Separation Processes. A thesis submitted to The University of Manchester for the degree of

Transcription

1 A Novel Approach to Fabricate Zeolite Membranes for Pervaporation Separation Processes A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Science 2013 Abdulaziz Ali Saleh Alomair School of Chemical Engineering and Analytical Science 1

2 To my beloved parents, wife, and siblings; whose support is abundant and love is nourishing. To my dear son, Ali; who has been a wonderful source of joy and learning. 2

3 List of Content Abstract Chapter 1: General Introduction Background Aims and Objectives Thesis Outline Chapter 2: Zeolites Introduction Historical Background Zeolite Applications Zeolite Structure Properties of Zeolites Aluminosilicate Ratios in Molecular Sieves Zeolite Catalysts Ion Exchange Zeolite Powder Synthesis Zeolite Precursor and Source Materials Ageing Process Crystallization Process Chapter 3: Membranes Introduction Historical Background Membrane Structures Porous membranes Membranes of molecular sieve materials and dense phases

4 3.3.3 Electrically charged membranes Thin-film Composite Liquid Membranes Membrane units and operations Reverse Osmosis Ultrafiltration Microfiltration Pervaporation Gas separation Membrane Transport Theory Membrane Material and Applications Organic or Polymeric membranes Inorganic Membranes or Ceramic Membranes Zeolite Membranes Zeolite Membranes Synthesis Methods Application of Zeolite Membrane in Separation Processes Gas Separation Liquid/Liquid Separation Carbon Membranes Introduction Carbon membrane Classification Carbon Precursor Selection Precursor Pre-treatment Pyrolysis/Carbonisation Membrane Post-treatment Chapter 4: Characterization Techniques X-ray diffraction

5 4.2 Scanning Electronic Microscope Gas Chromatography Dynamic Light Scattering (DLS) Chapter 5: Experimental Work Synthesis of Zeolite A Zeolite A using commercial source material Zeolite A prepared using kaolin,wbb UK and (ANK) Zeolite films and membranes preparations, using conventional methods Zeolite film/membrane supports Modification and pre-treatment of supports Using (MIM) with etched and oxidised non-porous supports Using (SGM) with etched and oxidised non-porous supports Zeolite membranes using SGM with oxidized, porous supports Membrane performance, testing and evaluation Repair of SGM membranes using the rubbing method and carbon properties Using rubbing method Using the properties of carbon Novel technique for fabricating carbon-zeolite membranes Zeolite A membrane Mordenite membrane Clinoptilolite membrane ZSM-5 membrane Chapter 6: Zeolite A Synthesis from Kaolin, Results and Discussion Introduction Synthesis of zeolite A Zeolite A, using commercial sources Zeolite A, Using Kaolin

6 Chapter 7: Carbon-Zeolite Membranes, Results and Discussion Introduction Zeolite films and membranes preparations, using conventional methods Zeolite film/membrane supports preparation Using the (MIM) With Etched and Oxidised Non-porous Supports Using the (SGM) with Etched and Oxidised Non-porous Supports Zeolite membranes using (SGM) with oxidized, porous supports Membrane performance testing and evaluation Repair of SGM membranes using the rubbing method and carbon properties Using rubbing post-treatment with zeolite A seed paste Using the properties of carbon Novel Technique for Fabricating Carbon-Zeolite Membranes Ethanol dehydration Separation of ethanol/cyclohexane mixture Separation of xylene isomers Removal of phenol from water Evaluation of quality and durability Chapter 8: Conclusions and Recommendation for Future Work Conclusions Recommendation References Appendix A: Extended Results and Calculations Appendix B: Repeatability Results Appendix C: ZSM-5 Symthesis amd Preparation Appendix D: Published Work

7 List of Figures Figure 2.1 Representation of [SiO 4 ] 4- or [AlO 4 ] 5- tetrahedral. 24 Figure 2.2 The major estimated usage of zeolite in the industrial applications. 28 Figure 2.3 Zeolite building units and frameworks. 30 Figure 2.4 Figure 2.5 Figure 2.6 Figure 2.7 The LTA framework type and Illustration of zeolite A pore size. The FAU framework type and Illustration of zeolite Y pore size. The MFI framework type and Illustration of ZSM-5 pore size. The HEU framework type and Illustration of Clinoptilolite pore size Figure 2.8 Product selectivity in the alkylation process of toluene to p- xylene. 37 Figure 2.9 Schematic representation of zeolite synthesis procedure. 40 Figure 2.10 Illustration of the autoclave used in the hydrothermal synthesis. 41 Figure 3.1 Schematic diagram of membrane morphologies and structures. 53 Figure 3.2 Figure 3.3 General classification of membranes separation process units according to the operated driving force. Illustration of membrane separation processes pore size and theoretical model principal Figure 3.4 Flow schematic laboratory scale reverse osmosis test system. 60 Figure 3.5 Figure 3.6 Ultrafiltration laboratory scale units (a) stirred batch cell (b) flow-through cells. Schematic illustration of (a) in-line filtration and (b) crossflow filtration using microfiltration membrane

8 Figure 3.7 Pervaporation process configurations. 67 Figure 3.8 Figure 3.9 Figure 3.10 Basic illustration of membrane used in gas separation process. Classification of diffusion transport mechanisms dependent on pore size. Illustration of the experimental procedure of in-situ and secondary growth methods Figure 3.11 Basic schematic illustration of pervaporation unit. 82 Figure 3.12 Classification of typical organic compounds. 87 Figure 3.13 Illustration carbon membrane fabrication steps. 92 Figure 3.14 Illustration of carbon membrane classifications. 92 Figure 3.15 Typical carbon s materials pore structure. 98 Figure 3.16 Carbon deposition forms on membrane pore walls. 103 Figure 4.1 Typical illustration of beams reflection from a lattice planes. 106 Figure 4.2 Illustration of the scanning electron microscope. 110 Figure 4.3 Schematic diagram of typical GC set up. 112 Figure 5.1 Figure 5.2 Figure 5.3 Figure 5.4 Figure 5.5 Figure 5.6 Figure 6.1 DLS image of (a) suspended seeds and (b) the ball mill slurry. Illustration of the autoclave used in the hydrothermal synthesis with the homemade Teflon holder and bowl. Illustration of the Teflon-lined autoclave (size 50 ml) used in this work and the homemade Teflon holder inside the autoclave. Schematic diagram of pervaporation unit with membrane module. Pervaporation used in this study and the membrane compartment. Tubular furnace used for the pyrolysis process, with the temperature system trend. SEM image of zeolite A particles at 10 and 5 µm spatial resolutions

9 Figure 6.2 Figure 6.3 Figure 6.4 Figure 6.5 Figure 6.6 Figure 6.7 Figure 6.8 Figure 6.9 Figure 6.10 Figure 7.1 Figure 7.2 Figure 7.3 Figure 7.4 Figure 7.5 Figure 7.6 Figure 7.7 Comparison of the synthesized and standard samples of zeolite A. Comparison of the ANK source before and after the hydroxylation process. Comparison of the WWB source before and after the hydroxylation process. Comparison of standard zeolite A and the synthesised sample of zeolite A from WBB. Comparison of the sample collected at the top of the crucible bowl and the sample from the bottom of the bowl. SEM images of top (a,b) and bottom (c,d) products of zeolite A from WBB using ball mill seeds. Comparison of the synthesized zeolite A before and after the washing process. SEM images of top (a,b) and bottom (c,d) products of zeolite A from ANK using ball mill slurry. Top sample of zeolite A compared with the commercial standard, using suspended solids and ball mill slurry. SEM image of (a) the non-porous stainless steel, (b) etched stainless steel, and (c) oxidised stainless steel. Comparison of the XRD patterns of zeolite A film using etched support and standard samples. Comparison of the XRD patterns of zeolite A film using oxidised support and standard samples. SEM of zeolite A layer (54.17 µm) on non-porous metal surface after the etching process: (a,b) top view and (c,d) edge view. SEM of zeolite A layer on non-porous metal surface after the oxidising process. Comparison between the zeolite formation layers on (a) etched (b) oxidised supports. SEM of zeolite A layer on non-porous metal surface after etching process using secondary growth method

10 Figure 7.8 Figure 7.9 Figure 7.10 Figure 7.11 Figure 7.12 Figure 7.13 Figure 7.14 Figure 7.15 Figure 7.16 Figure 7.17 Figure 7.18 Figure 7.19 Figure 7.20 Figure 7.21 Figure 7.22 SEM of zeolite A layer on non-porous metal surface after oxidising process using secondary growth method. Comparison of zeolite film and standard sample patterns using the secondary growth method on etched support. Comparison of zeolite film and standard sample patterns using the secondary growth method on oxidised support. SEM of zeolite A layer ( 100 µm) on porous metal surface after oxidising process using the secondary growth method: (a,b) top view and (c,d) edge view. Comparison of zeolite film and standard sample patterns using the secondary growth method on oxidised support. Illustration of comparison between membrane performances in terms of separation factors and fluxes at 96% feed of ethanol. Illustration of separation factors repeatability and fluxes repeatability at 96% feed of ethanol. Illustration of the flux dependence on the feed composition using zeolite membranes i.e., A1-A9. SEM of zeolite A layer on porous metal surface after applying rubbing post-treatment. Comparison of the membrane performances before and after applying the rubbing post-treatment for zeolite membranes A.1, A2 and A3 at 96% of ethanol in feed mixture. Comparison of the permeate fluxes before and after applying the rubbing post-treatment for zeolite membranes A.1, A2 and A3 at different feed compositions of water/ethanol. Comparison of the separation factors before and after applying the rubbing post-treatment for zeolite membranes. SEM image of carbon-zeolite composite layer using PFA at different spatial resolutions top views and edge view. SEM image of carbon-zeolite composite layer using sucrose at different spatial resolutions. Illustration of the flux dependence on the feed composition using zeolite membranes post-treated with PFA and sucrose

11 Figure 7.23 Illustration of the separation factor dependence on the feed composition using zeeolite membranes post-treated with PFA and sucrose. 185 Figure 7.24 Illustration of healing zeolite membrane using carbon. 187 Figure 7.25 Figure 7.26 Figure 7.27 Figure 7.28 Figure 7.29 Figure 7.30 Figure 7.31 Figure 7.32 Figure 7.33 Figure 7.34 Figure 7.35 Figure 7.36 SEM images of carbon-zeolite A composite layer using different concentrations of sucrose solution at different spatial resolutions; Z-S.1, Z-S.4 and sucrose formation after the carbonization process. General comparison between (a) zeolite membrane, (b) carbon-zeolite membrane with low concentrated sucrose solution and (c) carbon-zeolite membrane with high concentrated sucrose solution. Illustration of the overall flux dependence on the feed composition using zeolite membranes post-treated with sucrose. Illustration of the separation factor dependence on the feed composition using zeeolite membranes post-treated with sucrose. Comparison of the performances of the membranes prepared by Holmes et al and those in the current study at 25 o C using a mixture that contained 80 wt% ethanol. SEM image of carbon-mordenite composite layer using sucrose at different spatial resolutions. (a,b) top views and (c,d) edge view. Illustration of the overall flux dependence on the feed composition using mordenite membranes. Illustration of the separation factor dependence on the feed composition using mordenite membranes. Comparison of the performances of membranes prepared by Navajas et al and those prepared in our study. SEM images of carbon-zeolite clinoptilolite composite layer (1:1) concentrations of sucrose solution. Illustration of the flux dependence on the feed composition using zeolite membranes post-treated with sucrose. Illustration of the separation factor dependence on the feed composition using zeeolite membranes post-treated with sucrose

12 Figure 7.37 Figure 7.38 Figure 7.39 Illustration of permeate fluxes under different feed temperatures. Illustration of separation factors under different feed temperatures. Illustration of temperature effect on permeate fluxes using carbon - ZSM-5 membrane at constant feed composition of 50% wt Figure 7.40 SEM images of carbon-zeolite ZSM-5 composite layer (1:1) concentrations of sucrose solution. 209 Figure 7.41 Figure 7.42 Figure 7.43 Figure 7.44 Figure 7.45 Comparison of the performances of membranes prepared in this study and those prepared by Wenger et al. Comparison of the performances of the membranes prepared in this study at 50 o C with those of Yuan et al. Illustration of temperature effect on permeate fluxes using clinoptilolite-2 membrane. Comparison of the performances of membranes prepared in this study and those prepared by Pradhan et al. Illustration of the separation behaviour of carbon-zeolite A membrane at constant feed composition and temperature

13 List of Tables Table 2.1 Table 2.2 Table 3.1 Table 3.2 Table 3.3 Summary of the studies and developments relative to zeolites. Pores kinetic diameter, silicon to aluminum ratios and framework type of selected zeolite types. Summary of the studies and developments relative to membranes. Comparison of the advantages and disadvatages of the two microfiltration systems. Some examples of typical polymer types used in different application Table 3.4 Disadvantages of organic and inorganic membranes. 75 Table 3.5 Ethanol dehydration using zeolite A membrane. 88 Table 3.6 Ethanol removal using ZSM-5 membrane. 89 Table 3.7 Separation of organic / organic mixtures. 90 Table 3.8 Configuration of carbon membrane precursors. 93 Table 3.9 Table 3.10 Oxidation processes operating temperatures, according to different precursors. Pyrolysis process conditions using different types of precursors Table 3.11 Oxidation conditions using different types of precursors. 102 Table 5.1 Summary of the aluminosilicate source materials batches 115 Table 7.1 Table 7.2 Table 7.3 Evaluation of zeolite A membranes using dip-coating method at different feed compositions of water/ethanol mixture. Evaluation of zeolite A membranes after using rubbing posttreatment with SGM. Evaluation of carbon-zeolite membranes using PFA and sucrose as a carbon precursor after the SGM at different feed compositions of ethanol/water mixture

14 Table 7.4 Table 7.5 Table 7.6 Table 7.7 Table 7.8 Table 7.9 Table 7.10 Table 7.11 Table 7.12 Table 7.13 Table 7.14 Table 7.15 Table 7.16 Comparison between membrane performances, before and after involving CPFA at feed composition 96% ethanol. Evaluation of carbon-zeolite membranes using sucrose as a carbon precursor after the SGM at different feed compositions of water/ethanol mixture. Comparison between membrane performances, before and after involving sucrose at feed composition 96% ethanol. Evaluation of carbon-zeolite membranes using different sucrose solution concentrations (as a carbon precursor) after coating the porous support with zeolite A paste, at different feed compositions of ethanol/water mixture. Evaluation of mordenite membranes using sucrose as a carbon precursor at different feed compositions of water/ethanol mixture. Evaluation of carbon-zeolite membranes using clinoptilolite and sucrose as a carbon precursor with different feed compositions at room temperature. Evaluation of carbon-zeolite membranes using clinoptilolite and sucrose as a carbon precursor with different feed compositions at 40 o C. Evaluation of carbon-zeolite membranes using clinoptilolite and sucrose as a carbon precursor with different feed compositions at 60 o C. Evaluation of carbon-zeolite membranes using ZSM-5 after post-treatment with sucrose as a carbon precursor with different feed compositions at 25 o C. Evaluation of ZSM-5 membrane using sucrose solution of (1:1) concentration for xylene isomers separation at different feed temperatures. Comparison of the performances of membranes prepared in this study and those prepared by Wenger et al. Comparison of the performances of membranes prepared in this study and those prepared by Yuan et al. Evaluation of zeolite A and clinoptilolite membranes using sucrose solution of (1:1) concentration for phenol/water separation at 25 o C

15 Table 7.17 Table 7.18 Table 7.19 Evaluation of clinoptilolite membrane using sucrose solution of (1:1) concentration for phenol/water separation at different feed temperatures. Comparison of the performances of membranes prepared in this study and those prepared by Pradhan et al. Evaluation of carbon-zeolite A membrane using sucrose solution of (1:1) concentration for ethanol dehydration at 25 o C, and feed composition of 96% of ethanol

16 Abstract The production of zeolite membranes has developed over the last decade, and the membranes have been used extensively in pervaporation separation processes due to their resistance to chemical and thermal operating conditions. However, the conventional methods used in preparing anisotropic zeolite membranes, such as the secondary growth and in-situ crystallization methods, involve long and complex procedures that require the preparation of zeolite aluminosilicate gel prior to the fabrication process. Therefore, the aim of this study was to develop and test an easier, less expensive, and less time-consuming technique to fabricate different types of zeolite anisotropic membranes. Moreover, the fabrication of zeolite membranes using inexpensive kaolin raw materials taken straight out of the ground was taken into account and assessed. Within this framework, a novel technique of converting raw source alumina and silica, to a useful pure material of zeolite A was developed without any form of pre-treatment. Although this technique yielded a successful outcome in terms of the purity of the product, the later work conducted in fabricating membranes was focused on natural and commercial sources of zeolites rather than using the prepared products, to avoid the lengthy procedure. Anisotropic membranes of zeolite A, mordenite, and ZSM-5 were fabricated successfully using a simple, economical, and straight-forward technique. This technique made it possible to fabricate types of zeolite membranes that have been difficult to synthesise at the lab scale, where an anisotropic, clinoptilolite, thin membrane was fabricated for the first time in this study. All of the four membranes were subjected to different types of mixtures and provided promising results. 16

17 Declaration No portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. Abdulaziz Ali Saleh Alomair 19 th September

18 Copyright Statement i- The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the Copyright ) and s/he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. ii- Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. iii- The ownership of certain Copyright, patents, designs, trademarks and other intellectual property (the Intellectual Property ) and any reproductions of copyright works in the thesis, for example graphs and tables ( Reproductions ), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. iv- Further information on the conditions under which disclosure, publication and iv- commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see in any relevant Thesis restriction declarations deposited in the University Library, The University Library s regulations (see and in The University s policy on Presentation of Theses 18

19 Acknowledgment I would like to express my sincere gratitude to my supervisor, Dr. Stuart Holmes, for his encouragement and thoughtful guidance that lit the path towards accomplishing this project. I m deeply grateful to my beloved family, without whose patience and support this work could never have been achieved. My dearly loved parents, I will run out of words before I nearly complete my expression of thanks and appreciation for your generous support and endless love throughout my life. My brothers and sisters, I owe you all many thanks and much appreciation. My wife and son, I am so thankful for your kind assistance and patience, and thanks for believing in me and being a source of happiness during this demanding undertaking. Finally, I would like to thank the Kuwait Institute for Scientific Research and the Kuwait Cultural Office for their help and financial support. 19

20 Chapter 1. Chapter 1 General Introduction 1.1 Background Membrane technology has attracted considerable interest among many researchers due to its excellent performance and advantages, including the simplicity of the concept and low energy consumption [1,2,3]. In recent years, extensive efforts have been expended in the petrochemical-related industries to develop more economical separation methods to replace conventional methods because they require the consumption of excessive quantities of energy, e.g., azeotropic distillation [4]. The pervaporation process has been recognized as a potentially viable candidate to fulfil this task, because it is considered to be an economical technique that uses less energy than conventional techniques to separate liquid mixtures. Moreover, pervaporation overcomes numerous other separation difficulties, including the separation of azeotropic mixtures, the separation of mixtures in which the components have similar boiling points (especially in the case of isomeric components), and organic removal processes [5]. The term pervaporation is derived from its two basic steps, i.e., permeation and evaporation, where the permeate passes through the membrane and then evaporates into the vapour phase. In this process, many types of membranes are used, including polymeric and ceramic membranes. However, researchers have subjected ceramic/zeolite membranes to this process due to their uniform microstructures and their molecular sieving properties [6]. Moreover, the capability of zeolites to handle the extreme operating conditions required by some separation technologies is well known, e.g., in membrane reactors. Consequently, the zeolite industry is improving its products and 20

21 Chapter 1. opening new horizons in many applications. These applications include uses in the petrochemical industry and refining, as adsorbents, in the manufacturing of detergents, and in various agricultural processes. Zeolites are crystalline, aluminosilicate materials, and they are defined as a three-dimensional, aluminosilicate, crystalline framework structure that forms uniformly-sized pores of molecular dimensions [7]. The introduction of an anisotropic membrane that consisted of a very thin layer supported on a thicker, porous structure made a major contribution to membrane technology [8]. Zeolite membranes usually are fabricated using the conventional, hydrothermal, crystallization process that requires an initial step for the synthesis of the gel. This initial step, due to its complexity, time consumption, and energy consumption, is considered to be a significant obstacle in the way of developing and testing many types of zeolite membranes [2]. Therefore, a technique for synthesising a given type of zeolite membrane without first having to synthesise its gel would be a valuable breakthrough from both economic and technical perspectives. 1.2 Aims and Objectives The main aim of this study was to seek a straightforward, less time-consuming technique for fabricating membranes with numerous separation properties using natural and synthetic zeolites. Consequently, the key objectives are as follows: To investigate, without using a purification process, the feasibility of converting raw material to a useful pure zeolite to be utilized in membrane fabrication processes. 21

22 Chapter 1. To evaluate two different post-treatment methods for membrane synthesis by using zeolite seed paste and a carbon precursor to improve different types of membranes that have been fabricated by conventional methods. To define a novel, simple technique that incorporates a carbon precursor and can be used to fabricate the desired zeolite membranes that are suitable for use in the dehydration of organic liquids, the removal of organic materials from mixtures, and organic/organic separation processes. 1.3 Thesis Outline The work presented in this thesis consists of two major divisions, i.e., the synthesis of pure zeolite using raw sources and fabrication processes for zeolite membranes, and the thesis is divided into eight chapters. This chapter presents the important aspects that must be considered through a number of objectives. The literature and published studies that have been conducted concerning zeolites and membranes are mentioned in chapters two and three, respectively. Chapter four contains a general overview of all of the characterization techniques used in this study. The experimental work related to novel techniques for the synthesis of both zeolites and membranes is presented in chapter five. In chapter six, the results of converting kaolin raw material sources to pure zeolite A in a one-pot synthesis are presented and discussed. In chapter seven, the outcomes and test results are given for the posttreatment methods and the zeolite composite membranes that were fabricated. In chapter eight, the conclusions obtained from this work are listed, along with suggestions for future work in this area. 22

23 Chapter 2. Chapter 2 Zeolites 2.1 Introduction Zeolites are crystalline, aluminosilicate materials of elements in groups I and II. A zeolite is defined as a three-dimensional, aluminosilicate crystalline framework structure that forms uniformly-sized pores of molecular dimensions arising from a framework of [SiO 4 ] 4- and [AlO 4 ] 5-, as shown in Figure 2.1[7]. Natural zeolites are unique minerals that normally are formed from volcanic eruptions when the lava rocks and the ash go into water and form minerals with cage-like structures. Currently, many types of these zeolites can be produced synthetically. In general, zeolites are represented chemically by the empirical formula in Equation 2.1 in which (x) is equal to or greater than two (up to 10), (n) represents the valence of the cation, and (y) is the number of water molecules in the voids of the zeolite [9]. M 2/n O Al 2 O 3 xsio 2 yh 2 O 2.1 The main characteristic property of zeolite minerals is their porous, molecular structure that has regular arrays of spaces. This property is known as the molecular sieving property, which has attracted the interest of many scientists during the last few decades and is the basis for the first commercial success of zeolites [10]. Zeolites are considered to act as molecular sieves due to their microporous structure, and this capability is controlled by the dimensions of their channels. The terminology molecular sieves expresses the ability of zeolites to adsorb molecules that fit inside their pores and channels, while rejecting molecules that are too large based on the size-exclusion process. Such three-dimensional framework structures 23

24 Chapter 2. have been used extensively in many applications, including as laundry detergents, in the petrochemical industry, and in gas separations [11]. Currently, zeolites have many applications in petroleum refineries, and they are used extensively in industrial applications as adsorbents, catalysts, and ion exchangers [4]. The global price of zeolites depend mainly on their type / application; for example, the prices of zeolites used as catalysts and absorbents range from $1 to $100 per pound, whereas the prices are about $0.30 per pound for those used as detergents [12]. Si/Al Oxygen anions Figure 2.1: Representation of [SiO 4 ] 4- or [AlO 4 ] 5- tetrahedral 2.2 Historical Background In 1756, a Swedish mineralogist, Axel Cronstedt, first recognized zeolites as a new type of mineral. Cronstedt named this type of mineral Zeolite according to the Greek words zeo and lithos referring to boil and a stone [13]. Subsequently, many studies have been conducted and many developments have occurred relative to the synthesis of zeolites, as shown in Table

25 Chapter 2. Table 2.1. Summary of the studies and developments relative to zeolites. Year Scientists and Organizations Contribution 1756 Axel Cronstedt Identified a new type of minerals and named them Zeolites [14] Frank Fontana Described the surface adsorption mechanism [15] Dan Damour 1845 Schalfhaut 1850 Way and Thompson Described the reversible hydrationdehydration character of zeolites [16]. Emphasized the hydrothermal synthesis of quartz by heating a gel silica with water in an autoclave [7]. Clarified the nature of ion exchange in soils [17] Eichhorn Showed the reversibility of ion exchange on zeolite minerals [18] Deville Reported the hydrothermal synthesis of a zeolite [19] Friedel Identified the structure of dehydrated zeolites [20] Weigel and Steinhoff 1927 Leonard 1930 Taylor and Pauling Reported the first molecular-sieve effect [21]. Reported the first use of X-ray diffraction for identification of compounds formed in mineral synthesis Reactions [22]. Identified the first single-crystal structures of zeolite minerals [23, 24]. 25

26 Chapter McBain Established the molecular sieve term to define porous solid materials that act as sieves on a molecular scale [25] Barrer Worked in the areas of zeolite adsorption and synthesis and presented the first classification of the zeolites based on the consideration of molecular size [26] Barrer Achieved the first definitive synthesis of zeolites, including the synthetic analog of the zeolite mineral mordenite [27] Union Carbide Corporation Commercialized synthetic zeolites as a new class of industrial materials for separation and purification processes [9] Reed and Breck Reported the structure of the synthetic zeolite A [28] Union Carbide Corporation Commercialized zeolite Y as a based catalyst [9] Mobil Oil Introduced the use of synthetic zeolite X as a hydrocarbon cracking catalyst [13] W.R. Grace & Company Described the first alteration chemistry based on steaming zeolite Y to form an ultrastable form [13] Mobil Oil 1974 Henkel 1986 Flanigen and Wilson Reported the synthesis of ZSM-5 and the high silica zeolites beta [29]. Introduced zeolite A in detergents as an alternative for environmentallysuspect phosphates [30]. Synthesised aluminophosphate material and introduced new elements into the zeolite framework [31]. 26

27 Chapter Zeolite applications The applications of both natural and synthetic zeolites have increased significantly in many fields over the last two decades [32]. Many types of zeolites are in great demand in the global market, and the annual production of these synthetic and natural minerals has been estimated at approximately 1.8 and 5.5 million ton/yr, respectively. Among all types of zeolites, a huge commercial interest has developed in 9 molecular sieve structures that have been produced synthetically, i.e., LTA, FAU, MFI, MOR, BEA, FER, LTL, CHA, EDI [9]. The majority of synthetic zeolite production has been directed towards the detergent industry, odor removal, and plastic additives. Nevertheless, zeolites have been used extensively in the petrochemical-related industries as major catalytic components and as an octaneenhancing additive in the fluid catalytic cracking (FCC) process. Zeolites also have been utilized in adsorption and separation processes [33]. In addition, zeolites have been used in the electronic field and in ceramic compositions, and both areas show promise for continued market growth. Zeolites also have made important contributions to the efforts to mitigate environmental pollution, with zeolites with high silicon to aluminum ratios being used to remove volatile organic compounds (VOC) from waste streams [34]. Figure 2.2 shows the current relative percentages of zeolite usage by category. 27

28 Chapter 2. 13% other 8% adsorbents 70% detergents 9% catalysts Figure 2.2: The major estimated usage of zeolite in the industrial applications [9]. 2.4 Zeolite Structure As mentioned earlier, all types of zeolites consist of primary building units (PBUs) of tetrahedral shapes created from [SiO 4 ] 4- and [AlO 4 ] 5-. These tetrahedra are assembled to form what is known as secondary building units (SBUs). These SBUs lead to the formation of pores or rings (oxygen windows) that range from 0.3 to 0.9 nm in diameter and the framework for a given type of zeolite. The pore structure is of great importance because it affects the adsorption properties in terms of sieving selectivity and capacity, thereby affecting the applications in which the products can be used [7, 35]. Figure 2.3 shows the assembly of the final framework structure. The International Zeolite Association (IZA) has assigned three-letter Framework Type Codes that designate the specific structures of various zeolites. For example, Linde Type A zeolite is identified as LTA, faujasite zeolite is identified as FAU, and MFI. Many studies have indicated that the type of framework of a zeolite has a significant effect, defining the structure and properties of the zeolite. Therefore, scientists noticed very early that determining the framework structures of zeolites 28

