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Pyrolytic growth of polycyclic aromatic hydrocarbons by cyclopentadienyl moieties
Proceedings of the Combustion Institute, Volume 28, 2000/pp. 2593–2599
PYROLYTIC GROWTH OF POLYCYCLIC AROMATIC HYDROCARBONS BY CYCLOPENTADIENYL MOIETIES JAMES A. MULHOLLAND, MINGMING LU and DO-HYONG KIM Environmental Engineering Georgia Institute of Technology Atlanta, GA 30332-0512, USA
Polycyclic aromatic hydrocarbon (PAH) growth from cyclopentadiene (CPD) and indene (C9H8), which contains the five-member ring cyclopentadienyl moiety, was investigated experimentally in a four-second flow reactor over a temperature range of 550 to 850 ⬚C. Major products observed were grouped into two categories: ortho-fused six-member ring compounds formed from combinations of two reactants, and PAHs with one less carbon atom than the two reactants and containing one cyclopentadienyl moiety. Benzene and naphthalene were also formed by addition of one carbon atom to the reactants. Reaction pathways to these products were proposed. Addition of the cyclopentadienyl moiety to p bond produced a resonancestabilized radical, which further reacted by one of two unimolecular channels. Decomposition by b scission produced a biaryl intermediate, which then underwent two ring expansion sequences that were proposed for CPD-to-naphthalene conversion. In the case of indene and indene/CPD combinations, branching in the ring expansion steps led to three C18H12 isomers (chrysene, benz[a]anthracene, and benzo[c] phenanthrene) and two C14H10 isomers (anthracene and phenanthrene), respectively. Intramolecular addition competed with the b scission pathway to produce a 7-norbornenyl radical, which then decomposed to benzofluorenes (C17H12), fluorene, and benzindenes (C13H10), and indene from combinations of indenes, indene/CPD, and CPDs, respectively. This second reaction channel, in which one of the cyclopentadienyl moieties was retained, was observed at all temperatures and was most important at temperatures below 700 ⬚C. Reaction pathway analysis using the semiempirical PM3 method was used to explain both the preferential formation of chrysene and anthracene from combinations of two indenes and CPD/indene, respectively, by the first reaction channel, and the temperature dependence in the relative rates of the two reaction channels.
Introduction Formation of polycyclic aromatic hydrocarbons (PAHs) and soot in flames continues to be studied due to the complexity of the reaction mechanisms. These products of incomplete combustion are of interest because several PAHs are known to be potential mutagens and carcinogens and can be sorbed on fine soot particles and carried deep into the lung. The importance of cyclopentadienyl (CPDyl) moieties in PAH growth and soot formation has been postulated because these radicals are neutral (stablized by resonance) and ambident (reactive at different sites); for example, fuels containing CPDyl moieties have been found to have high sooting tendencies [1]. In addition, cyclopentadiene (CPD) is produced in flames by decomposition of monocyclic aromatic fuels, such as phenol [2], benzene, toluene, and xylenes [3,4]. Aromatic growth by combination of CPDyl radicals may also occur in postcombustion gas streams due to the stability of these structures. Naphthalene formation can occur by combination of CPDyl radicals [5–8]. A detailed chemical mechanism for CPD-to-naphthalene formation was
proposed by Melius et al. [9] based on quantum chemical modeling using BAC-MP2 and BAC-MP4 methods. Melius proposed dimerization of CPDyl radicals to form dihydrofulvalene, followed by conversion of five-member rings to six-member rings via radical rearrangements involving three-member ring closing and opening of resonance-stabilized radicals. While naphthalene formation from CPD has been thoroughly studied, there has been much less study of aromatic growth from its benzo derivatives, such as indene. Laskin and Lifshitz [10] studied the thermal decomposition of indene, but not carbon growth by indenyl radical addition. McEnally and Pfefferle [11] studied naphthalene formation from indene and methane but did not report on higher molecular weight products. Marinov et al. [12] proposed that phenanthrene could have been formed by indenyl and CPDyl combination in propane pyrolysis experiments. We recently extended this mechanism to the formation of three C18H12 isomers, chrysene, benz[a] anthracene, and benzo[c]phenanthrene, from indene [13]. Using semiempirical molecular orbital methods, which we found gave results similar to the
2593
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SOOT FORMATION AND DESTRUCTION ⳭC9H8
(a)
; C18H14 (CH3-C17H11) → C17H12 (2 benzofluorenes) Ⳮ C10H8 (benzofulvene, naphthalene) 2C9H8 ' C18H14 (biindenyl) → C18H12 (chrysene, benz[a]anthracene, benzo[c]phenanthrene)
(1)
(b)
ⳭC5H6
(a)
2C5H6
; C10H10 (CH3-C9H7) → C9H8 (indene) Ⳮ C6H6 (benzene) ' C10H10 (dihydrofulvalene) → C10H8 (naphthalene)
(2)
(b)
(a)
C9H8 Ⳮ C5H6
ⳭC9H8 or C5H6
; C14H12 (CH3-C13H9) → C13H10 (fluorene, 3 benzindenes) Ⳮ C10H8 or C6H6 ' C14H12 (C9H7-C5H5) → C14H10 (phenanthrene, anthracene)
(3)
(b)
ab initio results reported by Melius et al. [9] for the CPD reaction system, reaction pathway energetics could explain the distribution of C18H12 isomers as a function of temperature. In addition to this pathway, however, we observed a competing pathway resulting in the formation of C17H12 products and naphthalene. This pathway was dominant at temperatures below 700 ⬚C. We proposed that this occurs by indenyl radical addition to indene, followed by intramolecular addition to form a bridged intermediate (containing the 7-norbornenyl moiety). Competing with the formation of the bridged intermediate via addition is the formation of 1,2-biindenyl via b scission of H atom. To test implications of the results cited above, we studied the pyrolysis of CPD and indene individually and as a mixture. We postulated PAH growth from CPD and indene to occur by the overall equations shown previously. Routes labeled (a) represent the intramolecular addition pathways, and routes labeled (b) represent the b scission pathway. (See equation table above.) In this paper, we present experimental and computational results on aromatic growth from CPD and indene. First, we experimentally tested the hypothesis that indene and benzene are formed from three CPD molecules (equation 2a) by the mechanism proposed for benzofluorene and naphthalene formation from indene (equation 1a). Second, we experimentally studied the indene/CPD system (equation 3), proposing a detailed reaction mechanism based on the experimental results. Third, we computationally studied the distributions of C14H10 isomers and C14H10/C13H10 products as a function of temperature for comparison with the experimental results.
