Study of the oxidation of solutions of p-chlorophenol and p-nitrophenol on Bi-doped PbO2 electrodes by UV-Vis and FTIR in situ spectroscopy

The oxidation of p-chlorophenol (pcp) and p-nitrophenol (pnp) was studied at Bi-doped PbO₂ (Bi-PbO₂) electrodes. The mass balance between solution composition and flowing charge was obtained from deconvolution of UV-Vis spectra recorded during electrolysis at constant potential. It is shown that the time-dependent production of CO₂ is different for the oxidation of pcp and pnp, indicating different reaction mechanisms for the oxidation of each of these compounds. The reaction kinetics was also followed under thin layer conditions by SNIFTIRS; the similarly increasing signals associated to the generation of CO₂ obtained during oxidation of both compounds indicates that under conditions of restricted mass transfer the mineralization rates of pcp and pnp are similar. The results show that benzoquinone (bq) formed from oxidation of phenols desorbs prior to further oxidation to yield maleic acid (ma), in turn oxidizing further to CO₂.     

Effects of ultrasound on electrochemical oxidation mechanisms of p-substituted phenols at BDD and PbO2 anodes

The effects of low-frequency (40 kHz) ultrasound are investigated with regard to the effectiveness and mechanisms of electrochemical oxidation of p-substituted phenols (p-nitrophenol, p-hydroxybenzaldehyde, phenol, p-cresol, and p-methoxyphenol) at BDD (boron-doped diamond) and PbO₂ anodes. Although ultrasound improved the disappearance rates of p-substituted phenols at both the BDD and PbO₂ anodes, the degree of enhancement varied according to the type of p-substituted phenol and type of anode under consideration. At the BDD anode, the %Increase values were in the range 73-83% for p-substituted phenol disappearance and in the range 60-70% for COD removal. However, at the PbO₂ anode, the corresponding %Increase values were in the range 50-70% for disappearance of p-substituted phenols and only 5-25% for COD removal, much lower values than obtained at the BDD anode. Further investigations on the influence of ultrasound on the electrochemical oxidation mechanisms at BDD and PbO₂ anodes revealed that the different increase extent were due to the specialized electrochemical oxidation mechanisms at these two anodes. The hydroxyl radicals were mainly free at the BDD electrodes with a larger reaction zone, but adsorbed at the PbO₂ electrodes with a smaller reaction zone. Therefore, the enhancement due to ultrasound was greater at the BDD anode than at the PbO₂ anode.     

Solution and solid-state polymerizations of substituted p-quinodimethanes and p-quinone methides

This article describes the polymerization behavior of electron-accepting and electron-donating p-quinonoid monomers such as substituted p-quinodimethanes and p-quinone methides in solutions and solid states. In the solution polymerizations, the electron-accepting substituted p-quinodimethanes and p-quinone methides with same substituents such as cyano, ester, and sulfonyl groups at the exocyclic positions are not homopolymerizable, but copolymerizable with donor monomers like styrene in an alternating fashion. Their spontaneous polymerizations with donor monomers have been explained with the bond-forming initiation mechanism. The substituted p-quinodimethanes with ester groups are a first example to show an amphoteric behavior in alternating copolymerizations. Some electron-accepting substituted p-quinodimethanes and p-quinone methides with different substituents such as cyano, ester, acyl, alkylthio, and/or phenyl groups are homopolymerizable, and an anionic polymerization initiated with a butyllithium initiator proceeded in a living manner. Equilibrium polymerization behavior have been found in their radical homopolymerizations, and on the basis of the thermodynamic parameters determined for their polymerizations, it has been concluded that homopolymerizabilities of the electron-accepting substituted p-quinodimethanes and p-quinone methides are determined exclusively by a steric hindrance effect arising from the substituents at the exocyclic positions. A new concept for the radical alternating copolymerization have been proposed on the basis of the change in modes (random and alternating copolymerizations) for their copolymerizations with styrene and the cross-propagation step analysis by linear free energy relationship. The polymerizations of the electron-donating substituted p-quinodimethanes take place only in the presence of oxygen molecules. In the solid-state polymerizations, some electron-accepting substituted p-quinodimethanes with same ester substituents polymerized topochemically in vacuo, and the strict requirements of topochemical polymerization for substituted p-quinodimethane and p-quinone methide monomers have been determined on the basis of their crystallographic data. Topochemical alternating copolymerization with molecular oxygen in solid state was discovered for the first time.     

