Autocatalytic Kinetics Research Articles

Urease stabilization in urea–urease–H+ system and its influence on the clock reaction dynamics

AbstractThe inherent autocatalytic kinetics of the urea–urease–H+ system positions it as a promising candidate for the design of dynamic materials with time‐domain programmable functions. Nevertheless, the stability of the enzyme can markedly influence the temporal evolution dynamics of the system and curtail its widespread applicability. This work employs several kinds of enzyme stabilization methods, including chemical cross‐linking, physical coating, solvent stabilization, and solvent‐physical coating co‐modification, to systematically explore the impact of enzyme stabilization on clock reaction dynamics. Extensive experimental tests and analysis indicate that solvent and chemical cross‐linking stabilization methods can better preserve clock dynamics with sensitive switching ability. Nevertheless, due to significant pH changes in the reacting system, the reusability of the enzyme is better retained in the physical coating and solvent‐physical coating co‐modification methods.

Redox Reactivity of Nonsymbiotic Phytoglobins towards Nitrite.

Nonsymbiotic phytoglobins (nsHbs) are a diverse superfamily of hemoproteins grouped into three different classes (1, 2, and 3) based on their sequences. Class 1 Hb are expressed under hypoxia, osmotic stress, and/or nitric oxide exposure, while class 2 Hb are induced by cold stress and cytokinins. Both are mainly six-coordinated. The deoxygenated forms of the class 1 and 2 nsHbs from A. thaliana (AtHb1 and AtHb2) are able to reduce nitrite to nitric oxide via a mechanism analogous to other known globins. NsHbs provide a viable pH-dependent pathway for NO generation during severe hypoxia via nitrite reductase-like activity with higher rate constants compared to mammalian globins. These high kinetic parameters, along with the relatively high concentrations of nitrite present during hypoxia, suggest that plant hemoglobins could indeed serve as anaerobic nitrite reductases in vivo. The third class of nsHb, also known as truncated hemoglobins, have a compact 2/2 structure and are pentacoordinated, and their exact physiological role remains mostly unknown. To date, no reports are available on the nitrite reductase activity of the truncated AtHb3. In the present work, three representative nsHbs of the plant model Arabidopsis thaliana are presented, and their nitrite reductase-like activity and involvement in nitrosative stress is discussed. The reaction kinetics and mechanism of nitrite reduction by nsHbs (deoxy and oxy form) at different pHs were studied by means of UV-Vis spectrophotometry, along with EPR spectroscopy. The reduction of nitrite requires an electron supply, and it is favored in acidic conditions. This reaction is critically affected by molecular oxygen, since oxyAtHb will catalyze nitric oxide deoxygenation. The process displays unique autocatalytic kinetics with metAtHb and nitrate as end-products for AtHb1 and AtHb2 but not for the truncated one, in contrast with mammalian globins.

Two step model for crosslinking polymerization with account of diffusion

The proposed model includes two parallel reactions, initiation and propagation that respectively follow the reaction order and autocatalytic kinetics. The effect of diffusion is accounted for in both steps by introducing a term that depends linearly on conversion. The term is introduced into the exponential part of the rate constants. Determining the magnitude of this term allows for estimating the diffusion effect on the overall activation energy in the final stages of crosslinking polymerization. The model is tested on five previously published datasets. It appears to fit all data very well and yield physically meaningful fit parameters. Algorithms for successful fitting of the model are suggested. It is also demonstrated that disregarding the effect of diffusion gives rise to markedly inferior results. The model is quite general and expected to be applicable to any polymerization complicated by diffusion or other processes that involve the interplay between the reaction and diffusion kinetics.

Low viscosity and low temperature curing reactive POSS/epoxy hybrid resin with enhanced toughness and comprehensive thermal performance.

