Input from solid state 17O NMR into structural investigations on molecular and silica-grafted species relevant to olefin metathesis

Ce manuscrit décrit l’application de la RMN du solide de l’17O à l’étude structurale d’espèces oxo de tungstène bien définies en lien avec des catalyseurs industriels pour la métathèse des oléfines. Dans un premier temps, pour compenser la faible réceptivité de l’17O, des techniques d'amélioration de signal tels que DFS (Double Frequency Sweep) et HS (Hyperbolic Secant) ont été évaluées de façon critique pour ces systèmes comportant des interactions anisotropes d’amplitudes très diverses. La méthode DFS s’est avérée la plus robuste, avec un gain en signal de 2-2,4. Dans une seconde étape, des complexes oxo de tungstène moléculaires et supportés sur silice ont été étudiés par RMN MAS 17O, après enrichissement isotopique du groupement oxo. Les paramètres RMN de W=O sont très sensibles à la sphère de coordination du métal : combiné avec des calculs DFT, ceci permet une évaluation de la structure des espèces greffées. Le marquage sélectif en 17O de la surface de silice a également amené à une meilleure compréhension de ces systèmes, plus particulièrement en ce qui concerne les interactions métal-support. Des informations supplémentaires ont été obtenues grâce à l'application des méthodes de haute résolution (17O MQ MAS) et de corrélation hétéronucléaire (1H-17O HMQC). Quelques perspectives de ce travail sont avancées, plus particulièrement sur la chimie de surface du molybdène.This manuscript aims at the use of 17O solid state NMR for accessing the structure of well-defined silica-supported oxo-tungsten species related to industrial olefin metathesis catalysts. As a first step, to compensate for the low receptivity of 17O, signal enhancement techniques such as DFS (Double Frequency Sweep) and HS (Hyperbolic Secant) were critically assessed for such systems featuring large range of anisotropic interactions. DFS proved to be the most robust method, providing a signal enhancement of 2-2.4. In a second stage, series of molecular and silica-supported tungsten-oxo complexes have been studied by 17O MAS NMR, following isotopic enrichment of the oxo moiety. The W=O NMR parameters showed a high sensitivity to the metal coordination sphere, thus allowing structural assessment of grafted species when combined with DFT calculations. Silica-surface selective 17O labelling also afforded deeper understanding of these systems, most particularly regarding metal-support interactions. Further elements were obtained thanks to application of methods for high resolution (17O MQ MAS) and heteronuclear correlation (1H-17O HMQC). Some perspectives of this work are drawn, most particularly on the related molybdenum surface chemistry

On the use of manometry method for measurement of gas adsorption equilibria and characterization of clay texture with Derivative Isotherm Summation

International audienceGas adsorption is widely used for characterization of clays. Apart from “classical” experiment of nitrogen orother gases adsorption/desorption at cryogenic temperatures, allowing the determination of the specific surfacearea, partial mesopore and micropore volume and size distributions, gas adsorption can also be used for thecharacterization of clay surface texture (energetic heterogeneity) related to layered structure of clays. DerivativeIsotherm Summation applied to high-resolution gas adsorption isotherms was shown to be a powerful tool toaccess energetic characteristics and ratio between basal and lateral faces of clay particles. To date, the method isnot commonly employed for characterization of clays and other heterogeneous solids, probably due to specificityof high-resolution isotherms acquisition technique described in the original literature: quasi-equilibrium volumetry with gas injection by microleak. We illustrated in the present work a possibility to use widely availablestatic manometric commercial gas adsorption analysers for this purpos

Adsorption of methane and carbon dioxide by water-saturated clay minerals and clay rocks

International audienceUnderstanding the effects of water on gas adsorption in geological media is of high importance in order to efficiently control numerous subsurface engineering process operating at gas/rock interfaces. Due to preferential interaction with clay surfaces, water fills their porous body, greatly reducing CH4 and CO2 adsorption capacity. In order to quantitatively describe CH4 and CO2 adsorption by hydrated clay minerals, this work proposes to rely on the mechanism of gas uptake by dissolution in pre-adsorbed pore water. This approach was employed to characterise water-saturated porous media of increasing complexity: mesoporous silica SBA-15 and silica gel with different pore sizes and geometry, isolated illite and montmorillonite and natural clay-rich rock (the Callovo-Oxfordian formation – COx, France) in powdered and crushed states. It was found that the solubility in water can reliably explain the CO2 uptakes by hydrated pore systems regardless of their nature as well as the CH4 uptakes, but only for solids with large mesopores and montmorillonite mineral. A so called “adsorption enhanced gas uptake in pore water” for CH4, exceeding its solubility in bulk water by a factor of 5–8, was observed for the systems with narrow pore sizes, highlighting the impact of surface energy on gas uptake and the occurrence of the interaction of weakly-soluble methane with the surface, promoting its uptake in comparison to pure dissolution

