Etude des paramètres influençant la réactivité de bases inorganiques faibles (application à la catalyse hétérogène de transestérification)

Les enjeux actuels pour la valorisation de dérivés de la biomasse donnent à la catalyse hétérogène basique un intérêt nouveau. Dans la mesure où des relations complexes peuvent exister entre la performance d un catalyseur hétérogène basique et sa force déprotonante, une étude des critères qui influencent la réactivité des solides dans des réactions modèles de basicité et dans une réaction modèle de transestérification a été entreprise. Il apparaît que les réactions faisant intervenir des réactifs faiblement acides (dans notre cas, un alcène), ne peuvent être réalisées que sur des sites qui peuvent interagir fortement avec les acides de Brønsted. Ces sites ne peuvent pas être caractérisés par la force de leur interaction avec une sonde CO2, car nous avons montré que les échelles de basicité selon Lewis et selon Brønsted étaient différentes. En revanche, dans le cas de réactions faisant intervenir des acides plus forts, tels des alcools, plusieurs facteurs sont à considérer. Par exemple, dans la catalyse de la transestérification, la présence de sites pouvant réagir selon une voie acide peut augmenter la puissance catalytique des solides. Par ailleurs, cette étude a permis de dégager des tendances générales sur la structure et la réactivité basique de l oxyde de magnésium en fonction de son recouvrement en adsorbats hydroxyles et carbonates. Quatre types de carbonates adsorbés ont pu être identifiés, ce qui a permis de mettre en évidence la formation de plans (110) après certains prétraitements. Ces plans, très réactifs, sont particulièrement intéressants pour la catalyse hétérogène basique.The current challenges for the valorization of biomass derived products contribute to a growing interest for basic heterogeneous catalysis. Because of the complex connection between the catalytic performance of a basic heterogeneous catalyst and its deprotonating ability, a study of the parameters governing the reactivity of solids in model reactions of basicity and transesterification was undertaken. A reaction with slightly acidic reactants (in our case: an alkene) seems to be catalyzed only on active sites which can form strong interactions with Brønsted acids. These sites cannot be characterized by the interaction with CO2 as a basic probe, because we have found that the Lewis basicity scale and the Brønsted basicity scale are different. On the other hand, several parameters should be considered for reaction with stronger acids such as alcohols. For instance, in the catalysis of transesterification, active sites with an acidic behavior increase the catalytic performance of the solids. Furthermore, this study permitted us to reveal some general trends for the structure and the basic activity of magnesium oxide covered with OH group and carbonates. Four different kinds of adsorbed carbonates were identified, which led us to bring to light the formation of (110) planes after some pretreatments. These planes are very promising for the basic heterogeneous catalysis due to their enhanced activity.PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Influence of the Metallic Precursor and of the Catalytic Reaction on the Activity and Evolution of Pt(Cl)/δ-Al2O3 Catalysts in the Total Oxidation of Methane

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Etude de la réactivité basique de l'oxyde de Zinc

L objectif de ce travail est d identifier les paramètres influençant la réactivité basique de l oxyde de zinc. Les lacunes d oxygène de deux types de ZnO, kadox et ex-carbonate, ont été caractérisées par photoluminescence et Résonnance Paramagnétique Electronique (RPE). De plus, leurs adsorbats hydroxyles et carbonates ont été suivis par spectroscopies infra-rouge. L influence de l atmosphère de prétraitement à 500C sur les conditions de formation et de comblement des lacunes d oxygène sous atmosphères inerte et oxygénées respectivement ainsi que sur les adsorbats ont été étudiées. Si les hydroxyles et carbonates de surface n expliquent pas l influence des traitements sur la réactivité basique (conversion du MBOH), une corrélation a été établie entre lacunes d oxygène et niveau de conversion. L existence de deux types de sites est proposée : - le site Blacune présent sur les deux échantillons dont la basicité est exaltée par la présence de lacunes d oxygène ; - le site Bfaible uniquement présent sur le kadox qui est responsable de son activité résiduelle en absence de lacunes d oxygène. L effet relatif à la nature de l atmosphère de traitement est moindre sur la conversion de l éthanol du fait de la température supérieure de la réaction qui ne permet pas la même efficacité du comblement des lacunes par O2. Ces traitements n influencent pas la sélectivité en acétaldéhyde (62 %), éthylène (18 %) et produits lourds (7 %).PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Lewis acido-basic interactions between CO2 and MgO surface: DFT and DRIFT approaches

International audienceCombined experimental infrared (IR) and theoretical approaches have been carried out in an attempt to specify the actual structure of the CO2 species adsorbed on the magnesium oxide surface. The interaction of CO2 with regular sites of the MgO(100), (111) and (110) surfaces as well as MgO(100) defect sites (steps, corners, kinks and di-vacancies) has been investigated by mean of Density Functional Theory study. Theoretical IR frequencies compared with IR experiments show distinguishable carbonate species, adsorbed on different planes and defects, vibrating in different IR-frequency ranges. In addition, by mean of thermodynamic model, the stability of carbonates as a function of temperature have been calculated and compared to the experiment. Analyzing the nature of basic sites, the results show that the most active site versus CO2, which is a Lewis acid, is not the same that the strongest site for the deprotonating adsorption of Brønsted acids. The present work revisits and improves the understanding of carbonate species that could exist on the magnesium oxide surface and gives a picture of the accessible planes of magnesium oxide as well as their surface Lewis basicity