Experimental and Computational Methods A laminar flow, isothermal quartz tube reactor was used to study indene and CPD pyrolysis chemistry. Indene and the CPD dimer dicyclopentadiene are commercially available; dicyclopentadiene decomposes to CPD when heated above its boiling point (170 ⬚C). In single reactant experiments, indene and
CPD vapor were generated by heating 20 mg samples to 200 ⬚C prior to transport to the reactor by inert carrier gas (helium). In the indene/CPD mixture experiments, a syringe pump was used to introduce an equal molar mixture of indene and CPD to the vaporizer, where it was immediately vaporized and transported to the reactor. The residence time in the reactor of the gas stream containing 0.3 to 0.4 molar percent was 4 to 5 s. The entire product stream was immediately quenched at the outlet of the reactor and collected via a dichloromethane (DCM) dual trap system. Soot, defined as the DCM insoluble fraction, was separated by vacuum filtration and measured gravimetrically. DCM extracts were analyzed using a Varian 2000 GC/MS with DB5 ms column with cryogenic conditioning. Products were identified and quantified using chemical standards whenever possible. Biaryls and methyl substituted species were identified on the basis of elution order and spectral data (mass spectrometry and UV-vis) and quantified using gas-chromatography–mass spectrometry response factors for compounds of similar size and structure. A temperature range of 550 to 850 ⬚C was studied. Total carbon recovery ranged from 80% to 100%. At 500 ⬚C and below, aromatic growth from indene and CPD pyrolysis was less than 1%. Above 850 ⬚C, ring fragmentation was dominant, as evidenced by a broad aromatic product distribution that can be explained by acetylene addition pathways. For computational study, we used semiempirical molecular orbital methods because they are efficient and give results similar to those obtained by Melius et al. [9] using the ab initio BAC-MP2 method for the CPD reaction system. As described elsewhere [13], the PM3 method was found to correlate best with the BAC-MP2 methods (0.95 R-squared value) in predicting CPD-to-naphthalene system heats of formation. Results and Discussion Pyrolysis Product Yields Total aromatic yield and yields of major PAH products are shown in Figs. 1, 2, and 3 for pyrolysis
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Fig. 1. Aromatic product yields from indene pyrolysis. Squares denote total product yields. Circles denote compounds produced by equation 1b, and triangles denote compounds produced by equation 1a. Filled symbols are used for overall pathway products, and hollow symbols are used for pathway intermediates.
Fig. 2. Aromatic product yields from cyclopentadiene (CPD) pyrolysis. Squares denote total product yields. Circles denote compounds produced by equation 2b (top panel) and equation 3b (bottom panel). Filled symbols are used for overall pathway products, and hollow symbols are used for pathway intermediates.
Fig. 3. Aromatic product yields from pyrolysis of indene-CPD mixture. Squares denote total product yields, excluding indene, which is also a reactant. Circles denote compounds produced by equations 2b and 3b (top panel) and equation 1b (bottom panel), and triangles denote compounds produced by equations 2a and 3a (top panel) and equation 1a (bottom panel). Filled symbols are used for overall pathway products, and hollow symbols are used for pathway intermediates.
