Exploring fuel molecular structure effects on the pyrolysis chemistry of branched hexenes

3,3-Dimethyl-1-butene (NEC₆D3) and 2,3-dimethyl-2-butene (XC₆D2) are representative branched alkene components in gasoline. This work experimentally investigated the pyrolysis of NEC₆D3 and XC₆D2 in a flow reactor (T = 950–1350 K, P = 0.04 atm) and a jet-stirred reactor (T = 730–1000 K, P = 1 atm) using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC). A pyrolysis model of branched hexenes was proposed and validated against the new experimental data. The combined experimental observations and modeling analyses provide insights into the predominant fuel decomposition pathways and specific formation pathways of products under pyrolysis conditions. NEC₆D3 exhibits a much higher reactivity than XC₆D2 due to the existence of allylic CC bonds. Unimolecular decomposition reactions play the most crucial role in NEC₆D3 decomposition, while in XC₆D2 pyrolysis, fuel consumption is dominated by H-abstraction reactions and the H-assisted isomerization reaction. Fuel-specific pathways can remarkably influence the formation of pyrolysis products, especially the key C₁C₂ products, isomer pairs and dialkenes. Furthermore, the reactions involving propargyl radical dominate the formation of fulvene and aromatic products in the pyrolysis of both fuels, leading to more abundant production of C₆ and larger cyclic products in XC₆D2 pyrolysis.     

A combustion chemistry study of tetramethylethylene in a laminar premixed low-pressure hydrogen flame

The combustion chemistry of tetramethylethylene (TME) was studied in a premixed laminar low-pressure hydrogen flame by combined photoionization molecular-beam mass spectrometry (PI-MBMS) and photoelectron photoion coincidence (PEPICO) spectroscopy at the Swiss Light Source (SLS) of the Paul Scherrer Institute in Villigen, Switzerland. This hexene isomer with the chemical formula C₆H₁₂ has a special structure with only allylic CH bonds. Several combustion intermediate species were identified by their photoionization and threshold photoelectron spectra, respectively. The experimental mole fraction profiles were compared to modeling results from a recently published kinetic reaction mechanism that includes a TME sub-mechanism to describe the TME/H₂ flame structure. The first stable intermediate species formed early in the flame front during the combustion of TME are 2-methyl-2-butene (C₅H₁₀) at a mass-to-charge ratio (m/z) of 70, 2,3-dimethylbutane (C₆H₁₄) at m/z 86, and 3-methyl-1,2-butadiene (C₅H₈) at m/z 68. Isobutene (C₄H₈) is also a dominant intermediate in the combustion of TME and results from consumption of 2-methyl-2-butene. In addition to these hydrocarbons, some oxygenated species are formed due to low-temperature combustion chemistry in the consumption pathway of TME under the investigated flame conditions.     

Exploring fuel isomeric effects on laminar flame propagation of butylbenzenes at various pressures

This work reports an experimental and kinetic modeling investigation on the laminar flame propagation of three butylbenzene isomers (n-butylbenzene, iso-butylbenzene and tert-butylbenzene)/air mixtures. The experiments were performed in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 423 K, initial pressures of 1–10 atm, and equivalence ratios (?) of 0.7–1.5. The laminar burning velocities of butylbenzene/O₂/He mixtures were also measured at 423 K, 10 atm and ? = 1.5 to provide additional experimental data under conditions that the butylbenzene/air experiments are susceptible of cellular instability. Comparison among the laminar burning velocities of butylbenzenes including both the three isomers investigated in this work and sec-butylbenzene investigated in our recent work [Combust. Flame 211 (2020) 18–31] shows remarkable fuel isomeric effects, that is, iso-butylbenzene has the slowest laminar burning velocities, followed by n-butylbenzene and tert-butylbenzene, while sec-butylbenzene has the fastest laminar burning velocities. A kinetic model for butylbenzene combustion was developed to simulate the laminar flame propagation of butylbenzenes. Sensitivity analysis was performed to reveal important reactions in laminar flame propagation of butylbenzenes, including both small species reactions and fuel-specific reactions. Kinetic effects are concluded to result in the different laminar burning velocities of four butylbenzene isomers. Small species reactions control the laminar flame propagation under lean conditions, which results in small differences of laminar burning velocities. Chain termination reactions, especially fuel-specific reactions, have important contributions to inhibit the laminar flame propagation under rich conditions. The structural features of butylbenzene isomers can significantly affect the formation of some crucial radicals such as methyl, cyclopentadienyl and benzyl radicals under rich conditions, which leads to remarkable fuel isomeric effects on their laminar burning velocities, especially at high pressures.     

