Autocatalytic Reaction Mechanism Research Articles

Understanding and Modelling the Effect of Dissolved Metals on Solvent Degradation in Post Combustion CO2 Capture Based on Pilot Plant Experience

Oxidative degradation is a serious concern for upscaling of amine-based carbon capture technology. Different kinetic models have been proposed based on laboratory experiments, however the kinetic parameters included are limited to those relevant for a lab-scale system and not a capture plant. Besides, most of the models fail to recognize the catalytic effect of metals. The objective of this work is to develop a representative kinetic model based on an apparent auto-catalytic reaction mechanism between solvent degradation, corrosion and ammonia emissions. Measurements from four different pilot plants: (i) EnBW’s plant at Heilbronn, Germany (ii) TNO’s plant at Maasvlakte, The Netherlands; (iii) CSIRO’s plants at Loy Yang and Tarong, Australia and (iv) DONG Energy’s plant at Esbjerg, Denmark are utilized to propose a degradation kinetic model for 30 wt % ethanolamine (MEA) as the capture solvent. The kinetic parameters of the model were regressed based on the pilot plant campaign at EnBW. The kinetic model was validated by comparing it with the measurements at the remaining pilot campaigns. The model predicted the trends of ammonia emissions and metal concentration within the same order of magnitude. This study provides a methodology to establish a quantitative approach for predicting the onset of unacceptable degradation levels which can be further used to devise counter-measure strategies such as reclaiming and metal removal.

Curing kinetics, mechanical properties and thermal stability of epoxy/graphene nanoplatelets (GNPs) powder coatings

Epoxy/graphene nanoplatelets (GNPs) powder coatings were fabricated using ultrasonic predispersion of GNPs and melt-blend extrusion method. The isothermal curing kinetics of epoxy/GNPs powder coating were monitored by means of real-time Fourier transform infrared spectroscopy (FT-IR) with a heating cell. The mechanical properties of the epoxy/GNPs cured coatings had been investigated, by evaluating their fracture surfaces with field-emission scanning electron microscopy (FE-SEM) after three-point-bending tests. The thermal stability of the epoxy/GNPs cured coatings was studied by thermo-gravimetric analysis (TGA). The isothermal curing kinetics result showed that the GNPs would not affect the autocatalytic reaction mechanism, but the loading of GNPs below 1.0 wt % additive played a prompting role in the curing of the epoxy/GNPs powder coatings. The fracture strain, fracture toughness and impact resistance of the epoxy/GNPs cured coatings increased dramatically at low levels of GNPs loading (1 wt %), indicating that the GNPs could improve the toughness of the epoxy/GNPs powder coatings. Furthermore, from FE-SEM studies of the fracture surfaces, the possible toughening mechanisms of the epoxy/GNPs cured coatings were proposed. TGA result showed that the incorporation of GNPs improved the thermal stability of the cured coatings. Hence, the GNPs modified epoxy can be an efficient approach to toughen epoxy powder coating along with improving their thermal stability.

Study of Different Types of Carbon Fiber Oxidation Kinetics

Results are given for a study of the effect of treatment temperature on oxidation kinetics of carbon fibers of different structure. It is detected that carbon fiber oxidation by oxygen depends on the production process for fiber manufacture, and as a consequence on the nature of its structure, primarily the degree of crystallinity that governs the number of active centers emerging as oxidation initiators. It is established that oxidation of carbon fibers with a relatively low treatment temperature (up to 2000°C) is approximated by an autocatalytic reaction mechanism.

Mechanistic insights into lithium ion battery electrolyte degradation - a quantitative NMR study.

The changes in electrolyte composition on the molecular level and the reaction mechanisms of electrolyte degradation upon thermal aging are monitored by quantitative NMR spectroscopy, revealing similar rates of degradation for pristine and already aged electrolytes. The data analysis is not in favor of an autocatalytic reaction mechanism based on OPF3 but rather indicates that the degradation of LiPF6 in carbonate based solvents proceeds via a complex sequence of "linear" reactions rather than a cyclic reaction pattern which is determined by the amount of water present in the samples. All investigated electrolytes are reasonably stable at temperatures of up to 60 °C in the presence of minor amounts or absence of water hence indicating that chemical instability of electrolyte components against water is decisive for degradation and an increase in temperature ("thermal aging") just accelerates the degradation impact of water.