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of indene, CPD, and the indene/CPD mixture, respectively, over the temperature range 550 to 850 ⬚C. Data are presented on a percent carbon feed basis. The indene results (Fig. 1), which have been published previously [13], are presented for comparison with the CPD and indene/CPD mixture results. At 650 ⬚C, less than 3% of the indene was reacted; at 850 ⬚C, about 90% of the indene was reacted. Total aromatic product and soot yields increased with temperature. Unrecovered carbon, which did not exceed 15% for any experiment, was primarily due to unrecovered soot that could not be rinsed from the quartz tube wall at the point of quenching. For temperatures above 850 ⬚C, low carbon recovery and a broad range of aromatic products were observed, indicative of indene ring fragmentation. The yields of aromatic products shown in Fig. 1 represent 99% of the total aromatic yield. As expected from the CPDto-naphthalene mechanism, three C18H12 isomers, chrysene, benz[a]anthracene, and benzo[c]phenanthrene, were found, as were the 1,1- and 1,2-biindenyl intermediates (C18H14). Also formed were two C10H8 isomers, naphthalene and benzofulvene; two C17H12 isomers, benzo[a]fluorene and benzo[b] fluorene; and two additional C18H14 isomers, identified as methyl benzofluorenes. Formation of these compounds can be explained by indenyl radical addition to indene molecule, followed by intramolecular addition to yield a bridged structure containing the 7-norbornenyl moiety. The temperature dependence of the C18H12 isomer distribution and the C18H12 to C17H12 product distribution is discussed elsewhere [13]. In the case of CPD pyrolysis (Fig. 2), almost no dicyclopentadiene was recovered, indicating high conversion to CPD. CPD itself was not collected in the solvent trap. Aromatic growth from CPD occurred at lower temperatures than from indene, consistent with the ranking of Bruinsma et al. [14]. Major products were naphthalene, indene, and benzene. Four methyl indenes, intermediates in equation 2, were also detected. In addition to these compounds, significant amounts of anthracene, phenanthrene, and fluorene were found, and smaller amounts of four methyl fluorenes/benzindenes and three benzindenes were detected. As indicated by equation 3, these are the expected products from the indene/CPD mixture. Yields of these products were very low below 600 ⬚C, consistent with their formation from indene product. In pyrolysis of the indene/CPD mixture, all three sets of products expected from equations 1, 2, and 3 were identified. Shown in the top panel in Fig. 3 are those products expected from the CPD and CPD/indene combinations. Shown in the bottom panel are those products expected from combinations of indene. The isomer distributions of isomers
from the mixture are similar to those from the individual compounds, with chrysene the most abundant C18H12 isomer product and anthracene the more abundant C14H10 isomer product. The formation of fluorene, benzindenes, and benzofluorenes, which retain one of the CPDyl moieties, is evidence that intramolecular addition competes with b scission in the radical-molecule mechanism, as discussed next. Reaction Pathways Reaction pathways are proposed to account for the observed aromatic products from CPD/indene combinations in Fig. 4. In the top panel, the radicalradical pathway is shown. Formation of the CPDyl/ indenyl biaryl is followed by expansion of both fivemember rings via a pathway analogous to the CPDto-naphthalene mechanism of Melius et al. [9]. By this mechanism alone, phenanthrene is expected to be the major product based on reaction energetics, for the same reason that chrysene was the preferred C18H12 product from indene. Moreover, fluorene, benzindene, benzene, and naphthalene are not formed by this pathway. The radical-molecule pathways shown in the middle and bottom panels of Fig. 4 indicate how these compounds can be formed. Intramolecular addition instead of b scission yields a radical intermediate containing the 7-norbornenyl moiety, which decomposes to methyl fluorene and methyl benzindenes and then fluorene and benzindenes. The single carbon species released then combines with either CPD or indene to form fulvene and benzene or benzofulvene and naphthalene, respectively. The favored formation of anthracene can also be understood from the radical-molecule pathways. CPD is more reactive than indene, as demonstrated by our individual compound results as well as the work of Bruinsma et al. [14]. Therefore, the CPDyl/indene pathway (middle panel), which produces anthracene but not phenanthrene, is expected to be more important than the indenyl/CPD pathway (bottom panel), which produces both but phenanthrene preferentially. Last, partitioning between the b scission channel that leads to six-member ring only products and the intramolecular addition channel that leads to products that retain one CPD moiety is discussed, with temperature effects assessed computationally and compared with the experimental results. Partitioning between b Scission and Intramolecular Addition Pathways PM3 energy profiles (heats of formation) for the competing b scission and intramolecular addition reactions for the CPDyl/indene and indenyl/CPD systems are shown in Fig. 5. Energy profiles for the
PAH GROWTH FROM CYCLOPENTADIENYL MOIETIES
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Fig. 4. Pathways for aromatic growth by indene-CPD combinations. Radical-radical (top panel) and two radicalmolecule (middle and bottom panels) pathways are shown. In the two radical-molecule panels, intramolecular addition is shown at top and b scission at bottom.
formation of stereoisomers by intramolecular addition are similar. The profiles for the indenyl/indene and CPDyl/CPD systems (not shown) are also similar. The intramolecular addition thermodynamic barrier is 11.5 kcal/mol, and the kinetic barrier is approximately 40 kcal/mol; for b scission, the thermodynamic and kinetic barriers are 57 and 60 kcal/
mol, respectively. Thus, intramolecular addition is favored over b scission by 20 kcal/mol, consistent with addition being favored at low temperatures. Entropy differences, which were not calculated, favor b scission (bond breaking) over intramolecular addition (bond forming); thus, b scission should prevail at high temperatures.
