Catalytic oxidation of xanthine by the nanostructured poly(aniline-co-2,4-diaminophenol)

Poly(aniline-co-2,4-diaminophenol) (PADAP) was synthesized in a solution containing aniline, 2,4-diaminophenol (DAP) and sulfuric acid, using potentiostatic method. The image of a PADAP film is constructed of spherical particles with an average diameter of 50 nm, which was examined by both scanning electron microscope (SEM) and atomic force microscopy (AFM). The nanostructured PADAP can catalyze xanthine oxidation under a less positive potential of 0.31 V (vs. SCE), which was proved by cyclic voltammetry and amperometric method. The PADAP electrode has a very fast response for the determination of xanthine. The response current of the PADAP electrode increases with increasing xanthine concentration and applied potential. The catalytic mechanism for the oxidation of xanthine on the nanostructured PADAP electrode is similar to that of xanthine oxidase-catalyzed reaction. Experimental evidence for the electrocatalytic mechanism of xanthine oxidation on a PADAP electrode was demonstrated via measurements of the open-circuit potential and the in situ chemical-ESR spectra of PADAP in the solutions without and with xanthine, respectively.     

Electronic Effect of Substituent of Quinones on their Catalytic Performance in Hydrocarbons Oxidation

Quinones with electron-withdrawing F, Cl or Br groups and N-hydroxyphthalimide (NHPI) were used as catalysts in selective oxidation of hydrocarbons with molecular oxygen as oxidant. The catalytic activity in the selective oxidation of ethylbenzene to oxygenation products was in the following order: p-benzoquinone < tetrafluoro-p-benzoquinone ≈ tetrachloro-p-benzoquinone < tetrabromo-p-benzoquinone (p-TBBQ). Moderate electron-withdrawing power of substituent was suitable for quinone abstracting hydrogen from NHPI to generate reactive phthalimido-N-oxyl (PINO). The catalytic activity of p-TBBQ/NHPI, the best catalyst in our study, was also tested in the selective oxidation of alkylarenes, alkenes and alkanes.     

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.     

Tolyl-Substituted Siloles: Synthesis, Substituent Effects, and Aggregation-Induced Emission

The syntheses, spectroscopic characteristics, and electrochemical behavior of 1,1-dimethyl-2,5-diphenyl-3,4-bis(p-methylphenyl)silole and 1,1-dimethyl-2,3,4,5-tetra(p-methylphenyl)silole are reported. The compounds are weakly luminescent in dilute fluid solution but exhibit dramatic aggregation-induced emission (AIE). Absorbance, luminescence, and voltammetric characteristics are compared to 1,1-dimethyl-2,3,4,5-tetraphenylsilole and 1,1-dimethyl-3,4-diphenyl-2,5-bis(p-methylphenyl)silole, allowing a comparison of the effects of the position of the substituents on the silole ring. In addition, HOMO and LUMO energies and band gaps, derived from electrochemical, spectroscopic, and computational data, are reported. Substitution of the weakly electron-donating p-methyl groups on the peripheral aryl groups at the 2- and 5-positions of the silole ring results in slight red shifts in absorption and emission maxima, slight enhancement of luminescence quantum yields, slightly longer luminescence lifetimes, and more anodic oxidation potentials.     

The electrochemical copolymerization of aniline with 2,4-diaminophenol and the electric properties of the resulting copolymer

The copolymerization of aniline and 2,4-diaminophenol (DAP) was first carried out in an acidic solution under a constant potential of 0.75 V. The copolymerization rate was found to increase with an increasing ratio of monomer DAP to aniline in the mixture. The opposite is true of the electrochemical copolymerization of aniline and o-aminophenol or m-aminophenol. The difference is due to the two kinds of electron-donating groups in DPA. Poly(aniline-co-2,4-diaminophenol) (PADAP), when synthesized under optimal conditions, has a good redox activity from pH < 1 to 11.0 in a wide potential region. Its conductivity is 0.26 S cm⁻¹, which is very close to that of polyaniline synthesized under the same conditions in the absence of DAP. The pH dependence of PADAP conductivity was found to be better than that of polyaniline. PADAP that is synthesized under optimal conditions has unique magnetic resonance properties in NMR and ESR; molecules that are synthesized in the presence of other DAP to aniline concentration ratios have different properties. The monomer concentration ratio in the mixture and the applied potential strongly affect the properties of PADAP.     

