Abstract
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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Acknowledgements
This work was supported by the ARPA-E programme of the US Department of Energy under Award DE-AR0000338 (0670-3240) for MFI nanosheet synthesis and characterization; by the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0001015 for membrane fabrication and permeation testing; by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences under Award DEFG02-12ER16362 for the theoretical calculations; and by the Deanship of Scientific Research at the King Abdulaziz University D-003/433 for zeolite and membrane microstructure characterization. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC programme. SEM measurements were partially performed on a Hitachi 8230 provided by NSF MRI DMR-1229263. This research used the resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357. For the theoretical calculations we used the resources of the Minnesota Supercomputing Institute and of the Argonne Leadership Computing Facility (ALCF) at Argonne National Laboratory, which is supported by the Office of Science of the Department of Energy under contract DE-AC02-06CH11357.
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Contributions
P.S.L. performed the initial experiments and with M.T. conceived the project. M.Y.J. performed extensive synthesis optimization and time-dependent growth experiments. D.K. further improved nanosheet uniformity. M.Y.J. performed the initial TEM experiments; P.K. performed all the TEM experiments described in the Letter and with K.A.M. and M.T. analysed and interpreted the data. K.N., S.N.B., S.A.-T., M.S., M.Y.J., D.K., M.T., H.S.L. and W.X. performed the XRD measurements and other microstructural characterization including TGA, Ar porosimetry, AFM and SEM and analysed and interpreted the data. M.Y.J. and D.K. fabricated the membranes. B.E. and N.R. contributed to membrane preparation. M.Y.J., D.K., N.R. and B.E. evaluated membrane separation performance. P.B., E.O.F. and J.I.S. performed and analysed the theoretical calculations. R.T. and R.F.DJ. contributed to the theoretical estimation of membrane performance. H.J.C., P.S.L. and W.F. contributed to seed synthesis. D.K., P.K., M.Y.J. and M.T. prepared the manuscript with written contributions from all co-authors. M.T. directed all aspects of the project.
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Extended data figures and tables
Extended Data Figure 1 Structure of dC5 SDA and corresponding MFI/MEL intergrowth, and structure of TPAOH SDA and corresponding TPA-MFI seeds.
a, Structure of bis-1,5(tripropyl ammonium) pentamethylene diiodide (dC5). b, An SEM image of non-seeded growth of MFI nanosheets conducted with a precursor composition identical to that used for seeded nanosheet growth and a reaction time of 12 days. c, Schematic illustration of twining via MFI/MEL intergrowth. d, Structure of pentasil chain and projections of MEL and MFI structures along the c axes (yellow and red represent silicon and oxygen atoms, respectively). e, Structure of tetrapropylammonium hydroxide. f, HAADF-STEM image of MFI seed crystals with a distribution of projected areas (right top) and a schematic illustration of the crystal (right bottom). The average linear dimension of crystals, based on the projected area, is calculated to be about 30 nm.
Extended Data Figure 2 Crystallographic orientation and internal structure of TPA MFI seeds after secondary growth with dC5 (before nanosheet initiation).
a–f, BF-TEM images and the corresponding fast Fourier transforms of a seed after secondary growth with dC5 along the [100] direction (a, d); the [010] direction (b, e); and the [001] direction (c, f). Overlaid in yellow is the schematic of the seed crystal after secondary growth with dC5, where the longest dimension is along the b axis or [010] direction. g, High-resolution BF-TEM image (left panel) of grown seed (before the emergence of nanosheet) shown in Fig. 1c along the c axis. The coloured image (right panel) corresponds to a digitally processed HR-TEM image using a Sobel filter, followed by thresholding and binary colouring to highlight different fringe patterns in the HR-TEM image. Similarly coloured regions correspond to areas with the same crystallographic orientation/thickness. h, Demonstration of the procedure for obtaining dark-field images of the grown seeds. A BF-TEM image of a seed and its corresponding diffraction pattern are acquired. The bright diffraction spot is selected by placing the objective aperture in the diffraction plane. The dark-field TEM image formed using the selected spot shows domains of different crystallographic orientation/thickness within the seed as seen in g. i, Different seeds (before the emergence of nanosheets) imaged in dark-field mode using the method shown in h reveal regions of different crystallographic orientation/thickness within the seed.
Extended Data Figure 3 Thickness variation in nucleated nanosheet.
Panels I–IV show HAADF-STEM images of partially grown sheet from the seed. Marked in green, yellow and red are the carbon background, and the thin and thick regions of the nanosheet, respectively. The average intensity of these marked regions is tabulated. The ratio of intensity from the thick and thin regions after carbon background (from carbon support of the TEM grid) subtraction matches the corresponding ratio of either 5/4 or 5/3 pentasil chains.
