Formate Oxidase (FOx) from Aspergillus oryzae: One Catalyst Enables Diverse H2O2‐Dependent Biocatalytic Oxidation Reactions

An increasing number of biocatalytic oxidation reactions rely on H₂O₂ as a clean oxidant. The poor robustness of most enzymes towards H₂O₂, however, necessitates more efficient systems for in situ H₂O₂ generation. In analogy to the well‐known formate dehydrogenase to promote NADH‐dependent reactions, we here propose employing formate oxidase (FOx) to promote H₂O₂‐dependent enzymatic oxidation reactions. Even under non‐optimised conditions, high turnover numbers for coupled FOx/peroxygenase catalysis were achieved.     

Copper‐Catalyzed Oxidation of Alkanes with H2O2 under a Fenton‐like Regime

Copper complexes bearing readily available ligand systems catalyzed the oxidation of alkanes with H₂O₂ as the oxidant with high efficiency in remarkable yields (50–60 %). The reactions proceeded with unprecedented selectivity to give alkyl hydroperoxides as the major products. Detailed scrutiny of the reaction mechanism suggests the involvement of C‐centered and O‐centered radicals generated in a Fenton‐like fashion.     

Characterization of the FeV=O Complex in the Pathway of Water Oxidation

Hypervalent Fe^V=O species are implicated in a multitude of oxidative reactions of organic substrates, as well as in catalytic water oxidation, a reaction crucial for artificial photosynthesis. Spectroscopically characterized Fe^V species are exceedingly rare and, so far, were produced by the oxidation of Fe complexes with peroxy acids or H₂O₂: reactions that entail breaking of the O?O bond to form a Fe^V=O fragment. The key Fe^V=O species proposed to initiate the O?O bond formation in water oxidation reactions remained undetected, presumably due to their high reactivity. Here, we achieved freeze quench trapping of six coordinated [Fe^V=O,(OH)(Pytacn)]²⁺ (Pytacn=1‐(2′‐pyridylmethyl)‐4,7‐dimethyl‐1,4,7‐triazacyclononane) ( 2 ) generated during catalytic water oxidation. X‐ray absorption spectroscopy (XAS) confirmed the Fe^V oxidation state and the presence of a Fe^V=O bond at ≈1.60 Å. Combined EPR and DFT methods indicate that 2 contains a S=3/2 Fe^V center. 2 is the first spectroscopically characterized high spin oxo‐Fe^V complex and constitutes a paradigmatic example of the Fe^V=O(OH) species proposed to be responsible for catalytic water oxidation reactions.     

Identification of corona discharge-induced SF6 oxidation mechanisms using SF6/18O2/H2 16O and SF6/16O2/H2 18O gas mixtures

The absolute yields of gaseous oxyfluorides SOF₂, SO₂F₂, and SOF₄ from negative, point-plane corona discharges in pressurized gas mixtures of SF₆ with O₂ and H₂O enriched with¹⁸O₂ and H₂ ¹⁸O have been measured using a gas chromatograph-mass spectrometer. The predominant SF₆ oxidation mechanisms have been revealed from a determination of the relative¹⁸O and¹⁶O isotope content of the observed oxyfluoride by-product. The results are consistent with previously proposed production mechanisms and indicate that SOF₂ and SO₂F₂ derive oxygen predominantly from H₂O and O₂, respectively, in slow, gas-phase reactions involving SF₄, SF₃, and SF₂ that occur outside of the discharge region. The species SOF₄ derives oxygen from both H₂O and O₂ through fast reactions in the active discharge region involving free radicals or ions such as OH and O, with SF₅ and SF₄.     

Competition Between O2 and H2O2 in the Oxidation of Fe(II) in Natural Waters

The oxidation rates of nanomolar levels of Fe(II) in seawater (salinity S = 36.2) by mixtures of O₂ and H₂O₂ has been measured as a function of pH (5.8–8.4) and temperature (3–35∘C). A competition exists for the oxidation of Fe(II) in the presence of both O₂ (μ mol⋅L⁻¹ levels) and H₂O₂ (nmol⋅L⁻¹ levels). A kinetic model has been applied to explain the experimental results that considers the interactions of Fe(II) with the major ions in seawater. In the presence of both oxidants, the hydrolyzed Fe(II) species dominate the Fe(II) oxidation process between pH 6 and 8.5. Over pH range 6.2–7.9, the FeOH⁺ species are the most active, whereas above pH 7.9, the Fe(OH)⁰₂ species are the most active at the levels of CO²⁻₃ concentration present in seawater. The predicted Fe(II) oxidation rate at [Fe(II)]₀ = 30nmol⋅L⁻¹ and pH = 8.17 in the oxygen-saturated seawater with [H₂O₂]₀ = 50nmol⋅L⁻¹ (log ₁₀ k = −2.24s⁻¹) is in excellent agreement with the experimental value of log ₁₀ k = −2.29s⁻¹ ([H₂O₂]₀ = 55nmol⋅L⁻¹, pH = 8).     

