Fonctionnalisation de matériaux carbonés comme supports de métaux nobles pour la catalyse hétérogène en phase liquide

Carbon materials are used extensively as supports in heterogeneous catalysis, even if they are relatively inert when compared to the inorganic oxides used commonly. These materials are cheap, have a high specific surface area and can withstand high temperature and high moisture degree. However, carbonaceous materials are not well understood due to the difficulty in characterising them spectroscopically. In consequence, the preparation of carbon-based materials still relies on trials and errors. This study aims at showing that well-characterized catalysts, based on a carbon support, can be synthesized in a controlled manner. A step-by-step methodology was developped in order to improve the interaction between the metallic precursors and the support. The first step consists in modifying the carbon surface by adding oxygenated functional groups. These functions are then used as anchoring sites for the grafting of coordination compounds. It is clear that one of the challenges of this work stands in the characterization of the carbonaceous samples at each step of the synthesis. The first part of this work consisted in developping a method to functionalize the carbon supports, in order to obtain a set of supports with different surface properties. A detailled characterization of these supports – before and after the functionalization – was carried out. Two reactants were envisaged : nitric acid and hydrogen peroxide. It was shown that those reactants are capable of oxidazing the support. As the intensity of oxidation depends on the experimental conditions, supports with different or native amount of oxygenated functions were obtained. Some of these functions were removed selectively by a thermal treatment, to give thermally stable supports. In a second part, the grafting of a Pd complex onto these modified carbon supports was undertaken. The grafting mecanism was studied by XPS and it was shown to consist in ligands exchange. A tool was developped to assess the amount of grafted complex: it was suggested that the grafted amount is related to the proportion of PdII in the sample. The functionalization of the support preceeding Pd incorporation was proved to be essential. Indeed a stabilization of the complex on the functionalized supports was observed and it was shown to be linked to the surface acidity. However, this stabilization hinders the reduction during the activation process. Despite this fact, the obtained catalysts give rise to good catalytic performance in the hydrogenation of 2-méthyl-2-nitropropane into t-butylamine. The activity increases with the extend of functionalization. This is due to a decrease in particles sizes with the initial support acidity. However, the use of a support functionalized under harsh conditions is not beneficial, which can be attributed to a negative effect of the oxygenated functions on the catalytic activity. The last part consisted in reproducing the same grafting method for other metallic complexes, in order to synthesize bimetallic catalysts. In this way, the incorporation of Bi and Ru onto carbon supports was studied and seemed to be easily carried out. Bimetallic Pd-Bi/C and Pd-Ru/C were thus prepared using different methodologies (simultaneous, consecutive grafting or a method combining deposition and grafting). The obtained Pd-Bi/C catalysts were tested in the oxidation of glucose into gluconic acid but no improvement in catalytic performance or in terms of Bi losses in solution was obtained. The advantage of using a grafting process compared to the deposition method previously used was an increase of homogeneity in the samples. Small particles well dispersed on the support were, indeed, observed by TEM. The functionalization of the support allows to avoid agglomeration. Pd-Ru/C catalysts showed variable particles sizes in function of the degree of functionalization and the grafting pH. These catalysts were tested in the hydrogenation of MNP into TBA. It was shown that all the catalysts were very active in this reaction and that their activities may be linked to the initial acidity of the support but also, in a more pronounced manner, to the grafting pH.Ce travail s'inscrit dans le cadre de la préparation de catalyseurs supportés sur carbone. L'objectif est de mettre au point une méthodologie permettant un greffage réel de complexes métalliques sur le carbone, et de le prouver. Le greffage se définit au sens strict par la création d'un lien covalent entre les fonctions de surface du support et un complexe métallique. Une fonctionnalisation préalable de la surface du support carboné est cependant indispensable, pour introduire des groupes fonctionnels oxygénés qui serviront de sites pour le greffage de composés de coordination. La première partie du travail a consisté à mettre en oeuvre une méthode de fonctionnalisation des supports carbonés. En jouant sur les conditions opératoires, une série de supports présentant des caractéristiques de surface différentes ont pu être obtenus et les fonctions de surface caractérisées de manière qualitative et quantitative. Dans une seconde partie, la possibilité de greffer un complexe de Pd sur ces supports a été démontrée. L'étude du mécanisme de greffage a montré que celui-ci consiste en un échange de ligands. Après activation, les catalyseurs Pd/C obtenus se sont révélés actifs et présentent de fines particules de Pd bien dispersées. Les paramètres de synthèse de ces catalyseurs ont une influence sur les caractéristiques et l’activité obtenues. La méthode de synthèse a ensuite été reproduite pour d'autres complexes afin d'obtenir des catalyseurs bimétalliques Pd-Bi/C et Pd-Ru/C. Bien que le greffage du Bi et du Ru n'ait pu être clairement démontré, plusieurs indices laissent penser qu'il a bien lieu. De plus, la fonctionnalisation du support carboné préalable au greffage s'est avérée essentielle à l'obtention de catalyseurs présentant une répartition homogène des particules. Dans le cas Pd-Bi, celles-ci sont bimétalliques, tandis que dans le cas Pd-Ru, les deux métaux sont ségrégés à la surface.(CHIM 3) -- UCL, 201