Polycyclic Aromatic Hydrocarbons Precursors Research Articles

What on Earth Have We Been Burning? Deciphering Sedimentary Records of Pyrogenic Carbon.

Humans have interacted with fire for thousands of years, yet the utilization of fossil fuels marked the beginning of a new era. Ubiquitous in the environment, pyrogenic carbon (PyC) arises from incomplete combustion of biomass and fossil fuels, forming a continuum of condensed aromatic structures. Here, we develop and evaluate 14C records for two complementary PyC molecular markers, benzene polycarboxylic acids (BPCAs) and polycyclic aromatic hydrocarbons (PAHs), preserved in aquatic sediments from a suburban and a remote catchment in the United States (U.S.) from the mid-1700s to 1998. Results show that the majority of PyC stems from local sources and is transferred to aquatic sedimentary archives on subdecadal to millennial time scales. Whereas a small portion stems from near-contemporaneous production and sedimentation, the majority of PyC (∼90%) experiences delayed transmission due to "preaging" on millennial time scales in catchment soils prior to its ultimate deposition. BPCAs (soot) and PAHs (precursors of soot) trace fossil fuel-derived PyC. Both markers parallel historical records of the consumption of fossil fuels in the U.S., yet never account for more than 19% total PyC. This study demonstrates that isotopic characterization of multiple tracers is necessary to constrain histories and inventories of PyC and that sequestration of PyC can markedly lag its production.

Degradation of phytosterols in tobacco waste extract by a novel Paenibacillus sp.

Phytosterols have been demonstrated to be precursors of polycyclic aromatic hydrocarbons (PAHs) formed during biomass pyrolysis. Here, a novel Paenibacillus sp. was evaluated for its ability to degrade phytosterols in tobacco waste extract (TWE). The optimal conditions for cell growth and stigmasterol (a representative of phytosterols) degradation were 37 °C, pH 7.0, 1.0g/L yeast extract, and 6.0g/L glucose. Paenibacillus sp. could degrade stigmasterol under high concentrations of glucose (up to 130g/L) and tolerate wide pH (5.0-9.0) and temperature (25-42 °C) ranges. The new strain could degrade stigmasterol completely into CO2 and H2 O, and no intermediate steroids were detected during the degradation process. Phytosterol degradation in TWE was demonstrated by high-performance liquid chromatography-tandem mass spectrometry. Under optimal conditions (37 °C, pH 7.0, with the exponential-phase cells), the total degradation ratio of phytosterols reached 38.5% in TWE, including 45.2% of stigmasterol, 37.4% of β-sitosterol, 27.3% of campesterol, and 28.7% of cholesterol. These results showed that Paenibacillus sp. is a candidate for phytosterol degradation in TWE and other biomass and is potentially useful in reducing the PAHs generated from biomass pyrolysis.

Experimental and kinetic modeling study of laminar premixed decalin flames

Laminar premixed flames of decalin with equivalence ratios of 0.75, 1.0 and 1.8 were studied using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Dozens of flame species were identified and their mole fraction profiles were evaluated, including a series of free radicals. A detailed kinetic model of decalin combustion consisting of 538 species and 3218 reactions was developed and validated on the new flame data. The rate of production analysis at different equivalence ratios were performed to help understand the kinetics in the decompostion of decalin and the formation of some key intermediates. The H-atom abstraction reactions play an overwhelming role in the primary decomposition of decalin at flame conditions. Monocyclic aromatic hydrocarbons (MAHs) are mainly produced from the decomposition of decalin, while polycyclic aromatic hydrocarbons (PAHs) originate from mass growth process of PAH precursors. In addition, some oxygenated intermediates have important contributions to the formation of key MAHs and PAHs such as benzene (A1) and indene (C9H8). The direct production of PAH precursors in fuel decomposition process explains the higher sooting tendency of decalin than n-alkanes and benzene. Furthermore, the model was used to simulate literature data of decalin combustion, such as species profiles in jet stirred reactor oxidation of decalin and ignition delay times of decalin.

