A Study on the Hydrothermal Synthesis of the Zeolite DDR

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2 A Study on the Hydrothermal Synthesis of the Zeolite DDR A Thesis submitted to the Division of Graduate Studies and Research of the University of Cincinnati in partial fulfillment of the requirements of the degree of Master of Science in the Department of Chemical Engineering of the College of Engineering and Applied Science By Justin Provenzano Committee: Professor Junhang Dong (Chair) Professor Peter Smirniotis Professor Vesselin Shanov Professor Anastasios Angelopoulos Date of submission: November 7 th, 2011

3 Abstract Gas separation is a critical operation in chemical and energy industries. Currently, energy intensive and costly processes such as distillation, cryogenic fractionation, absorption, and adsorption are widely used. With technological advances, membranes processes have the potential to be a viable alternative. Zeolite membranes are a relatively new class of membranes that have attracted interest due to their outstanding stability and separation selectivity. The DDR-type zeolite is a pure silica zeolite with an ultra-small pore aperture of nm. This structure is promising for membranes capable of separating small molecule gas mixtures which is important to future clean power generation systems. While the effectiveness and mechanisms of gas separation by DDR membranes are current research subjects, the synthesis of DDR zeolite crystals and polycrystalline membranes is still a challenge with limited reported success. Furthermore, some of the literature results on DDR synthesis were unable to be reproduced by other researchers creating unsettled scientific controversies. This MS thesis research focused primarily on developing a highly reproducible procedure for DDR zeolite synthesis. The effects of the precursor composition, precursor preparation, and the synthesis conditions were extensively studied. Advanced materials characterization techniques such as SEM, XRD, and BET microporosimetry were employed to characterize the materials. A new two-step synthesis method has been established for rapid production of DDR-type zeolite crystals. The first step is the preparation of Sigma-1 seed crystals under a three-day stirred synthesis using a precursor molar ratio of 3 Na 2 O: 20 1-Adamantanamine (ADA): 1 Al 2 O 3 : 60 SiO 2 : 2400 H 2 O originally reported in literature. The second step is a one-day static synthesis of DDR-type zeolite using a molar ratio of ADA: Silica: Ethylenediamine: Water = 12:100:100:11240 with the assistance of ball-milled Sigma-1 as seed. This new process uses a total of 4 days to produce pure DDR crystals which is drastically shorter than days used in verified synthesis methods. The DDR zeolite was activated by 2

4 firing at 600 C for 8 hours. The activated DDR zeolite had a BET microporous surface area of 294 m 2 /g which is consistent with literature values. Based on the successful DDR zeolite particle synthesis, preliminary investigations were conducted on the fabrication of DDR membranes. A modified membrane synthesis procedure was developed in this research with a precursor containing NaOH. Membranes of DDR crystal phase were successfully obtained. However, a small amount of aluminum from the support was leached by the alkaline solution and incorporated into the zeolite structure. The membranes were further studied for their quality and gas permeation properties. A 10-micron thick membrane of DDR crystal phase was obtained by a three-day hydrothermal synthesis at 160 C and autogenous pressure. The membrane was free of large defects as evidenced by the very small N 2 permeance of mol/m 2 Pa s before template removal. However, the membrane exhibited post-firing permeance levels two orders of magnitude higher than reported literature which evidenced defect formation during the firing process. Further research is needed to investigate the structure evolution during the firing process to minimize defect formation. 3

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6 Acknowledgements I would like to sincerely thank my advisor, Dr. Junhang Dong, for the opportunity to work for him. His guidance and encouragement has greatly influenced me and enabled my success throughout this study. I would like to thank Dr. Anastasios Angelopoulos, Dr. Peter Smirniotis, and Dr. Vesslin Shanov for devoting their effort and time by serving on my committee. Additional gratitude is owed to all of my fellow colleagues from Dr. Dong s research group including Dr. Seok-Jhin Kim, Dr. Xiling Tang, Mr. Kurtis Remmel, Mr. Zhi Xu, Ms. Hongmin Jiang, Mr. Shaowei Yang, and Mr. Ruidong Yang. I would also especially like to thank Dr. Wenheng Jing, a visiting professor in our group. His guidance has immensely contributed to the success of my studies. I greatly appreciate all of the group s help and kindness during my time spent here. I am greatly indebted to my family, especially to my wife Katherine and to my parents, for the love and support they have shown throughout this study and my life. Without them, I would not have been able to accomplish this work. Lastly, I would like to thank the National Science Foundation (Grant CBET ) and the Ohio Air Quality Development Authority (Grant AY10-11-C34) for financially supporting this research. 5

7 Table of Contents Abstract... 2 Acknowledgements... 5 List of Figures... 7 List of Tables... 9 Chapter 1 Introduction Background The DD3R Zeolite DDR Zeolite Membrane Research Objectives Chapter 2 DDR Zeolite Synthesis DDR Zeolite Synthesis in Literature DDR Synthesis by Modified Method of This Work Activation of the DDR Zeolite Summary Chapter 3 DDR Membrane Synthesis DDR Membrane Synthesis by Literature Methods Modified Method for DDR Membrane Synthesis Single Gas Permeation Studies Summary Chapter 4 Summary References Cited

8 List of Figures Figure 1. Schematic showing the structure of the 1-adamantanamine molecule Figure 2. Cages and Crystal Structure of the DD3R zeolite [7] Figure 3. SEM image of DDR crystals synthesized by den Exter [3] Figure 4. SEM image of DDR crystals synthesized by Gascon [7] Figure 5. SEM image of a DDR membrane surface synthesized by Tomita [1] Figure 6. SEM image of a DDR membrane cross-section synthesized by Tomita [1] Figure 7. XRD of Sample 1, SGT[21], and DDR[22] Figure 8. SEM image of Sample Figure 9. SEM image of DDR crystals found in Sample Figure 10. XRD patterns of Sample-2 and SGT [21] Figure 11. XRD patterns of Sample-3, SGT [21], and DDR [22] Figure 12. Schematic showing the ball-milling assisted DDR zeolite synthesis Figure 13. XRD patterns of Sample-4, SGT [21], and DDR [22] Figure 14. SEM image of Sample Figure 15. XRD Patterns of Sample 4 and Sample Figure 16. SEM image of Sample Figure 17. XRD Pattern of Sample 6 and SGT[21] Figure 18. SEM image of Sample Figure 19. High magnification SEM image of Sample-7 showing DDR crystals as main products and a small amount of Sigma-2 large particles Figure 20. XRD patterns of as-synthesized Sample-7 and standard DDR [22] Figure 21. SEM image of as-synthesized Sigma Figure 22. SEM image of ball-milled Sigma Figure 23. XRD Pattern of as-synthesized Sigma

