Prof. Dr. Wolfgang Kleist

Industrial Chemistry – Nanostructured Catalyst Materials
Ruhr-Universität Bochum

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  • Increasing the Complexity in the MIL-53 Structure: The Combination of the Mixed-Metal and the Mixed-Linker Concepts
    Bitzer, J. and Teubnerová, M. and Kleist, W.
    Chemistry - A European Journal 27 (2021)
    The isoreticular mixed-component concept is a promising approach to tailor the material properties of metal–organic frameworks. While isoreticular mixed-metal or mixed-linker materials are commonly synthesized, the combination of both concepts for the development of isoreticular materials featuring both two metals and two linkers is still rarely investigated. Herein, we present the development of mixed-metal/mixed-linker MIL-53 materials that contain different metal combinations (Al/Sc, Al/V, Al/Cr, Al/Fe) and different linker ratios (terephthalate/2-aminoterephthalate). The possibility of changing the metal combination and the linker ratio independently from each other enables a large variety of modifications. A thorough characterization (PXRD, ATR-IR, TGA, 1H NMR, ICP-OES) confirmed that all components were incorporated into the framework structure with a statistical distribution. Nitrogen physisorption measurements showed that the breathing behavior can be tailored by adjusting the linker ratio for all metal combinations. All materials were successfully used for post-synthetic modification reactions with maleic anhydride. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/chem.202003304
  • Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal Metal–Organic Frameworks
    Bitzer, J. and Otterbach, S. and Thangavel, K. and Kultaeva, A. and Schmid, R. and Pöppl, A. and Kleist, W.
    Chemistry - A European Journal 26 (2020)
    Metal–organic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixed-metal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixed-metal metal–organic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixed-metal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder X-ray diffraction, X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixed-metal FeCuBTC material with HKUST-1 structure, which contained bimetallic Fe−Cu paddlewheels as well as monometallic Cu−Cu and Fe−Fe units under optimized synthesis conditions. The in-depth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixed-metal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixed-metal metal–organic framework materials and the thorough characterization thereof is particularly important to derive structure–property or structure–activity correlations. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/chem.201905596
  • Formic Acid-Assisted Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Bifunctional Pd Nanoparticles Supported on N-Doped Mesoporous Carbon
    Hu, B. and Warczinski, L. and Li, X. and Lu, M. and Bitzer, J. and Heidelmann, M. and Eckhard, T. and Fu, Q. and Schulwitz, J. and Merko, M. and Li, M. and Kleist, W. and Hättig, C. and Muhler, M. and Peng, B.
    Angewandte Chemie - International Edition (2020)
    Biomass-derived 5-hydroxymethylfurfural (HMF) is regarded as one of the most promising platform chemicals to produce 2,5-dimethylfuran (DMF) as a potential liquid transportation fuel. Pd nanoparticles supported on N-containing and N-free mesoporous carbon materials were prepared, characterized, and applied in the hydrogenolysis of HMF to DMF under mild reaction conditions. Quantitative conversion of HMF to DMF was achieved in the presence of formic acid (FA) and H2 over Pd/NMC within 2 h. The reaction mechanism, especially the multiple roles of FA, was explored through a detailed comparative study by varying hydrogen source, additive, and substrate as well as by applying in situ ATR-IR spectroscopy. The major role of FA is to shift the dominant reaction pathway from the hydrogenation of the aldehyde group to the hydrogenolysis of the hydroxymethyl group via the protonation by FA at the C-OH group, lowering the activation barrier of the C−O bond cleavage and thus significantly enhancing the reaction rate. XPS results and DFT calculations revealed that Pd2+ species interacting with pyridine-like N atoms significantly enhance the selective hydrogenolysis of the C−OH bond in the presence of FA due to their high ability for the activation of FA and the stabilization of H−. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202012816
  • Hard X-ray-based techniques for structural investigations of CO2methanation catalysts prepared by MOF decomposition
    Prinz, N. and Schwensow, L. and Wendholt, S. and Jentys, A. and Bauer, M. and Kleist, W. and Zobel, M.
    Nanoscale 12 (2020)
    Thermal decomposition of metal-organic framework (MOF) precursors is a recent method to create well-dispersed metal centers within active catalyst materials with enhanced stability, as required for dynamic operation conditions in light of challenges caused by the renewable energy supply. Here, we use a hard X-ray-based toolbox of pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) analysis combined with X-ray diffraction and catalytic activity tests to investigate structure-activity correlations of methanation catalysts obtained by thermal decomposition of a Ni(BDC)(PNO) MOF precursor. Increasing the decomposition temperature from 350 to 500 °C resulted in Nifcc nanoparticles with increasing particle sizes, alongside a decrease in Ni2+ species and strain-induced peak broadening. For lower temperatures and inert atmosphere, Ni3C and NiO phases co-existed. A graphitic shell stabilized the Ni particles. Compared to an inert atmosphere, reducing conditions led to larger particles and a faster decomposition of the MOF precursor. Catalytic studies revealed that the decomposition at an intermediate temperature of 375 °C in 5% H2/He is the best set of parameters to obtain high specific surface areas while maintaining particle sizes that feature many active Ni centers for the formation of CH4. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d0nr01750g
  • Post-synthetic Modification of DUT-5-based Metal Organic Frameworks for the Generation of Single-site Catalysts and their Application in Selective Epoxidation Reactions
    Yildiz, C. and Kutonova, K. and Oßwald, S. and Titze-Alonso, A. and Bitzer, J. and Bräse, S. and Kleist, W.
    ChemCatChem 12 (2020)
    New single-site catalysts based on mixed-linker metal-organic frameworks with DUT-5 structure, which contain immobilized Co2+, Mn2+ and Mn3+ complexes, have successfully been synthesized via post-synthetic modification. 2,2’-Bipyridine-5,5’-dicarboxylate linkers were directly metalated, while 2-amino-4,4’-biphenyldicarboxylate linkers were post-synthetically modified by their conversion to Schiff-base ligands and a subsequent immobilization of the metal complexes. The resulting materials were used as catalysts in the selective epoxidation of trans-stilbene and the activities and selectivities of the different catalysts were compared. The influence of various reaction parameters on conversion, yield and selectivity were investigated. Very low catalyst amounts of 0.02 mol % were sufficient to obtain a high conversion of trans-stilbene using molecular oxygen from air as the oxidant. For cobalt-containing MOF catalysts, conversions up to 90 % were observed and, thus, they were more active than their manganese-containing counterparts. Recycling experiments and hot filtration tests proved that the reactions were mainly catalyzed via heterogeneous pathways. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cctc.201901434
  • Tailoring the breathing behavior of functionalized MIL-53(Al,M)-NH2 materials by using the mixed-metal concept
    Bitzer, J. and Heck, S.-L. and Kleist, W.
