Spectroscopic Study of Hydrogen-Permeable Membranes Composed of Zeolite Wreckages

Zeolite nanoblock (ZNB), prepared by hydrochloric acid decomposition of Na-A type zeolite, works as a highly efficient hydrogen-permeable membrane. Si and Al K-edge X-ray absorption near edge structure (XANES) spectra were measured for ZNB to clarify the local structures. The Si K-edge XANES spectrum is similar to that of the Na-A type zeolite, but the Al K-edge XANES spectrum is interpreted as a mixture of those of Na-A type zeolite and AlCl3・6H2O. These results suggest that ZNB was mainly composed of a part of the framework of Na-A type zeolite, whose pores are partially filled with small clusters of SiO2 and AlCl3. As a result of it, the average pore size is much smaller than that of zeolite itself, only permeable to hydrogen molecules. The SR Center, Ritsumeikan University, Kusatsu, Shiga 525-8577 Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531 1.Introduction Zeolites are one of silicates in which part of Si atoms are substituted with Al atoms, so-called, ‘aluminosilicates’ and are porous material with a number of holes with fixed sizes, as shown in Fig. 1. These holes are sometimes occupied with water, which is easily desorbed by heating or evacuating. They have more than 150 kinds of crystal structures, depending on the composition of Si and Al, as natural products and synthesized ones. In the zeolite structure, Si is electrically neutral, but Al atoms are negatively charged due to tetrahedral coordination. To maintain the charge neutrality, zeolite crystals take in several kinds of cations in holes close to Al sites. These unique structures enable us to apply to molecular sieves, ion exchangers and catalysts. Among many kinds of zeolites, Na-A type zeolite is the most prototypical with Si : Al composition ratio of 1 : 1, in which both Si and Al atoms are tetrahedrally coordinated with O. These structural units constitutes four-, sixand eight-membered rings (see Fig. 1). Recently, Nishiyama et al. found that a membrane of Na-A type zeolite wreckages on a porous bycor glass prepared by spin-coating after decomposition of Na-A zeolite with hydrochloric acid works very effectively as a hydrogen filtering film [1, 2]. It is intriguing to investigate why such a film has a function of hydrogen filter and what kind of structural change is induced in the above process. The X-ray absorption fine structure (XANES) spectroscopy is a powerful method to provide information about local structures around X-ray absorbing atoms and can be applied to any materials which are difficult to study by X-ray diffraction (XRD) methods. The purpose of this report is to clarify the local structures around Si and Al atoms by measuring XANES spectra at the corresponding K-edges. Electron spectroscopy for chemical analysis (ESCA) spectra were also measured to determine the sample compositions. Figure 1. The framework model of Na-A type zeolite. Red balls and blue balls indicate Al atoms and Si atoms respectively. Gray rods are O atoms which connect adjacent an Al atom and a Si atom. eight-membered ring six-membered ring