The mechanical and high-temperature resistance properties of epoxy resins cured at low temperatures (Tcuring ≤ 100 °C) are often inferior, and the most toughening modification methods for epoxy resins tend to compromise thermal resistance, which significantly limit the practical applications of it. Therefore, this work reported a low viscosity and low-temperature curing epoxy hybrid resin system (OPEP), adopting E-51 as a resin matrix, liquid anhydride (MHHPA) as a curing agent, tertiary amine (DMBA) as a curing accelerator, and reactive octa-epoxy terminated polyhedral oligomeric silsesquioxane (OG-POSS) as a toughening modifier. Results demonstrated that the OPEP system has excellent processability with low viscosity and long processing window period and satisfies the practical requirements of low-temperature curing. The OG-POSS exhibits superior compatibility and reactivity with the resin matrix, and its addition slightly reduces the Eα of the curing reaction and has a certain promotive effect on the curing of epoxy resin. In addition, the curing reaction rate of the OPEP resin complies with the Šesták-Berggren autocatalytic kinetics model. The impact strength, flexural strength, tensile strength, and elongation at break of the OPEP resin reached a maximum of 15.55 kJ m-2, 121.65 MPa, 90.36 MPa, and 2.48%, representing increases of 55.97%, 3.1%, 64.68%, and 26.51% compared to those of the pure resin, respectively. Notably, due to the heat-resistant inorganic silicon cage structure of OG-POSS, the thermal decomposition temperature (Td5), glass transition temperature (Tg), and heat distortion temperature (THDT) of the OPEP0.02 resin were 313.2 °C, 123.7 °C, and 102.0 °C, showing increases of 13.0 °C, 2.3 °C, and 6.8 °C compared to the pure resin, respectively, which is difficult to achieve for the general thermosetting resin toughening modification method. This research utilized organic-inorganic nanohybrid materials (POSS) to optimize the toughness and thermal stability of the resin in a coordinated manner, providing guidance for the preparation of high-performance epoxy resins that cure at low temperatures.

A reaction-diffusion framework for hydrolytic degradation of amorphous polymers based on a discrete chain scission model

Hydrolytic degradation of polymers involves the scission of long chain molecules, leading to molecular weight reduction and mass loss. The precise degradation response however depends on the scission probability of individual bonds along the polymer backbone. In particular, bonds near the chain ends are considered to be more susceptible to hydrolysis than inner bonds. In this paper, we incorporate a discrete chain scission model that can handle arbitrary bond scission probabilities within a continuum reaction-diffusion framework. Overall hydrolysis kinetics (including autocatalysis) is described independently of the chain scission model. By decoupling the description of the chain scission mechanism from kinetics, our framework enables the identification of the chain scission mechanism from molecular weight reduction and mass loss curves commonly reported in experimental degradation studies. We further propose a reduced continuum model which is better suited for large-scale simulations while retaining the predictive capability of the full discrete-continuum model. The model capability is illustrated in representative case studies based on experimental data from the literature for different materials and geometries. Statement of significanceMany models have been proposed to predict the evolution of molecular weight and mass loss in biodegradable polymers undergoing hydrolytic degradation. However, existing models remain limited in their ability to describe the degradation mechanism, autocatalytic kinetics and short chains diffusion simultaneously. Moreover, existing models often rely on empirical relations and a large number of fitting parameters. Here, we propose a conceptually simple discrete-continuum mathematical framework with a small number of parameters which all have a clear physical meaning. Model calibration against experimental data is simplified, and further provides insights into the degradation mechanisms at play, namely random scission, chain-end scission, or a combination of both. The framework can serve as a basis for future generalisations, including a description of evolving crystallinity, or other degradation mechanisms, such as thermal oxidation or photo-degradation.

Behaviors of coking and kinetics of volatiles’ reaction during coal pyrolysis in a two-stage reactor

The reaction of volatiles, especially the coking of volatiles, is the core reaction in the thermal conversion of low-rank coal. In this work, a two-stage reactor is designed and used to investigate the coking behavior of volatiles by adjusting the temperature and residence time of volatiles in second-stage reactor. The variation of tar properties and coke structure is analyzed by Fourier transform infrared spectroscopy (FTIR), electron spin resonance (ESR), and Gel permeation chromatography (GPC). The results show that the temperature of tar condensation (540 °C) is 60 °C lower than that of coke formation (600 °C). The oxygen-containing functional groups in tar can inhibit the coking of volatiles, although oxygen content is higher in the initial coking. The coking behavior of volatiles can be expressed by first-order + autocatalytic kinetics with activation energy lower than 40 kJ/mol when the high temperature volatiles produced by the first-stage reactor directly enter the second-stage reactor to generate coke. The coke is categorized into two types, one suspended in the tar (coke-tar) and another deposited on the wall of reactor (coke-wall). The conversion of coke-tar to coke-wall lead to a big error in the first order + autocatalytic kinetics fitting.

