Radical Reaction Pathway Research Articles

Electrochemical synthesis of 1,2,3-trisubstituted pyrroles from β-dicarbonyl compounds, aldehydes and amines via radical addition reaction

An efficient electrochemical synthesis of 1,2,3-trisubstituted pyrroles was developed by one-pot three-component reaction of β-dicarbonyl compounds, aldehydes with amines. The target products could be obtained in good to excellent yields. Our investigations indicate that β-enamino ketone is a key intermediate in this transformation and the reaction mechanism may involve a radical addition reaction pathway.

Liquid-Phase Cyclohexene Oxidation with O2 over Spray-Flame-Synthesized La1-x Srx CoO3 Perovskite Nanoparticles.

La1−x Sr x CoO3 (x=0, 0.1, 0.2, 0.3, 0.4) nanoparticles were prepared by spray‐flame synthesis and applied in the liquid‐phase oxidation of cyclohexene with molecular O2 as oxidant under mild conditions. The catalysts were systematically characterized by state‐of‐the‐art techniques. With increasing Sr content, the concentration of surface oxygen vacancy defects increases, which is beneficial for cyclohexene oxidation, but the surface concentration of less active Co2+ was also increased. However, Co2+ cations have a superior activity towards peroxide decomposition, which also plays an important role in cyclohexene oxidation. A Sr doping of 20 at. % was found to be the optimum in terms of activity and product selectivity. The catalyst also showed excellent reusability over three catalytic runs; this can be attributed to its highly stable particle size and morphology. Kinetic investigations revealed first‐order reaction kinetics for temperatures between 60 and 100 °C and an apparent activation energy of 68 kJ mol−1 for cyclohexene oxidation. Moreover, the reaction was not affected by the applied O2 pressure in the range from 10 to 20 bar. In situ attenuated total reflection infrared spectroscopy was used to monitor the conversion of cyclohexene and the formation of reaction products including the key intermediate cyclohex‐2‐ene‐1‐hydroperoxide; spin trap electron paramagnetic resonance spectroscopy provided strong evidence for a radical reaction pathway by identifying the cyclohexenyl alkoxyl radical.

Peroxymonosulfate activation by a metal-free biochar for sulfonamide antibiotic removal in water and associated bacterial community composition

Antibiotic sulfamethoxazole (SMX) has been commonly found in various water matrices, therefore effective decontamination method is urgently needed. Metal-free pristine coconut-shell-derived biochar (CSBC), synthesized by thermochemical conversion at 700 °C, was used for activating peroxymonosulfate (PMS), an oxidant, to degrade SMX, a sulfonamide antibiotic, in water. SMX degradation, maximized at 0.05 mM concentration, was 85% in 30 min at pH 5.0 in the presence of 150 mg L−1 of CSBC. Remarkably, SMX removal reached 99% in a chloride-rich CSBC/PMS system. SMX degradation was mainly attributed to the role of CSBC in enhancing PMS activation to produce combined radical (SO4•−/HO•) and nonradical (1O2) reaction pathways. The most abundant genus in the CSBC/PMS system was Methylotenera, which belonged to the Proteobacteria phylum. Thus, from a perspective of biowaste-to-resource recycling and circular bioeconomy view point, CSBC is a potential catalytic activator of PMS for the removal of sulfonamide antibiotics from aqueous environments.

Metal-based catalysts for persulfate and peroxymonosulfate activation in heterogeneous ways: A review

A growing number of emerging organic contaminants associated with the potential risks for ecosystem and human health are frequently detected in environment. Sulfate radical-based advanced oxidation processes have attracted increasing attention during the past decades, owing to their high reactivity and oxidation capability of a wide range of toxic and non-biodegradable organic compounds. Metal-based materials have been proven to be effective heterogeneous catalysts for activating persulfate to degrade many refractory organic pollutants. This article reviews the up-to-date research progresses on various heterogeneous metal-based catalysts for persulfate (PS) and peroxymonosulfate (PMS) activation, including single metal and mixed metal catalysts. The activation mechanisms of these catalysts and their fundamental behaviors in PS/PMS activation are emphatically elucidated, in which contaminants can be eliminated through radical and/or non-radical reaction pathways. In addition, an in-depth discussion about metal composite namely metal spinel, metallic glasses and other metal hybrids in PS or PMS systems for organic degradation and possible environmental applications are presented. Finally, the perspectives on the metal-based PS/PMS activation method and their remediation potential for recalcitrant compounds in practical application are also proposed.

