Autocatalytic Kinetics Research Articles

Acidic permanganate oxidation of sulfamethoxazole by stepwise electron-proton transfer

Permanganate is a versatile chemical oxidant, and has undergone a dramatic evolution toward deep insight into its reaction mechanism. However, the hydrogen abstraction of the NH bond by permanganate remains unclear. We studied the permanganate oxidation of the emerging micropollutant sulfamethoxazole in acidic aqueous solution. The reaction followed autocatalytic kinetics and demonstrated first-order with respect to each reactant. The presence of HMnO4 accelerated the reaction rate, which was four orders of magnitude higher than that of MnO4−. Based on the identified products, the rate-limiting step was determined to be simple NH bond oxidation by metal-oxo species permanganate. The mechanism was then studied computationally by density functional theory (DFT) using ammonia as the simplest model. Results showed that the NH bond oxidation by MnO4− (32.86 kcal/mol) was a concerted mechanism similar to that of CH bond oxidation, whereas HMnO4 oxidation of the NH bond (10.44 kcal/mol) was a stepwise electron-proton transfer. This reminds us that coordination of Brønsted acid could not only produce the stronger electrophile but also change the reaction mode by avoiding the bond cleavage in electron transfer process.

Exploiting limited valence patchy particles to understand autocatalytic kinetics

Autocatalysis, i.e., the speeding up of a reaction through the very same molecule which is produced, is common in chemistry, biophysics, and material science. Rate-equation-based approaches are often used to model the time dependence of products, but the key physical mechanisms behind the reaction cannot be properly recognized. Here, we develop a patchy particle model inspired by a bicomponent reactive mixture and endowed with adjustable autocatalytic ability. Such a coarse-grained model captures all general features of an autocatalytic aggregation process that takes place under controlled and realistic conditions, including crowded environments. Simulation reveals that a full understanding of the kinetics involves an unexpected effect that eludes the chemistry of the reaction, and which is crucially related to the presence of an activation barrier. The resulting analytical description can be exported to real systems, as confirmed by experimental data on epoxy–amine polymerizations, solving a long-standing issue in their mechanistic description.

Homogeneous-heterogeneous reactions in flow of nanofluids near the stagnation region of a plane surface: The Buongiorno’s model

The homogeneous-heterogeneous reactions for nanofluids flow are developed incorporating the Buongiorno’s nanofluid model, in which the heterogeneous reactions given by isothermal cubic autocatalytic kinetics are assumed to take place on the wall surface, while the homogeneous reactions governed by first order kinetics occur in the ambient fluid. The classical problem of laminar flow in the stagnation region of a plane surface is illustrated as an example to testify the validity of this model. Multiple solutions of this state system are obtained for various physical parameters by means of hysteresis bifurcations. Results show that the ratio of Brownian and thermophoretic diffusivity plays an important role on the appearance of the bifurcation point of multiple solutions, the less this value is, the slower this bifurcation point appears. In other words, nanofluids are expected to alter the transport mechanisms of homogeneous-heterogeneous reactions and therefore have great potentials in applications of the related fields.

Behaviors of coking and stable radicals of a heavy oil during thermal reaction in sealed capillaries

This work studies thermal reaction of a heavy oil at temperatures of 250–500°C in sealed capillaries. The samples are extracted by chlorobenzene and toluene, and the insoluble matters are quantified in mass and in radical concentration. It is found that the chlorobenzene insolubles (CI) forms at temperatures of and higher than 440°C while the toluene insolubles (TI) forms at temperatures higher than 350°C. Both of these insoluble matters increase with time and temperature, and approach to similar asymptotic values. The CI formation can be fitted with a combination of the second-order and autocatalytic kinetics while the TI formation can be fitted with the second-order kinetics. Both CI and TI contain stable radicals measurable by electron spin resonance (ESR), and the radical concentration in CI is higher than that in TI. The kinetics of stable radical concentration in CI and TI are different from that in mass, except that of TI at a low temperature range (350–440°C). The changes in ESR line width of CI and TI are also studied. The above data are also discussed in terms of hard coke (CI insoluble matters) and soft coke (the difference between TI and CI), and in activation energy.

First, second and nth order autocatalytic kinetics in continuous and discontinuous reactors

Autocatalytic phenomena in chemical kinetics originate from various reasons, such as acid-catalytic effects of reaction products or change of the physical properties of the reaction mixture. The impact of autocatalytic kinetics on the chemical reactor design is crucially important leading to deviations from standard design rules. The impact of autocatalytic effects on first, second and nth order kinetics was investigated for batch, plug flow, backmixed, axial dispersion and recycled plug flow reactors was analyzed by classical approach and numerical simulations. Efficient numerical strategies were developed for the different reactor models. The results showed how an optimal degree of backmixing (Péclet number) and optimal recycle ratio can be determined for tubular reactors and how the reactor volume can be minimized for specific cases. Generic examples as well as hydrolysis of alkyl formate were considered as case studies.

