ZIF-8 mixed matrix membranes for gas separations

Journal Pre-proof Zn(II)-modified imidazole containing polyimide/ZIF-8 mixed matrix membranes for gas separations Yanfang Fan, Huiya Yu, Shan Xu, Qinchen Shen, Haimu Ye, Nanwen Li PII:

S0376-7388(19)33093-5

DOI:

https://doi.org/10.1016/j.memsci.2019.117775

Reference:

MEMSCI 117775

To appear in:

Journal of Membrane Science

Received Date: 6 October 2019 Revised Date:

20 December 2019

Accepted Date: 20 December 2019

Please cite this article as: Y. Fan, H. Yu, S. Xu, Q. Shen, H. Ye, N. Li, Zn(II)-modified imidazole containing polyimide/ZIF-8 mixed matrix membranes for gas separations, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117775. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Author statement Yanfang Fan: Conceptualization, Methodology, Supervision ,Writing- Reviewing and Editing; Huiya Yu, Shan Xu: Investigation, Data acquiring and analysis; Qinchen Shen: DMA Data acquiring; Haimu Ye: IR Data acquiring; Nanwen Li: Conceptualization, Supervision

Grapphic abstracct

1

Zn(II)-modified imidazole containing polyimide/ZIF-8 mixed

2

matrix membranes for gas separations

3

Yanfang Fan,a* Huiya Yu,a Shan Xu,b Qinchen Shen a, Haimu Ye, a Nanwen Lib*

4

a

5

Environment, China University of Petroleum-Beijing, Beijing, 102249, China

6

b

7

Chinese Academy of Sciences, Taiyuan, 030001, China

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering and

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry,

8 9

*Corresponding author.

10

Email: [email protected], [email protected]

11

Abstract: Finely tailoring the interfacial interaction to minimize the defective structure in

12

the hybrid membranes is a key to yield a mixed matrix membrane with enhanced gas

13

separation performance. Here, a highly selective mixed matrix membrane based on

14

imidazole containing polyimides and ZIF-8 fillers is reported. The ZIF-8 with imidazole

15

linker offers a more compatible interface with imidazole containing polyimides. At a

16

MOF loading of 20 wt. %, the membranes have a 2.2-fold increase in the gas

17

permeabilities over unfilled polymers with exceptional H2/CH4 and CO2/CH4 gas

18

selectivities of 224 and 58, respectively. The H2 and CO2 gas permeabilities are 78.5 and

19

20.3 Barrer, respectively. The Zn2+ post-modification enables the formation of metal

20

coordination crosslinking with enhanced polymer/ZIF-8 interaction, as indicated by

21

increased glass transition temperature, improved thermal stability and insoluble

22

properties in organic solvents. With the proper control of Zn2+ treatment conditions, gas 1

23

permeabilities of H2 and CO2 can be increased to 110.1 and 27.4 Barrer, respectively,

24

with the constant H2/CH4 and CO2/CH4 gas selectivity. For membranes treated at higher

25

Zn2+ concentration, an enhancement in H2/CH4 gas selectivity was observed with the

26

selectivity increasing from 223.9 to 318.3 and H2 permeability around 72.3 Barrer. The

27

separation performance of H2/CH4 for all Zn2+ modified membranes exceeds the 2008

28

Robeson upper bound. This facile approach to tune polymer/MOF interaction via metal

29

ion modification promotes the rational design of high-performance gas separation

30

membranes.

31

Key words: mixed matrix membrane, gas separation, ZIF-8, metal ion modification

32 33

1. Introduction Membrane-based gas separation as a cost-effective method is one of the most

34

promising technology for separating H2 and CO2 from various gas streams [1-3]. The

35

membrane as a key component plays a critical role of dictating the separation efficiency

36

and cost. Since commercial polymeric membranes based gas separator for H2 recovery

37

were developed by Permea Inc., the research on synthetic polymer membranes is growing

38

exponentially [4-6]. Polymer membranes such as polyimide, cellulose acetate,

39

polysulfone, and poly (dimethylsiloxane) have been applied for industrial applications

40

including natural gas sweetening, CO2 separation and H2 recovery [7, 8]. However, such

41

polymeric membranes suffer from trade-off relationships between permeability and

42

selectivity [9-12]. Moreover, the plasticization effects of polar gases seriously weaken the

43

gas separation performance under high pressure conditions. Long-term stability of

44

polymeric membranes is a concern [13]. Thus, researchers are dedicated to design

2

45

alternative membranes to achieve exceptional gas separation performance, finally

46

exceeding Robeson upper bound.

47

Mixed matrix membranes (MMMs) with a combination of low cost and solution-

48

processable polymers with molecular sieving fillers emerge as promising alternatives to

49

polymeric membranes [14-18]. In particular, metal organic frameworks (MOFs) based

50

MMMs with tunable texture structure and chemical properties have attracted intense

51

interest for gas separations. Generally, polyimides such as Matrimid®, 6FDA-based

52

polyimide [20], novel microporous polymers such as PIM-1 [21], Tröger base [18, 22]

53

and other permeable polymers of Pebax® [23] are selected as polymer matrix. The

54

Zeolitic Imidazolate Frameworks (ZIFs) with promising molecular sieving properties [24-

55

26], highly stable UIO-66 [20-21, 27], MIL-53 [18, 28], etc. are extensively investigated

56

as the dispersed phase. Bae et al. [19] have discussed in a recent review about the role of

57

a large number of traditional or emerging functional fillers in improving the CO2/CH4

58

separation performance of hybrid composite membranes. Although polymer matrix plays

59

a key role in determining the separation performance of hybrid membranes, proper filling

60

materials with optimum properties are needed to enhance the CO2/CH4 separation

61

performance of the resultant membranes.

62

Significant improvements in gas separation performance, especially enhanced gas

63

permeability, have been achieved in the majority of mixed matrix membranes. For

64

example, Tröger-base/NH2-MIL-53 MMMs shows 3-fold increase in CO2 gas

65

permeability with almost constant CO2/CH4 gas selectivity [18]. However, particle

66

aggregates lead to the abrupt drop of gas permeability at MOF loading up to 30 wt. %.

