Design and Development for Highly Permeable Mixed Matrix Membranes

Home , Membrane gas separation

Design and Development for Highly Permeable

Mixed Matrix Membranes (MMMs) for Gas Separation

Design und Entwicklung hoch permeabler Mixed-Matrix- Membranen (MMMs) für die Gastrennung

Der Technischen Fakultät

der Friedrich-Alexander-Universität

Erlangen-Nürnberg

zur

Erlangung des Doktorgrades Dr.-Ing

vorgelegt von

Bilal Haider, M.Sc.

aus Sialkot, Pakistan

Als Dissertation genehmigt

von der Technischen Fakultät

der Friedrich-Alexander-Universität Erlangen-Nürnberg

Tag der mündlichen Prüfung: 25-07-2017

Vorsitzender des Promotionsorgans: Prof. Dr.-Ing. Reinhard Lerch

Gutachter: Prof. Dr.-Ing. Malte Kaspereit

Assoc. Prof. Karel Friess, Ph.D.

To my beloved uncle

Ch. Muhammad Arshad (Late)

To my parents

Safia Begum and Ch. Muhammad Hanif

To my wife and son

Hina and Abdul Ahad

To my brothers

Usman and Salman

Acknowledgements I would like to extend my special gratitude to many wonderful people, who contributed generously to this work in many ways. First and foremost, I am thankful to my advisor Prof. Malte Kaspereit for his insightful supervision, academic and moral support, starting from very first day until completion of the thesis. His encouragement and help was not limited to my research related problems, but also to administrative and my personal matters. Prof. Kaspereit not only provided me financial assistance during last quarter of my research stay at FAU, but also supported me to meet my thesis submission deadlines. I express my sincere obligation to Prof. Wolfgang Arlt for accepting me as PhD student and facilitating me during my stay at the chair. I owe a great deal of appreciation for Prof. Wilhem Schwieger (CRT) for his cooperation in regards to provision of zeolite samples and access to testing facilities. I am very grateful to Dr. Alexander Günther for his support in designing and installation of experimental set up. I offer my heartfelt thanks to Jürgen for his help in gas analysis, company during conference trips, scientific discussions and help in troubleshooting software related issues. My colleagues from CRT deserve special appreciation for their help and support. I thank Marion, Benjamin, Florian, Ameen, Tobias, and Dr. Selvam for extending support through provision of zeolite membranes and samples, XRD analysis, TGA analysis, N2 adsorption measurements, SEM, density measurements and spin coating. I am indebted to my friend and colleague Atiq from Institute of Biomaterials, FAU for his help in SEM-EDX analysis. My sincere thanks also goes to Martin Kriesten (ECRC) for providing me training on FTIR and extending his support in analysis. As well, I would like to thank my Bachelor’s student Oliver and Master’s student Pinar for their contribution in this work. In my daily work, I always needed support from the workshop and I have great admiration for my workshop colleagues Matthias, Markus, Wolfgang and Schmacks (CRT), who provided all sort of support in setting up my apparatus as

3 well as in troubleshooting. I also thank the laboratory staff Petra Kiefer, Petra Koch and Roswitha for their help in various types of analysis and synthesis activities. I must also acknowledge the help and support received from Vera and Elisabeth in day to day official matters. I am so grateful to my fellow colleagues Detlef, Martin, Liudmilla, Karsten, Sebastian, Kathleen, Johannes, Carole, Ben, Danial, Kathrina, Christoph, Rabya, Patrick, Anatol, Thomas, Jonas, Andy, Andrea, Armin, Axel, Sebastian, and many more, for their collegial behavior and encouragement. I also thank my friends at FAU Atif, Lariab, Hassan, Ghufran, Azeem, Abbas, and Jehanzeb for their company and help in various matters. I express my deepest love for my mother and father, for providing me confidence, guidance, encouragement and support throughout my life. Whatever I have achieved, is the result of their untiring efforts and prayers. I also pay my special gratitude to my brothers Usman and Salman for their unconditional help and love. After spending a busy day at work, nothing was more joyful than spending time with my son Abdul Ahad. I love you more than anything and appreciate all your patience and emotional support during my Ph.D. studies. Finally, and most importantly, I would like to thank my wife Hina for her care, support, encouragement, and unwavering love which provided me courage and confidence to meet every challenge. I would also like to extend my sincere thanks to her father and siblings for their love and care.

Contents Acknowledgements ...... 3 Abstract ...... 7 Kurzfassung ...... 8 List of tables ...... 9 List of figures ...... 10 Nomenclature ...... 13 1 Introduction ...... 15 2 State of the Art ...... 19 2.1 History of membrane gas separation ...... 20 2.2 Types and features of gas separation membranes ...... 21 2.2.1 Dense membranes ...... 23 2.2.2 Porous membranes ...... 24 2.3 Transport mechanisms ...... 25 2.3.1 Transport mechanism for dense and porous membranes ...... 26 2.4 Mixed Matrix Membranes (MMMs) ...... 30 2.4.1 Structure and morphology ...... 30 2.4.2 Synthesis materials and methods ...... 32 2.5 Modeling gas permeability ...... 35 2.5.1 Maxwell Model ...... 35 2.5.2 Resistance Model ...... 36 3 Goals and Approach ...... 40 4 Experimental Procedures ...... 43 4.1 Materials ...... 43 4.1.1 Polymers ...... 43 4.1.2 Fillers ...... 46 4.1.3 Solvent and non-solvent additive ...... 49 4.2 Membrane synthesis methods ...... 50 4.2.1 Polymeric membranes ...... 50 4.2.2 Pure zeolite (SAPO-34) ...... 54 4.2.3 Mixed Matrix Membranes ...... 55 4.3 Characterization Methods ...... 58 4.3.1 Viscosity measurement ...... 58 4.3.2 Fourier Transform Infrared Spectroscopy (FTIR) ...... 58

4.3.3 Differential Scanning Calorimetry (DSC) ...... 60 4.3.4 Thermogravimetric Analysis (TGA) ...... 61 4.3.5 Scanning Electron Microscopy (SEM - EDX)...... 61 4.3.6 Methods for separation performance characterization ...... 62 4.3.7 Mean pore size and effective porosity measurements ...... 70 4.3.8 Pycnometric Density Measurement ...... 71 4.3.9 Nitrogen Adsorption ...... 71 5 Results and Discussion ...... 72 5.1 Fourier Transform Infrared Spectroscopy (FTIR) ...... 72 5.2 Differential Scanning Calorimetry (DSC) ...... 73 5.3 Thermogravimetric analysis (TGA) ...... 75 5.4 Viscosity ...... 78 5.5 Scanning Electron Microscopy (SEM - EDX) ...... 80 5.6 Gas separation performance characterization ...... 90 5.6.1 Dense rubbery polymeric membrane ...... 90 5.6.2 Dense glassy polymeric membranes ...... 97 5.6.3 Asymmetric polymeric membrane ...... 99 5.6.4 Pure zeolite (SAPO-34) asymmetric membranes ...... 105 5.6.5 Dense Mixed Matrix Membranes ...... 108 5.6.6 Asymmetric Mixed Matrix Membranes ...... 112 5.7 Modeling gas permeability ...... 124 6 Conclusions and Recomendations ...... 131 References ...... 135 Appendices ...... 140

Abstract Membrane gas separations are industrially attractive due to their low energy consumption, ease of control, and low environmental burden. Corresponding commercial processes utilize almost exclusively polymeric membranes. However, these face a significant performance trade-off between permeability and selectivity. Many industrial applications require membranes with high permeability at moderate selectivity. A promising approach in this context is to develop highly permeable Mixed Matrix Membranes (MMMs) that combine polymers with dispersed inorganic fillers to improve permeability and/or selectivity. This work aims at developing highly permeable MMMs on the basis of different polymers, membrane morphologies, and filler types. A rational development approach is applied by investigating and evaluating systematically different membrane synthesis materials, preparation methods and obtained morphologies, besides probing separation performance for the test gases (H2, He, CO2, CH4, and N2) and state-of-the art analytical techniques. As a starting point, dense flat sheet membranes are produced from different pure polymers by a casting technique. For this purpose both, a pure rubbery polymer (polydimethylsiloxane PDMS), as well as two glassy polymers (polysulfone PSF, and polyethersulfone PES), are applied and compared. In a next step asymmetric membranes are synthesized using PSF and PES. Two different synthesis techniques – single bath and dual bath coagulation – are compared. The resulting membranes show high permeability and (in part) Knudsen selectivity. The application of a thin layer of PDMS by spin-coating to seal potential micro-defects leads to solution-diffusion transport with different selectivities. Afterwards, dense as well as asymmetric MMMs are produced from the mentioned polymers and two different fillers, namely, the zeolite SAPO-34 and activated carbon. Depending on polymer, filler type, and morphology the membranes show different transport mechanisms. In particular the asymmetric MMMs achieve very high permeabilities. An additional PDMS layer again seals micro defects. The coated membranes produced this way outperform existing materials in terms of permeability and selectivity for relevant gas pairs. Finally, it is attempted to rationalize the findings on the basis of simple models.

Kurzfassung Gastrennungen mit Hilfe von Membranen sind aufgrund ihres niedrigen Energieverbrauchs, geringer Umweltbelastung sowie ihrer einfachen Bedienbarkeit industriell attraktive Prozesse. Fast ausschließlich werden in kommerziellen Prozessen Polymermembranen verwendet, die jedoch einen signifikanten Trade-off zwischen Permeabilität und Selektivität mit sich bringen. Viele industrielle Anwendungen benötigen Membranen mit hoher Permeabilität bei moderater Selektivität. Ein vielversprechender Ansatz in dieser Hinsicht ist die Entwicklung hoch permeabler Mixed-Matrix-Membranen (MMMs), welche Polymere mit dispersen anorganischen Füllstoffen kombinieren, um Permeabilität und/oder Selektivität zu verbessern. In dieser Arbeit sollen hoch permeable MMMs auf Basis verschiedener Polymere, Morphologien und Füllmaterialien entwickelt werden. Hierfür werden systematisch verschiedene Synthesematerialien und Präparationsverfahren getestet. Neben den erhaltenen Morphologien wird auch die Trennleistung für Testgase (H2, He,

CO2, CH4 und N2) mit Hilfe modernster Analysemethoden untersucht. Als Ausgangsmaterial werden dichte Flachmembranen aus verschieden reinen Polymeren hergestellt. Hierfür werden sowohl ein gummiartiges Polymer (Polydimethylsiloxan, PDMS), als auch zwei glasartige Polymere (Polysulfon, PSF und Polyethersulfon, PES) verwendet und verglichen. Im nächsten Schritt werden asymmetrische Membranen aus PSF sowie PES synthetisiert und zwei verschiedene Synthesetechniken miteinander verglichen – die Einbad- und Zweibadkoagulation. Die resultierenden Membranen zeigen eine hohe Permeabilität und (zum Teil) Knudsen-Selektivität. Um mögliche Mikrodefekte zu verschließen, wird eine dünne PDMS-Schicht durch Spin-Coating aufgetragen, was zu Lösungsdiffusionstransport mit unterschiedlichen Selektivitäten führt. Anschließend werden sowohl dichte, als auch asymmetrische MMMs aus den genannten Polymeren mit den zusätzlichen Füllstoffen SAPO-34 und Aktivkohle produziert. Abhängig von Polymer, Füllstoffart und Morphologie zeigen die Membranen verschiedene Transportmechanismen. Besonders asymmetrische MMMs erreichen hohe Permeabilitäten und Selektivitäten für relevante Gaspaare. Zuletzt werden die gewonnen Erkenntnisse, anhand einfacher Modelle rationalisiert.

List of tables

Table 4-1: Permeability of PDMS membrane for different gases [46] ...... 44 Table 4-2: The FTIR spectra functional group assignment for PDMS, PSF, PES and MMMs [60] ...... 60 Table 4-3: Gases used and their properties ...... 62 Table 4-4: Input quantities for evaluation of uncertainty ...... 70 Table 4-5: Combined standard uncertainty CU and expended uncertainty EU .... 70 Table 5.1: Glass transition temperature (Tg) for PDMS, PSF, PES and MMMs ... 74 Table 5.2: Viscosity of PES-NMP and PES-NMP-SAPO34 ...... 80 Table 5-3: Analysis of parameters equation 5.1 ...... 92 Table 5-5: Permeability, Solubility and Diffusivity parameters of PDMS ...... 94 Table 5-6: Pure gas selectivity of dense PDMS membrane at 10 bar ...... 95 Table 5-7: Comparison of permeability of dense Polysulfone membrane with literature ...... 98 Table 5-8: Comparison of permeance (GPU) at 5 bar of uncoated asymmetric PSF and PES membranes synthesized using dual bath and single bath coagulation 102 Table 5-9: Comparison of permeance (GPU) at 10 bar of PDMS coated asymmetric membranes PSF (dual bath coagulation) and PES (single bath coagulation) ...... 103 Table 5-10: Comparison of coated and uncoated asymmetric PES membrane selectivity with Knudsen Selectivity ...... 105 Table 5-11: Comparison of permeance (GPU) at 5 bar of asymmetric pure SAPO- 34 membranes supported on stainless steel support ...... 108 Table 5-12: Comparison of permeance rubbery and glassy polymer dense membranes ...... 111 Table 5-13: Comparison of permeance (GPU) at 1 bar of uncoated asymmetric PES SAPO-34 MMM with different zeolite concentrations ...... 115 Table 5-14: Comparison of permeance (GPU) at 1 bar of uncoated asymmetric activated carbon MMM with different zeolite concentrations ...... 116 Table 5-15: Comparison of permeance (GPU) at 5 bar for coated asymmetric PES SAPO-34 MMM with different zeolite concentrations ...... 119 Table 5-16: Comparison of permeance (GPU) at 5 bar of coated asymmetric activated carbon MMM with different zeolite concentrations ...... 120

List of figures

Figure 2.1 A typical membrane gas separation process ...... 19 Figure 2.2 Membrane classification on the basis of material of construction...... 22 Figure 2.3 Types of synthetic membranes, figure adapted from [29] ...... 23 Figure 2.4 Diffusion mechanism in porous membranes figure adapted from [7] . 25 Figure 2.5 Solution diffusion model, driving force gradients for permeation, figure adapted from [32] ...... 27 Figure 2.6: Solution diffusion model, driving force gradients for permeation, figure adapted from [32]...... 29 Figure 2.7 MMM morphologies on the basis of nature of particle distribution, figure adapted from [34] ...... 31 Figure 2.8 Different morphologies of MMM, figure adapted from [6] ...... 32 Figure 2.9 Schematic representation of an asymmetric membrane with a porous layer and a dense skin layer (left) and analogy with electric circuits (right). Figure adapted from [43] ...... 37 Figure 2.10 Schematic representation of composite membrane and analogy with electric circuits, figure adapted from [43]...... 38 Figure 4.1 Structural formula of PDMS ...... 44 Figure 4.2 Polysulfone repeating unit ...... 45 Figure 4.3 Polyethersulfone repeating unit...... 46 Figure 4.4 CHA crystal structure a) CHA building unit, b) framework structure and c) pore diameter, figure adapted from [56] ...... 48 Figure 4.5 ZUA 2000 Film Applicator ...... 50 Figure 4.6 Dense polymeric (rubbery) membrane synthesis process ...... 51 Figure 4.7 Dense polymeric (glassy) membrane synthesis process ...... 52 Figure 4.8 Asymmetric polymeric membrane synthesis process ...... 53 Figure 4.9 Process flow for synthesis of asymmetric MMM using polymer and filler ...... 54 Figure 4.10 Process flow for synthesis of dense MMM using DMS and filler ...... 56 Figure 4.11 Process flow for synthesis of dense MMM using glassy polymer and filler ...... 57 Figure 4.12 Process flow for synthesis of asymmetric MMM using polymer and filler ...... 58 Figure 4.13 Permeability measurement setup ...... 65 Figure 4.14 Flow scheme for permeability measurement...... 66 Figure 5.1 Effect of filler content on glass transition temperature Tg of uncoated asymmetric PES MMMs with SAPO-34 and activated carbon (AC) ...... 75 Figure 5.2 TGA for SAPO-34 and Activated Carbon...... 76 Figure 5.3 TGA for PES and PDMS pure membranes ...... 76 Figure 5.4 Effect of filler content on weight loss for PES and SAPO-34 MMMs .. 77 Figure 5.5 Determination of critical concentration of PES solution ...... 79 Figure 5.6 SEM pictures of an asymmetric pure PES polymeric membrane ...... 81 Figure 5.7 SEM surface of asymmetric PES-SAPO-34 30% MMM ...... 83

Figure 5.8 Membrane surface EDX map of an asymmetric PES-SAPO-34 30% MMM ...... 84 Figure 5.9 SEM picture (cross section) of an asymmetric PES-SAPO-34 30% MMM ...... 85 Figure 5.10 SEM picture of an asymmetric PES-AC 30% MMM ...... 86 Figure 5.11 Membrane surface EDX map of an asymmetric PES-AC-30% MMM membrane ...... 87 Figure 5.12 Membrane surface SEM image of stainless steel supported pure SAPO-34 membrane ...... 88 Figure 5.13 Surface and cross section SEM image of dense PSF-SAPO-30% MMM ...... 89 Figure 5.14 Cross section SEM image for dense PDMS-SAPO-30% MMM membrane ...... 90 Figure 5.15 Permeability vs. pressure for PDMS (35 µm) dense membrane ...... 91 ∞ Figure 5.16 Effect of gas solubility S [74], diffusivity P0 [74] and critical temperature Tc [78] on permeability P0 for the PDMS (35 µm) dense membrane. 93 Figure 5.17 Effect of pressure and membrane thickness on selectivity for dense PDMS membranes ...... 95

Figure 5.18 Robeson plot for CO2/CH4: Comparison of pure gas and mixed gas selectivity for PDMS-35 µm membrane at 10 bar, adapted with permission from [80]...... 96 Figure 5.19 Permeability of Pure dense polysulfone membrane ...... 98

Figure 5.20 Robeson plot for CO2/CH4: Comparison of pure gas selectivity for Dense PSF and PES, adapted with permission from [80] ...... 99 Figure 5.21 Permeance vs. pressure for an uncoated asymmetric polysulfone (PSF) membrane ...... 100 Figure 5.22 Permeance vs. pressure for uncoated asymmetric polyethersulfone (PES) membrane ...... 101 Figure 5.23 Permeance vs. pressure for coated asymmetric PES membrane .. 104 Figure 5.24 Effect of pressure on the permeance of a pure zeolite membrane with crystal size 2 and 6 µm ...... 106 Figure 5.25 Effect of pressure on the permeance of pure zeolite membrane with a crystal size ~ 500 nm...... 107 Figure 5.26 Permeability vs. pressure for PDMS and SAPO-34 (30%) dense MMM ...... 109 Figure 5.27 Permeability vs. pressure for PSF and SAPO-34 (30%) dense MMM ...... 110 Figure 5.28 Permeance vs. pressure for uncoated PES and SAPO-34 (30%) asymmetric MMM ...... 113 Figure 5.29 Permeance vs. pressure for uncoated PES activated carbon (30%) asymmetric MMM ...... 114 Figure 5.30 Permeance vs. pressure for coated PES SAPO-34 (30%) asymmetric MMM ...... 117

Figure 5.31 Permeance vs. pressure for coated PES activated carbon (30%) asymmetric MMM ...... 117 Figure 5.32 Effect of filler (SAPO-34) content on permeance of different gases at 5 bar ...... 118 Figure 5.33 Comparison of activated carbon composition on permeance of coated MMM at 5 bar ...... 118 Figure 5.34 Effect of SAPO-34 composition on selectivity of coated asymmetric PES-SAPO-34 MMM...... 121 Figure 5.35 Effect of SAPO-34 composition on selectivity of coated asymmetric PES-AC MMM ...... 121

Figure 5.36 Robeson plot for CO2/CH4: Comparison of coated asymmetric PES- SAPO and PES-AC with 30% filler loading gas pair at 20 bar, adapted with permission from [80] ...... 122

Figure 5.37 Robeson plot for H2/N2: Comparison of coated asymmetric PES- SAPO and PES-AC with 30% filler loading gas pair at 20 bar, adapted with permission from [80] ...... 123

Figure 5.38 Robeson plot for CO2/N2: Comparison of coated asymmetric PES- SAPO and PES-AC with 30% filler loading gas pair at 20 bar, adapted with permission from [80] ...... 124 Figure 5.39 Scheme of conductance in series and parallel model ...... 125

Figure 5.40 CO2 Permeability: Comparison different models for uncoated PES- SAPO-34 MMMs ...... 127

Figure 5.41: Experimental results: Effect of filler loading on CO2 Permeability for uncoated PES-SAPO-34 MMMs ...... 127

Figure 5.42 Asymmetric coated PES-SAPO-34 MMMs CO2 Permeability: Experimental results vs. Series Parallel and Series Maxwell model ...... 129

Figure 5.43 Comparison of CO2 permeability of uncoated and coated PES- SAPO-34 MMMs against filler loading ...... 130

Nomenclature List of Symbols

A cm2 Membrane surface area c mol/m3 Concentration of gas dissolved in the polymer D cm2/s Diffusivity coefficient

dm M Molecular diameter I A Current J gmol/cm2.s Molar flux

푙 Cm Membrane thickness m g/mol Molecular weight n Shape Factor P Barrer Permeability coefficient

푃̃ GPU Pure Gas Permeance p Bar Pressure

p1 bar Pressure on the feed side

p2 bar Pressure on the permeate side Δp bar Pressure difference across the membrane Q cm3/s Volumetric flow rate at STP R m3.Pa/K.mol Universal gas constant r Nm Mean pore radius S cm3(STP)/cm3.bar Solubility coefficient V Volts Voltage T K Temperature

O Tg C Glass transition temperature U Proportionality Constant

xi,yi Mole Fractions

푉̇ cm3/s Volumetric flow rate at ambient conditions V m/s Velocity v cm3/mol Molar Volume

