Detailed Reaction Mechanism Research Articles

Site-selective radical reactions of kinetically stable open-shell singlet diradicaloid difluorenoheteroles with tributyltin hydride and azo-based radical initiators.

We have demonstrated site-selective radical reactions of the kinetically stable open-shell singlet diradicaloids difluoreno[3,4-b:4',3'-d]thiophene (DFTh) and difluoreno[3,4-b:4',3'-d]furan (DFFu) with tributyltin hydride (HSn(n-Bu)3) and azo-based radical initiators. Treatment of these diradicaloids with HSn(n-Bu)3 induces hydrogenation at the ipso-carbon in the five-membered rings, while treatment with 2,2'-azobis(isobutyronitrile) (AIBN) induces substitution at the carbon atoms in the peripheral six-membered rings. We have also developed one-pot substitution/hydrogenation reactions of DFTh/DFFu with various azo-based radical initiators and HSn(n-Bu)3. The resulting products can be converted into substituted DFTh/DFFu derivatives via dehydrogenation. Theoretical calculations unveiled a detailed mechanism of the radical reactions of DFTh/DFFu with HSn(n-Bu)3 and with AIBN, and that the site-selectivity of these radical reactions is controlled by the balance of the spin density and the steric hindrance in DFTh/DFFu.

Theoretical study of the rearrangement reaction in bisorbicillinoid biosynthesis: insights into the molecular mechanisms involved.

Bisorbibutenolide and bisorbicillinolide are polyketide compounds with complex skeletons that are formed by the dimerization of sorbicillin. These compounds have long been of interest, with several reports of their biosynthesis, biological activity, and total synthesis. In this study, we theoretically investigated the detailed biosynthetic mechanism of the rearrangement reaction to form bisorbicillinolide. We showed that the presence of water molecules facilitates the intramolecular aldol reaction, determined the rate-limiting steps, and revealed that a cyclopropane intermediate is formed during the rearrangement process. Although computational chemistry has been widely applied to the carbocation chemistry present in terpene biosynthesis, it has seldom been used to investigate the carbonyl chemistry responsible for polyketide biosynthesis. This study shows that computational chemistry is a useful tool for studying anionic skeletal rearrangement reactions.

NOx Emissions of N-butanol and Methane in Low-speed Two Stroke Diesel Engines

This article firstly structures a detailed combustion reaction mechanism including methane N-butanol nitrogen oxide. According to two-stroke low-speed diesel engine’s parameter with the characteristics of engine firing, homogeneous charge compression ignition is selected as simulation model. At the same the output power and the excess air coefficient is 1.5, by means of the simulation calculation and analysis of reaction path and reaction mechanism, when N-butanol and methane are mixed in different proportions, the exhaust temperature in the reactor reduces with the increase of methane proportion in reactants. At the same time NO mole fraction increases with the proportion of CH4 in the mixed fuel decreases, but the NO2 mole fraction will increase with the proportion of CH4 in the mixed fuel increases.

Study on the Emission Characteristics in Renewable Energy Combustion under Different Working Conditions of Marine Two-Stroke Diesel Engine

In this paper, MAN 6S35ME-B9 two-stroke diesel engine is taken as the research object. By constructing a detailed combustion reaction mechanism including CH4, C4H10O, nitrides and other substances, CHEMKIN-PRO is used to simulate the same fuel mixing ratio and excess air coefficient. Under the condition of 1.5, the temperature, NO mole fraction and NH3 mole fraction in the reactor change and study the factors affecting the pollutant emission of marine diesel engine with the crank angle under different working conditions. The simulation shows that with the decrease of diesel engine speed, the maximum temperature of combustion reaction and the temperature at exhaust opening are obviously reduced. At the same time, mole fraction of NO and NH3 decreases with the decrease of rotational speed, and there is no nitride production in the combustion reaction at 25%.

Exploration of the reaction mechanism of the LaFeO3 oxygen carrier for chemical-looping steam methane reforming: a DFT study.

