Influence of temperature on the catalytic behaviors of Zn-modified HZSM-5 catalysts for the ethylene aromatization

Abstract

Zn-modified HZSM-5 catalyst has been widely used in the aromatization of ethylene, while the effect of reaction temperature on the product distribution remains unclear, a factor that is pivotal for the design of highly efficient aromatization catalysts and the optimization of process parameters. In this work, the structure, composition, and acid properties of various Zn-containing HZSM-5 catalysts prepared via ion exchange, impregnation, and physical mixing were analyzed by XRD, ICP, NH₃-TPD, and Py-FTIR. The ethylene aromatization reaction on various catalytic behaviors were carried out at 400−580 ℃. The results on HZSM-5 and ZnAl₂O₄-pure/HZSM-5 catalysts indicated that, with reaction temperature increasing, the aromatics selectivity and aromatics produced via the dehydrogenation route increased considerably. On Zn(IE)/HZSM-5 and ZnAl_1.5O/HZSM-5 catalysts, the aromatics selectivity increased rapidly and then remained basically unchanged. However, the aromatics selectivity increases at first and then remains basically unchanged, while the proportion of dehydrogenation route remains constant, on Zn(IM)/HZSM-5 and Zn(PM)/HZSM-5 catalysts. Combined with cyclohexane dehydrogenation kinetics experiments, it is confirmed that the introduction of Zn species is helpful in reducing the dehydrogenation activation energy. Furthermore, a linear relationship is observed between the dehydrogenation activation energy of the catalysts and its acid strength and type. Interestingly, due to the absence of catalytic activity for spinel-structured ZnAl₂O₄, the catalytic performance and dehydrogenation activation energy of the ZnAl₂O₄-pure/HZSM-5 catalyst closely resemble those of HZSM-5.