List of Greek Letters

ρ g/cm3 Density

η mPa.s Viscosity

α Selectivity

λ m Mean Free Path

γ Activity Coefficient

μ Joule/kg Chemical Potential

ε 1 /m Porosity

Abbreviations

MMM Mixed Matrix Membrane

PDMS Polydimethylsiloxane

PES Polyethersulfone

PSF Polysulfone

NMP N-Methyl-2-pyrrolidone

GPU Gas Permeation Units

FTIR Fourier Transform Infrared Spectroscopy

SEM Scanning Electron Microscopy

EDX Energy-dispersive X-ray spectroscopy

DSC Dynamic Scanning Calorimetry

TGA Thermogravimetric Analysis

STP Standard Temperature and Pressure

1 Introduction

Natural gas has the lowest carbon emissions when compared with conventional energy carriers like coal, heating oil and liquefied petroleum gas. It therefore has the potential to reduce carbon emissions by replacing coal for power generation and fossil fuels in transportation[1]. It is projected that utilization of gas and renewable energy resources will grow in the near future and that at some point renewables may even surpass natural gas. Thus, natural gas may replace coal and oil − which is phasing out even sooner − to a significant proportion [2]. These trends will drive the demand for cleaner energy sources and a rapid development of energy-efficient and environmentally friendly separation processes. Using natural gas for energy or the production of chemicals requires significant pre-processing to remove impurities like carbon dioxide (CO2), nitrogen (N2) and hydrogen sulfide (H2S). The exploited sources of natural gas contain increasing amounts of these impurities. For example, it is estimated that 50% of the currently known gas resources contain more than 2% of CO2 [3]. The removal of such amounts of impurities poses new challenges to the production environments. In this context, gas separation using membrane technology is already competing on overall economics with conventional separation technologies like cryogenic distillation, absorption and adsorption. Moreover, it also competes regarding process safety, environmental and technical aspects [4]. Particular advantages of membrane technology for gas separation are the low energy requirements, ease of operation and low space requirements. For example, in comparison to conventional amine absorption, membrane separation requires about 77% less energy per ton of CO2 removed from natural gas [5]. The advantages of potentially lower financial burden and environmentally friendlier processes make membrane separation plants attractive. Besides natural gas purification, membrane gas separation has a number of relevant applications including, but not limited to, the following: hydrogen recovery from waste gas streams, hydrogen purification, oxygen enrichment, and nitrogen production from air, biogas upgrading, fuel cells, removal of volatile organic compounds and on- board CO2 removal. The vast majority of gas separation membranes are made from polymers. Polymeric membranes are popular due to their ease of manufacturing, flexibility,

15 applicability under high pressure as well as their functional characteristics regarding hydrophobic or hydrophilic nature. Conventional polymers such as polysulfone, polyimide, cellulose acetate and polyamide are widely used, although their performance is limited given a significant trade-off between permeability and selectivity. Correspondingly, ongoing research effort is devoted tackle this typical trade-off by developing novel and more efficient membranes. Considering the two main types of membrane polymers ─ rubbery and glassy ─ one recognizes a largely different behavior. Rubbery polymers like polydimethylsiloxane (PDMS) exhibit a high permeability for CO2 but rather limited selectivity, which is related the high mobility of their polymer chains. Glassy polymers like polysulfone or polyimide exhibit a more rigid structure with lower chain mobility. These polymers show a higher selectivity but lower permeability as compared to rubbery polymers [6], [7]. A particularly promising approach to improve the performance of both types of polymers is the development of mixed matrix membranes (MMMs). These combine the ease of manufacturing of polymers and the excellent gas sieving properties of, for example, zeolites [6]. In the case of rubbery polymers, the selectivity may be enhanced using highly selective filler, while the permeability of glassy polymers can increase when using highly porous particles. During the last two decades the research focus towards composite materials has increased immensely and the development of MMMs is one example for this shift of paradigms. It was demonstrated that MMMs containing heterogeneous components, like nano-porous zeolites and polymers, may pose superior permeability, selectivity and stability compared to purely polymeric membranes [8]. This is due to their high free volumes and the selective sieving effect provided by the porous fillers. MMMs may be formed in two morphologies: symmetric and asymmetric. Symmetric membranes are dense thin layer membranes, which are frequently studied due to their ease of fabrication. Asymmetric nano-composite membranes consist of continuous separate layers of inorganic filler and polymer, which are embedded with each other by different mechanisms [6]. Various nano-porous fillers like zeolites, metal-organic frameworks (MOF), zeolitic imidazolate frameworks (ZIF), carbon molecular sieves (CMS), carbon nanotubes (CNT) and activated carbon have been studied for developing MMMs [9]. Among them, zeolites are particularly promising candidates for MMMs due to their

16 molecular sieving characteristics and large surface area. So far, various zeolites have been tested for their gas sieving and adsorption properties. Micro-porous zeolites are considered to be suitable for separation of CO2 from flue gas streams

[9]. Zeolite membranes (Y-type) were found to be potential candidates for CO2/N2 separations [10]. However, it remains a challenge to produce defect-free MMMs. This requires a uniform dispersion of the inorganic filler particles and a good interaction between the inorganic and the organic phase [6]. The particle loading of the inorganic filler cannot exceed particular limits in symmetric MMMs. Due to poor interaction and particle agglomeration voids can be formed between particle and polymer. This phenomenon is caused by restricting polymer chains to reach the particle surface or the gaps within agglomerates. These voids cause non-selective flow of gas through the spaces between particle and polymer which results in a strongly reduced selectivity of the membrane [6, 11]. The aforementioned issues have so far impeded the application of MMMs in industrial environments. This dissertation aims at contributing to more economic, safer and environmentally friendlier gas separation by the experimental preparation and characterization of high performance mixed-matrix membranes. An outcome- based membrane design and development strategy is applied that relies on rational selection of synthesis materials, membrane morphology and comparison of experimental data with modeling results. Different synthesis procedures, polymer matrix, membrane morphology and filler particles are applied to prepare novel membranes. Their performance is investigated in terms of permeability and selectivity under technically relevant conditions given by high operating pressures up to 20 bar. The findings are discussed along the results of various detailed optical and physico-chemical measurement techniques. An attempt is made to rationalize the findings by devising a suitable mathematical model that describes the transport through the membranes. The work is structured as follows: The present Chapter 1 describes the background and relevance of the research problem as well as the general objectives of the work. Chapter 2 discusses the state-of-the-art, including fundamental principles of membrane technology, transport mechanisms, important characteristics of materials and modeling tools. The specific research objectives and the approach followed in this work are laid down in Chapter 3. Chapter 4

17 elaborates details about methods and procedures for membrane synthesis as well as the techniques for characterization of the produced membranes. The results obtained are presented in Chapter 5, which is divided into sections devoted to dense rubbery polymeric membranes, dense glassy polymeric membranes, asymmetric polymeric membranes, pure zeolite asymmetric membranes, dense MMMs, asymmetric MMMs and modeling gas permeability. In each section the evaluated permeabilities are discussed along with the results of thermo-physical characterization, in order to establish a link between synthesized morphologies and obtained performance. Finally, Chapter 6 provides conclusions and further recommendations.

2 State of the Art

The most challenging engineering problem is the separation process, defined as, the operations that transform a mixture in to one or more products of distinct nature [12]. Membrane separations are becoming an emerging industrial separation process, which involve various fluid separation applications [7]. On the basis of their applications membrane processes may be categorized in six sub-groups e.g. reverse osmosis, ultraﬁltration, microﬁltration, pervaporation, electrodialysis and gas separation, [13]. Gas Separation is a challenging separation problem faced by the industry in the form of carbon capture, greenhouse gas separation, natural gas purification, air separation and gas separation from light hydrocarbons. A membrane gas separation process is carried out due to chemical potential gradient, a high pressure feed stream is divided in to retentate and a permeate stream at low pressure [13]. A typical membrane gas separation process is shown in the Figure 2.1 below:

Retentate

Feed

Permeate Figure 2.1 A typical membrane gas separation process

2.1 History of membrane gas separation

Membranes had been laboratory tool for development of various theories instead of industrial development through eighteenth and early twentieth centuries [13]. The earliest studies were performed by Nollet in 1752 [14] when he studied the water and ethanol mixture transport through pig’s bladder and coined the term of “osmosis” . The foundation of gas separation was laid down by Graham when he conducted first ever experiments on the gas permeability through polymers in 1829, further studies on transport of gases through natural rubber were performed [15]. Meanwhile the basic work on material transport through boundary layers was presented by Fick in 1855 [16]. A well-known Graham’s law of mass diffusion was presented in 1866 [15] and the concept of “solution diffusion” was emerged. The work by Graham and Fick provided understanding about gas transport mechanism. The phenomenon of Knudsen diffusion was presented in 1909 by Knudsen [17]. By the development of these elements of membrane science small and specialized industrial applications of membrane gas separation were observed by 1960. However, the widespread of gas separation membranes was restricted due to certain problems like low permeability, low selectivity, lack of reliability and high manufacturing costs [18]. In early 1960s Loeb–Sourirajan process was an important development which transformed membranes from laboratory scale to industrially common technology [13]. These integrally skinned asymmetric membranes were initially employed for reverse osmosis but later adopted by industry for gas separation. The technology for gas separation was capable and economically feasible by early 1980s when gas suppliers, petroleum refineries and chemical companies started to adopt this technology. The first industrial product “Prism A” intended for hydrogen separation from ammonia streams was produced by Monsanto in 1980 [19]. During the 1980s the composite materials attracted the focus of researchers and mixed matrix membranes (MMMs) emerged when Paul and Kemp (1973) first reported MMMs for gas separation [20]. PERMEA, Inc. obtained the patent in the field of asymmetric membranes for gas separation in 1988 [21]. In 1991 Robeson presented his very famous membrane performance trade-off curve known as “Robeson plot” an upper bound for evaluation of permeability and selectivity of polymeric membranes [22].

Application of zeolite for gas separation on the basis of their adsorption properties was reported by Yang et al. (1999) [23]. Use of Zeolite 4A to produce MMMs was reported by Mahajan and Koros in 2002 [8]. Conception of Metal Organic Framework (MOFs) for MMM synthesis was presented by Yoo, Lai, and Jeong group in 2009 [24]. For the application of carbon di-oxide separation from methane, the development of SAPO-34 membranes was reported by Li et al. in 2010 [25]. Mustafa et al. (2010) functionalized carbon nanotubes (CNTs) and synthesized MMM which increased the selectivity of CO2/CH4 for biogas up- gradation [26]. During the last two decades the focus towards composite materials has increased immensely and the development of mixed matrix membranes (MMMs) has emerged as potential solution for gas separation problems. Today the idea of MMMs is realized as solution which combines sieving features of inorganic porous materials and easy processability of polymers to constitute this new class of membranes [6].

2.2 Types and features of gas separation membranes

Membranes are thin films, composed of solid materials e.g. polymers having high surface area and good mechanical stability [27]. Membrane gas separation involves an interaction (physical or chemical) between membrane material and a gas. The ability of a membrane to permeate a gas is termed as permeability (product of permeance and membrane thickness). Another key characteristic of membranes is the selectivity which is defined as the ability of the membrane to separate two gases [7]. Membranes are classified in different ways, on the basis of material of construction they are classified as organic (polymeric) membranes and inorganic membranes [18], Figure 2.2 details the classification of gas separation membranes on the basis of material of construction. Membranes may also be classified as flat sheet and hollow fiber on the basis of module configuration.

Figure 2.2 Membrane classification on the basis of material of construction

Membranes are also classified on the basis of pore size e.g. microfiltration (0.1-2 µm), ultrafiltration (0.005-0.1µm), nanofiltration (0.0005-0.005µm) and reverse osmosis membranes (less than 0.5nm) [18, 28]. Based on membrane structure, synthetic membranes can be classified as isotropic (symmetrical) or anisotropic (non-symmetrical) membranes as shown in Figure 2.3.

Figure 2.3 Types of synthetic membranes, figure adapted from [29]

The membrane can provide a higher permeability and very low selectivity which can be attributed to a highly porous structure or defects in the membrane. . The pore size distribution dictates the selectivity of the membrane. Membrane having larger pore size has a greater flux while smaller pore size has less flux. The Robeson plot (log permeability vs. log selectivity) for dense membrane implies that as the permeability increases the selectivity of the membranes decreases and vice versa, and named as upper bound of membrane performances [27].

2.2.1 Dense membranes

Nonporous, dense membranes are composed of a dense film in which permeant is transported by diffusion under the driving force of pressure, concentration, or electrical potential gradient. Dense membranes are designed in such a way that molecules of different species have different permeability (product of the diffusion

23 coefficient and the sorption coefficient i.e. P = D × S where D is diffusion coefficient and S is sorption coefficient) [29]. The examples of dense membranes are polymeric (glassy or rubbery) membranes e.g. polysulfone, polydimethylsiloxane (PDMS) and palladium membranes for H2 separation. The transport mechanism in palladium membranes is different from dense polymeric membranes where in case of palladium membranes transport takes place due to proton exchange.

2.2.2 Porous membranes

Porous membranes contain rigid porous structure where interconnected pores are randomly distributed throughout the membrane. In porous membranes separation takes place due to difference in molecular size, with the condition that the molecular size should differ considerably. Porous membranes can provide very high permeance but low selectivity [18]. Porous membranes are further divided into micro, macro and meso-porous membranes on the basis of pore size. These may include asymmetric membranes, zeolite membranes, metallic porous membranes and metal organic frameworks (MOFs) membranes. The diffusion mechanism in porous membranes can be classified in to four categories, Knudsen diffusion, surface diffusion, capillary condensation and molecular sieving. These diffusion mechanisms through porous membranes are explained in the Figure 2.4:

Figure 2.4 Diffusion mechanism in porous membranes figure adapted from [7]

2.3 Transport mechanisms

Membranes are thin semi permeable barriers that selectively separate some compounds from others under the influence of a driving force (differential pressure and concentration gradient). All materials that form adequately stable thin film can be used as a membrane materials [18]. The permeation of a gas through a dense membrane takes place stepwise: I – absorption, II – diffusion and III – desorption

[7]. The permeability of the membrane is product of the solubility coefficient S

(thermodynamic parameter) and diffusion coefficient D (kinetic parameter). S depends upon the interaction between the membrane and the permeate species, where as D (kinetic parameter) depends on the membrane’s physical and chemical structure as well as the properties of permeant species.

P = S ∙ D 2.1

Permeability 푃 is defined the amount of permeate that passes through a particular thickness of membrane with unit area, in a given time for a particular pressure difference. It can also be expressed in the form of permeance 푃̃ as follows:

푃 푄 푃̃ = = 푙 퐴 ∙ ∆푝 2.2

P̃ , Permeance; Q , Volumetric flow rate at STP; P , Permeability 푙, Membrane thickness; A, Surface Area

∆p , Pressure difference; α , Selectivity

Commonly used unit for gas permeability P is Barrer which can be expressed as follows:

1 퐵푎푟푟푒푟 = 10 (푐푚 (푆푇푃)푐푚 푐푚 푠 푐푚퐻푔

In case of asymmetric membranes where thickness of membrane is difficult to describe Gas Permeation Unit (GPU) is used. It is expressed as 1 퐺푃푈 = 10 (푐푚 (푆푇푃) 푐푚 푠 푐푚퐻푔 [7]. A membrane having permeance of 1 GPU is equivalent to an intrinsic permeability of 1 Barrer and membrane thickness of 1 µm [30]. The ratio of permeability coefficients of pure gases i and j yields ideal selectivity.

푃 퐷 푆 훼 = = ( )( ) 2.3 푃 퐷 푆

Pure gas permeance 푃̃ can also be used to find idea selectivity as follows [30]: ̃ 푃 2.4 훼 = 푃̃

2.3.1 Transport mechanism for dense and porous membranes

The mechanism by which a membrane transports some gas across depends upon membrane material and composition of feed. The transport mechanisms through membranes are described by two famous models named solution diffusion model and pore flow model.

2.3.1.1 Transport through dense membranes - Solution Diffusion Model

Solution diffusion is a mechanism in which a gas diffuses through a membrane by random diffusion under the influence of a driving force. Gas dissolves in the membrane analogous to gas dissolution in a liquid and diffuses through the membrane, temporary pores / diffusion paths are produced which allow the gas to diffuse [31]. Mathematically membrane permeation can be based on the thermodynamic proposition that the driving forces like pressure, temperature, concentration and electromotive forces are related to each other, and overall driving force responsible for flux of a permeant is the gradient in itschemical potential µ [32].

Chemical Potential,

Pressure, p

Concentration, ci

Figure 2.5 Solution diffusion model, driving force gradients for permeation, figure adapted from [32]

The flux of a permeant i through a membrane is described by the following relationship:

dµ J = 푐 푉 = 푐 푈 dx 2.5

-2 -1 In the equation 2.5 above J (g mol cm s ) is the molar flux, 푐 is the concentration of component i , 푉 is the velocity of component i, and is the

chemical potential gradient while 푈 is the coefficient of proportionality which is not necessarily constant but links chemical potential gradient with velocity.

The chemical potential gradient in the form of concentration and pressure gradients can be represented as follows [29]:

dµ = RTdln(γ x ) + 푣 dp 2.6

Where as x is the mole fraction of component i, γ is the activity coefficient of component i, 푣 is the partial molar volume of the component i and p is the pressure. For the application of solution diffusion model it is assumed that the pressure does not vary inside the membrane and chemical potential gradient is described by concentration gradient of the gas molecules [32]. Combining the above two equations in the absence of pressure gradient gives following expression:

−RT 푐 푈 d (γ x ) J = ( ) ( ) 2.7 γ x 푑푥

By replacing 푛 by c in the above equation and using expression c = 푚 휌 푥 (푚 is the molecular weight and 휌 is the molar density of component i) then the above expression in terms of concentration gradient can be simplified as follows:

−RT 푈 d (γ 푐 ) J = ( ) ( ) 2.8 γ 푑푥

The above equation has the same form as Fick’s law while assuming γ constant and D = RT 푈 .

dc J = −D dx 2.9

Integrating the equation 2.8 with in the limit of membrane thickness gives following expression [29]:

퐷 (푐 푐 ) J = 2.10 푙

Where 푐 and 푐 are the concentrations of component i at membrane interface and at length l.

2.3.1.2 Transport through porous membranes - Pore Flow Model

Porous membranes have interconnected pores which are randomly distributed throughout the membrane in a rigid and highly voided structure. Separation depends upon membrane material, pore size distribution and molecular size of the gas to be separated. In case of porous membranes the pore diameter should be less than the mean free path. Gases having significantly different molecular size are separated by porous membranes [7]. In pore flow membranes gas is transported by pressure driven convective flow. It is assumed that concentration with the in membrane remains uniform and the pressure is responsible for the chemical potential gradient across the membrane as shown below in the Figure 2.6:

Chemical Potential,

Pressure, p

Concentration, ci

Figure 2.6: Solution diffusion model, driving force gradients for permeation, figure adapted from [32].

In case of pore flow model pressure difference across the membrane is responsible for the smooth pressure gradient [32]. Considering pore flow model the equations 2.5 and 2.6 can be combined in the absence of concentration gradient as follows:

푑푝 J = 푐 푈 푣 2.11 푑푥

When the above equation is integrated across the membrane length l and comparing 푘 with 푈 푣 it gives following form which is same as Darcy’s law:

푘 (푝 푝 ) J = 2.12 푙

2.4 Mixed Matrix Membranes (MMMs)

Polymeric membranes have been used for hydrogen separation, natural gas and air industries for gas separation since the 1980s. These membranes are facing challenges due to their compromise between gas permeability and selectivity [8]. Enormous efforts are undertaken to meet this challenge by the development of new materials including zeolites, metal organic frameworks (MOFs), carbon molecular sieves (CMS), carbon nanotubes (CNTs), and graphene. These microporous materials pose various challenges e.g. large scale production and processability. Mixed matrix membrane are composed of a polymeric phase and a dispersed filler phase (micro-porous), these membranes utilize the sieving properties of micro-porous fillers and the ability of polymer being scalable in the form of a suitable membrane module [6]. MMM are generally accepted as a solution because MMM, not only provide better separation characteristics but also provide opportunity for easy processability of polymers [33].

2.4.1 Structure and morphology

Extensive literature has been produced in last ten years on MMMs, which is the evidence that MMMs have become highly preferred morphology. MMMs have immense possible combinations of polymer and filler phase and a variety of morphologies can be designed and fabricated [33]. The objective of the MMMs development is to enhance the gas separation performance of pure polymeric membrane by utilizing separation properties of micro-porous fillers. This objective could be achieved through the development of a MMM which preferably transports the gas through the micro-porous filler. Hence, membrane morphology is the most important characteristic to consider during the development of MMM.

Mixed matrix membranes can be categorized on the basis of matrix interaction and nature of particle distribution as, a) ideal mixed matrix membranes, b) two phase MMM and c) multi-component MMM. Ideal MMM morphology is necessarily without interfacial defects. The two phase MMM morphology may have defects like pore blockage, interfacial voids and chain rigidification. In case of multi- component MMM, there can be two or more type of inorganic fillers can be used and interfacial defects may exist on interfaces e.g. polymer–polymer–inorganic interface or polymer–inorganic–inorganic interface [34].

Figure 2.7 MMM morphologies on the basis of nature of particle distribution, figure adapted from [34]

Due to polymer – filler interface defects four types of morphologies can exist e.g. a) sieve-in-a-cage, b) leaky interface, c) matrix rigidification and d) plugged sieves [35]. On the basis of membrane structure, There are two most common morphologies of MMMs i.e. symmetric (dense) MMMs and asymmetric MMMs [6]. These morphologies are shown in the Figure 2.8:

Figure 2.8 Different morphologies of MMM, figure adapted from [6]

Symmetric (dense) MMMs are the most studied membranes as they are easy to fabricate, however there is often a limit of particle loading termed as “percolation threshold”. In case of conventional MMMs, the transport of gas through membrane is mostly dominated by polymer properties. Dense MMMs also pose low permeability because of high thickness of membranes required for stability reasons which increases the resistance to flow of gas. This may starve the filler particle and a marginal enhancement of separation performance is expected this limits the application of these membranes in industrial environment [6]. Loeb– Sourirajan asymmetric membranes developed in early 1960s provide thin integrally skinned layer on highly porous sub-layer [13]. When asymmetric morphology is applied to MMMs overall membrane resistance is significantly reduced and cross membrane flux is increased, while transfer of inorganic filler particles to the surface is also proposed [6].

2.4.2 Synthesis materials and methods

The major focus of membrane research is high permeability and selectivity of membranes, while in the past polymeric membrane had limited applications due to their trade-off of permeability and selectivity. The prime objective of this focus is to

32 develop defect free interface of MMM while maintain desired separation characteristics of intended membranes. Due to this reason choice of inorganic filler and polymer are of utmost importance [35]. The forthcoming discussion briefly discusses various materials employed for fabrication of MMMs.

2.4.2.1 Polymers for MMMs fabrication Polymer phase intrinsic properties have detrimental effects on the performance of MMMs. There are a huge number of investigations on polymers as a basic starting material for membranes in last few decades. However the compatibility of polymers with inorganic fillers has been the topic of immense interest [6]. Polymeric membrane materials can be classified in two major categories e.g. rubbery ad glassy polymers [6, 36].

