Kinetics of β-picoline oxidation to nicotinic acid over vanadia-titania catalyst. 3. The oxidation of nicotinic acid

The oxidation of nicotinic acid (NA) over binary vanadia-titania catalysts with different contents of residual sulfate ions (0.2 and 2.8 wt. %) was studied at 250–300°C in the feed reaction mixture of the composition NA: O₂: H₂O = 0.01–0.17: 5–20: 0–20 (vol. %), with nitrogen as a balance. The kinetic parameters of the process were determined. It was shown that even a ten-fold increase in the content of residual sulfate ions in the samples has no significant effect on the kinetics of the NA oxidation.     

Kinetics and mechanism of oxidation of ferrocyanide by N-bromosuccinimide in aqueous acidic solution

The kinetics of oxidation of ferrocyanide by N-bromosuccinimide (NBS) has been studied spectrophotometrically in aqueous acidic medium over temperature range 20–35 °C, pH = 2.8–4.3, and ionic strength = 0.10–0.50 mol dm⁻³ over a range of [Fe²⁺] and [NBS]. The reaction exhibited first order dependence on both reactants and increased with increasing pH, [NBS], and [Fe²⁺]. The rate of oxidation obeys the rate law: d[Fe³⁺]/dt = [Fe(CN)₆]^4–[HNBS⁺]/(k ₂ + k ₃/[H⁺]). An outer-sphere mechanism has been proposed for the oxidation pathway of both protonated and deprotonated ferrocyanide species. Addition of both succinimide and mercuric acetate to the reaction mixture has no effect on the reaction rate under the experimental conditions. Mercuric acetate was added to the reaction mixture to act as scavenger for any bromide formed to ensure that the oxidation is entirely due to NBS oxidation.     

Oxidation of nanocrystalline lead sulfide in air

The oxidation of nanocrystalline PbS powders with a particle size of 10–20 nm and the oxidation of ∼100-nm-thick nanocrystalline PbS films with a crystallite size of 70–80 nm have been investigated in the temperature range from 293 to 773 K. The nanocrystalline powders and films were prepared by chemical deposition from aqueous solutions. The nanopowders with a particle size of 10–20 nm begin to oxidize at a 450 K lower temperature than bulk lead sulfide (870 K). The PbS nanofilms are more resistant to oxidation than the nanopowders and are stable in air up to 573 K. The oxidation behavior of the nanopowders and nanofilms is in agreement with the sulfate theory of PbS oxidation.     

Examination of the oxidation resistance of Cr–Mo–V tool steel by thermal analysis

In the present study, the oxidation behavior of Cr–Mo–V tool steel was examined at different temperatures in air. The examination was conducted by means of thermogravimetric analysis, scanning electron microscopy, and X-ray diffraction (XRD). After non-isothermal oxidation from ambient temperature to 1000 °C, it was revealed that the specimen begins to oxidize over 700 °C, while over 800 °C the oxidation rate increases significantly. Finally over 900 °C, this rate has a considerable value, and the specimen's oxidation resistance is inadequate. From these results, four different oxidation temperatures (805, 835, 865, and 895 °C) were selected for the isothermal test, as referred above, which correspond to different oxidation rates, to determine the oxidation activation energy of the Cr–Mo–V specimens. Energy dispersive X-ray spectroscopy (EDX) and XRD phase identification of the as-formed scales showed that in every case, it contains two distinguishable regions. The inner layer is a mixture of chrome and iron oxides and the outer layer contains iron oxides and is also characterized by high porosity. This phenomenon was explained by the different diffusion coefficients of every element in the steel matrix.     

Heterogeneous selective oxidation of formaldehyde on oxide catalysts:In situ FTIR study of formaldehyde surface species on a V-Ti-O catalyst and oxygen effect

The interaction of formaldehyde with a highly selective V-Ti-O catalyst for the oxidation of formaldehyde to formic acid is studied by Fourier-transform infrared (FTIR) spectroscopy at 70–200‡C. In a flow of formaldehyde/oxygen mixture and in a mixture without oxygen at optimal temperatures for formic acid formation (100–140‡C), methoxy groups and other oxygenates are formed in small amounts. These are two bidentate formates and covalently bound monodentate formate. The fact that similar oxygenates are observed independently of the presence of oxygen in the reaction mixture suggests the participation of the catalyst oxygen in their formation. Oxygen accelerates the desorption of bidentate formates. Bidentate formates of one type decompose in a flow of air at 100–150‡C, and bidentate formates of the other type decompose at 170–200‡C.     