The CO conversion is expected to be controllable for chemical-looping steam methane reforming. Herein, density functional theory (DFT) calculations were employed to systematically explore the detailed reaction mechanism of CO conversion over the LaFeO3 oxygen carrier. It is found that the FeO2-terminated surface could exhibit better activity for CO adsorption than the LaO-terminated surface. In addition, the FeO2-terminated surface is much more favorable for CO oxidation than the LaO-terminated surface and the Fe-O site is the main active site. The oxygen diffusion process is easier to proceed on the LaO-terminated surface compared with the FeO2-terminated surface. Four pathways for the reaction process between the FeO2-terminated surface and CO were proposed and oxygen diffusion was determined as the rate-limiting step. For the reaction of CO with the LaO-terminated surface, one pathway was considered and CO2 desorption is the rate-limiting step. Comprehensively, the reactivity of CO conversion over the FeO2-terminated surface is superior to that over the LaO-terminated surface. We could control the CO conversion by regulating the oxygen activity of LaFeO3. This work provides guidance for the rational design of LaFeO3 oxygen carriers in the CL-SRM process.

Germanium hydrides as an efficient hydrogen-storage material operated by an iron catalyst.

The use of metal hydrides such as NaBH4 as hydrogen-storage materials has recently received substantial research attention on account of the worldwide demand for the development of efficient hydrogen-production, -storage, and -transportation systems. Here, we report the quantitative production of H2 gas from a germanium hydride, Ph2GeH2, mediated by an iron catalyst at room temperature via dehydrogenative coupling, concomitant with the formation of (GePh2)5. Of particular importance is that Ph2GeH2 can be facilely recovered from (GePh2)5 by contact with 1 atm of H2 or PhICl2/LiAlH4 at 0 °C or 40 °C, respectively. A detailed reaction mechanism for the iron-catalyzed dehydrogenative coupling of Ph2GeH2 is proposed based on the isolation of four intermediate iron species.

Automating the generation of detailed kinetic models for halocarbon combustion with the Reaction Mechanism Generator

Originally developed to predict the chemical kinetics of hydrocarbon combustion via automated generation of detailed reaction mechanisms, Reaction Mechanism Generator (RMG) contains extensive thermokinetic data for C,H,O chemisty, and has more recently been expanded to nitrogen and sulfur. In this work, we present the addition of halogen (fluorine, chlorine, and bromine) chemisty to RMG to enable automated generation of detailed kinetic models for halocarbon combustion. RMG’s existing reaction templates are updated to include halogens, and 11 new reactions families are created specific to halogen chemistry. Notably, kinetics for more than 1000 elementary reactions are calculated via ab inito methods and transition state theory, and these kinetic data are combined with kinetics from literature sources to train rate rule decision tree estimators. Additionally, halogen groups are added to RMG’s statistical mechanics database, enabling model generation with RMG’s pressure dependence module and automated computation of microcanonical rate constants for unimolecular networks. Halogen groups are also incorporated in RMG’s transport database to provide estimated parameters for the Lennard-Jones potential, important for transport-dependent simulations including laminar flame speeds. To demonstrate RMG’s capability for predicting halocarbon combustion, RMG is used to build a flame suppression model for 2-BTP (CH2=CBrCF3) in methane flames. The laminar flame speeds of RMG’s 2-BTP model show good agreement with a published model under a variety of reaction conditions. Automating the generation of detailed kinetic models for halocarbon combustion will facilitate the exploration of previously unexplored reaction pathways, thereby accelerating the development of greener refrigerants and suppressants, as well as advancing the field of automated mechanism generation.

Insights into the mechanism and selectivity of the Rh(i)-catalyzed cycloisomerization reaction of benzylallene-alkynes involving C–H bond activation

The detailed reaction mechanisms of rhodium-catalyzed cycloisomerization of benzylallene-terminal alkynes involving Csp2−H bond activation on the benzyl phenyl ring have been investigated with the aid of density functional theory calculations.

Catalytic isomerization of glucose to fructose over organic ligands: a DFT study.

Isomerization processes between glucose and fructose catalyzed by four different organic ligands are investigated with quantum chemistry methods in this study. These organic ligands are the carboxylic pendant group, sulfonic pendant group, amino pendant group, and 1H-imidazole ligand. After guessing and verifying a variety of elementary reactions, transition states and energy barriers that are relevant to the optimum pathways have been confirmed. The effective barriers under the catalysis of the carboxylic pendant group, sulfonic pendant group, amino pendant group, and 1H-imidazole ligand are 97.5kJmol-1, 134.7kJmol-1, 146.7kJmol-1, and 167.7kJmol-1, respectively. Then, based on the conclusions of the non-solvation model, the effective barriers in solvents are briefly investigated. The implicit model predicts that solvents bring little improvement or setback to catalyzed reaction models. The explicit model shows that the proton transfer with the participant of water molecules can improve the catalytic performance of Lewis bases in these reactions. The detailed reaction mechanism combing and reliable reaction templates provided in this work will be useful for catalysis designs for glucose transformation to fructose. This work used the computational level of ωB97M-D3BJ/def2-SVP and the software package of ORCA 4.2. For solvent effects, energies of the gas phase were corrected by the combination of C-PCM and SMD.