Structure of an n-heptane/toluene flame: Molecular beam mass spectrometry and computer simulation investigations

Molecular beam mass spectrometry was used to measure mole fraction profiles of the reactants, major reaction products and intermediates, including precursors of polycyclic aromatic hydrocarbons, in a premixed fuel-rich (equivalence ratio of 1.75) n-heptane/toluene/O2/Ar flame stabilized on a flat burner at atmospheric pressure. The ratio of the liquid volumes in the n-heptane/toluene mixture was 7: 3. The chemical structure of the flame was modeled using a detailed mechanism of chemical reactions tested against experimental data of other authors on n-heptane/toluene flames and comprising the reactions of formation of polycyclic aromatic hydrocarbons. The mechanism was extended with cross-reactions involving derivatives of n-heptane and toluene. Overall, the new experimental data are in satisfactory agreement with the numerical simulation results; however, there are differences between the measured and calculated mole fraction profiles of some species. Analysis shows that in the n-heptane/toluene flame, the main reactions leading to the formation of low-aromatic compounds (benzene and phenyl) are reactions typical of the pure toluene flame.

Experimental and kinetic modeling investigation on decalin pyrolysis at low to atmospheric pressures

The pyrolysis of decalin was studied in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at the pressures of 4, 20, and 101kPa and the temperatures from 920 to 1500K. Dozens of pyrolysis species were identified and their mole fraction profiles were measured as a function of temperature, particularly several free radicals and aromatic species. Besides, a detailed kinetic model consisting of 484 species and 1774 reactions was proposed and used to simulate the present experimental data. The rate of production analysis and sensitivity analysis was performed in order to provide deep insight into the pyrolysis kinetics of decalin. The decomposition of decalin is initiated by the C–C bond dissociation reactions of both decalin and its C10H18 isomers, while the H-atom abstraction reactions consume most of decalin. Monocyclic aromatic hydrocarbons (MAHs) are mainly produced in the fuel decomposition process instead of the mass growth process from small molecules, indicating that the two fused six-membered ring structure offers a solid infrastructure to form MAHs. Polycyclic aromatic hydrocarbons (PAHs) are mainly produced in the mass growth process from PAH precursors such as benzyl radical (A1CH2), phenyl radical (A1-), and 1,3-cyclopentadienyl radical (CYC5H5). The relatively high degree of saturation of decalin facilitates ring-opening pathways in its decomposition. This enhances the formation of MAHs but limits the formation of PAHs, leading to the lower sooting tendency of decalin than tetralin. Furthermore, the model was also validated on the shock tube pyrolysis data of decalin in the literature.

Effect of CO2 Addition on the Structure of Premixed Fuel-Rich CH4/O2/N2 and C3H8/O2/N2 Flames Stabilized on a Flat Burner at Atmospheric Pressure

The effect of replacement of 15% of N2 by CO2 (on a mole basis) in unburnt mixtures on the structure of laminar premixed methane/oxygen/nitrogen and propane/oxygen/nitrogen flames stabilized on a flat burner at 1 atm is studied. The emphasis of this work is on the effect of CO2 addition on the mole fractions of intermediates that are potential precursors of polycyclic aromatic hydrocarbons (PAH). Mole fraction profiles of reactants (CH4, C3H8, O2, and CO2), major intermediates (CH3, CH4, CH2O, C2H2, C2H3, C2H4, C2H6, C3H3, and C3H4) and final products (H2O, CO, CO2, and H2) in fuel-rich (φ = 1.2) CH4/O2/N2, CH4/O2/N2/CO2, C3H8/O2/N2, and C3H8/O2/N2/CO2 flames were measured using flame-sampling molecular beam mass spectrometry and calculated using the PREMIX code and three detailed chemical kinetic mechanisms reported in the literature for combustion of small hydrocarbons: AramcoMech 1.3, Konnov’s mechanism, and GRI-Mech 3.0. Temperature measurements in the flames were carried out using the thermocouple te...

