Evolution of [Ga]+ and [GaH2]+ at Propane Dehydrogenation Conditions in MFI Zeolite: A Theoretical Investigation

Ga/ZSM-5 is among the most promising catalysts for propane dehydrogenation (PDH) to selectively produce propylene, which is one of the most important feedstocks in chemical industry. PDH over Ga/ZSM-5 operates at harsh conditions (T > 800 K), limiting the in-depth and in situ characterization of the catalysts. The Ga speciation and the structures of active sites on Ga/H-ZSM-5 in dehydrogenation have remained in active discussion as they have not been solved clearly. Furthermore, Ga species stabilized by mono-Al sites would be the most abundant Ga species; the PDH pathways over them would be different from those of Ga-oxo or reduced Ga species trapped by dual-Al sites in Ga/ZSM-5, and were reported to exhibit unexpectedly high performance. To bridge these gaps, the potential catalytic roles and evolution of [Ga]⁺, [GaH₂]⁺, and [Ga]³⁺ in the channel and on the surface of ZSM-5 in PDH were investigated with first-principles-based calculations. We showed that dynamically generated undercoordinated [GaH₂]⁺ (Sin-[GaH₂^]+) would exhibit superior catalytic performance as compared with other mononuclear reduced Ga species stabilized by mono-Al sites at the operation conditions. Though [Ga]⁺ is thermodynamically more plausible, [GaH₂]⁺ is also kinetically favored on PDH pathways. A catalytic cycle of PDH was proposed connecting the concerted pathway over [Ga]⁺ and the alkyl pathway over [GaH₂]⁺, showing the strong coupling between the evolution of Ga species and the conversion of propane. We also proposed that, competing with PDH and the interconversion, [Ga]⁺ and [GaH₂]⁺ may also evolve and transport to form [Ga]³⁺ in channels or on the surface of zeolites, and this transportation also changes the Ga/Al ratio, forming Ga species that are more active than [GaH₂]⁺ and [Ga]⁺ in situ and may account for the observed PDH performance of Ga/ZSM-5. The findings may help to rationalize the understanding of PDH performance of Ga/ZSM-5 and benefit the design of novel catalysts with superior PDH performance.