Accurate Activation Energies Research Articles

Kinetic verification and Comsol modeling of the Diels-Alder reaction of conjugated pentadiene with maleic anhydride in a continuous flow capillary reactor

Synthesis of 3/4-methyltetrahydrophthalic anhydride was carried out by the Diels-Alder reaction in a continuous flow capillary reactor. The efficient liquid–liquid reaction of pentadiene with molten maleic anhydride provided a theoretical basis for replacing the conventional batch reaction process. Different inner diameter sizes of the channel tubes were adjusted in the range of 0.50 ∼ 2.00 mm to find the optimal size to match the strong exothermic reaction. Under the capillary channel size of 0.8 mm, the conversion rate of C5 and MA reached 100 % when nC5: nMA = 1.08:1 at 65 °C for 8 min. The D-A reaction process is accompanied by the autopolymerization of diolefin in the batch reactor. Our results showed that choosing a continuous flow capillary reactor of appropriate size can directly improve the selectivity of the reaction and the quality of the product. The accurate reaction rate constant and activation energy were obtained by establishing a kinetic model, and the effects of temperature, reaction time and pipe diameter were designed using Comsol software.

Dynamic Monkey Bar Mechanism of Superionic Li‐ion Transport in LiTaCl6

AbstractThe LiTaCl6 solid electrolyte has the lowest activation energy of ionic conduction at ambient conditions (0.165 eV), with a record high ionic conductivity for a ternary compound (11 mS cm−1). However, the mechanism has been unclear. We train machine‐learning force fields (MLFF) on ab initio molecular dynamics (AIMD) data on‐the‐fly and perform MLFF MD simulations of AIMD quality up to the nanosecond scale at the experimental temperatures, which allows us to predict accurate activation energy for Li‐ion diffusion (at 0.164 eV). Detailed analyses of trajectories and vibrational density of states show that the large‐amplitude vibrations of Cl− ions in TaCl6− enable the fast Li‐ion transport by allowing dynamic breaking and reforming of Li−Cl bonds across the space in between the TaCl6− octahedra. We term this process the dynamic‐monkey‐bar mechanism of superionic Li+ transport which could aid the development of new solid electrolytes for all‐solid‐state lithium batteries.

Dynamic Monkey Bar Mechanism of Superionic Li-ion Transport in LiTaCl6.

The LiTaCl6 solid electrolyte has the lowest activation energy of ionic conduction at ambient conditions (0.165 eV), with a record high ionic conductivity for a ternary compound (11 mS cm-1 ). However, the mechanism has been unclear. We train machine-learning force fields (MLFF) on ab initio molecular dynamics (AIMD) data on-the-fly and perform MLFF MD simulations of AIMD quality up to the nanosecond scale at the experimental temperatures, which allows us to predict accurate activation energy for Li-ion diffusion (at 0.164 eV). Detailed analyses of trajectories and vibrational density of states show that the large-amplitude vibrations of Cl- ions in TaCl6 - enable the fast Li-ion transport by allowing dynamic breaking and reforming of Li-Cl bonds across the space in between the TaCl6 - octahedra. We term this process the dynamic-monkey-bar mechanism of superionic Li+ transport which could aid the development of new solid electrolytes for all-solid-state lithium batteries.

Entropic barrier of water permeation through single-file channels

Facilitated water permeation through narrow biological channels is fundamental for all forms of life. Despite its significance in health and disease as well as for biotechnological applications, the energetics of water permeation are still elusive. Gibbs free energy of activation is composed of an enthalpic and an entropic component. Whereas the enthalpic contribution is readily accessible via temperature dependent water permeability measurements, estimation of the entropic contribution requires information on the temperature dependence of the rate of water permeation. Here, we estimate, by means of accurate activation energy measurements of water permeation through Aquaporin-1 and by determining the accurate single channel permeability, the entropic barrier of water permeation through a narrow biological channel. Thereby the calculated value for triangle {S}^{{{ddagger}} } = 2.01 ± 0.82 J/(mol·K) links the activation energy of 3.75 ± 0.16 kcal/mol with its efficient water conduction rate of ~1010 water molecules/second. This is a first step in understanding the energetic contributions in various biological and artificial channels exhibiting vastly different pore geometries.

