Aromatic Ring Opening Research Articles

Biodegradation of aromatic hydrocarbons and phenols by bacteria isolated from caspian waters and soils

Experimental studies have been carried out on the degradability of monoaromatic hydrocarbons (benzene, toluene, and ethylbenzene) and phenols (phenol, pyrocatechol, hydroquinone, tetrachloropyrocatechol) by bacteria isolated from coastal waters and soils of the Absheron peninsula at the Caspian Sea. It has been shown that the degradation of pyrocatechol, in particular, by Pseudomonas sp. bacteria occurs with the cleavage of the aromatic ring in two directions (in the meta- and ortho-positions), and that of tetrachloropyrocatechol involves the cleavage of the aromatic ring in the ortho-position and the formation of tetrachloromuconic acid. By reversed-phase high-performance liquid chromatography, more than ten individual compounds were detected and identified in the biodegradation products under the test conditions. It has been also shown that mainly linear functional compounds formed by aromatic ring opening in the meta- or ortho-position are found in the case of pyrocatechol and hydroquinone.

Electrochemical treatment of wastewaters containing 4-nitrocathecol using boron-doped diamond anodes

The electrochemical oxidation of aqueous wastes polluted with 4-nitrocathecol has been studied on boron-doped diamond electrodes in an acidic medium. The voltammetric results showed that 4-nitrocathecol is oxidized in the potential region where the supporting electrolyte is stable. Galvanostatic electrolysis study showed that the oxidation of these wastes in a single-compartment electrochemical flow cell with boron-doped diamond anodes results in the complete mineralization of the organics. Cathecol, benzoquinone, 4-aminocathecol, maleic and oxalic acids have been detected as soluble organics, polymeric product as solid product at the cathode surface and NO3− as mineral product during the electrolysis of 4-nitrocathecol. The electrochemical oxidation of 4-nitrocathecol consists of a sequence of steps: release of NO2 and (or) hydroxylation of the aromatic ring; formation of quinonic compounds; oxidative opening of aromatic ring to form carboxylic acids; and oxidation of carboxylic acids to carbon dioxide. Both direct and mediated oxidation processes are involved in these stages.

NO binding to Mn-substituted homoprotocatechuate 2,3-dioxygenase: relationship to O2 reactivity

Iron(II)-containing homoprotocatechuate 2,3-dioxygenase (FeHPCD) activates O2 to catalyze the aromatic ring opening of homoprotocatechuate (HPCA). The enzyme requires Fe(II) for catalysis, but Mn(II) can be substituted (MnHPCD) with essentially no change in the steady-state kinetic parameters. Near simultaneous O2 and HPCA activation has been proposed to occur through transfer of an electron or electrons from HPCA to O2 through the divalent metal. In O2 reactions with MnHPCD-HPCA and the 4-nitrocatechol (4NC) complex of the His200Asn (H200N) variant of FeHPCD, this transfer has resulted in the detection of a transient M(III)-O2 (·-) species that is not observed during turnover of the wild-type FeHPCD. The factors governing formation of the M(III)-O2 (·-) species are explored here by EPR spectroscopy using MnHPCD and nitric oxide (NO) as an O2 surrogate. Both the HPCA and the dihydroxymandelic substrate complexes of MnHPCD bind NO, thus representing the first reported stable MnNO complexes of a nonheme enzyme. In contrast, the free enzyme, the MnHPCD-4NC complex, and the MnH200N and MnH200Q variants with or without HPCA bound do not bind NO. The MnHPCD-ligand complexes that bind NO are also active in normal O2-linked turnover, whereas the others are inactive. Past studies have shown that FeHPCD and the analogous variants and catecholic ligand complexes all bind NO, and are active in normal turnover. This contrasting behavior may stem from the ability of the enzyme to maintain the approximately 0.8-V difference in the solution redox potentials of Fe(II) and Mn(II). Owing to the higher potential of Mn, the formation of the NO adduct or the O2 adduct requires both strong charge donation from the bound catecholic ligand and additional stabilization by interaction with the active-site His200. The same nonoptimal electronic and structural forces that prevent NO and O2 binding in MnHPCD variants may lead to inefficient electron transfer from the catecholic substrate to the metal center in variants of FeHPCD during O2-linked turnover. Accordingly, past studies have shown that intermediate Fe(III) species are observed for these mutant enzymes.

