Contrôle de l'interaction polymère/particules dans les membranes à matrice mixte

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Contrôle de l'interaction polymère/particules dans les

membranes à matrice mixte

Thèse

Tien Binh Nguyen

Doctorat en génie chimique

Philosophiæ doctor (Ph. D.)

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Contrôle de l’interaction polymère/particules

dans les membranes à matrice mixte

Thèse

Tien-Binh. Nguyen

Sous la direction de: Serge Kaliaguine

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Résumé

Au cours des dernières décennies, la technologie membranaire a montré de grandes perfor-mances dans les séparations en phase liquide telles que la production d’eau potable à partir d’eau de mer. Beaucoup d’efforts ont été faits pour étendre son application aux séparations de gaz. La séparation des composants de l’air, des gaz industriels de raffineries, la sépara-tion et la récupérasépara-tion du CO₂du biogaz et du gaz naturel sont des exemples dans lesquels la technologie membranaire est appliquée au niveau industriel. La séparation par membranes a été substituée ou interfacée avec les méthodes conventionnelles telles que la distillation cryogénique pour produire de l’air enrichi en oxygène (fraction molaire≥ 30%) qui est in-jecté dans les brûleurs industriels pour obtenir une température plus élevée, avec moins de consommation de gaz. Il est également possible d’utiliser la technologie membranaire pour capturer et recycler le CO₂ émis par les gaz de combustion des centrales électriques et les aciéries pour résoudre le problème des gaz à effet de serre.

Les membranes pour la séparation des gaz peuvent être classées en deux catégories prin-cipales, basées sur le matériau, polymère et inorganique, dans lesquelles les membranes polymériques sont plus populaires. Par rapport aux matériaux inorganiques, les mem-branes polymères présentent une meilleure facilité de traitement, une résistance mécanique et une densité de remplissage plus élevée, ce qui les rend appropriées pour des applications à grande échelle. Elles ne peuvent cependant pas supporter des températures élevées ou des agents chimiques agressifs. Leurs propriétés de séparation (perméabilité et sélectivité) peu-vent être sévèrement compromises par les hydrocarbures condensables (C2+) lorsqu’elles sont appliquées dans les usines pétrochimiques, les raffineries et le traitement du gaz na-turel. Pour améliorer les performances des membranes polymères, le nouveau concept, de membrane à matrice mixte (MMM), a été réalisé en dispersant des particules nanométriques ou microscopiques de matériaux inorganiques dans une matrice polymère.

Dans ce travail, nous avons préparé de nouvelles MMMs en utilisant des polymères et des matériaux organo-métalliques (MOF) en tant que phases continue et dispersée, respective-ment. Nous avons développé plusieurs techniques pour surmonter la faible adhérence inter-faciale entre les deux phases qui diminue typiquement l’efficacité de séparation des MMM. Pour ce faire, dans la première partie de cette thèse (Chapitre3), nous avons synthétisé des

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particules de MOF comportant des fonctions −NH₂ et une série de polymères décorés de

−OH pour la préparation de MMMs. La liaison physique entre les deux groupes fonction-nels s’est avérée améliorer nettement l’adhérence polymère/charge des MMMs obtenues ainsi que leur performance de séparation des gaz. Dans la partie suivante (Chapitre4), nous avons introduit une modification post-synthétique pour former une liaison chimique entre le polymère et la charge dans les MMMs. Dans des conditions optimisées, un MOF fonc-tionnalisé portant des groupes réticulables a été amené à réagir avec un polymère déjà syn-thétisé contenant des extrémités de chaînes réactives pour produire, pour la première fois, des MMMs réticulées. Dans la dernière partie (Chapitre5), nous avons décrit une nouvelle technique pour obtenir in-situ la liaison chimique polymère-charge pendant la synthèse du polymère. Dans cette technique, les particules de MOF ont été directement introduites dans le milieu de polymérisation. L’importance des liens polymère-charge a été étudiée en fonc-tion du temps de polymérisafonc-tion. Cette étude a montré une forte relafonc-tion entre la qualité de l’interaction polymère-charge et les propriétés de séparation des gaz des MMMs.

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Abstract

In recent decades, membrane technology has shown its great performance in liquid phase separations such as production of drinking water from seawater. It has now attracted much scientific attention to expand its application to gas separations. Separation of air compo-nents, H₂ from refinery industrial gases, separation and recovery of CO₂ from biogas and natural gas are some examples in which the membrane technology is potentially applied at industrial level. The membrane based separation was either partially substituted or inte-grated with conventional methods like cryogenic distillation to product oxygen-enriched air (mole fraction≥ 30% ) that is injected into industrial burners to obtain higher temperature with less gas consumption. It is also possible to use membrane technology to capture and recycle CO₂ emitted from flue gas streams of power plants and steel mills in solving the greenhouse effect.

The membranes for gas separation can be classified in two main categories, based on mate-rial, polymeric and inorganic, in which polymeric membranes are more popular. Compared to the inorganic, the polymer membranes show better processability, mechanical strength and higher packing density, hence, being suitable for large-scale applications. They can-not, however, withstand high temperatures or aggressive chemical agents. Their separation properties (permeability and selectivity) may be severely affected by condensable hydrocar-bons (C2+) when they are applied in petrochemical plants, refineries and natural gas treat-ment. To enhance the performance of polymer membranes, a new concept, mixed matrix membrane (MMM), has been proposed by dispersing nano- or micro-sized particles of inor-ganic materials into a polymer matrix.

In this work, we have prepared novel MMMs using polymers and metal organic framework (MOF) as the continuous and dispersed phases, respectively. We have developed several techniques to overcome the weak interfacial adhesion between the two phases that typically decreases the separation efficiency of MMMs. To do so, in the first part of this thesis (Chap-ter3), we have synthesized a−NH₂included MOF particle and a series of−OH decorated polymers for MMM preparation. The physical bonding between the two functional groups was found to clearly improve the polymer/filler adhesion of the obtained MMMs as well as their gas separation performance. Then, in the following part (Chapter 4), we have

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in-troduced a post-synthetic modification to form chemical bonding between the polymer and filler within MMMs. Under optimized conditions, a functionalized MOF bearing crosslink-able groups was reacted with an as-synthesized polymer containing reactive chain-ends to produce, for the first time, crosslinked MMMs. In the final part (Chapter 5), we have de-scribed a novel technique to obtain in-situ the polymer-filler chemical bonding during the polymer synthesis. In this technique, the MOF particles were directly introduced into the polymerization medium. The extent of the polymer-filler link was studied as a function of polymerization time. This study has shown a strong relationship between the quality of polymer-filler interaction and the gas separation properties of the MMMs.

