DOCTORAL THESIS. Properties and Modeling of MFI Membranes FREDRIK JAREMAN

Transcription

1 2004:11 DOCTORAL THESIS Properties and Modeling of MFI Membranes FREDRIK JAREMAN Department of Chemical Engineering and Geosciences Division of Chemical Technology 2004:11 ISSN: ISRN: LTU - DT / SE

2 THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Properties and Modeling of MFI Membranes Fredrik Jareman Division of Chemical Technology Department of Chemical Engineering and Geosciences Luleå University of Technology S Luleå, Sweden April 2004

3 Doctoral thesis 2004:11 Properties and Modeling of MFI Membranes Fredrik Jareman Fredrik Jareman, 2004 ISSN: ISRN: LTU - DT / SE Division of Chemical Technology Department of Chemical Engineering and Geosciences Luleå University of Technology S Luleå, Sweden Universitetstryckeriet Luleå, Sweden 2004

4 Abstract The permeation properties of thin (<2 µm) film MFI molecular sieve membranes have been studied in the present work and a model has been developed. The synthesis of such materials has been studied to a smaller extent. The films have been grown on graded α-alumina microfiltration filters using a seeding method. Scanning electron microscopy and x-ray diffraction were used in addition to permeation measurements for characterization of the materials. In particular, a simple and unique model describing single component permeation was developed. The model is a combination of simple and basic equations for permeation and adsorption. The important defect distribution of the membrane and the properties of the support are measured in separate experiments. The model is unique since it is accounting for the effect of defects and support on the permeation properties. The model can adequately describe the performance of various MFI membranes. The model indicates mass transfer limitations of the supports that strongly affect, for instance, permeance ratios. It was also found that these ratios are dependent on crystallographic orientation, film thickness and experimental conditions in addition to the amount of defects. Permeance ratios can thus only be used to compare membranes with similar morphology and tested under similar conditions. It was found that defects formed in thicker films. Membranes prepared on masked substrates were of higher quality than membranes prepared on unmasked substrates. MFI membranes with low and varying aluminum content with similar material properties, such as defect distribution and thickness, were evaluated with multi-component hydrocarbon isomers permeation. The silicalite-1 membrane showed a minimum in separation selectivity between two C 6 isomers whereas the ZSM-5 membrane showed an almost constant selectivity, independent of temperature, but with lower permeances. The effect of the calcination rate on the membrane quality was investigated for silicalite-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 crystals in several layers were different from membranes comprised of one layer of larger crystals, although the quality of the membranes was similar. i

5 ii ABSTRACT ZSM-5 membranes with high aluminum content showed catalytic conversion of ethanol 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

6 Acknowledgements First I would like to acknowledge my supervisor, Associate Professor Jonas Hedlund for all the guidance, kick in 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 future work as president of Växjö University. I m also grateful to: Dr. Anton-Jan Bons, Mr. Marc Anthonis, Dr. Derek Creaser, Lic. Eng. Magdalena Lassinanti and M.Sc. Charlotte Andersson for their cooperation with some of the papers. Dr. Harry W. Deckman has given invaluable suggestions regarding single gas permeation measurements. Our former secretary Mrs. Ingrid Granberg for all the help 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, or have worked, at the division; Dr. Lubomira Tosheva, Dr. Qinghua Li, Lic. Eng. Olov Öhrman, Mr. Jonas Lindmark, M.Sc. Mattias Grahn, Dr. Valeri Naydenov, Mr. Olle Niemi, Lic. Eng Zheng Wang and Ms Maria Edin for your companionship, ideas and help, and the people at the department for interesting coffee and lunch breaks and some help in the laboratory. The Swedish Research Council (VR) is acknowledged for financial support of this work. Till slut skulle jag vilja tacka Min familj i Stockholm för ert stöd och engagemang under dessa år, som student och doktorand. Jag vill också tacka mina kamrater i Stockholm och Göteborg (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 det nu. Tack Nilla, hur skulle jag klara mig utan dig? :-) iii

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8 List of Papers This thesis is based on the work contained in the following papers, referred to in the text by Roman numerals. I II III IV V VI VII A Masking Technique for High Quality MFI Membranes Jonas Hedlund, Fredrik Jareman, Anton-Jan Bons and Marc Anthonis Journal of Membrane Science 222(1-2), pp , (2003) Modelling of Single Gas Permeation in Real MFI Membranes Fredrik Jareman, Jonas Hedlund, Derek Creaser and Johan Sterte Journal of Membrane Science, In press Single Gas Permeance Ratios in MFI Membranes: Effects of Material Properties and Experimental Conditions Fredrik Jareman and Jonas Hedlund Submitted to Microporous and Mesoporous Materials Permeation of H 2, N 2, He and SF 6 in Real MFI Membranes Fredrik Jareman and Jonas Hedlund Submitted to Microporous and Mesoporous Materials Effects of Aluminum Content on the Separation Properties of MFI Membranes Fredrik Jareman, Jonas Hedlund and Johan Sterte Separation and Purification Technology 32(1-3), pp , (2003) Influence of the Calcination Rate on Silicalite-1 Membranes Fredrik Jareman, Charlotte Andersson and Jonas Hedlund Submitted to Microporous and Mesoporous Materials Factors Affecting the Performance of MFI Membranes Jonas Hedlund, Fredrik Jareman and Charlotte Anderson Accepted for presentation and publication in the proceedings of the 14th International Zeolite Conference in Cape Town, South Africa v

9 vi LIST OF PAPERS VIII Silicalite-1 Membranes with Small Crystal Size Charlotte Andersson, Jonas Hedlund, Fredrik Jareman Accepted for presentation and publication in the proceedings of the 14th International Zeolite Conference in Cape Town, South Africa IX Preparation and Evaluation of Thin ZSM-5 Membranes Synthesized in the Absence of Organic Template Molecules Magdalena Lassinantti, Fredrik Jareman, Jonas Hedlund, Derek Creaser and Johan Sterte Catalysis Today 67(1-3), pp , (2001)

10 Contents List of Tables List of Figures xi xiii Part One: Introduction and Literature Survey 1 1 Introduction Background Scope of the Present Work Literature Survey Membranes Molecular Sieves and Zeolites Molecular Sieve Films MFI-Zeolite Membranes Substrates for Zeolite Membranes Defect Formation in Zeolite Films and Membranes Part Two: Permeation Theory and Model Development 15 3 Modeling of Diffusion and Adsorption in Zeolites Introduction Heterogenous Physical Adsorption Single Component Mass Transfer Condensation in the Pores Additional Equations for Diffusion in Zeolites vii

11 viii CONTENTS 4 Model Development Effect of Substrate Permeation in the Zeolite Film Defect distribution Part Three: Thesis summary 31 5 Experimental Masking Membrane Synthesis Permeation Measurements Additional Characterization Results and Discussion Morphology Defect Distribution from Porosimetry Data Mass Transfer Parameter Estimation Predicting Membrane Performance Effect of Substrate on the Permeance Ratios Preferred Orientation Influence of Applied Feed Pressure on the Permeance Ratios Influence of Defects on the Permeance Ratios Permeation Ratios in Various Membranes Small Crystal Size Influence of Calcination Rate Si/Al-Ratio Separation of the Ethanol/Water Azeotrope Temperature Stability 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

