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- ItemRelations between Structure, Activity and Stability in C3N4 Based Photocatalysts Used for Solar Hydrogen Production(Basel : MDPI, 2018-1-29) Sivasankaran, Ramesh P.; Rockstroh, Nils; Hollmann, Dirk; Kreyenschulte, Carsten R.; Agostini, Giovanni; Lund, Henrik; Acharjya, Amitava; Rabeah, Jabor; Bentrup, Ursula; Junge, Henrik; Thomas, Arne; Brückner, AngelikaSolar hydrogen production from water could be a sustainable and environmentally friendly alternative to fossil energy carriers, yet so far photocatalysts active and stable enough for large-scale applications are not available, calling for advanced research efforts. In this work, H2 evolution rates of up to 1968 and 5188 μmol h−1 g−1 were obtained from aqueous solutions of triethanolamine (TEOA) and oxalic acid (OA), respectively, by irradiating composites of AgIn5S8 (AIS), mesoporous C3N4 (CN, surface area >150 m2/g) and ≤2 wt.% in-situ photodeposited Pt nanoparticles (NPs) with UV-vis (≥300 nm) and pure visible light (≥420 nm). Structural properties and electron transport in these materials were analyzed by XRD, STEM-HAADF, XPS, UV-vis-DRS, ATR-IR, photoluminescence and in situ-EPR spectroscopy. Initial H2 formation rates were highest for Pt/CN, yet with TEOA this catalyst deactivated by inclusion of Pt NPs in the matrix of CN (most pronounced at λ ≥ 300 nm) while it remained active with OA, since in this case Pt NPs were enriched on the outermost surface of CN. In Pt/AIS-CN catalysts, Pt NPs were preferentially deposited on the surface of the AIS phase which prevents them from inclusion in the CN phase but reduces simultaneously the initial H2 evolution rate. This suggests that AIS hinders transport of separated electrons from the CN conduction band to Pt NPs but retains the latter accessible by protons to produce H2.
- ItemOrigins of high catalyst loading in copper(i)-catalysed Ullmann-Goldberg C-N coupling reactions(Cambridge : RSC, 2017) Sherborne, Grant J.; Adomeit, Sven; Menzel, Robert; Rabeah, Jabor; Brückner, Angelika; Fielding, Mark R.; Willans, Charlotte E.; Nguyen, Bao N.A mechanistic investigation of Ullmann-Goldberg reactions using soluble and partially soluble bases led to the identification of various pathways for catalyst deactivation through (i) product inhibition with amine products, (ii) by-product inhibition with inorganic halide salts, and (iii) ligand exchange by soluble carboxylate bases. The reactions using partially soluble inorganic bases showed variable induction periods, which are responsible for the reproducibility issues in these reactions. Surprisingly, more finely milled Cs2CO3 resulted in a longer induction period due to the higher concentration of the deprotonated amine/amide, leading to suppressed catalytic activity. These results have significant implications on future ligand development for the Ullmann-Goldberg reaction and on the solid form of the inorganic base as an important variable with mechanistic ramifications in many catalytic reactions.
- ItemImpact of Al Activators on Structure and Catalytic Performance of Cr Catalysts in Homogeneous Ethylene Oligomerization : A Multitechnique in situ/operando Study(Weinheim : Wiley-VCH Verlag, 2019) Grauke, Reni; Schepper, Rahel; Rabeah, Jabor; Schoch, Roland; Bentrup, Ursula; Bauer, Matthias; Brückner, AngelikaThe effect of different AlR3 activators (R=methyl, ethyl, isobutyl, n-octyl) has been studied in comparison to modified methylaluminoxane (MMAO) by operando EPR as well as by in situ UV-vis, ATR-IR and XANES/EXAFS spectroscopy during oligomerization of ethylene at 20 bar and 40 °C with a homogeneous Cr complex catalyst formed in situ upon mixing a Cr(acac)3 precursor, a Ph2PN(iPr)PPh2 ligand (PNP) and the activator. Coordination of PNP to Cr(acac)3 is initiated only in the presence of an activator. Highest 1-octene productivity (detected during operando EPR measurements) was obtained with MMAO which promotes bidentate coordination of the ligand to form an active (PNP)CrII(CH3)2 chelate complex. Rising bulkiness of R in AlR3 leads to only monodentate coordination of PNP to the Cr center by one P atom and increasing reduction to CrI to a maximum extend of around 30 % for AlOct3. This lowers the catalytic performance, which is mainly governed by the mode of PNP coordination rather than by the CrI content. ©2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemA selective route to aryl-triphosphiranes and their titanocene-induced fragmentation(Cambridge : RSC, 2019) Schumann, André; Reiß, Fabian; Jiao, Haijun; Rabeah, Jabor; Siewert, Jan-Erik; Krummenacher, Ivo; Braunschweig, Holger; Hering-Junghans, ChristianTriphosphiranes are three-membered phosphorus cycles and their fundamental reactivity has been studied in recent decades. We recently developed a high-yielding, selective synthesis for various aryl-substituted triphosphiranes. Variation of the reaction conditions in combination with theoretical studies helped to rationalize the formation of these homoleptic phosphorus ring systems and highly reactive intermediates could be isolated. In addition we showed that a titanocene synthon [Cp2Ti(btmsa)] facilitates the selective conversion of these triphosphiranes into titanocene diphosphene complexes. This unexpected reactivity mode was further studied theoretically and experimental evidence is presented for the proposed reaction mechanism. This journal is © The Royal Society of Chemistry.
- ItemPractical Catalytic Cleavage of C(sp3)−C(sp3) Bonds in Amines(Weinheim : Wiley-VCH, 2019) Li, Wu; Liu, Weiping; Leonard, David K.; Rabeah, Jabor; Junge, Kathrin; Brgckner, Angelika; Beller, MatthiasThe selective cleavage of thermodynamically stable C(sp3)−C(sp3) single bonds is rare compared to their ubiquitous formation. Herein, we describe a general methodology for such transformations using homogeneous copper-based catalysts in the presence of air. The utility of this novel methodology is demonstrated for Cα−Cβ bond scission in >70 amines with excellent functional group tolerance. This transformation establishes tertiary amines as a general synthon for amides and provides valuable possibilities for their scalable functionalization in, for example, natural products and bioactive molecules. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemTracing Active Sites in Supported Ni Catalysts during Butene Oligomerization by Operando Spectroscopy under Pressure(Washington, DC : ACS, 2016) Rabeah, Jabor; Radnik, Jörg; Briois, Valérie; Maschmeyer, Dietrich; Stochniol, Guido; Peitz, Stephan; Reeker, Helene; La Fontaine, Camille; Brückner, AngelikaSupported Ni catalysts have been studied during the dimerization of butenes by operando electron paramagnetic resonance (EPR) and in situ X-ray absorption spectroscopy (XAS) at 353 K and up to 16 bar. Single NiI/NiII shuttles were identified as active sites, whereby the conversion of initial NiI to NiII by oxidative addition of butene is obviously faster than the re-reduction of NiII to NiI by reductive elimination of the C8 product, rendering the equilibrium percentage of NiI small. At p ≤ 2 bar, NiI single sites form inactive Ni0 aggregates, while this is suppressed at higher pressure (∼12 bar). A reaction mechanism is proposed.