Controllable Growth of Zeolitic Imidazolate Framework ...Controllable Growth of Zeolitic Imidazolate Framework Composite Membranes for Gas Separation Ezzatollah Shamsaei M. Eng. (Chemical)

Controllable Growth of Zeolitic Imidazolate Framework Composite

Membranes for Gas Separation

Ezzatollah Shamsaei

M. Eng. (Chemical)

A thesis submitted for the degree of Doctor of Philosophy at

Monash University in 2016

Department of Chemical Engineering

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Copyright notice

© Ezzatollah Shamsaei (2016).

I certify that I have made all reasonable efforts to secure copyright permissions for third-party

content included in this thesis and have not knowingly added copyright content to my work

without the owner's permission.

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Dedicated to:

My parents, my sisters and brothers and my beloved wife whom I love the most because they

never lose faith in me and give me endless support and encouragement through my whole life.

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Abstract

As an emerging class of hybrid organic-inorganic nanoporous material, metal organic

frameworks (MOFs) with tunable pore size and chemistry are very attractive for integration into

membranes and thin films for gas separation applications. Zeolitic imidazolate frameworks (ZIFs),

a subfamily of MOFs, are known for their permanent porosity and exceptional chemical and

thermal stability. Among a number of available ZIF materials, ZIF-8 is particularly interesting

owing to its relative ease of synthesis as well as its great potential in separating small gas molecules.

However, the progress on the fabrication of ZIF-8 membranes with satisfactory gas separation

performance is very limited and there is no report of ZIF membranes being used in industrial scale

so far. Therefore, development of simple and more effective methods to fabricate high quality ZIF

molecular sieving membranes with high gas selectivity is still required. The new processing

approaches require the advantages of being rapid, reproducible, scalable, and economically and

environmentally viable while simultaneously producing high quality ZIF membranes.

The ultimate goal of this PhD research program is to address challenges that hinder the facile

synthesis of supported-ZIF membranes in a reproducible and scalable manner. In this thesis, three

new approaches are demonstrated to potentially address these challenges. First, a novel scalable

strategy of using vapor phase to chemically modify the polymer support for ZIF membrane

fabrication is developed. Such surface modification enabled fast formation of a continuous ZIF-8

ultrathin layer after only 3 minutes. The resulting ZIF-8 membranes exhibited exceptional H2

permeance as high as 2.05 ×10-6 mol m-2 s-1 Pa-1 with high H2/N2 and H2/CO2 selectivities (9.7 and

12.8, respectively). Next, based on the chemical vapor modification, a simple, effective, and

environmentally friendly method is described for the fabrication of high-quality ZIF-8 membranes

with controllable positioning on a polymer substrate in aqueous solution. The ZIF-8 membrane

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exhibited a propylene permeance of 1.50 × 10–8 mol m–2 s–1 Pa−1 and excellent selective

permeation properties; after post heat-treatment, the membrane showed ideal selectivities of

C3H6/C3H8 and H2/C3H8 as high as 27.8 and 2259, respectively. The new synthesis approach holds

promise for further development of the fabrication of high-quality polymer-supported ZIF

membranes for practical separation applications. Finally, a new concept for the use of one-

dimensional material (e.g. CNT) as nano-scaffolds and pseudo-seeds for the fabrication of

molecular sieving membranes supported on a porous substrate is introduced. To demonstrate the

potential for universal applicability of the proposed pseudo-seeding and nano-scaffolding method,

ZIF-8/CNTs membranes were prepared on both polymeric and inorganic substrates. At 25 °C and

1 bar, the ideal separation selectivities of H2/CO2, H2/N2, H2/CH4, H2/C3H6, and H2/C3H8 are 14,

18, 35, 52.4 and 950.1, respectively, with H2 permeance as high as 2.87 × 10−5 mol m−2 s−1 Pa−1.

This high hydrogen permselectivity combined with its mechanically reinforced structure shows

that the ZIF-8/CNT membrane is a promising candidate for hydrogen separation and purification.

Finally, it is anticipated that the novel strategies developed in this research may be further

developed for the fabrication of other MOF and zeolite molecular sieve membranes.

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Declaration

This thesis contains no material which has been accepted for the award of any other degree or

diploma at any university or equivalent institution and that, to the best of my knowledge and belief,

this thesis contains no material previously published or written by another person, except where

due reference is made in the text of the thesis

Signature:

Print Name: Ezzatollah Shamsaei

Date: 20/09/2016

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List of Publications

Journal Publications:

1. Shamsaei, E.; Lin, X.; Wan, L.; Tong, Y.; Wang, H. A One-Dimensional Material as a

Nano-Scaffold and a Pseudo-Seed for Facilitated Growth of Ultrathin, Mechanically

Reinforced Molecular Sieving Membranes. Chem Commun. 2016, 52 (95), 13764-13767.

2. Shamsaei, E.; Lin, X.; Low, Z.-X.; Abbasi, Z.; Hu, Y.; Liu, J. Z.; Wang, H. Aqueous Phase

Synthesis of ZIF-8 Membrane with Controllable Location on an Asymmetrically Porous

Polymer Substrate. ACS Appl. Mater. Interfaces 2016, 8, 6236-6244.

3. Shamsaei, E.; Low, Z.-X.; Lin, X.; Mayahi, A.; Liu, H.; Zhang, X.; Zhe Liu, J.; Wang, H.

Rapid Synthesis of Ultrathin, Defect-Free ZIF-8 Membranes Via Chemical Vapour

Modification of a Polymeric Support. Chem. Commun. 2015, 51, 11474-11477.

4. Shamsaei, E.; Low, Z.-X.; Lin, X.; Liu, Z.; Wang, H. Polysulfone and Its Quaternary

Phosphonium Derivative Composite Membranes with High Water Flux. Ind. Eng. Chem.

Res. 2015, 54, 3333-3340.

Oral and Poster Presentations:

1. E. Shamsaei, X. Lin, Z-X. Low, Z. Abbasi, Y. Hu, J.Z. Liu, H. Wang. Development of

Metal Organic Framework Composite Membranes for Improved Gas Separation Properties

(Received Best Oral Presentation Award by European Membrane Society (EMS)).

PERMEA & MELPRO Conference, Prague - Czech Republic, 15-19 May 2016.

2. E. Shamsaei, K. Wang, Z-X. Low, X. Lin, H. Wang. Enhanced water permeation through

nanoporous polymer membranes (Received Best Poster Presentation Award by

Membrane Society of Australasia (MSA)). 4th Membrane Society of Australasia Early

Career Researcher Symposium at Geelong – Victoria 19th to 21st of November 2014.

3. E. Shamsaei, Z-X. Low, H. Wang. High flux polysulfone-based ultrafiltration membrane.

Chemeca, Perth, Western Australia, Australia, 28th September to1st October 2014.

Other journal publications during enrolment:

1. Lin, X.; Kim, S.; Zhu, D.M; Shamsaei, E.; Xu, T.; Fang, X; Wang, H. Preparation of

porous diffusion dialysis membranes by functionalization of polysulfone for acid recovery.

J. Membr. Sci. 2016, 524, 557-564.

2. Abbasi, Z.; Shamsaei, E.; Leong, S.; Ladewig, B.; Zhang, X.; Wang, H. 2016,

Microporous Mesoporous Mater. 2016, 236, 28-37.

3. Zhao, Z.; Shamsaei, E.; Feng, Y.; Song, J.; Wang, H.; He, L. J. Membr. Sci. 2016, 518,

1–9.

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4. Lin, X.; Shamsaei, E.; Kong, B.; Liu, J. Z.; Zhao, D.; Xu, T.; Xie, Z.; Easton, C. D.; Wang,

H. J. Membr. Sci. 2016, 510, 437-446.

5. Lin, X.; Shamsaei, E.; Kong, B.; Liu, J. Z.; Hu, Y.; Xu, T.; Wang, H. J. Membr. Sci. 2016,

502, 76-83.

6. Wan, L.; Wei, J.; Liang, Y.; Hu, Y.; Chen, X.; Shamsaei, E.; Ou, R.; Zhang, X.; Wang, H.

RSC Advances. 2016; 6, 76575-81.

7. Lin, X.; Shamsaei, E.; Kong, B.; Liu, J. Z.; Xu, T.; Wang, H. J. Mater. Chem. A 2015, 3,

24000-24007.

8. Feng, Y.; Shamsaei, E.; Davies, C. H. J.; Wang, H. Mater. Chem. Phys.2015, 167, 209-

218.

9. Low, Z.-X.; Liu, Q.; Shamsaei, E.; Zhang, X.; Wang, H. Membranes 2015, 5, 136-149.

10. Moslehyani, A.; Mobaraki, M.; Ismail, A. F.; Matsuura, T.; Hashemifard, S.A., Othman,

M.H.D., Mayahi, A., DashtArzhandi, M.R., Soheilmoghaddam, M.; Shamsaei, E. React

Funct Polym. 2015, 95, 80-87.

11. Mayahi, A.; Ilbeygi, H.; Ismail, A. F.; Jaafar, J.; Daud, W. R. W.; Emadzadeh, D.;

Shamsaei, E.; Martin, D. J. Chem. Technol. Biotechnol. 2015, 90, 641-647.

12. Shamsaei, E.; Nasef, M. M.; H. Saidi; Yahaya, A. H. Radiochim. Acta 2014,102, 351-362.

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Acknowledgment

I would like to express my gratitude to my supervisor, Professor Huanting Wang for his great

support and guidance throughout my Ph.D. study.

I would also like to thank Dr Zhe Liu, my associate supervisor, Assoc Prof Xiwang Zhang,

Prof Jianfeng Yao and Dr Kun Wang for their help. Thanks to the members in our group including

Dr Xiaocheng Lin, Dr Ze-Xian Low, Dr Yi Feng, Dr Dongbo Yu, Dr Jing Wei, Dr Soo Kwan

Leong, Dr Seungju Kim, Li Wan, Ranwen Ou, Xiaofang Chen, Yan Liang, Yaoxin Hu, Kang Liu,

Dr Kha Tu, Zahra Abbasi, Dr Huacheng Zhang, and Prof Yuping Tong for their kind help.

Appreciation also goes to, Dr Meng Na, Huiyuan Liu for their continuous support and

companionship during my study. To my friends in the department, Dr Tahereh Hosseini, Shahrouz

Taranejoo, Soroush Shakiba, and Sajjad Asadi who encouraged me and supported me during the

PhD journey. I also wish to express my gratitude to the staffs in the Department of Chemical

Engineering, especially Kim Phu, Jill Crisfield and Lilyanne Price for their help during my study.

Special thanks to Sally El Meragawi and Joanne Tanner for proof reading my thesis and helping

me with my English.

I would like to acknowledge the financial support from Monash University. Without this

funding the research would not have been possible.

I would like to thank my beloved mother and father. Thanks to my dear sisters and brothers

to support me and believe me, without you I couldn’t achieve any success in my life. Finally, I

would like to thank my beautiful wife, Parisa, my best friend and best wife ever. You have

supported me and encouraged me during all difficult moments of my work. Without you and your

encouragement I couldn’t survive these past three and half years.

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Contents

ABSTRACT .............................................................................................................................................................. III

DECLARATION ........................................................................................................................................................ V

LIST OF PUBLICATIONS ..................................................................................................................................... VI

ACKNOWLEDGMENT ....................................................................................................................................... VIII

CONTENTS .............................................................................................................................................................. IX

LIST OF FIGURES .................................................................................................................................................. XI

LIST OF TABLES ................................................................................................................................................... XV

NOMENCLATURE .............................................................................................................................................. XVI

INTRODUCTION ................................................................................................................................ 1

BACKGROUND AND CHALLENGES .......................................................................................................... 1 RESEARCH AIMS ..................................................................................................................................... 6 THESIS STRUCTURE AND CHAPTER OUTLINE ........................................................................................... 7 REFERENCES ........................................................................................................................................... 9

LITERATURE REVIEW .................................................................................................................. 13

OVERVIEW ............................................................................................................................................ 13 POLYMERIC MEMBRANES ..................................................................................................................... 14 INORGANIC MEMBRANES ..................................................................................................................... 18 METAL ORGANIC FRAMEWORKS (MOFS) ............................................................................................. 22 2.4.1 MOF Materials and Fabrication ................................................................................................ 24 2.4.2 Synthesis of MOF-based membranes ......................................................................................... 30

2.4.2.1 Supported MOF membranes ..................................................................................................... 33 2.4.2.2 MOF-based mixed-matrix membrane ....................................................................................... 44

CONCLUSION AND PERSPECTIVES ......................................................................................................... 47 REFERENCES ......................................................................................................................................... 49

RAPID SYNTHESIS OF ULTRATHIN, DEFECT-FREE ZIF-8 MEMBRANES VIA

CHEMICAL VAPOUR MODIFICATION OF POLYMERIC SUPPORT ......................................................... 62

OVERVIEW ............................................................................................................................................ 62 INTRODUCTION ..................................................................................................................................... 62 EXPERIMENTAL .................................................................................................................................... 65 3.3.1 Materials .................................................................................................................................... 65 3.3.2 Synthesis of BPPO membrane and its EDA-vapour modification .............................................. 65 3.3.3 Growth of ZIF-8 Thin Film on modified BPPO Supports .......................................................... 67 3.3.4 Pure water flux and molecular weight cut off (MWCO) measurements ..................................... 67 3.3.5 Gas permeation experiments ...................................................................................................... 68 3.3.6 Characterization ........................................................................................................................ 69 RESULTS AND DISCUSSION ................................................................................................................... 70 3.4.1 Membrane support ..................................................................................................................... 71 3.4.2 Fast in Situ Seeding .................................................................................................................... 74

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3.4.3 Supported ZIF-8 membrane ....................................................................................................... 76 3.4.4 Single gas performance .............................................................................................................. 80 SUMMARY ............................................................................................................................................ 86 REFERENCES ......................................................................................................................................... 87

AQUEOUS PHASE SYNTHESIS OF ZIF-8 MEMBRANE WITH CONTROLLABLE

LOCATION ON AN ASYMMETRICALLY POROUS POLYMER SUBSTRATE .......................................... 94

OVERVIEW ............................................................................................................................................ 94 INTRODUCTION ..................................................................................................................................... 95 MATERIALS AND METHODS .................................................................................................................. 98 4.3.1 Chemicals ................................................................................................................................... 98 4.3.2 Sample preparation .................................................................................................................... 98 4.3.3 Characterization ...................................................................................................................... 100 4.3.4 Single gas permeation test ........................................................................................................ 101 4.3.5 Measurements of the support pore size .................................................................................... 102 RESULTS AND DISCUSSION ................................................................................................................. 103 4.4.1 Membrane support ................................................................................................................... 103 4.4.2 Supported ZIF-8 membrane ..................................................................................................... 107 4.4.3 Membrane prepared by conventional contra-diffusion ............................................................ 111 4.4.4 Single gas performance ............................................................................................................ 112 4.4.5 Effects of activation temperature on the ZIF-8 membranes ..................................................... 121 SUMMARY .......................................................................................................................................... 126 REFERENCES ....................................................................................................................................... 127

ONE-DIMENSIONAL MATERIAL AS NANO-SCAFFOLD AND PSEUDO-SEED FOR

FACILITATED GROWTH OF ULTRATHIN, MECHANICALLY REINFORCED MOLECULAR

SIEVING MEMBRANES ....................................................................................................................................... 133

OVERVIEW .......................................................................................................................................... 133 INTRODUCTION ................................................................................................................................... 134 MATERIALS AND METHODS ................................................................................................................ 137 5.3.1 Chemicals ................................................................................................................................. 137 5.3.2 Polydopamine modification of CNTs........................................................................................ 137 5.3.3 Preparation of ZIF-8/CNTs membrane on porous AAO disk ................................................... 138 5.3.4 Characterization ...................................................................................................................... 138 5.3.5 Gas permeation experiments .................................................................................................... 139 RESULTS AND DISCUSSION ................................................................................................................. 139 5.4.1 PDA-coated CNTs .................................................................................................................... 139 5.4.2 ZIF-8/CNT membrane .............................................................................................................. 141 5.4.3 CNTs coverage level ................................................................................................................. 144 5.4.4 Mechanical and structural stability of the ZIF-8/CNT membranes ......................................... 146 5.4.5 Synthesis time ........................................................................................................................... 148 5.4.6 Single gas performance ............................................................................................................ 149 5.4.7 Universal applicability ............................................................................................................. 154 SUMMARY .......................................................................................................................................... 156 REFERENCES ....................................................................................................................................... 157

CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK ................................. 162

CONCLUSIONS .................................................................................................................................... 162 RECOMMENDATIONS FOR FUTURE WORK ........................................................................................... 164

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List of figures

Figure 1-1. Olefin/paraffin experimental upper bound based on pure gas permeation data. Symbols:

(□) 100 °C; (■) 50 °C; (•) 35 °C; (▴) 30 °C; (♦) 26 °C [7]. ......................................................... 2

Figure 2-1. Schematic representation of various types of membranes [4]. .................................. 14

Figure 2-2. Relationship between the slope (n) of the upper bound and the difference between the

kinetic diameters of the gas pairs [12]. ......................................................................................... 17

Figure 2-3. Loss of selectivity occurred in polymeric membranes with increasing partial pressures

of CO2 [13]. Open points, pure gas. Closed points, mixed gas. .................................................... 18

Figure 2-4. a) Chemical structure of zeolite, b) Primary building unit of zeolite structure [28]. . 20

Figure 2-5. Silicon atoms are positioned at the intersections and linked by lines. (a) Sodalite cage;

(b) zeolite A, the sodalite cages are connected to each other by double 4-membered rings and form

an α-cage indicated by circle; (c) zeolite Y, the sodalite cages are linked by double 6-membered

rings and organized as in the diamond framework [27]. .............................................................. 21

Figure 2-6. Comparison of the effective pore sizes of various zeolites and the kinetic diameters of

common gas molecules [35]. ........................................................................................................ 23

Figure 2-7. Crystalline structure for three commonly used MOFs for separations. ..................... 24

Figure 2-8. Section of the crystal packing diagram of ZIF-8 [57]. ............................................... 25

Figure 2-9. Wire-frame model and ball-and-stick model of the crystal structure of (A) ZIF-L and

(B) ZIF-8 [67]. .............................................................................................................................. 26

Figure 2-10. SEM images of (a) ZIF-L nanoflakes, (b) ZIF-8 nanoparticles [67]. ...................... 26

Figure 2-11. Demonstration of UiO-66: (a) secondary building units (SBUs), (b) BDC ligand; (c)

crystal model, (d) a simplified form [74]; (e) SEM images of UiO-66 powders [73]. ................. 28

Figure 2-12. Schematic diagram of in-situ synthesis of pure UiO-66 membranes supported on

porous hollow fibers [73]. ............................................................................................................. 29

Figure 2-13. Fabrication methods for MOF-based membranes. ................................................... 30

Figure 2-14. Scheme of the fabrication methods for continuous MOF membranes [87]. ............ 31

Figure 2-15. (a) Diffusion cell for contra-diffusion preparation of ZIF-8 film and (b) the schematic

synthesis of ZIF-8 films on the sides of the nylon substrate by contra-diffusion of Hmim and Zn2+

through the porous nylon [56]....................................................................................................... 34

Figure 2-16. Synthesis illustration for the CuBTC@PSF membrane by in situ method and layer by

layer crystal deposition [99]. ......................................................................................................... 35

Figure 2-17. H2/CH4 (a) and H2/N2 (b) separation factors as a function of H2 permeance for ZIF-8

membranes in [100] as compared to those in the literature. ......................................................... 36

Figure 2-18. Schematic diagram of constituting layers of a supported-ZIF-8 membrane [102]. . 37

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Figure 2-19. SEM images of: a) the ZIF-90 seed-layer, b) top view and c, d) cross section views

of the polycrystalline ZIF-90 membrane after secondary growth [104]. ...................................... 38

Figure 2-20. a) Schematic representation of oriented synthesis of HKUST-1 crystals controlled by

surface functionalization [75]. b) A simplified model of anchoring a typical MOF-5 building unit

to a carboxylic acid-terminated self-assembled organic monolayers (SAM) [76]. ...................... 39

Figure 2-21. Scheme of the morphology and chemical structure of a MOF/PVDF membrane

prepared by a non-activation method [96]. ................................................................................... 40

Figure 2-22. (a) Optical images of the (1) pristine PAN hollow fiber, (2) hydrolyzed PAN hollow

fiber and (3) Cu3 (BTC)2–PAN hollow fiber membrane. (b) ,(c) proposed involved chemical

reactions of the PAN hollow fiber [97]......................................................................................... 41

Figure 2-23. Schematic illustration of (a) Zeolite Membranes on polymer−zeolite MM hollow fiber

supports [111] and (b) trinity MOF membranes preparation [112]. ............................................. 42

Figure 2-24. Scheme of pressure-assisted preparation of HKUST-1 layer on the surface of PVDF

hollow fibers substrate [113]. ....................................................................................................... 43

Figure 2-25. Schematic diagram of a typical MOF membrane and MOF based MMM. ............. 45

Figure 3-1(a) Schematic diagram of UF membrane fabrication via phase inversion, (b)

Experimental setup of vapour-phase EDA modification process. ................................................ 66

Figure 3-2. Schematic diagram of the preparation of BPPO polymer-supported ZIF-8 membrane.

....................................................................................................................................................... 70

Figure 3-3. FTIR ATR spectra of untreated BPPO support, BPPO modified with EDA-vapour for

16 h (MBPPO-16), MBPPO-16 supported ZIF-8 layer (ZIF-8-MBPPO-16), and synthesized ZIF-

8 powder........................................................................................................................................ 72

Figure 3-4. TGA curves (under air flow) of (1) untreated BPPO support, (2) MBPPO-16, (3) ZIF-

8-BPPO, (4) ZIF-8-MBPPO-16, and (5) synthesized ZIF-8 powder............................................ 73

Figure 3-5. Pure water flux and pore size of BPPO membranes as a function of exposure time to

EDA vapour. ................................................................................................................................. 74

Figure 3-6. XRD pattern (a), TEM image and SAED pattern (inset) (b) and nitrogen sorption

isotherm (c) of as-synthesized ZIF-8 nanocrystals. ...................................................................... 75

Figure 3-7. SEM images of (a) cross-section and (b) surface of the ZIF-8-MBPPO-16. ............. 76

Figure 3-8. EDS line scan across ZIF-8-MBPPO-16 cross-section for the zinc atoms. ............... 77

Figure 3-9. SEM images of the (a) surface and (b) cross section of the BPPO support, (c) surface

of the ZIF-8-BPPO, (d) surface of the ZIF-8-MBPPO-4, (e) surface of the ZIF-8-MBPPO-10, (f)

surface and (g, h) cross-section of the ZIF-8-MBPPO-16. ........................................................... 79

Figure 3-10. XRD patterns of the membranes and simulated ZIF-8 powder. .............................. 80

Figure 3-11. FTIR ATR spectra of (1) untreated BPPO support, BPPO modified with EDA-vapour

for (2) 4 h (MBPPO-4), (3) 10 h (MBPPO-10), (4) 16 h (MBPPO-16), (5) ZIF-8-MBPPO-16, (6)

synthesized ZIF-8 powder............................................................................................................. 80

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Figure 3-12. Single gas permeances of ZIF-8-MBPPO-16 as a function of kinetic diameter of gas

molecule. ....................................................................................................................................... 82

Figure 4-1. Digital photograph of a home-made contra-diffusion cell. ........................................ 99

Figure 4-2. Schematic diagram of gas permeation set-up. .......................................................... 101

Figure 4-3. Schematic diagram of the preparation of a BPPO polymer supported ZIF-8 membrane

using chemical vapour modification and subsequent contra diffusion synthesis. ...................... 103

Figure 4-4. FTIR ATR spectra of the untreated BPPO support, BPPO modified with EDA-vapor

(BPPO-EDA), BPPO-EDA supported ZIF-8 layer (ZIF-8-BPPO-EDA), and ZIF-8 powder. ... 104

Figure 4-5. Thermogravimetric analysis (under air flow) of untreated BPPO and BPPO-EDA

substrates. .................................................................................................................................... 105

Figure 4-6. SEM images of untreated BPPO (a), vapour-phase-EDA-modified BPPO (BPPO-EDA)

(b), ZIF-8@BPPO-EDA grown for 60 min (c), 90 min (d), and 120 min (e, f). ........................ 106

Figure 4-7. SEM images of ZIF-8 membranes grown for 2 h (a, b, c), 4 h (d, e, f), 6 h (g, h, i) via

conventional contra-diffusion method using untreated BPPO substrate. ................................... 107

Figure 4-8. XRD patterns of ZIF-8@BPPO-EDA membranes as a function of growth time. ... 108

Figure 4-9. SEM images of ZIF-8@BPPO-EDA grown for 120 min at different magnifications.

