2584
Energy & Fuels 2007, 21, 2584-2593
Model Compound Study of the Pathways for Aromatic Hydrocarbon Formation in Soot Randall E. Winans,*,†,‡ Nancy A. Tomczyk,† Jerry E. Hunt,† Mark S. Solum,§ Ronald J. Pugmire,§,| Yi Jin Jiang,| and Thomas H. Fletcher| Chemistry and X-ray Science DiVisions, Argonne National Laboratory, Argonne, Illinois 60439, Department of Chemistry and Department of Chemical Engineering, UniVersity of Utah, Salt Lake City, Utah 84112, and Department of Chemical Engineering, Brigham Young UniVersity, ProVo, Utah 84602 ReceiVed March 28, 2007. ReVised Manuscript ReceiVed July 3, 2007
As a follow-up of previous work on the flame pyrolysis of biphenyl and pyrene, a more detailed analysis of the pyrolytic products has been done using additional NMR data obtained on the whole soot sample correlated with detailed high-resolution and GC mass spectrometry data on the solvent-extracted portion of the same samples. These latter data complement the earlier NMR data with details of the pre-sooting structures, referred to as “young soot”, in pyrolyzed biphenyl samples collected at 1365, 1410, and 1470 K and pyrene at 1410 and 1470 K. The data reveal the roles played by free-radical-assisted polymerization reactions as well as the hydrogen-abstraction carbon-addition (HACA) reactions for the biphenyl pyrolysis. The mass spectroscopy data of pyrene describe a much different set of reactions due to polymerization which employs free-radical reactions of the pyrene due primarily to hydrogen abstraction followed by the formation of biaryl linkages at mass numbers up to five times that of the parent pyrene. Conceptual schema of reaction mechanisms are proposed to explain the formation pathways to materials detected in the soot extracts.
Introduction The formation of soot precursors and soot is the most complex chemical system in flames. Soot particles, containing several thousands of carbon atoms, are formed from simple fuel molecules “on timescales spanning pico- or nanoseconds for intramolecular processes to milliseconds for the formation of the first soot precursors”.1 The large number of molecules and particles, of different shape and size, involved by this process cannot be quantified. A representation of the soot formation process can be seen in Figure 1. Still, there are currently gaps in knowledge in the fields of aromatization, growth of polyaromatic hydrocarbons (PAHs), surface growth, and oxidation. Two types of species associated with the combustion of any organic fuel are grouped together as PAHs (the “young soots”, which are extractable) and the insoluble material, soot. PAHs are of particular concern since many are known to be mutagenic or carcinogenic. In 1991, Harvey’s first book on polycyclic aromatic hydrocarbons was published as part of the Cambridge Monographs on Cancer Research2 which were weighted heavily on the polyarenes because of their widespread environmental prevalence and the high carcinogenic potency of some members of this class. In a 1996 monograph published by NATO, Allmandola of the NASA Ames Research Center contributed a * Corresponding author. Fax: (630) 252-9288. E-mail:
[email protected]. † Chemistry Division, Argonne National Laboratory. ‡ X-ray Science Division, Argonne National Laboratory. § Department of Chemistry and Department of Chemical Engineering, University of Utah. | Department of Chemical Engineering, Brigham Young University. (1) Violi, A.; Voth, G. A.; Sarofim, A. F. Proc. Combust. Inst. 2005, 30 (1), 1343-1351. (2) Harvey, R. G. Polycyclic Aromatic Hydrocarbons: Chemistry and Carcinogenicity; Cambridge University Press: New York, 1991.
