hybrid materials, Angew. Chem., Int. Ed., 2006, 45, 3216â3251. [144] Slowing. I. I, Vivero-Escoto. J. L, Wu. C.W Lin. V. S. Y, Adv. Mesoporous silica nanoparticles.
Parimal Naik
Faculty of Bioscience Engineering Department of Microbial and Molecular Systems Centre for Surface Chemistry and Catalysis Leuven Chem&Tech Celestijnenlaan 200f box 2461 B-3001 Heverlee, Belgium www.biw.kuleuven.be/m2s/cok
ARENBERG DOCTORAL SCHOOL FACULTY OF BIOSCIENCE ENGINEERING
PREPARATION OF HIGH FLUX HYDROPHOBIC COMPOSITE MEMBRANES
PREPARATION OF HIGH FLUX HYDROPHOBIC COMPOSITE MEMBRANES
Parimal NAIK Supervisor: Prof. Ivo F.J. Vankelecom March 2016
Dissertation presented in partial fulfilment of the requirements for the degree of Doctor in Bioscience Engineering
March 2016
PREPARATION OF HIGH FLUX HYDROPHOBIC COMPOSITE MEMBRANES
Parimal NAIK
Supervisor: Prof. Ivo F.J. Vankelecom Dissertation presented in partial fulfillment of the requirements for the degree of Doctor in Bioscience Engineering
Members of the Examination Committee: Prof. Dirk Springael, KU Leuven (Chairman) Prof. Johan A. Martens, KU Leuven Prof. Bart Van der Bruggen, KU Leuven Dr. Pieter Verlooy, KU Leuven Dr. Pieter Vandezande, VITO
March 2016
Doctoraatsproefschrift nr.1344 aan de faculteit Bio-ingenieurswetenschappen van de KU Leuven
© 2016 KU Leuven, Science, Engineering & Technology Uitgegeven in eigen beheer, PARIMAL NAIK, LEUVEN Alle rechten voorbehouden. Niets uit deze uitgave mag worden vermenigvuldigd en/of openbaar gemaakt worden door middel van druk, fotokopie, microfilm, elektronisch of op welke andere wijze ook zonder voorafgaandelijke schriftelijke toestemming van de uitgever. All rights reserved. No part of the publication may be reproduced in any form by print, photoprint, microfilm, electronic or any other means without written permission from the publisher.
Dedicated to My beloved family
ACKNOWLDGEMENTS
First of all, I am grateful to God almighty for blessing me a good health and wellbeing that were necessary to complete this book after four years of scientific journey. I take this opportunity to express my deep gratitude and say thank you to a number of people who are (and were!) part of this journey directly or indirectly, without whom finalizing this PhD would not be possible. Although it is not possible to list all the people who helped me during these years, I have honestly attempted to accommodate most of them. My experience at KU Leuven and COK is full of pleasant memories which are unforgettable over the rest of my life. I would like to extend my deepest thanks and gratitude to my promoter, Prof. Ivo Vankelecom to offer an unique opportunity to discover and explore the field of membrane separation. His valuable guidance, suggestions and support helped me in developing a critical understanding of the subject. He is an exceptional mentor and an outstanding person. He is, in short, an ideal person one could ever ask for as a doctoral research promoter, and I am honored to be his student. I would like to thank my supervisory committee members, Prof. Johan Martens and Prof. Bart Van der Bruggen, for their timely guidance and support throughout this PhD in-spite of their busy scheduled. I would like to thank Dr. Pieter Vandezande (VITO) for accepting the invitation as an external examiner in my examination committee. My research carrier commenced 7 years before, at the National Chemical Laboratory, India. I owe a special thanks to Dr. Prakash Wadgaonkar who provided a platform to kick off my scientific career and cultivated the seeds of research interest in my mind. His devotion and enthusiasm for science and research makes him more respectable. He is an example of simple living and high thinking which is one of the important virtue I found in many researchers. I also take this opportunity to express my deep thanks to Dr. Venkat Iyer for their timely guidance and discussions which helped and motivated me to pursue the Ph.D. Being a fresher in the field of membrane technology, I faced several challenges in understanding the fundamentals that I could apply to my project. I wish to offer my sincere gratitude to members of the so called “COK Membrane crew” Prof. Asim Khan, Dr. Muhammad Roil Bilad and Dr. Pejman Ahamadianamini for providing me the fundamental and technical assistance during the initial and middle phase of the Ph.D. I am very grateful to Dr. Katrien Vanherck for providing the initial ideas and guidelines to plan my work efficiently. I would like to thank my colleagues Dr. Izabela, Dr. Katrien H, Dr. Sanne, Dr. Agnieska, Dr. Anna, Dr. Chalida, Dr. Xia, Dr. Yanbo, Dr. Lu, Dr. Waqas. I have always blessed with helpful and supportive lab mates during my stay at COK. I would like to thank other members of the membrane crew, Dr. Nithya, Lisendra, Stefaan, Elke, Maxime, Jeroen, Nick, Hanne, Matthias, Annelies, Cedric, Yun, Valerie. It has always been a great pleasure to work in the membrane group with all of you and I really enjoyed the relaxed and pleasant atmosphere in and outside of the lab. I am grateful to Maarten for his technical assistance to set up the high-throughput equipment and booking the SEM sessions without any hesitation. i
In the mid of my Ph.D., I discovered many new friends having diverse regional and geographical background through the Erasmus mundus and Marie curie Ph.D. program. The list is bit long, but have to mention some important ones, Dr. Sushumna, Veysi, Aylin, Dr. Abi, Dr. Salman, Joana, Dr. Cheryl, Violeta, Remigio, Daria, Shazia, Ahamd, Sardar, Satya, Maryam, thank you all. I also made some new Indian friends during their short stay at Leuven, Surya, Nakul, Rajeev, Nimisha, Jishna, Shruti, Dolika, Amit, I would like to thank them too. I would like to thank some important COK members outside the membrane crew, Dr. Lik, Dr. Stef K, Dr. Shanmugan, Dr. Pieter V, Dr. Sree, Dr. Warunee, Dr. Sambhu, Sam, Damiano, for their help in some characterization and discussions regarding my work and manuscripts. I would also like to thank COK technical team Stef U, Johan M and Dirk D. A special thanks to Werner for his technical support for my set-up. I would also like to thank the very efficient COK secretary staff, Lieve, Annelies, Ines, Birgit, Inge, Hilda for their administrative support. I would like to thank my colleagues and seniors at NCL, Pune, Dr. Mahesh, Dr. Nilakshi, Dr. Arvind, Dr. Arun, Dr. Anjana, Dr. Pandurang, Dr. Sharad, Dr. Nagendra, Dr. Mahesh, Dr. Mahadev, Dr. Ankush, Dr. Prakash, Dr. Savita, Kishore, Dr. Bhausaheb, Indravadan, Sharddha, Sayli, Deepshikha, Satish for their help in and outside of the lab. I would like to extend my appreciation and gratitude to all my Indian friends from ISAL family to whom I met in Leuven in past 4 years, Dr. Sagnik, Dr. Milind, Dr. Bharat, Eshwar, Vikas, Parveen, Shanish, Abhijit, Chandan, Manish, Aniket, Chaman, Pranjal, Chetan, Anjan, Niraj, Abhishek, who were always supportive and cheerful towards me. Your presence made my life outside the lab lively and enjoyable. Together with them, I enjoyed trips, movies, dinners, sports and some nice discussions. Four years seemed to be a long time, especially that to arrive to Leuven I needed to sacrifice my life with my parents and my sister. Finally, I would like to Thank specially to my parents (“Aai-Baba”) and sweet little sister “Pallavi” for their unconditional love and moral support. This degree is as much theirs as it is mine, for I would not be here if it were not for them. I can’t imagine my life without them.
Parimal V Naik
ii
ABSTRACT
Biomass is considered as one of the most promising renewable source of energy because of its sustainability. Ethanol produced from the fermentation of renewable biomass represents one of the important sources of renewable energy. In order to improve the efficiency of ethanol production, it is convenient to remove the produced ethanol from the fermentation broth periodically. Existing techniques applied for the recovery of ethanol from fermentation broth are vacuum distillation, solvent extraction, gas stripping and membrane pervaporation. Pervaporation is one of the most significant techniques for the recovery of ethanol from the fermentation broth. It has many advantages over other techniques as energy efficient, cost effective, and ecofriendly. The pervaporation process comprises of a separation of two components with different permeation rates by using either polymeric or inorganic membrane. Commonly, vacuum is applied at the downstream side of the membrane where an evaporative phase change occurs. The membrane is the key element in the pervaporation process. Polydimethylsiloxane (PDMS) is the promising membrane material considered for the purpose of recovering ethanol from dilute fermentation broth. PDMS membranes are hydrophobic in nature and have a high diffusivity for ethanol. Pure PDMS membranes are less efficient in the ethanol recovery, but their efficiency can be increased by incorporation of additives or fillers, or by preparing thin selective and highly permeable composite membranes. The main objective of this work is to improve the efficiency of PDMS-based PV-membranes by following approaches: In a first approach, MMM’s were fabricated by addition of hollow spheres (HS) covered with a shell of hydrophobic silicalite-1 crystals. These HS were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen physisorption and X-ray diffraction (XRD). The size of these spheres was approximately 1 μm with a shell thickness of 30 nm. High surface areas were obtained with presence of micro and mesopores. The PDMS-based Mixed matrix membranes were showing a uniform dispersion of the HS in the polymer matrix. Pervaporation of ethanol/water mixtures showed a strong increase in permeability and ethanol selectivity upon incorporation of the fillers. In continuation of the first approach , MMM were prepared by adding other types of fillers with core-shell morphology. Mesoporous silica spheres (MSS) were covered in this second approach with a shell of hydrophobic ZIF-8 or ZIF-71 crystals (designated as MSS-ZIF spheres). The MSS-ZIF spheres were characterized by scanning electron microscopy (SEM), high-angle annular dark-field imaging- scanning transmission electron microscope (HAADF-STEM and EDX), nitrogen physisorption and X-ray diffraction (XRD). The size of these spheres was approximately 2-3 μm. High surface areas were obtained with presence of micro and mesopores. The PDMS-based Mixed matrix membranes were characterized by SEM showing a uniform dispersion of the fillers in the polymer matrix. Pervaporation of ethanol/water mixtures in Mixed matrix membranes showed a strong increase in permeability and ethanol selectivity. It was observed that PDMS-membranes filled with MSS-ZIF-8 spheres gave higher selectivity, while membranes filled with MSSZIF-71 gave higher flux. In the third approach, MMM were fabricated by addition of newly synthesized polyoligosiloxysilane (PSS-2) nanoparticles. PSS-2 particles belong to the silicone family but with a much more rigid structure than PDMS. These PSS-2 particles were characterized by scanning electron microscopy (SEM), nitrogen physisorption and iii
X-ray diffraction (XRD) and thermogravimetrical analysis (TGA). The size of these particles was approximately 1-2 μm. PSS-2 particles showed a moderate surface area with presence of micro and mesopores. The PDMS-based Mixed matrix membranes were characterized by SEM showing a uniform dispersion of the PSS-2 particles in the polymer matrix. Pervaporation of ethanol/water mixtures in Mixed matrix membranes showed a strong increase in the permeability and ethanol selectivity. Lastly, a detailed study on preparing the composite membranes was performed, since self-supporting PDMSmembranes were primarily studied in the aforementioned approaches. The thickness of these membranes is around 100 µm, which is acceptable for lab-screening purposes, but which exclude them from practical use in real application. For this study, PDMS-coatings were applied without addition of fillers. In order to fabricate thin, defect-free, PDMS membranes on porous supports, several synthesis parameters were optimized. First, support membranes with different porosities and chemistries were prepared from selected polymer materials (polyimide, polysulfone, and polyvinylidene fluoride). The PDMS coating solution was optimized with respect to viscosity before coating the support, which is essential to prepare thin layers of PDMS on top of porous support layers. Coating conditions for the PDMS solution were optimized by using a custom-made automatic dip coating machine with different coating time.
iv
SAMENVATTING
Biomassa-energie wordt beschouwd als één van de meest veelbelovende hernieuwbare energiebronnen vanwege zijn duurzaamheid. Ethanol geproduceerd uit de vergisting van hernieuwbare biomassa is één van de belangrijke bronnen van hernieuwbare energie. Om de efficiëntie van de ethanolproductie verbeteren, is het noodzakelijk om de geproduceerde ethanol periodiek te verwijderen uit het fermentatiemedium. Bestaande technieken voor de afscheiding van ethanol uit fermentatiemedium zijn vacuüm destillatie, solventextractie, gasstripping en membraan pervaporatie. Pervaporatie (PV) is één van de belangrijkste technieken voor het terugwinnen van ethanol uit het fermentatiemedium. Deze techniek heeft vele voordelen ten opzichte van andere technieken, zoals energiebesparing, kosteneffectiviteit en milieuvriendelijkheid. Pervaporatie processen bestaan uit een scheiding van twee componenten met verschillende permeatie met behulp van polymerische of anorganische membranen. Gewoonlijk wordt een vacuüm aangebracht aan de permeaatzijde van het membraan waar een verdampende faseverandering optreedt. Het membraan is het belangrijkste element in een pervaporatie proces. Polydimethylsiloxaan (PDMS) wordt als een veelbelovende membraanmateriaal beschouwd met het oog op het terugwinnen van ethanol uit het verdunde fermentatiemedium. PDMS membranen zijn hydrofoob en hebben een hoge diffusiviteit voor ethanol. Pure PDMS membranen zijn slechts matig efficiënt in de afscheiding van ethanol maar hun efficiëntie kan verhoogd worden door de incorporatie van bepaalde additieven / vulstoffen of door het bereiden van dunne selectieve en hoog permeabele composiet membranen. Het belangrijkste doel van dit werk is om de efficiëntie van de PDMS-gebaseerde PV membranen te verbeteren door middel van volgende benaderingen: In een eerste benadering werden MMM's gesynthetiseerd door het toevoegen van kleine holle sferen (HS), bedekt met een schil van hydrofobe silicaliet-1 kristallen. Deze HS werden gekarakteriseerd met behulp van scanning elektronenmicroscopie (SEM), high- angle ringvormige donker - veld heen Beeldverwerking scanning transmissie-elektronenmicroscoop ( HAADF - STEM en EDX ), stikstof fysisorptie en X-stralen diffractie (XRD). De diameter van deze sferen was ongeveer 1 µm met een schildikte van 30 nm. Een hoog specifiek oppervlak werd verkregen door de aanwezigheid van micro- en mesoporiën. De PDMS gebaseerde MMM’s toonden een gelijkmatige dispersie van de HS in de polymeermatrix. Pervaporatie van een ethanol / watermengsel resulteerde in een sterke toename van de permeabiliteit en ethanol selectiviteit door het toevoegen van de vullers. In vervolg van de eerste benadering werden MMM’s vervaardigd door het toevoegen van andere types vullers met kern-schil morfologie. Mesoporeuze silica sferen werden in deze tweede methode bedekt met een schil van hydrofobe ZIF-8 of ZIF-71 kristallen (verder benoemd als MSS-ZIF sferen). De MSS-ZIF sferen werden gekarakteriseerd door scanning elektronenmicroscopie (SEM), transmissie elektronenmicroscopie (TEM), stikstof fysisorptie en X-stralen diffractie (XRD). De diameter van deze sferen was ongeveer 2-3 µm. Hoge specifieke oppervlakken werden verkregen door de aanwezigheid van micro- en mesoporiën. De PDMS gebaseerd Mixed matrix membranes werden gekarakteriseerd met behulp van SEM en toonden een gelijkmatige dispersie van de vulmiddelen in de polymere matrix. Pervaporatie van een ethanol / watermengsel in Mixed matrix membranes liet een sterke toename van de permeabiliteit en ethanol selectiviteit zien. Er werd
v
waargenomen dat PDMS membranen gevuld met MSS-ZIF-8 sferen een hogere selectiviteit gaf, terwijl membranen gevuld met MSS-ZIF-71 leidde tot een hogere flux. In een derde benadering werden MMM’s vervaardigd door de toevoeging van nieuw gesynthetiseerde polyoligosiloxysilaan (PSS-2) nanodeeltjes. PSS-2 deeltjes behoren tot een familie van siliconen met een meer rigide structuur dan PDMS. De PSS-2 deeltjes werden gekarakteriseerd door middel van scanning elektronenmicroscopie (SEM), Stikstof fysisorptie en X-stralen diffractie (XRD) en Thermogravimetrical analyse (TGA). De grootte van de deeltjes bedroeg ongeveer 1-2 µm. PSS-2 deeltjes vertoonden een matig specifiek oppervlak met aanwezigheid van micro- en mesoporiën. De PDMS gebaseerde MMM’s werden gekarakteriseerd met behulp van SEM en toonden een gelijkmatige dispersie van de PSS-2 deeltjes in de polymeermatrix. Pervaporatie van ethanol/watermengsel in MMM’s liet een sterke toename van de permeabiliteit en ethanol selectiviteit zien. Tenslotte werd een gedetailleerd onderzoek uitgevoerd omtrent de synthese van composiete membranen aangezien in bovengenoemde benaderingen voornamelijk zelfdragende PDMS membranen bestudeerd werden. De dikte van deze membranen is ongeveer 100 µm wat acceptabel is voor het screenen van membranen in het laboratorium maar niet voor industriële toepassing. Voor dit onderzoek werden PDMS-coatings aangebracht zonder de toevoeging van vullers. Om dunne, defect vrije PDMS membranen op poreuze steunlagen te synthetiseren werden verschillende parameters geoptimaliseerd. Eerst werden ondersteunende membranen met verschillende porositeiten en chemie gesynthetiseerd uit geselecteerde polymeren (polyimide, polysulfon en polyvinylideenfluoride). De PDMS coatingoplossing werd geoptimaliseerd met betrekking tot de viscositeit voor het coaten van de drager wat essentieel is voor het coaten van dunne lagen van PDMS bovenop een poreuze drager. Coating condities voor de PDMS oplossing zoals de coatingtijd werden geoptimaliseerd met behulp van een op maat gemaakte automatische dip coating machine met verschillende coating tijden.
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List of abbreviations
CTAB
cetyl trimethyl ammonium bromide
DMF
N,N‘-dimethylformamide
EDX
energy dispersive X-rays
EtOH
ethanol
FT-IR
Fourier transform infra-red
GS
gas separation
HAADF
high-angle annular dark-field imaging
HS
hollow sphere
MeOH
methanol
MF
microfiltration
MMM
mixed matrix membrane
MSS
mesoporous silica sphere
MOF
metal organic framework
NF
nanofiltration
NMP
N-methylpyrrolidon
NS
non solvent
PA
polyamide
PAN
polyacrylonitrile
PDMS
poly(dimethyl siloxane)
PE
polyethylene
PI
polyimide
POSS
polyhedral oligomeric silsesquioxane
PP
polypropylene
PSF
polysulfone
PSI
pervaporation separation index
PTMSP
poly[1-(trimethyl-silyl)-1-propyne] vii
PV
pervaporation
PVDF
polyvinylidine fluoride
RO
reverse osmosis
SEM
scanning electron microscopy
Si-1
silicalite-1
SRNF
solvent resistant nanofiltration
STEM
scanning transmission electron
TEM
transmission electron microscopy
TEOS
tetraethyl orthosilicate
TFC
thin film composite
TPAOH
tetrapropylammonium hydroxide
UF
ultrafiltration
XRD
X-ray diffraction
ZIF
zinc-imidazole framework
viii
List of Symbols
α
selectivity [-]
β
separation factor [-]
ε
membrane porosity [-]
θ
angle [-]
ρ
density [kg/m3]
λ
wavelength [Ao]
υ
kinematic velocity [m2/s]
A
membrane area [m2]
dh
hydraulic diameter [m]
din
inner diameter [-]
dout
outer diameter [-]
DE
effective thickness [-]
DN
nominal thickness [-]
J
flux [kg m-2 h-1]
l
length [m]
ɭ
thickness [µm]
Lp
membrane permeability [l m-2 h-1 bar-1]
Mw
molecular weight [-]
m
mass [kg]
P
permeance [-]
∆p
pressure [bar]
Re
Reynolds number [-]
T
temperature [oC]
t
time [h]
V
volume [L]
W
dry weight [g]
xA
weight fraction of ethanol in the feed or retentate [-]
xB
weight fraction of water in the feed or retentate [-]
ix
Table of contents ACKNOWLDGEMENTS…………………………………………………………………....i ABSTRACT………………………………………………………………………………….iii SAMENVATTING……………………………………………………………………...........v List of abbreviations…………………………………………………………………..........vii List of Symbols……………………………………………………………………………….ix Table of contents………………………………………………………………………...........x Chapter 1: General Introduction…………………………………………………………....1 1.1. Importance of biofuels…………………………………………………………………...2 1.2. Membrane technology…………………………………………………………………...4 1.3. Pervaporation………………………………………………………………………….....6 1.3.1. Historical Overview……………………………………………………………..6 1.3.2. Definition………………………………………………………………………..7 1.3.3. Mechanism of mass transport…………………………………………………...7 1.3.4. Separation performance of the membranes……………………………………..8 1.4. Applications…………………………………………………………………………......10 1.4.1. Dehydration of organic solvents using hydrophilic membranes………………10 1.4.2. Removal of dilute organics from water with hydrophobic membranes……….10 1.4.3. Separation of organic-organic mixtures with organoselective membranes……10 1.5. Pervaporation Membranes…………………………………………………………….11 1.5.1. Polydimethylsiloxane (PDMS) based membranes…………………………….12 1.5.2. Poly[1- (trimethylsilyl)-1-propyne] (PTMSP) based membranes…………….12 1.6. Mixed matrix membranes……………………………………………………………...13 1.6.1. Porous inorganic fillers………………………………………………………...14 1.6.1.1. Zeolites……………………………………………………………....14 1.6.2. Hollow fillers..…………………………………………………………............15 1.6.3. Fillers with core-shell morphology…………………………………………….16 1.6.3.1. MOF-silica composites………………………………………………16 1.6.4. Zinc-imidazole framework (ZIF)………………………………………………16 1.6.5. POSS/ Octameric silicate cubes / Caged silicate species……………………....17 x
1.7. Composite membranes………………………………………………………………....18 1.7.1. Role of support in composite membrane……………………………………....19 1.7.2. Effect of precrosslinking the PDMS coating solution………………………... 19 1.7.3. Coating techniques of the selective layer………………………………….......20 1.8. Literature review……………………………………………………………………….21 1.9. Scope of this thesis……………………………………………………………………...24 References…………………………………………………………………………………...25 Chapter 2: PDMS mixed matrix membranes containing hollow silicalite sphere for ethanol / water separation by pervaporation………………………………………………………..............41 Abstract……………………………………………………………………………..43 1. Introduction…………………………………………………………………….44 2. Experimental…………………………………………………………………...46 2.1. Chemicals…………………………………………………………………...46 2.2. HS synthesis………………………………………………………………...46 2.3. Synthesis of Silicalite-1……………………………………………………..46 2.4. Preparation of mixed matrix membranes…………………………………...46 2.5. Pervaporation……………………………………………………………….47 2.6. Sorption measurements……………………………………………………..48 3. Characterization………………………………………………………………..49 3.1. HS characterization………………………………………………………....49 3.1.1. Scanning electron microscopy (SEM)………………………………....49 3.1.2. Transmission electron microscopy (TEM)…………………………….49 3.1.3. N2 physisorption……………………………………………………….49 3.1.4. X-ray diffraction (XRD)……………………………………………….49 3.2. Membrane characterization………………………………………………....49 4. Results and Discussion………………………………………………………....50 4.1. HS characterization………………………………………………………....50 4.2. Membrane characterization………………………………………………....52 4.3. Membrane swelling………………………………………………………....54 4.4. Pervaporation……………………………………………………………….55 xi
4.5. Comparison with literature………………………………………….............56 5. Conclusion………………………………………………………………............58 References……………………………………………………………………………............59 Chapter 3: PDMS membranes containing ZIF-coated mesoporous silica spheres for efficient bioethanol recovery via pervaporation…………….............................................................65 Abstract…………………………………………………………………………......67 1. Introduction………………………………………………………………….....68 2. Experimental……………………………………………………………...........70 2.1. Chemicals……………………………………………………………...........70 2.2. Mesoporous silica sphere (MSS) synthesis………………………………....70 2.3. The preparation of MSS-ZIFs sphere……………………………………….70 2.4. Preparation of mixed matrix membranes…………………………...............71 2.5. Pervaporation………………………………………………………….........71 2.6. Sorption measurements…………………………………………………......71 3. Characterization……………………………………………………………......72 3.1. Scanning electron microscopy (SEM)………………………………...........72 3.2. Transmission electron microscopy (TEM)…………………………………72 3.3. N2 physisorption analysis…………………………………………………...72 3.4. X-ray diffraction (XRD)…………………………………………………....72 3.5. Fourier Transform Infra-red spectroscopy (FT-IR)………………………...72 4. Results and Discussion………………………………………………………....73 4.1. MSS-ZIF characterization………………………………………………......73 4.2. Membrane characterization………………………………………………....77 4.3. Membrane swelling………………………………………………………....78 4.4. Pervaporation…………………………………………………………….....79 4.5. Comparison with literature……………………………………………….....80 5. Conclusion……………………………………………………………………....82 References…………………………………………………………………………………....83 Chapter 4: PDMS mixed matrix membranes filled with novel PSS-2 nanoparticles for ethanol / water separation by pervaporation ……………………………………………………................89 xii
Abstract…………………………………………………………………………......91 1. Introduction…………………………………………………………………….92 2. Experimental…………………………………………………………………...93 2.1. Chemicals……………………………………………………………………….93 2.2. PSS-2 synthesis……………………………………………………………........93 2.3. Preparation of mixed matrix membranes………………………………….........93 2.4. Pervaporation…………………………………………………………………..93 2.5. Sorption measurements……………………………………………………........93 3. Characterization…………………………………………………………….....94 3.1. PSS-2 characterization………………………………………………...........94 3.1.1. Scanning electron microscopy (SEM)……………………………........94 3.1.2. N2 physisorption……………………………………………………….94 3.1.3. X-ray diffraction (XRD)……………………………………………….94 3.1.4. Thermal characterization………………………………………............94 3.2. Membrane characterization……………………………………………........94 4. Result and Discussion………………………………………………………......95 4.1. PSS-2 characterization……………………………………………………....95 4.2. Membrane characterization……………………………………………........97 4.3. Membrane swelling……………………………………………………........97 4.4. Pervaporation……………………………………………………………….98 5. Conclusion…………………………………………………………………......100 References………………………………………………………………………………......101 Chapter 5: Influence of support layer and PDMS coating conditions on composite membrane performance for ethanol/water separation by pervaporation ………….........................105 Abstract……………………………………………………………………………107 1. Introduction…………………………………………………………………...108 2. Experimental………………………………………………………………….110 2.1. Chemicals………………………………………………………………….110 2.2. Support synthesis………………………………………………………….110 2.3. Support porosity determination……………………………………………111 xiii
2.4. Viscosity measurement of PDMS coating solutions………………………111 2.5. Preparation of composite membranes by dip coating…………………......111 2.6. Pervaporation…………………………………………………..................112 3. Characterization……………………………………………………………....113 3.1. Pure water flux of supports………………………………………………..113 3.2. Scanning Electron Microscopy (SEM)………………………………........113 4. Results and Discussion……………………………………………………......114 4.1. Supports……………………………………………………………………114 4.1.1. Pure water fluxes of supports………………………………………........114 4.1.2. Support porosity……………………………………………………........114 4.1.3. Support surface morphology……………………………………….........115 4.1.4. Morphology of the support cross-sections…………………………........116 4.1.5. Support thickness………………………………………………………..117 4.2. Viscosity of the PDMS coating solutions……………………………………...117 4.3. Pervaporation……………………………………………………………..........118 4.3.1. Influence of support polymer type……………………………………118 4.3.2. Influence of polymer concentration in the support casting solution….118 4.3.3. Influence of the dip time of the supports in the PDMS solution……..121 4.4. Comparison with literature……………………………………………..123 5. Conclusion……………………………………………………………………..125 References…………………………………………………………………………………..126 Chapter 6. General conclusions and future challenges…………………………….........129 6.1. General Conclusions…………………………………………………………....129 6.2. Future challenges……………………………………………………………….132 Appendix……………………………………………………………...................................135 List of publications………………………………………………………………………...136
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CHAPTER 1 General Introduction
1
Chapter 1____________________________________________________________________________ 1.1. Importance of biofuels There is no simple, single solution to the energy challenge the world is facing today. By 2030, the world’s energy demand will be 50 to 60 percent higher than today and fossil fuels will continue to dominate the global energy market. At today’s rate of consumption, these reserves will only last for about 40 (oil and gas) to 100 (coal) years. [1] These trends would lead to increasing prices threatening economic growth, increasing concerns about energy security in the world. Burning fossil fuels is responsible for large emissions of carbon dioxide (CO2) as a by-product during combustion and for releasing pollutants that are naturally present in fossil fuels, such as sulphur and nitrogen. These emissions are related to environmental problems (such as air and water pollution, acid precipitation, ozone depletion, deforestation and biodiversity loss) and intensify the climate change. [2,3] High energy prices and the global warming issue encourage the growth of alternative energy sources such as solar energy, nuclear energy, wind-energy, geothermal energy, biomass-energy etc. Recently, the production of renewable biofuels derived from the biomass has been receiving increased attention. Among the variety of transportation fuels, ethanol accounts for the vast majority of liquid biofuel. (referred as bioethanol) [4,5,6,7] There has been an increasing interest in bio-resource derived ethanol due to many advantages as liquid transportation fuel. Bioethanol blended with gasoline extends crude oil utilization, reduces reliance on oil imports and helps to mitigate increasing oil prices. The higher oxygen content of ethanol results in a relatively cleaner combustion and has long been used as an additive in gasoline to reduce urban smog and other environmental pollution problems. A variety of biomass materials is available for production of liquid biofuels, intentionally grown materials for this purpose and a side product or waste material from another process which includes agricultural residues, fruit and vegetable processing wastes, plant trimmings, pulp and paper sludge, wood chips, cheese whey, and waste paper.[8–14] In the biomass-to-ethanol process, biomass is converted into sugars which are then fermented to ethanol (figure 1).
