design and fabrication of polymer based …methods for alcohol dehydration via pervaporation or...

DESIGN AND FABRICATION OF POLYMER BASED

MEMBRANES FOR ALCOHOL DEHYRATION VIA

PERVAPORATION OR VAPOR PERMEATION

HUA DAN

(B.Eng. & M.Eng., Xiamen University, China)

A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF PHILSOPHY

NUS GRADUATE SCHOOL FOR INTEGRATIVE

SCIENCES AND ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2016

i

DECLARATION

I hereby declare that the thesis is my original work and it has been written by me in its

entirety. I have duly acknowledged all the sources of information which have been

used in the thesis.

This thesis has also not been submitted for any degree in any university previously.

_________________

Hua Dan

01 Jan 2016

ii

ACKNOWLEDGEMENT

I would like to express the appreciation and thanks to all who have given me help

and support on my way to complete the challenging but wonderful and memorable

PhD life.

Firstly, I would like to show my great appreciation to my supervisor Professor

Chung Tai-Shung, who guided me to the fascinating membrane research field. His

valuable research directions, kind concern and enthusiastic encouragement inspired

me to gain the confidence to overcome all difficulties in the research process and

make the thesis research possible. Besides, Prof. Chung also provided concern and

guidance in my career and life. I have learned a lot from him and improved myself

during the PhD study, which will be good for my future life.

I also owe my thanks to my mentors Dr. Ong Yee Kang and Prof. Wang Yan, who

provided me detailed guidance patiently and gave me unselfish support and

encouragement. Besides, I would also like to extend my heartfelt appreciation to all

other group members and friends who have ever given me kind help and treasured

advice. Their discussions and sharing of research, module learning and daily life

made my PhD study in NUS wonderful and memorable.

iii

Special thanks are due to my thesis advisory committee (TAC) members, Professor

Zeng Hua Chun and Dr. Pallathadka Pramoda Kumari, who gave me valuable

guidance and helpful suggestions during my TAC meetings.

I would like to thank all lab technologists and staff members in NUS graduate

school for integrative sciences and engineering (NGS) and department of chemical

and biomolecular engineering, who helped me a lot in administrative matters,

laboratory services, characterization techniques and technical assistance.

In addition, I would like to thank NGS for providing my scholarship, and Singapore

National Research Foundation (NRF) for funding this research under its Competitive

Research Program for the project entitled, “New biotechnology for processing

metropolitan organic wastes into value-added products” (NUS grant number R-279-

000-311-281).

Finally, the warmest thanks must go to my parents in China for their everlasting and

unconditional love and support, Last but not least, I would like to thank my husband

Mr. Zhan Guowu, who is always there accompanying me with love and support, and

sharing the sorrows and joys along the way pursuing our study and life.

iv

TABLE OF CONTENTS

DECLARATION ............................................................................................................ i

ACKNOWLEDGEMENT ............................................................................................. ii

TABLE OF CONTENTS .............................................................................................. iv

SUMMARY ................................................................................................................. xii

NOMENCLATURE ................................................................................................... xvi

LIST OF TABLES ...................................................................................................... xxi

LIST OF FIGURES .................................................................................................. xxiv

CHAPTER 1 INTRODUCTION ................................................................................... 1

1.1. General introduction of membrane separation .................................................... 1

1.2. Introduction and history of pervaporation and vapor permeation ...................... 3

1.2.1. Introduction of pervaporation and vapor permeation .................................. 3

1.2.2. History of pervaporation .............................................................................. 5

1.2.3. History of vapor permeation ........................................................................ 6

1.3. Current applications of membrane pervaporation and vapor permeation ........... 7

1.3.1. Dehydration of organics ............................................................................... 9

1.3.2. Removal of organics from aqueous solutions ............................................ 10

1.3.3. Separation of organics mixtures................................................................. 11

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1.4. Key challenges in development of pervaporation and vapor permeation

membranes ............................................................................................................... 12

1.5. Research objectives and thesis organization ..................................................... 13

1.6. References ......................................................................................................... 17

CHAPTER 2 LITERATURE REVIEW ...................................................................... 26

2.1. Mass transport mechanism ................................................................................ 26

2.2. Evaluation of pervaporation/vapor permeation membranes ............................. 27

2.3. Pervaporation/vapor permeation membranes for alcohol dehydration ............. 30

2.3.1. Membrane materials................................................................................... 30

2.3.2. Membrane structure and configuration ...................................................... 38

2.3.3. Composite membranes for pervaporation/vapor permeation applications 41

2.4. References ......................................................................................................... 44

CHAPTER 3 EXPERIMENTAL ................................................................................. 59

3.1. Materials ........................................................................................................... 59

3.1.1. Polymers .................................................................................................... 59

3.1.2. Chemicals for synthesizing ZIF-90 nanoparticles ..................................... 60

3.1.3. Chemicals for surface modification ........................................................... 60

3.1.4. Chemicals for fabricating TFC membranes ............................................... 60

3.1.5. Organic solvents and other chemicals ........................................................ 61

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3.2. Membrane fabrication ....................................................................................... 61

3.2.1. Fabrication of flat sheet ZIF-90/P84 MMMs by knife casting .................. 61

3.2.2. Fabrication of flat sheet Teflon AF2400 dense film by ring casting ......... 64

3.2.3. Spinning of polymeric HF substrates ......................................................... 64

3.2.4. Composite HF membranes by aldehyde surface modification .................. 65

3.2.5. Composite HF membranes by interfacial polymerization ......................... 67

3.2.6. Composite HF membranes by Teflon AF2400 coating ............................. 68

3.2.7. Fabrication of HF modules ........................................................................ 68

3.3. Material and membrane characterization .......................................................... 69

3.3.1. Optical microscope .................................................................................... 69

3.3.2. Scanning electron microscopy (SEM) ....................................................... 69

3.3.3. Atomic force microscope (AFM)............................................................... 69

3.3.4. Wide angle X-ray diffraction (XRD) ......................................................... 70

3.3.5. Fourier transform infrared spectrometer (FTIR) ........................................ 70

3.3.6. X-ray photoelectron spectroscopy (XPS) .................................................. 70

3.3.7. Energy dispersive spectrometry (EDX) ..................................................... 70

3.3.8. Water contact angle measurement ............................................................. 70

3.3.9. Thermogravimetric analyses (TGA) .......................................................... 71

vii

3.3.10. Surface area measurement ....................................................................... 71

3.3.11. Density measurements ............................................................................. 71

3.3.12. Kinematic viscosity measurement of polymer solutions ......................... 72

3.3.13. Vapor sorption measurement ................................................................... 72

3.3.14. Swelling studies of membranes in high-temperature feed vapor ............. 73

3.3.15. Mechanical properties measurement ........................................................ 73

3.3.16. Slow beam positron annihilation spectroscopy (PAS)............................. 73

3.4. Pervaporation experiments................................................................................ 75

3.4.1. Pervaporation experiments for flat sheet membranes ................................ 75

3.4.2. Pervaporation experiments for HF membranes ......................................... 76

3.5. Vapor permeation experiments ......................................................................... 77

3.6. References ......................................................................................................... 78

CHAPTER 4 ZIF-90/P84 MIXED MATRIX MEMBRANES FOR

PERVAPORATION DEHYDRATION OF ISORPROPANOL ................................. 82

4.1. Introduction ....................................................................................................... 82

4.2. Experimental ..................................................................................................... 86

4.3. Results and discussions ..................................................................................... 86

4.3.1. Characterization of the ZIF-90 and the MMMs ......................................... 86

4.3.2. Effect of ZIF-90 loading on pervaporation performance ........................... 97

viii

4.3.3. Effect of the SPES primer on pervaporation performance ...................... 100

4.3.4. Effect of feed temperature on pervaporation performance ...................... 102

4.3.5. Comparison of present pervaporation performance with literature data . 105

4.4. Conclusions ..................................................................................................... 106

4.5. References ....................................................................................................... 108

CHAPTER 5 UNIVERSAL SURFACE MODIFICATION BY ALDEHYDES ON

POLYMERIC MEMBRANES FOR ISOPROPANOL DEHYDRATION VIA

PERVAPORATION .................................................................................................. 117

5.1. Introduction ..................................................................................................... 117

5.2. Experimental ................................................................................................... 120

5.3. Results and Discussion ................................................................................... 121

5.3.1. Torlon HF membranes modified by GA and HPEI mediated GA methods.

............................................................................................................................ 121

5.3.2. Chemical modifications by other aldehydes on Torlon HF substrates .... 130

5.3.3. Universal applications of the HPEI mediated aldehyde modification

method to other HF membranes. ........................................................................ 132

5.3.4. The effects of operation conditions on IPA dehydration performance. ... 134

5.4. Conclusions ..................................................................................................... 140

5.5. Reference ........................................................................................................ 140

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CHAPTER 6 THIN-FILM COMPOSITE TRI-BORE HOLLOW FIBER

MEMBRANES FOR ISOPROANOL DEHYDRATION BY PERVAPORATION 149

6.1. Introduction ..................................................................................................... 149

6.2. Experimental ................................................................................................... 152

6.3. Results and discussion .................................................................................... 155

6.3.1. Effect of TFC selective layer ................................................................... 155

6.3.2. The effects of spinning parameters on TbHF substrates and pervaporation

performance of TFC TbHFs............................................................................... 159

6.3.3. A comparison of TFC TbHF and TFC SbHF membranes ....................... 171

6.4. Conclusions ..................................................................................................... 174

6.5. References ....................................................................................................... 175

CHAPTER 7 TEFLON AF2400/ULTEM COMPOSITE HOLLOW FIBER

MEMBRANES FOR ALCOHOL DEHYDRATION BY HIGH-TEMPERATURE

VAPOR PERMEATION ........................................................................................... 186

7.1. Introduction ..................................................................................................... 186

7.2. Experimental ................................................................................................... 189

7.3. Results and discussion .................................................................................... 191

7.3.1. The characterization of the Ultem HF substrate ...................................... 191

7.3.2. The characterization of the Teflon AF2400 dense membrane. ................ 191

x

7.3.3. The morphologies and separation performance of Teflon AF2400/Ultem

composite HF membranes.................................................................................. 193

7.3.4. The effect of feed composition on the vapor permeation performance of

Teflon AF2400/Ultem composite HF for IPA dehydration ............................... 204

7.3.5. The vapor permeation performance of Teflon AF2400/Ultem composite

HF for other alcohol dehydration ....................................................................... 205

7.3.6. Benchmarking and long-term stability of the Teflon AF2400/Ultem

composite HF for alcohol dehydration via vapor permeation ........................... 206

7.4. Conclusions ..................................................................................................... 209

7.5. References ....................................................................................................... 209

CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS .............................. 218

8.1. Conclusions ..................................................................................................... 218

8.1.1. ZIF-90/P84 mixed matrix membranes for pervaporation dehydration of

isopropanol ......................................................................................................... 218

8.1.2. Universal surface modification by aldehydes on polymeric membranes

for isopropanol dehydration via pervaporation .................................................. 219

8.1.3. Thin-Film Composite Tri-bore Hollow Fiber Membranes for Isopropanol

Dehydration by Pervaporation ........................................................................... 220

8.1.4. Teflon AF2400/Ultem composite hollow fiber membranes for alcohol

dehydration by high-temperature vapor permeation .......................................... 220

xi

8.2. Recommendations ........................................................................................... 222

PUBLICATIONS ....................................................................................................... 224

xii

SUMMARY

Driven by concerns on energy security and global warming, pervaporation and vapor

permeation processes have progressively become promising separation technologies

for azeotropic or close boiling point mixtures owing to the advantages such as high

separation efficiency, lower energy consumption, small foot print, environmental

benignity and flexible process control. Membrane is the heart of pervaporation and

vapor permeation processes. Therefore, it is very important to develop appropriate

membranes with high productivity, good selectivity and long-term stability to make

these separation processes more competitive and prospective. In this thesis, several

types of polymer based membranes have been designed and fabricated via various

methods for alcohol dehydration via pervaporation or vapor permeation processes.

Firstly, zeolitic imidazolate framework-90 (ZIF-90)/P84 mixed matrix membranes

(MMMs) are fabricated by incorporating synthesized ZIF-90 nanoparticles into P84

barrier polymer matrix, and then applied to the pervaporation dehydration of

aqueous isopropanol (IPA). The effects of ZIF-90 loading as well as feed

temperature on separation performance of MMMs are studied. It is found that the

flux increases at higher ZIF-90 loading due to the enhanced fractional free volume

of MMMs, while the separation factor of water to IPA maintains at 5432 when the

ZIF-90 loading is less than 20 wt%, but reduces to 385 when the ZIF-90 loading

xiii

reaches 30 wt%. Interestingly, the usage of sulfonated polyethersulfone (SPES) as a

primer to ZIF-90 nanoparticles before fabricating MMMs consisting of 30 wt% ZIF-

90 recovers the separation factor without sacrificing the flux. The best MMM shows

a doubled flux of 109 g m-2 h-1 as compared to the pristine P84 membrane, and the

separation factor remains a high value of 5668 at 60 oC.

Secondly, various polymeric hollow fiber (HF) membranes such as Torlon, Ultem,

and PES HFs are modified by a proposed universal approach consisting of hyper-

branched polyethyleneimine (HPEI) pre-treatment and aldehyde modification, which

show greatly improved pervaporation separation performance. Characterization

results indicate that the HPEI pre-treatment both mitigates surface defects and

improves the membrane hydrophilicity, while the cross-linking reaction between

HPEI and aldehydes tightens polymeric chains of the selective layer and enhances

selectivity. Moreover, this work also fully investigates the effects of feed

temperatures and compositions on the separation performance of the designed

membranes. In addition, the Torlon HF modified via HPEI mediated glyoxylic acid

method shows a very stable separation performance with a flux of 1521 g m-2 h-1 and

a separation factor of 791 for the dehydration of aqueous IPA solutions (IPA/water

85/15 wt%) during the long-term pervaporation test at 50 oC with a monitoring

period longer than 200 hours.

xiv

Thirdly, novel thin-film composite tri-bore hollow fiber (TFC TbHF) membranes

are fabricated for pervaporation dehydration of IPA. The interfacial polymerization

conditions and pre-treatments of HF substrate are firstly studied to obtain a good

TFC dense selective layer. Moreover, the effects of spinning parameters such as

bore fluid composition, bore fluid flow rate, air gap and dope flow rate on TFC

TbHF membranes’ geometry, morphology and pervaporation performance are

systematically investigated. It is found that the bore fluid composition is the most

critical factor. Both fine and thick TbHF substrates obtained from spinning, the outer

diameters of which are around 1 mm and 2 mm, respectively. The TFC TbHFs made

from fine TbHF substrates exhibit both higher flux and separation factor than the

thick ones. The optimal TFC TbHF membrane shows a flux of 2650 g m-2 h-1 with a

separation factor (water/IPA) of 261 for the dehydration of an IPA/water (85/15

wt%) mixture at 50 oC. Most importantly, the optimal fine TFC TbHF membrane

not only shows enhanced separation performance and mechanical strength as

compared with the conventional TFC SbHF made from the identical spinning and

TFC layer fabrication conditions, but also exhibits satisfying long-term stability.

Fourthly, to develop membranes suitable for high-temperature vapor permeation

process that has a stringent requirement of membrane stability under harsh feed

environments, this work designs composite HFs with the combination of Teflon

AF2400 and Ultem materials, and applies them to alcohol dehydration. Fabrication

parameters such as Teflon concentration and coating time are systematically

xv

investigated. It is interesting to find that the fabricated composite HF membranes

possess an unusual Teflon surface with honeycomb-like microstructure patterns.

More importantly, only the Teflon AF2400/Ultem HFs with a Teflon layer

containing dense Teflon portion can exhibit a stable separation performance. The

optimal composite HF shows a flux of 4265 gm-2h-1 and a separation factor of 383

for the dehydration of IPA/water (95/5 wt%) feed vapor at 125 oC, which also

exhibits good long-term stability within 300-hour testing period. The composite HF

also performs well under extreme vapor feed compositions from 87 to 99 wt%

isopropanol. In addition, it exhibits impressive separation performance for the

dehydration of ethanol and n-butanol. This work may provide useful insights of

designing thermal-stable and high-performance composite polymeric membranes for

vapor permeation.

xvi

NOMENCLATURE

Abbreviations

AFM Atomic force microscope

BA Benzaldehyde

CTAB Cetyltrimethylammonium bromide

DBES Doppler broadening energy spectroscopy

DMF N, N-Dimethylformamide

ECN Energy research Centre of the Netherlands

EDX Energy dispersive spectrometry

FESEM Field-emission scanning electron microscope

FTIR Fourier transform infrared spectroscopy

GA Glyoxylic acid

GL Glucose

HF Hollow fiber

HPEI Hyper-branched polyethyleneimine

ICA Imidazolate-2-carboxyaldehyde

IEC Ion-exchange capacity

IPA Isopropanol

MbHF Multi-bore hollow fiber

MPD m-Phenylenediamine

MTR Membrane Technology and Research, Inc.

MOF Metal organic framework

xvii

NMP N-methyl-2-pyrrolidone

o-Ps Ortho-positronium

PAS Positron annihilation spectroscopy

PALS Positron annihilation lifetime spectroscopy

PDMS Polydimethylsiloxane

PEG Polyethylene glycol

PES Polyethersulfone

POMS Polyoctylmethyl siloxane

PVA Polyvinyl alcohol

PVDF Polyvinylidene fluoride

SDS Sodium dodecyl sulphate

SEM Scanning electron microscopy

SPES Sulfonated polyethersulfone

TbHF Tri-bore hollow fiber

TEA Trimethylamine

TGA Thermogravimetric analyses

TMC Trimesoyl chloride

TFC Thin-film composite

VP Vapor permeation

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

ZIF Zeolitic imidazolate framework

xviii

Symbols

A Effective surface area of the membrane (m2)

D Diffusivity (cm2 s-1)

JE Apparent activation energy of flux (kJ mol-1)

FFV Fractional free volume

J Flux of the membrane (g m-2 h-1)

0J Pre-exponential factor of Arrhenius equation (g m-2 h-1)

iJ Permeation flux of component i (g m-2 h-1)

l Membrane thickness (m)

Mi The molecular weight of component i (g mol-1)

tM Sorption uptake of solvent vapor into the sample (mg)

M Equilibrium sorption weight (mg)

P84m Weight of P84 (g)

ZIFm Weight of ZIF-90 (g)

iP Permeability of component i (mg m-1 h-1 KPa-1)

Pi/l Mole-based permeance of component i (mol m-2 h-1 kPa-1)

sat

ip Saturated vapor pressure of component i (kPa)

MMMP Total permeability within the MMMs (mg m-1 h-1 KPa-1)

fp The total pressure of the feed vapor (cm Hg)

xix

PP Experimental permeability of the polymer phase (mg m-1 h-1 KPa-1 or

cm Hg)

pp The total pressure at the permeate side (kPa)

ZIFP The permeability within the ZIF crystals (mg m-1 h-1 KPa-1)

Q Weight of the sample collected as permeate of the membrane (g)

T Pervaporation operating temperature (K)

t Time interval during the sample collection (h)

V Specific volume of the MMMs (cm3 g-1)

WV Van der Waals volume (cm3 mol-1)

ix Mole fraction of the component i in the permeate

wjwi xx , Weight fraction of the component i or j in the feed

iy Mole fraction of the component i in the permeate

wjwi yy , Weight fraction of the component i or j in the permeate

Mole-based selectivity (water/alcohol)

Separation factor (water/alcohol)

i Activity coefficient of component i

Density of the membrane sample

air Density of air

e Experimental density (g cm-3)

liq Density of liquid used

xx

t Theoretical density (g cm-3)

P84 Density of P84 (g cm-3)

ZIF Density of ZIF-90 (g cm-3)

ZIF Volume fraction of the ZIF-90 fillers

V Void volume fraction

i , j The fugacity coefficients of component i or j in the feed vapor

mixture

ZIF Theoretical ZIF-90 loading (wt%)

xxi

LIST OF TABLES

Table 1.1 Various membrane processes and applications. ............................................. 2

Table 1.2 Potential liquid separations by pervaporation and vapor permeation

process............................................................................................................................ 7

Table 1.3 Industrial suppliers of pervaporation and vapor permeation systems ........... 8

Table 1.4 Properties of alcohols and the azetropes of alcohol/water systems [50] ..... 10

Table 2.1 The kinetic diameter of water and various alcohols. ................................... 31

Table 2.2 The chemical structures and properties of polymers used in membranes

for alcohol dehydration by pervaporation .................................................................... 32

Table 4.1 Volume fraction, density, fractional free volume, and void volume of the

membranes. .................................................................................................................. 94

Table 4.2 Vapor sorption results of P84 and composite membranes with saturated

vapor at 45 oC............................................................................................................... 96

Table 4.3 Activation energies of pristine P84 membrane and MMMs for IPA

dewatering. ................................................................................................................. 105

Table 4.4 Comparison of pervaporation performance of IPA dehydration for various

MMMs. ...................................................................................................................... 106

Table 5.1 Spinning parameters for the polymeric HF substrates used in this work.a 121

Table 5.2 The separation performance of Torlon HFs before and after modification

for aqueous IPA (85/15 wt% IPA/water) dehydration at 50 oC. ................................. 123

Table 5.3 R parameters and selective layer thicknesses of Torlon HF substrate and

HFs modified by HPEI, GA, and HPEI mediated GA, respectively a. ...................... 125

Table 5.4 The separation performance of Torlon HF substrates and HFs modified by

various aldehydes or HPEI mediated aldehydes for aqueous IPA (85/15 wt%

IPA/water) dehydration at 50 oC. ............................................................................... 130

Table 5.5 The separation performance of other polymeric HFs modified by HPEI

mediated GA for aqueous IPA (85/15 wt% IPA/water) dehydration at 50 oC. .......... 132

Table 5.6 The calculated activation energies of total flux, water flux, and IPA flux

xxii

for the dehydration of IPA/water (85/15 wt%) mixtures. .......................................... 136

Table 5.7 Performance benchmarking of the HPEI mediated aldehyde modified HF

membranes with other polymeric HF membranes for pervaporative IPA dehydration

in literatures. .............................................................................................................. 139

Table 6.1 The spinning parameters for HFs spun in this study a. .............................. 153

Table 6.2 Effects of amine solution composition on IPA dehydration performance of

TFC SbHF a. ............................................................................................................... 156

Table 6.3 The IPA dehydration performance of TFC SbHF membranes with different

treatments a. ................................................................................................................ 158

Table 6.4 Effects of bore fluid composition on IPA dehydration performance of TFC

TbHF membranes a. .................................................................................................... 163

Table 6.5 R parameters, TFC layer thicknesses and positron lifetime results of the

fine and thick TFC TbHF membranes a. .................................................................... 164

Table 6.6 Effects of bore fluid rate on IPA dehydration performance of TFC TbHF

membranes a. .............................................................................................................. 167

Table 6.7 Effects of air gap on IPA dehydration performance of TFC TbHF

membranes a. .............................................................................................................. 169

Table 6.8 Effects of dope flow rate on IPA dehydration performance of TFC TbHF

membranes a. .............................................................................................................. 170

Table 6.9 Comparisons of mechanical properties and pervaporation performance of

TFC SbHF and TFC TbHF membranes a. .................................................................. 171

Table 6.10 Performance benchmarking of the fine TFC TbHF with polymeric

membranes for IPA pervaporative dehydration in literatures. ................................... 172

Table 7.1 The spinning parameters for Ultem HF substrates spun in this study a. .... 190

Table 7.2 The calculated fugacity coefficients of water and alcohols in the vapor

feed mixtures used in this work. ................................................................................ 190

Table 7.3 The swelling degree of dense membranes after immersing in an

isopropanol/water (95/5 wt%) feed vapor at 125oC for 10 days. ............................... 192

Table 7.4 Water contact angles of the Ultem HF substrate and Teflon AF2400/Ultem

composite HFs. .......................................................................................................... 198

Table 7.5 The vapor permeation performance of the Teflon AF2400/Ultem

xxiii

composite HF for C2-C4 alcohol dehydration at 125 °C. .......................................... 206

Table 7.6 A comparison of vapor permeation performance among the Teflon

AF2400/Ultem composite HF and other membranes for C2-C4 alcohol dehydration

in literatures. .............................................................................................................. 207

xxiv

LIST OF FIGURES

Figure 1.1 Schematic representation of membrane separation process. ............... 1

Figure 1.2 Schematic diagram of (a) pervaporation and (b) vapor permeation

processes. ............................................................................................................... 4

Figure 2.1 Schematic diagram of solution-diffusion transport model. ............... 26

Figure 2.2 Polymeric membranes classified by the membrane structure. .......... 39

Figure 2.3 Polymeric membranes classified by the membrane configuration. ... 41

Figure 2.4 A typical procedure of TFC membranes fabricated via interfacial

polymerization. .................................................................................................... 42

Figure 3.1 The chemical structures of (a) P84, (b) Torlon 4000T-MV, (c) Ultem

1010, (d) PES, (e) SPES with ion-exchange capacity (IEC) of 0.054 and 0.107

mequiv g-1, (f) Teflon AF2400. ............................................................................ 60

Figure 3.2 (a) Chemical structure of ZIF-90 and (b) a photo of a 30/70 (w/w)

ZIF-90/P84 membrane (15.2 cm2) on top of the NUS logo. ................................ 63

Figure 3.3 The scheme of a typical dry-jet wet spinning process. ...................... 65

Figure 3.4 The scheme of procedures for (a) aldehyde modification and (b)

HPEI mediated aldehyde modification of HF substrate. Note that: if there is no

‒H in the aldehyde molecule, the intermediated carbinolamines cannot be

dehydrated to give enamines, but form derivatives of amine, as described in

literatures [7, 8]. ................................................................................................... 66

Figure 3.5 The scheme of fabricating TFC HF with different methods: (1) The

central red box: the interfacial polymerization process; (2) The green box: pre-

coating HPEI and then interfacial polymerization; (3) The blue box: post-

PDMS coating after the interfacial polymerization. ............................................ 68

Figure 3.6 Flow diagram of the Sulzer pervaporation setup for the flat sheet

membrane testing. ................................................................................................ 76

Figure 3.7 Flow diagram of the pervaporation setup for the HF membrane

testing. .................................................................................................................. 77

Figure 3.8 Flow diagram of the vapor permeation experimental setup. ............. 78

xxv

Figure 4.1 (a) FESEM image, (b) particle size distribution, and (c) XRD

pattern of the as-synthesized ZIF-90 nanoparticles before and after the

hydrothermal test in IPA/water mixture (85/15 wt%, i.e. pervaporation feed) at

60 oC for 24 h. ...................................................................................................... 88

Figure 4.2 TGA curves of pristine P84, ZIF-90 and ZIF-90/P84 MMMs under

air atmosphere. ..................................................................................................... 89

Figure 4.3 FESEM images of cross-section morphologies of (a) pristine P84,

(b) 10/90 (w/w) ZIF-90/P84, (c) 20/80 (w/w) ZIF-90/P84, and (d) 30/70 (w/w)

ZIF-90/P84. .......................................................................................................... 90

Figure 4.4 (a) FESEM image and (b) the corresponding EDX element mapping

of Zn from the cross-section of the 30/70 (w/w) ZIF-90/P84 membrane, (c)

FESEM image and the corresponding EDX element mapping of Zn (d) and S

(e) from the MMM (primer). ............................................................................... 91

Figure 4.5 XRD patterns of ZIF-90/P84 MMMs with different particle

loadings. MMM (primer) represents the 30/70(w/w) ZIF-90/P84 MMM with

SPES (IEC = 0.054 mequiv g-1) as the primer. .................................................... 92

Figure 4.6 Vapor sorption curves of pristine P84 and composite membranes for

(a,b) water and (c,d) IPA saturated vapor at 45 oC. .............................................. 96

Figure 4.7 (a) Effects of ZIF-90 loading on the total flux and water

concentration in permeate and (b) IPA permeability, water permeability and

mole-based selectivity as a function of ZIF-90 loading for IPA dehydration

from an 85 wt% aqueous IPA solution at 60 oC. .................................................. 97

Figure 4.8 Correlations between total permeability and FFV. ............................ 99

Figure 4.9 Comparison of water permeability of ZIF-90/P84 MMMs among

experimental data, Maxwell and Bruggeman models’ predictions at 60 oC. ..... 100

Figure 4.10 The effect of SPES primer on (a) the total flux and separation

factor, and (b) total permeability and mole-based selectivity of MMMs from

dehydration of an 85 wt% IPA aqueous mixture in the temperature of 60 oC. .. 102

Figure 4.11 (a) Total flux, (b) separation factor, (c) total permeability, and (d)

the corresponding mole-based selectivity of pristine P84 membrane and 30/70

(w/w) ZIF-90/P84 membranes without and with the SPES primer (IEC = 0.054

mequiv g-1) using an 85 wt% IPA aqueous mixture as the feed at 60-90 C. .... 103

Figure 4.12 The Arrhenius plots of (a) total flux, (b) water flux, and (c) IPA

flux for pristine P84 membrane and 30/70 (w/w) ZIF-90/P84 with and without

xxvi

the SPES primer of IEC 0.054 mequiv g-1. ........................................................ 104

Figure 5.1 The molecular structures of aldehydes used to modify polymeric

hollow fibers. ..................................................................................................... 120

Figure 5.2 Water contact angles and FESEM images of the outer surfaces and

cross-sections of (a) Torlon HF substrate, and (b-d) HF membranes modified by

GA, HPEI and HPEI mediated GA, respectively. .............................................. 123

Figure 5.3 R parameter as a function of position incident energy of Torlon HF

substrates, and HFs modified by HPEI, GA, and HPEI mediated GA,

respectively. ....................................................................................................... 125

Figure 5.4 3-D AFM images (side and top views) of the outer surface

morphologies of (a) Torlon HF substrate, and (b-d) HF membranes modified by

GA, HPEI and HPEI mediated GA, respectively. .............................................. 127

Figure 5.5 The FTIR spectra of Torlon HF substrates and Torlon HF

membranes modified by GA, HPEI and HPEI mediated GA. ........................... 128

Figure 5.6 N 1s XPS spectra of (a) Torlon HF substrates and (b-d) Torlon HF

membranes modified by GA, HPEI, and HPEI mediated GA, respectively. ..... 129

Figure 5.7 FESEM images of outer surface of (a) Torlon HF substrate, and (b-

d) Torlon HF membranes modified by HPEI mediated BA, HPEI mediated GL

and HPEI mediated ICA, respectively. .............................................................. 131

Figure 5.8 N1s XPS spectra of Torlon HF membranes modified by HPEI

mediated BA, HPEI mediated GL, as well as HPEI mediated ICA. ................. 131

Figure 5.9 FESEM images of overall cross-section, enlarged cross-section near

the outer surface, outer surface as well as inner surface of (a) Torlon, (b) Ultem,

(c) PES HF substrates. ....................................................................................... 133

Figure 5.10 FESEM images of outer surface of (a, b) Ultem, PES HF

substrates, and (c,d) Ultem, PES HFs modified by HPEI mediated GA. .......... 134

Figure 5.11 The effects of feed temperature on separation performance of

HPEI mediated GA modified Torlon HF for aqueous IPA (85/15 wt%

IPA/water) dehydration. ..................................................................................... 135

Figure 5.12 The effects of feed composition on the separation performance of

HPEI mediated GA modified Torlon HF for aqueous IPA dehydration at 50 oC.137

Figure 5.13 Long-term pervaporation performance of the HPEI mediated GA

modified Torlon HF for the dehydration of aqueous IPA solutions (IPA/water

xxvii

85/15 wt%) at 50 oC. .......................................................................................... 138

