Entropy Of Activation Research Articles

Product specific thermal degradation kinetics of bisphenol F epoxy in inert and oxidative atmospheres using evolved gas analysis–mass spectrometry

Knowledge of the degradation kinetics for polymer materials is important for understanding thermal stability. Here, evolved gas analysis–mass spectrometry and pyrolysis gas-chromatography-mass spectrometry were evaluated for the potential to deliver additional insight into thermal degradation kinetics of diglycidal ether of bisphenol F (DGEBF) epoxy thermoset under inert and oxidative atmospheres. Degradation products of selected precursor ions were evaluated for their uniqueness to the specific precursor using extracted ion thermographs. Unique mass peaks, solely attributed to a single reaction pathway of a specific product, were determined from extracted ion thermographs and used to determine both activation energy (Ea) and pre-exponential factors for the specific primary reaction pathways. These primary reaction pathways for DGEBF epoxy degradation were then evaluated in the context of transition state theory (TST) and related transition state enthalpies (ΔH‡) and entropies (ΔS‡) of activation to further elucidate the degradation process. It was determined under pyrolysis conditions, as suggested by the Ea, the formation of bisphenol F monomer was the rate-limiting step toward the formation of xanthene and phenol. In contrast, under thermo-oxidative conditions, reactions involving oxygen containing species were identified as the rate-limiting step for all observed products based on the large negative ΔS‡ calculated from TST. This work demonstrates a powerful combination of technique and theory that can provide new insight into the degradation of polymer materials.

Malonic Anhydrides, Challenges from a Simple Structure.

After many years of unsuccessful attempts, monomeric malonic anhydrides were prepared by ozonolysis of ketene dimers, a procedure validated by model studies. The structure proof relied most heavily on IR absorption at 1820 cm-1 and a Raman band at 1947 cm-1. Malonic anhydrides are unstable, decomposing below room temperature to a ketene plus carbon dioxide. Surprisingly, according to kinetic studies, the dimethyl derivative is slightly less unstable than the parent, and the monomethyl is the fastest to decompose, with an enthalpy of activation of only 12.6 kcal/mol. Computations rationalize this behavior in terms of a concerted [2s + 2a] cycloreversion that requires a more highly organized transition state, as also manifested by a negative entropy of activation.

Breaking scaling relationships in alkynol semi-hydrogenation by manipulating interstitial atoms in Pd with d-electron gain

Pd catalysts are widely used in alkynol semi-hydrogenation. However, due to the existence of scaling relationships of adsorption energies between the key adsorbed species, the increase in conversion is frequently accompanied by side reactions, thereby reducing the selectivity to alkenols. We report that the simultaneous increase in alkenol selectivity and alkynol conversion is achieved by manipulating interstitial atoms including B, P, C, S and N in Pd catalysts. A negative linear relationship is observed between the activation entropies of 2-methyl-3-butyn-2-ol and 2-methyl-3-buten-2-ol which is highly related to the filling of d-orbital of Pd catalysts by the modification of p-block elements. A catalyst co-modified by B and C atoms has the maximum d charge of Pd that achieves a 17-fold increase in the turn-over frequency values compared to the Lindlar catalysts in the semi-hydrogenation of 2-methyl-3-butyn-2-ol. When the conversion is close to 100%, the selectivity can be as high as 95%.

Effect of γ-irradiation on thermally stimulated depolarization current spectra of polyethylene-grafted-poly(Acrylic acid)

The graft copolymerization process of polyethylene (PE) with acrylic acid is performed using a 60Co-gamma irradiator, and the samples are irradiated at different doses, 30, 50, 70 and 100 kGy with a dose rate of 0.55 Gy/s. Global thermally stimulated discharge current (TSDC) spectrum of irradiated and non-irradiated PE-g-AAc has been investigated from RT to ∼ 440K. The TSDC spectrum of non-irradiated PE-g-AAc exhibited two relaxation processes. The first process is located at ∼ 329 K, and the second one is at ∼390 K. These processes are called dipolar and space charge relaxation, i.e. α- and ρ-relaxation, respectively. On the other hand, the position of these processes is shifted towards a higher temperature for irradiated samples. The activation energy for α-relaxation process of non-irradiated samples is ∼ 0.21 eV, while for irradiated samples, it is 0.34 eV. The thermal sampling technique (TS) has been applied to decompose the global TSDC spectrum of all samples into its elementary peaks. The molecular parameters, such as activation energy (Ea ), pre-exponential factor (τ 0), enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG), are estimated for all samples. The compensation phenomenon has been verified for all samples in different ways, such as plotting ln τ 0 against Ea or entropy versus enthalpy.

