Phenol Radical Research Articles

Efficient catalytic removal of phenolic pollutants by laccase from Coriolopsis gallica: Binding interaction and polymerization mechanism

Laccase is a green catalyst that can efficiently catalyze phenolic pollutants, and its catalytic efficiency is closely related to the interaction between enzyme and substrates. To investigate the binding effects between enzyme and phenolic pollutants, phenol, p-chlorophenol, and bisphenol A were used as substrates in this study. We focused on the removal and catalytic mechanism of these pollutants in water using yellow laccase derived from Coriolopsis gallica. The enzymatic catalytic products were characterized using Ultraviolet-Visible Absorption Spectroscopy (UV–Vis), Fourier Transform Infrared Spectroscopy (FTIR), and High-Resolution Mass Spectrometry (HRMS), and the catalytic mechanism of laccase on phenolic pollutants was further explored by molecular docking. Based on the structural characterization and molecular docking results, the possible polymerization pathways of these phenolic compounds were speculated. Laccase catalyzed phenol to produce phenolic hydroxyl radicals, their para-radicals, and ortho-radicals, which polymerized to form oligomers linked by benzene‑oxygen-benzene and benzene-benzene. P-chlorophenol produced phenolic hydroxyl radicals and their ortho-radicals, polymerizing to form oligomers connected by benzene‑oxygen-benzene or benzene-benzene. The CC bond of the isopropyl group of bisphenol A broke to formed an intermediate product, which was further polymerized to formed a benzene‑oxygen-benzene linked oligomer.

Determination of Total Phenol and Flavonoids Contents in Five Red Peppers Variety along the Production Stages of Berbere in Ethiopia

The objective of the study was to evaluate the total phenolic, flavonoids and free radical scavenging activity of red pepper (Capsicum annum L.) along the production stages of berbere (Ethiopian spice). Melka awaze, melka zola, melka dehera, mareko fana and melka shote pepper varieties were harvested from wolkite. The samples were prepared with oven dried, destalked, washed and dried, faded, toasted, milled and boiled. Total phenolic, total flavonoids, and free radical scavenging activity during the production stages of berbere were studied by spectrophotometry. The result of total phenol and flavonoids were ranged from 74.87±2.28 up to 163.29mgGAE/g and 12.96up to 93.79mgQE/g respectively. Mareko fana, melka dehera and melka shote variety had the highest value of total phenol (128.58mgGAE/g), flavonoids (51.45mgQE/g) and free radical scavenging activity (88.23%) respectively. An appropriate ripening stage of pepper should be used for harvesting and dried at optimum temperature is recommended.

Optimization of bioactivities of solvent extracts of bark and heartwood of Pterocarpus marsupium and exploration of neuroprotective effect linking to the major Phytoconstituents

The study aims to evaluate the neuroprotective properties of Pterocarpus marsupium Roxb. The bioactive optimized hydroalcohol-extracted fractions of bark (PMBHA) and heartwood (PMHHA) at 300 and 400 mg/kg were examined in rats with scopolamine-induced neurotoxicity to study oxidative, neurochemical, and neuroinflammation biomarkers. Phenol and flavonoid-rich radical scavengers in contributing total antioxidant capacities of PMBHA and PMHHA quenched peroxyl radicals and protected the cellular membrane and lipoproteins from ROS in DPPH, ORAC, and CAP-e tests. In anticholinesterase activities, PMBHA inhibited AChE (IC50: 54.40 ± 0.98 µg/mL) and BChE (IC50: 50.95 ± 0.98 µg/mL) mixed-competitively, while PMHHA inhibited cholinesterase activities (AChE: 42.23 ± 0.54, and BChE: 47.19 ± 0.65 µg/mL) competitively. Dosage at 300 and 400 mg/kg of PMBHA and PMHHA improved serum and brain oxidative indicators (MDA, SOD, CAT, and GSH), and restored proinflammatory cytokines (TNF-α, and IL-6) leading to enhanced brain cholinesterase and β-secretase levels in neurotoxic rats. GC-MS reported antioxidant, anti-inflammatory, and neuroprotective compounds of PMBHA catechol, thiamet-G, d-Gala-l-ido-octonic amide, falcarinol, D-mannose, ribitol, 1,2,4-cyclopentanetrione, 3-(2-pentenyl)-, hexadecanoic acid methyl ester, n-hexadecanoic acid, oleic acid, and 3,5-Dimethoxybenzyl alcohol, α-epi-7-epi-5-Eudesmol, 1 R,4aR,7 R,8aR)-7-(2-Hydroxypropan-2-yl)-1,4a-dimethyldecahydro naphthalen-1-ol, arctiol, 1,1,4,7-tetramethyldecahydro-1 H-cyclopropa[e]azulene-4,7-diol, 2-Propen-1-ol, 3-(2,6,6-trimethyl-1-cyclohexen-1-yl), 4-t-Butyl-2-[4-nitrophenyl]phenol in PMHHA were responsible in neuroprotection in neurotoxic rats. However, heartwood demonstrated better neuroprotective activities than the bark of P. marsupium due to some of the key compounds (1 R,4aR,7 R,8aR)-7-(2-Hydroxypropan-2-yl)-1,4a-dimethyldecahydronaphthalen-1-ol, 2-Propen-1-ol, 3-(2,6,6-trimethyl-1-cyclohexen-1-yl)-, 4-t-Butyl-2-[4-nitrophenyl]phenol, 1,1,4,7-Tetramethyldecahydro-1 H-cyclopropa[e]azulene-4,7-diol.

