Solar photoassisted anodic oxidation of carboxylic acids in presence of Fe using a boron-doped diamond electrode

This paper reports a comparative study on the anodic oxidation of 2.5 l of 50 mg l⁻¹ TOC of formic, oxalic, acetic, pyruvic or maleic acid in 0.1 M Na₂SO₄ solutions of pH 3.0 with and without 1.0 mM Fe³⁺ as catalyst in the dark or under solar irradiation. Experiments have been performed with a batch recirculation flow plant containing a one-compartment filter-press electrolytic reactor equipped with a 20 cm² boron-doped diamond (BDD) anode and a 20 cm² stainless steel cathode, and coupled to a solar photoreactor. This system gradually accumulates H₂O₂ from dimerization of hydroxyl radical (OH) formed at the anode surface from water oxidation. Carboxylic acids in direct anodic oxidation are mainly oxidized by direct charge transfer and/or OH produced on BDD, while their Fe(III) complexes formed in presence of Fe³⁺ can also react with OH produced from Fenton reaction between regenerated Fe²⁺ with electrosynthesized H₂O₂ and/or photo-Fenton reaction. Fast photolysis of Fe(III)-oxalate and Fe(III)-pyruvate complexes under the action of sunlight also takes place. Chemical and photochemical trials of the same solutions have been made to better clarify the role of the different catalysts. Solar photoassisted anodic oxidation in presence of Fe³⁺ strongly accelerates the removal of all carboxylic acids in comparison with direct anodic oxidation, except for acetic acid that is removed at similar rate in both cases. This novel electrochemical advanced oxidation process allows more rapid mineralization of formic, oxalic and maleic acids, without any significant effect on the conversion of acetic acid into CO₂. The synergistic action of Fe³⁺ and sunlight in anodic oxidation can then be useful for wastewater remediation when oxalic and formic acids are formed as ultimate carboxylic acids of organic pollutants, but its performance is expected to strongly decay in the case of generation of persistent acetic acid during the degradation process.     

MoV-based catalysts in ethane oxidation to acetic acid: Influence of additives on redox chemistry

The formation of acetic acid and/or ethylene by oxidation of ethane is strongly dependent on X additives or Y promotor added to MoVO-based catalysts. MoV_0.4X_0.12YO_z (X = Nb; Y = Pd;  = 10⁻⁴) catalysts were prepared by the slurry method and their structural properties were studied by in situ (redox conditions) XRD, Raman and XPS techniques. The reactivity during reduction and reoxidation was analysed by thermal analysis (TGA/DSC). The oxidation of ethane was carried out in a conventional fixed bed microreactor with on line analysis by gas chromatography. Results show that Nb exerts mainly a structural effect as it is responsible for the stabilisation of molybdenum (VI) by formation of solid solutions with V, and that Pd modifies the rate of reduction of the solid catalysts. The increase of selectivity to acetic acid observed by Pd promotion is likely due to the transformation of ethylene to acetic acid occurring on neighboring Pd–V active sites.     

CO Oxidation on Gold Surfaces Studied on the Atomic Scale

The interaction of small gold crystal tips with oxygen gas and CO/O₂ gas mixtures was studied by means of field ion microscopy (FIM). High-resolution FIM-images of clean tips were obtained with hydrogen and neon as imaging gas. At temperatures between 300 and 450 K the exposure of a clean Au sample to O₂ gas at 100–1000 mbar, in the absence of an electric field, led to oxygen chemisorption and formation of a surface oxide. The presence of an electric field of 12–15 V/nm was found to enhance the oxidation process. Exposure to CO gas at 300 K led to the removal of the surface oxide. This was associated with the occurrence of a wave front which started in the apex centre and extended to the outskirts of the tip sample. The build-up of the surface oxide and its titration by carbon monoxide was completely reversible. Our results strongly suggest that pure gold crystals are active catalysts for the CO oxidation at 300 K.     

