Synchrotron Vacuum Ultraviolet Photoionization Mass Research Articles

Experimental and kinetic model studies on the pyrolysis of 2-furfuryl alcohol at two reactors: Flow reactor and jet-stirred reactor

2-Furfuryl alcohol, derived from non-edible biomass, is the significant component of bio-oil in the pyrolysis of lignocellulose biomass and a potential alternative fuel or fuel additive. To better optimize the pyrolysis model of lignocellulose to improve the properties of pyrolysis bio-oil and further understand the combustion characteristics of 2-furfuryl alcohol, the pyrolysis experiments were performed in a flow reactor at 30 Torr for T = 1006 – 1339 K and 760 Torr for T = 850 – 1131 K, and in a jet-stirred reactor at 760 Torr for T = 750 – 1050 K. The pyrolysis products, including radicals (methyl, propargyl, allyl, and cyclopentadienyl), isomers (furfural/2-ethyl furan, furan/vinyl ketene, and allene/propyne), and aromatics (benzene, toluene, indene, etc.) were identified and measured using synchrotron vacuum ultraviolet photoionization mass spectrometry. A comprehensive kinetic model for 2-furfuryl alcohol pyrolysis was developed and was validated against the experimental data in the present and previous work. Rate of production analysis indicated that the C–O bond dissociation reaction, H-abstraction reactions on the hydroxymethyl group, and the H-addition reactions on the furan-ring controlled the consumption of 2-furfuryl alcohol and the formation of primary pyrolysis products like 2-methyl furan, 2-ethyl furan, furfural, and furan. In addition, the OH-addition reaction on the furan-ring had a certain contribution to the consumption of 2-furfuryl alcohol, while the contribution of H-shift reactions was negligible. The present work systematically compared the effects of methyl, ethyl, and hydroxymethyl groups on the decomposition of furanic fuels and the formation of major products. The results showed that the substituent groups can reduce the decomposition temperature of the fuels, especially the ethyl and hydroxymethyl groups. The alkyl substituent group can promote the formation of small hydrocarbons, of which the mole fraction increased with the length of the carbon chain increasing. The hydroxymethyl group facilitated the formation of oxygenated products.

Pyrolysis study of iso-propylamine with SVUV-photoionization molecular-beam mass spectrometry

The pyrolysis of isopropylamine (IPA) was studied in a jet-stirred reactor at 1 atm within 670–1100 K. Pyrolysis products and intermediates were identified and quantified with both synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography analysis. Propiolonitrile, propanenitrile, 1-methylethenylamine, 3-butenenitrile, pyridine and benzonitrile were newly identified compared with previous studies of amines. HCN and 1-methylethenylamine were found to be the dominant N-containing species in IPA pyrolysis. The peak mole fractions and temperatures of C1-C2 hydrocarbons are generally higher than C3-C4 ones. The peak temperatures tend to rise with the increase of degree of unsaturation for hydrocarbons with the same C numbers. Based on the observations, a detailed kinetic model consisting of 429 species and 2407 reactions was developed with reasonable predictions against the measured data. Rate-of-production analysis indicates that IPA is mainly consumed by the H-abstraction at the Cα atom: (CH3)2CHNH2+H = (CH3)2CNH2+H2. Sensitivity analysis shows that the unimolecular decomposition via CC bond cleavage and H-abstraction at the Cα atom play promoting roles. The H-abstraction of propene by H-atom yielding allyl radical and self-combination of methyl radicals forming ethane exhibit inhibiting effects. Compared to the pyrolysis of n-propylamine (NPA), more amine- or imine-like species were generated during NPA pyrolysis while more nitriles were detected in IPA pyrolysis. Moreover, the peak temperatures of hydrocarbons in both NPA and IPA pyrolysis shift to higher values with the increase of degree of unsaturation. The results will enrich the understanding of IPA for further combustion kinetic studies of low sooting fuels.

