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- ItemCobalt-catalysed reductive C-H alkylation of indoles using carboxylic acids and molecular hydrogen(Cambridge : RSC, 2017) Cabrero-Antonino, Jose R.; Adam, Rosa; Junge, Kathrin; Beller, MatthiasThe direct CH-alkylation of indoles using carboxylic acids is presented for the first time. The catalytic system based on the combination of Co(acac)3 and 1,1,1-tris(diphenylphosphinomethyl)-ethane (Triphos, L1), in the presence of Al(OTf)3 as co-catalyst, is able to perform the reductive alkylation of 2-methyl-1H-indole with a wide range of carboxylic acids. The utility of the protocol was further demonstrated through the C3 alkylation of several substituted indole derivatives using acetic, phenylacetic or diphenylacetic acids. In addition, a careful selection of the reaction conditions allowed to perform the selective C3 alkenylation of some indole derivatives. Moreover, the alkenylation of C2 position of 3-methyl-1H-indole was also possible. Control experiments indicate that the aldehyde, in situ formed from the carboxylic acid hydrogenation, plays a central role in the overall process. This new protocol enables the direct functionalization of indoles with readily available and stable carboxylic acids using a non-precious metal based catalyst and hydrogen as reductant.
- ItemGeneral and selective deoxygenation by hydrogen using a reusable earth-abundant metal catalyst(Washington, D.C. : American Association for the Advancement of Science, 2019) Schwob, T.; Kunnas, P.; De, Jonge, N.; Papp, C.; Steinrück, H.-P.; Kempe, R.Chemoselective deoxygenation by hydrogen is particularly challenging but crucial for an efficient late-stage modification of functionality-laden fine chemicals, natural products, or pharmaceuticals and the economic upgrading of biomass-derived molecules into fuels and chemicals. We report here on a reusable earth-abundant metal catalyst that permits highly chemoselective deoxygenation using inexpensive hydrogen gas. Primary, secondary, and tertiary alcohols as well as alkyl and aryl ketones and aldehydes can be selectively deoxygenated, even when part of complex natural products, pharmaceuticals, or biomass-derived platform molecules. The catalyst tolerates many functional groups including hydrogenation-sensitive examples. It is efficient, easy to handle, and conveniently synthesized from a specific bimetallic coordination compound and commercially available charcoal. Selective, sustainable, and cost-efficient deoxygenation under industrially viable conditions seems feasible. © 2019 The Authors.
- ItemSelective Earth-Abundant System for CO2 Reduction: Comparing Photo- and Electrocatalytic Processes(Washington, DC : American Chemical Society, 2019) Steinlechner C.; Roesel A.F.; Oberem E.; Päpcke A.; Rockstroh N.; Gloaguen F.; Lochbrunner S.; Ludwig R.; Spannenberg A.; Junge H.; Francke R.; Beller M.The valorization of CO2 via photo- or electrocatalytic reduction constitutes a promising approach toward the sustainable production of fuels or value-added chemicals using intermittent renewable energy sources. For this purpose, molecular catalysts are generally studied independently with respect to the photo- or the electrochemical application, although a unifying approach would be much more effective with respect to the mechanistic understanding and the catalyst optimization. In this context, we present a combined photo- and electrocatalytic study of three Mn diimine catalysts, which demonstrates the synergistic interplay between the two methods. The photochemical part of our study involves the development of a catalytic system containing a heteroleptic Cu photosensitizer and the sacrificial BIH reagent. The system shows exclusive selectivity for CO generation and renders turnover numbers which are among the highest reported thus far within the group of fully earth-abundant photocatalytic systems. The electrochemical part of our investigations complements the mechanistic understanding of the photochemical process and demonstrates that in the present case the sacrificial reagent, the photosensitizer, and the irradiation source can be replaced by the electrode and a weak Brønsted acid. © 2019 American Chemical Society.