Cross-sectional view: (a, b, c); Top view: (d, e, f). ..................................................................... 109

Figure 4-10. EDS line scan across ZIF-8@BPPO-EDA-120 cross-section for the zinc atoms.. 110

Figure 4-11. SEM images of cross-section of ZIF-8 membrane synthesized by contra-diffusion (at

the reaction time of 2 h) after modification of the BPPO via immersion of the polymer in 60%

EDA aqueous solution at room temperature for 3 h. .................................................................. 112

Figure 4-12. Single gas permeances (a) and ideal selectivities (b) as a function of kinetic diameter

of gas molecules of the ZIF-8 membranes grown for 120 min and activated at 120 °C

(ZIF8@BPPO-EDA-120-120) and at 150 °C (ZIF8@BPPO-EDA-120-150). .......................... 113

Figure 4-13. Comparison of C3H6 permeability and C3H6/C3H8 selectivity of the membranes in the

present work with previously reported membranes. Closed and open symbols indicate separation

data obtained from single and binary gas permeation analysis, respectively. Hexagon: inorganic

supported ZIF-8 membranes [10]; pentagon: ZIF-8 mixed matrix membranes [15]; triangle:

polymer membranes [42]; circle: carbon membranes [43]; star: polymer supported ZIF-8

membranes in this study.............................................................................................................. 115

Figure 4-14. SEM images of ZIF-8@BPPO-EDA-120 membranes after activation at 150 ºC (a, b,

c) and 200 ºC (d, e, f). (a, d, e) top view and (b, c, f) cross-sectional view. ............................... 121

Figure 4-15. Room-temperature propylene/propane permeation properties of ZIF-8 membranes

grown for 120 min (ZIF-8@BPPO-EDA-120) as a function of activation temperatures. .......... 122

Figure 4-16. Heat-induced cross-linking of BPPO substrate. ..................................................... 123

Figure 4-17. FTIR ATR spectra of the BPPO support, BPPO support after being heated at 150 ºC

(BPPO-150) for 2h under air, EDA-vapour-modified BPPO (BPPO-EDA), EDA-vapour-modified

BPPO after being heated at 150 ºC (BPPO-EDA-150) for 2h under air..................................... 124

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Figure 4-18. (a) FTIR spectra and (b) XRD patterns of ZIF-8@BPPO-EDA membranes (grown

for 120 min) as a function of activation temperature (°C). ......................................................... 125

Figure 5-1. Schematic illustration of the preparation process of ZIF-8/CNT membrane through

deposition of modified CNTs on the support, followed by a contra-diffusion synthesis. .......... 139

Figure 5-2. (a) Photos of water dispersibility and the corresponding TEM images of CNTs (I) and

polydopamine-coated CNTs (II), XRD patterns (b) and FTIR spectrum (c) of CNTs (I) and

polydopamine-coated CNTs (II), (f) schematic illustration of the coated CNT and the chemical

structure of polydopamine. ......................................................................................................... 141

Figure 5-3. SEM (a, b) and optical (c) images of pristine (a, c1-c3) and modified (b, c4-c6) CNTs-

deposited on AAO. Detailed experimental: deposited pristine CNTs from (c1, c4) 1 mL (c2, c5) 3

mL and (c3, c6) 6 mL mother solution. Pristine CNTs mother solution: 10 mg CNTs in 200 mL

DDI water.................................................................................................................................... 142

Figure 5-4. SEM images of modified CNTs-deposited on AAO (a), ZIF-8/CNT membranes grown

for 5 min (b), 30 min (c), and for 60 min (d, e), and XRD patterns of ZIF-8 membranes as a function

of synthesis time (f). ................................................................................................................... 143

Figure 5-5. SEM images of ZIF-8 film prepared on (a, b, c) bare AAO and on (d, e, f) AAO

deposited with pristine CNTs. Zinc side: b, e; Hmim side: c, f. Synthesis time: 60 min. .......... 144

Figure 5-6. SEM images of bare AAO (a), as-prepared samples with insufficient (b, c), and excess

(d, e, f) deposition of modified CNTs on AAO before (b, d) and after (c, e, f) contra-diffusion

synthesis. The inset in f is a high magnification cross-sectional view. Detailed experimental:

deposited CNTs from (b) 1 mL and (d) 6 mL mother solution respectively. ............................. 145

Figure 5-7. SEM images (a, b) and XRD pattern (c) of the ZIF-8/CNTs membrane after sonication

for 2 h. ......................................................................................................................................... 146

Figure 5-8. Optical image of the free standing ZIF-8/CNT hybrid membrane floated in the sodium

hydroxide solution (a) and SEM images of Cross-sectional view (b, c) and surface edge (d) of

the corresponding free standing membrane. ............................................................................... 147

Figure 5-9. Single gas permeances of several gases through the ZIF-8/CNT membrane at 20 °C

and different feed pressures. ....................................................................................................... 149

Figure 5-10. FTIR ATR spectra of the AAO support deposited with modified CNTs, the ZIF-

8/CNTs membranes as a function of synthesis time, and ZIF-8 powder. ................................... 150


and 1 bar as a function of the kinetic diameter. The inset shows the ideal gas selectivity for H2

over other gases........................................................................................................................... 151

Figure 5-12. SEM images of (a, b) bare PES, (c, d) PES with deposited modified CNTs (6 ml of

mother solution) and (e, f) as prepared membrane after contra-diffusion synthesis (1 h). ......... 155

Figure 5-13. (a) XRD patterns and (b) FTIR spectra of pristine PES, supported ZIF-8/CNT

membrane and ZIF-8................................................................................................................... 156

xv

List of tables

Table 2-1. Primary membrane companies and polymeric membrane materials [9]. .................... 16

Table 3-1. Single gas permeances and ideal selectivities for the composite membranes at 25 ⁰C

and 1 bar. ....................................................................................................................................... 81

Table 3-2. Single gas permeances and ideal selectivities at 25 ⁰C and 1 bar of 3 tested ZIF-8-

MBPPO-16 membranes showing the reproducibility of membrane synthesis and testing. .......... 82

Table 3-3. Comparison of gas permeation properties (H2 permeance, H2/N2 and H2/CO2 selectivity)

of ZIF-8 membranes on inorganic and polymeric supports reported in recent literature. ............ 84

Table 4-1. Single gas permeances and ideal selectivities for the ZIF-8@BPPO-EDA-x-y (x:

crystallization time (min); y: heat treatment temperature (⁰C)) composite membranes at 25 ⁰C and

1 bar. ........................................................................................................................................... 113

Table 4-2. Comparison of gas permeation properties of the ZIF-8@BPPO-EDA composite

membrane in this work with other ZIF-8 membranes in the literature. ...................................... 116

Table 5-1. Single gas permeances and ideal selectivities for the ZIF-8@CNTs-t (t: crystallization

time (min), hybrid membranes at 20 ⁰C and 1 bar. E shows the sample prepared with an excess

use of CNTs (6ml of the mother solution). ................................................................................. 152

Table 5-2. Comparison of the synthesis parameters (time and temperature) and gas permeation

properties of the ZIF-8/CNTs hybrid membrane in this work with other ZIF-8 membranes from

the recent literature. .................................................................................................................... 153

Table 5-3. Single gas permeances and ideal selectivities of three ZIF-8/CNT-60 membrane

samples tested at 25 ⁰C and 1 bar. .............................................................................................. 154

xvi

Nomenclature

AAO: Anodized aluminum oxide

ATR: Attenuated total reflectance

BDC: 1, 4-benzenedicarboxylic acid

BET: Brunauer-Emmett-Teller

BPPO: Bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide)

CNF: Carbon nanofiber

CNT: Carbon nanotubes

CuBDC: Copper 1, 4-benzenedicarboxylate

DDI: Double-deionized water

DMF: Dimethylformamide

EDA: Ethylenediamine

EDX: Energy-dispersive X-ray spectroscopy

FTIR: Fourier transform infrared

HKUST: Hong Kong University of Science and Technology

Hmim: 2-methylimidazole

IRMOF: Isoreticular metal–organic framework

LbL: Layer-by-layer

LPE: Liquid-phase epitaxy

xvii

LTA: Linde type A

MIL: Matériaux de l'Institut Lavoisier

MMM: Mixed matrix membranes

MOF: Metal organic frameworks

MWCNT: Multiwall carbon nanotubes

MWCO: Molecular weight cut-offs

NMP: 1-methyl-2-pyrrolidinone

PAN: Polyacrylonitrile

PBI: Polybenzimidazole

PCP: Porous coordination polymers

Pd: Palladium

PDA: Polydopamine

PDMS: Polydimethylsiloxane

PEG: Polyethylene glycol

PEI: Polyetherimide

PES: Poly (ether sulfone)

PIM: Polymers of intrinsic microporosity

PSf: Polysulfone

PVDF: Polyvinylidene fluoride

xviii

RO: Reverse osmosis

RT: Room temperature

RTD: Rapid thermal deposition

SAED: Selected-area electron diffraction

SAM: Self-assembled monolayer

SBU: Secondary building units

SEM: Scanning electron microscopy

SOD: Sodalite

TEM: Transmission electron microscopy

TGA: Thermogravimetric analyses

TMP: Transmembrane pressure

TOC: Total organic carbon

TPIM-1: Triptycene-based ladder polymer

Tris: Tris (hydroxymethyl)aminomethane

UF: Ultrafiltration

UiO: University of Oslo

XRD: X-ray diffraction

ZIF: Zeolitic imidazolate frameworks

1D: One-dimensional

Chapter 1 Introduction

1

Introduction

Background and Challenges

Membranes have found an essential role in chemical technology and are used in a wide variety

of applications such as water purification and desalination, wastewater treatment, food and dairy,

medical and chemical production. The main characteristic of a membrane that is utilized is its

ability to regulate the permeation rate of a chemical species across the membrane. In separation

applications, the aim is to allow one component of a mixture to freely pass through the membrane,

while inhibiting permeation of other components. Therefore, a membrane is principally a thin

interface that controls the permeation of chemical species in contact with it [1]. In particular,

membrane-based separation methods are gaining increasing importance for energy efficient gas

separations and other molecular separations [2]. Currently, almost all of gas separation membranes

used commercially are polymeric. Polymer membranes can be readily processed into a number of

forms and modules (e.g. hollow fibers or anisotropic spiral-wound) and still maintain their

separation performance. However, polymer membranes have a number of limitations such as short

lifespans, low thermal and chemical stabilities [3], and a separation efficiency that is restricted by

the well-known trade-off relationship between selectivity and permeability of the polymer-based

membranes. More selective polymers are generally less permeable and vice versa [4, 5]. Based on

literature data, Robeson quantified the gas separation performance of a number of polymers for

the separation of O2/N2 and CO2/CH4 and constructed the so-called “upper bound” trade-off line

between permeability and selectivity [6]. Similar results were observed for the C3H6/C3H8

separation by Koros and coauthors [7]. The limitations of polymers in terms of the upper bound


2

line has also been addressed by Freeman [8]. The logical extension of these earlier studies lies in

the development of new membrane materials that go beyond this trade-off limit.

Figure 1-1. Olefin/paraffin experimental upper bound based on pure gas permeation data.

Symbols: (□) 100 °C; (■) 50 °C; (•) 35 °C; (▴) 30 °C; (♦) 26 °C [7].

Ceramic membranes and zeolites are of particular interest for gas separation due to their

advantages over polymers which include high thermal and chemical stabilities and promising

separation efficiency. However, these materials are more expensive, more complex in preparation,

less reproducible, and have lower mechanical resistance as compared to polymer membranes [9].

Carbon membranes have also been considered as promising non-traditional materials for

separation applications under high temperatures and harsh chemical environments. Then again,

carbon-based membranes have poor mechanical properties and the scale-up to industrial size is

problematic [10].

As emerging hybrid class of organic-inorganic nanoporous materials, metal organic

frameworks (MOFs) are very attractive option for integration into membranes and thin films for a

wide range of industrial applications including membrane-based gas separations. Their well-

defined pore structures can be rationally designed by the interplay of their building blocks, i.e.


3

metal ions and organic linkers, and their pore chemistry is readily tunable via a variety of methods

[11]. MOF synthesis is less energy intensive as compared to zeolites. For example, unlike zeolites,

fabrication of most MOFs is conducted at relatively low temperature and pressure conditions,

without the use of structure-directing agents, which eliminates the calcination step required in the

synthesis of zeolites. Furthermore, MOFs possess larger pore volumes and lower density than

zeolites, which make them more attractive for the preparation of composite membranes [12].

Several reports have discussed the synthesis and applications of MOF materials [13, 14];

however, relatively few reports exist addressing MOF membranes. This inequality is due to

challenges associated with the fabrication of MOF membranes. As the MOF layer does not hold

adequate mechanical strength as a self-supporting membrane, it is essential to be prepared on a

mechanically strong, porous support. Fabrication of supported-MOF membranes is accordingly

challenging due to the difficulty of directing nucleation and crystal growth onto the surface and

poor MOF-to-substrate adhesion.

MOF films/membranes are prepared by growing a thin MOF layer on a substrate via two

general techniques, i.e. in situ (direct) growth and secondary (seeded) growth. The in situ synthesis

is a simple method that allows for simultaneous nucleation, deposition and crystal growth by

immersing the substrate in a MOF precursor solution. Seeded growth involves the synthesis and

anchoring of seed crystals on substrates, followed by their crystallization. Although secondary

growth requires additional steps, seeded growth has been noted to more effectively induce

controlled MOF growth on the porous support.

The first MOF film was prepared in 2005 by selectively anchoring MOF-5 particles at the

carboxylate-terminated areas of the self-assembled monolayer (SAM) on the surface of a dense

substrate [15]. Similarly, an oriented growth of MOFs on SAM-functionalized metal substrates


4

has been reported [16]. In 2007, fabrication of microporous MOF film was investigated on porous

substrates [17]. It was found that the support surface properties and the synthesis route were

essential factors that affect the MOF film density. The growth of much denser MOF membranes

on a porous substrate was then achieved through a seeded method [18]. However, none of these

pioneering studies reported gas separation results, indicating it is critical yet

still challenging to fabricate a compact polycrystalline MOF membrane. Unlike MOF films which

are typically used for applications such as sensors, MOF membranes utilized in gas separation

applications require well-structured grain boundary and absence of pinholes or defects, minimizing

the nonselective intercrystalline diffusion. Very intimate contact between the MOF layer and the

support is also critical as to provide the sufficient mechanical stability when operating under harsh

environment in commercial applications. In 2009, the first MOF-based membrane was reported

for gas separation. Continuous MOF-5 membrane was prepared on a modified alumina disk by a

solvothermal synthesis. Shortly thereafter, continuous membranes of ZIF-7 and ZIF-8 were

prepared on alumina and titanium substrates, respectively, by a microwave-assisted solvothermal

preparation method. The absence of macroscopic defects in these reports were confirmed by

pressure dependent gas permeation measurements. These reports demonstrated the feasibility of

constructing supported MOF membranes for gas separations and other molecular discriminations.

To date, a number of porous organic (e. g PVDF, PES, and nylon) and inorganic (alumina, silica,

and porous ZnO) materials have been used as supports for the fabrication of MOF membranes.

Several innovative methods have been reported on the synthesis of MOF thin films and membranes

including contra-diffusion method [19], liquid-phase epitaxy [20], rapid thermal deposition (RTD)

[21], layer-by-layer deposition of crystals [22], and interfacial microfluidic processing [23]. Even

though much progress on the construction of supported-MOF membranes has been achieved, there is


5

still much research to be conducted for the facile fabrication of high quality MOF membranes before

robust synthesis strategies can be developed. It is also noted that there are no reports of supported-

MOF membranes being employed as separating membranes on the industrial scale so far.

Zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic frameworks (MOFs), are

porous crystalline hybrid materials consisting of imidazolate ligands (Im) bridging tetrahedral

metal ions (e.g., Zn, Co) [24]. They closely resemble the topologies of zeolites, due to the M-Im-

M (M = Zn, Co) bond angle of 145°, which is close to the T-O-T (T = Al, Si, P) angle (140−170°)

in zeolites [14, 25]. ZIFs show properties that combine the attractive features of both MOFs and

zeolites such as tunable pore size and chemistry, large internal surface area, and relatively good

thermal and chemical stability [26, 27]. These properties make ZIFs excellent candidates for the

fabrication of molecular sieving membranes for gas separation [28, 29]. Thus, fabrication of

supported-ZIF membranes has drawn a lot of attention in recent years [11].

Among a number of available ZIF materials, ZIF-8 is particularly interesting owing to its

relatively facile synthesis procedure as well as its great potential in separating small gas molecules.

ZIF-8 is made of zinc ions bridged by 2-methylimidazolate ligands, forming the sodalite-related

structure with a large cavity (11.6 Å) and small aperture size (3.4 Å). Several fabrication methods

for ZIF membranes have been published which can be categorized into two groups: direct growth

and secondary growth [30, 31]. Nagaraju et al. [22] and Cacho-Bailo et al. [32] grew ZIF-8 on a

porous polysulfone substrate using direct growth. However, although the direct synthesis is a

simple method that allows for simultaneous nucleation, deposition and crystal growth, it is not

very effective in preparing continuous ZIF membranes due to limited heterogeneous nucleation

sites on the substrate [33]. Alternatively, Ge at al. [34] used secondary seeded growth to fabricate

a continuous ZIF-8 film on an asymmetrically porous poly(ether sulfone) substrate. Secondary


6

seeded growth has been shown to effectively induce controlled ZIF growth on polymer supports,

but the resulting ZIF layer often suffers from weak adhesion to the support, leading to membrane

delamination. Furthermore, only a limited number of studies reported ZIF-8 membranes with

satisfactory gas separation performances [31]. Therefore, development of facile and more effective

methods to fabricate high quality ZIF molecular sieving membranes with high gas selectivity is

still required. The new processing approaches require the advantages of being rapid, reproducible,

scalable, and economically and environmentally viable while simultaneously producing high

quality ZIF membranes.

Research Aims

The overall aim of this project is to develop novel methods for fabrication of ultrathin ZIF-8

molecular sieve membranes with high selectivity performances. The specific aims are summarised

as:

Growing ZIF-8 on polymer substrate to achieve high-quality membranes at low cost.

Developing a novel scalable strategy of using vapour phase to chemically modify the polymer

support for ZIF membrane fabrication.

Investigating the role of the modifier in the mechanism of the growth.

Developing a simple, effective, and environmentally friendly method for the fabrication of

high-quality ZIF-8 membrane with controllable placement on a polymer substrate in aqueous

solution.

Utilizing one-dimensional material as a nano-scaffold and pseudo-seed for the fabrication of

molecular sieving membranes supported on a porous substrate, inspired by the success of the


7

nano-scaffolding technique in tissue engineering where biocompatible nanofibers are used as

nano-scaffold to promote tissue growth and provide mechanical support [35].

Thesis structure and chapter outline

This thesis is organised into six sections; the overview of each chapter is summarized below.

Chapter 1 (Introduction) contains a brief background for the motivation of this thesis, the

research aims and an outline of the thesis structure.

In chapter 2 (literature review), a short introduction to the structure and chemistry of several

MOFs is provided. Then focus is refined to the currently pursued strategies of supported-MOF

membrane fabrication, associated challenges and reported approaches addressing these problems.

Chapter 3 provides detailed experimental procedures for the rapid synthesis of ultrathin,

defect-free ZIF-8 membranes via chemical vapour modification of a polymeric support. The results

of the experiments are presented, analysed and discussed. The gas permeation test results for the

membranes are also given.

This chapter has been published as a journal article: E. Shamsaei, Z.-X. Low, X. Lin, A.

Mayahi, H. Liu, X. Zhang, J. Z. Liu, H. Wang, Chem. Commun. 2015, 51, 11474.

Chapter 4 demonstrates a simple, scalable, and environmentally friendly route for controllable

fabrication of continuous, well-intergrown ZIF-8 on a flexible polymer substrate via contra-

diffusion in conjunction with chemical vapor modification of the polymer surface. The results of

experiments are presented, analyzed and discussed. The gas permeation test results for the

membranes are also given.


8

This chapter has been published as a journal article: E. Shamsaei, X. Lin, Z.-X. Low, Z. Abbasi,

Y. Hu, J. Z. Liu, H. Wang, ACS Appl. Mater. Interfaces 2016, 8, 6236.

Chapter 5 presents a new concept for using one-dimensional material as a nano-scaffold and

pseudo-seed for the fabrication of molecular sieving membranes supported on a porous substrate.

This chapter provides detailed experimental procedures to utilize one-dimensional (1D) materials

such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs) to form a porous nano-scaffold

and pseudo-seed layer on the porous substrate for facilitated growth of ultrathin ZIF membranes

with mechanically reinforced structures. The results of experiments are presented, analyzed and

discussed. The gas permeation test results for the membranes are also given.

This chapter has been submitted as a journal article: E. Shamsaei, X. Lin, L. Wan, Y. Tong,

H. Wang, Chem Commun. 2016, 52 (95), 13764-13767.

Chapter 6 summarizes the major findings of this thesis, and recommendations are presented

for the future work.


9

References

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[2] N.N. Li, A.G. Fane, W.W. Ho, T. Matsuura, Advanced membrane technology and applications,

John Wiley & Sons, 2011.

[3] S. Qiu, M. Xue, G. Zhu, Metal–organic framework membranes: from synthesis to separation

application, Chem. Soc. Rev., 43 (2014) 6116-6140.

[4] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes,

J. Membr. Sci., 62 (1991) 165-185.

[5] S.A. Stern, Polymers for gas separations: the next decade, J. Membr. Sci., 94 (1994) 1-65.

[6] L.M. Robeson, The upper bound revisited, J. Membr. Sci., 320 (2008) 390-400.

[7] R.L. Burns, W.J. Koros, Defining the challenges for C3H6/C3H8 separation using polymeric

membranes, J. Membr. Sci., 211 (2003) 299-309.

[8] B.D. Freeman, Basis of permeability/selectivity trade-off relations in polymeric gas separation

membranes, Macromolecules, 32 (1999) 375-380.

[9] H. Strathmann, L. Giorno, E. Drioli, Introduction to membrane science and technology, Wiley-

VCH Weinheim, 2011.

[10] S. Lagorsse, F. Magalhaes, A. Mendes, Carbon molecular sieve membranes: sorption, kinetic

and structural characterization, J. Membr. Sci., 241 (2004) 275-287.

[11] M. Shah, M.C. McCarthy, S. Sachdeva, A.K. Lee, H.-K. Jeong, Current status of metal–

organic framework membranes for gas separations: promises and challenges, Ind. Eng. Chem. Res.,

51 (2012) 2179-2199.

[12] H.B. Tanh Jeazet, C. Staudt, C. Janiak, Metal-organic frameworks in mixed-matrix

membranes for gas separation, Dalton Trans., 41 (2012) 14003-14027.

[13] H. Furukawa, K.E. Cordova, M. O’Keeffe, O.M. Yaghi, The Chemistry and Applications of

Metal-Organic Frameworks, Science, 341 (2013).


10

[14] K.S. Park, Z. Ni, A.P. Côté, J.Y. Choi, R. Huang, F.J. Uribe-Romo, H.K. Chae, M. O’Keeffe,

O.M. Yaghi, Exceptional chemical and thermal stability of zeolitic imidazolate frameworks, Proc.

Natl. Acad. Sci. U. S. A., 103 (2006) 10186-10191.

[15] S. Hermes, F. Schröder, R. Chelmowski, C. Wöll, R.A. Fischer, Selective nucleation and

growth of metal−organic open framework thin films on patterned COOH/CF3-terminated self-

assembled monolayers on Au(111), J. Am. Chem. Soc., 127 (2005) 13744-13745.

[16] E. Biemmi, C. Scherb, T. Bein, Oriented growth of the metal organic framework Cu3 (BTC)

2 (H2O)3.H2O tunable with functionalized self-assembled monolayers, J. Am. Chem. Soc., 129

(2007) 8054-8055.

[17] M. Arnold, P. Kortunov, D.J. Jones, Y. Nedellec, J. Kärger, J. Caro, Oriented crystallisation

on supports and anisotropic mass transport of the metal‐organic framework manganese formate,

Eur. J. Inorg. Chem., 2007 (2007) 60-64.

[18] J. Gascon, S. Aguado, F. Kapteijn, Manufacture of dense coatings of Cu3(BTC)2 (HKUST-1)

on α-alumina, Microporous Mesoporous Mater., 113 (2008) 132-138.

[19] J. Yao, D. Dong, D. Li, L. He, G. Xu, H. Wang, Contra-diffusion synthesis of ZIF-8 films on

a polymer substrate, Chem. Commun., 47 (2011) 2559-2561.

[20] O. Shekhah, R. Swaidan, Y. Belmabkhout, M. du Plessis, T. Jacobs, L.J. Barbour, I. Pinnau,

M. Eddaoudi, The liquid phase epitaxy approach for the successful construction of ultra-thin and

defect-free ZIF-8 membranes: pure and mixed gas transport study, Chem. Commun., 50 (2014)

2089-2092.

[21] M.N. Shah, M.A. Gonzalez, M.C. McCarthy, H.-K. Jeong, An unconventional rapid synthesis

of high performance metal–organic framework membranes, Langmuir, 29 (2013) 7896-7902.

[22] D. Nagaraju, D.G. Bhagat, R. Banerjee, U.K. Kharul, In situ growth of metal-organic

frameworks on a porous ultrafiltration membrane for gas separation, J. Mater. Chem. A, 1 (2013)

8828-8835.

[23] A.J. Brown, N.A. Brunelli, K. Eum, F. Rashidi, J. Johnson, W.J. Koros, C.W. Jones, S. Nair,

Interfacial microfluidic processing of metal-organic framework hollow fiber membranes, Science,

345 (2014) 72-75.


11

[24] O.M. Yaghi, M. O'Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Reticular

synthesis and the design of new materials, Nature, 423 (2003) 705-714.

[25] K. Yamamoto, Y. Nohara, Y. Domon, Y. Takahashi, Y. Sakata, J. Plévert, T. Tatsumi,

Organic−inorganic hybrid zeolites with framework organic groups, Chem. Mater., 17 (2005) 3913-

3920.

[26] B. Chen, Z. Yang, Y. Zhu, Y. Xia, Zeolitic imidazolate framework materials: recent progress

in synthesis and applications, J. Mater. Chem. A, 2 (2014) 16811-16831.

[27] N. Rangnekar, N. Mittal, B. Elyassi, J. Caro, M. Tsapatsis, Zeolite membranes–a review and

comparison with MOFs, Chem. Soc. Rev., 44 (2015) 7128-7154.

[28] J. Yao, H. Wang, Zeolitic imidazolate framework composite membranes and thin films:

synthesis and applications, Chem. Soc. Rev., 43 (2014) 4470-4493.

[29] V.M.A. Melgar, J. Kim, M.R. Othman, Zeolitic imidazolate framework membranes for gas

separation: A review of synthesis methods and gas separation performance, J. Ind. Eng. Chem., 28

(2015) 1-15.

[30] Y. Liu, N. Wang, J.H. Pan, F. Steinbach, J.r. Caro, In situ synthesis of MOF membranes on

ZnAl-CO3 LDH buffer layer-modified substrates, J. Am. Chem. Soc., 136 (2014) 14353-14356.

[31] H.T. Kwon, H.-K. Jeong, In situ synthesis of thin zeolitic–imidazolate framework ZIF-8

membranes exhibiting exceptionally high propylene/propane separation, J. Am. Chem. Soc., 135

(2013) 10763-10768.

[32] F. Cacho-Bailo, B. Seoane, C. Téllez, J. Coronas, ZIF-8 continuous membrane on porous

polysulfone for hydrogen separation, J. Membr. Sci., 464 (2014) 119-126.

[33] W. Li, Q. Meng, X. Li, C. Zhang, Z. Fan, G. Zhang, Non-activation ZnO array as a buffering

layer to fabricate strongly adhesive metal–organic framework/PVDF hollow fiber membranes,

Chem. Commun., 50 (2014) 9711-9713.

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12

[35] R.M.A. Domingues, S. Chiera, P. Gershovich, A. Motta, R.L. Reis, M.E. Gomes, Enhancing

the biomechanical performance of anisotropic nanofibrous scaffolds in tendon tissue engineering:

reinforcement with cellulose nanocrystals, Adv. Healthcare Mater., 5 (2016) 1364-1375.

Chapter 2 Literature Review

13

Literature Review

Overview

Membranes can be categorized according to different viewpoints. The simplest classification

is by nature, i.e. synthetic or biological membranes. Membrane in this thesis only refers to

synthetic membranes, excluding all biological structures. Based on the type of materials, synthetic

membranes can be divided into inorganic (e.g. ceramic, silica, metals and oxides) membranes,

organic (polymeric or liquid) and composite membranes. Polymeric membranes, owing to their

relative ease of processing, low cost and large area per volume, have been developed for a vast

variety of industrial applications like ultrafiltration (UF), reverse osmosis (RO) and gas separation.

Inorganic membranes compared to organic counterparts have the advantages of high chemical and

thermal stability, ease of cleaning when fouling, and well-defined, stable pore structure.

Composite membranes, herein defined as combination of different materials (e.g.

organic/inorganic and organic/organic) in the form of membranes, have been explored in the recent

years. Owing to the synergistic effects of the materials, composite membranes have the potential

for solving the trade-off between permeability and selectivity in conventional membranes [1, 2].

In this dissertation, polymeric and inorganic membranes were selected as the base substrate and

metal organic frameworks (MOFs) were selected as active layer to prepare composite membranes,

aiming to attain high performance membranes for gas separation.