9 Figure 24. XRD Pattern of Ball-milled Sigma Figure 25. SEM image of Sample Figure 26. XRD Pattern of Sample-8 and standard DDR[22] Figure 27. XRD Patterns of the Calcined DDR Powder Samples Figure 28. XRD Patterns of the Membrane 1 and the standard SGT (Sigma-2) [21] Figure 29. XRD Pattern of the Calcined Membrane obtained by the modified synthesis method together with the spectrum of standard DDR zeolite [22] Figure 30. SEM image of the Membrane 2 Surface Figure 31. SEM image of the Membrane 2 Cross-Section Figure 32. SEM image of the Membrane 3 Cross-Section Figure 33. Image of DDR membrane 2 calcined at 600 C Figure 34. Schematic of the Transient Single Gas Permeation Setup Figure 35. Single Gas Permeance values at 301K ( ) and 373K ( ) of a DDR zeolite membrane synthesized by Tomita et al. [1]

10 List of Tables Table 1. Comparison of required DDR zeolite synthesis time between the new method developed in this work and those reported in literature Table 2. S BET of the Calcined DDR Samples Table 3. Literature Results for S BET of DDR Table 4. Reported Gas Mixture Separation Performance of DDR membranes Table 5. Single Gas Permeances for Membrane

11 Chapter 1 Introduction 1.1 Background Gas separation is a key operation in chemical and petrochemical industries. Currently, distillation, absorption, and adsorption processes are widely used for various chemical separations. These traditional separation technologies are energy intensive and complicated in practice [1]. Membrane processes can avoid or minimize phase changes in chemical separations which would reduce energy consumption and thus are actively sought as a viable alternative to the energy-intensive conventional processes. Along with this, membrane reactors have the ability to enhance the conversion of equilibrium limited reactions as well as the possibility of continuous operation which would otherwise not be accomplishable in traditional reactors [2]. Inorganic zeolite membranes have been an active research field due to the potential they have in industrial processes where polymeric membranes are inefficient in operation because of their limited thermal, chemical, radiological, and mechanical stability. The industrial use of polymeric membranes is limited to temperatures around 200 C and these membranes are not tolerant of harsh chemical environments [3]. Zeolite membranes, a relatively new family of inorganic membranes, have exceptional chemical and thermal stability due to their crystalline nature. Zeolite membranes are currently much more expensive than polymeric membranes and therefore their unique properties of thermal and chemical stability as well as size selectivity need to be properly exploited for competitiveness and success in practical applications [4]. Zeolites are crystalline microporous aluminosilicates built from corner-linked [SiO 4 ] 4- and [AlO 4 ] 5- tetrahedra. Zeolites are built in various connections which result in over 150 different zeolitic pore structures with various effective pore sizes. The electrovalence of the aluminum sites in the framework is balanced by the inclusion of an extra-framework counterion in the porosity. 10

12 The general chemical composition of zeolites is given by the following formula [5]: (M n+ ) 1/n Al O2 - xsio 2 yh 2 O Where M = counter-ion n = counter-ion valence x = silicon/aluminum ratio y = content of water hydrate Zeolitic pore apertures are well defined because of their crystalline nature. Due to their uniform pore sizes close to the dimensions of molecules, zeolite membranes are utilized as molecular sieves in chemical separations. The zeolite membranes can therefore control the transport of molecules based on size exclusion which leads to high separation selectivity. 1.2 The DD3R Zeolite DD3R (DDR, deca-dodecasil 3 Rhombohedral) is a pure silica ultramicroporous zeolite. The pioneering work on DDR synthesis and structure characterization was done by Gies [6]. The chemical composition of the as-synthesized DDR zeolite can be represented as (C 10 H 17 N) 6 (N 2 ) 9 [Si 120 O 240 ], where C 10 H 17 N (1-adamantanamine, Fig.1) located in the cage is used as the template molecule to direct the crystalline structure during the hydrothermal crystallization process. 11

13 Figure 1. Schematic showing the structure of the 1-adamantanamine molecule DDR zeolite possesses a trigonal crystal structure with the following cell parameters: a = Å, b = Å, c = Å, α = 90, β = 90, γ = 120 in hexagonal settings. The structure consists of window-connected cages forming the zeolitic pore system. DDR zeolite is a member of the group of clathrasils. The crystal structure consists of [SiO 4 ] 4- tetrahedra linked in the corners while sharing all of the oxygen atoms. These tetrahedra are connected to form rings of 4,5,6, and 8 Si atoms that form three different cages known as dodecahedron, decahedron, and 19-hedron. The latter is occupied by the 1- adamantanamine (ADA) template molecules during the synthesis [5]. Upon connecting these three types of cages, a 2-dimensional channel system is formed. The only accessible opening consists of the 8- member ring with a pore aperture of 0.36 nm 0.44 nm. Figure 2 shows the three types of cages that form the rhombohedral structure [7]. 12

14 Figure 2. Cages and Crystal Structure of the DD3R zeolite [7] The method most commonly used in literature for the preparation of DDR crystals was originally developed by den Exter et al. [3]. The synthesis solution normally has the molar composition of 1- Adamantanamine (ADA): silica: ethylenediamine: water = 47:100:404: The precursor solution is made by the following procedure: ADA is first dissolved in ethylenediamine; water is then added rapidly and the mixture is placed in a shaking machine for 1 hour of mixing; The solution is further stirred for 1 hour at 95 C, and then the mixture is cooled in an ice bath and the silica source is subsequently added drop wise under vigorous stirring. The mixture is then heated to 95 C again while stirring until the solution turned clear. The final solution is transferred to an autoclave where it undergoes a 25 day synthesis at 160 C while being rotated at 60 RPM. The SEM image in Figure 3 below shows the typical morphology of DDR zeolite crystals. 13