Insight into kinetic and Thermodynamic Analysis methods for lignocellulosic biomass pyrolysis

Pyrolysis is particularly attractive because it can convert lignocellulosic biomass into bio-oil, gases and solid charcoal in an efficient and environmentally friendly way. Determining the kinetic and thermodynamic parameters is fundamental to elucidate the reaction mechanisms and optimize biomass pyrolysis processes. The applicability and accuracy of kinetic and thermodynamic methods are systematically studied by analyzing theoretically simulated processes which simulate biomass pyrolysis processes and using statistical analyses. In terms of activation energy calculation, the Friedman method is capable of providing accurate activation energies, whereas the KAS and FWO isoconversional methods lead to some systematic errors in the resultant activation energies. The accuracy of the Kissinger method for estimating the frequency factor is dependent on the variation degree of kinetic parameters with conversion. The conventional method for determining thermodynamic parameters is based on the conversion rate peak temperature, which has no theoretical basis. Therefore, the concept of effective thermodynamic parameters is proposed, which are dependent on conversion and heating rate. And the thermodynamic analysis of the simulated processes is comprehensively investigated. Results revealed that the accuracy of the resulting thermodynamic parameters from the conventional method is dependent on the variation degree of kinetic parameters with conversion. It is expected that this study will provide an analysis framework and methodological guideline for the kinetic and thermodynamic analyses of biomass pyrolysis.

Application of Multiconfiguration Pair-Density Functional Theory to the Diels-Alder Reaction.

Transition states for Diels-Alder reactions are strongly correlated, as evidenced by high-to-very-high M diagnostics, and therefore they require treatment by multireference methods. Multiconfiguration pair-density functional theory (MC-PDFT) combines a multiconfiguration wave function with a functional of the electron density and the on-top pair density to calculate the electronic energy for strongly correlated systems at a much lower cost than wave function methods that do not employ density functionals. Here we apply MC-PDFT to the Diels-Alder cycloaddition reaction of 1,3-butadiene with ethylene, where two kinds of reaction paths have been widely studied: concerted synchronous paths and diradical stepwise paths. The lowest-energy reaction path is now known to be a concerted synchronous one, and a method's ability to predict this is an important test. By comparison to the best available theoretical results in the literature, we test the accuracy of MC-PDFT with several choices of on-top functional for geometries and enthalpies of stable structures along both paths and for the transition state geometries. We also calculate the Arrhenius activation energies for both paths and compare these to experiment. We also compare to Kohn-Sham density functional theory (KS-DFT) with selected exchange-correlation functionals. CAS-PDFT gives consistently good energies and geometries for both the concerted and stepwise mechanisms, but none of the KS-DFT functionals gives accurate activation energies for both. The stepwise transition state is very strongly correlated, and MC-PDFT can treat it, but KS-DFT (which involves a single-configuration treatment) has larger errors. The results confirm that using a multiconfigurational reference function for strongly correlated transition states can significantly improve the reliability and that MC-PDFT can provide good accuracy at a much lower computational cost than competing multireference methods.

Co-pyrolytic interactions, kinetics and products of biomass pyrolysis coke and rapeseed cake: Machine learning, DAEM and 2D-COS analysis

In this study, an efficient strategy for synergistic disposal of biomass pyrolysis coke (PC) and rapeseed cake (RC) was proposed to explore their clean utilization, with the purposes for evaluating the co-pyrolytic interactions, kinetics, mechanism and products. The results indicated that synergistic effect was strengthened by lower PC ratio and temperature. A multiple distributed activation energy model (MDAEM) was applied to obtain accurate activation energy distributions. Co-pyrolysis resulted in lower average activation energy and wider distribution, from which the activation energy E0 and σ were in the range of 103.99–214.16 kJ/mol and 3.18–38.09 kJ/mol. MDAEM was also innovatively combined with machine learning to describe and predict co-pyrolysis process, with the determination coefficient R2 beyond 0.999923. Sensitivity analysis and principal component analysis were applied, indicating RC was more sensitive to the heating rate, and preferred dominating reactions at high-temperature region. Temperature dependences of pyrolysates was obtained by two-dimensional correlation spectroscopy method.