Vibrational spectra of four polycyclic aromatic hydrocarbons under high pressure: implications for stabilities of PAHs during accretion

Infrared and Raman spectra of the polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, phenanthrene, and pyrene have been examined up to 10–55 GPa at 300 K, to probe structural changes in these materials under high-pressures, and to relate these to shock measurements on these materials. The goal is to develop an understanding of how such hydrocarbons might be processed during planetary accretion. A range of phase transitions in PAHs are observed and, in accord with previous investigations, these typically initiate at relatively low pressures (0.3–4.0 GPa): the lower-pressure transitions are likely associated with inter-molecular changes such as changes in symmetry and/or molecular orientation, charge transfer processes, or changes in π electron density, and are often sluggish. Higher-pressure (7–10 GPa) phase transitions in PAHs are likely associated with profound structural changes like dimerization, which are not always reversible. Laser-induced luminescence is encountered at pressures well below those at which PAHs amorphize, and a strong pressure-induced Fermi resonance is identified between the highest-lying inter-molecular modes and lowest-lying intra-molecular modes in each PAH examined. It is the increased strength of inter-molecular interactions under pressure that likely generates increasing overlap of π orbitals and leads to cross-linking (dimerization) of the molecules and the destruction of their planar symmetry. The first step in the amorphization of these compounds is likely dimerization, and amorphization occurs when long-range order is lost and a greater diversity of local structural environments is introduced into these materials, such as carbons being shared between rings, embayed structures, sp, sp2, and sp3 hybridized carbon atoms, a broad range of C–H bonding environments, and fewer residual resonance-stabilized C–C units. Our results are consistent with pressure producing amorphous, hydrogenated carbon material from PAH precursors: hence, impact phenomena, coupled with post-shock hydrogen loss, could provide an alternate pathway to produce amorphous carbon assemblages of the type observed within a range of meteorites. Additionally, smaller PAHs tend to be most stable under compression; as these are the most volatile of the PAHs, the combination of shock during accretion, coupled with trends in volatility, may limit the presence of PAHs within objects formed in the early solar system.

A computational study of ethylene–air sooting flames: Effects of large polycyclic aromatic hydrocarbons

An updated reduced gas-phase kinetic mechanism was developed and integrated with aerosol models to predict soot formation characteristics in ethylene nonpremixed and premixed flames. A primary objective is to investigate the sensitivity of the soot formation to various chemical pathways for large polycyclic aromatic hydrocarbons (PAH). The gas-phase chemical mechanism adopted the KAUST-Aramco PAH Mech 1.0, which utilized the AramcoMech 1.3 for gas-phase reactions validated for up to C2 fuels. In addition, PAH species up to coronene (C24H12 or A7) were included to describe the detailed formation pathways of soot precursors. In this study, the detailed chemical mechanism was reduced from 397 to 99 species using directed relation graph with expert knowledge (DRG-X) and sensitivity analysis. The method of moments with interpolative closure (MOMIC) was employed for the soot aerosol model. Counterflow nonpremixed flames at low strain rate sooting conditions were considered, for which the sensitivity of soot formation characteristics to different nucleation pathways were investigated. Premixed flame experiment data at different equivalence ratios were also used for validation. The findings show that higher PAH concentrations result in a higher soot nucleation rate, and that the total soot volume and average size of the particles are predicted in good agreement with experimental results. Subsequently, the effects of different pathways, with respect to pyrene- or coronene-based nucleation models, on the net soot formation rate were analyzed. It was found that the nucleation processes (i.e., soot inception) are sensitive to the choice of PAH precursors, and consideration of higher PAH species beyond pyrene is critical for accurate prediction of the overall soot formation.

