Simultaneous Enhancement of Aromatic Products and Catalytic Lifetime for Catalytic Re-forming of Lignite Pyrolysis Volatiles over a Self-Assembled Hierarchical Core–Shell ZSM-5@Silicalite-1 Zeolite

Catalytic re-forming of lignite pyrolysis volatiles to aromatics over zeolites is a promising method for value-added utilization of coal tar. However, to achieve a significant enhancement in both the yields of light aromatics and the catalytic lifetime by adjusting the structure of ZSM-5 zeolite and to understand its catalytic mechanism comprehensively are still great challenges. Herein, a self-assembled hierarchical core–shell ZSM-5@silicalite-1 with controlled mesopores and acidity as well as an ultrathin nonacidic protective shell was synthesized through a mild hydrothermal method of alkaline treatment with tetrapropylammonium hydroxide or tetraethylammonium hydroxide (TPAOH or TEAOH). Both the number and pore size distribution of mesopores can be regulated by adjusting the hydrothermal treatment time. These generated mesopores not only facilitate the mass transfer of reactants and products but also improve the utilization efficiency of the acid site. In addition, the newly fabricated nonacidic shell, which coats on the surface of ZSM-5 as a chain mail, plays a critical role in suppressing the coke formation by passivation of the acid sites on the external surface. In this work, the pore properties and acid distribution of ZSM-5 zeolite can be regulated synchronously through a desilication–recrystallization process. In addition, enhanced yields of light aromatics and catalytic lifetime were obtained simultaneously. The construction of this hierarchical core–shell zeolite with redistributed acid sites contributes to the highly efficient catalytic upgrading of macromolecular reactants.