Isomer-specific speciation behaviors probed from premixed flames fueled by acetone and propanal

Chemical structures of low-pressure premixed flames respectively fueled by two C₃ carbonyl isomers, acetone and propanal, at different equivalence ratios (1.0 and 1.5) were experimentally investigated in this work. Detailed speciation information was obtained by employing molecular-beam mass spectrometry with tunable synchrotron photoionization. A detailed kinetic model including the chemistry of acetone and propanal was developed and tested with the current flame speciation measurements. By combining experimental observations and modeling interpretations, comparisons were made regarding fuel-specific reaction pathways and the resulting different species pools. Some fuel-specific intermediates were detected and quantified in this work, such as ketene in acetone flames and methylketene in propanal flames. Particularly, the quantitative speciation measurements of ketene, an important primary intermediate of acetone, were satisfactorily predicted by the current model, which included an updated ketene sub-mechanism. Major efforts in this work were devoted to gaining some insights into the effects of the carbonyl position in fuel molecules on the speciation behaviors under premixed flame conditions. Carbonyl functionalities in the two C₃ carbonyl compounds are tightly bonded and preferably preserved in CO. Due to the different position of the CO bond in the two isomers, the oxidation of propanal leads to abundant ethyl as a chain carrier, while the acetone consumption easily results in a significant amount of methyl, an inhibitor on the fuel reactivity. As a result, higher reactivity of propanal was observed. More importantly, the different fuel consumption patterns also influence the speciation behaviors. Specifically, the larger concentration of benzene precursors such as allyl, was observed in the propanal flames. Besides, typical oxygenated emissions formaldehyde and acetaldehyde had more remarkable concentrations in acetone and propanal flames, respectively.     

On the laminar flame propagation of C5H10O2 esters up to 10 atm: A comparative experimental and kinetic modeling study

Biodiesel is a family of renewable engine fuels with carbon-neutral nature. In this work, three C₅H₁₀O₂ esters (methyl butanoate, methyl isobutanoate and ethyl propanoate), which can serve as model compounds of biodiesel and represent linear and branched methyl esters and linear ethyl esters, were investigated to characterize their laminar flame propagation characteristics up to 10 atm and unravel the effects of isomeric fuel structures. A high-pressure constant-volume cylindrical combustion vessel was used to achieve laminar burning velocity measurements at 1–10 atm, 423 K and equivalence ratios of 0.7–1.5, while comparative experimental work was performed on a heat flux burner at 1 atm, 393 K and equivalence ratios of 0.7–1.6 for methyl butanoate and ethyl propanoate. The laminar burning velocity generally decreases with increasing pressure and increases in the order of methyl isobutanoate, methyl butanoate and ethyl propanoate, which shows distinct fuel isomeric effects. A kinetic model of C₅H₁₀O₂ esters was developed and validated against the new data in this work and previous data in literature. Modeling analyses were performed to provide insight into the fuel-specific flame chemistry of the three esters isomers. Remarkable differences in radical pools of three ester isomers are concluded to be responsible for the observed fuel isomeric effects on laminar flame propagation. The feature of two ethyl groups connected to the ester group in ethyl propanoate facilitates the ethyl production and inhibits the methyl and allyl production, making it propagate fastest among the three isomers. The branched structure feature of methyl isobutanoate with methyl and i-propyl groups connected to the ester group prevents the ethyl formation and results in considerable CH₃ and allyl production, which decelerates its laminar flame propagation.     

