Identification of Five-to Seven-Ring Polycyclic Aromatic Hydrocarbons

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Identification of Five- to Seven-Ring Polycyclic Aromatic Hydrocarbons from the Supercritical Pyrolysis of n-Decane Sean P. Bagley and Mary J. Wornat* Louisiana State University, Department of Chemical Engineering, Baton Rouge, Louisiana 70803, United States

bS Supporting Information ABSTRACT: In order to study the formation of carbonaceous solid deposits from aviation fuels in the pre-combustion environment of next-generation high-speed aircraft, we have pyrolyzed the model fuel n-decane (critical temperature, 344.5 °C; critical pressure, 20.7 atm), an alkane component of jet fuel, under supercritical conditions at 570 °C, 100 atm, and 133 sec. The product polycyclic aromatic hydrocarbons (PAH), precursors to the solid deposits, have been analyzed by a two-dimensional high-pressure liquid chromatographic separation technique with ultraviolet-visible absorbance and mass spectrometric detection. The analyses reveal that there are 24 unsubstituted PAH products with molecular weights between 252 and 300. Of these, 12 are benzenoid: benzo[a]pyrene, benzo[e]pyrene, perylene, benzo[ghi]perylene, anthanthrene, benzo[b]chrysene, dibenz[a,c]anthracene, dibenz[a,h]anthracene, dibenz[a,j]anthracene, pentaphene, picene, and coronene; two are methylene-bridged derivatives of two of the benzenoid PAH: 11H-indeno[2,1,7-cde]pyrene and 4H-benzo[def]cyclopenta[mno]chrysene; five are fluoranthene benzologues: benzo[a]fluoranthene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, and indeno[1,2,3-cd]pyrene; and five are fluorene benzologues: dibenzo[a,h]fluorene, dibenzo[a,i]fluorene, naphtho[1,2-a]fluorene, naphtho[1,2-b]fluorene, and naphtho[2,1-a]fluorene. Eighty-three alkylated derivatives of these PAH have also been identified as products of n-decane pyrolysis. Fourteen of the unsubstituted PAH and all of the alkylated derivatives have never previously been identified as products of n-decane pyrolysis or combustion. The UV spectra establishing the identities of the 24 unsubstituted n-decane pyrolysis products are presented.

’ INTRODUCTION Hydrocarbon fuels used in future generations of high-speed aircraft will be exposed to increasingly high temperatures in the pre-combustion environment, due to the requirement that the fuels themselves be used as the primary coolant for the removal of excess heat from engine components. Projected requirements indicate that fuels used in this capacity may reach temperatures as high as 700 °C for periods on the order of minutes.1 Furthermore, high pressure (up to 150 atm) is necessary to maintain the fuel in a high-density state (conditions that are supercritical for most pure hydrocarbons as well as jet fuels2) so that sufficient heat transfer, not attainable by a low-density, gas-phase fluid, occurs. Aircraft fuels used in this capacity store heat not only by simple physical heating (accounting for the elevated temperature of the fuel) but also by undergoing endothermic chemical reactions that convert the fuel to higher-energy products.3 Unfortunately, parallel reaction pathways lead to the conversion of a small fraction of fuel molecules to undesirable solid deposits in this pre-combustion environment solids that can reduce heat transfer from the engine and clog fuel lines and injection nozzles, impairing engine performance and leading to eventual failure.1 Of particular interest are the reactions leading to polycyclic aromatic hydrocarbons (PAH), which can serve as precursors to these carbonaceous solid deposits.4 6 In order to better understand the reactions responsible for PAH formation under supercritical pyrolysis conditions, we have conducted supercritical pyrolysis experiments with the model fuel n-decane (critical temperature, 344.5 °C; critical pressure, 20.7 atm), an alkane component of jet fuels. A model fuel has the r 2011 American Chemical Society

advantage of reducing the number of reactions taking place by reducing the number of reactants in the reaction environment, thereby simplifying the elucidation of reaction pathways. A critical element in our experimental effort to discern PAH reaction pathways is the ability to analyze PAH with as much structural specificity as possible. We thus employ high-pressure liquid chromatography (HPLC) combined with diode-array ultraviolet-visible (UV) absorbance and mass spectrometric (MS) detection a technique ideally suited for isomer-specific PAH analysis. Previous work7,8 in our research group has shown that separation of a large number of PAH, ranging from two to ten aromatic rings, can be attained by HPLC with the appropriate column and solvent method and that good component resolution is essential for product identification by UV/MS. However, the products of supercritical n-decane pyrolysis present a significant challenge in this regard in that they consist not only of a large number of unsubstituted PAH, but also several alkylated derivatives of each of these PAH. The number of compounds is simply too many for a single HPLC column and method to resolve; therefore a two-dimensional chromatographic technique is used. Though the specific details of this technique were developed by our research group for the analysis of n-decane pyrolysis products, the concept was first established by Wise et al.9 and has been used extensively for the analysis of complex mixtures of polycyclic aromatic compounds.10 12 Received: May 30, 2011 Revised: August 5, 2011 Published: September 08, 2011 4517

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Energy & Fuels Application of the two-dimensional HPLC technique has enabled us to identify PAH with as many as nine fused aromatic rings in the products of supercritical n-decane pyrolysis. The identification of the lower-molecular-weight (less than 252) PAH products, while a necessary part of this work, is achievable with well-known, widely-practiced methods and consequently will not be detailed here. (Table S1 in the Supporting Information shows the structures of the unsubstituted two- to five-ring PAH identified13 in these lower-molecular-weight products.) Future articles will document the identification of PAH with molecular weights greater than 300, along with their alkylated derivatives. In this paper, we document the identification of 24 individual unsubstituted PAH with molecular weights between 252 and 300, as well as 83 of their alkylated derivatives, as products from our supercritical n-decane pyrolysis experiments. Of the 24 unsubstituted PAH products identified here, 14 have never before been reported as products of n-decane pyrolysis or combustion. It is also the first time that any of the 83 alkylated PAH are reported as products of n-decane pyrolysis or combustion.

