Free Radical Reaction Pathway Research Articles

Engineering atomic Rb-N configurations to tune radical pathways for highly selective photocatalytic H2O2 synthesis coupled with biomass valorization

Photocatalytic oxygen reduction for hydrogen peroxide (H₂O₂) synthesis presents a green and cost-effective production method. However, achieving highly selective H₂O₂ synthesis remains challenging, necessitating precise control over free radical reaction pathways and minimizing undesirable oxidative by-products. Herein, we report for the visible light-driven simultaneous co-photocatalytic reduction of O2 to H2O2 and oxidation of biomass using the atomic rubidium-nitride modified carbon nitride (CNRb). The optimized CNRb catalyst demonstrates a record photoreduction rate of 8.01 mM h−1 for H2O2 generation and photooxidation rate of 3.75 mM h−1 for furfuryl alcohol to furoic acid, achieving a remarkable solar-to-chemical conversion (SCC) efficiency of up to 2.27%. Experimental characterizations and DFT calculation disclosed that the introducing atomic Rb–N configurations allows for the high-selective generation of superoxide radicals while suppressing hydroxyl free radical formation. This is because the Rb–N serves as the new alternative site to perceive a stronger connection position for O2 adsorption and reinforce the capability to extract protons, thereby triggering a high selective redox product formation. This study holds great potential in precisely regulating reactive radical processes at the atomic level, thereby paving the way for efficient synthesis of H2O2 coupled with biomass valorization.

Operando Raman Spectroscopy Reveals Degradation Byproducts from Ionomer Oxidation in Anion Exchange Membrane Water Electrolyzers.

This work showcases the discovery of degradation mechanisms for nonplatinum group metal catalyst (PGM free) based anion exchange membrane water electrolyzers (AEMWE) that utilize hydroxide ion conductive polymer ionomers and membranes in a zero gap configuration. An entirely unique and customized test cell was designed from the ground up for the purposes of obtaining Raman spectra during potentiostatic operation. These results represent some of the first operando Raman spectroscopy explorations into the breakdown products that are generated from high oxidative potential conditions with carbonate electrolytes. We provide a unique design and fabrication method for three-dimensional (3D) printable flow cells that enable spatially resolved Raman spectra collection from the electrode surface into the bulk electrolyte. It is proposed that the generation of breakdown products from the hydroxide-conductive ionomers and membranes originates from a multistep, free radical reaction pathway resulting in chain scission of the poly aryl backbone. This hypothesis is backed by the detection of carboxylic and aromatic functional group Raman signals from small molecules that had dissolved and diffused into the bulk electrolyte.

Fe doping strategy induces peroxymonosulfate activation by ZIF-67 through a non-radical reaction pathway with dominant 1O2 generation

Zeolitic Imidazole Framework-67 (ZIF-67), which can be large-scale synthesized under mild conditions, has been widely reported as a promising and efficient catalyst in peroxymonosulfate (PMS)-based advanced oxidation process. However, ZIF-67 typically activates PMS through a monotonous free radical reaction pathway, which limits its reusability, resistance to environmental interference, and degradation selectivity. In this study, controlled doping of Fe atoms into the lattice structure of ZIF-67 (FexCo1-x-ZIF) was employed, resulting in the nearly complete conversion of PMS to 1O2 through a unique non-radical pathway. Series of experiments, characterizations, and theoretical calculations were employed to elucidate the micro-level mechanism of PMS activation. The results indicate that the tetrahedral coordinated Co-N4 sites in ZIF-67 are transformed into unsaturated Co-N2 sites due to the competitive coordination effect upon the introduction of Fe. PMS adsorption is enhanced as both oxygen atoms bind simultaneously to the Co-N2 site, inducing the removal of a hydrogen atom from PMS and electron transfer from PMS to the Co-N2 site, thus facilitating the generation of 1O2. The 1O2-dominated non-radical degradation reaction enhances the excellent degradation performance, reusability, and environmental tolerance of the FexCo1-x-ZIF/PMS system under diverse natural water conditions, presenting significant prospects for practical applications.