Coking propensity and reactive radical evolution during thermal reaction of heavy residue and SARA fractions at 250–400 °C

In this study, the coking propensity and reactive radical evolution of heavy residue and SARA fractions at 250–400 °C were studied to gain an insight into the coke induction process. The interactions among fractions for thermal coking were discussed. Results showed that residue started to form coke (i.e. toluene-insoluble at room temperature) at 350 °C for longer times while resins and asphaltenes have already extensively produced coke at 250 °C with no coke induction period. This indicates that the coke induction depends on the solubility gap of reactant and coke. The coking process can be fitted with the second-order and autocatalytic kinetics for residue while second-order kinetics for heavy fractions with initial coke precursor amount varying with temperature. Asphaltenes are the sole fraction that generated reactive radicals at 250 °C and responsible for the radicals of thermal residue. The explosion of reactive radicals began when the temperature exceeded 350 °C and became steady as the thermal reaction proceeded. The coke at low temperatures mainly originated from the molecules aggregation with faint radical emergence while that at high temperatures is strongly correlated with the radical evolution. The maltene subfractions could suppress the coking of asphaltenes possibly because of the physical solvation and chemical hydrogen donating behavior to cap reactive radicals. It can serve as the intrinsic immunity for coke inhibition of heavy residue during thermal partial upgrading.

New insight into the mechanism of ferric hydroxide-based heterogeneous Fenton-like reaction.

The heterogeneous Fenton-like reaction (HeFR) has always been a research focus for environmental applications. However, it has long been difficult to reach a consensus on the reaction mechanism because the process of metal ions dissolution and its role were not well understood. In this paper, we propose the courses of organics-mediated coordination or/and reduction dissolution of ferric hydroxide to initiate the autocatalytic kinetics of phenol degradation and illustrate it through density functional theory (DFT) and experiments. With the increase of hydrogen peroxide concentration, the degradation of phenol changes from autocatalytic kinetics to first-order kinetics. Furthermore, a novel "limit segmentation method" initiated by us indicates that homogeneous reaction plays a decisive role in the phenol degradation process. The dominant roles of the reactive organics in both iron dissolution and the iron cycle and of the homogeneous reaction in the whole degradation process in the ferric hydroxide-based HeFR system are brand-new insights that pave the pathway for future research.

A comprehensive physical insight of inclined magnetic field on the flow of generalized Newtonian fluid within a conduit with Homogeneous-heterogeneous reactions

Modern nanomaterials and their flow dynamism processes promotecomplex chemical reactions that are necessary for the accurate synthesis of bespoke geometries at high temperatures. Such flow processes are very intricate and involve viscous behavior along with mass and heat transfer. Such flows mechanism can be controlled by external magnetic fields. Mathematical models offer an inexpensive opening into the fundamental properties of these dynamical processes. The homogeneous-heterogeneous reactions for nanofluids flow are established by invokingthe Buongiorno's nanofluid model, in which the homogeneous reactions are regulated by first order kinetics occurring in the flowing liquid and the heterogeneous reactions are given by isothermal cubic autocatalytic kinetics. To testifythe feasibility of this model, the steady, laminar Jaffrey-Hamel flow problem in the converging conduitis extended to rheological model. The systemsteady states are evaluated under the scenario where the reactant and the catalyst's diffusion coefficients are equivalent. In order to investigate heat and mass transfer analysis, viscous dissipation affirmation, Joule heating, and homogeneous-heterogeneous reactions are incorporated. The mathematical model prevailing the dimensionless function, velocity for flow, temperature for heat, and nanoparticles volume fraction for concentration are simulated numerically by means of Runge-Kutta method. The numerical algorithm has been validated in comparison to previously published research with extremely good agreement. The acquisition and detailed discussion of distributions of flow structure, heat, concentrations, and average Nusselt, Sherwood number at a wide range of critical characteristics. The fluid velocity in the conduitcenter increases significantly as the Reynolds number rises. By intensifying the magnetic field, the flow reversal control is accomplished. Applications in the allied domains have enormous promise since the ratio of Brownian and thermophoretic diffusivity has a significant impact on the transport mechanisms of homogeneous-heterogeneous processes. Chemical species A∗ and B∗ behave in fundamentally distinct ways in the reduced concentrations.

