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UNIVERSITY OF ZARAGOZA/INSTITUTE OF NANOTECHNOLOGY ARAGON MASTER RESEARCH PROJECT Synthesis and characterization of zeolite A membranes for micro device applications AUTHOR: TOSHIYUKI GHANI SUPERVISOR :DR REYES MALLADA ERASMUS MUNDUS MEMBRANE ENGINEERING PROGRAM 2011-2013 ABSTRACT “In order to integrate zeolite membranes in micro devices, zeolite micro membranes were prepared on silicon grids to obtain self-supported membranes. The substrate was modified with PDDA, boehmite and different silanes to anchor the zeolite seeds for microwave secondary synthesis. The quality of the membranes was characterized by SEM to observe the effect of surface modification and synthesis parameters”. Page 2 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Acknowledgement I would like to Thank Dr Reyes Mallada for her support and suggestions as my supervisor throughout the research, also I would like to thank Dr Pilar Pina, Dr Miguel Urbitzondo and Dr Ismael Pellejero for their help during the research project. I would like to thank Carlos for his help with extensive SEM sampling and EM3E committee for giving me the opportunity to be a part of the EM3E program. Also I appreciate all the help, suggestions, feedback and training given by all of my colleagues at Institute of Nanotechnology Aragon, Spain. Page 3 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu DISCLAMER EN-This project has been funded with support from the European Commission. This publication reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein. BG-Този проект е финансиран с подкрепата на Европейската комисия.Тази публикация отразява само личните виждания на нейния автор и от Комисията не може да бъде търсена отговорност за използването на съдържащата се в нея информация. CS-Tento projekt byl realizován za finanční podpory Evropské unie. Za obsah publikací odpovídá výlučně autor. Publikace nereprezentují názory Evropské komise a Evropská komise neodpovídá za použití informací, jež jsou jejich obsahem. DA-Dette projekt er finansieret med støtte fra Europa-Kommissionen. Denne publikation forpligter kun forfatteren, og Kommissionen kan ikke drages til ansvar for brug af oplysningerne heri. DEDieses Projekt wurde mit Unterstützung der Europäischen Kommission finanziert. Die Verantwortung für den Inhalt dieser Veröffentlichung trägt allein der Verfasser; die Kommission haftet nicht für die weitere Verwendung der darin enthaltenen Angaben. ΕΛ-Το σχέδιο αυτό χρηματοδοτήθηκε με την υποστήριξη της Ευρωπαϊκής Επιτροπής. Η παρούσα δημοσίευση δεσμεύει μόνο τον συντάκη της και η Επιτροπή δεν ευθύνεται για τυχόν χρήση των πληροφοριών που περιέχονται σε αυτήν. ESEl presente proyecto ha sido financiado con el apoyo de la Comisión Europea. Esta publicación es responsabilidad exclusiva de su autor. La Comisión no es responsable del uso que pueda hacerse de la información aquí difundida. ET-Projekti on rahaliselt toetanud Euroopa Komisjon. Publikatsiooni sisu peegeldab autori seisukohti ja Euroopa Komisjon ei ole vastutav selles sisalduva informatsiooni kasutamise eest. Page 4 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu FIHanke on rahoitettu Euroopan komission tuella. Tästä julkaisusta (tiedotteesta) vastaa ainoastaan sen laatija, eikä komissio ole vastuussa siihen sisältyvien tietojen mahdollisesta käytöstä. FRCe projet a été financé avec le soutien de la Commission européenne. Cette publication (communication) n’engage que son auteur et la Commission n’est pas responsable de l’usage qui pourrait être fait des informations qui y sont contenues. GAMaoiníodh an tionscadal seo le tacaíocht ón gCoimisiún Eorpach. Tuairimí an údair amháin atá san fhoilseachán [scéala] seo, agus ní bheidh an Coimisiún freagrach as aon úsáid a d’fhéadfaí a bhaint as an eolas atá ann. HUAz Európai Bizottság támogatást nyújtott ennek a projektnek a költségeihez. Ez a kiadvány (közlemény) a szerzõ nézeteit tükrözi, és az Európai Bizottság nem tehetõ felelõssé az abban foglaltak bárminemû felhasználásért. ITIl presente progetto è finanziato con il sostegno della Commissione europea. L’autore è il solo responsabile di questa pubblicazione (comunicazione) e la Commissione declina ogni responsabilità sull’uso che potrà essere fatto delle informazioni in essa contenute. NLDit project werd gefinancierd met de steun van de Europese Commissie. De verantwoordelijkheid voor deze publicatie (mededeling) ligt uitsluitend bij de auteur; de Commissie kan niet aansprakelijk worden gesteld voor het gebruik van de informatie die erin is vervat. LTŠis projektas finansuojamas remiant Europos Komisijai. Šis leidinys [pranešimas] atspindi tik autoriaus požiūrį, todėl Komisija negali būti laikoma atsakinga už bet kokį jame pateikiamos informacijos naudojimą. LVŠis projekts tika finansēts ar Eiropas Komisijas atbalstu. Šī publikācija [paziņojums] atspoguļo vienīgi autora uzskatus, un Komisijai nevar uzlikt atbildību par tajā ietvertās informācijas jebkuru iespējamo izlietojumu. Page 5 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu MTDan il-proġett ġie finanzjat bl-għajnuna tal-Kummissjoni Ewropea. Din il-publikazzjoni tirrifletti (Dan il-komunikat jirrifletti) l-opinjonijiet ta’ l-awtur biss, u l-Kummissjoni ma tistax tinżamm responsabbli għal kull tip ta’ uzu li jista’ jsir mill-informazzjoni li tinsab fiha ( fih). PLTen projekt został zrealizowany przy wsparciu finansowym Komisji Europejskiej. Projekt lub publikacja odzwierciedlają jedynie stanowisko ich autora i Komisja Europejska nie ponosi odpowiedzialności za umieszczoną w niej zawartość merytoryczną. PTProjecto financiado com o apoio da Comissão Europeia. A informação contida nesta publicação (comunicação) vincula exclusivamente o autor, não sendo a Comissão responsável pela utilização que dela possa ser feita. ROAcest proiect a fost finanţat cu sprijinul Comisiei Europene.<0}Această publicaţie (comunicare) reflectă numai punctul de vedere al autorului şi Comisia nu este responsabilă pentru eventuala utilizare a informaţiilor pe care le conţine. SK-Tento projekt bol financovaný s podporou Európskej Komisie. Táto publikácia (dokument) reprezentuje výlučne názor autora a Komisia nezodpovedá za akékoľvek použitie informácií obsiahnutých v tejto publikácii (dokumente). SLIzvedba tega projekta je financirana s strani Evropske komisije. Vsebina publikacije (komunikacije) je izključno odgovornost avtorja in v nobenem primeru ne predstavlja stališč Evropske komisije. SVProjektet genomförs med ekonomiskt stöd från Europeiska kommissionen. För uppgifterna i denna publikation (som är ett meddelande) ansvarar endast upphovsmannen. Europeiska kommissionen tar inget ansvar för hur dessa uppgifter kan komma att användas Page 6 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu List of figures Figure 1: free standing membrane on silicon grid, Figure 2: patterned membrane on silicon oxide wafer ................................................................................................................................................... 9 Figure 3: steps for fabricating free standing membrane on silicon grids ................................................... 10 Figure 4:: Zeolite A formation from tetrahedron(a), Sodalite cage (b), secondary building unit(c), zeolite A crystal(d,e) (3) .......................................................................................................................................... 11 Figure 5: Synthesis routes for zeolite membranes(6) ................................................................................. 12 Figure 6: Applications of zeolite films in micro scale applications (21) ...................................................... 16 Figure 7: The Knoevenagel condensation reaction between benzaldehyde and ethyl acetoacetate(28) . 17 Figure 8: Yeung`s work zeolite A membrane deposited in micro channels on silicon substrate(27) ......... 