Efficient removal of sulfamethoxazole by activated peroxodisulphates using Fe/N co-doped biochar: The minimal role of high-valent iron species

The harmful effects of the widespread presence of sulfamethoxazole (SMX) in water on ecosystems have become an increasingly pressing concern, necessitating the urgent development of an effective decontamination strategy. In this work, iron and nitrogen co-doped biochar (Fe-N-BC) samples were synthesized using peanut shells as a precursor to activate peroxydisulfate for SMX degradation. The most effective Fe-N-BC had a 12 times higher SMX removal rate of 97.00% within 90 min compared to the pristine biochar. The excellent catalytic performance was achieved by the synergistic effect of the Fe and N active sites, and the system contained not only typical free radicals (SO4− and OH) but also high-valent iron species (Fe(IV)). Further quantitative experiments with active species revealed that the free radical reaction pathway played a predominant role, while the relative contribution of Fe(IV) was only 8.01%. Furthermore, the degradation pathways of SMX in the system were proposed, and toxicity assessment revealed that all intermediate products exhibited lower toxicity levels than SMX. This study clarifies the structure–activity relationships of Fe-N-BC for PDS activation and offers an innovative strategy for the treatment and utilization of agricultural waste biomass as the catalyst.

Scalable synthesis of MIL-88A(Fe) for efficient aerobic oxidation of cyclohexene to 2-cyclohexene-1-ol

Herein, we reported a facile and scalable approach for the synthesis of Fe-based MOF MIL-88A(Fe) via a simple liquid-phase route at atmosphere pressure, and its application as a heterogeneous catalyst for the efficient aerobic oxidation of cyclohexene to value-added 2-cyclohexene-1-ol. Over 130 g of spindle-shaped MIL-88A(Fe) nanocrystals with high crystallinity, monodispersed particle size, and large specific surface area were easily synthesized per batch according to our method. The resultant MIL-88A(Fe) exhibited a high catalytic activity (conversion: 81%) with 70% selectivity for 2-cyclohexene-1-ol in the aerobic oxidation of cyclohexene with O2 as the sole oxidant under mild reaction conditions (0.5 MPa of O2, 353 K for 8 h), outperforming the analogues reported in the literatures. The combined experimental and theoretical approach support the reaction mechanism of the oxidation of cyclohexene via both the free radical reaction pathway and Fe-mediated reaction pathway, resulting in the selective formation of 2-cyclohexene-1-ol.

Reaction kinetics and pathways of crotonic acid conversion in sub- and supercritical water for renewable fuel production

The degradation of an unsaturated lipid compound in water proceeds via two temperature-driven pathways – ionic and free radical reaction pathways.

Effective defect generation and dual reaction pathways for phenol degradation on boron-doped carbon nanotubes

ABSTRACT A new type of metal-free catalyst was successfully prepared by doping boron (B) in the carbon nanotube. The catalyst had 99.4% removal of phenol in 60 min at pH 7 by activating peroxymonosulfate (PMS). In order to explore the origin of the high catalytic activity, the samples were characterized by Raman and electron paramagnetic resonance (EPR), and the reactive oxygen species (ROS) in the process of catalytic degradation were investigated. The Raman results showed that the defect sites increased after doping, which indicated that the B doping increases the active sites on the surface of the carbon nanotubes. Identification experiments of ROS found that not only hydroxyl radicals (·OH) and sulfate radical (SO4 −•), but also singlet oxygen (1O2) exist in the system. The presence of multiple free radicals indicated the existence of free radical reaction pathway, and the presence of 1O2 confirmed the existence of non-radical reaction pathway. These results indicated that there were dual reaction pathways for the activation of persulfate by B-doped carbon nanotubes, which was the intrinsic nature for the high catalytic activity of the system.

Activation of Permonosulfate by Rhodamine B for BPA Degradation Under Visible Light Irradiation

Although the activation method of permonosulfate has been gradually developed, its practical application is severely restricted by the high cost and difficult recovery of the catalyst, thereby resulting in secondary pollution. In this study, the application potential of self-decolorization of dyes and degradation of other pollutants through persulfate(PS) activation was examined by building a self-decolorization system. The results showed that the dyes could activate PS under visible light irradiation, which could realize not only the self-decolorization of dyes, but also the degradation of other pollutants. The degradation rates of rhodamine B and bisphenol A could reach 80% and 90%, respectively. This process included both free radical reaction pathways and nonradical reaction pathways. The active oxidants produced in the system included superoxide radicals, sulfate radicals, hydroxyl radicals, and singlet oxygen. The self-decolorization efficiency of dyes was related to the type of dyes, initial concentration of the dyes, dosage of PS, and initial pH of the solution. Meanwhile, the initial concentrations of the dyes and other pollutants had a great influence on the degradation of other pollutants. This study provides a new idea for economic and environmental protection in the PS activation method, and has broad application prospects in the treatment of printing and dyeing wastewater.

Regioselective Chlorothiolation of Alkenes with Sulfonyl Chlorides.

