OOH Radicals Research Articles

Selective photooxidation of methane to C1 oxygenates by constructing heterojunction photocatalyst with mild oxidation ability

Selective photocatalytic aerobic oxidation of methane to value-added chemicals offers a promising pathway for sustainable chemical industry, yet remains a huge challenge owing to the consecutive overoxidation of primary products. Here, a type II heterojunction were constructed in Ag-AgBr/ZnO to reduce the oxidation potential of stimulated holes and prevent the undesirable CH4 overoxidation side reactions. For photocatalytic oxidation of methane under ambient temperature, the products yield of 1499.6 μmol gcat−1 h−1 with a primary products selectivity of 77.9% was achieved over Ag-AgBr/ZnO, which demonstrate remarkable improvement compared to Ag/ZnO (1089.9 μmol gcat−1 h−1, 40.1%). The superior activity and selectivity result from the promoted charge separation and the redox potential matching with methane activation after introducing AgBr species. Mechanism investigation elucidated that the photo-generated holes transferred from the valence band of ZnO to that of AgBr, which prevent H2O oxidation and enhance the selective generation of •OOH radical.

On the dual role of (+)-catechin as primary antioxidant and inhibitor of viral proteases

Natural antioxidants have become the subject of many investigations due to the role that they play in the reduction of oxidative stress. Their main scavenging mechanisms concern the direct inactivation of free radicals and the coordination of metal ions involved in Fenton-like reactions. Recently, increasing attention has been paid to non-covalent inhibition of enzymes involved in different diseases by the antioxidants. Here, a computational investigation on the primary antioxidant power of (+)-catechin against the •OOH radical has been performed in both lipid-like and aqueous environments, taking into account the relevant species present in the simulated acid-base equilibria at the physiological pH. Hydrogen Atom Transfer (HAT), Single Electron Transfer (SET), and Radical Adduct Formation (RAF) mechanisms were studied, and relative rate constants were estimated. The potential inhibitory activity of the (+)-catechin towards the most important proteases from SARS-CoV-2, 3C-like (Mpro) and papain-like (PLpro) proteases was also investigated by MD simulations to provide deeper atomistic insights on the binding sites.Based on the antioxidant and antiviral properties also unravelled by comparison with other molecules having similar chemical scaffold, our results propose that (+)-CTc satisfies can explicate a dual action as antioxidant and antiviral in particular versus Mpro from SARS-CoV-2.

Aerobic Oxidative Synthesis of Formamides from Amines and Bioderived Formyl Surrogates

AbstractHerein we present a new strategy for the oxidative synthesis of formamides from various types of amines and bioderived formyl sources (DHA, GLA and GLCA) and molecular oxygen (O2) as oxidant on g‐C3N4 supported Cu catalysts. Combined characterization data from EPR, XAFS, XRD and XPS revealed the formation of single CuN4 sites on supported Cuphen/C3N4 catalysts. EPR spin trapping experiments disclosed ⋅OOH radicals as reactive oxygen species and ⋅NR1R2 radicals being responsible for the initial C−C bond cleavage. Control experiments and DFT calculations showed that the successive C−C bond cleavage in DHA proceeds via a reaction mechanism co‐mediated by ⋅NR1R2 and ⋅OOH radicals based on the well‐equilibrated CuII and CuI cycle. Our catalyst has much higher activity (TOF) than those based on noble metals.

Primary Antioxidant Power and Mpro SARS-CoV-2 Non-Covalent Inhibition Capabilities of Miquelianin.

The antioxidant power of quercetin-3-O-glucuronide (miquelianin) has been studied, at the density functional level of theory, in both lipid-like and aqueous environments. In the aqueous phase, the computed pKa equilibria allowed the identification of the neutral and charged species present in solution that can react with the ⋅OOH radical. The Hydrogen Atom Transfer (HAT), Single Electron Transfer (SET) and Radical Adduct Formation (RAF) mechanisms were considered, and the individual, total and fraction corrected rate constants were obtained. Potential non-covalent inhibition of Mpro from SARS-CoV-2 by miquelianin has been also evaluated.

