Improved catalytic performance and decreased coke formation in post-treated ZSM-5 zeolites for methanol aromatization

The effects of the zeolite structure and acidity on the catalyst life and coke formation in the methanol-to aromatics (MTA) reaction were investigated using several MFI zeolites with different degrees of mesoporosity and acidity. These zeolites were prepared by post-synthesis desilication, dealumination, and combined desilication and dealumination processes. The influence of the post-synthesis on the pore characteristics, crystallinity, morphology and acidity of the ZSM-5 zeolites were studied by XRD, N-2-adsorption, n-octane temperature-programmed desorption experiments, (27)AI and Si-29 MAS NMR, SEM, TEM and NH3-TPD. Dealumination and/or desilication leads to an increase in the mesoporosity by widening the micropores and a decrease in the acid density. The MTA catalyst lifetime is increased by several times due to the enhanced mesoporosity and decreased acid density. The deactivated catalysts were characterized by thermogravimetry and N-2 adsorption/desorption experiments. More coke forms inside the micropores than on the external surfaces of the catalysts. Generalized and quantitative correlations between the mesoporosity and coke content and between the number of acid sites and coke content are given. (C) 2016 Elsevier Inc. All rights reserved.