C–H bond activation of propane on Ga2O22+ in Ga/H-ZSM-5 and its mechanistic implications

Abstract

Propane dehydrogenation (PDH) on Ga/H-ZSM-5 catalysts is a promising reaction for propylene production, while the detail mechanism remains debatable. Ga₂O₂²⁺ stabilized by framework Al pairs have been identified as the most active species in Ga/H-ZSM-5 for PDH in our recent work. Here we demonstrate a strong correlation between the PDH activity and a fraction of Ga₂O₂²⁺ species corresponding to the infrared GaH band of higher wavenumber (GaHHW) in reduced Ga/H-ZSM-5, instead of the overall Ga₂O₂²⁺ species, by employing five H-ZSM-5 supports sourced differently with comparable Si/Al ratio. This disparity in Ga₂O₂²⁺ species stems from their differing capacity in completing the catalytic cycle. Spectroscopic results suggest that PDH proceeds via a two-step mechanism: (1) C–H bond activation of propane on H-Ga₂O₂²⁺ species (rate determining step); (2) β-hydride elimination of adsorbed propyl group, which only occurs on active Ga₂O₂²⁺ species corresponding to GaHHW.