03 Fakultät Chemie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/4
Browse
38 results
Search Results
Item Open Access Synthese von Stickstoffliganden mit Phosphonatgruppen zur Immobilisierung von Kupferkomplexen auf γ-Alumina für kupferkatalysierte Atomtransfer-Radikaladditionsreaktionen (ATRA)(2024) Maier, Sarah; Estes, Deven (Jun. Prof. Dr.)Diese Arbeit beschreibt die Synthese und Charakterisierung von stickstoffhaltigen Liganden mit Phosphonat-Ankergruppen, die Immobilisierung ihrer Kupferkomplexe auf oxidischen Trägermaterialien, sowie den Einsatz dieser immobilisierten Komplexe als Katalysatoren für Atomtransfer-Radikaladditionsreaktionen.Item Open Access Structural characterization of surface immobilized platinum hydrides by sensitivity-enhanced 195Pt solid state NMR spectroscopy and DFT calculations(2024) Atterberry, Benjamin A.; Wimmer, Erik J.; Klostermann, Sina; Frey, Wolfgang; Kästner, Johannes; Estes, Deven P.; Rossini, Aaron J.Supported single-site platinum hydride compounds are promising heterogeneous catalysts for organic transformations. Few methods exist to describe the structures of single-site Pt catalysts with atomic resolution because of their disordered structures and low Pt loadings. Here, we study the compounds formed when bis(tri-tert-butylphosphino)platinum, Pt(PtBu3)2, is supported on dehydroxylated SiO2 or SiO2-Al2O3. First, we obtain magic angle spinning (MAS) 1H, 31P and 195Pt ssNMR spectra of four model Pt phosphine compounds with oxidation states of 0 or +2 and coordination numbers between 2 and 4. These compounds are analogs of potential structures present in the supported compounds. MAS 195Pt ssNMR spectra were obtained using 31P{195Pt} sideband selective J-resolved and J-HMQC experiments. The measured 1H and 31P chemical shifts, 31P-195Pt J-couplings and 195Pt chemical shift (CS) tensors are shown to be diagnostic of oxidation state and coordination number. Room temperature 1H ssNMR spectra of Pt(PtBu3)2 supported on SiO2 or SiO2-Al2O3 show diagnostic hydride NMR signals, suggesting that Pt(PtBu3)2 undergoes oxidative addition, resulting in surface hydrides and Pt–oxygen bonds to the support surface. MAS dynamic nuclear polarization (DNP) enables 31P{195Pt} correlation NMR experiments on the supported compounds. These experiments enable the measurement of the 31P-195Pt J-coupling constants and 195Pt CS tensors. Combined NMR and DFT analyses suggest that the primary surface platinum species are [HPt(PtBu3)2OSi] on SiO2 and [HPt(PtBu3)2]+[Si-O--Al] on SiO2-Al2O3. The Pt-oxygen bond length is dependent on the support and estimated as 2.1-2.3 Å and 2.7-3.0 Å for SiO2 and SiO2-Al2O3, respectively.Item Open Access Rapid aging as a key to understand deactivation of Ni/Al2O3 catalysts applied for the CO2 methanation(2021) Beierlein, Dennis; Häussermann, Dorothea; Traa, Yvonne; Klemm, EliasWe developed a rapid aging method for Ni/Al2O3 methanation catalysts mimicking the real aging in the actual application. The method is based on hydrothermal deactivation of the catalyst at 600 or 700 °C, which leads to a catalyst with nearly constant conversion after a much shorter time period compared to normal aging. The hydrothermally aged catalysts are characterized by N2 adsorption, X-ray powder diffraction, temperature-programmed reduction and H2 chemisorption. The catalytic performance of the aged catalysts is comparable to the one of a catalyst deactivated in a long-term measurement with up to 720 h on stream. The time needed for reaching a stable conversion can be diminished by rapid aging by a factor of 10. The investigations also showed that the long-term deactivation is caused by Ni particle sintering and that the support pores limit the Ni particle size.Item Open Access Untersuchung von Degradationsmechanismen an einlagigen Gasdiffusionselektroden für die elektrochemische Kohlenstoffdioxid-Reduktion(2023) Hildebrand, Joachim; Klemm, Elias (Prof. Dr.