29 Chapter 2. was of fundamental importance in understanding their physical and chemical characteristics [13]. A zeolite s framework is defined on the basis of the connectivity of its tetrahedral framework, including cages and subunits, such as α- cavity, β-cage, and pentasil units, as shown in Figure 2.3. Thus, the pore structure, the channel system, the volume of the cages, and their arrangement can be defined from the framework structure. In other words, the main properties of zeolites, i.e., ion exchange, catalytic activity, and sorption capacity, can be determined from their framework structures [9]. In the ion exchange process, selectivity depends on the number of cation sites, catalytic activity depends on the dimensions of the channel system and H +, and sorption properties depend on the size of the pores. However, chemical compositions are important for the elucidation of other specific properties. Therefore, many analytical techniques are used to investigate the crystalline structure of zeolites, such as solid state nuclear magnetic resonance (NMR), sorption experiments, powder diffraction, and electron microscopy [9,29]. 29

30 Chapter 2. (a) PBU (e) (d) (c) (b) SBU (f) (g) (h) Figure 2.3: (a) primary building unit (b) double 6-ring (c) pentasil unit (d) double 4- ring (e) β-cage (f) LTA framework type (g) SOD framework (h) FAU framework. 30

31 Chapter 2. LTA (Linde Type A) [36, 11, 37] The LTA framework can be expressed simply as a cubic arrangement that includes α-cages (Figure 2.4) that are attached through a single, eight-ring channel system. In this framework, β-cages are arranged and joined by four-ring sub-units, which form a three-dimensional system of α-cages in the center of the unit cell (Figure 2.4). The LTA framework includes numerous materials, e.g., zeolite A, Alpha, ITQ-29, LZ-215, SAPO-42, ZK-21 and ZK-22ZK-4. LTA, which includes eight-ring pores (free diameters of nm) with a framework density of 12.9 T atoms/1000å 3, has been used extensively in sieving and ion exchange separation applications. (a) (b) β-cage (sodalite cage) 4.1 α-cage (super cage) Figure 2.4: (a) The LTA framework type, (b) Illustration of zeolite A pore size. 31

32 Chapter 2. FAU (Faujasite) [11, 37] The FAU framework type consists of β-cages and double six-ring, as shown in Figure 2.5. In this framework type, the β-cages are arranged and joined to form a three-dimensional system of α-cages in the center of the unit cell through double six-rings. The FAU framework includes large pore voids of twelve-rings ( nm) with a framework density of 12.7 T atoms/1000å 3, which is even less than that of the LTA framework. Many materials have been classified within the FAU framework, e.g., CSZ-1, ECR-30, LZ-210, Li-LSX, SAPO-37, Siliceous Na-Y, ZSM-20, ZSM-3, Zeolite X, Zeolite Y, and Zincophosphate X. The properties and pore sizes of the FAU framework allow the materials to maintain thermal stability, which makes the FAU structure applicable for many catalytic applications. (a) (b) β-cage (sodalite cage) 7.4 double 6-ring Figure 2.5: (a) The FAU framework type, (b) Illustration of zeolite Y pore size. 32

33 Chapter 2. MFI (ZSM-5) [38, 9] Because of the number of atoms in the asymmetric unit of the MFI framework type, it is considered to be the most complex zeolite ever known. MFI can be expressed generally in terms of [5 4 ] units, and they are linked to create pentasil chains. These chains are connected by oxygen bridges to create a three-dimensional structure of wavy sheets with ten-rings with pores of ( Å), as shown in Figure 2.6. Zeolite Socony Mobil (ZSM-5), Silicalite, and ZKQ-1B are the most important types of this framework category, and they have been used in many commercial applications. MFI has several applications in refinery and petrochemical processes in which the normal, three-dimensional structure of MFI can be applied for many separation processes and acid-catalyzed reactions, such as hydrocarbon isomerisation. (a) (b) Pentasil unit Figure 2.6: (a) The MFI framework type, (b) Illustration of ZSM-5 pore size. 33

34 Chapter 2. HEU (Heulandite) [39, 40] HEU is a framework formed by (bre) building units (Figure 2.7). Clinoptilolite and Heulandite are the most common zeolite types of this framework, and they have been used extensively in many industrial, biological, and agricultural applications due to their strong adsorptive and ion exchange properties. Clinoptilolite zeolites are considered to be the most abundant and common minerals formed from natural conditions, and they can be easily found in sedimentary rocks. The structural shape of clinoptilolite zeolites consists of three channels, with one channel having a tetrahedral ring in the range of Å, while the other two are in the size range of Å and Å bre building unit Figure 2.7: The HEU framework type, (b) Illustration of Clinoptilolite pore size. 34

35 Chapter Properties of Zeolites The properties of zeolites are classified into physical and chemical properties. Their physical properties (e.g., density and pores size) are determined in nature when alkaline materials react with ash layers in igneous rocks. Their resulting chemical properties (e.g., chemical stability, ion exchange, and ph) determine the applications in which they can be used [41] Aluminosilicate Ratios in Molecular Sieves [42, 7, 43] Zeolites possess a range of uniform pores with molecular-sized dimensions created by the three-dimensional framework structure. The uniformity of the pores allows these minerals to act as sieves by excluding large molecules and adsorbing molecules that selectively fit into them. Also, zeolites possess a broad range of silicon to aluminum ratios (Si/Al). In general, there are three categories of Si/Al ratios in zeolites, i.e., low Si/Al zeolites (1-1.5); intermediate (1.5-7); and high (8 - ). Zeolite structures with low Si/Al ratios are mainly formed with four to eight tetrahedral rings. The intermediate and the high silicon to aluminum ratios are usually formed with ten tetrahedral rings. The Si/Al ratio is an important determinant of the properties of zeolite materials. For example, zeolites with low and high ratios are thermally stable at temperatures of C and C, respectively. Similarly, the acidity of zeolites increases as the Si/Al ratio increases. However, the surfaces of zeolites are highly hydrophilic at low ratios, and they are hydrophobic at high ratios. Ion exchange capability and cation concentrations decrease as the Si/Al ratio increases. Table 2.2 shows the kinetic diameter, silicon to aluminum ratios, and framework types of some types of zeolites that been used in industrial applications. 35

36 Chapter 2. Table 2.2: Pores kinetic diameter, silicon to aluminum ratios and framework type of selected zeolite types [13]. Zeolite Type (Si/Al) Ratio (Si/Al) Category *Pores Kinetic Diameter (Å) Framework Types A-Na 0.7 to 1.2 low 4.1 LTA Analcime Intermediate 2.6 ANA Beta 13 High BEA Brewsterite Intermediate 2.6 BRE Clinoptilolite Intermediate HEU Epistilbite Intermediate 2.6 EPI Gismondine low 2.6 GIS Gmelinite Intermediate 4.3 GME Harmotome Intermediate 2.6 PHI Heulandite Intermediate 2.6 HEU Laumontite Intermediate 2.6 LAU Mesolite Intermediate 2.6 NAT Mordenite Intermediate 3.9 MOR Mordenite (Large port) Intermediate 6.2 MOR Natrolite low 2.6 NAT Omega Intermediate 10 MAZ Phillipsite Intermediate 2.6 PHI Silicalite High 5.6 MFI Stilbite Intermediate 3.3 EPI Zeolite L Intermediate 8.1 LTL Zeolite X low 8.1 FAU Zeolite Y Intermediate 8.1 FAU ZK low/ Intermediate 4.3 LTA ZSM High 5.6 MFI * The indicated pore size range is due to the fact that the surfaces of some types of zeolites do not behave as well-defined walls and become flexible at high temperature. 36

37 Chapter Zeolite Catalysts [44, 30] Zeolites possess a very large internal surface area that has been estimated to be at least 20 times greater than the external surface area. Thus, many studies have shown that zeolites can be used as catalysts for chemical reactions that take place in their internal cavities. In 1959, zeolites were first used in catalysts when the Union Carbide Corporation used zeolite Y as an isomerisation catalyst. In 1962, Plank and Rosinski concluded that small amounts of zeolite will influence silica-alumina catalysts, consequently improving their performance in petroleum-cracking processes. Currently, most of these catalysts are used extensively in catalytic cracking processes, in which they act as acid-cracking catalysts. Generally, the catalytic process takes place inside the framework controlled by the free diameters of the windows (rings of framework oxygen atoms). This catalytic process is considered to be a diffusion-limited process that will admit certain reactant molecules to produce selected products. This selectivity process is known as shape selective catalysis (Figure 2.8), in which heterogeneous reactions can take place in the large internal surface area. Currently, the main commercial uses of zeolite catalysts are for catalytic cracking process, hydrocracking, selectoforming, hydroisomerization, xylene isomerization, and conversion of methanol to gasoline. Figure 2.8: Product selectivity in the alkylation process of toluene to p-xylene. 37

38 Chapter Ion Exchange [7,41,45] The ion exchange properties of many materials have led to interest because these properties are important in numerous applications in many different fields. Many types of crystalline minerals have good ion exchange properties, including clay minerals and feldspathoids (tectosilicate minerals). However, many studies have shown that zeolite frameworks do not change during the ion exchange process, because of the three-dimensional structure. Conversely, clay minerals, which have a two-dimensional structure, may shrink or swell during the ion exchange process. Thus, this property is vitally important in the commercialisation of zeolite minerals. The concept of this property can be expressed by the replacement of cations that exist in the aluminosilicate framework by other ions from an external solution. The ion exchange process can be functionalised either by cation exchange or by anion exchange. The cation exchange process involves the exchange of positively-charged ions, while the anion exchange process involves the exchange of negatively-charged ions. Many studies have shown that the behaviour of the ion exchange process depends upon the size, charge, and structure of the ions as well as the operational temperature. 2.6 Zeolite Powder Synthesis Zeolites are created under the influence of the hydrothermal conditions, where the crystallization and nucleation processes take place. The first attempt to create zeolites was conducted in 1845 by Schafhautle, who conducted his experiment using an autoclave to synthesise quartz by heating silica gel and water [9]. The first synthesis process of a zeolite was reported by Sainte-Claire Deville in Deville successfully synthesised levynite by hydrothermal treatment for the first time using 38

39 Chapter 2. an aqueous solution of potassium silicate and sodium aluminate and heating them in glass tubes [19]. In 1882, another zeolite synthesis was achieved by Schulten, who reported the synthesis of analcime. In 1940, Professor Barrer et al successfully achieved analcime synthesis and conducted the first characterization techniques using x-ray diffraction [46]. After that period, the synthesis of many types of zeolites was conducted in many studies and reported in the literature. The authors discussed many properties of these minerals, e.g., adsorption, molecular sieving, and ion exchange. These studies suggested that hydrothermal processes are the starting point for many synthesis preparations [47]. The early zeolite synthesis process is shown in Figure 2.9. Zeolite synthesis processes have been studied extensively and reported in many publications in the literature using several techniques with different operating conditions. The most notable techniques used in this respect are hydrothermal synthesis, solvothermal synthesis, and ionothermal synthesis [28]. Hydrothermal Synthesis The term hydrothermal process first was used in 1792 by Sir Roderick Murchison, a British geologist, to describe the behaviour of water at elevated pressure and temperature. The process can be defined as a crystal creation process in aqueous solution at elevated pressure and temperature that takes place in a pressure vessel known as an autoclave (Figure 2.10). The hydrothermal term refers to a reaction that involves aqueous solution at elevated temperature and pressure. Researchers suggested that the process takes place with conditions above 100 o C and 1 atm, but there are no limits for the operation conditions. The purpose of these conditions is to dissolve material that will not dissolve under ordinary conditions by creating high solvation power and compression, which lead to many benefits, e.g., crystal 39

40 Chapter 2. formation and growth of some inorganic compounds, formation of materials with defined structure (pore size and morphology), and transformation to a new phase of complex materials. The procedure of this technique includes mixing the source materials, during which the initial aluminosilicate homogenous gel is formed. Then, the homogenous gel is maintained at certain conditions at which the ageing period occurs. Afterwards, the homogenous gel is transferred into a sealed autoclave at specific conditions to conduct the crystallization process [48]. Alumina Alkali Hydroxide Ageing process Aluminosilicate gel Crystallization process Zeolite Silica Figure 2.9: Schematic representation of zeolite synthesis procedure. Solvothermal Synthesis [49, 28] The solvothermal synthesis technique is similar to hydrothermal synthesis processes, and it also is conducted in an autoclave. The main difference between these two techniques is the presence of organic solvents as reaction media rather than aqueous media. This technique first was used in 1985 by Bibby and Dale, who synthesized an all-silica form of sodalite (SOD) using ethylene glycol and propanol as the synthesis media. 40

41 Chapter 2. Ionothermal Synthesis [50] In 2004, Morris et al developed a new technique in which ionic liquids were used as the synthesis media rather than aqueous or organic media. This technique is known as ionothermal synthesis. Ionic liquids can be defined as salts with melting point below 100 o C and with high solvating property. One of the main advantages of the ionothermal technique is controlling the pressure with highly-elevated temperatures, due to their low autogenous pressure. Spring Stainless steel Lid Aqueous solution Teflon -lined Figure 2.10: Illustration of the autoclave used in the hydrothermal synthesis. 41

42 Chapter Zeolite Precursor and Source Materials Basic reactants and templates are used as starting components for synthesising zeolites. In the basic reactants, an aluminium source, a silicon source, and water are generally used as the reactants for the synthesis process [13]. In this process, as mentioned earlier, the zeolite framework depends mainly on the silica and alumina in building the primary units of the structure, while the water acts as the solvent during the synthesis process. Generally, water is used in these processes because it is capable of dissolving most of the ionic compounds that are used at the conditions of the process. The low viscosity of water makes it a good medium in which the reaction can take place, resulting in metastable phases with considerable crystal growth and various shapes [52]. However, some compounds are insoluble in water, even at supercritical conditions. Therefore, mineralizers, such as sodium carbonate, sodium borate, and sodium sulphide, are added to enhance the solubility of the solute. As mentioned earlier, this process is conducted using a pressure vessel (autoclave), because the temperature and pressure have major influences on the formation and growth of crystals. Many substances have been used extensively as sources of silica, e.g., alkali silicate solution (e.g., sodium silicate), colloidal solution, fumed silica, tetraalkylorthosilicate, and some minerals (e.g., kaolin and clays). Alumina sources can be provided by alkali aluminate solution (e.g., sodium aluminate), aluminium sulfate solution, and hydrous aluminium oxides (e.g., pseudo boehmite, aluminium alkoxides). In the chemical industry, numerous source materials are used to integrate the basic reactant of the synthesis preparation, e.g., templates. There are two categories of templates in the zeolite industry, i.e., organic and inorganic templates, and both are used to enhance the desired zeolite structure. The importance of the templates is evident in the crystallization process and in the 42

43 Chapter 2. final structure formation of the zeolite. Many reports have indicated that both aqueous alkaline solution and templates are very important in zeolite structure formation. Templates lead to a process that occurs during nucleation to provide the initial building blocks of the structure by allowing both organic and inorganic species to organise the oxide tetrahedra into a specific geometrical topologies around themselves. There are many types of templates including cations (e.g., Li +, Na +, K + and Ca + ) and organic templates (e.g.,tetrapropyl ammonium). The selection of the template to be used usually depends on the ability to remove the template without affecting the structure of the zeolite [51] Ageing process [53] Ageing is the period between the end of the preparation of aluminosilicate gel and the crystallization process to increase the crystal growth by increasing the number of nuclei present in the aluminosilicate gel. In other words, the ageing period allows the aluminosilicate gel to reorganize structurally and chemically, which leads to an increase in the nucleation sites necessary for zeolite nucleation. Slangen et al. investigated the effect of ageing time on the microwave synthesis of zeolite NaA [53]. They concluded that the short ageing time of five minutes led to an amorphous material, whereas complete synthesis of zeolite NaA was achieved successfully after an aging time of 20 hours by increasing the degree of crystallization Crystallization process [54,55,56] Crystallization can be described as a process in which solid crystals are formed from the gel that consists of the source materials. In this process, the molecules in the solution are transformed and arranged in a defined and organised pattern of crystals that can be controlled by the operating conditions. The crystallization 43

44 Chapter 2. process includes two major stages, i.e., the formation of crystals (nucleation) and the growth of the crystals. The nucleation process is initiated when unstable nuclei form in the solution of source materials, and these nuclei become larger and more stable over time, forming the crystals. There are two classifications of the nucleation step, i.e., heterogeneous and homogeneous mechanisms. Many studies have expressed the fact that homogenous nucleation is preferable over heterogeneous nucleation, despite the fact that heterogeneous nucleation occurs more commonly. The homogeneous mechanism, which is more difficult to achieve than the heterogeneous mechanism, refers to the coverage of this process through the surface and the interior structure of the substance. Conversely, the heterogeneous nucleation takes place at preferential sites, and it occurs more often than the homogeneous mechanism. The nucleation step is of a great importance, because crystal growth cannot be achieved without it. In other words, crystal growth can take place at any temperature below the melting point as long as nucleation exists. The operating conditions during the synthesis process have a major role in the final formation of the desired zeolite structure. In general, there are many common conditions that have been highlighted for the majority of zeolite types, i.e., aging period, crystallization period, temperature, and the operational pressure. In 2008, Bronić et al. conducted a study to investigate the mechanism of zeolite A crystallization. They concluded that both the aluminosilicate gel and the crystalline phase occurred in different amounts during the crystallization process and that the amorphous aluminosilicate was dissolved completely after its formation, depending on the crystallization temperature. Thus, the crystallization time depends mainly on the crystallization temperature, because it affects the mechanisms of nucleation and 44

45 Chapter 2. growth. Generally, the crystallization time decreases as the operating temperature increases. Therefore, there is an inverse relationship between the crystallization period and the operating temperature. In this work, the literature mentioned in this chapter will be utilized towards the synthesis of zeolite powder and membranes. As mentioned earlier, hydrothermal synthesis has no limits for its operating conditions. Therefore, both the ageing and crystallization processes will be tested at different intervals for zeolite A synthesis, presented in chapter 5. Moreover, the investigation of the synthesis of pure zeolite A using different source materials, including commercial and virgin sources, will be considered and evaluated as well. 45

46 Chapter 3. Chapter 3 Membranes 3.1 Introduction Membranes and their processes are not a human invention but they exist in our daily life and play major roles in many natural processes. Due to their importance, synthetic membranes have been developed for use in chemical technology e.g., water treatment, artificial organs and petrochemical industries (both liquid and gas phases) [57]. The vast majority of membrane technologies share the common advantages of energy conservation and are proven to have a low environmental impact. These advantages have attracted considerable interest among researchers, especially because of the excellent performance of these membranes over other convenient separation technologies such as, adsorption, distillation, extraction, flashing, gravitational and recrystallization processes [58]. Nevertheless, the longterm reliability of many membranes has not yet been established and requires further pre- or post treatments due to the influence of many factors including the concentration polarisation that affects the performance of these membranes, especially in the petroleum chemical-related industries [59]. Membranes with different properties, structural shapes or morphologies exist and currently, membrane properties can be synthetically adjusted and tailored by manipulating the synthesis operation conditions, to suit a given separation task. The selectivity of a given membrane depends mainly on two factors; the pores size uniformity (poreflow model) and diffusion rate difference [60]. However, the complexity of many different membrane structures and functions are still the subject of speculation, which makes the entire clarity of the definition of membranes and their functions 46

47 Chapter 3. rather difficult [61]. Nevertheless, membranes can be defined as a thin-layer barrier, consisting of either natural or synthetic material that separates two different regions in a selective manner, while restricting the transportation of different chemicals [62]. The structure of the layer can be solid or liquid, symmetric or asymmetric, and homogeneous or heterogeneous. A membrane can be negatively or positively charged, neutral, or bipolar. The thickness of a membrane may vary from a fraction of a micrometer to a few hundred micrometers [58]. In addition to the importance of membrane properties, the driving force for a given process plays a major role in determining the separation efficiency in terms of selectivity and flux. The driving force used in membrane processes may be pressure difference, temperature, electrical potential or concentration gradient across the membrane [63]. The wide range of membrane structures incorporated with different driving forces results in a number of different membrane processes, including, nanofiltration, ultrafiltration, microfiltration, reverse osmosis, pervaporation, dialysis, electrodialysis, gas separation, membrane distillation, membrane solvent extraction and membrane reaction. Currently, membrane processes are used in three main industrial areas i.e., water-related industries, molecular separation mixtures and artificial organ engineering [5]. In water-related industries, membranes are used extensively in desalination and purification processes and have the advantage of economy over the other convenient separation technologies. Separations processes, which exist in the petrochemical, food, and drugs industries, use membranes as clean technology. Membranes have surpassed other technologies in the area of artificial organ engineering, where there is no realistic alternative to membranes to date [61]. In all of the above mentioned applications, membrane performance is considered to be satisfactory and has been proven by long history of operating 47

48 Chapter 3. experience (e.g., reverse osmosis for water desalination processes). On the other hand, other separation processes such as gas separation, pervaporation and fuel cell separators are the subject of further improvements. Currently, there is a huge growth in petroleum chemical-related industries and this is being extensively explored by many researchers due to the huge market and business demand in this field [58]. 3.2 Historical Background The membranes industry has developed rapidly over the last four decades and many studies have been conducted to investigate how membranes can fulfil different tasks. The very first discovery in the phenomenon of a natural membrane was made in 1752 by a French physicist called Jean-Antoine Nollet, where he observed the relationship between membrane permeability and osmotic pressure in a pig bladder. Subsequently, many scientist and researchers conducted studies on membranes in many areas for many different applications, as shown in Table 3.1. Table 3.1: Summary of the studies and developments relative to membranes. Year Scientists / Organizations Contribution 1752 Jean-Antoine Nollet 1829 Thomas Graham 1850 Pfeffer et al 1855 Adolf Eugen Fick First to recognise the relationship between permeable membranes and osmotic pressure [64]. Assessed the rate of gas permeation and introduced Graham s law of diffusion [61]. Studied osmotic phenomena using ceramic membranes [61]. Deduced the diffusion equation in liquids as a function of concentration gradients [65]. 48

49 Chapter Thomas Graham 1864 Moritz Traube 1877 Wilhelm Pfeffer 1887 Van t Hoff 1888 Nernst 1890 Planck 1907 Bechhold H Frederick George Donnan Studied gas diffusion through different media and rubber permeability [66]. Introduced the first preparation of an artificial semipermeable membrane [67]. Developed a semiporous membrane to conduct fundamental studies of osmosis and mass transport [68]. Elucidated the thermodynamic aspects of the osmotic pressure of dilute solutions [69]. Introduced the flux formula that relates the concentration gradient to the electric gradient [70]. Developed the flux formula for electrolytes with relation to concentration gradient and the electric gradient [71]. Fabricated the first ultrafiltration membranes using nitro cellulose and coined the term ultrafiltration [72]. Introduced the relationship between membrane equilibria and potentials in the presence of electrolytes [73] Kober Coined the term pervaporation [74] Richard Zsigmondy 1926 Membranfilter GmbH 1927 Richard Zsigmondy Used nitrocellulose membranes for macromolecules/aqueous solution separation [75]. Commercialised collodion microfiltration membranes [58]. Developed microporous membranes for filtration processes [75] A.G. Horvath Coined the term reverse osmosis and patented its process [76]. 49

50 Chapter Sartorius GmbH 1944 Kolff 1950 Binning et al Produced nitrocellulose membranes with a wide range of pore sizes [61]. Introduced the large scale application of membranes in the biomedical area, due to the successful development of a functioning hemodialyser [77]. Published the first systematic study of the pervaporation process at American Oil [58] Staverman, Kedem and Schlögl Described the membrane transport properties comprehensively, based on thermodynamic processes. [78] 1958 K. S. Spiegler Studied the ion-exchange properties of membranes [58] Reid and Breton 1963 Loeb and Sourirajan 1963 General Atomics 1966 Ulrich Merten 1967 Du Pont 1968 Aptel et al 1970 Gelman 1972 Cadotte et al Developed the reverse osmosis membrane based on cellulose acetate, resulting in high fluxes with high salt rejection [79]. Developed a defect-free and high flux membrane (asymmetric cellulose acetate) for reverse osmosis processes [80]. Developed the first spiral-wound module membrane for reverse osmosis processes [58]. Described the membrane transport properties by postulating certain membrane transport models [81]. Commercialised the first hollow fibre module membranes for reverse osmosis processes. [82] Studied the basic principles of pervaporation [83]. Introduced the plated membrane cartridge [84]. Developed interfacial composite membranes [85]. 50

51 Chapter Late 1980s 1982 Henis J. M. Gesellchaft fur Trenntechnik mbh (GFT) Gesellchaft fur Trenntechnik mbh (GFT) Established the gas separation process [86]. Introduced commercial pervaporation systems for alcohol dehydration processes [87]. Constructed the first commercial pervaporation plant for ethanol dehydration processes [84] Dow Generon 1986 Nitto Denko Introduced and commercialized a nitrogen /air separation system [61]. Developed nanofiltration for low pressure fluids systems [88]. 3.3 Membranes structures [58, 61] The key success of any membrane separation process relies on the membrane itself, which represents the most important part of the entire process. Membranes have many different forms and possess different structures, functions and transport properties. Generally, the methods of fabricating membranes lead to these different structures. The manufacturing process can vary from a simple sinter technique to other, more complicated, methods such as irradiation and inversion methods. Strathmann reported that membranes can be classified into many different categories. The first noticeable classification of membranes is according to whether they are synthetic or biological. In addition, the morphology of membranes is of great importance in determining their application, and many studies have been conducted to determine structural classifications of membranes. Structurally, membranes are usually classified as either symmetric (isotropic) or asymmetric (anisotropic). Symmetric membranes can exist as porous, nonporous (dense membranes) or electrically charged membranes. The properties of symmetric 51

52 Chapter 3. membranes, such as the transport rates, are identical over the entire cross section as the membrane has a completely uniform structure. On the other hand, asymmetric membranes normally consist of at least two different layers, each with different thickness. In this type of membrane, the process of synthesising extremely thin layers is achievable, as one layer provides the required mechanical strength to support the other thinner layer. Many studies have employed these composite layers and revealed that the thickness of these membranes plays a major role in determining the mass transport and flux, where the transport rate increases with decreasing the thickness of a given membrane. In general, the thin layer in asymmetric membranes is 0.1 to 30μm thick, while the thickness of the supporting layer that provides the mechanical strength is usually above 200 micrometers (μm). In general there are three types of membranes; organic, inorganic, and hybrid. In industrial processes, mainly organic membranes are used. These are made of natural polymers (such as rubber and cellulose) or synthetic polymers (such as polyamide, polystyrene, and polytetrafluoroethylene (Teflon)). Membranes composed of inorganic or non-polymeric materials (for example, metals, ceramics, and zeolites) are also used. Recent years have seen the development of a combination of these two types of membranes, resulting in organic-inorganic or hybrid membranes. Figure 3.1 shows a schematic drawing of the main morphologies and materials used in the membrane synthesis industry. 52

53 Chapter 3. Membrane Structure symmetric (isotropic) porous dense electrically charged Asymmetric (anisotropic) thin-film composite supported liquid Figure 3.1: Schematic diagram of membrane morphologies and structures. 53

54 Chapter Porous membranes [61, 89] Porous membranes can be defined as a layer that consists of a highly voided, firm structure with a pore size of less than 10 μm. The main application of these membranes is in micro-filtration processes, where they remove contaminants from a desired fluid that can be either gas or liquid, by rejecting large particles or molecules and allowing particles that selectively fit into their pores to pass. The structure of this type of membrane can take several forms including flat, sheet, normal fibres and hollow fibres. This type of membrane was established in 1927 by Richard Adolf Zsimondy and was commercialised and introduced to the market a few years later in the water treatment and microbiology fields Membranes of molecular sieve materials and dense phases [90, 58] This type of membranes (e.g., zeolite and carbon membrane) consists of small pores (0.01 to 20 Å) in which the transport phenomenon takes place by diffusion under the driving force presence (for example, pressure, temperature, or concentration gradient). In this type of membrane, the diffusivity and solubility properties play major roles in determining the transport rate of a component, causing the separation behaviour to occur. Consequently, this technique is suitable for separating fluids with a difference in the size of their molecules. Therefore, microporous materials are considered as one of the best techniques used in separation processes where molecular sizes are close, and used extensively in processes like; gas separation and pervaporation systems. 54