Another classification of membranes is in terms of morphology or structure - for example

solid synthetic membranes can be divided into two types of membrane structures, symmetric and

asymmetric (anisotropic) membranes. Figure 2-1 schematically represents the various types of


14

membranes. A membrane structure may also fit more than one of these classes. For instance, a

membrane may have electrical charges and be microporous with an asymmetrical structure.

In terms of configurations, membranes can be categorized into hollow fiber membranes, spiral

membranes, tubular membranes and flat sheet membranes [3].

Figure 2-1. Schematic representation of various types of membranes [4].

Polymeric Membranes

Polymeric membranes, owing to their relative ease of processing, low cost and large area per

volume, have been developed for a vast variety of industrial applications like ultrafiltration (UF),

reverse osmosis (RO) and gas separation. By the influential discovery of Loeb and Sourirajan for

making highly permeable asymmetric RO membranes, membrane-based separation transformed

from a research laboratory to the industrial applications [5]. The technique involves precipitation


15

of a casting solution, consisting of one or more polymers in a proper solvent/solvents, by

immersion in a non-solvent (water) bath [6]. The membrane introduced by these workers was the

first integrally skinned asymmetric cellulose acetate [7]. The water flux of such membranes in RO

mode was 10 folds more than that of any other available membrane, which made RO applications

feasible. Subsequently, other viable processes for microfiltration, ultrafiltration and electro-

dialysis were all established. Early gas separation membranes were also adapted from the Loeb

and Sourirajan phase separation approach [8].

Although a large number of polymers have been studied to date as potential membrane

materials, only a handful of polymers, as presented in Table 2-1, have found actual application in

commercially viable processes [9, 10]. Properties such as cross-linking, functional groups, glass

transition temperature and degree of crystallinity result in membranes with different properties and

applicability. A robust separation membrane renders mechanical, thermal, and chemical stability,

good film-forming properties, absence of micro-defects and aging in selective layer (reduction of

flux in time). It stands to reason that the most of the technically used membranes (including support

membranes for hybrid membranes) are, and will continue to be, made from organic polymers. This

is mainly due to the inherently inexpensive and reproducible nature of polymer membrane

synthesis, as well as offering an efficient separation in a range of systems. From desalination to

biomedical applications, polymer membranes have demonstrated to be not only valuable and

efficient, but also, as in the case of membranes utilized for hemodialysis, outperform their

biological counterparts [11].


16

Table 2-1. Primary membrane companies and polymeric membrane materials [9].

In spite of all of the usefulness polymer membranes serve, some hurdles still exist for their

applications in a number of relevant gas separation. The most widely documented is known as

Robeson’s upper bound [12]. Due to the trade-off relationship between permeability and selectivity,

polymer membranes generally suffer an upper bound limitation. This upper bound suggests that

polymers with large segmental mobility and high sorption exhibit high permeability but a low

selectivity, and vice versa [11]. The severity of this effect was found to be proportionally

correlated to an increase in the difference between the kinetic diameters of the two permeating gas

molecules in the binary mixture. This is graphically illustrated in Figure 2-2 for binary system

from the list of H2, He, CH4, N2, O2, and CO2.


17

Figure 2-2. Relationship between the slope (n) of the upper bound and the difference between the

kinetic diameters of the gas pairs [12].

Robeson’s trade-off curve, however, is not the only prevention to the widespread polymer

membrane usage. Another major hindrance to the commercialization of polymeric membranes is

physical aging, which is instability of their permeability over time. Polymer aging, though still not

fully understood, it possibly occurs as a result of molecular rearrangements, depending on chain

mobility [13]. Plasticization is another major phenomenon that has extensive practical effects in

membrane based gas separation. Plasticization is also based upon molecular chain rearrangements

of a polymer in the presence of highly condensable molecules such as CO2. High pressure

performance data for polymer membranes are typically anticipated from low pressure experiments.

However, in the presence of highly condensable CO2, data achieved by the extrapolation to

relevant high pressure applications are uncertain [14]. The plasticizing effect of CO2 causes the

experimental deviation from theoretical extrapolations. The plasticizing effect results in a


18

significant loss in selectivity, as shown in Figure 2-3 the CO2/CH4 separation factor (mixed-gas

selectivity) in a triptycene-based ladder polymer (TPIM-1) dropped ∼60% in the mixed gas system

when CO2 partial pressure increases from 2 bar to 10 bar [13].

Figure 2-3. Loss of selectivity occurred in polymeric membranes with increasing partial

pressures of CO2 [13]. Open points, pure gas. Closed points, mixed gas.

Despite their inherent drawbacks, polymer membranes will undoubtedly be used to separate

gas mixtures, but in the longer term, novel membranes made from other materials such as ceramics

or organic/inorganic composites are likely to be required [10].

Inorganic Membranes

Since the prediction of the upper limit of performance (the upper bound) for polymer

membranes for gas separation in the early 1990s, only a few new materials could have separations

above this upper limit. The required performance level for most of practical applications is above

this limit of performance.


19

The efficiency of polymer membranes declines over time due to thermal instability, chemical

degradation, compaction and fouling. As a result of this inadequate thermal stability and

vulnerability to abrasion and also chemical attack, polymer membranes are not viable in separation

processes where high temperature reactive gases are encountered. All these weaknesses associated

with polymeric membranes triggered a shift of interest to inorganic membranes.

Owing to well-known chemical and thermal stabilities and usually having higher gas

permeability as compared to polymer membranes, inorganic membranes are increasingly being

investigated for the separation of gas mixtures [15-17].

Inorganic membranes can be basically divided into porous and dense (non-porous) membranes.

Ceramic membranes, such as silica, alumina, glass, and titania and porous metals, such as silver

and stainless steel are examples of commercially available porous inorganic membranes. These

membranes usually exhibits high gas flux, but low selectivity [18].

Dense inorganic membranes possess high selectivity in separating certain gases. For instance

palladium (Pd)-metal based membranes are highly hydrogen selective and their alloys have been

widely explored as potential membrane materials [19-22].

Recently, various techniques have been attempted to combine the high flux characteristics of

porous membranes with outstanding selectivities of the dense membranes by supporting dense

membranes on porous supports [23-25]. In addition, along with novel materials, other new

fabrication techniques are being devised and developed to make thinner selective layers and/or

narrower pore-sized membranes [26]. However, given the nature of this thesis, not all permutations

of the inorganic membranes are related and necessary to discuss. The main focus herein, therefore,

will be on microporous crystalline alumina-silicate membranes known as zeolites.


20

Zeolite membranes have open frameworks made of corner-shared TO4 (T = Al, Si, P)

tetrahedra with well-defined structures and channels of molecular dimensions. Figure 2-4

illustrates chemical structure and primary building unit of zeolite structure. The specific geometry

of zeolite pores enables discriminating guest molecules based on their size and/or shape. This

feature results in employing zeolites as molecular sieves [27]. The formation of tetrahedral metallic

(T-atoms)-oxygen clusters result in the defining feature of zeolites; their crystalline, rigid, and

porous framework. Zeolite frameworks can be generally classified into ultra large, large, medium,

and small pore materials. The number of tetrahedrons that makes the rings determine the pore size.

Ultra large pore frameworks have 14-, 18- or 20-membered rings, large structures have 12-

membered rings, medium pore zeolites have 10-membered rings, and small ones have 6-, 8-, or 9-

membered rings [29]. A representative illustration of some zeolite structures can be seen in

Figure 2-5.

To date, more than 40 unique natural types of zeolites from volcanic sources, and at least 150

different synthetic types are available. However, the commonly used zeolites for purification and

gas separation applications are limited to a handful of synthetic ones [30].

Figure 2-4. a) Chemical structure of zeolite, b) Primary building unit of zeolite structure [28].


21

Figure 2-5. Silicon atoms are positioned at the intersections and linked by lines. (a) Sodalite cage;

(b) zeolite A, the sodalite cages are connected to each other by double 4-membered rings and form

an α-cage indicated by circle; (c) zeolite Y, the sodalite cages are linked by double 6-membered

rings and organized as in the diamond framework [27].

Zeolitic membranes can be synthesized in pure phase (self-supported or symmetric

membranes) or on a variety of supports (supported or asymmetric membranes) [31]. Generally, the

preparation of self-supported symmetric zeolitic membranes introduces several drawbacks related

to the mechanical stability, dimension, and to the lack of uniform thickness. Therefore, as opposed

to most polymeric membranes, they are typically synthesized on either momentary support (to be

removed after the synthesis) such as Teflon sleeve and silver plate, or permanent (to form zeolite

composite membranes) such as active silica and porous ceramics [29].

In addition to their versatility, these microporous materials are also characterized by their

outstanding thermal and hydrothermal stability. Khodakov et al. [32] reported that zeolite types A,

X and Y were stable up to about 965°C, and more recently zeolite Y membranes was found to

remain unchanged in its pore volume following 4 hours in steam at 788 °C [33]. These outstanding

thermal and chemical stabilities are not expected when using polymer membranes and perfectly

demonstrate why these materials are necessary compliment to the polymer membranes. However,

zeolite membranes are not without their own set of challenges. As mentioned before, due to the


22

finite number of membered rings by which zeolitic structures can be formed, there are zeolites

with a limited and discontinuous array of available pore sizes. Therefore, zeolites of right pore

sizes may not be always available for separating gas mixtures of certain sizes [34]. The effective

pore sizes of some common zeolites and the kinetic diameters of several gases are shown in

Figure 2-6. Apart from the limited pore size and its chemical tailorability, the major disadvantage

of zeolite membranes is the high cost of production that hinders their wider applications in gas

separations. Zeolite membranes have also proven very difficult to make free of defects, and this is

more pronounced as the membrane size increases, while with the polymer membranes fabrication

is easy and reproducibility is very high.

Metal organic frameworks (MOFs), a rather new type of hybrid materials comprising of

inorganic and organic moieties in solid crystalline lattices, have the potential to overcome some of

the issues facing materials for membranes in gas separation.

Metal organic Frameworks (MOFs)

Metal–organic frameworks (MOFs) or porous coordination polymers (PCPs), are hybrid

porous crystalline materials formed by the coordination of metal ions/clusters and organic ligands.

MOF materials are relatively new and the systematic studies of their synthesis and applications

only initiated about two decades ago. With the pioneering work carried out by Hoskins [36, 37],

Zaworotko [38], Moore [39, 40], and Yaghi [41] in early 1990s, these materials were soon known

to be capable of integrating with designed structural, electrical, catalytic, optical, and magnetic

properties by an appropriate selection of metal ions and organic ligands [42]. Many scientists,

subsequently, joined this dynamic field and various synthetic approaches have been devised and

developed to form MOFs with different pore sizes, crystal structures and surface chemistry [43,

44].


23

Figure 2-6. Comparison of the effective pore sizes of various zeolites and the kinetic diameters of

common gas molecules [35].

Given the broad range of metal ions and linkers available, MOFs usually have a number of

unique features, such as exceptional large surface area, unusual adsorption affinities, tunable pore

sizes, structure diversity and facilely chemical tailorability [45]. These features make MOFs very

attractive for applications beyond the conventional areas of porous materials including molecule

storage and adsorption [46, 47], catalysis [48, 49], delivery [50] and separation [51, 52]. Similar

to the zeolites, MOFs can also be utilized for membrane constructions. However, due to the

relatively poor mechanical strength (brittleness) [53], MOFs are typically formed on the support

to attain continuous membranes or used as filler to obtain mixed matrix membranes (MMMs) [54,

55]. Different from the zeolitic membranes, where their pore structure can be only available (by

removing the surfactants) after a sintering process at elevated temperatures, the activation process

of MOF membranes is conducted at fairly lower temperatures. Furthermore, because of the organic


24

nature of the linkers in MOF materials they can interact with the polymer support and create a

stronger MOF-to-support adhesion, which in general enhances the selectivity [56].

2.4.1 MOF Materials and Fabrication

Over 20,000 different MOFs with typically high surface area values, range from 1000 to

10,000 m2g-1, have been reported and studied in the past decade [50]. However, like zeolites, only

a handful of MOF types are used in membrane separation. This is because a number of

considerations such as fabrication, activation, pore size, diffusivity, and solubility of the MOF

membranes should be taken into account when choosing a MOF type. In this section, we only

introduce MOFs that are widely studied for MOF-based membranes. The crystalline structure of

these MOFs are shown in Figure 2-7.

Figure 2-7. Crystalline structure for three commonly used MOFs for separations.

Yaghi׳s group has first reported the synthesis of zeolitic imidazolate frameworks (ZIFs) by

copolymerization of Zn (II) or Co (II) with imidazolate-type links [57]. Several synthesis methods,

afterwards, have been developed to obtain ZIF-8 in a large quantity, implementing simpler and

greener strategies [58-60]. They closely resemble the topologies of zeolites, due to the M-Im-M

(M = Zn, Co) bond angle of 145°, which is close to the T-O-T (T = Al, Si, P) angle in zeolites.

One of the most common and extensively studied member of ZIFs is ZIF-8, which consist of zinc

metal ions linked by 2-methylimidazole (Hmim). It has a sodalite (SOD) topology with a large


25

cavity of 11.6 Å (Figure 2-8) accessible through the theoretical small aperture (six-membered ring

window) of 3.4 Å (Figure 2-8) [57]. Owing to its permanent porosity, high thermal and chemical

stability, and excellent solvent resistance, ZIF-8 has attracted intensive interest in materials science

and chemistry [61]. Due to its small pore size (3.4 Å), ZIF-8 membranes gained considerable

interest for hydrogen separation. Zhang et al. [62] recently found that due to the flexibility of the

organic linker the effective pore size of ZIF-8 is in fact in the range of 4.0 to 4.2 Å, which is

considerably bigger than the XRD-derived value (3.4 Å). This opened up new opportunities for

ZIF-8 for separations that could not be economically accomplished by traditional microporous

materials such as synthetic zeolites. Very recently Li et al. [63] reported that propylene (∼4 Å)

diffuses in ZIF-8 100 times faster than propane (∼4.3 Å) due to its unexpected molecular sieving

effects and consequently Pan et al. [64] synthesized ZIF-8 membranes capable of separating

propylene from propane mixtures effectively. In addition, the surface structure and hydrophobic

pore of ZIF-8 likely repels water molecules and prevent the attack of ZnN4 units and

decomposition of the framework [57, 65], making ZIF-8 moisture resistant and a potential

candidate for separating humidified gas mixtures.

Figure 2-8. Section of the crystal packing diagram of ZIF-8 [57].

ZIF-L with a leaf-shaped morphology is comprised of zinc nitrate and Hmim, i.e. same

building blocks as ZIF-8 (Figure 2-9, Figure 2-10). It has a two-dimensional layered framework

with cushion-shaped cavities between layers with a dimension of 9.4 Å × 7.0 Å × 5.3 Å [66]. The


26

two-dimensional layers in ZIF-L are bridged by hydrogen bonds, unlike ZIF-8 in which the two

neighboring sod layers are bridged by Hmim (Figure 9). The layers are parallel to each other, 3.97

Å apart and stacked along the c direction [66] shows the crystal structures of ZIF-L and ZIF-8.

Figure 2-9. Wire-frame model and ball-and-stick model of the crystal structure of (A) ZIF-L and

(B) ZIF-8 [67].

Figure 2-10. SEM images of (a) ZIF-L nanoflakes, (b) ZIF-8 nanoparticles [67].

Attributed to the strong interactions between Hmim and CO2 molecules and the unique

cushion-shaped cavities, ZIF-L exhibits superior CO2 adsorption capacity (0.94 mmol g−1) and


27

CO2/CH4 adsorption selectivity (7.2) as compared with other ZIFs with large cages. ZIF-L crystals

with its unique cushion-shaped cavity and two-dimensional leaf-like morphology can be useful as

a kind of potential gas separation membrane/nanocomposite membrane materials [68, 69]. Wang

et al. [70] prepared ZIF-L membranes with two different orientations along their layered porous

structure, i.e., b-oriented and c-oriented membranes, and investigated their gas separation

properties. The c-oriented membrane due to the unique pore systems was found more favorable

than the b-oriented membrane respecting selectivity for H2/CO2 and H2/N2.

UiO-66 (UiO stands for University of Oslo), characterized by very high surface area and with

an unprecedented thermal stability, is the first generation of zirconium-based MOF that was

recently presented by Lillerud et al. [44] This robust, 3-dimensional porous structure is formed by

connecting hexanuclear zirconium clusters, as secondary building units (SBUs), with a commonly

available bridging ligand (1, 4-benzenedicarboxylic acid) (BDC) (Figure 2-11). Its outstanding

thermal and mechanical stability has been attributed to its high degree of strong coordination of

Zr–O metal moieties to the organic linkers [71]. Furthermore, revealed by repetitive

hydration/dehydration tests, porous UiO-66 switches reversibly between its dehydroxylated and

hydroxylated versions that further increase its thermal stability. The UiO-66 has a degradation

temperature over 500 °C and is resistance to several chemicals, and its crystalline structure remains

unchanged even after exposure to tons of external pressure [44]. Considering all these outstanding

features, therefore, it is expected that membranes constructed with UiO-66 would achieve various

promising applications including molecular separations. As estimated from crystallographic data,

the aperture size of UiO-66 is about 6.0 Å and thus capable of separating water molecules (∼2.8

Å) from hydrated ions (6.6−9.5 Å) [72]. Additionally, due to the specific interaction between


28

hydroxylated Zr6 cluster in the framework and CO2, UiO-66 can preferentially adsorb CO2 over

other gases [73].

Figure 2-11. Demonstration of UiO-66: (a) secondary building units (SBUs), (b) BDC ligand; (c)

crystal model, (d) a simplified form [74]; (e) SEM images of UiO-66 powders [73].

Although a number of pure MOF membranes, such as HKUST-1 [75], MOF-5 [76], ZIF-8

[77], MIL-53 [78], have been made and analyzed for gas separation, preparing continuous defect-

free UiO-66 membranes on porous supports by direct solvothermal synthesis is more challenging

due to the slow growth kinetics of this MOF [73]. UiO-66-type MOF thin films was initially grown

on flat silicon substrates using solvothermal approach [79] and on electrodes by electrochemical

deposition [80]. Very recently, Li and his co-workers successfully fabricated continuous UiO-66

polycrystalline membranes on alumina hollow fiber supports via an in situ solvothermal synthesis

approach (Figure 2-12) [73].

The obtained UiO-66 membrane exhibited high separation performance for multivalent ion

rejection (e.g., 98.0% for Mg2+, 99.3% for Al3+, and 86.3% for Ca2+) based on size-exclusion

mechanisms with moderate permeance of 0.14 L m−2 h−1 bar−1 and good permeability (0.28 L


29

m−2 h−1 bar−1 μm) in water desalination [73]. The membranes showed excellent recyclability due

to exceptional chemical stability of the UiO-66 material, which can be reasonably promising for

sea water desalination. Considering the wide-ranging of ligands used in UiO-66-type MOFs, it can

be expected that more pure UiO-66 membranes to be synthesized in the near future [74].

Figure 2-12. Schematic diagram of in-situ synthesis of pure UiO-66 membranes supported on

porous hollow fibers [73].

Besides the ZIF-8, ZIF-L and UiO-66, the MIL-101, MIL-53(Al), CAU-1, CAU-1-NH2, Bio-

MOF-14, Bio-MOF-13, Bio-MOF-1, ZIF-69, ZIF-22, ZIF-7, MOF-71, ZIF-78, ZIF-95, ZIF-90,

IRMOF-3, CuBTC etc. have also been utilized for constructing the MOF-based membranes. The

thermal and water stability of the employed MOFs are very essential for its applications. Some of

MOFs (MOF-5, IRMOF-3, CuBTC, etc.) are not stable in water or moisturized environments and

therefore these MOF membranes are not appropriate for use in humid or aqueous environments

[81-83]. Although MIL series of MOFs exhibit the high water and thermal stability, the properties

of the framework are changed upon the adsorption of water [84, 85]. ZIF series of MOFs, on the

other hand, usually display an excellent water and thermal stability and therefore these MOF


30

membranes have found a wide separation applications, such as organic solvent nanofiltration,

pervaporation, gas separation with steam, and dye water solution separation [86].

Figure 2-13. Fabrication methods for MOF-based membranes.

2.4.2 Synthesis of MOF-based membranes

Different methods have been reported for the fabrication of continuous MOF membranes on

various substrates. Figure 2-13 summarizes the fabrication methods of MOF-based membranes.

Considering the diffusion direction of the organic linker and metal ion during crystallization, the

synthesis procedures can be divided into three general categories: hydrothermal or solvothermal

method, contra-diffusion and interfacial synthesis method, and liquid phase epitaxy (layer by layer

or step-by-step) method (Figure 2-14).

In hydrothermal or solvothermal method, the substrate is directly immersed in a solution of

organic linker and metal ion (growth solution), so the same diffusion direction of metal ion and

linker is expected during the MOF film formation. This method can be subdivided into in situ

growth and secondary growth (seeded growth). In in situ growth the MOF layer is fabricated on

the substrate. Due to insufficient heterogeneous nucleation on substrate and poor adhesion between

the MOF layer and the substrate, support surface modification is usually introduced to create the

functional groups on the substrate that can be combined with metal ions or linkers. Hydrothermal

or solvothermal treatment can then be applied to form MOF layer on the modified substrate. In


31

contrast, in the secondary growth procedure, the substrate is initially coated by a seeded layer

before immersing into the precursor solution for hydrothermal or solvothermal treatment. The

seeded layer can be accomplished by either physical absorption (rubbing or dip coating) or by the

reaction between the substrate and precursor reactants. Compared with other methods,

hydrothermal or solvothermal method demonstrates to be applicable in a wider range. However, it

also has some drawbacks. The membrane synthesis process can be complex due to the employed

seeding or modification for improving the heterogeneous nucleation density on the substrates. In

addition, in this method a large amount of precursor solution is usually wasted due to the

homogenous formation of the MOF crystals in bulk solution and it is difficult to be scaled up.

Figure 2-14. Scheme of the fabrication methods for continuous MOF membranes [87].

In contra diffusion method, the substrate physically separates metal ions and ligand molecules

where they diffuse in opposite direction and crystallize at the interface. Since the crystallization

takes place at the interface the method is known as a self-limiting growth. This is because the


32

diffusion of precursor molecules is faster at the remaining defects than the already formed layer,

which is beneficial for a continuous, defect-free MOF film synthesis. This unique feature also

helps the formation of a film with uniform thickness. Owing to its simplicity and effectiveness in

the fabrication of ZIF films, this method has attracted many researchers attention, mainly in the

field of gas separation via molecular sieving. Yao et al. [56] prepared continuous ZIF-8 membrane

on a porous nylon with a moderate gas separation performance. Wasting the precursor solution is

less pronounced in this method as the metal ion and ligand solution is separated and the excess

solutions can be recycled. The method also can be employed in a membrane module directly and

thus displaying a superior scalability [88]. However, only a few MOF membranes, such as ZIF-7

[89], ZIF-71 [90], ZIF-8 [56], and Cu-BTC [91], have been fabricated by this method.

In MOF membrane synthesis by liquid phase epitaxy method [92, 93], the metal ion solution

and organic ligand solution are separately prepared. The substrate is immersed in one precursor

solution and after rinsing by the solvent, for removing the excess precursor, it is soaked into the

other precursor solution for MOF crystallization. Due to the non-consecutive crystallization,

precise control over the MOF film thickness is achievable, which is one of the outstanding

advantage of this method. Although the method also offers a simple, mild and controllable

synthesis approach, it is usually used for fabricating the MOF film rather than MOF membrane as

the continuous, defect-free MOF layer is difficult to achieve [87].

For MOF-based MMMs [94], the MOF particles are dispersed into polymer solution to make

the casting solution. The casting solution is then casted on the surfaces of a clean glass plate to

form the free standing MMMs or on the substrate to form the supported MMMs. The supported

MMMs usually possess higher permeability because they are usually thinner than their free

standing counterparts.


33

2.4.2.1 Supported MOF membranes

Owing to their relative ease of processing, low cost and large area per volume, polymeric

materials are attractive to be used as support in fabricating MOF-based membranes especially for

industrial separation application. Furthermore, due to the favourable chemical interaction between

the polymer and the MOF’s organic ligand, growth of MOF films on flexible polymeric substrates

is principally achievable. Hatton and co-workers for the first time demonstrated the feasibility of

growing MOF on polymer substrate [95]. They grew MIL-47 on the surface of polyacrylonitrile

(PAN) using microwave assisted solvothermal synthesis. There are several challenges in the

fabrication of polymer-supported MOF membranes. First, the polymer substrates should retain the

thermal stability and good solvent resistance as the substrates in the synthesis of MOF membranes

are usually soaked into the organic solvent, such as the methanol or DMF, at high temperature [96].

Second, the polymer materials usually swell in organic solvents and delamination or cracking of

MOF layers can occur as a results of the shrinking of the polymers if the MOF layers are formed

on the swelling polymer substrates [97]. Therefore, it is required to reduce the swelling of the

polymer substrates as much as possible. Third, due to the relative poor mechanical stabilities of

MOFs, the MOF layer tend to fall off from the elastic polymeric substrates when using it for

separation. This is due to the low stability and shrinkage of the polymer under hydrostatic

compression [96, 97]. However, several MOF membranes have been successfully fabricated on

polymer substrates. The polymer substrates used for the construction of MOF membranes can be

classified into flat polymer substrates and hollow fiber polymer substrates.

Yao et al. [56] prepared for the first time a polymer-supported ZIF-8 membrane on the porous

nylon membrane via contra-diffusion method (Figure 2-15). The employed nylon acts as the

substrate for growing ZIF-8 film. This strategy creates a concentration gradient of the ligand and


34

metal ion near the surface of the substrate, which promotes the heterogeneous formation of ZIF-8

on the substrate [98]. ZIF-8 film was formed on both side of the substrate with different

morphologies due to the different local molar ratio of Hmim and zinc nitrate. At the overall

Hmim/zinc ion molar ratio of 8, because of slower diffusion of organic ligand in nylon substrate

as compared to the metal ion, the Hmim/ zinc ion molar ratio at the zinc nitrate side of the substrate

was close to zero, resulting in large ZIF-8 crystals of 100–400 nm. While, on the ligand side the

local molar ratio of Hmim/zinc ion should be larger than the overall designed molar ratio of 8,

generating the ZIF-8 films made up of nanocrystals. ZIF-8 membranes prepared at room

temperature for 72 h showed a moderate ideal selectivity of hydrogen over nitrogen (4.3) with a

large hydrogen permeance of 19.7×10−7 mol m−2 s−1 Pa−1. The aqueous solution synthesis of ZIF-

8 films on a porous nylon substrate was further conducted with Hmim/zinc ion stoichiometric ratio

and the addition of ammonium hydroxide solution [98].

Figure 2-15. (a) Diffusion cell for contra-diffusion preparation of ZIF-8 film and (b) the schematic

synthesis of ZIF-8 films on the sides of the nylon substrate by contra-diffusion of Hmim and Zn2+

through the porous nylon [56].

