Propane dehydroaromatization on Ga-modified HZSM-5 catalyst: Brønsted/Lewis acid synergistic effect

Abstract

The HZSM-5 catalyst modified with Ga species exhibits excellent reaction performance for propane dehydroaromatization, a process that involves propane dehydrogenation to propylene, and further formation of aromatics via polymerization, cyclization, dehydrogenation, etc. The detailed mechanism of the synergistic effect between Ga species and Brønsted acid with each other during propane dehydroaromatization remains unclear due to the complexity and extreme branching of aromatization reaction pathways. In this study, the complete reaction process for propane dehydroaromatization on Ga-modified HZSM-5 is investigated based on the density functional theory (DFT) method. The results indicate that the Ga-modified HZSM-5 catalysts with [GaH]²⁺ species at all studied sites show superior thermodynamic and kinetic stability, with only [GaH]²⁺ species at the T3-T11 and T3-T5 sites exhibiting excellent dehydrogenation performance. In propane dehydroaromatization, the dehydrogenation reactions dominated by [GaH]²⁺ species tend to occur on the framework O atom surrounding [GaH]²⁺ species with stronger Lewis base strength, while polymerization and cyclization reactions dominated by Brønsted acid tend to occur on Brønsted acid with stronger acid strength. The synergistic effect between [GaH]²⁺ species and Brønsted acid is reflected in the fact that the [GaH]²⁺ species can indirectly improve the activity of propylene polymerization and C6 diene cyclization reactions on the Brønsted acid by enhancing the Brønsted acid strength, and in turn, the Brønsted acid enables the [GaH]²⁺ species to exist more stably kinetically by promoting the conversion of [GaH₂]⁺ species to [GaH]²⁺ species. This study contributes to the enriched understanding of the Brønsted/Lewis acid synergistic effect and the role of acidity-basicity during propane dehydroaromatization on Ga-modified HZSM-5.