Theoretical Study of the Adsorption of Ethylene on Alkali-Exchanged Zeolites

ABSTRACT: The structures of alkali-exchanged faujasite (X-FAU, X ϭ Li ϩ or Na ϩ ion) and ZSM-5 (Li-ZSM-5) zeolites and their interactions with ethylene have been investigated by means of quantum cluster and embedded cluster approaches at the B3LYP/6-31G (d, p) level of theory. Inclusion of the Madelung potential from the zeolite framework has a significant effect on the structure and interaction energies of the adsorption complexes and leads to differentiation of different types of zeolites (ZSM-5 and FAU) that cannot be drawn from a typical quantum cluster model, H 3 SiO(X)Al(OH) 2 OSiH 3 . The Li-ZSM-5 zeolite is predicted to have a higher Lewis acidity and thus higher ethylene adsorption energy than the Li-FAU zeolites (16.4 vs. 14.4 kcal/mol), in good agreement with the known acidity trend of these two zeolites. On the other hand, the cluster models give virtually the same adsorption energies for both zeolite complexes (8.9 vs. 9.1 kcal/mol). For the larger cation-exchanged Na-FAU complex, the adsorption energy (11.6 kcal/mol) is predicted to be lower than that of Li-FAU zeolites, which compares well with the experimental estimate of about 9.6 kcal/ mol for ethylene adsorption on a less acidic Na-X zeolite

The novel synthesis of microporous and mesoporous materials and their applications for hydrocarbon transformation and chiral recognition

Dans ce travail nous étudions l élaboration, la caractérisation et les applications de différents matériaux poreux. L étude est organisée en trois parties majeures: la synthèse de zéolithes micro/mesoporeux et leur application potentielle dans l industrie pétrochimique, l étude théorique de mécanismes réactionnels sur des zeolites microporeux, et le design de métaux mesoporeux avec une chiralité intrinsèque de leur surface interne. Ces matériaux poreux montrent des propriétés excellentes, notamment pour des applications potentielles en catalyse et comme interfaces chirales.In the present work, the elaboration, characterisation and applications of differentporous materials have been studied. Porous materials are divided into three categoriesdepending on the porous cavity size, namely microporous materials (pore diameter 50 nm). The thesis work is organized in three major parts: the synthesisof hierarchical micro/mesoporous zeolites and their potential application for thepetrochemical industry, the theoretical study of reaction mechanisms on microporouszeolite and the design of mesoporous metals with intrinsic chirality at their inner surface.The hierarchical micro/mesoporous zeolite, composed of microporous andmesoporous features, has been prepared using carbon-silica (C/SiO2) composites derivedfrom a pyrolysis of hydrocarbon gas on silica gel. Our findings demonstrate that not onlythe presence of a high surface area and porosity, but also an improved efficiency of thesematerials for many petrochemical processes such as n-butene isomerization, nhexadecanecatalytic cracking and hydrocracking. The novel synthetic method is expectedto be generalized for other types of zeolites, and is considered to be a promising methodfor creating hierarchical micro/mesoporous zeolites for potential catalytic applications,especially in the petrochemical industry.In addition to the study of practical catalytic aspects, a theoretical approach hasbeen used to investigate potential reaction mechanisms such as the selective isomerizationof 1-butene into isobutene. More specifically, the monomolecular skeletal isomerizationof 1-butene into isobutene on H-FER zeolite was theoretically studied by using theONIOM approach. This process was found to involve the transformation of adsorbed 1-butene through 2-butoxide, isobutoxide, and tert-butyl cation intermediates. The ratedeterminingstep is the conversion of isobutoxide into isobutene, in which the reactionhas to proceed through the primary isobutyl cation transition state. The shape selectivitydue to the nano-confinement effect of the zeolite framework strongly affects theadsorption, the stability of alkoxide species and carbenium ion, as well as the skeletalisomerization mechanism of 1-butene.Moreover, the microporous and mesoporous zeolite, the generation of chiralmesoporous metal and its enantioselective recognition properties have been studied.Molecular imprinting (MI) is a major approach for generating materials withenantioselective properties. In this work, a chiral imprinted mesoporous platinum hasbeen obtained by the electrochemical reduction of platinum salts in the simultaneouspresence of a lyotropic liquid crystal phase and chiral template molecules. The resultingmaterials exhibit not only a dramatic increase in active surface area due to theirmesoporosity, but also a significant discrimination between two enantiomers of a chiralprobe, confirmed by both electrochemical and enantioselective adsorption experiments.Most importantly the porous platinum retains its chiral character even after removal of thechiral template molecule. Our findings could lead to the development of new materials,which are of potential interest for applications in areas such as chiral synthesis, sensors,separation, purification and drug development.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Bulk properties of transition metals: a challenge for the design of universal density functionals