paper titled “PAHs, They’re Everywhere!”3 The author points out that PAHs are widespread throughout the interstellar medium. The author reviews the spectral evidence including a survey of studies on Solar System objects and extraterrestrial materials that have recently been shown to contain PAHs (Halley’s comet, meteorites, and interplanetary dust particles). In combustion systems, PAH formation plays a key role not only in environmental- and health-related effects but also in heat transfer processes. The latter are particularly important with regard to the optical constants of the gas-phase PAHs that not only control the radiative heat transfer but also produce spectral signatures that are integral to certain types of weapons systems. As seen in Figure 1, it is generally agreed that soot occurs as follows: (a) a complex process encompassing formation of radical precursors such as polycyclic aromatic hydrocarbons through radical reactions of small molecules and/or fragments; (b) molecular growth; (c) nucleation; (d) coagulation; and (e) oxidation.5 Numerous mechanisms have been proposed for the formation of precursors and growth of PAHs.6-8 For gaseous hydrocarbon fuels, reactions involving the addition of C2-C4 species9-11 have been shown to play a key role in PAH and (3) Allamandola, L. J. NATO ASI Ser., Ser. C 1996, 487 (Cosmic Dust Connection), 81-102. (4) Sarofim, A. F.; Longwell, J. P.; Wornat, M. J.; Mukherjee, J. Springer Ser. Chem. Phys. 1994, 59 (Soot Formation in Combustion), 485-499. (5) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26 (46), 565-608. (6) Bockhorn, H.; Fetting, F.; Wenz, H. W. Ber. Bunsen-Gesellschaft 1983, 87 (11), 1067-1073. (7) D’Anna, A.; Violi, A. Symp. (Int.) Combust., [Proc.] 1998, 27th (Vol. 1), 425-433. (8) Frenklach, M.; Clary, D. W.; Gardiner, W. C., Jr.; Stein, S. E. Symp. (Int.) Combust., [Proc.] 1985, 20th, 887-901. (9) Bittner, J. D.; Howard, J. B. Part. Carbon: Form. Combust. [Proc. Int. Symp.] 1981, 109-142. (10) Wang, H.; Frenklach, M. Combust. Flame 1997, 110 (1/2), 173221.
10.1021/ef070161p CCC: $37.00 © 2007 American Chemical Society Published on Web 08/21/2007
Models for Aromatic Hydrocarbon Formation in Soot
Energy & Fuels, Vol. 21, No. 5, 2007 2585
Figure 1. Schematic representation of the general types of reactants and products thought to be present in a combustion environment (modified from a scheme by A. F. Sarofim4).
soot formation reactions in a variety of combustion and pyrolysis systems. Other mechanisms might be expected to prevail, however, for certain liquid and solid fuels since aromatic units are inherent components of the fuels themselves. To explore the mechanisms for formation of aromatic hydrocarbons as precursors to soot, a model system using combustion of biphenyl (and to a lesser extent pyrene) in a fuelrich flame is studied. These two model structures were chosen because of the convenience of following the formation of bridging carbons (bridgehead and substituted) in their respective NMR spectra. The biphenyl and pyrene soots were acquired at three and two different temperatures, respectively. All samples were solvent extracted, and the extract was characterized by both GCMS and high-resolution mass spectrometry (HRMS). A description of the NMR results for these un-extracted soot samples has been published.12 The formation of most of the biphenyl products can be rationalized from the coupling of benzene and biphenyl radicals and subsequent aromatic species together with the addition of acetylenes to existing aromatic molecules. Early work by Badger on pyrolysis of hydrocarbons is used in developing these schemes.13 The data obtained from the two pyrene soots were not as rich in information as those obtained from the biphenyl samples. The reaction schemes that produce larger aromatic hydrocarbons will be discussed. These schemes are consistent with the work of Richter and Howard,5 who have discussed in detail potential reaction mechanisms in the formation of aromatics as precursors to soot. The product distributions from the aromatic fuels reveal that there are at least two types of reaction mechanisms at works (11) Pope, C. J.; Miller, J. A. Proc. Combust. Inst. 2000, 28, 15191527. (12) Solum, M. S.; Sarofim, A. F.; Pugmire, R. J.; Fletcher, T. H.; Zhang, H. Energy Fuels 2001, 15 (4), 961-971. (13) Badger, G. M. Progr. Phys. Org. Chem. 1965, 3, 1-40.
Table 1. Yields of Extracts from Soot Samples sample
temperature (K)
% extracted
pyrene
1410 1460
73.3 55
biphenyl
1365 1410 1470
15 13.4 not determined
the first involves polymerization of aryl (radical) species and cyclodehydrogenation; the second involves ring fragmentation and combination of fragment species. Experimental Soot samples from biphenyl were produced in a flat-flame burner, as described previously,12 at three temperatures (1365, 1410, and 1470 K), while the less reactive pyrene was collected at the two higher temperatures. The samples were extracted at room temperature with methylene chloride using sonication for 15 min followed by centrifugation with yields shown in Table 1. Desorption electron impact high-resolution mass spectra (DEIHRMS) of these extracts were taken on a 3-sector MS-50.14 Samples were heated on a probe from 200 to 700 °C at 200 °C/min directly in the source. Precise mass measurements were averaged from scans over the entire temperature range. Formulas were assigned, and the data were sorted via a procedure developed in-house.14 High-resolution mass spectrometry data are sorted by both heteroatom content and by hydrogen deficiency, which is also termed double-bond equivalents (dbe). From hydrogen deficiency, the size of aromatic clusters can be estimated. GCMS data were obtained using a Hewlett-Packard 6890 gas chromatograph with a 5973 quadrupole mass selective (14) Winans, R. E. AdV. Coal Spectrosc. 1992, 255-274. (15) Masonjones, M. C.; Lafleur, A. L.; Sarofim, A. F. Combust. Sci. Technol. 1995, 109 (1-6), 273-285. (16) Masonjones, M. C.; Mukherjee, J.; Sarofim, A. F.; Taghizadeh, K.; Lafleur, A. L. Polycyclic Aromat. Compd. 1996, 8 (4), 229-242.