55 Chapter Electrically charged membranes [91] Charged membranes can be defined as porous or dense membranes carrying charged ions, usually made from polymeric materials, and have been used extensively in electrodialysis, electrolysis and fuel cells for many industrial tasks. Many studies have revealed that this type of membrane can be classified into three main categories; positively charged membranes, negatively charged membrane and bipolar membranes. Positively charged membranes are referred to as anionexchange membranes as they bind to anions in the surrounding fluids, and are usually made from polymers that possess amine groups, such as quaternary ammonium polyelectrolyte and polethyleneimine materials. However, positively charged membranes are less preferred than other types of membrane, as they are easily blocked by negatively charged colloidal materials, especially in water softening processes. Negatively charged membranes are also known as cationexchange membranes and are normally made from polymers that have sulphonic acid groups (for example, sulphonated polyethersulphone). They are used extensively for separating elements with multivalent ions. Bipolar membranes consist of both anionic and cationic phases and can be described as a combination of both positively and negatively charged membranes. The most common method of preparing bipolar membranes is by incorporating polymers with sulphonic acid groups into positively charged membranes through an adsorption process. The concept of this process is influenced by the ion charge and concentration in the solution Thin-film composite [80] As indicated earlier, the thickness of a membrane deeply influences the transport rate and flux of components through it. Therefore, this type of fabrication 55

56 Chapter 3. membranes which consists of thin surface layer supported by a thicker, porous base was a remarkable milestone in the separation process industries. This type of membrane was fabricated by Sidney Loeb and Srinivasa Sourirajan in the early 1960s, when they successfully fabricated an ultrathin defect-free layer supported by a thick microporous structure. These membranes were applied to reverse osmosis processes and showed selective performances with high fluxes (ten times higher than the best membrane available at that time). This was one of the main pillars in the development of other types of membranes Liquid membrane [92] For the last 30 years, liquid membranes have been the subject of exploration for many scientists as the diffusivity in the liquid phase is higher than the solid. Liquid membranes basically consist of liquids, such as thin oil film, and used as a selective barrier between two regions of gases or solutions. They have also been used in ion separation. Liquid membranes are highly selective but are less stable than other types of membranes. These membranes can be either supported or not. There are three configurations of supported liquid membranes i.e., immobilised liquid membranes (ILM), supported liquid membranes (SLM) and contained liquid membranes (CLM). On the other hand, non-supported membranes can be divided into bulk liquid membranes (BLM) and emulsion liquid membranes (ELM). ILMs are considered to be the most compact form of liquid membrane types and operate by capillary force, where the liquid is located inside the support s pores. CLM and SLM share the same function but each has a different form and configuration. Where the CLM has porous supports on each side, the SLM comes with porous supports only on one side. The main advantage of CLM and SLM is the ability to replace and regenerate the phase of the membrane while it operates. Therefore 56

57 Chapter 3. membrane breakdown can be avoided using this type of liquid membrane (Baker, 2004). BLM is considered by many reviewers as the simplest form among other types of liquid membranes and has been used to study mass transfer behaviour, but has not been applied to large-scale processes due to their large thickness. ELM was invented by Li in 1968 and is used in contaminate extraction processes including, metals, inorganic species and acids. This type of membrane is being used widely in industrial waste treatment as they acquire high mass transfer interfacial areas and require small quantities of organic solvents. In this membrane, both extraction and stripping functions are combined. However, a main disadvantage of this type of liquid membrane is that they require a high level of stability to perform properly. 3.4 Membrane units and operations [61,93] As mentioned earlier, membrane separation processes have different operating principles and areas of application. However, the majority of these principle share the common characteristic that both the membrane properties and driving force determine the overall separation efficiency in a given separation process. Therefore, membrane separation process units are classified according to the operated driving force as listed in Figure 3.2 and the pore size of the most common established units as presented in Figure 3.3. In this section, only the pressure driven processes will be discussed as they are related to the scope of this study. 57

58 Chapter 3. Classifications of separation operations units reverse osmosis ultrafiltration Pressure driven process microfiltration nanofiltration pervaporation gas separation Concentration driven process process Electric potential process dialysis osmosis forward osmosis electrodialysis electrophoresis Figure 3.2: General classification of membranes separation process units according to the operated driving force. 58

59 Pore diameter (Å) Chapter 3. Microfiltration 1000 Pore-flow membrane ( 20 Å) 100 Ultrafiltration 10 1 Nanofiltration Finely microporous Pervaporation Reverse Osmosis Gas separation Intermediate pore-flow solutiondiffusion membrane (8-20 Å) Solution-diffusion ( < 8 Å) Figure 3.3: Illustration of membrane separation processes pore size and theoretical model principal. 59

60 Chapter Reverse Osmosis [94, 95] Reverse osmosis can be defined as a filtration technique that separates molecules and ions from solutions and mixtures by exerting pressure on the mixture on one side of a selective membrane. It is a technique mainly used in water desalination and purification processes. The name of this process came from the definition of the natural processes of osmosis, where the solvent normally tends to move from a region of low concentration to a region of higher concentration to equalise the overall concentration. Therefore, the osmotic process is reversed when applying an external pressure to the system to allow the pure solvent to pass while the solute is retained. This type of membrane operation was first described in the early of 1850s, when Pfeffer et al., studied the osmotic phenomena using ceramic membranes. The process was patented in A typical reverse osmosis system is illustrated in Figure 3.4. Pressure gauge P Water feed Flow meter Pump Membrane test cell Filtrate product Figure 3.4: Flow schematic laboratory scale reverse osmosis test system. 60

61 Chapter Ultrafiltration [96, 97] In this technique, separation is achieved using porous membranes with average pore sizes ranging from 50 to 1000 Å. Ultrafiltration and microfiltration can be considered as related processes, as the main difference between them lies in the pore size. Ultrafiltration has been widely applied for the separation of water and microsolutes from macromolecules and colloids. Generally ultrafiltration comes with three main geometries i.e., spiral wound module, tubular membrane and hollow fibre membrane. The first attempt at synthesising ultrafiltration membranes was in 1907, when Bechhold produced the first ultrafiltration membrane using nitro cellulose and then coined the term ultrafiltration. Currently, most ultrafiltration membranes are made using the Loeb-Sourirajan process with different materials including, polyacrylonitrile, polyacrylonitrile copolymers, polysulfone and cellulose acetate. In general, ultrafiltration units are too expensive to be used in large scale wastewater treatment applications unless the product value offsets the process cost. However, ultrafiltration is widely used in the treatment of concentrated waste streams before they are sent into large contaminated streams. For laboratory scale experiments, two types of system sets are used i.e., stirred batch cells or flowthrough cells (Figure 3.5). The stirred batch system is generally used in short experiments, while the flow-through cells (recirculation) system is used for regular experiments and is preferred, as the composition of the feed solution can be easily maintained. 61

62 Pressure source Chapter 3. Retentate Ultrafilration unit Filtrate product Bleed Ultrafiltration cell Pressure gauge P Heat exchanger Filtrate product Pump Figure 3.5: Ultrafiltration laboratory scale units (a) stirred batch cell (b) flowthrough cells. 62

63 Chapter Microfiltration [61, 98] Microfltration takes place between ultrafilitration membranes and conventional filters, as their pores are larger than ultrafiltration membranes and smaller than conventional filters. In general, these membranes are used in the separation of particles of 0.1 to 10μm in diameter. This type of membrane was first used in 1926 by Membranfilter Gmbh, who commercialised collodion microfiltration membranes. Microfiltration membranes are usually made from cellulose acetate and noncellulosic materials such as, polyamides, polyolefins and poly (tetrafluoroethylene). In 1970, Gelman introduced the pleated membrane cartridges, which was a breakthrough in this industry and allowed this type of membrane to be used for large scale applications. Currently, microfiltration processes have many applications in different industries including biological, pharmaceutical manufacturing, electronic processes and removal of trace contaminants/particles from solutions or wastewaters. In general there are three different operating systems of microfiltration membranes i.e., in-line filtration (dead-end), cross-flow filtration system and semi-dead-end filtration (Figure 3.6). The process of the in-line filtration (dead-end design) can be described as forcing the entire feed fluid flow through the membrane under a high pressure driving force. This system requires only simple equipment, but the membrane durability is limited due to the accumulation of particles on the membrane surface. On the other hand, in the cross-flow system the feed fluid flow is circulated around the membrane allowing one stream to pass (permeate). The equipment required for this system is far more complicated than for the in-line filtration system, however the durability of the cross-flow system is much better. The advantages and disadvantages of in-line filtration and cross-flow systems are 63

64 Chapter 3. listed in Table 3.2. The third operating system, semi-dead-end filtration, takes place somewhere between the two previously mentioned systems. Here, the system is operated in in-line filtration system mode, until the membrane becomes blocked by accumulating particles, the semi-dead-end filtration system switches to the crossflow system mode. (a) (b) Feed Feed Retentate Permeate Permeate Figure 3.6: schematic illustration of (a) in-line filtration and (b) cross-flow filtration using microfiltration membrane. Table 3.2: Comparison of the advantages and disadvatages of the two microfiltration systems. In-line Filtration Low capital cost Cross-flow Filtration High capital cost Membrane must be replaced after use high operating cost Membrane can acquire good lifetime if cleaned regularly reasonable operating cost. Usually applied to dilute solutions. Suited to high content solid solutions 64

65 Chapter Pervaporation [26,99] Pervaporation is a separation technology used to separate mixtures that are difficult to separate by other conventional technologies, especially azeotropes and closeboiling mixtures. This process began in the twentieth century; when the process was invented and coined by Kober in 1917 [74] and systematically studied later by Binning in 1950 [100]. The name of this process was derived from its two basic steps, i.e., permeation and evaporation, where the permeate passes through the membrane and then evaporates into the vapour phase. Pervaporation is a clean, economical and energy-saving technique that is an alternative to many energyconsuming conventional technologies. Moreover, this technique is considered to be the most well-known process for the separation of organic/organic mixtures due to its competence in separating azeotropic mixtures. Pervaporation applications are classified into three categories, i.e., organic /organic mixture separation, exclusion of organic compounds and aqueous/organic mixture dehydration. Organic dehydration using hydrophilic zeolite membranes led to the first application of zeolite membranes in commercial application [62]. Pervaporation can be described as a basic operating technique for liquid separation tasks. In this process the feed enters in the liquid phase then passes through the membrane and evaporates to exit in the permeate, while the stream that does not pass through the membrane exists as a retentate from the cell. For sample collection, the permeate (vapour) is partially condensed by a cold trap to produce a liquid product and any uncondensable vapour is purged from the system. The flow transport is induced by the pressure difference between the feed and permeate sides. This vacuum pressure can be easily maintained in the laboratory using a bench vacuum pump, but industrially the situation is far more complicated. In the first years of exploring this process, 65

66 Chapter 3. researchers assumed that it would be impossible to commercialise this unit, as the vacuum pump necessary would be unfeasibly large. However, a successful alternative to the pump was found to be cooling the permeate so that the condensation of the liquid generates vacuum pressure. The pervaporation design exists in many forms depending on the separation task and application. The most common designs used currently are presented in Figure 3.7 [61]. The first module design shown in Figure 3.7(a) is considered to be the simplest pervaporation design that is suitable for bench scale operation and is used in laboratories where the low vapour pressure on the permeate side is induced by a vacuum pump. Another very common module, called thermo pervaporation, includes heating and cooling units in the system for the condensation process to substitute the vacuum pump, as illustrated in Figure 3.7(b). This configuration is widely used in commercial scale systems to avoid the need for large vacuum pumps, as the driving force is due to the difference in the vapour pressure between the hot feed and cold permeate. If the permeate is of no value it can be discarded, the vacuum pressure can be higher by involving carrier gas in the counter-current flow to sweep the permeate side, as illustrated in Figure 3.7(c). This model can be improved by substituting the carrier gas with low-price steam if the permeate is water-immiscible, as it can recovered by a decantation unit, as shown in Figure 3.7(d). 66

67 Chapter 3. (a) Vacuum driven pervaporation (b) Temperature driven pervaporation Feed Feed Retentate Heat source Retentate Condenser Pump Permeate Permeate (c) Carrier gas pervaporation (d) Condensable and immiscible carrier Feed Feed Retentate Retentate Permeate Permeate Evaporator Carrier gas Permeate Decanter Immiscible Carrier gas Figure 3.7: Pervaporation process configurations. 67

68 Chapter Gas separation [101] The gas separation process is greatly involved in membrane separation industries, due to its product impact and application needs. The development of the membranes used in this process has accelerated over the last two decades. The process was first studied by Thomas Graham who assessed the rate of permeation for many gases in 1829, and introduced Graham s law of diffusion [102]. Another major contribution to this industry was the nitrogen air separation system that was commercialised by Dow Generon in 1982 [61]. Currently, around 10,000 nitrogen air separation system units are operating worldwide and the applications of this technology are quickly expanding. The module design of this process comes in many configurations depending on economic considerations. Generally, this process has two main configurations i.e., one-stage membrane systems and two-stage systems. Each system may contain many steps, depending on the desired application and tasks. However, the main operational concept of this process is illustrated in Figure 3.8, where a gas mixture is fed into a selectively permeable membrane at an elevated pressure. The membrane should be selective for one component of the gas feed mixture. The main current industrial applications for this process are the removal of nitrogen from air, hydrogen from petrochemical products, argon and methane in ammonia plants and volatile organic compounds (VOCs) from petrochemical vents. 68

69 Chapter 3. Feed Retentive Permeate Figure 3.8: Basic illustration of membrane used in gas separation process. 3.5 Membrane Transport Theory [60, 61, 103] The ability of membranes to separate components is controlled by the rate of permeation of these components. In general, there are two main models used to describe the mechanism of transporting theses species i.e., the solution-diffusion (surface diffusion) model and the pore-flow model. In the diffusion model, permeate dissolves in the membrane material and then diffuses through it. The separation in the solution-diffusion model occurs as a result of the differences in the material diffusion rates through a given membrane with small pores of 8Å in size. The term diffusion, describes thermal motion and transportation of a particle or matter by random movement from one region of the system to another down a concentration gradient and was first observed by Adolf Fick in Fick recognised that when two adjacent volumes with different concentrations are separated by an interface, molecules will move from the solution with a high concentration to the solution with a low concentration, simply because of the different number of molecules in each solution. Consequently, Fick developed a theoretical equation called Fick s law of diffusion: 69

70 Chapter 3. dø Ji = -D 3.1 dx - ( Ji ) represent the amount of component that flows through a given area over a certain time period and is known as the diffusion flux (g/cm 2.s) or (mol/m 2.s) - ( D ) is measures the mobility of the individual particles - ( dø/dx) is the concentration gradient of component i. The minus sign shown in Equation 3.1 expresses that the diffusion direction is towards the solution with the lower concentration, as described earlier. On the other hand, in the pore-flow model the permeates are transported through pressure-driven convective flow. In this model, separation takes place due to the exclusion of large molecules by membranes pores (usually 20Å) based on their atomic size, while the smaller molecules pass as permeates. This type of transport model was observed by Henry Darcy in 1856 and is usually described by capillary media using his equation: -K Ji = dp 3.2 μ -( Ji ) represents the amount of component that flows through a given area over a certain time period and is known as the diffusion flux (g/cm 2.s) or (mol/m 2.s) - (K) is the permeability of the given medium 70

71 Chapter 3. - (dp) is the pressure gradient - (k) is the intrinsic permeability of the media. -(μ) is the viscosity of component i. Therefore, membrane operating units can be classified into three main categories (Figure 3.3). The first class which is based on the pore flow includes; ultrafiltration, microfiltration and microporous membrane (Knudsen-flow). On the other hand, pervaporation and reverse osmosis usually operate using dense membranes and follow the solution-diffusion mechanism. In general, the diffusion mechanism permeant fluxes are much lower than those of the pore flow model. The third class, which falls between the two models, contains the membranes with a pores size of 8-20 Å and is considered to be intermediate between pore and diffusion flow, e.g., nanofiltration membranes. In general, molecules can transport in three different ways depending on the membrane pore diameter; i.e., molecular, Knudsen and configurational. For macropore membranes that have pores with a large diameter of around 1 µm, an interaction or collision takes place between the molecules more frequently than with the framework wall, leading to what is known as molecular transportation. The second regime, Knudsen, represents the diffusion mechanism in smaller pores when the number of interactions between the molecules and wall increases, and mobility starts to be influenced by the pore dimensions. In membranes with even smaller pores of 2nm or less, the molecule size becomes comparable to the pore size. Therefore, the interaction between the molecules and walls will be continuous, leading to configurational diffusion that usually takes place in all types of zeolite micropore structures. Figure 3.9 illustrates and summarises the different diffusion 71

72 Chapter 3. regimes, depending on the pore size of a given membrane. In zeolite membranes, molecules diffuse through channels causing a constant interaction between the diffusing molecules and the framework of a given zeolite type. Therefore, the motion and direction of the molecules are strongly influenced by the size and dimension of the framework channel in addition to the other operation conditions i.e., temperature and pressure. The interaction between molecules and channel walls creates the differences in diffusivity between these molecules. Pore diameter porous membrane regime >1µm Molecular nm Knudsen <2 nm Configuration Figure 3.9: Classification of diffusion transport mechanisms dependent on pore size. 72

73 Chapter Membrane Material and Applications [104] In general, there are two types of materials used for fabricating membranes in the petrochemical-related industries i.e., (1) Organic membranes (rubber, cellulose, or synthetic polymers) and (2) Inorganic membranes or non polymeric materials (metal, ceramic and zeolite) Organic or Polymeric membranes [105, 106] Many polymeric materials can be utilized for membrane fabrication processes. Organic or polymeric membranes are synthesised mainly by monomer polymerisation. However, there are still many types of polymers used in the synthesis of membranes, depending on the application of the membrane itself. For example, polycarbonate and polytetrafluoroethylene polymers are used for microfiltration processes, while polyoxadiazole and polytriazole polymers are used for gas separation processes. Table 3.3 presents some typical examples of polymeric membrane applications. Currently, the polymeric membrane market is strong due to its low capital cost, high surface area per module volume and simplicity. On the other hand, the instability of thermal and mechanical proprieties of this type of membrane is considered as a major disadvantage. 73

74 Chapter 3. Table 3.3: Examples of typical polymer types used in different application [106]. Application Polymer Used Microfiltration Polycarbonate, Poly ( Vinylidenefluoride) and Polytetrafluoroethylene. Ultrafiltration Cellulose ester, Polysulfone and Polyacrynitrile Gas Separation Polytriazoles and Polyoxadiaziles Reverse Osmosis Aromatic Polyamide Dialysis Cellulose Inorganic Membranes or Ceramic Membranes [107, 108] Inorganic membranes are considered as an artificial membrane consisting of inorganic materials including all ceramic and metallic membranes that do not contain carbon bonds. Currently, these membranes are involved in high temperature applications, thus the thermal stability factor has become a very important and necessary feature for many processes. Organic membranes barely meet the ultimate requirements of the latest industrial applications. Consequently, inorganic membranes have been widely investigated recently. In addition to the long-term stability of these membranes, inorganic membranes have other advantages, including their resistance in harsh environments and to high pressure drop. Inorganic membranes include; silica, oxide and non-oxide ceramic membranes, zeolite membranes, metal membranes. 74

75 Chapter 3. Both organic and inorganic materials can be used to form hybrid membrane. The introduction of hybrid membranes could allow the advantages of both ceramics and polymers to be utilized to overcome many problems related to those types of membranes, as listed in Table 3.4. As noted earlier, ceramic membranes have higher thermal stability and strength than polymers but are still brittle. On the other hand, the processing of polymers is much easier than ceramics, but with less thermal stability. Table 3.4: Advantages of organic and inorganic membranes [108]. Organic Membranes Inorganic Membranes Low capital costs Long-term stability at high temperatures Tensile structure Suitable to harsh environments Achieving elevated selectivity Easy cleanibility after fouling 3.7 Zeolite Membranes As mentioned earlier, zeolites are crystalline aluminosilicate materials with a broad range of silicon to aluminium ratios and well defined pores with diameters of several nanometres. Therefore, zeolites as membranes have been developed over the last decade for separating molecules based on the differences in their size. Many studies have been conducted on these membranes with the aim of separating many mixtures using much less energy than other conventional processes [1]. Moreover, zeolite membranes have many advantages, such as their high solvent resistance and stability in harsh temperature conditions [104]. Zeolite membranes are well suited 75

76 Chapter 3. to dehydrating organic compounds, removing organic compounds from water and separating organic mixtures. In the pervaporation process, polymeric membranes are also used. However, zeolite membranes offer several advantages over polymeric membranes for example, zeolite membranes do not swell, as is the case with polymeric membranes, and zeolite membranes are far more stable than polymeric membranes at high operating temperatures. Zeolite membrane synthesis was first conducted in the late 1980s, this membrane was embedded with polymer. However, this technique yielded a membrane with poor chemical stability, due to the polymer content [109]. Later in the 1990s, improvements were made by numerous of researchers in the synthesis of high quality zeolite membranes in terms of performance and durability [110]. To date, few types of zeolite membranes have been fabricated successfully, these include, ZSM-5, zeolite A, faujasite (X and Y), mordenite and zeolite P [30]. In the following section of this chapter, the conventional synthesis methods for preparing zeolite membranes are presented, as well as their applications Zeolite Membrane Synthesis Methods In general, the methods used in the synthesis of zeolite membranes, require the synthesised zeolite layer to be supported on another material to acquire mechanical strength [28]. The choice of support material is of great importance, since the properties of the support structure influence the characteristics of the nucleation growth that forms the zeolite layer [111]. The types of supports that are used most frequently in the zeolite membrane industry are alumina and stainless-steel supports, mainly because the structure and other properties of their surfaces are compatible with the zeolite gel [112]. Several methods for synthesising zeolite membranes have developed in order to replace other traditional separation 76

77 Chapter 3. techniques that require major energy and capital cost. Generally, the synthesising industry includes two well known methods for zeolite membrane of preparation i.e., hydrothermal in-situ synthesis, and secondary growth method. The final synthesised forms of zeolite membranes should be a thin film with coherent structure of (5 20) µm thickness. To enhance the mechanical structure of these membranes, metallic or ceramic supports are used i.e., stainless steel, alumina or silicon [108]. In-situ Crystallization [113] In this method, the sintered support is dipped into a synthesis gel (consisting of sodium aluminate, sodium hydroxide, water and a template) within the autoclave and through direct crystallization, the synthesised membrane is formed on the support s surface. The in-situ crystallization method is the most common method for zeolite membrane preparation. This method is distinctive in its simplicity and practicability. However, this method lacks controlled crystal growth or nucleation control and the support surface characteristics and properties influence the quality of the synthesised membrane. Many studies have shown a relationship between membrane characteristics and the synthesising conditions and have revealed that different synthesising conditions could result in different morphologies, leading to different separation performance and transportation properties of the film or membrane. Thus, preparation of high quality zeolite membranes using this method is considered to be a difficult task. Another major disadvantage of this method is the crystallization period, which often requires enough time for the formation of zeolite impurities. 77

78 Chapter 3. Secondary Growth Method The secondary growth method (SGM) was first proposed by Tsapatsis and coworkers. This method has the advantage of lower crystallization temperature and time over the in-situ crystallization method. Moreover, SGM has a better control over membrane microstructure in terms of thickness and crystals organisation [114]. The experimental part of this method involves the deposition and growth of seeds on the surface of the membrane or film before it is introduced to the synthesised aqueous alkaline solution or gel. This method allows a good distribution of the crystals over the surface of film or membrane. In general, most zeolite membranes are synthesised using one of these two methods [30] and their experimental procedures are summarised in Figure

79 CHARACTERIZATION Chapter 3. In-Situ Crystallization Secondary Growth Method Synthesis solution preparation Synthesis Solution Preparation Crystallization process of synthesis gel and support within the autoclave Seeds deposition on support surface Template removal by calcinations (if applicable) Crystallization process of synthesis gel and support within the autoclave Template removal by calcination (if applicable) XRD SEM Molecular Probing Figure 3.10: Illustration of the experimental procedure of in-situ and secondary growth methods. 79

80 Chapter Application of Zeolite Membranes in separation Processes As mentioned in chapter 2, section 2.5, zeolite possesses chemical and physical properties which can be utilized to carry out many different tasks and applications. However this section will present the applications which are relevant to the scope of this work i.e., gas separation and liquid/liquid separation Gas separation [30, 115] In industry, zeolite membranes are used to separate a range of gaseous mixtures. For binary mixtures, separation processes can be classified into three main categories based on the adsorption interaction between the zeolite framework and the permeating molecules. In other words, the binary gaseous mixtures can contain (1) two gases with strong adsorption properties, (2) two gases with weak adsorption properties or (3) both (a gas with strong and a gas with weak adsorption properties). For mixtures of gases both with weak adsorption properties, separation is simply determined by the difference in their diffusivity.. For weak-strong binary gaseous mixtures, one gas (component) will diffuse while the other will strongly adsorb. Therefore the operating conditions will cause the separation process to acquire either diffusion permeation or adsorption permeation. An example of this case is found in the study conducted by Dong et al, where the permeability of an eight component mixture including hydrogen and light hydrocarbons through a zeolite membrane of MFI type was tested at two different temperatures [116]. Lin and coworkers have observed that operating the experiment at a low temperature (below 100 o C) improved the adsorption of the hydrocarbons. On the other hand, testing the same membrane with the same mixture but with a higher temperature (above 500 o C) caused the H 2 to pass through the membrane. Dong et al have concluded that the diffusion model in the case of this mixture is characterised by the separation 80

81 Chapter 3. conditions. Other examples of weak-strong binary gaseous mixtures are N 2 /CO 2 and H 2 /CO 2. The final category of gas separation processes, a mixture of two strong components, is much more complicated than the two categories mentioned previously. The complexity of this type of mixture is basically due to the existence of both of the component/zeolite framework and component/component interactions. Examples of these components are hexane and octanes Liquid/liquid separation [1,62] Pervaporation is one of the most commonly used techniques and is considered to be a clean, economical and energy-saving technique that is an alternative to conventional high energy-consumption technologies. This technique has been widely used in petrochemical related industries due to its ability to separate mixtures that are difficult to separate by conventional technologies, especially azeotropes and close-boiling mixtures. On the other hand, azeotropic and extractive distillations are feasible and capable of separating these types of mixtures, but have a high capital cost and are complicated in their operating process. The basic process of the pervaporation unit is illustrated schematically in Figure 3.11, where the feed (in liquid phase) enters the membrane module, represented by a box with diagonal line (membrane) through it. Any part of the feed that passes through the membrane evaporates to exit in the permeate, while the part that is rejected and does not pass through the membrane exists as a retentate from the module. The permeate vapour is then condensed by a cold trap to produce a liquid product and any uncondensable vapour is purged from the system. 81

82 Chapter 3. feed Retentate Pump Permeate Figure 3.11: Basic schematic illustration of pervaporation unit In pervaporation, the transportation of molecules through zeolite pores is achieved by surface diffusion depending on the operating temperature. In general, surface diffusion is represented by the adsorption-diffusion mechanism [117], which can be summarised by the following steps: 1- Molecules diffuse from the bulk to the surface of the zeolite. 2- Molecules are adsorbed to the zeolite surface sites, then into the zeolite pores 3- The molecules and walls interact. 4- Driven by the concentration gradient, molecules diffuse along the surface within the pores of zeolite from site to site. 5- Molecules desorb from the external surface of the zeolite and diffuse into the bulk permeate side As discussed earlier, the interaction between the molecules and the zeolite structural wall follows the configurational diffusion model reported by Xiao and Wei in 1992, whereby molecules with a molecular diameter comparable to the zeolite pore 82

83 Chapter 3. diameter follow configurational diffusion transportation [103]. The configurational diffusion separation concept depends on the diameter difference of the molecules which leads to different diffusion rates in zeolite membranes. Alternatively, molecular sieving takes place when molecules of a given mixture contain molecules that can fit into the pores while other molecules are rejected. Adsorption plays an important role in molecule transportation and can be defined as the adhesion of molecules from any phase to a given absorbent surface. It can be described as a surface-based process, while desorption is the reverse process. There are two other similar terms used to describe the adsorption phenomena; absorption and sorption. The term absorption engages the whole volume of a material, while the sorption process describes a system that includes both adsorption and absorption. Generally, numerous mixtures are being separated using zeolite membranes according to adsorption differences, especially organic/water separations, due to the hydrophilicity of zeolites. Adsorption is classified into biological, chemical and physical systems. In our case, zeolite membranes were synthesised for the use in this process, considered as a process with physical adsorption as a result of the attractive force between the adsorbent and adsorbate [62]. In the membrane industry, adsorption is deeply influenced by the hydrophilicity and the concentration polarisation of the membrane. The hydrophilicity for a given type of zeolite membrane plays a major role in determining the permeate component, while silicon to aluminium content influences the interaction of mixtures with different polarities towards the membrane. In other words, hydrophilic or water loving compounds have an affinity for water due to their polar side groups that attract or dissolve water. In contrast, hydrophobic compounds repel water as they are not charged. The utilization of this property in zeolite membranes led to excellent 83

