(621ev) Ligand Effects on Gallium K Edge XANES: Implications for the Mechanism of Alkane Dehydrogenation in Gallium-Zeolite Catalysts
Acidic zeolites impregnated with gallium cations are industrial catalysts for the conversion of light alkanes into aromatics. In the absence of Brønsted acidity, the same cations on silica are highly selective catalysts for alkane dehydrogenation. A number of mechanisms have been proposed in literature for C-H bond activation over gallium cations. Theoretical calculations often favor non-redox, heterolytic C-H bond cleavage mechanisms. However, experimental results from X-ray Absorption Near Edge Spectroscopy (XANES) have long been interpreted as providing evidence for the existence of reduced gallium under reaction conditions, suggesting an oxidative addition/reductive elimination mechanism for C-H bond activation. We have prepared a series of model compounds designed to capture the structural features of possible reactive intermediates involved in C-H bond activation over gallium cations, compared their XANES spectra to spectra acquired on working catalysts under operando conditions. The results of this investigation demonstrate that shifts in the Ga K-edge XANES previously interpreted as evidence for reduction may instead be evidence for the presence of Ga(III) hydrides, the spectra of which are not readily distinguished from that of authentic Ga(I). These results call into question the only experimental evidence put forward to date for the existence of Ga(I) under reaction conditions in alkane aromatization catalysts, with consequent implications for the mechanism by which gallium cations activate C-H bonds.