Insight into fuel isomeric effects on laminar flame propagation of pentanones

Laminar flame propagation was investigated for pentanone isomers/air mixtures (3-pentanone, 2-pentanone and 3-methyl-2-butanone) in a high-pressure constant-volume cylindrical combustion vessel at 393–423 K, 1–10 atm and equivalence ratios of 0.6–1.5, and in a heat flux burner at 393 K, 1 atm and equivalence ratios of 0.6–1.5. Two kinds of methods generally show good agreement, both of which indicate that the laminar burning velocity increases in the order of 3-methyl-2-butanone, 2-pentanone and 3-pentanone. A kinetic model of pentanone isomers was developed and validated against experimental data in this work and in literature. Modeling analysis was performed to provide insight into the flame chemistry of the three pentanone isomers. H-abstraction reactions are concluded to dominate fuel consumption, and further decomposition of fuel radicals eventually produces fuel-specific small radicals. The differences in radical pools are concluded to be responsible for the observed fuel isomeric effects on laminar burning velocity. Among the three pentanone isomers, 3-pentanone tends to produce ethyl and does not prefer to produce methyl and allyl in flames, thus it has the highest reactivity and fastest laminar flame propagation. On the contrary, 3-methyl-2-butanone tends to produce allyl and methyl instead of ethyl, and consequently has the lowest reactivity and slowest laminar flame propagation.     

Ignition of non-premixed counterflow flames of octane and decane isomers

Ignition temperatures of non-premixed flames of octane and decane isomers were determined in the counterflow configuration at atmospheric pressure, a free-stream fuel/N₂ mixture temperature of 401 K, a local strain rate of 130 s^?1, and fuel mole fractions ranging from 1% to 6%. The experiments were modeled using detailed chemical kinetic mechanisms for all isomers that were combined with established H₂, CO, and n-alkane models, and close agreements were found for all flames considered. The results confirmed that increasing the degree of branching lowers the ignition propensity. On the other hand, increasing the straight chain length by two carbons was found to have no measurable effect on flame ignition for symmetric branched fuel structures. Detailed sensitivity analyses showed that flame ignition is sensitive primarily to the H₂/CO and C₁–C₃ hydrocarbon kinetics for low degrees of branching, and to fuel-related reactions for the more branched molecules.     

Probing the fuel-specific intermediates in the low-temperature oxidation of 1-heptene and modeling interpretation

In this work, low temperature (low-T) oxidation of 1-heptene was investigated in a jet-stirred reactor (JSR) over the temperatures of 450–800 K, 770 Torr and equivalence ratios of 0.5–2.0. The intermediates were identified and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography combined with mass spectrometry (GC–MS). The SVUV-PIMS experiment combined with quantum chemistry calculation of ionization energy enables the identification of fuel-specific intermediates, including C₇ alkenylperoxy radical and hydroperoxides, such as diolefinic-hydroperoxide, alkenylhydroperoxide, alkenyl-ketohydroperoxide and cyclic ether hydroperoxide. Among them, alkenylperoxy radical, diolefinic-hydroperoxide and cyclic ether hydroperoxide have not been detected in alkene oxidation before. In order to accurately identify and quantify other fuel-specific intermediates such as aldehyde and cyclic ether isomers, the GC–MS experiment was conducted under the same conditions as the SVUV-PIMS experiment. On the other hand, a detailed low-T oxidation model of 1-heptene was developed, which can reasonably capture the fuel oxidation rate and negative temperature coefficient behaviors observed in this work. The present model can not only interpret the formation of different kinds of hydroperoxides and predict their temperature windows, but also capture the formation of 2-heptenal, hexanal and heptanal, and branched tetrahydrofurans, which are derived from the H-abstraction by OH, OH addition and H addition reactions of 1-heptene, respectively, revealing that the competition between these reactions can be well characterized.     

Probing fuel-specific reaction intermediates from laminar premixed flames fueled by two C5 ketones and model interpretations