Improving quantification of hydrogen peroxide by synchrotron vacuum ultraviolet photoionization mass spectrometry

Hydrogen peroxide (H2O2) plays a key role in the autoxidation of fuels related to ignition in compression engines, and in the degradation of volatile organic compounds (VOC) in the atmospheric autoxidation. Advanced synchrotron vacuum ultraviolet radiation photoionization mass spectrometry (SVUV-PIMS) is widely used for in-situ analysis of H2O2 in the gas phase autoxidation process. However, considerable uncertainty has been observed in the quantification of H2O2 by SVUV-PIMS. This work improves the quantification of H2O2 by re-evaluating its photoionization cross section (PICS). Spray vaporization, combined with a low pressure flow reactor, was used to minimize decomposition and wall loss of H2O2 in the PICS measurement. It was found that the PICS of H2O2 obtained here was lower than that reported in the literature by a factor of two. Adopting this newly obtained PICS significantly improved quantification of H2O2, and will prove valuable in evaluating the reaction kinetics of H2O2 and the development of combustion and atmospheric autoxidation models.

Reaction mechanism of toluene decomposition in non-thermal plasma: How does it compare with benzene?

Non-thermal plasma (NTP) catalysis is considered one of the most promising technologies to address a wide range of energy and environmental needs, such as carbon dioxide (CO2) conversion, NH3 synthesis, and volatile organic compounds (VOCs) removal. A systematic approach to optimizing NTP systems benefits from understanding VOCs' fundamental NTP destruction behavior and analyzing the correlations between molecular structures and conversion and selectivity. Herein, the mechanical performance of the toluene destruction in NTP is examined and compared with benzene bearing a similar molecular structure. Different experimental and theoretical techniques are applied, including synchrotron vacuum ultraviolet photoionization mass spectrometry(SVUV-PIMS), thermochemistry, and quantum chemistry. Comparatively, toluene is more readily destroyed under the same NTP conditions than benzene. More intriguingly, the distribution of the decomposition species is significantly different. The theoretical calculations reveal that the abundant methyl radicals generated in toluene decomposition mainly lead to the various species distribution. These radicals promote some reactions, such as the decomposition of o-benzoquinone, one of the key intermediates, thus leading to new reaction pathways and products different from benzene. Finally, the critical mechanistic steps of toluene decomposition under the present non-thermal plasma conditions are established, which include the interactions between toluene and electrons or reactive radicals, the cleavage of the aromatic ring, and the various reaction pathways involving of methyl radicals. This study presents an effective approach to elucidate the distinct fundamental reaction mechanisms arising from subtle structural differences, offering new insights into the underlying plasma chemistry crucial for advancing various promising environmental and energy applications of non-thermal plasma systems.

Exploring low temperature oxidation of iso-octane under atmospheric pressure

Iso-octane is the dominant component of primary reference fuel (PRF), with high knock resistance during the combustion process. Understanding the low temperature oxidation chemistry of iso-octane aids in clarifying the auto-ignition properties of gasoline surrogates and developing their kinetic models. However, its low temperature oxidation performance has only been observed under high pressure conditions; no report in the literature includes detailed speciation, hindering a comprehensive examination of the iso-octane low temperature oxidation mechanism. In this work, low temperature oxidation behavior of iso-octane was clearly observed in jet-stirred reactors (JSRs) under an initial fuel mole fraction of 0.01, residence time of 2 s, equivalence ratio of 0.25, and pressure of 1 bar. The low temperature oxidation reaction networks of iso-octane were explored by measuring low temperature oxidation intermediates in three experimental approaches: synchrotron vacuum-ultraviolet photoionization mass spectrometry (SVUV-PIMS), gas chromatography (GC), and cw-cavity ring-down spectroscopy (cw-CRDS). More than 80 intermediates were qualitatively analyzed in combination with the photoionization efficiency (PIE) spectrum, quantum chemistry calculation of the ionization energies, and GC analysis. Hydroperoxides derived from iso-octane low temperature oxidation were reported experimentally for the first time, including keto-hydroperoxides (KHP) and/or hydroperoxyl cyclic ethers (HPCE). Approximately 40 products were quantified, including aldehydes, ketones, olefins, and carboxylic acids, etc. Kinetic models in the literature have been examined using the datasets obtained from this work and reaction pathways for the important intermediates discussed. Comparison indicated that the iso-octane models in the literature must be improved to accurately capture the performance of iso-octane low temperature oxidation.