Electrosynthesis of poly(aniline-co-p-aminophenol) having electrochemical properties in a wide pH range

Copolymerization of aniline and p-aminophenol in aqueous sulfuric acid solutions was electrochemically performed using cyclic voltammetry on platinum electrodes. The monomer concentration ratio can strongly affect the copolymerization rate and electrochemical property of the copolymer. The optimum conditions for the copolymerization are that the potential sweep covers the −0.20 to 0.95 V (vs. SCE) potential range, and that a solution contains 0.18 M aniline, 0.02 M p-aminophenol and 0.50 M H₂SO₄. A resulting copolymer synthesized under the optimum conditions has a good electrochemical activity in 0.50 M solutions of Na₂SO₄ with pH ≤ 10.0. IR and XPS spectra indicate that -OH groups and SO₄²⁻ ions are contained in the resulting copolymer. The SEM images reveal that the microstructure of the copolymer depends on the monomer concentration ratio during the electrolysis.     

Radical copolymerization of N-vinylcarbazole and p-bromostyrene: determination of monomer reactivity ratios by SEC-multidetection

N-Vinylcarbazole (A)/p-bromostyrene (B) copolymers were prepared by radical copolymerization. Size exclusion chromatography (SEC) equipped with a refractometer and UV-vis spectrophotometer was found to be a very convenient technique to follow copolymerizations and to determine monomer conversions, copolymer composition, average molecular weights, polydispersity indexes versus time. The monomer reactivity ratios r_A (N-vinylcarbazole) and r_B (p-bromostyrene) were determined by using the Finemann-Ross (FR), the inverted Finemann-Ross (IFR), the Kelen-Tüdos (KT), and the fitting curve graphical methods. The four methods were in good agreement and led to very different values for r_A (0.55) and r_B (12.3) which induces a preference for the incorporation of B in the copolymer structure. Eventually, with these results the influence of initial feed on the microstructure of the copolymer has been predicted.     

Electrosynthesis and characterization of a poly(paraphenylene) deriving from p-fluoroanisole

The electrochemical oxidation of p-fluoroanisole (p-FA) in the solvent acetonitrile leads to oligomers and polymers of poly(paraphenylene) type. The electropolymerization process involves coupling reactions of the cation radicals intermediates. The obtained polymers are separated according to their chain length by selective precipitation in cyclohexane and ether. The corresponding structures are characterized by NMR, MS, FTIR, UV and XR diffraction. A preliminary physical study shows that the polymers are photoluminescent with a maximum emission in the near infrared.     

Fabrication of tridoped p-ZnO thin film and homojunction by RF magnetron sputtering

Tridoping (Al–As–N) into ZnO has been proposed to realize low resistive and stable p-ZnO thin film for the fabrication of ZnO homojunction by RF magnetron sputtering. The tridoped films have been grown by sputtering the AlN mixed ZnO ceramic targets (0, 0.5, 1 and 2 mol%) on GaAs substrate at 450 °C. Here, Al and N from the target, and As from the GaAs substrate (back diffusion) takes part into tridoping. The grown films have been characterized by Hall measurement, X-ray diffraction, photoluminescence, time-of-flight secondary ion mass spectroscopy and X-ray photoelectron spectroscopy. It has been found that all the films showed p-conductivity except for 2 mol% AlN doped film. The obtained resistivity (8.6×10⁻² Ω cm) and hole concentration (4.7×10²⁰ cm⁻³) for the best tridoped film (1 mol% AlN) is much better than that of monodoped and codoped ZnO films. It has been predicted that [(As_Zn−2V_Zn)+N_O] acceptor complex is responsible for the p-conduction. The homojunction fabricated using the best tridoped ZnO film showed typical rectifying characteristics of a diode. The junction parameters have been determined for the fabricated homojunction by Norde's and Cheung's method.     