Rubbery polymers

Rubbery polymers have characteristics like low glass transition temperature (Tg), high free volume and flexible in nature. The separation of gases by rubbery polymers is affected by the condensability of the gases [36]. Rubbery polymers based on silicones are generally stable and maintain their properties even at harsh conditions e.g. silicones are stable from – 50 to + 300 OC. Mostly used rubbery polymers are polydimethylsiloxane (PDMS) and pyroline oxide-amide copolymers [7].Defect free polymer – filler interface is the most important feature of an ideal MMM and rubbery polymers due to their high chain mobility and flexible structure provide optimum interface. This important feature of high mobility also becomes a disadvantage because of high permeability of rubbery polymer the transport of gas is favored through polymer instead of filler. Due to this attribute the slight improvement of membrane’s separation performance is observed [6].

Glassy polymers

The glassy polymers are rigid polymers with high Tg, low free volume and separates the gases on the basis of size differences [36]. Due to the rigid structure of glassy polymers, the interaction between particle and polymer weakens which causes non-selective voids. Most commonly studied glassy

33 polymers are polysulfone, polyethersulfone, polyimides, cellulose acetate, polycarbonates, polyperfluorodioxoles and poly(phenylene oxide) [6, 7]. It is an established fact that most of glassy polymers undergo plasticization or swelling due to sorption of condensable gases / vapors at high pressure, which negatively affects the performance of membranes by enhancing the permeability and decreasing selectivity [37]. In the light of above discussion it is pertinent that the selection of polymer and membrane morphology for a successful MMM is very crucial task.

2.4.2.2 Inorganic fillers for MMMs fabrication

The last decade have been the era of the MMM materials development, the magnitude of research in the area of inorganic fillers has been remarkable. The conventional fillers like zeolites, carbon molecular sieves and nano-sized silicates have been the focus for MMM development. Such materials can enhance the separation performance of pure polymer by utilizing their molecular sieving ability, high surface area and functionality. Nano-porous zeolites are considered better candidate for MMMs due to their molecular sieving characteristics and surface area. So far, a huge number of zeolites have been tested for their gas sieving and adsorption properties. Micro-porous zeolites are considered to be suitable for separation of CO2 from flue gas streams [9]. Zeolite membranes (Y-type) are demonstrated as potential candidate for CO2/N2 separation [10]. SAPO-34 is microporous chabazite type zeolite having pore diameter 3.8 nm [7] and have high selectivity for CO2 [38]. IUPAC has classified the pore size of inorganic fillers in three categories e.g. microporous, mesoporous and macroporous having pore size respectively < 2nm, 2 nm – 50 nm and > 50 nm [39]. The particle size is also very important in determining the particle / polymer interaction. As particle size decreases, more surface area is available. It has been reported that fine particle size disrupts the polymer chain packing and enhances molecular transport [40], while ultra-fine particle size also pose problem of particle agglomeration. The pore size distribution of fillers also plays an important role in the selectivity of the membrane. Membrane having larger pore size has a greater flux while smaller pore size has less flux [27].

2.5 Modeling gas permeability

Mathematical modeling is the backbone of the design industry, such models provide basis for design and simulation of industrial processes. Because of technological importance of MMM the investigations based on theoretical models and validated by experiments are the basic need for further development in this area. Despite of the fact that transport through composite materials is very complex in nature but still the work by various scientists has led to the development and understanding of this problem in a great deal. Different models have been developed in the past for prediction of transport properties, these models include Maxwell model, Bruggeman model, Lewis Nielsen model and Pal and Felske model [41].

2.5.1 Maxwell Model

Originally the Maxwell model was applied to composite materials to predict their electrical conductivity. However since long time it has been accepted by membrane scientists as a tool to predict the permeability of MMMs with some modifications. When the Maxwell model is used for MMMs it is assumed that the transport properties of a membrane are similar to the dielectric properties of composite materials [6].

푛푃 + (1 − 푛)푃 + (1 − 푛)(푃 − 푃 )∅ 푃 = 푃 * + 2.13 푛푃 + (1 − 푛)푃 − 푛(푃 − 푃 )∅

In the above equation 푃 represents the effective permeability of MMM, 푃 is the permeability of continuous polymer phase, 푃 is the permeability of dispersed phase (inorganic filler), ∅ is the volume fraction of the dispersed phase and 푛 is the shape factor. This model is used for an ideal MMM interface and low particle loading, while for membranes with higher particle loading, where significant voids / defects can be present, further models e.g. Bruggeman model, Lewis Nielsen model, and the Pal and Felske model can be applied [42].

2.5.2 Resistance Model

The transport through asymmetric membranes can be modeled using the Henis and Tripodi model developed in 1981 [43], which is based on assuming flow resistances in series. It was reported that, if the defects in the skin layer are coated with rubbery polymer, the permeability of the coated membrane become similar to that of the solid polymer [19]. In the resistance model, an analogy is drawn between the permeability of a gas through a membrane and electric current flow through a conductor. Ohm’s law can be described as follows

푉 = 퐼 푅 2.14

Above relation correlates electric current I, voltage V and resistance R. From the equation of permeability we know:

푃 푄 = 2.15 푙 퐴 ∆푝

푉 ∆푝 푅 = ∝ 퐼 푄 2.16

By comparing above two equations we get following relationship of membrane resistance:

푙 푅 = 2.17 퐴 푃

Because an asymmetric membrane has a dense skin layer and a porous sub layer, the overall resistance of the membrane (Rt) may be described as a function of the resistance due to the dense (integrally skinned) part (R2), the resistance of the surface pores (R3), and the resistance of the porous support layer (R4). By following analogy of resistances of electrical circuit in parallel and series we get following relationship:

푅 푅 푅 = + 푅 2.18 푅 + 푅

This situation is illustrated in Figure 2.9. It is usually assumed that the resistance from the dense skin layer dominates all the resistances, while the transport through the defects or the pores can be assumed as Knudsen flow or viscous flow depending on the surface porosity of the membranes and defects present in the dense layer [43].

Figure 2.9 Schematic representation of an asymmetric membrane with a porous layer and a dense skin layer (left) and analogy with electric circuits (right). Figure adapted from [43]

In the case, when a very thin coating is applied at the surface of the membrane e.g. a PDMS coating, it is assumed that the pores or defects in the surface are filled with this highly permeable polymer and an additional resistance (R1) is added due to this coating, which is illustrated in Figure 2.10.

In this case, the total resistance Rt can be represented as:

푅 푅 푅 = 푅 + + 푅 2.19 푅 + 푅

Figure 2.10 Schematic representation of composite membrane and analogy with electric circuits, figure adapted from [43]

It is observed in practice that even a very high permeability polymer coating on asymmetric membrane can enhance the resistance in the order of 105- 106 times as compared to an uncoated membrane with the same area and depth. Assuming that the coating polymer does not penetrate into the pores and that it has the same thickness as the skinned layer ( ll23 ), and that the resistance for gas flow through the support layer (R4) is negligible. The total flux can be determined using a resistance model as follows:

∆푝 푄 = , 푙 푙 2.20 + 푃 , 퐴 푃 , 퐴 + 푃 , 퐴

Further simplifications can be made by assuming, that the coating layer is uniform and that AAA. If the membrane has fewer defects at the surface, then 1 2 3

AA32 . One may apply AA12 . Now, the above relationship simplifies to:

푄 푃 푙 푙 = ( ) = ( + ) 2.21 ∆푝 , 퐴 푙 푃 , 푃 , + 푃 , (퐴 퐴 )

The resistance model can be applied to membranes if surface porosity AA32, effective separation thicknessl2 , and coating thickness l1 are available from other characterization techniques.

3 Goals and Approach

Polymeric gas separation membranes face an inherent performance trade-off between their permeability and selectivity. Most of the industrial applications like natural gas purification and flue gas separation require processing of high volumes of feed gas. Ultimately, this requires membranes with high production capabilities along with appreciable separation performance. Due to the mentioned performance trade-off of conventional polymeric membranes there is an urgent need for the development of highly permeable membranes which provide better separation performance as well as are easy to synthesize utilizing cheap materials [19]. Mixed matrix membranes (MMMs) containing heterogeneous components (nano-porous zeolites and polymers) are an attractive approach, since they pose superior permeability, selectivity and stability over conventional polymeric membranes. However, these membranes still lack in performance due to voids and low permselectivity. The development of highly permeable MMMs requires membrane morphologies with least flow resistance coupled with high selectivity. In this regard, a polymer matrix having comparable permeability with that of the filler particles is a better choice as it allows utilization of sieving capabilities of filler particles. The conventional MMM morphology has problems of particles dispersion, void formation, polymer matrix rigidity and low permeability. Various approaches have been used to cope with these issues e.g. the use of silane coupling agents, particle surface modification, thermal annealing, and design of highly cross linked polymers. Each approach has its advantages and limitations; for example, in some lead to partially blocked pores of the dispersed particles, while others deliver thicker membranes accompanied with a reduction of membrane permeability. There are some attempts made by some of the membrane researchers to study simultaneously the effect of membrane morphology, selection of filler particles and polymer matrix on membrane performance. To achieve highly permeable MMMs the following goals are defined for the investigation in this work: I. To compare the dense and asymmetric polymeric morphology and selection of suitable polymer and solvent for development of membrane synthesis protocols.

II. To study the effect of filler type and surface coating on gas transport properties. III. To establish a procedure for the characterization of membrane structure, thermal analysis and gas separation performance for the novel membranes. IV. To predict of membrane permeability and comparison based on experimental data

To achieve above goals a step by step methodology is adopted, starting from development of pure polymeric membrane up to coated asymmetric MMM. Synthesis procedures are continuously improved through simultaneous membrane synthesis and characterization activities. For the development of the membrane synthesis protocol, in the first instance two types of polymers (rubbery and glassy) are used to form dense and asymmetric membranes as flat sheet membranes casted through a Doctor’s blade (tape casting). It is also pertinent to mention that the critical concentration of the polymer is determined using viscosity measurements. A concentration slightly above the critical concentration is employed to achieve polymer chain entanglement to obtain defect free asymmetric skin layer.

The gas separation performance of membranes is characterized using He, H2,

CO2, CH4, and N2. For this purpose a state of the art gas permeation setup based on the constant pressure variable volume method is designed and installed. This apparatus has the capability to test membranes using single gases or mixtures up to 20 bar pressure at ambient temperature. The apparatus employs flat sheet membranes and works in both dead end and cross flow modes. The permeability and corresponding transport mechanism of the membranes synthesized in this work are evaluated. Further, filler particles are also characterized in the form of asymmetric pure zeolite membrane whose permeability and transport mechanism are also investigated. Afterwards, MMMs are formed in three morphologies, e.g. dense rubbery MMMs, dense glassy MMMs, and asymmetric (PDMS coated and uncoated) MMMs. The transport properties of these three types of morphologies are compared on the basis of permeability, selectivity, and separation mechanism (solution diffusion or Knudsen diffusion). The potential morphology of membrane is chosen on the basis of high

41 permeability and selectivity for particular polymer – filler system. It is also considered that polymer matrix does not block the pores of filler particles and allow gas transport through filler particles too. After achieving the required morphology, effect of filler type on membrane gas separation performance is investigated. Further, filler concentration is varied from 10 – 40% to evaluate its effect on gas separation properties. To correlate the membrane structure with transport properties, SEM-EDX is used to determine the membrane structure, type of pores (sponge or fingerlike), state of integrally skinned layer and distribution of filler particles. The presence of impurities in the membrane, e.g. solvent / moisture and the composition of the membrane’s material are confirmed through FTIR measurements. Thermal analysis (TGA, DSC) is used to obtain insights into the effect of parameters like glass transition temperature, crystallinity, melting point, and degradation temperature on membrane performance. Mathematical modeling is the backbone of an engineering design approach of membrane-based processes, since such models provide the basis for process analysis and exploration of process specifications and performance limitations. The results obtained from the above characterization provided inputs for modeling the separation behavior of these membranes. The results of permeance measurements are also compared with the predicted results using Maxwell model and resistance model.

4 Experimental Procedures In the previous section the methodology has been laid down to achieve highly permeable mixed matrix membranes, while this section deals with the materials, methods and procedures to develop and characterize the required membranes.

4.1 Materials The selection of materials used for the synthesis of membranes is the key to successful development of defect free membranes. Physical and chemical properties of the materials as well as their gas transport features are the basis for the selection of membrane materials [44]. Following sections detail the key characteristics of the materials used for synthesis of MMMs:

4.1.1 Polymers The recent developments in the area of polymer synthesis and analysis has led to the precise design and well-ordered structures of polymeric membranes [37]. The basic criteria for the selection of polymers is the permeability, selectivity and ease of processability of the polymer to constitute a membrane. The polymers studied in this work can be classified as rubbery (PDMS) and glassy (polysulfone and polyethersulfone).

4.1.1.1 Poly(dimethylsiloxane) (PDMS) Poly(dimethylsiloxane) is a rubbery polymer and possess high permeability due to its high free volume, chain flexibility, rotational mobility and low glass transition temperature [45]. Therefore, it is widely utilized as a membrane synthesis material for various gas separation problems. Table 4-1 shows selected properties of PDMS membranes from the literature [46].

Table 4-1: Permeability of PDMS membrane for different gases [46] Penetrant Permeability Solubility Diffusivity [Barrer] [cm3 (STP) / cm3 . atm] [cm2 / s] × 10 6

H2 890 0.05 140

N2 400 0.09 34

CO2 3800 1.29 22

CH4 1200 0.42 22

Due to its low surface tension, PDMS has the ability to fill non-selective surface defects efficiently while maintaining high permeability and selectivity of the membrane. Therefore, it is applied in various industrial coatings [47].

In this work, PDMS being a rubbery polymer serves as a polymer matrix with high permeability and moderate selectivity. It is utilized for synthesis of PDMS pure polymer and dense MMMs as well as, it is also used as coating material to seal the surface defects of asymmetric MMMs.

The structural formula of the PDMS is shown in the Figure 4.1.

CH3 H3C H3C CH3 CH3 CH H3C 4 Si Si Si O O CH3 H3C n

Figure 4.1 Structural formula of PDMS

The PDMS (Elastosil RT 601 A / B) was supplied by Wacker AG (Burghausen, Germany) as two parts Elastosil RT 601 A and B, which was used as received. Elastosil RT 601 A / B is a pourable silicone rubber. It has excellent tensile strength and is transparent when vulcanized. The manufacturer recommended ratios of both parts in 9:1 are used [48].

4.1.1.2 Polysulfone Polysulfone is an amorphous thermoplastic glassy polymer having sulfone group as repeating unit. Polysulfone is rigid, thermally and chemically resistant, possess high glass transition temperature (Tg) and have good mechanical strength [49, 50]. In this study polysulfone being a glassy polymer is investigated as a polymer matrix due to its higher selectivity and low permeability as compared to rubbery PDMS. It is used to synthesize asymmetric pure polymer, dense MMMs and asymmetric MMMs. Due to its gas separation properties polysulfone is the most studied polymer for

CO2 / CH4 and H2 / N2 separation. Polysulfone bisphenol A is commercially used polymer mostly used in the form of hollow fiber bundles for gas separation applications [51]. The CO2 plasticization pressure for polysulfone is also higher than polyimide which is 34 bar and 14.8 bar respectively [52].

The structural formula of the PDMS is shown in the Figure 2.1.

Figure 4.2 Polysulfone repeating unit

Polysulfone (Ultrason S6010) was kindly provided by BASF (Ludwigshafen, Germany), it is high molecular weight glassy polymer with excellent chemical and stress crack resistance. It is applied for manufacturing of sanitary and heating parts as well as for membranes [53]. Ultrason S6010 was used after drying in an air circulating oven at a temperature of 105 °C for 2 days.

4.1.1.3 Polyethersulfone Polyethersulfone (PES) is a glassy polymer similar as polysulfone with higher glass transition temperature as compared to polysulfone as well as better long term stability at higher temperature.

The polyethersulfone shows better selectivity for various industrially important gas pairs like O2/N2, CO2/CH4, He/CH4 and H2/N2 as compared to polysulfone and cellulose acetate. It is reported that polyethersulfone has lower permeability, hence to make it competitive with other glassy polymers it is important to produce thinner defect free integral skin layer in asymmetric membranes [7]. In this study, polyethersulfone is utilized as an alternative glassy polymer because of its better selectivity and stability as compared to polysulfone. Polyethersulfone is used to synthesize asymmetric pure polymer and MMMs using both SAPO-34 and activated carbon. The molecular structure of polyethersulfone is shown in Figure 4.3. O

HO S O OH

O m

Figure 4.3 Polyethersulfone repeating unit

In this work, Ultrason® E 6020 P Polyethersulfone (PES) from BASF (Ludwigshafen, Germany) was used. The polymer has a molecular weight 75000 g/mol and a glass transition temperature (Tg) 225 °C [54]. The polymer was supplied in the form of flakes. Prior to further processing polymer was dried at least 2 days at 105 °C in an air circulating oven.

4.1.2 Fillers MMMs are conventionally produced using filler particles like zeolites, silica nanoparticles and carbon molecular sieves (CMS). Selection of the both polymer matrix and filler particles is essential for successful MMMs [9]. Two types of filler particles, zeolite (SAPO-34) and activated carbon (Darko® KB-G) are used in this study.

4.1.2.1 Zeolite - SAPO-34 Silicoaluminophosphates (SAPO) are the crystalline microsporous molecular sieve materials having properties of both zeolites and aluminophosphates. These

46 materials are applied as adsorbents and molecular sieves for separation and purification problems as well as for catalysis. SAPO materials exist in various framework structures including one of the important material structure chabazite (SAPO-34). The silicoaluminophosphates like SAPO-34 has a structure consisting of 8-rings pore openings of 4.3 Å [55]. In this study SAPO-34 is used as filler because it has a pore diameter 3.8 nm [7] and it provides high selectivity for CO2 because of molecular sieving [38]. It is used to synthesize both dense and asymmetric MMMs. The framework structure and building unit are shown in the Figure 4.4 [56].

a) Building Unit CHA

b) Framework viewed normal to [001]

c) 8 ring viewed normal to [001]

Figure 4.4 CHA crystal structure a) CHA building unit, b) framework structure

and c) pore diameter, figure adapted from [56]

In the current study a commercial SAPO-34 zeolite sample was used which was provided by Zeochem (Uetikon am See Switzerland). The SAPO-34 sample was calcined and characterized in-house for XRD, N2-adsorption, pycnomatic density, composition analysis and particle size distribution.

4.1.2.2 Activated Carbon Activated carbon is a low-cost, high surface area, highly porous material and a potential candidate for CO2 separation. Various adsorption based gas separation systems employ this material. Activated carbon has unique properties like pore structure, surface chemistry, adsorbent qualities and thermal stability which make this material attractive for gas separation application [57]. Because of its high surface area and affinity towards CO2 adsorption activated carbon is used in this work. In the current investigation a commercial activated carbon Darco® KB-G (CAS no: 7440-44-0) from Sigma Aldrich (St. Louis, USA) is used to synthesize dense as well as asymmetric MMMs. The material is characterized in-house for pore size, BET surface area, XRD, FTIR and pycnomatic density.

4.1.3 Solvent and non-solvent additive Membrane morphology and separation performance is influenced by the properties of solvent used for membrane synthesis. These solvent properties may include boiling point, density and viscosity. In case of MMMs solvents having high boiling point and viscosity may provide superior membrane performance [58]. The solvent selected for this study is N-Methyl-2-pyrrolidone (NMP) with 99.9% purity was purchased from VWR International Radnor, (USA). NMP has a boiling point of 202 °C and molecular weight of 99.13 g. mol -1. A non-solvent additive (NSA) effects a polymer-solvent system in various ways e.g. it effects solution properties, gas permeance and structure of the membranes [59]. Ethanol by Merck KGaA, Darmstadt Germany was used as NSA in the current study.

4.2 Membrane synthesis methods

To achieve the goal of highly permeable MMMs, one of the key aspects of research methodology is the design of a comprehensive membrane synthesis protocol. To formulate a highly permeable MMM a detailed study of various membrane morphologies starting from pure polymer membrane to coated asymmetric MMM morphology is performed. For the development of each membrane morphology, a unique membrane synthesis protocol is devised. All the membrane protocols can be categorized in three to five major steps e.g. 1) casting solution preparation, 2) membrane casting, 3) solvent displacement / curing, 4) membrane drying and incase of asymmetric membranes 5th step (PDMS) spin coating may also be involved.

The membrane casting (polymeric / MMMs) step is performed with the help of a manual film applicator from ZEHNTNER GmBH Testing Instruments (Model: ZUA- 2000.200). The film applicator has the capability to cast the uniform thickness of membranes on a suitable substrate with adjustable film thickness from 0 – 3000 µm. The film applicator is shown in the Figure 4.5.

Figure 4.5 ZUA 2000 Film Applicator

In the following sections detailed membrane synthesis procedures are presented for polymeric membranes, zeolite (SAPO-34) membranes, dense MMMs and asymmetric MMMs.

4.2.1 Polymeric membranes Polymeric membrane morphologies in the form of dense rubbery, dense glassy and asymmetric are synthesized and characterized utilizing membrane synthesis protocol as outlined in the later sections.

4.2.1.1 Dense rubbery polymeric membrane The synthesis procedure is carried out stepwise, in step – 1 (casting solution preparation), the polymer Elastosil RT 601 A (component A) and Elastosil RT 601 B (component B) are mixed together in a ratio of 9:1 (wt. / wt.) in a glass beaker. Both components are mixed vigorously using spatula for 20 minutes. Afterwards the solution along with beaker is placed in an ice bath to maintain the temperature of the solution to 0 °C, and further mixed for 20 minutes. After achieving the homogeneous polymer solution, the mixture is placed in an ultrasonic bath at room temperature for 15 minutes to remove the trapped air. In step – 2 (membrane casting), immediately after sonication the polymer solution is casted with a pre-set thickness using Zehntner film applicator on cleaned polyester sheets of thickness of 0.10 mm. In step – 3 (solvent displacement / curing), the casted film along with sheets are carefully transferred to an air circulating oven for 30 minutes at 70 °C for curing. Afterwards the sheets are removed from oven and placed in dark for further 24 hours for completion of curing. The process flow of the method is shown in Figure 4.6. The membranes are separated from sheet with hand and are cut in required size with the help of scissors.