Experimental and kinetic modeling investigations on low-temperature oxidation of 2-ethylfuran in a jet-stirred reactor

Biomass fuel, as a kind of alternative fuel, offers the possibility of a higher sustainability of the energy system. Furanic fuels that are proposed as next-generation biomass-derived fuels have become one of the targets of recent researches. Here the studies on the low-temperature oxidation of 2-ethylfuran (EF2) in a jet-stirred reactor were performed at atmospheric pressure, at temperature range of 600–900 K and at three equivalence ratios of 0.5, 1.0 and 2.0, respectively. Over 20 oxidation products were detected and measured by using synchrotron vacuum ultraviolet photoionization mass spectrometry, including hydrocarbons, such as methane, ethylene, acetylene, et al., and oxygenated species, such as formaldehyde, ketene, acetaldehyde, 2-methylfuran and 2-vinylfuran, especially the isomers (furan/vinyl ketene) et al. A detailed low-temperature oxidation kinetic model with 723 species and 3300 reactions based on the reported models and theoretical calculations was developed to demonstrate the experimental results in this work. Rate of production analysis shows that, in the low-temperature oxidation of EF2, the consumption of EF2 is dominated by hydrogen atom-abstraction to produce 1-(2-furyl)ethyl radical and by hydroxyl radical-addition onto C(2) of EF2 to produce 2,3-dihydro-2-hyroxyl-2-ethyl-3-furanyl radical at 50% EF2 consumption. The subsequent dominant consumption pathway of 1-(2-furyl)ethyl radical reacting with hydroperoxyl radical will generate 1-(2-furyl)ethyloxyl radical or 2,5-dihydro-5-ethyliden-2-furanyloxyl radical, while 2,3-dihydro-2-hyroxyl-2-ethyl-3-furanyl radical will subsequently go through β-scission to produce carbon monoxide + acetylene + formyl radical eventually. Sensitivity analysis indicates that reactions related to these four radicals, i.e., hydroxyl, 1-(2-furyl)ethyl, 2,5-dihydro-5-ethyliden-2-furanyloxyl and 2,3-dihydro-2-hyroxyl-2-ethyl-3-furanyl, are critical in the consumption of EF2. Hydroxyl radical is mainly produced by the decomposition of hydroperoxide at the equivalence ratio of 0.5 and 1.0, while at the equivalence ratio of 2.0, reactions between 1-(2-furyl)ethyl radical and hydroperoxyl radical, as well as that between methyl radical and hydroperoxyl radical, become more important.

An experimental and modeling study of ammonia oxidation in a jet stirred reactor

Ammonia is a promising carbon-free fuel and has attracted massive attention nowadays. However, there are still large uncertainties in the mechanism of ammonia oxidation that motivate further fundamental investigations. In this work, oxidation experiments of ammonia were carried out in a jet-stirred reactor (JSR) equipped with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry and gas chromatography. Ammonia/O2/Ar mixtures were tested under wide-range operating conditions covering an oxidation temperature of 700–1200 K and an equivalence ratio of ϕ = 0.1–1.0. Important intermediates and products (e.g., NO, NO2, N2O, N2, H2, and H2O) were detected and quantified. A chemical kinetic model for ammonia oxidation was proposed by integrating the rate constants of critical reactions from different models. The results show that ammonia oxidation reactivity is quite strong at ultra-lean conditions, manifesting lower oxidization temperatures and greater oxidization rates. However, this behavior becomes weakened rapidly with the elevation of equivalence ratio, such that ammonia oxidation characteristics are highly similar between ϕ = 0.5 and 1.0. Meanwhile, important intermediates like NO2 and N2O are only detected under ultra-lean conditions. Reaction pathway analysis for equivalence ratios shows that high oxygen concentration is beneficial to the formation of HO2, which leads to a huge amount of OH and promotes the dehydrogenation of ammonia. With the elevation of equivalence ratio, the dehydrogenation of ammonia becomes difficult and reactions NO+HO2=NO2+OH and NH2+NO2=N2O+H2O are suppressed, which answers for the vanishing of NO2 and N2O at ϕ = 0.5 and 1.0. Reaction pathway analysis for oxidation temperatures also emphasizes the significance of HO2 and its conversion pathway to OH, but they are also strongly influenced by equivalence ratios. The current work provides useful insights into ammonia oxidation and a reliable experimental database for modeling.

