Polycyclic Aromatic Hydrocarbons Chemistry Research Articles

A systematic analysis of chemical mechanisms for ethylene oxidation and PAH formation

Accurate prediction of soot formation and evolution remains a formidable challenge due to the complex interaction between gas-phase composition and solid-phase particles. Recent studies have shown that the choice of gas-phase mechanisms is of primary importance in affecting predictability. In this work, a systematic analysis of ethylene (C2H4) combustion mechanisms denoted as KAUST, Stanford, Aachen, Polimi, ABF, DLR/UT, Naples, Caltech, and SJTU, which have been widely used in the soot community, is performed to investigate their differences in fundamental chemistry, polycyclic aromatic hydrocarbon (PAH) chemistry, and soot prediction. It is found that the nine mechanisms exhibit large differences even in predicting canonical combustion properties (e.g., ignition delay time, laminar flame speed, and extinction strain rate), indicating significant variations in the fundamental chemistry. This is due to the fact that although most mechanisms share very similar dominant fuel-oxidation reactions, there are notable differences in the rate coefficients of sensitive reactions used in these mechanisms. Owing to the uncertainties in the fundamental chemistry, the predictions of C2H2 from the nine mechanisms show significant differences, which contributes to the differences in soot precursor prediction, in conjunction with the difference in benzene (A1) formation pathways demonstrated by the element flux analysis of the C atom. Furthermore, it is found that PAHs containing two rings play a dominant role in soot formation for most mechanisms. Moreover, it is observed that while Caltech and SJTU significantly under estimate soot formation, they reasonably reproduce its sensitivity to strain rate. These results indicate that despite substantial advances in the development of C2H4 oxidation and PAH formation chemistry, the various existing mechanisms lead to significant differences in predicting soot concentrations which can be traced back to considerable differences in both fundamental chemistry and PAH chemistry. This suggests that the fundamental chemistry should be calibrated or improved before further development of PAH chemistry and soot models.

Probing PAH Formation from Heptane Pyrolysis in a Single-Pulse Shock Tube

ABSTRACT To improve our capabilities to model surrogate fuels, particularly in respect to polycyclic aromatic hydrocarbons (PAHs) and soot formation, the pyrolysis of n-heptane is studied in a single-pulse shock across a wide temperature range (900–1700 K) at 20 bar nominal pressure and 4 ms residence time. Three different initial fuel mole fractions are considered, 103, 502, and 2000 ppm of n-heptane in argon. Fuel and intermediate species, including aromatics up to three-ring structures, are measured using gas chromatography and mass spectrometry diagnostics. An ongoing detailed chemical kinetic model for PAH chemistry has been updated to successfully capture the fuel decomposition, the formation of small hydrocarbons, and the concentration of the main PAH products. Major reaction pathways to PAHs are highlighted as well as the role of important intermediate species.

