分子催化最新文献

Cobalt catalysts obtained from the reduction of Co(acac)₂ with Al(Et)₃ have been studied by infrared spectroscopy (in the selective hydrogenation of multifunctional compounds). These solids prepared in situ were in suspension in a liquid mixture containing solvent and reagent. In order to obtain information on the preparation and the composition of the catalyst we carried out an FT-IR characterization using a special device warranting sample manipulation in air-free atmosphere. At room temperature there were instantaneous and reductive ligand-exchange between Al(Et)₃ and Co(acac)₂ and formation of Co⁰ particles, Co⁰ soluble complexes and Al(Et)₂(acac). The multistep process may be initiated through the formation of a donor-acceptor complex ((acac)Co(acac)→Al(Et)₃). The presence of CO in the gas phase (H₂) when heating the reaction mixture up to 180°C enhances the reduction of cobalt and probes Co⁰ in different coordination symmetries. Some of the Co⁰ species could be surrounded with cobalt alkoxide species and aluminium acetylacetonate.

The kinetics of the “classical” and the anthraquinone-2-sulfonate (AMS) catalysed alkaline oxidative degradation of lactose and related carbohydrates have been investigated. Batch experiments were carried out at initial sugar concentrations from 100 to 375 mol m⁻³, AMS concentrations from 0 to 5 mol m⁻³, di-oxygen concentrations from 0.28 to 1.38 mol m⁻³, a pH from 11.5 to 13.5 and temperatures at 293 and 303 K. A reaction network is presented that accounts for the main products formed. Regression analysis of the experimental data, using a multi-response Marquardt algorithm, allowed the experimental data to be described adequately by a reaction sequence consisting of different oxidation pathways starting from the sugar enediolates and having the formation of the latter as the common, most important, rate-determining step.

The effect of β-cyclodextrin (BCD) and its methyl and polymer derivatives on the stereoselective reduction of menthone was studied. Modification of the yield and the proportion of epimeric alcohols formed were found to be the salient features of this reaction. Reduction of menthone in aqueous solution in presence of heptakis-2,6-di-O-methyl-β-cyclodextrin (DMBCD) remarkably enhanced the yield to 68% (from 10%) resulting in a menthol/neomenthol ratio of 1:2.5. Aqueous DMF (1:1) as the solvent increased the yield from 14.0% in water to 76% in the presence of BCD, the menthol/neomenthol ratio being 1:3.6. Under phase transfer condition, DMBCD in water—benzene mixture gave 82.0% yield along with a good stereoselectivity compared to 47.0% in the absence of a phase transfer catalyst.

The activity of various 3d, 4d and 5d transition metal oxides in the selective reduction of nitrobenzene to nitrosobenzene has been investigated. An attempt has been made to find a correlation between these activities and several properties of the oxides investigated, such as their place in the Periodic Table, structure, reducibility, heat of formation and surface metal—oxygen bond strength. A weak volcano-shape correlation can be seen with the heat of formation and the surface metal—oxygen bond strength.

The catalytic reduction of CO₂ has been performed over Prussian blue-deposited TiO₂ particles, and the products were methanol, ethanol, acetaldehyde and acetone with a negligibly small amount of methane. The deposition of Prussian blue (PB) onto TiO₂ particles was accomplished under the irradiation of light, and on this occasion PB was further reduced to ES (reduced form of PB). The hydrogenation Of CO₂ taking place in dark was caused by the reaction with active hydrogen atoms generated in the redox reaction consisting of the oxidation of ES to PB and the reduction of H₂O. The deactivated catalyst was restored in KCl aqueous solution under the illumination of light by photo-rereducing PB to ES.

The effect of crown ethers as phase transfer catalysts (PTC) was investigated in the liquid-phase oxidation of 3,5-di-tert-butylcatechol (3,5-DtBC) using potassium permanganate. Both the liquid—liquid (aqueous liquid layer including KMnO₄/organic liquid layer including crown ether and 3,5-DtBC) and the solid—liquid (solid KMnO₄/organic liquid layer including crown ether and 3,5-DtBC) systems were studied under mild reaction conditions. The oxidations of 3,5-DtBC in both systems were promoted by using crown ethers as PTCs. In the liquid—liquid system, the influence of organic solvents, acid additives and the type of crown ethers utilized were investigated. The oxidation rate of 3,5-DtBC was increased by using a solvent with a low polarity such as n-hexane and/or by adding an organic or inorganic acid to the system. Crown ethers which have a cavity that conforms to the radius of the K⁺ ion, and were more lipophilic such as dicyclohexano-18-crown-6 (DC18C6) also caused an increase in the oxidation rate. The rate-determining step is thought to be the oxidation reaction step in the organic phase, rather than the phase transfer step of the crown ether—KMnO₄ complex. In the solid—liquid system, the influence of organic solvents and the type of crown ethers was investigated. The rate of 3,5-DtBC oxidation was increased by using a solvent with a high polarity such as chloroform. The complex stability between the crown ether and K⁺ was suggested to be one of the most important factors governing the oxidation rate in the solid—liquid system.

The chemisorption and decomposition of benzene, cyclohexene and cyclohexane on clean and carbided Fe(110) surfaces has been studied by electron impact-induced Auger electron spectroscopy. Distinct and characteristic carbon (1s VV) Auger line-shapes of the three cyclic hydrocarbons molecularly adsorbed on a clean Fe(110) surface are reported. The thermal decomposition pathway of both benzene and cyclohexene have been followed and found to be similar. Cyclohexane has been found to desorb from the clean surface at well below room temperature without undergoing decomposition.

An imidazole-containing polymer inprinted with a transition state analogue exhibited homogeneous esterolytic catalysis efficiently.

The scheme of the gas phase phenol acylation with acetic acid on a HZSM5 zeolite was established from the effect of contact time (hence of conversion) on the product distribution. Phenyl acetate and o-hydroxyacetophenone are primary products, O-acylation being much faster than C-acylation. At high conversion, part of the o-hydroxyacetophenone results from the acylation of phenol with phenyl acetate. The formation of p-hydroxyacetophenone which does not occur through phenol acylation involves the hydrolysis of p-acetoxyacetophenone selectively formed through the autoacylation of phenyl acetate. The ortho-selectivity of phenol acylation can be related to a pronounced stabilization of the transition state while the para-selectivity of phenyl acetate autoacylation could be due to a steric hindrance to the approach of the acetyl group in the ortho-position of phenyl acetate.

Several tungsten carbonyl complexes: W(CO)₆, [NEt₄] [W(CO)₅Cl], [W(CO)₄) (CH₃CN)₂], [W(CO)₄(pip)₂], [W(CO)₄(bpy)], [W(CO)₄(dppe)] and [WCl₂(CO)₃(PPh₃)₂] exhibit catalytic activity towards polymerization of phenylacetylene (PA) in the presence of ZrCl₄. They cause PA to polymerize under mild conditions, giving soluble, high molecular weight polymers in good yields.

An active carbene-containing catalyst can be generated from a simple carbonyl complex and free alkyne via the alkyne to vinylidene rearrangement.