51 results on '"Ive Hermans"'
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2. Insights into Ethanol Coupling over Hydroxyapatite using Modulation Excitation Operando Infrared Spectroscopy
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Ive Hermans, Melissa C. Cendejas, and Shao-Chun Wang
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Ethanol ,Butanol ,Organic Chemistry ,Infrared spectroscopy ,Photochemistry ,Catalysis ,Inorganic Chemistry ,Coupling (electronics) ,chemistry.chemical_compound ,Guerbet reaction ,chemistry ,Modulation ,Physical and Theoretical Chemistry ,Excitation - Published
- 2020
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3. UV–Vis and Photoluminescence Spectroscopy to Understand the Coordination of Cu Cations in the Zeolite SSZ-13
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Jamie N. Wheeler, Kerstin Hummer, Sabrina Conrad, Robert J. Hamers, Manos Mavrikakis, Georg Kresse, Philipp Müller, Alyssa M. Love, Florian Göltl, Samuel P. Burt, Patrick Wolf, and Ive Hermans
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Photoluminescence ,Materials science ,General Chemical Engineering ,Selective catalytic reduction ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Methane ,3. Good health ,0104 chemical sciences ,SSZ-13 ,chemistry.chemical_compound ,Ultraviolet visible spectroscopy ,chemistry ,Materials Chemistry ,Methanol ,0210 nano-technology ,Spectroscopy ,Zeolite - Abstract
The Cu-exchanged zeolite SSZ-13 is a highly active material in the selective catalytic reduction of nitrogen oxides and the conversion of methane to methanol. In this material, a distribution of ac...
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- 2019
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4. Why Boron Nitride is such a Selective Catalyst for the Oxidative Dehydrogenation of Propane
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Ive Hermans, Theodore O. Agbi, Zisheng Zhang, Juan M. Venegas, Anastassia N. Alexandrova, and William P. McDermott
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010405 organic chemistry ,Radical ,Vanadium ,chemistry.chemical_element ,General Chemistry ,010402 general chemistry ,Photochemistry ,01 natural sciences ,Catalysis ,Product distribution ,0104 chemical sciences ,chemistry.chemical_compound ,chemistry ,Propane ,Boron oxide ,Boron nitride ,Dehydrogenation - Abstract
Boron-containing materials, and in particular boron nitride, have recently been identified as highly selective catalysts for the oxidative dehydrogenation of alkanes such as propane. To date, no mechanism exists that can explain both the unprecedented selectivity, the observed surface oxyfunctionalization, and the peculiar kinetic features of this reaction. We combine catalytic activity measurements with quantum chemical calculations to put forward a bold new hypothesis. We argue that the remarkable product distribution can be rationalized by a combination of surface-mediated formation of radicals over metastable sites, and their sequential propagation in the gas phase. Based on known radical propagation steps, we quantitatively describe the oxygen pressure-dependent relative formation of the main product propylene and by-product ethylene. Free radical intermediates most likely differentiate this catalytic system from less selective vanadium-based catalysts.
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- 2020
5. Probing the Transformation of Boron Nitride Catalysts under Oxidative Dehydrogenation Conditions
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Juan M. Venegas, Michael P. Hanrahan, Aaron J. Rossini, Joseph T. Grant, Ive Hermans, Melissa C. Cendejas, Brijith Thomas, Samuel P. Burt, Sarah E. Specht, Alyssa M. Love, and William P. McDermott
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Alkane ,chemistry.chemical_classification ,Oxide ,chemistry.chemical_element ,General Chemistry ,010402 general chemistry ,Photochemistry ,01 natural sciences ,Biochemistry ,Oxygen ,Catalysis ,0104 chemical sciences ,chemistry.chemical_compound ,Colloid and Surface Chemistry ,chemistry ,Propane ,Boron nitride ,Dehydrogenation ,Boron - Abstract
Hexagonal boron nitride (h-BN) and boron nitride nanotubes (BNNT) were recently reported as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins in the gas phase. Previous studies revealed a substantial increase in surface oxygen content after exposure to ODH conditions (heating to ca. 500 °C under a flow of alkane and oxygen); however, the complexity of these materials has thus far precluded an in-depth understanding of the oxygenated surface species. In this contribution, we combine advanced NMR spectroscopy experiments with scanning electron microscopy and soft X-ray absorption spectroscopy to characterize the molecular structure of the oxygen functionalized phase that arises on h-BN and BNNT following catalytic testing for ODH of propane. The pristine BN materials are readily oxidized and hydrolyzed under ODH reaction conditions to yield a phase consisting of three-coordinate boron sites with variable numbers of hydroxyl and bridging oxide groups which is denoted B(OH)
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- 2018
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6. 2D Covalent Organic Frameworks as Intrinsic Photocatalysts for Visible Light-Driven CO2 Reduction
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Jier Huang, Ive Hermans, Peilei He, Xiaoyi Zhang, Wenhui Hu, Sizhuo Yang, Jian Zhang, Brian Pattengale, Xin Zhang, Cunming Liu, and Melissa C. Cendejas
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Absorption spectroscopy ,Diffuse reflectance infrared fourier transform ,Chemistry ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Rate-determining step ,Photochemistry ,Solar fuel ,01 natural sciences ,Biochemistry ,Catalysis ,0104 chemical sciences ,Colloid and Surface Chemistry ,Covalent bond ,0210 nano-technology ,Visible spectrum ,Covalent organic framework - Abstract
Covalent organic framework (COF) represents an emerging class of porous materials that have exhibited great potential in various applications, particularly in catalysis. In this work, we report a newly designed 2D COF with incorporated Re complex, which exhibits intrinsic light absorption and charge separation (CS) properties. We show that this hybrid catalyst can efficiently reduce CO2 to form CO under visible light illumination with high electivity (98%) and better activity than its homogeneous Re counterpart. More importantly, using advanced transient optical and X-ray absorption spectroscopy and in situ diffuse reflectance spectroscopy, we unraveled three key intermediates that are responsible for CS, the induction period, and rate limiting step in catalysis. This work not only demonstrates the potential of COFs as next generation photocatalysts for solar fuel conversion but also provide unprecedented insight into the mechanistic origins for light-driven CO2 reduction.
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- 2018
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7. Cover Feature: Highly Selective Carbon‐Supported Boron for Oxidative Dehydrogenation of Propane (ChemCatChem 16/2021)
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Ive Hermans, Theodore O. Agbi, Lesli O. Mark, Edgard A. Lebrón-Rodríguez, Natalie R. Altvater, Melissa C. Cendejas, Aaron J. Rossini, Rick W. Dorn, Jacob Jansen, and William P. McDermott
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Organic Chemistry ,chemistry.chemical_element ,Oxidative phosphorylation ,Heterogeneous catalysis ,Photochemistry ,Highly selective ,Catalysis ,Inorganic Chemistry ,chemistry.chemical_compound ,chemistry ,Propane ,medicine ,Dehydrogenation ,Physical and Theoretical Chemistry ,Boron ,Carbon ,Activated carbon ,medicine.drug - Published
- 2021
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8. Influence of Metal Doping on the Lewis Acid Catalyzed Production of Butadiene from Ethanol Studied by using Modulated Operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy and Mass Spectrometry
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Samuel P. Burt, Philipp Müller, Ive Hermans, and Shao-Chun Wang
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chemistry.chemical_classification ,Reaction mechanism ,Diffuse reflectance infrared fourier transform ,010405 organic chemistry ,Chemistry ,Organic Chemistry ,Inorganic chemistry ,010402 general chemistry ,Photochemistry ,01 natural sciences ,Aldehyde ,Catalysis ,0104 chemical sciences ,Inorganic Chemistry ,chemistry.chemical_compound ,Aldol condensation ,Lewis acids and bases ,Physical and Theoretical Chemistry ,Crotonaldehyde ,Bifunctional - Abstract
Due to an increasing gap between the demand and supply, the catalytic coupling of ethanol has regained the attention of the scientific and industrial community as an "on-purpose" production route to butadiene. The most promising systems are based on bifunctional catalysts, comprising of metal sites that can dehydrogenate ethanol to acetaldehyde, and Lewis acid sites that catalyze the aldol condensation between two aldehyde molecules, as well as the Meerwein-Ponndorf-Verley (MPV) reduction of the intermediate crotonaldehyde. Here, we investigate the role of Ag in an established Ag-Zr-BEA catalyst using modulated operando DRIFTS-MS experiments. We obtain insights into the complex reaction network that involves several consecutive and parallel reactions. Based on our investigations, we formulate suggestions for catalyst optimization.