    Microporous and Mesoporous Materials 308 (2020)
    Mixed-metal metal-organic frameworks with two or more metals distributed over crystallographically equivalent sites are gaining growing attention in various research areas. We have extended the range of available mixed-metal frameworks of the MIL-53-NH2 family by combining aluminum with scandium, vanadium, chromium or iron. In contrast to other already reported mixed-metal MIL-53 materials, the developed MIL-53(Al,M)-NH2 materials (M = Sc, V, Cr, Fe), which contain 2-aminoterephthalate as linker instead of terephthalate, will enable subsequent post-synthetic modifications. All developed materials have been thoroughly characterized with standard methods as well as X-ray absorption spectroscopy to prove the exclusive incorporation of the employed metals into the MIL-53-NH2 framework structure and to verify the absence of impurity phases. Nitrogen physisorption measurements revealed that the choice of the metal combinations and ratios had a strong influence on the breathing behavior. Furthermore, post-synthetic modification reactions of the amine groups with maleic anhydride showed that the metal combination had a dramatic impact on the resulting modification degrees. Thus, the flexibility of the pore systems can be tailored by a careful selection of suitable metal combinations for these materials. © 2020 Elsevier Inc.
    view abstract10.1016/j.micromeso.2020.110329
  • The introduction of functional side groups and the application of the mixed-linker concept in divalent MIL-53(Ni) materials
    Bitzer, J. and Titze-Alonso, A. and Roshdy, A. and Kleist, W.
    Dalton Transactions 49 (2020)
    Metal-organic framework materials with functional side groups are commonly used for various purposes like post-synthetic modification reactions or the tuning of pore geometries. Additionally, the mixed-linker concept, in which different linkers are statistically distributed over equivalent lattice positions, is a versatile approach to adjust the number of functional groups within the framework. However, neither functional side groups nor the mixed-linker approach have been used in MIL-53 materials with divalent metal ions yet. In the present work, we report on the synthesis of MIL-53(Ni)-Br(100), which contains only 2-bromoterephthalate as a linker molecule, and mixed-linker MIL-53(Ni)-Br(50) containing both terephthalate and 2-bromoterephthalate. These two materials represent the first functionalized and mixed-linker MIL-53 materials in combination with a divalent metal. Consequently, the possibilities to tailor the properties of the still rarely used divalent MIL-53 materials were expanded. © 2020 The Royal Society of Chemistry.
    view abstract10.1039/d0dt01222j
  • Continuous production of higher alcohols from synthesis gas and ethanol using Cs-modified CuO/ZnO/Al2O3 catalysts
    Walter, K.M. and Serrer, M.-A. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 585 (2019)
    The sustainable synthesis of higher alcohols is important for future energy scenarios, mobility applications and the production of chemicals. This study presents the synthesis of higher alcohols from synthesis gas and ethanol using Cs-doped Cu/ZnO catalysts supported on Al2O3 with focus on continuous operation. Ethanol was added to overcome the rate-limiting step of the reaction, which corresponds to the C1 to C2 chain growth step. Catalysts prepared by wet impregnation showed the highest selectivity and yield to higher alcohols during screening in batch reactors with 1-butanol as the main alcohol product. The highest yield and selectivity to higher alcohols in a continuous process was found for a space velocity of 19,400 L(STP)/(kgcat⋅h), a temperature of 593 K and an ethanol to CO ratio of 0.3:1. In general, alcoholic products were received from aldol-type couplings of C1 or C2 intermediates with adsorbed alcohol derivates. Two types of coupling paths were observed: (i) retention of the oxygen of the Cn intermediate (1-propanol, branched alcohols with a methyl side chain and 2-butanol) and (ii) retention of the oxygen of the adsorbed alcohol (1-propanol and 1-butanol). Thus, products derived from the homocoupling of ethanol (1-butanol/2-butanol) allowed the identification of the reaction path. The corresponding reaction path could be influenced by changing the ethanol to CO ratio resulting in a shift of the alcohol products mainly from reaction path (i), to a mixture of products derived from both paths (i) and (ii) via coupling of C1 and C2 intermediates and, finally, to products mostly derived from the homocoupling of ethanol via path (ii). Aldehydes and ketones were identified as reaction intermediates. Ester production competed with the formation of alcohols. Lower ethanol to CO ratios reduced the formation of esters via side reactions. © 2019
    view abstract10.1016/j.apcata.2019.117150
  • Enhancing the water splitting performance of cryptomelane-type α-(K)MnO2
    Antoni, H. and Morales, D.M. and Bitzer, J. and Fu, Q. and Chen, Y.-T. and Masa, J. and Kleist, W. and Schuhmann, W. and Muhler, M.
    Journal of Catalysis 374 (2019)
    Sustainable energy storage systems require the development of non-precious metal catalysts for water splitting. Cryptomelane-type α-(K)MnO2 is one of the few oxides of manganese with promising electrocatalytic activity for the oxygen evolution reaction (OER). We report a strategy to boost the performance of cryptomelane-type α-(K)MnO2 in OER electrocatalysis in alkaline electrolyte by thermal treatments in specific gas atmospheres. The thermal treatment of α-(K)MnO2 at 300 °C in He, H2O/He and air can lower the potential required to reach a current density of 10 mA cm−2 for the OER by up to 60 mV. We discuss the structural changes on the atomic level, by combining X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, transmission electron microscopy and electrochemical impedance spectroscopy. The simultaneous presence of three structural features for MnOx-based water oxidation catalysts is considered essential: (i) Mn3+ sites, (ii) structural defects, and (iii) mono- and di-µ-oxo bridges, which can be tailored on the surface and in the bulk. Notably, extended X-ray absorption fine structure measurements suggest that more Mn3+ sites are detected in the corner-sharing positions, which are suggested to be the favorable sites for the electrocatalytic water oxidation. © 2019
    view abstract10.1016/j.jcat.2019.05.010
  • Regulating the size and spatial distribution of Pd nanoparticles supported by the defect engineered metal-organic framework HKUST-1 and applied in the aerobic oxidation of cinnamyl alcohol
    Guo, P. and Fu, Q. and Yildiz, C. and Chen, Y.-T. and Ollegott, K. and Froese, C. and Kleist, W. and Fischer, R.A. and Wang, Y. and Muhler, M. and Peng, B.
    Catalysis Science and Technology 9 (2019)
    The functional composite of metal nanoparticles (NPs) and defect-engineered metal-organic frameworks (DE-MOFs), NPs@DE-MOFs, is an emerging field of MOF materials chemistry. Herein, we report on a series of novel Pd-NPs@DE-HKUST-1(Cu/Pd) catalysts containing both micro- and mesopores through the incorporation of the defect-generating linker 2,6-pyridyldicarboxylate (pydc). The Pd NPs are formed by partial reduction of the Pd2+ sites of the pristine mixed-metal DE-HKUST-1(Cu/Pd) with methanol. The size regime and the spatial distribution of the Pd NPs can be controlled by the amount of framework-incorporated pydc. The samples exhibit superior catalytic activity in the aerobic oxidation of cinnamyl alcohol as compared to the parent HKUST-1. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9cy00560a
  • Synthetic Strategies and Structural Arrangements of Isoreticular Mixed-Component Metal–Organic Frameworks
    Bitzer, J. and Kleist, W.
