610959 Multi-Scale Detection and Implication of 1D MEL Defects in 2D MFI Zeolite Nanosheets

Friday, November 20, 2020
Materials Engineering and Sciences Division (08) (PreRecorded+)
Prashant Kumar1, Neel Rangnekar2, Hao Xu3, Evgenii Fetisov4, J. Ilja Siepmann5, Traian Dumitrica6, Michael Tsapatsis7 and K. Andre Mkhoyan1, (1)Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, (2)ExxonMobil, (3)University of Minnesota, Minneapolis, MN, (4)Pacific Northwest National Lab, Richland, WA, (5)Department of Chemistry and Chemical Theory Center, University of Minnesota, Twin Cities, Minneapolis, MN, (6)Mechanical Engineering, University of Minnesota, Minneapolis, MN, (7)Department of Chemical Engineering and Materials Science, University of Minnesota, Twin Cities, Minneapolis, MN

The zeolite MFI is a widely used catalyst and adsorbent which also holds promise as a thin film membrane for the separation of hydrocarbon isomers and other difficult to separate mixtures. The discovery of nm-thick 2-dimensional (2D) MFI nanosheets has enabled methods for thin film zeolite fabrication that open new horizons for membrane science and engineering [1,2]. However, the crystal structure of 2D-MFI nanosheets at atomic-scale and its relationship to separation performance at macro-scale have remained elusive.

Herein, using aberration-corrected transmission electron microscopy, we find that one- to few-unit-cells wide intergrowths of the zeolite MEL exist within 2D-MFI. The symmetry of MFI framework along the a-direction is broken by insertions of near-single-unit-cell domains of MEL. Because the MEL insertions are extended only along the c-direction, we call them 1-D or near 1-D-MEL. Fast Fourier transforms (FFT) of images with such MEL insertions show elongated spots along the a*-direction as compared to a typical MFI spot pattern. We identify the planar distribution of these 1-dimensional (1D) or near-1D-MEL domains using custom-built computational pattern matching techniques. Furthermore, we show that a fraction of individual nanosheets have high (ca. 25% by volume) MEL content while the majority of nanosheets are MEL-free through quantitative electron- and x-ray diffraction analysis.

Atomistic simulations suggest that atomic scale knitting of 1D-MEL within 2D-MFI creates more rigid and highly selective pores as compared to those in pristine MFI nanosheets. These enhanced properties of nanosheets are translated to MFI-MEL membranes which show an unprecedented separation factor of 60 using an industrially relevant (undiluted 1 bar xylene mixture) feed [3].

References

[1] M. Choi et al, Nature 461, (2009), 246-249.

[2] K. Varoon et al, Science 334, (2011), 72-75.

[3] P. Kumar et al, Nature Materials 19, (2020), 443-449.


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