DOCTORAL THESIS

Department of Chemical Engineering and Geosciences Division of Chemical Technology

2004:11 • ISSN: 1402 - 1544 • ISRN: LTU - DT - - 04/11 - - SE

2004:11

Properties and Modeling of MFI Membranes

FREDRIK JAREMAN

T D D P

Properties and Modeling of MFIMembranes

Fredrik Jareman

Division of Chemical TechnologyDepartment of Chemical Engineering and Geosciences

Luleå University of TechnologyS- Luleå, Sweden

April

Doctoral thesis 2004:11

Properties and Modeling of MFI MembranesFredrik Jareman

© Fredrik Jareman, 2004

ISSN: 1402-1544ISRN: LTU - DT - - 04/11 - - SE

Division of Chemical TechnologyDepartment of Chemical Engineering and GeosciencesLuleå University of TechnologyS- Luleå, Sweden

UniversitetstryckerietLuleå, Sweden 2004

Abstract

The permeation properties of thin (<2 µm) film MFI molecular sieve membranes havebeen studied in the present work and a model has been developed. The synthesis of suchmaterials has been studied to a smaller extent. The films have been grown on gradedα-alumina microfiltration filters using a seeding method. Scanning electron microscopyand x-ray diffraction were used in addition to permeation measurements for characteri-zation of the materials.

In particular, a simple and unique model describing single component permeationwas developed. The model is a combination of simple and basic equations for per-meation and adsorption. The important defect distribution of the membrane and theproperties of the support are measured in separate experiments. The model is uniquesince it is accounting for the effect of defects and support on the permeation proper-ties. The model can adequately describe the performance of various MFI membranes.The model indicates mass transfer limitations of the supports that strongly affect, forinstance, permeance ratios . It was also found that these ratios are dependent on crys-tallographic orientation, film thickness and experimental conditions in addition to theamount of defects. Permeance ratios can thus only be used to compare membranes withsimilar morphology and tested under similar conditions.

It was found that defects formed in thicker films. Membranes prepared on maskedsubstrates were of higher quality than membranes prepared on unmasked substrates.

MFI membranes with low and varying aluminum content with similar material prop-erties, such as defect distribution and thickness, were evaluated with multi-componenthydrocarbon isomers permeation. The silicalite-1 membrane showed a minimum inseparation selectivity between two C6 isomers whereas the ZSM-5 membrane showed analmost constant selectivity, independent of temperature, but with lower permeances.

The effect of the calcination rate on the membrane quality was investigated forsilicalite-1 membranes. Based on a number of permeation characterization techniques,the membrane quality was independent of the calcination rate.

It was found that the permeation properties of membranes comprised of small crys-tals in several layers were different from membranes comprised of one layer of largercrystals, although the quality of the membranes was similar.

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ZSM-5 membranes with high aluminum content showed catalytic conversion ofethanol into diethylether and ethylene under simultaneous separation of the ethanol /water azeotrope. These membranes were not stable at high temperatures.

Keywords: MFI, membrane, Hydrocarbon isomers, Separation, Modeling,Defect distribution

Acknowledgements

First I would like to acknowledgemy supervisor, Associate Professor Jonas Hedlund for all the guidance, kickin the butt, cheerings and sick jokes that I have got during the years. Prof.Johan Sterte is acknowledged for giving me the opportunity to work in his

group and always listening to my crazy ideas. I wish you good luck with your futurework as president of Växjö University.

I’m also grateful to:Dr. Anton-Jan Bons, Mr. Marc Anthonis, Dr. Derek Creaser, Lic. Eng. MagdalenaLassinanti and M.Sc. Charlotte Andersson for their cooperation with some of thepapers. Dr. Harry W. Deckman has given invaluable suggestions regarding single

gas permeation measurements. Our former secretary Mrs. Ingrid Granberg for all thehelp with the administrative aspects and all the laughs. I acknowledge Sharon and Dr.Klaus Möller for help with the linguistic corrections. I thank the people that work, orhave worked, at the division; Dr. Lubomira Tosheva, Dr. Qinghua Li, Lic. Eng. OlovÖhrman, Mr. Jonas Lindmark, M.Sc. Mattias Grahn, Dr. Valeri Naydenov, Mr. OlleNiemi, Lic. Eng Zheng Wang and Ms Maria Edin for your companionship, ideas andhelp, and the people at the department for interesting coffee and lunch breaks and somehelp in the laboratory.

The Swedish Research Council (VR) is acknowledged for financial support of this work.

Till slut skulle jag vilja tackaMin familj i Stockholm för ert stöd och engagemang under dessa år, somstudent och doktorand. Jag vill också tacka mina kamrater i Stockholm ochGöteborg (varför måste ni bo på "baksidan"?), trots att vi inte ses och hörs så

ofta så har vi väldigt kul när väl gör det. Förhoppningsvis finns det mer tid över för detnu.

Tack Nilla, hur skulle jag klara mig utan dig? :-)

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List of Papers

This thesis is based on the work contained in the following papers, referred to in the textby Roman numerals.

I A Masking Technique for High Quality MFI MembranesJonas Hedlund, Fredrik Jareman, Anton-Jan Bons and Marc AnthonisJournal of Membrane Science 222(1-2), pp. 163-179, (2003)

II Modelling of Single Gas Permeation in Real MFI MembranesFredrik Jareman, Jonas Hedlund, Derek Creaser and Johan SterteJournal of Membrane Science, In press

III Single Gas Permeance Ratios in MFI Membranes: Effects of MaterialProperties and Experimental ConditionsFredrik Jareman and Jonas HedlundSubmitted to Microporous and Mesoporous Materials

IV Permeation of H2, N2, He and SF6 in Real MFI MembranesFredrik Jareman and Jonas HedlundSubmitted to Microporous and Mesoporous Materials

V Effects of Aluminum Content on the Separation Properties of MFIMembranesFredrik Jareman, Jonas Hedlund and Johan SterteSeparation and Purification Technology 32(1-3), pp. 159-163, (2003)

VI Influence of the Calcination Rate on Silicalite-1 MembranesFredrik Jareman, Charlotte Andersson and Jonas HedlundSubmitted to Microporous and Mesoporous Materials

VII Factors Affecting the Performance of MFI MembranesJonas Hedlund, Fredrik Jareman and Charlotte AndersonAccepted for presentation and publication in the proceedings of the 14thInternational Zeolite Conference in Cape Town, South Africa

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VIII Silicalite-1 Membranes with Small Crystal SizeCharlotte Andersson, Jonas Hedlund, Fredrik JaremanAccepted for presentation and publication in the proceedings of the 14thInternational Zeolite Conference in Cape Town, South Africa

IX Preparation and Evaluation of Thin ZSM-5 Membranes Synthesizedin the Absence of Organic Template MoleculesMagdalena Lassinantti, Fredrik Jareman, Jonas Hedlund, Derek Creaserand Johan SterteCatalysis Today 67(1-3), pp. 109-119, (2001)

Contents

List of Tables xi

List of Figures xiii

Part One:Introduction and Literature Survey 1

1 Introduction 31.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2 Scope of the Present Work . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Literature Survey 52.1 Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Molecular Sieves and Zeolites . . . . . . . . . . . . . . . . . . . . . . . 82.3 Molecular Sieve Films . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.4 MFI-Zeolite Membranes . . . . . . . . . . . . . . . . . . . . . . . . . 102.5 Substrates for Zeolite Membranes . . . . . . . . . . . . . . . . . . . . . 122.6 Defect Formation in Zeolite Films and Membranes . . . . . . . . . . . 13

Part Two:Permeation Theory and Model Development 15

3 Modeling of Diffusion and Adsorption in Zeolites 173.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.2 Heterogenous Physical Adsorption . . . . . . . . . . . . . . . . . . . . 183.3 Single Component Mass Transfer . . . . . . . . . . . . . . . . . . . . . 193.4 Condensation in the Pores . . . . . . . . . . . . . . . . . . . . . . . . 203.5 Additional Equations for Diffusion in Zeolites . . . . . . . . . . . . . . 22

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4 Model Development 274.1 Effect of Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2 Permeation in the Zeolite Film . . . . . . . . . . . . . . . . . . . . . . 284.3 Defect distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Part Three:Thesis summary 31

5 Experimental 335.1 Masking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.2 Membrane Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 345.3 Permeation Measurements . . . . . . . . . . . . . . . . . . . . . . . . 355.4 Additional Characterization . . . . . . . . . . . . . . . . . . . . . . . . 37

6 Results and Discussion 396.1 Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.2 Defect Distribution from Porosimetry Data . . . . . . . . . . . . . . . 416.3 Mass Transfer Parameter Estimation . . . . . . . . . . . . . . . . . . . 426.4 Predicting Membrane Performance . . . . . . . . . . . . . . . . . . . . 446.5 Effect of Substrate on the Permeance Ratios . . . . . . . . . . . . . . . 466.6 Preferred Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . 476.7 Influence of Applied Feed Pressure on the Permeance Ratios . . . . . . . 486.8 Influence of Defects on the Permeance Ratios . . . . . . . . . . . . . . 496.9 Permeation Ratios in Various Membranes . . . . . . . . . . . . . . . . 506.10 Small Crystal Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506.11 Influence of Calcination Rate . . . . . . . . . . . . . . . . . . . . . . . 516.12 Si/Al-Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526.13 Separation of the Ethanol/Water Azeotrope . . . . . . . . . . . . . . . . 536.14 Temperature Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

7 Conclusions 55

8 Future Work 57

Nomenclature 59

References 63

Part Four:Papers 69

I A Masking Technique for High Quality MFI Membranes 71

II Modelling of Single Gas Permeation in Real MFI Membranes 73

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III Single Gas Permeance Ratios in MFI Membranes: Effects of MaterialProperties and Experimental Conditions 75

IV Permeation of H2, N2, He and SF6 in Real MFI Membranes 77

V Effects of Aluminum Content on the Separation Properties of MFIMembranes 79

VI Influence of the Calcination Rate on Silicalite-1 Membranes 81

VII Factors Affecting the Performance of MFI Membranes 83

VIII Silicalite-1 Membranes with Small Crystal Size 85

IX Preparation and Evaluation of Thin ZSM-5 Membranes Synthesized inthe Absence of Organic Template Molecules 87

List of Tables

6.1 Calculated defect distribution for samples U17, M30, U30, M72 and U72. 426.2 Estimated intrinsic diffusion and adsorption coefficients. . . . . . . . . . . 446.3 Film thickness, experimental pressure drop, helium flux, and permeance ra-

tios for selected membranes. . . . . . . . . . . . . . . . . . . . . . . . . . 446.4 Experimental fluxes of samples M30 and M30∗, the latter is of lower quality.

Fluxes were simulated for M30∗. . . . . . . . . . . . . . . . . . . . . . . . 456.5 Absolute and relative errors between experimental and simulated fluxes of

sample U72. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.6 Measured and fabricated (10 times higher) defect distribution used for mem-

brane simulations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496.7 Separation selectivity of hexane and xylene isomers at T=390 ◦C for sample

M30 and M3×12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

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List of Figures

2.1 Illustration of a general membrane with separation mechanisms . . . . . . . 52.2 MFI-crystal with channel system and crystallographic axes and pore dimen-

sions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3 Basic concept of seed film method. . . . . . . . . . . . . . . . . . . . . . . 10

3.1 Schematic illustration of an adsorbed molecule within a slit shaped micropore. 21

4.1 General drawing of an asymmetric membrane. . . . . . . . . . . . . . . . . 274.2 A (very) defective membrane with two defect sizes and the corresponding

defect areas. The quadratic surface with the assumed mesh of defects. . . . . 29

5.1 Masking procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345.2 Porosimetry unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365.3 Principal setup of a Wicke-Kallenbach cell. . . . . . . . . . . . . . . . . . . 365.4 Separation performance testing unit. . . . . . . . . . . . . . . . . . . . . . 37

6.1 Side and top view SEM images of membranes prepared with 30 or 72 hhydrothermal treatment on masked substrates. . . . . . . . . . . . . . . . . 40

6.2 A membrane comprised of five layers of small crystals and a membrane withsimilar thickness comprised of one layer of crystals. . . . . . . . . . . . . . 41

6.3 Membrane quality evaluation by porosimetry and C6 isomer separation . . . 416.4 Experimental fluxes of thin masked MFI membranes versus applied film

pressure drop for helium and SF6 . . . . . . . . . . . . . . . . . . . . . . . 436.5 Experimental and simulated fluxes of H2, He, N2 and SF6 for sample U72. . 456.6 Permeance ratios of composite membranes as a function of MFI film thickness. 466.7 The effect of differences in crystallographic orientation on the N2/SF6 per-

meance ratio as a function of MFI film thickness. . . . . . . . . . . . . . . 476.8 Simulated permeance ratios as a function of membrane pressure drop for a

1 µm thick randomly oriented film. . . . . . . . . . . . . . . . . . . . . . . 486.9 The effect of defects on the N2/SF6 permeance ratios. . . . . . . . . . . . . 49

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6.10 n-Hexane porosimetry patterns for membranes prepared with small crystals. 506.11 Separation selectivity of a xylene isomer mixture as a function of temperature. 516.12 Butane isomers and hexane isomers separation selectivity of thin MFI mem-

branes as a function of temperature. . . . . . . . . . . . . . . . . . . . . . 536.13 Water/ethanol azeotrope separation properties . . . . . . . . . . . . . . . . 546.14 Temperature dependent SF6 permeation. . . . . . . . . . . . . . . . . . . . 54

Part One

Introduction and Literature Survey

Chapter 1

Introduction

1.1 Background

Industrial reactors convert the reactant 100 % into a single desired component. In fact,many reactions in industry are limited by kinetics and thermodynamics in a way thatis not favourable for the desired product. The resulting product stream is a mixture ofproducts, bi-products and reactants. A separation of the product stream is needed inorder to purify the products and recycle the reactants. This separation often involvesan energy consuming phase change. For instance, in the case of xylene production theproduct mixture is cooled down to the freezing point of p-xylene that crystallizes andmay thus be separated. The remaining liquid is subsequently vaporized and heated tothe reaction temperature (>300 ◦C) and recycled through the reactor. A more feasiblesolution would be to separate the mixture at the reaction temperature without an energyconsuming phase change. An example of a separation unit would be a thin and selectivemembrane for a specific component such as p-xylene. However, since industrial reactionsare often performed at high temperatures and pressures, available polymeric membraneswould be destroyed at these conditions. An inorganic membrane would thus be a betterchoice since inorganic membranes are more resistant to high temperatures and pressures.Unfortunately most inorganic membranes are not very selective due to the relativelylarge pore size of the membranes. Zeolites are a subgroup of molecular sieves. Thesematerials have well defined pore openings of molecular dimensions in addition to usefuladsorption and ion exchange capacity. Zeolites with a number of different structureshave been synthesized as well as defined powders or films. The properties of zeolites arewell-suited for membrane applications under harsh conditions. By using zeolite mem-branes in industrial processes, many energy demanding, or even impossible separationproblems, could be solved in a more economical and feasible way than what is possibletoday. However, defects in membranes are often a critical issue since they may deterio-rate the membrane separation performance. It is crucial that the amount of defects andthe defect distribution of the membrane is known in order to determine the quality of

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the membrane. Knowledge about defects is essential for successful optimization of thepreparation procedure of the materials, since the quality of the membranes is dependenton the amount of defects. The measured defect distribution may further be used for aprecise modeling of the permeation and separation performance of the membrane. Areliable model may also reduce the development cost of such materials, since the mem-brane separation performance may be predicted and the amount of experiments couldthereby be reduced. Most models described in the open literature neglect defects and donot account for the substrate. As will be shown later in this work the effect of defectsand substrate may be significant on the permeation properties of the membrane.

