Grafting strategy to develop single site titanium on an amorphous silica surface.

Titanium/silica systems were prepared by grafting a titanium alkoxide (titanium isopropoxide and titanium (triethanolaminate) isopropoxide) precursor onto amorphous silica. The grafting process, which consisted of the hydrolysis of the Ti precursor by the hydroxyl groups on the silica surface, yielded samples containing Ti-loadings of 1-1.6 wt %. The as synthesized and calcined TiO(2)-SiO(2) samples were characterized by UV-vis, FTIR, XPS, and XANES spectroscopic techniques. These systems were tested in the liquid-phase epoxidation of oct-1-ene with hydrogen peroxide reaction. Spectroscopic data indicated that titanium anchoring takes place by reaction between the alkoxide precursor and surface OH groups of the silica substrate. The nature of surface titanium species generated by chemical grafting depends largely on the titanium precursor employed. Thus, the titanium isopropoxide precursor yields tetrahedrally coordinated polymeric titanium species, which give rise to a low-efficiency catalyst. However, if an atrane precursor (titanium (triethanolaminate) isopropoxide) is employed, isolated titanium species are obtained. The fact that these species remain isolated even after calcination is due to the protective effect of the triethanolaminate ligand that avoids titanium polymerization. These differences in the titanium environment have a pivotal role in the performance of these systems in the epoxidation of alkenes with hydrogen peroxide.

A density functional theory study of the dissociation of H2 on gold clusters: importance of fluxionality and ensemble effects.

Density functional theory was employed to calculate the adsorption/dissociation of H2 on gold surfaces, Au(111) and Au(100), and on gold particles from 0.7 (Au14) to 1.2 nm (Au29). Flat surfaces of the bulk metal were not active towards H2, but a different effect was observed in gold nanoclusters, where the hydrogen was adsorbed through a dissociative pathway. Several parameters such as the coordination of the Au atoms, ensemble effects and fluxionality of the particle were analyzed to explain the observed activity. The effect of the employed functional was also studied. The flexibility of the structure, i.e., its adaptability towards the adsorbate, plays a key role in the bonding and dissociation of H2. The interaction with hydrogen leads to drastic changes in the structure of the Au nanoparticles. Furthermore, it appears that not only low coordinated Au atoms are needed because H2 adsorption/dissociation was only observed when a cooperation between several (4) active Au atoms was allowed.

Removal of PAH compounds from liquid fuels by Pd catalysts.

Palladium catalysts supported on gamma-alumina (AN, AS), amorphous silica-alumina (ASA), and beta-zeolite (betaZ) were prepared with the aim to reduce the content of polycyclic aromatic hydrocarbons (PAHs) in diesel fuels. The removal of PAH compounds was evaluated with a model feed (toluene, naphthalene, and dibenzothiophene)that approached the composition of diesel fuel. The catalysts were characterized by N2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy, transmission electron microscopy, temperature-programmed reduction/temperature-programmed oxidation, Fourier transform (FT) IR of absorbed CO, and diffuse reflectance FT spectroscopy of adsorbed NH3. When palladium was supported on ASA instead of AN or betaZ, the intrinsic activity improved considerably. Such behavior is discussed in terms of larger Brønsted acidity and the degree of reduction of the Pd species on Pd/ASA. For the 2.2 Pd/betaZ catalyst, interaction of the Pd clusters with zeolite protons led to formation of Pddelta+ species, which are not active in the hydrogenation of aromatics. All catalysts were resistant to poisoning by S compounds.

Silylation and surface properties of chemically grafted hydrophobic silica.

A commercial mesoporous silica (Grace Davison) was chemically grafted with trimethylsilyl chloride (TMSCl) and hexamethyldisilanaze (HMDS). The silylation process brought about some reduction in the specific BET area, the pore volume, and the pore sizes of the samples. Thermogravimetric studies of the silylated samples revealed that the grafting process is kinetically controlled at short reaction times. In the kinetic regime, increasing concentrations of the silylant agent up to 2 wt% in the solvent led to an increase of the extent of the silylated surface, although this limitation disappeared at higher concentrations. Silylation was confirmed by diffuse reflectance infrared Fourier transform (DRIFTS), (29)Si CP-MAS NMR, and photoelectron (XPS) spectroscopic techniques. Solid-state (29)Si MAS-NMR spectra of the silylated samples revealed the presence of -SiCH(3) groups (9.5 ppm) together with two resonances, Q3 (approximately equal to -104 ppm) and Q4 (approximately equal to -114 ppm), coming from siloxane [Qn approximately Si(OSi)n(OH)(4-n), n approximately 2-4] groups, the Q3 signal decreasing upon silylation. The DRIFT spectra of the silylated samples exhibited two well defined bands at 2970 and 2907 cm(-1), due to stretching vibration modes of the C-H bonds in surface -CH(3) groups formed during the silylation process, and also the disappearance of the band at 3740 cm(-1). This observation indicates the complete removal of terminal and geminal hydroxyl groups by grafting with the silylating agent. Similarly, high-resolution photoelectron spectra of the Si2p core levels showed a high binding-energy component (103.5 eV) in all the samples, coming from the Si coordinated with oxide anions in SiO(2), together with a second component at 102.1 eV, which is the fingerprint of Si coordinated by oxide anions and an organic group. Finally, the samples were ranked according to their hydrophobicity, as determined from the temperature-programmed desorption profiles of adsorbed water and 2-methylbutane.

Sulfonic acid-functionalized silica through quantitative oxidation of thiol groups.

A simple procedure for the preparation of amorphous silica containing thiol groups which quantitatively affords sulfonic acid groups has been developed, resulting in site densities and activity for the esterification of acetic acid with methanol greater than a commercial Nafion silica composite.

Highly effective epoxidation of alkenes with Ti-containing soluble polymers.

Ti-containing polysiloxane epoxidation catalysts have been prepared by controlled hydrolysis of titanium- and alkylsilane precursors. These polysilkoxanes exhibit very high yields to epoxides in the epoxidation reaction of primary alkenes with organic hydroproxides.