    Chemistry - A European Journal 25 (2019)
    In recent years, the synthesis of mixed-metal and mixed-linker metal–organic frameworks with multiple metals and/or linker molecules combined in one framework has become a growing field of interest. These mixed-component or multivariate metal–organic framework materials provide the possibility to introduce multiple functionalities inside one framework. The interaction of guest molecules with different functionalities in the same material is a promising approach in the fields of gas storage, separation, catalysis and drug delivery. Furthermore, the combination of different components may lead to synergistic effects that cannot be achieved otherwise. These mixed-component approaches open up new pathways to an even larger range of possible customizations in the field of metal–organic frameworks. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201803887
  • CuPd Mixed-Metal HKUST-1 as a Catalyst for Aerobic Alcohol Oxidation
    Guo, P. and Froese, C. and Fu, Q. and Chen, Y.-T. and Peng, B. and Kleist, W. and Fischer, R.A. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 122 (2018)
    Metal-organic frameworks (MOFs) featuring isolated coordinatively unsaturated metal sites (CUS) have enormous potential as single-site catalysts. In particular, mixed-metal MOFs may exhibit unique catalytic properties compared to their monometallic counterparts. Herein, we report a thorough fundamental study on the mixed-metal CuPd-HKUST-1 ([Cu 3-x Pd x (BTC) 2 ] n ; BTC = 1,3,5-benzenetricarboxylate) including the two-step synthesis, characterization, and catalytic performance evaluation. The combined results from a multitechnique approach provide solid evidence that the chemical properties of HKUST-1 can be tuned via successful incorporation of Pd-CUS into the framework, leading to the formation of new Cu-Pd and/or Pd-Pd dimers. The introduction of Pd occurs exclusively at the metal nodes in a controlled manner while retaining the structural integrity. The incorporated Pd ions have an oxidation state of +2, whereas no PdO or metallic Pd nanoparticles embedded inside MOFs are detected. These mixed-metal CuPd-MOFs exhibit superior catalytic activity and selectivity for the aerobic oxidation of benzyl alcohol to benzaldehyde, and the doped Pd 2+ -CUS species are identified as isolated single-active sites. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcc.8b05882
  • Operando Raman spectroscopy on CO2 methanation over alumina-supported Ni, Ni3Fe and NiRh0.1 catalysts: Role of carbon formation as possible deactivation pathway
    Mutz, B. and Sprenger, P. and Wang, W. and Wang, D. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 556 (2018)
    The methanation of CO2, as a part of the power-to-gas concept, was studied under various industrially relevant feed compositions with a focus on the formation and influence of carbonaceous species. For this purpose, 5 wt.% Ni/Al2O3, 5 wt.% Ni3Fe/Al2O3 and 3.4 wt.% NiRh0.1/Al2O3 catalysts were prepared and characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). During the methanation of CO2, the Ni3Fe catalyst emerged as the most active and selective catalyst in the mid-temperature regime (300–350 °C). At 400 °C, all three tested catalysts showed high conversion of CO2 (67–75%; Ni > Ni3Fe > NiRh0.1) and selectivity towards CH4 (95–98%). Operando Raman spectroscopy was applied to elucidate the possible influence of carbonaceous species on the performance of the catalysts. Notably, no carbon deposition was observed under various feed compositions, even in CO2 or CO2/CH4 mixtures, e.g. as provided by biogas plants. Only in pure CH4 atmosphere an intensive carbon deposition with graphitic structure occurred as uncovered by operando Raman spectroscopy. Experiments in the lab-scale reactor and a spectroscopic microreactor could be correlated and revealed a strong catalytic deactivation of the carbon covered catalysts including a pronounced shift of the selectivity towards CO. The initial activity could be recovered after reactivation in H2 at elevated temperatures, which led to a removal of the deposits especially from the metal particles. Raman spectroscopy, supported by the results from high-resolution transmission electron microscopy (HRTEM) and EELS, revealed that carbon remained on the support material. The latter did not have any significant influence on the catalytic activity and could be removed in an oxidizing atmosphere. © 2018
    view abstract10.1016/j.apcata.2018.01.026
  • Reactivity of Bismuth Molybdates for Selective Oxidation of Propylene Probed by Correlative Operando Spectroscopies
    Sprenger, P. and Stehle, M. and Gaur, A. and Gänzler, A.M. and Gashnikova, D. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 8 (2018)
    α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6 as target bismuth molybdate phases were prepared by hydrothermal synthesis and flame spray pyrolysis and tested for their catalytic performance in the selective oxidation of propylene. Their structure and reactivity during temperature-programmed reaction (TPR) and under reaction conditions were investigated by in situ and operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and Raman spectroscopy. To gain insight into amorphous and crystalline structures at the same time, XAS and XRD as well as XAS and Raman spectroscopy were combined in one experiment. TPR in propylene revealed that the reduction of Mo6+ to Mo4+ occurred at lower temperatures than from Bi3+ to Bi0 in scheelite-structured systems. In a reaction cycle, mainly reduction of molybdenum was observed and EXAFS fitting confirmed the removal of oxygen from MoO4 2- entities. Minor structural transformations were detected by XRD and Raman spectroscopy. The catalytic performance of aurivillius-structured systems was more diverse than for scheelite-based ones and ranged from highest to lowest observed acrolein yield, probably due to a synergy effect of two or more bismuth molybdate phases. For phase pure systems, bismuth was more easily reduced than molybdenum. In contrast, aurivillius structures with additional phases showed reduction and oxygen removal from both metal centers under steady-state conditions, but molybdenum was in most cases more easily reduced. A high catalytic activity mostly coincided with low reduction temperatures, except for the unselective pure γ-Bi2MoO6 that showed a facilitated reduction of bismuth compared to molybdenum. Hence, the combination of operando methods led to an understanding of the redox behavior of bismuth and molybdenum and their influence on the catalytic performance. © 2018 American Chemical Society.
    view abstract10.1021/acscatal.8b00696
  • Supported gold- and silver-based catalysts for the selective aerobic oxidation of 5-(hydroxymethyl)furfural to 2,5-furandicarboxylic acid and 5-hydroxymethyl-2-furancarboxylic acid
    Schade, O.R. and Kalz, K.F. and Neukum, D. and Kleist, W. and Grunwaldt, J.-D.
    Green Chemistry 20 (2018)
    The sustainable synthesis of two important intermediates relevant for the production of bio-based polymers, 2,5-furandicarboxylic acid (FDCA) and 5-hydroxymethyl-2-furancarboxylic acid (HFCA), via oxidation of 5-(hydroxymethyl)furfural (HMF) was investigated using supported gold- and silver-based catalysts in water with air as the oxidant. High yields and selectivities for the production of FDCA (89%) and HFCA (≥98%) were achieved under the optimized reaction conditions with Au/ZrO2 and Ag/ZrO2 catalysts, respectively. While FDCA was mainly formed in the presence of gold catalysts at a maximum productivity of 67 molFDCA h-1 molAu -1, silver catalysts showed a remarkably high activity in aldehyde oxidation producing HFCA in almost quantitative yields with a maximum productivity of 400 molHFCA h-1 molAg -1. By variation of the reaction parameters, the Au/ZrO2 catalyst could be tuned to produce also HFCA, whereas the Ag/ZrO2 catalyst exclusively produced HFCA in a wide range of reaction parameters. The observed differences in catalyst selectivities can be taken as a starting point for further mechanistic investigation on the oxidation of HMF, contributing to a fundamental understanding of this reaction which is particularly important for establishing the production of bio-based polymers. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8gc01340c
  • Aerobic oxidation of α-pinene catalyzed by homogeneous and MOF-based Mn catalysts
    Raupp, Y.S. and Yildiz, C. and Kleist, W. and Meier, M.A.R.