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.

Singlet oxygen-dominated activation of peroxymonosulfate by passion fruit shell derived biochar for catalytic degradation of tetracycline through a non-radical oxidation pathway

Waste-derived biochar has been emerged as promising catalysts to activate peroxymonosulfate (PMS) for the degradation of organic contaminants. Herein, passion fruit shell derived biochar (PFSC) was prepared by a one-pot pyrolysis method and used as a metal-free catalyst to activate PMS for the degradation of tetracycline hydrochloride (TC). The batch experiments indicated that the pyrolysis temperature could influence the efficiency of PFSC for the activation of PMS. In the PFSC-900 (prepared at 900 °C)/PMS system, the degradation rate of TC can reach 90.91%. The quenching test and electron paramagnetic resonance spectra revealed that the high catalytic performance of PFSC-900/PMS system was mainly attributed to the non-free radical reaction pathway containing a carbon bridge, and the TC degradation was controlled primarily by singlet oxygen-mediated oxidation. Moreover, the carboxyl group of ketones and the graphite-N atoms on PFSC-900 are the possible active sites of the non-free radical pathway including direct electron transfer or the formation of O2•-/1O2. This study not only shows a new type of biochar as an efficient catalyst for PMS activation but also provides a way of value-added reuse of passion fruit shell.

Visible-Light-Driven, Palladium-Catalyzed Heck Reaction of Internal Vinyl Bromides with Styrenes.

Functionalized 1,3-dienes were efficiently accessed from visible-light-driven, palladium-catalyzed Heck reaction of S,S-functionalized internal vinyl bromides with styrenes under mild conditions. This Heck reaction showcased tolerance of a wide array of functional groups, afforded the target products in moderate to excellent yields through a radical reaction pathway. The resultant diene products could be further transformed to highly functionalized trisubstituted furan derivatives.

Selective Transformations of Carbohydrates Inspired by Radical-Based Enzymatic Mechanisms.

Enzymes are a longstanding source of inspiration for synthetic reaction development. However, enzymatic reactivity and selectivity are frequently untenable in a synthetic context, as the principles that govern control in an enzymatic setting often do not translate to small molecule catalysis. Recent synthetic methods have revealed the viability of using small molecule catalysts to promote highly selective radical-mediated transformations of minimally protected sugar substrates. These transformations share conceptual similarities with radical SAM enzymes found in microbial carbohydrate biosynthesis and present opportunities for synthetic chemists to access microbial and unnatural carbohydrate building blocks without the need for protecting groups or lengthy synthetic sequences. Here, we highlight strategies through which radical reaction pathways can enable the site-, regio-, and diastereoselective transformation of minimally protected carbohydrates in both synthetic and enzymatic systems.

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.

Catalytic Boration of Alkyl Halides with Borane without Hydrodehalogenation Enabled by Titanium Catalyst

An unprecedented and general titanium-catalyzed boration of alkyl (pseudo)halides (alkyl-X, X = I, Br, Cl, OMs) with borane (HBpin, HBcat) is reported. The use of titanium catalyst can successfully suppress the undesired hydrodehalogenation products that prevail using other transition-metal catalysts. A series of synthetically useful alkyl boronate esters are readily obtained from various (primary, secondary, and tertiary) alkyl electrophiles, including unactivated alkyl chlorides, with tolerance of other reducing functional groups such as ester, alkene, and carbamate. Preliminary studies on the mechanism revealed a possible radical reaction pathway. Further extension of our strategy to aryl bromides is also demonstrated.

Catalytic Boration of Alkyl Halides with Borane without Hydrodehalogenation Enabled by Titanium Catalyst

AbstractAn unprecedented and general titanium‐catalyzed boration of alkyl (pseudo)halides (alkyl‐X, X=I, Br, Cl, OMs) with borane (HBpin, HBcat) is reported. The use of titanium catalyst can successfully suppress the undesired hydrodehalogenation products that prevail using other transition‐metal catalysts. A series of synthetically useful alkyl boronate esters are readily obtained from various (primary, secondary, and tertiary) alkyl electrophiles, including unactivated alkyl chlorides, with tolerance of other reducing functional groups such as ester, alkene, and carbamate. Preliminary studies on the mechanism revealed a possible radical reaction pathway. Further extension of our strategy to aryl bromides is also demonstrated.

