability of membranes for CO2 is more significant than those for N2, as

383

TDI increases from 10 wt% to 50 wt%. When excess TDI is added (more than 70 wt% in this

384

paper), the slight decrease in the grafted TDI molecules on the surface of GO-TDI70 and

385

GO-TDI90 causes the decrease in the gas permeability and the diffusivity coefficients for two

386

gases of membranes. 400

3 CO2 N2 D CO2 D N2

2.5

2

200

Diffusivity×10-8 /(cm2/s)

Permeability /Barrar

300

1.5

1 100

0.5

0

387

0 a

b

c

d

e

f

388

Figure 8 The gas permeability and the diffusivity coefficients for two gases of PI hybrid

389

membranes

390

a. PI polymer membrane; b. Hybrid membrane containing GO-TDI10; c. Hybrid membrane

391

containing GO-TDI30; d. Hybrid membrane containing GO-TDI50; e. Hybrid membrane

392

containing GO-TDI70; and f. Hybrid membrane containing GO-TDI90


Page 23 of 31

393


(The modified GO content in all hybrid membranes was 4.0 wt%.)

394

As a further explanation, the permeability selectivity values of different PI hybrid membranes

395

are shown in Figure 9. The changes in selectivity clearly show that the ideal selectivity and

396

solubility selectivity change slightly when the addition content of TDI increases from 30 wt% to

397

90 wt%. After modification with 30 wt% of TDI, 4.0 wt% of GO-TDI30 can distribute

398

homogeneously in the PI polymer matrix. When the addition content of TDI exceeds 30 wt%, the

399

distribution of GO-TDI in the PI polymer matrix remains constant. Because the solubility

400

selectivity is the main factor that affects the performance of membranes containing GO for CO2

401

separation, the selectivity of the hybrid membrane changes only slightly regardless of increases

402

in TDI addition from 30 wt% to 90 wt%. The changes in permeability and selectivity of different

403

membranes in pure gas system can be confirmed again by those in mixed gas system, as shown

404

in Figure S20 (SI). 14

60 SD SS

12

50

Ideal selectivity

Selectivity

40 8 30 6 20

CO2/N2 ideal selectivity

10

4 10

2

0

0 a

b

c

d

e

f

405 406 407

Figure 9 The permeability selectivity of different PI hybrid membranes a. PI polymer membrane; b. Hybrid membrane containing GO-TDI10; c. Hybrid membrane



408

containing GO-TDI30; d. Hybrid membrane containing GO-TDI50; e. Hybrid membrane

409

containing GO-TDI70; and f. Hybrid membrane containing GO-TDI90

410

411

(The modified GO content in all hybrid membranes was 4.0 wt%.)

SUPPORTING INFORMATION AVAILABLE

412

The structure of TDI and HMDI, Schematic diagram of reaction of isocyanate with hydroxyl

413

and carboxyl groups on GO surface, Schematic diagram of fabrication of GO/PI membrane by

414

insitu polymerization, and Schematic diagram of fabrication of modified-GO/PI membrane by

415

insitu polymerization were listed in Scheme S1-S4. TEM image and the SAED pattern of GO

416

samples, TEM images of modified GO samples, HRTEM images of different GO samples, XRD

417

patterns of different GO samples, XPS spectra of different GO samples, Digital photographs of

418

the dispersion of different GO samples in DMAC after a week, FTIR spectra of PI and different

419

PI hybrid membranes, The photographs of PI polymer and different hybrid membranes, SEM

420

images of surface and cross-section of PI hybrid membranes containing GO, TEM images of

421

different PI hybrid membranes, Zeta potential values of different PI hybrid membranes, Static

422

water contact angles of different PI hybrid membranes, TGA thermograms of different PI hybrid

423

membranes, Dependence of GO content on the gas permeability of different membranes, XRD

424

patterns of PI and different hybrid membranes, The diffusivity coefficients through PI hybrid

425

membranes with various GO contents, CO2-TPD results of GO and modified GO samples, CO2

426

and N2 sorption isotherms of PI and different hybrid membranes, The high-resolution XPS

427

spectra for the C1s region of different GO-TDI samples, and The gas permeability and

428

permeability selectivity for gas mixture of different hybrid membranes were listed in Figure


Page 24 of 31

Page 25 of 31


429

S1-S20. The atomic percentages of carbon, oxygen, nitrogen in different GO samples, XPS data

430

in different GO samples, The permeability parameters for different membranes, The diffusivity

431

and solubility for different membranes, The starting concentration of TDI and GO, the

432

percentages of carbon, oxygen, nitrogen in different GO-TDI samples, and XPS data of different

433

GO-TDI samples were listed in Tables S1-S7. This material is available free of charge via the

434

Internet at http://pubs.acs.org.

435

ACKNOWLEDGMENT

436

The authors gratefully acknowledge the financial support from the National Natural Science

437

Foundation of China Grants (Contracts 21376218 and 21076190), the Natural Science

438

Foundation of Zhejiang Province (Contracts LY14B060001) and Open Research Fund Program

439

of Collaborative Innovation Center of Membrane Separation and Water Treatment of Zhejiang

440

Province (Contract 2016YB08).

441

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