A sustainable process to utilize ferrous sulfate waste from titanium oxide industry by reductive decomposition reaction with pyrite

Ferrous sulfate waste has become a bottleneck in the sustainable development of the titanium dioxide industry in China. In this study, we propose a new method for the reductive decomposition of ferrous sulfate waste using pyrite. Thermodynamics analysis, tubular reactor experiments, and kinetics analysis were performed to analyze the reaction process. The results of the thermodynamic simulation showed that the reaction process and products were different when molar ratio of FeSO4/FeS2 was changed. The suitable molar ratio of FeSO4/FeS2 was 8–12. The reaction temperature of ferrous sulfate with pyrite was 580–770K and the main products were Fe3O4 and SO2. The simulation results agreed well with the experimental results. The desulphurization rate reached 98.55% and main solid products were Fe3O4 at 823.15K when mole ratio of FeSO4/FeS2 was 8. Nano-sized magnetite was obtained at this condition. The kinetic model was investigated by isoconversional methods. The average E value was 244.34kJmol−1. The ferrous sulfate decomposition process can be treated as autocatalytic reaction mechanism, which corresponded to the expanded Prout–Tompson (Bna) model. The reaction mechanism of autocatalytic reactions during the process of ferrous sulfate decomposition were explored, the products of Fe oxide substances are the catalyst components.

Characterization of the spherical intermediates and fibril formation of hCT in HEPES solution using solid-state 13C-NMR and transmission electron microscopy

Human calcitonin (hCT) is a 32-amino acid peptide hormone that contains an intrachain disulfide bridge between Cys1 and Cys7 and a proline amide at the C-terminus. hCT tends to associate to form a fibril precipitate of the same type as amyloid fibrils, and hence has been studied as a model of amyloid fibril formation. The fibrillation process in N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid (HEPES) solution was examined using transmission electron microscopy. The rate of hCT fibrillation in HEPES solution was much lower than in phosphate buffer and acetic acid solution. Spherical intermediate aggregates (nuclei) were observed during the early stage of fibril formation. Short proto-fibrils appeared on the surface of the spherical intermediates. Subsequently, the spherical intermediates transformed directly into long proto-fibrils, which then elongated into mature hCT fibrils. The fibrillation process was also examined using solid-state (13)C-NMR spectroscopy, which indicated that the fibril structure was a β-sheet in the central region and a mixture of random coils and β-sheets at the C-terminus. The kinetics of fibril formation was examined in terms of a two-step autocatalytic reaction mechanism. The first-step nucleation rate (k1) was lower in HEPES solution than in phosphate buffer and acetic acid solution because the half-life of the intermediates is significantly longer in HEPES solution. In contrast, the second-step fibril elongation rate (k2) was similar in HEPES solution and acidic solutions. Specific interaction of HEPES molecules with hCT may stabilize the spherical intermediates and consequently inhibit the fibril elongation process of hCT.

Inhibitory Mechanism of Pancreatic Amyloid Fibril Formation: Formation of the Complex between Tea Catechins and the Fragment of Residues 22–27

Islet amyloid polypeptide (IAPP) is a major component of pancreatic amyloid deposits associated with type 2 diabetes. Polyphenols contained in plant foods have been found to inhibit amyloid fibril formation of proteins and/or peptides. However, the inhibition mechanism is not clear for a variety of systems. Here the inhibition mechanism of green tea polyphenols, catechins, on amyloid fibril formation of the IAPP fragment (IAPP22-27), which is of sufficient length for formation of β-sheet-containing amyloid fibrils, was investigated by means of kinetic analysis. A quartz crystal microbalance (QCM) determined that the association constants of gallate-type catechins [epicatechin 3-gallate (ECg) and epigallocatechin 3-gallate] for binding to IAPP22-27 immobilized on the gold plate in QCM were 1 order of magnitude larger than those of the free IAPP22-27 peptide, and also those of epicatechin and epigallocatechin. Kinetic analysis using a two-step autocatalytic reaction mechanism revealed that ECg significantly reduced the rate constants of the first nucleation step of amyloid fibril formation, while the rate of autocatalytic growth was less retarded. (1)H nuclear magnetic resonance studies clarified that a IAPP22-27/ECg complex clearly forms as viewed from the (1)H chemical shift changes and line broadening. Our study suggests that tea catechins specifically inhibit the early stages of amyloid fibril formation to form amyloid nuclei by interacting with the unstructured peptide and that this inhibition mechanism is of great therapeutic value because stabilization of the native state could delay the pathogenesis of amyloid diseases and also the toxicity of the small oligomer (protofibril) is reported to be greater than that of the mature fibril.

