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Based on the nature of the deformation, molecular crystals can be defined as being plastically (irreversible) or elastically (reversible)bendable. The mechanical response of crystals is thereby directly related to they structure.The structural elements required for a specific mechanical behavior are known, so that compounds with these properties can be synthesized by applying the rules of crystal engineering.
Molecular crystalline materials are usually brittle and are prone to breaking when mechanically stressed. This fragility greatly limits they application in next the generation of adaptable, functional materials. The recent discovery of mechanical compliancy in molecular crystals has solved this problem.1 Based on the nature of the deformation, molecular crystals can be divided into being plastically (irreversibly) or elastically (reversibly) bendable. The plastic deformation is generally associated with anisotropic molecular arrangements and the existence of low energy slip planes which allow a permanent motion within the lattice. Here we report 4-bromo-6-[(6-chloropyridin-2-ylimino)methyl]phenol (CPMBP) as a promising candidate for future waveguide technologies.2 CPMBP has been found to have two different polymorphs with distinct optical and mechanical properties. The brittle crystals of Form I exhibits very weak emission at 605 nm (λex = 425 nm; photoluminescence quantum yield Φ = 0.4 %). In contrast, Form II has a large plastic regime together with a bright emission at 585 nm (λex = 425 nm; Φ = 8.7 %). Taking advantage of the favorable mechanical flexibility and optical properties, Form II was used as a shapeable optical waveguide. By changing the wavelength of the light source, active or passive waveguiding can be realized. CPMBP could thus be used as a flexible wavelength filter.
Mechanochemistry is widely applicable for the synthesis of inorganic, metal-organic, and organic compounds. It is known for short reaction times, nearly quantitative conversions, and decreasing amount of solvents, which opens the field to more environmentally friendly syntheses routes. Among organic syntheses, the Knoevenagel condensation is an important C-C bond forming reaction leading to α,β-unsaturated compounds. To gain more information on the underlying processes, we investigated the syntheses by a combination of different in situ investigation techniques, including synchrotron X-ray diffraction, Raman spectroscopy and thermography. This combination provides information on the structural changes and temperature influences during milling. Benzaldehyde derivates (nitro- and fluoro-derivates) reacted with malononitrile to the respective benzylidenemalononitriles. The in situ investigations show direct and quantitative conversions. In the case of the fluorinated benzaldehyde derivates we showed the possibility of using liquid substrates in mechanochemical organic synthesis. Surprisingly, after crystallization from a viscous state, the material was suitable for single-crystal X-ray analysis.
The exploration of metal phosphonates chemistry has gained great interest during the last decades, because of their structural diversity. Transition metal phosphonates are promising candidates for an application as electrocatalysts in oxygen evolution reactions (OER).
Here, we present the in situ investigation of mechanochemical syntheses of different manganese phosphonates by synchrotron X-ray diffraction. Nitrilotri(methylenephosphonic acid) and N,N-Bis(phosphonomethyl)glycine were chosen as ligands. The liquid-assisted milling process can be divided into three steps, including an amorphous stage. One of the products has not been obtained by classical solution chemistry before.
These metal phosphonates and/or their derivatives are considered to be active in electrochemical energy conversion. The verification of their applicability is one of the topics of our resent research.
Proton exchange membrane fuel cells (PEMFCs) are one of the most promising alternative green energy technologies that deliver high energy density without CO2 emissions. The proton conductivity of proton exchange membranes (PEM) contributes to the overall efficiency of a PEMFC. Materials being used as PEMs must exhibit high proton conductivity at the working conditions of the targeted PEMFC. To date, Nafion and Nafion-like polymers with acidic functionality are widely used as membrane materials due to their high proton conductivity in the range of 10-1 to 10-2 Scm-1 at higher relative humidity. However, these materials suffer from high costs, hazardous production process, and poor performance at high temperatures, limiting their versatility.
