Effective Reaction Rate Constants Research Articles

Detailed Kinetic Investigations on the Selective Oxidation of Biomass to Formic Acid (OxFA Process) Using Model Substrates and Real Biomass

A detailed kinetic analysis of the selective catalytic oxidation of different biogenic model substrates to formic acid (OxFA process) is presented using two different polyoxometalate catalysts, HPA-2 (H5PV2Mo10O40) and HPA-5 (H8PV5Mo7O40). By using high oxygen pressures (30–60 bar) and temperatures of 80–90 °C in aqueous solution, we were able to investigate the rate-determining substrate oxidation step catalyzed by the oxidized form of the polyoxometalates (POMs) by keeping the concentration of the catalytic active species constant. Under these conditions, kinetic parameters like effective reaction order and reaction rate constants could be determined by the differential method for model substrates like glucose, fructose, sorbitol, and gluconic acid. Furthermore, kinetic experiments using the two different catalysts showed a different behavior for various functionalities within the carbon framework of the substrate with the tendency of higher reaction rates with the HPA-5 catalyst. This effect was more p...

The study of antioxidant activity of the celery root extract

We prove promising use of supplements based on celery in the technology of frozen meat and fish products split. The aim of experimental research was to determine the celery antioxidant activity and to study the dynamics of change in storage. Antioxidant activity estimation method used colorimetry free radicals based on reaction of DPPH dissolved in ethanol with specimens antioxidants. The kinetics of spending in DPPH reaction with celery extract of different concentrations. When the concentration of extract of celery in a ditch more than 0.5%, the reaction proceeded immediately. Observation of the dynamics of the reaction was carried out at concentrations of plant extracts 0.5; 0.25; 0.1; 0.05; 0.01 and 0.005%. A quantitative assessment antioxidant activity of celery compared to previous studies strong herbal antioxidants (Jerusalem artichoke tubers and Stevia leaves). These anamorphosis kinetic curves show that the effective rate constants of reactions are: celery - 1.24, artichoke tubers - 1.73, the leaves of Stevia - 0.58, i.e quantitative assessment celery antioxidant activity 37% lower than the artichoke tubers, and 56% higher than the stevia leaves. Thus, the antioxidant properties of these plants are comparable, that celery extract antioxidant activity is high. Found that freezing does not affect the antioxidant activity of celery extract, the extract in storage for three months antioxidant activity it remains almost unchanged, storage more than four months antioxidant activity reduces by 12%, over six months - 30%. This allows us to conclude that the storage period of three months frozen minced celery additive in products able to keep their antioxidant properties. The data of the studies make it possible to redict that celery additive in minced products can help stabilize lipids meat and fish and have a direct impact on the shelf life of these products

Studies of the CO oxidation kinetic regularities on Cu–Co–Fe oxide catalysts, bulk and deposited on the carbon nanotubes

The effect of catalysts of the carbon nanotubes synthesis and of the technology of a Cu–Co–Fe active oxide mass deposition on the kinetic regularities of the CO oxidation reaction has been studied. It has been shown that the experimental data on both catalysts, bulk Cu–Co–Fe and deposited onto carbon nanotubes, agree with the Eley–Readil kinetic scheme. Based on the suggested mechanism, the effective reaction rate constants have been calculated by various methods. It has been shown that the use of Cu–Co–Fe catalysts deposited on carbon nanotubes offers considerable promise.

Water splitting at an anion-exchange membrane as studied by impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is largely used in studying water splitting reaction in bipolar membranes (BPMs). This reaction is also very important in monopolar membrane systems, where it constitutes one of the possible mechanisms of the overlimiting current conductance. However, despite of 60-year research history in this domain, there are only a few studies where EIS was applied. Apparently, it is due to poor understanding of AC response of monopolar membranes in overlimiting current range where charge transfer by electrodiffusion competes with current-induced convection and water splitting products transfer.For the first time, we propose a mathematical description of monopolar membrane’s impedance spectra associated with water splitting. We show that these spectra are of the Gerischer type, as in the case of BPMs. The boundary value problem takes into account the proton-transfer catalytic mechanism of water splitting. The solution of this problem is in a good accordance with experimental spectra found for an AMX-SB membrane at differenat biased DC currents. It is found that the effective rate constant of water splitting reaction increases with increasing DC current, similar, as in the case of BPMs.