Stable Electron Spin Pan on Aromatic Oxalic Acid Radical

Comprehensive SummaryThe stability of organic radicals in ambient condition is important for their practical application. During the development of organic radical chemistry, the electron‐withdrawing and steric hindrance groups are usually introduced to improve the stability of radicals via reducing the reactivity of radicals with oxygen in air. Herein, the electron‐withdrawing carbonyl groups are introduced to construct a planar aromatic oxalic acid radical (IDF‐O8) with two‐dimensional electron spin pan structure. Interestingly, IDF‐O8 exhibited a low optical bandgap of 0.91 eV in film, however, the multiple quinone resonance structures between electron‐withdrawing ketone and phenol radicals contribute to the high stability of open‐shell radical IDF‐O8 without protection of large steric hindrance groups. Under the irradiation of 808 nm (1.2 W·cm–2), IDF‐O8 reaches 147 °C in powder state. This work provides an efficient synthesis route for the open‐shell electron spin pan system, which is different from the famous fullerene, carbon nanotube and graphene. The electron spin pan can be extended to spin tube or spin sphere system based on the design strategy of aromatic inorganic acid radicals in future.

Silica Biomineralization with Lignin Involves Si-O-C Bonds That Stabilize Radicals.

Plants undergo substantial biomineralization of silicon, which is deposited primarily in cell walls as amorphous silica. The mineral formation could be moderated by the structure and chemistry of lignin, a polyphenol polymer that is a major constituent of the secondary cell wall. However, the reactions between lignin and silica have not yet been well elucidated. Here, we investigate silica deposition onto a lignin model compound. Polyphenyl propanoid was synthesized from coniferyl alcohol by oxidative coupling with peroxidase in the presence of acidic tetramethyl orthosilicate, a silicic acid precursor. Raman, Fourier transform infrared, and X-ray photoelectron spectroscopies detected changes in lignin formation in the presence of silicic acid. Bonds between the Si-O/Si-OH residues and phenoxyl radicals and lignin functional groups formed during the first 3 h of the reaction, while silica continued to form over 3 days. Thermal gravimetric analysis indicated that lignin yields increased in the presence of silicic acid, possibly via the stabilization of phenolic radicals. This, in turn, resulted in shorter stretches of the lignin polymer. Silica deposition initiated within a lignin matrix via the formation of covalent Si-O-C bonds. The silica nucleants grew into 2-5 nm particles, as observed via scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. Additional silica precipitated into an extended gel. Collectively, our results demonstrate a reciprocal relation by which lignin polymerization catalyzes the formation of silica, and at the same time silicic acid enhances lignin polymerization and yield.

Unravelling the Influence of Chlorogenic Acid on the Antioxidant Phytochemistry of Avocado (Persea americana Mill.) Fruit Peel.