Figure 6.1 The bottom view of tri-bore spinnerets (dimension unit: mm). ....... 153

Figure 6.2 The parameters (right) for describing the configuration of the TbHF

substrate (left). ................................................................................................... 154

Figure 6.3 FESEM images of SbHF outer surfaces made from different amine

solutions, (a) substrate, (b) 1st amine solution, (c) 2nd amine solution, (d) 3rd

amine solution. ................................................................................................... 156

Figure 6.4 FESEM images of cross-section and outer surface of TFC SbHFs:

(a) substrate, (b) only TFC, (c) TFC-PDMS, (d) HPEI-TFC. (The spinning

conditions of the SbHF substrate: bore fluid composition: NMP/water (95/5

wt%), bore fluid flow rate: 5 ml/min, air gap: 2.5 cm, dope flow rate: 5

ml/min). .............................................................................................................. 158

Figure 6.5 3-D AFM images and mean surface roughness of SbHF outer

surfaces (10×10 m) prepared under different coating methods (a) substrate (b)

only TFC (c) TFC-PDMS (d) only HPEI; (e) HPEI-TFC. (The spinning

conditions of the SbHF substrate: bore fluid composition: NMP/water (95/5

wt%), bore fluid flow rate: 5 ml/min, air gap: 2.5 cm, dope flow rate: 5

ml/min). .............................................................................................................. 159

Figure 6.6 The cross-section, outer surface as well as inner surface

morphologies of TbHF substrates spun from the bore fluid compositions of

NMP/water (80/20 wt%), NMP/water (95/5 wt%), NMP/BuOH (80/20 wt%),

NMP/BuOH (95/5 wt%). Other spinning conditions: bore fluid flow rate: 5

ml/min, air gap: 0.5 cm, dope flow rate: 5 ml/min. ........................................... 160

Figure 6.7 Schematic representation of the concept of phase inversion

processes for the formation of thick and fine TbHF substrates. ........................ 161


morphologies of TbHF substrates spun from the bore fluid flow rates of 3, 4

and 5 ml/min when using NMP/water 95/5 wt% as the bore fluid. Other

spinning conditions: air gap: 0.5 cm, dope flow rate: 5 ml/min. ....................... 165


morphologies of TbHF substrates spun from the bore fluid flow rates of 3, 4

and 5 ml/min when using NMP/BuOH 95/5 wt% as the bore fluid. Other

spinning conditions: air gap: 0.5 cm, dope flow rate: 5 ml/min. ....................... 166


morphologies of TbHF substrates spun at the air gap distances of 0.5, 2.5 and

xxviii

5.0 cm. Other spinning conditions: bore fluid composition: NMP/water (95/5

wt%), bore fluid flow rate: 5 ml/min, dope flow rate: 5 ml/min. ...................... 168


morphologies of TbHF substrates spun from dope flow rate of 5, 6 and 7

ml/min. Other spinning conditions: bore fluid composition: NMP/water (95/5

wt%), bore fluid flow rate: 5ml/min , air gap: 2.5 cm. ...................................... 170

Figure 6.12 Long-term pervaporation performance of the fine TFC TbHF for

the dehydration of aqueous IPA (IPA/water 85/15 wt%) at 50 oC. .................... 174

Figure 7.1 FESEM images of surface and cross-section morphologies of the

Ultem HF substrate. ........................................................................................... 191

Figure 7.2 FESEM images of (a) top surface and (b) cross-section of the Teflon

AF2400 dense membrane, as well as (c) a photo of the Teflon AF2400 dense

membrane after the vapor permeation experiment. ........................................... 192

Figure 7.3 FESEM images of the outer surface and cross-section morphologies

of (a) Ultem HF substrate, and Teflon AF2400/Ultem HF membranes coated by

Teflon AF2400 with a concentration of (b) 0.25 wt% , (c) 0.5 wt% , (d) 1 wt%

and (e) 2 wt% (coating time: 60 s). .................................................................... 194

Figure 7.4 3-D AFM images for the outer surfaces of the Ultem HF substrate

and Teflon AF2400/Ultem HF (coating solution: 1wt% Teflon AF2400, coating

time: 60s) from both the side and top views. ..................................................... 194

Figure 7.5 (a) SEM images and the corresponding EDX elemental maps of (b)

F and (c) C at Teflon AF2400 /Ultem HF (coating solution: 1 wt% Teflon

AF2400, coating time: 60s)................................................................................ 195

Figure 7.6 Schematic illustration of the membrane structure of Teflon

AF2400/Ultem composite HF membranes coated with different Teflon AF2400

concentrations. ................................................................................................... 197

Figure 7.7 Kinematic viscosities of the Galden solvent and Teflon

AF2400/Galden solutions with various Teflon AF400 concentrations. ............. 197

Figure 7.8 S parameter curves as a function of position incident energy of

Teflon AF2400/Ultem HF membranes coated with different Teflon AF2400

concentrations (coating time: 60 s). ................................................................... 199

Figure 7.9 Vapor permeation performance of Teflon AF2400/Ultem composite

HFs coated with different Teflon AF2400 concentrations and the coating time

of 60 s for the dehydration of IPA/water (95/5 wt%) vapor mixture at 125 oC. 200

xxix

Figure 7.10 FESEM images of the outer surface and cross-section

morphologies of Teflon AF2400/Ultem composite HF coated with 1 wt%

Teflon AF2400 solution as a function of (a) 5 s, (b) 30 s, (c) 60 s, (d) 120 s. ... 202

Figure 7.11 The vapor permeation performance of Teflon AF2400 /Ultem

composite HFs coated with 1 wt% Teflon AF2400 solution and different

coating time for the dehydration of IPA/water (95/5 wt%) vapor mixture at 125 oC........................................................................................................................ 203

Figure 7.12 The effects of the vapor feed composition on vapor permeation

performance of Teflon/Ultem composite HFs for aqueous IPA dehydration at

125 oC (coating solution: 1 wt% Teflon AF2400, coating time: 60s). ............... 205

Figure 7.13 Long-term vapor permeation performance of the Teflon

AF2400/Ultem composite HF for the dehydration of an isopropanol/water (95/5

wt%) vapor mixture at 125 oC. (coating solution: 1 wt% Teflon AF2400,

coating time: 60s). .............................................................................................. 208

Figure 7.14 The FESEM cross-section images of Teflon AF2400/Ultem HF (a)

before and (b) after long-term vapor permeation tests. (Coating solution: 1 wt%

Teflon AF2400, coating time: 60s). ................................................................... 208

Chapter 1 Introduction

1

CHAPTER 1 INTRODUCTION

1.1. General introduction of membrane separation

A membrane is a semi-permeable barrier between the feed stream for separation and

one product stream. It controls the relative rates of transport of various species

through itself and thus achieves separation by giving one product depleted in certain

components and a second product concentrated in these components [1]. A

schematic representation of membrane separation is given in Figure 1.1. Membrane

technology plays an important role in separation industry because it is

environmentally benign and has higher efficiency in energy/cost as compared with

conventional separation technologies such as distillation and extraction. Nowadays,

membrane processes are widely applied in various industrial fields related to

chemicals, food, gas, water and wastewater treatment, pharmaceutical, etc. [2].

Figure 1.1 Schematic representation of membrane separation process.

With the development of membrane science and technology, various membrane

processes have been employed in practice, including microfiltration, nanofiltration,

ultrafiltration, reverse osmosis, forward osmosis, gas separation, vapor permeation,


2

pervaporation, dialysis, electrodialysis, membrane distillation, membrane contactors

and liquid membranes [1, 3, 4]. Table 1.1 lists the membrane pore size, phase state,

driving force and major applications for each membrane process. Driven by

innovations and breakthrough in membrane research and design as well as the

increasing applications and demands, the global membranes market is projected to

grow at a compound annual growth rate of 9.47% from 2015 to reach a value of

USD 32.14 billion by 2020 [5].

Table 1.1 Various membrane processes and applications.

Membrane process Pore size Phase

1

Phase

2

Driving

force Applications

Microfiltration 0.1-20 µm L L ΔP1 Clarification, sterile filtration

Ultrafiltration 2-10 nm L L ΔP1 Separation of macromolecular

solutions

Nanofiltration 0.5-5 nm L L ΔP1

Separation of small organic

compounds and selected salts

from solutions

Reverse osmosis Dense L L ΔP1 Sea and brackish water

desalination

Forward osmosis Dense L L ΔP2 Seawater desalination, waste

water treatment

Gas separation Dense G G Δp Separation of gas mixture

Vapor permeation Dense G G Δp Separation of mixtures of

volatile liquids

Pervaporation Dense L G Δp Separation of volatile vapors

and gases

Dialysis 1-100 nm L L ΔE

Separation of microsolutes and

salts from macromolecular

solutions

Electrodialysis

Ion

exchange,

porous

L L ΔE Separation of ions from water

and non-ionic solutes

Membrane

contactors Microporous

L L Δc Both water and gas treatment,

including organic recovery

from water or gas, heavy metal

removal, etc.

G L Δc/Δp

L G Δc /Δp

Membrane

distillation

0.1-1.0

µm L L ΔT/Δp

Separation of water from non-

volatile solutes

Liquid membranes Microporous,

liquid carrier L L

Δc,

reaction

Separation of ions and solutes

from aqueous solutions

L: liquid phase, G: gas phase, ΔP1: hydrostatic pressure; ΔP2: osmotic pressure; Δp: gas or vapor

pressure, ΔE: electrical potential, Δc: concentration difference, ΔT: temperature difference.


3

Among all these membrane processes, pervaporation and vapor permeation are

relatively new technologies for liquid/liquid separations. In the following sections of

this chapter, these two membrane processes are specifically introduced, followed by

a survey of their current applications in industry and a summary on the main

challenges on these membrane technologies. Afterwards, the research objectives and

organization of this thesis are stated.

1.2. Introduction and history of pervaporation and vapor

permeation

1.2.1. Introduction of pervaporation and vapor permeation

As illustrated in Figure 1.2 (a), pervaporation is a combination process of

permeation and vaporization. In this process, the liquid feed directly contacts to a

membrane, thus the feed components sorb into the upstream of the membrane and

diffuse through it to the downstream and vaporize at vacuum condition. Afterwards,

the permeate vapor would be drawn by vacuum and captured using a condenser (a

cold trap immersed in liquid nitrogen is normally used in lab). Figure 1.2 (b) shows

the vapor permeation process, which is similar to pervaporation. The distinguishing

difference is that the state of feed stream in vapor permeation is vapor while it is

liquid in pervaporation. It is worth noting that the membrane for both pervaporation

and vapor permeation should be dense, which indicates the selective material only

has molecular-scale pores or pathways with diffusion as the primary means of

movement through the membrane [6].


4

Compared with conventional separation technologies for liquid mixture separations

such as distillation and extraction, pervaporation and vapor permeation have

advantages such as the ability to separate azeotropic or close-boiling mixtures, lower

energy consumption, small foot print, and flexible process control [7-10]. Besides,

vapor permeation is reported to possess some unique characteristics as compared

with pervaporation [11-17]: (1) No phase change occurs from the feed to the

permeation side; (2) Separation capacity can be increased by pressurizing the feed

vapor; (3) Hybrid with distillation can be effectively achieved; (4) Effects of

concentration polarization, swelling and even fouling (e.g., for fermentation broth

separation) in liquid phase separation could be reduced. Therefore, pervaporation

and vapor permeation have significant potential as alternatives to traditional

separation processes, especially for separations limited by poor vapor-liquid

separation properties.

Figure 1.2 Schematic diagram of (a) pervaporation and (b) vapor permeation

processes.


5

1.2.2. History of pervaporation

The origin of pervaporation dates back to the early 1900s when Kahlenberg reported

the observation the selective transport of hydrocarbon/alcohol mixtures through a

thin rubber sheet [18]. However, the term pervaporation was first introduced by

Kober in 1917 [19]. Farber made the first attempt to explore the usage of the

pervaporation technique in 1935 [20]. The first known quantitative work on

pervaporation was reported by Heisler et al. for the separation of water/ethanol

mixtures [21]. Then Binning and co-workers from the American Oil Company

established the principles of pervaporation technology and highlighted its potential

[22-24]. They carried out several experiments to separate various hydrocarbons by

using pervaporation experiments and even made a pilot plant consisting of 10 m2 of

membrane area to separate hydrocarbons. Meanwhile, academic studies on

pervaporation for separating azeotropic mixtures were carried out by Neel’s group

[25, 26]. However, after several years of work, this technology was not

commercialized because it was not economically useful due to low permeation flux.

A breakthrough was achieved in 1982, when a German company called G.F.T.

(taken over by Sulzer Chemtech in 1994) developed the first commercial

pervaporation membrane with a thin layer of cross-linked polyvinyl alcohol (PVA)

supported on a porous polyacrylonitrile (PAN) substrate for the dehydration of pre-

distilled ethanol [27, 28]. Since then, substantial work was done on widening the

application scope of many liquid mixtures and developing a variety of membranes.


6

1.2.3. History of vapor permeation

Studies on the permeability of vapor through a membrane could be traced to the 19th

century [29, 30]. However, studies of vapor permeability did not gain more attention

until the development of new polymers in the 1950s. The mechanism of diffusion

behaviour of low-molecular-weight compounds in polymers was investigated partly

because of the importance for packing materials [31]. There are roughly two

categories of vapor permeation developed in history [32]: one is the separation of

vapors from gas/vapor mixtures, which is the most important for removing volatile

organics from waste air. The other one was the separation of vapor mixtures without

permanent gases, which is a process alternative to pervaporation for separating

liquid/liquid mixtures since it is more economical and technically more appropriate.

Therefore, the historical development of the later vapor permeation separation is

briefly introduced as follows.

The separation of vapor mixtures using nonporous membranes was discussed by

Binning et al. in the late 1950s [22]. However, the more extensive investigations

started around 1985. With the experience and know-how that gained in the

development of pervaporation technology [33], the straightforward development of

vapor permeation from the laboratory to its first industrial application commercial-

scale vapour permeation plant for alcohol dehydration was realized by LURGI

GmbH in 1989 at Brüggemann Co. in Heilbronn, Germany [34]. Thereafter, more

work on vapor mixture separation was conducted on a variety of membranes for

various vapors separation [8, 12-14, 17, 35-44].


7

1.3. Current applications of membrane pervaporation and vapor

permeation

There are three major applications of pervaporation and vapor permeation: (1)

dehydration of organics; (2) removal of organics from aqueous mixtures; (3)

separation of organic mixtures. Table 1.2 provides some potential liquids separations

based on these three categories [9].

Table 1.2 Potential liquid separations by pervaporation and vapor permeation

process.

Dehydration of organics

Removal of

organics from

aqueous solutions

Organic/organic separation

•Alcohols:

EtOH,

IPA,

BuOH, etc.

•Acids

acetic acid

•Esters

•Ethers

•Tetrahydrofuran

•Ketones

acetone

•Others:

dimethyl acetyl,

phenol

ethylene glycol

methyl isobutyl ketone

triethylamine

•Alcohols (biofuels)

•Aromas

•Methylene dichloride

•Tetrachloroethylene

•Trichloroethylene

•Phenol

•Polar/non-polar:

MeOH/toluene or benzene

MeOH/cyclohexane

MeOH/methyl ester or ether

EtOH/benzene or toluene

EtOH/ethyl ether, IPA/toluene

•Aromatic/Aliphatic benzene/n-heptane or n-hexane,

toluene/n-hexane or isooctane,

styrene/ethylbenzene.

•Aromatic/Alicyclic

benzene/cyclohexane,

toluene/cyclohexane.

•Olefins/aliphatics

butadiene/butane, pentene or

isoprene/pentane,

styrene/ethylbenzene

•Isomeric

xylenes, C4-C8 alkanes, C3-C4

alcohols.

•Others:

carboxylic acid/ester/MeOH,

MeOH /acetone,

dimethyl carbonate/ MeOH,

MeOH/carbon tetrachloride,

ethyl acetate/carbon tetrachloride,

acetonitrile/carbon tetrachloride

MeOH: methanol, EtOH: ethanol, IPA: isopropanol, BuOH: butanol. Reused from Ref [9] with the

permission of Elsevier.


8

Besides, some industrial suppliers of pervaporation and vapor permeation systems,

membranes and applications are listed in Table 1.3, the information of which is

obtained from membrane-guide [45] and literatures [46, 47]. As shown, dehydration

of organics still possesses the widest application range in industry. There are also

more emerging commercial membranes for the removal of organics from aqueous

mixtures. In addition, separation of organic mixtures is still a growing sector. So far,

polymeric membranes are more widely used than inorganic membranes, which is

due to their lower cost, easier fabrication and flexibility.

Table 1.3 Industrial suppliers of pervaporation and vapor permeation systems.

Company Countries Application Membranes

I3 nanotec LLC USA

- Dehydration of organics

- Concentration of organics

and solvent recovery

- Organic-organic

separation

-Hydrophilic ZeoSep A

membrane (e.g. zeolite A, T)

- Hydrophobic or alkali and

acid-resistant membranes zeolite

membranes (e.g. zeolite ZSM-5

and Silicalite)

-Zeolite X,Y membranes

MTR USA - Solvent recovery PerVap® PV membranes: PDMS

based composite membranes

UBE America,

Inc. USA


(alcohols, esters and ethers)

UBE’s polyimide membranes for

high-temperature VP

Sulzer Chemtech

Ltd. Membrane

Technology

Switzerland

- Dehydration of solvents

and other organics

- Solvent recovery from

water

- Organic/organic

separations

- PVA crosslinked composite

membranes (e.g. PERVAP 2200,

2201, 2202, 2210, 2510).

- PDMS based composite

membranes (PERVAP 1060,

1070).

-PERVAP 2256 1/2

KÜHNI AG Switzerland

- Drying of solvents

- Recovery of complex

solvent mixtures

Both organic and inorganic

membranes for PV and VP

IBMEM

Industrial Biotech

Membranes

Germany - Solvent dehydration PV/VP membranes

Beroplan GmbH Germany - Solvent dehydration PV/VP membranes

PolyAn GmbH Germany

- Organic/organic

separations

(aromatics/aliphatics/alicycl

PolyAn organophilic PDMS

membranes for PV


9

ics, alcohols/hydrocarbons,

olefins/aliphatics, isomers)

Omniatex

Impianti Chimici

s.r.l.

Italia - Solvent dehydration Zeolite membranes for PV

ECN Netherland

- Dehydration of alcohols,

ketones, aldehydes, aprotic

solvents, organic acids, etc.

HbySi® membranes suitable for

both PV and VP

PERVATECH

BV Netherland


- Removal of organics

- Silica/Titania on ceramic

-PDMS/POMS on ceramic

Misui Japan - Dehydration of organics A-type zeolite membrane

Shanghai

Megavision

Membrane

Engineering &

Technology Co.,

Ltd.

China - Dehydration of organics

(eg. EtOH). PV membranes

ECN: Energy research Centre of the Netherlands, MTR: Membrane Technology and Research, Inc.

MeOH: methanol, EtOH: ethanol, PV: pervaporation, VP: vapor permeation, PVA: polyvinyl alcohol,

PDMS: polydimethylsiloxane. POMS: polyoctylmethyl siloxane.

1.3.1. Dehydration of organics

As shown in Table 1.3, to date, the dehydration of alcohols and other organics

remains the most important pervaporation and vapor permeation applications in

industry based on the number of installations, installed membrane area and

economical advantages. Membranes, modules and process are well developed,

allowing the installation and operation of industrial plants with large capacities. One

typical example of solvent dehydration is to remove water from concentrated

alcohol solution to produce or regenerate high-purity alcohols. As shown in Table

1.4, the alcohols and water can form azeotropic mixtures, rendering the purification

of alcohols by conventional methods such as distillation inefficient and uneconomic.

However, the membrane-based pervaporation and vapor permeation process can

easily overcome this issue because they can break azeotropes without adding any


10

other component such as entrainer that may cause cross-contamination [48, 49].

Moreover, they are more energy-saving as compared to other conventional

separation technologies such as distillation and adsorption [46].

Hydrophilic membranes are used for dehydration of organics. The most successful

commercial polymeric pervaporation/vapor permeation membrane to date is based

on PVA. More recently some polyimide based membranes have also become

available.

Table 1.4 Properties of alcohols and the azeotropes of alcohol/water systems [50].

Chemical

name

Density

(g cm-3)

Boling point

(oC)

Vapour pressure at

20 oC (hPa)

Azeotrope with

water (wt% water)

Water 1.00 100 23.34 -

Ethanol 0.79 78 59.5 4

Isopropanol 0.78 82 43.2 12.6

n-Propanol 0.80 97 19.9 28.3

2-Butanol 0.81 98 16.7 26.8

n-Butanol 0.81 118 5.3 42.5

t-Butanol 0.78 82 41 11.76

Reused from Ref [50] with the permission of Elsevier.

1.3.2. Removal of organics from aqueous solutions

Removal of organics from a dilute aqueous solution is a minority application. Some

application areas include: (1) removing of trace amount of volatile organic

compounds (VOCs) from waste water for environmental protection [51, 52]; (2) the

recovery of valuable organics from aqueous solutions. For example, natural aroma

recovery using organophilic pervaporation has great potential in food industry [53,

54]; (3) The removal of butanol from acetone-butanol-ethanol (ABE) fermentation

broth, which helps to improve the productivity by relieving the inhibitory effect due


11

to butanol and conducting a continuous production flow. This application has a

growing interest in recent years for the production of biofuels [10, 55].

Contrary to dehydration of organics, this application requires organophilic

membranes which prefer the permeation of organic compounds. At present, most of

the commercial available membranes for the removal of organics from aqueous

solutions are made from polydimethylsiloxane (PDMS) and related polymeric

materials. So far, there are some demonstration and industrial plants in operation,

but the process has still to prove its economic viability in larger capacities.

1.3.3. Separation of organics mixtures

Separation of organics by pervaporation/vapor permeation is the least developed

application, but is at a growing stage. Related membranes have been developed and

are reportedly tested on a pilot plant and industrial scale. Examples include the

recovery of methanol in the production of methyl-tert-butyl ether, the reduction of

benzene concentrations in gasoline, and the separation of aliphatics from aromatic

solvents, etc. [56].

Compared to the other two applications mentioned above, the membranes for the

separation of organics mixtures are more limited, due to the requirements of high

chemical resistance, sorption capacity and good mechanical strength of membrane

materials. Moreover, they should also have good affinity with one of the organic

solvent mixture for separation, thus the solubility parameter and membrane polarity

should be considered when selecting membrane materials. The development of


12

suitable and stable membranes is needed to make this application more competitive

and prospective in industrial application.

1.4. Key challenges in development of pervaporation and vapor

permeation membranes

Membrane is the heart of pervaporation and vapor permeation processes. Based on

the review above, one can find that one of the major hurdles for the expansion of

these processes is the lack of appropriate membranes with high productivity, good

selectivity and long-term stability. Without the development of membranes meeting

these requirements, pervaporation and vapor permeation are unlikely to serve well as

alternative separation techniques. To achieve this goal, there are two main aspects

when developing the membrane, namely, membrane materials selection and

membrane structure design.

Normally, membrane materials selected should possess not only satisfying intrinsic

separation performance, but also good mechanical properties, chemical resistance,

thermal stability and long-time durability, especially for high-temperature operation

and other harsh environments. Therefore, these constrains make the materials

suitable for pervaporation and vapor permeation very limited. Moreover, normally

one certain material normally can only show good separation performance for a

specific type of mixtures. Therefore, the separation of different types of mixtures

requires selection of different types of membrane materials.


13

The design of membrane structure also plays an important role in membrane

development because it would influence the membrane’s properties such as

separation performance, mechanical properties and durability a lot. Typically, an

asymmetric membrane structure composed of a very thin selective layer on a porous

supporting layer is desired to achieve high permeance. However, the challenges of

defects that normally formed at the skin layer of asymmetric membranes should be

overcome to maintain a good selectivity. Besides, the mechanical properties of

polymeric supports are also required to be enhanced to achieve a longer durability.

In addition, to make the pervaporation/vapor permeation processes be economic, the

ease of membrane fabrication and processability are also very important. Although

inorganic membranes exhibit higher separation performance and possess excellent

thermal and solvent stability, their application in industry is limited due to the

brittleness, complex processability and high cost. Polymeric membranes still

predominate in membrane market because they have better film forming properties

and lower costs, despite their lower separation performance. Researchers are also

working on integrating inorganic and organic components together to design

membrane materials with combined advantages and superior separation performance.

But the compatibility between polymeric and inorganic materials is a big problem

needed to be specially considered and overcome.

1.5. Research objectives and thesis organization


14

The objective of this work is to design various polymer based membranes based on

polyimide, polyamide, and perfluoropolymers for alcohol dehydration via

pervaporation or vapor permeation. The composite membranes are developed by the

combination of polymer and inorganic particles or the deposition of thin-film

polymer layer on various polymeric hollow fiber substrates via chemical bonding,

interfacial polymerization, or dip coating. The aims are to design and fabricate

membranes with improved separation performance and good stability for the

dehydration of alcohol, especially IPA, ethanol, and n-butanol. The detailed research

objectives are as follows:

(1) To study the flat-sheet zeolitic imidazolate framework-90 (ZIF-90)/P84

nanocomposite membranes for the enhancement of separation performance of P84

polyimide for the pervaporative dehydration of IPA under different operation

conditions.

(2) To design polymeric hollow fiber (HF) membranes via the hyper-branched

polyethyleneimine mediated aldehyde modification methods for IPA dehydration via

pervaporation.

(3) To fabricate thin film composite tri-bore hollow fiber membranes (TFC-TbHF)

by forming an ultra-thin polyamide layer via interfacial polymerization on Ultem tri-

bore hollow fiber substrates for pervaporative dehydration of IPA.


15

(4) To develop Teflon AF2400/Ultem composite HF membranes by dip coating the

perfluoropolymer on Ultem hollow fiber substrate for alcohol dehydration via high-

temperature vapor permeation.

The dissertation is organized into 8 chapters. Chapter 1 provides the introduction of

this thesis, including a brief introduction of membrane separation, pervaporation,

and vapor permeation, the historical development and major application of

pervaporation and vapor permeation, as well as the key challenges in the

development of pervaporation/vapor permeation membranes.

Chapter 2 presents the fundamental, transport mechanisms, and performance

evaluation parameters of pervaporation and vapor permeation processes. Besides, a

comprehensive literature review on membrane materials and modification methods

on pervaporation/vapor permeation membranes for alcohols dehydration is also

provided.

Chapter 3 describes the experimental details involved in the whole research,

including the materials used, the methods of fabricating and modifying membranes,

pervaporation/vapor permeation tests, as well as various characterization techniques.

Chapter 4 presents the fabrication of ZIF-90/P84 mixed matrix membranes (MMMs)

for pervaporative IPA dehydration by the synthesizing ZIF-90 nanoparticles and

embedding them into P84 polymeric membranes. Moreover, the sulfonated

polyethersulfone (SPES) is applied as a primer to ZIF-90 nanoparticles before

fabricating MMMs at high ZIF-90 loading to disperse the filler better. Fundamentals


16

of pristine P84, ZIF-90 and MMMs with different ZIF-90 loadings are investigated,

including the membrane or nanoparticle morphologies, crystallographic structure,

thermal properties, free volume properties, etc. The effects of ZIF-90 loading as well

as feed temperature on pervaporation performance of the MMMs were

systematically investigated. Vapor sorption measurements are employed to explain

the separation performance by revealing the mass transport behaviours of P84 and

MMMs.

Chapter 5 reports a novel and facile aldehyde modification method to transform

polymeric hollow fiber membranes for pervaporation dehydration of alcohols and

biofuels with much enhanced separation performance and chemical stability. A

universal approach consisting of using HPEI as a gutter layer and aldehyde

modification is proposed and successfully applied to various polymeric membranes

such as Torlon, Ultem, and PES hollow fibers to greatly improve separation

performance. The aldehyde modified HFs and the aldehyde modified HPEI/polymer

composite HFs are all characterized by positron annihilation spectroscopy, Fourier

transform infrared spectroscopy, X-ray photoelectron spectroscopy, and water

contact angle measurements to investigate their free-volume properties, chemistry

changes, and hydrophilicity, etc. The long-term separation performance of the

fabricated composite HF is also tested.

Chapter 6 states the fabrication of novel thin-film composite tri-bore hollow fiber

(TFC TbHF) membranes for pervaporation dehydration of IPA. The interfacial


17

polymerization conditions are first optimized using the conventional single-bore

hollow fiber (SbHF) substrate. Then, the effects of spinning parameters such as bore

fluid composition, bore fluid flow rate, air gap and dope flow rate on TFC TbHF

membranes’ geometry, morphology and pervaporation performance are

systematically investigated. Moreover, the separation performance and mechanical

strengths of the newly designed TFC TbHF membrane is also compared with the

conventional TFC SbHF spun from the identical spinning conditions. At last, the

long-term stability of the TFC TbHF is also investigated.

Chapter 7 presents the design of Teflon AF2400/Ultem composite HF membranes

for alcohol dehydration via high-temperature vapor permeation. The effects of

fabrication parameters such as Teflon concentration and coating time on the

membrane morphology and separation performance are systematically investigated.

Afterwards, the optimal composite HF is tested under extreme vapor feed

compositions from 87 to 99 wt% isopropanol. In addition, its vapor permeation

performance for other alcohols such as ethanol and n-butanol are also tested.

Chapter 8 draws the conclusions of the research work and proposes some

recommendations for the future work.

1.6. References

[1] K. Scott, in Handbook of industrial membranes (second edition), ed. K.

Scott, Elsevier Science, Amsterdam, 1998, pp. 3-185.


18

[2] V. Calabrò, A. Basile, in Advanced Membrane Science and Technology

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[3] M. Mulder, Basic principles of membrane technology, Springer Science

& Business Media, 1996.

[4] K. Scott, R. Hughes, Industrial membrane separation technology,

Springer Science & Business Media, 1996.

[5] Marketsandmarkets.com, Membranes market by type (polymeric

membranes, ceramic membranes, and others), by technology (MF, RO, UF,

pervaporation, gas separation, dialysis, NF, and others), by region (North

America, Europe, Asia-Pacific, the Middle East & Africa, and Latin

America), and by application - global forecast to 2020, (2015).

[6] L. M. Vane, Pervaporation and vapor permeation tutorial: membrane

processes for the selective separation of liquid and vapor mixtures,

Separation Science and Technology 48 (2012) 429-437.

[7] X. Feng, R. Y. M. Huang, Liquid separation by membrane

pervaporation: a Review, Industrial & Engineering Chemistry Research 36

(1997) 1048-1066.

[8] M. Y. Teng, K. R. Lee, S. C. Fan, D. J. Liaw, J. Huang, J. Y. Lai,

Development of aromatic polyamide membranes for pervaporation and vapor

permeation, Journal of Membrane Science 164 (2000) 241-249.


19

[9] L. Y. Jiang, Y. Wang, T. S. Chung, X. Y. Qiao, J. Y. Lai, Polyimides

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[10] G. Liu, W. Wei, W. Jin, Pervaporation membranes for biobutanol

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polystyrene membranes, Die Makromolekulare Chemie 190 (1989) 399-404.

[12] N. Tanihara, K. Tanaka, H. Kita, K.-i. Okamoto, A. Nakamura, Y.

Kusuki, K. Nakagawa, Vapor-permeation separation of water-ethanol

mixtures by asymmetric polyimide hollow-fiber membrane modules, Journal

of Chemical Engineering of Japan 25 (1992) 388-396.

[13] B. Will, R. N. Lichtenthaler, Comparison of the separation of mixtures

by vapor permeation and by pervaporation using PVA composite membranes.

I. Binary alcohol-water systems, Journal of Membrane Science 68 (1992)

119-125.