Nagaraju et al. [99] grew ZIF-8 and CuBTC on a porous asymmetric ultrafiltration

polysulfone using in situ (direct) growth followed by the layer by layer deposition at room

temperature (Figure 2-16). Among the prepared ZIF-8@PSF and CuBTC@PSF membranes,


35

CuBTC@PSF permeance dropped drastically after 7 crystallization cycles with an enhanced

H2/CO2 and H2/C3H6 selectivity of around 7.2 and 5.7, respectively.

Figure 2-16. Synthesis illustration for the CuBTC@PSF membrane by in situ method and layer

by layer crystal deposition [99].

More recently, Cacho-Bailo et al. [100] grew a 35 μm thick ZIF-8 layer on commercial porous

polysulfone by an alternating synthesize procedure using solutions that produced nano- and

micrometer-sized particles. A high excess of sodium formate (NaCOOH) was used in the synthesis

solution as ligand-deprotonator. Adding different ligand/metal ratios enabled the formation of

nano- or micro-sized ZIF-8 crystals. The prepared ZIF-8 membranes were able to separate H2/CH4

and H2/N2 mixtures with high separation factors of 10.5 and 12.4, respectively. These values, as


36

shown in Figure 2-17, were the highest among the previously reported selectivities for ZIF-8

membranes on polymer substrates.

Figure 2-17. H2/CH4 (a) and H2/N2 (b) separation factors as a function of H2 permeance for ZIF-8

membranes in [100] as compared to those in the literature.

Although the in situ synthesis is a simple method that allows for simultaneous nucleation,

deposition and crystal growth, it is not very effective in preparing continuous ZIF membranes due

to limited heterogeneous nucleation sites on the substrate. Alternatively, Ge at al. [101] used

secondary seeded growth to fabricate a continuous ZIF-8 film on an asymmetrically porous

polyethersulfone substrate. The prepared 7.2 μm thick ZIF-8 layer showed good affinity with the

PES substrate and displayed molecular sieving separation. At 333 K and 150 KPa, the H2

permeance reached about 4 × 10–7 mol m–2 s–1 Pa–1 with the ideal separation factors of 10.8, 9.7,

10.7, and 9.9 for H2/O2, H2/Ar, H2/CH4, and H2/N2, respectively. Long-term hydrogen permeance

and H2/N2 separation performance show the stable permeability of the derived membranes. Long-

term hydrogen permeance as well as H2/N2 separation analysis showed the stable permeability for

the prepared ZIF-8 membranes.


37

Secondary seeded growth has been shown to effectively induce controlled ZIF growth on the

polymer support, but the resulting ZIF layer often suffers from weak adhesion to the support,

leading to membrane delamination. To enhance the ZIF-to-substrate adhesion strength, Barankova

et al. [102] employed a tailor-made porous polyetherimide/zinc oxide mixed-matrix as the

substrate. A non-woven polyester material was first coated with a mixture solution of

polyetherimide (PEI, Ultem® 1000) and zinc oxide nanoparticles (Figure 2-18). After a polishing

step, a combination of rubbing and dip-coating was used for placing ZIF-8 seeds on the PEI/ZnO

substrate. Since the zinc ions in ZnO can act as a secondary source of metal for ZIF-8 formation,

the PEI/ZnO mixed matrix was beneficiary for the fabrication of continuous ZIF-8 crystals with

enhanced adhesion. After 36 hours of secondary growth at 45 °C a dense ZIF-8 layer with a

thickness of ∼1.5 μm was prepared. The ZIF-8 membranes were dried by applying solvent

exchange technique to avoid formation of cracks. The achieved hydrogen permeation of 1.6 × 10−6

mol m−2 s−1 Pa−1 was about 4 times higher than that reported for polyethersulfone-supported ZIF-

8 membrane due to its thinner ZIF layer (1.5 μm versus 7.2 μm in [101]), but two folds less than

the one synthesized on a nylon substrateby the contra-diffusion method [56]. However, the

membrane showed a relatively high H2/C3H8 ideal selectivity of 22.4 [102].

Figure 2-18. Schematic diagram of constituting layers of a supported-ZIF-8 membrane [102].


38

Hollow fiber polymer membranes are advantageous over flat sheets due to their large

membrane surface area per unit volume [103]. These fibers are readily used to produce modules

with large membrane areas exceeding 1000 m2 m-3 of module volume [104].

Brown et al. [104] prepared a hollow fiber polymer-supported MOF membrane for the first

time by growing ZIF-90 on poly (amide-imide) Torlon hollow fiber in a technologically scalable

low-temperature synthesis procedure. Torlon was chosen as an appropriate polymeric substrate for

separation applications due to its high pressure endurance (up to 2000 psia) with no plasticization,

chemical resistance and ease of processing. A dense layer of uniform ZIF-90 seed crystals was

deposited on the surface of the hollow fiber by dip-coating technique. A continuous 5 μm thick

(Figure 2-19) polycrystalline ZIF-90 membranes were obtained at 65 °C for 4 h by secondary

growth. The membrane showed a CO2/CH4 selectivity of 1.5 which was interestingly lower than

the CO2/N2 selectivity of 3.5. This behavior was attributed to the fact that ZIF-90 and other ZIF

materials are known to have high CO2 adsorption capacities, and typically also adsorb CH4 more

strongly than N2.

Figure 2-19. SEM images of: a) the ZIF-90 seed-layer, b) top view and c, d) cross section views

of the polycrystalline ZIF-90 membrane after secondary growth [104].


39

Figure 2-20. a) Schematic representation of oriented synthesis of HKUST-1 crystals controlled by

surface functionalization [75]. b) A simplified model of anchoring a typical MOF-5 building unit

to a carboxylic acid-terminated self-assembled organic monolayers (SAM) [76].

To strengthen the adhesion between the support and MOF layer in inorganic-supported MOF

membranes, support surface modification with organic ligands [105, 106] or organosilane

molecules (Figure 2-20) have been widely applied [75, 76, 107, 108]. However, the process of the

surface modification with linkers is generally time-consuming [87]. In order to eliminate this step

and yet achieve a MOF membrane with excellent adhesion, Li et al. [96] reported a new method

applying a non-activation ZnO array as a buffering layer on polyvinylidene fluoride (PVDF)

hollow fibers (Figure 2-21). To produce the non-activation (NA) ZnO array, 2-methyl-imidazole,

zinc nitrate hexahydrate and sodium formate were employed. After the successful growth of NA-

ZnO array on the PVDF hollow fiber, the crack-free and uniform MOF (HKUST-1, ZIF-7 and

ZIF-8) membranes were fabricated by directly immersing the NA-ZnO/ PVDF into the MOF

precursor for crystallization. The prepared MOF/PVDF membranes possess excellent hollow fiber

structures and displayed exceptional hydrogen permselectivity. For the ZIF-7/PVDF membrane,

the ideal separation factors for H2/CO2 and H2/N2 were 18.43 and 20.27, respectively, with high

H2 permeance of 23.54 × 10−7 mol s−1 m−2 Pa−1. Due to these properties the NA-ZnO array is an

excellent buffering layer for fabricating MOF membranes, and the prepared ZIF/PVDF


40

membranes are potentially promising candidates for industrial hydrogen separation. Furthermore,

the strong MOF-to-substrate adhesion was confirmed by ultrasonic treatment test in which no

crystal was exfoliated from the hollow fiber after 60 min of sonication.

Figure 2-21. Scheme of the morphology and chemical structure of a MOF/PVDF membrane

prepared by a non-activation method [96].

Though the flexibility of polymeric substrates can be advantageous over brittle inorganic

materials, highly flexible polymers are not desirable for fabrication of polymer-supported MOF

membranes. In other words, to enhance the performance of a MOF membrane supported on a

polymeric substrate, the flexibility of the polymer substrate must be reduced to avoid MOF layer

cracking. Polyacrylonitrile (PAN) hollow fibers is a good choice to meet this requirement (low

flexibility) in addition to its commendable adhesion [97]. PAN with a chain of carbon connected

to each other is a hard polymer. More importantly, it has abundant nitrile groups (–CN) which can

crosslink together upon heating and dramatically increase the stiffness and mechanical and

chemical stability of the polymer [109, 110]. Li et al. prepared a continuous well intergrown ZIF-

8 and Cu3 (BTC)2 membrane on PAN hollow fibers by solvothermal treatment. Dehydrogenation,

cyclization and crosslinking reactions (Figure 2-22) significantly improved the support stiffness

and its compression strength. The prepared Cu3 (BTC)2–PAN composite membrane achieved a


41

high hydrogen permeance of 705×10−7 mol m−2 s−1 Pa−1 and good H2/CO2 separation factor of 7.14

for binary mixture.

Figure 2-22. (a) Optical images of the (1) pristine PAN hollow fiber, (2) hydrolyzed PAN hollow

fiber and (3) Cu3 (BTC)2–PAN hollow fiber membrane. (b) ,(c) proposed involved chemical

reactions of the PAN hollow fiber [97].

Ge et al. [111] imbedded zeolite crystals in the polymer hollow fiber to make polymer–zeolite

mixed-matrix hollow fiber membranes and used as substrate for the growth of a zeolite layer

(Figure 2-23a). The imbedded zeolite crystals act as seeds for the growth of zeolite membrane and

also anchor the zeolite layer to the polymer support to improve the zeolite membrane adhesion. A

similar strategy was applied to fabricate MOF membranes on polymer hollow fiber substrates with

strong adhesion and good separation performance (Figure 2-23b) [112]. The MOF crystals were

first blended with polydimethylsiloxane (PDMS) in the presence of catalyst and cross-linking

agent. The resultant solution then was deposited on the surface of PSf hollow fiber by drop coating.

The obtained polymer-MOF composite provides seeds for MOF growth, increase the adhesion of

the MOF layer, enhances the gas separation performance, and reduces the mass transfer resistance

compared with MMMs or polymer membranes. Second, the PSf hollow fiber with the PDMS/MOF

layer was placed into the Teflon autoclave filled with the MOF precursor to crystallize. The


42

obtained 20 μm thick trinity Cu3(BTC)2 membrane exhibited an excellent separation performance

with hydrogen permeance of 48.5×10−7 mol m−2 s−1 Pa−1 and N2/CO2 and H2/CO2 ideal

selectivity of 21.03 and 7.23, respectively. The membrane also maintained excellent performance

under different pressures (from 1 bar to 3 bar), helpful for industrial applications [112].

Figure 2-23. Schematic illustration of (a) Zeolite Membranes on polymer−zeolite MM hollow fiber

supports [111] and (b) trinity MOF membranes preparation [112].

Reducing the growth temperature and minimizing the use of organic solvents are the two

critical factors for the scalable fabrication of continuous MOF layer on polymer hollow fibers.

Mao et al. [113] developed a pressure-assisted growth strategy to fabricate compact HKUST-1

films on PVDF hollow fiber in 40 min at room temperature (Figure 2-24). First, a mesoporous

copper hydroxide nanostrands (CHNs) layer was deposited on PVDF hollow fiber by filtering, to

serve as the copper source. Then, without turning off the pump, the PVDF substrate with CHN

layer was immersed into a linker ethanol/water solution (volume ratio of 1:1) for MOF layer

growth. The fabricated HKUST-1/PVDF membrane showed a good separation performance with


43

high hydrogen permeance of 20.1×10−7 mol m−2 s−1 Pa−1 and H2/CH4, H2/N2, H2/CO2 separation

factors of 5.4, 6.5, 8.1, respectively [113].

Figure 2-24. Scheme of pressure-assisted preparation of HKUST-1 layer on the surface of PVDF

hollow fibers substrate [113].

Contra-diffusion synthesis has also been applied for the scale-up synthesis of the hollow fiber

polymer-supported MOF membranes. Based on contra-diffusion synthesis concept, Brown et al.

[88] recently reported a methodology, interfacial microfluidic membrane processing (IMMP), for

the scalable fabrication of molecular sieving MOF membranes on polymeric hollow fibers. The

method also enable the control over the ZIF membrane position by employing an oil/ water system,

in which crystals grow at the interfaces between the two immiscible solvents. Different from the


44

previously reported methods, the MOF precursor solution flew within the hollow fiber bore.

Therefore, the MOF membrane can be directly produced on the hollow fiber substrate that was

installed into the module and excess reactants can be readily recycled into the hollow fiber bore.

The high quality ZIF-8 membranes made in water-octanol system by IMMP method could achieve

H2/C3H8 ideal selectivity of more than 600 with the permeance of H2 around

5×10−7 mol m−2 s−1 Pa−1 and demonstrated an excellent stability within 35 days of operation.

Similarly, Biswal et al. [103] developed a simple and scalable room temperature interfacial

approach for growing ZIF-8 and CuBTC. The method allowed the MOF growth on either the outer

or inner side of a polybenzimidazole (PBI) based hollow fiber support surface in a controlled

manner. An immiscible pair of low boiling solvents (isobutyl alcohol (IBA), CHCl3 and water)

was employed for the fabrication of MOF@membrane composite. As compared to other high

boiling solvents such as octanol, which is usually used for the interfacial fabrication of MOFs [88,

114], the employed solvents have the benefit of the easy exclusion of these solvents from the MOFs

and membrane with a mild activation procedure.

2.4.2.2 MOF-based mixed-matrix membrane

MOF-based mixed matrix membranes (MMMs) consisting of a dispersed MOF nanocrystals

in a polymer matrix are another important family of MOF membranes. Unlike supported MOF

membranes, MMMs need not form the continuous MOF layer. Figure 2-25 schematically

compares supported MOF membranes with MOF-based MMMs. MMMs have increasingly

attracted the attention of researchers over the last decades because they usually combine the

advantages of polymers and MOFs [115-118].


45

Figure 2-25. Schematic diagram of a typical MOF membrane and MOF based MMM.

A good dispersion of the filler with an excellent interaction with the polymer chains

(composite interface) is extremely important for the optimum MMM performance [119]. Due to

the organic linker of MOFs, a good interaction between MOF and polymer matrix is usually

achieved. This can result in a good interfacial contact between MOF and the polymer and reduce

the micro-gaps, which is usually an issue in inorganic/polymer MMMs. Therefore a higher optimal

filler loading is achievable in MOF-based MMMs compared with inorganic (e.g. zeolite or silica)-

based MMMs, which usually have the optimal loading below 10 wt. % [120]. However, the void

spaces between the polymer and filler still exist for some MOF-based MMMs. To overcome this

challenge and minimize the interfacial defects and also strengthen the adhesion between the

polymer phase and dispersion phase, enrichment of interfacial interaction between continuous

phase and inorganic phase of the MOF, inhibition of particle agglomeration and proper choice of

MOF/polymer pair are important aspects to take into account [120, 121]. Additionally, the

separation performance of gas mixture is usually affected by the diffusivity and solubility of the

gas components in MMMs [122]. In most of the polymer membranes the diffusivity selectivity

term favours permeation of the smaller gas molecule, H2, while the solubility selectivity term

favours permeation of the more condensable component, CO2 [123]. This causes the challenge of


46

separating H2/CO2 mixtures using polymer membranes, which remains undissolved in the case of

MMMs [124]. The gas permeability in MMMs is relatively low as compared with continuous

supported MOF membranes.

Since MMMs possess a lot of advantages such as flexibility, easy of processing, relatively

large permeability (as compared to polymer membranes) and low cost, many recent studies have

focused on the development of the MOF-based MMMs. In the following section, we will discuss

the MOF-based MMMs for gas separation.

Ordonez et al. [125] prepared ZIF-8/Matrimid® MMMs with loadings up to 80 wt. %.These

loadings, as mentioned, are much greater than the usual loadings attained with selected zeolite

materials. To examine the quality of the ZIF-8/Matrimid® MMMs, gas permeation experiments

were carried out for C3H8, CH4, N2, O2, CO2, H2 and gas mixtures of CO2/CH4 and H2/CO2. The

permeability initially increased with increasing the ZIF-8 loading, however at loadings above 40

wt. %, the permeability of all gases decreased. It is known that nanoparticles can interrupt chain

packing in glassy polymers, leading to an increase in the polymer free volume and its permeability

[126, 127]. For the work done by Ordonez et al., the addition of ZIF-8 nanocrystals to the

Matrimid® polymer matrix results in an increase in the polymer chain-to-chain distance, creating

more polymer free volume. Loadings above 50 wt. % increased selectivities which demonstrated

the influence of the ZIF additive and a transition from a polymer-governed to a ZIF-8-governed

gas transport process, where at higher loadings the ZIF-8 sieving effect becomes dominant.

Sonication is commonly used for a homogenous dispersion of MOF particles in the polymer

matrix. It was shown that sonication causes substantial changes in the structure, shape and size

distribution of ZIF-8 nanocrystals dispersed in an organic solvent in membrane processing [128].


47

However, data obtained from powder X-ray diffraction and nitrogen physisorption indicated that

losses in microporosity and crystallinity are minor.

In a parallel study, 60 nm ZIF-8 nanocrystals with surface area of 1300–1600 m2 g−1 were

added into a Matrimid® polymer via the solution mixing method [129]. An excellent dispersion

of nanocrystals, up to 30 wt. % loading, was obtained with a good interfacial interaction between

nanocrystals and polymer matrix, as confirmed by SEM and gas sorption studies. Single gas

analysis were conducted for CH4, N2, O2, CO2 and H2. The gas permeability considerably

increased by increasing the ZIF-8 loading while the selectivity remained relatively unchanged as

compared to the pristine polymer membrane. The porous nature of ZIF-8 was also utilized to

enhance the permeability of the polybenzimidazole (PBI) membrane [124, 130, 131]. The addition

of ZIF-8 into the polymer matrix resulted in a hundred times enhancement in hydrogen

permeability of the resultant MMMs, which reached about 105.4 Barrer without any significant

reduction in H2/CO2 selectivity (12.3) compared to the pure polymer membrane [124]. ZIF-8 was

also used in combination with zeolite in polysulfone (PSf) membranes for the separation of CO2/N2,

CO2/CH4, O2/N2, and H2/CH4 gas mixtures [132]. However, it was shown that the ZIF-8 and

silicalite-1 combination, ZIF-8/S1C-PSF MMM, could not outperform either S1C-PSF or ZIF-8-

PSF MMMs towards CO2/N2 and CO2/CH4 gas mixtures. This was attributed to the relatively large

S1C crystals which might not be intercalated between small ZIF-8 nanocrystals (ca. 100 nm).

Silicalite-1 also has a lower affinity towards CO2.

Conclusion and perspectives

In summary, membranes provide an eco-friendly and energy efficient alternative to traditional

separation protocols. MOF materials, due to their tunable pore size and chemistry, are promising


48

candidates for constructing membranes capable of separating small gases (e.g. hydrogen) from

other larger gases (e.g. N2, CH4) and also gas mixtures (e.g., C3H6/C3H8) that other materials

such as polymer-based membranes only show low separation performances. ZIFs are particularly

attractive for membrane-based gas separations due to their high thermal and chemical stability.

Despite several synthesis method being developed, the highly reproducible fabrication of

MOF membranes is particularly challenging owing to the difficulty of directing nucleation and

crystal growth onto the surface and the tendency for growth into unfavourably large crystals and

thick MOF layers. Various synthesis methods targeted at addressing these challenges have been

discussed.

To date, a number of metal−organic frameworks with diverse structures have been synthesized

that can be assembled into membranes for many commercially challenging separation applications.

Hence, intensive research efforts for developing facile MOF membrane fabrication will continue

to be critical and many new MOF membranes are expected to be reported.

It is also important to note that due to similar mechanisms of crystallizing of MOFs and

zeolites, the key techniques for fabrication zeolite membranes have been commonly modified to

fabricate MOF membranes. However, considering that the coordination chemistry of MOFs is

basically different from the covalent chemistry of zeolites, novel fabrication methods are expected

to afford utilizing distinctive features of MOFs for not only traditional gas separations but also for

the new area of enantioselective and chiral separations. The new processing approaches require

advantages of being rapid, reproducible, scalable, and economically and environmentally viable

and at the same time produce high quality of MOF membranes.


49

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Chapter 3 Rapid synthesis of ultrathin, defect-free ZIF-8 membranes via chemical

vapour modification of polymeric support

62

Rapid Synthesis of

Ultrathin, Defect-Free ZIF-8

Membranes via Chemical Vapour

Modification of Polymeric

Support

Overview

In this chapter, ultrathin ZIF-8 membranes with a thickness of around 200 nm were

prepared by chemical vapour modification of surface chemistry and nanopores of

asymmetric bromomethylated poly (2, 6-dimethyl-1, 4-phenylene oxide) (BPPO)

substrate. The resulting ZIF-8 membranes exhibited exceptional H2 permeance as high

as 2.05 × 10-6 mol.m-2.s-1.Pa-1 with high H2/N2 and H2/CO2 selectivities (9.7 and12.8,

respectively). This chapter has been reformatted from the following published

manuscript: Shamsaei, E., Low, Z.X., Lin, X., Mayahi, A., Liu, H., Zhang, X., Liu, J.Z.

and Wang, H. Rapid synthesis of ultrathin, defect-free ZIF-8 membranes via chemical

vapour modification of a polymeric support. Chemical Communications, 2015, 51(57),

pp.11474-11477.

Introduction

Zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic frameworks

(MOFs), are hybrid porous crystalline materials composed of metal ions (e.g., Zn, Co)

bridged by imidazolates [1, 2]. Remarkably, they exhibit permanent porosity and

relatively high chemical and thermal stability [3, 4], which make them very promising



63

candidate materials for numerous applications such as catalysis [5], molecular separation

[6], chemical sensing [7], gas adsorption and storage [8]. In particular, the preparation

of MOFs into membranes and thin films is desirable for gas separation [9]. The well-

defined porous structures of ZIFs allow them to achieve gas separation with high

selectivity via molecular sieving. ZIF membranes are prepared by growing a thin ZIF

layer on porous substrate via two general techniques, i.e. in situ (direct) growth and

secondary (seeded) growth [10-12]. In situ growth, a method used for direct growth of a

ZIF layer on a bare porous substrate, has been widely studied for the fabrication of ZIF

membranes. However, due to limited heterogeneous nucleation sites on the support and

poor compatibility between ZIF and support, this method may result in defective ZIF

films with intercrystalline voids [13, 14]. Chemical modification can effectively

overcome the issue by providing anchors to ligands or metal ions. Compared to other

methods such as microwave-assisted solvothermal synthesis [15], rapid thermal

deposition (RTD) [16], layer-by-layer deposition of crystal [17], and liquid-phase

epitaxy (LPE) [18], chemical modification not only provides a faster and energy-

efficient route but also indirectly improves the thermal stability and chemical resistance

of the composite.

So far, most supports for growing a ZIF layer are ceramic-based materials such as

alumina. To favour ZIF formation and adhesion, these supports are functionalized with

organosilane molecules or other functional groups with amine group [12, 19-24]. Caro

and co-workers prepared a continuous ZIF-90 membrane by using 3-

aminopropyltriethoxysilane (APTES) as a covalent linker [23]. The amine groups of the

APTES were shown to react with the aldehyde groups of the ZIF precursor and promote

the nucleation and growth of the ZIF-90 at these fixed sites on the surface of the porous

ceramic substrate. A similar surface modification method and its influence on



64

heterogeneous nucleation have been also demonstrated with other MOF materials by

other groups [25-27]. However, the poor scalability and high cost of the inorganic

materials limit the applications of ZIF membranes [14, 28-31]. Growing ZIF on a

polymer substrate, on the other hand, has great potential to achieve high-quality

membranes at a low cost. To date, there have been only a few reports on successful in

situ growth of ZIF on a polymer surface [13, 14, 17, 18]. To effectively use a polymer

membrane as a substrate, the polymer membrane needs to be chemically modified. Li et

al. successfully grew a continuous and well integrated ZIF-8 layer on a polyvinylidene

fluoride (PVDF) substrate treated in ammonia or ethanediamine solution [14]. The same

group also successfully prepared ZIF-8-polyacrylonitrile (PAN) membrane by

hydrolysing PAN substrate to produce deprotonated carboxyl groups [32]. The modified

polymer substrates exhibited increased stiffness due to cross-linking [14, 32].

Nevertheless, solution phase chemical modification for the preparation of polymer-

supported ZIF membranes can cause uneven swelling and adversely affect the polymeric

membrane morphology and separation performance [33, 34]. Also, the contaminants in

chemical modifier solution increase with substrate modification cycle and need to be

treated before reuse. Furthermore, the substrates after modification need to be rinsed and

dried before growing ZIF. At a lab scale, fresh modifiers can be used and the substrate

can be left at room temperature until completely dry; but at larger scale, these steps need

to be improved on the basis of production cost and time.

Here we report a novel scalable strategy of using vapour phase ethylenediamine

(EDA) to chemically modify the polymer support for ZIF membrane fabrication. An

asymmetric bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO)

ultrafiltration membrane was fabricated by non-solvent induced phase separation, and

used as the support for growing a thin ZIF-8 layer via a rapid in situ route after chemical



65

vapour modification. BPPO is a common polymer with a high glass transition

temperature, high mechanical strength and excellent hydrolytic stability [35]. It attains

superior membrane formation and functionalizable characteristics due to the abundant

highly reactive -CH2Br groups. EDA-vapour modification of BPPO results in the

reduction of pore size of support and also provides a large number of nucleation sites.

When combined with the rapid, in situ growth of ZIF, a submicron-thin and defect-free

ZIF-8 membrane with high gas separation performance can be attained.

Experimental

3.3.1 Materials

BPPO was kindly supplied by Tianwei Membrane Co. Ltd., Shandong of China.

Zinc acetate dihydrate (Zn(CH3COO)2.2H2O, 98%), 2-methylimidazole (Hmim, C4H6N2,

99%), ethylenediamine (EDA, 99.5%) and ammonium hydroxide solution (NH3, 28–30%

aqueous solution) were purchased from Sigma-Aldrich, Australia and used as received.

Methanol (absolute) was purchased from Merck, Australia.

3.3.2 Synthesis of BPPO membrane and its EDA-vapour modification

The flat sheet BPPO support ultrafiltration membranes were fabricated via non-

solvent induced phase inversion (also known as the immersion precipitation technique).

Dope solution was prepared by dissolving 18 wt. % of BPPO in NMP at around 25 °C

for 24 h with mechanical stirring at 200 rpm. The homogenous solution was left stagnant

until no bubbles were observed. Subsequently, the polymer solution was cast on a

cleaned glass plate using a casting knife (Paul N. Gardner Co., Inc. USA) with a gap of

150 µm at room temperature (21–23 °C) and 30-35 % humidity and immediately

immersed in a coagulation bath of deionized water (Figure 3-1a). After peeling off from



66

the glass plate, the membranes were removed from the coagulation bath, washed and

kept in water bath for at least one day to thoroughly remove the residual solvents. The

thickness of the prepared membrane was about 70 μm.

The vapour-phase EDA modification was conducted in a custom-made container as

illustrated in Figure 3-1b. 20 mL of EDA was allowed to vaporize, and stabilized for 1

h. Based on the Antoine equation [36], the EDA vapour pressure at 25 °C was estimated

to be 12.0 mm of Hg and the air of the closed chamber consists of 1.6 % v/v EDA vapour.