Analyse des hétérogénéités de surfaces d’argiles à partir d’isothermes d’adsorption d’Ar : détermination probabiliste de paramètres énergétiques,

International audienc

Smectite fraction assessment in complex natural clay rocks from interlayer water content determined by thermogravimetric and thermoporometry analysis

International audienceThe smectite content is a key parameter to be determined for various applications of clays and clay-rich rocks. The quantity of interlayer water characteristic of swelling domains can be used to assess the smectite content in clays. We propose in this study to use a simple approach to determine water distribution in clays (mainly between pores and interlayers) by means of thermoporometry and thermogravimetric analysis. Provided the interlayer water does not freeze at low temperature upon thermoporometry experiments, the difference between water quantities determined by the two techniques is assigned to interlayer water. Single-phase model clays and complex natural clay rocks and their composites in water-saturated state are characterized by this approach. The open question is the application of available thermoporometry models developed for simple pore geometry to characterize the complex pore network of clays. Depending on the approach used, different pore sizes were obtained highlighting the limit of a simplified model to describe the complex porous network. The results are more coherent when quantifying the amount of interlayer water, further used for smectite content estimation. Good agreement was obtained between smectite fraction contents deduced from the results of thermal analysis and those measured by conventional mineralogical techniques

A direct experimental evidence of the influence of clay textural properties on CH4 and CO2 adsorption

International audienc

Direct Experimental Evidence of the Effects of Clay Particles’ Basal-to-Lateral Surface Ratio on Methane and Carbon Dioxide Adsorption

International audienceThe amount and size of the charge-balancing cations on the exposed faces of clay particles are supposed to be one of the key factors affecting the specificity of adsorption of gases at clay surfaces. However, the trends characterizing the thermodynamics of gas adsorption, reported for different members of 2:1 phyllosilicate phases, do not systematically follow neither the difference in their layer charge, determining the number of charge-balancing cations, nor the nature of the latter. To better understand the specificity of CH4 and CO2 molecular interactions with different clay phases, the adsorption isotherms were measured for isoionic pure-phase illite and montmorillonite up to pressures of 9 MPa at ambient temperature. For both gases, higher adsorption capacities per unit of specific surface area for montmorillonites in comparison to illites could not be explained by the existing theory relying on the properties of exposed charge-balancing cations on the surface. Instead, the slopes of the isotherms perfectly correlate with the shape of clay particles, characterized by their basal-to-lateral faces’ aspect ratio, which is assessed by derivative isotherm summation (DIS) using argon adsorption at 77 K. Montmorillonite clays, characterized by higher fractions of high-energy hydroxylated particles’ edges (∼45%) in the total specific surface area, featured stronger adsorption interactions with CO2 and CH4 in comparison to illites, for which the contribution of edges to the surface area is only 20%. This introduces a new important factor controlling the mechanism of CH4 and CO2 adsorption by clay minerals

Thermodynamic data of adsorption reveal the entry of CH4_4 and CO2_2 in a smectite clay interlayer

International audienceThe ability of smectite clays to incorporate gases in their interlayers is shown to be conditioned by interlayer spacing, depending, in turn, on phyllosilicate layer composition and effective size of the charge-balancing cations. As illustrated by earlier in situ X-ray diffraction and spectroscopic characterization of the gas/clay interface, most smectites with small-size charge-balancing cations, such as Na+ or Ca2+, accommodate CO2 and CH4 in their interlayers only in a partially hydrated state resulting in the opening of the basal spacing, above a certain critical value. In the present study CH4 and CO2 adsorption isotherms were measured for Naand Mg-exchanged montmorillonite up to 9 MPa using a manometric technique. The process of dehydration of these clays was thoroughly characterized by thermogravimetric analysis and powder X-ray diffraction. A dramatic decrease in specific surface area and methane and carbon dioxide adsorption capacities for fully dehydrated samples in comparison to partially dehydrated ones is assigned to the shrinkage of interlayer spacing resulting in its inaccessibility for the entry of CH4 and CO2 molecules. This observation is direct evidence of CH4 and CO2 adsorption capacity variation depending on the opening of smectite clay interlayer spacing.</p

Anodic deposit from respiration metabolic pathway of Escherichia coli

International audienceno abstrac

31P NMR applied to vanadium phosphate catalysts: crucial influence of 51V decoupling strategies

International audienc