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Energy & Fuels 1999, 13, 1092-1096
Detection of Benz[f]indene among the Pyrolysis Products of Coal and Anthracene Mary J. Wornat,* Celina J. Mikolajczak, Brian A. Vernaglia, and Meredith A. Kalish Princeton University, Department of Mechanical & Aerospace Engineering, Princeton, New Jersey 08544 Received April 14, 1999. Revised Manuscript Received June 8, 1999
Using high-pressure liquid chromatography with diode-array ultraviolet-visible detection, we have identified benz[f]indene (C13H10) among the pyrolysis products of coal and the model compound anthracene, a three-ring compound representative of the aromatic moieties in coal. The coal pyrolysis experiments are conducted at 700-1000 °C in a fluidized bed reactor; the anthracene experiments, at 900-1000 °C in a laminar flow reactor. This is the first time that benz[f]indene has been identified as a product of either fuel, coal or anthracene. Oxidative ring rupture is proposed as the mechanism responsible for the formation of benz[f]indene and other indene benzologues in these products.
Introduction The identification of the products evolved during the pyrolysis of coal is crucial in determining the reaction pathways responsible for polycyclic aromatic hydrocarbons (PAH) and other organic emissions.1,2 The complexity of the mixture of hydrocarbons produced during coal pyrolysis, however, presents substantial difficulties in identifying individual species and even classes of species.3 One class of PAH derived from coal is composed of indene (C9H8) and its benzologues, all of which contain one partially unsaturated five-membered ring fused to one or more fully aromatic six-membered rings. One indene benzologue, fluorene, has long been known to be produced during coal pyrolysis,4 and the chemical structure of fluorene (benz[b]indene) suggests that at least three additional C13H10 isomers may existsbenz[e]indene, benz[f]indene, and benz[g]indene. These three benzindene isomers, however, have not previously been identified as products of coal. (The C13H10 isomer phenalene is not considered here since it is not an indene benzologue.) In this paper, we present evidence of the identification of the C13H10 isomer benz[f]indene in the pyrolysis products of both coal and the model compound anthracene, a three-ring PAH representative of the aromatic moieties in coal. In both cases, the identification * Author to whom correspondence should be addressed at Princeton University, Mechanical & Aerospace Engineering Department, Engineering Quadrangle, Room D329, Princeton, NJ 08544-5263. Telephone: (609) 258-5278. (1) Nelson, P. F.; Tyler, R. J. Symp. (Int.) Combust., [Proc.] 21st 1986, 427-435. (2) Wornat, M. J.; Vernaglia, B. A.; Lafleur, A. L.; Plummer, E. F.; Taghizadeh, K.; Nelson, P. F.; Li, C.-Z.; Necula, A.; Scott, L. T. Symp. (Int.) Combust., [Proc.] 27th 1998, 1677-1686. (3) Vernaglia, B. A.; Wornat, M. J.; Li, C.-Z.; Nelson, P. F. Symp. (Int.) Combust., [Proc.] 26th 1996, 3287-3294. (4) Clar, E. J. Polycyclic Hydrocarbons; Academic Press: New York, 1964.
of benz[f]indene follows from compositional analysis of the pyrolysis products by high-pressure liquid chromatography (HPLC) coupled with ultraviolet-visible (UV) diode-array detection, a technique which is optimally suited for the analysis of complex mixtures of aromatic components. In this paper we also propose a mechanism for benz[f]indene formation in a pyrolytic reaction environment. Experimental Materials and Procedures The coal of this study is Yallourn coal, a low-ash (1.1%) Australian brown coal of the Latrobe Valley, with elemental composition 67.4% C, 4.6% H, 0.6% N, 0.3% S, and 27.1% O (by difference) on a dry, ash-free (daf) basis.5 It yields 53% proximate volatile matter in the Australian standard test. To allow for future comparisons with ion-exchanged coals, this study is conducted with acid-washed coal, which is prepared by contact of the raw (as mined) Yallourn coal with a 1.3 N HCl solution over mild heat.6 The vacuum-dried coal (of particle size 75-106 µm) is pyrolyzed in nitrogen at temperatures of 700-1000 °C. The fluidized bed apparatus, similar to that used by Tyler,7 is described elsewhere.3 The reactor effluent passes through a liquid nitrogen cooled glass trap, fitted with a Soxhlet thimble, for collection of the char and condensed-phase products. The condensed-phase products, which include the PAH, are recovered by Soxhlet extraction with dichloromethane.3 In the model compound experiments, anthracene is vaporized in a stream of nitrogen and pyrolyzed at 9001000 °C (for residence times on the order of 1 s) in a high-temperature laminar flow reactor described elsewhere.8,9 The PAH reaction products are quenched, (5) Schafer, H. N. S.; Wornat, M. J. Fuel 1990, 69, 1456-1458. (6) Schafer, H. N. S. Fuel 1970, 49, 197-213. (7) Tyler, R. J. Fuel 1979, 58, 680-686.
