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2 Nijmeijer, Arian Hydrogen-Selective Silica Membranes for Use in Membrane Steam Reforming Thesis University of Twente, Enschede With ref. With summary in Dutch ISBN Copyright 1999 by A. Nijmeijer, The Netherlands Printing and binding by Printpartners Ipskamp, Enschede Cover illustration: J.R. Rostrup-Nielsen, Catalytic Steam Reforming, Springer Verlag, Berlin (1984).

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5 Here we stand Like an Adam and an Eve Waterfalls The Garden of Eden Two fools in love So beautiful and strong The birds in the trees Are smiling upon them From the age of the dinosaurs Cars have run on gasoline Where, where have they gone? Now, it s nothing but flowers (Nothing but) flowers, David Byrne, The Talking Heads

6 The work described in this thesis was financially supported by the EU in the framework of the Brite-Euram program.

7 &RQWHQWV Chapter 1 1 Introduction Chapter 2 15 The process technology of (membrane) steam reforming Chapter 3 37 Colloidal processing of ceramic membrane supports. General introduction Chapter 4 53 Colloidal processing of ceramic membrane supports. The experimental part Chapter 5 69 The preparation and properties of hydrothermally stable γ-alumina membranes Chapter 6 85 Preparation, characterisation and properties of microporous silica membranes Chapter Low temperature CVI modification of γ-alumina membranes Chapter 8, 115 The thermal dehydrogenation of H 2 S in a membrane reactor Chapter Evaluation and recommendations

8

9 &KDSWHU,QWURGXFWLRQ 1. Concepts for membrane reactors With the development of new, highly selective ceramic membranes the possibility of using them in high temperature membrane reactors came into scope. Hence, a large amount of research is currently being done on a world-wide scale on these reactors. An extensive review of current research on membrane reactors was given in a recent paper by Bredesen [1] and by Saracco et al. In somewhat older work, Saracco and Specchia [3] and Zaman and Chakma [4] provided good reviews on the use of membrane reactors as well. Saracco and Specchia focused on catalytic membrane reactors while Zaman and Chakma paid more attention to membrane synthesis. Armor [5] gave a good review on membrane catalysis and defined the current problems. In this chapter an overview is given on the general ideas of membrane reactors and types of reactions under investigation in the world. Membrane reactors can be used to shift the equilibrium in thermodynamically limited reactions. Several types of membrane reactors are currently under investigation, especially for dehydrogenation reactions such as the dehydrogenation of propane to propene [6] or of ethylbenzene to styrene [7]. Also the dehydrogenation of H 2 S has been studied in membrane reactors [8,9]. Reforming reactions have been studied in membrane reactors as well. Most well-known is the steam-reforming of various hydrocarbons [10-13], especially methane steam-reforming which is the major source of hydrogen in the world [14]. Some research has been performed on CO 2 reforming of methane [15] and also a considerable amount of effort has been put in performing the water-gas shift reaction in a membrane reactor [16,18]. In all of the above reactions, the selective removal of hydrogen from the reaction zone is the main role of the membrane, hence, such membranes must be hydrogen selective. Several types

10 2 Chapter 1 of membranes can be used and their advantages and disadvantages will be discussed in section 2 of this chapter. In general, problems with the existing membrane concepts are either an insufficient selectivity or permeance. Also the thermochemical stability of membranes is often insufficient for the process conditions used [17]. Another type of membrane reactors involves the reactors where the reactant is fed in a controlled manner to the reactor via a membrane. Examples are the oxidative coupling of methane [19-21] and ethylene hydrogenation [22]. Especially in the case of dense perovskite-type membranes large problems with respect to materials stability are to be expected in the reactive environments. Also, because of the very high temperatures ( ºC) used in this type of reactors, the high temperature sealing technology that should be developed involves a large technological risk. A novel type of membrane reactor, emerging presently, is the pervaporation reactor. Conventional pervaporation processes only involve separation and most pervaporation set-ups are used in combination with distillation to break azeotropes or to remove trace impurities from product streams, but using membranes also products can be removed selectively from the reaction zone. Next to the polymer membranes, microporous silica membranes are currently under investigation, because they are more resistant to chemicals like Methyl Tertair Butyl Ether (MTBE) [23-24]. Another application is the use of pervaporation with microporous silica membranes to remove water from polycondensation reactions [25]. A general representation of such a reaction is: R(COOH) x + R (OH) y o RCOOR + H 2 O Clearly, in such a reaction the removal of the produced water will lead to an enhanced conversion. Commercially available polymer membranes cannot withstand the severe operation and cleaning conditions for this process ( ºC) and microporous silica membranes again come into the picture. 2. Hydrogen selective membranes When considering membrane reactors for dehydrogenation and reforming reactions, three types of membrane are of most interest: dense palladium or palladium composite membranes,

11 Introduction 3 silica membranes produced by Chemical Vapour Infiltration (CVI) and silica membranes produced by sol-gel techniques. These membrane types will now be discussed. 2.1 Palladium membranes Dense palladium membranes are the most investigated membranes for hydrogen separation. They are investigated by several research groups as the group of Gryaznov [26-28] and the group of Uemiya and Kikuchi [29,30]. However, a large problem when pure palladium membranes are used is the phase transition between the α- and β-form in hydrogen-containing environments. This phase transition can be suppressed by doping the palladium with other metals. Silver is used most and Pd-alloy membranes with 23-25% silver are produced commercially by Johnson Matthey [31,32]. These membranes show increased stability and slightly higher hydrogen permeance compared with the conventional, undoped, palladium membranes. Johnson Matthey claims a hydrogen permeance of 1 * 10-6 mol/m 2 spa at 500ºC. Currently, palladium and palladium composite membranes are used only when ultra-pure hydrogen is needed and only in the separation step. First, the hydrogen is produced, for example with methanol steam reforming [32] after which the produced H 2 /CO 2 mixture is fed to the palladium membrane. The chemical stability of the palladium membranes, however, remains a large problem. They are very sensitive towards sulphur and chlorine and also the stability towards CO might be problematic. It has been reported that a CO concentration of only 0.2 vol-% gives a significant reduction in hydrogen flux [33,34]. The poisoning effects of these gases, however, depend largely on the type of alloy [35] and can therefore be limited by choosing the right type of alloy for a specific gas mixture. 2.2 CVI-silica membranes Much research has been performed on silica membranes produced by Chemical Vapour Infiltration for hydrogen separation purposes. In chapter 7, CVI experiments are described and a concise literature review is provided as well. Below some highlights will be presented briefly. In CVI, the pores of a porous medium are plugged by the reaction product of a precursor and an oxidising agent. For the preparation of silica layers, the precursor is a gaseous or volatile

12 4 Chapter 1 silica compound, such as silane [36-38] or Tetra Ethyl Ortho Silicate [39-42] and the oxidising agent is usually pure oxygen, air or steam. The porous materials used for CVI are commonly α-alumina supports [39], γ-alumina membranes [40] or porous Vycor glass [36,37]. In theory CVI membranes are very promising. Especially membrane stability is expected to be very good, because the separative layer is located inside the support, where it is protected against mechanical damage and chemical attack. In addition, measured permselectivities of CVI-silica membranes are very high: for H 2 :N 2 values as high as 3000 have been measured [36,37]. However, large-scale commercial production of CVI-membranes is hindered by the complicated equipment needed for their synthesis and the associated high costs. Furthermore the permeance of CVI membranes (1*10-7 mol/m 2 spa or lower) are still too low. Maybe these low permeances can be increased using different precursors or other reaction conditions, but a large improvement is not to be expected. 2.3 Sol-gel silica membranes A third type of membrane is the sol-gel microporous silica membrane. This type of membrane is of major importance in this thesis. Below, a short overview will be provided of state-of-theart silica membranes at the start of the project (1995). This has been the starting point from which the new membranes described in this thesis were developed Synthesis At the start of the project microporous sol-gel silica membranes were under investigation in various research groups. An extensive literature review is provided in the introduction of the thesis of De Vos [43]. Common supports for sol-gel silica membranes are mesoporous γ-alumina membranes [44-47]. These mesoporous membranes are either home made [43-45] or obtained from commercial sources [46,47]. A standard membrane as prepared by de Lange [45] consisted of a die-pressed α-alumina support, fired at 1360ºC with a pore diameter of 160 nm on which a γ-alumina membrane was coated with a home-prepared boehmite sol. The coated γ-alumina layer was calcined at 600ºC, had a thickness of 7 µm with a pore diameter of ~5 nm. On top of this mesoporous membrane,

13 Introduction 5 a silica top-layer was coated with a home-prepared polymeric silica sol. The silica layer was fired at 400ºC and had a pore-diameter of around 3Å. Some defects were however present in the microporous silica layer resulting in relatively low permselectivities. For example the H 2 /CO 2 permselectivity was only 5, which is very close to the ideal Knudsen selectivity of 4.7. An SEM micrograph of a supported silica membrane is provided in Figure 1. Because of the very small thickness (~30 50 nm) of the silica layer, this layer is not visible in the micrograph. Figure 1: SEM micrograph of a supported silica membrane Properties At the start of the project (1995) state-of-the-art microporous silica membranes as prepared by de Lange [45] and described above had a permselectivity of 43 of hydrogen towards methane and a hydrogen permeance of 1.6*10-6 mol/m 2 spa. By performing the synthesis of the membranes under cleanroom conditions later, De Vos [48,49] showed the influence of particle contamination on the integrity of silica membrane layers. Furthermore, the firing temperature of the silica was increased to 600ºC, which resulted

14 6 Chapter 1 in a decrease of the hydroxyl group concentration on the pore-surface in the silica layer and a significant decrease in the CO 2 permeance [50]. Therefore, the H 2 /CO 2 permselectivity improved by more than a factor 10, while the hydrogen permeance remained high (6 * 10-7 mol/m 2 spa at 300ºC). Gas transport properties through silica membranes have not been extensively studied. Especially the resistance of gas transport of small molecules like H 2 through the thin SiO 2 layer are currently such that the resistance in the supporting layers should not be ignored or might even dominate the transport properties of the final membrane, see for example [50]. In macro- and mesoporous membrane layers the nature of the flow is determined by the relative magnitude of the mean free path λ of the molecules and the pore size d p. When the mean free path of the gas molecules is much larger than the pore size, i.e. λ >> d p, collisions of molecules with the pore walls are predominant and the mass transport takes place by the wellknown selective Knudsen diffusion process. If the pore radius is much larger than the mean free path of the molecules and a pressure difference over the membrane exists the mass transport takes place by non-selective viscous flow. Studies with many types of porous media have shown that for the transport of a pure gas the Knudsen diffusion and viscous flow are additive (Present and DeBethune [52] and references therein). When more than one type of molecules is present at intermediate pressures there will also be momentum transfer from the light (fast) molecules to the heavy (slow) ones, which gives rise to non-selective mass transport. For the description of these combined mechanisms, sophisticated models have to be used for a proper description of mass transport, such as the model presented by Present and DeBethune or the Dusty Gas Model (DGM) [53]. In the DGM the membrane is visualised as a collection of huge dust particles, held motionless in space. Benes and Verweij provide a thorough theoretical description of the multi-component mass transport in microporous systems [54]. Lately, some systematic gas transport data has been obtained for different microporous membranes in our group [50], but more extensive measurements are necessary to get a good insight in the detailed transport properties of the different types of silica membranes. A good description of the transport phenomena in the membrane systems studied here may result in the possibility to develop quality estimators for membrane units. Such quality estimators can be used in process industry to evaluate compositions of permeate and retentate

15 Introduction 7 streams from the reactors. In this way, it is possible to control product streams by the use of easy measurable quantities, such as feed and sweep flows, total pressure and temperature. By doing this, the number of expensive and maintenance consuming gas chromatographs in process streams can be reduced. 3. Project description and objectives The goal of the present study is the development of a high temperature membrane reactor for steam reforming of natural gas (methane), which occurs by the following reaction: CH 4 + 2H 2 O o 4H 2 + CO 2 Actually this is a combination of steam reforming by: CH 4 + H 2 O o 3H 2 + CO and the shift conversion reaction by: CO + H 2 O o CO 2 + H 2 The removal of H 2 from the reaction zone in a membrane reactor under equilibrium conditions, enables three possible changes, or a combination of these changes: 1. By keeping temperature and conversion constant one is able to reduce the catalyst volume. 2. By keeping the catalyst volume and the temperature constant one can increase the conversion of the reaction. 3. By keeping the catalyst volume and the conversion constant one can lower the reaction temperature. It is mainly this last option that is considered in this thesis. Improvements suggested above have a large positive environmental effect. When environmental regulations regarding CO 2 emissions become stricter, a reactor that utilises one or more of the above options might even be cost-effective compared with the conventional steam reformers. 3.1 General objectives The development of a novel membrane reactor requires considerable effort, so a European consortium of universities, institutes and industry was formed. The complete consortium consists of seven partners including the University of Twente. The development of a new micro-

16 8 Chapter 1 porous silica membrane, stable under steam reforming conditions, is the main objective of the University of Twente in the consortium. The results of this development are described in this thesis, while a detailed list of objectives is provided in section 3.2. The other tasks are described briefly below. After preparation of newly developed membranes, high temperature permeation measurements were performed by VITO in Belgium. Steam treatment and membrane material characterisation were performed at SINTEF in Norway and reactor testing together with kinetic modelling of the reactor at IRC in France. The development of a high temperature test module for this reactor testing was the task of Velterop BV in the Netherlands. Clearly, for steam reforming at lower temperatures, other catalysts are necessary, so the choice of a catalyst and the testing of this catalyst under real process conditions are important. This task has been a cooperation between IRC and Norsk Hydro in Norway. To get more insight in the economics of the project and the merits and drawbacks of the membrane reactor with respect to a conventional steam-reformer, also a techno-economic evaluation (TEE) was performed. For the use of the produced hydrogen two cases were considered. The hydrogen can be used for the production of NH 3 with ammonia being the feedstock for fertiliser production (case 1, Norsk Hydro) or for the production of electricity by using the hydrogen as fuel for a gas turbine (case 2, KEMA, the Netherlands). Some results of the TEE, together with some general ideas about reactor engineering and how to operate a membrane steam reformer are provided in chapter 2 of this thesis. 3.2 Derived objectives for membrane development The main task for the University of Twente was the development of a hydrogen selective microporous silica membrane for use at high temperatures. The membrane should be suitable for use in a membrane reactor for steam reforming of natural gas. The target goals stated in the project were to develop a membrane with a H 2 flux equal to 1 * 10-5 mol/m 2 spa with a separation factor > 50 with respect to the other gas components like CH 4, CO and CO 2 for 1000 hours at 600 C in Simulated Ambient Steam Reforming Atmosphere (SASRA). SASRA conditions are: 30 bar total pressure with CH 4 : H 2 O = 1:3. Apart from the stability towards the high temperature, mainly the stability in steam-containing environments was expected to be a large problem.

17 Introduction 9 4. Problem definition In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the γ-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. For the preparation of high-quality membranes, also high quality supports are needed. It was decided that the project would start with the development of colloidal filtrated flat supports and centrifugal cast tubular supports, which have a higher degree of homogeneity than conventional die-pressed, tape-cast and extruded supports. The development of these new supports is described in chapter 4 of this thesis. 5. Thesis outline The purpose of the thesis is to provide a detailed description of the improvements that have been made since the project start-up on silica membranes. In chapter 2, some basic ideas about steam reforming in conventional and membrane reactors are worked out. In this chapter the operation of conventional steam-reformers is compared with possible membrane steam-reformers. In this chapter also a techno-economic evaluation of a membrane reactor compared with the conventional process is provided. The boundary conditions imposed by process technology and the techno-economic evaluation result in the formulation of requirements for the development of the membranes, i.e. selectivity, flux, tube length, operating pressure, etc. The improvements that have been made in the preparation of molecular sieving silica membranes started with the development of high quality membrane supports, because quality of the supporting system is of crucial importance for the quality of the final molecular sieving membrane. To this end, the synthesis of the supports was performed by means of colloidal proc-

18 10 Chapter 1 essing. A literature review of the basic concepts of the preparation of colloidal suspensions is provided in chapter 3, whereas the actual preparation is dicussed in chapter 4. On top of the newly developed supports a steam-stable intermediate layer was coated. The preparation of these layers is treated in detail in chapter 5. After this, the permselective silica layer was applied, which should be resistant against high temperature and steam-containing environments as well. The experimental procedure together with some transport and Rutherford BackScattering (RBS) studies are described in chapter 6. Apart from the silica membranes prepared by dipcoating, also Chemical Vapour Infiltration (CVI)-type membranes have been prepared. Chapter 7 is dedicated to this type of membrane. In chapter 8 a new project has been formulated for the use of membrane reactors for the thermal dehydrogenation of H 2 S. Compared to the conventional Claus process, the application of a membrane reactor in the thermal H 2 S might have some large advantages. Finally, in chapter 9, conclusions are drawn and suggestions made for further research on (steam-stable) molecular sieving silica membranes or mesoporous γ-alumina membranes. Though not all of the project objectives were obtained, progress was made in the synthesis of micro- and mesoporous membranes. Especially the development of steam stable membranes may be a large step forward in the development of ceramic membranes. 6. References 1. R. Bredesen, Key Points in the Development of Catalytic Membrane Reactors Paper no. A7.0 in Proc. 13 th Int. Congr. Chem. Process Eng., August , Praha, Czech Republic. 2. G. Saracco, H.W.J.P. Neomagus, G.F. Versteeg and W.P.M. van Swaaij, High-Temperature Membrane Reactors: Potential and Problems, Chem. Eng. Sci., (1999). 3. G. Sarracco and V. Specchia, Catalytic Inorganic Membrane Reactors: Present Experience and Future Opportunities, Catal. Rev. Sci. Eng., 36 [2] (1994). 4. J. Zaman and A. Chakma, Inorganic Membrane Reactors, J. Membrane Sci., (1994). 5. J.N. Armor, Membrane Catalysis: Where is it Now, What Needs to be Done, Catal. Today, (1995). 6. Y. Yildirim, E. Gobina and R. Hughes, An Experimental Evaluation of High-Temperature Composite Membrane Systems for Propane Dehydrogenation J. Membrane Sci., (1997).

19 Introduction J.C.S. Wu, T.E. Gerdes, J.L. Pszczolkowski, R.R. Bhave, P.K.T. Liu and E.S. Martin, Dehydrogenation of Ethylbenze to Styrene Using Commercial Ceramic Membranes as Reactors, Sep. Sci. Tech., 25 [13-15] (1990). 8. T. Kameyama, M. Dokiya, M. Fujihige, H. Yokokawa and K. Fukuda, Production of Hydrogen from Hydrogen Sulfide by Means of Selective Diffusion Membranes, Int. J. Hydrogen Energy, 8 [1] 5-13 (1983). 9. T. Kameyama, M. Dokiya, M. Fujishige, H. Yokokawa and K. Fukuda, Possibility for Effective Production of Hydrogen from Hydrogen Sulfide by Means of a Porous Vycor Glass Membrane, Ind. Eng. Chem. Funam., (1981). 10. S.L. Jorgensen, P.E.H. Nielsen and P. Lehrmann, Steam Reforming of Methane in a Membrane Reactor, Catal. Today, (1995). 11. S. Uemiya, N. Sato, H. Ando, T. Matsuda and E. Kikuchi, Steam Reforming of Methane in a Hydrogen Permable Membrane Reactor", Appl. Catal., (1991). 12. M. Chai, M. Machida, K. Eguchi and H. Arai, Promotion of Hydrogen Permeation on Metal-Dispersed Alumina Membranes and its Application to a Membrane Reactor for Methane Steam Reforming, Appl. Catal. A, (1994). 13. A.M. Adris, S.S.E.H. Elnashaie and R. Hughes, A Fluidized Bed Membrane Reactor for the Steam Reforming of Methane, Can. J. Chem. Eng., (1991). 14. T. Johansen, K.S. Raghuraman and L.A. Hacket, Trends in Hydrogen Plant Design Steam Reforming will Continue to be the Main Source of H 2, Hydrocarbon Processing, [8] (1992). 15. E. Kikuchi and Y. Chen, Low-Temperature Syngas Formation by CO 2 Reforming of Methane in a Hydrogen Permselective Membrane Reactor, Stud. Surf. Sci. Catal (1997). 16. E. Kikuchi, S. Uemiya, N. Sato, H. Inoue, H. Ando and T. Matsuda, Membrane Reactor Using Microporous Glass-Supported Thin Film of Palladium. Application to the Water Gas Shift Reaction, Chem. Lett., (1989). 17. C.H. Chang, R. Gopalan and Y.S. Lin, A Comparative Study on thermal and Hydrothermal Stability of Alumina, Titania and Zirconia Membranes, J. Membrane Sci., (1994). 18. S. Uemiya, N. Sato, H. Ando and E. Kikuchi, The Water Gas Shift Reaction Assisted by a Palladium Membrane Reactor, Ind. Eng. Chem. Res (1991). 19. J.E. ten Elshof, H.J.M. Bouwmeester and H. Verweij, Oxidative Coupling of Methane in a Mixed- Conducting Perovskite Membrane Reactor, Appl. Catal. A, (1995). 20. A.M. Ramachandra, Y. Lu, Y.H. Ma, W.R. Moser and A.G. Dixon, Oxidative Coupling of Methane in Porous Vycor Membrane Reactors, J. Membrane Sci., (1996). 21. K. Omata, S. Hashimoto, H. Tominaga and K. Fujimoto, Oxidative Coupling of Methane using a Membrane Reactor, Appl. Catal., 52 L1-L4 (1989). 22. A.F.Y. Al-Shammary, I.T. Caga, J.M. Winterbottom, A.Y. Tate and I.R. Harris, Palladium-Based Diffusion Membranes as Catalysts in Ethylene Hydrogenation, J. Chem. Tech. Biotech., (1991). 23. M. Asaeda, Preparation of thin Porous Silica Membranes for Separation of Non-Aqueous Organic Solvent Mixtures by Pervaporation, Ceram. Trans., (1993).