[14] W. Kujawski, Application of pervaporation and vapor permeation in

environmental protection, Polish Journal of Environmental Studies 9 (2000)

13-26.


20

[15] T. Uragami, S. Yamamoto, T. Miyata, Dehydration from alcohols by

polyion complex cross-linked chitosan composite membranes during

evapomeation, Biomacromolecules 4 (2003) 137-144.

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separation process, Industrial & Engineering Chemistry Research 49 (2010)

3760-3768.

[17] Y. Fujita, M. Yoshikawa, Vapor permeation of aqueous ethanol

mixtures through agarose membranes, Journal of Membrane Science 459

(2014) 114-121.

[18] L. Kahlenberg, On the nature of the process of osmosis and osmotic

pressure with observations concerning dialysis, The Journal o f Physical

Chemistry 10 (1905) 141-209.

[19] P. A. Kober, Pervaporation, perstillation and percrystallization, Journal

of the American Chemical Society 39 (1917) 944-948.

[20] L. Farber, Applications of pervaporation, Science 82 (1935) 158.

[21] E. G. Heisler, A. S. U. Hunter, ANN S., J. Siciliano, R. H. Treadway,

Solute and temperature effects in the pervaporation of aqueous alcoholic

solutions, Science 124 (1956) 77-79.

[22] R. Binning, R. Lee, J. Jennings, E. Martin, Separation of liquid

mixtures by permeation, Industrial & Engineering Chemistry 53 (1961) 45-

50.


21

[23] R.C. Binning, F.E. James, Permeation: a new commercial separation

tool, Petroleum Management 30 (1958) C14.

[24] R.C. Binning, J.F. Jennings, E.C. Martin, Process for removing water

from organic chemicals, US Patent 3035060 (1962).

[25] P. Aptel, N. Challard, J. Cuny, J. Neel, Application of the

pervaporation process to separate azeotropic mixtures, Journal of Membrane

Science 1 (1976) 271-287.

[26] J. Neel, Q. T. Nguyen, R. Clement, L. Le Blanc, Fractionation of a

binary liquid mixture by continuous pervaporation, Journal of Membrane

Science 15 (1983) 43-62.

[27] G. F. Tusel, H. E. A. Brüschke, Use of pervaporation systems in the

chemical industry, Desalination 53 (1985) 327-338.

[28] F. U. Baig, in Advanced Membrane Technology and Applications , John

Wiley & Sons, Inc., 2008, pp. 469-488.

[29] J. K. Mitchell, On the penetrativeness of fluids, J. Roy. Inst. 2 (1831)

301.

[30] T. Graham, LV. On the absorption and dialytic separation of gases by

colloid septa, Philosophical Magazine Series 4 32 (1866) 401-420.

[31] D. A. Blackadder, J. S. Keniry, The measurement of the permeability of

polymer membranes to solvating molecules, Journal of Applied Polymer

Science 16 (1972) 2141-2152.


22

[32] Y. Cen, R. N. Lichtenthaler, in Membrane Science and Technology, eds.

D. N. Richard and S. A. Stern, Elsevier, 1995, pp. 85-112.

[33] U. Sander, P. Soukup, Practical experience with pervaporation systems

for liquid and vapour separation, Proc. 3rd. Int. Conf. on Pervaporation in

the Chemical industry, France, Sept 19-22, 1988, Bakish Materials

Corporation, Englewood (1988) 508-518.

[34] U. Sander, H. Janssen, Industrial application of vapour permeation,

Journal of Membrane Science 61 (1991) 113-129.

[35] K. Okamoto, N. Tanihara, H. Watanabe, K. Tanaka, H. Kita, A.

Nakamura, Y. Kusuki, K. Nakagawa, Vapor permeation and pervaporation

separation of water-ethanol mixtures through polyimide membranes, Journal

of Membrane Science 68 (1992) 53-63.

[36] T. Uragami, T. Morikawa, Permeation and separation characteristics of

alcohol–water mixtures through poly(dimethyl siloxane) membrane by

pervaporation and evapomeation, Journal of Applied Polymer Science 44

(1992) 2009-2018.

[37] J.-Y. Lai, R.-Y. Chen, K. R. Lee, Polyvinyl alcohol γ-ray grafted nylon

4 membrane for pervaporation and evapomeation, Separation Science and

Technology 28 (1993) 1437-1452.

[38] M. S. Schehlmann, E. Wiedemann, R. N. Lichtenthaler, Pervaporation

and vapor permeation at the azeotropic point or in the vicinity of the LLE


23

boundary phases of organic/aqueous mixtures, Journal of Membrane Science

107 (1995) 277-282.

[39] J. Fontalvo, P. Cuellar, J. M. K. Timmer, M. A. G. Vorstman, J. G.

Wijers, J. T. F. Keurentjes, Comparing pervaporation and vapor permeation

hybrid distillation processes, Industrial & Engineering Chemistry Research

44 (2005) 5259-5266.

[40] C.-M. Bell, I. Huang, M. Zhou, R. W. Baker, J. G. Wijmans, A vapor

permeation processes for the separation of aromatic compounds from

aliphatic compounds, Separation Science and Technology 49 (2014) 2271-

2279.

[41] G. Dudek, A. Strzelewicz, R. Turczyn, M. Krasowska, A. Rybak, The

study of ethanol/water vapors permeation through sulfuric acid cross -linked

chitosan magnetic membranes, Separation Science and Technology 49 (2014)

1761-1767.

[42] G. Gong, J. Wang, H. Nagasawa, M. Kanezashi, T. Yoshioka, T. Tsuru,

Fabrication of a layered hybrid membrane using an organosilica separation

layer on a porous polysulfone support, and the application to vapor


[43] Y. Guan, S. Hu, Y. Wang, P. Qin, M. N. Karim, T. Tan, Separating

isopropanol from its diluted solutions via a process of integrating gas

stripping and vapor permeation, RSC Advances 5 (2015) 24031-24037.


24

[44] N. Itoh, J. Ishida, Y. Kikuchi, T. Sato, Y. Hasegawa, Continuous

dehydration of IPA-water mixture by vapor permeation using Y type zeolite

membrane in a recycling system, Separation and Purification Technology

147 (2015) 346-352.

[45] http://www.membrane-guide.com/.

[46] A. Jonquières, R. Clément, P. Lochon, J. Néel, M. Dresch, B. Chrétien,

Industrial state-of-the-art of pervaporation and vapour permeation in the

western countries, Journal of Membrane Science 206 (2002) 87-117.

[47] A. Rozicka, W. Kujawski Application of pervaporation in acetone,

butanol and ethanol recovery from binary aqueous mixtures, International

scientific conference on pervaporation, vapor permeation and membrane

distillation (2013).

[48] S. Matsui, D. R. Paul, Pervaporation separation of aromatic/aliphatic

hydrocarbons by a series of ionically crosslinked poly(n-alkyl acrylate)

membranes, Journal of Membrane Science 213 (2003) 67-83.

[49] W. Xu, D. R. Paul, W. J. Koros, Carboxylic acid containing polyimides

for pervaporation separations of toluene/iso-octane mixtures, Journal of

Membrane Science 219 (2003) 89-102.

[50] P. D. Chapman, T. Oliveira, A. G. Livingston, K. Li, Membranes for

the dehydration of solvents by pervaporation, Journal of Membrane Science

318 (2008) 5-37.


25

[51] L. M. Vane, F. R. Alvarez, Full-scale vibrating pervaporation

membrane unit: VOC removal from water and surfactant solutions, Journal


[52] L. Hitchens, L. M. Vane, F. R. Alvarez, VOC removal from water and

surfactant solutions by pervaporation: a pilot study, Separation and

Purification Technology 24 (2001) 67-84.

[53] F. Lipnizki, J. Olsson, G. Trägårdh, Scale-up of pervaporation for the

recovery of natural aroma compounds in the food industry. Part 1:

simulation and performance, Journal of Food Engineering 54 (2002) 183-195.

[54] J. H. A. Willemsen, B. H. Dijkink, A. Togtema, Organophilic

pervaporation for aroma isolation - industrial and commercial prospects,

Membrane Technology 2004 (2004) 5-10.

[55] L. M. Vane, A review of pervaporation for product recovery from

biomass fermentation processes, Journal of Chemical Technology &

Biotechnology 80 (2005) 603-629.

[56] B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna, Separation of

organic-organic mixtures by pervaporation - a review, Journal of Membrane

Science 241 (2004) 1-21.

Chapter 2 Literature review

26

CHAPTER 2 LITERATURE REVIEW

2.1. Mass transport mechanism

The transport of a real pervaporation/vapor permeation process involving multiple

feed components is complicated. The separation performance is influenced by

various factors, including (1) the physicochemical properties of feed mixtures and

their interactions, (2) the affinities of feed components to the membrane material,

and (3) the membrane morphology and structure.

For both pervaporation and vapor permeation, the most accepted transport

mechanism is the solution-diffusion model based on a non-porous membrane [1, 2].

As shown in Figure 2.1, this model consists of the following steps: (1) Sorption of

the molecules into the membrane on the upstream (feed) side. (2) Diffusion through

the membrane. (3) Desorption into vapor phase on the downstream (permeate) side

of the membrane.

Figure 2.1 Schematic diagram of solution-diffusion transport model.


27

The first two steps are the rate-controlling steps. Therefore, the difference in

solubility and diffusivity of each feed component permeating through the membrane

determines the final separation performance. An ideal membrane should have both

high solubility and diffusivity to one of the feed components. The solubility of a

feed component is related to the chemical nature of the membrane material and

interaction between membrane material and the permeating component, which could

be estimated qualitatively using solubility parameter [3]. Whereas the diffusivity is

greatly influenced by the penetrant size and shape, the fractional free volume

properties of the membrane, as well as the mutual interactions between the penetrant

and membrane [4].

2.2. Evaluation of pervaporation/vapor permeation membranes

The performance of the pervaporation and vapor permeation processes for

separating a binary mixture is usually characterized by two parameters, namely, the

flux and the separation factor. The flux J can be calculated by the following

equation:

tA

QJ

(2.1)

Wherein, Q, A and t are the weight of the permeate sample collected, the effective

membrane surface area, and the time interval during the sample collection,

respectively. The separation factor β is defined by the equation below:


28

wjwi

wjwi

xx

yy

/

/ (2.2)

Wherein, wy and wx are the weight fractions of the component in the permeate and

feed, respectively; the subscripts i and j refer to components of water and alcohol in

this work, respectively.

The intrinsic properties of a dense membrane could be reflected better by the driving

force normalized terms; namely, permeability and selectivity [5]. Therefore, the flux

and separation factor were converted to the permeability and selectivity based on the

solution-diffusion model. The permeability Pi can be calculated by:

pf

ii

ii

pp

lJP

(2.3)

Wherein, l is the membrane thickness. iJ is the flux of component i . While

f

ip

andp

ip are the partial vapor pressure of component i in feed and permeate,

respectively.

In pervaporation, the feed is in liquid state. So the f

ip is the partial vapor pressure

of component i in a hypothetical vapor phase in equilibrium with the feed liquid,

which can be calculated using the Raoult’s Law:

satf

iiii pxp (2.4)

Wherein, ix is the mole fraction of component i in the feed. i and

sat

ip are the

activity coefficient and the saturated vapor pressure of component i , respectively.


29

Both i and

sat

ip pisat were calculated using the AspenTech Process Modelling

software (version 7.2) based on the Wilson equation and the Antoine equation,

respectively.

While in vapor permeation, the feed is vapor state, thus f

ip is calculated by:

ff pxp iii (2.5)

Wherein,i is the fugacity coefficient of component i at the vapor feed side, and pf

is the total feed pressure of the feed vapor. The fugacity coefficient i is determined

by the Peng-Robinson equation of state with the aid of Thermo Solver software [6].

Besides, the partial vapor pressure of component i in the permeatep

ip is expressed by:

pp pyp ii (2.6)

Wherein, iy refers to the mole fraction of component i in the permeate.

pp , the total

pressure at the permeate side, was assumed to be zero due to the vacuum condition.

Therefore, the value ofp

ip is also assumed to be zero.

By combining eq.(2.3)-(2.6), the permeability in pervaporation and vapor

permeation could be expressed using eq.(2.7) and eq. (2.8), respectively：

sat

iii

ii

px

lJP

(2.7)

fpx

lJP

ii

ii

(2.8)


30

In addition, for asymmetric membranes, there is difficulty in obtaining the accurate

thickness of dense selectively layer. Therefore, the mole-based permeance (Pi/l) is

frequently used in this case:

sat/

iiii

ii

pxM

JlP

(2.9)

f/

pxM

JlP

iii

ii

(2.10)

Wherein, iM is molecular weight of component i. Then, the mole-based selectivity of

the membrane ( ) was defined as the ratio of the mole-permeance of components i

and j:

)//(/ lPlP ji ）（ (2.11)

2.3. Pervaporation/vapor permeation membranes for alcohol

dehydration

2.3.1. Membrane materials

The membranes materials for fabricating pervaporation and vapor permeation

should possess good chemical stability, strong mechanical strength and solvent

resistance towards the target feed mixture. Besides, to obtain good water selectivity

for alcohol dehydration, either high solubility selectivity or diffusion selectivity of

water to the alcohol of the membrane is also required. Hence, rigid chain polymers

that are capable of ion-dipole interactions or hydrogen bonding with water are


31

usually used. In more detail, two aspects should be considered when choosing a

membrane material. One is the hydrophilicity of the membrane. Hydrophilic

materials are usually used due to their good affinity to water. Thus the solubility

selectivity of membrane could be high. However, highly hydrophilic materials also

have the problem of severe swelling caused by water, which leads to the increase of

flux but deterioration of separation factor. Therefore, a proper balance of

hydrophilicity and hydrophobicity is needed when choosing membrane material.

The other is the free-volume property of the membrane material, such as the

fractional free volume or pore radius. Normally a higher fractional free volume leads

to the higher flux. Besides, to obtain good diffusivity selectivity, the membrane

material with pore radius that is larger than the kinetic diameter of water but smaller

than that of alcohols is desired. The kinetic diameters of water and various alcohols

are shown in Table 2.1.

Table 2.1 The kinetic diameter of water and various alcohols [7].

Chemicals Kinetic diameter (Å)

Water 2.96

Methanol 3.80

Ethanol 4.30

1-Propanol 4.69

Isopropanol 4.70

n-Butanol 5.05

Reused from Ref [7] with the permission of Elsevier.

The materials available for fabricating pervaporation/vapor permeation membranes

mainly include polymeric and inorganic materials. Generally, inorganic membranes

offer good mechanical stability and high performance, but their wide applications


32

are often limited by their expensive costs and fragility. On the other hand, polymeric

membranes that are relatively inexpensive and easy to fabricate usually encounter

the trade-off between permeability and selectivity. There are several reviews that

introduce the membrane material for pervaporation and vapor permeation process

[8-11], so the demonstration here mainly focuses on some recent developments.

2.3.1.1. Polymeric materials

Several types of polymeric materials that are explored for fabricating

pervaporation/vapor permeation membranes for alcohol dehydration are summarized

in Table 2.2.

Table 2.2 Chemical structures and properties of polymers used in membranes for

alcohol dehydration by pervaporation.

Polymeric Materials Chemical structure Properties

Poly(vinyl alcohol)

(PVA)

Highly hydrophilic, high-

abrasion resistance, tensile

strength, flexibility

Poly(acrylic acid)

Highly hydrophilic

Chitosan

Highly hydrophilic, has -OH

and -NH2, modifiable

Alginate

Highly hydrophilic

Polyimides

High mechanical strength,

heat and chemical resistance

Polyamides

Relative high heat resistance

Perfluoropolymers

e.g. ,

Super hydrophobic,

extremely high mechanical,

heat and chemical resistance


33

Polyelectrolytes

e.g.

Negatively charge: cation-

exchange; Positively charge:

anion-exchange.

Polysulfone (PSF)

Hydrophobic, excellent

mechanical, electrical

properties and improved

chemical resistance,

relatively high heat

resistance

Polyacrylonitrile

(PAN)

Hydrophilic, relatively high

heat resistance;

Among them, some types of polymeric materials are highly hydrophilic, including

poly(acrylic acid), PVA, sodium alginate and chitosan, etc. However, they lack

mechanical strength and stability in aqueous solutions due to severe swelling in

water. As a result, cross-linking is normally needed to enhance their stability in feed

solutions. As already mentioned in Chapter 1, most of commercial available

polymeric membranes are made from PVA currently because they have very high

selectivity towards water with reasonably satisfying flux. However, PVA based

membranes are not stable in feed with high water content especially under high

temperature and may disintegrate over time [12]. Therefore, the applications of

membranes made from highly hydrophilic polymers may be limited to dehydrating

feed with high alcohol content at low temperature.

Polyimides have emerged as potential alternative materials for solvent dehydration

via both pervaporation [7, 10, 13-17] and vapor permeation [18-20]. This is because

they have excellent thermal, chemical and mechanical stabilities, high water

selectivity, and less degree of swelling compared with hydrophilic polymers such as

polyvinyl alcohol [13, 14, 21]. Commercially available polyimides materials such as


34

P84, Matrimid, Ultem and Torlon have been studied a lot during recent years. Some

researchers find that though the swelling degrees of polyimides are lower than the

polymers mentioned above, the swelling still reduces their performance greatly [7,

15]. Therefore, crosslinking is also used to control the swelling and stability,

especially in harsh environments. The detailed procedures include photocrosslinking,

thermal induced crosslinking, and chemical crosslinking by diamines or other

chemicals, etc. [10, 14, 17]. At present, the commercial available polyimide

membrane for alcohol dehydration is developed by UBE America Inc.

Polyamides known as nylon are also a group of heat resistant materials with good

chemical and mechanical stability. They can be synthesized by step-growth

polymerization, such as interfacial polymerization. However, these dense

membranes exhibit extremely low permeability. Thus, the polyamide can only work

well in form of an ultra-thin film, which helps to reduce the transport resistance [22].

Therefore, polyamide material is normally used in fabricating thin film composite

membranes via interfacial polymerization, which will be introduced in next section.

Amorphous, solvent-processable perfluoropolymers (PFPs) such as Teflon AF,

Cytop and Hyflon AD are another family of promising membrane materials

developed in the past 30 years [23-30]. They have extraordinary thermal and

chemical resistance, and are unaffected by most chemicals including acids, bases,

organic solvents, oils and strong oxidizers [31]. Though PFPs are very hydrophobic,

they have been explored for solvent dehydration based on the size exclusion


35

mechanism [12, 32-35]. Studies showed that PFPs membranes have a very low

sorption of alcohol/water mixtures, thus the water permeance and water/alcohol

selectivity of their membranes are essentially independent of feed water/ethanol

composition. Usually, PFPs materials show high permeabilities due to their large

free volumes, but their selectivities are relatively low, especially for high-

temperature vapor permeation. Taking Teflon AF2400 as an example, its selectivity

for the vapor permeation dehydration of ethanol/water (62/38 wt %) mixture is only

6.9 at 120 oC [34]. In recent years, pervaporation composite membranes based on

PFPs have been developed by Membrane Technology and Research Inc. for

dehydration of alcohols at high temperature [12, 35].

Besides, other materials such as polyelectrolytes are used for membrane fabrication

to improve the flux of dehydration membranes. However, the stability of

polyelectrolytes in aqueous solution is an issue due to the possible weakening of the

ionic bonding between positively charged and negatively charged polyelectrolytes

by hydrogen bonding between water and the polyelectrolytes.

2.3.1.2. Inorganic materials

The separation mechanism of inorganic membrane is mainly based on size exclusion.

The inorganic membranes developed for alcohol dehydration include zeolite [36, 37],

silica [38-40], ceramic [41, 42], carbon [43, 44], graphene oxide [45-48], etc. In the

market, most of the commercialized inorganic membranes are made from zeolites

and silica, which has been summarized in Chapter 1, Table 1.3. Other inorganic

materials are still at the research stage.


36

Normally the commercial inorganic membrane possess several times higher flux

than polymeric membranes and also a good selectivity [39, 40, 49]. However, they

are more expensive than polymeric membranes and some of them cannot withstand

in acid environment, such as zeolite membranes.

Carbon membranes from polymeric precursors have shown some promising

performance in alcohol dehydration. However, their fragility and poor mechanical

strength may hinder their further development. Graphene oxide membrane have

drawn more and more attention during these years since it is reported to process

ultra-high water permeability with almost no permeation of organic vapor. Several

issues need to be overcome for further development of graphene oxide membranes

such as the fabrication of an ultra-thin defect-free film to achieve good separation

performance, its stability in the feed, especially at high temperature or flowing mode.

Besides these materials, metal organic frameworks (MOFs) are also a family of new

emerging inorganic membrane materials since they have large porosity and tuneable

window size for molecules to permeate through. Therefore, it has become a hot topic

to develop MOF membranes for gas separation. In recent years, they have also been

extended to pervaporation and vapor permeation process [50, 51]. However, some of

the MOFs membranes are not stable in water, so very few research works focus on

developing pure MOF membranes for alcohol-water separation. Nevertheless, MOFs

are still very popular membrane materials, which can also be used as fillers to

enhance the polymeric membranes.


37

2.3.1.3. Polymer-inorganic hybrid materials

In order to combine the advantages of both polymeric and inorganic membranes in

energy and environment related separations, the concept of hybrid polymer and

inorganic materials together to fabricate mixed matrix membranes (MMMs) was

proposed by Kulprathipanja et al. in 1988 [52]. Afterwards, various types of

inorganic fillers have been incorporated into polymeric matrix to enhance the

dehydration separation performance of polymeric membranes, including zeolites [53,

54], silicalite [55, 56], metal oxide [57], carbon nanotubes [58], MOFs [59], etc. The

effects of the inorganic fillers on the MMMs are various. Some studies report an

enhancement of flux with a sacrificed separation factor, while others observe

enhancement in both flux and separation factor. The different results are attributed

to several aspects, including the interfacial voids between inorganic fillers with the

polymeric matrix, the hydrophilicity nature of fillers, the pore size and porosity of

fillers, and the rigidification of polymer chains by fillers, etc. Normally, larger voids

result in higher permeability but lower selectivity of water, while the right pore size

of the filler and good compatibility between fillers and polymer matrix leads to

better separation performance. Achieving a homogeneous dispersion of inorganic

fillers in polymer matrix is a big challenge in developing MMMs since

agglomeration of inorganic fillers causes defects in membranes and thus the

selectivity would deteriorate. To improve the dispersion of fillers in polymeric

matrix, several methods could be tried: 1) Controlling the size of filler to be less

than 100 nm [60]; 2) Modifying the filler with agents that can have a strong bonding


38

with polymer matrix [61]; 3) Coating the filler with a thin-film of charged polymer

[62]; 4) Using less rigid fillers which contains both organic and inorganic

components such as polyhedral oligomeric silsesquioxane [63, 64] and MOFs [59,

65]. The incorporation of MOFs is a simple and promising method because MOFs

have tuneable pores, high porosity, and are compatible with polymer matrix.

2.3.2. Membrane structure and configuration

Base on the membrane structure, pervaporation/vapor permeation membranes can be

categorized as dense, asymmetric and composite membranes as illustrated in Figure

2.2. Dense membranes are made from the same material(s) with homogeneous

structure, which are usually developed via solvent evaporation method to study the

intrinsic property of membrane materials. Dense membranes are scarcely used for

industrial application due to their low permeation flux and large transport resistance

caused by their large thickness. To achieve membranes with high flux and good

separation factor demanded by industry, asymmetric membranes are developed to

reduce the membrane thickness. A desired asymmetric membrane possesses an

ultra-thin dense film as selective layer and a porous substrate as the mechanical

support. Most of the wholly integral asymmetric membranes are fabricated via non-

solvent phase inversion process [66]. During this process, a homogeneous polymer

solution is precipitated in a non-solvent coagulation bath to form a continuous

porous solid phase and a dense skin layer. The non-solvent phase inversion is a

complicated process, thus several detailed fabricating conditions and combined


39

effects needs to be considered to design an asymmetric membrane with ultra-thin

defect-free dense selective layer and a porous substructure. The other type of the

asymmetric membrane is the composite membrane, which consists of multi-layers

made of different materials. The composite membrane gives more freedom to design

and engineer high performance membranes. For example, researchers can choose a

small amount of expensive materials with excellent separation performance as the

ultra-thin selective layer, while use cheap materials for the porous support.

Therefore, the composite membranes are more cost-effective and have great

potential for industrialization.

Figure 2.2 Polymeric membranes classified by the membrane structure.

Based on the configuration, the polymeric membrane can be categorized as flat sheet,

hollow fiber (HF) or tubular membrane as shown in Figure 2.3. Among them, the

polymeric HF membrane has been widely applied in various separation processes

due to its superior advantages such as higher surface area, higher packing density,

excellent flexibility, self-support property, and ease of fabrication as well as scale-


40

up [11, 67, 68]. The conventional HF membrane has single-bore geometry.

Extensive researches have been focused on the design and fabrication of single-bore

hollow fiber (SbHF) membranes. Besides, a new generation of multi-bore hollow

fiber (MbHF) membranes is emerging recently [69-78]. The new membranes

possess improved mechanical properties as compared with traditional SbHFs owing

to the presence of spokes as the mechanical reinforcement, and thus reduce fiber

breakage and increase operation reliability. At present, all the commercialized HF

membranes for pervaporation are SbHFs. The MbHFs have been commercialized

only for ultrafiltration (UF) applications. They are (1) Multibore® polyethersulfone

(PES) HF membranes consisting of seven bores developed by inge GmbH [79] and

(2) tri-bore PES or polyvinylidene fluoride (PVDF) HF membranes launched by

Hyflux [80].


41

Figure 2.3 Polymeric membranes classified by the membrane configuration.

2.3.3. Composite membranes for pervaporation/vapor permeation applications

As just mentioned above, the composite membranes are more cost-effective and

have great potential for industrialization. Therefore, the development of composite

membranes with satisfying performance and stability has drawn much attention.

There are two ways to fabricate composite membranes. One is to fabricate the

porous membrane substrate first, and then to deposit the dense selective layer on the

substrate via casting, coating, interfacial polymerization, layer by layer method, etc.

The other is to simultaneously fabricate both the top selective layer and porous

support by dual layer spinning/casting method. Some typical technologies for

fabricating the composite membranes for pervaporation/vapor permeation are

introduced below.

2.3.3.1 Coating technologies

The coating technologies are widely used for fabricating composite

pervaporation/vapor permeation membranes, including spin coating, dip coating,

spray coating and so on [81-83]. The intrusion of the top selective layer into the

substrate should be minimized during coating either by pre-wetting of the substrate

or coating a permeable gutter layer on the substrate first to stop the intrusion. The

coating conditions are also very important to achieve a uniform and thin dense

selective layer. Some typical technologies for fabricating the composite membranes

for pervaporation/vapor permeation are introduced below.


42

2.3.3.2 Interfacial polymerization

As demonstrated in Figure 2.4, during interfacial polymerization process, two

monomers are dissolved in two immiscible solvents, respectively. Then the

polymerization of the two monomers takes place at the interface of the two liquid

phases on top of the substrate, thus an ultrathin layer would form. The thin-film

composite (TFC) membranes fabricated via interfacial polymerization have drawn

attention in recent years for pervaporation due to its excellent separation

performance and ease of fabrication [69-71, 84-87]. The morphology, hydrophilicity,

as well as free-volume properties of the formed thin-film selective layer can be

adjusted via using different types of monomer, tuning the monomers’ concentrations,

changing the solvent types, adding surfactants or catalyst, or adjusting the surface

morphology of the support. Nevertheless, TFC membranes fabricated from this

technology is prospective since the thickness of dense selective layer could be

controlled in molecular level easily and thus can achieve both high flux and

separation factor.

Figure 2.4 A typical procedure of TFC membranes fabricated via interfacial

polymerization.


43

2.3.3.3 Layer-by-layer technology

Layer-by layer technology is also used for developing composite membranes, in

which the thin film of selective layer is normally formed by depositing alternating

layers of oppositely charged polyelectrolyte materials with wash steps in between

based on multiple attractive forces [88, 89]. However, the long stability of this kind

of composite membrane in aqueous solution is an issue. Therefore, some researchers

have also tried to use other materials via layer-by-layer technology based on

covalent bond interaction, which has been proved to possess a stable performance

for alcohol dehydration [90]. Even though, this method involved multiple steps

including assembly, washing and drying after each assembly, which might incur

additional costs and prolong production duration.

2.3.3.4 Dual-layer spinning

In recent years, Chung’s group have developed series of dual-layer composite HF

membranes for pervaporation via the co-extrusion spinning technology in just one

step [7, 16, 63, 91-94]. Without intensive post treatment such as annealing or

chemical crosslinking, these HF membranes intrinsically possess a good separation

performance for alcohol dehydration. Though this dual-layer spinning technique is

complicated since the material selection is very important and several spinning

parameters need to be optimized to achieved good separation performance, it may

open a new prospective for the development of next-generation of pervaporation

membranes.


44

To summarize, several criteria needs to be considered when fabricating composite

membranes: 1) Good compatibility between the materials used for the two layers so

that there is no delamination; 2) The solvent for the dense selective layer should not

damage the substrate; 3) The intrusion of the top selective layer into the substrate

should be minimized to decrease the substrate resistance. 4) An ultra-thin selective

layer and a porous substrate are desired to achieve high productivity.

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Chapter 3 Experimental

59

CHAPTER 3 EXPERIMENTAL

3.1. Materials

3.1.1. Polymers

P84 polyimide powders were purchased from HP Polymer GmbH, Austria.

Commercial Torlon 4000T-MV polyamide-imide and Radel® A polyethersulfone

(PES) were supplied by Solvay Advanced Polymers, while commercial Ultem 1010

polyetherimide was purchased from SABIC Innovative Plastics. Sulfonated

polyethersulfone (SPES) granules were provided by Dr. Weber of BASF via direct

route with various contents of sulfonated units [1]. Teflon AF2400 powder is an

amorphous glassy perfluoropolymer containing 87 mol% 2,2-bistrifluoromethyl-4,5-

difluoro-1,3-dioxole and 13 mol% tetrafluoroethylene, which was supplied by Du

Pont. The polymers were dried overnight at 120 oC under vacuum prior to be used

for fabricating membranes. The chemical structures of all polymers used are shown

in Figure 3.1.


60

Figure 3.1 The chemical structures of (a) P84, (b) Torlon 4000T-MV, (c) Ultem

1010, (d) PES, (e) SPES with ion-exchange capacity (IEC) of 0.054 and 0.107

mequiv g-1, (f) Teflon AF2400.

3.1.2. Chemicals for synthesizing ZIF-90 nanoparticles

Zinc acetate dihydrate (reagent grade, Sigma-Aldrich) and imidazolate-2-

carboxyaldehyde (ICA, reagent grade, Alfa Aesar) are the metal source and ligand

used for synthesizing ZIF-90. N, N-Dimethylformamide (DMF, HPLC grade, Fisher)

was used as reaction medium.

3.1.3. Chemicals for surface modification

Several aldehydes, including glyoxylic acid monohydrate (GA), benzaldehyde (BA),

and glucose (GL) purchased from Sigma-Aldrich, as well as imidazolate-2-

carboxyaldehyde (ICA) provided by Alfa Aesar were used to modify polymeric HF

membranes. A hyper-branched polyethyleneimine (HPEI, 50 wt% in aqueous

solutions, Sigma Aldrich) with a molecular weight of 60 kg/mol was acquired to

prepare solutions for the pretreatment of membrane surface. Sylgard®184 silicone

elastomer and its curing agent for polydimethylsiloxane (PDMS) coating were

purchased from Dow Corning Singapore Pte. Ltd.