18 Figure 9: SEM images of zeolite membrane on micro reactor walls (27) ................................................... 18 Figure 10: micro channel fabrication on silicon substrate(37) ................................................................... 19 Figure 11: PDDA Molecule, schematic representation of PDDA layer on silicon oxide surface ................. 21 Figure 12: left Boehmite structure, Right schematic representation of Boehmite layer on silicon oxide . 21 Figure 13: schematic of function of organo silane, (53) ............................................................................. 22 Figure 14: left structure of trialkoxy-silanes, right APTES attachment on silicon oxide surface ................ 22 Figure 15: left and right, complex mechanism of APTES orientation on silicon oxide ............................... 23 Figure 16: an ideal mechanism of APTES attachment on silicon oxide ...................................................... 24 Figure 17: Different modification of substrates for zeolite attachment(74) .............................................. 25 Figure 18: left silicon oxide layer on silicon wafers; 4 inch wafer cut into small squares for synthesis ..... 26 Figure 19: grid sizes description (a grid 1000_100) .................................................................................... 27 Figure 20: Cross section of silicon grid ........................................................................................................ 27 Figure 21: dip coating device ...................................................................................................................... 29 Figure 22: thermal expansion co-efficient of some zeolites(89) ................................................................ 31 Figure 23: Experimental setup for silanization process Left for small wafer, right for 3inch wafers. ........ 33 Figure 24: TOP SEM image sizing data and Bottom DLS results for zeolite 4 ............................................. 36 Figure 25: Silicon oxide without modification: seed concentration 1%,2% AND 3% SEM images of seeding left, right after 30 minute microwave synthesis ......................................................................................... 37 Figure 26: Silicon oxide without modification :seed concentration 3%,4% AND 5% SEM images of seeding left, right after 30 minute microwave synthesis ......................................................................................... 38 Figure 27: seeding and synthesis of PDDA modified surface using spin coating 1%, 2% and 3% seed concentration .............................................................................................................................................. 42 Figure 28: seeding and synthesis of PDDA modified surface using spin coating 4%, 5% and 6% seed concentration .............................................................................................................................................. 43 Figure 29: Effect of spin coating conditions ................................................................................................ 44 Figure 30: effect of PDDA solution mixed with zeolite seeds in water and spin coated ............................ 45 Figure 31: Dip coating of seeds on PDDA modified substrate .................................................................... 46 Figure 32: top Cross section of Boehmite and membrane layer, bottom microwave synthesis growth with time ............................................................................................................................................................. 48 Figure 33: Dip coating of Boehmite and zeolite seeds, seeding and synthesis .......................................... 48 Page 7 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 34: Spin coating of Boehmite and zeolite seeds. Followed by 30 minute synthesis ....................... 49 Figure 35: SEM images of seeded Boehmite modified grids ...................................................................... 49 Figure 36: SEM images of Boehmite modified grids, evolution with time ................................................. 50 Figure 37: Boehmite 9% solution mixed with zeolite seeds 5% solution (1:1 ratio) spin coated ............... 51 Figure 38: different seeding solvents and seeding conditions on APTES modified support .................................................................................................................................................................... 54 Figure 39: different concentration of APTES in toluene during surface modification 1mM, 6mM, 30mM55 Figure 40: different concentration of APTES in toluene during surface modification 50mM, 90mM, 120mM ........................................................................................................................................................ 56 Figure 41: Evolution of thickness of membrane on APTES modified substrate ......................................... 57 Figure 42: Growth of membrane with synthesis time, ethanol seeded substrate ..................................... 57 Figure 43: Spin coating and dip coating sample on APTES modified substrates ........................................ 58 Figure 44: Samples for APTES modified substrates, tested under same conditions for reproducibility test .................................................................................................................................................................... 59 Figure 45:Covalent linkage (epoxy-Amine) ................................................................................................. 61 Figure 46: Covalent linkage using Halogen propyl -oxy silanes .................................................................. 62 Figure 47: GRID PREPARATION ................................................................................................................... 71 List of tables Table 1: List of substrates used for zeolite A membrane synthesis(14) ..................................................... 15 Table 2: cleaning procedure for silicon oxide substrates ........................................................................... 28 Table 5: Thermal expansion coefficient values ........................................................................................... 31 Table 7: silanization process ....................................................................................................................... 32 Table 3: zeolite seeds size data ................................................................................................................... 35 Table 4: list of PDDA experiments .............................................................................................................. 39 Table 6: Boehmite experiments .................................................................................................................. 47 Table 8: experiments performed for silanes ............................................................................................... 53 Page 8 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu TABLE OF CONTENT 1. OBJECTIVE OF THE PROJECT ............................................................................................................ 9 2. INTRODUCTION TO ZEOLITE MEMBRANES .................................................................................... 11 2.1 Zeolite ............................................................................................................................................ 11 2.2 Applications of zeolite membrane ................................................................................................. 11 2.3 Zeolite membrane synthesis .......................................................................................................... 