The effects of unburned-gas temperature and pressure on the unstable behavior of cellular-flame fronts generated by intrinsic instability in hydrogen-air lean premixed flames under adiabatic and non-adiabatic conditions: numerical simulation based on the detailed chemical reaction model

ABSTRACT The effects of unburned-gas temperature and pressure on the unstable behavior of cellular-flame fronts in hydrogen-air lean premixed flames are elucidated by numerical simulations. The detailed chemical reaction mechanism with seventeen reversible reactions of eight reactive species and a diluent is adopted for hydrogen-air combustion and heat loss is taken into account. The burning velocity of a planar flame increases as the unburned-gas temperature increases, and it decreases as the unburned-gas pressure and heat loss increase. In the dispersion relation, the growth rate increases and the unstable range widens when the unburned-gas temperature increases. The former decreases and the later narrows when the heat loss becomes larger. The normalized growth rates become large under the non-adiabatic conditions. The normalized burning velocity of a cellular flame is large when the pressure is high and heat loss is large, and it decreases as the temperature increases at high pressure. This shows that the high unburned-gas temperature at high pressure weakens the instability in hydrogen-air lean premixed flames. With increasing the domain size, the burning velocity of a cellular flame increases, and the cellular-flame front becomes more unbalance. This is an aspect of the long-wavelength disturbance to the unstable behavior of cellular-flame front.

The effect of mixing and heat transfer on regression rate of TAGN-based fuel in a segregated AP/TAGN solid motor

The effect of mixing and heat transfer on regression rate of TAGN-based fuel of a segregated AP/TAGN solid motor is studied in the present study. A regression rate model of TAGN-based fuel under coupled combustion is developed and validated based on detailed reaction mechanism. The two-dimensional mixing characteristics in boundary layer and the effect on regression rate of TAGN-based fuel are analyzed in detail. The simulation results show that the increase of initial temperature of oxidizer gas leads to better mixing in the axial direction and higher temperature of burning surface. However, the mass concentration of O2 decreases greatly due to the increasing consumption by primary combustion reactions of AP-derived gaseous species. Therefore, the regression rate of burning surface decreases slightly. With inflow Mach number increasing from 0.32 to 0.75, mixing in the boundary layer is greatly enhanced. Therefore, both the temperature and regression rate increase with inflow Mach number. As pressure increases from 4 MPa to 10 MPa, the primary reactions of AP-derived gas species are also enhanced. Therefore, the mass concentration of O2 decreases rapidly, which leads to oxygen-lean combustion in the combustion chamber. Therefore, the temperature of burning surface decreases while the regression hardly varies despite of the better mixing near the burning surface at high pressure. The results show that the coupled combustion reactions between inflow oxidizer gas and fuel gas mainly occur in a narrow belt-like region of the boundary layer. The results show that the regression rate of solid fuel in the segregated solid motor is controlled by the competition between primary combustion and coupled combustion.

Synthesis of Indole-Fused Dihydrothiopyrano Scaffolds via (3 + 3)-Annulations of Donor-Acceptor Cyclopropanes with Indoline-2-Thiones.

A new methodology for the synthesis of N-haloindole-fused dihydrothiopyrano derivatives via (3 + 3)-annulation of donor-acceptor cyclopropanes (DACs) with indoline-2-thiones in the presence of Sc(OTf)3 as a Lewis acid catalyst has been developed. This protocol provides a variety of indole-fused dihydrothiopyrano molecules in good to excellent yields, which architecturally resemble other indole-fused tricyclic molecules having potential medicinal value. In addition, we have described a detailed reaction mechanism and transformation of the furnished product into N-fused thiazino indole molecule.

Luminescence turn-off and turn-on interaction mechanism of optical probe with hydrogen peroxide

The mechanism of interaction between luminescent positively charged Carbon dots (CDs) and manganese dioxide (MnO2) based optical probes and hydrogen peroxide (H2O2) has been investigated. The optical probe has the mechanism of luminescence turn-off and on, which enables the detection of H2O2. MnO2 nanosheets can quench CDs luminescence due to spectral overlap between CDs emission and MnO2 absorption. As a result of the underlying detailed redox reaction mechanism, the initial quenched probe exhibits luminescence activation upon exposure to H2O2 solution. The quenched probe begins to regain the luminescence that increases as the concentration of H2O2 increases (0.1–12μ M). The Johnson-Mehl-Avrami-Kolmogorov model was used to investigate the luminescence turn-on mechanism further.