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UNDERGRADUATE EDUCATION Reactions ShareShare onFacebookTwitterWechatLinked InRedditEmail C&EN, 2023, 101 (7), p 3February 27, 2023Cite this:C&EN 101, 7, 3Letters to the editorOrganic chemistryAs a footnote to Mark M. Green’s letter on how to make organic chemistry “be part of understanding the wider world, to be worth knowing” (Jan. 2, 2023, page 3), I wish to illustrate my approach to “a great story to be told.”For over 50 years I have incorporated five special topics in my sophomore organic chemistry courses starting on day 1. “Chemical Evolution” looks at the simple chemicals and chemical reactions occurring in our universe, Stanley Miller’s exotic prebiotic syntheses of amino acids and peptides, and the simple organic compounds recently found on comets and meteorites. “Insect Pheromones” is introduced during the discussion of alkanes and alkenes, with examples like muscalure, the long-chain alkene of the housefly sex pheromone (a Wittig reaction synthesis), and illustrated by the syntheses of the queen bee pheromone and the Japanese beetle sex pheromone. Halogen addition reactions of alkenes that occur in the ocean via bromonium ions are examples of the natural halogenated alkanes produced by seaweeds (the smell of the ocean), including 1,2-bromochloroalkanes formed when oceanic chloride attacks a bromonium ion generated by bromoperoxidase. “Pesticides and Herbicides” includes the natural insecticides rotenone, caffeine, pyrethrins, and cocaine. Pyrethrins are naturally occurring cyclopropanes, and physostigmine, the “drug of divine justice,” is a natural carbamate. The preparation of DDT and DDD, which includes the cancer drug mitotane, involves electrophilic aromatic substitution and carbonium ion chemistry and illustrates ortho and para ring selectivity. Vietnam defoliants, presented alongside pesticides and herbicides, involve the syntheses of 2,4-D and 2,4,5-T, which illustrate nucleophilic aromatic substitution, benzyne generation, and dioxin formation. “Chemical Carcinogenesis” comprises the metabolism and chemistry of polycyclic aromatic hydrocarbons, benzene versus toluene, aromatic amines, azo dyes, nitrosamines, and natural carcinogens (such as aflatoxin). Protein and nucleic acid chemistry includes “Chemical Warfare,” featuring tear gas, chlorine, phosgene, mustard gas, and organophosphates (such as nerve gas) and the inhibition of acetylcholinesterase. The reaction of pepper spray exemplifies the Michael reaction with nucleic acids, as do the environmental pollutants acrylamide and acrolein.Each special topic occupies 1–3 lectures of the 30–32 course lectures. The special topics and accompanying slides are intermeshed with these conventional organic chemistry topics: alkene and alkyne electrophilic addition, nucleophilic substitution (SN1, SN2), aromatic electrophilic and nucleophilic addition, aryne formation, reductive amination, carbonyl addition and condensation, conjugate addition (Michael reaction), haloform reaction, and Diels- Alder cycloaddition.It will take some work, but it’s worth the effort.Gordon W. GribbleLebanon, New HampshireMuch has already been said about the fiasco surrounding the firing of professor Maitland Jones Jr. by New York University as the result of a student petition claiming that his organic chemistry class was too difficult. Without wishing to comment on this case myself, I am reminded of a related aspect of teaching organic chemistry that I suspect others may also have experienced.Organic chemistry seems to hold a special place in the hearts and minds of most practicing physicians. Some have been left with very positive feelings about this subject, while others have very negative feelings. Both groups of doctors keenly remember how important it was for them to receive good grades in organic chemistry to be admitted to medical school.Today, when I tell my doctor that I’ve taught organic chemistry, this always draws an immediate response. Some light up with a smile and tell me how much they’ve enjoyed this class, while others frown and offer words to the contrary. When the latter happens, this does give me pause for thought. Is this the right doctor for me? On one occasion, only moments before I was anesthetized for a colonoscopy, my gastroenterologist asked me what class I was teaching at my university. When I told her it was organic chemistry, her face lit up with a smile. Oh, what a relief that was.Steven L. RegenBethlehem, Pennsylvania CorrectionsFeb. 13, 2023, page 8: In the news story about Croda’s acquisition of Solus BioTech, the caption incorrectly labels parts of the ceramide NP structure. The sphingosine group is red and the fatty acid is blue, not the other way around.Feb. 13, 2023, page 13: The news story about Alondra Nelson misidentifies the organization she is returning to. It is the Institute for Advanced Study, not the Institute for Advanced Studies. And the institute is in Princeton, New Jersey, not part of Princeton University.