12 CONTENTS ix III Single Gas Permeance Ratios in MFI Membranes: Effects of Material Properties and Experimental Conditions 75 IV Permeation of H 2, N 2, He and SF 6 in Real MFI Membranes 77 V Effects of Aluminum Content on the Separation Properties of MFI Membranes 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 in the Absence of Organic Template Molecules 87

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14 List of Tables 6.1 Calculated defect distribution for samples U17, M30, U30, M72 and U Estimated intrinsic diffusion and adsorption coefficients Film thickness, experimental pressure drop, helium flux, and permeance ratios for selected membranes Experimental fluxes of samples M30 and M30, the latter is of lower quality. Fluxes were simulated for M Absolute and relative errors between experimental and simulated fluxes of sample U Measured and fabricated (10 times higher) defect distribution used for membrane simulations Separation selectivity of hexane and xylene isomers at T=390 C for sample M30 and M xi

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16 List of Figures 2.1 Illustration of a general membrane with separation mechanisms MFI-crystal with channel system and crystallographic axes and pore dimensions Basic concept of seed film method Schematic illustration of an adsorbed molecule within a slit shaped micropore General drawing of an asymmetric membrane A (very) defective membrane with two defect sizes and the corresponding defect areas. The quadratic surface with the assumed mesh of defects Masking procedure Porosimetry unit Principal setup of a Wicke-Kallenbach cell Separation performance testing unit Side and top view SEM images of membranes prepared with 30 or 72 h hydrothermal treatment on masked substrates A membrane comprised of five layers of small crystals and a membrane with similar thickness comprised of one layer of crystals Membrane quality evaluation by porosimetry and C 6 isomer separation Experimental fluxes of thin masked MFI membranes versus applied film pressure drop for helium and SF Experimental and simulated fluxes of H 2, He, N 2 and SF 6 for sample U Permeance ratios of composite membranes as a function of MFI film thickness The effect of differences in crystallographic orientation on the N 2 /SF 6 permeance ratio as a function of MFI film thickness Simulated permeance ratios as a function of membrane pressure drop for a 1 µm thick randomly oriented film The effect of defects on the N 2 /SF 6 permeance ratios xiii

17 xiv LIST OF FIGURES 6.10 n-hexane porosimetry patterns for membranes prepared with small crystals Separation selectivity of a xylene isomer mixture as a function of temperature Butane isomers and hexane isomers separation selectivity of thin MFI membranes as a function of temperature Water/ethanol azeotrope separation properties Temperature dependent SF 6 permeation

18 Part One Introduction and Literature Survey

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20 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 that is not favourable for the desired product. The resulting product stream is a mixture of products, bi-products and reactants. A separation of the product stream is needed in order to purify the products and recycle the reactants. This separation often involves an energy consuming phase change. For instance, in the case of xylene production the product mixture is cooled down to the freezing point of p-xylene that crystallizes and may thus be separated. The remaining liquid is subsequently vaporized and heated to the reaction temperature (>300 C) and recycled through the reactor. A more feasible solution would be to separate the mixture at the reaction temperature without an energy consuming phase change. An example of a separation unit would be a thin and selective membrane for a specific component such as p-xylene. However, since industrial reactions are often performed at high temperatures and pressures, available polymeric membranes would be destroyed at these conditions. An inorganic membrane would thus be a better choice since inorganic membranes are more resistant to high temperatures and pressures. Unfortunately most inorganic membranes are not very selective due to the relatively large pore size of the membranes. Zeolites are a subgroup of molecular sieves. These materials have well defined pore openings of molecular dimensions in addition to useful adsorption and ion exchange capacity. Zeolites with a number of different structures have been synthesized as well as defined powders or films. The properties of zeolites are well-suited for membrane applications under harsh conditions. By using zeolite membranes in industrial processes, many energy demanding, or even impossible separation problems, could be solved in a more economical and feasible way than what is possible today. However, defects in membranes are often a critical issue since they may deteriorate the membrane separation performance. It is crucial that the amount of defects and the defect distribution of the membrane is known in order to determine the quality of 3

21 4 INTRODUCTION the membrane. Knowledge about defects is essential for successful optimization of the preparation procedure of the materials, since the quality of the membranes is dependent on the amount of defects. The measured defect distribution may further be used for a precise modeling of the permeation and separation performance of the membrane. A reliable model may also reduce the development cost of such materials, since the membrane separation performance may be predicted and the amount of experiments could thereby be reduced. Most models described in the open literature neglect defects and do not account for the substrate. As will be shown later in this work the effect of defects and 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 sieve membranes. The model should account for the effect of defects and the support. The defect distribution is determined in a separate experiment and permeation measurements are subsequently carried out. The model is applied on permeation of light inorganic molecules. The effects of experimental conditions on permeance ratios, in particular, are studied. Further objectives were to study how material properties such as Si/Al ratio, crystal size and orientation affect the permeation and separation of linear and branched hydrocarbons. To a smaller extent, the work also includes development of synthesis procedures of molecular sieve (MFI) membranes.

22 Chapter 2 Literature Survey 2.1 Membranes Membranes have been successfully utilized in several commercial applications, such as waste water treatment and desalination of sea water [1, 2]. A membrane [3] is able to separate components in gas or liquid phase with the aid of a driving force. The driving force for flow through the membrane is a difference in chemical potential as illustrated in 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 the following groups: biological, synthetic- organic and inorganic. Synthetic organic and inorganic membranes are discussed in this literature survey. For the sake of simplicity, the word 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 a porous membrane may either be straight, as illustrated in Figure 2.1, or tortuous. Porous membranes may either be microporous, mesoporous or macroporous. IUPAC defines these 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. 5

23 6 LITERATURE SURVEY 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 various porous materials at the boiling point of nitrogen at atmospheric pressure [4]. For a porous membrane three main separation mechanisms may occur: sieving, preferential adsorption and separation due to different diffusivities. The sieving mechanism, illustrated in Figure 2.1(a), only allows particles/molecules smaller than the pores of the membrane to permeate. In preferential adsorption, one species is more strongly adsorbed. In this case, larger components may permeate more effectively than smaller components if both components are sufficiently small to fit the pores. Separation due to differences in diffusivities of the permeating species may also occur, for instance when a molecule with a larger molecular weight has a lower diffusion coefficient than a molecule with a lower molecular weight. This mechanism depends on the transport mechanism and is only applicable for Knudsen flow, see section Terminology The terms feed, retentate, permeate, flux and permeance are commonly used in membrane science. The feed is the stream that is fed to the membrane for separation and the retentate is the flow rejected by the membrane. The permeate is the flow passing through the membrane. Fluxes and permeances may be based on mass-, molar- or volumetric flows. The flux is defined as flow through the membrane per unit area and the permeance is calculated from the flux by dividing with the partial pressure gradient: Π i = J i P i (2.1) Permeance ratio ( α Perm) (or ideal selectivity /permselectivity) is commonly used to describe the performance of the membrane. The quantity is calculated from single gas permeances, measured at certain experimental conditions such as room temperature, using: α Perm i,j = Π i Π j (2.2) Permeance ratio should not be confused or compared with the separation factor, or separation selectivity, for a mixture. The separation selectivity for a mixture is calculated with the well-known formula: α i,j = (x i/x j ) Permeate (x i /x j ) Feed (2.3) The relation describes the ability of a certain membrane to separate two components in a mixture under certain conditions.

