EXAFS studies on MFI-type gallosilicate molecular sieves

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. Te mplate burning (at 773 K) causes little disturbance to the zeolite framewor k, and most of the gallium remains tetrahedrally coordinated. However, afte r a heat treatment at 923 K the apparent coordination number was found to d ecrease, 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.