Electrochemical incineration of p-cresol and o-cresol in the filter-press-type FM01-LC electrochemical cell using BDD electrodes in sulfate media at pH 0

This work shows a comparative study of the incineration of 2-mM p-cresol and o-cresol in 1 M-H₂SO₄ in aqueous media. Microelectrolysis studies indicated that both the p-cresol and o-cresol oxidation were carried out via hydroxyl radicals (OH) formed by water oxidation in the boron-doped diamonds (BDD)-H₂O-H₂SO₄-p-cresol and o-cresol interface. In both cases, the potential and current density ranges, where great amounts of OH are formed, were between 2.3 V ≤ E ≤ 2.75 V versus SHE and J = 10 mA cm⁻². Electrolyses in an undivided FM01-LC reactor were performed at different Reynolds values 27,129 ≤ Re ≤ 42,631, and at J = 10 mA cm⁻². For p-cresol and o-cresol, the rate of degradation was slow, however it increases slightly as a function of the Re, indicating that the oxidation involves a complex pathway; current efficiency also rises as a function of the Re. For p-cresol, the mineralization at Re = 42,631 reached 90%, with 71% current efficiency and an energy consumption of 7.84 kWh m⁻³; whereas o-cresol was mineralized to 84%, with 67% current efficiency and an energy consumption of 6.56 kWh m⁻³. The results obtained in this work demonstrated that o-cresol is more recalcitrant than p-cresol.     

Synthesis and gas permeability of ester substituted poly(p-phenylene)s

Polymerizations of various ester substituted 2,5-dichlorobenzoates [substituent: linear alkyl groups (1a-f), branched alkyl groups (1g-l), cyclohexyl groups (1m-o), phenyl groups (1p-r), and oxyethylene units (1s-v)] were investigated with Ni-catalyzed/Zn-mediated system in 1-methyl-2-pyrrolidone (NMP) at 80 °C. Most of monomers bearing linear and branched alkyl groups successfully polymerized to give relatively high-molecular-weight polymers (M_n = 10,000-20,800). However, the molecular weight of the polymer having eicocyl groups was low because of steric hindrance of long alkyl chain. The polymerizations of cyclohexyl 2,5-dichlorobenzoate and phenyl 2,5-dichlorobenzoate produced low-molecular-weight polymers, while the polymerizations of monomers with alkyl cyclohexyl and alkyl phenyl groups proceeded to afford polymers with relatively high-molecular-weights. The polymers possessing oxyethylene units were obtained, but the molecular weights were low when the oxyethylene chains were long. The gas permeability of membranes of poly(p-phenylene)s with alkyl chains increased as increasing the length of alkyl chain. The membranes of poly(p-phenylene)s with phenyl groups and oxyethylene units exhibited high densities and relatively low gas permeability. However, the CO₂/N₂ separation factor of membrane of poly(p-phenylene) having oxyethylene units was as large as 73.6.     

Electrochemical synthesis of belt-like polyaniline network on p-phenylenediamine functionalized glassy carbon electrode and its use for the direct electrochemistry of horse heart cytochrome c

Three-dimension (3D) belt-like polyaniline (PAN) network has been prepared via electrochemical polymerization of aniline on p-phenylenediamine (PDA) functionalized glassy carbon electrode (GCE) using a three-step electrochemical deposition procedure. PDA was covalently binded on GCE via the formation of carbon-nitrogen bond between amine cation radical and the aromatic moiety of GCE surface using electrochemical oxidation procedure. X-ray photo-electron spectroscopy (XPS) and cyclic voltammetry have been performed to characterize the attachment of PDA on GCE. The images of scanning electron microscope (SEM) show that the 3D belt-like PAN network is uniform. The width and thickness of the PAN belt varies in the range of 1.5-5.5 μm and 0.1-0.8 μm, respectively. The distance between the belt-contacts ranges from 2.5 to 15 μm. The 3D belt-like PAN network modified GCE (PAN-PDA/GCE) exhibits an improved electro-activity of PAN at an extended pH up to 7.0. The PAN-PDA/GCE not only immobilizes but also leads to a direct electrochemical behavior of cytochrome c (Cyt c). The immobilized Cyt c maintains its activity, showing a surface-controlled electrode process with the electron-transfer rate constant (k_s) of 14.8 s⁻¹ and electron-transfer coefficient (α) of 0.48, and could be used for the electrocatalytic reduction of hydrogen peroxide (H₂O₂).     