24 2.1 MEMBRANES Organic Membranes Organic membranes may be divided into two subgroups: liquid and polymeric membranes. A liquid membrane is simply a thin film of liquid that is immiscible with the liquids on the retentate and permeate side of the membrane [5]. The liquid film may either be self-supported or supported by a porous material that contains the liquid membrane. Polymeric membranes are the second and more widely used type of organic membranes. These membranes are fabricated by organic polymers of varying molecular weight and cross-linking of the polymeric chains. Polymers commonly used for membrane applications are, among others, cellulose acetate, fluorocarbon polymers and aromatic polyamides [3]. The preparation methods for polymeric membranes are, for instance, sintering, stretching and sol-gel processes. A disadvantage with organic membranes is their limited thermal stability. A deeper discussion of organic membranes is out of the scope of this literature survey. Polymeric membranes are used in several industrial applications such as desalination of sea water and dehydration of solvents Inorganic Membranes Inorganic membranes have several advantages over organic membranes, such as thermal and chemical stability. They may be either dense or porous as polymeric membranes. Zeolite membranes are porous inorganic membranes. They will be treated separately in subsequent sections. The first large breakthrough for inorganic membranes was in the early 1950s. The process of interest was to separate the two uranium isotopes in the form of UF 6 gas by a membrane. This particular process is still the largest application for inorganic membranes. A membrane with quite large pores is used in the process and the separation mechanism is based on differences in the Knudsen diffusivity, see section Palladium membranes are dense and the flux is low. On the other hand, since only hydrogen may be absorbed in the palladium film, an infinite selectivity for hydrogen is expected for a perfect palladium membrane. Pervoskite membranes are also dense inorganic membranes and are selective for oxygen instead of hydrogen [6]. Thin metal membranes may, for instance, be prepared using physical or chemical vapour deposition or electroplating methods [7]. Carbon membranes are porous inorganic membranes, prepared by a pyrolysis of organic materials such as polymeric membranes [8]. These membranes are selective for hydrogen 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 nitrogen [9]. Separation of hydrogen from various process streams is also of special interest for hydrogen recovery and purification of methane [9] instead of conventional methods that may involve condensation of the gas streams. Silica membranes are also important porous inorganic membranes. These membranes 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 technique from polymeric SiO 2 sols with varying amount of polymer branching. The branch-

25 8 LITERATURE SURVEY a-direction b-direction b a c Figure 2.2: MFI-crystal with channel system and crystallographic axes and pore dimensions (Ångström units) in the a- and b- directions. ing of the polymers affects the final pore size of the membrane; less branching results in narrower 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 reported to a much lesser extent than the previously described membranes they have been left out in 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 mixture on the basis of molecular size and shape. A subset of molecular sieves is the group of zeolites. Zeolites are crystalline aluminum silicates with well-defined pores. The pores in varying zeolites are within the range 3 Å to 13 Å [12], i.e. micropores. The micropores can be utilized for gas phase separation of molecules at high temperatures. The zeolite structure is built by a three-dimensional network of [AlO 4 ] 5 and [SiO 4 ] 4 tetrahedras. The tetrahedras are linked by sharing oxygen atoms and form a threedimensional framework [13]. A representative formula of a general zeolite structure may be written as: M x/n [(AlO 2 ) x (SiO 2 ) y ] wh 2 O M is a cation of valence n, w is the number of water molecules and y/x is the silicon/aluminum ratio for the zeolite. The counterion may be a metal-, ammonium- or alkylammonium cation [13]. Over 130 different zeolite framework types [14] are known today. The most frequently studied zeolite framework types are MFI, LTA and FAU. Two well-known molecular sieves of MFI type are silicalite-1 and ZSM-5. Figure 2.2 shows a MFI-crystal with typical habit and the channel system and crystallographic axes. The sinusoidial channels run along the a-direction in the crystal and have the dimensions 5.5 Å 5.1 Å. The straight elliptical channels run along the b-direction within the crystal and have the dimensions 5.6 Å 5.3 Å. A difference between silicalite-1 and ZSM-5 is the aluminum

26 2.3 MOLECULAR SIEVE FILMS 9 content. The silicon/aluminum ratio for silicalite-1 is > 200, and for ZSM-5 the ratio is in the range of 10 to 200 [12]. Aluminum affects the properties of the zeolite, by adding 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 containing a silica-, alumina- and alkali source. The alkali source may be an alkali hydroxide and/or organic base. In a number of cases, such as in the synthesis of silicalite-1, an organic additive is necessary in order to crystallize the desired molecular sieve. Tetramethylammoniumion [TMA] + and tetrapropylammonium [TPA] + ion are among the most frequently used additives. The additive is denoted template molecule or shortly template. Unfortunately, the template remains within the pores after synthesis and blocks the pores. The template has to be removed in order to open up the microporous framework. The template removal may either be carried out by ion exchange, if the pore size admits transport of the template molecules, or by oxidation at high temperature. The latter 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 selectivity towards a desired component in a structured catalyst. In the case of membranes, a thin, perhaps less than 200 nm [15], film is necessary in order to reduce mass transport resistance. 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 synthesis. A substrate is treated with an appropriate synthesis solution under hydrothermal conditions. The method relies on the occurrence of both nucleation and crystal growth on the surface in order to facilitate film growth. Both supported and non-supported films have been synthesized with the method, where supported films dominate due to the higher mechanical strength. The vapour phase transport method, first described by Xu et al. [16], utilizes a dry gel containing the aluminum and silica source. The gel is hydrothermally treated with vapours of triethylamine, ethylendiamine and water in order to crystallize the zeolite film. Seeding methods have been used for synthesis of thin films of zeolite on almost any type of substrate. A substrate with pre-attached seed crystals is hydrothermally treated

27 LITERATURE SURVEY (a) (b) (c) (d) Figure 2.3: Basic concept of seed film method. in an appropriate synthesis solution. Different methods have been used to attach the seed crystals on the substrate surface. One method is to dip the substrate in a solution containing the crystals [17 19]. Another method, used to a lesser extent, is to rub the substrate with a powder of small zeolite crystals [20]. Previous work at the division has concerned 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 to reverse 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 is illustrated 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) shows the 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 of the crystalline material in the film. The orientation depends on several factors, such as shape and size of seed crystals and synthesis conditions, which has been investigated by Hedlund 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 zeolite on a porous and mechanically stable substrate. In this thesis the terms substrate and support are used interchangeably. This substrate is commonly referred to as support in the literature. A large disadvantage with self-supported membranes is that a substantial film thickness is necessary for mechanical strength. The mass transport resistance will be large in the narrow zeolite pores and a low flux through an unsupported membrane will result. However self-supported membranes have a few advantages compared to supported zeolite membranes. Leaching of the substrate and stress in the film due to differences in thermal expansion coefficient between zeolite and support is eliminated. Leaching may result in incorporation of unwanted species in the zeolite film. The thermal expansion issue 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-

