Terms Of Reaction Rate Research Articles

Chiral phosphinoferrocene carboxamides with amino acid substituents as ligands for Pd-catalysed asymmetric allylic substitutions. Synthesis and structural characterisation of catalytically relevant Pd complexes

An extensive series of chiral amino acid amides prepared from 1'-(diphenylphosphino)ferrocene-1-carboxylic acid (Hdpf) or its planar-chiral isomer, 2-(diphenylphosphino)ferrocene-1-carboxylic acid, have been tested as ligands for Pd-catalysed asymmetric allylic substitution reactions. In alkylation of 1,3-diphenylallyl acetate as a model substrate with dimethyl malonate the ligands performed well in terms of both reaction rate and enantioselectivity, achieving up to 98% ee. In contrast, the reactions of the same substrate with other nucleophiles proceeded either slowly and with poor ee's (amination with benzylamine) or not at all (etherification with benzyl alcohol). In order to rationalise the influence of the ligand structure on the reaction course, three model complexes, viz. [(η(3)-methallyl)PdCl(L-κP)], [(η(3)-methallyl)Pd(L-κ(2)O,P)]ClO(4) and [(η(3)-methallyl)Pd(L-κP)(2)]ClO(4) have been prepared from the achiral amide Ph(2)PfcCONHCH(2)CO(2)Me (L; fc = ferrocene-1,1'-diyl) and structurally characterised. The coordination study showed that the amido-phosphines readily form 1 : 1 complexes as O,P-chelates where the amino acid chirality is brought close to the Pd atom. At higher ligand-to-metal ratios, however, simple P-monodentate coordination prevails, minimising the influence of the chiral amino acid pendant.

10.2478/s11814-009-0168-9

Two sized fractions (<75 μm and 150–250 μm) of Ban Pu lignite A and Lampang subbituminous B coals were pyrolyzed in a drop tube fixed bed reactor under nitrogen atmosphere at 500–900 °C. Gasification of coal chars with excess carbon dioxide was then performed at 900–1,100 °C. The result was analyzed in terms of reactivity index, reaction rate and activation energy. It was found that chars at lower pyrolysis temperature had highest carbon conversion, and for chars of the same sized fraction and at the same pyrolysis temperature, reactivity indices increased with gasification temperature. The lower rank Ban Pu lignite A had higher R s values than higher rank Lampang subbituminous B coals. Smaller chars from both coals had higher R s values, due to the higher ash content. At present, it can be concluded that, within the gasification temperature range studied, gasification rates of chars obtained at various pyrolysis temperatures showed a linear correlation with temperature. However, additional experiment is needed to verify the correlation.

Structured catalysts for photo-Fenton oxidation of acetic acid

In this work photo-Fenton oxidation of acetic acid, was carried out on perovskites based structured catalysts, in the presence or in the absence of low amounts of Pt. Homogeneous photo-Fenton reaction by ferrioxalate complex has been also performed. The comparison of homogeneous and heterogeneous photo-Fenton oxidation indicates that the use of a heterogeneous structured catalyst greatly improves the total organic carbon (TOC) removal and leads to a more effective use of H 2O 2. The rate of TOC removal during the runtime decreased because of the occurring side H 2O 2 decomposition to O 2 that subtracts the oxidant to the photo-Fenton reaction. Moreover it allows enlarging the pH range of operation without the formation of sludge or significant metal leaching. LaFeO 3 and Pt/LaMnO 3 resulted the best catalysts for this process in terms of reaction rate.

