Frédéric Lefebvre

A study by in situ laser Raman spectroscopy of VPO catalysts for n-butane oxidation to maleic anhydride I. Preparation and characterization of pure reference phases

An in situ laser Raman spectroscopy (LRS) cell has been constructed in order to study the evolution of the local structure of the vanadium phosphate catalysts for n-butane oxidation to maleic anhydride in catalytic conditions. The LRS cell is described with the on line disposal for detection of the evolving gases. The first issue concerns the preparation and the physicochemical characterization of the reference phases of the VPO system: (VO)2P2O7, αII, β, γ, and δ VOPO4. Their purity was controlled by X-ray diffraction and 31P and 51V solid NMR and their LRS spectra were studied in the 800–1200 cm−1 range characteristic of the PO and VO bonds. Among these phases only δ VOPO4 is partly transformed (into αII VOPO4) in the catalytic conditions. From the evolution of their respective Raman spectra with temperature and with conditions of hydration, new proposals for the structure of γ VOPO4 are given. They are in agreement with the solid state NMR data. Raman spectra of the phases exhibit features specific enough to allow identification of the different VPO structures.

Stable polyoxometalate insertion within the mesoporous metal organic framework MIL-100(Fe)

Successful encapsulation of polyoxometalate (POM) within the framework of a mesoporous iron trimesate MIL-100(Fe) sample has been achieved by direct hydrothermal synthesis in the absence of fluorine. XRPD, 31P MAS NMR, IR, EELS, TEM and 57Fe Mossbauer spectrometry corroborate the insertion of POM within the cavities of the MOF. The experimental Mo/Fe ratio is 0.95, in agreement with the maximum theoretical amount of POM loaded within the pores of MIL-100(Fe), based on steric hindrance considerations. The POM-MIL-100(Fe) sample exhibits a pore volume of 0.373 cm3 g−1 and a BET surface area close to 1000 m2 g−1, indicating that small gas molecules can easily diffuse inside the cavities despite the presence of heavy phosphomolybdates. These latter contribute to the decrease in the overall surface area, due to the increase in molar weight, by 65%. Moreover, the resulting Keggin containing MIL-100(Fe) solid is stable in aqueous solution with no POM leaching even after more than 2 months. In addition, no exchange of the Keggin anions by tetrabutylammonium perchlorate in organic media has been observed.

Catalytic Cleavage of the C-H and C-C Bonds of Alkanes by Surface Organometallic Chemistry: An EXAFS and IR Characterization of a Zr-H Catalyst

The catalytic cleavage under hydrogen of the C & singlebond;H and C & singlebond;C bonds of alkanes with conventional catalysts requires high temperatures. Room-temperature hydrogenolysis of simple alkanes is possible on a well-defined and well-characterized zirconium hydride supported on silica obtained by surface organometallic chemistry. The surface structure resulting from hydrogenolysis of (≡SiO)Zr(Np)3 (Np, neopentyl) was determined from the extended x-ray absorption fine structure (EXAFS) and 1H and 29Si solid-state nuclear magnetic resonance and infrared (IR) spectra. A mechanism for the formation of (≡SiO)3Zr-H and (≡SiO)2SiH2 and the resulting low-temperature hydrogenolysis of alkanes is proposed. The mechanism may have implications for the catalytic formation of methane, ethane, and lower alkanes in natural gas.

Catalytic oxidation of light alkanes (C1-C4) by heteropoly compounds.

(C1−C4) by Heteropoly Compounds Miao Sun,*,† Jizhe Zhang,‡ Piotr Putaj, Valerie Caps, Fred́eŕic Lefebvre, Jeremie Pelletier, and Jean-Marie Basset* †Research and Development Center, Saudi Aramco Oil Company, Dhahran 31311, Saudi Arabia ‡Division of Chemical and Life Sciences and Engineering, and KAUST Catalysis Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia Laboratoire de Chimie Organomet́allique de Surface, CPE Lyon, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne, France

Immobilization of multifunctional Schiff base containing ruthenium complexes on MCM-41

Abstract Two solid catalysts in which a Schiff base containing ruthenium complex has been covalently anchored on MCM-41 have been prepared and characterized. The catalytic behavior of these materials in ring-closing metathesis (RCM), ring-opening metathesis polymerization (ROMP), Kharasch addition, atom transfer radical polymerization (ATRP) and vinylation reactions as well as the recycling characteristics in RCM reactions are presented to exemplify the effectiveness and the multifunctional character of these catalytic systems. For ROMP the influence of the polymerization solvent on the performance of the catalysts was checked. For the RCM reactions both reaction time and reaction temperature were varied in order to determine the influence of these parameters on the RCM activity of our catalysts. The activity of our catalytic systems in radical reactions such as ATRP and the Kharasch addition is promising. Our results demonstrate that the ATRP process of styrene proceeds in a controlled fashion allowing the synthesis of well-defined polymers. Furthermore, we were able to demonstrate that the Kharasch activity of our catalysts shows a pronounced temperature dependence. In addition to all this, we also postulated a mechanism that explains for the observed selectivities in the vinylation experiments. The host–guest interaction of these hybrid catalytic systems is studied by XRD, XRF, ICP/MS, BET, FT-Raman and solid state NMR analysis.