28 2.4 MFI-ZEOLITE MEMBRANES 11 son for this interest is the large potential in terms of selectivity and flux for this type of membrane. Zeolite membranes are able to separate molecules not only by molecular sieving [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 expected. However, when applying a mixture, n-butane was the faster permeating species at steady state permeation. The reason for this peculiar behaviour is the variation in the strength of adsorption between different species. n-butane blocks the pores, making them inaccessible for hydrogen permeation. This phenomenon is less important at higher 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 stability since zeolites may be stable up to at least 500 C [25]. This should be compared with the considerably lower thermal stability of an organic membrane. The temperature stability opens up for the use of zeolite membranes in membrane reactors. Membrane reactors combine two unit operations in one, saving both investment and operational costs. Another advantage is the possibility to overcome equilibrium limitations. An example is the ethylbenzene dehydrogenation to styrene [34]. If hydrogen would be selectively removed from the reactor, the reaction would produce more styrene, since equilibrium would be avoided. 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 considerably lower fluxes and the films were almost three orders of magnitude thicker than the best membranes reported today [17, 27]. However, the performance was sufficient for measurement of the permeation properties and the samples provided important data on the separation potential of zeolite membranes Criteria for membrane quality Various groups have postulated quality criteria in order to enable comparison of membranes. Funke et.al. [37] postulated that a high quality MFI membrane should have a permselectivity between nitrogen and sulphurhexafluoride (SF 6 ) greater than 80 at room temperature. Nitrogen was chosen due to the relatively small diameter of the molecule compared with the MFI-pore diameter. SF 6, on the other hand, have a critical diameter 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 MFI membrane. Kapteijn et.al. [38] postulated that a good quality MFI membrane should have a permeance ratio between the two butane isomers higher than 10. n-butane is the faster permeating species. However, one aspect of the membrane quality is seldom mentioned. 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 Properties of Zeolite Membranes MFI-type membranes have successfully been synthesized using the three different methods given in section 2.3. The group of Noble and Falconer [39] has successfully synthesized MFI membranes on both alumina forms (α and γ) with good performance. N 2 /SF 6

29 12 LITERATURE SURVEY 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 butane isomers of 25. Matsukata et al. [32] used the vapour phase method for preparing MFI membranes and reported a N 2 /SF 6 permselectivity of 13. The group of Tsapatsis et. al. used a seeding technique for preparing MFI type membranes [17]. The result was a b-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 high quality [27] on substrates with low mass transfer resistance. These membranes show the combination 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 substrate might 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 substrates have been reported [40]. Depending on film thickness, the pore size of the substrate at the zeolite/support interface must be sufficiently small. In that case, even a thin zeolite film is sufficient to close the pores of the substrate. Substrates may either be symmetrical or asymmetrical. Symmetrical membrane substrates consist of a single layer with a well-defined pore size distribution with an average pore size usually in the range of 60 nm to 200 nm. Asymmetrical substrates consist of two or more layers with different pore size, which may reduce mass transfer resistance since the dominating part could be manufactured with a coarse pore size. Thus only a thin layer with small pore size is needed as a support for the zeolite film and possible flux resistances from the substrate may thereby be minimized. Porous stainless steel substrates have successfully been used for zeolite membrane synthesis [41, 42]. The main advantage of stainless steel substrates is the straightforward sealing with the membrane module at high temperatures by using copper washers. The disadvantage is the larger differences in thermal expansion coefficients between the substrate and the film, see section section 2.6. Another disadvantage is the relatively large pore size of the substrate that requires a thick film in order to close the pores of the substrate. Alumina substrates have been used in the majority of work reported in scientific publications. Both the α and γ forms have successfully been used. The well-defined pore size, >5 nm, of alumina substrates is ideal for membrane preparation. Alumina has a better conformity in thermal expansion coefficients with zeolite, compared with stainless steel. The α form of alumina is also relatively inert and shows a low tendency to leach aluminum into the alkaline synthesis solution. However, this is not the case for the γ form which has to be protected during synthesis [39]. If not protected, aluminum will be incorporated in the growing zeolite film. Sealing is unfortunately not as straightforward as in the case of stainless steel substrates, especially at high temperatures, but graphite gaskets can be used up to 450 C.

30 2.6 DEFECT FORMATION IN ZEOLITE FILMS AND MEMBRANES Defect Formation in Zeolite Films and Membranes In a membrane application defects have to be kept at a minimum in order to obtain an effective separation. Defects are pathways through the film with a width greater than the pores 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 formation mechanisms have been put forward. The first work was reported by Geus et.al. [43], who investigated the degradation of TPA within the MFI-framework and crack formation in large 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 noted that a shrinkage in the a-direction and an expansion in the b-direction took place during the process. Dong et.al. [44] investigated changes of the microstructure in a MFI membrane during calcination. The authors claim that cracks are formed by a compression tension. The tension could be caused by changes in cell parameters during template removal and is dependent on the type of substrate. Should the film be bonded to the substrate before template removal, defects would form during removal of template and remain during cooling. However if the film is not chemically bonded to the substrate until after the removal, defects would form during cooling of the sample due to shrinkage of the substrate. Possible orientational effects on crack formation in silicalite-1 films were investigated by den Exter et.al. [45]. In that work it was postulated that the shrinkage in the a-direction and expansion in the b-direction could have a major effect on the crack formation within oriented MFI films. Open grain boundaries and pinholes have not been investigated in the same manner as crack formation, simply due to insufficient measurement techniques. Open grain boundaries are assumed to arise during synthesis, due to lack of space to add another building block between the two growing crystals, or by the mechanism described by Dong et.al. [44], previously mentioned in the part of crack formation. It is therefore possible 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 incomplete and/or uneven seeding.

