Inorganic Microporous Membranes for Gas Separation in Fossil Fuel ...

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2 Theoretical background membranes (Figure 4). The supported zeolite membranes are by far the most extensively studied. The most investigated support material is α-alumina. Stainless Steel, γ-alumina and titania are examples of other support materials suitable for zeolite membranes. 43,44 A B C Figure 4 Schematic drawing of polymer – zeolite composite membrane (A), self supported zeolite membrane (B) and a (α-Al2O3) supported zeolite membrane (C). Ideal zeolite membranes are gastight directly after the hydrothermal crystallisation. A common nitrogen permeation test is applied to test the gas tightness. Zeolite membranes must be activated, or in more detail, the water and organics, such as structure directing agents (SDA’s), must be removed in order to obtain an open and interconnected microporous structure. Removing these SDA’s can be done via: o Chemical leaching o Ion exchange techniques which is applicable for the larger pore zeolites (e.g. 12ring) o Thermal treatment at specified temperatures, heating rate, atmosphere and duration. The heating rate and cooling rate can be key factors for removing SDA without destroying the zeolite layers due to different expansion coefficient of templated and template free zeolite structures o Low temperature ozone treatment. 43 The thermal expansion coefficient difference of the membrane layer and support can cause stress, undesired cracks and peeling off. Porous intermediate layers or thin zeolite layers with randomly ordered crystals are recommended in order to reduce the thermal stresses. The preparation methods for different supported zeolite membranes are known and include (a) in situ hydrothermal synthesis in the presence of a substrate, (b) “dry or wet gel conversion method” and (c) the application of two stage (phase) secondary growth methods. The seeds in the secondary growth methods can be colloidal zeolites that are deposited by dip coating, spin coating or sputtering 38 . 17
2 Theoretical background In situ hydrothermal synthesis The most widely reported and successful route to prepare zeolite membranes is in situ hydrothermal synthesis on a substrate. The substrate is immersed into the zeolite precursor mixture which is contained in an autoclave, and then heated for a predetermined time, see Figure 5. Both, nucleation and growth of the zeolite crystals, occurs on the support surface. Figure 5 Schematic arrangement for the in-situ hydrothermal synthesis of a zeolite membrane. The support for the zeolite film is immersed in the precursor mixture within the autoclave during the hydrothermal synthesis. These synthesis routes are used for many zeolite types. However, there is little scope for the control of the microstructure of the final films. Intercrystal defects can be difficult to avoid when using in situ growth routes and this limits the applications in gas separation. 45 The gel compositions, which result in zeolite layers, usually are less concentrated than the optimal composition for making powders. 43 Synthesis routes in which the zeolite is formed within the pores of the support are studied in order to overcome the problems of the defects and to increase the strength. The preferential nucleation and growth is dependent on many factors such as time and temperature. Generally, the growth is slow at low temperatures and is observed mostly within the pores. Fast growth and the formation of an outer layer occur at higher temperatures (190 °C). Several processes have been studied to minimise the amount of layer defects such as Chemical vapour deposition (CVD) and microwave heating. 45 18
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4 Results and discussion Intensity
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4 Results and discussion ~50nm TiO2
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5 Conclusions and recommendations m
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6 References 22. Shao, Z., Dong, H.
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6 References 65. Van den Berg, A. W
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6 References 108. Wen, T. L., Heber
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Curriculum Vitae - George van der D
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Last but not least, I would like to
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Band | Volume 8 ISBN 978-3-89336-52
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