Membrane Curing Component A Component B casting

Sonication Stirring

Step - 1 Step - 2 Step - 3

Figure 4.6 Dense polymeric (rubbery) membrane synthesis process

4.2.1.2 Dense glassy polymeric membrane For synthesis of dense polymeric membrane similar procedure is adopted as in section 4.2.1.1 with variations in casting solution preparation (step-1) and solvent displacement / curing (step-3). In step-1, polymer and solvent (NMP) are weighed in required quantities separately. Solvent is added to a 250 ml bottle with air tight cap. Polymer is added to the bottle slowly in small portions while stirring the

51 mixture until complete mixing of whole of the polymer. It took about 3 days for complete mixing of the polymer. Then sonication of the polymer solution was carried out at room temperature for 30 minutes to remove trapped air. In step-2 (membrane casting), immediately after sonication the polymer solution is casted in preset thickness using Zehntner film applicator on already cleaned glass sheets. Solvent is evaporated in step – 3 (solvent displacement / curing) , the film along with glass sheets are carefully transferred to vacuum oven at 10 mbar pressure and 65 °C temperature for 24 hours. After the evaporation of solvent the glass sheets are removed from oven and placed in a desiccator for cooling. After cooling the membrane are separated carefully from glass sheet with hand and are cut in required size with the help of scissors. The process flow of the synthesis method is shown in Figure 4.7.

Solvent Polymer Solvent Membrane casting displacement

Sonication Stirring

Step - 1 Step - 2 Step - 3

Figure 4.7 Dense polymeric (glassy) membrane synthesis process

4.2.1.3 Asymmetric polymeric membrane Asymmetric polymeric membranes are synthesized by wet/dry phase inversion method. Step-1 (casting solution preparation) is the same as section 4.2.1.2 with the addition of non-solvent additive (NSA). After polymer solution preparation is achieved, membrane is casted in step-2 with film applicator as discussed in section 4.2.1.2. In step – 3 (solvent displacement) after casting the membrane the sample is left for dry phase inversion in ambient air for 60 seconds and then proceeded further for coagulation. For coagulation two routes can be followed e.g. a) single bath coagulation and b) dual bath coagulation. In single bath coagulation casted membrane is carefully placed in distilled water bath for 24 hours afterwards membrane is placed in methanol bath for further 24 hours. In dual bath coagulation, first membrane is placed in 50/50 water – NMP coagulation bath for 30 seconds and then placed in methanol bath for 24 hours. After coagulation is

52 complete in step -4 (membrane drying) the membranes are dried in ambient air for 4 hours and then placed in vacuum oven at 10 mbar pressure and 65 °C temperature for 24 hours, then membranes are shifted to desiccator for cooling and storage. Finally, in step – 5 (spin coating) membranes are spin coated with PDMS. The details of spin coating procedure are given in section 4.2.1.4. For permeation measurements the membranes are cut with the help of scissors in required size. The above process flow is presented in the Figure 4.10.

Non-solvent Solvent Polymer Solvent Membrane casting displacement Single bath coagulation Sonication Dual bath Stirring coagulation

Step - 1 Step - 2 Step - 3

Spin coating Membrane drying

Vacuum oven

Step - 5 Step - 4

Figure 4.8 Asymmetric polymeric membrane synthesis process

4.2.1.4 Spin coating with PDMS In case of asymmetric (polymeric / mixed matrix) membranes to seal surface defects PDMS spin coating was applied. PDMS solution was prepared by adding a cross linker (Elastosil®RT601B) to PDMS (Elastosil®RT601A) in a ratio 1 to 9, the mixture was mixed thoroughly for 15 minutes at room temperature and then n- Hexane (EMSURE Merck KGaA) was added as the solvent to prepare a 5 wt% polymer solution. Further the solution was mixed with magnetic stirrer for overnight. Spin coater (Model WS-65OHZ-23NPP/LITE from Laurell Technologies Corporation) was used for spin coating. The membranes coating was achieved by two steps spin coating process. First the PDMS solution was dispensed

53 dynamically on the membrane at 500 RPM for 30 s, followed by 1500 RPM for 90 s. Afterwards the membranes were put in an oven at 70 °C for 30 minutes for curing. Further the membranes are stored in desiccator for 24 hours in dark.

Figure 4.9 Process flow for synthesis of asymmetric MMM using polymer and filler

4.2.2 Pure zeolite (SAPO-34) The pure zeolite membranes supported on stainless steel support were synthesized by the research group of porous materials and hierarchal systems at Institute of Chemical Reaction Engineering and generously provided for further testing for gas permeation at a pressure range of 1-20 bar. The in-situ hydrothermal synthesis procedure was followed for synthesis of SAPO-34 membranes. The membranes contained small (up to 200 nm) to medium (2 – 5 µm) sized zeolite crystals. Different method was employed for each smaller and medium sized crystal membranes. In both cases the porous stainless steel support was masked with a Teflon tape at one side and placed in a PTFE lined autoclave along with synthesis solution. A specific PTFE cylindrical holder was used to position the masked porous support in the vertical position in the autoclave. For the synthesis of smaller sized crystal SAPO-34 membranes, first of all in a 300 ml beaker tetraetyhlammonium hydroxide (TEAOH) 35% (wt. / wt.) was added. The solution in the beaker is continuously stirred with a stirrer at 450 rpm,

54 while Aluminum isopropoxide is added slowly and then stirring was continued for further 2 hours. Phosphoric acid 85% (wt. / wt.) is added slowly with a dropping funnel. Again the solution is stirred for a further 30 min and then pH of the solution is determined. Transferred approximately 20-30 g of the synthesis solution into an autoclave for hydrothermal synthesis at 195 °C for 48 h. For the synthesis of membrane with medium sized SAPO-34 crystals, in a 300 ml beaker, a solution of phosphoric acid 65% (wt. / wt.) is added and stirred at 800- 900 rpm with magnetic stirrer. Afterwards, added required amount of aluminum isopropoxide while stirring. TEAOH was added to the solution and stirring was continued for 30 minutes at 500 rpm. Then added 3g of Ludox HS 40 and 1 g of distilled water then further stirring was continued for 3 days at 500 rpm. Upon completion of stirring, 20-30 g of the synthesis solution is transferred into an autoclave for hydrothermal synthesis at 195 °C for 48 h. After hydrothermal process is complete, the membranes are thoroughly washed with distilled water, afterwards membranes are dried at 100 °C in air circulating oven for 24 hours. After drying membranes are calcined at a heating rate of 0.2 K min-1 up to 400 °C followed by a holding time of 16 h, the cooling was performed at a cooling rate of 0.3 K min-1 and the flow rate of air was maintained at 50 ml min-1.

4.2.3 Mixed Matrix Membranes MMMs are synthesized in two morphologies e.g. dense MMMs and asymmetric MMMs. The synthesis procedures are given in following sections for each morphology.

4.2.3.1 Dense mixed matrix membranes For synthesis of rubbery dense MMM and glassy dense MMM different methods are used. In case of rubbery dense MMM, in step – 1 (casting solution preparation), the polymer Elastosil RT 601 A (component A) and Elastosil RT 601 B (component B) are mixed together in a ratio of 9:1 (wt. / wt.) in a glass beaker. Both components are mixed vigorously using spatula for 15 minutes, then filler particles are added to the polymer mixture and the mixture is further mixed for 5 minutes. Afterwards the solution along with beaker is placed in an ice bath to maintain the temperature

55 of the solution to 0 °C, and further mixed for 25 minutes. After achieving the homogeneous polymer – filler mixture it is placed in an ultrasonic bath at room temperature for 15 minutes to remove the trapped air. The further procedural steps of membrane casting and membrane curing are similar to the procedure discussed earlier in section 4.2.1.1 (Dense rubbery polymeric membrane). The process flow of the method is shown in Figure 4.12.

Component B Membrane Curing Component A Filler casting

Sonication

Step - 1 Step - 2 Step - 3

Figure 4.10 Process flow for synthesis of dense MMM using DMS and filler

In case of glassy dense MMM, in Step – 1, polysulfone and solvent (NMP) are weighed in required quantities separately. Solvent is added to a 250 ml bottle with air tight cap. Polysulfone is added to the bottle slowly in small portions while stirring the mixture until complete mixing of whole of the polymer. It took about 3 days for complete mixing of the polymer. The polymer solution is sonicated for 30 minutes followed by addition of required amount of filler particles. The polymer – filler mixture is periodically mixed for 24 hours. Before casting the membrane the mixture is sonicated at room temperature for 30 minutes to remove the trapped air. Further steps of membrane casting and solvent displacement are similar to the procedure discussed earlier in section 4.2.1.24.2.1.1 (Dense glassy polymeric membrane). The process flow of the synthesis method is shown in Figure 4.11.

Solvent Membrane Solvent Polymer Filler casting displacement

Sonication

Step - 1 Step - 2 Step - 3

Figure 4.11 Process flow for synthesis of dense MMM using glassy polymer and filler

4.2.3.2 Asymmetric mixed matrix membranes The synthesis procedure for asymmetric MMM is a 5 step procedure, starting from casting solution preparation to spin coating of membrane. In step – 1, solvent and non-solvent are added to a 250 ml bottle with air tight cap and mixed. Afterwards polymer is added to the bottle in small portions while stirring the mixture until complete mixing of the polymer. The mixing of polymer took about 3 days till homogeneous mixture. The polymer solution is sonicated for 30 minutes followed by addition of required amount of filler particles. The polymer – filler mixture is periodically mixed with spatula for 24 hours. Before casting the membrane the mixture is sonicated again at room temperature for 30 minutes to remove the trapped air. Further steps of membrane casting, solvent displacement (coagulation), membrane drying and spin coating are similar to the procedure discussed earlier in section 4.2.1.3 4.2.1.1(Asymmetric polymeric membrane). The process flow of the synthesis method is shown in Figure 4.12.

Non-solvent

Solvent Membrane Solvent Polymer Filler casting displacement Single bath coagulation Sonication Dual bath coagulation

Step - 1 Step - 2 Step - 3

Spin coating Membrane drying

Vacuum oven

Step - 5 Step - 4

Figure 4.12 Process flow for synthesis of asymmetric MMM using polymer and filler

4.3 Characterization Methods

4.3.1 Viscosity measurement Viscosity measurements were done for polymer solutions of polysulfone, polyethersulfone and polysulfone – zeolite mixtures. For sample preparation 10 wt%, 15 wt%, 20 wt% and 25 wt% of polymer were dissolved in NMP as solvent. An ultrasonic bath was used to remove the air from the polymer solution. An Anton Paar MCR 102 rheometer (Graz, Austria) was used for viscosity measurements. Viscosity was measured over a shear rate interval of 0.1-100 s-1 at 25 °C by using the cone and plate configuration and a sample volume of 0.5 ml was used.

4.3.2 Fourier Transform Infrared Spectroscopy (FTIR) FTIR measurements are performed to determine the composition and configuration of chemical groups within the membrane structure. These measurements served for the composition analysis of membranes and confirmation of any impurities (e.g. solvent / non-solvent). FTIR provides a fast

58 and easy technique to identify functional groups in the test material. Infrared measurements were done with the help of JASCO FT/IR-4100 LE spectrometer (Oklahoma, United States). The program Spectra Manager was used for data analysis. First of all background correction was performed then sample measurements were done in the mid-infrared region (4000-400 cm-1). The assignment of peaks for different functional groups is presented in Table 4-2.

Table 4-2: The FTIR spectra functional group assignment for PDMS, PSF, PES and MMMs [60] Wavenumber Component Functional group assignment /cm-1 aliphatic methyl group C-H stretching 2870 (sym) aliphatic methyl group C-H stretching 2960 PSF, PES, (asym) PDMS aliphatic methyl group C-H bending 1380 (sym) aliphatic methyl group C-H bending 1475 (asym) aromatic C-H stretching 3100-3000 aromatic overtone and combination 2000-1700 bands PSF, PES aromatic C=C stretching 1600-1430 aromatic “In-plane” C-H bending 1275-1000 aromatic “Out-of-plane” C-H bending 900-690 C-S stretching 700-600 SO stretching (sym) 1190-1120 PSf,PES 2 SO2 stretching (asym) 1390-1290 S=O stretching 1060-1020 O-H stretching 3600, 3300-2500 C=O stretching 1700 aliphatic ketone C=O stretching 1730-1700 PSF, PES C-O-H in -plane bending 1430 C-O-H out-of-plane bending 930 C-O-C stretching 1240, 1100 Si-CH3 bending (sym) 1280-1250 Si-O stretching 825-600 PDMS Si-O-Si stretching 1130-1000 C – H stretching 2870 Activated C – C stretching 700-400 carbon SAPO-34 Si–O stretching 1120-1000 NMP >C=O stretching of the hydrogen bond 1673

4.3.3 Differential Scanning Calorimetry (DSC) The glass transition temperature and degree of crystallization of polymers are the important parameters obtained from DSC. Using the heat flux versus temperature plot, the shift in the baseline can be detected which provides glass transition temperature while degree of crystallization is obtained from the area under the

60 curve. To analyze change in physical properties of polymer with temperature against time DSC is an easy technique. Tg can also be related with permeability and mechanical behavior of the polymers. This technique involves measurement of heat flux of a sample and a reference as a function of temperature during heating or cooling cycles while keeping both samples at the same temperature. During glass transition and melting heat is absorbed, resulting in difference of heat flux through sample and reference. In this study the DSC 200F3 (NETZSCH) is utilized. To specify measurement procedure, temperature (°C), heating rate (Kmin-1) and time (min) were set for measurement. The heating rate of 10 Kmin-1 was used while samples were heated till 450 °C.

4.3.4 Thermogravimetric Analysis (TGA) TGA is used to analyze the change in sample mass versus temperature. It can be utilized for the analysis of thermal, oxidative and decomposition behavior of the sample. It can also be used for composition analysis like determination of moisture contents, volatile matter and presence of solvents. In this technique the thermal curve which is presented as weight percentage (or weight) versus temperature (or time). TGA was performed using TA instruments AutoTGA 2950HR V5.4A . A sample weight of about 2 mg samples was introduced to the pre-tarred platinum pans and heated from 25 °C or 35 °C and finally to 700°C at a rate of 20 °C/min in N2 atmosphere. The TGA thermograms are used for the analysis of thermal degradation of membrane and determination of residual solvent.

4.3.5 Scanning Electron Microscopy (SEM - EDX) To evaluate the membrane morphology SEM - EDX by LEO 435VP, Carl Zeiss AG (Oberkochen, Germany) was used. The membrane samples (cross-section) for SEM – EDX were prepared cryogenically using liquid nitrogen and coated with gold to prevent charging of the sample using sputter coater Q150/ S by Quorum Technologies (Lewes, United Kingdom). SEM – EDX provides information about surface topography and inner structure of membrane. It also provides elementary analysis of species present in the structure [61]. SEM performed for the zeolite

61 sample and an open source software ImageJ is used for determination of particle size distribution optically.

4.3.6 Methods for separation performance characterization The characterization of membrane separation performance is the core of membrane research and can be expressed by three fundamental parameters i.e. Permeability, b) diffusion and c) solubility. Permeation measurements are performed using gases like helium, hydrogen, methane, carbon dioxide and nitrogen. The permeation measurements are performed in the order He, H2, CH4, CO2 and N2. All test gases used were obtained from Linde AG (Munich, Germany). The gases, their purity and properties are listed in Table 4-3.

Table 4-3: Gases used and their properties

Molecular Lennard- Gas weight Jones kinetic Purity Level Purity / Vol.% diameter [62]

H2 2 2.89 5.0 99.999 He 4 2.60 4.6 99.996

CH4 16 3.80 2.5 99.500

CO2 44 3.30 4.5 99.995

N2 28 3.64 5.0 99.999

Two methods are generally employed for permeation measurements, constant volume (CV) variable pressure method and constant pressure (CP) variable volume method [7].

4.3.6.1 Constant Volume Variable Pressure Method The constant volume method employs a system in which a cylinder with pre- calibrated volume is pressurized to predetermined pressure and connected to the upstream of the membrane module while another calibrated cylinder under vacuum is connected at permeate side. As the permeation starts the pressure starts rising in the cylinder at permeate side and automatic sensitive sensors recorders are used to record the pressure after defined intervals. This increase in

62 pressure is correlated using model equations to determine permeability of the membrane.

4.3.6.2 Constant Pressure Variable Volume Method This method, which is applied in this work, relies on permeation measurement at constant trans-membrane pressure. A predefined pressure is applied and maintained at upstream of membrane while the permeate side remains at constant pressure i.e. atmospheric pressure. Suitable pressure control and measurement devices are used for controlling and measuring upstream pressure while a bubble flow meter is used for measurement of volumetric flow rate of permeate at atmospheric pressure.

4.3.6.3 Apparatus for Permeation Measurements As part of this work, to perform permeation measurements, a state-of-the-art apparatus based on constant pressure variable volume method was designed and installed. The apparatus is capable of measuring the permeability of flat sheet polymeric / mixed matrix membranes (dia. 18 mm / 85 mm ) as well as pure zeolite membranes supported on stainless steel porous support (dia. 18 mm). The apparatus can be used to measure pure gas (H2, He, N2, CO2 and CH4) permeance as well as mixed gas permeance at a pressure of 1-20 bar at room temperature. The setup for permeation measurement is shown in Figure 4.13 and the flow scheme is illustrated in the Figure 4.14. The flow scheme is divided in 6 major portions 1) pure gas supply line, 2) feed mixing vessel, 3) feed pressure regulator and preheater, 4) membrane module, 5) permeate line and 6) retentate line. The membrane is mounted on a highly porous sintered stainless steel support in the membrane module using special sealing arrangement including seals and Viton O-rings. The membrane module is tightened from opposite side screws turning them each time approximately 90O till the complete tightening of the module. At the setup all the valves and pressure regulators are manually operated and electronic devices are only used for data acquisition. Kiethley 2700 data acquisition device (Ohio, USA) is used for data acquisition from pressure sensors. Pressure and temperature sensors are calibrated before installation. For the

63 permeate flow rate measurements A-Class bubble flow meters 1, 5, 10, 25 and 50 ml are used. The waste / excess flow streams are routed to the exhaust system using flexible PTFE pipes. Rupture discs up to 60 bar are installed for feed vessel protection while feed gas inlet line (20 bar), GC inlet line (2 bar) and carrier / purge gas line (5 bar) are secured by installation of safety valves. Further, in the event that flammable gas (H2 and CH4) leakage takes place, the lab is equipped with a gas detection and alarm system.

Figure 4.13 Permeability measurement setup

T G

I F

P P

Figure 4.14 Flow scheme for permeability measurement

4.3.6.4 Single gas permeance

Single gas measurements are performed using helium, hydrogen, methane, carbon dioxide and nitrogen one by one in the order He, H2, CH4, CO2 and N2. First of all membrane is mounted in the module as described in section 4.3.6.3, then the whole set up is purged with the test gas sufficiently before commencement of permeation. The required temperature of feed gas is maintained with the help of preheater and required gas pressure is also set at the feed side using pressure regulators. The temperature and pressure is maintained until steady state is achieved, then pressure difference (∆p) across the membrane, feed gas temperature (푇) and permeate volumetric flow rate (푄) are measured. For the measurements at higher pressure, the pressure is increased using feed pressure regulator and temperature is maintained throughout the measurement. Measurements are performed at each predefined pressure interval and data is recorded. Before starting the measurement with new gas the whole setup is depressurized and purged again with test gas as described earlier. Equation 2.1 is used to convert the measured volumetric flow rate to STP. Permeance and permeability can be calculated using equation 2.1 and the selectivity can be calculated using equation 2.4.

273.15퐾 4.1 푄 = 푉̇ 273.15퐾 + 푇

Where 푉̇ is the volumetric flow rate at ambient conditions in (cm3∙s-1)

4.3.6.5 Mixed gas permeance

Mixed gas measurements are performed in the same way as single gas measurements, except preparation of feed gas mixture and analysis of permeate composition. The feed mixture is prepared in required proportion on the basis of partial pressure of test gases. A sample gas mixture of CO2 and CH4 is studied in this work. Before preparation of feed mixture the whole setup is purged with one of the test gas and then the gases are filled to required pressure one by one. The system is left for 30 minutes to achieve homogeneous composition throughout the cylinder and feed supply line. The whole system is depressurised except the feed cylinder. Afterwards the gas supply and retentate lines are purged with the feed

67 gas. Before starting the experiment the sample of feed gas for composition analysis is filled in the multi-port valve attached at the analysis line. The required temperature of feed gas is maintained with in the preheater and required gas pressure is also set at the feed side using pressure regulators. The temperature and pressure is maintained until steady state is achieved. Back pressure regulators are used to keep a constant pressure and flow of retentate (10 ml / minute). Permeate and analysis lines are purged sufficiently for 10 – 30 minutes depending on the permeability of the membrane. After purging the lines, the pressure difference (∆p) across the membrane, feed gas temperature (푇) and permeate volumetric flow rate (푄) are measured. Afterwards the three-way valve is moved in the direction of permeate and the designated port of multi-port valve is filled with permeate sample. For the measurements at higher pressure, the pressure is increased using feed pressure regulator and the same procedure is repeated. Measurements are performed at each predefined pressure interval and data is recorded.

The mixed gas permeance 푃̃ can be calculated using equation 4.2. 4.2 y Q 푃̃ = A(p x − p y ) x and y are the mole fraction of carbon dioxide or methane in the feed and permeate. The pressure at the feed side is denoted by p (cmHg) and the atmospheric pressure on the permeate side by p (cmHg). All other variables are the same as for single gas measurements. The mixed gas composition analysis for permeate and feed was carried out by an Agilent 6890 Series Gas Chromatograph (California, USA). TCD detector and Restek’s (PA, USA) ShinCarbon St Micro-packed GC Column were used, while helium was used as the mobile phase at 5 bar pressure. The program Chemstation (Agilent Technologies) is used for data acquisition and analysis.

4.3.6.6 Analysis of Measurement uncertainty for permeation measurement

Measurement uncertainty is a parameter associated with a measurement and characterize the dispersion of values observed during the measurement process [63]. The Uncertainty calculations are performed to determine the deviation of permeance results. Uncertainty measurements are performed using He as a test

68 gas. In measurement uncertainty repeatability and reproducibility are the major factors contributing to the deviation. The uncertainty factors can contribute to deviation in the form of repeatability and reproducibility, to calculate the overall uncertainty these factors are evaluated using statistical analysis Repeatability measurements are carried out using the same operating conditions over a short period of time and while reproducibility measurements are performed using the same sample and measurement device on different days. The standard deviation of each contributor is calculated and further divided with its probability distribution factor to calculate standard uncertainty. The standard uncertainties of each contributor are squared and added together, further the square root of the sum is obtained which is the combined standard uncertainty CU. Further the expanded uncertainty EU is determined by multiplying the combined standard uncertainty with the coverage factor of two [63]. The input quantities for the determination of repeatability and reproducibility are given in Table 4-4. Combined standard uncertainty and the expanded uncertainty are presented in Table 4-5.