Nickel oxide redox processes with oxide ion conductor-supported nickel oxide in dry and humidified methane: Effect of oxide ion conductors on induction period in nickel oxide reduction and subsequent hydrogen production

NiO redox processes and hydrogen production from dry and humidified methane were investigated by chemical looping combustion (CLC) and chemical looping reforming (CLR) using NiO as an oxygen carrier. To achieve a lower operating temperature in the NiO reduction process, oxide ion conductors such as gadolinia doped ceria (GDC) and strontium doped lanthanum aluminate (LSA) were investigated as the support materials of NiO. Hydrogen rich gas products were obtained by the CLC and CLR processes but a prolonged induction period for the NiO reduction by methane (i.e., induction period for CO2 formation from methane) was observed at relatively low operating temperatures in both processes. NiO/GDC and NiO/LSA lead to a decrease in the induction period of NiO reduction by comparison with the reference NiO/ZrO2. Upon repeating the NiO reduction–oxidation cycles, we also found that the induction period for NiO/GDC became shorter with an increase in the redox cycle number. However, for NiO/LSA this time became longer, which may result from the strong interaction between Ni and the support materials. This NiO reduction behavior on the oxide ion conductors is discussed together with the autocatalytic reaction mechanism for NiO reduction, which provides new insights into the efficient use of lattice oxygen in CLC and CLR.

Quantum-chemical investigation of isocyanate reactions with linear methanol associates: IV. Mechanism of autocatalytic reaction of methyl isocyanate with linear methanol associates

It was shown by quantum-chemical method B3LYP/6-311++G(df,p) that in the autocatalytic reaction the molecules of methyl carbamate formed prereaction complexes with the monomer and dimer of methanol. The complexes possess higher electron-donor properties than free alcohol molecules thus increasing the activity of complexes in the reaction with isocyanates. The products of the autocatalytic reaction are new molecules of carbamate and azomethinenol. The conversions occur through concerted asymmetric late transition states. The isomerization of azomethinenols into carbamates is catalyzed by molecules of alcohols and their associates. The autocatalytic reaction of isocyanates with alcohols becomes possible owing to the ability of alcohols to catalyze the mentioned isomerization.

Kinetics and Mechanism of the Permanganate-Induced Autocatalytic Dehydration Reaction of L-Y-Amino-N-Butyric Acid to Give 2-Pyrrolidone through a Radical Intermediate in Moderately Concentrated Acidic Medium

The reaction kinetics and mechanism of the permanganate ion oxidation of L-y-amino-butyric acid (GABA) in moderately concentrated sulfuric acid medium has been investigated spectrophotometrically.The product of this reaction is 2-pyrrolidone, which is a 5-membered lactam structure compound (gamma-butyrolactam). Conclusive evidence indicates autocatalytic activity for Mn(II) in this reaction, analogous to that of some a- amino acids. It has been shown that such activity appears when a certain concentration ratio of Mn(II) to GABA is built up in the medium, termed the “critical ratio”. The magnitude of this ratio depends on the sulfuric acid concentration. In considering the “delayed autocatalytic behaviour” of Mn(II) ions, rate equations satisfying our observations for both the catalytic and non- catalytic routes have been presented. The reaction shows a first-order dependence on permanganate ion concentration both in the catalytic and non-catalytic pathways, and an apparent first-order dependence on Mn2+ions in the catalytic pathway. Also, it was found that the variation in the apparent rate constants of the catalytic and non-catalytic pathways versus the GABA concentration follows saturation curves. The correspondence of the pseudo-order rate constants of the catalytic and non-catalytic pathways to the Arrhenius and Eyring laws have verified the “critical ratio” as well as the “delayed autocatalytic behaviour” concepts.The activation parameters associated with both pathways have been computed and discussed. Mechanisms for both catalytic and non-catalytic routes involving radical intermediates, as well as a product having a lactam skeleton, are reported.

STABILITY OF KETOPROFEN COATED BY CHITOSAN-GUAR GUM GEL

The coating stability of ketoprofen by chitosan-guar gum gel has been studied. Into 228.6 mL of 1.75% (w/v) chitosan solution in 1% (v/v) acetic acid, 38.1 mL of guar gum (gg) solution was added with concentration variation of 0.35, 0.55, and 0.75% (w/v) for ketoprofen microcapsules, and stirred with magnetic stirrer until homogenous. Afterwards, 7.62 mL of glutaraldehyde (glu) was added slowly under stirring, with concentrations varied: 3, 3.5, and 4% (v/v). All mixtures were shaked for 20 min for homogenization. Into each microcapsule mixture for ketoprofen, a solution of 2 g of ketoprofen in 250 mL of 96% ethanol was added. Every mixture was then added with 5 mL of 2% Tween-80 and stirred with magnetic stirrer for an hour at room temperature. Conversion of suspension into fine powders/granules (microcapsules) was done by using spray dryer. Every microcapsule formula was packed into capsules, as much as 100 g per capsule. The capsules were contained in 100-mL dark bottles and the bottles were kept in climatic chamber at (40 ± 2) °C and RH (75 ± 5) % for 3 months. The microcapsule stabilities were tested chemically and physically. The result showed that formulation of ketoprofen preparation composed of 1.75% (w/v) chitosan, 0.35% (w/v) gg, and 3.50% (v/v) glu, was relatively the best, with ketoprofen percentage left in microcapsule after 3 months, degradation rate constant, and shelf life of 80.33%, 0.0351 % week-1, and 18.92 months, respectively. Reaction kinetic model for this formula followed Prout-Tompkins equation and the degradation of ketoprofen was seem to follow autocatalytic reaction mechanism controlled by the formation and growth of reaction core. Keywords: Ketoprofen, chitosan-guar gum gel

