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

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 SiO2/Al2O3 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.