Polymerization of ε-caprolactone in the presence of tin(II) chloride complexes

The bulk polymerization of e-caprolactone at 110 and 155°C initiated by the tin complexes SnCl2 · 2H2O, SnCl4 · 5H2O, and SnCl2 · C4H8O2 (1,4-dioxane) has been compared to that initiated by tin(II) 2-ethyl hexanoate (octanoate). With 1H NMR spectroscopy, the monomer conversion and numberaverage degree of polymerization of poly(e-caprolactone) samples have been determined and the effective rate constants of reaction have been calculated. All the studied initiators provide a conversion of 98.0–99.5%. The polymers with the highest molecular masses were obtained in the presence of SnCl4 · 5H2O at 155°C and in the presence of the tin(II) chloride complex with dioxane at 110°C.

Para-hydrogen and helium cluster size distributions in free jet expansions based on Smoluchowski theory with kernel scaling

The size distribution of para-H2 (pH2) clusters produced in free jet expansions at a source temperature of T0 = 29.5 K and pressures of P0 = 0.9-1.96 bars is reported and analyzed according to a cluster growth model based on the Smoluchowski theory with kernel scaling. Good overall agreement is found between the measured and predicted, Nk = A k(a) e(-bk), shape of the distribution. The fit yields values for A and b for values of a derived from simple collision models. The small remaining deviations between measured abundances and theory imply a (pH2)k magic number cluster of k = 13 as has been observed previously by Raman spectroscopy. The predicted linear dependence of b(-(a+1)) on source gas pressure was verified and used to determine the value of the basic effective agglomeration reaction rate constant. A comparison of the corresponding effective growth cross sections σ11 with results from a similar analysis of He cluster size distributions indicates that the latter are much larger by a factor 6-10. An analysis of the three body recombination rates, the geometric sizes and the fact that the He clusters are liquid independent of their size can explain the larger cross sections found for He.

Kinetics of heterogeneous methanolysis of sunflower oil with CaO∙ZnO catalyst: Influence of different hydrodynamic conditions

The kinetics of heterogeneous methanolysis of sunflower oil was studied at 60?C using mechanochemically synthesized CaO?ZnO as catalyst. Influence of agitation speed, catalyst amount and methanol to oil molar ratio on the rate of reaction was analyzed. The rate of the process depends on the two resistances - mass transfer of triglycerides to the catalyst surface and chemical reaction on the catalyst surface, which are defined as the values of the overall triglyceride volumetric mass transfer coefficient, kmt,TG, and the effective pseudo first-order reaction rate constant, k, respectively. These kinetic parameters actually determine the value of the apparent reaction rate constant, kapp, whose change with time is defined with the change of triglyceride (TG) conversion. The kinetic model was proposed and the model parameters determined.

Kinetic measurements of the C2H5O2 radical using time-resolved cavity ring-down spectroscopy with a continuous source

We report on the design of a time-resolved, high duty-factor cavity ring-down apparatus utilizing a continuous laser and detail a technique for the accurate and precise measurement of effective reaction rate constants with it. This report complements an earlier paper concerning the measurement of the absolute absorption cross-sections, σP, of reactive intermediates. To demonstrate the performance of the new technique, we have measured the decay rate of ethyl peroxy radicals by monitoring the Ã←X̃ origin band of the G-conformer of these species. A measured value kobs∕σP = 1.827(45) × 10(7) cm/s was determined and it, along with the previously measured value of σP, was used to derive the value of kobs = 9.66(44)×10(-14) cm(3)/s, for the effective rate constant for ethyl peroxy self-reaction (all uncertainties are 1 σ). The present value of kobs is compared to those previously reported, and sources of systematic errors and their impact are discussed.

Kinetics of the metal-ligand exchange of cadmium rhodo- and pyrroporphyrins with cobalt and zinc chlorides in organic solvents

Experimental data on electron absorption spectra (EASs) and the kinetics of substitution of Co2+ for the central Cd2+ ion in rhodoporphyrin complexes (CdRodP) in the reaction with CoCl2 in acetonitrile (AN) and ZnCl2 in dimethyl sulfoxide (DMSO) and the substitution of Zn2+ for Cd2+ in pyrroporphyrin complexes (CdPyrP) in the reaction with ZnCl2 in DMSO are reported and discussed. The evolution of EASs in the reaction of metal-ligand exchange and the effective and true rate constants of the exchange reaction are reported. The activation energies and activation entropies are estimated.

Effect of pH on the reactivity of dipeptides and α-amino acids in the N-acylation

The reaction kinetics of Gly, L-α-Ala, Gly-Gly, L-α-Ala-L-α-Ala and β-Ala-β-Ala with picryl benzoate in water (40 wt %)-2-propanol was investigated. At pH = 4–8 the rate constants of N-acylation of the anionic form of dipeptides are less than those of the corresponding amino acid anions, in agreement with their basicity, whereas the relative effective rate constants of reactions depend on pH: in acidic, neutral and slightly alkaline media the kef values are higher for the dipeptides, and in a strongly alkaline medium, for the amino acids. These differences are due to the changes in the concentrations of reactive forms of amino acids and dipeptides in the system at varying the medium pH.