Newly developed sulfonyl chloride-based regioselective chlorothiolation of alkenes has been disclosed; the reaction is compatible with a variety of functional groups and can be scaled up to the gram scale with no loss in yield. The employment of readily available reactants, mild reaction conditions, and high regioselectivity makes this process very practical. Mechanistic studies revealed a possible free radical reaction pathway.

Sonochemical Preparation of Dipicolinamide Mn‐complexes and Their Application as Catalysts Towards Sono‐synthesis of Ketones

A series of non‐heme Mn‐complexes has been synthesized by the sonication of manganese (II)chloride and bis‐amides (condensation products of 2‐picolinic acid and o‐phenylenediamines). The Mn‐complexes effectively promote the oxidation of unactivated aliphatic and benzylic C─H and N‐bearing heterocycles substrates with low catalyst loading using eco‐friendly hydrogen peroxide in the presence of acetic acid as additive under ultrasonic irradiation. Chromatographic studies revealed that the corresponding ketones are the only detectable products. Noteworthy, the presence of electron donors in the catalyst structure significantly increased the reaction yields. The substantial lowering of the oxidation reaction yields by adding ionol (2,6‐di‐tert‐butyl‐4‐methylphenol) as a free radical trap suggesting a free radical reaction pathway.

Titanium-catalyzed hydrosilylation of olefins: A comparison study on Cp2TiCl2/Sm and Cp2TiCl2/LiAlH4 catalyst system

Hydrosilylation of olefins catalyzed by Cp2TiCl2/Sm (Cp = cyclopentadienyl) under solvent free conditions have been investigated. By using Cp2TiCl2/Sm as catalyst system, β-adducts and hydrogenation products were detected. Hydrosilylation of olefins catalyzed by Cp2TiCl2/LiAlH4 under room temperature has also been studied. The influence of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) on Cp2TiCl2/Sm and Cp2TiCl2/LiAlH4, respectively, indicated that hydrosilylation of olefins catalyzed with Cp2TiCl2/Sm went through a free radical reaction pathway while a coordination mechanism was applied for Cp2TiCl2/LiAlH4 catalyst system.

A reactive force field molecular dynamics simulation of nickel oxidation in supercritical water

The atomistic mechanism of nickel (Ni) oxidation in supercritical water (SCW) is investigated using molecular dynamics simulations with a reactive force field. The oxidation kinetics of Ni surfaces in SCW at 300−800°C and 26−164kg/m3 are simulated. The adsorption, dissociation, and deprotonation processes of water, and the hydroxylation and oxidation processes of Ni are tracked. The oxidation potential of SCW is found to be higher than that of water at normal state. Charge analysis of deprotonation reveals that the dissociation of water is more likely to be a homolytic reaction instead of a heterolitic one under the supercritical state of relatively high temperatures and low densities. Oxidation in SCW prefers the free radical reaction pathway to the ionic pathway, which appears a gas/steam oxidation scope instead of aqueous oxidation. Reaction rate increases with increased temperature and density of SCW.

Aerosol Fragmentation Driven by Coupling of Acid-Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals.

A key uncertainty in the heterogeneous oxidation of carboxylic acids by hydroxyl radicals (OH) in aqueous-phase aerosol is how the free-radical reaction pathways might be altered by acid-base chemistry. In particular, if acid-base reactions occur concurrently with acyloxy radical formation and unimolecular decomposition of alkoxy radicals, there is a possibility that differences in reaction pathways impact the partitioning of organic carbon between the gas and aqueous phases. To examine these questions, a kinetic model is developed for the OH-initiated oxidation of citric acid aerosol at high relative humidity. The reaction scheme, containing both free-radical and acid-base elementary reaction steps with physically validated rate coefficients, accurately predicts the experimentally observed molecular composition, particle size, and average elemental composition of the aerosol upon oxidation. The difference between the two reaction channels centers on the reactivity of carboxylic acid groups. Free-radical reactions mainly add functional groups to the carbon skeleton of neutral citric acid, because carboxylic acid moieties deactivate the unimolecular fragmentation of alkoxy radicals. In contrast, the conjugate carboxylate groups originating from acid-base equilibria activate both acyloxy radical formation and carbon-carbon bond scission of alkoxy radicals, leading to the formation of low molecular weight, highly oxidized products such as oxalic and mesoxalic acid. Subsequent hydration of carbonyl groups in the oxidized products increases the aerosol hygroscopicity and accelerates the substantial water uptake and volume growth that accompany oxidation. These results frame the oxidative lifecycle of atmospheric aerosol: it is governed by feedbacks between reactions that first increase the particle oxidation state, then eventually promote water uptake and acid-base chemistry. When coupled to free-radical reactions, acid-base channels lead to formation of low molecular weight gas-phase reaction products and decreasing particle size.