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List of Tables

2.1 CO₂ permeability and CO₂/CH₄ selectivity of some neat polymers widely

used for MMMs [20]. . . 12

2.2 Some filler materials and their important properties for MMMs. . . 15

2.3 Properties of major zeolite types [29].. . . 16

2.4 Physical characteristics of some commonly used MOFs for MMMs . . . 20

3.1 Designation of copolymers. . . 44

3.2 Physical properties of MOFs. . . 47

3.3 Density and glass transition temperature (Tg) of homo- and co-polyimides and their MMMs. . . 49

3.4 CO₂and CH₄ permeability, diffusivity, solubility coefficients and their ratios in pure gas measurement at 35 °C and 150 psia. . . 55

3.5 CO₂/CH₄separation property of some MOF-based MMMs. . . 57

4.1 Mechanical properties[1]_{of the neat and PIM-1 based MMMs as a function of} filler loading, tested at ambient temperature. . . 72

4.2 Pure-gas permeabilities and ideal selectivities of PIM-1/MOF-74 membranes and other reported MMMs made with PIM-1. . . 73

5.1 Averaged permeabilities and selectivities at 25◦C and 2 bar, for the pristine PIM-1 and the derived MMMs with 20 wt.% of MOFs.. . . 91

5.2 Diffusivity (10−8_cm2_s−1_{) and solubility (10}−2_cm3₍_STP₎_cm3_atm−1_{) coefficients,} diffusivity selectivity (αD) and solubility selectivity (αS) for neat PIM-1 and the MMMs with 20 wt.% of MOFs. . . 93

5.3 CO₂permeability, CH₄ permeability and CO₂/CH₄ideal selectivity at differ-ent aging periods for PIM-1 and MMMs.. . . 98

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List of Figures

1.1 Schematic separation of gas mixture by a polymer membrane . . . 3 1.2 Robeson’s trade-off curve for CO₂/CH₄gas pair (TR, thermally rearranged) [5]. 4 2.1 (a) Schematic configuration of a MMM including particles dispersed in a

poly-mer membrane; (b) cross-sectional surface of a real MMM [14].. . . 10 2.2 Repeat units of several polymers employed for MMMs. . . 13 2.3 Schematic representation of chemical grafting of 6FDA-6FpDA/4MPD/DABA

to the zeolite particle surface [27,28].. . . 14 2.4 "Sieve-in-cage" morphology illustrating a poor adhesion between zeolite

par-ticles and glassy polymer [31].. . . 17 2.5 Illustration for molecular sieving effect of CMS. . . 18 2.6 Topologies of ZIF-8 and zeolite A. . . 21 2.7 SEM images of (a) as-synthesized ZIF-8 nanocrystals and (b and c)

cross-section of Matrimid®–ZIF-8 MMMs. (b) Example of poor dispersion using dried ZIF-8 nanoparticles (20 wt% loading), (c) example of good dispersion (20 wt% ZIF-8) using as synthesized ZIF-8 nanoparticles. Scale bars are 500

nm [42].. . . 22 2.8 Front view and side view of HKUST-1 crystal [44]. . . 23 2.9 SEM images of hollow-fiber MMMs filled with HKUST-1: (a, b) cross-section,

(c) outer surface, (d) surface of finger-like void, and (e) sponge-like structure

[45]. . . 24 2.10 Breathing behavior occurring in MIL-53 (Al, Cr), (left) MIL-53 hydrated with

narrow pore, (right) MIL-53 dehydrated with large pore [48].. . . 25 2.11 Gas transport behavior in (a) ideal interface morphology and (b-e) four

non-ideal interface morphologies [11]. . . 28 2.12 Illustration of crystal growth using modulator. Modulated nucleation is

stim-ulated by the introduction of modulators having the same functionality as

organic linkers to impede the metal-organic linker coordination [72]. . . 31 2.13 Illustration of the ZIF-8 nanocage before and after the incorporation of RTILs.

The cut-off size shifts from the aperture size of six-membered ring to the re-duced effective cage size by confinement of [bmim][Tf₂N] in a ZIF-8’s SOD

cage [75]. . . 31 2.14 (a) post-synthetic modification of UiO-66-NH₂ by chemical grafting of

chlo-ride and anhydchlo-ride agents with the -NH₂group; (b) interactions between the

Matrimid®polymer and phenyl acetyl grafted UiO-66-NH₂[84].. . . 33 2.15 Schematic diagram of HKUST-1 decoration with IL [89]. . . 34

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2.16 (a) Preparation of a "grafting from" based MMM via photo-induced polymer-ization; (b) Modification of UiO-66-NH₂with methacrylic anhydride and

sub-sequent polymerization with butyl methacrylate (BMA) under UV light [102]. 35 3.1 Synthesis and structure of hydroxyl-functionalized copolyimide FDH-xy. . . 43 3.2 SEM images of (a) MIL−53, (b) NH₂−MIL−53, (c) particle size distributions

of MIL−53 and NH₂−MIL−53, respectively, determined from a suspension

in ethanol and acidic buffer using DLS analysis. . . 46 3.3 FTIR spectra of NH₂-MIL-53 and MIL-53. . . 47 3.4 FTIR spectra between 3200 and 3800 cm−1for NH₂-MIL-53 and the

correspond-ing MMMs.. . . 49 3.5 XRD patterns of (a) neat FDH-11; (b) 10 wt%; (b) 15 wt% NH₂-MIL 53/FDH-11

MMMs and (d) neat NH₂-MIL-53. . . 50 3.6 XRD patterns of (a) 15 wt% MIL-53/FDH-11 and (b) neat MIL-53. . . 50 3.7 SEM micrographs of hydroxyl-copolyimide FDH-11 based MMMs as

illustra-tion for particle spatial distribuillustra-tion at loadings of (a) 10 wt%, (b) 15 wt% and

(c) 20 wt% NH₂-MIL-53. . . 51 3.8 Illustration for particle agglomeration in a non-hydroxyl polyimide-based MMM

containing 10 wt% NH₂-MIL-53. . . 52 3.9 SEM micrographs of (a) 10 wt% and (b) 15 wt% MIL-53/FDH-11 MMMs. . . 52 3.10 (a) CO₂permeability and (b) CO₂/CH₄separation factor of the MMMs based

on FD and three FDH copolyimides filled with NH₂-MIL-53 as a function of

MOF loading, in gas mixture of CO₂:CH₄= (50:50) at 35◦C and 150 psi. . . . 53 3.11 (a) CO₂permeability and (b) CO₂/CH₄separation factor of the MMMs based

on FD and three FDH copolyimides filled with MIL-53 as a function of MOF

loading, in a gas mixture of CO₂:CH₄= (50:50) at 35◦C and 150 psi. . . 55 3.12 CO₂/CH₄separation performance of homo-, co-polyimides and their MMMs

filled with NH₂-MIL-53. . . 57 3.13 CO₂/CH₄separation performance of homo-, co-polyimides and their MMMs

filled with MIL-53. . . 58 4.1 Illustration of gas transport for: (a) neat PIM-1 showing inter-connected pores

with bottle-necked slit, (b) MMMs without polymer-filler cross-link, hatched areas indicate fillers working as secondary pathway to get gas passed the bottle-necked slit, (c) and (d) MMMs with polymer-filler crosslink, inner pores of fillers directly connected with the micropore network of PIM-1, accelerating

gas transport. . . 65 4.2 Schematic procedure to prepare (a) MOF-74, (b) PIM-1, (c) molecular

cross-link MOF-74/PIM-1 MMMs and (d) Optical images for the MMMs with

vari-ous filler loadings, showing their flexibility. . . 67 4.3 (a)SEM image of pristine MOF-74 particles, (b) adsorption isotherms of CO₂

(◦) and CH₄() on MOF-74 at 298 K.. . . 69 4.4 SEM cross-section images of (a) M-10 and (b) M-15 membranes containing 10

and 15 wt% MOF-74, respectively. . . 70 4.5 (a) Optical image of membrane before and after acidic treatment (M-20 and

treated M-20, respectively); SEM cross-section images of (b) M-20 and (c) treated

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4.6 (a) XRD patterns and (b) ATR-FTIR for the neat MOF-74 and MMMs. (Mix10∗)

is a physical mixture of PIM-1 powder and 10 wt% MOF-74. . . 71 4.7 (a) Pure-gas permeabilities for PIM-1 and MMMs made with MOF-74 as a

function of kinetic diameter measured at 2 bar and 25◦C, (b) CO₂/CH₄ideal

selectivity compared with the Robeson’s upper bounds. . . 73 4.8 Diffusivity (filled symbols) and solubility (open symbols) of CO₂(4) and CH₄

