Effect of Zeolitic Hydroxyl Nests on the Acidity and Propane Aromatization Performance of Zinc Nitrate Impregnation-Modified HZSM‑5 Zeolite

A defective HZSM-5 zeolite (SiO₂/Al₂O₃ = 55, crystal size = 300 nm) with a high density of hydroxyl nests and its patched-up counterpart obtained by pretreating the defective zeolite with (NH₄)₂SiF₆ were employed to investigate the effect of zeolitic hydroxyl nests on the acidity and propane aromatization performance of zinc nitrate-impregnated HZSM-5 zeolite. Catalyst characterizations show that, in the case of impregnating the defective HZSM-5 zeolite, zinc ions were preferentially introduced into the zeolitic hydroxyl nests, forming subnanomeric ZnO clusters as Lewis acid sites (showed 1451 cm^–1 infrared (IR) adsorption feature when adsorbed by pyridine). By contrast, when the hydroxyl nests of the defective zeolite were patched by (NH₄)₂SiF₆ pretreatment, the zinc ions were mainly ushered in as isolated Zn²⁺ and bulky ZnO particles, the former species were the known Lewis acid sites (showed 1455 cm^–1 IR adsorption feature when adsorbed by pyridine) located at the counter-cation positions of zeolitic bridging hydroxyls. Propane aromatization operando infrared spectroscopic study and pulse microreactor evaluation indicated that the zeolitic hydroxyl nests-stabilized subnanomeric ZnO clusters had clear advantage over the known isolated Zn²⁺ Lewis acid sites in the dehydroaromatization of propane.