20 12 Chapter M. Asaeda, P. Uchytil, T. Tsuru, T. Yoshioka, M. Ootani and N. Nakamura, Pervaporation of Methanol/MTBE Mixture by Porous Silica-Zirconia (10%) Membranes, pp in Proc. ICIM 5 June 22-28, Nagoya, Japan (1998). 25. J.W. Bakker, Application of Ceramic Pervaporation Membranes in Polycondensation Reactions, pp in Proc. ICIM 5 June 22-28, Nagoya, Japan (1998). 26. V.M. Gryaznov Hydrogen Permeable Palladium Membrane Catalysts, Platinum Metals Rev., 30 [2] (1986). 27. V.M. Gryaznov, O.S. Serebryannikova, Y.M. Serov, M.M. Ermilova, A.N. Karavanov, A.P. Mischenko and N.V. Orekhova, Preparation and Catalysis over Palladium Composite Membranes Appl. Catal. A, (1993). 28. V.M. Gryaznov, Platinum Metals as Components of Catalyst-Membrane Systems, Platinum Metals Rev., 36 [2] (1992). 29. S. Uemiya, N. Sato, H. Ando, T. Matsuda and E. Kikuchi, Promotion of Methane Steam Reforming by Use of Palladium Membrane, Sekiyu Gakkaishi, 33 [6] (1990). 30. E. Kikuchi, Palladium/Ceramic Membranes for Selective Hydrogen Permeation and Their Application to Membrane Reactor, Catal. Today, (1995). 31. J.E. Philpott, Hydrogen Diffusion Technology, Commercial Application of Palladium Membranes, Platinum Metals Rev., 29 [1] (1985). 32. J.E. Philpott, The On-Site Production of Hydrogen, A Mobile Generator for Meteorological and Industrial Purposes, Platinum Metals Rev., (1975). 33. F. Sakamoto, Y. Kinari, F.L. Chen and Y. Sakamoto. Hydrogen Permeation Through Palladium Alloy Membranes in Mixtures Gases of 10% Nitrogen and Ammonia in the Hydrogen, Int J. Hydrogen Energy, 22 [4] (1997). 34. F.L. Chen, Y. Kinari, F. Sakamoto, Y. Nakayama and Y. Sakamoto, Hydrogen Permeation through Palladium-Based Alloy Membranes in Mixtures of 10% Methane and Ethylene in the Hydrogen, Int. J. Hydrogen Energy, 21 [7] (1996). 35. H. Yoshida, S. Konishi and Y. Naruse, Effects of Impurities on Hydrogen Permeability through Palladium Alloy Membranes at Comparatively High Pressures and Temperatures, J. Less-Common Metals, (1983). 36. G.R. Gavalas, C.E. Megiris and S.W. Nam, Deposition of H 2 -Permselective SiO 2 Films, Chem. Eng. Sci., 44 [9] (1989). 37. S.W. Nam and G.R. Gavalas, Stability of H 2 -Permselective SiO 2 Films Formed by Chemical Vapor Deposition, AIChE Symp. Series, 85 [268] (1989). 38. S. Kitao and M. Asaeda, Gas Separation Performance of Thin Porous Silica Membrane Prepared by Sol- Gel and CVD Methods, Key Eng. Mater., 61 & (1991). 39. S. Morooka, S. Yan, K. Kusakabe and Y. Akiyama Formation of Hydrogen-Permselective SiO 2 Membrane in Macropores of α-alumina Support Tube by Thermal Decomposition of TEOS, J. Membrane Sci., (1995).

21 Introduction C.L. Lin, D.L. Flowers and P.K.T. Liu, Characterization of Ceramic Membranes II. Modified Commercial Membranes with Pore Size under 40 Å, J. Membrane Sci., (1994). 41. S. Yan, H. Maeda, K. Kusakabe, S. Morooka and Y. Akiyama, Hydrogen-Permselective SiO 2 Membrane Formed in the Pores of Alumina Support Tube by Chemical Vapor Deposition with Tetraethyl Orthosilicate, Ind. Eng. Chem. Res., (1994). 42. J.C.S. Wu, D.F. Flowers and P.K.T. Liu, High-Temperature Separation of Binary Gas Mixtures Using Microporous Ceramic Membranes, J. Membrane Sci., (1993). 43. R.M. de Vos, High-Selectivity, High-Flux Silica Membranes for Gas Separation, PhD Thesis, University of Twente, R.J.R. Uhlhorn, M.H.B.J. Huis in t Veld, K. Keizer and A.J. Burggraaf, High Permselectivities of Microporous Silica-Modified γ-alumina Membranes, J. Mater. Sci. Lett., (1989). 45. R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Permeation and Separation Studies on Microporous Sol-Gel Modified Ceramic Membranes, Microporous Mater., (1995). 46. C.J. Brinker, T.L. Ward, R. Sehgal, N.K. Raman, S.L. Hietala, D.M. Smith, D.W. Hua and T.J. Headley, Ultramicroporous Silica-Based Supported Membranes, J. Membrane Sci., (1993). 47. N.K. Raman and C.J. Brinker, Organic Template Approach to Molecular Sieving Silica Membranes, J. Membrane Sci., (1995). 48. R.M. de Vos and H. Verweij, High Selectivity, High Flux Silica Membranes for Gas Separation, Science, (1998). 49. R.M. de Vos and H. Verweij, Improved Performance of Silica Membranes for Gas Separation, J. Membrane Sci., 143 [1] (1998). 50. N.E. Benes, A. Nijmeijer and H. Verweij, Microporous Silica Membranes, to be published in Recent Advances in Gas Separations by Microporous Membranes, N. Kannellopoulos ed. 51. R.M. de Vos, W.F. Maier and H. Verweij, Hydrophobic Silica Membranes for Gas Separation, J. Membrane Sci., (1999). 52. R.D. Present and A.J. DeBethune, Separation of a Gas Mixture Flowing Through a Long Tube at Low Pressure, Phys. Rev., 75 [7] (1949). 53. E.A. Mason and A.P. Malinauskas, Gas Transport in Porous Media: The Dusty-Gas Model, Chem. Eng. Monographs, (1983). 54. N.E. Benes and H. Verweij, Comparison of Macro- and Microscopic Theories Describing Multicomponent Mass Transport in Microporous Media, accepted for publication in Langmuir.

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23 &KDSWHU 7KHSURFHVVWHFKQRORJ\RI PHPEUDQHVWHDPUHIRUPLQJ 1. Introduction Hydrogen is one of the most important industrial chemicals and energy carriers. Today hydrogen is mostly produced using the steam reforming process [1-4]. In this process the overall reaction is: CH 4 + 2H 2 O o CO 2 + 4H 2 At thermodynamic equilibrium conditions H 2 conversion may be far from complete, and hence a high temperature and high steam to carbon ratios are needed to obtain sufficient conversion. Normally the process is carried out at C and a pressure of bar, resulting in a conversion of 90%. In order to obtain the same conversion at a lower temperature, hydrogen must be removed selectively from the reaction zone during the process. This can be done in a hydrogen selective membrane reactor. Such a reactor, provided with a membrane having a separation factor >50 for H 2 towards CO 2 /CO/CH 4 /H 2 O would give the same conversion at 600 C as obtained at 900 C in the conventional process [5]. The use of a membrane reactor in steam reforming has several advantages. Because of the lower temperature operation, the energy consumption of the process is reduced which results in lower emission of CO 2. The lower temperature also requires less expensive catalyst, tubing and other reactor materials. Since hydrogen of sufficient purity is produced directly from the reformer, the downstream shift conversion can be omitted. Moreover, the dimensions of the CO 2 removal and final purification units can be reduced. Hence, significant savings in equipment costs can be expected. If the membrane surface area in the reactor is sufficiently high, conversion depends only on the selectivity of the membrane. In this case, all the natural gas that is not lost by transfer through the membrane will be converted in the reactor. Because of the large costs of the high

24 16 Chapter 2 surface area needed, this option is not of industrial interest. A surface area comparable to the heat exchange area will be more realistic for industrial purposes. To obtain a conversion of 90% with this membrane surface area, the membrane must have a separation factor >50 and a H 2 permeance of 1 mol/m 2 s bar under steam reforming conditions [5]. The objective for the project, on which this thesis is based, is the extension of the applicability of H 2 selective microporous silica membranes to higher temperatures and harsh environments by improvement of the material properties. Compared to the 1994 state-of-the-art, both H 2 permeance as well as selectivity towards H 2 had to be increased. In the present chapter, the conventional process is discussed first for the sake of comparison together with several catalyst issues. The membrane process is analysed after that and a comparison between both approaches is made. 2. Conventional Process The present treatment of the conventional process will be based on the process diagram presented in Figure 1, which represents a steam reformer coupled with an ammonia synthesis plant [6]. This is one of the two cases, which were considered in the project. The other was the use of the produced hydrogen as fuel in combined cycle gas turbines. In this chapter, the steam reforming part will be treated only, but some comments on the ammonia plant will be made, in view of the composition of the product stream leaving the steam reformer. 2.1 Feed gas purification The nickel-based reforming catalysts which are commonly used in steam reforming are quite sensitive to sulphur, halogen and heavy metal poisons. Since these elements may all be found in natural gas, a feed gas purification section is normally required. Of the mentioned catalyst poisons, sulphur is by far the most important [6]. In process industry, quite a number of processes are available for the removal of sulphur from gaseous feedstock. In steam reforming hot or cold zinc oxide beds are generally used for that purpose. Zinc oxide is not only effective in removing the sulphur compounds but removes some chlorides as well. H 2 S reacts irreversibly to the solid ZnS when it is led through the ZnO-bed. The spent bed must be discarded afterwards. The overall reaction is:

25 The process technology of (membrane) steam reforming 17 ZnO + H 2 S o ZnS + H 2 O Figure 1: Process diagram for natural-gas-based steam reformer with a connected ammonia plant [6]. Other organic sulphur compounds that are not easily removed by zinc oxide can be hydrogenated to H 2 S first by reacting with the hydrogen over a cobalt or nickel molybdenate catalyst. A conventional zinc oxide bed as described above can then remove the formed H 2 S. If the chloride content of the natural gas feed is too high, a modified alumina catalysts that can irreversibly absorb the chloride can be used.

26 18 Chapter Primary and secondary reforming In the primary and secondary reformer the following steam reforming reaction takes place: CH 4 + H 2 O o CO + 3H 2 The catalyst for this reaction is normally nickel on a refractory or aluminate support. The steam reforming reaction is highly endothermic (- H = -206 kj/mol) and high temperature, low pressure and high steam-carbon ratios (3-4 is commonly used) favour conversion [1]. Primary reforming The primary reformer is a process furnace in which fuel is burned with air to provide the heat of reaction to the catalyst contained within tubes. This area of the furnace is usually referred to as the radiant section, so named because radiation is the primary mechanism for heat transfer at the high ( C) temperatures required by the process. Reforming pressures in the range 3-4 MPa in the reactor provide a reasonable compromise between costs and downstream recompression requirements. Carbon formation (coking) in the primary reformer must be prevented (as is discussed further in paragraph 4). State-of-the-art primary reformer designs differ in the arrangement of tubes and burners, tube material, and feed gas distribution and reformer gas collector systems. A primary reformer contains typically between 40 and 400 tubes. The internal tube diameter is in the range mm with a tube thickness of mm. The heated length is 6-12 meter depending on the furnace type. The tubes are made from high alloy nickel chromium steel by centrifugal casting. In this casting process sections of ca. 6 meter length are produced which are welded together to the required tube length. The practical limit on the primary reformer exit temperature is determined by tube metallurgy considerations. One of the numerous possible configurations is provided in Figure 2. Secondary reforming The reforming process is completed in the authothermic secondary reformer, which is a refractory lined vessel containing a fixed-bed catalyst. The remainder of the endothermic heat requirement is provided by the combustion of part of the primary reformer effluent directly with air. This allows much higher process temperatures, of the order of 1000ºC, to be attained at the secondary reformer exit and consequently low methane slips in the range of 0.2-

27 The process technology of (membrane) steam reforming vol-%. The secondary reformer catalyst is similar to that used in the primary reformer. Because the amount of air added to the secondary reformer is determined by the nitrogen requirements of the downstream ammonia synthesis, the split between the primary and secondary reformers obey in a heat balance consistent with equipment design temperatures [6]. Figure 2: Side-fired primary reformer [6]. 2.3 Shift conversion Carbon monoxide, which is formed in the steam reforming reaction, deactivates the ammonia synthesis catalyst and must be removed by means of the exothermic water-gas shift reaction, which also maximises hydrogen production. To this end, CO is converted first to more easily removable CO 2 : CO + H 2 O o H 2 + CO 2

28 20 Chapter 2 Initially, the bulk of CO is shifted to CO 2 in a high temperature shift (HTS) converter operating at C to take advantage of the faster reaction kinetics at those temperatures. The HTS converter is operated at a temperature much lower than in the secondary reformer to protect the used catalyst. The gases are cooled and the remaining CO is shifted to CO 2 in a low temperature shift (LTS) converter, operating at about 220ºC to achieve almost complete CO conversion due to more favourable equilibrium conditions. HTS catalysts consist of magnetite (Fe 3 O 4 ) crystals stabilised by chromium oxide. Phosphorus, arsenic acid and sulphur are poisoning catalyst. The LTS catalyst is normally copper oxide supported by zinc oxide and alumina. After LTS the product stream contains some CO, only 0.25 to 0.4 vol-% 2.4 Carbon dioxide removal The effluent gases from the shift converters contain about vol-% (dry) carbon dioxide, which is ultimately reduced to a few ppm by bulk CO 2 removal, using an absorber-stripper configuration. Three configurations are used in industry, illustrated by the examples in the subsequent paragraphs: 1. Pure reaction 2. Combined reaction with physical adsorption 3. Pure physical adsorption systems The choice of a specific CO 2 removal system depends on the overall ammonia plant design and process integration. Important considerations include: CO 2 slip permitted, CO 2 partial pressure in the synthesis gas, presence of sulphur, process energy demands, investment cost, availability of solvent, and CO 2 recovery requirements. Alkanolamine process In this case, carbon dioxide reacts reversibly in the adsorber with aqueous alkaline solutions to form a carbonate adduct (configuration 1). This adduct decomposes in the stripper upon heating. In early ammonia plants, an aqueous solution of wt % monoethanolamine (MEA) was always standard for removing CO 2. Primary alkanolamine solutions, however, require a relatively high heat of regeneration so that, nowadays, secondary and tertiary ethanol amines are mainly used. Hence, activated tertiary amines such as triethanolamine (TEA) and methyl diethanolamine (MDEA) have now gained wide acceptance for CO 2 removal. These materials require very

29 The process technology of (membrane) steam reforming 21 low regeneration energy because of the weak CO 2 -amine interaction energy, and do not form corrosive compounds. Activated carbonate process The activated carbonate process is based on absorption of CO 2 by potassium carbonate to give potassium bicarbonate (configuration 2). When potassium bicarbonate is heated it releases CO 2 while potassium carbonate is formed back again. The original hot carbonate process was found too corrosive for carbon steel reactor walls. Nowadays, however, improvements in additives and optimisation of operation have made activated carbonate processes competitive with state-of-the-art MDEA systems. Water stripping A third method is CO 2 removal by physical absorption in a (sea)water scrubber (configuration 3). Because of the low costs of (sea)water, large quantities can be used and a stripping section is not necessary because the water is discarded. PSA-unit In modern plants, a Pressure Swing Adsorption (PSA) unit replaces the complete LTS, the CO 2 stripping section and the final purification. 2.5 Final purification Oxygen-containing compounds (CO, CO 2, H 2 O) contaminate the ammonia synthesis catalyst and must be removed or converted to inert species before entering the ammonia synthesis. The presence of CO 2 in the synthesis gas may lead to the formation of ammonium carbamate, which may cause fouling and compressor breakdown due to corrosion. Most ammonia plants use a methanation process to convert carbon oxides to methane, while cryogenic processes that are suitable for purification of synthesis gas have been developed as well. Methanation The methanation reactions used are the reverse of reforming and shift reactions: CO + 3H 2 o CH 4 + H 2 O CO 2 + 4H 2 o CH 4 + 2H 2 O

30 22 Chapter 2 The methanator catalyst is nickel, supported by alumina, kaolin or calcium aluminate cement. After methanation the CO and CO 2 content of the treated gas is of the order of a few ppm. A methanator typically operates in the temperature range of C. Methanation reactions are strongly exothermic and hence the CO and CO 2 concentrations at the inlet of the methanator should be carefully monitored, to avoid thermal runaway. Dehydration The use of molecular sieve dryers for removal of the remaining carbon oxides and water in the synthesis gas to levels of < 1 ppm levels has gained prominence in low-energyconsumption ammonia plant designs. Instead of molecular sieves so-called knockout drums (high pressure vessels to remove traces of liquids) can be used as well. 3. Catalyst The choice of the catalyst is of large influence on the behaviour of the reforming process. Ni-based catalyst are most common, but recently more advanced catalysts have been developed as well. As indicated before, one of the advantages of a membrane reactor is that it can be operated at much lower temperatures but with the consequence that state-of-the-art catalysts might not be sufficiently active anymore. In this paragraph, an overview is provided on commonly used catalysts and some of the problems that may be encountered [7]. 3.1 Nickel-based catalysts The process design for steam reforming is based on the minimisation of the costs of hydrogen production. As catalyst costs are high, their activity and stability play a critical role. Because of the relatively low surface area of steam reforming catalysts, a high surface coverage of the active nickel component is required to achieve an acceptable catalytic activity per unit weight of catalyst. Consequently, the active nickel crystallites are situated close to each other, however insufficient adherence to the carrier may lead to severe sintering (loss of nickel surface area) during catalyst pre-treatment or actual operation. On active Ni-based catalysts, coke formation is apt to occur. The primary site of carbon formation is the acidic metal-promoted supporting oxide [7]. This catalytically active oxide is however necessary for the majority of catalytic reactions and is essential for high steam

31 The process technology of (membrane) steam reforming 23 reforming activity. So the metal oxide site confers activity and imparts unselective carbonreforming properties if not correctly moderated. Modification of the highly active, carbon forming catalyst site is generally accomplished by the introduction of basic species to partially neutralise the active acid sites. In general, the level of basic moderator is chosen such that the super active sites are neutralised, leaving the medium activity sites unaffected to obtain the required process activity. Typical basic additives to catalyst formulations are usually one or more of the metal oxides of sodium, potassium, lithium, cesium, calcium, barium, strontium, magnesium, lanthanum and cerium. Doping with alkali elements Potassium is one of the most common constituents of basic additives that reduce carbon formation. Potassium salts are highly soluble, however, and mobile at relatively low temperatures and therefore prone to migration and loss from the catalyst surface. This might lead to downstream deposits and potential process upsets. Andrew [8] stated, however, that the presence of an adequate quantity of mobile alkali appears to be the key factor to enable a supported nickel-reforming catalyst to operate successfully at low steam to carbon ratios. In addition, it is well documented that potassium has an activity-moderating effect on steam reforming catalysts [7,8,9], so that more catalyst has to be used. Another disadvantage of the use of potassium in a membrane steam reformer is the possibility of reaction with the separative silica layer of the membrane. At the envisaged steam reforming conditions this may lead to the formation of a crystalline keatite phase [10,11]. Doping with lanthanum To avoid the problems encountered with potassium, that additive can be replaced by lanthanum oxide 1 [12]. Lanthanum oxide is a high melting point oxide with strong basic properties. It neutralises carbon forming acidic sites and does not suffer from surface migration or enhanced mobility at the catalyst surface as potassium does under influence of steam. Contrary to potassium, lanthanum additions have a positive effect on catalyst activity and it promotes the reduction of nickel as required to obtain sufficient steam reforming activity. Moreover, no reactions of lanthanum with silica at steam reforming conditions are known to occur. 1 Product information Dycat international, Mandeville, Louisiana, USA.