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32 Part Two Permeation Theory and Model Development

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34 Chapter 3 Modeling of Diffusion and Adsorption in Zeolites 3.1 Introduction Along with the increasing amount of permeation results in the field of zeolite membranes comes a desire to explain the data with a model. The diffusion in zeolites may either be described by a micro- or macro-scale model. Common to micro-scale models based on molecular dynamics, force field simulations, Monte-Carlo simulations, etcetera, is that the diffusion is modelled on an atomic scale and that quantum mechanical effects may be considered. A major disadvantage with the microscopic models is that they are very cumbersome with regard to computation. Keil et.al. [46] showed with simple calculations that the computing time, on a Cray Y-MP super computer, necessary for obtaining a reliable value of the self diffusivity of benzene in silicalite-1 would be about 5600 h. The macro-scale models used for diffusion have the advantage of being much less computer demanding. A famous macro-scale model for mass transfer is Fick s law, here written with a concentration gradient as the driving force for diffusion. The chemical potential may also be used as the driving force. J i = D ij dc i dz (3.1) A disadvantage by using macro-scale models is that they assume the diffusing components to be a continuum. However this is not the case when considering diffusion in zeolites, where the pores are of molecular dimensions and the crystals and film thicknesses 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 frequently used 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 used macro-scale models for modeling zeolite diffusion. They will be described briefly later in 17

35 18 MODELING OF DIFFUSION AND ADSORPTION IN ZEOLITES this section. It should also be mentioned that one of the most common, and also very critical, assumptions regarding most membrane modeling is that the membrane is defect free. However recent reports [47 49] present methods to estimate the flux through defects in the membrane and calculations of the intrinsic diffusion coefficient, that is the diffusion coefficient of the zeolite without the defects. This diffusion coefficient is vital since it may be used together with a model describing defects in order to predict separation performance for a real zeolite membrane. Real membranes will always contain some defects. 3.2 Heterogenous Physical Adsorption This section describes the two correlations used for estimating the amount adsorbed on the zeolite surface Henry s law Physical adsorption on heterogenous surfaces is distinguished from chemisorption on the basis that no change in molecular state (i.e. no association or dissociation) occurs in the first case. A uniform heterogenous surface (adsorbent) surrounded by a fluid (liquid or gas phase) containing the adsorbing species (adsorbate) in low concentrations and with negligible intra adsorbate interactions have a linear equilibrium relationship between the fluid phase and the adsorbed matter. This relationship is commonly referred to as Henry s law [50]: C = KP (3.2) Langmuir adsorption isotherm At higher adsorbate concentrations, the linear relationship will no longer be valid due to increasing intra adsorbate interactions. With a development of a complete adsorption isotherm for the adsorbate on the adsorbent, a mathematical relation describing surface concentration versus gas pressure may be formulated. The simplest theoretical model for monolayer adsorption on a heterogenous surface is the Langmuir isotherm. The assumptions 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: C = θ = bp C Sat 1 + bp (3.3)

36 3.3 SINGLE COMPONENT MASS TRANSFER Single Component Mass Transfer This section presents the theory needed in the present work for the development of the model of real MFI membranes. The model is described in chapter Intrinsic zeolite diffusion Fick s law, as given by Ruthven [50], relates the flux of a single component to the chemical potential: J = BC dν dz (3.4) B is the mobility and C is the concentration, dependent of the length coordinate z, of the diffusing species in the molecular sieve. If ideal behaviour of the diffusing gas molecule is assumed, the chemical potential may be written as: This yields: ν = ν 0 + RT ln(p) (3.5) d(ln P) J = BRTC dz If it is assumed that Henry s law, equation (3.2), is valid then equation (3.6) becomes: J = BRTK P δ (3.6) (3.7) In the case of nonlinear adsorption, i.e. Henry s law is not valid, the Langmuir adsorption isotherm is used when developing the flux equation. Assuming that the adsorption isotherm, section 3.2, is invertible and is a function of pressure i.e. c = f (P) then equation (3.6) becomes: J = BRT d (ln P) dc d (ln C) dz (3.8) In isothermal systems BRT is combined to D 0. It should also be noted that equation (3.8) may also be developed from the Maxwell-Stefan equation (3.34) or the generalized Fick s law (3.35a) for a single component Knudsen diffusion Knudsen diffusion occurs within pores where the mean free path of the diffusing molecule is similar to the pore diameter. This results in a high probability for collisions between a specific molecule and the wall. A general form of Knudsen diffusion coefficient may be written [4]: D K = RT T 3 K 0 πm = 194K 0 M (3.9)

37 20 MODELING OF DIFFUSION AND ADSORPTION IN ZEOLITES K 0 is a structural parameter describing the pore size and structure. If cylindrical capillaries may be assumed: D K = 97r T M (3.10) In this r is the circular pore radius. By using the Fick s law and assuming ideal gas behaviour, the Knudsen flux of a single component may then be written as: J = D K dp RT dz (3.11) Poiseuille flow Poiseuille flow or viscous flow occurs when a pressure gradient is applied over a small capillary. In contrast to Knudsen diffusion, the mean free path in this case is smaller than the capillary width or diameter. The Poiseuille diffusion coefficient may be written as [4]: D P = B 0P µ (3.12) B 0 is a structural parameter describing the pore width and structure. If circular capillaries may be assumed: D P = r2 P 8µ (3.13) By using Fick s law and assuming ideal gas behaviour, Poiseuille flux of a single component may then be written as : J = D P dp RT dx (3.14) 3.4 Condensation in the Pores Defects in the form of micropores, mesopores and macropores may occur in a real zeolite film. In the present work, these defects are measured in a separate experiment involving condensation of a hydrocarbon in the defects. Correlations for pore size and required pressure of the hydrocarbon for condensation is thus needed. However, no single model is capable of estimating pore size for the entire range of defects that may occur in the membrane. Thus, two models were used in the present work Micropores A schematic representation of a molecule adsorbed on a surface within a slit shaped micropore is illustrated in Figure 3.1. Horvath and Kawazoe [51] showed that the potential

38 3.4 CONDENSATION IN THE PORES 21 d 0 d S z Adsorbate molecule Surface atoms d A d i =(2d-d S ) 2d Figure 3.1: Schematic illustration of an adsorbed molecule within a slit shaped micropore. energy for such a molecule could be written as: φ(z) = N SA S + N A A A 2σ 4 [ ( ) 10 ( σ σ + d + z d z ) 10 ( ) 4 ( ) ] 4 σ σ d + z d z (3.15) The parameter σ is the distance where the interaction energy is zero and is defined by: ( ) 1/6 2 d S + d A σ = 5 2 ( ) 1/6 2 = d 0 (3.16) 5 This relation was derived by setting the potential describing the interaction energy between a molecule and a single lattice plane to zero, i.e: φ(z) = 10 [ ] 1 ( σ ) 10 3 ε 1 ( σ ) 4 SLP = 0 (3.17) 5 z 2 z The pre-potential term in equation (3.15) may be regarded as the adsorption energy for one molecule, since it is describing the energy depth of the potential. This statement is further supported by Ruthven [50], who performed theoretical calculations of the heat of adsorption on the basis of Lennard-Jones potentials. The potential equation (3.15) may now be written: φ(z) = H Ads N Av [ ( ) 10 ( ) 10 ( ) 4 ( ) ] 4 σ σ σ σ + d + z d z d + z d z (3.18)

39 22 MODELING OF DIFFUSION AND ADSORPTION IN ZEOLITES An average of the interaction energy was obtained by integration over the free space within the slit shaped pore; i.e.: d d 0 φ(z)dz d+d φ(z) = 0 d d 0 dz d+d 0 The average interaction energy for one molecule is then: φ(z) = [ H Ads σ 10 N Av (d d 0 ) 9d 9 0 σ4 3d 3 0 ] σ 10 9 (2d d 0 ) 9 + σ 4 3 (2d d 0 ) 3 (3.19) (3.20) The average interaction energy was further related to the change in free energy: ( ) P RT ln = N Av φ(z) (3.21) P0 The slightly modified Horváth-Kawazoe equation, with equations (3.20) and (3.21) thus becomes: ( ) P RT ln = 2 H [ Ads σ 10 P0 (2d 2d 0 ) 9d 9 0 σ4 3d 3 0 ] σ 10 9 (2d d 0 ) 9 + σ 4 3 (2d d 0 ) 3 (3.22) This equation relates the pressure P at which condensation occurs to the width d of the micropore Mesopores Several different relations that describe a cylindrical pore with a certain diameter as a function of partial pressure of adsorbing species have been proposed. One of the most widely used is the Kelvin equation [4]. For a condensation process in an open capillary the Kelvin equation is written as: r = γv m RT ln(p/p 0 ) (3.23) The equation is only valid for mesopores and not for micropores. This arises from the assumption 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 modeling of 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.