In this context, crystalline porous materials are recognized as promising proton conductors for the proton exchange membrane (PEM) in fuel cell technology, owing to their tunable framework structure. However, it is still challenging bulk synthesis for real-world applications of these materials. Herein, we present mechanochemical gram-scale synthesis of series of mixed ligand metal organic frameworks (MOFs) and metal hydrogen‐bonded organic frameworks (MHOFs) using phenylene diphosphonic acid and 1-hydroxyethylidene-1,1-diphosphonic acid with different bipyridyl type of ligands, respectively. In all cases, the existence of extensive hydrogen bonds with amphiprotic uncoordinated phosphonate hydroxyl and oxygen atoms, the frameworks exhibited high proton conductivity. The study demonstrates the potential of green mechanosynthesis for preparations of framework-based proton conducting materials in bulk scale for green energy generation.
As one of nature's largest carbon sources with an annual production of around 20 billion tonnes, lignin is the third most abundant biopolymer on the planet. It becomes available as technical lignin, which is produced as a by-product in the pulp and paper industry and in smaller quantities in second generation biofuel refineries. Current estimates suggest that less than 10% of all technical lignin is reused. The high polydispersity, complex heterogeneous structure and uncertain reactivity are the major limiting factors for further processing. The most common applications for various technical lignins without extensive modifications are for example: Surface active substances, additives in bitumen, cement and animal feed. One way to make lignin usable is to break the structure into oligomer units and thus reduce the polydispersity and average molar mass. In addition, it is advantageous to introduce new functionalities such as hydroxyl or carbonyl groups when splitting the high-molecular-weight (HMW) fractions, or to convert existing functionalities. In this study, a mechanochemical method is presented that can degrade and modify technical kraft lignin by means of sodium percarbonate (SPC).
Metal phosphonates are promising materials for applications in fuel cells, due to their high proton conductivity and higher chemical and thermal stability compared to the industry standard (e.g. Nafion®). Additionally, metal phosphonates are precursors to porous carbon materials with evenly distributed centers for ORR catalysis. As a fast and sustainable synthesis, mechanochemistry is the synthesis method of choice. Thorough characterization is carried out by XRD, MAS-NMR, XAS, BET, and DVS.
We report the scale-up of a batch solid synthesis of zeolitic imidazolate framework-8 (ZIF-8) for reactive extrusion. The crystalline product forms in the extruder directly under the mixture of solid 2-methylimidazole and basic zinc carbonate in the presence of a catalytic amount of liquid. The process parameters such as temperature, liquid type, feeding rate, and linker excess were optimized using the setup specifically designed for in situ Raman spectroscopy. Highly crystalline ZIF-8 with a Brunauer–Emmett–Teller (BET) surface area of 1816 m2 g–1 was quantitatively prepared at mild temperature using a catalytic amount of ethanol and a small excess of the linker. Finally, we developed a simple and comprehensive approach to evaluating the environmental friendliness and scalability of metal–organic framework (MOF) syntheses in view of their large-scale production.
Optimizing the Green Synthesis of ZIF-8 by Reactive Extrusion Using In Situ Raman Spectroscopy
(2023)
ZIF-8 is a prominent member of the zeolitic imidazolate frameworks (ZIFs) subfamily of MOFs which possesses high thermal, chemical, and mechanical stabilities. Different routes have been explored to achieve the large-scale production of ZIF-8. However, these synthetic procedures are often inconsistent with the principles of sustainable chemical manufacturing. Aimed at developing scalable and greener production of ZIF-8, we adapted our previously reported in-batch „mix and wait“ synthesis[2] to continuous extrusion. To optimize the process, in-situ Raman spectroscopy was applied. Finally, we developed a simple and comprehensive approach to evaluating the environmental friendliness and scalability of MOF syntheses in view of their large-scale production.
