Zerovalent palladium and rhodium catalysts have been dispersed in conducting anion-exchange polymers coated on carbon-felt electrodes by oxidative electropolymerization of pyrrole monomers containing alkylammonium or pyridinium groups. Incorporation of metal catalysts in polymeric films was effected either by impregnation of PdCIz-or RhCIi-complexes followed by an electroreductive precipitation to PdO or Rho (method A), or by electroreduction of polymer-coated electrodes immersed in K,PdCI, or Na,RhCI, aqueous solutions (method B). A study of the electrocatalytic activity in hydrogenation of isophorone of various electrodes differing in loading level and incorporation method of metal catalysts was performed. Their activity increased with increasing amounts of metal in the polymer. The most efficient electrodes were obtained by incorporation (method A) of rhodium in poly(pyrro1e-alkylammonium) films. Scanning electron microscopy (SEM) showed a dispersion of metal particles from 100-200 nm (method A) up to 2000 nm (method B) on the polymer surface and a large accumulation of metal at the carbon substrate/polymer interface. These results were confirmed by Auger electron spectroscopy (AES) depth profile analysis, which also showed that metal catalyst is dispersed throughout the polymer overlayer. Differences in catalytic activities of method A and method B cathodes could be due to a difference in metal particle sizes.
Recebido em 13/7/01; aceito em 28/10/01 SYNTHETIC METHODS FOR PREPARATION OF SUBSTITUTED 2,2'-BIPYRIDINES. The 2,2'-bipyridine has been entitled as the most widely used ligand. Nowadays there is a large variety of known molecules comprising at least two 2,2'-bipyridine units and the number of applications in many areas such as catalysis, new materials, optoeletronics and electrochemistry have increased very much in the past decades. Nevertheless, there is no article that gives an overview of the main synthetic methods for obtaining the substituted 2,2'-bipyridines, generally non available. This article presents a synthetic discussion about the three different methods (coupling reaction, ciclo-functionalization and functionalization of the heteroaromatic rings of 2,2'-bipyridine) for preparing these heterocyclic compounds and also provides a practical and fundamental guide, for obtaining more than eighty different symmetric and unsymmetrical substituted 2,2'-bipyridines, shown in a table with the corresponding references.
Recebido em 13/2/02; aceito em 17/4/02 CHIRAL ALCOHOLS: CHEMICAL AND CATALYTIC METHODS OF PREPARATION BY ASYMMETRIC REDUCTION. The importance of chiral alcohols as starting materials for the production of fine chemicals and as useful chirons for the building of several interesting molecules or natural products is reported. The useful and common methods of asymmetric reduction such as the chemical (with organoboron or organoaluminum reagents) and the catalytic ones (with ruthenium or rhodium complexes) for preparation of chiral alcohols are described; even the newer and much more rare electrocatalytic methods are reported.
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