Scielo RSS <![CDATA[South African Journal of Chemistry]]> vol. 69 num. lang. pt <![CDATA[SciELO Logo]]> <![CDATA[<b>Partial oxidation of <em>n</em>-pentane over vanadium phosphorus oxide supported on hydroxyapatites</b>]]> The selective oxidation of M-pentane to value-added products, maleic anhydride or phthallic anhydride by vanadium phosphorus oxide loaded on hydroxyapatites as catalysts and oxygen as oxidant was investigated. Hydroxyapatite (HAp) and cobalt-hydroxyapatite (Co-HAp) were prepared by the co-precipitation method and VPO with varying weight percentages (2.5-15.0 %) were loaded on the hydroxyapatite supports by the wet impregnation technique. The catalyst materials were characterized by surface area measurements, elemental analysis, powder X-ray diffraction (XRD), infrared spectroscopy (IR) and temperature-programmed reduction (TPR). VPO is present in two phases, viz.( VO)2P2O7 and VOPO4. With increase in the VPO loading on the hydroxyapatites, the (VO)2P2O7 phase also increased. From catalytic results, a conversion of 75 % of n-pentane and selectivity towards maleic anhydride, about 50 % and phthalic anhydride, about 25 %, were consistently achieved with loadings of 5.0 and 7.5 wt. % VPO at 360 °C for GHSVs of 1900 and 2300 h-1. Under optimum conditions, product yields of up to 40 % maleic anhydride and 20 % phthalic anhydride were obtained. It is proposed that the products formed through the diene intermediate. <![CDATA[<b>A preliminary investigation into the stability of inorganic arsenic species in laboratory solutions simulating sediment pore water</b>]]> A simple method to preserve arsenic species in simulated pore water was investigated. Synthetic pore water containing high levels of Fe, Mn and S (as sulfide, S2-) were synthesized and spiked with different arsenic species. Arsenite [As(III)], arsenate [As(V)], dimethylarsinate [(CH3)2AsO2-, DMA], monomethylarsonate [(CH3)AsO3²", MMA], monothioarsenate(V) species (AsO3S3-, MTA) and tetrathioarsenate(v) species (AsS4(3-), TTA) in different combinations were used. Disodium ethylenediamine tetraacetate (EDTA) and temperature was used to preserve speciation. Solutions were analyzed at intervals of one day, week, month and 2 months after preparation. Samples were analyzed by HPLC-ICP-MS using an ion exchange column and ammonium carbonate buffer. Samples containing Fe and Mn spiked with As(III), As(V), DMA, MMA showed adequate species preservation for two months when EDTA was added. The total As in samples containing Fe, Mn and S2- was preserved over 60 days however, speciation was not preserved. Samples spiked with synthesized mono- and feira-thioarsenate(V) species showed immediate degradation of the mono- and tetra-thioarsenic species into unidentified As-S species. The results show that while EDTA may be adequate to preserve As species containing high Fe and Mn concentrations, the combination of EDTA and temperature was not successful in preserving As speciation in samples containing sulfides. <![CDATA[<b>The synthesis of carbon nanomaterials using chlorinated hydrocarbons over a Fe-Co/CaCO<sub>3</sub> catalyst</b>]]> The effect of chlorine on the morphology of carbon nanotubes (CNTs) prepared from a Fe-Co/CaCO3 catalyst was investigated using chlorobenzene (CB), dichlorobenzene (DCB), trichlorobenzene (TCB), dichloroethane (DCE), trichloroethane (TCE) and tetrachloroethane (TTCE) as chlorine sources using a catalytic chemical vapour deposition (CCVD) method. Toluene was used as a chlorine-free carbon source for comparison. Multi-walled carbon nanotubes (MWCNTs) were successfully synthesized. The physicochemical properties of the CNTs were studied using transmission electron microscopy (TEM), Raman spectroscopy, thermal gravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD) spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. The inner and outer diameters of the MWCNTs increased with an increase in the number of chlorine atoms contained in the reactant. Chlorine incorporation into the MWCNTs was observed by EDS analysis for all reactants. Formation of'bamboo-like' structures for the MWCNTs