84 Chapter 3. separation efficiency in the water/organic industry. Zeolites have both hydrophobic and hydrophilic properties depending on which type of zeolite is being used. Zeolite A is typically hydrophilic and has been used widely in alcohol dehydration processes [117], while silicalite-1 is considered as a hydrophobic type of zeolite and is used for the removal of organic compounds from organic/water mixtures, despite the fact that organic molecules are larger than water. The second aspect that influences the adsorption and permeability properties is the concentration polarisation. The concentration polarisation is represented by a boundary layer formed due to the accumulation of those molecules rejected by membrane during the process. This phenomenon leads to a reduction in the driving force and consequently reduces the permeate flux of a given process [118]. Organic Dehydration Organic dehydration using hydrophilic zeolite membranes was the first application used in commercial industry [119]. Zeolite A is considered as one of the best choices for organic dehydration processes. Zeolite A pores, which includes eightring pores (free diameters of nm) with a framework density of 12.9 T atoms/1000å 3 are larger than water molecules. Moreover, the pore size of zeolite A is smaller than many organic molecules. In addition, zeolite A membranes have the advantage of being highly hydrophilic with a low- silica ratio (Si/Al=1), allowing the permeation of water over almost all organic compounds. In 1997, Kondo et al. synthesised NaA membranes on tubular porous supports of mullite, alumina and cristobalite for ethanol/water separation [120]. The study was conducted at temperatures of 50, 70 and 120 o C, reaching a flux of 2.35 kg/m 2 h and a separation factor >5000. Kondo et al, concluded that selectivity increases with higher alumina content, of up to 70%wt. In 2000, Caro et al. fabricated a titanosilicate molecular 84

85 Chapter 3. sieve membrane of LTA on porous alumina supports for ethanol/water separation. Caro et al, reported that the water flux increases with the water content in the feed and with increasing temperature [104]. Holmes and co-workers, synthesised zeolite A membranes supported by porous stainless steel discs using a secondary growth method for H 2 O/EtOH separation at temperatures of o C [121]. The performance of these membranes were evaluated and yielded a separation factor of 180 and a total flux of 200 g/m 2 h. In 2001, Okamoto et al, achieved a separation factor of using zeolite NaA membranes (30 μm in thickness) on porous alumina support tubes. The study revealed a similar conclusion to Caro et al, in which both the permeation flux and separation factor (Equation 5.6, chapter 5) increase proportionally with temperature. The separation factors and fluxes of these studies and others are listed in Table 3.5. Table 3.5: Ethanol dehydration using zeolite A membrane. Support Sintered SS steel Membrane Thickness (μm) 5 Mullite/Al 2 O 10 T ( o C) Feed (wt.% EtOH) 85 Flux (g/m 2 h) Separation factor Reference [121] [120] Al 2 O [131] TiO [132]

86 Chapter 3. Organic compound removal Sano et al and Bowen et al have revealed that the separation factor of the organic removal process is lower than the dehydration fluxes using hydrophobic membranes such as silicalite-1 and ZSM-5, as the diffusivity favours water permeation due to the larger size of organic molecules compared to water [62,122]. However, zeolite minerals are capable of overcoming this hurdle due to the silanol groups on their surface that have the ability to adsorb organic compounds. In 2003, Xiao Lin et al. prepared a silicate membrane with a thickness of 10 to 20 µm, resulting in remarkable performance with flux and separation factors of 900 g/m 2 h and 106 respectively [123]. The study was carried out with in-situ crystallization using colloidal silica and seeding method, concluding that in-situ crystallization had a higher separation selectivity in silicalite membrane preparation. Another attempt at alcohol removal from water was carried out by Falconer et al using a ZSM-11 membrane on a stainless steel support. The study was carried out at 333K reaching a flux of 0.95 Kg/m 2 h for an ethanol aqueous solution, with the highest separation factor being 330 for an acetone/water mixture. Falconer et al concluded that the fluxes decreased as the carbon number increased [124]. In 2002, Tuna et al prepared a ZSM-5 membrane on a porous stainless steel support for methanol, ethanol and propanol (1 and 2) removal from water. The study was conducted at 333 K and resulted in separation factors of 78, 9.4, 7.8 and 7, respectively [125]. Table 3.6 presents results from the literature of different types of membranes for separation of a range of organic mixtures. 86

87 Chapter 3. Table 3.6: Ethanol removal using ZSM-5 membrane. Membrane/ support membrane thickness (μm) T ( o C) Feed (wt.% EtOH) Flux (g/m 2 h) Separation factor Reference Silicalite-1 / Al 2 O [123] ZSM-11 / Al 2 O [120] B-ZSM-11 / SS [133] B-ZSM-11/ SS [133] B-ZSM-5 / NA NA [125] Silicalite-1 /SS 10 to [122] Silicalite-1 /SS NA [134] Organic / organic separation Separation of organic/organic mixtures has been widely investigated using the pervaporation process due to its efficiency in the separation of close-boiling mixtures. The separation of organic mixtures is classified into four main categories; polar/non-polar, aromatics/aliphatic, aromatic/alicyclic and aromatic isomers[126]. The organic classifications of some components are illustrated in Figure In 1960, Jennings and Binning tested the separation of polar/non-polar mixtures using 87

88 Organic Categories Chapter 3. hydrophobic materials such as polyethylene (PE) and polypropylene (PP) [127]. These materials did not yield sufficient selectivity as they lacked the functional groups that create the interaction between the components. On the other hand, many studies have revealed that methanol/mtbe provides the most successful separation efficiency using zeolite membranes, as some of zeolite types are aluminium-rich materials possessing localised electrostatic poles [126,128,129]. In 2001, Kita et al prepared NaX and NaY zeolite membranes on a ceramic support [130]. These membranes yielded high selectivity using different organic mixtures as shown in Table 3.7. Kita et al observed that the separation factor showed an inverse relationship with the methanol feed concentration. Table 3.7 presents the results of some studies results using different types of membranes for a range of organic mixtures. Polar Non-Polar Methanol, Ethanol and i-propanol Benzene, Toluene, MTBE and ETBE Aromatic Aliphatic Alicyclic Benzene and Toluene i-octane, n-heptane, n-octane and n-heptane Cyclohexane and Cyclohexane Isomers Isomeric xylenes, n/i- Heptane and Propanol n/i- Figure 3.12: Classification of typical organic compounds. 88

89 Chapter 3. Table 3.7: Separation of organic / organic mixtures [130]. Membrane System (A/B) 10%A Membrane Thickness (µm) T ( o C) Flux (g/m 2 h) Separation Factor NaX NaX NaY NaY NaA H 2 O/ EtOH MeOH/ MTBE H 2 O/ EtOH MeOH/ MTBE H 2 O/ EtOH Carbon membranes In this study, the main aim of introducing the carbon membranes industry and incorporating its precursors in the fabrication of zeolite membranes was to use them as a healing material to block any pinholes and defects in the zeolite layer (chapter 7). The key variables that will be investigated are the types of carbon precursors, pyrolysis temperature, pyrolysis soak-time, and the post-treatment process. Therefore, a brief literature review is presented in this section related to carbon precursors, operating conditions, and techniques used in the membrane industry Introduction In the last two decades, much effort has focused on developing carbon membranes with molecular sieving properties for several applications. Carbon molecular sieve membranes (CMSM) are produced by the pyrolysis process to form a carbon porous 89

90 Chapter 3. film from the original precursor that is able to discriminate between molecules in a mixture [135]. The pore size of the CMSM ranges from (0 to 20 Å), depending on the precursor used [136]. For example, Foley and co-workers used a polyfurfuryl alcohol precursor to prepare a nanoporous carbon membrane [137]. In general CMSM are being chosen over polymeric membranes in more separation processes, as they are more thermally stable and selective. CMSM possess excellent separation properties, and Kyotani and Ismail and have demonstrated their selectivity, corrosion resistance, and thermal stability [138, 139]. The essential parameters for the fabrication of CMSM are classified into four steps (shown in Figure 3.13); 1) Selection of the Carbon Precursor 2) Carbon Precursor Pre-treatment, 3) the Pyrolysis Process, and 4) Post-treatment of the Membrane [135]. The manipulation of these parameters determines the final properties (including pore size) and performance of the CMSM, as will be discussed later in this section Carbon membranes classification [140] In general, carbon membranes can be classified into two categories; supported and unsupported. There are two configurations of supported carbon membranes, i.e., flat membranes and tubular membranes. On the other hand, unsupported carbon membranes can exist in three formations, i.e., flat membranes, hollow fibre membranes, and capillary membranes. The carbon membrane classification is summarised in Figure

91 Chapter 3. Selection of Carbon Precursor Pre-treatment of Carbon Precursor Carbon Membrane Membrane Post-treatment Pyrolysis Process Figure 3.13: Illustration carbon membrane fabrication steps. Carbon Membrane Supported Flat Membranes Tubular Membrane Unsupported Flat Membranes Hollow Fiber Membranes Capillary Membranes Figure 3.14: Illustration of carbon membrane classifications. 91

92 Chapter Carbon Precursor Selection [141] Many studies have reported that the selection of the carbon precursor is of great importance as it can lead to different structures and properties in the final carbon membrane, and suitable precursors lead to crack-free carbon membranes. In general, carbon layers are produced by material carbonisation via the pyrolysis process under a vacuum or inert atmosphere. The most common precursors used in the carbon membrane industry are polyfurfuryl alcohol (PFA), polyacrylonitrile (PAN), polyetherimide (PEI), polyimide derivatives (Kapton polyimide and Polyetherimide (PEI)), phenolic resin and cellulose. As mentioned earlier, there are two main types of carbon membranes i.e., supported and unsupported. The precursors that form the supported membranes are; PEI, phenolic resin, polyimide, PVDC AC and phenol formaldehyde. On the other hand, unsupported membranes include; PFA, PAN and cellulose. The configurations of these membranes are discussed in more detail in Table 3.8. Table 3.8: Configuration of carbon membrane precursors Precursor polyetherimide (PEI) phenolic resin Polyimide (PVDC AC) Supported Membranes Unsupported Membranes Flat/Film Tubular Flat/Film Hollow Fiber phenol formaldehyde polyfurfuryl alcohol (PFA) polyacrylonitrile (PAN) Cellulose 92

93 Chapter 3. Polyfurfuryl alcohol has been used widely as a precursor for nanoporous carbon membrane (NPC) preparation [142]. In addition to PFA s flexible polymerisation, it is a very attractive precursor due to the simplicity of its structure formation mechanism and its application in simulation studies. PFA is used as a support film in the carbon membrane industry as it exists in the liquid state at room temperature. PFA has desirable properties i.e., distribution of its narrow pore size and its chemical stability [143]. Foley and co-workers proved the ability of PFA to be used in nanoporous applications by demonstrating its use in the preparation of NPC membranes using the porous surface of a stainless steel disk for support. They successfully achieved large separation factors of 600, 45, 17 and 14 for H 2 /CH 4, CO 2 /CH 4, N 2 O/N 2 and H 2 /CO 2, respectively [135]. In 2010, Wang et al fabricated a carbonaceous polyfurfuryl alcohol (CPFA) layer supported on commercial polysulfone support by partial carbonisation of PFA to be used for water desalination processes and achieving ratios of water permeability of 1.54, 2.01 and 0.17 L μm m -2 h -1 bar -1. Wang et al concluded that the CPFA structure exhibits hydrophilic properties and contributes to a largely improved flux compared to PFA membranes [144]. Another type of polymer used in this industry called Polyacrylonitrile (PAN) which is a semicrystalline polymer with the chemical formula (C 3 H 3 N) n, formed from a complex of monomers. PAN has many different applications in fibre production used in reverse osmosis, textile production and high-quality carbon fibre production; 90% of the carbon fibre in the world is produced from this polymer [145]. Fibres produced from PAN have attracted considerable attention due to their advantages which include a high melting point, thermal stability and vast yields of carbon fibre with considerable mechanical properties. Polyimides are also known in fabricating carbon membranes and they 93

94 Chapter 3. are formed by the condensation of dianhydrides with diamines. They are well known for their stability at high temperatures and decomposition before reaching their melting point without going through the melting phase transition. Recently, many studies have emphasised the use of this polyimide as a precursor in glassy carbon production as it has shown remarkable performance in terms of both mechanical and separation properties. One of the most common derivatives of this polymer is Kapton polyimide which is used widely in CMS films after carbonisation at 800 o C. Polyetherimide (PEI) is an amorphous polyimide with chemical resistance properties used in membranes in gas separation processes. Another well-known polymer, phenolic polymer, is a mixture of chemical compounds i.e., aldehyde and phenol. Phenolic polymers are used in a wide variety of applications including the manufacture of carbon membranes which have advantages such as molecular sieve properties, high carbon yield, low cost and corrosion resistance. Finally, cellulose is an organic natural polymer with the chemical formula (C 6 H 10 O 5 ), which is considered as a low-cost precursor and the most common organic compound found in almost all types of plants [138] Precursors pre-treatment Many studies have showed that pre-treatment is an essential step with a substantial influence on the maintenance of precursor stability during pyrolysis, as the polymeric membrane tends to shrink under pyrolysis conditions [146]. This process is divided into chemical and physical methods. The chemical method includes the exposure of the polymeric precursor to different chemical reagents to acquire the desired separation properties of the carbon membrane. In general, there are two well-known classifications of the chemical pre-treatment; oxidation and chemical 94

95 Chapter 3. pre-treatment. On the other hand, physical pre-treatment is achieved by the stretching of these membranes [135]. Oxidation pre-treatment [147] It is well-known that oxidation treatment has a great influence over the preservation of membrane structure during the pyrolysis process to increase the precursor s carbon yield in its final form. In 1997, Kusuki et al observed the poor performance of a few membranes that had not undergone this pre-treatment step before being carbonised. Therefore, this process is of great importance to many membrane precursors and can be applied at different temperatures, depending on the precursor being used, as shown in Table 3.9. Table 3.9: Oxidation processes operating temperatures, according to different precursors. Precursor Temperature Range ( o C) Exposure Duration (hr) Reference Polyacrylonitrile [146] Phenol resin [148] Polyfurfuryl alcohol 90 3 [135] Polyimide [135] 95

96 Chapter 3. Chemical pre-treatment [149] Treating the precursor with chemicals (e.g., HCl and NH 4 Cl) could enhance the distribution and uniformity of the pores during the pyrolysis process. In the chemical pre-treatment, the subjected membrane is exposed to or fully immersed in the chemical solution and, following drying, is washed before undergoing the separation process. Stretching [145] Unlike the two previous methods, this technique is a physical pre-treatment process applied to the hollow fibre precursors and was adapted from the spinning treatment. The spinning or stretching method makes the surface defects accessible in order that they can be removed and healed to form stiff fibres. Moreover, this method enhances the molecular orientation resulting in a stiff and balanced fibre structure Pyrolysis / carbonisation Pyrolysis or carbonisation is a process where the precursor is heated at an elevated temperature with specific heating rates in a vacuum or inert atmosphere. The main aim of this process is to produce micropore carbon fibres with molecular sieve properties [150]. The pyrolysis process removes any heteroatoms such as bromine, chlorine, nitrogen, sulphur, iodine, or phosphorus from the precursor macromolecules to produce a cross-linked carbon structure, in order to prevent the formation of graphite-like crystals that could lead to tapered pores. A superior understanding of the operating conditions of this process i.e., pyrolysis temperature, heating rates, thermal soaking period and gas flow, allow the achievement of tailored pores within the membrane. In other words, carrying out a pyrolysis process for a given precursor, under proper conditions, leads to a membrane with suitable 96

97 Chapter 3. pore size that is of great importance to its performance in terms of selectivity and permeability [151]. In general, carbon membranes consist of a non-homogeneous pore system i.e., their size and shape depend on the precursor type and the pyrolysis operating conditions, as shown in Figure (3.15). Figure (3.15) shows a typical carbon pore structure, where ultramicropores with a diameter (D1) less than 10Å applies the sieving properties for a given molecule. In contrast, micropores (D2) with diameter of 5-20 Å allow the diffusion of molecules through the structure. Thus, the carbon membrane has the ability to perform with a considerably higher flux of permeated molecules than the sieving property alone [152]. d1 D1 <10Å d2 D2 (5-20) Å d3 D3 >20 Å Figure 3.15: Typical carbon s materials pore structure [152]. 97

98 Chapter 3. Pyrolysis s temperature effect Depending on the type of the given precursor, the pyrolysis process is usually conducted at a temperature of o C in a controlled atmosphere to prevent any undesired burn off that could ruin the precursor. As stated before, this process contributes to the removal of the heteroatoms to form a cross-linked porous carbon structure [141]. Many studies concluded that the temperature has a strong affect on the fabricated carbon structure including the separation performance selectivity and permeability, and have revealed that an increase in temperature of the pyrolysis leads to higher crystallinity and smaller spacing between the layers of the carbon structure [153]. Table 3.10 summarises numerous studies showing the pyrolysis temperatures for different precursors. 98

99 Chapter 3. Table 3.10: Pyrolysis process conditions using different types of precursors [135]. Precursor Type Heat Rate Temperature ( o K) Inert Gas Dwell Period PFA 10 C/min He or N 2 2 hr PFA 5 C/min He or N hr PFA 6 C/min 723 He 2 hr PAN 1 C/min 1223 N 2 3 hr PAN 9 C/ min N 2 3 hr Polyimide 5 C/min N 2 N.A. Polyimide 6 C/min Ar N.A. Kapton and matrimid PEI 0.5 C/min 0.5 C/min Vacuum 1 hr 1073 Vacuum 1 hr Phenolic resin 25C/min 1173 N 2 1 hr Phenolic resin 50 C/min 1073 N 2 N.A. Cellulose Cellulose 0.5 C/min 1-10 C/ min Ar N.A Ar N.A. Heating rate effect During the pyrolysis process, a range of products with different volatilities are formed and cause a final weight reduction which can be controlled by the heating rate sequence [154]. The heating rate contributes to the formation of the final shape in terms of pore size by determining the evolution rate of the volatile components. In general, a range of 1-10 o C/min has been widely used in many studies. It was concluded that lower heating rates are more efficient for the formation of small pores and increase the carbon crystallinity, leading to improved selectivity. On the 99

100 Chapter 3. other hand higher heating rates lead to the formation of a structure with low separation selectivity as it may contain pinholes, cracks and deformations [155]. Thermal dwell period The thermal dwell period plays an important role in the rearrangement of the microstructures, which affects the distribution of pores in the final form of the membrane. The period of thermal dwell depends on the pyrolysis temperature. In general, the separation selectivity of a given membrane increases when applying longer thermal soak periods [156]. Gas flow effect The main aim of calibrating the gas flow in the pyrolysis process is to prevent any undesired burn off that could negatively affect the membrane formation. In general, the pyrolysis process can be operated using either a vacuum or an inert atmosphere. Vu reported that a vacuum leads to a less permeable structure but greater separation selectivity than an inert atmosphere [157]. In the case of the inert gas, the flow is of great importance as it affects the membrane permeability performance without affecting the selectivity. It is believed that the increase in gas flow enhances the permeability of the carbon membrane [158] Membrane post-treatment [159] As stated earlier, the pyrolysis process is a key player in the formation of pore shape in the membrane fabrication process. However, some studies have revealed the ability of further thermochemical post- treatments to adjust pore distribution and repair any defects or cracks in the carbon membrane. Carbon membrane posttreatment includes methods such as post-oxidation and chemical vapour deposition 100

101 Chapter 3. (CVD). However, post-oxidation is preferable and been widely used in this industry, contributing to increased average pore size of the carbon membrane. Post-oxidation The oxidation or activation process is simply carried out after the pyrolysis step, by exposing the membrane to an oxidising atmosphere to alter the average pore size. Kusakable et al have successfully increased the permeability of polyimide carbon membrane towards CO 2 without damaging it [160]. Generally, the post-oxidation technique is the preferred method among and is performed using oxygen or oxidising agents at elevated temperatures [161]. Many research studies have been conducted at different temperatures and durations (see Table 3.11) and have revealed that the permeance of all gases increase with increasing temperature, but this also leads to a reduction in selectivity [162]. Table 3.11: Oxidation conditions using different types of precursors. Precursor Oxidation Temperature ( o C) Duration (min) Reference Cellulose 400/air 15 [163] Phenol formaldehyde 800/CO 2 60 [164] Phenol resin 300/air 30 [165] Phenol resin 75/air 30 [166] Polyimide 300/O [160] Polyimide 250/air 12 [167] 101

102 Chapter 3. Chemical vapour deposition (CVD) Oxidation or activation post-treatment processes have been proven to increase the permeability of a given carbon membrane. On the other hand, Hayashi et al [162] demonstrated the ability to improve membrane selectivity through the implementation of the chemical vapour deposition (CVD) process. The CVD is carried out by introducing organic species into the membrane pore system and channels [168]. The most common organic substances used in this method are ethane, propane, ethylene and benzene. The selection of the organic species is of great importance in accomplishing this technique, as they should produce a sufficient amount of deposition at the pores. In general, the CVD technique leads to three different deposition forms on membrane walls, as shown in Figure (3.16) i.e., homogeneous, adlayer and inlayer depositions [169]. (a) (b) (c) Figure 3.16: carbon deposition forms on membrane pore walls (a) homogeneous carbon deposition, (b) inlayer carbon deposition, and (c) adlayer carbon deposition [161]. 102

103 Chapter 3. The literature presented in this chapter will be applied to fabricate zeolite anistropic membranes using in-situ crystallization and secondary-growth methods. In addition, the ability to tailor small pores of carbon material to block any defects in the fabricated zeolite membrane will be considered to produce a composite membrane with high performance in terms of productivity and selectivity. The process will be conducted by defining the precursor options, and then by manipulating certain conditions, i.e., selection and pre-treatment of the carbon precursor, a pyrolysis process, and post-treatment of the membrane (chapter 7). 103

104 Chapter 4. Chapter 4 Characterization Techniques 4.1 X-Ray Diffraction (XRD) [170, 171] XRD is a characterization tool used to examine and identify a given crystalline sample. The analysis of this instrument lies on the atomic arrangement in the crystal lattice. The main aim of the X-ray diffraction is to generate information regarding the properties of the crystalline structure. In general, crystalline materials are made of a regular array of atoms that have a high atomic and electronic density. Therefore, every crystalline substance has its own pattern or fingerprint, and a definite type of crystalline substance should always generate the same pattern. The fact that 95% of all solid materials are classified as crystalline materials (atoms are arranged in a regular pattern), makes the XRD tool important and it is used widely in various types of research. The application of X-ray diffraction was discovered in 1912 at the University of Munich by Laue. XRD is based on diffraction analysis that creates form scattering phenomena. When x-rays hit an atom of the crystalline substance, an electronic cloud is produced and the movement of these electrons reradiates waves with the same frequency as the incoming beam (elastic scattering phenomenon). As the atoms in the crystalline substance are arranged in a well defined pattern, the x-ray beams leaving the tested sample in various directions will be unique and can be defined according to the material that is being tested. Figure 4.1 shows two fixed wavelength of X-ray beams (P1 and P2) projected at a certain incident angle on a pair of parallel crystal lattice planers with an inter-planar spacing of (d) and diffracted beams are leaving at an angle equal to that of the incident beam. In general, crystalline materials consist of a countless number of 104

105 Chapter 4. lattice planers and numerous spacing of the atoms (d). These factors i.e., wavelength, incident beam and distance between crystal lattice planar, are described by Bragg s law (Equation 4.1). Bragg s law was presented by Sir William Lawrence Bragg in 1912, when he irradiated the surface of cleaved mica using a thin beam of X-rays and observed that the modification of the incidence angle changes the diffraction angle equally. n λ=2d sin θ (4.1) P1 λ P2 d d 2θ θ Figure 4.1: typical illustration of beams reflection from a lattice planes. 105

106 Chapter 4. As shown in Figure 4.1, P2 clearly travels a longer distance than P1 and the ray paths is: AB+BC (4.2) In order to have a constructive interference, the paths length must be equal to any integer value (order of reflection) of the wavelength: AB+BC=nλ (4.3) Where: - n is an integer (order of reflection) determined by the order given - λ is the wavelength As AB is opposite to θ (Figure 4.1) and taking d as the hypotenuse of the triangle ABB, we can relate the distance of the ray paths (AB+BC) to L and θ, as shown in Equation 4.4 AB= d sin θ (4.4) Because AB = BC Equation 4.3 becomes: n λ=2ab (4.5) Finally Bragg s law is concluded by substituting Equation 4.4 into Equation 4.5; n λ =2d sin θ (4.6) 106

107 Chapter 4. XRD works by emitting rays from the X-ray tube (generator) by electron deceleration, where streams of electrons are released into a vacuum from the cathode to be collected by the anode target. When the x-ray beams strike the tested sample, while moving in a circular path, they will be diffracted in all directions, as described by Bragg s law. Ideally the distribution of all possible h, k, l planes (miller indices) should have parallel reflections in the case of crystalline substances. The distance from the X-ray detector to the tested sample is the same as the distance between the sample and the X-ray tube. Therefore the tested sample acts as a mirror and only crystallites will affect the reflected intensity as they reflect the beams in parallel from the sample to the detector. The intensity of the reflected beam depends on the electron density (distribution) around the atom. In other words, h, k, l planes with a high electron density will yield high intensities (reflect strongly) while h, k, l planes with a low electron density will generate low intensities. The resulting pattern indicates structural information such as, material type, as every crystalline material has its own distinctive XRD pattern and can generally be identified by comparison to standard samples provided in the literature. 4.2 Scanning Electron Microscope (SEM) [172,173]. Scanning electron microscope (SEM) is a type of electron microscope used to study the surface topography and composition of a specimen using an electron beam interaction. This instrument is of great importance and has been used widely in different fields, as it can scan the surface of a given specimen with a large depth of focus to provide the required information. The ability to image the solid sample at a high magnification was an important goal, and the history and development of this method began in the 1930s, and the first dedicated SEM was developed in 1942 by 107

108 Chapter 4. Zworykin et al [174]. The SEM consists of an electron gun, vacuum chamber, lenses and a sample chamber (Figure 4.2). In general, the SEM works by producing a beam of electrons in the electron gun which are sent by anodes through a series of electro-magnets to finely focused the electrons (from 50μm to 100nm) on the specimen surface within a vacuum. The aim of using a vacuum system is to avoid the interference of air particles that will block the path of the electrons. The electrogun is usually located at the top of the SEM column or at the very bottom in some designs, to generate large electron beams (0.1-30eV). Typically, there are two main types of electron guns used within the SEM; thermionic and field emission guns. Thermionic electron guns are the most common, and are made of tungsten filament which creates electrons by applying thermal energy to the filament. The second type of electron gun, the field emission gun, provides a stronger electron emission using conventional thermionic emitters (e.g., lanthanum hexaboride). The electron lenses are made of a series of electron-magnets to produce a magnetic field to focus the electron beam. Generally, there are two types of electron lenses; condensers and objective lenses. More details about the operation of electron lenses can be found in the work published by Goldstein et al [172]. 108

109 Chapter 4. Electron gun Electron detector Anode Electron beam Electron detector Condenser lens Specimen Secondary electron Figure 4.2: Illustration of the scanning electron microscope When the electron beam hits the specimen, two types of electron/sample interactions take place i.e., secondary electrons (SE) and back scattered electrons (BSE). In the secondary electrons interaction, specimen s atoms absorb the energy of the electron beam and release their own electrons which are collected by a positively charged detector (300v). On the other hand, back scattered electrons reflect off the surface of the atoms and usually come from a deeper layer of the specimen. The detector used to collect the SE and converts the energy of electrons into a photon by scintillation in a photomultipier tube. Then the photon s quantum energy is converted again into electrons and the output is amplified and presented on the screen of the SEM. The screen can be modified to show the features of the surface of the specimen. 109

110 Chapter Gas Chromatography (GC) [175, 176] Gas chromatography (GC) is a method of separating volatile liquid mixtures for the analysis of compounds with boiling points up to 700 K. GC is considered to be one of the most important analytical techniques used for this purpose, and has been used widely in many industries including the petrochemical manufacture, environmental and food contaminant industries. This method was invented in 1952 by Martin and James [177]. As in all chromatographic methods, there is a mobile phase in which a gas (typically helium) carries components of the mixture over a stationary phase (column). The column or the stationary phase is usually packed with solids or coated with a liquid with a high boiling point. The general principle of GC relies on the interaction between the vapourised compounds and the stationary phase. The columns possess a high partition coefficient of solubility to the vapourised components through a series of adsorbing and desorbing steps. Then, components of the mixture leave the stationary phase in order of volatility i.e., the most volatile first. A schematic diagram of GC is shown in Figure 4.3. During the process, samples should be introduced to the column in low volumes, as large samples will usually lead to a low resolution result. Therefore, the sample is injected using a microsyringe through a rubber septum port at the head of the column with an injector temperature of around 50 o C (higher than the boiling point of the lowest volatile sample). The volume of the sample mixture injected is between ten and 20 microlitres in the case of packed columns. Packed columns consist of finely inert, solid material coated with stationary phase liquid and are usually designed to be 1 to 10 m in length with an inner diameter of 2 to 5 millimetres. Capillary columns are considered to be more efficient than packed columns, as they require much less sample (around 10-3 microlitres). 110

