Rate-determining Reaction Research Articles

Chemical reaction rates and non-equilibrium pressure of reacting gas mixtures in the state-to-state approach

Viscous gas flows with vibrational relaxation and chemical reactions in the state-to-state approach are analyzed. A modified Chapman–Enskog method is used for the determination of chemical reaction and vibrational transition rates and non-equilibrium pressure. Constitutive equations depend on the thermodynamic forces: velocity divergence and chemical reaction/transition affinity. As an application, N2 flow with vibrational relaxation across a shock wave is investigated. Two distinct processes occur behind the shock: for small values of the distance the affinity is large and vibrational relaxation is in its initial stage; for large distances the affinity is small and the chemical reaction is in its final stage. The affinity contributes more to the transition rate than the velocity divergence and the effect of these two contributions are more important for small distances from the shock front. For the non-equilibrium pressure, the term associated with the bulk viscosity increases by a small amount the hydrostatic pressure.

Integrated solvent and process design exemplified for a Diels–Alder reaction

A new kind of solvent descriptor obtained from quantum chemical calculations is introduced. Group contributions to each solvent descriptor are regressed for 71 UNIFAC groups. A reaction kinetic model is built by correlating a set of experimentally determined reaction rate constants in various solvents with the corresponding theoretical solvent descriptors. Based on the kinetic model and the developed group contribution method, a computer‐aided molecular design problem is formulated and optimal solvents to achieve highest reaction rates are identified. For considering the multiple and complicated effects of solvents on a chemical process, an integrated solvent and process design is performed. Solvent molecular structures and process operations are simultaneously optimized by the formulation and solution of a mixed‐integer nonlinear program. The proposed design methodology is exemplified for a selected Diels–Alder reaction. © 2014 American Institute of Chemical Engineers AIChE J, 61: 147–158, 2015

Theoretical Insight into the Mechanism of an Efficient ʟ-Proline-catalyzed Transamidation of Acetamide with Benzylamine

-proline and pyrrolidine catalyzed transamidation of acetamide withbenzylamine have been investigated using density functional theory (DFT) calculations. Our calculated resultsshow: (1) the mechanisms of two catalytic cycle reactions are similar. However, the rate-determining steps oftheir reactions are different for the whole catalytic process. One is the intramolecular nucleophilic additionreaction of 1-COM, the other is hydrolysis reaction of 2-C. (2) COOH group of

Kinetics of Forward Extraction of V(IV) in V(IV)-SO2- 4(H+, NA+)-Cyanex 302-Kerosene System Using Lewis Cell Technique

Kinetics of V(IV) extraction from acidic sulfate medium by Cyanex 302 (2,4,4-trimethylpentylmonothio phosphinic acid, [H2A2](o)) dissolved in kerosene were investigated using a Lewis cell operated at 3 Hz and flux (F) method of data treatment. The F (kmol/m2s) is inversely proportional to (1 + 0.01 [V(IV)]−1), (1 + 2000 [H+]), and (1 + 0.089 ) and directly proportional to (1 + 0.2 []). The activation energy (E a ) depends on the temperature region and is a function of reactants (f(R)). When log f(R) <−1.0, E a > 48 kJ/mol and when log f(R)>−0.3, E a < 20 kJ/mol. Within log f(R) values of −1.0 to −0.3, E a varies within 20–50 kJ/mol. The rate constant (k) at 293 K is 10−7.335 kmol/m2 s. The entropy changes in activation (ΔS ‡) were measured as negative value always. The complicated empirical rate equation was analyzed to provide extraction mechanisms in different parametric conditions, being supported by E a values. In most cases, the extraction process is either diffusion controlled or mixed controlled; a mixed-controlled process can be converted to a diffusion-controlled process at a lower temperature region and to a chemical-controlled process at a higher temperature region. For systems with log f(R) <−1.0, the process is completely chemical controlled. The rate-determining chemical reaction steps in order are: → ; and → at lower and higher concentration regions of . The negative ΔS ‡ values suggest that the rate-determining chemical reaction steps occur by SN2 mechanisms.