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Fig. 5. Energy diagrams (PM3 heats of formation) for competing b scission and intramolecular addition reactions from radical-molecule combinations of the two indeneCPD routes shown in the middle and bottom panels of Fig. 4. The endo addition pathways are shown because these are favored over exo pathways. Fig. 6. Relative rates of b scission to intramolecular addition in pyrolysis of indene, CPD, and indeneCPD mixtures. Circles denote indene combinations from indene and indene-CPD pyrolysis experiments, triangles denote CPD combinations from CPD pyrolysis experiments, and squares denote CPD-indene combinations from CPD and indene-CPD pyrolysis experiments. Error bars are shown for indene pyrolysis data. Curves represent fits to the experimental data.
The experimental results qualitatively agree with these findings, with intramolecular addition favored at low temperatures. Shown in Fig. 6 are the ratios of rates of b scission to intramolecular addition based on pathway product yields. Data are included from all three sets of experiments (pure indene, pure CPD, and indene/CPD mixture) and three sets of reaction products (indenyl/indene, CPDyl/CPD, and both CPDyl/indene and indenyl/CPD). Two curves are fit to the data, one for the CPDyl/CPD, CPDyl/indene, and indenyl/CPD pathways and another for the indenyl/indene pathway. The experimental results suggest that intramolecular addition is favored enthalpically by 10 kcal/mol for the former pathways and 25 kcal/mol for the indenyl/indene pathway, and that b scission is favored entropically by 12 cal/mol/K for the former pathways and 25 cal/mol/K for the indenyl/indene pathway. The entropy differences correspond to relative preexponential factors of 350 to 300,000, respectively. The computed temperature dependence appears to be greater than that observed experimentally, particularly for the CPD and CPD/indene systems. One possible explanation is that the radical-radical pathway, in which the formation of a norbornenyl intermediate does not occur, may be important at low
temperatures. Further study, including the use of higher-order molecular orbital theory, is needed.
Conclusions Our results offer further support that addition of resonance-stabilized CPDyl radicals may be important in the growth of aromatic hydrocarbons and soot. We extended the mechanism of naphthalene formation from two CPDyl radicals to indene and CPD/indene systems, accounting for the distribution of isomer products containing only ortho-fused six-member rings. Modifying the mechanism to include radical-molecule pathways, we propose an intramolecular addition channel for the formation of polycyclic compounds that contain odd numbers of carbon atoms and one CPDyl moiety. Partitioning between b scission and intramolecular addition reactions that result in the two sets of products was studied computationally. Computational and experimental results indicate that intramolecular addition products are favored at low temperature (below 700 ⬚C), and b scission products are favored at high temperatures.
PAH GROWTH FROM CYCLOPENTADIENYL MOIETIES
Limited experiments performed under oxidative conditions show a similar aromatic product distribution, with much lower overall yields. All experiments were performed in the absence of non-cyclic hydrocarbons, such as acetylene, which are known to contribute to aromatic growth and soot formation in flames, and at temperatures below which ring fragmentation becomes significant. The relative contribution of these pathways was not addressed. Acknowledgment The support of the National Science Foundation under grant no. CTS-9457028 is gratefully acknowledged. REFERENCES 1. Gomez, A., Sidebotham, G., and Glassman, I., Combust. Flame. 58:45–57 (1984). 2. Roy, K., Horn, C., Slutsky V. G., and Just, T., Proc. Combust. Inst. 27:329–336 (1998). 3. Brezinsky, K., Butler, R. G., and Glassman I., in Symposium on the Mechanisms and Chemistry of Pollutant Formation and Control from Internal Combustion Engines, American Chemical Society, Washington, DC, 1992, pp. 1467–1472.
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4. Tregrossi, A., Ciajolo, A., and Barbella, R., Combust. Flame 117:553–561 (1997). 5. Spielmann, R., and Cramers, C. A., Chromatographia 5:295–300 (1972). 6. Manion, J. A., and Louw, R., J. Phys. Chem. 93:3563– 3574 (1989). 7. Castaldi, M. J., Marinov, N. M., Melius, C. F., Huang, J., Senkan, S. M., Pitz, W. M., and Westbrook, C. K., Proc. Combust. Inst. 26:693–702 (1996). 8. Marinov, N. M., Pitz, W. J., Westbrook, C. K., Castaldi, M. J., and Senkan, S. M., Combust. Sci. Technol. 116– 117:211–287 (1996). 9. Melius, C. F., Colvin, M. E., Marinov, N. M., Pitz, W. J., and Senkan, S. M., Proc. Combust. Inst. 26:685–692 (1996). 10. Laskin, A., and Lifshitz, A., Proc. Combust. Inst. 27:313–320 (1998). 11. McEnally, C. S., and Pfefferle, L. D., Combust. Sci. Technol. 131:323–344 (1998). 12. Marinov, N. M., Castaldi, M. J., Melius, C. F., and Tsang, W., Combust: Sci. Technol. 128:295–342 (1997). 13. Lu, M., and Mulholland, J. A., Chemosphere, 42:189– 197 (2001). 14. Bruinsma, O. S., Tromp, P. J., Nolting, H. J., and Moulijn J. A., Fuel 67:334–340 (1988).