’ EXPERIMENTAL EQUIPMENT AND PROCEDURES The supercritical n-decane pyrolysis experiments are conducted in an isothermal, isobaric flow reactor designed by Davis,14 illustrated elsewhere,5 and used, in its original form, by Stewart et al.,4,15 Ledesma et al.,5 McClaine et al.,6 and Somers et al.7,16 for supercritical pyrolysis of other model fuels. The reactor has subsequently been modified to accommodate higher temperatures. Prior to an experiment, the n-decane fuel is sparged with nitrogen for three hours to remove any dissolved oxygen that could introduce autooxidative effects to the reactor system.17 Once the oxygen is removed, the sparged fuel is loaded into a high-pressure syringe pump for continuous delivery of the fuel through the reactor. The reactor is a silica-lined stainless-steel tube (length, 53 cm; inner diameter, 2.16 mm; outer diameter, 3.17 mm) immersed in a temperature-controlled fluidized alumina bath. The silica lining prevents wallcatalyzed deposit formation that would occur if reactants were to come into contact with bare stainless steel.18 The fluidized bath maintains the temperature inside the stainless-steel tubing (the reaction environment) and ensures isothermality throughout the reactor length. The reactor tubing passes through a water-cooled (25 °C) shell-andtube heat exchanger immediately before entering and after exiting the heated area, ensuring a controlled thermal history for the reactant. After exiting the reaction zone and passing through the heat exchanger, the quenched reaction products (and any unreacted n-decane) pass through a stainless-steel filter (hole size, 10 μm) to trap any solids that may have formed inside the reactor. They then pass through a back-pressure regulator, which maintains constant pressure inside the reactor. Upon leaving the back-pressure regulator, the pyrolysis products and unreacted fuel proceed to the liquid- and gas-phase product collection apparatus, where they are separated by phase for later analyses. Two sets of six pyrolysis experiments, each at a residence time of 133 sec, have been performed with n-decane (critical temperature, 344.5 °C; critical pressure, 20.7 atm). In the first set, pressure is held constant at 100 atm, and experiments are conducted at the six temperatures of 530, 540, 550, 560, 565, and 570 °C. In the second set, temperature is held constant at 570 °C, and experiments are conducted at the six pressures of 40, 60, 70, 80, 90, and 100 atm. Gas-phase products and liquid-phase aliphatic and single-ring aromatic products are analyzed by gas chromatography coupled to flameionization detection and mass spectrometry. This paper documents the analysis of PAH products in the liquid phase by high-pressure liquid chromatography (HPLC) coupled to ultraviolet-visible spectroscopy

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Table 1. Contents of the 13 Normal-Phase HPLC Fractions of an n-Decane Product Mixture fraction number

primary constituentsa

1

2-ring, C10H8

2 3

3-ring, C13H10 3-ring, C14H10

4

4-ring, C16H10

5

4-ring, C18H12; 5-ring, C18H10

6


7


8

6-ring, C22H12

9


10 11

6-ring, C24H14 7-ring, C26H14

12

8-ring, C28H14

13


a

The given molecular formula is for the unsubstituted PAH. In most cases, alkylated derivatives of such PAH are also within the fraction.

and mass spectrometry (UV/MS) from the experiment conducted at the most severe conditions, 570 °C and 100 atm, which corresponds to 90% n-decane conversion and the onset of solids production. All PAH produced at experiments conducted at lower temperatures or pressures are also produced at this condition. To obtain good component resolution among the very large number of PAH and alkylated PAH produced during supercritical n-decane pyrolysis resolution that is essential for product identification by UV/MS a two-dimensional chromatographic technique is used. In the first dimension of separation, products are separated with a normal-phase cyano HPLC column. The lone pair of electrons on each cyano functional group of the stationary phase interacts directly with the pibonding electrons of the PAH analyte but interacts very little with any alkyl substituents. Therefore the cyano column separates unsubstituted and alkylated PAH into groups based on aromatic structure only: benzene (molecular weight 78) and alkylbenzenes elute together first, followed by naphthalene (molecular weight 128) and alkylnaphthalenes, followed by anthracene and phenanthrene (molecular weight 178) and their alkylated derivatives, and so on to higher-molecular-weight aromatic compounds. Unsubstituted PAH of a particular molecular weight and their alkylated derivatives are collected as individual “fractions” over different ranges of elution time on the normal-phase HPLC column. In the second dimension of separation, individual component resolution is achieved on a reversed-phase octadecylsilica (C18) HPLC column, which is sensitive both to the aromatic structure and to any alkyl substituents of the PAH that might be present. Once products are fully resolved by the C18 column, they are identified by UV/MS. More specifically, the normal-phase HPLC fractionation consists of injection of the liquid-phase products of n-decane pyrolysis, without any additional preparation, onto an Agilent model 1200 HPLC and separation by two in-series Restek Pinnacle II Cyano columns, each with length, 250 mm; inner diameter, 4.6 mm; and particle size, 5 μm. The columns are maintained at 11 °C, and hexane is pumped through, as mobile phase, at 1 mL/min. Altogether, 13 fractions are obtained each fraction corresponding to PAH of a common ring number or isomer group along with alkylated derivatives of those PAH. Table 1 presents the characteristics of the main constituents of the 13 fractions. Due to of orders-of-magnitude differences in concentration between the PAH that elute in the first five fractions and those that elute in the last eight fractions, two distinct fractionation methods are used. The earlyeluting PAH in the first five fractions, along with the mobile-phase solvent hexane, are simply collected in flasks as they elute off the column, with 4518