Systematic study of neutron capture including the compound, pre-equilibrium, and direct mechanisms

The neutron capture reaction is investigated. The three major reaction mechanisms, namely, compound-nucleus capture (CNC), pre-equilibrium capture (PEC), and direct capture (DIC), are considered on the basis of the Hauser--Feshbach model, the exciton model, and potential model, respectively. The three mechanisms are treated simultaneously and consistently, i.e, they are obtained on the basis of the same nuclear ingredients, such as the optical potential and nuclear-level densities. In this framework, the three components are calculated on the same footing and represent partial fluxes of the same total reaction cross section. The total neutron-capture cross sections and astrophysical reaction rates are calculated within the updated modern reaction code talys for about 8000 nuclei with $8\ensuremath{\le}Z\ensuremath{\le}110$ lying between the proton and neutron drip lines. The nuclear-structure ingredients involved in the calculation are determined from experimental data whenever available and, if not, from global microscopic nuclear models. For the targets with mass number $Ag26$, a fair agreement between the computed total-capture cross sections and experimental data is found but, for the lightest nuclei, only the predicted DIC cross sections reproduce the experimental results satisfactorily. Significant and even dominant contribution to the total reaction rate comes from the DIC component for neutron-rich nuclei, especially in the $Z=50--70$ region. The impact of the newly determined reaction rates on the r process abundances resulting from the ejection of matter in neutrino-driven winds or the decompression of neutron star matter is investigated.

Kinetic studies of inverse electron demand Diels–Alder reactions (iEDDA) of norbornenes and 3,6-dipyridin-2-yl-1,2,4,5-tetrazine

Inverse electron demand Diels–Alder additions (iEDDA) between 1,2,4,5-tetrazines and olefins have recently found widespread application as a novel ‘click chemistry’ scheme and as a mild technique for the modification of materials. Norbornenes are, due to their straightforward synthetic availability, especially interesting in the latter context. Therefore, the reactivity of different norbornene-based compounds was compared with unsubstituted norbornene and other alkenes using UV-vis measurements for the determination of reaction rates under pseudo first order conditions. Thereby, exo,exo-5-norbornene-2,3-dimethanol was found to be almost as reactive as unsubstituted norbornene whereas (±)-endo,exo-dimethyl-5-norbornene-2,3-dicarboxylate reacted only insignificanty faster than unstrained alkenes.

Alpha induced reaction cross section measurements on 162Er for the astrophysical γ process

The cross sections of the Er162(α,γ)Yb166 and Er162(α,n)Yb165 reactions have been measured for the first time. The radiative alpha capture reaction cross section was measured from Ec.m.=16.09 MeV down to Ec.m.=11.21 MeV, close to the astrophysically relevant region (which lies between 7.8 and 11.48 MeV at 3 GK stellar temperature). The Er162(α,n)Yb165 reaction was studied above the reaction threshold between Ec.m.=12.19 and 16.09 MeV. The fact that the Er162(α,γ)Yb166 cross sections were measured below the (α,n) threshold at first time in this mass region opens the opportunity to study directly the α-widths required for the determination of astrophysical reaction rates. The data clearly show that compound nucleus formation in this reaction proceeds differently than previously predicted.

Constant Growth Rate Can Be Supported by Decreasing Energy Flux and Increasing Aerobic Glycolysis

Fermenting glucose in the presence of enough oxygen to support respiration, known as aerobic glycolysis, is believed to maximize growth rate. Weobserved increasing aerobic glycolysis during exponential growth, suggesting additional physiological roles for aerobic glycolysis. We investigatedsuch roles in yeast batch cultures by quantifying O2 consumption, CO2 production, amino acids, mRNAs, proteins, posttranslational modifications, and stress sensitivity in the course of nine doublings at constant rate. During this course, the cells support a constant biomass-production rate with decreasing rates of respiration and ATP production but also decrease their stress resistance. As the respiration rate decreases, so do the levels of enzymes catalyzing rate-determining reactions of the tricarboxylic-acid cycle (providing NADH for respiration) and of mitochondrial folate-mediated NADPH production (required for oxidative defense). The findings demonstrate that exponential growth can represent not a single metabolic/physiological state but a continuum of changing states and that aerobic glycolysis can reduce the energy demands associated with respiratory metabolism and stress survival.

