Hierarchical Nanostructures of Metal-Organic Frameworks Applied in Gas Separating ZIF-8-on-ZIF-67 Membranes.

Membranes from metal-organic frameworks (MOFs) are highly interesting for industrial gas separation applications. Strongly improved performances for carbon capture and H2 purification tasks in MOF membranes are obtained by using highly reproducable and very accuratly, hierarchically grown ZIF-8-on-ZIF-67 (ZIF-8@ZIF-67) nanostructures. To forgo hardly controllable solvothermal synthesis, particles and layers are prepared by self-assembling methods. It was possible for the first time to confirm ZIF-8-on-ZIF-67 membrane growth on rough and porous ceramic supports using the layer-by-layer deposition. Additionally, hierarchical particles are made in a fast RT synthesis with high monodispersity. Characterization of the hierarchical and epitaxial grown layers and particles is performed by SEM, TEM, EDXM and gas permeation. The system ZIF-8@ZIF-67 shows a nearly doubled H2 /CO2 separation factor, regardless of whether neat membrane or mixed-matrix-membrane in comparison to other MOF materials.

Continuous Separation of Light Olefin/Paraffin Mixtures on ZIF-4 by Pressure Swing Adsorption and Membrane Permeation.

In this study, two zeolitic imidazolate frameworks (ZIFs) called ZIF-4 and ZIF-zni (zni is the network topology) were characterized by sorption studies regarding their paraffin/olefin separation potential. In particular, equilibrated pure and mixed gas adsorption isotherms of ethane and ethene were recorded at 293 K up to 3 MPa. ZIF-4 exhibits selectivities for ethane in the range of 1.5-3, which is promising for continuous pressure swing adsorption (PSA). ZIF-4 shows high cycle stability with promising separation potential regarding ethane, which results in purification of the more industrial desired olefin. Furthermore, both ZIF materials were implemented in Matrimid to prepare a mixed matrix membrane (MMM) and were used in the continuous separation of a propane/propene mixture. The separation performance of the neat polymer is drastically increased after embedding porous ZIF-4 crystals in the Matrimid matrix, especially at higher feed pressures (3-5 barg). Due to the smaller kinetic diameter of the olefin, the permeability is higher compared to the paraffin.

An Untrodden Path: Versatile Fabrication of Self-Supporting Polymer-Stabilized Percolation Membranes (PSPMs) for Gas Separation.

The preparation and scalability of zeolite or metal organic framework (MOF) membranes remains a major challenge, and thus prevents the application of these materials in large-scale gas separation. Additionally, several zeolite or MOF materials are quite difficult or nearly impossible to grow as defect-free layers, and require expensive macroporous ceramic or polymer supports. Here, we present new self-supporting zeolite and MOF composite membranes, called Polymer-Stabilized Percolation Membranes (PSPMs), consisting of a pressed gas selective percolation network (in our case ZIF-8, NaX and MIL-140) and a gas-impermeable infiltrated epoxy resin for cohesion. We demonstrate the performance of these PSPMs by separating binary mixtures of H2 /CO2 and H2 /CH4 . We report the brickwork-like architecture featuring selective percolation pathways and the polymer as a stabilizer, compare the mechanical stability of said membranes with competing materials, and give an outlook on how economic these membranes may become.

Sprayable, Large-Area Metal-Organic Framework Films and Membranes of Varying Thickness.

Despite their huge potential for efficient molecular separation, the fabrication of membranes from metal-organic frameworks (MOFs) remains a major challenge. The powders obtained by the conventional solvothermal MOF syntheses are difficult to process, and as a result the fabrication of well-performing, large-area MOF-based membranes is still awaiting success. The deposition of MOF thin films suited for membrane applications is demonstrated by employing a step-by-step spray method. This method can be scaled up to obtain industrially relevant membrane areas and a continuous process is also possible. The performance of sprayed HKUST-1-based membranes by the separation of a binary H2 /CO2 mixture is also demonstrated. Furthermore, this approach enables the control of the MOF film thickness, and thus controlling the permeance and the selectivity of the membrane.

Inverted Fuel Cell: Room-Temperature Hydrogen Separation from an Exhaust Gas by Using a Commercial Short-Circuited PEM Fuel Cell without Applying any Electrical Voltage.

A short-circuited PEM fuel cell with a Nafion membrane has been evaluated in the room-temperature separation of hydrogen from exhaust gas streams. The separated hydrogen can be recovered or consumed in an in situ olefin hydrogenation when the fuel cell is operated as catalytic membrane reactor. Without applying an outer electrical voltage, there is a continuous hydrogen flux from the higher to the lower hydrogen partial pressure side through the Nafion membrane. On the feed side of the Nafion membrane, hydrogen is catalytically split into protons and electrons by the Pt/C electrocatalyst. The protons diffuse through the Nafion membrane, the electrons follow the short-circuit between the two brass current collectors. On the cathode side, protons and electrons recombine, and hydrogen is released.

Antibody Profiling: Kinetics with Native Biomarkers for Diagnostic Assay and Drug Developments.