2586 Energy & Fuels, Vol. 21, No. 5, 2007
Winans et al.
detector. The injection system is a CDS pyrolysis injector. The solvent-extracted samples were placed in a quartz tube, and the solvent was allowed to evaporate prior to analysis. The samples were pyrolyzed at 600 °C while a stream of helium was passed over the sample and onto the column. The column used is a 60 meter J&W DB-17HT with a 0.25 mm i.d. and a film thickness of 0.25 µm. The oven was held at 40 °C for 1 min, then ramped at 6 °C/min to 280 °C and held there for 10 min. The quadrupole was held at 150 °C, while the source was at 230 °C. The LD-TOF mass spectrometer used in these studies is a modified version of a commercial MALDI instrument (Kratos MALDI III). A 337 nm 10 µs pulse duration nitrogen laser focused in the range of 1-4 µm diamter achieves sub-megawatt power densities. The repetition rate of the laser was 10 Hz. The desorbed ions were extracted at 18 kV. Mass separation of the ions was achieved using a 1.2 meter flight path fitted with a reflectron. The ions were detected by an electron multiplier. The dichloromethane extracts were evaporated onto the stainless steel probe to form thin films. Most of the samples were soluble under these conditions. LD spectra, each accumulated over 100 shots, were recorded for each sample. Standard samples were recorded under similar conditions to verify the efficiencies and ionization yields.
Results The discussion is limited to the structure and formation of only the most abundant aromatic hydrocarbons (relative abundance >10). All of these species can be rationalized from the pyrolysis of biphenyl.5,12,15,16Growth to larger molecules occurs by dimerization of aromatics and by acetylene addition. Both of these growth mechanisms have previously been suggested for soot formation.6-8 Analyses of these data suggest that both pathways are occurring along with a small amount of methylation and insertion of methylenes. The structures are deduced from the precursors and from molecular formulas. In some cases, especially in C2 addition, multiple isomers probably exist and a representation is shown. Only in the lowest-temperature sample is biphenylene observed which can be formed from biphenyl by the loss of two hydrogens. The preparation of biphenylene in a high-temperature plasma has previously been reported.17-19 The NMR data, which describe the unextracted sample, increase in average size of the aromatic clusters and in the number of cross-links per cluster, were observed with increasing temperature. These data are presented in Figure 2 and Table 2. The ring size distribution for the three temperatures, derived from the mass spectroscopy data, is shown in Figure 3. The products from the highest pyrolysis temperature exhibit the largest number of rings but at the two lower temperatures they were similar in average size, an observation that is consistent with the NMR results. However, at 1410 K there was a greater abundance of 4-, 5-, 6-, and 7-ring structures compared to either the cooler and hotter sample and the higher ring species (>9) are absent. It should be noted that the HRMS data contained information only on the extracts (which represented ∼15% of the entire sample), while the NMR data represented the unextracted soot samples. It is useful to note that the estimated average ring size in an anthracene 1400 K aerosol extract (9.5% w/w) was ∼15 aromatic carbons per cluster while that of the residue was very large (estimated to be g100 carbons per cluster).20 (17) Suhr, H.; Weiss, R. I. Angew. Chem., Int. Ed. Engl. 1970, 9 (4), 312. (18) Suhr, H. Angew. Chem., Int. Ed. Engl. 1972, 11 (9), 781-792. (19) Munoz, R. H.; Charalampopoulos, T. T. Symp. (Int.) Combust., [Proc.] 1998, 27th (Vol. 1), 1471-1479. (20) Solum, M. S.; Veranth, J. M.; Jiang, Y.-J.; Orendt, A. M.; Sarofim, A. F.; Pugmire, R. J. Energy Fuels 2003, 17 (3), 738-743.
Figure 2. Examples of dipolar dephased 13C CP/MAS NMR spectra of aerosol samples obtained at the indicated temperatures. The dephasing time was selected so as to eliminate signals from protonated carbons. The dotted lines highlight the key spectral areas for substituted, and two different types of bridgehead carbons which can be used to describe the general types of aromatic structures present.