40 3.5 ADDITIONAL EQUATIONS FOR DIFFUSION IN ZEOLITES Maxwell Stefan equations The generalized Maxwell-Stefan equations, developed from the theory of irreversible thermodynamics, describe multi-component bulk diffusion of non-ideal fluids. The fluids may either be liquids, gases or electrolytes etc. In the case of a non-ideal fluid the general Maxwell-Stefan equation in three dimensions may be written as [52]: x i RT ν i = n j=1 j i x i J j x j J i C t Ð ij (3.24) The chemical potential, assuming non-ideal gases, may be written as: ν i = ν 0 i + RT ln(f i ) (3.25) Taylor and Krishna [52] has shown how the chemical potential gradient for a component in a multi-component mixture could be expressed in terms of molar fraction gradients: x i RT ν n 1 i = Γ ij x j (3.26) j=1 Where Γ ij is the thermodynamical correction factor, defined as: ln f i Γ ij = δ ij + x i x j T, P, Σ (3.27) The term Σ states that the partial derivative should be evaluated with the constraint Σx i = 1. Equations (3.24) and (3.26) together yields: n 1 Γ ij x j = j=1 n j=1 j i x i J j x j J i C t Ð ij (3.28) The generalized Maxwell-Stefan equation was applied to multi-component surface diffusion by Krishna [53], where the zeolite matrix was assumed to be the (n + 1)th component and the fractional occupancies to be analogous with molar fractions. In that case equation (3.24) becomes: ρ θ i RT ν i = n j=1 j i θ j J i θ i J j Θ Sat Ð ij + θ n+1 J i Θ Sat Ð i,n+1 (3.29) The Maxwell Stefan diffusivity of species i in the zeolite is defined to be: Ð i Ð i,n+1 θ n+1 (3.30) This definition is due to occupancy of the zeolite that is undefined when considering the interaction between species i and the zeolite. The Ð i diffusivity is regarded to be

41 24 MODELING OF DIFFUSION AND ADSORPTION IN ZEOLITES the single component diffusivity and it may be mechanistically described by a molecule jumping from site to site in the zeolite framework. The cross diffusivity Ð ij could physically be interpreted as a counter-exchange coefficient for two components adsorbing on a site in the zeolite. The net effect of this cross exchange is that faster diffusing species are hindered by slower and/or more strongly adsorbed species. In practice the cross diffusivity is estimated by using the Vignes correlation as suggested by Krishna [53]. Ð ij = [Ð i ] θ i θ i +θ j [ Ðj ] θ j θ i +θ j (3.31) This effect of the cross diffusivity was investigated by Kapteijn et.al. [33] as discussed in section 2.4. The chemical potential is also related to the fractional occupancies by introducing a thermodynamical correction term: θ i RT ν i = n Γ ij θ j (3.32) j=1 The thermodynamic correction term, as given below, relates the gas phase pressure to the amount adsorbed on the surface of the given species. Γ ij = θ i ln f i θ j (3.33) By rewriting equation (3.29) with the definitions of the chemical potentials and the thermodynamic factor, the Maxwell-Stefan equations for multi-component diffusion in zeolites and zeolite membranes may then be written as: ρ n Γ ij θ j = j=1 n j=1 j i θ j J i θ i J j Θ Sat Ð ij + J i Θ Sat Ð i (3.34) Generalized Fick s law The generalized Fick s law, based on irreversible thermodynamics may also be used for describing surface diffusion and diffusion in zeolites. The Onsager formalism may be used for describing the flux of a component in a multi-components system [54]: J i = J i = n L ij ν j j=1 n D ij q j j=1 (3.35a) (3.35b) Equation (3.35a) is used for describing the flux if the chemical potential is used and equation (3.35b) is used if the adsorbed amounts in the zeolite are used as driving force

42 3.5 ADDITIONAL EQUATIONS FOR DIFFUSION IN ZEOLITES 25 for the diffusion. The cross diffusivities are given by: n ln P j D ij = RT L ij (3.36) q j j=1 The Onsager phenomenological coefficients relate interaction between diffusing species and how temperature gradients affect the flux of a given species among others. By comparing equations (3.29), (3.35a) and (3.36), it may be seen that the Maxwell-Stefan diffusivity and the Fick s diffusivity are related as: D ij = Γ ij Ð ij (3.37) Chen and Yang [55] proposed a method based on irreversible thermodynamics for obtaining the cross-correlation coefficients thermodynamics from main term coefficients in a binary mixture. The Onsager reciprocal relation was used and an interaction parameter λ, where λ 1, was added: L ij = λ L ii L jj (3.38) The parameter λ was estimated by combining equations (3.35a), (3.35b) and (3.36) with a suitable multi-component adsorption isotherm Activated diffusion Xiao and Wei [56] proposed a model for single-component activated diffusion, where the diffusing species jump between equilibrium positions within the zeolite only if a certain activation energy for transport has been exceeded. Fick s law was used to describe the flux in the zeolite as given in equation (3.1). A combined diffusivity relation, valid for 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 = kT Z πm αe RT E (GT ) (3.40a) D = υ e α 2 e E RT (SV ) (3.40b) In the GT model, the diffusion mechanism could be described by the fact that molecules are moving in a potential field of a periodic lattice. When the molecule acquires an energy equal to or greater than the activation energy, it diffuses away from the potential well to a new equilibrium position adjacent to the old position. This model is similar to the Maxwell-Stefan model for a single component, as previously described. Molecules diffusing according to the SV model lose their gaseous entity due to the strong interaction with the framework of the zeolite. The potential field in the SV model could be considered as a harmonic oscillator, where the molecules are strongly bonded to the framework in the channel and are vibrating with a frequency υ e.

