Mecanismes de reacció de sistemes catalítics amb èmfasi en carbens de Fischer i quantificació de l'aromaticitat en petites molècules inorgàniques cícliques i planes.

El tema principal del treball és sobre l'estudi teòric de l'estructura i reactivitat en carbens de Fischer de la forma (CO)5Cr=C(X)R (X= OH, NH2, OMe, NMe2 i R= CHCH2 i Ph). Particularment, el nostre interès va sorgir del tipus de reaccions de cicloadicció que donen lloc a la síntesi de productes naturals i fàrmacs de gran valor afegit. Hem estudiat els mecanismes de reacció dels casos més comuns de cicloanul•lació: la reacció de benzanul•lació de Dötz i ciclopentanulacions que es troben en competència amb el primer cas, derivats de l'inserció de acetilè i fenilacetilè. En l'últim pas de les reaccions que comporten la formació d'un sistema de més d'un anell, hi tenim una migració del complex metàl•lic de crom d'un extrem a un altre anomenat com rearranjament haptotròpic. Aleshores, hem investigat sobre els mecanismes de migració haptotròpica de Cr(CO)3 sobre hidrocarburs aromàtics policíclics analitzant l'efecte de la mida i la curvatura del sistema així com la complexació d'un segon fragment metàl•lic a la manera de coordinació als anells. D'una altre banda, vam estudiar l'aromaticitat en 54 cúmuls cíclics de molecules inorgàniques mitjançant metodologia desenvolupada al nostre grup de recerca i altres. Vam proposar la tècnica del scan-NICS com nova mesura quantitativa d'aromaticitat i reportar l'escassa correlació entre els distints índexs d'aromaticitat a la literatura. Finalment, com resultat de col•laboracions en estades de recerca, he desenvolupat propostes de mecanismes de reacció en sistemes catalítics de isonitrils i fosfinetà-amides i en dímers de gassos nobles.

Els manlleus en la revista “Time Out Barcelona”

El present treball constitueix un estudi i una anàlisi sobre les dades obtingudes del buidatge dels manlleus recollits en dos números de la revista catalana Time Out Barcelona. L’objectiu és aprofundir en el tema dels préstecs i extreure algunes conclusions sobre els tipus d’unitats manllevades i el seu tractament.

Contaminação por crômio de águas de rios proveniente de curtumes em Minas Gerais

In order to evaluate the chromium contamination from tannery discharges into rivers in the State of Minas Gerais, samples of water and suspended material were collected and submitted to chemical analysis. The total content of chromium in the samples was measured by flame atomic absorption spectrophotometry. Water samples were analysed by standard addition method, while chromium concentration in suspended materials was determined by calibration curves. Localities investigated were Ipatinga, Matias Barbosa, Dores de Campo, Ressaquinha, Ubá and Juiz de Fora. Samples from a not-industrialized area were also analysed to obtain regional background values. Metal inputs were related to effluent discharges into the rivers. Suspended material transported Cr downriver. Chromium concentration in river water exceeded 656 times the value of the Brazilian Environmental Standards, while its concentration in suspended material ranged from 15 to 11066 µg g-1.

Coatings on Composites

Polymer based composite materials coated with thin layers of wear resistant materials have been proposed as replacements for steel components for certain applications with the advantage of reduced mass. Magnetron sputtered coatings can be successfully deposited on composite materials. Nevertheless there are number of issues which must be addressed such as limited temperature, which the composite can withstand because of the epoxy binder which is used, the adhesion of the coating to the composite and the limited mechanical support, the hard coating can obtain from the relatively soft epoxy. We have investigated the deposition of chromium nitride, titanium carbide and titanium doped DLC coatings on carbon fibre reinforced composites and various polymers. The adhesion of the coatings has been studied by the pull-off adhesion tester. In general, the failure mechanism has been noticed to be due to the cohesive failure for a wide range of conditions. The wear behavior of the coatings has been noticed to be complicated. Wear tests on coated composites have shown that where the reinforcing fibres are near the surface, the composite samples do not perform well due to breakage of the fibres from the polymer matrix. A fibre free surface has been noticed to improve the wear resistance.