An experimental and modeling study of tetramethyl ethylene pyrolysis with polycyclic aromatic hydrocarbon formation

Iso-olefins, in the C5–C8 range can potentially be blended with renewable gasoline fuels to increase their research octane number (RON) and octane sensitivity (S). RON and S increase with the degree of branching in iso-olefins and this is a desirable fuel anti-knock quality in modern spark-ignited direct-injection engines. However, these iso-olefins tend to form larger concentrations of aromatic species leading to the formation of polycyclic aromatic hydrocarbons (PAHs). Thus, it is important to understand the pyrolysis chemistry of these iso-olefins. In this study, a new detailed chemical kinetic mechanism is developed to describe the pyrolysis of tetramethyl ethylene (TME), a symmetric iso-olefin. The mechanism, which includes the formation of PAHs, is validated against species versus temperature (700–1160 K) measurements in a jet-stirred reactor at atmospheric pressure and in a single-pulse shock tube at a pressure of 5 bar in the temperature range 1150–1600 K. Synchrotron vacuum ultraviolet photoionization mass spectrometer (SVUV-PIMS) and gas chromatography (GC) systems were used to quantify the species in the jet-stirred reactor and in the single-pulse shock tube, respectively. The mechanism derives its base and PAH chemistry from the LLNL PAH sub-mechanism. The predictions are accurate for most of the species measured in both facilities. However, there is scope for mechanism improvement by understanding the consumption pathways for some of the intermediate species such as isoprene. The formation of 1, 2, and 3-ring aromatic species such as benzene, toluene, naphthalene and phenanthrene measured experimentally is analyzed using the chemical kinetic mechanism. It is found that the PAH formation chemistry for TME under pyrolysis conditions is driven by both propargyl addition reactions and the HACA mechanism.

Polarizability of Kekulene, Septulene, and Nearest Non-Planar Polycyclic Aromatic Hydrocarbons

The polarizability of polycyclic aromatic hydrocarbons (PAHs) is an important property that relates to their abundance in natural environments. To assess the differences in the mean polarizability of planar and non-planar polycyclic aromatic hydrocarbons (PAHs), we computationally studied the PAH series of circulenes (kekulene C48H24, [13]circulene C52H26, and septulene C56H28), a number of the nearest helicenes, and their “expanded” isomers. We mean under “nearest” a close number of aromatic rings: 11 (C46H26), 12 (C50H28), 13 (C54H30), and 14 (C58H32). For these PAHs, we performed the quantum chemical calculations of thermodynamic and polarizability parameters with the PBE/3ζ density functional theory method, which is widely used in the theoretical chemistry of fullerenes and PAHs. The calculated mean polarizabilities (in Å3) ranged from 80.1 for [11]helicene to 135.5 for septulene, and while the circulenes and expanded helicenes had similar values, the mean polarizability of the normal helicenes was markedly lower. In all four pairs of helical PAHs, the expanded helicene was energetically considerably more favorable than its standard helicene isomer. Herewith, the ratio of their polarizabilities was equal to 1.3.

Probing the influence of hydrogen cyanide on PAH chemistry

Hydrogen cyanide (HCN) is an important and highly toxic intermediate in coal, nitrogen-rich biomass, and ammonia combustion. It may play an important role in polycyclic aromatic hydrocarbons (PAH) and soot reduction, but the influence of HCN on PAH chemistry is unclear due to the lack of experimental data. With this motivation, the experimental and numerical investigations were performed in this work. Key intermediates were detected, identified, and quantified by the combination of Linear Trap Quadropole (LTQ) Velos Orbitrap mass spectrometer and gas chromatography-mass spectrometer in a jet-stirred reactor fueled by C2H2/HCN/N2 in the temperature range of 800-1200 K. The results reveal that the formation of benzene increases with the addition of HCN, but the PAH formation decreases. PAH reduction can be attributed to the formation of N-containing PAH (NPAH) via HCN-PAH interaction reactions, which were investigated by quantum chemistry and the Rice–Ramsperger–Kassel–Marcus theory with solving the master equation (RRKM-ME). Reaction rate comparison suggests that the HCN addition pathways to NPAH compete with the C2H2 addition pathways to PAH. The product yields in the systems of 1-naphthyl radical + C2H2 and 1-naphthyl radical + HCN were evaluated (T=800-2500 K and p=0.1-100 atm). The results indicate that the increasement of a new aromatic ring via cyclization is difficult in the 1-naphthyl radical + HCN system due to the high energy barrier, and the growth of larger NPAH is limited since the saturated N atoms in the heterocycle rings and CN functional group inhibit further carbon addition.