32 24 Chapter Non-nickel catalysts Instead of nickel, other catalytically active metals are used as well. Rhodium and ruthenium, for example, show an activity that is about ten times higher than that of nickel, platinum and palladium [6]. The addition of small amounts of copper to the conventional nickel catalyst is reported to improve the activity of nickel at high temperatures [13]. Complications with in desulphurising heavy feedstocks have also lead to attempts to use nonmetallic catalysts for steam reforming, but their activity is still inferior to that of nickel catalysts [14,15]. 3.3 Catalyst poisoning Sulphur is the most severe poison for steam reforming catalysts. A detailed study of sulphur contamination is provided in [7]. On the other hand, sulphur may have a positive effect too, because it may depresse coke formation on nickel catalysts [16]. A second important poison is As 2 O 3 but its poisoning effect is much less than that of sulphur [17]. The mechanism of As 2 O 3 -poisoning is based on the formation of an alloy with nickel. The arsenic typically originates from the solutions used in carbon dioxide wash of the catalyst or is present as an impurity in some zinc oxide sulphur removal beds. Also silica is mentioned as a pore mouth poison by physically blocking the entrance to the pore system by which the catalyst activity is decreased [18]. 4. Coking and process conditions As mentioned before, coke formation is apt to occur in the primary reformer, which is highly undesirable because the catalyst conversion rate is then reduced significantly. A detailed discussion of the mechanism behind coking and how to avoid coking at process conditions is provided in [7]. Carbon formation on catalyst materials is discussed in paragraph 3 of this chapter and a proper choice of catalyst, depending on the feedstock used for reforming, can solve many coking problems. A good choice of process conditions, however, may also help to minimise coke formation and if the right catalyst is chosen one can operate a steam reformer for ten years without extensive coking problems [7].

33 The process technology of (membrane) steam reforming 25 Carbon may be formed from carbon monoxide and methane by the following reversible reactions [6,12,18]. 2CO o C + CO 2 CH 4 o C + 2H 2 (Boudouard reaction, H = -173 kj/mol) (Decomposition of methane, H = 75 kj/mol) Depending on the operation conditions three different types of carbon can be formed: whisker-like carbon, encapsulating carbon and pyrolytic carbon. Whisker-like carbon is formed by diffusion of carbon through the Ni-crystal. After nucleation, the whisker grows further with a Ni-crystal on top. This mechanism does not deactivate the catalyst, but causes breakdown of the catalysts after some time. Whiskers are formed at temperatures > 450ºC. Encapsulating carbon consists of carbon polymers, which encapsulate the complete catalyst particles. This type of carbon is formed at temperatures <500ºC. Encapsulating carbon formation results in a progressive deactivation of the catalyst. Pyrolytic carbon is formed by thermal cracking of the hydrocarbon feed. It will encapsulate the catalyst as well finally resulting in deactivation of the catalyst and an increased pressure drop over the reactor. Apart from the catalyst type and its modification, the steam to carbon ratio has the largest influence on coke formation. Formation may be expected below a certain, critical, steam to hydrocarbon ration. The critical ratio was found to increase rapidly with temperature and to be influenced by the type of hydrocarbon and by catalyst [6]. To avoid carbon deposition, the steam-to-carbon ratio is normally kept between , but processes exist too where a steam to methane ratio is used, as high as 4.0 [6]. The hydrogen content in the gas influences the coke formation rate [7] as well. In a recent article Hou et al. [19] showed the influence of the removal of hydrogen on coking rates in a membrane steam reformer using palladium membranes. The need of a minimum concentration of hydrogen is of special importance when operating a membrane steam reformer, because it limits the process conditions at which such a reactor can be operated. A minimum hydrogen concentration is not only required to minimise coke formation, but it is also important to avoid oxidation of the used catalyst [9,20]. Mostly steam to hydrogen ratios of approximately 10 are used [20].

34 26 Chapter 2 Thirdly, also a minimum hydrogen concentration is required for inhibiting H 2 S poisoning of the used catalyst [19]. Poisoning takes place by reactive adsorption of H 2 S on the nickel of the catalyst surface: H 2 S + Ni = NiS + H 2 Of course, this reactive adsorption is favoured by removal of hydrogen from the reaction zone. When 80% of the hydrogen is removed in the membrane reactor, the H 2 S tolerance of the catalyst is about halve the tolerance when no hydrogen is removed from the reaction zone. A higher degree of sulphur removal from the feed stream should be accomplished when operating a membrane steam reformer. 5. Membrane reactor process In this paragraph two of the most appropriate concepts for steam reforming employing a membrane will be discussed, namely Membrane Steam Reforming (MSR) and Gas Heated Reforming (GHR) with enriched air. The cases are described below, and are based on a Techno-Economic Evaluation prepared by KEMA, SINTEF and Norsk Hydro [21]. 5.1 Membrane Steam Reforming The proposed process design for Membrane Steam Reforming (MSR) is shown in Figure 3. The natural gas feed is depressurised first from 100 bar to 30 bar. The depressurised natural gas feed is then heated in heat exchanger, passed through a water saturation column and heated further before mixing with process steam to meet the selected steam to carbon ratio. The mixed feed stream is heated up to 430ºC and fed to the catalytic membrane steam reformer. Hydrogen formed by the steam reforming and water-gas-shift reactions is then selectively removed from the reaction zone through the membrane. The high-pressure gas (retentate) stream leaves the membrane steam reformer at 625 C and 30 bar, while the H 2 -rich permeate stream leaves the membrane steam reformer at 555 C and 1.5 bar. Pure nitrogen from an air separation plant is supplied as a sweep gas on the permeate side of the membrane.

35 The process technology of (membrane) steam reforming 27 The hydrogen content of the retentate stream is too small to make hydrogen recovery economically feasible. On the other hand, the heat content of the retentate stream is reused. The stream is cooled by heat exchange with part of the reformer feedstock and subsequently used for preheating the water feed of the saturation column. Figure 3: Process layout of a membrane steam reformer [21]. The permeate (product) stream is split into two streams, providing heat to the natural gas feed and for producing LP steam (not drawn in Figure 3). After a further temperature decrease down to 25ºC by using cooling water, the product stream is compressed in three steps from 1.4 bar to 36.7 bar using repeated inter-stage cooling to 25 C and water knockout drums after each compressor. CO is methanised, as described in paragraph 2.5 and residual water is removed either by molecular sieves or knockout drums. The ammonia synthesis gas (N 2, H 2 ) is finally compressed to 100 bar before being used for the ammonia synthesis. 5.2 Gas heated reforming using enriched air Another possible concept for membrane steam reforming is Gas Heated Reforming (GHR). A flowsheet of this process is provided in Figure 4.

36 28 Chapter 2 The natural gas feed is depressurised again from 100 to 30 bar. The heated gas stream is saturated in a column by counter-current scrubbing with hot water. The saturated gas stream is heated further, before being mixed with additional steam to obtain the required steam to carbon ratio of 3.0. The mixed feed stream is given a final preheating to 430ºC, and fed to the gas-heated reformer, where the feedstock is partially converted to synthesis gas by conventional membrane steam reforming (paragraph 5.1). The partially reformed gas leaves the gas-heated reformer and is fed to the secondary reformer together with enriched air and hence partially combusted. Figure 4: Process layout for a gas heated reformer [21]. The product gas leaves the secondary reformer at a temperature of 885 C and is heatexchanged in the primary membrane reformer. After that, the product gas leaving the gasheated reformer is utilised for preheating of the natural gas feed, heating of circulating water in the saturator loop and generation of LP steam at 3 bar. Finally, after a temperature decrease to 265ºC the gas is fed to a shift converter, after which again methanation takes place and removal of CO 2 and traces of water.

37 The process technology of (membrane) steam reforming Membrane design For operation in a steam reformer, membranes must be found with a proper balance between permeance and selectivity. Ideally, a membrane with both high selectivity and high permeance is required, but one may expect on forehand that, typically, attempts to maximise one are compromised by a reduction in the other. State-of-the-art hydrogen-selective membranes were already discussed in chapter 1 and the reader is referred to that for more information on suitable membrane types. The membrane surface area in the reactor has to be optimised with respect to the number of membrane tubes. There are, however, two important boundary conditions: The length of the used tubes. With current technology it is not possible to produce proper membrane tubes longer than 2 m. Requirements on both membrane quality and microstructural homogeneity along the tube determine the maximum allowable length. On the other hand, two or more complete membrane tubes might be sealed head to head to obtain longer lengths. The location of the catalyst. A membrane steam reformer contains two types of tubes, the reformer tubes and the membrane tubes. The membrane tubes are placed inside the reformer tubes. There are two possibilities for both the location of the separative layer on the membrane tube and the location of the catalyst. The separating layer can be located at the inside or at the outside of the support tube. Membrane tubes with their separative layer inside are less sensitive towards operational and handling damage. The catalyst can be placed inside the membrane tubes or between the membrane and the reformer tubes. Unfortunately, both possibilities have specific disadvantages: If the catalyst is placed inside the membrane tubes and also the separative layer is coated at the inside of the tube, any compounds such as potassiumoxide from the catalyst might react with the silica separative layer to form keatite. This will destroy the molecular sieving properties of the silica toplayer, see paragraph 3.1. Additionally there is a risk that catalyst loading will damage the membrane layer. Positioning of the catalyst between the membrane and the reformer tubes can result in a lower hydrogen flux through the membrane in comparison with the configuration in which the catalyst is placed near the separative layer at the inside of the tube. This decrease will be due to gas transport limitations through the support, because the pores of the support might be partly blocked by molecules from the feed stream. This effect will

38 30 Chapter 2 not occur when the feed-stream is at the side of the separative layer, because the molecules, which may cause blocking, cannot pass the separative layer. In view of the above-mentioned phenomena, the best and most reliable configuration will be the one with the catalyst in the annular space between the outer reformer tube and the inner membrane tube. The separative layer can be located best at the inside of the membrane tube. This configuration is shown in Figure 5. Figure 5: Cross-section of a reformer tube in a membrane steam reformer. The separative layer is located at the inside of the membrane tube. 5.4 Sealing The membrane tubes must be sealed, possibly to each other and to the collector plate of the reactor vessel. The sealing has to be adherent and mechanically strong but also gas-tight and thermally resistant, up to temperatures of at least 700 C. These high demands make sealing one of the most important problems in current high temperature membrane technology. State-of- the-art high temperature sealing materials are based on glass and glass-ceramic [22-25]. The major disadvantage of such materials in steam reforming environments is their possible limited resistance against the high temperatures and corrosive environments occurring in the steam reforming reactor space.

39 The process technology of (membrane) steam reforming 31 Compared to glass, glass-ceramics are mechanically stronger, more resistant to chemical attack and have a wider range of thermal expansion coefficients. Complex non-linear thermal expansion characteristics can be achieved, resulting in very close thermal expansion matching to a variety of metals and alloys, including those with non-linear behaviour [24]. A good sealing not only dictates the properties of the sealing material, but also some requirements on the tube design. In the ideal case the tubes are perfectly round, which enables the application of a very thin seal in the annular space between the tube and the collector plate. This, in its turn, is advantageous, because the chance of seal cracking during heating up of the reactor is then largely reduced. To obtain tubes with a superior roundness compared to conventional ones, centrifugal cast tubes have been developed. These are discussed further in chapter Comparison of the different processes In this paragraph, a sensitivity analysis is made of the costs of the different processes. It must be noted in advance, however, that it is very difficult to provide really accurate quantitative cost estimations, because of the lack of information on, for example, membrane selectivity and life-time and the costs of supported membranes and sealing. An attempt to provide yet a quantitative cost analysis has been made in [21]. 6.1 Tube length The costs of a membrane reactor is highly dependent on the number of tubes used. In [21] a comparison is provided of the (membrane) tube length against the investment. By using longer tubes, the costs of burners and tube collector plates decrease considerably. Conventional steam reformers contain tubes with a length of meters. According to [21], a membrane reactor consisting of 12 meter tubes would have investment costs which are 50% lower than a membrane reactor with 2 meter tubes. For ceramic membranes, however, a length of 12 meter is not realistic today. Therefore, an option can be to prepare tubular membranes with a length of 2 m and to seal them together to a length of 12 m. In this case only the sealing costs increase, but the advantage of a large tube length remains. Sealing two tubes together is a completely new technique, however, and will need a large amount of development work.

40 32 Chapter 2 The costs of the GHR concept are somewhat less sensitive to the number of tubes, because a GHR does not contain burners. 6.2 Temperature The reaction temperature has a significant influence on operating costs. When using a membrane reactor it might be possible to operate the system at a much lower temperature, enabling the use of less expensive tubing materials. As a starting point for the project, we chose 600ºC as reaction temperature. The economic evaluation prepared during the project [21], however, uses a reaction temperature of 700ºC. 6.3 Membrane selectivity The selectivity of the used membranes is of large influence as well. A selectivity larger than 500 would significantly reduce the amount of impurities in the permeate stream and thereby downstream processing of the synthesis gas. 6.4 Permeate pressure The largest inherent weakness of membrane reactors is the low pressure permeate streams which should be recompressed for further use. This compression represents 30 to 40% of the total annual operating costs. 6.5 Comparison of the different concepts An attempt has been made to compare the different concepts on the basis of a cost estimation. The results obtained in [21] are summarised in Table 1. They have been calculated with a simulation spreadsheet, prepared by SINTEF [26]. For reasons of comparison, the results from [21] have been compared with designs in which 12 meter (sealed) tubes are assumed together with an operation temperature of 600ºC. Please note that in that case the total costs are highly dependent on the costs of sealing. Moreover, sealing membrane tubes one to another might be unrealistic. It is evident, however, from the comparison, that there is a large uncertainty in cost-estimations for membrane reactors and that with only slight changes in the input parameters of the cost-model, membrane steam reforming can be cost-effective. When

41 The process technology of (membrane) steam reforming 33 better membranes with higher permeances and selectivities become available and when sealing technology is getting more developed and cheaper, membrane steam reforming remains worthwhile studying. Moreover the possibility to operate membrane steam reformers at lower temperatures than conventional steam reformers might make them even more costeffective when in future environmental regulations become more strict. These environmental regulations might provide a large drive towards energy effective processes, which favours membrane techniques. Implementation of membrane steam reforming in process industry in future largely depends on the above mentioned factors. Total annual costs Investment costs Conventional reforming MSR (700ºC, 2 meter) GHR (700ºC, 2 meter) MSR (600ºC, 12 meter) GHR (600ºC, 12 meter) Table 1: Cost-comparison of steam reformers. All costs are given in million Euro. 7. Conclusions Replacing conventional steam reformers by membrane steam reformers is an interesting option providing the possibility of lower operating temperature, thereby creating a more energy efficient process. Another advantage is the possibility to at least partly omit the methanation and CO 2 removal section. On the other hand, the use of lower temperatures might involve serious problems with reformer operation. Coking problems might arise and this effect might even become more outspoken since hydrogen is removed from the reaction zone. In the worst case a lowered hydrogen concentration might even lead to (partial) oxidation and sulphur poisoning and thereby extended deactivation of the catalyst. Cost calculations for a membrane reactor are very cumbersome. Numerous uncertainties and assumptions have to be made for a large number of parameters. Tube length and sealing costs are very important and up to now it is not even sure whether a sealing material can be developed that is able to withstand the severe operation conditions. This makes a proper

42 34 Chapter 2 estimation of sealing costs and hence total costs very difficult. Besides that, it was found that the cost estimation is rather sensitive towards assumptions for the design. This led us to the conclusion that, membrane steam reforming might be cost effective with the GHR concept as the most interesting option. And last but not least costs are found to depend largely on different membrane properties and first of all a suitable membrane has to be developed. The development of such a membrane is described in the remaining chapters of this thesis. 8. References 1. S.L. Jorgensen, P.E.H. Nielsen and P. Lehrmann, Steam Reforming of Methane in a membrane Reactor, Catal. Today, (1995). 2. T. Johansen, K.S. Raghuraman and L.A. Hacket, Trends in Hydrogen Plant Design Steam Reforming will Continue to be the Main Source of H 2, Hydrocarbon Processing, [8] (1992). 3. F.W. Hohmann, Improve Steam Reformer Performance, Hydrocarbon Processing, [3] (1996). 4. J.M. Abrardo and V. Khurana, Hydrogen Technologies to Meet Refiners Future Needs, Hydrocarbon Processing, [2] (1995). 5. Proposal to the EC Framework Programme IV, Membrane Reactor for Cost-Effective Environmental- Friendly Hydrogen Production, Brite Euram, no. BE (1995). 6. J.R. Rostrup-Nielsen, Catalytic Steam Reforming, Springer Verlag, Berlin (1984). 7. J.R. Rostrup-Nielsen, Steam Reforming Catalysts, Danish Technical Press Inc., Copenhagen (1975). 8. S.P.S. Andrew, Catalysts and Catalytic Processes in the Steam Reforming of Naphta, Ind. Eng. Chem. Prod. Res. Develop., (1969). 9. J.R. Rosrup-Nielsen, Activity of Nickel Catalysts for Steam Reforming of Hydrocarbons J. Catal., (1973). 10. P.P. Keat, A New Crystalline Silica, Science, (1954). 11. J. Shropshire, P.P. Keat and P.A. Vaughan, The Crystal Structure of Keatite, a New Form of Silica, Z. Kristall., (1959). 12. J.R. Rostrup-Nielsen, T.S. Christensen and I. Dybkjær, Steam Reforming of Liquid Hydrocarbons, Stud. Surf. Sci. Catal., (1998). 13. J. Barcicki, A. Denis, W. Grzegorizyk, D. Nazimek and T. Borowiecki, Promotion of Nickel Catalysts for the Steam Reforming of Methane, React. Kinet. Catal. Lett., 5 [4] (1976). 14. T. Tomita and M. Kitagawa, Ein Neues Steam Reforming-Verfahren für Hochsiedende Kohlenwasserstoffe, Chem. Ing. Tech., 49 [6] (1977) 15. T. Tomita, A. Moriya, T. Shinjo, K. Kikuchi and T. Sakamoto, J. Jap. Petrol. Inst., The Influence of Steam on Coking Rates in Steam Reforming, 23 [2] (1980).

43 The process technology of (membrane) steam reforming J.R. Rostrup-Nielsen, Sulphur-Passivated Nickel Catalysts for Carbon-Free Steam Reforming of Methane, J. Catal., (1984). 17. G.W. Bridger and W. Wyrwas, Steam Reforming of Liquid Hydrocarbons, Chem. Process Eng., 48 [11] (1967). 18. T.S. Christensen, Adiabatic Prereforming of Hydrocarbons an Important Step in Syngas Production, Appl. Catal. A, (1996). 19. K. Hou, M. Fowles and R. Hughes, Potential Catalyst Deactivation Due to Hydrogen Removal in a Membrane Reactor Used for Methane Steam Reforming, Chem. Eng. Sci., (1999). 20. J.D. Rankin and J.G. Livingstone, Catalysts: a Recipe for Longer Life, Ammonia Plant Saf (1981). 21. R. Meijer, D. van der Vlist, F. Janssen, A. Anundskås, T. Pettersen and T. Strøm, Membrane Reactor for Cost Effective Environmental-Friendly Production of Hydrogen Techno-economic Evaluation, Internal Report, (1997). 22. I.W. Donald, Preparation Properties and Chemistry of Glass- and Glass-Ceramic-to-Metal Seals and Coatings, J. Mater. Sci., (1993). 23. C. Günther, G. Hofer and W. Kleinlein, The Stability of the Sealing Glass AF45 in H 2 /H 2 O and O 2 /N 2 Atmospheres, Electrochem. Proc., 97 [18] (1997). 24. M.A. Ritland, D.W. Ready, R.N. Kleiner and J.D. Sibold Method for Sealing a Filter, US Patent, (1997). 25. F.M. Velterop, Method of Connecting Ceramic Material to Another Material, US Patent, (1992). 26. Sintef Applied Chemistry, Simulation and Cost Estimation of Membrane Steam Reformers, Microsoft Excel spreadsheet (1996).