A coupled biological and photocatalysis pretreatment system for the removal of crystal violet from wastewater

The efficiency of a coupled photocatalytic–biological system for removing crystal violet (CV) from an aqueous solution was assessed. Initial experiments demonstrated that the optimal operating parameters for the photoreactor were a 1.5-h reaction time, pH 7.0, and a 2.0-min retention time. Under these conditions, the photocatalytic reaction reduced the toxicity of the CV solution by 94%. Subsequent evaluation of the performance and characteristics of the coupled photocatalytic–biological system in terms of CV removal revealed that the coupled system successfully removed and efficiently mineralized CV in a semi-continuous mode when the CV concentration was <150mgL−1. Based on our analysis of the degradation products, CV degradation in this coupled system involved stepwise demethylation and aromatic ring opening. Phylogenetic analysis of the bioreactor effluent showed that the predominant phyla were Proteobacteria, followed by Bacteroidetes and Actinobacteria, suggesting that this coupled system is conducive for such processes as demethylation, aromatic ring opening, carbon oxidation, and nitrification. These results were verified in a GC–MS analysis. To our knowledge, this is the first report on CV removal using a coupled system.

Structural Modification of Lignin and Characterization of Pretreated Wheat Straw by Ozonation

Ozonolysis is potentially an effective method for pretreating lignocellulosic biomass to improve the production of fermentable sugars via enzymatic hydrolysis. Further understanding of the ozonolysis process and identifying specific lignin structural changes are crucial for improving the pretreatment process. Investigation into pretreatment of wheat straw using ozonolysisis is reported in this paper, with special emphasis on selective modification/degradation of lignin subunits. The ozonolysis was performed for 2 h with less than 60 mesh particles in order to achieve maximum lignin oxidation. The results showed that the lignin structure was significantly modified under these conditions, leading to higher sugar recovery of more than 50% which increased from 13.11% to 63.17% corresponding to the control and ozone treated samples, respectively. Moisture content was found to be an important parameter for improving sugar recovery. Ninety percent (w/w) moisture produced the highest sugar recovery. The concentration of acid soluble lignin in the ozone treated sample increased from 4% to 11% after 2 h treatment. NMR analysis revealed that the S2/6 and G2 lignin units in the wheat straw were most prone to oxidation by ozone as the concentration of aromatic units decreased while the carboxylic acids became more abundant. The experimental data suggest the degradation of β-O-4 moieties and aromatic ring opening in lignin subunits. The pyrolysis-gas chromatography/mass spectrometry results revealed that the rate of lignin unit degradation was in the following order: syringyl > guaiacyl > p-hydroxyphenyl. Long ozone exposure resulted in few condensed lignin structure formation. In addition, the formation of condensed units during this process increased the activation energy from ASTM-E, 259.74 kJ/mol; Friedman-E, 270.08 kJ/mol to ASTM-E, 509.29 kJ/mol; Friedman-E, 462.17 kJ/mol. The results provide new information in overcoming lignin barrier for lignocellulose utilization.

Photocatalytic activity of a nitrogen-doped TiO2 modified zeolite in the degradation of Reactive Yellow 125 azo dye

In this study, the hybrid material based on natural zeolite modified with nitrogen-doped TiO2 photocatalyst (Z-TiO2-N) was synthesized by solid-state reaction under microwave-assisted hydrothermal conditions. The surface characterization of hybrid material has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analysis, and FT-IR spectroscopy. The photocatalytic activity of Z-TiO2-N photocatalyst under UV and VIS irradiation was determined for the degradation of Reactive Yellow 125 (RY 125) dye solution. Also, UV–VIS diffuse reflectance (DRUV–VIS) spectroscopy was used to determine the light absorption properties of the hybrid material. The operational conditions using photocatalyst dose of 1g/L at pH 3 were established as optimal conditions for photocatalytic application in RY 125 dye solution degradation. The experimental studies revealed that the degradation efficiency expressed in terms of discoloration, aromatic ring-opening and mineralization depended strongly on the initial RY 125 dye solution concentrations. The photocatalyst exhibited a good performance for discoloration and aromatic-ring opening processes at all studied concentrations (25, 50 and 100mg/L) but an effective mineralization was reached only for 25mg/L RY 125 dye solution. Also, the degradation of low concentrations of RY 125 dye solution was achieved using Z-TiO2-N catalyst under VIS irradiation.