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- 2017
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9. Selective Oxidation ofn-Butane and Isobutane Catalyzed by Boron Nitride
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Juan M. Venegas, Carlos A. Carrero, Joseph T. Grant, William P. McDermott, Ive Hermans, Samuel P. Burt, and Jack Micka
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Chemistry ,Organic Chemistry ,Kinetics ,Inorganic chemistry ,Hexagonal boron nitride ,Butane ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Catalysis ,0104 chemical sciences ,Inorganic Chemistry ,chemistry.chemical_compound ,Adsorption ,Boron nitride ,Isobutane ,Dehydrogenation ,Physical and Theoretical Chemistry ,0210 nano-technology - Abstract
Hexagonal boron nitride (hBN) is presented as an outstanding catalyst for the selective production of C4 olefins by the oxidative dehydrogenation of n-butane and isobutane. Unlike catalysts reported previously, hBN limits the amount of undesired COx and instead forms C2 and C3 olefins as the main side products. Kinetic experiments suggest a mechanism in which the rates of n-butane and isobutane consumption are dependent on O2 adsorption. Kinetic and spectroscopic insights are used to formulate mechanistic hypotheses for the formation mechanisms of C2–C4 olefins.
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- 2017
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10. Selective oxidative dehydrogenation of propane to propene using boron nitride catalysts
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Carlos A. Carrero, Ive Hermans, Sarah E. Specht, Alessandro Chieregato, Juan M. Venegas, Philipp Mueller, F. Goeltl, Joseph T. Grant, Samuel P. Burt, and William P. McDermott
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chemistry.chemical_classification ,Exothermic reaction ,Multidisciplinary ,Alkene ,Inorganic chemistry ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,0104 chemical sciences ,Catalysis ,Propene ,chemistry.chemical_compound ,chemistry ,Propane ,Boron nitride ,Dehydrogenation ,0210 nano-technology ,Selectivity - Abstract
Boron nitride catalysis Propene is one of the highest-volume organic chemicals produced. Propene has mainly been made from naphtha, but changes in the global supply chain are creating shortages. Direct conversion from propane, a component of natural gas, via reaction with oxygen is an attractive alternative, but existing approaches produce a large fraction of unwanted CO and CO 2 . Grant et al. report that boron nitride, normally an unreactive material, has high selectivity to catalyze the production of propene (77%) and ethene (13%). Science , this issue p. 1570
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- 2016
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11. Influence of Hydrophilicity on the Snβ-Catalyzed Baeyer-Villiger Oxidation of Cyclohexanone with Aqueous Hydrogen Peroxide
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Philipp Müller, Patrick Wolf, Ive Hermans, Sabrina Conrad, and Hailey Orsted
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Aqueous solution ,010405 organic chemistry ,Organic Chemistry ,Cyclohexanone ,Substrate (chemistry) ,010402 general chemistry ,Photochemistry ,01 natural sciences ,Catalysis ,0104 chemical sciences ,Baeyer–Villiger oxidation ,Inorganic Chemistry ,chemistry.chemical_compound ,Adsorption ,chemistry ,Organic chemistry ,Molecule ,Physical and Theoretical Chemistry ,Hydrogen peroxide - Abstract
Snβ zeolites are amongst the most effective heterogeneous catalysts for the Baeyer-Villiger (BV) oxidation of ketones with aqueous hydrogen peroxide (H2O2). The high selectivity is rooted in the activation of the carbonyl substrate through interaction with the isolated SnIV-sites. However, these sites are also accessible to other molecules in the reaction mixture (in particular the co-solvent and product). In this contribution, we report on the impact of competitive adsorption on the Snβ-catalyzed BV oxidation of cyclohexanone with aqueous H2O2. We furthermore prepared a series of Snβ zeolites with varying amounts of framework silanols and quantified their hydrophilicities with water adsorption and IR experiments. By correlating the results with catalytic data, we show that Snβ zeolites with an intermediate hydrophilicity achieve the highest activity. Our adaptable post-synthetic synthesis allows us to tune the material synthesis, resulting in enhanced activity compared to conventional hydrothermal Snβ.
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- 2016
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12. Measurement of intrinsic catalytic activity of Pt monometallic and Pt-MoOx interfacial sites over visible light enhanced PtMoOx/SiO2 catalyst in reverse water gas shift reaction
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Canan Sener, Samuel P. Burt, James A. Dumesic, George W. Huber, Ive Hermans, Thomas M. Stadelman, Juan M. Venegas, Madelyn R. Ball, and Insoo Ro
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Order of reaction ,Chemistry ,Nanotechnology ,02 engineering and technology ,Activation energy ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Catalysis ,Water-gas shift reaction ,0104 chemical sciences ,symbols.namesake ,Chemisorption ,symbols ,Photocatalysis ,Physical and Theoretical Chemistry ,0210 nano-technology ,Raman spectroscopy ,Visible spectrum - Abstract
Supported Pt-Mo catalysts were prepared with different Mo contents by a controlled surface reaction (CSR) method and studied for the reverse water gas shift (RWGS) reaction under dark and visible light irradiation conditions. Characterization results from Raman spectroscopy, scanning transmission electron microscopy (STEM), CO chemisorption, and inductively coupled plasma-absorption emission spectroscopy (ICP-AES) indicate that selective Mo deposition onto Pt was achieved at low Mo loading (Mo/Pt ratio
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- 2016
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13. Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts
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Juan M. Venegas, William P. McDermott, Joseph T. Grant, and Ive Hermans
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Alkane ,chemistry.chemical_classification ,Reaction mechanism ,010405 organic chemistry ,Chemistry ,Inorganic chemistry ,Oxide ,General Chemistry ,010402 general chemistry ,Photochemistry ,01 natural sciences ,0104 chemical sciences ,Catalysis ,Metal ,Chemical kinetics ,chemistry.chemical_compound ,Propane ,visual_art ,visual_art.visual_art_medium ,Oxygenate - Abstract
Heterogeneous metal oxide catalysts are widely studied for the aerobic oxidations of C1–C4 alkanes to form olefins and oxygenates. In this review, we outline the properties of supported metal oxides, mixed-metal oxides, and zeolites and detail their most common applications as catalysts for partial oxidations of light alkanes. By doing this we establish similarities between different classes of metal oxides and identify common themes in reaction mechanisms and research strategies for catalyst improvement. For example, almost all partial alkane oxidations, regardless of the metal oxide, follow Mars–van Krevelen reaction kinetics, which utilize lattice oxygen atoms to reoxidize the reduced metal centers while the gaseous O2 reactant replenishes these lattice oxygen vacancies. Many of the most-promising metal oxide catalysts include V5+ surface species as a necessary constituent to convert the alkane. Transformations involving sequential oxidation steps (i.e., propane to acrylic acid) require specific reacti...
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- 2017
14. Electron Transfer-Initiated Epoxidation and Isomerization Chain Reactions of β-Caryophyllene
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Dirk De Vos, Jan Fransaer, Bart Steenackers, Ive Hermans, and Nicolò Campagnol
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Polycyclic Sesquiterpenes ,Reaction mechanism ,Molecular Structure ,Organic Chemistry ,Epoxide ,Electrons ,General Chemistry ,Photochemistry ,Catalysis ,Dioxetane ,chemistry.chemical_compound ,Electron transfer ,Isomerism ,chemistry ,Radical ion ,Triplet oxygen ,Solvent effects ,Sesquiterpenes ,Isomerization - Abstract
The abundant sesquiterpene β-caryophyllene can be epoxidized by molecular oxygen in the absence of any catalyst. In polar aprotic solvents, the reaction proceeds smoothly with epoxide selectivities exceeding 70 %. A mechanistic study has been performed and the possible involvement of free radical, spin inversion, and electron transfer mechanisms is evaluated using experimental and computational methods. The experimental data-including a detailed reaction product analysis, studies on reaction parameters, solvent effects, additives and an electrochemical investigation-all support that the spontaneous epoxidation of β-caryophyllene constitutes a rare case of unsensitized electron transfer from an olefin to triplet oxygen under mild conditions (80 °C, 1 bar O2 ). As initiation of the oxygenation reaction, the formation of a caryophyllene-derived radical cation via electron transfer is proposed. This radical cation reacts with triplet oxygen to a dioxetane via a chain mechanism with chain lengths exceeding 100 under optimized conditions. The dioxetane then acts as an in situ-formed epoxidizing agent. Under nitrogen atmosphere, the presence of a one-electron acceptor leads to the selective isomerization of β-caryophyllene to isocaryophyllene. Observations indicate that this isomerization reaction is a novel and elegant synthetic pathway to isocaryophyllene.