15 Figure 3. SEM image of DDR crystals synthesized by den Exter [3]. The current state of the art DDR synthesis procedure was developed by Gascon et al [7]. This method consists of a 2-step process in which the crystals obtained from the 25-day synthesis method of den Exter [3] are used as seed in a 2 day synthesis of DDR zeolite particles. The same precursor solution and synthesis conditions are used for the second step in the seeded synthesis. The SEM image of the DDR crystals obtained by this improved synthesis procedure is shown below in Figure 4, which shows the crystal morphology identical to that found by den Exter. 14

16 Figure 4. SEM image of DDR crystals synthesized by Gascon [7]. The synthesis of the DDR zeolite was also studied by the Materials Research Laboratory of NGK Insulators in Japan. Nakayama et al. [8] reported the following precursor molar composition for synthesizing pure DDR crystals: 1-Adamantanamine (ADA): silica: ethylenediamine: water = 25:100:200: In this reported synthesis approach, 0.57g of ADA was dissolved in 1.80g of ethylenediamine to form the first solution. A second solution was obtained by adding 73.45g of water to 3.0g of Snowtex S silica sol (30% by mass) and the resultant mixture was gently stirred. The second solution was then mixed with the first solution giving the resultant solution an ADA/SiO 2 ratio of 0.25, water/sio 2 ratio of 280, and an ethylenediamine/ada ratio of 8. The precursor solution was then placed in a shaker to mix at 500RPM for one hour. Then, 0.1mg of DDR type zeolite powder, which was prepared according to the method of den Exter et al.[3], was added to the above prepared precursor solution and then placed in an ultrasonic bath at 65 C for 5 minutes for dispersion. This final seeded 15

17 solution was transferred to an autoclave to conduct the hydrothermal reaction for 5 days at 160 C for DDR zeolite formation. Potapova [5] reviewed the different methods of DDR synthesis that are previously discussed in this chapter. It was concluded in this review that the method of Nakayama [8] was not appropriate for DDR crystallization while the method of den Exter et al.[3] was effective in producing DDR zeolite with the addition of Sigma-1 (an aluminosilicate structure analog of DDR) crystals as seed. However, this method still requires the lengthy 25 day synthesis as well as the sigma-1 synthesis. This critical review was conducted prior to the development of the procedure by Gascon [7] so that method was not studied. 1.3 DDR Zeolite Membrane The current method in the literature for the synthesis of a DDR zeolite membrane consists of a secondary growth process in which a porous substrate is coated with DDR seed crystals prior to the hydrothermal synthesis [1]. DDR membranes have been successful in several types of separations for molecules with small kinetic sizes. Unlike most aluminosilicate zeolites, DDR is all-silica making its surface hydrophobic. This reduces pore blocking by water sorption at the internal surface and enables selective transport of organo-compounds through preferential adsorption-diffusion mechanisms. Kuhn et al. [9] have demonstrated the effectiveness of a DDR membrane to separate water from organic solvents (mainly ethanol) under pervaporation conditions with ethanol preferentially permeating through the membrane. DDR has been shown to be effective in the separations of CO 2 /CH 4 which has important applications in natural gas upgrading [1]. The DDR membrane also exhibited good selectivity in the separation of propylene/propane mixtures [7]. The result of DDR membrane separation for the separation of propylene/propane mixtures is very encouraging because traditional distillation separation is very 16

18 energy intensive. Distillation has been the primary source of separation for hydrocarbons. The columns used to separate the olefins from the paraffins of the C2, C3, and C4 are among the most energy intensive operations in oil refining [7]. The olefin/paraffin separation makes up about 7% of the total distillation energy demand in the United States [10]. Also with the continuing rise in crude oil prices along with the move to reduce greenhouse gas emissions, it is essential that industry becomes more energy efficient. Zeolite membranes are potential energy-efficient alternatives to many distillation separations of these kinds [11, 12]. The ultramicropore zeolites like DDR, NaA and SAPO-34, which have pore sizes around 0.4 nm, are appropriate for separation of small molecules such as O 2, N 2, H 2, CO 2, and CH 4, etc. based on both competitive adsorption-diffusion and size discrimination effects. Recently, the ultramicropore NaA zeolite membranes have shown good selectivity for O 2 separation from N 2 [13]. The separation of O 2 /N 2 in the NaA membranes (pore dia. d p =0.41nm) relies on the difference of molecular mobility between O 2 and N 2 as both gases are weakly adsorbing in the zeolite. The O 2 molecule (size of 0.38nm 0.28nm) is smaller than N 2 (size of 0.42nm 0.32nm) and thus O 2 diffuses much faster than N 2 in the critically sized zeolitic pores. The sensitive dependence of the diffusivity on the ratio ( ) of molecular size (d k ) to pore diameter (d p ) ( =d k /d p ) has been established by molecular dynamic theory and verified by experimental data [14, 15]. Theoretical models predict that O 2 /N 2 selectivity as high as 37 can be achieved in NaA zeolite membranes [16]. However, NaA zeolites are highly hydrophilic which causes severe decline of O 2 selectivity and permeance when handling humid gases. The water sorption-induced degradation of separation properties is common for ultramicropore zeolite membranes. For example, severe decrease in CO 2 /N 2 separation performance occurs in the hydrophilic SAPO-34 zeolite membrane (d p =0.38nm) for humid feed gases [17, 18]. The all silica DDR zeolite membrane thus may be ideal for separation of O 2 from air due to its hydrophobic surface. 17