Detailed influences of ethanol as fuel additive on combustion chemistry of premixed fuel-rich ethylene flames

To better understand potential pollutant formations during combustion of conventional hydrocarbon fuels blended with oxygenated fuels, detailed influences of ethanol as fuel additive on small polycyclic aromatic hydrocarbons (PAHs) precursors, aldehydes, ketene and other important intermediate species in premixed fuel-rich low-pressure ethylene flames are distinguished among dilution, thermal and chemical effects of additives. Dominant effects of ethanol addition on each species are underlined respectively. Ethylene oxidation process is delayed when ethylene is substituted by ethanol. The influence of ethanol dilution and thermal effects on ethylene consumption are larger than chemical effects. CO mole fractions slightly decrease mainly as a result of dilution and thermal effects of added ethanol. The reductions in small PAHs precursors (acetylene and propargyl) are attributed to dilution and thermal effects of ethanol addition instead of chemical effects. The ethanol chemical effects promote formations of hazardous pollutants formaldehyde and acetaldehyde, and especially are responsible for the significant increase of acetaldehyde. C2H6, C4H2 and C4H4 mole fractions decrease in a similar way with acetylene and propargyl as ethanol is added. Ethanol used here only serves as a prototype of oxygenated additive, and identification method in this work is more universal which can be easily extended for analyzing other fuel blends of hydrocarbon and oxygenated fuels.

Top-down formation of fullerenes in the interstellar medium.

Fullerenes have been recently detected in various circumstellar and interstellar environments, raising the question of their formation pathway. It has been proposed that they can form at the low densities found in the interstellar medium by the photo-chemical processing of large polycyclic aromatic hydrocarbons (PAHs). Following our previous work on the evolution of PAHs in the NGC 7023 reflection nebula, we evaluate, using photochemical modeling, the possibility that the PAH C66H20 (i.e. circumovalene) can lead to the formation of C60 upon irradiation by ultraviolet photons. The chemical pathway involves full dehydrogenation of C66H20, folding into a floppy closed cage and shrinking of the cage by loss of C2 units until it reaches the symmetric C60 molecule. At 10" from the illuminating star and with realistic molecular parameters, the model predicts that 100% of C66H20 is converted into C60 in ~ 105 years, a timescale comparable to the age of the nebula. Shrinking appears to be the kinetically limiting step of the whole process. Hence, PAHs larger than C66H20 are unlikely to contribute significantly to the formation of C60, while PAHs containing between 60 and 66 C atoms should contribute to the formation of C60 with shorter timescales, and PAHs containing less than 60 C atoms will be destroyed. Assuming a classical size distribution for the PAH precursors, our model predicts absolute abundances of C60 are up to several 10-4 of the elemental carbon, i.e. less than a percent of the typical interstellar PAH abundance, which is consistent with observational studies. According to our model, once formed, C60 can survive much longer (> 107 years for radiation fields below G0 = 104) than other fullerenes because of the remarkable stability of the C60 molecule at high internal energies. Hence, a natural consequence is that C60 is more abundant than other fullerenes in highly irradiated environments.

The effect of methyl pentanoate addition on the structure of premixed fuel-rich n-heptane/toluene flame at atmospheric pressure

The effect of adding methyl pentanoate (MP) on the species pool in a rich premixed flame fueled by n-heptane/toluene blend (7/3 by volume of liquids) at atmospheric pressure is investigated. The emphasis of this work is on the effect of MP on the concentrations of intermediates, which are precursors of polycyclic aromatic hydrocarbons (PAH), in order to understand the processes responsible for reduction of concentration of PAH when biodiesel is added to diesel fuel in combustion devices. Two premixed fuel-rich n-heptane/toluene/О2/Ar (2.29/1.36/21.36/75%) and n-heptane/toluene/MP/О2/Ar (1.26/0.75/2.00/21.355/75%, 50% of liquid MP in liquid fuel mixture of n-heptane and toluene) fuel-rich flames with the same equivalence ratio φ=1.75 were stabilized on a flat burner at 1atm. Molecular beam mass spectrometric measurements of mole fraction profiles of reactants, the major products and many intermediate species were performed. The experimental profiles were compared with those calculated using a detailed chemical kinetic mechanism, which was a combination of two detailed mechanisms proposed earlier in the literature for combustion of n-heptane/iso-octane/toluene mixture and for MP oxidation, respectively. Addition of MP was found to reduce mole fractions of many intermediates, which play an important role in formation of PAH, specifically, benzene, cyclopentadienyl, acetylene, propargyl, and vinylacetylene. Analysis of the reaction pathways responsible for formation of naphthalene, a typical representative of small PAH, was performed in order to elucidate the chemical effect of MP addition on its formation. To ascertain the effect of MP addition on the primary reactions of consumption of n-heptane and toluene, the analysis of the relative contributions of these reactions to the total rate of consumption of the fuels was carried out.

