Search results for " Catalysis"

Density functional theory study of the Lewis acid-catalyzed Diels-Alder reaction of nitroalkenes with vinyl ethers using aluminum derivatives

2002

The inverse electron demand Diels–Alder reaction of nitroalkenes with vinyl ethers catalyzed by Lewis acids based on aluminum metal, AlMe3, Al(OMe)3 and AlCl3 was studied using density functional theory methods. A continuum model was selected to represent the effects of dichloromethane used as solvent. For this cycloaddition reaction two reactive channels corresponding to the endo and exo approach modes of the vinyl ether to the Lewis acid-coordinated nitroalkene were studied. Coordination of the aluminum metal to an oxygen atom of the nitroalkene increases the electrophilicity of the heterodiene, decreasing the activation energy of the cycloaddition. The substitution effect on the aluminum…

Exploring the potential of some yeast strains in the stereoselective synthesis of aldol reaction products and its reduced 1,3-dialcohol derivatives

2013

Abstract The behavior of two yeast strains has been studied under different conditions. Both microorganims catalyzed the aldol reaction between activated aldehydes and acetone when a large amount of the latter was present in the reaction medium producing, with moderate stereoselectivity, the aldol product with the R configuration. No reduction of any of the products present in the medium was detected. On the other hand, the carbonyl group of the racemic aldol was reduced to produce chiral 1,3-dialcohol derivatives when water was employed as the only solvent. In this case, the resolution of the racemic starting material was also possible with one of the biocatalysts, and the aldol was recove…

Gold catalysts supported on CeO2 and CeO2–Al2O3 for NO reduction by CO

2006

Abstract The reduction of NO x by CO was studied over gold catalyst supported on ceria and ceria–alumina. The mixed supports with different CeO 2 /Al 2 O 3 ratios were prepared by co-precipitation. The catalysts were characterized by means of XRD, TPR, XPS and Raman spectroscopy. The addition of alumina led to a slight enlargement of the gold particles, while the ceria particle size was decreased. Deeper oxygen vacancies formation in the presence of alumina was detected by TPR, XPS and Raman spectroscopy, compared to the pure ceria support. The samples exhibited a high and stable activity and 100% selectivity towards N 2 was reached at 200 °C.

V-containing MCM-41 and MCM-48 catalysts for the selective oxidation of propane in gas phase

2001

Well-organised V-containing MCM-41 and -48 (0.3‐1 wt.% of V content) have been synthesised by one-pot synthesis or by grafting using VOSO4 or VOCl3 as V sources, respectively. The samples before and after the calcination step have been characterised by several physicochemical techniques (Ar and N2 adsorption, XRD, diffuse reflectance-UV‐VIS (DR-UV‐VIS) spectroscopy, temperature-programmed reduction). It was found that V species in the as-prepared catalysts were mainly as vanadyl ions (VO 2C ), while highly dispersed V 5C species with tetrahedral coordination were observed in the calcined materials. The catalytic behaviour of the calcined materials for the gas phase oxidation of propane has …

ChemInform Abstract: Selectively Deprotectable Carbohydrates Based on Regioselective Enzymatic Reactions.

2010

Phase analysis and oxygen strorage capacity of ceria-lanthana-based TWC promoters prepared by sol-gel routes

2002

Ceria–lanthana-based promoters of three-way catalysts are synthesized by two different sol–gel routes, involving nitrate precursors. The oxygen uptake ability of these compounds is measured by O2 chemisorption. The specific surface area is determined by N2 adsorption (BET). X-ray diffraction data are analyzed by Rietveld refinement, demonstrating that lanthanum forms solid solution with CeO2; its total amount in ceria depends on the competitive formation of La–Al mixed oxides and on the synthetic method. The O2 uptake ability is essentially determined by the La content in the ceria–lanthana solid solution, while it is independent on the surface area and on the CeO2 particle size. The O2 upt…

A Liquid-Liquid Biphasic Homogeneous Organocatalytic Aldol Protocol Based on the Use of a Silica Gel Bound Multilayered Ionic Liquid Phase

2012

An innovative two stage liquid–liquid biphasic homogeneous protocol for the asymmetric organocatalytic aldol reaction is proposed, based on the use of the cis-ion-tagged proline 8 dissolved in the liquid film of a multilayered ionic liquid covalently bonded to silica gel 4. The resulting catalytically active material 9 is first soaked with cyclohexanone in the presence of water, resulting in a semi-transparent gel, then the aldehyde is added and the mixture stirred at RT. In the first stage, 4 acts as a catalyst reservoir that delivers 8 to the cyclohexanone phase allowing the reaction to take place homogeneously. In the second stage, cyclohexanone is removed under vacuum and the resulting …

Inorganic materials acting as heterogeneous photocatalysts and catalysts in the same reactions

2016

General considerations on the differences and similarities between heterogeneous photocatalysis and thermal catalysis are presented. Some research papers are reviewed where a reaction has been carried out in the presence of an inorganic material used both as catalyst and photocatalyst. The existing literature often compares catalytic reactions undertaken with the contemporaneous presence of radiation, showing only that photocatalytic reactions can occur under milder experimental conditions and at much lower temperatures. Nevertheless, differences in mechanistic aspects, conversions and selectivities between catalytic and photocatalytic reactions should also be highlighted. These are due to …

Using theozymes for designing transition-state analogs for the intramolecular aldol reaction of δ-diketones

2001

Two theozymes for the intramolecular aldol reaction of δ-diketones have been studied using ab initio methods. The presence of both acid/base residues favors several steps of the aldol reaction. The appropriate positioning of these residues can accelerate one of two diastereromeric reaction pathways, the catalyzed aldol reaction being highly stereoselective. Analysis of the geometrical parameters, charge distribution, and the shape of molecular electrostatic potential for the corresponding acid/base catalyzed transition structure allows us to design adequate transition-state analogs to favor a reactive channel of this intramolecular aldol reaction. © 2001 John Wiley & Sons, Inc. Int J Quant …

Heterogeneous vs Homogeneous Palladium Catalysts for Cross-Coupling Reactions

2012

A large number of immobilized-Pd-catalysts for cross-coupling reactions have been introduced in the last decade. Are the observed catalyzed reactions truly heterogeneous or are they homogeneous due to leached palladium? This account critically addresses the leaching issue by selectively referring to some of the newly developed catalytic systems in an attempt to evaluate said systems based on uniform criteria. The report is concluded by identifying the relevant chemical and structural challenges in the field.