67

Kharul et al. fabricated ZIF-8/substituted polybenzimidazole (DBzPBI-BuI) membranes 3

68

for olefin/paraffin separation [29]. The imidazole moieties in polymer interact with ZIF-8,

69

thus enhancing phase compatibility. An enhancement in propylene permeability by 7.5

70

times and ideal selectivity of propylene/propane by 4.3 times was achieved in this

71

membrane. However, some key issues of particle agglomeration, polymer chain

72

rigidification, and formation of defective voids persist in these membranes. The key to

73

solve these problems is finely tuning interfacial interaction in MMMs to improve phase

74

compatibility, thus yielding uniform dispersion of MOFs in polymers with good interface

75

adhesion.

76

Various approaches including in-situ polymerization [27, 30], cross-linking reaction

77

[30], functionalization of MOFs [18, 32] and in-situ MOFs assembly [33] have been

78

utilized to enhance interfacial adhesion in mixed matrix membranes. For example,

79

Coronas et al. [33] synthesized MIL-68(Al) crystals in polymer solutions via in-situ

80

assembly technique and then casted membrane without MOF purfication. The

81

dispersibility of MOFs is found to be uniform across entire membranes and no detectable

82

voids are present in the membrane. Shao et al. [31] incorporated reactive isopropenyl-

83

functionalized UIO-66-MA in crosslinked polyethylene oxide matrix with the formation

84

of bonded interface. UIO-66-MA reacted with methacrylate-capped polyethylene oxide

85

during UV-induced polymerization, considerably enhancing interfacial interaction.

86

Recently, Li et al. [32] developed amino functionalized ZIF-7/crosslinked polyethylene

87

oxide hybrid membranes, wherein metal sites chelate with ether bonds in polymers,

88

leading to improved interfacial adhesion. Accordingly, CO2 gas permeability increased

89

from 105 to 240 Barrer and CO2/CH4 selectivity increased from 21 to 54. All these

90

studies confirmed that good interfacial compatibility in mixed matrix membranes is 4

91

necessary to attain a membrane with simutaneous enhancements in gas permeability and

92

selectivity.

93

In this work, we demonstrate a new approach that uses metal chelating capability

94

with nitrogen atom of imidazole groups to engineer MOF-polymer interaction. The

95

imidazole containing polyimide polymers were synthesized as polymer matrix (6FDA-BI,

96

Figure 1) and then combined with ZIF-8 via conventional physical mixing methods.

97

Sodalite type ZIF-8 nanocrystals composed of zinc and 2-methylimidazole (2mIM)

98

linkers have a small pore window size of 0.34 nm and cavity size of 1.16 nm with good

99

molecular sieving properties. The reasons for choosing this polymer-filler combination

100

are: (i) 6FDA-BI with imidazole groups are speculated to be highly compatible with ZIF-

101

8 as both of them contain the imidazole units with high structural similarity; (ii) ZIF-8 is

102

readily available with excellent stability and uniform particle size distribution; (iii) The

103

electron donor of nitrogen atom in the imidazole moiety of 6FDA-BI can interact with the

104

metal ion to form metal-N complex.

105

A series of mixed matrix membranes (MMMs) with various ZIF-8 loading from 0 to

106

30 wt. % were fabricated and further evaluated for their gas transport properties. The

107

mixed matrix membrane with 20 wt. % ZIF-8 was then post-treated in zinc (II)

108

acetylacetonate [Zn(acac)2] solutions of different concentrations. The Zn2+ could

109

efficiently chelate with imidazole groups in polymers and ZIF-8 with the formation of

110

metal ion crosslinked networks. The resultant membranes have enhanced ZIF-8-polymer

111

interaction as reflected by the inhibited polymer chain mobility in several

112

characterizations. In comparison to pristine 6FDA-BI membranes, the Zn2+ modified

113

hybrid membranes prepared under optimized conditions have improved gas separation 5

114

performance for H2/CH4 and CO2/CH4 with simultaneous increase of gas permeabilities

115

and selectivities. Although many works [34-37] on metal modified polymer membranes

116

have been reported for gas separations, the Zn2+ modified MOF containing MMMs are

117

reported firstly in this work. The concepts of creating metal-N complex can be extended

118

to other MOFs based MMMs to tune interfacial microstructure so that gas separation

119

performance can be improved.

120

2. Experimental

121

2.1 Material synthesis

122

The synthesis mechanism of 6FDA-based polyimide is shown in Figure 1. 2-(4-

123

Aminophenyl)-1H-benzimidazol-5-amine (BI, 4.4852 g, 0.02 mol, CAS: 7621-86-5,

124

purchased from Changzhou Sunshine Pharmaceutical Co., Ltd.) was added to 100 mL

125

flask with stirrer bar and nitrogen inlet, and then to NMP (45 mL). 4,4’-

126

(Hexafluoroisopropylidene)diphthalic anhydride (6FDA, 8.8848 g, 0.02 mol) was added

127

at low temperature after BI was completely dissolved. The reaction mixture was stirred at

128

room temperature for 24 h to form corresponding polyamic acid. After that, 3-

129

methylpyridine (1.9 mL, 0.02 mol) and acetic anhydride (18.9 mL, 0.2 mol) were added

130

to the reaction mixture and stirred for 24 h to induce complete imidization of polyamic

131

acid to form polyimide. The polyimide was precipitated in methanol, washed several

132

times and dried in vacuum at 100 °C for 24 h to obtain 6FDA-BI polyimide.

133 134

Figure 1 Synthesis scheme of 6FDA-BI polyimide. 6

135

ZIF-8 crystals were synthesized according to a literature procedure [38]. 1.5 g (5

136

mmol) Zn(NO3)2·6H2O and 3.3 g (40 mmol) MeIm were dissolved separately in 70 mL

137

methanol. The two solutions mixed when they were completely dissolved and then stirred

138

at room temperature for 24 h. White powders were acquired by centrifugation at 8000

139

rpm for 5 min. The product was washed in 50 mL methanol for 3 times. The resulting

140

solids were dried in a vacuum oven at 80 °C for 12 h.