10.1021/ef990065r CCC: $18.00 © 1999 American Chemical Society Published on Web 07/22/1999
Benz[f]indene in Pyrolysis Products of Coal and Anthracene
collected on a Balston filter, and dissolved into dichloromethane with ultrasonic agitation. The product solution is then concentrated in a Kuderna-Danish apparatus. The coal and anthracene PAH product solutions are exchanged into dimethyl sulfoxide and analyzed by HPLC using a Hewlett-Packard Model 1050 chromatograph with a diode-array UV detector. The chromatographic separation method utilizes a reverse-phase Vydac 201-TP octadecylsilica column with time-programmed mobile phases of water/acetonitrile, acetonitrile, and dichloromethane (1.5 mL/min). This method resolves individual PAH species,10,11 which are then unequivocally identified by comparison of their unique UV absorbance spectra with those of reference standards. Our reference standard of benz[f]indene, not commercially available, comes from Professor Mark McLaughlin, whose paper12 documents the synthesis procedure of benz[f]indene as well as the authentication of its structure by nuclear magnetic resonance spectroscopy and melting point. The spectral data from this reference standard also match those published elsewhere13 for an independently synthesized reference standard of benz[f]indene.
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Figure 1. UV absorbance spectra of the reference standard of benz[f]indene and of a Yallourn brown coal pyrolysis product component with the same HPLC elution time.
Results and Discussion Figure 1 displays the UV absorbance spectra of the reference standard of benz[f]indene and a component of the coal pyrolysis products that has the same HPLC elution time. The strong match of these two spectra and the equivalent elution times confirm the identity of this coal pyrolysis product as benz[f]indene. Figure 2 portrays the analogous spectral match between the benz[f]indene standard and one of the pyrolysis products of anthracene. This is the first time that benz[f]indene has been identified as a product from either coal or anthracene pyrolysis. Figures 3 and 4 present portions of the HPLC chromatograms of the products of coal and anthracene, respectively. Eluting at approximately 15 min in both chromatograms is the newly identified benz[f]indene, the C13H10 indene benzologue whose structure is the third boxed compound in Figures 3 and 4. The other three boxed structures in each chromatogram correspond to indene (C9H8) and the other two of its benzologues identified in these samples, fluorene (C13H10) and benzo[a]fluorene (C17H12). The remaining labeled compounds in Figures 3 and 4 belong to different classes of PAH and are discussed in more detail elsewhere.2,9,14,15 (8) Mikolajczak, C. J.; Livesey, C. E.; Guran, S.; Wornat, M. J. A High-Temperature Laminar-Flow Reactor for the Pyrolysis of Polycyclic Aromatic Hydrocarbons. Poster presentation at the Twenty-Seventh International Symposium on Combustion, Boulder, Colorado, August, 1998. (9) Mikolajczak, C. J., Ph.D. Thesis, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, in preparation. (10) Wornat, M. J.; Sarofim, A. F.; Lafleur, A. L. Symp. (Int.) Combust., [Proc.] 24th 1992, 955-963. (11) Wornat, M. J.; Lafleur, A. L.; Sarofim, A. F. Polycyclic Aromatic Compounds 1993, 3, 149-161. (12) Becker, C. L.; McLaughlin, M. L. Synlett 1991, 1991, 642. (13) Photoelectric Spectroscopy Group, London, and Institut fu¨r Spectrochemie und Angewandte Spektroskopie, Dortmund. UV Atlas of Organic Compounds, Vol. II; Plener Press: New York, 1966. (14) Vernaglia, B. A., M. S. E. Thesis, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, 1997.
