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Product Selectivity (product + selectivity)
Selected AbstractsChemInform Abstract: Efficient Oxidation of Hantzsch 1,4-Dihydropyridines with Tetrabutylammonium Peroxomonosulfate Catalyzed by Manganese(III) Schiff Base Complexes: The Effect of Schiff Base Complex on the Product Selectivity.CHEMINFORM, Issue 15 2010Masoud Nasr-Esfahani Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Deposition of Gold Clusters on Porous Coordination Polymers by Solid Grinding and Their Catalytic Activity in Aerobic Oxidation of AlcoholsCHEMISTRY - A EUROPEAN JOURNAL, Issue 28 2008Tamao Ishida Dr. Gold clusters were deposited in a narrow size distribution on porous coordination polymers (PCPs) by solid grinding with volatile dimethyl AuIII acetylactonate. The mean diameter could be minimized down to 1.5,nm for Al-containing PCP. Gold clusters on PCPs showed noticeably high catalytic activity in the aerobic oxidation of alcohols. Product selectivity was tunable by the selection of PCP supports in benzyl alcohol oxidation. [source] Pyrolysis of liquefied petroleum gas assisted by radicals desorbed from mesh catalyst surfaceINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2003Eugene B. H. Quah The purpose of this study is to understand the reactions on the catalyst surface and in the gas phase during the catalytic pyrolysis of light hydrocarbons. To avoid the complexity of internal pore diffusion and heat transfer limitation, nickel mesh without pore structure was used as a catalyst for the catalytic pyrolysis of a commercial liquefied petroleum gas (LPG) sample in a quartz tube reactor and in a wire-mesh reactor over a temperature range of 600,850°C. With a Ni mesh catalyst, no catalyst deactivation associated with coke formation was observed at high gas flow rate. Our experimental results indicate that the desorption of radicals from the catalyst surface is an important process in the catalytic pyrolysis of LPG using the Ni mesh catalyst. The desorption of radicals across the gas,catalyst interface is greatly facilitated by increasing gas flow rate passing through the mesh. The desorbed radicals would initiate and/or enhance the gas-phase radical chain reactions and lead to improved reaction rates for the pyrolysis of LPG although the product selectivities remained unchanged. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 637,646, 2003 [source] Performance features of Pt/BaO lean NOx trap with hydrogen as reductantAICHE JOURNAL, Issue 3 2009Robert D. Clayton Abstract The performance of a model Pt/BaO/Al2O3 monolith catalyst was studied using H2 as the reductant. The dependence of product selectivities on operating parameters is reported, including the durations of regeneration and storage times, feed composition and temperature, and monolith temperature. The data are explained in terms of a phenomenological model factoring in the transport, kinetic, and spatio-temporal effects. The Pt/BaO catalyst exhibits high cycle-averaged NOx conversion above 100°C, generating a mixture of N2 and byproducts NH3 and N2O. The cycle-averaged NOx conversion exhibits a maximum at about 300°C corresponding to the NOx storage maximum. The N2 selectivity exhibits a maximum at a somewhat higher temperature, at which point the NH3 selectivity exhibits a minimum. This trend conveys the intermediate role of NH3 in reacting with stored NOx. Both N2 and N2O are also formed during the storage steps from the oxidation of NHx species produced during the regeneration. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Simulating cyclohexane millisecond oxidation: Coupled chemistry and fluid dynamicsAICHE JOURNAL, Issue 6 2002R. P. O'Connor Cyclohexane partial oxidation over a 40-mesh Pt,10% Rh single-gauze catalyst can produce ,85% selectivity to oxygenates and olefins at 25% cyclohexane conversion and 100% oxygen conversion, with cyclohexene and 5-hexenal as the dominant products. A detailed 2-D model of the reactor is solved using density-functional theory (with 35 reactions among 25 species) and computational fluid dynamics. Rapid quenching in the wake of the wires allows highly nonequilibrium species to be preserved. The simulations show that the competition between cyclohexyl and cyclohexylperoxy radicals is crucial in determining product selectivities. At high temperatures and low pressures, the cyclohexyl radical is favored, leading to high selectivities to cyclohexene. At lower temperatures or high pressures, cyclohexylperoxy radicals are favored, allowing the formation of parent oxygenates to dominate. Numerical simulations suggest ways to tune reactor operation for desired product distributions and allow the investigation of dangerous or costly operating conditions, such as high pressure. [source] Protein