PYROLYTIC GROWTH OF POLYCYCLIC AROMATIC HYDROCARBONS BY CYCLOPENTADIENYL MOIETIES JAMES A. MULHOLLAND, MINGMING LU and DO-HYONG KIM Environmental Engineering Georgia Institute of Technology Atlanta, GA 30332-0512, USA
Polycyclic aromatic hydrocarbon (PAH) growth from cyclopentadiene (CPD) and indene (C9H8), which contains the five-member ring cyclopentadienyl moiety, was investigated experimentally in a four-second flow reactor over a temperature range of 550 to 850 ⬚C. Major products observed were grouped into two categories: ortho-fused six-member ring compounds formed from combinations of two reactants, and PAHs with one less carbon atom than the two reactants and containing one cyclopentadienyl moiety. Benzene and naphthalene were also formed by addition of one carbon atom to the reactants. Reaction pathways to these products were proposed. Addition of the cyclopentadienyl moiety to p bond produced a resonancestabilized radical, which further reacted by one of two unimolecular channels. Decomposition by b scission produced a biaryl intermediate, which then underwent two ring expansion sequences that were proposed for CPD-to-naphthalene conversion. In the case of indene and indene/CPD combinations, branching in the ring expansion steps led to three C18H12 isomers (chrysene, benz[a]anthracene, and benzo[c] phenanthrene) and two C14H10 isomers (anthracene and phenanthrene), respectively. Intramolecular addition competed with the b scission pathway to produce a 7-norbornenyl radical, which then decomposed to benzofluorenes (C17H12), fluorene, and benzindenes (C13H10), and indene from combinations of indenes, indene/CPD, and CPDs, respectively. This second reaction channel, in which one of the cyclopentadienyl moieties was retained, was observed at all temperatures and was most important at temperatures below 700 ⬚C. Reaction pathway analysis using the semiempirical PM3 method was used to explain both the preferential formation of chrysene and anthracene from combinations of two indenes and CPD/indene, respectively, by the first reaction channel, and the temperature dependence in the relative rates of the two reaction channels.
Introduction Formation of polycyclic aromatic hydrocarbons (PAHs) and soot in flames continues to be studied due to the complexity of the reaction mechanisms. These products of incomplete combustion are of interest because several PAHs are known to be potential mutagens and carcinogens and can be sorbed on fine soot particles and carried deep into the lung. The importance of cyclopentadienyl (CPDyl) moieties in PAH growth and soot formation has been postulated because these radicals are neutral (stablized by resonance) and ambident (reactive at different sites); for example, fuels containing CPDyl moieties have been found to have high sooting tendencies [1]. In addition, cyclopentadiene (CPD) is produced in flames by decomposition of monocyclic aromatic fuels, such as phenol [2], benzene, toluene, and xylenes [3,4]. Aromatic growth by combination of CPDyl radicals may also occur in postcombustion gas streams due to the stability of these structures. Naphthalene formation can occur by combination of CPDyl radicals [5–8]. A detailed chemical mechanism for CPD-to-naphthalene formation was
proposed by Melius et al. [9] based on quantum chemical modeling using BAC-MP2 and BAC-MP4 methods. Melius proposed dimerization of CPDyl radicals to form dihydrofulvalene, followed by conversion of five-member rings to six-member rings via radical rearrangements involving three-member ring closing and opening of resonance-stabilized radicals. While naphthalene formation from CPD has been thoroughly studied, there has been much less study of aromatic growth from its benzo derivatives, such as indene. Laskin and Lifshitz [10] studied the thermal decomposition of indene, but not carbon growth by indenyl radical addition. McEnally and Pfefferle [11] studied naphthalene formation from indene and methane but did not report on higher molecular weight products. Marinov et al. [12] proposed that phenanthrene could have been formed by indenyl and CPDyl combination in propane pyrolysis experiments. We recently extended this mechanism to the formation of three C18H12 isomers, chrysene, benz[a] anthracene, and benzo[c]phenanthrene, from indene [13]. Using semiempirical molecular orbital methods, which we found gave results similar to the
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SOOT FORMATION AND DESTRUCTION ⳭC9H8
(a)
; C18H14 (CH3-C17H11) → C17H12 (2 benzofluorenes) Ⳮ C10H8 (benzofulvene, naphthalene) 2C9H8 ' C18H14 (biindenyl) → C18H12 (chrysene, benz[a]anthracene, benzo[c]phenanthrene)
(1)
(b)
ⳭC5H6
(a)
2C5H6
; C10H10 (CH3-C9H7) → C9H8 (indene) Ⳮ C6H6 (benzene) ' C10H10 (dihydrofulvalene) → C10H8 (naphthalene)
(2)
(b)
(a)
C9H8 Ⳮ C5H6
ⳭC9H8 or C5H6
; C14H12 (CH3-C13H9) → C13H10 (fluorene, 3 benzindenes) Ⳮ C10H8 or C6H6 ' C14H12 (C9H7-C5H5) → C14H10 (phenanthrene, anthracene)
(3)
(b)
ab initio results reported by Melius et al. [9] for the CPD reaction system, reaction pathway energetics could explain the distribution of C18H12 isomers as a function of temperature. In addition to this pathway, however, we observed a competing pathway resulting in the formation of C17H12 products and naphthalene. This pathway was dominant at temperatures below 700 ⬚C. We proposed that this occurs by indenyl radical addition to indene, followed by intramolecular addition to form a bridged intermediate (containing the 7-norbornenyl moiety). Competing with the formation of the bridged intermediate via addition is the formation of 1,2-biindenyl via b scission of H atom. To test implications of the results cited above, we studied the pyrolysis of CPD and indene individually and as a mixture. We postulated PAH growth from CPD and indene to occur by the overall equations shown previously. Routes labeled (a) represent the intramolecular addition pathways, and routes labeled (b) represent the b scission pathway. (See equation table above.) In this paper, we present experimental and computational results on aromatic growth from CPD and indene. First, we experimentally tested the hypothesis that indene and benzene are formed from three CPD molecules (equation 2a) by the mechanism proposed for benzofluorene and naphthalene formation from indene (equation 1a). Second, we experimentally studied the indene/CPD system (equation 3), proposing a detailed reaction mechanism based on the experimental results. Third, we computationally studied the distributions of C14H10 isomers and C14H10/C13H10 products as a function of temperature for comparison with the experimental results.
