Thèse de doctorat en Chimie appliquée
Sous la direction de Yannick Pouilloux.
Soutenue en 2002
à Poitiers , en partenariat avec Université de Poitiers. UFR des sciences fondamentales et appliquées (autre partenaire) .
Control of the chemioselectivity of the CoSn or RuSn catalysts in the reduction of methyl oleate into insaturated alcohols
Unsaturated alcohols are important materials for the manufacture of heavy-duty liquid detergents, cosmetics, pharmaceuticals and toiletries. One of the synthesis methods is the hydrogenolysis of unsaturated fatty methyl esters (methyl oleate ) which requires the control of the chemioselectivity. The study of new CoSn supported over zinc oxide catalysts leads to the determination of the active species (CoSn2) in the reduction of methyl oleate in unsaturated alcohols. In order to increase their dispersion, several factors were studied such as the pre-treatment temperature, the effect of cobalt and tin content, and the effect of the precursors as well as the preparation method. These studies have shown that the prereduction temperature has no significant effect on the CoSn/ZnO catalysts, whereas the cobalt and tin content have a strong effect on the catalytic properties. It seems that, the most the active centers are well dispersed and with a small size, the most the selectivity in unsaturated alcohol is high. The precursors used are also important and a catalyst prepared from cobalt carbonyl leads to the formation of 66% of unsaturated alcohols after only 21 hours of reaction. This result is similar to the one obtained in the presence of RuSn supported over alumina catalysts. On the other hand, the deposition in gas phase did not allow to increase the catalytic properties of CoSn supported catalysts because of a low metal loading. Two secondary reactions can occur during the reduction of methyl oleate: the transesterification between the ester and the alcohol formed and the cis/trans isomerization of methyl oleate in methyl elaidate. It appears that the transesterification reaction occur on the isolated tin oxides species whereas the isomerization reaction occur on the cobalt species (especially cobalt oxides). Moreover, tin species inhibit the cis/trans isomerization by limiting the hydrogen activation. The study of the RuSn catalysts has shown that the coimpregnation in acid media leads to the formation of Ru3Sn7 species which seem to be the active centers in the reduction of methyl oleate in unsaturated alcohols in the presence of RuSn solids. Nevertheless, the use of mesoporous supports did not reduce the formation of heavy esters (products from the transesterification reaction) as it was expected.