Quasi‐isothermal DSC testing of epoxy adhesives using initial fast heating rates

ABSTRACTThree major factors decrease the accuracy of the cure measurement in standard‐isothermal testing using differential scanning calorimetry (DSC). First, cure occurs during the heating step. Second, data are lost during the stabilization period between the dynamic and isothermal step. Third, the baseline selection requires a modification to the protocol. An alternative, which is explored in this study, is the use of fast ramps, which decrease the heating time, but this has been avoided due to overshoot that occurs between the dynamic and isothermal step, which is troublesome for systems with autocatalytic kinetics. By mitigating these factors, a quasi‐isothermal protocol was developed. Therefore, more complete cure kinetics were captured with the implementation of fast DSC to decrease the ramp time and through the optimization of furnace parameters to decrease stabilization time and temperature overshoot. The data suggested this quasi‐isothermal analysis more accurately measured the isothermal curing kinetics of a commercial epoxy adhesive at 110, 115, and 120 °C for fast ramps of 175, 350, and 500 K/min compared to the traditional ramp of 5 K/min. The enthalpy spike at the dynamic to isothermal transition remains an issue; however, an empirical shift can be used to compensate for the enthalpy signal lag. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45425.

Unsteady flow of carbon nanotubes with chemical reaction and Cattaneo-Christov heat flux model

Present analysis examines boundary layer flow of carbon nanotubes over a curved stretching surface. Instead of classical Fourier law we employed Cattaneo-Christov heat flux theory. The heterogeneous reaction taking place on the wall surface are given by isothermal cubic autocatalytic kinetics. The homogeneous reaction occurring in the ambient fluid are governed by first order kinetics. Appropriate transformations are employed to obtain system of nonlinear ordinary differential equations. Convergent series solution are obtained. Single and multi wall carbon nanotubes are used. Water is taken as a base fluid. Fluid flow, temperature, concentration, skin friction coefficient and Nusselt number are examined and analyzed for different involved parameters.

The effect of graphene oxide on UV curing kinetics and properties of SU8 nanocomposites

AbstractIn the present study SU8 nanocomposites were prepared by incorporating graphene oxide (GO), and its effect on the UV curing kinetics, morphology, electrical, hardness and thermal properties of the nanocomposites were investigated at different loading levels of GO (0.1 − 3 wt%). Studying the reaction kinetics of the UV curing process by means of real‐time infrared spectroscopy showed that the polymerization rate and the final conversion of epoxy groups was related to the loading level of GO in the nanocomposites. An autocatalytic kinetics model of the curing reaction confirmed the effect of GO nanoparticles on the curing rate constant (k), the order of the initiation reaction (m) and the ultimate conversion of the UV‐cured nanocomposites. Appropriate experimental observations indicated that dispersion of GO within the resin plays a critical role on the cure kinetics and final conversion. The results of the kinetics modeling and morphological observations showed that the curing rate constant of the nanocomposites is highly dependent on the GO content and its dispersion state, indicating that GO prevents epoxy resin crosslinking by photoinitator deactivation. Moreover, oxygen functionalities, such as hydroxyl and carboxyl groups, on the surface of GO facilitate interfacial interactions between epoxy groups from the matrix and GO. Electrical conductivity measurements demonstrated that the UV‐induced photo‐cured GO filled resins are conductive SU8 nanocomposites. It was observed that the thermal stability of the nanocomposites is enhanced due to the dispersion of GO in the matrix. Moreover, the microhardness analysis showed that addition of GO to neat SU8 increases the mechanical hardness of the nanocomposite. © 2016 Society of Chemical Industry

Coke formation during thermal reaction of tar from pyrolysis of a subbituminous coal

The reaction of volatiles in a coal pyrolysis reactor determines the yield and quality of tar. This paper studies the yield, group composition and radical concentration of tars obtained from pyrolysis of a subbituminous coal, mainly on the effect of the tars' residence time in the pyrolysis reactor and in post-pyrolysis heating. The latter operation simulates the environment of tars, if they were in a large fast pyrolysis reactor. It is found that an increase in the volatiles' residence time in the pyrolysis reactor reduces the yield and increases the radical concentration of tars. The pitch fraction of the tars is mainly responsible for coke formation which occurs significantly at temperatures higher than 420°C. The tars contain radicals, mainly in their coke and pitch fractions. The coke concentration in the tars increases with increasing heating time, linearly at 420 and 450°C but in a sigmoidal pattern at higher temperatures. The coking behavior within 10min can be expressed by the zero-order or zero-order+autocatalytic kinetics with activation energies of 128–138kJ/mol.

