分子催化最新文献

Zirconia/silicas were prepared by three methods, namely complexing-agent assisted sol—gel, coprecipitation and hydrogel kneading. The relationship between the preparation methods and properties of the zirconia/silicas was investigated by X-ray powder diffraction, solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR), ammonia temperature-programmed desorption (TPD) and nitrogen adsorption. Among the three methods, the complexing-agent assisted sol—gel method gave the most homogeneous zirconia/silicas and the ZrOSi bond formation. The specific surface area of the zirconia/silicas depended on the complexing agents used in the preparations and the Zr/Si ratios. In contrast, the kneaded zirconia/silicas were relatively heterogeneous and their properties did not depend on the Zr/Si ratio. In the dependence of properties of zirconia/silicas on the Zr/Si ratio, the coprecipitation zirconia/silicas are intermediate between the complexing-agent assisted sol—gel and kneaded ones.

Cyclohexanol (CHL) conversion was measured under low-pressure on-stream conditions on HZSM-5 zeolites and on H-boralite using small amounts of catalysts. Catalytic runs were followed by TPD of the surface species, which also were studied using FTIR spectroscopy. It was found that the individual reaction steps, low-temperature dehydration (yielding cyclohexene), skeletal isomerization (ring contraction to methylcyclopentene) and reactions involving H-transfer (formation of methylcyclopentane, substituted aromatics and simple olefins) were conditioned by the presence of strong acid sites whose number needed for these reactions increased from the dehydration to the H-transfer reactions. The CHL reaction was compared with the reactions of cyclohexene, and the effect of Lewis acid sites present in some cases together with the Brønsted sites was discussed. Ammonia was found to block the active centers, reduce the isomerization and especially the reactions involving H-transfer. Contrary to ammonia, methanol reacted with the CHL surface complexes yielding more substituted aromatics and more olefins than CHL or methanol alone (and/or simple sum of CHL and methanol products).

Hydrogenation of anthraquinones at the oxygen is a key step in the industrial production of hydrogen peroxide. The reaction is very fast in the presence of palladium supported catalysts. However, these catalysts also promote the hydrogenation of the aromatic rings of the anthraquinone molecules giving place to substances not participating in the formation of hydrogen peroxide and causing a loss of active quinone. 2-ethylanthraquinone can also give tautomerization to oxanthrone and derivatives with a further loss of active quinone. The kinetics of the hydrogenation of the aromatic rings of 2-ethylanthraquinone are studied. A dual site reaction mechanism well explains the experimental findings. On the basis of this mechanism, kinetic expressions have been developed and the kinetic parameters determined. The contribution of tautomerization to the kinetic pattern has also been ascertained.

In order to understand the role of metal—support interactions in determining the structure and catalytic chemistry of multimetallic catalysts, kinetic and calorimetric studies of FeRh particles supported on a number of refractory oxide supports were conducted. Kinetic studies (1-butene hydroisomerization) showed that selectivity and activity were influenced by a number of parameters, particularly support identity, but also reduction temperature, and time on stream. To explain the observed kinetics it is postulated: (i) catalysis takes place both at metal sites and at acid sites on the surface, and that the selectivity of each site type is very different, (ii) the presence of metal enhances the rate of reaction at the acid sites by providing hydrogen atoms via a spillover mechanism, (iii) the metal loses activity more rapidly than the acid sites and (iv) due to the relative strengths of iron and rhodium interactions with the support, rhodium is preferentially reduced. Microcalorimetric studies of the surface composition support the postulate that rhodium is preferentially reduced and found at the surface. In sum, this study clearly shows that the structural and hence catalytic behavior of refractory oxide supported FeRh is distinctly different than that observed for the same metals on a graphitic support, thus demonstrating the critical importance of metal—support interactions in determining the catalytic character of multimetallic particles.

Efficient epoxidation of olefinic compounds is achieved by using soluble polymerizable β-ketoesterate complexes. Iron(III), cobalt(II) and nickel(II) catalytic centres are utilized in 1,2-dichloroethane at ambient conditions. Oxygen or air and a branched aldehyde are employed as the oxidant and reductant respectively (Mukaiyama's conditions).