Efficient Oxidative Debromination of Decabromodiphenyl Ether by TiO2-Mediated Photocatalysis in Aqueous Environment

Direct evidence was first demonstrated for the oxidative degradation of decabromodiphenyl ether (BDE209) in aqueous TiO(2) dispersions under UV irradiation (λ > 340 nm). BDE209 was hardly debrominated over TiO(2) in UV-irradiated acetonitrile dispersions, but the addition of water into the dispersions greatly enhanced its photocatalytic oxidative debromination. The debromination efficiency of BDE209 as high as 95.6% was achieved in aqueous TiO(2) dispersions after 12 h of UV irradiation. The photocatalytic oxidation of BDE209 resulted in generation of aromatic ring-opening intermediates such as brominated dienoic acids, which were further degraded by prolonging UV irradiation time. The photocatalytic oxidative debromination of BDE209 was further confirmed by the observation that the BDE209 degradation in water-acetonitrile mixtures with different water contents was positively correlated with the formation of •OH radicals, but not photogenerated electrons. The use of water not only avoided the scavenging of reactive radicals by organic solvent but also enhanced the adsorption of BDE209 on the surface of TiO(2), both of which favor the contact of BDE209 with photogenerated holes and •OH species. The confirmation of efficient oxidative degradation and debromination of BDE209 is very important for finding new ways to remove polybrominated diphenyl ethers from the environment.

Ozonation of metoprolol in aqueous solution: ozonation by-products and mechanisms of degradation

This study investigated the degradation pathway of metoprolol, a widely used β-blocker, in the ozonation via the identification of generated ozonation by-products (OPs). Structure elucidation of OPs was performed using HPLC coupled with quadrupole time-of-flight high-resolution mass spectrometry. Seven OPs were identified, and four of these have not been reported elsewhere. Identified OPs of metoprolol included aromatic ring breakdown by-products; aliphatic chain degraded by-products and aromatic ring mono-, di-, and tetrahydroxylated derivatives. Based on the detected OPs, metoprolol could be degraded through aromatic ring opening reaction via reaction with ozone (O3) and degradation of aliphatic chain and aromatic ring via reaction with hydroxyl radical (•OH).

Chemical effects of 1,2,4-trimethyl benzene addition in counterflow gaseous diffusion flames

The structure of gaseous counterflow diffusion flames perturbed with the addition of hundreds of ppm of prevaporized 1,2,4-trimethyl benzene (TMB) is studied in two distinct flame environments: a blue methane flame and an incipiently sooting ethylene flame. The two flames provide well defined temperature-time histories and chemical environments to investigate the behavior of complex fuels and complement other reacting environments lacking the coupling of kinetics and transport that is typical of flame environments. Profiles of critical pyrolysis products and of some stable soot precursors are determined from GC/MS analysis of gas samples extracted from the flames and compared with results from the OPPDIFF model using a semi-detailed chemical mechanism. Experimentally, because of the presence of aliphatic fragments, TMB reactivity is enhanced in these flames with the onset of TMB decay beginning at relatively modest temperatures, on the order of 800K. The dominant path to stable species is driven by H radical attack. It leads in sequence to xylenes, toluene (through benzyl radical) and benzene formation. This enhanced reactivity is captured reasonably well by the model in the methane flame, but not in the ethylene flame, in the presence of a richer, more complex mixture. The model does not reproduce accurately the pathway yielding C3 and some C4 species from TMB cracking. Aromatic ring opening is the bottleneck in the TMB cracking process in the methane flame but not in the ethylene one. Indene, an important soot precursor for monoaromatic fuels since the second aromatic ring formation is considered to be a bottleneck in the process, is measured in the ethylene flame in poor agreement with the model predictions. The dataset presented here and available supplemental data online may help identifying improvements to the chemical kinetic mechanism of this reference fuel.

Examination of benzoquinone electrooxidation pathway as crucial step of phenol degradation process

The electrochemical oxidation of benzoquinone on expanded graphite (EG) electrode was conducted. Electrochemical process was realized by cyclic voltammetry technique in alkaline solution. The main part of this work was devoted to examination of benzoquinone oxidation proceeding through the opening of aromatic ring accompanied by the formation of carboxylic acids. Water samples containing benzoquinone and short chain mono- and dicarboxylic acids were analyzed qualitatively and quantitatively with high performance liquid chromatography (HPLC). The validation of chromatographic method was determined for benzoquinone, oxalic acid, malonic acid, acetic acid, maleic acid, succinic acid and fumaric acid.

ChemInform Abstract: Catalytic Hydrocracking—Mechanisms and Versatility of the Process

AbstractReview: 114 refs.