The support membranes were quickly placed inside the containment with the top layer

exposed and suspended above the EDA solution. After surface modification at room

temperature for 4-16 h, the surface modified membranes were removed from the

containment and immediately washed with pure water to completely remove the residual

EDA. The resultant membranes were denoted as MBPPO-4, MBPPO-10, and MBPPO-

16, where numbers show EDA exposure time.

Figure 3-1(a) Schematic diagram of UF membrane fabrication via phase inversion, (b)

Experimental setup of vapour-phase EDA modification process.



67

3.3.3 Growth of ZIF-8 Thin Film on modified BPPO Supports

Modified BPPO supports were immersed vertically in the solution of zinc acetate

dehydrate (0.22 g) in 9.6 g methanol and sonicated for 3 min to fix the Zn2+. A solution

of Hmim (0.164 g) in 9.6 g methanol was added to the above solution followed by

dropwise addition of ammonia hydroxide solution (0.12 g) and the mixture was then

ultrasonically treated for another 3 min. After crystallization, the composite membranes

were washed with methanol and dried. The ZIF-8 nanocrystals were also separated from

the solution by centrifugation and washed several times with methanol, and dried at

60 °C overnight. For comparison, BPPO was also used for ZIF-8 membrane growth

under the same condition.

3.3.4 Pure water flux and molecular weight cut off (MWCO)

measurements

Pure water flux of the membranes was determined at room temperature (21–23 °C)

using a Sterlitech HP4750 dead-end stirred cell (Sterlitech Corporation, USA) with an

inner diameter of 49 mm and an effective membrane area of 14.6 cm2. The cell has a

volume capacity of 300 ml and is attached to a 5.0 L dispensing vessel. To attain stable

flux data, each membrane was first pre-compacted at 150 kPa for about 60 min, and then

the pure water flux was measured at a trans-membrane pressure drop 100 kPa. Pure water

flux was measured constantly by collecting the permeate on a digital balance (PA2102C,

Ohaus) interfaced with a computer. The data from the balance was logged to a computer

using a program written in LabVIEW. Polyethylene glycol (PEG) with a molecular

weight of 10, 20, 35, 100, 200 and 300 kDa (analytical grade, Sigma-Aldrich) was

dissolved in deionized water to prepare 1 g L-1 aqueous solutions for the estimation of

MWCO and solute rejection. Rejection measurements were performed at a pressure of

100 kPa. 20 ml of permeate was collected. The permeate and feed solution were both



68

diluted by 10 times and then the concentration of each solution was measured via a total

organic carbon analyser (TOC-LCSH, Shimadzu, Japan). The PEG rejection was

calculated from the measured feed (Cf) and permeate (Cp) concentrations by

𝑅 = (1 −𝐶𝑝

𝐶𝑓) × 100

All results represent average values for at least three repeated experiments with less

than ±5% deviation. The pore size of the membrane was defined as the hydrodynamic

diameter of PEG. The hydrodynamic radius of PEG can be calculated from the MWCO

of the membrane by the following equation [37]:

𝑆𝑜𝑙𝑢𝑡𝑒 𝑅𝑎𝑑𝑖𝑢𝑠 (𝑛𝑚) = 0.0262√𝑀𝑊 − .03

where MW is the lowest molecular weight of the PEG molecule which has a rejection of

90% in the ultrafiltration measurements.

3.3.5 Gas permeation experiments

The gas permeation test is carried out as previously reported [13]. The composite

membranes were attached to a stainless steel stand with pore size ~200 nm, which was

fixed in a sample holder with Torr Seal epoxy resin (Varian). The film was dried at

100 °C for 2 h to remove H2O. Gas permeation tests were performed at 20 °C for pure

H2, CO2 and N2. Between each measurement, the system was evacuated for 30 min prior

to introduction of the next gas. The pressure increase of the permeate stream was

measured and the permeance Pi of each gas calculated by:

𝑃𝑖 =𝑁𝑖

∆𝑃𝑖𝐴



69

where Ni is the permeating flow rate of component i (mol/s); ΔPi is the transmembrane

pressure difference of component i (Pa), and A is the membrane area (m2). The ideal

selectivity Sij is defined as the ratio of the two permeances Pi and Pj.

3.3.6 Characterization

Fourier Transform Infrared (FTIR) spectra of the membranes were recorded using

an attenuated total reflectance (ATR) FTIR (Perkin Elmer, USA) in the range of 500-

4000 cm-1 at an average of 32 scans with a resolution of 4 cm-1. Thermogravimetric

analyses (TGA) were carried out on a SETARAM (TGA 92) device from 30 to 800 °C

at a heating rate of 10 °C min-1 under air flow. Scanning electron microscopy (SEM)

(FEI Nova NanoSEM 450) with a X-ray detector (Bruker Nano GmbH, Germany) was

used for imaging the surface and cross-sectional morphologies of membranes. Energy-

dispersive X-ray sepectroscopy (EDS) line-scan analysis of the membrane samples was

conducted using EDX equipped in Nova NanoSEM 450 (Quantax 400 X-ray analysis

system, Bruker, USA). The membranes were fractured in liquid nitrogen, fixed on stubs

with double-sided carbon tape and then sputter coated with roughly 2 nm iridium (Ir)

layer to ensure good electrical conductivity. The images were recorded at an accelerating

voltage of 5 kV with different magnifications. Transmission electron microscopy (TEM)

micrographs were obtained using a JEOL JEM- 2100F instrument operating at 200 kV.

Selected-area electron diffraction (SAED) patterns were taken using the same instrument.

The ZIF-8 samples were dispersed on a copper-supported carbon grid for TEM

observation. Powder X-ray diffraction (XRD) patterns were measured using a Miniflex

600 diffractometer (Rigaku, Japan) with Cu Kα radiation (15 mA and 40 kV) at a scan

rate of 2° min-1 with a step size of 0.02°. The XRD studies were carried out at room

temperature. The crystallite size was estimated by using the Scherrer equation:



70

𝑑 =𝑘𝜆

𝛽 cos 𝜃

where β is the full-width at half-maximum in radian for the peak (011); k is Scherrer

constant (0.94); λ is the X-ray wavelength, 0.1541 nm for Cu Kα; θ is the angle of the

diffraction peak in degree.

Nitrogen (N2) adsorption–desorption isotherms were measured using physisorption

analyser (Micromeritics ASAP 2020, USA) at liquid nitrogen temperature (77 K). All

the samples were degassed at 100 °C for 12 h prior to analysis.

Figure 3-2. Schematic diagram of the preparation of BPPO polymer-supported ZIF-8

membrane.

Results and Discussion

The method developed in this work involves two steps, as shown in Figure 3-2. First,

the physicochemical properties of the top layer of BPPO support were modified by using

EDA-vapour. Then, ZIF-8 was grown inside the pores and on the surface of the support

via a rapid, in-situ seeding method [38]. The modified BPPO is denoted as MBPPO-X

(X: modification reaction time). EDA-vapour modification is surface-limiting with

minimal swelling effect to the polymer support, and this process can be carried out at

room temperature [33, 34]. Unlike liquid phase modification, EDA is directly reusable



71

in the vapour modification. Therefore, amine functional groups for the coordination with

Zn2+ ions can be covalently attached on the top layer of BPPO support without affecting

the sublayer structure. For formation of ZIF-8 layer, the vapour-phase-EDA-modified

BPPO support was immersed vertically in the solution of zinc acetate dehydrate (0.22 g)

in 9.6 g methanol and sonicated for 3 min. A solution of Hmim (0.164 g) in 9.6 g

methanol was added to the above solution, followed by dropwise addition of ammonia

hydroxide solution (0.12 g) and the mixture was then ultrasonically treated for another

3 min. The final precursor solution had a Zn: Hmim: NH3: CH3OH molar composition

of 1: 2: 2: 300 and was kept constant in our study. After crystallization at room

temperature, the ZIF-8/BPPO composite membranes were washed with methanol and

dried. The following simultaneous amination and crosslinking reactions may take place

during the surface modification [39].

R–CH2Br + NH2CH2CH2NH2 → R–CH2NHCH2CH2NH2 + HBr− (1)

R–CH2Br + R–CH2NHCH2CH2NH2 → R–CH2NHCH2CH2NHCH2R + HBr− (2)

Reactions (1) and (2) are typical amination and crosslinking reactions. Reaction (1)

takes place where one end of a diamine molecule reacts with bromine groups (−CH2Br).

Reaction (2) occurs where EDA reacts at both ends of the diamine molecule to form

either interchain or intrachain crosslinks.

3.4.1 Membrane support

Pure water flux and PEG rejection of the membranes were analysed to evaluate the

influence of the chemical modification on the membrane permeability in correlation with

the change of membranes surface microstructure after EDA-vapour modification. There

was a pronounced drop in pore size from 17.5 nm for BPPO to 11.5 nm for the membrane

following 16 hours EDA-vapour treatment (MBPPO-16). It is worth noting that the



72

values of the pure water flux and the pore size show that all the membranes fall in the

ultrafiltration range. Membranes with such porosity are desirable as support for ZIF

membranes since they provide a platform for the growth of ZIFs with no resistance or

interruption to gas permeation.

Figure 3-3. FTIR ATR spectra of untreated BPPO support, BPPO modified with EDA-

vapour for 16 h (MBPPO-16), MBPPO-16 supported ZIF-8 layer (ZIF-8-MBPPO-16),

and synthesized ZIF-8 powder.

The chemical reaction between the BPPO support and EDA vapour during the

modification was determined using the FTIR-ATR technique and the results are shown

in Figure 3-3. The pristine BPPO has IR bands at around 586 cm-1 and 633 cm-1, which

are attributed to the benzyl bromide (−CH2Br) groups (C−Br stretching). After EDA-

vapour modification, these bands almost disappear and a new broad band in the range of

~3100-3600 cm-1 emerges, which is ascribed to the N−H stretching and confirms the

amination of BPPO. The ZIF-8 characteristic band at 421 cm-1 (Zn–N stretching) is

observed in the BPPO-supported ZIF-8 membrane (ZIF-8-MBPPO-16). Furthermore,

TGA results (Figure 3-4) suggest higher degradation temperature for main chains of

BPPO after EDA vapour-phase modification, which could be attributed to the presence



73

of partial crosslinking in the modified BPPO substrate. TGA measurements of the

untreated BPPO support show a two-step degradation pattern primarily due to the weight

loss commencing at ~ 240 °C associated with the degradation of bromomethyl side

groups (−CH2Br) followed by the decomposition of the aromatic main chains at ~ 468 °C.

In contrast, MBPPO shows a slight mass loss (about 4 wt. %) at lower temperatures due

to the loss of absorbed water. The loading amounts of ZIF-8 can be roughly estimated

from the zinc oxide (ZnO) residue at 800°C in TG curves. The results show the ZIF-8

loading amounts increase from 1.9 % for ZIF-8-BPPO to 13.8 % for ZIF-8-MBPPO-16.

These results clearly show the importance of EDA modification in the growth of ZIF-8

on the supporting membranes. This was further verified by reduced water flux and

increased rejection of the support after the modification, since the crosslinking causes

tightening of the polymer network which increases the membrane dimensional stability

and reduces the pore size of BPPO substrate (Figure 3-5). This reduces the flexibility of

the polymeric substrate, which is favourable for reducing ZIF layer cracking [9].

Figure 3-4. TGA curves (under air flow) of (1) untreated BPPO support, (2) MBPPO-

16, (3) ZIF-8-BPPO, (4) ZIF-8-MBPPO-16, and (5) synthesized ZIF-8 powder.



74

Figure 3-5. Pure water flux and pore size of BPPO membranes as a function of exposure

time to EDA vapour.

3.4.2 Fast in Situ Seeding

For ZIF-8, the nucleation rate controls the crystallization process [40, 41], which is

crucial to the particle sizes. Due to the generation of localized extremely high

temperatures and pressures, the fast in situ seeding method [38], employed in this study,

results in a high nucleation rate and subsequently in small-sized crystals. The

introduction of ammonium hydroxide, in addition, can deprotonate organic ligands and

thereby accelerate ligand exchange reactions, resulting in an even higher nucleation rate

and consequently in a smaller final crystal size. As shown in Figure 3-6a, the XRD

pattern of the particles collected after 1h seeding is exactly same as the simulated SOD-

type ZIF-8 structure, which confirms the formation of pure crystalline ZIF-8 phase. The

average crystal size was ~ 20 nm estimated from the full width at half maximum of the

(011) peak using the Scherrer’s equation. The formation and size of the ZIF-8 crystals

were further confirmed by TEM, as shown in Figure 3-6b. The diffraction rings of the

different planes, shown in inset of Figure 3-6b, are in good agreement with the XRD

peaks of ZIF-8. Spherical particles of ~20 nm observed in the TEM image was also



75

consistent with crystallite size obtained from XRD patterns. It should be noted that the

small ZIF-8 nanocrystals are easily damaged in the high energy of the electron beam of

a TEM [42]. Type I nitrogen sorption isotherms (Figure 3-6c) were observed

representing the microporous nature of the as-synthesized ZIF-8 crystals. The second

step (at P/Po > 0.8) observed in the isotherm with an obvious adsorption–desorption

hysteresis loop is attributed to interparticle mesopores. The micropore volume of the

ZIF-8 nanocrystals is 0.74 cm³/g, and the BET and Langmuir surface areas are 1146 and

1715 m²/g, respectively.

Figure 3-6. XRD pattern (a), TEM image and SAED pattern (inset) (b) and nitrogen

sorption isotherm (c) of as-synthesized ZIF-8 nanocrystals.



76

Figure 3-7. SEM images of (a) cross-section and (b) surface of the ZIF-8-MBPPO-16.

3.4.3 Supported ZIF-8 membrane

After the substrate modification, an ultrathin ZIF-8 membrane of about 200 nm is

formed on top of the polymer support (Figure 3-7). From the membrane cross section

(Figure 3-7a), it can also be observed that a fraction of ZIF-8 crystals formed inside the

porous polymer support. This can be due to the diffusion of the EDA vapour into the

polymer sublayer which creates additional active sites for ZIF-8 nucleation. Energy-

dispersive X-ray spectroscopy (EDS) line-scan analysis confirmed the existence of ZIF-

8 within the support sublayer as zinc was detected up to ~150 nm underneath the

membrane surface (Figure 3-8). Growth of the crystals partially inside the support could

improve the membrane structural integrity. The top-view image (Figure 3-7b) shows that

the support surface was covered entirely with a continuous and compact ZIF-8 layer

without any visible defects such as pinholes or cracks. This very thin, dense and defect-

free ZIF-8 membrane on the support with large pore size is desirable for high-

performance gas separation. By contrast, separate ZIF-8 crystals and crystal islands

formed if the support surface was not modified with EDA before ZIF-8 crystallization

(Figure 3-9). This difference in the morphology between ZIF-8-MBPPO and ZIF-8-

BPPO provides strong evidence of the important role of EDA modification in growing

a thin and compact ZIF-8 layer. The intensity of the XRD (Figure 3-10) peaks of ZIF-8



77

in ZIF-8-MBPPO is much higher than those in ZIF-8-BPPO, indicating that the ZIF-8

layer has higher crystallinity and better surface coverage. Benzyl bromine groups

(−CH2Br) in the BPPO are readily transformed to primary amine groups (−NH2) in the

modification process. The obtained primary amine groups can subsequently react with

Zn2+, as reported by Liu et al. [43], where stable zinc complexes are formed from Zn2+

coordinated with monoamine. The FTIR results also show that primary amine groups

generated by the EDA treatment have been consumed in the reaction with Zn2+ during

ZIF-8 nucleation step where the broad band ascribed to the N−H stretching disappeared

(Figure 3-3).

Figure 3-8. EDS line scan across ZIF-8-MBPPO-16 cross-section for the zinc atoms.

On the basis of experimental results described above, it is clear that EDA can act as a

covalent link between the ZIF-8 crystals and support, providing a large number of

nucleation sites for the growth of the ZIF-8 layer. The existence of a strong interaction

between ZIF-8 and MBPPO support was also confirmed by TGA results (Figure 3-4) in

which a considerable shift from 310°C to 358°C is observed between the degradation

peaks of MBPPO and ZIF-8-MBPPO; whereas the BPPO and its counterpart ZIF-8-



78

BPPO membrane show only a slight difference in their degradation peaks. To gain an

insight into the effect of EDA on the growth of ZIF-8 on the BPPO support, ZIF-8-

MBPPO-4 and ZIF-8-MBPPO-10 were prepared and their SEM images were compared

to that of ZIF-8-MBPPO-16 (Figure 3-9). These images reveal that the shorter EDA

exposure resulted in ZIF-8 membranes with larger defects and pinholes. This can be

explained by considering the multiple roles of EDA. By increasing the EDA exposure

time, the reaction extent increases, as confirmed by FTIR (Figure 3-11), resulting in

aforementioned a higher number of nucleation sites (amine groups). In addition,

ethylenediamination induces a change in the membrane microstructure. The effect of

pore size of porous support on the formation of ZIF membranes was previously reported

[44, 45]. Increasing the EDA modification time leads to increased crosslinking and

reduced substrate pore size, which is essential for the formation of a thin and defect-free

ZIF-8 membrane. The corresponding pore size of MBPPO-16 is 11.4 nm.



79

Figure 3-9. SEM images of the (a) surface and (b) cross section of the BPPO support,

(c) surface of the ZIF-8-BPPO, (d) surface of the ZIF-8-MBPPO-4, (e) surface of the

ZIF-8-MBPPO-10, (f) surface and (g, h) cross-section of the ZIF-8-MBPPO-16.



80

Figure 3-10. XRD patterns of the membranes and simulated ZIF-8 powder.

Figure 3-11. FTIR ATR spectra of (1) untreated BPPO support, BPPO modified with

EDA-vapour for (2) 4 h (MBPPO-4), (3) 10 h (MBPPO-10), (4) 16 h (MBPPO-16), (5)

ZIF-8-MBPPO-16, (6) synthesized ZIF-8 powder.

3.4.4 Single gas performance

To further evaluate the quality of the ZIF-8 membranes, single gas (H2, N2 and CO2)

permeation experiments were carried out, and the results are summarized in Table 3-1.

ZIF-8-BPPO without modification showed no gas separation property because no



81

continuous ZIF-8 film was formed. The result also confirms the formation of a

continuous and compact ZIF-8 film on the modified BPPO substrate when it is

sufficiently aminated. Lower amination time led to a low-quality ZIF-8 layer on the

BPPO substrate (ZIF-8-MBPPO-4 and ZIF-8-MBPPO-10). ZIF-8-MBPPO composite

membranes prepared with longer amination times show excellent gas selectivities. ZIF-

8-MBPPO-16 exhibited H2/CO2 and H2/N2 ideal selectivities of 12.8 and 9.7,

respectively; it also had H2 permeance as high as 2.05 × 10-6 mol.m-2.s-1.Pa-1

(Figure 3-12). Furthermore, all H2 permeances and ideal selectivities of H2/CO2 and

H2/N2 are similar for the membranes obtained from different synthesis batches

(Table 3-2), indicating the good reproducibility of the reported synthesis strategy.

Table 3-1. Single gas permeances and ideal selectivities for the composite membranes

at 25 ⁰C and 1 bar.

Sample

Permeance (10-7 mol.m-2.s-

1.Pa-1)

Ideal selectivity

H2 H2/N2 H2/CO2

BPPO 80.8 1.6 2.1

MBPPO 40.0 2.0 2.9

ZIF-8@BPPO 75.5 1.8 2.2

ZIF-8@MBPPO-4 32.7 2.2 3.0

ZIF-8@MBPPO-10 31.2 2.5 3.4

ZIF-8@MBPPO-16 20.5 9.7 12.8



82

Table 3-2. Single gas permeances and ideal selectivities at 25 ⁰C and 1 bar of 3 tested

ZIF-8-MBPPO-16 membranes showing the reproducibility of membrane synthesis and

testing.

Sample

Permeance (10-7

mol.m-2.s-1.Pa-1)

Ideal selectivity

H2 H2/N2 H2/CO2

Original membrane 1 20.5 9.7 12.8

Membrane 2 20.0 9.8 13

Membrane 3 20.3 9.7 12.5

This membrane is amongst the best ZIF-8 membranes reported previously (Table 3-3).

For instance, at a similar H2/N2 selectivity, the ZIF-8 membrane prepared in this study

had two orders of magnitude higher H2 gas permeance than those prepared by the epitaxy

method [18, 46].

Figure 3-12. Single gas permeances of ZIF-8-MBPPO-16 as a function of kinetic

diameter of gas molecule.



83

It is worth mentioning that the N2 permeance is higher than CO2 permeance despite

CO2 having a smaller kinetic diameter than N2. This behaviour can be attributed to the

peculiar structure of ZIF-8 and to the combined linear structure and permanent dipole

moment of CO2 [6, 47]. The small CO2 permeance may also be related to partially

coordinated organic ligand (Hmim) molecules present in the ZIF-8 crystals as it was

previously shown that this molecule is able to strongly coordinate with CO2 gas [47].

The similar behaviour was also observed in other ZIF materials [48-51]. The high

permeance for hydrogen as well as high hydrogen selectivity in this study is due to the

ultrathin ZIF layer (~ 200 nm) and the absence of pinholes or defects, which are

attributed to vapour phase amination of asymmetrically porous BPPO support. Another

possible reason for the high permeance may be the highly porous and asymmetric

structure of the support, which minimises the overall hydraulic resistance of the

permeate flow through the membrane structure. The formation of a thin and compact

ZIF-8 layer in this study is essentially attributed to the enriched heterogeneous

nucleation density and the reduction of the support pore size via EDA-vapour

crosslinking. Thus, this work demonstrates a new strategy that can be applied to the

formation of polymer-supported ZIF membranes through rational design and chemical

modification of the thin polymer supports.



84

Table 3-3. Comparison of gas permeation properties (H2 permeance, H2/N2 and H2/CO2

selectivity) of ZIF-8 membranes on inorganic and polymeric supports reported in recent

literature.

Support

Synthesis

Method

Thickness

(µm)

T

[°C]

Permeance

(10-7mol/m2.s.Pa)

Selectivity

Ref. H2 N2 CO2 H2/N2 H2

/CO2

Polymeric support

BPPO Surface

chemistry

and pore

structure

modification

~0.3 Room 20.5 2.1 1.6 9.7 12.8 This

work

PVDF Chemical

modification

~30 Room 24.4 1.7 2 14.3 12.1 [14]

Nylon Contra-

diffusion

16 25 19.7 4.6 NR* 4.3 NR [13]

Nylon Contra-

diffusion

2.5 25 11.3 2.5 NR 4.6 NR [52]

Torlon Interfacial

microfluidic

membrane

processing

(IMMP)

~9 25 8.5 NR NR NR NR [53]

PES Secondary

growth

7.2 60 4 0.4 NR 9.9** NR [54]

PSf In situ

followed by

layer-by-

layer

10 25 3.98 NR 1.06 NR 3.8 [17]

PAN Surface

chemical

modification

NR 20 3.05 NR 0.44 NR 6.85** [32]



85

Support

Synthesis

Method

Thickness

(µm)

T

[°C]

Permeance

(10-7mol/m2.s.Pa)

Selectivity

Ref. H2 N2 CO2 H2/N2 H2

/CO2

Polymeric support

PSf In situ 35 35 2 0.16 NR 12.4** NR [55]

Inorganic support

Alumina/PTFE Seeded

growth

2 25 76.8 8.1 NR 9.4 NR [56]

Alumina

hollow

fiber

Hot support

seeding

20 25 7.3 0.79 1.35 9.2 5.4 [57]

Alumina

hollow

fiber

Repeated

growth

6 25 5.2 2.1 0.16 2.5 32.2 [58]

Alumina tube APTES and

cycling

precursors

2 Room 4.3 0.35 1.2 11.1 3.6 [59]

Alumina Surface

chemical

modification

12 25 1.7 0.15 0.44 11.4 3.8 [12]

γ-Al2O3 Surface

chemical

modification

20 Room 1.4 0.14 0.33 10 4.2 [10]

AAO Fast in situ

seeding and

secondary

growth

0.5 25 1.34 0.32 0.21 4.19 6.38 [38]

Al2O3 tube Repeated

synthesis

25 100 1.2 NR 0.058 NR 20.7 [60]

Titania Direct

synthesis

30 25 0.6 0.052 0.13 11.5 4.5 [61]

Alumina LBL*** ~1.5 35 0.19 0.019 0.041 11 5 [18]

*NR: Not reported; **Mixed gas separation; ***LBL: Layer-by-Layer



86

Summary

In summary, we have successfully prepared a compact, ultra-thin ZIF-8 layer on an

asymmetric polymeric substrate by chemical vapour modification of the surface

chemistry and pore structure. The thickness of the membrane fabricated by simultaneous

modification of surface chemistry and pore structure is one of the thinnest ever reported.

In addition, we have also demonstrated the influence of the surface microstructure and

chemical composition of the polymer substrate on the formation of a continuous ZIF-8

layer. Vapour-phase EDA has been used to simultaneously tailor the chemical nature

and pore size of the surface of BPPO for the successful growth of ZIF-8 membrane. The

EDA treatment produced a large number of nucleation sites and modified the BPPO pore

structure, promoting the formation of a thin ZIF-8 layer. The ZIF-8 membrane exhibits

ideal selectivities (H2/CO2: 12.8; H2/N2: 9.8) and permeance (2.05× 10-6 mol.m-2.s-1.Pa-

1) which is among the highest reported so far. The proposed chemical vapour

modification followed by fast in situ synthesis provides a rapid, convenient and effective

route for preparing thin yet continuous and defect free ZIF membranes on the surface of

polymeric substrates.



87

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Chapter 4 Aqueous phase synthesis of ZIF-8 membrane with controllable location on an

asymmetrically porous polymer substrate

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Aqueous Phase Synthesis of

ZIF-8 Membrane with Controllable

Location on an Asymmetrically

Porous Polymer Substrate

Overview

In chapter 3, ultrasonication was utilized to generate intense, localized heating to promote

ZIF-8 growth. Although the prepared membrane showed excellent separation properties, such

ultrasonication-assisted method may not be applicable for large-scale membrane sample

preparation. In this chapter, we have demonstrated a simple, scalable, and environmentally friendly

route for controllable fabrication of continuous, well-intergrown ZIF-8 on a flexible polymer

substrate via contra-diffusion method in conjunction with chemical vapour modification of the

polymer surface. The combined chemical vapour modification and contra-diffusion method

resulted in controlled formation of a thin, defect-free and robust ZIF-8 layer on one side of the

support in aqueous solution at room temperature. The ZIF-8 membrane exhibited propylene

permeance of 1.50×10-8 mol m-2 s-1 Pa and excellent selective permeation properties; after post

heat-treatment, the membrane showed ideal selectivities of C3H6/C3H8 and H2/C3H8 as high as 27.8

and 2259, respectively. The new synthesis approach holds a promise for further development for

the fabrication of high-quality polymer-supported ZIF membranes for practical separation

applications. This chapter has been reformatted from the following published manuscript:

Shamsaei, E., Lin, X., Low, Z.X., Abbasi, Z., Hu, Y., Liu, J.Z. and Wang, H.. Aqueous phase



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synthesis of ZIF-8 membrane with controllable location on an asymmetrically porous polymer

substrate. ACS Applied Materials & Interfaces, 2016, 8(9), pp.6236-6244.