12 2.3.1 Microwave synthesis ....................................................................................................... 13 2.4 Zeolite membranes in Microscale applications ............................................................................. 16 2.5 Zeolite membrane synthesis on silicon substrates ........................................................................ 19 2.6 Surface modification of silicon oxide substrate ............................................................................. 20 2.6.1 Polydiallyldimethylammonium-chloride (PDDA) ............................................................ 20 2.6.2 Boehmite (aluminum oxide hydroxide / γ-AlO(OH) ........................................................ 21 2.6.3 Silanization ...................................................................................................................... 21 3 EXPERIMENTAL .............................................................................................................................. 26 3.1 Materials ........................................................................................................................................ 26 3.2 Silicon substrates ........................................................................................................................... 26 3.3 Preparation of zeolite A nanoparticles for seeding ....................................................................... 28 3.4 Surface modification by PDDA solution ......................................................................................... 29 3.5 Surface modification by Boehmite solution .................................................................................. 30 3.6 Surface modification by silanes ..................................................................................................... 32 3.7 Preparing microwave synthesis solution ....................................................................................... 33 4 RESULTS AND DISCUSSION ............................................................................................................ 35 4.1 Characterization of zeolite seeds ................................................................................................... 35 4.2 Seeding and synthesis on bare silicon oxide wafers ...................................................................... 36 4.3 PDDA modified surface and synthesis .................................................................................... 39 4.4 Boehmite surface modification and synthesis ........................................................................ 47 4.5 Silanization surface modification and synthesis ..................................................................... 52 5 Conclusion and future work suggestions ....................................................................................... 63 References .................................................................................................................................................. 65 Appendix ..................................................................................................................................................... 68 Page 9 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu 1. OBJECTIVE OF THE PROJECT The goal of this Final Master Project is to integrate zeolite membranes in micro devices. The zeolite micro membrane will be prepared in a silicon grid to obtain a self-supported membrane explained in figure 1. Alternatively zeolite layers on silicon wafers could be patterned for integration in a micro device as shown in figure 2. Figure 1: free standing membrane on silicon grid, Figure 2: patterned membrane on silicon oxide wafer To obtain the zeolite membrane; the secondary growth method that consists of two steps; seeding and hydrothermal synthesis will be applied. This method is well established for the synthesis of zeolite membranes and layers on porous supports such as alumina, stainless steel or cordierite. The goal is to develop a protocol for the secondary growth on silicon wafers. There are two main challenges, first compared to the traditional supports mentioned before, silicon wafers are flat surfaces, where the anchoring of the seeds will be difficult, and in this sense a proper surface modification to anchor the seeds will be necessary. Secondly silicon could be etched under the high alkalinity of the zeolite synthesis gel, to avoid this silicon oxide layers will be deposited and fast synthesis methods such as microwave heating will be employed. The main objective is to combine a practical seeding approach with microwave synthesis. To achieve this; following tasks will be done (explained in figure 3).  Synthesis of zeolite A nanocrystals  Surface modification of silicon oxide surface  Homogeneous seeding of modified surface  Microwave synthesis of zeolite A layer Page 16 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu 2.4 Zeolite membranes in Microscale applications Apart from conventional large scale separation and synthesis a lot of focus is also based on micro scale applications ranging from sensors to micro reactors which utilize zeolite membrane in the micro scale for detection, catalysis, separation and adsorption. Previously our group from Institute of Nanotechnology of Aragon published a very detailed review on micro scale applications of zeolite membranes(18) and detail of some application is given in figure 6. Figure 6: Applications of zeolite films in micro scale applications (21) Sensors Zeolites have been applied in sensing devices to increase sensitivity and selectivity of detection of various molecules. There are many application reported in literature based on different mechanism from modification of traditional electrodes in liquid detection to modified cantilever in gas sensing. Some of the reasons for their functionality and use in sensors are based on its adsorption, high surface area, porosity, presence of mobile ions, Page 17 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu catalytic activity (19, 20). also a filter based on zeolite membrane have been previously proposed to act as a gas pre-filter(21). Zeolites have also been applied to improve conventional electrode sensors and are used to improve selectivity and sensitivity by modifying electrodes, they have been used in humidity sensors, ethanol sensors, NO, SO2, water sensing applications. They have also been used in cantilever based sensors where the cantilever acts as microbalance and can detect adsorption of specific components and hence has been applied to detect humidity based on this mechanism.(22) also gas sensors for explosive detection have been reported in literature based on zeolite material coating(23). More detailed information on zeolite based sensors are reported in the following sources which explain in detail the mechanism, fabrication and functionality of gas sensors based on Zeolites.