generated from TCE and TTCE was also observed, facilitated by the presence of the high percentage of chlorine in these reactants. Numerous MWCNTs revealed the presence of small carbon nanostructures that grew on top of the dominant CNTs, suggesting an unexpected secondary carbon growth mechanism. <![CDATA[<b>Oxyhalogen-sulfur chemistry: Kinetics and mechanism of oxidation of n-acetylthiourea by aqueous bromate and acidified bromate</b>]]> The oxidation of N-acetylthiourea (ACTU) by acidic bromate has been studied by observing formation of bromine in excess bromate conditions. The reaction displays an induction period before formation of bromine. The stoichiometry of the reaction was determined to be 4:3:4BrO3- + 3(CH3CO)NH(NH2)C=S + 3H2O ―> 4Br- + 3(CH3CO)NH(NH2)C=O + 3SO4(2-) + 6H+ (A) witha complete desulfurization of ACTU to its urea analogue. In excess bromate conditions the stoichiometry was 8:5: 8BrO3- + 5(CH3CO)NH(NH2)C=S + H2O ―> 4Br2 + 5(CH3CO)NH(NH2)C=O + 5SO4(2-) + 2H+ (B). Bromine is derived from an extraneous reaction in which bromide from stoichiometry (A) reacts with excess acidic bromate. The oxidation of ACTU by aqueous bromine gave stoichiometry (C):4Br2(aq) + (CH3CO)NH(NH2)C=S + 5H2O ―> 8Br- + (CH3CO)NH(NH2)C=O + SO4(2-) + 10H+. Reaction (C) is much faster than reactions (A) and (B), with a lower limit bimolecular rate constant of 2.1 X10(5) M-1 s-1 such that appearance of bromine signals complete consumption of ACTU. We were unable to trap any intermediate sulfur oxo-acids of ACTU on its oxidation pathway to N-acetylurea. As opposed to other substituted thioureas, none of its intermediates were stable enough to be isolated and detected. <![CDATA[<b>Lack of Co-crystal Formation with Cyclotriphosphazenes: A Cautionary Tale</b>]]> The attempted formation of co-crystals with a series of cyclotriphosphazene derivatives has been investigated. Despite numerous attempts, only one co-crystal was obtained. The crystal structure of this material, [hexakis(4-pyridyloxy)-cyclotriphos-phazene][terephthalic acid]2y is presented here. The crystal structures of 2,2-bis(4-formylphenoxy)-4,4,6,6-bis[spiro(2',2"-dioxy-1',1"-biphenylyl]cyclo-triphosphazene and hexakis(4-cyanophenoxy)cyclotriphosphazene are also reported for the first time. The extremely low rate of co-crystal occurrence in these materials cannot be explained, despite the consideration of several possibilities. This serves as a cautionary tale - co-crystal formation is not necessarily straightforward. <![CDATA[<b><i>In vitro </i></b><b>investigation of the antimicrobial activity of a series of lipophilic phenols and naphthols</b>]]> Five groups of phenols/naphthols (42 compounds in total) were synthesized and screened against Gram-positive Staphylococcus aureus and Bacillus subtilis, Gram-negative Escherichia coli and Klebsiella pneumoniae, and the fungus Candida albicans. Whereas compounds were found inactive against Gram-negative bacteria, potent activities against Gram-positive bacteria were observed. The activities correlate with the ability of molecules to form quinone methides, suggesting potential new modes of action. <![CDATA[<b>Synthesis, physical and antimicrobial studies of ferrocenyl-<i>N</i>-(pyridinylmethylene)anilines and ferrocenyl-<i>N</i>-(pyridinylmethyl)anilines</b>]]> Ferrocenyl-N-(pyridinylmethylene)anilines Schiff bases were synthesized by reaction of 3- or 4-ferrocenylaniline with either 2-, 3-, or 4-pyridinecarboxaldehyde under solvent-free conditions via mechanochemistry technique. Products were obtained in excellent yields within 10 min of grinding. The reactions afforded a melt or gummy semi-solid that solidified to the desired Schiff bases within a short time. These Schiff bases were reduced to their corresponding amines, ferrocenyl-N-(pyridinylmethyl)ani-lines, with NaBH4 over neutral Al2O3 solid support via grinding. Amines were obtained in excellent yields after intermittent grinding for approximately 1 h. Herein, five novel ferrocenyl-N-(pyridinylmethylene)anilines (compounds 3, 4, 6-8) and six ferrocenyl-N-(pyridinylmethyl)anilines (compounds 9-14) are reported. Compounds were