The flame chemistry was explored for two C₅ ketones with distinct structural features, cyclopentanone (CPO) and diethyl ketone (DEK). Quantitative information for numerous species, including some reactive intermediates, was probed from fuel-rich (?= 1.5) laminar premixed flames fueled by the ketones with a photoionization molecular-beam mass spectrometer (PI-MBMS). Furthermore, a new kinetic model was proposed aimed at interpreting the high-temperature combustion chemistry for both ketones, which could satisfactorily predict the current flame speciation measurements. Experimental observations in combination with modeling analyses were used to reveal the similarities and differences between the compositions of the species pools of the two flames, with emphasis on the effects of the carbonyl functionality on pollutants formations. Besides some primary species which preserve fuel-specific features produced from initial steps of fuel consumptions, basic C₁C₄ intermediates also differ much between the two flames. More abundant intermediates were observed in the CPO flame because the cyclic fuel structure enables ring-opening processes followed by formations of C₃ and C₄ hydrocarbons which cannot be easily produced from the two isolated ethyl moieties in DEK under flame conditions. The consumptions of C₃C₄ hydrocarbons in the CPO flame further lead to larger C₅C₆ species which were under the detection limit in the DEK flame. In both flames, the tightly bonded carbonyl groups in the fuels tend to be preserved, leading to carbon monoxide through α-scissions of fuel-related acyl radicals. The carbonyl moieties in most detected C₁C₃ aldehydes and ketones form through oxidations of hydrocarbon species rather than directly originating from the fuels.     

Exploring the low-temperature oxidation chemistry of 1-butene and i-butene triggered by dimethyl ether

In order to better understand the low-temperature oxidation chemistry of alkenes, 1-butene and i-butene oxidation experiments triggered by dimethyl ether (DME) were conducted in a jet-stirred reactor at 790 Torr, 500–725 K and the equivalence ratio of 0.35. Low-temperature oxidation intermediates involved in alcoholic radical chemistry and allylic radical chemistry were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). To better interpret the experimental data, a kinetic model was proposed based on our low-temperature oxidation model of DME and comprehensive oxidation models of 1-butene and i-butene in literature. Based on present experimental results and modeling analysis, alcoholic radical chemistry initiated by OH addition is mainly responsible for the low-temperature chain propagation of butenes, since the Waddington mechanism plays a dominant role compared with the chain-branching pathways through the second O₂ addition. Allylic radical+HO₂ reactions producing alkenyl hydroperoxides and fuel+O₂ serve as the major chain-branching and chain-termination pathways, respectively, and they are competitive in the negative temperature coefficient (NTC) region. In contrast, chain-branching pathways originating from allylic radical+O₂ and alkyl-like radical+O₂ reactions have little contribution to the OH formation. Comparison with the simulation results of butane/DME mixtures demonstrates that butenes can largely inhibit the reactivity of DME at low temperatures due to its reduced low-temperature chain-branching process. However, in the NTC region, butenes may not be good OH absorbents since the allylic radicals can convert HO₂ to OH and consequently enhance the oxidation reactivity.     

Exploring the pyrolysis chemistry of 1,3,5-trimethylcyclohexane with insight into fuel isomeric and multiple substitution effects

To reveal insights into the combustion mechanism of multiple alkyl substituent cycloparaffins, this work reports an experimental and modeling study of 1,3,5-trimethylcyclohexane (T135MCH) pyrolysis in an extended flow reactor at low and atmospheric pressures. More than 30 species were detected and quantified employing synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry, and a detailed kinetic model developed based on reaction classes and update kinetic data was validated against the measured species profiles with a reasonable agreement. The reaction flux analyses were performed to reveal the key pathways of the fuel decomposition, intermediates production and aromatics formation. For the primary decomposition, the branching ratios of reaction types show strong dependence on changes of pressures and temperatures, including unimolecular methyl elimination, unimolecular ring-opening isomerization and H-abstraction. Besides the direct dissociation channels, major intermediate hydrocarbons are formed via stepwise dehydrogenation, recombination with ĊH₃ radical or “formally direct” chemically activated reactions triggered by Ḣ atom addition. Monocyclic aromatic hydrocarbons such as benzene and toluene can be produced by traditional H-abstraction/β-C-H scission sequence, cyclopentadiene-related pathways, or recombination mechanism from small linear products. The formations of indene and naphthalene are controlled by C₅+C₅ and C₅+C₄ mechanism respectively. The comparison work of species profiles combined with theoretical calculations of bond dissociation enthalpies (BDEs) was performed to reveal the multiple CH₃-group substituent and isomeric effects of methylcyclohexane (MCH), 1,2,4-trimethylcyclohexane (T124MCH) and T135MCH on pyrolysis activity and ethylene/benzene formation. Besides the increased reaction active sites, the added CH₃-group and ortho-substitution can both weaken the strength of CC and CH bonds, leading to the promoting decomposition activity. The different formation tendencies of products are caused by different BDEs, length of carbon skeleton, as well as complex fuel-specific pathways.     