19 Currently, there are very few publications in the literature on DDR zeolite particles or membranes due to the challenges of the synthesis. NGK Insulators of Japan is the only place to have shown success and reproducibility in synthesizing pure DDR phase membranes [1] but a detailed synthesis procedure is not disclosed. Further research is needed both to develop a better synthesis route for DDR membrane production and to extensively study the potential of DDR membranes for the important separations of hydrogen, oxygen, and carbon dioxide from industrial gas mixtures. The following is the method published by NGK Insulators. According to Tomita [1], DDR membranes were prepared by a multiple-step procedure including (i) the synthesis of DDR seed crystals by hydrothermal crystallization using precursors containing structural directing agents (SDA), (ii) coating seed layer of the DDR crystals on the porous substrate, (iii) secondary growth of the seed layer to form a continuous polycrystalline film free of pinholes, and (iv) removal of the SDA molecules from the zeolitic cavities by thermal decomposition and oxidization at high temperature [18]. Like for other zeolite membranes, the resultant membrane quality and performance in gas separation is affected by each of the steps as well as post-synthesis modifications [15, 19]. In their specific approach, DDR zeolite crystals were first prepared according to the 25-day synthesis method of den Exter [3]; A porous alumina tube was then immersed in a solution of pulverized DDR crystals in deionized water; The tube was then dried at 323K for 30 minutes. The membrane precursor mixture molar ratio of ADA: silica: ethylenediamine: water = 9:100:150:4000 was prepared in a perfluoro container. Tetramethyl Orthosilicate (TMOS) was used as the silica source. The seeded porous alumina tube with was then immersed in the precursor mixture. The mixture was then heated in a pressure vessel at 423K for 48h. The membrane was then calcined at 973K for 5 hours. The following SEM images show the surface and cross section of the developed DDR membrane. 18

20 Figure 5. SEM image of a DDR membrane surface synthesized by Tomita [1] Figure 6. SEM image of a DDR membrane cross-section synthesized by Tomita [1]. 19

21 1.4 Research Objectives The goal of this research is to develop a new process for the synthesis of pure DDR zeolite crystals with high reproducibility. The main emphasis of this work will be to significantly reduce the extremely long synthesis time (>25 days) required by the currently established procedures which is a major hurdle to the practical consideration for this unique type of zeolite material both as adsorbents and membranes. To accomplish this, the following parameters of the synthesis procedure will be studied: 1) Precursor Chemical Composition 2) Precursor Preparation Techniques 3) Synthesis Conditions (Temperature, Duration, and Agitation) Upon achieving successful synthesis of DDR zeolite crystals, activation of the small pore structure by high temperature template removal will be investigated. Exploratory research will then be conducted focusing on the fabrication of thin DDR membranes on practical porous ceramic substrates. 20

22 Chapter 2 DDR Zeolite Synthesis The DDR type zeolite is a pure silica zeolite with a pore aperture of nm. This ultramicropore structure is promising for membranes capable of separating small molecule gases such as O 2, N 2, H 2, and CO 2, etc. However, the lengthy synthesis time of 25 days or longer and the limited reported cases of successful DDR zeolite synthesis in the literature show that further research is needed to extensively improve the hydrothermal synthesis method. In this work, the existing synthesis methods were investigated and the findings were used to direct the development of more effective synthesis procedures. The chemical composition, precursor preparation, and synthesis duration were superficially studied which led to the establishment of a rapid and reproducible synthesis procedure. 2.1 DDR Zeolite Synthesis in Literature In the method of Nakayama et al [8], a molar composition of ADA: silica: ethylenediamine: water = 25:100:200:27218 was used for this synthesis. This method was slightly modified due to the requirement of DDR crystals as seed. Sigma-1 (aluminosilicate structure analog of DDR) seed crystals were substituted for DDR since they are of the same crystal structure and can be easily obtained. The actual synthesis procedure is described below: (i) ADA (Sigma Aldrich, 97%) was dissolved in ethylenediamine (Sigma Aldrich, 99.5%) in a Teflon Flask (ii) Water and Ludox Sm-30 Silica Sol were gently mixed in a second Teflon Flask (iii) The mixtures were then combined and stirred vigorously for 1 hour at 65 C (iv) As-synthesized Sigma-1 seed was added and the mixture was stirred another 10 minutes at 65 C (v) The mixture was then transferred to an autoclave and the hydrothermal synthesis was conducted for 5 days at 160 C. 21

23 The final solid product, denoted as Sample-1, was examined by XRD to identify its crystal phase and by SEM to observe the particle morphology. The XRD pattern given in Figure 7 indicates that Sample-1 was of crystalline nature but the crystal phase was of the competing 6-member ring structure Sigma-2 type (SGT phase) rather than the 8-member ring DDR phase. The SEM image in Figure 8 clearly shows the typical crystal morphology of Sigma-2 which is very different from that of the DDR crystals reported in the literature [3, 20]. This experimental result agrees with the conclusion made by Potapova [5] that the method of Nakayama has poor reproducibility. However, a small amount of DDR crystals were also formed and are seen in the SEM images. The formation of a small amount DDR phase is also verified by the weak XRD peaks characteristic to DDR, primarily at 2θ angle 17 o. Figure 7. XRD of Sample 1, SGT[21], and DDR[22]. 22

24 Figure 8. SEM image of Sample-1. Figure 9. SEM image of DDR crystals found in Sample 1 23

25 The method of den Exter et al. [3] involves some operation conditions that are hard to reproduce precisely or quantitatively. In this method, the experimental procedure includes: (i) ADA (Sigma Aldrich, 97%) was dissolved in ethylenediamine (Sigma Aldrich, 99.5%) in a Teflon flask and deionized water was added rapidly. (ii) The mixture was then stirred vigorously for 1 hour instead of being placed in a shaking machine due to the limit of facility in our laboratory. (iii) Next, the mixture was placed in an oil bath kept at 95 C on a magnetic hotplate. It was stirred for 1 hour. During this heating step, ADA separated from solution and stuck to the walls of the flask as well as on the inside of the cap. (iv) After being removed from the oil bath, the solution was cooled with running tap water and shaken by hand until all of the ADA went back into solution. (v) The mixture was then placed in an ice bath for 15 minutes under light stirring. Tetramethyl orthosilicate (TMOS Sigma Aldrich, 99%) was then added drop wise while stirring vigorously in the ice bath. (vi) The solution was then again placed in the 95 C oil bath again until the solution turned clear. The mixture was then transferred to an autoclave and the synthesis was carried out under various conditions. It is important to note that the reported synthesis procedure using 60 RPM could not be replicated in our laboratory. The highest rotation able to be achieved during synthesis was approximately 10RPM. 24