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Energy & Fuels later-eluting fractions channeled to a waste collection vessel. Multiple injections and subsequent fraction collection are required to yield sufficient material for analysis by our identification techniques. Afterwards, the fractions are condensed with a Kuderna-Danish apparatus to remove excess hexane and then exchanged into dimethylsulfoxide (DMSO), a solvent compatible with the reversed-phase mobile-phase solvents. The identification of PAH in Fractions 1 5 will not be described here, as these fractions consist of products for which reference standards are readily available and their analysis is relatively straightforward. (Table S1 in the Supporting Information lists the 15 unsubstituted PAH that have been identified in Fractions 1 5, along with the number of identified singly-methylated derivatives of each. Of the unsubstituted PAH in Table S1 in the Supporting Information, acephenanthrylene and cyclopenta[cd]pyrene have never before been reported as products of n-decane pyrolysis or combustion.) The present paper focuses on the analysis of Fractions 6 9, and future papers will detail the separation and identification of PAH products in Fractions 10 13. To obtain enough material for Fractions 6 13, a “pre-fractionation” step is used in which injection of the liquid-phase products onto the cyano column (20 injections of 100 μL) removes the lighter material and isolates and concentrates the heavier PAH. Although a 100-μL injection volume is too large to achieve good separation between individual fractions, it is perfectly acceptable for the purpose of collecting all of Fractions 6 13 together. This material is then condensed down, first in a Kuderna-Danish apparatus, then by blowing nitrogen to evaporate almost all of the remaining solvent. The remaining solution of five- to nine-ring PAH (in approximately 200 μL of hexane) is injected back onto the cyano column in 20-μL increments, separating the heavier PAH into Fractions 6 13. By adding the initial pre-fractionation step, 2000 μL of the liquid-phase products can be fractionated with approximately 30 injections onto the cyano column, as opposed to the 100 that would be necessary if this step were not taken. The eight fractions are then concentrated in a Kuderna-Danish apparatus and exchanged into DMSO for analysis by reversed-phase HPLC/UV/MS to achieve compositional analysis of the six- to nine-ring PAH products. Tests of the entire analytical procedure with a mixture of PAH reference standards representative of the supercritical n-decane pyrolysis products demonstrate overall individual PAH recovery rates of 91 to 97%. Each of the eight product fractions is injected onto an Agilent model 1100 HPLC, coupled to a diode-array ultraviolet-visible (UV) absorbance detector in series with a mass spectrometer (MS). The HPLC utilizes a Restek Pinnacle II PAH (C18) reversed-phase HPLC column with length, 250 mm; inner diameter, 2.1 mm; and particle size, 4 μm. The column is maintained at 30 °C, and the mobile phase is pumped through the column at 0.2 mL/min. The mobile phase, initially a 15/85 water/acetonitrile mixture, is ramped at a constant rate to pure acetonitrile over 80 min, then held for 100 min. UV absorbance spectra are taken every 0.8 sec, and mass spectra are taken every 1 sec of the separated components as they exit the column. The MS employs an atmosphericpressure photo-ionization (APPI) source equipped with a krypton discharge lamp, operates in the positive-ion mode, and monitors mass-to-charge ratios from 200 to 700. Optimized to achieve the strongest signal for product PAH, the following MS parameters are employed: vaporizer and drying gas temperatures of 350 °C, a capillary voltage of 3000 V, a drying gas flow rate of 13 L/min, and a nebulizer pressure of 60 psig. The fragmentor is set to 125 V, the gain to 1.5, the stepsize to 0.1, and the signal level threshold to 25. The APPI mass spectrum of each PAH consists chiefly of a single primary ion with a mass-to-charge ratio corresponding to the molecular weight of the PAH, so the CxHy formula of a PAH product component is readily determined. A UV absorbance spectrum is unique to each PAH and acts as a “fingerprint” for the compound. Once the UV spectrum is obtained, the product is identified by matching its spectrum with that of the appropriate reference standard. In cases in which reference standards

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are not available, product identities are established by matching UV spectra with those published in the literature. In certain instances, HPLC retention time evidence is also used to aid product identification. For PAH with no substituent groups, the UV spectrum alone is sufficient to establish the exact isomer-specific identity. If a PAH has alkyl substituent(s), the UV spectrum looks almost exactly like that of the parent PAH, only shifted a few nanometers to higher wavelengths the position(s) and length(s) of the substituent(s) dictating the details of the shift.19,20 Thus the UV spectrum also establishes the aromatic structure of an alkylated PAH. For PAH that have a multitude of sites at which substituents can be located, one must have reference standards or UV spectra of all possible positional isomers in order to be certain of the exact position of the alkyl substituent a condition rarely met for large PAH or PAH with multiple alkyl groups. Consequently, for most of the alkylated PAH products reported here, the exact position(s) of the alkyl substituent(s), are not known. However, for each one of the alkylated PAH products, the exact structure of the aromatic portion of the PAH is known (from the UV spectrum), the molecular formula is known (from the mass spectrum), and the total number of carbons associated with the alkyl substituent(s) is known.

’ RESULTS AND DISCUSSION As detailed elsewhere,13 the primary products of the supercritical n-decane pyrolysis experiments are n-alkanes and olefin. R-olefins composed of nine or fewer carbons. The next highest-yield product classes, cyclic alkanes and alkenes, as well as branched alkanes, are also observed among the products in significant amounts, followed by the single-ring aromatics and then the PAH. Future papers will report individual PAH product yields as functions of temperature and pressure for the two sets of supercritical n-decane pyrolysis experiments conducted in this study. For purposes of the present paper, we note that PAH yields are 1%, on a mass basis, at 570 °C and 100 atm, the condition of focus here. Even these relatively low PAH yields are significant with regard to solids formation, however, as more severe conditions producing PAH yields >1% also produce solids so abundantly that the reactor clogs.13 The first step in examining the role of PAH as precursors to solids in the supercritical n-decane pyrolysis environment is the determination of the PAH product structures. Future papers will document the identification of the unsubstituted PAH products with molecular weights greater than 300 and their alkylated derivatives (Fractions 10 13 in Table 1). The present paper details the separation and identification of the 24 unsubstituted supercritical n-decane pyrolysis product PAH with molecular weights between 252 and 300 (Fractions 6 9) and their 83 alkylated derivatives. The alkyl groups of these five- to seven-ring PAH consist of up to three carbons (i.e., the substituents of the most highly alkylated compounds are propyl, methylethyl, or trimethyl). Figures 1 4 present the reversed-phase HPLC chromatograms of Fractions 6 9, obtained after first separating the supercritical n-decane pyrolysis products (from the experiment at 570 °C and 100 atm) on the normal-phase, cyano stationaryphase HPLC column. The chromatograms of Figures 1 4 are labeled with the aromatic structures of the identified products, with the colors indicating the degree of alkylation: unsubstituted PAH structures are shown in black; singly-methylated PAH, red; dimethylated or ethylated PAH, blue; and trimethylated, methylethylated, or propylated PAH, green. C20H12 Benzopyrenes and Their Derivatives. Figure 1 presents the reversed-phase HPLC chromatogram of Fraction 6 of 4519


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Figure 1. Reversed-phase HPLC chromatogram of Fraction 6 of the products of n-decane pyrolysis at 570 °C and 100 atm. This fraction primarily contains the C20H12 benzopyrenes and their alkylated derivatives. Black structures represent unsubstituted PAH, red structures represent singly-methylated PAH, and blue structures represent dimethylated or ethylated PAH.

