Hierarchical Nanomanufacturing: From Shaped Zeolite Nanoparticles to High-Performance Separation Membranes

Exceptionally Fast Separation of Xylene Isomers with Zeolitic Nanotube Array Membranes

The separation of xylene isomers is of vital importance in chemical industry but remains challenging due to their similar structure and overlapping physiochemical properties. Membrane-based separations using the zeolite MFI, graphene oxide, and metal-organic frameworks have been intensively studied for this application, but the performance is limited by the well-known rule that the filtrate permeance scales inversely with the membrane thickness. We propose a novel membrane design that is capable of breaking this rule, based on an array of recently discovered zeolite nanotubes. Each zeolite nanotube possesses a 3.6-nm-wide central channel, connecting to dense, uniform 0.8-nm-wide holes on its wall that act as selective pores. Comprehensive molecular dynamics simulations show that this membrane exhibits permeance exceeding current state-of-the-art membranes by at least an order of magnitude while simultaneously maintaining an acceptable selectivity. In particular, a thicker membrane featuring longer zeolite nanotubes exhibits a higher permeance due to the presence of more selective pores. The proposed membrane design is expected to be broadly applied to other gas separations and even desalination as long as zeolitic nanotubes with customized pores are available.

Effects of aging and hydrothermal treatment on the crystallization of ZSM-5 zeolite synthesis from bentonite

In the present study, Lam Dong bentonite clay was utilized as a novel resource to effectively synthesize microporous ZSM-5 zeolite (Si/Al ∼ 40). The effects of aging and hydrothermal treatment on the crystallization of ZSM-5 were carefully investigated. Herein, the aging temperatures of RT, 60, and 80 °C at time intervals of 12, 36, and 60 h, followed by high temperature hydrothermal treatment (170 °C) for 3-18 h were studied. Techniques such as XRD, SEM-EDX, FTIR, TGA-DSC, and BET-BJH were applied to characterize the synthesized ZSM-5. Bentonite clay showed great benefits as a natural resource for ZSM-5 synthesis and is cost efficient, environment friendly, and has a large reserves. The form, size, and crystallinity of ZSM-5 were greatly influenced by aging and hydrothermal treatment conditions. The optimal ZSM-5 product had high purity, crystallinity (∼90%), and porosity (BET ∼380 m² g^-1) as well as thermal stability, which are beneficial for adsorptive and catalytic applications.

Recent Advances in Continuous MOF Membranes for Gas Separation and Pervaporation

Metal-organic frameworks (MOFs), a sub-group of porous crystalline materials, have been receiving increasing attention for gas separation and pervaporation because of their high thermal and chemical stability, narrow window sizes, as well as tuneable structural, physical, and chemical properties. In this review, we comprehensively discuss developments in the formation of continuous MOF membranes for gas separation and pervaporation. Additionally, the application performance of continuous MOF membranes in gas separation and pervaporation are analysed. Lastly, some perspectives for the future application of continuous MOF membranes for gas separation and pervaporation are given.

CO2 Capture from High-Humidity Flue Gas Using a Stable Metal–Organic Framework

The flue gas from fossil fuel power plants is a long-term stable and concentrated emission source of CO2, and it is imperative to reduce its emission. Adsorbents have played a pivotal role in reducing CO2 emissions in recent years, but the presence of water vapor in flue gas poses a challenge to the stability of adsorbents. In this study, ZIF-94, one of the ZIF adsorbents, showed good CO2 uptake (53.30 cm3/g), and the calculated CO2/N2 (15:85, v/v) selectivity was 54.12 at 298 K. Because of its excellent structural and performance stability under humid conditions, the CO2/N2 mixture was still well-separated on ZIF-94 with a separation time of 30.4 min when the relative humidity was as high as 99.2%, which was similar to the separation time of the dry gas experiments (33.2 min). These results pointed to the enormous potential applications of ZIF-94 for CO2/N2 separation under high humidity conditions in industrial settings.

Carbon nanotube supported oriented metal organic framework membrane for effective ethylene/ethane separation

Zeolitic imidazolate framework 8 (ZIF-8) is effective for C₃H₆/C₃H₈ separation because of the "sieving effect" of a six-membered (6-M) window. Here, we demonstrate that ZIF-8 is a versatile material that could effectively separate C₂H₄ from C₂H₆ via its 4-M window along the <100> direction. We established a facile and environmentally friendly carbon nanotube (CNT)-induced oriented membrane (CNT-OM) approach to fabricate a {100}-oriented ZIF-8 membrane (100-M). In this approach, 2-methyimidazole was anchored onto the CNT surface followed by 3-hour in situ growth in aqueous solution at room temperature. The obtained 100-M, whose 4-M window is aligned along the transport pathway, showed ~3 times higher C₂H₄/C₂H₆ selectivity than a randomly oriented membrane. Thus, this work demonstrates that the membrane orientation plays an important role in tuning selectivity toward different gas pairs. Furthermore, 100-M exhibited excellent mechanical stability that could sustain the separation performance after bending at a curvature of ~109 m^-1.