Systematic evaluation of the accuracy of exchangecorrelation functionals is essential to guide scientists in their choice of an optimal method for a given problem when using density functional theory. In this work, accuracy of one Generalized Gradient Approximation (GGA) functional, three meta-GGA functionals, one Nonseparable Gradient Approximation (NGA) functional, one meta-NGA, and three hybrid GGA functionals was evaluated for calculations of the closest interatomic distances, cohesive energies, and bulk moduli of all 3d, 4d, and 5d bulk transition metals that have face centered cubic (fcc), hexagonal closed packed (hcp), or body centered cubic (bcc) structures (a total of 27 cases). Our results show that including the extra elements of kinetic energy density and Hartree−Fock exchange energy density into gradient approximation density functionals does not usually improve them. Nevertheless, the accuracies of the Tao−Perdew−Staroverov−Scuseria (TPSS) and M06-L meta-GGAs and the MN12-L meta-NGA approach the accuracy of the Perdew−Burke−Ernzerhof (PBE) GGA, so usage of these functionals may be advisable for systems containing both solid-state transition metals and molecular species. The N12 NGA functional is also shown to be almost as accurate as PBE for bulk transition metals, and thus it could be a good choice for studies of catalysis given its proven good performance for molecular species

Adding pieces to the CO/Pt(111) puzzle: the role of dispersion

The so-called CO/Pt(111) puzzle, the experimentally demonstrated preference of CO to adsorb on the top site on the Pt(111) surface rather than the 3-fold hollow sites predicted by standard density functional theory (DFT) methods, was alleged to be solved by properly leveling the CO frontier molecular orbitals. However, the subtle energy difference between the top and hollow sites is of the same order of magnitude as the possible contribution of dispersive forces to this interaction. Herein, the role of dispersion in this system has been investigated by considering the PBE, PBEsol, RevPBE, RPBE, and SOGGA11 generalized gradient approximation (GGA) based exchange-correlation functionals; nonseparable functionals such as N12; and the TPSS and M06-L meta-GGA-type functionals, together with D2, D3, D3BJ, and MBD dispersion corrections. The results reinforce the recommendation of using M06-L for a correct description of CO adsorption-site preference even if including dispersion leads to a change of site and a noticeable overestimation of the adsorption energy indicating the presence of error-compensation effects. The present results also highlight the fact that dispersion contributes in bridging the preference gap between the top and hollow sites when other functionals are used. Dispersive forces play a role in site preference for CO on Pt(111), and it is likely that a similar situation is encountered for other late transition metals. Therefore, dispersion must be considered to reach a completely unbiased description of CO adsorption on metals. Nevertheless, including dispersion leads to adsorption energy values that overestimate the experimental value, indicating the limitations of the existing, widely used density functionals

Tuning adsorption properties of GaxIn2−xO3 catalysts for enhancement of methanol synthesis activity from CO2 hydrogenation at high reaction temperature

Light olefins can be produced from CO2 hydrogenation in a single reactor using a combination of a methanol synthesis catalyst and a methanol-to-olefin (MTO) catalyst. However, commercial methanol synthesis catalysts are active at low temperatures (200–260 °C), while MTO reaction is feasible at higher temperatures (>300 °C). Herein, we report the CO2 hydrogenation to methanol at high temperatures (320–400 °C) over GaxIn2−xO3 catalysts. By tuning the Ga/In ratios, phase, crystallinity, pore structure, morphology, electronic properties as well as adsorptive properties of GaxIn2−xO3 catalysts can be modified. At the lowest temperature (320 °C), the pure In2O3 shows the highest methanol yield. However, the maximum methanol yield declines significantly with increasing reaction temperatures. Incorporation of Ga into the In2O3 crystal lattices at x = 0.4 (Ga0.4In1.6O3) maximizes the methanol yield at higher reaction temperatures of 340–360 °C. This enhancement can be attributed to an increased binding energy of adsorptive molecules with the catalyst surface to promote the hydrogenation of CO2 to methanol. Further increasing Ga content (x > 0.4) leads to greatly strengthen the binding for adsorptive molecules, resulting in a lower methanol yield and the formation of methane. The surface chemisorbed oxygen is found to be a key factor determining the CO yield

Tuning interactions of surface‐adsorbed species over Fe−Co/K−Al2O3 catalyst by different K contents: selective CO2 hydrogenation to light olefins

Selective CO2 hydrogenation to light olefins over Fe−Co/K−Al2O3 catalysts was enhanced by tuning bonding strengths of adsorbed species by varying the content of the K promotor. Increasing the K/Fe atomic ratio from 0 to 0.5 increased the olefins/paraffins (O/P) ratio by 25.4 times, but then slightly raised upon ascending K/Fe to 2.5. The positive effect of K addition is attributed to the strong interaction of H adsorbed with the catalyst surface caused by the electron donor from K to Fe species. Although the Fe−Co/K−Al2O3 catalyst with K/Fe=2.5 reached the highest O/P ratio of 7.6, the maximum yield of light olefins of 16.4 % was achieved by the catalyst promoted with K/Fe of 0.5. This is explained by the considerable reduction of amount of H2 adsorbed on the catalyst surface with K/Fe=2.5

Fonctionnalisations de nanaotubes de carbone (étude expérimentale et théorique)

Dans ce travail nous avons exploré des nouvelles voies pour synthétiser et fonctionnaliser des nanotubes de carbone. Des tubes de diamétre et de longueur bien définie, ont été obtenu en utilisant une apprcohe "template" pour la synthèse. Par la suite des nanotubes ont été solubilisés en utilisant une approche à la fois simple et originale. Enfin nous avons pu mettre au point une technique permettant de fonctionnaliser sélectivement une extrémité de ces nanotubes avec une couche métallique en utilisant le concept d'électrochimie bipolaire. Le travail a été complété par une étude théorique de la réactivité de ces objets.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceThailandFRT