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High Reaction Rate (high + reaction_rate)
Selected AbstractsEnantioselective Synthesis of Chiral Tetrahydroisoquinolines by Iridium-Catalyzed Asymmetric Hydrogenation of EnaminesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 18 2009Pu-Cha Yan Abstract Chiral iridium complexes based on spiro phosphoramidite ligands are demonstrated to be highly efficient catalysts for the asymmetric hydrogenation of unfunctionalized enamines with an exocyclic double bond. In combination with excess iodine or potassium iodide and under hydrogen pressure, the complex Ir/(Sa,R,R)- 3a provides chiral N -alkyltetrahydroisoquinolines in high yields with up to 98% ee. The L/Ir ratio of 2:1 is crucial for obtaining a high reaction rate and enantioselectivity. A deuterium labeling experiment showed that an inverse isotope effect exists in this reaction. A possible catalytic cycle including an iridium(III) species bearing two monophosphoramidite ligands is also proposed. [source] Photoinitiated dispersion polymerization of methyl methacrylate: A quick approach to prepare polymer microspheres with narrow size distributionJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2008Jia Chen Abstract Photoinitiated dispersion polymerization of methyl methacrylate was carried out in a mixture of ethanol and water as dispersion medium in the presence of poly(N -vinylpyrrolidone) (PVP) as the steric stabilizer and Darocur 1173 as photoinitiator. 93.7% of conversion was achieved within 30 min of UV irradiation at room temperature, and microspheres with 0.94 ,m number,average diameter and 1.04 polydispersity index (PDI) were obtained. X-ray photoelectron spectroscope (XPS) analysis revealed that only parts of surface of the microspheres were covered by PVP. The particle size decreased from 2.34 to 0.98 ,m as the concentration of PVP stabilizer increased from 2 to 15%. Extra stabilizer (higher than 15%) has no effect on the particle size and distribution. Increasing medium polarity or decreasing monomer and photoinitiator concentration resulted in a decrease in the particle size. Solvency of reaction medium toward stabilizer, which affects the adsorption of stabilizer on the particle surface, was shown to be crucial for controlling particle size and uniformity because of the high reaction rate in photoinitiated dispersion polymerization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1329,1338, 2008 [source] Solid-State Synthesis of Nanocrystalline BaTiO3: Reaction Kinetics and Powder PropertiesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2008Maria Teresa Buscaglia The formation of BaTiO3 nanoparticles by a solid-state reaction between nanocrystalline raw materials BaCO3 and TiO2 was studied as a function of temperature (400°,800°C), time (1,24 h), and titania particle size (15 and 30 nm). The reaction starts at 500°C and a high reaction rate is already observed at 600°C for the finest titania, with up to 90% conversion after 2 h. Two main reaction stages were observed at 600°,700°C. The first step is dominated by nucleation and growth of BaTiO3 at the TiO2,BaCO3 contact points and at the TiO2 surface. Surface diffusion of BaCO3 is, most likely, the prevailing mass transport mechanism responsible for the rapid formation of BaTiO3, even in the absence of a significant contribution from lattice diffusion. The second stage begins when the residual TiO2 cores are completely covered by the product phase. For longer times, the reaction can only proceed by the slower lattice diffusion, resulting in a strong decrease of the reaction rate. Single-phase BaTiO3 nanopowders with a specific surface area of 12,15 m2/g, an average particle size of 70,85 nm, a relative density of 96.5%,98.3%, and a tetragonality of 1.005 were obtained by calcination at 700°,800°C. Critical parameters in the preparation of ultrafine powders by solid-state reactions are the particle size of both raw materials, the absence of large hard agglomerates, and the homogeneity of the mixture. The use of fine raw materials and optimization of the reaction conditions make mechanical activation unnecessary. [source] Comparison of Selective Gas Phase- and Liquid Phase Hydrogenation of (Cyclo-)Alkadienes towards Cycloalkenes on Pd/Alumina Egg-Shell CatalystsCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 12 2006N. Wuchter Abstract The hydrogenation of dienes such as 1,3-butadiene, cyclooctadiene, and of acetylenic hydrocarbons on Pd catalysts shows high reaction rates and consequently, a strong influence of mass transfer on the selectivity of the intermediate alkene or cycloalkene product. 100,% selectivity towards (cyclo)-alkene hydrogenation is achieved for the gas phase when the Thiele modulus is , where L is the thickness of the active layer and Deff is the effective diffusion coefficient of the diene. The interdependencies expressed by this formula were studied in detail using model catalysts with regular pores of uniform length and diameter and perpendicular to the surface. These catalysts were prepared by anodic oxidation of aluminium wires and immobilization of the active Pd. For the liquid phase procedure of selective hydrogenation, a reaction mass transfer model has been derived in order to compare the gas phase and liquid phase procedures, in particular with respect to the selectivity. The hydrogenation of 1,3-cyclooctadiene and of 1,3-butadiene were studied for both procedures employing the same catalyst. The rate of hydrogenation can be represented for both cases by the identical kinetic equation r1,=,k1,cH2. This result is interpreted by assuming that the access of hydrogen to the surface through the dense layer of adsorbed diene is the rate determining step. [source] | |||||||||||||