Table 4-4: Input quantities for evaluation of uncertainty

Press Input quantities Repeatability ure ∆U/ ∆T/° V̅/c ∆V/ P̅ /GP /bar U̅/V T̅/°C t/̅ s σ /s He σ /GPU V C m3 cm3 ̅ U ̅ 3.99 1.58 0.00 23.70 0.10 0.50 0.02 14.00 0.13 42.77 1.76

9.96 3.98 0.01 23.80 0.10 1.00 0.02 10.97 0.06 43.71 0.91

15.99 6.40 0.01 24.00 0.10 1.00 0.02 6.34 0.04 47.09 1.00

19.90 7.97 0.01 24.00 0.10 1.00 0.02 4.82 0.02 49.83 1.02

Reproducibility

3.99 1.58 0.00 23.30 0.27 0.50 0.02 14.78 0.77 40.85 2.67

9.96 3.98 0.00 23.43 0.23 1.00 0.02 11.81 0.42 40.64 1.65

15.98 6.40 0.00 23.57 0.26 2.00 0.02 13.95 0.45 42.89 1.45

19.90 7.97 0.00 23.57 0.26 2.00 0.02 10.71 0.43 44.88 1.85

Table 4-5: Combined standard uncertainty CU and expended uncertainty EU

Pressure/bar CU/GPU EU/GPU 3.99 3.20 6.40 9.96 1.88 3.77 15.98 1.76 3.52 19.90 2.12 4.24

4.3.7 Mean pore size and effective porosity measurements In this work permeation measurements are performed by varying the feed gas pressure, these measurements are used to calculate the mean pore size and effective porosity of the membranes. In this method gas flux and pressure data is used to relate the gas transport with Poiseulle and Knudsen diffusion using “Dusty

Gas Model” [64] . The permeance (푃̃ ) is plotted against mean pressure (푝 ) and equation 4.3 is used, where α and β are intercept and slope of the straight line and obtained by linear fitting of pressure and permeance data. Using these two

70 parameters the mean pore radius (푟 ) and effective porosity (ε) can be calculated using following equations.

푃̃ = 훼 + 훽 푝 4.3

Mean pore radius and effective porosity can be calculated using following relation:

16 훽 8 푅 푇 . 푟 = ( ) ( ) µ 3 훼 휋 푀 4.4

훳 8 푅 푇 µ 휀 = = 훽 퐿 푟 4.5

Where 푅 universal gas constant, 푇 represents the temperature, 푀 molecular weight of the gas and µ is the viscosity of the gas, while 훳 and 퐿 are actual porosity and effective pore length respectively.

4.3.8 Pycnometric Density Measurement The solid density of zeolite sample and polymeric membranes is measured by instrument called Pycnomatic ATC by Thermo Fisher Scientific (Massachusetts, USA). The instrument determines the density of the material by helium displacement method. Vacuum dried samples are used for density measurements. The device has built-in system for recoding and analyzing the measured data and can operate in a range of 18-35 °C ± 0.01 °C.

4.3.9 Nitrogen Adsorption To investigate BET surface area, pore volume and pore size of zeolite and activated carbon N2-Adsorption measurements are performed. The N2- physisorption was studied at 77 K using Quadrasorb™ SI gas adsorption analyzer. Prior to the analysis, the pre-treatment of the samples was carried out for 12 h at 250 °C in vacuum.

5 Results and Discussion

This chapter presents the results of the membrane structure and performance analysis, and discusses the results obtained from the characterization of polymeric, zeolite and mixed matrix membranes developed in this study. The transport properties of both polymer and filler play an important role in the development of MMMs. To achieve the desired effect of the filler phase within a MMM, the polymer phase needs to allow the transport through the filler phase [65]. The gas transport in conventional dense MMM is mostly influenced by the polymer phase (highly permeable) due to which the separation properties of filler particles cannot be fully utilized to enhance the separation performance of the membrane [6]. On the other hand selection of highly impermeable polymer phase can starve the filler of the gas resulting in compromise in membrane performance [66]. To develop a synergy between polymer and filler phase, the investigation of both dense and asymmetric polymeric membranes are analyzed and further utilized as basis of MMMs development. In this context, results of fundamental characterizations and gas separation performance have a strong link, which is established through cross referencing among different sections throughout this chapter. It is difficult to characterize thickness of skin and porous layer in case of asymmetric membranes, which makes it difficult to calculate the permeability of such membranes in Barrer. Due to this reason, it is convention to use GPU as permeance unit in literature. Hence, following this convention throughout this chapter, permeance of asymmetric membranes is expressed in GPU units, while for dense morphologies, permeability is given in Barrer.

5.1 Fourier Transform Infrared Spectroscopy (FTIR) Membrane permeability and microstructure is affected by residual solvent available in membrane during the synthesis process [67]. The solvent present in a membrane may also act as plasticizer and affect membrane pore size and pore size distribution, which influences membrane performance [68], [69]. FTIR serves to analyze the composition of membranes as well as presence of impurities. Further information on the FTIR measurement method is given in section 4.3.2.

The determined FTIR spectrum of dense PDMS membrane (Appendix-A) shows characteristic peak at 784 cm-1 (Si–O stretching) and 1006 cm-1(Si–O–Si asymmetric stretching). The Si-CH3 group gives a sharp peak at 1256 cm-1, while a small peak at 2962 cm-1 is assigned as stretching of C – H group from methyl. These characteristic peaks confirm the formation of PDMS and absence of other impurities. The spectra of pure polysulfone membrane shows strong bands at 1146 cm−1 (C– −1 −1 SO2–C symmetric stretching), 1234 cm (C–O–C stretching), 1293 cm (S=O −1 −1 stretching), 1485 cm (CH3–C–CH3 stretching), C-S stretching at 688 cm and C=C aromatic ring stretching at 1583 cm−1. The measured spectra of pure polyethersulfone −1 membrane shows peaks at 1147 cm (C–SO2–C symmetric stretching), 1236 −1 −1 −1 cm (C–O–C stretching), 1296 cm (S=O stretching), 1484 cm (CH3–C– −1 CH3 stretching), C-S stretching at 699 cm and C=C aromatic ring stretching at 1576 cm−1. In both cases the characteristic peak of NMP (solvent) at 1673 cm-1 (stretching of the hydrogen bonded >C=O) is absent in the spectra, which shows that membrane is free of solvent. Asymmetric MMMs are also characterized for confirmation of PDMS coating on membrane. The strong distinctive IR bands at 1484 cm−1 and 1576 cm−1 are observed in all PDMS coated asymmetric PES MMMs, simultaneously a short peak at 1010 cm-1(Si–O–Si asymmetric stretching) of PDMS was also present. The strong peaks of PES at 1484 cm−1 and 1576 cm−1 bands confirms the presence of PES at surface along with a very thin layer of PDMS. The FTIR analysis of zeolite (SAPO-34) and activated carbon were also performed which has good correlation with literature, further no impurities were observed in these materials.

5.2 Differential Scanning Calorimetry (DSC) Differential scanning calorimetry is used to characterize the thermodynamic state of the membranes through glass transition temperature. The glass transition temperature (Tg) does not occur on a fixed temperature, but it occurs in a range of temperatures. Glass transition temperature is important for characterization of polymer dimensional stability as well as rigidity against the parameters of time and temperature [70]. The glass transition temperature of PDMS, PSF, PES, and MMMs was determined and presented in Table 5.1, while the DSC thermograms

73 of various membranes are attached as Appendix – B. The method for Tg measurements is explained in section 4.3.3. It is shown in the Table 5.1 that the glass transition temperature of pure polymer membranes is comparable with the values reported in the literature. It is observed that glass transition temperature of MMM is lower as compared to pure polymer membrane, which is due to the addition of filler particles. The filler particles may act as a plasticizer and reduce the glass transition temperature of the membrane by increasing the free volume in the polymer [70].

Table 5.1: Glass transition temperature (Tg) for PDMS, PSF, PES and MMMs

Glass transition Literature Tg Membrane temperature (OC) [70] Tg (OC)

Pure PDMS -122 -123

Asymmetric PSF 196 195

Asymmetric PES 223 220

PDMS-SAPO-30% -120 NR

Dense PES-SAPO-30% 226 NR

Dense PSF-SAPO-30% 194 NR

Uncoated asymmetric PES-SAPO- 183 NR 30%

Uncoated asymmetric PES-AC-30% 196 NR *NR – Not reported

Figure 5.1 shows the effect of addition of filler contents (SAPO-34 and activated carbon) on the glass transition temperature of PES. It is observed that the value of Tg reduces with increasing amount of filler. The glass transition temperature of membranes with activated carbon appears to be slightly higher at filler concentration above 20%, which can be attributed to better interaction of PES with activated carbon as compared to SAPO-34.

250

240 PES-SAPO-34

230 PES-AC

220

210

C) O

200 Tg ( Tg 190

180

170

160

150 0 20 40 60 % Filler (SAPO-34)

Figure 5.1 Effect of filler content on glass transition temperature Tg of uncoated asymmetric PES MMMs with SAPO-34 and activated carbon (AC)

5.3 Thermogravimetric analysis (TGA) Thermogravimetric analysis is used to determine the thermal degradation, physical and chemical changes in membranes. TGA gives information about evaporation, sublimation, desorption, adsorption, decomposition, and solid-gas reactions for the sample under investigation. In this study, TGA measurements (see section 4.3.4 for experimental details) are performed for the membrane synthesis materials SAPO-34, activated carbon, as well as pure polymer and MMMs.

110

100 SAPO-34

Activated Carbon 90

70 Weight (%) Weight

50 0 100 200 300 400 500 600 700 800 Temperature (OC)

Figure 5.2 TGA for SAPO-34 and Activated Carbon In the Figure 5.2, the initial weight loss due to hygroscopic water observed until 105 OC in SAPO-34 and activated carbon is 17% and 5% respectively. After this temperature no significant weight loss for SAPO-34 is observed which is stable up to 700 OC. In case of activated carbon, the weight remains stable up to about 335 OC after wards a continuous weight loss is observed which could be due to the degradation of fixed carbon.

120 PES PDMS 100

Weight (%) Weight 40

0 0 100 200 300 400 500 600 700 800 Temperature (OC)

Figure 5.3 TGA for PES and PDMS pure membranes * During the measurement for PES a disturbance occurred, due to which data between 37-450 OC is excluded.

Figure 5.3 shows the thermogravimetric curve for PES. It is depicted that decomposition of PES starts at 450 OC and ends at 600 OC, afterwards the ash is left. The similar thermogravimetric curve for PES is also reported by Ping et al [71]. In contrast, the TGA curve for PDMS shows that it is stable up to only 190 OC; at higher temperatures, PDMS continuously degrades up to 650 OC.

PES-SAPO-0% PES-SAPO-10% 120 PES-SAPO-20% PES-SAPO-30% 100 PES-SAPO-40%

Weight (%) Weight 40

0 0 100 200 300 400 500 600 700 Temperature (OC)

Figure 5.4 Effect of filler content on weight loss for PES and SAPO-34 MMMs

The effect of filler content on thermal degradation of PES and SAPO-34 MMMs is demonstrated in Figure 5.4. There is no significant weight loss observed for the pure PES membrane, while a maximum weight loss of 8 % due to water was observed in the PES-SAPO-40% membrane for T< 110 OC. Addition of SAPO-34 appears to increase the thermal stability of the membranes as pure PES membrane starts to degrade at 450 OC, while MMM with 40% SAPO-34 apparently at 500 OC. This comparison of the TGA curve of pure PES and MMMs confirms that there are interactions between PES and SAPO-34 which lead to the increase in the decomposition temperature of MMM.

5.4 Viscosity The viscosity of polymer dope solution is affected by the solvent-polymer ratio. The viscosity of the dope solution is an important parameter as it affects the membrane structure. Viscosity is measured using the method described in section 4.3.1 for polymer dope solutions with NMP to determine the critical concentration of the dope. Furthermore, viscosity measurements are also performed for polymer and zeolite solution. Figure 5.5 shows the effect of polymer concentration on viscosity of dope solution for PES-NMP system in the concentration range of 10, 15, 20 and 25%. Obviously, the viscosity of polymer solution increases exponentially with increasing the polymer concentration. The curve shows that for PES concentration of 20%, there is a significant increase in viscosity which is caused due to polymer chain entanglement. Higher chain entanglement causes a denser skin layer along with smaller pore size in asymmetric membranes, The use of a highly viscous dope solution affects the diffusion of solvent and non-solvent, which causes fast phase separation (coagulation step) at surface layer inducing a denser skin layer and simultaneously, more closed sub-layer pores in the bottom layer [72]. In the figure below, the critical polymer concentration is estimated at the intersection of the tangents at about 20%. Using polymer concentrations above this critical value during synthesis will help in formation of a dense skin layer while creating least defects at the surface even for membranes.

6000

5000 y = 5.8089e0.2729x

R² = 0.996

)

4000

mPa.s

(

η 3000

2000 Viscosity, Viscosity, 1000

0 8 13 18 23 28 Polymer Concentration (wt. %)

Figure 5.5 Determination of critical concentration of PES solution

Table 5.2 shows the viscosity of dope solution of PES-NMP and dope solution of PES with filler (SAPO-34). It is observed that viscosity of dope solution increases with increasing the concentration of PES and SAPO-34. Furthermore, the viscosity of dope solutions with SAPO-34 is appreciably higher than for pure polymer dope solutions. It is expected that the higher viscosity of PES and SAPO- 34 dope will help to improve the skin layer of asymmetric membranes.

Table 5.2: Viscosity of PES-NMP and PES-NMP-SAPO34

PES-25% and PES Average Average Viscosity SAPO-34 Viscosity (mPa.s) (wt%) O O (mPa.s) at 20 C conc. 20 C

10 99.6 SAPO-10% 7497.3

15 442.0 SAPO-20% 7844.4

20 1377.9 SAPO-30% 8605.4

22 3032.9 -- --

25 6552.9 -- --

5.5 Scanning Electron Microscopy (SEM - EDX) The characterization of membrane structure is important to correlate the gas transport properties with separation performance. The SEM-EDX analysis (see section 4.3.3) is performed on pure polymer as well as MMMs at surface and cross-section of membranes. EDX helps to identify different constituents of MMMs and the interaction between the particle and polymer. Further, SEM-EDX analysis is also used to identify the thickness of asymmetric membranes for both, the skin layer and porous the porous layer. Figure 5.6 shows the SEM image of an asymmetric PES polymeric membrane synthesized through single bath method and dope solution containing polymer and NMP-Ethanol (mixed solvent see synthesis method in section 4.2.1.3). The morphology of the membrane constitutes a top dense skin layer of thickness of 4- 7 µm and a porous bottom porous layer. The porous bottom layer contains fingerlike pores as well as closed cellular pores. The concentration of fingerlike pores is reduced due to slow diffusion of mixed solvent (NMP-Ethanol) and anti- solvent (water) in the coagulation bath. The top skin layer has no visual defects because a thicker skin layer was achieved as compared to the other approaches followed in the literature [73].

Dense skin layer

Top layer

Figure 5.6 SEM pictures of an asymmetric pure PES polymeric membrane

The top surface membrane morphology of a PDMS coated asymmetric PES- SAPO-34 MMM with filler content 30% is shown in Figure 5.7. The image at 1000x shows that the membrane surface is dense and no pin holes are visible. The SAPO-34 appears to be distributed evenly in the membrane structure. Agglomerates or lumps are not observed at the membrane surface. The filler particles are also oriented (in circle) on the surface of the membrane. This is

81 confirmed also by EDX as presented in Figure 5.8. The EDX mapping also confirms the presence of thin PDMS coating on the surface of the membrane which is represented by Si colored profile and the FTIR results as shown in section 5.1 also confirm this observation. However, the Sulfur (S) color profile is also present which is indicative of Sulfone group of PES. From Figure 5.7 and Figure 5.8 it can be concluded that the intended morphology of asymmetric MMM is achieved with surface oriented SAPO-34 and absence of surface defects as well as a thin layer of PDMS coating at the surface.

Surface oriented SAPO-34

Figure 5.7 SEM surface of asymmetric PES-SAPO-34 30% MMM

SAPO-34

Figure 5.8 Membrane surface EDX map of an asymmetric PES-SAPO-34 30% MMM The cross sectional SEM images of an asymmetric PES-SAPO-34 (filler content 30%) MMM are presented in Figure 5.9. It can be observed that SAPO-34 crystals are distributed throughout the membrane i.e. they are found in the top dense skin layer as well as in the bottom porous layer. It is also observed that the zeolite crystals have good contact with polymer phase. The image at 10,000x indicates attachment of the PES porous structure at the crystal boundaries. It is also observed that the crystals near the region of finger like pores compress the pore

84 and macroscopically the size of the fingerlike pores is reduced with the addition of SAPO-34. This phenomenon can be attributed to the higher viscosity of the dope solution after the addition of filler particles, which is also confirmed by the viscosity measurements of the dope solutions.

Top side

Bottom side

Figure 5.9 SEM picture (cross section) of an asymmetric PES-SAPO-34 30% MMM The Figure 5.10 shows SEM images of the surface and the cross section of an uncoated asymmetric PES-AC 30% MMM membrane. The surface of the membrane has certain roughness due to the high concentration (30%) of surface oriented activated carbon particles. Surface defects or pin holes are not observed in the membrane surface. The cross sectional image shows a good contact between PES and activated carbon particles. Only a few finger like pores are

85 observed in the bottom porous layer. It is also observed that the activated carbon particles are slightly moved upwards in the surface layer.

Surface Image

Top side of cross section

Figure 5.10 SEM picture of an asymmetric PES-AC 30% MMM

The EDX mapping of asymmetric PES-AC-30% MMM membrane is presented in Figure 5.11, the map confirms the presence of surface oriented activated carbon as given by C (red color) and S (purple color) profiles, respectively. The presence of the PDMS layer is indicated by Si (light blue color) profile. These results are in accordance with already reported results of FTIR.

PDMS lump at surface

Activated carbon particles

Figure 5.11 Membrane surface EDX map of an asymmetric PES-AC-30% MMM membrane

Figure 5.12 Membrane surface SEM image of stainless steel supported pure SAPO-34 membrane

The SEM image of a pure zeolite (SAPO-34) membrane is presented in Figure 5.12, It shows that the SAPO-34 crystals are inter grown and inside the macro pores of the stainless steel support. Further, the membrane surface is partially covered with the zeolite crystals. The morphology of a dense PSF-SAPO-34 MMM is illustrated by the SEM image in Figure 5.13. It is shown that a dense thin membrane of up to 30 µm thickness is

88 synthesized. During the evaluation of surface image it is observed that surface voids or defects are invisible, while the cross section shows voids at the interface of particle and polymer, which is difficult to decide either these voids are generated during membrane synthesis or due to the stress caused through cryogenic SEM sample preparation method.

Surface Image

Void

Cross section

Figure 5.13 Surface and cross section SEM image of dense PSF-SAPO-30% MMM

The cross section of dense PDMS and SAPO-34 MMM was also analyzed with SEM as shown in Figure 5.14. A good compatibility between PDMS and SAPO-34 is observed and there were no micro-voids observed in this morphology.

Figure 5.14 Cross section SEM image for dense PDMS-SAPO-30% MMM membrane

5.6 Gas separation performance characterization The gas permeation measurements are performed using setup described in section 4.3.6.3 for pure gases in the order: He, H2, CH4, CO2 and N2, using constant pressure variable volume method, up to 20 bar pressure and ambient temperature (see section 4.3.6.4 for details). Mixed gas measurements are performed for CO2 / CH4 (50/50) mixture following the procedure as per section 4.3.6.5. Corresponding values of permeance, permeability and selectivity are calculated using equations 2.2, 2.3 and 2.4.

5.6.1 Dense rubbery polymeric membrane Flat sheet dense PDMS membranes were synthesized in the thickness range of 20 – 50 µm using synthesis procedure discussed in section 4.2.1.1. For the sake of comparison 18 and 35 µm thickness are selected. Transport mechanism and

90 separation performance for He, H2, CH4, CO2 and N2 for these membranes is investigated to analyze suitability of this polymer as a candidate for MMM development. Another motivation for this is to investigate the applicability of PDMS as a coating layer for asymmetric MMM. Figure 5.15 represents the permeability of pure PDMS dense membrane having a thickness of 35 µm for pressures up to 20 bar. From the figure, it is evident that highest permeability is achieved for CO2 while N2 has the least permeability. A slight decrease in permeability is observed with increasing pressure. The reason for this slight decrease is discussed hereafter.

10000

CO2

CH4 H2 He 1000 N2

100 Permeability Coefficient, P [ Barrer]

10 0 5 10 15 20 25 Δp [bar]

Figure 5.15 Permeability vs. pressure for PDMS (35 µm) dense membrane

Transport of gases through dense PDMS membrane follows solution-diffusion mechanism i.e. P = S ∙ D (see equation 2.1). The permeability (푃) depends linearly on the pressure difference and pressure difference 훥푝 across [74, 75]. Therefore it holds [76]: 퐿표푔 푃 = 퐿표푔 푃 ( 훥푝 = 0 ) + 푚푥 5.1

Table 5-3: Analysis of parameters equation 5.1

18 µm 35 µm Literature [77]

Gas P at P at P at 0 m 0 m 0 m Δp=0 Δp=0 Δp=0 [Barrer/bar] [Barrer/bar] [Barrer/atm] [Barrer] [Barrer] [Barrer]]

CO2 3085 -73.93 2678 -59.87 4691 51.95

CH4 906 -25.25 739 -17.64 1629 -6.831

H2 676 -19.99 554 -15.31 1313 -23.15

He 335 -8.91 291 -7.87 NR NR

N2 210 -3.94 166 -2.76 626.7 -8.408 *NR – Not reported

It is shown in the Table 5-3 that the value of slope m is negative for both membranes and all penetrants, which indicates that permeability of all penetrants decreases with increasing pressure. This trend can be attributed to the reduction of polymer matrix free volume due to slight compression of the polymer matrix by the increased pressure [74]. The permeability of 18 µm membrane is observed to be about 15% higher as compared to the 35 µm membrane, which confirms a reverse relationship between thickness and permeability. It is also shown that permeabilities of all the gases have the same order as per literature [77]. They are, however, about 30-35% lower, which could have been caused by a different of degree of cross linking during membrane preparation of plasticization effect.