Electrooxidation of chlorophenols at a glassy carbon electrode in a pH 11 buffer

The electrochemical oxidation of three chlorophenols (CPs), namely 2-chlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP), and 2,4,6-trichlorophenol (2,4,6-TCP) at a glassy carbon electrode in a pH 11 carbonate/hydrogen carbonate buffer electrolyte has been studied by cyclic voltammetry (CV) and impedance measurements. The three CPs show in CV an irreversible oxidation peak whose potential decreased linearly with increasing pK_a of the CPs, showing that the oxidation is the more facile the less acidic is the CP. Although the oxidation is not under diffusion control, i.e., the peak potential was not directly related to the rate constant, the same slope was obtained when the potential at which the integrated charge began to increase significantly was plotted instead of the peak potential. A reversible process near −0.45 V is observed for 2,4,6-TCP from the second positive scan, which was attributed to the formation of a soluble (since this couple nearly disappeared upon solution stirring) benzoquinone. Impedance data showed that for 2-CP and 2,4-DCP the formation of the polyphenolic film begins already at 0.0-0.1 V, and that the film is non-porous and non-conducting. On the contrary, with 2,4,6-TCP polyphenol formation began near 0.15 V, and the film is so porous that it allows the oxidation of 2,4,6-TCP to continue.     

Scale-up of a fixed bed electrochemical reactor consisting of parallel screen electrode used for p-aminophenol production

The behavior of a fixed bed consisting of amalgamated copper screens has been investigated for the electrolytic reduction of nitrobenzene to p-aminophenol under potentiostatic condition (controlled potential). The preparative electrolysis of nitrobenzene was carried out using supporting electrolytes consisting of 2 M H₂SO₄ in a solution of 50% 2-propanol/50% water (v/v). The criterion for scale-up (?_n) was determined through application of one-dimensional model. The polarization curves that describe the reduction of nitrobenzene to p-aminophenol were obtained experimentally by using a pilot scale for different nitrobenzene concentrations and flow rates of catholyte.It was found that the effectiveness factor (?_n) increases with increasing flow rate, and decreasing nitrobenzene concentration. An optimum thickness of bed equal to 0.6 cm was obtained, in which the effectiveness factor not less than 0.588, to ensure a well distribution of current and potential.     

Optimization of aqueous p-aminophenol degradation by external-loop airlift sonophotoreactor using response surface methodology

The combination of sonolysis and photolysis in the presence of hydrogen peroxide (H₂O₂) in a 7-L external-loop airlift sonophotoreactor was used to treat the aqueous solution of p-aminophenol. The central composite design (CCD) and response surface methodology (RSM) were employed to evaluate the interaction effects of the initial H₂O₂ concentration (x₁ = 100–900 mg/L), the ultrasonic power (x₂ = 25–65 W), the air flow rate (x₃ = 1–5 L/min), and the initial concentration of p-aminophenol (x₄ = 10–50 mg/L) on the p-aminophenol degradation and total organic carbon (TOC) reduction efficiencies as well as to optimize operating conditions. The coefficients of determination (R²) and adjusted-R² obtained from the analysis of variance (ANOVA) were 0.9900 and 0.9812 for the p-aminophenol degradation; and 0.9742 and 0.9516 for the TOC removal, respectively, ensuring a satisfactory adjustment of the quadratic regression model with experimental results. The linear, square, and interaction effects of x₁, x₂, x₃, and x₄ were also calculated. Genetic algorithm optimization was employed to maximize the mineralization efficiency. 79% TOC reduction efficiency after 90 min and 86.5% p-aminophenol removal efficiency after 30 min were achieved under recirculating batch mode at operating conditions of x₁ = 740 mg/L, x₂ = 65 W, x₃ = 5 L/min, and x₄ = 24 mg/L.     