() estimated from single gas permeation by the time lag method.. . . 74 4.9 Mixed gas separation factors for PIM-1 and MMMs made with MOF-74 as a

function of CO₂partial pressure (measured with CO₂:CH₄, 1:1 mixture at 25

°C). . . 75 4.10 Mixed-gas permeabilities of (a) CO₂and (b) CH₄for PIM-1 and MMMs made

with MOF-74 as a function of CO₂partial pressure (measured with CO₂:CH₄,

1:1 mixture at 25◦C). . . 75 5.1 Synthesis route for PIM-1. TTSBI = 5,5’,6,6’

tetrahydroxyl-3,3,3’,3’-tetramethyl-1,10-spirobisindance and DCTB = 1,4-dicyanotetrafluorobenzene. The

condi-tions are: DMF at 70◦C for 72 h [156] or DMAc at 155◦C for 1 h [146]. . . 81 5.2 Optical photos of: (a) PIM-1 membrane and (b) UiO-66-NH2powder in a test

with deionized water drops, clearly indicating the hydrophobicity of PIM-1 in

contrast with the hydrophilic nature of UiO-66-NH2. . . 82

5.3 Cross interface linking between UiO-66-NH2 and PIM-1 obtained during the

in situ polymerization (method B). . . 82 5.4 Schematic diagram of the two preparation methods for MMMs. . . 84 5.5 (a) UiO-66-NH2 modification with DCTB monomer, (b) UiO-66-NH2

modi-fication with PIM-1 polymerization, the modified MOFs were washed sev-eral times with chloroform to remove non-crosslinked PIM-1 polymer, (c,e) XRD patterns, (d,f) ATR-FTIR spectra for the neat UiO-66, UiO-66-NH2 and

the modified MOFs. . . 86 5.6 SEM images of: (a) as-synthesized UiO-66-NH2; (b,c) DCTB grafted MOFs for

6 and 12 h modification; (d,e) PIM-1 grafted MOFs for 12 and 72 h modifica-tion and an optical image of PIM-1 membrane. The inserts show contact angle values measured for a water drop on the pellet-pressed samples of MOF

pow-ders and PIM-1 membrane. . . 87 5.7 N₂ isothermal adsorption-desorption (closed-opened symbols), measured at

77 K for (a) as-synthesized UiO-66-NH2 and DCTB-grafted MOFs; (b)

PIM-1 powder and PIM-PIM-1-grafted MOFs; (c) TGA curves, the values written in brackets report the sample weight at 450◦C to estimate the amount of PIM-1

deposited on the MOF surfaces. . . 88 5.8 (a)1HNMR spectra of the neat and DCTB modified MOFs. The samples were

digested in a mixture of HF and DMSO (0.5:100, v-v). The squares and as-terisks stand for NH2-BDC ligands in neat UiO-66-NH2and modified ligands

in DCTB-grafted MOFs, respectively; (b)13C-MAS NMR spectra of PIM-1,

as-synthesized UiO-66-NH2and PIM-1 grafted MOFs. . . 89

5.9 TGA curves of neat PIM-1 membrane and MMMs. The temperature was

ramped from 50◦C to 700◦C (10◦C / min), in air flow. . . 90 5.10 Average normalized permeability and ideal selectivity for selected gas pairs. 91 5.11 Cross-section images for MMMs prepared by method A using PIM-1 with 20

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5.12 Cross-section images of MMMs prepared by method B where PIM-1 was syn-thesized in the presence of UiO-66-NH2 filler. The polymerization time was

varied from (a,b) 12 h to (c,d) 72 h. . . 93 5.13 Binary gas permeation tested with a mixture of CO₂:CH₄= 50:50 (v:v) at 25◦C

for PIM-1 and some MMMs, (a) CO₂/CH₄selectivity, (b) CO₂permeability, (c)

CH₄permeability. . . 94 5.14 Permeability and selectivity tested with a CO₂-CH₄mixture (v:v = 1:1), at 25

◦_{C and p}

total =4 bar, for PIM-1 and MMMs prepared by two different methods. 95

5.15 Comparison of CO₂/CH₄ separation performance of PIM-1 and MMMs pre-pared by method A (non-crosslinked PIM/UiO-66-NH2) and method B

(in-situ crosslinked PIM-co-UiO-66) relative to Robeson’s 2008 upper bound [5].. 96 5.16 Aging behavior tracked over 400 days. (a,b) Absolute permeability; (c,d)

rel-ative permeability and (e) ideal selectivity for MMMs loaded with 20 wt.% of filler namely PIM-co-UiO-6672h (square) and PIM/UiO-66-NH2 (triangle),

which were prepared by method A and B respectively, compared with PIM-1 (circle). All the membranes were aged at ambient conditions and the

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Acknowledgments

At first, I would like to express my deep gratitude to Prof. Serge Kaliaguine and Prof. De-nis Rodrigue, my supervisors, for their academic guidance, support and encouragement throughout my doctoral program at Laval University. They gave me great freedom of pursu-ing my own interest but always have time to discuss new ideas and troubleshoot problems. I would like to thank my big brother, my colleague, Dr. Hoang Vinh Thang, who helped me preparing MOFs. This thesis would not have finished without his valuable contribution. I would like to thank Dr. Xiao Yuan Chen for teaching techniques of polymerization as well as characterization to me.

The findings described this thesis were found thanks to assistance of many other people. I would like to thank Dr. Richard Janvier for his help with nice SEM images. Thanks to Yann Giroux for his great help for many instruments in Prof. Denis Rodrigue lab.

I would also like to thank the Research Center on Advanced Materials (CERMA) and Natural Science and Engineering Research Council of Canada (NSERC) for technical and financial support for this study.

I would like to show my appreciation to all members in Prof. Kaliaguine research group. I have received a lot of useful experience from them since I initially started as a PhD candidate. Thanks to Thanh-Binh Nguyen who instructed me about BET analysis. I would also like to thank Luc Charbonneau, my Québécois friend, for access to his bookshelf with full of reference books. I would also like to thank the Chemical Engineering department staffs for all the technical and administrative assistance that I received during the entire period of my study.

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Chapter 1

Introduction

1.1 Introduction of biogas purification and membrane technology

Energy demand is increasing globally with the rapid development of the world economy and population. Though fossil fuels have been playing the main role in the world’s fuel sup-ply, they are nonrenewable sources thus becoming increasingly scarce and expensive, posing the tough challenge of searching alternative energy sources for the world’s sustainable de-velopment. Biogas, a gas mixture produced from a complex anaerobic digestion of biological wastes (e.g. kitchen waste, landfill sites, and animal waste), is a renewable energy source. It typically contains 55-60 mol% methane, 38-40 mol% carbon dioxide and small amount of hydrogen sulfide, generating a calorific value of 35-44 kJ/g, comparable to kerosene, petrol, diesel and LPG [1]. Thus, biogas could become a potential source of clean and cheap al-ternative fuel. Indeed, CH₄ in biogas could be applied for household cookers or heating, automobile fuel and power generation. However, biogas supply is now restricted to the neighborhood areas, where it is generated.

In order to widen the biogas supply, CO₂and other contaminants such as H₂S must be re-moved from biogas so that it can be delivered to distant customers by either a gas pipeline grid or compressed cylinders. Currently, water scrubbing is widely used for biogas purifi-cation because this technology can remove both CO₂and H₂S at the same time due to their higher solubility in water than CH₄. Low purity (5-10% CO₂ remaining) is the main draw-back of this technique. Polyethylene glycol absorption, is a similar but more advanced tech-nology than water scrubbing, where water is substituted by higher-CO₂-soluble solvents. In contrast, pressure swing adsorptions employ porous materials such as activated carbon and carbon molecular sieves to selectively adsorb CO₂over CH₄at high pressure. CO₂removal from biogas can also take place by fractional distillation at low temperature, this technique is called cryogenic separation.

