48 research outputs found
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 CâC 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 rĂ©action d'hydrogĂ©nation du benzĂšne a Ă©tĂ© suivie par spectroscopie infrarouge pour des solides PtNaY, PtNaX et PtCsX. Les rĂ©sultats montrent que pour tous les Ă©chantillons, la rĂ©partition des molĂ©cules de benzĂšne entre les deux sites d'adsorption de la faujasite (les cations et les fenĂȘtres dodĂ©cagonales « 12-R ») au cours de la rĂ©action dĂ©pend de l'Ă©quilibre obtenu pour l'adsorption, c'est Ă dire de la quantitĂ© de benzĂšne n'ayant pas rĂ©agi et prĂ©sente Ă l'intĂ©rieur des cavitĂ©s. Cette Ă©tude suggĂšre Ă©galement qu'au moins dans le cas du solide PtCsX, la vitesse de retour Ă l'Ă©quilibre d'adsorption des molĂ©cules de benzĂšne sur les diffĂ©rents sites de la zĂ©olite est supĂ©rieure Ă celle de la rĂ©action d'hydrogĂ©nation. Ces rĂ©sultats 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 propriĂ©tĂ©s Ă©lectroniques induites par ce cation
3H-1,2-Benzodithiole-3-thione
The almost planar (r.m.s. deviation = 0.034â
Ă
) title compound, C7H4S3, was synthesized by reacting 2,2-dithiodibenzoic acid with phosphorus pentasulfide in xylene solution. In the crystal, short S...S [3.3727â
(14), 3.3765â
(13) and 3.4284â
(13)â
Ă
] contacts and aromatic ÏâÏ stacking [shortest centroidâcentroid separation = 3.618â
(2)â
Ă
] 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 â2.1â
(1)° for the endocyclic CâCâSâS fragment. The SâS and C=O bond lengths are 2.0521â
(8) and 1.212â
(3)â
Ă
, respectively. The endocyclic SâCâC angle sustained by the carbonyl group, 110.94â
(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 2,2âČ-(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 81.0â
(2)°. In the crystal, slipped ÏâÏ 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 âsilica supported zirconium hydroxideâ
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