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- 2014
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15. Enhancing the Deperoxidation Activity of Cobalt(II)Acetylacetonate by the Addition of Octanoic Acid
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Ive Hermans and Eyal Spier
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inorganic chemicals ,Kinetics ,chemistry.chemical_element ,Homogeneous catalysis ,Photochemistry ,Atomic and Molecular Physics, and Optics ,Catalysis ,Homolysis ,chemistry ,Catalytic cycle ,Polymer chemistry ,Physical and Theoretical Chemistry ,Cobalt ,Bond cleavage - Abstract
The homolytic scission of peroxides with catalytic amounts of cobalt(II) complexes is used in several industrial oxidation processes. In this contribution, we report that addition of small amounts of octanoic acid significantly enhances the catalytic deperoxidation activity of the cobalt(II)acetylacetonate complex. We attribute this to the stabilization of the Co--OOR bond upon coordination of octanoic acid, preventing the unimolecular scission. As such, the cobalt peroxo intermediate is forced to enter an alternative catalytic cycle which causes its rapid conversion to the highly reactive cobalt hydroxy. This shift in catalytic cycle results in a higher pre-exponential rate factor, over-compensating the higher barrier of the new rate-determining step.
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- 2013
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16. Thermal Restructuring of Silica-Grafted TiClxSpecies and Consequences for Epoxidation Catalysis
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René Verel, Ive Hermans, Philipp Mania, Ceri Hammond, and Florian Jenny
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Organic Chemistry ,chemistry.chemical_element ,General Chemistry ,Nuclear magnetic resonance spectroscopy ,respiratory system ,Grafting ,Heterogeneous catalysis ,Photochemistry ,Catalysis ,Silanol ,chemistry.chemical_compound ,chemistry ,Thermal ,Titanium - Abstract
TiCl4 grafted to dehydrated silica is an industrially applied catalyst for the epoxidation of propylene. As with many heterogeneous catalysts, the precise nature of the surface species is not yet fully known, prohibiting reliable structure-activity correlations. In this study, the speciation and restructuring of site-isolated Ti(IV) Lewis acid centers was carefully investigated by using a variety of techniques. The initially formed ≡SiOTiCl3 species were found to restructure upon heating through the transfer of Cl ligands to the silica surface, eventually leading to tripodal (≡SiO)3 TiCl species. The superior activity and stability of such tripodal species is demonstrated for catalytic olefin epoxidation under continuous flow conditions.
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- 2013
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17. Hydrogen Transfer Processes Mediated by Supported Iridium Oxide Nanoparticles
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Ceri Hammond, Sabrina Conrad, Ive Hermans, and Martin T. Schümperli
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inorganic chemicals ,Cerium oxide ,organic chemicals ,Organic Chemistry ,Oxide ,chemistry.chemical_element ,Heterogeneous catalysis ,Photochemistry ,Transfer hydrogenation ,Catalysis ,Inorganic Chemistry ,chemistry.chemical_compound ,chemistry ,Benzyl alcohol ,Alcohol oxidation ,Iridium ,Physical and Theoretical Chemistry - Abstract
Homogeneous iridium catalysts have demonstrated exceptional catalytic activity for a number of hydrogen transfer reactions. Herein, we demonstrate the synthesis of a heterogeneous iridium catalyst supported on nanoparticulate cerium oxide and investigate its application for the aerobic oxidation of benzyl alcohol and the Meerwein–Ponndorf–Verley transfer hydrogenation of cyclohexanone. Along with the optimisation of the activity of the catalyst, the kinetic parameters have been examined to unravel the elementary reaction steps mediated by this catalyst and further rationalise the observed structure–activity relationships. Both spectroscopic and catalytic investigations suggest that iridium oxide nanoparticles, most likely Ir2O3, mediate these reactions via the formation of metal hydroxide species, which are subsequently reoxidised with either a molecular oxygen or a ketone. In contrast to many other metal- or metal oxide-based catalysts, this catalyst can perform the selective oxidation of alcohols in the absence of a base, at mild temperatures and at a low metal loading.
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- 2013
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18. Molecule-Induced Peroxide Homolysis
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Natascia Turrà, Ive Hermans, and Ulrich Neuenschwander
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chemistry.chemical_classification ,Free Radicals ,Radical ,Temperature ,chemistry.chemical_element ,Photochemistry ,Peroxide ,Oxygen ,Hydrocarbons ,Atomic and Molecular Physics, and Optics ,Dissociation (chemistry) ,Peroxides ,Homolysis ,Kinetics ,chemistry.chemical_compound ,chemistry ,Molecule ,Physical and Theoretical Chemistry ,Oxidation-Reduction ,Algorithms ,Alkyl ,Bond cleavage - Abstract
The homolytic cleavage of peroxide bonds, leading to the formation of free radicals, plays an important role in the (spontaneous) oxidation of a wide variety of hydrocarbons in the presence of oxygen. Such aerobic oxidations can be desired (e.g. for industrially applied autoxidations) or undesired (e.g. food deterioration). In this contribution we provide experimental and computational evidence for a molecule-induced homolytic dissociation mechanism between alkyl peroxide and compounds featuring weakly bonded H atoms such as (di)unsaturated hydrocarbons.
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- 2013
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19. Overview of Radical Chain Oxidation Chemistry
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Ive Hermans
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Chain (algebraic topology) ,Autoxidation ,010405 organic chemistry ,Chemistry ,Radical ,010402 general chemistry ,Photochemistry ,01 natural sciences ,Redox ,Mechanism (sociology) ,0104 chemical sciences - Published
- 2016
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20. Insights into the Cobalt(II)-Catalyzed Decomposition of Peroxide
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Ive Hermans, Eyal Spier, and Ulrich Neuenschwander
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Chemistry ,Kinetics ,chemistry.chemical_element ,Homogeneous catalysis ,General Chemistry ,Photochemistry ,Heterogeneous catalysis ,Decomposition ,Peroxide ,Catalysis ,chemistry.chemical_compound ,Organic chemistry ,Cobalt - Published
- 2012
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21. Cavitation-induced radical-chain oxidation of valeric aldehyde
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Ive Hermans, Ulrich Neuenschwander, and Jürg Neuenschwander
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Free Radicals ,Acoustics and Ultrasonics ,Radical ,chemistry.chemical_element ,Photochemistry ,Peroxide ,Oxygen ,Aldehyde ,Sonochemistry ,Inorganic Chemistry ,chemistry.chemical_compound ,Chemical Engineering (miscellaneous) ,Environmental Chemistry ,Organic chemistry ,Ultrasonics ,Radiology, Nuclear Medicine and imaging ,chemistry.chemical_classification ,Aldehydes ,Aqueous solution ,Chemistry ,fungi ,Organic Chemistry ,Temperature ,food and beverages ,Substrate (chemistry) ,Homolysis ,Oxidation-Reduction - Abstract
The application of high-amplitude ultrasound to liquids triggers cavitation. By the collapse of the thereby appearing vacuum cavities, high temperatures can be reached in a transient manner. The high temperatures in these hot-spots can lead to homolytic scission of chemical bonds. The thereby generated radicals are usually utilized in aqueous systems for the degeneration of organic pollutants. In this contribution, we demonstrate that the radicals can also be used for synthetic purposes: under an oxygen atmosphere, they trigger the oxidation of an aldehyde substrate.
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- 2012
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22. Catalytic Epoxidations with Peroxides: Molybdenum Trioxide Species as the Origin of Allylic Byproducts
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Ive Hermans, Emanuel Meier, and Ulrich Neuenschwander
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inorganic chemicals ,Allylic rearrangement ,Radical ,Organic Chemistry ,chemistry.chemical_element ,General Chemistry ,Photochemistry ,Peroxide ,Catalysis ,Molybdenum trioxide ,chemistry.chemical_compound ,chemistry ,Molybdenum ,Reagent ,Trioxide - Abstract
Molybdenum(VI)peroxide species, formed in the reaction of MoVI complexes with peroxides, are able to epoxidize >CC< double bonds heterolytically. In this study, theoretical and experimental evidence is provided for a kinetically competing reaction reaction of such molybdenum(VI)peroxide species with additional peroxide reagent, leading to molybdenum(VI)trioxide species, which easily decompose into radicals. Under epoxidation conditions, those radicals will reduce the selectivity, due to the formation of allylic byproducts. The involved reaction pathways are characterized by DFT calculations, providing kinetic parameters that are in good agreement with the experimental observations.