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gas separation as well as biogas purification. Separation processes based on polymer mem-branes show obvious advantages over the above-mentioned ones namely high energy effi-ciency, ease of operation, and small footprint, therefore, very suitable to install in remote areas, where biogas is produced. Membrane based separations involve the application of polymer membranes as thin barriers that selectively allow the intended components to pass through but inhibit the others to permeate. Any separation process is driven by a suitable force such as concentration or pressure difference that is applied to facilitate transport of one or more intended components across the membrane. Basically, the permeability and selectivity are the two most important criteria to evaluate the separation performance of a membrane. The permeability measures the ability of a component to permeate through a membrane while the selectivity is the ratio of the more permeable component’s permeability to that of the less permeable. The higher the permeability the smaller amount of membrane area required to treat a given throughput thereby decreasing capital cost for membrane units, while the greater the selectivity the higher the product purity is. Therefore, membranes pos-sessing both high permeability and selectivity are always preferable.

Currently, membrane gas separation is utilized worldwide for air separation (>99.5% nitro-gen production and oxynitro-gen-enrichment), hydronitro-gen recovery from ammonia purge stream. Other potential applications include separation of hydrogen from synthesis gas (syngas, H₂/CH₄, H₂/CO₂), from ammonia plants (H₂/N₂, H₂/CH₄) and from petroleum refining processes (H₂/CO₂); purification of methane from biogas (CO₂/CH₄) and from natural gas (CO₂/CH₄); removal of the water vapor from natural gas and other gases (H₂O/CH₄); CO₂ capture (CO₂/N₂); as well as to recover helium from rejected gas streams during natural gas processing (He/N₂).

A feed gas mixture is driven by a pressure difference across the membrane. A feed mixture is separated into one or more gases, thus generating a specific gas-enriched permeate or retentate (as illustrated in Fig.1.1). For gases, membranes are generally used in the form of thin-film composite flat sheet spiral wound modules or hollow fiber membranes. The latter are typically utilized for industrial applications because of their high surface area per unit volume

1.2 Fundamentals of gas transport in polymer membranes

In membrane-based separations, a feed gas mixture is pressurized on the feed side of the membranes. Due to differences in physical properties such as kinetic diameter and condens-ability, the other side of the membrane will release a specific gas-enriched mixture called the permeate. The gas transport behavior through polymer membranes is generally explained by the solution-diffusion model [2]. Gas molecules are absorbed in the membranes on the high pressure side (feed side), the absorbed molecules then diffuse through the membranes

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Figure 1.1.Schematic separation of gas mixture by a polymer membrane

with a driving force being the concentration gradient. They finally desorb on the lower pres-sure side (permeate side). The permeability coefficient (or permeability), P, of a gas is a product of its diffusivity, D, and solubility, S, in the membrane as:

P=D×S (1.1)

The permeability of an individual component (for example gas A) through the membrane is defined by:

PA=

NA×l p2−p1

(1.2) where NA is the steady state gas flux through the film, p2and p1are upstream and down-stream pressures, respectively, and l is the film thickness. Permeability is generally re-ported with units of mol·m·m−2_·_s−1_·_Pa−1 _{or more conveniently in Barrer (1 Barrer =} 10−10·cm−3· (STP) ·cm·cm−2·s−1·cmHg−1). The permeability of a polymer to gases is dependent of the physical properties of the permeant gases (molecular size, shape, and po-larity), membrane properties (physical and chemical structures) and interaction between the permeant gases and the polymer. If a polymer membrane is used to separate a gas mixture containing two components (A and B), where component A is more permeable than B, its separation efficiency can be characterized by the ideal selectivity, α_A/Bgiven by:

α_A/B = PA PB = DA DB × SA SB (1.3)

The ideal selectivity can be separated into the diffusivity selectivity and solubility selectivity as shown in Eqn.1.3. For a given gas pair, the solubility selectivity is almost unchanged for a wide range of chemically different polymers, while diffusivity selectivity is highly func-tion of polymer chain rigidity [3]. Membrane researches, therefore, focus on designing new polymers having both high diffusivity and high diffusivity selectivity.

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1.3 Mixed matrix membranes

Membrane-based technologies are developing rapidly in recent decades for molecular scale separations owing to their advantages such as low cost, high energy efficiency, good mechan-ical property and small footprint. For gas separation, polymeric membranes are dominant because they are much cheaper than inorganic membranes, easy to manufacture in the form of hollow fiber or spiral wound modules. From material viewpoint, polymers for gas sepa-ration membranes should comply with the following requirements: good mechanical prop-erties, thermal/chemical resistance, plasticization resistance and physical aging endurance which ensure a long-time stability for membranes working under harsh practical conditions. Several limitations of the membrane-based separation, related to intrinsic polymer proper-ties, are poor resistance to contaminants, low chemical and thermal stability, inverse relation-ship between permeability and permselectivity – Robeson upper bounds [4,5]. An example of the Robeson’s upper bound for CO₂/CH₄gas pair is shown in Fig.1.2.

Figure 1.2.Robeson’s trade-off curve for CO₂/CH₄gas pair (TR, thermally rearranged) [5]. Although several attempts have been made in tailoring polymer structures to improve their separation properties, further progress surpassing the trade-off line seems to be a tough challenge in the near future. The published studies showed that when a polymer with sepa-ration properties near this limit is chemically modified based on its natural structure, these polymers show permeability and selectivity close to the trade-off line instead of exceeding it [6,7].

On the other hand, inorganic membranes such as zeolite and carbon molecular sieve (CMS) have extremely high permeability and selectivity, well above the trade-off lines. The excep-tional separation performance is due to their highly defined pore structures and narrow pore size distribution. Furthermore, their excellent thermal stability and chemical resistant make

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them attractive for practical applications. The wide application of inorganic membranes is, however, hindered by the insufficient technology to manufacture continuous, defect-free membranes and their inherent brittleness. To address this issues, researchers are seeking approaches to combine these benefits of polymeric and inorganic materials in developing a new class of membranes, the mixed matrix membrane, which was first patented in the mid-1980s [8,9].

Mixed matrix membranes (MMMs), hybrid materials, formed by dispersing inorganic par-ticles in a bulky and continuous polymer matrix offers a new approach towards combining the low cost and easy processability of polymers with high selectivity and permeability of inorganic materials [10,11]. Thanks to the incorporated inorganic phases, the MMMs inherit some of inorganic properties, specially their superior separation performances, while the mechanical drawback of inorganic materials like fragility can be compensated by the flex-ibility of polymeric membranes themselves. A number of reports on MMMs using molec-ular sieving materials such as zeolites, carbon molecmolec-ular sieves (CMS), and metal organic frameworks (MOFs) exhibiting attractive transport properties have been published. How-ever, only a few showed separation performance surpassing the Robeson upper bounds [12]. MOFs, a structured material, are constructed from some selected organic linkers and metalic salts creating 1D, 2D or 3D micropore networks. They have been receiving wide research attention due to their remarkable properties, such as high surface area, controllable porosity and selective adsorption capacity. When a MOF is used as a filler in MMMs, two important effects could be expected: (i) a significantly increased permeability, contributed by the ex-ceptional high porosity of MOFs (SBET up to thousands per gram, for example 5300 m2/g for MIL-101); (ii) a higher selectivity, this is brought by either the polarized surface of MOFs, which could selectively adsorb CO₂over CH₄, or the tunable aperture diameters of MOFs to discriminate gases by molecular sieving.