Improvement of the Thermal Insulation Performance of Silica Aerogel by Proper Heat Treatment: Microporous Structures Changes and Pyrolysis Mechanism

A simple heat treatment method was used to optimize the three-dimensional network structure of the hydrophobic aerogel, and during the heat treatment process at 200–1000 °C, the thermal conductivity of the aerogel reached the lowest to 0.02240 W/m·K between 250 °C and 300 °C, which was mainly due to the optimization of microstructure and pyrolysis of surface groups. Further Fluent heat-transfer simulation also confirmed the above results. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used to finely measure the pyrolysis process of aerogels, and the pyrolysis process of aerogel was divided into four stages. (I) Until 419 °C, as the temperature continued to rise, surface methyl groups were oxidized to form hydroxyl. (II) As the temperature reached to 232 °C, the oxidation proceeded. In addition, inside the aerogel, because of lacking oxygen, the reaction produced CH4 and C–Si bonds would form. (III) After 283 °C, Si–OH groups began to condense to form Si–O–Si, which optimized the three-dimensional network structures to be beneficial to improve the thermal insulation performance of silica aerogel. (IV) When it reached 547 °C, the chemical reaction was terminated, and all the primary particles gradually fused into secondary particles and sintered to form clusters.

Experimental and kinetic modeling study of the homogeneous chemistry of NH3 and NOx with CH4 at the diluted conditions

Homogeneous oxidation of CH4/NH3/NO/NO2 mixtures in the temperature range from 600 to 1200 K was investigated in this work. Seven gas mixtures were designed to reveal the influence of CH4 and NO/NO2 ratio during the reaction process. The experiments were carried out in a laminar flow reactor at atmospheric pressure. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to achieve comprehensive detection of major products and critical hydrocarbon, carbonyl and nitrogenous intermediates. An improved kinetic model, which incorporates a detailed C1-C2/NOx/NH3 sub-mechanism was developed to complement the experiments. In general, the experimental results are in good agreement with prediction of the kinetic model. Rate of production analysis and sensitivity analysis were performed to interpret the experimental observations. The results show that in the oxidation of NH3/NOx mixtures, the NO/NO2 ratio can affect the conversion of NO through the reaction NH2 + NO2 = H2NO + NO. In the presence of CH4, the oxidation reactivity of NH3/NOx mixture is significantly promoted. As the added amount of CH4 increases, the production of H2O is enhanced while N2 formation is inhibited. Beside, the addition of CH4 also promotes the production of carbonyl and nitrogenous species. In the oxidation of CH4/NH3/NOx mixtures, the change of NO2/NO ratios only has little effect on the formation of major products and intermediates.

Variable pressure JSR study of low temperature oxidation chemistry of n-heptane by synchrotron photoionization mass spectrometry

Low temperature oxidation chemistry is crucial in the auto-ignition process of internal combustion engines. The development of laboratory based reactors and diagnostic systems promotes understanding of the low temperature oxidation mechanism. This work develops a variable pressure jet-stirred reactor (VP-JSR) platform working from one to ten bar. Compared to previous high pressure studies which routinely use gas chromatography to analyze low temperature oxidation products, the difference in this work is the possibility of probing intermediates by synchrotron vacuum ultraviolet photoionization mass spectrometry via molecular beam sampling. The setup was validated by repeating n-heptane low temperature oxidation experiments at one and ten bar, respectively. Good agreement was observed between the data in this work and the data in the literature. A much more detailed species pool was probed compared to the literature study, including H2O2, carbonyl acids, alkylhydroperoxides, and keto-hydroperoxides. As a first demonstration of the usefulness of this design, the low temperature oxidation of n-heptane with initial fuel mole fraction of 0.005, residence time of 2 s, and equivalence ratio of 1.0 was studied at one, five, and ten bar. The preliminary results, including reactants, final products, and the initial low temperature oxidation intermediates, are discussed. The VP-JSR system developed in this work is valuable for study of the fuel chemistry from one to ten bar, to develop and examine chemical kinetic models, and to guide the development of the reaction system at even higher pressures, such as those in engine function and supercritical conditions.