On the relationship between the basicity of a surface and its ability to catalyze transesterification in liquid and gas phases: the case of MgO

International audienceGas or liquid phase transesterification reactions are used in the field of biomass valorization to transform some platform molecules into valuable products. Basic heterogeneous catalysts are often claimed for these applications but the role of basicity in the reaction mechanism depending on the operating conditions is still under debate. In order to compare the catalyst properties necessary to perform a transesterification reaction both in liquid and gas phases, ethyl acetate and methanol, which can be easily processed both in these two phases, were chosen as reactants. The catalyst studied is MgO, known for its basic properties and its ability to perform the reaction. By means of appropriate thermal treatments, different kinds of MgO surfaces, with different coverages of natural adsorbates (carbonates and hydroxyls groups), can be prepared and characterized by means of CO2 adsorption followed by IR spectroscopy and hept-1-ene isomerization model reaction. New results on the basicity of the natural MgO surface (covered by carbonate and hydroxyl groups) are first given and discussed. The catalytic behavior in the transesterification reaction is then determined as a function of the adsorbate coverage. It is shown that the transesterification activity in the liquid phase is directly correlated with the kinetic basicity of the surface in agreement with the mechanism already proposed in the literature. On the reverse, no direct correlation with the basicity of the surface was established with the transesterification activity in the gas phase. A very high activity, in the gas phase, was observed and discussed for the natural surface pre-treated at 623 K. Preliminary DFT modeling of ester adsorption and methanol adsorption capacity determination were performed to investigate plausible reaction routes

How Surface Hydroxyls Enhance MgO Reactivity in Basic Catalysis: The Case of Methylbutynol Conversion

International audienceThis combined experimental and theoretical study aims at understanding why surface hydroxyl groups may enhance catalytic reactivity of MgO surfaces in basic catalysis, whereas hydroxyls are weakly deprotonating groups. We investigated that reactivity enhancement in the catalytic conversion of 2-methyl-but-3-yn-2-ol (MBOH). Reaction kinetics was experimentally determined on partially hydroxylated MgO: active sites were saturated with the reactant and catalyzed its conversion with an activation energy of 85 kJ·mol–1. Reaction pathways were calculated over fully hydroxylated, partially hydroxylated, and dehydroxylated MgO surfaces by means of first-principles simulations. To highlight the effect of reactant coverage, we also calculated the reaction pathway on MgO precovered with MBOH molecules. The results show that the surface OH groups generated by dissociative adsorption of water induce a lowering in the activation energy barriers when they keep a bare Mg2+-O2– pair available in the vicinity for MBOH to adsorb and react. Interestingly, OH groups do not directly interact with MBOH converting on the surface, but they modify the basic properties of the vicinal bare Mg2+-O2– pair on which MBOH adsorbs and converts. A similar effect is predicted when MBOH converts on a bare Mg2+-O2– pair in the vicinity of a second adsorbed MBOH molecule. The beneficial effect of coadsorbates on the reactivity of active sites is analyzed by means of a thermodynamic model and an electronic structure analysis

Nanosized Layered TOT Magnesium-Silicates: Equilibrium Morphologies and Surface Speciation, a Computational and Experimental Study

International audienceThe morphology, surface speciation, NMR, and IR spectroscopic properties of nanosized layered magnesium silicate isostructural to talc at equilibrium in an aqueous environment were computed from first principles. The theoretical predictions were successfully compared with experimental results obtained on a commercial magnesium silicate hydrate, revealing insights relevant for understanding the catalytic and other surface properties of such materials of promising industrial applications

Thermodynamic brønsted basicity of clean MgO surfaces determined by their deprotonation ability: Role of Mg2+–O2 pairs

The deprotonation ability of clean (i.e., carbonate- and hydroxyl-free) MgO surfaces of various morphologies towards protic molecules, such as propyne and methanol, is evaluated by following the adsorption modes and amount of dissociated species by FTIR. Photoluminescence is used to identify the nature of the oxide ions involved in deprotonation. The same trend is observed for propyne and methanol: (i) the amount of dissociated species increases with the relative concentration of oxide ions of low coordination OLC 2, (ii) MgLC 2+ also are involved in deprotonation. Only 0.5% of OLC 2 are implied in propyne deprotonation. Quantitative adsorption and IR measurements suggest that the deprotonating Mg2+O2 pairs involve 3- and 4- coordinated ions: Mg3CO4C and Mg4CO3C. The pairs able to lead to deprotonation have to combine a strong Brønsted basic site OLC 2, able to abstract a proton and a strong acidic Lewis site able to stabilize the anion generated by deprotonation. The deprotonation equilibrium position is related to the thermodynamic Brønsted basicity. The choice of protic molecules used to identify the catalytically active basic sites was justified a posteriori by the correlation between the relative amount of dissociated propyne and methanol species and the catalytic conversion of 2-methylbut-3-yn-2-ol (MBOH)