New method for controlled synthesis of polylactide block copolymers: organoborane/p-quinone system and reversible-deactivation radical polymerization

We developed a new approach to obtain polylactide hybrid block copolymers with vinyl monomers (styrene, methyl methacrylate, methyl acrylate) through the realization of a reaction sequence using triethylborane and various p-quinones. The method offered includes two stages. In the first stage, a chain-transfer agent was obtained by borylation of the terminal hydroxyl groups of polylactide. The second stage was vinyl monomer radical polymerization in the presence of p-quinone accompanied by S_H2-substitution at the boron atom.1,4-Naphthoquinone, 2,3-dimethyl-1,4-benzoquinone, duroquinone and 2,5-di-tert-butyl-1,4-benzoquinone were used as synthetic polymer chain growth mediators. It is shown that 1,4-naphthoquinone and 2,3-dimethyl-1,4-benzoquinone, similar in their characteristics, are effective agents providing the realization of reversible-deactivation radical polymerization. Realization of reversible-deactivation radical polymerization was proved with the analysis of the kinetics of block copolymerization, molecular weight characteristics and compositional homogeneity of block copolymers as well as its further capability to elongate the polymer chain. Synthesized block copolymers have a high thermal stability compared to the initial borylated polylactide. © 2021 Society of Industrial Chemistry.     

Enhanced electrochemical treatment of phenolic pollutants by an effective adsorption and release process

Phenolic compounds are the most prevalent pollutants in industrial wastewater. Here we report on a novel approach: using adsorption and release process to improve the efficiency of electrochemical oxidation of phenolic pollutants. To illustrate this innovative technique, three phenolic compounds, phenol, p-nitrophenol (p-NPh) and p-cresol, were selected as model pollutants. The adsorption studies were carried out by adsorbing the three pollutants onto a hyper-cross-linked resin (Purolite Macronet MN-200). The adsorption isotherms, adsorption kinetics and the effect of temperature on adsorption were systematically investigated. Our experimental results have shown that the MN-200 resin exhibits a high removal efficiency of the three model pollutants and the adsorbed phenolic compounds can be efficiently released in a NaOH solution. The preconcentrated phenolic compounds were further treated using electrochemical oxidation at the Ti/SnO₂-Sb₂O₅-IrO₂ electrode. The first-order model fits the kinetics data of the electrochemical oxidation very well, showing that the degradation rate constant decreases in the order of p-NPh > phenol > p-cresol. Our study has demonstrated that the integration of the effective preconcentration process and the advanced electrochemical oxidation offers a promising approach for the efficient elimination of phenolic pollutants in wastewater.     

Electrochemical oxidation of hydrogen peroxide at a bromine adatom-modified gold electrode in alkaline media

Bromine (Br)-adatom (Br_(ads)) was in situ fabricated onto polycrystalline gold (Au (poly)) electrode in Br⁻-containing alkaline media. The surface coverage of Br_(ads) (Γ_Br) varied only in the submonolayer coverage within the investigated potential window under potentiodynamic condition because of the coadsorption of hydroxyl ion (OH⁻) in alkaline media. The in situ fabricated Br_(ads)-submonolayer-coated Au (poly) electrode was successfully used for the electrochemical oxidation of hydrogen peroxide (H₂O₂). About five times higher oxidation current was achieved at the modified electrode as compared with the bare electrode. The enhancement of the electrode activity towards the electrochemical oxidation of H₂O₂ was explained based on the enhanced electrostatic attraction between the anionic HO₂⁻ molecules and Br_(ads)-adlayer-induced positively polarized Au (poly) electrode surface.     

Unexpected Hydrogenation of C?C-Double Bonds with tert-Butyl Iodide

Heating of 3-isobutylidene-2,5-diketopiperazines 1 or 4 , maleinimide or 2,3-dichloro-5,6-dicyano-p-benzoquinone with tert-butyl iodide in toluene gave rise to hydrogenation of the conjugated C C-double bond affording 3-isobutyldiketopiperazines 2(rac) and 3(rac) , succinimide, or 2,3-dichloro-5,6-dicyanohydroquinone, respectively. Furthermore, an interesting N O migration of a benzoyl group as well as reductive aromatization to pyrazines 5 and 6 , respectively, were observed.     

Quinones and phenols in the defensive secretions of neotropical opilionids

Three quinones (2,3-dimethyl-1,4-benzoquinone; 2,5-dimethyl 1,4-benzoquinone; and 2,3,5-trimethyl-1,4-benzoquinone) and two phenols (2,3-dimethylphenol and 2-methyl-5-ethylphenol) were isolated from the defensive secretions of four species of opilionids (suborder Laniatores) from the Canal Zone. Phenols had not previously been reported from opilionids, and 2-methyl-5-ethylphenol was not known as a natural product. The secretions, which are mixed with regurgitated enteric fluid upon discharge, are administered by the animals by leg dabbing.Paper no. 53 of the seriesDefense Mechanisms of Arthropods.     

