Search results for "catalysts"

Expanded micro-particles by supercritical antisolvent precipitation: Interpretation of results

2008

Abstract Supercritical antisolvent (SAS) micronization has been used to obtain nanoparticles and micro-particles of several kinds of materials. Sometimes hollow expanded micro-particles have also been obtained. This work is focused on the analysis of this last morphology. We organized literature data and our previous experiments and we added new experiments on previously tested compounds and on compounds never tested before. As a result, expanded micro-particles using several compounds belonging to different categories and precipitated from different solvents in laboratory and pilot scale plants were obtained with diameters between about 10 and 180 μm. They also showed different sub-structu…

Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3-loaded TiO2 catalysts

2015

Abstract The photocatalytic oxidation of trans-ferulic acid to vanillin in water has been studied by using various TiO2 and WO3-loaded TiO2 samples as catalysts. Different values of selectivity were obtained depending on the physico-chemical properties of the single samples and a vanillin selectivity of 10% was reached in the presence of the commercial TiO2 Merck. Higher selectivity values were obtained by impregnation of TiO2 with H2WO4 followed by calcination. The increased production of vanillin exhibited by the obtained WO3-loaded TiO2 powders was attributed to a reduced further oxidation of the aldehyde caused by the presence of the practically inactive tungsten trioxide hydrate on the…

(Co)polymerization behavior of supported metallocene catalysts. I. Ligand and substituent effect

2005

Ethylene polymerization and its copolymerization with 1-hexene with a set of supported metallocene catalysts were studied. As a carrier, the complex of mag- nesium chloride with tetrahydrofuran, which was previously pretreated with a triiso- butylaluminium (TIBA), was used. The investigated metallocene compound differs in the metal type (Zr or Ti), the nature of the alkyl substituent in the cyclopentadienyl ring, and the type of ligand (Cp or Ind). The effect of catalyst composition on the anchored metal content, catalyst activity, comonomer reactivity, and polymer proper- ties was investigated. The results obtained with supported catalysts were compared with those obtained with their homog…

(Co)polymerisation Behaviour of Supported Metallocene Catalysts: Carrier Effect

2006

The polymerisation and copolymerisation of ethylene with 1-hexene over metallocene catalysts L 2 ZrCl 2 / MAO (L = Cp, n-BuCp, t-BuCp, i-PrCp, Me 5 Cp) supported on different types of carriers (MgCl 2 (MeOH) 6 or silica with CH 3 surface groups obtained in the sol-gel process) were studied. It was demonstrated that both the metallocene structure and the type of inorganic support affected catalyst activity and polymer properties such as melting point, molecular weight and molecular weight distribution. The metallocene structure also determined comonomer incorporation, both for homogeneous and supported catalytic systems. When a catalyst is anchored on a support, it becomes less effective at …

Asymmetric Aldol Reaction with Polystyrene-Supported Proline-Based Catalysts

2009

Die herstellung einiger gekennzeichneter, aliphatischer azoverbindungen und ihre anwendung als initiatoren der polymerisation

1970

Es wird die Herstellung von vier Azoverbindungen beschrieben, die sich zum Teil vom α.α′-Azo-(isobuttersaurenitril) ableiten. Zwei Verbindungen geben bei der Polymerisation von Methacrylsauremethylester und Styrol Endgruppen, die in den Polymeren photometrisch leicht zu bestimmen sind; fur eine wird gezeigt, das sie die Polymerisation von Styrol startet, ohne das storende Nebenreaktionen auftreten. Die beiden anderen Azoverbindungen sind wegen ihrer hohen Zersetzungspunkte geeignet, radikalische Polymerisationen bei hoheren Temperaturen auszulosen (160°C). The preparation of four derivatives of α.α′-azo-bis-isobutyronitrile is described. The polymerization of methylmethacrylate and styrene …

l-Arabinose Conformers Adsorption on Ruthenium Surfaces: A DFT Study

2012

Adsorption of 5 L-arabinose tautomers – one acyclic and four cyclic (α and β, pyranose and furanose) species – on a ruthenium surface was studied as a precursor-process of the, nowadays more and more, industrially important sugar catalytic hydrogenation on metal surfaces in water medium. The study was mostly referred to a 37-atom metal catalyst fragment, even though border-effects on the adsorption processes were also checked employing a 61-atom metal fragment. In order to figure out conformational effects on the title process the tautomer flexibility was, at first, investigated by the genetic-algorithm based code Balloon, considering the conformational spaces of the different aquo tautomer…

Proximity Effect using a Nanocage Structure: Polyhedral Oligomeric Silsesquioxane-Imidazolium Tetrachloro- palladate Salt as a Precatalyst for the Su…

2016

A polyhedral oligomeric silsesquioxane-imidazolium tetrachloropalladate salt (POSS-Imi-PdCl4) was prepared by the reaction of a polyhedral oligomeric silsesquioxane-imidazolium chloride salt (POSS-Imi-Cl) with PdCl2 and used as a pre-catalyst for the Suzuki-Miyaura reaction in water at 100 °C at a low loading (0.08-0.16 mol %). Biphenyl compounds were isolated in high to excellent yields. A comparison of the POSS-based catalyst with the corresponding catalyst without the nanocage structure (i.e., 1-butyl-3-methylimidazolium tetrachloropalladate) highlighted the role of the POSS structure to reach higher yields in the Suzuki-Miyaura reaction. This result is ascribed to a proximity effect of …

Applications and stereoselective syntheses of P-chirogenic phosphorus compounds.

2016

International audience; Phosphorus compounds bearing chirality on the P-center are usually qualified as P-chirogenic or P-stereogenic. This chemical class concerns natural products, agrochemistry, molecular materials, biology and pharmacy, although it is certainly in coordination chemistry and in asymmetric catalysis using chiral transition metal complexes that P-chirogenic phosphorus compounds are the most used. The chiral phosphine ligands and their uses in asymmetric metal-catalyzed reactions have been widely reviewed in literature. However, an overview covering the applications as well as the stereoselective syntheses of all classes of phosphorus compounds has not yet been provided. Thi…

Increased conformational rigidity of humic substances by oxidative biomimetic catalysis

2005

A synthetic water-soluble meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate of iron(III) chloride, Fe(TDCPPS)Cl, was employed as a biomimetic catalyst in the oxidative coupling of terrestrial humic materials. High-performance size-exclusion chromatography (HPSEC), solid-state nuclear magnetic resonance (CPMAS-(13)C NMR), electron paramagnetic resonance (EPR), and diffuse reflectance infrared spectroscopy (DRIFT) were used to follow conformational and structural changes brought about in different humic materials by the oxidative coupling. Increase in apparent weight-average molecular weight (Mw(a)) occurred invariably for all humic substances with the oxidative polymerization catalyzed …