Aimery De Mallmann

Nickel-silicide colloid prepared under mild conditions as a versatile Ni precursor for more efficient CO2 reforming of CH4 catalysts.

Preparing highly active and stable non-noble-metal-based dry reforming catalysts remains a challenge today. In this context, supported nickel nanoparticles with sizes of 1.3 ± 0.2 and 2.1 ± 0.2 nm were synthesized on silica and ceria, respectively, via a two-step colloidal approach. First, 2-nm nickel-silicide colloids were synthesized from Ni(COD)(2) and octylsilane at low temperature; they were subsequently dispersed onto supports prior to reduction under H(2). The resulting catalysts display high activity in dry reforming compared to their analogues prepared using conventional approaches, ceria providing greatly improved catalyst stability.

SILICA-SUPPORTED TANTALUM CATALYSTS FOR ASYMMETRIC EPOXIDATIONS OF ALLYL ALCOHOLS

Tantalum good, titanium bad: This appears to be the case for silica-supported catalysts for the asymmetric epoxidation of allyl alcohols. Complexes such as [SiO-Ta(OEt)(4)] were prepared from silica and [Ta(=CHCMe(3))(CH(2)CMe(3))(3)], and in the presence of a tartrate and an alkyl hydroperoxide, these surface tantalum compounds lead to efficient and convenient catalysts for the asymmetric epoxidation of 2-propen-1-ol (R=H) and trans-2-hexen-1-ol (R=nPr; see reaction).

On the track to silica-supported tungsten oxo metathesis catalysts: input from 17O solid-state NMR.

The grafting of an oxo chloro trisalkyl tungsten derivative on silica dehydroxylated at 700 °C was studied by several techniques that showed reaction via W-Cl cleavage, to afford a well-defined precatalyst for alkene metathesis. This was further confirmed by DFT calculations on the grafting process. (17)O labeling of the oxo moiety of a series of related molecular and supported tungsten oxo derivatives was achieved, and the corresponding (17)O MAS NMR spectra were recorded. Combined experimental and theoretical NMR studies yielded information on the local structure of the surface species. Assessment of the (17)O NMR parameters also confirmed the nature of the grafting pathway by ruling out other possible grafting schemes, thanks to highly characteristic anisotropic features arising from the quadrupolar and chemical shift interactions.

Asymmetric Epoxidation of Allylic Alcohols Catalyzed by (≡SiO)xTa(OEt)3-x(Dialkyl Tartrate Diolate): Influence of the Reaction Conditions

Silica-supported chiral tantalum alkoxides are active catalysts for the asymmetric epoxidation of propenol and trans hex-2-en-1ol. The influence of different parameters on their catalytic performance was followed: the impregnation duration by a tartrate (step in their preparation); the nature of the solvent (CH2Cl2, pentane, toluene) and of the oxidant (TBHP, CHP, H2O2); poisoning effects by water or t-butanol; the reaction temperature and the substrate concentration.

Low temperature hydrogenolysis of waxes to diesel range gasoline and light alkanes: Comparison of catalytic properties of group 4, 5 and 6 metal hydrides supported on silica–alumina

A series of metal hydrides (M = Zr, Hf, Ta, W) supported on silica–alumina were studied for the first time in hydrogenolysis of light alkanes in a continuous flow reactor. It was found that there is a difference in the reaction mechanism between d0metal hydrides of group 4 and d0 ↔ d2metal hydrides of group 5 and group 6. Furthermore, the potential application of these catalysts has been demonstrated by the transformation of Fischer–Tropsch wax in a reactive distillation set-up into typical gasoline and diesel molecules in high selectivity (up to 86 wt%). Current results show that the group 4 metal hydrides have a promising yield toward liquid fuels.

Small changes have consequences: lessons from tetrabenzyltitanium and -zirconium surface organometallic chemistry.

Homoleptic benzyl derivatives of titanium and zirconium have been grafted onto silica that was dehydroxylated at 200 and 700 °C, thereby affording bi-grafted and mono-grafted single-site species, respectively, as shown by a combination of experimental techniques (IR, MAS NMR, EXAFS, and elemental analysis) and theoretical calculations. Marked differences between these compounds and their neopentyl analogues are discussed and rationalized by using DFT. These differences were assigned to the selectivity of the grafting process, which, depending on the structure of the molecular precursors, led to different outcomes in terms of the mono- versus bi-grafted species for the same surface concentration of silanol species. The benzylzirconium derivatives were active towards ethylene polymerization in the absence of an activator and the bi-grafted species displayed higher activity than their mono-grafted analogues. In contrast, the benzyltitanium and neopentylzirconium counterparts were not active under similar reaction conditions.

Well-Defined Molybdenum Oxo Alkyl Complex Supported on Silica by Surface Organometallic Chemistry: A Highly Active Olefin Metathesis Precatalyst

The well-defined silica-supported molybdenum oxo alkyl species (≡SiO-)MoO(CH2tBu)3 was selectively prepared by grafting of MoO(CH2tBu)3Cl onto partially dehydroxylated silica (silica700) using the surface organometallic chemistry approach. This surface species was fully characterized by elemental analysis and DRIFT, solid-state NMR, and EXAFS spectroscopy. This new material is related to the active species of industrial supported MoO3/SiO2 olefin metathesis catalysts. It displays very high activity in propene self-metathesis at mild (turnover number = 90 000 after 25 h). Remarkably, its catalytic performance outpaces those of the parent imido derivative and its tungsten oxo analogue.

Tantalkatalysatoren auf Kieselgel für die asymmetrische Epoxidierung von Allylalkoholen

Tantal erfolgversprechend – Titan erfolglos: Dies scheint fur auf Kieselgel als Tragermaterial fixierte Katalysatoren zu gelten, die fur die asymmetrische Epoxidierung von Allylalkoholen entwickelt wurden. Komplexe wie [SiO−Ta(OEt)4] wurden aus Kieselgel und [Ta(=CHCMe3)(CH2CMe3)3] synthetisiert. In Gegenwart eines Tartrats und eines Alkylhydroperoxids bilden diese Materialien, an deren Oberflache sich Tantalzentren befinden, effiziente und leicht handhabbare Katalysatoren fur die asymmetrische Epoxidierung von 2-Propen-1-ol (R=H) und trans-2-Hexen-1-ol (R=nPr; siehe Reaktionsgleichung).

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

A new tantalum-hydride supported on zirconium hydroxide [(triple bond SiO)(2)Zr(H)-O-Ta(H)(x)-(OSi triple bond)] (x = 1 or 3) was prepared using surface organometallic chemistry and its catalytic properties in the propane metathesis reaction were assessed showing improved activity and selectivities in comparison to the tantalum-hydride supported on silica.

Grafting Reaction of Organotin Complexes on Silica Catalyzed by Tungstic Heteropolyacids

The grafting reaction of tetramethyltin on silica is catalyzed by H(4)SiW(12)O(40) preliminary impregnated on the support. While the reaction proceeds at temperatures higher than 150 degrees C on silica alone, the presence of the polyacid allows the grafting at room temperature. A study as a function of the polyacid coverage has shown that there is a direct correlation between the reaction rate and the number of highly acidic sites on the support, probing that there is a reaction of the tetraalkyltin with them (limiting step) followed by a migration of the grafted fragment on the silica surface. Not only monografted species (as observed on silica) but also multigrafted tin species are formed because of further reactions of the grafted fragments.