Hierarchische Nanofertigung: von geformten Zeolithnanopartikeln zu hochleistungsfähigen Trennmembranen

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.

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.

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.

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.

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

The control of membrane morphology and microstructure is crucial to improve the separation performance of molecular-sieve membranes. This can be enabled by making thin, dense, and uniform seed-crystal coatings, which are then intergrown into continuous membranes. Herein, we show a novel and simple floating particle coating method can give closely packed monolayers of zeolite nanosheets on nonporous or porous supports. The zeolite nanosheet monolayer is formed at the air-water interface in a conical Teflon trough. As the water in the trough is drained, the monolayer is deposited on a support placed below. Membranes prepared by gel-free secondary growth of the nanosheets deposited by this method show unprecedented ultra-selective performance for separation of para- from ortho-xylene (separation factor >10 000).

Topotactic conversion of layered silicate RUB-15 to silica sodalite through interlayer condensation in N-methylformamide

Layered silicate RUB-15 was topotactically converted to silica sodalite through interlayer condensation by refluxing in N-methylformamide.

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.

Confined Polymerization in Highly Ordered Mesoporous Organosilicas

Hybrid mesoporous organosilica exhibiting crystal-like order in the walls provided an ideal channel reaction vessel for the confined polymerization of acrylonitrile (PAN). The resulting high-molecular-mass PAN fills the channels at high yield and forms an ordered nanostructure of polymer nanobundles enclosed into the hybrid matrix. The in situ thermal transformation of PAN into rigid polyconjugated and, eventually, into condensed polyaromatic carbon nanofibers, retains the periodic architecture. Simultaneously, the matrix evolves showing the fusion of the p-phenylene rings and the cleavage of carbonsilicon bonds: this gives rise to graphitic-carbon/silica nanocomposites containing hyper-oxydrylated silica nanophases. Interestingly, the 3D hexagonal mesostructure survives in the carbonaceous material. The exploitation of porous materials of high capacity and a hybrid nature, for polymerization in the confined state, followed by high temperature treatments, allowed us to achieve unique and precisely fabricated nanostructures, thus paving the way for the construction of fine-tuned electronic and light-harvesting materials.