Oxidative stress is pivotal in the pathology of many diseases. This study investigated the antioxidant phytochemistry of avocado (Persea americana Mill.) peel. Different solvent extracts (dichloromethane, ethyl acetate, methanol, and water) of avocado peel were subjected to total phenol and flavonoid quantification, as well as in vitro radical scavenging and ferric reducing evaluation. The methanol extract was subjected to gradient column chromatographic fractionation. Fraction 8 (eluted with hexane:chloroform:methanol volume ratio of 3:6.5:0.5, respectively) was subjected to LC-MS analysis. It was assessed for cellular inhibition of lipid peroxidation and lipopolysaccharide (LPS)-induced ROS and NO production. The DPPH radical scavenging mechanism of chlorogenic acid was investigated using Density Functional Theory (DFT). The methanol extract and fraction 8 had the highest phenol content and radical scavenging activity. Chlorogenic acid (103.5 mg/mL) and 1-O-caffeoylquinic acid (102.3 mg/mL) were the most abundant phenolics in the fraction. Fraction 8 and chlorogenic acid dose-dependently inhibited in vitro (IC50 = 5.73 and 6.17 µg/mL) and cellular (IC50 = 15.9 and 9.34 µg/mL) FeSO4-induced lipid peroxidation, as well as LPS-induced ROS (IC50 = 39.6 and 28.2 µg/mL) and NO (IC50 = 63.5 and 107 µg/mL) production, while modulating antioxidant enzyme activity. The fraction and chlorogenic acid were not cytotoxic. DFT analysis suggest that an electron transfer, followed by proton transfer at carbons 3'OH and 4'OH positions may be the radical scavenging mechanism of chlorogenic acid. Considering this study is bioassay-guided, it is logical to conclude that chlorogenic acid strongly influences the antioxidant capacity of avocado fruit peel.

AsIII-enhanced oxidation by coexisting MnIII-phenolic complexes during arsenic contaminated groundwater treatment by MnO2

Manganese oxides (MnO2) are widely utilized in arsenic(AsIII)-contaminated groundwater remediation; however, As(III) oxidation is greatly affected by co-occurring substances. This study discusses the oxidation pathways of As(III) oxidation-adsorption in presence of co-solutes (e.g., Mn(II), Fe(II), and 2,4-dichlorophenol (DCP). Mn(II) species competed with As(III) for adsorption and oxidation sites at MnO2 surface, resulting in a decrease in As(III) removal. As(III) oxidation in Mn(II) and Mn(II)/MnO2 systems was primarily occurred via electron-transfer with Mn(III) intermediate, however, Mn(III) species were rapidly oxidized into the less reactive oxidant Mn(IV). In the presence of DCP, As(III) oxidation was enhanced due to stabilization of Mn(III) species by complexation with DCP. Fe(II) enhanced the adsorption of Arsenic and DCP by MnO2, as Fe-(hydr)oxides formed, creating extra adsorption sites on MnO2. Three DCP byproducts were also identified by means of LC/MS analysis in the Mn(II)/MnO2-DCP system, with reaction As(III) oxidation pathways are proposed. The enhanced As(III) oxidation by Mn(II)/MnO2 system, in presence of DCP resulted due to a series of chemical reactions with hydroxyl and phenolic radicals generated in system, in addition to electron transfer with DCP-Mn(III) complexes. These results provide new insights into the environmental behavior of As(III) within mixed water systems containing metals (Mn/Fe) and phenolics.

Significant nuclear quantum effect of proton in the reaction of phenol and hydroxyl radical

The reaction between phenol and hydroxyl radical, leading to the formation of phenoxy radicals, is analyzed using DFT calculations at the multi-component M06-2X level that accounts for the nuclear quantum effects (NQEs) of hydrogen nuclei. The NQEs affect not only the relative energy diagram of the reaction but also the geometric parameters of the stationary point structures. In the energy minimum structures, the OH covalent bonds have lengthened, while the OH…O hydrogen bonds have shortened due to the NQEs. Furthermore, the NQEs also affect the transition state structure, where the distance between carbon and oxygen atoms is strongly influenced by the NQEs of hydrogen nuclei through a hydrogen bond network formed by surrounding water molecules. Different NQEs of hydrogen nuclei have been observed in the ipso/ortho and the para stepwise reaction pathways due to differences in the driving force of the reaction.

