Light hydrocarbons to BTEX aromatics over Zn-modified hierarchical ZSM-5 combined with enhanced catalytic activity and stability

A series of Zn modified hierarchical ZSM-5 catalysts with Si/Al ratios of 25 and 40, and Zn loading amounts of 0, 0.5, 1, and 2 wt% was designed to pursue a highly efficient catalyst with excellent catalytic activity and stability toward BTEX formation in the aromatization of light hydrocarbons. The Zn modified hierarchical ZSM-5 catalysts with intracrystalline mesopores were prepared by alkali treatment and Zn impregnation. The effects of the alkali treatment and Zn modification on the textural property, acidity, catalytic performance, and coke formation were investigated in detail via various characterization techniques. It was found that the catalytic stability of the alkali-treated catalysts notably improved. This can be mainly attributed to the fact that the alkali treatment of the pre-synthesized ZSM-5 generated a large amount of mesopores, which could alleviate the diffusion limitations. Moreover, the Zn modification remarkably improved the initial BTEX yield, which was due to the improved Lewis/Bronsted (L/B) ratio in the prepared catalysts. However, as the Zn loading amount increased from 0 to 2 wt%, the catalyst stability dramatically decreased, especially for the 2 wt% case. It seems that the balance between the Lewis and Bronsted acid sites (i.e., the suitable L/B ratio) is the key to obtaining both high BTEX yield and catalyst stability, and the optimum Zn loading amount is determined to be 0.5 wt% with an L/B ratio of around 1.