Exploring NH3 and NOx Interaction Chemistry With CH4 and C2H4 at Moderate Temperatures and Various Pressures

The oxidation of CH4/C2H4/NH3/NO/NO2 gas mixtures was studied aiming to explore the homogenous chemistry of exhaust gas from lean-operated natural gas engine. With respect to this goal, experiments were carried out with a laminar flow reactor under engine-relevant (diluted and lean) conditions over the temperature range of 600–1400 K. Four gas mixtures were designed to evaluate the effects of NO/NO2 ratio (1, 4) and pressure (0.04 and 1.0 atm) on the interaction chemistry of NH3/NOx with CH4 and C2H4. By using synchrotron vacuum ultraviolet photoionization mass spectrometry, fingerprint products for revealing interaction chemistry were identified and quantified, e.g., nitrogenous and oxygenated intermediates. The experimental results show that the NO concentrations are significantly affected by adding CH4/C2H4, changing NO/NO2 ratio and pressure. Besides, the promotion of DeNOx reactions and narrower temperature windows of NO reduction are unexpectedly observed in the presence of CH4/C2H4. To interpret the experimental observations, a detailed kinetic model was developed by integrating hydrocarbons/NH3/NOx interaction chemistry. Rate of production and sensitivity analyses indicate that the active radical pool is enriched and additional chain-branching pathways regarding NO/NO2 interconversion are activated with the addition of hydrocarbons. In the presence of both CH4 and C2H4, reaction C2H3 + O2 = CH2CHO + O was demonstrated as a crucial reaction that drives the reactivity of CH4/C2H4/NH3/NO/NO2 mixture. This is explained by the production of CH2CHO, whose dissociation generates CH2O and ultimately leads to the abundant production of active OH via the reaction sequence CH2O → HCO → HO2 → OH. The conversion kinetics of hydrocarbons, NO and NH3 under different NO/NO2 ratios and pressure, as well as the formation kinetics of oxygenated and nitrogenous intermediates was also analyzed in this work.

Conformation-dependent low-temperature oxidation chemistry of methylcyclohexane: First oxygen addition and chain-branching

Conformational effect is an important feature of the low-temperature (low-T) oxidation chemistry of cycloalkanes. In this work, both theoretical calculation and conformational analysis were performed for the first oxygen addition in methylcyclohexane (MCH) oxidation to better understand the influence of the spatial orientation and the steric hindrance of substituted functional groups. Based on synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) measurements and adiabatic ionization energy calculations, reactive hydroperoxides, highly oxygenated molecules and stable products were identified in the jet-stirred reactor (JSR) oxidation of 1.0% and 0.5% MCH at 1.04 bar, 500–900 K and the equivalence ratio of 0.25. The abundant formation of 1-methylcyclohexene and the observation of alkenylhydroperoxides and their decomposition products verify the existence of conformation-dependent pathways in first and second oxygen addition in low-T MCH oxidation, respectively. A MCH oxidation model incorporated with conformation-dependent pathways was constructed to consider the conformational effects of cyclohexane ring and sidechain, and was validated against experimental data over a wide range of conditions in this work and literature. The consideration of conformational effects of cyclohexane ring and sidechain can generally better predict the measured species profiles and ignition delay time under low-T conditions. Based on modeling analysis, conformational effects of cyclohexane ring were found to play important roles in first oxygen addition and chain-branching in low-T MCH oxidation. Compared with the situation in low-T cyclohexane oxidation, the methyl substitution on cyclohexane ring reduces its conformational effects on chain-branching process.

Investigation on n-pentylbenzene combustion at various pressures: Insight into effects of side-chain length on alkylbenzene combustion

This work investigated the flow reactor pyrolysis of n-pentylbenzene using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at 0.04 and 1 atm, and measured the laminar burning velocities (LBVs) of n-pentylbenzene in a high-pressure constant-volume cylindrical combustion vessel at 473 K, 1–10 atm and equivalence ratios (ϕ) of 0.7–1.5. A kinetic model of n-pentylbenzene combustion was developed from our previous models of n-butylbenzene and validated against the present experimental data. The unimolecular decomposition reactions and H-abstraction reactions are the dominant consumption pathways of n-pentylbenzene under pyrolysis conditions. Aromatic intermediates with unsaturated side-chains play crucial roles in indene and naphthalene formation, while phenyl and benzyl are precursors of unfused polycyclic aromatic hydrocarbons (PAHs) which in turn convert to fused tricyclic PAHs. In the laminar flame propagation, small species reactions are crucial under lean conditions, while chain-termination reactions, especially fuel-specific reactions, show important inhibition effects on the laminar flame propagation under rich conditions. Furthermore, the effects of side-chain length on the pyrolysis and laminar flame propagation of alkylbenzenes were also investigated for toluene, ethylbenzene, n-propylbenzene, n-butylbenzene and n-pentylbenzene. It is concluded that alkylbenzene smaller than n-butylbenzene tends to decompose at low temperature as the side-chain length increases, while n-butylbenzene and n-pentylbenzene have very close decomposition temperature regions. For indene and naphthalene formation, decomposition/cyclization reaction sequences of C9 and C10 alkenylbenzenes have rapidly increasing importance in the pyrolysis of large alkylbenzenes. Benzyl is concluded to play an important role in inhibiting laminar flame propagation of alkylbenzenes, which is an important reason for the observations that toluene and ethylbenzene have the lowest and highest LBVs, respectively.

