Synergic Effect of GaO ⁺ /Brønsted Acid in Hierarchical Ga/Al-ZSM-5 Bifunctional Catalysts for 1-Hexene Aromatization

Abstract

Nano-Ga/Al-ZSM-5 zeolites are synthesized by the in situ isomorphous substitution method (I-series) and a secondary synthesis method in a Ga(NO3)3/NH4F solution (S-series). The effects of the Ga incorporation approach on the texture, acidity, and catalytic behavior of 1-hexene aromatization are studied in this work. The reaction stability of the Ga/Al-ZSM-5 catalysts is notably enhanced compared with that of the pristine ZSM-5; this difference is mainly due to the improved mesoporosity and weakened Brønsted acidity. Moreover, the uniform intracrystalline mesopores generated in the secondary synthesized samples effectively promote active GaO+ species in closer proximity to the acidic protons, thus leading to stronger Lewis acid sites (LAS). Therefore, the S-2 sample prepared by secondary synthesis in a high concentration of Ga(NO3)3/NH4F solution exhibited higher BTX selectivity because of the greater synergic effect and higher dehydrogenation activity of the GaO+/Brønsted acid coordination centers in 1-hexene aromatization.