Uso de irradiação de microondas na determinação espectrofotométrica de cromo com EDTA

Cr(III) slowly forms a violet complex with EDTA at pH 2.5-5.0 under normal conditions. The complex formation can be accelerated by irradiating the reacting mixture with microwave energy. The complex Cr(III)-EDTA is completely formed within 3 minutes of microwave irradiation and followed by a simple and rapid method for spectrophotometric determination of chromium. The method may be successfully applied to determination of chromium in catalyst.

Determinação direta de crômio em açúcar e leite por espectrometria de absorção atômica com atomização eletrotérmica em forno de grafite

A fast and direct method for the determination of Cr in milk and cane sugar suspensions using graphite furnace atomic absorption spectrometry with Zeeman-effect background correction is described. No sample pre-treatment was necessary, minimizing the risk of contamination. The concentration of chromium in cane sugar was evaluated using Cr reference solutions prepared in 1% v/v HNO3 solution. The milk samples were introduced into the furnace with a mixture of amines for avoiding the autosampler blockage and foaming of milk. Chromium determination in milk was based on the standard additions method (SAM). The limit of detection and characteristic mass for cane sugar sample (30 muL) were 0,13 ng/ml and 4,3 pg, and for milk sample (10 muL) were 0,23 ng/ml and 7,8 pg, respectively. The graphite tube lifetime was 300 firings for sugar-cane sample and 100 firings for milk sample. The heating program was implemented in 68 s.

Construção e aplicação de uma cela espectrofotométrica de camada delgada para análises em fluxo

A low cost spectrophotometric cell for use in flow analysis was manufactured in acrylic and adapted to a commercial spectrophotometer. The application of this cell was performed in the determination of chromium (VI) in steel samples using the reaction with the alkaloid brucine in presence of oxalic acid and 0.6 mol L-1 sulfuric acid. The cell allows an enlarged analytical range, diminishing the extension of dilutions, which is useful for on-line monitoring of industrial processes.

O impacto do crômio nos sistemas biológicos

The impact of biological chromium's activity may be beneficial or not. This review presents the most relevant chemical aspects of these "two faces" of chromium by covering first, the efforts toward a clearer understanding of the carcinogenic properties of chromium compounds. The biomimetic chemistry of Cr(V) complexes illustrates the interactions of the intermediates formed by Cr(VI) reduction with DNA or oligonucleotides. The importance of trivalent chromium as an essential element is also emphasized by summarizing the recent results of the investigations on trinuclear chromium complexes as models of the cofactor responsible for the insulin activity, as well as good candidates for new nutritional supplements.

Remoção de metais pesados de efluentes aquosos pela zeólita natural escolecita - influência da temperatura e do pH na adsorção em sistemas monoelementares

Cation exchange capabilities of a Brazilian natural zeolite, identified as scolecite, were evaluated for application in wastewater control. We investigated the process of sorption of chromium(III), nickel(II), cadmium(II) and manganese(II) in synthetic aqueous effluents, including adsorption isotherms of single-metal solutions. The natural zeolite showed the ability to take up the tested heavy metals in the order Cr(III) > Cd(II) > Ni(II) > Mn(II), and this could be related to the valence and the hydration radius of the metal cations. The influence of temperature (25, 40 and 60 ºC) and initial pH value (from 4 to 6) was also evaluated. It was found that the adsorption increased substantially when the temperature was raised to 60 ºC and that maximum adsorption capacity was observed at pH 6. These results demonstrate that scolecite can be used for removal of heavy metals from aqueous effluents, under optimized conditions.