Experimental and Computational Methods A laminar flow, isothermal quartz tube reactor was used to study indene and CPD pyrolysis chemistry. Indene and the CPD dimer dicyclopentadiene are commercially available; dicyclopentadiene decomposes to CPD when heated above its boiling point (170 ⬚C). In single reactant experiments, indene and
CPD vapor were generated by heating 20 mg samples to 200 ⬚C prior to transport to the reactor by inert carrier gas (helium). In the indene/CPD mixture experiments, a syringe pump was used to introduce an equal molar mixture of indene and CPD to the vaporizer, where it was immediately vaporized and transported to the reactor. The residence time in the reactor of the gas stream containing 0.3 to 0.4 molar percent was 4 to 5 s. The entire product stream was immediately quenched at the outlet of the reactor and collected via a dichloromethane (DCM) dual trap system. Soot, defined as the DCM insoluble fraction, was separated by vacuum filtration and measured gravimetrically. DCM extracts were analyzed using a Varian 2000 GC/MS with DB5 ms column with cryogenic conditioning. Products were identified and quantified using chemical standards whenever possible. Biaryls and methyl substituted species were identified on the basis of elution order and spectral data (mass spectrometry and UV-vis) and quantified using gas-chromatography–mass spectrometry response factors for compounds of similar size and structure. A temperature range of 550 to 850 ⬚C was studied. Total carbon recovery ranged from 80% to 100%. At 500 ⬚C and below, aromatic growth from indene and CPD pyrolysis was less than 1%. Above 850 ⬚C, ring fragmentation was dominant, as evidenced by a broad aromatic product distribution that can be explained by acetylene addition pathways. For computational study, we used semiempirical molecular orbital methods because they are efficient and give results similar to those obtained by Melius et al. [9] using the ab initio BAC-MP2 method for the CPD reaction system. As described elsewhere [13], the PM3 method was found to correlate best with the BAC-MP2 methods (0.95 R-squared value) in predicting CPD-to-naphthalene system heats of formation. Results and Discussion Pyrolysis Product Yields Total aromatic yield and yields of major PAH products are shown in Figs. 1, 2, and 3 for pyrolysis
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Fig. 1. Aromatic product yields from indene pyrolysis. Squares denote total product yields. Circles denote compounds produced by equation 1b, and triangles denote compounds produced by equation 1a. Filled symbols are used for overall pathway products, and hollow symbols are used for pathway intermediates.
Fig. 2. Aromatic product yields from cyclopentadiene (CPD) pyrolysis. Squares denote total product yields. Circles denote compounds produced by equation 2b (top panel) and equation 3b (bottom panel). Filled symbols are used for overall pathway products, and hollow symbols are used for pathway intermediates.
Fig. 3. Aromatic product yields from pyrolysis of indene-CPD mixture. Squares denote total product yields, excluding indene, which is also a reactant. Circles denote compounds produced by equations 2b and 3b (top panel) and equation 1b (bottom panel), and triangles denote compounds produced by equations 2a and 3a (top panel) and equation 1a (bottom panel). Filled symbols are used for overall pathway products, and hollow symbols are used for pathway intermediates.