Structural Modulation on NiCo2S4Nanoarray by N Doping to Enhance 2e-ORR Selectivity for Photothermal AOPs and Zn−O2Batteries**

As a H₂O₂ generator, a 2e⁻ oxygen reduction reaction active electrocatalyst plays an important role in the advanced oxidation process to degrade organic pollutants in sewage. To enhance the tendency of NiCo₂S₄ towards the 2e⁻ reduction reaction, N atoms are doped in its structure and replace S²⁻. The result implies that this weakens the interaction between NiCo₂S₄ and OOH*, suppresses O−O bond breaking and enhances H₂O₂ selectivity. This electrocatalyst also shows photothermal effect. Under photothermal heating, H₂O₂ produced by the oxidation reduction reaction can decompose and releaseOH, which degrades organic pollutants through the advanced oxidation process. Photothermal effect induced by the advance oxidation process shows obvious advantages over the traditional Fenton reaction, such as wide pH adaptation scope and low secondary pollutant due to its Fe²⁺ free character. With Zn as anode and the electrocatalyst as cathode material, a Zn−O₂ battery is assembled. It achieves electricity generation and photothermal effect induced by the advance oxidation process simultaneously.     

Reoxidation of Transition‐metal Catalysts with O2

Transition‐metal catalyzed oxidation reactions are central components of organic chemistry. On behalf of green and sustainable chemistry, molecular oxygen (O₂) has been considered as an ideal oxidant due to its natural, inexpensive, and environmentally friendly characters, and therefore offers attractive academic and industrial prospects. In recent years, some powerful organic oxidation methods have been continuously developed. Among them, the use of molecular oxygen (O₂) as a green and sustainable oxidant has attracted considerable attentions. However, the development of new transition metal‐catalyzed protocols using O₂ as an ideal oxidant is highly desirable but very challenging because of the low standard electrode potential of O₂ to reoxidize the transition‐metal catalysts. In this Account, we highlight some of our progress toward the use of transition‐metal catalyzed aerobic oxidation reactions. Through the careful selection of ligand and the acidic additives, we have successfully realized the reoxidation of Cu, Pd, Mn, Fe, Ru, Rh, and bimetallic catalysts under O₂ or air atmosphere (1 atm) for the oxidative coupling, oxygenation reactions, oxidative C‐H/C‐C bond cleavage, oxidative annulation, and olefins difunctionalization reactions. Most of the reactions can tolerate a range of functional groups. These methods provide new strategies for the green synthesis of alkynes, (α ‐keto)amides/esters, ketones/diones, O/N‐heterocycles, β ‐azido alcohols, and nitriles. The high efficiency, low cost, and simple operation under air make these methodologies very attractive and practical. We will also discuss the mechanisms of these reactions which might be useful to promote the new type of aerobic oxidative reaction design.     

Second-sphere effects on H2O2 activation by non-heme FeII complexes: role of a phenol group in the [H2O2]-dependent accumulation of FeIVO vs. FeIIIOOH

Redox metalloenzymes achieve very selective oxidation reactions under mild conditions using O₂ or H₂O₂ as oxidants and release harmless side-products like water. Their oxidation selectivity is intrinsically linked to the control of the oxidizing species generated during the catalytic cycle. To do so, a second coordination sphere is used in order to create a pull effect during the activation of O₂ or H₂O₂, thus ensuring a heterolytic O–O bond cleavage. Herein, we report the synthesis and study of a new non-heme Fe^II complex bearing a pentaazadentate first coordination sphere and a pendant phenol group. Its reaction with H₂O₂ generates the classical Fe^IIIOOH species at high H₂O₂ loading. But at low H₂O₂ concentrations, an Fe^IVO species is generated instead. The formation of the latter is directly related to the presence of the 2nd sphere phenol group. Kinetic, variable temperature and labelling studies support the involvement of the attached phenol as a second coordination sphere moiety (weak acid) during H₂O₂ activation. Our results suggest a direct Fe^II → Fe^IVO conversion directed by the 2nd sphere phenol via the protonation of the distal O atom of the Fe^II/H₂O₂ adduct leading to a heterolytic O–O bond cleavage.