A novel electrocatalytic filtration system with carbon nanotube supported nanoscale zerovalent copper toward ultrafast oxidation of organic pollutants

In this study, we designed an integrated electrochemical filtration system for catalytic activation of peroxymonosulfate (PMS) and degradation of aqueous microcontaminants. Composites of carbon nanotube (CNT) and nanoscale zero valence copper (nZVC) were developed to serve as high-performance catalysts, electrode and filtration media simultaneously. We observed both radical and nonradical reaction pathways, which collectively contributed to the degradation of model pollutants. Congo red was completely removed via a single-pass through the nZVCCNT filter (τ <2 s) at neutral pH. The rapid kinetics of Congo red degradation were maintained across a wide pH range (from 3.0–7.0), in complicated matrixes (e.g., tap water and lake water), and for the degradation of a wide array of persistent organic contaminants. The superior activity of nZVCCNT stems from the boosted redox cycles of Cu2+/Cu+ in the presence of an external electric field. The flow-through design remarkably outperformed the conventional batch system due to the convection-enhanced mass transport. Mechanism studies suggested that the carbonyl group and electrophilic oxygen of CNT served as electron donor and electron acceptor, respectively, to activate PMS to generate •OH and 1O2via one-electron transport. The electron-deficient Cu atoms are prone to react with PMS via surface hydroxyl group to produce reactive intermediates (Cu2+-O-O-SO3−), and then 1O2 will be generated by breaking the coordination bond of the metastable intermediate. The study will provide a green strategy for the remediation of organic pollution by a highly efficient and integrated system based on catalytic oxidation, electrochemistry, and nano-filtration techniques.

Application of Catalytic Ozonation Process Using a Novel Fe3O4/Mg(OH)2/4A-Zeolite Catalyst for Swift Treatment of Dairy Effluent

The present study investigated the feasibility of employing a catalytic ozonation process by a novel Fe3O4/Mg(OH)2/4A zeolite catalyst to facilitate the treatment of real dairy effluent. The Fe3O4/Mg(OH)2 magnetic nanoparticles supported onto 4A zeolite (Fe3O4/Mg(OH)2/4AZ) were synthesized and characterized by extensive techniques (XRD, FE-SEM, TEM, EDS, VSM, and BET). Reaction rate, operating conditions, mechanism, and thermodynamics of the dairy effluent treatment by the Fe3O4/Mg(OH)2/4AZ-catalyzed ozonation process were investigated. Catalytic ozonation experiments indicated that synchronous use of Fe3O4/Mg(OH)2/4AZ and O3 considerably accelerated the rate of COD reduction, compared to sole ozonation. Results of the designed experiments by response surface methodology (RSM) revealed that approximately 80% of COD was reduced within 30 min at natural pH of solution (≈ 7) when the catalyst and ozone dosages were 5 g L−1 and 9 mg min−1, respectively. Exploration of the mechanism disclosed that the process mainly followed a hydroxyl radical reaction pathway. Thermodynamics studies demonstrated that the process was endothermic and temperature sensitive, so that it was spontaneous at the temperature of 313 K. Overall, this study supplies a bold and quick manner as well as environmental friendliness (no secondary pollution) to eradicate refractory substances and wastewater treatment.

General Mechanism for Sulfate Radical Addition to Olefinic Volatile Organic Compounds in Secondary Organic Aerosol.

Previous laboratory studies have suggested that sulfate radical addition to olefinic biogenic volatile organic compounds (BVOCs) is a potential formation mechanism for some organosulfates detected in ambient secondary organic aerosol (SOA). However, these studies propose conflicting reaction products, possibly because laboratory dissolved oxygen levels did not accurately reflect atmospheric conditions. Additionally, these studies used analytical methods that could not definitively identify and quantify the structurally specific products. Here, we describe a method that allows for the study of the reaction of sulfate radicals and several olefinic precursors, including allyl alcohol (AA), methyl vinyl ketone (MVK), 2-methyl-3-buten-2-ol (MBO), and methacrolein (MA), with careful control of dissolved oxygen levels and using the isomer-specific nuclear magnetic resonance (NMR) method to definitively identify and quantify the reaction products. Specific mechanisms for each olefinic precursor were developed, as well as a generalized mechanism that can be used to predict the sulfate radical reaction pathways for any olefin. The product yield results indicate that this mechanism is dominated by carbon backbone fragmentation pathways: 61, 83, 79, and 100% for AA, MVK, MBO, and MA, respectively. Several of the observed organosulfate products have also been detected in field observations of SOA, which indicates the potential relevance of this mechanism in the atmosphere.