111 Chapter 4. Detector Injector Recorder Column Carrier gas Figure 4.3: Schematic diagram of typical GC set up. In general, there are two designs for capillary columns; wall-coated open tubular (WCOT) and support-coated open tubular (SCOT) columns, both of which have an inner diameter of a few tenths of a millimetre. The main difference between the two designs is that WCOT is coated with the liquid stationary phase, while the SCOT is covered with a thin layer of adsorbent material. The column temperature is of great importance and its choice depends on the sample boiling point. According to rule of thumb, operating the GC column with a temperature slightly above the average boiling point of a given sample will lead to a sample elution time of 2 to 30 min. On the other hand, using a lower temperature will yield better resolution results but will require a longer elution time. In most cases, when a sample contains a wide range of boiling points programming can be used and the column temperature can be increased from the initial to final temperatures, as separation proceeds. 111

112 Chapter Dynamic Light Scattering (DLS) [178] Dynamic light scattering DLS is a technique used to determine the size profile and distribution of particles and suspension solids. DLS is an important technique used in physics, chemistry and biology. Moreover, unlike many other electron microscopy techniques, this technique provides the advantage of examining the dynamics of a given sample in a safe environment without the risk of damaging the sample. This method was introduced in 1869, when Rayleigh and Tyndall combined their work to present the basis of scattering analysis by observing the light scattering through aerosols. Later, many studies were conducted into the development of this technique. The operation principle of DLS lies in the interaction between the emitted light and electrons. In other words, when the DLS emits light which hits a particle, the light will scatter randomly with certain intensity in all directions. The intensity and angles of the scattered light depends on the particle size, therefore the size of particles can be determined and derive.in other words, larger particles scatter light at narrow angles and with a high intensity, while smaller particles scatter light at wider angles with less intensity. In addition to the particle size, the intensity is also affected by optical properties or the refractive index (how light is passing through the particle). Detail on the operation of DLS is beyond the scope of this work, however more details can be found in relevant review by [178]. In this work a zetasizer 3000 HAS model was used to determine the particle size of zeolite A suspensions and slurry. The mentioned characterization techniques were used in this study, where the zeolite analysis was carried out using an XRD instrument obtained from RIGAKU, model MiniFlex with a diffraction angle of 3-50 θ (as zeolite features have been found within diffraction angles of 3 to 50 θ). As for the SEM, both powder samples 112

113 Chapter 4. and membranes were imaged using a FEI Quanta 200 SEM and their chemical composition were analysed with an energy-dispersive X-ray spectroscope (EDAX) that was attached to the SEM. The membrane performances were evaluated using a Variant model GC instrument to investigate different sample mixtures obtained from the pervaporation test. Two types of column were used i.e., Poraplot Q-HT (10m x 0.32mm) and DB-5MS (30m x 0.25mm). The Q-HT column was used to analyse ethanol/water and ethanol/cyclohexane mixtures, while DB-5MS was used to carry out the analysis of phenol removal and xylene isomer separation. For both columns, helium was used as a carrier gas at a constant flow rate of 33 cm/sec. The detector type used within this GC model was FID04. Both injector and detector temperatures were 50 o C. For all experiments, the analysis was carried out by injecting a standard solution to obtain a calibration curve, which includes the mixture concentration in the peak area. 113

114 Chapter 5. Chapter 5 Experimental work 5.1. Synthesis of Zeolite A The main goal of the work presented in this section is to fabricate zeolite membranes using different, inexpensive materials, including commercial materials, kaolin from WBB UK and virgin kaolin. Therefore, this section presents the experimental preparation and evaluation of a synthesis technique for zeolite A powder as a first step, after which this technique will be used to fabricate zeolite membranes. Then, in chapter 6, the outcome and results of the XRD, SEM, and DLS assessments are discussed and compared to standard samples prepared by the commercial sector Zeolite A using commercial materials First, zeolite A was synthesised using a standard conventional procedure so that it could be assessed and compared with zeolite A produced by the novel synthesis method that will discussed later. Therefore, a conventional, hydrothermal method (discussed in section 2.6) was used to prepare zeolite A powder, and this method can be described by the reaction shown in Equation 5.1. Alumina + Alkali + Silica+H 2 O Aluminosilicate Zeolite (5.1) hydroxide gel crystals The source materials were deionized water, sodium aluminate solid (50% Al 2 O 3, 40% Na 2 O), sodium hydroxide pellets (99% NaOH), and sodium silicate solid (47% SiO 2, 52% Na 2 O). The molar composition of the sample of zeolite A is described in Equation

115 Chapter SiO 2 : Al 2 O 3 : 3Na 2 O: 180 H 2 O (5.2) Sodium hydroxide pellets were dissolved in the required amount of water, and the solution was divided into equal portions. Sodium aluminate was added to one of the portions, and sodium silicate was added to the other portion. Then, the two portions were agitated separately after which they were mixed to form homogenous aluminosilicate gel. The batches and quantities of the source materials used for the aluminosilicate gel preparation are as listed below in table 5.1: Table 5.1: Summary of the aluminosilicate source materials batches. Source material Quantity Sodium hydroxide (99%NaOH) 0.67 Sodium aluminate (50%Al 2 O 3, 40%Na 2 O) 2.21 Sodium silicate (47%SiO 2, 52%Na 2 O) 1.18 Deionized water Then the solution was mixed and then aged for 24 h at room temperature. After that, the aluminosilicate gel was transferred to an autoclave in which the crystallization process took place for 3h at 100 o C. After each run, the product (powder sample) was washed with deionised water using a low vacuum system to obtain a ph less than 8, and the product was retained for XRD and SEM analyses. The results of the SEM analyses are presented in the next chapter (chapter 6). 115

116 Chapter Zeolite A prepared using kaolin obtained from WBB UK and Ahoko Nigerian kaolin (ANK) In general, synthetic zeolites are prepared using different, expensive, commercial materials, which results in expensive products. This problem can be solved by using cheap raw materials to produce zeolite, and this approach has attracted the attention of the zeolite production community [179, 180]. However, there are some difficulties associated with using kaolin raw material because they usually contain several impurities, e.g., quartz, iron, and feldspar [181]. Studies have emphasized the significant importance of removing these impurities because they reduce the commercial value of the mineral that is produced [ ], and suggested that kaolin clay may be a potential candidate to replace the precursors of micro-porous materials because it is readily available and is more environmentally-friendly than other precursors. Kaolin production in Nigeria alone was estimated to have been more than 88,000 metric tons in 2007 [186], so, in this work, another approach for synthesising zeolite A was investigated in which kaolin was used as the raw material to serve as the main source of alumina and silica precursors. The Si:Al ratio in kaolin is approximately 1, which makes it an ideal choice as a low-silica, zeolite precursor [180, 187]. However, the quartz content of kaolin is considered to be a major obstacle to its use in synthesising zeolite. The novelty of the work in this section is the conversion of a raw material, which is abundant and can be taken directly from the ground, to a pure material that has many applications in several important areas, such as the catalyst, ion exchange, and adsorption industries. This was achieved by transferring Si and Al ions from the aqueous gel (prepared using kaolin) to a seeded crucible bowl located inside the stainless-steel autoclave, where 116

117 Chapter 5. the formation of pure zeolite takes place (Figure 5.1). Any impurities will remain at the bottom of the stainless-steel autoclave, since most of them have high densities. Autoclave Teflon sleeve Crucible bowl Aqueous solution Figure 5.1: Illustration of the autoclave used in the hydrothermal synthesis with the homemade Teflon holder and bowl. In this study, kaolin from two different sources, i.e., kaolin obtained from WBB and Ahoko Nigerian kaolin (ANK), was used to synthsise zeolite A. The kaolin from both sources underwent the same procedure, which began with the dehydroxylation or metakaolinisation process. This process increased the reactivity of kaolin by exposing it to elevated temperatures, since kaolin in its natural state only can form sodalite and cancrinites in the presence of aqueous bases (e.g., sodium hydroxide) [188]. Kaolin is an aluminosilicate material that contains Al 3+ that can be either in IV or VI coordinates, whereas it must be in the IV coordination for zeolite formation [189]. This can be addressed by the dehydroxylation or metakaolinisation since this reaction involves the loss of hydroxyl groups (oxygen atoms connected by a covalent bond to hydrogen atoms), as shown in Equation 5.4. Thus, rearrangement 117

118 Chapter 5. from octahedral to tetrahedral orientation takes place [189]. Moreover, this process has been used to remove some impurities, such as mica o C 2Al 2 Si 2 O 5 (OH) 4 2Al 2 Si 2 O 7 + 4H 2 O (5.4) Metakaolin Therefore, in this study, the dehydroxylation or metakaolinization process was conducted at 650 o C for 10 min for kaolin from both sources, i.e., WBB and ANK, to increase the kaolin reactivety, after which the following procedure was implemented: Zeolite synthesis gel using WBB commercial kaolin - The synthesis solution was prepared using the following molar composition: 3.75 Na 2 O:Al 2 O 3 :2.1SiO 2 :243.7 H 2 O (5.5) g of sodium hydroxide were placed in 81 g of deionised water and stirred for 10 min to create a homogeneous solution. - After stirring, 4.7 g of WBB metakaolin were added, and the mixture was stirred for 24 h to obtain the final, aqueous solution. Zeolite synthesis gel using ANK virgin kaolin - The synthesis solution was prepared using the same molar composition described in Equation 5.5. The percentage of quartz in the raw ANK was 77.3 wt%, so 5.7 g of sodium hydroxide were placed in 81 g of deionised water and stirred for 10 min to create a homogeneous solution. 118

119 Chapter 5. - After stirring, 20.6 g of ANK metakaolin were added, and the mixture was stirred for 24 h to obtain the final aqueous solution. Zeolite A seed preparation Zeolite A seeds were prepared from commercial zeolite A (BDH Chemicals, Ltd., Poole, England) to be utilized in the conversion of kaolin to pure zeolite A. The preparation of the seeds is described below: g of commercial zeolite A (average particle size of nm, shown by DLS) were transferred along with 200 ml of wetting agent (deionised water) to a 500-ml, plastic bottle (radius = 3.5 cm). - The contents were milled by ceramic zircon oxide beads ( D = 1 mm), using a ball milling machine for 12 h with a speed of 160 rpm. - After the ball milling process, the ceramic grinding media were separated from the mixture and removed, leaving a slurry of zeolite A. - The slurry was sonicated for 3 h and kept for 3 d to segregate the heaver particles from the colloidal suspensions and the colloidal solution. The seeds and slurry that were suspended by the ball mill process were shown by DLS to have an average particle size of 42.4 nm (Figure 5.2). 119

120 Chapter 5. Figure 5.2: DLS image of (a) suspended seeds and (b) the ball mill slurry, with average particle size of 42.4 and nm, respectively. Zeolite synthesis process The prepared zeolite gel (using WBB kaolin sources) was transferred to a 100-ml, Teflon-lined, stainless-steel autoclave where it was left for 24 h, allowing the heavier particles to settle. A 0.15-gram portion of the colloidal solution was transferred into a crucible bowl and treated in an ultrasonic bath for 2 h to be attached to the surface by electrostatic force and act as a nucleation core for zeolite growth. The colloidal solution in the crucible bowl was located on the Teflon holder within the aqueous solution in the Teflon-lined, stainless-steel autoclave, as shown in Figure 5.1. The solution was crystallized at different ageing and crystallization 120

121 Chapter 5. periods i.e., 3, 12 and 24 h. After each synthesis run, the autoclaved samples were removed from the oven and allowed to cool to room temperature. The powder samples were washed with deionised water in a vacuum filtration system to obtain a ph less than 8, after which they were stored for later XRD and SEM analyses. 5.2 Zeolite films and membranes preparations, using conventional methods. This section presents all of the experimental work performed in fabricating zeolite films and membranes, including the pre-treatments of the stainless-steel support using the conventional method that has been used extensively in many studies and research efforts. In this work, two conventional methods were used to prepare zeolite membranes i.e., the modified in-situ method (MIM) and the secondarygrowth method (SGM) Zeolite film/membrane supports Since these methods require that the synthesised zeolite layer be supported on another material to acquire mechanical strength (discussed in chapter 3, page 76), porous and non-porous stainless-steel metals were involved in the synthesis procedure. The choice of support material is of great importance, since many studies have indicated that the properties of the support structure influences the characteristics of the nucleation growth that forms the zeolite layer [190]. However, the types of supports that are used most frequently in the zeolite membrane industry are alumina and stainless-steel supports, mainly because the structure and properties of their surfaces are compatible with the zeolite gel [112]. Stainless-steel supports were chosen in this study due to the following advantages they offer: 121

122 Chapter 5. - Corrosion resistance: stainless steel can resist corrosion in many acids and alkaline solutions. - Heat resistance: stainless steel has excellent heat conductivity, which makes it favourable for use with many reaction processes. - Fabrication simplicity: stainless steel can be easily cut and fabricated to membrane model. - Cost prospective: considering the long-term costs, stainless steel is probably the cheapest option. However, in the preliminary stage of this work, low-priced supports (non-porous, stainless-steel metals) were used to fabricate zeolite films to investigate the effects of both crystallization and ageing time on the formation of the zeolite structure before using high-priced, porous, stainless-steel supports. The non-porous supports were used to prepare the zeolite films, and the zeolite membranes were fabricated using porous supports Modification and pre-treatment of supports For the fabrication of the zeolite A films, non-porous, stainless-steel metal, obtained from Multi-Metal Service, Ltd., Lancashire, UK, was cut into 20 x 20-mm pieces that were 1.5 mm thick. As for the membranes, the support metals were circular, porous, 20-mm diameter, stainless-steel pieces that were obtained from Aegis Advanced Materials, Ltd., UK. In general, it was very important for the surfaces of the supports to be clean, because impurities (e.g., grease and organic materials) have a negative effect on the growth of zeolites and prevent nucleation from taking place on the support. Therefore, the porous and non-porous supports were soaked in a mild detergent solution (Dish-Bac liquid detergent) and sonicated with deionised 122

123 Chapter 5. water for 3 h at room temperature. Then, the supports were left to dry overnight at room temperature. Hang Chau and co-workers have expressed the potential effect of modifying the support surface and condition to acquire better adhesion and interaction between the layers by altering both the structural and chemical properties of the support [111]. This was achieved by using a chemical pre-treatment before proceeding with the crystallization process (nucleation). Therefore, two methods were used in this study to create some kind of roughness on the metal surfaces of the supports to achieve better interaction and adhesion between the surfaces and the zeolite synthesis gel. Chemical pre-treatment techniques (i.e., surface etching and oxidation) were studied by Hang Chau and Davies, who proved that such treatment had the effect of promoting bonding between the supports and the zeolite synthesis gel. The two chemical pre-treatment techniques are: - Surface etching: a strong acid is used to create roughness in the metal surface and ensure better adhesion. - Surface Oxidation: the support surface is exposed to high temperatures in the range of 120 to 650 o C, causing oxidation of the surface, which extensively influences the chemical properties of the supports. In the etching process, the non-porous metals were soaked in a 2.2-M KOH solution with heating and stirring for 10 min, after which the metals were treated 37% HCl. The final stage of the etching process involves washing the metals with a 1-M NaOH solution and leaving them to dry overnight at50 o C. In the oxidising process, the metal heated to a temperature of 650 o C for 10 h, after which it was allowed to cool to room temperature overnight. 123

124 Chapter Using the Modified In-situ Method (MIM) With Etched and Oxidised Non-porous Supports As mentioned in chapter 3, page 77, the MIM method involves the combination of both the zeolite synthesis mixture and the supports. The non-porous, etched/oxidised support metal was washed after the pre-treatment process described in the previous section, and, then, it was transferred with the synthesis mixture of zeolite A to a 50-ml, Teflon-lined autoclave. The non-porous supports were held by a homemade Teflon holder inside the autoclave, as shown in Figure 5.3. The zeolite A synthesis mixture was prepared according to Equation 5.2 using deionised water, sodium aluminate (50% Al 2 O 3, 40% Na 2 O), sodium hydroxide pellets (99% NaOH), and sodium silicate solid (47% SiO 2, 52% Na 2 O) as source materials. Then, the powder sample product was washed with deionised water using a low vacuum system to obtain a ph less than 8, and the product was retained for XRD and SEM analyses. The results of the SEM analyses are presented in the next chapter (chapter 6). The Teflon-lined autoclaves were cleaned using a 10-wt% NaOH solution at 170 o C for 7 h after each experiment. Figure 5.3: Illustration of the Teflon-lined autoclave (size 50 ml) used in this work and the homemade Teflon holder inside the autoclave. 124

125 Chapter Using the Secondary Growth Method (SGM) with Etched and Oxidised Non-porous Supports. As stated in chapter 3, page 78, the main difference between this method and the MIM was the implementation of seed crystal growth of the zeolite phase using the seed-coating method. Therefore, this method is considered to be advanced over the MIM. After pre-treating the non-porous, stainless-steel metal supports by the etching and oxidizing process, the supports were subjected to the zeolite A seed-dip coating method, which can be summarized in the following steps: - A 200-ml sample of commercial zeolite A (BDH Chemicals, Ltd., Poole, England) was crushed gently by an agate mortar and transferred along with 200 ml of a wetting agent to a 500-mL plastic bottle (radius in). - The content was milled by ceramic zircon oxide beads ( D = 1 mm), using a ball milling machine for 12 hours at a speed of 160 RPM. - The ceramic grinding media were removed and separated from the mixture after the ball milling process, leaving a water-zeolite A slurry. - The slurry was sonicated for 3 h and kept for five days to segregate the heavier particles from the colloidal suspension. - The non-porous, stainless-steel metals were seeded by the dip-coating method, in which the supports were immersed in the colloidal suspension for 10 min and left to dry at room temperature for 20 min. - Both the etched and oxidized supports were held vertically by a homemade, Teflon holder inside the autoclave and immersed within the synthesis mixture. Since the SGM involves two steps, i.e., the dip coating and crystallization processes, both of the seeded supports were transferred with the aluminosilicate 125

126 Chapter 5. synthesis gel to an autoclave to conduct the crystallization process. The synthesis gel was prepared using Equation 5.2 using deionised water, sodium aluminate (50% Al 2 O 3, 40% Na 2 O), sodium hydroxide pellets (99% NaOH), and sodium silicate solid (47% SiO 2, 52% Na 2 O) as source materials. The quantity of 0.67 g of sodium hydroxide was added to 35 g of deionised water, and the solution was divided into two parts. A quantity of g of sodium aluminate was dissolved in the first part, and 1.18 g of sodium aluminate were dissolved in the second part. Then, the two solutions were mixed and aged for 24 h at room temperature. Both of the pretreated, seeded supports were transferred with the aluminosilicate gel into an autoclave. The supports were held vertically by the homemade, Teflon holder inside the autoclave, as shown in Figure 5.3. The crystallization process was conducted initially for 3 h, and zeolite formation began to occur in the bulk phase of the aluminosilicate gel and on the supports (porous and non-porous stainless steel). Unlike the MIM, the formation of zeolite began to occur after 6 h due to absence of the seeding process. Therefore, in the SGM, the process was conducted for 6 h at 100 o C Fabrication of zeolite membranes using Secondary Growth Method (SGM) with oxidized, porous supports. As mentioned earlier, the aim of synthesising zeolite films using different approaches was to determine the most appropriate method for fabricating zeolite membranes using porous, stainless-steel metal as supports. In the fabrication of zeolite films, SGM had the advantage of shorter duration with higher crystallinity than Modified in-situ Method (MIM), because the seeds have a major role in the growth of crystals to form zeolite. Both pre-treatment methods generated good formation of zeolite layers, but the oxidised supports yielded better (thicker) layers 126

127 Chapter 5. of zeolite than the etched supports (see chapter 7). Therefore, it was concluded that the SGM method with oxidising pre-treatment was more effective for fabricating zeolite membranes. Consequently, in this section, the fabrication of zeolite A membranes using the SGM method, which involves the use of supports that have been oxidized in a pretreatment process, is discussed. The support metal used in fabricating the zeolite membranes were circular, porous, stainless-steel metal disks with diameters of 20 mm, thicknesses of 1.5 mm and porosity of 0.5 µ. These disks were obtained from Aegis Advanced Materials Ltd., UK. The porous metal disks were washed by soaking them in a mild detergent solution (Dish-bac liquid detergent), after which they were sonicated in deionised water for 3 h at room temperature. Then, the washed metals were left to dry overnight at room temperature. The porous metal disks were pre-treated by an oxidation process, and then the SGM method was used for fabricating the zeolite membranes. As mentioned in chapter 3, page 78, this method involves two steps, i.e., seed-dip coating and the crystallization process. Both zeolite A seeds and the preparation of the aluminosilicate gel were conducted in a procedure similar to that discussed in section Then, the seeded, porous supports were transferred along with the aluminosilicate synthesis gel to an autoclave for the crystallization (hydrothermal) process, which was conducted for 6 h at 100 o C. The fabrication of the membranes was repeated 11 times to evaluate the repeatability performance. Then to assess different methods of developing the membranes. 127

128 Chapter Membrane performance testing and evaluation The performance of the 11 zeolite A membranes prepared by the SGM using oxidised support metals was tested by the pervaporation system. The theoretical developments of this process are presented in chapter 3, page 81. In this study, the pervaporation rig used to evaluated carbon-zeolite membranes consisted of six major elements, as shown in Figure 5.4 i.e., a feed tank, liquid pump, heater, membrane compartment, permeate traps, and vacuum pumps. The prepared feed mixture was taken into a well-sealed container to prevent heat loss and to avoid evaporation of the mixture. For some experiments, heating of the feed mixture was required which was achieved by heating the feed container using a water bath and magnetic stirrer. The control of temperature was of great importance, as there was heat loss from the connection tubes. In order to maintain a uniform and constant temperature during the experimental run, the feeding process was subjected to a circulation system (between the external feed container and the membrane compartment) using a liquid pump. The liquid pump was used to take the feed mixture from the feed tank to the membrane compartment at a constant volume rate, and to return the retentate streams back to the feed tank. The design of the membrane compartment used in this study contained two hemispherical glass sections, as shown in Figure 5.5. The prepared disc-shaped membranes were fixed in the membrane compartment by gluing on a nonporous metal washer using twocomponent adhesive epoxy araldite rapid obtained from Fisher Scientific UK, Ltd. However, this type of glue was inadequate for the long, repeated experiments, and it dissolved after about 10 days. Therefore, this glue was used only for testing the membranes until the first time they had to be detached for further post-treatments. In order to acquire the desired performance of a given membrane, the original glue 128

129 Chapter 5. was replaced by a two-component, J-B weld, epoxy adhesive, which lasted for more than a month of experiments conducted in ethanol/water mixtures.then, membranes attached to the washers were fitted between two Teflon rings and sealed with vacuum grease between the hemispherical glass parts. Finally the hemispherical glass parts were clamped together using clamp ring (Figure 4.5) Feed Tank 2- Water Compartment (plastic bottle) 3- Heater / Magnetic Stirrer 4- Liquid Feed Pump ( 130 ml/min) 5- Stirrer 6- Membrane Compartment 7- Permeate Traps 8- Liquid Nitrogen 9- Vacuum Pump Dewar flask Liquid nitrogen Figure 5.4: Schematic diagram of pervaporation unit with membrane module. 129

130 Chapter Figure 5.5: Pervaporation used in this study and the membrane compartment. (1) feed tank, (2) water compartment, (3) heater / magnetic stirrer, (4) stirrer, (5) membrane compartment, (6) Permeate Traps, and (7) Vacuum Pump. 130

131 Chapter 5. In this process the feed is in the liquid phase while the permeate is a vapour resulting in a pressure difference across the membrane that causes the pressure driving force. In other words, the feed side pressure is normally high to ensure that there is a difference with the feed components vapour pressure. Accordingly, the permeate pressure should be adjusted to be significantly lower than the permeate components vapour pressure. Therefore, the permeate side of the pervaporation system was evacuated using a vacuum pump. The reason for choosing the vacuum pump over the sweep gas was the need to recover the permeate, which is difficult when using a sweep gas system. In this work, the pervaporation process was conducted with different feed compositions with total volume of 120 ml, at atmospheric pressure on the feed side. On the permeate side, the pressure was set to 8 Pa, and the total permeate of 0.15g (vapour phase) was collected by condensing it with liquid nitrogen trapping system. Therefore, the change in the feed concentration was assumed to be negligible. The very low pressure in the permeate side leads to the need for very low dew points (condensation temperature), for example, the boiling point of ethanol is o C at 0.5 Pa, therefore, liquid nitrogen was required to fulfil this task [191]. Liquid nitrogen boils at -196 o C at atmospheric pressure and was an ideal freezing medium for all of the mixture components used in this study. However, liquid nitrogen might trap explosive liquid oxygen by condensing oxygen from the atmosphere, as liquid oxygen has a boiling point of -183 o C. This could accrue if the low temperature system is vented to air. Therefore, it was of a great importance to ensure that there was no air leak in the system by monitoring the pressure gauge constantly. In this study, three traps were included in the pervaporation design, as shown in Figure 4.4. The first trap was for sample collection, while the second and 131

132 Chapter 5. third traps were used for safety reasons and precautions. At the end of each experiment, the first trap was kept under vacuum conditions and the liquid nitrogen Dewar flask was removed to allow the permeate sample to defrost. Then, the liquid nitrogen Dewar was attached to the tip of the liquid trap glass and removed several times to ensure total collection of the permeated sample.the sample was then removed from the trap using a glass pipette, weighed and kept in the refrigerator for further analysis. The performance of each membrane used in the process described above was estimated in terms of separation factor and total flux. The separation factor (αi,j) αi,j= W P,i. W F,j (5.6) W P,j. W F,i Where: - i and j are the binary components of the mixture - W P and W F are the weight composition of permeate and feed, respectively. Total permeate flux m s F= (5.7) A t - m s is the weight of the collected permeate sample - A is activated membrane surface area 132

133 Chapter 5. - t experiment duration Initially, these membranes were tested with ethanol / water solution. The difficulty of separating this azeotropic mixture by other conventional processes makes this mixture one of the most classic mixtures used in the pervaporation process. The feed mixture was injected to the feed side of the membrane compartment at room temperature with feed compositions of 4, 6, and 20 wt% water. The permeate was collected and weighted to determine the overall fluxes, after which it was stored in a refrigerator for the GC analyses to estimate the separation factors. In theory, a zeolite A membrane should allow water to pass through preferentially, while restricting the transport of ethanol through the membrane [62]. Despite the fact that the pores of zeolite A are very close to the kinetic diameter of ethanol, the separation mechanism should be based on the affinity and selective adsorption and diffusion rate due to zeolite A hydrophilicity. However the results obtained in this work indicated that these prepared membranes did not yield high performances (chapter 7) due to pinholes and defects in their structures. Therefore, these membranes were post-treated using the two methods described in the next section. 5.4 Repair of SGM membranes using the rubbing method and carbon properties [192] Using rubbing post-treatment with zeolite A seed paste The element of improving zeolite membranes by applying rubbing post-treatment to the membranes after the dip-coating and crystallization processes was conducted in this study. The dip-coating method, which is considered to be the main part of the SGM method provides a good distribution of the seeds on the support membrane, but there is weak coverage over any pinholes. On the other hand, the rubbing 133

134 Chapter 5. treatment provides coverage over pinholes, but it lacks the ability to create surface uniformity. Therefore, the surfaces of three membranes that were prepared earlier in this work using the SGM (three out eleven membranes) subsequently were rubbed with seed paste of zeolite A. The paste was prepared by mixing commercial zeolite A obtained from BDH Chemicals, Ltd., Poole, England with same amount of deionised water by weight. Then, a 0.28-g portion of the paste was applied and gently rubbed on the surface of the membrane that had been prepared by the SGM method. Afterwards, these membranes were exposed to low vacuum suction for 5 min to acquire better adhesion to the surface, and then they were left overnight at temperature of 50 o C. These membranes were tested with an ethanol/water mixture in order to compare them with the earlier results obtained using the SGM method (discussed in chapter 7) and indicated an improvement in the separation factor as expected but the overall fluxes of the three membranes were lower than the fluxes before the rubbing post-treatment was applied. Therefore, another approach was evaluated that involved the incorporation of carbon properties Using the properties of carbon As mentioned in chapter 3, page 54, the carbon materials had very narrow pores ( Å) that adsorb molecules based on size and shape exclusion, and they have been used extensively in the gas separation industry and in water treatment processes to meet different objectives and separation requirements. In this part of this study, the aim was to acquire a carbon precursor that possessed pores that were smaller than those of the binary mixture that was tested to block any pinholes in the structure of the zeolite layer. Therefore, polyfurfuryl alcohol (PFA) was used as a carbon precursor to form the carbon layer in the zeolite membrane. PFA was chosen because it is considered to be a very typical thermosetting resin, i.e., it softens easily 134