Experimental and kinetic modeling study of styrene combustion

The aim of this work is to report new experimental data on styrene combustion and develop a kinetic model for styrene combustion. Three laminar premixed flames of styrene with equivalence ratios of 0.75, 1.00 and 1.70 were studied at low pressure (0.0395atm) using synchrotron vacuum ultraviolet photoionization mass spectrometry. The JSR oxidation of styrene/benzene mixtures with three equivalence ratios of 0.50, 1.00, and 1.50 was studied at atmospheric pressure using gas chromatography. The mole fraction profiles of flame species and oxidation species were measured. A kinetic model of styrene combustion with 290 species and 1786 reactions was developed from our recently reported toluene model, and was validated on the new experimental data from this work and previous ignition delay time data. The O-atom attack reactions, H-atom abstraction reactions and other H-atom attack reactions were found to dominate the consumption of styrene. In particular, the O-atom attack reactions on the double bond of styrene play an important role in the JSR oxidation and the lean and stoichiometric flames, while the pyrolytic reactions have increasing contributions as the equivalence ratio increases. The phenyl radical plays a crucial role in the decomposition of styrene since it affords most of the reaction fluxes from styrene. In the aromatics growth process, phenylacetylene, the 1-phenylvinyl radical, the phenyl radical, and styrene serve as major precursors of polycyclic aromatic hydrocarbons (PAHs), leading to the high sooting tendency of styrene compared to toluene and ethylbenzene.

Absolute evaporation rates of non-rotating neutral polycyclic aromatic hydrocarbon clusters

Context. Clusters of polycyclic aromatic hydrocarbons (PAHs) have been proposed as candidates for evaporating very small grains, which are thought to be precursors of free-flying PAHs. Evaporation rates have been calculated so far only for species containing up to a few 100-C atoms, whereas interstellar PAH clusters could contain up to ~1000 C atoms.

Chemical kinetic modeling for the effects of methyl ester moiety in biodiesel on PAHs and NO x formation

In order to investigate the effects of methyl ester moiety on polycyclic aromatic hydrocarbons (PAHs) and NO x formation in biodiesel combustion, the combined models were developed based on detailed methyl butanoate (MB) oxidation model and n-butane model. Also, PAHs detailed reaction mechanism and NO x formation mechanism were added to the detailed models to form the combined models. The combined models were used to compare the combustion of n-butane and MB in a shock tube simulation to understand the effects of methyl ester moiety. The results indicated that compared with n-butane, the methyl ester moiety in MB leads to different reaction pathways, more CO and CO2 formation and less formation of PAHs precursors such as ethylene and acetylene. In addition, a better chemical insight into the effects of methyl ester moiety on NO x formation was given, which will help to understand the combustion process of biodiesel.