Elimination of Grain Boundary Defects in Zeolitic Imidazolate Framework ZIF-95 Membrane via Solvent-Free Secondary Growth

Metal-organic framework membranes are usually prepared by in situ or secondary growth in a solution/hydrogel. The use of organic solvents may cause safety and environmental problems and produce solvent-induced defects. Here, highly oriented and permselective ZIF-95 membranes are prepared for the first time via a solvent-free secondary growth method. The solvent-free growth is not only helpful to control the membrane microstructure and thickness, but also to reduce the intercrystalline defects. In case of solvent-free growth, a perfectly oriented structure leads to an outstanding reduction of intercrystalline defects and transport resistances. For the separation of equimolar binary gas mixtures by using the highly oriented ZIF-95 membrane at 25 °C and 1 bar, the mixture separation factors of H₂ /CO₂ and H₂ /CH₄ are 184 and 140, respectively, with H₂ permeance of over 1.9×10^-7  mol m^-2  s^-1  Pa^-1 which are much higher than those of the randomly oriented ZIF-95 membrane.

Electrochemical Deposition of Cu Metal-Organic Framework Films for the Dual Analysis of Pathogens

Metal-organic framework (MOF) thin films with flexible nature and prominent qualities have opened doors to new technological applications in different fields. Herein, we propose an electrochemical biosensor for the dual detection of Staphylococcus aureus based on the electrodeposition of Cu metal-organic framework (Cu-MOF) thin films. The promising sensing layer with features of good electronic conductivity and enhanced electron-transfer property can not only identify S. aureus through specific micrococcal nucleases in the supernatant but also detect the pathogen directly via aptamer recognition. The dual analysis design ensures the accuracy of this method for S. aureus detection in the range of 7-7 × 10⁶ cfu/mL with the limits of detection of 1.9 and 5.2 cfu/mL. Moreover, the analytical method validation confirmed that the biosensor could efficiently work in complex biological samples, showing good selectivity and specificity and great potential for clinical diagnosis. More importantly, the current proposed strategy is simple and easy to implement without the need for extra signaling elements, which is convenient for timely clinical detection.

Direct Synthesis of Nanosheet-Stacked Hierarchical "Honey Stick-like" MFI Zeolites by an Aromatic Heterocyclic Dual-Functional Organic Structure-Directing Agent

Soft template designing is the most promising strategy for the synthesis of zeolite nanosheets. MFI nanosheets directed by soft templates (containing long-chain alkyl groups or aromatic groups as hydrophobic component) can be found frequently; however, so far, MFI nanosheets synthesized by soft templates with aromatic heterocycle groups (e. g., s-triazine groups) are rare. Herein, a nanosheet-stacked hierarchical MFI zeolite (NSHM) has been synthesized by using a triply branched s-triazine-based surfactant as a bifunctional organic structure-directing agent. On the basis of a geometrical match relationship, a formation model has been proposed. Synthesized NSHM had abundant mesopores stacked by nanosheets and exhibited a high surface area (430 m²  ⋅ g^-1 ). The 1 wt% Pd/NSHM attained a significant increase in yield of cyclohexanol/cyclohexanone mixture (from 66 to 85 %) in the oxidation of cyclohexane compared with Silicalite-1 and SBA-15 as supports.

Anisotropic Gas Separation in Oriented ZIF-95 Membranes Prepared by Vapor-Assisted In-Plane Epitaxial Growth

Control of the microstructure grain orientation, grain boundaries and thickness are crucial for MOF membranes. We report a novel synthesis strategy to prepare highly c-oriented ZIF-95 membranes through vapor-assisted in-plane epitaxial growth. In a mixed DMF/water vapor atmosphere, in-plane epitaxial growth of a ZIF-95 seeds layer was achieved to obtain an oriented and well-intergrown ZIF-95 membrane with a thickness of only 600 nm. Demonstrated by both experimental and simulation studies, the c-oriented ZIF-95 membrane displayed superior separation performance because a perfectly oriented structure resulted in a notable reduction of intercrystalline defects and transport pathways. For the separation of equimolar binary mixtures at 100 °C and 1 bar, the mixture separation factors of H₂ /CO₂ and H₂ /CH₄ were 32.2 and 53.7, respectively, with an H₂ permeance of over 7.9×10^-7  mol m^-2  s^-1  Pa^-1 , which was 4.6 times higher than that of a randomly oriented ZIF-95 membrane.