Synthesis of 20-deoxoluteone

20-Deoxoluteone has been synthesized from sclareol. Sclareol was brominated with phosphorus tribromide to form a mixture of primary allyl bromides, from which, by the malonic synthesis, a mixture of bicyclogeranylgeranylacetic acids was obtained which was cyclized with fluorosulfonic acid to form a mixture of two diastereomeric -lactones. The predominating lactone was converted by successive reduction with lithium tetrahydroaluminate, oxidation with oxalyl chloride in dimethyl sulfoxide, reaction with methylmagnesium iodide, and oxidation by the chromium trioxide/pyridine complex into 20-deoxyluteone.Institute of Chemistry, Moldavian SSR Academy of Science, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 346–353, May–June, 1990.     

A selective determination of norepinephrine on the glassy carbon electrode modified with poly(ethylenedioxypyrrole dicarboxylic acid) nanofibers

A glassy carbon electrode modified with poly(3,4-ethylenedioxypyrrole-2,5-dicarboxylic acid) nanofibers (PEDOPA-NFs) was prepared for the determination of norepinephrine (NE) in phosphate buffer saline. The modified electrode demonstrated an improved sensitivity and selectivity toward the electrochemical detection of NE and could detect separately ascorbic acid (AA), uric acid (UA), and NE in their mixture. The separations of the oxidation peak potentials of NE–AA and NE–UA were 160 and 150 mV, respectively. Meanwhile, the modified electrode showed higher sensitivity and selectivity toward NE than dopamine and epinephrine. Using differential pulse voltammetry, the oxidation peak current of NE was found to be linearly dependent on its concentration within the range of 0.3–10 μM, and the detection limit of the NE oxidation current was 0.05 μM at a signal-to-noise ratio of 3. The PEDOPA-NFs promoted the electron transfer reaction of NE, while the PEDOPA-NFs, acting as a negatively charged linker, combined with the positively charged NE to induce NE accumulation in the NFs at pH under 7.4. However, the PEDOPA-NFs restrained the electrochemical response of the negatively charged AA and UA due to the electrostatic repulsion. The result indicates that the modified electrode can be used to determine NE without interference from AA and UA and selectively in the mixture of catecholamines.     

Chemical synthesis of tungsten-copper nanocomposite powder

In this study homogeneous powders of CuWO₄ and WO₃ was produced from ammonium para-tungstate (APT) and copper nitrate. Then, the product was used to prepare nano-sized W-Cu powder. Hence, a mixture of ammonium paratungstate and copper nitrate with predetermined weight proportion was made in distilled water, while the content of the beaker was being stirred at a certain speed to reach the desired composition of W-20 wt % Cu. Mixture was heated to 80–100°C for 6 h. Also, pH range was adjusted at about 3–4. The mixture was then evaporated and dried in the air. To reach W-Cu composite powder, the precursor powders burned out at 520°C for 2 h in the air to form W-Cu oxide powder and then were ball milled and reduced in H₂ atmosphere to convert it into W-Cu composite powder. The resulting powders were evaluated using scanning electron microscopy, X-ray diffraction, thermogravimetric analysis and differential thermal analysis techniques. The results showed that homogeneous powders of W-Cu with particle size of around 100 nm and a nearly spherical shape could be obtained by this process. Each particle include smaller parts with size of around 20–30 nm.     

Activated carbon cloth as adsorbent and oxidation catalyst for the removal of amitrole from aqueous solution

Removal of amitrole from water was studied by adsorption on an activated carbon cloth and by oxidation with hydrogen peroxide using the same activated carbon cloth as catalyst. Study variables included the solution pH, ionic strength, and temperature in the adsorption process and the solution pH and the surface chemistry of the activated carbon cloth in the oxidation process. Results showed that amitrole adsorption on activated carbon cloth was not adequate to remove amitrole from water due to the high solubility and low aromaticity of the herbicide, which reduced its adsorption on the carbon. A higher amitrole removal rate was obtained with the activated carbon/H₂O₂ system. The best results were obtained on basic activated carbon surfaces at pH 7–10, when hydroxyl radical formation is favored, achieving the removal of 35–45% of the AMT, compared with 20–25% under the best adsorption conditions. Importantly, oxygen fixed on the carbon surface during AMT oxidation must be removed by heat treatment in order to regenerate the surface basicity of the carbon before its reutilization in another oxidation cycle.     