Extended Data Figure 4 Partial wrap of sheets around seeds.
a, b, Two representative BF-TEM images of partial wraparound of sheets around seeds. The thicker seeds appear darker and the thinner large-area nanosheets appear lighter. Overlaid is a schematic of the partial wraparound of the sheet (yellow) around the seed (red). c, BF-TEM image of partial wrap of b-axis-oriented sheet around the a-axis-oriented seed shown in Fig. 1g. The edges of the growing diamond-shaped sheet are terminated by (101) crystallographic planes. Overlaid in colour are the crystallographic planes forming different faces of the nanosheet. d, Fast Fourier transform of HR-TEM image of the sheet in c, confirming the crystallographic directions marked in c. e, Unit cell of the MFI crystal illustrating the atomic connectivity of planes forming the edges of the sheet shown in c.
Extended Data Figure 5 Thickening of MFI nanosheets by epitaxial island nucleation.
a, b, SEM images of MFI nanosheets synthesized at 140 °C for 4 days. Additional layers formed continuously on top of the primary nanosheet, resulting in thickening of the central part. The darker contrast at the centre is attributed to charging caused by absence of good contact with support. Both scale bars represent 500 nm. c, Multislice simulated BF-TEM image (accelerating voltage Vo = 300 kV, spherical aberration coefficient Cs = 2 mm, defocus df = 100 nm) of MFI crystal with increasing thickness (left to right) in the b-axis direction. The MFI crystal shown here has a slight mis-tilt (1.7° about the [101] axis) from the [010] zone axis to match experimental imaging conditions. The effect of a small mis-tilt is amplified in the HR-TEM image owing to an increase in thickness from left to right. Regions with 0–10 nm thickness demonstrate a typical [010] zone axis pattern while those of 20–70 nm thickness exhibit complex HR-TEM image patterns. d, BF-TEM images of a thick island growing epitaxially on top of the nanosheet. Images taken at defocus ∆f and ∆f + 33 nm represent imaging conditions with the nanosheet in focus (note the clear appearance of the MFI structure in the background) and the thick island in focus, respectively. e, Comparison of two different image sections, one taken from within the thick island HR-TEM image in d and the other from multislice simulations in c, shows the presence of regions with thickness 10–40 nm varying along the b-axis direction. f, Model of a thick island with steps 6–10 nm in height along the b-direction on a 5-nm-thick nanosheet. The corresponding multislice simulation (Vo = 300 kV, Cs = 2 mm, df = 100 nm) of the MFI crystal model shows a HR-TEM pattern similar to those observed in d, thus confirming the epitaxial growth of thick regions on a thin MFI nanosheet.
Extended Data Figure 6 Crystallographic relationship between seed and nanosheet orientation.
a, BF-TEM imaging of partial wrap of nanosheet around the seed. The white circle marked as region (I) encloses the seed, and the corresponding diffraction pattern confirms the orientation of the seed along the a axis. Regions marked as (II), (III), (IV) and (V) enclose the nanosheet and the corresponding diffraction patterns confirm the b-axis orientation of the nanosheet. b, Bragg-filtered HR-TEM image of the MFI nanosheet showing the typical [010] zone-axis pattern. c, d, HR-TEM images of the seed oriented along [100] zone-axis taken at under-focus (c) and over-focus (d) imaging conditions. The data support the idea that the seed and sheet are connected by an a/b twin crystallographic relationship.
Extended Data Figure 7 Powder XRD data and analysis (top), and electronic structure calculation (bottom) for strain in dC5 MFI nanosheets.
XRD patterns for the MFI nanosheet (i) and seed crystals after (ii) and before (iii) growth with dC5. The initial seed crystal (iii) was prepared with TPAOH, resulting in a conventional MFI structure. Seed crystals after growth with dC5 (ii) and MFI nanosheets (i) exhibit a- and c-axis lattice parameters comparable to those of the seed crystal (iii), which indicates that dC5 yields non-strained MFI materials (at the resolution of the current experiment). a–f, Model MFI/SDA systems obtained from density functional theory calculations. The membrane framework consists of 1 × 3 × 1 unit cells of the MFI structure, with the two {010} surfaces truncated and 30 Å of vacuum space (not shown in figure) added to the b-axis direction. The empty framework (a) therefore contains five layers of pentasil chains. In addition, up to six dC5 SDA cations per simulation cell were considered: one (b) or two (c) dC5 molecules located in the straight channels and sandwiching the centre pentasil layer; four dC5 molecules, either all located in the straight channels sandwiching the second and fourth pentasil layers (d), or with two in the straight channels as in c and the other two in the zigzag channels (e); and six dC5 molecules saturating the straight channels with four of them partially extending beyond the outer surfaces (f). Geometry optimizations were performed first using an empirical force field and then via density functional theory calculations. The branching sites in dC5 molecules can only be accommodated at channel intersections, whose centre-to-centre separations are about 10 Å, while the distance between the two N atoms of dC5 is roughly 7.6 Å in the all-trans conformation of the pentamethylene chain. Despite this incompatibility of length scales and the substantial amount of strain present in adsorbed SDA molecules, relatively minor energetic differences were found for the arrangement of SDA molecules in the straight versus zigzag channels; for example, e is higher in energy than d by less than 0.2 kJ per mole of SiO2 units. Furthermore, the strain affects SDA molecules more than the inorganic framework itself, and consequently, the a and c lattice parameters lengthen by only 0.1%–0.4%, while the b lattice parameter shrinks by only 0.02%–0.24%.
