(660a) Multi-Scale Detection and Implication of 1D MEL Defects in 2D MFI Zeolite Nanosheets

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.