Facile creation of hierarchical nano-sized ZSM-5 with a large external surface area via desilication–recrystallization of silicalite-1 for conversion of methanol to hydrocarbons

Abstract

Diffusion limitations caused by micropores of ZSM-5 result in its deactivation in the methanol to hydrocarbons reaction. Introducing mesopores could shorten the length of micropores and increase the external surface area, which could reduce coke formation in the micropores and increase coke capacity. In this work, mesopores and acid sites were fabricated synchronously for ZSM-5 through the desilication–recrystallization process of silicalite-1 and a series of nano-sized ZSM-5 zeolites with large external surface areas of over 200 m² g⁻¹ were obtained, which were much higher than previously reported ones. Based on the precise regulation of hydrothermal synthesis time, it was found that desilication and recrystallization occurred simultaneously and their different levels determined the pore structure and acidity. The ²⁷Al MAS NMR results revealed that five-coordinated Al and octahedral coordinated extra-framework Al convert into tetrahedral coordinated framework Al during the desilication–recrystallization process. The catalytic studies indicated that the lifetime and product selectivity were dependent on the pore structure and the acidic properties of the catalysts. The external surface area of the ZSM-5 prepared by 24 h treatment was 206 m² g⁻¹ and the acidity was 0.58 mmol g⁻¹ which resulted in a longer lifetime of 164 h and a high aromatics selectivity of 74.5%. Interestingly, the regenerated catalyst exhibited a 2.5-fold longer lifetime as compared to the fresh catalyst, which was ascribed to the maintained large external surface area and the decreased acid sites after the reaction and regeneration. This excellent regeneration performance was rarely reported. This work would provide an excellent example for preparation of highly stable ZSM-5 based catalysts for some diffusion-limited reactions.