Prenucleation chemistry of aromatics: A two-ring precursor?

Prenucleation reaction chemistry of polycyclic aromatic hydrocarbons was examined in detailed, site-specific kinetic Monte-Carlo simulations, tracking a single aromatic structure evolving from naphthalene in soot environments of two laminar premixed flames. The numerical results offer a possible reconciliation of a recent flame-based suggestion that one- or two-ring aromatics are the precursors to soot with the otherwise accumulating evidence that soot nucleation begins with mid-size aromatics. The resolution rests in the numerical observation of rapid destruction of the two-ring aromatic molecule, naphthalene, and its replacement with cyclopenta-added derivative structures. The analysis of the reaction events identified important contributions to aromatics evolution from methyl addition to edge five-membered rings. The computed curvature of the growing aromatics exhibited a similar dependence on the molecular size in two substantially different flame environments, thus raising a possibility of universality in aromatic-structure evolution.

Combined high-pressure experimental and kinetic modeling study of cyclopentene pyrolysis and its reactions with acetylene

A combined experimental and kinetic modeling study is presented to improve the understanding of the formation of polycyclic aromatic hydrocarbons (PAHs) from neat cyclopentene and cyclopentene/acetylene mixtures. High-pressure experiments are conducted for the first time over a temperature range covering 930–1650 K using a single-pulse shock tube coupled to gas chromatography/gas chromatography-mass spectrometry (GC/GC–MS) techniques. Several updates and inclusions, mainly regarding the reactions involving C5 molecules and radicals, are made in our on-going PAH kinetic model, which shows satisfactory predictive performances for the speciation measurements obtained in the current work and in the literature. On the basis of the experimental observations and modeling analyses, the reaction pathways active during the pyrolysis of cyclopentene are illustrated and the effects of acetylene addition as co-reactant on the PAH chemistry are assessed. In all of the cases investigated, it is noted that the cyclopentadienyl radical largely participate in the formation of mono-aromatic hydrocarbons (benzene and styrene) and PAHs (indene, naphthalene and phenanthrene).

Recent Advances in Emission, Analysis and Remediation technique of Carcinogenic Polycyclic Aromatic Hydrocarbons: A Review

<p>Polycyclic Aromatic Hydrocarbons (PAHs) are aromatic hydrocarbons with three or more fused benzene rings that include carbon and hydrogen. These are thought to be harmful to human health, and we cannot avoid being exposed to them because they are widely disseminated throughout the environment. PAHs enter our bodies through the air, water, food, and soil. The developments in different sources of generation, analysis, and remediation approaches are reviewed in this paper based on recent available literature. The chemistry of PAHs, their exposure routes, and their health implications are also discussed. In this manuscript future scope of work as well as removal and reduction of carcinogenic PAHs are also discussed briefly. These types of review article are one of the most promising solutions to reduce air pollution issues from environment. This study could be one of the best studies to reduce carcinogenic pollutants from our environment.</p>

Quantitative Resonance Theory Based on the Clar Sextet Model.

Despite its great explanatory power in understanding the chemistry of polycyclic aromatic hydrocarbons (PAHs) and related systems, the Clar sextet rule still remains an intuitive and qualitative model with notable exceptions in some cases. Here we develop a quantitative theory of chemical resonance based on semilocalized Clar-type resonance structures (named the Clar resonators) consisting of variable numbers of Clar sextets and C═C bonds. The constructed wave functions of the Clar resonators are used to expand the actual wave function of the π-conjugated system obtained from a DFT or Hartree-Fock calculation. The resultant weights and one-electron energies of the Clar resonators can serve as a quantitative measure of the importance of these resonators. Implementing the theory in our open-source python code EzReson and applying it to over a thousand PAH molecules of different sizes and shapes, we show that the weight of the Clar resonators increases exponentially with increasing number of sextets and that their energy decreases linearly with the latter, thus confirming the general validity of the Clar rule. On the basis of such a large-scale resonance analysis, we propose three extended Clar rules, along with a unified quantitative model, that are able to evaluate the importance of all Clar resonators and the ring aromaticity for PAHs. Using the present theories, we uncover the essential role that the minor Clar resonators may play in correctly understanding the resonance stabilization and local aromaticity of rings, which was totally overlooked in the original Clar model.

