Speciation and interconversion of atomically dispersed extra-framework Ga in ZSM-5 zeolite

Abstract

Ga/ZSM-5 has been studied for a long time for its efficiency in catalytic dehydrogenation and conversion of light alkanes. Atomically dispersed extra-framework Ga species, such as [Ga]+(I), [GaH2]+(III), [GaO]+(III), [GaHOH]+(III), [Ga(OH)2]+(III) and [GaH]2+(III), etc. were proposed as reaction centers for such processes. To rationale the observed catalytic performances of Ga/ZSM-5, we investigated the electronic structures of these Ga species and pathways for their interconversion in zeolite channels by first-principles based calculations. Results from thermodynamics calculations at conditions relevant to experiments show that [Ga]+(I) is plausible at high temperature and low PH2O, and [Ga(OH)2]+(III) is plausible at low temperature and high PH2O. On pathways for interconversion of proposed Ga species, the calculated energy and free energy barriers for H2 dissociation and H2O formation are among the highest and correlate with involvement of Ga centers and their coordination with H and O-containing ligands. The coordination of framework O to Ga center are rather flexible, making dynamic formation of undercoordinated Ga species feasible at operation conditions and these species may attend subsequent chemical processes. We expect the findings would help to understand the formation and evolution of atomically dispersed reaction centers in zeolite channels under operation conditions.