1.2 Scope of the Present Work

The scope of the present work is to develop a model for permeation in molecular sievemembranes. The model should account for the effect of defects and the support. Thedefect distribution is determined in a separate experiment and permeation measurementsare subsequently carried out. The model is applied on permeation of light inorganicmolecules. The effects of experimental conditions on permeance ratios, in particular, arestudied.

Further objectives were to study how material properties such as Si/Al ratio, crystalsize and orientation affect the permeation and separation of linear and branched hydro-carbons. To a smaller extent, the work also includes development of synthesis proceduresof molecular sieve (MFI) membranes.

Chapter 2

Literature Survey

2.1 Membranes

Membranes have been successfully utilized in several commercial applications, such aswaste water treatment and desalination of sea water [1, 2]. A membrane [3] is able toseparate components in gas or liquid phase with the aid of a driving force. The drivingforce for flow through the membrane is a difference in chemical potential as illustratedin Figure 2.1. The chemical potential could be a result of differences in total pressure,partial pressure, concentration or electrical potential. Membranes may be divided in thefollowing groups: biological, synthetic- organic and inorganic. Synthetic organic andinorganic membranes are discussed in this literature survey. For the sake of simplicity, theword synthetic will not be used hereafter, all membranes that are discussed are synthetic.

Organic and inorganic membranes may either be porous or dense. The pores in aporous membrane may either be straight, as illustrated in Figure 2.1, or tortuous. Porousmembranes may either be microporous, mesoporous or macroporous. IUPAC definesthese terms as:

∆µ

(a) (b)

Figure 2.1: A membrane with indicated driving force for diffusion and the sieving (a)and preferential adsorption (b) separation mechanisms.

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• Micropores d < 2 nm

• Mesopores 2 nm < d < 50 nm

• Macropores d > 50 nm

The classification is arbitrary and based on nitrogen adsorption measurements on vari-ous porous materials at the boiling point of nitrogen at atmospheric pressure [4]. Fora porous membrane three main separation mechanisms may occur: sieving, preferen-tial adsorption and separation due to different diffusivities. The sieving mechanism,illustrated in Figure 2.1(a), only allows particles/molecules smaller than the pores ofthe membrane to permeate. In preferential adsorption, one species is more stronglyadsorbed. In this case, larger components may permeate more effectively than smallercomponents if both components are sufficiently small to fit the pores. Separation due todifferences in diffusivities of the permeating species may also occur, for instance when amolecule with a larger molecular weight has a lower diffusion coefficient than a moleculewith a lower molecular weight. This mechanism depends on the transport mechanismand is only applicable for Knudsen flow, see section 3.3.2.

2.1.1 Terminology

The terms feed, retentate, permeate, flux and permeance are commonly used in membranescience. The feed is the stream that is fed to the membrane for separation and theretentate is the flow rejected by the membrane. The permeate is the flow passing throughthe membrane. Fluxes and permeances may be based on mass-, molar- or volumetricflows. The flux is defined as flow through the membrane per unit area and the permeanceis calculated from the flux by dividing with the partial pressure gradient:

Πi =Ji

∆Pi(2.1)

Permeance ratio(αPerm

)(or ideal selectivity /permselectivity) is commonly used to de-

scribe the performance of the membrane. The quantity is calculated from single gaspermeances, measured at certain experimental conditions such as room temperature, us-ing:

αPermi,j =

Πi

Πj(2.2)

Permeance ratio should not be confused or compared with the separation factor, or sep-aration selectivity, for a mixture. The separation selectivity for a mixture is calculatedwith the well-known formula:

αi,j =(xi/xj)Permeate

(xi/xj)Feed(2.3)

The relation describes the ability of a certain membrane to separate two components ina mixture under certain conditions.

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2.1.2 Organic Membranes

Organic membranes may be divided into two subgroups: liquid and polymeric mem-branes. A liquid membrane is simply a thin film of liquid that is immiscible with theliquids on the retentate and permeate side of the membrane [5]. The liquid film mayeither be self-supported or supported by a porous material that contains the liquid mem-brane.

Polymeric membranes are the second and more widely used type of organic mem-branes. These membranes are fabricated by organic polymers of varying molecularweight and cross-linking of the polymeric chains. Polymers commonly used for mem-brane applications are, among others, cellulose acetate, fluorocarbon polymers and aro-matic polyamides [3]. The preparation methods for polymeric membranes are, for in-stance, sintering, stretching and sol-gel processes. A disadvantage with organic mem-branes is their limited thermal stability. A deeper discussion of organic membranes is outof the scope of this literature survey. Polymeric membranes are used in several industrialapplications such as desalination of sea water and dehydration of solvents.

2.1.3 Inorganic Membranes

Inorganic membranes have several advantages over organic membranes, such as thermaland chemical stability. They may be either dense or porous as polymeric membranes.Zeolite membranes are porous inorganic membranes. They will be treated separately insubsequent sections. The first large breakthrough for inorganic membranes was in theearly 1950s. The process of interest was to separate the two uranium isotopes in theform of UF6 gas by a membrane. This particular process is still the largest applicationfor inorganic membranes. A membrane with quite large pores is used in the processand the separation mechanism is based on differences in the Knudsen diffusivity, seesection 3.3.2.

Palladium membranes are dense and the flux is low. On the other hand, since onlyhydrogen may be absorbed in the palladium film, an infinite selectivity for hydrogenis expected for a perfect palladium membrane. Pervoskite membranes are also denseinorganic membranes and are selective for oxygen instead of hydrogen [6]. Thin metalmembranes may, for instance, be prepared using physical or chemical vapour depositionor electroplating methods [7].

Carbon membranes are porous inorganic membranes, prepared by a pyrolysis of or-ganic materials such as polymeric membranes [8]. These membranes are selective forhydrogen in a hydrogen/nitrogen mixture and oxygen in a oxygen/nitrogen mixture [8].The latter separation is important in a low cost separation of air in order to obtain nitro-gen [9]. Separation of hydrogen from various process streams is also of special interestfor hydrogen recovery and purification of methane [9] instead of conventional methodsthat may involve condensation of the gas streams.

Silica membranes are also important porous inorganic membranes. These mem-branes are highly selective for hydrogen and carbon dioxide in mixtures with methane.These materials are thus useful for purification of methane or recovery of hydrogen.Silica membranes are microporus and amorphous and are prepared by the Sol-Gel tech-nique from polymeric SiO2 sols with varying amount of polymer branching. The branch-

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a

b

c

5.5

5.1

a-direction

5.6

5.3

b-direction

Figure 2.2: MFI-crystal with channel system and crystallographic axes and pore dimen-sions (Ångström units) in the a- and b- directions.

ing of the polymers affects the final pore size of the membrane; less branching results innarrower pores [6].

Other porous inorganic membranes are built from MCM-48 [10] and SAPO-34[11] crystals to name a few. Since membranes from these materials have been reportedto a much lesser extent than the previously described membranes they have been left outin this literature survey. They may show high selectivities for organics in water solutions[10].

2.2 Molecular Sieves and Zeolites

Molecular sieves is a class of materials, capable of separating components in a mixtureon the basis of molecular size and shape. A subset of molecular sieves is the group ofzeolites. Zeolites are crystalline aluminum silicates with well-defined pores. The pores invarying zeolites are within the range 3 Å to 13 Å [12], i.e. micropores. The microporescan be utilized for gas phase separation of molecules at high temperatures.

The zeolite structure is built by a three-dimensional network of [AlO4]5− and [SiO4]4−

tetrahedras. The tetrahedras are linked by sharing oxygen atoms and form a three-dimensional framework [13]. A representative formula of a general zeolite structuremay be written as:

Mx/n[(AlO2)x(SiO2)y] · wH2O

M is a cation of valence n, w is the number of water molecules and y/x is the sili-con/aluminum ratio for the zeolite. The counterion may be a metal-, ammonium- oralkylammonium cation [13].

Over 130 different zeolite framework types [14] are known today. The most fre-quently studied zeolite framework types are MFI, LTA and FAU. Two well-known molec-ular sieves of MFI type are silicalite-1 and ZSM-5. Figure 2.2 shows a MFI-crystal withtypical habit and the channel system and crystallographic axes. The sinusoidial channelsrun along the a-direction in the crystal and have the dimensions 5.5 Å × 5.1 Å. Thestraight elliptical channels run along the b-direction within the crystal and have the di-mensions 5.6 Å × 5.3 Å. A difference between silicalite-1 and ZSM-5 is the aluminum

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content. The silicon/aluminum ratio for silicalite-1 is > 200, and for ZSM-5 the ratiois in the range of 10 to 200 [12]. Aluminum affects the properties of the zeolite, byadding charge to the framework. The charged framework results in catalytic activity,hydrophilicity and an ion exchange capacity.

Molecular sieves are synthesized by hydrothermal treatment of a solution or gel con-taining a silica-, alumina- and alkali source. The alkali source may be an alkali hydroxideand/or organic base. In a number of cases, such as in the synthesis of silicalite-1, anorganic additive is necessary in order to crystallize the desired molecular sieve. Tetram-ethylammoniumion [TMA]+ and tetrapropylammonium [TPA]+ ion are among themost frequently used additives. The additive is denoted template molecule or shortly tem-plate. Unfortunately, the template remains within the pores after synthesis and blocksthe pores. The template has to be removed in order to open up the microporous frame-work . The template removal may either be carried out by ion exchange, if the pore sizeadmits transport of the template molecules, or by oxidation at high temperature. Thelatter method is denoted calcination.

2.3 Molecular Sieve Films

Thin films of molecular sieves are interesting for several novel applications, such as:

• Catalysts

• Membranes

• Sensors

The film thickness may be used to control properties such as catalytic activity and selec-tivity towards a desired component in a structured catalyst. In the case of membranes, athin, perhaps less than 200 nm [15], film is necessary in order to reduce mass transportresistance.

Molecular sieve films are often prepared using one of the three methods given below:

• In-situ crystallisation (direct synthesis)

• Vapour phase transport method

• Seeding method

One of the most frequently used methods for synthesis of zeolite films is direct synthe-sis. A substrate is treated with an appropriate synthesis solution under hydrothermalconditions. The method relies on the occurrence of both nucleation and crystal growthon the surface in order to facilitate film growth. Both supported and non-supportedfilms have been synthesized with the method, where supported films dominate due tothe higher mechanical strength. The vapour phase transport method, first described byXu et al. [16], utilizes a dry gel containing the aluminum and silica source. The gel is hy-drothermally treated with vapours of triethylamine, ethylendiamine and water in orderto crystallize the zeolite film.

Seeding methods have been used for synthesis of thin films of zeolite on almost anytype of substrate. A substrate with pre-attached seed crystals is hydrothermally treated

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(a)

++++

++

++

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++++

++

++

++++

++

++

++++

++

++

++++

+

(b)

++++

++

++

---------------------------------------

++++

++

++

++++

++

++

++++

++

++

++++

+

(c) (d)

Figure 2.3: Basic concept of seed film method.

in an appropriate synthesis solution. Different methods have been used to attach theseed crystals on the substrate surface. One method is to dip the substrate in a solutioncontaining the crystals [17–19]. Another method, used to a lesser extent, is to rub thesubstrate with a powder of small zeolite crystals [20]. Previous work at the division hasconcerned the development of a versatile seeding method, "The seed film method" [21].A negatively charged substrate is treated with a cationic polymer solution in order toreverse the surface charge. Since the surface charge of the colloidal seeds is negative,an attractive force between the substrate and seeds is accomplished. The technique isillustrated in Figure 2.3, for a negatively charged substrate. The substrate is shown in(a) and (b) illustrates the substrate after application of the cationic polymer, (c) showsthe seeded substrate and the film formed after the hydrothermal treatment is shown in(d). During growth of the seed crystals it is possible to obtain different orientation ofthe crystalline material in the film. The orientation depends on several factors, such asshape and size of seed crystals and synthesis conditions, which has been investigated byHedlund et.al. [22–24] for MFI type films.

2.4 MFI-Zeolite Membranes

Zeolite membranes could either be self-supported zeolite films or a thin film of zeoliteon a porous and mechanically stable substrate. In this thesis the terms substrate and sup-port are used interchangeably. This substrate is commonly referred to as support in theliterature. A large disadvantage with self-supported membranes is that a substantial filmthickness is necessary for mechanical strength. The mass transport resistance will be largein the narrow zeolite pores and a low flux through an unsupported membrane will re-sult. However self-supported membranes have a few advantages compared to supportedzeolite membranes. Leaching of the substrate and stress in the film due to differences inthermal expansion coefficient between zeolite and support is eliminated. Leaching mayresult in incorporation of unwanted species in the zeolite film. The thermal expansionissue is further discussed in section 2.6.

Zeolite membranes have been studied extensively during the last decades [6, 15, 25],which may be illustrated by the increasing number of patents and articles [6]. One rea-

2.4 MFI-Z M 11

son for this interest is the large potential in terms of selectivity and flux for this typeof membrane. Zeolite membranes are able to separate molecules not only by molecularsieving [26–28] but also by preferential adsorption [26, 27, 29–32]. Kapteijn et al. [33]showed this by feeding a mixture of hydrogen and n-butane to a silicalite-1 membrane.In the case of single gas permeation, hydrogen was the faster permeating species as ex-pected. However, when applying a mixture, n-butane was the faster permeating speciesat steady state permeation. The reason for this peculiar behaviour is the variation inthe strength of adsorption between different species. n-Butane blocks the pores, mak-ing them inaccessible for hydrogen permeation. This phenomenon is less important athigher temperatures, due to the temperature dependence of the adsorption coefficients,see section 3.2.

