Five-step Mechanism Research Articles

Enhancing biochar oxidation reaction with the mediator of Fe2+/Fe3+ or [formula omitted]/[formula omitted] for efficient hydrogen production through biochar-assisted water electrolysis

The biochar-assisted water electrolysis (BAWE) is a promising method for clean biomass utilization and hydrogen production. But it is limited by the slow anode biochar oxidation reaction (BOR). To enhance the BOR, the mediator of Fe2+/Fe3+ or NO2-/NO3- was added to the pyrolytic biochar (PB) and tested using electrochemical methods. Both Fe2+ and NO2- showed stronger oxidation activity than Fe3+ and NO3-, and thereby presented more significant enhanced role on BOR, including the reduction in onset potential and the enlargement of current density. Fe2+/Fe3+ existence resulted in the five-step mechanism such as adsorption, redox reaction in solution, oxidation at electrode surface, and passive films formation on the biochar surface. 0.5 mol/L or less of Fe2+/Fe3+ was suggestable, while the lower acid concentration was necessary to avoid side reaction with NO2- existence. The Fe2+/Fe3+ acted as catalysts due to the almost unchanged total iron ions during continuous BAWE process, whereas a high consumption of NO2- occurred. The cathode hydrogen production was less influenced by mediator and showed comparable with the theoretical value of Faraday’s law. In addition, the activation energy of BOR was significantly reduced with Fe2+ and NO2- mediation. The present work provides a favorable strategy for improving BOR and energy-saving production of hydrogen via BAWE process.

One-Pot Multicomponent Synthesis of 2H-Chromene Derivative, Kinetics, and Thermodynamic Studies Using a Stopped-Flow Technique in Combination with Theoretical Computation Methods

The diethyl 6-bromo-2H-chromene-2,3-dicarboxylate (4) was successfully synthesized by the reaction between triphenylphosphine (1), diethyl acetylenedicarboxylate (DEAD, 2) and 5-bromo-2-hydroxybenzaldehyde acid (3) in the presence of dichloromethane. The 2H-chromene’s compound were characterized using IR, 1H, 13C, and 31P NMR spectroscopy. The kinetics and mechanism of the reaction were experimentally investigated by stopped-flow technique and UV–vis spectrophotometry. The first step in the proposed five-step mechanism, the reaction between 1 and 2 for the formation of intermediate 1 (I1), was fast. The reaction of I1 and 3 was slow and followed first-order kinetics. The experimental kinetic data proved that the step4 of the proposed mechanism is a rate-determining step. To make a further understanding about the proposed mechanism, theoretical investigations were carried out at the B3LYP/6-31++g(d,p) level of theory. The calculations showed that the step4 (Δ 82.1 kJ/mol) is in terms of energy undesirable and difficult in line with experimental results, and is an endothermic process. It also cleared that why I3 with diethyl groups in step4 tends to convert to the I4 very slowly through a four-member ring formation in comparison with other steps and ultimately it becomes impossible process (step4) when t-But groups are replaced instead of diethyl groups within the structure of I3. The experimental and theoretical data indicated that the overall reaction rate follows second-order kinetics and depends on the concentrations of compounds 1 and 2.

Numerical optimization of laboratory combustor geometry for NO suppression

In this article, geometry optimization of a jet stirred reactor (JSR) combustor has been carried out for minimum NO emissions in methane oxidation using a combined numerical algorithm based on computational fluid dynamics (CFD) and differential evolution (DE) optimization. The optimization algorithm is also used to find a fairly accurate reduced mechanism. The combustion kinetics is based on a five-step mechanism with 17 unknowns which is obtained using an optimization DE algorithm for a PSR–PFR reactor based on GRI-3.0 full mechanism. The optimization design variables are the unknowns of the five-step mechanism and the cost function is the concentration difference of pollutants obtained from the 5-step mechanism and the full mechanism. To validate the flow solver and the chemical kinetics, the computed NO at the outlet of the JSR is compared with experiments. To optimize the geometry of a combustor, the JSR combustor geometry is modeled using three parameters (i.e., design variables). An integrated approach using a flow solver and the DE optimization algorithm produces the lowest NO concentrations. Results show that the exhaust NO emission for the optimized geometry is 10.3% lower than the original geometry, while the inlet temperature of the working fluid and the concentration of O2 are operating constraints. In addition, the concentration of CO pollutant is also much less than the original chamber.