    Applied Catalysis A: General 546 (2017)
    Manganese catalysts were investigated for the liquid-phase aerobic oxidation of α‐pinene. First, the influence of reaction parameters such as time, solvent, temperature, oxidant flow rate and catalyst concentration on conversion, yield and selectivity were evaluated using Mn(III) acetate as homogeneous catalyst. Mn(III) acetate afforded pinene oxide as the main product along with small amounts of verbenol and verbenone. The optimized reaction conditions were likewise applied to a novel mixed-linker metal-organic framework (MOF) based on MIL-53-NH2(Al), which was successfully synthesized and modified in a two-step post-synthetic reaction using maleic anhydride and Mn(III) acetate. The performance of the Mn-containing MOF catalyst was directly compared to its homogeneous counterpart Mn(III) acetate, showing very similar activity in a mixture of diethyl carbonate/dimethylformamide (DEC/DMF) as solvent. In both cases, a conversion of 31% and the formation of 17% pinene oxide (55% selectivity) were observed after 6 h. The heterogeneous MOF catalyst was easily removed from the reaction mixture by filtration and reused for at least five catalytic cycles without significant loss of activity. A hot filtration test showed that the catalysis mainly proceeded heterogeneously, although minor contributions of homogeneous species could not be completely excluded. © 2017 Elsevier B.V.
    view abstract10.1016/j.apcata.2017.07.047
  • Continuous Synthesis of γ-Valerolactone in a Trickle-Bed Reactor over Supported Nickel Catalysts
    Hengst, K. and Ligthart, D.A.J.M. and Doronkin, D.E. and Walter, K.M. and Kleist, W. and Hensen, E.J.M. and Grunwaldt, J.-D.
    Industrial and Engineering Chemistry Research 56 (2017)
    Various Ni-based catalysts were tested in the continuous liquid phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) in a trickle-bed reactor using water as solvent with the aim to develop an economic and environmentally friendly way for the GVL synthesis. For this purpose, various synthesis methods were used to prepare Ni-based catalysts, which were first screened in batch reactors. Characterization by X-ray diffraction, temperature-programmed reduction, electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy showed that slow precipitation using urea resulted in a good Ni dispersion. The dispersion also improved at lower Ni loading, and smaller Ni particles mostly showed an enhanced catalytic performance for the synthesis of GVL. 5 wt % Ni/Al2O3 prepared by wet impregnation showed the highest specific activity for the hydrogenation of LA to GVL (90% LA conversion and 75% GVL yield) featuring an average Ni particle size of 6 nm. Some deactivation of the catalysts was observed, probably due to transformation of γ-Al2O3 to boehmite and sintering of the Ni particles. In addition, reoxidation of Ni particles may additionally lead to deactivation as concluded by comparison with screening studies in batch reactors. (Graph Presented). © 2017 American Chemical Society.
    view abstract10.1021/acs.iecr.6b03493
  • Potential of an alumina-supported Ni3Fe catalyst in the methanation of CO2: Impact of alloy formation on activity and stability
    Mutz, B. and Belimov, M. and Wang, W. and Sprenger, P. and Serrer, M.A. and Wang, D. and Pfeifer, P. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 7 (2017)
    A promising bimetallic 17 wt % Ni3Fe catalyst supported on γ-Al2O3 was prepared via homogeneous deposition-precipitation for the application in the methanation of CO2 to gather more detailed insight into the structure and performance of the catalyst compared to state-of-the-art methanation systems. X-ray diffraction (XRD) analysis, detailed investigations using scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy analysis (EDX) of single particles as well as larger areas, high-resolution transmission electron microscopy (HRTEM) imaging, temperature-programmed reduction (H2-TPR), and in-depth interpretation of Raman bands led to the conclusion that a high fraction of the Ni and Fe formed the desired Ni3Fe alloy resulting in small and well-defined nanoparticles with 4 nm in size and a dispersion of 24%. For comparison, a monometallic catalyst with similar dispersion using the same preparation method and analysis was prepared. Using a fixed-bed reactor, the Ni3Fe catalyst showed better low-temperature performance compared to a monometallic Ni reference catalyst, especially at elevated pressures. Longterm experiments in a microchannel packed bed reactor under industrially relevant reaction conditions in competition with a commercial Ni-based methanation catalyst revealed an improved performance of the Ni3Fe system at 358°C and 6 bar involving enhanced conversion of CO2 to 71%, selectivity to CH4 > 98%, and most notably a high stability. Deactivation occurred only at lower temperatures, which was related to carbon deposition due to an increased CO production. Kinetic measurements were compared with literature models derived for Ni/Al2O3 catalysts, which fit well but underestimate the performance of the Ni3Fe system, emphasizing the synergetic effect of Ni and Fe. © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b01896
  • Recent Advances in Selective Propylene Oxidation over Bismuth Molybdate Based Catalysts: Synthetic, Spectroscopic, and Theoretical Approaches
    Sprenger, P. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 7 (2017)
    The selective oxidation of propylene to acrolein is an important reaction in the chemical industry which has been extensively studied over the last few decades. Today, spectroscopic, computational, and synthetic approaches allow a renewed view of this established and well-understood catalytic process at a fundamental level. Consequently, a revised mechanistic pathway for the selective propylene oxidation over bismuth molybdates has been suggested recently. Furthermore, studies concerning the local interaction of specific surface entities as well as concepts from semiconductor science have provided valuable information to describe the operation mode of oxidation catalysts. New synthetic methods can be used not only to tune the specific surface area and surface species of a catalyst but also to give direct access to distinct metal oxide phases or specific crystalline phases with a synergetic interplay on the nanoscale. Since complex multicomponent systems, which exhibit both higher selectivity and activity in comparison to pure bismuth molybdates, are used for industrial applications, it is important to transfer the research concepts from such model systems to those more complex systems. This also involves operando characterization techniques on multiple length scales. Recent research activities shine a renewed light on this well-studied reaction, which therefore may become one of the drivers in selective oxidation catalysis to apply and further establish new tools that have been developed in theory, modeling, synthesis, and operando spectroscopy. (Chemical Equation Presented). © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b01149
  • Surface oxidation of supported Ni particles and its impact on the catalytic performance during dynamically operated methanation of CO2
    Mutz, B. and Gänzler, A.M. and Nachtegaal, M. and Müller, O. and Frahm, R. and Kleist, W. and Grunwaldt, J.-D.
    Catalysts 7 (2017)
    The methanation of CO2 within the power-to-gas concept was investigated under fluctuating reaction conditions to gather detailed insight into the structural dynamics of the catalyst. A 10 wt % Ni/Al2O3 catalyst with uniform 3.7 nm metal particles and a dispersion of 21% suitable to investigate structural changes also in a surface-sensitive way was prepared and characterized in detail. Operando quick-scanning X-ray absorption spectroscopy (XAS/QEXAFS) studies were performed to analyze the influence of 30 s and 300 s H2 interruptions during the methanation of CO2 in the presence of O2 impurities (technical CO2). These conditions represent the fluctuating supply of H2 from renewable energies for the decentralized methanation. Short-term H2 interruptions led to oxidation of the most reactive low-coordinated metallic Ni sites, which could not be re-reduced fully during the subsequent methanation cycle and accordingly caused deactivation. Detailed evaluation of the extended X-ray absorption fine structure (EXAFS) spectra showed surface oxidation/reduction processes, whereas the core of the Ni particles remained reduced. The 300-s H2 interruptions resulted in bulk oxidation already after the first cycle and a more pronounced deactivation. These results clearly show the importance and opportunities of investigating the structural dynamics of catalysts to identify their mechanism, especially in power-to-chemicals processes using renewable H2. © 2017 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/catal7090279
  • Dynamic transformation of small Ni particles during methanation of CO2 under fluctuating reaction conditions monitored by operando X-ray absorption spectroscopy
    Mutz, B. and Carvalho, H.W.P. and Kleist, W. and Grunwaldt, J.-D.