Figure 1. A typical biomass-to-ethanol process with implementation of a PV process (adapted from [70]).
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Cellulose, hemicellulose, and lignin are the most common components in lignocellulosic biomass. The most common sugars produced from these materials are glucose (C6H12O6) and xylose (C5H10O5) which can be converted fermentatively to ethanol. Standard baker’s yeast strains, genetically engineered species, and other microorganisms can be employed to ferment sugars into ethanol. An efficient process is required in order to convert biomass materials to biofuels such as ethanol or butanol in a cost effective manner at a small scale with a variable feed source composition. Fermentation is an attractive process for producing ethanol from renewable biomass [7]. To improve the productivity of ethanol, it is beneficial to remove the produced ethanol continuously from the fermentation broth. This will help in reducing the inhibitory effect of high ethanol concentration. This approach would also allow a continuous fermentation to be conducted. Selective separation and recovery of these biofuels from other undesired components from the fermentation broth is an essential task in the biorefinery process. Conventionally, recovery of the biofuels from the fermentation broth has been carried out by distillation. On a larger scale, distillation is economically and energetically efficient. However, as the scale of the operation is reduced, the advantages of distillation reduces as compared to the other competing separation technologies. Thus, technologies such as gas stripping, [9,15-18] liquid–liquid extraction, [16-20] vacuum stripping, [17,18,21-23] membrane distillation, [24] vacuum membrane distillation (VMD), [16,25] sorption, [16-18] and pervaporation [26-37] have emerged periodically. The overall purpose is to explore a large variety of potential separation approaches and technologies which might help to reduce the overall bioethanol production cost, and improve the overall techno-economic feasibility of the biorefinery.
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Chapter 1____________________________________________________________________________ 1.2. Membrane technology Membrane technology is an interdisciplinary field which comprises of material synthesis, characterization and modification along with module design and process engineering which makes membrane separation as an integral part of the industrial processes. [38] Membrane technology has been accepted in various industries due to technical achievements occurred in the last few years. Membrane processes has significant advantages such as, no additives, no thermal damage of the final product, easy recovery of the valuable products with less energy consumption and low operational cost. [38,39] Various membrane techniques can be combined with conventional separation techniques, such as distillation and evaporation to develop a hybrid system for a wide range of industrial applications. From the past few decades, membrane separation has been attracting research in the separation technology field. Membrane separation processes are used in molecular separations, controlled release of compounds, membrane reactors and energy storage devices. Membrane processes have been emerged as a primary solution in the water desalination and waste water treatment industries. Membrane processes are leading in the field of fuel cells and hemodialysis. The use of membranes in industrial gas separations and chemical processing is growing. Membrane separation involves separating a feed containing a mixture of two or more components through a semipermeable barrier (membrane) wherein one or more components moves faster than other. “A membrane is a thin layer of natural or synthetic material that covers a surface and is permeable to a certain component in the solution.” [40-42] In general, a feed stream will be separated into a permeate (which passes the membrane) and a retentate (which cannot pass the membrane) under the influence of a driving force. This is schematically represented in figure 2. The various processes are differentiated by the nature of the driving force employed, characteristics of the solute and the mixture to be separated and the type of membrane used. The driving force can be a gradient in pressure, concentration, temperature or electrical potential, which exists over the membrane. Under influence of this force, mass transfer through the membrane occurs.
Figure 2. Illustration of a membrane process.
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The main membrane separation technologies are listed below [40].
Microfiltration (MF) Ultrafiltration (UF) Hyperfiltration (HF) or reverse osmosis (RO) Electro dialysis (ED) Nanofiltration (NF) Gas separation (GS) Pervaporation (PV) Membrane distillation (MD) Separation by liquid membranes (LM)
Advantages of the membrane technology can be summarized as follows:
Continuous process Mild operating conditions Compatible with other separation processes Up-scaling is easy No additives are required Adjustable membrane properties Compact, modular design Lower energy consumption Flexible designing system Clean technology with operational ease Meet various separation demands
The main drawbacks of membrane technology are:
Fouling (deposition of solutes or suspended solids onto the membrane) Limited solvent stability Limited resistance to harsh chemical, thermal and pressure conditions
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Chapter 1____________________________________________________________________________ 1.3. Pervaporation 1.3.1. Historical Overview Separations of hydrocarbon and alcohol mixtures through rubber membranes were first studied by Kahlenberg in 1906. In 1917 Kober introduced the term ‘pervaporation’ in his study on the selective permeation of water from aqueous solutions of albumin and toluene through cellulose nitrate films. The potential of pervaporation was recognized in the concentration of very dilute protein and enzyme solutions with the simultaneous removal of salts and for the concentration of aqueous and aqueous-glycerol solution using cellophane. [43-45] In 1956, Heisler reported the first quantitative study on the separation of water-ethanol mixtures using a cellulose membrane. The first systematic study on the principles and potential of pervaporation was carried out by Binning and co-workers in 1965 at American Oil Company (Amoco) and by Néel and coworkers in Nancy (France). [46] However, due to the low permeation rate and selectivity of the membranes, pervaporation was unsuccessful for commercial use. In the 1970s, development of high permeation asymmetric membranes by Loeb and Sourirajan transformed the potential of pervaporation as there membrane types could be applied as support material for the PV-membranes. A major breakthrough in pervaporation came in the early 1980s with the development of thin film polyvinylalcohol-polyacrylonitrile (PVA/PAN) composite membranes by GFT (Gesellschaft fur Trenntechnik GmBH, since 1997 Sulzer Chemtech). In 1980 and 1990, more than 50 plants with large capacities (ranging from 2,000 to 15,000 liters per day) were installed by GFT in Europe and the USA for the dehydration of ethanol and other organic solvents. In 2000, inorganic NaA zeolite membranes were introduced and installed in a large-scale plant for the dewatering of ethanol. [47] Hence, industrial pervaporation was mainly focused on the dehydration of organic solvents using hydrophilic membranes. Meanwhile, commercial organophilic membranes (polydimethylsiloxane, PDMS) were also developed, which enabled the extraction of organic compounds from aqueous streams. [48-50] Spiral-wound PDMS membranes were used in California, USA (1993) for the removal of trichloroethylene from groundwater. Membrane Technology and Research (MTR, USA) established a few systems for the extraction of volatile organic components from water. [45] A third new type of membrane (i.e. organoselective membrane) was developed for the separation of purely organic mixtures by the end of the 20th century. In Europe, these membranes were aimed for the enrichment of ethyl tert-butyl ether (ETBE), used as a fuel octane enhancer to replace formerly applied lead derivatives. This led to the commercialization of the first organoselective membrane (PERVAP® 2256) for the removal of methanol or ethanol from purely organic mixtures. [44,45,51] Currently, dehydration of azeotropic, close-boiling or heat-sensitive organic mixtures are the most important industrial applications. [45,47,50,52,53] Research indicates that it is still important to further improve the pervaporation technology in general, but especially for difficult organic-organic separations for the pharmaceutical and (petro)chemical industry. [51]
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1.3.2. Definition “Pervaporation is a membrane separation process wherein the desired component from the liquid feed mixture is partially vaporized and selectively permeated through a dense membrane.” [50,54] In pervaporation, the feed (upstream side) is kept at atmospheric or slightly elevated pressure and the permeate (downstream) is recovered in the vapor form by means of a vacuum or an sweep gas. The permeating component undergoes a phase transition from liquid to vapor during transport and the permeate is collected as a liquid, after condensation of these vapors. Figure 3 presents schematically the fundamental modules of the pervaporation process.
Figure 3. Schematic representation of pervaporation modules: (a) dead-end and (b) crossflow. 1.3.3. Mechanism of mass transport The solution-diffusion model developed by Graham was introduced to pervaporation by Binning et al. [46]. According to this model, the membrane is considered to be nonporous and transport occurs by diffusion. Transport of a component from the feed solution through the membrane takes place in 3 steps: (1) sorption onto the membrane, (2) diffusion through the membrane and (3) desorption from the membrane. The diffusion in the solution-diffusion model is commonly described by either the Fick model or the Maxwell-Stefan approach. [55] The driving force is a difference in chemical potential (due to a partial pressure or activity difference) between feed and permeate side. Since desorption is considered to be a fast step [37], it is usually not taken into account. Figure 4 schematically presents the solution-diffusion mechanism. In this model, the permeability is considered as the amount of penetrant sorbed by the membrane under equilibrium conditions, whereas the diffusivity is the speed of a penetrant moving through the membrane. The penetrant’s diffusivity depends on its geometry (shape and size), hence any increase in molecular size reduces the diffusion. Transport through the membrane involves interactions between permeating components, as well as between the components and the membrane material. Hence, the ideal solution-diffusion approach can experience some deviations due to these coupling and dragging effects. [56-59]
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Chapter 1____________________________________________________________________________
Figure 4: Transport phenomenon in pervaporation. 1.3.4. Separation performance of the membranes [54] The separation performance of a membrane is generally determined by interactions between membrane and feed components. The actual separation is mostly achieved because of differences in sorption and diffusion of the respective components in and through the membrane. The permeate is concentrated with the preferentially permeating component, while the retentate is enriched by non-permeating components. The performance of pervaporation membranes is typically described in terms of permeate fluxes (J) and separation factors (β). The total permeate flux (J) can be determined by equation (1) where (m) is the total amount of material that flows through a unit area of membrane (A) per unit time (t). J is generally expressed in kg m-2 h-1. 𝑚
J=
𝐴.𝑡
(1)
The membrane separation factor (β) is the ratio of the molar component concentrations in the fluids on either side of the membrane. It can be determined from equation 2. x ( A)
β=
xB permeate x ( A) xB feed
(2)
Where x is the weight fraction. In a bioethanol pervaporation with a hydrophobic membrane, xA represents ethanol (the preferential component) and xB then stands for water. Fluxes and separation factors are function of intrinsic membrane properties which changes with the experimental conditions (feed concentration, permeate pressure, feed temperature). Thus, pervaporation performance of the membranes can be describe more effectively by replacing flux by permeability (P) and separation factor by selectivity (α).
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Membrane permeability (Pi) is a component flux, normalized for membrane thickness and driving force which can be determined from equation (3), 𝑃𝑖 = 𝑗𝑖 𝑝
ℓ 𝑖 𝑜 −𝑝𝑖 ℓ
(3)
Where, ji is component flux, ℓ is the membrane thickness, pio and piℓ are the partial pressures of the component i on either side of the membrane. Permeability (P) is most commonly reported as Barrer (1 Barrer 1×10−10 cm3(STP) cm/cm2 s cm Hg). Membrane selectivity (αij) is the ratio of the permeabilities which can be determined from equation (4), 𝑃
𝛼𝑖𝑗 = 𝑃 𝑖
𝑗
(4)
Where Pi and Pj are the permeabilities of the components i and j respectively. Frequently, membrane separation factors are called membrane selectivities and are given the symbol α instead of β which often leads to the confusion.
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Chapter 1____________________________________________________________________________ 1.4. Applications It is briefly mentioned already in the historical overview that, there are three viable applications of pervaporation [44,45] 1.4.1. Dehydration of organic solvents using hydrophilic membranes Commonly used solvents in many chemical syntheses are economically and environmentally important, so their recovery is inevitable. [60-62] Pervaporative dehydration is an effective process to recover these compounds when earlier contacted in the process with water. Pervaporative dehydrations of (bio)ethanol and isopropanol are the best-developed separation processes. [63-65] 1.4.2. Removal of dilute organics from water with hydrophobic membranes Organic removal from water is a well-established issue for environmental protection. Crosslinked polydimethylsiloxane (PDMS) membranes have found wide use in this application [66-68], since they show high affinity and low transport resistance for organics, and are very stable. Other rubbery materials were also attempted for the application. Industrial applications of organophilic pervaporation are limited. However, promising results were obtained in the following fields [6979]: Removal of organic traces (chloroform, acetone, phenol, acetic acid, toluene) from ground and drinking water Removal of ethanol from wine and beer Recovery of aroma compounds (esters, alcohols, ketones, aldehydes, amines, ethers) in the food industry Separation of ethanol and butanol from fermentation broth in biotechnology 1.4.3. Separation of organic-organic mixtures with organoselective membranes Owing to their extensive industrial importance, separation of organic mixtures attained a lot of research interest in order to develop an efficient separation method. [80] It is well known that components of these mixtures have similar physicochemical properties and hence it is very difficult to separate them effectively. [51,81]. However, research studies indicate that pervaporation has a great potential for difficult organic-organic separations in the pharmaceutical and (petro)chemical industry. Attempts have been made already, but it is difficult to find a suitable and tailor-made membrane for the specific process. The processes are commonly classified into four categories [51]:
Polar / non-polar: methanol-toluene, methanol-MTBE, ethanol-hexane Aromatic / alicyclic: benzene-cyclohexane Aromatic / aliphatic: toluene-hexane, benzene-heptane Isomers: isomeric xylenes, n- / i-heptane
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1.5. Pervaporation Membranes For pervaporation, dense membranes preferably with an asymmetric or composite structure are required. A thin dense top layer is then applied on a porous sublayer made from the same or a different material. The first layer enables an effective separation while maintaining a high flux. The second sublayer gives the membrane the desired mechanical strength. It can be a single plate, tube, hollow-fiber or honeycomb structure. The major requirement for the sublayer is an open structure to minimize resistance to vapor transport and to avoid capillary condensation. Currently, available membranes on the market are prepared either from polymers, inorganic materials (i.e., ceramics, zeolites, glass, metal) or a combination of both. [38,64,65] Polymeric and ceramic membranes play an important role in order to develop the membrane technology. Polymeric membranes have a tendency to undergo swelling or cracking which can be overcome by different approaches such as grafting, blending, cross-linking, or adding inorganic fillers, viz. zeolites, metal oxide or silica, to optimize their performance [45]. In addition to their good separation characteristics and long lifetime, ceramic materials show very high chemical, thermal and mechanical stability. However, ceramic membranes are unstable in strong acidic or alkaline conditions despite their better resistance to organic solvents. In order to combine advantages of both polymeric and ceramic materials, hybrid membranes were developed which are discussed further in this work. Hence, selecting a good membrane is difficult and requires a sound knowledge of both the application and the different membrane structures. Table 1 presents a brief overview of the membranes materials suitable for selected pervaporation processes. Table 1: Overview of the membrane materials suitable for pervaporation processes.
Application
Dehydration of organic solvents
Removal of diluted organics
Membrane type
Membrane materials
Polymeric
Polyvinylalcohol (PVA), Polyimide (PI), Cellulose and derivatives, Sodium Alginate, Chitosan
Inorganic
Microporous silica, Titania, Zirconia, ZSM-5, Mordenite, A-, X-, Y- and T- type zeolite membranes
Polymeric
Polydimethylsiloxane (PDMS), Polyurethane (PU), Poly (ether-block-amide) (PEBA), Nitrile-butadiene rubber (NBR), Styrene-butadiene rubber (SBR), Supported ionic liquid membranes (SILM)s
Inorganic
Organic-organic separation
Polymeric
Inorganic
Silicalite-1 membranes Polyimide copolymer membranes, Polyetherimide segmented copolymer, Polyimide-copolypheneylenediamine, Blended cellulose acetate and polyphosphonates, Polypropylene (PP), Polyvinylidine fluoride (PVDF), Polymethyl methacrylate (PMMA) Silicalite-1 membranes 11
Chapter 1____________________________________________________________________________ 1.5.1. Polydimethylsiloxane (PDMS) based membranes Many membrane materials have been studied for the purpose of recovering organic compounds, e.g. ethanol from water by pervaporation. Membrane materials having higher selectivities and permeabilities are required for the selective removal of ethanol from the fermentation broth. Water content must be reduced to obtain the fuel-grade ethanol which requires a hydrophobic membrane material. The current benchmark hydrophobic pervaporation membrane material is poly(dimethyl siloxane) [PDMS], often stated as ‘silicone rubber’. PDMS is an elastomeric material which is familiar as the most alcohol-permselective membrane material for the removal of alcohol from aqueous solutions at low alcohol concentrations. The free rotation of the Si-O bond (figure 5) increases the diffusivity in the PDMS membrane. PDMS membranes can be fabricated as hollow fiber, tubular, unsupported sheet, or thin layer supported sheet. In the literature, the reported ethanol–water separation factor for ‘pure’ PDMS membranes ranges from 4.4 to 10.8. The broad range of ethanol–water separation factors for PDMS membranes is due to the different conditions used. This deviation in separation performance arises due to different factors such as, source of the PDMS polymer, the method of casting the film, the cross-link density, the thickness of the selective layer, the porous support material, and the test conditions. [70] Over the years, several companies have manufactured PDMS membranes, such as SolSep (Apeldoorn, Netherlands) [82] Pervatech (Rijssen, Netherlands), [83] Sulzer Chemtech (Neunkirchen, Germany), [84] and Celanese (NC, United States). [85] At present, Membrane Technology and Research, (MTR) (CA, United States) [86] is the leading supplier, manufacturing spiral wound modules out of their supported silicone rubber membranes. PDMS is also a very useful polymer for nanofiltration and gas separation, since it is stable in all organic solvents when it is fully crosslinked. As it has a very low polarity, [87] it is preferably used in apolar solvents such as alkanes and low-polarity alcohols. [54] PDMS is a chemically, thermally and mechanically very stable polymeric material.