26 The greatest challenge associated with the synthesis of pure phase DDR is the low solubility of the template ADA. This is evident during the synthesis procedure when the template precipitated from the solution when being heated before adding TMOS. Upon the second heating step after adding TMOS, the ADA does not come out of solution due to the methanol produced from the hydrolysis of TMOS because methanol increased the solubility of ADA in the aqueous solution. However, if the solution is allowed to rest for a short period of time, it can be seen that the precursor becomes unstable and the ADA will start to segregate from the solution. The low solubility of the template ADA leads to phase separation giving undesirable reactant compositions which lead to the formation of the denser 6-membered ring Sigma-2 (SGT) phase. Along with this, the addition of organic alcohol (methanol from the hydrolysis of tetramethyl orthosilicate) affects the thermodynamic properties of the system, such as internal energy, and contributes to the production of the unwanted competing phase of SGT [23]. The following samples 2 and 3 were from the same precursor solution using TMOS prepared by the method of den Exter et al [3]. They were each synthesized for 25 days at 160 C in the same oven. The difference is that Sample-2 was obtained under static reaction condition and Sample-3 was synthesized with the reactor being rotated on our homemade rotation system built into the oven. This rotation system operates at less than 10 RPM while the method used by den Exter operates at 60 RPM. As shown by the following results, the degree of agitation clearly has a great impact on the outcome of the product when using this procedure. Figure 10 compares the XRD pattern of sample-2 (static) to that of the standard SGT pattern. 25

27 Figure 10. XRD patterns of Sample-2 and SGT [21]. Figure 10 shows that there is clearly a separation of phases here giving a lower concentration of template dissolved in the silica-rich aqueous phase leading to the unwanted 6-membered ring product of Sigma-2 (SGT). This demonstrates that proper agitation/mixing is needed in order to ensure a reasonably homogeneous reaction mixture needed for the formation of DDR zeolite with high purity. Figure 11 below compares the XRD pattern of Sample-3 (obtained under rotated condition) to that of the standard DDR and SGT pattern. As can be seen in Figure 11, the XRD pattern of Sample-3 is very different than that of Sample-2. The rotation/mixing appeared to help keep the ADA dissolved or uniformly distributed in the solution but the mixing was likely not strong enough to achieve the homogeneity of reactant solution necessary for producing pure DDR crystals. The XRD pattern of sample-3 clearly indicates the coexistence of DDR and SGT phase. 26

28 Figure 11. XRD patterns of Sample-3, SGT [21], and DDR [22]. To increase agitation effectiveness, zirconia beads ( 6-mm) were added to the solution while undergoing the rotated synthesis. The zirconia bead assisted agitation/milling-mixing reaction condition is depicted in Figure

29 Cylindrical vessel Zirconia milling beads Rotatory gear Figure 12. Schematic showing the ball-milling assisted DDR zeolite synthesis. The thus synthesized solid product is denoted as Sample-4. SEM observations of the resultant solids revealed some very unique morphologies. The following samples were synthesized using the recipe of den Exter as described before, but the hydrothermal synthesis duration was only 12 days to observe the product morphology in the middle stage of crystallization. Figure 13. XRD patterns of Sample-4, SGT [21], and DDR [22]. 28

30 Figure 13 shows that once again both DDR and SGT phases are present and some interesting type of polymorph was formed. However, there is a peak around 6 which is not characteristic of either of the structures. The low angle peaks indicate that there is a much larger pore or mesostructure in this material different than that of the 6-member ring SGT or the 8-member ring DDR. The most interesting thing about this sample is the nanowire bundle structure produced shown in Figure 14 below. It is not characteristic of typical crystal shape for either of these phases. Figure 14. SEM image of Sample-4. In an effort to reproduce the nanowire structure, the synthesis was repeated under the same conditions. However, a different type of structure was produced in this trial (Sample-5). The comparison of the XRD patterns for this Sample-5 and the previous Sample-4 is shown below. 29

31 Figure 15. XRD Patterns of Sample 4 and Sample 5. It can be seen that all of the peaks from Sample-5 appear in Sample-4 but a large peak at about 14 missing in Sample-5. This suggests a variation in crystalline structure for the two samples made under the same conditions. The results demonstrate that the crystal structure and morphology is very sensitive to subtle changes in the reaction system. There is a dramatic difference in this morphology compared to the nanowires. The SEM picture of Sample-5 is shown below in Figure

32 Figure 16. SEM image of Sample-5. As can be seen in Figure 16, an inter-grown plate structure is obtained in Sample-5 instead of the nanowires in Sample-4. Several additional attempts were made to reproduce both of these interesting structures for possible new applications to take advantage of their unique morphologies and nanostructures. However, it was found that these products were not readily reproducible. In an attempt to obtain pure DDR, more rigorous agitation was used to achieve the mixing level used in the method of den Exter et al [3]. Stirred syntheses were attempted in an oil bath on top of a magnetic hotplate. A stir bar was placed in the vessel and the oil bath was held at 160 C with moderate (300 RPM) to high (800 RPM) stirring used. The same method of precursor preparation was used with TMOS as previously discussed. To avoid the lengthy synthesis times, seeded syntheses were conducted using Sigma-1 (the aluminosilicate structural analog of DDR) particles as seeds. The syntheses were conducted for 3 days. The final product is named Sample-6. However, it was found that this was not an effective way to obtain DDR as SGT phase was primarily formed as shown in Figure 17 below. 31

33 Figure 17. XRD Pattern of Sample 6 and SGT[21]. Based on the findings that the previous method using TMOS as the silica source was not effective, efforts were focused on using Ludox SM-30 Silica Sol as the silica source to produce DDR. To avoid the lengthy synthesis times, Sigma-1 seeds were employed. It is important to note that when using silica sol as the silica source, there are issues with the low ADA template solubility. During the second step of heating the solution, the ADA was found to separate from the solution and stick to the walls of the flask. This does not occur when using TMOS due to the presence of methanol which helps keep the ADA dissolved or more uniformly dispersed. Sample-7 was prepared using Ludox silica sol by the same procedure and conditions as the TMOS samples following the method of den Exter et al [3]. During the second heating step, the solution turned slightly clear but a significant amount of the ADA separated from solution and stuck to the walls of the flask and the inside of the cap. The cap was then removed for 5 minutes while stirring and the solution became completely clear. As-synthesized Sigma-1 crystals (0.2% by mass) were then added and 32