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of indene, CPD, and the indene/CPD mixture, respectively, over the temperature range 550 to 850 ⬚C. Data are presented on a percent carbon feed basis. The indene results (Fig. 1), which have been published previously [13], are presented for comparison with the CPD and indene/CPD mixture results. At 650 ⬚C, less than 3% of the indene was reacted; at 850 ⬚C, about 90% of the indene was reacted. Total aromatic product and soot yields increased with temperature. Unrecovered carbon, which did not exceed 15% for any experiment, was primarily due to unrecovered soot that could not be rinsed from the quartz tube wall at the point of quenching. For temperatures above 850 ⬚C, low carbon recovery and a broad range of aromatic products were observed, indicative of indene ring fragmentation. The yields of aromatic products shown in Fig. 1 represent 99% of the total aromatic yield. As expected from the CPDto-naphthalene mechanism, three C18H12 isomers, chrysene, benz[a]anthracene, and benzo[c]phenanthrene, were found, as were the 1,1- and 1,2-biindenyl intermediates (C18H14). Also formed were two C10H8 isomers, naphthalene and benzofulvene; two C17H12 isomers, benzo[a]fluorene and benzo[b] fluorene; and two additional C18H14 isomers, identified as methyl benzofluorenes. Formation of these compounds can be explained by indenyl radical addition to indene molecule, followed by intramolecular addition to yield a bridged structure containing the 7-norbornenyl moiety. The temperature dependence of the C18H12 isomer distribution and the C18H12 to C17H12 product distribution is discussed elsewhere [13]. In the case of CPD pyrolysis (Fig. 2), almost no dicyclopentadiene was recovered, indicating high conversion to CPD. CPD itself was not collected in the solvent trap. Aromatic growth from CPD occurred at lower temperatures than from indene, consistent with the ranking of Bruinsma et al. [14]. Major products were naphthalene, indene, and benzene. Four methyl indenes, intermediates in equation 2, were also detected. In addition to these compounds, significant amounts of anthracene, phenanthrene, and fluorene were found, and smaller amounts of four methyl fluorenes/benzindenes and three benzindenes were detected. As indicated by equation 3, these are the expected products from the indene/CPD mixture. Yields of these products were very low below 600 ⬚C, consistent with their formation from indene product. In pyrolysis of the indene/CPD mixture, all three sets of products expected from equations 1, 2, and 3 were identified. Shown in the top panel in Fig. 3 are those products expected from the CPD and CPD/indene combinations. Shown in the bottom panel are those products expected from combinations of indene. The isomer distributions of isomers
from the mixture are similar to those from the individual compounds, with chrysene the most abundant C18H12 isomer product and anthracene the more abundant C14H10 isomer product. The formation of fluorene, benzindenes, and benzofluorenes, which retain one of the CPDyl moieties, is evidence that intramolecular addition competes with b scission in the radical-molecule mechanism, as discussed next. Reaction Pathways Reaction pathways are proposed to account for the observed aromatic products from CPD/indene combinations in Fig. 4. In the top panel, the radicalradical pathway is shown. Formation of the CPDyl/ indenyl biaryl is followed by expansion of both fivemember rings via a pathway analogous to the CPDto-naphthalene mechanism of Melius et al. [9]. By this mechanism alone, phenanthrene is expected to be the major product based on reaction energetics, for the same reason that chrysene was the preferred C18H12 product from indene. Moreover, fluorene, benzindene, benzene, and naphthalene are not formed by this pathway. The radical-molecule pathways shown in the middle and bottom panels of Fig. 4 indicate how these compounds can be formed. Intramolecular addition instead of b scission yields a radical intermediate containing the 7-norbornenyl moiety, which decomposes to methyl fluorene and methyl benzindenes and then fluorene and benzindenes. The single carbon species released then combines with either CPD or indene to form fulvene and benzene or benzofulvene and naphthalene, respectively. The favored formation of anthracene can also be understood from the radical-molecule pathways. CPD is more reactive than indene, as demonstrated by our individual compound results as well as the work of Bruinsma et al. [14]. Therefore, the CPDyl/indene pathway (middle panel), which produces anthracene but not phenanthrene, is expected to be more important than the indenyl/CPD pathway (bottom panel), which produces both but phenanthrene preferentially. Last, partitioning between the b scission channel that leads to six-member ring only products and the intramolecular addition channel that leads to products that retain one CPD moiety is discussed, with temperature effects assessed computationally and compared with the experimental results. Partitioning between b Scission and Intramolecular Addition Pathways PM3 energy profiles (heats of formation) for the competing b scission and intramolecular addition reactions for the CPDyl/indene and indenyl/CPD systems are shown in Fig. 5. Energy profiles for the
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Fig. 4. Pathways for aromatic growth by indene-CPD combinations. Radical-radical (top panel) and two radicalmolecule (middle and bottom panels) pathways are shown. In the two radical-molecule panels, intramolecular addition is shown at top and b scission at bottom.
formation of stereoisomers by intramolecular addition are similar. The profiles for the indenyl/indene and CPDyl/CPD systems (not shown) are also similar. The intramolecular addition thermodynamic barrier is 11.5 kcal/mol, and the kinetic barrier is approximately 40 kcal/mol; for b scission, the thermodynamic and kinetic barriers are 57 and 60 kcal/
mol, respectively. Thus, intramolecular addition is favored over b scission by 20 kcal/mol, consistent with addition being favored at low temperatures. Entropy differences, which were not calculated, favor b scission (bond breaking) over intramolecular addition (bond forming); thus, b scission should prevail at high temperatures.