44

45 &KDSWHU &ROORLGDOSURFHVVLQJRI FHUDPLFPHPEUDQHVXSSRUWV *HQHUDOLQWURGXFWLRQ 1. Introduction In the preparation of multi-layered ceramic membranes, the quality of the support is of crucial importance to the integrity of the membrane layers that are applied in the subsequent preparation steps. First, the surface roughness and homogeneity of the support will determine the integrity of these membrane layers, and, second the surface roughness determines the minimal thickness of the membrane layer for complete surface coverage. In this work, two support shapes are of particular interest: tubular and flat supports, which are currently the most used supports in membrane research. Apart from these shapes also ceramic multi-bore tubes and honeycomb structures are produced for membrane applications and recently α-alumina hollow fibre supports were developed as well [1]. Though the majority of this chapter is related to the production of tubular supports, a few comments on the preparation of flat supports will be made. Emphasis is put on the stabilisation of the suspensions used in the preparation of the supports. Experimental procedures are provided in more detail in chapter 4 of this thesis; the present chapter mainly provides the basic knowledge for suspension preparation and shaping techniques. 1.1 Flat supports Flat supports can be produced in various ways. Die pressing (or dry pressing) is most often used, but also tape casting can be applied. Both methods have disadvantages due to the extensive use of binding agents and other additives. In the case of die pressing, mainly PolyVi-

46 38 Chapter 3 nylalcohol (PVA) is used as a binding agent, while in tape casting very complicated slurries are used, consisting of plasticisers, binders and anti-foaming agents [1,3]. The use of extensive amounts of these additives might result in inhomogeneities in the support, which certainly influence the quality of the subsequent membrane layers. A rather new technique for the preparation of flat membrane supports is colloidal filtration. In this technique an aqueous suspension is made of high purity α-alumina particles with a narrow size distribution using a very low concentration of electrostatic or electrosteric stabiliser. The suspension is homogenised using ultrasound, which results in the break-up of the large agglomerates in the powder. The suspension is poured out in moulds after which the liquid in the suspension is removed by vacuum filtration and the particles are stacked in a controlled manner. The resulting cake (green cast) is dried overnight, released from the mould and fired to the final product. Due to the homogeneous stacking, the product can be shaped and polished with relative ease to a surface roughness necessary for applying membrane layers. A more detailed synthesis procedure for the supports used in this study is provided in chapter 4 and information on the strength of these supports as a function of synthesis conditions can be found in [4]. Some general information about the colloidal filtration method can be found in [5,6]. 1.2 Tubular supports For research on stability, permeability and separation characteristics of the membranes, flat membranes are perfectly suitable. For the use in process industry, however, tubular membranes are necessary, because of the ease of sealing tubes into modules compared to stacked flat plate modules. The conventional way of preparing ceramic tubes is extrusion but a problem of extruded ceramic tubes may be the inhomogeneity of the surfaces due to inhomogeneous distribution of binders and other extrusion additives and stresses induced during processing. Inhomogeneities often lead to an enhanced roughness, which may be detrimental to the quality of subsequently applied layers. Moreover, the minimal thickness of an applied layer has to be higher than the surface roughness of the supporting system, which means that if the surface roughness is reduced, subsequent membrane layers may be thinner, thereby reducing the resistance to flow.

47 Colloidal processing of ceramic membrane supports. General introduction 39 For preparing tubular ceramic parts, centrifugal casting of a colloidal suspension can be used. In this process a suspension of particles with a particle size distribution as narrow as possible is used to diminish segregation of the prepared suspension. The suspension is obtained by dispersing the powder in a suitable liquid by milling or ultrasound, after which it is poured or injected in a mould-tube, which is placed in a horizontal or vertical centrifuge. The mould-tube is rotated for a certain time at a typical rotational speed of to rpm and released from the centrifuge. Afterwards the remaining liquid is poured or sucked out of the mould and the mould-tube containing the wet green compact is dried in an atmosphere with controlled temperature and humidity. The compact can be released from the mould after drying and fired at a suitable temperature. With centrifugal casting one can prepare homogeneous tubes with a very smooth inside surface without the need of polishing. The importance of a low surface roughness has already been pointed out. While the preparation of flat supports by colloidal filtration is rather simple and robust, the preparation of tubes by centrifugal casting offers more challenges. Especially during mould release and drying cracking and warping of the green cast often occurs. These problems can be eliminated by the use of additives in the starting suspension or the use of release agents on the inner surface of the mould-tube. However, these techniques also result in inhomogeneities in the final product and its use should therefore be limited. The stability of the starting suspension should also be controlled carefully. If the starting suspension is too stable, the final sediment will remain fluid-like, so that an actual compact is not formed and redispersion will occur as soon as rotation ends. On the other hand, a less stable suspension might give rise to attraction of the particles in the suspension and to flocculation, which influences the homogeneity and the surface roughness of the final product. Controlled flocculation, however, might be advantageous for mould release, because in this case the particles are not completely close-packed which gives rise to some shrinking during drying. This shrinkage enables easy release of the green compact from the mould-tube. Most literature on centrifugal (slip) casting deals with the preparation of dense and homogeneous flat ceramic parts from colloidal suspensions by centrifuging [7-15]. This method utilises centrifugal forces, but also makes use of forces involved in conventional slip casting to produce higher green densities. The above mentioned articles contain valuable information

48 40 Chapter 3 about stability of very different kinds of colloidal suspensions. Another application of centrifugal casting can be found in the production of steel tubes, as described in [16] and [17]. One of the most important articles on the centrifugal casting of ceramic tubes is from the group of Bachmann [18]. They developed a method to synthesise silica tubes for the production of optical telecommunication fibres, consisting of a commercial centrifuge equipped with an injection tube. Using this technique, it was possible to produce a silica tube, layer by layer. Because the doping of the silica could be changed during the process, axial varying optical properties were obtained in the resulting tubes. An identical technique was used in the present work to prepare multilayer tubular membrane supports. Results are described in section 9 of chapter Theoretical background 2.1 The DLVO-theory The DLVO-theory is named after Derjaguin, Landau, Verwey and Overbeek and predicts the stability of colloidal suspensions by calculating the sum of two interparticle forces, namely the Van der Waals force (usually attraction) and the electrostatic force (usually repulsion) [19]. The van der Waals force between atoms consists of three different dipole induced forces, the Keesom interaction, the Debye interaction and the London interaction. Keesom interaction occurs when a permanent molecular dipole creates an electric field, which orients other permanent dipoles in such a way that they will attract each other. Debye interaction occurs when a permanent dipole induces a dipole in a polarisable atom or molecule. The induced dipole is oriented in such a way that attraction occurs. London interaction occurs by fluctuations in the electrons in atoms or molecules in such a way that instantaneous dipoles are formed. This effect leads to attraction between the two induced dipoles. The origin of the electrostatic force is the surface charge that solid particles acquire when they are immersed in a liquid that contains a sufficient amount of ions. Possible charging mechanisms are ionisation, ion adsorption and ion dissolution, which are now discussed.

49 Colloidal processing of ceramic membrane supports. General introduction 41 Ionisation Colloidal metal-oxide particles, with hydroxyl groups at their surface, may undergo proton association or dissociation depending on the ph of the solution. At low ph, a metal-oxide particle will be charged positively and at high ph negatively. The ph at which the net charge is zero, is the iso-electric point. Ion adsorption A net surface charge can be acquired by the adsorption of ions on the surface of the particle. Ion adsorption may be positive or negative. Surfaces, which are already charged (e.g. by ionisation), usually show a tendency to adsorb counter-ions. It is possible that counter-ion adsorption causes a reversal of charge. Surfaces in contact with aqueous media are more often negatively charged than positively. This is a consequence of the fact that cations are usually more hydrated than anions and therefore have a greater tendency to reside in the bulk of the aqueous medium. The smaller, less hydrated and more polarising anions have the greater tendency to be specifically adsorbed. The interaction between the counter-ion and the surface is not only physical; chemical interaction may occur as well. Chemical interaction is not only influenced by the valence and size of the ions, but also by the chemical composition of the surface and the ion. This type of adsorption is called specific adsorption. Depending on the concentration of the counter-ions a layer can be formed on the surface. If the surface is positively charged, first a layer of negatively charged counter-ions will adsorb, present within the inner Helmholtz-plane. On this layer of specifically adsorbed negatively charged ions, a layer of positively charged ions can again be adsorbed, which are enclosed by the outer Helmholtz plane, or simply the Helmholtz plane. The total layer of specifically and non-specifically adsorbed ions is called the Stern layer. Another layer of counter-ions will also form a diffuse layer surrounding the Stern layer, which is called the Gouy layer. The Gouy and Stern layers as well as the Helmholtz plane are shown in Figure 1.

50 42 Chapter 3 Ion dissolution Figure 1: The different layers due ion adsorption on a particle. Ionic substances can acquire a surface charge by unequal dissolution of the ions of which they are composed. An example is silver iodide particles in an aqueous suspension, which are in equilibrium with a saturated solution. With excess I - - ions, the silver iodide particles are negatively charged. With an excess of Ag + -ions, the particles will be positively charged. The DLVO-theory In the DLVO-theory, the force between two particles is seen as a summation of the attractive force imposed by the van der Waals force and the repulsive force imposed by the electrostatic interaction. For two spheres with radius R s at a distance D, the force is given by: ARs FDLVO = Fattr + Frep. = + 64N ktz 2 12D 0 2 ( κd) exp In which, A is the Hamaker constant, N 0 the number of ions, k the Boltzmann constant, T the temperature and κ the Debye length. Z is given by: ze Z = tanh 4kT ψ 0 In which z is the valence of the ions surrounding the sphere, e the electronic charge and ψ 0 the surface potential. The behaviour of the interaction energy between two particles as a function of the surface separation (distance between the surfaces) is plotted in Figure 2.

51 Colloidal processing of ceramic membrane supports. General introduction 43 In several cases, however, the DLVO-theory has shown to be inadequate, due to the occurrence of other inter-particle forces that may be present in colloidal suspensions. These phenomena are summed up below: Hydrophobic forces Hydrophobic forces are long-range attractive forces between macroscopic, hydrophobic surfaces in water. The force is significantly stronger than the Van der Waals attraction and can still be measured at surface separations as large as 70 nm. Solvation forces When two surfaces are immersed in a liquid, the force between them can be greatly affected by the interaction of the liquid with the surface. In this case the surface may be solvated in a particular way. An isolated surface will thereby modify the structure of the liquid adjacent to it. The nature and thickness of the solvation layer depend on properties of surface and liquid. The solvation force in water is called the hydration force. Figure 2: Schematic diagram of the variation of interaction free energy with surface separation according to DLVO-theory. The net energy (solid line) is given by the sum of the double layer repulsion (-.-.-.) and the Van der Waals attraction ( ). Two different situations are depicted, one with a low salt concentration (a) and one with a high salt concentration (b) [20]. Capillary forces It is well known that a vapour is apt to condense in a narrow gap, capillary or between two solid surfaces in close proximity. Such a condensed meniscus between two particles results in a strong attractive force between these particles due to surface tension.

52 44 Chapter 3 Steric repulsion forces Surfactants or polymers adsorbed on the particle surface are able to keep particles that far appart that the Van der Waals attraction cannot become effective. This phenomenon is called steric stabilisation. Gravitational force Gravitational forces might influence the stability of the suspension especially for larger particles (larger than 100 µm). Brownian motion Brownian motion, which is a result of random collisions between the particles in the suspension, might influence suspension stability especially for smaller particles (smaller than 100 µm). 2.2 Stabilising a colloidal suspension Stabilising a colloidal suspension implies that the total interparticle potential decreases with increasing inter particle distance. The different kinds of stabilisation all use some of the above-mentioned interparticle forces. There are mainly six different methods of stabilisation, as summed up by Everett [21], see Figure Electrostatic stabilisation. Repulsion between the double layers formed by non-adsorbing counter-ions causes stabilisation. Simple salts can be used which do not have a steric effect. 2. Electrosteric stabilisation. By using larger molecules with charge-carrying groups electrostatic and steric stabilisation are combined. The thicker the layer that is formed, the larger the steric part of the stabilisation. Polyelectrolytes with a high number of -COO - groups are mostly used for this type of stabilisation [23, 24]. 3. Steric stabilisation. Particles with large molecules adsorbed on the surface are repelled by each other because the freedom for chain movement decreases if particles approach (this

53 Colloidal processing of ceramic membrane supports. General introduction 45 would imply a decrease of entropy). For small enough particles, Brownian motion is then sufficient to keep the particles suspended for an indefinite time. Note that the polymer must not adsorb too strong on the particles because if polymer segments stick strongly and irreversibly to the first area of the surface that they encounter, further polymer adsorption is hindered and a comparatively poor polymer coverage of the surface may result. 4. Depletion stabilisation. This type of stabilisation is related to steric stabilisation and arises only when a high concentration of non-adsorbing large molecules is dissolved in the suspension. In this case, there would be an effect arising from the entropic penalty arising from compressing the free polymer chains between the particles if not all polymer chains are excluded from the gap between the particles. This effect would result in a repulsive force between the particles and thereby stabilisation. This last idea is however not generally accepted and it therefore remains a question whether the depletion stabilisation effect occurs in practice. 5. Hydration stabilisation. Due to the polarisability of the water molecules, even in a deionised aqueous solution, stabilisation may occur. Positively charged alumina particles, for example, bind preferentially to the negative oxygen of the water molecule. As a result, a double layer is formed, similar to ionic electrostatic repulsion. In ionic solutions, the hydration repulsive force occurs simultaneously with the electrostatic force, while the proportion of the two forces depends on the ionic concentration [22]. 6. Masking of the Van der Waals forces. By choosing a suspending medium with dielectric properties as close as possible to those of the solid (particularly at optical and UV frequencies), the van der Waals force is minimised.

54 46 Chapter 3 Figure 3: Methods of stabilising colloidal suspensions [21]. Clearly steric, electrosteric or depletion stabilisation as described above may occur, but attraction, leading to destabilisation, can also be the result, as can be seen in Figure 4 [20]. Destabilisation by polymer addition Stabilisation of a suspension by just adding a polymer solution can have a beneficial, but also a detrimental effect. When a polymer chain is very long, the possibility arises that one polymer molecule can adsorb to more than one particle, thereby forming a link between the particles in suspension. This effect is known as bridging flocculation and results in the destabilisation of the suspension. The use of block copolymers can overcome the problem of bridging flocculation. These block copolymers consist of two different polymer chains connected end to end. If a block copolymer is dissolved in a liquid that is a good solvent for one of its ends but a poor solvent for the other, the latter end will have strong tendency to adsorb to the particles, while the remainder of the molecule extends into the solvent. In this way a coat of non-adsorbing and non-bridging polymer is attached to the particle, thereby effectively preventing other particles from approaching.

55 Colloidal processing of ceramic membrane supports. General introduction 47 Figure 4: Different effects of adding polymers [20]. When using polyelectrolytes even more complicated effects can arise. The addition of large polyelectrolytes to a suspension of opposite charged particles is likely to lead to bridging with again one polymer chain attached to two or more particles, in this way forming a necklace. Because of restrictions on the number of possible configurations, non-adsorbing polymers tend to stay out of a region near the surfaces of the particles, known as the depletion layer. As two particles approach, the polymers in the solution are repelled from the gap between the surfaces of the particles. In effect the polymer concentration in the gap is decreased and is increased in the solution. As a result, an osmotic pressure difference is created which tends to push the particles together. The resulting attractive force is the reason for depletion flocculation. In contrast to this, depletion stabilisation has been mentioned above. 3. Stabilising alumina suspensions Stable suspensions are necessary to obtain homogeneous casts by colloidal filtration. Different kinds of stabilisation mechanisms for alumina suspensions are used in literature. Pure electrostatic stabilisation can be obtained using HNO 3, while electrosteric stabilisation is obtained using polyelectrolytes like PolyMethylAcrylicAcid (PMAA) and PolyAcrylicAcid (PAA). In

56 48 Chapter 3 the following section an overview will be given of several studies on the stabilisation of alumina-containing suspensions with different stabilisation agents. 3.1 Stabilisation with HNO 3 Hidber et al. [9] described the colloidal processing of wet-milled α-alumina suspensions by centrifugal casting from a 80 wt-% suspension at ph 4.3. The mean particle diameter of the α-alumina powder was 0.3 µm and the BET surface area 8.59 m 2 /g *. Nitric acid or ammonia was used for electrostatic stabilisation. Very dense ceramics could be obtained with relative densities up to 99.9%. Also Huisman et al. [13] used nitric acid to obtain stable alumina/magnesia suspensions. The powder had an average particle diameter of 0.46 µm and a surface area of 9.4 m 2 /g. The resulting ph was Again high sintered densities up to 99.7% were reached. α-alumina/zirconia composites were made by Chang et al. [26]. AKP-50 powder was used as alumina source. This powder has an average particle diameter of 0.2 µm and a surface area of 9.9 m 2 /g. For the zirconia, TZ-3Y powder with a mean particle diameter of 0.30 µm and a BET surface area of 7.7 m 2 /g was used. Chang et al. prepared three slurries, a slurry dispersed by electrostatic forces at ph 4, a flocculated slurry at ph 9 without salt addition and a coagulated (weakly flocculated) slurry at ph 4 with 2M NH 4 Cl. For the second and third system, ph adjustments were made after sonification. After centrifugation, the coagulated system gave the highest green density (58.0%), while the flocculated suspension gave the lowest green density (42.4%). The addition of large concentration of electrolytes (the coagulated slurry case) can significantly increase the viscosity of an otherwise dispersed slurry. Hereby the mass segregation of the α-alumina and zirconia parts of the suspension was prevented. The conclusion is therefore that coagulated slurries with large salt additions are beneficial for preparing homogeneous dense ceramic composites. * Reynolds Aluminium Co, Arkansas, USA. Ceralox, Tuscon, Arizona, USA. AKP series, Sumitomo, Tokyo, Japan. Tosoh, Tokyo, Japan.

57 Colloidal processing of ceramic membrane supports. General introduction 49 Roeder et al. [14] used dilute mixtures (8 vol-% solids) for the preparation of dense alumina composites with CeO 2 -ZrO 2 and Al 2 O 3 -platelets. As main alumina powder (the matrix), AKP-30 was used with an average particle diameter of 0.4 µm and a surface area of 6.5 m 2 /g. The resulting specimens were sintered at 1600 C. No further densification behaviour was mentioned. In this case segregation behaviour during consolidation is even more pronounced than in the work of Chang et al. [26] In the work of Roeder [14] prevention of segregation is described in terms of particle drafting during consolidation. In the case of particle drafting one particle carries one or more other particles by which a joint effect in particle movement is created during the consolidation step. 3.2 Stabilisation with polyelectrolytes In two articles, J. Cesarano III et al. [23, 24] described the stability of aqueous α-alumina suspensions stabilised with PMAA and PAA polyelectrolytes. The powders used were AKP-20 with a mean particle diameter of 0.52 µm and a surface area of 4.5 m 2 /g and AKP-30 as described above. The electrolytes used were the Na-salt of PMAA with an average molecular weight of g/mol and PAA of various molecular weights (1800, 5000 and g/mol). The structural formulas of the polymers are shown in Figure 5.

58 50 Chapter 3 Figure 5: Structural formulas of PMAA and PAA [23]. Figure 6: Stability map of PMAA/alumina suspension as a function of the ph for 20 vol-% AKP-30 [23]. The effect of the ph on suspension stability was measured and is shown in a so-called stability map, see Figure 6. Using polyelectrolytes, powder loadings as high as 60 vol-% were obtained in stable suspensions, which is extremely high. A possible drawback of the use of polyelectrolytes, however is the inherent pollution with sodium when using the sodium salt of PMAA. For obtaining dense ceramics this seems to be no problem, because 99% dense AKP-30 based ceramics were obtained after sintering at 1350 C. However it might be better to use the ammonium salt of PMAA, NH + 4 PMAA (here called APMA). This component can be burned out completely during sintering and thereby any contamination of the sintered compact avoided. Steinlage et al. [25] indeed used electrosteric stabilisation with APMA for α-alumina suspensions to prepare dense tube- and gear-shaped ceramic components by centrifugal slipcasting. The aqueous suspensions contained vol-% solids and 8 vol-% APMA ** and were brought at ph 9.5 with NH 4 OH after which the slurries were treated with ultrasound. 3.3 Conclusions All in all, the results from the research mentioned above show that it is possible to obtain good compacts by suspension processing using a centrifugal force. However, centrifugation has been limited to dense systems up to now and for preparing porous systems challenges arise as ** Darvan C, R.T. Vanderbilt Co. Inc., Norwalk, CT, USA.