3.1.4. Chemicals for fabricating TFC membranes

The reaction monomers for the interfacial polymerization, namely, m-

phenylenediamine (MPD, reagent grade, Tokyo Chemical Industry CO., LTD) and

trimesoyl chloride (TMC, reagent grade, Sigma Aldrich) were dissolved in aqueous


61

and organic phases, respectively. In addition, sodium dodecyl sulphate (SDS,

reagent grade, Fluka), triethylamine (TEA, reagent grade, Sigma Aldrich), and

cetyltrimethylammonium bromide (CTAB, reagent grade, Sigma Aldrich) were

procured as additives for the interfacial polymerization.

3.1.5. Organic solvents and other chemicals

The solvents acquired for preparing polymer solution include N-methyl-2-

pyrrolidone (NMP, analytical grade, Merck), DMF and Galden HT55 (analytical

grade, APP Systems Services Pte. Ltd.). While polyethylene glycol 400 (PEG,

analytical grade, Merck), n-butanol (analytical grade, Fisher), ethanol (analytical

grade, Fisher), and deionized water were acquired as non-solvents for the

preparation of spinning solutions. Besides, NMP, ethanol, n-butanol and deionized

water were also used to prepare the bore fluids for spinning. Methanol (reagent

grade, Merck) and n-hexane (analytical grade, Fisher) were employed as solvents for

both reaction and solvent exchange procedure. Isopropanol (IPA, reagent grade,

Fisher), ethanol, n-butanol and deionized water were used to prepare the feed

mixture for pervaporation and vapor permeation studies. An ultra-high bond epoxy

(Blaze Technology Pte Ltd) was specially used for fabricating membrane modules

for high-temperature vapor permeation.

3.2. Membrane fabrication

3.2.1. Fabrication of flat sheet ZIF-90/P84 MMMs by knife casting


62

To fabricate the MMMs, ZIF-90 nanoparticles were firstly synthesized used the

following procedure. ICA (0.845 g) was first added to DMF (50 ml) and heated at

65 oC for 1 h followed by filtration using a 1.0 μm PTFE syringe membrane filter.

After cooling down to room temperature, a mixture of zinc acetate dihydrate (0.241

g) dissolved in DMF (50 ml) was rapidly poured into the ICA/DMF solution and

stirred for 4 h to form a stable nanoparticle suspension. After the completion of

reaction, the product was recovered by centrifugation (12000 rpm, 60 min) and

further purified by washing with fresh DMF to remove the unreacted components.

The yield of ZIF-90 during the aforementioned batch synthesis was around 90%,

which was higher than most of the reported data [2, 3]. The crystalline structure of

ZIF-90 is illustrated in Figure 3.2(a), with tetrahedral Zn sites in blue, N atoms in

green, C atoms in black and O atoms in red (wherein, H atoms and bonds are

omitted for clarity) [3].

After washing and the second centrifugation, the wet-state ZIF-90 particles were re-

dispersed in a fresh DMF solvent under sonication for at least 60 min to ensure a

good dispersion. Then, the ZIF-90 suspension and a P84/DMF solution (25 wt%)

were directly mixed, and vigorously stirred for 12 h. The total weight fraction of

polymer and ZIF-90 particles in DMF was 17 wt%. After degassing for 4 h, the ZIF-

90/P84/DMF mixture was cast onto a glass plate using a 250 m casting knife. The

as-cast membrane and the glass plate were placed in an oven and the solvent was

then evaporated at 60 °C for 13 h. Thereafter, the membrane was peeled off from the

glass plate and solvent-exchanged with methanol for 24 h to effectively remove the


63

DMF trapped inside ZIF-90 pores. The membrane was further dried under vacuum

at 60 °C for 12 h in order to remove the residual solvents. The thickness of the

membranes was in the range of 19-22 m measured by a Mitutoyo micrometer.

The theoretical ZIF-90 loading in the composite membrane was defined as:

100P84ZIF

ZIFZIF

mm

m (3.1)

Wherein, ZIF is the particle loading,

ZIFm and P84m represent the weights of ZIF-

90 and P84, respectively. The particles loading in the present study was

predetermined at 10 wt%, 20 wt% and 30 wt%, which were defined as 10/90 (w/w)

ZIF-90/P84, 20/80 (w/w) ZIF-90/P84, 30/70 (w/w) ZIF-90/P84, respectively. Figure

3.2 (b) depicts the appearance of a 30/70 (w/w) ZIF-90/P84 MMM on top of the

NUS logo, which shows high transparency of the membrane at a high ZIF-90

loading.

Figure 3.2 (a) Chemical structure of ZIF-90 and (b) a photo of a 30/70 (w/w) ZIF-

90/P84 membrane (15.2 cm2) on top of the NUS logo.


64

In addition, the SPES was incorporated as a primer to fabricate MMMs based on the

method reported by Li et al. with minor alterations [4]. In a typical procedure, the

ZIF-90/DMF mixture (i.e. 0.5 g ZIF-90 and 4.64 g DMF) was first sonicated for 60

min. Then, 0.05 g of SPES granules were added to the ZIF-90 mixture and the

solution was further stirred for 18 h. Next, the aforementioned solution was poured

into a polymer solution containing 1.17 g of P84 and 3.5 g of DMF and then

subjected to vigorous stirring for another 18 h. The remaining steps were similar

with the preparation of MMMs without the primer. Two batches of SPES with the

IEC of 0.054 and 0.107 mequiv g-1 were used in this study.

3.2.2. Fabrication of flat sheet Teflon AF2400 dense film by ring casting

The Teflon AF2400 polymer solution was prepared by dissolving 1 wt% Teflon

AF2400 powder in the Galden solvent via stirring at room temperature, followed by

filtering it with a 1.0 m polytetrafluoroethylene syringe membrane to remove

impurities. Afterwards, the polymer solution was poured onto a silicon wafer sealed

with a metal ring. The volatile solvent was slowly evaporated at room temperature.

The resultant transparent Teflon AF2400 film was peeled off and dried at 120 oC

under vacuum with a heating rate of 0.6 oC/min, and held there for 24 h to remove

the residual solvents.

3.2.3. Spinning of polymeric HF substrates

The HF substrates were obtained via a dry-jet wet spinning process [5, 6]. As shown

in Figure 3.3, the dope solution was poured into an ISCO syringe pump and


65

degassed overnight. The bore fluid was stored in another syringe pump shortly

before the spinning started. During spinning, the dope solution and bore fluid were

co-extruded via the spinneret with controlled flow rates. The nascent fiber travelled

a certain air-gap distance and then entered a water coagulation tank for phase

inversion. The HF substrate was then collected by a rolling drum with a controlled

take up speed. The obtained HF substrates were cut and rinsed in tap water for at

least 3 days to remove residual solvents. Afterwards, they were solvent exchanged

with methanol followed by hexane for three consecutive times to further remove

residual solvents within the membrane. Finally, they were air dried for further

characterizations, post-modifications and pervaporation/vapor permeation tests.

Figure 3.3 The scheme of a typical dry-jet wet spinning process.

3.2.4. Composite HF membranes by aldehyde surface modification

Firstly, the aldehyde solutions were prepared by dissolving specific aldehydes in

methanol with a concentration of 0.05 mol/L. Figure 3.4(a) illustrates the aldehyde

modification scheme on Torlon HF substrates using an aldehyde solution at 65 oC

for 4 h because aldehyde molecules would react with the secondary amino groups (-


66

NH-) of Torlon. Then the modified HF was taken out, rinsed by fresh methanol, and

air dried. Figure 3.4(b) elucidates the HPEI mediated aldehyde scheme so that the

aldehyde modification scheme could be applicable to HF membranes made from

other materials. Firstly, the primary amino groups (-NH2) were grafted to the

membrane surface by immersing the HF substrate in a 1 wt% HPEI aqueous solution

at 60 oC for 1 h. Then, the HF was wiped with tissue papers and subsequently

immersed into an aldehyde solution via a similar aldehyde modification scheme. As

a result, aldehyde molecules would react easily with the primary amino groups of

HPEI, and modify the membrane surface.

Figure 3.4 The scheme of procedures for (a) aldehyde modification and (b) HPEI

mediated aldehyde modification of HF substrate. Note that: if there is no ‒H in the

aldehyde molecule, the intermediated carbinolamines cannot be dehydrated to give

enamines, but form derivatives of amine, as described in literatures [7, 8].


67

3.2.5. Composite HF membranes by interfacial polymerization

The TFC HF membranes were prepared by conducting interfacial polymerization on

the HF substrates, where the monomer MPD in the aqueous phase reacts with

another monomer TMC in the organic phase to form a polyamide selective layer on

the outer surface of the substrates [9-12]. The aqueous phase consists of 2 wt%

MPD in deionized water with or without 0.1 wt% SDS (or CTAB) and 0.5 wt%

TEA in certain conditions; while the organic phase has 0.15 wt% TMC in hexane.

The TbHF substrates with one end sealed by epoxy were immersed into the aqueous

solution for 3 min at room temperature. They were then blotted with tissue paper

and dipped into the organic solution for 1 min for the interfacial polymerization.

Finally, the resultant fibers were annealed at 65 oC for 15 min to stabilize the

selective TFC layer. In the case of applying HPEI as the pre-treatment (referred to as

HPEI-TFC), the substrate was first immersed into an aqueous 1 wt% HPEI solution

for 2 min, and then dried at 65 oC for 10 min. Thereafter, the polyamide selective

layer was synthesized as described above. The resultant sample is referred to as

HPEI-TFC. In the case of PDMS post-treatment (referred to as TFC-PDMS), the

TFC-HF was immersed into a 3 wt% PDMS/hexane solution for 1 min and then the

coated fiber was cured for at least 24 h in air. The scheme of all methods to fabricate

TFC-HF membranes is illustrated in Figure 3.5.


68

Figure 3.5 The scheme of fabricating TFC HF with different methods: (1) The

central red box with solid line: the interfacial polymerization process; (2) The green

box with dashed line: pre-coating HPEI and then interfacial polymerization; (3) The

blue box with dotted line: post-PDMS coating after the interfacial polymerization.

3.2.6. Composite HF membranes by Teflon AF2400 coating

The composite HF membranes were fabricated by dip coating a Teflon AF2400

layer on the outer surface of Ultem HFs. First, the coating solutions with different

Teflon AF2400 concentrations were prepared. Then the Ultem HF substrates with

one end sealed by epoxy were dip coated into the prepared Teflon AF2400 solution

for some time. Afterwards, the coated composite HFs were dried in air overnight

before further testing and characterizations.

3.2.7. Fabrication of HF modules

The HF modules for pervaporation were prepared by loading the HF membranes

into a module holder assembled from two Swagelok stainless steel male run tees and

a 3/8 inch perfluoroalkoxy tube, and both ends of the male run tee were sealed with

epoxy. It’s worth noting that when fabricating HF modules for vapor permeation, the


69

perfluoroalkoxy tube was changed to stainless steel tube to achieve good sealing.

Each module contained one HF with an effective length within the range of 14-17

cm. All modules were cured for 48 h at ambient temperature before evaluation.

3.3. Material and membrane characterization

3.3.1. Optical microscope

The dimensions of HFs, especially the tri-bore HFs were determined by an optical

microscope (microscope: Olympus, SZX16; digital camera: Olympus, CMAD3)

3.3.2. Scanning electron microscopy (SEM)

The morphologies of ZIF-90 nanoparticles, flat sheet MMMs, HF substrates and

composite HF membranes were observed by using scanning electron microscopy

(SEM JEOL JSM-5600LV) and field emission scanning and electron microscopy

(FESEM JEOL JSM-6700LV). To observe the cross-sections of membranes, the

samples were prepared by fracturing the pre-dried membranes in liquid nitrogen and

coated with platinum using a JEOL JFC-1200 ion sputtering device.

3.3.3. Atomic force microscope (AFM)

A Nanoscope IIIa atomic force microscope (AFM) from Bruker Dimension ICON

was used to study the topology and roughness of the resultant composite HF

membrane surface. The mean roughness (Ra) was calculated based on the AFM

measurement.


70

3.3.4. Wide angle X-ray diffraction (XRD)

The crystallographic properties of the ZIF-90 nanoparticles and MMMs were

investigated using powder X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα

radiation, λ = 1.5406 Å).

3.3.5. Fourier transform infrared spectrometer (FTIR)

The chemical structures of the HFs before and after the surface modification were

analyzed by using Fourier transform infrared spectroscopy (FTIR, Perkin-Elmer

Spectrum 2000 FTIR spectrometer) under the attenuated total reflectance mode.

3.3.6. X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS, Kratos AXIS His spectrometer) was

employed to characterize the variation of N element-containing functional groups at

the surface of HFs before and after chemical modifications. The binding energy of C

1s at 284.5 eV was taken as the reference.

3.3.7. Energy dispersive spectrometry (EDX)

To observe the distribution of ZIF-90 nanoparticles in MMMs and to confirm the

Teflon coating layer on the HF surface, elemental mapping was conducted using an

energy-dispersive X-ray spectroscopy (EDX) apparatus along with the SEM.

3.3.8. Water contact angle measurement


71

The surface hydrophilicity of HF substrates and composite HFs after modifications

were determined by water contact angle measurement using a Sigma 701

Tensiometer from KSV Instruments Limited. Each sample was measured at least 3

times.

3.3.9. Thermogravimetric analyses (TGA)

The actual ZIF-90 nanoparticles loadings in MMMs were determined by

thermogravimetric analysis (TGA) using a thermobalance (DTG-60AH/TA-

60WS/FC-60A, Shimazu) under air atmosphere at a heating rate of 10 °C min-1 from

50 oC to 800 oC.

3.3.10. Surface area measurement

The specific surface areas of ZIF-90 nanoparticles were determined by measuring

nitrogen (N2) adsorption-desorption isotherms using a Quantachrome NOVA-3000

system at 77 K. Prior to the N2 physisorption measurements, the samples were

degassed at 100 oC for 4 h under N2 flow.

3.3.11. Density measurements

The density measurements of MMMs were carried out on an analytical balance

(ML204, Mettler Toledo) with a density kit (ML-DNY-43, Zurich). The density of

the sample was calculated based on Eq. (3.2) in accordance with the Archimedean

principle by measuring the weights of the membrane in both air and liquid [13]:


72

airairliq

liqair

air

mm

m (3.2)

Wherein, , air and liq are the densities of the sample, air and the applied liquid

(i.e., ethanol), respectively. airm and liqm represent the measured sample weights in

air and liquid, respectively.

3.3.12. Kinematic viscosity measurement of polymer solutions

The kinematic viscosities of polymer solutions with various concentrations are

measured using capillary viscometer from SCHOTT Instruments with ViscoSystem®

AVS 360. Each sample was tested more than 3 times and their average value was

reported.

3.3.13. Vapor sorption measurement

A gravimetric sorption setup built in our laboratory was employed to carry out the

dynamic vapor sorption tests on membranes and nanoparticles, a detailed

introduction of which has been demonstrated elsewhere [14, 15]. Samples were

dried in vacuum overnight prior to test. Then 12-15 mg of the sample weighted by a

microbalance was hung on a sensitive quartz spring (Deerslayer, US) with a spring

constant of 0.21 mm/mg. After the whole system was vacuumed, the solvent vapor

was generated from 10 L liquid vessel by nitrogen at 45oC, and then the saturated

vapor together with nitrogen carrier was gently introduced to the sorption chamber

until the pressure reached 6 psi. The mass uptake in the polymer samples as a


73

function of time was automatically recorded based on the quartz spring displacement

monitored by a Basler camera during sorption tests.

3.3.14. Swelling studies of membranes in high-temperature feed vapor

The dense membranes with known initial areas were loaded above the liquid feed in

autoclaves separately. Then the autoclaves were sealed and heated to 125oC, and

thus the generated vapor would directly contact with the membrane inside. The

membranes were taken out after a certain time and the areas were measured. The

degree of swelling is calculated by the ratio of the membrane area change after

immersing in the vapor feed for several days to the initial membrane area.

3.3.15. Mechanical properties measurement

The membrane mechanical properties of the fabricated HFs were characterized using

an Instron tensiometer (Model 5542, Instron Corp.) at room temperature, where the

HFs were clamped at the both ends with an initial gauge length of 50 mm and a

testing rate of 10 mm/min.

3.3.16. Slow beam positron annihilation spectroscopy (PAS)

Slow beam positron annihilation spectroscopy (PAS) is a powerful characterization

tool for analyzing the layer structures and free-volume properties of composite

membranes [16]. In this research, two positron annihilation spectrometers were

installed in this beam for this study: Doppler energy spectroscopy (DBES) and

positron annihilation lifetime spectroscopy (PALS).


74

DBES was employed to qualitatively investigate free volume depth profile of

composite membranes and quantitatively determine the selective layer thickness of

composite membranes. The DBES spectra were measured as a function of depth

using the slow positron beam (0-30 keV) under an ultra-high vacuum of ~10-7 torr.

Two of the characteristic parameters of DBES spectra, S parameter and R parameter

were employed in this research. S parameter was defined as the ratio of the

integrated counts between energy 510.3 keV and 511.7 keV to the total counts after

the background was properly subtracted [17]. S parameter is mainly influenced by

three factors, including the amount of free volume, the size of free volume and the

chemical environment of the membrane. R parameter was defined as the ratio of the

total counts from an energy width between 364.2 and 496.2 keV (due to ortho-

positronium (o-Ps) 3γ annihilation) to from the 511 keV peak region with a width

504.35 and 517.65 keV (due to 2 annihilation) [17]. It can be used to analyse larger

pores in polymeric membranes. Moreover, to quantitatively determine the selective

layer thickness, the resultant S and R parameter data against positron incident

energy were fitted using the well-established VEPFIT computer program with a

three-layer model [18].

PALS was applied to quantitatively determine the free volume properties. The

obtained PALS data were analyzed and fitted using a three lifetime components

model with the aid of the PATFIT program [19], from which the third lifetime

component τ3 corresponding to o-Ps annihilation and its intensity I3 were determined.


75

Normally τ3 for polymeric membranes is within the range of 1-5 ns, which is usually

adopted to calculate the mean free volume radius and fractional free volume.

3.4. Pervaporation experiments

3.4.1. Pervaporation experiments for flat sheet membranes

Pervaporation experiments for flat sheet membranes testing were carried out using a

lab-scale Sulzer pervaporation unit [15, 20], the flow diagram of which is shown in

Figure 3.6. The effective membrane area was 15.2 cm2. A 2 L of liquid feed mixture

was circulated in the system with a flow rate of 80-85 L/h at a certain temperature.

The permeate pressure was maintained less than 1 mbar by a vacuum pump. The

system was stabilized for 2 h before the samples collection. Thereafter, permeate

samples were collected by a cold trap immersed in liquid nitrogen. The samples

were then weighted and their compositions were analyzed by gas chromatography

(Hewlett-Packard GC 7890, HP-INNOWAX column, thermal conductivity detector.

The feed concentration varied less than 0.5 wt% during the entire experiment and

can be therefore considered as constant during the experiment because of the large

quantity of the feed solution compared to the permeate sample. Moreover, at least

three permeate samples were collected to ensure the reliability of the results and at

least two identical membranes were tested for each of the pervaporation condition.

Afterwards, the average value was reported as the pervaporation performance.


76

V1: Three-way ball valve, V2-V5: ball valves, V6: Pressure relief valve, P1: pressure gauge, P2:

Digital vacuum gauge, T1: temperature gauge; T2: temperature sensor.

Figure 3.6 Flow diagram of the Sulzer pervaporation setup for the flat sheet

membrane testing.

3.4.2. Pervaporation experiments for HF membranes

A laboratory scale pervaporation unit was employed to evaluate the HF membrane

performance [6]. The schematic of the HF pervaporation apparatus is illustrated in

Figure 3.7. A 2 L of liquid feed solution was circulated through the shell side of the

module with a flow rate of 30 L/h at a certain feed temperature, while the lumen side

(permeate side) of the module was vacuumed at a pressure below 1 mbar throughout

the experiments. Thereafter, the permeate sample collection and sample analysis

steps were the same with those in pervaporation experiments for the flat sheet

membrane testing.


77

V1: Pressure relief valve, V2-4, 6-21: ball valves, V5: needle valve, P1-5: pressure gauge, P6: Digital

vacuum gauge, T1: temperature gauge; T2-3: temperature sensor.

Figure 3.7 Flow diagram of the pervaporation setup for the HF membrane testing.

3.5. Vapor permeation experiments

The apparatus to evaluate the vapor permeation performance of membranes is

shown schematically in Figure 3.8. The vapor generator tank contains 5 L alcohol-

water mixture with a certain composition. The tank was heated to 125 oC by an oil

bath. Both the testing chamber and pipelines were preheated to 125 oC after

installing two sets of HF modules in parallel in the chamber. Then the generated

feed vapor was delivered and contacted to the shell side of the membranes with a

controlled flow rate around 0.25-0.3 kg h-1 and a vapor pressure of approximately 3

bar for isopropanol/water and ethanol/water vapor mixtures. For the n-butanol/water

mixture its vapor pressure could only be controlled around 1 bar at this flow rate.

The pressure at the lumen side of the membranes was kept less than 2 mbar. The


78

retentate was condensed by a chiller and stored in a retentate tank. After the system

was stabilized for 1 hour, the experimental steps of sample collection and analysis

were the same with those for pervaporation experiments. This vapor permeation

system can also be used to flat sheet membrane by changing the hollow fiber

modules into flat sheet membrane modules.

V1-3, 5-10, 12-14: ball valve, V4: pressure relief valve, V11: needle valve, P1: pressure gauge,

P2,3,4: pressure transducer, T1-5: temperature sensor, T6: temperature gauge.

Figure 3.8 Flow diagram of the vapor permeation experimental setup.

3.6. References


79

[1] N. Widjojo, T. S. Chung, M. Weber, C. Maletzko, V. Warzelhan, A

sulfonated polyphenylenesulfone (sPPSU) as the supporting substrate in thin

film composite (TFC) membranes with enhanced performance for forward

osmosis (FO), Chemical Engineering Journal 220 (2013) 15-23.

[2] T. X. Yang, T. S. Chung, Room-temperature synthesis of ZIF-90

nanocrystals and the derived nano-composite membranes for hydrogen

separation, Journal of Materials Chemistry A 1 (2013) 6081-6090.

[3] W. Morris, C. J. Doonan, H. Furukawa, R. Banerjee, O. M. Yaghi,

Crystals as molecules: postsynthesis covalent functionalization of zeolitic

imidazolate frameworks, Journal of the American Chemical Society 130

(2008) 12626-12627.

[4] Y. Li, W. B. Krantz, T. S. Chung, A novel primer to prevent nanoparticle

agglomeration in mixed matrix membranes, AIChE Journal 53 (2007) 2470-

2475.

[5] T. S. Chung, X. Hu, Effect of air-gap distance on the morphology and

thermal properties of polyethersulfone hollow fibers, Journal of Applied

Polymer Science 66 (1997) 1067-1077.

[6] R. Liu, X. Qiao, T. S. Chung, The development of high performance P84

co-polyimide hollow fibers for pervaporation dehydration of isopropanol,


[7] J. E. Fernandez, G. B. Butler, The reaction of secondary amines with

formaldehyde, The Journal of Organic Chemistry 28 (1963) 3258-3259.


80

[8] M. A. Sprung, A summary of the reactions of aldehydes with amines,

Chemical Reviews 26 (1940) 297-338.

[9] J. E. Cadotte, Interfacially synthesized reverse osmosis membrane, US

Patent 4,277,344 (1981).

[10] R. B. Hodgdon, Using all aliphatic polyamides from polyamino

compounds and compounds having two or more acid halide groups, US

Patent 5,616,249 (1997).

[11] P. S. Singh, S. V. Joshi, J. J. Trivedi, C. V. Devmurari, A. P. Rao, P. K.

Ghosh, Probing the structural variations of thin film composite RO

membranes obtained by coating polyamide over polysulfone membranes of

different pore dimensions, Journal of Membrane Science 278 (2006) 19-25.

[12] A. K. Ghosh, E. M. V. Hoek, Impacts of support membrane structure

and chemistry on polyamide-polysulfone interfacial composite membranes,


[13] L. Hao, P. Li, T. X. Yang, T. S. Chung, Room temperature ionic

liquid/ZIF-8 mixed-matrix membranes for natural gas sweetening and post-

combustion CO2 capture, Journal of Membrane Science 436 (2013) 221-231.




586.


81

[15] Y. Tang, N. Widjojo, G. M. Shi, T.-S. Chung, M. Weber, C. Maletzko,

Development of flat-sheet membranes for C1–C4 alcohols dehydration via

pervaporation from sulfonated polyphenylsulfone (sPPSU), Journal of

Membrane Science 415-416 (2012) 686-695.

[16] Y. C. Jean, W.-S. Hung, C.-H. Lo, H. Chen, G. Liu, L. Chakka, M.-L.

Cheng, D. Nanda, K.-L. Tung, S.-H. Huang, K.-R. Lee, J.-Y. Lai, Y.-M. Sun,

C.-C. Hu, C.-C. Yu, Applications of positron annihilation spectroscopy to

polymeric membranes, Desalination 234 (2008) 89-98.

[17] H. Chen, W. S. Hung, C. H. Lo, S. H. Huang, M. L. Cheng, G. Liu, K.

R. Lee, J. Y. Lai, Y. M. Sun, C. C. Hu, R. Suzuki, T. Ohdaira, N. Oshima, Y.

C. Jean, Free-volume depth profile of polymeric membranes studied by

positron annihilation spectroscopy: layer structure from interfacial

polymerization, Macromolecules 40 (2007) 7542-7557.

[18] A. van Veen, H. Schut, J. de Vries, R. A. Hakvoort, M. R. Ijpma,

Analysis of positron profiling data by means of ‘‘VEPFIT’’, AIP Conference

Proceedings 218 (1991) 171-198.

[19] P. Kirkegaard, N. J. Pederson, M. Eldrup, PATFIT package, Roskilde:

Riso National Laboratory, 1989.

[20] X. Qiao, T. S. Chung, K. P. Pramoda, Fabrication and characterization

of BTDA-TDI/MDI (P84) co-polyimide membranes for the pervaporation

dehydration of isopropanol, Journal of Membrane Science 264 (2005) 176-

189.

Chapter 4 ZIF-90/P84 mixed matrix membranes

82

CHAPTER 4 ZIF-90/P84 MIXED MATRIX

MEMBRANES FOR PERVAPORATION

DEHYDRATION OF ISORPROPANOL

4.1. Introduction

Driven by concerns on energy security and global warming, there is an urgent need

to explore and develop both energy-saving and environmentally benign processes

for chemical industries. Previous studies indicate that separation alone currently

accounts for 60 to 80% of the process cost in most mature chemical processes [1].

Distillation, gas stripping and molecular sieve adsorption are the conventional unit

operations applied to separate and purify alcohols from alcohol/water mixtures.

However, these processes often require large footprints and intensive energy [2].

Moreover, it is costly to achieve a complete dewatering of alcohol via single-step

ordinary distillation process due to the formation of azeotropic mixtures between

alcohols and water. In contrast, the pervaporation process (a combination of

permeation and evaporation processes) is an economic and eco-friendly separation

process. It has been considered as one of potential candidates to outperform the

conventional separation processes in dehydration of alcohols as it has the flexibility

in process design and integration, lower energy consumptions coupled with

simplicity in the process control [3, 4]. For instance, the energy demand for

purification of isopropanol (IPA) from aqueous solutions may be lowered by 87%

when comparing pervaporation vs. azeotropic distillation [5]. In addition, the risk of


83

cross-contamination using the entrainer in azeotropic distillation can be eliminated

by using the pervaporation process [6, 7]. Hence, pervaporation is a promising

technology for the dehydration of alcohols from alcohol/water mixtures [2-5, 8].

In the past decades, substantial works have been devoted to explore high

performance pervaporation membranes for alcohol separation. An ideal

pervaporation membrane should have superior permeability and selectivity,

excellent durability, high mechanical stability, and economic viability. Generally,

inorganic membranes offer good mechanical stability and high performance, but

their wide applications are often limited by their expensive costs and fragility. On

the other hand, polymeric membranes that are relatively inexpensive and easy to

fabricate usually encounter the trade-off between permeability and selectivity.

Therefore, heterogeneous mixed matrix membranes (MMMs) consisting of

nanometric inorganic fillers embedded in a polymeric matrix may provide

synergistic characteristics of both organic and inorganic membranes in energy and

environment related separations [9, 10]. Since the compatibility between the

inorganic and organic phases as well as the elimination of interface defects are

among the essential aspects in the development of MMMs for pervaporation

processes, significant researches have been focused on elucidating these issues [4,

10, 11].

Zeolitic imidazolate frameworks (ZIFs) are a subfamily of metal-organic

frameworks (MOFs), which have been demonstrated as promising membrane


84

materials due to their extremely large surface areas as well as superior hydrothermal

and chemical stabilities [12-14]. In general, ZIFs are regarded to possess flexible

structures that have a better affinity with polymer chains, as compared with

traditional inorganic particles (e.g. zeolites, silica, carbon nanotubes, etc.) with

“rigid” frameworks [8, 13, 15]. Therefore, MMMs with the incorporation of ZIFs as

additives (fillers) are expected to exhibit enhanced flux and possibly selectivity

when comparing to the pristine polymeric membrane. A variety of MMMs

consisting of ZIF materials have recently been studied in order to explore their

prospects for gas separation [15-25]. However, only limited studies are for

pervaporation.

Shi et al. prepared ZIF-8/PBI mixed matrix membranes for pervaporation

dehydration of C2-C4 alcohols and studied their fundamental properties of sorption

and permeability [26, 27]. Liu et al. fabricated an organophilic ZIF-8/polymethyl-

phenylsiloxane membrane to recover bioalcohols by taking the advantage of

ultrahigh adsorption selectivity of ZIF-8 nanoparticles [8]. Recently, their group

employed a novel ‘‘plugging-filling’’ method to fabricate ZIF-8-silicone rubber

nanocomposite membranes with interesting performance [28]. Kang et al. also

designed ZIF-7/chitosan MMMs by incorporating microporous ZIF-7 particles into

chitosan membranes [29]. They found that ZIF-7 particles have a strong affinity

with the chitosan due to the nature of organic linkers within ZIF-7. Jin et al.

immobilized the organophilic ZIF-71 particles into polyether-block-amide (PEBA)

membranes for the recovery of biobutanol via pervaporation [30]. Since ZIF-71


85

particles with an appropriate loading of ≤20 wt% exhibited excellent compatibility

with the polymer matrix, the resultant separation performance was significantly

improved.

Among various ZIFs, Zinc(2-carboxyaldehyde imidazolate)2 (ZIF-90) has gained

particular attention since the pioneering work by Yaghi et al. in 2008 [31]. The

imidazole linker of ZIF-90 contains a carbonyl group, which has a favorable

noncovalent interaction with the polymer matrix. Therefore, ZIF-90 based MMMs

may have the advantage to suppress the delamination between the filler and polymer

phases. In addition to better affinity with the polymer phase, ZIF-90 possesses the

free aldehyde groups in the framework which can be covalently functionalized with

amine groups through the imine condensation reaction. Huang et al. employed 3-

aminopropyltriethoxysilane (APTES) as a covalent linker between the ZIF-90 layer

and Al2O3 support via imine condensation and the resultant ZIF-90 membranes

exhibited excellent separation of H2 from CO2, N2, CH4, and C2H4 [32]. All these

salient features facilitate their promising applications in the field of membrane

separation.

To our best knowledge, there are limited reports regarding the fabrication of ZIF-90

based MMMs for gas-separation [17, 33] but no study of ZIF-90 MMMs for

pervaporation. Moreover, the particle sizes of ZIF-90 reported in literatures are

relative big (>100 nm), which are unfavorable in the fabrication of MMMs [11].

