Showing papers on "Reaction rate published in 2007"

Structure of a spatially developing turbulent lean methane–air Bunsen flame

Abstract: Direct numerical simulation of a three-dimensional spatially developing turbulent slot-burner Bunsen flame has been performed with a new reduced methane–air mechanism. The mechanism, derived from sequential application of directed relation graph theory, sensitivity analysis and computational singular perturbation over the GRI-1.2 detailed mechanism is non-stiff and tailored to the lean conditions of the DNS. The simulation is performed for three flow through times, long enough to achieve statistical stationarity. The turbulence parameters have been chosen such that the combustion occurs in the thin reaction zones regime of premixed combustion. The data is analyzed to study possible influences of turbulence on the structure of the preheat and reaction zones. The results show that the mean thickness of the turbulent flame, based on progress variable gradient, is greater than the corresponding laminar flame. The effects of flow straining and flame front curvature on the mean flame thickness are quantified through conditional means of the thickness and by examining the balance equation for the evolution of the flame thickness. Finally, conditional mean reaction rate of key species compared to the laminar reaction rate profiles show that there is no significant perturbation of the heat release layer.

Efficient Acid-Catalyzed Hydrolysis of Cellulose in Ionic Liquid

Abstract: A novel method for cellulose hydrolysis catalyzed by mineral acids in the ionic liquid 1-butyl-3-methylimidazolium chloride ([C(4)mim]Cl) has been developed that facilitates the hydrolysis of cellulose with dramatically accelerated reaction rates at 100 degrees C under atmospheric pressure and without pretreatment.

High-pressure laminar flame speeds and kinetic modeling of carbon monoxide/hydrogen combustion

Abstract: Laminar flame speeds were accurately measured for CO/H 2 /air and CO/H 2 /O 2 /helium mixtures at different equivalence ratios and mixing ratios by the constant-pressure spherical flame technique for pressures up to 40 atmospheres. A kinetic mechanism based on recently published reaction rate constants is presented to model these measured laminar flame speeds as well as a limited set of other experimental data. The reaction rate constant of CO + HO 2 → CO 2 + OH was determined to be k = 1.15 × 10 5 T 2.278 exp(−17.55 kcal/ RT ) cm 3 mol −1 s −1 at 300–2500 K by ab initio calculations. The kinetic model accurately predicts our measured flame speeds and the non-premixed counterflow ignition temperatures determined in our previous study, as well as homogeneous system data from literature, such as concentration profiles from flow reactor and ignition delay time from shock tube experiments.

Water catalysis of a radical-molecule gas-phase reaction.

Abstract: There has been considerable speculation about the role of water and water complexes in chemical gas-phase reactions, including the conjecture that water may act as a molecular catalyst through its ability to form hydrogen bonds. Here, we present kinetic studies in which the effect of water on the rate of the reaction between hydroxyl radicals and acetaldehyde has been measured directly in Laval nozzle expansions at low temperatures. An increasing enhancement of the reaction rate by added water was found with decreasing temperatures between 300 and 60 kelvin. Quantum chemical calculations and statistical rate theory support our conclusions that this observation is due to the reduction of an intrinsic reaction barrier caused by specific water aggregation. The results suggest that even single water molecules can act as catalysts in radical-molecule reactions.

An extremely efficient three-component reaction of aldehydes/ ketones, amines, and phosphites (Kabachnik-Fields Reaction) for the synthesis of α-aminophosphonates catalyzed by magnesium perchlorate

Abstract: Commercially available magnesium perchlorate is reported as an extremely efficient catalyst for the synthesis of α-aminophosphonates. A three-component reaction (3-CR) of an amine, an aldehyde or a ketone, and a di-/trialkyl phosphite (Kabachnik−Fields reaction) took place in one pot under solvent-free conditions to afford the corresponding α-aminophosphonates in high yields and short times. The use of solvent retards the rate of the reaction and requires a much longer reaction time than that for neat conditions. The reactions involving an aldehyde, an aromatic amine without any electron-withdrawing substituent, and a phosphite are carried out at rt. The reactions involving cyclic ketones, aromatic amines with an electron-withdrawing substituent, and aryl alkyl ketone (e.g., acetophenone) require longer reaction times at rt or heating. Magnesium perchlorate was found to be superior to other metal perchlorates and metal triflates during the reaction of 4-methoxybenzaldehyde, 2,4-dinitroaniline, and dimethy...