(24, 25). Micro reactors Process intensification and miniaturization is increasing for the goal to achieve more efficient reactions, to achieve higher conversion of reactants and to reduce the quantity of by products and to improve quality of the products. Zeolite membranes have been used as catalysts as a well as selective barriers for removal of products or harmful by products(18). In most cases the combined effect of selectivity and catalysis has improved many reactions of fine chemicals where selective catalysis is required to improve conversion and to reduce unwanted byproduct formation(4, 26). Other micro scale applications include use of zeolite membranes to remove volatile organic compounds from air and water and to recover catalysts from reactions(22). A detailed review of zeolite membrane reactors is described in the following publication by Kapteijn(27). Figure 7: The Knoevenagel condensation reaction between benzaldehyde and ethyl acetoacetate(28) Yeung and his colleagues have carried out extensive research in Knoevenagel reactions where 2 different layers of zeolite membranes are used in micro reactors for Knoevenagel reactions; one of the reactions is explained in figure 7. The first layer is used to remove water Page 18 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu which is the by product in the reaction and the 2nd layer acts as a selective catalyst only catalyzing the reactants as they are smaller in size to reach to the catalysts exchanged membranes as shown in figure 7 and 8. They etched different substrates silicon, stainless steel to design the micro channels and then using mercapto-3-propyltrimethoxysilane 50mM in ethanol for modifying the silicon substrate. Then they deposited the zeolite seeds using vacuum on the modified silicon, alumina or stainless steel substrate. Figure 8: Yeung`s work zeolite A membrane deposited in micro channels on silicon substrate(27) Figure 9: SEM images of zeolite membrane on micro reactor walls (27) Yeung used zeolite A as the water removing membrane layer and zsm-5 exchanged with catalyst to perform the catalysis of the Knoevenagel reaction as explained in figure 9. Page 19 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu 2.5 Zeolite membrane synthesis on silicon substrates Many research has been published on zeolite membrane growth on various supports, most of the work is conducted on porous supports which provide structural support to the seeds and for the membrane to grow like porous alumina, porous stainless steel, porous ceramic supports and polymeric supports(29-33).Yang did an extensive review of microwave synthesis of zeolite and the table of supports used in microwave synthesis of zeolite LTA is listed in table 1. Our focus is to grow zeolite LTA membranes on flat nonporous support, previous works performed on flat surfaces include work by Frontera(34) where they modified silicon wafer with polyelectrolyte as mention in previous method by Decher(35), details of the modification were not published. Other work which included MFI zeolite membrane growth on silicon wafer is work by Mintova(36) where they tested the effect of different binders silica, TEOS and colloidal alumina as binder additives and different solvents and their effect on MFI membrane growth on silicon wafers. Yeung(37) applied zeolite A membrane for micro reactor application on silicon wafer with etched channels by using mercapto silane as the binder. Tsapatsis(38) tried different surface, seeding, zeolite modification and deposition on silicon wafers for zeolite A membranes he concluded that dip coating provided more close packing compared to spin coating, but spin coating provided more surface coverage, Tsapatsis also concluded that the electrostatic deposition did not create well intergrown membrane because the seed packing was too dense for the nutrients to go in the deposited seed. Ozburk and Akata(39) used e-beam lithography to pattern zeolite A nanoparticles on silicon wafers, they used IPA, toluene as solvents and they used ultrasound aided dip coating method, spin coating and direct attachment method previously reported by Yoon(40) and they concluded that Yoon`s method provided better result for making nano patterns of zeolite A nano crystals. Figure 10: micro channel fabrication on silicon substrate(37) Page 20 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Most of Yeung`s work on micro reactor are on porous stainless steel or porous alumina substrates which are pre-etched with micro channels and then zeolite seeds are deposited followed by zeolite synthesis. Only one of his publication reports using silicon substrate of micro reactor application with zeolite membrane as shown in figure 10. He etched the channel pattern on the silicon using photolithography process; the silicon wafer was coated with a silicon nitride layer. KOH was used to etch the pattern. Silicalite seeding was performed using mercapto silanes and grown by traditional oven method for 110C at 24 hours. 2.6 Surface modification of silicon oxide substrate Silicon oxide surface after cleaning has OHgroups on the surface making it hydrophilic and the OHgroups give the surface a negative charge. The zeolite A seed suspension have a negative charge in the solution, the similar charges cause repulsion and nonuniform seeding of zeolite crystals on silicon oxide surface. Different modifications were performed to attach zeolite seeds onto the silicon oxide surface. The important parameter for all coating is to ensure that the residual water is evaporated from the silicon surface by heating it above 100C; our samples were heated at 150C for more than 1 hour to remove water from the surface and to improve adhesion of modification and seeds. 2.6.1 Polydiallyldimethylammonium-chloride (PDDA) Polydiallyldimethylammonium-chloride (polyDADMAC/PDDA) is a cationic polymer with a positive charge. It has been previously used to modify surfaces through self assembly of polymer to form monolayer and multilayer of opposite charge polymers. Caro used PDDA to modify porous alumina support to perform seeding free synthesis of zeolite A membrane, the positive charge attracted the nutrients onto the alumina surface modified by PDDA and was able to get better gas separation compared to untreated surface(41). Many other works of self assembly of PDDA on surfaces have been reported previously by Decher and also by Yang.(35, 42-44). Structure and attachment on silicon oxide surface is explained in figure 11. Page 21 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 11: PDDA Molecule, schematic representation of PDDA layer on silicon oxide surface 2.6.2 Boehmite (aluminum oxide hydroxide / γ-AlO(OH) Boehmite is an aluminum oxide hydroxide ore and the reason to use Boehmite is to act as a aluminum source to feed the zeolite growth and also it has a low coefficient of thermal expansion, which helps reduce stress during processing and hence reducing cracks and defects during synthesis. Boehmite solution was spin coated 8 times because coverage was difficult during one spin coating process caused by repulsion between OH groups in Boehmite and silicon oxide layers. Preparation procedure for boehmite solution is reported in the annex. The structure of Boehmite is explained in figure 12. Figure 12: left Boehmite structure, Right schematic representation of Boehmite layer on silicon oxide 2.6.3 Silanization Organo-silanes are molecules which contain a backbone of Si-C and have more than one functional group as shown in figure 13. Generally organo-functional alkoxy-silane are the most commonly used organo silanes as they alkoxy group reacts with hydroxyl (OH) groups on substrate and form covalent bond. There is another functional group for example Page 22 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu amine, ethoxy, halogen; alkyl group reacts with a different material. In this way the silanes are used as coupling agents to bind different materials like organic and inorganic materials, they are also used to modify surface characteristics, reducing agents, to functionalize surfaces and many other applications. It is widely used in rubber, glass fiber, composites, paints, inks, adhesives. (45-49) Figure 13: schematic of function of organo silane, (53) In literature organo silanes have been used for improving metal adhesion on silicon substrate(50), modification of silica particles using different silanes(51), patterning of silicon oxide over silicon and using silane for gold patterning(52), modification of pores of porous silica membranes(53),nano patterning on silicon using silanes(54), using silane to apply dicarboxylic group on silicon surface(55). Below in figure 14 is a simple explanation of APTES and how it can