34 the solution was stirred for another 5 minutes. The seed-containing solution was then poured off without filtering and a small amount of the excess ADA remaining on the walls of the flask also entered into the synthesis vessel. The extra ADA that entered the vessel exceeded the soluble amount making the solution heterogeneous. This is believed to have contributed to the formation of the large SGT crystals in addition to the much smaller DDR crystals shown in Figure 18 below. Sample-7 was synthesized for 3 days under static conditions at 160 C. Figure 18. SEM image of Sample-7. 33

35 Figure 19. High magnification SEM image of Sample-7 showing DDR crystals as main products and a small amount of Sigma-2 large particles. Figure 20. XRD patterns of as-synthesized Sample-7 and standard DDR [22]. 34

36 When comparing the two XRD patterns in Figure 20, the peak at 2- value of 6.5 o is missing from the XRD spectrum of Sample-7. This peak however appears upon calcination which will be shown later in this thesis. The XRD pattern for Sample-7 also contains a very small peak around 2- value of 9 o which indicates the existence of a small amount of Sigma-2 (also see SEM images in Figures 18 and 19). The formation of Sigma-2 is believed to be caused by a small amount of the precipitated ADA entering the synthesis vessel. Most importantly, this sample verifies that a static synthesis is capable of producing DDR zeolite particles with reasonable purity provided that the right concentration of organic materials (both the template ADA and ethylenediamine) is carefully controlled and a clear precursor solution is obtained. 2.2 DDR Synthesis by Modified Method of This Work The above study on DDR zeolite synthesis based on the method of den Exter et al. [3] has revealed that a large amount of template precipitates from the solution and sticks to the walls of the flask during the second heating step. The precipitated organic phase can provide conditions for the formation of the undesirable Sigma-2 phase. To improve the effectiveness of DDR synthesis, the synthesis method has been modified using a reduced amount of organic material. It was found through a series of experiments that ¼ of the amount of ADA and ethylenediamine used in the traditional method of den Exter et al. was sufficient to produce the zeolite DDR. It was found through our research that the shaking machine, the first heating step, and the ice bath are not needed when using Ludox silica sol as the silica source. This significantly simplifies the synthesis process and enhances the reproducibility of the current modified DDR zeolite synthesis method by avoiding the less controllable operations. 35

37 The modified method used the following molar ratio for the DDR zeolite synthesis precursor: ADA: Silica: Ethylenediamine: Water = 12:100:100: The detailed synthesis process is described below: (i) In a Teflon flask, ADA was dissolved in ethylenediamine. Water was then added and the solution was stirred at RT for 5 minutes. (ii) Ludox SM-30 silica sol was then added and the solution was stirred at RT for 5 minutes. (iii) The solution was then placed in an oil bath kept at 95 C on a magnetic hotplate. It was stirred until the solution turned clear. A small amount of ADA precipitated and stuck to the walls of the flask. (iv) The cap of the flask was then removed for 5 minutes while stirring. During this step, the solution became exceptionally clear as any excess organic material is allowed to evaporate. (v) Ball-milled Sigma-1 seed crystal (0.2% by mass) was then added and the solution was stirred for another 5 minutes with the cap off. (vi) The solution was then filtered with filtration paper when being transferred to the autoclave to prevent any excess precipitated ADA from entering the solution. (vii) The hydrothermal synthesis was conducted at 160 C for 20 hours. The product was then recovered by centrifugation and washed with DI water. 36

38 The Sigma-1 particles used as seed were prepared by a stirred hydrothermal synthesis method. A synthesis precursor with a molar ratio of 3Na 2 O: 20ADA: 1Al 2 O 3 : 60SiO 2 : 2400H 2 O was produced according to the procedure reported by Stewart et al. [24]. The resulting mixture was stirred on a hotplate set at 80 C for 1 hour. The solution was then transferred to an autoclave along with a magnetic stir bar. The autoclave was then emerged into an oil bath on a magnetic hotplate. The temperature of the oil bath was maintained at 180 C. The synthesis was conducted under moderate stirring for 3 days. The resulting product was washed thoroughly with DI water and dried at 80 C. After drying, Sigma-1 was ball-milled in water for 2 hours at 250 RPM using a Retsch Planetary Ball Mill PM 100. The effect of ball-milling the Sigma-1 seed crystal was found to be critical. Figure 21 below shows the SEM image of the as-synthesized Sigma-1 particles. The crystals are large with very ununiform size distribution before ball-milling. After ball-milling, the SEM image in Figure 22 confirms that Sigma-1 particles have been crashed down to sub-micron size. Figure 23 presents the XRD pattern of the as-synthesized Sigma-1 particles. Figure 21. SEM image of as-synthesized Sigma-1 37

39 Figure 22. SEM image of ball-milled Sigma-1 Figure 23. XRD Pattern of as-synthesized Sigma-1. 38

40 After ball-milling Sigma-1, X-ray diffraction was done again to verify that the structure was still intact. Figure 24 below shows that there was no damage to the crystal structure when crashing down the particle size. Figure 24. XRD Pattern of Ball-milled Sigma-1. Ball-milling the Sigma-1 seed has a very big impact on the outcome of the product. With the small particle size and high surface energy, the crystals are able to stay suspended throughout the solution giving a more uniform reaction condition with more crystallization sites allowing for a very rapid synthesis. Therefore, DDR can be synthesized in less than 1 day and this synthesis method produces a smaller DDR crystal size. Sample-8 was synthesized by the above described modified synthesis approach. As can be seen from the particle morphology in SEM picture (Figure 25) and XRD pattern (Figure 26), pure DDR phase crystals were obtained within 1 day of hydrothermal reaction. The synthesis process was found to be highly reproducible by 8 independent synthesis trials. 39

41 Figure 25. SEM image of Sample 8 Figure 26. XRD Pattern of Sample-8 and standard DDR[22]. 40

42 Table 1 below summarizes all of the reported DDR Synthesis Procedures in comparison to the work done in this study. The overall synthesis duration required for the method developed in this research is significantly shorter than those reported in the literature. Table 1. Comparison of required DDR zeolite synthesis time between the new method developed in this work and those reported in literature. Summary of DDR Synthesis Procedures Reference Seeded Method Seed Synthesis Duration (days) DDR Synthesis Duration (days) Total Duration (days) [3] No [7] Yes [5] Yes [8] Yes This Study Yes Activation of the DDR Zeolite The cages and porosity of the DDR zeolite are occupied by the ADA template molecules. To make the zeolite useful as adsorbents or gas permeation membranes, the template molecules must be removed to empty the pores. Such template removal for zeolite activation is accomplished by firing at high temperatures. To find out the appropriate firing temperature and duration, the calcination process was conducted at various temperatures. A heating rate of 2 C/min was used for all of the calcination trials. A Micromeritics ASAP 2020 Accelerated Surface Area and Porosimetry Analyzer was used for the nitrogen adsorption and desorption measurements (BET surface measurement) to verify the removal of 41