Studies on the cure kinetics and networks properties of neat and polydimethylsioxane modified tetrafunctional epoxy resins

AbstractThe cure kinetics of tetrafunctional epoxy resins with three different backbone structures and their modification by polydimethylsioxane (PDMS) were studied by means of differential scanning calorimetry with dynamic approach. The development of epoxy networks was characterized by dynamic viscoelastic measurements. Results showed that all the epoxy resins obeyed the autocatalytic reaction mechanism with a reaction order of about 3. Epoxy resin with softer aliphatic backbone demonstrated a higher cure reactivity and stronger tendency towards autocatalysis, as well as lower crosslinking density. The PDMS‐modified epoxy resins showed higher early cure reactivity and a lower crosslinking density due to the plasticization and restriction effect of the dispersed PDMS phase, respectively. Based on cure kinetics and dynamic viscoelastic results, the αm was found to be an effective precursor for describing the developing of epoxy networks during the course of cure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Mechanism of the Asymmetric Autocatalytic Soai Reaction Studied by Density Functional Theory

The mechanism of the Soai reaction has been thoroughly investigated at the M05-2X/6-31G(d) level of theory, by considering ten energetically distinct paths. The study indicates the fully enantioselective catalytic cycle of the homochiral dimers to be the dominant mechanism. Two other catalytic cycles are shown to both be important for correct understanding of the Soai reaction. These are the catalytic cycle of the heterochiral dimer and the non-enantioselective catalytic cycle of the homochiral dimers. The former has been proved to be not really competitive with the principal cycle, as required for the Soai reaction to manifest chiral amplification, whereas the latter, which is only slightly competitive with the principal one, nicely explains the experimental enantioselectivity observed in the reaction of 2-methylpyrimidine-5-carbaldehyde. The study has also evidenced the inadequacy of the B3LYP functional for mechanistic investigations of the Soai reaction.

Cure kinetics of epoxy resin-natural zeolite composites

The cure kinetics of epoxy resin and epoxy resin containing 10 mass% of natural zeolite were investigated using differential scanning calorimetry (DSC). The conformity of the cure kinetic data of epoxy and epoxy-zeolite system was checked with the auto-catalytic cure rate model. The results indicated that the hydroxyl group on the zeolite surface played a significant role in the autocatalytic reaction mechanism. This group was able to form a new transition state between anhydride hardener and epoxide group. The natural zeolite particles acted as catalyst for the epoxy system by promoting its curing rate.

Kinetics of Reduction of Cu Ions in MFI Zeolite Investigated by H2-TPR Method

The temperature programmed reduction with hydrogen (H2-TPR) profiles of the Cu-MFI zeolite matrix were measured. The effect of the Cu ions loading and the heating rate on the H2-TPR profiles was investigated. The obtained TPR profiles were evaluated by different methods in order to obtain kinetic parameters and the reduction process mechanism. It was observed that the results given by convenient methods based on the simple power-law kinetics cannot be used for evaluation of at least the reduction of monovalent copper. Based on the reaction rate dependence on the degree of conversion, an autocatalytic reaction mechanism model was suggested for monovalent copper reduction. The autocatalytic effect was ascribed to nanoclusters of metallic copper, which is formed during the reduction. The parameters of this model were obtained by fitting the reaction rate equation to experimental data. The simulated TPR profiles well describe the position and relative height of peaks in experimental TPR profiles. Using our approach to evaluation of kinetic parameters we found the activation energies for reduction of Cu2+, direct reduction of Cu+and autocatalytic reduction of Cu+equal to 91, 67 and 38 kJ mol-1, respectively. All the mentioned processes are controlled by the kinetics of order ca. 1.5.