The synthesis of ZIF-8 was performed using a twin-screw extruder ZE 12 HMI equipped with an automatic volumetric feeder ZD 12B (Three-Tec GmbH, Switzerland) and peristaltic pump BT-L (Lead Fluid, China). The process was monitored in six different zones using a Raman RXN1TM analyzer (Kaiser Optical Systems, France) with a non-contact probe head. PMMA screw-in parts, which are transparent to Raman laser radiation, were specially manufactured to provide the laser focus within the barrel. PXRD, TGA, N2 adsorption measurements, and SEM were used as complementary techniques to characterize the extrudates.
The batch ‘mix and wait’ synthesis of ZIF-8, consisting of bringing solid basic zinc carbonate and 2-methylimidazole in contact in a closed vial, was successfully adapted to reactive extrusion. The crystalline ZIF-8 continuously forms in the extruder under the mixing of solid reagents in the presence of a catalytic amounts of H2O or EtOH. The temperature, type of liquid, feeding rate, and excess of linker were optimized using in situ Raman spectroscopy. Pure and highly crystalline ZIF-8 was isolated at 40 °C by adding a catalytic amount of EtOH and a linker excess of 25%. The resulting material has excellent porosity with the BET surface area slightly exceeding that of the reference Basolite® Z1200 (1816 vs. 1734 m2 g–1). The reaction could yield ~ 3 kg d–1 assuming a continuous operation, with a space-time yield of ca. 67,000 kg m–3 d–1. The present method was compared to the published pathways based on Green Chemistry principles and proved to have the highest potential for large-scale production of ZIF-8.
Crystalline molecular materials are usually brittle and are prone to break upon external mechanical force. This fragility poses challenges for their application in next-generation technologies, including sensors, synthetic tissues, and advanced opto-electronics. The recent discovery of mechanical flexibility in single crystals of molecular materials has solved this problem and enable the design of smart flexible device technologies. Mechanical flexibility of organic crystals can be tuned by altering the weak interactions in the crystal structure, for examples through polymorphism. Here we report 4-bromo-6-[(6-chlorolpyridin-2-ylimino)methyl]phenol (BCMPMP) as a promising candidate for future waveguide technologies. It turns out that BCMPMP has two different polymorphs with distinct optical and mechanical properties. Form I shows brittle behavior under mechanical stress and exhibits very weak emission at 605 nm (λex = 425 nm) together with a low fluorescence quantum yield (Φ = 0.4 %).In contrast, Form II has a large plastic (irreversible bending) regime and a bright emission at 585 nm (λex = 425 nm; Φ = 8.7 %). Making use of favorable mechanical fexiblity and optical properties, form II was explored as a bendable optical waveguide. Light was successfully propagated through a straight-shaped and mechanically deformed BCMPMP crystal. Depending on the light source, active or passive waveguiding could be achieved. So BCMPMP can also be used as a flexible wavelength filter.
Mechanochemistry offers a unique opportunity to modify or synthesize new crystal forms. Although the method is very promising, little is known about the mechanochemical means to control the synthesis of a solid form. Using an polymorphic organic cocrystal system, we show here that mechanochemistry can be used to obtain a polymorph transformation under the apparently conventional (thermal) transition point.
Metal-organic framework-based biocomposites (MOF-biocomposites) are promising materials for biosensing, biocatalysis, and delivery of biopharmaceuticals. One of the most studied MOFs for bioapplications is ZIF-8 (zeolitic imidazolate framework 8) due to its high surface area, high thermal and chemical stability, and low cytotoxicity. The conventional synthesis of ZIF-8-biocomposites called biomimetic mineralization includes mixing selected biomolecules 2-methylimidazole, and soluble Zn2+ source in water. Despite the high efficiency of the method, it does not allow for large-scale production and is restricted to hydrophilic biomolecules. Aimed at developing a scalable and versatile approach, we adapted our recently-reported ZIF-8 reactive extrusion for biocomposite production.