59 Colloidal processing of ceramic membrane supports. General introduction 51 discussed in chapter 4 of this thesis, which treats the synthesis of flat and tubular membrane supports using colloidal techniques. 4. References 1. J. Smid, C.G. Avci, V. Günay, R.A. Terpstra and J.P.G.M. van Eijk, Preparation and Characterisation of Microporous Ceramic Hollow Fibre Membrane J. Membrane Sci., (1996). 2. R. Moreno, The Role of Slip Additives in Tape-Casting Technology: Part I - Solvents and Dispersants, Am Ceram. Soc. Bull., 71 [10] (1992). 3. R. Moreno, The Role of Slip Additives in Tape-Casting Technology: Part II - Binders and Plasticizers, Am Ceram. Soc. Bull., 71 [11] (1992). 4. P.M. Biesheuvel and H. Verweij, Ceramic Membrane Supports, Permeability, Tensile Strength and Stress, J. Membrane Sci (1999). 5. F.F. Lange and K.T. Miller, Pressure Filtration, Consolidation Kinetics and Mechanics, Am. Ceram. Soc. Bull., (1987). 6. F.F. Lange, Powder Processing Science and Technology for Increased Reliability, J. Am. Ceram. Soc., (1989). 7. W. Huisman, T. Graule and L.J. Gauckler, Centrifugal Slip Casting of Zirconia (TZP), J. Europ. Ceram. Soc., (1994). 8. G. Steinlage, R. Roeder, K. Trumble, K. Bowman, S. Li and M. McElfresh, Preferred Orientation of BSCCO via Centrifugal Slip Casting, J. Mater. Res., 9 [4] (1994). 9. P. Hidber, F. Baader, Th. Graule and L.J. Gauckler, Sintering of Wet-Milled Centrifugal Cast Alumina, J. Europ. Ceram. Soc., (1994). 10. F.F. Lange, Forming a Ceramic by Flocculation and Centrifugal Casting, United States Patent, (1986). 11. A. Yamakawa, Y. Doj, M. Miyake, Formation of a Ceramic Composite by Centrifugal Casting, United States Patent, (1993). 12. J.S. Moya, A.J. Sánchez-Herencia, J. Requena, R. Moreno, Functionally Gradient Ceramics by Sequential Slip Casting, Mater. Lett (1992). 13. W. Huisman, T. Graule and L.J. Gauckler, Alumina of High Reliability by Centrifugal Casting, J. Europ. Ceram. Soc., (1995). 14. R.K. Roeder, G.A.Steinlage, K.P. Trumble and K.J. Bowman, Preventing Segregation during centrifugal Consolidation of particle suspensions: Particle Drafting, J. Am. Ceram. Soc., 78 [9] (1995). 15. W. Huisman, T. Graule and L.J. Gauckler, High Quality Ceramics by Centrifugal Slip Casting, Third Euro-Ceramics, V (1993), P. Durán ed. 16. A. Royer, Horizontal Centrifugation: A Technique of Foundry Well Adapted to the Processing of High Reliability Pieces, J. Mater. Shaping Techn (1988).

60 52 Chapter L. Northcott and V. Dickin, The Influence of Centrifugal Casting (Horizontal Axis) upon the Structure and Properties of Metals, J. Inst. Metals, (1944). 18. P.K. Bachmann, P. Geittner, E. Krafczyk, H. Lydtin and G. Romanowski, Shape Forming of Synthetic Silica Tubes by Layerwise Centrifugal Particle Deposition, Ceram. Bull., 68 [10] (1996). 19. D.J. Shaw, Introduction to Colloid and Surface Chemistry, 3 rd edition, Butterworths, London, R.G. Horn, Particle Interactions in Suspensions, pp in: Ceramic Processing, R.A. Terpstra, P.P.A.C. Pex and A.H. de Vries (eds.), (1995). 21. D.H. Everett, Basic Principles of Colloid Science, 2 nd ed., John Wiley & Sons, Inc., New York (1995). 22. P. Somasundaran, B. Markovic, S. Krishnakumar, and X. Yu, Colloid Systems and Interfaces Stability of Dispersions Through Polymer and Surfactant Interaction, Ch. 14 in Handbook of Surface and Colloid Chemistry, K.S. Birdi (ed.), CRC Press, Boca Raton, Florida (1997). 23. J. Cesarano III and I.A. Aksay, Processing of Highly Concentrated Aqueous α-alumina Suspension Stabilized with Polyelectrolytes, J. Am. Ceram. Soc., 71 [12] (1988). 24. J. Cesarano III, I.A. Aksay and A. Bleier, Stability of Aqeous α-al 2 O 3 Suspensions with Poly(methacrylic acid) Polyelectrolyte, J. Am. Ceram. Soc., 71 [4] (1988). 25. G.A. Steinlage, R.K. Roeder, K.P. Trumble and K.J. Bowman, Centrifugal Slip Casting of Components, Am. Ceram. Soc. Bull., 75 [5] (1996). 26. J.C. Chang and B.V. Velamakanni, F.F. Lange and D.S. Pearson, Centrifugal Consolidation of Al 2 O 3 and Al 2 O 3 /ZrO 2 Composite Slurries vs Interparticle Potentials: Particle Packing and Mass Segregation, J. Am. Ceram. Soc., 74 [9] (1991).

61 &KDSWHU &ROORLGDOSURFHVVLQJRI FHUDPLFPHPEUDQHVXSSRUWV 7KHH[SHULPHQWDOSDUW This chapter is split in two parts. The first part will briefly treat the preparation of flat ceramic membrane supports by colloidal processing. In our laboratory, these supports are used to study stability and gas separation properties of microporous silica membranes because they are easy to prepare and demand less complex testing equipment. The second part of this chapter will treat in detail the, more cumbersome, preparation of high quality tubular membrane supports. This geometry is necessary for upscaling to process industry. Therefore some research has been performed on the production of high quality tubular membrane supports. This part has been published in a concise form [1]. 3DUW7KHSUHSDUDWLRQRI IODWVXSSRUWV 1. Introduction As already mentioned in chapter 3, several methods exist to produce flat membrane supports, as die pressing and tape casting. Relatively new is colloidal filtration that is discussed in this section. For the preparation of α-alumina supports, a colloidal suspension is obtained by dispersing the suitable powder in an aqueous solution using ultrasound. The resulting suspension is poured out in moulds and the solution is separated from the powder particles using a pressure difference as driving force. The resulting cake is dried, released from the mould and sintered at the desired temperature. The advantage of colloidal filtration is the ease of preparation and the high degree of homogeneity in the resulting cast, which makes them excellent supports for molecular sieving silica membranes.

62 54 Chapter 4 2. The experimental part The starting α-al 2 O 3 powders were AKP-30 and AKP-15 * with a mean particle diameter of 0.40 and 0.62 µm and a BET surface of 6.2 m 2 /g and 3.5 m 2 /g respectively. Both powders have narrow particle size distributions of (1.5 wt-%<0.25 µm + 95 wt-%<1 µm) and (1.5 wt-%<0.27 µm + 89 wt-%<1 µm), respectively and a chemical purity of >99.99% as stated by the producer. Because of the above-mentioned properties, the powders are excellent starting materials for the preparation of membrane supports. A 50 wt-% suspension is obtained by dispersing the α-alumina powder either in a 0.02M nitric acid solution for the AKP-30 powder or a 0.02M nitric acid solution, mixed with Poly Vinyl Alcohol [PVA ] (5 g/l) for the AKP-15 powder and using ultrasonic treatment for 15 minutes. From the resulting suspension, the liquid is removed by filtration using water-jet evacuation. Polyester filters, consisting of a biological mixture of cellulose nitrate and cellulose acetate, with a pore diameter of 0.8 µm are used. The resulting filter cake (cast) is dried overnight at ambient temperature and fired at 1100 C [AKP-30] or 1150 C [AKP-15] for 1 hour with a heating/cooling rate of 2 C/min. After firing, the supports are machined to the required dimensions and polished until a shiny surface is obtained. Before use the supports are cleaned in ethanol by ultrasonic treatment to remove any loose particles in the pores caused by machining of the samples. After cleaning the supports are fired at 800 C to remove any ethanol left in the pores and to release stresses induced by machining and polishing. Typical dimensions are a diameter of 39.0 mm and a thickness of 2.0 mm. 3. Properties The porosity of the resulting flat supports was measured after firing of AKP-30 and AKP-15 at respectively 1100ºC and 1150ºC with the Archimedes method by immersion in mercury. * Sumitomo Chemical Company, Ltd, Japan. E. Merck, Darmstadt, Germany. Model 2520 Sonifier, Branson Ultrasonics Corporation, Danbury, USA. ME 27, Schleicher & Schuell, Dassel, Germany.

63 Colloidal processing of ceramic membrane supports. The experimental part 55 The sintered compacts had a porosity of 32%. No difference in porosity was found between the AKP-30 and AKP-15 supports. Their pore-size distributions, measured by mercury porosimetry *, are given in Figure 1. The mean pore-radii were found to be 40 and 80 nm respectively. Pore-size distribution flat supports pore volume (cc/g) radius (nm) AKP-15 AKP-30 Figure 1: Pore size distribution of AKP-30 and AKP-15 supports made by colloidal filtration. Hydrogen permeances were measured following the dead-end permeation method and showed to be 8*10-7 mol/m 2 spa at 500ºC for AKP-30 supports and 2*10-6 mol/m 2 spa for the AKP-15 supports. No pressure testing was performed, but the tensile strength was measured in a four point bending set-up [2]. Results are that the flat ceramic supports are strong enough to withstand typical pressures used in lab-scale testing of the resulting membranes (i.e. at least up to 5 bars pressure difference). 4. Conclusions Colloidal filtration showed to be a very convenient way of preparing high quality flat membrane supports. These supports are extremely homogeneous as can be seen from the very * Series 200, Carlo Erba, Milan, Italy.

64 56 Chapter 4 sharp pore size distribution. As already stated in chapter 3 the homogeneity of the supporting system is very important because it will result in a very narrow pore-size distribution. This is necessary for a good control of the dip coating process, because of the capillary forces involved in this process. During dip coating a broad size distribution might influence the formation of the layer and thereby the integrity of the final membrane. Moreover, a homogeneous support will exhibit a very low surface roughness after polishing, so that very thin layers can be coated on the support. The above mentioned advantages make the supports very suitable for the preparation of flat microporous silica membranes for lab-scale tests. However, due to the almost perfect particle packing, the hydrogen permeance may be too low for application in process industry. For stability testing, on the other hand, the permeance of the membranes is of a far lower importance than the selectivity of the layer under investigation. More information about stability testing can be found in chapter 5 and 6 for the γ-alumina and the silica layer respectively. 3DUW7KHSUHSDUDWLRQRI WXEXODUPHPEUDQHVXSSRUWV 5. Introduction on centrifugal casting Porous α-al 2 O 3 tubes are frequently used as support for inorganic membranes. The conventional way of producing such tubes is by extrusion or isostatic pressing followed by sintering. These techniques are fully accepted for the production of dense ceramic tubes, but may be less suitable for the production of porous membrane supports. Especially the occurrence of unroundness, inhomogeneities and a considerable surface roughness may impose problems in both cases. For the application of defect-poor meso- and micro-porous membrane layers for gas separation [1,4] a very smooth inner surface together with a narrow pore-size distribution of the membrane support tube is needed [5]. Ceramic tubes can also be prepared by centrifugal casting (CC) of colloidal particles [6-8]. In this process, a powder is dispersed in a liquid with a stabilising agent, followed by rotating for some time in a cylindrical mould around its axis. The resulting cast is dried, released from the mould and slightly sintered. If particles are used with a narrow size distribution and a low degree of agglomeration one may expect the formation of a nearly random close packed (RCP) green compact [9]. This requires the use of a proper colloidal stabiliser at a concentration such

65 Colloidal processing of ceramic membrane supports. The experimental part 57 that the particles stay well-dispersed in the liquid but form a coherent rigid structure in the compact. Examples of possible stabilisers are nitric acid [10-12] or polyacrylate-based products [8, 13, 14]. If the concentration of stabiliser is too low the particles will already flock in the liquid and form a low-density compact that will exhibit a rough surface. At higher stabiliser concentrations the dispersion may become too stable so the compact remains fluid-like [15] and redispersion might occur as soon as the rotation stops. At optimum conditions the compact shape will closely follow the cylindrical mould shape which can be made with roundness close to perfection. In addition the surface roughness of the inside surface of the compact can be expected to be of the order of the particle size. Sintering mainly serves to obtain sufficient strength by the formation of necks without significant grain growth and shrinkage. In this section process optimisation of porous membrane support tubes by CC of high-quality α-al 2 O 3 particles is described. 6. Experimental The starting α-al 2 O 3 powders were the same as for the preparation of flat supports, AKP-30 and AKP-15. To obtain tubes with 2 mm wall thickness and ~20 mm diameter, 120 gram of powder was mixed with different amounts of APMA (Ammonium PolyMethAcrylate aqueous solution, Darvan C * ) and distilled water. The mixture of water and APMA, 120 ml in total, was brought on ph = 9.5 by adding (~1.5 ml) concentrated ammonia. The resulting suspension was ultrasonically treated for 15 minutes using a frequency of 20 khz and a transducer output power of 100 W. This suspension was used to prepare tubes with three different lengths: short, 6&10 cm, tubes in a home-built apparatus, using steel moulds and long tubes (16 cm) in a commercial centrifuge using Delrin ** moulds. The inner diameter of the tubes was ~20 mm diameter. Before pouring the suspension into the moulds, the moulds were coated at the inside with a solution of Vaseline in petroleum ether (boiling range C) * R.T. Vanderbilt Company, Inc., Norwalk, USA. E. Merck, Darmstadt, Germany. Model 250 Sonifier, Branson Ultrasonics Corporation, Danbury, USA. CEPA, GLE, Carl Padberg GmbH, Lahr, Germany. ** Du Pont de Nemours, Dordrecht, The Netherlands. Elida Fabergé, Bodegraven, The Netherlands.

66 58 Chapter 4 to obtain easy mould release. The tubes were centrifuged for 20 minutes at rpm and the remaining liquid was poured out of the moulds afterwards. The green tubes were horizontally dried inside the moulds in a climate chamber * for two days at 30 C and 60% relative humidity. After drying the green tubes were removed from the moulds and sintered horizontally on a flat support at 1150 C for 1 hour with a heating/cooling rate of 1 C/min. To study the influence of the amount of APMA on the drying and sintering behaviour of the AKP-30 tubes a series with different APMA concentrations in the suspension was made. Figure 2: AKP-30 tubes made by centrifugal casting: 1,3 with sinter warping and cracking defects ([APMA] = 417 kg/m 3 ) and 2,4 without visible processing defect ([APMA] = 167 kg/m 3 ). * Heraus Vötch, Ballingen, Germany.

67 Colloidal processing of ceramic membrane supports. The experimental part Results The suspension mixtures of 120 gram powder and 120 ml stabilising liquid were sufficient for two tubes with a length of 16 cm (and a wall thickness of 2 mm). The wall thickness of the supports could be varied from 1 to 2 mm at least, depending on solid concentration. It was found that porous tubes, visually free of processing defects could be obtained only with a certain, optimum, APMA concentration, [APMA], in the suspension. With [APMA] below the optimum, drying [APMA] Observations (kg/m 3 ) 0 No suspension possible 8 Low green strength; green tube difficult to release 42 Better green strength; some surface roughness 83 Good green strength; some surface roughness 100 Ibid 167 No visible processing defects 250 Ibid 292 Reasonable quality; some sinter-cracking 333 Some sinter-cracks; some surface roughness 417 Considerable cracks after sintering, warping and surface roughness Table 1: Influence of liquid phase [APMA] on the quality of sintered porous AKP-30 tubes. cracks were observed after drying. At [APMA] higher than the optimum, typical defects were obtained such as surface corrugation and excessive warping and cracking during sintering. Examples are given in Figure Influence of binder concentration The results of the study on the influence of the APMA concentration on the quality of AKP-30 tubes after sintering are summarised in Table 1. It was found that [APMA] = 167 kg/m 3 (addition of 20 ml APMA) gave optimal results. The AKP-30 results enabled us to use a less extended optimisation procedure for AKP-15, resulting in an optimum of [APMA] = 83 kg/m 3 (10 ml APMA).

68 60 Chapter 4 Pore size distribution tubular supports pore volume (cc/g) AKP-30 AKP radius (nm) Figure 3: Pore-size distribution of AKP-30 and AKP-15 tubes made by CC at optimum conditions. 7.2 Properties The porosity of AKP-30 (AKP-15) tubes, made with optimum [APMA] was 42.5% (43.2%) * after firing at 500 C, measured with the Archimedes method by immersion in mercury. The sintered compacts had a porosity of 34.8% (34.5%). Their pore-size distributions, measured by mercury porosimetry are given in Figure 3. The mean pore radius was found to be 60 (92) nm. Figure 4: Roundness diagram of an AKP-30 centrifuged tube. * The numbers between parentheses refer to AKP-15 tubes. Series 200, Carlo Erba, Milan, Italy.

69 Colloidal processing of ceramic membrane supports. The experimental part 61 The surface roughness * of the tubes was found to be ~0.25 µm for the inside and ~0.9 µm for the outside. The mean unroundness (deviation from a perfect circle) was ~0.025 mm, based on a 100 point measurement; the unroundness diagram is shown in Figure 4. In this diagram the drawn line around the concentrical circles gives the deviation from a perfectly round object. Figure 4 shows a slight elliptic deformation, possibly caused by gravitational stress during sintering. For comparison: the unroundness of a typical extruded α-al 2 O 3 tube was measured to be 0.16 mm as shown Figure 5. The surface roughness of this tube was ~6 µm. Figure 5: Roundness diagram for a typical extruded α-al 2 O 3 tube. 8. Discussion and conclusion With the CC technique excellent tubular membrane supports can be prepared with a very low surface roughness (0.25 µm). The roundness of tubes was found to be 6 better than that of a typical extruded tube. The roundness can possibly be improved further if special attention is paid to mould roundness and drying and/or sintering is done vertically or in a rotating set-up. This roundness is very important for application in reactors. If the tubes are glass-soldered in ring-shaped machined flanges, a good roundness may result in a minimal and evenly solderfilled space between the tube and the flange. This, in turn, will generally result in a better sealing process, quality and stability. If deformable (graphite) gaskets are used in a removable flange, a large unroundness will result in a radially inhomogeneous stress distribution in the tubes near the sealing, increasing the risk of brittle fracture. The wall-thickness of 2 mm provides the CC tubes with sufficient mechanical strength to withstand gas and liquid pressures that are common in membrane technology. The measured pore diameters of 120 and 184 nm are well in the range used for mesoporous membrane preparation and it is expected that γ-alumina layers can be applied on the supports by conven- * Mitutoyo Surftest III, Mitutoyo Mfg C0., Ltd., Tokyo, Japan. MC 850, Carl Zeiss, Oberkochen, Germany.

70 62 Chapter 4 tional dip-coating and without the need of further intermediate layers [4]. The minimum membrane thickness that can be obtained defect-free can be expected to be of the order of the support roughness. This leads to the conclusion that the thickness of membranes on CC tubes can be 24 less than those on extruded tubes. This, in turn, may result in a large flow increase for gases and liquids. The best tube quality was obtained with an optimum APMA concentration that is proportional to the specific surface area of the powders. In the present experiments (with α-al 2 O 3 powders) the optimum ratio between [APMA] and specific surface area was found to be ~0.03 kg 2 /m 5. An [APMA] of 8 kg/m 3 only, showed to be sufficient for electrosteric stabilisation and a rather stable suspension but also resulted in some drying cracks and roughness on the inside tube surface. This is likely to be caused by the fact that the suspension is partly flocked, leading to a poor particle packing that densifies significantly during drying. In addition it was found that the green strength was insufficient at low [APMA]. This can be ascribed to poor particle packing too, but it is more likely that APMA acts as a polymeric binder. At optimum [APMA], tubes can be prepared with sufficient green handling strength, which exhibit no surface roughness or cracking during drying or sintering. With higher [APMA], the green state shows no visual defects, but significant warping and cracking is obtained during sintering. This observation can be explained best by the presence of internal stresses in the green state caused by green state handling, or thermal processing. These stresses neither relax nor lead to cracks in the green state because of the combined effect of particle packing, close to RCP and a significant amount of interparticle bonding. The occurrence of drying cracks is quite familiar in ceramic technology. On the other hand, warping and sintering cracks due to internal stresses is less frequently reported. However it may be expected, that both phenomena are actually quite common in many practical ceramic processes. In such processes, compact homogeneity is often lower than in the present case so that the identification of processing defects as mentioned, may become obscured. Less dense areas in the green state cause (local) drying cracks; more dense areas cause (local) sintering stress. The observations made can be generalised for all ceramic suspension processing since they are not limited to shapes made with APMA and may be reproduced, for instance, when HNO 3 together with PVA is used as a stabiliser.

71 Colloidal processing of ceramic membrane supports. The experimental part Centrifugal injection casting Next, experiments are discussed using the so-called Centrifugal Injection Casting (CIC) technique [16]. Here, the same centrifuge as for conventional centrifugal casting is used, but the mould is not filled with the suspension before the experiment, but the suspension is sprayed into the mould during centrifuging. This method was developed by Bachmann et al. [6] for the production of silica tubes for optical glass fibres. In this method it is possible to change the suspension composition during spraying which results in property changes of the produced tubular part over the cast radius. For membrane support applications, this might be a favourable option, because one can start with a very coarse porous outside layer and end up with a layer with smaller pores at the inside of the tube. In this way one can combine very high fluxes (coarse porous layer) with a very smooth surface (inside layer) which is a suitable support for coating high quality membrane layers. 9.1 Experimental Two types of tubes have been prepared, single powder tubes (AKP-30 tubes) and tubes prepared with two different powders (AKP-30 and CR-1 * ). This CR-1 powder has a broad particle size distribution 5% < 0.6 µm and 95% < 3 µm with a mean particle diameter of 1.5 µm and a BET specific surface area of 4 m 2 /g as stated by the producer. For the single powder AKP-30 tube the following recipe was used. AKP-30 and 0.02M HNO 3 were mixed in a 1:1 weight ratio. After mixing, the suspension was treated with 175 Watt ultrasound for 15 minutes. The obtained suspension was filtered through a 200 µm steel filter and under magnetic stirring more 0.02 M nitric acid is added until a final weight composition of AKP-30:HNO 3 = 1:5. Because the suspension is not very stable, stirring had to be continued until the end of the preparation, including the injection process. Injection is started in an empty mould when the final rotational speed of rpm is reached. The movement of the injection tube is controlled by a commercial stepmotor system, this to ensure a smooth and reproducible movement of the injector tube. A typical axial * Baikolox CR-1, Baikowski Chimie, Annecy, France. Festo BV, Delft, The Netherlands.