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- 2012
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23. Thermal and catalytic formation of radicals during autoxidation
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Ulrich Neuenschwander and Ive Hermans
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Olefin fiber ,Reaction mechanism ,Autoxidation ,Chemistry ,Radical ,Kinetics ,Thermal ,Physical and Theoretical Chemistry ,Photochemistry ,Catalysis ,Homolysis - Abstract
The aerobic autoxidation of hydrocarbons proceeds through a complicated reaction mechanism, mediated by free radicals. Most reported autoxidation catalysts enhance the radical formation rate via homolytic activation of hydroperoxide products. Whereas our knowledge of the product formation mechanisms has significantly improved over the last couple of years, the chain-initiation is still poorly understood. In this contribution, the thermal and catalytic initiation rate for the oxidation of the renewable olefin α-pinene is quantified, thereby providing evidence for a substrate-assisted thermal initiation. The kinetics of the catalytic initiation is in good agreement with previous studies under model conditions.
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- 2012
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24. Autoxidation Chemistry: Bridging the Gap Between Homogeneous Radical Chemistry and (Heterogeneous) Catalysis
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Pierre Jacobs, Ive Hermans, and Jozef Peeters
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chemistry.chemical_classification ,Ketone ,Cyclohexane ,Autoxidation ,Radical ,Kinetics ,Alcohol ,General Chemistry ,Photochemistry ,Heterogeneous catalysis ,Catalysis ,chemistry.chemical_compound ,chemistry - Abstract
During the autoxidation of cyclohexane, abstraction of the αH-atom of the hydroperoxide product by chain-carrying peroxyl radicals produces both the desired alcohol and ketone products, as well as the majority of by-products. Rationalizing the impact of this reaction, one should aim for a (catalytic) destruction of this hydroperoxide without the intervention of peroxyl chain-carriers. Starting from these new insights in the molecular mechanism, attempts for rational catalyst design are initiated.
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- 2008
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25. The Formation of Byproducts in the Autoxidation of Cyclohexane
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Ive Hermans, Pierre Jacobs, and Jozef Peeters
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Reaction mechanism ,Time Factors ,Free Radicals ,Molecular Structure ,Autoxidation ,Cyclohexane ,Chemistry ,Adipates ,Radical ,Organic Chemistry ,Cyclohexanol ,Cyclohexanone ,General Chemistry ,Photochemistry ,Catalysis ,Glutarates ,Solvent ,Lactones ,chemistry.chemical_compound ,Cyclohexanes ,Hydroxy Acids ,Selectivity ,Caproates ,Oxidation-Reduction - Abstract
In this work, a complementary experimental and theoretical approach is used to unravel the formation of byproducts in the autoxidation of cyclohexane. The widely accepted vision that cyclohexanone would be the most important precursor of undesired products was found inconsistent with several experimental observations. However, the propagation reaction of cyclohexyl hydroperoxide, which we recently put forward as the missing source of cyclohexanol and cyclohexanone, is now unambiguously identified also as the dominant path leading to byproducts. Indeed, this overlooked reaction produces large amounts of cyclohexoxy radicals, able to ring-open via a beta-C--C cleavage to omega-formyl radicals. The pathway by which these radicals are converted into the observed and quantified byproducts is derived in this work. In this liquid-phase reaction, solvent cages were found very important, steering the fate of nascent species.
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- 2007
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26. Silica-Immobilized Chromium Colloids for Cyclohexane Autoxidation
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André Maes, Guido Maes, Eric Breynaert, Bert Lambie, Pierre Jacobs, Jozef Peeters, and Ive Hermans
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Chromium ,chemistry.chemical_compound ,Colloid ,Autoxidation ,chemistry ,Cyclohexane ,Radical ,Inorganic chemistry ,chemistry.chemical_element ,General Chemistry ,General Medicine ,Photochemistry ,Catalysis - Published
- 2006
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27. Enhanced Activity and Selectivity in Cyclohexane Autoxidation by Inert H-Bond Acceptor Catalysts
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Pierre Jacobs, Ive Hermans, and Jozef Peeters
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chemistry.chemical_compound ,Cycloalkane ,Autoxidation ,Cyclohexane ,chemistry ,Radical ,Cyclohexanol ,Cyclohexanone ,Physical and Theoretical Chemistry ,Photochemistry ,Beta scission ,Atomic and Molecular Physics, and Optics ,Catalysis - Abstract
Herein, we demonstrate that the chain-initiating dissociation of cyclohexyl hydroperoxide, CyOOH, is substantially accelerated by H-bond acceptors (e.g. Teflon), which assist O-O bond breaking by stabilising the leaving *OH radical. This is a completely new approach to boost the chain-propagating radical concentration. Indeed, up to now, literature has remained focussed on transition metal catalysis. In addition to this initiation effect, we demonstrate how inert perfluorinated compounds are also able to steer the selectivity at the molecular level, by promoting the conversion of the intermediate cyclohexyl hydroperoxide to the most desired end-product, cyclohexanone. This effect is explained by an enhanced, H-bond-assisted, hydroperoxide propagation. This hitherto overlooked hydroperoxide propagation was recently presented by us as the dominant cyclohexanone and cyclohexanol source. We herein thus confirm our previously reported autoxidation scheme, and illustrate its usefulness as a solid basis for designing new catalytic systems.
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- 2006
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28. Understanding the autoxidation of hydrocarbons at the molecular level and consequences for catalysis
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Pierre Jacobs, Jozef Peeters, and Ive Hermans
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chemistry.chemical_classification ,Ketone ,Adipic acid ,Cyclohexane ,Autoxidation ,Process Chemistry and Technology ,Radical ,Cyclohexanol ,Cyclohexanone ,Photochemistry ,Catalysis ,chemistry.chemical_compound ,Cycloalkane ,chemistry ,Organic chemistry ,Physical and Theoretical Chemistry - Abstract
In this article, we present a thorough study on the autoxidation of cyclohexane, a model substrate for other (saturated) hydrocarbons. Despite the industrial impact of autoxidation reactions, a detailed mechanism is still missing. We present a combined experimental and computational study on the formation of both the major products (cyclohexylhydroperoxide, cyclohexanol and cyclohexanone), and the formation of ring-opened side-products. Up to now, these by-products, mainly adipic acid, were thought to originate from cyclohexanone. However, we found strong evidence that the subsequent propagation of ketone is much slower than assumed, and can only account for some 25% of ring-opened products. On the other hand, the hitherto completely overlooked propagation of the hydroperoxide, via fast αH-abstraction by chain-carrying peroxyl radicals, is identified as the major source of not only alcohol and ketone, but also by-products. In the case of N-hydroxyphthalimide (NHPI) catalysed oxidations, where mostly phthalimide N-oxyl (PINO ) radicals are propagating the chain, the situation is slightly different, as PINO reacts more selectively with the alkane substrate than peroxyl radicals. This results in an increase in hydroperoxide selectivity. Lowering of the ROOH concentration by its, e.g. cobalt-catalyzed decomposition, leads to an enhanced catalytic efficiency, as a result of the shift in the ROO + NHPI ⇌ ROOH + PINO equilibrium to the more efficient PINO chain carrier.