(Invited) Impact of Surface Chemistry on the Electrochemical Performance of Perovskite Cathodes

Surface chemistry and reactivity of electrodes in electrochemical devices play a key role for electrochemical performance and cell durability. In this work, we use spatially resolved scanning photoelectron microscopy and electrochemical measurements to study in situ the surface chemistry of perovskite catalysts in SOFC cathode model cells during high temperature operation at 500-550°C in oxygen containing environment (10-6mbar) at OCV or under oxygen flux. Oxygen ion flux and electric field cause dynamic changes of catalyst and electrolyte surface chemistry, including redox reaction, surface segregation and long range surface diffusion. In LSM cathodes, oxidizing environment was found to promote transition metal-rich cathode surface termination with high surface oxygen content and a variety of oxygen surface species, while reducing environment produced surface segregation of strontium and low level of surface oxygen. The electrochemical response provided the rate-determining reaction steps of the operating model cell and could directly be compared to the spectroscopic observations.

Decomposition kinetics of MTBE, ETBE and, TAEE in water and wastewater using catalytic and photocatalytic ozonation

The results of an extensive kinetic study on the heterogeneous ozonation of three different fuel-oxygenates: Methyl-t-butyl-ether (MTBE), ethyl-t-butyl-ether (ETBE) and t-amyl-ethyl-ether (TAEE), over an immobilised TiO2 layer in dark (catalytic ozonation) and in the presence of UVA irradiation (photocatalytic ozonation) are shown in the present paper. A falling film reactor was utilised to handle the oxidation processes in this work. The study was conducted in both deionised water and a real wastewater sample. The decomposition of the three chosen model compounds showed good agreement with pseudo first-order kinetics using both catalytic and photocatalytic ozonation in water and wastewater. The increase of ozone concentration and the concentrations of MTBE, ETBE and TAEE in their aqueous solutions led to a linear increase in the initial removal rates of these compounds by photocatalytic ozonation. The removal rates of MTBE, ETBE and TAEE by photocatalytic ozonation were about 10 times, 2.5 times and 2.2 times more than those by catalytic ozonation, respectively. The determined reaction rate constants of MTBE, ETBE and TAEE with ozone were 0.231M−1s−1, 0.785M−1s−1 and, 0.960M−1s−1, respectively. The determined adsorption level of model compounds on the photocatalyst surface was in the following order: MTBE>ETBE>TAEE.

Diffusion of Oxygen in Ceria at Elevated Temperatures and Its Application to H2O/CO2 Splitting Thermochemical Redox Cycles

Determination of reaction and oxygen diffusion rates at elevated temperatures is essential for modeling, design, and optimization of high-temperature solar thermochemical fuel production processes, but such data for state-of-the-art redox materials, such as ceria, is sparse. Here, we investigate the solid-state reduction and oxidation of sintered nonstoichiometric ceria at elevated temperatures relevant to solar thermochemical redox cycles for splitting H2O and CO2 (1673 K ≤ T ≤ 1823 K, 3 × 10–4 atm ≤ pO2 ≤ 2.5 × 10–3 atm). Relaxation experiments are performed by subjecting the sintered ceria to rapid oxygen partial pressure changes and measuring the time required to achieve thermodynamic equilibrium state. From such data, we elucidate information regarding ambipolar oxygen diffusion coefficients through comparison of experimental data to a numerical approximation of Fick’s second law based on finite difference methods. In contrast to typically applied analytical approaches, where diffusion coefficients a...

Cross section and reaction rate of determined from thick target yield measurements

For the better understanding of the astrophysical gamma-process the experimental determination of low energy proton- and alpha-capture cross sections on heavy isotopes is required. The existing data for the 92Mo(p,gamma)93Tc reaction are contradictory and strong fluctuation of the cross section is observed which cannot be explained by the statistical model. In this paper a new determination of the 92Mo(p,gamma)93Tc and 98Mo(p,gamma)99mTc cross sections based on thick target yield measurements are presented and the results are compared with existing data and model calculations. Reaction rates of 92Mo(p,gamma)93Tc at temperatures relevant for the gamma-process are derived directly from the measured thick target yields. The obtained rates are a factor of 2 lower than the ones used in astrophysical network calculations. It is argued that in the case of fluctuating cross sections the thick target yield measurement can be more suited for a reliable reaction rate determination.