Development of a symmetrical micro-beam minimizing horizontal drift for indentation and scratch

In this work, a symmetric micro-beam was developed to eliminate horizontal drift of the tip during indent and scratch processes, which was inevitable in the usage of a traditionally one-end-fixed cantilever. Using the finite element simulation, a database involving the maximum stress and spring constant depending on the size parameters (length and inertial moment) of the symmetric beam was established to help in optimizing the beam design. A real symmetric micro-beam was fabricated based on the database, and the loading accuracy was checked through the calibration of the spring constant. The reliability of the symmetric beam was experimentally verified through the fabrication of the micro-channel and micro-dot, showing that the horizontal drift was completely suppressed. This technology can potentially be applied in micro/nanoscale research and manufacturing to increase the accuracy of detection and processing, such as the applications in probe-based lithography to improve the loading accuracy at high load condition and enhance the stability of the beam system at high sliding speed.

Wetting Properties and Thin-Film Quality in the Wet Deposition of Zeolites

Zeolites are microporous crystalline materials widely used in catalysis and adsorption applications. The fabrication of zeolite thin films and membranes has also opened up the possibility of using zeolites in electronic devices and membrane separations. The existing approach to growing zeolite films involves exposing the substrate to a high-pH environment; however, this process is applicable to only specific types of substrates. Our group has developed the direct wet deposition of zeolites via ultrasonic nozzle spray deposition to address this issue; however, the relationship between wetting properties and thin-film quality has yet to be investigated. In this study, we prepared zeolite CHA (Si:Al:P = 3:10:20) suspensions using different solvents and surfactants in various concentrations. We then examined the relationships among the composition of the cast solution, their wetting behavior on the glass substrate, and the uniformity of the resulting thin films. We found that using ethanol as a solvent with zeolite crystals in low concentrations with added surfactant yielded zeolite films of high quality. We were also able to produce low-haze zeolite coatings on glass. The zeolite coatings with high hydrophilicity and adsorption capacity presented excellent antifogging capability.

Silicalite-1 zeolite membrane: Synthesis by seed method and application in organics removal

Silicalite-1 (S-1) zeolite membrane synthesized by seed method with superior features attracts intensive attentions, while the influences of key parameters during synthesis process and its applications for organics removal require further investigation. This study revealed the morphology and the structure of the prepared membranes under different crystallization temperatures and seed concentrations by using a suite of characterization methods. The as-prepared membrane under optimal condition (crystallization temperature of 175 °C and seed concentration of 1.0 wt. %) possessed high membrane integrity, with ideal separation factor of 4.0. It also exhibited outstanding performance for organics removal, with dyes retention of 99.9% and 99.2% for 500 mg L^-1 neutral red and 500 mg L^-1 methyl blue, respectively. Excellent antifouling property of the synthesized membrane was also proved. Results of this work can guide the characteristic improvement of the S-1 zeolite membrane by adjusting key parameters and broaden its applications in dye wastewater treatment.

Additive-free synthesis of house-of-card faujasite zeolite by utilizing aluminosilicate gel memory

The gel “memory” effect of aluminosilicate sols was used to tune the mesopore size of hierarchical house-of-card faujasite in the absence of additives, following earlier findings by Khaleel et al. demonstrating the use of pre- and post-nucleation trajectories for the synthesis of high FAU content faujasite nanocrystals.

Ultrafast synthesis of zeolites: breakthrough, progress and perspective

An ultrafast route was established to synthesize industrially important zeolites in several minutes, which represents a breakthrough in the field of zeolite synthesis.

Two-Step to One-Step Nucleation of a Zeolite through a Metastable Gyroid Mesophase

The importance of nonclassical nucleation pathways in the formation of complex crystals has become apparent in recent years. Nonclassical pathways were unraveled for, among others, the crystallization of proteins, colloids, and clathrates. In those cases, the formation of a metastable fluid with density close to the crystal decreases the crystallization barrier. Recent simulations indicate that mesophases can facilitate the nucleation of zeolites. Here, we use molecular simulations to investigate the role of a gyroid mesophase on the crystallization of a model zeolite from liquid. The nucleation pathway is always nonclassical. At warmer temperatures, the mechanism proceeds in two well-defined steps: nucleation of a metastable gyroid followed by its crystallization into a zeolite. At colder temperatures, the second barrier becomes negligible, and the crystallization occurs in one step. This second scenario is also nonclassical, as the critical nucleus for the crystallization has the structure of the gyroid and seamlessly transforms into a zeolite as it grows past its critical size. To our knowledge, this is the first report of a nonclassical mechanism of crystallization mediated by a mesophase.