Characteristics of small molecule compounds produced from the co-pyrolysis of cotton stalk and coal

Pyrolysis experiments of cotton stalk (CS) and Shenmu coal (SM) were conducted in a tubular furnace. The pyrolysis temperature was 600 °C at 5 °C/min and sustained for 15 min. The water-soluble small molecule compounds (WSMC) were derived from the liquid products obtained during pyrolysis with the methods of toluene entrainment and ultrasonic extraction. The compositions of WSMC were further characterized by gas chromatography–mass spectrometry (GC-MS). The components of the syngas were analyzed by gas chromatography (GC). The results showed that the phenol yield was promoted by the interaction of CS and SM during co-pyrolysis. Moreover, the co-pyrolysis interaction blocked the radical reaction pathway that produces amides and accelerated the formation of pyridines. Because the ester yield increased, the esterification was clearly enhanced and the yield of carboxylic acids in WSMC was reduced during co-pyrolysis. In addition, the inhibition of furan generation resulted in an increased yield of C2–C4 hydrocarbons in the co-pyrolysis syngas. The maximal yields of C2–C4 hydrocarbons all occurred at a 20/100 ratio of CS/SM. Lastly, the formation mechanisms of small molecule compounds were proposed.

Degradation mechanisms of simple aliphatic amines under ozonation: a DFT study.

Aliphatic amines as common constituents of dissolved organic nitrogen (DON) exhibit high reactivity during ozonation; however, our understanding of their degradation mechanisms is very limited. In this study, methylamine (MA) and ethylamine (EA), as well as their secondary and tertiary amines (DMA, DEA, TMA and TEA) were chosen as aliphatic amine models and their degradation mechanisms during ozonation were investigated by using the DFT method. The oxygen-transfer reaction occurs initially and rapidly in the ozonation of all the above amines with a ΔG≠ value of 8-10 kcal mol-1 in great agreement with the experimental rate constant of 104 to 107 M-1 s-1. Moreover, N-oxide as the main degradation product for tertiary amines directly forms after oxygen-transfer, while nitroalkanes as main products for secondary and primary amines are yielded after a series of reactions mediated by hydroxylamine and nitrosoalkane with a ΔG≠ value of 10-13 kcal mol-1. Regarding the minor N-dealkylated products for all amines, alkylamino alcohol is an important intermediate possibly generated via a radical reaction pathway with a ΔG≠ value of 21-34 kcal mol-1. Additionally, comparison of the reactivity of aliphatic amines, hydroxylamines and alkylamino alcohols with ozone was made and elucidated in this study. The results are expected to expand our understanding of the degradation mechanisms for nitrogenous compounds during ozonation.

Atmospheric Processing of Nitrophenols and Nitrocresols From Biomass Burning Emissions

AbstractWe present the analysis of the atmospheric budget of nitrophenols and nitrocresols, a class of nitroaromatics that raise great ecosystem and health concerns due to their phytotoxic and genotoxic properties, during the spring wheat harvest season in Eastern China. Significant quantities with maximum concentrations over 100 pptv and distinct diurnal patterns that peak around midnight and maintain low levels throughout the day were observed, in coincidence with the extensive open crop residue burning activities conducted in the vicinity. An observationally constrained zero‐dimension box model was constructed to assess the relative importance of various production and removal pathways at play in determining the measured surface concentrations. The NO3‐initiated dark chemistry, in concert with meteorological variations predominantly dilution and entrainment, exerts major controls over the observed diurnal behaviors of nitrophenols and nitrocresols. Structural isomerism is predicted to have a significant impact on the multiphase partitioning and chemistry of nitrophenol isomers. Furthermore, simulations show that an appreciable amount of nitrophenols is present in the aerosol water, thereby representing an important source of water‐soluble brown carbon in atmospheric aerosols under the humid subtropical weather prevailing during the campaign. Sensitivity analysis performed on the model parameterizations of reaction schemes helps to further understand the chemistry underlying the diurnal cycles. Implementing NO‐dependent yields of cresols from toluene photooxidation improves the model predictions of nitrocresols at low NO ranges (&lt;1 ppb), thereby underscoring the complexity of the peroxy radical reaction pathways from toluene photooxidation under atmospheric relevant conditions.

