EXAFS spectroscopy shows that gallium atoms incorporated into the framework of MFI-type ZSM-5 gallosilicates have a typical Ga-O bond length of 1.82 +/- 0.01 Angstrom corresponding to a tetrahedral coordination. Accordingly, the coordination number, N, was found to take the value N = 4.2 +/- 0.3. Template burning (at 773 K) causes little disturbance to the zeolite framework, and most of the gallium remains tetrahedrally coordinated. However, after a heat treatment at 923 K the apparent coordination number was found to decrease, as well as the corresponding Ga-O bond length. These results were interpreted in terms of formation of extra-framework gallium oxide species which appear to be EXAFS silent. The thermal stability of the gallosilicate framework was found to increase with increasing Si/Ga ratio.
EXAFS studies on MFI-type gallosilicate molecular sieves
LAMBERTI, Carlo;BORDIGA, Silvia;ZECCHINA, Adriano;
1999-01-01
Abstract
EXAFS spectroscopy shows that gallium atoms incorporated into the framework of MFI-type ZSM-5 gallosilicates have a typical Ga-O bond length of 1.82 +/- 0.01 Angstrom corresponding to a tetrahedral coordination. Accordingly, the coordination number, N, was found to take the value N = 4.2 +/- 0.3. Template burning (at 773 K) causes little disturbance to the zeolite framework, and most of the gallium remains tetrahedrally coordinated. However, after a heat treatment at 923 K the apparent coordination number was found to decrease, as well as the corresponding Ga-O bond length. These results were interpreted in terms of formation of extra-framework gallium oxide species which appear to be EXAFS silent. The thermal stability of the gallosilicate framework was found to increase with increasing Si/Ga ratio.
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