Rotational Isomerization of Carbon-Carbon Single Bonds in Ethyl Radical Derivatives in a Room-Temperature Solution.

The rotational isomerization of 1,2-disubstituted ethyl radical derivatives, reaction intermediates often found in the reaction of 1,2-disubstituted ethane derivatives, has never been measured because of their short lifetime and ultrafast rotation. However, the rotational time constant is critical for understanding the detailed reaction mechanism involving these radicals, which determine the stereoisomers of compounds produced via the intermediates. Using time-resolved infrared spectroscopy, we found that the CF2BrCF2 radical in a CCl4 solution rotationally isomerizes with a time constant of 47 ± 5 ps at 280 ± 2 K. From this value and the rotational barrier heights of related compounds, CH3CH2 and CH3CH2CHCH3 radicals in CCl4 were estimated to rotationally isomerize within 1 ps at 298 K, considerably faster than ethane and n-butane, which rotationally isomerize with time constants of 1.8 and 81 ps, respectively. The time constant for the rotational isomerization was similar to that calculated using transition state theory with a transmission coefficient of 0.75.

Improving the thermophotovoltaic performance of a hydrogen-fueled planar combustor with four-corner inlets via partially inserting porous medium

Micro thermophotovoltaic (TPV) generator is a potentially efficient approach to achieve microscale fuel-to-electricity conversion with high-energy–density, high-efficiency, and small-scale portability, where micro combustor provides the high-temperature continuous combustion process as well as a heat source of thermal radiation. In this study, a hydrogen-fueled planar combustor with four-corner inlets by partially inserting porous medium is proposed and investigated by a three-dimensional numerical model of porous medium combustion with detailed reaction mechanism and local thermal non-equilibrium. The combustor performance with and without partially inserting porous medium under various inlet velocities Vin and equivalence ratios ϕ are compared and analyzed to reveal the inherent strengthening mechanism. Moreover, the effects of various porous parameters on the energy conversion performance are examined: the porosity ɛ, thermal conductivity ks, and porous medium filled volume. The results demonstrate that the inserted porous medium can significantly enhance combustion efficiency, combustor wall temperature and its uniformity, and the largest increase in mean wall temperature reaches 79.7 K at Vin = 9 m/s, resulting in a 35.8 % increase in micro-TPV system efficiency. As ɛ and ks decrease, the combustor energy conversion performance increases while the temperature uniformity decreases. A moderate channel width L = 2.0 mm is good for further improving temperature uniformity and energy conversion efficiency. This study paves the way for the improvement of efficient, practical, and portable microscale generators.

DNS predictions of NOx production in developing turbulent mixing layers with non-premixed hydrogen–air combustion

ABSTRACT Direct numerical simulations of three-dimensional compressible mixing layers with non-premixed hydrogen–air combustion were performed using a detailed chemical reaction mechanism with production. Flow fields with three types of initial disturbances were simulated to investigate the relationship between developing vortical structures and formation. The amounts of and produced in the simple shear layer were smaller than those in the two- and three-dimensional mixing layers with vortical structures. In the mixing layers, the formation and expansion of the combustion region by the roller vortices and the baroclinic torque had a significant impact on production, while the relatively low-temperature combustion region formed by the three-dimensional developed rib vortices in the blade regions between the roller vortices had a large effect on the production. It was found that a two-dimensional simulation can estimate the production, while the information on a three-dimensional mixing layer is necessary to predict the production.