The structure of toluene-doped counterflow gaseous diffusion flames

The structure of gaseous counterflow diffusion flames perturbed with the addition of hundreds of ppm of prevaporized toluene is studied in two distinct flame environments: a blue methane flame stabilized on the fuel side of the gas stagnation plane and an incipiently sooting ethylene flame stabilized on the oxidizer side. The goal is to provide a well-defined testbed in terms of temperature–time history, major species and part of the radical pool, for the examination of reference fuels that are critical components of practical fuel blends. Gas samples are extracted from the flame with fused silica microprobes for subsequent GC/MS analysis and thermocouples and thin filament pyrometry are used to characterize the temperature field. Profiles of critical toluene pyrolysis products and stable soot precursors are compared with computational models using two semi-detailed chemical mechanisms. Results show that in the methane flame some oxygen containing radicals like O and OH are contributing early on to the toluene destruction path. In the incipiently sooting ethylene flame, the primary attack is from H alone. This finding confirms the different challenges that such flames pose to the validation of a chemical kinetic mechanism. The onset of toluene decay in these flames begins at relatively modest temperatures, on the order of 800K. This reactivity is captured reasonably well by both chemical mechanisms in the methane flame, in the absence of reactants larger than C2, but not so in the ethylene flame, in the presence of a richer, more complex mixture. The aromatic ring opening mechanisms are not adequately modeled in either case. This discrepancy has implications for the modeling of practically relevant fuel blends with both aliphatic and aromatic compounds. The dominant species larger than toluene in the doped methane flame is ethylbenzene, which at least one of the mechanisms reproduces quite well. The largest measured species in the incipiently sooting flame is indene, whose concentration increase due to toluene addition is properly captured by one of the models. The experimental dataset reported here may help identifying future improvements to chemical kinetic mechanisms and complement other reactor datasets lacking the coupling of kinetics and transport of flame environments.

Catalytic Hydrocracking—Mechanisms and Versatility of the Process

AbstractHydrocracking of saturated hydrocarbons can proceed by means of four distinctly different mechanisms. On bifunctional catalysts comprising hydrogenation/dehydrogenation and Brønsted acid sites alkenes and carbocations occur as intermediates. The current mechanistic views of bifunctional hydrocracking of long‐chain n‐alkanes are discussed in detail with emphasis on the now widely accepted concept of ideal hydrocracking. Other mechanisms are hydrogenolysis and Haag–Dessau hydrocracking which proceed, respectively, on monofunctional metallic and acidic catalysts. Even without a catalyst, thermal hydrocracking occurs in chain reactions via radicals. The chemistry of hydrocracking naphthenes on bifunctional catalysts resembles that of alkanes. A peculiarity, however, is the pronounced reluctance of cyclic carbenium ions to undergo endocyclic β‐scissions. The effect manifests itself in the so‐called paring reaction, which, in turn, forms the basis for measuring the Spaciousness Index for characterizing the effective pore width of zeolitic catalysts. Hydrocracking on bifunctional catalysts is among the very important processes in modern petroleum refining. It is primarily used for converting heavy oils into diesel and jet fuel. Besides, hydrocracking is appreciated for its pronounced versatility: numerous process variants exist which help to meet specific requirements in refineries or petrochemical plants. Two recent developments are briefly discussed in this review, viz. the conversion of surplus aromatics, e.g., in pyrolysis gasoline, into a synthetic feedstock for steam crackers, and quality enhancement of diesel fuel by selective ring opening of polynuclear aromatics.

Water as an additive to enhance the ring opening of naphthalene

Use of water as a reaction medium or additive to enhance reaction efficiency is an important topic in green chemistry, and ring opening and contraction reactions of aromatics are crucial for upgrading diesels. In this work, we investigated the effect of water on the yields of ring opening and contraction reactions of naphthalene. A series of catalysts, such as Rh2O3/HY zeolite, Mo–Ni oxide and their physical mixtures, were used as the catalysts. The influences of the amount of water, hydrogen pressure, reaction temperature and reaction time on the yields of the ring opening and contraction products (ROCP) were studied. It was found that Rh2O3/HY and Mo–Ni oxide showed an excellent synergistic effect for catalyzing the reaction, and water could be used as a green and efficient additive for enhancing the yield of the ROCP. At the optimized conditions, the yield of the ROCP could be as high as 63.3%. The mechanism for the effect of water on the reactions was discussed on the basis of control experiments.

Photodegradation of commercial dye, CI Reactive Blue 160 using ZnO nanopowder: Degradation pathway and identification of intermediates by GC/MS

The ZnO-mediated photocatalysis process has been successfully applied to degradation of commercial dye, CI Reactive Blue 160 (RB 160). Eleven reaction intermediates were separated, identified and characterized by the GC–MS techniques, giving insight into mechanistic details of the ZnO-mediated catalysis process under UV irradiation and the pathways of degradation process. The degradation process take place through competitive reactions such as destruction of chromophoric group, hydroxylation of the aromatic ring, substitution on aromatic ring, desulphonation, decarboxylation and further ring opening to give aliphatic compounds. Smaller aliphatic compounds formed during the process were identified by IC, while the triazine group and naphthalene group containing compounds remains intact even after decolorization for a prolonged time due to high resistance to oxidation. The probable degradation pathways were proposed and discussed.