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- 2006
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29. Autoxidation of Cyclohexane: Conventional Views Challenged by Theory and Experiment
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Pierre Jacobs, Jozef Peeters, Thanh Lam Nguyen, and Ive Hermans
- Subjects
chemistry.chemical_classification ,Chain propagation ,Ketone ,Cyclohexane ,Cyclohexanol ,Free-radical reaction ,Cyclohexanone ,Hydrogen atom abstraction ,Photochemistry ,Atomic and Molecular Physics, and Optics ,Reaction rate ,chemistry.chemical_compound ,chemistry ,Physical and Theoretical Chemistry - Abstract
In spite of its industrial importance, the detailed reaction mechanism of cyclohexane autoxidation by O2 is still insufficiently known. Based on quantum chemical potential energy surfaces, rate coefficients of the primary and secondary chain propagation steps involving the cyclohexylperoxyl (CyOO) radical were evaluated using multiconformer transition-state theory. Including tunneling and hindered-internal-rotation effects, the rate coefficient for hydrogen-atom abstraction from cyclohexane (CyH) by CyOO was calculated to be k(T)= 1.46 x 10(-11) x exp(-17.8 kcal mol(-1)/ RT) cm3s(-1) (300-600K), close to the experimental data. A "Franck-Rabinowitch cage" reaction between the nascent cyclohexylhydroperoxide (CyOOH) and cyclohexyl radical, products from CyOO + CyH, is put forward as an initially important cyclohexanol (CyOH) formation channel. alphaH abstraction by CyOO. from cyclohexanone was calculated to be only about five times faster than that from CyH, too slow to explain all the observed side products. The a-hydrogen (alphaH) abstractions from CyOH and CyOOH by CyOO. are predicted to be about 10 and 40 times faster, respectively, than the CyOO. +CyH reaction. The very fast CyOO.+CyOOH reaction proceeds through the unstable Cy-alphaH .OOH radical that decomposes spontaneously into the ketone (Q=O) plus the OH radical; the "hot" .OH is found to produce the bulk of the alcohol via a second, "activated cage" reaction analogous to that above. It is thus shown how the very reactive CyOOH intermediate is the predominant source of ketone and alcohol, while it also leads to some side products. The alpha-hydroxycyclohexylperoxyl radical formed during the moderately fast oxidation of CyOH is shown to decompose fast into HO2 + cyclohexanone in a rapidly equilibrated reaction, which constitutes a smaller, second ketone source. These two fast cyclohexanone forming routes avoid the need for unfavorable molecular routes hitherto invoked as ketone sources. The theoretical predictions are supported and complemented by experimental findings. The newly proposed scheme is also largely applicable to the oxidation of other hydrocarbons, such as toluene, xylene, and ethylbenzene.
- Published
- 2005
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30. Kinetics of α-Hydroxy-alkylperoxyl Radicals in Oxidation Processes. HO2•-Initiated Oxidation of Ketones/Aldehydes near the Tropopause
- Author
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Pierre A. Jacobs, Ive Hermans, Jozef Peeters, Thanh Lam Nguyen, and Jean-François Müller
- Subjects
chemistry.chemical_classification ,Ketone ,Autoxidation ,Chemistry ,Radical ,Kinetics ,Ab initio ,Formaldehyde ,Photochemistry ,Aldehyde ,Decomposition ,chemistry.chemical_compound ,Computational chemistry ,Physical and Theoretical Chemistry - Abstract
A comparative theoretical study is presented on the formation and decomposition of alpha-hydroxy-alkylperoxyl radicals, Q(OH)OO* (Q = RR'C:), important intermediates in the oxidation of several classes of oxygenated organic compounds in atmospheric chemistry, combustion, and liquid-phase autoxidation of hydrocarbons. Detailed potential energy surfaces (PESs) were computed for the HOCH2O2*==HO2* + CH2O reaction and its analogues for the alkyl-substituted RCH(OH)OO* and R2C(OH)OO* and the cyclic cyclo-C6H10(OH)OO*. The state-of-the-art ab initio methods G3 and CBS-QB3 and a nearly converged G2M//B3LYP-DFT variant were found to give quasi-identical results. On the basis of the G2M//B3LYP-DFT PES, the kinetics of the approximately equal to 15 kcal/mol endothermal alpha-hydroxy-alkylperoxyl decompositions and of the reverse HO2*+ ketone/aldehyde reactions were evaluated using multiconformer transition state theory. The excellent agreement with the available experimental (kinetic) data validates our methodologies. Contrary to current views, HO2* is found to react as fast with ketones as with aldehydes. The high forward and reverse rates are shown to lead to a fast Q(OH)OO*==HO2* + carbonyl quasi-equilibrium. The sizable [Q(OH)OO*]/[carbonyl] ratios predicted for formaldehyde, acetone, and cyclo-hexanone at the low temperatures (below 220 K) of the earth's tropopause are shown to result in efficient removal of these carbonyls through fast subsequent Q(OH)OO* reactions with NO and HO2*. IMAGES model calculations indicate that at the tropical tropopause the HO2*-initiated oxidation of formaldehyde and acetone may account for 30% of the total removal of these major atmospheric carbonyls, thereby also substantially affecting the hydroxyl and hydroperoxyl radical budgets and contributing to the production of formic and acetic acids in the upper troposphere and lower stratosphere. On the other hand, an RRKM-master equation analysis shows that hot alpha-hydroxy-alkylperoxyls formed by the addition of O(2) to C(1)-, C(2)-, and C(3)-alpha-hydroxy-alkyl radicals will quasi-uniquely fragment to HO2* plus the carbonyl under all atmospheric conditions.
- Published
- 2005
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31. Computational study of the stability of α-hydroperoxyl- or α-alkylperoxyl substituted alkyl radicals
- Author
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Ive Hermans, Thanh Lam Nguyen, Luc Vereecken, and Jozef Peeters
- Subjects
inorganic chemicals ,Chemistry ,Radical ,General Physics and Astronomy ,Alkyl radicals ,Beta scission ,Photochemistry ,Dissociation (chemistry) ,Theory analysis ,chemistry.chemical_compound ,Coupled cluster ,Hydroperoxyl ,Physical and Theoretical Chemistry ,Bond cleavage - Abstract
Degradation mechanisms of peroxide-containing compounds, either stable or transients in the degradation of volatile organic compounds, are still poorly understood, despite their importance in atmospheric chemistry. A theoretical DFT and Coupled Cluster theory analysis of the stability of alkyl radicals with hydroperoxyl- (HOO–) or alkylperoxyl (ROO–) substituents on the radical carbon atom revealed that such radicals are unstable, dissociating by O–O scission to a carbonyl compound and a hydroxy or alkoxy radical even for multiple substituted compounds. This dissociation occurs sequentially after completion of the reaction forming the α-peroxyl-substituted radicals, such that the latter is independent of the O–O scission.
- Published
- 2004
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32. Biomimetic oxidation with Fe-ZSM-5 and H2O2? Identification of an active, extra-framework binuclear core and an FeIII-OOH intermediate with resonance-enhanced Raman spectroscopy
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Ive Hermans, Nikolaos Dimitratos, Ceri Hammond, Hammond, Ceri, Hermans, Ive, and Dimitratos, Nikolaos
- Subjects
Inert ,Chemistry ,methane oxidation ,Inorganic chemistry ,Organic Chemistry ,Photochemistry ,Resonance (chemistry) ,biomimetic catalysi ,Methane ,Catalysis ,Inorganic Chemistry ,chemistry.chemical_compound ,symbols.namesake ,Anaerobic oxidation of methane ,symbols ,Methanol ,ZSM-5 ,zeolite ,Physical and Theoretical Chemistry ,Raman spectroscopy ,C-H activation ,insitu analysi - Abstract
Developing inorganic materials that can mimic nature's ability to selectively oxidise inert C-H bonds remains a topic of intense scientific research. In recent years, zeolitic materials containing Fe and/or Cu have been shown to be highly active, heterogeneous catalysts for the selective oxidation of alkanes (including methane), amongst a range of other related oxidation challenges. By using resonance-enhanced Raman spectroscopy, we demonstrate that, following high-temperature pre-treatment (activation), Fe-containing ZSM-5 possesses an active binuclear core, and forms a key Fe-OOH intermediate upon activation with H2O2. Both factors are reminiscent of biological oxidation catalysts, and may account for the unique ability of this material to selectively oxidise methane to methanol at low temperature. A pre-ferryl cat: Through insitu resonance-enhanced Raman spectroscopy, we identify an active, binuclear Fe-O(H)-Fe core and an FeIII-OOH intermediate in Fe-containing ZSM-5 following activation with H2O2. The pre-ferryl nature of this biomimetic intermediate may account for the unique ability of this solid catalyst to selectively oxidise methane to methanol under mild conditions.