English

Computational fluid dynamics has reached a stage where flow field in practical situation can be predicted to aid the design and to probe into the fundamental flow physics to understand and resolve the issues in fundamental fluid mechanics The study examines the computation of reacting flows After exploring the conservation equations for species and energy, the methods of closing the reaction rate terms in turbulent flow have been examined briefly Two cases of computation where combustion-flow interaction plays important role, have been discussed to illustrate the computational aspects and the physical insight that can be gained by the reacting flow computation

Statistical Analysis of Scalar Dissipation Rate Transport in Turbulent Partially Premixed Flames: A Direct Numerical Simulation Study

Statistically planar turbulent premixed and partially premixed flames for different initial turbulence intensities are simulated for global equivalence ratios = 0.7 and = 1.0 using three-dimensional Direct Numerical Simulations (DNS) with simplified chemistry. For the simulations of partially premixed flames, a random distribution of equivalence ratio following a bimodal distribution of equivalence ratio is introduced in the unburned reactants ahead of the flame. The simulation parameters in all of the cases were chosen such that the combustion situation belongs to the thin reaction zones regime. The DNS data has been used to analyse the behaviour of the dissipation rate transports of both active and passive scalars (i.e. the fuel mass fraction Y F and the mixture fraction ξ) in the context of Reynolds Averaged Navier–Stokes (RANS) simulations. The behaviours of the unclosed terms of the Favre averaged scalar dissipation rates of fuel mass fraction and mixture fraction (i.e. $\widetilde {\varepsilon }_Y =\overline {\rho D\nabla Y_F^{\prime \prime } \cdot \nabla Y_F^{\prime \prime } } /\overline{\rho }$ and $\widetilde {\varepsilon }_\xi =\overline {\rho D\nabla \xi ^{\prime \prime }\cdot \nabla \xi ^{\prime \prime }} /\overline {\rho })$ transport equations have been analysed in detail. In the case of the $\widetilde {\varepsilon }_Y $ transport, it has been observed that the turbulent transport term of scalar dissipation rate remains small throughout the flame brush whereas the terms due to density variation, scalar–turbulence interaction, reaction rate and molecular dissipation remain the leading order contributors. The term arising due to density variation remains positive throughout the flame brush and the combined contribution of the reaction and molecular dissipation to the $\widetilde {\varepsilon }_Y $ transport remains negative throughout the flame brush in all cases. However, the behaviour of scalar–turbulence interaction term of the $\widetilde {\varepsilon }_Y $ transport equation is significantly affected by the relative strengths of turbulent straining and the straining due to chemical heat release. In the case of the $\widetilde {\varepsilon }_\xi $ transport, the turbulent transport term remains small throughout the flame brush and the density variation term is found to be negligible in all cases, whilst the reaction rate term is exactly zero. The scalar–turbulence interaction term and molecular dissipation term remain the leading order contributors to the $\widetilde {\varepsilon }_\xi $ transport throughout the flame brush in all cases that have been analysed in the present study. Performances of existing models for the unclosed terms of the transport equations of $\widetilde {\varepsilon }_Y $ and $\widetilde {\varepsilon }_\xi $ are assessed with respect to the corresponding quantities obtained from DNS data. Based on this exercise either suitable models have been identified or new models have been proposed for the accurate closure of the unclosed terms of both $\widetilde {\varepsilon }_Y $ and $\widetilde {\varepsilon }_\xi $ transport equations in the context of Reynolds Averaged Navier–Stokes (RANS) simulations.

Extending Ru-BINAP Catalyst Life and Separating Products from Catalyst Using Membrane Recycling