Mechanisms and energetics in the early stages of solvent-assisted low-temperature sintering of ZnO

The Hydro/Solvothermal Sintering (HSS) is a technique enabling the low temperature sintering of ceramics and composites, by using a solvent and uniaxial pressure. This energy-efficient technique also opens new perspectives in designing new composites with tailored functional properties. While the solvent plays a key role in reducing the sintering temperature, the modeling and deep understanding of sintering mechanisms remain an open field of investigation. This study unveils the energetics and mechanisms involved in the first stage of HSS. The strategy highlighted in this paper includes: (i) the building of a hydrothermal sintering device equipped with a dilatometer to monitor the shrinkage in situ, (ii) numerical stress calculations at the contact between particles to show the suitability of the two-particle kinetic equation, and (iii) the use of anisothermal and non-conventional stepwise isothermal methodologies for the investigation of mechanisms and energetics. The model material, ZnO, was densified to high relative densities with acetic acid as a solvent, and a pressure and temperature of 320 MPa and 150 °C, respectively. With these experimental conditions, the kinetic analysis obtained from the two methodologies is consistent and implies a dissolution reaction of the material as the rate controlling mechanism with possible coupling to grain boundary sliding. The activation energy of 90 kJ.mol—1 is determined with the different analysis methodologies. The interest of the stepwise isothermal methodology in obtaining accurate activation energy is also discussed.

Toward Accurate Formation Routes of Complex Organic Molecules in the Interstellar Medium: The Paradigmatic Cases of Acrylonitrile and Cyanomethanimine

The investigation of reaction mechanisms in the interstellar medium requires the evaluation of reaction rates and branching ratios, which can be effectively obtained in the framework of the ab-initio transition state/master equation formalism. However, the reliability of this approach relies on the computation of accurate reaction enthalpies and activation energies for all the paths characterizing the potential energy surface under investigation. Two effective yet reliable parameter-free model chemistries are introduced to obtain accurate energies of all stationary points, with structural determination performed using double-hybrid density functionals. After their validation, these model chemistries have been employed to analyze the competition between hydrogen abstraction and addition/elimination in the reaction between the CN radical and ethylene or methanimine. The energetics has then been complemented by a kinetic study. The results provide new information about important reactive channels operative in different regions of the interstellar medium and in the atmospheres of exoplanets. These further extend the recent general addition/elimination mechanism for the formation of “complex imines” from the reaction of methanimine with a small radical species.

On the various-heating-rates method for evaluating the activation energies of thermoluminescence peaks

The well-known various-heating-rates (VHR) method for evaluating the activation energy of thermoluminescence (TL) peaks is revisited. The method hinges on the shift of the TL peak with changing heating rate, and is based on the properties of first-order curves. Several works have shown that the same method yields a good approximation of the activation energy when general-order peaks are involved. A recent work by Maghrabi has presented a heuristic explicit expression for the magnitude of the shift between two heating rates within the first-order kinetics framework. In the present work, we address two related points. We show how the expression suggested by Maghrabi can be reached by making a very reasonable approximation of the original equation yielding the maximum condition. We also present an alternative expression which yields the amount of shift of the TL maximum with changing heating rate and with less approximation. The other point dealt with involves the results of a numerical study of the evaluation of the activation energy by the use of various heating rates in the more general one-trap-one-recombination-center (OTOR) situation. The results show that even in this general case, the various heating rates method yields very good results. The same is true for the "mixed-order" kinetics. The numerical results are accompanied by an analytical account which shows that the method yields very accurate activation energies in the rather general OTOR situation.