    Journal of Physics: Conference Series 712 (2016)
    A 10 wt.-% Ni/Al2O3 catalyst with Ni particles of about 4 nm was prepared and applied in the methanation of CO2 under dynamic reaction conditions. Fast phase transformations between metallic Ni, NiO and NiCO3 were observed under changing reaction atmospheres using operando X-ray absorption spectroscopy (XAS). Removing H2 from the feed gas and, thus, simulating a H2 dropout during the methanation reaction led to oxidation of the active sites. The initial reduced state of the Ni particles could not be recovered under methanation atmosphere (H2/CO2 = 4); this was only possible with an effective reactivation step applying H2 at increased temperatures. Furthermore, the cycling of the gas atmospheres resulted in a steady deactivation of the catalyst. Operando XAS is a powerful tool to monitor these changes and the behavior of the catalyst under working conditions to improve the understanding of the catalytic processes and deactivation phenomena.
    view abstract10.1088/1742-6596/712/1/012050
  • Bismuth molybdate catalysts prepared by mild hydrothermal synthesis: Influence of pH on the selective oxidation of propylene
    Schuh, K. and Kleist, W. and Høj, M. and Trouillet, V. and Beato, P. and Jensen, A.D. and Grunwaldt, J.-D.
    Catalysts 5 (2015)
    A series of bismuth molybdate catalysts with relatively high surface area was prepared via mild hydrothermal synthesis. Variation of the pH value and Bi/Mo ratio during the synthesis allowed tuning of the crystalline Bi-Mo oxide phases, as determined by X-ray diffraction (XRD) and Raman spectroscopy. The pH value during synthesis had a strong influence on the catalytic performance. Synthesis using a Bi/Mo ratio of 1/1 at pH ≥ 6 resulted in γ-Bi<inf>2</inf>MoO<inf>6</inf>, which exhibited a better catalytic performance than phase mixtures obtained at lower pH values. However, a significantly lower catalytic activity was observed at pH = 9 due to the low specific surface area. γ-Bi<inf>2</inf>MoO<inf>6</inf>synthesized with Bi/Mo = 1/1 at pH = 6 and 7 exhibited relatively high surface areas and the best catalytic performance. All samples prepared with Bi/Mo = 1/1, except samples synthesized at pH = 1 and 9, showed better catalytic performance than samples synthesized with Bi/Mo = 2/3 at pH = 4 and 9 and γ-Bi<inf>2</inf>MoO<inf>6</inf> synthesized by co-precipitation at pH = 7. At temperatures above 440 °C, the catalytic activity of the hydrothermally synthesized bismuth molybdates started to decrease due to sintering and loss of surface area. These results support that a combination of the required bismuth molybdate phase and a high specific surface area is crucial for a good performance in the selective oxidation of propylene. © 2015 by the authors; licensee MDPI, Basel, Switzerland.
    view abstract10.3390/catal5031554
  • Catalytic hydrodeoxygenation of guaiacol over platinum supported on metal oxides and zeolites
    Hellinger, M. and Carvalho, H.W.P. and Baier, S. and Wang, D. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 490 (2015)
    Hydrodeoxygenation of guaiacol over Pt-based catalysts was studied as a representative for phenolic compounds in pyrolysis oil. Screening of various Pt-based catalysts supported on different oxides and using different preparation methods resulted in 1%Pt/SiO2 and platinum supported on zeolites, such as 1% Pt/H-MFI-90, as the most promising catalysts in a temperature range up to 200 ° C. Thereby conversions of 86% and 100% were received, respectively. Particularly, selectivities to cyclohexane above 90% were achieved for 1% Pt/H-MFI-90. X-ray absorption near edge structure (XANES) uncovered that mild reduction temperatures were sufficient for the reduction of 1%Pt/SiO2 (up to 150°C) and 1%Pt/H-MFI-90 (up to 40°C) while 1%Pt/Al2O3 required a higher temperature of at least 320 °C. The average particle size obtained for Pt/SiO2 was 2-3 nm as unraveled by scanning transmission electron microscopy (STEM) and extended X-ray absorption fine structure (EXAFS). The deoxygenation ability of the catalysts was improved if the Pt particles were deposited on an acidic H-MFI zeolite (&gt;130 μmol acid sites per gram) as support. 1%Pt/SiO2 showed the highest selectivity towards deoxygenation at 50 °C, whereas for 1% Pt/H-MFI-90 temperatures of about 150 °C were required to achieve a high selectivity to cyclohexane. For the latter catalyst a longer reaction time was beneficial to maximize the selectivity towards cyclohexane. The hydrogen pressure did not have significant influence on the reaction rate. The results are in agreement with a hydrodeoxygenation mechanism over Pt/zeolite catalysts at temperatures up to 200 °C that comprises hydrogenation in the first step and acid catalyzed dehydration combined with hydrogenation in the second step. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apcata.2014.10.043
  • Continuous catalytic hydrodeoxygenation of guaiacol over Pt/SiO2 and Pt/H-MFI-90
    Hellinger, M. and Baier, S. and Mortensen, P.M. and Kleist, W. and Jensen, A.D. and Grunwaldt, J.-D.
    Catalysts 5 (2015)
    Hydrodeoxygenation of guaiacol in the presence of 1-octanol was studied in a fixed-bed reactor under mild conditions (50–250 °C) over platinum particles supported on silica (Pt/SiO2) and a zeolite with framework type MFI at a Si/Al-ratio of 45 (Pt/H-MFI-90). The deoxygenation selectivity strongly depended on the support and the temperature. Both guaiacol and octanol were rapidly deoxygenated in the presence of hydrogen over Pt/H-MFI-90 at 250 °C to cyclohexane and octane, respectively. In contrast, Pt/SiO2 mostly showed hydrogenation, but hardly any deoxygenation activity. The acidic sites of the MFI-90 support lead to improved deoxygenation performance at the mild temperature conditions of this study. Significant conversions under reaction conditions applied already occurred at temperatures of 200 °C. However, during long-term stability tests, the Pt/H-MFI-90 catalyst deactivated after more than 30 h, probably due to carbon deposition, whereas Pt/SiO2 was more stable. The catalytic activity of the zeolite catalyst could only partly be regained by calcination in air, as some of the acidic sites were lost. © 2015 by the authors; licensee MDPI, Basel, Switzerland.
    view abstract10.3390/catal5031152
  • Design of highly porous single-site catalysts through two-step postsynthetic modification of mixed-linker MIL-53(Al)
    Gotthardt, M.A. and Schoch, R. and Brunner, T.S. and Bauer, M. and Kleist, W.
    ChemPlusChem 80 (2015)
    Defined mixed-linker metal-organic frameworks based on MIL-53-NH2(Al) were successfully synthesized under ambient pressure, thus, enabling easy scaling up of the synthesis. The two-step postsynthetic modification reaction of these materials with, first, maleic anhydride and, second, palladium(II) acetate resulted in immobilized palladium(II) complexes at the side chain of the linker molecules. The high porosity of the mixed-linker materials was retained throughout the modification process, which was in contrast to a previous study with pure MIL-53-NH2(Al). The novel palladium-containing materials were applied in Heck-type C-C coupling reactions of bromo- or chlorobenzene with styrene, in which they exhibited high catalytic activity and selectivity. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201402123
  • Effect of the addition of ethanol to synthesis gas on the production of higher alcohols over Cs and Ru modified Cu/ZnO catalysts
    Walter, K.M. and Schubert, M. and Kleist, W. and Grunwaldt, J.-D.
