Formation of polycyclic aromatic hydrocarbons and their growth to soot—a review of chemical reaction pathways
Introduction
Combustion processes used in transportation, manufacturing, and power generation are major sources of airborne species of current health concern. In epidemiological studies [1], [2], air pollution was positively associated with death from lung cancer and cardiopulmonary disease. Fine particles—defined by a diameter equal to or below 2.5 μm—are thought to pose a particularly great risk to health because they are more likely to be toxic than larger particles and can be breathed more deeply into the lungs [3]. A possible explanation of the hazardous health effects of atmospheric aerosols is their association with polycyclic aromatic hydrocarbons (PAH). The amounts of different PAH associated with different aerosol size fractions were measured in order to obtain a better understanding of their fate and human exposure [4]. Many PAHs present in aerosols have been found to be mutagenic or tumorigenic [5], [6], [7], [8], [9], [10] and a molecular biological pathway linking one of them—benzo[a]pyrene—to human lung cancer has been established recently [11].
The need to control the emission of combustion products of environmental and health concern while also promoting more efficient utilization of fossil energy resources requires the development of cleaner and more economic combustion equipment which in turn requires a better physical and chemical understanding of combustion processes. A significant research effort on PAH and soot has been undertaken during recent years [12], [13], [14], [15]. Although many important details of PAH and soot formation remain poorly understood, there is considerable agreement on the general features of the processes involved, which are summarized below as introductory overview and shown schematically in Fig. 1.
(a) Formation of molecular precursors of soot: The molecular precursors of soot particles are thought to be heavy PAHs of molecular weight 500–1000 amu. The growth process from small molecules such as benzene to larger and larger PAH appears to involve both the addition of C2, C3 or other small units, among which acetylene has received much attention, to PAH radicals, and reactions among the growing aromatic species, such as PAH–PAH radical recombination and addition reactions. The relative contribution of the different types of growth reactions seems to depend on the fuel. In the case of aromatic fuels such as benzene, acetylene and other active reactants for aromatics formation are formed in relatively large concentrations in the breakdown of the fuel, whereas in the case of aliphatic fuels such as acetylene, ethylene or methane, the first aromatic ring must be formed from fuel decomposition products by a sequence of elementary reactions in which the active ring formation reactants are in lower concentrations than in the aromatics flames. This picture is consistent with a trend of increasing ease of soot formation from paraffin to mono- and di-olefins, benzenes and naphthalenes [16], [17], [18]. Much progress toward a better understanding of the chemistry of PAH formation has been made in the recent years.
(b) Nucleation or inception of particles from heavy PAH molecules: In this process mass is converted from molecular to particulate systems, i.e. heavy PAH molecules form nascent soot particles with a molecular mass of approximately 2000 amu and an effective diameter of about 1.5 nm. Chemical details of the formation of nascent soot particles are relatively poorly understood, mostly because of experimental difficulties. Efficient identification of species produced at different stages of the growth process is limited to molecular masses less than about 300 amu using gas chromatography while the observation and counting of soot particles by high resolution electron microscopy has been limited to particle diameters of larger than about 1.5 nm. The analysis of larger PAH by means of liquid chromatography and mass spectrometry as well as the increase of the resolution of electron microscopic techniques are objectives of ongoing research. Additional information has been obtained using optical techniques and contributes to an increasing body of experimental data.
(c) Mass growth of particles by addition of gas phase molecules: After the formation of the nascent soot particles their mass is increased via the addition of gas phase species such as acetylene and PAH, including PAH radicals. These reactions are believed to involve radical sites on the soot particles in the case of stable reactants such as acetylene and stable PAH but not necessarily so in the case of PAH radicals. This process of course does not affect the number of soot particles. The relative contributions of acetylene and PAH is the subject of current discussions in the combustion community.
(d) Coagulation via reactive particle–particle collisions: Sticking collisions between particles during the mass growth process significantly increases particles size and decreases particle number without changing the total mass of soot present. The continuation of substantial molecular addition of gas phase species after the early formation of composite particles via sticking particle–particle collisions, partly hides the identity of primary particulate units in electron microscopy images of soot particles.
