Oxidação de hidrocarbonetos com peroxido de hidrogenio catalisada por renio em fase homogenea e heterogenea

AUTOR(ES)
DATA DE PUBLICAÇÃO

1999

RESUMO

Nowadays, there has been an increasing number of studies on the oxidation of organic compounds such as olefins, phenols, anisols, alcohols and alkanes catalyzed by rhenium complexes. In the first part of this work, the oxidation of hydrocarbons with anhydrous hydrogen peroxide, catalyzed by CH3ReO3, using acetonitrile as a solvent at 80°C was studied. In the oxidation of aromatic compounds, such as benzene and toluene, the main products (phenols and cresols) were formed by ring hydroxilation. In the oxidation of cyclohexane, the principal product was cyclohexylhydroperoxide, followed by cyclohexanol and cyclohexanone. The addition of 2-pyrazinecarboxylic acid (PCA) as a co-catalyst increased the activity of the system, probably due to the formation of a chelated complex with the PCA bonded to the rhenium through N and O atoms. The addition of water, or the use of t-butylhydroperoxide as an oxidant, drastically decreased the yield of products. The oxidation of cis- and trans-decaline and the selectivity for the oxidation at different carbons, in their structures was also studied. The ratio of oxidation products for the tertiary to the secondary carbons was 26 and 5, respectively for cis- and trans-decaline. These high selectivities are similar to those obtained with peroxo type complexes. The oxidation of n-heptane also showed high selectivities for secondary carbons. Taking into account these results, a mechanism based on peroxo complexes was proposed. In the second part of this work, the epoxidation of olefins catalyzed by O3Re-O-AI2O3 and O3Re-O-SiO2 AI2O3 was studied. The effect of the Re loading (3 to 18 wt. %) in the catalysts and of the Al2O3 loading (0 to 100%) in the support, on the activity of these systems was evaluated. The stability of the catalysts with respect to the leaching was also studied. The reactions were carried out using anhydrous hydrogen peroxide and ethyl acetate as a solvent at 80°C. In the epoxidation of cyclooctene catalyzed by O3Re-O-AI2O3 (12 wt.%ReO4), a conversion of 13% and a selectivity of 99% for the epoxide was obtained after 60 min, giving a turnover number of 68. This catalyst proved to be stable towards leaching and was recycled twice without any loss in the activity .When cyclohexene was used as the substrate a higher conversion (35%) was obtained after 30 min, but the selectivity decreased due to the lower stability of the epoxide formed. The addition of bases, such as pyridine, increased the selectivity of the systems for the epoxides, but decreased ther activity. When supports such as SiO2-Al2O3 were used, leaching of the active species was observed, as Re binds stronger to the AI than to the Si. Catalysts with Re loading higher than 12 wt. % were not stable towards leaching.

ASSUNTO(S)

renio catalisadores oxidação

ACESSO AO ARTIGO

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