Another reason for the great interest in zeolite membranes is the temperature stabilitysince zeolites may be stable up to at least 500 ◦C [25]. This should be compared with theconsiderably lower thermal stability of an organic membrane. The temperature stabilityopens up for the use of zeolite membranes in membrane reactors. Membrane reactorscombine two unit operations in one, saving both investment and operational costs. An-other advantage is the possibility to overcome equilibrium limitations. An example is theethylbenzene dehydrogenation to styrene [34]. If hydrogen would be selectively removedfrom the reactor, the reaction would produce more styrene, since equilibrium would beavoided.

In early work on zeolite membranes by Geus et.al. [35] and Tsikoyiannis et.al. [36],both unsupported and supported membranes were described. The membranes had con-siderably lower fluxes and the films were almost three orders of magnitude thicker thanthe best membranes reported today [17, 27]. However, the performance was sufficientfor measurement of the permeation properties and the samples provided important dataon the separation potential of zeolite membranes.

2.4.1 Criteria for membrane quality

Various groups have postulated quality criteria in order to enable comparison of mem-branes. Funke et.al. [37] postulated that a high quality MFI membrane should have apermselectivity between nitrogen and sulphurhexafluoride (SF6) greater than 80 at roomtemperature. Nitrogen was chosen due to the relatively small diameter of the moleculecompared with the MFI-pore diameter. SF6, on the other hand, have a critical diam-eter similar to the MFI-pore diameter and would preferably permeate through defects.A low permeance and a high ideal selectivity would thus indicate a good quality MFImembrane. Kapteijn et.al. [38] postulated that a good quality MFI membrane shouldhave a permeance ratio between the two butane isomers higher than 10. n-Butane is thefaster permeating species. However, one aspect of the membrane quality is seldom men-tioned. In order to have a commercially applicable membrane, high flux is a necessity.The combination of high flux with high selectivity defines a high quality membrane.

2.4.2 Properties of Zeolite Membranes

MFI-type membranes have successfully been synthesized using the three different meth-ods given in section 2.3. The group of Noble and Falconer [39] has successfully synthe-sized MFI membranes on both alumina forms (α and γ) with good performance. N2/SF6

12 L S

permselectivities in the range of 300 were reported. The group of Moulijn et al. [38]used porous stainless steel substrates and reported a permselectivity between the butaneisomers of 25. Matsukata et al. [32] used the vapour phase method for preparing MFImembranes and reported a N2/SF6 permselectivity of 13. The group of Tsapatsis et. al.used a seeding technique for preparing MFI type membranes [17]. The result was ab-oriented MFI film with very good separation performance of the xylene isomers.

Our group has developed a method for preparation of very thin membranes of highquality [27] on substrates with low mass transfer resistance. These membranes show thecombination of high selectivity, between hydrocarbon isomers such as xylene isomers,and high fluxes under industrial conditions.

2.5 Substrates for Zeolite Membranes

For mechanical stability, a porous substrate must support a thin film. The substratemight be made from a wide range of materials such as stainless steel, α− or γ− alumina.Flat discs or tubular substrates are frequently used and even monolith type substrateshave been reported [40]. Depending on film thickness, the pore size of the substrate atthe zeolite/support interface must be sufficiently small. In that case, even a thin zeolitefilm is sufficient to close the pores of the substrate.

Substrates may either be symmetrical or asymmetrical. Symmetrical membrane sub-strates consist of a single layer with a well-defined pore size distribution with an averagepore size usually in the range of 60 nm to 200 nm. Asymmetrical substrates consist oftwo or more layers with different pore size, which may reduce mass transfer resistancesince the dominating part could be manufactured with a coarse pore size. Thus only athin layer with small pore size is needed as a support for the zeolite film and possible fluxresistances from the substrate may thereby be minimized.

Porous stainless steel substrates have successfully been used for zeolite membranesynthesis [41, 42]. The main advantage of stainless steel substrates is the straightforwardsealing with the membrane module at high temperatures by using copper washers. Thedisadvantage is the larger differences in thermal expansion coefficients between the sub-strate and the film, see section section 2.6. Another disadvantage is the relatively largepore size of the substrate that requires a thick film in order to close the pores of thesubstrate.

Alumina substrates have been used in the majority of work reported in scientific pub-lications. Both the α and γ forms have successfully been used. The well-defined poresize, >5 nm, of alumina substrates is ideal for membrane preparation. Alumina has abetter conformity in thermal expansion coefficients with zeolite, compared with stain-less steel. The α form of alumina is also relatively inert and shows a low tendency toleach aluminum into the alkaline synthesis solution. However, this is not the case forthe γ form which has to be protected during synthesis [39]. If not protected, aluminumwill be incorporated in the growing zeolite film. Sealing is unfortunately not as straight-forward as in the case of stainless steel substrates, especially at high temperatures, butgraphite gaskets can be used up to ∼450 ◦C.

2.6 D F Z F M 13

2.6 Defect Formation in Zeolite Films and Membranes

In a membrane application defects have to be kept at a minimum in order to obtain aneffective separation. Defects are pathways through the film with a width greater than thepores of the zeolite. Several different kinds of defects exist, such as:

• Cracks

• Open grain boundaries

• Non-closed film (pinholes)

Cracks are believed to form in the film mainly during calcination of the synthesized film.This process has been studied carefully by several groups and several crack formationmechanisms have been put forward. The first work was reported by Geus et.al. [43], whoinvestigated the degradation of TPA within the MFI-framework and crack formation inlarge MFI-crystals during calcination. They identified the following important steps:

• The dehydration of the framework in the early stages of TPA degradation

• Elimination reactions of TPA-degradation intermediates

• Coke formation of TPA residues during degradation

The unit cell parameters during calcination were measured in that work and it was notedthat a shrinkage in the a-direction and an expansion in the b-direction took place duringthe process.

Dong et.al. [44] investigated changes of the microstructure in a MFI membraneduring calcination. The authors claim that cracks are formed by a compression tension.The tension could be caused by changes in cell parameters during template removaland is dependent on the type of substrate. Should the film be bonded to the substratebefore template removal, defects would form during removal of template and remainduring cooling. However if the film is not chemically bonded to the substrate until afterthe removal, defects would form during cooling of the sample due to shrinkage of thesubstrate.

Possible orientational effects on crack formation in silicalite-1 films were investi-gated by den Exter et.al. [45]. In that work it was postulated that the shrinkage inthe a-direction and expansion in the b-direction could have a major effect on the crackformation within oriented MFI films.

Open grain boundaries and pinholes have not been investigated in the same manneras crack formation, simply due to insufficient measurement techniques. Open grainboundaries are assumed to arise during synthesis, due to lack of space to add anotherbuilding block between the two growing crystals, or by the mechanism described byDong et.al. [44], previously mentioned in the part of crack formation. It is thereforepossible that open grain boundaries are always present to some extent in the membrane.Defects classified as pinholes are a result of insufficient film thickness or incompleteand/or uneven seeding.

Part Two

Permeation Theory and ModelDevelopment

Chapter 3

Modeling of Diffusion andAdsorption in Zeolites

3.1 Introduction

Along with the increasing amount of permeation results in the field of zeolite membranescomes a desire to explain the data with a model. The diffusion in zeolites may either bedescribed by a micro- or macro-scale model. Common to micro-scale models basedon molecular dynamics, force field simulations, Monte-Carlo simulations, etcetera, isthat the diffusion is modelled on an atomic scale and that quantum mechanical effectsmay be considered. A major disadvantage with the microscopic models is that they arevery cumbersome with regard to computation. Keil et.al. [46] showed with simplecalculations that the computing time, on a Cray Y-MP super computer, necessary forobtaining a reliable value of the self diffusivity of benzene in silicalite-1 would be about5600 h. The macro-scale models used for diffusion have the advantage of being much lesscomputer demanding. A famous macro-scale model for mass transfer is Fick’s law, herewritten with a concentration gradient as the driving force for diffusion. The chemicalpotential may also be used as the driving force.

Ji = −DijdCi

dz(3.1)

A disadvantage by using macro-scale models is that they assume the diffusing compo-nents to be a continuum. However this is not the case when considering diffusion inzeolites, where the pores are of molecular dimensions and the crystals and film thick-nesses may be in the range of the mean free path of the molecules.

Today, and probably in the near future, macro-scale models are the most frequentlyused for diffusion in zeolites and zeolite membranes. The Maxwell-Stefan equations,the generalized Fick’s law and an activated diffusion model are the most frequently usedmacro-scale models for modeling zeolite diffusion. They will be described briefly later in

17

18 M D A Z

this section. It should also be mentioned that one of the most common, and also verycritical, assumptions regarding most membrane modeling is that the membrane is defectfree. However recent reports [47–49] present methods to estimate the flux throughdefects in the membrane and calculations of the intrinsic diffusion coefficient, that isthe diffusion coefficient of the zeolite without the defects. This diffusion coefficient isvital since it may be used together with a model describing defects in order to predictseparation performance for a real zeolite membrane. Real membranes will always containsome defects.

3.2 Heterogenous Physical Adsorption

This section describes the two correlations used for estimating the amount adsorbed onthe zeolite surface.

3.2.1 Henry’s law

Physical adsorption on heterogenous surfaces is distinguished from chemisorption on thebasis that no change in molecular state (i.e. no association or dissociation) occurs in thefirst case. A uniform heterogenous surface (adsorbent) surrounded by a fluid (liquid orgas phase) containing the adsorbing species (adsorbate) in low concentrations and withnegligible intra adsorbate interactions have a linear equilibrium relationship betweenthe fluid phase and the adsorbed matter. This relationship is commonly referred to asHenry’s law [50]:

C = KP (3.2)

3.2.2 Langmuir adsorption isotherm

At higher adsorbate concentrations, the linear relationship will no longer be valid due toincreasing intra adsorbate interactions. With a development of a complete adsorptionisotherm for the adsorbate on the adsorbent, a mathematical relation describing surfaceconcentration versus gas pressure may be formulated. The simplest theoretical modelfor monolayer adsorption on a heterogenous surface is the Langmuir isotherm. Theassumptions stated when developing the model are [50]:

• Molecules are adsorbed on well-defined localized sites

• Each site can hold only one adsorbate molecule

• Adsorption sites are energetically equivalent

• No interactions between adsorbed species on neighbouring sites

Under these assumptions the Langmuir isotherm may be written as:

CCSat

= θ =bP

1 + bP(3.3)

3.3 S C M T 19

3.3 Single Component Mass Transfer

This section presents the theory needed in the present work for the development of themodel of real MFI membranes. The model is described in chapter 4.

3.3.1 Intrinsic zeolite diffusion

Fick’s law, as given by Ruthven [50], relates the flux of a single component to the chem-ical potential:

J = −BCdν

dz(3.4)

B is the mobility and C is the concentration, dependent of the length coordinate z, of thediffusing species in the molecular sieve. If ideal behaviour of the diffusing gas moleculeis assumed, the chemical potential may be written as:

ν = ν0 + RT ln(P) (3.5)

This yields:

J = −BRTCd(ln P)

dz(3.6)

If it is assumed that Henry’s law, equation (3.2), is valid then equation (3.6) becomes:

J = −BRTK∆Pδ

(3.7)

In the case of nonlinear adsorption, i.e. Henry’s law is not valid, the Langmuir adsorp-tion isotherm is used when developing the flux equation. Assuming that the adsorptionisotherm, section 3.2, is invertible and is a function of pressure i.e. c = f (P) thenequation (3.6) becomes:

J = −BRTd (ln P)d (ln C)

dCdz

(3.8)

In isothermal systems BRT is combined to D0. It should also be noted that equa-tion (3.8) may also be developed from the Maxwell-Stefan equation (3.34) or the gener-alized Fick’s law (3.35a) for a single component.

3.3.2 Knudsen diffusion

Knudsen diffusion occurs within pores where the mean free path of the diffusing moleculeis similar to the pore diameter. This results in a high probability for collisions between aspecific molecule and the wall. A general form of Knudsen diffusion coefficient may bewritten [4]:

DK =43

K0

√8000RT

πM= 194K0

√TM

(3.9)

20 M D A Z

K0 is a structural parameter describing the pore size and structure. If cylindrical capillar-ies may be assumed:

DK = 97r

√TM

(3.10)

In this r is the circular pore radius. By using the Fick’s law and assuming ideal gasbehaviour, the Knudsen flux of a single component may then be written as:

J = −DK

RTdPdz

(3.11)

3.3.3 Poiseuille flow

Poiseuille flow or viscous flow occurs when a pressure gradient is applied over a smallcapillary. In contrast to Knudsen diffusion, the mean free path in this case is smallerthan the capillary width or diameter. The Poiseuille diffusion coefficient may be writtenas [4]:

DP =B0Pµ

(3.12)

B0 is a structural parameter describing the pore width and structure. If circular capillariesmay be assumed:

DP =r2P8µ

(3.13)

By using Fick’s law and assuming ideal gas behaviour, Poiseuille flux of a single compo-nent may then be written as :

J = −DP

RTdPdx

(3.14)

3.4 Condensation in the Pores

Defects in the form of micropores, mesopores and macropores may occur in a real zeolitefilm. In the present work, these defects are measured in a separate experiment involvingcondensation of a hydrocarbon in the defects. Correlations for pore size and requiredpressure of the hydrocarbon for condensation is thus needed. However, no single modelis capable of estimating pore size for the entire range of defects that may occur in themembrane. Thus, two models were used in the present work.

