How can Microwave Heating Contribute to the Development of Zeolite Membranes
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How can Microwave Heating Contribute to the Development of Zeolite Membranes Anne Julbe*, Julius Motuzas, Christophe Charmette, Christian Guizard Institut Européen des Membranes (UMR 5635 CNRS), UM II- CC47, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France * contacting author
ABSTRACT In this work the growth of both a zeolite (MFI) and a zeolite-like material (SOD) were investigated in/on αAl2O3 tubular support, using hydrothermal conditions and microwave (MW) heating. The method reveals efficient for the rapid synthesis of MFI/αAl2O3 membranes although gentle conditions were required in order to limit the thermal degradation of the template. Promising results were also obtained for directly growing SOD in/on the support. In both MFI and SOD MW synthesis the chemical dissolution of the αAl2O3 support influences the final membrane characteristics. In the case of SOD synthesis, this phenomena which increases with both temperature and support size, alters the membrane homogeneity (composition and structure). In order to get round it, MWs were used to prepare SOD small crystals which were deposited on/in the αAl2O3 support and submitted to a secondary growth. Homogeneous membranes were then obtained whose ideal selectivities αH2/N2 and αHe/N2 reaches respectively 4.5 at 20°C and 6.2 at 115°C (these selectivities are lower than 2 for a ZSM-5 membrane in similar conditions). INTRODUCTION Zeolite membranes are the subject of intensive researches for applications in fluid separation, chemical synthesis, pollution abatement but also for sensors, electrodes and opto-electronic devices. MFI membranes were the most largely investigated although LTA, FAU, MOR, FER, KFI, ANA, AlPO, SAPO and MCM membranes were also reported. Specific characteristics have still to be developed or improved for these membranes: crystal orientation, smaller thickness, lower number of intercrystalline defects, phase purity, possible de-alumination or insertion of specific active sites such as basic oxides or metal ions, rapidity and reproducibility of synthesis, possibly without templates, secondary growth from various seeds… Other structures are also required for specific applications (silica rich phases; small channel structures such as ZSM-39 or SOD…) but the synthesis of such optimized membranes, is either difficult or very long in classical conditions. Among the latest development in zeolite membrane synthesis, microwave (MW) heating is very promising for answering several of the above needs. Energy transfer from MW to material results in rapid heating and causes simultaneous, abundant nucleation and fast dissolution of precursors. These advantages enhance the crystallization rate in a very short time, yielding small particles with narrow size distribution and high purity. It seems now clear that the observed rate acceleration in MW synthesis is caused by inhomogeneous heating and superheating. An intrinsic MW effect, defined as a non-temperature effect, is unlikely [1]. Although MWs are used since about 15 years for zeolite syn
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