Brønsted acid-base and -polybase complexes as electrolytes for fuel cells under non-humidifying conditions

A novel series of Brønsted acid-base complexes were prepared by the combination of 2,5-diphenyl-1,3,4-oxadiazole (DOD) with a super strong acid bis(trifluoromethane sulfonyl)imide (HTFSI) at various molar ratios. The DOD-HTFSI complexes showed high ionic conductivities and were electroactive for H₂ oxidation and O₂ reduction at a Pt electrode. The complexes could serve as fuel cell electrolytes under entirely non-humidifying conditions and at elevated temperatures. The concept of the DOD-HTFSI complexes was applied to a polybase, poly[p-phenylene-2,5-(1,3,4-oxadiazole)] (POD), and the ion-conduction behavior of the POD-HTFSI complexes was discussed with a view to realizing anhydrous solid-state fast proton conductors.     

CHEMICAL DOPING OF TRIPHENYLAMINE-BENZALDEHYDE POLYMERS

A series of different benzaldehydes-4-tolyldiphenylamine polymers has been oxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and tetrachloro-1,4-benzoquinone (p-chloranil), and also doped by 10-camphorsulfonic acid in order to increase their conductivity. The doped polymers were characterized by ¹H NMR spectroscopy, cyclic voltammetry, UV, and IR spectroscopy. It was found by ¹H NMR spectroscopy that the oxidation yield increased with the increased amount of oxidation agent. It was shown that optical and electrochemical bandgaps of the polymers decreased after oxidation. Conductivity measurements revealed that the applied oxidation and doping procedure resulted in decreased electrical resistance of their initial polymers, down to the level of semiconducting materials. An increased thermal stability of the polymer after oxidation was confirmed by differential scanning calorimetry.     

Anodic oxidation of parabens in acetic acid�Cacetonitrile solutions

The electrochemical properties of esters of p-hydroxybenzoic acid, called parabens, were investigated on a carbon fiber microelectrode and on a glassy carbon macroelectrode in glacial acetic acid containing 20?% acetonitrile (v/v) and sodium acetate as a supporting electrolyte. The anodic oxidation of parabens in this medium proceeds in a single stage giving well-shaped peaks or waves in the same potential region of above 1.0?V (vs. Ag/AgCl). The electrode process was characterized as being quasireversible, diffusion-controlled, and proceeds with the exchange of one electron and one proton. Phenoxyl radicals as products of the electrode process are chemically unstable and participate in the successive irreversible homogenous reactions resulting in electroinactive products (E _q C _i mechanism). Smaller oxidation potentials for parabens in comparison to such unsaturated fatty acids as oleic, linoleic, and linolenic ones indicate that these compounds can show antioxidative properties. Since parabens are often used as preservatives in many products, the results presented can be found useful in the determination of their total content in real samples.     

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.     

Electrooxidation of 2,4-dichlorophenol and other polychlorinated phenols at a glassy carbon electrode

The electrooxidation of 2,4-dichlorophenol (2,4-DCP) on a rotating GC disk electrode has been studied by cyclic voltammetry and chronoamperometry at different pH values. A dual mechanism where one pathway yields a quinone-like species and the other one leads to insoluble polymers that passivate the electrode surface is proposed. From a comparative study of the electrooxidation of several DCPs at pH 2.2, it is concluded that for the same number of chlorine atoms the ortho isomer is oxidized at a lower potential than the para isomer. The anodic peak potential for electrooxidation of the chlorophenols (CPs) on glassy carbon increases with increasing pK_a, with a slope of 35 mV (pK_a unit)⁻¹, illustrating the correlation of one chemical parameter of the CPs, namely their acidity constant, with their electrochemical reactivity.     

Electrochemical investigation of platinum electrode in solid electrolyte cell

Electro-oxidation of platinum film electrode deposited on yttria-stabilized zirconia (YSZ) is examined in situ using potential programmed voltammetry at 450 °C in oxygen containing atmosphere. Under prolonged anodic oxidation different sorts of oxidized species are formed which are consumed subsequently during a linear cathodic potential scan resulting in three distinct reduction peaks, one of them being fairly reversible while the two others strongly irreversible. Higher oxidation potential and longer time of polarization favor the irreversible processes. The coexistence of three electrochemical processes is explained with the extension of the triple phase boundary. The rapid first process is identified as formation of PtO_x at the electrode/metal interface, the second process - a much slower, parallel one - is related to the phenomenon of oxygen backspillover at the metal/gas interface, and the slowest third process - consecutive to the first one - is attributed to growth of the PtO_x layer at the electrode/metal interface toward the bulk of the metal by analogy to electro-oxidation of platinum in aqueous liquid electrochemistry.     