Transesterification of poultry fat with methanol using Mg–Al hydrotalcite derived catalysts

Abstract: The synthesis of biodiesel from poultry fats provides a way to convert the by-product of a renewable resource to a very important value-added biofuel. In this work, the use of heterogeneous base catalysts derived from Mg–Al hydrotalcite was investigated for the conversion of poultry lipids to biodiesel. This solid base showed high activity for triglyceride (TG) transesterification with methanol without signs of catalyst leaching. Catalytic performance was significantly affected by pretreatment and operating conditions. Calcination at optimum temperature was key in obtaining the highest catalyst activities. Rehydration of the calcined catalyst before reaction using wet nitrogen decreased catalytic activity for the transesterification of poultry fat, opposite to what has been reported for condensation reactions. Also, methanol had to be contacted with the catalyst before reaction; otherwise, catalyst activity was seriously impaired by strong adsorption of triglycerides on the active sites. Both temperature (60–120 °C) and methanol-to-lipid molar ratio (6:1–60:1) affected the reaction rate in a positive manner. The use of a co-solvent (hexane, toluene, THF), however, gave rise to a change in TG conversion profile which cannot be explained solely by a dilution effect. The catalyst underwent significant deactivation during the first reaction cycle probably due to deactivation of the strongest base sites. Subsequent reaction cycles showed stable activity. By re-calcination in air, complete catalyst regeneration was achieved.

Reduction Kinetics of Cu-, Ni-, and Fe-Based Oxygen Carriers Using Syngas (CO + H2) for Chemical-Looping Combustion

Abstract: The reactivity of three Cu-, Fe-, and Ni-based oxygen carriers to be used in a chemical-looping combustion (CLC) system using syngas as fuel has been analyzed The oxygen carriers exhibited high reactivity during reduction with fuel gases present in syngas (H2 and CO), with average values in the range 8−30% min-1 No effect of the gas products (H2O, CO2) on the reduction reaction rate was detected The kinetic parameters of reaction with H2 and CO have been determined by thermogravimetric analysis The grain model with spherical or platelike geometry in the grain was used for the kinetic determination, in which the chemical reaction controlled the global reaction rate The activation energies determined for these reactions were low, with values ranging from 14 to 33 kJ mol-1 The reaction order depended on the reacting gas, and values from 05 to 1 were found Moreover, the reactivity of the oxygen carriers when both H2 and CO are simultaneously present in the reacting gases has been analyzed, both at atm

Detailed chemical kinetic modeling of cyclohexane oxidation.

Abstract: A detailed chemical kinetic mechanism has been developed and used to study the oxidation of cyclohexane at both low and high temperatures. Rules for reaction rate constants are developed for the low-temperature combustion of cyclohexane. These rules can be used for in chemical kinetic mechanisms for other cycloalkanes. Because cyclohexane produces only one type of cyclohexyl radical, much of the low-temperature chemistry of cyclohexane is described in terms of one potential energy diagram showing the reaction of cyclohexyl radical with O2 through five-, six-, and seven-membered-ring transition states. The direct elimination of cyclohexene and HO2 from RO2 is included in the treatment using a modified rate constant of Cavallotti et al. (Proc. Combust. Inst. 2007, 31, 201). Published and unpublished data from the Lille rapid compression machine, as well as jet-stirred reactor data, are used to validate the mechanism. The effect of heat loss is included in the simulations, an improvement on previous studies ...

Mechanism of the oxidation of alcohols by oxoammonium cations.

