Theoretical investigation on propane dehydrogenation over extraframework Ga hydride species trapped at Al-pairs in MFI zeolite

Abstract

Ga/ZSM-5 is widely used to catalyze propane dehydrogenation (PDH). For the harsh operation conditions, the detailed active species, and the plausible reaction pathways that are responsible for the observed superior PDH performance remain unresolved for Ga/ZSM-5. Reduced extraframework Ga hydride species trapped at zeolite framework Al-pairs, including ^b[GaH]²⁺, [Ga]⁺-BAS, [GaH₂]⁺-BAS, etc. are an important kind of Ga species known as active species for PDH. In this work, the PDH pathways these over these Ga species were investigated theoretically at temperatures and pressures relevant to experiments. We showed that dynamically generated undercoordinated ^b[GaH]²⁺ would exhibit significantly higher catalytic activity at PDH conditions as compared with other Ga hydride species. The most plausible PDH pathway over ^b[GaH]²⁺ is the carbenium pathway. The Ga species and BAS act in concert in promoting the H₂ elimination and β-H transfer, and in turning PDH efficient. Ga hydride species may also form as intermediates along the PDH pathways as active sites for subsequent activation and conversion of propane, suggesting the evolution of Ga hydride species trapped by Al-pairs in Ga/ZSM-5 at operation conditions. The findings are expected to pave the way for better understanding of the experimentally observed operation condition dependent PDH performance of Ga/ZSM-5.