1.4 Objectives and research contributions

This thesis aims at preparing MOF-based MMMs with high gas separation based on using high performance polymers and MOFs and techniques enhancing the polymer-filler com-patibility. In detail, the objectives are specified as follows:

— Developing new MMMs based on functionalized polymers and MOF fillers, study the effect of polymer structure on gas separation of the obtained MMMs

— Developing various strategies using either physical or chemical bonding to eliminate the polymer-filler interface defects

By now, substantial research efforts have been directed to enhance the polymeric membranes performance for use in gas separation and overcome Robeson’s trade-off limits. MOFs-based mixed matrix membranes, as new hybrid materials, combine the advantages of MOFs and polymer membranes, such as high adsorption selectivity of the MOFs, good process ability

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and mechanical properties of the polymer. So far, numerous MOFs-based MMMs have been reported about gas separation in the past several years, there still exist some challenges in membrane performance optimization and mechanism development. In this work, the most advanced polymers and fillers will be selected and paired together to obtain novel MMMs.

1.5 Structure of this thesis

Chapter1Introduction

This chapter introduces some fundamental concepts related to this thesis and briefly presents the main findings as well as contributions to the research field.

Chapter2Literature review

This chapter shows an overview of the membrane separation field, MOFs and MOFs based mixed matrix membrane

Chapter3Polymer Functionalization to Enhance Interface Quality of Mixed Matrix Membranes for High CO₂/CH₄Gas Separation

This chapter studies the relation between the gas separation of MMMs and the quality of polymer-filler adhesion. To do so, the in-teraction between MOF particles and poly-mer chains was controlled by adjusting both the polymer polarity and the filler function-alization. Hydroxyl (-OH) groups were in-troduced into the polymer chain by chang-ing monomer components durchang-ing the poly-mer synthesis while the filler employed was

either non-functional or amine-functionalized MOFs (MIL-53 or NH₂-MIL-53, respectively). The physical interfacial interaction between the MOFs and polymers were confirmed by FTIR and SEM. As for permeability and selectivity of the mixed matrix membranes, those were found to clearly depend on the quality of polymer-filler interaction. Mixed matrix membrane with excellent interface between micrometer-sized MOF crystals and polymeric matrix demonstrates an increase in both permeability and selectivity.

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Chapter4Crosslinked MOF-polymer to enhance gas separation of mixed matrix membranes

In this chapter, both polymer and MOF filler were selected so that they can directly link together via chemical bondings without adding any third component. This aims to further improve the polymer-filler interaction of the resulting MMMs. To do so, a novel porous filler (MOF-74) is prepared. Besides offering very high CO₂/CH₄adsorption selectivity, the key feature of this MOF is that it also inherently owns crosslinkable groups (−OH) on its surface. The polymer phase, on the other hand, was made from a microporous polymer (PIM-1 with (SBET≈700 m2/g) that shows extremely high gas permeabilities. The polymer-filler chemical bonding was facilitated under mild conditions (in chloroform solvent, 65 °C and 24 h) where the−OH groups of MOF filler react with the fluoride chain-ends of PIM-1 polymer. Such polymer-filler crosslinking effectively helped removing interfacial defects. Thanks to the polymer-filler chemical

bond-ing, the polymer chains are spatially re-arranged on MOF-74 surface to link with

−OH groups forming an inter-connected micropore network throughout the MMMs. As a result, the crosslinked MMMs exhibit excellent gas separation performance, not only very high permeabilities which origi-nated from PIM-1 polymer but also high

se-lectivities especially for CO₂/CH₄given by MOF-74.

This chapter proposes a general method of preparing crosslinked MMMs via using some polymers and fillers both containing crosslinkable groups, which are then exposed to react together in a suitable solvent (such as chloroform) under optimized temperature and time. This chapter was published in J. Memb. Sci. 2016, 520, 941-950.

Chapter5In-Situ Cross Interface Linking of PIM-1 Polymer and UiO-66-NH₂For Outstanding Gas Separation and Physical Aging Control

This chapter introduces a novel crosslinked MMM derived from in-situ polymerization of PIM-1 on the filler surface. The MOF, UiO-66-NH₂, was selected because of the high chemical stability, high porosity (SBET > 1000 m2/g) and more importantly its

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amino-surfaced groups. Since the polymer-MOF linking was generated during the polymerization, the extent as well as quality of the crosslinking was enhanced being better than that de-scribed in Chapter4. The developing of PIM-1 polymer on MOF surface was tracked with polymerization times and confirmed by 1H- and 13C-NMR, FTIR. The good adhesion be-tween MOF and in-situ synthesized PIM-1 was also observed clearly in SEM. The separation performance of crosslinked MMMs was well above the 2008 Robeson upper bound.

This chapter was published in J. Memb. Sci. 2018, 15, 429-438.

Chapter6Conclusions and recommendations for future work

This chapter summarizes the most important findings from this work and propose some recommendations for future work.

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Chapter 2

Literature review

This chapter reviews the existing literatures of MOFs based mixed matrix membranes (MMM) for gas separation. It begins with an introduction to MMM (preparations and proposed mechanism for gas transport in MMM). Then a short overview of polymer and filler mate-rials recently reported for MMM will be given in Section2.1. The application of MOF filler in MMMs for gas separation will be extensively discussed in Section2.3. Section2.4focuses the main factors of MMMs affecting their separation properties such as polymer-filler selec-tion, filler morphology and polymer/filler interfacial defects. Section2.5 introduces some up-to-date progresses to modify MMMs for better gas separation. Finally, Section2.6 gives some future trend and outlook on MOF-based MMMs

2.1 Mixed matrix membranes (MMMs)

The concept of MMMs was first applied by Kemp and Paul [13] who mixed zeolite 5A into silicon rubber to improve the low selectivity of the polymer. A schematic configuration and a SEM image of MMM are shown in Fig.2.1. The main aim of MMM fabrication is to utilize the superior gas transport properties of inorganic adsorbents without losing the flexibility of polymer matrix. As a result, mixed matrix membranes combine the advantages of both inor-ganic particles and polymer membranes, including high permeability and selectivity of filler particles, good processability and mechanical properties of the polymer. It is documented that the separation performance of MMM is decided by not only the intrinsic properties of polymer and particles but also by the polymer-filler interaction.

2.1.1 Preparation of mixed matrix membranes

MMMs could be easily obtained by the solution casting method involving 4 main steps: (1) preparing filler particles (MOFs, zeolites...), (2) preparing polymer, (3) dissolving the poly-mer in a suitable solvent, adding the filler and stirring carefully to obtain a suspension, (4) casting the suspension on a leveled surface, as all solvent evaporates, a MMM is formed. The

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membrane is soaked in methanol and then dried in a vacuum oven to completely remove some solvent trapped inside the filler. Following this method, the interaction between poly-mer and particles is poly-merely based on some weak physical forces such as hydrogen bonding, van der Waals contacts, or electrostatic forces. Therefore, the filler agglomeration or sedi-mentation might occur at high loadings thus decreasing the performance of the MMM.

Figure 2.1. (a) Schematic configuration of a MMM including particles dispersed in a polymer mem-brane; (b) cross-sectional surface of a real MMM [14].

Alternatively, MMMs could be obtained by in-situ polymerization. In this method, the parti-cles are mixed well with monomers, and then the monomers are polymerized. On the surface of filler particles, there are often some functional groups such as hydroxyl, carboxyl which can generate initiating radicals, cations or anions under high-energy radiation, plasma or other circumstances to initiate the polymerization of the monomers on their surface. In the in situ polymerization method, inorganic particles with functional groups can be connected with polymer chains by covalent bonds preventing the formation of interfacial defects.

