Where Are the Active Sites in Zeolites? Origin of Aluminum Zoning in ZSM-5

We visualized the aluminum zoning in TPA + -templated crystals of ZSM-5 by EDX mapping of cross sections obtained by milling of the crystal with a focused ion beam. Aluminum was preferentially located in the rim, while silicon and oxygen were well distributed. Because of its limited penetration, EDX of the large uncut crystals failed to detect the enrichment. Desilication and dealumination by base and acid leaching supported the results of FIB/EDX mapping, showing that the crystals have a dissolved interior or etched rims. To understand the origin of the heterogeneous distribution of aluminum within the crystal, the location of aluminum and silicon after the synthesis of crystals of TPA + -templated ZSM-5 was determined by analyzing the crystals at different stages of synthesis. The crystals had different sizes, and all showed aluminum zoning. The catalytically active sites are located almost exclusively at the rim of the zeolite crystals. These results have improved our understanding of the mechanism of aluminum incorporation into the ZSM-5 framework. Our results may contribute to the controlled locating of active sites within zeolite crystals.