비등온 TGA을 이용한 절연재료의 활성화에너지 신뢰도 향상

Measuring the accurate activation energy of the insulating materials used in the HVDC environment is an essential condition for reliable prediction of the accelerated life of the insulating materials, so it is important to select various measurement models and analyze them. The activation energy of PE and PP was measured through five measurement methods through TGA, and the reliability was improved by comparing the results of each condition of the heating rate, the degree of conversion and the reaction order. Excluding the conditions that cause errors, the measured activation energy was measured to be 2% lower than the existing measurement methods, and it is thought that it will be used as a useful basic data in the field of reliability.

Proposed Model for Shale Compaction Kinetics

Shales are the most abundant class of sedimentary rocks, distinguished by being very fine-grained, clayey, and compressible. Their physical and chemical properties are important in widely different enterprises such as civil engineering, ceramics, and petroleum exploration. One characteristic, which is studied here, is a systematic reduction of porosity with depth of burial. This is due increases in grain-to-grain stress and temperature. Vertical stress in sediments is given by the overburden less the pore fluid pressure, σ, divided by the fraction of the horizontal area which is the supporting matrix, (1−φ), where φ is the porosity. It is proposed that the fractional reduction of this ratio, Λ, with time is given by the product of φ4m/3, (1−φ)4n/3, and one or more Arrhenius functions Aexp(−E/RT) with m and n close to 1. This proposal is tested for shale sections in six wells from around the world for which porosity-depth data are available. Good agreement is obtained above 30–40 °C and fractional porosities less than 0.5. Single activation energies for each well are obtained in the range 15–33 kJ/mole, close to the approximate pressure solution of quartz, 24 kJ/mol. Values of m and n are in the range 1 to 0.8, indicating nearly fractal water-wet pore-to-matrix interfaces at pressure solution locations. Results are independent of over- or under-pressure of pore water. This model attempts to explain shale compaction quantitatively. For the petoleum industry, given porosity-depth data for uneroded sections and accurate activation energy, E, paleo-geothermal-gradient can be inferred and from that organic maturity, indicating better drilling prospects.

Mechanically Responsive Crystals: Analysis of Macroscopic Strain Reveals “Hidden” Processes

Mechanical response of single crystals to light, temperature, and/or force-an emerging platform for the development of new organic actuating materials for soft robotics-has recently been quantitatively described by a general and robust mathematical model ( Chem. Rev . 2015 , 115 , 12440 - 12490 ). The model can be used to extract accurate activation energies and kinetics of solid-state chemical reactions simply by tracking the time-dependent bending of the crystal. Here we illustrate that deviations of the macroscopic strain in the crystal from that predicted by the model reveal the existence of additional, "hidden" chemical or physical processes, such as sustained structural relaxation between the chemical transformation and the resulting macroscopic deformation of the crystal. This is illustrated with photobendable single crystals of 4-hydroxy-2-(2-pyridinylmethylene)hydrazide, a photochemical switch that undergoes E-to-Z isomerization. The irreversible isomerization in these crystals results in amorphization and plastic deformation that are observed as poor correlation between the transformation extent and the induced strains. The occurrence of these processes was independently confirmed by X-ray diffraction and differential scanning calorimetry. An extended mathematical model is proposed to account for this complex mechanical response.

Adsorption behavior of methyl iodide on a silver ion-exchanged ZSM-5

Abstract An organic iodide, especially methyl iodide (CH3I), is one of the most safety concerns against a nuclear power plant severe accident because it is highly toxic and easily inhaled. Due to its high volatility, the dry filtration methods are generally employed to capture the gas phase CH3I. Although it is important to accurately assess the performance of dry filter in a postulated severe accident situation, no sufficiently accurate adsorption kinetic behaviors have been reported yet. In this study, we designed the highly accurate experimental set-up, thermogravimetric analysis-gas chromatography coupled system, which overcame the limitations of previous studies, and performed the CH3I adsorption tests on a silver ion-exchanged ZSM-5 (Ag-ZSM-5) zeolite at a wide temperature range (50–150 °C) under the postulated severe accident condition. The fresh and adsorbed Ag-ZSM-5 samples were characterized by powder XRD, elemental analysis, and SEM-EDS, and the further CH3I adsorption behaviors were discussed in depth. From the overall results, we obtained the accurate adsorption rates and activation energies, and an understanding of chemisorption and physisorption behaviors. Our findings provide the useful fundamentals for a model development of dry filter performance assessment.