72 64 Chapter 4 speed of the injector is 3.4 cm/s, while a peristaltic pump * is used to inject the suspension in the mould. Injection can be terminated when the desired amount of suspension has been injected. With the powder loading of the suspension one can easily calculate the thickness of the formed compact. Drying and sintering is essentially the same as described in section 6 for the conventional casted (CC) tubes. With the same method also tubes consisting of a CR-1 outside layer and an AKP-30 inside layer were prepared. The CR-1 suspension was produced in essentially the same way as described above for the AKP-30 suspension. Injection also took place in the same way as the single powder AKP-30 tube. The process started with injecting the CR-1 suspension and afterwards the AKP-30 suspension was injected. An SEM photograph of the resulting tube is shown in Figure 6. Figure 6: SEM micrograph of CIC-tube consisting of an outer CR-1 part (light, striped) and an inner AKP-30 part (grey, even). * Masterflex, model , Barnant, Barrington, UK. Jeol, JSM 5800, Tokyo, Japan.

73 Colloidal processing of ceramic membrane supports. The experimental part 65 Because the CR-1 powder has a rather broad particle size distribution, also some segregation in the subsequent sprayed layers can be found. This segregation is nicely shown in Figure 7, which is a close-up of the CR-1 (outside) part of the SEM picture shown in Figure 6. Figure 7: Close-up of the CR-1 part of the combined AKP-30/CR-1 tube. 9.2 Results AKP-30 tubes made by CIC had a porosity of 28%, which is somewhat lower than the conventional cast AKP-30 tubes. This is most probably due to the way of stabilising the suspension. In the case of conventional casting the suspension is stabilised by a relatively large amount of polymeric stabiliser and the suspension is most likely to be partly flocked. This flocculation will cause some extra porosity in the green compact. This porosity is not removed during the firing process and a more porous support will result. In the case of nitric acid stabilisation the suspension might be less flocked. The resulting green and sintered density of the compacts shall therefore be higher. The surface roughness of CIC tubes is somewhat larger (by optical examination) than the roughness of the CC tubes. This is most probably due to the

74 66 Chapter 4 spraying action of the injection tube. This spraying causes some turbulence during centrifuging which disturbs the formation of a smooth inside surface of the tube in production. 9.3 Conclusions on Centrifugal Injection Casting With CIC, it is possible to produce tubes without visual defects, but when using only a single powder, the conventional centrifugal casted tubes showed not only a higher porosity, but also a somewhat smoother inside surface. However, the largest advantage of using the CIC technique is the possibility of creating multilayer tubes in just one production step. One is able to build up tubes which have a very coarse outside, ensuring high permeability and a smooth inside surface, ensuring the right conditions for coating high quality membrane layers. Moreover with the CIC technique one has the possibility to apply layers with different properties, like catalytic active layers. So, depending on the application of the tubes, one can choose either to use the CC technique to obtain a very homogeneous, rather porous single powder tube or to use the CIC technique to obtain graded tubes or layered tubes with special properties of a somewhat lower quality. 10. Perspectives It may be questioned whether the high degree of perfection of the CC tubes justifies the higher costs in mass-scale production and the limitation to circular shapes when compared to extrusion processes. Extrusion processes are cheap, continuous and enable more complex shapes such as multi-bore tubes. On the other hand, the CC technique allows a radially varying concentration and morphology of composition and particle morphology. The use of a suspension that consists of largely different sizes of particles may automatically result in specific radial variations that can be predicted quantitatively on basis of the method described in [17]. However, these suspensions might be more difficult to stabilise and in general there are less degrees of freedom for varying properties of the produced tubes. With the CIC technique it is possible to inject small amounts of suspension layerwise so that all thinkable radial distributions can be realised. Suspensions with large particles and suspensions with smaller particles can be stabilised seperately, which will be easier than stabilising a suspension with a large variation in particle size. Multilayer membranes and solid fuel cell

75 Colloidal processing of ceramic membrane supports. The experimental part 67 structures can be made in this way with radial variations of transport properties and catalytic activity to obtain innovative solutions for dedicated reactor and separation problems. 11. Acknowledgement The author wishes to express his thanks to Dr. P. Geittner (Philips Forschungslabor Aachen) for the fruitful discussions on centrifugal (injection) casting. 12. References 1. A. Nijmeijer, C. Huiskes, N.G.M. Sibelt, H. Kruidhof and H. Verweij, Centrifugal Casting of Tubular Membrane Supports, Am. Ceram. Soc. Bull. 77 [4] (1998). 2. P.M. Biesheuvel, H. Verweij, Design of Ceramic Membrane Supports: Permeability, Tensile Strength and Stress, J. Membrane Sci. 156 [1] (1999). 3. R.J.R. Uhlhorn, M.H.B.J. Huis in t Veld, K. Keizer and A.J. Burggraaf, Synthesis of Ceramic Membranes. Part 1. Synthesis of Non-Supported and Supported Gamma-Alumina Membranes without Defects, J. Mater. Sci., 27, (1992). 4. R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Formation and Characterization of Supported Microporous Ceramic Membranes by Sol-Gel Modification Techniques, J. Membrane Sci., 99, (1995). 5. R.M. de Vos and H. Verweij, Improved Performance of Silica Membranes for Gas Separation, J. Membrane Sci., 143 [1] (1998). 6. P.K. Bachmann, P. Geittner, E. Krafczyk, H. Lydtin and G. Romanowski, Shape Forming of Synthetic Silica Tubes by Layerwise Centrifugal Deposition, Am. Ceram. Soc. Bull., 68 [10] (1989). 7. P.K Bachmann, P. Geittner, H. Lydtin, G. Romanowski and M. Thelen, Preparation of Quartz Tubes by Centrfugational Deposition of Silica Particles, pp in: Proc. 14 th. Europ. Conf. Optical Commun. Vol.1 Inst. Electr. Eng., London, Sept (1988). 8. G.A. Steinlage, R.K. Roeder, K.P. Trumble and K.J. Bowman, Centrifugal Slipcasting of Components, Am. Ceram. Soc. Bull., 75 [5] (1996). 9. E.A. Barringer and H.K. Bowen, Formation, Packing, and Sintering of Monodisperse TiO 2 Powders, J. Am. Ceram. Soc., 62 [12] C199-C201 (1982). 10. W. Huisman, T. Graule and L.J. Gauckler, Alumina of High Reliability by Centrifugal Casting, J. Europ. Ceram. Soc., (1995). 11. P. Hidber, F. Baader, T. Graule and L.J. Gauckler, Sintering of Wet-Milled Centrifugal Cast Alumina, J. Europ. Ceram. Soc., (1994).

76 68 Chapter J.C. Chang, B.V. Velamakanni, F.F. Lange and D.S. Pearson, Centrifugal Consolidation of Al 2 O 3 and Al 2 O 3 /ZrO 2 Composite Slurries vs Interparticle Potentials: Particle Packing and Mass Segregation, J. Am. Ceram. Soc., 74 [9] (1991). 13. J. Cesarano III, I.A. Aksay and A. Bleier, Stability of Aqueous α-alumina Suspensions with Poly(methacrylic acid) Polyelectrolyte, J. Am. Ceram. Soc., 71 [4] (1988). 14. J. Cesarano III, I.A. Aksay and A. Bleier, Processing of Highly Concentrated Aqueous α-alumina Suspension Stabilized with Polyelectrolytes, J. Am. Ceram. Soc., 71 [12] (1988). 15. C.P. Cameron and R. Raj, Better Sintering through Green-State Deformation Processing, J. Am. Ceram. Soc., 73 [7] (1990). 16. C. Huiskes, A. Nijmeijer, H. Kruidhof and H. Verweij, Porous Ceramic Supports by Centrifugal Injection Casting, to be submitted to J. Europ. Ceram. Soc. 17. P.M. Biesheuvel, A. Nijmeijer and H. Verweij, A Theory of Batchwise Centrifugal Casting, AIChE J., 44 [8] (1998).

77 &KDSWHU 7KHSUHSDUDWLRQDQGSURSHUWLHVRI K\GURWKHUPDOO\VWDEOHγDOXPLQDPHPEUDQHV 1. Introduction There is a growing interest in the application of inorganic membranes in high temperature gas separation and membrane reactors. Microporous amorphous silica membranes are of particular interest for H 2 -separation in processes like steam reforming, the water-gas shift reaction, dehydrogenation and coal gasification [1-3]. Selective removal of hydrogen in numerous equilibrium-restricted processes (e.g. dehydrogenation processes) may lead to a significant increase in conversion and yield, provided membrane permeance and selectivity are high. In addition, by selectively removing hydrogen, strongly endothermic equilibrium-limited processes may be operated at temperatures that are significantly lower than those in conventional reactors without loss in conversion (e.g., steam-reforming of methane). In most of the above-mentioned processes, high operation temperatures are necessary while the reaction atmospheres usually contain considerable amounts of steam because water is one of the reactants, or because water is added to reduce coke formation. Also in food processing and medical applications, steam is often used for sterilisation. Thus, in many applications the membranes must be sufficiently stable in environments of both increased temperature and containing steam. In this work, this is called hydrothermal stability. Somewhat surprisingly, however, only a very limited amount of literature is available on hydrothermal stability of even the most commonly applied mesoporous membrane type, namely γ-alumina membranes on α-al 2 O 3 supports. These mesoporous γ-alumina membranes are the common supports for the microporous silica membranes to be used in membrane steam reformers. In the investigations that finally led to the present study, delamination of the γ-alumina membrane from the α-al 2 O 3 supports in hot steam was found to be a major compli-

78 70 Chapter 5 cation. In a few studies [4,5] a rather large change in pore size under hydrothermal conditions has been reported without mentioning delamination from the support. At high temperatures changes in pore-size of the γ-alumina membrane can be related to sintering and conversion of γ-alumina to other transition alumina forms (with increasing temperature δ-alumina and θ-alumina) and α-alumina as described by Wefers and Misra [6]. Most authors conclude the conversion from one low temperature phase to another, for example γ-alumina to δ-alumina from X-ray diffraction data. These alumina forms and their transitions, however, are not well defined and it may be difficult to distinguish between the different types. Most reliable data are therefore available for the overall change of the transition aluminas (γ, δ and θ) to α-alumina, which shows distinct peaks in the X-ray diffraction spectrum. A well-known method to improve the thermal, and to a certain degree the hydrothermal stability of γ-alumina is the use of dopants. Several authors [7-16] made detailed studies of the effects of lanthanum doping, which generally increases pore stability and impedes the phase transition to α-alumina. A large number of doping elements was investigated by Vereshchagin et al. [17], and, contrary to other reports, they found that lanthanum enhanced the conversion of transition aluminas to α-al 2 O 3. On the other hand the same authors reported that elements like scandium, cerium, calcium and strontium retarded conversion. The discrepancies in the literature made us to investigate, besides lanthanum, the effects of gadolinium and calcium on pore stability under hydrothermal conditions. In refractory industry, phosphate bonding is a well-known method for bonding ceramic powders together [18-21] in either a reactive or non-reactive manner. Phosphate bonding may yield strong compacts at temperatures far below the normal sintering temperature of the powders. Much research has been conducted on the use of MonoAluminumPhosphate (MAP, Al(H 2 PO 4 ) 3 ) for bonding alumina powders. In addition to MAP, other phosphor-containing compounds like orthophosphoric acid (H 3 PO 4 ) can be used as well. In the work described here, phosphate bonding with MAP precursor solutions has been used to anchor the γ-alumina membranes to the α-al 2 O 3 support to inhibit delamination in the interface. The purpose of the present paper is to describe the procedures for and effects on hydrothermal stability of Ladoping and phosphate bonding of γ-alumina membranes on α-al 2 O 3 supports.

79 The preparation and properties of hydrothermally stable γ-alumina membranes Experimental All stability tests were performed on γ-alumina membranes coated on flat α-al2o3 supports. These supports were prepared as follows: A 50 wt-% suspension of α-alumina powder (AKP-30) * in a 0.02M nitric acid solution was treated with ultrasound for 15 in a specially designed beaker (a so-called Glass Rosett cell) using a frequency of 20 khz and a transducer output power of 100W. The resulting suspension was filtered on polyester with 0.8 µm pore size. The resulting filter cake (cast) was dried overnight at ambient temperature and sintered at 1100 C for 1 hour using a heating/cooling rate of 2 C/min. After firing, the supports were machined to the required dimensions of Ø 39 mm, 2 mm thickness and polished until a shiny surface was obtained. Before use the supports were cleaned in ethanol by ultrasonic treatment to remove any debris that remained in the pores after machining. After cleaning the supports were fired at 800 C to remove ethanol left in the pores. The typical thickness of a final flat membrane support was 2 mm. The sintered compacts had a porosity of 32%. A MAP layer was coated on the supports according to the following procedure: A commercial 50 wt-% MAP solution was diluted either 10 or 20 times, further indicated as MAP10 and MAP20, respectively. The shiny surface of a flat support was brought in contact with this solution for 3 seconds, after which it was dried. Next to this pre-treatment, the supports were coated under class 100 clean room conditions with either pure or doped 0.5M boehmite sols. Sols were prepared by reacting 0.5 mole of aluminium-tri-sec-butoxide ** (ATSB) with 70 moles of double-distilled water of 90ºC [22]. The ATSB was added drop-wise under a nitrogen flow to avoid premature hydrolysis. The temperature of the reaction mixture should at least be 80ºC to avoid the formation of any Bayerite (Al(OH) 3 ) [23]. After the addition of ATSB, the mixture was kept at 90ºC for at least one hour to evaporate off the butanol formed. * Sumitomo, Tokyo, Japan. Model 2520 Sonifier, Branson Ultrasonics Corporation, Danbury, USA. ME 27, Schleicher & Schuell, Dassel, Germany. Alfa, Johnson Matthey GmbH, Karslruhe, Germany. ** Acros, 97% purity, Geel, Belgium.

80 72 Chapter 5 The mixture was subsequently cooled down to ~60ºC and peptised with 1M HNO * 3 at a ph of about 2.5. During the synthesis, the sol was stirred vigorously. The peptised mixture was refluxed for 20 hours at 90ºC, resulting in a very stable 0.5 molar boehmite sol with a clear white/blue appearance. Doping of this sol was performed by thorough mixing with the appropriate amount of a 0.3M metal nitrate solution. The mixing was done directly before coating to avoid possible ageing effects that have been reported in the literature, for example by Lin and Burggraaf [11]. No such ageing studies were, however, performed in the present work. A dip-coating solution was obtained by diluting 30 ml of (doped) boehmite sol with 20 ml of a solution of 30 g PVA/l in 0.05M HNO 3. Both solutions were filtered before use through a 0.8 µm cellulose acetate filter to remove any large particles from the solutions. The shiny surface of a polished α-al 2 O 3 support was then brought in contact with dip-coating sol for 3 seconds, using a dedicated dip-coating apparatus and class 100 cleanroom conditions. After dip-coating, the membranes were dried and fired in air at temperatures between 600ºC and 1000ºC for three hours with a heating/cooling rate of 1ºC/min. Unsupported bulk membrane material was obtained for specific surface area and XRD-characterisation by drying the dipcoating solutions and subsequently firing the dried gels at the above-mentioned conditions. To detect any possible reaction of MAP with La-doped γ-alumina, powder XRD samples were prepared by mixing doped γ-alumina that was fired at 1000ºC and pure MAP that was fired at 300ºC in 50 wt-% ratio. The mixture was subsequently fired at 1000ºC for 3 hours. The pore-size of the resulting mesoporous membranes was determined by permporometry. This method and the home-built equipment that we used for the measurements have been described in detail elsewhere [24]. During the measurements the following processes took place: 1. By capillary condensation the mesopores of the membranes became filled with cyclohexane. 2. Starting with all pores filled, the cyclohexane partial pressure above the membrane was reduced in intervals, gradually emptying the pores. 3. At each cyclohexane vapour pressure level, the oxygen permeance through the open pores of the membrane was measured. * E. Merck, Darmstadt, Germany. FP030/50, Green rim, Schleicher & Schuell GmbH, Dassel, Germany. Velterop BV, Delden, The Netherlands.

81 The preparation and properties of hydrothermally stable γ-alumina membranes 73 The measurements were performed at 20ºC and the oxygen concentration was determined with a gas chromatograph *. From the oxygen permeance data as a function of cyclohexane partial pressure the pore-size distribution was calculated with the Kelvin equation [25]. After the pore-size was established, the membranes were treated in a steel reactor with a Simulated Ambient Steam Reforming Atmosphere (SASRA) for 100 hrs at 600 C with H 2 O/CH 4 = 3/1 (by volume) at 2.5 MPa total pressure. Heating and cooling was performed in an argon atmosphere at the same total pressure at a rate of 1 C/min. In a few experiments a pure steam treatment was carried out at 0.2 MPa total pressure at 150ºC or 300ºC in the same manner as for SASRA treatment. A pure CO 2 treatment was done likewise, but at 500ºC at 1.2 MPa pressure. The surface area of the membrane materials was measured before and after SASRA treatment by a single point BET instrument with a TC-detector. Three samples were run in parallel and the amount of adsorbed nitrogen on the sample surface was measured and used for calculating the surface area. The phase composition was characterised by XRD. After SASRA treatment the adherence of the membranes to the support was tested by the Scotch Tape Test [26]. In this test, a piece of Scotch Tape was applied firmly with the sticky side onto the membrane surface and torn off rapidly. If the membrane layer was torn off together with the tape, it was concluded that delamination had occurred. For membranes that showed no sign of delamination, the pore-size was measured with permporometry for a second time. Specific surface area measurements were performed with the unsupported bulk membrane material before and after SASRA, to obtain information about the stability of (doped) γ-alumina under steam-reforming conditions. * Model 3300, Varian, Sugarland (TX), USA. Siemens D5000, Cu Kα radiation, Essen, Germany.

82 74 Chapter 5 Figure 1a: SEM micrograph of a delaminated conventional mesoporous γ-alumina membrane after SASRA treatment. Figure 1b: Cross section showing that the γ-alumina layer has detached from the α-alumina support.

83 The preparation and properties of hydrothermally stable γ-alumina membranes Results Standard γ-alumina membrane-layers on α-al 2 O 3 supports not treated with MAP and prepared as described in [10] always came off in the Scotch Tape Test after SASRA treatment. As shown in Figure 1 (a and b), in these membranes a crack is formed in the membrane-support interface leading to delamination. When the support was treated with MAP, however, after steam treatment no delamination was observed. We suggest that the beneficial effect of MAP treatment resulted from chemical bonding between the membrane-layer and the support. To investigate independently the effect of the surface morphology of the support on delamination, without chemical phosphate bonding, the roughness of the support was varied by changing the grinding procedure. It was found that increasing the roughness of the α-alumina supports did not help avoiding delamination of the γ-alumina layer from the support during SASRA treatment. The concentration of the MAP solution was found to be critical. Treatment with 5 wt-% MAP-solution gave good adherence, while a 2.5 wt-% solution resulted in some delamination, possibly due to insufficient phosphate on the surface of the supports. Table 1 summarises the most important results from the investigation of metal doping. In this table the results of MAP treatment are combined with effects of firing temperature and doping. As can be seen in Table 1, γ-alumina membranes with pore radii as low as 2.0 nm (Kelvin radius) may be obtained after firing at 600 C. Note that an instrumental standard error of 0.5 nm (90% reliability) is common in permporometry. This technique should therefore only be used for comparison purposes and to obtain a qualitative impression of the pore-size and pore-size distribution of the material under investigation.

84 76 Chapter 5 Membrane Support Treatment T calc (ºC) Test conditions r Kelvin (nm) γ None 600 None 2.0 γ None 825 None 3.6 γ None 1000 None 8.7 γ MAP None 4.2 γ MAP SASRA 6.2 γ MAP SASRA 7.5 γ +3La None 825 None 3.3 γ + 3La MAP None 8.4 γ + 3La MAP SASRA 9.3 γ + 6La MAP None 6.0 γ + 6La MAP SASRA 6.1 γ + 6La MAP ºC steam 6.0 γ + 6La MAP ºC steam 6.0 γ + 6La MAP CO γ + 9La MAP None 8.6 γ + Ca MAP None 7.8 γ + Ca MAP SASRA 13.2 γ + Gd MAP None 10.3 γ + Gd MAP SASRA 15.8 Table 1: Influence of support treatment, γ-alumina doping, membrane firing temperature and SASRAtreatment on the pore-size of γ-alumina. MAP 10 indicates a 10 times diluted standard MAP solutions, which results in an effective MAP concentration of 5 mol-%., 3La indicates a 3 mol-% La-doped membrane, 6La indicates a 6 mol-% La-doped membrane.