Failure of Alloy 20 nozzles used for spraying sulfuric acid in an incineration furnace

Alloy 20 nozzles, which were used for spraying sulfuric acid into an incineration furnace of a petrochemical plant, failed prematurely after 7days of service at approximately 1453K. The material for manufacturing the nozzles complied with Alloy 20 (ASTM B473 UNS N08020) specification. Investigation by optical microscopy, scanning electron microscopy and metallographic analysis showed that the nozzles failed by erosion corrosion action of concentrated sulfuric acid. Five-step mechanisms of erosion corrosion damage were proposed. Using SiC or Si3N4 as materials for coating or fabricating the nozzle part could potentially alleviate erosion corrosion problem.

DFT study of the single electron transfer mechanisms in Ni-Catalyzed reductive cross-coupling of aryl bromide and alkyl bromide

Ni-catalyzed reductive cross-coupling reactions of electrophilic regents provide an important method to form C–C bonds. The present study explored several single electron transfer mechanisms for Ni-catalyzed reductive cross-coupling of aryl bromide and secondary alkyl bromide using Density Functional Theory (DFT) calculations. The results showed that two of the proposed mechanisms were feasible. One was a six-step catalytic cycle including oxidative addition, reduction, radical production, radical addition, reductive elimination and catalyst regeneration. The other was a five-step mechanism involving radical production, reduction, oxidative addition, radical addition, and reductive elimination. The rate-limiting step for both mechanisms was the radical addition step with the energy barrier of 10.42 kcal/mol. All DFT calculations were implemented in the gas phase.

Catalytic Mechanism of the Glycyl Radical Enzyme 4-Hydroxyphenylacetate Decarboxylase from Continuum Electrostatic and QC/MM Calculations

Using continuum electrostatics and QC/MM calculations, we investigate the catalytic cycle of the glycyl radical enzyme 4-hydroxyphenylacetate decarboxylase, an enzyme involved in the fermentative production of p-cresol from tyrosine in clostridia. On the basis of our calculations, we propose a five-step mechanism for the reaction. In the first step, the substrate 4-hydroxyphenylacetate is activated by an unusual concerted abstraction of an electron and a proton. Namely, Cys503 radical abstracts an electron from the substrate and Glu637 abstracts a proton. Thus in total, a hydrogen atom is abstracted from the substrate. In the second step, the carboxylic group readily splits off from the phenoxy-acetate radical anion to give carbon dioxide. This decarboxylation step is coupled to a proton transfer from Glu637 back to the phenolic hydroxyl group which leads to a p-hydroxybenzyl radical. The remaining steps of the reaction involve a rotation of the Cys503 side chain followed by a proton transfer from Glu505 to Cys503 and a hydrogen atom transfer from Cys503 to the p-hydroxybenzyl radical to form p-cresol. The calculated mechanism agrees with experimental data suggesting that both Cys503 and Glu637 are essential for the catalytic function of 4-hydroxyphenylacetate decarboxylase and that the substrate requires a hydroxyl group in para-position to the acetate moiety.

A Global Kinetic Model for the Combustion of the Evolved Gases in Wildland Fires

The analysis of combustion kinetics in the gas-phase is decisive for wild land fire behavior modeling. However, the use of detailed reaction mechanisms, which involves a large number of species and reactions, is impractical due to large computational time requirements. The present work proposes a five-step chemical kinetic mechanism to simulate the gas phase combustion processes taking place in wildland fires. Both experimental data and data from simulations run using the PSR code from the CHEMKIN-II package with a detailed kinetic mechanism (GDF-kin 3.0) have been used to calibrate and evaluate the global model under typical wild land fire conditions in terms of the inlet mixture composition, equivalence ratio, and range of temperatures.

Single Turnover Kinetics of Tryptophan Hydroxylase: Evidence for a New Intermediate in the Reaction of the Aromatic Amino Acid Hydroxylases

Tryptophan hydroxylase (TrpH) uses a non-heme mononuclear iron center to catalyze the tetrahydropterin-dependent hydroxylation of tryptophan to 5-hydroxytryptophan. The reactions of the TrpH.Fe(II), TrpH.Fe(II).tryptophan, TrpH.Fe(II).6MePH(4).tryptophan, and TrpH.Fe(II).6MePH(4).phenylalanine complexes with O(2) were monitored by stopped-flow absorbance spectroscopy and rapid quench methods. The second-order rate constant for the oxidation of TrpH.Fe(II) has a value of 104 M(-1) s(-1) irrespective of the presence of tryptophan. Stopped-flow absorbance analyses of the reaction of the TrpH.Fe(II).6MePH(4).tryptophan complex with oxygen are consistent with the initial step being reversible binding of oxygen, followed by the formation with a rate constant of 65 s(-1) of an intermediate I that has maximal absorbance at 420 nm. The rate constant for decay of I, 4.4 s(-1), matches that for formation of the 4a-hydroxypterin product monitored at 248 nm. Chemical-quench analyses show that 5-hydroxytryptophan forms with a rate constant of 1.3 s(-1) and that overall turnover is limited by a subsequent slow step, presumably product release, with a rate constant of 0.2 s(-1). All of the data with tryptophan as substrate can be described by a five-step mechanism. In contrast, with phenylalanine as substrate, the reaction can be described by three steps: a second-order reaction with oxygen to form I, decay of I as tyrosine forms, and slow product release.