Therefore, this study aims to synthesize ZIF-90 nanoparticles with a relatively small


86

particle size and a narrow particle size distribution via a modified room-temperature

synthesis method. The ZIF-90 nanoparticles were subsequently applied to fabricate

high performance ZIF-90/P84 MMMs for dehydration of IPA/water mixtures via

pervaporation. Moreover, the sulfonated polyethersulfone (SPES) primer method

was adopted during the membrane fabrication process to further enhance the

separation performance of the MMMs. The newly developed ZIF-90/P84 MMMs

show great potentials for pervaporation applications.

4.2. Experimental

ZIF-90 nanoparticles were firstly synthesized and incorporated into P84 polymeric

matrix to fabricate MMMs as illustrated in Chapter 3. The crystallographic structure

and specific surface areas of ZIF-90 nanoparticles were determined by XRD and

BET, respectively. The morphology of ZIF-90 nanoparticles and MMMs were

characterized by FESEM. The distribution of ZIF-90 in MMMs was studied via N

elemental mapping using EDX, while the actual ZIF-90 loading in MMMs was

determined by TGA. The densities of MMMs with different ZIF-90 loading were

measured to study the free volume variation. The mass transport of the membranes

was evaluated by vapor sorption and pervaporation tests.

4.3. Results and discussions

4.3.1. Characterization of the ZIF-90 and the MMMs

4.3.1.1. ZIF-90 characterization


87

Figure 4.1(a) shows the morphology of the synthesized ZIF-90 nanoparticles was

observed by FESEM. A statistical evaluation of 124 particles indicates that the

average particle size of ZIF-90 particles is around 55 nm with a standard deviation

of 14 nm, as shown in Figure 4.1(b). To our best knowledge, this may be the first

report on the synthesis of ZIF-90 with such a small particle size and narrow size

distribution, which are favorable for MMM fabrication [11]. There are many factors

that would influence the particle size and size distribution of ZIF-90, including the

type of solvent, zinc sources, reaction temperature and time, the presence of a

catalyst, molar ratio of Zn to the ligand, etc. In fact, these parameters determine the

size via influencing the rates of nucleation and growth process of particles during

the reaction [34]. Normally, if the nucleation process is dominant (i.e., fast nuclei

formation via homogeneous nucleation), particles with smaller size and narrow size

distribution could be obtained. In this work, the substitution of commonly used zinc

nitrate with zinc acetate is the key factor that leads to the fast nucleation rate and

yields very small ZIF-90 particles. The crystal phase of the synthesized ZIF-90 was

characterized by XRD. As shown in Figure 4.1(c), all diffraction peaks match well

with those in literature [31], suggesting the presence of a highly crystalline ZIF-90

structure (i.e., sodalite (SOD) topology) without other impurities. Moreover, the

hydrothermal stability should be considered for the application of ZIF materials [35].

As a result, the ZIF-90 nanoparticles were kept in hot IPA/water mixture (85/15

wt%, i.e. pervaporation feed) at 60 oC for 24 h to evaluate the hydrothermal stability.

As shown in Fig. 2c, the ZIF-90 particles can retain the crystal structure, showing it


88

possesses a good stability at the pervaporation condition in this work. Besides,

Huang et al. also demonstrated the super hydrothermal stability of ZIF-90

membranes in the presence of steam (3 mol%) at 200 oC and 1 bar [32]. Moreover,

the surface area was determined by the N2 physisorption measurements. The

calculated surface area for ZIF-90 was 1379 m2 g-1 by the Langmuir model and 1106

m2 g-1 by the Brunauer-Emmett-Teller (BET) model, which are in agreement with

the published results [31].

Figure 4.1 (a) FESEM image, (b) particle size distribution, and (c) XRD pattern of

the as-synthesized ZIF-90 nanoparticles before and after the hydrothermal test in

IPA/water mixture (85/15 wt%, i.e. pervaporation feed) at 60 oC for 24 h.

4.3.1.2. TGA characterization of MMMs

Figure 4.2 depicts the TGA results of pristine P84, ZIF-90 nanoparticles and MMMs

under air atmosphere. All the TGA curves exhibit no weight loss before 200 °C,

which indicate that all the guest molecules were evacuated from the pores of ZIF-90

after post treatments (i.e., solvent exchange and drying) [36]. In addition, the TGA

analysis shows that the ZIF-90 structure remains stable up to 300 oC, whereupon it

starts to gradually decompose. In the case of MMMs, there are two stages of weight

loss that can be observed upon 300 oC. The first weight loss at about 300 °C is due


89

to the onset of ZIF-90 decomposition, while the second stage of weight loss at about

400 oC is the decomposition of P84. At 800 oC, the residual weight (i.e. ash) can be

corresponded to zinc oxide (ZnO). As a result, the actual loading of ZIF-90 in

MMMs can be calculated based on the stoichiometry amount of remaining ZnO. The

actual loadings of ZIF-90 in 10/90 (w/w) ZIF-90/P84, 20/80 (w/w) ZIF-90/P84,

30/70 (w/w) ZIF-90/P84 MMMs were determined to be 10.1, 22.3 and 31.5 wt%,

respectively. These results are quite similar to the original ZIF-90 loadings during

the membrane fabrication, suggesting that most ZIF-90 nanoparticles have been

incorporated into the membranes. Moreover, The percentage of zinc (Zn) element in

ZIF-90 nanoparticles is around 25.7 wt%, which is consistent with the value

calculated from the structure of (Zn(C4H3N2O)2).

Figure 4.2 TGA curves of pristine P84, ZIF-90 and ZIF-90/P84 MMMs under air

atmosphere.

4.3.1.3. FESEM and EDX characterizations of MMMs


90

FESEM was used to observe the membrane cross-section morphology as well as to

probe the particle-polymer interface. The representative FESEM images with high

and low magnifications are displayed in Figure 4.3. The cross-section morphology

of the dense membrane shows a homogeneous dispersion of ZIF-90 nanoparticles in

the polymer matrix and good adhesion between the ZIF-90 and polymer matrix. No

apparent particle aggregates and voids were observed even in the case of MMMs

with the highest ZIF-90 loading (i.e., 30/70 (w/w) ZIF-90/P84). These observations

indicate that the polymer matrix and ZIF-90 are compatible. Since the Zn element

only exists in ZIF-90 nanoparticles, the uniform dispersion of ZIF-90 nanoparticles

in P84 matrix was further verified by EDX mappings of Zn element in Figure 4.4.

Moreover, EDX element mappings of Zn and S in the 30/70(w/w) ZIF-90/P84

MMM with SPES (ion-exchange capacity (IEC) = 0.054 mequiv g-1) as the primer

are also given in Figure 4.4, indicating both the ZIF-90 and primer were evenly

distributed in the membrane.

Figure 4.3 FESEM images of cross-section morphologies of (a) pristine P84, (b)

10/90 (w/w) ZIF-90/P84, (c) 20/80 (w/w) ZIF-90/P84, and (d) 30/70 (w/w) ZIF-

90/P84.


91

Figure 4.4 (a) FESEM image and (b) the corresponding EDX element mapping of

Zn from the cross-section of the 30/70 (w/w) ZIF-90/P84 membrane, (c) FESEM

image and the corresponding EDX element mapping of Zn (d) and S (e) from the

MMM (primer).

4.3.1.4. XRD patterns of MMMs

XRD characterizations were used to determine the crystalline structure of ZIF-90 in

MMMs. Figure 4.5 presents the XRD patterns of the pristine P84 membrane and

MMMs. Due to its amorphous structure, the XRD pattern of the pristine P84

membrane shows a broad peak from the diffraction angle (2θ) of 5o to 35o. When the

ZIF-90 loading increases, the XRD pattern representing P84 diminishes and shifts

towards that of ZIF-90 structure with enhanced characteristic peaks. These results

suggest that ZIF-90 nanoparticles have been successfully incorporated into the P84

matrix and the ZIF-90 structure remains stable after membrane fabrication and post-

treatments.


92

Figure 4.5 XRD patterns of ZIF-90/P84 MMMs with different particle loadings.

MMM (primer) represents the 30/70(w/w) ZIF-90/P84 MMM with SPES (IEC =

0.054 mequiv g-1) as the primer.

4.3.1.5. Density measurements of MMMs

In order to investigate the alteration of polymer chain packing and the free volume

in MMMs after the incorporation of ZIF-90 nanoparticles, the densities of the

membranes were experimentally determined and compared with their theoretical

values. For the pristine P84 membrane, the fractional free volume (FFV) was

estimated with the aid of Eq. (4.1) [37]:

/

3.1/ W

M

VMFFV

(4.1)

Where, M is the molar weight of P84 monomer (423.6 g mol-1), is the density of

the membrane, and WV is the van der Waals volume which was calculated to be

194.5 cm3 mol-1 by the group contribution method. In the case of MMMs, the

corresponding FFV was calculated as follows [37]:


93

V

M

VV

FFV

ZIFZIFZIF

W /13.1

(4.2)

Wherein, V is the specific volume of the composite membrane which can be

obtained from the density measurement, ZIF is the volume fraction of the ZIF-90,

which can be calculated based on Eq. (4.3).

ZIFZIFP84ZIF

ZIFZIFZIF

//)1(

/

(4.3)

Wherein, ZIF is the weight fraction of ZIF-90 nanoparticles in MMMs, P84 and

ZIF refer to the densities of P84 (1.317 g cm-3) and ZIF-90 (1.511 g cm-3),

respectively. Moreover, the void volume fraction ( V ) of membranes was calculated

using Eq. (4.4).

e

etV

(4.4)

Wherein, e is the experimental density determined by the Archimedean principle,

t is the theoretical density calculated from Eq. (4.5).

ZIFZIFP84ZIF

t//)1(

1

(4.5)

As shown in Table 4.1, an enhancement in FFV is observed with an increasing in

ZIF-90 loading. Specifically, the FFV of 30/70 (w/w) ZIF-90/P84 shows a 10.7%

increment compared with the pristine P84. Furthermore, the experimental density

values of MMMs are lower than the theoretical values, revealing the existence of


94

voids volume in the composite membranes. As depicted in Table 4.1, the void

volume fraction ( V ) also exhibits a similar trend with FFV such that V gradually

increases with the increase of ZIF-90 loading. Moreover, the MMM fabricated using

the SPES primer method shows the same FFV but a slightly lower V as compared

with MMM without the primer.

Table 4.1 Volume fraction, density, fractional free volume, and void volume of the

membranes.

Membrane

ID

Weight

fraction

(%)

Volume

fraction

(%)

Experimental

density

(g cm-3)

Theoretical

density

(g cm-3)

Fractional

free

volume

(FFV)

Void

volume

( V )

Pristine P84 0 0 1.317 1.317 0.214 0.000

10/90 (w/w)

ZIF-90/P84 10.1 8.9 1.293 1.334 0.221 0.032

20/80 (w/w)

ZIF-90/P84 22.3 20.0 1.268 1.355 0.226 0.068

30/70 (w/w)

ZIF-90/P84 31.5 28.6 1.240 1.372 0.237 0.107

MMM

(primer)a 29.7 26.9 1.241 1.372 0.237 0.103

a MMM (primer) is the 30/70(w/w) ZIF-90/P84 MMM with SPES (IEC = 0.054 mequiv g-1) as the

primer.

4.3.1.6. Vapor sorption studies

To characterize and understand the interactions of water/IPA within the membranes,

vapor sorption measurements were conducted on MMMs [38]. In general, the

sorption curves could be sufficiently described by the Fickian sorption curves.

Therefore, it is possible to calculate the vapor diffusivity into the membrane based

on the initial vapor uptakes (M

M t<0.5), according to the following diffusion

equation [39]:


95

π

4 tD

lM

M t

(4.6)

Where tM is the solvent vapor uptake by the sample during a particular period,

M is the equilibrium sorption weight, l is the membrane thickness, D is the

diffusivity and t is the time. Fig. 4.6 displays the vapor sorption curves and Table

4.2 summarizes the calculated sorption and diffusion selectivity. The water sorption

increases in the order of 30/70 (w/w) ZIF-90/P84 < pristine P84 < MMM (primer)

whereas the IPA sorption follows the order of pristine P84 < 30/70 (w/w) ZIF-

90/P84 < MMM (primer). The addition of ZIF-90 nanoparticles into the P84

membrane does not significantly improve water sorption capability. This

observation can be attributed to the hydrophilic/hydrophobic nature of ZIF materials.

A previous simulation study indicated that ZIF-90 does not have favorable

functional groups for water adsorption, thus it exhibits a slightly hydrophobic

character [40]. Moreover, the affinity of pure ZIF materials to alcohol and water

could also be estimated from vapor sorption [41]. Herein, the single-component

sorption behaviors of the water and IPA in pure ZIF-90 were also tested. The results

show the saturated water and IPA sorption in ZIF-90 at 45 oC were 14.4 and 230.8

mg/gparticles, respectively. The hydrophobic nature of ZIF-90 is consistent with the

vapour sorption results of (30/70 w/w) ZIF-90/P84 MMM. However, MMMs

prepared with the SPES primer show a significantly higher water sorption, which

may be associated with the presence of hydrophilic sulfonic acid groups. Therefore,

the MMM with the SPES primer has the highest water/IPA sorption selectivity,


96

while the pristine P84 has the highest water/IPA diffusivity selectivity. Moreover,

the IPA diffusivity is three orders of magnitude slower than water diffusivity, this

suggests the penetrant size exerts a strong influence on diffusivity, because the

kinetic diameter of water is much smaller than that of IPA (kinetic diameter: 2.65 vs.

4.5 Å) [42].

Figure 4.6 Vapor sorption curves of pristine P84 and composite membranes for (a,b)

water and (c,d) IPA saturated vapor at 45 oC.

Table 4.2 Vapor sorption results of P84 and composite membranes with saturated

vapor at 45 oC.

Membrane

ID

Water

sorption

(mgwater

/gmembrane)

IPA

sorption

(mgIPA

/gmembrane)

Sorption

selectivity

(Water/IPA)

H2O

diffusivity

(×10-10

cm2 s-1)

IPA

diffusivity

(×10-13

cm2 s-1)

Diffusion

selectivity

(Water/IPA)

Pristine

P84

20.1 18.7 1.07 7.89 4.91 1606

30/70

(w/w) ZIF-

90/P84

16.7 20.2 0.83 4.96 6.36 780

MMM

(primer)a

63.5 26.4 2.41 2.14 8.36 256

a MMM (primer) is the 30/70(w/w) ZIF-90/P84 MMM with SPES (IEC = 0.054 mequiv g-1) as the

primer.


97

4.3.2. Effect of ZIF-90 loading on pervaporation performance

Figure 4.7 shows the effect of ZIF-90 loading on pervaporation performance of the

newly synthesized MMMs for dehydration of IPA from an 85 wt% IPA aqueous

solution at 60 oC. Both total flux and water permeability show significant

enhancements with an increase in ZIF-90 content in MMMs. The highest flux of 114

g m-2 h-1 and the highest water permeability of 0.166 mg m-1 h-1 kPa-1 are achieved

for the MMM with a 30 wt% ZIF-90 loading. At the same time, the water

concentration in permeate is maintained at >99.9% for MMMs with ZIF-90 loadings

less than 20 wt%. However, a slight reduction of water concentration in permeate

(98.4 wt%) can be observed for MMMs with a ZIF-90 loading of 30 wt%. These

results indicate that the separation factor of MMMs reduces as the ZIF-90 loading

increases to 30 wt%, probably due to the presence of void volume in MMMs as

shown in Table 4.1. In terms of IPA permeability, all the membranes show a

negligible value, suggesting an excellent separation performance.

Figure 4.7 (a) Effects of ZIF-90 loading on the total flux and water concentration in

permeate and (b) IPA permeability, water permeability and mole-based selectivity as

a function of ZIF-90 loading for IPA dehydration from an 85 wt% aqueous IPA

solution at 60 oC.


98

The correlation between permeability and FFV of a polymer can be expressed as

following [43]:

FFVP

B- expA (4.7)

Wherein, both A and B are constants. As illustrated in Figure 4.8, the permeability

of MMMs is inversely correlated to FFV with a good linear relationship (R2 = 0.978).

Clearly, a higher FFV resulted from a higher ZIF-90 loading is the major cause for

the permeability enhancement of the MMMs.

The Maxwell model (Eq. 4.8) and Bruggeman model (Eq. 4.9) have been widely

used to predict the permeation behavior of MMMs [10, 11, 18, 44, 45].

ZIFPZIFPZIF

ZIFPZIFPZIFPMMM

2

22

PPPP

PPPPPP

(4.8)

ZIF

PZIF

PZIFPMMM

3/1

P

MMM 1/1

//

PP

PPPP

P

P (4.9)

In these expressions, MMMP is the total permeability of MMMs,

PP is the

experimental permeability of the polymer phase (i.e. P84), ZIFP is the permeability

of ZIF-90 crystals. ZIF is the volume fraction of ZIF-90 nanoparticles dispersed

within the polymer phase. Since there are no reported water and IPA permeability

for ZIF-90 membranes, and it is difficult to predict their values via the product of

sorption and diffusion due to the lack of experimental sorption and especially

diffusion data in literatures, the experimental permeability of MMMs (i.e., PMMM)


99

and pristine P84 membrane (i.e., PP ) were employed to estimate the water

permeability of ZIF-90 membranes using the aforementioned models [46]. As

indicated in Figure 4.9, the water permeability of ZIF-90 membranes can be

predicted as 1.851 mg m-1 h-1 kPa-1 at 60 oC by fitting the experimental data through

a nonlinear regression analysis with an error minimization technique. Moreover,

both models can be applied to estimate water permeation properties across MMMs

with a higher particles loading. For example, the water permeability of MMMs with

a 60% volume fraction of ZIF-90 is predicted as 0.504 mg m-1 h-1 kPa-1 at 60 oC

based on the Bruggeman model and 0.325 mg m-1 h-1 kPa-1 based on the Maxwell

model. Generally, the Bruggeman model provides more accurate predictions than

Maxwell model when the fillers loading is higher than 20 wt% [45, 47]. It is

important to note that the procedure of back-calculation of ZIF permeability from

MMM permeability data via any model might lead to improper result in some cases,

as recently reported by Singh et al. [47].

Figure 4.8 Correlations between total permeability and FFV.


100

Figure 4.9 Comparison of water permeability of ZIF-90/P84 MMMs among

experimental data, Maxwell and Bruggeman models’ predictions at 60 oC.

4.3.3. Effect of the SPES primer on pervaporation performance

It was reported that the SPES primer can play dual roles in the fabrication of MMMs:

(1) it provides a good dispersion of particles by preventing the agglomeration via

electrostatic repulsion, (2) it improves the affinity between the polymer matrix and

filler phases and enhances their interfacial interactions [48]. Therefore, two batches

of SPES with different IEC (0.054 and 0.107 mequiv g-1) are adopted as primers in

the preparation MMMs with a 30wt% particle loading. It is important to note that

the fabrication of MMMs using SPES as primer is a physical blending process, and

the XRD characterization results (show in Figure 4.5) indicate the ZIF-90 crystal

structure was unperturbed after adding SPES primer. Therefore, herein, the SPES

primer could adhere around the ZIF-90 surface and help to achieve a good


101

dispersion via the electrostatic repulsion force. Moreover, using SPES primer would

also increase the hydrophilicity of the MMMs (shown in Table 4.2).

As shown in Figure 4.10, the total flux and total permeability are not considerably

affected by the presence of the primer. However, both of the separation factor and

mole-based selectivity show a dramatic enhancement through the incorporation of

the primer. When the SPES with an IEC value of 0.054 mequiv g-1 is used as the

primer in the MMMs fabrication, the separation factor and mole-based selectivity

improve from 385 to 5668, and 449 to 6621, respectively, as compared with the

MMMs without the primer. Also, the total flux is maintained between 114 and 109 g

m-2 h-1. This observation is consistent with the results from water/IPA vapor sorption

shown in Table 4.2, which shows that the water uptake is significantly enhanced in

the case of MMMs using SPES as the primer. However, an inverse effect is

observed when the SPES with a higher IEC value of 0.107 mequiv g-1 is applied as

the primer, the resultant MMMs exhibit a lower separation factor and mole-based

selectivity. This phenomenon can be attributed to the fact that ZIF materials might

partially decompose when suspending in an acidic solution [49]. However, under

this condition, the defects caused by the partial decomposition could only obviously

affect the separation factor but not the flux. The reason is due to the size of the

defects which might be still within a molecular size. Therefore, the SPES with an

appropriate IEC value of 0.054 mequiv g-1 is used as the primer throughout this

study.


102

Figure 4.10 The effect of SPES primer on (a) the total flux and separation factor,

and (b) total permeability and mole-based selectivity of MMMs from dehydration of

an 85 wt% IPA aqueous mixture in the temperature of 60 oC.

4.3.4. Effect of feed temperature on pervaporation performance

The pervaporation performance of the pristine P84 membrane and MMMs at the

operating temperatures of 60, 70, 80 and 90 oC are shown in Figure 4.11. The flux is

observed to increase with an increase in feed temperature. By decoupling the

permeation flux into permeability, the water permeability decreases with an increase

in operating temperature, which is primarily caused by the sharp increment in feed

side fugacity for the IPA/water system. Interestingly, the separation factor and mole-

based selectivity for both of the pristine P84 membrane and MMMs are nearly

constant throughout the entire test range. This indicates that the diffusivity

selectivity and solubility selectivity for the water and IPA pair could maintain a

balance with the variations of operating temperatures. In the case of MMMs using

SPES as the primer, the flux monotonically increases along with temperature and

reaches to 267.6 g m-2 h-1 at 90 oC with almost no reduction in mole-based

selectivity (6488) for IPA dehydration.


103

Figure 4.11 (a) Total flux, (b) separation factor, (c) total permeability, and (d) the

corresponding mole-based selectivity of pristine P84 membrane and 30/70 (w/w)

ZIF-90/P84 membranes without and with the SPES primer (IEC = 0.054 mequiv g-1)

using an 85 wt% IPA aqueous mixture as the feed at 60-90 C.

In addition, the temperature dependency of pervaporation flux can be described

through the Van’t Hoff-Arrhenius equation:

T

EJJ

R

- exp J

0 (4.10)

Wherein, J is the flux, 0J is the pre-exponential factor, R is the universal gas

constant, T is the operating temperature, and JE is the apparent activation energy of

flux. Figure 4.12 presents the plots of ln(flux) versus operation temperature for

pristine P84, 30/70 (w/w) ZIF-90/P84 with and without the SPES primer. The

calculated activation energies of total flux, water flux and IPA flux across the

MMMs (30 wt%) and P84 membranes are listed in Table 4.3. The apparent


104

activation energy of water permeation through the MMMs primed with SPES is 29.5

kJ mol−1, showing a 29.9% reduction of energy barrier as compared with the pristine

P84 membrane. Since previous researchers [50] reported that the apparent activation

energies of water permeation through the 13X/P84(20 wt%) and 5A/P84(20 wt%)

membranes, were 30.8 and 34.6 kJ mol−1, respectively. Therefore, the 30% ZIF-90

loading outperforms 20 wt% loadings of 13X and 5A zeolites in P84 based MMMs

in terms of lowering the activation energy of flux for the dehydration of IPA-water

mixtures.

Figure 4.12 The Arrhenius plots of (a) total flux, (b) water flux, and (c) IPA flux for

pristine P84 membrane and 30/70 (w/w) ZIF-90/P84 with and without the SPES

primer of IEC 0.054 mequiv g-1.


105

Table 4.3 Activation energies of pristine P84 membrane and MMMs for IPA

dewatering.

Membranes EJ,total (kJ mol-1)

(total flux)

EJ,water (kJ mol-1)

(water flux)

EJ,IPA (kJ mol-1)

(IPA flux)

Pristine P84 42.1 41.8 41.8

30/70 (w/w) ZIF-90/P84 28.8 28.6 27.7

MMM (primer)a 29.5 29.5 29.5 a MMM (primer) is the 30/70(w/w) ZIF-90/P84 MMM with SPES (IEC = 0.054 mequiv g-1) as the

primer.

4.3.5. Comparison of present pervaporation performance with literature data

Table 4.4 shows a comparison of pervaporation performance of the ZIF-90/P84

membranes with various MMMs reported in literatures. The total flux of ZIF-90/P84

membranes at 60 oC is comparable with most literature data. In terms of selectivity,

the incorporation of ZIF-90 benefits for higher water selectivity. Particularly when

SPES is used as the primer in fabricating MMMs, both separation factor and mole-

based selectivity are higher than 5600. This superior performance not only

demonstrates the potential of incorporating ZIF-90 nanoparticles into the P84

polymeric matrix for IPA dehydration but also open up new opportunities for the

fabrication and application of ZIF-90 based membranes.


106

Table 4.4 Comparison of pervaporation performance of IPA dehydration for various

MMMs.

Polymer Filler

Loadin

g

(wt%)

Temp.

(oC)

Feed

IPA

conc.

(wt%)

MMM performance

Ref. Total

flux Jtotal

(g m-2 h-

1)

Separa

tion

factor

(H2O/

IPA)

Water

permeabi

lity totalPPtotal

(mg m-1

h-1 KPa-1)

Mole-

based

selecti

vity

(H2O

/IPA)

Sodium

alginate silicalite-1 10 30 90 27 >5000 0.537 >5000 [51]

Matrimid cyclodextri

n 5 22.4 86 50 >5000 0.553 >5000 [52]

Matrimid zeolite 4A 15 30 90 21 >5000 0.418 >5000 [53]

Matrimid MgO 15 100 82 120 1200 0.067 1513 [54]

Polyimide

(BAPP–

BODA)

zeolite

13X 33.3 25 90 150 272 4.215 249 [55]

PBI ZIF-8 33.7 60 85 103 1686 0.358 1969 [26]

PVA silicalite-1 10 30 90 69 2241 1.367 2062 [51]

PVA aluminosili

cate 6 40 87.7 109.8 73 1.041 76 [56]

P84 zeolite

13X 40 60 85 95 2800 0.132 3269 [50]

P84 zeolite 5A 20 60 85 40 4200 0.056 4904 [50]

P84 ZIF-90 30 60 85 114 385 0.166 449 this work

P84 ZIF-90-

SPES 30 60 85 109

5668 0.163 6621 this

work

4.4. Conclusions

We have molecularly designed MMMs containing ZIF-90 nanoparticles and P84

polymer with superior performance for pervaporation dehydration of IPA/water

mixtures. The best MMM shows the separation performance with a flux of 109 g m-2

h-1 and a separation factor of 5668 at 60 oC. The following conclusions can be drawn

from this work:


107

(1) A significantly small size (~55 nm) and narrow size distribution of ZIF-90

nanoparticles with a yield of 90% has been synthesized through a modified synthesis

scheme. The resultant MMMs show uniform particle dispersion without any visible

agglomeration.

(2) The incorporation of ZIF-90 into P84 can effectively enhance flux while

maintaining separation factor similar to the pristine P84 membrane if the ZIF-90

loading is ≤20 wt%. The separation factor reduces when the ZIF-90 loading

achieves 30 wt%.

(3) Using SPES with an IEC value of 0.054 mequiv g-1 to prime ZIF-90

nanoparticles could effectively recover the separation factor of the 30/70 (w/w) ZIF-

90/P84 MMM from 385 to 5668 due to the enhanced affinity between ZIF-90

particles and P84 as well as the preferential sorption of water over IPA.

(4) Both Maxwell and Bruggeman models were used to estimate water permeation

properties of MMMs with higher ZIF-90 loadings. The water permeability of the

pristine ZIF-90 membrane was estimated to be 1.851 mg m-1 h-1 kPa-1 at 60 oC.

(5) The apparent activation energy of water permeation through the SPES primed

30/70 (w/w) ZIF-90/P84 MMM is 29.5 kJ mol−1, showing a 29.9% reduction of

energy barrier compared with the pristine P84 membrane.


108

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Chapter 5 Surface modification by aldehydes on polymeric membranes

117

CHAPTER 5 UNIVERSAL SURFACE MODIFICATION

BY ALDEHYDES ON POLYMERIC MEMBRANES FOR

ISOPROPANOL DEHYDRATION VIA

PERVAPORATION

5.1. Introduction

Pervaporation is a promising membrane technique for alcohol dehydration and

biofuel production due to its high separation efficiency, ability to break the

azeotrope and low energy consumption [1-5]. As the heart of pervaporation

processes, the ideal membrane must possess high productivity, good selectivity and

long-term stability. Extensive researches have been conducted toward it via various

molecular engineering of membrane materials and fabrication [2-7]. Polymers are of

particular interest because they own many advantages over inorganic materials such

as reasonable costs, easy fabrication and scale up [5]. However, most commercially

available polymeric materials have some drawbacks such as limited separation

performance and/or poor anti-swelling properties. To overcome these material

weaknesses, two strategies are available; namely, syntheses of novel polymers and

modifications of the existing materials. Compared with novel material syntheses,

modifications of the existing polymers provide benefits of simplicity and low risk

[8].


118

Among numerous modification methods, chemical grafting and crosslinking have

shown effectiveness to augment pervaporation membranes with enhanced separation

factor and durability [2-5, 9, 10]. For instance, aldehyde-induced crosslinking

modifications have successfully improved the physicochemical properties of early-

generation pervaporation membranes made of poly(acrylic acid), poly(vinyl alcohol)

(PVA), sodium alginate and chitosan for dehydration. These hydrophilic materials

originally have weak mechanical properties and swell severely in aqueous solutions

[2-4, 11].

Recently, polyimides and polyamide-imides have emerged as potential

pervaporation materials for alcohol dehydration due to their excellent thermal,

chemical and mechanical stabilities and high water selectivity [5, 8, 12, 13]. Though

these two materials have lower degrees of swelling than the aforementioned

polymers, swelling reduces their performance too [5, 14]. Therefore, various

crosslinking modification methods have been proposed to control their swelling and

stability, especially in harsh environments [2, 5, 15]. However, using aldehydes as

cross-linkers to modify polyamide-imides (as well as polyimides) is rarely reported.

Aldehydes are organic compounds containing aldehyde groups, which are highly

reactive in various chemical reactions. For example, formaldehyde and

glutaraldehyde have worked as cross-linkers to modify polymers containing

hydroxide groups such as PVA, chitosan, alginate, and cellulose [2-4, 11, 16, 17].

Aldehydes also react with both primary amines (RNH2) and secondary amines


119

(R2NH) to form carbinolamines, which then dehydrate to form substituted imines

and enamines, respectively [18-20]. Several aldehydes were reported to crosslink

proteins and chemicals containing amino groups [21-25]. In addition, formaldehyde

was found to crosslink polyurethane with its free -NHCO groups [26]. With such

high reactivity, aldehydes inspire us to explore the use of them to modify

polyamide-imides and other polymeric membranes for dehydration pervaporation of

alcohols. Therefore, the objectives of this manuscript are to investigate (1) if we

could use aldehydes to directly modify polyamide-imide membranes for

pervaporation and (2) if we could develop a universal approach to modify

polyamide-imides, polyimides as well as other polymeric membranes using

aldehydes.

To meet the 1st objective, a Torlon 4000T-MV polyamide-imide was chosen because

(i) Torlon contains abundant secondary amino groups (-NH-), (ii) Torlon owns high

thermal stability, good mechanical properties and solvent resistance which are

critical for industrial applications. Several types of aldehydes, including glyoxylic

acid (GA), benzaldehyde (BA), glucose (GL) and imidazolate-2-carboxyaldehyde

(ICA) shown in Figure 5.1 were chosen to modify the surface of Torlon hollow fiber

(referred to as HF thereafter) membranes.