- ItemEffect of synthesis catalyst on structure of nitrogen-doped carbon nanotubes and electrical conductivity and electromagnetic interference shielding of their polymeric nanocomposites(New York, NY [u.a.] : Pergamon Press, 2016) Arjmand, Mohammad; Chizari, Kambiz; Krause, Beate; Pötschke, Petra; Sundararaj, UttandaramanDifferent catalysts including Co, Fe, and Ni were used to synthesize nitrogen-doped carbon nanotubes (N-CNTs) by chemical vapor deposition technique. Synthesized N-CNTs were melt mixed with a polyvinylidene fluoride (PVDF) matrix using a small scale mixer at different concentrations ranging from 0.3 to 3.5 wt%, and then compression molded. The characterization techniques revealed significant differences in the synthesis yield and the morphological and electrical properties of both N-CNTs and nanocomposites depending on the catalyst type. Whereas Co and Fe resulted in yields comparable to industrial multiwalled CNTs, Ni was much less effective. The N-CNT aspect ratio was the highest for Co catalyst, followed by Ni and Fe, whereas nitrogen content was the highest for Ni. Raman spectroscopy revealed lowest defect number and highest N-CNT crystallinity for Fe catalyst. Characterization of N-CNT/PVDF nanocomposites showed better dispersion for N-CNTs based on Co and Fe as compared to Ni, and the following order of electrical conductivity and electromagnetic interference shielding (from high to low): Co > Fe > Ni. The superior electrical properties of (N-CNT)Co nanocomposites were ascribed to a combination of high synthesis yield, high aspect ratio, low nitrogen content and high crystallinity of N-CNTs combined with a good state of N-CNT dispersion.
- ItemCatalyst-free site-specific surface modifications of nanocrystalline diamond films via microchannel cantilever spotting(London : RSC Publishing, 2016) Davydova, Marina; de los Santos Pereira, Andres; Bruns, Michael; Kromka, Alexander; Ukraintsev, Egor; Hirtz, Michael; Rodriguez-Emmenegger, CesarThe properties of nanocrystalline diamond (NCD) films offer great potential for the creation of various sensing and photonic devices. A great challenge in order to materialize such applications lies in achieving the micrometrically resolved functionalization of NCD surfaces. In the present work, we introduce a facile approach to meet this challenge employing the novel strain-promoted alkyne–azide cycloaddition “click” chemistry reaction, a catalyst-free ligation protocol compatible with biomolecules. The ability to achieve well-resolved multicomponent patterns with high reproducibility is demonstrated, paving the way for the fabrication of novel devices based on micropatterned NCD films.
- ItemHydration of nitriles using a metal-ligand cooperative ruthenium pincer catalyst(Cambridge : RSC, 2019) Guo, Beibei; de Vries, Johannes G.; Otten, EdwinNitrile hydration provides access to amides that are important structural elements in organic chemistry. Here we report catalytic nitrile hydration using ruthenium catalysts based on a pincer scaffold with a dearomatized pyridine backbone. These complexes catalyze the nucleophilic addition of H2O to a wide variety of aliphatic and (hetero)aromatic nitriles in tBuOH as solvent. Reactions occur under mild conditions (room temperature) in the absence of additives. A mechanism for nitrile hydration is proposed that is initiated by metal-ligand cooperative binding of the nitrile. This journal is © The Royal Society of Chemistry.
- ItemEmulsion soft templating of carbide-derived carbon nanospheres with controllable porosity for capacitive electrochemical energy storage(Cambridge : Royal Society of Chemistry, 2015) Oschatz, Martin; Zeiger, Marco; Jaeckel, Nicolas; Strubel, Patrick; Borchardt, Lars; Reinhold, Romy; Nickel, Winfried; Eckert, Jürgen; Presser, Volker; Kaskel, StefanA new approach to produce carbide-derived carbon nanospheres of 20-200 nm in diameter based on a novel soft-templating technique is presented. Platinum catalysis is used for the cross-linking of liquid (allylhydrido)polycarbosilane polymer chains with para-divinylbenzene within oil-in-water miniemulsions. Quantitative implementation of the pre-ceramic polymer can be achieved allowing precise control over the resulting materials. After pyrolysis and high-temperature chlorine treatment, resulting particles offer ideal spherical shape, very high specific surface area (up to 2347 m^2/g^-1), and large micro/mesopore volume (up to 1.67 cm^3/g^-1). The internal pore structure of the nanospheres is controllable by the composition of the oil phase within the miniemulsions. The materials are highly suitable for electrochemical double-layer capacitors with high specific capacitances in aqueous 1 M Na2SO4 solution (110 F/g^-1) and organic 1 M tetraethylammonium tetrafluoroborate in acetonitrile (130 F/g^-1).
