(376r) A Combined Seeding Approach for High-Flux Zeolitic-Imidazolate Framework ZIF-67 Membranes for Olefin/Paraffin Separation

Jingze Sun¹, Chen Yu¹, Kumar Varoon Agrawal², and Hae-Kwon Jeong*^1,3

¹Artie McFerrin Department of Chemical Engineering and ³Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843-3122

²Laboratory of Advanced Separations (LAS), École Polytechnique Fédérale de Lausanne (EPFL) CH-1951, Sion, Swizerland

* Corresponding author: hjeong7@tamu.edu

Having been extensively researched as potential membrane materials, zeolite-imidazole frameworks (ZIFs) are famous for their robust synthesis protocols, chemical and thermal stabilities, and well-defined ultra-micropores (~ 2 to 5 Å).^{1, 2} Composed of zinc nodes and 2-methylimidazole linkers, ZIF-8 has been shown very promising for propylene/propane separation, because of its effective aperture size lying between that of propylene and propane..^{3, 4} Due to its slightly smaller aperture, ZIF-67 (Co-substituted ZIF-8)^{5, 6} seems promising for improving propylene/propane separation performances as compared with ZIF-8. Previous works from our group for ZIF-8/ZIF-67/ZIF-8 multi-layer membranes has exhibited propylene/propane separation factors as high as 209.⁷ Unfortunately, no ZIF-67 membranes have been reported primarily due to the transformation of ZIF-67 seed crystals into a double hydroxide phase upon the secondary growth.

Here, we are glad to present the first phase-pure ZIF-67 membranes as thin as ~ 300 nm. The resulting ultrathin ZIF-67 membranes resulted from microwave-assisted (MW) seeding followed by Electrophoretic Nuclei Assembly (ENACT), by which high-quality densely-packed ZIF-8 seed layers prepared.⁸ Much increased propylene permeances were observed from these ultra-thin ZIF-67 membranes as compared to ZIF-8 membranes. To obtain phase-pure ZIF-67 membranes, it was found critical 1) to have densely-packed ZIF-67 seed layers by combining MW and ENACT seeding and 2) to lower secondary growth temperature. This MW/ENACT seeding technique was demonstrated effective for ZIF-8 and ZIF-90 membranes on alumina supports as well.

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