Kinetic study of plasma-assisted n-dodecane/O2/N2 pyrolysis and oxidation in a nanosecond-pulsed discharge

The present study investigates the kinetics of low-temperature pyrolysis and oxidation of n-dodecane/O2/N2 mixtures in a repetitively-pulsed nanosecond discharge experimentally and numerically. Time-resolved TDLAS measurements, steady-state gas chromatography (GC) sampling, and mid-IR dual-modulation Faraday rotation spectroscopy (DM-FRS) measurements are conducted to quantify temperature as well as species formation and evolution. A plasma-assisted n-dodecane pyrolysis and oxidation kinetic model incorporating the reactions involving electronically excited species and NOx chemistry is developed and validated. The results show that a nanosecond discharge can dramatically accelerate n-dodecane pyrolysis and oxidation at low temperatures. The numerical model has a good agreement with experimental data for the major intermediate species. From the pathway analysis, electronically excited N2* plays an important role in n-dodecane pyrolysis and oxidation. The results also show that with addition of n-dodecane, NO concentration is reduced considerably, which suggests that there is a strong NO kinetic effect on plasma-assisted low-temperature combustion via NO-RO2 and NO2-fuel radical reaction pathways. This work advances the understandings of the kinetics of plasma-assisted low-temperature fuel oxidation in N2/O2 mixtures.

Electrochemical Nonacidic N‐Nitrosation/N‐Nitration of Secondary Amines through a Biradical Coupling Reaction

AbstractAn acid‐free N‐nitrosation/nitration of the N−H bonds in secondary amines with Fe(NO3)3 ⋅ 9H2O as the nitroso/nitro source through an electrocatalyzed radical coupling reaction was developed. Cyclic aliphatic amines and N‐heteroaromatic compounds were N‐nitrosated and N‐nitrated, respectively, under mild conditions. Control and competition experiments, as well as kinetic studies, demonstrate that N‐nitrosation and N‐nitration involve two different radical reaction pathways involving N+ and N. radicals. Moreover, the electrocatalysis method enables the preferential activation of the N−H bond over the electrode and thus provides high selectivity for specific N atoms. Finally, this strategy exhibits a broad scope and provides a green and straightforward approach to generate useful N‐nitroso/nitro compounds in good yields.magnified image

Enhanced surface activation process of persulfate by modified bagasse biochar for degradation of phenol in water and soil: Active sites and electron transfer mechanism

Carbon-based catalysis for green and sustainable degradation of phenol in water and soil has attracted extensive concern. In this study, mesoporous biochar produced by bagasse calcination with KOH and CaCl2 modified activation (Ca/BS-800-KOH) was used to stimulate persulfate (PS) for the surface oxidation degradation of phenol, which exhibited excellent removal rate of 90% in 60 min (k = 0.0404 min−1) with a relatively low dosage (0.066 g/L). Compared with untreated materials (BS-800, 27% removal rate in 180 min), after KOH and CaCl2 treatment, the functional groups (COH), increased porosity, defective structure and the more efficient electron transfer of Ca/BS-800-KOH were probably responsible for its good adsorptive and catalytic performance, which also closely related to the inactivation of Ca/BS-800-KOH. In the (Ca/BS-800-KOH)/PS/phenol system, PS was successfully activated in the presence of catalyst, and radical and non-radical reaction pathways were firstly found, in which the non-radical pathway like 1O2 accelerated quickly the oxidative degradation of phenol. While in radical pathway involving SO4·− and ·OH, ·OH was the main oxidative species and also played an irreplaceable role in the degradation of phenol. The Ca/BS-800-KOH as a medium promotes the electron transfer between phenol (electron donor) and PS (electron acceptor), in which Ca accelerates the electron transfer rate on the surface of Ca/BS-800-KOH, thereby causing oxidative degradation of phenol. The superior removal efficiencies of common environmental pollutants (all more than 93%) and phenol in water (100%) and soil samples were found. This research proposed an insightful mechanism of the low cost and feasible carbon-material-based PS-AOPs in degrading organic pollutants.