Synthesis, crystal structures and catalytic activity of Cu(II) and Mn(III) Schiff base complexes: Influence of additives on the oxidation catalysis of cyclohexane and 1-phenylehanol

The complexes of copper [Cu(κONN’-HL)(NO3)(DMF)](NO3)∙H2O (1) and [Cu(κONN’-HL)Cl2]∙½DMSO (2), and of manganese [Mn(κON-HL)2Cl2]Cl (3) and [Mn(κON-HL)2(NO3)2](NO3)∙H2O (4) were synthesized by reactions of the respective chloride or nitrate salt with a non-aqueous solutions of the Schiff base aminoalcohol HL (product of condensation of salicylic aldehyde and aminoethylpiperazine) and characterized by X-ray diffraction analysis. The catalytic investigations disclosed a prominent activity of the copper compounds 1 and 2 towards oxidation of cyclohexane with hydrogen peroxide in the presence of various promoters (nitric, hydrochloric, oxalic acids and pyridine), under mild conditions. The unusual promoting effect of pyridine on the catalytic activity of the copper catalysts allowed to achieve yields up to 21% based on cyclohexane. Chromatographic studies revealed that cyclohexyl hydroperoxide is a main reaction product and chlorocyclohexane (in the presence of HCl as promoter) was also detected, suggesting a free radical reaction pathway with hydroxyl radicals as attacking species. Complexes 1 and 2 act also as catalysts in the oxidation of 1-phenylethanol with tert-butylhydroperoxide, showing acetophenone yields up to 62% and TON (turnover numbers) up to 620 in the presence of the K2CO3 promoter.

(Invited) Tetrapyrrole-Based Metal Complexes Designed for Bio-Inspired Catalysis and Solar Fuel Production

Light-induced substrate transformations in artificial photosynthetic devices and functional enzyme model systems strongly depend on the feasibility of multielectron transfer catalysis [1,2]. The crucial advantage of coupling multiple redox equivalents and proton transfer steps in photocatalysis is to avoid free radical reaction pathways and energetic constraints due to charge accumulation, as these processes can decrease both the long-term stability and the efficiency of the corresponding systems. In this context, certain coordination compounds including tetrapyrrole-based derivatives such as porphyrin, corrole and phthalocyanine complexes have been shown to achieve an efficient fusion of the complementary functions of a light-harvester, a photoredox interface, a substrate recognition site and a mediator for chemical reactions [3-6]. To further improve the performance of such systems, the design of long-term stable processes for artificial photosynthetic energy storage and green synthetic chemistry using artificial (photo)enzymes has to be achieved with sustainable, environmentally benign and earth-abundant bulding blocks. Another crucial aspect in the context of solar chemistry and catalysis is not only the requirement for using visible light, but also to shift the threshold wavelength of the photosensitizers as far as possible to the red and NIR-spectral regions. Some of our recent developments in this direction will be presented in this contribution. [1] G. Knör, Chem. Eur. J. 2009, 15, 568 [2] G. Knör, Coord. Chem. Rev. 2015, 304-305, 102 [3] M. Hajimohammadi, C. Schwarzinger, G. Knör, RSC Advances 2012, 2, 3257 [4] C. Uslan, K. T. Oppelt, L. M. Reith, B. Ş. Sesalan, G. Knör, Chem. Commun. 2013, 49, 8108 [5] K. T. Oppelt, E. Wöß, M. Stiftinger, W. Schöfberger, W. Buchberger, G. Knör, Inorg. Chem. 2013, 52, 11910 [6] M. Ertl, E. Wöß, G. Knör, Photochem. Photobiol. Sci. 2015, 14, 1826

Radical Pathways for the Prebiotic Formation of Pyrimidine Bases from Formamide.