Abstract: The mechanism of the oxidation of primary and secondary alcohols by the oxoammonium cation derived from 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) has been investigated computationally at the B3LYP/6-31+G* level, along with free energies of solvation, using a reaction field model. In basic solution, the reaction involves formation of a complex between the alkoxide anion and the oxoammonium cation in a pre-oxidation equilibrium wherein methoxide leads to a much larger formation constant than isopropoxide. The differences in free energy of activation for the rate-determining hydrogen transfer within the pre-oxidation complexes were small; the differences in complex formation constants lead to a larger rate of reaction for the primary alcohol, as is observed experimentally. In acidic solution, rate-determining hydrogen atom transfer from the alcohol to the oxoammonium cation had a large unfavorable free energy change and would proceed more slowly than is observed. A more likely path involves a hydride tran...

Genesis of coordinatively unsaturated palladium complexes dissolved from solid precursors during heck coupling reactions and their role as catalytically active species

Abstract: The Forum Article critically summarizes investigations and discussions on the nature and role of potential active species in C−C coupling reactions of the Heck type using catalyst systems with “ligand-free” inorganic salts, simple inorganic complexes, and supported and nonsupported (colloidal) Pd particles. From a series of experiments and reports, it can be concluded that the “active species” is generated in situ in catalytic systems at higher temperature conditions (>100 °C). In all heterogeneous systems with solid Pd catalysts, Pd is dissolved from the solid catalyst surface under reaction conditions by a chemical reaction (complex formation and/or oxidative addition of the aryl halide), forming extremely active coordinatively unsaturated Pd species. Pd is partially or completely redeposited onto the support at the end of the reaction when the aryl halide is used up. The Pd dissolution−redeposition processes correlate with the reaction rate and are strongly influenced by the reaction conditions. Skille...

Debromination of decabrominated diphenyl ether by resin-bound iron nanoparticles.

Abstract: Nanoscale zerovalent iron, n-ZVI, was found to be highly effective in reductively debrominating decabromodiphenyl ether (BDE209) at ambient conditions and without the catalysis of noble metals. A method was developed to immobilize n-ZVI particles on a cation-exchange resin. The n-ZVI coated resin was then mixed with BDE209 in a water/acetone (1:1) solution, and the reaction was allowed to proceed for up to 10 days. The first-order rate constant of BDE209 disappearance was estimated to be 0.28 ± 0.04 h-1. The debromination was found to be stepwise, and less-brominated congeners were produced with increasing reaction time. Dechlorination of decachlorobiphenyl (PCB209) was also investigated, but the reaction rate was much slower than the debromination of BDE209. Identification of the reaction products was highly challenging and was assisted by regression equations between experimental and reference gas chromatographic relative retention times, with confirmation by high-resolution mass spectrometry and refere...

Steam reforming of ethanol on supported nickel catalysts

Abstract: The catalytic behavior of Ni/Al2O3 catalysts modified with La and Ag was investigated in the steam reforming of ethanol. The catalysts were characterized by SBET, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and Fourier transform infrared spectroscopy (FTIR) of CO adsorption. The reaction rate for noncatalytic decomposition of ethanol in the homogeneous gas phase becomes significant only at high temperatures (T ≥ 890 K). FTIR results revealed that Ag strongly modifies the Ni surface, decreasing the intensity ratio of the bands for adsorbed CO in the bridging mode at low frequency (LF) and the linear mode at high frequency (HF). Similar but smaller effect was observed in the La-containing catalyst. The activity and stability against carbon depostion for steam reforming of ethanol of Ni/Al2O3 catalyst was only slightly sensitive to Ag but the activity was strongly dependent on the presence of La. The reaction data suggest that at temperatures lower than 650 K the Ni/Al2O3 catalyst was active for decomposition of ethanol via the acetaldehyde intermediate, showing high selectivity to methane and CO. The rate of steam reforming of ethanol became significant at temperature higher than 650 K. Comparing Ni/Al2O3 and Ni/La–Al2O3 catalysts the results pointed out that the Ni became more susceptible to modification by water in La-containing Ni catalyst. The Ni/La–Al2O3 catalysts become inactive at low temperatures, and the activity could be regenerated with reduction of NiO by ethanol on raising the reaction temperature. Differently from the steam reforming of methane, the coking cannot be suppressed by Ag promotion in the case of steam reforming of ethanol. Inversely, the La has a positive effect to decreasing coking on Ni catalysts. A scheme for ethanol reactions with H2O on Ni surfaces is proposed based on reaction tests.