141 142

2.2 MMM preparation ZIF-8 and 6FDA-BI were dried at 80 °C and 150 °C overnight prior to membrane

143

casting. Then 0.1 g ZIF-8 was dispersed in dimethylacetamide (DMAc) and stirred

144

overnight. The stirred ZIF-8 solution was sonicated for 3 min to form a homogeneous

145

suspension. A desired amount of 6FDA-BI was dissolved in DMAc and then mixed with

146

ZIF-8 solution to obtain priming solution. The primed dispersion was stirred for 30 min

147

and then sonicated for 3 min. The remaining 6FDA-BI were added to the above solution

148

in three batches to get the desired mixed matrix membrane composition, and the stirring

149

and sonication steps were repeated three times. Finally, mixed solution was stirred

150

overnight, and then poured on a pre-heated glass plate at 60 °C. A wet film applicator

151

with adjusted heights was used to obtain a film with thickness about 40-70 µm. The wet

152

film was dried at 60 °C in an oven for 2-3 hours. After the solvent was completely

153

evaporated, the film was scraped off the glass plate, soaked in a methanol solution for 12

154

hours, and then dried in a vacuum oven at 150 °C for 12 hours. Zn2+ modified membranes

155

were prepared by immersing the membranes into methanol for 6 h and then soaked in

156

various concentration of Zn(acac)2 solutions in methanol from 0.004 g/mL to 0.01 g/mL

157

for 12 h. 7

158 159

2.3 Material characterization methods N2 physisorption isotherm at 77 K were acquired on a Micrometrics ASAP 2460

160

instrument after degassing samples at 150 °C under vacuum overnight. Particle size and

161

membrane morphology were examined on a Zeiss Gemini Ultra-55 Analytical Scanning

162

Electron Microscope. The membrane samples were carefully fractured in liquid nitrogen.

163

The thin layers of gold were coated on sample surfaces via the subsequent sputter coating.

164

The X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer

165

using Cu Kα radiation (λ = 0.154 nm). All samples were measured using a zero-

166

background sample holder. The glass transition temperature (Tg) of membranes were

167

measured using dynamic mechanical analysis (DMA Q800) on multi-frequency-strain

168

modes with a heating rate of 2 °C/min from 30 °C to 430 °C. The amplitude was 15 µm,

169

the reload force was 0.03 N, tracking force was 150%, and the frequency was 1 Hz.

170

Thermogravimetric analysis (TGA) were performed on a NETZSCH STA 409 TG

171

analyzer under flowing Argon from 35 °C to 900 °C at a constant heating rate of

172

10 °C/min. Bruker Tensor II IR spectrometer was employed to obtain membrane infrared

173

spectra in the wavenumber range of 600-2000 cm-1. X-ray photoelectron spectroscopy

174

(XPS) of ZIF-8 and MMMs were performed using Thermo Scientific K-Alpha. CO2

175

adsorption experiment was carried out using custom-built pressure decay sorption

176

instrument at 35oC. 1 g polymer samples were wrapped in a porous stainless steel filter to

177

load in the sample cell. Prior to sorption tests, the samples were degassed at 120 °C

178

overnight.

8

179

2.4 Gas permeation measurements

180

Pure gas permeability test in MMMs was carried out at 35 °C using a custom

181

designed constant-volume/variable-pressure apparatus [18, 39]. Tests were conducted

182

with different single gases (H2, N2, CH4 and CO2) at feed pressure of 4 bar. A membrane

183

coupon sandwiched between aluminum tapes were taped on the permeation cell. The

184

membrane edge was sealed with epoxy resin to ensure its tightness. Prior to permeation

185

test, the entire permeation system was evacuated to make sure downstream pressure

186

below 0.005 Torr. Then gases with constant gas pressure were introduced into the

187

permeation cell upstream. Upstream and downstream pressures were recorded by

188

pressure transducers. The leak rate of the permeation system is below 5×10-5 torr/s.

189

Membrane permeation data was reproduced at least twice using different batches of

190

membranes. The selectivity mentioned in the paper is ideal selectivity.

191

The gas permeability is calculated used the following equation: P=

22414

192

where P is the permeability of the gas through the membrane, with unit of Barrer (1

193

Barrer = 10-10 cm3 (STP) cm cm-2 sec-1 cmHg-1), V is the downstream volume (cm3 ), A is

194

the effective membrane area (cm2), T is the absolute temperature (K), R is the universal

195

gas constant, l is the membrane thickness (cm), p is the feed pressure (cmHg), dp/dt is the

196

permeation rate (cmHg s-1).

197

The ideal selectivity of a membrane for gas A to gas B was evaluated as follows:

9

/

198

=

Maxwell model is commonly used to calculate the gas permeability of the

199

polymer/molecular sieves nanocomposite membranes at low filler contents [24]. Maxwell

200

model can be expressed by the equation: =

+2 −2 ( − +2 + ( −

) )

201

where Peff is the effective permeability of the composite membrane, PC and PD

202

represent polymer permeability and dispersed phase permeability (ZIF-8), respectively. Φ

203

is the volume ratio of the dispersed phase.

204

3. Results and Discussion

205

3.1 Membrane characterization of 6FDA-BI/ZIF-8

206

The microstructure of ZIF-8 and corresponding hybrid membranes was

207

characterized by N2 isotherms, XRD and SEM. ZIF-8 crystals were successfully

208

synthesized with an average particle size of ~80 nm (Figure 2). The BET surface area

209

obtained from N2 isotherms (Figure S1) was 1443 m²/g and pore volume was 0.64 cm3/g.