Estimation of Effective Moisture Diffusivity in Starchy Materials Following Hydrothermal Treatments

Two hydrothermal treatment processes (DV-HMT and DIC treatment) were investigated on standard maize starch for three processing temperatures; 100, 110 and 120°C. The gravimetric change of starch powder during the treatment was analyzed by a simultaneous water diffusion and starch reaction model. The effective diffusivity coefficient (Deff) and reaction rate constant (k) were estimated by minimizing the error between experimental and analytical results. The values of Deff and k clearly increased with temperature. The degree of starch melting was evaluated for the two treatments using the first-order reaction model as a function of processing time. The results suggest that the absorption process is controlled by water–starch reactivity that induces melting phenomenon of starch crystallites, which progresses when temperature increases. The two hydrothermal treatments considerably differ: DIC being more prone to water absorption as demonstrated by the values of Deff and k.

Kinetics and mechanisms of the catalytic reactions of formaldehyde with copper oxides and a copper ion complex in aqueous alkali

The kinetics of the autocatalytic reactions of formaldehyde with copper(II) and copper(I) oxides and with the Cu2+ ion of the copper EDTA complex, as well as formaldehyde disproportionation in the presence of copper metal, have been investigated in aqueous solutions of sodium hydroxide. Two likely reaction mechanisms are presented. The difference between these mechanisms does not alter the observed kinetics of the processes, whose rate is determined by their first, slow step, namely, the oxidation of the methylene glycol anion adsorbed on the copper surface into formic acid. In the slow step of the first mechanism, a hydride ion is abstracted from the methylene glycol anion and is transferred to copper. In the slow step of the second mechanism, the methylene glycol anion undergoes anodic oxidation, releasing a hydrogen atom and an electron. In the rapid steps of the first mechanism, the hydride ion undergoes anodic oxidation to hydrogen, the copper compound undergoes cathodic reduction to copper metal, and, simultaneously, the electron and hydrogen are transferred to a nonionized formaldehyde molecule to yield methanol. Mathematical models are suggested for the reactions. The effective rate constants and activation energies of the slow steps of the reactions have been determined. The effective rate constants of the noncatalytic reduction reactions of the copper compounds and the ratios of the rates of the rapid hydrogen and methanol formation reactions have been estimated.

A Kinetic Model of the Monocarboxylate Transporter MCT1 and its Interaction with Carbonic Anhydrase II

The enzyme carbonic anhydrase isoform II (CAII), catalyzing the hydration and dehy-dration of CO2, enhances transport activity of the monocarboxylate transporter isoform I (MCT1, SLC16A1) expressed in Xenopus oocytes by a mechanism that does not require CAII catalytic activity. In the present study, we have investigated the mechanism of the CAII induced increase in transport activity by using electrophysiological techniques and mathematical modeling of the MCT1 transport cycle. The model consists of six states arranged in cyclic fashion and features an ordered, mirrorsymmetric, binding mechanism, where binding and unbinding of the proton to the transport protein is considered to be the rate limiting step under physiological conditions. An explicit rate expression for the substrate flux is derived using model reduction techniques. By treating the pools of intra-and extracellular MCT1 substrates as dynamic states, the time dependent kinetics are obtained by integration, using the derived expression for the substrate flux. The simulations were compared with experimental data obtained from MCT1-expressing oocytes injected with different amounts of CAII. The model suggests that CAII increases the effective rate constants of the proton reactions, possibly by working as a proton antenna.

Extracellular diffusion and permeability effects on NO–RBCs interactions using an experimental and theoretical model