10000 Permeability Solubility 1000 Diffusivity Critical Temp. 100

0.1

0.01 CO2 CH4 H2 He N2 CO2 CH4 H2 He N2 ∞ Figure 5.16 Effect of gas solubility S [74], diffusivity P0 [74] and critical temperature Tc [78] on permeability P0 for the PDMS (35 µm) dense membrane.

∞ The interplay of gas solubility S , diffusivity D0, critical temperature Tc and permeability P0 for dense PDMS membranes is demonstrated in Figure 5.16. It is shown that the penetrant having higher solubility has higher permeability. Similarly penetrants with higher critical temperature (higher condensability and, thus, solubility) also show higher permeability. Owing to these trends it can be inferred that the highest permeability of CO2 can be attributed to higher solubility coefficient of CO2 for PDMS, which is related to its higher critical temperature [74]. The following order of permeabilities for all gases was observed for both membranes (18 and 35 µm):

푃 > 푃 > 푃 > 푃 > 푃

The permeability parameters P0 of the penetrants, investigated in this study are also compared with the solubility and diffusivity parameters of PDMS reported by Merkel et al. [74]. As shown in the Table 5-4, it is observed that the solubility parameter for CO2 is the highest while the diffusivity parameter for H2 is the highest for all membranes investigated. However the permeability of CO2 is the highest which again endorses the effect of solubility on permeability. It is also evident that permeability of smaller size molecules like H2 and He is lower than

CH4 and CO2. Such membranes are denoted as “reverse selective”. The same

93 order of permeability, solubility and diffusivity is also reported for PDMS by Sadrzadeh et al [77].

Table 5-4: Permeability, Solubility and Diffusivity parameters of PDMS

Gas CO2 CH4 H2 He N2

Molecular weight, g mol-1 44.01 16.043 2.016 4.003 28.014

Critical temperature T (K) c 304.12 190.56 33.25 5.2 126.2 [78]

Critical volume V c 94.07 98.6 65 57.3 90.1 (cm3/mol) [78]

Permeability P0 at 3085 906 676 335 210 Δp=0 [Barrer]

Solubility [74] S∞ 1.29 0.42 0.05 0.03a 0.09 [cm3 (STP)/cm3 atm]

Diffusivity [74] × 103 22 22 140 11.2b 34 [cm2/s] a Value of solubility obtained from [79]. b Value calculated using permeability and solubility coefficients.

The pure gas selectivity (α) using equation 2.3 was calculated from pure gas permeability data. From the Figure 5.17, it can be deduced that there is no significant effect of feed gas pressure on the selectivity of gas pair CO2/CH4 and

CH4/He, while for the gas pairs H2/N2 and CO2/N2 the selectivity reduces up to a pressure of 5 bar and then becomes constant.

Figure 5.17 Effect of pressure and membrane thickness on selectivity for dense PDMS membranes

Pure gas selectivities for the gas pairs CO2 / CH4, H2 / N2, CH4 / He and CO2 / N2 are reported in Table 5-5 for a pressure of 10 bar. The highest selectivity (14.65) is observed for CO2 / N2 and the selectivity of the 35 µm membrane is higher than for 18 µm membrane.

Table 5-5: Pure gas selectivity of dense PDMS membrane at 10 bar

Membrane CO2 / CH4 H2 / N2 CH4 / He CO2 / N2 18 µm 3.57 2.90 2.69 13.96 35 µm 4.92 2.98 2.56 14.65

The permeability and selectivity of dense PDMS membranes produced in this study are plotted in a Robeson plot, which provides a comparison of permeability

95 and selectivity utilizing equation 5.2, where 푃 is the permeability of fast moving gas and 훼 is the selectivity of gas pair [80].

푃 = 푘 훼 5.2 The Figure 5.18 summarizes the performance of a dense PDMS membrane (35 µm). It shows that the membranes have a high permeability, while its selectivity is somewhat lower than the prior upper bound of 1991. Also shown in the diagram is the measured mixed gas selectivity determined for a 1:1 mixture of CO2 / CH4. The mixed gas selectivity with a value of 1.88 is expectedly lower than the pure gas selectivity.

Figure 5.18 Robeson plot for CO2/CH4: Comparison of pure gas and mixed gas selectivity for PDMS-35 µm membrane at 10 bar, adapted with permission from [80].

In view of above results and discussion, it can be concluded that the applied synthesis method delivers PDMS membranes with high permeability, which is an important characteristic of rubbery polymers as well as reasonable selectivity. The membranes produced using this are dense and show a transport behavior according to the solution-diffusion mechanism. Permeability and selectivity was

96 insignificantly affected by pressure. The PDMS membranes showed highest permeability for CO2 and a reverse selectivity for non-condensable gases like H2,

He, and N2. Condensability of the penetrant is dependent on the critical temperature, hence a penetrant having higher critical temperature is more soluble in PDMS. The PDMS dense membrane with 35 µm showed higher selectivity as compared to an 18 µm membrane. Overall, the characteristics of PDMS make it a suitable candidate as membrane synthesis material. Specifically, the high permeability and good selectivity for CO2 make it an attractive material for CO2 selective membranes. Furthermore, it’s of high permeability and the possibility to form dense membranes can also be exploited by using it as coating material for development of highly permeable MMMs, as will be shown in section 5.6.5.

5.6.2 Dense glassy polymeric membranes Dense membranes of polysulfone and polyethersulfone were synthesized as per section 4.2.1.1 and permeability and selectivity of these membranes were measured for the pure gases He, H2, CH4, CO2, and N2. The dependence of permeability on pressure is also studied. It is shown that for both PSF and PES, the order of permeability follows the penetrant size:

푃 > 푃 > 푃 > 푃 > 푃

This finding confirms the argument that – in contrast to rubbery polymers – in glass polymers the diffusivity, which depends on the molecular size of the penetrant, dominates transport [81]. The effect of pressure on permeability is summarized in the Figure 5.19, the trend shown in the figure indicates that the permeabilities of He, H2, CH4 and N2 remain constant when increasing pressure. The permeability of CO2 decreases with pressure. This was also suggested by Freeman et al. [44]. Who argue that CO2 follows a dual-mode of sorption at higher pressures in glassy polymers which leads to such behavior of CO2 permeability. The fact that the permeability for non- condensable gases is very low and remains unchanged even at higher pressures like 20 bar, confirms the solution-diffusion transport through the membrane. The overall relatively low permeability values and the higher glass transition

97 temperatures of the membrane materials (see Table 5.1) confirm the rigid form of the polymers. 100 CO2 H2 10

He CH4 N2

[ Barrer] 1

Permeability Coefficient, P 0 5 10 15 20 25 Δp [bar] Figure 5.19 Permeability of Pure dense polysulfone membrane

The permeability and selectivity of a dense PSF membrane is compared with literature data in Table 5-6. The pure gas permeabilities and the selectivity values are close to those from the literature. It should be noted that polysulfone is more permeable to CO2 while the selectivity for the CO2 / CH4 gas pair is higher for polyethersulfone. Table 5-6: Comparison of permeability of dense Polysulfone membrane with literature

Polyethersulfone Polysulfone Literature Permeability, P Literature Gas Permeability, [82] [Barrer] [83] P [Barrer]

CO2 6.57 7.53 4.1 3.4

CH4 0.35 0.39 0.16 0.12

H2 9.05 NR 11.3 NR He 8.56 NR 9.6 8.0

N2 0.25 0.26 0.14 NR Pure gas selectivity CO / 25.63 2 18.77 19.30 28.33 CH4 CO / 29.28 2 26.8 28.96 NR N2 *NR – Not reported

Figure 5.20 compares the performance of the dense polysulfone and polyethersulfone membranes in a Robeson plot. From the figure the achieved performance of the membranes may be judged as average. Both membranes perform below the prior upper bound and fail to cross the permeability and selectivity tradeoff.

Figure 5.20 Robeson plot for CO2/CH4: Comparison of pure gas selectivity for Dense PSF and PES, adapted with permission from [80]

When considering the potential use of the prepared dense membranes for

CO2/CH4 separations, the relatively high selectivities (e.g. a maximum selectivity of 25.6 for PES) are in contrast to the limited permeabilities for CO2 (6.57 and 4.1 Barrer for PSF and PES, respectively). Based on this it is concluded that the use of these polymers in a dense morphology is not very attractive when aiming at developing highly permeable MMMs.

5.6.3 Asymmetric polymeric membrane Since gas transport through dense PSF and PES morphologies was found to be limited (see previous section), this section deals with the investigation of uncoated

99 and coated asymmetric membranes synthesized of the same polymers. The goal of this study is to determine an ideal asymmetric membrane morphology that is highly permeable and through which gases are transported by a solution diffusion mechanism. Asymmetric membranes were prepared using dual bath coagulation (PSF) and by single bath coagulation (PES) using the procedures described in section 4.2.1.3.

Figure 5.21 and Figure 5.22 show the effect of pressure on the permeance of the gases He, H2, CH4, CO2, and N2. In case of PSF, it is observed that the permeance of all gases increases slightly with increasing feed pressure. This indicates the presence of porous structures in the membrane. The order of permeance is as follows: :

푃 > 푃 > 푃 > 푃 > 푃

45 CO2

CH4

] 35

30 H2 GPU , [ 25

nce He 20

15 N2 Permea 10

0 0 5 10 15 20 25 Δp [bar]

Figure 5.21 Permeance vs. pressure for an uncoated asymmetric polysulfone (PSF) membrane

100

In the case of PES, the order of permeances:

푃 > 푃 > 푃 > 푃 > 푃 Figure 5.22 shows that here the permeance of the gases increases much stronger with pressure. This is due to the highly porous structure of the membrane (see SEM Figure 5.6), which is also confirmed by the mean pore size and effective porosity values determined by method discussed in section 4.3.7, presented in Table 5-7. The mean pore size for PSF membrane and PES membrane is 378.31 nm and 44.79 nm, while the effective porosity is 0.002 m-1 and 8.800 m-1,

respectively. PES has a much higher permeance for N2 (182.94 GPU) than PSF (14.72 GPU). The higher porosity of the PES membrane shows that it has a comparatively thinner skin layer than the PSF membrane. Further, the bottom structure of the PES membrane also contains finger like pores which are shown in the SEM analysis in Figure 5.6.

1000 CO2 900

800 CH4

] 700 H2

GPU 600 , [

nce 500 He

400

N2 Permea 300

200

100 0 5 10 15 20 25 Δp [bar]

Figure 5.22 Permeance vs. pressure for uncoated asymmetric polyethersulfone (PES) membrane

101

The strong effect of pressure on permeance shown in Figure 5.25 indicates that gas transport takes place due to viscous flow and Knudsen diffusion. Table 5-7 compares the permeance of an uncoated asymmetric PSF membrane (dual bath coagulation) and PES membrane (single bath coagulation). It is shown that PES with NMP-Ethanol mixed solvent has a higher permeance for all gases as compared to PSF with pure NMP. This increase of permeance is due to thinner skin layer formed by PES due to instantaneous de-mixing. A ternary phase diagram attached as Appendix-C shows the mechanism of phase inversion for PES, NMP-Ethanol and water system. It is presented that mixing even small amounts of Ethanol with NMP helps to accelerate the phase separation and lets the solution enter the two phase region, which leads to the formation of a highly porous membrane structure.

Table 5-7: Comparison of permeance (GPU) at 5 bar of uncoated asymmetric PSF and PES membranes synthesized using dual bath and single bath coagulation

PES with NMP- PSF with dual Gas Ethanol mixed bath solvent

CO2 14.72 182.94

CH4 0.93 284.53

H2 36.2 645.2 He 34.2 422.12 200.35 N 0.93 2 332.49 [84] Mean Pore radius 378.31 44.79 (nm) Effective Porosity 0.002 8.800 (1/m)

In a next step, PDMS coating layers were applied to asymmetric PSF and PES membranes to seal voids / defects as indicated above results. Table 5-8 compares the permeance and selectivity of corresponding membranes. Higher permeances are achieved for the PDMS coated PES membrane than for the corresponding PSF membrane. Although the PDMS coating reduced the permeance in

102 comparison to the uncoated PES membrane, the relative difference of permeances between the two polymers is still pronounced after coating.

Table 5-8: Comparison of permeance (GPU) at 10 bar of PDMS coated asymmetric membranes PSF (dual bath coagulation) and PES (single bath coagulation)

PES with NMP-Ethanol Gas PSF with dual bath mixed solvent

CO2 14.62 26.04

CH4 0.99 17.63

H2 33.29 49.52

He 29.02 38.59

N2 0.2 9.55

Pure gas selectivity 14.76 1.47

Mixed gas selectivity NM 1.06

*NM – Not Measured

The effect of pressure on the permeance of the test gases for a PDMS coated asymmetric PES membrane is shown in Figure 5.23. The feed gas pressure increases the permeance of all the gases. However, the permeance of CO2 is highly affected by increasing pressure, which is due to the higher solubility of CO2 for PDMS [74], While the N2 is the least affected by increase of pressure, which could represent the solution diffusion through the membrane. When considering that the permeability of CO2 in dense PDMS is an order of magnitude higher than for the other gases (Figure 5.15), it is not surprising that the CO2/CH4 selectivity of the PES membrane reverses after coating with PDMS (see also discussion below). Finally, N2 transport is least affected by an increase of pressure, which may indicate a solution diffusion transport through the membrane.

103

100

90 CO2

CH4

] 70

60 H2 GPU , [ 50

nce He 40

30 N2 Permea 20

0 0 5 10 15 20 25 Δp [bar]

Figure 5.23 Permeance vs. pressure for coated asymmetric PES membrane

The pure gas selectivity of coated and uncoated PES membranes is compared with Knudsen selectivity in Table 5-9. One observes that the selectivity for

CO2/CH4 (0.64) of an uncoated PES membrane corresponds to the theoretical Knudsen selectivity (0.60), which indicates transport by Knudsen diffusion. Similar

is the case for the gas pair CO2/N2, which also indicates a flow through the uncoated membrane due to Knudsen diffusion [85]. On the other hand, the coated asymmetric PES membrane shows higher selectivities than the theoretical Knudsen selectivities for all gas pairs. This indicates that the gas molecules have more interaction with pore walls and the permeance takes place due to surface diffusion.

104

Table 5-9: Comparison of coated and uncoated asymmetric PES membrane selectivity with Knudsen Selectivity

Selectivity of Selectivity of Knudsen Gas Pair uncoated PES coated PES Selectivity Membrane Membrane

CO2 / CH4 0.64 1.47 0,60 3.22 5.19 H2/N2 3,73 1.48 2.19 He/CH4 2,00

CO2/N2 0.91 2.73 0,80

The results and discussion in this section leads to the conclusion that the asymmetric PES membranes synthesized using single bath coagulation are synthesized with high effective porosity and permeance as compared to asymmetric PSF membrane synthesized using dual bath coagulation. Further, the transport through uncoated asymmetric PES membranes takes place due to Knudsen diffusion while through coated asymmetric PES membranes through solution diffusion. Simultaneous correlation of gas transport properties and SEM characterization (Figure 5.6) suggests morphology of the PES membranes with a thin dense skin layer and a highly porous bottom structure. This morphology is well suited for the desired synthesis of highly permeable membranes in which the bottom structure has the least resistance to gas transport.

5.6.4 Pure zeolite (SAPO-34) asymmetric membranes When developing mixed matrix membranes, the properties of the filler material is of obvious interest. A particularly interesting option is to compare the performance of an MMM with that of a membrane consisting of the pure filler material only. Membranes prepared from pure zeolite SAPO-34 were kindly provided by the group of Prof. Wilhelm Schwieger at the Institute of Chemical Reaction Engineering in Erlangen. The corresponding synthesis procedure is summarized in section 4.2.2. These membranes were characterized for their gas transport properties for the pure He, H2, CH4, CO2, and N2. Mixed gas measurements were also performed for 50/50 CO2/CH4 gas mixtures.

105

Figure 5.24 shows the effect of pressure on the permeance for a membrane with

crystal size between 2 and 6 µm. H2 and CO2 show a decreasing permeance with

increasing pressure while the permeance of He and N2 remains constant. However, the permeance of methane increases with pressure. The order of

permeance 푃 > 푃 > 푃 > 푃 > 푃 is according to the kinetic diameter of gases and indicates that diffusion takes place by a molecular sieving.

140 CO2 120 CH4

100 ] H2

GPU 80 , [

nce He 60

40 N2 Permea

0 0 5 10 15 20 25 Δp [bar]

Figure 5.24 Effect of pressure on the permeance of a pure zeolite membrane with crystal size 2 and 6 µm

A SAPO-34 membrane with a smaller crystal size of about 500 nm (Figure 5.28) showed a strong effect of pressure on the permeance for all gases. The

membrane exhibits Knudsen selectivity for the gas pair CO2/CH4, which indicates that the diffusion takes place due to Knudsen diffusion. The order of permeance

for the gases is 푃 > 푃 > 푃 > 푃 > 푃 .

106

800 CO2 700

600 CH4

]

500 H2 GPU , [ 400

nce He 300 N2

Permea 200

100

0 0 5 10 15 20 25 Δp [bar]

Figure 5.25 Effect of pressure on the permeance of pure zeolite membrane with a crystal size ~ 500 nm.

The comparison of permeance and selectivity of both SAPO-34 membranes in Table 5-10 shows that the membrane with crystal size 2 – 6 µm has the highest selectivity while the permeance is higher for membrane with 500nm crystal size. When estimating the permeance for the membrane with the larger particles by

assuming a thickness of 2 µm, the permeance for CO2 at 1 bar is, for example, about 91.8 Barrer. This is a very low value for SAPO-34, for which permeances of 10,000 Barrers were reported in the literature [86]. The reason for the low permeances observed here remains unclear so far and would require further investigation. However, based on the literature and the selectivities observed, the assumption that SAPO-34 is a selective and highly permeable potential filler material appears justified.

107

Table 5-10: Comparison of permeance (GPU) at 5 bar of asymmetric pure SAPO- 34 membranes supported on stainless steel support

Asymmetric Asymmetric SAPO-34 SAPO-34 Gas membrane membrane (crystal size: 2 – 6 (crystal size: ~ µm) 500 nm) CO2 41.63 134.75 CH4 14.57 219.14

H2 86.11 463.54 He 34.96 309.71

N2 11.54 157.90 Pure gas selectivity 2.85 0.61

Mixed gas 1.07 0.96 selectivity

The above discussion shows that the SAPO-34 membrane with a crystal size of 2 – 6 µm as a relatively high selectivity along with moderate permeance. The latter indicates that the macro pores of the stainless steel support were completely closed. This is also confirmed by the SEM characterization shown in Figure 5.12, where crystals are observed to pack the pores properly. In contrast, the order of permeance for the SAPO-34 membrane with 500 nm crystals indicates that there are defects at Knudsen level that promoted permeance but reduced the selectivity. The gas transport properties of both asymmetric SAPO-34 and asymmetric PES membranes let expect synergies when aiming at the synthesis of highly permeable mixed matrix membranes that exploit the molecular sieving capabilities of SAPO-34.

5.6.5 Dense Mixed Matrix Membranes

In this section, membrane characterization is carried out to investigate the dense MMM morphology and its effect on gas transport behavior of MMMs synthesized through rubbery (PDMS) and glassy (PSF and PES) polymers. The dense MMMs are fabricated using a polymer matrix (PDMS, PSF, and PES) and 30% filler (SAPO-34) on dry polymer weight basis and activated carbon by

108 the procedure given in section 4.2.3.1. Permeability and selectivity measurements of the pure gases He, H2, CH4, CO2, and N2 are reported. Also mixed gas measurements were performed for a 50:50 CO2/CH4 gas mixture.

10000

CO2

CH4 H2 He 1000 N2

100 Permeability Coefficient, P [ Barrer] 10 0 5 10 15 20 25 Δp [bar]

Figure 5.26 Permeability vs. pressure for PDMS and SAPO-34 (30%) dense MMM

Figure 5.29 shows the permeability of the test gases for a PDMS-SAPO-34 (30%) MMM. For the membrane a similar effect of pressure is observed as for the pure PDMS membrane (Figure 5.15) and also the order of permeability is similar. However, the permeability is considerably reduced when compared to pure PDMS, e.g. for CO2 at 10 bar from 3552.17 to 2612.64 Barrer. This could be caused by the effect of tortuosity diffusion in the membrane due to addition of filler [87].

푃 > 푃 > 푃 > 푃 > 푃

109

10000

CO2

CH4 H2 He 1000 N2

100 Permeability Coefficient, P [ Barrer] 10 0 1 2 3 4 5 6 Δp [bar]

Figure 5.27 Permeability vs. pressure for PSF and SAPO-34 (30%) dense MMM

Figure 5.27 displays the effect of pressure on permeability for a PSF-SAPO-34 (30%) MMM. It is observed that permeability slightly increases with increasing the pressure (Knudsen diffusion). The order of permeabilities is 푃 > 푃 > 푃 >

푃 > 푃 . The same order and a similar gas transport behavior is observed in case of PES- SAPO-34 (30%) MMM.

The comparison of dense rubbery and glassy polymer MMM is presented in the Table 5-11. The data reveal that the rubbery polymer dense membrane shows higher permeability and selectivity than a glassy polymer MMM. The higher permeability of PDMS MMM is a sign for the inherent high permeability of its polymer matrix while the improved selectivity indicates that there is good compatibility between the polymer and the filler phase. This notion was also confirmed earlier by the SEM picture in Figure 5.14. In contrast to the PDMS- based MMM, the MMMs with glassy polymers (PSF and PES) achieved a lower permeability for CO2 and CH4 while displaying a similar or higher permeability for the smaller molecules H2 and He. It is assumed that this is because the smaller

110 molecules can escape through micro-voids between the polymer and the filler phase. The selectivity of glassy MMMs is in the range of theoretical Knudsen selectivity which confirms that transport through these membranes takes place due to Knudsen diffusion, while in the case of the PDMS MMM permeability is rather independent of pressure which indicates some solution diffusion influence.

