Fabrication of a core–shell MFI@TON material and its enhanced catalytic performance for toluene alkylation†
Core–shell MFI@TON composites were designed and synthesized as a highly shape-selective catalyst for toluene alkylation with methanol by passivating the nonselective acid sites and tuning the diffusion behavior. The synthesis parameters were comprehensively investigated, indicating the importance of the Si/Al ratio compatibility of the ZSM-5 and ZSM-22 components on the formation of a core–shell structure. The synthesis process was systemically traced, which allowed the formulation of a crystallization mechanism involving the oriented crystal growth and selective fusion steps during the secondary crystallization. As a result, the MFI zeolites as the core were fully covered by the TON zeolites as the shell, yielding spherical morphology. When applied to toluene alkylation with methanol, the core–shell MFI@TON composite exhibited significantly improved para-xylene selectivity in comparison with the original, unattached, and physically mixed catalysts. The enhanced catalytic behaviors of the core–shell MFI@TON composite could be ascribed to the effective suppression of para-xylene isomerization as a result of the passivated acid sites on the external surface and the improved diffusion time and distance for the intermediates inside the channels due to the unique structure. The synthesis method for the MFI@TON composite described herein may provide a generic platform for the design of core–shell zeolites with potentially broader applicability to other porous materials with advanced applications.