Summary Optimum catalyst is discussed for two-step catalytic sequence in case of multi-centered adsorption. Contrary to the case of
single site adsorption intermediate it is shown, that the optimum coverage is not equal to the value of Polanyi parameter. 相似文献
The present work comprises a detailed investigation of a complex reaction system, revealing features of reaction mechanisms that are general for asymmetric heterogeneous catalysis. Heterogeneous enantioselective hydrogenation of 1-phenyl-1,2-propanedione was studied over cinchonidine modified Pt catalysts producing (R)-1-hydroxy-1-phenylpropanone as the main product with an enantiomeric excess (ee) of 65% at maximum yield, which could be further increased above 90% due to kinetic resolution. The results of kinetic studies in batch and continuous reactors, catalyst screening and characterization results, as well as quantum chemical calculations, are summarized, and pertinent mechanistic aspects are discussed. 相似文献
Structures and conformational behavior of several cinchona alkaloid O-ethers in the solid state (X-ray), in solution (NMR and DFT), and in the gas phase (DFT) were investigated. In the crystal, O-phenylcinchonidine adopts the Open(3) conformation similar to cinchonidine, whereas the O-methyl ether derivatives of both cinchonidine and cinchonine are packed in the Closed(1) conformation. Dynamic equilibria in solutions of the alkaloids were revealed by combined experimental-theoretical spin simulation/iteration techniques for the first time. In the (1)H NMR spectra in CDCl3 and toluene-d8 at room temperature, Closed(1) conformation was observed for the O-silyl ethers as a separate set of signals. For O-methyl ether derivatives Closed(1) could be separated only at -30 degrees C in CDCl3 or toluene-d8 and for O-phenylcinchonidine at -70 degrees C in CDCl3/CD2Cl2. The ratio between the Closed(2) and Open(3) conformers was estimated by analyzing the vicinal coupling constant (3)J(H9,H8) at ambient and low temperatures. The observed conformational equilibria of O-(tert-butyldimethylsilyl)cinchonidine in CDCl 3 and toluene-d8 are in good agreement with the theoretically estimated equilibrium populations of the conformations according to Boltzmann statistics. The conformational equilibria of four cinchona alkaloid O-ether solutes in CDCl3 and toluene-d8 are discussed in the light of their relevance to the mechanism of 1-phenyl-1,2-propanedione (PPD) hydrogenation over cinchona alkaloid modified heterogeneous platinum catalysts. It was demonstrated that the conformation found to be abundant in the liquid phase has no direct correlation with the enantioselectivity of the PPD hydrogenation reaction. 相似文献
Citral transformation to menthols was conducted in liquid phase over Ni supported H-MCM-41 and H-Y. The highest selectivity to four different stereoisomers of menthol and to (±)-menthols over Ni-H-MCM-41 were 54 and 38%, respectively. The effect of different metals (Ni, Ir, Ru, Pd) was studied over H-MCM-41. The selectivity to menthols decreased in the following order: Ni > Pd > Ru > Ir. 相似文献
The purpose of this review was to show the versatility of gas-phase microreactors for the determination of kinetics of chemical systems. Precise kinetic models were demonstrated for two industrially relevant cases: (a) the continuous selective catalytic NO reduction by hydrocarbons in the excess oxygen at 150–550°C and (b) the catalytic production of ethylene oxide, one of the most important intermediates in the world. 相似文献
Oxidation of a mixture of glucose and arabinose over Au particles deposited on porous carbons, N-doped carbons and carbon nitrides was investigated at 70 °C, under constant pH of 8, and oxygen partial pressure 0.125 atm. In particular, Au deposited on nitrogen-containing carbon-based mesoporous structures demonstrated activity in the oxidation of the sugars to the corresponding aldonic acids higher than gold deposited on undoped carbon supports (conversion of glucose up to ca. 60%, arabinose–ca. 30% after 200 min). The results can be explained by the basic nature of the supports leading to an increase in the polarity of the carbon surface and the oxygen activation. Glucuronic acid (with selectivity ca. 10–93.5%) together with gluconic acid was formed as a result of glucose oxidation, while arabinose was selectively oxidized to arabinonic acid.
The influence of the synthesis route and the chemical nature of tin precursors on the catalytic properties of supported sol-gel Ru-Sn/SiO2 catalysts were studied. It was demonstrated that introduction of tin afforded better selectivities than a monometallic sol-gel catalyst in the liquid-phase hydrogenation of cinnamaldehyde; however, the chemical nature of the tin precursor did not influence the catalyst performance. Sol-gel catalyst properties depended significantly upon the preparation method used and on the activation temperature. The selectivity to unsaturated alcohol increased with conversion, which is indicative of the in-situ creation of active sites selective in the carbonyl bond hydrogenation. 相似文献
A method is proposed to get information about carrier distribution function in superlattices and multiple quantum-well structures from the analysis of the vertical transport experiments in a transverse magnetic field. The method was applied to the GaAs/AlGaAs superlattices with wide quantum wells in strong (B=0–7 T) magnetic fields. It was shown that the distribution function of electron is nonequilibrium Boltzmann-like, with electronic temperature T=10–20 K. 相似文献