A decoupled modeling approach and experimental measurements for pyrolysis of C6-C10 saturated fatty acid methyl esters (FAMEs)

Biodiesels are promising renewable fuels that can aid in the transition to carbon neutrality. The high molecular weight and complex composition of real biodiesel fuels complicate development of compact kinetic models needed for engine simulations. Our group previously proposed the functional group approach (FGMech) to model real-fuel combustion based on the identification of intrinsic relationships between fuel molecular structure and model parameters. Establishing these relationships requires a database consisting of the model parameters of pure fuels for training. In this work, we selected five fatty acid methyl esters (FAMEs) as target fuels, including methyl pentanoate (MPE), methyl hexanoate (MHX), methyl heptanoate (MHP), methyl octanoate (MO) and methyl nonanoate (MN). To facilitate development of an FGMech reaction scheme, a decoupling model approach is adopted here for model construction. Lumped reaction mechanisms are developed to describe the (oxidative) pyrolysis of fuels while a detailed model is used for describing the conversion of pyrolysis intermediates. To validate the present model, pyrolysis experiments for these FAMEs are conducted in a jet-stirred reactor (JSR) at 1 atm and over 790–1120 K. Both the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC)/GC–MS are applied for measuring pyrolysis intermediates. The fuels, primary hydrocarbon and oxygenated products, secondary products including various aromatic compounds, are identified and quantified for model validation. The present model well-predicts the temperature window of fuel decomposition, and reasonably predicts the yields of most pyrolysis products under both present atmospheric conditions and high pressure conditions in literature. The agreement between the measured and predicted results indicates that the present decoupling methodology can accurately describe fuel decomposition and the evolution of intermediates under pyrolysis conditions. In addition, it is found that increasing alkyl CH2 groups in C6 to C10 FAMEs has little influence on the yields of primary oxygenated products; however, increasing yields of hydrocarbon products with increasing alkyl CH2 groups indicates that alkane chemistry becomes more important moving from MPE to MN.

Experimental and kinetic modeling study of di-n-propyl ether and diisopropyl ether combustion: Pyrolysis and laminar flame propagation velocity

To explore the fuel isomeric effect on ether combustion characteristics, pyrolysis, and laminar flame propagation of di-n-propyl ether (DPE) and diisopropyl ether (DIPE) were investigated. A new kinetic model of DIPE was developed based on our recent DPE model [Fuel 298 (2021) 120797]. The pyrolysis experiments were carried out in two jet-stirred reactors at near-atmospheric pressure with good agreement on the measurements using synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography/mass spectrometry. The decomposition profile of DIPE showed a faster trend than that of DPE, which can be attributed to the faster alcohol elimination reaction of DIPE. The dominant roles of alcohol elimination reaction and H-abstraction reactions in fuel consumption explain the production of fuel-specific oxygenated species, i.e. n-propanol and propanal in DPE and i-propanol, acetaldehyde and acetone in DIPE. The laminar burning velocities of DPE and DIPE were also measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 373 K and pressures of 1–10 atm. It was found that the linear DPE propagated faster than DIPE with the branched structure under all investigated conditions. Rate of production analysis and sensitivity analysis were also performed to elucidate the key radicals and reactions responsible for the remarkable reactivity of isomeric fuels. Fuel structures have a great influence on the distribution of radical pools, resulting in the easy formation of active radicals in DPE flames such as vinyl and ethyl and stable radicals in DIPE flames like methyl and allyl. The successive decomposition reactions of these dominant radicals promote the DPE flame and inhibit the DIPE flame propagation respectively, which explains the higher laminar burning velocities and reactivity of DPE than that of DIPE. Furthermore, the present model was also examined against the literature data, including pyrolysis and oxidation in the jet-stirred reactor and flow reactor.

