Neel Rangnekar

Oriented MFI membranes by gel-less secondary growth of sub-100 nm MFI-nanosheet seed layers

A zeolite membrane fabrication process combining 2D-zeolite nanosheet seeding and gel-free secondary growth is described. This process produces selective molecular sieve films that are as thin as 100 nm and exhibit record high permeances for xylene- and butane-isomers.

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

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).

Open‐Pore Two‐Dimensional MFI Zeolite Nanosheets for the Fabrication of Hydrocarbon‐Isomer‐Selective Membranes on Porous Polymer Supports

Two-dimensional zeolite nanosheets that do not contain any organic structure-directing agents were prepared from a multilamellar MFI (ML-MFI) zeolite. ML-MFI was first exfoliated by melt compounding and then detemplated by treatment with a mixture of H2 SO4 and H2 O2 (piranha solution). The obtained OSDA-free MFI nanosheets disperse well in water and can be used for coating applications. Deposits made on porous polybenzimidazole (PBI) supports by simple filtration of these suspensions exhibit an n-butane/isobutane selectivity of 5.4, with an n-butane permeance of 3.5×10(-7)  mol m(-2)  s(-1)  Pa(-1) (ca. 1000 GPU).

2D zeolite coatings: Langmuir-Schaefer deposition of 3 nm thick MFI zeolite nanosheets

Stable suspensions of zeolite nanosheets (3 nm thick MFI layers) were prepared in ethanol following acid treatment, which partially removed the associated organic structure-directing agent. Nanosheets from these suspensions could then be dispersed at the air-water interface and transferred to silicon wafers using Langmuir-Schaefer deposition. Using layer-by-layer deposition, control on coating thickness was demonstrated. In-plane X-ray diffraction (XRD) revealed that the deposited nanosheets contract upon calcination similar to bulk MFI crystals. Different methods for secondary growth resulted in preferentially oriented thin films of MFI, which had sub-12-nm thickness in certain cases. Upon calcination, there was no contraction detectable by in-plane XRD, indicating well-intergrown MFI films that are strongly attached to the substrate.

Nanoscale Control of Homoepitaxial Growth on a Two-Dimensional Zeolite

Nanoscale crystal growth control is crucial for tailoring two-dimensional (2D) zeolites (crystallites with thickness less than two unit cells) and thicker zeolite nanosheets for applications in separation membranes and as hierarchical catalysts. However, methods to control zeolite crystal growth with nanometer precision are still in their infancy. Herein, we report solution-based growth conditions leading to anisotropic epitaxial growth of 2D zeolites with rates as low as few nanometers per day. Contributions from misoriented surface nucleation and rotational intergrowths are eliminated. Growth monitoring at the single-unit-cell level reveals novel nanoscale crystal-growth phenomena associated with the lateral size and surface curvature of 2D zeolites.

Structural Rearrangement of 2-D Zeolite Nanosheets under Electron Beam

Two-dimensional (2-D) zeolites and zeolite nanosheets are porous silicate frameworks desirable for catalytic uses involving bulky molecules [1], thin film separation membranes [2], and low-k dielectric materials [3]. Functionality of such zeolites is highly dependent on their crystal structure, thickness and pore dimensions. Low-dose transmission electron microscopy (TEM) studies at an optimum accelerating voltage have proven particularly useful in crystallographic structure determination of these electron beam sensitive materials [4]. However, it is known that zeolites undergo amorphization through radiolysis at low accelerating voltages, as well as both sputtering and amorphization through knock-on at high accelerating voltages [5]. Being inherently destructive, the examination of zeolites in TEM cannot perfectly reveal as-synthesized structure.

Characterization of MEL defects in 2 - Dimensional MFI nanosheets

MFI-type zeolite is a microporous silicon-oxygen framework that has been important for several decades due to its o-/p-xylene separation properties in petroleum industry. In 1980s, significant TEM characterization work was done on large-sized MFI-type zeolite crystals, leading to imaging and electrondiffraction studies of its crystal structure, identification of defects and detection of MEL intergrowth within the MFI framework [1-2]. Since then, development of novel synthesis techniques has enabled the creation of 2-dimensional MFI nanosheets [3-4], while development of aberration corrected TEM has pushed the resolution limit of electron microscopes. Capitalizing on these scientific advancements, we revisit the problem of MEL intergrowth within MFI framework by characterizing this defect at the atomic scale in 2-dimensional nanosheets using aberration corrected TEM.

Observation of MEL stacking faults in two-dimensional MFI zeolite nanosheets

Two-dimensional (2D) MFI-type zeolite nanosheet has gained widespread recognition due to its commercial viability in several applications such as separations of difficult to separate hydrocarbon isomers like xylenes, selective reactions like sugar isomerizations and, dehydrations for the production of renewable fuels and chemicals [1-2]. These nanosheets are porous frameworks through which molecules of only appropriate size can pass through. Presence of any kind of defects distorts the pore structure thereby affecting their performance as separation membranes and catalysts. While, presence of stacking faults have previously been reported in MFI-type zeolite bulk crystals [3], no such observations have yet been made in 2D MFI frameworks. This study reports the observation of a different type zeolite i.e. MEL-type inter-grown within the MFI framework of the nanosheets, which appears as stacking faults (Figure 1).