Anodic oxidation and electro-Fenton treatment of rotenone

Rotenone, a widely used botanical insecticide submitted to strong restrictions regarding its environmental hazards, was studied as a target compound for electro-Fenton (EF) treatment in aqueous-acetonitrile mixture (70:30) of pH 3.0. In this system, the degradation of organic pollutants occurs by attack of hydroxyl radicals (OH) which are produced from the reaction of added ferrous catalyst (Fe²⁺) and hydrogen peroxide (H₂O₂) electrogenerated by oxygen reduction at carbon felt cathode. The degradative efficiency of EF system was comparatively studied versus anodic oxidation method (AO) in absence and presence of H₂O₂. It was found that only EF is sufficiently powerful to induce fast and efficient mineralization of rotenone and its degradation intermediates.The mineralization of rotenone was found to depend largely on organic solvent type, metal ion catalyst, applied current and initial rotenone concentration. The best operative conditions are achieved using aqueous-acetonitrile mixture of pH 3.0 in the presence of 0.2 mM Fe²⁺ catalyst with a current intensity of 100 mA. Under these optimized conditions, 30 min were sufficient to completely degrade rotenone in 100 mL of a 20 mg L⁻¹ solution. A nearly complete mineralization (∼96% of COD removal) was achieved after 8 h treatment.Rotenone removal kinetic was found to obey the pseudo-first order model and the absolute second order rate constant (k_Rot = 2.49 × 10⁹ M⁻¹ s⁻¹) for the reaction between the substrate and OH was derived.HPLC-MS and HPLC-DAD analysis were applied to identify and follow the evolution of rotenone oxidation products. Three stable aromatic intermediates were observed and two of these were identified as 12aβ-hydroxyrotenone and hydroquinone. Subsequent attack of these intermediates by OH radicals leads to the formation of aliphatic carboxylic acids such as succinic, acetic, oxalic and formic, quantified by ion-exclusion chromatography.     

Synthesis,characterization and catalytic properties of two novel vanado-aluminosilicates with EU-1 and ZSM-22 structures

The synthesis in the presence of alkali ions of two novel vanadium containing zeolites V-Al-EU-1 and V-Al-ZSM-22 is reported. Both V⁴⁺- and V⁵⁺-ions are present in as-synthesized samples. Cyclic voltammograms of the samples reveal the presence of two types of V⁵⁺ in the calcined samples. A weak shoulder at 980 cm^–1 is observed in the IR spectra of the calcined samples. On extraction with acid a sharp band appears at 960 cm^–1. The acid washed samples are more active in the hydroxylation of phenol and the oxidation of toluene (with H₂O₂) than the calcined samples.     

Galvanostatic oxidation of formaldehyde-methanol solutions on Ti/Ru0.3Ti0.7O2 electrodes using a filter-press cell

The galvanostatic oxidation of methanol-containing formaldehyde solutions, under conditions of simultaneous oxygen evolution, in 0.5 M H₂SO₄ acid was studied using a Ti/Ru_0.3Ti_0.7O₂ dimensionally stable anode (DSA^®), in a filter-press cell. The reaction products detected were HCOOH, CO₂ and CO₃ ^2–. The CO₃ ^2– species is formed from the oxidation of HCOOH and subsequently decomposes in solution to CO₂. Conversely CO₂ is also formed electrochemically from the electrooxidation of formaldehyde and methanol. A mechanism, which considers the active and non-active nature of the electrode, is suggested. First-order kinetics, with respect to the variation of formaldehyde and methanol, are displayed and two linear regions observed. This is interpreted as being due to the presence of the reaction products of oxidation inhibiting the oxidation of formaldehyde at the electrode surface. Further, a mechanism is proposed considering the species present in solution.     

The electrochemical oxidation of dimethyl disulfide—Anodic methylsulfanylation of phenols and aromatic ethers

The electrochemical oxidation of dimethyl disulfide was investigated in acetonitrile and dichloromethane. The nature of the oxidation strongly depends on the nucleophilicity of the solvent. In acetonitrile a one-electron oxidation is observed and the consecutive species did not exhibit any reactivity towards aromatic compounds subject to electrophile substitution. On the contrary, the oxidation of dimethyl disulfide in methylene chloride afforded a two-electron process with the formation of a species consistent with CH₃S⁺. Its reactivity towards phenols and aromatic ethers was confirmed and showed a selective monomethylsulfanylation in most of the cases.     

Redox Behaviors of Magnesium Vanadate Catalysts During the Oxidative Dehydrogenation of Propane

In order to examine the mobility of lattice oxygen in magnesium vanadates, these catalysts were employed for the oxidative dehydrogenation of propane in the absence of oxygen for 2.25h, followed by the addition of gaseous oxygen into the feedstream. Depending on the degree of the abstraction of lattice oxygen from these catalysts during the oxidation in the absence of the gaseous oxidant, oxygen in the effluent was detected at approximately 1.4 and 9min with Mg₃V₂O₈ and Mg₂V₂O₇ respectively, after the addition of gaseous oxygen under the present reaction conditions. However, no oxygen was detected with MgV₂O₆ even after 18.5min from the addition of gaseous oxygen. ⁵¹V MAS NMR was also employed for the observation of redox behaviors of vanadium species in these catalysts during the reaction.     