To meet the 2nd objective, a hyper-branched polyethyleneimine (HPEI) gutter layer

would be firstly formed on the membrane surface before the aldehyde-induced

crosslinking reaction. The HPEI pre-treatment would not only introduce abundant


120

primary amino groups on the membrane surface for the subsequent aldehyde

modification but also improve the overall hydrophilicity as well as effectively seal

the membrane defects [27-30]. A covalent assembly of HPEI-glutaraldehyde

multilayer on hydrolyzed polyacrylonitrile HF membranes had been fabricated by

the layer-by-layer method for alcohol dehydration [31], but this method needed

vacuum assistance during assembly and involved multiple steps including washing

and drying after each assembly, which might incur additional costs and prolong

production duration. Since the HPEI mediated aldehyde modification method

(including HPEI pre-treatment and then aldehyde modification) can be easily

extended to other polymeric membranes such as Ultem, PES and others, this study

may provide useful insights of using aldehydes to design next generation

membranes for pervaporation applications.

Figure 5.1 The molecular structures of aldehydes used to modify polymeric hollow

fibers.

5.2. Experimental

Three home-made polymeric HFs were used as substrates in this work. The details

of spinning parameters for fabricating all the polymeric HF membranes used were


121

listed in Table 5.1. The detailed description of spinning process can be found in

chapter 3.

Table 5.1 Spinning parameters for the polymeric HF substrates used in this work.a

Spinning

conditions Torlon HF substrate Ultem HF substrate PES HF substrate

Dope

formulation Torlon/PEG/ NMP

(17/15/68 wt%) Ultem/ethanol/NMP

(23/5/72 wt%) PES/PEG/water/NMP

(20/25/5/50 wt%)

Bore fluid NMP/water

(95/5 wt%) NMP/butanol

(95/5 wt%) NMP/water

(90/10 wt%) Dope flow rate

(ml/min) 4.0 4.0 6.0 Bore fluid flow

rate (ml/min) 3.0 3.0 4.0

Air gap (cm) 5.0 5.0 3.0 Take up speed

(m/min) 15 15 22 a All HFs were spun at ambient temperature around 23 oC. The dimension of single bore spinneret

used is 0.44/0.6/1.14/1.3/1.6 mm.

Moreover, the morphologies and surface topology of HF substrates and surface-

modified HFs were observed FESEM and AFM respectively. The surface

hydrophilicity and the variations in free volume of HFs were determined by water

contact angle measurement and PAS DBES. Moreover, the chemical changes of HFs

after surface modification of HFs were studied by FTIR. The separation

performance of HFs for IPA dehydration was evaluated via pervaporation process

described in Chapter 3.

5.3. Results and Discussion

5.3.1. Torlon HF membranes modified by GA and HPEI mediated GA methods.


122

The pervaporation performance of the pristine Torlon HF substrate was studied for

the dehydration of an aqueous IPA solution (i.e., 85/15 wt% IPA/water) at 50 oC. As

shown in Table 5.2, its separation factor is only 50 because the unmodified HF

substrate is not defect-free, as observed by FESEM on its outer surface in Figure

5.2(a). After individual GA and HPEI modifications, the separation factors are

greatly enhanced but the fluxes of the resultant HFs drop. This is normal because

both modifications reduce the surface defects on Torlon HF substrates, which is

clearly observed from the outer surface and cross-section morphologies in Figure

5.2(a), (b) and (c). Interestingly, the HPEI modified HF has a higher flux but a lower

separation factor than the GA modified one. This may be attributed to the fact that

the former has higher hydrophilicity than the latter (i.e., contact angle: 67.4±3.8o vs.

84.8±3.2o) because HPEI possesses abundant hydrophilic amino groups.

Encouragingly, the separation factor of the HPEI pretreated and then GA modified

HF (referred to as the HPEI mediated GA modified HF thereafter) can be further

raised to 791 with a minor flux drop as compared with HFs modified by GA or

HPEI alone. As a result, the permeate contains almost pure water of ~99.3 wt%. In

addition, the contact angle of the resultant HF is 72.1±4.4o (Figure 5.2(d)) which is

between those HFs modified by GA and HPEI, separately. Clearly, an

interpenetration layer between HPEI and GA molecules has been formed on the

Torlon outer surface with synergic effects for water transport across the membrane.

Not only does it have a less defective selective layer to enhance the separation factor


123

but also possess greater hydrophilicity to augment the solubility selectivity of

water/IPA during the penetrant transports.

Figure 5.2 Water contact angles and FESEM images of the outer surfaces and cross-

sections of (a) Torlon HF substrate, and (b-d) HF membranes modified by GA,

HPEI and HPEI mediated GA, respectively.

Table 5.2 The separation performance of Torlon HFs before and after modification

for aqueous IPA (85/15 wt% IPA/water) dehydration at 50 oC.

Membrane Water in

permeate (wt %)

Flux (gm

-2h

-1)

Separation factor

(water/IPA)

Torlon HF substrate 89.8 3947 50 GA modified Torlon HF 97.5 1744 225 HPEI coated Torlon HF 94.9 2085 108

HPEI mediated GA modified Torlon HF 99.3 1521 791


124

The differences in dense selective layer among these HFs were further verified by

PAS DBES. Figure 5.3 shows R parameter curves of HFs as a function of positron

incident energy, which determines the penetration depth of positrons into the

membrane surface [32, 33]. For all the HF membranes, the R parameter near the

surface is observed to decrease rapidly with increasing positron incident energy,

which is typical due to the back diffusion and scattering of positronium. With an

increase in positron incident energy to around 1.2-3.0 keV, the R parameter quickly

reaches the minimum value, which can be interpreted as the dense selective layer of

the HF membranes [33]. Thereafter, the R parameter starts to increase as the

positron incident energy increases, indicating the evolution of large pores in the

membrane structure and the transition from the dense layer structure to the porous

substrate structure. From the comparison in Figure 5.3, it can be seen the HF

modified by HPEI mediated GA has the smallest value of the minimum R parameter,

suggesting it has the densest selective layer. Moreover, its increasing trend of R

parameter starts the latest, implying it owns the thickest selective layer. Furthermore,

to quantitatively analyse the results, the R parameters are fitted by the VEPFIT

program with a three-layer model. The fitted R values are also plotted in Figure 5.3,

which achieve a good match with the experimental data. Table 5.3 summarizes the

fitted R parameter and thickness of the selective layer (R1 and L1). As shown, the

Torlon HF substrate has the largest R1 and smallest L1, indicating it possesses a loose

and thin selective layer, and thus leads to its high flux and low separation factor. On

the contrary, the other three modified HFs show significant thickness increments,


125

which cause their flux declines. Moreover, the lowest R1 value of the HPEI-

mediated GA modified HF indicates that it has the densest selective layer and the

highest separation factor.

Figure 5.3 R parameter as a function of position incident energy of Torlon HF

substrates, and HFs modified by HPEI, GA, and HPEI mediated GA, respectively.

Table 5.3 R parameters and selective layer thicknesses of Torlon HF substrate and

HFs modified by HPEI, GA, and HPEI mediated GA, respectively a.

Sample R1 L

1 (nm)

Torlon HF Substrate 0.4232 ± 0.0003 99 ± 8

HPEI treatment 0.4211 ± 0.0004 307 ± 37

GA modification 0.4207 ± 0.0003 372 ± 50

HPEI mediated GA modification 0.4182 ± 0.0004 358 ± 55 a R1 and L1 are the R parameter and thickness of the first layer (i.e. selective layer) estimated by

fitting the resultant R parameter data against positron incident energy using the VEPFIT computer

program with a three-layer model.


126

Figure 5.4 compares the outer surface morphology of pristine and modified Torlon

HFs measured by AFM. It is observed the outer surface becomes smoother after

either GA or HPEI modification. However, the surface roughness increases from

20.0 to 26.3 nm after the HPEI mediated GA modification because of forming sharp

mountain-like nodules on its surface. Nevertheless, the concaves on the surface of

the Torlon HF disappear and become smoother, which could be observed from the

top view of AFM images. These nodules might be generated by the interpenetration

reaction between the primary amino groups of HPEI and the aldehyde groups of GA.

To confirm this, both unmodified and modified Torlon HF membranes were further

characterized by FTIR and XPS.


127

Figure 5.4 3-D AFM images (side and top views) of the outer surface morphologies

of (a) Torlon HF substrate, and (b-d) HF membranes modified by GA, HPEI and

HPEI mediated GA, respectively.

As shown in Figure 5.5, two kinds of functional groups are present in the FTIR

spectrum of the Torlon HF substrate; namely, imide groups and amide groups. The

two peaks of the imide group appear at around 1770 cm-1 and 1315 cm-1

corresponding to the C=O stretching and C-N stretching, respectively. while the

peaks of the amide group occur at around at around 1650 cm-1 and 1540 cm-1

representing the C=O stretching and N-H bending, respectively [34-36]. The HF

modified by GA shows an enhanced stretching of C-N at around 1315 cm-1. Since

the HPEI coating offers free primary amino groups on the substrate, a broad peak

around 3400-3500 cm-1 is observed on the HPEI modified HF due to the N-H

stretching band [37]. In contrast, the HPEI mediated GA modification introduces a

new -N=C peak at around 1615-1710 cm-1 [38], but this peak is overlapped with the

peak for C=O stretching. Therefore, XPS was adopted to further characterize the

modified Torlon HFs.


128

Figure 5.5 The FTIR spectra of Torlon HF substrates and Torlon HF membranes

modified by GA, HPEI and HPEI mediated GA.

High resolution N 1s core level XPS spectra of the Torlon HF substrate and

chemical modified HF membranes were compared in Figure 5.6. The Torlon

substrate has a single peak at binding energy of 399.8 eV, which is ascribed to N

atoms of N-C=O in amide and imide groups of Torlon. After the GA modification

there is still a single peak representing N-C=O. Nevertheless, the successful grafting

of GA on the Torlon substrate is evidenced by the changes of surface element

concentrations. The O, N, C mass ratios change from the initial values of 19.5%,

6.6%, and 73.9% on the unmodified HF substrate to 22.5%, 5.4%, and 72.1%,

respectively, after the GA modification. In the case of the HPEI modified HF, Figure

5.6(c) shows a strong peak at binding energy of 398.2 eV due to the introduction of

C-NH2 from HPEI [39]. The binding energy of N atoms in N-C=O is higher than

that in C-NH2 because the former has a stronger electron-withdrawing environment


129

induced by -C=O. Based on the ratio of N species in different states, the N atoms in

the form of C-NH2 cover 82% of the total surface, indicating a successful HPEI

coating on the HPEI modified HF.

Interestingly, the surface compositions relative to the N species alter a lot after the

HPEI mediated GA modification. As depicted in Figure 5.6(d), a new peak emerges

at a binding energy of 401.0 eV emerges besides the N-C=O peak (399.8 eV) and C-

NH2. This new C=N-C peak is formed via the imine condensation reaction between

primary amino groups of HPEI and aldehyde groups of GA. Quantitative XPS

analyses on the HPEI mediated GA modified HF indicate that the atomic

percentages of surface N in the C-NH2, N-C=O, C=N-C are 38.4%, 35.7% and

25.9%, respectively.

Figure 5.6 N 1s XPS spectra of (a) Torlon HF substrates and (b-d) Torlon HF

membranes modified by GA, HPEI, and HPEI mediated GA, respectively.


130

5.3.2. Chemical modifications by other aldehydes on Torlon HF substrates

To verify the general application of aldehyde modification methods, several other

aldehydes such as BA, GL and ICA were used to modify the Torlon HF substrate.

As shown in Table 5.4, all modified HFs display much higher separation factors

than the unmodified HF substrate, indicating that both aldehyde and HPEI mediated

aldehyde modification methods are really effective. Especially, the HPEI mediated

aldehyde modifications can further bring the separation factor higher than 300 and

maintain the flux of around 2000 gm-2h-1 for the dehydration of aqueous IPA (85

wt%) solutions at 50 oC. Consistent with the surface change after the HPEI mediated

GA modification shown in Figure 5.2, the outer surfaces of Torlon HFs modified by

these three new aldehydes also become much denser than the pristine Torlon HF

substrate, as illustrated in Figure 5.7.

Table 5.4 The separation performance of Torlon HF substrates and HFs modified by

various aldehydes or HPEI mediated aldehydes for aqueous IPA (85/15 wt%

IPA/water) dehydration at 50 oC.

Modification Water in permeate

(wt %) Flux

(gm-2

h-1

) Separation

factor

(water/IPA) Torlon HF substrate 89.8 3947 50

BA modification 97.4 2078 221

HPEI mediated BA modification 98.2 2285 316

GL modification 96.7 1989 171 HPEI mediated GL modification 98.6 2055 413

ICA modification 96.8 2282 172 HPEI mediated ICA modification 98.7 1978 432


131

Figure 5.7 FESEM images of outer surface of (a) Torlon HF substrate, and (b-d)

Torlon HF membranes modified by HPEI mediated BA, HPEI mediated GL and

HPEI mediated ICA, respectively.

High resolution N 1s core level XPS spectra also confirm the hypotheses that the

imine condensation reaction between primary amino groups of HPEI and aldehyde

groups forms the new C=N-C bond in all three cases. As illustrated in Figure 5.8, the

newly formed N atoms have a strong peak at binding energy of around 400.8 eV,

while the N atoms with binding energies of ~399.4eV and ~398.5 eV belong to N-

C=O in the Torlon HF substrate and C-NH2 in HPEI, respectively. Likewise,

quantitative XPS analyses indicate that the atomic percentages of surface N in C=N-

C are 29.7%, 33.7% and 25.2% after the BA, GL and ICA modifications,

respectively.

Figure 5.8 N1s XPS spectra of Torlon HF membranes modified by HPEI mediated

BA, HPEI mediated GL, as well as HPEI mediated ICA.


132

5.3.3. Universal applications of the HPEI mediated aldehyde modification method

to other HF membranes.

Even though the aldehyde modification can only be applicable to polymeric

membranes containing amino or hydroxide groups, the HPEI mediated aldehyde

modification may break this limitation because the HPEI pre-treatment can

inherently graft primary amino groups onto the membrane surface. To prove our

hypotheses, Ultem and PES HF substrates without any amino or hydroxide groups

were chosen and studied. Table 5.5 shows the separation performance of Ultem and

PES HF substrates as well as their modified ones. The Ultem and PES HF substrates

are very porous and have larger surface pores than the Torlon HF (see Figure 5.9).

As a result, they have high fluxes and low separation factors. After the HPEI

modification, both HFs show enhanced separation factors because HPEI not only

seals parts of surface defects but also improves the hydrophilicity [27-30]. The HPEI

modified PES HF still has a relatively low separation factor because the PES

substrate is very porous and has almost no selectivity for IPA/water separation.

Table 5.5 The separation performance of other polymeric HFs modified by HPEI

mediated GA for aqueous IPA (85/15 wt% IPA/water) dehydration at 50 oC.

Polymeric HF

material used Modification Water concentration in

permeate (wt%)

Flux (gm

-2h

-1)

separation

factor

Ultem Nil 66.5 4351 11

HPEI 93.7 2634 86 HPEI mediated GA 97.2 2192 202

PES Nil 16.9 41430 1

HPEI 45.8 11989 5 HPEI mediated GA 86.8 5008 38


133

Figure 5.9 FESEM images of overall cross-section, enlarged cross-section near the

outer surface, outer surface as well as inner surface of (a) Torlon, (b) Ultem, (c) PES

HF substrates.

After the HPEI mediated GA modification, the separation factors of both HF

membranes were greatly enhanced. For the modified Ultem HF, it has a satisfactory

pervaporation performance with a separation factor of 209 and a total flux of 2192

gm-2h-1. For the modified PES HF, it exhibits a greatly improved separation factor of

38 and a very high total flux of 5008 gm-2h-1. This separation factor can be further

improved if better PES substrates with smaller pores are used. The changes of outer-

surface morphologies of Ultem and PES HFs after the HPEI mediated GA

modification were shown in Figure 5.10. The successes of these two examples

further prove that the HPEI mediated aldehyde modification is universally


134

applicable to modify polymeric HF membranes with enhanced pervaporation

performance.

Figure 5.10 FESEM images of outer surface of (a, b) Ultem, PES HF substrates, and

(c,d) Ultem, PES HFs modified by HPEI mediated GA.

5.3.4. The effects of operation conditions on IPA dehydration performance.

To expand the potential of the HPEI mediated aldehyde modification for alcohol

dehydration, the effects of feed temperature and composition as well as long term

stability on IPA dehydration were investigated and the HPEI mediated GA modified

Torlon HF was chosen for the study.

5.3.4.1. Effects of feed temperature on pervaporation performance.

As illustrated in Figure 5.11, permeate flux increases dramatically while separation

factor drops with an increase in feed temperature. However, the water concentration


135

in the permeate remains as high as 98% at 80oC. The flux increase at high feed

temperatures is ascribed to two factors [3, 10, 40-42]: (1) increased driving forces

for IPA and water transports, and (2) enhanced chain motion and expanded chain-

chain distance. Since IPA has a larger molecular size than water, the expanded

chain-chain distance at high temperatures will facilitate IPA transport more

effectively than water transport. As a result, the separation factor drops. Table 5.6

shows the apparent activation energies of water and IPA transports across the

membrane by plotting their fluxes against the reciprocal of temperature using the

Van’t Hoff-Arrhenius equation. The water transport has a much lower activation

energy than that of IPA, indicating that this modified HF membrane is highly water-

selective.

Figure 5.11 The effects of feed temperature on separation performance of HPEI

mediated GA modified Torlon HF for aqueous IPA (85/15 wt% IPA/water)

dehydration.


136

Table 5.6 The calculated activation energies of total flux, water flux, and IPA flux

for the dehydration of IPA/water (85/15 wt%) mixtures.

Flux EJ (kJ/mol)

Total flux 37.4 Water flux 37.0 IPA flux 66.5

5.3.4.2. Effects of feed composition on pervaporation performance.

Figure 5.12 depicted the effects of feed composition on separation performance.

Three trends can be observed: (1) total flux decreases with increasing IPA

concentration, (2) separation factor also increases with IPA concentration but

reaches the highest value at the feed IPA concentration of 85 wt% then drops, and (3)

water content in the permeate is always higher than 95 wt% even at the feed IPA

concentration of 95 wt%.

The rise of feed IPA concentration results in total flux decline is a typical

phenomenon in pervaporative dehydration because of less water-induced membrane

swelling at higher IPA concentrations and the lower driving force for water

permeation [17, 25, 41-43]. The water content in the permeate is constantly high

(99.3 wt%) when the feed IPA concentration varies from 70 to 85 wt% but drops

slightly at higher IPA concentrations. This is due to the fact that a higher IPA feed

concentration results in a greater driving force for isopropanol transport across the

membrane [25]. However, the water content in the permeate always stays higher

than 95 wt% even at the feed IPA concentration of 95 wt%, indicating the modified

HF is chemically stable and has a high selectivity for IPA/water separation.


137

Figure 5.12 The effects of feed composition on the separation performance of HPEI

mediated GA modified Torlon HF for aqueous IPA dehydration at 50 oC.

5.3.4.3. Long-term stability for IPA dehydration.

Figure 5.13 displays the long-term stability of the HPEI mediated GA modified

Torlon HF membrane for more than 200 h at 50 oC. Both the flux and water

concentration in the permeate stay almost constant, evidencing good chemical and

mechanical stability for dehydration applications.


138

Figure 5.13 Long-term pervaporation performance of the HPEI mediated GA

modified Torlon HF for the dehydration of aqueous IPA solutions (IPA/water 85/15

wt%) at 50 oC.

5.3.5 Benchmarking for the HF membranes modified by HPEI mediated

aldehyde.

Table 5.7 compares the pervaporation performance of the HPEI mediated aldehyde

modified HF membrane with other polymeric HF membranes for pervaporative IPA

dehydration available in literatures. The newly molecularly designed HPEI mediated

aldehyde modified Torlon HF possesses both high permeance and high selectivity

among the reported data, indicating their great potential for industrial applications.


139

Table 5.7 Performance benchmarking of the HPEI mediated aldehyde modified HF

membranes with other polymeric HF membranes for pervaporative IPA dehydration

in literatures.

Torlon HF

Membrane

Temperature

(oC)

Feed

(IPA

wt%)

Flux

(gm-

2h

-1)

Separation

factor

(water/IPA)

mole based

water

permeance

(mol m-2h-

1kPa-1)

mole-based

selectivity

(water/IPA)

Ref

Heat treated polyacrylonitrile (PAN)

HF 25 90 186 1116 5.5 1022 [41]

Chitosan/polyacrylonitrile (PAN)

composite HF 25 90 145 2991 4.3 2740 [42]

Cellulose/polysulfone dual-layer HF 25 95 4 94981 0.2 66731 [44]

Torlon single-layer HF 60 85 801 7 1.7 8 [5]

Heat treated Torlon /Ultem dual-layer

HF 60 85 765 1944 3.0 2199 [5]

Torlon Single-layer HF 60 85 850 15 2.3 15 [45]

PXD Crosslinked Torlon /P84 blend

single-layer HF 60 85 1000 185 3.8 222 [45]

P84 co-polyimide HF 60 85 883 10585 3.4 13227 [46]

Heat treated P84/PES dual layer HF 60 85 570 125 2.1 145 [47]

(HPEI/GA)4/PAN composite HF 50 95 426 18981 4.7 13626 [31]

MPD-TMC polyamide TFC/Torlon

dual-layer HF 50 85 1374 53 7.8 62 [35]

HPEI-TMC polyamide TFC/Torlon

dual-layer HF 50 85 1280 624 8.0 729 [35]

HPEI-HGOTMS polyamide TFC/

Ultem HF 50 85 3519 278 21.8 324 [48]

HPEI/MPD-TMC polyamide

TFC/Ultem tri-bore HF 50 85 2647 261 15.8 294 [30]

Nexar Block Copolymer/Ultem 50 85 2060 509 12.9 595 [49]

HPEI mediated GA modified Torlon

HF 50 85 1521 791 9.5 899

This

work

HPEI mediated BA modified Torlon

HF 50 85 2285 316 14.2 353

This

work

HPEI mediated GL modified Torlon

HF 50 85 2055 413 12.8 461

This

work

HPEI mediated ICA modified Torlon

HF 50 85 1978 432 12.3 488

This

work

HPEI mediated GA modified Ultem

HF 50 85 2192 202 13.4 230

This

work


140

5.4. Conclusions

In summary, not only we have demonstrated that aldehydes can directly modify

Torlon polyamide-imide hollow fibers for improved dehydration of IPA solutions,

but more importantly, a novel universal modification method based on the

synergistic combination of HPEI and aldehyde has been discovered and successfully

applied to various polymeric membranes, such as Ultem and PES, with enhanced

pervaporative dehydration. It was found that the HPEI pre-treatment can mitigate

surface defects and enhance membrane hydrophilicity towards water transport. In

addition, the reaction between HPEI and aldehydes at the outer surface of the HF

membranes enhances selectivity and chemical resistance. The HPEI mediated

aldehyde modified hollow fibers possess satisfactory separation performance in

wide ranges of feed temperature and composition ranges, and also exhibit a good

long-term stability, suggesting their potential for industrial IPA dehydration

applications.

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Chapter 6 Thin film composite tri-bore hollow fiber membranes

149

CHAPTER 6 THIN-FILM COMPOSITE TRI-BORE

HOLLOW FIBER MEMBRANES FOR ISOPROANOL

DEHYDRATION BY PERVAPORATION

6.1. Introduction

Alcohols are of great importance in industry because they are widely used as good

solvents, cleaning agents, raw materials or chemical intermediates for organic

synthesis, and potentially clean liquid fuels [1]. Accordingly, the dehydration of

alcohol/water mixtures is a critical issue in the production and recycle of alcohols.

However, the alcohol (ethanol, isopropanol, or butanol) and water can form an

azeotropic mixture, rendering the purification of alcohols by conventional methods

such as distillation inefficient and uneconomic. Considering the requirements for

energy-saving and environmentally friendly processes in the chemical industry

nowadays, the pervaporation process is of greater interest due to its high separation

efficiency, ability to break azeotropes, lower energy consumption, as well as flexible

process control and module fabrication [2-5].

The pervaporation membrane is the heart of pervaporation processes. Based on its

configuration, it can be categorized as flat sheet, hollow fiber (HF) or tubular

membrane. Among them, the polymeric HF membrane has been widely applied in

various separation processes due to its superior advantages such as higher surface

area, higher packing density, excellent flexibility and ease of fabrication as well as

scale-up [5-7]. Polymeric pervaporation HF membranes including single-layer


150

asymmetric HFs and dual-layer composite HFs are usually fabricated by means of

spinning via non-solvent induced phase inversion process [7-13]. Since heat

treatment, silicone coating and chemical crosslinking are often needed to improve

the separation efficiency which incur extra costs and the dual-layer spinning

technique is complicated, the thin-film composite hollow fiber (TFC HF) membrane

fabricated via interfacial polymerization has drawn attention in recent years for

pervaporation due to its excellent separation performance and ease of fabrication

[13-19].

The conventional HF membrane has single-bore geometry. Extensive researches

have been focused on the design and fabrication of single-bore hollow fiber (SbHF)

membranes [20]. Fine polymeric SbHFs are found to suffer from the tendency of

fiber breakage during continuous operations and thus lack long-term stability [21,

22]. To enhance the mechanical properties, mixed matrix SbHF membranes were

fabricated by the incorporation of inorganic fillers such as carbon nanotubes,

graphene based nanomaterials, and layered silicate into the polymer matrix [23-25].

Besides, a new generation of multi-bore hollow fiber (MbHF) membranes is

emerging recently [11, 26-34]. The new membranes also possess improved

mechanical properties as compared with traditional SbHFs owing to the presence of

spokes as the mechanical reinforcement, and thus reduce fiber breakage and increase

operation reliability. Comparing with the mixed matrix SbHFs, the polymeric

MbHFs are free from issues such as filler agglomeration and poor compatibility

between inorganic fillers and polymer matrix.


151

Two MbHF membranes have been commercialized for ultrafiltration (UF)

applications. They are (1) Multibore® polyethersulfone (PES) HF membranes

consisting of seven bores developed by inge GmbH [35] and (2) tri-bore PES or

polyvinylidene fluoride (PVDF) HF membranes launched by Hyflux [36]. Both of

them claimed greater durability and mechanical strength while maintaining the same

permeate production rate as compared with the respective SbHF membranes [37, 38].

Moreover, some showed extra benefits such as a ~75% reduction in energy

consumption compared with the traditional UF membrane in the pretreatment of

reverse osmosis plants for seawater desalination [39, 40].

Besides UF, the applications of MbHFs were also extended to other membrane

processes. Chung and his co-workers fabricated a series of MbHF membranes with

various dimensions, configurations and pore sizes for UF, membrane distillation

(MD) and forward osmosis (FO) applications [35, 36, 41-43]. The detailed MbHF

spinneret designs, membrane formation mechanisms and spinning parameters were

fully explored. Their MbHFs for MD not only showed high permeation fluxes and

energy efficiency but also possess superior stability and robustness. In addition, TFC

MbHFs for FO were fabricated via spinning process followed by interfacial

polymerization. These fibers showed comparable separation performance with other

FO membranes. Spruck et al. also fabricated TFC seven-bore HF membranes which

showed good mechanical strengths and acceptable performance in nanofiltration for

desalination and water softening applications [36].


152

In view of the aforementioned advantages of MbHF membranes and TFC

membranes, we aim to design and explore thin-film composite tri-bore hollow fiber

(TFC TbHF) membranes for the application of IPA dehydration. To our best

knowledge, this is a pioneering work to fabricate polymeric outer-selective TFC

TbHF membranes for pervaporation dehydration. Firstly, the effects of interfacial

polymerization conditions on pervaporation performance of TFC HF membranes

were studied by using conventional SbHF substrates. A most suitable procedure for

the subsequent fabrication of TFC TbHF membranes was then identified based on

the aforementioned study. Afterwards, the effects of spinning conditions on

membrane morphology of the TbHF substrates and their TFC TbHF performance for

pervaporation dehydration of IPA were systematically investigated. The outcome of

this study may provide useful insights towards the design of next generation HF

membranes for pervaporation applications.

6.2. Experimental

Both traditional SbHF and TbHF substrates were prepared via the dry-jet wet

spinning process demonstrated in Chapter 3. Note that the spinneret for fabricating

TbHF substrates was a specifically designed tri-bore spinneret with blossom

geometry by our group [35], the bottom view of which is shown in Figure 6.1. The

detailed spinning parameters of all HF substrate used in this study were listed in

Table 6.1. Then, the HF substrates were fabricated into TFC HF membranes via

interfacial polymerization which has also been described in Chapter 3.


153

Figure 6.1 The bottom view of tri-bore spinnerets (dimension unit: mm).

Table 6.1 The spinning parameters for HFs spun in this study a.

Membranes configuration Bore fluid

(wt%)

Dope flow rate

(ml/min)

Bore fluid flow

rate (ml/min)

Air gap

(cm)

0 SbHF NMP/water

(95/5) 5.0 5.0 2.5

1 TbHF NMP/water

(80/20) 5.0 5.0 0.5

2 TbHF NMP/water

(95/5) 5.0 5.0 0.5

3 TbHF NMP/n-butanol

(80/20) 5.0 5.0 0.5


(95/5) 5.0 5.0 0.5

5 TbHF NMP/water

(95/5) 5.0 4.0 0.5

6 TbHF NMP/water

(95/5) 5.0 3.0 0.5


(95/5) 5.0 4.0 0.5


(95/5) 5.0 3.0 0.5

9 TbHF NMP/water

(95/5) 5.0 5.0 2.5

10 TbHF NMP/water

(95/5) 5.0 5.0 5.0

11 TbHF NMP/water

(95/5) 6.0 5.0 2.5

12 TbHF NMP/water

(95/5) 7.0 5.0 2.5

a Spinning temperature: around 23 oC, dope formulation: Ultem/ethanol/NMP (22/5/73 wt%); take up

speed: free fall.


154

The morphologies of TbHF and SbHF membranes were observed by an optical

microscope and FESEM. The dimensions of TbHF membranes could be determined

from the microscopic images, including the outer diameter, inner diameter, wall

thickness, as well as spoke thickness as shown in Figure 6.2. The shaded area

represents the polymeric cross-section area. The membrane surface topology was

examined using AFM. Besides, both the water contact angle and mechanical

properties of HF membranes were measured and compared. In addition, the depth

profile of the selective layer and the free volume of TFC TbHF membranes were

studied with the aid of DBES and PALS (DBES) respectively. The membrane

separation performance was evaluated for IPA dehydration via pervaporation

process shown in Chapter 3.

Figure 6.2 The parameters (right) for describing the configuration of the TbHF

substrate (left).


155

6.3. Results and discussion

6.3.1. Effect of TFC selective layer

Since the polyamide selective layer plays an important role in determining the

overall separation performance, identifying a suitable interfacial polymerization

protocol for pervaporation dehydration of IPA is necessary. To search for the proper

protocol, SbHFs spun from the same dope were used prior to applying to the novel

TbHFs.

6.3.1.1. Interfacial polymerization conditions

Interfacial polymerizations with and without TEA additive (i.e., a reaction

accelerator) and surfactant (i.e., a wetting agent) have been reported for

pervaporation dehydration of alcohols [37, 38]. Accordingly, three different amine

solutions were conducted on SbHF substrates to compare their effects on the

polyamide selective layer for pervaporation dehydration of IPA. As shown in Table

6.2, the TFC-SbHF interfacially polymerized from the 1st amine solution and TMC

has greater dehydration performance in terms of flux and separation factor than

those TFC-SbHFs synthesized from TMC with the 2nd and 3rd amine solutions.