Exploring the combustion chemistry of anisole in laminar counterflow diffusion-flames under oxy-fuel conditions

Biomass combustion under oxy-fuel conditions, i.e., burning biomass by using CO2/O2 mixtures as oxidizer instead of air, is a promising approach to mitigate climate change by recycling CO2 from the exhaust gas. Understanding oxy-fuel biomass combustion chemistry can further help to achieve higher efficiency and lower emissions in future design concepts. In this work, we investigated the combustion chemistry of anisole, a potential component of surrogates for biomass and volatiles, in two selected laminar counterflow diffusion flames under oxy-fuel conditions. A time-of-flight molecular-beam mass-spectrometer (ToF-MBMS) and a gas chromatograph with a mass spectrometer (GC–MS) were used to analyze the chemical compositions of the gaseous samples at various flame positions. The combined measurements allowed us to identify and quantify over 40 species, including many polycyclic aromatic hydrocarbons (PAH) and oxygenated polycyclic aromatic hydrocarbons (OPAH). Comparing the experimental results to numerical simulations using the latest kinetic model from Yuan et al. (Combust. Flame 2019, 201, 187–199), we found that some of the aromatic hydrocarbons (e.g., phenylacetylene, styrene, and ethylbenzene), PAH molecules, (e.g., naphthalene and acenaphthylene), and OPAH molecules (e.g., cresol, benzofuran, and dibenzofuran) were overpredicted by the kinetic model. The model analysis indicated that the reaction rate coefficients in PAH and OPAH chemistry may be responsible. Tentative kinetic model updates showed improvement on the predictions of benzofuran, naphthalene, and acenaphthylene. In summary, this work provides comprehensive speciation datasets from two measurement techniques, examines the latest anisole kinetic model in laminar counterflow diffusion-flames, and provides novel insights for biomass combustion chemistry under oxy-fuel conditions.

Experimental and kinetic modeling study of α-methyl-naphthalene pyrolysis: Part II. PAH formation

α-Methyl-naphthalene plays an important role as a functional material in petrochemical industries and as a precursor of soot particles. The formation chemistry of polycyclic aromatic hydrocarbons (PAHs) from α-methyl-naphthalene, therefore, warrants detailed investigations. In this work, we studied PAH formation from its pyrolysis using experiments and kinetic models. Flow reactor pyrolytic experiments at low and atmospheric pressures (30 and 760 Torr) were performed using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). A kinetic model was then developed to predict PAH formation from α-methyl-naphthalene. According to the kinetic analysis of the proposed model, naphth-1-yl-methyl, benzo-fulvenallene, and benzo-fulvenallenyl are three critical intermediates in the formation of large PAHs. Other than the traditional H-abstraction acetylene-/vinylacetylene-addition mechanisms, three prototypical PAH formation pathways are identified in α-methyl-naphthalene pyrolysis: 1) addition and cyclization reactions of naphth-1-yl-methyl and naphth-1-yl radicals; 2) recombination of resonance stabilized radicals (indenyl, benzo-fulvenallenyl, phenalenyl, etc.) and the subsequent ring expansion reactions; 3) sequential propargyl addition reactions.