Modeling and Simulation of the Autocatalytic Kinetics of Haemoglobin SS Polymerization: Onset of Polymerization

We report a fresh and simpler approach to the modelling of the kinetics of the polymerization of Hb SS in sickle cell patients that couples the kinetics and the hydrodynamics of blood flow in mechanistic understanding of the process. The well-known two-step autocatalytic reaction scheme was used for the polymerization reaction with the assumption of simpler first-order reaction scheme for each stage. In addition, the forces acting on a particle in motion were also introduced to account for compelling settling of the red cells that lead to vessel occlusion (vaso-occlusion). A first attempt on the prediction of vessel blockage was made using this novel model. The time for the onset of the polymerization reaction was derived from hydrodynamic considerations and kinetics while the kinetic rate constants were obtained from the autocatalytic nature of the reaction. Experimental data for model validation were obtained from recruited SS patients and in vitro data of Hofrichter. Over 100 volunteers were recruited for participation in this work but less than 40% met the inclusion criteria. Participants were of age range 13 - 43 (with a mean of 26 ± 8 years) for SCD patients and 18 - 43 (with a mean of 28 ± 7 years) for control participants. Blood indices and Transcranial Doppler (TCD) test parameters of all participants were the principal parameters used for model validation. Constant k2/k1 ratios was obtained for individual in vivo/in vitro system. This ratio is unique for any individual, independent on protein sequence and also suggests the degree of expression of the symptoms of Sickle Cell Disease (SCD) with higher values reflecting greater propensity to pain crisis. Delay time, tD, was found to have an inverse relationship with the kinetic constant for the residual reaction, k1. Therefore, long delay times calculated, offer insight on why SCD patients are not in perpetual crises because enough time is provided the cells to escape microcirculation while keeping the residual reaction at the minimum. Sensitivity analysis was carried out to obviate the limitations encountered in the course of the work. Results showed the onset of occlusion to be most sensitive to the diameter of the blood vessel.

A solid control strategy for preparation of silver nanoparticles in aqueous medium

Uniformly distributed, spherically shaped, mild concentrated silver nanoparticles with single-digit to hundreds nm size have been prepared by reducing silver nitrate with popular reducers like sodium borohydride or hydrazine in the presence of ordinary stabilizers such as SDS, PVP, Polysorbates and ultrasonication. Uv-visble spectroscopic analysis, particle size analysis, and particle-imaging through transmission electron microscopy (TEM) were used for nanoparticle characterization. Higher temperature accelerated the reduction rates, which follows the typical autocatalytic kinetics. Particularly, ultrasonication helped to facilitate the crucial stage of reduction phase to result in excellent quality of nanosolution, such as narrow distribution of particles and size uniformity. We found that initial location or arrangement of silver ions in clouds of stabilizers and ‘effectve mixing’ in the stage of reduction were vital for successful preparation of silver nanosolution. Also, reagent/stabilizer ratio, reducer input, solution environment such as pH, temperature, and stabilizer properties were optimized and discussed in detail. Proper selection of stabilizer and molar ratio to reagent and effective ‘mixing’ for preventing grain growth needs to be investigated more in the future work.

Thermogravimetric Monitoring of Crude Oil and Its Cuts in an Oil Refinery

The thermal degradation profile for any type of oil-based sample under thermogravimetric analysis (TGA) conditions exhibits three distinct stages, namely, vaporization from room temperature to 340–350 °C, cracking from 340–350 °C to 480–500 °C, and char oxidation from 500 °C to 570 °C. The former two stages occur in both inert (nitrogen) and oxidative (air) environments, whereas the latter only occurs in the presence of oxygen. Deconvoluting thermogravimetric data allows one to estimate the composition of oil derivatives with a view to expeditiously obtaining useful information from a refining process. To this end, thermal degradation of crude oil and its main refining cuts were modeled here by using the smallest possible number of representative pseudo-components. In order to ensure accurate fitting of thermogravimetric results, mass losses were interpreted in terms of autocatalytic kinetics. Fitting to an nth-order kinetics was useful below 350 °C (vaporization), but not above this temperature, because of cracking with fast mass losses in the vicinity of certain temperatures. This modeling scheme for thermogravimetric results afforded the following conclusions: (i) atmospheric gas-oil typically contains 10% residual kerosene fraction; (ii) atmospheric residue still contains 35–45% distillable compounds; (iii) the main component of visbreaking feed (nearly 66%) degrades at a similar temperature as asphaltenes; (iv) visbreaking residue is similar to feed at high temperatures but contains light components similar to naphtha or gas-oil, which vaporize at low temperatures; and (v) simulating crude oil allowed us to estimate the potential production of distillates (62.93% from heavy crudes and 79.61% from light crudes). Such useful information can be used by process engineers to assess the performance of equipment such as distillation columns or visbreaking units, and also to estimate the quality of some streams, such as atmospheric gas-oil or visbreaking feed.

