Abstract
Zeolites—microporous crystalline aluminosilicates—are widely used in petrochemistry and fine-chemical synthesis1,2,3 because strong acid sites within their uniform micropores enable size- and shape-selective catalysis. But the very presence of the micropores, with aperture diameters below 1 nm, often goes hand-in-hand with diffusion limitations3,4,5 that adversely affect catalytic activity. The problem can be overcome by reducing the thickness of the zeolite crystals, which reduces diffusion path lengths and thus improves molecular diffusion4,5. This has been realized by synthesizing zeolite nanocrystals6, by exfoliating layered zeolites7,8,9, and by introducing mesopores in the microporous material through templating strategies10,11,12,13,14,15,16,17 or demetallation processes18,19,20,21,22. But except for the exfoliation, none of these strategies has produced ‘ultrathin’ zeolites with thicknesses below 5 nm. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on the mesoporous and microporous length scales simultaneously and thus yield MFI (ZSM-5, one of the most important catalysts in the petrochemical industry) zeolite nanosheets that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The large number of acid sites on the external surface of these zeolites renders them highly active for the catalytic conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-to-gasoline conversion. We expect that our synthesis approach could be applied to other zeolites to improve their performance in a range of important catalytic applications.
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Acknowledgements
This work was mainly supported by the National Honor Scientist Program of the Ministry of Education, Science and Technology in Korea. Electron microscopic studies were performed with help from the measurement and analysis team at the National NanoFab Center, from the research supporting team at KAIST, and also at the Electron Microscopy Center (EMC) at Stockholm University with support from the Knut and Alice Wallenberg Foundation. Y.S. and O.T. thank the Swedish Research Council (VR) and the Japan Science and Technology Agency (JST), respectively.
Author Contributions R.R. initiated and managed the whole project, including the intuitive design of the structure-directing agents. M.C. and K.N. did synthesis and characterization (SEM, TEM, X-ray diffraction and adsorption). K.N. synthesized the multilamellar zeolite, and M.C. synthesized the unilamellar zeolite later. J.K. performed the catalytic testing. Y.S. and O.T. did high-resolution SEM and TEM investigations.
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Choi, M., Na, K., Kim, J. et al. Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts. Nature 461, 246–249 (2009). https://doi.org/10.1038/nature08288
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DOI: https://doi.org/10.1038/nature08288
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