2.1.2 Gas transport in mixed matrix membranes

As mentioned above, the permeability (P) of a gas through a membrane is proportional to the solubility (S) and diffusivity (D) of the gas in the membrane (P = D×S). Thus, adding inorganic nanofillers may affect the gas separation in two ways: the interaction be-tween polymer-chain segments and nanofillers may disrupt the polymer-chain packing and increase the voids (free volume) between the polymer chains, and thus enhance gas diffu-sion; the hydroxyl and other functional groups on the surface of the inorganic phase may interact with polar gases such as CO₂ and SO₂, improving the penetrants’ solubility in the nanocomposite membrane. The gas transport in most of MOF-based MMMs mainly follows the diffusion-solution mechanism, the MOF effect is explained by some proposals described below.

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Free fractional volume increased mechanism

This mechanism is applied for MMMs made from glassy polymers and nano-sized fillers. An important property of glassy polymer membrane strongly affecting gas separation prop-erties is packing density of polymer chains. The packing density of glassy polymer is quan-titatively expressed by free fractional volume that measures the free volume existing within the membrane but unoccupied by polymer molecules. FFV is calculated by the following equation:

Vf =Vsp−1.3×Vw (2.1)

FFV = Vf

Vsp

(2.2) where Vf is the free volume, Vsp is the specific volume derived from the polymer density and Vwis the specific van de Waals volume calculated using the group contribution method of Bondi [15]. For neat polymer membranes, free fractional volume (FFV) is an intrinsic property of the polymer structure characterized by chain stiffness, local motion of polymer chains, side groups and chain bulkiness. It is known that any small increase in FFV results in strong enhancement in gas diffusivity.

Merekl et al. [16] and Winberg et al. [17] studied the permeability and diffusivity of some MMMs with nano-silica particles (loading = 0-50 wt.%). They observed a clear correlation be-tween N₂permeability and the FFV of the nanocomposite membranes, additionally the per-meability increased with increasing filler loading. The authors concluded that the nanofillers may disrupt the polymer-chain packing and increase the free volume between the polymer chains, enhancing gas diffusion and, in turn, increasing gas permeability. This mechanism is consistent with many experimental observations of other groups [18,19].

Solubility increased mechanism

The solubility increased mechanism is based on the interaction between the penetrant gases and the particles. Functional groups, such as hydroxyl, on the surface of the inorganic filler may interact with polar gases, such as CO₂ and SO₂, and increase the solubility in the nanocomposite membranes and, in turn, increase the gas permeability. This mechanism is widely applied for MOF-based MMMs because almost all MOF materials have high ad-sorption capacity of the polar over non-polar gases such as N₂, O₂and CH₄.

2.2 Material selection for MMMs

2.2.1 Polymer materials

In MMMs, the polymer plays as the continuous phase while the filler particle will be the dispersed phase. The selections of polymers and inorganic particles are both crucial in the

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MMMs fabrication. Generally, polymers with higher selectivity result in MMMs with better separation performance. Consequently, glassy polymers with high selectivity are preferred to rubbery polymers which have high permeability but low selectivity. Table2.1shows CO₂ permeability and CO₂/CH₄selectivity of some common-used polymeric membranes while their chemical structures are given in Fig. 2.2.

Normally glassy polymers have rigid structures, adhesion between inorganic phase and polymeric matrix is still a problem in the MMMs preparation, for voids form due to poor interaction between fillers and polymer. Gas separation performance and interaction with inorganic filler are two factors that should be considered when choosing the polymer matrix.

Table 2.1.CO₂permeability and CO₂/CH₄selectivity of some neat polymers widely used for MMMs [20].

Polymers PCO₂ (Barrer) Selectivity (CO2/CH4) Polymer type

Polyethersulfone (PES) 2.8 28 Glassy Polysulfone (PSF) 3.7 23 Cellulose acetate 6 29 Polyimide Matrimid® 6.5 34 Polyimide (6FDA-ODA) 14.4 44.1 Polyimide (6FDA-DAF) 24.1 51 Polyimide (6FDA-6FpDA) 63.9 39.9 Poly(4-methyl-1-pentene) 63.5 5.7

Poly(p-phenylene oxide) (PPO) 90 16.7

Polyimide (6FDA-DAM) 370 21

Poly(tert-butylacetylene) 1020 8.5

Silicone rubber (PDMS) 4553 3.4 Rubbery

Two types of polymeric membranes are widely used commercially for gas separations: glassy and rubbery polymers. Glassy polymers are rigid and glass-like and operate below their glass transition temperatures (Tg). They have low chain intra-segmental mobility and long relaxation times. On the other hand, rubbery polymers are flexible and soft, and they oper-ate above their Tg. The most critical problem for polymer moper-aterial selection for MMMs is to ensure a good interaction between the polymer and the fillers. Compared to the glassy counterparts, rubbery polymers can generate a strong polymer-particle adhesion due to their high degree of chain mobility yielding defect-free interfacial membranes. However, the main drawback of rubbery polymers is also related to their flexible chains which make the

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resul-tant MMMs highly permeable, since the gas transports within the MMMs will be mainly dominated by the rubbery polymer low selectivity and only a small portion of overall trans-port is contributed by the dispersed phase. In consequence, the filler contribution to gas separation of rubbery-based MMMs is quite limited and not much better than the neat poly-mers.

Figure 2.2.Repeat units of several polymers employed for MMMs.

Rubbery polymers

Rubbery polymer composites were first investigated by Jia et al. [21]. The MMMs pre-pared from rubbery PDMS polymer and silicalite-1 zeolite were tested for various gas trans-port. The permeability of He, H₂, O₂ and CO₂was observed to increase while those of N₂, CH₄ and C₄H₁₀ were found to decrease upon adding the filler (up to 50 wt.%). They con-cluded that the silicalite-1 acted as a molecular sieve to discriminate those gases although its pore size is bigger than their kinetic diameters. This means that the shape exclusion effect was governed by both kinetic adsorption and diffusion of gas molecules in the zeo-lite channels. Duval et al. [22] investigated CO₂/CH₄ separation through several rubbery polymers, namely PDMS, ethylene-propylene rubber (EPDM) and nitrile butadiene rubber (NBR) incorporated with zeolites. They found that the addition of silicalite-1, zeolite 13X and KY improved MMM gas separation performance, which was attributed to both CO₂ selective adsorption and molecular sieve effects induced by the zeolites. On the contrary, zeolites 3A, 4A, 5A were totally ineffective in improving the CO₂/CH₄selectivity of the rub-bery polymers because of the smaller pore sizes of zeolites A inhibited diffusion of absorbed molecules from zeolites to the polymer phase. An example of rubbery polymers combined with MOFs in MMM was tested for H₂, N₂, O₂, CH₄and CO₂permeations [23]. Two types of MOFs, Cu₃(BTC)₂– copper(II)-benzen-1,3,5-tricarboxilate and Mn(HCOO)₂– manganese (II) formate were dispersed in PDMS polymer with loadings ranging from 0 to 40 wt%, how-ever no clear improvement in gas-pair selectivities for all composites was reported. Recently, Hussain et al. [24] failed to improve the performance of rubbery-based MMMs when zeolite ZSM-5 was added in PDMS for CO₂, N₂, C₃H₈, and CH₄gas permeations. With increasing zeolite loadings to 25 wt.%, unchanged ideal selectivities (for CO₂/N₂ and CO₂/CH₄) and slightly increased permeabilities of all the tested gases were reported for the MMMs.