Table 5-11: Comparison of permeance rubbery and glassy polymer dense membranes

PSF - PES - PDMS- Pure PDMS SAPO-34 SAPO-34 Gas permeability SAPO-34 dense (30%) (30%) [Barrer] (30%) dense membrane dense dense MMM MMM MMM

CO2 3552.17 2612.64 193.48 147.99

CH4 904.37 707.11 387.06 239.54

H2 533.27 544.87 762.33 387.74

He 180.62 272.01 363.05 258.64

N2 174.43 156.36 268.86 179.32

Pure gas 3.92 3.69 0.50 0.62 selectivity

Mixed gas 1.88 2.76 0.96 0.98 selectivity

In the light of the above results it can be concluded that PDMS MMMs show an order of permeability and a selectivity which is close to pure PDMS membranes. The gases here prefer to pass through the polymer instead of the filler due to high inherent permeability of the polymer phase. In contrast, for the glassy polymer MMMs the rigid structure of the polymer matrix appears to develop a sieve-in- cage morphology that does not allow the gases to pass through SAPO-34 but rather through micro-voids between polymer and filler [8]. Despite the interesting fact that the PDMS-based MMM shows a better selectivity than a dense PDMS membrane, it is concluded that both dense MMM morphologies – rubbery and

111 glassy – fail to improve the permeability of the polymer phase to achieve the goal of highly permeable MMMs with good selectivity.

5.6.6 Asymmetric Mixed Matrix Membranes

The investigations reported in this work so far on dense and asymmetric pure polymer membranes, on pure zeolite membranes, and on dense MMMs have helped for the selection of a suitable polymer matrix and a membrane morphology that could allow for the development of highly permeable MMMs. Correspondingly, focus of this section is on the development of asymmetric PES-based MMMs. For these membranes, the effect of filler type is investigated by using SAPO-34 and activated carbon (AC) at various filler loadings. Furthermore, the influence of an additional PDMS coating on the gas transport properties of such MMMs is elucidated. The asymmetric MMMs were synthesized using SAPO-34 and activated carbon by the procedure in section 4.2.3.2. Filler loadings of 10, 20, 30, and 40% were used on dry polymer weight basis. Permeance and selectivity for the pure gases

He, H2, CH4, CO2, and N2 for both uncoated and coated MMMs were determined.

Also mixed gas measurements were performed for 50/50 CO2/CH4 gas mixtures.

112

30000 CO2 CH4 25000

] He 20000

GPU N2

[

nce 15000 Permea 10000

5000

0 0 1 2 3 4 5 Δp [bar]

Figure 5.28 Permeance vs. pressure for uncoated PES and SAPO-34 (30%) asymmetric MMM

Figure 5.28 and Figure 5.29 compare the results of pure gas permeance and the effect of feed gas pressure on permeance for uncoated PES/SAPO-34 and PES/AC membranes with 30% filler loadings each. Overall, very high permeances were achieved for these uncoated MMMs. Due to this the measurements had to be performed at relatively low pressures since the obtained flow rates were otherwise too high for the flow rate measurement procedure.

For both MMMs the order of permeance observed is 푃 > 푃 > 푃 >

푃 > 푃 , while the highest permeances for H2 are 24075 GPU (PES/SAPO) and 10654 GPU (PES/AC), respectively. The feed gas pressure is found to affect the permeance of light gases stronger. Both membranes show a pure gas selectivity for CO2/CH4 that is comparable to theoretical Knudsen selectivity. This indicates that the size of pores or voids lies in the range of 2 to 50 nm. PES- SAPO-34 and PES-AC (30% filler loading) show a mean pore size (42.97 and 39.94 nm) and effective porosity (415.09 and 115.08) respectively, as reported in Table 5-12 and Table 5-13. Correlating the values of mean pore size and effective porosity with the permeance results leads to the assumption that there is a better interaction between PES and activated carbon particles than between PES and

113

SAPO-34. Furthermore, the SEM images of both membranes as shown in Figure 5.9 and Figure 5.10 confirm that asymmetric PES-SAPO-34 MMM shows wider finger like pores in the bottom layer while these pores are suppressed in the case of asymmetric PES-AC MMM. These variations in bottom porous layer cause asymmetric PES-SAPO-34 MMM to achieve higher permeances.

14000 CO2 CH4 12000

] 10000 He

GPU N2 [

8000 nce 6000

Permea 4000

2000

0 0 1 2 3 4 5 6 Δp [bar]

Figure 5.29 Permeance vs. pressure for uncoated PES activated carbon (30%) asymmetric MMM

The correlation between filler particle loading, porosity measurements, and transport properties for both asymmetric PES-SAPO-34 and PES-AC MMMs is presented in Table 5-12 and Table 5-13. It is shown that the mean pore size of PES-SAPO-34 MMMs remains at 40-45 nm from 0 to 30% filler loading while it increases abruptly up to 66 nm at filler loading 40%. In case of the PES-AC MMM similar values are observed up to a filler loading of 20%, but for filler loading of 30% the mean pore size increases to 88.38 nm. These high values of mean pore size could have been caused due to the defects or voids between filler and polymer interphase as the pore size of both fillers (SAPO-34, 3.8 Å and activated

114 carbon, 1.007 nm) is much smaller than the mean pore size observed for the membranes.

Table 5-12: Comparison of permeance (GPU) at 1 bar of uncoated asymmetric PES SAPO-34 MMM with different zeolite concentrations

PES- PES- PES- PES- PES- Gas SAPO- SAPO-34- SAPO-34- SAPO-34- SAPO-34- 34-0% 10% 20% 30% 40%

CO2 145,4 1721,55 2602,8 6478,25 16531,1

CH4 218,92 2660,51 4262,6 10285,9 25766,1

H2 555,17 7129,51 9616,6 24075,4 57800 He 362,75 4093,72 6607,4 16296 38785,5

N2 148,03 1832,59 2953,7 7301,92 18470,8 Pure gas selectivity 0,66 0,65 0,61 0,63 0,64

Mean Pore 44.79 39.94 40.88 42.97 66.00 radius (nm) Effective Porosity 8.80 115.08 177.43 415.09 684.93 (1/m)

115

Table 5-13: Comparison of permeance (GPU) at 1 bar of uncoated asymmetric activated carbon MMM with different zeolite concentrations Activated Activated Activated Activated Gas carbon 0% carbon 10% carbon 20% carbon 30%

CO2 145,4 587,33 1721,6 3249,35

CH4 218,92 930,89 2660,5 4975,33

H2 555,17 2185,8 6098,1 10654,2 He 362,75 1439,12 4093,7 6980,09

N2 148,03 542,01 1832,6 3416,79 Pure gas selectivity 0,66 0,63 0,65 0,65

Mean Pore 44.79 46.94 39.94 88.38 radius (nm) Effective Porosity 8.800 30.45 115.08 91.31 (1/m)

The effect of PDMS coating on gas separation performance of asymmetric PES- SAPO-34 and PES-AC MMM is presented in Figure 5.30 and Figure 5.31. As expected, in both cases the PDMS coating has decreased the permeance for all the gases. Furthermore, the order of permeance has changed to 푃 > 푃 >

푃 > 푃 > 푃 . This order of permeance is different from that of pure PES membranes as well as from that of pure SAPO-34 membranes. Both types of MMMs showed the highest permeance for CO2, which indicates that the PDMS coating has promoted the

CO2 permeance due to the high affinity of CO2 towards PDMS. Furthermore, the permeance of H2 and He is higher than for CH4 which does not correspond to the inherent pure PDMS and PES permeance. It is also noticed that permeance for all the gases is relatively independent of pressure (solution diffusion), with the exception of CO2, for which permeance increases strongly with pressure. When these results are correlated with the FTIR and SEM-EDX measurements, it supports the argument that the PDMS coating has successful closed micro voids at the membrane surface, while the surface oriented filler particles helped to improve the permeance and selectivity of the membrane.

116

700 CO 2 600 CH4

500 ] H2

GPU 400 , [ He

nce 300 N2

200 Permea 100

0 0 5 10 15 20 25 Δp [bar] Figure 5.30 Permeance vs. pressure for coated PES SAPO-34 (30%) asymmetric MMM

350 CO 2 300 CH

] 250 H2

GPU 200

, [ He

150 nce N2 100

Permea 50

0 0 5 10 15 20 25 Δp [bar] Figure 5.31 Permeance vs. pressure for coated PES activated carbon (30%) asymmetric MMM

117

Figure 5.32 and Figure 5.33 shows the effect of filler loading on permeance of various gases for coated asymmetric MMMs. It is noted that in case of PES- SAPO-34 MMMs the highest permeance is achieved for a filler loading of 30% while at 40% it reduces slightly. A similar trend is observed for the PES-AC MMM. Yet, the permeance of the PES-AC MMM is lower than for the PES-SAPO-34 MMM. This could be confirmation for that the PES-SAPO-34 MMM has a more porous bottom structure than the PES-AC MMM.

1000

CO2 H2 100 He CH4 N2

10 Permeance[GPU] 1 0% 10% 20% 30% 40% SAPO-34 Content

Figure 5.32 Effect of filler (SAPO-34) content on permeance of different gases at 5 bar

1000 CO2 H2 100 He CH4

10 N2 Permeance[GPU] 1 0% 10% 20% 30% Activated Carbon Content

Figure 5.33 Comparison of activated carbon composition on permeance of coated MMM at 5 bar

118

The pure gas permeance and selectivity of coated asymmetric PES-SAPO-34 and PES-AC MMMs are compared in Table 5-14 and Table 5-15, where also the mixed gas selectivity for CO2/CH4 is presented. It is shown that highest permeance is achieved for CO2 by the PES-SAPO-34 (30%) MMM, while the highest pure gas selectivity (6.20) of CO2/CH4 is found for PES-AC (30%). It is also noticed that in all membranes the pure gas selectivity is significantly higher than the mixed gas selectivity.

Table 5-14: Comparison of permeance (GPU) at 5 bar for coated asymmetric PES SAPO-34 MMM with different zeolite concentrations

PES- PES- PES- PES- PES-SAPO- Gas SAPO-34- SAPO- SAPO-34- SAPO-34- 34-0% 10% 34-20% 30% 40%

CO2 19,47 127,8 240,58 516,95 418,18

CH4 11,86 40,81 44,57 128,65 94,69

H2 44,96 140,95 166,23 228,55 176,58

He 35,82 115,45 127,13 172,22 121,41

N2 7,31 14,77 15,57 44,16 36,57

Pure gas selectivity 1,64 3,13 5,40 4,02 4,42

Mixed gas selectivity 1,05 1,79 2,8 2,43 --

119

Table 5-15: Comparison of permeance (GPU) at 5 bar of coated asymmetric activated carbon MMM with different zeolite concentrations

Activated Activated Activated Activated carbon carbon Gas carbon carbon content content content content 0% 10% 20% 30%

CO2 19,47 153,31 214,98 238,17

CH4 11,86 43,95 45,79 38,39

H2 44,96 160,33 175,04 188,05

He 35,82 127,79 142,81 147,57

N2 7,31 17,94 16,85 11,81

Pure gas selectivity 1,64 3,49 4,69 6,20

Pure gas selectivity 1,05 -- 2,972 3,99

The effect of filler concentration on pure gas selectivity is shown in Figure 5.34 and Figure 5.35. It is observed that in case of coated PES-SAPO-34 MMM the highest selectivity is achieved at filler loading of 20%. Although this finding needs some additional verification, the decrease of selectivity observed for higher filler loadings could well be attributed to an increasing number of voids caused by larger filler concentration via, for example, the agglomeration or an unfavorable dispersion of the particles. In case of the coated asymmetric PES-AC MMM the selectivity increases smoothly with increasing the concentration of activated carbon. This indicates that there is a better interaction between PES and the activated carbon particles. Also the higher adsorption capacity of activated carbon for CO2 helps to improve the selectivity of the membrane [88].

120

3 Selectivity Selectivity 2 CO2/CH4

0 0 10 20 30 40 50 Filler loading % (SAPO-34) Figure 5.34 Effect of SAPO-34 composition on selectivity of coated asymmetric PES-SAPO-34 MMM

5 Selectivity CO2/CH4 4

Selectivity 3

0 0 10 20 30 40 Filler loading % (activated carbon)

Figure 5.35 Effect of SAPO-34 composition on selectivity of coated asymmetric PES-AC MMM

Finally, the performance of the coated PES-SAPO-34 (30%) and PES-AC (30%) MMMs is analyzed by plotting the data in Robeson diagram together with the

121 upper bound curves for the gas pairs CO2/CH4, H2/N2, and CO2/N2 at a pressure of 20 bar [80]. From Figure 5.36, which shows the performance for the CO2/CH4 gas pair, it is noted that the pure gas permeability and selectivity for both membranes is reaches or exceeds prior upper bound, while for the gas mixture performance is below the prior upper bound. The PES-AC MMM performs better than the PES-SAPO-34 MMM.

Figure 5.36 Robeson plot for CO2/CH4: Comparison of coated asymmetric PES-SAPO and PES-AC with 30% filler loading gas pair at 20 bar, adapted with permission from [80]

* Permeability is calculated using complete thickness of asymmetric membranes.

The Figure 5.37 represents the performance of PES-SAPO-34 and PES-AC

MMMs for the pure H2/N2 gas pair at 20 bar. It can be seen that both membranes perform above the present upper bound. Also here, the PES-AC MMM performs better than the PES-SAPO-34 MMM.

122

Figure 5.37 Robeson plot for H2/N2: Comparison of coated asymmetric PES-SAPO and PES-AC with 30% filler loading gas pair at 20 bar, adapted with permission from [80] * Permeability is calculated using complete thickness of asymmetric membranes.

The membrane performance for the pure CO2/N2 gas pair for both MMMs is compared in Figure 5.38. Both membranes exceed the present upper bound and, again, the PES-AC MMM performs better than the PES-SAPO-34 MMM.

123

Figure 5.38 Robeson plot for CO2/N2: Comparison of coated asymmetric PES-SAPO and PES-AC with 30% filler loading gas pair at 20 bar, adapted with permission from [80]

* Permeability is calculated using complete thickness of asymmetric membranes.

In conclusion of the above results, it can be stated that a design method and corresponding synthesis procedures for MMMs were developed that lead to membranes with morphologies capable of attaining very high permeances and good selectivities that are beyond the upper bound in the corresponding Robeson plots. The approach is seen as promising in particular for applications that require very high permeances.

5.7 Modeling gas permeability

Development of MMMs requires a rational selection of dispersed phase and continuous phase. Theoretical prediction of gas separation properties based on pure gas transport properties has become an important aspect of membrane research. Depending on membrane morphology efforts are made by various researcher for prediction of MMM performance [42].

124

In this section, an attempt is made to model the permeability using simple models based on conductance in series and parallel are compared with more sophisticated Maxwell model. The modeling is accomplished for CO2 permeability of uncoated and coated asymmetric PES-SAPO-34 MMMs. The analogy is made between conductance and permeability, and a conductance in series model is applied for determining the 푃 (effective permeability) [89, 90].

Flow direction Flow direction

Continuous phase

Continuous Dispersed phase Phase

Dispersed Phase

Series model Parallel model

Figure 5.39 Scheme of conductance in series and parallel model

A minimum value of 푃 is achieved when applying following series model in the form of two layers in series.

푃 푃 푃 = ∅ 푃 + ∅ 푃

For highest value of 푃 , parallel model is assumed which apply two layers in parallel, it is represented by following equation [89, 90]:

푃 = ∅ 푃 + ∅ 푃 5.3

In the above equation 푃 represents the effective permeability of MMM, 푃 is the permeability of continuous polymer phase, 푃 is the permeability of dispersed

125 phase (filler), ∅ is the volume fraction of the dispersed phase and ∅ is the volume fraction of the continuous phase. Maxwell model is widely used by membrane researchers for prediction of gas transport through MMMs. Following form of Maxwell model is achieved when shape factor n = 1 / 3 is used in the equation 2.15 [89]:

푃 + 2 푃 − 2 ∅ (푃 − 푃 ) 푃 = 푃 * + 5.4 푃 + 2 푃 + ∅ (푃 − 푃 )

For all the permeability predictions, only the dense skin layer thickness of asymmetric MMM is used for calculating the permeabilities, while the bottom porous layer is not considered due to presence of fingerlike pores.

126

18000 Series 16000

14000 Parallel 12000 Maxwell 10000 8000 6000

Permeability, Barrer Permeability, 4000 2000 0 0 10 20 30 40 50 60 70 80 90 100 Filler Loading %

Figure 5.40 CO2 Permeability: Comparison different models for uncoated PES-SAPO-34 MMMs

50000

45000

40000 Experimental 35000 30000 25000 20000 15000

10000 Permeability Barrer(Exp.),Permeability 5000 0 0 10 20 30 40 50 Filler Loading %

Figure 5.41: Experimental results: Effect of filler loading on CO2 Permeability for uncoated PES-SAPO-34 MMMs

127

The comparison of different models to predict permeability of CO2 for uncoated PES-SAPO-34 MMM is presented in Figure 5.40. It is shown that parallel model presents higher permeability because of high permeability of dispersed phase which is assumed to be in parallel to continuous phase, while series model predicts lowest permeability because low permeability of continuous phase dominates the transport. However Maxwell model predicts slightly higher permeability than series model because Maxwell model assumes that the filler particles are dispersed uniformly in the continuous phase. The experimental results for this membrane is shown in Figure 5.41, it shows a sudden increase in permeability after the filler loading of 20%, the permeabilities are very high, even higher than predicted by series model, which indicates presence of voids / defects at the polymer – filler interface. During the investigation of SEM-EDX images, it was found that filler particles are oriented at the surface of membrane and voids between these surface particles and polymer may lead to such an increase of permeability. The results discussed hereafter for PDMS coated asymmetric PES- SAPO-34 MMMs confirm this observation.

For modeling 푃 through PDMS coated asymmetric PES-SAPO-34 MMMs, coating of PDMS is conceptualized as layer having resistance 푅 which is laminated on uncoated PES-SAPO-34 MMM having resistance푅 . Using

PDMS permeability and 푃 already predicted for uncoated asymmetric MMMs (Series and Maxwell model) the resistances of both membranes are calculated. After calculation of both resistances separately, Resistances in Series model is applied to calculate total resistance 푅 , which is further used to calculate total permeability, 푃 .

푅 = 푅 + 푅 5.5

Where the resistance R can be calculated using following relation: 푙 푅 = 5.6 퐴 . 푃

128

7050 Series Parallel

6050 Series-Maxwell Experimental 5050

4050

3050

Permeability, Barrer Permeability, 2050

1050

50 0 10 20 30 40 50 60 70 80 90 100 Filler Loading %

Figure 5.42 Asymmetric coated PES-SAPO-34 MMMs CO2 Permeability: Experimental results vs. Series Parallel and Series Maxwell model

Figure 5.42 compares combined permeability of PDMS coating and MMM named as total permeability (푃 ) for CO2 by applying Series – Parallel and Series – Maxwell model along with experimental results. It is observed that Series – Parallel model predicts much higher values, on the other hand Maxwell shows close agreement up to filler concentration of 20%. This close agreement at 20% filler loading indicates that the PDMS coating has closed filler-polymer voids at surface oriented particles. Filling of these micro voids helped to obtain a surface morphology close to the morphology assumed in original Maxwell model. Highest deviation is observed at 30% loading which could be due to the development of voids / defects in the sub-skin layer structure. At 40% filler loading the permeability decreases slightly, which could be attributed to the reduction in size of finger like pores due to increased viscosity of the dope solution, this effect is also noticed in SEM images.

129

Exp. Coated 50000 y = 912.9e0.0973x

Exp. Uncoated

5000

y = 94.581e0.0689x

500 Permeability, Barrer Permeability,

50 0 10 20 30 40 50 Filler Loading %

Figure 5.43 Comparison of CO2 permeability of uncoated and coated PES- SAPO-34 MMMs against filler loading

The effect of filler loading on the permeability of uncoated and coated asymmetric PES-SAPO-34 MMM is presented in Figure 5.43. It is noticed that the permeability of the coated membranes is significantly lower than the permeability of uncoated membranes. Exponential function is tried to fit to the experimental data of both uncoated and coated MMMs. It is shown that for uncoated membranes fit is close to the experimental results, while for coated MMMs the outliers are significant at the lowest and highest concentration which could have been bent due to variations in the thickness of coating, skin layer and highly porous sub-skin layer.

130

6 Conclusions and Recomendations

Mixed Matrix Membranes have a great innovation potential in terms of simplifying membrane synthesis procedure, improving morphology, enhancing separation performance, and increasing productivity.

This work focuses on a systematic evaluation of membrane synthesis materials, preparation methods, and morphologies for the development of highly permeable MMMs. For this purpose, the gas transport results of permeation measurements for five different test gases at pressures of up to 20 bar are compared with the obtained membrane morphologies and the results of physio-chemical analysis techniques. The latter include, among others, measurements by FTIR, TGA, DSC, and SEM.

