Browsing by Subject "Heterogeneous catalysis"

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    ItemOpen Access
    Bimetallic hydroxide catalysts for aerobic C-H activation
    (2024-01) Erdivan, Beyzanur
    The increasing interest in the oxidation of sp3 C-H and O-H bonds has garnered tremendous attention due to its potential for facile production of oxygenated organics. Precious metal-free bimetallic hydroxide-based materials are commonly employed in various applications such as batteries and photocatalysts. However, their prospects in C-H activation reactions have been poorly explored. This research focuses on the development and evaluation of a bimetallic Fe-Mn hydroxide catalyst for aerobic C-H activation and O-H oxidation reactions without the need for an initiator. The Fe-Mn hydroxide catalyst was synthesized and carefully optimized to enhance its catalytic efficiency in the direct oxygenation of a wide scope of alkylarene compounds through C-H functionalization and oxidation of benzylic alcohols. A series of Fe-Mn bimetal hydroxides with different Fe/Mn ratios were synthesized using a customized chemical co-precipitation method. These catalysts were then tested for the catalytic oxidation of fluorene to fluorenone using molecular oxygen as the sole oxidant, with the Fe0.6Mn0.4(OH)y-12S catalyst demonstrating the best performance. Under mild reaction conditions, the catalyst exhibited remarkable performance in activating C-H bonds using molecular oxygen as the oxidant. Various substrates, including alkylarenes and alcohols, were investigated, consistently yielding high yields of oxygenated products with minimal catalyst loadings. XRD, XPS, XANES, ICP-MS, BET, and TGA were employed to gain insights into the structural features of the catalyst. Our findings indicate that the following structural properties of the optimized Fe0.6Mn0.4(OH)y-12S catalyst could be responsible for the currently observed enhanced catalytic reactivity: i) unique Mn oxidation state (ca. Mn2.6+), ii) Fe cationic sites containing a mixture of Fe2+ and Fe3+ species, where Fe3+ species are the dominating species, iii)realtively low specific surface area of 68 m2/g, iv) relatively disordered and defective crystal structure comprised of bimetallic hydroxides as well as additional oxide/oxyhydroxide phases, v) residual Na+ surface species enabling electronic promotion of the cationic active sites via electron donation.
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    ItemOpen Access
    Highly-dispersed iridium catalysts with sub-nanometer diameters for carbon monoxide oxidation
    (2021-09) Hosseini, Seyedsaber
    Novel catalytic architectures composed of catalytic centers with sub-nanometer diameters for CO oxidation reaction were designed, synthesized, and characterized. Accordingly, well-dispersed iridium precious metal active sites were supported on various catalytic support materials. Namely, magnesium oxide (MgO), ceria (CeO2), lanthana-zirconia (La2O3–ZrO2) and titania-zirconia (TiO2–ZrO2) systems were chosen as different support systems. The favorable catalytic effect of highly-dispersed Ir active sites with sub-nanometer diameters were demonstrated in flow-mode catalytic performance tests, where the lower loadings of highly dispersed Ir sites showed comparable catalytic activity in CO oxidation to that of bigger Ir clusters with higher metal loading. Furthermore, influence of the catalyst pre-treatment conditions (e.g., reduction in H2, oxidation in O2, and calcination in air) on the catalyst structure and performance were also studied via XRD, Raman, BET, XPS, TEM, EDX, and in-situ FTIR spectroscopy techniques. Our results indicate that in all the catalytic systems, high-dispersion Ir sites can be generated on supports where Ir exists as small clusters with < 1 nm particle size. Moreover, catalyst pretreatment conditions revealed noticeable alterations in the catalyst structure in terms of average support particle size, reduction extent of the support, specific surface area, pore volume, pore size, and Ir oxidation state. Finally, catalytic performance results indicated that under reaction conditions yielding close to 100% CO conversion, 0.2 and 0.5 wt.% Ir catalysts led to comparable performance to that of 1 wt.% Ir catalyst demonstrating the advantage of catalytic systems with highly dispersed sub-nanometer diameter active sites with extremely low metal loading.