3.4.1 Micropores

A schematic representation of a molecule adsorbed on a surface within a slit shaped mi-cropore is illustrated in Figure 3.1. Horvath and Kawazoe [51] showed that the potential

3.4 C P 21

Adsorbate molecule

Surface atoms di=

(2d-d

S)

2d

dA

d0

z

dS

Figure 3.1: Schematic illustration of an adsorbed molecule within a slit shaped microp-ore.

energy for such a molecule could be written as:

φ(z) =NSAS + NAAA

2σ4

[(σ

d + z

)10

+(

σ

d − z

)10

−(

σ

d + z

)4

−(

σ

d − z

)4]

(3.15)

The parameter σ is the distance where the interaction energy is zero and is defined by:

σ =(

25

)1/6 dS + dA

2=(

25

)1/6

d0 (3.16)

This relation was derived by setting the potential describing the interaction energy be-tween a molecule and a single lattice plane to zero, i.e:

φ(z) =103

εSLP

[15

(σ

z

)10− 1

2

(σ

z

)4]= 0 (3.17)

The pre-potential term in equation (3.15) may be regarded as the adsorption energy forone molecule, since it is describing the energy depth of the potential. This statement isfurther supported by Ruthven [50], who performed theoretical calculations of the heatof adsorption on the basis of Lennard-Jones potentials. The potential equation (3.15)may now be written:

φ(z) =∆HAds

NAv

[(σ

d + z

)10

+(

σ

d − z

)10

−(

σ

d + z

)4

−(

σ

d − z

)4]

(3.18)

22 M D A Z

An average of the interaction energy was obtained by integration over the free spacewithin the slit shaped pore; i.e.:

φ(z) =

d−d0∫−d+d0

φ(z)dz

d−d0∫−d+d0

dz

(3.19)

The average interaction energy for one molecule is then:

φ(z) =∆HAds

NAv (d − d0)

[σ10

9d90

− σ4

3d30

− σ10

9 (2d − d0)9 +

σ4

3 (2d − d0)3

](3.20)

The average interaction energy was further related to the change in free energy:

RT ln(

PP0

)= NAvφ(z) (3.21)

The slightly modified Horváth-Kawazoe equation, with equations (3.20) and (3.21) thusbecomes:

RT ln(

PP0

)=

2∆HAds

(2d − 2d0)

[σ10

9d90

− σ4

3d30

− σ10

9 (2d − d0)9 +

σ4

3 (2d − d0)3

](3.22)

This equation relates the pressure P at which condensation occurs to the width d of themicropore.

3.4.2 Mesopores

Several different relations that describe a cylindrical pore with a certain diameter as afunction of partial pressure of adsorbing species have been proposed. One of the mostwidely used is the Kelvin equation [4]. For a condensation process in an open capillarythe Kelvin equation is written as:

r =−γVm

RT ln(P/P0)(3.23)

The equation is only valid for mesopores and not for micropores. This arises from theassumption of a continuous surface of condensed fluid.

3.5 Additional Equations for Diffusion in Zeolites

As a background, this section describes other commonly used models used for modelingof diffusion in zeolites. These equations are not used in the present work. However,these equations may be used in future work on multi-component permeation.

3.5 A E D Z 23

3.5.1 Maxwell Stefan equations

The generalized Maxwell-Stefan equations, developed from the theory of irreversiblethermodynamics, describe multi-component bulk diffusion of non-ideal fluids. Thefluids may either be liquids, gases or electrolytes etc. In the case of a non-ideal fluid thegeneral Maxwell-Stefan equation in three dimensions may be written as [52]:

xi

RT∇ν i =

n∑j=1j 6=i

xi J j − xj J i

CtÐij(3.24)

The chemical potential, assuming non-ideal gases, may be written as:

ν i = ν0i + RT ln(fi) (3.25)

Taylor and Krishna [52] has shown how the chemical potential gradient for a componentin a multi-component mixture could be expressed in terms of molar fraction gradients:

xi

RT∇ν i =

n−1∑j=1

Γij∇xj (3.26)

Where Γij is the thermodynamical correction factor, defined as:

Γij = δij + xi∂ln fi∂xj

∣∣∣∣T , P, Σ

(3.27)

The term Σ states that the partial derivative should be evaluated with the constraintΣxi = 1. Equations (3.24) and (3.26) together yields:

n−1∑j=1

Γij∇xj =n∑

j=1j 6=i

xi J j − xj J i

CtÐij(3.28)

The generalized Maxwell-Stefan equation was applied to multi-component surface dif-fusion by Krishna [53], where the zeolite matrix was assumed to be the (n + 1)th com-ponent and the fractional occupancies to be analogous with molar fractions. In that caseequation (3.24) becomes:

−ρθi

RT∇νi =

n∑j=1j 6=i

θj J i − θi J j

ΘSatÐij+

θn+1 J i

ΘSatÐi,n+1(3.29)

The Maxwell Stefan diffusivity of species i in the zeolite is defined to be:

Ði ≡Ði,n+1

θn+1(3.30)

This definition is due to occupancy of the zeolite that is undefined when consideringthe interaction between species i and the zeolite. The Ði diffusivity is regarded to be

24 M D A Z

the single component diffusivity and it may be mechanistically described by a moleculejumping from site to site in the zeolite framework. The cross diffusivity Ðij could phys-ically be interpreted as a counter-exchange coefficient for two components adsorbing ona site in the zeolite. The net effect of this cross exchange is that faster diffusing speciesare hindered by slower and/or more strongly adsorbed species. In practice the crossdiffusivity is estimated by using the Vignes correlation as suggested by Krishna [53].

Ðij = [Ði]θi

θi+θj[Ðj] θj

θi+θj (3.31)

This effect of the cross diffusivity was investigated by Kapteijn et.al. [33] as discussed insection 2.4.

The chemical potential is also related to the fractional occupancies by introducing athermodynamical correction term:

θi

RT∇νi =

n∑j=1

Γij∇θj (3.32)

The thermodynamic correction term, as given below, relates the gas phase pressure to theamount adsorbed on the surface of the given species.

Γij = θi∂ ln fi∂θj

(3.33)

By rewriting equation (3.29) with the definitions of the chemical potentials and thethermodynamic factor, the Maxwell-Stefan equations for multi-component diffusion inzeolites and zeolite membranes may then be written as:

−ρn∑

j=1

Γij∇θj =n∑

j=1j 6=i

θj J i − θi J j

ΘSatÐij+

J i

ΘSatÐi(3.34)

3.5.2 Generalized Fick’s law

The generalized Fick’s law, based on irreversible thermodynamics may also be used fordescribing surface diffusion and diffusion in zeolites. The Onsager formalism may beused for describing the flux of a component in a multi-components system [54]:

J i = −n∑

j=1

Lij∇νj (3.35a)

J i = −n∑

j=1

Dij∇qj (3.35b)

Equation (3.35a) is used for describing the flux if the chemical potential is used andequation (3.35b) is used if the adsorbed amounts in the zeolite are used as driving force

3.5 A E D Z 25

for the diffusion. The cross diffusivities are given by:

Dij = RTn∑

j=1

Lij∂ ln Pj

∂qj(3.36)

The Onsager phenomenological coefficients relate interaction between diffusing speciesand how temperature gradients affect the flux of a given species among others. By com-paring equations (3.29), (3.35a) and (3.36), it may be seen that the Maxwell-Stefandiffusivity and the Fick’s diffusivity are related as:

Dij = ΓijÐij (3.37)

Chen and Yang [55] proposed a method based on irreversible thermodynamics forobtaining the cross-correlation coefficients thermodynamics from main term coefficientsin a binary mixture. The Onsager reciprocal relation was used and an interaction param-eter λ, where |λ| ≤ 1, was added:

Lij = λ√

LiiLjj (3.38)

The parameter λ was estimated by combining equations (3.35a), (3.35b) and (3.36) witha suitable multi-component adsorption isotherm.

3.5.3 Activated diffusion

Xiao and Wei [56] proposed a model for single-component activated diffusion, wherethe diffusing species jump between equilibrium positions within the zeolite only if acertain activation energy for transport has been exceeded. Fick’s law was used to describethe flux in the zeolite as given in equation (3.1). A combined diffusivity relation, validfor the gaseous, Knudsen, liquid and solid diffusion regimes, was used:

D = guLe−E

RT (3.39)

It was postulated that the diffusion in the zeolite could be described with two models,the gas translational model (GT) or the solid vibration model (SV):

D =1Z

√8000kT

πMαe−

ERT (GT ) (3.40a)

D = υeα2e−

ERT (SV ) (3.40b)

In the GT model, the diffusion mechanism could be described by the fact that moleculesare moving in a potential field of a periodic lattice. When the molecule acquires an en-ergy equal to or greater than the activation energy, it diffuses away from the potentialwell to a new equilibrium position adjacent to the old position. This model is similar tothe Maxwell-Stefan model for a single component, as previously described . Moleculesdiffusing according to the SV model lose their gaseous entity due to the strong inter-action with the framework of the zeolite. The potential field in the SV model couldbe considered as a harmonic oscillator, where the molecules are strongly bonded to theframework in the channel and are vibrating with a frequency υe.

Chapter 4

Model Development

This chapter describes the model developed in the present work for flow through a realzeolite membrane. Flow through zeolite pores and defects will be considered and theeffect of the substrate will be accounted for. High flux through the membrane may causea significant pressure drop over the substrate, although the pores in the substrate are sub-stantially larger than the zeolite pores. Figure 4.1 shows a zeolite composite membraneon a graded alumina substrate. The top layer z is the zeolite film, layer S1 is a 30 µmthick layer with 100 nm pores, and S2 is a 3 mm thick layer with 3 µm pores.

4.1 Effect of Substrate

Layer S1 is modelled with a combination of Knudsen and Poiseuille diffusion since themean free path of diffusing gases is similar to the pores of layer S1. Due to the large poresize of layer S2, Poiseuille flow was assumed to be the dominating diffusion mechanism.

Zeolite film, z 0.3 - 2 µm

30 µm

3 mm

Narrow pore layer, S1

PFeed

Large pore layer, S2

PPermeate

P1

P2

∆PT

ot

Figure 4.1: General drawing of an asymmetric membrane.

27

28 M D

Pressures P1 and P2 indicated in Figure 4.1 are the pressures in the interfaces betweenthe layers. The fluxes through layer S1 and S2 may now be written as:

JS1 =εS1

τS1(DK ,S1 + DP,S1)

1RT

dPdz

(4.1a)

JS2 =εS2

τS2DP,S2

1RT

dPdz

(4.1b)

In the integrated form, with the expressions for the Knudsen and Poiseuille diffusioncoefficients inserted, the equations are:

JS1 =εS1

τS1

(194KS1 +

BS1(P1 + P2)2µ

)1

RT∆PS1

δS1(4.2a)

JS2 =εS2

τS2

BS2(P2 + PPermeate)2µ

1RT

∆PS2

δS2(4.2b)

4.2 Permeation in the Zeolite Film

If the total membrane area is written ATotal and the area of a certain defect size is denotedAi, the area of non-defective film AZ is then estimated as AZ = ATotal −

∑Ai. It is

assumed that defects only have a number of discrete widths. For high quality membranes,AZ may be approximated with ATotal , since the sum of the defect areas is small. If bothKnudsen diffusion and Poiseuille flow occurs in defects, then the following equationdescribes the single gas flux through a zeolite film with defects:

JZ = − AZ

ATotalBRT

d ln Pd ln C

dCdz

−∑

i

Ai

ATotal

(97ri

√TM

+r2i P8µ

)1

RTdPdz

(4.3)

In the event that Henry’s law is valid, then equation (4.3) becomes:

JZ =AZ

ATotalD0K

∆PFilm

δ

+∑

i

Ai

ATotal

(97ri

√TM

+r2i (PFeed + P1)

16µ

)1

RT∆PFilm

δ(4.4)

Where ∆PFilm = PFeed −P1. If Langmuir adsorption mechanisms may be assumed, thenequation (4.3) becomes:

JZ =AZ

ATotal

D0CSat

δln(

1 + bPFeed

1 + bP1

)+∑

i

Ai

ATotal

(97ri

√TM

+r2i (PFeed + P1)

16µ

)1

RT∆PFilm

δ(4.5)

4.3 D 29

Equations (4.4) and (4.5) together with equations (4.2a) and (4.2b) was used for calcula-tions of the defect distribution from porosimetry data, estimating the intrinsic propertiesof the zeolite and simulating the permeation behaviour.

4.3 Defect distribution

The defect areas were estimated by using equations (4.4), (4.2a) and (4.2b) on eachmeasurement in the porosimetry experiment, see section 5.3.2. The estimated areas werefurther used for determining the distance between two defects of the same defect width.This may be accomplished by assuming that the defects form a mesh over a quadraticsurface with an area equal to the circular membrane, as illustrated in Figure 4.2.Thelength of the defect may be written as:

l2 =πd2

G

4⇒ l = dG

√π

4(4.6)

The defects are further assumed to have a length equal to the side of the quadratic surfaceas shown in Figure 4.2(c). With the length, width and total defect area, the total amountof defects for a certain defect size was calculated from:

ni =Ai

2ril(4.7)

It is assumed that the defect width is equal to the Kelvin diameter or the Horváth-Kawazoe width of the defect. The width between defects on the membrane surface maynow be calculated using:

wi =2lni

(4.8)

dG

A2

A1

AZ

(a)

lATot

(b)

l

w1

w2

(c)

Figure 4.2: A (very) defective membrane with two defect sizes and the correspondingdefect areas (a), and the quadratic surface (b) with the assumed mesh of defects (c).

Part Three

Thesis summary

Chapter 5

Experimental

Asymmetric α-alumina microfiltration filters were used as substrates for the zeolite mem-branes in the present work. The substrates consist of two layers with different pore sizes:a 30 µm thick top layer with 100 nm pore size and a 3 mm thick bottom layer with3 µm pore size. Graded supports reduce the mass transport resistance compared with asubstrate consisting of a single layer of small pores, as described in section 2.5.

These substrates were used directly for membrane synthesis or pretreated with a novelmethod termed masking [27]. The masking procedure is described in detail in section 5.1whereas film synthesis details are given in section 5.2.

5.1 Masking

A previous research project at the division aimed at developing a new strategy for prepa-ration of thin MFI-type membranes [27]. The idea was to reduce the growth of zeoliteinto the substrate in order to avoid support invasion, i.e. deposition of zeolite or siliceousspecies in the interior of the support. Growth into the substrate may increase the masstransfer resistance and may increase defect formation at higher temperatures. A synthe-sis procedure where the interior of the substrate is filled with wax was developed. Inprevious work [27], the outcome of the masking procedure was only exemplified for amembrane with a thickness of 500 nm. Subsequent work was dedicated to investigatethe effect of masking as the film thickness was varied, see P I.

The masking procedure is described in detail in P I and is outlined in Figure 5.1.The initial support is shown in (a). The substrate is rinsed with acetone and filtered99.9 vol% ethanol. The filtering is carried out in order to remove dust and other parti-cles. A protective layer of polymethylmetacrylat (PMMA) has been added on top of thesubstrate in (b). A solution of PMMA in acetone is applied through a 0.1 µm filter withthe aid of a syringe and a needle. The solution has a rather high viscosity in order toprevent it from penetrating into the substrate. The PMMA is allowed to dry very care-fully by increasing the temperature slowly in a programmable oven. The PMMA covered

33

34 E

(a) (b) (c) (d)

Figure 5.1: Masking procedure.

substrates are immersed in molten wax, with the PMMA layer facing downwards. Thepressure is lowered and the air in the substrate is removed and replaced with molten wax(c). The PMMA layer is removed by dissolving the film in acetone after cooling thesubstrate to room temperature (d). The substrate is now protected by wax with the topsurface available for seed deposition.