Evaluation on activation energy of deposited clay based on thermo-gravimetric analysis (TGA) and four kinetics models

In order to evaluate and compare the accurate activation energy variations of two kinds of deposited clays (i.e. non-organic clay and organic clay), thermo-gravimetric analysis (TGA) was firstly conducted to study the reaction characteristics of the two clay samples. Two periods of weight losses were observed according to each TGA curve for both clay samples. The first period happened due to water evaporation, while the second period was mainly caused by the composition reactions of clay minerals, especially influenced by the de-hydroxylation of kaolinite. An obvious phenomenon is that the organic clay with a high soil organic matter (SOM) has a higher activity energy. Then, four common kinetics models [i.e. the Coats-Redfern method, the Doyle integral method, the maximum rate method, and the distributed activation energy model (DAEM)] were used to calculate and compare the activation energy values. And the Doyle integral method and the DAEM method are considered as two recommended models to analyze the activation energy performance during thermal treatment. Some interesting conclusions can be summarized as: (1) The multi-period reactions of deposited clays during the heating process are common, especially for organic clay. It would be beneficial knowing its biomass resource feasibility concerning the second weight loss; (2) The high SOM has a significant influence on the potential energy unitization of deposited clays, while the variations of activity energy for deposited clays are influenced by the mineral compositions and microstructure features; (3) It is possible to evaluate the accurate activity energy of deposited clays combined with TGA tests and two recommended models (i.e. the Doyle integral method and DAEM method).

Thermogravimetric Characteristics and Non-isothermal Kinetics of Macro-Algae With an Emphasis on the Possible Partial Gasification at Higher Temperatures

Pyrolysis of Turbinaria ornata was realised in a thermogravimetric analyzer. The process was crudely classified into primary and secondary reaction zones. In the primary reaction zone, the thermal decompositions of the low-thermal stable components produced volatiles and biochar. The solid products obtained from the primary decomposition reactions contained inorganics with heavy metals. At mild-to-high temperatures, the catalytic effects accompanied gasification using oxygen, which was partially supplied by the oxygen carriers present in the solids and evolved gases. In order to study the pyrolytic conversion, combined and multiple reaction schemes were employed. While the model-free methods helped to provide the accurate activation energies and the initial value of the pre-exponential factor, the non-linear regression optimized the chosen model parameters. In this study, a simple order-based model was compared with the versatile Sestak-Berggren (SB) model considering combined and multiple reactions. The application of a multiple reaction scheme to the primary and secondary reaction zones concluded that a simple order-based model suffices for the kinetic analysis. The secondary decomposition was shown to start with a high activation energy, which decreased appreciably when the conversion proceeded towards completion.

Comparative kinetic study of the pyrolysis of mandarin and pineapple peel

This study focuses on the pyrolysis processes of fruit peels as energy sources. These pyrolysis processes for mandarin and pineapple peels were analyzed by profiles of conversion vs. temperature and DTG curves. Five kinetic stages were observed, each with a different activation energy calculated by utilizing the Coats and Redfern method and assuming a first order model. The mandarin peel process showed a higher activation energy during the first and second stages of the pyrolysis. Consequently, mandarin peel was selected over pineapple peel for further studies. Other models such as diffusion and nucleation also fitted the data and as a result, the nonlinear integral isoconversional method was implemented to get more accurate activation energies. In the range of 350 and 460°C, there was a rapid decrease in activation energy that was attributed to the radical recombination as the dominant mechanism. Experiments of pyrolysis with the mandarin peel were carried out in the pyrolytic reactor to verify these findings. The pyrolysis oils obtained at 460 and 600°C were analyzed by GC⿿MS and FTIR Spectroscopy. The results showed that the pyrolysis oil obtained at 460°C had a higher average degree of unsaturation. Once the pyrolysis went through temperatures higher than 460°C, the average degree of unsaturation decreased. Although the oil obtained at 460°C showed a higher average degree of unsaturation, it was not enough to show a substantially higher calorific value than the oil obtained at 600°C. The significant difference in the calorific values could be potentially observed in the biochars obtained at the temperatures already mentioned.