Figure 3. Reversed-phase HPLC chromatogram of Fraction 8 of the products of n-decane pyrolysis at 570 °C and 100 atm. This fraction primarily contains the C22H12 PAH isomer family and its alkylated derivatives. Black structures represent unsubstituted PAH, red structures represent singly-methylated PAH, and blue structures represent dimethylated or ethylated PAH.

Figure 2. Reversed-phase HPLC chromatogram of Fraction 7 of the products of n-decane pyrolysis at 570 °C and 100 atm. This fraction primarily contains the C20H12 fluoranthene benzologues, the C21H14 fluorene benzologues, and their alkylated derivatives. Black structures represent unsubstituted PAH, and red structures represent singly-methylated PAH.

Figure 4. Reversed-phase HPLC chromatogram of Fraction 9 of the products of n-decane pyrolysis at 570 °C and 100 atm. This fraction primarily contains the C24H12 coronene, the C22H14 PAH isomer family, and their alkylated derivatives. Black structures represent unsubstituted PAH, red structures represent singly-methylated PAH, blue structures represent dimethylated or ethylated PAH, and green structures represent trimethylated, methylethylated, or propylated PAH.

the products of supercritical n-decane pyrolysis. This fraction primarily contains five-ring C20H12 benzenoid PAH and their alkylated derivatives. Figure 5a displays the UV spectrum of the n-decane pyrolysis product eluting at 17.7 min in the chromatogram of Figure 1, as well as the UV spectrum of a reference standard of benzo[e]pyrene. The coincidence of the two spectra confirms that the n-decane pyrolysis product eluting at 17.7 min is benzo[e]pyrene and the corresponding chromatographic peak in Figure 1 has been appropriately labeled. The mass spectrum of this product, displayed in the inset to Figure 5a, shows that this compound has a molecular weight of 252, consistent with the identification made from the UV spectra. Note the weak but observable secondary mass signal of 266 in the inset of Figure 5a

(as well as additional mass signals in all four mass spectra in Figure 5), indicating the presence of a minor product or products eluting at the same time as benzo[e]pyrene. Co-eluting products and their consequences for product identification will be discussed at greater length below. Figure 5b displays the UV spectrum of the n-decane pyrolysis product eluting at 26.3 min in the chromatogram of Figure 1 as well as the UV spectrum of a reference standard of benzo[e]pyrene. The product’s UV spectrum, shifted to higher wavelengths relative to but still maintaining the same absorbance features as the UV spectrum of benzo[e]pyrene, establishes that this product is an alkylated derivative of benzo[e]pyrene. Furthermore, the 4520


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Figure 5. Comparisons of the UV spectra of n-decane pyrolysis products (solid lines) from the chromatogram of Fraction 6 (Figure 1) to that of a reference standard of benzo[e]pyrene (dashed lines). Comparisons are shown for the products identified as (a) benzo[e]pyrene, eluting at 17.7 min, (b) a methylated benzo[e]pyrene, eluting at 26.3 min, (c) a dimethylated or ethylated benzo[e]pyrene, eluting at 30.7 min, and (d) 11H-indeno[2,1,7cde]pyrene and a methylated benzo[e]pyrene, co-eluting at 24.2 min in Figure 1. The mass spectrum of each product component is displayed as an inset to the respective figure. The structures of each identified product are shown with each comparison. The structures in (b) and (c) contain unattached methyl groups that represent the degree of alkylation in the identified products, although the product in (c) could also be an ethylated benzo[e]pyrene. Note that relative to the UV spectrum of benzo[e]pyrene, the spectra of the two alkylated and one methylene-bridged benzo[e]pyrene products are shifted by a few nm to higher wavelengths.

mass spectrum of this product, displayed in the inset to Figure 5b, shows that this compound has a molecular weight of 266, corresponding to the molecular formula C20H11-CH3, establishing that the product is a singly-methylated benzo[e]pyrene. Likewise, the UV and mass spectra of five additional products in Figure 1 have established these products’ identities as singlymethylated benzo[e]pyrenes. In accordance with our labeling scheme, the six chromatographic peaks corresponding to the six identified singly methylated benzo[e]pyrenes have been labeled with red benzo[e]pyrene structures in Figure 1. Four products identified as either dimethylated or ethylated benzo[e]pyrenes are labeled with their aromatic structures colored blue in Figure 1. Each of these has a UV spectrum with the same absorbance features as that of benzo[e]pyrene (but shifted to higher wavelengths), and therefore must be an alkylated derivative of benzo[e]pyrene. Each also has a molecular weight of 280, as determined by mass spectrometry, and therefore must have either two methyl substituents (C20H10-(CH3)2) or one ethyl substituent (C20H11-C2H5). (For a given aromatic structure, the APPI mass spectrum of a dimethyl-substituted derivative is indistinguishable from that of an ethyl-substituted derivative.) Figure 5c displays the comparison of the UV spectrum of one of these dimethylated or ethylated benzo[e]pyrenes, eluting at 30.7 min in the chromatogram of Figure 1, to the spectrum of a reference standard of benzo[e]pyrene. The mass spectrum of this compound is displayed as an inset to the figure.

