Ru catalysts for levulinic acid hydrogenation with formic acid as a hydrogen source

International audienceThe catalytic hydrogenation of levulinic acid (LA) with formic acid (FA) as a hydrogen source into [gamma]-valerolactone (GVL) is considered as one of the crucial sustainable processes in today's biorefinery schemes. In the current work, we investigated the modification of Ru/C as efficient catalysts for both formic acid decomposition and levulinic acid hydrogenation in comparison with Pd and Pt catalysts. In order to better understand what features are responsible for high catalytic performance, we combined experimental tests, DFT calculations together with extensive material characterization. In LA hydrogenation with FA as a hydrogen source, the intermediate surface formate inhibits at least partially the LA hydrogenation. In addition, the FA decomposition is highly sensitive to the kind of the preparation method of the Ru/C catalyst: (i) the process looks structure sensitive favored on larger particles and (ii) residual chlorine decreases significantly the FA decomposition rate

Molecular organization of n-cyanobiphenyl liquid crystals on a molybdenite surface

The alignement and anchoring of liquid crystals on solid surfaces is a key problem for modern device technology that until now has been treated empirically, but that can now be tackled by atomistic computer simulations. Molecular dynamics (MD) simulations were used in this thesis work to study two films of 7 and 8 n-alkyl-4’cyanobiphenyl (7CB and 8CB) liquid crystals , with a thickness of 15 nm, confined between two (001) surfaces of MoS2 (molybdenite). The isotropic and nematic phases of both liquid crystals were simulated, and the resulting structures characterized structurally. A new force field was designed to model the interactions between the liquid crystal (LC) molecules and the surface of molybdenite, while an accurate force field developed previously was used to model the 7CB and 8CB molecules. The results show that the (001) molybdenite surface induces a planar orientation in both the liquid crystals. For the nematic phase of 8CB, one of the two solid/LC interfaces is composed of a first layer of molecules aligned parallel to the surface, followed by a second layer of molecules aligned perpendicular to the surface (also called, homeotropic). The effect of the surface appears to be local in nature as it is confined to the first 15 Angström of the LC film. Conversely, for the nematic phase of 7CB, a planar ordering is established into the LC film. The LC molecules at the interface with the molybdenite appear to align preferentially their alkyl chains toward the solid substrate. The resulting tilt angle of molecules was found to be in good agreement with experimental measurements available in literature. Despite the fact that the MD simulations spanned a time range of more than 100 ns, the nematic phases of both 7CB and 8CB were found not to be completely formed. In order to confirm the findings presented in this thesis, we propose to extend the current study

Amination sélective d'alcools : une étude théorique pour le design de catalyseurs hétérogènes innovants

Les polyamines sont des intermédiaires industriels très importants, cependant leur production dépend du pétrole, d’un apport en hydrogène et de produits toxiques (HCN, HCl). Il est donc important de changer de matière première. Dans cette optique, nous visons ici à aminer des alcools biosourcés. Nous avons donc réalisé des calculs DFT afin de comprendre les étapes clés de l’amination d’alcools, pour en maitriser les principes et résultats.En particulier, nous avons montré que l’environnement chimique jouait un grand rôle pendant cette réaction : expérimentalement, le Nickel (Ni) est plus actif que le Palladium (Pd). Cependant, les modèles de surfaces nues les prédisent aussi actifs. En prenant en compte un recouvrement de 1/9 monocouche en ammoniaque sur les surfaces, nous avons prédit une réactivité correcte : Ni plus actif que Pd.Nous avons aussi étudié les effets de l’environnement chimique sur l’hydrogénation de l’acide levulinique par l’acide formique, et sur l’isomérisation de l’isosorbide. Nous avons montré qu’il jouait un rôle sur la stabilité d’intermédiaires, modifiant l’activité des catalyseurs.Polyamines are important industrial intermediates. Their production however relies on petroleum, external hydrogen input and toxic products (HCN, HCl). It becomes therefore important to change the feedstock. In that optic, we aim here at aminate biosourced alcohols.We have performed a DFT study in order to understand alcohols amination key steps, to master its principle and outcomes.In particular, we showed that the chemical environment was playing an important role during that reaction : experimentally, Nickel (Ni) is more active than Palladium (Pd). However bare surfaces models were not predicting that fact. Taking a 1/9-monolayer ammonia coverage into account allowed to retrieve the correct reactivity.We studied the effects of the chemical environment on levulinic acid hydrogenation by formic acid, and on isosorbide isomerization. We showed that it was playing a key role on intermediates stability, modifying the catalysts activity

Hydrogenolysis and β–elimination mechanisms for C S bond scission of dibenzothiophene on CoMoS edge sites