Mechanism of nitric oxide induced deamination of cytosine

A five-step mechanism is proposed for the NO -induced deamination of cytosine. It has been investigated using DFT calculations, including both explicit water molecules and a bulk solvent model to mimic an aqueous environment. According to this mechanism, cytosine first undergoes tautomerization with the assistance of a water molecule from the bulk. A NO(+) cation produced by the autooxidation of NO is subsequently added to the exocyclic imino group of the cytosine imine tautomer. The resulting adduct is able to undergo a tautomerization step with the participation of a water molecule to produce a cytosine in which a -N(2)OH group is attached to carbon C4. Protonation of the oxygen of the latter gives a water molecule which dissociates instantaneously, leading to a pyrimidinic diazonium cation. This constitutes the rate-determining step of the mechanism with an activation free energy of 92.6 kJ mol(-1). The last step, which is highly exergonic, represents the driving force of the reaction. It is the substitution of the -N(2)(+) terminal group by a water molecule which simultaneously allows the transfer of one of the two hydrogens to the bulk. Thus, the two products of the reaction consist of a nitrogen molecule and the enol tautomer of uracil in equilibrium with the keto form.

Proton transfer across a liquid/liquid interface facilitated by phospholipid interfacial films

The five-step mechanism proposed earlier for the formation of phospholipid films at aqueous/organic interfaces and for their behaviour involving transfer of protons from the aqueous to the organic phase was further studied by cyclic voltammetry. A theoretical model was created for this mechanism and compared with the experimental results. Both the experimental and theoretical results support the proposed mechanism and evaluate the importance of the individual steps. It seems that the decisive step is the regeneration of the zwitterionic form of the phospholipid at the interface, ensuring stability of the film in time and its continuous functioning. In view of the complexity of the system, a wider selection of methods will be needed for a further study of this system; the theoretical model will then gradually be made more sophisticated.

Adsorption and ion-pairing interactions of phospholipids in the system of two immiscible electrolyte solutions. Part II: The formation and behaviour of a lecithin layer at the water/1,2-dichloroethane interface in the presence of multivalent anions in the aqueous phase

The present paper investigates voltammetrically the properties of the layers formed, whereas Part I of this series studied the electrochemical formation of l-α-lecithin (dipalmitoyl, DPPC) adsorbed layers at the water/1,2-dichloroethane interfaces. These are primarily determined by ion pairing between the positively charged layer and the anions present on the aqueous side of the interface. Cerium(IV) sulfate has been used as the source of the model anion and a five-step mechanism proposed for the formation and behaviour of DPPC layers. For the sake of comparison, the same experiments have been carried out with trimethyloctadecylammonium cation (TODA+) which have helped in elucidation of certain parts of the mechanism. In the absence of anions of higher charges, both DPPC and TODA+ form monolayers at the interface and the process involves simple free diffusion transport of the layer components through the bulk of the solution. In their presence, both DPPC and TODA+ form multilayers, the transport of the components is slower and is given by lateral diffusion along the layer surface and diffusion through the layer. The passage of ions across the interface is virtually not hindered by the monolayers, unless the ions are very large. However, DPPC and TODA+ multilayers strongly slow down or even prevent passage of larger ions (trimetylammonium (TMA+), PF6- and K+ ions assisted by a crown compound were used in the study).

Adsorption and ion-pairing interactions of phospholipids in the system of two immiscible electrolyte solutions Part I. The behaviour of lecithin at the water/1,2-dichloroethane interface, compared with that of trimethyloctadecylammonium cation

Abstract The behaviour of l -α-lecithin (DPPC) at the interface of water and 1,2-dichloroethane electrolyte solutions was studied by cyclic voltammetry and surface tension measurements. Model calculations of the cyclic voltammograms were carried out for the conditions employed in the experiments and comparative voltammetric measurements were performed with the trimethyloctadecylammonium cation (TODA + ). A five-step mechanism has been proposed and substantiated for the lecithin adsorption/desorption processes, involving sorption, acid–base and association/dissociation interactions. The TODA + behaviour is simpler, independent of the pH and in principle it corresponds to a single adsorption/desorption step at the interface from the organic phase.