Elucidation of the pre-nucleation phase directing metal-organic framework formation

Metal-organic framework (MOF) crystallization is governed by molecular assembly processes in the pre-nucleation stage. Yet, unravelling these pre-nucleation pathways and rationalizing their impact on crystal formation poses a great challenge since probing molecular-scale assemblies and macroscopic particles simultaneously is very complex. Herein, we present a multimodal, integrated approach to monitor MOF nucleation across multiple length scales by combining in situ optical spectroscopy, mass spectrometry, and molecular simulations. This approach allows tracing initial metal-organic complexes in solution and their assembly into oligomeric nuclei and simultaneously probing particle formation. During Co-ZIF-67 nucleation, a metal-organic pool forms with a variety of complexes caused by ligand exchange and symmetry reduction reactions. We discriminate complexes capable of initiating nucleation from growth species required for oligomerization into frameworks. Co 4 -nuclei are observed, which grow into particles following autocatalytic kinetics. The geometric and compositional variability of metal-organic pool species clarifies long-debated amorphous zeolitic imidazolate framework (ZIF)-particle nucleation and non-classic pathways of MOF crystallization. Simultaneous probing of metal-organic complexes, oligomeric nuclei, and MOF particles Metal-organic pool forms as pre-equilibrium with geometry diverse complexes “Nucleation” and “growth” complexes are discriminated, which assemble into nuclei Stable nuclei are observed, growing into MOF particles with autocatalytic kinetics The molecular pathway that governs metal-organic framework formation is challenging to grasp. Filez et al. establish a multi-scale characterization approach combined with theory to link the somewhat different worlds of “molecular” metal-organic ligand assembly and MOF “particle” crystallization.

Gravity-driven hydromagnetic flow of couple stress hybrid nanofluid with homogenous-heterogeneous reactions

This investigation describes the hydromagnetic flow of gravity-driven couple stress hybrid nanofluid past a heated plate. The carbon nanotubes (CNTs) are used to characterize the hybrid nanofluid. The heated plate is placed vertically with an application of homogenous-heterogeneous reactions to the assumed flow system. The homogeneous reaction governs by isothermal cubic autocatalytic kinetics while the heterogeneous reaction governs by the first order kinetics. For current study the couple stress hybrid nanofluid is presumed to be conducted electrically with impact of non-uniform magnetic effects. An appropriate set of dimensionless quantities has employed to governing equations and then has solved by homotopy analysis method. The influence of emerging parameters encountered in this work has discussed in detail with the help of graphs. In this study it has examined that, flow of fluid reduces with upsurge in magnetic parameter and volumetric concentrations, whereas thermal and concentration characteristics augment with increase in volumetric concentrations. Moreover, growth in Prandtl number leads to a reduction in thermal characteristics and growth in Schmidt number result a reduction in concentration profile. The impact of various emerging parameters has also studied numerically upon physical quantities. It has established that, with augmentation in values of buoyancy parameter there is a growth in the values of skin friction. A comparison has also carried out between current and established results with a fine agreement in both results.

On thermal decomposition kinetics of poly(ethylene-alt- tetrafluoroethylene) using an autocatalytic model

The thermal decomposition of a commercial poly(ethylene-alt-tetrafluoroethylene) (ETFE) Fluon®C-88AXP, in nitrogen atmosphere has been carried out using thermogravimetric analysis (TG) with dynamic method at different heating rates. The widely used Kissinger equation, Friedman's and Ozawa analysis are used to estimate the activation energy of the degradation reaction. Method of fitting the dynamic derivative thermogram (DTG) data directly using isothermal kinetic model is adopted and the model parameters have been obtained which could not be acquired by using the conventional methods. The TG data of Fluon®C-88AXP could not be described properly using n-order kinetic model but the autocatalytic Kamal model fits the data perfectly. The standard deviations of the calculated mass loss rates from the experimental results decrease from 1.497 × 10−6 to 0.138 × 10−6, with a decrease of 91 %, when Kamal model was used instead of n-order kinetics. This indicates that the thermal decomposition of ETFE in nitrogen atmosphere obeys an autocatalytic kinetics.