characterized through FT-IR, ¹H-NMR, 13C-NMR, HRMS and SC-XRD techniques. These compounds show visible solvatochromism, when UV-Vis absorption was measured in polar and nonpolar solvents. In changing solvent from polar to non-polar, the Schiff bases exhibited a blue shift while the amines portrayed a red shift. Electrochemical studies on these compounds reveal that redox behaviour of the iron centre is influenced by the position imine or amine groups. Antimicrobial properties of these compounds were studied for Escherichia coli, Staphylococcus aureus, Salmonella typhimirium and Candida albicans. Highest activity was recorded against Gram-positive bacteria and fungi. <![CDATA[<b>Speciation of chromium and vanadium in medicinal plants</b>]]> In this study, Cr(VI) and V(V) were determined in medicinal plants collected from the farm located between two smelters. Chromium(VI) and vanadium(V) were leached from medicinal plants with 0.1 M Na2CO3 prior their determination by ETAAS. The concentration of Cr(VI) in medicinal plants varied between 3.1 ± 0.5 g-1 and 9.4 ± 0.2 μg g-1 and that of V(V) ranged from 1.1 ± 0.08μgg-1 to 17 ± 1.7μ.gg-1. It was established that element species taken up by plants do not depend on metal content in soil but is a function of plant properties. Results of the determination of water-soluble (hot water) Cr(VI) and V(V) concentrations showed that consumption of three cups of tea prepared from medicinal plants from Zizameleni farm is safe since recommended maximum limits is not exceeded through normal consumption of tea infusion. <![CDATA[<b>Organocatalyzed Mannich reactions on minocycline: Towards novel tetracycline antibiotics</b>]]> Herein, we report the development of a mild synthetic route towards novel minocycline derivatives using the proline-catalyzed three-component Mannich reaction. The reaction conditions were optimized and was then screened for its tolerance to the other popular organocatalysts as well as variation of the ketone and aldehyde substrates. The Mannich adducts were evaluated for their in vitro antibacterial efficacies against Gram-negative and Gram-positive bacteria. <![CDATA[<b>Anodized Edge-plane pyrolytic graphite for electroanalysis of pantoprazole in tablet dosage forms and human urine samples</b>]]> Electroanalytical parameters of different graphitic carbon-based electrode materials were compared to select the best one for the pantoprazole electroanalysis. Such parameters include sensitivity, repeatability, residual background current, and signal-to-background current ratio of the analytical response and such electrodes include conventional carbon-based electrodes such as glassy carbon (GC), carbon paste, edge-plane pyrolytic graphite (EPPG), and basal-plane pyrolytic graphite electrodes and film-coated modified GC electrodes with graphitic carbon-based materials such as carbon nanotube, nanographene, carbon black and graphite powder. The EPPG electrode, after applying a simple electrochemical anodization, showed more acceptable analytical performances compared with the other electrodes. Raman spectroscopy was employed to study the surface structural changes that occurred dring the anodic activation. Calibration plot with a good linearity was obtained in the concentration range of 0.2-25 and 0.02-8.5 μΜ, and the detection limit was estimated to be 0.055 and 0.0041 μΜ using cyclic voltammetry and differential pulse voltammetry techniques, respectively. Finally, the electrochemically activated EPPG electrode was used successfully for the determination of pantoprazole in tablet dosage forms and human urine samples with satisfactory results. <![CDATA[<b>QuEChERS method development for bio-monitoring of low molecular weight polycyclic aromatic hydrocarbons in South African carp fish using hplc-fluorescence: An initial assessment</b>]]> Matrix components in the analysis of polycyclic aromatic hydrocarbons (PAHs) in fish require analytical methods with high selectivity and sensitivity. A quick, easy, cheap, effective, rugged and safe (QuEChERS) sample treatment method was optimized and applied, using HPLC-fluorescence for quantification