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Zeolite MFI is a widely used catalyst and adsorbent that also holds promise as a thin-film membrane. The discovery of nanometre-thick two-dimensional (2D) MFI nanosheets has facilitated methods for thin-film zeolite fabrication that open new horizons for membrane science and engineering. However, the crystal structure of 2D-MFI nanosheets and their relationship to separation performance remain elusive. Using transmission electron microscopy, we find that one- to few-unit-cell-wide intergrowths of zeolite MEL exist within 2D-MFI. We identify the planar distribution of these 1D or near-1D-MEL domains, and show that a fraction of nanosheets have high (~25% by volume) MEL content while the majority of nanosheets are MEL-free. Atomistic simulations show that commensurate knitting of 1D-MEL within 2D-MFI creates more rigid and highly selective pores compared to pristine MFI nanosheets, and permeation experiments show a separation factor of 60 using an industrially relevant (undiluted 1 bar xylene mixture) feed. Confined growth in graphite is shown to increase the MEL content in MFI nanosheets. Our observation of these intergrowths suggests strategies for the development of ultra-selective zeolite membranes.
Bibliographical noteFunding Information:
This work was primarily supported by the National Science Foundation (CBET-1705687). XRD was performed at Argonne National Laboratory supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC and NNIN programs (DMR-1420013). The FPMD simulations used resources of the Argonne Leadership Computing Facility, which is a DOE Office of Science User Facility supported under contract no. DE-AC02-06CH11357. Additional computer resources were provided by the Minnesota Supercomputing Institute. P.K. and E.O.F. acknowledge support from a Doctoral Dissertation Fellowship received from the Graduate School at the University of Minnesota. H.X. and T.D. acknowledge support from NSF 1332228. Q.X. acknowledges support from the National Natural Science Foundation of China (21471131).
© 2020, The Author(s), under exclusive licence to Springer Nature Limited.
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