Preparation of graphene supported copper composite materials and study on the catalytic activity of phenol hydroxylation

The hydroxylation of phenol to synthesize dihydroxybenzene (DHB) has recently attracted considerable attention due to its inherent advantages, including its simplicity in industrial applications, environmental friendliness, and remarkable selectivity towards DHB. The hydrothermal method was used to prepare graphene supported copper composites (Cu/GNS) with excellent catalytic properties, which were applied in the phenol hydroxylation for DHB preparation. Cu/GNS with controllable copper content were successfully obtained, achieving the homogeneous dispersion of copper ions on the graphene surface while preventing graphene agglomeration. During catalysis, graphene serves as an electron transport conduit, enhancing the exposure of reactive active sites and facilitating the generation of hydroxyl radicals. The π-π interactions between graphene and phenol enhance the propensity for collision between phenol and hydroxyl radicals. The total selectivity of DHB reached 98.4%, and the phenol conversion reached 85.1%. The composites exhibited high stability, with 80.5% phenol conversion and 93.7% DHB total selectivity even after five reuse cycles.

Construction of copper-manganese based aminoclays with significant laccase-like activity and its prominent degradation performance towards bisphenol A

In recent years, nanozymes have become promising alternatives to natural enzymes owing to their high catalytic efficiency, substrate specificity, and mild reaction conditions. The catalytic performance of nanozymes can be further improved by doping heterogeneous metals to provide high redox potentials. However, nanozymes for the degradation of endocrine-disrupting chemicals (EDCs) have been scarcely studied, and the degradation mechanisms remain unclear. Herein, a series of aminoclays (ACs) containing various metal dopants were prepared and employed for the degradation of phenolic compounds. The as-synthesized ACs with a Cu-Mn molar ratio of 1:7 (CuMnAC-1–7) showed excellent stability and superior activity for oxidizing bisphenol A (BPA) than natural laccases owing to the electron transport between Cu2+/Cu+ and Mn4+/Mn3+ in the nanozyme. The catalytic oxidation process of BPA consists of a non-free radical pathway mediated by Mn3+, as well as a free radical pathway facilitated by superoxide radicals (O2·-) and singlet oxygen (1O2). Specifically, Mn3+ deprived the electrons of the BPA benzene ring to form phenolic hydroxyl radicals, which were then polymerized to BPA dimers. Cu+ in the structure reduced Mn4+ to Mn3+, accelerating the electron deprivation of BPA and promoting the polymerization reaction. Finally, the high-charge-density region on the dimeric benzene ring tended to lose electrons and form free radicals, facilitating the polymerization into trimers and tetramers. The CuMnAC-1–7 could remove 95% of BPA at a concentration of 200 mg/L within 1 h under acidic conditions. This study provides a new strategy to design high-performance laccase nanozymes for EDCs degradation.

Molecular insights into substrate promiscuity of CotA laccase catalyzing lignin-phenol derivatives

CotA laccases are multicopper oxidases known for promiscuously oxidizing a broad range of substrates. However, studying substrate promiscuity is limited by the complexity of electron transfer (ET) between substrates and laccases. Here, a systematic analysis of factors affecting ET including electron donor acceptor coupling (ΗDA), driving force (ΔG) and reorganization energy (λ) was done. Catalysis rates of syringic acid (SA), syringaldehyde (SAD) and acetosyringone (AS) (kcat(SAD) > kcat(SA) > kcat(AS)) are not entirely dependent on the ability to form phenol radicals indicated by ΔG and λ calculated by Density Functional Theory (SA < SAD ≈ AS). In determined CotA/SA and CotA/SAD structures, SA and SAD bound at 3.9 and 3.7 Å away from T1 Cu coordinating His419 ensuring a similar ΗDA. Abilities of substrate to form phenol radicals could mainly account for difference between kcat(SAD) and kcat(SA). Furthermore, substrate pocket is solvent exposed at the para site of substrate's phenol hydroxyl, which would destabilize binding of AS in the same orientation and position resulting in low kcat. Our results indicated shallow partially covered binding site with propensity of amino acids distribution might help CotA discriminate lignin-phenol derivatives. These findings give new insights for developing specific catalysts for industrial application.