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    ItemEmbargo
    Na-promoted bimetallic hydroxide nanoparticles for aerobic c-h activation catalyst design principles and insights into reaction mechanism
    (American Chemical Society, 2024-10-25) Erdivan, Beyzanur; Çalıkyılmaz, Eylül; Bilgin, Suay; Erdali, Ayşe Dilay; Gül, Damla Nur; Ercan, Kerem Emre; Türkmen, Yunus Emre; Özensoy, Emrah
    A precious metal-free bimetallic Fe x Mn1-x (OH) y hydroxide catalyst was developed that is capable of catalyzing aerobic C-H oxidation reactions at low temperatures, without the need for an initiator, relying sustainably on molecular oxygen. Through a systematic synthetic effort, we scanned a wide nanoparticle synthesis parameter space to lay out a detailed set of catalyst design principles unraveling how the Fe/Mn cation ratio, NaOH(aq) concentration used in the synthesis, catalyst washing procedures, extent of residual Na+ promoters on the catalyst surface, reaction temperature, and catalyst loading influence catalytic C-H activation performance as a function of the electronic, surface chemical, and crystal structure of Fe x Mn1-x (OH) y bimetallic hydroxide nanostructures. Our comprehensive XRD, XPS, BET, ICP-MS, 1H NMR, and XANES structural/product characterization results as well as mechanistic kinetic isotope effect (KIE) studies provided the following valuable insights into the molecular level origins of the catalytic performance of the bimetallic Fe x Mn1-x (OH) y hydroxide nanostructures: (i) catalytic reactivity is due to the coexistence and synergistic operation of Fe3+ and Mn3+ cationic sites (with minor contributions from Fe2+ and Mn2+ sites) on the catalyst surface, where in the absence of one of these synergistic sites (i.e., in the presence of monometallic hydroxides), catalytic activity almost entirely vanishes, (ii) residual Na+ species on the catalyst surface act as efficient electronic promoters by increasing the electron density on the Fe3+ and Mn3+ cationic sites, which in turn, presumably enhance the electrophilic adsorption of organic reactants and strengthen the interaction between molecular oxygen and the catalyst surface, (iii) in the fluorene oxidation reaction the step dictating the reaction rate likely involved the breaking of a C-H bond (k H /k D = 2.4), (iv) reactivity patterns of a variety of alkylarene substrates indicate that the C-H bond cleavage follows a stepwise PT-ET (proton transfer-electron transfer) pathway.
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    ItemOpen Access
    Precious metal-Free LaMnO3 perovskite catalyst with an optimized nanostructure for aerobic C–H bond activation reactions: alkylarene oxidation and naphthol dimerization
    (American Chemical Society, 2021-02-03) Şahin, Yeşim; Sika-Nartey, Abel Tetteh; Ercan, Kerem Emre; Koçak, Yusuf; Senol, Sinem; Özensoy, Emrah; Türkmen, Yunus Emre
    In this article, we describe the development of a new aerobic C–H oxidation methodology catalyzed by a precious metal-free LaMnO3 perovskite catalyst. Molecular oxygen is used as the sole oxidant in this approach, obviating the need for other expensive and/or environmentally hazardous stoichiometric oxidants. The electronic and structural properties of the LaMnO3 catalysts were systematically optimized, and a reductive pretreatment protocol was proved to be essential for acquiring the observed high catalytic activities. It is demonstrated that this newly developed method was extremely effective for the oxidation of alkylarenes to ketones as well as for the oxidative dimerization of 2-naphthol to 1,1-binaphthyl-2,2-diol (BINOL), a particularly important scaffold for asymmetric catalysis. Detailed spectroscopic and mechanistic studies provided valuable insights into the structural aspects of the active catalyst and the reaction mechanism.