Industrial process monitoring based on optimal active relative entropy components

The multivariate statistic method has been widely applied, but no clear mapping relationship exists between the latent variables and the fault information, which leads to various information in different latent variables. The traditional methods cause the loss of useful fault information and the decline of monitoring performance. In this paper, a process monitoring method based on the optimal active relative entropy component is proposed. Based on global and local preserving projection, relative entropy is selected as the monitoring statistic. The influence of latent variable selection on relative entropy calculation is analyzed, and a relative entropy activity index is proposed to select the relative entropy component beneficial to process monitoring. The parameters are optimized to generate the optimal active relative entropy component for process monitoring using the mayfly algorithm. Finally, a numerical case, Tennessee Eastman (TE) process, and three-phase flow facility are used to verify the superiority of the monitoring algorithm.

Resolving Activation Entropy of CO Oxidation under the Solid–Gas and Solid–Liquid Conditions from Machine Learning Simulation

Resolving Activation Entropy of CO Oxidation under the Solid–Gas and Solid–Liquid Conditions from Machine Learning Simulation

P(HMA-co-ATU) hydrogel synthesis via gamma radiation and its use for in situ metal nanoparticle preparation and as catalyst in 4-nitrophenol reduction

Hydrogels with reactive thiourea functional groups were prepared by radiation-induced crosslinking of N-(Hydroxymethyl) acrylamide (HMA) and N-allyl thiourea (ATU) at different ATU content and irradiation doses. P(HMA-co-ATU) hydrogels were then utilized as template for in situ metal nanoparticle (MNP) preparation by the reduction of Ni2+, Co2+, and Cu2+ ions within the hydrogel matrix using sodium borohydride (NaBH4) as reducing agent. These MNP@p(HMA-co-ATU) hydrogel composites (MNP = Co, Ni, and Cu) were further used as catalysts in the reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (AP). Various parameters such as the effect of ATU content, the total dose used in the hydrogel preparation, MNPs type, and temperature on the catalytic activity of hydrogel composites were investigated. The activation energy, enthalpy, and entropy of Ni@p(HMA-co-ATU) hydrogels for the reduction reaction of 4-NP to 4-AP were calculated as 42.5 kJ mol−1, 38.9 kJ mol−1, and −187.3 J mol−1 K−1, respectively.

Macromolecular Crowding Slows Glutamate Dehydrogenase Kinetics

Most of our understanding of enzyme kinetics is based on experiments conducted in dilute solution, which does not accurately represent crowded cellular environments. Cells contain high concentrations (300‐400g/L) of various macromolecules such as proteins, carbohydrates and ribosomes. This crowding reduces the volume of solvent available for other molecules in the solution and has previously been shown to impact the behavior of enzymes. To study these potential consequences, the Michaelis‐Menten kinetics of glutamate dehydrogenase (GDH) was monitored in the presence of high concentrations of “crowding agents” such as dextran, a glucose polymer. Given this enzyme’s central role at interface of nitrogen and carbohydrate metabolism, GDH is highly regulated. Our results suggest that crowding may influence this regulation. For example, leucine is a less effective activator of GDH in the presence of dextran. Furthermore, crowding enhances substrate inhibition of GDH in a pH‐dependent manner. The resulting effects from crowding were substrate dependent such that the presence of dextran decreased the Vmax more with the NADP+ cofactor than with NAD+. Both dextran and BSA decreased the Km of NADP+ but did not affect NAD+. The effects of crowding on the enthalpy (ΔH҂) and entropy (ΔS҂) of activations determined from Eyring plots also were also substrate and pH‐dependent. Overall, these results highlight the importance of studying enzymes especially heavily regulated ones like GDH under crowded conditions that more realistically mimic the crowded cellular environment.