135 Temperature o C Chapter 5. when heated and hardens when it is cooled, and provides high yields of carbon formation [193,194]. Moreover, furfuryl alcohol has the ability to produce carbon that has desirable properties, i.e., very narrow pores with considerable chemical stability. The preparation of carbonaceous polyfurfuryl alcohol was conducted in two steps, i.e., polymerization and carbonization. Therefore, 0.3 g of furfuryl alcohol, 99% (obtained from Sigma-Aldrich Co., Limited) was applied to the surfaces of the synthesised zeolite membranes (two out the eleven membranes) discussed in the previous section, after which they were placed into a low vacuum system for 10 min and then left overnight to dry. Then, the membranes were immersed in 1 M HCl aqueous solution at 80 o C for 24 h, where the polymerization took place. The zeolite membrane covered with polymerized furfuryl alcohol was placed in a nitrogen gas pyrolysis system (for carbonization) located in tubular furnace at a heating rate of 5 o C/min with a thermal-soaking period of 4 h at 800 o C and at a nitrogen flow rate of 4 l/min (Figure 5.6). At this stage, four of the membranes that were made earlier by the SGM method were subjected to posttreatment with polyfurfuryl alcohol, to examine the repeatability of the performance of these membranes and the results are discussed in chapter min 5 o C/ min 155 Time (min) 395 Figure 5.6: tubular furnace used for the pyrolysis process, with the temperature system trend. 135

136 Chapter 5. Although PFA produced drastic improvement in the overall fluxes with competitive separation behaviour, the distribution of the carbon layer and its coherence over the zeolite membrane was not easy to achieve using PFA (chapter 7). For that reason, a sucrose precursor was chosen because it was simple to prepare and did not require any pre-treatment. Moreover, sucrose has the advantage of being a natural resource that does not require any extensive usage of energy in its production. In this study, the two remaining membranes from the SGM were tested using carbon posttreatment, with a sucrose solution providing the carbon precursor. The sucrose solution was prepared by dissolving 2.5 g of sucrose (obtained from Fisher Scientific UK, Ltd.) in 5 g of deionised water. After 10 min of mixing, 0.45 g of sucrose solution was applied on the surfaces of the zeolite membranes prepared by SGM under low vacuum pressure for 5 min and left for to dry for 1 h. For the carbonization process, the membranes covered with the sucrose solution underwent the same procedure discussed before with carbonizing PFA. After the pyrolysis process was finished, the thick, crescent-shaped carbon layer that had formed over the membrane was removed very gently. 5.5 Novel Technique for Fabricating Carbon-Zeolite Membranes To date, the fabrication of zeolite membranes using conventional methods requires a long procedure that starts with the synthesis of zeolite gels. Moreover, many studies have expressed the difficulty of synthesising many types of zeolites in laboratory conditions, despite the fact that many types of zeolites exist in abundance in nature, e.g., clinoptilolite [2]. Thus, the aim of this work was to develop a simple, inexpensive, and less time-consuming process for the production of zeolite membranes. The approach to achieving this task was to avoid the complexity of the zeolite gel preparation process, the lengthy preparation time required, and the 136

137 Chapter 5. thermal crystallization process by replacing them with direct coating of synthetic or natural zeolites. The novel fabrication method introduced in this study was simply initiated by rubbing the paste of the desired type of zeolite on a stainless-steel support and then applying the carbon precursor before the carbonization process. At this stage, the aim was to test zeolite A membranes synthesised by this novel method and compare their performances with those of SGM and carbon post-treated membranes Zeolite A membrane This section describes the fabrication of a zeolite A membrane using zeolite A paste that was prepared by mixing deionised water and commercial zeolite A (BDH Chemicals Ltd., Poole, England) in the ratio of 1:2 by weight. A quantity of 0.3 g of the prepared paste was applied on the porous, stainless-steel supports, which, like the supports used before, were porous, stainless-steel disks that were 20 mm in diameter and 1.5 mm thick (obtained from Aegis Advanced Materials Ltd., UK). After the paste was applied to the metal disks, it was allowed to dry for one hour at room temperature, after which 0.45 g of sucrose solution was prepared with different concentration ratios by weight (0.5:1, 0.7:1, 1:1, and 3:1) and applied over the zeolite paste layer in a low vacuum system. After that, the membranes were placed in the tubular furnace for the carbonization process according to the procedure discussed in section Mordenite membrane As described in chapter 7, the results showed that those membranes prepared by the short, novel method were synthesised successfully with comparable separation factors to those of the previous membranes and relatively good fluxes., Therefore, 137

138 Chapter 5. another attempt was taken to evaluated different type of zeolite membrane (Mordenite membrane) for ethanol dehydration using the same novel fabrication technique for comparison purposes. Thus, a Mordenite paste was prepared by mixing deionised water and commercial Mordenite (obtained from Zeolyst International) in the same ratio and quantities used for zeolite A. These membranes were tested at different operational conditions and different feed concentrations of the ethanol/water mixtures. (The results are discussed in chapter 7.) Clinoptilolite membrane The ability to fabricate any type of zeolite membrane using the novel method presented in this work would be a step forward in the zeolite membranes synthesis landscape, since it was achieved with easy procedure and requires a much shorter fabrication time than the conventional method. Chi and Sand noted the difficulty of synthesising many types of zeolite, including clinoptilolite, and they observed that this difficulty was a major obstacle for the fabrication of anisotropic membranes of these zeolites using conventional methods, e.g., in-situ and secondary-growth methods [2]. Therefore, the feasibility of fabricating clinoptilolite membrane was considered using the same novel method with zeolite powder that was acquired from natural and commercial sources. Thus, equal amounts of natural clinoptilolite (obtained from Holistic Valley) and deionised water were mixed to form the paste, and, then, 0.33 g of the prepared paste was applied over the porous, stainless-steel disc and left to dry for 1h at room temperature. As for the carbon precursor, a low concentration ratio (0.5:1 by weight) sucrose solution was used because, as discussed in chapter 7, a high concentration did not yield good separation behaviour. Then, 0.6 g of the sucrose solution was applied on the dried zeolite paste under low vacuum pressure for 5 min. Afterwards, the carbonization process was 138

139 Chapter 5. conducted in a nitrogen gas pyrolysis system in a tubular furnace at a heating rate of 5 o C/min with a thermal soaking period of 4 h at 800 o C with a nitrogen flow rate of 4 l/min. After the pyrolysis process, the carbon-clinoptilolite membrane was glued onto a non-porous metal washer and fitted in the membrane cell using (twocomponent adhesive epoxy araldite rapid obtained from Fisher Scientific UK, Ltd.), allowing it to be detached for post treatment required (as mentioned in section 5.3 and described in section 4.3). Carbon-clinoptilolite membrane was evaluated for separating different mixtures at different operating conditions. The tasks used to evaluate the clinoptilolite membrane were the separation of the ethanol/cyclohexane mixture, removal of phenol from water, and separation of the xylene isomers ZSM-5 membranes The novel method introduced in this work for fabricating zeolite membranes with hydrophilic properties (low Si:Al ratio) was successful for all membranes, and short times were required to fabricate the membranes. In this section, the feasibility of fabricating hydrophobic membranes using the novel method is evaluated. Due to the importance of separating a mixture of xylene isomers in petrochemical-related industries, this mixture was an excellent candidate for use in testing hydrophobic membranes, such as ZSM-5 membranes, in a pervaporation system. The literature indicated that ZSM-5 has been used extensively in organic separation processes due to its hydrophobic property and its pore size (0.6 nm). These key properties make ZSM-5 suitable for numerous of organic separation applications. Xylene isomers, i.e., p-xylene, m-xylene, and o-xylene have similar boiling points, i.e., 138.5, 139.3, and144 o C, respectively. This makes them difficult to separate by conventional methods, e.g., distillation. In this mixture, p-xylene is expected to permeate through the membrane since its kinetic diameter is estimated to be 0.58 nm, while both m- 139

140 Chapter 5. xylene and o-xylene have kinetic diameters of 0.68 nm. In order to obtain clear and reliable results, p-xylene and o-xylene were chosen because of the greater difference in their boiling points (5.5 o C). This eased the analysis of the products using gas chromatography, which was expected to yield well-distinguished peaks in the product s pattern. The process of fabricating ZSM-5 membranes using the novel method introduced in this study was performed using an aqueous sucrose solution (1:1 by weight) as a carbon precursor. Equal weights of ZSM-5 (obtained from Eka Nobel) and deionised water were mixed to form the ZSM-5 paste. Then, 0.4 g of the ZSM-5 paste was applied on porous, stainless-steel disc (discussed in section 5.2.1), and the disc was placed in a low vacuum system for 5 min. Then, the porous metal coated with ZSM-5 paste was allowed to dry for 1 h. A sucrose solution was prepared by mixing 2 g of sucrose (obtained from Fisher Scientific UK, Ltd.) with 2 g of distilled water. A 0.45 g quantity of the sucrose solution was used to cover the ZSM-5 paste that had been in the low vacuum system for 5 min, and then the disc was subjected to the pyrolysis process for 4 h at a heating rate of 5 o C/min in the tubular furnace discussed in section The carbon-zeolite membrane (ZSM-5) that was produced was glued onto a non-porous, stainless-steel support using (twocomponent adhesive epoxy araldite rapid obtained from Fisher Scientific UK, Ltd.) as a preliminary stage, allowing it to be detached, if required, for post treatment (as mentioned in section 5.3). After that, Teflon rings were used to fit the membrane between the two compartments of the membrane cell, and the membrane was clamped. The o-/p-xylene mixture was injected to the membrane at different feed concentrations and temperatures. 140

141 Chapter 6. Chapter 6 Zeolite A Synthesis from Kaolin, Results and Discussion 6.1 Introduction In this section, the results and outcome of zeolite A preparation using cheap raw materials, including a commercial source from WBB UK and kaolin obtained from virgin kaolin are discussed and presented. The novelty of converting virgin Nigerian kaolin to pure zeolite A at low temperature without any pre-treatment was a preliminary stage of this work and aimed to be achieved before moving to membrane synthesis. As the goal behind converting raw materials (virgin Nigerian kaolin) to pure zeolite powder, is to utilize this novelty into fabricating zeolite membranes. Although there are some difficulties associated with using these raw materials because they usually contain several impurities, e.g., quartz, iron, and feldspar [181]. But it was of a great importance to remove these impurities because they negatively affect the commercial value of these products. 6.2 Synthesis of Zeolite A Zeolite A using commercial source material. Before testing the feasibility of using raw materials to synthesise zeolite A, a standard, conventional procedure and commercial sources were used for comparison purposes. Therefore, materials from commercial sources, i.e., deionized water, sodium aluminate, sodium hydroxide pellets, and sodium silicate, were used to prepare the synthesis gel of zeolite A (as described earlier in section 5.1.1). Then, the prepared aluminosilicate gel was transferred to an autoclave in which the 141

142 Intensity Chapter 6. crystallization process took place for 3 h at 100 o C. The results of the SEM analyses are presented in Figure 6.1, which shows zeolite morphology at a special resolution of 10 and 5 µm. XRD was used to evaluate each sample, and the key peaks that were obtained showed good agreement with the commercial, standard sample (Figure 6.2). 10 μm 5 μm Figure 6.1: SEM image of zeolite A particles at 10 and 5 µm spatial resolutions Degree 2 theta Zeolite A standard Zeolite A sample Figure 6.2: Comparison of the synthesized and standard samples of zeolite A. 142

143 Chapter Zeolite A prepared using kaolin obtained from WBB UK and Ahoko Nigerian kaolin (ANK) As it was mentioned in the literature by [179], kaolin clay may be a potential candidate to replace the precursors of micro-porous materials because it is readily available and is more environmentally-friendly than other precursors. Moreover, the Si:Al ratio in kaolin is approximately 1, which makes it an ideal choice as a lowsilica, zeolite precursor. Therefore, the approach of synthesising pure zeolite A was investigated using raw kaolin as the raw material to serve as the main source of alumina and silica. In this work, two different sources of kaolin were used to synthesis zeolite A i.e., WBB commercial kaolin (purified) and Ahoko Nigerian kaolin (ANK). Before applying these sources to the synthesis process, both sources underwent the dehydroxylation or metakaolinisation process to remove some impurities, such as mica and to increase the reactivity of kaolin by exposing it to elevated temperatures to form zeolite with low Si:Al ratios. The synthesis procedures of both sources are described in section Both the WBB and raw ANK kaolin samples were analysed by XRD and after the hydroxylation process, it was evident that the mica impurities were removed from both the WBB kaolin and the ANK kaolin (first peak in Figure 6.3 and 6.4).First, WBB purified kaolin that consisted of 4.6% quartz was used to synthesise pure zeolite A at different crystallization intervals, i.e., 3, 12 and 24 h. After the synthesis of zeolite A, two samples were taken and analysed by XRD, i.e., the samples were collected in a crucible bowl (pure zeolite without quartz), and the materials remaining at the bottom of the autoclave consisted of zeolite A and impurities. The zeolite samples that were synthesised using WBB kaolin at different intervals are shown in Figure 6.5. Another comparison also was conducted between the samples collected at the 143

144 Intensity Intensity Chapter 6. top of the crucible bowl and the materials at the bottom of the autoclave (Figure 6.6). High purity zeolite A was formed in the bowl with short crystallization intervals, and impurities began to form as the crystallization intervals became longer. The SEM images for both products i.e., top and bottom samples of zeolite A after crystallization at 100 o C for 3 h, are shown in Figure 6.7, which clearly shows pure crystals of zeolite A in the sample taken from the crucible bowl Hydroxylation process of ANK Raw Kaolin Metakaolin Degree 2 theta Figure 6.3: Comparison of the ANK source before and after the hydroxylation Hydroxylation process of WWB Raw kaolin metakaolin Degree 2 theta Figure 6.4: Comparison of the WWB source before and after the hydroxylation. 144

145 Intensity Intensity Intensity Chapter Degree 2 theta Standard-A S-T-3 (a) Degree 2 theta Standard-A S-T-12 (b) Degree 2 theta Standard-A S-T-24 (c) Figure 6.5: Comparison of standard zeolite A and the synthesised sample of zeolite A from WBB at the top at different crystallization intervals (a) 3,(b) 12 and (c) 24 h. 145

146 Intensity Intensity Intensity Chapter (a) S-B-3 S-T Degree 2 theta 2500 (b) S-B-12 S-T Degree 2 theta 3000 (c) S-B-24 S-T Degree 2 theta Figure 6.6: Comparison of the sample collected at the top of the crucible bowl and the sample from the bottom of the bowl at different crystallization intervals (a)3, (b)12 and (c)24 h using WBB. 146

147 Chapter 6. (a) (b) 20 μm 10 μm (c) (d) 20 μm 5 μm Figure 6.7: SEM images of top (a,b) and bottom (c,d) products of zeolite A from WBB. 147

148 Chapter 6. After the successful synthesis of zeolite A using purified WBB kaolin, it was considered to take the work to the next level by using a virgin source of kaolin ANK. The preparation of the gel using ANK is described in the previous chapter (section 5.1.2), but, after preparing the synthesis gel, the final gel was cloudy, and it was expected that it would contaminate the top product, even after allowing 24 h for the heavier particles to settle. Next, the crystallization process was conducted at100 o C for 3 h (as it yielded to the best result in case of using WBB). The XRD analysis showed a small amount of impurities in the final product, which was expected because there is a greater percentage of quartz in the ANK kaolin than in the WBB kaolin, leading to the formation of the cloudy solution that contaminated the colloidal solution in the crucible bowl. Therefore, the ANK was washed before it was used in the synthesis gel. The washing process involved a brief stirring of 50 g of ANK kaolin with 300 ml of deionised water for 10 min. Then, the mixture was left for 24 h to allow the heavier particles to be segregated from the suspensions by settling. Afterwards, the excess water was decanted and removed from the slurry, and the product was dried overnight at 60 o C. The washing process was repeated twice to ensure that the suspended impurities were removed. Even though the impure, decanted water contained 5% kaolin, the benefits it provided over other methods, e.g., the flocculation process, were worth the energy and the time. The new procedure, which involved pre-treatment by washing, yielded a clear synthesis gel and the formation of pure zeolite powder in the crucible bowl after the crystallization process. Figure 6.8 compares the product before and after the washing process. The SEM images of the top and bottom samples presented in Figure 6.9, show that zeolite A with the distinctive, cubic structure that measure 2 μm on each side, indicating that crystal growth took place on the colloidal seeds (42 148

149 Intensity Intensity Chapter 6. nm). For the bottom sample, Figure 6.9 shows the non-uniform structure of quartz without any crystal formation. (a) Degree 2 theta Before washing Zeolite Standard Top sample (b) Degree 2 theta After washing Zeolite Standard Top sample Figure 6.8: Comparison of the synthesized zeolite A (a) before and (b) after the washing process. 149

150 Chapter 6. (a) (b) 2μm 1μm (c) (d) 20μm 20μm Figure 6.9: SEM images of top (a,b) and bottom (c,d) products of zeolite A from ANK using ball mill slurry. 150

151 Chapter 6. From these results, it can be concluded that the synthesis of pure zeolite A was achieved successfully. However, another approach was considered to assess the possibility of eliminating the time- and energy-intensive sonication step by using the slurry produced during ball milling (see 5.1.2). This allowed us to overcome the sonication of the seeds to the crucible bowl. Therefore, the ball mill slurry, an average particle size of 744 nm was applied directly by rubbing it on the crucible bowl, and crystallization occurred at the same conditions as before. The slurry was superior to the suspended solids because the seeds were not suspended in the aqueous phase and were not mixed with the bottom product. The XRD results for zeolite A using suspended solids and the slurry are shown in Figure From this chapter of this work, it can be concluded that the aim of synthesising pure zeolite A using a cheap raw material, i.e., virgin kaolin, extracted directly from the source was accomplished successfully using an economical and straightforward technique. However, the amount of pure zeolite A product in the bowl was insufficient for use with the membrane fabricating process. Consequently, future work related to the fabrication of zeolite membranes will be focused on using commercial sources of zeolites rather than using the purified products that were discussed earlier in this chapter. 151

152 Intensity Intensity Chapter Degree 2 theta Degree 2 theta (a) Suspended solids Zeolite Standard Top sample (b) Slurry Zeolite Standard Top sample Figure 6.10: Top sample of zeolite A compared with the commercial standard, using (a) suspended solids and (b) ball mill slurry. 152

153 Chapter 7. Chapter 7 Novel Technique for Fabricating Carbon-Zeolite Membranes, Results and Discussion 7.1 Introduction This section presents all of the results and outcomes associated with fabricating the zeolite films and membranes, including the conventional method of treating the stainless-steel supports, which has been used extensively in many studies and research efforts (repeatability tests and result details are given in appendix A and B, respectively). Initially and for comparison purposes, the conventional modified insitu method (MIM) and the conventional secondary-growth method (SGM) were used to prepare the anisotropic zeolite membranes. Then, a novel method was used to fabricate different types of zeolite membranes. 7.2 Zeolite films and membranes preparations, using conventional methods Zeolite film/membrane supports preparation Stainless steel was used in the synthesis procedure because it had the properties required to fulfill the operational tasks and needs (see section 5.2.1). First, a less expensive stainless steel, i.e., non-porous stainless steel was used to evaluate the formation of the zeolite layer on the metal support. As mentioned in chapter 5, section 5.2.2, the stainless steel underwent two types of treatments, i.e., surface etching and surface oxidation (see section 5.4.2). After subjecting the metal to both pre-treatment processes, there was a noticeable difference in the SEM between the two stages, as shown in Figure 7.1, which shows that the etched support has 153

154 Chapter 7. changed, roughness has increased, and it is expected to promote better nucleation. The colour of the oxidised support metal became dark brown, which indicated that oxidation had occurred. Then, both the films and the membranes were fabricated using Teflon-lined, steel autoclaves, as mentioned in chapter 2 section 2.6, and they were cleaned using a 10-wt% NaOH solution at 170 o C for 7 h after each experiment. (a) (b) 200 μm 200 μm (c) 100 μm Figure 7.1: SEM image of (a) the non-porous stainless steel, (b) etched stainless steel, and (c) oxidised stainless steel. 154

155 Chapter Using the Modified In-situ Method (MIM) With Etched and Oxidised Non-porous Supports The MIM was conducted using the procedure discussed in section in chapter 5. Initially, the crystallization process was conducted for 3 h, and zeolite started to form in the bulk phase of the aluminosilicate gel (prepared using Equation 5.2), but there was no sign of its formation on the non-porous, stainless-steel supports. Therefore, the crystallization process was conducted several times at different durations, and the first formation of zeolite started to occur on the support surface after 6 h. Therefore, the crystallization process was conducted for 9 h at 100 o C to ensure better formation. The XRD analysis of the excess zeolite powder (Figures 7.2 and 7.3) showed resulting peak intensities (at 2θ=7.28) of 800 and 1100, for etched and oxidised supports, respectively. The morphologies of the zeolite layers that were formed on both of the pre-treated non-porous supports (etched and oxidised) were observed using SEM, and the results are presented in Figures 7.4 and 7.5, respectively. The SEM analysis indicated that zeolite A layers had formed on both the etched and oxidised metals with thicknesses of approximately 50 and 200 µm, respectively. Figure 7.6 shows that the non-porous stainless-steel metal that was treated by the oxidisation process had better coverage of zeolite A than the etched support. Therefore, it can be concluded that the oxidation process yielded a thicker zeolite A layer and better adhesion between the zeolite layer and the support with the MIM. 155

156 Intensity Intensity Chapter Zeolite A standard Zeolite A sample Degree 2 theta Figure 7.2: Comparison of the XRD patterns of zeolite A film using etched support and standard samples Zeolite A standard Zeolite A sample Degree 2 theta Figure 7.3: Comparison of the XRD patterns of zeolite A film using oxidised support and standard samples. 156

157 Chapter 7. (a) 20 μm (b) 200 μm zeolite A layer support metal Figure 7.4: SEM of zeolite A layer (54.17 µm) on non-porous metal surface after the etching process: (a) top view and (d) edge view. 157

158 Chapter 7. (a) 20 μm (b) 400 μm zeolite A layer support metal Figure 7.5: SEM of zeolite A layer ( µm) on non-porous metal surface after the oxidising process: (a) top view and (b) edge view. 158

159 Chapter 7. (a) (b) 100 μm 100 μm Figure 7.6: Comparison between the zeolite formation layers on (a) etched (b) oxidised supports Using the Secondary Growth Method (SGM) with Etched and Oxidised Nonporous Supports. Secondary Growth Method (SGM) has the advantage of zeolite seed crystal growth implementation, which is achieved by seed-coating method. This additional step is expected to yield a better growth of zeolite layer over the stainless steel support. Therefore, both of etched and oxidised supports were treated with zeolite seed-dip coating method as described in section in chapter 5. Initially, the crystallization process was conducted for 3 h, and zeolite formation began to take place in the bulk phase of the zeolite synthesis gel and on the non-porous, stainlesssteel supports. On the other hand, in the MIM, zeolite formation began after 6 h due to absence of the seeding. Therefore, in the SGM, the process was conducted for 6 h at 100 o C to ensure better formation. The SEM results were in accord with the previous MIM synthesis method, because the oxidation process yielded a better intergrowth thickness of 309 µm, whereas the thickness on the etched supports was 121 µm, as shown in Figures 7.7 and 7.8. The XRD patterns of the zeolite A layer that coated the supports are shown in Figures 7.9 and 7.10, with the resulting peak 159

160 Chapter 7. intensities (at 2θ=7.28) of 1800 and 1700 for the etched and oxidised supports, respectively. The XRD analysis indicated greater crystallization of zeolite with a duration of six hours, which was less than the modified in-situ method Figures 7.2 and 7.3. To summarize the work in this section, the oxidising pre-treatment of the stainless-steel metal support yielded better adhesion than etching the supports since the thickness of the zeolite A layer was greater than that on the etched supports. As for the MIM and SGM methods, SGM showed better growth and faster formation of zeolite A on both the etched and oxidised supports than MIM. Moreover, the SGM method had the advantage of having the crystallization occur in a shorter time than the MIM. The longer time required by the MIM method also affects the properties of the zeolite, because zeolites are metastable materials, and longer crystallization times can transform a given type of zeolite into more dense, narrower structure, following Ostwald s law, eventually leading to the conversion of the type of zeolite (e.g., transformation of mordenite to analcime). Therefore, it was decided to proceed with the SGM method, which uses pre-treated, oxidised supports to fabricate zeolite membranes, as discussed in the next section. 160

161 Chapter 7. (a) 20 μm (b) support metal zeolite A layer 200 μm Figure 7.7: SEM of zeolite A layer (121 µm) on non-porous metal surface after etching process using secondary growth method: (a) top view and (b) edge view. 161

162 Chapter 7. (a) 200 μm (b) support metal zeolite A layer 500 μm Figure 7.8: SEM of zeolite A layer ( µm) on non-porous metal surface after oxidising process using secondary growth method: (a) top view and (b) edge view. 162

163 Intensity Intensity Chapter Zeolite A standard Zeolite A sample Degree 2 theta Figure 7.9: Comparison of zeolite film and standard sample patterns using the secondary growth method on etched support Zeolite A standard Zeolite A sample Degree 2 theta Figure 7.10: Comparison of zeolite film and standard sample patterns using the secondary growth method on oxidised support. 163

164 Chapter Fabrication of zeolite membranes using Secondary Growth Method (SGM) with oxidized, porous supports. The main goal of the work described in the previous section was to determine the most appropriate method for fabricating zeolite membranes, i.e., the MIM method or the SGM method. From the results obtained from using the zeolite films, it was decided to use the SGM to fabricate the zeolite membranes, using high-priced, porous, stainless-steel supports. Consequently, the zeolite membranes were fabricated using the same procedure that was used to fabricate zeolite films, including the seed-coating method and the surface-oxidation process. Then, the seeded, porous supports and the aluminosilicate synthesis gel were transferred to an autoclave for the crystallization process, which was conducted for 6 h at 100 o C, resulting in a 100-µm-thick zeolite layer, as shown in Figure Although the layer was thicker in the case of zeolite films, zeolite layers were formed successfully on both the membranes and films, and expected to yield a separation factor. The XRD analyses showed good matching of the key peaks with the standard sample (Figure 7.12). After the first successful synthesis of the zeolite membranes using porous, stainless-steel supports, the fabrication process was repeated several times to evaluate the repeatability of their performances. Then, these membranes were subjected to different post-treatment techniques to develop them and enhance their performance from the standpoints of their fluxes and separation factors. This is discussed in more detail in the next section of this chapter. 164

165 Chapter 7. (a) 50 μm (b) 20 μm 165

166 Chapter 7. (c) 300 μm support metal zeolite A layer (d) support metal 100 μm zeolite A layer Figure 7.11: Edge view SEM of zeolite A layer ( 100 µm) on porous metal surface after oxidising process using the secondary growth method (a,b) top view and (c,d) edge view. 166

167 Intensity Chapter Degree 2 theta Zeolite A standard Zeolite A sample Figure 7.12: Comparison of zeolite film and standard sample patterns using the secondary growth method on oxidised support Membrane performance testing and evaluation The performances of the 11 zeolite A membranes (similarly prepared earlier by the SGM using oxidised porous, stainless-steel supports) were tested by the pervaporation process described in section 5.3 of chapter 5. The pervaporation process was conducted at room temperature with different feed compositions of an ethanol/water mixture (i.e., 4, 6, and 20 wt% water) at atmospheric pressure on the feed side and a pressure of 8 Pa on the permeate side. According to Bowen et al [62], this type of membrane (zeolite A) should allow the water to permeate preferentially over the ethanol. Although the pores of zeolite A are very close to the kinetic diameter of ethanol, the separation mechanism was affected by the hydrophilicity of zeolite A (chapter 3, page 83). Therefore, zeolite A was expected to allow water to permeate through its pathways faster than any organic component. However, after conducting the experiment for 7 h in this study, the analysis of the permeates showed that two out of 11 membranes were leaking due to the lack of coverage and control of the formation of the zeolite 167