Electrochemical and spectral properties of thienylene-polyparaphenylenevinylene derivative stereoisomers

Four new compounds: 1,4-dimetoxy-2,5-bis[2-(tien-2-yl)ethenyl]benzene), 1,4-dietoxy-2,5-bis[2-(tien-2-yl)ethenyl]benzene), 1,4-isopropyloxy-2,5-bis[2-(tien-2-yl)ethenyl]benzene) and 1,4-dietoksy-2,5-bis[2-(5-methylthiophen-2-yl)ethenyl]benzene are synthesized. Three steroisomers ZZ, EZ and EE are isolated from the reaction mixture for the first two of them. Third compound is fully converted to the most stable EE form. Polymerization of all isomers leads to identical polymeric product. Mechanism of polymerization is recognized by using model molecule with methyl substituents blocking α-, α′-sites. All seven stereoisomers have photoluminescent properties. Detailed spectral and electrochemical studies reveal isomerization phenomena during oxidation or at light exposure. Published in Russian in Elektrekhimiya, 2006, Vol. 42, No. 12, pp. 1401–1408. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes.” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Influence of medium on the kinetics of oxidation of iron(II) ion with t-butyl hydroperoxide

The oxidation of iron(II) with tert-butyl hydroperoxide was investigated in the absence of oxygen in water, methanol, and the dichloromethane—methanol solvent mixture (φ_r = 2:1). The oxidation rate depends on solvent polarity; measured in the presence of SCN⁻ at constant 0.8 mmol dm⁻³ HCl, the rate constant increases with the polarity decrease passing from water and methanol to the dichloromethane—methanol solvent mixture. Further, in non-aqueous solutions at this acid concentration the rate constant was higher than the rate constant in the presence of Cl⁻ only. The oxidation rate measured in the [FeCl]²⁺ complex in dichloromethane—methanol was slow in acidic medium and increased by decreasing the acid concentration. Approaching the physiological pH conditions the rate constant attained the value of an order of magnitude of 10³ dm³ mol⁻¹ s⁻¹, while very little alteration of stoichiometry of the oxidation reaction was observed. The rate constant measured in the presence of Cl⁻ strongly depends on electrolyte concentration at concentrations less than 0.5 mmol dm⁻³ HCl, both in MeOH and the solvent mixture. Based on these results, a possible mechanism of the influence of solvent, acidity, and ligand type on the rate constant is discussed. We assume that the oxidation proceeds by an inner-sphere mechanism considering that the breakdown of the successor inner-sphere complex forming reactive alkoxyl radicals is probably the rate-limiting step. Presented at the 20th International Conference on the Coordination and Bioinorganic Chemistry organized by the Slovak Chemical Society, Slovak University of Technology, Comenius University, and the Slovak Academy of Sciences, Smolenice Castle, 5–10 June 2005.     

Thermogravimetric characteristics and kinetic of co-pyrolysis of olive residue with high density polyethylene

The co-pyrolytic behaviour of olive residue/high-desity polyethylene mixture was examined with a thermogravimetric analyser. The experiments were done over the temperature range of room temperature to 1273 K at various heating rates (2, 10, 20 and 50 K min⁻¹) and in a nitrogen atmosphere. The results indicated that mass loss process of mixture consists of three distinct stages and the increase of the heating rate shifts in the maximum rate loss to higher temperature. The difference of mass loss (Δm) between experimental and theoretical, calculated as algebraic sums of the mixture for different heating rates of 2, 10, 20 and 50 K min⁻¹, is about 7–11% at 740–900 K. These experimental results indicate a significant synergistic effect during co-pyrolysis of olive residue with high-density polyethylene. In addition, a kinetic analysis was performed to fit thermogravimetric data, the mixture is considered as multi-stage process. A reasonable fit to the experimental data was obtained for all materials and their mixture by isoconversional Friedman method.     