Effect of Constant‐Rate Reduction on the Performance of a Ternary Cu/ZnO/Al2O3 Catalyst in Methanol Synthesis

AbstractA multi‐functional flow set‐up was developed for the rate‐ and temperature‐controlled reduction of copper catalysts, their application in high‐pressure methanol synthesis and the determination of the copper surface area by N2O frontal chromatography. The influence of constant‐rate reduction on the catalytic properties of a ternary Cu/ZnO/Al2O3 catalyst was investigated. The temperature during the constant‐rate reduction was found to decrease, indicating autocatalytic kinetics, but no significant catalytic effect of the milder reduction conditions was observed compared with a slow linear heating ramp.

Involvement of ferryl in the reaction between nitrite and the oxy forms of globins.

The reaction between nitrite and the oxy forms of globins has complex autocatalytic kinetics with several branching steps and evolves through chain reactions mediated by reactive species (including radicals) such as hydrogen peroxide, ferryl and nitrogen dioxide, starting with a lag phase, after which it proceeds onto an autocatalytic phase. Reported here are UV-Vis spectra collected upon stopped-flow mixing of myoglobin with a supraphysiological excess of nitrite. The best fit to the experimental data follows an A→B→C reaction scheme involving the formation of a short-lived intermediate identified as ferryl. This is consistent with a mechanism where nitrite binds to oxy myoglobin to generate an undetectable ferrous-peroxynitrate intermediate, whose decay leads to nitrate and ferryl. The ferryl is then reduced to met by the excess nitrite. DFT calculations reveal an essentially barrierless reaction between nitrite and the oxy heme, with a notable outer-sphere component; the resulting metastable ferrous-peroxynitrate adduct is found to feature a very low barrier towards nitrate liberation, with ferryl as a final product-in good agreement with experiment.

Oxyhalogen–Sulfur Chemistry: Kinetics and Mechanism of Oxidation of Chemoprotectant, Sodium 2-Mercaptoethanesulfonate, MESNA, by Acidic Bromate and Aqueous Bromine

The oxidation of a well-known chemoprotectant in anticancer therapies, sodium 2-mercaptoethanesulfonate, MESNA, by acidic bromate and aqueous bromine was studied in acidic medium. Stoichiometry of the reaction is: BrO3(-) + HSCH2CH2SO3H → Br(-) + HO3SCH2CH2SO3H. In excess bromate conditions the stoichiometry was deduced to be: 6BrO3(-) + 5HSCH2CH2SO3H + 6H(+) → 3Br2 + 5HO3SCH2CH2SO3H + 3H2O. The direct reaction of bromine and MESNA gave a stoichiometric ratio of 3:1: 3Br2 + HSCH2CH2SO3H + 3H2O → HO3SCH2CH2SO3H + 6Br(-) + 6H(+). This direct reaction is very fast; within limits of the mixing time of the stopped-flow spectrophotometer and with a bimolecular rate constant of 1.95 ± 0.05 × 10(4) M(-1) s(-1). Despite the strong oxidizing agents utilized, there is no cleavage of the C-S bond and no sulfate production was detected. The ESI-MS data show that the reaction proceeds via a predominantly nonradical pathway of three consecutive 2-electron transfers on the sulfur center to obtain the product 1,2-ethanedisulfonic acid, a well-known medium for the delivery of psychotic drugs. Thiyl radicals were detected but the absence of autocatalytic kinetics indicated that the radical pathway was a minor oxidation route. ESI-MS data showed that the S-oxide, contrary to known behavior of organosulfur compounds, is much more stable than the sulfinic acid. In conditions where the oxidizing equivalents are limited to a 4-electron transfer to only the sulfinic acid, the products obtained are a mixture of the S-oxide and the sulfonic acid with negligible amounts of the sulfinic acid. It appears the S-oxide is the preferred conformation over the sulfenic acid since no sulfenic acids have ever been stabilized without bulky substituent groups. The overall reaction scheme could be described and modeled by a minimal network of 18 reactions in which the major oxidants are HOBr and Br2(aq).