Figure 5: Chemical structure of PDMS. 1.5.2. Poly[1- (trimethylsilyl)-1-propyne] (PTMSP) based membranes Exploration of new polymeric materials with better ethanol–water separation performance than PDMS is a topic of interest. Poly[1-(trimethylsilyl)-1-propyne] or ‘PTMSP’ has received significant attention. It is a high free volume polymer displaying a permeability greater than PDMS. [35,88-92] The ethanol-water separation factor for PTMSP has been reported to be higher than that of PDMS, ranging from 9 to 26. [93] However, PTMSP membranes periodically show unstable performance due to declining flux and selectivity. [94] Cross-linked PTMSP shows a better physical stability. [95,96]
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Figure 6: Chemical structure of PTMSP. 1.6. Mixed matrix membranes In-spite of its useful properties, pure PDMS shows limited performance in pervaporation. However, the performance can be improved by incorporating some porous inorganic fillers into the PDMS matrix. These are well known as mixed Matrix Membranes Mixed matrix membranes. In mixed matrix membranes, the fillers are selected on the basis of their surface properties, so that they can interact with the PDMS matrix to induce additional properties. Furthermore, these fillers have a very defined pore structure which increases the membrane selectivity through size exclusion without affecting the flux. The idea of mixed matrix membranes was originated in the field of membrane based gas separation. In 1991, Robeson reported that, the separation performance of the polymeric membranes cannot be enhanced further once they reach to a certain threshold limit. [97] Usually, there exists a trade-off between flux and selectivity where membranes with high selectivities show low permeabilities and the other way round. Ceramic membranes can overcome this threshold limit but their production is rather difficult and expensive, also they are brittle compared to polymer membranes and less versatile in applications. [38] Thus, a membrane can be fabricated by incorporating these ceramic (nano)particles into a polymeric matrix which combines the advantages of both, if the combination of materials is worthy. The polymeric matrix keeps the membrane flexible and reduces the material costs, while the ceramic particles provide a high selectivity and high permeability. mixed matrix membranes can be divided into two categories. In conventional mixed matrix membranes, porous inorganic particles are dispersed in a polymer matrix, to enhance both the selectivity and the permeability of the membrane. In the second category, the inorganic particles may be non-porous and their main objective is to enhance the stability of the membrane (e.g. by crosslinking) or to modify the polymer chain packing in order to obtain a better performance. [98]
Figure 7 : Graphical illustration of a mixed matrix membrane. 13
Chapter 1____________________________________________________________________________ Following guideline could be followed while preparing the mixed matrix membranes. [115, 200] 1. Particles should be well-dispersed and agglomerations disrupted. Sonication is the most useful method. 2. The surface chemistry of particles may need to be altered. Consider acid and/or steam treatment and/or silylation. 3. Avoid moisture when preparing membranes because water may alter the surface chemistry of the particles or interfere with reactions. Particles and/or solvents should be dry prior to use. 4. Pre-crosslinking of the silicone rubber may be advantageous, but is not necessary to achieve the best results-likely dependent on the silicone rubber system used. 5. Higher particle loadings are not always necessary to achieve the best results. 6. Particle loadings as high as 77 wt% are possible, but some systems may be limited to a maximum of 30 wt% due to particle agglomeration and defect formation. 7. The use of the smallest diameter particles intuitively leads to the ability to make the thinnest possible defect-free membranes. However, the tendency of particles to agglomerate (and with it the tendency to form defects and reduce separation performance) is inversely related to particle size. 1.6.1. Porous inorganic fillers 1.6.1.1. Zeolites Zeolites are crystalline, microporous alumino-silicates formed by a continuous network of SiO4 or AlO4. [99] Because of their microporous structure, zeolites act as molecular sieves, i.e. they have a very regular pore structure of molecular dimensions, which enables them to sort molecules selectively based on size exclusion. Moreover, zeolites are also shape-selective. Since a lot of progress has been made in the synthesis methods of zeolites, a very high amount of zeolite structures now exist. A comprehensive catalogue of zeolite framework types is available in the International Zeolite Association (IZA, [100]). In membrane technology, zeolites are often used as a filler in mixed matrix membranes because of their microporosity and molecular sieving properties which enables them to provide a higher selectivity and enhanced membrane fluxes. [98,101-106] Other filler types often used in mixed matrix membranes include amorphous [107] and mesoporous silica [108], carbon black [107], metal-organic frameworks [109], metal nanoparticles [110] and polymer beads. [111]
Figure 8: Zeolite MFI network. 14
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It is possible to create thinner membranes by using smaller zeolites which enhances the membrane flux. [112-118] This can lead to a stronger polymer-zeolite interaction due to their increased surface contact which results in decreased permeability of mixed matrix membranes. [98] Moreover, these fillers can agglomerate within the polymer matrix leading to an inhomogeneous distribution, which can induce defects in the membrane. This is especially problematic at higher filler loadings. [98,115] To overcome these situations, various solutions have been reported, such as using primers, increasing the viscosity of the PDMS solution by prepolymerization, modifying the filler surface, an intensive ultrasonic treatment, or combinations thereof. [98,115] Homogeneity of the PDMS-filler solution can be increased by dispersing the filler aggregates through intensive stirring and ultrasonic treatment. A probe type ultrasonic device is more efficient than an ultrasonic bath. [115] In order to obtain a defect-free PDMS membrane, it is favorable to pre-crosslink the zeolite-PDMS solution. [118] Pre-crosslinking can increase the viscosity of the PDMS solution, thereby slowing down the precipitation and aggregation of the fillers. Moreover, pre-crosslinking can prevent the intrusion of the polymer chains in the pores when fillers with large pores are present. [108] To improve the compatibility between filler and polymer, three important parameters have been identified: a uniform particle dispersion, a high zeolite loading and the zeolite size. [115] During MMM fabrication, some unselective voids might appear due to inadequate interaction between the zeolite surface and the polymer matrix. [98] Possibilities to overcome this issue are either by modifying the zeolite surface or using a primer to enhance the zeolite compatibility with the polymer matrix. [119-132] 1.6.2. Hollow fillers Hollow fillers are a recently developed category of porous inorganic fillers comprising of an open core surrounded by a shell made from the actual material. [133,134] In general, the core part is amorphous mesoporous, while the shell is microporous and crystalline in nature. Hollow fillers can be synthesized by many different techniques, of which self-assembly and layer-by-layer (L-bL) deposition are the most common. [135-138] A template approach using micelles, [139] microemulsion droplets [140] or vesicular structures has been developed. [141] The rich variety of aggregates of amphiphilic surfactants as template can be explored for making hollow silica structures of various curvatures. [142] Zornoza et al. [136] reported for the first time the incorporation of hollow fillers in mixed matrix membranes for the separation of CO2/N2 and other gaseous mixtures. They observed the improvement in the performance of the mixed matrix membranes which was attributed to the spherical shape of hollow fillers leading to excellent dispersion and good filler/polymer interaction. Moreover, the hollow nature of hollow fillers permitted the gas molecules to pass through more easily. Vanherck et al. reported that, hollow fillers can be used as a filler in mixed matrix membranes for nanofiltration. They observed that incorporation of hollow fillers in PDMS matrix increased the flux and retention of the membranes. Additionally, mixed matrix membranes showed improved resistance against swelling in various organic solvents. [135]
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Chapter 1____________________________________________________________________________ 1.6.3. Fillers with core-shell morphology 1.6.3.1. MOF-silica composites Silica nanoparticles and nanostructures have attracted a considerable attention because of their excellent material properties and ability to achieve various nanoscale functions in applications such as catalysis, separation, and drug release. [143-146] Silica nanomaterials can be integrated with MOFs to combine their exceptional properties which can be useful in novel applications. There are two main approaches to achieve these MOF-silica composites. In the first approach, a MOF shell can be grown in-situ within the MOF precursor solution on a pre-formed silica sphere. In the second approach, a silica shell can be coated on the surface of MOFs or microporous MOF particles can be grown throughout the porous silica supports. Compared to the original MOFs, these composites can exhibit enhanced adsorption properties due to the presence of meso and micropores. The properties of these core-shell structures can be easily tailored by varying the organic building blocks and metal centers for specific applications. Seed coating and chemical modification are effective methods for growth of MOFs on the surface of silica spheres. [147-150] The thickness of the MOF shell within the microspheres can be controlled by regulating the amount of MOF precursors and altering the amount of silica spheres. [151] 1.6.4. Zinc-imidazole framework (ZIF) Zeolitic imidazolate frameworks (ZIFs) are porous crystals with extended 3D structures constructed from tetrahedral metal ions (e.g. Zn) bridged by imidazolate linkers (Im). As the M– Im–M angle in ZIF is similar to the Si–O–Si angle (145o) in zeolites, thus it is possible to synthesize a large number of ZIFs with zeolite-type tetrahedral topologies. [152,153] ZIFs exhibit permanent porosity and high thermal and chemical stability, which makes them attractive candidates for many applications such as adsorption, separation and gas storage. During the past decade, more than 90 different ZIF structures have been synthesized which include ZIF-7, ZIF-8, ZIF-22, ZIF-69, ZIF-71 and ZIF-90 etc. [154-161] ZIFs have gained a great attention as fillers for the generation of ZIF based mixed matrix membranes , due to their molecular sieving effect, facile synthesis and good compatibility with polymers. In pervaporation, ZIF-8 and ZIF-71 are the most studied structures because of their hydrophobic nature. [162,163] ZIF-8 in particular has been one of the most extensively studied porous solids. It has a sodalite (SOD) like topology with outstanding thermal stability and hydrophobic character. [164] ZIF-8 has been studied as a potential filler for solvent resistant nanofiltration (SRNF) [165], for gas separation, [166-173] pervaporation, [174] catalysis,[175] and sensing [176], and as fillers [162,177]. Simultaneous spray self-assembly of a ZIF-8–PDMS nanohybrid membrane with an extremely high loading showed excellent biobutanol-selective pervaporation performance. [162] ZIF-8/PBI mixed matrix membranes have been prepared for pervaporation to dehydrate ethanol, isopropanol (IPA) and butanol. [178] Very recently, Yaghi and co-workers synthesized a new 3-D porous MOF using zinc acetate and 4,5-dichloroimidazole (dcIm) called ZIF-71. [179,180] ZIF-71 is a hydrophobic material [181183] and it can be synthesized at room temperature. [26] ZIF-71 has a RHO type topology with small window size. [184] According to Li et al, adding nano-sized ZIF-71 particles to the PDMS 16
____________________________________________________________________________ Chapter 1
membranes significantly improved both flux and separation factor. [163] New mixed matrix membranes were prepared by incorporating ZIF-71 particles into polyether-block-amide (PEBA) for biobutanol recovery from an acetone–butanol–ethanol (ABE) fermentation broth by pervaporation. In addition to an excellent separation, the membrane exhibited a stable performance in the real ABE fermentation broth for more than 100 hours [184,185]
Figure 9 : Topology (a) SOD-ZIF-8; (b) RHO-ZIF-71. 1.6.5. POSS/ Octameric silicate cubes / Caged silicate species Polyhedral oligomeric silsesquioxanes, abbreviated as POSS, are compound with the general formula [RSiO3/2]n, where n = 6–12 and (R= H, OH etc.). POSS is a nanostructured chemical that bridges the gap between ceramic and organic materials. POSS have a cage-like structure, which is often cubic, consisting of hexagonal, octagonal, decagonal, or dodecagonal prisms. POSS is very flexible and can be functionalized with various groups attached to the apex silicon atoms which are suitable for polymerization or grafting them to polymer chains. It provides a good compatibility with the diverse polymer matrices at molecular level by forming chemical/physical interactions. The chemical diversity of POSS technology is very broad and a large number of POSS monomers and polymers are currently available and under development. POSS is easy to use and available in both liquid and solid form. POSS is soluble in most common solvents and can be used as common additives. POSS can be incorporated into nearly all polymer types (glassy, elastomeric, rubbery, semicrystalline and crystalline). The most studied POSS is the octa-POSS, [186,187] its reactive functional groups and nano-size also make it a promising and attractive material. POSS can be readily introduced into a polymer without altering its intrinsic mechanical properties. As an effective nanoparticle filler, the incorporation of POSS into polymers enhances the physical and mechanical properties. These outstanding characteristics make POSS a promising candidate for numerous applications, like catalysis [188], waveguides [189] and lasers. [190] Moreover, POSS have also been investigated as permeable fillers for mixed matrix membranes in gas separation [191-197] and pervaporation. [198,199] An increase in permselectivities was observed for most gas pairs, including He/CH4, CO2/CH4 and O2/N2. Incorporation of POSS nanoparticles significantly improved the performance of the mixed matrix membranes in the separation of ethanol/water mixture by pervaporation. Incorporating POSS particles into the Pebax 2533 polymer enhanced both total flux and separation factor even at low (2 wt%) loading.
17
Chapter 1____________________________________________________________________________ 1.7. Composite membranes In order to develop thin and high-flux membranes, it is advantageous to introduce asymmetricity into the membrane structure. [87] All industrially important membranes are structurally asymmetric. These membranes generally have a thin, dense skin layer supported on a microporous substrate which offers substantially enhanced permeation flux. One of the advantages of composite membranes is that different polymers can be used as the barrier layer and the porous support, which allows a combination of different properties which may not be available in a single material. [201]
Figure 10. Composite membrane structure. Composite membranes are fabricated in two major steps: a. Casting of the porous support b. Deposition of the selective dense layer (barrier) on the surface of the microporous support Several methods have been developed to prepare composite membranes i. Casting of the barrier layer and membrane support separately followed by lamination, ii. Direct coating of a polymer solution onto a support either by dip coating, spin coating or spray coating etc. followed by an appropriate post- treatment. iii. In-situ formation of the barrier layer on a microporous support film e.g. in case of interfacial polymerization method a barrier layer is formed in-situ on the microporus support by reaction of organic phase with aqueous phase. Currently, direct coating of a polymer solution onto a microporous support is widely used for producing composite membranes. It has to be emphasized that the support layer of the composite membrane should be highly porous so that its resistance to mass transport is small. However, the pores must be sufficiently small in order to prevent the coating solution from filling into the pores because the pore sealing will decrease the permeation flux of the resulting membrane. [54] Several parameters affect the coating technique for the preparation of composite membranes, such as (i) The composition, viscosity, and surface tension of the coating solution, (ii) The pretreatment of the support, (iii) The choice of appropriate coating methods (dip coating, spray coating, spin coating etc.) (iv) The drying conditions. 18
____________________________________________________________________________ Chapter 1
1.7.1. Role of support in composite membrane Several studies have demonstrated that the support layer can have significant effects on pervaporation. [200-220] A more detailed discussion is included in the Chapter 5. An important issue in preparing composite PDMS membranes is to control the intrusion of the polymer into the support. In gas separation, the resistance of the interface layer is either described by two resistances in parallel (221,222) or by one overall resistance, which effectively combines the two resistances. (223) Due to this resistance, flux is divided into a flux through the support and a flux through the pores of the support which are filled by the selective polymer layer (pore intrusion). Gudernatsch et al. (201,224) adopted this approach for pervaporation, it has been assumed that there are two different mass transport paths with different resistances through a composite membrane: Path I : selective layer Path II : selective layer
interface layer
pores of support
support material
pores of support
The flow through a composite membrane can be determined by interactions between permeating components and support layer. The mass transport of permeate through the porous support layer is based on convective flow. Hence, a pressure gradient can be observed between the interface layer and the permeate channel, which can influence the driving force of the mass transport: The pressure drop depends on the mass transport through the porous structure. An approach to estimate the influence of this effect is the capillary model. In this model, the porous structure is assumed to consist of similar sized and parallel capillaries. Depending on the pore diameter and the mean free path of the permeate, two different types of flow through these capillaries can be distinguished: 1. Viscous flow, which is dominated by molecular collisions between penetrating molecules 2. Molecular or Knudsen flow, which refers to collisions between the pore walls and the gas molecules. 1.7.2. Effect of precrosslinking the PDMS coating solution The transport of organic molecules through PDMS dense membranes is based on the solubility and diffusivity of the penetrants into the polymer [225-228]. By introducing extra cross-links in the PDMS network, the membrane swelling can be restricted and the diffusivity of the penetrant through the PDMS can be altered. Stafie et al. used poly(acrylonitrile) (PAN)/PDMS membranes prepared at pre-polymer/crosslinker weight ratio of 10/1 (the recommended ratio by the supplier General Electric, The Netherlands). This ratio corresponds to the stoichiometry of the reaction between the pre-polymer of vinyl-type and the cross-linker of hydrosilane-type. The hydrosilylation reaction relies on the ability of the hydrosilane bond of the cross-linker (Si-H) to add across a carbon-carbon double bond that belongs to the prepolymer in the presence of Pt
19
Chapter 1____________________________________________________________________________ catalyst. [229,230] Ideally, the Si-H reacts only with the CH=CH2 groups along the pre-polymer chains, allowing a good control over the cross-links distribution.
Figure 11. The assumed chemical composition of the RTV 615 compounds. Nguyen et al. [231] performed pervaporation of water-ethyl acetate mixtures through PDMS crosslinked membranes at different conditions (pre-polymer/cross-linker ratios and temperatures). They observed a decrease of flux, with the increase in crosslinker/polymer ratio or in crosslinking temperature which partly attributed to the decrease of sorption of the components in the membrane. PDMS coating solution of well-defined properties is necessary to fabricate a defect- free composite membrane. The viscosity of the PDMS solution is important factor in this case and it should be sufficient to achieve good quality coating. Dilute PDMS solutions have very low viscosity and are not suitable to be applied directly as coatings. To achieve a high solution viscosity at low PDMS concentrations it is necessary to pre-crosslink the coating solution [232]. Dutczak et al. [233] studied the SRNF performance of the composite capillary membranes made from alumina support and a selective poly (dimethylsiloxane) (PDMS) top layer. Pre-crosslinking behaviour of PDMS solution was investigated and it was found that viscosity changes as a function of crosslinking reaction time for different silicone systems. Since controlling the viscosity of a coating solution is very important to obtain a good quality coating, it is desirable to have a solution with relatively low PDMS concentration, to obtain thin selective layer, but of relatively high viscosity, to avoid pore intrusion and defects. Thus viscosity of the diluted PDMS solutions can be adjusted and it is independent of the concentration. In fact, a pre-crosslinking procedure enables preparation of carefully tailored PDMS solutions of various concentrations and of any viscosity. 1.7.3. Coating techniques for the selective layer Different coating techniques have been introduced such as dip coating, film casting, coating on declined surfaces, etc. these methods are applied commercially for preparation of different membranes. Differences in the coating methods results in membranes with different separation properties. In order to coat the PDMS solutions on porous supports, different methods such as film casting, spin coating, dip coating or spray techniques have been reported. [234] In “dip-coating”, the top layer is formed by immersing the substrate in an appropriate polymer solution. During dipping, the polymer solution gets accumulated and deposited on the support surface, while in the support withdrawal step, an adhering polymer layer is formed by the drag force exerted by the support during withdrawal from the solution. In this step, a tangential flow of solution against the support affects the membrane formation by sweeping away weakly attached polymer chains. [235] Some studies have investigated the effects of PDMS coating conditions, such as concentration of coating solution, solvent type, and number of coatings on performance of the prepared composite membranes. [236,237].
20
____________________________________________________________________________ Chapter 1
1.8. Literature review Since PDMS is the main benchmark material available for hydrophobic pervaporation, a list of alcohol–water separation data from literature for PDMS-based membranes is assembled in Table 2. The list is divided into composite membranes (1-11) and mixed matrix membranes (12-26) for better understanding. It can be seen that the reported ethanol–water separation factor for PDMS membranes ranges from 2 to 37. The variations in ethanol–water separation factors for PDMS membranes arise from a variety of factors, such as, source of the polymer starting materials (although called ‘PDMS’, there are often differences), method of casting the PDMS film, PDMS cross-linking density, thickness of the selective layer, the use of different types of porous support material (if any), addition and loading of fillers/additives, feed ethanol concentration and the test conditions. In the case of composite PDMS membranes (1-11), both hollow fiber and flat sheet membranes have been used as support. Hollow fibers were prepared by using ceramic materials such as zirconia or alumina, while flat sheet supports were prepared using polymers. Hydrophilic polymers, such as CA, PVA, and hydrophobic polymers, such as PVDF or PSF were used to prepare the supports. In some studies, chemical cross-linking of the support layer was achieved in order to improve the support layer properties. Several groups have investigated the potential of mixed matrix membranes consisting of silicalite1 particles dispersed in PDMS (12-15). The range of ethanol–water separation factors for these membranes varies from 9–37. The performance of these membranes is additionally dependent on the silicalite-1 loading, the size of the particles, the source of silicalite-1, and the membrane casting conditions. Apart from silicalite-1, ZSM-5 is another frequently used filler material to prepare PDMS-based mixed matrix membranes (16-20). Their ethanol–water separation factors vary from 5 to14, mainly depending on the Si/Al ratio. It can be seen that the mixed matrix membranes prepared with ZSM5 generally show lower separation factors than when silicalite-1 is incorporated, as attributed to the increased hydrophilicity of ZSM-5 due to presence of aluminum in the zeolite framework. Other fillers, such as carbon black or MOFs, were also screened in order to improve the separation performance of the membranes (21-26). However, these membranes didn’t show any satisfactory improvement in their performance.
Table 2. Overview of the pervaporation performance of the PDMS-based membranes for the separation of ethanol–water mixtures.
21
Chapter 1____________________________________________________________________________ Entry No.
PDMS source
Filler
Filler loading (wt%)
Thickness of PDMS layer (µ)
Feed ethanol conc. (wt%)
Support
Permeate pressure [Pa]
Temperature [oC]
PV Separation Flux factor [kg/m2h] (β)
Reference
Unfilled membranes
1
α-Ω- PDMS Mn = 80,000.
-
-
6
PSF
5
260
40
4-0.6
1.79-5.2
238
2
α-Ω- PDMS Mn = 5000
-
-
< 10
ZrO2/Al2O3
4-6
460
40-60
5-20
5-8
239
3
RTV 107
-
-
5
PSF/ PA blend
2-5
100
30-40
0.6-1.6
5-11
240
-
-
N.A
PSF
8
100
50
0.5-0.2
1-6
241
-
-
N.A
ZrO2/Al2O3
0.2
400
37
0.6-1
5-6
242
-
-
10
PVDF
5
100
40
0.5
8
243
-
-
35-100
PVDF
10-70
-
RT
8
2-10
244
100
30-50
0.5-0.8
3-2
245
4 5 6
α-Ω-PDMS 2550 Pa. S α-Ω- PDMS Mn= 60,000 α -Ω-PDMS Mn = 69,856
7
α-Ω-PDMS, Mn = 600
8
α-Ω-PDMS
9
α-Ω-PDMS Mn = 60,962
-
-
10
ZrO2/Al2O3
5
500
40
1.6
8
246
10
α- Ω -PDMS Mn = 60,000
-
-
10
ZrO2/Al2O3
5
400
40
0.8
14
247
11
RTV 615
-
-
12.5
Crosslinked PI
3-9
100
40
0.12-0.13
4.6
248
170
50
0.1
20-30
249
-
-
8
CA
3
Mixed matrix membranes 12
RTV 615
Silicalite-1
67
85-150
Unsupported
22
5
____________________________________________________________________________ Chapter 1
13
RTV 615
Nano silicalite-1
30
200
Unsupported
6
300
35
0.08
15.7
250
14
RTV 615
Silicalite-1
30
200
Unsupported
6
300
35
0.35
10
251
15
RTV
Silicalite-1
77
100-200
PEI
5
-
22
0.15
35
252
16
vinyl-PDMS PMHSDMS
ZSM-5 (Si/Al)=137
65
85
PVDF
5
500
50-70
0.2
37
253
17
RTV 615
ZSM-5 (Si/Al)=280
60
100 / 5
PVA
5
300
50
0.4
39
254
18
RTV 615
ZSM-5 (Si/Al)=275
30
200
Unsupported
6
300
35
0.25
5
251
19
RTV 615
ZSM-5 (Si/Al)=280
60
150-300
Unsupported
5
300
50
0.1-0.3
23-13
255
20
RTV 107
ZSM-5 (Si/Al)=300
20
N.A
CA
10
-
40
0.35
14.1
256
21
RTV 615
Zeolite Y
30
200
Unsupported
6
300
35
0.75
4.5
251
22
RTV
Zeolite
30
200
Unsupported
10
200
25
0.15
12
257
23
α- Ω -PDMS 10,000 m Pa
Fumed silica
5
18
CA
10
20
40
0.197
11.5
258
24
RTV 615
Carbon black
10
200
Unsupported
6
300
35
0.05
9
259
25
RTV
[CuII2 (bza)4 (pyz]n
3
300
Unsupported
5
-
25
0.02
6.5
260
26
RTV 615
ZIF-71
30
7-15
PVDF
5
100
50
1.3
9.9
261
23
Chapter 1____________________________________________________________________________ 1.9. Scope of this thesis Mixed matrix membranes have demonstrated an improved performance with moderate increase in their production cost which led to an increasing interest in the preparation of mixed matrix membranes. From table 2 it can be seen that, most of the pervaporation studies were carried out by using membranes filled with the more conventional fillers, such as silicalite-1 and ZSM-5. Advantage of using these fillers is their smaller size, which makes it possible to prepare thinner membranes but there is an enhanced possibility of particle agglomeration which could deteriorate the membrane performance and subsequently can’t overcome the traditional flux-separation factor trade off. Eventually, shape of the filler is also an important factor which might help to improve the membrane performance. It is thus necessary to explore new types of fillers with more attractive structures and surface properties not only to enhance separation performances but also to improve filler dispersion during membrane preparation. The goal of this thesis was to synthesize PDMS-based high-flux membranes and to test their performance in the sepration of a 6 wt% aqueous ethanol solutions, as a mimic for the conditions encountered in the biorefinery industries. Two main types of membranes viz. self-supporting mixed matrix and unfilled composite membranes were considered in this work. In the case of mixed matrix membranes, three different types of newly developed inorganic porous fillers were explored and incorporated into the PDMS polymer. Support structure and chemical composition, properties of the PDMS coating solution and its coating conditions were investigated in the case of composite membranes. This thesis thus deals with one of the major problems considered in the preparation of successful membranes, known as the trade-off between flux and selectivity. This thesis is divided into six chapters including this introduction (chapter 1).
Chapter 2 describes the synthesis and performance of hollow sphere filled PDMS membranes. Hollow spheres were designed in order to allow the fast flow of permeating components through its hollow core, while the crystalline shell of silicalite-1 primarily helps in selectively passing ethanol and crosslinking the PDMS matrix, as well in increasing the hydrophobicity of the membranes.
Chapter 3 deals with the synthesis of mesoporous silica spheres coated with ZIF nanocrystals (MSS-ZIF) as a core-shell filler for the PDMS membranes. ZIF-8 and ZIF-71 were investigated as shell forming materials owing to their hydrophobic and crystalline nature.
Chapter 4 describes the synthesis of a new class of silicate species, termed as PSS-2. These PSS2s were incorporated as a filler into the PDMS polymer. The PSS-2s are characterized by a cagelike structure with a size of 3µm.
Chapter 5 covers a detailed study on the fabrication of PDMS-based composite membranes. Supports with similar pore sizes were prepared from 3 different polymers to study the role of support chemistry and structure. PDMS coating solution properties were optimized and an automated dip coating machine was used to coat the PDMS solution with variable dip times.
Finally, general conclusions and future work are discussed in Chapter 6.
24
____________________________________________________________________________ Chapter 1
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CHAPTER 2 PDMS mixed matrix membranes containing hollow silicalite sphere for ethanol / water separation by pervaporation Based on: Parimal V. Naik, Stef Kerkhofs, Johan A. Martens, Ivo F. J. Vankelecom, PDMS mixed matrix membranes containing hollow silicalite sphere for ethanol/water separation by pervaporation, Journal of Membrane Science, 502 (2016), 48-56.
