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11.
The heterogeneous enantioselective hydrogenation of activated ketones over chirally modified platinum is reviewed with emphasis on identifying the role of the various species observed in this catalytic system. The past years have witnessed a continuous broadening of the scope of this catalytic system including new reactants and modifiers affording over 97% ee. New reaction pathways have been uncovered and the kinetic and mechanistic studies have been faced with a number of complicating factors caused by spectator species and interactions in solution and on the Pt surface. The previously proposed mechanistic models are critically assessed in the light of these new findings. 相似文献
12.
Epoxidation of 2-cyclohexen-1-ol with TBHP and TiO2–SiO2, Fe2O3–SiO2 and SiO2 aerogels has been studied. The influence of surface silanol groups and transition metal impurities in titania–silica on the product distribution has been analyzed. The results, supported by literature data, indicate that high surface area silica cannot generally be regarded as an inert matrix in epoxidation catalysts. Contribution of Ti-free silica limits the selectivity of Ti- and Si-containing catalysts in demanding epoxidation reactions. The activity of silica – together with the background oxidation of the olefin – may lead to overestimated intrinsic activities when the reaction rate is related to the Ti content of the catalyst. 相似文献
13.
A Penzkofer 《International journal of molecular sciences》2012,13(7):9157-9183
The blue-light photo-reduction (conversion of oxidized flavin quinone via flavin semiquinone to fully reduced flavin hydroquinone) and dark re-oxidation of the flavins riboflavin and lumiflavin in starch (α-amylose) films was studied by absorption and luminescence spectroscopy. Blue-light photo-excitation caused an absorption, fluorescence, and phosphorescence decrease which recovered in the dark. The photo-reduction dark-oxidation cycle could be repeated. The efficiency of photo-reduction decreased with exposed excitation energy, and the speed of re-oxidation in the dark slowed down with time after excitation. The absorption did not fully recover. The fluorescence efficiency after a long time of storage in the dark increased beyond the initial flavin quinone fluorescence efficiency. Flavin photo-excitation is thought to cause starch-flavin restructuring (static fluorescence quenching center formation), enabling enhanced photo-induced starch to flavin electron transfer with subsequent flavin reduction and starch oxidation. In the dark, after light switch-off, thermal reversion of flavin reduction and starch oxidation occurred. 相似文献
14.
Modified FeCrAl coatings were studied with respect to their capability to form a thin protective oxide scale in liquid lead environment. They were manufactured by low pressure plasma spraying and GESA surface melting, thereby tuning the Al content. The specimens were exposed for 900 h to liquid lead containing 10?6 and 10?8 wt.% oxygen, respectively, at various temperatures from 400 to 550 °C. Threshold values for an Al content that guarantees the formation of thin protective Al-rich oxide scales are determined, dependent on the respective chromium content, on the presence of yttrium in the modified coating, and on the exposure conditions. 相似文献
15.
Heterogeneous catalytic epoxidation of functionalized olefins in the liquid phase has been reviewed, focusing on catalyst performance and its interrelation with the crucial parameters of the catalytic systems. Efficient catalysts include supported and mixed oxides, framework-substituted (“redox”) molecular sieves, layered-type materials, heterogenized homogeneous catalysts, and some others. Among the various substrates, allylic and homoallylic alcohols, and unsaturated carbonyl compounds have received most attention so far. The great variety of available catalysts enables selective epoxidation of most substituted olefins. The mechanistic understanding of heterogeneous catalytic epoxidation is still underdeveloped, rendering catalyst design rather empirical. A considerable potential for future development lies in the area of “heterogenization” of successful homogeneous catalysts especially for asymmetric epoxidation. Crucial requirements in the development of heterogeneous catalytic epoxidation catalysts are, besides good catalytic performance and cheap oxidant, recyclability and resistance to leaching of the active component. Some of the examples shown in the literature do not fulfill the latter requirement. 相似文献
16.