Feature extraction and artificial neural networks for the on-the-fly classification of high-dimensional thermochemical spaces in adaptive-chemistry simulations

Abstract The integration of Artificial Neural Networks (ANNs) and Feature Extraction (FE) in the context of the Sample- Partitioning Adaptive Reduced Chemistry approach was investigated in this work, to increase the on-the-fly classification accuracy for very large thermochemical states. The proposed methodology was firstly compared with an on-the-fly classifier based on the Principal Component Analysis reconstruction error, as well as with a standard ANN (s-ANN) classifier, operating on the full thermochemical space, for the adaptive simulation of a steady laminar flame fed with a nitrogen-diluted stream of n-heptane in air. The numerical simulations were carried out with a kinetic mechanism accounting for 172 species and 6,067 reactions, which includes the chemistry of Polycyclic Aromatic Hydrocarbons (PAHs) up to C $ {}_{20} $ . Among all the aforementioned classifiers, the one exploiting the combination of an FE step with ANN proved to be more efficient for the classification of high-dimensional spaces, leading to a higher speed-up factor and a higher accuracy of the adaptive simulation in the description of the PAH and soot-precursor chemistry. Finally, the investigation of the classifier’s performances was also extended to flames with different boundary conditions with respect to the training one, obtained imposing a higher Reynolds number or time-dependent sinusoidal perturbations. Satisfying results were observed on all the test flames.

Experimental and numerical study of polycyclic aromatic hydrocarbon formation in ethylene laminar co-flow diffusion flames

Recent literature kinetic studies revealed the importance of new mechanisms for polycyclic aromatic hydrocarbon (PAH) and soot inception beyond hydrogen–abstraction–acetylene–addition (HACA) and hydrogen–abstraction–vinylacetylene–addition (HAVA) mechanisms in the combustion of ethylene and other hydrocarbons. Co-flow diffusion flame is a canonical flame used to investigate the interaction between fluid dynamics and PAH chemistry. In this study, supersonic molecular beam sampling technique was utilized for the first time with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) to measure laminar co-flow diffusion flame at atmospheric pressure. We report quantitative measurement of precursor radicals as well as critical intermediates and odd carbon number PAH species. A custom-designed computational code, based on OpenFOAM and Cantera, was adopted to simulate laminar co-flow diffusion flames with literature kinetic model. Chemical kinetic analyses show that addition reactions of odd carbon number species provide considerable contribution to PAH formation processes beside HACA and HAVA mechanisms. Reasonable mass growth reactions are postulated for aromatic species with odd carbon numbers, such as ethynyl-indene, fluorene, benzo-indene, which need further investigations. Reactions of resonantly stabilized radicals followed by ring expansion are shown to be critical for both odd and even carbon number aromatics, and are suggested to be included in future PAH models.

Estimation of NOx and soot emission from a constant volume n-butanol/n-dodecane blended spray using unsteady flamelet model based on n-dodecane/n-butanol/NOx/PAH chemistry

A 246 species and 1062 reactions containing skeletal n-dodecane/n-butanol/NOx/Polycyclic Aromatic Hydrocarbon (PAH) combustion mechanism (Mix246) was developed for n-dodecane/n-butanol blend combustion under diesel engine-like conditions. The Mix246 mechanism was validated using ignition delay and species concentration profile data from various n-dodecane and n-butanol combustion experiments as well as by comparisons with predictions from the parent mechanisms. Subsequently, the Mix246 mechanism was coupled with a 3D Computational Fluid Dynamic (CFD) solver to simulate turbulent combustion of n-butanol/n-dodecane blended sprays in a constant volume combustor using an unsteady flamelet model. The CFD model for the 100% n-dodecane case was benchmarked against the non-reacting and reacting Spray-A conditions of the Engine Combustion Network (ECN). Finally, a parametric study was conducted to investigate the effects of various diesel engine-like conditions such as fuel injection pressure, initial ambient pressure and temperature conditions and varying n-butanol/n-dodecane blend ratios on NOx and soot emissions. Simulations show that the turbulent spray combustion evolves through distinct temporal stages and that the fully developed flame have spatially separated zones of reactive physics. Combustion properties and emission profiles for pure n-dodecane and blends with up to 20% n-butanol were seen to be nearly identical suggesting that n-butanol is a suitable alternative biofuel for CI engine combustion.