The products identified as benzo[e]pyrene and its alkylated derivatives all have mass spectra that include additional, weaker mass signals showing the presence of other co-eluting products. For example, such signals are clearly visible in each of the mass spectra displayed in the insets of Figure 5. Virtually all of the 107 products identified in this paper exhibit some additional mass signals in their mass spectra, indicating the presence of minor co-eluting products, but evidence regarding the aromatic structures of these co-eluting products is not always definitive. Such a co-eluting product may have the same aromatic structure as the identified product component but a different degree of alkylation a reasonable conclusion when the product UV spectrum exhibits only those absorbance features exhibited by the spectrum of the reference standard. It is conceivable, however, that a co-eluting product could have an aromatic structure different from that of the identified product but just be present in too small of an amount to noticeably affect the UV spectrum of the identified product. Since our analytical techniques do not allow us to determine with certainty the identities of such lower-yield co-eluting compounds, identifications are made based on the UV spectrum and the single strongest mass signal in the mass spectrum of any given chromatographic peak. With few exceptions, this convention is used throughout this work. One of these exceptions in Figure 1 is a product eluting at 24.2 min, the same elution time as that of an identified methylated benzo[e]pyrene. The mass spectrum associated with this product, 4521


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Figure 6. Comparisons of the UV spectra of n-decane pyrolysis products (solid lines) from the chromatogram of Fraction 6 (Figure 1) to that of a reference standard of benzo[a]pyrene (dashed lines). Comparisons are shown for the products identified as (a) benzo[a]pyrene, eluting at 24.9 min, and (b) 4H-benzo[def]cyclopenta[mno]chrysene along with some alkylated derivatives of benzo[a]pyrene, co-eluting at 32.2 min in Figure 1. The mass spectrum of each product component is displayed as an inset to the respective figure. The structures of benzo[a]pyrene and 4H-benzo[def]cyclopenta[mno]chrysene are displayed in (a) and (b), respectively.

shown in the inset to Figure 5d, includes mass signals of 266 (the methylated product) as well as 264. The height of the secondary 264 signal is more than one-third that of the primary 266 signal, indicating that a co-eluting product is present in a yield of the same order of magnitude as that of the methylated benzo[e]pyrene, but the composite UV spectrum of the two products in Figure 5d is very similar to the spectrum of benzo[e]pyrene. Therefore these two products must have very similar UV spectra, and by extension the aromatic portions of their structures must be the same, that of benzo[e]pyrene. The only compound with a molecular weight of 264 and the same aromatic structure as benzo[e]pyrene is its C21H12 methylene-bridged derivative, 11H-indeno[2,1,7-cde]pyrene. The elution time of this product in Figure 1 (just before benzo[a]pyrene, the n-decane product whose identification is presented next) is also consistent with its identification as 11H-indeno[2,1,7-cde]pyrene: During the separation of PAH products of the gas-phase pyrolysis of catechol with the same C18 stationary phase and mobile-phase solvents as used to separate the present n-decane pyrolysis products 11H-indeno[2,1,7-cde]pyrene has been shown21 to elute shortly before benzo[a]pyrene. Consistent with the placement of the black benzo[a]pyrene structure in Figure 1, the supercritical n-decane pyrolysis product eluting at 24.9 min in the chromatogram in Figure 1 has been identified as the unsubstituted C20H12 PAH benzo[a]pyrene by the close matching of the product’s UV spectrum, in Figure 6a, to that of a reference standard of benzo[a]pyrene. Additionally, the mass spectrum of the product, the inset of Figure 6a, shows a molecular weight of 252, consistent with the identification from the UV spectra. As indicated by the seven red and ten blue benzo[a]pyrene structures in Figure 1, seven singly-methylated and ten dimethylated or ethylated derivatives of benzo[a]pyrene have also been identified, in Fraction 6, by their UV and mass spectra. Analogous to the case of the identified alkylated benzo[e]pyrenes, each of the 17 identified alkylated benzo[a]pyrenes has a UV spectrum with the same absorbance features as the spectrum of benzo[a]pyrene, only shifted to higher wavelengths, indicating an alkylated benzo[a]pyrene, and each of these compounds displays a mass signal of 266 (methylated) or 280 (dimethylated or ethylated), indicating the degree of alkylation. As shown in the inset to Figure 6b, the product eluting at 32.2 min in the chromatogram of Figure 1, identified as one of the

seven methylated benzo[a]pyrenes, includes several mass signals in its mass spectrum in addition to the strongest, 266, that of the identified methylated benzo[a]pyrene. The UV spectrum of this product (Figure 6b) does not include absorbance features other than those of benzo[a]pyrene; it is only shifted a few nanometers to higher wavelengths, indicating that the compounds responsible for this chromatographic peak all have the same aromatic structure as benzo[a]pyrene. As displayed in the inset to Figure 6b, the mass spectrum exhibits signals at 278, 280, and 294 none of which lead to a definitive conclusion regarding the identities of the products responsible. However, the other mass signal, at 264, does lead to a definitive conclusion: only one compound has both a molecular weight of 264 and the same aromatic structure as benzo[a]pyrene, namely 4H-benzo[def]cyclopenta[mno]chrysene, the methylene-bridged derivative of benzo[a]pyrene. The elution time relative to benzo[a]pyrene is also consistent with the elution behavior observed for these products in the aforementioned catechol pyrolysis study.21 Altogether, Fraction 6 (Figure 1) contains two unsubstituted C20H12 PAH, benzo[a]pyrene and benzo[e]pyrene; their two respective C21H12 methylene-bridged derivatives, 11H-indeno[2,1,7-cde]pyrene and 4H-benzo[def]cyclopenta[mno]chrysene; six singly-methylated derivatives of benzo[e]pyrene; seven singlymethylated derivatives of benzo[a]pyrene; four dimethylated or ethylated derivatives of benzo[e]pyrene; and ten dimethylated or ethylated derivatives of benzo[a]pyrene. The two unsubstituted benzopyrenes in this fraction have been reported before as products of n-decane pyrolysis,22 but the 27 alkylated and two methylene-bridged derivatives are reported here for the first time as n-decane pyrolysis products. C20H12 Benzofluoranthenes and Perylene and Their Alkylated Derivatives. Figure 2 presents the reversed-phase HPLC chromatogram of Fraction 7 of the products of supercritical n-decane pyrolysis. As indicated by the black product structures in Figure 2, Fraction 7 primarily contains five-ring C20H12 fluoranthene benzologues and five-ring C21H14 fluorene benzologues along with methylated derivatives of the fluoranthene benzologues, as indicated by the red structures of Figure 2. A small amount of benzo[e]pyrene, primarily found in Fraction 6, is also present in this fraction. (During fractionation, benzo[e]pyrene elutes off the cyano column slightly later than benzo[a]pyrene in Fraction 6 but partially with the benzofluoranthenes of Fraction 7.) 4522


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Figure 7. Comparisons of the UV spectra of n-decane pyrolysis products (solid lines) from the chromatogram of Fraction 7 (Figure 2) to reference spectra (dashed lines). Comparisons are shown for the products identified as (a) benzo[a]fluoranthene, eluting at 15.6 min, (b) benzo[j]fluoranthene, eluting at 16.7 min, (c) benzo[b]fluoranthene, eluting at 18.2 min, and (d) benzo[k]fluoranthene, eluting at 21.1 in Figure 2. The reference UV spectrum in (a) of benzo[a]fluoranthene is taken from the literature;23 all other reference spectra are those of standards.