-Ing.)Item Open Access Asymmetric Rh diene catalysis under confinement : isoxazole ring‐contraction in mesoporous solids(2024) Marshall, Max; Dilruba, Zarfishan; Beurer, Ann‐Katrin; Bieck, Kira; Emmerling, Sebastian; Markus, Felix; Vogler, Charlotte; Ziegler, Felix; Fuhrer, Marina; Liu, Sherri S. Y.; Kousik, Shravan R.; Frey, Wolfgang; Traa, Yvonne; Bruckner, Johanna R.; Plietker, Bernd; Buchmeiser, Michael R.; Ludwigs, Sabine; Naumann, Stefan; Atanasova, Petia; Lotsch, Bettina V.; Zens, Anna; Laschat, SabineCovalent immobilization of chiral dienes in mesoporous solids for asymmetric heterogeneous catalysis is highly attractive. In order to study confinement effects in bimolecular vs monomolecular reactions, a series of pseudo‐C2‐symmetrical tetrahydropentalenes was synthesized and immobilized via click reaction on different mesoporous solids (silica, carbon, covalent organic frameworks) and compared with homogeneous conditions. Two types of Rh‐catalyzed reactions were studied: (a) bimolecular nucleophilic 1,2‐additions of phenylboroxine to N‐tosylimine and (b) monomolecular isomerization of isoxazole to 2H‐azirne. Polar support materials performed better than non‐polar ones. Under confinement, bimolecular reactions showed decreased yields, whereas yields in monomolecular reactions were only little affected. Regarding enantioselectivity the opposite trend was observed, i. e. effective enantiocontrol for bimolecular reactions but only little control for monomolecular reactions was found.Item Open Access Untersuchung von aluminiumreichen Zeolithkatalysatoren ohne große Hohlräume zur Dehydratisierung von Milchsäure zu Acrylsäure(2023) Häussermann, Dorothea; Traa, Yvonne (apl. Prof. Dr.)Um erdöl-basierte Grundchemikalien zukünftig bio-basiert herzustellen, müssen bio-basierte Plattformchemikalien effizient umgesetzt werden können. Acrylsäure könnte beispielsweise aus bio-basierter Milchsäure hergestellt werden. Dazu fehlt bislang noch ein geeigneter Katalysator. In dieser Arbeit sollen Zeolithkatalysatoren für diese Reaktion systematisch untersucht werden. Für eine belastbare Bestimmung der katalytischen Leistung werden vergleichende Voruntersuchungen durchgeführt. Dadurch soll sichergestellt werden, dass die katalytischen Messungen zuverlässig sind. Die katalytischen Untersuchungen zeigen, dass die aluminiumreichen 12-Ring-Zeolithkatalysatoren eher ungeeignet für die Milchsäuredehydratisierung sind. Die ausschließlich Lewis-sauren 12-Ring-Zeolithkatalysatoren zeigen einen schnellen Rückgang der Acrylsäure-Selektivität, aufgrund der starken Adsorption von Milchsäure. Die 12-Ring-Katalysatoren, bei denen Brønsted-Säurezentren vorhanden sind, haben eine sehr hohe Acetaldehyd-Selektivität und sind daher ebenfalls ungeeignet. Die mittelporigen Zeolithkatalysatoren sind vielversprechende Katalysatoren. Mit abnehmendem Aluminiumgehalt steigt die Acrylsäure-Selektivität. Die Katalysatoren zeigen ein stationäres Reaktionsverhalten. Der Austausch von Natrium gegen Kalium steigert die Acrylsäure-Selektivität signifikant. Bei den engporigen Zeolithkatalysatoren findet die katalytische Reaktion auf der äußeren Oberfläche statt und nicht im Porensystem. Bei einem ausgeglichenen Säure-Base-Verhältnis kann Acrylsäure auf der äußeren Oberfläche gebildet werden. Allerdings wird auch 2,3-Pentandion gebildet, da es keine dirigierenden Effekte durch die Poren gibt. Ist der Katalysator jedoch zu nukleophil, dann wird überwiegend Acetaldehyd gebildet. Na-SAPO-42 ist für die Milchsäuredehydratisierung ungeeignet, da die schwachen Säurezentren und die stark nukleophilen Gittersauerstoffatome nicht die entsprechende Balance erreichen, um Milchsäure zu dehydratisieren. An amorphem Silica-Alumina kann keine Acrylsäure gebildet werden, da überwiegend starke Lewis-Säurezentren vorhanden sind, die die Decarbonylierung katalysieren. Am Beispiel des engporigen Zeoliths PHI kann gezeigt werden, dass die Syntheseparameter einen deutlichen Einfluss auf den synthetisierten Katalysator und seine katalytische Leistung haben. Bei einer dichten Struktur muss die Reaktion auf der äußeren Oberfläche stattfinden, dadurch finden weniger Folgereaktionen statt und die gebildete Acrylsäure kann die Katalysatoroberfläche schnell wieder verlassen. Auch hier wird deshalb 2,3-Pentandion gebildet. Ist das enge Porensystem dagegen zugänglich beziehungsweise teilweise zugänglich ist, dann können aufgrund der langsamen Diffusion Folgereaktionen ablaufen, und die Acrylsäure-Selektivität ist gering.Item Open Access Herstellung, Charakterisierung und Testung von Zeolithkatalysatoren für die Alkoholumsetzung(2024) Dittmann, Daniel; Dyballa, Michael (PD Dr.)Item Open Access Sulfur‐composites derived from poly(acrylonitrile) and poly(vinylacetylene) : a comparative study on the role of pyridinic and thioamidic nitrogen(2023) Kappler, Julian; Klostermann, Sina V.; Lange, Pia L.; Dyballa, Michael; Veith, Lothar; Schleid, Thomas; Weil, Tanja; Kästner, Johannes; Buchmeiser, Michael R.Sulfurized poly(acrylonitrile) (SPAN) is a prominent example of a highly cycle stable and rate capable sulfur/polymer composite, which is solely based on covalently bound sulfur. However, so far no in‐depth study on the influence of nitrogen in the carbonaceous backbone, to which sulfur in the form of thioketones and poly(sulfides) is attached, exists. Herein, we investigated the role of nitrogen by comparing sulfur/polymer composites derived from nitrogen‐containing poly(acrylonitrile) (PAN) and nitrogen‐free poly(vinylacetylene) (PVac). Results strongly indicate the importance of a nitrogen‐rich, aromatic carbon backbone to ensure full addressability of the polymer‐bound sulfur and its reversible binding to the aromatic backbone, even at high current rates. This study also presents key structures, which are crucial for highly cycle and rate stable S‐composites.Item Open Access Comparison of electrochemically deposited Bi and Sn catalysts onto gas diffusion electrodes for the electrochemical CO2 reduction reaction to formate(2023) Manolova, Mila; Hildebrand, Joachim; Hertle, Sebastian; Sörgel, Şeniz; Kassner, Holger; Klemm, EliasIn this publication, we report about the selectivity and stability of bismuth (Bi)- and tin (Sn)-based electrocatalysts for the electrochemical CO2 reduction reaction (eCO2RR) for formate production. Bismuth and tin were successfully electrodeposited using the pulse plating technique on top of and inside of the gas diffusion layers (GDLs). The distribution of the catalyst throughout the thickness of the gas diffusion electrodes (GDEs) was investigated by using scanning electron microscopy and computer tomography; it was found that the catalyst morphology determines the performance of the electrode. Inhomogeneous deposits, with their enlarged catalyst surface area, provide more active centres for the eCO2RR, resulting in increased Faraday efficiency (FE) for formate. The initial electrochemical characterisation tests of the bismuth- and tin-loaded GDEs were carried out under laboratory operating conditions at an industrially relevant current density of 200 mA·cm-2; complete Sn dissolution with a subsequent deformation of the GDL was observed. In contrast to these results, no leaching of the electrodeposited Bi catalyst was observed. An FE of 94.2% towards formate was achieved on these electrodes. Electrodes based on an electrodeposited Bi catalyst on an in-house prepared GDL are stable after 23 h time-on-stream at 200 mA·cm-2 and have very good selectivity for formate.Item Open Access Oxo‐bridged Zr dimers as well‐defined models of oxygen vacancies on ZrO2(2023) Wimmer, Erik J.; Klostermann, Sina V.; Ringenberg, Mark; Kästner, Johannes; Estes, Deven P.While ZrO2 is known to have a large effect on the activity and selectivity of the Cu/ZrO2 catalyst for methanol synthesis, its role in this process is poorly understood. Surface defects such as oxygen vacancies could play a role in the strong metal-support interaction (SMSI) between Cu and ZrO2. However, due to the complexity of the surfaces, the exact molecular nature of this interaction is not at present known. Here, we make well-defined models of both reduced and coordinatively unsaturated surface oxygen vacancies on ZrO2 using the molecular precursor [Cp2ZrCl]2(μ2-O) (1). Complex 1 can be reduced to form a complex (2) containing one Zr(III) center and a bridging hydride ligand (according to EPR and IR spectroscopy) derived from C-H activation of either thf or the Cp ring. Complex 2 reacts with CO2 to largely produce CO, suggesting that surface defects with similar structures probably do not play a role in the industrial catalyst. Halide abstraction from complex 1 results in the Lewis acidic species 3, which has similar Lewis acid properties to acidic defects on the ZrO2 surface. Similarities of both of these model species to real surface oxygen vacancies and their role in the catalytic reaction are discussed.