engineering of Bacillus megaterium CYP102FEBS JOURNAL, Issue 10 2001The oxidation of polycyclic aromatic hydrocarbons Cytochrome P450 (CYP) enzymes are involved in activating the carcinogenicity of polycyclic aromatic hydrocarbons (PAHs) in mammals, but they are also utilized by microorganisms for the degradation of these hazardous environmental contaminants. Wild-type CYP102 (P450BM-3) from Bacillus megaterium has low activity for the oxidation of the PAHs phenanthrene, fluoranthene and pyrene. The double hydrophobic substitution R47L/Y51F at the entrance of the substrate access channel increased the PAH oxidation activity by up to 40-fold. Combining these mutations with the active site mutations F87A and A264G lead to order of magnitude increases in activity. Both these mutations increased the NADPH turnover rate, but the A264G mutation increased the coupling efficiency while the F87A mutation had dominant effects in product selectivity. Fast NADPH oxidation rates were observed (2250 min,1 for the R47L/Y51F/F87A mutant with phenanthrene) but the coupling efficiencies were relatively low (< 13%), resulting in a highest substrate oxidation rate of 110 min,1 for fluoranthene oxidation by the R47L/Y51F/A264G mutant. Mutation of M354 and L437 inside the substrate access channel reduced PAH oxidation activity. The PAHs were oxidized to a mixture of phenols and quinones. Notably mutants containing the A264G mutation showed some similarity to mammalian CYP enzymes in that some 9,10-phenanthrenequinone, the K -region oxidation product from phenanthrene, was formed. The results suggest that CYP102 mutants could be useful models for PAH oxidation by mammalian CYP enzymes, and also potentially for the preparation of novel PAH bioremediation systems. [source] Selective Hydrogenation of 5-Ethoxymethylfurfural over Alumina-Supported Heterogeneous CatalystsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 18 2009Erik-Jan Ras Abstract We report here the synthesis and testing of a set of 48 alumina-supported catalysts for hydrogenation of 5-ethoxymethylfurfural. This catalytic reaction is very important in the context of converting biomass to biofuels. The catalysts are composed of one main metal (gold, copper, iridium, nickel, palladium, platinum, rhodium, ruthenium) and one promoter metal (bismuth, chromium, iron, sodium, tin, tungsten). Using a 16-parallel trickle-flow reactor, we tested all 48 catalyst combinations under a variety of conditions. The results show that both substrate conversion and product selectivity are sensitive towards temperature changes and solvent effects. The best results of >99% yield to the desired product, 5-ethoxymethylfurfuryl alcohol, are obtained using an iridium/chromium (Ir/Cr) catalyst. The mechanistic implications of different possible reaction pathways in this complex hydrogenation system are discussed. [source] Highly Selective Oxidation of Alkylphenols to Benzoquinones with Hydrogen Peroxide over Silica-Supported Titanium Catalysts: Titanium Cluster Site versus Titanium Single SiteADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2009Oxana Abstract Titanium-silica catalysts have been prepared by supporting titanium(IV) precursors with different nuclearity {mononuclear titanocene dichloride Ti(Cp)2Cl2, dinuclear titanium diethyl tartrate and the tetranuclear titanium peroxo complex (NH4)8[Ti4(C6H4O7)4(O2)4],8,H2O} onto the surface of silica materials with different textural characteristics. The supported catalysts have been explored as highly active and reusable catalysts for the oxidation of 2,3,6-trimethylphenol (TMP) and 2,6-dimethylphenol (DMP) to 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, vitamin E key intermediate) and 2,6-dimethyl-1,4-benzoquinone (DMBQ), respectively, using aqueous hydrogen peroxide as green oxidant. Catalysts prepared by grafting mononuclear Ti(Cp)2Cl2 revealed a strong dependence of the product selectivity on the surface concentration of titanium active centers. Mesoporous materials with titanium surface concentration in the range of 0.6,1.0,Ti/nm2 were identified as optimal catalysts for the transformation of alkylphenols to benzoquinones. Catalysts having <0.6,Ti/nm2 produced a mixture of benzoquinones and dimeric by-products. Conversely, when di-/tetranuclear titanium precursors were employed for the catalyst preparation, a diminution of the titanium surface concentration had no impact on the benzoquinone selectivity, which was typically as high as 96,99%. DR-UV spectroscopic studies revealed that the catalysts capable of producing alkylbenzoquinones with nearly quantitative yields possess titanium dimers and/or subnanometer-size clusters homogeneously distributed on a silica surface. On the contrary, catalysts with isolated titanium sites give a considerable amount of