    Industrial and Engineering Chemistry Research 54 (2015)
    The addition of ethanol to synthesis gas and its influence on the production of higher alcohols (HA) was investigated over Cs- and Ru-Cu/ZnO catalysts at 320 °C and an initial pressure of 5.0 MPa in a batch reactor. A change in the reaction path from aldol-type condensation of C 1 -intermediates to homocoupling of ethanol was found upon increase of the ethanol to CO ratio. Furthermore, the productivity toward HA was enhanced for higher ethanol to CO ratios. The production of HA was maximized with the side products in an acceptable range for n EtOH :n CO = 0.5. Excess of ethanol gave lower production rates and leaching of the metals was observed. Cesium (0.3-1.0 mol %) proved to be a better dopant. Excess of Cs (3 mol %) led to a lower catalyst performance, probably due to a blockage of the active sites and a change in the reducibility of the catalyst. © 2015 American Chemical Society.
    view abstract10.1021/ie504066f
  • Methanation of CO2: Structural response of a Ni-based catalyst under fluctuating reaction conditions unraveled by operando spectroscopy
    Mutz, B. and Carvalho, H.W.P. and Mangold, S. and Kleist, W. and Grunwaldt, J.-D.
    Journal of Catalysis 327 (2015)
    The methanation of CO<inf>2</inf> as a relevant strategy for energy storage has been studied by operando X-ray absorption spectroscopy under dynamic H<inf>2</inf>/CO<inf>2</inf> and CO<inf>2</inf> reaction atmospheres. A typical CO<inf>2</inf> conversion of 81% was reached at 400 °C with a 23 wt.-% Ni/CaO-Al<inf>2</inf>O<inf>3</inf> catalyst, yielding 80% of CH<inf>4</inf>. The operando XAS experiment under working conditions revealed pronounced structural changes, e.g., a fast bulk oxidation of the Ni particles after removal of H<inf>2</inf> from the H<inf>2</inf>/CO<inf>2</inf> (4:1) gas stream. A lower performance of the catalyst was observed in the subsequent methanation cycle due to the presence of a residual oxidized fraction of Ni. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jcat.2015.04.006
  • Synthesis and characterization of bimetallic metal-organic framework Cu-Ru-BTC with HKUST-1 structure
    Gotthardt, M.A. and Schoch, R. and Wolf, S. and Bauer, M. and Kleist, W.
    Dalton Transactions 44 (2015)
    The bimetallic metal-organic framework Cu-Ru-BTC with the stoichiometric formula Cu2.75Ru0.25(BTC)2·xH2O, which is isoreticular to HKUST-1, was successfully prepared in a direct synthesis using mild reaction conditions. The partial substitution of Cu2+ by Ru3+ centers in the paddlewheel structure and the absence of other Ru-containing phases was proven using X-ray absorption spectroscopy. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c4dt02491e
  • Synthesis and post-synthetic modification of amine-, alkyne-, azide- and nitro-functionalized metal-organic frameworks based on DUT-5
    Gotthardt, M.A. and Grosjean, S. and Brunner, T.S. and Kotzel, J. and Gänzler, A.M. and Wolf, S. and Bräse, S. and Kleist, W.
    Dalton Transactions 44 (2015)
    Functionalized 4,4′-biphenyldicarboxylic acid molecules with additional amine, alkyne, azide or nitro groups were prepared and applied in the synthesis of novel metal-organic frameworks and mixed-linker metal-organic frameworks isoreticular to DUT-5. The properties of the frameworks could be tuned by varying the number of functional groups in the materials and the amine groups were employed in post-synthetic modification reactions without changing the framework structure or significantly decreasing the porosity of the materials. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c5dt02276b
  • Synthesis of γ-valerolactone by hydrogenation of levulinic acid over supported nickel catalysts
    Hengst, K. and Schubert, M. and Carvalho, H.W.P. and Lu, C. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 502 (2015)
    Ni/Al2O3 catalysts were tested for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) as an important bio-based platform molecule for chemical products based on renewable feedstocks. The catalysts were prepared by wet impregnation, incipient wetness impregnation, precipitation, and flame spray pyrolysis; both the influence of different solvents (monovalent alcohols and water) as well as solvent free reaction conditions were screened in batch autoclaves. Whereas alcohols led to a number of side reactions that could only be suppressed by high hydrogen pressures (&gt;20 bar), water as solvent resulted in a GVL selectivity of 100%. The GVL yields reached 57%. Further improvement was achieved without any solvent, whereby the GVL yield increased to 92% at 100% LA conversion. Reuse of the Ni catalysts resulted in a significant drop in activity. The catalysts were thoroughly characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), linear combination analysis of X-ray absorption near edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS). The results indicated that incorporated Ni2+, as present in flame-derived catalysts, was less active for GVL synthesis compared to supported Ni particles, as present in the wet impregnated catalyst. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apcata.2015.05.007
  • Systematic study on the influence of the morphology of α-MoO3 in the selective oxidation of propylene
    Schuh, K. and Kleist, W. and Høj, M. and Jensen, A.D. and Beato, P. and Patzke, G.R. and Grunwaldt, J.-D.
    Journal of Solid State Chemistry 228 (2015)
    Abstract A variety of morphologically different α-MoO<inf>3</inf> samples were prepared by hydrothermal synthesis and applied in the selective oxidation of propylene. Their catalytic performance was compared to α-MoO<inf>3</inf> prepared by flame spray pyrolysis (FSP) and a classical synthesis route. Hydrothermal synthesis from ammonium heptamolybdate (AHM) and nitric acid at pH 1-2 led to ammonium containing molybdenum oxide phases that were completely transformed into α-MoO<inf>3</inf> after calcination at 550 °C. A one-step synthesis of α-MoO<inf>3</inf> rods was possible starting from MoO<inf>3</inf>·2H<inf>2</inf>O with acetic acid or nitric acid and from AHM with nitric acid at 180°C. Particularly, if nitric acid was used during synthesis, the rod-like morphology of the samples could be stabilized during calcination at 550°C and the following catalytic activity tests, which was beneficial for the catalytic performance in propylene oxidation. Characterization studies using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy showed that those samples, which retained their rod-like morphology during the activity tests, yielded the highest propylene conversion. © 2015 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jssc.2015.04.011
  • One-step synthesis of bismuth molybdate catalysts via flame spray pyrolysis for the selective oxidation of propylene to acrolein
    Schuh, K. and Kleist, W. and Høj, M. and Trouillet, V. and Jensen, A.D. and Grunwaldt, J.-D.
    Chemical Communications 50 (2014)
    Flame spray pyrolysis (FSP) of Bi(iii)- and Mo(vi)-2-ethylhexanoate dissolved in xylene resulted in various nanocrystalline bismuth molybdate phases depending on the Bi/Mo ratio. Besides α-Bi2Mo3O12 and γ-Bi2MoO6, FSP gave direct access to the metastable β-Bi2Mo2O9 phase with high surface area (19 m2 g-1). This phase is normally only obtained at high calcination temperatures (&gt;560 °C) resulting in lower surface areas. The β-phase was stable up to 400 °C and showed superior catalytic performance compared to α- and γ-phases in selective oxidation of propylene to acrolein at temperatures relevant for industrial applications (360 °C). This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c4cc07527g
  • Selective oxidation of propylene to acrolein by hydrothermally synthesized bismuth molybdates
    Schuh, K. and Kleist, W. and Høj, M. and Trouillet, V. and Beato, P. and Jensen, A.D. and Patzke, G.R. and Grunwaldt, J.-D.
