Release of full catalytic capacity of desilicated ZSM-5 in MTH reaction: Al migration along mesopore introduction and post engineering

Abstract

Alkali-treatment is an effective method for improving ZSM-5 lifetime in methanol to hydrocarbon (MTH) reaction. In this paper, the effect of alkali-treatment on framework Al migration in ZSM-5 was firstly studied by 27Al 2D MQMAS NMR experiment. The result clearly demonstrates that part of framework Al in ZSM-5 zeolite is migrated into extra-framework position along with the mesopore introduction. Such Al migration is the reason of Lewis acidity formation in zeolite during alkali-treatment. Lewis acidity, together with internal hydroxyl group, partly neutralizes the benefit of mesopore introduction in MTH reaction. Post-treatment of mesoporous ZSM-5 sample in FeF3/NH4HF2 can selectively diminish the Lewis acidity and internal hydroxyl group. The improvement of catalyst lifetime by such post-treatment clearly proves the negative effects of Lewis acidity and internal hydroxyl group in MTH reaction, and the benefits of mesopore in MTH reaction is potentially overlooked.