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Chapter 2____________________________________________________________________________ Contributions All experimental work and writing of the article was done by P.V. Naik. SEM analysis of the hollow silicalite spheres was carried out by S. Kerkhofs. The editing of the article text was done by S. Kerkhofs and I. Vankelecom.
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Abstract Separation of ethanol/water mixtures through classical separation methods, like distillation, is very energy consuming. Pervaporation is an alternative membrane separation process with much lower energy demand, but still lacking high performance membranes with sufficient flux-selectivity properties. In this study, polydimethylsiloxane (PDMS) based mixed matrix membranes were developed and their pervaporation performance investigated. The fillers consist of hollow spheres (HS) covered with a shell of silicalite-1 crystals. These HS were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen physisorption and X-ray diffraction (XRD). The spheres were approximately 1 μm in size with a shell thickness of 30 nm. Nitrogen physisorption revealed the micro- and mesoporous nature of the spherical shells, with a BET surface area of over 800 m2/g. The hollow silicalite spheres were then uniformly distributed in the PDMS membrane to increase the membrane permeability since the hollow core of the HS allows very fast flow of the permeating compound. Furthermore, the zeolitic shell improves the ethanol selectivity through its specific pore structure and hydrophobicity, while additional crosslinking of the HS with the PDMS matrix further increases the selectivity of the polymer matrix, thus reaching a separation factor and flux value of 16 and 3.8×10-6 g.m/m2.s (1.4 kg/m2.h) respectively for a 6 % aqueous ethanol solution at 40 oC.
43
Chapter 2____________________________________________________________________________ 1. Introduction The production of renewable fuels has been receiving increased attention due to phase-out of reliable non-renewable fossil fuel sources and their problematic effects on the earth’s climate. Ethanol currently accounts for the majority of liquid biofuel, but a process which can convert biomass materials to ethanol in an efficient, cost and energy efficient manner is still necessary to adopt. [1,2] Most biomass-to-ethanol conversion processes involve the fermentative production of ethanol from sugars released from the biomass. Concentration of ethanol in the fermentation broth can generally range from 1 to 15 wt % depending on biomass source, yeast type and hydrolysis procedures. [3] In order to produce fuel-grade ethanol, the water content must later on be reduced to less than 1-3 wt %. [4] Distillation is the traditional technology for performing the bulk separation of ethanol from these dilute biomass fermentation broths with molecular sieve adsorption used to remove the water left in the final azeotropic mixture. However, distillation is a highly energy intensive technology, thereby opening the door for other technologies such as gas stripping, liquid–liquid extraction, vacuum stripping, membrane distillation and adsorption. Another interesting technique to separate the water/ethanol mixture is via hydrophobic pervaporation. Pervaporation is a membrane separation technique in which a liquid mixture is partially vaporized during transport through a membrane by means of vacuum or sweep gas at the permeate side. [5,6] A lot of research is currently carried out in order to develop membranes that combine high flux with high selectivity. The most common membrane polymer used for hydrophobic pervaporation is polydimethylsiloxane (PDMS), an elastomer which is also widely utilized in gas separation and solvent resistant nanofiltration, because of its chemical stability, high permeability and highly hydrophobic character. [7,8] Much effort has been spent searching for polymeric materials with better ethanol–water separation performance than PDMS. [9-17] Polymeric membranes are limited in their performance due to a general trade-off between flux and selectivity. [18] Inorganic membranes based on hydrophobic zeolites overcome this boundary, but large-scale defect-free preparation is less straightforward. [19] To overcome these limitations, mixed matrix membranes have been introduced, [8,19-21] originally in the field of gas separation. They consist of an organic polymer (bulk phase) and inorganic particles (dispersed phase), having the potential to combine the performance of the inorganic membranes with the processability and low price of polymeric membranes. Zeolites, metal organic frameworks (MOFs) and carbon molecular sieves are the most attractive particles used in mixed matrix membranes, as their very defined pore structure can cause an increase in selectivity. [22-26] Zeolite-Filled PDMS membranes have been developed for nanofiltration in solvents, [27-31] where the porous zeolites reduced swelling of PDMS in these solvents without lowering the intrinsic fluxes. [24,27,28,32,33] The flux through mixed matrix membranes is often limited because a zeolite-filled PDMS top-layer requires a certain minimal thickness, several times the size of the particles, to remain defect-free. The use of smaller zeolite particles is problematic, as it becomes more difficult to get a good particle dispersion. [34] 44
____________________________________________________________________________Chapter 2
In the present work, the pervaporation performance of PDMS-based mixed matrix membranes for separation of ethanol/water mixtures was investigated. Mixed matrix membranes were fabricated by incorporating hollow silicalite spheres (HS) of 1 μm size, covered with a thin shell of microporous silicalite-1 zeolite crystals. Silicalite-1 is a pure silicon oxide polymorph of ZSM-5, a zeolite with MFI framework topology. [35,36] MFI type zeolites are microporous with 3D pore channels of ca. 5.5Å. [37-39] Many different synthesis methods exist to obtain hollow zeolite spheres with a variety of sizes. [40-52] The general aim for adding these porous fillers is to create pathways for molecular transport with a lower mass transfer resistance to increase permeability and to introduce a shell with well-defined pore structure which can discriminate between two permeating molecules, thus simultaneously increasing the membrane selectivity. Previously, hollow silicalite spheres were used as a filler for gas separation [53], while hollow titania spheres were studied as filler in dehydration of isopropanol. [54] In the present work, a simple method of self-assembly was used to synthesize the HS, which is novel and unique within itself. HS synthesized using this method have been studied in solvent resistant nanofiltration (SRNF) already. [,55] However, their potential separation performance for ethanol/water separation using hydrophobic pervaporation has not been studied yet. Incorporation of these hollow zeolitic spheres was expected to increase permeability and simultaneously improve selectivity towards ethanol. The former is achieved through the hollow core of these particles, allowing a fast flow of the permeating compounds while latter can be achieved by a well-defined pore structure of the shell. The mixed matrix membranes with HS can indeed be represented schematically as in Fig.1. The effective thickness of the membrane (DEFF) is larger than the nominal or total thickness (DN) when spheres are solid. For HS, the hollow parts reduce the effective thickness of the filled polymer toplayer. DEFF will then be equal to the sum of the shell thicknesses of the packed hollow particles plus the thickness of the polymer matrix forming that same cross-section (shown as a + b + c in Fig.1) An additional crosslinking effect of the PDMS by the spheres, together with the hydrophobicity and the specific pore structure of the selected zeolite, i.e. silicalite-1[36] provide the higher selectivity. [55]
Fig. 1. HS concept in mixed matrix membranes. 45
Chapter 2____________________________________________________________________________ 2. Experimental 2.1. Chemicals Tetraethylorthosilicate (TEOS), cetyl trimethyl ammonium bromide (CTAB) were purchased from Acros. Tetra propyl ammonium hydroxide 40% (TPAOH) from Alfa Aesar, polydimethylsiloxane (PDMS, RTV-615, comprising two components A and B) from GE silicones (Belgium), while toluene and ethanol were supplied by VWR. 2.2. HS synthesis HS were prepared according to the synthesis procedure described by Vanherck et al. [50] An equal volume of 5 wt% cetyl trimethylammonium bromide (CTAB) in ethanol was added drop-wise to a clear solution of TEOS: TPAOH: H2O (molar concentration of 25:9: 400) whilst stirring vigorously. The resulting solution was transferred to a Teflon liner and sealed inside a stainless steel autoclave. The autoclave was kept in an oven at 90°C under hydrothermal conditions. After 6 days, a white precipitate was formed, which was Buchner filtrated, repeatedly washed with ethanol and dried overnight at 60 oC. The spheres were calcined in a muffle oven for 6 h at 550°C (heating rate 0.5 °C. min-1) before use. The calcinated samples were left to cool slowly at ambient temperature in the calcination oven. 2.3. Synthesis of Silicalite-1 Silicalite-1 fillers were prepared by reacting a clear solution of TEOS:TPAOH:H2O (molar ratio 25:9:400) at 120oC in a Teflon liner, sealed inside a stainless steel autoclave for 24 h. The resulting silicalite-1 suspension was centrifuged and washed repeatedly in distilled water, freeze-dried and calcined in a muffle oven for 5 h at 550 o C (heating rate 1 o C. min -1) The calcinated samples were left to cool slowly at ambient temperature in the calcination oven. 2.4. Preparation of mixed matrix membranes To form the mixed matrix membranes, two components of the PDMS (RTV-615 A and B, prepolymer and cross-linker respectively) were dissolved separately in anhydrous toluene. The HS were dispersed ultrasonically in toluene for 1h to disaggregate clusters. The prepolymer and crosslinker solutions were mixed with the HS dispersion and stirred at 60o C for 4 h in order to partially crosslink the solution to obtain a reasonable viscosity. The filler fraction was expressed as : Filler fraction =
(weight of sphere) × 100 (wt %) (weight of sphere) + (weight of polymer)
The resulting pre-polymerized PDMS solution was poured into a glass Petri dish and covered with a funnel in order to achieve slow evaporation of the solvent. The petri dish was subsequently kept at 110 oC for at least 1h in order to complete the crosslinking. After crosslinking, the membrane was detached from the petri dish and stored in a dust-free environment. Membranes with different
46
____________________________________________________________________________Chapter 2
PDMS:filler ratios were prepared, while PDMS membranes without HS and membranes with dense silicalite-1 zeolite fillers were prepared for comparison. The thickness of each membrane was measured using a Mitutoyo disk micrometer (series 369) at 6 points across the surface and averaged. Thickness of all membranes were in the range of 120 to 200 µm.
Fig. 2. (a) Assumed chemical composition of the RTV 615 compounds and their reaction; (b) assumed chemical crosslinking between surface silanols of HS and the Si-H compound. [24,33,34] 2.5. Pervaporation A cross-flow pervaporation module was used in the experiments, as represented in figure 3. A feed pump was used to circulate the feed through the cells at a speed of 1L/min to avoid concentration polarization and it was confirmed by the Reynolds number of 5780 calculated for the system by using equation (1). 𝑅𝑒 =
𝜌𝜐𝑑ℎ 𝜇
(1)
Where, Re is Reynolds number, ρ is density of the fluid (kg/m3), υ is the kinematic velocity (m2/s), dh is the hydraulic diameter of the pipe (m) and μ dynamic viscosity of the fluid (Pa.s). The temperature was kept constant at 40 oC. During pervaporation, a vacuum (< 1 mbar) was applied on the permeate side to ensure a constant driving force for the separation. The active membrane area was 0.001589 m2. Membranes were left to reach a steady state for at least 12 h. The samples were then collected for 3 hours. Experiments were carried out with 6 wt% aqueous ethanol. This concentration was found to be a good representation of ethanol concentration in the feed used by other researchers in similar separations alcohol/water mixtures. [19,20,25,36] Obtained permeates were collected as a function of time in round bottom glass containers using liquid nitrogen in a dewar flask as a cooling trap. The concentration of ethanol in the permeate samples was determined using a refractometer (ATAGO RX-7000α). Three replicates of each MMM were prepared and measured in parallel. The permeate samples were collected twice after overnight equilibration for each membrane and an average value with standard deviation was calculated out of these 6 observations.
47
Chapter 2____________________________________________________________________________ Thickness normalized flux (J) was calculated (g.m/m2s) using equation (2) with the mass of the sample, m (g); the active membrane area A, (m2); the collection time t, (s) and thickness of the membrane 𝑙 (m). m
J = A.t × 𝑙
(2)
The separation factors of the membranes were calculated as follows (3): x ( A)
β=
xB permeate x ( A) xB feed
(3)
where x is the weight fraction, xA represents the preferential component (ethanol) and xB stands for water.
Fig. 3. Pervaporation cross-flow set-up with 3 membrane cells in series. 2.6. Sorption measurements Sorption of pure liquids in the membranes was determined on membrane strips at room temperature. The membrane strips were dried in the oven at 120 oC before measuring the sorption by immersion in the pure liquids for at least 24h. The amount adsorbed was determined by weight. The membrane surface was wiped quickly before weighing so as to minimize evaporation of the liquids.
48
____________________________________________________________________________Chapter 2
3. Characterization 3.1. HS characterization 3.1.1 Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) micrographs were obtained with a Nova NanoSEM450 (FEI, Eindhoven) operating at 1 kV. Powder samples were dispersed on carbon tape and measured without coating. 3.1.2 Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) was performed on a Philips FEG CM200 operated at 200 kV. The samples were dispersed in ethanol and loaded on 50 nm 300-mesh carbon-coated copper grids. 3.1.3 N2 physisorption Nitrogen physisorption isotherms at -196 °C were recorded on an Autosorb-1 instrument (Quantachrome, USA). The samples were evacuated at 300 °C under vacuum. The specific surface area and pore distribution were determined using the Brunauer–Emmett–Teller (BET) method [54] and Density Functional Theory (DFT) analysis, respectively. The external surface area and micropore volume were determined using the t-plot method. [55] 3.1.4 X-ray diffraction (XRD) Powder XRD was performed with a Philips high throughput STOE stadi P diffractometer (flat plate sample holder, Bragg-Bretano geometry) using Cu Kα1 radiation (λ= 1.5418Ao).
3.2. Membrane characterization 3.2.1 SEM SEM was used to take images of membrane cross-sections (obtained by breaking membranes while submerged in liquid nitrogen). Pictures were acquired at 10.0 kV on a Philips XL 30 FEG-SEM. Samples were mounted onto SEM sample holders and coated with a 1.5-2 nm thick gold layer to reduce sample charging under the electron beam.
49
Chapter 2____________________________________________________________________________ 4. Results and discussion 4.1. HS characterization TEM images (Fig.4) of the particles show the mesoporous core covered with a shell of crystalline silicalite-1 particles. The thickness of the shell is approximately 30 nm.
Fig. 4. TEM micrographs of calcined HS. SEM images (Fig.5) show that the HS are covered entirely with silicalite-1 crystals. The size of the spheres is about 1-2 μ and the size of the silicalite-1 particles is around 180 nm.
Fig. 5. SEM micrographs of calcined HS at different magnitudes. The nitrogen sorption isotherms for HS and silicalite-1 materials are quite different (Fig. 6) and show different hysteresis as well. The isotherm of the conventional silicalite-1 crystals is characteristic for a microporous material with presence of some less pronounced interparticle (meso)porosity at higher relative pressures. These large pores are not material specific, but rather originate from the interparticle porosity due to the presence of interstitial pores between the submicron-size crystallites. [35] The tailing hysteresis during pore desorption at reduced P/P0 can be 50
____________________________________________________________________________Chapter 2
due to the presence of some slit-shaped pores or pore blocking. The silicalite-1 particles have a BET surface area and microporous volume of 577 m2 g-1 and 0.16 ml g-1, respectively. On the other hand, the HS material is hierarchal, containing micropores (inside the silicalite-1 crystals) and a large fraction of mesopores in the range of 2-4 nm (in-between the silicalite-1 crystals). This hierarchal pore distribution is reflected in the slope of the adsorption branch of the isotherm. The HS isotherm also shows some hysteresis, characteristic for slit-shaped pores. These HS have a BET surface area of approximately 840 m2g-1, a micropore volume of 0.11 ml g-1 and mesopores with an average pore size of 3.2 nm. [56] The difference in particle size and morphology is responsible for the difference in the hysteresis. Furthermore, the difference in pore structure is emphasized by the pore size distribution shown as an inset. Using the t-plot method [57], the microporous and total pore volume for both silicalite-1 and the hollow spheres were calculated. These HS exhibit a higher microporous volume compared to the pure silicalite-1 crystals. Moreover, additional larger pores contribute 0.053 ml/g of pore volume for the hollow spheres, whereas for the silicalite-1 material almost the entire pore volume is presented by micropore volume.
Fig. 6. Adsorption (●) and desorption (○) branches of nitrogen physisorption isotherms: (A) hollow spheres with silicalite-1 shell and (B) silicalite-1 sub-micron-sized crystals. Inset: DFT model of the pore size distribution derived from the adsorption branch.
51
Chapter 2____________________________________________________________________________ Figure 7 shows the XRD spectra of pure silicalite-1 and HS, where HS clearly show the presence of crystalline regions, originating from the silicalite-1 shell.[52,57] It is apparent that HS retain some crystallinity, resembling the silicalite-1 structure, however, the intensity of the diffraction peaks is much weaker compared to the pure silicalite-1 material.
Fig. 7. X-ray diffraction patterns of calcined HS and silicalite-1.
4.2. Membrane characterization SEM cross-section images of the prepared mixed matrix membranes show dense and homogenous morphologies (Fig. 8). All micrographs show that HS are well-dispersed in the PDMS rubbery matrix and no bulky agglomeration is observed which indicates a good interaction between the two different materials. As shown in the close-up SEM images, voids or defects were nearly absent at the polymer–zeolite interface, due to a proper choice of the solvent to prepare casting solutions and well optimized synthesis conditions.
52
____________________________________________________________________________Chapter 2
Fig. 8. SEM cross-section micrographs at different magnifications (left to right: higher magnifications) of mixed matrix membranes containing HS at different loadings (a-c 10 wt% loading, d-f 15 wt% loading, g-i20 wt% loading and j-l 30 wt% loading).
53
Chapter 2____________________________________________________________________________ 4.3. Membrane swelling The sorption capacity of a membrane is the sum of the sorption in the filler and in the polymer. Possibly also voids present in the membrane, most commonly observed at the filler-polymer interface, can contribute to this overall measured sorption. Fig. 9 shows the sorption tendency for unfilled and HS-filled PDMS membranes. Incorporation of HS leads to a drastically reduced overall swelling of the membrane by more than 1 order of magnitude. This can be attributed to the crosslinking action of the silicalite-1 shell on the polymer chains. The chemical crosslinking could take place through the reaction between Si-H groups present on the cross-linker and surface silanols present on the outer surface of the HS. [58] The possible reaction mechanism is presented in fig 2 (b). Presumably, a more densified polymer region might have formed around the filler as a consequence of this crosslinking. [24,33,34] In addition, physical crosslinking could be due to three reasons. First, PDMS-chain can sorb on the outer sphere wall through Van der Waals interactions. Second, the mesopores observed in the N2-physisorption experiment (fig. 6) could allow intrusion of PDMS-chains through the shell, further into the hollow part of the HS. PDMS chains could thus get entrapped inside the particle and even partly fill up the HS. Third, PDMS chains could also partly enter the zeolite pores and get entrapped in them. The sorption trend for the silicalite-1 filled membranes was almost similar to that of HS filled membranes.
Fig. 9. Sorption in water, ethanol and the feed mixture for unfilled PDMS membranes (inset) HS and silicalite-1 filled PDMS mixed matrix membranes with different loadings.
54
____________________________________________________________________________Chapter 2
4.4. Pervaporation Figure 10 shows the pervaporation performance of the PDMS membranes filled with the HS and the reference silicalite-1 as a function of filler loading. The incorporation of both filler types (HS and silicalite-1) into the PDMS network causes an important increase in the ethanol selectivity. The positive effect of the incorporation of the HS as compared to the silicalite-1 is especially striking for the flux. This is conform the expectations: the hollow core can provide a fast passage of the compound that passed the shell (i.e. ethanol). The conversion of the x-axis from vol% to wt% is not obvious as the shell thickness is hard to determine exactly. Moreover, it is not sure that the hollow parts would not contain any loose synthesis debris. Calculating with a HS density of 1.39 g/ml (corresponding to a 30 nm shell for a completely empty 135 nm sphere), the 30 wt% HS-loading is converted into a 24 vol% loading, while it corresponds to a 20 wt% loading for silicalite-1 (assuming a 1.76 g/ml density). [36] The selectivities of the mixed matrix membranes with both fillers almost coincide. This is indeed logical, since the extra selectivity upon filler incorporation is attributed to the good PDMS-filler interactions (see 4.3) and the ethanol-selective sorption in the zeolite pores. These effects should not be significantly different for the two filler types.
Fig. 10. Effect of filler loading (a) in weight %, (b) in volume % on the pervaporation performance of PDMS mixed matrix membranes (6 wt% ethanol/water feed, T=40 oC).
55
Chapter 2____________________________________________________________________________ 4.5. Comparison with literature A comparison of the pervaporation performance of PDMS-based mixed matrix membranes containing different fillers reported in the literature and tested under comparable conditions, is given in Table 1. Herein, separation factors and fluxes were all converted to selectivities and permeabilities. This conversion is preferred in order to make a fair comparison of the diverse data sets available in the literature, since separation factor and flux are strongly dependent on the operating conditions. [65] Reported ethanol–water selectivities of the membranes broadly range from 0.2 to 2.8. It should be emphasized that proper comparison with literature is difficult due to variations arising from different preparation conditions, such as source and type of PDMS (although always called ‘PDMS’, there are often significant differences with respect to presence of functional groups or degree of crosslinking), filler type, crystallinity degree of the filler, filler loading, pretreatment of the fillers, post-treatment of the membranes, method of casting the PDMS film, use of support material, thickness of the selective layer, etc. This can all still substantially influence results, despite the above mentioned conversion of all data to selectivities and permeabilities. Several groups have investigated the potential of mixed matrix membranes consisting of MFI-type zeolites particles (i.e. silicalite or ZSM-5) dispersed in PDMS. The ethanol–water selectivities for these membranes range from 0.3 to 2.8. The performance of these membranes is additionally dependent on the particle loading, the size of the particles and the Si/Al ratio of the zeolite (the higher this Si/Al ratio, the more hydrophobic the filler). In principle, only Al-free MFI-structures should be called silicalite-1, while all others should be referred to as ZSM-5, even though this is not rigorously followed in literature. Higher fluxes were generally achieved at higher temperature, as expected from an activated process, and in some cases with higher filler loadings as well. Somehow surprising, it can be seen that mixed matrix membranes prepared with ZSM-5 (hence a more hydrophilic filler) show selectivities comparable to those of silicalite-1 filled membranes. The ref.22-entry clearly shows the best selectivity-permeability combination, possibly due to the very high zeolite loading that was realized, obviously without introduction of defects. The lowest selectivity of 0.2 was observed for mixed matrix membranes consisting of zeolite-Y due to the strongly hydrophilic nature of this filler, as also reflected in the high water permeability. Other fillers, such as carbon black and MOFs, did not manage to improve selectivities as much as silicalite-1 and ZSM-5 upon incorporation. The mixed matrix membranes from this work both show a selectivity larger than 1, proving an improved performance as compared to mere distillation. An overall good comparable performance was obtained with the literature data, but surely not the best possible selectivity/permeability combinations. mixed matrix membranes with a loading of 30 wt% HS showed the most pronounced improvement in performance, but in order to achieve consistently high membrane performance, higher loadings upto 60 wt% may probably be needed. Table 1. Pervaporation performance of the PDMS membranes with different fillers for the separation of ethanol–water mixtures. 56
____________________________________________________________________________Chapter 2 Feed concentration (wt%)
Feed temperature (oC)
Thickness normalized flux (×10-6 g.m/m2.s)
Separation factor (βew)
20
5
22
0.8
30
100
6
35
Silicalite-1
50
100
6
Nano silicalite-1
40
100
Silicalite-1
60
Filler loading (wt%)
Thickness (µm)
Silicalite-1
77
Silicalite-1
Filler
Permeability (Barrer)
Selectivity (αew)
Reference
Ethanol
Water
34
25300
9800
2.5
22
9.7
10
65500
92500
0.7
23
35
1.4
7
7000
15000
0.5
23
6
35
2.2
15.7
19000
17500
1.1
34
100
5
22.5
13.8
16.5
334000
320000
1
66
30
100
6
35
6.9
5
28000
84500
0.3
23
60
250
5
50
6.3
30
81000
39000
2.1
62
60
300
5
50
5.5
39
77000
27000
2.8
63
60
87
5
50
5.1
25
50800
28000
1.8
64
60
90
5
50
4.8
37
58000
23000
2.5
65
Zeolite Y
30
100
6
35
20
4.5
77000
265000
0.3
23
Carbon black
10
100
6
35
1.4
9
8500
14000
0.6
33
3
300
5
25
1.9
6.2
25000
85000
0.3
60
ZSM-5 (Si/Al)= 275 ZSM-5 (Si/Al) =280 ZSM-5 (Si/Al) =280 ZSM-5* (Si/Al) =137 ZSM-5* (Si/Al) =137
[CuII2 (bza)4 (pyz]n ZIF-71
40
5
5
50
1.8
9.9
3700
5300
0.7
61
HS
30
196
6
40
3.9
15.3
51000
40000
1.3
This work
Silicalite-1
30
198
6
40
2.8
14.9
37000
33000
1.1
This work
unfilled PDMS
0
120
6
40
0.9
8.7
10800
32000
0.3
This work
*PDMS supported on PVDF support
57
Chapter 2____________________________________________________________________________ 5. Conclusions Hollow spheres (HS) in mixed matrix membranes can significantly improve the membrane performance in pervaporation. Micron-sized HS covered with silicalite-1 crystals were obtained and well dispersed in the PDMS matrix with loadings up to 30 wt%. A significant increase in the fluxes and ethanol selectivities was observed for the membranes filled with the HS. This flux enhancement could be attributed to the presence of a hollow core allowing fast permeation of the selectively sorbed component. The high selectivity was attributed to the selective sorption of ethanol in the zeolite pores and the good adhesion between the polymer and the filler. The described method for mixed matrix membranes can be adopted for other systems consisting of hollow particles with a selective shell for the preparation of mixed matrix membranes in order to improve membrane performance. Improvements would even become more significant if intrusion of the polymer chains into the hollow part of the spheres could be excluded by for instance partly prepolymerising the PDMS.