Reaction network and kinetic modeling of wet oxidation of phenol catalyzed by activated carbon

A detailed kinetic model for the catalytic wet oxidation (CWO) of phenol with a commercial activated carbon as catalyst has been proposed. Experimental data have been obtained under kinetic control at steady state in a three-phase fixed bed reactor with concurrent up flow. The kinetic model is able to predict the appearance and disappearance of phenol and the cyclic organic intermediates formed along the phenol oxidation progress. As some cyclic oxidation intermediates—such as hydroquinone and p-benzoquinone—are two orders of magnitude more toxic than phenol, this detailed model is required to design a CWO process achieving the detoxification of the influent. Influences of oxygen pressure and temperature have been quantified (in the ranges 127- and 3.4-16 bar, respectively). This model also considers the formation of refractory compounds to the CWO (short-chain acids as, i.e., acetic, maleic and formic acids, which are biodegradable). Besides, a constant value of the overall fractional yield for the oxidation of phenol, as target pollutant to CO₂, has been obtained. The discriminated kinetic model fits quite well the experimental data for the whole range of variables used.     

Removal of Hg from flue gases in wet FGD by catalytic oxidation with air - An experimental study

About 46% of global mercury emissions are due to fossil fuel combustion for electrical and thermal energy production. Since more stringent emission standards are expected, important research efforts are being focused on the development of mercury removal technologies, mainly directed to two alternative approaches: (i) the enhancement of homogeneous oxidation in the flue gases of Hg⁰ to water soluble Hg²⁺ by the addition of chlorides or bromides to the boiler or; (ii) the adsorption of Hg²⁺ and Hg⁰ on impregnated activated carbon (AC). The latter may require the treatment of the entire gas volume of the thermal power plant and constantly consumes relatively large quantities of AC.A third option gaining more attention lately is based on the oxidation and retention of dissolved Hg⁰ in the wet flue gas desulphurisation (FGD) system. A series of chemical oxidants, such as halogens, hydrogen peroxide, sulphur and oxygen, are theoretically able to oxidize Hg⁰ in the wet FGD system. Most chemical oxidants when applied in the FGD, however, are non-selective and are largely consumed by SO₂ absorbed from the flue gas. The less expensive oxidant, non-selective as well, is oxygen (as air) which is already being dispersed into FGD absorbing suspension for the conversion of into .The experimental evidence of the present work showed that Hg⁰ present in the gaseous phase can be dissolved and oxidized to a high degree (70-90%) by air together with in wet FGD solutions. Transition metals such as Fe²⁺ and Mn²⁺ act as catalysts, chloride enhances the reaction, while some oxosulphur compounds, e.g. tetrathionate, inhibit the oxidation. A combination of several catalysts at a concentration of sulphite () below 100 mg L⁻¹ and an adequate redox potential of the solution can assure reasonable mercury removal even in the presence of oxidation inhibiting compounds.The main competitive reactions that govern final Hg⁰ removal in the FGD are as follows: (1) oxidation of Hg⁰ together with SO₂ with air, enhanced by catalysts; (2) removal of catalysts by precipitation in the form of Fe(OH)₃ and eventually as MnO₂ (to overcome this problem continuous addition of catalysts to the solution is required); (3) reduction of Fe³⁺ by tetrathionate to Fe²⁺ which (4) may reduce Hg²⁺ to Hg⁰ and probably (5) the complexation of Hg²⁺ by anions present which may play an important role in the mechanism by complexing the product(s) of the Hg⁰ oxidation reaction.     

On the electrolysis of aqueous bromide solutions to bromate

The behaviour of aqueous active bromine solutions prepared by the anodic oxidation of bromide at a Ti/RuO₂ anode has been investigated. Bromate is formed chemically through the oxidation of hypobromite by hypobromous acid and vice versa:

The rate of the first reaction is at least 10 times higher than the second.The electrolytic formation of bromate occurs with the simultaneous evolution of oxygen. The following stoichiometry is suggested:

This is analogous to the anodic chlorate formation. Because the oxidation potential for Br^? is lower than for active bromine, the anodic formation of bromate takes place at current densities lower than the limiting value at Ti/RuO₂ anodes.     