Sensitive synchronous spectrofluorimetric methods for the determination of naproxen and diflunisal in serum

The interaction of diflunisal and naproxen with several surfactants was studied. Spectrofluorimetric methods were developed for the determination of both drugs in sodium dodecylsulfate micellar medium. The mixture of these drugs was resolved by synchronous fluorescence spectrometry using two scans. At Δλ = 20 nm, only naproxen yields a detectable signal that is unaffected by the presence of diflunisal. At Δλ = 110 nm the signal of diflunisal is not influenced by the presence of an up to 3-fold excess of naproxen. Mixtures containing naproxen/ diflunisal in ratios from 50:1 to 1:50 were analyzed with good results. The linear calibration ranges of both drugs were ca 0.02–2.0 μg mL^–1. The method has satisfactorily been applied to a mixture of both drugs in serum. Received: 7 October 1997 / Revised: 3 December 1997 / Accepted: 7 December 1997     

Role of CH, CH3, and OH Radicals in Organic Compound Decomposition by Water Plasmas

Decomposition of acetone, methanol, ethanol, and glycerine by water plasmas at atmospheric pressure has been investigated using a direct current discharge. At torch powers of 910–1,050 W and organic compound concentrations of 1–10 mol%, the decomposition rate of methanol and glycerine was over 99%, while those of acetone and ethanol was 95.4–99%. The concentrations of H₂ obtained were 60–80% in the effluent gas for any compounds by pyrolysis. Based on the experimental results, the decomposition mechanism of organic compounds in water plasmas was proposed and the roles of intermediate species such as CH, CH₃, and OH have been investigated; CH radical generated from organic compounds decomposition was easily oxidized to form CO; incomplete oxidation of CH₃ leads to C₂H₂ generation as well as soot formation; and negligible amount of soot observed from glycerine decomposition even at high concentration indicated that oxidation of CH_×(×:1–3) was enhanced by OH radical.     

Thermal analysis of polyesters containing oxirane groups

The preliminary studies of the thermal behaviour of polyester obtained in polycondensation process of cyclohex-4-ene-1,2-dicarboxylic anhydride and ethylene glycol and its new epoxidized form have been performed. The thermal characterization of initial polyester and its completely oxidized form was done by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). The non-isothermal DSC was applied to determine the influence of time and the temperature on the chemical modification of initial polyester using 38-40% solution of peracetic acid. On the basis of DSC profiles it has been found that the endothermic transition, due to the degradation process of initial polyester was characteristic feature under controlled heating program. The two characteristic transitions for the new epoxidized polyester, the exothermic peak corresponded to the thermal crosslinking of epoxidized polyester (322.8–336.4°C) and the endothermic decomposition peak of the cured material (363.8–388.9°C) were observed. The peak maximum temperatures (Tmax) and the heat of cross-linking reaction (ΔH_c) for epoxypolyester prepared at 20–60°C under 1–4 h were evaluated. The Tmax1 were almost independent from epoxidation conditions, while, the values of ΔH_c were dependent from conditions of synthesis. The ΔH_c values of this process decreased when time of oxidation increased. The highest values of ΔH_c at 40°C were obtained. Additionally, TG experiments confirmed two separated degradation steps of the new epoxidized polyester indicating the ester (370–380°C) and ether (450–460°C) bond breakdown.     

Catalytic oxidation of furan and hydrofuran compounds 8. Synthesis of 5-ethoxycarbonyl-4-hydroxy-3-oxo-2(3H)-furanone by the oxidation of furfural in the system aqueous H2O2-VOSO4-ethanol

The special features of the oxidation of furfural by aqueous hydrogen peroxide in the presence of vanadyl sulfate and ethanol has been studied for the first time. It has been established that this reaction proceeds with the formation of previously unknown oxidation products of furan compounds, one of which was isolated from the reaction mixture as the ethyl ester. It was established by spectral methods and qualitative reactions that this product is 5-ethoxycarbonyl-4-hydroxy-3-oxo-2(3H)-furanone. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 386–392, March, 2008.     