Tuning of the neutral state color of the π-conjugated donor-acceptor-donor type polymer from blue to green via changing the donor strength on the polymer

Two donor-acceptor-donor types of π-conjugated monomers were synthesized using Stille coupling reaction. Both monomers were found to produce electroactive polymers upon electrochemical oxidation. The effects of different donor substituents on the polymers' electrochemical and spectroelectrochemical properties were examined. Optical characterization revealed that the band gaps of poly(2-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-3-(2,3-dihydrobenzo[b][1,4]dioxin-7-yl)-5,8-di(thiophen-2-yl)quinoxaline) (PDBQTh) and poly(2-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-3-(2,3-dihydrobenzo[b][1,4]dioxin-7-yl)-5-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-8-(2,3-dihydrothieno[3,4-b][1,4]dioxin-7-yl)quinoxaline) (PDBQEd) were 1.5 eV and 1.3 eV, respectively. PDBQEd reveals two distinct absorption bands as expected for this type of donor-acceptor-donor polymer at 423 and 738 nm, while PDBQTh has a single absorption band at 630 nm. The colorimetry analysis revealed that while PDBQTh has a blue color, PDBQEd showed a green color in the neutral state. PDBQEd revealed reversible n-doping.     

Catalytic wet oxidation of p-chlorophenol over supported noble metal catalysts

Heterogeneous catalytic wet oxidation of aqueous p-chlorophenol (p-CP) solution was investigated at 453 K and 2.6 MPa in a slurry reactor. The performance of noble metal catalysts was compared with that of traditional manganese catalysts. Activated carbon supported catalysts showed significant higher activities for total organic carbon (TOC) reduction than those supported on alumina or cerium oxide. Pt was found to be the most active metal for p-chlorophenol oxidation. The activities of noble metal catalysts were found to correlate with heat of formation of metal oxides. CO₂ is the predominant oxidation product with formation of minor p-benzoquinone and acetic acid as intermediate compounds. Possible reaction pathway was also discussed.     

Electrocatalytic activity of 2,3,5,6-tetrachloro-1,4-benzoquinone/multi-walled carbon nanotubes immobilized on edge plane pyrolytic graphite electrode for NADH oxidation

This work reports the electrocatalytic activity of 2,3,5,6-tetrachloro-1,4-benzoquinone (TCBQ)/multi-walled carbon nanotubes (MWCNT) immobilized on an edge plane pyrolytic graphite electrode for nicotinamide adenine dinucleotide (NADH) oxidation. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) were used to confirms the presence of chloro after the nanotube modification with 2,3,5,6-tetrachloro-1,4-benzoquinone. The surface charge transfer constant, k_s, and the charge transfer coefficient for the modified electrode, α, were estimated as 98.5 (±0.6) s⁻¹ and 0.5, respectively. With this modified electrode the oxidation potential of the NADH was shifted about 300 mV toward a less positive value, presenting a peak current much higher than those measured on an unmodified edge plane pyrolytic graphite electrode (EPPG). Cyclic voltammetry and rotating disk electrode (RDE) experiments indicated that the NADH oxidation reaction involves 2 electrons and a heterogenous rate constant (k_obs) of 3.1 × 10⁵ mol⁻¹ l s⁻¹. The detection limit, repeatability, long-term stability, time of response and linear response range were also investigated.     

Oxidation of p-methoxyphenol on SnO2–Sb2O5 electrodes: Effects of electrode potential and concentration on the mineralization efficiency

The oxidation of p-methoxyphenol in aqueous solution on antimony-doped tin oxide has been studied, and the effects of applied potential and initial PMP concentration upon the oxidation rate have been identified. The concentration decay of PMP during its electrooxidation follows first-order reaction kinetics. Analysis solution during electrolysis using UV–Vis spectroscopy revealed that under some experimental conditions partial oxidation of PMP occurs. The principal products were p-benzoquinone and aliphatic (maleic and oxalic) acids. The Faradaic efficiencies for oxidation at different applied potentials were determined from the UV–Vis spectra obtained. It is shown that production of CO₂ was very low at potentials below 2.3 V with respect to the saturated calomel electrode, and that at more positive potentials mineralization to CO₂ decreased as the concentration of PMP in solution increased.