Enhanced catalytic activity of ZnWO4 by nickel-doping in oxygen evolution reactions

The development of efficient and low-cost metal materials is indispensable in clean energy technology. Metal tungstates have previously been reported as catalysts for oxygen evolution reactions (OER). However, ZnWO4 alone does not exhibit significant catalytic activity for OER, it requires 330 mV to drive the reaction at 10 mA cm−2. To address this issue, a self-supporting electrode mixed with tungstate is synthesized by a hydrothermal method. Notably, the Ni-doped ZnWO4 self-supporting material demonstrates improved OER catalytic activity. Specifically, when comparing ZnWO4 to ZnNiWO4, it exhibits a reduced Tafel slope by 190 mV·dec−1, resulting in an overpotential of only 110 mV at 10 mA cm−2, with long-term stability for at least 20 h. Furthermore, the d-band center of ZnNiWO4 is modeled using Materials Studio and simulated through projected density of states (PDOS) calculations. These calculations reveal that the presence of Ni activates the metal ion as an OER active site with strong orbital overlap with the reaction intermediate. This adjustment in the electronic structure of the Ni–Zn metal active site during the oxygen evolution reaction alters the adsorption energy. Consequently, doping of Ni enhances the metal's ability to adsorb *OOH radical, shifting the d-band center of ZnNiWO4 further away from the Fermi level, and facilitates rapid oxygen release.

Computational insights into the zeolite-supported gold nanocluster-catalyzed ethanol dehydrogenation to acetaldehyde.

Zeolite-supported gold nanoclusters play increasingly important roles in heterogeneous catalysis and exhibit unique catalytic properties for ethanol dehydrogenation to acetaldehyde. Nevertheless, the reaction mechanism and potential roles of the zeolite-encapsulated gold nanoclusters during the catalytic process remain unclear. Herein, computational studies were carried out to gain mechanistic insights into ethanol dehydrogenation to acetaldehyde under both aerobic and anaerobic conditions catalyzed by a silicalite-1 zeolite-encapsulated Au3 cluster cation (Au3+-S1). The presence of O2 can significantly promote the ethanol dehydrogenation catalyzed by Au3+-S1. A feasible mechanistic pathway could be initiated via the O2 induced H-atom transfer (HAT) step from the hydrogen of the hydroxyl group to afford ethoxy and OOH radical species. Subsequently, the OOH induced second HAT from α-C-H of the ethoxy intermediate could follow to afford the acetaldehyde product. Moreover, the possible confinement and stabilization effect of the zeolite channels on the ethanol dehydrogenation reaction was discussed.

Deciphering the origin of the first steps in the degradation of azo dyes: a computational study.

Azo dyes find applications across various sectors including food, pharmaceuticals, cosmetics, printing, and textiles. The contaminating effects of dyes on aquatic environments arise from toxic effects caused by their long-term presence in the environment, buildup in sediments, particularly in aquatic species, degradation of pollutants into mutagenic or mutagenic compounds, and low aerobic biodegradability. Therefore, we theoretically propose the first steps of the degradation of azo dyes based on the interaction of hydroperoxyl radical (•OOH) with the dye. This interaction is studied by the OC and ON mechanisms in three azo dyes: azobenzene (AB), disperse orange 3 (DO3), and disperse red 1 (DR1). Rate constants calculated at several temperatures show a preference for the OC mechanism in all the dyes with lower activation energies than the ON mechanism. The optical properties were calculated and because the dye-•OOH systems are open shell, to verify the validity of the results, a study of the spin contamination of the ground [Formula: see text] and excited states [Formula: see text] was previously performed. Most of the excited states calculated are acceptable as doublet states. The absorption spectra of the dye-•OOH systems show a decrease in the intensity of the bands compared to the isolated dyes and the appearance of a new band of the type π → π* at a longer wavelength in the visible region, achieving up to 868nm. This demonstrates that the reaction with the •OOH radical could be a good alternative for the degradation of the azo dyes.

Electro-assisted methane oxidation to formic acid via in-situ cathodically generated H2O2 under ambient conditions

Direct partial oxidation of methane to liquid oxygenates has been regarded as a potential route to valorize methane. However, CH4 activation usually requires a high temperature and pressure, which lowers the feasibility of the reaction. Here, we propose an electro-assisted approach for the partial oxidation of methane, using in-situ cathodically generated reactive oxygen species, at ambient temperature and pressure. Upon using acid-treated carbon as the electrocatalyst, the electro-assisted system enables the partial oxidation of methane in an acidic electrolyte to produce oxygenated liquid products. We also demonstrate a high production rate of oxygenates (18.9 μmol h−1) with selective HCOOH production. Mechanistic analysis reveals that reactive oxygen species such as ∙OH and ∙OOH radicals are produced and activate CH4 and CH3OH. In addition, unstable CH3OOH generated from methane partial oxidation can be additionally reduced to CH3OH on the cathode, and so-produced CH3OH is further oxidized to HCOOH, allowing selective methane partial oxidation.

Reactive molecular dynamics simulations of multicomponent models for RP-3 jet fuel in combustion at supercritical conditions: A comprehensive mechanism study

The aeroengine combustors frequently operate at temperatures and pressures higher than the critical values of the fuels. In this work, the combustion mechanisms and kinetic parameters of two multicomponent models for RP-3 jet fuel are investigated using reactive force field molecular dynamics (ReaxFF-MD). This study reveals that CC bond dissociation, H-abstraction, and isomerization dominate the initial combustion, with the main products of C1–C3 species, •OH and •OOH radicals. The reactions related to the formation and consumption of the radicals play a crucial role in the fuel combustion process. Some radicals rupture into smaller species by releasing C2H4 species. The cracked small molecules are further oxidized and react with radicals to produce large amounts of H2O molecules. The apparent activation energies of the two models are predicted to be 36.40 and 38.04 kcal/mol, judged by the variations in the number of total reactants at different temperatures. The simulation results are in good agreement with the experimental data at high pressures. Finally, the reactants and major product distributions of the two- and three-component fuels under fuel-rich, stoichiometric and fuel-lean combustion are studied. A comprehensive mechanism is developed by simulating the supercritical combustion of two surrogate fuels.

Nearly 100% selective and visible-light-driven methane conversion to formaldehyde via. single-atom Cu and Wδ+

Direct solar-driven methane (CH4) reforming is highly desirable but challenging, particularly to achieve a value-added product with high selectivity. Here, we identify a synergistic ensemble effect of atomically dispersed copper (Cu) species and partially reduced tungsten (Wδ+), stabilised over an oxygen-vacancy-rich WO3, which enables exceptional photocatalytic CH4 conversion to formaldehyde (HCHO) under visible light, leading to nearly 100% selectivity, a very high yield of 4979.0 μmol·g−1 within 2 h, and the normalised mass activity of 8.5 × 106 μmol·g-1Cu·h−1 of HCHO at ambient temperature. In-situ EPR and XPS analyses indicate that the Cu species serve as the electron acceptor, promoting the photo-induced electron transfer from the conduction band to O2, generating reactive •OOH radicals. In parallel, the adjacent Wδ+ species act as the hole acceptor and the preferred adsorption and activation site of H2O to produce hydroxyl radicals (•OH), and thus activate CH4 to methyl radicals (•CH3). The synergy of the adjacent dual active sites boosts the overall efficiency and selectivity of the conversion process.

Metal-free oxidized carbon nitride for efficient sunlight-driven photocleavage of lignin β-O-4 bond

Benefitting from mild reaction conditions and good reusability, photocatalysis has industrial potential for high-value utilization of lignin waste. Commonly used metal sulfide photocatalysts present the risk of contamination of aromatic products by heavy metal ions due to photocorrosion. Here, a metal-free oxidized g-C3N4 photocatalyst was developed to selectively cleave the β-O-4 bond of lignin. Simple high-temperature oxidation treatment reduced carbon nitride size and scavenged the terminal NH2 groups, resulting in higher oxygen adsorption capacity and valence band potential. In the photoelectric effect, oxidized g-C3N4 exhibited lower carrier migration resistance and a longer photogenerated electron lifetime. The high oxygen adsorption and specific surface area led to a 160 % increase in superoxide radicals. Two routes for the photocatalytic cleavage β-O-4 bond in g-C3N4 are proposed. The catalytic activity of oxidized g-C3N4 is 8.5 times higher than that of untreated g-C3N4, the selective photocleavage reactivity attributed mainly to more OOH radicals and higher valence band potential. This work will provide a green and efficient strategy for the high-value conversion of lignin.

Computational investigation on the antioxidant activities and on the Mpro SARS-CoV-2 non-covalent inhibition of isorhamnetin.

In the present work, we report a computational study on some important chemical properties of the flavonoid isorhamnetin, used in traditional medicine in many countries. In the course of the study we determined the acid-base equilibria in aqueous solution, the possible reaction pathways with the •OOH radical and the corresponding kinetic constants, the complexing capacity of copper ions, and the reduction of these complexes by reducing agents such as superoxide and ascorbic anion by using density functional level of theory Density Functional Theory. Finally, the non-covalent inhibition ability of the SARS-CoV-2 main protease enzyme by isorhamnetin was examined by molecular dynamics (MD) and docking investigation.

Possible Role of Metal-Ions in the Chemistry of Prochirality and the Origin of Chirality in the Interstellar Medium

Propylene oxide (c-C3H6O) is the first chiral molecule detected in the interstellar medium (ISM). Its recent detection has opened up newer avenues to be explored in interstellar chemistry. On earth, metal-ions play a preeminent role in the chemistry of chirality and prochirality. Taking a cue from this idea, we computationally investigate the possible role of metal-ions in (a) the origin of chirality in the ISM and (b) advancing interstellar prochiral chemistry; noting that prochirality is a severely understudied concept in astrochemistry. The gas-phase reactions of ethylene, and propylene with OOH radical to lead to the formation of ethylene oxide and propylene oxide respectively were noted to be excellent model reactions, and have been chosen to examine the role of the metal-ions. The interstellar metal-ions being studied (viz. Al+, Mg+, and Mg2+) are considered in three different forms─as bare metal-ions, or as hydrated metal-ions bound to one molecule of water, and as part of a neutral molecule. Carefully calibrated electronic structure calculations (CCSD(T) and DFT) were used to obtain the relevant saddle points (minimum energy structures and transition states). One key conclusion from this work is that, metal-ions, regardless of what form they are in, lead to the barrierless formation of the epoxides, thereby facilitating the generation of interstellar chirality. The key chemical, and astrochemical facets involved in the chemistry of metal-ions in the circumstellar envelope of IRC+10216 are also discussed herein in detail. This work further recommends the detection of four molecules in the ISM, namely, aziridine, methyl aziridine, 2-amino-3-hydroxy propane, and 2-hydroxy-3-amino propane. Lastly, a foremost point to emerge out of this study and related contemporary works probing the chemistry of metal-ions in the ISM is that metal-ions may act as the glue connecting interstellar gas-phase chemistry with the surface chemistry occurring on ice-grains, and could aid in the unravelling of complex and new aspects in astrochemistry.

In Situ-Generated CuI Catalytic System for Oxidative N-Formylation of N-Heterocycles and Acyclic Amines with Methanol.

The development of a sustainable and simple catalytic system for N-formylation of N-heterocycles with methanol by direct coupling remains a challenge, owing to many competing side reactions, given the sensitivity of N-heterocycles to many catalytic oxidation or dehydrogenation systems. This work concerns the development of an in situ-generated CuI catalytic system for oxidative N-formylation of N-heterocycles with methanol that is based on the case study of a more typical 1,2,3,4-tetrahydroquinoline as substrate. Aside from N-heterocycles, some acyclic amines are also transformed into the corresponding N-formamides in moderate yields. Furthermore, a probable reaction mechanism and reaction pathway are proposed and extension of work based on some findings leads to a demonstration that the formed ⋅O2 - and ⋅OOH radicals in the catalytic system is related to the formation of undesired tar-like products.

Atomically Dispersed Nickel Anchored on a Nitrogen-Doped Carbon/TiO2 Composite for Efficient and Selective Photocatalytic CH4 Oxidation to Oxygenates.

Direct photocatalytic oxidation of methane to liquid oxygenated products is a sustainable strategy for methane valorization at room temperature. However, in this reaction, noble metals are generally needed to function as cocatalysts for obtaining adequate activity and selectivity. Here, we report atomically dispersed nickel anchored on a nitrogen-doped carbon/TiO2 composite (Ni-NC/TiO2 ) as a highly active and selective catalyst for photooxidation of CH4 to C1 oxygenates with O2 as the only oxidant. Ni-NC/TiO2 exhibits a yield of C1 oxygenates of 198 μmol for 4 h with a selectivity of 93 %, exceeding that of most reported high-performance photocatalysts. Experimental and theoretical investigations suggest that the single-atom Ni-NC sites not only enhance the transfer of photogenerated electrons from TiO2 to isolated Ni atoms but also dominantly facilitate the activation of O2 to form the key intermediate ⋅OOH radicals, which synergistically lead to a substantial enhancement in both activity and selectivity.

Atomically Dispersed Nickel Anchored on a Nitrogen‐Doped Carbon/TiO2Composite for Efficient and Selective Photocatalytic CH4Oxidation to Oxygenates

AbstractDirect photocatalytic oxidation of methane to liquid oxygenated products is a sustainable strategy for methane valorization at room temperature. However, in this reaction, noble metals are generally needed to function as cocatalysts for obtaining adequate activity and selectivity. Here, we report atomically dispersed nickel anchored on a nitrogen‐doped carbon/TiO2composite (Ni−NC/TiO2) as a highly active and selective catalyst for photooxidation of CH4to C1 oxygenates with O2as the only oxidant. Ni−NC/TiO2exhibits a yield of C1 oxygenates of 198 μmol for 4 h with a selectivity of 93 %, exceeding that of most reported high‐performance photocatalysts. Experimental and theoretical investigations suggest that the single‐atom Ni−NC sites not only enhance the transfer of photogenerated electrons from TiO2to isolated Ni atoms but also dominantly facilitate the activation of O2to form the key intermediate ⋅OOH radicals, which synergistically lead to a substantial enhancement in both activity and selectivity.

Copper Cocatalyst Modulated Radical Generation for Selective Heterogeneous Photosynthesis of α-Haloketones.

The α-haloketones are important precursors for synthetic chemistry and pharmaceutical applications; however, their production relies heavily on traditional synthetic methods via halogenation of ketones that are toxic and environmentally risky. Here, we report a heterogeneous photosynthetic strategy of α-haloketone production from aromatic olefins using copper-modified graphitic carbon nitride (Cu-C3N4) under mild reaction conditions. By employing NiX2 (X = Cl, Br) as the halogen source, a series of α-haloketones can be synthesized using atmospheric air as the oxidant under visible-light irradiation. In comparison with pristine carbon nitride, the addition of Cu as a cocatalyst provides a moderate generation rate of halogen radicals and selective reduction of molecular oxygen into •OOH radicals, thus leading to a high selectivity to α-haloketones. The Cu-C3N4 also exhibits high stability and versatility, rendering it a promising candidate for solar-driven synthetic applications.

Molecular oxygen enhances H2O2 utilization for the photocatalytic conversion of methane to liquid-phase oxygenates

H2O2 is widely used as an oxidant for photocatalytic methane conversion to value-added chemicals over oxide-based photocatalysts under mild conditions, but suffers from low utilization efficiencies. Herein, we report that O2 is an efficient molecular additive to enhance the utilization efficiency of H2O2 by suppressing H2O2 adsorption on oxides and consequent photogenerated holes-mediated H2O2 dissociation into O2. In photocatalytic methane conversion over an anatase TiO2 nanocrystals predominantly enclosed by the {001} facets (denoted as TiO2{001})-C3N4 composite photocatalyst at room temperature and ambient pressure, O2 additive significantly enhances the utilization efficiency of H2O2 up to 93.3%, giving formic acid and liquid-phase oxygenates selectivities respectively of 69.8% and 97% and a formic acid yield of 486 μmolHCOOH·gcatalyst−1·h−1. Efficient charge separation within TiO2{001}-C3N4 heterojunctions, photogenerated holes-mediated activation of CH4 into ·CH3 radicals on TiO2{001} and photogenerated electrons-mediated activation of H2O2 into ·OOH radicals on C3N4, and preferential dissociative adsorption of methanol on TiO2{001} are responsible for the active and selective photocatalytic conversion of methane to formic acid over TiO2{001}-C3N4 composite photocatalyst.

Mactanamide and lariciresinol as radical scavengers and Fe2+ ion chelators – A DFT study

A DFT based kinetic study of OOH radical scavenging potency of mactanamide (MA) and lariciresinol (LA), two natural polyphenols, indicates their nearly equal potential via the proton coupled electron transfer (PCET) mechanism in lipid media. Contribution of C–H bond breaking to this potency is negligible compared to O–H bond breaking, contrary to recent claims. The predicted potency of both compounds is not sufficient to protect biological molecules from oxidative damage in lipid media. In aqueous media, the scavenging potency of MA and LA phenoxide anions via the single electron transfer (SET) mechanism is much higher and may contribute to the protection of lipids, proteins, and DNA from OOH radical damage. Also, MA and LA have the potential to chelate catalytic Fe2+ ions, thus suppressing the formation of dangerous OH radicals via Fenton-type reactions. The monoanionic species of MA and LA show stronger monodentate chelating ability with Fe2+ ion compared to its neutral form. The dianionic specie LA2− exhibited the highest chelation ability with Fe2+ ion via bidentate 1:2 coordination. However, direct radical scavenging and metal chelation could be rarely operative in vivo because MA and LA presumably achieve very low concentrations in systemic circulation.

Primary and secondary antioxidant properties of scutellarin and scutellarein in water and lipid-like environments: A theoretical investigation

The scavenging capability of scutellarin flavonoid against the OOH radical has been studied, at Density Functional level of theory, by considering different reaction mechanisms in both lipid-like and water media. In water phase, the neutral and charged species present in solution, identified from the computed pKa equilibria, have been determined. Furthermore, the complexation ability of Scutellarin and its metabolite scutellarein with respect to the Cu2+ and Fe3+ ions has been determined at the same level of theory. From the obtained rate constants results that scutellarin is a better Fe(III) ligand while scutellarein show a revers behavior. The glucuronidation process affect both the primary and secondary antioxidant properties of the systems.