Experimental and kinetic modeling studies of 2-acetylfuran pyrolysis at atmospheric pressure

Furan-based derivatives are promising biomass fuels that may be used directly in internal combustion engines as alternative fuels or as additives to fossil fuels. In this work, the experimental studies of 2-acetylfuran (AF2) pyrolysis were carried out in a jet-stirred reactor in the temperature range of 770 – 1130 K and at the pressure of 760 Torr, using synchrotron vacuum ultraviolet photo-ionization mass spectrometry, coupled with gas chromatography. Key pyrolysis intermediates such as methane, ethylene, acetylene, ketene, vinylacetylene, cyclopentadiene, ethynylketene, 2-methylfuran, furfural, carbon monoxide and isomers such as allene/propyne, furan/vinylketene, benzene/fulvene, etc., were identified and quantified. The potential energy surfaces of AF2 unimolecular decomposition reactions were calculated at CBS-QB3 level. The temperature- and pressure-dependent rate constants of the relevant reactions were calculated by solving the master equation based on Rice-Ramsperger-Kassel-Marcus theory. A detailed pyrolysis kinetics of AF2 was developed based on the previous combustion models of 2-methylfuran and was validated against the current experimental results. Rate of production and sensitivity analysis showed that the main consumption pathways of AF2 pyrolysis under atmospheric pressure are unimolecular decomposition reactions, leading to 2-furyl + acetyl and furoyl + methyl, and hydrogen atom-addition reaction to C(5), leading to 2,3-dihydro-5-acetyl-3-furyl, accounting at 1070 K for 25.1%, 13.2% and 20.7% of AF2 consumption, respectively (the remaining 41.0% are due to minor reaction channels contributing < 10% each). At this temperature, the total AF2 consumption is 73%. The contributions of ipso-substitution reactions by methyl and by hydrogen atom to AF2 consumption were comparable.

Comprehensive study of the low-temperature oxidation chemistry by synchrotron photoionization mass spectrometry and gas chromatography

Comprehensive analysis of the low-temperature oxidation intermediates, including the reactive intermediates and isomers, is crucial to develop the low-temperature oxidation models of the fuels. In this aim, the complementary analysis by different analytic methods is needed. Furthermore, the cross check by different analytic methods increases the fidelity of the experiment data. In this work, we developed a jet-stirred reactor (JSR) system that coupled to the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC) analysis. The JSR was designed to couple the SVUV-PIMS and GC, and the temperature homogeneity and mixing of the JSR was examined by a CFD simulation. The SVUV-PIMS system was developed to have a mass resolution of ca. 5000 at m/z 100. The GC system included a flame ionization detector (FID), thermal conductivity detector (TCD), and a mass spectrometer (MS). This newly developed setup was validated by repeating the oxidation data of n-heptane in the literature. A good agreement was observed between the two datasets. The low-temperature oxidation of n-butane was then studied to show the advantage of this setup, i.e., the simultaneous and comprehensive measurement of the low-temperature oxidation products. Specifically, the SVUV-PIMS enables the separation of the C/H/O composition of the intermediates, the measurement of formaldehyde and the peroxide intermediate, while the GC analysis gives a better separation of the oxygenated isomers and the quantification of the species that the SVUV-PIMS has difficult in quantifying. The experimental method developed in this work is valuable to study the fuel low-temperature oxidation chemistry, to reveal the species pool, and to provide reliable mole fraction data for the validation and development of combustion models.

