Publications Search Results

Now showing 1 - 7 of 7
  • Publication
    Selectivity of Grignard Reagent Formation: From Semibatch to Continuous Lab and Pilot Scale
    ( 2023-12-04) ;
    Dahms, Kai
    ;
    ;
    Ziegenbalg, Dirk
    ;
    Menges-Flanagan, Maria Gabriele
    The formation of Grignard reagents from metallic magnesium and a halide is often accompanied by the formation of the Wurtz coupling product, an undesired side product formed by the reaction of a Grignard reagent molecule with a halide molecule. By using a scale-up approach from semibatch type synthesis to continuous lab and pilot scale for various Grignard reagents, it is demonstrated that a continuous production process can improve Grignard reagent selectivity and reduce Wurtz coupling.
  • Publication
    Impact of residence time distributions in reacting magnesium packed beds on Grignard reagent formation - selectivity of Grignard reagent formation (part 2)
    ( 2023-06-28) ; ;
    Menges-Flanagan, Maria Gabriele
    ;
    Ziegenbalg, Dirk
    Grignard reagents are used as intermediates in the production of complex molecules, since they can be used to form new carbon-carbon bonds, e.g. in the formation of active pharmaceutical ingredients. Side product formation like Wurtz coupling diminishes the selectivity in Grignard reagent formation and therefore side product formation needs to be reduced. It was found that the different pumping behaviours of a syringe pump, a valveless rotary piston pump and a micro annular gear pump and the obtained residence time distributions have an impact on the selectivity of the Grignard reagent formation. Selectivity can also be enhanced by the available magnesium surface and by choosing a tubular flow reactor instead of a batch reactor, showing the importance of choosing the right equipment and parameters for the specific reaction system.
  • Publication
    Impact of residence time distributions in reacting magnesium packed beds on Grignard reagent formation - pump-induced flow behaviour in non-reacting magnesium beds (part 1)
    Grignard reagent formation in continuously operated magnesium packed-bed reactors can be influenced by fine tuning the residence time distribution within the magnesium packing. By decreasing the magnesium turning size and increasing the packing density, narrower residence time distributions and therefore improved Bodenstein numbers can be obtained. The utilized pump system and its induced flow behaviour also have an impact on the residence time distributions in packed-bed reactors. By using oscillatory flow rates instead of pulsation-free pumps, Bodenstein numbers within a magnesium filled reactor cartridge can be increased by 25% for fine magnesium turnings and by 70% for coarse magnesium turnings, resulting in minimized backmixing and approaching plug flow behaviour.
  • Publication
    Activating a [FeFe] Hydrogenase Mimic for Hydrogen Evolution under Visible Light**
    ( 2022)
    Buday, Philipp
    ;
    Kasahara, Chizuru
    ;
    Hofmeister, Elisabeth
    ;
    Kowalczyk, Daniel
    ;
    Farh, Micheal K.
    ;
    Riediger, Saskia
    ;
    Schulz, Martin
    ;
    Wächtler, Maria
    ;
    Furukawa, Shunsuke
    ;
    Saito, Masaichi
    ;
    Ziegenbalg, Dirk
    ;
    Gräfe, Stefanie
    ;
    Bäuerle, Peter
    ;
    Kupfer, Stephan
    ;
    Dietzek‐Ivanšić, Benjamin
    ;
    Weigand, Wolfgang
    Inspired by the active center of the natural [FeFe] hydrogenases, we designed a compact and precious metal-free photosensitizer-catalyst dyad (PS-CAT) for photocatalytic hydrogen evolution under visible light irradiation. PS-CAT represents a prototype dyad comprising π-conjugated oligothiophenes as light absorbers. PS-CAT and its interaction with the sacrificial donor 1,3-dimethyl-2-phenylbenzimidazoline were studied by steady-state and time-resolved spectroscopy coupled with electrochemical techniques and visible light-driven photocatalytic investigations. Operando EPR spectroscopy revealed the formation of an active [FeIFe0] species-in accordance with theoretical calculations-presumably driving photocatalysis effectively (TON≈210).
  • Publication
    Strongly affected photocatalytic CO2 reduction by CO2 adsorbed to the surface of Ba2(In1.8Cr0.2)O5·(H2O)d powders
    ( 2020)
    Yoon, Songhak
    ;
    Nikoee, Sheler
    ;
    Ranjbar, Maryam
    ;
    Ziegenbalg, Dirk
    ;
    Widenmeyer, Marc
    ;
    Nanocrystalline Ba2(In1.8Cr0.2)O5·(H2O)d powders were synthesized via a polymerizable-complex method in two different ways, one based on citric acid and the other additionally on 2,6-pyridinedicarboxylic acid, 2,6-diaminopyridine, and polyethylene glycol. Crystal structure, surface area, thermochemical and optical properties were analyzed. The photocatalytic activity was measured by the reduction of CO2 under H2 gas flow. CO2 adsorption at the catalyst surface is found to be a critical factor strongly affecting the reactivity. With this study, we demonstrate the importance of the surface states as a crucial parameter when developing better CO2 photo-reduction catalysts.
  • Publication
    Methanol oxidation reaction on core-shell structured Ruthenium-Palladium nanoparticles: Relationship between structure and electrochemical behavior
    ( 2018)
    Kübler, Markus
    ;
    Jurzinsky, Tilman
    ;
    Ziegenbalg, Dirk
    ;
    In this work the relationship between structural composition and electrochemical characteristics of Palladium(Pd)-Ruthenium(Ru) nanoparticles during alkaline methanol oxidation reaction is investigated. The comparative study of a standard alloyed and a precisely Ru-core-Pd-shell structured catalyst allows for a distinct investigation of the electronic effect and the bifunctional mechanism. Core-shell catalysts benefit from a strong electronic effect and an efficient Pd utilization. It is found that core-shell nanoparticles are highly active towards methanol oxidation reaction for potentials â¥0.6 V, whereas alloyed catalysts show higher current outputs in the lower potential range. However, differential electrochemical mass spectrometry (DEMS) experiments reveal that the methanol oxidation reaction on core-shell structured catalysts proceeds via the incomplete oxidation pathway yielding formaldehyde, formic acid or methyl formate. Contrary, the alloyed catalyst benefits from the Ru atoms at its surface. Those are found to be responsible for high methanol oxidation activity at lower potentials as well as for complete oxidation of CH3OH to CO2 via the bifunctional mechanism. Based on these findings a new Ru-core-Pd-shell-Ru-terrace catalyst was synthesized, which combines the advantages of the core-shell structure and the alloy. This novel catalyst shows high methanol electrooxidation activity as well as excellent selectivity for the complete oxidation pathway.
  • Publication
    Optimization of a split and recombine micromixer by improved exploitation of secondary flows
    ( 2018)
    Hermann, Pascal
    ;
    ;
    Hoffmann, Marko
    ;
    Schlüter, Michael
    ;
    ; ;
    Ziegenbalg, Dirk
    Flow pattern in two split and recombine (SAR) micromixers were experimentally studied by -LIF measurements to validate corresponding CFD simulations. The original design of the investigated micromixer type is based on the repetition of identical SAR segments of a specific geometry. Aside a micromixer following this original design, the second micromixer investigated alternates two SAR segments versions, one being the original one, the other being an inverted and rotated version of it. The results of the simulations indicate that the secondary flows developing in the original design reduce the mixing performance for Re < 700. This knowledge and the improved understanding of the mixing principle in this specific kind of SAR micromixer was used to develop the design of an optimized version of this split and recombine micromixer type for Re < 700.