Organic solvent nanofiltration (OSN) is a new membrane technology with many applications in pharmaceutical and fine chemicals development and manufacture, from laboratory through production scales. One application of particular industrial relevance is the ability to recover and recycle homogeneous catalysts, in particular asymmetric, organometallic, homogeneous transition metal catalysts. Industrial application of this group of catalysts is often limited due to the cost of applying these catalysts to single reactions and the subsequent product yield losses associated with removal of the catalyst from solution. OSN provides a technique that can maximise the value of the catalyst through catalyst recycle from one reaction batch to the next, whilst minimising the concentration of catalyst present in the reaction product. This contribution describes the development and demonstration of OSN catalyst recycle on an example catalytic system, the homogeneous, asymmetric hydrogenation of dimethyl itaconate (DMI) to dimethyl methylsuccinate (DMMS) with Ru-BINAP. The first step of the development process was to demonstrate that Ru-BINAP was sufficiently stable that the process concept, i.e., catalyst recycling, was feasible. A dilute solution (0.8 wt %) of DMI in methanol with a high catalyst loading (S/C = 500) was used, and 14 successive reactions were carried out with constant reaction rate and without yield or enantiomeric excess (ee) reduction. In industrial practice, the goal is always to operate a reaction at the highest, practical S/C ratio, and thus the next stage of development was to optimise the S/C ratio. The limiting, practical S/C ratio was found to be 7000. The process was then run with the optimised dilute reaction system, which allowed 10 reaction cycles to be carried out with only 20% addition of the initial (reaction 1) mass of catalyst to reactions 2−10 to maintain reaction rate, conversion, and ee. This leads to a 5 times increase in the overall S/C ratio. In addition, this optimisation reduced the amount of metal present in the product solution from 130 μg Ru per gram of product to <6 μg Ru per gram of product. The process was then scaled up to an industrially relevant substrate concentration (20 wt % DMI in methanol), and it was found that both the reaction (in terms of reaction rate, conversion, and yield) and the ability to recycle the catalyst were unaffected by the scale-up in concentration.

Design of Pt–Sn catalysts on mesoporous titania films for microreactor application

A new generation of nanostructured Pt–Sn/TiO 2 catalytic thin films has been developed by deposition of Pt–Sn mixed-metal precursors from organic solvents on mesoporous TiO 2/Ti films with a thickness of 200–300 nm. The titania sol was obtained by templating a TiO 2 precursor with Pluronic F127 surfactant. The films were prepared on Ti substrates by spin-coating. The influence of the F127/Ti ratio in the range between 0.006 and 0.050, the pH of the titania sol between 1.5 and 2.0, and the aging time between 8 and 240 h on the morphology and porous structure of titania films was investigated. A TiO 2 film with the highest degree of the long-order structure was obtained at a surfactant/Ti molar ratio of 0.009, a pH of 1.5, and an aging time of 24 h. This film has a hexagonal pore structure with a mean pore size of 3.5 nm and a porosity of 25%. A powder titania support with a similar chemical composition and morphology was also produced and used for optimization of an active component deposition. The Pt–Sn carbonyl [Pt 3(CO) 3(SnCl 3) 2(SnCl 2·H 2O)] n −2 n clusters were synthesized separately from monometallic precursors. They were loaded onto the TiO 2 supports by impregnation or adsorption. The adsorption of the Pt–Sn precursor for 24 h from an ethanol solution with concentrations of Pt and Sn of 2.0 and 1.2 mg/ml, respectively, followed by a vacuum treatment at 463 K, resulted in Pt–Sn nanoparticles embedded in the mesoporous titania network. An average size of bimetallic nanoparticles was 1.5–2 nm with a narrow particle size distribution. A reaction rate in terms of TOF between 0.2 and 3.3 min −1 was observed in the hydrogenation of citral over the Pt–Sn/TiO 2 catalysts. The selectivity to the unsaturated alcohols was as high as 90% at a citral conversion above 95%.

CO2 gasification of Thai coal chars: Kinetics and reactivity studies

Two sized fractions (<75 µm and 150-250 µm) of Ban Pu lignite A and Lampang subbituminous B coals were pyrolyzed in a drop tube fixed bed reactor under nitrogen atmosphere at 500-900 o C. Gasification of coal chars with excess carbon dioxide was then performed at 900-1,100 o C. The result was analyzed in terms of reactivity index, reaction rate and activation energy. It was found that chars at lower pyrolysis temperature had highest carbon conver- sion, and for chars of the same sized fraction and at the same pyrolysis temperature, reactivity indices increased with gasification temperature. The lower rank Ban Pu lignite A had higher Rs values than higher rank Lampang subbitumi- nous B coals. Smaller chars from both coals had higher Rs values, due to the higher ash content. At present, it can be concluded that, within the gasification temperature range studied, gasification rates of chars obtained at various py- rolysis temperatures showed a linear correlation with temperature. However, additional experiment is needed to verify the correlation.

