X-ray absorption spectroscopic study on the structure and crystallization of Ga-containing MFI-type zeolites

Abstract

The local environment of the Ga atoms in a series of gallosilicates with MFI structure as well as in the dried synthesis gel has been investigated by GaK XANES and EXAFS. In fully crystallized gallosilicate MFI samples, Ga is tetrahedrally coordinated by oxygen atoms with a GaO bond distance of 1.83 Å. Lack of order beyond the first coordination shell is in agreement with Ga located in several different crystallographic sites of the zeolite-type framework. For the gel, the same oxygen coordination is observed for the first coordination shell. As expected for the amorphous gel, no order beyond the first coordination shell is detected. Thus, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) cannot differentiate between tetrahedrally coordinated Ga atoms in a gel phase and Ga atoms incorporated into the framework. Inspection of a series of samples prepared with different crystallization times and with Si Ga ratios>40 show that during crystallization framework and extra-framework Ga remain in tetrahedral coordination. A sample with a low Si Ga ratio of 19 in the gel, a value far below the so far observed maximum Ga framework content of Si Ga ≈40 , contains also octahedral Ga after prolonged hydrothermal treatment. A quantitative evaluation shows that extra-framework Ga in gallosilicate MFI samples occurs in both tetrahedral and octahedral coordination. This result on the nature of extra-framework Ga is of importance, since these Ga species are believed to play a decisive role in catalytic processes on gallosilicate MFI-type solids.