Reducing the effects of noise in the calculation of activation energy by the Friedman method

The Friedman method for analysis of kinetic parameters of solid-state reactions has been widely used. One of the drawbacks of this method is its sensitivity to noise which can be caused by both experimental error and the intrinsic inaccuracy of the differential methods, particularly for non-isothermal kinetic data obtained by means of thermogravimetric analysis (TGA). This paper proposes a modified version of the Friedman method in order to decrease the effect of noise and consequently obtain more accurate activation energy values. The new method is based on the idea of considering, for a given value of the extent of conversion α, not only information corresponding to α but also data in its neighbourhood. To check the procedure, it is applied to simulated data and the activation energy obtained is compared with the preset value and that given by the Friedman method. Moreover, the proposed method is used to determine the activation energy on the thermal degradation of γ-Ti(PO4)(NH4HPO4) compound as a function of the extent of conversion.

C-2020: Determination of membrane permeability of porcine adipose derived-stem cell

Adipose-derived stem cells (ADSCs), as an abundant and accessible source of adult stem cells, self-renew and differentiate into various cell lines and have promising potential in tissue engineering and as an alternative to the use of regenerative medicine strategies. In order to meet clinically urgent demand of ADSCs, the cell cryopreservation was applied to the ADSCs long-term preservation. To optimize the cryoprotective agents (CPAs) addition, removal and the freezing as well as thawing processes, we measured the transport properties of porcine adipose-derived stem cells (pADSCs). In this study, the microperfusion system was used to measure transport properties of porcine adipose-derived stem cells (pADSCs) membrane. The membrane permeability coefficient to water (Lp), the membrane permeability coefficient to Me2SO, EG and glycerol (Ps), and the activation energies and reference membrane coefficient have been investigated at 28, 18, 8 and −2 °C, respectively. We determined the inactive cell (Vb), the hydraulic conductivity (Lp) and the CPAs permeability (Ps) at various temperature in 2-paramerter (2 − p) model. No significant difference in the Me2SO and EG permeability was obtained at experimental temperature and the effect of the temperature on the transport properties of the pADSC membrane was analyzed. However, the glycerol permeability of pADSCs was too small to be precisely measured. We obtained the parameter of pADSCs with 1.5 M glycerol at 32 °C and determined the accurate activation energies of glycerol permeability and the referent of glycerol permeability. Membrane permeability of porcine Adipose Derived-Stem cell quantitatively measured was used to minimize the excessive volume shrinkage and the toxicity with CPAs addition.

Negative surface ionization electron affinities and activation energies of SFn

Sulfur hexafluoride (SF6 ) is the most frequently used standard for anion mass spectrometry because of its large temperature-independent cross-section for electron attachment. However, the kinetic and thermodynamic properties--the products of the reactions, and the number of negative ion states of the SF(n)--are presently in dispute. The electron affinities for SF(n) (n = 1-6) are predicted by assuming dissociation energies and ground-state electron affinities based on literature values. The temperature dependence of the 2012 negative surface ionization mass spectrometer data, and other negative ionization mass spectrometry, electron capture detector, and beam and magnetron data, are analyzed using a kinetic model. More precise and accurate activation energies for thermal electron attachment and electron affinities of SF6 , SF5 and SF4 are reported. The largest experimental electron affinities of SF, SF2 and SF3 are assigned to the ground states. Ionic Morse potentials for multiple states are calculated. A mechanism for the formation of the ionic products observed in negative surface ionization thermal electron attachment to SF6 is presented. Negative surface ionization on a hot filament with mass spectrometry is a relatively simple and effective method for determining electron affinities similar to the electron capture detector, magnetron, swarm and beam procedures. A new method of predicting the number of negative ion states from dissociation limits establishes targets for the data analysis. Pseudo one-dimensional anionic Morse potentials illustrate the mechanism for the reaction of thermal electrons with SF6 and the consecutive dissociation pathways.