Silver-Cobalt bimetallic nanoparticles to the generation of hydrogen from formic acid decomposition

The fabrication of cost effective heterogeneous catalysts for the release of hydrogen from hydrogen storage materials is the key technological challenge to the fuel-cell based hydrogen economy. Ag-Co bimetallic NPs were fabricated by using sodium dodecyl sulphate (SDS) and sodium borohydride as stabilizing and reducing agents, respectively. Catalytic activity of SDS-Ag-Co was higher than that of SDS free Ag-Co and monometallic (Ag and Co) due to the electron interactions and synergistic effect between the Ag and Co. Ag 10 -Co 90 exhibited the superior catalytic activity, with rate constant of 5.2 × 10 −4 s −1 at 303 K, activation energy of 46 kJ/mol, activation enthalpy of 44 kJ/mol, activation entropy of −165 J/K/mol, and turn over frequency of 240 h −1 . The hydrogen generation rate increased and decreased with in increasing temperature, molar concentration of formic acid, presence of sodium formate, and pH, respectively. The kinetic rate equation can be stated as: rate = -d[formic acid]/dt = k obs [formic acid] 0.99 [Ag 50 Co 50 ] 0.98 , k obs = 2.48 × 10 7 exp (−5637.96/RT).

Al2O3 microrods supported Pd catalysts for semi-hydrogenation of acetylene: Acidic properties tuned reaction kinetics behaviors

The acidic properties of the support show remarkable effects on Pd-catalyzed reactions including the semi-hydrogenation of acetylene. Herein, Al2O3 microrods with distinct acidic properties are synthesized as the supports for Pd catalysts to address the acidic properties dominated kinetics behavior of acetylene hydrogenation. The acidic properties of Al2O3 microrods decrease with the calcination temperature, in the order of Al2O3-1400 < Al2O3-1200 < Al2O3-800, in which the numbers denote the temperature for calcination process. Catalytic performance show that the hydrogenation activities of the corresponding Pd catalysts increase by Pd/Al2O3-1400 < Pd/Al2O3-800 < Pd/Al2O3-1200, while the selectivities to the target ethylene decrease by Pd/Al2O3-1400 > Pd/Al2O3-1200 > Pd/Al2O3-800. A quantitative analysis combined with structure characterizations are further performed to elucidate the underlying origin of such acidic-property dependent performances. It is demonstrated that the different electronic structures of Pd caused by the acidic properties contribute to distinct adsorption properties for the reactant and the target products, which can be quantitatively described by the entropy of activation (ΔS*).

Kinetic insights into the effect of promoters on Co/Al2O3 for Fischer-Tropsch synthesis

The fundamentals of promoter effects in the Fischer − Tropsch synthesis over Co-based catalysts were systematically investigated by employing the Rh, Ir, Sb and Ga as promoters of Co/Al2O3. Various characterizations showed the higher dispersion and reducibility over Rh- and Ir- promoted catalysts would cause a promoted adsorption property, while the Sb- and Ga- promoted catalysts showed a contrary result because the formation of alloy suppressed the accessibility of Co species. Based on the experimental results and kinetic calculations, a linear relationship for the activation energies of these catalysts as a function of activation entropies was proposed. Besides, steady state isotopic transient kinetic analysis (SSITKA) revealed that promoters mainly affected the surface CHx concentration to alter the CO reaction rate, and the chain growth rate constants would affect the product distribution. Since CoRhA and CoIrA greatly enhanced the CHx concentration and inhibited the chain growth ability, they owned a higher CO reaction rate and CH4 selectivity. While the CoSbA and CoGaA suppressed the CHx concentration and intrinsic activity, but enhanced the chain growth ability, thereby revealing a lower CO reaction rate and CH4 selectivity. Furthermore, the reactivity and selectivity difference were attributed to the modification of adsorption strength dictated by the enthalpy of activation on cobalt catalysts with different promoters. These findings provide kinetic insights into the promoter effects and pave the way for optimizing catalysts.