210

A series of MMMs with various ZIF-8 loading from 10 wt. % to 30 wt. % were prepared

211

by solution casting methods. As shown in SEM cross-section images in Figure 2, ZIF-8

212

particles were evenly distributed in polyimide matrix up to 20 wt. % loading. Moreover,

213

the good optical transparency was achieved in the membranes, indicating good

214

compatibility between ZIF-8 and 6FDA-BI as shown in Figure 2. With further increasing

215

of ZIF-8 content to 30 wt. %, partial ZIF-8 particle agglomerated together to form 10

216

clusters, which could deteriorate gas selectivities of the membranes. XRD profiles

217

(Figure 3) showed the complete diffraction peaks of pure ZIF-8 in all MMMs, confirming

218

the intact crystal morphology of ZIF-8 upon incorporation into MMMs.

219 220

Figure 2 SEM of ZIF-8 and MMMs. (a) 6FDA-BI; (b) 6FDA-BI/10% ZIF-8; (c) 6FDA-

221

BI/20% ZIF-8; (d) 6FDA-BI/25% ZIF-8; (e) 6FDA-BI/30% ZIF-8; (f) ZIF-8. The inset

222

images are membrane pictures to show good optical transparency of mixed matrix

223

membranes.

11

Intensity [a.u.]

(f) (e) (d)

(c)

(b) (a)

5

224

10

15

20

25

30

2θ (Deg.)

225

Figure 3 XRD profiles of MMMs. (a) 6FDA-BI; (b) 6FDA-BI/10% ZIF-8; (c) 6FDA-

226

BI/20% ZIF-8; (d) 6FDA-BI/25% ZIF-8; (e) 6FDA-BI/30% ZIF-8; (f) ZIF-8.

227

The glass transition temperature (Tg) was measured by DMA instruments (Figure

228

S2). As expected, the inclusion of ZIF-8 in pure 6FDA-BI led to the increase of Tg from

229

382 °C to 388 °C (Table 1), indicating the polymer chain mobility is inhibited upon ZIFs

230

inclusion. This fact confirmed the good interaction between ZIF-8 and 6FDA-BI again.

231

TGA tests were performed to evaluate the thermal stability of MMMs (Figure S3). As

232

indicated by thermal decomposition temperature Td,5% in Table 1, 6FDA-BI/10% ZIF-8

233

was much more thermally stable over pure polymer membranes as contributed by ZIF-

234

8/polymer interaction. Reduced Td,5% in 6FDA-BI/20% ZIF-8 is likely owing to the

235

disrupted polymer chains interaction induced by MOFs. The reduced thermal stability

236

was also seen in a study on UIO-66 incorporation into PEG and PVDF [40].

237

12

238

Table 1 Physical properties of 6FDA-BI, MMMs and Zn2+ modified MMMs.

ρe

Tg

φD

Td, 5%

g/cm3

°C

vol. %

°C

6FDA-BI

1.54±0.09

382

0

413

6FDA-BI/10% ZIF-8

1.43±0.10

386

16.1

495

6FDA-BI/20% ZIF-8

1.29±0.03

388

27.2

383

6FDA-BI/20% ZIF-8 (0.007 Zn2+ )

1.36±0.05

406

36.0

500

Name

239 240

FTIR analysis was performed to analyze the chemical structural changes occurred in

241

MMMs. As both 6FDA-BI and ZIF-8 have the imidazole groups, most of absorption

242

bands associated with the vibration of imidazole units coincide in their spectra as shown

243

in Figure 4 and Table S1. For example, the bands at 1143 cm-1 and 1376 cm-1

244

corresponding to C-N stretching vibrations in the imidazole groups [41-43] appeared in

245

the spectra of pure ZIF-8 and 6FDA-BI membranes. There are some characteristic bands

246

ascribed to ZIF-8 that can be distinguished from IR spectra of MMMs compared to

247

unfilled polymer membranes. These bands at 694 cm-1 (ring out of plane bending

248

vibration of Hmim), 754 cm-1 (C-H bending mode) and 991 cm-1 (C-N bending vibration)

249

for ZIF-8 became more intense as the filler loading increased, indicating ZIF-8 crystals

250

are successfully incorporated into the polymers and retain the chemical structure as well.

251

Overall, XRD and FTIR analysis confirmed the integrity of the ZIF-8 crystals at any

252

loading of MOFs into the polymers.

13

Absorbance (a.u.)

694 754

991

1143, C-N

1376, C-N

MMM 20%

MMM 10%

6FDA-BI

ZIF-8

600

800

1000

1200

1400

1600

Wavenumber (cm-1) 253 254

Figure 4 FT-IR spectra of 6FDA-BI, ZIF-8 and MMMs.

255

3.2 Membrane characterization of Zn2+ modified 6FDA-BI/ZIF-8

256

Zn2+ containing polymeric membranes have been reported in previous work [34-37]

257

to improve CO2 separation performance. As revealed by the research on previous metal

258

ion modified membranes and MOF-based MMMs [44, 45], the unsaturated metal sites

259

can selectively adsorb certain gases such as CO2, C2H4, etc. Thus, the incorporation of

260

transition metal ions in MMMs could be an efficient method to further enhance the

261

separation performance. We select Zn (II) as the incorporated metal ion based on several

262

factors including their selective affinity on CO2 gases and the chelating capability with

263

imidazole groups. And zinc (II) acetylacetonates [Zn(acac)2] was employed as the metal

264

sources [46] according to its simple structure, basicity nature (ZIF-8 is quite stable in this

265

solution), and good solubility in the methanol. The 6FDA-BI/20% ZIF-8 hybrid

266

membranes were used to perform post modification to prepare Zn2+ treated hybrid

267

membranes. The ZIF-8 containing membranes were soaked in various concentration of 14

268

Zn(acac)2 solutions in methanol from 0.004 g/mL to 0.01 g/mL to gain post-modified

269

MMMs, which are named as 6FDA-BI/20% ZIF-8 (x Zn2+), with x representing

270

concentration of Zn(acac)2.

271

Figure 5 compares the SEM images and XRD profiles of Zn2+ modified 6FDA-

272

BI/20% ZIF-8 with untreated ones. As seen from SEM images, ZIF-8 particles evenly

273

dispersed in the membranes and XRD profiles for Zn2+ modified MMMs were consistent

274

with that of ZIF-8. No visible changes were found for these membranes upon Zn2+

275

treatment and the morphology of ZIF-8 was unaltered in Zn2+ modified MMMs.