5.2 Membrane Synthesis

Substrates were rinsed with a 0.1 M NH3 solution filtered through a 0.1 µm filter inorder to remove dust. Subsequently, the samples were treated in a 0.4 w% cationic poly-mer solution and rinsed with a filtered 0.1 M NH3 solution. This treatment results ina positively charged surface which is necessary for adsorption of the negatively chargedseed crystals. A final rinse with a 0.1 M NH3 solution was carried out in order to removeexcess seed crystals.

5.2.1 Organic template assisted synthesis

In all work, except that described in P IX, all films were grown by hydrothermaltreatment directly after seeding. The synthesis was conducted in an oilbath with thetemperature 100 ◦C at atmospheric pressure with reflux in a synthesis solution with thefollowing molar composition: 3TPAOH: 25SiO2: 1500H2O: 100EtOH. A synthesissolution with the molar composition: 3TPAOH: 0.125Na2O: 0.125Al2O3: 25 SiO2:1500H2O: 100EtOH was used to grow ZSM-5 films in the same manner as the silicalite-1 membranes. The film thickness was varied by varying the duration of the hydrothermaltreatment. Synthesis solution residues and aggregates were removed from the membranesby rinsing in a 0.1 M NH3 solution. Calcination removed the TPA and opened up themicroporous framework.

5.3 P M 35

5.2.2 Synthesis of template free films

The membrane preparation differs slightly in this case, see P IX. Unmasked sub-strates were used and a calcination procedure of the seeded substrate was performed inorder to remove TPA from the seed crystals. The calcined seeded substrates were im-mersed in a synthesis solution with the molar composition: 30Na2O: Al2O3: 100SiO2:4000H2O and hydrothermally treated for 12 h at 180 ◦C in a teflon lined autoclave.This synthesis results in ZSM-5 films with very high aluminum content, Si/Al=10 [57].Residues of synthesis solution and crystallite aggregates were removed by rinsing themembranes in a 1 M NH3 solution.

5.3 Permeation Measurements

5.3.1 Single Gas Measurements

Single gas experiments were carried out directly after calcination and the membraneswere removed from the furnace when the temperature had reached 110 ◦C. This pro-cedure was used for all work except that described in P IX, where the single gasexperiments were carried out after drying at 110 ◦C. A flow of dry nitrogen was usedduring mounting of the membrane in a stainless steel cell in order to avoid adsorptionof water, etc. Rubber gaskets were used during single gas experiments for sealing.

The driving force for the flow through the membrane was a difference in total pres-sure across the membrane. The feed pressure was varied from 1.1 bar to 5 bar absolutepressure, while the permeate side was always kept at 1 bar absolute pressure. The perme-ance was calculated from the volumetric flow rate of the permeate. Helium, nitrogen,hydrogen and SF6 were used as probe molecules during the single gas experiments.

5.3.2 Porosimetry

Porosimetry [27, 58] was used as a tool to characterize the size and amount of defects inthe membranes, see P II & V. The steady state permeance of helium was measuredas a function of relative pressure of either n-hexane or p-xylene in the feed stream at roomtemperature. With increasing relative pressure, pores smaller than the Horváth-Kawazoewidth, see equation (3.22), or Kelvin radius, see equation (3.23), will be closed by thecondensable species.

Figure 5.2 shows a principal drawing of the porosimetry unit used in the presentwork. Two mass flow controllers were used to adjust the relative pressure of the con-densable species present in saturators. The gas mixture is fed to the membrane, whichis situated in a stainless steel cell, at an appropriate pressure. The permeate was keptat 1 bar absolute pressure and the flow was measured using a soap bubble flow meter.A condenser, connected on the permeate side of the membrane, removed most of thecondensable species from the permeate prior to flow measurement.

5.3.3 Mixture Separation

Characterization by mixture separation was performed in a Wicke-Kallenbach setup,schematically shown in Figure 5.3. The main idea is to use a partial pressure gradient

36 E

T

Gas

Cell

MFC

T Thermocouple

PM

FM Flow meter

Pressure meter

Pressure regulator

T

T

FM

PM

MFC

MFC Mass flow controller

Figure 5.2: Porosimetry unit.

as driving force for diffusion. This is in contrast to both the single gas permeationand porosimetry that both employed a gradient in total pressure. The partial pressuregradient was maintained by using a sweep gas that removed permeated species fromthe permeate side of the membrane. The membranes were mounted in a stainless steelcell and graphite gaskets were used for sealing. These gaskets can be used up to about450 ◦C. A thermocouple mounted in the membrane cell was used for online temperaturemonitoring.

A principal drawing of the separation test facility is shown in Figure 5.4. Gases werefed to the membrane cell through a valve manifold via mass flow controllers. Two massflow controllers were used for preparing the feed and one for the sweep gas. Liquids werefed in two different ways. In the work described in P IX, the liquids were fed to avaporizer by a syringe pump and further mixed with a carrier gas, whereas in the otherstudies, saturators filled with the desired components were used. Helium was used asboth carrier- and sweep-gas in all liquid component measurements.

The test facility permits independent setting of the feed and permeate pressure viaregulating valves. These valves are controlled by PID regulators connected to pressuretransmitters. However in this work, pressures were set to 1 bar on each side of the

T

Feed

Retentate

Sweep gas

Permeate

Membrane

Figure 5.3: Principal setup of a Wicke-Kallenbach cell.

5.4 A C 37

Sw

eep g

as

Gas 1 B

ubble

r

Liq

uid

Cell

P

GC

T

Heated zone

Furnace

Valv

e-

manifold

Evaporator

P

Gas 2

T

On/off valve

Regulating valve

Pressure transmitter

Thermocouple

P

MFC Mass flow controller

MFC

MFC

MFC

Figure 5.4: Separation performance testing unit.

membrane. An online connected gas chromatograph (GC) equipped with both a ther-mal conductivity (TCD) and flame ionization detector (FID) was used for compositionanalysis.

5.4 Additional Characterization

Scanning electron microscopy (SEM), Philips XL30, was used for film thickness and filmmorphology investigations. The microscope was equipped with a LaB6 electron emissionsource. High magnification SEM images may also be used to study film morphology. X-ray diffraction patterns were collected with a Siemens D5000 powder diffractometer.The diffractometer records X-ray intensity as a function of beam deflection angle (2θ).According to Braggs law, high x-ray intensity will occur at specific deflection angles de-pending on the d-value. In a powder sample containing randomly oriented crystals acharacteristic diffractogram with certain relative intensities will result. However in azeolite film, crystals may be oriented and the relative intensities will therefore deviatesignificantly from randomly oriented crystals. This information may be used to deter-mine the crystal orientation of the crystals in the zeolite membrane. This orientationshould be taken into account when modeling flow through zeolite membranes, since itis believed that different orientations of the crystals within the film alters the perme-ation properties of the zeolite membrane. XRD may also be used for estimating the filmthickness by comparing the area of the peaks.

Chapter 6

Results and Discussion

In order to better appreciate the permeation properties of a zeolite membrane, detailedknowledge of the material is necessary. Parameters such as crystal size and orientation,film thickness, defects and silicon/aluminum ratio may affect the permeation properties.Several of these parameters have been varied and the effect on the permeation propertieshas been investigated in the present work. The results are summarized in this chapter.

The sample labelling in this chapter follows a specific pattern. The letter M or Udesignates if a Masked or Unmasked substrate has been used for the membrane prepara-tion. After the first letter there is a combination of numbers that has three appearances:xx specifies a hydrothermal treatment for xx hours, z×x designates a hydrothermal treat-ment for x hours that was repeated z times with intermediate seeding, x.x-y specifies aheating cooling rate of x.x ◦C min−1 and y indicates a replicate number (1 or 2).

6.1 Morphology

The morphology of all samples was characterized using SEM and XRD. Figure 6.1 showsside and top view SEM images of two silicalite-1 membranes with different film thicknessprepared on masked substrates, see P I. The samples are labelled M30 and M72.According to Figure 6.1(a) and (b) the film thickness is about 500 nm and 1100 nm,respectively. Figure 6.1(b) also shows that the film has grown with a competitive growthmechanism; the winning crystals are wider at the surface. This is also indicated byFigure 6.1(c) and (d), sample M72 has much larger crystals on the surface. Competitivegrowth results in preferred orientation of the crystals. Preferred orientation in MFI filmshas previously been studied using XRD by various researchers [17, 59] and our group inparticular [22–24, 57, 60]. According to XRD measurements (P I) it was found thatthe crystals in sample M30 are weakly a-oriented whereas the crystals in sample M72 aremore a-oriented.

More defects were found in thicker films as illustrated by Figure 6.1(d). The sametrend was also observed for samples prepared on unmasked substrates. These samples

39

40 R D

500 nm

(a) M30 Side view

500 nm

(b) M72 Side view

500 nm

(c) M30 Top view

500 nm

(d) M72 Top view

Figure 6.1: Side and top view SEM images of membranes prepared with 30 or 72 hhydrothermal treatment on masked substrates.

contained more defects than membranes with similar film thickness prepared on maskedsubstrates. This is believed to be due to growth of zeolite into the substrate duringhydrothermal treatment.

The effect of grain boundaries on the permeation properties of MFI membranes wasstudied (P VIII) by fabricating membranes with small crystals in multiple layers us-ing a multi step synthesis method. Membranes with varying film thickness but smallcrystal size were obtained by repeating the synthesis procedure with seeding and a shorthydrothermal treatment (12 h). These films consist only of small crystals and thereforemore grain boundaries. Figure 6.2 shows a SEM image of a membrane with small crys-tals and a film thickness of about 850 nm. The Figure shows that the surface consists ofsmall crystals with a diameter of about 120 nm. This morphology was also found for theother membranes prepared using the multi step method, see P VIII. The morphol-ogy of sample M5×12 differs significantly from a single synthesis step membrane, M96with similar film thickness, as shown in Figure 6.2(b). The M96 sample has approxi-mately five times larger crystals on the surface than M5 and consequently fewer grain

6.2 D D P D 41

500 nm

(a) M5×12

500 nm

(b) M96

Figure 6.2: A membrane comprised of five layers of small crystals, (M5×12)and a mem-brane with similar thickness comprised of one layer of crystals (M96).

boundaries. The permeation properties of the multi-layer membranes will be discussedfurther in section 6.10.

6.2 Defect Distribution from Porosimetry Data

According to SEM investigations it was found that thicker films contained more de-fects. In order to verify these findings a series of permeation experiments was performed.The study included porosimetry, gas phase separation of hydrocarbon isomers and singlegas permeation measurements. In total five sample types, U17; M30; U30; M72 andU72, with varying film thickness and amount of defects, were selected for the series ofpermeation experiments.

0 0.2 0.4 0.6 0.8 1 1.210

-2

10-1

100

101

102

P/P0

He

pe

rm

ea

nce

/[1

0-7 m

ol m

-2 s

-1 P

a-1] U17 M30 U30 M72 U72

(a)

100 150 200 250 300 350 400 45010

0

101

102

103

n-C

6/2

,2-dm

b s

eparation s

ele

ctivity

Temperature /[°C]

U17 M30 U30 M72 U72

(b)

Figure 6.3: Porosimetry patterns (a) and n-hexane / 2,2-dmb separation factor (b) forselected MFI membranes of varying type.

42 R D

Table 6.1: Calculated defect distribution for samples U17, M30, U30, M72 and U72.The width between defects is given in [µm] and di represents the width of the defect.

Width between defects [µm]P/P0 di [nm] U17 M30 U30 M72 U72

0.025 1.27 12.0 168 4.0 3.0 1.00.25 2.65 1.07 · 103 1.67 · 103 17.9 12.5 6.390.85 9.18 8.07 · 104 1.56 · 105 628 3.94 · 104 97.40.99 100 1.06 · 106 2.66 · 106 5.22 · 105 1.83 · 106 6.13 · 103

The measured n-hexane porosimetry patterns of the selected membranes are givenin Figure 6.3(a). According to the Figure, M30 would be of highest quality since thissample has the overall lowest helium permeances for P/P0 > 0 for n-hexane and U72 isof lowest quality. Defect distributions were calculated from poroimetry data by solvingequations (4.4), (4.2a) and (4.2b), the results are given in Table 6.1. The calculateddefect distributions show that sample M30 is superior in comparison with the othersamples. For instance the smallest defects with a width of 1.3 nm are distributed with adistance of about 170 µm if the defects are assumed to be distributed in a mesh over themembrane. For sample U72 this distance is only 1 µm . The porosimetry patterns forsample U72 also shows that it is of lowest quality and that very large defects are present inthe membrane since it is not completely blocked by n-hexane in the experiment, even atthe highest partial pressure of n-hexane. The findings for sample U72 are consistent withthe SEM investigation, see P I. It was found that wide defects propagate through thefilm and into the support. The calculated defect distribution shows that smaller defectsare more common than larger defects for all samples. According to the Table, the widestdefects are separated by more than a meter for some of the membrane types. This isunrealistic; these defects are probably point defects and not distributed in a mesh.

The separation performance for hydrocarbon isomers was also measured. Figure 6.3(b)shows the separation selectivity between n-hexane and 2,2-di methyl butane (DMB) as afunction of temperature. The results agree with the porosimetry data and the calculateddefect distribution. Based on the separation performance, M30 is the best membranefollowed by U17. Samples U72 and U30 are of equally low quality according to the sep-aration measurements. However, the calculated defect distributions indicate that U30is better than U72, but the overall quality is so low that the separation performance isunaffected by these small differences.