Introduction

Zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic frameworks (MOFs), are

porous crystalline hybrid materials consisting of imidazolate ligands (Im) bridging tetrahedral

metal ions (e.g., Zn, Co) [1]. They closely resemble the topologies of zeolites, due to the M-Im-M

(M = Zn, Co) bond angle of 145°, which is close to the T-O-T (T = Al, Si, P) angle (140-170°) in

zeolites [2, 3]. ZIFs show properties that combine the attractive features of both MOFs and zeolites

such as tunable pore size and chemistry, large internal surface area and relatively good thermal

and chemical stability [4, 5]. These properties make ZIFs excellent candidates for the fabrication

of molecular sieving membranes for gas separation [6-8]. ZIF-8 membranes, for example, have

been reported to be capable of molecularly discriminating propylene (~4.0 Å) from propane (~

4.3 Å) since the effective pore aperture size of ZIF-8 falls in the range of 4.0-4.2 Å (larger than its

crystallographic value of 3.4 Å, owing to the swaying effect of the ligands) [2, 9-11].

ZIFs have been widely used to fabricate the so-called mixed matrix membranes (MMMs,

consisting of pre-synthesized ZIF particles dispersed in a polymeric matrix) to afford a solution to

go beyond the Robeson's upper-bound trade-off limit of the polymeric membranes [6, 12-15].

While MMMs have been shown to enhance the permeation properties of polymeric membranes,

further enhancements were made using in-situ synthesized ZIF membranes [16]. For example,

ZIF-8 supported membranes prepared by Pan et al. [10] showed superior propylene/propane

permselectivity (permeability of propylene up to 200 barrers and a propylene/propane separation

factor up to 50) compared to those of ZIF-8 MMMs, as for instance the ZIF-8/ 6FDA–DAM



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polyimide (a propylene permeability of 56.2 barrer and propylene/propane ideal selectivity of 31.0)

reported by Zhang et al. [15]. To date, several synthesis methods have been reported for the

formation of ZIF films on various substrates [6, 17]. In particular, polymer-supported ZIF

membranes are of great interest as they potentially combine the advantages of both polymer

membranes (e.g. easy processing and low cost) and ZIFs (e.g. high selectivity). In principle, the

growth of ZIF films on flexible polymeric substrates can be easily achieved due to favourable

chemical interaction between the polymer and the organic ligand of ZIFs. Nagaraju et al. [18] and

Cacho-Bailo et al. [19] grew ZIF-8 on a porous polysulfone using in situ (direct) growth. However,

although the in situ synthesis is a simple method that allows for simultaneous nucleation,

deposition and crystal growth, it is not very effective in preparing continuous ZIF membranes due

to limited heterogeneous nucleation sites on the substrate [20]. Alternatively, Ge at al. [21] used

secondary seeded growth to fabricate a continuous ZIF-8 film on an asymmetrically porous

polyethersulfone substrate. Secondary seeded growth has been shown to effectively induce

controlled ZIF growth on the polymer support, but the resulting ZIF layer often suffers from weak

adhesion to the support, leading to membrane delamination [22]. The surface modification has

been commonly used to functionalize the support, thereby promoting heterogeneous nucleation

and enhancing the ZIF-to-substrate adhesion strength [23-25].

Very recently, we successfully developed a new strategy of vapour phase modification to

introduce amine groups and reduce surface pore sizes of the polymer support; such surface

modification enabled fast formation of a continuous ZIF-8 ultrathin layer in the presence of

ammonium hydroxide (as a deprotonating agent) under sonication for only 3 minutes [23].

However, the sonication-induced crystallization offers limited control over the ZIF-8 crystal sizes

and intergrowth, and thus membrane properties. Our group was one of the first groups to report



97

contra-diffusion synthesis, which self-limits growth of ZIF films on porous substrate, and has great

potential to offer better control in the membrane fabrication [11, 26-28]. However, the growth of

ZIF films via contra diffusion method depends on the surface properties and porous structure of

support; the formation of ZIF layer on both sides of support or within porous channels of support

has been reported [26, 27]. To achieve better control over the membrane position, Brown et al.

recently introduced an interfacial synthesis approach [29]. The control over the membrane position

relies on employing an oil/ water system, in which crystals grow at the interfaces between the two

immiscible solvents. The resulting ZIF-8 membrane exhibited high gas separation performance

with H2/C3H8 and C3H6/C3H8 separation factors as high as 370 and 12, respectively.

In this work, we report a simple, effective and environmental friendly method for the

fabrication of high-quality ZIF-8 membrane with controllable location on a polymer substrate in

aqueous solution. Our synthesis method is based on contra-diffusion (CD) concept in conjunction

with chemical vapour modification (hereafter chemical vapour modification-contra diffusion

method). A flat sheet asymmetric bromomethylated poly (2, 6-dimethyl-1, 4-phenylene oxide)

(BPPO) ultrafiltration membrane was prepared via phase inversion and employed as the support

for growing a thin ZIF-8 layer via contra-diffusion after modification. We have selected BPPO

for ZIF-8 growth because of its outstanding membrane formation and mechanical properties as

well as excellent hydrolytic stability [30]. It can also be easily functionalized and crosslinked due

to the abundant highly reactive –CH2Br groups. Using vapour-phase ethylenediamine (EDA), we

have previously shown that amine functional groups can be covalently attached selectively on the

top layer of the support without affecting the sublayer structure [23]. The presence of the covalent

link (amine groups) can be a driving factor for maintaining a high concentration of the metal ions

selectively near the support surface. When combined with the slow diffusion of the ligand in contra



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diffusion process, the approach can lead to well-controlled crystal growth in the vicinity of the

support surface. This results in the formation of thin, defect-free and robust ZIF-8 layer on one

side of the support at room temperature without the addition of deprotonating agents, which has

proven to be challenging when using other reported synthesis procedures [11, 26, 31, 32].

Materials and Methods

4.3.1 Chemicals

BPPO, BPPO was kindly supplied by Tianwei Membrane Co. Ltd., Shandong of China.

Ethylenediamine (EDA, 99.5%),1-methyl-2-pyrrolidone (NMP, 99.5%), zinc acetate dihydrate

(Zn(CH3COO)2.2H2O, 98%) and 2-methylimidazole (Hmim, C4H6N2, 99%) were purchased from

Sigma-Aldrich, Australia and used as received. Methanol (absolute) was purchased from Merck,

Australia. The water used for the experiments was purified with a water purification system (Milli-

Q integral water purification system, Merck Millipore) with a resistivity of 18.2 MΩ/cm. Distilled

water was obtained from a laboratory water distillation still (Labglass Aqua III).

4.3.2 Sample preparation

BPPO support ultrafiltration membranes were prepared via non-solvent induced phase

separation at room temperature [33]. The casting solution was prepared by dissolving 15 wt. % of

BPPO in NMP for 12 h with mechanical stirring at 200 rpm. The homogenous solution was left to

degas for 10 h before use. Subsequently, the solution was cast on a clean glass plate using an

adjustable micrometre film applicator (Paul N. Gardner Co., Inc. USA) with a gap of 200 μm at

room temperature (22 ± 2 °C) and immediately immersed in a coagulation bath of deionized water.

After peeling off from the glass plate, the membranes were removed from the bath, washed and

kept in fresh deionized water (DI) for at least one day to thoroughly remove the residual solvents.



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The vapour-phase EDA modification was conducted in a custom-made container according to

method developed in chapter 3 [23]. In brief, 20 mL of EDA was allowed to vaporize, and

stabilized for 1 h. The support membranes were quickly placed inside the containment with the

top layer exposed and suspended above the EDA solution. After surface modification at room

temperature for 16 h, the surface modified membranes were removed from the containment and

immediately washed with pure water to completely remove the residual EDA. The resultant

membrane were denoted as BPPO-EDA.

Figure 4-1. Digital photograph of a home-made contra-diffusion cell.

For preparation of the BPPO supported ZIF-8 membranes, the modified BPPO supporting

membrane was cut into 32 mm diameter discs, which were then mounted on a home-made setup

(Figure 4-1), where the zinc acetate solution and Hmim solution were separated by the supporting

membrane. Zinc acetate solution was prepared by dissolving 0.09 g of Zn (CH3COO)2.2H2O

(0.5 mmol) in 20 mL of deionized water, and Hmim solution was prepared by adding 0.649 g of

Hmim (8 mmol) in 20 mL of deionized water. The designed Hmim: Zn2+ molar ratios in the system



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was 16 and was kept constant in our study. After crystallization at room temperature (22 ± 2 °C)

for 60–120 min, the membrane samples were taken out and rinsed with DI water several times.

Finally, the composite membranes were dried in ambient conditions for 24 h, followed by heating

at 120-200 °C for 2 h before tests. The resulting samples were denoted as ZIF-8-BPPO-EDA-t-T,

where “t” and “T” denote the crystallization time and heat treatment temperature, respectively.


Fourier Transform Infrared (FTIR) spectra of the membranes were recorded using an

attenuated total reflectance (ATR) FTIR (Perkin Elmer, USA) in the range of 400-4000 cm-1 at an

average of 20 scans with a resolution of 4 cm-1. Scanning electron microscopy (SEM; FEI Nova

NanoSEM 450) with an X-ray detector (Bruker Nano GmbH, Germany) was used for imaging the

surface and cross-sectional morphologies of membranes. Energy-dispersive X-ray spectroscopy

(EDS) line-scan analysis of the membrane samples was conducted using EDX equipped in Nova

NanoSEM 450 (Quantax 400 X-ray analysis system, Bruker, USA). The membranes were

fractured in liquid nitrogen, fixed on stubs with double-sided carbon tape and then sputter coated

with roughly 2 nm iridium (Ir) layer to ensure good electrical conductivity. The images were

recorded at an accelerating voltage of 5 kV with different magnifications. Thermogravimetric

analyses (TGA) were carried out on a SETARAM (TGA 92) device from 30 to 800 °C at a heating

rate of 10 °C min-1 under air flow. Powder X-ray diffraction (XRD) patterns were measured using

a Miniflex 600 diffractometer (Rigaku, Japan) with Cu Kα radiation (15 mA and 40 kV) at a scan

rate of 2° min-1 with a step size of 0.02°. The XRD studies were carried out at room temperature.



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Figure 4-2. Schematic diagram of gas permeation set-up.

4.3.4 Single gas permeation test

The single gas permeation of composite membranes was measured using the pressure rise

method [34]. The schematic of the single gas permeation setup is shown in Figure 4-2. To measure

the gas permeation flux, the composite membrane (16 mm diameter disc) was attached to a porous

stainless steel holder (pore size ~200 nm) using epoxy resin (Torr seal, Varian), and then placed

inside a larger Pyrex tube and connected to a sensitive pressure transducer (MKS 628B Baratron)

and a vacuum line. The effective remaining membrane area was 1 cm2. For each single gas

measurement, the pure single gas was fed to one side (feed) of the membrane while the other side

(permeate) of the membrane was under vacuum. Since the feed side was at ambient pressure, a

pressure difference of 1 atm was maintained between the permeate side and the feed side during

permeation measurements. After allowing enough time to achieve a steady state conditions, the

permeate side was shut off from vacuum and the pressure build-up of the permeating gas was

measured by the pressure transducer and continuously recorded in a computer. To accomplish a



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single test, the pressure was allowed to reach a few Torr. To repeat an analysis, the permeate side

was evacuated again and then shut off from vacuum so as to record the pressure rise. All the gas

permeation tests were performed at room temperature. The molar flow rate of the permeating gas

was calculated based on the recorded pressure. The permeance, Pi, of each gas was calculated

according to the following equation,

Pi= (V/RTAΔp) (dp/dt)

where, V is the volume of the permeate side that was obtained by calibration using a bubble

flowmeter (m3) , R is the ideal gas constant (m3 Pa K−1 mol−1), T is the temperature (K), ∆p is the

pressure difference across the membrane (Pa), A is the effective membrane area (m2), and dp/dt is

the rate of pressure rise in the permeate side (Pa/s). The ideal selectivity Sij is defined as the ratio

of the two permeances Pi and Pj. Permeation data are average values recorded from at least three

samples, which were prepared from different batches.

4.3.5 Measurements of the support pore size

Polyethylene glycol (PEG) with a molecular weight of 300, 200, 100, 35, 20, and 10 kDa

(analytical grade, Sigma-Aldrich) was dissolved in deionized water to prepare 1 g L-1 aqueous

solutions for the estimation of molecular weight cut off (MWCO) and solute rejection. Rejection

analysis were performed at a pressure of 100 kPa. 20 ml of permeate was collected. The permeate

and feed solution were both diluted by 10 times and then the concentration of each solution was

measured via a total organic carbon analyser (TOC-LCSH, Shimadzu, Japan). The PEG rejection

was calculated from the measured feed (Cf) and permeate (Cp) concentrations by

𝑅 = (1 ‒ 𝐶𝑝/𝐶𝑓) × 100

https://en.wikipedia.org/wiki/Cubic_metre

https://en.wikipedia.org/wiki/Pascal_(unit)



103

All results represent average values for at least three repeated experiments with less than ±5%

deviation. The pore size of the membrane was defined as the hydrodynamic diameter of PEG. The

hydrodynamic radius of PEG can be calculated from the MWCO of the membrane by the following

equation [35]:

𝑆𝑜𝑙𝑢𝑡𝑒 𝑅𝑎𝑑𝑖𝑢𝑠 (𝑛𝑚) = 0.0262 𝑀𝑊 ‒ 0.03

where MW is the lowest molecular weight of the PEG molecule which has a rejection of 90% in

the ultrafiltration measurements.


4.4.1 Membrane support

Figure 4-3 illustrates the synthesis of dense and defect-free polymer supported ZIF-8

membrane using chemical vapour modification-contra diffusion method.

Figure 4-3. Schematic diagram of the preparation of a BPPO polymer supported ZIF-8 membrane

using chemical vapour modification and subsequent contra diffusion synthesis.



104

As illustrated in the figure (step (1)), the surface chemistry and pore size of the top layer of

the BPPO are modified by using EDA-vapour. Substitution of bromide functional group with

amine groups during EDA-vapour modification was confirmed by FTIR (Figure 4-4).

Figure 4-4. FTIR ATR spectra of the untreated BPPO support, BPPO modified with EDA-vapor

(BPPO-EDA), BPPO-EDA supported ZIF-8 layer (ZIF-8-BPPO-EDA), and ZIF-8 powder.

Upon modification, the peak at 586 cm-1 and 633 cm-1 attributed to the benzyl bromide (–CH2Br)

groups (C–Br stretching) almost disappear and a new broad band in the range of 3100–3600 cm-1

emerges, which is attributed to the N–H stretching and confirms the amination of BPPO. TGA

results (Figure 4-5) also show that when the temperature is below 150 ºC, BPPO-EDA have more

weight loss (~4%) than BPPO due to higher moisture residual in the membrane as a result of the

introduction of hydrophilic amine groups. Additionally, the decomposition of EDA initiates at ~

180 °C, which is higher than the boiling point (117 °C) of the EDA, indicating that there is an

interaction between the EDA molecules and BPPO. Furthermore, SEM images (Figure 4-6a, b)

show an obvious decrease in the size of the nanopores at the top surface of the membrane after

EDA vapour-phase modification. The reduction in the pore size of the support (from 25.5 to 15

nm) after its modification was further confirmed by TOC analysis (see measurements of the

support pore size in section 4-3-5). The changes in the surface microstructure can be attributed to



105

the partial cross-linking effect of the EDA, since the crosslinking causes tightening of the polymer

network which reduces the pore size of the BPPO substrate. In addition, the final decomposition

of the BPPO-EDA in the TGA results (Figure 4-5) is much slower than BPPO, which indicates a

higher thermal stability due to partial crosslinking of the BPPO substrate. Note that partial

crosslinking reduces the flexibility of the polymer support, which is favourable for avoiding ZIF

layer cracking [6].

Figure 4-5. Thermogravimetric analysis (under air flow) of untreated BPPO and BPPO-EDA

substrates.



106

Figure 4-6. SEM images of untreated BPPO (a), vapour-phase-EDA-modified BPPO (BPPO-EDA)

(b), ZIF-8@BPPO-EDA grown for 60 min (c), 90 min (d), and 120 min (e, f).



107

4.4.2 Supported ZIF-8 membrane

The ZIF-8 membrane is formed on the pre-treated support by applying contra diffusion

synthesis, in which the metal precursor solution and ligand (Hmim) solution are separated by the

modified BPPO substrate (step (2) in Figure 4-3), at room temperature for various crystallization

times. As demonstrated in chapter 3, the direct heterogeneous nucleation and growth of a dense

ZIF-8 layer on untreated BPPO surface was unsuccessful (Figure 4-7). In fact, due to the fast

diffusion of zinc ions through the pores of the unmodified support, crystallization occurs

constantly inside the support channels, where there can be a high concentration of the reactant

solutions, until the entire path through the ZIF-8 layer becomes “plugged”. A similar phenomenon

was observed by Hara et al. in preparing copper-benzene tricarboxylate (Cu-BTC) or ZIF-8

membranes using porous α-alumina capillary substrate by applying a typical contra diffusion

method [28, 32].

Figure 4-7. SEM images of ZIF-8 membranes grown for 2 h (a, b, c), 4 h (d, e, f), 6 h (g, h, i) via

conventional contra-diffusion method using untreated BPPO substrate.



108

Figure 4-8. XRD patterns of ZIF-8@BPPO-EDA membranes as a function of growth time.

After modification of the BPPO with EDA-vapour before contra diffusion synthesis, a

compact ZIF-8 layer was selectively formed on only one side (pre-treated side) of the support.

Figure 4-6 and Figure 4-8 show the SEM images and XRD patterns of the ZIF-8 membranes grown

for different lengths of time. As shown in Figure 4-6c and Figure 4-8, a large amount of ZIF-8

crystals with clear facets is observed on the modified support even after 60 min of the contra

diffusion synthesis at room temperature. Nanopores of the skin layer of the support are still

observable through inter-crystalline gaps between the ZIF-8 crystals in the high magnification

image (inset in Figure 4-6c). With increasing the reaction time to 90 min, a dense and continuous

ZIF-8 layer is formed on the modified BPPO skin layer, as shown in Figure 4-6e. Eventually, after

120 min of reaction, a layer of well intergrown ZIF-8 crystals with rhombic dodecahedron



109

morphology and a thickness of about 2 µm fully covered the support surface without any visible

defects such as pinholes or cracks (Figure 4-6e, f and Figure 4-9).

There are only few studies reporting such a thin continuous polycrystalline ZIF-8 film [24, 36]

and most of the membranes prepared by the conventional in situ methods are too thick (in the range

of tens of micrometers), showing lower gas flux through the membranes [11, 37]. The continuous

thin ZIF-8 membranes remained unchanged even with further growth, demonstrating the self-

limiting crystal growth, in which the crystals continue to grow only if the metal ions and the ligand

molecules are in contact.

Figure 4-9. SEM images of ZIF-8@BPPO-EDA grown for 120 min at different magnifications.

Cross-sectional view: (a, b, c); Top view: (d, e, f).



110

Figure 4-10. EDS line scan across ZIF-8@BPPO-EDA-120 cross-section for the zinc atoms.

Another important observation is that unlike the conventional contra diffusion method in

which the crystals grow along the whole thickness of the support, ZIF-8 crystals can be observed

only at the very outermost section of the EDA-vapour-modified BPPO support (Figure 4-6f).

Energy-dispersive X-ray spectroscopy (EDS) line-scan analysis (Figure 4-10) further confirmed

the presence of ZIF-8 within the support sublayer as zinc was detected up to about 1 µm underneath

the support surface. This means that the heterogeneous nucleation and crystal growth occur on the

skin layer of the pre-treated support, which is contrary to what was observed for the untreated

BPPO, where the nucleation and crystal growth happened all along the support channels with a

non-continuous film, if any, on the surface. As explained in our previous work, EDA can

simultaneously create amino groups, which can coordinate to the free zinc ions and provides a

large number of nucleation sites; and reduce the substrate pore size induced by its crosslinking



111

effect [23]. In the present work, therefore, the reduction in the surface pore size and the

coordination interaction between the support surface and zinc ions can lead to a decrease of the

diffusion rate of Zn2+ and provide a relatively high precursor concentrations at the support surface

and restricting the reaction zone in this vicinity (Figure 4-3). This high precursor concentration

and the large number of previously formed nucleation sites result in the faster and thinner crystal

growth in the vicinity of the support skin layer as compared to the slow and undirected crystal

growth along the channels of untreated BPPO.

4.4.3 Membrane prepared by conventional contra-diffusion

Figure 4-11 presents ZIF-8 membranes synthesized by contra diffusion (at the reaction time of 2h)

after modification of the BPPO via immersion of the polymer in 60% EDA aqueous solution at

room temperature for 3h. It is obvious that the crystals are grown along the whole thickness of the

polymer. As shown, ZIF-8 crystals not only block the polymer micro-channels but also grow

within the whole porous structure of the polymer as the interface between the ZIF-8 and the

polymer matrix is hardly distinguishable. This is because the immersion of the polymeric support

in the nucleophilic diamine solution can result in an extremely high degree of modification (a large

number of nucleation sites) within the bulk polymer and the subsequent formation of ZIF-8 crystals

when applying contra diffusion synthesis. This shows that crystal formation within the whole

polymer support is unavoidable when modifying the BPPO via solution immersion method.

Instead, as already shown, contra-diffusion method in conjunction with vapour phase modification

of the support offers more degrees of freedom in directing the formation of ZIF-8 membranes.

It is worth mentioning that growing MOFs inside the pores of the support can be very attractive

for molecular separations and such membranes have been shown to outperform mixed matrix

membranes (MMM) for organic solvent nanofiltration (OSN) applications, as demonstrated by



112

Livingston et al [16, 37, 38]. Although the resulting ZIF-8 membrane (Figure 4-11) was too brittle

to allow permeation experiments, this work demonstrates a new methodology for in situ growth of

ZIFs predominantly inside the supports, which needs to be improved further for practical

applicability.

Figure 4-11. SEM images of cross-section of ZIF-8 membrane synthesized by contra-diffusion (at

the reaction time of 2 h) after modification of the BPPO via immersion of the polymer in 60%

EDA aqueous solution at room temperature for 3 h.


To further evaluate the quality of the obtained ZIF-8 membranes, single-component gas

permeation experiments were conducted, and the results are summarized in Figure 4-12 and

Table 4-1.



113

Figure 4-12. Single gas permeances (a) and ideal selectivities (b) as a function of kinetic diameter

of gas molecules of the ZIF-8 membranes grown for 120 min and activated at 120 °C

(ZIF8@BPPO-EDA-120-120) and at 150 °C (ZIF8@BPPO-EDA-120-150).

Table 4-1. Single gas permeances and ideal selectivities for the ZIF-8@BPPO-EDA-x-y (x:

crystallization time (min); y: heat treatment temperature (⁰C)) composite membranes at 25 ⁰C and

1 bar.

Sample ID Permeance

(10-7 mol.m-2.s-

1.Pa-1)

Ideal selectivity

H2 C3H6 H2/CO2 H2/N2 H2/CH4 H2/C3H8 C3H6/C3H8

BPPO 448.3

±19.5

155.5

±16.5

1.1

±0.1

1.2

±0.2

1.4

±0.2

2.9

±0.2

1

±0.2

BPPO-EDA 214.8

±19.0

96.6

±14.5

2.1

±0.6

1.8

±0.4

2.1

±0.3

2.3

±0.1

1

±0.3

ZIF-8@BPPO-

EDA-30

104.1

±6.0

48.2

±7.4

1.4

±0.2

1.2

±0.1

1.5

±0.1

2.2

±0.3

1.02

±0.3

ZIF-8@BPPO-

EDA-90

19.1

±2.8

0.67

±0.1

6.5

±1.0

5

±0.8

7.7

±2.7

32.9

±3.5

1.1

±0.4

ZIF-8@BPPO-

EDA-120

7.5

±0.2

0.15

±0.01

5.1

±0.2

8.3

±0.2

9.1

±0.2

833.3

±202.6

16

±4.4

ZIF-8@BPPO-

EDA-120-150

6.1

±0.1

0.075

±0.004

5.5

±0.1

9.2

±0.3

10.2

±0.3

2259

±159.4

27.8

±3.6

ZIF-8@BPPO-

EDA-120-200

10

±0.2

1.3

±0.1

4.3

±0.3

4.5

±0.3

5.8

±0.4

35.7

±3.3

4.5

±0.9



114

In comparison, single gas permeation of the untreated BPPO and its vapour phase modified

counterpart were tested. Due to their large pores (pore size of 25.5 and 15 for untreated and treated

supports, respectively), none of these membranes showed any obvious gas selectivity. However,

upon modification, H2 permeance was decreased by more than half when compared to the

untreated support. This is attributed to the reduced pore size of the support induced by partial

crosslinking effect of the EDA modification, which increases the support dimensional stability and

surface tightness. This also lessens the flexibility of the polymeric substrate, which is beneficial

for reducing ZIF layer cracking [24, 25]. Membranes started to display molecular sieve

performance, favouring the smaller molecules, with a moderate H2 permselectivity after 90 min of

the crystal growth (H2/C3H8 ideal selectivity of 32.9 compared to Knudsen diffusion selectivity of

4.7). However, no clear C3H6/C3H8 ideal separation selectivity was observed at this point,

indicating the presence of grain boundary micro-defects. As crystallization time is extended, more

pronounced molecular sieving behaviour with an obvious increase in the propylene/ propane ideal

selectivity can be observed which reaches as high as 16 after 120 min reaction time. It is worth

mentioning here that except for a few membranes [11, 29, 32, 39, 40], majority of ZIF-8

membranes reported so far have not shown any decent C3H6/ C3H8 selectivity [41]. Figure 4-13

depicts C3H6/ C3H8 separation performance of ZIF-8 membranes developed in this study in

comparison to those reported in the literatures. As can be seen, our membranes not only overtake

polymeric and ZIF-8 mixed-matrix membranes in terms of C3H6/C3H8 selectivity and C3H6

permeance they are also amongst the best ZIF-8 membranes previously reported (Table 4-2). For

instance, the high quality ZIF-8 membranes made in water-octanol system by interfacial

microfluidic membrane processing (IMMP) method could achieve H2/C3H8 ideal selectivity of

more than 600 and the permeance of H2 around 50 ×10-8 mol m-2 s-1 Pa-1 [29]. While in the current



115

study, the H2/C3H8 selectivity and H2 permeance are considerably enhanced, with H2/C3H8

selectivity of 833.3 and H2 permeance of 75×10-8 mol m-2 s-1 Pa-1. The enhancement in permeance

in this study is in agreement with the reduction in thickness of the ZIF-8 layer (~2 versus ~9 µm

in [29]) and also the highly porous and asymmetric structure of the support, which minimizes the

overall hydraulic resistance of the permeate flow through the membrane structure; whereas the

enhanced selectivity is mainly due to the improved membrane quality, such as the well-structured

grain boundary and absence of pinholes or defects.