Despite remarkable progress in applied Surface Plasmon Resonance (SPR)-based methods, concise monitoring of kinetic properties for native biomarkers from patient samples is still lacking. Not only are low concentrations of native targets in patient samples, often in the pM range, a limiting and challenging factor, but body fluids as complex matrices furthermore complicate measurements. The here-described method enables the determination of kinetic constants and resulting affinities for native antigens from patients' cerebrospinal fluid (CSF) and sera binding to antibodies. Using a significantly extended target-enrichment step, we modified a common sandwich-assay protocol, based on a primary and secondary antibody. We successfully analyze antibody kinetics of native targets from a variety of origins, with consistent results, independent of their source. Moreover, native neurofilament light chain (NFL) was investigated as an exemplary biomarker. Obtained data reveal antibodies recognizing recombinant NFL with high affinities, while showing no, or only significantly weakened binding to native NFL. The indicated differences for recombinant vs. native material demonstrate another beneficial application. Our assay is highly suitable for gaining valuable insights into characteristics of native biomarkers, thus impacting on the binder development of diagnostic reagents or pharmaceutical drugs.

Metal-Organic Framework UiO-66 Layer: A Highly Oriented Membrane with Good Selectivity and Hydrogen Permeance.

The 3D metal-organic framework (MOF) structure UiO-66 [Zr6O4(OH)4(bdc)6], featuring triangular pores of approximately 6 Å, has been successfully prepared as a thin supported membrane layer with high crystallographic orientation on ceramic α-Al2O3 supports. The adhesion of the MOF layer to the ceramic support was investigated in different taxing conditions. Furthermore, by coating this UiO-66 membrane with a thin polyimide (Matrimid) top layer, we prepared a multilayer composite. Said membranes have been evaluated in the separation of hydrogen (H2) from different binary mixtures at room temperature. H2 as the smallest molecule (2.9 Å) should pass the UiO-66 membrane preferably since the kinetic diameters of all the other gases under study are larger. The gas mixture separation factors for the neat UiO-66 membrane were indeed found to be H2/CO2 = 5.1, H2/N2 = 4.7, H2/CH4 = 12.9, H2/C2H6 = 22.4, and H2/C3H8 = 28.5. The coating with Matrimid led to a sharp cutoff for gases with kinetic diameters greater than 3.7 Å, resulting in increased separation performance.

On the Better Understanding of the Surprisingly High Performance of Metal-Organic Framework-Based Mixed-Matrix Membranes Using the Example of UiO-66 and Matrimid.

Metal-organic frameworks feature a certain framework flexibility, mainly due to a linker mobility inside the lattice. The latter is responsible for effects like breathing or gate-opening, thus making predictions of the sorption and diffusion behavior quite difficult. Permeation measurements on supported UiO-66 membranes at low temperatures and on polymer-coated UiO-66 membrane layers as well as 2H NMR line shape studies and nitrogen sorption measurements of UiO-66 with deuterated linkers in Matrimid as mixed-matrix membranes (MMM) indicate that the 2-site 180° flips (π-flips) of the aromatic ring are hindered by the presence of (i) the surrounding polymer Matrimid and (ii) residual solvent molecules, thus giving profound insights into the molecular understanding of gas transport through metal-organic framework-based MMMs.

Comparative Study of MIL-96(Al) as Continuous Metal-Organic Frameworks Layer and Mixed-Matrix Membrane.

MIL-96(Al) layers were prepared as supported metal-organic frameworks membrane via reactive seeding using the α-alumina support as the Al source for the formation of the MIL-96(Al) seeds. Depending on the solvent mixture employed during seed formation, two different crystal morphologies, with different orientation of the transport-active channels, have been formed. This crystal orientation and habit is predefined by the seed crystals and is kept in the subsequent growth of the seeds to continuous layers. In the gas separation of an equimolar H2/CO2 mixture, the hydrogen permeability of the two supported MIL-96(Al) layers was found to be highly dependent on the crystal morphology and the accompanied channel orientation in the layer. In addition to the neat supported MIL-96(Al) membrane layers, mixed-matrix membranes (MMMs, 10 wt % filler loading) as a composite of MIL-96(Al) particles as filler in a continuous Matrimid polymer phase have been prepared. Five particle sizes of MIL-96(Al) between 3.2 µm and 55 nm were synthesized. In the preparation of the MIL-96(Al)/Matrimid MMM (10 wt % filler loading), the following preparation problems have been identified: The bigger micrometer-sized MIL-96(Al) crystals show a trend toward sedimentation during casting of the MMM, whereas for nanoparticles aggregation and recrystallization to micrometer-sized MIL-96(Al) crystals has been observed. Because of these preparation problems for MMM, the neat supported MIL-96(Al) layers show a relatively high H2/CO2 selectivity (≈9) and a hydrogen permeance approximately 2 magnitudes higher than that of the best MMM.

Aerogels from CdSe/CdS Nanorods with Ultra-long Exciton Lifetimes and High Fluorescence Quantum Yields.

Hydrogels are fabricated from CdSe/CdS seeded nanorod building blocks by the addition of hydrogen peroxide and converted to aerogels by supercritical drying. The aerogels show higher photoluminescence quantum yields and longer lifetimes than the hydrogels and the nanoparticle solutions. A model for this observation is derived.

Aerogels: Aerogels from CdSe/CdS Nanorods with Ultra-long Exciton Lifetimes and High Fluorescence Quantum Yields (Adv. Mater. 40/2015).

The fabrication of gels from semiconductor nanoparticles by means of a controlled and optimized destabilization process is investigated by N. C. Bigall and co-workers on page 6152. Aerogels with high photoluminescence quantum yield and ultra-long radiative lifetimes are fabricated from CdSe/CdS seeded nanorods. It is shown that excited electrons can be delocalized within the aerogel monolith while, at the same time, holes stay confined in the CdSe cores. This type of assembly of nanoparticles shows novel properties in comparison to those of the nanoparticle building blocks and of the bulk material.