Efeito do cromo nas propriedades catalíticas da MCM-41

The effect of chromium on the catalytic properties of MCM-41 was evaluated in order to develop new catalysts for the trimethylbenzene transalkylation with benzene to produce ethylbenzene, a high-value aromatic in the industry. It was found that chromium decreases the specific surface area but increases the acidity, turning MCM-41 into an active and selective catalyst for ethylbenzene and toluene production. The coke produced on the catalyst is hydrogenated and mainly located outside the pores and thus can be easily removed. The catalyst is more active and selective than mordenite, a commercial catalyst, and thus more promising for commercial applications.

Caracterização por XPS de filmes passivos formados sobre aços de baixa liga em meio de bicarbonato

Passive films formed in bicarbonate solutions on carbon steel, chromium steel and high speed steel have been characterized by XPS. The passive films formed on chromium and high speed steels showed superior protective properties than those formed on carbon steel. It was confirmed by XPS that the steel composition influences the passive film composition. Chromium oxide and hydroxide, as well as molybdenum and tungsten oxides and hydroxides are present in the passive film of chromium steel and high speed steel, respectively, besides iron oxide and hydroxide. The more complex composition of the oxide film on high speed steel explains its electrochemical behaviour and highest corrosion resistance.

Resíduos sólidos de curtumes como adsorventes para a remoção de corantes em meio aquoso

The tanning process in the leather industry generates very high quantities of chromium-containing solid waste ("wet blue" leather). Environmental concerns and escalating landfill-costs are becoming increasingly serious problems for the leather industry and an alternative disposal is needed. In this work, we are presenting a novel application for this solid waste, which is the removal of organic contaminants from aqueous-solution. The adsorption isotherm of "wet blue" leather waste from the AUREA tanning company in Erechim-RS (Brazil) showed that this material presents high adsorption capacities of the reactive textile dyes.

Distribuição e biodisponibilidade de crômio em solos contaminados por resíduos de couro

Samples of soil, water and sediment were collected and analyzed in order to evaluate chromium contamination due to deposition of tannery residues onto soils under different management regimes. The results showed that soils used for sugar cane cultivation were not adversely impacted. However, in the case of mango plantations, variable concentrations of chromium were measured in the soil profile, with 22.2% of values being higher than permitted legal limits, and 38.9% being at levels requiring remediation. Concentrations of bioavailable chromium were lower than the detection limit of the method (0.01 mg of chromium kg-1 of soil), indicating that all of the chromium present in the samples was either complexed or in an insoluble form. Chromium concentrations measured in samples of water and sediments were indicative of low mobility of the metal in soils. The main cause of differences found between soil samples obtained from different cultivations was the type of soil management.

Estudio preliminar sobre la determinación de elementos traza en cervezas venezolanas por ICP-OES

The levels of several chemical elements were determined simultaneously in ten different beers of the Venezuelan market by ICP-OES. With the exception of chromium, beer does not provide important amounts of nutritional oligoelements. The average chromium content found (33 µg/L) is higher than one reported for USA's beer (10 µg/L) but smaller than another reported for Brazilian beer (66 µg/L). The average percentage of chromium RDA by beer consumption found for Venezuela (6.3%) is similar to that reported for Belgium (5%). Regarding toxic elements, one of the samples contained an elevated amount of aluminium, probably dissolved from the aluminium can.

Pré-concentração de Cr(III) e análise de especiação de cromo empregando montmorilonita saturada com íons potássio

The clay mineral montmorillonite-K10 (MT), treated under acidic medium and saturated with potassium ions (MTK), was employed in sorption and desorption studies aiming the preconcentration of Cr(III) and the speciation analysis of chromium. The sorption process of Cr(III) was close to 100%, suggesting that MTK was a good material for Cr(III) preconcentration, although, the maximum recovery in HNO3 solution was near 89%. On the other hand, Cr(VI) practically was not retained in MTK, suggesting this material as an appropriate mineral phase to be used in speciation analysis of chromium in aqueous medium.