Figure 4-13. Comparison of C3H6 permeability and C3H6/C3H8 selectivity of the membranes in the

present work with previously reported membranes. Closed and open symbols indicate separation

data obtained from single and binary gas permeation analysis, respectively. Hexagon: inorganic

supported ZIF-8 membranes [10]; pentagon: ZIF-8 mixed matrix membranes [15]; triangle:

polymer membranes [42]; circle: carbon membranes [43]; star: polymer supported ZIF-8

membranes in this study.



116

Table 4-2. Comparison of gas permeation properties of the ZIF-8@BPPO-EDA composite

membrane in this work with other ZIF-8 membranes in the literature.

Support Synthesis

method/

Synthesis

media

Synthesis

T[°C]

ZIF

thickness

/μm

Permeance

(10-7 mol/m2.s.Pa)

Selectivities Ref

H2 C3H6

Polymeric supports

BPPO Contra

diffusion in

conjunction

with chemical

vapour

modification

(CD-CVM)/

H2O

Room ~2 7.5 0.15 8.3

Η2/Ν2

5.1

Η2/CΟ2

9.1

Η2/CΗ4

833

Η2/C3Η8

16

C3Η6/C3Η8

This

work

Torlon Interfacial

microfluidic

membrane

processing

(IMMP)/

C8H17OH-

H2O

Room ~9 5.8 0.077 682

H2/C3H8

9.2

C3H6/C3H8

[29]

PAN Double-zinc-

source

method/

H2O-NaOH

Room 0.9 1.23 _ 26

H2/C3H8

[44]

PSf In situ growth

followed by

LBL

deposition/

CH3OH-

NaOH

Room ~10 3.97 1.05 3.8

Η2/CΟ2

[18]



117

Support Synthesis

method/

Synthesis

media

Synthesis

T[°C]

ZIF

thickness

/μm

Permeance

(10-7 mol/m2.s.Pa)

Selectivities Ref

H2 C3H6

Polymeric supports

PEI/ZnO secondary

seeding

method/ H2O

45 1.5 16 _ 22.4

H2/C3H8

[45]

PES Metal based

gel

deposition/

CH3OH

80 20 1.11 _ 22.7*

Η2/Ν2

5.2*

Η2/CΟ2

[46]

PES Microfluidic/

CH3OH-

NaCOOH

Room ~3.6 0.05 _ 18.3*

Η2/Ν2

2.6*

Η2/CΟ2

17.2*

Η2/CΗ4

[36]

PSf In situ

synthesis /

CH3OH-

NaCOOH

Room 35 2 _ 12.4*

Η2/Ν2

10.5*

Η2/CΗ4

[19]

PVDF Chemical

modification/

CH3OH-

NaCOOH

110 ~30 24.4 _ 14.3

Η2/Ν2

12.1 Η2/CΟ2

[25]

PES Secondary

growth/

CH3OH

90 7.2 4 _ 9.9 Η2/Ν2

9.1 Η2/CΗ4

9.7 Η2/Ar

10.8 Η2/O2

[21]



118

Support Synthesis

method/

Synthesis

media

Synthesis

T[°C]

ZIF

thickness

/μm

Permeance

(10-7 mol/m2.s.Pa)

Selectivities Ref

H2 C3H6

Polymeric supports

PAN Contra-

diffusion/

CH3OH

Room 16 19.7 _ 4.3 Η2/Ν2

[26]

Nylon Contra-

diffusion/

H2O-NH4OH

Room 2.5 11.3 _ 4.3 Η2/Ν2

[27]

BPPO Chemical

modification/

CH3OH-NH3

Room ~0.2 20.5 _ 9.7 Η2/Ν2

12.8

Η2/CΟ2

[23]

Inorganic supports

α-

alumina

Contra-

diffusion/

CH3OH

50 60 _ 0.011 135

C3Η6/C3Η8

[31]

α-

alumina

Contra-

diffusion/

CH3OH

50 80 0.91 0.025 10 Η2/Ν2

2000

Η2/C3Η8

59

C3Η6/C3Η8

[32]

α-

alumina

Secondary

growth/

H2O

30 2.2 _ 0.3 50*

C3Η6/C3Η8

[10]

α-

alumina

Secondary

growth/

CH3OH-

NaCOOH

100 12 ~1 _ 15 Η2/CΗ4

300

Η2/C3Η8

[47]



119

Support Synthesis

method/

Synthesis

media

Synthesis

T[°C]

ZIF

thickness

/μm

Permeance

(10-7 mol/m2.s.Pa)

Selectivities Ref

H2 C3H6

Inorganic supports

α-

alumina

Microwave-

assisted

seeding and

secondary

growth/

CH3OH-

NaCOOH

100 W for

1.5 min

1.5 _ 0.208 40*

C3Η6/C3Η8

[48]

α-

alumina

Contra-

diffusion -

based in situ

method/

CH3OH-

NaCOOH

120 1.5 _ ~0.2 50*

C3Η6/C3Η8

[11]

α-

alumina

Rapid thermal

deposition

(RTD)/ N,N-

dimethyl

acetamide

(DMA)- H2O

200 5-20 _ 0.07 30*

C3Η6/C3Η8

[40]

α-

alumina

Contra-

diffusion-

based in situ

method/

CH3OH-

NaCOOH

120 ~1 _ ~0.27 70*

C3Η6/C3Η8

[49]

Inorganic supports

Alumina

hollow

fiber

Hot support

seeding/

CH3OH-

NaCOOH

120 20 7.3 _ 9.2

Η2/Ν2

5.4

Η2/CΟ2

[50]



120

10.8

Η2/CΗ4

7.1

Η2/O2

γ-

alumina

Surface

chemical

modification/

CH3OH-

NaCOOH

100 25 1.4 _ 10

Η2/Ν2

4.2

Η2/CΟ2

12.5

Η2/CΗ4

[51]

Titania Direct

synthesis/

CH3OH-

NaCOOH

100 ~30 0.6 _ 11.6

Η2/Ν2

4.5

Η2/CΟ2

12.6

Η2/CΗ4

[52]

*Gas mixture separation.

Highly improved grain boundary structure, on the other hand, can be related to the confinement

effects of the crystals within the porous support as the confinement can increase the compactness

of the grain boundary structure [49]. Another possible reason for the high quality grain boundary

structure can be the well-intergrown ZIF-8 crystals with no preferred orientation as a result of the

aqueous synthesis. It was found that water, being less acidic, in comparison with organic solvents

can more easily deprotonate the organic ligand on the growing surface, leading to growth occurring

in more directions, resulting in a better crystals intergrowth and formation of denser ZIF

membranes [39, 53]. Attributed to the formation of high heterogeneous nucleation density in the

vicinity of the support surface, the EDA vapour modification is helpful for the controlled synthesis

of thin, defect-free and reproducible ZIF-8 membranes. In summary, the enhanced gas permeation

properties strongly suggest that the chemical vapour modification-contra diffusion method



121

provides a new route for preparing high quality ZIF-8 membranes having superior grain boundary

structure as compared to those prepared by other methods.

Figure 4-14. SEM images of ZIF-8@BPPO-EDA-120 membranes after activation at 150 ºC (a, b,

c) and 200 ºC (d, e, f). (a, d, e) top view and (b, c, f) cross-sectional view.

4.4.5 Effects of activation temperature on the ZIF-8 membranes

Finally, we investigated the effects of activation temperature on the morphology and

performance of ZIF-8 membranes. Membranes (grown for 120 min) were further exposed to 150

and 200 ºC for 2 h under oxidative conditions (air). For the membranes activated at 150 ºC, a

compact well-intergrown ZIF-8 grains of rhombic dodecahedral shape with no defects (i.e.

pinholes or cracks) in the entire membrane surface can be observed (Figure 4-14). A very intimate

contact between ZIF-8 and the support is also observed in the cross section view of the membranes

as the interface between ZIF-8 and the support is hardly distinguishable.

Figure 4-15 shows the room-temperature C3H6/C3H8 permeation properties of ZIF-8

membranes activated at different temperatures.



122

Figure 4-15. Room-temperature propylene/propane permeation properties of ZIF-8 membranes

grown for 120 min (ZIF-8@BPPO-EDA-120) as a function of activation temperatures.

As shown in the figure, by increasing the activation temperature from 120 to 150 ºC the

C3H6/C3H8 ideal selectivity is significantly increased, with minimal effect on the permeance. This

unique behaviour indicates that the ZIF-8@BPPO-EDA membrane becomes even denser with

probably more compact grain boundary structure when activated at a higher temperature. This

positive result is likely attributed to the fact that the BPPO and BPPO-EDA form crosslinking

structure by heating (Figure 2-16) [54], which can further increase the stability and tightness of

the support. This was further supported by the 20 (±4) % reduction observed in the hydrogen

permeability of the BPPO-EDA substrate upon heating at 150 °C for 2 h.



123

Figure 4-16. Heat-induced cross-linking of BPPO substrate.

Since a fraction of ZIF-8 is formed within the support porous structure, it may subsequently

enhance the interfacial interaction between the ZIF-8 and the support and also increase the

compactness of the grain boundary structure, thereby minimizing the non-selective intercrystalline

diffusion and leading to improved separation performance. Thermally induced cross-linking

reaction was verified by FTIR result (Figure 4-17) where bands attributed to the benzyl bromide

(CH2Br) and amine groups for BPPO and BPPO-EDA, respectively, disappeared upon their

thermal treatment at 150 ºC. Another possible reason for the observed improvement in the

membrane selectivity activated at a higher temperature can be the complete removal of residual

solvent molecules from the ZIF-8 layer [55]. However, further analysis is required to fully

understand the effect of annealing temperature on the grain boundary and subsequent gas

performance. Further increasing the activation temperature up to 200 ºC results in a significant

increase in the propylene permeance (more than one order of magnitude) with a drop in C3H6/C3H8

ideal selectivity from 27.8 to 4.5, indicating the grain boundary structure of the membranes was

compromised. While the XRD patterns in Figure 4-18 indicate that the ZIF-8-BPPO-EDA samples

did not undergo obvious structural alterations in the studied temperature range compared to the

simulated ZIF-8 pattern, the FTIR shows a drop in the intensity of the Zn-N peak for the membrane

activated at 200 °C. The activation process at the elevated temperature caused some degradation



124

of the ZIF-8 layer (Figure 4-14d, e, and f). However, as can be seen from the cross sectional view

(Figure 4-14f), the degradation was apparently restricted to the membrane surface, resulting in still

reasonable C3H6/C3H8 ideal selectivity of 4.5 (compared to the Knudsen propylene/propane

selectivity of ∼1.02). Similar degradation behaviour of ZIF-8 membranes upon activation process

at high temperatures were recently reported and was correlated to the corrosion action of water or

methanol on ZIF-8 grains [49, 56]. These results indicate that the activation temperature plays a

critical role in determining the gas permeation properties of the ZIF-8 membranes. However, an

elaborate choice of activation conditions (temperature, duration, and environment) is required in

order to achieve ZIF-8 membranes with the best performance.

Figure 4-17. FTIR ATR spectra of the BPPO support, BPPO support after being heated at 150 ºC

(BPPO-150) for 2h under air, EDA-vapour-modified BPPO (BPPO-EDA), EDA-vapour-modified

BPPO after being heated at 150 ºC (BPPO-EDA-150) for 2h under air.



125

Figure 4-18. (a) FTIR spectra and (b) XRD patterns of ZIF-8@BPPO-EDA membranes (grown

for 120 min) as a function of activation temperature (°C).

It should be noted that the propylene/propane selectivity obtained in this study is the best ever

obtained for ZIF-8 membrane on polymeric supports, but it is still lower than those obtained with

inorganic-supported ones. For example, alumina-supported ZIF-8 membrane prepared by an in

situ counter-diffusion method exhibited both higher C3H6/C3H8 selectivity (∼50) and propylene

permeability (∼200 ×10-10 mol m-2 s-1 Pa-1) than the best membrane obtained in this work with the

C3H6/C3H8 selectivity and propylene permeability of 27.8 and 75×10-10 mol m-2 s-1 Pa-1,

respectively [11]. However, since the contra diffusion method is dependent on the surface

properties and porous structure of support, the performance of the membranes could be further

improved by investigating the effect of the modification reaction condition. Optimizing activation

conditions could also further improve the performance of the ZIF-8 membranes.

In addition, it is worth mentioning that the synthesis procedure developed here offers a number

of advantages over those reported in the literature. First of all, almost all of previously reported

ZIF membranes were made by using organic solvents (e.g. methanol, dimethyl formamide, octanol)

and/or alkaline additives (e.g. sodium formate, ammonia) under non-ambient conditions (i.e. high

temperature, high pressure) [11, 27, 52, 57]. In many other cases the use of seed crystals and a

long reaction time were unavoidable [26, 39, 47, 58]. The high quality ZIF-8 membranes in this

study were made in aqueous solution under ambient conditions (room temperature, atmospheric



126

pressure) in a relatively short period of time (less than 2 h), without any additives or seed crystals.

The synthesis process requires significantly smaller amounts of metal salt and organic ligand

reagents. For example, ∼ 41-90% savings in the usage of the reagents (per cm2 of permeable area)

could be achieved compared to ZIF-8 membranes fabricated by the microfluidic experimental

approach [36].

Summary

In summary, we reported a novel strategy, contra-diffusion based synthesis in conjunction

with vapour modification, for room temperature synthesis of high-quality ZIF-8 membranes on an

asymmetric polymeric substrate in aqueous solution. The ZIF-8 membranes have shown excellent

gas permeation properties (e.g. propylene/propane ideal selectivity of 16 with propylene

permeance of 150×10-10 mol m-2 s-1 Pa-1), intensely indicating impressively enhanced membrane

microstructure (in particular enhanced grain boundary structure). More importantly, by increasing

the activation temperature from 120 to 150 ºC, the propylene/propane selectivity was further

increased (almost two-fold), without compromising the high permeance of propylene, indicating

the important role of thermal activation conditions (in particular activation temperature) in

microstructures of ZIF-8 membranes. With efficient synthesis conditions, the strategy developed

here provides an effective and environmentally friendly route for preparing high-quality ZIF

membranes on the surface of polymeric support.



127

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Chapter 5 One-dimensional Material as Nano-scaffold and Pseudo-seed for Facilitated

Growth of Ultrathin, Mechanically Reinforced Molecular Sieving Membranes

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One-dimensional Material

as Nano-scaffold and Pseudo-seed for

Facilitated Growth of Ultrathin,

Mechanically Reinforced Molecular

Sieving Membranes

Overview

In chapter 4, we combined the vapour phase modification strategy, described in chapter 3,

with a simple contra-diffusion method for the scalable fabrication of polymer-supported ZIF-8

membranes in aqueous solution at room temperature. The method offers an environmentally

friendly route for controllable fabrication of high-quality polymer-supported ZIF membranes, but

the development of a simpler and more versatile method that can construct ZIF layer on various

substrates without substrate modification is desired. In this chapter, we utilize one-dimensional

(1D) materials such as carbon nanotubes (CNTs) to form a porous nano-scaffold layer on the

porous substrate for facilitated growth of ultrathin ZIF membranes with mechanically reinforced

structures. CNTs with surface enriched coordination sites create a uniform pseudo-seeding matrix

layer that facilitates rapid nucleation and crystal growth during membrane formation. The

submicron-thick ZIF-8 hybrid membrane (100-200 nm), consisting of a CNT network integrated

in a ZIF-8 matrix, exhibits good mechanical and structural stability whose performance is amongst

the best ZIF membranes studied so far. This chapter has been reformatted from the following



134

submitted manuscript: Shamsaei, E., Lin, X., Wan, L., Tong, Y., and Wang, H. One-dimensional

material as nano-scaffold and pseudo-seed for facilitated growth of ultrathin, mechanically

reinforced molecular sieving membranes. Chem Commun. 2016, 52 (95), 13764-13767.

Introduction

Zeolitic imidazolate frameworks (ZIFs), Membrane-based separation methods are gaining

increasing importance for energy-efficient gas separations and other molecular discriminations [1].

Among many porous materials, metal-organic frameworks (MOFs), owing to their permanent

porosity with a diverse structure, chemistry and relative ease of preparation, have been extensively

investigated for their application as molecular sieving membranes. MOF membranes not only offer

an alternative to overcome the traditional permeability-selectivity trade-off of polymer membranes

[2-4], but also they are seen as promising tools in the new area of enantioselective and chiral

separations [5]. The major requirement for large-scale, efficient separations is to develop scalable

preparation of robust, ultrathin and defect -free MOF membranes. It is well known that the highly

reproducible fabrication of such membranes is particularly challenging owing to the difficulty of

directing nucleation and crystal growth onto the surface and the tendency for growth into

unfavourably large crystals and thick MOF layers. Among numerous fabrication methods being

developed, the seeded method, in which the porous substrates are coated with MOF crystals for

secondary crystal growth, has been proven to be one of the most effective ways to grow MOF

membranes [6-8]. However, seeding route often relies on the MOF features and the support surface

chemistry and pore size distribution, requiring elaborate preparation of the crystal seed layer. The

use of nanosheets as seeds, which allows for seeding of substrates with large pores and rough

surfaces, has been recently investigated as a practical solution to the problems associated with the

conventional seeding routes; such a strategy has been demonstrated to be effective for controllable



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growth of ultrathin molecular sieving membranes [9, 10]. For instance, our group has very recently

applied two dimensional ZIF-8/ graphene oxide (2D ZIF-8/GO) hybrid nanosheets as seeds to the

synthesis of ultrathin ZIF-8 membranes with a thickness of around 200 nm and superior CO2/N2

selectivity [11]. However, the efficient synthesis of the nanosheet seeds remains a key task for

adoption of such a seeding technique for fabrication of molecular sieving membranes from a wide

variety of MOF structures.

In this chapter, a new concept for using one-dimensional material as nano-scaffold for

fabrication of molecular sieving membranes supported on a porous substrate is described, inspired

by the great success of nano-scaffolding technique in tissue engineering where biocompatible

nanofibers are used as nano-scaffold to promote tissue growth and provide mechanical support

[12]. We propose to utilize one-dimensional (1D) materials such as carbon nanotubes (CNTs) and

carbon nanofibers (CNFs) to form a porous nano-scaffold layer on the porous substrate for

facilitated growth of ultrathin MOF membranes with mechanically reinforced structures. Owing

to their high surface area, outstanding mechanical strength, and thermal and chemical stability, 1D

carbon materials (CNTs and CNFs) have been widely employed for the synthesis of novel

membranes both as direct filters and effective reinforcing fillers [13, 14]. A number of methods,

such as surface oxidation and coating, have been introduced for the chemical modification and

dispersion of these 1D carbon nanomaterials, making it possible to prepare a variety of carbon

based composite materials [15]. Particularly, hybrid composites based on CNTs and MOFs have

recently attracted enormous interest due to their unique synergistic effects [16]. Several MOF-

CNT hybrids, such as MOF-5/CNT [17, 18], HKUST-1/CNT [19], ZIF-67/CNT [20], ZIF-8/CNT

[21, 22], have been reported, clearly showing the feasibility of using 1D carbon materials as nano-

scaffold for MOF growth and mechanical toughening.



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To demonstrate our new concept, polydopamine (PDA)-coated CNTs are utilized as nano-

scaffold for construction of ultrathin MOF membranes on porous substrates. The high-surface-

area CNTs provide superior contact with MOF precursors during crystallization; and the surface

functionalization with PDA endows CNTs with the metal-chelating ability of catecholamine

moieties present in the PDA structure [23]. It is anticipated that the PDA-coated CNTs functions

as the so-called ‘‘pseudo-seeds’’ to bind MOF building blocks onto various surfaces. These

flexible, highly-dispersible hybrid nanotubes allow for the uniform ‘‘pseudo-seeding’’ of

substrates via a simple coating method, without substrate modification and/or the complicated

seeding processes usually needed in conventional membrane preparation. Importantly, this new

concept has great potential for further development as a versatile platform technique for the

scalable fabrication and mechanical reinforcement of ultrathin molecular sieve membranes.

Zeolitic imidazolate framework-8 (ZIF-8), Zn (Hmim)2 (Hmim = 2-methylimidazolate), was

selected as an example to illustrate our new technique. ZIFs possess the attractive features of both

zeolites and MOFs such as tunable pore size and chemistry, high chemical and thermal stability,

and as such are increasingly being explored for water treatment, chemical sensors, and gas

separation applications [24-27]. Owing to its small aperture size (∼3.4 Å) and sodalite-related

structure, defect-free ZIF-8 membranes effectively separate H2 (molecular size ∼2.9 Å) from

larger molecules such as CO2 (3.3 Å), N2 (3.6 Å), CH4 (3.8 Å), C3H6 (4 Å), and C3H8 (4.3 Å), and

therefore they are promising candidates for hydrogen separations.



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Materials and Methods

5.3.1 Chemicals

All chemicals were used as received. Anopore aluminium oxide (AAO, GE Healthcare Life

Sciences; diameter: 25 mm, pore size: 0.1 µm) membranes, with annular polypropylene ring were

used as support. The polyethersulfone (PES) membrane with diameter of 47 mm and 0.03 micron

pore size was provided by Sterlitech Corporation. Carbon nanotubes, (multi-walled O.D. × I.D. ×

L 10-20 nm × 5-10 nm × 0.5-200 μm, purity ≥95%) were purchased from Sigma Aldrich.

Dopamine hydrochloride (98 %, Sigma-Aldrich) and tris(hydroxymethyl)aminomethane (Tris,

ACS reagent, ≥99.8%, Sigma-Aldrich) were used for CNT coating layer modification. Zinc nitrate

hexahydrate (Zn(NO3)2.6H2O, 98%) and 2-methylimidazole (Hmim, C4H6N2, 99%) supplied from

Sigma-Aldrich were used for ZIF-8 preparation. Methanol (absolute) was purchased from Merck,

Australia. The water used for the experiments was purified with a water purification system (Milli-

Q integral water purification system, Merck Millipore) with a resistivity of 18.2 MΩ/cm.

5.3.2 Polydopamine modification of CNTs

CNTs were modified with polydopamine by an ethanol-mediated oxidative dopamine

polymerization process as reported previously [28]. Briefly, 10 mg of CNTs was dispersed in a

solution of water (15 mL) and ethanol (20 mL) under sonication, followed by the addition of 40

mg dopamine. 10 mL of Tris aqueous solution (25 mM) was then added with magnetic stirring.

After 2 h of reaction at room temperature, the modified CNTs were refined from the solution by

centrifugation (5000 rpm for 5 min) and washed with deionized water three times. The modified

CNTs were dispersed by 200 ml fresh deionized water and kept as mother solution.



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5.3.3 Preparation of ZIF-8/CNTs membrane on porous AAO disk

The above mother solution was sonicated for 60 min to yield a stable and uniformly dispersed

nanotubes. 3 ml of the solution was then deposited onto the AAO porous membranes by vacuum

filtration. Contra-diffusion method was conducted to fabricate the ultrathin ZIF-8/CNT

membranes [29]. The CNTs deposited supports were mounted on a homemade setup, where the

zinc nitrate solution and Hmim solution were separated by the supporting membrane. The zinc

nitrate solution, prepared by dissolving 0.293 g of Zn(NO3)2.6H2O I n 20 mL of methanol, was

added to the CNTs deposited side of the support and Hmim solution, prepared by adding 0.649 g

of Hmim in 20 mL of methanol, was immediately added to the other side of the support. The

designed Hmim: Zn2+ molar ratios in the system was 8 and was kept constant throughout the study.

After crystallization at room temperature (20 ± 2 °C) for up to 60 min, the obtained membranes

were thoroughly rinsed with methanol and dried at 50 °C for 6 h. For comparison, ZIF-8

membranes were also prepared on a bare AAO and on a pristine CNTs deposited AAO, following

the same preparation method described above for 60 min growth.


Powder X-ray diffraction (XRD) patterns were recorded at room temperature using a Miniflex

600 diffractometer (Rigaku, Japan) with Cu Kα radiation (15 mA and 40 kV) at a scan rate of 2°

min–1 and a step size of 0.02°. Scanning electron microscopy (SEM; FEI Nova NanoSEM 450)

was used for imaging the surface and cross-sectional morphologies of membranes. The membranes

were sputter coated with roughly 2 nm iridium (Ir) layer to ensure good electrical conductivity.

Transmission electron microscopy (TEM) images were taken by a FEI Tecnai G2 T20 operated at

an accelerating voltage of 200 kV. Fourier Transform Infrared (FTIR) spectra of the samples were



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taken by an attenuated total reflectance (ATR) FTIR (PerkinElmer, U.S.A.) at an average of 20

scans with a resolution of 4 cm–1.

5.3.5 Gas permeation experiments

The single gas permeation of hybrid membranes was measured using the pressure rise method

[30], as described in detail in chapter 4. The membrane samples were attached to a porous stainless

steel holder using a vacuum sealant (Torr seal, Varian). The gas permeation tests were performed

at room temperature (20 ± 2 °C) on H2, CO2, N2, and CH4. The pressure rise of the permeating gas

was measured using a pressure transducer (MKS 628B Baratron).


5.4.1 PDA-coated CNTs

The method developed in this work is quite simple, comprising three steps as illustrated in

Figure 5-1.

Figure 5-1. Schematic illustration of the preparation process of ZIF-8/CNT membrane through

deposition of modified CNTs on the support, followed by a contra-diffusion synthesis.



140

CNTs were first coated by a layer of PDA in a freshly prepared dopamine solution via a very

simple process as reported elsewhere [28]. The CNTs coated with PDA show considerably better

water dispersibility as compared to the pristine CNTs (Figure 5-2a), indicating an obvious change

in surface properties. In high resolution transmission electron microscopy (HRTEM) images

(Figure 5-2a), a layer of lower contrast, surrounding the CNTs can be easily identified,

demonstrating a conformal PDA coating on CNTs. Fourier transform infrared spectroscopy (FTIR)

analysis further confirmed that PDA was successfully coated on CNTs (Figure 5-2c). Upon coating

with PDA, new absorption bands at around 1633 and 3438 cm-1 appear, which are assigned to the

aromatic rings and catechol –OH groups [28], respectively, confirming the successful deposition

of PDA on the surface of the CNTs. For the minimal changes in the intrinsic properties of CNTs,

specifically its flexibility and diameters, an extremely thin coating layer (< 2 nm) of PDA was

formed on the CNTs. The introduction of thin PDA coating on the CNTs made no difference in

the X-ray diffraction patterns (XRD) (Figure 5-2b) as compared with the pristine CNTs. After the

successful conformal coating, a dilute suspension of polydopamine-coated CNTs was simply

vacuum-filtered onto a porous substrate such as an anodized aluminum oxide (AAO) disk. The

homogeneity of the deposited layer is guaranteed by high water dispersibility of the nanotubes

(Figure 5-3). Interestingly, PDA-coated carbon nanotubes tend to recline with ultimate overlap as

they accumulate onto the substrate, producing an ultrathin film with maximal mechanical integrity.

A nanoporous structure is also created by the interstices between the CNTs within the film. These

can have an important implication for the next step for successful fabrication of continuous

ultrathin ZIF-8 membrane with reinforced microstructure. Finally, the ZIF-8/CNT membrane was

synthesized via a simple room-temperature contra-diffusion method [31].