6.3 Mass Transfer Parameter Estimation

In order to perform simulations of single component permeation several parameters,such as the mass transfer resistance of the support and diffusion and adsorption coeffi-cients of the zeolite, have to be known. Mass transfer parameters of the supports wereestimated by fitting equations (4.2a) and (4.2b) to permeation data on substrates priorto film growth. The measurements were conducted in the same manner as single gaspermeation measurements for zeolite membranes, but at lower pressure drops. The ob-

6.3 M T P E 43

0 1 2 30

0.5

1.0

1.5

2.0

2.5

∆P /[105 Pa]

Flu

x /

[mo

l m

-2 s

-1]

Total flux vs ∆PTot

Intrinsic flux vs ∆PTot

Intrinsic flux vs ∆PFilm

Effect of

substrate

Effect of

defects

(a) Helium

0 1 2 3 40

0.1

0.2

0.3

0.4

0.5

Flu

x /

[mo

l m

-2 s

-1]

∆P /[105 Pa]

Total flux vs ∆PTot

Intrinsic flux vs ∆PTot

Intrinsic flux vs ∆PFilm

Effect of substrate

Effect of

defects

(b) SF6

Figure 6.4: Experimental fluxes of thin masked MFI membranes versus applied filmpressure drop for helium and SF6

tained mass transfer parameters agreed with previously reported values [61] for similarmicrofiltration filters.

With the knowledge of the mass transfer resistance in the support and the amountand size of defects in the film, diffusion and adsorption coefficients for the zeolite filmwithout defects could be estimated from single gas permeation data. These parametersare denoted intrinsic parameters in this work. Figure 6.4 illustrates single gas fluxes ofhelium (a) and SF6 (b) as a function of applied pressure drop. The helium flux increaseslinearly with pressure. Hydrogen and nitrogen also had a similar flux dependence withpressure. The linear increase in flux indicates that the adsorption is linear, i.e. Henry’slaw prevails, and equation (4.4) may be used for parameter estimation. On the otherhand, SF6 showed a non-linear dependence. Equation (4.5) was used in this case forestimation of the diffusion coefficient and the adsorption coefficient. In all cases equa-tions (4.2a) and (4.2b) were used for determining the pressure drop over the substrate inorder to obtain the correct driving force for diffusion.

From experimental data as shown in Figure 6.4 intrinsic diffusion coefficients wereestimated for six samples, see P II. Three samples had a film thickness of 500 nmand three samples 1100 nm. The diffusivity was determined for all probe molecules.In the case of hydrogen, helium and nitrogen, the adsorption coefficient in Henry’s lawwas taken from literature data. The Langmuir adsorption coefficient for SF6 was alsoestimated. The estimated parameters are given in Table 6.2. No significant differencewas observed in the diffusion coefficient for hydrogen, helium and nitrogen. This wasnot the case for SF6; a lower diffusion coefficient was observed for the thicker films.This difference is attributed to the stronger a-orientation of the crystals in the thickerfilms . Since the pores running in the a-direction are slightly smaller than those in theb-direction, a lower diffusion coefficient is expected for the large SF6 molecule. It is alsonotable that the obtained diffusion coefficients of hydrogen, helium and nitrogen aresimilar to their Knudsen values. Xiao et al. [56] postulated that Knudsen flow wouldprevail in the MFI pores for these small molecules.

44 R D

Table 6.2: Estimated intrinsic diffusion and adsorption coefficients.

H2 He N2 SF6

Sample D0 /[10−7 m2 s−1

]D0 /

[10−10 m2 s−1

]b /[10−5 Pa−1

]M36-1 7.74 6.60 0.56 0.51 11.1M36-2 6.90 6.07 0.48 0.45 11.1M36-3 7.42 6.42 0.50 0.49 11.1

M96-1 7.32 7.03 0.47 0.27 11.1M96-2 6.92 6.77 0.42 0.22 11.1M96-3 8.29 7.99 0.52 0.26 11.1

Intrinsic fluxes have also been included in Figure 6.4. The film pressure drop PFeed −P1 was obtained from equation (4.2a). Figure 6.4(a) shows that the effect of the substrateis significant and alters the helium permeance more than the defects. In fact, the modelpredicted that the pressure drop for hydrogen in the support could be as high as 50 %of the total pressure drop, see P III. This finding is consistent with the measuredpressure drop of the substrates. Figure 6.4(b) shows that the effect of the substrate onthe SF6 flux is small and the flux is more affected by defects, as expected. However, theeffect of the support is still larger than the effect of defects.

6.4 Predicting Membrane Performance

A model is only useful if it is able to predict data for a variety of samples without adjust-ment of parameters, such as diffusion and adsorption coefficient. The versatility of themodel was illustrated (P IV) by prediction of experimental single gas permeationdata for samples U17, U30, M72 and U72 given in Table 6.3. Table 6.3 gives filmthickness, experimental pressure drop over the membrane, helium flux and experimental

Table 6.3: Film thickness, experimental pressure drop, helium flux [mol s−1 m−2], andpermeance ratios for selected membranes.

Sample

U17 M30 U30 M72 U72 Knudsen

δ [nm] 350 500 900 1100 1800 -∆P [Bar] 0.5 0.7 1 2.2 1.5 -He flux. 0.69 0.6 0.74 0.93 0.93 -H2/He 2.3 2.6 2.2 2.6 1.9 1.4H2/N2 2.0 1.7 1.8 1.2 2.1 3.7He/N2 0.88 0.65 0.8 0.46 1.12 2.6H2/SF6 11 15 17 38 20 8.5He/SF6 4.9 5.6 7.5 16 11 6.0N2/SF6 5.6 8.9 9.5 29 9.6 2.3

6.4 P M P 45

Table 6.4: Experimental fluxes of samples M30 and M30∗, the latter is of lower quality.Transport parameters were taken from M30. Fluxes were simulated for M30∗. Fluxesare given in [mol s−1 m−2].

M30 M30∗

Exp. Flux Exp. Flux Sim. Flux Rel. Err. [%]

H2 1.57 1.77 1.84 -4He 0.60 0.68 0.71 -5N2 0.95 1.02 1.10 -7SF6 0.11 0.12 0.13 -12

permeance ratios of the samples U17, M30, U30, M72, and U72. The predictions wereperformed using the model and the calculated defect distribution for the samples. Trans-port parameters (D0 and b) were taken from sample M30. The fluxes were calculated aspreviously described using the experimental pressure drop given in Table 6.3 as the driv-ing force for diffusion. In addition, a simulation was carried out for another membrane(M30∗) of type M30 of lower quality (more defective). The experimental fluxes of M30and M30∗ and predicted fluxes for M30∗ are given in Table 6.4. The model predicts thefluxes of M30∗ very well with only minor deviations. Experimental and estimated fluxesof hydrogen, helium, nitrogen and SF6 as a function of film thickness for sample U72are illustrated in Figure 6.5 and Table 6.5. The experimental fluxes are predicted reason-ably. Similar predictability was found for the other sample types as well. The absoluteand relative errors between experimental and simulated flux are given in Table 6.5. Fromthe Table it can be concluded that the model is able to predict the experimental data.A relatively large deviation (93 %) in SF6 flux was observed, see P IV, if the SF6

0

1

2

3

4

5

6

7

H2

He

N2

SF6

0.1 1 10

Flu

x /

[mo

l m

-2 s

-1]

Film thickness /[µm]

Flux ErrorExp. Abs. Rel. [%]

H2 1.75 -0.05 -3He 0.93 0.23 24N2 0.84 -0.67 -79SF6 0.09 -0.08 -92SF∗6 0.09 -0.01 -14

Figure 6.5: Experimental and simulatedfluxes of H2, He, N2 and SF6 for sampleU72.

Table 6.5: Absolute and relative errors be-tween experimental and simulated fluxesof sample U72. D0 values are as forM30 except for SF∗6 which is as for M96-2. Fluxes and absolute errors are given in[mol s−1 m−2]

46 R D

diffusivity was set to 0.37× 10−10 m2 s−1 as for sample M30. This deviation may beattributed to the different preferred orientation of sample U72 where a lower diffusioncoefficient would be appropriate. If the SF6 diffusivity was set to 0.22× 10−10 m2 s−1

as for sample M96-2 a much better prediction is obtained, with a deviation of 14 %.The obtained errors in model predictions are in line, or even better, than predictionspreviously reported with more advanced models such as the Maxwell-Stefan equations[49, 62, 63].

6.5 Effect of Substrate on the Permeance Ratios

It was shown in section 6.3 that a significant part of the total mass transfer resistancemay be caused by the substrate. Various substrates may influence the permeance ratiosof the composite membranes differently due to variations in the mass transfer resistance.In order to study these effects, permeance ratios were estimated using the model forflux through composite membranes with various supports, see P III. Three types ofsupports were investigated:

A: Asymmetric microfiltration filter as those used for membrane fabrication in thiswork, i.e. a 30 µm thick top layer with 100 nm pore size and a 3 mm thick bottomlayer with 3 µm pore size.

B: A homogenous substrate with 100 nm pore size and a thickness of 3 mm.

C: A homogenous substrate with 200 nm pore size and a thickness of 3 mm.

Ideal membranes were assumed, the defect contribution in equations (4.4) and (4.5) wasset to zero. The simulations are valid for a pressure drop over the membrane of 1 bar.The calculated H2/He and N2/SF6 permeance ratios as a function of film thickness areillustrated in Figure 6.6. The substrate type and film thickness have a large impact onthe results. In the case of H2/He permeance ratio and substrate A, the ratio is reduced

10-1

100

101

102

103

1

1.5

2

2.5

3

Film thickness /[µm]

Knudsen

Zeolite

A BC

Perm

eance r

atio

(a) H2/He

10-1

100

101

102

103

0

5

10

15

20

Film thickness /[µm]

Perm

eance r

atio

Knudsen

Zeolite

A BC

(b) N2/SF6

Figure 6.6: Permeance ratios of composite membranes as a function of MFI film thick-ness for membranes of type A, B and C.

6.6 P O 47

significantly due to mass transfer resistance in the support for films thinner than 15 µm(less than 2 % reduction for thicker films). For substrates B and C the correspondingfilm thicknesses are over 1 mm and 0.5 mm respectively. The difference between com-posite membranes on substrate B and C, in H2/He permeance ratio is due to a highercontribution of viscous flow for the mass transfer resistance in substrate C.

The N2/SF6 permeance ratio shows several interesting features. The two single layersubstrates, B and C, have permeance ratios lower than the Knudsen ratio for film thick-nesses less than 6 µm and 2 µm respectively. This is explained by a significant contribu-tion of Poiseuille flow in the substrate to the total mass transfer resistance, as discussedpreviously. It is also remarkable that the permeance ratio is reduced due to mass transferresistance in the support, even for a film with a thickness of 1 mm in the case of substrateB and C. Composite membranes using substrate A show a reduced permeance ratio forfilm thicknesses lower than 50 µm. Even though this substrate possessed a much lowermass transfer resistance, it is evident that this type of substrate also affects the permeationproperties of the molecules studied in this work.

6.6 Preferred Orientation

As was discussed previously in section 6.1 and P I, a thicker film contained more a-oriented crystals. The pores in the a-direction in the MFI structure are slightly narrowerthan the pores in the b-direction. A molecule with a size similar to the pore size isexpected to be sensitive to the preferred orientation.

SF6 is an important probe molecule with dimensions similar to the pore size of theMFI structure. It was shown earlier that the diffusion coefficient for SF6 was lower fora more a-oriented MFI film, see Table 6.2. An oriented film with the same amountof defects will therefore have a higher N2/SF6 permeance ratio than a film with weakerpreferred orientation. In order to investigate this further, the N2/SF6 permeance ratiowas calculated for defect free membranes on either substrate A or B as a function of

10-1

100

101

102

103

0

5

10

15

20

25

Film thickness /[µm]

AB B

A

Perm

eance r

atio

Figure 6.7: The effect of differences in crystallographic orientation on the N2/SF6 per-meance ratio as a function of MFI film thickness. Dashed line and solid line representrandomly and a-oriented crystals respectively, the letters A and B correspond to the twosubstrate types.

48 R D

film thickness, see P III. Two diffusion coefficients for SF6, one corresponding toa weakly a-oriented film and the other with stronger a-direction were used. Figure 6.7shows the results of the simulations. As expected, large differences in the N2/SF6 perme-ance ratio between the two orientations were observed. It should also be noted that thepermeance ratio for the membrane on substrate B is reduced significantly even for a filmthickness of 1 mm.

6.7 Influence of Applied Feed Pressure on the Permeance Ratios

The permeance of any component that adsorbs non-linearly will be affected by changesin pressure. For instance the N2/SF6 permeance ratio will increase with increasing pres-sure due to the decrease in SF6 permeance with increasing pressure. In the case of apermeance ratio between two species that adsorb linearly in the zeolite, such as hydro-gen and helium, the ratio for a zeolite film should be independent of applied pressuresince the permeance is constant with increasing pressure. Any deviation from this can beassigned to other mass transfer resistances and transport mechanisms, such as Poiseuilleflow in the support.

The effect of pressure drop on the permeance ratios was simulated for a 1 µm thickweakly a-oriented film on a support of type A with the measured defect distributionof sample M30, see P IV. The permeance ratios of linearly adsorbing species areillustrated in Figure 6.8(a) and ratios with SF6 in the denominator are illustrated in (b).Figure 6.8(a) shows that the ratios of the linearly adsorbing species are slightly pressuredependent, due to pressure dependency of Poiseuille flow in the support. The permeanceratios with SF6 increase with increasing pressure due to non-linear SF6 adsorption. Forinstance the N2/SF6 ratio increases by a factor of two when the pressure drop over themembrane is increased from 1 bar to 4 bar.

0 1 2 3 40

0.5

1

1.5

2

2.5

3

3.5

∆P /[105 Pa]

Pe

rm

ea

nce

ra

tio

H2/He

He/N2

H2/N

2

(a)

0 1 2 3 40

5

10

15

20

25

30

35

∆P /[105 Pa]

Pe

rm

ea

nce

ra

tio

H2/SF

6

He/SF6

N2/SF

6

(b)

Figure 6.8: Simulated permeance ratios as a function of membrane pressure drop for a1 µm thick randomly oriented film.

6.8 I D P R 49

6.8 Influence of Defects on the Permeance Ratios

The effects of defects on the N2/SF6 permeance ratio was investigated (P III) and theresults are summarized below. Since SF6 has a critical diameter similar to the diameterof the pores of the MFI structure, the permeance ratio should be reduced by defects.Two defect distributions were used in the simulations. One distribution was calculatedfrom porosimetry data of a 500 nm thick film of high quality on substrate A. The otherdistribution was fabricated from the measured distribution by multiplying it by ten.The defect distributions are given in Table 6.6 and Figure 6.9 illustrates the results.The defects decrease the N2/SF6 ratio as expected. The difference in permeance ratiobetween the real and ideal membrane increases with increasing film thickness, due tolarger relative mass transfer resistance in the film. The total pressure drop is constant butthe relative pressure drop over the film is increasing with increasing film thickness, thus alarger driving force for diffusion through the defects is applied. Defects in thick MFI filmwill consequently have a larger effect on the total mass transfer through the membrane,while the support has a stronger influence for thin films. The rather small effect of themeasured defect distribution on the selectivity is due to the small amount of defects inthis film of high quality. However, even a defect concentration one order of magnitudelarger than the measured may affect the permeance ratio less than the support. Note thatfor film thicknesses less than 30 µm on substrate B, a very small difference between adefect free membrane and a defective membrane is observed.

Width between defects /[µm]di/[nm] Measured Fabricated

1.08 33.7 3.371.27 149 14.92.65 5.25 · 103 5.25 · 102

9.18 1.65 · 104 1.65 · 103

100 3.64 · 106 3.64 · 105

10-1

100

101

102

103

0

5

10

15

20

Film thickness /[µm]

Perm

eance r

atio

A

B

Table 6.6: Measured and fabricated (10times higher) defect distribution used formembrane simulations, see P II.

Figure 6.9: The effect of defects on theN2/SF6 permeance ratios. Solid line,dashed line and dotted line represent idealmembrane, membrane with measured de-fect distribution and membrane with fabri-cated defect distribution respectively. Theletters A and B correspond to the two sub-strate types.

50 R D

6.9 Permeation Ratios in Various Membranes

It is now easy to explain all experimentally observed permeance ratios for the variousmembranes in Table 6.3. Equal H2/He ratios were observed for M72 and M30, althoughthe former membrane was of lower quality based on porosimetry and mixture separationfactors. This is explained by the fact that the film is thicker in M72 and this sample wastested at higher pressure. Both factors will increase the permeance ratio. Since M72 wasmore defective, which reduces the ratio, equal ratios were observed.

The high quality membrane M30 has quite low H2/N2 and He/N2 ratios, which isexplained by stronger adsorption of N2, than He and H2 in the zeolite. Furthermore,the Knudsen permeance ratios are higher than permeance ratios in the zeolite, whichincreases the ratios for more defective membranes.

Guided by Figures 6.6, 6.7 and 6.8, it is easy to explain the high gas/SF6 ratiosthat were observed experimentally for M72 and U72, despite the low quality of thesemembranes. The films in these samples are thick and oriented and the experimentalmeasurements were carried out at high ∆P, all resulting in high ratios. All other ratioscan be explained by a similar discussion.

6.10 Small Crystal Size

Grain boundaries are believed to alter the permeation properties of zeolite membranes[6], see section 2.6. The change would be a result of a different mass transfer mechanismalong the grain boundaries. The effect of grain boundaries on the separation performancewas reported in P VIII and the morphology on these samples has been discussed insection 6.1. The samples were characterized with porosimetry and separation of hydro-carbon isomers. The results were compared with membranes of similar thickness butcomprised of larger crystals. Figure 6.10 illustrates the n-hexane porosimetry patternsfor samples prepared with varying number of layers of small crystals. The data showsthat a dense film was not obtained for samples M1×12 and M2×12. Sample M3×12

0 0.2 0.4 0.6 0.8 1 1.210

-3

10-2

10-1

100

101

102

103

He

pe

rm

ea

nce

/[1

0-7 m

ol m

-2 s

-1 P

a-1]

P/P0

M1×12

M2×12

M3×12

M4×12 M5×12

α

M30 M3×12

n-hexane/DMB 227 11p-/o-xylene 16 1.4

Figure 6.10: n-Hexane porosimetry pat-terns for membranes prepared with smallcrystals.

Table 6.7: Separation selectivity of hexaneand xylene isomers at T=390 ◦C for sampleM30 and M3×12

6.11 I C R 51

and M4×12 are of high quality since the porosimetry patterns are similar to that of sam-ple M30 shown in Figure 6.3(a). Membrane M5×12 has more defects than M3×12and M4×12. The lower quality may be related to the thickness of the film as describedearlier. Table 6.7 compares the separation performance of hexane and xylene isomersfor sample M3×12 and M30. Even though the porosimetry patterns are very similarfor the two membranes, they separate very differently. The difference was attributed tomore grain boundaries in sample M3×12. Alternatively, not all defects are detected inthe porosimetry experiment. These effects are not yet fully understood.

6.11 Influence of Calcination Rate

The calcination procedure is often carried out using very low heating and cooling rates.The reason for this is that it is commonly believed that a fast procedure would increasethe amount of defects due to stress in the materials. However a slow procedure is timeconsuming and a quicker process is desirable. The influence of calcination rate on thequality of thin silicalite-1 membranes was reported in P VI. Membranes with athickness of 500 nm were calcined at 500 ◦C for 6 h with heating cooling rates vary-ing between 0.2 ◦C min−1 and 5 ◦C min−1. Single gas permeation, porosimetry andxylene isomer separation were used for determining the membrane quality. Accordingto porosimetry and single gas characterization the quality is independent of calcinationrate. The results varied slightly randomly. The separation factor of p-xylene and o-xylenein a p-xylene / o-xylene / helium mixture as a function of temperature is illustrated inFigure 6.11. Data from M30 is also included in the Figure as a reference. No corre-lation between separation performance and the calcination rate is observed. This maybe concluded since the separation factor at, for instance, 200 ◦C varies randomly withvarying calcination rate. The separation factors should be compared at a relatively lowtemperature. Coke formation at a higher temperature might affect the separation selec-tivity of the membrane. This was observed for membranes M0.2-2 and M2.0-1, where

0

5

10

15

20

25

30

α p

-/o

-xyle

ne

100 200 300 400

Temperature /[°C]

M0.2-1

M1.0-1

M30

M0.2-2

M1.0-2

(a)

0

5

10

15

20

25

30

α p

-/o

-xyle

ne

100 200 300 400

Temperature /[°C]

M2.0-1M5.0-1

M2.0-2

M5.0-2

(b)

Figure 6.11: Separation selectivity of a xylene isomer mixture as a function of tempera-ture.

52 R D

the selectivity increased above 350 ◦C. This increase in selectivity is probably due tocoke formation on the membrane surface that close small defects in the membrane. Thecoke deposits on the membrane are visible as a light grey color on the samples aftermeasurements.

The separation factor for M5.0-1 increased with increasing temperature up to about250 ◦C. This is expected since the dominating separation mechanism of the isomers ischanging with temperature from adsorption to molecular sieving [27]. At a temperatureof 250 ◦C the separation factor decreased slightly, and at 300 ◦C an abrupt decrease ofthe separation factor was observed. The abrupt decrease in separation factor was due toa mechanical failure of the sample. When the cell was opened after the experiment, themembrane was found in two pieces. This is probably associated with the extreme heatingrate of the membrane during drying, 5.0 ◦C min−1. The second sample (M5.0-2) wasdried with a lower heating rate (1.0 ◦C min−1) prior to porosimetry and separation andthe experiment was carried out with a heating rate of 0.5 ◦C min−1. No mechanicalfailure during the separation experiment was observed in this case.

6.12 Si/Al-Ratio

It is expected that several properties of the zeolite are dependent on the Si/Al ratio. Theeffect of small amounts of aluminum in the zeolite on the hydrocarbon isomer separa-tion performance was thus investigated, see P V. Two MFI membranes with varyingSi/Al ratio but with very similar film thickness, crystal orientation and defect distributionwere selected for the study. The ZSM-5 membrane had been prepared using a synthesissolution with the silicon aluminum ratio equal to 100. The defect distribution was mea-sured using porosimetry and the separation properties were investigated by separationof butane and hexane isomers as a function of temperature. The results are illustratedin Figure 6.12. A decrease in selectivity with increasing temperature is observed for thebutane isomers. This phenomenon has previously been reported for MFI membranesprepared by seeded growth [27, 64]. The decrease is a result of decreasing adsorptionstrength of the butanes as the temperature increases. At higher temperatures molecu-lar sieving would be the dominating separation effect due to weaker adsorption effects.Since the critical diameter for both n-butane and iso-butane is smaller than the porediameter of the MFI structure, a low separation selectivity between butane isomers athigh temperatures is expected. In fact, literature data [54] suggests that the diffusivityof n-butane is approximately 3 times higher than for i-butane. If the butane isomersare permeating independently through the membrane, n-butane would have three timeshigher flux than i-butane which is in agreement with the measured separation factor at150 ◦C for both membranes.

Figure 6.12(b) shows separation selectivity for hexane isomers as a function tem-perature for the selected membranes. In contrast to butane isomer separation, a largedifference between the two membrane types was observed. The silicalite-1 membraneshowed a minimum in selectivity at about 150 ◦C. The ZSM-5 membrane showed analmost constant separation selectivity throughout the entire temperature range. A higherseparation selectivity between n-hexane and DMB at 400 ◦C compared with 100 ◦Chas previously been reported [27] for silicalite-1 membranes. An explanation for thisbehaviour could be that adsorption effects are decreasing with increasing temperature,

6.13 S E/W A 53

0 50 100 1500

2

4

6

8

10

n-/

i-b

uta

ne

se

pa

ratio

n s

ele

ctivity

Temperature /[°C]

ZSM-5

Sil-1

(a) Butane isomers

100 150 200 250 300 350 4000

20

40

60

80

100

n-C

6/2

,2-D

MB

se

pa

ratio

n s

ele

ctivity

Temperature /[°C]

ZSM-5

Sil-1

(b) Hexane isomers

Figure 6.12: Butane isomers and hexane isomers separation selectivity of thin MFI mem-branes as a function of temperature. The lines are a guide for the eye.

resulting in a dominance of molecular sieving effects at higher temperature. Literaturedata [54, 65] suggest that the Si/Al-ratio only affects the diffusivities modestly indicatingthat the difference in separation properties of hexane isomers between the silicalite-1 andthe ZSM-5 membrane are due to differences in adsorption characteristics.

With the findings observed and discussed in this section, it is believed that the ob-served differences in butane and hexane isomers separation between the two selectedmembranes are due to differences in Si/Al ratio. This may be concluded since two verysimilar membranes in terms of defect distribution and preferred orientation were selectedfor the study.

6.13 Separation of the Ethanol/Water Azeotrope

ZSM-5 membranes with a high aluminum content are water selective in an ethanol/watersystem due the polar nature of the zeolite. Thus, the main separation mechanism in thesystem is differences in adsorption characteristics rather than molecular sieving sinceboth water and ethanol are small enough to penetrate the pores of the MFI structure.Figure 6.13 shows separation selectivity between water and ethanol as a function of tem-perature, see P IX. Only separation was observed below 150 ◦C for a helium dilutedethanol/water azeotrope. At higher temperatures it was found that the membrane cat-alyzed the dehydration of ethanol to form diethylether and ethylene, causing an apparentincrease in measured water/ethanol selectivity.

In order to compensate for the reaction a corrected selectivity was calculated. Thecorrected separation selectivity is given in Figure 6.13 as a dashed line. As may be seenwith the aid of the corrected selectivity, a decrease in separation selectivity between waterand ethanol was found throughout the temperature range investigated. The decreasein separation performance with increasing temperature is expected, due to the decreas-ing adsorption strength with increasing temperature. At sufficently high temperatures,molecular sieving is assumed to be the separation mechanism and a low separation per-formance is expected since both water and ethanol are able to permeate through the MFIpores.

54 R D

80 100 120 140 160 180 200 2200

5

10

15

20

25

H2O

/EtO

H s

eparation s

ele

ctivity

Temperature /[°C]

Figure 6.13: Ethanol separation performance. Dashed lines show a corrected, due tocatalytic activity, water/ethanol selectivity

The possible catalytical activity of the substrate was investigated by testing a substrateunder the same experimental conditions as the membrane. In this case no catalyticactivity was observed.

6.14 Temperature Stability

The temperature stability of ZSM-5 membranes prepared without template moleculeswas investigated by studying the permeance of SF6 at varying temperatures, see P IX.Figure 6.14 shows the SF6 permeance as a function of temperature, where the permeanceremained low up to about 250 ◦C and increased dramatically at higher temperatures.The sample was cooled to ambient temperature and the SF6 permeance remained veryhigh, as indicated by the arrow in Figure 6.14. In order to confirm that this behaviourwas a result of crack formation, a SEM investigation was carried out. It was confirmedthat large cracks, in the range of 50 nm to 200 nm, had formed.

0 50 100 150 200 250 30010

-3

10-2

10-1

100

101

Temperature /[°C]

SF

6 P

erm

eance /[1

0-7 m

ol m

-2 s

-1 P

a-1]

Figure 6.14: Temperature dependent SF6 permeation.

Chapter 7

Conclusions

A simple and unique model describing single component permeation in real zeolitemembranes was developed. Simple and basic relations describing permeation and ad-sorption were combined in the model. The important defect distribution of the mem-brane is calculated from porosimetry data and the properties of the support are measuredin a separate permeation measurement. Since the model accounts for the effect of defectsand support it is unique. Intrinsic diffusion coefficients of hydrogen, helium, nitrogenand SF6 were estimated from single gas permeation data. In the case of the non-linearlyadsorbing SF6 molecule, the Langmuir adsorption coefficient was also estimated. Theestimated parameters agreed with previously reported values. The model can adequatelydescribe the performance of various membranes under a wide variety of experimentalconditions. The model indicates that in many cases the supports strongly affect thepermeance ratios. It was also shown by simulations that these ratios are dependent oncrystallographic orientation, film thickness and experimental conditions in addition tothe amount of defects. Single gas permeance ratios can thus only be used to comparemembranes with similar morphology tested under similar conditions. On the otherhand, the porosimetry experiment is much more reliable for assessment of membranequality.

It was found that thicker membranes had a higher tendency to form defects. Mem-branes prepared on masked substrates were generally of higher quality than membranesprepared on unmasked substrates.

MFI membranes with low and varying aluminum content with similar material prop-erties, such as defect distribution and thickness, showed differences in hydrocarbon iso-mer separation experiments. The silicalite-1 membrane showed a minimum in sepa-ration selectivity between two C6 isomers whereas the ZSM-5 membrane showed analmost constant selectivity, independent of temperature, but with lower permeances. Itis believed that this difference is due to changes in adsorption properties since literaturedata indicate that the diffusivity does not change significantly with the silicon aluminumratio.

55

56 C

It was found that the quality of 500 nm thick silicalite-1 membranes was independentof the calcination rate according to single gas permeation, porosimetry and separation ofxylene isomers experiments, although the difference between the highest and the lowestheating and cooling rate was a factor of 25.

Membranes comprised of small crystals in several layers separated hydrocarbon iso-mers less effectively than membranes with larger crystals in one layer although they wereof similar quality according to porosimetry. It is believed that this difference was due toa higher concentration of grain boundaries in the film due to the smaller crystal size.