Figure 7 presents the UV spectra of the n-decane pyrolysis products eluting at 15.6, 16.7, 18.2, and 21.1 min in Figure 2, along with the reference spectra of the appropriate benzofluoranthene standards: benzo[a]fluoranthene, in Figure 7a; benzo[j]fluoranthene, in Figure 7b; benzo[b]fluoranthene, in Figure 7c; and benzo[k]fluoranthene, in Figure 7d. The reference UV spectrum of benzo[a]fluoranthene is taken from the literature;23 the reference spectra of the other three benzofluoranthenes are those of reference standards. The close matching of the UV spectra of each product/standard pair in Figure 7 establishes the identity of each of the four n-decane pyrolysis products as the respective benzofluoranthene shown. The mass spectra of the four products also confirm that each has a molecular weight of 252, consistent with these benzofluoranthenes. UV and mass spectral evidence also confirms, among the products of Fraction 7, the presence of eight singly-methylated derivatives of the benzofluoranthene products: four methylated benzo[j]fluoranthenes, three methylated benzo[k]fluoranthenes, and one methylated benzo[b]fluoranthene. Eight of the nine red structures in Figure 2 reflect these identifications. Figure 8a presents the UV spectrum of the product component (eluting at 18.8 min in Figure 2) identified as perylene along with the UV spectrum of a reference standard establishing the identity of this product. Mass spectrometry confirms that this product has a molecular weight of 252, consistent with the identification from the UV spectra, in Figure 8a, and also reveals that the additional absorbance features apparent in the UV spectrum of the product component (between wavelengths 260 and 325 nm) are the result of co-eluting product(s) with a molecular weight of 280. One product, eluting at 23.6 min in the chromatogram in Figure 2, is also identified as a singly-methylated perylene by its UV and mass spectra. Note, from Figure 2, the co-elution of this product with one of the methylated benzo[k]fluoranthenes. These two products are identified by comparing features of their combined UV spectra to those of the individual spectra of reference standards of perylene and benzo[k]fluoranthene.

C21H14 Fluorene Benzologues. Figures 8b f presents the UV spectra of the n-decane pyrolysis products eluting at 19.9, 24.0, 25.2, 39.2, and 41.6 min in Figure 2, along with the published spectra of the appropriate synthesized reference standards: naphtho[1,2-a]fluorene,24 in Figure 8b; dibenzo[a,i]fluorene,25 in Figure 8c; naphtho[1,2-b]fluorene,26 in Figure 8d; naphtho[2,1-a]fluorene,27 in Figure 8e; and dibenzo[a,h]fluorene,28 in Figure 8f. The close matching of the UV spectra of each product/ standard pair in Figures 8b f establishes the identity of each of the five n-decane pyrolysis products as the respective naphtho- or dibenzofluorene shown. The mass spectra of the five products confirm that each has a molecular weight of 266, consistent with the identifications from their UV spectra. The mass spectra also reveal that the additional absorbance features in the product component UV spectra of Figure 8b, 8c, and 8f are due to unidentified coeluting compounds of molecular weights 252, 280, and 280 along with 294, respectively. In total, Fraction 7 (Figure 2) contains four unsubstituted C20H12 fluoranthene benzologues and eight of their singlymethylated derivatives, one unsubstituted C20H12 benzenoid PAH and one of its singly-methylated derivatives, and five unsubstituted C21H14 fluorene benzologues. Four of the C20H12 PAH benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, and perylene have been reported22 before as products of n-decane pyrolysis; however, benzo[a]fluoranthene, the eight singly methylated benzofluoranthenes, and the singly methylated perylene, are reported here, as n-decane pyrolysis products, for the first time. The five C21H14 fluorene benzologues naphtho[1,2-a]fluorene, naphtho[1,2-b]fluorene, naphtho[2,1-a]fluorene, dibenzo[a,h]fluorene, and dibenzo[a,i]fluorene have also never before been reported as products of n-decane pyrolysis. It should be noted that in addition to the three naphthofluorenes and two dibenzofluorenes identified in Figure 8, there are eleven other isomers in the C21H14 fluorene benzologue family. Even though these other eleven have not been identified in Fraction 7, one or more of them could be present but just masked by potentially co-eluting 4523


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Figure 8. Comparisons of the UV spectra of n-decane pyrolysis products (solid lines) from the chromatogram of Fraction 7 (Figure 2) to reference spectra (dashed lines). Comparisons are shown for the products identified as (a) perylene, eluting at 18.8 min, (b) naphtho[1,2-a]fluorene, eluting at 19.9 min, (c) dibenzo[a,i]fluorene, eluting at 24.0 min, (d) naphtho[1,2-b]fluorene, eluting at 25.2 min, (e) naphtho[2,1-a]fluorene, eluting at 39.2 min, and (f) dibenzo[a,h]fluorene, eluting at 41.6 min in Figure 2. Additional absorbance features in the product component spectra in (a), (b), (c), and (f) are due to unidentified co-eluting compounds of molecular weights 280, 252, 280, and 280 along with 294, respectively, as determined by mass spectrometry. The reference spectrum in (a) of perylene is that of a reference standard. Taken from the literature are the reference spectra of (b) naphtho [1,2a]fluorene,24 (c) dibenzo[a,i]fluorene,25 (d) naphtho[1,2b]fluorene,26 (e) naphtho[2,1a]fluorene,27 and (f) dibenzo[a,h]fluorene.28