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Glassy polymers

The glassy polymer first used for MMMs was PES having a high glass transition temper-ature (Tg) of 225 °C [25]. In that study, MMMs of PES filled with hydrophilic zeolites 4A and 13X were prepared for gas transport investigation of O₂, N₂, Ar, CO₂ and H₂. They found that permeabilities were decreased at first and then increased at zeolite loading of 42-50 wt%, compared to the neat membranes. Nevertheless, the changed performances of MMMs were mainly due to the interface defects, which originated from the zeolite-polymer incompatibility rather than zeolite impacts. Subsequently, Duval et al. [26] investigated the formation of interfacial voids resulting from the low adhesion of glassy polymers and zeolite surfaces. Various commercial glassy polymers with increasing backbone rigidities, charac-terized by Tgvalues, namely poly(4-methyl-1-pentene) (TPX) (Tg= 36 °C), CA (Tg= 80 °C), PSF (Tg= 190 °C), PEI (Tg = 210 °C), PPO (Tg = 210 °C) and PI (Tg = 315 °C) were used as the continuous phase and combined with silicalite-1. Despite the changes in polymer chain rigidities, the poor zeolite-polymer interaction remained unimproved, which was confirmed by SEM micrographs of the cross-sectional MMMs indicating similar interface defects, ex-isting between the polymer and particles. The existence of interfacial voids consequently induced the increased gas permeabilities, but decreased or maintained selectivities for all the composite membranes. The authors concluded that there are three phase co-existing in the MMMs, including zeolite, polymer, and interface voids. Furthermore, the study in-dicated that high temperature treatments clearly improve the interface interaction and gas separation of MMMs, but polymer degradation was accelerated.

Figure 2.3. Schematic representation of chemical grafting of 6FDA-6FpDA/4MPD/DABA to the ze-olite particle surface [27,28].

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Mahajan et al. [27,28] prepared copolyimides 6FDA-6FpDA/4MPD/DABA with their back-bones containing carboxylic groups which can make hydrogen or even covalent bonds with surface groups of zeolite particles providing a better interface interaction (see Fig.2.3). The SEM micrographs indicated that compared to non-functional copolyimide based MMMs, those incorporated with the carboxylic groups show a much better contact with the zeo-lite 4A surface resulting in an enhanced O₂/N₂selectivity for the MMM. The success of their studies introduced an alternative route to improve the polymer-particle adhesion using poly-mers integrated with chemical linkers able to react with the functional groups located on the particle external surface.

2.2.2 Filler materials

The selection of inorganic fillers in MMMs is mainly based on several factors, such as adsorp-tion performance for the desired gas, particle size, surface chemistry and funcadsorp-tional groups. Table 2.2 summarizes some advantages and limitations of filler materials used in MMMs fabrications.

Table 2.2.Some filler materials and their important properties for MMMs.

Filler materials Important properties

Zeolite

High diffusivity and selectivity as compared to polymer mate-rial, but difficult to fabricate MMMs because of weak interaction with polymer.

Carbon molecular sieve High adsorption performance for gas separation, narrow pore size, but weak adhesion with polymer.

Mesoporous silica

A wide range of pore size (2–50 nm). The pore size is too big to discriminate gases by molecular sieving effect. Functional mod-ification of pores is necessary to achieve selective adsorption.

Nonporous silica

Addition of this filler can modify the molecular packing of poly-mer chains; improved permeability and selectivity could be ob-tained.

Metal oxide High specific area, but nano-sized particles are highly recom-mended to avoid sedimentation and phase separation.

Carbon nanotube

Improve gas permeability because of super high gas diffusion within the tunnels as well as enhance tensile modulus and break stress for MMMs, but very expensive.

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Zeolites

Zeolites are the most common inorganic materials used as dispersed phase in MMMs. Ze-olites, crystalline aluminosilicates constructed from AlO₄and SiO₄ tetrahedral, have an in-terconnected channel network and cavities with pore size varying in the range of molecular dimensions from 0.3 to 1.0 nm. The idea of using zeolites as fillers in polymer membranes was because of their significantly higher diffusivity and selectivity than polymer materials. The main driving force for adsorption in zeolites is the highly polar surface within the pores. This unique characteristic distinguishes zeolites from other commercially available adsor-bents, enabling an extremely high adsorption capacity for water and other polar components likes CO₂even at very low concentrations. Zeolites are, therefore, extensively employed as catalysts, adsorbents and ion-exchange media.

For gas separation, gas molecules transport through zeolites by first adsorbing into the pores, then diffusing along the pore surface and finally desorbing. Once introduced into mem-branes, zeolite phase can affect the gas separation of MMMs either by size-shape discrimina-tion or selective adsorpdiscrimina-tion, depending on the pore sizes or selective adsorpdiscrimina-tion properties. For example, zeolite 4A with a pore size of 3.8 Å is more appropriate for the separation of O₂/N₂ and CO₂/CH₄ gas mixture based on the molecular sieving effect. The structure properties of the some commonly applied zeolites are summarized in Table2.3.

Table 2.3.Properties of major zeolite types [29]. Zeolites Structural type Structural dimension Pore size (Å)

4A LTA 3D 3.8 5A LTA 3D 4.3 ITQ-29 LTA 3D 4 13X Faujasite 3D 7.4 NaY Faujasite 3D 7.4 ZSM-2 Faujasite 3D 7.4 L LTL 2D 7.1

Beta BEA 3D (5.5×5.5) and (6.4×7.6)

Silicalite-1 MFI 2D (5.1×5.5) and (5.3×5.6)

ZSM-5 MFI 2D (5.1×5.5) and (5.3×5.6)

SSZ-13 CHA 3D 3.8

SAPO-34 CHA 3D 3.8

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more desirable gas separation properties than those made with rubbery polymers. How-ever, the MMMs prepared by glassy polymers and zeolites often suffer from void formation at the interface of zeolite and polymer. The interfacial voids, which is called “sieve-in-a-cage” morphology, appear because of the rigid structure of glassy polymers resulting in the poor adhesion with the zeolite particles (Fig.2.4). This interface defects may increase perme-ability of all gases but reduce the selectivity. In order to solve the poor adhesion problem between zeolites and glassy polymers, a wide range of modifications has been applied, such as coating of a diluted solution of a highly permeable silicone rubber, surface modification of zeolites and adding a plasticizer to increase the flexibility of the polymeric matrix. The SEM images showed that the interface quality was improved however gas permeability of those MMMs decreased significantly and no improvement in selectivity was observed [30]. This was attributed to zeolite pore blockage as a result of modifications.

Figure 2.4. "Sieve-in-cage" morphology illustrating a poor adhesion between zeolite particles and glassy polymer [31].

Silica

Silica is another class of filler materials in MMMs fabrication which received much atten-tion. To obtain a good dispersion and adhesion with the polymer matrix, nano-sized silica particles are usually employed to create MMMs. The nano-silica is prepared from a sol-gel reaction with hydrolyzation of silica precursors, for example with tetraethyl orthosilicate (TEOS) in acidic medium [32]. Thanks to the nano-size, the silica particles might act as molecular spacers placing between polymer chains hence increasing FFV of the membrane, consequently, modifying permeability and selectivity. Ahn et al. prepared MMMs adding fumed silica nano-particle into polysulfone [33] or PIM-1 [18]. In both cases, the polymer inter-chain space and free volume were increased upon the addition of silica. The increased FFV significantly enhanced the diffusion and solubility coefficients of MMM and improved the permeability of all tested gases. However, the permeability of large gases was more enhanced by the addition of silica, resulting in a reduction in selectivity.