Synchronization of chemical noise-sustained structures in asymmetrically coupled differential-flow reactors

The differential-flow-induced chemical instability is investigated in the context of two coupled reactors with cubic autocatalytic kinetics (the Gray-Scott model). Previous results for master-slave arrangement [Izús, Deza, and Sánchez, J. Chem. Phys. 132, 234112 (2010)] are extended in this study to include bidirectional coupling between reactions. Numerical simulations in the convectively unstable regime show that synchronized noise-sustained structures are developed in both reactors due to the selective amplification of noise. A theoretical analysis shows that the nature of the synchronization and the stability of the synchronized manifold are related with the properties of the critical modes.

Thermogravimetric monitoring of oil refinery sludge

The present work has two dimensions: analytical and environmental. On the one hand we proved that thermogravimetric analysis can be used to perform fast characterization of oil refinery sludge. To this end, thermogravimetric curves were deconvoluted by using autocatalytic kinetics to take into account acceleratory phases in a thermal degradation performed in oxygen-containing atmosphere or at high heating rates. Based on thermogravimetric results, oil refinery sludge was modeled in terms of various fractions (pseudo-components) which degrade as major oil cuts. On the other hand, as an alternative to landfill, we have seen that Soxhlet extraction allows recovery almost half of the weight of sludge as a mixture of hydrocarbons, similar to gas–oil, which burns without residue. This ensures both, waste inerting and significant reduction in sludge volume.

Photocure kinetics and control of LC microdroplets for acrylate-based polymer dispersed liquid crystal with different methacrylate contents

This paper reports on the relationship between the photocure kinetics and the morphological properties of UV-curable acrylate-based polymer dispersed liquid crystal (PDLC) systems with the various 2-ethylhexylacrylate (2-EHA) and 2-ethylhexylmethacrylate (2-EHMA) contents using photo-differential scanning calorimetry (photo-DSC), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). Photo-DSC analysis using an autocatalytic kinetics model demonstrated that the cure extent and cure rate gradually decreased and then leveled off as the concentration of 2-EHMA increased up to 25 wt%, which was well confirmed by using FTIR spectroscopy. The morphological observations revealed that increasing the 2-EHMA content increased the size of the LC droplets within the acrylate-based polymer matrix. Notably, there was a marked increase in the number of LC droplets larger than 10 μm in PDLC composites containing more than 20 wt% 2-EHMA, suggesting that the addition of more than 20 wt% 2-EHMA to the acrylate-based PDLC compounds is not desirable. The presence of 2-EHMA clearly affected the photocure behaviors of acrylate-based PDLC compounds, which interestingly means that the slow reactivity of methacrylate monomer could be simply used to control the phase separation rate and the microstructures of LC droplets of acrylate-based PDLC systems. Open image in new window

Acid hydrolysis of O-acetyl-galactoglucomannan

Acid hydrolysis of O-acetyl-galactoglucomannan (GGM), the main non-cellulosic polysaccharide in coniferous wood, was studied at 90 °C in the pH range of 0.5–2.0 using different acids (hydrochloric acid, sulfuric acid, trifluoroacetic acid, oxalic acid, and formic acid) as homogeneous catalysts and cation exchange resins (Smopex 101 and Amberlyst 15) as heterogeneous catalysts. The hydrolysis rate was exclusively dependent on the pH of the solution, but independent of the concentration and type of acid. Analysis of the formation rates of oligomers with DP 2–5 revealed that GGM hydrolysis occurs rather randomly. The final products were mannose, galactose, and glucose. In the beginning of the reaction, the formation of galactose was slightly faster than those of other sugars, since galactose is a side substituent to the glucose–mannose main chain of GGM. However, with longer reaction times, the formation rates of sugar monomers were correlated with their initial abundances in the polysaccharide. Autocatalytic kinetics was observed in the presence of the heterogeneous catalysts: an induction time was observed, after which the reaction proceeded with the same rates as with homogeneous acids. The autocatalytic effect is probably explained by the increase of the hydrolysis rate constant, as the GGM chain length decreases and the molecule becomes more accessible for the active sites on the outer surface of the catalyst.

Heterogeneous reaction kinetics of epoxide-functionalized regenerated cellulose membrane and aliphatic amine

The reaction kinetics of an epoxide-functionalized regenerated cellulose membrane with excess aliphatic amine are examined using differential scanning calorimetry. Both isothermal and dynamic experiments are analyzed. The reaction is found to follow first-order autocatalytic kinetics. The activation energy based on five different methods ranges from 77.0 to 79.7 kJ/mol, indicating that the apparent activation energy for the heterogeneous epoxy/aliphatic amine reaction is considerably higher than the values reported in the literature for the homogeneous reaction.