276 277

Figure 5 SEM (a) and XRD profiles (b) of 6FDA-BI/20% ZIF-8 and Zn2+ modified

278

membranes. 15

279

The solubility tests (Figure 6c) showed that all membranes treated by Zn(acac)2

280

became largely insoluble in DMAc while untreated membranes dissolved easily. This fact

281

provides a strong evidence of metal coordination cross-linking in Zn2+ treated membranes.

282

Such strong interaction enhanced the Tg from 388 oC for untreated membranes to 406 oC

283

for 6FDA-BI/20% ZIF-8 (0.007 Zn2+), as summarized in Table 1. In addition, the

284

remarkably enhanced thermal stability as indicated by higher Td over untreated one could

285

also be a sign of enhanced MOF-polymer interaction induced by Zn2+ chelating with

286

imidazole units. In a control experiment, pure polymers were treated in a similar manner

287

using Zn2+ solution concentration of 0.007 g/mL. Pure 6FDA-BI with Zn2+

288

functionalization easily dissolved in DMAc (Figure S4). This fact indicates that the

289

majority Zn2+ crosslinking occurs between ZIF-8 and polymers not within polymer itself

290

as proposed in Figure 6d.

291

In subsequent research, we will add Zn(acac)2 directly into membrane casting

292

solution to study Zn2+ functionalization effects on membrane performance. We propose

293

that this casting method will create much high metal coordination crosslinking degree. In

294

preliminary experiments, we found that the membrane of 6FDA-BI-20% ZIF-8

295

incorporated with Zn(acac)2 during casting step cannot dissolve in DMAc.

296

Next, XPS and FTIR were performed to detect in depth chemical interactions

297

occurred in these membranes. The chemical states of Zn and N in these membranes were

298

characterized by XPS. Figure 6 shows the Zn and N spectra of the 6FDA-BI/20% ZIF-8

299

and Zn2+ treated ones. Compared to untreated 6FDA-BI/20% ZIF-8, XPS spectrum of N

300

1s in Zn2+ treated samples did not show distinctive changes. The Zn 2p1/2 and Zn 2p3/2

301

core levels in Zn modified membranes has 0.5 ev red shift, which ascribes to the 16

302

chelation of Zn with C-N moieties. Moreover, the Zn2+ intensity increased significantly,

303

demonstrating successfully integration of Zn2+ in the membranes. One or two Zn2+

304

interacts with imidazole groups in 6FDA-BI and ZIF-8 to form mono bridged complex

305

[34] and metal-coordination crosslinking network (Figure 6d). The similar behavior has

306

been previously reported with ZIF-7 and ZIF-11 embedded in polybenzimidazole where

307

metal sites interact with imidazole units in polymers [29, 47].

308

IR spectrum in Figure 7 demonstrated that the absorption bands at 1143 cm-1and

309

1376 cm-1 corresponding to C-N stretching vibrations in imidazole rings [42] shifted to

310

lower wavenumber upon Zn2+ treatment. The intensity of bands at 695 cm-1 and 756 cm-1

311

associated with ZIF-8 reduced. All these clues suggest the weakening of C-N bonds in

312

imidazole units which is caused by the coordination of nitrogen atom to the Zn2+ [48]. No

313

clear evidence of new bonds formation between fillers and polymers was found from IR

314

spectra but the chemical interaction between two phases was clearly visible. To sum up,

315

all these results support the postulation that ZIF-8 could efficiently interact with the

316

imidazole units in 6FDA-BI via Zn2+ post treatment with the formation of Zn-N

317

crosslinking network.

(b)

(a)

1016

318

6FDA-BI/20% ZIF-8 6FDA-BI/20% ZIF-8 (0.007g/ml Zn2+)

6FDA-BI/20% ZIF-8 (0.007g/mL Zn2+)

1044

1026

1036

1046

1056

394

Binding energy (eV)

399.8

N 1s

399.8

Intensity (a.u.)

Zn 2p1/2 1043.5

Zn 2p3/2

1020.9

Intensity (a.u.)

1020.4

6FDA-BI/20% ZIF-8

396

398

400

402

Binding energy (eV)

17

404

319 Figure 6 (a) Zn 2p1/2 XPS spectra and 2p3/2; (b) N1s XPS spectra of Zn2+ modified

321

MMMs; (c) comparison of solubility tests of Zn2+ modified MMMs in solvent DMAc, 1.

322

20% MMM, 2. 0.004 g/ml Zn2+, 3. 0.007 g/ml Zn2+, 4. 0.01 g/ml Zn2+; (d) schematic of

323

metal-N coordination crosslinking in Zn2+ modified membranes.

Absorbance (a.u.)

320

1143, C-N *

1376, C-N *

6FDA-BI/20% ZIF-8(Zn2+)

6FDA-BI/20% ZIF-8

6FDA-BI

600

800

1000

1200

1400

Wavenumber (cm-1)

1600

324 325

Figure 7 Comparison of IR spectra of MMMs and Zn2+ modified MMMs

18

326 327

3.3 Gas transport properties of MMMs The nanocrystals of ZIF-8 with particle size of ~80 nm were dispersed into 6FDA-

328

BI solutions in various loading of 10 to 30 wt. % to prepare MMMs. Pure gas permeation

329

experiments were carried out at pressure of 4 bar and 35 oC based on time-lag technique.

330

A series of gases H2, N2, CH4, and CO2 were tested in sequence.