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Fig. 5. Energy diagrams (PM3 heats of formation) for competing b scission and intramolecular addition reactions from radical-molecule combinations of the two indeneCPD routes shown in the middle and bottom panels of Fig. 4. The endo addition pathways are shown because these are favored over exo pathways. Fig. 6. Relative rates of b scission to intramolecular addition in pyrolysis of indene, CPD, and indeneCPD mixtures. Circles denote indene combinations from indene and indene-CPD pyrolysis experiments, triangles denote CPD combinations from CPD pyrolysis experiments, and squares denote CPD-indene combinations from CPD and indene-CPD pyrolysis experiments. Error bars are shown for indene pyrolysis data. Curves represent fits to the experimental data.
The experimental results qualitatively agree with these findings, with intramolecular addition favored at low temperatures. Shown in Fig. 6 are the ratios of rates of b scission to intramolecular addition based on pathway product yields. Data are included from all three sets of experiments (pure indene, pure CPD, and indene/CPD mixture) and three sets of reaction products (indenyl/indene, CPDyl/CPD, and both CPDyl/indene and indenyl/CPD). Two curves are fit to the data, one for the CPDyl/CPD, CPDyl/indene, and indenyl/CPD pathways and another for the indenyl/indene pathway. The experimental results suggest that intramolecular addition is favored enthalpically by 10 kcal/mol for the former pathways and 25 kcal/mol for the indenyl/indene pathway, and that b scission is favored entropically by 12 cal/mol/K for the former pathways and 25 cal/mol/K for the indenyl/indene pathway. The entropy differences correspond to relative preexponential factors of 350 to 300,000, respectively. The computed temperature dependence appears to be greater than that observed experimentally, particularly for the CPD and CPD/indene systems. One possible explanation is that the radical-radical pathway, in which the formation of a norbornenyl intermediate does not occur, may be important at low
temperatures. Further study, including the use of higher-order molecular orbital theory, is needed.
Conclusions Our results offer further support that addition of resonance-stabilized CPDyl radicals may be important in the growth of aromatic hydrocarbons and soot. We extended the mechanism of naphthalene formation from two CPDyl radicals to indene and CPD/indene systems, accounting for the distribution of isomer products containing only ortho-fused six-member rings. Modifying the mechanism to include radical-molecule pathways, we propose an intramolecular addition channel for the formation of polycyclic compounds that contain odd numbers of carbon atoms and one CPDyl moiety. Partitioning between b scission and intramolecular addition reactions that result in the two sets of products was studied computationally. Computational and experimental results indicate that intramolecular addition products are favored at low temperature (below 700 ⬚C), and b scission products are favored at high temperatures.
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Limited experiments performed under oxidative conditions show a similar aromatic product distribution, with much lower overall yields. All experiments were performed in the absence of non-cyclic hydrocarbons, such as acetylene, which are known to contribute to aromatic growth and soot formation in flames, and at temperatures below which ring fragmentation becomes significant. The relative contribution of these pathways was not addressed. Acknowledgment The support of the National Science Foundation under grant no. CTS-9457028 is gratefully acknowledged. REFERENCES 1. Gomez, A., Sidebotham, G., and Glassman, I., Combust. Flame. 58:45–57 (1984). 2. Roy, K., Horn, C., Slutsky V. G., and Just, T., Proc. Combust. Inst. 27:329–336 (1998). 3. Brezinsky, K., Butler, R. G., and Glassman I., in Symposium on the Mechanisms and Chemistry of Pollutant Formation and Control from Internal Combustion Engines, American Chemical Society, Washington, DC, 1992, pp. 1467–1472.
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4. Tregrossi, A., Ciajolo, A., and Barbella, R., Combust. Flame 117:553–561 (1997). 5. Spielmann, R., and Cramers, C. A., Chromatographia 5:295–300 (1972). 6. Manion, J. A., and Louw, R., J. Phys. Chem. 93:3563– 3574 (1989). 7. Castaldi, M. J., Marinov, N. M., Melius, C. F., Huang, J., Senkan, S. M., Pitz, W. M., and Westbrook, C. K., Proc. Combust. Inst. 26:693–702 (1996). 8. Marinov, N. M., Pitz, W. J., Westbrook, C. K., Castaldi, M. J., and Senkan, S. M., Combust. Sci. Technol. 116– 117:211–287 (1996). 9. Melius, C. F., Colvin, M. E., Marinov, N. M., Pitz, W. J., and Senkan, S. M., Proc. Combust. Inst. 26:685–692 (1996). 10. Laskin, A., and Lifshitz, A., Proc. Combust. Inst. 27:313–320 (1998). 11. McEnally, C. S., and Pfefferle, L. D., Combust. Sci. Technol. 131:323–344 (1998). 12. Marinov, N. M., Castaldi, M. J., Melius, C. F., and Tsang, W., Combust: Sci. Technol. 128:295–342 (1997). 13. Lu, M., and Mulholland, J. A., Chemosphere, 42:189– 197 (2001). 14. Bruinsma, O. S., Tromp, P. J., Nolting, H. J., and Moulijn J. A., Fuel 67:334–340 (1988).