分子催化最新文献

XPS line decomposition of carbon residues on Pt-black and EUROPT-1 reveal the presence of a graphitic, hydrocarbon polymer and oxidized carbon overlayers on both catalysts. Usual regeneration pretreatment with oxygen and hydrogen at reaction temperatures (ca. 600 K) does not remove all carbon from Pt-black or EUROPT-1 but an O₂ treatment of EUROPT-1 at 673 K removes most of the carbon. A ‘coke sensitivity’ is suggested as an additional factor influencing the ‘selectivity’ (the 2-methylpentane/n-hexane (2MP/nH) ratio) of ring opening of methylcyclopentane, the absence of oxidized carbonaceous entities being necessary to produce low 2MP/nH ratios.

A PDHCl catalytic system is highly active and selective in the hydrogen transfer from H₂OCO to the olefinic double bond of the unsaturated γ-ketoacid PhCOCHCHCOOH to PhCOCH₂CH₂COOH. Typical reaction conditions are: P_CO: 20–30 atm; Pd/substrate/H₂O/HCl = 1/400–1000/800–3000/100–1000 (mol); temperature: 100–110°C; [Pd]: 10⁻³ to 10⁻² M; solvent: dioxane; reaction time: 1–2 h. High yields are obtained only when the palladium catalyst is used in combination with HCl. When a palladium(II) catalyst precursor is employed, extensive decomposition to palladium metal occurs. Pd/C shows also high activity. The proposed catalytic cycle proceeds through the following steps. (i) Addition of HCl to the olefinic double bond of the starting substrate gives the chloride PhCOCH₂CHClCOOH, which oxidatively adds to “reduced palladium”, with formation of a catalytic intermediate having a Pd[CH(COOH)CH₂COPh] moiety. “Reduced palladium” is the metal coordinated by other atoms of palladium, and/or by carbon monoxide. (ii) H₂O and CO react on the metal center of this species giving an intermediate having also a carbohydroxy ligand, (HOOC)Pd[CH(COOH)CH₂COPh]. (iii) β-hydride abstraction from the carbohydroxy ligand gives a hydride HPd[CH(COOH)CH₂COPh], with evolution of CO₂. (iv) Finally, reductive elimination of the product PhCOCH₂CH₂COOH returns the catalyst to the catalytic cycle. Alternatively, protonolysis of the intermediate formed in the first step yields directly the final product and a Pd(II) species, which is reduced by CO and H₂O to palladium metal back into the catalytic cycle. This is supported by the fact that when PhCOCHCHCOOH is allowed to react with a stoichiometric amount of Pd/C, in the presence of HCl and of CO and in the absence of H₂O, PhCOCH₂CH₂COOH is formed in a significant amount.

Two industrially important dehydrogenation reactions were catalyzed with Cu/γ-Al₂O₃ catalyst prepared by the electroless copper plating technique. One is the dehydrogenation of cyclohexanol and the other is the dehydrogenation of isopropanol to realize the catalytic activity of these catalysts on cyclic and noncyclic alcohols. The dehydrogenation activity strongly depends on the number of exposed copper sites and the dehydration activity clearly influences the effective acid sites of the catalysts. The copper surface area increases with loading up to ca. 15 wt.% Cu; above this loading the reduction of copper surface area was observed. The efficiency of the acid site is almost same and is independent of the preparation method up to the lower copper loading range. But at high copper content the efficiency of the acid site is relatively low when compared with the catalysts prepared by the urea hydrolysis procedure. Though the acid amount of the catalysts prepared by the electroless method was higher than that of the catalysts prepared by the urea hydrolysis procedure, the selectivity to cyclohexene was relatively low.