A new Fe^II complex with a phenol group attached as a second coordination sphere moiety activates H₂O₂ to yield Fe^IVO following a mechanism reminiscent of peroxidase enzymes.     

On-Demand Synthesis of H2O2 by Water Oxidation for Sustainable Resource Production and Organic Pollutant Degradation

H₂O₂ is a versatile and environmentally friendly chemical involved in water treatment, such as advanced oxidation processes. Anthraquinone oxidation is widely used for large-scale production of H₂O₂, which requires significant energy input and periodic replacement of the carrier molecule. H₂O₂ production should be customized considering the specific usage scenario. Electrochemical synthesis of H₂O₂ can be adopted as alternatives to traditional method, which avoids concentration, transportation, and storage processes. Herein, we identified Bi₂WO₆:Mo as a low-cost and high-selectivity choice from a series of Bi-based oxides for H₂O₂ generation via two-electron water oxidation reaction. It can continuously provide H₂O₂ for in situ degradation of persistent pollutants in aqueous solution. Clean energy from H₂ can also be produced at the cathode. This kind of water splitting producing sustainable resources of H₂O₂ and H₂ is an advance in environmental treatment and energy science.     

High temperature oxidation of HP40 alloy under H2–H2O and air atmospheres: a surface study

In the present work, high temperature oxidation of HP40 alloy was carried out at 1050 °C under H₂–H₂O and air atmospheres; the influence of atmosphere on surface morphology and composition was studied. Octahedral crystals with considerable spalled regions are present on the surface of alloy oxidized under air, the oxide scale composes of MnCr₂O₄, Cr₂O₃ and (Fe, Ni)Cr₂O₄ and spalled regions exhibit base alloy and SiO₂‐rich regions. The surface of alloy oxidized under H₂–H₂O is fully covered by small granular crystals and blade‐type structures without spallation, and the oxide scale composes of MnCr₂O₄ and Cr₂O₃. Moreover, X‐ray photoelectron spectroscopy analysis shows considerable difference in chemical valence states of Mn, Cr and O elements on both alloy surfaces, and hydroxyl compounds exist on the alloy oxidized under H₂–H₂O atmosphere. Copyright © 2017 John Wiley & Sons, Ltd.     

Interaction of aniline and benzidine with layered solids M n+(UO2XO4) n − ·z H2O[M = H3O+, Cu2+, VO2+, Fe2+; X = P,As]

The layered acid solids of formula H₃OUO₂XO₄· 3 H₂O (X = As, P) intercalate aniline and benzidine arylamines, by protonation of the guest molecules. The intercalates maintain the original laminar structure.The insertion of aniline and benzidinein the metal derivativesM(UO₂XO₄)₂·n H₂O (M = Cu, VO, Fe) requires rather drastic conditions. Near and medium infrared spectra of intercalates in which M = VO²⁺ and Cu²⁺, indicate that the polymerization and/or oxidation of sorbed amines occurs; however, the guest-host reactions for Fe²⁺-derivatives are of the acid-base type.     

Crystal structure and heat capacity of the mixed-valence trinuclear iron monoiodoacetate complex, [Fe(III)2Fe(II)O(O2CCH2I)6(H2O)3][Fe(III)3O(O2CCH2I)6(H2O)3]I

The crystal structure of a trinuclear iron monoiodoacetate complex was determined. Although it has been incorrectly characterized as [Fe₃O(O₂CCH₂I)₆(H₂O)₃], the correct chemical formula turned out to be [Fe(III)₂Fe(II)O(O₂CCH₂I)₆(H₂O)₃]-[Fe(III)₃O(O₂CCH₂I)₆(H₂O)₃]I (1). The two kinds of Fe₃O molecules (Fe(III)₂Fe(II)O and Fe(III)₃O) are crystallographically indistinguishable. All the Fe atoms are crystallographically equivalent because of a crystallographic threefold symmetry. Heat capacities of 1 seem to exhibit no thermal anomaly in the temperature range 5.5–309 K, although the valence detrapping phenomenon has been observed in this temperature range. This fact indicates that the valence-detrapping phenomenon in 1 occurs without any phase transition, leading 1 to a glassy state, probably because the crystal of 1 is just like a solid solution of distorted mixed-valence Fe(III)₂Fe(II)O molecules and permanently undistorted Fe(III)₃O molecules which may act as an inhibitor for a cooperative valence-trapping.     