43 ADA template. The XRD examination (Figure 27) proved that the pure silica DDR zeolite is structurally stable up to at least 900 o C. Table 2 below presents the results of the BET surface area values for samples fired at different temperatures in a range of C and Table 3 summarizes the BET surface area values of the calcined DDR zeolite reported in the literature. The comparison of results in Table 2 and Table 3 shows that the BET surface areas for the DDR zeolite of this study are consistent with the reported literature values. Figure 27. XRD Patterns of the Calcined DDR Powder Samples. 42

44 Table 2. S BET of the Calcined DDR Samples Sample Temperature ( C) Duration (hours) S BET (m 2 /g) DDR DDR DDR DDR DDR Table 3. Literature Results for S BET of DDR Reference Firing Temperature ( C) Duration (hours) S BET (m 2 /g) [5] [25] [26] (Ozonicated) [27]

45 2.4 Summary It was found that the previously reported procedures were of poor reproducibility for the synthesis of DDR zeolite and involved a very lengthy synthesis duration (>25 days). A modified two-step synthesis method has been developed for the rapid production of DDR-type zeolite crystals with good reproducibility and excellent purity. The developed method uses ¼ of the organic material compared to the traditional method preventing phase separation which contributes to the formation of the undesired product Sigma-2. The first step in this process is the preparation of Sigma-1 seed crystals by a stirred three-day hydrothermal synthesis. The second step is a one-day static synthesis of DDR zeolite with the assistance of ball-milled Sigma-1 as seed. Ball-milling as-synthesized Sigma-1 was found to be critical in the synthesis process. With the small particle size and high surface energy, the crystals are able to stay suspended throughout the solution giving a more uniform reaction condition and more crystallization sites allowing for increased reaction kinetics. The new process developed in this study uses a total of 4 days for the synthesis of pure DDR zeolite which is drastically shorter than the 25 to 30 day duration used in verified synthesis methods in the literature. It was found that the DDR zeolite can be activated by high temperature firing at 600 C in air for 8 hours. The activated DDR zeolite had a BET microporous surface area of 294 m 2 /g which is consistent with literature values. 44

46 Chapter 3 DDR Membrane Synthesis Exploratory research was performed on the synthesis of DDR zeolite membranes on porous alumina supports. A seeded secondary growth method previously reported in the literature was investigated. Gas permeation studies were conducted using a transient Single Gas Permeation Setup. mechanisms: In general, molecular separations through the DDR zeolite membranes depend on two distinct (i) Preferential adsorption-diffusion mechanism. The all silica DDR zeolite possesses a hydrophobic surface that can preferentially adsorb organic molecules over inert gases or water. Examples of separations based on this mechanism include the high permeation selectivities for methanol and ethanol over water [9] and CO 2 over air. (ii) Size exclusion effect or competitive diffusion governed separation for small molecular size gases. The DDR zeolite has a pore opening size of about 0.4 nm which provides a large difference in diffusivity for molecules with similar kinetic sizes close to the pore opening. Examples of separations based on molecular size discrimination include the high selectivity for H 2 /i-c 4 H 10, CO 2 /CO, CO 2,/CH 4 and N 2 /CH 4, etc. [1,2,4,28]. The following Table summarizes the separation performance of DDR membranes for various gas mixtures reported in literature. 45

47 Table 4. Reported Gas Mixture Separation Performance of DDR membranes. Reference Thickness (µm) H 2 /CO 2 H 2 / Isobutane CO 2 / CO CO 2 / CH 4 N 2 / CH 4 CO 2 / Air O 2 / N 2 Permeance (10-8 mol/s.m 2.Pa) [1]* α ~ 220 at 373K P CO2 ~ 7.0 P CH4 ~ 0.1 [2]* S o > P H2 ~ 4.5 at 773K [4]* α ~ 3000 at 220K α ~ 45 at 235K α ~ 400 at 225K α ~ 2 at 220K P CO2 ~ 11 P CH4 ~ P H2 ~ 9 [28]* S o >600 S o ~ P ISO ~.015 at K at 303K P CO2 ~ 12 [29]* 10 S o ~ P H2 ~ 2.24 at 773K [30] S o ~ P H2 = 0.5 at 823K α = Separation Factor, S o = Ideal Selectivity * Membrane synthesized by NGK Insulators. 46

48 3.1 DDR Membrane Synthesis by Literature Methods In the method of Tomita et al [1] of NGK Insulators, a two-step secondary growth process was used to synthesize pure phase DDR membranes. DDR zeolite crystals were first prepared according to the 25-day synthesis method of den Exter [3]. A porous alumina tube was immersed in a solution of pulverized DDR crystals in deionized water and was then allowed to dry for 30 minutes. The membrane precursor molar ratio of 1-adamantanamine: silica: ethylenediamine: water = 9:100:150:4000 was prepared using tetramethyl orthosilicate (TMOS) as the silica source. The synthesis was conducted at 150 C for 48 hours. This method was slightly modified in this study using ball-milled Sigma-1 crystals as seed instead of DDR. The use of ball-milled Sigma-1 proved to be effective in producing the DDR zeolite in the developed DDR powder synthesis procedure above. The precursor preparation conditions may have varied due to the fact a detailed procedure was not reported. The synthesis procedure used is described below: (i) ADA (Sigma Aldrich, 97%) was dissolved in ethylenediamine (Sigma Aldrich, 99.5%) in a Teflon flask and deionized water was added. (ii) The mixture was then stirred vigorously for 1 hour. (iii) Next, the mixture was placed in an oil bath kept at 95 C on a magnetic hotplate. It was stirred for 1 hour. During this heating step, ADA separated from solution and stuck to the walls of the flask as well as on the inside of the cap. (iv) After being removed from the oil bath, the flask was cooled with running tap water and shaken by hand until all of the ADA went back into solution. 47