Nitric oxide (NO) is a potent vasodilator and its homeostasis depends on interaction with RBCs. A key factor in understanding NO–RBC interactions in vascular lumen is a comprehensive analysis of product identification and quantification. In this context, administration of NO during in vitro NO–RBC interactions becomes a crucial variable. In this study, we designed a bioreactor that maintains a precise NO concentration in the headspace that diffuses to RBCs suspension to study the quantitative effect of NO concentration and hematocrit (Hct) on NO–RBC interactions. The products of NO–RBC reaction (nitrite and total nitrogen species (total NOx)) were measured by chemiluminescence assay. A mathematical model simulating NO biotransport to a single RBC was developed to (1) estimate NO–RBC reaction rate constant; (2) predict the NO concentrations in the bulk RBC suspension and at the RBC membrane for RBC membrane NO permeability ( P m) values of 0.0415–40 cm/s. Measured nitrite and total NOx concentrations increased with increase in headspace NO concentration whereas nitrite concentrations decreased with hematocrit and total NOx concentrations increased with hematocrit. This indicates that the extracellular resistance is a controlling factor for RBC uptake of NO. Modeling results showed that the effective reaction rate constant ( k eff) for NO–RBC interactions was 2.32 × 10 4–1.08 × 10 6 M − 1 s − 1 . Results also predict that the membrane permeability in the range of 0.0415–0.4 cm/s is required to maintain physiologically relevant levels of NO at the smooth muscle cell layer. The effective reaction rate constant increased with increase in P m and magnitude of increase was higher at 45% Hct. For all P m values, the k hb/ k eff ratios were lower for 45% Hct as compared to 5% Hct indicating extracellular resistance is important for RBC NO uptake. Our experimental and mathematical analyses of NO–RBC interactions indicate that both unstirred layer and RBC membrane have a significant effect on NO transport to RBCs. In addition, the membrane permeability in the range of 0.0415–0.4 cm/s is required to maintain sufficient NO concentrations at the smooth muscle cell layer.

REACTION ROUTES IN THE CO-H{sub 2}CO-d{sub n} -CH{sub 3}OH-d{sub m} SYSTEM CLARIFIED FROM H(D) EXPOSURE OF SOLID FORMALDEHYDE AT LOW TEMPERATURES

Grain surface reaction has been expected to be a key process for deuterium enrichment in interstellar molecules. We focus on formaldehyde, which is predicted to be formed on cold grain surface in astrophysical models and is known to be deuterium-enriched in a molecular cloud. Reaction routes and effective reaction rate constants are experimentally investigated when H{sub 2}CO and D{sub 2}CO are exposed to D and H atoms on amorphous solid water (ASW) at 10-20 K, respectively. For D + H{sub 2}CO on ASW, H{sub 2}CO was converted to HDCO and D{sub 2}CO by the H-D substitution reactions. Although CD{sub 3}OD was slightly observed, doubly and triply deuterated methanol, CH{sub 2}DOD and CHD{sub 2}OD, were not observed. This implies that D addition to H{sub 2}CO (formation of deuterated methanol) is a minor reaction route. On the other hand, for H + D{sub 2}CO, H addition reactions to form CHD{sub 2}OH proceed at a significant rate. Simultaneously, a competitive reaction, the substitution reaction by H atoms and subsequent H addition (D{sub 2}CO {yields} HDCO {yields} H{sub 2}CO {yields} CH{sub 3}OH) also proceed at a significant rate. However, no H addition to HDCO was observed. The effective surface reaction routes when CO ismore » exposed to H and D atoms are summarized using the present experimental results and the previous results of our group.« less

REACTION ROUTES IN THE CO-H2CO-dn-CH3OH-dmSYSTEM CLARIFIED FROM H(D) EXPOSURE OF SOLID FORMALDEHYDE AT LOW TEMPERATURES

Grain surface reaction has been expected to be a key process for deuterium enrichment in interstellar molecules. We focus on formaldehyde, which is predicted to be formed on cold grain surface in astrophysical models and is known to be deuterium-enriched in a molecular cloud. Reaction routes and effective reaction rate constants are experimentally investigated when H2CO and D2CO are exposed to D and H atoms on amorphous solid water (ASW) at 10–20 K, respectively. For D + H2CO on ASW, H2CO was converted to HDCO and D2CO by the H–D substitution reactions. Although CD3OD was slightly observed, doubly and triply deuterated methanol, CH2DOD and CHD2OD, were not observed. This implies that D addition to H2CO (formation of deuterated methanol) is a minor reaction route. On the other hand, for H + D2CO, H addition reactions to form CHD2OH proceed at a significant rate. Simultaneously, a competitive reaction, the substitution reaction by H atoms and subsequent H addition (D2CO → HDCO → H2CO → CH3OH) also proceed at a significant rate. However, no H addition to HDCO was observed. The effective surface reaction routes when CO is exposed to H and D atoms are summarized using the present experimental results and the previous results of our group.