Characterization of atenolol transformation products in ozonation by using rapid resolution high-performance liquid chromatography/quadrupole-time-of-flight mass spectrometry

This experiment investigated the transformation pathway of atenolol, a widely used β-blocker, in the ozonation process through the identification of generated intermediate compounds. In order to characterize the contribution of ozone and hydroxyl radical (•OH) in the transformation of atenolol, ozonation were performed at pH 2, 7 and 9. 15 major transformation products have been identified based on the chromatographic behavior of these compounds and the information obtained from accurate mass MS and MS/MS spectra. By comparing to the elution pattern of the identified transformation products and its fragmentation pattern in the MS/MS spectrum, a variety of isomers of the transformation products were characterized.Identified transformation products of atenolol are including its mono-, di and tri-hydroxylated derivatives as well as the aliphatic and aromatic ring breakdown products. Transformation of atenolol in the ozonation involved hydroxylation reaction, aromatic ring opening reaction, oxidation and cleavage of 2-hydroxy-3-(isopropylamino)propoxy group of atenolol. In ozonation, aromatic ring of atenolol was transformed through the reaction with both ozone and •OH whereas the aliphatic chain of atenolol was degraded mainly through the reaction with •OH. The results also indicated that both •OH and ozone involved in the aromatic ring opening reaction.

Polymerization of 1,2,4,5-tetramethylbenzene (durene) in direct-current discharge

Polymer films possessing dielectric properties were prepared from 1,2,4,5-tetramethylbenzene (durene) at the cathode in a dc discharge. The chemical structure of the films was studied by IR and UV spectroscopy. It was shown that the polymer synthesized from durene in a dc discharge contains basically polyene structures produced via the opening of aromatic rings.

ChemInform Abstract: Aromatic Ring Opening of Pyridinium Salts Using Carbanions Formed from Malononitrile and Ethyl Cyanoacetate.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Measurement of phenols dearomatization via electrolysis: The UV-Vis solid phase extraction method

Dearomatization levels during electrochemical oxidation of p-methoxyphenol (PMP) and p-nitrophenol (PNP) have been determined through UV-Vis spectroscopy using solid phase extraction (UV-Vis/SPE). The results show that the method is satisfactory to determine the ratio between aromatic compounds and aliphatic acids and reaction kinetics parameters during treatment of wastewater, in agreement with results obtained from numerical deconvolution of UV-Vis spectra. Analysis of solutions obtained from electrolysis of substituted phenols on antimony-doped tin oxide (SnO 2 Sb) showed that an electron acceptor substituting group favored the aromatic ring opening reaction, preventing formation of intermediate quinone during oxidation.

Comparison of degradation mechanism of electrochemical oxidation of di- and tri-nitrophenols on Bi-doped lead dioxide electrode: Effect of the molecular structure

In the present work, the effect of molecular structures of di- and tri- nitrophenols on electrochemical degradation has been investigated on Bi-doped lead dioxide electrodes in terms of cyclic voltammetry and bulk electrolysis. The results of SEM and AFM displayed a porous structure with small-sized crystal particles and compact crystalline structure of Ti/Bi-PbO2. These nitrophenols were believed to be mainly degraded by means of indirect oxidation due to the low anodic oxidation currents observed in cyclic voltammetries and large amount of oxidants. And the absence of polymeric adhesive compounds formed and deposited on the surface of electrodes indicated the high electrocatalytic activity of Ti/Bi-PbO2 for decomposing organics. Within the present experimental conditions used, almost complete elimination of nitrophenols was achieved. The electrochemical oxidation of them followed in the order: 2,6-dinitrophenol>2,5-dinitrophenol>2,4-dinitrophenol>2,4,6-trinitrophenol. The relationship between electrochemical oxidation rate and the molecular structure of nitrophenols was discussed. The results of LC/MS and HPLC suggested that three kinds of intermediates were generated, i.e., polyhydroxylated intermediates, reduction products of nitrophenols and carboxylic acids. The possible degradation pathways of nitrophenols were proposed. The denitration and substitution by hydroxyl radicals on aromatic rings seemed to be the first stage. As a consequence, the formation of polyhydroxylated intermediates took place. These compounds were successively oxidized into catechol, resorcinol and hydroquinone, as well as reduced to aminophenols, followed by the opening of aromatic rings and the formation of a series of carboxylic acids. Finally, these carboxylic acids were oxidized into CO2 and H2O.