(e) Carbonization of particulate material: At higher residence times under pyrolytic conditions in the postflame zone, the polyaromatic material comprising the yet formed particles undergoes functional group elimination, cyclization, ring condensation and ring fusion attended by dehydrogenation and growth and alignment of polyaromatic layers. This process converts the initially amorphous soot material to a progressively more graphitic carbon material, with some decrease in particle mass but no change in particle number. Recently, additional interest in this process has been motivated by the observation of soot containing curved or fullerenic layers.
(f) Oxidation: Oxidation of PAH and soot particles is a process competing with the formation of these species. It decreases the mass of PAH and soot material through the formation of CO and CO2. Depending upon flame type, oxidation may occur simultaneously with formation as in premixed aromatics flames and well-mixed combustors, or it may occur subsequent to formation as in diffusion flames or staged combustors. The main oxidation reactants are OH, O and O2, the largest contributor in general being OH under fuel-rich conditions and O2 under fuel-lean conditions.
Since the early work of Street and Thomas [19] a more and more detailed characterization of soot formation behavior, including the effects of parameters like fuel type, global and local equivalence ratio, temperature, pressure and the presence of additives has been attained. Assisted by the availability of more and more sophisticated experimental techniques, this evolution facilitated improved insight into the mechanistic processes involved in soot and PAH formation, which is described in excellent review articles [18], [20], [21], [22], [23], [24], [25]. Due to the tremendous body of available data no systematic attempt is undertaken here to retrace the history leading to the present state of the knowledge. In addition to the experimental work, the increase of computational power has allowed the critical testing of chemical reaction networks, in particular for premixed flames, well stirred, plug flow reactors and shock tubes. Suitable and easy to use software [26], [27], [28], [29], [30], [31], [32], [33] has been developed and applied to more and more complex combustion systems [28], [34], [35], [36]. Many thermodynamic and kinetic property values have been determined by means of increasingly sophisticated molecular-mechanical and in particular quantum-mechanical techniques. The discovery of fullerenes [37], [38] and fullerenic nanostructures [39] in fuel-rich hydrocarbon flames presents an additional challenge to the mechanistic understanding to PAH and particle formation.
The present review focuses on the chemical understanding of the formation of PAH and soot. The elucidation of reaction networks requires the investigation of well-defined and physically easy to describe experimental systems. Therefore mainly results from easy to model combustion systems such as premixed flames, well-stirred and plug-flow reactors are discussed here. In particular the following aspects are covered:
- 1.
The first sequences of the growth process beginning with different compounds are described. Pathways leading to the first aromatic rings are discussed. Different reaction sequences forming benzene from compounds containing between one and four carbon atoms are described. The decay of small aromatic fuel components such as benzene, which is an important source of growth reactants such as acetylene, and the role of C5-moities in the growth process is addressed.
- 2.
The pre-particle or molecular chemistry leading to soot precursors, i.e., species forming nascent soot particles in their subsequent reaction step, is discussed. Different forms of the hydrogen-abstraction/acetylene-addition mechanism are described. The roles of small PAH as reactants in the formation of larger PAH and soot are assessed.
- 3.
The formation of fullerenes and of fullerenic nanostructures, especially in connection—or in competition—with soot formation is addressed. The recent observation of fullerenes appearing to behave as soot growth reactants is discussed.
- 4.
In a brief review of soot formation mechanisms alternative to the above described—and today widely accepted—route, the possible roles of ionic species and polyacetylenes are discussed.
- 5.
Methods for the determination of thermodynamic properties are discussed, and evidence for thermodynamic effects on the concentrations of PAH at different residence times are summarized.
- 6.
Possible future directions for PAH and soot formation research, such as experimental and computational approaches which could provide deeper insight into key elementary reaction steps are discussed. Currently used theoretical techniques for the determination of pressure-dependent rate constants, and limitations of these methods, are discussed.