168 Chapter 7. A layer on the oxidised support using SGM. The other nine membranes i.e., A.1 through A.9, had different performance behaviours in terms of fluxes and separation factors, as listed in Table 7.1 and Figure The data shows that the highest separation factor achieved by the SGM was with permeate fluxes of g/m 2 h. In general, these membranes had low and modest performances in terms of fluxes and separation factors. The low fluxes can be justified since the high thicknesses of these membranes lower the diffusion rate of the permeate component, while the low separation factors likely are due to pinholes and to the long duration of the runs where the close sizes of the pores of the zeolite A the kinetic diameter of ethanol have a negative role in narrowing the diffusion preferences. Where, the kinetic diameter of ethanol is estimated to be 0.43 nm, and the pores of zeolite A have diameters of 0.41 nm. Each experiment was repeated two times at feed composition of 96% to ensure the quality of the results, which are compared in Figure The dependence of permeate flux (g/m 2 h) on the composition of the feed at constant temperature is illustrated in Figure Although, as mentioned earlier, the separation factors were relatively low, Figure 7.15 underlines the fact that water diffusion was faster than ethanol diffusion. The Figure shows that the permeate fluxes increase as the amount of water in the feed mixture increases, and the relation between the fluxes and the feed component of water was very close to being linear, especially for membrane A.1. In general, it can be concluded from the results listed in Table 7.1 that the attempts to fabricate zeolite membranes of high quality using the SGM method were not achieved due to the low performances of these membranes in terms of separation factors and fluxes. Basically, this was due to the unavoidable pinholes/ defects that existed because of the lack of control in the SGM method over the coverage of the 168

169 Chapter 7. zeolite layer on the porous supports. Therefore, testing these membranes with different temperatures was not conducted at this stage; rather, the use of different post-treatment techniques was considered to minimize the pinholes/defects in the membranes, and this is discussed in the next section of this chapter. 169

170 Chapter 7. Table 7.1: Evaluation of zeolite A membranes using SGM method at different feed compositions of water/ethanol mixture. Membrane Zeolite A.1 Zeolite A.2 Zeolite A.3 Zeolite A.4 Zeolite A.5 Zeolite A.6 Feed (wt%) Permeate (wt%) Permeate Water Ethanol Water Ethanol Flux (g/m 2 h) Separation Factor Zeolite A Zeolite A Zeolite A

171 Permeate Flux (g/m2h) Separation Factor Chapter Separation Factor Permeate Flux (g/m2h) Figure 7.13: Illustration of comparison between membrane performances in terms of separation factors and fluxes at 96% feed of ethanol st Run 2nd Run 3rd Run st Run 2nd Run 3rd Run Figure 7.14: (a) Illustration of separation factors repeatability and (b) fluxes repeatability at 96% feed of ethanol. 171

172 Permeate Flux (g/m2h) Permeate Flux (g/m2h) Permeate Flux (g/m2h) Chapter membrane A.1 membrane A.2 membrane A Water in feed wt% Water in feed wt% Water in feed wt% membrane A.4 membrane A.5 membrane A.6 membrane A.7 membrane A.8 membrane A.9 Figure 7.15: Illustration of the flux dependence on the feed composition using zeolite membranes i.e., A1-A9. 172

173 Chapter Repair of SGM membranes using the rubbing method and carbon properties Using rubbing post-treatment with zeolite A seed paste As mentioned earlier, the main aim of this work was to seek an efficient method of preparing and healing zeolite membranes. Two different post-treatment methods were tested for their ability to improve the zeolite membranes. The first method, i.e., the rubbing method, was conducted on three membranes that were prepared earlier in this work using the SGM (A.1, A.2, and A.3). The use of the rubbing method is described in chapter 5, section 5.4. The thickness of one of the posttreated membranes (membrane A.1) was measured by a SEM, which indicated that the thickness of zeolite layer was 301 μm, as shown in Figure In other words, the thickness of this particular membrane increased from 100 to 300 μm after the rubbing post-treatment. These membranes were tested with an ethanol/water mixture in order to compare them with the earlier results obtained using the SGM method. These membranes were tested with three different feed compositions i.e., 4, 6, and 20 wt% water, and the results indicated an improvement in the separation factor, which reached 35.15, as shown in Table 7.2. However, the overall fluxes of the three membranes were as expected lower than the fluxes before the rubbing post-treatment was applied. This was not surprising, and it clearly was due to the thicknesses of the zeolite layer, which minimized the number of pinholes and lowered the diffusion rate, leading to lower overall fluxes. The separation behaviours and performances of the zeolite membranes before and after applying the rubbing post-treatment were compared, and the results are presented in Figures 7.17, 7.18, and

174 Chapter 7. (a) 20 μm (b) 10 μm 174

175 Chapter 7. (c) support metal μm zeolite A layer 500 μm (d) 100 μm Figure 7.16: SEM of zeolite A layer (301.28µm) on porous metal surface after applying rubbing post-treatment (a,b) top view and (c,d) edge view. 175

176 Water/ Ethanol selectivity Chapter 7. Table 7.2: Evaluation of zeolite A membranes after using rubbing post-treatment with SGM. Membrane Zeolite A.1 Zeolite A.2 Zeolite A.3 Feed (wt%) Permeate (wt%) Permeate Water Ethanol Water Ethanol Flux (g/m 2 h) Separation Factor Separation Factor (a) Permeate Fluz (g/m2h) Total Permeability SGM (b) Rubbing treatment Figure 7.17: Comparison of the membrane performances before and after applying the rubbing treatment at (a) at 96% (b) at 96, 94 and 80 of ethanol in feed mixture. 176

177 Permeate Flux (g/m2h) Permeate Flux (g/m2h) Permeate Flux (g/m2h) Chapter Membrane A SGM SGM+Rubbing Water in feed wt% Water in feed wt% Membrane A.2 SGM SGM+Rubbing Water in feed wt% Membrane A.3 SGM SGM+Rubbing Figure 7.18: comparison of the permeate fluxes before and after applying the rubbing post-treatment for zeolite membranes A.1, A2 and A3 at different feed compositions of water/ethanol. 177

178 separation factor separation factor separation factor Chapter Membrane A SGM SGM+Rubbing Water in feed wt% Water in feed wt% Water in feed wt% Membrane A.2 SGM SGM+Rubbing Membrane A.3 SGM SGM+Rubbing Figure 7.19: Comparison of the separation factors before and after applying the rubbing post-treatment for zeolite membranes A.1, A2 and A3 at different feed compositions of water/ethanol. 178

179 Chapter 7. From the results, it can be concluded that the combination of the SGM method and the rubbing post-treatment improved the separation performance significantly by distributing the seeds over most of the defects; however, the low fluxes and the time required to prepare these membranes are likely to suppress the interest in this lengthy procedure. Moreover, these membranes were tested for a longer period and showed a reduction in their separation behaviour after approximately one week of experiments. Therefore, another approach was evaluated that involved the incorporation of carbon Using the properties of carbon The initial attempt to use carbon to improve the zeolite membranes involved the use of polyfurfuryl alcohol (PFA) because it has the ability to form small-pore-size carbon structure that should block any pinholes in the zeolite layer. Therefore, four of the membranes that were made earlier by the SGM method were subjected to post-treatment with polyfurfuryl alcohol i.e., A4, A.5, A6, and A7. These membranes were tested with an ethanol/water mixture with different feed compositions at room temperature for comparison purposes. After conducting the experiment, two of these membranes leaked, which probably was attributable to the harsh acid treatment and to the surface wetting between PFA and the hydrophilic structure of zeolite A. The two remaining membranes yielded separation factors of and with fluxes >1000 g/m 2 h. These membranes had considerably higher fluxes than the membranes prepared only by SGM, and they also had competitive separation factors (Table 7.3). The results of the high fluxes and the separation factors of the carbon-zeolite membrane could be described as a result of the thin layer of the membrane. It is well known that the thickness of the membrane has major role in determining mass transport and flux since it has been proven that 179

180 Chapter 7. transport rate is inversely proportional to the thickness of a given membrane. However, an attempt was made to determine the thickness of these membranes by SEM, but, as shown in Figure 7.20, no clear boundaries were found. able 7.3: Evaluation of carbon-zeolite membranes using PFA as a carbon precursor after the SGM at different feed compositions of ethanol/water mixture. Membrane C-Z (4) C-Z (6) Feed (wt%) Permeate (wt%) Permeate Water Ethanol Water Ethanol Flux (g/m 2 h) Separation Factor Leaking high 1 Table 7.4: comparison between membrane performances, before and after involving CPFA at feed composition 96% ethanol Membrane Zeolite A.4 Zeolite A.5 Zeolite A.6 Zeolite A.7 Fabrication Method Permeate (wt%) Water Ethanol Permeate Flux (g/m 2 h) Separation Factor SGM PFA SGM PFA Leaking SGM PFA SGM PFA Leaking 180

181 Chapter 7. (a) (b) 1 mm 50 μm (c) (d) 20 μm 400 μm support metal Figure 7.20: SEM image of carbon-zeolite composite layer using PFA at different spatial resolutions (a,b and c) top views and (d) edge view. As shown by the results listed in Table 7.4, the PFA produced drastic improvement in the overall fluxes with separation behavior that was competitive with that of the previous work. However, the distribution of the carbon layer and its coherence over the zeolite membrane were difficult to achieve when PFA was used. In other words, the repeatability results were very inconsistent and unreliable, since two of the membranes from the first run leaked, and the other two lasted only a short time (9 h) before they also began to leak. Based on the results achieved when PFA was used, it was replaced by sucrose as the carbon precursor. Sucrose was chosen due to 181

182 Chapter 7. its advantages, such as its being a natural source and the simplicity of the preparation process, which negated the need for any pre-treatment. The experimental part of this work included the two remaining membranes prepared using the SGM in section (7.2.4), and the sucrose precursor was prepared using the procedure described in section (5.4.2). After involving the sucrose precursor with the pyrolysis process, the thick, crescent-shaped carbon layer that formed over surface of the membrane was removed very gently. The SEM images of the surfaces of the carbon-zeolite composite membranes using the sucrose precursor are presented in Figure These images show that the chamfered-edged morphology of the zeolite crystals was not covered or blocked by the carbon layer. The carbonzeolite membranes that were prepared with sucrose were tested with the ethanol/water solution for comparison purposes, and they showed comparable results with the CPFA membranes, as shown in Table 7.5. A comparison was made to assess the improvement of these membranes before and after carbon posttreatment (sucrose) was applied, and the results in Table 7.6 show similar enhancement to the membranes repaired by PFA. As for the separation factors, Table 7.6 presents the performance of both SGM and carbon post-treatment for the same membranes, and it can be seen clearly that the separation factors have improved as well. The dependences of the flux and the separation factors on the feed composition using SGM post-treated with carbon precursors (PFA and sucrose) are shown in Figures 7.22 and 7.23, respectively. Therefore, it can be concluded at this stage that carbon incorporation generally has yielded considerable improvement in fluxes. However, the sucrose precursor was better than PFA in terms of its simplicity and the performance repeatability it provided. 182

183 Chapter 7. Table 7.5: Evaluation of carbon-zeolite membranes using sucrose as a carbon precursor after the SGM at different feed compositions of water/ethanol mixture. Membrane C-Z (8) C-Z (9) Feed (wt%) Permeate (wt%) Permeate Separation Water Ethanol Water Ethanol Flux (g/m 2 h) Factor Table 7.6: comparison between membrane performances, before and after involving sucrose at feed composition 96% ethanol. Membrane Zeolite A.8 Zeolite A.9 Fabrication method Permeate (wt%) Water Ethanol Permeate Flux (g/m 2 h) Separation Factor SGM Sucrose SGM Sucrose

184 Chapter 7. (a) (b) 500 μm 20 μm (c) (d) 500 μm 500 μm Figure 7.21: SEM image of carbon-zeolite composite layer using sucrose at different spatial resolutions (a,b) top views and (c,d) edge view 184

185 Separation Factor Permeate Flux (g/m2h) Chapter PFA C-Z (4) PFA C-Z (6) Sucrose C-Z (8) Sucrose C-Z (9) Water in feed wt% Figure 7.22: Illustration of the flux dependence on the feed composition using zeolite membranes post-treated with PFA and sucrose PFA C-Z (4) PFA C-Z (6) Sucrose C-Z (8) Sucrose C-Z (9) Water in feed wt% Figure 7.23: Illustration of the separation factor dependence on the feed composition using zeeolite membranes post-treated with PFA and sucrose. 185

186 Chapter 7. It can be concluded from this section that preparation of such a membrane of high fluxes with comparable separation factors was successfully achieved using posttreated SGM with the sucrose precursor. The relatively high fluxes are referred to the formation of thin layers of zeolite A, i.e., thicknesses less than 1 μm, as shown in Figures 7.20 and Baker (2004) indicated that the thicknesses of these membranes determine the overall performance including mass transport and flux i.e., transport rate is inversely proportional to the thickness of a given membrane. From the previous results, it is apparent that the sucrose carbon precursor improved the performance of zeolite A by blocking the formation of a large number of pinholes and other defects after the carbonization step. Thus, when the excess carbon that formed was removed gently, a membrane was produced with less pinholes and defects. Figure 7.24 shows the predicted illustration of curing the zeolite membrane and minimizing its thickness by removing the excess carbon. The three main steps involved in the process of healing zeolite membranes using carbon are listed below: - Use SGM to prepare the zeolite membranes - Apply a sucrose solution on the surface of the zeolite layer under low vacuum pressure to allow the carbon solution to pass through the pinholes to a certain extent. The purpose of using low vacuum suction is to limit the distribution of the precursor carbon solution through the zeolite to block the formation of pinholes without affecting the zeolite layer that could lower the overall fluxes and block the entire surface. - Gentle removal of the carbon layer, leaving a thin zeolite layer with less defects. Therefore, the most significant contributions of this work was proving that the entire fabrication process could be simplified by minimizing the time required to 186

187 Chapter 7. prepare the membranes, minimizing the energy consumption of the process, and, most importantly, the zeolite gel synthesis preparation. (a) Pinholes Zeolite layer Porous support (b) Carbon layer Zeolite layer Porous support (c) (d) Figure 7.24: Illustration of healing zeolite membrane using carbon: (a) zeolite membrane was prepared with SGM forming zeolite layer with inescapable defects /pinholes; (b) carbon precursor was added and absorbed through the zeolite layer using a low vacuum system; (c) the carbon layer was removed, leaving a thin zeolite layer with less defects as shown in (d). 187

188 Chapter Novel Technique for Fabricating Carbon-Zeolite Membranes In this section, the results of preparing zeolite membranes using a novel, simple, inexpensive, less time-consuming process are presented. The main advantage of this process was to avoid the lengthy and complex preparation of the zeolite gel. In other words, the novel process replaced the conventional, thermal-crystallization process with direct coating of the synthetic or natural zeolites on stainless-steel supports and then applying the carbon precursor before the carbonization process (chapter 5, section 5.5). In this study, the novel technique was used to fabricate different types of zeolite membranes for use in different separation tasks, i.e., ethanol dehydration, ethanol/cyclohexane, phenol removal, and xylene isomers Ethanol dehydration As an initial attempt to use this novel technique, it was used to synthesize a zeolite A membrane so the technique could be compared with the technique used previously in this work. After the membrane was prepared using the procedure described in section (chapter 5), it was evaluated using various ethanol/water mixtures, i.e., 4, 6, and 20 wt% water. The results of the experiments, which are presented in Table 7.7, indicated that the membranes prepared by the novel method had been synthesized successfully and that they had comparable separation factors to those of the previous membranes and relatively good fluxes. Although the fluxes were lower than the previous techniques, they reflect better separation performance. This is probably due to the better control of spreading zeolite paste over the porous support than was possible during the hydrothermal crystallization step. The effect of the carbon precursor solution was tested and evaluated at three different concentrations, as shown in Table 7.7. The results indicated that the concentration of the carbon precursor solution had significant influence on the performance of the 188

189 Chapter 7. membranes in terms of fluxes and separation factors, and, to a certain extent, the membranes with higher concentrations of the carbon precursor provided better separation and, consequently, lower fluxes. However, the membrane prepared with the highly concentrated sucrose solution (3:1) did not show a performance that was consistent with those of the other three membranes. This clearly indicated the difficulty of passing the concentrated sucrose solution all the way through the narrow pinholes to reach the bottom of the zeolite paste layer on the support metal. The SEM analyses presented in Figure 7.25 show that membranes with high concentrations acquired inconsistent distribution of carbon formation and accumulated only on part of the zeolite layer (as in the case of the (membrane Z- S.4)). On the other hand, for the membranes prepared with the low-concentration solution of carbon precursor, the zeolite layer was presented clearly, and carbon formation did not appear as they should have taken place within the pinholes and defects (as in the case of the (membrane Z-S.1)). In Figure 7.26, both of the membranes prepared with low- and high-concentration carbon precursor solution were compared with a pure zeolite membrane prepared by SGM to indicate the carbon coverage. The performances of membranes Z-S.1 to Z-S.4 were evaluated using three different feed compositions of ethanol (i.e.,96, 94, and 80 wt%), and its influence on both of the permeate fluxes and separation factors is illustrated in Figures 7.27 and 7.28, respectively. On the other hand, a sucrose membrane was prepared without involving zeolite to assess its permeability. This sucrose membrane did not allow any component to permeate within 12 h, which fulfils the initial aim of incorporating carbon precursor in this fabrication process. As stated before, the main objective of involving carbon in the synthesis process is to heal and block as many defects as possible, and the results presented in this section 189

190 Membrane Chapter 7. indicate that this objective was achieved. Therefore, it can be concluded from this section that the concentration of the carbon precursor solution is proportional to the separation selectivity and inversely related to the overall fluxes. In addition, the choice of the concentration of the solution is of great importance and should be made based on the desired separation behaviour of a given membrane. Table 7.7: Evaluation of carbon-zeolite membranes using different sucrose solution concentrations (as a carbon precursor) after coating the porous support with zeolite A paste, at different feed compositions of ethanol/water mixture. Sucrose/ Water Ratio (Weight) Feed (wt%) Permeate (wt%) Permeate Flux Water EtOH Water EtOH (g/m 2 h) Separation Factor Z-S.1 0.5: Z-S.2 0.7: Z-S : Z-S :

191 Chapter 7. Z-S.1 Z-S.1 50 μm 20 μm Z-S.4 Z-S.4 20 μm 50 μm Sucrose Sucrose 20 μm 5 μm Figure 7.25: SEM images of carbon-zeolite A composite layer using different concentrations of sucrose solution (sucrose: water) at different spatial resolutions; Z-S.1 (0.5:1), Z-S.4 (3:1), and sucrose formation after the carbonization process. 191

192 Chapter 7. (a) (b) Pure zeolite A membrane prepared by SGM. Composite carbon-zeolite membrane prepared with low concentrated carbon precursor solution (Z-S.1) 200 μm 100 μm (c) 100 μm Composite carbon-zeolite membrane prepared with high concentrated carbon precursor solution (Z-S.2) Figure 7.26: General comparison between (a) zeolite membrane, (b) carbon-zeolite membrane with low concentrated sucrose solution and (c) carbon-zeolite membrane with high concentrated sucrose solution. 192

193 Separation Factor Permeate Flux (g/m2h) Chapter Z-S.1 Z-S.2 Z-S.3 Z-S Water in feed wt% Figure 7.27: Illustration of the overall flux dependence on the feed composition using zeolite membranes post-treated with sucrose Water in feed wt% Z-S.1 Z-S.2 Z-S.3 Z-S.4 Figure 7.28: Illustration of the separation factor dependence on the feed composition using zeolite membranes post-treated with sucrose. 193

194 (g/m 2 h) and α Chapter 7. Although the results showed an improvement of the zeolite membrane in terms of performance and the simplicity of preparation, the separation factors of the membranes prepared in this work were lower than those in the literature provided by Holmes et al [121]. Holmes and co-workers achieved a separation factor of at 25 o C with a feed of 80 wt% ethanol. However, our work has the advantages of a shorter preparation time and higher fluxes. Holmes and co-workers fabricated membranes using the SGM after a crystallization process of four days, resulting in a flux of 58 (g/m 2 h), whereas the novel technique used in this study was conducted for less than one day (10 h). A comparison was conducted at 25 o C with a feed composition of 80 wt% ethanol, and the results of the comparison are shown in Figure Current study Holmes et al., separation factor 58 permeate flux Figure 7.29: Comparison of the performances of the membranes prepared by Holmes et al and those in the current study at 25 o C using a mixture that contained 80 wt% ethanol. 194

195 Chapter 7. Another experiment was conducted in which the ethanol/water mixtures were separated using a mordenite membrane. The experiment was conducted at 20 o C with ethanol/water mixtures that consisted of 2 and 6 wt% water. The mordenite membrane was prepared as described in section in chapter 5. The SEM images Figure 7.30 show a defined boundary of the edge of the metal support and the mordenite layer of approximately 60 μm. The uniform boundary explains the relatively high separation factor and low fluxes that were obtained with this membrane compared to those of the zeolite A membrane. The apparent morphology of the zeolite crystals presented in Figure 7.30 indicates that the surface of the membrane was not covered or blocked by the carbon layer. First, the membrane was prepared using sucrose:water (0.5:1) solution, but this membrane provided no separation. Therefore, a higher concentration of sugar was used (1:1), and the results of the experiment showed that this membrane had good performance (Table 7.8). The separation factor was high compared to the zeolite A membrane used in this study, reaching 634, and the fluxes were competitive (15 g/m 2 h). Figures 7.31 and 7.32 show that the feed compositions of ethanol have a significant influence on both the permeate fluxes and the separation factors. The membranes of this current study had better separation factors than those reported in the literature by Navajas et al using 10 wt% ethanol [195]. Although Navajas et al achieved greater permeate fluxes (200 g/m 2 h), the method they used, i.e., the secondary growth method (SGM), took a long time and required the preparation of the zeolite gel. Table 7.8 presents the outcomes of the current study and those of Navajas and co-workers, and the outcomes are compared in Figure

196 Chapter 7. (a) 20 μm (b) 10 μm 196

197 Chapter 7. (c) 500 μm Mordenite Support metal (d) 300 μm Mordenite layer 60 μm Figure 7.30: Edge view SEM image of carbon-mordenite composite layer using sucrose at different spatial resolutions (a,b) top views and (c,d) edge view 197

198 Permeate Flux (g/m2h) Chapter 7. Table 7.8: Evaluation of mordenite membranes using sucrose as a carbon precursor at different feed compositions of water/ethanol mixture. Membrane sucrose/ water ratio (weight) Feed (wt%) Permeate (wt%) Water EtOH Water EtOH Permeate Flux (g/m 2 h) Separation Factor Mordenite 1: Water in feed wt% Mordenite membrane Figure 7.31: Illustration of the overall flux dependence on the feed composition using mordenite membranes. 198

199 (g/m 2 h) or α Separation Factor Chapter Water in feed wt% Mordenite membrane Figure 7.32: Illustration of the separation factor dependence on the feed composition using mordenite membranes Current study Navajas et al., separation factor 15.7 permeate flux (g/m 2 h) Figure 7.33: Comparison of the performances of membranes prepared by Navajas et al [195] and those prepared in our study 199

200 Chapter Separation of ethanol/ cyclohexane mixture In this section, an attempt was made to test the feasibility of fabricating another type of membrane using the same novel method with different types of zeolites that were acquired from natural sources. At this stage, clinoptilolite, due its abundance in nature, seemed to be a good candidate to fulfill this objective. Clinoptilolite has pore sizes of about 0.6 nm, which makes it suitable for many separation applications. Moreover, the fabrication of this type of zeolite anisotropic membrane has not been reported in the literature due to the complexity of its synthesis. Therefore, the ability to develop a successful membrane made of clinoptilolite would be a significant accomplishment of this work, resulting in important added value to this field. After clinoptilolite membranes were prepared using the procedure discussed in section of chapter 5, they were evaluated using ethanol/cyclohexane mixtures. The main reason this mixture was used was to take advantage of the difference between the properties of the mixture in terms of polarity and the molecules dimensions. Methanol and ethanol are less polar than water, but, compared to other organic solvents, they can be considered as polar molecules [196]. In this section, ethanol was chosen over methanol because it is less polar than methanol and is miscible in cyclohexane. Moreover, the ethanol-cyclohexane mixture offers the advantage of different size molecules, i.e., the kinetic diameters of ethanol and cyclohexane are 0.43 and 0.60 nm, respectively. The results of the experiments showed that the first membrane, which was prepared with a concentration of sucrose to water of 0.5:1 by weight, did not provide any separation (as was the case for the mordenite membrane), therefore this membrane was subjected to further treatment using the same carbon precursor at the higher concentration of 1:1. The 200

201 Chapter 7. performance of the membrane after treatment is listed in Table 7.9, and it reached a separation factor of with high fluxes (around 200 g/m 2 h), and this result underlined the benefit of subjecting any leaking membrane fabricated using this method to subsequent treatment by applying a suitable concentration of the carbonprecursor solution (sucrose). For comparison, the fabrication of a clinoptilolite membrane was repeated, starting with a sucrose-solution concentration of (1:1) and conducting analyses with an SEM, as shown in Figure The clinoptilolite s morphology can be observed clearly in the SEM images, and the sucrose carbons were assumed to have filled the pinholes since the EDAX analysis indicated their existence. However, the boundary layers of both the clinoptilolite and the support metal were difficult to observe from the edge view. The performance of the second membrane was slightly better than that of the first one in terms of the overall fluxes, and this was due to the repeated treatment by the sucrose solution in the first membrane. The relation between the separation behaviour in terms of separation factors and fluxes and the feed composition is illustrated in Figures 7.35 and 7.36, respectively. In order to have a better understanding, these membranes were evaluated further at different operating temperatures, i.e., 40 and 60 o C, on the feed side of the membrane cell. As indicated in Tables 7.10 and 7.11, the temperature influences the overall fluxes to a great extent, but it did not make a noticeable difference in the separation factors. The effects of temperature on the permeate flux and the separation factor are shown in Figures 7.37 and 7.38, respectively. 201

202 Chapter μm 20 μm 50 μm 300 μm Figure 7.34: SEM images of carbon-zeolite clinoptilolite composite layer (1:1) concentrations of sucrose solution at different spatial resolutions (a,b and c) top views and (d) edge view. 202

203 Chapter 7. Table 7.9: Evaluation of carbon-zeolite membranes using clinoptilolite and sucrose as a carbon precursor with different feed compositions at room temperature. Membrane Sucrose/ Water Ratio (Weight) Feed (wt%) Permeate (wt%) EtOH Chx EtOH Chx Permeate Flux (g/m 2 h) Separation Factor C-S.1 0.5: Leaking C-S.1 1: C-S.2 1: Table 7.10: Evaluation of carbon-zeolite membranes using clinoptilolite and sucrose as a carbon precursor with different feed compositions at 40 o C. Membrane Sucrose/ Water Ratio (Weight) C-S.2 1:1 Feed (wt%) Permeate (wt%) EtOH Chx EtOH Chx Permeate Flux (g/m 2 h) Separation Factor Table 7.11: Evaluation of carbon-zeolite membranes using clinoptilolite and sucrose as a carbon precursor with different feed compositions at 60 o C. Membrane sucrose/ water ratio (weight) C-S.2 1:1 Feed (wt%) Permeate (wt%) EtOH Chx EtOH Chx Permeate Flux (g/m 2 h) Separation Factor

204 Separation Factor Permeate Flux (g/m2h) Chapter Clinop 1 Clinop EtOH in feed wt% Figure 7.35: Illustration of the flux dependence on the feed composition using zeolite membranes post-treated with sucrose EtOH in feed wt% Clinop 1 Clinop 2 Figure 7.36: Illustration of the separation factor dependence on the feed composition using zeeolite membranes post-treated with sucrose. 204

205 Separation factor) Permeate Flux (g/m2h) Chapter C 40 C 60 C EtOH in feed wt% Figure 7.37: Illustration of permeate fluxes under different feed temperatures EtOH in feed wt% 25 C 40 C 60 C Figure 7.38: Illustration of separation factors under different feed temperatures. 205

206 Chapter Separation of xylene isomers From the earlier results, it can be concluded that the novel method introduced in this work for fabricating zeolite membranes with hydrophilic properties (low Si:Al ratio) was successful for all membranes, and the fabrication times were significantly shorter. This section presents the feasibility of fabricating hydrophobic membranes, i.e., ZSM-5, using the same novel method described above. Bowen et al stated that ZSM-5 has been used extensively in organic separation processes due to its pore size (0.56 nm) in addition its hydrophobic property [62]. These key properties make ZSM-5 preferable for many organic separation applications. However, a long time is required to prepare the synthesis gel of ZSM-5, and the procedure is complex, as prepared and discussed in Appendix C. Therefore, the ability to fabricate ZSM-5 membranes using the short, straightforward, novel method would contribute significant added value to this work. A mixture of xylene isomers was used due to the importance of obtaining pure isomers in industrial processes, where pure p- xylene and m-xylene are used for phthalic anhydride and isophthalic acid production, respectively [ ]. The ZSM-5 membranes prepared using the procedure described in chapter 5 (section 5.5.4) did not provide any separation due to a leakage in its structure caused by the incomplete coverage of carbon over the defects and pinholes. The affinity of a given zeolite layer for the sucrose solution had an important role in blocking the defects in its structure, because it determines the distribution of the carbon precursor in the zeolite structure. Therefore, it was anticipated that the novel method of synthesizing zeolite membranes with hydrophobic structure (e.g., ZSM- 5) would be more difficult to accomplish than fabricating hydrophilic membranes. However, from the first attempt, the fabricated carbon-zeolite membrane (ZSM-5) 206