- Published
- 2015
33. Front Cover: Boron and Boron-Containing Catalysts for the Oxidative Dehydrogenation of Propane (ChemCatChem 19/2017)
- Author
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William P. McDermott, Juan M. Venegas, Carlos A. Carrero, Samuel P. Burt, Ive Hermans, Joseph T. Grant, Jack Micka, and Somphonh P. Phivilay
- Subjects
Chemistry ,Organic Chemistry ,Inorganic chemistry ,chemistry.chemical_element ,Photochemistry ,Catalysis ,Inorganic Chemistry ,chemistry.chemical_compound ,Front cover ,X-ray photoelectron spectroscopy ,Propane ,Boron containing ,Dehydrogenation ,Physical and Theoretical Chemistry ,Boron - Published
- 2017
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34. Inside Back Cover: Selective Oxidation of n -Butane and Isobutane Catalyzed by Boron Nitride (ChemCatChem 12/2017)
- Author
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Carlos A. Carrero, Ive Hermans, Jack Micka, Juan M. Venegas, William P. McDermott, Joseph T. Grant, and Samuel P. Burt
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Inorganic Chemistry ,chemistry.chemical_compound ,chemistry ,Boron nitride ,Organic Chemistry ,Inorganic chemistry ,Isobutane ,Butane ,Dehydrogenation ,Physical and Theoretical Chemistry ,Photochemistry ,Catalysis - Published
- 2017
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35. The strained sesquiterpene β-caryophyllene as a probe for the solvent-assisted epoxidation mechanism
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Luc De Cooman, Dirk De Vos, Alexander Neirinckx, Ive Hermans, and Bart Steenackers
- Subjects
Solvent ,Aqueous solution ,Bicyclic molecule ,Chemistry ,Hydrogen bond ,Moiety ,Reactivity (chemistry) ,Physical and Theoretical Chemistry ,Solvent effects ,Photochemistry ,Atomic and Molecular Physics, and Optics ,Catalysis - Abstract
In our attempt to synthesize β-caryophyllene oxide in food-compatible conditions, we observed the uncatalyzed and highly selective epoxidation of β-caryophyllene, a strained bicyclic sesquiterpene, in ethanol with aqueous H2 O2 under radical-suppressing conditions without the addition of a catalyst. The unusual reactivity of β-caryophyllene allowed us to use it as a probe for the mechanism of the solvent-assisted epoxidation in a wide range of organic solvents. A kinetic study was performed to investigate the epoxidation mechanism; an excellent correlation was found between the observed epoxidation rates in different solvents and the Abraham's hydrogen bond formation parameters of these solvents. By means of computational analysis, it was found that the main role of the solvent consists of the stabilization of the elongated OO bond of H2 O2 in the transition state through hydrogen-bond donation to the leaving OH moiety of H2 O2 . α-Humulene was found to possess similar reactivity as β-caryophyllene whereas isocaryophyllene-the unstrained isomer of β-caryophyllene-was unreactive.
- Published
- 2013
36. Peculiarities of β-pinene autoxidation
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Ulrich Neuenschwander, Ive Hermans, and Emanuel Meier
- Subjects
Reaction mechanism ,Allylic rearrangement ,Ketone ,General Chemical Engineering ,Epoxide ,Photochemistry ,chemistry.chemical_compound ,Bridged Bicyclo Compounds ,Isomerism ,Environmental Chemistry ,General Materials Science ,Bicyclic Monoterpenes ,chemistry.chemical_classification ,Olefin fiber ,Autoxidation ,Singlet oxygen ,Temperature ,Substrate (chemistry) ,Ketones ,Peroxides ,Kinetics ,General Energy ,chemistry ,Alcohols ,Monoterpenes ,Epoxy Compounds ,Quantum Theory ,Oxidation-Reduction - Abstract
The thermal oxidation of the renewable olefin β-pinene with molecular oxygen was experimentally and computationally investigated. Peroxyl radicals abstract weakly bonded allylic hydrogen atoms from the substrate, yielding allylic hydroperoxides (i.e., myrtenyl and pinocarvyl hydroperoxide). In addition, peroxyl radicals add to the C=C bond of the substrate to form an epoxide. It was found that a relatively high peroxyl radical concentration, together with the high rate of peroxyl cross-reactions, make radical-radical reactions surprisingly important for this particular substrate. Approximately 60 % of these peroxyl cross-reactions lead to termination (radical destruction), keeping a radical chain length of approximately 4 at 10 % conversion. Numerical simulation of the reaction-based on the proposed reaction mechanism and known or predicted rate constants-demonstrate the importance of peroxyl cross-reactions for the formation of alkoxyl radicals, which are the precursor of alcohol and ketone products.
- Published
- 2011
37. Mechanism of the catalytic deperoxidation of tert-butylhydroperoxide with cobalt(II) acetylacetonate
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Ulrich Neuenschwander, Jozef Peeters, Ive Hermans, Alfons Baiker, and Natascia Turrà
- Subjects
Autoxidation ,Cyclohexane ,Organic Chemistry ,chemistry.chemical_element ,General Chemistry ,Photochemistry ,Chain termination ,Decomposition ,Catalysis ,chemistry.chemical_compound ,Reaction rate constant ,chemistry ,Catalytic cycle ,Cobalt - Abstract
The Co(II)/Co(III)-induced decomposition of hydroperoxides is an important reaction in many industrial processes and is referred to as deperoxidation. In the first step of the so-called Haber-Weiss cycle, alkoxyl radicals and Co(III)-OH species are generated upon the reaction of the Co(II) ion with ROOH. The catalytic cycle is closed upon the regeneration of the Co(II) ion through the reaction of the Co(III)-OH species with a second ROOH molecule, thus producing one equivalent of the peroxyl radicals. Herein, the deperoxidation of tert-butylhydroperoxide by dissolved cobalt(II) acetylacetonate is studied by using UV/Vis spectroscopy in situ with a noninteracting solvent, namely, cyclohexane. Kinetic information extracted from experiments, together with quantum-chemical calculations, led to new mechanistic hypotheses. Even under anaerobic conditions, the Haber-Weiss cycle initiates a radical-chain destruction of ROOH propagated by both alkoxyl and peroxyl radicals. This chain mechanism rationalizes the high deperoxidation rates, which are directly proportional to the cobalt concentration up to approximately 75 μM at 333 K. However, at higher cobalt concentrations, a remarkable decrease of the rate is observed. The hypothesis put forward herein is that this remarkable autoinhibition effect could be explained by the hitherto overlooked chain termination of two Co(III)--OH species. The direct competition between the first-order Haber-Weiss initiation and the second-order termination can indeed explain this peculiar kinetic behavior of this homogeneous deperoxidation system.
- Published
- 2010
38. Mechanism of the aerobic oxidation of alpha-pinene
- Author
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Ive Hermans, Florian Guignard, and Ulrich Neuenschwander
- Subjects
Allylic rearrangement ,Ketone ,General Chemical Engineering ,chemistry.chemical_element ,Epoxide ,Alcohol ,Photochemistry ,Oxygen ,chemistry.chemical_compound ,Environmental Chemistry ,General Materials Science ,Hydrogen peroxide ,Bicyclic Monoterpenes ,chemistry.chemical_classification ,Autoxidation ,Temperature ,Hydrogen Peroxide ,Ketones ,Chain termination ,Carbon ,Peroxides ,General Energy ,chemistry ,Alcohols ,Monoterpenes ,Epoxy Compounds ,Quantum Theory ,Oxidation-Reduction ,Hydrogen - Abstract
A combined experimental and theoretical approach is used to study the thermal autoxidation of alpha-pinene. Four different types of peroxyl radicals are generated; the verbenyl peroxyl radical being the most abundant one. The peroxyl radicals propagate a long radical chain, implying that chain termination does not play an important role in the production of the products. Two distinct types of propagation steps are active in parallel: the abstraction of allylic H atoms and the addition to the unsaturated C=C bond. The efficiency for both pathways appears to depend on the structure of the peroxyl radical. The latter step yields the corresponding epoxide product, as well as alkoxyl radicals. Under the investigated reaction conditions the alkoxyl radicals seem to produce both the alcohol and ketone products, the ketone presumably being formed upon the abstraction of the weakly bonded alphaH atom by O2. This mechanism explains the predominantly primary nature of all quantified products. At higher conversion, co-oxidation of the hydroperoxide products constitutes an additional, albeit small, source of alcohol and ketone products.