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Carbon molecular sieve

Carbon molecular sieve (CMS) produced from the pyrolysis of polymeric materials have shown to be very effective for gas separation in adsorption applications since 1980s. CMS can be obtained by pyrolysis of many thermosetting polymers such as poly(vinylidene chloride) (PVDC), poly(furfuryl alcohol) (PFA), cellulose, cellulose triacetate, polyacrylonitrile (PAN), phenol formaldehyde and various coals such as coconut shell. The pore dimensions of CMS depend not only on the morphology of the organic precursor, but also on the chemistry of pyrolysis. The size of the pore opening of CMS is of the same magnitude as the kinetic diameter of gas molecules, therefore gas transport within CMS is governed by molecular sieving as shown in Fig.2.5.

Figure 2.5.Illustration for molecular sieving effect of CMS.

The pore system consists of relatively wide openings but narrow constrictions. The openings contribute the major part of the pore volume and are thus responsible for the adsorption capacity, while the constrictions are responsible for the stereo-selectivity of pore penetration by host molecules and for the kinetics of penetration. Hence, the diffusivity of gases in CMS change abruptly depending on their size and shape because the CMS has pores size close to dimension of gas molecules. When the pore size diameters of CMS are relatively larger than gas molecules, probably in range of 5-7 Å their gas discrimination is given by selective adsorption/surface diffusion mechanism. Thereby, weakly adsorbable gases (O₂, N₂, and CH₄) can be separated from adsorbable gases, such as NH₃, SO₂, H₂S, and CO₂.

Typical preparation of CMS/MMM involves the programed pyrolysis of polymer precur-sors and ball-milling the pyrolyed products to obtain the CMS particles, followed by adding those particles into a polymer matrix. Vu et al [34] introduced CMS into some commercial polymers (Ultem® and Matrimid®) and observed a clear increase in selectivity ( 40% for CO₂/CH₄and 20% for O₂/N₂) without suffering any losses of permeabilities. Furthermore, SEM images showed that the CMS particles have good adhesion with polymer matrix. The main limitation of CMS based MMMs is that the pyrolysis to obtain CMS is unpredictable and difficult to control. Moreover, the particle size of CMS is relatively large (few microme-ters) causing the MMMs to be very fragile.

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Carbon nanotubes

Carbon nanotubes (CNTs) have been recognized as novel filler materials of MMMs due to their outstanding physical and chemical properties. Carbon nanotubes with diameters of 10-60 nanometer and hundreds nanometer long, can offer a promising opportunity to make membranes with high flux and selectivity. As with other inorganic fillers, the polymer-filler incompatibility still challenges the introduction of CNTs into polymer matrix, namely the weak adhesion with polymer matrix or agglomeration of CNTs. For the CNTs based MMMS, the voids caused by the agglomeration of CNTs can weaken the separation performance. Therefore, improving the dispersion of CNTs in polymeric matrix is important. The most possible solution is to introduce functional groups (such as hydroxyl and carboxyl groups) to the CNT surface through oxidation by strong inorganic acids such as concentrated H₂SO₄or HNO₃. Kim et al. [35] first reported the gas separation of MMMs made from polysulfone and functionalized CNTs (loadings up to 15 wt.%). Overall, diffusivity and permeability of all tested gases increased with CNTs loadings which was explained by a higher gas diffusivity within the CNTs than the polymer matrix. However, no improvement in selectivity was observed because the CNTs used had a pore diameter of 15 Å that was far greater than kinetic diameter of gas molecules(3-4 Å). The SEM images, also, showed many dense cluster of CNTs indicating they had poor interaction with polymer.

2.2.3 Conclusions for material selections

The limitation of neat polymer membranes for gas separation expressed by the Robeson’s upper bound is an undeniable fact, considered as an inherent property of polymer materials which is hard to change. The concept of making MMMs by introducing inorganic fillers into polymer matrix is regarded as an effective approach to enhance the gas separation perfor-mance of polymers. However, the inorganic materials reviewed above still have their own limitations in MMMs fabrication. To be applied for MMMs filler materials should have high surface area, adsorption properties, controllable cage dimension and more importantly a modifiable surface to improve the interaction with the polymer matrix. Searching for novel fillers for high performance MMMs is still following that way.

2.3 Metal organic frameworks (MOFs) as fillers for MMMs

Metal–organic frameworks (MOFs),a novel generation of porous materials, are constructed from inorganic metal clusters and organic linkers forming one-, two-, and three-dimensional porous but flexible frameworks. From an easy synthesis, we can obtain MOFs in high purity and crystallinity. MOFs have several unique structural properties and advantages: high spe-cific surface area, adjustable pore size by substituting the organic linkers and tunable surface property, good thermal and chemical stability. The surface area and porosity of MOFs are generally higher than those of zeolites, silica, and activated carbons. The most important

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advantage of MOFs over traditional porous materials is that the pore structure and pore size can be tailored to specific applications through simply changing the combination of metals and organic ligands. For example, pore size of IRMOF series can be tuned from 0.38 to 2.88 nm by selecting linkers [36]. This section summarizes some of the most commonly studied MOFs used for MMMs, particularly ZIF-8, HKUST-1, MIL-53, MIL-101, MOF-74, and UiO-66. The physical characteristics and chemical compositions of those MOFs are introduced in Table2.4.

Table 2.4.Physical characteristics of some commonly used MOFs for MMMs

Common name Metal clusters Organic ligand Pore aperture (Å) BET surface area (m2/g) ZIF-8 Zn 3.4 1800 HKUST-1 Cu 9 1500-2100 MIL-53 Al 8.5 1100-1500 MIL-101 Cr 29, 34 2800-4200 MOF-74 Mn, Mg, Fe, Co, Ni, Zn 12 1300-2000 UiO-66 Zr 6 1000

The surface chemistry of MOFs offers more advantages than zeolites since it could be tunned from natural to basic by introducing functional groups (-NH₂) on the linkers. Due to these

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physical and chemical properties, MOFs have shown great potential in a wide range of ap-plications, including gas adsorption and separation, gas storage, catalysis, drug delivery. Compared to zeolites or carbons, the organic linkers in MOFs provide a tunable feature to their structure, enabling potentially better interaction with polymers, thereby reducing non-selective defects at the MOF–polymer interface. Therefore, new design strategies have been developing rapidly to form MOF-based MMMs to optimize gas diffusion and selectivity. To date, thousands of MOFs have been prepared and investigated for their gas transport perfor-mance. Only several MOFs were applied for MOF-based MMMs because the MOFs should meet some requirements on chemical stability, gas selective adsorption as well as pore size diameters.

2.3.1 ZIF-8

In 2006, Yaghi et al. [37] reported that they had successfully synthesized a series of crystals, named zeolitic imidazolate frameworks (ZIFs). ZIF-8 is a member of ZIFs which are synthe-sized from imidazolate linkers tetrahedrally coordinated to metalic ions (typically Zn, Co, Cd, Li, B...). This tetrahedral arrangement is very similar to the zeolite structure, therefore, they were named “zeolitic” which originates from similarities of topological and bond an-gles between these materials and zeolites. For example, an analogous topology of ZIF-8 and zeolite A with sodalite structure could be seen in Fig.2.6. ZIF-8 is easily prepared from low-cost available precursors and has better thermal stability (up to 400 °C) and smaller pore sizes than other MOFs. ZIF-8 has a pore diameter of 3.8 Å that is beneficial to selectively absorb small gas molecules (H₂ and CO₂) over the bigger ones (such as CH₄, O₂ and N₂). Thanks to its outstanding properties and the fact that it is commercially available, ZIF-8 has been extensively investigated for gas separation.

Figure 2.6.Topologies of ZIF-8 and zeolite A.