Section snippets
The first reaction steps
Even in the very early stages of combustion research, hypotheses have been advanced [40], [41], [42] regarding potential reaction pathways for the growth of the initial fuel to “carbon”, nowadays in general called soot. Correlations between soot formation tendency and fuel structure have been established and qualitative conclusions have been suggested about possible chemical reaction sequences leading to soot. As summarized by Palmer and Cullis [20], sooting tendencies were shown to decrease in
Formation of the first aromatic ring
After the early work of Berthelot [54] who suggested the formation of benzene via direct polymerization of acetylene and Bone and Coward [55] hypothesized a potential role for CH2 and CH fragments [55], a great number of different chemical pathways have been critically discussed. Reaction sequences involving stable species as in the case of Diels–Alder addition of 1,3-butadiene to ethylene [56], [57] or the alkene trimerization into rings [58], and pathways involving at least one radical have
Further growth process
Without excluding the possibility of other “building bricks” such polyacetylene or ionic species, the importance of polycyclic aromatic hydrocarbons (PAH) in the growth process leading to soot particles has received more and more attention in the combustion community [114], [115]. Quantitative testing of the possible contribution of the different reaction pathways has become possible with the increasing computational power which allows more and detailed kinetic modeling of combustion processes
Roles of acetylene and PAH in the growth process
The formation of larger and larger PAH and the nucleation and growth of soot particles has been described mainly using different variations of H-abstraction/C2H2-addition sequences. After the qualitative assessment of the contribution of this pathway [59], [66], [69] the increasing use of kinetic modeling required reliable rate constants for the elementary reactions. Kinetic data of key reactions involving species with one aromatic ring have been deduced but only few studies have investigated
Formation of fullerenes and their relation to PAH and soot formation
Fullerenes, a class of all-carbon, polyhedral, closed shells were identified as ionic species in fuel-rich flat premixed acetylene- and benzene–oxygen low-pressure flames using molecular beam sampling coupled to on-line mass spectrometry [167], [168]. Later, the solvent extraction of soot generated in premixed laminar low-pressure benzene flames allowed the identification of macroscopic amounts of different fullerenes such as C60 and C70 [169], [170], [171], [172], [173] but also of larger ones
Other soot nucleation models
Although there is considerable evidence that PAH are key reactants in soot formation and that PAH growth is through free radical intermediates, historically other species have also been considered key soot formation reactants and some of these are still of interest. Calcote and Gill [197], [198] advocate the role of ionic species and point out experimental evidence that the concentration of PAH under some conditions continues to increase after the rate of soot formation has gone to zero [197].
Impact of thermodynamics
Detailed kinetic modeling requires the use of reversible elementary reaction steps with the ratio of the rate constants of forward and reverse reaction determined by the equilibrium constant, i.e. by the thermochemical properties of the species involved.
Forward and reverse reaction fluxes of elementary reactions contributing to aromatics mass growth are often tightly balanced [93] and it has been shown by Frenklach and Warnatz [63] that uncertainties in the thermochemical data can affect
Future routes of research
The development in the recent years of increasingly sophisticated kinetic models has stimulated significant qualitative and quantitative advances in the understanding of flame chemistry in general and of PAH and soot growth processes in particular. Simultaneously, new experimental measures in different combustion environments such as premixed and diffusion flames, shock tubes, well-stirred and plug-flow reactors have enlarged the data base available for the testing and improvement of kinetic
Conclusions
Considerable progress in the qualitative as well as quantitative understanding of the growth steps occurring in combustion processes and leading finally to the formation of soot particles has been made during recent decades. An increasing body of experimental data has become available and the use of kinetic modeling has allowed the quantitative testing of the role of suggested reaction pathways. Thus, the importance of propargyl (C3H3) for benzene formation and of cyclopentadienyl (C5H5) for
Acknowledgements
We are grateful for financial support from the Chemical Sciences Division, Office of Basic Energy Sciences, Office of Energy Research, US Department of Energy under grant DE-FG02-84ER13282, and from the National Institute of Environmental Health Sciences, MIT Center for Environmental Health Sciences and the US Environmental Protection Agency, MIT Center on Airborne Organics.
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