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Figure 5-2. (a) Photos of water dispersibility and the corresponding TEM images of CNTs (I) and

polydopamine-coated CNTs (II), XRD patterns (b) and FTIR spectrum (c) of CNTs (I) and

polydopamine-coated CNTs (II), (f) schematic illustration of the coated CNT and the chemical

structure of polydopamine.

5.4.2 ZIF-8/CNT membrane

Well-grown ultrathin ZIF-8 membranes were prepared in only one hour of synthesis time. As

shown in the top-view scanning electron microscope (SEM) images (Figure 5-4) the support

surface was fully covered with well-grown and compact ZIF-8 nanocrystals devoid of any defects

such as pinholes. From the cross-sectional view, an ultrathin well intergrown ZIF-8 layer of 100-

200 nm is observable. Another important observation is that the interface between ZIF-8 layer and

the AAO is hardly distinguishable and also the CNTs are completely embedded within the ZIF-8

crystal matrix, indicating tight contacts between ZIF- 8, CNTs, and the support. This exceptionally

thin, compact and defect-free ZIF-8 layer on the porous substrate is expected to favour gas

separations.



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Figure 5-3. SEM (a, b) and optical (c) images of pristine (a, c1-c3) and modified (b, c4-c6) CNTs-

deposited on AAO. Detailed experimental: deposited pristine CNTs from (c1, c4) 1 mL (c2, c5) 3

mL and (c3, c6) 6 mL mother solution. Pristine CNTs mother solution: 10 mg CNTs in 200 mL

DDI water.

Interestingly, ZIF-8 layer formed only on one side (i.e. nanotubes-deposited side) of the

support, leaving the other side almost intact and only some scattered single crystals are attached

on the AAO channel walls. This is very important from both synthesis and application points of

view. In the synthesis of supported MOF membranes by conventional modification or seeded

growth one side of the support is often required to be shielded [32], making MOF membrane

synthesis complicated, otherwise MOF layer grows on both sides and/or within the support porous

structure, causing an undesirable increase in the overall hydraulic resistance [33]. In the present

strategy, the strong chelating activity of catechol components in PDA and the attractive

hydrophobic interaction between organic ligands and aromatic units of PDA, in addition to the

high surface area of the CNTs, all represent an ideal reaction environment for the rapid



143

heterogeneous nucleation and growth of ZIF-8 on CNT-deposited side of the substrate, affording

unique ultrathin ZIF-8 hybrid membrane. A high degree of crystallinity was revealed by XRD

analysis, and the ZIF-8 membrane patterns match well with those of the simulated ZIF-8 powder

(Figure 5-4f).

Figure 5-4. SEM images of modified CNTs-deposited on AAO (a), ZIF-8/CNT membranes grown

for 5 min (b), 30 min (c), and for 60 min (d, e), and XRD patterns of ZIF-8 membranes as a function

of synthesis time (f).

On sharp contrast, distinct ZIF-8 nanocrystals and crystal islets were notable on both sides

and within the support if the support surface was not deposited with the modified CNTs

(Figure 5-5a-c). In fact, due to the uninterrupted fast contra-diffusion of metal ions and ligand

molecules through the large pores of the support and the week interaction between the species and

the support, a non-continuous ZIF-8 layer is formed. The situation is similar when the pristine

CNTs are attempted (Figure 5-5d-f). Filtration of non-modified CNTs suspension usually results

in a deposition of huge agglomerates and bundles on the substrate as a result of their poor aqueous

dispersibility and high tendency to bundle up [34]. Subsequently, such a non-uniform deposition

does not effectively contribute to the formation of a continuous MOF film; as seen in Figure 5-5d,

scattered crystals along with islands composed of a mixture of CNTs and ZIF-8 crystals is



144

observable. It is also important to note that ZIF-8 and CNTs are phase-separated as opposed to the

fully embedded CNTs in ZIF-8 when PDA-coated CNTs are used (Figure 5-5e). On the other hand,

the homogenous and sufficient coverage of the entire surface is also an important factor in

preparing a continuous ZIF-8 layer.

Figure 5-5. SEM images of ZIF-8 film prepared on (a, b, c) bare AAO and on (d, e, f) AAO

deposited with pristine CNTs. Zinc side: b, e; Hmim side: c, f. Synthesis time: 60 min.

5.4.3 CNTs coverage level

The coverage level is easily regulated, with nano-scale precision, through the suspension

volume filtered and concentration of the CNTs [35]. When PDA-coated CNTs coverage is

insufficient (see materials and methods), only non-continuous and imperfect growth of ZIF-8 on

the support can be obtained (Figure 5-6). Meanwhile, using an excess volume of the suspension

(see materials and methods) results in a membrane with rough surface morphology after the contra-

diffusion synthesis (Figure 5-6). However, high-magnification top view and cross-sectional SEM

images reveal that the rough morphology at the top is indeed composed of an underlying compact

and defect-free intercalating layer. The excessive volume of the suspension results in a multi-layer

CNTs network, which by generating a nanoporous structure with high surface area per unit volume



145

provides an ideal medium for ZIF-8 crystallization. However, the crystallization rate on the CNTs

closer to the surface of AAO is much faster due to the presence of higher ligand concentration [31].

Subsequently, owing to the self-limiting crystal growth mechanism in the contra-diffusion process

[36], the underlying CNT network is sealed prior to further growth across the CNTs film, leaving

an ultrathin continuous and compact ZIF-8/CNT membrane with a rough and larger size ZIF-8/

CNTs composite on the surface. This mechanism is further confirmed by the fact that increasing

synthesis time affects neither the membrane morphology nor its thickness. This nanoscale surface

morphology can provide a better contact between the feed gas and the hybrid membrane,

potentially leading to a higher gas permeation through the membrane [37]. Indeed, the resulting

membrane exhibits higher gas permeances (shown below).

Figure 5-6. SEM images of bare AAO (a), as-prepared samples with insufficient (b, c), and excess

(d, e, f) deposition of modified CNTs on AAO before (b, d) and after (c, e, f) contra-diffusion

synthesis. The inset in f is a high magnification cross-sectional view. Detailed experimental:

deposited CNTs from (b) 1 mL and (d) 6 mL mother solution respectively.

A long reaction time was noted for the preparation of ZIF-8 layer on the supports directly

modified with PDA and the reported MOF membranes had thicknesses in the rage of tens of

microns [38], adversely affecting gas flow through the membranes. In our study The formation of



146

considerably thinner (more than two order of magnitude) membrane in a shorter synthesis time (1

hr. versus 1 day in reference [38]) is attributed to the considerably larger nucleation area available

on the PDA-coated CNTs deposited support resulted from the subsequent larger PDA content,

considering the high surface area of the deposited CNTs as compared to the corresponding bare

support, as well as to the self-terminating contra-diffusion synthesis process. These results

explicitly demonstrate the essential role of PDA-coated CNTs hybrid and that a sufficient and

uniform coverage of nanotubes on the support is required for the formation of ultrathin and

compact ZIF-8 layer on support.

5.4.4 Mechanical and structural stability of the ZIF-8/CNT membranes

Remarkably, the ultrathin hybrid membrane is composed of CNTs and finely intergrown small

sized ZIF-8 crystals (≤ 50 nm), in which the CNTs act as a nano-scaffold for the construction of a

densely packed reinforced ZIF-8 film. Such a reinforced nanostructure is anticipated to enhance

the mechanical integrity of the ZIF-8 layer, which is one of the central concerns for practical

applications. Indeed, the mechanical integrity test using the ultrasonication method revealed that

the ZIF-8 layer retained its morphology without substantial degradation, deconstruction or

detachment, even after 2 h of intensive ultrasonication (Figure 5-7), strongly demonstrating the

exceptional mechanical integrity of the hybrid ZIF-8/CNT membranes.

Figure 5-7. SEM images (a, b) and XRD pattern (c) of the ZIF-8/CNTs membrane after sonication

for 2 h.



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Figure 5-8. Optical image of the free standing ZIF-8/CNT hybrid membrane floated in the sodium

hydroxide solution (a) and SEM images of Cross-sectional view (b, c) and surface edge (d) of

the corresponding free standing membrane.

The integrity of the CNT/ZIF-8 hybrid membrane was further tested by soaking the supported

membrane in a weak sodium hydroxide solution (NaOH). The base solution selectively etched and

removed the AAO support and an ultrathin free standing ZIF-8/CNT hybrid membrane floated in

the solution (Figure 5-8). ZIF-8/CNTs membranes also showed a greater structural stability against

strong electron beams as compared to pure ZIF-8 films. While pure ZIF-8 films underwent

structural deformation and local cracking immediately upon exposure to electron beam irradiation

(Figure 5-5), the ZIF-8/CNTs membranes retained their structure and integrity as shown in high

magnification SEM images (the inset in Figure 5-4d). The structural robustness of the ZIF-8 hybrid

membranes upon exposure to electron beam, could be attributed to the ability of the integrated

CNTs to dissipate electrostatic charges and subsequently preserves the structure. The enhancement

in the structural stability of MOF materials after integration with carbon materials was also

observed for other MOF/carbon systems such as MOF-5/CNTs and HKUST-1/GO [18, 39]. This

experimental observation not only validates the high mechanical and structural stability of the

hybrid membrane, it also suggest that the method explained here can lead to a versatile route for

the development of ultrathin free-standing MOF/CNTs hybrid membranes. Carbon nanotubes



148

(CNTs), having low wettability, low thermal expansion coefficient (CTE) and high gas adsorption

selectivity [40], are considered as an ideal reinforcing material and the resulting free-standing

MOF hybrid should be able to retain good thermal and mechanical stability under practical

applications.

5.4.5 Synthesis time

To gain further insight into the membrane formation mechanism, different synthesis times

were conducted. Figure 5-4 shows the XRD patterns and SEM images of the ZIF-8 films formed

in different growth times. CNTs entirely embedded with phase-pure ZIF-8 crystals can be observed

in just 5 min of contra diffusion synthesis (Figure 5-4b), strongly demonstrating the presence of a

perfect environment on the support surface for the rapid heterogeneous nucleation and crystal

growth. Remaining interstitial gaps within ZIF-8 embedded CNTs are observable in the high

magnification image. However, an obvious reduction in the size and population of these nanopores

is noted when the synthesis time was increased to 30 min (Figure 5-4c). This was also evidenced

by the observed reduction in the H2 permeance of the membrane. Eventually, after 60 min, the

adjacent ZIF-8 crystals grow together and form a uniform and compact ZIF-8 membrane

(Figure 5-4d). The absence of cracks and defects was further confirmed by transmembrane

pressure-dependent permeation measurements (Figure 5-9). XRD and FTIR spectroscopy were

also employed to track the growth process of ZIF-8 on the surface of CNTs deposited substrates.

As shown in Figure 5-4f and Figure 5-10, the characteristic peaks of ZIF-8 increase with increasing

synthesis time, further confirming the complete coverage of ZIF-8 on the substrate to form a

uniform membrane structure. Our proposed mechanism is that the rapid crystal growth on the

surface of the nanotubes progressively bridge the interstices between the homogenously deposited

CNTs and soon acts as an operative barrier between the metal and the ligand precursor solutions



149

and effectively confines the synthesis within the nanotubes matrix. Once the CNTs interstitial

spaces have been fully sealed through the entire film, termination of the reaction is achieved as

there would be no more reagents (ZIF-8 building blocks) in contact to react with each other.


and different feed pressures.


Following its successful synthesis, the supported ZIF-8/CNT hybrid membranes were further

investigated for their single component permeation properties for H2, CO2, N2, CH4, C3H6, and

C3H8. Owing to its large diameter (~ 100 nm) straight channels, AAO support shows very high and

relatively close permeances for all the gases, indicating its minimal effect on the overall membrane

separation properties. Surprisingly, an increase in the permeances of all gases was noticed upon

homogeneous deposition of modified nanotubes. For example, H2 permeance of blank AAO

increased from 1267 to 1343 × 10−7 mol m−2 s−1 Pa−1 when a moderate amount (See materials and

methods) of the nanotubes were deposited on its surface. This is attributed to the increased porosity



150

and surface area per unit volume of the CNT-deposited support and its subsequent superior contact

with the feed gases.

Figure 5-10. FTIR ATR spectra of the AAO support deposited with modified CNTs, the ZIF-

8/CNTs membranes as a function of synthesis time, and ZIF-8 powder.

Figure 5-11 represents the single gas permeances across the ZIF-8/CNT membrane versus the

molecular diameter of the permeating gas at room temperature and atmospheric pressure. As

shown in the Figure and Table 5-1, there is an explicit cut-off between H2 permeance and CO2

(287 × 10−7 and 20.5 × 10−7 mol m−2 s−1 Pa−1, respectively) and other larger gases, indicating that

the ZIF-8/CNT hybrid membrane displays high hydrogen permselectivity. The ideal selectivity of

H2 over CO2, N2, CH4, C3H6, and C3H8 are 14, 18, 35, 52.4 and 950.1 which considerably surpass

their corresponding Knudsen coefficients (4.7, 3.7, 2.8, 4.6 and 4.7). This ideal separation factors

are in good agreement (except for H2/CO2) with those obtained from ZIF-8 membrane prepared

on PDA directly modified alumina support. However, the ZIF-8 hybrid membrane synthesized in

this study has more than two orders of magnitude higher H2 permeance than those synthesized on

PDA-modified alumina disc. The observed higher H2/CO2 selectivity in this study (14 vs 10.3 in



151

reference [38]) can be attributed to the presence of CNTs network integrated in ZIF-8 matrix. An

enhanced CO2 adsorption capacity and higher CO2/N2 selectivity was previously observed by

integration of CNTs into ZIF-8 as compared to pure ZIF-8 [21, 22]. This was further confirmed

when analysing ZIF-8 membranes prepared with an excess use of CNTs, in which a decrease in

the permeance of CO2 was observed. Surprisingly, the membrane also showed an enhanced H2

permeance (304 (± 8.717)× 10−7 mol m−2 s−1 Pa−1) and greater H2 selectivity over CO2 (16.3 ± 0.87),

which could be attributed to the resultant new nano-scale surface morphology, which can allow

better contact between the ZIF-8 crystals and highly mobile H2 molecules [37]. Further, the

developed ultrathin ZIF-8/CNT hybrid membranes show superior hydrogen permselectivity as

compared to the recently reported high quality ZIF-8 membranes such as, GO [11], TiO2 [37], and

EDA assisted ZIF-8 ultrathin membranes (Table 5-2) [41]. These results show that the strategy

developed in this study is a powerful approach for increasing the hydrogen permeance without

compromising the selectivity.


and 1 bar as a function of the kinetic diameter. The inset shows the ideal gas selectivity for H2

over other gases.



152

Table 5-1. Single gas permeances and ideal selectivities for the ZIF-8@CNTs-t (t: crystallization

time (min), hybrid membranes at 20 ⁰C and 1 bar. E shows the sample prepared with an excess

use of CNTs (6ml of the mother solution).

Sample ID Permeance

(10-7 mol/

m2.s.Pa)

Selectivity

H2 CO2 Η2/CΟ2 Η2/Ν2 Η2/CΗ4 Η2/C3Η8 C3Η6/C3Η8

Bare AAO 1267

±60.2

998

±55.8

1.3

±0.1

1.5

±0.1

1.5

±0.1

3.1

±0.1

1.1

±0.1

AAO-CNTs 1343

±63.3

1002

±58.8

1.3

±0.1

1.4

±0.1

1.2

±0.2

2.8

±0.3

1

±0.1

AAO- ZIF-

8/CNTs-5

954

±40.6

381

±10.1

2.5

±0.2

2.2

±0.1

2.5

±0.4

3.2

±0.4

1.1

±0.1

AAO- ZIF-

8/CNTs-30

560

±25.4

181

±8.7

3.7

±0.4

3.8

±0.4

4.1

±0.6

5.2

±0.6

1.3

0.1±

AAO- ZIF-

8/CNTs-60

287

±7.5

20.5

±1.1

14

±1.1

18

±0.9

35

±1.0

950.1

±118.2

16.7

±2.1

AAO- ZIF-

8/CNTs-60-

E

304

±9.2

18.6

±1.2

16.3

±0.8

16.5

±0.8

30.3

±1.5

916

±98.8

16.1

±1.8

The outstanding hydrogen permselectivity in this study can be due to the ultrathin ZIF-8 layer

(~200 nm) which is defect-free, densely packed and reinforced within a network of CNTs. Another

possible reason for the clear cut-off between H2 and larger gases can be the presence of the CNTs



153

network in the matrix of ZIF-8. It was previously shown that the presence of carbonaceous

materials such as GO can effectively slow down the permeance of larger gases through the ZIF-8

membrane by reducing the nonselective intercrystalline defect pathways and also by its possible

constriction effect on the flexibility of ZIF-8 lattices, thus restricting larger molecules to go into

the pores [42]. Since the PDA modification of CNTs and their subsequent vacuum deposition are

readily controllable, the preparation of ZIF-8/CNT membrane was found to be highly reproducible

(Table 5-3).

Table 5-2. Comparison of the synthesis parameters (time and temperature) and gas permeation

properties of the ZIF-8/CNTs hybrid membrane in this work with other ZIF-8 membranes from

the recent literature.

Membrane/

Growth

Facilitator

MOF

Thick-

ness

(μm)

Synthesis

T (°C)/t

(hr)

H2

Permeance

(10-7 mol/

m2.s.Pa)

Selectivity Ref

Η2/

CΟ2

Η2/

Ν2

Η2/

CΗ4

Η2/

C3Η8

ZIF-8/APTES

functionalized

α-alumina

particles

~ 2 150/5 573 17.05 15.4 _ _ [43]

ZIF-8/PDA-

coated

surface

30 85/24 266 8.8 15.4 24.6 442.5 [44]

ZIF-8/APTES

functionalized

TiO2 layer

Less

than 1

RT/5 201 7.04 7.8 8.6 _ [37]

ZIF-8/Nil 16 RT/24 126.2 _ 3.7 _ _ [29]

ZIF-8/EDA

modified

surface

0.2 RT/3 min 20.5 12.8 9.7 _ _ [41]

ZIF-8/EDA

modified

surface

~2 RT/2 7.5 5.1 8.3 9.1 833 [33]

ZIF-8/ZnO

nanorods

6 100/5 1.04 7.6 12.3 12.7 _ [45]

ZIF-8/PDA-

coated

surface

20 85/24 2.17 10.3 17.6 34.8 905.1 [38]



154

ZIF-8@GO/

PDA-coated

surface

20 85/16 1.45 22.4 102.8 198.3 5870.1 [42]

ZIF-8/ZnAl-

CO3 LDH

buffer layers

20 100/12 1.4 4.2a 10a 12.1 _ [46]

ZIF-8/Nil

0.9 30/9 1.23 _ _ _ 370 [36]

ZIF-8/ 2D

ZIF-8/GO

hybrid

nanosheets

0.1 RT/3 0.546 1.6 11.1 11.2 405 [47]

ZIF-8/ZIF-L

seed crystals

3.5 RT/6 ~ 6000

Barrerb

~ 4.5 ~ 6.5 ~ 6 _ [48]

ZIF-8/PDA-

modified

CNTs

Less

than 0.2

RT/1 287 14 18 35 950 This

work

a)Mixture separation factor; b)1 Barrer = 3.348 × 10−16 mol m m−2 s−1.

Table 5-3. Single gas permeances and ideal selectivities of three ZIF-8/CNT-60 membrane

samples tested at 25 ⁰C and 1 bar.

Membrane Permeance

(10-7 mol/ m2.s.Pa)

H2/ CO2

selectivity

Average

selectivity

Standard deviation

of selectivity

H2

CO2

M1 295 19.5 15.1

14

1.1 M2 280 21.7 12.9

M3 286 20.3 14.1

5.4.7 Universal applicability

Finally, to demonstrate the potential for universal applicability of our pseudo-seeding method,

ZIF-8/CNTs membrane was prepared on a porous polymeric substrate. In particular, a commercial



155

porous polyethersulfone filtration membrane (PES, 0.03 µm, 47 mm, STERLITECH) was selected

as the support material. A densely packed ZIF-8/CNTs hybrid membrane was formed on only CNT

deposited side of the polymer within 1h (Figure 5-12), following the same preparation method

described above for AAO supported ZIF-8/CNTs membrane but having replaced the AAO

substrate with PES. The XRD patterns of the PES supported ZIF-8/CNTs match well with those

of the simulated ZIF-8 powder (Figure 5-13). FTIR analysis further confirmed the presence of a

thin ZIF-8 layer on the support as the characteristic peaks of both ZIF-8 and PES are detected

(Figure 5-13b). A long synthesis time (≥ 16h) and formation of ZIF-8 layer on both side of the

substrate through the same contra-diffusion method were noticed when a bare polymer substrate

was used in our previous study [31].

Figure 5-12. SEM images of (a, b) bare PES, (c, d) PES with deposited modified CNTs (6 ml of

mother solution) and (e, f) as prepared membrane after contra-diffusion synthesis (1 h).



156

Figure 5-13. (a) XRD patterns and (b) FTIR spectra of pristine PES, supported ZIF-8/CNT

membrane and ZIF-8.

Summary

In conclusion, in the present study we have successfully synthesized a new ZIF-8/CNT hybrid

membrane with outstanding hydrogen permselectivity. The fabrication method is simple,

reproducible, and adaptable that consist of deposition of PDA-coated CNTs on the supports

followed by contra-diffusion synthesis. PDA and CNTs jointly provide an ideal environment for

the rapid heterogeneous nucleation and growth of ZIF-8 on the deposited support. It yields ultrathin

yet defect free and reinforced ZIF-8 hybrid membranes whose performance is amongst the best

ZIF membranes studied so far. At 25 °C and 1 bar, the ideal separation selectivities of H2/CO2,

H2/N2, H2/CH4, C3H6, and C3H8 are 14, 18, 35, 52.4 and 950.1, respectively, with H2 permeance as

high as 287 × 10−7 mol m−2 s−1 Pa−1. We have also observed that the H2 permeance could be even

higher by altering the CNTs content in the ZIF-8 matrix and subsequent surface morphology. This

high hydrogen permselectivity combined with its mechanically reinforced structure recommend

the developed ZIF-8/CNT membrane as a promising candidate for hydrogen separation and

purification. Finally, we anticipate that one-dimensional carbonaceous materials assisted

crystallization strategy may be further adapted for the fabrication of other MOF and zeolite

molecular sieve membranes.



157

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Chapter 6 Conclusions and Recommendations for Future Work

162

Conclusions and

Recommendations for Future

Work

Conclusions

This dissertation aims to address challenges that hinder the facile synthesis of

supported-ZIF membranes with improved gas separation performances in a reproducible

and scalable manner. In summary, novel methods for the fabrication of ZIF-8

membranes was developed, and the obtained membranes showed impressive gas

separation performance. The key findings are briefly summarized as follows.

In chapter 3, a compact, ultrathin ZIF-8 layer on an asymmetric polymeric substrate

was successfully prepared via the chemical vapour modification of surface chemistry

and pore structure. In addition, we have also demonstrated the influence of the surface

microstructure and chemical composition of the polymer substrate on the formation of a

continuous ZIF-8 layer. Vapour-phase EDA has been used to simultaneously tailor the

chemical nature and pore size of the surface of the polymer substrate for the successful

growth of the ZIF-8 membrane. The EDA treatment produced a large number of

nucleation sites and modified the polymer pore structure, promoting the formation of a

thin ZIF-8 layer. The ZIF-8 membrane exhibits ideal selectivities (H2/CO2: 12.8; H2/N2:

9.8) and permeance (2.05 × 10−6 mol m−2 s−1 Pa−1) and are among the highest reported

so far. The proposed chemical vapour modification method followed by fast in situ

synthesis provides a rapid, convenient and effective route for preparing thin yet

continuous and defect-free ZIF membranes on the surface of polymeric substrates.


163

In chapter 4, we reported a novel strategy, contra-diffusion based synthesis in

conjunction with vapor modification, for the room temperature synthesis of high-quality

ZIF-8 membranes on an asymmetric polymeric substrate in aqueous solution. The ZIF-

8 membranes have shown excellent gas permeation properties (e.g., propylene/propane

ideal selectivity of 16 with propylene permeance of 150 × 10–10 mol m–2 s–1 Pa–1),

indicating impressively enhanced membrane microstructure (in particular enhanced

grain boundary structure). More importantly, by altering the activation temperature, the

propylene/propane selectivity was further improved (almost 2-fold), without

compromising the high permeance of propylene, indicating the important role of thermal

activation conditions (in particular activation temperature) in microstructures of ZIF-8

membranes. With efficient synthesis conditions, the strategy developed here provides an

effective and environmentally friendly route for the preparation of high-quality ZIF

membranes on the surface of polymeric support.

In chapter 5, we have successfully developed a nano-scaffolding and pseudo-

seeding strategy for the fabrication of ultrathin ZIF-8/CNT hybrid membrane with

excellent mechanical stability and hydrogen selectivity. The fabrication method is

simple, reproducible, and adaptable, consisting of the deposition of PDA-coated CNTs

on the supports followed by contra-diffusion synthesis. PDA and CNTs collectively

provide an ideal environment for the rapid heterogeneous nucleation and growth of ZIF-

8 on the support. At 25 °C and 1 bar, the ideal separation selectivities of H2/CO2, H2/N2,

H2/CH4, C3H6, and C3H8 are 14, 18, 35, 52.4 and 950.1, respectively, with H2 permeance

as high as 2.87 × 10−5 mol m−2 s−1 Pa−1. We have also observed that the H2 permeance

could be further improved by altering the CNTs content in the ZIF-8 matrix and

subsequent surface morphology. This high hydrogen permselectivity combined with its

mechanically reinforced structure shows the ZIF-8/CNT membrane is a promising


164

candidate for hydrogen separation and purification. Finally, we anticipate that our new

strategy may be further developed for the fabrication of other MOF and zeolite molecular

sieve membranes.

Recommendations for Future Work

In the course of this project, various areas are identified where further research is

needed. Th areas of further research include the following:

All of the membranes in this study have been investigated for their single gas

permeation; mixed gases also need to be explored to obtain an overall gas separation

performance of each membrane. In addition, a performance analysis can be conducted

under various conditions such as operating temperature and pressure, as they can play

significant roles in gas separation performance. This can also result in an evaluation of

the stability and separation properties of the membranes under harsh industrial

conditions.

The methods developed in this study could be used for the construction of other

ZIF membranes as the synthesis of ZIFs is principally smilar, which consists of bridging

the metal ions by imidazoles to build up ZIF structure.

The method explained in chapter 5 can lead to a versatile route for the development

of ultrathin free-standing MOF/CNTs hybrid membranes. Carbon nanotubes (CNTs),

having low wettability, low thermal expansion coefficient (CTE) and high gas adsorption

selectivity, are considered as an ideal reinforcing material and the resulting free-standing

MOF hybrid should be able to retain good thermal and mechanical stability under

practical applications. However, an elaborate study on the free-standing membranes is

required before it can be used for practical applications.


165

Constructing MOF membranes on polymeric hollow fiber substrates are highly

attractive for large-scale industrial applications and since the presented work included

membranes synthesized on only flat supports the next step is to prepare membranes on

hollow fibers.

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