Vibrational Analysis of 2,3-Dimethyl-1-Butene

Infrared and Raman spectra were obtained for 2,3-dimethyl-1-butene. The spectra showed the presence of two stable conformations. Vibrational assignments were made for both conformers with the aid of normal coordinate calculations. Values for the force constants that were obtained will be used in the future as the initial values for other substituted 1-alkenes, such as 2-isopropyl-3-methyl-1-butene.     

Experimental and modeling study of the high-temperature combustion chemistry of tetrahydrofurfuryl alcohol

Lignocellulosic tetrahydrofuranic (THF) biofuels have been identified as promising fuel candidates for spark-ignition (SI) engines. To support the potential use as transportation biofuels, fundamental studies of their combustion and emission behavior are highly important. In the present study, the high-temperature (HT) combustion chemistry of tetrahydrofurfuryl alcohol (THFA), a THF based biofuel, was investigated using a comprehensive experimental and numerical approach.Representative chemical species profiles in a stoichiometric premixed methane flame doped with ～20% (molar) THFA at 5.3 kPa were measured using online gas chromatography. The flame temperature was obtained by NO laser-induced fluorescence (LIF) thermometry. More than 40 chemical products were identified and quantified. Many of them such as ethylene, formaldehyde, acrolein, allyl alcohol, 2,3-dihydrofuran, 3,4-dihydropyran, 4-pentenal, and tetrahydrofuran-2-carbaldehyde are fuel-specific decomposition products formed in rather high concentrations. In the numerical part, as a complement to kinetic modeling, high-level theoretical calculations were performed to identify plausible reaction pathways that lead to the observed products. Furthermore, the rate coefficients of important reactions and the thermochemical properties of the related species were calculated. A detailed kinetic model for high-temperature combustion of THFA was developed, which reasonably predicts the experimental data. Subsequent rate analysis showed that THFA is mainly consumed by H-abstraction reactions yielding several fuel radicals that in turn undergo either β-scission reactions or intramolecular radical addition that effectively leads to ring enlargement. The importance of specific reaction channels generally correlates with bond dissociation energies. Along THFA reaction routes, the derived species with cis configuration were found to be thermodynamically more stable than their corresponding trans configuration, which differs from usual observations for hydrocarbons.     

Dissociative electron attachment to the explosive detection taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB)

A detailed study on dissociative electron attachment (DEA) to 2,3-dimethyl-2,3-dinitrobutane (DMNB) in the gas phase is presented. Ion yields as a function of the incident electron energy from about 0 to 14 eV have been measured for the most dominant fragments including anions such as NO₂ ⁻, [M-NO₂]⁻ or N₂O₄ ⁻. To help identifying which anion and neutral fragments are formed upon electron attachment we calculated the thermodynamic thresholds using the G4(MP2) method.     

Role of CH2O moiety on laminar burning velocities of oxymethylene ethers (OMEn): A case study of dimethyl ether,OME1 and OME2

Oxymethylene ethers (OME_n) are an important family of e-fuels that can be produced sustainably from carbon dioxide and hydrogen via renewable electricity. In this work, laminar flame propagation of dimethyl ether (DME, which can be deemed as OME₀), dimethoxymethane (OME₁) and methoxy(methoxymethoxy)methane (OME₂) was investigated in a constant-volume cylindrical combustion vessel. Laminar burning velocities (LBVs) of the three fuels were derived at 423 K, 1–10 atm and equivalence ratios of 0.7–1.5. A kinetic model for the high-temperature oxidation of the three fuels was developed with the isomerization and decomposition reactions of OME₂ radicals theoretically calculated. Reasonable predictions can be achieved by the present model during the validation against the new data in this work and previous data in literature. Based on the modeling analysis, fuel-specific flame chemistry of the three fuels was analyzed, especially for the key formation pathways of major intermediates including formaldehyde, methyl formate and CH₃. Special attentions were paid on the role of CH₂O moiety, which is demonstrated by the variation of LBV and flame chemistry with the ratio (α) of CH₂O moiety to the rest moiety in the fuel molecule (α = 1, 2 and 3 for DME, OME₁ and OME₂). It is observed from the experimental and simulated results that as α increases, the LBV profile has close peak values and peaks towards rich conditions, which results in the crossings of profiles and ascending LBV values under the richest conditions. Reactions involving fuel-specific radicals HCO and CH₃ result in the peak shift of H profile and different LBV values, especially under the richest conditions. Furthermore, extended α values at 0 and ∞ by using methane and formaldehyde respectively were also explored with kinetic modeling to provide more insight into the effects of fuel molecular structures.     