Figure 2. UV absorbance spectra of the reference standard of benz[f]indene and of an anthracene pyrolysis product component with the same HPLC elution time.
Evident from Figures 3 and 4 is the remarkable similarity in the identity of the PAH produced from both coal and anthracene, a model compound chosen to represent the aromatic moieties of coal. Of the twentyfour PAH products of coal shown in Figure 3, nineteen are present among the anthracene pyrolysis products in Figure 4 and three of the remaining five are produced from anthracene at higher pyrolysis temperatures.10,16 Conversely, of the twenty-one anthracene pyrolysis products shown in Figure 4, all but two (2-ethynylan(15) Vernaglia, B. A.; Wornat, M. J.; Li, C.-Z.; Nelson, P. F. In preparation. (16) Wornat, M. J.; Vriesendorp, F. J. J.; Lafleur, A. L.; Plummer, E. F.; Necula, A.; Scott, L. T. The Identification of New EthynylSubstituted and Cyclopenta-Fused Polycyclic Aromatic Hydrocarbons in the Products of Anthracene Pyrolysis. Polycyclic Aromatic Compounds, in press.
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Figure 3. Reverse-phase HPLC chromatogram of products of Yallourn brown coal pyrolyzed at 1000 °C in a fluidized bed. Boxed structures correspond to indene benzologues. Identified components (from left to right) are indene, 9-fluorenone, naphthalene, acenaphthylene, 2-ethynylnaphthalene, 1-methylnaphthalene, 2-methylnaphthalene, 1-ethynylacenaphthylene, fluorene, benz[f]indene, phenanthrene, anthracene, acephenanthrylene, aceanthrylene coeluting with fluoranthene, 1-methylanthracene, pyrene, benzo[c]phenanthrene, 2-methylanthracene, cyclopent[hi]acephenanthrylene, benzo[a]fluorene, cyclopenta[cd]fluoranthene, benzo[ghi]fluoranthene coeluting with cyclopenta[cd]pyrene.
thracene and 2-ethylanthracene) appear as coal products in Figure 3, and one of the exceptions (2-ethynylanthracene) appears among the pyrolysis products of another coal.17 Consistent with our previous findings,2,10,16 the similarity of the product species in Figures 3 and 4 clearly demonstrates the appropriateness of anthracene as a model compound for coal in these pyrolysis studies. The use of anthracene as a model compound grants us insight into the formation pathway of indene and its benzologues. A nonoxidative pyrolytic ring rupture mechanism, applied to anthracene and proceeding via β scission, will not lead to benz[f]indene. Even though our anthracene experiments are run under nominally nonoxidative conditions, however, the nitrogen carrier gas can contain up to 20 ppm O2,18 and this oxygen can participate in the pyrolysis reactions. By analogy with the mechanism developed19 for cyclopentadiene formation from benzene, we propose that the indene and its benzologues in our anthracene pyrolysis products are formed via oxidative ring rupture. For benz[f]indene, the sequence involves formation of the anthryl radical in either the 1 or 2 position of anthracene, attack of the anthryl radical by oxygen, elimination of CO, and (17) Ledesma, E. B.; Kalish, M. A.; Nelson, P. F.; Wornat, M. J.; Mackie, J. C. Observation of Cyclopenta-Fused and Ethynyl-Substituted PAH During the Partial Oxidation of Primary Tar from a Bituminous Coal. Submitted to Energy & Fuels. (18) Murphy, S. L., M. S. E. Thesis, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, 1982. (19) Brezinsky, K. F. Prog. Energy Combust. Sci. 1986, 12, 1-24.