58
____________________________________________________________________________Chapter 2
References [1] Shapouri. H, Duffield. J.A, Grabonski. M.S, Estimating the net energy balance of corn ethanol, (1995), US Department of Agriculture, Agricultural Economic Report # 721. [2] Shapouri. H, Duffield. J.A, Wang. M, The energy balance of corn ethanol: an update, (2002), US Department of Agriculture, Agricultural Economic Report # 814. [3] Jeffries. T.W, Schartman. R, Bioconversion of secondary fiber fines to ethanol using counter-current enzymatic saccharification and Co-fermentation, Appl. Biochem. Biotechnol, (1999), 78,(1-3), 435-444. [4] Standard Specification for Denatured Fuel Ethanol for Blending with Gasolines for Use as Automotive Spark-ignition Engine Fuel, (2004), D4806-04a, ASTM International, West Conshohocken, PA. [5] Mulder. M, Basic Principles of membrane technology, ed. M. Mulder, Kulwer academic Publishers, (2004), Dordrecht, 2nd edn, ch, 3, 71 - 156, 474 – 478. [6] Vane. L.M, A review of pervaporation for product recovery from biomass fermentation processes, J. Chem. Technol. Biotechnol, (2005), 80, 603-629. [7] Vandezande. P, Gevers. L.E.M, Vankelecom. I.F.J, Solvent resistant nanofiltration: separating on a molecular level, Chem. Soc. Rev, (2008), 37, 365-405. [8] Basu. S, Khan. A.L, Cano-Odena. A, Liu. C Vankelecom. I. F. J, Membrane-based technologies for biogas separations, Chem. Soc. Rev, (2010), 39, 750–768. [9] Schmidt. S.L, Myers. M.D, Kelley. S.S, McMillan. J.D, Padukone. N, Evaluation of PTMSP membranes in achieving enhanced ethanol removal from fermentations by pervaporation, Appl. Biochem. Biotechnol, (1997), 63–65 (1), 469-482. [10] Lopez-Dehesa. C, Gonzalez-Marcos. J.A, Gonzalez-Velasco. J.R, Pervaporation of 50 wt % ethanol–water mixtures with poly(1-trimethylsilyl-1-propyne) membranes at high temperatures, J. Appl. Polym. Sci, (2007), 103 (5), 2843-2848. [11] Nagase. Y, Sugimoto. K, Takamura. Y, Matsui. K, Introduction of fluoroalkyl group into poly(1-trimethylsilyl-1-propyne) and the improved ethanol permselectivity at pervaporation, J. Appl. Polym. Sci, (1991), 43 (7), 1227-1232. [12] Kang. Y.S, Shin. E.M Jung. B, Kim. J.J, Composite membranes of poly(1-trimethylsilyl1-propyne) and poly(dimethyl siloxane) and their pervaporation properties for ethanol– water mixture, J. Appl. Polym. Sci, (1994), 53 (3), 317-323. [13] Uragami. T, Doi. T, Miyata. T, Control of permselectivity with surface modifications of poly[1-(trimethylsilyl)-1-propyne] membranes, Int. J. Adhes. Adhes, (1999), 19, 405-409. [14] Li. S, Tuan. V.A, Falconer. J.L, Noble. R.D, Properties and separation performance of Ge-ZSM-5 membranes, Micropor. Mesopor. Mat, (2003), 58(2), 137-154 [15] Bowen. T.C, Noble. R.D, Falconer. J.L, Fundamentals and applications of pervaporation through zeolite membranes, J. Membr. Sci, (2004), 245(1-2), 1-33. [16] Lin. X, Kita. H, Okamoto. K, Silicalite membrane preparation, characterization, and separation performance, Ind. Eng. Chem. Res, (2001), 40, 4069-4078.
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Chapter 2____________________________________________________________________________ [17] Matsuda. H, Yanagishita. H, Negishi. H, Kitamoto. D, Ikegami. T, Haraya. K, Nakane. T, Idemoto. Y, Koura. N, Sano. T, Improvement of ethanol selectivity of silicalite membrane in pervaporation by silicone rubber coating, J. Membr. Sci, (2002), 210(2), 433-437. [18] Robeson. L. M, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci, (1991), 62(2), 165-185. [19] Chung. T. S, Jiang. L. Y, Li. Y, Kulprathipanja. S, Mixed matrix membranes () comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci, (2007), 32, 483-507. [20] Zimmerman. C. M, Singh. A, Koros. W. J, Tailoring mixed matrix composite membranes for gas separations, J. Membr. Sci, (1997), 137(1-2), 145-154. [21] Mahajan. R, Vu. D. Q, Koros. W. J, Mixed matrix membrane materials: an answer to the challenges faced by membrane based gas separations today?, J. Chin. Inst. Chem. Engrs, (2002), 33(1), 77-86. [22] Jia. M.D, Pleinemann. K.V, Behling. R.D, Preparation and characterization of thin-film zeolite–PDMS composite membranes, J. Membr. Sci, (1992), 73 (2–3), 119-128. [23] Vankelecom. I.F.J, Depre. D, Beukelaer. S.D, Uytterhoeven. J.B, Influence of Zeolites in PDMS Membranes: Pervaporation of Water/Alcohol Mixtures, J. Phys. Chem,(1995), 99 (35), 13193-13197. [24] Adnadjevid. B, Jovanovid. J, Gajinov. S, Effect of different physicochemical properties of hydrophobic zeolites on the pervaporation properties of PDMS-membranes J. Membr. Sci, (1997), 136, (1–2), 173-179. [25] Tang. X.Y, Wang. R, Xiao. Z.Y, Shi. E, Yang. J, Preparation and pervaporation performances of fumed-silica-filled polydimethylsiloxane–polyamide (PA) composite membranes, J. Appl. Polym. Sci, (2007), 105, (5), 3132-3137. [26] Takamizawa. S, Kachi-Terajima. C, Kohbara. M, Akatsuka T, Jin T, Alcohol-Vapor Inclusion in Single-Crystal Adsorbents [MII2(bza)4(pyz)]n (M=Rh, Cu): Structural Study and Application to Separation Membranes, Chem. Asian. J, (2007), 2 (7), 837-848. [27] Gevers. L. E. M, Aldea. S, Vankelecom. I. F. J, Jacobs P. A, Optimization of a lab-scale method for preparation of composite membranes with a filled dense top-layer, J. Membr. Sci, (2006), 281(1-2), 741-746. [28] Gevers. L. E. M, Vankelecom. I. F. J, Jacobs. P. A, Zeolite filled polydimethylsiloxane (PDMS) as an improved membrane for solvent-resistant nanofiltration (SRNF), Chem. Commun, (2005), 2500-2502. [29] Gevers. L. E. M, Vankelecom. I. F. J, Jacobs. P. A, Solvent-resistant nanofiltration with filled polydimethylsiloxane (PDMS) membranes, J. Membr. Sci, (2006), 278(1-2), 199204. [30] Kazuse. Y, Iwama. A, Chikura. S, (1986),US Pat. 4590098. [31] Bitter. J. G. A, Haan. J. P, Rijkens. H. C, (1998), US Pat. 4748288. [32] Vankelecom. I. F. J, Scheppers. E, Heus. R, Uytterhoeven. J. B, Parameters Influencing Zeolite Incorporation in PDMS Membranes, J. Phys. Chem,(1994), 98(47), 12390-12396. 60
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[33] Vankelecom. I.F.J, De Kinderen. J, Dewitte. B.M, Uytterhoeven. J.B, Incorporation of Hydrophobic Porous Fillers in PDMS Membranes for use in Pervaporation, J. Phys. Chem. B, (1997),101(26), 5182-5185. [34] Moermans. B, De Beuckelaer. W, Vankelecom. I.F.J, Ravishankar R., Martens. J.A, Jacobs P.A, Incorporation of nano-sized zeolites in membranes, Chem. Commun., (2000), 2467 -2468. [35] Ravishankar. R, Kirschhock. C, Schoeman. B. J, Vanoppen. P, Grobet. P. J, Storck. S,| Maier. W. F, Martens. J. A, De Schryver. F. C, Jacobs. P. A, Physicochemical Characterization of silicalite-1 nanophase material, J. Phys. Chem. B, (1998), 102, 26332639. [36] Flanigen. E. M, Bennett. J. M, Grose. R. W, Cohen. J. P, Patton. R. L, Kirchner. R. M Smith. J. V, Silicalite, a new hydrophobic crystalline silica molecular sieve, Nature (1978), 271, 512 – 516. [37] Kokotailo. G.T, Lawton. S.L, Olson. D.H, Meier. W.M, Structure of synthetic zeolite ZSM-5, Nature, (1978), 272, 437-438. [38] Olson. D.H, Kokotailo. G.T, Lawton. S.L, Meier. W.M, Crystal Structure and StructureRelated Properties of ZSM-5, J. Phys. Chem, (1981), 85, 2238-2243. [39] van Koningsveld. H, van Bekkum. H, Jansen. J.C, On the location and disorder of the tetrapropylammonium (TPA) ion in zeolite ZSM-5 with improved framework accuracy, Acta Crystallogr, (1987), B43, 127-132. [40] Vankelecom. I.F.J, Dotremont. C, Morobe. M, Uytterhoeven. J. B Vandecasteele. C, Zeolite-Filled PDMS Membranes. 1. Sorption of Halogenated Hydrocarbons, J. Phys. Chem. B, (1997), 101(12), 2154-2159. [41] Naik. S. P, Chiang. A. S. T, Thompson. R. W Huang. F. C, Formation of Silicalite-1 Hollow Spheres by the Self-assembly of Nanocrystals, Chem. Mater, (2003), 15(3), 787792. [42] Kulak. A, Lee. Y. J, Park. Y. S, Kim. H. S, Lee. G. S Yoon. K. B, Anionic Surfactants as Nanotools for the Alignment of Non-spherical Zeolite Nanocrystals, Adv. Mater, (2002), 14(7), 526-529. [43] Wang. H. T, Huang. L. M, Wang. Z. B, A. Mitra and Y. S. Yan, Hierarchical zeolite structures with designed shape by gel-casting of colloidal nanocrystal suspensions, Chem. Commun, (2001), 1364-1365. [44] Yang. W. L, Wang. X. D, Tang. Y, Wang. Y. J, Ke. C Fu. S. K, Layer-by-layer assembly of nanozeolite based on polymeric microsphere: zeolite coated sphere and hollow zeolite sphere, J. Macromol. Sci. Pure Appl. Chem, (2002), 39(6), 509-526. [45] Wang. X. D, Yang. W. L, Tang. Y, Wang. Y. J, Fu. S. K Gao. Z. Fabrication of hollow zeolite spheres, Chem. Commun, (2000), 2161-2162. [46] Lee. S. J, Shantz. D. F, Modifying zeolite particle morphology and size using water/oil/surfactant mixtures as confined domains for zeolite growth, Chem. Commun, (2004), 6, 680-681. 61
Chapter 2____________________________________________________________________________ [47] Lee. S. J Shantz. D. F, Zeolite Growth in Nonionic Microemulsions: Synthesis of Hierarchically Structured Zeolite Particles, Chem. Mater, (2005), 17 (2), 409–417. [48] Xiong. C. R, Coutinho. D Balkus. K. J, Fabrication of hollow spheres composed of nanosized ZSM-5 crystals via laser ablation, Micropore. Mesopore. Mater, (2005), 86, 1422. [49] Song. W, Kanthasamy. R, Grassian. V. H Larsen. S. C, Hexagonal, hollow, aluminiumcontaining ZSM-5 tubes prepared from mesoporous silica templates, Chem. Commun, (2004), 17, 1920-1921. [50] Dong. A. G, Wang. Y. J, Tang. Y, Wang. D. J, Ren. N, Zhang. Y. H Gao. Z, Hydrothermal Conversion of Solid Silica Beads to Hollow Silicalite-1 Sphere, Chem. Lett, (2003), 32(9) 790-791. [51] Dong. A. G, Ren. N, Yang. W. L, Wang. Y. J, Zhang. Y. H, Wang. D. J, Hu. H. H, Gao. Z Tang. Y, Preparation of Hollow zeolite spheres and three-dimensionally ordered macroporous zeolite monoliths with functionalized interiors, Adv. Funct. Mater, (2003), 13, 943-948. [52] Navascues. N, Tellez. C, Coronas. J, Synthesis and adsorption properties of hollow silicalite-1 spheres, Micropore. Mesopore. Mater, (2008), 112, 561-572. [53] Zornoza. B, Esekhile. O, Koros. W.J, Téllez. C, Coronas. J, Hollow silicalite-1 spherepolymer mixed matrix membranes for gas separation, Sep. Purif. Technol, (2011), 77, 137– 145. [54] Liu. G, Jiang. Z, Wang. Y, Yang. D, Novel Hollow Titania Spheres-Chitosan Hybrid Membranes with High Isopropanol Dehydration Performance, Chem. Eng. Technol. (2013), 36, ( 2), 332–338. [55] Vanherck. K, Aerts. A, Martens. J.A, Vankelecom. I.F.J, Hollow filler based mixed matrix membranes, Chem. Commun, (2010), 46, 2492-2494. [56] Brunauer. S, Emmett. P. H, Teller. E, Adsorption of Gases in Multimolecular Layers, J. Am. Chem. Soc, (1938), 60(2), 309–319. [57] de Boer. J.H, Linsen. B.G, van der Plas. Th, Zondervan. G.J, Studies on pore systems in catalysts: VII. Description of the pore dimensions of carbon blacks by the t method J. Catal, (1965), 4, 649-653. [58] Beck. J. S, Vartuli. J. C, Roth. W. J, Leonowicz. M. E, Kresge. C. T, Schmitt. K. D, Chu. C. TW, Olson. D. H, Sheppard. E. W, McCullen. S. B, Higgins. J. B, Schlenker. J. L, A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates, J. Am. Chem. Soc. (1992), 114, 10834-10843. [59] K. Vanherck, Membranes with improved performance in solvent resistant nanofiltration, (2011), Ph.D. thesis, KU Leuven, Leuven, Belgium. [60] Takamizawa. S, Kachi-Terajima. C, Kohbara. M, Akatsuka. T, Jin. T, Alcohol-vapor inclusion in single-crystal adsorbents [M2II (bza)4(pyz)]n (M=Rh, Cu): structural study and application to separation membranes, Chem. Asian. J, (2007), 2 (7): 837.
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[61] Wee. L.H, Li. Y, Zhang. K, Davit. P, Bordiga. S, Jiang. J, Vankelecom. I.F.J, Martens. J.A, Submicrometer-sized ZIF-71 filled organophilic membranes for improved bioethanol recovery: mechanistic insights by Monte Carlo simulation and FTIR spectroscopy Adv. Funct. Mater, (2015), 25, 516–525. [62] Offeman. R, Ludvik. C. N, Poisoning of mixed matrix membranes by fermentation components in pervaporation of ethanol, J. Membr. Sci, (2011), 367, 288–295. [63] Offeman. R, Ludvik. C. N, A novel method to fabricate high permeance, high selectivity thin-film composite membranes, J. Membr. Sci, (2011), 380, 163– 170. [64] Vane. L.M, Namboodiri. V.V, Meier. R.G, Factors affecting alcohol–water pervaporation performance of hydrophobic zeolite–silicone rubber mixed matrix membranes, J. Membr. Sci, (2010), 364 102–110. [65] Vane. L.M , Namboodiri. V.V, Bowen. T.C, Hydrophobic zeolite–silicone rubber mixed matrix membranes for ethanol–water separation: Effect of zeolite and silicone component selection on pervaporation performance, J. Membr. Sci, (2008), 308 230–241. [66] Hennepe te. H.J.C, Bargeman. D, Mulder. M.H.V, Smolders. C.A, Zeolite–filled silicone rubber membranes part 1. Membrane pervaporation and pervaporation results, J. Membr. Sci, (1987), 35(1) 39–55.
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CHAPTER 3 PDMS membranes containing ZIF-coated mesoporous silica spheres for efficient bioethanol recovery via pervaporation Based on: Parimal V. Naik, Lik H. Wee, Maria Meledina, Stuart Turner, Yanbo Li, Sara Bals, Gustaaf Van Tendeloo, Johan A. Martens, Ivo F. J. Vankelecom, submitted for publication.
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Chapter 3____________________________________________________________________________ Contribution All experimental work was done by P.V. Naik. Writing of the article was done by P.V. Naik and L.H. Wee. M. Meledina, S. Turner, S. Bals, G. Van Tendeloo carried out the TEM analysis of the MSS-ZIF-71 nanoparticles. The editing of the article text was done by L.H. Wee and I. Vankelecom.
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Abstract The design and development of functional micro- and mesostructured composite materials is significantly important for many advanced separation processes. Mesoporous silica cores are attractive materials for the reduction of diffusional limitation, while the microporous zeolitic imidazolate frameworks (ZIFs) outer coating is beneficial in the selective adsorption and separation of a desirable molecule from a mixture. In this work, homogeneous growth of ZIF-71 and ZIF-8 nanocrystals on mesoporous silica spheres (MSS) is reported via control over the crystallization kinetics in dimethylformamide (DMF) mediated secondary growth synthesis. The monodispersed MSS-ZIF-71 and MSS-ZIF-8 spheres have a particle size of 3-4 μm in diameter. These hierarchical MSS-ZIF spheres are uniformly dispersed into a polydimethylsiloxane (PDMS) matrix, as confirmed by SEM, and evaluated in the pervaporation of a water ethanol mixture. The pervaporation results reveal that the MSS-ZIF filled Mixed matrix membranes substantially improve the bioethanol recovery in both flux and separation factor, outperform the unfilled PDMS membranes and the conventional carbon and zeolite filled Mixed matrix membranes. As expected, the mesoporous silica core allows very fast flow of the permeating compound, while the hydrophobic ZIF coating enhances the ethanol selectivity through its specific pore structure and surface chemistry. ZIF-8 mainly has a positive impact on selectivity, while ZIF-71 enhances fluxes more significantly.
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Chapter 3____________________________________________________________________________ 1. Introduction Mixed-matrix membranes (mixed matrix membranes), fabricated by the combination of the favorable features of polymers and particles, have proven to be promising materials for pervaporation aiming at higher selectivities and permeabilities. These mixed matrix membranes are composed of porous inorganic particles such as zeolites, [1,2,3-7] silica’s, [8,9] or carbonaceous particles, [10,11] dispersed in a polymer matrix. However, membranes filled with these particles possess some issues which limits their practical application. The main issue is defect-formation in the membranes during the membrane fabrication and drying, due to (i) incompatibility between the inorganic and polymeric phases and (ii) agglomeration of fillers resulting into an inhomogeneous dispersion within the polymer matrix, especially at higher filler loadings. [12-15] Therefore, further development of new and efficient porous fillers for the fabrication of high-flux, defect-free mixed matrix membranes is required. Metal-organic frameworks (MOFs), a new class of crystalline porous materials constructed from inorganic metal ions and organic linkers have emerged as potential fillers for the fabrication of mixed matrix membranes. [16-25] MOFs offer great advantages over other conventional inorganic porous materials mostly due to: (i) the better compactibility of the polymer chains with MOFs due to their partial organic nature, and (ii) their tunable hydrophobicity by selection of appropriate ligands used in the synthesis. Within the MOF family, zeolitic imidazolate frameworks (ZIFs) are porous crystals with extended 3D structures constructed from tetrahedral metal ions (e.g. Zn) bridged by imidazolate linkers (Im). [26] ZIFs exhibit permanent porosity and high thermal and chemical stability, which makes them attractive candidates for many applications such as separation and gas storage. [26-28] Over the last decade, more than 90 different ZIF-topologies have been reported. [26] Amongst them, ZIF-8 is one of the most extensively studied ZIFs. It has a sodalite (SOD) topology with a small pore window of 3.4 Å. [27] The outstanding thermal stability and hydrophobic characteristic of ZIF-8 are ideally suited as potential filler for applications in mixed matrix membranes solvent resistant nanofiltration (SRNF) [28,29], gas separation, [30-37] and pervaporation. [38-41] For instance, simultaneous spray self-assembly of a ZIF-8-PDMS nanohybrid membrane with high loading (40 wt%) showed excellent biobutanolselective pervaporation performance. [38] ZIF-8/PBI mixed matrix membranes have been prepared for pervaporation to dehydrate ethanol, isopropanol (IPA) and butanol. [39] ZIF-71 is yet another hydrophobic material which possesses a RHO topology with a pore window size of 4.8 Å. [42-45] It has been reported that ZIF-71 is a potential filler to separate bioalcohols from water through pervaporation. [46-49] For instance, ZIF-71 filled polyether-block-amide (PEBA) mixed matrix membranes were reported for biobutanol recovery from an acetone-butanol-ethanol (ABE) fermentation broth by pervaporation. In addition to an excellent separation, the membrane exhibited a stable performance in the real ABE fermentation broth over the course of more than 100 hours. [47] In recent years, micro- and mesostructured composite materials have attracted a great deal of scientific attention as promising candidates for catalysis, [50] photoluminescence, [51] drug delivery, [52] separations, [53] electrode materials [54] and fuel cells [55] owing to their attractive physical characteristics such as lower density, high specific surface area, delivering ability and 68
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high surface permeability. The objective of micro- and mesostructures design is to combine the benefits of a microporous molecular sieve shell possessing a high adsorption capacity and separation power with a mesostructure core which is expected to facilitate diffusion. Mesoporous silica nanoparticles possessing a high specific surface area and a large pore volume in addition to their large mesopores, are suitable candidates as the core material, whereas the microporous hydrophobic ZIFs having high adsorption capacity and molecular sieving property are ideally suited for the outer shell coating. Therefore, the combination of micro- and mesostructured composite material is expected to improve the overall separation process with respect to an improved flux and separation factor. Seed coating and chemical modification are effective methods for MOFs coatings. [56-60] For example, Coronas and co-workers have reported the preparation of MSS-ZIF-8 core-shell spheres via in-situ seeding and secondary growth and used then for the fabrication of mixed matrix membranes for the dehydration of ethanol. [59,61] In the present study, solvent mediated secondary growth synthesis of uniform, homogeneous and monodispersed MSS-ZIF-71 coated spheres is reported. These MSS-ZIF-71 and MSS-ZIF-8 coated are used for the fabrication of PDMS-based mixed matrix membranes and evaluated for the pervaporation of ethanol from water:ethanol mixtures.
Fig. 1. Concept of MSS-ZIF core-shell structures in mixed matrix membranes.
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Chapter 3____________________________________________________________________________ 2. Experimental 2.1 Chemicals Sodium metasilicate, cetyltrimethyl ammonium bromide (CTABr) and 2-methylimidazole were purchased from Acros, while zinc nitrate hexahydrate, hexane, methanol, dimethyl formamide (DMF) and zinc acetate were purchased from Sigma-Aldrich. 4,5-dichloroimidazole was purchased from Alfa Aesar, ethyl acetate from VWR, polydimethylsiloxane (PDMS) RTV-615 from GE silicones (Belgium). 2.2 Mesoporous silica sphere (MSS) synthesis MSS were prepared according to the synthesis procedure described by Sorribas et al. [59] The synthesis mixture has a molar concentration of 1.5Na2SiO3:1CTABr:361H2O:7.4 CH3COOC2H5. CTABr and Na2SiO3 were dissolved in distilled water. Ethyl acetate was then added under stirring for 30s. After mixing the reactants, the solution was kept undisturbed for 5h, followed by heating at 90 °C for 50 h. The resulting white precipitate was filtered, washed with water and ethanol, dried overnight at 60 °C and calcined in a muffle oven at 600 °C for 8h with a heating ramp of 0.5 o C/min. 2.3 The preparation of MSS-ZIFs sphere The synthesis of MSS-ZIF-8 spheres was carried out as reported by Corona and co-workers. [59] The method consists of in-situ seeding and secondary crystal growth. The seeding process is required to obtain MSS with a homogeneous and completely covered ZIF layer. The details of the linker and the solvents used for the ZIF nanocrystal formation are listed in Table. 1. Coating of ZIF-71 nanoparticles on MSS spheres was carried out according to the recipe reported by Wee et al. [49] DMF was used for both seeding and secondary growth synthesis. Reaction condition for MSS-ZIF-71 were optimized by varying different solvent ratios and reaction time. Table 1. ZIF nanoparticle synthesis compositions.
