2 resultados para Micro-arc oxidation

em Biblioteca Digital da Produção Intelectual da Universidade de São Paulo (BDPI/USP)


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This work reports on a distinct experimental procedure conceived to closely approach the question of development of crystallization in lead oxyfluoroborate glasses in the presence of an electric field. After proposing earlier that this phenomenon should involve occurrence of redox-type electrochemical reactions occurring at the electrodes. it was in fact recently shown that a direct contact of the glasses with both the cathode and anode revealed essential, provided that crystallization did not develop when ions migration to these electrodes became frustrated. The present study demonstrates that. even in Pt,Ag/Glass/YSZ:PbF(2)/Ag,Pt-type electrochemical cells subjected to electric field action, where YSZ:PbF(2) represents composite-like mixtures (formed by Y(2)O(3)-doped ZrO(2) and PbF(2)) placed between the glass and anode. crystallization was observable in given cases. In summary, supported by (micro)structural and electrical characterizations, clear evidence is provided here that, besides Pb(2+) reduction at the cathode, crystallization really involves simultaneous F(-) oxidation at the anode, completing thus the whole redox electrochemical reaction so far postulated. In these cases, F(-) migration to the anode was achievable following PbF(2) percolative-like paths through the YSZ:PbF(2) mixtures. (C) 2010 Elsevier B.V. All rights reserved.

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The use of whole cells of micro-organisms to bring about the biotransformation of an organic compound offers a number of advantages, but problems caused by enzymatic Promiscuity may be encountered upon With Substrates hearing more than one functional group. A one-pot screening method, in which whole fungal cells were incubated with a Mixture of 4-rnethylcyclohexanone I and phenyl methyl Sulfide 2, has been employed to determine the chemoselectivity of various biocatalysts. The hyphomycetes, Aspergillus terreus CCT 3320 and A. terreus URM 3571, catalysed the oxidation of 2 accompanied by the reduction of I to 4-methylcyclohexanol 1a and, for strain A. terreus CCT 3320, the Baeyer-Villiger oxidation of 1. The Basidomycetes, Trametes versicolor CCB 202, Pycnoporus sanguineus CCB 501 and Trichaptum byssogenum CCB 203, catalysed the oxidation of 2 and the reduction 1, but no Baeyer-Villiger reaction products were detected. In contrast. Trametes rigida CCB 285 catalysed the biotransformation of 1 to 1a, exclusively, in the absence of any detectable Sulfide oxidation reactions. The chemoselective reduction Of (+/-)-2-(phenylthio)cyclohexanone 3 by T. rigida CCB 285 afforded exclusively the (+)-cis-(1R,2S) and (+)-trans-(1S,2S) diastereoisomers of 2-(phenylthio)cyclohexan-1-ol 3a in moderate yields (13% and 27%, respectively) and high enantiomeric excesses (>98%). Chemoselective screening for the reduction of a ketone and/or the oxidation Of a Sulfide group in one pot by whole cells of micro-organisms represents an attractive technique with applications in the development of synthesis of complex molecule hearing different functional groups. (C) 2008 Published by Elsevier Ltd.