interact with a silicon oxide surface. Figure 14: left structure of trialkoxy-silanes, right APTES attachment on silicon oxide surface Literature on silanization of surfaces Up to date many works have been published to study the structure and reaction of organosilanes on silicon and silicon oxide surface. Some of the works include Vandenberg`s study on the structure of APTES on silicon oxide(56) where he did extensive study on effect of solvent, heat, tome and different curing conditions, Manifar(57) studied in detail the effect on solvent on deposition Octadecyltrichlorosilane(OTS) on silicon Page 23 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu wafers, Hu(58) studied in detail the structure of MPTMS on silicon oxide and effect of water, concentration of silane and the surface of SiO2. Petri(59) studied the assembly of APTES on silicon oxide by different characterization and then used the monolayer of silane to improve adhesion of Gold without using chromium which is used in conventional coating of silicon oxide with gold, Howarter(60) studied the deposition of APTES on silicon wafers and studied the effect of temperature. Zhang(61) studied the chemical vapor deposition of 3 different silanes including APTES and used a dye adsorption to test the density of amines and concluded that vapor phase deposition provides better monolayer coverage. Pasternack(62) studied the effect of solution temperature on APTES deposition on silicon oxide surfaces. Kim(63) studied the effect of post curing conditions on the structure and the stability of APTES on silicon oxide layers and found that curing does not have any significant effect on the silane structure. Zhu(64) performed a detailed literature review and experiments to obtain the best reproducibility and stability of different silanes on silicon wafers, and concluded the vapor phase deposition is more reproducible and ethoxy silanes are less effected by water concentration compared to methoxy silanes and hence are more reproducible. Gu(65) studied the deposition of APTES on silicon wafers and tested the effect of wear by friction on the APTES layer. Graf(66) experimented with the optimization of cleaning procedure for silicon wafers to be used for modification by silanes. Li(54) deposited APTES on silicon wafer using micro printing to form micro patterns by silanization. Kim(67) studied the effect of solvent Toluene and water on APTES deposited on silicon wafers by different characterization and concluded that Toluene deposition of APTES has more reactive surface amino groups compared to aqueous deposition. Wang(68) studied the effect of different vapor and liquid deposition technique on the formation of APTES silanization on glass slides. Figure 15: left and right, complex mechanism of APTES orientation on silicon oxide Page 24 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu All the work agrees on the fact that amount of water is the important parameter in silanization as excess water can cause self-polymerization of silanes while lack of water can cause poor coverage of the substrate. Other parameters include temperature; increase in temperature increases reactivity of the silanes and faster deposition of silane takes place at higher temperature. Another important parameter is silanization time, as with time the thickness and roughness of silane layer increases. So our main focus was to control water content by drying the silane system, and the substrate properly and uniformly to ensure reproducibility of the experiment. Figure 16: an ideal mechanism of APTES attachment on silicon oxide Silanization of substrates for zeolite membranes Organo silanes have been previously used to modify substrates to attract zeolite nutrients from the synthesis solution to promote nucleation and growth and they have also been used to modify surface for seeding of Zeolites. Tsapatsis has modified zeolite seeds with APTES(38), Caro also extensively studied silanization of substrates for membrane growth which include APTES modified porous alumina support to grow zeolite FAU membrane without seeding(69), the seeding free synthesis of zeolite A membrane on porous alumina support using Page 25 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu CPTMS(70), Multilayer zeolite A synthesis on porous alumina support using APTES as interlayer(71), using APTES for seeding free zeolite A growth(72).Yoon has done extensive study of glass modification with silanes to bind zeolite seeds; he also explains various possible silanization processes to bind zeolites on substrates with hydroxyl groups one of them is explained in figure 17 (40), Zhang used silanes to attach zeolite catalysts on the stainless steel micro reactor channels(73). Yeung used 3-Mercapto-alkylmethoxysilane to attach zeolite A seeds on silicon, alumina and stainless steel substrates(37). Figure 17: Different modification of substrates for zeolite attachment(74) Page 32 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu 3.6 Surface modification by silanes For the preparation of silanization process, we dried the reflux chamber, the flasks and all other glassware in the oven at 150C over night to ensure no moisture is available on the surface. When the experiment is run, the glassware is taken out from the oven and the wafer on holder from the hot plate which is dried at 150C overnight is introduced into the hot reflux chamber and sealed with nitrogen flow. The chamber is purged for 30minutes before we introduce any solvents. Toluene is injected into the chamber using the syringe, and heated to the required reflux temperature. Finally the silane is added to the system. The solution is refluxed to the desired time, then solution is drained with the help of a syringe in the nitrogen environment, fresh toluene is added to wash the substrate, then drained, then for ethanol seeding procedure fresh absolute ethanol is added to clean the system twice and then finally zeolite seed suspension in ethanol is added and refluxed at 85C. Seeding is performed both under reflux of the solvent, sonication or a combination of both can be used. After seeding the sample, it is dried on a hot plate at 100-110C overnight to remove solvent. Table 7 explains the whole general process of silanization. Table 4: silanization process Silanization process Reason Description 1 Complete drying of reflux sysmem To remove any moisture 150C dried overnight 2 silicon substrate drying to remove moisture from surface 110-150c Hotplate 3 Substrate placed on a teflon holder placed vertically in chamber put nitrogen flow 4 venting to remove atmosphere from chamber N2 flow for 30minutes 5 Toluene added in the system solvent fixed amount (50ml) 6 System heated to reflux 110C heat to increase reaction temperature over stirrer/hot plate 7 Silane added required conc and type silane added silane addition 8 reflux solution refluxed for required time 9 draining solution silane sol removed from chamber 10 fresh toluene wash to remove unreacted silane wash 11 fresh ethanol wash X2 only for ethanol seeding wash 12 addition of seed solution reflux or sonication or combination to attach seed to silane 13 wash in the same solvent ( as used in seeding) 14 dried 110c remove solvent and cure Page 33 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Different silanes were used for the surface modification of silicon oxide wafers and grids. APTES was tested in great detail because of its stability and wide application in different silicon oxide modifications to attach different types of functional groups. We also used more reactive silanes capable of making covalent linkages as previously explained in the introduction section on silanes. We used Halogen based silanes to modify substrates and try to form covalent bonds between the modified substrate and our zeolite nano particles in order to try to improve anchoring of the zeolite seeds. In order to ensure reproducibility we used fixed volume of solvent for all experiments which was 50ml this is to ensure that the water content in the solvent is the same and we prepared samples in batches of two samples for each run. Figure 23: Experimental setup for silanization process Left for small wafer, right for 3inch wafers. 