Electrochemical determinations of 6-mercaptopurine on the surface of a carbon nanotube-paste electrode modified with a cobalt salophen complex

A mixture of multi-walled carbon nanotube/graphite paste electrode modified with a salophen complex of cobalt was prepared and was applied for the study of the electrochemical behavior of 6-mercaptopurine (MP) using cyclic and differential pulse voltammetry (DPV). An excellent electrocatalytic activity toward the oxidation of MP was achieved, which led to a considerable lowering in the anodic overpotential and remarkable increase in the response sensitivity in comparison with unmodified electrode. Utilizing DPV method, a linear dynamic range of 1–100 μM with detection limit of 0.1 μM was obtained in phosphate buffer of pH 3.0. The electrochemical detection system was very stable, and the reproducibility of the electrode response, based on the six measurements during 1 month, was less than 3.0% for the slope of the calibration curves of MP. The electrochemical method as a simple, sensitive, and selective method was developed for the determination of MP in pharmaceutical dosage form and human plasma without any treatments.     

Quantitative HPLC Determination and Stability Studies of Pyridostemin in Extracts and Water Dispersible Granule Formulations of Stemona curtisii

A reversed-phase high-performance liquid chromatographic method has been developed and validated for the determination of pyridostemin, the major pesticidal alkaloid found in Stemona curtisii. This methodology was applied to the investigation of plant extracts and water dispersible granule formulations. Stability indicating procedures have also been carried out. The chromatographic separation was on a C₁₈ column with a mixture of acetonitrile–water–triethylamine (30:70:0.12, v/v/v), using UV detection at 300 nm. Validation procedures showed that the method was specific, accurate and precise. The response was linear over a range of 5–25 μg mL⁻¹ with recoveries in the range of 98.28–102.85%. The RSD for intra- and inter-day precision were <0.72 and <1.29%, respectively. Extraction of plant material with dichloromethane gave a significantly higher pyridostemin content in the crude extracts when compared with extractions in methanol. Partial purification of the crude extracts by silica gel column chromatography was used to concentrate the mixture about fourfold. Degradation behavior of pyridostemin in the partially purified extracts followed first-order kinetics. The main pathways for its decomposition were base hydrolysis and oxidation.     

Catalytic pyrolysis of propane-butane hydrocarbon mixture on the composite ceramic materials in an open system

Catalytic pyrolysis of propane-butane hydrocarbon mixture on the composite ceramic materials with the surface modified with zinc-, cadmium- phosphorus- and silicon-containing compounds in an open system was studied in the temperature range 500–850°C, at the rate of the gas mixture flow 20–200 ml min⁻¹, contact duration 0.75–155 s, and the values of the heterogeneity factor 2.0 to 2.9×10⁵ cm⁻¹. The catalytic activity of the systems under similar conditions was compared, the influence of various factors on the yield of ethylene and propylene was investigated and the sooting was estimated.     

TEMPO-mediated Oxidation of Native Cellulose: SEC–MALLS Analysis of Water-soluble and -Insoluble Fractions in the Oxidized Products

Linter cellulose was suspended in water and oxidized by the NaClO/NaBr/2,2,6,6-tetramehylpiperidine-1-oxy radical (TEMPO) system at pH 10.5 (TEMPO-mediated oxidation), and the oxidized products were separated into several fractions by filtration and centrifugation, depending on their particle sizes and apparent water-solubility. The major fraction (>ca. 80 mass % of the original linter cellulose) is the filter paper-trapped fibers, which can form inter-fiber hemiacetal linkages when handsheets are prepared thereof. Size-exclusion chromatographic analysis with multi-angle laser light scattering detection (SEC–MALLS) of these fibrous fractions dissolved in 0.5% LiCl/N,N-dimethylacetamide (DMAc) showed that some depolymerization occurred on cellulose chains during the TEMPO-mediated oxidation. On the other hand, the apparently water-soluble fractions (<ca. 20 mass % of the original linter cellulose) in the TEMPO-oxidized linter cellulose consisted of small amounts of colloidal particles having the cellulose I crystal structure, which came off from linter cellulose by the TEMPO-mediated oxidation and were mixed in the apparently water-soluble fraction even after filtration using 0.45 μm membrane. The presence of such colloidal cellulose crystals in the water-soluble fractions of the TEMPO-oxidized linter cellulose brings about anomalous bimodal SEC-elution patterns and extremely large molecular-mass values calculated from the SEC–MALLS data. Truly water-soluble cellouronic acid and/or over-oxidized compounds having glucuronic acid and hexeneuronic acid units are also present in the water-soluble fractions.