ZIF
Zn source
Imidazole linker
Solvents
Reaction time
ZIF-8
Zn(NO3)2
2-methylimodazole
H2O-seeding H2O/methanol-growth
2h
ZIF-71
Zn(CH3COO)2
4,5dichloroimidazole
DMF for seeding and growth
4h
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Seeding process: A molar ratio of Zn:imidazole= 1:70 was used. 0.1 g of MSS was mixed with 3.78 g of the respective imidazole linker in 13.3 g of solvent. Additionally, another 0.1 g of MSS was mixed with 0.195 g of the respective zinc salt in 1.3 g of solvent. Both suspensions were stirred and sonicated for 10 min. The zinc salt solution was then mixed with the imidazole solution under stirring for 10 min. The product was collected by centrifugation, washed and dried overnight. Secondary growth of MSS-ZIFs spheres: 0.14 g of the seeded MSSs and 1 g of the imidazole linker were dispersed in 20 mL of the respective solvent. Additionally, 0.47 of zinc salt was dissolved in 10 mL of the respective solvent (molar ratio of Zn: imidazole=1:7.6). These two suspensions were stirred and sonicated separately for 10 min followed by vigorous stirring (Table 1). The final product was recovered by centrifugation at 4000 rpm for 5 minutes, washed many times with solvent and dried overnight. 2.4. Preparation of mixed matrix membranes Mixed matrix membranes were prepared as described in the Chapter 2, section 2.4. 2.5. Pervaporation Pervaporation experiments were carried out as described in the Chapter 2, section 2.5. Thickness normalized flux (J) was calculated (kg/m2h) using equation (1): m
J = A.t
(1)
Where, m (kg) mass of the sample, A (m2) active membrane area and t (h) collection time. The separation factors of the membranes were calculated as follows (2): x ( A)
β=
xB permeate x ( A) xB feed
(2)
where x is the weight fraction, xA represents the preferential component (ethanol) and xB stands for water. 2.6. Sorption measurements Sorption measurements were carried out as described in the Chapter 2 section 2.6.
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Chapter 3____________________________________________________________________________ 3. Characterization of MSS-ZIF core-shell spheres and mixed matrix membranes 3.1 Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) micrographs were obtained from a Philips XL 30 FEG-SEM acquired at 10.0 kV. Prior to imaging, the powder samples were dispersed on carbon tape and coated with a 1.5-2 nm thick gold layer to reduce sample charging under the electron beam. For the membrane cross-sectional SEM characterization of the mixed matrix membranes, the membranes were fractured while submerged in liquid nitrogen. 3.2 Transmission electron microscopy (TEM) HAADF-STEM imaging and EDX mapping was carried out for MSS-ZIF nanoparticles on a FEI Tecnai Osiris microscope, operated at 200 kV and equipped with a wide solid angle “super-X” EDX detector. The convergence semi-angle used was 10 mrad, the inner ADF detection angle was 68 mrad. 3.3 N2 physisorption analysis N2 physisorption isotherms of MSS-ZIF nanoparticles were measured using a Micromeritics TriStar II (Micromeritics instrument Corporation, Norcross, Georgia). The measurements were performed at -196 °C and all samples were degassed at 400 °C for 10h under nitrogen flow prior to analysis. The pore diameters (nm) were calculated from the adsorption branches of the nitrogen isotherms using the Barrett–Joyner–Halenda model (BJH). 3.4 X-ray diffraction (XRD) Powder XRD of MSS-ZIF nanoparticles was performed with a Philips high throughput STOE stadi P diffractometer (flat plate sample holder, Bragg-Bretano geometry) using Cu Kα1 radiation (λ= 1.5418Ao). 3.5 Fourier Transform Infra-red spectroscopy (FT-IR) Fourier-transform infrared (FT-IR) spectra of nanoparticles were recorded on a Bruker IFS 66v/s infrared spectrophotometer using KBr pellets
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4. Results and discussion 4.1. MSS-ZIF characterization The SEM image in Fig. 2a shows the calcined MSS. The MSS have an average particle size of 2.2 μm with a rough external surface, which makes it an ideal core template for the coating of the ZIF. Fig. 2b & c shows the MSS-ZIF-8 core-shell spheres. The entire MSS was fully covered with ZIF8 nanocrystals. The average particle size in the ZIF-8 shell was about 170 nm as evidenced from the magnified SEM image (Fig. 2c), reassembling a core-shell sphere of 2.4 µm in diameter. The observed enlarged particle size of the spheres after coating of ZIF-8 confirmed the successful growth of ZIF-8 shell on the surface of the MSS particle upon seeding and secondary growth of the MOF nanocrystals and its reproducibility.
Fig. 2. SEM micrographs of (a) calcined MSS, and (b) &c calcined MSS-ZIF-8.
ZIF-71 is a three-dimensional porous material with accessible cages of 1.68 nm, interconnected through a pore window of 0.48 nm. Recent molecular simulations and experimental results have confirmed the potential of ZIF-71 for the selective adsorption of ethanol from aqueous solution. [45] To improve the pervaporation performance of the mixed matrix membranes, MSS-ZIF-71 core-shell spheres were thus synthesized by a similar method used for the preparation of MSSZIF-8. Unfortunately, preparation of monodispersed and homogeneous MSS-ZIF-71 core-shell spheres was not successful due to the simultaneous bulk crystallization and aggregation of the ZIF71 nanoparticles in methanol, which can be observed in the SEM image in Fig. 3a. The particle size of ZIF-71 was relatively large, in the range of 600-900 nm. This could be attributed to the difference in crystallization kinetics of ZIF-8 and ZIF-71. [49,66-68] ZIF-71 can be synthesized in either methanol or DMF. [45,46] Recently, a simple mixed-solvent approach was reported, focusing on the influence of solvent on the crystallization kinetics of ZIF-71 leading to a finetuning of the ZIF-71 particle size [49]. Thus, in order to obtain homogeneous growth of ZIF-71 nanoparticles on the surface of the MSS, further experiments were performed. The influence of solvent on the secondary growth of the ZIF-71 shell was studied by replacing 1/3 of the methanol volume with DMF. It can be observed from Fig. 3b that some side-crystallization was still present, but the particle size of the bulk ZIF-71 was significantly reduced . At a higher volume of DMF to methanol (2/3=v/v), side aggregation of ZIF-71 was reduced while a more homogeneous growth of ZIF-71 on the MSS spheres was noted, as evidenced from the SEM images in Fig. 3c. With only 73
Chapter 3____________________________________________________________________________ DMF as solvent, smooth and homogeneous monodispersed MMS-ZIF-71 core-shell spheres were obtained without any ZIF-71 side crystallization (Fig. 3d). The ZIF-71 nanocrystals forming the shell layer were much smaller than those crystallized in the bulk, mainly due to the steric hindrance induced by the competitive growth of the ZIF-71 crystals in the shell with respect to the bulk crystallization [69]. By increasing the synthesis time to 4h, homogeneous MSS-ZIF-71 core-shell spheres were obtained having a particle size of about 2.3 µm (Fig. 3e). ZIF-71 nanocrystals with typical morphology were observed. [49,67,70] A prolonged synthesis time (24 h) resulted in a cracked ZIF-71 shell layer, as shown in Fig. 3f. The results suggest that DMF reduces the surface free energy on the outer surface of the MSS allowing a spontaneous self-nucleation followed by controlled growth of ZIF-71 nanocrystals. [71,72]
Fig. 3. SEM micrographs of MSS-ZIF-71 at different magnitude synthesized in different solvent medium (a) Methanol, 2h, (b) Methanol-DMF (2:1), 2h, (c) Methanol-DMF (1:2), 2h, (d) DMF 2h, (e) DMF, 4h, (f) DMF, 24h.
In order to further confirm the coating of ZIF-71 on the surface of MSS spheres, the composite material was characterized by HAADF-STEM and EDX analysis. The HAADF-STEM overview image (Fig. 4a) and the corresponding EDX. elemental maps for Si, O, Zn and CI are shown in Fig. 4b-d. As can be seen in Fig. 4b and e, both zinc (green) and chloride (blue) are uniformly distributed on the surface of MSS spheres. Both Zn and Cl originate from the Zn coordinated 4,5dichloroimidazolate linkers, indicating the presence of ZIF-71 nanoparticles on the surface of MSS.
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Fig. 4. (a) HAADF-STEM overview image showing a MSS-ZIF-71 spheres together with EDX elemental maps for (b) Si and Zn, (c) O and (d) Cl.
Figure 5 shows the adsorption-desorption isotherms of MSS, MSS-ZIF-8 and MSS-ZIF-71. MSS material gave a Type IV [73] isotherm, characteristic for a mesoporous material. The BJH pore size distribution curve of MSS shows a bimodal distribution which suggests the presence of small mesopores (2.2 nm) and larger mesopores (10.5 nm), both are typical for an MSS material. [7376] The N2 adsorption-desorption isotherms of both MSS-ZIF8 and MSS-ZIF71 spheres revealed a high N2 uptake at low relative pressure (P/P0=0.05) indicating the presence of microporosity which is related to the ZIF shells, in addition to the mesoporosity of the MSS core. As expected, the deposition of the microporous ZIF material onto the shells gives rise to a Type I isotherm. The calculated BET specific surface areas of MSS-ZIF-8 and MSS-ZIF-71 were 892 and 804 cm2 g-1, respectively. After coating ZIFs onto the surface of the MSS sphere, it was observed from the BJH pore size distribution that the small mesopores of the MSS disappeared and that the large mesopore system was reduced. This phenomenon is mainly due to the fact that the filled micropores of the shell have blocked the accessibility of the N2 towards the internal pores of the core-shell material. [59,77] A similar phenomenon has also been reported for MSS-ZIF-8 [59] and MSS-zeolite. [77] Nevertheless, a considerable pore volume of the MSS-ZIFs materials was preserved. The total pore volumes of MSS-ZIF-8 and MSS-ZIF-71 materials were 0.58 and 0.35 cm3 g-1, respectively versus 0.74 cm3 g-1 for the MSS material. The presence of both Type I and IV isotherms for MSS-ZIF-8 and MSS-ZIF-71 materials further confirmed the preserved mesostructured silica core and newly formed microstructured ZIF shells, respectively.
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Fig. 5. N2 Physisorption isotherms and BJH pore size distribution (Inset). The XRD patterns of MSS, MSS-ZIF-71, ZIF-71, MSS-ZIF-8 and ZIF-8 are presented in Fig. 6a. The characteristic peaks of ZIF-71 and ZIF-8 are observed in the XRD patterns of MSS-ZIF-71 and MSS-ZIF-8 respectively, which further confirmed the presence of an extra-crystalline phase due to the deposition of ZIF nanocrystals on the MSS shells. [48,59] The FT-IR spectra of MSS, MSS-ZIF-8, MSS-ZIF-71, ZIF-71 and ZIF-8 are presented in Fig. 6b. For MSS, the peak at 1069 cm-1 can be assigned to the Si–O–Si bonds and a broad peak at 3400-3600 cm-1 is due to unreacted surface silanol groups on the outer surface of the MSS. This peak shows a considerable reduction in the case of MSS-ZIF-71 and MSS-ZIF-8. For MSS-ZIF-71, the characteristic peaks appear at 665 cm-1 (C–Cl vibration) and 1050 cm-1 (C–N vibration), while for MSS-ZIF-8, a peak appears at 1145 cm-1 (C–N vibration). These results confirm that the MSS are covered with ZIF-8 and ZIF71 crystals. [48]
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Fig. 6. (a) X-ray diffraction patterns, and (b) FT-IR spectra of the MSS-ZIF spheres and of the constituting compounds as reference.
4.2. Membrane characterization Fig. 7 and 8 show the cross-sectional SEM micrographs of the MSS-ZIF-8 and MSS-ZIF-71 filled mixed matrix membranes at different loadings. The SEM images reveal that the MSS-ZIF coreshell spheres were homogeneously dispersed in the PDMS matrix. No large particle agglomerations were observed over the sample, even at higher SEM magnifications. The results indicate a good compatibility between the filler and the polymer. Some small voids can be observed between the filler and polymer at the higher magnifications. This is possibly due to the removal of residual solvent or fracturing of the membrane samples in liquid nitrogen.
Fig. 7. Cross-sectional SEM micrographs of mixed matrix membranes filled with MSS-ZIF8 core-shell sphere. (a-c) 10 wt%, (d-f) 15 wt%, and (g-I) 20 wt% loading viewed at different magnifications.
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Fig. 8. Cross-sectional SEM micrographs of mixed matrix membranes filled with MSS-ZIF71 core-shell sphere: (a-c) 10 wt%, (d-f) 15 wt %, and (g-I) 20 wt% loading viewed at different magnifications.
4.3. Membrane swelling Fig. 9 shows the sorption tendency for unfilled and MSS or MSS-ZIF-filled PDMS membranes. In the case of water, all membranes showed a very restricted sorption, confirming the hydrophobic nature of the materials. PDMS on its own has a clearly more pronounced affinity for ethanol, as also reflected in the intermediate swelling in the feed solution. When incorporating fillers, all sorption values decrease by at least 2 orders of magnitude, clearly indicating a very strong fillerPDMS interaction, most probably even with a strong intrusion of polymer inside the filler. Among, the fillers, incorporation of uncoated MSS still allows the largest liquid sorption, in line with its larger pore volume (Fig. 5). Nevertheless, it should be mentioned that the sorption values are all very low for the filled membranes, and experimental error thus quite high.
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Fig. 9. Sorption in water, ethanol and the feed mixture for unfilled PDMS membranes (inset) and for MSS, MSS-ZIF filled PDMS mixed matrix membranes with different filler loadings.
4.4. Pervaporation The incorporation of the MSS into the PDMS network significantly improved the membrane flux, but, little effect on the selectivity compared to the pure PDMS membranes. The incorporation of MSS can enhance the sorption capacity as shown in Fig. 10, due to the introduction of extra pore volume. However, the mesopores present in the MSS might lead to a higher permeability not only for ethanol but also for water. As for the MSS-ZIF filled membranes, the increase in ethanol selectivity is very pronounced compared to the unfilled PDMS membrane and much better than the MSS filled membranes. As known from the adsorption isotherms, the bigger pores of the MSS were sealed by the ZIF shell. The hydrophobic nature of ZIF-71 and ZIF-8 shells can prevent water to adsorb and permeate. Meanwhile, higher ethanol adsorption and permeation can result from the higher affinity of ethanol for ZIF-71 and ZIF-8. It can be observed from Fig.10 that the normalized total fluxes for the membranes filled with MSSZIF-71 were higher than the membranes filled with MSS-ZIF-8, while the selectivity is higher for membranes filled with MSS-ZIF-8. This trend can be explained on the base of the pore size of ZIF-8 which is smaller (0.34 nm) [78] than for ZIF-71 (0.48 nm) [79].
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Fig. 10. Effect of MSS-ZIF loading on pervaporation performance of PDMS mixed matrix membranes for 6 wt% ethanol/water feed at 40 oC. 4.5. Comparison with literature Fig. 11 comprises the pervaporation performance of the MSS-ZIF filled PDMS membranes prepared in this work with literature data using different kinds of fillers. It can be observed that the mixed matrix membranes prepared in this work show a better performance. The better performance of the mixed matrix membranes in this work can be correlated with the core-shell morphology of the filler, where the mesoporous core thus increases the overall permeation of the specific component that is preferentially sorbed in the selective microporous ZIF shells.
Fig. 11. Comparison of the pervaporation performance of the MSS-ZIF-8 and MSS-ZIF-71 filled PDMS mixed matrix membranes with literature data (6 wt% ethanol/water at 40 oC; 80
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filler loadings, 10 wt% for carbon black and 30 wt% for all others). Dotted lines show an increasing pervaporation performance for the membranes prepared in this thesis from the reference unfilled PDMS membrane upto membranes with MSSZIF loadings (either ZIF-8 or ZIF-71)-10,15 and 20 wt%. As the MSS-ZIF filler concentration increases upto 20 wt% separation factors systematically increase (denoted by the dotted lines starting from unfilled PDMS to MSSZIF-8-20 and MSSZIF71-20). In addition to the increased separation factors, MSS-ZIF-71 filled membranes also show an increasing flux with increasing filler loadings. It is thus clear that with respect to MSS-ZIF fillers, ZIF-8 is the preferred shell material when a high selectivity (i.e.β= 15) is necessary, while ZIF-71 is the better choice when higher fluxes (upto 1kg/m2h) are desired at a slight expense of selectivity (β= 13).
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Chapter 3____________________________________________________________________________ 5. Conclusions Uniform hierarchical micro- and mesostructures of MSS-ZIF-71 composites were prepared via control over crystallization kinetics in DMF solution. The introduction of dual micro-and mesostructures into the matrix of PDMS-based mixed matrix membranes via MSS-ZIFs composite spheres incorporation significantly improved both the fluxes and separation factors in the pervaporation of water ethanol mixtures, compared to the unfilled and the MSS filled membranes. The mixed matrix membranes filled with MSS-ZIF-71 gave excellent fluxes, while the MSS-ZIF8 systematically gave higher separation factors for similar loadings but at lower fluxes somewhat. The results clearly prove the potential of MSS-ZIFs filled mixed matrix membranes for liquid phase separation processes, such as purification of bioethanol from fermentation broths.
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[54] Diamant. Y, Chappel. S, Chen. G.S, Melamed. O, Zaban. A, Core-shell nanoporous electrode for dye sensitized solar cells: the effect of shell characteristics on the electronic properties of the electrode, Coordination. Chem. Rev. 248, (2004), 1271–1276. [55] Strasser. P, Koh. S, Anniyev. T, Greeley. J, More. K, Yu. C, Liu. Z, Kaya. S, Nordlund. D, Ogasawara. H, Toney. M. F, Nilsson. A, Lattice-strain control of the activity in dealloyed core-shell fuel cell catalysts, Nature Chemistry, 2, (2010), 454–460. [56] Biemmi. E, Scherb. C and Bein. T, Oriented growth of the metal organic framework Cu3(BTC)2(H2O)3·xH2O tunable with functionalized self-assembled monolayers, J. Am. Chem. Soc, 129, (2007), 8054–8055. [57] Huang. A, Bux. H, Steinbach. F Caro. J, Molecular-sieve membrane with hydrogen permselectivity: ZIF-22 in LTA topology prepared with 3-aminopropyltriethoxysilane as covalent linker, Angew. Chem. Int. Ed, 49, (2010), 4958–4961. [58] Tanaka. K, Muraoka. T, Hirayama. D, Ohnish. A, Highly efficient chromatographic resolution of sulfoxides using a new homochiral MOF-silica composite, Chem. Commun, 48, (2012), 8577–8579. [59] Sorribas. S, Zornoza. B, Tellez. C and Coronas. J, Ordered mesoporous silica–(ZIF-8) core-shell spheres, Chem. Commun, 48, (2012), 9388–9390. [60] Jo. C, Lee. H. J Oh. M, One-Pot Synthesis of silica@coordination polymer core–shell microspheres with controlled shell thickness, Adv. Mater, 23, (2011), 1716–1719. [61] Kudasheva. A, Sorribas. S, Zornoza. B, Téllez. C, Pervaporation of water/ethanol mixtures through polyimide based mixed matrix membranes containing ZIF-8, ordered mesoporous silica and ZIF-8-silica core-shell spheres, J. Coronas, J. Chem. Technol. Biotech., 90, (2015), 669–677. [62] Chung. T. S, Jiang. L. Y, Li. Y, Kulprathipanja. S, Mixed matrix membranes comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci, 32, (2007), 483-507. [63] Zimmerman. C. M, Singh. A Koros. W. J, Tailoring mixed matrix composite membranes for gas separations, J. Membr. Sci, 137, (1997), 145-154. [64] Adnadjevic. B. Jovanovic. J. Gajinov. S , Effect of different physicochemical properties of hydrophobic zeolites on the pervaporation properties of PDMS-membranes, J. Membr. Sci., 136, (1997), 173-179. [65] Vankelecom. I.F.J, Dotremont. C, Morobe. M, Uytterhoeven. J. B, Vandecasteele C, Zeolite-Filled PDMS Membranes. 1. Sorption of Halogenated Hydrocarbons, J. Phys. Chem. B, 101, (1997), 2154-2159. [66] Venna. S. R, Jasinski. J. B, Carreon. M. A, Structural Evolution of Zeolitic Imidazolate Framework-8, J. Am. Chem. Soc, 132, (2010), 18030–18033. [67] Cravillon. J, Schröder. C. A, Bux. H, Rothkirch. A, Caro. J, Wiebcke. M, Formate modulated solvothermal synthesis of ZIF-8 investigated using time-resolved in situ X-ray diffraction and scanning electron microscopy, Cryst. Eng. Comm, 14, (2012), 492-498.
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CHAPTER 4 PDMS mixed matrix membranes filled with novel PSS-2 nanoparticles for ethanol / water separation by pervaporation Based on: Parimal V. Naik, Pieter L.H. Verlooy, Sam Smet, Johan A. Martens, Ivo F. J. Vankelecom, Ready for submission.
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Chapter 4____________________________________________________________________________ Contribution All experimental work was done by P.V. Naik. Synthesis and characterization of the PSS-2 nanoparticles was done by P.L.H. Verlooy and S. Smet. Writing of the article was done by P.V. Naik. The editing of the article text was done by P.L.H. Verlooy, S. Smet and I. Vankelecom.
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Abstract A new class of silicate species termed as poly oligosiloxysilicone (PSS-2) was synthesized and characterized by scanning electron microscopy (SEM), nitrogen physisorption, X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The approximate size of these PSS-2 crystals was around 3 µm. Nitrogen physisorption revealed the microporous nature of the particles with a BET surface area of 356.98 m2/g and with a microporous volume of 0,1384 cm³/g. These PSS-2 crystals were then added to the PDMS polymer to fabricate mixed matrix membranes for the separation of ethanol from water. SEM showed a uniform dispersion of the PSS-2 particles in the polymer matrix. An increase in ethanol selectivity and flux was observed compared to unfilled PDMS membrane attributed to the hydrophobic nature, specific pore structure of the filler.
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Chapter 4____________________________________________________________________________ 1. Introduction Recently, mixed matrix membranes consisting of inorganic nanoparticles with definite structural and separation properties have attracted much attention. The combination of the individual properties of polymer and particles creates mixed matrix membranes with synergistic characteristics. These mixed matrix membranes are promising materials for membrane separations such as gas sepration, [1-5] pervaporation [6,7,17], nanofiltration. [8-11] The properties and characteristics of the fillers vary depending on the applications. Common inorganic particles used to fabricate mixed matrix membranes for pervaporation comprise of zeolites [12-22], mesoporous silica [23], carbonaceous particles [24,25] and MOFs [26-31]. These particles enhance the pervaporation performance of the membranes by facilitating the transport of the desired components. However, severe agglomeration of the small particles, affecting their selective function, is still a matter of concern which might result in defective membranes [19,32,33]. Thus, it is important to constantly search for a class of nanoparticles with various chemical and molecular designs to fabricate the ideal mixed matrix membranes. A new class of materials closely related to PDMS (polydimethylsiloxane) was invented recently and termed as polyoligosiloxysilicone (PSS-2). [34] In general, silicones are inert synthetic compounds with a variety of forms and applications. Their quite flexible polymer backbones (or chains) however are undesirable for particular applications. PSS-2 is a member of a new family of silica-based materials where robust silicate oligomers are interconnected by short flexible dimethylsiloxane bridges. A typical PSS-2 synthesis involves a three-step procedure. In a first synthesis step, cyclosilicate hydrate crystals containing cyclosilicate oligomers, organic template and water are formed. In a second step, those cyclosilicate hydrate crystals are dried and in a final step dichlorodimethylsilane is added and short dimethylsiloxane bridges between the cyclosilicate oligomers are formed. These PSS-2 particles possess similar properties of the PDMS polymer (often used in hydrophobic pervaporation) and porous silica as well, which is the base material for most of the filler such as mesoporous silica, zeolites etc. In addition, these PSS-2 crystals show porous and hydrophobic characteristics which are of outmost importance in hydrophobic pervaporation. In the present work, pervaporation performance of the PDMS-based mixed matrix membranes for separation of ethanol/water mixtures was investigated. Mixed matrix membranes were fabricated by incorporating PSS-2 particles of 3 μm size in order to enhance both the flux and the selectivity of the mixed matrix membranes.
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2. Experimental 2.1. Chemicals PSS-2 powder (pre-synthesized) and polydimethylsiloxane (PDMS) RTV-615 was purchased from GE silicones (Belgium). Hexane and ethanol were supplied by VWR. 2.2. PSS-2 synthesis PSS-2 was synthesized by mixing the amino templates with water and TEOS (ratios) in a PP-bottle and stirring everything at room temperature for 21 days. The crystals were recovered by filtration. The procedure for the POSiSil transformation: First the crystals are dried under vacuum. After 3 days of drying, the silane is added (still under reduced pressure) in the gas phase and allowed to react for 3 days. The reaction was stopped by adding ammonia and water. The crystal structure of the LTA-cyclosilicate hydrate crystals is structurally similar to zeolites of LTA topology. In contrast to the LTA zeolites, the silicate octameric cubes in LTA-cyclosilicate hydrates are not connected with each other by direct siloxane bonds but by hydrogen bonds between hydroxyl groups and the silicate octameric cubes. The template molecule inside the pores of the PSS-2 were removed through several wash cycles with water and acetone followed by a calcination in air. (1 °C/min to 400 °C for 1 hour) 2.3. Preparation of mixed matrix membranes Mixed matrix membranes were prepared as described in the Chapter 2, section 2.4. 2.4. Pervaporation Pervaporation experiments were carried out and thickness normalized fluxes (J), separation factors of the membranes (β) were calculated from equation 1 and 2 as described in the Chapter 3, section 2.5. 2.5. Sorption measurements Sorption measurements were carried out as described in the Chapter 2 section 2.6.