ZSM-5 membranes with a high aluminum content showed catalytic conversion ofethanol into diethylether and ethylene at temperatures exceeding 150 ◦C under simulta-neous separation of the ethanol / water azeotrope. However these membranes sufferedfrom instability at high temperatures.

Chapter 8

Future Work

The model developed in the present work can predict single gas fluxes. A natural stepwould be to expand the model to multi-component mass transfer in zeolite membranes.The Maxwell-Stefan equation or the generalized Ficks’s Law would be suitable to usein the expanded model. Membrane preparation has not yet been fully optimized, forinstance the masking procedure may be improved. Silicalite-1 and ZSM-5 membranesmay be modified by various methods for separation tasks other than the separation ofhydrocarbon isomers. The masking procedure may also be utilized in the developmentof membranes from other types of zeolites such as LTA and FAU.

57

Nomenclature

Roman Letters

AA Adsorbate energy dispersion coefficient (J)AS Adsorbent energy dispersion coefficient (J)Ai Area of defect size i (m2)ATotal Total membrane area (m2)AZ Non defective zeolite area (m2)B Molecular mobility (m2 mol J−1 s−1)b Langmuir gas adsorption coefficient (Pa−1)B0 Poiseuille flow structural parameter (m2)BS1 Poiseuille flow structural parameter of substrate

layer S1(m2)

BS2 Poiseuille flow structural parameter of substratelayer S2

(m2)

C Surface concentration (mol m−3)Ct Total concentration (mol m−3)CSat Saturation surface concentration (mol m−3)Ci Concentration of species i (mol m−3)Ði,n+1 Maxwell-Stefan diffusivity describing inter-

change between i and pseudo species n+1(m2 s−1)

Ðij Maxwell-Stefan diffusivity describing inter-change between i and j

(m2 s−1)

Ði Maxwell-Stefan diffusivity of component i in ze-olite

(m2 s−1)

d Slit pore half width (m)d0 Adsorbent, adsorbate average molecular diame-

ter(m)

dA Adsorbate molecular diameter (m)DK Knudsen diffusivity (m2 s−1)

59

60 N

DP Poiseuille flow diffusivity (m2 s−1)dS Adsorbent molecular diameter (m)DK ,S1 Knudsen diffusivity of substrate layer S1 (m2 s−1)DP,S1 Poiseuille flow diffusivity of substrate layer S1 (m2 s−1)DP,S2 Poiseuille flow diffusivity of substrate layer S2 (m2 s−1)D0 Corrected diffusivity (m2 s−1)dG Gasket diameter (m)Dij Fickian diffusivity between component i and j (m2 s−1)E Activation energy for diffusion (J mol−1)fi Fugacity coefficient of component i (Pa)g Geometrical constant (−)J Flux (mol m−2 s−1)JZ Flux through zeolite film (mol m−2 s−1)JS1 Flux through substrate layer S1 (mol m−2 s−1)JS2 Flux through substrate layer S2 (mol m−2 s−1)J i Molar flux in three dimensions of component i (mol m−2 s−1)Ji Molar flux of component i (mol m−2 s−1)J j Molar flux in three dimensions of component j (mol m−2 s−1)K Henry law gas adsorption coefficient (mol m−3 Pa−1)k Boltzmann’s constant (J K−1)K0 Knudsen structural parameter (m)KS1 Knudsen structural parameter of substrate layer

S1(m)

D Fick single component diffusivity (m2 s−1)L Average distance of diffusion jumps (m)l Length of defect (m)Lii Main term phenomenological coefficient of

component i(mol2 m−2 J−1 s−1)

Lij Phenomenological coefficient (mol2 m−2 J−1 s−1)Ljj Main term phenomenological coefficient of

component j(mol2 m−2 J−1 s−1)

M Molecular weight (g mol−1)NA Adsorbate molecular area (m2)NS Adsorbent molecular area (m2)NAv Avogadro number (mol−1)ni Number of defect for size i (−)P Gas phase partial pressure (Pa)P0 Saturation pressure (Pa)P1 Pressure at zeolite and S1 layer interface (Pa)P2 Pressure at S1 and S2 layer interface (Pa)PPermeate Permeate pressure (Pa)PFeed Feed pressure (Pa)Pj Gas phase partial pressure of component j (Pa)R Gas constant (J mol−1 K−1)r Capillary radius (m)ri Radius of defect size i (m)

N 61

T Temperature (K)u Molecular velocity (m s−1)Vm Molar volume (m3 mol−1)wi Width between defects for size i (m)xi Molar fraction of component i (−)xj Molar fraction of component j (−)z Mass transfer coordinate (m)z Slit pore coordinate (m)

Greek Letters

α Distance between adjacent equilibrium sites (m)αPerm

i,j Ideal selectivity between components i and j (−)αi,j Separation selectivity between i and j (−)δ Mass transport path, i.e. membrane thickness (m)δij Kronecker delta (−)∆HAds Isosteric heat of adsorption (J mol−1)∆Pi Gas phase partial pressure difference of compo-

nent i(Pa)

∆PFilm Zeolite film pressure drop (Pa)∆PS1 Pressure drop over substrate layer S1 (Pa)∆PS2 Pressure drop over substrate layer S2 (Pa)δS1 Thickness of substrate layer S1 (m)δS2 Thickness of substrate layer S2 (m)εS1 Porosity of substrate layer S1 (−)εS2 Porosity of substrate layer S2 (−)εSLP Minimum interaction energy between a

molecule and a single lattice plane(J)

φ Average interaction energy (J)φ(z) Interaction energy potential (J)γ Surface tension (N m−1)Γij Thermodynamic correction factor (−)λ Phenomenological coefficient interaction pa-

rameter(−)

µ Viscosity (Pa s−1)νi Chemical potential of component i (J mol−1)νj Chemical potential of component j (J mol−1)ν0

i Chemical potential at reference state of compo-nent i

(J mol−1)

Πi Permeance of component i (mol m−2 s−1 Pa−1)Πj Permeance of component j (mol m−2 s−1 Pa−1)qj Adsorbed loading of component j (J mol−1)ρ Zeolite density (kg m−3)σ Zero interaction energy distance (m)τS1 Tortuosity of substrate layer S1 (−)τS2 Tortuosity of substrate layer S2 (−)

62 N

Θ Saturation fractional coverage (−)θi Surface fractional coverage of component i (−)θj Surface fractional coverage of component j (−)θn+1 Surface fractional coverage of pseudo species n+1 (−)υe Vibrational frequency of molecule inside the ze-

olite(Hz)

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Part Four

Papers

Paper I

A Masking Technique for High Quality MFI Membranes

Jonas Hedlund, Fredrik Jareman, Anton-Jan Bons and Marc AnthonisJournal of Membrane Science 222(1-2), pp. 163-179, (2003)

Abstract

A procedure for the preparation of high quality zeolite membranes was developed. Thisprocedure relies on a masking approach that fills all support pores withwaxwhile leavingthe top surface free for deposition of the zeolite film, thus, protecting the support fromthe synthesis mixture. Zeolite films of different thickness were grown on masked andnon-masked supports using a seeded growth method. The zeolite-coated supports werecalcined in order to remove the wax from the support and the template molecules fromthe zeolite. The membranes were characterized by SEM, XRD, single gas and multi-component permeation measurements. Support masking reduces the zeolite membranethickness and the width of the cracks in the zeolite film. Thicker films, especially thoseprepared without masking, are defective. Masked membranes with a film thickness of500 nm show no cracks or pinholes.

Paper II

Modelling of Single Gas Permeation in Real MFI Membranes

Fredrik Jareman, Jonas Hedlund, Derek Creaser and Johan SterteJournal of Membrane Science, In press

Abstract

A novel permeation model for flow through defects and zeolite pores in real MFI mem-branes, also accounting for substrate effects has been developed. Defect distributionsfor two types of MFI membranes were determined from porosimetry data using themodel, which incorporated the Horvath Kawazoe (micropores) or the Kelvin equation(mesopores). The narrowest (1.08 nm) and also most common defects were found to beseparated with a distance of 10-40 µm according to the model. Diffusion coefficients forhydrogen, helium, nitrogen and SF6 in the zeolite were further determined from singlegas permeation data using the model using the independently determined defect distri-bution. The coefficients are consistent with values previously reported in the literature.

Paper III

Single Gas Permeance Ratios in MFI Membranes: Effects ofMaterial Properties and Experimental Conditions

Fredrik Jareman and Jonas HedlundSubmitted to Microporous and Mesoporous Materials

Abstract

A previously developed mathematical model with parameters fitted to experimental datawas used to study effects of material properties and experimental conditions on single gaspermeance ratios of MFI membranes. It was shown that single gas permeance ratios arehighly dependent on substrate morphology, feed pressure, crystallographic orientationof the zeolite film and defects in the film.

It was found that the pore size and the thickness of the substrate affected perme-ance ratios, due to mass transfer resistance in the substrate. The applied feed pressurealso had a significant effect on the permeance ratios. This is due to differences in masstransfer resistance of the substrate and adsorption characteristics with varying feed pres-sures. The crystallographic orientation of the zeolite film also affected permeance ratiosdue to changes in diffusivity with varying orientation of the film. Finally, the effect ofdefects was investigated. As expected, it was found that the permeance ratios decreasedwhen more defects were added in the model. However, if the membrane is not verydefective, the permeance ratio is much more affected by the substrate and by variationin pressure drop than by defects. The results in the present work show that single gaspermeance ratios cannot be used directly as a benchmark of membrane quality unless allother parameters are kept constant.

Paper IV

Permeation of H2, N2, He and SF6 in Real MFI Membranes

Fredrik Jareman and Jonas HedlundSubmitted to Microporous and Mesoporous Materials

Abstract

The present work shows that previously developed single gas permeation models for realMFI membranes is capable of predicting single gas fluxes with the aid of intrinsic masstransfer parameters and a measured defect distribution. Deviations in SF6 flux for thickand oriented films is explained with lower diffusion coefficient for the narrower pores inthe a-direction of the MFI crystals.

Variations in previously reported single gas permeance ratios for selected membranesis explained in terms of experimental conditions, film thickness and defect distribution.It was found that high feed pressures and thick oriented films resulted in large valuesof single gas permeance ratios containing SF6 even though these membranes containedmore defects than thinner membranes with randomly oriented crystals. In general allpermeance ratios with hydrogen, helium, nitrogen and SF6 has a large dependence ofmaterial and experimental properties. Thereby these ratios may only be used when com-paring membranes with similar morphology and characterized under identical condi-tions.

Paper V

Effects of Aluminum Content on the Separation Properties ofMFI Membranes

Fredrik Jareman, Jonas Hedlund and Johan SterteSeparation and Purification Technology 32(1-3), pp. 159-163, (2003)

Abstract

MFI-membranes with almost identical film thickness and defect distribution but differ-ent Si/Al ratio were evaluated by separation of butane and hexane isomers. Film thicknesswas evaluated by SEM and defect distribution by porosimetry. When the temperaturewas varied, the membranes showed similar separation trends for butanes, but clear dif-ferences were observed for hexane separation. The hexane separation factor varied withtemperature for the silicalite-1 membrane but was constant for the ZSM-5 membrane.It is believed that this difference may be a result of differences in adsorption properties.

Paper VI

Influence of the Calcination Rate on Silicalite-1 Membranes

Fredrik Jareman, Charlotte Andersson and Jonas HedlundSubmitted to Microporous and Mesoporous Materials

Abstract

Silicalite-1 membranes with a thickness of 500 nm were calcined at 500 ◦C with heatingand cooling rates varying between 0.2 ◦C/min and 5.0 ◦C/min. The membranes werecharacterized with single gas permeation, porosimetry, and xylene isomer separation ex-periments. It was found that the quality of the prepared membranes was independentof the heating/cooling rate according to the single gas permeation and porosimetry char-acterization. Xylene isomer separation data was found to vary between the samples, butnone of the variations could be attributed to the heating/cooling rate during calcinationsince the variations did not follow a trend but occurred randomly. It is thus concludedthat the calcination rate does not influence the quality of these membranes.

Paper VII

Factors Affecting the Performance of MFI Membranes

Jonas Hedlund, Fredrik Jareman and Charlotte AndersonAccepted for presentation and publication in the proceedings of the 14th InternationalZeolite Conference in Cape Town, South Africa

Abstract

Thin MFI membranes with varying morphology have been prepared using high flux alu-mina supports using an advanced synthesis procedure employing support masking andseeding. Evaluation of membrane quality by physical characterization and permeationmeasurements revealed a number of factors affecting the membrane performance. In thepresent work, the effects of film thickness, support type, preferred orientation and cal-cination rate are discussed. Some quality criteria for zeolite membranes are also debatedand the porosimetry technique is discussed.

Paper VIII

Silicalite-1 Membranes with Small Crystal Size

Charlotte Andersson, Jonas Hedlund, Fredrik JaremanAccepted for presentation and publication in the proceedings of the 14th InternationalZeolite Conference in Cape Town, South Africa

Abstract

Silicalite-1 membranes with small crystal size were prepared using a multiseeding method,where the support was repeatedly seeded and exposed to a short hydrothermal treat-ment up to five times. The films were characterized using SEM, single gas permeation,porosimetry and mixture separation experiments. Films with three or four layers were ofhigh quality i.e with minor defects according to the porosimetry experiments but showedpoor separation of binary mixtures. This result may be attributed to the small crystal sizeand/or large amount of grain boundaries in the films.

Paper IX

Preparation and Evaluation of Thin ZSM-5 MembranesSynthesized in the Absence of Organic Template Molecules

Magdalena Lassinantti, Fredrik Jareman, Jonas Hedlund, Derek Creaser and Johan SterteCatalysis Today 67(1-3), pp. 109-119, (2001)

Abstract

Porous α-alumina supports with a pore size of 100 nm were seeded with colloidal TPA-silicalite-1 crystals with a size of 120 nm. The seeded supports were calcined and treatedin a synthesis solution free from organic template molecules to form ZSM-5 films onthe supports. According to SEM images, the films were about 2 µm thick and no de-fects could be found on the as-synthesized membranes. Single gas permeation data wascollected and good quality membranes (defined as having a non detectable permeanceof SF6 after drying at 100 ◦C) were further evaluated using binary/ternary gas mixtures.The selectivity for n-butane/i-butane had a maximum value of 17.8 at 220 ◦C. Waterwas selectively separated from a helium-diluted vaporized water/ethanol azeotrope witha maximum selectivity of 12.4.