- ItemSelective cobalt nanoparticles for catalytic transfer hydrogenation of N-heteroarenes(Cambridge : RSC, 2017) Chen, Feng; Sahoo, Basudev; Kreyenschulte, Carsten; Lund, Henrik; Zeng, Min; He, Lin; Junge, Kathrin; Beller, MatthiasNitrogen modified cobalt catalysts supported on carbon were prepared by pyrolysis of the mixture generated from cobalt(ii) acetate in aqueous solution of melamine or waste melamine resins, which are widely used as industrial polymers. The obtained nanostructured materials catalyze the transfer hydrogenation of N-heteroarenes with formic acid in the absence of base. The optimal Co/Melamine-2@C-700 catalyst exhibits high activity and selectivity for the dehydrogenation of formic acid into molecular hydrogen and carbon dioxide and allows for the reduction of diverse N-heteroarenes including substrates featuring sensitive functional groups.
- ItemContinuous synthesis of diethyl carbonate from ethanol and CO2 over Ce–Zr–O catalysts(London : RSC Publ., 2015) Prymak, Iuliia; Kalevaru, Venkata Narayana; Wohlrab, Sebastian; Martin, AndreasCexZr1−xO2 (x = 0, 0.2, 0.5, 0.8 and 1.0) solids were prepared by a citrate method and characterized by various techniques such as N2-adsorption (BET-SA), XRD, XPS, TEM, H2-TPR, NH3- and CO2-TPD. The catalytic performance of these solids was evaluated for the direct synthesis of diethyl carbonate (DEC) from ethanol and CO2 in continuous mode using a plug-flow reactor (PFR). According to thermodynamic data, the reaction is favourable at low reaction temperatures and high reaction pressures. Thus, the catalytic experiments were carried out at reaction temperatures ranging from 80 to 180 °C and at reaction pressures from 80 to 180 bar. The CexZr1−xO2 catalysts exhibited significant differences in their performance mainly depending on (i) their Ce : Zr ratio and (ii) the different acid–base characteristics. Among the series Ce0.8Zr0.2O2 (C80Z) and Ce0.5Zr0.5O2 (C50Z) catalysts displayed the most efficient performance. Moreover, C80Z, pretreated at 700 °C, yielded DEC at the equilibrium conversion level of YDEC ~ 0.7% at 140 °C and 140 bar at a CO2 : ethanol ratio of 6 : 1 at a LHSV of 42 Lliq kgcat−1 h−1.
- ItemEffects of synthesis catalyst and temperature on broadband dielectric properties of nitrogen-doped carbon nanotube/polyvinylidene fluoride nanocomposites(New York, NY [u.a.] : Pergamon Press, 2016) Ameli, A.; Arjmand, M.; Pötschke, Petra; Krause, Beate; Sundararaj, U.This study reports on nitrogen-doped carbon nanotube (N-CNT)/polymer nanocomposites exhibiting relatively high and frequency independent real permittivity (ϵ′) together with low dielectric loss (tan δ). N-CNTs were synthesized by chemical vapor deposition, and their nanocomposites were prepared by melt-mixing with polyvinylidene fluoride (PVDF). In the synthesis of N-CNTs, three catalysts of Co, Fe and Ni, and three temperatures of 650, 750 and 950 °C were employed. The morphology, aspect ratio, synthesis yield, remaining residue, nitrogen content, nitrogen bonding type, and powder conductivity of N-CNTs, and the morphology, polar crystalline phase, and broadband dielectric properties of N-CNT/PVDF nanocomposites were investigated. The results revealed that by proper selection of synthesis catalyst (Fe) and temperature (650 °C and 950 °C), nitrogen doping generated polarizable nanotubes via providing local polarization sites, and resulted in nanocomposites with favorable dielectric properties for charge storage applications at N-CNT loadings as low as 1.0 wt%. As a result, 3.5 wt% (N-CNT)Fe/950°C/PVDF nanocomposites exhibited an insulative behavior with ϵ' = 23.12 and tan δ = 0.05 at 1 kHz, a combination superior to that of PVDF, i.e., ϵ' = 8.4 and tan δ = 0.03 and to those of percolative nanocomposites, e.g., ϵ' = 71.20 and tan δ = 63.20 for 3.5 wt% (N-CNT)Fe/750°C/PVDF. Also, the relationships between the dielectric properties, N-CNT structure, and nanocomposite morphology were identified.