Knitting Patterns of Pentasil Chains reveal MFI-MEL Heterostructures in Two-Dimensional 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. However, the crystal structure of 2D-MFI nanosheets and its relationship to separation performance remain elusive. Using transmission electron microscopy, we find that one- to few-unit-cells wide intergrowths of the zeolite MEL exist within 2D-MFI. We identify the planar distribution of these 1-dimensional (1D) or near-1D-MEL domains, and show that a fraction of individual nanosheets have high MEL content while the majority of nanosheets are MEL-free. Atomistic simulations suggest that commensurate knitting of 1D-MEL within 2D-MFI creates more rigid and highly selective pores as compared to those in pristine MFI nanosheets and permeation experiments show an unprecedented separation factor of 60 using an industrially relevant (undiluted 1 bar xylene mixture) feed.