Zeolite NaA (LTA) membranes supported by an alumina porous support tube are useful tools for dehydration of organic liquids.
1For design not only of synthesis methods of the LTA membrane but also of the optimum dehydration process, it is very important to understand a certain relationship between the fine structure of the LTA membrane and the dehydration performance. So far, many researchers have characterized zeolite membranes used for pervaporation (PV) dehydration process by using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and so on.1 However, since those methods provide only information on mm -μm scale, the fine structure of the membranes which must affect PV dehydration performance has not been clarified well.Among the characterization methods of zeolite materials, it is already well known that transmission electron microscopy (TEM) is a useful method to study fine structure. [2][3][4][5] We already reported a TEM observation method with focused ion beam (FIB) thin-layer specimen preparation technique for the characterization of the fine structure of LTA membranes.6,7 In our previous work, 8 it was also reported that Fourier transform infrared attenuated total reflectance method (FTIR-ATR) with a diamond prism as the waveguide can be used conveniently to determine ethanol/water PV dehydration performance (water selectivity) using Si-O asymmetric stretching vibration spectrum of the LTA membrane. As described there, since the FTIR-ATR method is the most useful and attractive method to characterize nondestructively tubular LTA membrane surface, for pursuing our FTIR-ATR study more deeply, it is very important to compare the Si-O spectrum with TEM images of fine structure and to grasp their relationships.Therefore, in this study, the tubular LTA membrane for dehydration of ethanol was characterized by both the TEM with FIB thin-layer specimen preparation technique and the FTIR-ATR. Three LTA membrane samples synthesized under different hydrothermal conditions were used to check performance of the combined method. Using such results, we could study the thickness of surface LTA crystals, grain boundary, LTA/alumina interface structure and crystallinity and density of materials inside of the alumina porous support. Consequently, fine structure changes of LTA membranes corresponding to hydrothermal synthesis conditions could be sensitively detected.
ExperimentalTubular LTA membrane sample and PV test of ethanol/water mixture Figure 1(a) shows a schematic diagram of tubular LTA membrane. Porous alumina tubes with dimensions of 12 ± 0.3 mm o.d., 9 ± 0.3 mm i.d. and 800 mm length were used as a substrate.9,10 Average pore size and porosity of the substrate were 1.3 μm and 40 -47%, respectively. 9,10 LTA membranes were gel-synthesized hydrothermally on the substrates according to our previous work. 9,10 In this study, hydrothermal synthesis times of 3 -5 h were tested. The thicknesses of surface layer of the LTA membranes used in this study were approximat...
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