    Applied Catalysis A: General 482 (2014)
    Hydrothermal synthesis has been used as a soft chemical method to prepare bismuth molybdate catalysts for the selective oxidation of propylene to acrolein. All obtained samples displayed a plate-like morphology, but their individual aspect ratios varied with the hydrothermal synthesis conditions. Application of a high Bi/Mo ratio during hydrothermal synthesis afforded γ-Bi2MoO6 as the main phase, whereas lower initial bismuth contents promoted the formation of α-Bi2Mo 3O12. Synthesis with a Bi/Mo ratio of 1:1 led to a phase mixture of α- and γ-bismuth molybdate showing high catalytic activity. The use of nitric acid during hydrothermal synthesis enhanced both propylene conversion and acrolein yield, possibly due to a change in morphology. Formation of β-Bi2Mo2O9 was not observed under the applied conditions. In general, the catalytic performance of all samples decreased notably after calcination at 550 °C due to sintering. © 2014 Elsevier B.V.
    view abstract10.1016/j.apcata.2014.05.038
  • Post-synthetic immobilization of palladium complexes on metal-organic frameworks-a new concept for the design of heterogeneous catalysts for Heck reactions
    Gotthardt, M.A. and Beilmann, A. and Schoch, R. and Engelke, J. and Kleist, W.
    RSC Advances 3 (2013)
    MIL-53-NH2(Al) was modified in a two-step post-synthetic reaction using maleic anhydride and palladium acetate. The resulting Pd-containing frameworks were used as catalysts in Heck-type CC coupling reactions where they exhibited high conversion and selectivity. The results of leaching tests indicate that the reaction is partly catalyzed heterogeneously. © The Royal Society of Chemistry 2013.
    view abstract10.1039/c3ra42105h
  • Salen-based coordination polymers of manganese and the rare-earth elements: Synthesis and catalytic aerobic epoxidation of olefins
    Bhunia, A. and Gotthardt, M.A. and Yadav, M. and Gamer, M.T. and Eichhöfer, A. and Kleist, W. and Roesky, P.W.
    Chemistry - A European Journal 19 (2013)
    Treatment of N,N'-bis(4carboxysalicylidene)ethylenediamine (H 4L), with MnCl2·(H2O)4, and Ln(NO3)3·(H2O)m (Ln=Nd, Eu, Gd, Dy, Tb), in the presence of N,N-dimethylformamide (DMF)/pyridine at elevated temperature resulted (after work up) in the formation of 1D coordination polymers {[Ln2(MnLCl)2(NO3)2(dmf) 5]·4 DMF}n (1-5). In these coordination polymers the rare earth ions are connected through carboxylate groups from Mn-salen units in a 1D chain structure. Thus, the Mn-salen complex acts as a "metalloligand" with open coordination sites. All compounds were used as catalysts in the liquid-phase epoxidation of trans-stilbene with molecular oxygen, which resulted in the formation of stilbene oxide. Since the choice of the lanthanide had virtually no influence on the performance of the catalyst, only the manganese-gadolinium was studied in detail. The influence of solvent, catalyst concentration, reaction temperature, oxidant, and oxidant flow rate on conversion, yield, and selectivity was analyzed. A conversion of up to 70 %, the formation of 61 % stilbene oxide (88 % selectivity), and a TON of 84 were observed after 24 h. A hot filtration test confirmed that the reaction is mainly catalyzed through a heterogeneous pathway, although a minor contribution of homogeneous species could not be completely excluded. The catalyst could be reused without significant loss of activity. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/chem.201203636
  • Aerobic epoxidation of olefins catalyzed by the cobalt-based metal-organic framework STA-12(Co)
    Beier, M.J. and Kleist, W. and Wharmby, M.T. and Kissner, R. and Kimmerle, B. and Wright, P.A. and Grunwaldt, J.-D. and Baiker, A.
    Chemistry - A European Journal 18 (2012)
    A Co-based metal-organic framework (MOF) was investigated as a catalytic material in the aerobic epoxidation of olefins in DMF and exhibited, based on catalyst mass, a remarkably high catalytic activity compared with the Co-doped zeolite catalysts that are typically used in this reaction. The structure of STA-12(Co) is similar to that of STA-12(Ni), as shown by XRD Rietveld refinement and is stable up to 270 °C. For the epoxidation reaction, significantly different selectivities were obtained depending on the substrate. Although styrene was epoxidized with low selectivity due to oligomerization, (E)-stilbene was converted with high selectivities between 80 and 90%. Leaching of Co was low and the reaction was found to proceed mainly heterogeneously. The catalyst was reusable with only a small loss of activity. The catalytic epoxidation of stilbene with the MOF featured an induction period, which was, interestingly, considerably reduced by styrene/stilbene co-epoxidation. This could be traced back to the formation of benzaldehyde promoting the reaction. Detailed parameter and catalytic studies, including in situ EPR and EXAFS spectroscopy, were performed to obtain an initial insight into the reaction mechanism. Aim for the STAs: The metal-organic framework catalyst STA-12(Co) showed remarkable activity in the aerobic epoxidation of E-stilbene in DMF (see figure) and could be efficiently recycled. High selectivity (80-90%) for stilbene oxide was observed. The induction period of the reaction could be reduced by styrene/stilbene co-epoxidation. Detailed parameter studies combined with spectroscopic investigations were performed to obtain an initial insight into the reaction mechanism. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201101223
  • Identification of the active species generated from supported Pd catalysts in heck reactions: An in situ quick scanning EXAFS investigation
    Reimann, S. and Stötzel, J. and Frahm, R. and Kleist, W. and Grunwaldt, J.-D. and Baiker, A.
    Journal of the American Chemical Society 133 (2011)
    Quick scanning extended X-ray absorption fine structure (QEXAFS) studies in the subsecond time scale have been performed to gain insight into the reaction mechanism of Heck-type C-C coupling reactions in the presence of supported Pd-based catalysts. Using a specially designed in situ EXAFS cell, both the solid catalyst and the liquid reaction mixture during the reaction of phenyl bromide (PhBr) with styrene were monitored. Soluble Pd species were only, but rapidly, detected in the liquid reaction phase once the reaction temperature of 150 °C was reached. At the same time, the conversion of PhBr started, and during the following "active phase" of the catalyst hardly any changes in the corresponding EXAFS and XANES spectra were observed. The present species could be identified as colloidal Pd0 clusters with a size of ∼2 nm estimated from the corresponding EXAFS spectra. The QEXAFS mode not only allowed monitoring rapid changes in the second time scale but also permitted minimization of effects caused by the heterogeneity of the systems. When the reaction rate started to decrease, pronounced changes in the EXAFS spectra were observed, which were attributed to an increased formation of bromo-palladates ([PdBr4]2-, [Pd2Br6]2-). In addition to the liquid-phase species, significant changes were observed for the solid catalyst that was also probed in situ during the reaction. The originally oxidized Pd catalyst was efficiently reduced upon heating. Additionally, growth of the supported Pd particles was observed by both EXAFS and STEM. The above results confirm the role of the soluble molecular Pd species as the catalytically active species and clarify their conjunction with the in situ formed Pd colloids. Furthermore, the investigation demonstrates the potential of the QEXAFS not only for monitoring rapid changes during catalysis but also for gaining deeper insight into the mechanism of such complex industrially important systems under relevant reaction conditions. © 2011 American Chemical Society.
    view abstract10.1021/ja108636u
  • Synthesis, structural properties, and catalytic behavior of Cu-BTC and mixed-linker Cu-BTC-PyDC in the oxidation of benzene derivatives
    Marx, S. and Kleist, W. and Baiker, A.