43

44 Chapter 4 Model Development This chapter describes the model developed in the present work for flow through a real zeolite membrane. Flow through zeolite pores and defects will be considered and the effect of the substrate will be accounted for. High flux through the membrane may cause a significant pressure drop over the substrate, although the pores in the substrate are substantially larger than the zeolite pores. Figure 4.1 shows a zeolite composite membrane on a graded alumina substrate. The top layer z is the zeolite film, layer S1 is a 30 µm thick 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 the mean free path of diffusing gases is similar to the pores of layer S1. Due to the large pore size of layer S2, Poiseuille flow was assumed to be the dominating diffusion mechanism. P Feed P 1 Zeolite film, z µm Narrow pore layer, S1 30 µm P Tot P 2 Large pore layer, S2 3 mm P Permeate Figure 4.1: General drawing of an asymmetric membrane. 27

45 28 MODEL DEVELOPMENT Pressures P 1 and P 2 indicated in Figure 4.1 are the pressures in the interfaces between the layers. The fluxes through layer S1 and S2 may now be written as: J S1 = ɛ S1 (D K,S1 + D P,S1 ) 1 dp τ S1 RT dz J S2 = ɛ S2 1 dp D P,S2 τ S2 RT dz (4.1a) (4.1b) In the integrated form, with the expressions for the Knudsen and Poiseuille diffusion coefficients inserted, the equations are: J S1 = ɛ ( S1 194K S1 + B ) S1(P 1 + P 2 ) 1 P S1 (4.2a) τ S1 2µ RT δ S1 J S2 = ɛ S2 B S2 (P 2 + P Permeate ) 1 τ S2 2µ RT P S2 δ S2 (4.2b) 4.2 Permeation in the Zeolite Film If the total membrane area is written A Total and the area of a certain defect size is denoted A i, the area of non-defective film A Z is then estimated as A Z = A Total A i. It is assumed that defects only have a number of discrete widths. For high quality membranes, A Z may be approximated with A Total, since the sum of the defect areas is small. If both Knudsen diffusion and Poiseuille flow occurs in defects, then the following equation describes the single gas flux through a zeolite film with defects: J Z = A Z BRT d ln P dc A Total d ln C dz i A i A Total ( 97r i T M + r2 i P 8µ In the event that Henry s law is valid, then equation (4.3) becomes: ) 1 dp RT dz (4.3) J Z = A Z D 0 K P Film A Total δ + i A i A Total ( 97r i T M + r2 i (P Feed + P 1 ) 16µ ) 1 P Film RT δ (4.4) Where P Film = P Feed P 1. If Langmuir adsorption mechanisms may be assumed, then equation (4.3) becomes: J Z = A ( ) Z D 0 C Sat 1 + bpfeed ln A Total δ 1 + bp 1 ( ) T 97r i M + r2 i (P Feed + P 1 ) 1 P Film (4.5) 16µ RT δ + i A i A Total

46 4.3 DEFECT DISTRIBUTION 29 Equations (4.4) and (4.5) together with equations (4.2a) and (4.2b) was used for calculations of the defect distribution from porosimetry data, estimating the intrinsic properties of 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 each measurement in the porosimetry experiment, see section The estimated areas were further 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 quadratic surface with an area equal to the circular membrane, as illustrated in Figure 4.2.The length of the defect may be written as: l 2 = πd G 2 π 4 l = d G 4 (4.6) The defects are further assumed to have a length equal to the side of the quadratic surface as shown in Figure 4.2(c). With the length, width and total defect area, the total amount of defects for a certain defect size was calculated from: n i = A i 2r i l (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 may now be calculated using: w i = 2l n i (4.8) A 2 w 1 A 1 A Z d G A Tot l w 2 l (a) (b) (c) Figure 4.2: A (very) defective membrane with two defect sizes and the corresponding defect areas (a), and the quadratic surface (b) with the assumed mesh of defects (c).

47

48 Part Three Thesis summary

49

50 Chapter 5 Experimental Asymmetric α-alumina microfiltration filters were used as substrates for the zeolite membranes 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 with 3 µm pore size. Graded supports reduce the mass transport resistance compared with a substrate 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 novel method termed masking [27]. The masking procedure is described in detail in section 5.1 whereas film synthesis details are given in section Masking A previous research project at the division aimed at developing a new strategy for preparation of thin MFI-type membranes [27]. The idea was to reduce the growth of zeolite into the substrate in order to avoid support invasion, i.e. deposition of zeolite or siliceous species in the interior of the support. Growth into the substrate may increase the mass transfer resistance and may increase defect formation at higher temperatures. A synthesis procedure where the interior of the substrate is filled with wax was developed. In previous work [27], the outcome of the masking procedure was only exemplified for a membrane with a thickness of 500 nm. Subsequent work was dedicated to investigate the effect of masking as the film thickness was varied, see PAPER I. The masking procedure is described in detail in PAPER I and is outlined in Figure 5.1. The initial support is shown in (a). The substrate is rinsed with acetone and filtered 99.9 vol% ethanol. The filtering is carried out in order to remove dust and other particles. A protective layer of polymethylmetacrylat (PMMA) has been added on top of the substrate in (b). A solution of PMMA in acetone is applied through a 0.1 µm filter with the aid of a syringe and a needle. The solution has a rather high viscosity in order to prevent it from penetrating into the substrate. The PMMA is allowed to dry very carefully by increasing the temperature slowly in a programmable oven. The PMMA covered 33

51 34 EXPERIMENTAL (a) (b) (c) (d) Figure 5.1: Masking procedure. substrates are immersed in molten wax, with the PMMA layer facing downwards. The pressure 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 the substrate to room temperature (d). The substrate is now protected by wax with the top surface available for seed deposition. 5.2 Membrane Synthesis Substrates were rinsed with a 0.1 M NH 3 solution filtered through a 0.1 µm filter in order to remove dust. Subsequently, the samples were treated in a 0.4 w% cationic polymer solution and rinsed with a filtered 0.1 M NH 3 solution. This treatment results in a positively charged surface which is necessary for adsorption of the negatively charged seed crystals. A final rinse with a 0.1 M NH 3 solution was carried out in order to remove excess seed crystals Organic template assisted synthesis In all work, except that described in PAPER IX, all films were grown by hydrothermal treatment directly after seeding. The synthesis was conducted in an oilbath with the temperature 100 C at atmospheric pressure with reflux in a synthesis solution with the following molar composition: 3TPAOH: 25SiO 2 : 1500H 2 O: 100EtOH. A synthesis solution with the molar composition: 3TPAOH: 0.125Na 2 O: 0.125Al 2 O 3 : 25 SiO 2 : 1500H 2 O: 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 hydrothermal treatment. Synthesis solution residues and aggregates were removed from the membranes by rinsing in a 0.1 M NH 3 solution. Calcination removed the TPA and opened up the microporous framework.

