Search results for " Catalysis"
showing 10 items of 585 documents
Carbocatalytic Oxidative Dehydrogenative Couplings of (Hetero)Aryls by Oxidized Multi‐Walled Carbon Nanotubes in Liquid Phase
2019
HNO3-oxidized carbon nanotubes catalyze oxidative dehydrogenative (ODH) carbon-carbon bond formation between electron-rich (hetero)aryls with O-2 as a terminal oxidant. The recyclable carbocatalytic method provides a convenient and an operationally easy synthetic protocol for accessing various benzofused homodimers, biaryls, triphenylenes, and related benzofused heteroaryls that are highly useful frameworks for material chemistry applications. Carbonyls/quinones are the catalytically active site of the carbocatalyst as indicated by model compounds and titration experiments. Further investigations of the reaction mechanism with a combination of experimental and DFT methods support the compet…
Diels-Alder Reactions of 2-Azabutadienes with Aldehydes: Ab Initio and Density Functional Theoretical Study of the Reaction Mechanism, Regioselectivi…
1997
The Diels−Alder reaction of 2-azabutadiene with aldehydes has been studied using high level ab initio molecular orbital and density functional methods. Multiconfigurational calculations were carried out on the concerted and stepwise mechanisms. At the CASPT2F/6-31G*//CASSCF/6-31G* level of theory, the [π4s + π2s]-cycloaddition of 2-azabutadiene with formaldehyde is predicted to be a concerted reaction, in good agreement with the experimental evidence. The regio- and stereoselectivity of the reaction was studied at the HF/6-31G*, MP2/6-31G*, and Becke3LYP/6-31G* levels of theory. The density functional calculations appears to give a good description of the basic features of the reaction. The…
A DFT Study of the Molecular Mechanisms of the Diels−Alder Reaction between Cyclopentadiene and 3-Phenyl-1-(2-pyridyl)-2-propen-1-one − Role of the Z…
2002
The molecular mechanism of the Diels−Alder reaction between cyclopentadiene (1) and 3-phenyl-1-(2-pyridyl)-2-propen-1-one (2) in the absence and in the presence of a Zn2+ Lewis acid catalyst has been studied by quantum mechanical calculations at the B3LYP/6-31G* level of theory. A continuum model was selected to represent the effects of the water as solvent. For the uncatalyzed process, two channels, endo and exo, were characterized, and the mechanism corresponded to an asynchronous concerted reaction associated with a [4+2] process. The presence of a Lewis acid catalyst changed the mechanism drastically, the reaction taking place by a polar stepwise mechanism. In the first step, a C−C sigm…
Hydrogenation of acetylene in ethylene rich feedstocks: Comparison between palladium catalysts supported on pumice and alumina
1998
Abstract The activity–selectivity patterns of Pd/pumice catalysts are compared with industrial and home prepared Pd/Al2O3 catalysts in the hydrogenation of acetylene in ethylene rich feedstocks (front-end and tail-end cuts). The iso-kinetic relationship (IKR) approach and a new mathematical model, surface site evolution model (SSEM), are employed in this comparison. Pumice and alumina supported Pd catalysts show different metal redox properties. A similar reaction mechanism is adequate to describe the reaction pathway independently by the catalysts and the gas mixtures considered. This mechanism involves the formation of surface polymers during the catalytic reactions and different catalyti…
Oxygen-assisted hydroxymatairesinol dehydrogenation: a selective secondary-alcohol oxidation over a gold catalyst.
2013
Selective dehydrogenation of the biomass-derived lignan hydroxymatairesinol (HMR) to oxomatairesinol (oxoMAT) was studied over an Au/Al(2)O(3) catalyst. The reaction was carried out in a semi-batch glass reactor at 343 K under two different gas atmospheres, namely produced through synthetic air or nitrogen. The studied reaction is, in fact, an example of secondary-alcohol oxidation over an Au catalyst. Thus, the investigated reaction mechanism of HMR oxidative dehydrogenation is useful for the fundamental understanding of other secondary-alcohol dehydrogenation over Au surfaces. To investigate the elementary catalytic steps ruling both oxygen-free- and oxygen-assisted dehydrogenation of HMR…
Theoretical study of the mechanism of branching rearrangement of carbenium ions
1996
Abstract Owing to the practical interest of the acid catalyzed isomerization reactions of hydrocarbons, the mechanism of the branching rearrangements of C 4 H 9 + and C 5 H 11 + carbenium ions has been studied theoretically using ab initio methods which include electron correlation and extended basis sets. It has been found that the protonated cyclopropane-type species does not appear as a common intermediate for these reactions, since it is a transition state and not a minimum on the potential energy surfaces studied. In the case of C 4 H 9 + cation, the protonated methyl-cyclopropane ring is the transition state for the carbon scrambling reaction in the secondary n-butyl cation, while the…
Kinetics of homogeneous catalytic chlorination of acetic acid
2007
The kinetics of the chlorination of acetic acid (HAc) to monochloroacetic acid (MCA) and the physical solubility of chlorine in HAc and its mixtures with MCA were investigated in a laboratory scale semibatch reactor. The temperature dependence of the Henry's constant was determined from solubility data obtained at temperatures between 70°C and 110°C. The kinetic experiments were performed at 75–100°C with 3–13 mol % acetyl chloride (AcCl) and 0–5 weight % H2SO4 added as catalysts. Gas chromatography was used to determine the concentrations of HAc, MCA, dichloroacetic acid and AcCl during the experiments. The results indicate that the reaction rate is independent of the concentrations of HAc…
Unraveling the Role of the Rh–ZrO2 Interface in the Water–Gas-Shift Reaction via a First-Principles Microkinetic Study
2018
The industrially important water–gas-shift (WGS) reaction is a complex network of competing elementary reactions in which the catalyst is a multicomponent system consisting of distinct domains. Herein, we have combined density functional theory calculations with microkinetic modeling to explore the active phase, kinetics, and reaction mechanism of the WGS over the Rh–ZrO2 interface. We have explicitly considered the support and metal and their interface and find that the Rh–ZrO2 interface is far more active toward WGS than Rh(111) facets, which are susceptible to CO poisoning. CO2 forming on the zirconia support rapidly transforms into formate. These findings demonstrate the central role of…
Surface phase transitions at O and CO catalytic reaction on Pd(111)
2006
Abstract The model has been proposed to simulate numerically the reaction O + CO → CO2 and occurring phase transitions on Pd(1 1 1) surface. We calculate the phase diagram for this system by kinetic Monte Carlo method. It shows the phase transitions p ( 2 × 2 ) O → 3 × 3 R 30 O ° and p ( 2 × 2 ) O → 3 × 3 R 30 O ° → p ( 2 × 1 ) O with increase of CO coverage for room and intermediate temperatures, respectively, while in the low temperature limit the direct p(2 × 2)O → p(2 × 1)O phase transition is observed. We demonstrate that the reaction rate is the crucial factor determining the occurrence of the p(2 × 1)O phase and vanishing of the 3 × 3 R 30 O ° with decrease of temperature. The result…
Sex steroids, carcinogenesis, and cancer progression
2004
The relationship between sex steroids and cancer has been studied for more than a century. Using an original intact cell analysis, we investigated sex steroid metabolism in a panel of human cancer cell lines, either hormone responsive or unresponsive, originating from human breast, endometrium, and prostate. We found that highly divergent patterns of steroid metabolism exist and that the catalytic preference (predominantly reductive or oxidative) is strictly associated with the steroid receptor status of cells. We explored intra-tissue concentrations and profiles of estrogens in a set of human breast tumors as compared to normal mammary tissues, also in relation to their estrogen receptor s…