We selected bovine serum albumin (BSA) as an inexpensive model biomacromolecule for the preparation of biocomposites. The synthesis of BSA@ZIF-8 was performed using a twin-screw extruder ZE 12 HMI (Three-Tec Gmbh) at a mild temperature of 40 °C. Automatic volumetric feeder ZD 12B (Three-Tec GmbH) was used to supply the reagent mixture consisting of 2-methylimidazole, zinc source, and BSA. To initiate the reaction, a catalytic amount of EtOH was added using a peristaltic pump BT-L (Lead Fluid, China). Powder X-Ray diffraction (PXRD), thermogravimetric analysis (TGA), FTIR, and N2 adsorption were used to characterize the extrudates.
Highly crystalline and pure BSA@ZIF-8 with different BSA loadings was isolated after washing the extrudate with EtOH and sodium dodecyl sulfate. The EtOH feeding rate was optimized by following the protein encapsulation efficiency at a BSA mass fraction of 10%. A continuous extruder operation under optimized conditions showed good reproducibility and capability of producing biocomposites on the kilograms scale. These results provide highly valuable information for cheap and large-scale production of ZIF-8-based biocomposites. Due to the lack of restrictions on molecule size and solubility, our proof-of-concept study may significantly expand the selection of biomolecules for immobilization in ZIF-8, making the method applicable to various functional applications
In this contribution we describe an in-situ study of the crystallization of simvastatin in three solvents. The studies were carried out by solvent evaporation at the µSpot beamline using acoustically levitated solution droplets in combination with simultaneous X-ray diffraction, Raman spectroscopy, and imaging analysis.
Coating materials are nowadays often required to deliver not only sufficient barrier performance and suited optical appearance but a broad range of other functional properties. The incorporation of inorganic nanoparticles (NPs) is known to improve many key characteristics and provide new functionalities in polymer materials. Presented work aims to prepare and characterize an organic-inorganic coating material designed to bring together advantageous properties of hybrid materials and reinforcement effect delivered from the inorganic NPs embedment.
Siloxane-based hybrid resins hold great advantages as coating materials as their properties can be tuned between those of polymers and those of glasses, thus, the compositions with superior thermal and mechanical properties can be achieved. We used Cycloaliphatic Epoxy Oligosiloxane (CEOS) resin as a polymeric matrix where the network formation was achieved by UV induced cationic polymerisation. Boehmite Alumina (BA) nanoparticles were added to CEOS resin as a reinforcing agent and resultant material was processed into films either by bar-coating or by spin-coating depending on further characterization procedure. Two different types of BA NPs, hydrophilic and organophilic, were used in order to assess the impact of particles surface on the resin characteristics. CEOS synthesis by condensation reaction was confirmed using 13C and 29Si NMR. Changes in CEOS photocuring process, resulting from particles incorporation, were monitored by real-time IR spectroscopy. At the same time, the thermal behaviour was evaluated by DSC and TGA methods. Morphology of the coatings was investigated by means of SEM operated in transmission mode.
It was observed that BA presence increased the epoxy conversion degree and glass transition temperature. Material formulations providing best film characteristics were determined with regard to the particle type and loading. Compared to the hydrophilic nanoparticles, organophilic BA NPs yield superior overall performance of the foils.
Nanoparticles (NPs) have become important materials for a variety of chemical technologies, including catalysis. One of the main challenges is the reduction of green house gases, such as CO2. One opportunity besides the capturing is the conversion to synthesis gas via the reverse water-gas shift reaction.
A facile and efficient method is described for the solvothermal synthesis of size-tunable, stable, and uniform NiCu core-shell NPs. The diameter of the NPs can be tuned in a range from 6 nm to 30 nm and the Ni:Cu ratio from 30:1 to 1:1. The NPs are structurally characterized with combination of transmission electron microscopy, anomalous small-angle X-ray scattering, X-ray photoelectron spectroscopy, and X-ray absorption fine structure. Using these analytical methods, a core-shell-shell structure their chemical composition is elucidated. A depletion from the core to the shell is observed, with the core consisting of NiCu alloy, surrounded by an inner Ni-rich shell and an outer NiO shell. The SiO2-supported NiCu core-shell NPs show pronounced selectivity of >99% for CO in the catalytic reduction of CO2 to CO using hydrogen as reactant (reverse water–gas shift reaction).
