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Les Hydroxyapatites, un système basique atypique modulable par la synthèse : vers l'identification des sites actifs

Abstract : Hydroxyapatites (HAp) are basic materials whose atypical catalytic properties, in particular their high selectivity in n-butanol from ethanol's conversion, are yet to be rationalised. Modulation of their synthesis parameters as well as their ability toward substitution allowed us to use their versatility in terms of composition, stoechiometry and morphology in order to highlight the relationships existing between structure and acid-base properties. Indeed, at a macroscopic level, stoechiometry - represented by the bulk Ca/P ratio- and morphology are the two key parameters controlling the basicity of those materials. Even if the difference of composition between the bulk and the surface is yet to be made, the decrease in the number of OH groups, whether related to under-stoechiometry or to the occurrence of a mix between Hap and another calcium phosphate OH-deprived phase, leads to a fall-off in basic reactivity. This points toward a central part of these species at a molecular level. To identify the nature of the superficial sites involved in the catalytic process, we implemented a procedure, notably though isotopic marking , so as to discriminate the IR and NMR spectroscopic signatures of the bulk and surface OH and PO-H groups. Then, the implication of those surperfical species was investigated by the adsorption of acidic probe molecules such as CO2, acetylene, propyne and basic probe molecules: CO and pyridine. If CO2 leads to a surface and bulk carbonation of the material, surface OH and PO-H groups do interact with protic probes. On the opposite, those adsorptions highlighted that on the one hand cation are not always accessible on the surface and on the other hand PO4 groups do not seem to interact with protic probe molecules. Those investigations led us to propose the implication of an atypical OH/PO-H pair which, associated with the important proton mobility existing in HAp, could explain the odd propensity of HAp to selectively form n-butanol from ethanol in the Guerbet reaction.
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Sarah Diallo Garcia. Les Hydroxyapatites, un système basique atypique modulable par la synthèse : vers l'identification des sites actifs. Science des matériaux [cond-mat.mtrl-sci]. Université Pierre et Marie Curie - Paris VI, 2012. Français. ⟨tel-00828591⟩

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