Electrochemical behavior of p-tert-butyl calix[8]arene in CH2Cl2

The electrochemical behavior of p-tert-butyl calix[8]arene has been investigated by cyclic voltammetry. The result shows that there is an irreversible electrochemical oxidative wave when the potential ranges from −0.3 to 1.6 V versus Ag/0.1 M AgNO₃ in acetonitrile (Ag/Ag⁺). At 25 °C, the peak potential is ca. 1.43 V (versus Ag/Ag⁺) at scan rate of 0.05 V s⁻¹. The number of the electrons transferred in the electrochemical reaction is four. The diffusion coefficient of p-tert-butyl calix[8]arene is 2.8 × 10⁻⁵ cm² s⁻¹. The diffusion activation energy is 12.3 kJ mol⁻¹.     

Potential controlled anion absorption in a novel high surface area composite of Cladophora cellulose and polypyrrole

The electrochemical properties of a novel composite paper material of high surface area consisting of polypyrrole (PPy) deposited on cellulose derived from Cladophora sp. algae have been investigated in electrolytes containing different concentrations of nitrate, chloride and p-toluene sulfonate, as well as in solutions containing both p-toluene sulfonate and chloride. The oxidation mechanism and the dependence of the oxidation behavior of the polypyrrole, which was obtained by oxidation of pyrrole with iron(III) chloride, on the anion type and concentration have been studied. Current nucleation maxima, appearing at different times depending on the anion concentration, were obtained during the oxidation of the reduced polymers as a result of the combined action of the formation and growth of conducting polymer strands and anion diffusion. No loss of capacity was seen during repeated oxidation and reduction of the polymer indicating that trapping of anions in the reduced polymer did not limit the electroactivity of the present material. The latter can be explained by the thin polymer layer present on the cellulose substrate. During the oxidation of the polymer, the anions most likely first cover most of the surface of the composite before diffusing into the bulk of the polymer. The estimated distance between these surface sites was also found to match the size of the anions. For electrolytes containing a mixture of anions, the oxidation charge depends on the concentration and size of the different anions.The combination of the thin polymer coating and the large specific surface area of the composite give rise to a high ion absorption capacity even for large anions. Hence, the investigated material should be well-suited for use in biotechnological applications involving, e.g., desalting and extraction of proteins and DNA from biological samples.     

Prediction of the electrochemical oligomerization of p-tert-butyl anisole by analyses of the electrolysed solution

The objective of this paper is the study of the electrochemical oxidation of p-tert-butyl anisole in acetonitrile solution and its subsequent electrodeposition on platinum anode. In dry medium, potentiostatic deposition at potential more anodic than these of the two first overlapped waves, which requires relatively high initial substrate concentration, provided an electroactive film. It is shown that whilst the formation of coated electrode at relatively low concentration is difficult in dry medium, it became possible in the presence of an excess amount of water. In the voltammetric study, we observe that the addition of water to the monomer, in the same concentrations leads to the appearance of several new consecutive redox systems, at potential region more anodic than the two overlapped waves. The number of these processes, which are attributed to oxidation of the products of the two first steps, is found to be increased with the water concentration. Combining cyclic voltammetry along with high performance liquid chromatography (HPLC), the effect of the analytical concentration of the monomer and this of the addition of water, on macroscale electrolyses is discussed. Possible reaction mechanisms as well as probable structures for oligomeric products are proposed.     

