Olefin Metathesis by Group VI (Mo, W) Metal Compounds

Olefin metathesis is an important reaction not only in petroleum chemistry but also in fine chemistry. Professors Grubbs, Schrock, and Chauvin obtained the Nobel Prize in 2005 for the development of this reaction (determination of the mechanism and synthesis of homogeneous catalysts). This reaction can be described as the redistribution of carbon chains of olefins via a breaking of their C═C double bonds. It is catalyzed by metal carbenes and the catalytic cycle passes through a metallacyclobutane. The purpose of this chapter is to give an overview of catalysts based on tungsten or molybdenum active for this reaction. Numerous tungsten and molybdenum organometallic complexes displaying a carbene functionality were synthesized. Some of them are highly active in olefin metathesis. Industrially, tungsten oxide on silica is used as a precursor of the propene production by olefin metathesis of but-2-ene and ethylene. However, the active sites are not well known but they can be modeled by grafting, via surface organometallic chemistry, perhydrocarbyl complexes of molybdenum or tungsten on oxide surfaces. After a review of the complexes used in homogeneous catalysis, a review of the industrial catalysts and their models will be given

Modifications du benzene par adsorption sur des faujasites echangees par des ions alcalins

SIGLECNRS T 59109 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Influence des proprietes acido-basiques de faujasites et de l'A1PO_4-5 sur l'adsorption d'hydrocarbures aromatiques et sur la reactivite du platine supporte

SIGLEINIST T 70664 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Lettre à la rédaction

La réaction d'hydrogénation du benzène a été suivie par spectroscopie infrarouge pour des solides PtNaY, PtNaX et PtCsX. Les résultats montrent que pour tous les échantillons, la répartition des molécules de benzène entre les deux sites d'adsorption de la faujasite (les cations et les fenêtres dodécagonales « 12-R ») au cours de la réaction dépend de l'équilibre obtenu pour l'adsorption, c'est à dire de la quantité de benzène n'ayant pas réagi et présente à l'intérieur des cavités. Cette étude suggère également qu'au moins dans le cas du solide PtCsX, la vitesse de retour à l'équilibre d'adsorption des molécules de benzène sur les différents sites de la zéolite est supérieure à celle de la réaction d'hydrogénation. Ces résultats montrent aussi que le solide PtCsX est moins actif que PtNaY ou PtNaX ce qui indique une forte influence de la taille du cation ou/et des propriétés électroniques induites par ce cation

3H-1,2-Benzodithiole-3-thione

The almost planar (r.m.s. deviation = 0.034 Å) title compound, C7H4S3, was synthesized by reacting 2,2-dithiodibenzoic acid with phosphorus pentasulfide in xylene solution. In the crystal, short S...S [3.3727 (14), 3.3765 (13) and 3.4284 (13) Å] contacts and aromatic π–π stacking [shortest centroid–centroid separation = 3.618 (2) Å] are observed

4,5-Dichloro-3H-1,2-dithiol-3-one

The crystallization from an ethyl acetate solution of the title compound, also known as dichloro-1,2-dithiacyclopentenone, C3Cl2OS2, leads to a monoclinic system with P21/n space group. The molecule displays an almost planar geometry, with a torsion angle of −2.1 (1)° for the endocyclic C—C—S—S fragment. The S—S and C=O bond lengths are 2.0521 (8) and 1.212 (3) Å, respectively. The endocyclic S—C—C angle sustained by the carbonyl group, 110.94 (16)°, deviates from the expected value of 120° for an sp2-hybridized C atom. In the crystal, short S...S, S...Cl, S...O and Cl...Cl contacts are observed

Diethyl 2,2′-(trisulfane-1,3-diyl)dibenzoate

The title compound, C18H18O4S3, was synthesized in the presence of chloridoauric acid from 3H-1,2-benzodithiole-3-thione as starting material. The asymmetric unit comprises one half of the molecule, the complete molecule being generated by the application of twofold rotation symmetry. The two benzene rings are inclined by 81.0 (2)°. In the crystal, slipped π–π stacking interactions are observed between the benzene rings of neighbouring molecules

Synthesis, characterization and propane metathesis activity of a tantalum-hydride prepared on high surface area “silica supported zirconium hydroxide”

International audienc

Catalytic Reduction of NOx With NH3 Over CeO2 and SiO2 Supported Tungstophosphoric Acid: Promoting Effects of Ceria Support and Cobalt Proton Substitute

International audienceGrafted tungstophosphoric acid (H3PW12O40/ HPW) is applied as a catalyst in the selective catalytic reduction of NOx with NH3 (NH3-SCR). The HPW/CeO2 catalyst has been found to be effective in the SCR reaction compared to HPW/SiO2. The cobalt-exchanged HPW/CeO2 improved NOx conversion, which reached 98% over a wide operating temperature window (280-500 °C). The better NOx performance of the CoPW/CeO2 catalyst is mainly due to the higher dispersion of tungsten on the CeO2 support, the formation of oxygen vacancies and the production of nitrogenous species. Incorporation of vanadium decreased the acidity of HPW, which reduced SCR activity on the HPVW/CeO2 catalyst