Figure 6.3 compares their surface morphology. The membrane synthesized from the

1st amine solution and TMC has a dense polyamide layer consisting of “nodular

structure”. However, the TFC HFs manufactured from TMC and amine solutions

containing surfactants have “flake-like” surface morphology. This is because

surfactants facilitate the diffusion of MPD to the organic phase, enlarge the contact


156

area and enhance the reaction. The resulting polyamide layer with this morphology

was reported to have a larger free volume and loose structure [39, 40]. As a result, it

has a lower separation factor compared to the TFC HF synthesized from the

condition 1. Therefore, the condition 1 was applied to the subsequent sections for

the interfacial polymerization of TbHF substrates.

Table 6.2 Effects of amine solution composition on IPA dehydration performance of

TFC SbHF a.

Condition Composition of the aqueous

amine solution

Total flux

(g m-2 h-1)

Water

concentration at

the permeate

(wt%)

Separation

factor

(Water/IPA)

Substrate nil 6172 ± 404 37.3 ± 2.6 3.4 ± 0.4

1 MPD (2 wt%) 2682 ± 37 90.4 ± 0.3 51.2 ± 1.4

2 MPD(2 wt%)+SDS(0.1wt%)b 2501 ± 61 84.1 ± 1.0 30 ± 2.1

3 MPD (2 wt%)+CTAB(0.1 wt%)b 2637 ± 38 83.0 ± 1.0 28 ± 2.0

a Substrate spinning conditions: bore fluid composition: NMP/ water (95/5wt%), bore fluid flow rate:

5 ml/min, air gap: 2.5 cm, dope flow rate: 5 ml/min; b TEA was used as the base source with a weight

amount of 0.5 wt%.

Figure 6.3 FESEM images of SbHF outer surfaces made from different amine

solutions, (a) substrate, (b) 1st amine solution, (c) 2nd amine solution, (d) 3rd amine

solution.


157

6.3.1.2. Effect of HPEI or PDMS coating

To further improve the pervaporation performance of TFC HF membranes, two

treatments were explored to alleviate defects in the polyamide selective layer. Figure

6.4 shows the morphologies of four types of fibers; namely, (1) the hollow fiber

substrate, (2) TFC hollow fiber (i.e., interfacial polymerization under condition 1),

(3) HPEI-TFC (i.e., HPEI pre-treatment and then interfacial polymerization), and (4)

TFC-PDMS (i.e., interfacial polymerization and then PDMS coating), while Table

6.3 summarizes their pervaporation performance. The HPEI-TFC hollow fiber has

the most impressive pervaporation performance. Compared to the TFC hollow fiber,

it has about fourfold separation factor without significantly compromising the flux.

This is likely due to the fact that the HPEI coated substrate has a hydrophilic smooth

surface with smaller pores [41, 42]. As a consequence, not only does the modified

substrate facilitate the absorbance of MPD for interfacial polymerization but also

promote the formation of a less defective polyamide layer. The surface contact angle

of the original HF substrate reduced from 88.4±2.2o to 40.4±2.9o after the HPEI

coating, which proves the enhancement of surface hydrophilicity. The AFM images

of these membrane surfaces further confirm our hypotheses. As shown in Figure 6.5,

the surface roughness decreases after the HPEI coating. Moreover, the amine groups

of HPEI can react with TMC, minimize the interstitial space and thus further

increase the separation factor [41, 43]. Similarly, the roughness of the TFC hollow

fiber decreases after the PDMS coating. However, since there is no chemical

reaction between the TFC and PDMS, the defects in the rough polyamide layer may


158

not be fully sealed by the PDMS coating. Thus, the TFC-PDMS hollow fiber does

not show much improvement in separation factor. As a result, the HPEI-TFC

method was chosen to fabricate the TFC TbHF membranes in the following study.

Figure 6.4 FESEM images of cross-section and outer surface of TFC SbHFs: (a)

substrate, (b) only TFC, (c) TFC-PDMS, (d) HPEI-TFC. (The spinning conditions of

the SbHF substrate: bore fluid composition: NMP/water (95/5 wt%), bore fluid flow

rate: 5 ml/min, air gap: 2.5 cm, dope flow rate: 5 ml/min).

Table 6.3 The IPA dehydration performance of TFC SbHF membranes with

different treatments a.

Coating conditions Total flux

(g m-2 h-1)

Water concentration at

the permeate

(wt%)

Separation factor

(Water/IPA)

Nil (substrate) 6172 ± 404 37.3 ± 2.6 3.4 ± 0.4

TFC (condition 1) 2682 ± 37 90.4 ± 0.3 51.2 ± 1.4

TFC-PDMS 2480± 57 90.8± 0.5 55.9 ± 3.6

HPEI-TFC 2356 ± 26 97.1 ± 0.2 192.2 ± 12.1

a Substrate spinning conditions: bore fluid composition: NMP/water (95/5wt%), bore fluid flow rate:

5 ml/min, air gap: 2.5 cm, dope flow rate: 5 ml/min.


159

Figure 6.5 3-D AFM images and mean surface roughness of SbHF outer surfaces

(10×10 m) prepared under different coating methods (a) substrate (b) only TFC (c)

TFC-PDMS (d) only HPEI; (e) HPEI-TFC. (The spinning conditions of the SbHF

substrate: bore fluid composition: NMP/water (95/5 wt%), bore fluid flow rate: 5

ml/min, air gap: 2.5 cm, dope flow rate: 5 ml/min).

6.3.2. The effects of spinning parameters on TbHF substrates and pervaporation

performance of TFC TbHFs

In order to fabricate TbHF substrates with a desirable pore-size distribution and

open-cell substructure to maximize the separation performance of TFC TbHFs, it is

necessary to investigate the key spinning factors for the formation of TbHFs [25].

Therefore, the effects of bore fluid composition, bore fluid flow rate, air-gap

distance as well as dope flow rate on TbHF morphology and its pervaporation

performance after interfacial polymerization were systematically investigated.

6.3.2.1. Bore fluid composition

Figure 6.6 depicts the morphologies of TbHF substrates spun from bore fluids with

different compositions. The bore fluid composition has significant impact toward the

fiber dimension and morphology. The outer diameter, wall thickness, and spoke

thickness of TbHFs spun from a strong bore fluid (namely, NMP/water (80/20 wt%)


160

which contains 20 wt% water (strong non-solvent)) are much larger than those

TbHFs spun from three other weaker bore fluids consisting of more NMP (solvent)

or BuOH (weak non-solvent). As a result, the former is a large and thick TbHF,

while the latter three are fine TbHFs. All the fine TbHFs show partial deformed

spokes. However, they have a denser outer surface and a much more porous inner

surface compared with the thick TbHF.

Figure 6.6 The cross-section, outer surface as well as inner surface morphologies of

TbHF substrates spun from the bore fluid compositions of NMP/water (80/20 wt%),

NMP/water (95/5 wt%), NMP/BuOH (80/20 wt%), NMP/BuOH (95/5 wt%). Other

spinning conditions: bore fluid flow rate: 5 ml/min, air gap: 0.5 cm, dope flow rate:

5 ml/min.


161

The mechanism of forming fine and thick TbHFs can be elucidated from Figure 6.7.

During the dry-jet wet-spinning process, the phase inversion begins at the lumen

side of the nascent fiber once the bore fluid contacts with the polymer dope solution

at the exit of the spinneret. Meanwhile, a partial phase inversion occurs at the outer

surface of the nascent fiber due to the moisture induced phase inversion in the air

gap region. The nascent fiber would have a complete phase inversion at its outer

surface once it is fully immersed into the external coagulation bath of water [26, 44,

45].

Figure 6.7 Schematic representation of the concept of phase inversion processes for

the formation of thick and fine TbHF substrates.


162

Since NMP/water (95/5 wt%), NMP/BuOH (80/20 wt%) and NMP/BuOH (95/5

wt%)) are weak bore fluids, while water is a strong coagulant, these result in a much

slower phase inversion at the lumen side than at the shell side of the fiber [24, 25,

46]. The resultant TbHF substrate has a porous inner surface and a dense outer

surface. The slow phase inversion in the air-gap region also makes the nascent fiber

stretchable by gravity. As a consequence, fine TbHFs are produced. It is worth

noting that the coagulation strength of BuOH is weaker than water [24]; therefore,

the inner surface of TbHFs spun from NMP/BuOH has a more porous structure than

those spun from NMP/water at the same composition.

On the contrary, a faster phase inversion occurs at the lumen side of the fiber when a

stronger bore fluid (i.e. NMP/water (80/20 wt%)) is applied. It not only quickly

solidifies the inner dimension of the nascent fiber but also tightens its inner skin

structure. As a result, the resultant dry-jet wet-spun TbHF has a large diameter and a

dense inner skin. Moreover, since the solidifying rate of the lumen side is faster than

that at the shell side, the net mass transfer direction of the solvent has the tendency

to move towards the shell side of the fiber, yielding a TbHF substrate with a denser

inner surface and a more porous outer surface.

As shown in Table 6.4, the thick TFC TbHF membrane has a much lower flux

compared with the fine ones due to its less porous structure (typically at inner

surface) and thicker walls as mentioned above. Moreover, the lower separation

factor may be ascribed to the relatively large pores at the fiber outer surface, which


163

is unfavorable for forming a uniform and defect-free TFC layer [25]. The difference

in TFC layer between thick and fine TbHFs has been further verified by PAS DBES

and PALS. Firstly, the R parameter curves of TFC TbHF membranes as a function

of positron incident energy were obtained from PAS DBES. By fitting these curves

using the VEPFIT program with a three-layer model, the R1 and thickness L1 of the

selective layer of TFC membranes could be obtained [11]. As presented in Table 6.5,

the TFC selective layer of the fine TFC TbHF is much thinner than that of the thick

TFC TbHF (104 ± 4 vs. 199 ± 12 nm), thus the former has a higher flux than the

latter.

Table 6.4 Effects of bore fluid composition on IPA dehydration performance of TFC

TbHF membranes a.

Bore fluid composition

Total flux

(g m-2 h-1)

Water

concentration at

the permeate

(wt%)

Separation factor

(Water/IPA)

NMP/water (80/20 wt%) 943 ± 11 77.8 ± 0.5 20.1 ± 0.6

NMP/water (95/5 wt%) 2475 ± 80 97.1 ± 0.2 197.0 ± 14.6

NMP/BuOH (80/20 wt%) 2730 ± 47 95.1 ± 0.3 113.0 ± 6.5

NMP/BuOH (95/5 wt%) 3080 ± 112 98.0 ± 0.3 274.0 ± 40.5

a Other spinning conditions: bore fluid flow rate: 5 ml/min , air gap: 0.5 cm, dope flow rate: 5 ml/min.

Since the o-Ps lifetime τ3 and its intensity I3 obtained from PALS analyses

correspond to the free volume size and concentration, respectively, the mean free

volume radius (R) and fractional free volume (FFV) can be calculated according to

established semi-empirical correlation equations from the pick-off annihilation [47-

49]. Table 6.5 shows the calculated results and indicates that the fine TFC TbHF

have a smaller R and FFV than the thick TFC TbHF. The smaller R implies that the


164

fine TFC TbHF has a higher rejection against IPA than the thick TFC TbHF. In

addition, the smaller FFV does not affect the fine TFC TbHF because it has a thinner

selective layer. As a result, the fine TbHF substrates spun from weak bore fluids

were more preferred for fabricating TFC membranes with good overall

pervaporation performance.

Table 6.5 R parameters, TFC layer thicknesses and positron lifetime results of the

fine and thick TFC TbHF membranes a.

Sample R1 L1 (nm) τ3 (ns) I3 (%) R (Å) FFV (%)

Thick

TbHF 0.4194±0.0004 199±12 2.234±0.025

12.85±0.1

2

3.06±0.0

2

2.78±0.0

7

Fine TbHF 0.4190±0.0003 101±4 1.595±0.037 10.53±0.2

7

2.45±0.0

4

1.17±0.0

8 a The TbHF substrates for the thick and fine TFC TbHFs were spun from bore fluids of NMP/H2O

(80/20 wt%) and NMP/H2O (95/5 wt%), respectively. R1 is the R parameter obtained from VEPFIT;

L1 is the thickness of the top selective layer obtained from VEPFIT; τ3 is the o-Ps lifetime obtained

from PALS; I3 is the intensity of τ3 obtained from PALS; R is the mean free volume radius obtained

from PALS; FFV is the fractional free volume obtained from PALS.

6.3.2.2. Bore fluid flow rate

The influences of bore fluid flow rate on TbHF morphology were investigated and

shown in Figures 6.8 and 6.9. An increase in bore fluid flow rate results in an

enlargement in outer diameter and inner diameter, as well as a reduction in wall

thickness. In addition, the large surface pores on the inner skin become small and

scattered at a higher bore fluid flow rate, which may be due to the fact that these

large inner surface pores are formed because of non-solvent intrusion from the outer

surface [50, 51]. This is proven by Figures 6.8 and 6.9 where the enlarged outer

cross-section and inner surface morphologies change as a function of bore fluid flow

rate. The outward-pointed macrovoids (i.e., the tails of macrovoids face to the outer

surface [52]) across the fiber walls become smaller at a higher bore fluid flow rate.


165

As a result, a high bore-fluid flow rate would retard the non-solvent intrusion and

reduce the pore sizes. Moreover, a higher bore fluid flow rate also causes more

severe deformation of the spokes because of slow phase inversion and stress

unbalance [52].


TbHF substrates spun from the bore fluid flow rates of 3, 4 and 5 ml/min when

using NMP/water 95/5 wt% as the bore fluid. Other spinning conditions: air gap: 0.5

cm, dope flow rate: 5 ml/min.


166


TbHF substrates spun from the bore fluid flow rates of 3, 4 and 5 ml/min when

using NMP/BuOH 95/5 wt% as the bore fluid. Other spinning conditions: air gap:

0.5 cm, dope flow rate: 5 ml/min.

The flux and separation factors of fine TFC TbHFs prepared under various bore

fluid flow rates are shown in Table 6.6. A higher bore fluid flow rate leads to an

increase in flux, which is consistent with aforementioned observations because of

thinner wall and less transport resistance. A higher bore fluid flow rate also

enhances the separation factor by radially expanding the nascent fiber, which may

result in an increase in hoop elongation and induce molecular orientation along the

circumference [24]. In addition, the effects of bore fluid flow rate on pervaporation

performance are valid for both TFC TbHFs when using NMP/water (95/5 wt%) and


167

NMP/BuOH (95/5 wt%) as bore fluids. Even though the bore fluid of NMP/BuOH

(95/5 wt%) produces TFC TbHFs with better separation performance than the

NMP/water (95/5 wt%), the former has weaker or more bended spokes than the

latter because water is a much stronger coagulant than BuOH. As a result, the TbHF

substrate spun from the bore fluid of NMP/ water (95/5 wt%) at a flow rate of 5

ml/min is preferred.

Table 6.6 Effects of bore fluid rate on IPA dehydration performance of TFC TbHF

membranes a.

Bore fluid

composition

Bore rate

(ml/min)

Total flux

(g m-2 h-1)

Water

concentration at

the permeate

(wt%)

Separation

factor

(Water/IPA)

NMP/H2O

(95/5 wt%)

3 2075 ± 34 88.9 ± 0.5 45.4 ± 2.2

4 2348 ± 29 95.1 ± 0.2 110.7 ± 3.8

5 2475 ± 80 97.1 ± 0.2 197.0 ± 14.6

NMP/BuOH

(95/5 wt%)

3 2099 ± 54 95.4 ± 0.3 120.7 ± 8.2

4 2515 ± 74 97.6 ± 0.4 233.6 ± 39.1

5 3080 ± 112 98.0 ± 0.3 274.0 ± 40.5

a Other spinning conditions: air gap: 0.5 cm, dope flow rate: 5 ml/min.

6.3.2.3. Air gap

Figure 6.10 shows the effects of air gap on cross-section, outer and inner skin

morphologies of the TbHF substrates. The increase in air gap results in TbHFs with

a smaller outer diameter and a thinner wall due to the elongational stretch induced

by the gravitational force [22, 53]. The spokes of TbHF become severely deformed

at the air gap of 5 cm possibly due to unbalanced flow stresses and slow phase

inversion. Nevertheless, a dense outer layer and a porous inner layer are observed

for all the TbHFs.


168

Figure 6.10 The cross-section, outer surface as well as inner surface morphologies

of TbHF substrates spun at the air gap distances of 0.5, 2.5 and 5.0 cm. Other

spinning conditions: bore fluid composition: NMP/water (95/5 wt%), bore fluid flow

rate: 5 ml/min, dope flow rate: 5 ml/min.

As shown in Table 6.7, the flux of the TFC TbHF continuously increases at a higher

air gap due to the decrease of wall thickness and consequently reduced transport

resistance. The up and down trend of separation factor with air gap distance is

caused by two competing factors: the elongational stress induced by the gravity may

bring about molecular orientation at the skin (i.e., selective) layer and enhance the

separation factor, while a large air gap may overstretch the skin and create defects

that favor flux but impair the separation factor [49, 54, 55]. Therefore, the TbHF

substrate spun from an air gap distance of 2.5 cm is selected for the subsequent

studies considering its separation performance and morphology in maintaining the

mechanical integrity of the TbHFs.


169

Table 6.7 Effects of air gap on IPA dehydration performance of TFC TbHF

membranes a.

Air gap

(cm)

Total flux

(g m-2 h-1)


the permeate

(wt%)

Separation factor

(Water/IPA)

0.5 2475 ± 80 97.1 ± 0.2 197.0 ± 14.6

2.5 2647 ± 49 97.8 ± 0.2 260.7 ± 26.9

5 2877 ± 110 96.9 ± 0.3 180 ± 19.4

a Other spinning conditions: bore fluid composition: NMP/H2O (95/5 wt%), bore fluid flow rate: 5

ml/min, dope flow rate: 5 ml/min.

6.3.2.4. Dope flow rate

The dope flow rate also influences the TbHF morphology as demonstrated in Figure

6.11. The thicknesses of both wall and spokes increase with an increase in dope flow

rate. At the optimum air gap of 2.5 cm, the spokes of all TbHFs are able to maintain

their geometry. However, a higher dope flow rate results in a less porous inner

surface because a thick wall may retard non-solvent intrusion from the outer surface.

As a consequence, the flux of the TFC TbHF decreases at a higher dope flow rate

due to the increased wall thickness and less porous inner wall (shown in Table 6.8).

The separation factor also decreases possibly because of higher substructure

resistance [51] and slight defects created by the overstretch of polymer chains when

the dope flow rate exceeds a critical value [49, 56, 57].


170

Figure 6.11 The cross-section, outer surface as well as inner surface morphologies

of TbHF substrates spun from dope flow rate of 5, 6 and 7 ml/min. Other spinning

conditions: bore fluid composition: NMP/water (95/5 wt%), bore fluid flow rate:

5ml/min , air gap: 2.5 cm.

Table 6.8 Effects of dope flow rate on IPA dehydration performance of TFC TbHF

membranes a.

Dope flow rate

(ml/min)

Total flux

(g m-2 h-1)


the permeate

(wt%)

Separation factor

(Water/IPA)

5 2647 ± 49 97.8 ± 0.2 260.7 ± 26.9

6 2334 ± 81 93.6 ± 0.5 81.8 ± 7.5

7 2081 ± 75

90.8 ± 0.3 55.5 ± 2.1

a Other spinning conditions: bore fluid composition: NMP/water (95/5 wt%), bore fluid flow rate:

5ml/min , air gap: 2.5 cm.

Based on the above studies on TbHFs, the fine TbHF spun from the bore fluid

composition of NMP/water (95/5 wt%), air gap of 2.5 cm, the dope flow rate of 5

ml/min is the most preferred membrane for IPA dehydration, owing to its excellent

separation performance and desirable geometry.


171

6.3.3. A comparison of TFC TbHF and TFC SbHF membranes

Table 6.9 compares the fine and thick TFC TbHF membranes with the conventional

TFC SbHF membranes in three aspects including geometries, mechanical properties

as well as the pervaporation performance. In terms of geometry, the outer diameters

of the fine TbHF and SbHF are similar, which are smaller than that of the thick

TbHF. However, the fine TFC TbHF membrane outperforms the other two in all

aspects in terms of mechanical strength and IPA dehydration performance. It has the

highest tensile strength, Young’s Modulus, total flux and separation factor. In

contrast, the thick TFC TbHF shows the worst pervaporation performance. Clearly,

designing the TbHF substrate to have a proper morphology is essential to ensure the

resultant TFC TbHF membranes with superior performance.

Table 6.9 Comparisons of mechanical properties and pervaporation performance of

TFC SbHF and TFC TbHF membranes a.

Comparisons SbHF thick TbHF fine TbHF

Geometries polymeric cross-section area (mm2) 0.33 1.53 0.30

Outer diameter (mm) 1.00 2.18 1.02

Mechanical

properties

Elongation at break (%) 42.8 ± 2.8 42.7 ± 3.2 50.0 ± 2.9

Maximum tensile Stress(MPa) 11.8 ± 0.5 14.9 ± 0.3 15.0 ± 0.7

Young's Modulus (MPa) 350.5 ± 9.0 411.4 ±

10.8 426.2 ± 9.1

PV

performance

Total flux (gm-2h-1) 2356 ± 26 1008 ± 56 2647 ± 49

Separation factor 192.2 ± 12.1 22 ± 0.6 260.7± 26.9

a Spinning conditions: dope flow rate: 5 mLmin-1; bore fluid flow rate: 5 mLmin-1; air gap: 2.5 cm;

coagulant bath: water; bore fluid composition NMP/water (95/5 wt%) for the SbHF and fine TbHF

and NMP/water (80/20 wt%) for the thick TbHF; take up speed: free fall.


172

3.4 Benchmarking and long-term pervaporation performance of the fine TFC

TbHF

Table 6.10 compares the pervaporation performance of the fine TFC TbHF with

other pervaporation membranes for IPA dehydration available in literatures. The

fabricated fine TFC TbHF shows a high permeance with a reasonable selectivity

among the reported data. Moreover, it is worth mentioning that the performance of a

membrane with a low permeance but a very high selectivity is reported to easily fall

into the pressure-ratio-limited region and thus increases the capital investment due

to the requirement of a larger membrane area [14, 47]. As a result, the newly

fabricated fine TFC TbHF membrane with a high permeance and a moderate

selectivity is desirable for industrial applications in comparison to those membranes

with low permeance and very high separation factors.

Table 6.10 Performance benchmarking of the fine TFC TbHF with polymeric

membranes for IPA pervaporative dehydration in literatures.

Membrane T/o

C

Feed

(IPA

wt%)

Flux/

g m-

2h-1

Separatio

n factor

(water/IP

A)

mole

based

water

permeanc

e (mol m-

2h-1kPa-1)

mole-

based

selectivi

ty

(water/I

PA)

Ref.

Heat treated

polyacrylonitrile (PAN) HF 25 90 186 1116 5.5 1022 [58]

P84 co-polyimide hollow

fiber 60 85 883 10585 3.4 13227 [47]

Heat treated P84/PES dual

layer HF 60 85 570 125 2.1 145 [14]

Chitosan/polyacrylonitrile

(PAN) composite HF 25 90 145 2991 4.3 2739 [59]

Torlon/P84 blended HF 60 85 1000 185 3.8 216 [60]

Matrimid HF 80 84 1800 132 2.7 155 [61]

Torlon 4000T-MV/Ultem

dual layer HF 60 85 765 1944 3.0 2275 [62]

6FDA-ODA-NDA/Ultem 60 85 480 2332 1.8 2750 [63]


173

HF

Cellulose/polysulfone dual-

layer HF 25 95 4.4 94981 0.2 66731 [64]

MPD-TMC polyamide

TFC/torlon HF 50 85 1374 53 7.8 62 [13]

HPEI-TMC polyamide

TFC/ torlon HF 50 85 1980 349 12.3 407 [14]

HPEI-HGOTMS polyamide

TFC/ Ultem HF 50 85 3519 278 21.8 324 [15]

EDA-TMC

polyamide/modified PTFE

flat sheet membrane

70 70 1720 177 3.5 364 [16]

TAEA- TMC polyamide

TFC/mPAN 70 70 340 1150 0.7 2394 [17]

EDA-TMC polyamide TFC/

mPAN TFC flat sheet

membrane

25 90 213 105 5.8 96 [18]

MPD-TMC polyamide TFC

/CPA-5 flat sheet membrane

with heat treatment

16 90 140 40 11.0 59 [19]

HPEI/MPD-TMC

polyamide TFC/Ultem1010

TbHF

50 85 2647 261 15.8 294 This

work

Moreover, the long-term performance stability is also an important parameter to

evaluate the membrane. Therefore, the dehydration of aqueous IPA using the fine

TFC TbHF membrane was carried out and the permeate compositions were

continuously monitored for more than 200 h at 50 oC. From the results shown in

Figure 6.12, a slight decline in flux but constant product purity are observed,

evidencing a relatively stable performance of the fine TFC TbHF throughout the

monitored period. The slight decline in flux is possibly due to the stabilization of the

TFC layer or the TbHF substrates. It will be studied in the future.


174

Figure 6.12 Long-term pervaporation performance of the fine TFC TbHF for the

dehydration of aqueous IPA (IPA/water 85/15 wt%) at 50 oC.

6.4. Conclusions

We have molecularly designed TFC TbHF membranes with excellent pervaporation

performance for IPA dehydration and superior mechanical strength to traditional

SbHF membranes. The optimal TFC TbHF membrane shows a flux of 2.65 kg m-2 h-

1 with a separation factor of 246 for water/IPA separation at 50 oC using 85/15 wt%

IPA/water as the feed. The following conclusions can be drawn from this study:

(1) The interfacial polymerization conditions play important roles in determining the

morphology of the TFC layer and its performance for pervaporation dehydration.

The pre-treatment of HPEI on substrates not only smoothens the substrate surface

but also produces the TFC layer with fewer defects.


175

(2) The effects of spinning conditions on the geometry and morphology of TbHFs

have been determined. Bore fluid composition plays the most important role. To

prepare high-performance TFC TbHFs for pervaporation dehydration of IPA, the

preferred conditions to prepare TbHF substrates are as follows: spinning from a

dope made of Ultem/ethanol/NMP (23/5/72 wt%) with a bore fluid composition of

NMP/ water (95/5 wt%), a bore fluid flow rate of 5 ml/min, an air gap of 2.5 cm,

and a dope flow rate of 5 ml/min.

(3) In comparison with the TFC SbHF with a similar outer diameter, the fine TFC

TbHF shows superior pervaporation performance and mechanical strength.

(4) The newly developed fine TFC TbHF shows both satisfactory separation

performance and long-term stability. It may have potential for the industrial IPA

dehydration application.

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Chapter 7 Teflon AF2400/Ultem composite hollow fiber membranes

186

CHAPTER 7 TEFLON AF2400/ULTEM COMPOSITE

HOLLOW FIBER MEMBRANES FOR ALCOHOL

DEHYDRATION BY HIGH-TEMPERATURE VAPOR

PERMEATION

7.1. Introduction

Membrane-based technologies for solvent dehydration possess several advantages

such as the ability to separate azeotropic or close-boiling mixtures, lower energy

consumption, and flexible process control. Therefore, a variety of studies have been

conducted on solvent dehydration via pervaporation and vapor permeation [1-9].

Compared with pervaporation, vapor permeation is reported to have following

characteristics [7, 10-15]: (1) No phase change occurs from the feed to the

permeation side; (2) Separation capacity can be increased by pressurizing the feed

vapor; (3) Hybrid with distillation can be effectively achieved; (4) Effects of

concentration polarization, swelling and even fouling (e.g., for fermentation broth

separation) in liquid phase separation could be reduced.

The development of membranes with high permeability, good selectivity, and

durability is a key to the success of the vapor permeation process. Among numerous

polymer materials for membranes, polyimides have excellent thermal, chemical and

mechanical stabilities, high water selectivity, and less degree of swelling compared

with hydrophilic polymers such as polyvinyl alcohol, and thus they have emerged as


187

potential membrane materials for solvent dehydration via both pervaporation [3, 4,

16-20] and vapor permeation [11, 21, 22]. However, similar to other membranes,

polyimide membranes also suffer the problem of swelling which severely reduces

the separation performance. Thus, modifications of polyimide membranes such as

crosslinking treatments are still needed to control the swelling and improve the

stability, particularly in harsh environments [3, 4, 20, 23, 24].

Amorphous, solvent-processable perfluoropolymers (PFPs) such as Teflon AF,

Cytop and Hyflon AD are another family of promising membrane materials

developed in the past 30 years [25-32]. They have extraordinary thermal and

chemical resistance, and are unaffected by most chemicals including acids, bases,

organic solvents, oils and strong oxidizers [33]. Though PFPs are very hydrophobic,

they have been explored for solvent dehydration based on the size exclusion

mechanism [8, 34-37]. Studies showed that PFPs membranes have a very low

sorption of alcohol/water mixtures, thus the water permeance and water/alcohol

selectivity of their membranes are essentially independent of feed water/ethanol

composition. Usually, PFPs materials show high permeabilities due to their large

free volumes, but their selectivities are relatively low, especially for high-

temperature vapor permeation. Taking Teflon AF2400 as an example, its selectivity

for the vapor permeation dehydration of ethanol/water (62/38 wt %) mixture is only

6.9 at 120 oC [36].


188

To better utilize expensive PFP materials for membranes, Huang et al. fabricated flat

sheet composite membranes consisting of a PFP as the top protective layer and a

hydrophilic polymer as the selective underlayer [8, 37]. The fabricated composite

membranes were supposed to have the superior stability of PFP combined with the

high permeance and good selectivity of hydrophilic polymers for alcohol

dehydration. When coating Hyflon AD on hydrophilic membranes, they found that

the Hyflon AD coating could effectively alleviate the swelling and improve the

overall selectivity in pervaporation. However, the permeance was reduced by more

than 50% due to the additional mass transfer resistance.[37] Using a similar

approach, they also designed a novel vapor permeation membrane called Aquarius

for ethanol/water separation at 120 oC with required separation performance and free

of short-term stability issues [8].

At present, there is no systematic study on the fabrication of PFP coated polymeric

hollow fiber (HF) membranes for vapor permeation. Therefore, we aim to design

Teflon AF2400/Ultem composite HF membranes for high-temperature vapor

permeation for the first time. In this work, a PFP named as Teflon AF2400 was

chosen as the protective layer because its dense film shows stable performance for

high-temperature ethanol/water vapor permeation [36]. Moreover, it has a lower

selectivity but a higher permeability as compared with Hyflon AD in pervaporation

[8], thus the permeance drop due to the additional Teflon AF2400 coating may not

be so high as the Hyflon AD coating. Besides, Ultem polyetherimide was chosen as

the HF substrate because it not only owns good thermal stability and mechanical


189

properties, but also exhibit satisfying separation performance for alcohol

dehydration [38-40]. Therefore, our strategy is to combine the advantages of these

two materials by dip-coating a Teflon AF2400 layer on top of the Ultem HF

substrates.

The effects of Teflon AF2400 concentration in coating solutions and the coating

time on membrane morphology and separation performance of the fabricated

composite membranes for IPA dehydration will be firstly studied for IPA

dehydration via high-temperature vapor permeation. Then the fabricated Teflon

AF2400/Ultem composite HF membranes will be applied to IPA dehydration under

different feed concentrations. In the end, the dehydration of other alcohols such as

ethanol and n-butanol will be investigated in order to reveal the full potential of

Teflon AF2400/Ultem composite hollow fiber membranes for alcohol dehydration.

7.2. Experimental

To investigate the thermal stability of Teflon AF2400 material in high-temperature

feed vapor, its dense membrane was prepared via ring casting method as illustrated

in Chapter 3. Besides, the protocols for fabricating HF substrates and Teflon

AF2400/Ultem composite HFs have also been demonstrated in Chapter 3. The

details of spinning parameters are listed in Table 7.1. The morphologies and surface

topology of the HF membranes were observed by a FESEM and AFM, respectively.

The elemental mapping of composite HFs was conducted using EDX to confirm the

formation of Teflon layer. The surface hydrophilicity and depth profile of HF


190

membranes were studied by water contact angle measurement and PAS DBES,

respectively. The separation performance of the flat sheet Teflon dense membrane

and fabricated composite membranes were evaluated by alcohol dehydration using

high-temperature vapor permeation process described in Chapter 3. During the

calculation of membrane permeance, the fugacity coefficients of components at the

feed side were needed, which were obtained by the Peng-Robinson equation of state

with the aid of Thermo Solver software [41] and summarized in Table 7.2.

Table 7.1 The spinning parameters for Ultem HF substrates spun in this study a.

Spinning parameters Ultem HF substrate

Dope formulation (wt%) Ultem1010/NMP/ethanol (22/73/5)

Dope flow rate (ml/min) 6.67

Bore fluid composition (wt%) NMP/ethanol (95/5)

Bore fluid flow rate (ml/min) 2.67

Air gap (cm) 5.0

Take up speed (m/min) 21.8

Coagulant bath water

a The HF substrates were spun at ambient temperature around 23 oC. The dimension of single bore

spinneret used is 0.8/1.0/1.6 mm.

Table 7.2 The calculated fugacity coefficients of water and alcohols in the vapor

feed mixtures used in this work.

Vapor feed concentration T (oC) pf (bar) i

(i: water)

j

(j: alcohol)

95.0 wt% IPA 125 3 0.9854 0.9457

90.5 wt% IPA 125 3 0.9838 0.9461

91.3 wt% ethanol 125 3 0.9815 0.9572

90.0 wt% ethanol 92 1 0.9922 0.9813

76.7 wt% n-butanol 125 1 0.9942 0.9755


191

7.3. Results and discussion

7.3.1. The characterization of the Ultem HF substrate

The structure of HF substrates is essential to form defect free composite membranes

with minimum substrate resistance. As shown in Figure 7.1, the Ultem HF substrate

spun in this study possesses a cross-section with finger-like macrovoids and an inner

surface full of open pores, which provide low transport resistance and thereby

enhance the permeation rate. Moreover, its dense outer-surface would prevent the

intrusion of Teflon AF2400 during dip coating, and thus facilitates the deposition of

the protective layer.

Figure 7.1 FESEM images of surface and cross-section morphologies of the Ultem

HF substrate.

7.3.2. The characterization of the Teflon AF2400 dense membrane.

The fabricated Teflon AF2400 dense membrane is colorless and transparent. Its

surface and cross-section images are shown in Figure 7.2(a) and (b), from which the


192

thickness of the membrane is estimated to be about 20 m. The fabricated Teflon

AF2400 dense membrane shows very stable vapor permeation performance for the

dehydration of IPA/water (95/5 wt%) vapor at 125 oC with a high flux of 2973 gm-

2h-1 and a separation factor of 11. Moreover, Figure 7.2(c) qualitatively shows that

there is no severe swelling of the dense membrane after the vapor permeation

experiment since no wrinkles caused by swelling in the fixed space were observed.

Moreover, Table 7.3 shows that the swelling degree of Teflon AF2400 dense

membrane after immersing in the isopropanol/water feed vapor for 10 days is 0,

which is much less than that of the Ultem dense membrane (5.2%). Therefore,

Teflon AF2400 is an ideal material for protecting the Ultem HFs in the vapor

permeation separation of high-temperature alcohol/water mixtures.

Figure 7.2 FESEM images of (a) top surface and (b) cross-section of the Teflon

AF2400 dense membrane, as well as (c) a photo of the Teflon AF2400 dense

membrane after the vapor permeation experiment.

Table 7.3 The swelling degree of dense membranes after immersing in an

isopropanol/water (95/5 wt%) feed vapor at 125oC for 10 days.

Dense membrane Teflon AF2400 Ultem

Initial area A0 (cm2) 6.61 6.58

Area after immersing in feed vapor A (cm2) 6.61 6.92

Swelling degree* (%) 0 5.20

*The swelling degree is calculated by 100%× (A-A0)/A0


193

7.3.3. The morphologies and separation performance of Teflon AF2400/Ultem

composite HF membranes

During the fabrication of Teflon AF2400/Ultem composite HF membranes, coating

conditions such as Teflon AF2400 concentration in the coating solution and coating

time play important roles in the formation of Teflon layer, and thus may influence

the separation performance of composite HFs. Therefore, the effects of these two

factors on membrane morphology and vapor permeation performance are fully

investigated for the dehydration IPA/water (95/5 wt%) vapor at 125 °C .

7.3.3.1. The effect of Teflon AF2400 concentration

Figure 7.3 provides a comparison of the evolutional changes in terms of outer

surface and cross-section morphologies from the Ultem substrate to the Teflon

AF2400/Ultem composite HFs fabricated using different Teflon AF2400 coating

concentrations. It is interesting to notice that all Teflon AF2400 layers of the

fabricated composite HF membranes possess a honeycomb-like microstructure

pattern. Figure 7.4 compares the AFM images of the Ultem HF substrate and the

Teflon AF2400/Ultem HF coated by a 1 wt% Teflon AF2400 solution for 60 s. The

outer surface of the HF membrane becomes much rougher after coating. The

roughness Ra increases from 16 to 108 nm and the honeycomb-like microstructure

pattern has pores of around 1 m size on its surface. Besides, the EDX elemental

maps in Figure 7.5 also confirm the formation of honeycomb-like composite

membranes, where the F element only appears in the Teflon AF2400 region.


194

Figure 7.3 FESEM images of the outer surface and cross-section morphologies of (a)

Ultem HF substrate, and Teflon AF2400/Ultem HF membranes coated by Teflon

AF2400 with a concentration of (b) 0.25 wt% , (c) 0.5 wt% , (d) 1 wt% and (e) 2 wt%

(coating time: 60 s).

Figure 7.4 3-D AFM images for the outer surfaces of the Ultem HF substrate and

Teflon AF2400/Ultem HF (coating solution: 1wt% Teflon AF2400, coating time:

60s) from both the side and top views.


195

Figure 7.5 (a) SEM images and the corresponding EDX elemental maps of (b) F

and (c) C at Teflon AF2400 /Ultem HF (coating solution: 1 wt% Teflon AF2400,

coating time: 60s).

The formation of this interesting honeycomb-like microstructure pattern might be

ascribed to two reasons. Firstly, the boiling point of the Galden solvent for

dissolving Teflon AF2400 is very low (55 oC). Therefore, during the dip coating

process, the evaporation of this solvent from the HF outer surface would take place

very quickly and result in a concentration gradient. Then the concentration gradient

leads to the surface tension-driven Marangoni instability that normally generates the

honeycomb-like microstructure pattern [42-44]. Secondly, air humidity and

evaporative cooling favor the formation of water droplets in the Teflon AF2400

solution, which serve as soft templates and lead to the formation of a well-ordered

and honeycomb-like pattern [45].

When the Teflon AF2400 concentration is lower (i.e., 0.25 and 0.5 wt%), the Teflon

AF2400 layer cannot fully cover the surface of Ultem HF substrates, as shown in


196

Figure 7.3. However, when the Teflon AF2400 concentration is increased to 1 wt%,

a Teflon AF2400 layer composed of a top honeycomb-like porous portion and a

bottom dense portion is formed on the outer surface of the Ultem substrate. The

thickness of Teflon AF2400 layer (especially the dense portion) is further increased

as the Teflon AF2400 concentration reaches 2 wt%. Figure 7.6 elucidates the

membrane structures of these composite membranes as a function of Teflon AF2400

concentration. The formation of the Teflon dense portion at the concentrations of 1%

and 2% may be ascribed to following reasons. As indicated in Figure 7.7, the

kinematic viscosity of the Teflon AF2400 solution increases sharply when the

Teflon concentration reaches 1%, thus the amount of Teflon AF2400 solution

deposited on the Ultem HF surface also increases a lot. Therefore, when the top

portion of the Teflon solution forms the honeycomb-like microstructure pattern, a

certain amount of the liquid-state Teflon solution still remains at the bottom. The

solvent evaporation rate of this bottom Teflon solution is slow due to the

solidification of the top surface, leading to form a dense bottom Teflon layer.


197

Figure 7.6 Schematic illustration of the membrane structure of Teflon

AF2400/Ultem composite HF membranes coated with different Teflon AF2400

concentrations.

Figure 7.7 Kinematic viscosities of the Galden solvent and Teflon AF2400/Galden

solutions with various Teflon AF400 concentrations.


198

Table 7.4 tabulates the contact angles of the Ultem HF membrane and Teflon

AF2400/Ultem composite HFs. The contact angle increases with increasing Teflon

AF2400 concentration and reaches the highest value of around 156o when the

concentration is 1 or 2 wt%, indicating that a higher Teflon AF2400 concentration

promotes the formation of the dense portion of Teflon AF2400 layer on the Ultem

HF substrate.

Table 7.4 Water contact angles of the Ultem HF substrate and Teflon

AF2400/Ultem composite HFs.

Membrane Teflon AF2400 solution

concentration (wt%)

Water contact angle (o)

Ultem HF substrate 0 86 ± 3

Teflon AF2400/Ultem

composite HF

0.25 150 ± 6

0.5 152 ± 8

1 156 ± 7

2 156 ± 5

Note: The coating time for fabricating Teflon AF2400/Ultem composite HF is 60s.

Afterwards, the depth profiles of the selective layers among these composite HFs

with different Teflon AF2400 concentrations were further verified by PAS DBES.

Figure 7.8 shows S parameter curves of HFs as a function of positron incident

energy, which determines the penetration depth of positrons into the membrane

surface [38, 40, 46]. For all the HF membranes, the S parameter near the surface is

observed to decrease rapidly with increasing positron incident energy, which is

typical due to the back diffusion and scattering of positronium. With an increase in

positron incident energy, the S parameter quickly reaches the minimum value, which

can be interpreted as the dense selective layer of the HF membranes [40]. Thereafter,


199

the S parameter starts to gradually increase to a flat region, indicating the transition

from the top selective layer to the Ultem porous substrate. As compared, the

composite HFs coated with higher Teflon AF2400 concentrations (i.e. 1 and 2 wt%)

possess much thicker selective layers, which is consistent with the FESEM results in

Figure 7.3.

Figure 7.8 S parameter curves as a function of position incident energy of Teflon

AF2400/Ultem HF membranes coated with different Teflon AF2400 concentrations

(coating time: 60 s).

Figure 7.9 shows the separation performance of Teflon AF2400/Ultem composite

HFs coated with different Teflon AF2400 concentrations for aqueous IPA (95 wt%)

dehydration. The total flux decreases with an increase in Teflon AF2400

concentration because a thicker protective layer is formed at a higher Teflon

AF2400 concentration. In contrast, the water concentration in the permeate increases


200

with increasing Teflon AF2400 concentration, indicating a rise in separation factor.

The Teflon AF2400/Ultem HFs fabricated from Teflon AF2400 solutions with low

concentrations (i.e., 0.25 wt% and 0.5 wt%) show relatively low separation factors

because the Teflon AF2400 layer could not fully cover the Ultem HF surface, and

thus cannot perform a good protective function. While the Teflon AF2400/Ultem

HFs fabricated from Teflon AF2400 solution with high concentrations (i.e., 1 wt%

and 2 wt%) show greatly improved selectivities because a complete dense portion of

Teflon AF2400 layer is formed on the Ultem HF substrate. Not only does it protect

the Ultem HF from direct contact with the high temperature feed vapor, but also

reduce the swelling degree of the Ultem HF [8, 47].

Figure 7.9 Vapor permeation performance of Teflon AF2400/Ultem composite HFs

coated with different Teflon AF2400 concentrations and the coating time of 60 s for

the dehydration of IPA/water (95/5 wt%) vapor mixture at 125 oC.


201

The variation of error bars shown in Figure 7.9 also indicates that a dense Teflon

AF2400 layer completely covering the Ultem HF substrate is critical for

membrane’s performance stability. Large variations in separation performance can

be observed for AF2400/Ultem HFs coated with 0.25 and 0.5 wt% AF2400, while

those HFs coated with 1 and 2 wt% AF2400 exhibit very small variations in terms of

flux and separation factor. Since the HF coated with 2 wt% Teflon AF2400 has a

much lower flux due to a much thicker Teflon AF2400 dense layer formed on the

Ultem surface, the composite HF coated with 1 wt% Teflon AF2400 has the most

impressive and balanced vapor permeation performance.

7.3.3.2. The effect of coating time

Figure 7.10 shows the surface and cross-section morphologies of the Teflon

AF2400/Ultem composite HFs fabricated from the 1 wt% Teflon AF2400 coating

solution as a function of coating time. All fabricated HFs have a Teflon AF2400

layer composed of a top honeycomb-like porous portion and a bottom dense portion.

When the coating time is too short (i.e., 5 s), the Teflon AF2400 layer is very thin,

especially the bottom Teflon AF2400 dense portion (only ~150 nm), indicating this

layer might have some defects. As the coating time increases, the thickness of the

Teflon AF2400 layer increases, and a clearly dense portion can be observed, which

suggests fewer defects in this layer and it can work as a protective layer. Therefore,

a sufficient coating time is required to form a Teflon AF2400 layer containing a

dense portion on the Ultem HF substrate.


202

Figure 7.10 FESEM images of the outer surface and cross-section morphologies of

Teflon AF2400/Ultem composite HF coated with 1 wt% Teflon AF2400 solution as

a function of (a) 5 s, (b) 30 s, (c) 60 s, (d) 120 s.

Figure 7.11 illustrates the separation performance of Teflon AF2400/Ultem HFs as a

function of coating time. The total flux slightly decreases with an increase in coating

time due to the enhanced transport resistance in thicker and more uniform Teflon

AF2400 layers. In contrast, the water concentration in the permeate increases with

increasing coating time from 5 s to 60 s, leading to an increment in separation factor.

However, the separation factor improves little when the coating time further

increases from 60 to 120 s, indicating that a coating time of 60s is sufficient to form

a perfect Teflon AF2400 protective layer on the Ultem HF substrate. Similar to

Figure 7.9, the error bars of separation performance in Figure 7.11 vary large if the

coating time is too short, but become very small if the coating time reaches 60 s,


203

indicating that a coating time of no shorter than 60 s is needed to ensure the resultant

composite membranes with good performance stability.

Figure 7.11 The vapor permeation performance of Teflon AF2400 /Ultem

composite HFs coated with 1 wt% Teflon AF2400 solution and different coating

time for the dehydration of IPA/water (95/5 wt%) vapor mixture at 125 oC.

Overall, the Teflon AF2400/Ultem composite HF membrane coated with 1 wt%

Teflon AF2400 solution for 60 s possesses satisfying flux, high separation factor,

and stable performance. The fluxes and separation factors of the composite

membrane fabricated from this condition and the Ultem HF substrate are converted

to permeances and selectivities for comparison. It’s worth noting that this composite

HF shows a tripled mole-based selectivity (125 vs 368) and a drop of around 30% in


204

permeance (7.0 to 4.9 molm-2h-1kPa-1) as compared with the Ultem HF substrate.

However, this permeance drop caused by the Teflon AF2400 coating is much

smaller than that of the Hyflon AD coating in Huang’s work (i.e. 30 vs. >50%) [37].

Therefore, this condition is selected for the following studies.

7.3.4. The effect of feed composition on the vapor permeation performance of

Teflon AF2400/Ultem composite HF for IPA dehydration

Figure 7.12 depicts the effects of vapor feed composition on separation performance.

As observed, the total flux decreases with an increase in IPA concentration, which is

a typical phenomenon in pervaporative dehydration because of the lower driving

force for water permeation at higher IPA concentrations [48, 49]. Similarly, both the

water content in the permeate and separation factor gradually decreases when the

IPA concentration increases, especially at an extreme high IPA concentration such

as of 99 wt%. This is caused by the greatly increased driving force for IPA transport

across the membrane due to its high partial pressure of IPA. Nevertheless, the

Teflon AF2400/Ultem composite HF still shows a good separation factor of 347

even at this extreme high IPA concentration of 99 wt%.


205

Figure 7.12 The effects of the vapor feed composition on vapor permeation

performance of Teflon/Ultem composite HFs for aqueous IPA dehydration at 125 oC

(coating solution: 1 wt% Teflon AF2400, coating time: 60s).

7.3.5. The vapor permeation performance of Teflon AF2400/Ultem composite HF

for other alcohol dehydration

To explore the potential application of the Teflon AF2400/Ultem composite HF

membrane for dehydration of various alcohol/water mixtures, the newly fabricated

composite HF was also applied to ethanol and n-butanol dehydration. Table 7.5 lists

its vapor permeation performance of Teflon AF2400 coated Ultem substrates HFs

for different alcohols/water dehydration. During the experiments, the alcohol

concentration in the original feed liquid was fixed at around 90 wt% in the vapor

generator tank, but the generated feed vapor composition differs due to their

different liquid-vapor equilibria at 125 °C, especially for the n-butanol-water


206

mixture. Nevertheless, the Teflon AF2400/Ultem composite HF shows satisfying

vapor permeation performance for all the investigated C2-C4 alcohols at this high

temperature.

Table 7.5 The vapor permeation performance of the Teflon AF2400/Ultem

composite HF for C2-C4 alcohol dehydration at 125 °C.

Alcohol Vapor feed

concentration

(wt%)

Water concentration in

the permeate

(wt%)

Flux

(gm-2h-1)

Separation

factor

Ethanol 91.3 91.1 5311 108

IPA 90.5 98.0 7650 480

n-butanol 76.7 98.5 10505 214

Note: The Teflon AF2400/Ultem HF was fabricated by coating 1 wt% Teflon AF2400 solution for

60s.

7.3.6. Benchmarking and long-term stability of the Teflon AF2400/Ultem

composite HF for alcohol dehydration via vapor permeation

Table 7.6 compares the vapor permeation performance of the Teflon AF2400/Ultem

composite HF with other reported membranes for alcohol dehydration at high

temperatures. All the data are reported using permeance and selectivity because

these terms are normalized by driving force and thus the results obtained at different

conditions can be fairly compared. As shown, the newly designed Teflon

AF2400/Ultem composite HF possesses both high permeance and high selectivity

among the reported data, especially when compared with polymeric membranes.


207

Table 7.6 A comparison of vapor permeation performance among the Teflon

AF2400/Ultem composite HF and other membranes for C2-C4 alcohol dehydration

in literatures.

Membranes Temperature

(oC)

Alcohol

Vapor feed

concentration

(Alcohol

wt%)

Mole-based water

permeance

(molm-2h-1kPa-1)

Mole-based

selectivity

(water/alcohol)

Ref.

Polyimide HF from UBE

company 105 ethanol 88 1.2 472 [11]

Polyetherimide/

polyvinylpyrrolidone HF 85 n-propanol 50 5.1 384 [22]

Aquarius membrane 125 ethanol 88 4.2 248 [8]

Crosslinked agarose

membrane 92 ethanol 90 0.4 67 [15]

Flat sheet

organosilica/polysulfone

composite membrane

105 isopropanol 90 3.3 299 [50]

Inorganic NaA

membrane 120 isopropanol 95 7.0 5680 [51]

Inorganic NaY

membrane 130 isopropanol 80 15.1 35 [52]

Teflon AF2400/Ultem

HF 125 isopropanol 95 4.9 368

This

work

Teflon AF2400/Ultem

HF 125 isopropanol 90.5 5.6 462

This

work

Teflon AF2400/Ultem

HF 125 ethanol 91.3 4.8 106

This

work

Teflon AF2400/Ultem

HF 125 n-butanol 76.7 10.9 215

This

work

Besides the separation performance, the long-term stability is also a very important

parameter to evaluate the membrane. Therefore, a long-term vapor permeation

experiment for the dehydration of isopropanol/water (95/5 wt%) feed vapor using

the Teflon AF2400/Ultem composite HF at 125oC was carried out, the details of

which were described in Supplementary Material SM-4. As shown in Figure 13,

both the flux and permeate water concentration keep nearly constant, evidencing a

good stability of the composite membranes throughout the testing period of 300

hours. In addition, the cross-section morphology of the composite HF after long-

term testing in Figure 14 shows no delamination between the Teflon AF2400 layer


208

and Ultem HF. This further confirms good compatibility between Teflon AF2400

and Ultem HF as well as good stability of the composite membrane.

Figure 7.13 Long-term vapor permeation performance of the Teflon AF2400/Ultem

composite HF for the dehydration of an isopropanol/water (95/5 wt%) vapor mixture

at 125 oC. (coating solution: 1 wt% Teflon AF2400, coating time: 60s).

Figure 7.14 The FESEM cross-section images of Teflon AF2400/Ultem HF (a)

before and (b) after long-term vapor permeation tests. (Coating solution: 1 wt%

Teflon AF2400, coating time: 60s).


209

7.4. Conclusions

In summary, we have designed the Teflon AF2400/Ultem composite HF membrane

with good vapor permeation separation performance and also excellent thermal

stability for alcohol dehydration by a dip coating method. Interestingly, a

honeycomb-like microstructure pattern was observed on the top of Teflon AF2400

coating layers. However, only membranes containing a thin dense Teflon AF2400

layer fully covering the Ultem HF substrate exhibit impressive separation

performance. The optimal Teflon/Ultem composite HF was fabricated by coating 1

wt% Teflon AF2400 solution for 60 s. Compared to the Ultem HF substrate, it has

stable separation performance with a tripled separation factor of 383 with a flux of

4265 gm-2h-1for the dehydration of IPA/water (95/5 wt%) vapor mixture at 125 oC.

This membrane also performs well when the IPA vapor composition increases from

87 to 99 wt%. Satisfactory vapor permeation performance for ethanol and n-butanol

dehydration has also been obtained, demonstrating its great potential for alcohol

dehydration applications.

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Chapter 8 Conclusions and recommendations

218

CHAPTER 8 CONCLUSIONS AND

RECOMMENDATIONS

8.1. Conclusions

The design and fabrication of membranes with both good separation performance

and stability are very important for the development of pervaporation and vapor

permeation processes. Currently there is no membrane that is suitable for all

conditions of the pervaporation/vapor permeation dehydration process. Therefore,

this PhD study focuses on the design of polymeric composite membranes to improve

the membrane separation performance or stability, including the dense

nanocomposite membrane which is a hybrid of inorganic nanomaterials and

polymeric materials and composite hollow fiber (HF) membranes that combine the

advantages of different polymeric materials via covalent reaction, interfacial

polymerization, or coating. The conclusions based on the aforementioned studies are

drawn as follows.

8.1.1. ZIF-90/P84 mixed matrix membranes for pervaporation dehydration of

isopropanol

In this work, Zeolitic imidazolate framework-90 (ZIF-90) nanoparticles with an

average particle size of 55 nm are synthesized and embedded into P84 polymeric

membranes with excellent dispersion to fabricate mixed matrix membranes

(MMMs). The effects of ZIF-90 loading as well as feed temperature on


219

pervaporation performance for isopropanol (IPA) dehydration of the mixed matrix

membranes (MMMs) are systematically investigated. The flux of MMMs increases

with increasing ZIF-90 loading due to the enhanced fractional free volume, while

the separation factor of water/IPA can be maintains at 5432 when the ZIF-90

loading is less than 20 wt%, but reduced to 385 when the ZIF-90 loading is 30 wt%.

Interestingly, the application of sulfonated polyethersulfone (SPES) as a primer to

ZIF-90 nanoparticles before fabricating MMMs consisting of 30 wt% ZIF-90

recovers the separation factor without sacrificing the flux. The best MMM shows the

separation performance with a doubled flux of 109 g m-2 h-1 and a separation factor

of 5668 at 60 oC. Therefore, ZIF-90 nanoparticles promising fillers for enhancing

the permeability of barrier materials like P84.

8.1.2. Universal surface modification by aldehydes on polymeric membranes for

isopropanol dehydration via pervaporation

This work proposes a universal approach consisting of hyper-branched

polyethyleneimine (HPEI) pre-treatment and aldehyde modification to modify

various polymeric membranes such as Torlon, Ultem, and PES hollow fibers, which

greatly improves separation performance for IPA dehydration. Results show the

HPEI pre-treatment not only mitigates surface defects but also augments membrane

hydrophilicity favoring water transport, while the cross-linking reaction between

HPEI and aldehydes tightens polymeric chains and enhances selectivity. Besides, the

newly molecularly designed membranes not only display satisfactory separation


220

performance at various feed temperatures and compositions but also shows good

stability during long-term pervaporation test of more than 200 hours.

8.1.3. Thin-Film Composite Tri-bore Hollow Fiber Membranes for Isopropanol

Dehydration by Pervaporation

Novel thin-film composite tri-bore hollow fiber (TFC TbHF) membranes for

pervaporation dehydration of isopropanol (IPA) are designed and fabricated in this

study. The investigation of interfacial polymerization conditions shows that the pre-

treatment of HPEI on substrates not only smoothens the substrate surface but also

produces the TFC layer with fewer defects. And the systematic study on the effects

of several spinning parameters on TFC TbHF membranes’ geometry, morphology

and pervaporation performance reveals the bore fluid composition is the most

critical factor. The optimal TFC TbHF membrane shows a flux of 2.65 kg m-2 h-1

with a separation factor of 261 for the dehydration of aqueous IPA (85 wt%) at 50

oC. Most importantly, the newly designed TFC TbHF membrane not only shows

enhanced separation performance and mechanical strengths as compared with the

conventional TFC SbHF spun from the identical spinning conditions, but also

exhibits satisfying long-term stability, indicating a great potential for pervaporation

applications.

8.1.4. Teflon AF2400/Ultem composite hollow fiber membranes for alcohol

dehydration by high-temperature vapor permeation


221

In this work, Teflon AF2400/Ultem composite hollow fiber (HF) membranes are

designed for alcohol dehydration via high-temperature vapor permeation, which has

a stringent requirement of membrane stability under harsh feed environments.

Fabrication parameters such as Teflon concentration and coating time are

systematically investigated. Interestingly, the fabricated composite HF membranes

possess an unusual surface with honeycomb-like microstructure patterns. However,

only membranes with a thin dense Teflon AF2400 layer fully covering the Ultem

HF substrate exhibit impressive separation performance. Owing to the Teflon

protective layer, the newly developed composite HF shows a promising and very

stable separation performance with a flux of 4265 gm-2h-1 and a separation factor of

383 for 95% isopropanol dehydration at 125 oC. The composite HF also performs

well under extreme vapor feed compositions from 87 to 99 wt% isopropanol. In

addition, it exhibits impressive separation performance for the dehydration of

ethanol and n-butanol. This work may provide useful insights of designing thermal-

stable and high-performance composite polymeric membranes for vapor permeation.

To sum up, among the four research works above, the ZIF-90/P84 MMMs are dense

membranes, so it is only a study of intrinsic property of membranes, which is still

far away from real industry application. The HFs modified via aldehydes method

possess both high separation factor and good stability, while the TFC TbHF has both

high flux and separation factor, so both of them are promising for pervaporative IPA

dehydration, but their performance for ethanol dehydration is relatively poor due to

the more severe swelling effects caused by ethanol. The Teflon AF2400/Ultem HF

performs well and stably for the dehydration of several alcohols via high-

temperature vapor permeation. Moreover, the fabrication method of this membrane

is the simplest. Therefore, Teflon AF2400/Ultem HF membrane is considered to


222

have the greatest potential for the industrial alcohol dehydration application.

Nevertheless, the findings in developing the other three types of membrane are also

very meaningful in developing next generation of membranes with good separation

performance, stability and mechanical properties for pervaporation and vapor

permeation.

8.2. Recommendations

Based on the experimental results analysis, discussions as well as the conclusions

from this research, the following recommendations are proposed, which may

provide further insight for future studies on developing pervaporation and vapor

permeation membranes.

1) Covalent modify ZIF nanoparticles to improve the hydrophilicity, and then

incorporate them into barrier polymeric materials for pervaporation.

Enhancement in both diffusivity selectivity and sorption selectivity of water may

be achieved.

2) Develop composite HF membranes using ZIF based MMMs as the top dense

selective layer by dual-layer spinning or coating. The MMM based composite

HF membranes may have an improved flux as compared with polymeric HF

membranes.

3) Further improve the separation performance of TFC membranes either by

modifying monomers with inorganic agents or adding inorganic materials such

as ZIFs or graphene oxide into the polyamide layer during interfacial


223

polymerization to increase the fractional free volume and hydrophilicity of the

TFC layer.

4) The TeflonAF2400/Ultem composite membrane has a good and stable

separation performance for alcohol dehydration. However, the honeycomb-like

Teflon portion at the surface of Teflon layer is not desired for the membrane

since it will consumes more expensive Teflon without offering separation and

protection functions. Some methods may help to overcome this so that a thinner

and dense Teflon layer could be formed at lower concentrations. One is to slow

down the evaporation rate of solvent by using less volatile solvent; the other is to

modify the HF substrate to be more hydrophobic.

5) Explore the possibility of TFC membranes and composite HF membranes

modified by HPEI mediated aldehyde method in vapor permeation process.

Since the operation temperature of vapor permeation is higher than

pervaporation, the thermal-stability of these composite membranes may need to

be enhanced by further modification such as high-degree of crosslinking.

6) Extend the application of developed composite membrane to the dehydration of

other solvents such as methanol, acetone, phenol, etc.

224

PUBLICATIONS

Journal papers:

1. D Hua, GM Shi, C Fang, TS Chung, Teflon AF2400/Ultem composite hollow

fiber membranes for alcohol dehydration by high-temperature vapor permeation,

AIChE Journal 62 (2016), 1747-1757. (IF: 2.748)

2. D Hua, TS Chung, Universal surface modification by aldehydes on polymeric

membranes for isopropanol dehydration via pervaporation, Journal of

Membrane Science 492 (2015), 197-208. (IF:5.056)

3. D Hua, YK Ong, P Wang, TS Chung, Thin-film composite tri-bore hollow fiber

(TFC TbHF) membranes for isopropanol dehydration by pervaporation, Journal

of Membrane Science 471(2014), 155-167. (IF:5.056)

4. D Hua, YK Ong, Y Wang, T Yang, TS Chung, ZIF-90/P84 mixed matrix

membranes for pervaporation dehydration of isopropanol, Journal of Membrane

Science 453(2014), 155-167. (IF:5.056)

Conference presentations:

1. D Hua, TS Chung, Aldehyde modification of polymeric hollow fiber membranes

for pervaporative isopropanol Dehydration, The 9th Conference of Aseanian

Membrane Society, 19-21 Jul 2015, Taipei, Taiwan. (Oral presentation)

2. D Hua, YK Ong, P Wang, TS Chung, Design and Fabrication of Thin-Film

Composite Tri-bore Hollow Fiber (TFC TbHF) Membranes for pervaporative

225

Isopropanol Dehydration, 2014 AIChE Annual Meeting, 16-21 Nov 2014,

Atlanta, GA, USA. (Oral presentation)

3. D Hua, YK Ong, Y Wang, T Yang, TS Chung, Zeolitic imidazolate framework

(ZIF-90) filled P84 mixed matrix membranes for the dehydration of isopropanol

via pervaporation, The 10th International Congress on Membrane and

Membrane Processes, 20-25 Jul 2014, Suzhou, Jiangsu, China. (Oral

presentation).

design and fabrication of polymer based …methods for alcohol dehydration via pervaporation or...

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