XAFS study of H-ZSM5 catalysts modified with gallium

Abstract

H-ZSM5 zeolites were impregnated by the incipient wetness method with gallium amounts varying between 2 and 10 wt.%, and were submitted to reductive and oxidative pre-treatments. The samples were analyzed by XAFS spectroscopy; the 2, 5 and 10 wt.% catalysts were analyzed after ex situ reduction followed by oxidation, while the 3 wt.% catalyst was analyzed after impregnation and after each pre-treatment. Quantitative analysis of the XAFS spectra suggested the presence of three gallium species: (i) bulk β-Ga 2O 3 probably located on the external surface of the grains; (ii) highly dispersed gallium species, probably dimers, located in the zeolite channels; (iii) gallium clusters, probably tetramers, also located in the zeolite channels. The results show that the increase in gallium content systematically increases the proportion of β-Ga 2O 3 and tends to form larger clusters (tetramers). On the other hand, the reductive and oxidative pre-treatments reduce the proportion of β-Ga 2O 3 and tend to generate smaller clusters (dimers). Catalytic activity depends on the proportions of these species.