A Family of Highly Ordered Mesoporous Polymer Resin and Carbon Structures from Organic−Organic Self-Assembly

The syntheses of a family of highly ordered mesoporous polymers and carbon frameworks from organic−organic assembly of triblock copolymers with soluble, low-molecular-weight phenolic resin precursors (resols) by an evaporation induced self-assembly strategy have been reported in detail. The family members include two-dimensional hexagonal (space group, p6m), three-dimensional bicontinuous (Ia3d), body-centered cubic (Im3m), and lamellar mesostructures, which are controlled by simply adjusting the ratio of phenol/template or poly(ethylene oxide)/poly(propylene oxide) in the templates. A five-step mechanism from organic−organic assembly has been demonstrated. Cubic FDU-14 with a gyroidal mesostructure of polymer resin or carbon has been synthesized for the first time by using the copolymer Pluronic P123 as a template in a relatively narrow range. Upon calcination at 350 °C, the templates should be removed to obtain mesoporous polymers, and further heating at above a critical temperature of 600 °C transfor...

Computational Study of the Deamination Reaction of Cytosine with H2O and OH-

The mechanism for the deamination reaction of cytosine with H(2)O and OH(-) to produce uracil was investigated using ab initio calculations. Optimized geometries of reactants, transition states, intermediates, and products were determined at RHF/6-31G(d), MP2/6-31G(d), and B3LYP/6-31G(d) levels and for anions at the B3LYP/6-31+G(d) level. Single-point energies were also determined at B3LYP/6-31+G(d), MP2/GTMP2Large, and G3MP2 levels of theory. Thermodynamic properties (DeltaE, DeltaH, and DeltaG), activation energies, enthalpies, and free energies of activation were calculated for each reaction pathway that was investigated. Intrinsic reaction coordinate analysis was performed to characterize the transition states on the potential energy surface. Two pathways for deamination with H(2)O were found, a five-step mechanism (pathway A) and a two-step mechanism (pathway B). The activation energy for the rate-determining steps, the formation of the tetrahedral intermediate for pathway A and the formation of the uracil tautomer for pathway B, are 221.3 and 260.3 kJ/mol, respectively, at the G3MP2 level of theory. The deamination reaction by either pathway is therefore unlikely because of the high barriers that are involved. Two pathways for deamination with OH(-) were also found, and both of them are five-step mechanisms. Pathways C and D produce an initial tetrahedral intermediate by adding H(2)O to deprotonated cytosine which then undergoes three conformational changes. The final intermediate dissociates to product via a 1-3 proton shift. Deamination with OH(-), through pathway C, resulted in the lowest activation energy, 148.0 kJ/mol, at the G3MP2 level of theory.

Application of genetic algorithms and thermogravimetry to determine the kinetics of polyurethane foam in smoldering combustion

In this work, the kinetic parameters governing the thermal and oxidative degradation of flexible polyurethane foam are determined using thermogravimetric data and a genetic algorithm. These kinetic parameters are needed in the theoretical modeling of the foam's smoldering behavior. Experimental thermogravimetric mass-loss data are used to explore the kinetics of polyurethane foam and to propose a mechanism consisting of five reactions. A lumped model of solid mass loss based on Arrhenius-type reaction rates and the five-step mechanism is developed to predict the polyurethane thermal degradation. The predictions are compared to the thermogravimetric measurements, and using a genetic algorithm, the method finds the kinetic and stoichiometric parameters that provide the best agreement between the lumped model and the experiments. To date, no study has attempted to describe both forward and opposed smolder-propagation with the same kinetic mechanism. Thus, in order to verify that the polyurethane kinetics determined from thermogravimetric experiments can be used to describe the reactions involved in polyurethane smoldering combustion, the five-step mechanism and its kinetic parameters are incorporated into a simple species model of smoldering combustion. It is shown that the species model agrees with experimental observations and that it captures phenomenologically the spatial distribution of the different species and the reactions in the vicinity of the front, for both forward and opposed propagation. The results indicate that the kinetic scheme proposed here is the first one to describe smoldering combustion of polyurethane in both propagation modes.

A theoretical study on the behaviour of a neutral 2-(amino)-1-cyclopentene-1-dithiocarboxylate compound: Rotamers and hydrogen transfer effects

Isomerisation of 2-(amino)-1-cyclopentene-1-dithiocarboxylate was observed through a five-step mechanism of intermolecular hydrogen transfer and isomerisation. Hydrogen transfer was studied at the B3LYP/6-31+G(d,p) level of theory, both dissociative and rotational pathways were identified for the proton exchange. Three low energy conformers were identified for the title compound below the 50 kJ/mol energy limit. The optimised minimum is in good agreement with the observed solid-state structure; with an RMS overlay error of less than 0.077 Å.

