Formation of polycyclic aromatic hydrocarbons and their growth to soot—a review of chemical reaction pathways

Chemical reactions and physical processes responsible for the formation of polycyclic aromatic hydrocarbons and soot in hydrocarbon flames are reviewed. The discussion is focused on major elements in the present understanding of the phenomena, clarification of concepts central to the present state of the art, and a summary of new results.

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Reaction mechanism of soot formation in flames

A detailed kinetic modeling study of aromatics formation in laminar premixed acetylene and ethylene flames

Over the last two decades, our understanding of soot formation has evolved from an empirical, phenomenological description to an age of quantitative modeling for at least small fuel compounds. In this paper, we review the current state of knowledge of the fundamental sooting processes, including the chemistry of soot precursors, particle nucleation and mass/size growth. The discussion shows that though much progress has been made, critical gaps remain in many areas of our knowledge. We propose the roles of certain aromatic radicals resulting from localized π electron structures in particle nucleation and subsequent mass growth. The existence of these free radicals provides a rational explanation for the strong binding forces needed for forming initial clusters of polycyclic aromatic hydrocarbons. They may also explain a range of currently unexplained sooting phenomena, including the large amount of aliphatics observed in nascent soot formed in laminar premixed flames and the mass growth of soot in the absence of gas-phase H atoms. While the above suggestions are inspired, to an extent, by recent theoretical findings from the materials research community, this paper also demonstrates that the knowledge garnered through our longstanding interest in soot formation may well be carried over to flame synthesis of functional nanomaterials for clean and renewable energy applications. In particular, work on flame-synthesized thin films of nanocrystalline titania illustrates how our combustion knowledge might be useful for developing advanced yet inexpensive thin-film solar cells and chemical sensors for detecting gaseous air pollutants.

Formation of nascent soot and other condensed-phase materials in flames

We present the fifth release of the UMIST Database for Astrochemistry (UDfA). The new reaction network contains 6173 gas-phase reactions, involving 467 species, 47 of which are new to this release. We have updated rate coefficients across all reaction types. We have included 1171 new anion reactions and updated and reviewed all photorates. In addition to the usual reaction network, we also now include, for download, state-specific deuterated rate coefficients, deuterium exchange reactions and a list of surface binding energies for many neutral species. Where possible, we have referenced the original source of all new and existing data. We have tested the main reaction network using a dark cloud model and a carbon-rich circumstellar envelope model. We present and briefly discuss the results of these models.

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The UMIST database for astrochemistry 2012

Analyse thermodynamique chimique de molecules d'hydrocarbures de 1500° a 3000°K pour les especes C 2n H 2m (n=1-21, m=1-8)

High-temperature stabilities of hydrocarbons

We present a study of the unimolecular steps in a pathway leading from propargyl radicals to benzene. Specifically, reactions of 1,5-hexadiyne, a recombination product of propargyl radicals, were examined in two different reactors. In atmospheric pressure flow experiments, the sole product at 250°–400°C was 1,2-dimethylenecyclobutene, with k/s −1 =10 11.7 exp(−35.5 kcal/RT). At 460°–550°C this product isomerized to benzene and fulvene, with k/s −1 =10 12.9 exp(−50.0 kcal/RT). In very low pressure flow experiments, all three cyclic products were formed in parallel. Differences in the two experimental results originate from “chemical activation” in the low pressure experiments. These results imply that recombination products of propargyl radicals in flames irreversibly generates benzene and fulvene. An analysis of Westmoreland's low pressure acetylene flame data 4 indicates that this channel can fully account for reported rates of benzene formation. It is also consistent with this channel being the first stage in the formation of larger aromatic substances.

A new path to benzene in flames

Laser photolysis-kinetic spectroscopy, end-product analysis, and detailed modeling were used to investigate three reaction systems in detail. Included were vinyl-vinyl, vinyl-methyl, and vinyl-methyl-hydrogen atoms. Product ratios, as a function of the rate constants could be approximated by using simple expressions. This greatly simplified the precise numerical modeling of these complex systems and also permitted a realistic error analysis. Reactions and rate constants (obtained at 25C, 100 Torr of He) in units of cm{sup 3} (molecule s){sup {minus}1} are summarized as follows: CH{sub 3} + CH{sub 3} {yields} C{sub 2}H{sub 6}, (0.52 {plus minus} 0.05) {times} 10{sup {minus}10}; C{sub 2}H{sub 3} + C{sub 2}H{sub 3} {yields} C{sub 4}H{sub 6}, (1.2 {plus minus} 0.2) {times} 10{sup {minus}10}; C{sub 2}H{sub 3} + C{sub 2}H{sub 3} {yields} C{sub 2}H{sub 2} + C{sub 2}H{sub 4}, (0.24 {plus minus} 0.05) {times} 10{sup {minus}10}; CH{sub 3} + C{sub 2}H{sub 3} {yields} C{sub 3}H{sub 6}, (1.2 {plus minus} 0.3) {times} 10{sup {minus}10}; CH{sub 3} + C{sub 2}H{sub 3} {yields} CH{sub 4} + C{sub 2}H{sub 2}, (0.34 {plus minus} 0.07) {times} 10{sup {minus}10}; C{sub 2}H{sub 3} + H {yields} C{sub 2}H{sub 4}, (2.0 {plus minus} 0.8) {times} 10{sup {minus}10}. The cross-combination rate constant obtained for vinyl-methyl is not twicemore » the geometric mean of the vinyl-vinyl and methyl-methyl rate constants. This observation is explained on the basis that the vinyl-vinyl rate constant is faster than predicted by simple correlation arguments. The effect of bound low-lying triplet states of the product on the vinyl combination kinetics is discussed.« less

Reaction rate determinations of vinyl radical reactions with vinyl, methyl, and hydrogen atoms

Reactions and Kinetics of Unsaturated C2 Hydrocarbon Radicals

Rates of vinylation and phenylation of ethyne, ethene and benzene have been determined relative to vinyl recombination in a Knudsen cell flow reactor. Phenylvinylsulfone and divinylmercury were used as radical initiators and studies were done over the temperature range 1000–1330 K. Assuming a recombination rate constant of 1010.3 M−1s−1 for vinyl and 109.5 for phenyl, we obtain the following rate constants (M−1s−1) at 1100 K: k(R=vinyl) k(R=phenyl) R|+C2H2→R−C2H+H| 2.0±.4×108 2.6±.6×108 R|+C2H4→R−C2H3+H| 1.4±.2×108 1.5±.4×108 R|+C6H6→R−C6H5+H| 4.5±1.2×107 6.0±2.0×107 When combined with room temperature addition rates from the literature, we obtain (M−1s−1): k(vinyl+ethyne→vinylacetylene+H)=109.3 exp{−5.0 kcal/RT} k(phenyl+benzene→biphenyl+H)=108.6 exp{−4.0 kcal/RT}

Askar Fahr

Papers

High-temperature stabilities of hydrocarbons

A new path to benzene in flames

Reaction rate determinations of vinyl radical reactions with vinyl, methyl, and hydrogen atoms

Reactions and Kinetics of Unsaturated C2 Hydrocarbon Radicals

Reactions of vinyl and phenyl radicals with ethyne, ethene and benzene