Review of Recent Developments in Solid Acid, Base, and Enzyme Catalysts (Heterogeneous) for Biodiesel Production via Transesterification

Development of new and effective solid catalysts (solid acid base to immobilized enzyme) is necessary for renewable and green energy production. The various choices of heterogeneous solid catalysis were reported recently, and the specific research outcomes of trans-esterification on heterogeneous catalysis are discussed in the present article. The studies on biodiesel production are mainly focused on the following aspects: (i) examining the appropriate oil source, (ii) establishing the adequate alcohol to oil molar ratio, and (iii) evaluating the available solid acid catalysts, solid base catalysts, and lipase immobilized enzyme catalysts. Although homogeneous catalysis are superior in terms of reaction rate compared to heterogeneous catalysis, the focus of research on development of solid catalysts for heterogeneous catalytic transesterification is directed from the point of easy process and possible adoption for large scale production. The scope of this presentation is to examine these aspects in detail...

A novel scrape-applied method for the manufacture of the membrane–electrode assembly of the fuel–cell system

This study investigates the transfer of the scrape-applied method from the electrodes of a lithium battery to the membrane–electrode assembly of fuel cells, including Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cell. Three methods are commonly used to manufacture lithium battery electrodes: the roller-applied method, the spraying-applied method, and the scrape-applied method. This study develops novel scrape-applied equipment for lithium battery electrodes. This method is novel and suitable for producing fuel cell, better than other traditional methods. In this study, the stability of coating process was tested by measuring the weight and thickness of a dry electrode. The stability and reproducibility of electrode fabrication were examined by systematic data analysis. Finally, the study used a specially designed single cell composed of 16 conductive segments, which are insulated locally. The current passing through each segment was measured using Hall Effect sensors connected to the segment compartments. Based on the measured distribution of the local current in a segmented single cell, the influence of flooding and stoichiometry variation of feed gas was discussed in terms of electrochemical reaction rate. The experimental results serve as an important basis for future research in this field, which hold potential benefits to the academia and the industry.

Mechanistic Study of Aldopentoses in the Maillard Reaction

In the present comparative study, we investigated the relative reactivity of three aldopentoses (ribose, xylose, and arabinose) and two hexoses (glucose and fructose) in the Maillard reaction (55°C, pH 6.5) with a hydrolysate produced on an industrial scale from marine by-products. Results revealed the prevailing propensity of pentoses over hexoses to react in the Maillard reaction, with a distinguishable behavior for the ribose-hydrolysate system compared to the other aldopentoses. All the aldopentoses tested appeared to differ mainly in terms of reaction rate: molecular rearrangements studied using SEC‐FPLC exhibited similar chromatographic patterns after 6 h of incubation with ribose, compared to 17 and 24 h of incubation with xylose and arabinose, respectively. No cytotoxicity was observed when incubation was performed at neutral pH and mild temperature. To conclude, the glycation reaction conducted under controlled conditions can be considered as a promising way to upgrade marine hydrolysates without generating deleterious compounds.

Conversion of carbon dioxide to cyclic carbonates using diimine Ru(II) complexes as catalysts

AbstractCationic diimine Ru(II) complexes were synthesized and tested as catalysts for the formation of cyclic organic carbonates from CO2 and liquid epoxides (propylene oxide, epichlorohydrine, 1,2‐epoxybutane and styrene oxide) which served as both reactant and solvent. The reaction rates not only depended on the type of ligand, but also on reaction conditions such as temperature, pressure, base, the epoxide substrates and the use of an additional solvent. Reaction rates in terms of turnover frequencies up to 4050 molproduct molcat.−1 h−1 at 99% selectivity were achieved by optimizing the diimine ligand as well as the reaction temperature and CO2 pressure. Consistent with CV measurements, the electron donating group on the p‐position of the aryl ring accelerated the reaction rate. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Interaction between two nearby diffusion-controlled reactive sites in a plane

The complete series solution for two diffusion-controlled chemically reactive sites, each of radius a, located on an inert plane an arbitrary center-to-center distance d apart, is presented. Simple analytical forms are developed to calculate the site reaction rates in terms of da. The impact of these results on a number of important problems in chemical physics is discussed.