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Chapter 4____________________________________________________________________________ 3. Characterization 3.1. PSS-2 characterization 3.1.1. Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) micrographs of PSS-2 were obtained with a Nova NanoSEM 450 (FEI) at 2kV. 3.1.2. N2 physisorption N2 physisorption isotherms were obtained using a Micromeritics TriStar II (Micromeritics instrument Corporation, Norcross, Georgia). The measurements were performed at –196 oC and in order to remove any adsorbed species the PSS-2 sample was outgassed at 300 °C for 2 hours prior to the measurement. The total surface area was calculated using the Brunauer, Emmet and Teller model (BET model). [38] The total pore volume is directly derived from the adsorption isotherm (P/P0 = 0.95), in which P and P0 are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption. The pore diameters (nm) were calculated from the desorption branches of the nitrogen isotherms using the Barrett–Joyner–Halenda model (BJH). [39] This model is based on the Kelvin equation to describe the pore filling through capillary condensation. 3.1.3. X-ray diffraction (XRD) Powder XRD was performed with a Philips high throughput STOE stadi P diffractometer (flat plate sample holder, Bragg-Bretano geometry) using Cu Kα1 radiation (λ= 1.5418Ao) and an image plate detector with a linear range of 70°. 3.1.4. Thermal characterization Thermo gravimetrical analysis was performed on a TGA-Q500 TA instrument using an nitrogen flow and a heating rate of 20 K min-1. 3.2. Membrane characterization 3.2.1. SEM SEM was used to take images of membrane cross-sections (obtained by breaking membranes while submerged in liquid nitrogen). Pictures were acquired at 10.0 kV on a Philips XL 30 FEG-SEM. Samples were mounted onto SEM sample holders and coated with a 1.5-2 nm thick gold layer to reduce sample charging under the electron beam.
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4. Results and discussion 4.1. PSS-2 characterization In order to get some more information about the origin of the meso- and marcopores, SEM images were taken on a novoNova NanoSEM at 2kV ( fig. 1). These SEM images shows particles of up to about 3µm with a large portion of meso- and macro-pores in the form of cracks at the outside of the particles and a sponge-like structure at the inside.
Fig. 1. SEM micrographs of PSS-2 particles. Nitrogen physisorption isotherm of the PSS-2 nanoparticles is shown in fig. 2. A type I adsorption isotherm with H1 hysteresis loop showing the presence of micropores. The BET surface area of the PSS-2 was 356.98 m²/g, with a micropore area of 331.52 m²/g, an external area of 25.46 m²/g, and a micropore volume of 0.138 cm³/g remarkably the hysteresis loop does not close completely. This would indicate that the material changed its porosity slightly during the measurement which could be an indication of a flexible material.
Fig. 2. N2 Physisorption isotherms of PSS-2 particles.
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Chapter 4____________________________________________________________________________ In the X-ray diffraction pattern of PSS-2 particles (fig. 3) a limited amount of especially long range order can be seen. This lack of sharp diffraction peaks is expected for a material with high degree of flexibility. In PSS-2, such flexibility can be expected due to the dimethylsiloxane bridges interconnecting the silicate oligomers.
Fig. 3. X-ray diffraction pattern of PSS-2 particles showing some degree of long range order. As synthesized PSS-2 nanoparticles still contains template. Hence, to obtain a porous material, this template should be removed without destroying the structure and affecting the hydrophobic dimethylsiloxane bridges. The thermogravimetric analysis of PSS-2 nanoparticles shown in fig. 4 with 5 distinct weight losses when the particles were heated up to 750 °C in N2-atmosphere. The weight losses around 50 °C, 130 °C, 260 °C and 550 °C corresponds to the loss of physisorbed water, the removal of template molecules (both 130 and 260 °C) and the removal of the methyl groups on the dimethylsiloxane bridges respectively. Based on this thermogravimetric analysis a calcination temperature of 400 °C was chosen to remove the organic template molecules without affecting the dimethylsiloxane bridges and retaining the overall structure.
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Fig. 4. Thermogravimetric analysis (TGA and DTGA) of PSS-2 particles. 4.2. Membrane characterization SEM cross section images of the prepared mixed matrix membranes (Fig. 5) show well-dispersed PSS-2 particles embedded in the PDMS matrix.
Fig. 5. SEM cross-section micrographs of mixed matrix membranes with 3 different PSS-2 loadings. 4.3. Membrane swelling The sorption capacity of a mixed matrix membrane is the sum of the sorption in the filler and in the polymer. Possibly, also voids present in the membrane, most commonly observed at the fillerpolymer interface, can contribute to this overall measured sorption. Fig. 6 shows the sorption tendency for unfilled and PSS-2-filled PDMS membranes. Incorporation of PSS-2 leads to a drastically reduced overall swelling of the membrane compared to unfilled membranes. It can be seen that water sorption in the PSS-2 filled membranes was almost zero. Sorption of pure ethanol and 6 wt% aqueous ethanol were nearly similar. Thus, it can be assumed that any increase in the selectivity is a direct consequence of the higher sorption of the PSS-2 filled membranes towards ethanol.
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Chapter 4____________________________________________________________________________ Generally, ethanol sorption in the mixed matrix membranes should increase with increasing PSS2 loadings. However, it can be seen that ethanol sorption doesn’t increase but remains fairly constant. A clear explanation of this trend is yet unknown due to limited information about structure and properties of the PSS-2 nanoparticles which need to be study in detail.
Fig. 6. Sorption in water, ethanol and the PV feed mixture for unfilled PDMS membranes (inset) and PDMS mixed matrix membranes (right) with different PSS-2 loadings. 4.4. Pervaporation The incorporation of the PSS-2 particles into the PDMS network caused an important increase in the total flux and at the same time an increased ethanol selectivity as shown in fig. 7. This increasing trend remained intact for loadings up to 20 wt%. From the sorption studies, it is clear that the PSS-2 filled PDMS membranes do not adsorb water, while the ethanol adsorption is quite good. The presence of meso and macro pores in the PSS-2 particles probably explain the increase in the flux of the MMM, while micropores and the highly hydrophobic nature of the material helps in improving the selectivity of the membranes.
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Fig. 7. Effect of PSS-2 loading on the pervaporation performance of PDMS mixed matrix membranes. In order to check the effect of increased filler loading, membranes with high filler loading (>20 wt%) was tried, but the membrane became more brittle, possibly due to more rigidity of the material which limits its loading upto 20 wt%.
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Chapter 4____________________________________________________________________________ 5. Conclusions A new class of material termed as poly oligosiloxysilicone (PSS-2) was prepared. These PSS-2s have a rigid structure and are hydrophobic nature. PDMS membranes filled with PSS-2 particles which showed a notable improvement in the flux and in the ethanol selectivity as well. The increase in the flux could be attributed to the mesopores and marcopores present in the PSS-2 nanoparticles, while presence of micropores and the hydrophobic nature of the particles are most likely to cause for the improved selectivity. PSS-2 filled mixed matrix membranes can significantly improve the performance of PDMS-membranes for pervaporation e.g. purification of bioethanol from fermentation broths.
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CHAPTER 5 Influence of support layer and PDMS coating conditions on composite membrane performance for ethanol/water separation by pervaporation Based on: Parimal V. Naik, Roy Bernstein, Ivo F. J. Vankelecom, submitted for publication.
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Chapter 5____________________________________________________________________________ Contribution All experimental work and the writing of the article was done by P.V. Naik. The editing of the article text was done by R. Bernstein and I. Vankelecom.
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Abstract A systematic study was performed on the combination of support properties and polydimethylsiloxane (PDMS) coating conditions for the lab-scale fabrication of a defect-free, thin film composite membrane for organophilic pervaporation. Supports were prepared from 3 glassy polymers with different chemistries (PVDF, PSF, PI) by non-solvent induced phase inversion. The idea was to obtain supports having similar pore sizes but different chemistries to study the exact role of support chemistry on the deposition of the PDMS coating (i.e. wetting and intrusion) and the final membrane performance (i.e. effect on mass transfer of the permeants). The crosslinking behavior of the PDMS solution was studied by viscosity measurements to optimize the coating procedure with respect to coating layer thickness, support intrusion and wetting. Using automated dip coating, the dip time for coating the PDMS solution on the supports was varied. Scanning electron microscopy (SEM) was used to investigate the composite membrane structure and define the thickness of the deposited PDMS layer. The performance of the synthesized membranes was tested in the separation of ethanol/water mixtures by pervaporation.
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Chapter 5____________________________________________________________________________ 1. Introduction Several studies have demonstrated that the support layer can have significant effects on pervaporation. [1-22] Some studies focused on inorganic materials as support, e.g. ceramic [2022], while polyacrylonitrile (PAN) is one of the most common polymer supports used in pervaporation. [3,4] In the hydrophobic pervaporation of multicomponent mixtures, support materials should be hydrophobic to improve the fluxes and selectivities of the organic compounds, [5] Intrusion of PDMS in pre-treated polysulfone (PSF) and polyvinylidene fluoride (PVDF) support layers changed the normalized fluxes of the composite membrane, while the selectivity remained the same. It was also noted that without support pre-treatment (filling the pores with water), it was not possible to prevent the PDMS intrusion into the support, [14] and that the separation performance was strongly related to the silicone rubber composition i.e. nature of the Si- substituents. [15] Asymmetric cross-linked polyimide (PI) supports were coated with a dense PDMS layer where the skin-layer prevented the intrusion of the PDMS solution into the pores. [6] In PDMS/PSF composite membranes, the evaporation time of the NMP-based solution PSF before immersion in the non-solvent bath proved to influence the mass transfer in pervaporation. [16] PAN supports were found to be less suitable to coat PDMS top-layers due to their high surface roughness. [17] In addition, it was difficult to reduce the pore size of PAN membranes due to the poor solubility of PAN, preventing casting from concentrated solutions. [23] The rather poor mechanical strength of PAN also often led to leaks. Even though many types of PAN polymers and copolymers exist, it is thus not obvious to turn PAN into a good support material for PDMSbased pervaporation membranes. Apart from the good quality of the support, it is essential to prepare an appropriate PDMS coating solution to obtain good quality composite membranes. In general, the viscosity of the PDMS solution should be sufficiently high to make a defect-free coating. For most types of PDMS, it is then necessary to pre-crosslink the coating solutions to achieve the appropriate viscosity. [25] When the coating solution is highly viscous, a thick PDMS layer is obtained, whereas pore intrusion and defects are obtained when not viscous enough. [24,26] It is thus desirable to have a solution with a relatively low PDMS concentration to have the layer thin enough, but with adequate viscosity to prevent pore intrusion and defect creation. In order to coat at lab-scale PDMS solutions on porous supports, different methods such as film casting, spin coating, dip coating or spray techniques have been reported. [27] In “dip-coating”, the top layer is formed by immersing the substrate in an appropriate polymer solution. During dipping, the polymer solution gets accumulated and deposited on the support surface, while in the support withdrawal step, an adhering polymer layer is formed by the drag force exerted by the support during withdrawal from the solution. In this step, a tangential flow of solution against the support affects the membrane formation by sweeping away weakly attached polymer chains. [28] Some studies have investigated the effects of PDMS coating conditions, such as concentration of coating solution, solvent type, and number of coatings on performance of the prepared composite 108
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membranes. [29,30] However, the effect of variation in dip time for coating the PDMS solution has not been reported so far. In this study, PSF, PI and PVDF will be screened as alternative polymeric supports for PAN. Rationale behind choosing these polymers to prepare supports was the difference in their chemical structure, hence surface tension and hydrophilicity: PVDF is hydrophobic, PSF has a moderate hydrophilicity and PI is relatively more hydrophilic than PSF. [31] They all thus have different functional groups to interact with the PDMS. All these polymers are easy to process, readily available and possess good mechanical, thermal, and chemical properties. Moreover, it is generally easy to prepare asymmetric membranes by the phase inversion method from these materials. [3237] Next, an appropriate combination of support and coating conditions was searched for at labscale to fabricate thin, defect-free composite PDMS membranes for pervaporation. Dip time of the support in the PDMS solution as well as pre-crosslinking of the PDMS solution were optimized in order to obtain a thin, defect-free coating solution using an automated dip coating machine.
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Chapter 5____________________________________________________________________________ 2. Experimental 2.1 Chemicals Polysulfone (PSF, UDEL P-1835) and polyvinylidene fluoride (PVDF, Solef 6010) were kindly supplied by Solvay (Belgium), Polyimide (PI, Matrimid® 9725) was kindly provided by Huntsman (Switzerland). All polymers were dried in an oven overnight at 110 oC. Polydimethylsiloxane (PDMS, RTV-615, comprising two components A and B) was obtained from GE silicones (Belgium), N-methyl pyrrolidinone (NMP, 99%) and dimethylformamide (DMF, 99%), were obtained from Acros Organics. Ethanol (99.9%) and hexane (99%) were purchased from VWR. All solvents were used as received. The non-woven polypropylene/polyethylene fabric Novatexx 2471 was generously supplied by Freudenberg filtration technologies (Germany). 2.2 Support synthesis Asymmetric supports were prepared via the non-solvent induced phase separation process. Casting solutions were prepared as presented in Table 1 by dissolving the polymers in respective solvents at room temperature. After dissolution, these casting solutions were kept undisturbed for at least 24 h in order to remove the air bubbles. The polymer solutions were then cast at a speed of 1.2 m/min on a PP/PE non-woven fabric using an automated casting knife (Braive Instruments, Belgium) set at a gap of 250 μm. In order to prevent excessive penetration of the polymer solution, the non-woven fabric was first impregnated with solvent and then wiped dry before coating. The resulting film was immediately immersed in the non-solvent coagulation bath (de-ionized water) at room temperature for 10 minutes. Three replicate supports were cast from each solution. These membranes were stored in distilled water until use.
Table 1. Casting solution and non-solvent bath composition. Polymer
Solvent
PSF
NMP
PI
DMF
PVDF
DMF
Concentration (wt%)
Non-solvent
16, 18, 20
Distilled water
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2.3 Support porosity determination The porosity ε (%) for the standalone supports was calculated gravimetrically from equation (1), as reported by Li et al. [38] It is defined as the volume of the pores divided by the total volume of the microporous membrane, determining the weight of liquid (ethanol) contained in the membrane pores.
ε=
(m1 − m2 )⁄ ρE × 100% (m1 − m2 )⁄ m2 ρE + ⁄ρP
(1)
where m1 is the weight of the wet support (g); m2 is the weight of the dry support(g); ρE is the ethanol density (0.789 g/cm3); and ρP is the polymer density (PSF=1.24g/cm3, PI=1.20 g/cm3, PVDF=1.765 g/cm3). [39-41] 2.4 Viscosity measurement of PDMS coating solutions Pre-crosslinking behavior of the PDMS solutions was studied as a function of reaction time by viscosity measurements performed on a stress- controlled rheometer (Anton Paar MCR501) with a cone-plate geometry and a solvent trap. PDMS solutions of three concentrations ( 5%, 10% and 20 wt%) were prepared in n-hexane. The solutions were stirred at 60 oC and 300 rpm for crosslinking in a closed glass bottles to avoid solvent evaporation. The viscosity of each solution was measured each hour at the shear rate of 100 s-1 till it reached to a viscosity of 100 [mPa.s]. 2.5 Preparation of composite membranes by dip coating PDMS composite membranes were prepared by coating a thin layer of PDMS onto the supports using an automated dip coating machine shown in fig. 1 (HTML, Belgium). [42] In order to avoid collapsing of the pores and intrusion of PDMS solution inside the pores, these supports were treated with different solvent exchange baths of ethanol, iso-propanol and finally hexane, prior to the coating. The PDMS coating solution was prepared by slightly modifying the procedure described by Stafie et al. [26] A 10 wt% PDMS (RTV615A:RTV615B = 10:1) in hexane was precross-linked at 60 oC for 4 h with stirring at 300 rpm in a closed glass bottles to avoid solvent evaporation and in order to have a sufficiently viscous solution. This solution was then coated on the supports by immersing them in the solution bath, all supports were pre-soaked in hexane before coating the PDMS solution. Dip time was varied from 1 to 4 min while a removal speed of 0.010 m/s was maintained. After coating, the membranes were kept at room temperature for 30 min to evaporate most of the hexane and then kept in the oven at 110 oC for 1 h in order to complete the crosslinking. All the composite membranes were prepared in a same way and stored in dust-free environment.
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Fig. 1. Automatic dip coating machine. [42]
2.6 Pervaporation set-up Pervaporation experiments were carried out and thickness normalized flux (J), separation factors of the membranes (β) were calculated from equation 1 and 2 as described in the Chapter 3, section 2.5. To integrate membrane flux and separation factor, the pervaporation separation index (PSI) was calculated using equation (2): PSI = J × (β − 1) where J is expressed in kg m−2 h−1, α = separation factor.
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(2)
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3. Characterization 3.1. Pure water flux of supports To evaluate the porosity of the supports, pure water fluxes were measured by using a standard Amicon® cell [43] at room temperature. Three coupons were cut from each membrane strip and per coupon 3 samples were analyzed. Membrane permeance (Lp) was calculated using equation (3) as follows: 𝐿𝑝 =
𝑉 𝐴𝑡∆𝑃
(3)
where V is the permeate volume (L), A is the membrane area (m2), t is the time (h) and ΔP is the applied pressure (bar) 3.2. Scanning Electron Microscopy (SEM) Scanning electron microscopy (SEM) was used to take images of surface sections of the supports and composite membrane cross-sections (obtained by breaking membranes submerged in liquid nitrogen). Pictures were acquired at 10.0 kV on a Philips XL 30 FEG-SEM. Samples were mounted onto SEM sample holders and coated with a 1.5-2 nm thick gold layer to reduce sample charging under the electron beam.
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Chapter 5____________________________________________________________________________ 4. Results and discussion 4.1. Supports From the three different chemistries that were selected, a set of supports was prepared first in order to obtain supports with comparable surface porosities for each chemistry. Pure water flux, bulk properties and pore morphologies of the supports were determined for three supports cast from solutions with different polymer concentrations for each chemistry. 4.1.1. Pure water fluxes of supports Pure water permeances of the supports prepared from the 3 different polymers and for each from 3 different casting solution concentrations, are shown in fig. 2. The values fall within the range of ultrafiltration (20-70 LMHB for supports prepared from 16 and 18 wt% casting solutions) to nanofiltration (2-10 LMHB for 20 wt%). PVDF supports show in general slightly higher permeances than PSF and PI supports when cast from solutions with the same polymer concentration, even though the standard deviations (mostly obtained from analysis of 3 permeate samples per coupon, using 3 coupons per membrane type cast in triple, hence n = 27 ) are often rather large.
Fig. 2. Pure water permeance of the supports as a function of polymer type and polymer concentration in the casting solution. 4.1.2. Support porosity The bulk porosities of the supports are shown in fig. 3. Porosity values fall within the range of 60 to 83%. PVDF membranes generally show a somewhat higher porosity (75 to 83%) than PI (60 to 73%) and PSF (70 to 80 %) membranes, but differences are again rather small. As anticipated, porosities decreased as the concentration of polymer increased, resulting in somewhat less porous structures of the supports. [32-35,38]
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Fig. 3. Bulk porosity of supports as a function of polymer type and polymer concentration in the casting solution. 4.1.3. Support surface morphology Fig. 4 presents the SEM surface micrographs of PSF (fig 4 a-c), PVDF (fig 4 d-f) and PI (fig 4 gi) supports. As expected, the porosity of the supports decreased with increasing polymer concentration from 16 to 20 wt% in the casting solution.
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Chapter 5____________________________________________________________________________ Fig. 4. SEM surface micrographs of support membranes (top: PSF, middle: PVDF and bottom: PI; with from left to right: 16 wt%, 18 wt% and 20 wt % polymer concentration in the casting solution respectively). 4.1.4. Morphology of the support cross-sections As expected, all supports had an asymmetric structure with a thin denser active layer over a thick more open sub-layer. It was observed that the surface pore morphology and the support bulk porosities (section 4.1.2) changed somewhat over the different chemistries and casting solution concentrations. SEM cross sections confirmed this trend, but also revealed important changes in morphology. SEM pictures of all these PSF supports showed the presence of macrovoids (fig. 5ac). In the case of PVDF supports (fig. 5d-f ), similar macrovoids were observed for 16 and 18 wt% polymer concentrations in the casting solution, but a clearly less open structure was observed for 20 wt%. Each time, a quite thick spongy substructure appeared at the bottom of the support layer in contrast with the non-woven. In PI supports, a membrane cast from a polymer concentration of 16% showed elongated macrovoids, but for 18 and 20 wt%, a spongy structure appeared (fig. 5gi). In general, the relatively high porosity of the support layer is expected to ensure a negligible resistance for the composite membrane during the pervaporation, even though the exact structure of the denser skin-layer cannot be visualized with SEM.
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Fig. 5. SEM cross-sectional micrographs of composite membranes (a to c: PDMS/PSF, d to f: PDMS/PVDF and g to i: PDMS/PI, with from left to right 16 wt%,18 wt%, 20 wt % polymer concentration in the casting solution respectively). 4.1.5. Support thickness Although the wet casting thickness was similar for all membranes (250 μm), the dry thicknesses differed significantly, ranging from PI (65-105 μm) to PSf (85-99 μm) and PVDF (28-45 μm). During phase inversion, polymer films always shrink significantly, since, upon coagulation, solvent diffuses out of the polymer film and the polymer solidifies, which lowers the volume and thus the thickness of the film. [44] This shrinkage obviously depends strongly on the polymer type. 4.2. Viscosity of the PDMS coating solutions Fig. 6 shows the changes in viscosity as a function of the crosslinking reaction time for three PDMS solutions with different concentrations. For a 20 wt% PDMS solution, the viscosity reaches 10 [mPa s] after 2 h, which is sufficient for applying a good coating layer. After 3 h, viscosity already jumped to almost 100 [mPa s] which made the solution too viscous to apply good coatings from. For a 10 wt% PDMS solution, it took almost 3 h to reach a viscosity value of 10 [mPa s]. For the 5 wt% PDMS solution, no significant increase in viscosity could be observed at all, not even after 5 h. Due to its less critical changes in viscosity in short time intervals, the 10 wt% PDMS solution with 3 h cross-linking time was selected for subsequent coatings.
Fig. 6. Crosslinking time vs. viscosity for PDMS solutions at 60 oC with 3 different concentrations in hexane.
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Chapter 5____________________________________________________________________________ 4.3. Pervaporation 4.3.1. Influence of support polymer type The pervaporation fluxes of the composite membranes on the 9 different supports reported above, can be arranged in the order PI >PVDF > PSF, while ethanol selectivity follows the order PSF > PVDF > PI (fig. 7, 8 and 9). This trend can be explained as follows. a. Support pore structure and morphology Depending on the pore size of the supports, various kinds of flows are possible. In the large pores, viscous flow of the permeating vapors occurs and the support does not interfere with mass transport. However, if the pores are small enough, the local vapor pressure can exceed the critical condensation pressure due to excessive resistance in the support towards removal of vaporized molecules by the vacuum pump, as commonly applied at lab-scale, resulting in capillary condensation. This capillary condensation reduces membrane performance by decreasing the driving force for pervaporation. [5] Hence, to avoid additional mass transfer resistance against the permeating compounds, the porosity of the supports should be high enough. [2,3] The SEM images in fig. 4 revealed that an increasing polymer concentration in the casting solution decreased the surface pore size. It even changed the pore morphology in cross-sections (fig. 5) from finger-like to sponge-like for PI. This change in support structure affected the composite membrane performance as well (fig. 7-9). It can be seen that both flux and selectivity of the composite membranes decreased for supports cast from 18 and 20 wt% polymer concentration. In the case of PI, the mass transfer resistance is more pronounced than for PSF and PVDF, consisting with the transition of a macrovoid structure to a spongy one. This would thus indicate, at least for the polymer and support structures studied here, that macrovoid-containing support layers would be most preferred. b. Support hydrophobicity/hydrophilicity From the contact angle values, the supports can be arranged in the order PVDF (95o) > PSF (81o) > PI (63o), as anticipated. It is striking in fig. 7-9 that PI-supports generally lead to lower membrane selectivities. It can thus be assumed that the better interaction of PI with water results in an increased water content in the permeate. More hydrophobic support materials thus seem more favorable for ethanol/water separations 4.3.2. Influence of polymer concentration in the support casting solution Fluxes of the composite and unsupported membranes were normalized to a thickness of 10 µm for the selective layer. Even though the error on the PDMS-layer thickness determination could be significant (see for instance fig. 13). All normalized fluxes of the composite membranes are much lower than the one of the unsupported reference membrane. This could be due to a variety of factors, such as too limited support (surface) porosity (discussed in the sections 4.1.3 and 4.1.4),
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intrusion of the PDMS-layer in the support, or simply reduced swelling possibilities of the PDMSlayer when adhered to the support (discussed in the section 4.3.3). a. PSF support The PV performances of the PSF-supported composite PDMS-membranes is shown in fig. 7. Normalized fluxes were obtained in the range of 0.064 to 0.035 kg/m2h and selectivities were in the range of 3 to 9.6. The highest selectivity of 9.6 was obtained when using a support cast from a 16 wt% PSF solution using a dip time of 4 min. This high selectivity is unfortunately combined with a rather low normalized flux of 0.036 kg/m2h. Composite membranes with supports prepared from 18 and 20 wt% PSF-solutions showed reduced selectivities probably due to the decreased porosities of the supports.