Flow reactor studies and kinetic modeling of the H2/O2/NOX and CO/H2O/O2/NOX reactions

Flow reactor experiments were performed over wide ranges of pressure (0.5–14.0 atm) and temperature (750–1100 K) to study H₂/O₂ and CO/H₂O/O₂ kinetics in the presence of trace quantities of NO and NO₂. The promoting and inhibiting effects of NO reported previously at near atmospheric pressures extend throughout the range of pressures explored in the present study. At conditions where the recombination reaction H + O₂ (+M) = HO₂ (+M) is favored over the competing branching reaction, low concentrations of NO promote H₂ and CO oxidation by converting HO₂ to OH. In high concentrations, NO can also inhibit oxidative processes by catalyzing the recombination of radicals. The experimental data show that the overall effects of NO addition on fuel consumption and conversion of NO to NO₂ depend strongly on pressure and stoichiometry. The addition of NO₂ was also found to promote H₂ and CO oxidation but only at conditions where the reacting mixture first promoted the conversion of NO₂ to NO. Experimentally measured profiles of H₂, CO, CO₂, NO, NO₂, O₂, H₂O, and temperature were used to constrain the development of a detailed kinetic mechanism consistent with the previously studied H₂/O₂, CO/H₂O/O₂, H₂/NO₂, and CO/H₂O/N₂O systems. Model predictions generated using the reaction mechanism presented here are in good agreement with the experimental data over the entire range of conditions explored. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 705–724, 1999     

Boosting Molecular Complexity with O2: Iron-Catalysed Oxygenation of 1-Arylisochromans through Dehydrogenation,Csp3−O Bond Cleavage and Hydrogenolysis

Oxidative cleavage of the Csp³−O bond in 1-arylisochromans with stoichiometric oxidants, such as CrO₃/H₂SO₄, has been practiced for decades in synthetic chemistry. Herein, we report that a structurally well-defined Fe^II–pyridyl(bis-imidazolidine) catalyst promotes the aerobic oxygenation of 1-arylisochromans, affording highly selectively 2-(hydroxyethyl)benzophenones, compounds of potential for neuroprotective agents. Key intermediates have been isolated, indicating that the reaction proceeds through dehydrogenative oxygenation of the isochromans at the 1-position, Csp³−O bond cleavage at the iron centre and hydrogenolysis of the resulting Fe−O bond with the H₂ generated from the dehydrogenation step. In the absence of H₂ but under iron catalysis, the peroxide intermediate is converted into an unexpected ketal compound, which transfers into a 2-(hydroxyethyl)benzophenone when both O₂ and H₂ are admitted. The unique ability of the iron catalyst for oxygenation and hydrogenation in the same catalytic process under mild conditions allows for the stepwise preparation of a variety of isolable oxygenated products on a preparative scale, circumventing the need for using wasteful and/or toxic oxidants.     

Carbonized Hemoglobin Nanofibers for Enhanced H2O2 Detection

Electrospun hemoglobin (Hb) microbelts were used as a novel precursor to produce a new class of carbon nanofibers (Hb‐CNFs) containing Fe species (Fe₂O₃ and/or Fe‐N₄ moiety). The Hb‐CNFs modified glassy carbon electrode (Hb‐CNFs/GCE) exhibits significant oxidation/reduction towards H₂O₂. The observed H₂O₂ oxidation/reduction starting at ca. +0.26 V and +0.15 V (vs. Ag/AgCl) are significantly lower than the values observed at other CNFs modified GCE. The Hb‐CNFs/GCE was also applied to the amperometric detection of H₂O₂ and the results showed fast response, high sensitivity, excellent reproducibility, good selectivity, and wide dynamic range with good limit of detection.     

