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

Abstract

A zeolite with structure type MFI 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. As with all zeolites, strategies to tailor them for specific applications include controlling their crystal size and shape. Nanometre-thick MFI crystals (nanosheets) have been introduced in pillared and self-pillared (intergrown) architectures, offering improved mass-transfer characteristics for certain adsorption and catalysis applications. Moreover, single (non-intergrown and non-layered) nanosheets have been used to prepare thin membranes that could be used to improve the energy efficiency of separation processes. However, until now, single MFI nanosheets have been prepared using a multi-step approach based on the exfoliation of layered MFI, followed by centrifugation to remove non-exfoliated particles. 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).

para‐Xylene Ultra‐selective Zeolite MFI Membranes Fabricated from Nanosheet Monolayers at the Air–Water Interface

“…Following this demonstration, improvements in exfoliation [21] and gel-free secondary growth [22,23] led to better separation performances,while the discovery of non-thermal de-templation by piranha treatment allowed selective membrane formation on porous polymer supports. [28] Membranes prepared by gel-free secondary growth of these nanosheets showed improved performance,c ompared to other membranes,w ith separation factors of p-xylene over o-xylene of 2000-8000. [28] Membranes prepared by gel-free secondary growth of these nanosheets showed improved performance,c ompared to other membranes,w ith separation factors of p-xylene over o-xylene of 2000-8000.…”

“…[28]), the rectangular crystals have an a-out-of-plane orientation. [28] Such thin, continuous,a nd oriented MFI membranes fabricated on porous supports have been shown to serve as excellent molecular sieve membranes. However, these pin-holes do not run through the membrane,a s confirmed by very low N 2 permeance of the as-synthesized membrane (ca.…”

“…[28,29] We hypothesize that these higher separation factors could be achieved by further reducing non-selective paths in the membranes. [28,29] We hypothesize that these higher separation factors could be achieved by further reducing non-selective paths in the membranes.…”

“…[28,29] We hypothesize that these higher separation factors could be achieved by further reducing non-selective paths in the membranes. [28] were fabricated from nanosheet coatings prepared by av acuum-assisted filtration method. [28] were fabricated from nanosheet coatings prepared by av acuum-assisted filtration method.…”

“…[28] This direct synthesis method can provide large-size MFI nanosheets,asitdoes not rely on the exfoliation process that results in the fracture of nanosheets.F igure 1a is ar epresentative SEM image of the MFI faceted nanosheets with approximately 2-mml ength. [28] This direct synthesis method can provide large-size MFI nanosheets,asitdoes not rely on the exfoliation process that results in the fracture of nanosheets.F igure 1a is ar epresentative SEM image of the MFI faceted nanosheets with approximately 2-mml ength.…”