Effect of the modification of alumina supports with chloride on the structure and catalytic performance of Ag/Al2O3 catalysts for the selective catalytic reduction of NOx with propene and H2/propene

The effect of the modification of an alumina support with chloride on the structure and the catalytic performance of Ag/Al2O3 catalysts (SA) was investigated for the selective catalytic reduction (SCR) of NO using C3H6 or H2/C3H6 as reductants. The Ag/Al2O3 catalyst and Cl−-modified Ag/Al2O3 catalysts (SA-Cl) were prepared by a conventional impregnation method and characterized by X-ray diffraction, Brunauer-Emmett-Teller isotherm analysis, electron probe microanalysis, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and hydrogen temperature-programmed reduction. The catalytic activities in the C3H6-SCR and H2/C3H6-SCR reactions were evaluated, and the reaction mechanism was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy and synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS). We found that Cl− modification of the alumina-supported Ag/Al2O3 catalysts facilitated the formation of oxidized silver species (Agnδ+) that catalyze the moderate-temperature oxidation of hydrocarbons into partial oxidation products (mainly acetate species) capable of participating in the SCR reaction. The low-temperature promoting effect of H2 on the C3H6-SCR (“hydrogen effect”) was found to originate from the enhanced decomposition of strongly adsorbed nitrates on the catalyst surface and the conversion of these adsorbed species to –NCO and –CN species. This “H2 effect” occurs in the presence of Agnδ+ species rather than the metallic Ag0 species. A gaseous intermediate, acrylonitrile (CH2CHCN), was also identified in the H2/C3H6-SCR reaction using SVUV-PIMS. These findings provide novel insights in the structure-activity relationship and reaction mechanisms of the SA-catalyzed HC-SCR reaction of NO.

On-line photoionization mass spectrometric study of the catalytic pyrolysis of acrylonitrile-butadiene-styrene copolymer over HZSM-5, HUSY and Al-MCM-41

Three molecular sieve-based catalysts, HZSM-5, HUSY, and Al-MCM-41, were employed in the catalytic pyrolysis of acrylonitrile-butadiene-styrene (ABS) copolymer. The on-line synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) together with pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was used to evaluate the performances of these catalysts in the catalytic pyrolysis processes, especially their denitrogenation ability on the pyrolysis oil. Based on the Py-GC/MS results, the formation of N-containing oligomers can be significantly suppressed with the presence of HZSM-5 and HUSY, over which a high content of Brønsted acid sites were detected. In contrast, although large amounts of polycyclic aromatic hydrocarbons can be formed over Al-MCM-41, little effect was exhibited on the yields of major products owing to its inferior acidity. Given the relative production of light nitrogenous compounds, it can be concluded that HUSY is more applicable to the denitrogenation than HZSM-5. On account of the temperature-evolved profiles of the aromatic products acquired with SVUV-PIMS in real time, two different reaction paths were proposed for the catalytic pyrolysis over zeolite catalysts, by which the formation of valuable compounds can be clearly demonstrated, including benzene, toluene, ethylbenzene, and styrene. Moreover, it has been directly evidenced that the framework structure of HUSY can facilitate intermolecular hydrogen transfer to a great extent. This work highlights the superiority of SVUV-PIMS in the study of catalytic decomposition mechanisms of complex copolymers.

A comprehensive study on low-temperature oxidation chemistry of cyclohexane. II. Experimental and kinetic modeling investigation

Low-temperature oxidation of cyclohexane is investigated in two jet-stirred reactors (JSRs) at 1.04 bar and the equivalence ratio of 0.25. Reactive hydroperoxides and highly oxygenated molecules are detected using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The isomers of C6H10O (5-hexenal, cyclic ethers and cyclohexanone) are separated using gas chromatography combined with mass spectrometry (GC–MS). Detection of characteristic hydroperoxides verifies that the conventional two-stage oxygen addition channels and recently reported third oxygen addition channels both contribute to the low-temperature oxidation of cyclohexane. Conformation-dependent channels theoretically investigated in Part I of this work are found correlated with the experimental observations of ketohydroperoxide (KHP) and alkenyl-hydroperoxide (AnHP) intermediates. A new detailed kinetic model of cyclohexane oxidation is constructed with consideration of the investigated conformation-dependent pathways in Part I and the experimental revisit of OH attack reactions over 889–1301 K and 1.22–1.84 bar. The model is validated against the newly measured oxidation data in this work and previous experimental data over a variety of pressure, temperature and equivalence ratio conditions. Modeling analysis reveals that the KHP channel and AnHP channel dominate the chain-branching process under the investigated conditions. The third oxygen addition channels and bimolecular reaction channels are found to play less important roles under the investigated conditions, while these reactions can provide more significant contributions to OH formation under high-pressure and lean conditions.