Effect of acidity and diffusibility on coke deactivation over nano-sized HZSM-5 zeolites

Abstract

Four series of nano-sized HZSM-5 zeolite catalysts were prepared with different steaming and HNO3 leaching processes. These samples were studied by XRD, NH3-TPD, FT-IR, n-hexane and cyclohexane adsorption, and fixed-bed aromatization of C4 liquefied petroleum gas (C4 LPG). Results show that, with the steaming temperature increasing, the acidity of catalyst decreased gradually, which was attributed to the removal of Al species from the framework of the zeolite. When steaming is combined with acid leaching, we found that acid leaching-steam catalysts exhibit stronger acidity than steam-acid leaching catalysts. Besides, acid leaching also played an important role in enhancing the diffusibility of catalysts. Catalysts which underwent acid leaching twice exhibit the best microporous diffusibility. Through correlating the acidity and diffusibility of the catalysts with their aromatization performance, it is found that the product distribution of aromatization is mainly determined by catalyst acidity rather than diffusibility. On the other hand, the average coking speed of the catalysts is not only influenced by the acidity of the catalyst, but also by its microporous diffusibility. The improvement of microporous diffusibility remarkably favors suppressing the coking deactivation of nano-sized ZSM-5 zeolite in C4 LPG aromatization.