dimeric by-products. This is the first example which clearly demonstrates the advantages of titanium cluster-site catalysts over titanium single-site catalysts in hydrogen peroxide-based selective oxidation reaction. [source] Selective Oxidation of Alcohols to Carbonyl Compounds and Carboxylic Acids with Platinum Group Metal CatalystsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2003Ross Anderson Abstract The use of platinum group metal (PGM) catalysts for the selective oxidation of various primary and secondary alcohols under mild conditions is described. High throughput screening (HTS) techniques have been used to identify trends in catalyst activity and product selectivity. Using air as oxidant and water as solvent 5% Pt, 1% Bi/C has been identified as an efficient catalyst for the transformation of 2-octanol to 2-octanone and 1-octanol to octanoic acid. To improve aldehyde selectivity the promotion of Pt/Al2O3 and Ru/C catalysts has been investigated. The use of H2O2 as oxidant has been demonstrated as a suitable alternative to air. [source] Theoretical elucidation of the rhodium-catalyzed [4 + 2] annulation reactionsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2008Cai-Yun Geng Abstract The reaction mechanism of the Rh-catalyzed [4 + 2] annulation of 4-alkynals with isocyanates is unraveled using density functional calculations. The reaction mechanisms of the model system and the real substituted system have been investigated and the results are compared. From our theoretical results based on the model and real substituted system, it is shown that (a) the rate-determining step is the Rh-H addition to the alkyne, (b) the formation of the cyclopentenone G and glutarimide K represents a severe competition, and (c) the product selectivity should be controlled by the amount of the isocyanates. In addition, it is demonstrated that there exist steric effects in the real substituted system, but missed in model system. Our calculations also show that although the results obtained on the model system could explain the mechanism in principle, the real substituted system could reflect the mechanism more exactly and make the reaction proceed with regioselectivity. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Millisecond catalytic reforming of monoaromatics over noble metalsAICHE JOURNAL, Issue 4 2010C. M. Balonek Abstract The millisecond autothermal reforming of benzene, toluene, ethylbenzene, cumene, and styrene were independently studied over five noble metal-based catalysts: Pt, Rh, Rh/,-Al2O3, Rh,Ce, and Rh,Ce/,-Al2O3, as a function of carbon-to-oxygen feed ratio. The Rh,Ce/,-Al2O3 catalyst exhibited the highest feedstock conversion as well as selectivities to both synthesis gas and hydrocarbon products (lowest selectivities to H2O and CO2). Experimental results demonstrate a high stability of aromatic rings within the reactor system. Benzene and toluene seem to react primarily heterogeneously, producing only syngas and combustion products. Ethylbenzene and cumene behaved similarly, with higher conversions than benzene and toluene, and high product selectivity to styrene, likely due to homogeneous reactions involving their alkyl groups. Styrene exhibited low conversions over Rh,Ce/,-Al2O3, emphasizing the stability of styrene in the reactor system. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Solvent effects in the hydrodechlorination of 2,4-dichlorophenol over Pd/Al2O3AICHE JOURNAL, Issue 3 2010Santiago Gómez-Quero Abstract Solvent effects in the liquid phase (0.1 MPa; 303 K) hydrodechlorination (HDC) of 2,4-dichlorophenol have been established over Pd/Al2O3. In the absence of secondary reactions, catalyst deactivation, and transport limitations, a stepwise HDC yields 2-chlorophenol and phenol, where product selectivity was insensitive to the nature of the solvent. In contrast, the initial HDC rates exhibited a marked dependence on the reaction medium and increased in the order: benzene < THF < n -hexane < cyclohexane < alcohols < water. Higher rates result from the concomitant effect of an increase in the dielectric constant (,) and a decrease in the molar volume ( ) of the solvent, where the major (ca. 80%) contribution is due to ,. We attribute this response to the increased solvent capacity to stabilize the arenium intermediate at higher/lower ,/, an effect that extends to reaction in water + organic combinations. We provide, for the first time, a reliable quantification of solvent effects that can be potentially applied to other catalytic hydrogenolysis systems. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Gas-solids flow behavior: CFB riser vs. downerAICHE JOURNAL, Issue 9 2001H. Zhang Comparisons are made in a circulating fluidized-bed riser/downer system between a 15.1 m high, 0.10 m ID riser and a 9.3 m high, 0.10 m ID downer, based on the measurements of the radial distributions of the local solids holdups and local particle velocities along the two columns. Although the core-annulus flow structures exist in both the riser and downer, the radial flow structure in the downer differs largely from that in the riser. The radial distributions of solids holdup and particle velocity in the downer are much more uniform than those in the riser, thus ensuring the low back mixing and the narrow particle residence time distribution in the downer. The axial flow structure in the downer is also more uniform than that in the riser. Due to the high particle acceleration and the high particle velocity in the downer, the overall solids holdup is significantly lower than that in the riser. The microflow structure in the downer, characterized by the low intermittency indices, is also more uniform than that in the riser. These key properties of the downer make it a very promising candidate for industrial applications where short reaction times and high product selectivity are required. [source] Characterization of downflowing high velocity fluidized bedsAICHE JOURNAL, Issue 3 2000Chunshe Cao A downer-riser circulating high velocity fluidization apparatus was developed to study the fundamentals of downflowing gas-solid particle mixtures. The acceleration and deceleration of solids due to the influences of the entrance and exit sections result in a relatively uniform axial solids distribution. Radial solid density profiles detected with an X-ray imaging system in the downer show the existence of a core-annulus flow with a dilute core surrounded by a denser wall region. Local solids flux profiles were obtained with an aspirating probe device and the solid velocity profile obtained from the two measured quantities. These confirm that the majority of solids segregates in a wall region that flows faster than the dilute core region. Thus, the shorter residence time in the high-speed downer wall region is coupled with faster reaction rates due to the accompanying high concentration of catalyst, while the dilute core has slower reaction rates with longer residence time due to the lower catalyst concentration and flow velocity. This results in much more uniform reaction extent over the cross-sectional area of the downer and, therefore, should improve the product selectivity. [source] A novel approach for the kinetic and mechanistic modeling of acid-catalyzed degradation of polymersASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2009Y.-H. Lin Abstract A new approach is presented that combines kinetic and mechanistic considerations which take into account chemical reactions and catalyst deactivation in the modeling of the catalytic degradation of polymers. Though acid-catalyzed hydrocarbon cracking reactions involve a large number of compounds, reactions and catalyst deactivation and are very complex, the model gives a good representation of experimental results from the degradation of polypropylene over fluidized acidic catalysts. This model provides the benefits of product selectivity for the chemical composition, such as alkanes, alkenes, aromatics and coke, in relation to the effect of structurally different polymer feeds, and the performance of the catalyst used. It is an improvement of the currently available empirical ,lumping' techniques which usually are severely restricted in terms of product definitions. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Substrate Supply for Effective BiocatalysisBIOTECHNOLOGY PROGRESS, Issue 1 2007Pei-Yi Kim Using biocatalysis for some chemical synthesis steps has unique advantages such as achieving higher product selectivity under ambient process conditions. However, a common limitation with such systems is the inhibition or toxicity posed by the starting substrate as well as limited aqueous solubility in many cases. In this review, we discuss the supply of substrate to bioconversions. The delivery of substrate via an auxiliary, which may be water-miscible, or a second phase such as a water-immiscible organic solvent, adsorbing resin, or a gas, is examined through recent examples in the field. Finally, guidelines for experimental planning and process considerations are suggested to facilitate the choice of substrate delivery method and accelerate process development. [source] ChemInform Abstract: Gold-Catalyzed Domino Reactions Consisting of Regio- and Stereoselective 1,2-Alkyl Migration.CHEMINFORM, Issue 43 2010Wenbo Li Abstract A cationic gold(I)-catalyzed domino reaction involving highly regio- and stereoselective 1,2-alkyl migration and heterocyclization or oxygen transfer is developed, in which the product selectivity is controlled by the counteranion of the gold catalyst. [source] | ||||||||||||||||||||||