A comprehensive investigation of the ability of Naƒon to influence catalytic reactions has been undertaken using the [Pd( 1,10-phen)₂]²⁺ species immobilized in Naƒon—H⁺. The effects of catalyst loading, solvent, substrate, temperature and pressure on catalytic activity have been studied. We have found that whilst the homogeneous [Pd(1,10-phen)₂][p-tolyl-SO₃]₂ model complex dimerizes ethene and propene, and isomerizes 1-butene, Naƒon supported [Pd(1,10-phen)₂]²⁺ effectively dimerizes ethene only. The ethene dimerization activity of the supported catalyst has been found to be highly dependent on catalyst loading and solvent. Rate limitations are overcome at very low catalyst loadings and activities as high or higher than homogeneous activities were obtained. It appears likely that ethene solubility in the reaction solvent, ethene diffusion through Naƒon, Naƒon morphology, and possible steric constraints on the catalyst at high loadings are important factors. The use of water as the reaction solvent resulted in a dramatic increase in the activity of the supported catalyst and adds weight to the argument that cation-anion separation and a non-coordinating anion are important aspects of catalysis using cationic Pd(II) species. Tests with propene and 1 -butene as the feed indicate that the activity of the supported catalyst is limited by the diffusion of these substrates in Naƒon.

Methyltrioxorhenium CH₃ReO₃ (MTO, 1) is one of the most active catalysts of the Baeyer-Villiger oxidation of cyclic ketones by use of H₂O₂ as oxidant. In contrast to the chemistry of established molybdenum and tungsten peroxo complexes, the bis(peroxo) complex [CH₃ReO(O₂)₂·H₂O] reacts stoichiometrically with ketones. The electronic character of the active species was determined by spectroscopy and by application of an oxygen transfer probe.

This paper reviews the use of Spanish sepiolites as supports for Pd catalysts employed for the reduction of variously substituted alkenes, the selective semi-hydrogenation of phenylacetylene and the reduction of acetophenone, both with gaseous hydrogen and by hydrogen transfer with cyclohexene as donor. The performance of Pd/sepiolite catalysts is compared with that of synthesized catalysts supported over SiO₂, SiO₂AlPO₄, AlPO₄ and AlPO₄Al₂O₃, which we have used extensively, as well as with that of several commercially available catalysts from Fluka, Merck and Degussa.

The oxidation of aldehydes, ketones, sulfides, alcohols and alkanes is achieved by using a soluble β-ketoesterate complex of iron (III), nickel (II) or cobalt (II) in the presence of a branched aldehyde and molecular oxygen (or air) at room temperature and atmospheric pressure (Mukaiyama's conditions).

A cyanomethylated polybenzimidazole coordinated to Pd(II) has been employed as a Wacker-type alkene oxidation catalyst in aqueous ethanol. Starting with alk-1-enes isomerisation to the more thermodynamically stable internal alkenes is very much faster than oxidation. Indeed after only a short time no alk-1-ene is detectable e.g. by nuclear magnetic resonance analysis. Almost certainly, however, traces of the alk-1-ene do exist in equilibrium. Irrespective of whether the starting alkene is oct-1-ene, t-oct-2-ene or t-oct-4-ene the same three products are obtained: octan-2-one, -3-one and -4-one. In the case of oct-1-ene and t-oct-2-ene the composition of the ketone product mixture is very similar, although with t-oct-4-ene a significant increase in the proportion of the 4-one is observed. The major product in all cases however in the 2-one. The latter almost certainly arises from rapid oxidation of a small stationary concentration of alk-1-ene, with shift of the alkene equilibria maintaining the latter. Direct oxidation of the higher alkenes to the higher ketones occurs more slowly, but contrary to other reports this is significant.

The kinetics of dissolution and oxygen catalysis of RuO₂·xH₂O samples, thermally activated at different temperatures in the range ambient to 100°C, are reported. The results of a kinetic study show that increasing the temperature of activation causes an increasingly thick, dissolution-inert and diffuse layer to form on the outside of the particles. The observed decrease in activity towards dissolution with increased annealing temperature is mirrored an by increase in the activity of the samples as catalysts for the oxidation of water to O₂ by the BrO⁻₃ ions present. The results of this work re-enforce the crucial link between the water content of a hydrated sample of ruthenium dioxide and its activity as an oxygen catalyst.

Supported rhenium tricarbonyls were prepared by reductive carbonylation of K₂ReCl₆ adsorbed on MgO powder. K₂ReCl₆ on MgO was treated at 350°C in CH₃OH-saturated CO, leading to the formation Of [Re(CO)₃{HOMg}_x{OMg}_3-x], where the braces denote groups terminating the bulk MgO, as evidenced by infrared spectra. The synthesis is the simplest known for preparation of supported rhenium subcarbonyls.