Acylation of p-xylene over zeolites

Zeolites differing in structure and acidity were tested in liquid phase acylation of p-xylene. Hexanoyl chloride, propionic anhydride and isobutyric anhydride were used as acylating agents. The highest conversions of acylating agent were achieved over large pore zeolites USY (66.3%) and Beta (58.2%). It was found that acylation of p-xylene proceeds only over large pore zeolites Beta and USY. Selectivities to monoacylated p-xylene obtained over USY zeolite decreased in the order: propionic anhydride 78.0% > hexanoyl chloride 67.2% > isobutyric anhydride 33.1%. Diacylated product was formed over zeolite USY with all acylating agents tested but only with hexanoyl chloride over zeolite Beta. It was found that the optimum Si/Al ratio of zeolite Beta for p-xylene acylation with propionic anhydride is 25, while for isobutyric anhydride is 19. Conversion of isobutyric anhydride decreased with increasing isobutyric anhydride concentration and increased with increasing amount of catalyst.     

Cyclic voltammetry studies of n-type polymers with non-alternant fluoranthene units

Poly(p-fluoranthenevinylenes) and their dithiocarbamate precursors have been deposited on indium-tin oxide electrodes and electrochemical properties of the obtained films have been investigated by means of cyclic voltammetry studies in acetonitrile solutions containing 0.1 M (n-C₄H₉)₄NBF₄ as supporting electrolyte. It has been found that all investigated polymers display well pronounced n-doping processes. Electrochemical reduction of the dithiocarbamate precursors seems to be associated with C-S bond cleavage with elimination of -SC(S)N(C₂H₅)₂ group. In view of UV-vis spectroscopic data the obtained products, tentatively identified as polymers containing fluoranthene units connected by -CH₂-CH₂- bridges, are somewhat less conjugated than the corresponding poly(p-fluoranthenevinylenes). Reversible electrochemical reduction of poly(p-fluoranthenethanes) occurs at potentials only somewhat (ca. 0.1 V) more negative as found for their poly(p-fluoranthenevinylenes) analogues suggesting relatively weak coupling between fluoranthene kernels in both kinds of investigated polymers.     

Electrochemical regeneration of activated carbon loaded with p-nitrophenol in a fluidized electrochemical reactor

This paper described a novel electrochemical process for the regeneration of activated carbon (AC) loaded with p-nitrophenol (PNP), aiming to reduce regeneration time and improve cost-effectiveness of the process by adoption of a novel non-active anode of modified lead dioxide and operation of AC in a fluidized mode. The regeneration parameters such as current density, liquid flow rate, NaCl concentration, pH of the solution and regeneration time were systematically investigated. Under the optimum conditions, the regeneration efficiency of AC could reach 90% in 1.5 h, and no significant declination was observed after five-times continuous adsorption-regeneration cycles, confirming the reuse feasibility of the regenerated AC. The adsorption of organic pollutants was confirmed occurring in the micropore of AC, and AC regeneration was mainly due to the decomposition of organics by the attack of active species such as hydroxyl radical that were generated by electrochemical oxidation. The time-space production for AC regeneration has been greatly improved in the present modified process, indicating this regeneration process is much more potentially cost-effective for application.     

Novelties of kinetics and mechanism of liquid-liquid phase transfer catalysed reduction of p-nitroanisole to p-anisidine

Anisidines, which are valuable intermediates in dyestuff industry, are synthesised from nitroanisoles. The reduction of nitroanisoles and other substituted nitroaromatics to the corresponding amines can be affected by aqueous inorganic sulphides and polysulphides and the rates of these biphasic reductions are amenable to intensification under phase transfer catalysis (PTC). The selectivity in the reduction of substituted nitroaromatics is greatly affected by the type and number of phases involved in the PTC process. The kinetics and mechanisms of complex L-L PTC processes are not properly understood. This paper delineates novelties of the kinetics and mechanism of L-L PTC reduction of the model compound p-nitroanisole to p-anisidine wherein the sulphide ions get transformed into bisulphide in the aqueous phase and are extracted as ion pairs in the organic phase. A series of reactions of the bisulphide occur to reduce the nitroaromatic to the corresponding amino compound and the Q⁺HSO₃⁻ ion pair is transferred across the interface to react with S²⁻ leading to the formation of the Q⁺SH⁻ ion pair and S₂O₃²⁻. The effects of different parameters are studied to establish this mechanism. The selectivity to the product is 100%.