(and more abundant) methylated benzofluoranthenes and perylenes, which elute in the same fraction and have mass spectra that are indistinguishable from those of the C21H14 fluorene benzologues. C22H12 and C24H12 PAH and Their Alkylated Derivatives. Figure 3 presents the reversed-phase HPLC chromatogram of Fraction 8 of the products of supercritical n-decane pyrolysis. As indicated by the structures in Figure 3, Fraction 8 primarily contains six-ring C22H12 PAH (two benzenoid and one fluoranthene benzologue) along with their alkylated derivatives. Figures 9a c presents the UV spectra of the three unsubstituted C22H12 PAH products of n-decane pyrolysis in this fraction, along with the UV spectra of the appropriate PAH reference standards, which confirm the product identities as: benzo[ghi]perylene, in Figure 9a; indeno[1,2,3-cd]pyrene, in Figure 9b; and anthanthrene in Figure 9c. The mass spectra of these products confirm that their molecular weights are 276, consistent with these identifications. In addition to the three unsubstituted C22H12 products, UV and mass spectral evidence also confirms, among the products of Fraction 8, the presence of 14 singly-methylated and 12 dimethylated or ethylated PAH: five singly-methylated benzo[ghi]perylenes, eight singly-methylated indeno[1,2,3-cd]pyrenes, one singly-methylated anthanthrene, five dimethylated or ethylated benzo[ghi]perylenes, five dimethylated or ethylated indeno[1,2,3-cd]pyrenes, and two

dimethylated or ethylated anthanthrenes. Both benzo[ghi]perylene and anthanthrene have been reported22 before as products of n-decane pyrolysis, but indeno[1,2,3-cd]pyrene and the 26 alkylated derivatives of the three C22H12 PAH are reported here for the first time. Figure 4 presents the reversed-phase HPLC chromatogram of Fraction 9 of the products of supercritical n-decane pyrolysis. As indicated by the structures in Figure 4, Fraction 9 primarily contains the seven-ring C24H12 coronene and its alkylated derivatives as well as six five-ring C22H14 benzenoid PAH and their alkylated derivatives. (Indeno[1,2,3-cd]pyrene is present in this fraction as well as in Fraction 8.) Figure 9d presents the UV spectrum of the product component identified as coronene along with the UV spectrum of a reference standard establishing the identity of this product. The mass spectrum of this product confirms that its molecular weight is 300, consistent with this identification. As indicated by the red, blue, and green structures of Figure 4, 1-methylcoronene along with seven dimethylated or ethylated and six trimethylated, methylethylated, or propylated coronenes have also been identified in Fraction 9 from the appropriate UV and mass spectral evidence. (As there is only one isomer of methylcoronene, the position of the methyl group is known for this one product.) Coronene has been identified before22 as a product of n-decane 4524


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Figure 9. Comparisons of the UV spectra of n-decane pyrolysis products (solid lines) from the chromatograms of Fractions 8 and 9 (Figures 3 and 4) to those of reference standards (dashed lines). Comparisons are shown for the products identified as (a) benzo[ghi]perylene, eluting at 34.8 min, (b) indeno[1,2,3-cd]pyrene, eluting at 37.7 min, and (c) anthanthrene, eluting at 51.1 min in Figure 3, and (d) coronene, eluting at 65.1 min in Figure 4.

Figure 10. Comparisons of the UV spectra of n-decane pyrolysis products (solid lines) from the chromatogram of Fraction 9 (Figure 4) to those of reference standards (dashed lines). Comparisons are shown for the products identified as (a) dibenz[a,c]anthracene, eluting at 20.2 min, (b) dibenz[a,j] anthracene, eluting at 23.6 min, (c) pentaphene, eluting at 28.1 min, (d) dibenz[a,h]anthracene, eluting at 30.6 min, (e) benzo[b]chrysene, eluting at 43.9 min, and (f) picene, eluting at 50.6 min in Figure 4. Additional absorbance features between 210 and 270 nm in the product component spectrum in (f) are due to unidentified co-eluting compound(s) of molecular weight 292, as determined by mass spectrometry. 4525


Energy & Fuels pyrolysis, but all 14 of its alkylated derivatives are reported here for the first time. C22H14 PAH and Their Alkylated Derivatives. Figure 10 presents the UV spectra of the n-decane pyrolysis products eluting at 20.2, 23.6, 28.1, 30.6, 43.9, and 50.6 min in Figure 4, along with the UV spectra of the appropriate reference standards: dibenz[a,c]anthracene, in Figure 10a; dibenz[a,j]anthracene, in Figure 10b; pentaphene in Figure 10c; dibenz[a,h]anthracene in Figure 10d; benzo[b]chrysene in Figure 10e; and picene in Figure 10f. The close matching of the UV spectra of each product/standard pair in Figure 10 establishes the identity of each of the six n-decane pyrolysis products as the respective C22H14 PAH shown. The mass spectra of the six products confirm that each has a molecular weight of 278, consistent with the identifications from their UV spectra. As indicated by the red structures of Figure 4, UV and mass spectral evidence also confirms among the products the presence of seven singlymethylated derivatives of these five-ring PAH: four singlymethylated dibenz[a,c]anthracenes, one singly-methylated dibenz[a,j]anthracene, one singly-methylated pentaphene, and one singly-methylated dibenz[a,h]anthracene. None of these C22H14 PAH or their methylated derivatives have ever before been identified as products of n-decane pyrolysis. It should be noted that in addition to the six products of Figure 10, the C22H14 PAH isomer family contains six other members, all of which have available UV spectra. Since none of these UV spectra match any of the product spectra in Figure 10 and since the chromatogram of Fraction 9 (Figure 4) contains only six peaks with mass spectra corresponding to C22H14 PAH we can definitively say that these other C22H14 PAH are either not produced in the supercritical n-decane pyrolysis environment or are produced in amounts significantly lower than the C22H14 products identified in Figure 10. There are certain structural peculiarities of these “missing” six isomers that distinguish them, as a class, from the six C22H14 PAH that have been identified in the supercritical n-decane pyrolysis products. The details of these structural distinctions, along with the mechanistic implications of this observation, will be discussed in a future paper on reaction mechanisms.