- Published
- 2009
39. Mechanism of thermal toluene autoxidation
- Author
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Jozef Peeters, Ive Hermans, Pierre Jacobs, and Luc Vereecken
- Subjects
Models, Molecular ,Reaction mechanism ,Autoxidation ,Free Radicals ,Radical ,Temperature ,Free-radical reaction ,Photochemistry ,Toluene ,Atomic and Molecular Physics, and Optics ,Peroxides ,Benzaldehyde ,Oxygen ,chemistry.chemical_compound ,Kinetics ,chemistry ,Models, Chemical ,Benzyl alcohol ,Benzaldehydes ,Organic chemistry ,Physical and Theoretical Chemistry ,Benzoic acid ,Benzyl Alcohol ,Hydrogen - Abstract
Aerobic oxidation of toluene (PhCH3) is investigated by complementary experimental and theoretical methodologies. Whereas the reaction of the chain-carrying benzylperoxyl radicals with the substrate produces predominantly benzyl hydroperoxide, benzyl alcohol and benzaldehyde originate mainly from subsequent propagation of the hydroperoxide product. Nevertheless, a significant fraction of benzaldehyde is also produced in primary PhCH3 propagation, presumably via proton rather than hydrogen transfer. An equimolar amount of benzyl alcohol, together with benzoic acid, is additionally produced in the tertiary propagation of PhCHO with benzylperoxyl radicals. The "hot" oxy radicals generated in this step can also abstract aromatic hydrogen atoms from PhCH3, and this results in production of cresols, known inhibitors of radical-chain reactions. The very fast benzyl peroxyl-initiated co-oxidation of benzyl alcohol generates HO2* radicals, along with benzaldehyde. This reaction also causes a decrease in the overall oxidation rate, due to the fast chain-terminating reaction of HO2*with the benzylperoxyl radicals, which causes a loss of chain carriers. Moreover, due to the fast equilibrium PhCH2OOH+HO2* right harpoon over left harpoonPhCH2OO* + H2O2, and the much lower reactivity of H2O2 compared to PhCH2OOH, the fast co-oxidation of the alcohol means that HO2* gradually takes over the role of benzylperoxyl as principal chain carrier. This drastically changes the autoxidation mechanism and, among other things, causes a sharp decrease in the hydroperoxide yield.
- Published
- 2007
40. Diazo chemistry controlling the selectivity of olefin ketonisation by nitrous oxide
- Author
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Bart Moens, Ive Hermans, Jozef Peeters, Pierre Jacobs, and Bert F. Sels
- Subjects
Ketone ,Nitrogen ,Surface Properties ,Side reaction ,Molecular Conformation ,Nitrous Oxide ,General Physics and Astronomy ,Oxadiazole ,Alkenes ,Photochemistry ,Medicinal chemistry ,chemistry.chemical_compound ,Elementary reaction ,Physical and Theoretical Chemistry ,chemistry.chemical_classification ,Oxadiazoles ,Molecular Structure ,Chemistry ,Diazomethane ,Alkene ,Chemistry, Physical ,Ketones ,Models, Theoretical ,Cycloaddition ,Carbon ,Oxygen ,Models, Chemical ,Diazo - Abstract
The thermal reaction of olefins with nitrous oxide was recently put forward as a promising synthetic ketone source. The 1,3-dipolar cycloaddition of N(2)O to the C=C double bond, forming a 4,5-dihydro-[1,2,3]oxadiazole intermediate, was predicted to be the first elementary reaction step. This oxadiazole can subsequently decompose to the desired carbonyl product and N(2)via a hydrogen shift. In this contribution, Potential Energy Surfaces are constructed at the reliable G2M level of theory and used to evaluate thermal rate constants by Transition State Theory. Compelling theoretical and experimental evidence is presented that an oxadiazole intermediate not only can undergo a hydrogen shift, but eventually also a methyl- or even an alkyl-shift. Special emphasis is also given on a hitherto neglected decomposition of the oxadiazole via a concerted C-C and N-O cleavage. For some substrates, such as internal olefins, this diazo route is negligibly slow, compared to the ketone path, leaving no marks on the selectivity. For cyclopentene the diazo cleavage was however found to be nearly as fast as the desired ketone route. However, the diazo compound, viz. 5-diazopentanal, reconstitutes the oxadiazole much faster upon ring-closure than it is converted to side-products. Therefore, a pre-equilibrium between the diazoalkanal and the oxadiazole is established, explaining the high ketone yield. On the other hand, for primary alkenes, such a concerted C-C and N-O cleavage to diazomethane is identified as an important side reaction, producing aldehydes with the loss of one C-atom. For these substrates, the bimolecular back-reaction of the C(n-1) aldehyde and diazomethane is too slow to sustain an equilibrium with the oxadiazole; diazomethane rather reacts with the substrate to form cyclopropane derivatives. The overall selectivity is thus determined by a combination of H-, methyl- or alkyl-shift, and the eventual impact of a diazo cleavage in the oxadiazole intermediate.
- Published
- 2007
41. Autoxidation of ethylbenzene: the mechanism elucidated
- Author
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Jozef Peeters, Ive Hermans, and Pierre Jacobs
- Subjects
chemistry.chemical_classification ,chemistry.chemical_compound ,Reaction mechanism ,Ketone ,Autoxidation ,Cyclohexane ,Chemistry ,Concerted reaction ,Organic Chemistry ,Cyclohexanol ,Cyclohexanone ,Photochemistry ,Ethylbenzene - Abstract
Using a combined experimental and theoretical approach, we elucidated the mechanism of ethylbenzene autoxidation, at about 420 K. The generally accepted literature mechanism indeed fails to explain basic experimental observations, such as the high ketone to alcohol ratio. The hitherto overlooked propagation of 1-phenyl-ethylhydroperoxide, the primary chain product, is now unambiguously identified as the source of acetophenone as well as of 1-phenylethanol via a subsequent activated cage reaction. A similar mechanism allowed rationalizing of the cyclohexanone and cyclohexanol formation in the autoxidation of cyclohexane. The primary hydroperoxide product is found to react about 10 times faster than the arylalkane substrate with the chain carrying peroxyl radicals, whereas in cyclohexane autoxidation, this reactivity ratio is as high as 55. In combination with a lower efficiency of the above-mentioned cage reaction, this results in a rather high 1-phenyl-ethylhydroperoxide yield and causes a high ketone/alcohol ratio. Radicals are shown to be predominantly generated via a concerted bimolecular reaction of the hydroperoxide with the arylalkane substrate, producing alkyl and hydrated alkoxy free radicals. In this autoxidation system, no reaction product exhibits a major initiation-enhancing autocatalytic effect, as is the case with cyclohexanone in cyclohexane autoxidation. As a result, the conversion rate increases less sharply in time compared to cyclohexane autoxidation. In fact, even some slight inhibition can be observed, due to the formation of chain-terminating HO2* radicals in the alcohol co-oxidation. At 418 K, the chain length is estimated to be about 300-500 for conversions up to 10%.
- Published
- 2007
42. Autoxidation catalysis with N-hydroxyimides: more-reactive radicals or just more radicals?
- Author
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Jozef Peeters, Ive Hermans, and Pierre Jacobs
- Subjects
Alkane ,chemistry.chemical_classification ,Autoxidation ,Free Radicals ,Molecular Structure ,Radical ,General Physics and Astronomy ,Substrate (chemistry) ,Nitroxyl ,Photochemistry ,Imides ,Catalysis ,chemistry.chemical_compound ,chemistry ,Alkanes ,Molecule ,Reactivity (chemistry) ,Nitrogen Oxides ,Physical and Theoretical Chemistry ,Oxidation-Reduction - Abstract
A first-principles analysis reveals that the catalytic efficiency of nitroxyl radicals in alkane oxidations is not simply correlated with their reactivity toward the substrate.