Sources and health risks of ambient polycyclic aromatic hydrocarbons in China

Polycyclic aromatic hydrocarbons (PAHs) in the environment are of significant concerns due to their high toxicity to human health. PAHs measurements at limited air quality monitoring stations alone are insufficient to gain a complete understanding of ambient levels and public exposure of PAHs in China. This study simulated the concentrations of PAHs in China, identified the source contributions, and estimated the health risks. Anthropogenic emissions of 16 priority PAHs directly associated with health risks were generated from the global high-resolution PKU-FUEL-2007 inventory. Open biomass burning emissions were generated from the Fire Inventory from NCAR (FINN). PAHs concentrations in January, April, July, and October 2013 were simulated using the Community Multiscale Air Quality (CMAQ) model after incorporation of chemistry, partitioning, and deposition of PAHs. Predicted PAHs were in good agreement with seasonal and annual averaged observations from previous studies. The surface concentrations of 16-PAHs were higher in winter, with population weight average of 0.8 μg/m3 and peak value of 2.0 μg/m3 in urban areas in the North China Plain (NCP) and the Yangtze River Delta (YRD). Summer and spring exhibited lower concentrations of approximately 0.2 μg/m3 in most areas. The most important sources to PAHs were biomass burning and coal combustion in winter and industrial processes and oil and gas activities in summer. The cancer risk due to inhalation exposure of naphthalene (NAPH) and seven carcinogenic PAHs was significant, with the incremental lifetime cancer risk (ILCR) of >5 × 10−4 in many urban and industrial areas. Exposure to PAHs was estimated to result in 15,198 excess lifetime cancer cases in China. Oil and gas burning associated with transport, residential and commercial activities were major contributors to ILCR in China. Coal combustion was predominant in Shanxi but less important in other regions.

The C5 chemistry preceding the formation of polycyclic aromatic hydrocarbons in a premixed 1-pentene flame

The formation of small polycyclic aromatic hydrocarbons (PAHs) and their precursors can be strongly affected by reactions of C5 species. For improving existing combustion mechanisms for small PAH formation, it is therefore valuable to understand the fuel-specific chemistry of C5 fuels. To this end, we provide quantitative isomer-resolved species profiles measured in a laminar premixed (ϕ = 1.8) low-pressure (4 kPa) flame of 1-pentene with photoionization molecular-beam mass spectrometry (PI-MBMS) using tunable synchrotron vacuum-ultraviolet (VUV) radiation. These experimental results are accompanied with numerical simulations, starting from models from the literature by Wang et al. [JetSurF version 2.0 (2010)] and Healy et al. [Energy Fuels 24 (2010) 1521–1528] that were developed for different fuels, but which include 1-pentene as an intermediate, and by Narayanaswamy et al. [Combust. Flame 157 (2010) 1879–1898] focusing on the small PAH chemistry. Taking observed discrepancies between experimental results and simulations into consideration, a mechanism for C5 chemistry was newly developed including PAH formation pathways, and its performance analyzed in detail. Special emphasis was placed on the initial fuel consumption of 1-pentene as well as on formation pathways of small aromatics. The mechanisms show differences regarding fuel decomposition and hydrocarbon growth reactions. These contribute to noticeable differences between the simulations with different models on one hand, and deviations between model predictions and experimental results on the other. While the new model presents overall satisfactory capabilities to predict the mole fraction profiles of common combustion intermediates, the predictive capability of the literature models was not fully satisfying for some intermediate species, including C4H6, C7H8, and C10H8. The results indicate that the fuel-specific C5 reaction routes as well as the mechanism for small PAH formation need further investigation.