Thomas Bürgi Fachri Atamny Axel Knop‐Gericke Michael Hävecker Thomas Schedel‐Niedrig Robert Schlögl Alfons Baiker 《Catalysis Letters》2000,66(3):109-112
The adsorption of ethyl pyruvate on Pt(111) has been studied by in situ XANES measurements in the presence and absence of
hydrogen. Depending on the hydrogen and ethyl pyruvate pressure, the C and O K‐edge spectra exhibit distinctly different angular
dependence. Without hydrogen ethyl pyruvate is oriented preferentially perpendicular to the surface, indicating bonding via
the O lone pairs. In the presence of hydrogen the mean orientation is more tilted towards the surface. Likely, ethyl pyruvate
also interacts with Pt via its π system under these conditions. The observed angle‐dependent shift of the energy of the π*
and σ* resonances indicates the coexistence of differently adsorbed ethyl pyruvate species. The experimental findings demonstrate
the importance of the in situ approach for unraveling the adsorption mode of ethyl pyruvate in the enantioselective hydrogenation
over cinchona‐alkaloid‐modified Pt.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
Alfons Radunz Georg H. Schmid 《European Journal of Lipid Science and Technology》2000,102(12):734-738
The seeds of the evergreen Buxus‐tree, Buxus sempervirens, contain a yellowish oil which represents up to 42% of the dry weight. The oil consists, in comparison to the lipids of jojoba fruits (Simmondsia chinensis), of only 3.6% wax esters. 95% of the oil consist of triglycerides, the typical storage substances of oil fruits. Phospholipids occur with 0.4% and glycolipids with 0.13%. The fatty acid patterns of these lipids correspond to the typical fatty acid compositions of the respective lipid classes. In the wax esters monoenoic fatty acids and saturated fatty acids with 16 and 18 carbon atoms prevail. The glycolipid and phospholipid fractions are characterized by a high portion of dienoic and monoenoic as well as saturated fatty acids having 16 and 18 carbon acids. 相似文献
18.
Progress in the use of ATR-IR spectroscopy to improve the understanding of liquid-phase heterogeneous catalytic reactions
is illustrated using the example of the oxidation of benzyl alcohol over Pd/Al2O3 and Bi–Pd/Al2O3. The in situ studies performed in both batch and continuous reactor cells provide rich information on the reaction pathway
and important facets of the mechanism, such as the nature of active Pd sites and the effect of the Bi-promoter. The combination
of CO site blocking prior to reaction and isotopic labeling suggests that alcohol dehydrogenation occurs uniformly over Pd
nanoparticles, but only selected sites may allow desorption of the product benzaldehyde thus providing the required selectivity.
Promotion of Pd/Al2O3 using bismuth produces infrared spectra free of adsorbed CO. This information demonstrates that Bi is deposited on selected
adsorption sites (terraces rather than defects) and simultaneously confirms that open terraces favor product decomposition.
Experiments performed in the continuous reactor cell using different catalyst film thickness show that reactions can be studied
under kinetic or mass transfer limited conditions depending on catalyst film thickness. This allowed to study the alcohol
oxidation under conditions of oxygen diffusion limitation, which are preferably applied in praxis in order to prevent catalyst
deactivation by over-oxidation. 相似文献
19.
A magnetic Pt/SiO2/Fe3O4 catalyst consisting of chirally modified platinum supported on silica coated magnetite nanoparticles was prepared using an easy synthetic route and successfully applied for the enantioselective hydrogenation of various activated ketones. The magnetic catalyst modified with cinchonidine showed a catalytic performance (activity, enantioselectivity) in the asymmetric hydrogenation of various activated ketones in toluene comparable to the best known Pt/alumina catalyst used for these reactions. The novel catalyst can be easily separated from the reaction solution by applying an external magnetic field and recycled several times with almost complete retention of activity and enantioselectivity. 相似文献
20.
A model system for the study of structural and chemical properties of monolayers and multilayers of vanadium oxide immobilized on titania is presented. Investigation of the planar oxide-oxide interface by XP, UV and IS spectroscopy indicated that vanadium immobilized by a single impregnation step exists as an incomplete heterogeneous layer containing well dispersed V4+ species. Increase of the vanadia loading by multiple impregnations led to vanadia agglomerates with higher apparent oxidation state of the vanadium. TD spectroscopy with O2 and CO2 as probe molecules revealed that the chemical reactivity of the vanadia surface species depends on their structure. The surface containing well-dispersed vanadia species exchanged oxygen more easily and showed pronounced interactions with CO2. 相似文献