Dependence of the effective rate constants for the hydrogenation of CO on the temperature and composition of the surface

Experiments on the hydrogenation of CO on pure CO and CO–H 2O mixed ice have been performed at temperatures between 8 and 20 K. We obtained temperature and compositional dependence of the effective reaction rate constants. Results indicate that hydrogenation proceeds efficiently on pure solid CO and CO–H 2O mixed ice at temperatures below 10 and 20 K, respectively. Rate constants for pure CO decreased significantly at 12 K compared to those obtained with CO–H 2O mixed ice. Hydrogenation of CO at temperatures greater than 12 K were catalyzed by the H 2O adjacent to the CO. The importance of the experimental results for some relevant astrophysical environments has also been outlined.

Influence of poly(acrylic acid) xerogel structure on swelling kinetics in distilled water

The basic structural properties of xerogels of crosslinked poly(acrylic acid) were defined and determined: xerogel density (ρxg), xerogel volume fraction in the equilibrium-swollen state (v2), the number average molar mass between network crosslinks $(\bar{M}_{c})$ , the crosslink density (ρc) and the distance between macromolecular chains (d). A crosslinking ratio (X) increase leads to a linear increase in the values for ρxg and ρc, while the values $\bar{M}_{c}$ and d decrease. The isothermal swelling kinetic curves of four samples of structurally different poly(acrylic acid) xerogels in bidistilled water at different temperatures ranging from 25 to 45 °C were determined. It is shown that isothermal kinetic swelling curves could not be described with the model of first-order reaction kinetics in entire. It was found that these curves could be described by the Johanson-Mampel-Avrami (JMA) equation. For all of the investigated xerogel samples, the initial swelling rate (vin), effective reaction rate constant (k) and equilibrium swelling degree increased with swelling temperature increase. Based on the determined values of the vin and k, the activation energy (Ea) and pre-exponential factor (lnA) were determined. It was concluded that the activation energy linearly increased with increasing distance between macromolecular chains (d) and molar mass between the network crosslinks $(\bar{M}_{c})$ . The relationship between the activation energy changes with pre-exponential factor (compensation effect) caused by xerogel structural properties was established. Isothermal swelling kinetics could be completely described by the kinetics of phase transition of the xerogel transformation from glassy to rubbery state, i.e. with the JMA kinetic equation.

Spatial modeling of dimerization reaction dynamics in the plasma membrane: Monte Carlo vs. continuum differential equations

Bimolecular reactions in the plasma membrane, such as receptor dimerization, are a key signaling step for many signaling systems. For receptors to dimerize, they must first diffuse until a collision happens, upon which a dimerization reaction may occur. Therefore, study of the dynamics of cell signaling on the membrane may require the use of a spatial modeling framework. Despite the availability of spatial simulation methods, e.g., stochastic spatial Monte Carlo (MC) simulation and partial differential equation (PDE) based approaches, many biological models invoke well-mixed assumptions without completely evaluating the importance of spatial organization. Whether one is to utilize a spatial or non-spatial simulation framework is therefore an important decision. In order to evaluate the importance of spatial effects a priori, i.e., without performing simulations, we have assessed the applicability of a dimensionless number, known as second Damköhler number ( Da), defined here as the ratio of time scales of collision and reaction, for 2-dimensional bimolecular reactions. Our study shows that dimerization reactions in the plasma membrane with Da ∼> 0.1 (tested in the receptor density range of 10 2–10 5/μm 2) require spatial modeling. We also evaluated the effective reaction rate constants of MC and simple deterministic PDEs. Our simulations show that the effective reaction rate constant decreases with time due to time dependent changes in the spatial distribution of receptors. As a result, the effective reaction rate constant of simple PDEs can differ from that of MC by up to two orders of magnitude. Furthermore, we show that the fluctuations in the number of copies of signaling proteins (noise) may also depend on the diffusion properties of the system. Finally, we used the spatial MC model to explore the effect of plasma membrane heterogeneities, such as receptor localization and reduced diffusivity, on the dimerization rate. Interestingly, our simulations show that localization of epidermal growth factor receptor (EGFR) can cause the diffusion limited dimerization rate to be up to two orders of magnitude higher at higher average receptor densities reported for cancer cells, as compared to a normal cell.

The Kinetics of Plasma-Induced Degradation of Organic Pollutants in Sewage Water

The processes of treatment of actual surface water and model solutions in a low-temperature dielectric-barrier discharge (DBD) plasma were studied. The general evaluation of the efficiency of the plasma treatment procedure for sewage purification was made, and a conclusion on the reasonability of its use in combination with conventional treatment methods was drawn. Schemes of conversion processes for organic compounds upon DBD treatment of surface water were proposed, and the effective rate constants of corresponding reactions were estimated (for phenol, k = 0.46 × 10–5 s–1). It was found that the reaction of oxidation of pollutants in solutions modeling surface runoff occurs in the diffusion region.