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    ItemOpen Access
    Surface decoration of Pt nanoparticles via ALD with TiO2 protective layer on polymeric nanofibers as flexible and reusable heterogeneous nanocatalysts
    (Nature Publishing Group, 2017) Celebioglu, A.; Ranjith, K. S.; Eren, H.; Bıyıklı, Necmi; Uyar, T.
    Coupling the functional nanoheterostructures over the flexible polymeric nanofibrous membranes through electrospinning followed by the atomic layer deposition (ALD), here we presented a high surface area platform as flexible and reusable heterogeneous nanocatalysts. Here, we show the ALD of titanium dioxide (TiO2) protective nanolayer onto the electrospun polyacrylonitrile (PAN) nanofibrous web and then platinum nanoparticles (Pt-NP) decoration was performed by ALD onto TiO2 coated PAN nanofibers. The free-standing and flexible Pt-NP/TiO2-PAN nanofibrous web showed the enhancive reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) within 45 seconds though the hydrogenation process with the degradation rate of 0.1102 s-1. The TiO2 protective layer on the PAN polymeric nanofibers was presented as an effective route to enhance the attachment of Pt-NP and to improve the structure stability of polymeric nanofibrous substrate. Commendable enhancement in the catalytic activity with the catalytic dosage and the durability after the reusing cycles were investigated over the reduction of 4-NP. Even after multiple usage, the Pt-NP/TiO2-PAN nanofibrous webs were stable with the flexible nature with the presence of Pt and TiO2 on its surface.
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    ItemOpen Access
    Two-dimensional bimetallic hydroxide nanostructures for catalyzing low-temperature aerobic C–H bond activation in alkylarene and alcohol partial oxidation
    (American Chemical Society, 2022-12-08) Sika-Nartey, Abel Tetteh; Sahin, Yesim; Ercan, Kerem Emre; Kap, Zeynep; Kocak, Yusuf; Erdali, Ayşe Dilay; Erdivan, Beyzanur; Türkmen, Yunus Emre; Ozensoy, Emrah
    Two-dimensional (2D) bimetallic NixMn1–x(OH)y layered double hydroxide (LDH) nanostructures were synthesized and optimized as a remarkably active catalytic platform for low-temperature aerobic C–H bond activation in alkylarenes and partial oxidation of alcohols using a wide substrate (i.e., reactant) and diverse solvent scope. The NixMn1–x(OH)y structure consists of nonprecious and earth-abundant metals that can effectively operate at low catalyst loadings, requiring only molecular oxygen as the stoichiometric oxidant. Structurally diverse alkylarenes as well as primary and secondary alcohols were shown to be competent substrates where oxidation products were obtained in excellent yields (93–99%). Comprehensive catalyst structural characterization via XRD, ATR-IR, TEM, EDX, XPS, BET, and TGA indicated that the ultimately optimized Ni0.6Mn0.4(OH)y-9S catalyst possessed not only particular rotational faults in its β-Ni0.6Mn0.4(OH)y domains but also distinct α/β-Ni0.6Mn0.4(OH)y interstratification disorders, in addition to a relatively high specific surface area of 125 m2/g, a 2D platelet morphology, and an average Mn oxidation state of +3.5, suggesting the presence of both Mn3+ and Mn4+ species in its structure working in a synergistic fashion with the Ni2+/x+ cations (the latter is justified by the lack of catalytic activity in the monometallic LDH catalysts Ni(OH)2 and Mn(OH)2). Kinetic isotope effect studies carried out in the fluorene oxidation reaction (kH/kD = 5.7) revealed that the rate-determining step of the catalytic oxidation reaction directly involved the scission of a C–H bond. Moreover, the optimized catalyst was demonstrated to be reusable through the application of a regeneration protocol, which can redeem the full initial activity of the carbon-poisoned spent catalyst in the fluorene oxidation reaction.