Figure 3. Possible non-alternate free radicals that could be formed as intermediate structures from pyrolysis of biphenyl. Table 2. Average Structural Information Derived from Data on Un-extracted Aerosol Samples
13C
NMR
pyrolysis temperture (K)
average aromatic cluster size (C per cluster)
average cross-links per cluster
biphenyl
300 1365 1410 1470
6 9 11 20
1 2.2 2.5 4.7
pyrene
300 1410 1470
16 18 23
0 2.6 3.7
molecule
NMR Results. Dipolar dephased spectra (42 µs dephasing time) of the three biphenyl (1365, 1410, and 1470 K) and two pyrene (1410 and 1460 K) un-extracted aerosols are shown in
Models for Aromatic Hydrocarbon Formation in Soot
Energy & Fuels, Vol. 21, No. 5, 2007 2587
Figure 4. Distribution of ring sizes determined from HRMS analysis. Figure 5. Distribution of o-, m-, and p-terphenyl structures from GCMS data. Table 3. Unpaired Electron Spin Concentrations of the Biphenyl and Pyrene Aerosols sample
Ne 1019 spins/g
biphenyl 1365 K biphenyl 1410 K biphenyl 1470 K
0.06 0.2 4.7
pyrene 1410 K pyrene 1460 K
2.0 4.3
Figure 2 and can be used to define the approximate size of the aromatic ring structures and the number of cross-links per cluster (see Table 2). These NMR spectra represent only carbons from nonprotonated aromatic systems. There are three major regions of interest where key structural features can be found. The peaks or shoulders highlighted by the three vertical dotted lines define the general chemical shift regions for alkyl/aryl substituted aromatic carbons (∼139 ppm), the cata-condensed or outer bridgehead aromatic carbons (∼130-131 ppm) and the pericondensed, or inner-bridgehead carbons (∼124 ppm). All five spectra exhibit a distinctive peak in the region of the catacondensed outer bridgehead aromatic carbons. Since these types of carbons are not present in the original biphenyl sample, they must have been created during pyrolysis. The shoulder at 139140 ppm is persistent in all of the samples studied and represents the presence of aromatic carbons in biaryl linkages or with aliphatic substituents. This structural feature is original only to the biphenyl molecule and is present in each of the samples studied. The appearance of relatively weak shoulders in the pyrene samples indicates that at least modest amounts of biaryl linkeages are present in these samples, as the mass spectroscopy data confirm (vida infra). Peaks at this chemical shift are most uniquely prominent in the two lower-temperature biphenyl samples and identify the presence of polyphenyl structures (e.g., m- and p-terphenyl, quaterphenyl, etc., vida infra) The presence of polybiphenyl structures as large as the pentamers and hexamers have been reported by Sarofim et al.16 This peak is still present in the much broader 1470 K biphenyl sample with only a small shoulder evident in each of the pyrene spectra. The third area of interest (124 ppm), arising from the presence of inner or peri-condensed bridgehead carbons, is originally present only in the pyrene starting material. The intensity of this region in the two lower-temperature biphenyl spectra is very small. While the GCMS results indicate that pyrene is the most abundant product observed in the extractable material (representing 13.4% of the sample) from the 1410 K sample, the NMR data suggest that the relative amount of peri-condensed structures in the entire soot sample is less abundant than that suggested from the extracted material. However, the NMR data in Figure 2 suggest that the peri-condensed PAH structure is much higher in the 1470 K sample. These data on the un-extracted sample
indicate that the 1470 K biphenyl has undergone a major structural rearrangement to form larger aromatic ring systems with inner bridgeheads. This is in contrast to the two lowertemperature samples where polymerization of phenyl and biphenyl units is more dominant when one considers the unextracted sample. When considering the whole sample, the formation of the more condensed aromatic units does not appear to be as dominant as the polymerized structures in the 13651410 K temperature region. ESR Results. The unpaired electron spin concentrations have been measured for the three biphenyl and two pyrene aerosols (un-extracted), and the results are given in Table 3. The unpaired spin concentrations are consistent with data obtained on other aerosol samples examined. Unpaired electrons in these types of carbonaceous systems are thought to be of the odd-alternate type.21 Several of the mass peaks from the HRMS experiment could be of this type. Examples of the possible types of structures derived from the biphenyl samples are shown in Figure 3. Mass Spectrometry Results. Biphenyl. The distribution of ring sizes from the HRMS data for biphenyl aerosols is presented in Figure 4. The detail results for the three temperatures are given in Table 4. The structures in this table represent identified molecules where there are GCMS data and possible structures for HRMS. The details for the formation of the significant ones are discussed in the following text. At each of the three temperatures, the distribution in ring size passes through a maximum at 5-6. Note the small amount of 1-3 ring structures (