Energetic materials (explosives, propellants, pyrotechnics, and gas generators; EM) release large amounts of energy when initiated by mechanical loading and have many technological applications including in energy storage and propulsion. The accidental initiation of an EM – particularly in the wrong setting – has the potential to be catastrophic. Unfortunately, there is little understood about what determines the sensitivity of a given EM. This poses severe restrictions on our ability to design new and safer EMs.
Aiming to better understand the initiation mechanisms of EMs, we here investigate the reactivity of simple, isomorphous azides (MN3 M=Li, Na). Both metal azides contain the same explosophoric azido anions, but differ significantly in their reactivity, presumably owing to different bonding interactions between the anion and the metal cation. This interaction offers a promising probe for X-ray spectroscopy and quantum chemical simulations. In noting that mechanical initiation results from mechanical impact – with high local pressures – we are particularly interested in identifying how this bonding interaction changes as a function of pressure.
This interaction offers a promising probe for X-ray spectroscopy and quantum chemical simulations. In noting that mechanical initiation results from mechanical impact – with high local pressures – we are particularly interested in identifying how this bonding interaction changes as a function of pressure.
Here we show results from DFT simulations that indicate a shift in electronic structure and changes in the metal-azide bond with increasing pressure, which is further investigated through experimental XAS spectra. Together, our results show promising insights into the behaviour of simple metal azide EMs.
Previous work in the Dunne Group targeted dispersibility of metal oxide nanoparticles, which had been synthesised via an aqueous sol-gel route. Dispersibility was attained by solvothermal surface modification of the particles with trifluoro acetic acid (TFA). Part of the studies were tin oxide particles, which is known for its predominant rutile phase. Besides the dispersibility in acetone of the particles, an unexpected peak splitting of the (110) reflection was observed, which could be an unknown phase of SnO2 . For crystal growth long-term reaction studies on the tin oxide particles were performed. Interestingly these studies exhibited a time dependent extend of the peak splitting observed in XRD characterisation. Further extended analysis using multinuclear solid-state MAS-NMR spectroscopy indicate a size dependent structure change due to partial fluorination of the particles during the solvothermal treatment.
Synchrotron based absorption edge tomography for the analysis of 3D printed polymer embedded MOF
(2019)
Absorption edge tomography, also known as differential tomography at absorption edges, is a method which exploits the sudden change of the attenuation coefficient, when the photon energy crosses the absorption edge of an element. Synchrotron radiation is the best source for absorption edge tomography, because of its small bandwidth, high intensity and easily adjustable photon energy. The synchrotron beamline BAMline at the synchrotron radiation facility BESSY II in Berlin, which is operated by the Bundesanstalt für Materialforschung und -prüfung (BAM), provides a monochromatized beam in a photon energy range from 5 keV up to 80 keV with a bandwidth of 2%, when the double multilayer monochromator is used. Together with the microtomography setup, this enables differential tomography with submicron resolution at the K edge of the elements from chromium up to the lanthanides, and up to uranium, when the L edges are used as well.
In this work, metal organic frameworks (MOFs) embedded in polymer are characterized using differential tomography. MOFs are microporous structures of metal ions, coordinated by organic linker molecules, that can be used in a broad field of applications, especially in gas storage and catalysis. In this work, polymer embedded MOFs were extruded into filaments, which were subsequently used for 3d-printing to profit from the specific properties of the MOFs in polymeric materials combined with the arbitrary shapes provided by 3d-printing. For the extrusion, different polymer classes like ABS, polyester- and polyetherurethanes, as well as different MOFs (ZIF-8, ZIF-67, HKUST-1) were used to create MOF containing filaments with a nominal diameter of 3.0 mm. Differential tomography at the edges of the Zn, Co, and Cu was then used to find the distribution of the corresponding MOF in the filament and to analyze the shape of the inclusions.