Propane dehydrogenation over binuclear Ga-oxo species stabilized by paired Al-Sites in Ga/ZSM-5: A First-Principles investigation

Ga/ZSM-5 is highly efficient for the direct dehydrogenation of propane (PDH) and binuclear Ga-oxo species were recently proposed to account for the observed PDH performance. However, it is still challenging to determine the active sites and reaction mechanisms at the harsh operation conditions. In this work, the thermodynamic stability and PDH mechanisms of binuclear Ga-oxo species stabilized by paired Al- sites in Ga/ZSM-5, including [Ga2O2]2+, [Ga2O2H2]2+, [HGaOGaOH]2+, [GaOGa]2+, [Ga]+-[Ga(OH)2]+, etc. were investigated with extensive first-principles based calculations. [Ga]+ not only exists stably when isolated but also interacts with neighboring oxidized Ga species to stabilize them. This enhanced stability can be attributed to the change of Ga coordination and formation of Ga-O-Ga bridge. The most plausible PDH mechanisms over these Ga-oxo species are all alkyl mechanisms. The calculated free energy barriers for PDH steps can all be correlated with the thermodynamic stability of the Ga-oxo species. Further to these, [HGaOGaOH]2+ would exhibit superior performance in PDH for the fine balance between the thermodynamics stability and activity. There are intermediate Ga-oxo species with better thermostability formed during PDH over these Ga-oxo species, making the regeneration of the active sites demanding. However, some of these intermediate Ga-oxo species are also eligible as active sites for PDH, such as [HGaOGaOH]2+. We expect the findings would help to rational the knowledge on PDH over Ga-oxo species, as well as the operation condition driven evolution of reaction sites for catalysis in harsh conditions and pave the way for design and fabrication of Ga-based catalysts of superior performance for conversion of light alkanes.