An experimental and kinetic modeling investigation on a rich premixed n-propylbenzene flame at low pressure

A rich premixed flame of n-propylbenzene (ϕ=1.79) was investigated at low pressure. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to detect flame species including a lot of radicals, isomers and polycyclic aromatic hydrocarbons (PAHs) and measure their mole fraction profiles. A preliminary kinetic model of n-propylbenzene combustion was developed from recently reported toluene and ethylbenzene models [Y.Y. Li et al., Proc. Combust. Inst. 33 (2011) 593–600, 617–624] and validated by the experimental results. Rate constants of the important pathways of n-propylbenzene consumption, i.e. the H-abstraction reactions by H atom and the benzylic C–C bond dissociation, were calculated theoretically and included in this model. Based on the rate of production analysis and experimental observations, styrene, benzyl and benzene are confirmed as significant intermediates in the n-propylbenzene flame. For the formation pathways of PAHs, n-propylbenzene flame has a fuel specific pathway to form indene compared to smaller alkylbenzene, which increases the concentrations of indene and indenyl radical. It is concluded that high concentrations of important PAH precursors such as benzyl and indenyl radicals result in the enhanced PAHs formation in this flame compared with smaller alkylbenzenes.

Reduction of PAH and Soot Precursors in Benzene Flames by Addition of Ethanol

A one-dimensional premixed flame model (PREMIX) and schemes resulting from the merging of validated kinetic schemes for the oxidation of the components of the present mixtures (benzene and ethanol) were used to investigate the effect of oxygenated additives on aromatic species, which are known to be soot precursors, in fuel-rich benzene combustion. The specific flames were low-pressure (45 mbar), laminar, premixed flames at an equivalence ratio of 2.0. The blended fuels were formed by incrementally adding 4% wt of oxygen (ethanol) to the neat benzene flame and by keeping the inert mole fraction (argon) and the equivalence ratio constants. Special emphasis was directed toward the causes for the concentration-dependent influence of the blends on the amount of polycyclic aromatic hydrocarbons (PAHs) formed. The effects of oxygenate addition to the benzene base flame were seen to result in interesting differences, especially regarding trends to form PAH. The modeling results indicated that the concentration of acetylene and propargyl radicals, the main PAH precursors, as well as the PAH amounts were lower in the flame of the ethanol-benzene fuel mixture than in the pure benzene flame and that all of the formed PAHs were issued from the phenyl radical. Finally, the modeling results provided evidence that the PAH reduction was a result of simply replacing "sooting" benzene with "nonsooting" ethanol without influencing the combustion chemistry of the benzene.

Role of fluorine in distortion from planarity of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are a large class of compounds that pose a cancer risk to humans. Previously, it has been shown that increased molecular distortion decreases the amount of PAH precursors that are transformed into carcinogenic metabolites through cytochrome P450‐mediated oxidation of Benzo[c]phenanthrene (BcPh) derivates to DNA‐alkylating diol epoxides. The presence of substituent groups in and around the bay or fjord regions has been shown to influence out‐of‐plane distortion. In our studies, we analyzed the effect of the introduction of Fluorine atoms into the angular ring of chrysene and BcPh. Chrysene has a less hindered bay region and is very planar, while BcPh is the smallest PAH with a highly hindered fjord region and has been shown to be distorted 26.7° between the outermost rings. The 3‐D structures of 5‐FluoroBcPh, 6‐FluoroBcPh, 5‐Fluorochrysene, Benzo[g]chrysene and 10‐Fluorobenzo[g]chrysene were determined by single‐crystal X‐ray diffraction with Mo radiation using the Bruker Smart APEX II diffractometer. Results reveal a decrease in distortion from planarity upon the introduction of Fluorine atoms. The angle between the outermost rings was found to be decreased from 26.7° in BcPh to 24.3° and 23.6° in the 6‐Fluoro‐ and 5‐FluoroBcPh respectively. The angle between the outermost rings in 10‐Fluorobenzo[g]chrysene was found to be 33.33°, which is a decrease from 36.04° in its non‐fluorinated precursor. Future studies on these and other similar compounds will clarify the role of substituent groups on the out‐of‐plane distortion and measure the relationship between distortion and DNA alkylating ability. Research is supported by a grant from CNR for Short Term Mobility Funding (FC) and an NSF grant for the Bruker X‐ray diffractometer (CHE 0521237).