International audienceUnraveling the mechanisms of hydrodesulfurization (HDS) of dibenzothiophene (DBT) and the corresponding active sites represents a scientific challenge to improve the intrinsic performances of Co-promoted MoS2 (CoMoS) catalysts. By using density functional theory calculations, we compare two historical mechanisms for the Csingle bondS bond scission of DBT (direct desulfurization): direct hydrogenolysis of DBT and β–elimination of α,β–dihydro-diobenzothiophene (α,β–DHDBT) on four relevant sites of the two CoMoS M- and S-edges. On the Co promoted M-edge, the α,β–DHDBT is formed through dihydrogenation which is kinetically competing with hydrogenolysis (both exhibiting activation free energies, ΔG‡, smaller than +1.24 eV). On the S-edge, both dihydrogenation and hydrogenolysis exhibit higher ΔG‡ (>+1.78 eV). Interestingly, on the S-edge, the β–elimination (E2 type) on the α,β–DHDBT is found to be kinetically competing (ΔG‡ = +1.14 eV). The elimination of Hβ atom involves a S2 dimer close to the S-vacancy site where DHDBT is adsorbed. Since this leaving Hβ atom is distinct from the one added at dihydrogenation step, this may explain why direct desulfurization of 4,6-alkyl substituted DBT compounds is hampered according to the elimination mechanism. We finally discuss the possible synergy between the two edges of CoMoS for HDS of DBT

Parameter identifiability of ductile fracture criterion for DP steels using bi-level reduced surrogate model

Generally, a large number of fracture tests are required to characterize the material parameters of the modified Mohr-Coulomb (MMC) fracture criterion. It takes enormous amount of time and expenses to prepare specimens and analyze experimental measurements. However, manufacturing industries are keen to seek fast and cost effective approaches. Therefore, a bi-level reduced surrogate model has been proposed as an alternative method for the parameter identification. Using this approach, the identification process becomes feasible and it requires only a limited number of experimental tests. The surrogate model is used in the current framework to empirically capture the non-linear evolution of material parameters for the fracture onset under the uniaxial loading condition. It assembles local critical elements associated with the global 3D finite element (FE) models. The surrogate model of fracture strain is constructed using Diffuse Approximation and the local elements to identify the unknown parameters. Then, global fracture simulations which consider the identified parameters, are preformed to update the target fracture strain and to compute the corresponding failure onset displacement. Satisfactory results are obtained by adopting the concept of successive Design of Experiments (DOE). The identification protocol is validated, and it is capable of calibrating ductile fracture parameters for different DP steels. Finally, a parametric study is performed to analyze the sensitivity of each free parameter and the influence of the used polynomial basis within the identification approach

A novel development of bi-level reduced surrogate model to predict ductile fracture behaviors

In this paper, a bi-level reduced surrogate model is developed and presented to identify the material parameters of ductile fracture criterion. Using this method, the identification process becomes feasible with only a limited number of experimental tests. The method assembles local critical elements associated with global models. The surrogate model of fracture strain constructed using Diffuse Approximation and the local elements, reduces the computational effort for searching identified material parameters. Global fracture simulations are preformed to update the target fracture strain and to compute the corresponding failure onset displacement. Convincing results are obtained via successive applications of Design of Experiments (DOE) and enhanced design space transformation algorithms. The proposed identification protocol is validated for the modified Mohr-Coulomb fracture criterion using DP590 steel. Robustness of this method is confirmed with different initial guess. It orients a new direction for material parameters identification based on surrogate model which can effectively be implemented to predict the fracture behaviors

Micromechanical modeling of the effect of phase distribution topology on the plastic behavior of dual-phase steels

In this paper, a topology optimization based numerical method is proposed to investigate the micromechanical plastic behavior of dual-phase (DP) steels. Representative volume elements (RVEs) are constructed using the topology optimization based artificial microstructures. Micromechanical behavior under various loading conditions are predicted for the RVEs to investigate the plasticity, strain localization and strengthening mechanism affected by the microstructural characteristics of DP steels. Plastic strain patterns including shear band are found during the deformation. Due to the twisting movement of martensite grains, the direction of the strain localization bands in the shear loading case is 0° or 90° to the loading direction, while it is 45° in the tensile and compressive cases. Moreover, the effective flow behavior of the material under shear loading is lower than those found in tensile and compressive cases. The influence of various microstructural features, such as, martensite fraction, distribution of each phase, on the effective flow properties and the local strain partitioning has also been identified. Both of the effective flow properties and strain localization exhibit the tendency to be strengthened with the increase of martensite phase fraction. Furthermore, the RVE with more uniform martensite distribution leads to the decrease of effective flow properties and strain localization. Longer martensite-ferrite interface results from the clustering of martensite, which increases the strain localization effect during the plastic deformation

An Efficient Theoretical Tool for Screening Promising Heterogeneous Catalysts in Alcohols Selective Aminations

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