Mechanistic insight to acidity effects of Ga/HZSM-5 on its activity for propane aromatization

Abstract

Ga-modified HZSM-5 precursors, containing 1 wt% Ga, were first prepared using the incipient wetness impregnation method, and then subjected to one or three-times consecutive treatment by cycles of reduction in hydrogen and re-oxidation in air. The resulting Ga/HZSM-5 catalysts were characterized by N₂ physical adsorption, ICP-AES, DRIFT, Py-FTIR, NH₃-TPD, H₂-TPR, XPS, DRIFT-TPSR and MS-TPSR techniques to obtain clear mechanistic details that the acidity of these Ga/HZSM-5 catalysts affected their activity for propane aromatization. The characterization data suggested that the impregnated introduction of Ga to ZSM-5 zeolite and subsequent reduction–oxidation treatment led to a great decrease in the number of Brønsted acid sites (BAS) and promoted the formation of strong Lewis acid sites (LAS) attributed to highly dispersed Ga species. The formed strong LAS specifically promoted the dehydrogenation steps during propane aromatization, whereas the original BAS were responsible for the whole aromatization process. The TPSR results suggested that propane was converted to propylene through the dehydrogenation on BAS and strong LAS, and simultaneously converted to ethylene through the β-scission on BAS at a low temperature. With the elevation of temperature, the generated propylene and ethylene on the strong BAS and the strong LAS were further converted into BTX aromatics accompanied by hydrogen release. It was plausible that the highly efficient synergy between BAS and strong LAS could result in lower aromatization temperatures and more product of benzene (β state) over the Ga/HZSM-5 catalysts than that over the Ga-modified HZSM-5 precursors.