Quantitative appraisal of the interfacial anchoring state of polyaromatic hydrocarbons during the formation of C/C composites

The preferred anchoring state for polycyclic aromatic hydrocarbons (PAHs) pyrolyzed onto carbon substrates has been studied by semi-quantitative methods, examining their interfacial lattice arrangements. The samples were prepared by decomposing petroleum pitch inside carbon nanotubes resulting in a variety of crystallinities forming one-dimensional C/C composites. Studies indicate that the preferred anchoring state of PAH molecules depends on the nature of the substrate. Accordingly the PAHs in mesophase pitch should exhibit a face-on orientation on the carbonaceous substrates, including a graphite sheet, glassy carbon, and pyrolytic carbon. However, it showed that the anchoring state (face-on or edge-on) of PAH units can be altered even on carbonaceous substrates. The results demonstrated that in C/C composites the anchoring state is predominantly determined by the relative degree of crystallinity of the pitch/carbon substrate, and can be semi-quantitatively estimated using the I D/ I G ratio from Raman spectra. Face-on anchoring is preferred when the I D/ I G ratio of substrate is smaller (higher crystallinity) than that of the pyrolyzed precursor, whereas edge-on anchoring is favored when it is larger. Such an estimation method is useful in tailoring microstructures in the fabrication of C/C composites using PAH precursors.

Formation of polycyclic aromatic hydrocarbons from tobacco: The link between low temperature residual solid (char) and PAH formation

The formation of condensed ring polycyclic aromatic hydrocarbons (PAHs) from the pyrolysis of ground tobacco in helium over the temperature range of 350–600 °C was investigated. PAH yields in the ng/g range were detected and the maximum yields of all PAHs studied including benzo[a]pyrene (B[a]P) and benzo[a]anthracene (B[a]A) occurred between 500 and 550 °C. The pathway to PAH formation in the 350–600 °C temperature range is believed to proceed via a carbonization process where the residual solid (char) undergoes a chemical transformation and rearrangement to give a more condensed polycyclic aromatic structure that upon further heating evolves PAH moieties. Extraction of tobacco with water led to a two fold increase in the yields of most PAHs studied. The extraction process removed low temperature non-PAH-forming components, such as alkaloids, organic acids and inorganic salts, and concentrated instead (on a per unit weight basis) tobacco components such as cell wall bio-polymers and lipids. Hexane extraction of the tobacco removed lipophilic components, previously identified as the main source of PAH precursors, but no change in PAH yields was observed from the hexane-extracted tobacco. Tobacco cell wall components such as cellulose, hemicellulose, and lignin are identified as major low temperature PAH precursors. A link between the formation of a low temperature char that evolves PAHs upon heating is established and the observed ng/g yields of PAHs from tobacco highlights a low temperature solid phase formation mechanism that may be operable in a burning cigarette.

Synthesis of carbon nanotubes using polycyclic aromatic hydrocarbons as carbon sources in an arc discharge

The influence of starting carbon materials on the synthesis of carbon nanotubes (CNTs) is investigated. Comparisons are made between graphite rods and polycyclic aromatic hydrocarbons (PAHs) as carbon sources in helium arc discharge. The major parameters are also evaluated in order to obtain high-yield and high-quality carbon nanotubes. The cathode deposits are examined using scanning electron microscopy (SEM) to determine the microstructure of carbon nanotubes. The SEM investigation of the carbon nanotube deposits formed on the cathode provides evidence that PAHs can serve as building blocks for nanotube formation. The high-temperature graphitization process induced by the arc plasma enables the hydrocarbons to act as carbon sources and changes the aromatic species into the layered graphite structure of CNT. These polycyclic aromatic hydrocarbons not only act as the precursors but also enhance the production rate of carbon nanotubes. The PAH precursors thus play an important role in the mass production of carbon nanotubes.