Hierarchical Polymer Structures Using Templates and the Modified Breath Figure Method

Hierarchical structures are commonly observed in nature and possess unique properties. The fabrication of hierarchical structures with well-controlled sizes in different length scales, however, is still a great challenge. To further understand the morphologies and properties of the hierarchical structures, here we present a novel strategy to prepare hierarchical polymer structures by combining the modified breath figure method and the template method. Poly(methyl methacrylate) (PMMA) honeycomb films with regular micropores are first prepared using the modified breath figure method by dipping PMMA films into mixtures of chloroform and methanol. The polymer chains on the honeycomb films are then annealed and wetted into the nanopores of anodic aluminum oxide templates via capillary forces, resulting in the formation of hierarchical polymer structures. The morphologies of the polymer structures, which can be controlled by the molecular weights of the polymers and the concentrations of the polymer solutions, are characterized by scanning electron microscopy. The surface wettabilities of the polymer structures are also examined by water contact angle measurements, and the hierarchical structures are observed to be more hydrophobic than the flat films and honeycomb films. This work not only provides a feasible approach to fabricate hierarchical polymer structures with controlled sizes but also gives a better understanding of the relationship between surface morphologies and properties.

Carbon Nanotube Networks as Nanoscaffolds for Fabricating Ultrathin Carbon Molecular Sieve Membranes

Carbon molecular sieve (CMS) membranes have shown great potential for gas separation owing to their low cost, good chemical stability, and high selectivity. However, most of the conventional CMS membranes exhibit low gas permeance due to their thick active layer, which limits their practical applications. Herein, we report a new strategy for fabricating CMS membranes with a 100 nm-thick ultrathin active layer using poly(furfuryl alcohol) (PFA) as a carbon precursor and carbon nanotubes (CNTs) as nanoscaffolds. CNT networks are deposited on a porous substrate as nanoscaffolds, which guide PFA solution to effectively spread over the substrate and form a continuous layer, minimizing the penetration of PFA into the pores of the substrate. After pyrolysis process, the CMS membranes with 100-1000 nm-thick active layer can be obtained by adjusting the CNT loading. The 322 nm-thick CMS membrane exhibits the best trade-off between the gas permeance and selectivity, a H₂ permeance of 4.55 × 10^-8 mol m^-2 s^-1 Pa^-1, an O₂ permeance of 2.1 × 10^-9 mol m^-2 s^-1 Pa^-1, and an O₂/N₂ ideal selectivity of 10.5, which indicates the high quality of the membrane produced by this method. This work provides a simple, efficient strategy for fabricating ultrathin CMS membranes with high selectivity and improved gas flux.

Supramolecular Organization in Confined Nanospaces

Empty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies - from small molecules to quantum dots or luminescent species - are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions.

Synthesis of Engineered Zeolitic Materials: From Classical Zeolites to Hierarchical Core-Shell Materials

The term "engineered zeolitic materials" refers to a class of materials with a rationally designed pore system and active-sites distribution. They are primarily made of crystalline microporous zeolites as the main building blocks, which can be accompanied by other secondary components to form composite materials. These materials are of potential importance in many industrial fields like catalysis or selective adsorption. Herein, critical aspects related to the synthesis and modification of such materials are discussed. The first section provides a short introduction on classical zeolite structures and properties, and their conventional synthesis methods. Then, the motivating rationale behind the growing demand for structural alteration of these zeolitic materials is discussed, with an emphasis on the ongoing struggles regarding mass-transfer issues. The state-of-the-art techniques that are currently available for overcoming these hurdles are reviewed. Following this, the focus is set on core-shell composites as one of the promising pathways toward the creation of a new generation of highly versatile and efficient engineered zeolitic substances. The synthesis approaches developed thus far to make zeolitic core-shell materials and their analogues, yolk-shell, and hollow materials, are also examined and summarized. Finally, the last section concisely reviews the performance of novel core-shell, yolk-shell, and hollow zeolitic materials for some important industrial applications.

Directing Aluminum Atoms into Energetically Favorable Tetrahedral Sites in a Zeolite Framework by Using Organic Structure-Directing Agents

The Al location in zeolites can have massive influences on the zeolite properties because it directly correlates with the cationic active sites. Herein, the synthesis of IFR zeolites with controlled Al distribution at different tetrahedral sites (T sites) is reported. The computational calculations suggest that organic structure-directing agents (OSDAs) used for zeolite synthesis can alter the energetically favorable T sites for Al. Zeolite products synthesized under identical conditions but with different OSDAs are found to have altered fractions of Al at different T sites in accordance with the energies derived from the zeolite-OSDA complexes. Our finding thus provides evidence for the ability of OSDAs to direct Al into more energetically favorable T sites, thereby offering rational synthetic guidelines for the selective placement of Al into specific crystallographic sites.