3.7 Preparing microwave synthesis solution The gel composition for microwave synthesis has the molar ratio 1:1.8:7.2:108 Al2O3: SiO2: NaOH: H2O. First the sodium silicate solution 30gm is mixed with 60gm of water and stirred. For aluminum source 17gm sodium hydroxide pellets and 17gm of water is stirred and heated between 50-70c and then 11.4 gram of aluminum hydroxide is added and stirred for 30minutes till it is dissolved into a transparent solution then 43.6 gm of water is added and stirred for at least 1 hour. Then drop by drop aluminum solution is added to silica solution under stirring. During the addition the PH of the silica solution becomes more basic Page 34 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu going to gelation of the solution we need to ensure the stirring is increased to make sure we prevent large aggregates. After the solution is completely mixed it is aged for at least 3 hours before use for microwave secondary synthesis of zeolite A membrane. In order to prevent the aggregates to get into the microwave synthesis solution, the aged solution was extruded through a syringe with a needle which breaks any large aggregates in the solution. The synthesis was carried out in a CEM microwave system using a fiber optic thermocouple to measure the reaction temperature and the maximum power setting used was 140watts. A Teflon autoclave was used for the synthesis to hold the sample and synthesis solution with a stirrer and the synthesis was carried out at 90C.(90, 91) Page 35 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu 4 RESULTS AND DISCUSSION 4.1 Characterization of zeolite seeds The characterization for the zeolite A seeds were conducted using the following equipment; the pH was tested using a pH meter EUTECH instruments model cyber scan pH2100, the particle size was analyzed using a DLS size analyzer Brookhaven instruments corporation 90Plus particle size analyzer and by using scanning electron microscope FEI instruments. The concentration of the zeolite A crystals was measured by using a microbalance Radwag Mya5/2Y. The zeolite seed data is give in table 5(molar mass of zeolite A-Na 2190g/mol). Table 5: zeolite seeds size data BATCH AGEING SYNTHESIS SYNTHESIS DLS POLYDISPERSITY SEM ZETA PH mass TIME TEMPERATURE TIME SIZE SIZE POTEN in yield HOURS C HOURS Nm nm Mv grams % 1 90 100 15 199.5±23 0.104 186±17.9 -37 10.2 1.896 86.5 2 20 95 15 129.5±19.5 0.036 127±21.3 -29 9.9 1.529 69.8 3 48 100 15 119.6±24.1 0.124 112±21.7 -33 10.4 1.78 81.2 4 30 90 22 95.3±6.2 0.048 97±9.1 -43 11.4 1.341 61.2 5 24 80 48 79±8.4 0.034 77.5±6.3 -49 9.9 0.978 44.6 We observed during our synthesis that ageing time significantly increases the yield. But at the same time it increases the size of the zeolite crystals. Also we observe that when the synthesis temperature is reduced we are able to obtain smaller crystal size but with lower yield. Hence to obtain smaller crystals with higher yield; we can age the solution longer, and synthesize at lower temperature to obtain the smallest size of zeolite A seeds with a suitable yield. We also observe good agreement between the DLS and SEM results for the zeolite particle sizing. Page 36 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 24: TOP SEM image sizing data and Bottom DLS results for zeolite 4 4.2 Seeding and synthesis on bare silicon oxide wafers The zeolite seeds were spin coated at 100rpm 1 min and 3000 rpm 0.5 min. The seeding of bare silicon oxide wafers were performed at different seed concentrations and we can clearly see from the SEM images Figure 25 and 26 that the number of zeolite seeds increase with increasing concentration of seed solution, but due to similar charges and repulsion the coverage by the zeolite seeds was incomplete. Also this is further obvious after synthesis that most of the zeolite seeds on the surface are displaced during synthesis and do not take part in the membrane growth. Because of this we need to modify the surface to attract and possibly find ways to improve the membrane growth and attachment on the surface of silicon oxide wafers. 0 5 10 15 20 25 SEM results Zeolite 4 Page 37 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 25: Silicon oxide without modification: seed concentration 1%,2% AND 3% SEM images of seeding left, right after 30 minute microwave synthesis Page 38 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 26: Silicon oxide without modification :seed concentration 3%,4% AND 5% SEM images of seeding left, right after 30 minute microwave synthesis Page 39 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu 4.3 PDDA modified surface and synthesis From the seeding images from figure 27 and 28 it is observed that the positively charged PDDA layer is attracting more zeolite seeds as seen in the SEM images. The insets in figure 27 and 28 show the zoomed out images for the synthesis sample. This increases the probability to attach seeds electrostatically to the substrate during synthesis. It is also observed that during synthesis we have well defined morphology which could be due to the fact that the PDDA attracts nutrients from the microwave synthesis solution. Seed concentration was varied from 1% to 6% using the spin conditions specified by Castro. We can clearly see that the thickness of seed deposition can be controlled by seed concentration, instead of multiple spin coating procedures. Table 6: list of PDDA experiments PDDA experiments Conditions Results Figure 1 Effect of seed concentration 170rpm 1.5 minutes; 4000 rpm 1min PDDA; zeolite seeds in water at 100rpm 1 min and 3000 rpm 0.5 min 27, 28 Seed concentration 1-6%.(optimum 2%) 2 Effect of spin coating conditions Normal condition: 170rpm 1.5 minutes; 4000 rpm 1min PDDA; zeolite seeds in water at 100rpm 1 min and 3000 rpm 0.5 min 6000 rpm drop by drop PDDA; 6000 rpm drop by drop seed 2% 29 normal PDDA; 6000rpm seeds drop by drop 3 Effect of PDDA in seed solution 300rpm 30 seconds ; 3000 rpm 1min 30 concentration of PDDA (2%, 5%, 10% by vol of PDDA 20% solution ) in 2% seed solution 4 Comparison of dip coating dip coating 6 times PDDA solution; seed solution 2% 4 times 31 and spin coating Page 40 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu From the figure 27 and 28 we can observe that higher concentration of seeds causes more layers of seed deposition, after synthesis we observe that excessive seed deposition causes poor adhesion of the synthesized layer. This can be due to the fact that the top layer of deposited seed starts growing into the membrane blocking nutrients to the seed layer close to the substrate surface causing inadequate growth of zeolite seeds, which causes poor adhesion. According to the seed deposition effect we observed the best results for PDDA modified surface was obtained by using 2% seed concentration. Another important observation was the uneven distribution of the zeolite seeds because of the electrostatic attraction of the zeolite seeds to the PDDA layer, as previously mentioned by Tsapatsis(38). The normal seeding conditions was 100rpm 1 min and 3000 rpm 0.5 min, this gives the zeolite seeds enough time to deposit on the substrate in multi layers and also unevenly. In order to provide better seeding we tried to deposit the seed at high revolutions and by dropping the seed solution on the spinning substrate, we called this process “drop by drop” process. As seen in figure 29. The spinning speed was 6000rpm for 1 minute and during the first 30 seconds 1ml of seed solution was deposited drop by drop on the spinning substrate. We