On catalyst activation and reaction mechanisms in propane aromatization on Ga/HZSM5 catalysts

Ga/HZSM5 catalysts prepared by incipient wetness impregnation of HZSM5 with gallium nitrate solutions and submitted to reductive and oxidative pre-treatments were studied by adsorbed pyridine FTIR, in situ DRIFTS and had their catalytic activity tested in propane aromatization reactions at 703 K with different initial hydrogen partial pressures. It is shown that dihydridogallium ions formed under hydrogen at aromatization temperatures, slowly decompose under inert gas flow and rapidly decompose when hydrogen is replaced by propane. During reduction under propane, no dihydridogallium species are observed, only alkylgallium species. Dehydrogenation reactions were inhibited and hydrogenolysis activities went through a maximum with increasing hydrogen partial pressure. The results showed that dihydridogallium ions cannot be the active sites for alkane activation and that, while taking part in hydrogenolysis reactions, hydrogen competes with propane for the active sites. A possible reaction scheme for catalyst activation and the first steps in the aromatization reaction is proposed.

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► Hydridogallium ions cannot be the active sites for alkane aromatization on Ga/HZSM5.
► Hydrogen competes with propane for hydrogenolysis sites on Ga/HZSM5.
► Pre-formed hydridogallium ions on Ga/HZSM5 disappear rapidly under propane flow.
► Hydridogallium ions are formed Ga/HZSM5 upon reduction by hydrogen but not by propane.