Molecular sieve membrane-based separation is a potentially energy-efficient alternative to conventional separation processes (e.g., distillation). In particular, the all silica zeolite with structure type MFI has been of interest, as, in addition to its superior thermal and chemical stability, its pore dimensions are close to the sizes of many valuable chemicals. The fabrication of high-performance zeolite MFI separation membranes requires control of microstructure that can be achieved by the secondary growth of a MFI nanosheet coating.
We have developed a direct synthesis method, which readily provides MFI nanosheets with increased lateral dimensions (~2 μm) and similar nano-scale thicknesses (5 nm) compared to the prior stste-of-the-art.2 This was enabled by seeded growth with bis-1,5(tripropyl ammonium) pentamethylene iodide (denoted as dC5) as a structure-directing agent. This dC5 SDA was reported to be able to yield high-aspect-ratio MFI crystals with thin dimensions along their b-axes.3 However, the formation of rotational intergrowths prohibits the formation of high aspect ratio flat nanosheets.4 We utilized a seeded growth method to suppress the rotational intergrowths in MFI crystals and to grow the MFI crystal in a single nanosheet morphology. Time-dependent growth investigations with TEM established that a nanosheet forms from a corner of the seed crystal and encircles the seed crystal. A single rotational intergrowth appears to trigger a morphology transition from a cylinder to a nanosheet. No additional rotational intergrowth was observed, allowing to achieve high-aspect-ratio MFI nanosheets with ~2-μm lateral dimensions. Upon the fabrication of seed coating with a filtration method, followed by a gel-free secondary growth, the membranes exhibit unprecedented combination of high p-xylene permeance (~5×10-7 mol Pa-1 s-1 m-2) and separation factor (2,000) for xylene isomer separation.2 The separation factor of the membrane was further improved (>10,000), when the membrane was fabricated from dense and uniform nanosheet coatings prepared from the monolayer transfer method.5
(1) Varoon et al., Science 2011, 334, 72–75.
(2) Jeon et al., Nature 2017, 543, 690–694.
(3) Bonilla et al., Chem. Mater. 2004, 16, 5697–5705.
(4) Chaikittisilp et al., Angew. Chem. Int. Ed. 2013, 52, 3355–3359.
(5) Kim et al., under review.