331

The permeation data in Table 2 revealed that gas permeabilities of all membranes

332

decreased with increasing gas molecule size in an order of H2 (2.89 Å) > CO2 (3.30 Å) >

333

N2 (3.60 Å) > CH4 (3.80 Å). Thus, all membranes exhibit strongly molecular sieving

334

properties. Pure polymer membranes had H2 and CO2 permeabilities of 33.4 and 9 Barrer

335

with decent H2/CH4 and CO2/CH4 selectivities (278.2, 75.3). For MMMs, the gas

336

permeabilities increased significantly upon incorporation of ZIF-8 as clearly shown in

337

Figure 8. The H2 and CO2 gas permeabilities increased from 33.4 and 9 Barrer for pure

338

polymer up to 174.8 and 50.9 Barrer, respectively, when ZIF-8 content increased from 0

339

to 30 wt. %. The improvement of gas permeability is expected as the ZIF-8 inclusion

340

raised the free volume of hybrid membranes as reflected by reduced membrane density

341

(Table 1). The free volume increments majorly come from the cumulative porosity of the

342

ZIF-8 and the possible voids created at the interfacial region. As shown in Figure 8, the

343

gas selectivities for H2/CH4, CO2/CH4 and CO2/N2 in MMMs gradually reduced with

344

increasing ZIF-8 content. Among them, the selectivity of CO2/N2 is less influenced by

345

increased ZIF-8 loading compared to H2/CH4 and CO2/CH4. This is likely due to much

346

better intrinsic selective adsorption properties of ZIF-8 for CO2/N2 over H2/CH4 and

347

CO2/CH4 [49].

19

300

P (Barrer)

250 H2 CO2 P(H2)/P(CH4) P(CO2)/P(CH4) P(CO2)/P(N2)

120 80

200 150 100

40

50

30

w

t.%

t.% 25

w

t.% w 20

w 10

w 0

348

t.%

0

t.%

0

Ideal Selectivity

160

349

Figure 8 Pure gas separation performance of MMMs with different loadings of ZIF-8 at

350

35 °C and 4 bar.

351

Figure 9 demonstrates gas permeability (P), diffusivity (D) and solubility (S)

352

changes in these membranes as a function of gas kinetic diameter and critical temperature.

353

The corresponding gas diffusivity and solubility data are summarized in Table S2. The

354

gas permeabilities of various gases followed similar trends with gas diffusivities, which

355

are dependent on gas kinetic diameters. As the loading of ZIF-8 increased, the diffusivity

356

of all gases increased along with the enhancement of diffusivity selectivity, mainly

357

contributing to enhanced gas permeability in MMMs. When ZIF-8 loading is ≥ 25 wt. %,

358

the gas diffusivities increased abruptly with a sharp reduction in diffusivity selectivity,

359

indicating the appearance of non-selective voids in membranes. Gas solubilities in all

360

membranes increased in an order of CO2 > CH4 > N2, which correlates well with critical

361

temperature of gas molecules CO2 (304.19 K) > CH4 (190.9 K) > N2 (126.3 K) (Figure

362

9c). The CO2/CH4 and CO2/N2 sorption selectivity reduced with the ZIF-8 incorporation. 20

363

As a result, gas selectivities of hybrid membranes dropped with increasing ZIF-8 loading

364

although no obvious defects were found in hybrid membranes with low ZIF-8 content.

160

P (Barrer)

N2

CO2

H2

CH4

(b)

120

350

CO2

80 40

CH4

N2

6FDA-BI 6FDA-BI/10% ZIF-8 6FDA-BI/20% ZIF-8 6FDA-BI/25% ZIF-8 6FDA-BI/30% ZIF-8

300

6FDA-BI 6FDA-BI/10% ZIF-8 6FDA-BI/20% ZIF-8 6FDA-BI/25% ZIF-8 6FDA-BI/30% ZIF-8

D´1010 (cm2/s)

(a)

250 200 150 100 50

0 2.8

0 3.0

3.2

3.4

3.6

3.8

3.3

Kinetic diameter (Å)

365

S [cm3(STP)/(cm3. cmHg)]

(c) 0.25

3.4

3.5

3.6

3.7

3.8

Kinetic diameter (Å) CH4

N2

CO2

6FDA-BI 6FDA-BI/10% ZIF-8 6FDA-BI/20% ZIF-8 6FDA-BI/25% ZIF-8 6FDA-BI/30% ZIF-8

0.20 0.15 0.10 0.05 0.00 110

150

190

230

270

310

Critical temperature (K)

366 367

Figure 9 P (a), D (b) and S (c) of gases with various kinetic diameters for MMMs. D and

368

S for H2 were not included as time lag is less than 4 s.

369

Table 2 Pure gas permeabilities and ideal selectivities of pure 6FDA-BI and 6FDA-

370

BI/ZIF-8 MMMs at 35 °C and 4 bar. Permeability (Barrer)

Ideal Selectivity

Membrane H2

CO2

CH4

N2

CO2/CH4

CO2/N2

H2/CH4

6FDA-BI

33.4±0.97

9.0±0.61

0.12±0.021

0.32±0.024

75.3±8.5

28.2±0.5

278.2±38.6

10% ZIF-8

45.7±1.89

11.8±0.51

0.19±1.5E-04

0.44±0.020

62.3±4.1

26.9±0.3

240.3±15.2

20% ZIF-8

78.5±5.84

20.3±2.03

0.35±0.033

0.78±0.076

57.9±0.02

25.9±0.2

223.9±16.7

21

25% ZIF-8

79.4±12.3

25.4±0.91

0.56±0.036

0.99±0.055

45.6±4.3

25.7±0.6

142.6±29.5

30% ZIF-8

174

50.9

1.24

2.38

41.1

21.4

140.9

371

The experimental data is compared with gas permeation data predicted by Maxwell

372

model. The intrinsic gas permeabilities of H2, CO2, N2 and CH4 for ZIF-8 based on

373

Song’s work [24] are 5411, 1192, 466, 430 Barrer, respectively, and the density of ZIF-8

374

is 0.95 g/cm3. As clearly seen in Figure 10, when ZIF-8 volume fraction is lower than 27

375

vol. %, gas permeabilities predicted by model matched well with experimental data with

376

deviation less than 10%, implying good compatibility between ZIF-8 and polymers. At

377

extra high MOF loading over 27 vol. %, the experimental permeabilities were

378

distinctively higher than predicted values, as the defective structure in MMMs occurred

379

with high fillers addition mentioned above. The inefficient packing of polymer chains at

380

high ZIF-8 loading caused the formation of defective voids in the membrane, thus non-

381

ideal deviation from the predicted trend.