Investigation of small-scale processes in the rhizosphere of Lupinus albus using micro push-pull tests

Rhizosphere processes affect the mobility, phytoavailability and toxicity of solutes in soil. To study reactions in the rhizosphere under quasi in situ conditions, we recently developed the “micro push-pull test” (μPPT) method, combining micro-suction cups with the principle of the “push-pull test” method known from groundwater applications. Here we report the application of μPPT to investigate rhizosphere reactions in situ, i.e. degradation of deuterated citrate (citrate-d4) in the rhizosphere of Lupinus albus grown in sand-filled rhizoboxes. In a μPPT, a solution containing reactive (citrate-d4) and non-reactive solutes (bromide) is injected into a porous medium and shortly thereafter, the pore water solution is re-extracted from the same location. Concentration (“breakthrough”) curves of extracted reactants can be compared to those of the non-reactive solute, allowing the determination of reaction rates. We applied the μPPT in rhizoboxes with Lupinus albus and sampled different types of micro-habitats: bulk soil, rhizosphere of normal roots and rhizosphere of cluster roots of different ages. Breakthrough curves of citrate-d4 varied considerably between tests adjacent to cluster roots and normal roots, and in bulk soil. Degradation of citrate-d4 in bulk soil and adjacent to normal roots was below detection, while we found strong degradation of citrate-d4 adjacent to 4 to 5-days old cluster roots. In situ degradation rate constants for citrate-d4 around cluster roots were found to be in the range from 0.38 to 0.71 h−1. We successfully applied the μPPT to the rhizosphere. The μPPT is useful to investigate local processes in microcosms and to monitor processes also over time (e.g., during cluster-root development) due to its non-destructive nature.

Successful downsizing for high-throughput ¹³C-MFA applications.

(13)C label-based metabolic flux analysis is a powerful technique for the determination of intracellular reaction rates and is used in such different research fields as quantitative physiology, metabolic engineering, and systems biology. Metabolic fluxes can be determined at high quality using (pseudo)-steady-state cultures and advanced mathematical models for data interpretation. Here, we describe a protocol for parallel metabolic flux analysis that consists of downsized microbial (yeast) cultivation, miniaturized sample preparation, and semiautomated analytics and data evaluation. With this protocol dozens of metabolic flux analyses can be carried out in 1 week, thereby enabling for example the analysis of genetic and environmental perturbations on the operation of metabolic networks.

The use of benthic metabolic processes as indicators for environmental quality assessment in coastal lagoons

The review highlights the various methods used for assessing environmental quality in Mediterranean coastal lagoons, with emphasis on benthic parameters and processes. The application of indices based on benthic macrofauna, extensively used in coastal areas, may fail in discerning between natural and anthropogenic pressures over naturally stressed coastal lagoons. Sediment can play an important regulatory role over the overlying water composition through the storage capacity for organic matter and pollutants, regeneration of nutrients or its buffering capacity. Descriptive classical measurements like sedimentary organic matter, Chlorophyll α and nutrient content are commonly included in monitoring efforts. However, other more complex indicators like primary production, sediment-water solute fluxes, solute sorption dynamics or microbial reaction rate determinations, have not been fully implemented for environmental quality assessment in coastal lagoons. These could offer crucial information on current and projected anthropogenic influence on ecosystem functioning. Irruption of novel techniques in benthic biogeochemistry like Excitation-Emission-Matrix (EEM) fluorescence for the study of dissolved organic matter dynamics shows high potential in combination with biological quality elements and other metabolic measurements for the evaluation of the environmental quality in coastal lagoons.

The use of benthic metabolic processes as indicators for environmental quality assessment in coastal lagoons

The review highlights the various methods used for assessing environmental quality in Mediterranean coastal lagoons, with emphasis on benthic parameters and processes. The application of indices based on benthic macrofauna, extensively used in coastal areas, may fail in discerning between natural and anthropogenic pressures over naturally stressed coastal lagoons. Sediment can play an important regulatory role over the overlying water composition through the storage capacity for organic matter and pollutants, regeneration of nutrients or its buffering capacity. Descriptive classical measurements like sedimentary organic matter, Chlorophyll α and nutrient content are commonly included in monitoring efforts. However, other more complex indicators like primary production, sediment-water solute fluxes, solute sorption dynamics or microbial reaction rate determinations, have not been fully implemented for environmental quality assessment in coastal lagoons. These could offer crucial information on current and projected anthropogenic influence on ecosystem functioning. Irruption of novel techniques in benthic biogeochemistry like Excitation-Emission-Matrix (EEM) fluorescence for the study of dissolved organic matter dynamics shows high potential in combination with biological quality elements and other metabolic measurements for the evaluation of the environmental quality in coastal lagoons.