also tried the same procedure for PDDA solution which is figure 29 a in which PDDA solution and seed solution were deposited by the drop by drop method and we can see that it did not have significant difference from figure 29 b which is only seeding with this procedure. So we conclude that the PDDA deposition is homogeneous during the normal spin coating process but the seeds due to electrostatic interaction deposit in many layers hence a high speed spin coat for seed solution is essential to deposit uniform seed layer. Another approach was to reduce the number of spin coating steps and to improve the wet ability of the samples and to increase viscosity of the spin coated solution; we decided to incorporate PDDA solution in the seeding solution. The idea was to help incorporate the zeolite seeds within the polymer network that would help suspend the particles, increase the solution viscosity and also help improve the adhesion of the membrane by eliminating a boundary layer of PDDA between the silicon oxide and the membrane layer. From our results in figure 30, we observe some aggregates of zeolite seeds but the sample was fully covered with the PDDA seed solution. The main limitation that we see is during microwave synthesis, is that the PDDA coats the Page 41 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu zeolite seeds and prevents the nutrients from reaching them, causing slow and more rounded crystal growth. Dip coating was also used to compare spin coating with other methods, we observed that dip coating (figure 31) had better packing density of the zeolite seeds. However the membrane after synthesis showed poor adhesion to the substrate. The observed average thickness of membrane growth on PDDA modified surface was around 1.5 microns for 30 minutes microwave synthesis, as seen in figure 31, we can see the different seeding techniques for the zeolite seeds, spin coating and dip coating and membrane after synthesis and washing procedure, we observe that spin coated samples had better membrane adhesion to the substrate compared to dip coated samples. Calcination was also used to remove excess PDDA from the surface and try to sinter the particles on the silicon oxide surface a similar technique is used by Aguado (92) to modify surface only by PDDA on a porous substrate. Page 48 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 32: top Cross section of Boehmite and membrane layer, bottom microwave synthesis growth with time Figure 33: Dip coating of Boehmite and zeolite seeds, seeding and synthesis 0 0.5 1 1.5 2 0 20 40 60 80 thickness (microns) time (minutes) Boehmite microwave synthesis (growth with time) Page 49 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 34: Spin coating of Boehmite and zeolite seeds. Followed by 30 minute synthesis Figure 35: SEM images of seeded Boehmite modified grids Page 50 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 36: SEM images of Boehmite modified grids, evolution with time Page 51 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 37: Boehmite 9% solution mixed with zeolite seeds 5% solution (1:1 ratio) spin coated In order to incorporate the seeds and the Boehmite in one single step to cover the substrate and try to prevent the layer of remaining boehmite as seen in figure 32 to cause adhesion problems, we incorporated Boehmite and zeolite seeds in one solution. We observed increase in viscosity because of the change in pH, the boehmite solution has a pH of 3.5 while the seed solution has a pH of around 9-10. This pH changes causes either aggregation of zeolite seeds or the particles in the Boehmite solution causing the increase in viscosity. Due to the high viscosity the solution was spin coated at around 6000rpm for 1 min to help spread and reduce the deposited layer. After synthesis we observe good morphology but in some areas attachment was poor. Suggesting that uniform boehmite deposition is essential to obtain a defect free membrane using boehmite modification. Page 52 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu 4.5 Silanization surface modification and synthesis Toluene was chosen as the solvent for silanization process as it has been reported in almost all of the silanization process using APTES as explained in section 2.6.3. Also experiments by Manifar(57) discuss various solvents and good results are observed from toluene as a solvent. In order to select the right conditions to test the effect of silane concentration, first we tested different seeding techniques using 120mM APTES. Figure 38 shows the results for different seeding solvent and different techniques for seeding including reflux, spin coating and dip coating on APTES modified support. Also a detailed comparison between spin and dip coated seeds on APTES modified surface is report in figure 43. We observed that since the seeds are more stable in ethanol suspension they seed the surface uniformly compared to toluene as in toluene we observe aggregates due to the drying process used to change the solvents. Although toluene is the solvent of choice, the seed stability is the problem that could be improved to improve seeding in toluene suspension. Because of these results we choose ethanol seed suspension as our seeding suspension to test the APTES concentration effect on seeding. Test for different APTES concentration on seeding was observed at 1,6, 30, 50, 90, 120 mM solution and the results are shown in figure 39 and 40. Since no major differences were observed we choose 120mM as our standard for all further experiments. Grids were used to provide the structural support to try to fabricate a free standing defect free membrane. Variation of synthesis time was observed as seen in figure 42. And we can clearly observe the evolution of the membrane growth with time, it can also be seen that 30min is required to completely cover and obtain a defect free membrane. During our experiments we observed that reproducibility of the samples was a major limitation in silanization, as the variation in seeding and hence growth of the membrane was observed. To observe the reproducibility several samples on silicon oxide wafer and grid samples were conducted and are shown in figure 44. Our explanation for variation in the synthesis is that the moisture content is varying during silanization process, causing variation in the deposition of the silane layer causing changes in seeding. In order to further improve silanization and to obtain reproducibility we need to control the reaction and sample handling environment in a climate controlled glove box where humidity, temperature and atmosphere conditions can be reproduced. Page 53 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Table 8: experiments performed for silanes SILANIZATION experiment Description figure 1 effect of seeding solvent seeds in ethanol, APTES toluene, APTES modified 36 substrate seeds in toluene, 120mM APTES 2 effect of spin coating of seeds effect of dip coating of seeds wafer APTES 120mM, reflux 110c, 2 hour, spin coat 250rpm 30 sec, 2500 rpm 1min 36 On APTES modified substrate 3 effect of APTES concentration seeding in ethanol grid 1,6,30,50,90,120 mM 37,38 concentration 4 effect of microwave synthesis time wafer 10,20,30 minutes 39 APTES cross sections 40 5 COVALENT LINKAGE grid 120mM GPDMS substrate, 120mM APTES seeds in toluene and ethanol EPOXY+APTES 45 6 effect of sonication spin coat 250rpm, 30 sec effect of relux wafer 2500rpm 1min, sonication 5-30minutes 41 Page 54 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 38: different seeding solvents and seeding conditions on APTES modified support Page 55 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 39: different concentration of APTES in toluene during surface modification 1mM, 6mM, 30mM Page 56 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 40: different concentration of APTES in toluene during surface modification 50mM, 90mM, 120mM Page 57 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Figure 41: Evolution of thickness of membrane on APTES modified substrate Figure 42: Growth of membrane with synthesis time, ethanol seeded substrate 0 0.5 1 1.5 0 10 20 30 THICNKESS (microns) MICROWAVE SYNTHEISS TIME (minutes) Ethanol seeding, membrane thickness Page 64 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Future work is needed to improve adhesion of zeolite layer to the surface and reproducibility, several strategies are proposed:  The main limitation of all silanization process reproducibility could be due to change in moisture conditions. 