Synergistic Catalysis of Brønsted Acid, Al-Lewis Acid, and Zn-Lewis Acid on Steam-Treated Zn/ZSM-5 for Highly Stable Conversion of Methanol to Aromatics
Tingjun Fu, Chuntao Cao, Liangliang Zhang, Liangliang Zhang, Li Zhang, Li Zhang, Ma Qian, Zhenjun Xu, Ran Wang, Han Li, Zhong Li
Abstract
The development of stable and selective ZSM-5 catalysts for methanol aromatization remains an ongoing challenge. Here, we demonstrate highly stable aromatization on ZSM-5 by preliminary steam treatment before Zn introduction. Steam treatment destroyed part of frameworks, which decreased acid density from 0.5 to 0.25 mmol g –1 and increased Al-Lewis acid sites (Al-LAS) at the expense of Brønsted acid sites (BAS). However, aromatization activity was further increased and the lifetime was still kept at 100 h. A comparative experiment found that increasing the SiO 2 /Al 2 O 3 ratio from 60 to 240 also decreased acid density to 0.25 mmol g –1, but BAS-based acid sites aggravated coke formation and reduced lifetime to 50 h. This reflected that Al-LAS and BAS on steam-treated ZSM-5 provided synergistic catalysis for stable aromatization. Introducing Zn into steam-treated ZSM-5 further transformed part of BAS to Zn-LAS but not changed acid density, which determined increased aromatics selectivity and a quite different well-maintained stability.