Synthesis and characterization of zirconium containing mesoporous silicas: I. Hydrothermal synthesis of Zr-MCM-41-type materials

Abstract Zirconium containing MCM-41-type has been synthesized by a near neutral route. Various solids, with Si/Zr molar ratios ranging from 20 to ∞, were prepared. All these solids have high surface area and good crystallinity. Addition of zirconium results in a linear increase of both the Lewis and Bronsted acidities. At least one-third of zirconium is at the surface or near the surface, probably in the form of [(≡Si–O)3Zr–O−] species. As a consequence, introduction of zirconium on the surface results in the appearance of a negative charge, compensated by sodium ions which can be exchanged.

Surface organometallic chemistry: some fundamental features including the coordination effects of the support

Abstract The study of the various kinds of reactions between organometallic complexes and the surfaces of inorganic oxides, metals or zeolites constitutes a new aspect of the coordination chemistry on surfaces. In this non-exhaustive and short review article, we would like to try to answer a few questions regarding this area of coordination (or organometallic chemistry) which may have some future impact in the field of catalysis. The questions that we would like to answer are the following: Are the basic rules of molecular organometallic and coordination chemistry valid when one tries to apply them to surfaces? One can wonder whether or not the functionalities which are present at the surface of an oxide, M x O y (M–OH groups, strained M–O–M groups and MO, aso) have a chemical reactivity which can be predicted on the basis of molecular chemistry. A few selected examples will be given about the reactivity of tin, rhenium or zirconium alkyls with the silanol groups of partially dehydroxylated silica. Can we obtain reliable and precise informations when some selected tools of surface science and molecular organometallic chemistry are applied simultaneously to elucidate the structure of surface organometallic fragments? One can reasonably expect that the way the surface organometallic fragments coordinate to the surface can be rationalized on the simple rules of coordination chemistry (electron counting, formal oxidation state). A few examples will be given regarding the surface structure of silica-supported zirconium hydrides or rhodium allyls. Is it possible that a well chosen surface organometallic fragment represents an intermediate in heterogeneous catalysis? If one can study the reactivity of a well chosen surface organometallic fragment, then one is in a position to demonstrate some elementary steps of heterogeneous catalysis. In this review we shall consider the surface reactivity of supported rhodium allyls or tin alkyls. What kind of mobility can we expect from surface organometallic fragments? In sharp contrast with discrete ligands of molecular chemistry, surfaces of oxides obviously provide a so called “pool of oxygens” which binds the surface organometallic fragments in a localized manner. However, due to its almost infinite structure, such a “pool” is obviously responsible for surface mobility, which is also a key parameter in certain catalytic processes (sintering, diffusion processes, reconstructions, leaching,⋯). Examples will be given on the mobility of Rh I (CO) 2 grafted onto a silica surface. The organometallic fragments are also mobile around the metal carbon bonds and this phenomenon can be evidenced by solid-state nuclear magnetic resonance (NMR) and can have applications in molecular separations and on the reactivity of the organometallic complexes. In each case, the role of the support as a coordinating ligand is a key factor of this chemistry.

CO-induced disintegration of rhodium aggregates supported in zeolites: In situ synthesis of rhodium carbonyl clusters

Y-type zeolites exchanged with [Rh(NH3)5Cl]2+ ions and subjected to different treatments have been studied by radial electron distribution derived from X-ray scattering and by infrared spectroscopy. O2 treatment at 620 K of the RhY zeolite leads to rhodium oxide clusters and rhodium cations. Metal aggregates smaller than 1 nm are formed upon H2 reduction at 470 K. The adsorption of CO at 300 K produces a complete disintegration of the Rh aggregates into monomeric species identified as Rh1(CO)2 by IR spectroscopy. In the presence of CO:H2O mixture, the Rh1(CO)2 species condense into polynuclear carbonyl clusters.

Gas separation with a zeolite filter, application to the selectivity enhancement of chemical sensors

The poor selectivity of most of the current chemical sensors is a major problem that limits their application fields. A molecular filter disposed ahead of those sensors can enhance this property. We have realised such filter with zeolite and some tests have been conducted with semiconductor and optical chemical sensors.

Microcalorimetric study of the acidity of tungstic heteropolyanions

The number and strength of the acid centres of tungstic heteropolyacids have been determined by absorption calorimetry of ammonia. The initial heats are in the order H3PW12O40>H4SiW12O40>H6P2W21O71(H2O)3>H6P2W18O62, varying from 200 to 155kJ mol–1. An increase in the number of protons in Keggin heteropolyanions decreases the acidic strength. Moreover, the influence of the activation temperature on the acidity has been studied and confirms that there is a drastic modification of the solid at ca. 350 °C. The heat capacities of the heteropolyanions and of the corresponding ammonium salts, the thermokinetic parameters of ammonia absorption and the heat capacities of the acids and ammonium salts have been related to the porosity of the various samples.