The critical role of the active [GaH]2+ site in n-heptane dehydrocyclization over Ga/H-ZSM-5 zeolite†

Abstract

The Ga/H-ZSM-5 zeolite has long been acknowledged as an effective catalyst for the aromatization of light alkanes. In the present work, a three-coordinated carbenium ion is identified as a crucial intermediate in the formation of the [GaH]²⁺ active site. In particular, the involvement of the carbenium ion as a bridge facilitates the kinetic unhindered creation and restoration of the [GaH]²⁺ active site. Using density functional theory calculations, comprehensive reaction pathways for n-heptane, encompassing its conversion to toluene via the C1–C6 ring closure, were explored for both H-ZSM-5 and Ga/H-ZSM-5 zeolites. Compared to the BAS, the [GaH]²⁺ active site significantly lowers the activation barrier for the C–H bond cleavage. Furthermore, Bader charge and crystal orbital Hamilton population analysis confirmed that the [GaH]²⁺ active site facilitates the activation of the C–H bond of n-heptane while impeding C–C bond cleavage in the aromatization process.