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Page 68 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu Appendix 1. Zeolites Zeolites can be defined as Inorganic crystalline frameworks that consist of Silica (SiO2) and depending on the type of zeolite some Silicon(Si4+) atoms are replaced by Aluminum ions (Al3+) in the framework giving the framework a negative overall charge which is balanced by a cation that is stay inside the porous cavity by electrostatic attraction to balance the charge, hence neutralizing and balancing the overall charge(2). 2. Applications of zeolites 2.1 Gas separation Gas separation is one of the largest researched areas for zeolite membrane application because of its potential to reduce separation and purification costs and is termed as green separation as the focus is to reduce energy and cost of separation and purification of gases. Some of the applications of Zeolites in gas separation are to remove water from gas streams, to remove trace amounts of impurity gases, to separate different bulk gases. The gas separation through zeolite membrane is based on 3 main mechanism, size exclusion, thermodynamic selectivity which is the preferential adsorption of some components over the other and kinetic selectivity which is the ability of some components to diffuse through pores(25)Hydrogen purification is also one of the major growth areas for future application of zeolite as much research is now focusing on renewable energy sources and significant research is being conducted on hydrogen fueled vehicles which will increase the need for hydrogen production and also improving the efficiency of the process. One of major hydrogen production is by steam reforming of light hydrocarbons and carbon dioxide is the major impurity in the hydrogen produced by this method. So improving CO2 removal by membranes is also an important area of research for gas separation(4). and selective oxidation of CO simulated in the hydrogen production from steam reforming process using zeolite/pt catalysts have been reported(93). Page 69 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu 2.2 Liquid separation Zeolite membranes are used for a wide variety of liquid/liquid separation, some of the main reasons for application of zeolite for liquid separation is lower energy requirements and hence lower cost and higher separation factors. One of the major areas of application and research is dehydration/drying using zeolite membranes to remove water from important industrial solvents like ethanol from their azeotropic mixtures and also from various other organic liquids. Another major area is of separating hydrocarbons from their isomers or byproducts, some examples are Isoprene which is used in artificial rubber production, isoprene needs to be purified from other similar hydrocarbons especially isoprene`s azeotropic mixture with n-pentane. Another important industrial separation is p-xylene which is used in synthetic plastic manufacturing and needs to be purified from its isomers (p-xylene, 0-xylene, m-xylene and ethlybenzene) and is an expensive purification process, Zeolites are being researched to reduce cost of purification. (4) Other applications in liquid separation include water removal from acid solutions requiring a stable membrane in highly acidic conditions which is fulfilled by zeolite membranes. Also organic pollutants can be removed from water using zeolite membranes. Another important organic separation is of methyl-tert-butyl ether, which is a fuel additive and needs to be purified from methanol impurities. [(26)] Detailed information on organic production and separation based on zeolites is described by Herman.(94, 95). 3. Zeolite LTA/ Zeolite A Zeolite A is a low Si/Al ratio zeolite structure; it was first discovered by union carbide in 1950. The framework code for Zeolite A given by the International Zeolite Association is LTA (Linde Type A). Linde was division of the Union Carbide company that discovered this Zeolite. Si/Al ratio for zeolite A is around 1 to 1.5 and they are hydrophilic in nature. The general formula for Zeolite A is Na2O/Al2O3/SiO2/H2O. The Zeolite A structure is a 4-4 structure of secondary building unit of sodalite or beta cage which is formed by the primary building block the tetrahedron which is combined in 4-4 oxygen ring pattern. This structure gives the highest cation exchange concentration to zeolite A and the optimum adsorption properties in terms of capacity and pore size. It can be used to remove water from organic rich mixtures and also as catalyst. 70% of the total zeolite produced is Zeolite A and its main application is in detergents Page 70 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu where is acts as ion exchanger.(96) There have been many lab scale application of zeolite A micro membranes for sensor, liquid separation, gas separation applications in micro reactors, membranes and other applications. The only known industrial application of zeolite A membrane is in Ethanol dehydration to purify ethanol from the azeotropic mixture of ethanol and water. Mitsui engineering were the first to commercialize zeolite A membranes for industrial application in ethanol dehydration and other manufacturers are listed by Bowen in his publication(97). Another important application of zeolite A is in micro reactors where it is used as a selective barrier to remove water from reactions, micro reactor walls are lined with zeolite A where it can remove water or adsorb and desorbs water in closed system, a lot of micro reactors lined with zeolite A membrane has been done by Yeung and his team.(28, 37, 73, 98, 99) . A detailed list of organic synthesis were Zeolites are used are listed in the following publications with numerous applications where zeolite A are used as water absorbers or catalysts and these can be further used in form of zeolite membranes in micro reactors for these chemical reactions(94). 4. Boehmite solution preparation Boehmite preparation previously reported by Yoldas(100) was performed to obtain the required Boehmite solution. 100ml of distilled water is heated at 80C in a beaker with stirring closed from air with nitrogen purge to prevent moisture from air, 25ml of Aluminum-tri-sec-butoxide (Al[OCH(CH3)C2H5]3, 97 wt% Aldrich) is added from the nitrogen environment to prevent polymerization. After drop by drop addition of aluminum-tri-secbutoxide is completed, the solution is stirred for 1 hour and the alcohol smell disappears indicating the hydrolysis of aluminum-tri-sec-butoxide is completed. Then temperature is raised to 90C and then 7ml of HNO3 of 1 MOLAR concentration added drop by drop. Then solution was transferred to Rota vapor equipment to age the sample at 90c for 24 hours and to prevent water evaporation during that time. The temperature was set to 90C at 80RPM. After the ageing of the solution is completed a 9% concentration of Boehmite solution is obtained. (after the synthesis the solution can be centrifuged at 3000rpm for 10 minutes to remove any big particles or it can be filtered). Page 71 The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities; www.em3e.eu 5. Preparation of silicon grids Figure 47: GRID PREPARATION