EPR investigation of plasma-chemical resist etching in O2 and O2/CF4 discharges

During the etching of AZ 1350 photoresist in O₂ and O₂/CF₄ discharges, ground-state concentrations of atoms (O, F, and H), and small radicals (OH, HO₂, RO₂) were measured in the discharge afterglow by EPR spectroscopy. In the case of CF₄/O₂ discharges, the dependence of O and F atom concentrations on the etch time reflects both surfäce oxidation and fluorination reactions in accordance with existing etch models. In the case of high-rate resist etching in pure O₂ discharges, high concentrations of product radicals (H, OH and HO₂) were detected and compared with resist free O₂/H₂O discharges. Kinetic modeling of the afterglow reactions reveals that the mean lifetime and, accordingly, the diffusion length of the etchant species O(³P) is drastically reduced in rapid reactions with OH and HO₂. The results are used to simulate both etch homogeneity and the loading effect in a simple etch model.     

Beiträge zum Chemischen Transport oxidischer Metallverbindungen. II. Bemerkungen zum Transport von α-Fe2O3 über monomeres Eisen(III)-chlorid

Transport of α? Fe₂O₃ with HCl via monomeric iron(III) chloride according to Fe₂O_3(s) + 6 HCl_(g) = 2FeCl_3(g) + 3 H₂O_(g); T₂ → T₁ between T₂ = 1000°C and T₁ = 800°C in the region of diffusion produced crystals which contained, in dependence of total pressure, different amounts of divalent iron. By addition of oxygen to the transport gas stoichiometric crystals of hematite by otherwise unchanged conditions were obtained. The necessary amount of oxygen was calculated from the phase diagram Fe? O, and an explanation of the gas phase reactions is given. Dependence of the transport rate of hematite on total pressure in the region of diffusion (0.009 to 6 atm) is reported.     

Trapping of a Highly Reactive Oxoiron(IV) Complex in the Catalytic Epoxidation of Olefins by Hydrogen Peroxide

The generation of a nonheme oxoiron(IV) intermediate, [(cyclam)Fe^IV(O)(CH₃CN)]²⁺ ( 2 ; cyclam=1,4,8,11‐tetraazacyclotetradecane), is reported in the reactions of [(cyclam)Fe^II]²⁺ with aqueous hydrogen peroxide (H₂O₂) or a soluble iodosylbenzene (sPhIO) as a rare example of an oxoiron(IV) species that shows a preference for epoxidation over allylic oxidation in the oxidation of cyclohexene. Complex 2 is kinetically and catalytically competent to perform the epoxidation of olefins with high stereo‐ and regioselectivity. More importantly, 2 is likely to be the reactive intermediate involved in the catalytic epoxidation of olefins by [(cyclam)Fe^II]²⁺ and H₂O₂. In spite of the predominance of the oxoiron(IV) cores in biology, the present study is a rare example of high‐yield isolation and spectroscopic characterization of a catalytically relevant oxoiron(IV) intermediate in chemical oxidation reactions.     

Understanding LiOH Chemistry in a Ruthenium‐Catalyzed Li–O2 Battery

Non‐aqueous Li–O₂ batteries are promising for next‐generation energy storage. New battery chemistries based on LiOH, rather than Li₂O₂, have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru‐catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e⁻ oxygen reduction reaction, the H in LiOH coming solely from added H₂O and the O from both O₂ and H₂O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li₂O₂, LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long‐lived battery. An optimized metal‐catalyst–electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.     

Modified graphitized carbon black as transducing material for reagentless H2O2 and enzyme sensors

Direct electron transfer between redox enzymes and electrodes is the basis for the third generation biosensors. We established direct electron transfer between quinohemoprotein alcohol dehydrogenase (PQQ-ADH) and modified carbon black (CBs) electrodes. Furthermore, for the first time, this phenomenon was observed for pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase (PQQ-GDH). Reagentless enzyme biosensors suitable for the determination of ethanol, glucose and sensors for hydrogen peroxide were designed using CB electrodes and screen-printing technique. Aiming to create an optimal transducing material for biosensors, a set of CB batches was synthesized using the matrix of Plackett-Burman experimental design. Depending on the obtained surface functional groups as well as the nano-scale carbon structures in CBs batches, the maximal direct electron transfer current of glucose and ethanol biosensors can vary from 20 to 300 nA and from 30 to 6300 nA for glucose and ethanol, respectively. Using modified CB electrodes, an electrocatalytic oxidation of H₂O₂ takes place at more negative potentials (0.1-0.4 V versus Ag/AgCl). Moreover, H₂O₂ oxidation efficiency depends on the amount and morphology of fine fraction in the modified CBs.