Hydroxycinnamaldehyde-derived benzofuran components in lignins.

Lignin is an abundant polymer in plant secondary cell walls. Prototypical lignins derive from the polymerization of monolignols (hydroxycinnamyl alcohols), mainly coniferyl and sinapyl alcohol, via combinatorial radical coupling reactions and primarily via the endwise coupling of a monomer with the phenolic end of the growing polymer. Hydroxycinnamaldehyde units have long been recognized as minor components of lignins. In plants deficient in cinnamyl alcohol dehydrogenase, the last enzyme in the monolignol biosynthesis pathway that reduces hydroxycinnamaldehydes to monolignols, chain-incorporated aldehyde unit levels are elevated. The nature and relative levels of aldehyde components in lignins can be determined from their distinct and dispersed correlations in 2D 1H-13C-correlated nuclear magnetic resonance (NMR) spectra. We recently became aware of aldehyde NMR peaks, well resolved from others, that had been overlooked. NMR of isolated low-molecular-weight oligomers from biomimetic radical coupling reactions involving coniferaldehyde revealed that the correlation peaks belonged to hydroxycinnamaldehyde-derived benzofuran moieties. Coniferaldehyde 8-5-coupling initially produces the expected phenylcoumaran structures, but the derived phenolic radicals undergo preferential disproportionation rather than radical coupling to extend the growing polymer. As a result, the hydroxycinnamaldehyde-derived phenylcoumaran units are difficult to detect in lignins, but the benzofurans are now readily observed by their distinct and dispersed correlations in the aldehyde region of NMR spectra from any lignin or monolignol dehydrogenation polymer. Hydroxycinnamaldehydes that are coupled to coniferaldehyde can be distinguished from those coupled with a generic guaiacyl end-unit. These benzofuran peaks may now be annotated and reported and their structural ramifications further studied.

Evolution of lignin pyrolysis heavy components through the study of representative lignin monomers

Seven typical lignin monomers with different number of methoxyl groups and different 4-substituted patterns were pyrolyzed at 500 °C and 700 °C for 120 s to simulate the secondary polymerization and side chain conversion reactions of primary pyrolytic lignin monomers. The pyrolytic heavy oil components were identified at molecular scale with Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR-MS) and analyzed with Kendrick mass defect (KMD) and van Krevelen diagrams. The detected heavy components were typically phenolic oligomers formed dominantly through the radical coupling reactions of phenol radicals (C6H4O) and methoxyl radicals (OCH2). Model compounds with CC type 4-substituted groups were prone to produce more phenolic oligomers with more aromatic units. For the CO type model compounds, the number of methoxyl groups have significant influence on the generation of heavy components, thereby affecting the char yields. Rising temperature promotes the generation of heavy components through three main routes, namely the additions of CH2, OCH2 and C6H4O. These routes were mapped on van Krevelen diagrams, shedding light on the evolution patterns of both polymerization and side chain conversion reactions during the secondary pyrolysis of lignin.

The atmospheric oxidation mechanism of acetophenone initiated by the hydroxyl radicals

Acetophenone (C8H8O, AP) is a common indoor air component but with no information on its atmospheric oxidation. The atmospheric oxidation mechanism of AP in gas phase, initiated by its reaction with the OH radical, is investigated here using quantum chemistry and chemical kinetics calculations. The reaction starts mainly by OH additions to the aromatic ring, and the predicted rate coefficient suggests a lifetime of ∼8 days in the atmosphere. The additions form adducts AP-i-OH (Ri) (i = 1, 2, 3, 4, the position on the aromatic ring). The adducts Ri would react with O2 or undergo unimolecular decompositions. The adduct R1 reacts with O2 rather slowly with an effective rate coefficients of <10−20 cm3 molecule−1 s−1, and it would alternatively decompose to phenol and acetyl radical. The dominate fate of R2, R3, and R4 in their reaction with O2 proceeds via direct hydrogen-atom abstraction, forming hydroxyacetophenones, and only R3 would have a fraction of bicyclic radical formation as in oxidation of alkyl benzenes. The main products from oxidation of AP include phenol and three isomers of hydroxyacetophenones, suggesting that oxidation of AP by OH would reduce its unpleasant effect to human beings.