Exploration of cubic autocatalysis and thermal relaxation in a non-Newtonian flow field with MHD effects

This paper relates to the study of cubic autocatalysis in non-Newtonian Casson fluid with magnetohydrodynamic (MHD) effects. The homogeneous reaction is supposed to be provided by isothermal cubic autocatalytic kinetics and the heterogeneous reaction by first order kinetics. The boundary layer flow is discussed in case when both reactant and auto-catalyst have equal diffusion coefficients. The thermal relaxation and heat transfer on the flow of Casson fluid is examined through Cattaneo–Christov heat diffusion under the influence of heat generation. The fluid flow is taken over the stretching cylinder. The independent variables in a coupled partial differential system are reduced through similarity transformation. The numerical results for the reduced coupled system are found by means of Runge–Kutta Fehlberg method along with shooting scheme. The fluid temperature trend declines when uplifting thermal relaxation parameter. The fluid concentration trend decreases when increasing the strength of homogeneous and heterogeneous reaction parameter. Also, the heat transfer rate for thermal relaxation parameter is examined numerically.

Mixed convection in gravity-driven thin nano-liquid film flow with homogeneous–heterogeneous reactions

A modified nanofluid model for homogeneous–heterogeneous reactions in a gravity-driven liquid film is proposed based on the assumptions of the homogeneous reaction governed by isothermal cubic autocatalytic kinetics and the heterogeneous reaction given by first-order kinetics. The Buongiorno model is introduced to describe behaviors of nanofluids with the Boussinesq approximation being used for simplification of the buoyancy term. Multiple solutions are captured, whose stabilities are checked and discussed based on the theory of Sturm–Liouville. The influence of various physical parameters on important physical quantities is presented for different cases, including buoyancy assisting, no buoyancy, and buoyancy opposing. Different from previous studies on chemical reactions in which reactants are presumed in nanometer scale, we assume that the chemical species are of regular size, which react with each other in a nanofluid. This configuration makes our model physically more realistic than previous ones.

Selection from a pool of self-assembling lipid replicators

Replication and compartmentalization are fundamental to living systems and may have played important roles in life’s origins. Selection in compartmentalized autocatalytic systems might provide a way for evolution to occur and for life to arise from non-living systems. Herein we report selection in a system of self-reproducing lipids where a predominant species can emerge from a pool of competitors. The lipid replicators are metastable and their out-of-equilibrium population can be sustained by feeding the system with starting materials. Phase separation is crucial for selective surfactant formation as well as autocatalytic kinetics; indeed, no selection is observed when all reacting species are dissolved in the same phase. Selectivity is attributed to a kinetically controlled process where the rate of monomer formation determines which replicator building blocks are the fittest. This work reveals how kinetics of a phase-separated autocatalytic reaction may be used to control the population of out-of-equilibrium replicators in time.

Effects of heterogeneous catalysis in porous media on nanofluid-based reactions

This paper studies a new type of homogeneous(HOM)-heterogeneous(HET) reactions in Al2O3-water-based nanofluid flowing through porous media over a stretching plate. Prior investigators have focused mainly on the catalytic effects on the plate, we model the influence of heterogeneous catalysis in porous media on these reactions. The HET reactions on the surfaces of porous media and plate are both governed by the first-order kinetics, while the HOM reaction in the fluid is given by the isothermal cubic autocatalytic kinetics. In addition, the thermal conductivity of four distinct shapes of nanoparticle, sphere, brick, cylinder, and platelet, is taken into consideration with the Hamilton-Crosser model. The obtained nonlinear differential systems simplified by using similarity transformations are numerically calculated by the bvp4c algorithm. Results demonstrate that the increase of interfacial area of porous media enhances the rate of surface-catalyzed reaction and therefore porous media can greatly shorten the chemical reaction time. Moreover, we find that platelet nanoparticles exhibit the highest convective heat transfer capacity.

