Preparação, caracterização e avaliação de catalisadores heterogeneos a base de Pt na hidrogenação enantiosseletiva do piruvato de metila em presença de cinconidina

AUTOR(ES)
DATA DE PUBLICAÇÃO

2000

RESUMO

Platinum based catalysts were prepared by impregnation on different supports, namely C, Al303, TiO2 and La203. The commercial activated carbon was oxidized by different methods in order to modify its surface. The main surface groups identified were carboxylic acid, when HN03 is the oxidizing agent, and carbonyl, phenol and quinone, when the oxidation is made with O2. The chemical nature of these functional groups determined the acidic-basic character of the support and revealed a strong influence on the [PtCl6]-2 adsorption process. The formation of a complex between the metal ions and the oxygen from the non-protonated surface groups is suggested. The oxygenated functional groups showed a negative effect on dispersion. For the catalyst Pt/La203, it was observed, through XRD and XPS, that it is a crystalline compound after reduction at 723 K. This deep contact between the metal and the support is attributed to the support dissolution due to the acidic solution of the precursor during impregnation. However, the active sites nature in this catalyst did not seem suitable to enantioselective hydrogenation. The catalyst Pt/TiO2 is highlighted due to its promising enantioselectivity,60%, and high activity. This performance is attributed to the interaction between partially reduced species TiOx and the a-carbonyl group of the substrate. The modifier showed to act as an agent responsible for forming a complex with the substrate in a configuration suitable for the formation of a specific enantiomer. Due to the interaction TiOx-carbonyl, the mechanism whereby the substrate: modifier complex is formed does not seem crucial to the enantiodifferentiation. The use of Pt/C catalysts revealed that the oxygen concentration on the support affects the catalytic activity. This effect can be related to the system hydrophobicity and the carbonyl activation via hydrogen bond between the oxygen atom and the hydrogen chemisorbed on the support. On the other hand, the existence of the oxygenated groups does not seem to influence the interaction between the modifier and the substrate with the catalyst surface.The low enantioselectivity of these systems is attributed to the carbon microporosity

ASSUNTO(S)

alcaloides enantiomeros hidrogenação catalisadores de platina catalise heterogenea

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