49 (v) The mixture was then placed in an ice bath for 15 minutes under light stirring. Tetramethyl orthosilicate (TMOS Sigma Aldrich, 99%) was then added drop wise while stirring vigorously in the ice bath. (vi) The solution was then again placed in the 95 C oil bath again until the solution turned clear. The mixture was then transferred to an autoclave containing a dip-coated support. The synthesis was conducted at 150 C for 48 hours. It was found that this method was not easily reproducible by these synthesis conditions as DDR phase membranes were not formed. Several different silica sources were studied including tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), Ludox silica sol, and fumed silica. It was found that TMOS and TEOS led to the production of SGT phase due to the addition of organic alcohols, methanol and ethanol respectively, as discussed by Narita [23]. This same issue of impurity phase formation was seen previously in the particle synthesis in the last chapter. Figure 28 below shows the XRD pattern of Membrane 1 that was synthesized using TMOS according to this procedure. As can be seen, it is primarily Sigma-2 (SGT). 48

50 Figure 28. XRD Patterns of the Membrane 1 and the standard SGT (Sigma-2) [21]. Ludox SM-30 silica sol was studied next as the silica source since no alcohol is present. However, it was found that the Ludox silica sol is not effective for membrane formation as no film was produced on several attempts. Therefore, fumed silica was investigated. It was found that upon the final heating step the fumed silica would not stay dissolved. This led to the proposed membrane synthesis procedure which involves the addition of sodium hydroxide to keep the fumed silica dissolved. 49

51 3.2 Modified Method for DDR Membrane Synthesis The modified membrane synthesis procedure explored in this thesis research is similar to the modified DDR powder synthesis procedure described in the previous chapter. The only difference between the particle and membrane synthesis solution is the addition of NaOH to the membrane synthesis solution. The addition of NaOH is needed to dissolve the fumed silica which is used as the silica source. The presence of NaOH and the change of alkalinity of the solution are expected to influence the zeolite crystallization. The following molar ratio was used in the synthesis solution for membrane preparation: ADA: Silica: Ethylenediamine: NaOH: Water = 12:100:100:82: The solution was prepared as follows: In a Teflon flask, NaOH (Sigma Aldrich, %) was dissolved in deionized water and fumed silica (Sigma Aldrich, 0.014μm particle size) was added. This mixture was then stirred on a hotplate set at 125 C until it turned clear (approximately 30 minutes). In a second Teflon flask, ADA was dissolved in ethylenediamine. The mixtures were then combined and the resultant mixture was stirred for 30 minutes at RT. It was then placed in an oil bath maintained at 95 C on top of a magnetic hotplate. It was stirred until the solution turned fairly clear. During this step of mixing at elevated temperature, a small amount of ADA precipitated and stuck to the walls of the flask. The cap of the flask was then removed for 10 minutes while stirring. During the stirring with an open lid, the solution became completely clear as any excess organic material was allowed to evaporate. The solution was then filtered with filtration paper to prevent any excess precipitated ADA from entering the synthesis vessel upon transferring to the autoclave that contained a dip coated support. A homemade α-alumina disc substrate was dip-coated with a layer of ball-milled Sigma-1 seed crystals. The dip coating suspension consisted of ball-milled Sigma-1 crystals dispersed in DI water. This support was placed facing up in the bottom of the vessel before the precursor solution was transferred. 50

52 The autoclave was then placed in an oven set at 165 C for 20 hours. This produced a membrane thickness of approximately 4-5 microns. The homemade membrane substrates used in this study were porous α-alumina discs (2mm thick and 25 mm diameter) produced from Alcoa SG16 α-alumina by a dry pressing and sintering process. The porosity and mean pore size of the substrates were approximately 27% and 0.1μm, respectively [31]. The substrates were polished with 500 then 800 mesh SiC sand paper until the surface was very shiny. The substrates were then cleaned individually using an ultrasonic bath to remove any loose particles on the surface. They were then dried in an oven at 40 C for at least two days. The results from several experiments show that the modified procedure for membrane preparation with the use of fumed silica and the addition of NaOH was easily reproducible. However, when SEM characterization was done after gas permeation studies, it was determined by EDAX analysis that the membrane formed was not all-silica and had a Si/Al ratio of approximately 25 near the surface of the zeolite layer. The incorporation of aluminum into the structure is due to the strong alkalinity of the solution which dissolved part of the alumina substrate [12, 32]. Therefore, it was found that the proposed membrane procedure was not effective in forming the all-silica DDR structure but it is very effective for forming the Sigma-1 structure. Even though the membrane formed was not all-silica, several parameters such as permeation, activation and film thickness could still be studied since the pore structure is the same. The following membrane was synthesized according to the modified membrane procedure for 3 days at 160 C. It was then calcined for 8 hours at 600 C using a heating and cooling rate of 0.25 C/min. The XRD pattern in Figure 29 confirms the Sigma-1 structure is formed. The extra peaks in the membrane 2 spectrum are from the α-alumina substrate. The SEM images of the surface and cross section are shown in Figure 30 and Figure

53 Substrate Peaks Figure 29. XRD Pattern of the Calcined Membrane obtained by the modified synthesis method together with the spectrum of standard DDR zeolite [22]. Figure 30. SEM image of the Membrane 2 Surface. 52

54 Figure 31. SEM image of the Membrane 2 Cross-Section. As can be seen from Figure 31, the membrane thickness is larger than 10 microns from a 3 day synthesis which is greater than desired. It was determined by further studies that a 20 hour synthesis at 165 C was sufficient to produce a membrane with a thickness of 4-5 microns. Membrane 3 in the following SEM image in Figure 32 was synthesized according to this 20 hour procedure and the reduced thickness is confirmed. This membrane was not subject to calcination. 53

55 Figure 32. SEM image of the Membrane 3 Cross-Section. In Figure 33 below, membrane 2 is shown after calcination. This image confirms that 600 C is a sufficient calcination temperature for the activation of this structure. The surface is completely white indicating the complete removal of the template. The surface would be grey or black if there was any evidence of a significant amount of carbon deposits in the zeolite porosity. Figure 33. Image of DDR membrane 2 calcined at 600 C. 54