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

A direct synthesis of high-aspect-ratio microporous zeolite nanosheets and the use of such nanosheets in separation membranes are described. Ultra-slim zeolite membranes Zeolites—naturally occurring porous crystalline aluminosilicates that are also produced industrially on a large scale—are used commercially as selective adsorbents. Zeolite membranes can be used for selective dehydration, but more general separations, for example of hydrocarbon isomers, are challenging because they require thin membranes with highly oriented pores. At present, such thin membranes are produced by an expensive and low-yielding exfoliation process. Here the authors produce nanometre-thick zeolite nanosheets using a bottom-up seeded growth method that retains the pore structure and avoids rotational intergrowths. Using xylene isomer separation as a benchmark, the authors found that their compact membranes had higher selectivities and flux rates than previous zeolite membranes. A zeolite with structure type MFI 1 , 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 adsorbents 3 , 4 , 5 . As with all zeolites, strategies to tailor them for specific applications include controlling their crystal size and shape 5 , 6 , 7 , 8 . Nanometre-thick MFI crystals (nanosheets) have been introduced in pillared 9 and self-pillared (intergrown) 10 architectures, offering improved mass-transfer characteristics for certain adsorption and catalysis applications 11 , 12 , 13 , 14 . Moreover, single (non-intergrown and non-layered) nanosheets have been used to prepare thin membranes 15 , 16 that could be used to improve the energy efficiency of separation processes 17 . However, until now, single MFI nanosheets have been prepared using a multi-step approach based on the exfoliation of layered MFI 9 , 15 , followed by centrifugation to remove non-exfoliated particles 18 . 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