85 The preparation and properties of hydrothermally stable γ-alumina membranes 77 All membranes without MAP treatment delaminated after SASRA treatment. Therefore no permporometry results for these membranes are presented. The pore-growth of undoped γ-alumina strongly depended on temperature showing a large increase in pore-size between 825 and 1000ºC. The MAP-treated membranes had somewhat larger pores after firing at 825ºC. The cause of this effect is not clear yet. For undoped γ-alumina membranes, the pores grew during SASRA from 4.2 to 6.2 nm, and after a second SASRA treatment to 7.5 nm. Thus, it appears that the pore-growth continues within the time scale of our SASRA treatment experiments. Compared to undoped materials, 3 mol-% lanthanum doping gave hardly any beneficial effects on stability (Table 1). A significant improvement was found, however, for 6 mol-% lanthanum doping. For this case a pore-size of only 6.0 nm was found after firing at 1000ºC and no pore growth during SASRA treatment was observed at all. Additionally, after SASRA treatment, the pore-size distribution of a 6 mol-% doped γ-alumina membrane was still very narrow, as can be seen in Figure 2. # of pores 6E+17 5E+17 4E+17 3E+17 2E+17 1E Kelvin radius (nm) Figure 2: Pore size distribution of a SASRA-treated γ-alumina membrane. The support was treated with 5 mol-% MAP (MAP 10). The γ-alumina was doped with 6 mol-% La and sintered at 1000ºC for three hours.

86 78 Chapter 5 In an attempt to investigate the reactions between the γ-alumina membrane material and other components, powder mixtures of γ-alumina doped with 6 and 9 mol-% La fired at 1000 C were studied. In Figure 3 XRD patterns of these powders as prepared, and after SASRA treatment at 600 C are shown. After firing at 1000 C the patterns did not reveal the presence of any α-al 2 O 3, but several broad peaks that can be ascribed to transition alumina. From comparison with the XRD pattern for LaAlO 3, it can be concluded that traces of LaAlO 3 may be present. After SASRA treatment, the presence of LaAlO 3 is more outspoken, suggesting that the SASRA treatment promoted formation of this phase. Intensity theta values Figure 3: XRD patterns. 1. γ-alumina with 6 mol-% La-doping after calcination at 1000 C, 2. Same powder as in 1 after additional SASRA treatment at 600 C, 3. γ-alumina with 9 mol-% La-doping after calcination at 1000 C, 4. Same powder as in 3, but after additional SASRA treatment at 600 C, 5. Reference LaAlO 3 powder. In Figure 4 XRD patterns of powders mixtures of γ-alumina with 6 mol-% La and MAP are shown. After firing at 1000 C the XRD pattern suggests that the powder consists mainly of AlP 3 O 9 and AlPO 4 (orthorhombic phase). After SASRA treatment of the powder at 600 C a clear change in the XRD pattern could be observed: a single dominating phase of AlPO 4 (trigonal, Berlinite phase) is observed together with traces of a LaPO 4 phase. The SASRA treatment at 600 C thus induced structural changes in the Al-P-O phases.

87 The preparation and properties of hydrothermally stable γ-alumina membranes 79 Intensity c c c c c c c c c d d b a a a ba ab a+b aa a a a a a a a a a theta values 2 1 Figure 4: XRD patterns of: 1. MAP + γ-alumina with 6 mol% La-doping after calcination at 1000 C, 2. Same material as in 1 after additional SASRA treatment. a = Al 3 P 3 O 9, b = AlPO 4, c = AlPO 4 (Berlinite), d = LaPO 4. Table 2 shows results of specific surface area measurements of the same powders as mentioned in Figure 3 and Figure 4. Powders of undoped γ-alumina fired at 1000 C had a specific surface area close to 100 m 2 /g. Addition of 6 mol-% La reduced the surface area with about 20%, while the reduction is about twice as large with a 9 mol-% lanthanum addition. For all these materials the surface area showed an increase after SASRA treatment at 600 C. The increase was largest for 9 mol-% La addition and smallest for pure γ-alumina. Addition of MAP reduced the surface area significantly, and opposite to the previous observations, the SASRA treatment gave a further reduction. The remarkable effect of MAP in reduction of surface area is possibly due to the formation of some reaction products of the MAP with the γ-alumina. The effect of CO 2 on the stability of the membranes was tested, because of the possible instability of lanthanum compounds towards CO 2 (formation of La 2 (CO 3 ) 2 ). CO 2 treatment at 600ºC for 100 hours did not result in a measurable pore-growth: the pore-size increased from 6.0 nm to 6.3 nm after treatment, which is within the measurement error.

88 80 Chapter 5 Composition T, C SASRA treatment, 600 C Surface area, m 2 /g γ-alumina 1000 no 96 γ-alumina 1000 yes 99 γ-alumina + 6 mol% La 1000 no 82 γ-alumina + 6 mol% La 1000 yes 89 γ-alumina + 9 mol% La 1000 no 60 γ-alumina + 9 mol% La 1000 yes 70 MAP + γ-alumina + 6 mol% La 1000 no 12 MAP + γ-alumina + 6 mol% La 1000 Yes 7.6 Table 2: Specific surface area of different unsupported membrane materials. The treatment of the above mentioned membranes in pure steam for 100 hours at 150ºC and 300ºC did not induce any pore-growth either. This is a very important result, because such a treatment is common in steam-sterilisation. The encouraging results that were found for the 6 mol-% lanthanum doped samples could not be reproduced for the materials with 9 mol-% lanthanum doping. These samples had already a Kelvin radius of 8.6 nm before SASRA treatment. Hence no further investigations were performed with such highly-doped membranes. Compared to the 6 mol-% La-doped membranes, calcium and gadolinium-doped membranes showed larger pores and more pore growth during SASRA treatment. This indicates that the stabilising effect of the latter metal ions is not of the same quality as that of lanthanum. These findings could be of interest, however, for the preparation of membranes with specific poresizes. 4. Discussion & Conclusions A clear and beneficial effect of treating the α-al 2 O 3 membrane support with a MAP precursor solution on the stability of γ-alumina membrane/α-al 2 O 3 support structure was observed.

89 The preparation and properties of hydrothermally stable γ-alumina membranes 81 BET and permporometry results show, however, that unsupported γ-alumina sintered during SASRA treatment even with lanthanum stabilisation. Since it can be assumed that sintering is more pronounced in the membrane than in the support, the tensile stresses may build up in the γ/α-alumina interface. This stress manifests itself clearly in conventional membrane structures where the γ-alumina membrane blisters off in flakes from the support after SASRA treatment. As shown in this study, however, a pretreatment of the support with a sufficiently concentrated MAP solution together with 6 mol-% La-doping of the γ-alumina results in a highly steam stable membrane-support combination. Because steam stability is largely improved going from a 2.5 wt-% to a 5 wt-% MAP solutions, it seems reasonable to suggest that the number of phosphate bonds is critical in order to overcome the interfacial stress. The phase changes in the Al-P-O phases as observed by XRD studies of powder mixtures seem to have no effect on the mechanical stability of the phosphate bonded membrane. This could mean that the bonding is maintained even if structural changes occur or that locally, the aluminium phosphate phase condition has reached a saturated state. From an industrial point of view, the MAP treatment appears very promising for stabilising layered membrane structures that are applied in processes where water vapour is present at high temperatures. The possible large impact of steam stable (mesoporous) membranes in industry gave rise to a patent application of the described process [27]. The second issue to address, besides bonding, is the stability of the mesopores. Our results show that doping with lanthanum can prevent pore growth to a large extent. The effect of Ladoping on γ-alumina has been subject to many studies, particularly in catalysis. The mechanisms of pore-growth and phase stabilisation, however, are not fully understood. It is usually assumed that the presence of lanthanum on the alumina surface leads to formation of microdomains of lanthanum aluminate on the alumina surface, which immobilises the surface ions and thereby reduces sintering of the material and impedes the phase transition to α-alumina [13,14]. The interested reader is referred to the paper of Oudet et al. [13] who provided a detailed description of the mechanism behind lanthanum doping of γ-alumina. A larger amount of La-doping, e.g., the 9 mol-% doped membranes, possibly gives rise to completely crystalline LaAlO 3 clusters. The formation of such crystalline clusters during sintering may destroy the mesoporous structure of the material due to stresses involved. This is a tentative explanation for the relatively large pore-size of the 9 mol-% lanthanum-doped membranes. The influence of doping with Ca 2+ ions is discussed in a paper by Burtin et al. [15]

90 82 Chapter 5 who found a CaAl 12 O 19 phase in the doped material. No explanation is given for the stabilising effect of calcium, but essentially the same mechanisms as for lanthanum doping are likely to be present. The fact that no reports are available on the behaviour of microporous amorphous silica membranes under high temperature steam containing atmospheres is not surprising in the light of the instability of γ-alumina which is by far the most commonly used intermediate layer for such membranes. With the availability of more hydrothermally stable supporting structures, however, a systematic study of the intrinsic properties of microporous silica membranes in water vapour containing atmospheres at high temperatures has recently been initiated in our group. Additionally, we expect significant improvements in membrane stability under milder steam conditions than applied in this study, as often encountered in steam sterilisation and pervaporation. As one can see from Table 1, a spin-off result of this work is a list of recipes for the preparation of membranes with different amounts of doping, covering a complete range of pore-sizes with a resolution of 1-2 nm. This shows that we are now able to produce membranes with a tailor-made pore-size, which may be important for retaining certain large molecules by highflux nanofiltration. 5. Acknowledgement The author wishes to thank Dr. R. Bredesen (SINTEF) for performing the SASRA treatment and the XRD and specific surface area measurements and for the fruitful discussions about hydrothermal stability of intermediate layers. 6. References 1. R. Bredesen, Key Points in the Development of Catalytic Membrane Reactors, Paper no. A7.0 in Proc. 13th Int. Congr. Chem Process Eng., August , Praha, Czech Republic. 2. G. Sarraco and V. Specchia, Catalytic Inorganic Membrane Reactors: Present Experience and Future Opportunities, Catal. Rev. Sci. Eng., 36 [2] (1994). 3. J. Zaman and A. Chakma, Inorganic Membrane Reactors, J. Membrane Sci., (1994). 4. G.R. Gallaher, and P.K.T. Liu, Characterization of Ceramic Membranes. I. Thermal and Hydrothermal Stabilities of Commercial 40Å Membranes, J. Membrane Sci., (1994).

91 The preparation and properties of hydrothermally stable γ-alumina membranes C.H. Chang, R. Gopalan and Y.S. Lin, A Comparative Study on Thermal and Hydrothermal Stability of Alumina, Titania and Zirconia Membranes, J. Membrane Sci., (1994). 6. K. Wefers and C. Misra, Oxides and Hydroxides of Aluminium, Alcoa Technical Paper No19 (1987). 7. H. Schaper and L.L. van Reijen, The Influence of Dopants on the Stability of Gamma Alumina Catalyst Supports, Mater. Sci. Monographs, (1982). 8. H. Schaper, E.B.M. Doesburg and L.L. van Reijen, The Influencce of Lanthanum Oxide on the Thermal Stability of Gamma Alumina Catalyst Supports, Appl. Catal (1983). 9. H. Schaper, E.B.M. Doesburg, P.H.M. de Korte, L.L. van Reijen, Thermal Stabilisation of High Surface Area Alumina, Solid State Ionics, (1985). 10. Y.S. Lin, K.J. de Vries and A.J. Burggraaf, Thermal Stability and Its Improvement of the Alumina Membrane Toplayers Prepared by Sol-Gel Methods, J. Mater. Sci., (1991) 11. Y.S. Lin and A.J. Burggraaf, Preparation and Characterisation of High-Temperature, Thermally Stable Alumina Composite Membrane, J. Am. Ceram. Soc., 74 [1] (1991). 12. M.F.L. Johnson, Surface Area Stability of Aluminas, J. Catal., (1990). 13. F. Oudet, P. Courntine, and A. Vejux, Thermal Stabilisation of Transition Alumina by Structural Coherence with LnAlO 3 (Ln = La, Pr, Nd), J. Catal., (1988). 14. B. Beguin, E. Garbowski and M. Primet, Stabilisation of Alumina by Addition of Lanthanum, Appl. Catal (1991). 15. P. Burtin, J.P. Brunelle, M. Pijolat, and M. Soustelle, Influence of Surface Area and Additives on the Thermal Stability of Transition Alumina Catalyst Supports. I: Kinetic Data, Appl. Catal., (1987). 16. D. Lafarga, A. Lafuente, M. Menéndez and J. Santamaría, Thermal Stability of γ-al 2 O 3 /α-al 2 O 3 Mesoporous Membranes, J. Membrane Sci., (1998). 17. V.I. Vereshchagin, V.Y. Zelinskii, T.A. Khabas and N.N. Kolova, Kinetics and Mechanism of Conversion of Low-Temperature Forms of Alumina into α-al 2 O 3 in the Presence of Additives, J. Appl. Chem. USSR, (1983). 18. W.H. Gitzen, L.D. Hart and G. MacZura, Phosphate-Bonded Alumina Castables: Some Properties and Applications, Ceram. Bull., 35 [6] (1956). 19. J.E. Lyon, T.U. Fox and J.W. Lyons, An Inhibited Phosphoric Acid for Use in High-Alumina Refractories, Ceram. Bull., 45 [7] (1966). 20. C. Toy and O.J. Whittemore, Phosphate Bonding with Several Calcined Aluminas, Ceram. Int., (1989). 21. M.J. O Hara, J.J. Duga and H.D. Sheets Jr., Studies in Phosphate Bonding, Ceram. Bull., 51 [7] (1972). 22. R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Formation and Characterization of Supported Microporous Ceramic Membranes by Sol-Gel Modification Techniques, J. Membrane Sci., (1995). 23. B.E. Yoldas, Hydrolysis of Aluminium Alkoxides and Bayerite Conversion, J. Appl. Biotech., (1973).

92 84 Chapter H.W. Brinkman, Ceramic Membranes by (Electro)Chemical Vapour Deposition, PhD thesis, University of Twente, F.P. Cuperus, D. Bargeman and C.A. Smolders, Permporometry. The Determination of the Size Distribution of Active Pores in UF Membranes, J. Membrane Sci., (1992). 26. S. Krongelb, Environmental Effects on Chemically Vapour-Plated Silicon Dioxide, Electrochem. Tech., 6 [7-8] (1968). 27. A. Nijmeijer, H. Kruidhof and H. Verweij, Scheidingsinrichting met Keramisch Membraan Dutch Patent Application (in Dutch), no (1998).

93 &KDSWHU 3UHSDUDWLRQFKDUDFWHULVDWLRQDQGSURSHUWLHVRI PLFURSRURXVVLOLFDPHPEUDQHV 1. Introduction Microporous inorganic membranes are expected to be suitable for use in gas separation and pervaporation at high temperatures (up to 600ºC) and harsh environments. State-of-the-art microporous silica membranes show high fluxes for small gas molecules like H 2 and high selectivities with respect to larger molecules [1,2]. These membranes can be used in applications like natural gas purification, molecular air filtration and selective CO 2 removal. High temperature gas separation can be performed in either stand-alone operation or in direct combination with reaction in high temperature membrane reactors. In that case reactions of interest include steam reforming, the water-gas shift process, the dehydrogenation of hydrocarbons and coal gasification [3,4]. Amorphous microporous silica membranes as discussed here, consist of a macroporous α-alumina support (pore diameter ~100 nm) with a mesoporous γ-alumina intermediate layer (Kelvin radius of 2.5 nm) and a microporous silica top layer (pore diameter ~4 Å) [1,2]. In this chapter several experiments are described in which the silica was either modified by increasing the firing temperature, or by doping the silica with foreign ions or atoms. The firing temperature was increased to obtain a better hydrothermal stability, while doping with foreign ions or atoms was done for two reasons. First, partly doping of silica structures may enhance (hydro)thermal stability of the structure, as described below, and second, doping the silica structure may improve the transport properties of the silica membrane: e.g., by doping with palladium, hydrogen transport through the membrane might be promoted. The influence of firing temperature on the stability and transport properties of microporous silica membranes can largely be explained on the basis of the concentration of Si-OH groups

94 86 Chapter 6 in the network. As described by Imai et al. [5] the densification of silica structures can be influenced by the number of OH groups present in the structure. We therefore assume that when these OH-groups are already removed from the silica structure during synthesis, the hydrothermal stability under process conditions will be much improved. It is well-known from literature [6-8] that after firing at 800ºC only traces of OH-groups are left in the silica structure. The influence of the addition of Al 3+ - and Mg 2+ - ions to the silica structure is described by Fotou et al. [9]. This addition improved the hydrothermal stability of unsupported microporous silica material considerably. The positive effect of doping silica with Al 2 O 3 on the hydrothermal stability of the material was also recognised by Lin et al. [10], who investigated the thermal behaviour of alumina-doped silica xerogels. Because doping with large amounts of Al 2 O 3 hindered the viscous sintering, the mesoporous structure remained unchanged even after firing at 1000ºC. In this study modified silica layers were applied on undoped, flat γ-al 2 O 3 membranes, as described in chapter 5. Rutherford Backscattering (RBS) on some selected doped membranes was used to reveal the location of the dopants in the membrane structure. Until now, flat silica membranes were only tested at relatively low temperatures (up to 300ºC) because of limitations in thermal stability of the polymer sealing rings [2]. However, with the use of dense alumina rings together with carbon sealing it is possible now to measure membrane properties (e.g. permeance) at much higher temperatures (up to 600ºC), which will be described below. Next to the experiments with modified silica on flat supports, also coating experiments of commercially available tubular α-alumina supports with state-of-the-art silica are described. The intention of this chapter is to provide an overview of the research performed on silica membranes and membrane materials during the project. Due to the fact that subjects like the preparation of tubular supports and a hydrothermally stable intermediate layer were given a higher priority, most of the results are preliminary and much more research will be needed to get a clear picture of the behaviour of (doped) silica membranes under SASRA conditions. Therefore the results provided should be seen as the result of preliminary work and might serve as a basis for future research.