Prevailing surface reactions in the plasma-catalytic ammonia synthesis with Ru/CeO2 and Ru/Ti-CeO2

Sustainable plasma-catalytic NH3 synthesis is receiving an increasing amount of interest as a greener alternative for the conventional Haber-Bosch (HB) process. However, the best energy efficiency for the plasma synthesis reported so far is still low compared to the one of HB. This research was conducted to experimentally investigate one of the plasma process rate-limiting steps, which is N2 bond breaking. This study aimed at identifying the enhancing-surface reactions as well as limiting surface reactions such as H-inhibition. Ru/CeO2 and Ru/Ti-CeO2 were tested in a packed-bed DBD reactor to investigate the influence of TiO2 addition, which possibly prevented H-inhibition, and simultaneously enhanced different surface reactions. Variations of feed gas ratios and designed operation schemes were studied to elucidate the effect of the catalyst composition. Results showed that Ru/Ti-CeO2 outperformed Ru/CeO2 in terms of energy consumption and a N2-rich atmosphere was needed to obtain minima of 84.5 MJ mol−1 and 126.5 MJ mol−1, respectively. The addition of TiO2 in the catalysts provided more surface sites for H2 to adsorb on, which was identified as main reason for improving the energy efficiency of the plasma-catalytic NH3 synthesis.Three detailed reaction mechanisms were proposed to elaborate on the behavior between N2-rich plasma and Ru/Ti-CeO2.

Numerical study on extinction characteristics of counterflow flame in premixed NH3/CH4/air mixtures under normal temperature and pressure

Use of ammonia fuel is of particular interest because it does not release carbon dioxide during combustion. However, ammonia flameout is easy to occur in practical applications due to its poor combustion intensity and instability. It has been suggested to mix methane with ammonia to enhance its combustion performance. In this study, the Okafor mechanism was selected as the most suitable detailed chemical reaction mechanism for numerical simulation at normal temperature and pressure (NTP) by comparing and analyzing the extinction stretch rate of pure ammonia with air flame under different reaction mechanisms. The NH3-CH4-air laminar premixed counterflow flame at NTP was numerically studied to reveal variations in flame extinction stretch rate over different conditions. It was obtained that the flame extinction stretch rate was increased by increasing CH4 mole fraction in NH3/CH4 mixed fuel, as expected, and that the flame extinction limit was the greatest under lean burn condition (φ = 0.9). When the extinction limit was achieved and then exceeded, the elementary reactions that significantly contributed to the flame heat release rate at different blending ratios were obtained. By comparing the pathway analysis of double ammonia blending ratios, the elementary reaction and radical associated with addition of methane to ammonia and its effects on the flame extinction characteristics were studied.

Exergy analysis and investigation on effect of inlet valve closing temperature and hydrogen enrichment in syngas composition in an HCCI engine

This study presents the parametric investigation of syngas fuelled homogeneous charge compression ignition (HCCI) engine. The chemical kinetic simulation of the syngas HCCI engine is performed using the stochastic reactor model (SRM). This study first compares and evaluates the performance of different reaction mechanisms for the HCCI engine. The experimental combustion pressure data is compared with numerically investigated combustion pressure to validate the reaction mechanisms. Results indicate that a numerically simulated combustion pressure using a detailed reaction mechanism consisting of 173 reactions and 32 species (CRECK-2014) is well-matched with the profile of experimental combustion pressure. The validated reaction mechanism is further used to investigate the HCCI engine at different engine speeds for various equivalence ratio (ϕ), syngas composition (hydrogen addition (H2:CO)) and inlet valve closing temperature (Tivc). The sensitivity analysis is performed to find the prominent reactions in the oxidation reaction mechanism. Based on the combustion characteristics, syngas HCCI maps are developed. The effect of engine operating conditions on combustion and emission characteristics is also investigated. Furthermore, the result of engine operating parameters on converting fuel energy into physical exergy, chemical exergy, work exergy, and heat loss exergy is also briefly discussed.

Oxidation and degradation of (epi)gallocatechin gallate (EGCG/GCG) and (epi)catechin gallate (ECG/CG) in alkali solution

The oxidative decomposition/degradation of two main tea flavanols, EGCG/GCG and ECG/CG, was studied in alkaline solution under ultrasonic-assisted thermal conditions. The study employed HPLC–ESI-ToF-MS to identify the products generated by atmospheric oxygen oxidation and various base-catalyzed reactions. Strong basic condition led to accelerated hydrolysis and oxidation of EGCG/GCG and ECG/CG and yielded gallic acid, de-galloyl flavanols and corresponding o-quinone derivatives. Meanwhile, peroxidation or base-catalyzed cleavage and rearrangement occurred extensively on C- and B-rings of flavanol and generated various simpler aldehydes or acids. Besides, a number of dimers/trimers were produced. This contribution provides empirical proof of oxidative degradation of flavanols under strong alkaline condition. Meanwhile, detailed reaction mechanisms of C-/B-ring degradation and dimerization/polymerization phenomena are proposed to help understand the structural changes of flavanols under strong alkaline conditions.