The prebiotic formation of nucleobases, the building blocks of RNA/DNA, is of current interest. Highly reactive radical species present in the atmosphere under irradiation have been suggested to be involved in the prebiotic synthesis of nucleobases from formamide (FM). We studied several free radical reaction pathways for the synthesis of pyrimidine bases (cytosine, uracil, and thymine) from FM under cold conditions. These pathways are theoretically determined using density functional theory (DFT) computations to examine their kinetic and thermodynamic feasibilities. These free radical reaction pathways share some common reaction types such as H-rearrangement, (•)H/(•)OH/(•)NH2 radical loss, and intramolecular radical cyclization. The rate-determining steps in these pathways are characterized with low energy barriers. The energy barriers of the ring formation steps are in the range of 3-7 kcal/mol. Although DFT methods are known to significantly underestimate the barriers for addition of (•)H radical to neutral species, many of these reactions are highly exergonic with energy release of -15 to -52 kcal/mol and are thus favorable. Among the suggested pathways for formation of cytosine (main route, routes 7a and 1a), uracil (main route, routes 7b and 1b), and thymine (main route and route 26a), the main routes are in general thermodynamically more exergonic and more kinetically favored than other alternative routes with lower overall energy barriers. The reaction energies released following formation of cytosine, uracil, and thymine from FM via the main radical routes amount to -59, -81, and -104 kcal/mol, respectively. Increasing temperature induces unfavorable changes in both kinetic and thermodynamic aspects of the suggested routes. However, the main routes are still more favored than the alternative pathways at the temperature up to the boiling point of FM.

Free-radical conversion of a lignin model compound catalyzed by Pd/C

Benzyl phenyl ether can be decomposed into phenol and toluene effectively by using Pd/C as the catalyst in the presence of sodium carbonate and N-methyl-2-pyrrolidone via a free-radical reaction pathway.

Chemical properties of superfine pulverized coal particles. Part 1. Electron paramagnetic resonance analysis of free radical characteristics

Superfine pulverized coal combustion is a new pulverized coal combustion technology with lots of advantages. A mechanochemical effect exists during the comminution process, which changes the chemical properties of coal significantly. Free radical concentrations and certain functional groups would increase with the decrease of particle sizes. In this paper, we combined electron paramagnetic resonance (EPR) and 13C solid-state nuclear magnetic resonance (NMR) techniques to study the free radical characteristics of superfine pulverized coal thoroughly. The final results indicate that the EPR spectra of coal are the superimpositions of several lines induced by different paramagnetic centers, which can be fitted by 1 Gaussian and 3 Lorentzian lines. The influences of coal maturities and particle sizes on EPR parameters, such as g-values, linewidths, and spin concentrations, are analyzed in detail. It is shown that with the decrease of particle sizes, more free radicals are induced through bond cleavages. Mechanical forces initiate the accumulation of free radicals in the fractures and inner pore surfaces of coal. Furthermore, the influence of particle sizes on oxygen-containing radicals (i.e., Lorentzian 1 types) is the greatest. This work provides a primary picture of the occurrence modes and spatial distributions of free radicals in superfine pulverized coal. The findings will help form the basis and provide guidance for further studies on revealing the correlations between the free radical reaction pathways and NOx formation mechanisms.

Single-site Sn-grafted Ru/TiO2 photocatalysts for biomass reforming: Synergistic effect of dual co-catalysts and molecular mechanism

This work offers an engineering guide to obtaining highly efficient photocatalysts for hydrogen production. Synergetic enhancement of photocatalytic hydrogen evolution from biomass/water solution is achieved by co-modifying anatase TiO2 with single-site Sn-oxo species and RuO2 nanoparticles. Detailed characterization and analysis clearly reveal that such TiO2-based composites can function as photoelectrolysis cells, where RuO2 and SnOx species serve, respectively, as an anode and a cathode and TiO2 is mainly responsible for the conversion of photons into electrical energy and the OH formation. Electron paramagnetic resonance and infrared spectroscopy studies suggest a free radical reaction pathway triggered by the hole oxidation for the photocatalytic reforming of biomass. The CxHyOz renewables undergo one or more processes for the sequential oxidation of alcohol to aldehyde, acid, and finally CO2 and CO. The activity results indicate that proton reduction is the controlling-rate step of the overall photoreforming reaction.

The pyrolysis chemistry of a β-O-4 type oligomeric lignin model compound

The pyrolysis behavior of a β-O-4 type oligomeric lignin model compound is studied at a temperature range from 250 °C to 550 °C. Thermogravimetric analysis (TGA) indicates the model compound has three distinct thermal decompositions peaks when a slow temperature ramp is applied while only one decomposition peak is observed when a higher temperature ramp is used. The pyrolysis behavior of this model compound is similar to that of lignin prepared by enzymatic hydrolysis of Maplewood. 1H-NMR shows that the β-O-4 linkage thermally decomposes at a temperature between 250 °C and 350 °C with the formation of solid products at 350 °C. This solid product undergoes transformation to a polyaromatic from 350 °C to 550 °C. Around twenty five volatile compounds are quantified by pyroprobe-GC-MS with vanillin and 2-methoxy-4-methyl phenol being the most abundant monomeric products. A free radical reaction pathway is proposed to explain the product chemistry for pyrolysis of the lignin model compound. Char is most likely formed by random repolymerization of the radicals.