PDMS dense pure polymer membranes showed a solution-diffusion mechanism with high intrinsic permeabilities for all test gases and an order of permeabilities

CO2 > CH4 > H2 > He > N2, which is a typical behavior of PDMS. The dense film forming capability of PDMS and its low resistance to gas flow is an attractive aspect that is later exploited by using this material for coating of asymmetric membranes as well as for developing dense rubbery MMMs. Dense PSF and PES membranes prepared exhibit very low permeabilities. For example, a dense PES membrane has a CO2 permeability of 4.1 Barrer while this value is as high as 3552 Barrer for a dense PDMS membrane. However, considering the advantage of the higher selectivities of dense polymeric PSF and PES membranes, these polymers are later evaluated to prepare dense glassy MMMs. PSF and PES are also used to form asymmetric morphologies which are an attractive approach in membrane development. The asymmetric morphology has the advantage of a very thin skin layer, which helps reducing the transport resistance. Uncoated asymmetric PSF and PES membranes showed Knudsen flow with selectivities similar to theoretical Knudsen selectivity. Although SEM images does not show pin holes in the surface skin layer, micro voids cannot be ruled out, which may have caused the flow in the Knudsen regime. To improve the selectivity by closing potential voids in the skin layer, a coating with a very thin

131 layer of highly permeable PDMS is applied. Here spin coating was chosen as it can help in applying very thin layers. The formation of a thin layer at the surface is confirmed through SEM-EDX scans and FTIR results. The asymmetric PES membranes coated this way showed slightly increasing permeability with pressure for all gases except CO2, which is an indication that the non-condensable gases

H2, He, and N2 have low solubilities in PDMS through which they are transported by a solution diffusion mechanism. However, CO2 shows an increasing trend of permeability with pressure due to its higher solubility. The order of permeabilities for the PDMS coated asymmetric membranes is H2 > He > CO2 > CH4> N2, and the ideal selectivity for the CO2/CH4 pair is 1.8. Because of its higher permeability at moderate selectivity this PDMS coated asymmetric morphology is found promising for synthesis of highly permeable MMMs. In the context of selecting filler materials for MMMs, pure zeolite (SAPO-34) asymmetric membranes could be investigated that were provided by the group of Wilhelm Schwieger at CRT in Erlangen. A SAPO-34 membrane with medium size crystals between 2 and 6 µm shows a permeance for CO2 of 41.6 GPU and a selectivity for CO2/CH4 of 1.07. The permeance for nitrogen was independent of pressure, indicating a very compact structure of the membrane. As the permeance of these membranes is closely related to that of asymmetric PES membranes and also literature data indicate a high potential of SAPO-34 as filler material, it can be advantageous to combine both materials to achieve the goal of higher permeability of MMMs. In the next step, dense MMMs are prepared from the different polymers by different techniques using SAPO-34 as filler material. A PDMS/SAPO-34 MMM shows an order of permeability and selectivity that is close to that of pure PDMS membranes, indicating gas flow through the polymer phase instead of the filler phase. In the case of glassy polymer MMMs the permeabilities were significantly higher. For example, for PES-SAPO-34 a CO2 permeability of 148 Barrer is found. However, the selectivity for these membranes is similar to the theoretical Knudsen selectivity, which may indicate interfacial voids between filler and polymer in their morphology. Overall, neither rubbery nor glassy dense morphologies are found here suitable for achieving highly permeable MMMs. Finally, asymmetric PES MMMs are synthesized using SAPO-34 and activated carbon. Filler particle loading between 0 and 40 wt% is used for these

132 membranes. Furthermore, these membranes, uncoated and PDMS coated membranes. These membranes are prepared by tape casting and a dry/wet phase inversion method for solvent displacement. The uncoated MMMs obtained show very high permeabilities with a strong pressure dependence and Knudsen selectivity. In contrast, the order of permeances for the coated asymmetric PES MMM with 30% SAPO-34 loading was at 5 bar, CO2 (516.95 GPU) > H2 (228.55) > He (172.22) > CH4 (128.65) > N2

(44.16), while the CO2/CH4 selectivity was 4.02 and the selectivity of CO2/N2 was

11.7 and for H2/N2 5.17. It is observed that increasing the filler content up to 30% increases the permeability for all gases and largely also the CO2/CH4 selectivity. However, for 40% filler content the permeance is somewhat lower. This decrease could be due to the compression of finger like pores (observed in SEM images) in the bottom structure due to the higher filler concentration. Further investigations on this aspect appear desirable. Similar transport behavior and order of permeances is also achieved with uncoated and coated asymmetric PES MMMs that used activated carbon as filler. A slight increase of selectivity is found for the coated membranes compared to the coated asymmetric PES SAPO-34 MMMs. A decrease of permeance for high filler loadings is not observed. The improvement in gas separation performance by coating the asymmetric MMMs suggests that the filler particles distributed in the surface skin layer and bottom layer provided extra channels for gas flow leading to the drastic increase of permeance observed for the uncoated membranes. The thin PDMS coating obviously helped to seal such surface voids while promoting selectivity.

Overall, in comparison to coated asymmetric pure PES membrane, the 30 times increase of permeance for CO2 achieved by coated asymmetric PES-SAPO-34

MMMs and the three-fold improvement of the CO2/CH4 selectivity – together with the good performance achieved for other gas pairs – suggests that the set target of developing highly permeable MMMs is achieved here in the form of membranes with thickness in the micron range that show stability at high pressures.

In this study, one glassy polymer (PES) is tested along with two types of filler particles for the development of asymmetric MMM morphology. It is shown that

133 this ambitious route offers good potential for developing highly permeable MMMs. Further studies are needed to optimize and understand such membranes and their preparation methods. A variety of questions is related to, for example, the optimal size and distribution of filler particles and thickness of the membrane layers. Of particular interest is to investigate other combinations of polymers and fillers to study their effect on membrane morphology, interfacial voids, and separation performance. Filler materials like metal-organic frameworks (MOFs), carbon nanotubes (CNTs), and graphene are potential candidates for such further developments. Also different types of surface coatings can also provide room for further improvement of permeability and selectivity.

134

References

1. Sharafian, A., et al., A review of liquefied natural gas refueling station designs. Renewable and Sustainable Energy Reviews, 2017. 69: 503-513. 2. Qin, Y., et al., Air quality, health, and climate implications of China’s synthetic natural gas development. Proceedings of the National Academy of Sciences, 2017. 114(19): p. 4887- 4892. 3. Rufford, T.E., et al., The removal of CO2 and N2 from natural gas: A review of conventional and emerging process technologies. Journal of Petroleum Science and Engineering, 2012. 94–95: p. 123-154. 4. Paola Bernardo, G.C., 30 Years of Membrane Technology for Gas Separation. 2013, The Italian Association of Chemical Engineering, vol. 32, 2013. 5. Khoo, H.H. and R.B.H. Tan, Life Cycle Investigation of CO2 Recovery and Sequestration. Environmental Science & Technology, 2006. 40(12): p. 4016-4024. 6. Dong, G., H. Li, and V. Chen, Challenges and opportunities for mixed-matrix membranes for gas separation. Journal of Materials Chemistry A, 2013. 1(15): p. 4610-4630. 7. Ismail, A.F., K. Khulbe, and T. Matsuura, Gas Separation Membranes: Polymeric and Inorganic. 2015: Springer Dordrecht London. 8. Mahajan, R. and W.J. Koros, Mixed matrix membrane materials with glassy polymers. Part 1. Polymer Engineering & Science, 2002. 42(7): p. 1420-1431. 9. Goh, P.S., et al., Recent advances of inorganic fillers in mixed matrix membrane for gas separation. Separation and Purification Technology, 2011. 81(3): p. 243-264. 10. Bernardo, P., E. Drioli, and G. Golemme, Membrane Gas Separation: A Review/State of the Art. Industrial & Engineering Chemistry Research, 2009. 48(10): p. 4638-4663. 11. Hussain, M., Mixed Matrix Membranes for Gas Separation, Phd Thesis, 2012, Universität Erlangen-Nürnberg: Erlangen. p. 137. 12. King, C.J., Separation Processes: Second Edition. 2013: Dover Publications, New York. 13. Baker, R.W., Membrane Technology and Applications. 2012: Wiley, West Sussex, United Kingdom. 14. Nollet, J.A., Investigations on the causes for the ebullition of liquids. Journal of Membrane Science, 1995. 100(1): p. 1-3. 15. Graham, T., On the Absorption and Dialytic Separation of Gases by Colloid Septa. Phil. Trans. R. Soc. Lond., 1866. 156: p. 399-439. 16. Fick, A., Über Diffusion. Annalen der Physik, 1855. 170(1): p. 59-86. 17. Knudsen, M., Eine Revision der Gleichgewichtsbedingung der Gase. Thermische Molekularströmung. Annalen der Physik, 1909. 336(1): p. 205-229. 18. Pandey, P. and R.S. Chauhan, Membranes for gas separation. Progress in Polymer Science, 2001. 26(6): p. 853-893. 19. Henis, J.M.S. and M.K. Tripodi, A Novel Approach to Gas Separations Using Composite Hollow Fiber Membranes. Separation Science and Technology, 1980. 15(4): p. 1059-1068. 20. Paul, D.R. and D.R. Kemp, The diffusion time lag in polymer membranes containing adsorptive fillers. Journal of Polymer Science: Polymer Symposia, 1973. 41(1): p. 79-93. 21. Kesting, R.E., et al., Asymmetric gas separation membranes having graded density skins. 1988, EP0257012 A2. 22. Robeson, L.M., Correlation of separation factor versus permeability for polymeric membranes. Journal of Membrane Science, 1991. 62(2): p. 165-185. 23. Yang, M., et al., The hindering effect of adsorbed components on the permeation of a non-adsorbing component through a microporous silicalite membrane: the potential barrier theory. Journal of Membrane Science, 1999. 156(1): p. 1-9. 24. Yoo, Y., Z. Lai, and H.-K. Jeong, Fabrication of MOF-5 membranes using microwave- induced rapid seeding and solvothermal secondary growth. Microporous and Mesoporous Materials, 2009. 123(1–3): p. 100-106.

135

25. Li, S., et al., Scale-up of SAPO-34 membranes for CO2/CH4 separation. Journal of Membrane Science, 2010. 352(1–2): p. 7-13. 26. Mustafab, A., et al., The Effect of Functionalization Carbon Nanotubes (CNTs) on the Performance of PES-CNTs Mixed Matrix Membrane International Journal of Science and Engineering (IJSE), 2010. 1(1): p. 15-20. 27. Gin, D.L. and R.D. Noble, Designing the Next Generation of Chemical Separation Membranes. Science, 2011. 332(6030): p. 674-676. 28. Caro, J. and M. Noack, Zeolite membranes – Recent developments and progress. Microporous and Mesoporous Materials, 2008. 115(3): p. 215-233. 29. Baker, R.W., Membrane technology and applications. 2004, John Wiley & Sons, Ltd., West Sussex, England. 30. Matteucci, S., et al., Transport of Gases and Vapors in Glassy and Rubbery Polymers, in Materials Science of Membranes for Gas and Vapor Separation. 2006, John Wiley & Sons, Ltd. p. 1-47. 31. Baker, R.W. and B.T. Low, Gas Separation Membrane Materials: A Perspective. Macromolecules, 2014. 47(20): p. 6999-7013. 32. Wijmans, J.G. and R.W. Baker, The Solution–Diffusion Model: A Unified Approach to Membrane Permeation, in Materials Science of Membranes for Gas and Vapor Separation. 2006, John Wiley & Sons, Ltd. p. 159-189. 33. Li, P., et al., Recent developments in membranes for efficient hydrogen purification. Journal of Membrane Science, 2015. 495: p. 130-168. 34. Bakangura, E., et al., Mixed matrix proton exchange membranes for fuel cells: State of the art and perspectives. Progress in Polymer Science, 2016. 57: p. 103-152. 35. Rezakazemi, M., et al., State-of-the-art membrane based CO2 separation using mixed matrix membranes (MMMs): An overview on current status and future directions. Progress in Polymer Science, 2014. 39(5): p. 817-861. 36. Jusoh, N., et al., Current Development and Challenges of Mixed Matrix Membranes for CO2/CH4 Separation. Separation & Purification Reviews, 2016. 45(4): p. 321-344. 37. Kanehashi, S., S. Sato, and K. Nagai, Synthesis and Gas Permeability of Hyperbranched and Cross-Linked Polyimide Membranes, in Membrane Gas Separation. 2010, John Wiley & Sons, Ltd. Chichester, UK. 38. Belhaj Messaoud, S., et al., Mixed matrix membranes using SAPO-34/polyetherimide for carbon dioxide/methane separation. Separation and Purification Technology, 2015. 148: p. 38-48. 39. Rouquerol, J., et al., Recommendations for the characterization of porous solids (Technical Report). 1994. 66(8): p. 1739–1758. 40. Chung, T.-S., et al., Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation. Progress in Polymer Science, 2007. 32(4): p. 483-507. 41. Hashemifard, S.A., A.F. Ismail, and T. Matsuura, A new theoretical gas permeability model using resistance modeling for mixed matrix membrane systems. Journal of Membrane Science, 2010. 350(1–2): p. 259-268. 42. Vinh-Thang, H. and S. Kaliaguine, Predictive Models for Mixed-Matrix Membrane Performance: A Review. Chemical Reviews, 2013. 113(7): p. 4980-5028. 43. Henis, J.M.S. and M.K. Tripodi, Composite hollow fiber membranes for gas separation: the resistance model approach. Journal of Membrane Science, 1981. 8(3): p. 233-246. 44. Freeman, B., Y. Yampolskii, and I. Pinnau, Materials Science of Membranes for Gas and Vapor Separation. 2006: Wiley, West Sussex, England. 45. Li, N.N., et al., Advanced Membrane Technology and Applications. 2011: Wiley Hoboken, New Jersey. 46. Reddy, B.S.R. and U. Senthilkumar, Prospects of Siloxane Membrane Technology for Gas Separation - A Review. Journal of Scientific & Industrial Research, 2003. 62(7): p. 666-677.

136

47. Ghannam, M.T. and M.N. Esmail, Rheological Properties of Poly(dimethylsiloxane). Industrial & Engineering Chemistry Research, 1998. 37(4): p. 1335-1340. 48. Technical data sheet for ELASTOSIL® RT 601 A/B, W.C. AG., 2014: München, Germany. 49. Mark, H.F., Encyclopedia of Polymer Science and Technology, Encyclopedia of Polymer Science and Technology, Third Edition. 2003: Wiley, New York. 50. Massey, L.K., Permeability Properties of Plastics and Elastomers: A Guide to Packaging and Barrier Materials. 2003: Elsevier Science, New York, USA. 51. Fried, J.R., Molecular Simulation of Gas and Vapor Transport in Highly Permeable Polymers, in Materials Science of Membranes for Gas and Vapor Separation. 2006, John Wiley & Sons Ltd., New York, p. 95-136. 52. Scholes, C.A., S.E. Kentish, and G.W. Stevens, The Effects of Minor Components on the Gas Separation Performance of Polymeric Membranes for Carbon Capture, in Membrane Gas Separation. 2010, John Wiley & Sons, Ltd. p. 201-226. 53. Product Information Ultrason® S 6010 PSU (Polysulfone), B. 2016: Wyandotte-MI. 54. BASF, Product Data Sheet ULTRASON® E 6020 P POLYARYLETHERSULFONE. 2015. 55. Lok, B.M., et al., Silicoaluminophosphate molecular sieves: another new class of microporous crystalline inorganic solids. Journal of the American Chemical Society, 1984. 106(20): p. 6092-6093. 56. Baerlocher, C., L.B. McCusker, and D.H. Olson, CHA - Rm, in Atlas of Zeolite Framework Types (Sixth Edition). 2007, Elsevier Science B.V.: Amsterdam. p. 96-97. 57. Singh, V.K. and E. Anil Kumar, Measurement and analysis of adsorption isotherms of CO2 on activated carbon. Applied Thermal Engineering, 2016. 97: p. 77-86. 58. Adams, F.V., et al., The influence of solvent properties on the performance of polysulfone/β-cyclodextrin polyurethane mixed-matrix membranes. Journal of Applied Polymer Science, 2013. 130(3): p. 2005-2014. 59. Wang, D., K. Li, and W.K. Teo, Polyethersulfone hollow fiber gas separation membranes prepared from NMP/alcohol solvent systems. Journal of Membrane Science, 1996. 115(1): p. 85-108. 60. Stuart, B.H., Infrared Spectroscopy: Fundamentals and Applications. 2004: Wiley. New York, USA. 61. JEOL. Invitation to the SEM World. 2006; Available from: http://www.jeolusa.com/. 62. Robeson, L.M., Correlation of separation factor versus permeability for polymeric membranes, in Journal of Membrane Science. 1991. vol. 62, p. 165-185. 63. Accreditation, A.A.f.L., G104 - Guide for Estimation of Measurement Uncertainty In Testing 2014, A2LA, USA. 64. Calvo, J.I., et al., Porosity, in Encyclopedia of Membrane Science and Technology. 2013, John Wiley & Sons, Inc New York. 65. Davey, C.J., D. Leak, and D.A. Patterson, Hybrid and Mixed Matrix Membranes for Separations from Fermentations. Membranes, 2016. 6(1): p. 17. 66. Mixed matrix membranes: Important and challenging! Membrane Technology, 1999. 1999(105): p. 6-8. 67. Zhang, L., W. Fang, and J. Jiang, Effects of Residual Solvent on Membrane Structure and Gas Permeation in a Polymer of Intrinsic Microporosity: Insight from Atomistic Simulation. The Journal of Physical Chemistry C, 2011. 115(22): p. 11233-11239. 68. Fu, Y.-J., et al., Effects of residual solvent on gas separation properties of polyimide membranes. Separation and Purification Technology, 2008. 62(1): p. 175-182. 69. Ismail, A.F. and A.R. Hassan, Effect of additive contents on the performances and structural properties of asymmetric polyethersulfone (PES) nanofiltration membranes. Separation and Purification Technology, 2007. 55(1): p. 98-109. 70. International, A. and S. Lampman, Characterization and Failure Analysis of Plastics. 2003: ASM International.

137

71. Ping Qu, H.T., Yuan Gao, Li-ping Zhang, Siqun Wang, Polyethersulfone Composite Membrane Blended with Cellulose Fibrils. BioResources, 2010. 72. H. Sofiah, A.N.a.a.M.A.M., The Influence of Polymer Concentration on Performance and Morphology of Asymmetric Ultrafiltration Membrane for Lysozyme Separation. Journal of Applied Sciences, 2010. 10: p. 3325-3330. 73. Prajapati, P.K., A.M. Kansara, and P.S. Singh, Preparation and characterization of an oxygen permselective polydimethylsiloxane hollow fibre membrane. RSC Advances, 2016. 6(92): p. 88943-88953. 74. Merkel, T.C., et al., Gas sorption, diffusion, and permeation in poly(dimethylsiloxane). Journal of Polymer Science Part B: Polymer Physics, 2000. 38(3): p. 415-434. 75. Stern, S.A., Mullhaupt, J. T. and Gareis, P. J. , The effect of pressure on the permeation of gases and vapors through polyethylene. Usefulness of the corresponding states principle. AIChE J., 1969. 15: p. 64–73. 76. Stern, S.A., V.M. Shah, and B.J. Hardy, Structure-permeability relationships in silicone polymers. Journal of Polymer Science Part B: Polymer Physics, 1987. 25(6): p. 1263-1298. 77. Sadrzadeh, M., K. Shahidi, and T. Mohammadi, Synthesis and gas permeation properties of a single layer PDMS membrane. Journal of Applied Polymer Science, 2010. 117(1): p. 33-48. 78. Poling, B.E., J.M. Prausnitz, and J.P. O'Connell, The properties of gases and liquids. 2001: McGraw-Hill, USA. 79. Tremblay, P., et al., Gas permeability, diffusivity and solubility of nitrogen, helium, methane, carbon dioxide and formaldehyde in dense polymeric membranes using a new on-line permeation apparatus. Journal of Membrane Science, 2006. 282(1–2): p. 245-256. 80. Robeson, L.M., The upper bound revisited. Journal of Membrane Science, 2008. 320(1–2): p. 390-400. 81. Murphy, T.M., G.T. Offord, and D.R. Paul, Fundamentals of Membrane Gas Separation, in Membrane Operations. 2009, Wiley-VCH Verlag GmbH & Co. KGaA. Weinheim, Germany p. 63-82. 82. Reid, B.D., et al., Gas Permeability Properties of Polysulfone Membranes Containing the Mesoporous Molecular Sieve MCM-41. Chemistry of Materials, 2001. 13(7): p. 2366-2373. 83. Ghosal, K., R.T. Chern, and B.D. Freeman, Gas permeability of radel a polysulfone. Journal of Polymer Science Part B: Polymer Physics, 1993. 31(7): p. 891-893. 84. Ismail, A.F., et al., Preparation and characterization of hyperthin-skinned and high performances asymmetric polyethersulfone membrane for gas separation. Desalination, 2011. 273(1): p. 93-104. 85. Isobe, T., et al., Gas separation using Knudsen and surface diffusion II: Effects of surface modification of epoxy/porous SiO2 composite. Journal of Asian Ceramic Societies, 2014. 2(3): p. 190-194. 86. Li, S. and C.Q. Fan, High-Flux SAPO-34 Membrane for CO2/N2 Separation. Industrial & Engineering Chemistry Research, 2010. 49(9): p. 4399-4404. 87. Giel, V., et al., Gas Transport Properties of Polybenzimidazole and Poly(Phenylene Oxide) Mixed Matrix Membranes Incorporated with PDA-Functionalised Titanate Nanotubes. Nanoscale Research Letters, 2017. 12: p. 3. 88. Tamnanloo, J., S. Fatemi, and A. Golmakani, Binary Equilibrium Adsorption Data and Comparison of Zeolites with Activated Carbon for Selective Adsorption of CO2 from CH4. Adsorption Science & Technology, 2014. 32(9): p. 707-716. 89. Vu, D.Q., W.J. Koros, and S.J. Miller, Mixed matrix membranes using carbon molecular sieves: II. Modeling permeation behavior. Journal of Membrane Science, 2003. 211(2): p. 335-348. 90. Gonzo, E.E., M.L. Parentis, and J.C. Gottifredi, Estimating models for predicting effective permeability of mixed matrix membranes. Journal of Membrane Science, 2006. 277(1–2): p. 46-54.

138

91. Telal, P., Preparation and Characterization of Polymeric Multilayer Thin Film Composite Membranes for Gas Separation, in Chemie-und Bioingenieurwesen. 2015, Friedrich- Alexander-Universität Erlangen-Nürnberg: Erlangen.

139

Appendices Appendix – A: Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectra of pure PDMS membrane

FTIR spectra of asymmetric PDMS coated PSF membrane

140

FTIR spectra of asymmetric PDMS coated PES

FTIR spectra of SAPO-34

FTIR spectra of activated carbon

FTIR spectra of Dense PDMS-SAPO34 30% MMM

141

FTIR spectra of asymmetric PDMS coated PES-AC 30% MMM

FTIR spectra of asymmetric PDMS coated PES-SAPO34 30% MMM

142

Appendix – B: Differential scanning calorimetry

DSC thermogram for PES-SAPO-34 Uncoated MMM: effect of SAPO concentration on Tg

DSC thermogram for PES-SAPO34 Coated MMM: effect of SAPO concentration on Tg

143

Appendix – C: Ternary Phase Diagram

Phase diagram for PES, Ethanol, NMP and water system [91]

144

Appendix – D: The EDX map of asymmetric PES-SAPO-34 MMM membrane

EDX of asymmetric PES-SAPO-34 MMM membrane

145

Appendix – E: Effect of filler concentration on permeance of asymmetric MMMs

Effect of filler composition on permeance of uncoated PES-SAPO-34 MMM

Effect of filler composition on permeance of coated PES-SAPO-34 MMM

146

Appendix – F: Gas Adsorption Isotherms

CO2 Sorption Isotherm for activated carbon at 30 OC

CO2 Sorption Isotherm for Pure PDMS at 30 OC

147

Appendix – G: Particle Size Distribution SAPO-34

1.0 0.40

0.9 0.35 0.8 0.30 0.7 0.25

0.6 ]

0.5 0.20 -1

[-]



Q [

0.4 0

0.15 q 0.3 0.10 0.2 0.05 0.1

0.0 0.00 0 1 2 3 4 5 6 7 8 9 10 Length [m]

10 Count 8

0 0 10 20 30 40 50 60 70 80 90 100 Length [m]

Particle Size Distribution SAPO34

148

Appendix – H: X-ray powder diffraction (XRD)

XRD-Activated Carbon

149

Counts 100 200 0 Stand1_2-50°_SAPO-34_kom 10 20 Position [°2Theta] (Copper (Cu)) 30 40

XRD-SAPO-34

150