Effect of morphology on the isothermal decomposition kinetics of nitroguanidine

The thermal stability of nitroguanidine (NQ) with different morphologies was studied using a self-developed isothermal decomposition gas manometric device at both high and low temperatures. At 443.15–463.15 K, the total decomposition of needle-like NQ (NNQ) and hexagonal block-shaped NQ (HNQ) proceeded in two stages: a fast decomposition stage after heating, and a much slower decomposition stage later. It was indicated that all the models for NNQ and HNQ at the first stage followed the Avrami-Erofeev equation but with different values of n, while all the models at the second stage conformed to Parabola order. The decomposition rate of HNQ was faster than NNQ after heating, showing a better stability of NNQ. At 373.15–413.15 K, the initial decomposition kinetics were calculated. The activation energy calculated by mode-fitting method was 146.33 kJ/mol and 182.71 kJ/mol for NNQ and HNQ, respectively, which was consistent with the results obtained through model-free method. The average activation free energy (ΔG≠), enthalpy (ΔH≠) and entropy (ΔS≠) for NNQ were 148.30 kJ/mol, 142.03 kJ/mol, and -15.81 J/(mol•K), respectively, while they were 139.61 kJ/mol, 184.42 kJ/mol, and 107.14 J/(mol•K) for HNQ. The storage life of NNQ and HNQ at 298.15 K were extrapolated to 21.58 and 29.44 years, respectively.

Excess properties and activation properties for viscous flow of amino acid ionic liquids with H2O binary mixtures

Amino acid ionic liquids (ILs) 1-butyl-2,3-dimethylimidazolium glycine ([C4Dmim][Gly]), 1-butyl-2,3-dimethylimidazolium alanine ([C4Dmim][Ala]) and 1-butyl-2,3-dimethylimidazolium threonine ([C4Dmim][Thr]) were prepared. Over the temperature from (288.15–318.15) K, the density, surface tension and viscosity for binary system mixtures of ILs with H2O were measured, and excess thermodynamic properties and activation properties for viscous flow of these binary system mixtures were systematically studied. Secondly, the variation trend between excess molar volume (VE)/excess molar surface Gibbs energy (gsE) and mole fraction for ILs + H2O binary system mixtures was investigated. With the increase of mole fraction, the VE and gsE values of binary system mixtures increased firstly and then decreased, and they were positive value, which may be due to the weak chemical interaction between ILs and H2O in the mixture. Furthermore, according to the thermodynamic formula, the Gibbs energy of activation for viscous flow of the relative viscosity (ΔG≠r), activation entropy (ΔS≠r) and activation enthalpy (ΔH≠r) were calculated. Over the entire concentration range, the ΔG≠r values for binary system mixtures of ILs with H2O were negative value, and the result was in good agreement with relative viscosity (ηr).

Temperature dependence of the formation of the g ~ 5 EPR signal in the oxygen evolving complex of photosystem II.

The temperature dependence of the formation of the g ~ 5 S2 state electron paramagnetic resonance (EPR) signal in photosystem II (PSII) was investigated. The g ~ 5 signal was produced at an illumination above 200K. The half inhibition temperature of the formation of the g ~ 5 EPR signal was approximately 215K. The half inhibition temperature is close to that of the transition from the S2 state-to-S3 state in the untreated PSII, and not to that of the transition from S1 state -to-S2 state in the untreated PSII. The upshift of the half inhibition temperature of the transition from the S1 state -to-S2 state (g ~ 5) reflects the structural change upon transition from the S1 state to the S2 state. The activation energy of the g ~ 5 state formation was estimated as 40.7 ± 4.4kJ/mol, which is comparable to the reported activation energy for the S2 formation in the untreated PSII. The activation enthalpy and entropy were estimated to be 39.0 ± 4.4kJ/mol and -103 ± 19J/mol K at 210K, respectively. Based on these parameters, the formation process of the g ~ 5 state is discussed in this study.