of five low molecular weight PAHs: naphthalene (Nap), acenaphthene (Ace), phenathrene (Phe), fluoranthene (Flu) and pyrene (Pyr) in muscle of carp fish. Important in this study was the testing of different sorbents for clean-up in QuEChERS and comparison with Soxhlet extraction. Polymers and magnetite modified with γ-methacryloxypropyltrimethoxysilane (γ-MPS) were equally as selective towards PAHs as primary secondary amine (PSA) and multiwalled carbon nanotubes were the least selective. The concentration of PAHs in carp fish from Hartebeespoort Dam using QuEChERS extraction method was comparable to Soxhlet extraction and ranged from 0.8-739 μg kg-1. In this study, the minimum concentration of PAHs in carp fish was 0.8 μg kg-1 for fluoranthene and the maximum was 739 μg kg-1 for Ace using QuEChERS method. The study revealed that polymers and magnetite modified with γ-MPS could be used as alternative clean-up sorbent in QuEChERs and that the concentration of PAHs in carp fish in the dam are a source of concern. <![CDATA[<b>Towards a mechanistic understanding of rheological behaviour of water-in-oil emulsion: Roles of nanoparticles, water volume fraction and aging time</b>]]> The viscosity of water-oil emulsions plays an important role in oil production and transportation. The objective of this study was to improve the basic understanding of the influence of nanoparticles on the viscosity of water-in-oil emulsions. Using crude oil and different industrial nanomaterials, the droplet size distribution, droplet mean size, and rheological models of emulsions were investigated. Experimental results show that the addition of nanoparticles increases the crude oil viscosity; however, the Newtonian flow behaviour of oil is not affected by nanoparticles. It is observed that the viscosity of crude oil increased from 36.5 to 49 cP when the nanoparticle concentration was elevated from 0 to 0.1 wt%. From the results of rheological experiments, it can be concluded that the influence of nanoparticles on the emulsion viscosity is mainly affected by the type and amount of nanoparticles, water/oil-ratio and aging time. Mean droplet diameter decreased from 5.68 to 4.11 micrometre when 0.1 wt% nanoparticles were added to emulsion. The results also suggest that the properties of stabilized water-in-oil emulsions are significantly time-dependent, and the droplet size and viscosity of emulsions is reduced by time. Most of previously published correlations have huge errors and could not precisely predict the apparent viscosities of non-solid stabilized and solid-stabilized emulsions. None of the previously utilized equations did ever consider the effect of added solids to the emulsion. <![CDATA[<b>A review of using spray pyrolysis through sol-gel materials in the synthesis of cathode materials for lithium-ion batteries</b>]]> Over the years, new synthesis routes of the cathode electrochemical active material for lithium-ion batteries have improved remarkably to optimize their capacity and cycle life performance. This review study focused on the use of some techniques to synthesize the common cathode materials (LiCoO2, LiMn2O4, LiFePO4). The most common and simplest synthesis method was the mixing of powders in their solid-state form and heating them at relatively high temperatures over long periods. Other methods included the formation of sol-gel products that could be either heat-treated more or could be used directly by means of a spray pyrolysis method producing the desired active material. The spray pyrolysis method showed that suitable cathode oxide materials formed in shorter periods, resulting in small homogenous particles with narrow particle size distribution. The spray pyrolysis method allowed for making doped or coated cathode materials easily of the various base forms LiCoO2, LiMn2O4 and LiFePO4, with doping elements such as Zr, Mn, Ni, Co, B or Mg. Coating of the particles could also be done with materials such as glassy lithium boride oxide, TiO2 or carbon. These additives to the cathode material improved the active material's physical morphology and electrochemical properties.