Methane oxidation to acetic acid catalyzed by Pd2+ cations in the presence of oxygen

Synthesis of acetic acid from methane catalyzed by Pd²⁺ cations dissolved in sulfuric acid was investigated to determine the effects of reaction conditions and the mechanism. Acetic acid yield was found to be a strong function of CH₄ and O₂ partial pressures. High O₂/CH₄ ratio and high total pressure delivered the highest yield of acetic acid (14.2 turnovers of Pd²⁺) and the highest retention of Pd²⁺ in solution (96%). Byproducts were sulfur containing compounds (most notably methyl bisulfate) and CO_x, but the acetic acid selectivity was maximized (82%) by lowering the reaction temperature. Methane is activated by Pd(OSO₃H)₂, forming (CH₃)Pd(OSO₃H). CO, generated from the oxidation of methyl bisulfate, inserts into the CH₃Pd bond creating a (CH₃CO)Pd(OSO₃H) species. Reaction of this complex with H₂SO₄ produces acetic acid. Pd²⁺ is reduced to Pd⁰ during the oxidation of methyl bisulfate or CO, and Pd⁰ is reoxidized to Pd²⁺ by H₂SO₄ and O₂.     

Electrochemical destruction of chlorophenoxy herbicides by anodic oxidation and electro-Fenton using a boron-doped diamond electrode

The degradation of herbicides 4-chlorophenoxyacetic acid (4-CPA), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aqueous medium of pH 3.0 has been comparatively studied by anodic oxidation and electro-Fenton using a boron-doped diamond (BDD) anode. All solutions are totally mineralized by electro-Fenton, even at low current, being the process more efficient with 1 mM Fe²⁺ as catalyst. This is due to the production of large amounts of oxidant hydroxyl radical (OH) on the BDD surface by water oxidation and from Fenton’s reaction between added Fe²⁺ and H₂O₂ electrogenerated at the O₂-diffusion cathode. The herbicide solutions are also completely depolluted by anodic oxidation. Although a quicker degradation is found at the first stages of electro-Fenton, similar times are required for achieving overall mineralization in both methods. The decay kinetics of all herbicides always follows a pseudo first-order reaction. Reversed-phase chromatography allows detecting 4-chlorophenol, 4-chloro-o-cresol, 2,4-dichlorophenol and 2,4,5-trichlorophenol as primary aromatic intermediates of 4-CPA, MCPA, 2,4-D and 2,4,5-T, respectively. Dechlorination of these products gives Cl⁻, which is slowly oxidized on BDD. Ion-exclusion chromatography reveals the presence of persistent oxalic acid in electro-Fenton by formation of Fe³⁺-oxalato complexes, which are slowly destroyed by OH adsorbed on BDD. In anodic oxidation, oxalic acid is mineralized practically at the same rate as generated.     

Ethene oxidation over supported heteropoly acids

Heteropoly acids of composition H_3+x PMo_12+x V _x O₄₀ supported on silica and-alumina catalysed the ethene oxidation with high selectivity to acetic acid. Acidic properties of the catalytic system, heteropoly anion structure and surface area affect the oxidation reaction.     

Mercury removal from coal combustion by Fenton reactions – Part A: Bench-scale tests

This series of papers describes the development of technology to convert Hg(0) to Hg(II) in coal-derived flue gas based on the well-known Fenton reactions so that a Hg control strategy can be implemented in a wet scrubber. This effort consists of both bench-scale and pilot-scale work. This first paper reports on the bench-scale tests. The bench-scale results showed that Hg(0) oxidation can be achieved by the Fenton reactions and the oxidation rate is quantitatively dependent on the residence time of the Hg stream in the solution. An average of 75% oxidation of Hg(0) was achieved. Iron-based Fenton-type additives gave much more promising results compared to Cu-based Fenton-like additives for Hg(0) oxidation. The pH value of the sorbent solution also had a significant effect on the oxidation of Hg(0) and a suitable pH window was found to lie between 1.0 and 3.0 for this application. This may be attributed to the chain reaction mechanisms of Fe³⁺/H₂O₂ for Fenton reactions, i.e., the decomposition of H₂O₂ for the production of OOH radicals in the Fe³⁺/H₂O₂ system which is kinetically favoured under a wide range of conditions at pH values of 3 or less. At higher pH values, H₂O₂ is converted to H₂O instead of OOH radicals in the presence of Fe³⁺.     