Calculation of Multidimensional Flames Using Large Chemical Kinetics

A time-dependent, two-dimensional, detailed-chemistry computational fluid dynamics model, known as UNICORN (unsteady ignition and combustion using reactions), is used for solving complex flame problems. The unique features incorporated in UNICORN for handling extremely large chemical kinetics with ease and efficiency are discussed. A submixture concept that is used for evaluating transport properties is described. This concept increases the computational speed by a factor of five for a 208-species mechanism and is expected to have even higher efficiency with larger mechanisms. An implicit treatment for certain reaction-rate terms applied during the solution of species-conservation equations is described. Moving the reaction-rate source terms to the left-hand side of the partial differential equations eases the stiffness problem that is typically associated with combustion chemical kinetics. Computational speeds are further improved in UNICORN by completely integrating the chemical-kinetics mechanisms with the solution algorithm. A software-generated computational fluid dynamics approach is used to avoid the tedious and near-impossible task of manually integrating a large chemical-kinetics mechanism into a computational fluid dynamics code. Several calculations demonstrating the abilities of the UNICORN code are presented. Chemical-kinetics mechanisms up to 366 species and 3700 reaction steps are incorporated, and simulations for unsteady multidimensional flames are performed on personal computers. Making use of the robustness and efficiency of the UNICORN code, detailed chemical mechanisms developed for JP-8 fuel are tested for their accuracy, and a parametric study on the role of parent species of a surrogate mixture in predicting flame extinguishment is performed. Ease of changing chemical kinetics in the UNICORN code is demonstrated through the investigation of effects of additives in JP-8 fuel.

Kinetic study on the Maillard reaction. Consideration of sugar reactivity

The aim of the present study was to compare five reducing sugars (ribose, xylose, arabinose, glucose and fructose) with respect to their relative reactivity in the Maillard reaction (MR) (55 °C, pH 6.5) with a shrimp hydrolysate. For each system, the extent of the MR was assessed for a 24-h period by monitoring browning intensity, free amino group disappearance and sugar consumption. Results revealed the prevailing propensity of pentoses over hexoses to react in the MR, with a distinguishable behaviour for the ribose-hydrolysate system. Interestingly, pentoses were shown to mainly differ in terms of reaction rate. Complementary data on the chemical composition of the MR products (MRPs) were provided by size-exclusion chromatography, thus demonstrating the occurrence of some molecular rearrangements detected at 294 nm. The modification of protein substrates through the MR could represent a key step in the formation of new molecules and constitute a promising means to produce high value-added ingredients with biological and techno-functional properties.

Comparative Study on the Kinetic Modeling of the Oxidative Coupling of Methane in Laboratory and Bench Scales

The aim of this work is to develop and compare kinetic models for the oxidative coupling of methane (OCM) based on the gas hourly space velocity (GHSV) value and CH4/O2 ratio in two scales: laboratory and bench. The experiments were carried out in tubular fixed bed reactors at 1023 K, using 0.7-1.5 g and 30 g of perovskite titanate as the reaction catalyst for laboratory and bench scales, respectively. The various GHSVs (8000, 12000, 17000 h-1) and (3400, 4300, 5200 h-1) and methane to oxygen ratios (1, 2, 3, 4, 7.5) and (2, 2.5, 3) were selected for laboratory and bench scales, respectively. We have proposed a mechanism in which the consumption rate of methane is always twice of production rate of C2. A power law model was assumed for rate of reaction in terms of partial pressure of oxygen and methane. Using a linear regression analysis, the kinetic models were determined. Comparison of the calculated rate of reaction with the experimentally measured data confirmed the accuracy and applicability of the developed model for both scales.