Analysis of 6- and 7-ring PAH and other non-EPA PAH by atmospheric pressure laser ionization – mass spectrometry (APLI-MS) in environmental certified reference materials NIST 1941b, NIST 1649b, BAM CC013a and IRMM BCR 535

Proceedings in environmental analytical chemistry of polycyclic aromatic hydrocarbons (PAH) in the past decades cover improvements in laboratory methods as well as new insights in the toxicology of single PAH compounds. A re-evaluation of the established list of 16 priority PAH published by the U. S. Environmental Protection Agency seems overdue. In this study we have applied PAH analysis by atmospheric pressure laser ionization – mass spectrometry (APLI-MS) to NIST 1941b, NIST 1649b, BAM CC013a and IRMM BCR 535 giving further references in addition to the certificates of analysis. Gas chromatography enables to distinguish between different alkylated PAH derivatives whereas liquid chromatography additionally allows detection of PAH tentatively with six or more aromatic rings on the level of chemical formula. For an optimal isomer-specific differentiation, LC-APLI-MS should be used in combination with fluorescence detection. Distribution patterns of 6 and 7-ring PAH are presented for urban dust, marine and harbor sediments and the soil sample. Additionally, a set of 14 nitrogen, sulfur and oxygen-containing polycyclic aromatic compounds were analyzed by APLI and quantified in all reference materials for the first time.

Capturing aromaticity in automatic mechanism generation software

Better modeling of polycyclic aromatic hydrocarbon (PAH) formation has the potential to greatly aid in predicting soot formation. Automatic mechanism generation provides a powerful platform for comprehensively exploring reaction sequences implied by known chemistry and identifying the ones that are important in various systems. Reaction Mechanism Generator (RMG), a software developed on our group, has been successfully used to model a variety of chemistries, but has previously been unprepared to properly handle the challenge of aromatics. To accurately model PAH chemistry, several improvements were needed to core RMG algorithms: more robust resonance structure generation for PAHs and more flexible reaction generation algorithms for aromatics. Resonance structures are an integral part of the reaction generation algorithm, so representation accuracy is an important consideration. Clar structures were introduced as a concise and accurate representation for PAHs to replace Kekulé structures. This combined with a refactoring of the overall resonance algorithm also provided significant performance gains. Another major improvement was enabling reactivity of benzene bonds, which allows much greater flexibility in designing rate rules to differentiate aromatic and aliphatic reactions. As a test case, a model was generated for co-pyrolysis of iodonaphthalene and acetylene and compared with literature data, demonstrating the much-improved ability of RMG in modeling aromatic species.

Growth of polycyclic aromatic hydrocarbons (PAHs) by methyl radicals: Pyrene formation from phenanthrene

The formation of polycyclic aromatic hydrocarbons (PAHs) in high temperature environments is an ongoing area of research due to the mismatch between the experimental and simulated concentration profiles of PAHs in several flames. In this work, the role of methyl radicals in the growth of PAHs is identified by developing a detailed reaction mechanism for the conversion of phenanthrene to pyrene by methyl radicals. The reaction energetics are obtained through quantum calculations using B3LYP and M06-2X functionals along with 6-311++G(d,p) basis set, and are used to compare the competing channels for pyrene formation. The transition state theory is used to determine the reaction kinetics. Through kinetic simulations, the most preferred path for pyrene formation from phenanthrene is determined. To quantify the contribution of the newly found reactions in PAH growth in the presence of other competing PAH mechanisms, the reactions are merged with a recently developed and well-validated mechanism for C1C4 hydrocarbons with detailed PAH chemistry. Three premixed laminar aliphatic flames were simulated with the updated mechanism. It was observed that the new reactions did not appreciably influence the computed profiles of phenanthrene and pyrene. Through the rate-of-production analysis, it was found that the phenanthrene radicals created through H-abstraction were more susceptible to attack by C2H2 than by methyl radicals. The indirect role of methyl radicals in PAH formation and growth is shown.