Thermal energy statistics for Jeffery fluid flow regime: A generalized Fourier’s law outcomes

This paper is about Jeffery fluid with thermal stratification effects at a stagnation point. The thermal energy and heat transfer is studied through Cattaneo–Christov heat flux model. The flow is induced by the stretching cylinder. The homogeneous–heterogeneous chemically reactive species are taken into account. The homogeneous reaction is provided by isothermal cubic autocatalytic kinetics and the heterogeneous reaction by first order kinetics. The steady state boundary layer flow is discussed in case when both reactants and auto-catalyst have equal diffusion coefficients. The concerned mathematical problem is developed by laws of conservation of mass, momentum and energy which provides a coupled partial differential equations. The order of these equations is reduced through similarity transformation. Later on, the set of reduced coupled equations are sorted out numerically by means of Runge–Kutta Fehlberg technique with shooting algorithm. The fluid velocity is decreasing at Deborah number. The influence of thermal relaxation parameter and thermal stratification parameter on fluid temperature trend is decreasing. The numerical values for skin friction coefficient is examined and found excellent match with existing literature.

Kinetic Study of Curing Bisphenol A Dicyanate Ester with Ionic Liquid Additive

ABSTRACTA kinetic study of the trimerization reaction of bisphenol A dicyanate ester with an aromatic imidazolium‐based ionic liquid (IL) as additive is performed using dynamic and isothermal differential scanning calorimetry. The reaction follows second‐order autocatalytic kinetics, and a slight acceleration effect is observed in the presence of the aromatic IL relative to the neat resin. The activation energy also increases with the IL additive, whereas the glass transition temperature (Tg) is depressed, consistent with the Fox equation and a homogeneous one‐phase material. A model incorporating diffusion effects is able to describe the dynamic and isothermal curing data for both the neat resin system and that containing aromatic IL. A comparison with aliphatic‐based IL additive indicates that the reaction is more accelerated with aliphatic IL than with the aromatic IL in spite of the fact that the aliphatic additive phase separates during cure. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1315–1324

The influence of MHD and heat generation/absorption in a Newtonian flow field manifested with a Cattaneo–Christov heat flux model

The purpose of this report is to emphasize magnetohydrodnamics, heat generation/absorption and slip effects over a Newtonian fluid flow with homogeneous–heterogeneous chemical reactions induced by the rotating disk. The homogeneous reaction is provided by isothermal cubic autocatalytic kinetics and the heterogeneous reaction by first order kinetics. The concerned steady state boundary layer flow is discussed in the case when both reactants and auto-catalyst have equal diffusion coefficients. The Cattaneo–Christov approach is proposed to derive the energy equation and heat transfer phenomena. The consequential partial differential equations descend to ordinary differential equations by using similarity transformation. Further, these equations are sorted numerically using the Runge–Kutta–Fehlberg technique with a shooting scheme. The influences of arising parameters on fluid velocity, temperature and concentration are observed through graphs. Further, the numerical results for the skin friction coefficient and local Nusselt number are examined and compared with existing literature.

Behavior of coking and stable radicals formation during thermal reaction of an atmospheric residue

This work studies the thermal reaction of Maoming atmospheric residue at a temperature range 400–500 °C in sealed tube reactor. It is found that the CS2 insoluble (coke) is formed at 420 °C and above and it increases with time and temperature, which can be fitted with a combination of the second-order and autocatalytic kinetics. The loss of heteroatoms in the coke such as sulfur atom accompanied with the condensation of coke. The concentration, g value and linewidth of stable radicals in residue are measured by electron spin resonance (ESR) and discussed to explore the transformation of coke structure. The formation of stable radicals can be expressed by a combination of the two first-order kinetics including CS2 soluble (oil) cracking and coke condensation with activation energies of 168 and 98 kJ/mol, respectively. More than half of stable radicals exist in oil (or in the coke presented in oil) when the coke contents are no >3.4%, while about 90% of the total stable radicals can be found in the coke when coke contents are >8.0%. In addition, the variation of g value and linewidth of stable radicals in coke are also studied.