207 Chapter 7. was far thinner than it had been at the beginning (before conducting the carbonization step, see figure 7.24). So, it was decided that applying the sucrose solution again over the leaking membrane under low vacuum pressure would be more effective as the ZSM-5 layer became thinner, making the adsorption of the sucrose solution easier and making the pinholes more accessible to the sucrose solution. Thus, the carbon-zeolite membrane (ZSM-5) was subjected to further posttreatment using the same sucrose solution and carbonization procedure. After conducting the first experiment with a 95% o-xylene feed, the membrane showed separation behaviour with low separation factors. In other words, the performance of the carbon-zeolite membrane (ZSM-5) was improved from leaking to a separation factor of 5.35 with the 95% of o-xylene mixture. For comparison purposes, the post-treated, carbon-zeolite membrane was tested at four different concentrations, i.e., 95, 90, 80, and 50 wt% o-xylene, and the results are listed in Table The temperate effect was evaluated as well by using a constant feed composition of50 wt%. Table 7.13 and Figure 7.39 show that the temperature was proportional to the permeate flux, while the separation performance did not show any noticeable change in that range. The SEM images of the post-treated membrane are presented in Figure 7.40, and the zeolite crystals can be seen clearly, but no defined boundaries of the ZSM-5 layer are evident. 207

208 Permeate Flux (g/m2h) ZSM-5 ZSM-5 Chapter 7. Table 7.12: Evaluation of carbon-zeolite membranes using ZSM-5 after posttreatment with sucrose as a carbon precursor with different feed compositions at 25 o C. Feed (wt%) Permeate (wt%) Permeate P-xylene O-xylene P-xylene O-xylene Flux (g/m 2 h) Separation Factor Table 7.13: Evaluation of ZSM-5 membrane using sucrose solution of (1:1) concentration for xylene isomers separation at different feed temperatures. T Feed (wt%) Permeate (wt%) Permeate Separation ( O C) Flux P-xylene O-xylene P-xylene O-xylene Factor (g/m 2 h) Feed Temperature Figure 7.39: Illustration of temperature effect on permeate fluxes using carbon - ZSM-5 membrane at constant feed composition of 50% wt. 208

209 Chapter μm 20 μm 300 μm 300 μm Figure 7.40: SEM images of carbon-zeolite ZSM-5 composite layer (1:1) concentrations of sucrose solution (a,b) top views and (c,d) edge view 209

210 Chapter 7. Comparing the outcome of the ZSM-5 membrane prepared in this study with the membranes prepared by Wegner et al [200], our membranes had better performance in terms of separation selectivity, overall fluxes, and the simplicity of preparation. Wenger et al used ZSM-5 membranes to separate a mixture of p-xylene/o-xylene at 26 and75 o C, resulting in separation factors of 0.94 and 0.96, respectively. However, the fluxes obtained from this study were lower than ours, as shown in Table 7.14 and Figure Our results also were compared with the results of Yuan et al s work. They used a silicalite-1 membrane to separate the p-xylene/oxylene mixture at 50 o C. The present study showed higher fluxes for pure o-xylene, but lower selectivity, as shown in Table 7.15 and Figure Nevertheless, the procedure presented in this study of preparing hydrophopic membranes was much easier than the procedures used in the two studies that were mentioned, i.e., Wegner et al and Yuan et al [200,201], which involved the preparation of the zeolite gel prior to the fabrication of the membrane. Table 7.14: Comparison of the performances of membranes prepared in this study and those prepared by Wenger et al. Membrane Reference T( o C) ZSM-5 ZSM-5 ZSM-5 ZSM-5 Wegner et al., 1999 Current study Wegner et al., 1999 Current study Feed (wt%) p- xylene o- xylene Permeate Flux (g/m 2 h) Separation Factor 26 N.A N.A

211 (g/m 2 h) and α (g/m 2 h) and α Chapter Current study at 25 C Wegner et al., 1999 at 26 C separation factor permeate flux Current study at 70 C Wegner et al., 1999 at 75 C separation factor permeate flux Figure 7.41: Comparison of the performances of membranes prepared in this study and those prepared by Wenger et al. 211

212 (g/m 2 h) and α Chapter 7. Table 7.15: Comparison of the performances of membranes prepared in this study and those prepared by Yuan et al. Membrane Silicalite-1 Reference Yuan et al, 2004 T ( o C) Feed (wt%) p- xylene o- xylene Permeate Flux of o-xylene (g/m 2 h) Separation Factor ZSM-5 Current study Current study Yuan et al, separation factor 4 permeate flux Figure 7.42: Comparison of the performances of the membranes prepared in this study at 50 o C with those of Yuan et al. 212

213 Chapter Removal of phenol from water Phenol has been used as a raw material for the production of many chemicals including caprolactam, which is consumed in the production of nylon fiber. Therefore, A large quantity of aqueous waste streams containing phenols are discharged by numerous of industries. Consequently, removing phenol from water is of great importance in the wastewater treatment industry [202,203]. In this section, results are presented for the testing and evaluation of the previously-made zeolite A and clinoptilolite membranes with the phenol-water mixture. Both of these membranes had the appropriate polarity and pore-dimension properties to carry out this task. Zeolite A and clinoptilolite are classified as hydrophilic zeolites because their silicon to aluminum ratios are 1.1 and 5.25, respectively, based on the results of EDAX tests. Therefore, both of these membranes possess a hydrophilic affinity for water molecules. Moreover, the kinetic diameter of phenol molecules (0.66 nm) is larger than the pores of both zeolite A (0.41 nm) and clinoptilolite (0.60 nm). Since the solubility of phenol in water is 8.3 g/100 ml at 20 o C, a solution of 5 wt% phenol was prepared by mixing 5 g of phenol (obtained from Sigma Aldrich) with 95 g of deionised water at room temperature; this solution was fed to the membrane cell using zeolite A and clinoptilolite membranes. In general, the observed performances of these membranes showed that water was highly preferred over phenol for both of these membranes, as listed in Table However, zeolite A had a better separation factor than clinoptilolite, but the fluxes in case of clinoptilolite were much greater because the dimensions of its pores were larger. This part of this study was preceded by testing the repeatability of the fabricated clinoptilolite membranes, and their performances were evaluated at operating temperatures of 25, 40, and 60 o C for comparison purposes. The separation behaviours at different 213

214 Clinoptilolite-2 Chapter 7. temperatures are presented in Table 7.17 and the data indicate that they all had similar performances. The overall fluxes increased as the temperature increased, as shown in Figure Table 7.16: Evaluation of zeolite A and clinoptilolite membranes using sucrose solution of (1:1) concentration for phenol/water separation at 25 o C. Membrane Feed (wt%) Permeate (wt%) Permeate Separation Flux Phenol Water Phenol Water Factor (g/m 2 h) Zeolite A Clinoptilolite Table 7.17: Evaluation of clinoptilolite membrane using sucrose solution of (1:1) concentration for phenol/water separation at different feed temperatures. T ( O C) Feed (wt%) Permeate (wt%) Permeate Flux Phenol Water Phenol Water (g/m 2 h) Separation Factor

215 Permeate Flux (g/m2h) Chapter Feed Temperature Figure 7.43: Illustration of temperature effect on permeate fluxes using clinoptilolite-2 membrane. The results achieved in this work were compared with those in the literature provided by Pradhan et al [202] in a study in which they separated phenol/water mixtures using two different types of membranes, i.e., polyimide and polyimide with lithium chloride (LiCl) at40 o C. Table 7.18 and Figure 7.44 show that this current study had better performance in terms of separation factor and total fluxes. Table 7.18: Comparison of the performances of membranes prepared in this study and those prepared by Pradhan et al. Membrane Reference T( o C) Polyimide Polyimide with LiCl Clinoptilolite Pradhan et al Pradhan et al Current study Feed (wt%) Water Phenol Permeate Flux (g/m 2 h) Separation Factor

216 (g/m 2 h) and α Chapter Current study (clinoptilolite) Pradhan et al.,2002 (polyimide) Pradhan et al., 2002(polyimide+Licl) 0 separation factor permeate flux Figure 7.44 : Comparison of the performances of membranes prepared in this study and those prepared by Pradhan et al. 216

217 Chapter Evaluation of quality and durability The durability of the carbon-zeolite membranes that were synthesized earlier was tested and assessed. After submerging the carbon-zeolite A in the ethanol/water mixture for eight weeks at room temperature, the membrane was tested after different intervals using the pervaporation process for 30 min at 25 o C and at a feed concentration of 96% of ethanol. The results shown in Table 7.19 and Figure 7.45 indicate that the novel method of fabricating zeolitea introduced in this study yielded a membrane that had stable performance up to two months. Since the other types of zeolie membranes followed the same fabrication procedure, they are expected to have similar durabilities. Table 7.19: Evaluation of carbon-zeolite A membrane using sucrose solution of (1:1) concentration for ethanol dehydration at 25 o C, and feed composition of 96% of ethanol. Intervals Feed (wt%) Permeate (wt%) Permeate Flux Water Ethanol Water Ethanol (g/m 2 h) Separation Factor Week Week Week Week

218 Permeate flux (g/m2h) separation factor Chapter Separation factor number of weeks number of weeks Permeate Flux (g/m2h) Figure 7.45: Illustration of the separation behaviour of carbon-zeolite A membrane at constant feed composition and temperature. 218

219 Chapter 8. Chapter 8 Conclusions and Recommendation for Future Work 8.1 Conclusions The main objective of the work conducted in this study was to investigate two key processes, i.e., the synthesis of a pure type of zeolite using cheap raw materials (virgin kaolin) and the use of a novel technique to incorporate this pure product in the fabrication of zeolite membranes. The results of the work indicated that the objectives were accomplished successfully to a great extent and that positive impacts were possible in both industries. The work conducted to synthesize zeolite A using virgin kaolin included several novel aspects that have significant implications for the zeolite synthesis industry. In all previous studies concerned with converting kaolin to zeolite, the procedures included a process for removing quartz prior to the synthesis procedure. However, even with such a process, the purified kaolin that was produced contained quartz, and it also was present in the synthesized zeolite. Thus, this work presents a novel method of removing quartz impurities during the process for synthesizing pure zeolite A. As was discussed in chapter 6, section 6.2.2, the first approach that was attempted to accomplish this goal was to use pre-treated kaolin, which we obtained from WBB Minerals, to assess the feasibility of accomplishing this task using the proposed technique. After conducting the crystallization process at 100 o C for different operating intervals, the results indicated that the crystallization process has a significant effect on the formation of impurities in the zeolite. It also was noted 219

220 Chapter 8. that the impurities began to form as the intervals used in the crystallization process became longer. Therefore, it was concluded that the technique could achieve the desired results if the crystallization intervals were short enough. The work that followed involved a source of virgin kaolin, i.e., Ahoko Nigerian kaolin ANK, in which the kaolin was taken directly from the ground. After the preparation process, the synthesized gel was cloudy even after allowing a full day for the heavier particles to settle, and it was expected that it would contaminate the top product. This occurred because many of the suspended impurities were so small that they would not settle. However, this problem was solved by using a washing process to remove these contaminants. The washing process removed 5% of the total mass of the raw material, but we felt that this loss would be more than offset by the development of an economical method of producing purified zeolite A, which was our priority at this stage. Therefore, it was concluded that pure zeolite A was synthesized successfully using the virgin kaolin. However, the initial aim of incorporating the purified kaolin raw material into the preparation of membranes was not considered after this point due to the insufficient amount of pure zeolite that was produced from a single run. Therefore, it was decided to focus on the process of fabricating the membranes rather than considering the preparation of the zeolite gel and other sources of materials. Another novel aspect was presented in this study. A novel, simple, inexpensive, and less time-consuming process for fabricating zeolite membranes was developed. The production of anisotropic membranes that consist of an extremely thin layer of zeolite supported on a thicker material was considered to be a major breakthrough in the membrane industry because these thin membranes provided higher fluxes [61]. However, many studies have reported the difficulty of synthesizing many types of zeolites in laboratory 220

221 Chapter 8. conditions despite the fact that many such zeolites exist in abundance in nature, e.g., clinoptilolite [2]. The novel method presented in this study for fabricating zeolite membranes is not complex, does not require a lengthy process for the preparation of zeolite gel, and does not require a thermal crystallization process. The initial objective related to the fabrication of zeolite membranes was to investigate the possibility of healing and repairing membranes that have been prepared using conventional methods, i.e., the secondary growth method (SGM) or the modified, in-situ crystallization method. The first attempt to treat membranes prepared by SGM involved rubbing their surfaces with zeolite A seed paste. Although this method showed a noticeable improvement in selectivity, the overall fluxes of the treated membranes decreased. This was expected because the thickness of the zeolite layer was increased by a factor of three by the rubbing treatment, which increased the path lenght and led to a lower overall permeation rate. The durability of these post-treated membranes was tested, and they were found to lose their selectivity performance after a week. Therefore, another approach was considered, in which we sought to identify a carbon precursor that had smaller pores than the molecules of the binary mixture that was being tested (e.g., ethanol/water). Consequently, SGM membranes were subjected carbonization post-treatment using polyfurfuryl alcohol (PFA). After conducting the pervaporation experiment, two out of four membranes leaked, which probably was attributable to the harsh acid treatment at elevated temperature that was involved in the procedure prior to the carbonization process. The two remaining membranes delivered a dramatic increase in the fluxes with slight improvements in selectivity. Therefore, it was concluded that PFA yielded a strong improvement in the fluxes and provided competitive separation behavior, however, the unreliable performance and the lack of desired 221

222 Chapter 8. levels of repeatability in the results were major obstacles to proceeding with PFA. Then, it was decided to evaluate another precursor that was much easier to prepare, and sucrose was considered to be a good candidate. The membranes post-treated with sucrose showed a considerable improvement in their performance in terms of selectivity and fluxes. Therefore, the significant contributions of this work to this field was simplifying the conventional fabrication process, minimizing energy consumption, and reducing the preparation time required by avoiding the preparation of the zeolite gel. The novel technique introduced in this work was initiated simply by rubbing the paste of the desired type of zeolite on a stainlesssteel support and then applying the carbon precursor before the carbonization process. Four types of zeolite membranes were fabricated successfully using this method, i.e., zeolite A, mordenite, clinoptilolite, and ZSM-5. These membranes were intended for different separation tasks for the purposes of comparison. Zeolite A, mordenite, and ZSM-5 were fabricated to evaluate the technique for fabricating membranes that had different hydrophobicity/hydrophilicity properties. Clinoptilolite was fabricated to assess the feasibility of fabricating an anisotropic type of membrane with thin layer that had not been reported before due to the complexity of preparing clinoptilolite synthesis gel. The sucrose precursor was tested in all of the membranes and evaluated at different concentrations to indicate the best possible performance that can be achieved. After the short preparation discussed in chapter 5, page 139, zeolite A and mordenite membranes were tested with ethanol/water mixtures, and they had performances that were competitive with those presented in the literature i.e., Holmes et al[121] and Navajas et al[195]. The ZSM-5 membrane was prepared and assessed using a mixture of xylene isomers. Although the membrane yielded a better performance than that mentioned by 222

223 Chapter 8. Wenger et al [200], the membrane preparation had to be repeated because of the difficulties due to its hydrophobic structure; it had less affinity than hydrophilic membranes towards the precursor aqueous solution (see chapter 7, page 213). In this study, the results obtained using a clinoptilolite membrane to remove phenol from water provides one of the major finding of this work, because the fabrication of a clinoptilolite anisotropic membrane has not been previously reported in the literature due to it s the difficulty of its synthesis. Clinoptilolite membrane prepared in this study had greater fluxes and greater selectivity than the polyimide membranes prepared by Pradhan et al [202]. In summary, the work presented in this study has resulted in the development of a method that can be used to convert raw materials, taken from the ground, into a pure zeolite material in a one-pot process (publication in appendix D). In addition, in this study, the synthesis of anisotropic clinoptilolite membranes was achieved successfully for the first time using a novel technique. This proposed technique is suitable for preparing all types of zeolite membranes by avoiding having to prepare the synthesis gel and instead, replacing it with the direct use of synthetic or natural zeolites. Thus, the approaches proposed in this thesis should be very useful and have significant implications for the future development of the zeolite membranes industry. 223

224 Chapter Recommendations for Future Work As discussed earlier, the insufficient amount of pure zeolite A produced from the virgin kaolin raw material was a major obstacle to the use of the process to fabricate zeolite membranes. This problem can be solved by using a larger, Teflon-lined, stainless-steel autoclave and by a thorough investigation of the proper crystallization conditions. In this study, the pores of the membranes that were fabricated were characterized using the molecular probing method, which involves the adsorption of molecules that can fit inside the pores of the membrane, while rejecting molecules that are too large, i.e., using the size-exclusion process. However, N 2 adsorption characterization method would provide a clear picture of the pores and the surface area of the membrane. The novel technique presented in this study was conducted on circular, stainlesssteel, support metals to evaluate the simplicity of the technique and to examine the thin layer that was fabricated rather than considering the properties of the support material. However, the technique can be compared with other techniques that have been described in the extant literature, which use other types of supports, including tubular shapes. Moreover, the support properties (e.g., hydrophobicity and hydrophilicity) can be incorporated and used for different tasks and applications. As it was mentioned in chapter 3, section 3.8.5, the pyrolysis conditions, i.e., the heating rate, the thermal-soaking period, and gas flow, have a major role in determining the final form of the carbon structure. Although the conditions used in this study were suitable for the sucrose precursor, other conditions could lead to the same results while consuming less energy and time. 224

225 Chapter 8. The fabrication of hydrophobic membranes was achieved successfully in this work, but the nature of the carbon precursor was not very suitable for these membranes, and additional post-treatment was required. Therefore, it is recommended that other carbon precursor materials be investigated to determine which of them have the appropriate properties for incorporation with membranes that have hydrophobic properties. The laboratory-scale pervaporation system was successfully designed and assembled in this work. However, the system can be modified by making it more compactable to ease the cleaning process. Moreover, the membrane compartment had not yet been developed, and the synthesized membrane had to be attached to the system by a two-component epoxy adhesive, which lasted for about a month of experiments. Although the adhesive had reasonably good performance, this approach is not reliable for all chemicals and operating conditions. Therefore, it would be useful to improve the membrane compartment in the pervaporation system by substituting a more reliable method for the epoxy adhesive. Finally, the influence of the carbon precursor concentration in fabricating zeolite A membrane was demonstrated using four different concentrations in this study. However, a further investigation is recommended to verify this approach for other types of membranes as it plays a key role in manipulating both selectivity and fluxes. 225

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238 Appendix A. Appendix A Extended Results and Calculation 238

239 lllllllappendix A. Table A.1: Evaluation of zeolite A membranes using dip-coating method at different feed compositions of water/ethanol mixture. Membrane Zeolite A.1 Membrane Zeolite A.2 Membrane Zeolite A.3 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 (mol/m 2 h) Factor h) Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 (mol/m 2 h) Factor h)

240 lllllllappendix A. Membrane Zeolite A.4 Membrane Zeolite A.5 Membrane Zeolite A.6 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 (mol/m 2 h) Factor h) Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux (g/m 2 (mol/m 2 h) Factor Water Ethanol Water Ethanol Water Ethanol Water Ethanol h)

241 lllllllappendix A. Membrane Zeolite A.7 Membrane Zeolite A.8 Membrane Zeolite A.9 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 (mol/m 2 h) Factor h) Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 (mol/m 2 h) Factor h)

242 lllllllappendix A. Table A.2: Evaluation of zeolite A membranes after using rubbing post-treatment with SGM. Membrane Zeolite A.9 Membrane Zeolite A.9 Membrane Zeolite A.9 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Water Ethanol Water Ethanol Water Ethanol Water Ethanol Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux (g/m 2 (mol/m 2 h) Factor Water Ethanol Water Ethanol Water Ethanol Water Ethanol h)

243 lllllllappendix A. Table A.3: Evaluation of carbon-zeolite membranes using PFA and sucrose as a carbon precursor after the SGM at different feed compositions of ethanol/water mixture. Membrane C-Z (4) Membrane C-Z (6) Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux (g/m 2 (mol/m 2 h) Factor Water Ethanol Water Ethanol Water Ethanol Water Ethanol h) Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor

244 lllllllappendix A. Table A.4: Evaluation of carbon-zeolite membranes using sucrose as a carbon precursor after the SGM at different feed compositions of water/ethanol mixture. Membrane C-Z (8) Membrane C-Z (9) Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 h) 244 Permeate Flux (mol/m 2 h) Separation Factor Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 (mol/m 2 h) Factor h)

245 lllllllappendix A. Table A.5: Evaluation of carbon-zeolite membranes using different sucrose solution concentrations (as a carbon precursor) after coating the porous support with zeolite A paste, at different feed compositions of ethanol/water mixture. Membrane Z-S.1 Z-S.2 Z-S.3 Z-S.4 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 (mol/m 2 h) Factor h)

246 lllllllappendix A. Table A.6: Evaluation of mordenite membranes using sucrose as a carbon precursor at different feed compositions of water/ethanol mixture. Membrane mordenite Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 (mol/m 2 h) Factor h) Table A.7: Evaluation of carbon-zeolite membranes using clinoptilolite and sucrose as a carbon precursor with different feed compositions at room temperature. Membrane C.S.1 C.S.2 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux EtOH ChX EtOH ChX EtOH ChX EtOH ChX (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor

247 lllllllappendix A. Table A.8: Evaluation of carbon-zeolite membranes using clinoptilolite and sucrose as a carbon precursor with different feed compositions at 40 o C. Membrane C-S.2 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux EtOH ChX EtOH ChX EtOH ChX EtOH ChX (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Table A.9: Evaluation of carbon-zeolite membranes using clinoptilolite and sucrose as a carbon precursor with different feed compositions at 60 o C. Membrane C-S.2 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux EtOH ChX EtOH ChX EtOH ChX EtOH ChX (g/m 2 (mol/m 2 h) Factor h)

248 lllllllappendix A. Table A.10: Evaluation of zeolite A and clinoptilolite membranes using sucrose solution of (1:1) concentration for phenol/water separation at 25 o C. Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Membrane Flux Water Phenol Water Phenol Water Phenol Water Phenol (g/m 2 (mol/m 2 h) Factor h) Clinoptilolite Zeolite A Table A.11: Evaluation of clinoptilolite membrane using sucrose solution of (1:1) concentration for phenol/water separation at different feed temperatures. Membrane C-S.2 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Permeate Flux Separation Flux Water Phenol Water Phenol Water Phenol Water Phenol (g/m 2 (mol/m 2 h) Factor h)

249 lllllllappendix A. Table A.12: Evaluation of carbon-zeolite membranes using ZSM-5 after post-treatment with sucrose as a carbon precursor with different feed compositions at 25 o C. Membrane ZSM-5 Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) p- o- p- o- p- o- p- o- xylene xylene xylene xylene xylene xylene xylene xylene Permeate Flux (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Table A.13: Evaluation of ZSM-5 membrane using sucrose solution of (1:1) concentration for xylene isomers separation at different feed temperatures. Membrane ZSM-5 Feed (wt%) p- xylene o- xylene Feed (mol%) p- o- xylene xylene Permeate (wt%) p- xylene o- xylene Permeate (mol%) p- o-xylene xylene Permeate Flux (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor

250 lllllllappendix A. Table A.14: Evaluation of carbon-zeolite A membrane using sucrose solution of (1:1) concentration for ethanol dehydration at 25 o C, and feed composition of 96% of ethanol. Membrane Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Week Week Week Week

251 Appendix B. Appendix B Repeatability Results and Calculations 251

252 Appendix B. Table B.1: Ethanol dehydration using zeolite A prepared by novel technique with sucrose solution (1:1) Membrane Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Zeolite A Zeolite A (repeated) Standard deviation of the permeate flux is 9.98 (within 1.08%) -Standard deviation of the separation factor is 1.97 (within 4.05%) Table B.2: Ethanol dehydration using mordenite prepared by novel technique with sucrose solution (1:1) Membrane Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Ethanol Water Ethanol Water Ethanol Water Ethanol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor mordenite mordenite (repeated) Standard deviation of the permeate flux is 0.33 (within 2.15%) -Standard deviation of the separation factor is (within 5.79%) 252

253 Appendix B. Table B.3: Ethanol / cyclohexane separation using mordenite prepared by novel technique with sucrose solution (1:1) Membrane Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux EtOH Chx EtOH Chx EtOH Chx EtOH Chx (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Clinoptilolite Clinoptilolite (repeated) Standard deviation of the permeate flux is (within 14.6%) -Standard deviation of the separation factor is 1.27 (within 12.77%) Table B.4: Phenol removal using zeolite A prepared by novel technique with sucrose solution (1:1) Membrane Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Phenol Water Phenol Water Phenol Water Phenol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Zeolite A Zeolite A (repeated) Standard deviation of the permeate flux is 2.49 (within 3.44%) -Standard deviation of the separation factor is 1.13 (within 6.53%) 253

254 Appendix B. Table B.5: Phenol removal using zeolite A prepared by novel technique with sucrose solution (1:1) Membrane Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) Permeate Flux Water Phenol Water Phenol Water Phenol Water Phenol (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor Clinoptilolite Clinoptilolite (repeated) Standard deviation of the permeate flux is 2.62 (within 0.87%) -Standard deviation of the separation factor is 0.42 (within 2.74%) Table B.6: Separation of xylene isomers using ZSM-5 prepared by novel technique with sucrose solution (1:1) Membrane Feed (wt%) Feed (mol%) Permeate (wt%) Permeate (mol%) p- xylene o- xylene p- xylene o- xylene p- xylene o- xylene p- xylene o- xylene Permeate Flux (g/m 2 h) Permeate Flux (mol/m 2 h) Separation Factor ZSM ZSM-5 (repeated) Standard deviation of the permeate flux is 2.00 (within 2.58%) -Standard deviation of the separation factor is 0.3 (within 5.46%) 254

255 Appendix C. Appendix C ZSM-5 Synthesis and Preparation The synthesis of this type of zeolite was achieved using a 1.0 M solution of tetrapropylammonium hydroxide (TPA-OH), sodium aluminate (50% Al 2 O 3, 40% Na 2 O), sodium silicate solution (Ludox-40), and sodium hydroxide pellets (99% NaOH) as source materials with the following molar composition: 23.4 Na 2 O: Al 2 O 3 : 83.4SiO 2 : 4.2 (TPA) 2 O: 3750 H 2 O A 1.0 M solution of TPA-OH was used, so 4,200 ml of TPA-OH solution (4,250.4 g) were required to yield mol of water. Sodium aluminate contains 50% aluminium oxide and 40% sodium oxide, so, for a basis of 10 g of sodium aluminate, mol ( g) of sodium aluminate and 1.32 mol of sodium oxide are needed. Therefore, mol of sodium oxide are required from the sodium hydroxide to fit the molar equation. A basis of 10 g of sodium hydroxide results in mol of pure sodium hydroxide. From equation (5.9), mol ( g) of sodium hydroxide ( g) are maintained. The fourth source, colloidal silica, consists of 40% of silicon dioxide and 60% water. Taking 10 g of colloidal silica source as a basis, yielding mol ( g) of colloidal silica and mol of deionised water. As a result, the total water required is mol ( g). Scaling down, (dividing by and multiplying by 35), the final amounts of sodium hydroxide, sodium aluminate, colloidal silica, TPA-OH, and water, were 1.089, 0.125, 6.532, 2.597, and 35 g, respectively. ZSM-5 was prepared using at two crystallization periods i.e., 48 and 72 h. The SEM images are presented in Figures C.1 and C.2, which show the 255

256 Appendix C. morphology of the zeolite at 5 and 2 µm of spatial resolution, respectively. The SEM analyses clearly showed that the 72 h crystallization period yielded chamfered-edged morphology. XRD was used to analyse both prepared samples and there were similarities and good matching between the key peaks of the prepared samples and the standard samples. (a) (b) 5μm 2μm Figure C.1: SEM image of ZSM-5 particles at (a) 5 µm and (b) 2 µm spatial resolution (crystallisation of 48 h). (a) (b) 10μm 5μm Figure C.2: SEM image of the repeated sample of ZSM-5 particles at (a) 10 µm(a) and (b) 5 µm spatial resolution (crystallization of 72 hours). 256

257 Appendix C. Appendix D Published Work 257

258 Appendix C. 258

259 Appendix C. 259

260 Appendix C. 260