52 5.3 PERMEATION MEASUREMENTS Synthesis of template free films The membrane preparation differs slightly in this case, see PAPER IX. Unmasked substrates were used and a calcination procedure of the seeded substrate was performed in order to remove TPA from the seed crystals. The calcined seeded substrates were immersed in a synthesis solution with the molar composition: 30Na 2 O: Al 2 O 3 : 100SiO 2 : 4000H 2 O 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 the membranes in a 1 M NH 3 solution. 5.3 Permeation Measurements Single Gas Measurements Single gas experiments were carried out directly after calcination and the membranes were removed from the furnace when the temperature had reached 110 C. This procedure was used for all work except that described in PAPER IX, where the single gas experiments were carried out after drying at 110 C. A flow of dry nitrogen was used during mounting of the membrane in a stainless steel cell in order to avoid adsorption of 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 pressure across the membrane. The feed pressure was varied from 1.1 bar to 5 bar absolute pressure, while the permeate side was always kept at 1 bar absolute pressure. The permeance was calculated from the volumetric flow rate of the permeate. Helium, nitrogen, hydrogen and SF 6 were used as probe molecules during the single gas experiments Porosimetry Porosimetry [27, 58] was used as a tool to characterize the size and amount of defects in the membranes, see PAPERS II & V. The steady state permeance of helium was measured as a function of relative pressure of either n-hexane or p-xylene in the feed stream at room temperature. With increasing relative pressure, pores smaller than the Horváth-Kawazoe width, see equation (3.22), or Kelvin radius, see equation (3.23), will be closed by the condensable species. Figure 5.2 shows a principal drawing of the porosimetry unit used in the present work. Two mass flow controllers were used to adjust the relative pressure of the condensable species present in saturators. The gas mixture is fed to the membrane, which is situated in a stainless steel cell, at an appropriate pressure. The permeate was kept at 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 the condensable species from the permeate prior to flow measurement 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

53 36 EXPERIMENTAL MFC Mass flow controller Pressure regulator PM PM Pressure meter FM Flow meter T Thermocouple MFC T Cell FM Gas MFC T T Figure 5.2: Porosimetry unit. as driving force for diffusion. This is in contrast to both the single gas permeation and porosimetry that both employed a gradient in total pressure. The partial pressure gradient was maintained by using a sweep gas that removed permeated species from the permeate side of the membrane. The membranes were mounted in a stainless steel cell and graphite gaskets were used for sealing. These gaskets can be used up to about 450 C. A thermocouple mounted in the membrane cell was used for online temperature monitoring. A principal drawing of the separation test facility is shown in Figure 5.4. Gases were fed to the membrane cell through a valve manifold via mass flow controllers. Two mass flow controllers were used for preparing the feed and one for the sweep gas. Liquids were fed in two different ways. In the work described in PAPER IX, the liquids were fed to a vaporizer by a syringe pump and further mixed with a carrier gas, whereas in the other studies, saturators filled with the desired components were used. Helium was used as both carrier- and sweep-gas in all liquid component measurements. The test facility permits independent setting of the feed and permeate pressure via regulating valves. These valves are controlled by PID regulators connected to pressure transmitters. However in this work, pressures were set to 1 bar on each side of the Membrane Sweep gas Feed T Permeate Retentate Figure 5.3: Principal setup of a Wicke-Kallenbach cell.

54 5.4 ADDITIONAL CHARACTERIZATION 37 Sweep gas Gas 1 Valvemanifold MFC MFC MFC Bubbler On/off valve Regulating valve P Pressure transmitter T Thermocouple MFC Mass flow controller Gas 2 Evaporator Furnace Liquid GC Heated zone P P Cell T Figure 5.4: Separation performance testing unit. membrane. An online connected gas chromatograph (GC) equipped with both a thermal conductivity (TCD) and flame ionization detector (FID) was used for composition analysis. 5.4 Additional Characterization Scanning electron microscopy (SEM), Philips XL30, was used for film thickness and film morphology investigations. The microscope was equipped with a LaB 6 electron emission source. 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 depending on the d-value. In a powder sample containing randomly oriented crystals a characteristic diffractogram with certain relative intensities will result. However in a zeolite film, crystals may be oriented and the relative intensities will therefore deviate significantly from randomly oriented crystals. This information may be used to determine the crystal orientation of the crystals in the zeolite membrane. This orientation should be taken into account when modeling flow through zeolite membranes, since it is believed that different orientations of the crystals within the film alters the permeation properties of the zeolite membrane. XRD may also be used for estimating the film thickness by comparing the area of the peaks.

55

56 Chapter 6 Results and Discussion In order to better appreciate the permeation properties of a zeolite membrane, detailed knowledge 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 properties has 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 U designates if a Masked or Unmasked substrate has been used for the membrane preparation. 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 treatment for x hours that was repeated z times with intermediate seeding, x.x-y specifies a heating 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 shows side and top view SEM images of two silicalite-1 membranes with different film thickness prepared on masked substrates, see PAPER 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 growth mechanism; the winning crystals are wider at the surface. This is also indicated by Figure 6.1(c) and (d), sample M72 has much larger crystals on the surface. Competitive growth results in preferred orientation of the crystals. Preferred orientation in MFI films has previously been studied using XRD by various researchers [17, 59] and our group in particular [22 24, 57, 60]. According to XRD measurements (PAPER I) it was found that the crystals in sample M30 are weakly a-oriented whereas the crystals in sample M72 are more a-oriented. More defects were found in thicker films as illustrated by Figure 6.1(d). The same trend was also observed for samples prepared on unmasked substrates. These samples 39

57 40 RESULTS AND DISCUSSION 500 nm 500 nm (a) M30 Side view (b) M72 Side view 500 nm 500 nm (c) M30 Top view (d) M72 Top view Figure 6.1: Side and top view SEM images of membranes prepared with 30 or 72 h hydrothermal treatment on masked substrates. contained more defects than membranes with similar film thickness prepared on masked substrates. This is believed to be due to growth of zeolite into the substrate during hydrothermal treatment. The effect of grain boundaries on the permeation properties of MFI membranes was studied (PAPER VIII) by fabricating membranes with small crystals in multiple layers using a multi step synthesis method. Membranes with varying film thickness but small crystal size were obtained by repeating the synthesis procedure with seeding and a short hydrothermal treatment (12 h). These films consist only of small crystals and therefore more grain boundaries. Figure 6.2 shows a SEM image of a membrane with small crystals and a film thickness of about 850 nm. The Figure shows that the surface consists of small crystals with a diameter of about 120 nm. This morphology was also found for the other membranes prepared using the multi step method, see PAPER VIII. The morphology of sample M5 12 differs significantly from a single synthesis step membrane, M96 with similar film thickness, as shown in Figure 6.2(b). The M96 sample has approximately five times larger crystals on the surface than M5 and consequently fewer grain

58 6.2 DEFECT DISTRIBUTION FROM POROSIMETRY DATA nm 500 nm (a) M5 12 (b) M96 Figure 6.2: A membrane comprised of five layers of small crystals, (M5 12)and a membrane with similar thickness comprised of one layer of crystals (M96). boundaries. The permeation properties of the multi-layer membranes will be discussed further in section Defect Distribution from Porosimetry Data According to SEM investigations it was found that thicker films contained more defects. In order to verify these findings a series of permeation experiments was performed. The study included porosimetry, gas phase separation of hydrocarbon isomers and single gas permeation measurements. In total five sample types, U17; M30; U30; M72 and U72, with varying film thickness and amount of defects, were selected for the series of permeation experiments. He permeance /[10-7 mol m -2 s -1 Pa -1 ] 10 2 U17 M30 U30 M72 U P/P 0 (a) n-c 6 /2,2-dmb separation selectivity U17 M30 U30 M72 U Temperature /[ C] (b) Figure 6.3: Porosimetry patterns (a) and n-hexane / 2,2-dmb separation factor (b) for selected MFI membranes of varying type.