An experimental kinetic model for the oxidative coupling of methane using a bench‐scale reactor

AbstractThe aim of this work is to develop a kinetic model for the oxidative coupling of methane (OCM) based on the gas‐hourly space velocity (GHSV) and methane‐to‐oxygen ratio. The experiments were carried out in a bench‐scale tubular‐fixed bed reactor, using 30 g of perovskite titanate as the reaction catalyst. Different GHSVs (3000–6000 h−1) and methane‐to‐oxygen ratios (2,2.5 and 3) were selected. A mechanism was assumed in which the consumption rate of methane is always twice the production rate of C2 hydrocarbons. A power‐law model was assumed for rate of reaction in terms of partial pressure of oxygen and methane. Using a linear regression analysis, the kinetic model was determined. Comparison of the calculated rate of reaction with the experimentally measured data confirmed the accuracy and applicability of the developed model under various reaction conditions. Copyright © 2008 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Effect of methyl branching of C8H18 alkanes and water activity on lipase-catalyzed enantioselective esterification of ibuprofen

The purpose of this research was to study the effect of the methyl branching of a high log P alkane solvent and the water activity in the organic medium on the initial rate and the enantioselectivity of ibuprofen esterification catalyzed by Candida rugosa lipase. Resolution of ibuprofen is important because S-(+)-ibuprofen has the desired pharmacological activity, whereas the R-(-)-enantiomer causes much of the side effects. The Candida rugosa lipase-catalyzed reaction in isooctane at 40oC and 0.73 water activity gave the best results, both in terms of the initial reaction rate and the enantioselectivity of the reaction. An increase in water activity allowed a higher reaction rate and enantiomeric excess in each of the four solvents. An increase in methyl branching did not necessarily increase the initial reaction rate, but it allowed a higher enantioselectivity, evidenced by an increase in the substrate enantiomeric excess

A Novel Adsorption/Saturation Approach to Ultrasonic Degradation of Phenol

AbstractThe study describes development and application of a kinetic model based on the local concentration of solutes and hydroxyl radicals to predict the rate of ultrasonically induced oxidative degradation of phenol at 300 kHz. The results reveal that the degradation follows saturation kinetics. The rate is slow at low concentrations of the solute, increases with concentration and reaches a maximum at what we defined as “the critical concentration”. At higher concentrations (than the “critical”) the rate slows down and stabilizes at a plateau as it becomes concentration-independent. Owing to the similarity of sonochemical reactions in water to chemical reactions in heterogeneous systems, a kinetic model based on Langmuir adsorption isotherm is developed to predict the rate of reaction in terms of the initial concentration of the solute. The similarity of the model to models describing catalytic and adsorptive reaction kinetics is explained by reduced mass transfer limitations under ultrasonic irradiation, and localization of the solute at the bubble-liquid interface as a function of concentration.

Direct formation of β-glycosides of N-acetyl glycosamines mediated by rare earth metal triflates

A direct, mild and efficient protocol for the preparation of beta-glycosides of N-acetyl glucosamine (GlcNAc) and N-acetyl galactosamine (GalNAc) has been developed using peracetylated beta-GlcNAc and beta-GalNAc as donors. All rare Earth metal triflate promoters screened were found to promote glycosylation with Sc(OTf)(3) being superior in terms of reaction rate. Simple alcohol glycosylation was found to proceed smoothly in refluxing dichloromethane, whereas higher temperatures under microwave conditions were needed to attain acceptable yields with less reactive, carbohydrate based glycosyl acceptors. The protocol developed was applied to provide the first example of direct chemical formation of a disaccharide using both GlcNAc as a glycosyl donor and acceptor. The alpha-acetate donor was found to be significantly less reactive than the corresponding beta-anomer necessitating higher reaction temperatures under which glycoside anomerisation was found to occur. It was established, that the anomerisation only took place in the presence of both Sc(OTf)(3) and acetic acid.