Study on the oxidation of C4-phenolic-1,4-dihydropyridines and its reactivity towards superoxide radical anion in dimethylsulfoxide

Electrochemical characterization on glassy carbon electrode (GCE) and reactivity with superoxide radical anion in aprotic medium of three new synthesized C4-phenolic-1, 4-dihydropyridines is reported.Voltammetry, coulometry, controlled-potential electrolysis (CPE), UV-vis spectroscopy, ¹H NMR techniques were employed for the characterization of title compounds.The oxidation mechanism involves initially an oxidation process on the phenol moiety with the formation of the corresponding quinone followed by a second one affecting the dihydropyridine ring to give the pyridine derivative. Both processes appeared irreversible in character.Cyclic voltammetry was used to generate O₂⁻ by reduction on GCE of molecular oxygen in DMSO. The reactivity of DHPs towards O₂⁻ was directly measured by the anodic current decay of the radical in the presence of increasing concentration of tested 1,4-dihydropyridines and compared with the reaction of the reference antioxidant, Trolox. The linear correlations obtained between the anodic current of O₂⁻ and compound concentrations in the range between 0.01 mM and 1.00 mM allowed the determination of both the DHP antioxidant index (AI) and the concentrations needed to consume 50% of O₂⁻. Synthesized C4-phenolic 1,4-dihydropyridines exhibited significant scavenging capacity towards superoxide radical anion higher than Trolox and tested commercial 1,4-dihydropyridines.     

Reaction sequence for the mineralization of the short-chain carboxylic acids usually formed upon cleavage of aromatics during electrochemical Fenton treatment

Electrochemical Fenton treatment of aromatic pollutants in aqueous medium always leads to the formation of short-chain carboxylic acids, which account for the slower degradation rate at the final stages of the process. In order to gain further insight into the fate of such compounds, bulk electrolyses of 200 ml aqueous solutions of eleven C₁-C₄ carboxylics, namely formic, glyoxylic, oxalic, acetic, glycolic, pyruvic, malonic, maleic, fumaric, succinic and malic acid have been carried out by electro-Fenton process with 0.1 mM Fe²⁺ as catalyst, at room temperature and pH 3.0, applying a constant current and using an open and undivided cell equipped with a carbon-felt cathode and a Pt anode. In situ cathodic electrogeneration of Fenton's reagent leads to the formation of the very oxidizing species hydroxyl radical (OH) in the medium, allowing the degradation and total mineralization of all carboxylics studied. Various goals have been accomplished: (a) identification of the degradation intermediates for each carboxylic acid and study of their time course, (b) discussion and proposal of the reaction mechanisms under the action of OH/O₂, (c) analysis of the decay kinetics and determination of the absolute rate constants, which agree well with those available in literature for processes involving OH, (d) verification of the great oxidation ability of the process to degrade mixtures containing all the carboxylics, upon variation of some experimental parameters such as current, concentration and cathode dimensions and, finally, (e) elucidation of a detailed reaction sequence for their mineralization, indicating the plausible pre-eminent pathways.     

High yields of synthesis gas by millisecond partial oxidation of higher hydrocarbons

Cyclohexane, n-hexane, and isooctane were reacted with air in a Rh-monolith reactor and converted into synthesis gas (H₂+CO) in yields exceeding 90%, with >95% conversion of fuels and 100% conversion of oxygen. The best catalyst was an 80 ppi washcoated alumina monolith containing 5 wt% Rh. There was a small effect of catalyst contact time on syngas selectivity and conversions for gas space velocities from 3×10⁵ to 3×10⁶ h^–1. Preheating the feed enhances syngas selectivities slightly, but no reactor preheat is necessary provided the fuel remains vaporized. Addition of 25 mol% toluene to isooctane also produces syngas, olefins, and methane with 90% yield, including 70% yield to syngas. Partial oxidation of gasoline–air mixtures was attempted but the catalysts were poisoned after several hours, probably by sulfur and/or metals.     

Oxidation of cyclohexanone to adipic acid with a cobalt acetate/oxygen/acetic acid system

The liquid phase oxidation of cyclohexanone to adipic acid at 378 K using oxygen as the oxidising agent and cobalt acetate as the catalyst in an acetic acid medium was investigated both at atmospheric pressure and at a pressure of 0.5MN m⁻². The effects of catalyst concentration, solvent concentration and partial pressure of oxygen, were studied at a constant stirrer speed of 535 rev min⁻¹. Increasing the solvent concentration and decreasing the catalyst concentration (up to 0.113×10⁻³ kmol m⁻³) had positive effects on the overall first order reaction rate constant. It was also found to vary linearly with gas flow rate.