Fig. 7. Effect of PSF concentration in the support casting solution and of dip time in the PDMS (10%) solution on the PV performance of PDMS–PSF membranes (left: flux normalized to a thickness of 10 µm; right: selectivity). b. PVDF support The PV performances of the PVDF supported PDMS composite membranes is shown in Fig. 8. Normalized fluxes were in the range of 0.12 to 0.03 kg/m2h and selectivities varied from 1.7 to 6.4. The fluxes were generally higher than for the PDMS/PSF composite membranes, probably due to the more hydrophobic character in combination with the more open pore structure of the PVDF support which can be linked to the higher pure water fluxes. A maximum selectivity of 6.4 was obtained for a 20 wt% PVDF concentration and a dip time of 4 min.
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Fig. 8. Effect of PVDF concentrations in the support casting solution and of dip time in the PDMS (10%) solution on the PV performance of PDMS–PVDF membranes (left: flux normalized to a thickness of 10 µm; right: selectivity). c. PI support The PV performances of the composite PDMS/PI membranes is shown in Fig. 9. Normalized fluxes of the membranes were in the range of 0.2 to 0.02 kg/m2h and selectivities varied from 2.1 to 5.5. The maximum selectivity value of 5.5 was obtained for PI 16 wt% with a dip time of 4 min. Higher fluxes but lower selectivities were generally observed compared to the above described composite membranes which might be due to the hydrophilic nature of PI.
Fig. 9. Effect of PI concentrations in the support casting solution and of dip time in the PDMS (10%) solution on the PV performance of PDMS–PI membranes (left: flux normalized to a thickness of 10 µm; right: selectivity).
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4.3.3. Influence of the dip time of the supports in the PDMS solution The change in dip time of the supports in the PDMS solution was studied in order to form more uniform, defect-free layers. Fig. 10 shows the thickness of the PDMS top-layer as a function of dip time on the different types of supports
Fig. 10. Thickness of the PDMS top layer as a function of dip time on the different types of supports (3 different supports cast from 3 different casting solution concentrations: 16, 18 and 20 wt%). It can be seen that the thickness of the PDMS layer clearly increases with increasing polymer concentration in the casting solution of the support and with increasing dip time. The first effect can most probably be related to intrusion of the PDMS solution in support, which will decrease systematically with increasing polymer concentration, since support porosities and surface pore sizes tend to decrease. Babaluo et al. developed a model to predict the effects of variable dip time and withdrawal speed of porous supports on the thickness of the top layer. [45] According to the model, for a fixed combination of coating solution and porous support, the top layer thickness increases linearly with the square root of the dip time (t0.5) at a constant withdrawal speed of the support. Fig. 11 shows the thickness of the PDMS top-layer as a function of the square root of the dip time for the different types of supports.
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Fig. 11. Thickness of the PDMS top layer as a function of the square root of dip time of the different types of supports (3 different supports cast from 3 different casting solution concentrations: 16, 18 and 20 wt%) in the PDMS solution (10 wt% PDMS in hexane, 3h precrosslinked at 60 oC) at a constant withdrawal speed of 0.01m/s. It can be seen that, all lines in the fig. 11 are in accordance with the model. The increase in the thickness of the PDMS layer as a function of dip time, i.e. the slope of the lines, follows the order of PSF >> PVDF > PI. The Hildebrand-parameters of the different polymers utilized in this work are listed below : 1. PDMS 14.9 (Mpa1/2) 2. PSF 20.2 (Mpa1/2) 3. PVDF 23.2 (Mpa1/2) 4. PI 24.3 (Mpa1/2) Considering the Hildebrand-parameters of the different polymers, it is clear that this order follows the order of decreasing interactions between the support material and PDMS. A better chemical interaction with the support thus leads to enhanced adsorption of polymer chains from the coating solution during the dip time, and hence the deposition of a thicker PDMS-layer. It is clear from fig. 12 that the selectivities of the composite membranes increased with increasing dip time, while the fluxes declined gradually, due to an increased thickness of the PDMS selective layer. These thickness values were derived from SEM images, as typically presented in fig. 13. Thus, composite membranes prepared with increased dip time result in the uniform coating of defect-free PDMS solutions to create membranes with a good selectivity.
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Fig. 12. Effect of dip time on performance in PV for PDMS composite membranes prepared on PSF, PVDF and PI supports cast from their 16 wt % casting solutions (left: normalized flux; right: selectivity).
Fig. 13. SEM cross section micrographs of PDMS/PI (20 wt % casting solution) composite membranes showing the effect of increasing dip time (a= 1 min, b= 2 min, c= 4 min). 4.4. Comparison with literature The pervaporation performances of different PDMS supported membranes reported by other research groups for comparable ethanol-water mixture separations are listed in Table 2. The composite PDMS membranes prepared in this work on different supports and with PDMS top layer thicknesses varying from 0.79 µm to 2.8 µm, showed total fluxes from 20 to 200 g/m 2.h and selectivities from 1.7 to 9.6 for the separation of a 6 wt% aqueous ethanol solution at a feed temperature of 40 oC. In comparison with the literature data, the selectivities and fluxes obtained in present work were not the highest, but a good comparable performance was obtained. It should be emphasized that proper comparison with literature is difficult as different preparation conditions are adopted by other research groups e.g. source and type of PDMS [26], support casting conditions [27], posttreatment on the support [17] coating conditions of the PDMS layer. [17,26-28] In addition, operational conditions during PV (e.g. feed concentration) can also substantially influence results.
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Chapter 5____________________________________________________________________________ Table 2. PV performance of the PDMS coated composite membranes for the separation of 6 wt% ethanol/water mixture (all measured at 40 oC). Normalized flux
Separation factor
(µm)
Feed concentration(w t%)
(g/m2h)
(α)
PSF
6
5
600-4000
PSF
1
2-5
PVDF
10
Crosslinked PI
Support
Thickness of PDMS layer
PSI
Reference
1.79-5.2
2.5-3.2
27
160
5
0.64
28
5
500
8.3
3.6
26
12.5
3-9
120-130
4.6
0.47
17
PSA/PAa
5
2-5
300
11
3
28
PSF
0.86 to 2.65
6
64-35
3-9.6
0.13-0.3
This study
PVDF
0.79 to 1.87
6
120-30
1.7-6.4
0.08-0.2
This study
PI
0.93 to 2.88
6
200-20
2.1-5.5
0.22-0.1
This study
PDMS unsupported
120
6
320
7.7
2.1
This study
a
PSA/PA- Polysulfonamide blend with polyamide
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5. Conclusions Both flux and selectivity for PDMS coated PV membranes in the removal of ethanol from aqueous feeds are clearly influenced by the support layer. Three polymers with different chemical composition were used in this work to prepare support layers having comparable surface porosities to obtain the PDMS composite membranes. Resistance of the support layers increased by increasing the polymer concentration in the casting solutions of the supports. This could be attributed to a decreased pore size in the support, as indicated by SEM images, and a gradual decrease in pure water flux. For the current separation, a hydrophobic support material with macrovoid structure was found optimal. For the composite membranes, a certain pre-crosslinking time of the dilute PDMS coating solution prior to dipping was found essential. Enhanced performance, especially in the selectivity of the composite membranes, was realized by increasing the dip time in the PDMS coating solution. SEM analysis of the composite membranes showed that this leads to a minor increase in the thickness of the PDMS top layer. For a certain PDMS coating solution and porous support, the top layer thickness increased linearly with the square root of the dip time (t0.5) at a constant withdrawal speed of the support.
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Chapter 5____________________________________________________________________________ References [1] Ping. P, Baoli. S, Yongqiang. L, A Review of Membrane Materials for Ethanol Recovery by Pervaporation, Sep. Sci. Technol, (2011), 46(2), 234-246. [2] Gudernatsch. W, Menzel. Th, Strathmann H, Influence of composite membrane structure on pervaporation, J. Membr. Sci, (1991), 61, 19-30. [3] Lipnizki. F, Olsson. J, Wu. P, A. Weis G. Tragardh, R. W. Field, Hydrophobic pervaporation: influence of the support layer of composite membranes on the mass transfer, Sep. Sci. Technol, (2002), 37(8), 1747-1770. [4] Claes. S, Vandezande. P, Mullens. S, Leysen. R, Sitter. K. De, Andersson. A, Maurer. F.H.J, Van den Rul. H, Peeters. R, Van Bael. M.K, High flux composite PTMSP-silica nanohybrid membranes for the pervaporation of ethanol/water mixtures, J. Membr. Sci, (2010), 351 160–167. [5] Trifunovic. O, Tragardh. G, The influence of support layer on mass transport of homologous series of alcohols and esters through composite pervaporation membranes, J. Membr. Sci, (2005), 259(1-2), 122-134. [6] Dobrak. A, Figoli. A, Chovau. S, Galiano. F, Simone. S, Vankelecom. I.F.J, Drioli. E, Van der Bruggen. B, Performance of PDMS membranes in pervaporation: Effect of silicalite fillers and comparison with SBS membranes, J. Colloid Interface Sci, (2010), 346(1), 254264. [7] Nijhuis. H. H, Mulder. M. H.V, and Smolders. C.A, Removal of trace organics from aqueous solutions: Effect of membrane thickness, J. Membr. Sci, (1991), 61, 99-111. [8] Heintz. A, Stephan. W, A generalized solution-diffusion model of the pervaporation process through composite membranes Part II. Concentration polarization, coupled diffusion and the influence of the porous support layer, J. Membr. Sci, (1994), 89(1-2), 153-169. [9] Bai J, Fouda. A.E, Matsuura. T, Hazlett. J.D, A study on the preparation and performance of polydimethylsiloxane coated polyetherimide membranes in pervaporation, J. Appl. Polym. Sci, (1993), 48(6), 999-1008. [10] Rautenbach. R, Helmus. F.P, Some considerations on mass-transfer resistances in solution-diffusion-type membrane processes, J. Membr. Sci, (1994), 87(1-2), 171-180. [11] Bode. E, Hoempler. C, Transport resistances during pervaporation through a composite membrane: experiments and model calculations, J. Membr. Sci, (1996), 113(1), 43-56. [12] Liu. M.G, Dickson. J.M, Cote. P, Simulation of a pervaporation system on the industrial scale for water treatment Part I: Extended resistance-in-series model, J. Membr. Sci, (1996), 111(2), 227-241. [13] Smart. J, Schucker. R.C, Lloyd. D.R, Pervaporative extraction of volatile organic compounds from aqueous systems with use of a tubular transverse flow module: Part I. Composite membrane study, J. Membr. Sci, (1998), 143(1-2), 137-157. [14] Vankelecom. I.F.J, Moermans. B, Verschueren. G, Jacobs. P.A, Intrusion of PDMS top layers in porous supports, J. Membr. Sci, (1999), 158(1-2), 289-297. 126
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[15] Han. X, Wang. L, Li. J, Zhan. X, Chen. J, J. Yang, Separation of ethanol from ethanol/water mixtures by pervaporation with silicone rubber membranes: Effect of silicone rubbers, J. Appl. Polym. Sci, (2011), 119(6), 3413-3421. [16] Tan. S, Li. L, Zhang. Z, Wang. Z, The influence of support layer structure on mass transfer in pervaporation of composite PDMS–PSF membranes, Chem. Eng. J, (2010), 157(2-3), 304-310. [17] Shi. E, Huang. W, Xiao. Z, Li. D, M. Tang, Influence of binding interface between active and support layers in composite PDMS membranes on permeation performance J. Appl. Polym, (2007), 104(4), 2468–2477. [18] Gevers. L. E.M, Aldea. S, Vankelecom. I.F.J, Jacobs. P. A, Optimization of a lab-scale method for preparation of composite membranes with a filled dense top-layer, J. Membr. Sci, (2006), 281(1-2), 741-746. [19] Claes. S, Vandezande. P, Mullens. S, Sitter. K. D, Peeters. R, Van Bael. M.K, Preparation and benchmarking of thin film supported PTMSP-silica pervaporation membranes, J Membr. Sci, (2012), 389 265–271. [20] W. Wei, S. Xia, G. Liu, X. Dong, W. Jin, N. Xu, Effects of polydimethylsiloxane (PDMS) molecular weight on performance of PDMS/ceramic composite membranes, J. Membr. Sci, (2011), 375, 334–344. [21] Liu. G, Xiangli. F, Wei. W, Liu. S, Jin. W, Improved performance of PDMS/ceramic composite pervaporation membranes by ZSM-5 homogeneously dispersed in PDMS via a surface graft/coating approach, Chem. Eng. J, (2011), 174, 495–503. [22] Xiangli. F, Chen. Y, Jin W, and Xu N, Polydimethylsiloxane (PDMS)/Ceramic Composite Membrane with High Flux for Pervaporation of Ethanol-Water Mixtures, Ind. Eng. Chem. Res. (2007), 46, 2224-2230. [23] Kim. I.C, Yun. H G, Lee. K.H, Preparation of asymmetric polyacrylonitrile membrane with small pore size by phase inversion and post-treatment process, J Membr. Sci, (2002), 199(1-2), 75-84. [24] Stafie. N, (2004), Poly(dimethyl siloxane)-based composite nanofiltration membranes for non-aqueous applications, Ph.D. thesis, University of Twente, Enschede, The Netherlands. [25] Stafie. N, Stamatialis. D.F, Wessling. M, Insight into the transport of hexane-solute systems through tailor-made composite membranes, J. Membr. Sci, (2004), 228(1), 103116. [26] Stafie. N, Stamatialis. D.F, Wessling. M, Effect of PDMS cross-linking degree on the permeation performance of PAN/PDMS composite nanofiltration membranes, Sep. Purif. Technol, (2005), 45, 220-231. [27] Wang. R, Shan. L, Zang. G, Ji. S, Multiple sprayed composite membranes with high flux for alcohol permselective pervaporation, J. Membr. Sci, (2013), 432, 33-41. [28] Zhu. J, Fan. Y, Xu. N, Modified dip-coating method for preparation of pinhole-free ceramic membranes, J. Membr. Sci, (2011), 367 (1-2), 14-20.
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Chapter 5____________________________________________________________________________ [29] Peng. F, Liu. J, and Li. J, Analysis of the gas transport performance through PDMS/PS composite membranes using the resistances-in-series model, J. Membr. Sci, (2003), 222(12), 225-234. [30] Madaeni. S.S, Mohammadi Sarab Badieh. M, Vatanpour V, Effect of coating method on gas separation by PDMS/PES membrane, Polym. Eng. Sci, (2013), 53(9), 1878-1885. [31] Qtaishata. M, Khayet M, Matsuura. T, Novel porous composite hydrophobic/ hydrophilic polysulfone membranes for desalination by direct contact membrane distillation, J. Membr. Sci, (2009), 341(1-2),139-148. [32] Holda. A. K, Aernouts. B, Saeys. W, Vankelecom. I.F.J, Study of polymer concentration and evaporation time as phase inversion parameters for polysulfone-based SRNF membranes, J. Membr. Sci, (2013), 442, 196–205. [33] Holda. A.K, Vankelecom. I.F.J, Understanding and guiding the phase inversion process for synthesis of solvent resistant nanofiltration membranes, J. Appl. Polym. Sci. (2015), 132, 42130-42147. [34] Vandezande. P, Gevers. L.E.M, Jacobs. P. A, Vankelecom I.F.J, Preparation parameters influencing the performance of SRNF membranes cast from polyimide solutions via SEPPI, Sep. Purif. Technol, (2009), 66, 104–110. [35] Vandezande. P, Li. X, Gevers. L.E.M, Vankelecom I.F.J, High throughput study of phase inversion parameters for polyimide-based SRNF membranes, J. Membr. Sci, (2009), 330, 307–318. [36] Bottino. A, Camera-Rodab. G, Capannelli. G, Munari S, The formation of microporous polyvinylidene difluoride membranes by phase separation, J. Membr. Sci, (1991), 57, l-20. [37] Liu. F, Hashim. N. A, Liu. Y, Moghareh Abed. M.R, Li. K, Progress in the production and modification of PVDF membranes, J. Membr. Sci, (2011), 375, 1–27. [38] Lia. Q, Xua. Z.L, and Liub. M, Preparation and characterization of PVDF microporous membrane with highly hydrophobic surface, Polym. Adv. Technol, (2011), 22(5), 520-531. [39] http://www.solvayplastics.com/sites/solvayplastics/EN/specialty_polymers/Markets/Me mbranes/Pages/sulfone-polymers-membranes.aspx accessed on July 13 2014. [40] http://catalog.ides.com/Datasheet.aspx?I=92041&U=0&FMT=PDF&E=111432 accessed on July 13 2014. [41] http://polymer-additives.specialchem.com/product/a-huntsman-matrimid-9725 accessed on July 14 2014. [42] http://www.html-membrane.be accessed on June 31 2013. [43] http://www.emdmillipore.com/US/en/product/Stirred-Cell-Model-8050%2C50%C2%A0mL,MM_NF-5122 accessed on August 15 2014. [44] Hendrix. K, Koeckelberghs. G, Vankelecom. I. F.J, Study of phase inversion parameters for PEEK-based nanofiltration membranes, J. Membr. Sci, (2014), 452, 241-252. [45] Babaluo. A. A, Kokabi. M, Manteghian. M, Sarraf-Mamoory. R, A modified model for alumina membranes formed by gel-casting followed by dip-coating, J. Eur. Ceram. Soc, (2004), 24, 3779–3787. 128
CHAPTER 6 General conclusions and future challenges
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Chapter 6____________________________________________________________________________ 6.1 General Conclusions Pervaporation (PV) membrane processes represent great potential for future industrial and environmental applications. This work has contributed to one of the most fundamental problems in pervaporation, and in fact in membrane technology in general, namely to overcome the traditional trade-off between flux and selectivity. Three different types of inorganic porous fillers were introduced in order to prepare mixed matrix membranes aiming towards increasing the membrane fluxes while at least retaining but preferentially also enhancing their selectivities. The longer term aim is to apply these membranes in bio-fermentation pervaporation processes, exemplified in this work by the removal of ethanol from synthetic aqueous feeds. The inorganic porous fillers considered in this work are versatile and can possibly be useful for many other membrane processes as well. PDMS-based mixed matrix membranes were prepared by incorporating porous inorganic fillers with diverse morphologies, such as hollow spheres, core-shell spheres and octameric silicate species (PSS-2). Unfilled PDMS composite membranes were prepared by coating them on support layers after optimization of the support layer chemistry and morphology, the viscosity of the PDMS coating solution and the dip coating conditions, in order to achieve thin, defect-free selective layers. Pervaporation performance of these lab-made PDMS-based membranes was studied in the separation of 6 wt% ethanol/water mixtures at 40°C with a downstream pressure below 0.7 mbar.
Fig. 1. Comparison of the pervaporation performance of PDMS-based membranes. Fig.1 shows the overall comparison of the pervaporation performance of the PDMS-based filled membranes prepared in this work. These membranes were not the best with respect to separation factors, but they did manage to surpass the literature flux/selectivity trade-off curve but showed an improved performance in comparison with literature membranes. A considerable improvement in 130
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the membrane fluxes was indeed observed while retaining the separation factors. The separation factors can possibly be further improved by increasing the filler loadings, since, filler loadings only upto 30 wt% was achieved in the present work. Preparation of self-supporting PDMS membranes with higher filler loadings >30 wt% for hollow spheres and 20 wt% for ZIF-coated mesoporous silica spheres were tried but they were too brittle to handle. Thus, composite membranes with thinner PDMS top layer and with higher filler loadings should be prepared in order to enhance the separation factors. Hollow spheres in mixed matrix membranes significantly improved the membrane performance. These micron-sized HS were characterized by a hollow core covered with a shell of well-grown silicalite-1 crystals. These HS could be well dispersed in the PDMS matrix with loadings up to 30 wt%. A significant increase in fluxes and selectivities was observed for these membranes upon filler incorporation. The enhanced flux values could be attributed to the presence of a hollow core allowing fast permeation of the selectively sorbed component. The high selectivity was attributed to the selective sorption of ethanol in the zeolite pores and the good adhesion between the polymer and the filler. The described method for mixed matrix membranes can be adopted for other systems consisting of hollow particles with a selective shell for the targeted compounds . Improvements would even become more significant if intrusion of the polymer chains into the hollow part of the spheres could be excluded by for instance partly prepolymerising the PDMS or making fully covered spheres. Novel inorganic porous fillers with a core-shell morphology were prepared through the seeding and regrowth mechanism. Micron-sized mesoporous silica spheres were thus coated with a shell formed by ZIF-8 and ZIF-71 nanocrystals. A significant increase in the flux with increased ethanol selectivity was observed for the membranes filled with MSS-ZIFs. Membranes filled with MSSZIF-8 gave systematically higher selectivities, while membranes filled with MSS-ZIF-71 gave higher fluxes for same loadings. Thus, these MSS-ZIF filled mixed matrix membranes can significantly improve the performance of PDMS-membranes for pervaporation as mesoporous core increases the overall permeation of that specific component that is preferentially sorbed by the selective microporous ZIF shells in analogy with the hollow spheres. It is clear that with respect to MSS-ZIF fillers, ZIF-8 is the preferred shell material when a high selectivity is necessary, while ZIF-71 is the better choice when higher fluxes are desired. A new class of silicate species termed as polyoligosiloxysilicone (PSS-2) were introduced as filler for making the mixed matrix membranes. The approximate size of these posisils was around 3μm. They exhibited a microporous nature with a BET surface area of 357 m2/g. The posisils could be added to the PDMS polymer to prepare defect-free membranes with loadings upto 20 wt%. An increase in ethanol selectivity was observed compared to unfilled PDMS membrane, attributed to the hydrophobic nature and specific pore structure of these fillers. Characterization of the mixed matrix membranes by SEM showed a uniform dispersion of the fillers in the polymer matrix. Support layer properties were found to influence both flux and selectivity significantly for PDMS coated PV membranes in the removal of ethanol from aqueous feeds. Three polymers with different chemical composition were used in this work to prepare support layers having comparable morphology to obtain the PDMS composite membranes. Resistance of the support 131
Chapter 6____________________________________________________________________________ layers increased by increasing the polymer concentration in the casting solutions of the supports. This could be attributed to the changes in the pore morphology of the support, as indicated by SEM images, and a gradual decrease in clean water permeance. For the current separation, hydrophobic support materials with macrovoid structure were found optimal. It was found that a certain precrosslinking time of the dilute PDMS coating solution prior to coating on the supports is essential to improve the separation performance of the composite membranes. Increase in the dip time for coating the PDMS coating solution improved the composite membrane performance. While support layers should only offer mechanical stability but not influence separations, especially enhancement in the selectivity of the composite membranes was thus realized reflecting less defects. For a certain PDMS coating solution and porous support, the top layer thickness increased linearly with the square root of the dip time (t0.5) at a constant withdrawal speed of the support. In general, a beneficial effect on the normalized permeate flux and an increased membrane selectivity of the PDMS-based mixed matrix membranes was found for all inorganic porous fillers applied in this work in comparison to unfilled PDMS membranes. A good dispersion of fillers was obtained in all membranes. In the case of supported PDMS membranes, a higer selectivity was achieved with polysulphone (PSF) as support, while a higher flux was obtained with polyimide (PI). A polyvinylidine fluoride (PVDF) support showed an intermediate effect on the composite membrane performance.
6.2 Future challenges From the present study, it is well understood that new research scenarios can still be investigated for the preparation of mixed matrix membranes. Different combinations of coated fillers, such as MOFzeolite, MOF-MOF, zeolite-zeolite and several others, including ordered mesoporous materials, nonporous dense silica etc., could be examined still with selection based on open character of the core and selectivity for the preferentially permeating compound of the shell Eventually, there is a wide scope to still investigate several other approaches such as:
Studying effect of filler size on the membrane performance. Preparation of asymmetric mixed matrix membranes by using the phase inversion process Use of the polymers such as PVDF, PSF, PTMSP etc., which also show hydrophobicity. Employing hollow fiber mixed matrix membranes to create membrane modules with more active area per module volume.
In the specific case of hollow sphere based PDMS mixed matrix membranes, improvements in the membrane performance can still be realized if intrusion of the PDMS chains into the hollow part of the spheres could be excluded by for instance partly prepolymerising the PDMS solution. The synthesis procedure of the hollow spheres can still be modified in order to make them smaller (