95 Preparation, characterisation and properties of microporous silica membranes Theoretical background on Rutherford Backscattering Rutherford Backscattering Spectrometry (RBS) is a non-destructive (sub)-surface analysis technique for solid systems. In principle atomic composition and depth distribution can be obtained for a sub-surface layer of a few microns. 2.1 Principle of method A small beam of high energy protons or 4 He + ions is directed to the surface of the sample ( target ) under investigation. The primary ions, with mass M 1 and energy E 1 (typically in the range of 1-2 MeV) can collide with a target atom with mass M 2. In the elastic scattering process (Figure 1), the incident ion transfers energy and momentum to the target atom. The energy of the scattered primary particle depends on the mass ratio, M 2 /M 1, and the scattering angle, θ: E = E 3 1 cosθ M M 1+ M 1 / M 2 1 = KE where K is the kinematic factor. Hence the energy loss of the scattered ion, measured at an angle θ, is a measure for the atomic weight of the target atom and consequently for its chemical nature (assuming no isotope interference). 2 sin θ 2 1 Figure 1: Schematic representation of the scattering event. Figure 2: Schematic representation of different pathways of scattered primary ions. Along its trajectory through the target the primary particle will also loose energy through inelastic interactions with the target electrons (free and bound) and nuclei. For a pure element target this energy loss over a distance x is given by:

96 88 Chapter 6 x E E1'= E1 I d dx= E1 N ε x dx 0 Here it is assumed that (for small distances from the sample surface) the energy loss per unit length is independent of the particle energy and can be replaced by a constant, Nε, where ε is the stopping cross-section and N is the atomic density of the target. It should be noted that the energy loss is indirectly related to the length of the path but directly to the number of atoms (per unit area) encountered. The stopping cross-sections have been tabulated for most elements. For composite samples the total stopping cross-section can be calculated from the stopping cross-sections of the elements using Bragg s rule. E.g. for a compound A m B n : AB A B ε = mε + nε The inelastic energy loss of the scattered primary ion thus serves as a measure of the distance between the surface and the target atom. But one has to take into account the angle, α, the primary beam makes with the surface (see Figure 2): ε t E1' = E1 S1l1 = E1 N sinα c where t is the depth with respect to sample surface and ε c the composite stopping crosssection for the target. Similarly the energy loss of the scattered primary particle leaving the sample depends on the travelled path: ε t E3" = E3' S3l3 = E3' N sinβ c where β is the angle between the detector normal and the surface of the sample, with the scattering angle: θ = α+β. Another important parameter is the yield. The yield of particles detected by a solid state detector subtending a small solid angle, Ω, (typically Ω < 10-2 sr) is given by: Y = Q Nt d σ dω Ω Q is the number of primary ions, Nt the number of atoms per unit area, N, in a layer of thickness t. dσ/dω is the average differential scattering cross-section. For small values of Ω the average scattering cross-section is given by:

97 Preparation, characterisation and properties of microporous silica membranes 89 =% 2 2 ZZe & ( % 1 2 Ω ' )* 4 &K 4E sin θ 'K dσ d For M 2 >M 1 this reduces to: =% 2 ZZe & ( 1 2 Ω ' 4 )* dσ d E 2 sin M M [ 1 ( sin θ) ] 2 + cosθ 1 4 θ 2 M M2 1 [ 1 ( sin θ) ] ( )K *K Hence the yield measured by the detector provides quantitative information on the number of atoms per unit area, N t (for pure elements). The energy spread of the primary ions is small (typically < 2keV), but due to the statistical nature of the inelastic interactions with the target this energy spread increases. This phenomenon, called energy straggling, reduces the accuracy of the mass and depth determination of the atoms at increasing distance from the target surface. The energy resolution of the detector (typically 15 kev) also places a limit on the mass resolution, which becomes most distinct for the heavy elements. Each particle entering the detector causes an electronic pulse with a height proportional to the particle energy. Each detector event is analysed and stored, with respect to the measured energy, in a Multi Channel Analyser (MCA) with a typical channel width of 4keV. With 512 channels an energy spectrum range can be recorded of just over 2 MeV. The position of the surface atoms depends on the ion beam energy, E 1, and the kinematic factor for the specific atom. Hence the heaviest atoms appear in the high energy side of the spectrum. The peak will extend to lower energies depending on the thickness of the sample or the concentration profile of the atom in the sample. 2

98 90 Chapter 6 Figure 3: Examples of RBS measurements and results for different types of samples. In Figure 3 three schematic examples are presented. Figure 3A shows the energy spectrum for a thin foil of composition AB where A has the highest atomic number. For a thin foil two separate peaks will be observed, each limited at the high-energy side by K X E 1. In Figure 3B the case of a stack of layers is presented. The energy edge for element B is shifted to lower energy as the primary and scattered particles must travel through foil A, loosing energy through inelastic interactions. When a thick sample of composition AB is analysed then the contribution from the light element B is added to the spectrum of the heavy element (Figure 3C). For thin film samples the analysis of the RBS spectra is generally straightforward, especially when the peaks are well separated. For bulk samples and samples with layers of different compositions the spectrum will be a complicated sum of the individual element spectra. For analysis, a model spectrum is generated based on assumptions about the elemental composition and element distribution in the sample. The model parameters are (manually or by a minimisation routine) adjusted until a satisfactory agreement with the measured spectrum is obtained. Typical standard RBS analysis conditions [11,12] are: 4He+ or H+ beam energy: 1.5 to 2.3 MeV, energy spread < 2 kev Beam spot size: 1 to 4 mm 2, beam current: 10 to 50 na Ion dose to accumulate one spectrum: 10 to 40 µc Incidence angle: 5 to 10 to the sample normal Incident beam divergence: better than 3 (full angle) Target vacuum: better than 10-4 Pa (10-6 Torr)

99 Preparation, characterisation and properties of microporous silica membranes 91 Detector angle: scattering at 165 to 170 to the incident beam Detector solid angle: 3 to 5 msr, surface barrier detector area: 25 to 300 mm2 Detector resolution: 15 kev, spectrum channel width: 4 kev Analyser data storage per spectrum: 512 channels Typical accumulation time: 5 to 20 minutes 2.2 Application to supported silica membranes A remaining question is the morphology of the interface between the silica top-layer and the intermediate γ-al 2 O 3 support layer. In the dip-coating process the silica layer may either form an almost flat layer on top of the polished support surface or partly infiltrate the pores in the γ-layer. With conventional techniques (SEM-EDS, or TEM) the microscopic geometry of this interface cannot be elucidated. With RBS analysis a more clear indication can be obtained on the depth distribution of the silicon atoms with respect to the γ-alumina surface. 3. Experimental 3.1 Membrane synthesis Supported silica membranes were prepared according to the sol-gel method first described by De Lange et al. [13]. In the experiments on flat membranes the supporting system consisted of a colloidal filtrated α-alumina support with a γ-alumina intermediate layer. The support AKP-30 supports were prepared as described in chapter 4. After firing the supports were machined to the required dimensions and polished until a shiny surface was obtained. For low temperature measurements, up to 300ºC, the supports were used without any further sealing. The membranes were sealed with Kalrez * or PTFE O-rings in a K250 testing cell. For the * Du Pont Company, Wilmington, Delaware, USA. Eriks, Alkmaar, The Netherlands.

100 92 Chapter 6 high-temperature measurements, the polished porous supports were glass-soldered to dense alumina rings for application in the K500 high temperature membrane testing cell *. The γ-alumina intermediate layer For the application of the mesoporous γ-alumina layer, essentially the same recipe as developed by de Lange et al. [13] was used. The complete synthesis route is described in chapter 5. Dip coating was performed under class 100 cleanroom conditions in order to minimise particle contamination of the membrane layer [1,2]. After coating, the membranes were dried in a climate chamber at 40ºC and 60% relative humidity for 3 hours in air, because it has been shown [15] that the drying rate of the boehmite layer under such conditions is sufficiently low to avoid crack formation. After drying the membranes were fired at various temperatures between 650 and 1000ºC for 3 hours with a heating/cooling rate of 1ºC/min, as described in chapter 5. The total γ-alumina layer thickness is in the order of 3 µm with an average Kelvin radius of nm, depending on the firing temperature. The Kelvin radius was determined by permporometry [16]. The (doped) silica toplayer The microporous silica layer was coated on the γ-alumina membrane surface by dip-coating under class 100 cleanroom conditions. A standard silica dip-coating [13] was prepared by reacting 21 ml of Tetra Ethyl Ortho Silicate (TEOS) with 8 ml concentrated HNO 3 and 3 ml double distilled water. As solvent, 21 ml ethanol was used. The reaction was performed under refluxing at 60ºC for 3 hours. After reaction, the resulting polymeric silica sol was diluted 19 times with ethanol to obtain the dip coating solution. After coating, the silica membranes were fired at various temperatures ( ºC) for three hours with a heating/cooling rate of 0.5ºC/min. The silica sol was doped by adding the required concentration of dopant to the sol during synthesis after 2 hours of refluxing. The solutions of ionic dopants were prepared by dissolv- * Velterop BV, Delden, The Netherlands. Heraeus Vötsch, Ballingen, Germany. Adrich Chemical Company Inc., Milwaukee (WI), USA. E. Merck, Darmstadt, Germany.

101 Preparation, characterisation and properties of microporous silica membranes 93 ing the required amount of metal nitrates in a mixture of 90 vol-% ethanol and 10 vol-% double distilled water. The sol was doped by metallic platinum by adding the required amount of platinum in the form of 1 mg/ml colloidal platinum in 20% HCl * to the sol after 2 hours of reaction. For various systems unsupported silica membrane material has been prepared by drying the (doped) silica sol and subsequent firing at the desired temperatures with a heating/cooling rate of 0.5ºC/min. Special 0.5 mol-% K-doped samples were prepared for studying the effect of potassium on the silica membrane material. This is of special importance because the catalyst used in steam reforming contains some potassium. Of the different unsupported silica samples the specific surface area and XRD -spectra were measured before and after a treatment in a Simulated Ambient Steam Reforming Atmosphere (SASRA). The conditions of such a SASRA treatment are: 100 hours in 3:1 steam/methane atmosphere at 600ºC and 2.5 MPa total pressure. Tubular membranes For the preparation of tubular silica membranes, commercially available mesoporous membranes [17] are used. These tubular supports have a total length of 25 cm and are enamelled at both ends, required for a gas-tight sealing with carbon seals to the reactor, so that an effective porous length of 20 cm remains. The tube consists of 4 layers. Layer 1, 2 and 3 consist of α-alumina with a thickness of 1.5 mm, 40 and 20 µm and a pore diameter of 12, 0.9 and 0.2 µm respectively. Layer 4 consists of γ-alumina with a thickness of 3-4 µm a Kelvin radius of 4 nm. A schematic drawing of the cross-section of a mesoporous support tube is provided in Figure 4. * Alfa, Johnson Matthey GmbH, Karlsruhe, Germany. Siemens D5000, Cu Kα radiation, Essen, Germany. T1-70, SCT/US Filter Membralox, Bazet, France.

102 94 Chapter 6 For the coating of the tubes a standard silica dip-coating sol was used, prepared as described above. This sol was either used undiluted, or was diluted 10 times to account for the longer contact time during coating, compared to the coating of flat membranes. The tubes were filled with sol under class 1000 cleanroom conditions, and left standing for 1 or 2 min. after which they were emptied. After coating, the membranes were fired at 650ºC for 3 h. with a heating/cooling rate of 0.5ºC/min. Often, this procedure was repeated Figure 4: Schematic drawing of a cross-section of the used commercial tubes. to obtain a second silica layer. After permeance measurements, the tubes were often coated again to repair any defects, either already present in the structure or formed during the measurements. 3.2 Permeance and selectivity measurements Gas permeance through the membranes was measured in the pressure-controlled dead-end mode [18]. The disc-shaped membranes were placed in the commercially permeance cells, K250 and K500 as mentioned before. Maximum operation temperatures were 300 and 600ºC respectively. The membrane was fitted in the cell with the microporous top-layer at the gas feed side. The pressure difference over the membrane was adjusted by an electronic pressure controller *. The gas flow through the membrane was measured by electronic mass flow meters. A schematic representation of the permeance set-up is given in Figure 5. * Model 5866, Brooks Instrument B.V., Veenendaal, The Netherlands. Model 5850TR, Brooks Instrument B.V., Veenendaal, The Netherlands.

103 Preparation, characterisation and properties of microporous silica membranes 95 Figure 5: Schematic diagram of the experimental set-up for permeance measurements. The tubular supports were measured in a membrane reactor, which could also serve for steamreforming experiments when applicable (Figure 6). The tubes were sealed with carbon sealing at the enamelled ends of the tubes. Permeance measurements were performed at 500ºC. copper seal enameled surface opposite male ridges graphite seal porous surface Figure 6: IRC membrane reactor. 4. Results 4.1 Permeance and selectivity measurements To obtain quantitative data about the integrity of the prepared membranes, both permeance and selectivity measurements were performed. It was found that high quality flat membranes

104 96 Chapter 6 could be prepared which showed a very high permselectivity. An overview of the most interesting results is provided in Table 1. The experiments on the tubular supports were more cumbersome. The increase in selectivity was not as high as normally obtained for the flat membranes. With an undiluted sol, however, it was possible to increase the H 2 /CH 4 permselectivity considerably above the Knudsen permselectivity (2.8). The results of two representative measurements on tubular supports are summarised in Table 2. Material Firing T Measuring T H 2 Permeance Permselectivity Permselectivity (ºC) (ºC) (mol/m 2 spa) H 2 /CO 2 H 2 /CH 4 St. silica * >1000 St. silica * St. silica * St. silica * Si + Pt * >2000 Si + Mg/Al * >2000 Table 1: Permeance and permselectivity properties for different flat silica membranes. St. silica indicates standard silica, Si + Pt = standard silica with 2.5 mol-% Pt metal, Si + Mg/Al = standard silica with 0.5/0.5 mol-% Mg 2+ and Al 3+. Sample no Nr. of SiO 2 layers Coating time Dilution N 2 permeance Permselectivity (min) (mol/m 2 spa) H 2 /CH x 25* x 3* Undil. 2* Undil 10* Undil. 5* Table 2: Results of permeance and permselectivity measurements for tubular silica membranes.

105 Preparation, characterisation and properties of microporous silica membranes RBS measurements In Figure 7 two RBS spectra are shown, one for an untreated α/γ-al 2 O 3 membrane and one for a silica-coated membrane. The two spectra are virtually identical, except for a small step at the high-energy side of the spectrum of the silica-coated membrane. This small step in the spectrum indicates that a silica layer is formed on top of the gamma layer. This can be seen more clearly in an enlargement of the highenergy region in Figure 8. Also a simulation curve is shown (dashed line). For the simulation the membrane was divided into four layers, a silica top-layer (about 8 nm thick), an intermediate silica/γ-alumina layer of approximately 45 nm and an γ-alumina layer of about 1 µm thick. The fourth layer is formed by the α-al 2 O 3 support. Figure 9 shows the total spectrum for a Ptmodified silica membrane. The first (highenergy) peak of Pt occurs exactly at the surface energy edge and is followed by a second peak with a tail with decreasing energy. Also a clear and separate Si peak is found at the surface indicating a silica top-layer with little penetration in the γ-alumina layer, as evidenced by the onset of the Al-edge at an energy substantially lower than the edge for surface Al. Figure 7: Figure 8: RBS spectra for a treated and an untreated α/γ-al 2 O 3 membrane. Enlargement of the high-energy region of Figure 7. Figure 9: RBS spectrum of a Pt-doped silica membrane.

106 98 Chapter Specific surface area measurements The results of specific surface area measurements of different unsupported (doped) silica membrane materials is provided in Table 3. Unfortunately due to time limitations no measurements were performed on Mg/Al-doped material fired at lower temperatures. Material Before SASRA (m 2 /g) After SASRA (m 2 /g) SiO 2 (400ºC) SiO 2 (600ºC) SiO 2 (825ºC) SiO 2 (Mg/Al 825ºC) 0.06 <0.06 Table 3: Specific surface area of different membrane materials. The temperature between brackets indicates the firing temperature of the material. 4.4 XRD measurements XRD-diagrams do not show any phase transformation in the SASRA treatment for the undoped samples and the samples doped with Pd, Al/Mg. All these XRD-spectra showed an amorphous silica phase, for the undoped material the diagrams are shown in Figure 10. In the case of K-doping, however, a phase change to a keatite [19,20] phase occurred during SASRA. As shown in the XRD-diagram of Figure 11, a more detailed description of the results will be provided in [21].

107 Preparation, characterisation and properties of microporous silica membranes I n t e n s I t y Si Pure silica material Theta Figure 10: XRD-diagrams of pure unsupported silica material. Diagrams 1, 2 and 3 are before and 4, 5 and 6 after SASRA treatment, respectively. 1 and 4 is silica fired at 400ºC, 2 and 5 silica fired at 600ºC and 3 and 6 silica fired at 825ºC I n t e n s I t y Silica with 0.5%K Theta 2 1 Figure 11: XRD-diagrams of 0.5 mol-% K-doped unsupported silica material, prepared at 825ºC. Curve 1 before and curve 2 after SASRA. The peaks observed in curve 2 indicate a keatite phase.

108 100 Chapter 6 5. Discussion and conclusions As the permeance and permselectivity measurements show, it is possible to prepare highquality doped silica membranes with excellent properties. Moreover it was possible to perform permeance and permselectivity measurements at temperatures up to 600ºC on flat membranes. To the author s knowledge these are the first reliable measurements ever performed on flat membranes at such a high temperature. A more detailed discussion of the permeance and permselectivity results follows. It must however be noted that the relatively low hydrogen permeances obtained for the described membranes were at least partly due to the used AKP-30 supports, which had a bare-support hydrogen permeance of ~8*10-7 mol/m 2 spa. Standard silica membranes The very high H 2 /CO 2 permselectivity for the 825ºC fired standard silica membrane is remarkable. It is even more remarkable that the H 2 /CH 4 selectivity is lower, which is contrary to the common observation that the H 2 /CO 2 selectivity is lower than the H 2 /CH 4 selectivity. The excellent separation properties of silica membranes prepared at temperatures as high as 825ºC enables their use for high temperature applications, such as the dehydrogenation of H 2 S (chapter 8). Unfortunately no hydrothermal stability of the prepared layers could be tested because the mesoporous intermediate layer was not hydrothermally stable, but an indication of the hydrothermal stability of the unsupported material could be obtained from the specific surface area and XRD measurements. These measurements did not show any structural change in the material during SASRA treatment, which is a very hopeful result for the operation of real, supported, membranes at high temperatures and high pressures. High temperature measurements reveal a somewhat lower permselectivity of the membranes, but the permselectivities measured are still far above the Knudsen permselectivity. We assume that the lower permselectivity is due to defects caused by the sealing of the porous substrate to the dense alumina ring necessary for the application of the carbon sealing. The coating of the membrane layers on the supports was performed after sealing the dense alumina ring to the porous substrate. When the adherence of one of the layers to the glass-seal is not perfect due to different adherence, some defects might arise on the interface between the seal and the porous substrate.

109 Preparation, characterisation and properties of microporous silica membranes 101 Doped membranes It was shown that silica membranes doped with Mg/Al and with Pt showed very high selectivity towards methane and good selectivity towards CO 2. From specific area and XRD measurements it is clear that after firing at 825ºC, no large structural changes occur in doped silica material during a SASRA treatment. For the moment it is not clear whether Mg/Al doping has a positive effect of hydrothermal stability of silica membranes. To get real insight in a possible increase in hydrothermal stability, also experiments with material fired at lower temperatures should be performed, as was done for undoped silica. Silica membrane material doped with potassium, however, showed in SASRA treatment the formation of a crystalline keatite phase. One should keep this result in mind when deciding on the design of a membrane steam reformer. Regarding the amount of potassium in the common used catalysts in steam reforming (see chapter 2) and the mobility thereof under steam reforming conditions, it might be wise to avoid spill-over of potassium from the catalyst to the silica layer. Thus, when applying the separative silica layer on the inside of the tube, the catalyst should be loaded in the annular space between reactor tube and membrane tube. A detailed description of reactor design is provided in chapter 2. Tubular membranes Not much research was performed on coating of tubes in this project. Results on the coating of commercial tubes show however that it is rather difficult to coat sufficiently defect-free membranes on these supports. As was already stated in chapter 4, the surface roughness of the used tubes was possibly too high to coat high quality silica layers. Another possibility is that here the same problem occurs as was encountered with the sealed flat membranes. Some defects in the membrane layer might result from a bad adherence of the coated layer at the enamel/membrane interface. First results are, however, rather positive. It was possible to coat silica layers with a selectivity, which was somewhat above the Knudsen selectivity. This shows the presence of at least some degree of microporosity with, fortunately, still a relatively high N 2 permeance compared to that of the centrifugal cast tubes described in chapter 4. Coating with just 1 silica layer from an undiluted silica coating solution showed to give best results. Coating with a second layer, did not improve selectivity, it did only reduce the N 2 permeance of the membrane with

110 102 Chapter 6 a factor of 2. This is an indication that the defects in the coated layer are of such large size that they can not be repaired anymore by just coating another layer. It might, however, also support the suggestion that the reduction in selectivity is due to bad adherence of the coating to the enamel, thereby creating large defects on the enamel/membrane interface. As can be seen from Table 2, it was not possible to coat microporous silica layers starting with a 10 times diluted coating sol. The silica concentration in this sol might be too low to get the joint effect (entanglement) of the silica polymers during coating, which is normally preventing the silica polymers from intruding the pores in the γ-alumina layer. Silica polymers will therefore intrude in the pores of the support, as is shown by the decrease of N 2 permeance by a factor of 10 by coating an extra silica layer on a support which was already coated with 2 silica layers. RBS measurements RBS analysis revealed the existence of a very thin silica layer on top of a γ-al 2 O 3 membrane. An interesting feature that was observed in the RBS measurements is the increased oxygen/aluminium ratio in the γ-alumina compared to the α-al 2 O 3 supports. Assuming a stoichiometric ratio of 3/2 for the α-al 2 O 3 support, the model used for fitting the RBS data seems to indicate rather a 4/2 ratio for the γ-layer. This most likely indicates that the surface of the γ- Al 2 O 3 particles in the γ-al 2 O 3 layer consists of Al(OH) 3. A rough estimate shows that, with a particles of about 5 to 10 nm diameter covered by an Al(OH) 3 surface layer would have an avarage O to Al ratio between 3.5 and 4. Furthermore, the model used for fitting the RBS data indicates that the top-layer contains half of the silica. The other half is mixed in with the first part of the γ-layer. As the amount of silica present in these membranes is rather small, the modelling parameters present rather rough estimates. For the Pt-doped membranes, the data suggests that the Pt is deposited mostly in the gamma layer and on top of the silica layer. The deposition of platinum on top of the silica layer is confirmed by SEM on a 5% Pt-doped membrane (Figure 12). Moreover it seems that in the case of the platinum doped membranes the silica layer is much more located on top of the γ-alumina layer than in the case of the undoped membranes. The reason for this is still unclear.

111 Preparation, characterisation and properties of microporous silica membranes 103 Figure 12: SEM micrograph of the surface of a 5% Pt-doped silica membrane at a magnification of 5500x. The white areas indicate surface platinum clusters. 6. Acknowledgements The author wishes to express his thanks to: Dr. H. Weyten (VITO) for performing the high temperature measurements, Dr. R. Bredesen (SINTEF) for performing SASRA treatments, XRD-measurements and specific surface area measurements, Dr. J.A. Dalmon (IRC) for performing permeance and permselectivity measurements on the tubular membranes and Dr. A. Vredenburg (University of Utrecht) for performing the RBS measurements. These people are also kindly thanked for fruitful discussions on their respective measurments. 7. References 1. R.M. de Vos and H. Verweij, High-Selectivity, High-Flux Silica membranes for Gas Separation, Science, (1998).