Thermal Behavior of Heavy Oil Catalytic Pyrolysis and Aquathermolysis

There is still considerable controversy surrounding the mechanisms, thermodynamics, and kinetics of heavy oil aquathermolysis and pyrolysis processes. The present paper aims to widen our knowledge about the effect of iron tallates on pyrolysis and aquathermolysis of Cuban heavy oil. The obtained SARA (S: saturates, A: aromatics, R: resins, A: asphaltenes) analysis has shown a significant increase in light hydrocarbon content during aquathermolysis. Moreover, the elemental analysis has indicated an increase in C and H content by almost 4% and 6%, respectively, with a significant decrease in S and O content by up to 23% in the presence of iron tallates. These results have been further confirmed by infrared spectrometry. The obtained IR data indicated that asphaltene and resin compounds transform into light hydrocarbons after aquathermolysis. On another hand, the activation energy of heavy oil pyrolysis decreased in the presence of the utilized catalyst; meanwhile, the reaction rate increased, especially in the temperature range of 200–480 °C, which may validate a significant effect of the used catalyst in real conditions. Moreover, the obtained thermodynamic data showed a decrease in the enthalpy and entropy of activation of oil pyrolysis in the presence of iron tallates. Our results are encouraging in terms of energy consumption, optimization, and process control and should be validated by a larger sample size.

Synthesis and characterization of ZnO nanoparticles for the production of biodiesel by transesterification: Kinetic and thermodynamic studies

Under sol–gel conditions, a stable zinc oxide powder with nanoparticle morphology was synthesized utilizing zinc nitrate as a precursor, EDTA as a complexing agent, and citric acid as a stabilizing agent. Zinc oxide nanoparticles were characterized utilizing XRD, SEM, BET, EDXRF, FT-IR, and UV–Vis techniques. The catalytic activity of the ZnO nanoparticle catalyst was successfully evaluated by carrying out a transesterification reaction in sunflower oil (SFO). To establish the optimal conditions for biodiesel synthesis, the transesterification process was carried out under various reaction parameters. Several oil-to-metal ratios (1:10, 1:20, and 1:30) were investigated at various times ranging from 1 to 4 h in a reaction operated at temperatures of 60, 65, and 70 °C. The results indicate that ZnO catalysts show good performance in biodiesel production from sunflower oil and provide a biodiesel yield of 71%. under optimal reaction conditions, i.e., methanol to oil molar ratio of 20:1, catalyst amount of 4.7 wt%, and a reaction temperature of 70 °C for 3 h. The catalytic activity can be attributed to the existence of an optimal number of catalytically active sites on the surface of the catalyst. The composition analysis of the synthesized biodiesel was studied using different characterization techniques, such as GC–MS and FT-IR, to identify the FAMEs. The kinetics of the transesterification reaction indicate a pseudo-first-order reaction, and the rate of the reaction is calculated. Thermodynamic functions such as activation energy, heat content, entropy, and Gibbs free energy were determined. The physical and chemical properties of synthetic biodiesel, such as density, viscosity, flash point, pour point, acid value, and water content, were studied and compared against international standards.

Study of the Interaction Between Reduced Graphene Oxide and NO2Gas Molecules via Density Functional Theory (DFT)

Electronic properties such as density of state, energy gap, HOMO (the highest occupied molecular orbital) level, LUMO (the lowest unoccupied molecular orbital) level and density of bonds, as well as spectroscopic properties like infrared (IR), Raman scattering, force constant, and reduced masses for coronene C24, reduced graphene oxide (rGO) C24O5and interaction between C24O5and NO2gas molecules were investigated. Density functional theory (DFT) with the exchange hybrid function B3LYP with 6-311G** basis sets through the Gaussian 09 W software program was used to do these calculations. Gaussian view 05 was employed as a supplementary software to investigate the geometrical structure of C24, C24O5and the interaction of C24O5with NO2gas molecule. It shows the energy gap of coronene C243.5 eV because the effect of quantum confinement and the Coulomb interaction geometry greatly influence the quasi-particle band gap and C24Oxwhere x = 1–7 was from 0.89 eV to 1.6862 eV a function of number of oxygen atoms and compared with the experiment value of graphene oxide which was between 1 eV and 2.2 eV. The spectroscopic properties were compared with the experiment value of graphene, graphene oxide and NO2longitudinal optical (LO) modes of 1 585, 1 582 and 1 600 cm−1, respectively. The transition state of the interaction of rGO with nitrogen dioxide and Gibbs energy, enthalpy, activation entropy and reaction at various temperatures between 25°C and 100°C were calculated. The activation energy of C24O5with nitrogen dioxide decreases with increasing temperature.