Chemical process of multiphase system in lignin-biochar co-pyrolysis for enhanced phenol recovery

Carbon materials prepared by biomass pyrolysis have been studied for conversion of lignin into phenols. In this study, the control mechanism of free radicals on phenolic products selectivity was studied using guaiacol as a model chemical. The results showed that gaseous radicals or carbon-centered radicals of carbon materials could promote phenols formation through different mechanisms. In gas-phase system, the selectivity of phenols was mainly controlled by methyl radicals which led to a theoretical selectivity of phenols lower than 50%. In the heterogeneous system, methyl radicals and phenolic radicals were easily and rapidly adsorbed on the carbon edge sites. The o-hydroxyphenoxy radical was the key intermediate and was firstly absorbed on the carbon center radical sites where phenols produced at the same position after undergoing desorption and hydrogenation. In conclusion, the present study concluded that conversion of lignin to phenols can proceed rapidly as long as there are carbon-centered radicals distributed at zigzag edge on the carbon surface.

Response of NaCl stress to morphology and content of phenolic compounds, flavonoids, and antioxidant activity in clove seeds (Syzycum aromaticum.L)

NaCl stress in plants affects various physiological and biochemical processes that result in reduced biomass production. This study aims to determine the aspects of morphological and physiological changes of plants with indications of the induction of secondary metabolism in survival, which is indicated by an indication of an increase in bioactive compounds, including phenolic compounds, flavonoids, single oxygen (O2), hydroxyl radicals (OH-), radicals ( O-2), and superoxide. The clove seed pot experiment under NaCl stress used a randomized block design with three replications. Data are presented as mean ± standard deviation. The significance of the difference between the mean values was tested by analysis of variance, if there was a significant effect, then Duncan Multiple Range Test (DMRT). The results of the NaCl stress study on clove seeds showed fluctuating phenolic and flavonoid content, almost all NaCl treatment with the optimum plant value obtained at a NaCl concentration of 200 mM, with a content of (1.18μg GAE/g) and flavonoids (0.158 g QE/g) ( 1.18μg GAE/g). The antioxidant activity of DPPH, Hydroxyl activity, Superoxide is also affected by the concentration of NaCl

Biomimetic oxidative copolymerization of hydroxystilbenes and monolignols

Hydroxystilbenes are a class of polyphenolic compounds that behave as lignin monomers participating in radical coupling reactions during the lignification. Here, we report the synthesis and characterization of various artificial copolymers of monolignols and hydroxystilbenes, as well as low-molecular-mass compounds, to obtain the mechanistic insights into their incorporation into the lignin polymer. Integrating the hydroxystilbenes, resveratrol and piceatannol, into monolignol polymerization in vitro, using horseradish peroxidase to generate phenolic radicals, produced synthetic lignins [dehydrogenation polymers (DHPs)]. Copolymerization of hydroxystilbenes with monolignols, especially sinapyl alcohol, by in vitro peroxidases notably improved the reactivity of monolignols and resulted in substantial yields of synthetic lignin polymers. The resulting DHPs were analyzed using two-dimensional NMR and 19 synthesized model compounds to confirm the presence of hydroxystilbene structures in the lignin polymer. The cross-coupled DHPs confirmed both resveratrol and piceatannol as authentic monomers participating in the oxidative radical coupling reactions during polymerization.

Abatement of Aromatic Contaminants from Wastewater by a Heat/Persulfate Process Based on a Polymerization Mechanism.

A novel approach to the abatement of pollutants consisting of their conversion to separable solid polymers is explored by a heat/persulfate (PDS) process for the treatment of high-temperature wastewaters. During this process, a simultaneous decontamination and carbon recovery can be achieved with minimal use of PDS, which is significantly different from conventional degradation processes. The feasibility of this process is demonstrated by eight kinds of typical organic pollutants and by a real coking wastewater. For the treatment of the selected pollutants, 30.2-91.9% DOC abatement was achieved with 24.8-91.2% carbon recovery; meanwhile, only 5.2-47.0% of PDS was consumed compared to a conventional degradation process. For the treatment of a real coking wastewater, 71.0% DOC abatement was achieved with 66.0% carbon recovery. With phenol as a representative compound, our polymerization-based heat/PDS process is applicable in a wide pH range (3.5-9.0) with a carbon recovery of >87%. Both SO4•- and HO• can be initiators for polymerization, with different contribution ratios under various conditions. Phenol monomers are semioxidized to form phenolic radicals, which are polymerized via chain transfer or chain growth processes to form separable solid phenol polymers, benzenediol polymers, and cross-linked polymers.

