Aluminum fluoride (AlF3)-modified HZSM-5 obtained by simple mechanical mixing and calcination was used to enhance the conversion of the lignite pyrolysis volatiles to light aromatics. The results revealed that the appropriate amount of AlF3 modification can simultaneously dealuminize and aluminize to enhance the catalytic performance, in which dealumination considerably enlarged the pores while realumination generated more midstrength acid sites to compensate for the loss of acid sites caused by dealumination. HZ-2 (HZSM-5 mixed with 2 wt % AlF3) obtained the maximum light aromatics yield of 31.0 mg/g as a result of its abundant mesopores and suitable midstrength acid sites, especially the yield of benzene increased by 6.3 mg/g in comparison with that of the original HZSM-5. However, mixing with only a small amount of AlF3 generated the most serious carbon depositions because of the acidity enhancement by the critical role of alumination. Moreover, excessive AlF3 etched more framework Al that dramatically reduced the midstrength acid sites, inhibiting the formation of light aromatics, although the enlargement of pore size facilitated mass transfer. This one-step modification combining the expansion of the pore size and the optimization of acid sites provides a promising strategy for upgrading the lignite pyrolysis volatiles.

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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.

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