Extended Data Figure 8 Grain-size comparison of MFI membranes prepared from different seeds.
a–c, SEM top-view images for MFI membranes prepared by secondary growth of MFI nanocrystals (a), exfoliation-based MFI nanosheets (b), and dC5 MFI nanosheets (c). Lateral dimensions of MFI grains of the membrane fabricated from dC5 MFI nanosheets are much larger than those fabricated from MFI nanocrystals and exfoliation-based MFI nanosheets. All scale bars indicate 500 nm.
Extended Data Figure 9 TEM and electron diffraction of MFI nanosheet membrane after gel-free secondary growth.
a, b, Low-magnification SEM (a) and BF-TEM images (b) of focused-ion-beam milled section. c, Schematic of the area highlighted in red in b. The spherical particles are Stöber silica particles coated onto the porous support. The edge of a silica fibre from the main body of the porous support is also visible. The intergrown membrane layer does not exceed 1 μm. d, High-magnification BF-TEM image of highlighted area shown in b. Plate-like intergrown grains with thickness of 100–200 nm form a top layer of about 200 nm, followed by a less intergrown deposit that penetrates the Stöber silica layer. e, f, BF-TEM image (e) of highlighted area shown in b tilted near to the < 101 > zone axis of a nanosheet grain with the corresponding electron diffraction pattern (f). Scale bars in a and b are 2 μm, in d and e are 400 nm and in f is 1 nm−1.
Extended Data Figure 10 Xylene isomer separation performance.
a, Potentials of mean force for p-xylene and o-xylene along the straight channel, W(ξ), were obtained from FP molecular dynamics simulations for a 3-nm MFI film at T = 573 K and from FF molecular dynamics simulations for bulk MFI at T = 423 K. Based on the potentials of mean force, the p-xylene/o-xylene self-diffusion coefficient ratios are estimated as 1.2 × 104 (FF molecular dynamics; ΔW ≈ 33 kJ mol−1) and 4 × 104 (FP molecular dynamics; ΔW ≈ 50 kJ mol−1). FF-based Monte Carlo simulations for bulk MFI and 5-nm film indicate that adsorption of o-xylene is slightly favoured, but the adsorption selectivity is less than 2 at low loading. b, Comparison of p-xylene/o-xylene separation performances of the membrane reported here and other reports. Thin (<1 μm) MFI membranes have been reported to exhibit high permeances15,26,27,28,29,49,50,51,52,53, and our membranes show remarkable combinations of high permeances and high separation factors (S.F.), which were enabled by use of high-aspect-ratio dC5 MFI nanosheets (1–2 μm lateral dimensions and 5 nm thicknesses) as seeds. c, Representative permeation data with p- and o-xylene partial pressures in feed and permeate. Feed is an approximately equimolar mixture of p- and o-xylene in a carrier gas (helium), and permeate is a stream composed of p-xylene and o-xylene passing through the membrane and a carrier gas (helium). The second 150 °C measurement was performed after 39 days of testing (from when the first 150 °C measurement was performed). Tests in between these two measurements included single (p-xylene), binary (p- and o-xylene) and multicomponent (p-, o-, m-xylene, TMB, EB mixture) measurements and temperature cycling from 150 °C to about 50 °C twice. p/psat was calculated using psat for p-x: 4,328 Pa at 50 °C and 136,424 Pa at 150 °C and for o-x: 3,375 Pa at 50 °C and 116,768 Pa at 150 °C; psat is the saturated vapour pressure. d, p-xylene separation performance measured for a multi-component mixture (p-x: p-xylene, o-x: o-xylene, m-x: m-xylene, EB: ethylbenzene, TMB: trimethylbenzene).
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Jeon, M., Kim, D., Kumar, P. et al. Ultra-selective high-flux membranes from directly synthesized zeolite nanosheets. Nature 543, 690–694 (2017). https://doi.org/10.1038/nature21421
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DOI: https://doi.org/10.1038/nature21421
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