Excitonic nature of the dual fluorescence of 2,3-diazabicyclo[2.2.2]oct-2-ene and 1,4-dimethyl-(2,3-diazabicyclo[2.2.2]oct-2-ene)

Absorption, fluorescence excitation, and fluorescence spectra and the dependence of the degree of fluorescence polarization on the emission wavelength are measured for glass-like ethanol solutions of 2,3-diazabicyclo[2.2.2]oct-2-ene and 1,4-dimethyl-(2,3-diazabicyclo[2.2.2]oct-2-ene) at a temperature of 77 K. The analysis of the spectral polarization data shows that two excited electronic states S₁ and S₂ that contribute to the emission of the compounds are related to the exciton splitting and correspond to the symmetric (S₂) and antisymmetric (S₁) wave functions.     

Microwave spectrum of 3,4-epoxy-1-butene

The microwave spectrum of 3,4-epoxy-1-butene has been studied in the region 26.5–40 GHz. For the ground-state molecule, 170 lines have been assigned up to J = 34. From these the rotational constants and the centrifugal distortion constants were determined by least-squares fitting. The rotational constants are (in MHz): A = 17367.284 ± 0.011, B = 3138.186 ± 0.004, C = 3043.697 ± 0.004. The dipole moment has been determined from the Stark effect as (in Debye): μ_a = 0.72 ± 0.01, μ_b = 1.688 ± 0.003, μ_c = 0.39 ± 0.02, μ = 1.875 ± 0.005. The rotational constants and dipole moment components indicate that the assigned conformer is the s-trans form. A rotational assignment has also been made for the first excited state of the torsional mode. The fundamental frequency of the torsional mode has been estimated as 142 ± 20 cm^?1 from relative intensity measurement.     

DSC studies of neohexanol and its isomers

DSC method has been used to establish a solid state polymorphism of four dimethyl butanols having CH₃ side groups: 2,2-dimethyl-1-butanol [CH₃CH₂C(CH₃)₂CH₂OH)], 3,3-dimethyl-1-butanol [(CH₃)₃CCH₂CH₂OH], 3,3-dimethyl-2-butanol [(CH₃)₃CCH(OH)CH₃] and 2,3-dimethyl-2-butanol [(CH₃)₂CHCOH(CH₃)₂]. Three isomers appear to be glass formers. Glass of liquid phase has been observed for 3,3-DM-1-B while glass of plastic phase for 2,2-DM-1-B and 3,3-DM-2-B. For 2,3-DM-2-B on cooling only crystallization has been found. Influence of location of the OH group in molecule on polymorphism is discussed. IR-spectra, measured in the liquid phase, have revealed hydrogen bonds in all isomers.     

Synthesis and spectroscopic and electrochemical properties of binuclear complexes of Au(III) bridged by 6,7-dimethyl-2,3-di(2-pyridyl) quinoxaline

A method of synthesis of new diimine complexes of Au(III) with a four-dentate bridging ligand 6,7-dimethyl-2,3-di(2-pyridyl) quinoxaline (Ddpq), [Au₂(μ-Ddpq)Cl₄]Cl₂ and [(AuN^N)₂(μ-Ddpq)](NO₃)₆, where N^N is ethylenediamine, 2,2′-bipyridyl, or 1,10-phenanthroline, is described and the composition, structure, and properties of these complexes are studied. The coordination-induced chemical shifts in the ¹H NMR spectra are determined, as well as the spectral-luminescent and electrochemical parameters of the complexes. The nature of the energetically lowest spin-allowed ¹(π-d*)-and spin-forbidden ³(π-π*) states is established.