Indications of ion dehydration in diffusion-only and pressure-driven nanofiltration

Recent insights into the permeation of small ions through polyamide membranes highlighted the role of ion dehydration in the transport and selectivity observed. However, such insights were a by-product of studies exploring different transport phenomena without a systematic methodology that focuses on ion dehydration itself. In this study, we quantified the intrinsic permeability and its underlying Eyring's enthalpy and entropy of activation for a systematic set of cations and anions in polyamide nanofiltration membranes to gain better understanding of ion dehydration. Our results in a diffusion-only system expose a distinct correlation between the hydration strength of the ions and the enthalpic barrier they experience during permeation, with a permeability order that is similar to the mobility order in solution. Smaller entropic losses are observed for smaller bare ions, indicating their higher freedom of motion after dehydration compared to larger bare ions. Comparing the measured activation enthalpies to quantum chemical calculations for the ions' hydration in solution suggests that ions are only partially dehydrated. Last, our simulated ion permeability at higher temperatures and experimental results in a pressure-driven system expose higher permeation rates with reversed ion-ion selectivity, indicating the activation of ion transport in the form of ion dehydration when additional energy is supplied. • Transport of monovalent ions is explored in diffusion-only and pressurized NF. • More strongly hydrated ions face higher enthalpic barrier during permeation. • Smaller bare ions face lower entropic losses during permeation. • Experimental barriers correlate to simulated stripping of 1–3 water molecules. • Diffusion-only and pressurized filtration expose reversed ion-ion selectivity.

Kinetics and characterization of preparing conductive nanofibrous membrane by In-situ polymerization of Polypyrrole on electrospun nanofibers

Polypyrrole (PPy) is a conductive polymer that can be fabricated into various structures via in-situ polymerization for different applications. However, a kinetics study of its in-situ polymerization is needed for a better understanding of the mechanisms of pyrrole polymerization. This paper develops two different kinetics models based on existing PPy polymerization mechanisms. The most suitable mechanism is first identified by comparing the model results with experimental data. Then, in-situ polymerizations of PPy on electrospun fibrous polyacrylonitrile (PAN) templates were produced at temperatures ranging from 273 to 285 K. The results show that the overall reaction rate of the in-situ polymerization process in the presence of electrospun fibrous template is faster than that without the template. Further investigation confirms that the increase in the overall reaction rate is attributed to the enhanced reactions between oxidized pyrrole oligomers and neutral pyrrole monomers, as the corresponding reaction rate constant and activation entropy increased by 158.2–209.6% and 37.7%, respectively. The reason is that the considerable number of available reaction sites between oxidized oligomers and neutral monomers are created by the fibrous template. In addition, the composite membrane obtained from the in-situ polymerization exhibited well-developed fibrous nanostructure, good electrical conductivity (0.29 ± 0.04 S∙m−1), sufficient mechanical strength (UTS of 8.7 MPa) and relatively stable thermal properties (up to 250 °C).

Aziridine strategy for stereospecific synthesis of 1′-alkyl/aryl-5′-aryl-2′,5′-dihydropyrrolofullerene-2′-carboxylates and NMR study of hindered 5′-aryl group rotation

A reaction of aziridines with fullerene C60 is shown to provide a stereospecific route to fulleropyrrolidines for a diverse range of substituents. The important limitation of the approach is shown to be the synthesis of N-unsubstituted fulleropyrrolidines, where the imine route described elsewhere suits better. For 2-arylsubstituted fulleropyrrolidines the activation free Gibbs energy, enthalpy and entropy of a hindered aryl group rotation are determined based on the line-shape analysis of temperature dependent NMR spectra. ortho-Fluorophenyl substituted pyrrolofullerenes are shown to have the activation barrier of ca. 16 kcal/mol, which poses these compounds as possible hindered rotors in molecular machines application.