- Published
- 2007
43. Cover Picture: Silica-Grafted SnIVCatalysts in Hydrogen-Transfer Reactions (ChemCatChem 20/2015)
- Author
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Ive Hermans, Sabrina Conrad, Ceri Hammond, Florian Göltl, René Verel, and Patrick Wolf
- Subjects
Meerwein–Ponndorf–Verley reduction ,Organic Chemistry ,chemistry.chemical_element ,Hydrogen transfer ,Grafting ,Photochemistry ,Catalysis ,Lewis acid catalysis ,Inorganic Chemistry ,Adsorption ,chemistry ,Organic chemistry ,Cover (algebra) ,Physical and Theoretical Chemistry ,Tin - Published
- 2015
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44. To the core of autocatalysis in cyclohexane autoxidation
- Author
-
Pierre Jacobs, Ive Hermans, and Jozef Peeters
- Subjects
chemistry.chemical_classification ,Reaction mechanism ,Ketone ,Autoxidation ,Concerted reaction ,Radical ,Organic Chemistry ,Cyclohexanol ,Cyclohexanone ,General Chemistry ,Photochemistry ,Catalysis ,Autocatalysis ,chemistry.chemical_compound ,chemistry ,Cyclohexanes ,Oxidation-Reduction - Abstract
Despite their industrial importance, the detailed reaction mechanism of autoxidation reactions is still insufficiently known. In this work, complementary experimental and theoretical techniques are employed to address the radical-chain initiation in the autoxidation of cyclohexane with a particular focus on the "lighting-off" of the oxidation by (added) cyclohexanone. We used a newly developed method to quantify the intrinsic rate of chain initiation as well as the rate enhancement by cyclohexanone and several other (oxygenated) molecules. On the basis of first principles, the hitherto assumed perhemiketale mechanism was found to be many orders of magnitude too slow to account for the observed initiation enhancement by the ketone. Instead, it is shown that the pronounced chain-initiation enhancement by the ketone is attributable to a newly proposed concerted reaction between cyclohexyl hydroperoxide and cyclohexanone, in which the (.)OH radical breaking away from the hydroperoxide abstracts an alphaH atom from the ketone, thereby energetically assisting in the cleavage of the RO--OH bond. This reaction is highly efficient in generating radicals as it quasi-excludes geminate in-cage recombination. As a result, the ketone oxidation product at a level of 1 mol % increases the initiation rate by one order of magnitude, and so acts as a highly efficient "autocatalyst" in autoxidation reactions. An analogous reaction with cyclohexanol, although estimated to be even faster, has only a marginal effect on the overall kinetics, owing to the fast subsequent formation of HO(2) (.) radicals that very rapidly terminate with other ROO(.) radicals. Finally, solid evidence is presented that, also in absence of oxygenates, ROOH initiation is actually a bimolecular reaction, involving concerted H abstraction from the alkane substrate by the nascent (.)OH.
- Published
- 2006
45. Tropopause chemistry revisited: HO2*-initiated oxidation as an efficient acetone sink
- Author
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Thanh Lam Nguyen, Jozef Peeters, Ive Hermans, and Pierre Jacobs
- Subjects
Chemistry ,Radical ,General Chemistry ,Reaction intermediate ,Photochemistry ,Biochemistry ,Catalysis ,Troposphere ,chemistry.chemical_compound ,Colloid and Surface Chemistry ,Reaction rate constant ,Atmospheric chemistry ,Acetone ,Organic chemistry ,Tropopause ,Stratosphere - Abstract
Acetone is known to be a key species in the chemistry of the Upper Troposphere and Lower Stratosphere. In this theoretical study, using amply validated methodologies, the hitherto overlooked reaction of acetone with HO2* radicals is found to lead to a fast equilibrium (CH3)2C=O + HO2* right harpoon over left harpoon (CH3)2C(OH)OO*. At room temperature, this is shifted entirely to the left and thus of no consequence. However, near the tropopause (T/= 220 K), the equilibrium is shown to shift to the right to such an extent that the subsequent reaction of (CH3)2C(OH)OO* with (partly air-traffic-generated) NO becomes an effective acetone sink. This process finally results in acetic acid, thus explaining the great abundance of this important organic acid observed near the tropopause.
- Published
- 2004
46. Reactivity of α-amino-peroxyl radicals and consequences for amine oxidation chemistry
- Author
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Ive Hermans, Martin T. Schümperli, and Ceri Hammond
- Subjects
Solvent ,chemistry.chemical_compound ,chemistry ,Radical ,Imine ,Peroxyl radicals ,General Physics and Astronomy ,Substrate (chemistry) ,Reactivity (chemistry) ,Amine gas treating ,Physical and Theoretical Chemistry ,Photochemistry ,Redox - Abstract
A comparative theoretical study is presented on the formation and fate of α-amino-peroxyl radicals, recently proposed as important intermediates in the aerobic oxidation of amines. After radical abstraction of the weakly bonded αH-atom in the amine substrate, the α-amino-alkyl radical reacts irreversibly with O(2), forming the corresponding α-amino-peroxyl radical. HO(2)˙-elimination from various types of α-amino-peroxyl radicals (forming the corresponding imine) and the kinetically competing substrate H-abstraction (forming the α-amino-hydroperoxide) were computationally characterized. Polar solvents were found to reduce the HO(2)˙-elimination barrier, but increase the barrier for H-abstraction. Depending on the reaction conditions (gas or liquid phase, amine concentration, nature of the solvent, and temperature), either of the two mechanisms is favored. The consequences for aerobic amine oxidation chemistry are discussed.
- Published
- 2012
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47. Cover Picture: Mechanism of the Aerobic Oxidation of α-Pinene (ChemSusChem 1/2010)
- Author
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Florian Guignard, Ulrich Neuenschwander, and Ive Hermans
- Subjects
Pinene ,chemistry.chemical_compound ,General Energy ,chemistry ,Autoxidation ,General Chemical Engineering ,Environmental Chemistry ,Organic chemistry ,General Materials Science ,Cover (algebra) ,Photochemistry ,Quantum chemistry ,Mechanism (sociology) - Published
- 2010
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48. Autoxidation of α-pinene at high oxygen pressure
- Author
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Ive Hermans and Ulrich Neuenschwander
- Subjects
chemistry.chemical_classification ,Reaction mechanism ,Double bond ,Autoxidation ,Atmospheric pressure ,Chemistry ,General Physics and Astronomy ,chemistry.chemical_element ,Partial pressure ,Photochemistry ,Oxygen ,Product distribution ,Reaction rate constant ,Physical and Theoretical Chemistry - Abstract
The liquid-phase oxidation of the renewable olefin alpha-pinene with molecular oxygen yields several valuable compounds for the fine-chemical industry. The most important products are verbenol/-one and alpha-pinene oxide. Following our previous work on the radical autoxidation at atmospheric pressure, this contribution addresses the influence of the oxygen pressure on the reaction mechanism and the product distribution. Trapping of the radical epoxide-precursor by O(2) causes a decrease of the epoxide selectivity, as well as the formation of a thermally unstable dialkylperoxide. This dialkylperoxide accelerates the rate significantly, due to an enhancement of the radical initiation. Although this causes a decrease of the radical chain-length, the amount of products produced in the chain-termination can still be neglected compared to the amount produced in the chain-propagations. Parallel to this, the ketone to alcohol ratio increases at higher oxygen pressure, due to the reaction of alkoxyl radicals with O(2), as well as a reaction of O(2) with the addition product of the alkoxyl radicals and the C=C double bond of the substrate. For O(2) partial pressures of 1 to 80 bar, rate constants of important reactions are extracted from the experimental observations via differential modelling, and confronted with literature values and/or quantum-chemical predictions. The derived mechanism is supported at the molecular level and provides a reliable description of the experimental observations.
- Published
- 2010
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49. Cover Picture: The Formation of Byproducts in the Autoxidation of Cyclohexane (Chem. Eur. J. 3/2007)
- Author
-
Ive Hermans, Jozef Peeters, and Pierre Jacobs
- Subjects
Reaction mechanism ,Cyclohexane ,Autoxidation ,Radical ,Organic Chemistry ,Kinetics ,General Chemistry ,Photochemistry ,Catalysis ,chemistry.chemical_compound ,chemistry ,Organic chemistry ,Cover (algebra) ,Selectivity - Published
- 2007
- Full Text
- View/download PDF
50. Mechanism of the catalytic oxidation of hydrocarbons by N-hydroxyphthalimide: a theoretical studyElectronic supplementary information (ESI) available: all discussed TS and important intermediates (geometries, energies, ZPE, rotational constants and frequencies). See http://www.rsc.org/suppdata/cc/b4/b401050g
- Author
-
Jozef Peeters, Ive Hermans, Pierre Jacobs, and Luc Vereecken
- Subjects
Quantum chemical ,Chemistry ,Metals and Alloys ,chemistry.chemical_element ,General Chemistry ,N-hydroxyphthalimide ,Photochemistry ,Oxygen ,Catalysis ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Catalytic oxidation ,Materials Chemistry ,Ceramics and Composites ,Organic chemistry ,Mechanism (sociology) - Abstract
The mechanism of the recently proposed catalytic oxidation of hydrocarbons by oxygen in the presence of N-hydroxyphthalimide (NHPI) was established by quantum chemical calculations, consistent with experiments.
- Published
- 2004
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