The Mechanism of Ethylene Dimerization with the Ti(OR′)4/AlR3 Catalytic System: DFT Studies Comparing Metallacycle and Cossee Proposals

The mechanism of ethylene dimerization to 1-butene, as promoted by the Ti(OR′)4/AlR3 catalyst system, has been explored with the aid of density functional theory (DFT) to determine which mechanistic proposal, metallacycle or Cossee–Arlman, is most likely. The theoretical studies predict that the Cossee mechanism has the lowest rate-determining reaction barrier and also that this mechanism is more likely to lead to selective dimerization. In contrast, for the metallacycle mechanism, a higher likelihood of 1-hexene formation is predicted. The possibility of isomerization or codimerization of 1-butene has also been studied according to a Cossee mechanism, with the results obtained in good agreement with previous experiments. As a result of this study and recent experimental results, a Cossee mechanism of dimerization with this catalyst, proceeding via a titanium-hydride intermediate, is considered the most probable route.

Kinetic studies on chromium-catalyzed conversion of glucose into 5-hydroxymethylfurfural in alkylimidazolium chloride ionic liquid

5-Hydroxymethylfurfural (HMF) is a promising green platform chemical derived from biomass. Kinetic studies were performed on chromium chloride-catalyzed conversion of glucose into HMF in alkylimidazolium chloride ionic liquids. The main by-products are disaccharides, fructose, glyceraldehyde, formic acid, and humins. The formation of HMF is strongly affected by reaction temperature and initial glucose concentration. The reaction is second order in glucose, with an activation energy of 134.9kJmol−1. The order in chromium is first, indicating that the rate-determining isomerization reaction is catalyzed by a mononuclear chromium species. The observed glucose conversion rate constant decreases as initial glucose concentration increases, suggesting that the catalytic activity of the chloride anion is significantly restrained by the hydrogen bonding with hydroxyl groups. A simplified kinetic model is developed to describe the behaviors of glucose conversion and HMF formation. This model is in good agreement with the experimental results.

Experiments on screening effect in deuteron fusion reactions at extremely low energies

The enhanced electron screening effect in nuclear reactions taking place in dense astrophysical plasmas is extremely important for determination of stellar reaction rates in terrestrial laboratories as well as in prediction of cross sections enhancement in interiors of stars such as White and Brown Dwarfs or Giant Planets. This effect resulting in reduction of the nuclear Coulomb potential by the atomic electrons has been confirmed in many laboratory experiments. Unfortunately, experimental screening energies are much higher than the theoretical predictions and the reason for that remains unknown. Here, we present absorbing results of the experiment studying d + d nuclear reactions in different deuterized metallic targets under ultra high vacuum conditions. The total cross sections and angular distributions of the 2H(d,p)3H and 2H(d,n)3He reactions have been measured using a deuteron beam of energies between 8 and 30 keV provided by the electron cyclotron ion source. The atomic cleanness of the target surface has been secured by combining Ar sputtering of the target and Auger electrons spectroscopy. Due to application of an on-line analysis method, the homogeneity of the implanted deuteron densities could be continuously monitored. We will discuss probable causes of the large discrepancy between theoretical and experimental data.

The Mechanism of Glucose Isomerization to Fructose over Sn‐BEA Zeolite: A Periodic Density Functional Theory Study

The isomerization of glucose to fructose in the presence of Sn-containing zeolite BEA (beta polymorph A) was studied by periodic DFT calculations. Focus was placed on the nature of the active site and the reaction mechanism. The reactivities of the perfect lattice Sn(IV) site and the hydroxylated SnOH species are predicted to be similar. The isomerization activity of the latter can be enhanced by creating an extended silanol nest in its vicinity. Besides the increased Lewis acidity and coordination flexibility of the Sn center, the enhanced reactivity in this case is ascribed to the reaction environment that promotes activation of the confined sugar intermediates through hydrogen bonding. The resulting multidentate activation of the substrate favors the rate-determining hydrogen-shift reaction. These findings suggest the important role of defect lattice sites in Sn-BEA for catalytic glucose isomerization.