    Journal of Catalysis 281 (2011)
    Mixed-linker metal-organic frameworks based on the Cu-BTC structure have been synthesized in which the benzene-1,3,5-tricarboxylate (BTC) linkers have been partially replaced by pyridine-3,5-dicarboxylate (PyDC). X-ray-based techniques (powder XRD and XAS), thermal analysis, and infrared spectroscopy proved that a desired amount of PyDC (up to 50%) can be incorporated without changing significantly the crystal structure. The pyridine unit can be seen as a defect site in the local coordination environment of the dimeric copper units, which is significantly altering their electronic structure and the catalytic properties. Both Cu-BTC and the mixed Cu-BTC-PyDCs catalyze the demanding direct hydroxylation of toluene both in acetonitrile and in neat substrate. Different selectivity toward the desired ortho- and para-cresol and other oxidation products (benzaldehyde, benzyl alcohol, methylbenzoquinone) was observed for Cu-BTC and the Cu-BTC-PyDCs, respectively. Leaching tests and comparison with homogeneously dissolved Cu catalysts indicate mainly a heterogeneous reaction pathway. © 2011 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jcat.2011.04.004
  • Effect of dehydration on the local structure of framework aluminum atoms in mixed linker MIL-53(Al) materials studied by solid-state NMR spectroscopy
    Jiang, Y. and Huang, J. and Marx, S. and Kleist, W. and Hunger, M. and Baiker, A.
    Journal of Physical Chemistry Letters 1 (2010)
    The present study features 1H and 27Al MAS NMR spectroscopic investigations on mixed ligand metal-organic frameworks (MOFs) of MIL-53(Al) type with benzene-1,4-dicarboxylate (BDC) and 2-aminobenzene-1,4- dicarboxylate (ABDC) linkers. The excellent resolution of the 1H spectra allowed an elegant and facile quantitative analysis of the organic linkers using solid-state NMR. The actual molar fraction of ABDC in the dehydrated mixed linker MOFs was determined by evaluating the intensity of the -NH2 signal at 5.6 ppm. The incorporation of amine groups led to higher field shifts of the corner-sharing AlOH signals and a more homogeneous charge distribution in the local structure of framework aluminum atoms corresponding to a decrease of the quadrupole coupling constant by ∼1 MHz compared to that of aluminum coordinated to BDC. Upon rehydration, the local structures of the framework aluminum atoms exhibited a much lower symmetry, as indicated by an increase of the 27Al quadrupole coupling constant by up to 3 MHz. © 2010 American Chemical Society.
    view abstract10.1021/jz1010835
  • Flame-made MgAl2-xMxO4 (M = Mn, Fe, Co) mixed oxides: Structural properties and catalytic behavior in methane combustion
    van Vegten, N. and Baidya, T. and Krumeich, F. and Kleist, W. and Baiker, A.
    Applied Catalysis B: Environmental 97 (2010)
    Spinel-like oxides with the general formula MgAl2-xMxO4 (M = Mn, Fe, Co) were synthesized in a single step by flame-spray pyrolysis. High surface area materials were obtained, which showed improved resistance to sintering upon increasing the Al-content. XPS showed surface enrichment of the transition metal constituent, which favorably increased the fraction of transition metal exposed to the environment. By tuning the fraction of transition metal in the mixed oxide, materials were obtained with a good balance between thermal stability and activity in the catalytic combustion of methane. The activity of the materials increased in the order Fe &lt; Co &lt; Mn. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apcatb.2010.04.026
  • MOF-5 based mixed-linker metal-organic frameworks: Synthesis, thermal stability and catalytic application
    Kleist, W. and Maciejewski, M. and Baiker, A.
    Thermochimica Acta 499 (2010)
    Based on the well-known metal-organic framework material MOF-5 we developed a new route for the synthesis of highly porous mixed-linker metal-organic frameworks (MIXMOFs) where 5% and 10% of the benzene-1,4-dicarboxylate linkers have been substituted by a functionalized linker, namely 2-aminobenzene-1,4-dicarboxylate. The thermal stability of the materials decreased with increasing degree of substitution. However, all materials showed thermal stability up to at least 350 °C in oxidizing atmosphere which renders the MIXMOFs promising for catalytic applications. Choosing the optimum ratio of the two linker molecules both the number of active sites and thermal stability of the resulting catalysts could be tuned. The amino group at the functionalized linker proved to be beneficial for the immobilization of Pd species. The Pd loading achieved by equilibrium adsorption could be controlled by the number of NH 2 groups in the material. Although the thermal stability of the organic framework was affected to some extent in the presence of Pd, the Pd/MIXMOF materials could successfully be applied as catalysts in the oxidation of CO at elevated temperatures which was chosen as a test reaction. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tca.2009.11.004
  • Platinum nanoparticles: The crucial role of crystal face and colloid stabilizer in the diastereoselective hydrogenation of cinchonidine
    Schmidt, E. and Kleist, W. and Krumeich, F. and Mallat, T. and Baiker, A.
    Chemistry - A European Journal 16 (2010)
    The preparation of stable metal nanoparticles requires a strong interaction between the (organic) stabilizer and the metal surface that might alter the catalytic properties. This behavior has been described as "poisoning" since the stabilizer normally decreases the catalytic activity due to site blocking. Here we show a striking influence of the stabilizer on the selectivity in the hydrogenation of cinchonidine (CD) over poly(acrylic acid) (PAA)-stabilized Pt nanoparticles with well-defined shape distributions. In the hydrogenation of the heteroaromatic ring of cinchonidine in toluene, the diastereomeric excess of the (S)-hexahydrocinchonidine increased upon increasing Pt{111}/Pt{100} ratio, but this distinct shape selectivity was observed only after the oxidative removal of PAA at 473 K. The use of the as-prepared nanoparticles inverted the major diastereomer to R, and this isomer was formed also in acetic acid. This striking change in the diastereoselectivity indicates that poly(acrylic acid), which remains on the Pt surface after preparation, interacts with CD during hydrogenation almost as strongly as the solvent acetic acid. The PAA stabilizer plays a dual role: it allows one to control the size and shape of the nanoparticles during their synthesis, and it affects the rate and diastereoselectivity of the hydrogenation of CD probably through a "surface-localized acidification". © 2010 Wiley-VCH Verlag GmbH & Co. KGaA,.
    view abstract10.1002/chem.200902517
  • Tuning functional sites and thermal stability of mixed-linker MOFs based on MIL-53(Al)
    Marx, S. and Kleist, W. and Huang, J. and MacIejewski, M. and Baiker, A.
    Dalton Transactions 39 (2010)
    The MIXMOF concept has been applied to the metal-organic framework compound MIL-53(Al). The random incorporation of two different linker molecules (benzene-1,4-dicarboxylate and 2-aminobenzene-1,4-dicarboxylate) in the framework structure and its influence on thermal stability were proven using several complementary techniques (XRD, ATR-IR, MAS-NMR and TG). © 2010 The Royal Society of Chemistry.
    view abstract10.1039/c002483j
  • catalysis

  • heterogeneous catalysis

  • metal organic framework

  • synthesis

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