分子催化最新文献

SnZnAl₂O₄ mixed supports were prepared by coprecipitation method with Sn contents of 0.5, 1.6 and 3 wt.%. Later Pt (1 wt.%) was incorporated by impregnation and samples reduced in hydrogen at 823 and 1073 K. Chemisorption data showed a decrease in hydrogen as temperature increases which is attributed to PtSn alloy formation. Catalytic activity in isobutane dehydrogenation and n-heptane dehydrocyclization seems to be dependent on the amount of metallic Pt phase. Pt/SnZnAl₂O₄ catalysts were thermally more stable than Pt/ZnAl₂O₄ preparation. Tin alters the ZnAl₂O₄ properties towards a higher thermal stability, but if added in more than 1 wt.%, it alloys with platinum inhibiting catalytic performance.

Human placental alkaline phosphatase has been chemically immobilized on a hydrophilic cross-flow microfiltration membrane made of polyvinylidene difluoride (PVDF) chemically labelled with 1-carbonylimidazole groups. Physicochemical characterization of this immobilized biocatalyst focused especially on attributes such as the immobilization isotherms (Langmuir type) of the enzyme to the support, the stability of the catalytic activity, the effects of pH and temperature on this activity and the existence of limitations of external diffusion for H⁺, substrate and/or products. Regarding enzyme stability and its dependence on different experimental conditions, patterns of hysteresis or memory are proposed.

Human placental alkaline phosphatase was chemically immobilized on a hydrophilic cross-flow microfiltration membrane made of polyvinylidene difluoride (PVDF) chemically labelled with 1-carbonylimidazole groups and characterized physicochemically (in Part I, J. Mol. Catal. (Vol. 93, 85–104)) The intrinsic kinetic behaviour (rate vs. substrate concentration) in the absence of diffusional constraints was analysed graphically and numerically, by non-linear regression and use of the F statistical test for model discrimination, postulating a minimum rational rate equation of 2:2 degree in substrate concentration. According to the results found, a mechanism for the catalytic kinetic action has been postulated.

Kinetic studies of a series of alkene epoxidations (ethylene, propylene and 1-butene) with intact cells and cell-free extracts of Methylosinus trichosporium OB3b were carried out. For the epoxidation with intact cells, V_max values were independent of the type of alkenes. K_m values, however, were remarkably different, and the order was ethylene < propylene < 1-butene. The results suggest that the reactivity of alkene epoxidation with intact cells of M. trichosporium OB3b strongly depends on the affinity of alkene and methane monooxygenase. For cell-free extracts, V_max values for ethylene and propylene were almost the same. V_max value for 1-butene was extremely small. K_max values were remarkably different, and the order was ethylene < propylene < 1-butene. On the comparison of intact cells and cell-free extracts of M. trichosporium OB3b, the K_max value of ethylene with intact cells was much smaller than that with cell-free extracts, indicating that ethylene is more easily incorporated into the cells. On the other hand, propylene and 1-butene are not easily incorporated on comparison of the K_max values of intact cells with those of the cell-free extracts. These results indicate that the cell membrane may act to avoid higher alkenes.

Nickel-based, complex reducing agent (NiCRA) prepared by reacting Ni(OAc)₂, NaH and t-BuONa in THF were studied by transmission electron microscopy. Nickel is in the form of 1 nm-large clusters. In presence of bipyridyl ligands particles as small as 0.5 nm are detected. The formation and stability of nickel clusters is attributed to the strong reducing potential of the NaH—t-BuONa mixture complexing the nickel clusters. Bipyridyl ligands exert an additional stabilization so that clusters containing less than 10 atoms could be present.

Structural and catalytic properties of Ti^IV/K10-montmorillonite and fine size anatase activated at different temperatures are compared. EXAFS spectra at the Ti K edge and X-ray diffraction patterns show the formation of small TiO₂ particles in the preparative process, which grow up with the calcination temperature. The low catalytic activity of the fine size anatase clearly shows that the activity of the exchanged clay is due to the exchanged Ti^IV into the K10-montmorillonite. XANES indicates that some of the titanium exchanged atoms would present a tetrahedral structure and consequently behave as a strong Lewis acid.

Reactivity, regioselectivity and stereoselectivity of the addition reaction between tetrachloromethane and C₅ to C₁₀ cycloalkenes in the presence of copper, ruthenium, molybdenum, cobalt and iron complexes as catalysts and dibenzoyl peroxide as an initiator have been examined in the temperature range 22 to 80°C. Relative reactivities of the cycloalkenes decreased in sequence C₈>C₅>C₇>C₆>C₁₀. These rates were not strongly affected by the catalysts indicating that the reactivities of cycloalkenes are mainly controlled by I-strain in the cycloalkene molecule. Transannular addition was observed in the reactions of cis-cyclooctene and cis-cyclodecene. The ratio of transannular to vicinal addition was significantly lowered by [RuCl₂(PPh₃)₃] and [CuCl(NHEt₂)₂], in comparison with dibenzoyl peroxide. A strong catalyst effect on the stereoselectivity of the addition reaction of cyclohexene was also recorded. The trans:cis isomer ratio of the adduct was significantly affected by ruthenium catalysts and, to lesser extent, by copper, molybdenum, cobalt and iron catalysts. The catalyst effect on regioselectivity and stereoselectivity strongly support a non-chain mechanism involving coordination of reactants and product formation at the metal centre in contrast to the classical initiation of chain reactions by organic peroxides.

A complex RhH₂(Ph₂N₃) (PPh₃)₂ 1 has been shown to be an efficient catalyst for the homogeneous hydrogenation of unsaturated compounds (styrene, acrylonitrile, cinnamic acid, cinnamic alcohol and cinnamonitrile) in dimethyl sulfoxide (DMSO) under mild conditions. DMSO appears to play an important role in generating the active catalytic species for hydrogenation of styrene. No catalytic activity for styrene hydrogenation was observed for 1 in solvents such as benzene, toluene, acetone, tetrahydrofuran and N,N-dimethylformamide. The results are discussed in relation to the mechanism of styrene hydrogenation.

The role of chromium as a promoter of butadiene selectivity in n-butane oxidative dehydrogenation on ZnCr_xFe_2-xO₄ (0<x<1.09) catalysts was studied. Catalysts have a spinel structure with Cr³⁺ replacing Fe³⁺ in octahedral sites. Mössbauer spectroscopy studies showed that the substitution of iron by chromium modifies the electron density of the iron nuclei in the ZnFe₂O₄ structure. Iron nuclei showed the lowest electron density for x=0.5–0.6, hence the electron density of oxygen in FeO bond has to be enhanced. n-Butane oxidative dehydrogenation also showed a maximum in butadiene selectivity at x=0.5–0.6. These results suggest that chromium increases butadiene and CO₂ selectivities, with simultaneous decrease of butenes and cracking products explained by an enhancement of the lattice oxygen basicity, which promotes the acid-base type dissociation of the CH bond during butene activation.