Scheme 1. Oxidative Ring Rupture Mechanism Leading from Anthracene to Benz[f]indene
hydrogen abstraction. The sequence of steps from anthracene to benz[f]indene, via the 1-anthryl radical, is illustrated in Scheme 1. In the brown coal pyrolysis experiments, attack of the fuel by O2 is not necessary since oxygen is available in the coal itself. Twenty-seven percent of the coal mass is oxygen, of which 25% is phenolic.20-22 Abstraction of the phenolic hydrogen would yield the phenolic type
Benz[f]indene in Pyrolysis Products of Coal and Anthracene
Energy & Fuels, Vol. 13, No. 5, 1999 1095
Figure 4. Reverse-phase HPLC chromatogram of products of anthracene pyrolyzed at 920 °C in a high-temperature laminar flow reactor. Boxed structures correspond to indene benzologues. Identified components (from left to right) are indene, 9-fluorenone, naphthalene, acenaphthylene, 2-ethynylnaphthalene, 1-methylnaphthalene, 2-methylnaphthalene, fluorene, benz[f]indene, phenanthrene, unreacted anthracene, acephenanthrylene, aceanthrylene coeluting with fluoranthene, 1-methylanthracene, pyrene, 2-ethynylanthracene, 2-ethylanthracene, 2-methylanthracene, benzo[a]fluorene, and cyclopenta[cd]pyrene.
radical shown in Scheme 1 for O2 attack on the anthryl radical. Formation of indene and its benzologues from phenolic-type ring structures in the coal would then follow via CO elimination. Further support for this mechanism is found in the model compound pyrolyses of phenol23 and naphthol,24 which yield cyclopentadiene and indene, respectively, along with CO. Consistent with our proposed oxidative ring rupture mechanism are the following observations: (1) When pyrolyzed under the same conditions as the brown coal, a bituminous coal,25 containing only 8% oxygen by mass, produces only a trace amount of benz[f]indene, whereas the brown coal (27% oxygen) produces a significant amount, as illustrated in Figure 3. (2) In anthracene pyrolysis experiments9 conducted with 250 ppm O2, the relative proportions of indene benzologue products to other PAH are severalfold higher than in the experiments with O2 concentrations of e20 ppm. (3) Anthracene pyrolysis experiments conducted with argon (in which O2 is virtually absent) produce only a small trace of either fluorene or benz[f]indene.10 (20) Perry, G. J.; Allardice, D. J.; Kiss, L. T. In The Chemistry of Low-Rank Coals; Schobert, H. H., Ed.; ACS Symposium Series 264; American Chemical Society: Washington, DC, 1984; Chapter 1. (21) Charlesworth, J. M. Fuel 1980, 59, 859-864. (22) Schafer, H. N. S. Fuel 1980, 59, 302-304. (23) Cypres, R.; Bettens, B. Tetrahedron 1974, 30, 1253-1260. (24) Bredael, P.; Vinh. T. H.; Braekman-Danheux, C. Fuel 1983, 62, 1193-1198. (25) Ledesma, E. B.; Kalish, M. A.; Nelson, P. F.; Wornat, M. J.; Mackie, J. C. Formation and Fate of PAH During the Partial Oxidation of Coal Pyrolysis Tars. Submitted to Fuel.
Table 1. Yields of Indene Benzologues and Naphthalene from Brown Coal Pyrolysis (mass percent of daf coal as given product) Pyrolysis Temperature (°C) indene fluorene benz[f]indene benzo[a]fluorene naphthalene a
C9H8 C13H10 C13H10 C17H12 C10H8
700
800
900
1000
0.022 nda nda nda 0.041
0.135 0.153 0.079 0.031 0.271
0.130 0.220 0.064 0.054 0.389
0.062 0.203 0.027 0.032 0.673
nd, compound not detected.
Table 1 presents the yields of indene and its benzologues as products from the coal pyrolysis experiments at temperatures from 700 to 1000 °C. The yield of naphthalene, a benzenoid PAH, is included for comparison. As Table 1 demonstrates, the yields of all of the indene benzologues illustrate maxima at intermediate temperatures (800 or 900 °C), in contrast to naphthalene, whose yield increases steadily with pyrolysis temperature. Since, in the brown coal pyrolysis experiments, the oxygen is supplied by the fuel itself, the production of indene benzologues could be viewed to parallel the release of oxygen-containing structures from the coal, as chromatographic1,3 and spectroscopic (infrared26-28 and nuclear magnetic resonance29) results have indicated. (26) Wornat, M. J.; Nelson, P. F. Energy Fuels 1992, 6, 136-142. (27) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1987, 1, 431-437. (28) Wornat, M. J.; Nelson, P. F. Symp. (Int.) Combust., [Proc.] 23rd 1990, 1239-1245.
