Creation of Hollow Zeolite Architectures by Controlled Desilication of Al-Zoned ZSM-5 Crystals

Abstract

Zeolites are crystalline aluminosilicates that possess an exceptional combination of properties such as high thermal stability, Brønsted acidity, and microporosity. Accordingly, these materials are frequently practiced in industrial processes, for example, fluid catalytic cracking (Y), cumene production (mordenite), and toluene disproportionation and xylene isomerization .The presence of regular micropores of molecular dimensions is responsible for the zeolites' unequaled shape selectivity in catalytic conversions and separations, but this feature also poses diffusional limitations due to the restricted transport of molecules to and from the active sites. As a consequence, generation of mesoporosity in microporous crystals is attracting significant research interest. 1,2 The created mesopores facilitate physical transport and should lead to a more effective use of these materials. Desilication, that is, extraction of framework Si, is an efficient methodology to create intracrystalline mesoporosity in MFI-type zeolites. 3,4 The Si extraction and related mesopore formation is controlled by framework Al, and an optimal bulk Si/Al ratio of 25-50 has been identified. 5 A high framework Al concentration prevents Si extraction, while a low Al concentration leads to excessive extraction and formation of large pores. However, how the porosity is distributed throughout the crystals and the effect of crystal size is practically ignored. This information is indispensable to rationally devise potential applications of alkaline-treated zeolites.The occurrence of Al zoning, that is, an Al-rich external surface as compared to the bulk aluminum concentration, is well-known in large ZSM-5 crystals. 6,7 Accordingly, it can be anticipated that this Al gradient will affect the Si extraction throughout the zeolite particle. Dessau et al. 8 have observed the nonuniform dissolution of large ZSM-5 crystals (5-10 µm) upon prolonged treatment at high concentrations of alkaline solution. This was speculatively attributed to the nonuniform concentration of Al in the crystals. However, a systematic study on the impact of Al gradients and the effect of crystal size to tailor the porosity development under optimized treatment conditions has not yet been accomplished.Here we show that by controlled desilication of both large and small ZSM-5 crystals we have been able to address and make use of the impact of Al-zoning on the mesoporosity development. The presented approach provides a thus-far unused method to tailor the fabrication of hollow zeolite crystals, offering clear advantages to other methodologies pursuing hollow zeolite entities. 9,10 Our results are of fundamental interest in materials science and may open auspicious uses of alkaline-treated zoned ZSM-5 zeolites.Two procedures were applied to synthesize ZSM-5 zeolite with small (SZ) and large (LZ) crystals in the optimal bulk Si/Al range of 25-50. The alkaline treatment was performed as described in the Supporting Information. The chemical composition and textural properties of the n...

Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

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Multimodal Zr-Silicalite-1 zeolite nanocrystal aggregates with interconnected hierarchically micro-meso-macroporous architecture and enhanced mass transport property

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