’ SUMMARY AND CONCLUSIONS In order to better understand the formation of solids in the pre-combustion environment of future high-speed aircraft, we have performed supercritical pyrolysis experiments with the model fuel n-decane in a flow reactor at temperatures between 530 and 570 °C and pressures between 40 and 100 atm. The PAH products of these experiments likely precursors to solid deposits have been separated by a two-dimensional HPLC technique and identified by UV/MS. Product analysis reveals 24 unsubstituted PAH with molecular weights between 252 and 300, and another 83 of their alkylated derivatives. Of these 24 unsubstituted PAH, 12 are benzenoid, two are methylene-bridged derivatives of two of the benzenoid PAH, five are fluoranthene benzologues, and five are fluorene benzologues. Ten of the 24 unsubstituted PAH have been identified before as products of n-decane: benzo[a]pyrene, benzo[e]pyrene, perylene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, benzo[ghi]perylene, anthanthrene, picene, and coronene. The remaining fourteen are identified here for the first time as products of n-decane pyrolysis or combustion: benzo[a]fluoranthene, 11H-indeno[2,1,7-cde]pyrene, 4H-benzo-

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[def]cyclopenta[mno]chrysene, naphtho[1,2-a]fluorene, naphtho[1,2-b]fluorene, naphtho[2,1-a]fluorene, dibenzo[a,h]fluorene, dibenzo[a,i]fluorene, indeno[1,2,3-cd]pyrene, benzo[b]chrysene, dibenz[a,c]anthracene, dibenz[a,h]anthracene, dibenz[a,j]anthracene, and pentaphene. In addition, none of the 83 identified alkylated derivatives of these products have ever been previously identified as products of n-decane pyrolysis or combustion. Furthermore, none of the 107 PAH identified here have ever before been reported as products of supercritical pyrolysis of any pure alkane reactant. Isomer-specific identification of the PAH precursors to solid deposits is critical to determining the reaction mechanisms that govern aromatic compound formation, growth to higher-ringnumber species, and eventual precipitation from fuel pyrolyzed at supercritical conditions. The investigation of these reaction mechanisms is a primary focus of our ongoing work, and the results from these investigations will be reported in future papers.

’ ASSOCIATED CONTENT

bS

Supporting Information. Table of the name, structure, chemical formula, and number of observed methylated derivatives of each of the two- to five-ring PAH identified13 as products of supercritical n-decane pyrolysis. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Address: Louisiana State University, Department of Chemical Engineering, South Stadium Drive, Baton Rouge, LA 70803. Phone: (225) 578-7509. Fax: (225) 578-1476. E-mail: mjwornat@ lsu.edu.

’ ACKNOWLEDGMENT The authors gratefully acknowledge the United States Air Force Office of Scientific Research for support of this research: Grants FA9550-05-1-0253, FA9550-07-1-0033, FA9550-08-1-0281, and FA9550-10-1-0056. ’ REFERENCES (1) Edwards, T. Combust. Sci. Technol. 2006, 178, 307–334. (2) Yu, J.; Eser, S. Ind. Eng. Chem. Res. 1995, 34, 404–409. (3) Lander, H.; Nixon, A. C. J. Aircraft 1971, 8, 200–207. (4) Stewart, J. F. Supercritical Pyrolysis of the Endothermic Fuels Methylcyclohexane, Decalin, and Tetralin. Ph.D. Thesis, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, 1999. (5) Ledesma, E. B.; Wornat, M. J.; Felton, P. G.; Sivo, J. A. Proc. Combust. Inst. 2005, 30, 1371–1379. (6) McClaine, J. W.; Wornat, M. J. J. Phys. Chem. C 2007, 111, 86–95. (7) Somers, M. L.; Wornat, M. J. Polycyclic Aromat. Compd. 2007, 27, 261–280. (8) Thomas, S.; Wornat, M. J. Proc. Combust. Inst. 2009, 32, 516–522. (9) Wise, S. A.; Chesler, S. N.; Hertz, H. S.; Hilpert, L. R.; May, W. E. Anal. Chem. 1977, 49, 2306–2310. (10) Niles, R.; Tan, Y. L. Anal. Chim. Acta 1989, 221, 53–62. (11) Bouloubassi, I.; Saliot, A. Fresenius. J. Anal. Chem. 1991, 339, 765–771. (12) Saravanabhavan, G.; Helferty, A.; Hodson, P. V.; Brown, R. S. J. Chromatogr., A 2007, 1156, 124–133. 4526


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(13) Bagley, S. P. Supercritical Pyrolysis of n-Decane. Ph.D. Thesis, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA, 2010. (14) Davis, G. D. An Experimental Study of Supercritical Methylcyclohexane Pyrolysis. M.S.E. Thesis, Princeton University, Princeton, NJ, 1994. (15) Stewart, J. F.; Brezinsky, K.; Glassman, I. Combust. Sci. Technol. 1998, 136, 373–390. (16) Somers, M. L.; McClaine, J. W.; Wornat, M. J. Proc. Combust. Inst. 2007, 31, 501–509. (17) Darrah, S. Jet Fuel Deoxygenation, AFWAL-TR-88-2081, 1988. (18) Altin, O.; Eser, S. Ind. Eng. Chem. Res. 2001, 40, 596–603. (19) Jones, R. N. J. Am. Chem. Soc. 1945, 67, 2127–2150. (20) Fetzer, J. C. Large (C g 24) Polycyclic Aromatic Hydrocarbons: Chemistry and Analysis; Wiley-Interscience: New York, 2000. (21) Thomas, S.; Wornat, M. J. Polycycl. Aromat. Compd. to be submitted for publication. (22) Badger, G. M.; Novotny, J. Aust. J. Chem. 1963, 16, 613–622. (23) Clar, E. Polycyclic Hydrocarbons; Academic Press: New York, 1964; Vol. 2. (24) Friedel, R. A. Ultraviolet Spectra of Aromatic Compounds; John Wiley and Sons, Inc.: New York, 1951. (25) Bergmann, E. D.; Fischer, E.; Hirshberg, Y.; Lavie, D.; Sprinzak, Y.; Szmuszkovicz, J. Mem. Presentes Soc. Chim. 1953, 798–809. (26) Mayneord, W. V.; Roe, E. M. F. Philos. R. Soc. London, Ser. A: Math., Phys. Eng. Sci. 1937, 158, 634–650. (27) Mayneord, W. V.; Roe, E. M. F. Philos. R. Soc. London, Ser. A: Math., Phys. Eng. Sci. 1935, 152, 299–324. (28) Zander, M.; Franke, W. H. Chem. Ber. 1965, 4033–4035.

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