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The yields in Table 1 and the chromatograms in Figures 3 and 4 demonstrate that of the two C13H10 isomers, fluorene is produced much more abundantly than benz[f]indene, in both the coal and anthracene pyrolysis experiments. This predominance may reflect the preference for anthryl radical formation in the 9 position (which would yield fluorene) as opposed to the 1 or 2 position (which would yield benz[f]indene). Herndon30 and Pullman and Pullman31 have shown that indeed the 9 position is the preferred position for radical formation from anthracene. The dominance of fluorene may also reflect a greater thermal stability of this C13H10 isomer, compared to benz[f]indene. The presence of two C13H10 isomers, fluorene and benz[f]indene, among our pyrolysis products suggests the possible presence of the two other isomers, benz[e]indene and benz[g]indene, as well. To our knowledge, no reference standard or UV absorbance spectrum of benz[g]indene exists, so we do not know whether this compound is present among our coal or anthracene pyrolysis products. In theory, benz[g]indene could be formed by oxygen attack on a 3- or 4-phenanthryl radical, but steric hindrance might preclude attachment of the oxygen in those positions. No such steric hindrance would be posed by oxygen attack on a 1- or 2-phenanthryl radical, however, to produce benz[e]indene. A UV absorbance spectrum of benz[e]indene has been published,32 but none of our pyrolysis products have UV spectra consistent with that one. Therefore, benz[e]indene’s apparent absence in the anthracene pyrolysis experiments may simply result from the fact that phenanthrene is present in much lower concentrations than anthracene. In the coal pyrolysis experiments, anthracene and phenanthrene are produced in comparable amounts, as Figure 3 reveals, so benz[e]indene’s apparent absence may be due to other reasons, e.g., its possible coelution with a major product species. (29) Collin, P. J.; Tyler, R. J.; Wilson, M. A. Fuel 1980, 59, 819820. (30) Herndon, W. C. Tetrahedron 1982, 38, 1389-1396. (31) Pullman, B.; Pullman, A. In Progress in Organic Chemistry; Cook, J. W., Ed.; Vol 4, Butterworth: London, 1958; Chapter 2. (32) Kotlyarevskii, I. L.; Zanina, A. S. Zh. Obshch. Khim. 1961, 31, 3206-3214.
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In any case neither benz[e]indene nor benz[g]indene could be a major product in our coal or anthracene pyrolysis experiments because there are no large unidentified peaks with appropriate elution times, in Figures 3 and 4. Summary and Conclusions Using HPLC with diode-array UV detection, we have analyzed the pyrolysis products of both brown coal and anthracene, a three-ring model compound representative of the aromatic moieties of coal. The remarkable similarity of the product species from the two fuels suggests that anthracene is indeed an excellent model compound for coal, in the context of PAH formation chemistry. Among our pyrolysis products is benz[f]indene (C13H10), identified here for the first time as a product of either coal or anthracene. In the anthracene pyrolysis experiments, benz[f]indene appears to be formed via oxygen attack on the 1- or 2- anthryl radical, followed by CO elimination and H abstraction. The more abundant C13H10 isomer fluorene would be formed analogously by oxygen attack on the more preferred 9-anthryl radical. In the brown coal pyrolysis experiments, benz[f]indene and the other indene benzologues can form by a similar CO elimination mechanism, but from phenolictype structures already present in the oxygen-rich, lowrank coal. These results illustrate the usefulness of model compounds in providing insight into the reaction mechanisms involved with complex fuels such as coal. Acknowledgment. The authors gratefuly acknowlege the National Science Foundation for support of this research. They thank Dr. Mark McLaughlin and Mr. Ted Gauthier, of Louisiana State University, for generously providing the reference standard of benz[f]indene. The authors express their appreciation to Dr. Peter Nelson and Dr. Chun-Zhu Li, of the CSIRO, Australia, for providing the coal pyrolysis product sample. They also thank Dr. Elmer Ledesma for helpful discussions on pyrolysis reaction mechanisms. EF990065R
