Sebastian Friebe

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.

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.

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.

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.

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.

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.

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.

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.