Reduced methanol kinetic mechanisms for combustion applications

Reduced chemical kinetic mechanisms for methanol combustion were investigated by evaluating ignition delay magnitudes and combustion in a continuously stirred reactor. Unsteady computations were made to study the characteristics of the kinetic mechanisms proposed in the literature and to compare the dependence of various parameters on methanol combustion. All computations were done under isobaric conditions, and, to capture the influence of all the reactions involved in the mechanism, a very small time step was used. Finite-difference methods were used to solve the coupled differential equations. The five-step mechanism developed by C.M. Mueller and N. Peters [in: N. Peters, B. Rogg (Eds.), Reduced Kinetic Mechanisms for Applications in Combustion Systems, Springer-Verlag, New York, 1993, pp. 143–155] for premixed flames and both the five-step mechanism and the four-step mechanisms developed by C.M. Mueller, K. Seshadri, J.Y. Chen [ibid, pp. 284–307] for non-premixed flames were considered. It was found that the Mueller et al. five-step mechanism, with some modifications, best supported the spontaneous ignition and continuous stirred reactor combustion. The results were validated by comparing calculated ignition delays with available experimental data of C.T. Bowman [Combust. Flame 25 (1975) 343–354], and calculated final steady-state concentrations with chemical equilibrium calculations [J.-Y. Chen, Combust. Sci. Technol. 78 (1991) 127]. Initial temperature and concentration and the operating pressure of the system have a major effect on the delay of methanol ignition. The residence time of the continuous stirred reactor affects ignition delay and also changes the transient characteristic of chemical composition of the fuel–vapor mixture. The computations are intended to guide and explain many combustion studies that require a methanol kinetic mechanism.

How is N–N bond formation facilitated by P450 NO reductase? A DFT study

The breaking of the very strong NO bond by the enzyme nitric oxide reductase has been explored using density functional theory calculations of a model cluster. A five-step mechanism is proposed. We find a novel mechanism for the formation of the N–N bond in N 2O, involving the addition of NO to the intermediate Fe–N(H)OH, followed by a conformational change on a flat potential energy surface to give the strong N–N bond.

A DFT‐Based Theoretical Investigation of the Mechanism of the PtCl2‐Mediated Cycloisomerization of Allenynes

The mechanism of Pt(II)-catalyzed intramolecular cycloisomerization of allenyne systems has been extensively investigated by DFT calculations. Different mechanistic schemes have been proposed and discussed, including the Alder-ene reaction. The free energy results suggest that the kinetically preferred reaction pathway for precursors that are tri- and tetrasubstituted on the allene moiety should proceed by a five-step mechanism. This would involve formation of a platina(IV)cyclopentene intermediate by selective engagement of the external pi bond of the allene, which would undergo regioselective beta-H elimination from the equatorially disposed methyl group. A metal-induced H migration leads to a second octahedral Pt(IV)-chelate complex, which would yield the expected bicyclic system through an intramolecular migratory insertion step. Therefore, depending on the conformation of the initial eta(4)-reactant complex for trisubstituted patterns, two possible intermediates can be formed that would evolve through different paths. In these cases, the regio- and stereochemical outcomes predicted by the mechanistic scheme proposed agree with experimental data. Substituted precursors on the alkyne moiety follow a distinct, four-step, mechanism also involving an oxidative cyclometalation process to an octahedral Pt(IV) intermediate complex. Theoretical results reveal the kinetic preference for beta-H elimination from the allylic group rather than from the gem-dimethyl group, which should account for the observed regioselectivity.

Hydrogen storage in LiAlH4: Predictions of the crystal structures and reaction mechanisms of intermediate phases from quantum mechanics

We use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH(4). First, we explore the decomposition mechanism of monoclinic LiAlH(4) into monoclinic Li(3)AlH(6) plus face-centered cubic (fcc) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH(4) and Li(3)AlH(6) agree well with experimental data. On the other hand, the alternative decomposition of LiAlH(4) into LiAlH(2) plus H(2) is predicted to be unstable with respect to that through Li(3)AlH(6). Next, we investigate thermal decomposition of Li(3)AlH(6) into fcc LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH(6) to form the stable Li-H-Li-H complex. Then, two sequential H(2) desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li(3)AlH(6)(monoclinic)-->3 LiH(fcc)+Al(fcc)+3/2 H(2) is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s)+Al(s)-->LiAl(s)+12H(2)(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl.