Ultra-selective high-flux membranes from directly synthesized zeolite nanosheets

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Abstract

A direct synthesis of high-aspect-ratio microporous zeolite nanosheets and the use of such nanosheets in separation membranes are described.

A zeolite with structure type MFI1,2 is an aluminosilicate or silicate material that has a three-dimensionally connected pore network, which enables molecular recognition in the size range 0.5-0.6 nm. These micropore dimensions are relevant for many valuable chemical intermediates, and therefore MFI-type zeolites are widely used in the chemical industry as selective catalysts or adsorbents3,4,5. As with all zeolites, strategies to tailor them for specific applications include controlling their crystal size and shape5,6,7,8. Nanometre-thick MFI crystals (nanosheets) have been introduced in pillared9 and self-pillared (intergrown)10 architectures, offering improved mass-transfer characteristics for certain adsorption and catalysis applications11,12,13,14. Moreover, single (non-intergrown and non-layered) nanosheets have been used to prepare thin membranes15,16 that could be used to improve the energy efficiency of separation processes17. However, until now, single MFI nanosheets have been prepared using a multi-step approach based on the exfoliation of layered MFI9,15, followed by centrifugation to remove non-exfoliated particles18. This top-down method is time-consuming, costly and low-yield and it produces fragmented nanosheets with submicrometre lateral dimensions. Alternatively, direct (bottom-up) synthesis could produce high-aspect-ratio zeolite nanosheets, with improved yield and at lower cost. Here we use a nanocrystal-seeded growth method triggered by a single rotational intergrowth to synthesize high-aspect-ratio MFI nanosheets with a thickness of 5 nanometres (2.5 unit cells). These high-aspect-ratio nanosheets allow the fabrication of thin and defect-free coatings that effectively cover porous substrates. These coatings can be intergrown to produce high-flux and ultra-selective MFI membranes that compare favourably with other MFI membranes prepared from existing MFI materials (such as exfoliated nanosheets or nanocrystals).

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