A new model for single-route, steady-state reactions on nonuniform catalytic surfaces is presented. Giving up some assumptions of the Temkin model and making use of a common mean-value theorem result in the introduction of the concept of kinetic insensitivity. The model rigorously accounts for the existence of a broad class of reactions the evolution of which is accurately described by “classical kinetics” (derived on ideal, uniform surfaces). The connections with structure sensitivity/insensitivity are examined in relation with the frequently invoked effect of surface restructuring. The application to two-step sequences leads to the classical Temkin model (as a particular case) and reveals some of its shortcomings, not yet reported.

Lipase from Candida cylindracea has been covalently immobilized on trichlorotriazine activated supports (alumina, silica and two types of controlled pore glass). The optimum conditions of the activation process have been determined (pretreatment, solvent, gram of activating agent/gram of support ratio and reaction time). The influence of the enzyme concentration and of the temperature on the immobilization process has been evaluated. The immobilized derivatives on silica and alumina exhibited greater residual activity and were more resistant to inactivation by temperature (50°C) than their immobilized counterpart on controlled pore glasses. The derivatives obtained on alumina and silica have been used in the hydrolysis of (R,S) ethyl 2-phenylpropionate only yielding the S(+) acid. The influence of Na^I and Ca^II on the lipase activity is discussed. The immobilized derivative on silica, stored at 50°C, was 37 times more stable than the native enzyme and displayed 80% residual activity after 336 h of operating time

The hydroformylation of 1-aryl-1-(2-pyridyl)ethenes 1 can be accomplished in good yield (85%) using HRh(CO) (PPh₃)₃ with rather high catalyst to substrate molar ratios (1/80) under standard conditions. Other cobalt, platinum and rhodium complexes exhibit lower catalytic activity towards hydroformylation, the hydrogenation of the substrate being in most cases the main reaction. The formation of the more branched aldehyde 3 occurs in the presence of HRh(CO) (PPh₃)₃ regiospecifically. The formation of the complex 9 from [Rh(CO)₂Cl]₂ and olefin 1a, in which only the pyridine nitrogen is coordinated to the metal, enlightens on the role played by the heteroatom in determining both chemo- and regioselectivity of the reaction.

Kinetic studies were performed over silica-alumina and acidic zeolites to determine the effects of temperature (from 600 to 700 K) and reactant pressures ( < 80 Torr) on the rates of methylamine synthesis from methanol and ammonia. Related kinetic studies were conducted of methylamine disproportionation reactions, methanol dehydration, and reactions of methanol and dimethyl ether with methylamines. The results suggest that the reactive species for methylamine synthesis are adsorbed nitrogenous bases, while methoxy groups are reactive species for dimethyl ether production. Disproportionation reactions may be important pathways for the production of the higher-substituted amines during methylamine synthesis. The combination of microcalorimetric measurements, reported elsewhere, and kinetic studies suggests that acid sites are required in these reactions for the strong adsorption of ammonia and methylamines, and weak adsorption sites are required to facilitate desorption of adsorbed amine species from the acid sites.

Particle and solubilized cobalt(II) phthalocyanines (PCs) have been investigated for their catalytic activity on the oxidative erythrosine decomposition with hydrogen peroxide. Particle PCs show much higher activity than solubilized PCs as a result of a reaction of particle PCs with oxygen molecules that cannot take place with monomeric PCs in solution where a slower reaction with hydrogen peroxide dominates erythrosine decomposition. The high activity of particle PCs seems to be that particle PCs provide a surface where both an oxygen molecule and an erythrosine molecule can coordinate to neighboring cobalt(II) ions, and then the multinuclear cobalt(II) phthalocyanine particle can trigger the decomposition reaction by helping electron transfer from the dye to oxygen. Erythrosine decomposition with particle PCs obeyed Michaelis—Menten kinetics. The activation energies for the catalysis obtained from Arrhenius plots were significantly smaller than those without PCs, that is, particle PCs can be good catalysts for erythrosine oxidation.