A. Choplin

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.

From supported homogeneous catalysts to heterogeneous molecular catalysts

Abstract Numerous methods are now available for the synthesis of a large variety of heterogeneous molecular catalysts. Their specific domain of application, and the main advantages and drawbacks of these solids are briefly analyzed here.

A surface organometallic approach to the synthesis of rhenium-based catalysts for the metathesis of olefins: CH3ReO3/Nb2O5☆

Abstract The reaction of methyltrioxorhenium with the surface of niobium oxide leads to an active catalyst for the metathesis of acyclic olefins. The activity is related to the niobia surface acidity and to a methylrhenium surface species that resonates at 25 ppm on the 13 C MAS NMR spectrum.

Surface supported metal cluster carbonyls. Chemisorption, reactivity, and decomposition of Ru3(CO)12 on silica

Abstract Ru3(CO)12 supported on silica is oxidised by surface SiOH groups in presence of water and/or dioxygen to form oxidised species, probably incorporated into the silica surface, such as RuII(CO)n(OSi )2 (n = 2, 3). The presence of air (involving both water and dioxygen) greatly accelerates this oxidative process. When supported in total absence of dioxygen, Ru3(CO)12 reacts with surface silanol groups to produce the grafted cluster HRu3(CO)10(OSi ), which has been characterized by chemical methods and by infrared and Raman spectroscopies. The grafted cluster is not very stable; it is an intermediate in the formation, by controlled thermal decomposition, of both small metallic particles and some RuII(CO)n(OSi )2 (n = 2, 3) carbonyl surface species (this oxidation by surface protons is also evidenced by the formation of molecular hydrogen). The formation of metallic ruthenium accounts for the production of hydrocarbons during the thermal decomposition.

The characterization and thermal stability of a cluster grafted on silica surface

Abstract Ru 3 (CO) 12 , supported on silica in the absence of oxygen, reacts with silanol groups of the surface to produce a grafted cluster , which has been characterized by IR and Raman spectroscopy; the molecular formula of this cluster is in agreement with the stoichiometric balance of CO evolved during its formation from Ru 3 (CO) 12 . The grafted cluster is an intermediate step to produce by thermal decomposition small metallic ruthenium particles of 14 A together with some Ru(II) carbonyl species encapsulated in the silica surface.

Palladium(0)-catalyzed substitution of allylic substrates in perfluorinated solvents

Abstract Palladium(0)-catalyzed alkylation reactions of allylic substrates can be performed using the new concept of fluorous biphasic system, allowing a very easy recycling of the catalyst.

Group 4 alkyl complexes as precursors of silica anchored molecular catalysts for the reduction of ketones by hydrogen transfer

Abstract The silica anchored mononuclear isopropoxides of the elements of group 4, SiOM(O i Pr) 3 , M=Ti, Zr, Hf, were synthesized via a two-step procedure, comprising: (i) the reaction of the tetraneopentyl complexes MNp 4 with a silica partially dehydroxylated at 500°C; (ii) the reaction of the surface complexes thus obtained with isopropanol. The supported Ti complex is totally inactive for the reduction of ketones by isopropanol; furthermore a significant fraction of titanium leaches into the solution. On the contrary, the zirconium and hafnium complexes are efficient catalysts for the same reaction. Their properties are dependent upon the nature of the ketone, the nature of the solvent and are sensitive to the presence of water. Under all conditions so far tested, the supported hafnium catalyst exhibits a higher activity than the zirconium one. The very similar catalytic behaviors of SiOHf(O i Pr) 3 and SiOHf(OH) 3 , a complex obtained by mild hydrolysis of SiOHfNp 3 , are interpreted by the easy substitution of the hydroxy ligands by isopropanol, evidenced by in situ IR spectroscopy; unexpectedly, the reverse reaction is much more difficult. Finally, the better performances of SiOZr(O i Pr) 3 when compared to those of a formally similar solid synthesized from Zr(O i Pr) 4 and silica highlights the importance of the choice of the precursor and of the surface state of silica to obtain a stable, mononuclear active species.

A novel heterogeneous molecular catalyst for the Meerwein-Ponndorf-Verley and Oppenauer reactions

Abstract The silica anchored mononuclear (tris)isopropoxyzirconium, (Sio)Zr(OiPr) 3 , is an efficient catalyst for the reduction of ketones and the oxidation of alcohols by hydrogen transfer. Easy separation of the products and recyclability of the catalyst are demonstrated.

Surface organometallic chemistry on zeolites: A tool for modifying the sorption properties of zeolites

Abstract The shape selective properties of zeolites can be modified by reaction of their surface functional groups with hydride, alkyl or alkoxy complexes of main group elements. The different methods are reviewed and their effects analyzed.

Palladium(0) allylic alkylation in a two-phase system or with a supported aqueous phase catalyst

Abstract The reaction of allylic carbonates with various acyclic and cyclic carbonucleophiles is catalyzed by the system Pd(OAc)2 and P(C6H4-m-SO3Na)3 (or tppts) in a two-phase liquid medium H2O-nitrile, the activity of the catalyst depending mainly on the nature of the nitrile, the temperature of the reaction and the ratio palladium/tppts. The same system Pd(OAc)2 and P(C6H4-m-SO3Na)3 supported on silica catalyzes also this reaction. The formation of the active palladium species in the two cases is followed by 31 P NMR spectroscopy and discussed.