Quantitative evaluation of the actual hydrogen atom donating activities of O-H bonds in phenols: structure-activity relationship.

The H-donating activity of phenol and the H-abstraction activity of phenol radicals have been extensively studied. In this article, the second-order rate constants of 25 hydrogen atom transfer (HAT) reactions between phenols and PINO and DPPH radicals in acetonitrile at 298 K were studied. Thermo-kinetic parameters ΔG ≠o(XH) were obtained using a kinetic equation [ΔG ≠ XH/Y = ΔG ≠o(XH) + ΔG ≠o(Y)]. Bond dissociation free energies ΔG o(XH) were calculated by the iBonD HM method, whose details are available at https://pka.luoszgroup.com/bde_prediction. Intrinsic resistance energies ΔG ≠ XH/X and ΔG ≠o(X) were determined as ΔG ≠o(XH) and ΔG o(XH) were available. ΔG o(XH), ΔG ≠ XH/X, ΔG ≠o(XH) and ΔG ≠o(X) were used to assess the H-donating abilities of the studied phenols and the H-abstraction abilities of phenol radicals in thermodynamics, kinetics and actual HAT reactions. The effect of structures on these four parameters was discussed. The reliabilities of ΔG ≠o(XH) and ΔG ≠o(X) were examined. The difference between the method of determining ΔG ≠ XH/X mentioned in this study and the dynamic nuclear magnetic method mentioned in the literature was studied. Via this study, not only ΔG o(XH), ΔG ≠ XH/X, ΔG ≠o(XH) and ΔG ≠o(X) of phenols could be quantitatively evaluated, but also the structure-activity relationship of phenols is clearly demonstrated. Moreover, it lays the foundation for designing and synthesizing more antioxidants and radicals.

Highly selective conversion of tetrabromobisphenol A epoxy resin waste to high-purity phenolic chemicals by subcritical water-CuO process

Tetrabromobisphenol A epoxy resin (TBBPA-ER) is the most important nonmetallic composition of waste printed circuit boards (PCBs). Currently, the TBBPA-ER waste is mainly sent to incineration or landfills. The great obstacle of the recovery of the TBBPA-ER waste is the low value-added products obtained by existing technologies. In this study, a novel resource conversion strategy was developed to produce high-purity phenolic chemicals from the TBBPA-ER waste by subcritical water-CuO (SubCW-CuO) process. The high selectivity of the SubCW-CuO process for the recovered products from the TBBPA-ER waste could be attributed to the regulating effect of CuO. The CuO could preferentially trigger the rupture of C-O and C-Ph bonds in the TBBPA-ER chain, resulting in the producing of high-purity phenol (91.47 %) at 300 °C. The reaction between phenol and methyl radical (derived from β-scission of ternary carbon chain groups in the TBBPA-ER waste) resulted in the recovery of high-purity p-cresol (90.30 %) at 350 °C. The optimal conditions of the SubCW-CuO process were solid-liquid ratio of 1:30 g/mL, M(TBBPA-ER):M(CuO) ratio of 10:1 g/g, and 60 min. The conversion ratio of the TBBPA-ER waste could reach 70 % under the optimal conditions. Approximately 3 % of the bromine was transferred into the oil phase products after the SubCW-CuO process at 250 °C, while no organic bromine could be found in the oil phase products at 300 and 350 °C. More than 70 % of the bromine was fixed in the solid residue after the SubCW-CuO process at 300 and 350 °C. The bromine contained in the solid residue mainly existed in the form of CuBr and the Cu species had efficient capture ability for the bromine released from the molecular chain of the TBBPA-ER. It is believed that the SubCW-CuO process developed in this study is a promising alternative technology for the high-efficiency resource conversion of the TBBPA-ER waste.