Curing behavior and structure of an epoxy/clay nanocomposite system

AbstractThe curing behavior of an epoxy/clay nanocomposite system composed of a bifunctional epoxy resin with an aromatic amine curing agent and an organically modified clay was investigated. Differential scanning calorimetry (DSC) was used to investigate the curing behavior of the epoxy/clay nanocomposite system. The curing rate of the nanocomposite system increased with increasing clay content. A kinetic equation, considering an autocatalytic reaction mechanism, could describe fairly well the curing behavior of the epoxy/clay nanocomposite system. The reaction kinetic parameters of the kinetic equation were determined by fitting DSC conversion data to the kinetic equation, using a nonlinear numerical method. Dynamic mechanical analysis was used to investigate the thermomechanical properties of the epoxy/clay nanocomposite system. The glass transition temperature of the epoxy/clay nanocomposite system increased slightly with increasing clay content. The structure of the nanocomposite system was characterized by X‐ray diffraction analysis and transmission electron microscope imaging. The formation of intercalated structures was observed dominantly in the epoxy/clay nanocomposites, together with some exfoliated structures. POLYM. ENG. SCI., 46:1318–1325, 2006. © 2006 Society of Plastics Engineers

An In Situ ATR-FTIR Study on Palladium Displacement Reaction on Hydrogen-Terminated Silicon Surface

We have studied the mechanism of Pd autocatalytic displacement reaction on a silicon surface in both and baths. Using in situ and ex situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) experiments, it has been shown that in the electrolyte is reduced to metallic Pd particles by a displacement reaction and an compound is generated during the Pd activation in electrolyte. From our results, we have concluded that different mechanisms of displacement reactions in each electrolyte result in the different characteristics of Pd growth. Hydrofluoric acid, which is generated in the Si etching reaction, is directly dissolved to during the Pd displacement reaction. The in the electrolyte plays a role as the Si etchant, whereas bonds with ion in the electrolyte. These different mechanisms result in the different of Pd growth speeds and characteristics.

The reaction of benzothiazole sulfenamide with (TMS) 3SiH: An example of degenerate-branched chain process

The reaction of sulfenamide 3 with (TMS) 3SiH initiated by the decomposition of AIBN at 76 °C has been studied in some detail. The reaction is a rare example of a radical chain-branching process. The two main products are dialkylamine 4 and the thiosilane 5. It is also established that 2-mercaptobenzothiazole ( 2) is formed in a substantial yield as one of the by-products. The mechanism of this chain autocatalytic reaction is complex due to a mix of different radical chain reactions and some discussion is provided. The amine obtained in a quantitative yield can arise from two independent routes of attack of (TMS) 3Si radical on sulfenamide 3. The minor route affords thiol 2 that can act as a catalyst for the major route during the reaction course and then gives a salt with secondary amine, which precipitates upon cooling. The origin of autocatalysis is discussed in some detail.

Polyaniline-modified electrode as an amperometric ascorbate sensor

Electrochemical oxidation of ascorbate at polyaniline-modified electrode has been studied. The autocatalytic reaction mechanism has been proposed for anodic oxidation of ascorbate in nearly pH-neutral buffered solutions: protons liberated during ascorbate oxidation cause a local pH drop within a thin polyaniline layer, turning it into a proton-doped conducting form that shows electrocatalytic activity towards oxidation of ascorbate. The proposed mechanism explains the ability of polyaniline to electrocatalyze the anodic oxidation of ascorbate even in pH-neutral solutions, where polyaniline presents in its undoped and non-conducting form. It has been shown that polyaniline-modified electrodes can be used for ascorbate assay within a pH range of 5.5–7.2, operating potential window of 0.1–0.3 V versus Ag/AgCl, showing a linear range of response up to 0.6 mM, and a lower detection limit of 0.05 mM.

Polymerization kinetics and thermal properties of dicyanate/clay nanocomposites

AbstractThe polymerization kinetics and thermal properties of dicyanate/clay nanocomposites were investigated. A type of organically modified clay was used as nanometer‐size fillers for the thermosetting dicyanate resin. Differential scanning calorimetry (DSC) was used to study the curing behavior of the dicyanate/clay nanocomposite systems. The polymerization rate of the nanocomposite systems increased with increasing clay content. An autocatalytic reaction mechanism could adequately describe the polymerization kinetics of the dicyanate/clay nanocomposite systems. The polymerization kinetic parameters were determined by fitting the DSC conversion data to the proposed kinetic equation. The glass‐transition temperature of the dicyanate/clay nanocomposites increased with increasing clay content. The thermal decomposition behavior of the dicyanate/clay nanocomposites was investigated by thermogravimetric analysis. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1955–1960, 2004