(a) 180 140

2.4

H2

Maxwell predicted H2

120

N2 CH4 Maxwell predicted N2 Maxwell predicted CH4

2.0

Maxwell predicted CO2

P (Barrer)

P (Barrer)

(b)

H2 CO2

160

100 80 60

CO2

1.6 1.2 0.8

40

0.4

20 0

382

0

10

20

30

0.0

40

0

10

20

30

40

MOF vol. %

MOF vol. %

383

Figure 10 Comparison of experimental gas permeabilities with the predicted data by

384

Maxwell model. Lines: model prediction, symbols: experimental data.

22

385 386

3.4 CO2 sorption behavior of MMMs In order to further understand the role of ZIF-8 on the improved gas separation

387

performance, direct CO2 sorption behavior was investigated in representative 6FDA-

388

BI/20% ZIF-8 membranes and Zn2+ treated membranes using pressure decay methods. As

389

expected, the addition of ZIF-8 into 6FDA-BI distinctively boosted CO2 sorption capacity

390

in hybrid membranes (Figure 11). For example, at pressure of 4 bar, CO2 sorption

391

amounts increased from 17.07 cm3 (STP)/cm3 for pure membranes to 34.69 cm3

392

(STP)/cm3 for 6FDA-BI/20% ZIF-8. More importantly, the weighted average CO2

393

sorption capacities based on pristine 6FDA-BI membranes and ZIF-8 crystals from 0 to 7

394

bar is consistent with the measured CO2 sorption capacities of MMMs with 20 wt. %

395

ZIF-8 loading. It suggests no pore blockage of ZIF-8 by polymer chains. The increasing

396

CO2 sorption amount is beneficial for improved gas permeability of CO2 in the MMMs.

ZIF-8 6FDA-BI 6FDA-BI/20% ZIF-8 6FDA-BI/20% ZIF-8(0.007g/ml Zn2+) Calculated

C (cm3(STP)/cm3)

250 200 150 100 50 0

0

2

4

6

8

10

12

14

16

18

Pressure (bar)

397 398

Figure 11 Isothermal CO2 sorption curves of ZIF-8, 6FDA-BI and MMMs.

23

399 400 401

3.5 Optimized permeation performance of MMMs through Zn2+ post modification The positive effects from the addition of Zn2+ on gas separation performance of

402

6FDA-BI/ 20% ZIF-8 MMMs may be found from several aspects: (I) Zn2+ could

403

efficiently interact with imidazole groups in polymer and ZIF-8 to create metal-N

404

coordination complexes. Such complex could enhance gas selectivity; (II) the π

405

complexation reactions between Zn2+ and CO2 [34] greatly favor CO2 permeation, thereby

406

increasing CO2 permeability; (III) presence of extra Zn2+ from solution provides higher

407

exclusion of less permeable gas.

408

Figure 12 shows the trends of gas permeabilities and selectivities in Zn2+ modified

409

membranes. The permeation data is summarized in Table 3. A significant enhancement in

410

H2 and CO2 permeabilities was observed with increasing Zn2+ solution concentration from

411

0 to 0.007 g/mL. For the membranes treated by 0.007 g/mL Zn(acac)2 solution, H2 and

412

CO2 gas permeabilities increased by around 40% compared to untreated ones. The gas

413

selectivities of CO2/CH4 and H2/CH4 remained unchanged with values of around 56 and

414

225, respectively. In contrast, pure polymers were treated in a similar manner in Zn2+

415

solution concentration of 0.007 g/mL. The gas separation performance for pure polymer

416

membrane with the Zn2+ modification did not change significantly (Table 3).

417

The enhanced gas permeability is dominantly determined by the increased

418

diffusivity in the case of membrane 6FDA-BI/20% ZIF-8 (0.007 Zn2+ ) (Table S3).

419

Diffusivity increment may indicate partial facilitated transport mechanism [37]. With

420

further increase of Zn2+ solution concentration to 0.01 g/mL, the gas permeability slightly

421

reduced whereas the gas selectivity of CO2/CH4 increased from 57.9 to 70.2 and H2/CH4 24

422

selectivity increased from 223.9 to 318.3. The reduction of gas diffusivity caused

423

decreasing gas permeability, indicating strongly inhibited polymer chain mobility

424

induced by the formation of metal complex crosslinking network. The increase of gas

425

selectivity of CO2/CH4 is mainly contributed to increasing solubility selectivity, which

426

increases from 3.7 to 4.5 (Table S3). In the membrane treated by high concentration Zn2+

427

solution, π complexation reactions between Zn2+ and CO2 [34] likely occurs during gas

428

transport, thus improving CO2/CH4 selectivity. As D and S data for H2 is not available

429

due to short time lag for H2 permeation, the selectivities of D and S were not analyzed in

430

detail. Presumably, the enhancement of H2/CH4 selectivity with largest gas size

431

differences is majorly contributed by metal coordination crosslinking, which may

432

decrease polymer chain spacing. The presence of extra Zn2+ from solution provides

433

higher methane exclusion than small penetrants of H2, thereby increasing H2/CH4

434

selectivity. This need to be validated in future.

435

Aforementioned, the improved membrane thermal stability, increased Tg, and non-

436

dissolved Zn2+ functionalized MMMs in DMAc suggest Zn2+ functionalization enhanced

437

interaction of polymer-ZIF-8 whereas metal coordination crosslinking is insufficient to

438

completely eliminate the interfacial defects in hybrid membranes. As a result, diffusivity

439

selectivity of CO2/CH4 did not change much. The similar phenomenon is also seen in

440

previous studies [27, 51]. For instance, Jin et al.’s work [51] reported that interfacial

441

design by polydopamine coating of ZIF-8 enhanced phase compatibility with a slight

442

increase of ideal selectivity whereas diffusivity selectivity did not show an increasing

443

trend. In future, the addition of metal sources in the casting solution is expected to

444

improve the resultant membrane selectivity. 25

445

The significant enhancement in CO2 solubility is not seen in Zn2+ treated membranes

446

as shown by the direct sorption results (Figure 11) which does not meet our expectation.

447

It could be only a small amount of metal ion interacting with polymer chains. Thus the

448

enhancement effects of metal ions on CO2 adsorption is not seen from CO2 isothermal

449

sorption results. However, Zn2+ treated membranes indeed exhibit either enhanced gas

450

permeability or improved gas selectivities over untreated ones.

P (Barrer)

90

300

P(H2)/P(CH4) P(CO2)/P(CH4) P(CO2)/P(N2)

250

60

50 30

0

0

451 452

0.007

0.004

0.01

Ideal Selectivity

120

350

H2 CO2

0

Zn2+ concentration (g/ml)

Figure 12 Gas separation performance of Zn2+ treated 6FDA-BI/20% ZIF-8.

453 454 455 456 457 26

Table 3 Gas permeabilities of Zn2+ modified MMMs.

458

Permeability (Barrer)

Ideal Selectivity

Membrane H2

CO2

CH4

N2

CO2/CH4

CO2/N2

H2/CH4

6FDA-BI

33.4±0.97

9.0±0.61

0.12±0.02

0.32±0.02

75.3±8.5

28.2±0.5

278.2±38.6

P/ Zn2+

31.8

9.1

0.11

0.36

79.8

25.6

278.0

20% ZIF-8

78.5±5.84

20.3±2.03

0.35±0.03

0.78±0.08

57.9±0.02

25.9±0.2

223.9±16.7

Zn2+ (0.004) 88.2

22.8

0.38

0.86

60.3

26.4

233.4

Zn2+ (0.007) 110.1±9.2

27.4±2.49

0.49±0.05

1.10±0.09

56.1±0.89

25.1±0.2

225.2±5.4

Zn2+ (0.01)

15.9

0.23

0.60

70.2

26.7

318.3

72.3

459 460

3.6 Performance Comparison with Robeson upper bound

461

Gas separation performance of all membranes was compared with Robeson upper

462

bound for CO2/CH4 and H2/CH4 in Figure 13. Firstly, the addition of ZIF-8 into 6FDA-BI

463

led a stepwise increase of gas permeabilities. The separation performance of CO2/CH4

464

was gradually approaching 1991 upper bound with MOF loading increasing. Notably, all

465

Zn2+ modified membranes were located on the 1991 Robeson upper bound for CO2/CH4.

466

In the case of H2/CH4, gas separation performance of 6FDA-BI/20% ZIF-8 and all Zn2+

467

treated ones exceeds the 2008 Robeson upper bound. When compared to other polymer

468

based membranes and MMMs in literature [6, 50-51, 53-56], gas selectivities of

469

membranes developed in this work are among the highest value for H2/CH4 with

470

moderate H2 gas permeability (Table S4). Particularly, the membrane demonstrated the 27

471

comparable H2/CH4 separation capability with recently developed boron nitride

472

nanosheets/thermally arranged PI membranes [56], achieving exceptionally high

473

selectivity. To further improve gas permeability of hybrid membranes, enhancing the

474

intrinsic permeability of imidazole containing polyimide membranes and increasing ZIFs

475

loading could be efficient strategies to yield a membrane with both superior permeability

476

and selectivity, which is the scope of our future work.

(a)

CO2/CH4

2008

Zn2+ modified MMMs

102

7 0

1

1991

5 6 2 3

4

101 101

477

102

PCO2 (Barrer)

28

(b)

103 2008

Zn2+ modified MMMs 7

H2/CH4

1991

102

101 101

0

6FDA-BI/ZIF-8/Zn2+(0.007) vs Boron nitride 1%@PI

56 1 2 3

4

102

103

104

PH2 (Barrer)

478 479

Figure 13 CO2/CH4 and H2/CH4 separation performance of MMMs with upper bounds

480

defined in 1991 and 2008. 0-4 represent the membranes with MOF loading from 0 to 30

481

wt. % (blue square), 5-7 represent 6FDA-BI/20% ZIF-8 treated with different

482

concentrations of Zn2+ (0.004 g/ml, 0.007 g/ml, 0.01 g/ml) (red star), grey close circle

483

symbols represent data derived from references.

484 485

4. Conclusions The addition of ZIF-8 into imidazole containing 6FDA-BI yielded hybrid

486

membranes with good compatibility. The integration of Zn2+ provided much stronger

487

interaction between polymer and ZIFs, thereby improving thermal stability of membranes

488

and gas separation performance. The best performing membranes have H2 and CO2 gas

489

permeabilities of 110.1 and 27.4 Barrer, respectively, with high H2/CH4 and CO2/CH4 gas

490

selectivity of 223 and 56, respectively. The gas transport performance surpasses the

491

H2/CH4 upper bound of state of art membranes, positioning the results among highly

492

selective polymer based membranes. The results in this work evidence the important role 29

493

of metal-N complex in enhancing gas transport properties. The combination of structural

494

similarity in polymer-filler and metal ion induced crosslinking network could be utilized

495

to enhance MMMs performance for gas separations.

496

Supporting Information

497

Detailed N2 isotherm, TGA, DMA, gas permeation data and extra IR spectra peak

498

assignments are given in supporting information. These materials are available free of

499

charge via the internet.

500

ACKNOWLEDGEMENT

501

The authors gratefully acknowledge the financial support of the Nation Natural

502

Science Foundation of China (Grant No.21978321, U1510123), the Fund of China

503

University of Petroleum (Grant No. 2462015YJRC017). Dr. Li would like to thanks the

504

support of the Hundred Talents Program of the Shanxi Province

505

AUTHOR INFORMATION

506

Corresponding Authors

507

E-mail: [email protected], [email protected]

508 509

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Highlights: 1. Imidazole groups present in polyimide and ZIF-8 favored phase compatibility 2. The Zn2+ post modification on mixed matrix membranes enhanced polymer/MOF interaction 3. The membranes with exceptionally high H2/CH4 and CO2/CH4 gas selectivity of 224 and 58 4. The H2/CH4 separation performance well above the 2008 Robeson upper bound.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: