Pyrolytic hydrocarbon growth from cyclopentadiene

Article abstract of DOI:10.1021/jp106749k

Aromatic hydrocarbon growth from cyclopentadiene (CPD) was studied using a laminar flow reactor operating in the temperature range 550-950 °C without oxygen. Benzene, indene, and naphthalene were the major products, which is in agreement with the previous computational studies on the reaction pathways from CPD. A crossover of indene and naphthalene yields around 775 °C was also observed, which further supports the results of the computational studies. Although the specific intermediates in the proposed pathways from CPD were not detected, the high selectivity of products and the observation of other methylindene and dihydronaphthalene intermediates suggest that the recombination of two CPDs via radical-molecule and/or radical-radical pathways to form indene and naphthalene is the dominant formation pathway. In addition to the products from the CPD-CPD reactions, the products from the reactions of CPD with indene, naphthalene, and acenaphthylene were also observed, which demonstrate the importance of CPD in carbon growth.

Full text of DOI:10.1021/jp106749k

J. Phys. Chem. A 2010, 114, 12411–12416
12411
Pyrolytic Hydrocarbon Growth from Cyclopentadiene
Do Hyong Kim,*,† James A. Mulholland,† Dong Wang,‡ and Angela Violi§
School of CiVil and EnVironmental Engineering, Georgia Institute of Technology,
Atlanta, Georgia 30332, United States, Department of Chemistry, UniVersity of Utah,
Salt Lake City, Utah 84112, United States, and Department of Mechanical Engineering, UniVersity of
Michigan, Ann Arbor, Michigan 48109, United States
ReceiVed: July 20, 2010; ReVised Manuscript ReceiVed: September 24, 2010
Aromatic hydrocarbon growth from cyclopentadiene (CPD) was studied using a laminar flow reactor operating
in the temperature range 550-950 °C without oxygen. Benzene, indene, and naphthalene were the major
products, which is in agreement with the previous computational studies on the reaction pathways from CPD.
A crossover of indene and naphthalene yields around 775 °C was also observed, which further supports the
results of the computational studies. Although the specific intermediates in the proposed pathways from CPD
were not detected, the high selectivity of products and the observation of other methylindene and
dihydronaphthalene intermediates suggest that the recombination of two CPDs via radical-molecule and/or
radical-radical pathways to form indene and naphthalene is the dominant formation pathway. In addition to
the products from the CPD-CPD reactions, the products from the reactions of CPD with indene, naphthalene,
and acenaphthylene were also observed, which demonstrate the importance of CPD in carbon growth.
Introduction
flame experiment. Mulholland and co-workers26-28 investigated
experimentally and computationally detailed pathways to PAH
The formation of five-membered rings in combustion is of
particular interest due to the high reactivity1,2 and toxicity of
many compounds that contain these moieties.3,4 Polycyclic
aromatic hydrocarbons (PAHs) with peripherally fused five-
membered rings demonstrate a greater facility to undergo certain
kinds of reactions, such as dimerization and isomerization
involving intramolecular rearrangement, than PAHs without
these moieties.1,2,5-7 Although the hydrogen-abstraction and
acetylene-addition (HACA) mechanism has been shown to be
an important mechanism of PAH formation in flames,8-13 the
HACA mechanism alone cannot explain the formation of many
large PAHs in flames that are thermodynamically unfavorable
or that are too slow kinetically.14 Violi and co-workers6,15,16
suggested that high molecular mass structures, soot inception,
and particulate mass growth occur in oxidative environments
predominantly through fast reactions of small PAH rather than
acetylene.
Among many potenital precursors for aromatic growth, the
cyclopentadienyl (CPDyl) radical is considered to be an
important intermediate in PAH formation due to its multiple
reaction sites and its ability of self-recombination,17,18 which
also resulted in increased formation of aromatic products.19 The
CPDyl radical contribution to PAH growth has been observed
from naphthalene formation in flames.14,20-22 In addition,
computational studies23,24 proposed a formation pathway to
naphthalene from CPDyl radicals. A detailed chemical mech-
anism by Melius et al.24 proposed dimerization of CPDyl radicals
to form 9,10-dihydrofulvalene, followed by conversion of five-
membered rings to six-membered rings. Ritcher et al.25 sug-
gested the self-recombination of CPDyl radicals to be the major
pathway to naphthalene in benzene flames from their premixed
growth from cyclopentadiene and indene in the postflame
conditions. Butler and Glassman29 also reported that indene and
naphthalene were major products from CPD combustion in a
plug flow reactor at 1150 K.
Recently, various pathways leading to naphthalene, indene,
and benzene from the addition of the CPDyl radical to
cyclopentadiene (CPD) were studied at the B3LYP/6-31G**
level.15 Proposed intermediates included 4- and 7-methylindenes
and 9,10-dihydronaphthalene. Kislov and Mebel30 reported ab
initio Gaussian 3 (G3)-type calculations of reaction routes from
CPD to naphthalene, azulene, and fulvalene. They suggested
additional routes leading to naphthalene via the azulene-
naphthalene isomerizartion. Later, Kislov and Mebel31 also
calculated the potential energy surface for possible rearrange-
ments of reaction products of CPDyl radical recombination and
the intermolecular addition of the CPDyl radical to CPD and
proposed the detailed picture of reaction pathways from CPD
to indene, naphthalene, azulene, and fulvalene.
Although the theoretical investigations provided more detailed
information on both stable and radical species in the CPD
reaction pathways, the CPD chemistry in a real combustion
system is expected to be much more complicated. In addition
to the radical-radical/radical-molecule reactions of two CPDs,
reactions between CPD and other species in the system and other
competitive pathways than those suggested are possible, which
would result in different reaction products and their yields.
Therefore, hypothesized CPD reaction routes need to be tested
and refined on the basis of experimental work. In this paper,
we present the observed aromatic hydrocarbon products from
CPD using a laminar flow reactor. This experimental study
complements the theoretical investigations of the CPD chemistry
and provides a better understanding of the CPD chemistry in
carbon growth.
* Corresponding author. Phone: (404) 657-8685. Fax: (404) 651-9425.
E-mail: dr.dohyongkim@gmail.com.
† Georgia Institute of Technology.
‡ University of Utah.
§ University of Michigan.
10.1021/jp106749k  2010 American Chemical Society
Published on Web 11/05/2010

12412 J. Phys. Chem. A, Vol. 114, No. 47, 2010
Kim et al.
Supporting Information). The gas temperatures in the remaining
31 cm of the reactor, defined as a reaction zone, are ap-
proximately constant within (10 °C of the set value. Radial
variation of temperature was negligible.
Dicyclopentadiene (DCPD) with 99.9% purity (Chem Service,
Inc., West Chester, PA, U.S.) was heated to liquid form (34
°C) and fed by a syringe pump into a glass vessel. The glass
vessel was wrapped with a heat tape to ensure the temperature
would be 20 °C above the boiling point of DCPD (180 °C); the
temperature in the glass vessel was monitored by a thermo-
couple. At the outlet of the glass vessel, DCPD was more than
99.9% decomposed to CPD. Nitrogen was used as a carrier gas,
and the gas stream entering the reactor consisted of 0.7% molar
CPD vapor in nitrogen. Experiments were conducted at tem-
peratures ranging from 550 to 950 °C; triplicate experiments
were conducted at 825 °C, the temperature at which total
aromatic product yield was greatest. A nominal residence time
of 3 s was maintained; the mass flow rate was held constant so
the actual residence time varied slightly with reactor tempera-
ture. The product stream was quenched at the outlet of the
reactor and collected in a dual ice-cooled dichloromethane
(DCM) trap.
Figure 1. Overall reaction profile from CPD pyrolysis.
Experimental Methods
The laminar flow reactor system used for CPD pyrolysis
consists of a Thermolyne Model 21100 furnace, a digital
temperature controller, and a quartz tube of 48 cm long and
1.7 cm in diameter (Figure S1, Supporting Information).26
Temperature profiles inside the quartz tube reactor were
measured using a thermocouple for different temperature
settings, and measurements of the reactor temperature profiles
indicate that temperatures at the top 8 cm and bottom 9 cm of
the quartz tube are lower than the set value (Figure S2,
After each experiment, the quartz tube, all fittings, and
collection trap were also rinsed with DCM to remove any
products deposited on the surface. Rinse samples were found
to contain no identifiable products and were combined with the
TABLE 1: Percent Yields (Carbon Basis) of Products from CPD Pyrolysis
temperature (°C)
750 775
550
600
650
700
725
800
825a
850
900
950
cyclopentadiene
(dihydrofulvene)b
1,3-cyclohexadiene
benzene
toluene
(p-xylene)
(phenylethyne)
styrene
DCPD
C5H6
C6H8
C6H8
C6H6
C7H8
C8H8
C8H8
C8H8
100
90.7
88.0
84.9
52.8
45.7
27.5
16.8
0.341
0.245
15.7
3.55
0.139
0.255
3.51
0.000
21.7
8.93
0.005
0.007
15.0
0.850
0.001
0.001
23.7
5.76
0.149
0.492
3.59
0.310
0.300
0.023
0.989
0.958
0.416
0.415
0.468
0.529
0.074
0.792
0.639
2.11
0.877
0.704
3.00
0.685
0.073
0.015
0.820
0.227
6.53
0.343
0.653
0.361
1.191
4.72
0.873
0.744
5.46
0.736
0.580
8.64
20.4
2.13
26.6
0.114
0.244
1.48
2.43
4.50
0.139
0.071
1.75
0.150
11.8
0.269
0.407
0.261
1.142
9.25
0.222
0.191
2.66
0.066
14.7
0.155
1.066
0.070
0.818
0.240
0.462
3.75
0.028
15.1
0.075
0.000
0.269
0.698
5.52
0.168
0.957
0.679
1.303
2.77
0.515
0.838
0.188
0.022
C10H12
C9H8
0.083
0.031
0.002
0.002
0.001
0.005
0.004
0.948
0.205
0.003
0.012
0.004
0.032
0.038
0.129
0.980
0.071
0.257
0.142
0.304
0.617
indene
8.81
1.86
0.063
2-methylindene
3-methylindene
(diydrobenzofulvene)
1,2-dihydronaphthalene
naphthalene
1-methylnaphthalene
2-methylnaphthalene
biphenyl
(ethenylnaphthalene)
acenaphthylene
acenaphthene
(1H-phenalene)
fluorene
(benzindene)
(benzindene)
(benzindene)
(methylfluorene)
(methylfluorene)
(methylfluorene)
phenanthrene
C10H10
C10H10
C10H10
C10H10
C10H8
C11H10
C11H10
C12H10
C12H10
C12H8
C12H10
C13H10
C13H10
C13H10
C13H10
C13H10
C14H12
C14H12
C14H12
C14H10
C14H10
C15H10
0.104
0.131
0.050
0.410
0.004
0.114
28.6
0.003
30.6
14.7
23.7
20.4
13.7
0.006
0.006
0.029
0.028
0.066
0.065
0.011
0.052
0.075
0.007
0.164
0.176
0.179
0.063
0.586
0.012
0.011
2.35
0.082
0.069
0.254
0.089
0.285
0.404
3.25
0.121
0.117
0.02
0.002
0.938
0.002
0.002
1.86
0.012
0.007
0.029
0.017
0.003
0.022
3.66
0.084
0.054
0.047
0.004
1.51
0.002
0.001
0.004
0.002
1.59
0.001
0.017
0.001
0.075
0.004
0.169
0.007
0.006
0.308
0.035
0.032
0.070
0.080
0.103
0.219
0.377
0.516
0.022
0.301
0.000
0.003
0.771
0.015
0.011
0.040
0.081
0.064
0.292
1.074
0.839
0.031
0.004
0.070
0.013
0.012
0.020
0.018
0.048
0.076
0.098
0.128
0.004
0.736
0.010
0.008
0.017
0.010
0.000
0.003
0.006
0.006
0.006
2.90
1.71
0.115
1.002
0.167
0.003
anthracene
3.46
0.202
2.26
0.132
(4H-cyclopenta[def]
phenanthrene)
(methylphenanthrene)
(methylanthracene)
fluoranthene
(acephenanthrylene)
(aceanthrylene)
benzo[b]fluorene
soot - assumed
as all carbon
C15H12
C15H12
C16H10
C16H10
C16H10
C17H12
0.008
0.009
0.036
0.038
0.028
0.035
0.003
0.001
0.001
0.125
0.171
0.171
0.069
0.009
0.069
4.92
0.026
0.051
0.168
0.016
0.023
0.002
4.11
0.010
0.289
0.037
0.104
0.167
0.007
0.023
4.06
7.64
a Average of triplicate at 825 °C. b Parentheses denote compounds identified on the basis of published mass spectra.

Hydrocarbon Growth from Cyclopentadiene
J. Phys. Chem. A, Vol. 114, No. 47, 2010 12413
Filtered samples were analyzed by gas chromatography
coupled with mass spectroscopy (GC/MS). The GC column
employed was a HP-5MS capillary column with 30 m × 0.25
mm with a 0.25 µL film of cross-linked 5% siloxane. Occasion-
ally, samples underwent the evaporative concentration process
with nitrogen for better identification and detection of products
with low concentrations. The GC column oven temperature was
programmed as follows: an initial hold time of 2 min at 32 °C,
32 to 250 °C at a rate of 5 °C/min, 250 to 280 °C at a rate of
8 °C/min, and a final hold time of 3 min. Due to the high
volatility of CPD and its coelution with the solvent, CPD
concentrations were separately measured from gas samples by
direct injection of the gas samples into the GC-MS column.
Measurements of CPD in the gas sample were in triplicate at
600 °C.
Products were identified mainly by comparing the MS spectra
and retention times with data for commercially available
chemical standards including cyclopentene, 1,3-cyclohexadiene,
1-methylcyclopentadiene, 1-, 2- and 3-methylindene, 1,2-
dihydrofulvalene, and naphthalene. Response factors were
obtained through four dilutions of chemical standards. When
standards were not available, product structures were identified
on the basis of published MS spectra and elution order. Yields
of the observed products were calculated with the response
factors and peak areas expressed in a percent carbon input. For
Figure 2. Yields of benzene, indene, and naphthalene from CPD
pyrolysis.
DCM in the collection trap. The combined samples were filtered
with a polytetrafluoroethylene membrane filter of 0.25 µL pore
diameter by vacuum filtration to remove soot, defined as the
DCM-insoluble fraction. Soot yields were determined gravi-
metrically. For calculation of total carbon recovery, soot was
assumed to be pure carbon.
Figure 3. Ion chromatograms and mass spectra of selected C10H10 products from CPD pyrolysis at 775 °C. Dihydrobenzofulvene was not confirmed
with chemical standard.

12414 J. Phys. Chem. A, Vol. 114, No. 47, 2010
Kim et al.
the products without standards, yields were estimated using
response factors of compounds having similar structures.
Results and Discussion
Pyrolytic Product Yields. The yields of total carbon
recovery, aromatic compounds, CPD, and soot over the tem-
perature range 550-950 °C are shown in Figure 1 expressed
as a percent of carbon fed to the reactor. The yields of observed
products over the temperature range studied are reported in Table
1; soot was assumed to be pure carbon. Compounds enclosed
in parentheses in Table 1 were identified on the basis of
published mass spectra. Carbon recoveries of 80% and above
were obtained for temperatures up to 850 °C. Lower carbon
recoveries for temperatures higher than 875 °C are primarily
attributed to unrecovered soot that could not be rinsed from
the quartz tube wall. PAH product yields ranged from 2.7% of
carbon input at 600 °C to 82% at 850 °C. The yields of CPD
were greater than 85% up to 700 °C. Above 700 °C, CPD begins
to react in significant amounts with more than 90% of the CPD
reacted at 825 °C. Soot formation was observed starting at 850
°C and increasing with temperature.
The major products from CPD pyrolysis were benzene,
indene, and naphthalene, and their yields are shown in Figure
2. The amount of indene produced exceeded that of naphthalene
at temperatures lower than 775 °C. Between 775 and 900 °C,
naphthalene was the main product, and above 900 °C, benzene
formation becomes predominant. The high yields of benzene,
indene, and naphthalene (more than 90% of total product yield)
suggest that the self-recombination route of CPDyl radicals24
and/or the CPDyl radical addition to CPD route15,26-29 are
dominant reaction channels until 850 °C. Overall, carbon is
conserved in the conversion of 2 CPDs to naphthalene and 3
CPDs to indene and benzene. The latter route involves the
formation of methyl radicals; methane was not measured in these
experiments. At the lowest three temperatures studied (550, 575,
and 600 °C), yields of C6 compounds exceeded indene (C9)
yields by greater than a 9:6 ratio (Table 1); with yields expressed
on a carbon basis, a 9:6 ratio corresponds to carbon conservation
via 3 CPDs being converted to benzene and indene. Butler and
Glassman29 noted higher methane concentrations than expected
in CPD combustion experiments, consistent with our finding
of higher C6 product yields than expected at the lowest
temperatures. We did not measure methane in these experiments,
however. At 600 °C and below, product yields were very low
and impurities in the DCPD feed (99.9% purity) might account
for the excess in C6 product yields. At 725 °C and above, the
lack of full carbon recovery might be due to methane production.
The observation of high yields of benzene, indene, and
naphthalene from CPD pyrolysis and the crossover of indene
and naphthalene yields around 775 °C appear to be consistent
with the computational studies.15,26-28,31 However, the specific
intermediates leading to indene and naphthalene from CPD in
the proposed pathways were not observed over the temperatures
studied. Ion chromatograms and mass spectra of observed C10H10
compounds from CPD pyrolysis are shown in Figure 3. On the
basis of the elution times and mass spectra of available
standards, three of the observed C10H10 compounds were
identified as 2- and 3-methylindenes and 1,2-dihydronaphtha-
lene.
Figure 4. Yields of major aromatic products and their derivatives from
CPD pyrolysis.
not dihydrofulvalene (
), a precursor for naphthalene
formation from CPD in the proposed mechanism suggested by
Melius et al.24 The abundance order of principal m/z peaks
associated with this hydrocarbon (128 > 130 > 129) in the mass
spectrum is not consistent with the published order of principal
m/z peaks in a dihydrofulvalene (129 > 128 > 130).32 Moreover,
this hydrocarbon does not have a distinctive signature of the
dihydrofulvalene with a big half-ion peak at 65 m/z whereas
both 1,1- and 1,2-biindenyl containing a dihydrofulvalene moiety
have a distinguished half-ion peak at 115 m/z.33
Yields of benzene, indene, and naphthalene and their deriva-
tives from CPD pyrolysis are compared in Figure 4. Yields of
1,3-cyclohexadiene and another C6H8 compound, possibly
dihydrofulvene, were greater than the yield of benzene below
675 °C. Similarly, the ratio of 1,2-dihydronaphthalene to
naphthalene was highest at low temperatures and decreased with
increasing temperature. These results indicate that cyclohexa-
diene and dihydronaphthalene might be intermediates for
benzene and naphthalene formation, respectively. On the other
hand, indene yield was much greater than yields of methylin-
denes and dihydrobenzofulvene at low temperatures, suggesting
that these products might be formed from indene. Thus, our
results suggest that, in addition to the CPD-to-naphthalene
mechanism,24 naphthalene might be also formed from indene
via methylation followed by a ring expansion of methylindenes,
analogous to the conversion of the methylcyclopentadienyl
radical to benzene.34,35
Although one C10H10 compound, denoted as (c) in Figure 3,
was not identified by chemical standard, the mass spectrum and
elution time of this compound relative to methyl indenes
Minor products observed include toluene, styrene, fluorene,
phenanthrene, anthracene, and acenaphthylene (Table 1). Trace
amounts of xylene, phenylethyne, methyl- and ethylnaphtha-
suggested that this compound is dihydrobenzofulvene (
),

Hydrocarbon Growth from Cyclopentadiene
J. Phys. Chem. A, Vol. 114, No. 47, 2010 12415
Figure 5. Observed PAH growth from CPD pyrolysis.
lenes, and fluoranthene were also found at temperatures above
750 °C. Yields of most of the minor products, except acenaph-
thylene, reached a maximum between 825 and 850 °C and then
decreased. Phenanthrene reached a maximum at 850 °C with a
yield of 3.5% carbon feed and declined, whereas anthracene
reached a maximum at 825 °C with the yield of 3.3% and
declined. The maximum yield of fluorene was also achieved at
825 °C. Fluorene, phenanthrene, and anthracene have been
proposed to be products of the reaction between CPD and
indene,26,28 which is supported here by the fact that their yields
are proportional to the indene yield over the temperature range
studied.
Toluene, styrene, and other C8 species appear to be formed
from the reactions involving CPD, benzene, and acyclic
fragments. Unlike other minor products, acenaphthylene yield
keeps increasing as temperature increases. Acenaphthylene has
been suggested to be an important product supporting PAH
growth by the carbon addition.7,16 As naphthalene becomes a
dominant product and more acyclic fragments are available at
high temperatures, acenaphthylene is expected to be formed
from the addition of acetylene to naphthalene,11,12 which is
further supported by the observation of ethenylnaphthalene. The
observation of these products suggests that growth via carbon
fragments (e.g., the HACA mechanism) likely becomes an
important route for PAH growth at high temperatures.
PAH Growth from CPD: Comparison of Experimental
and Computational Results. Observed PAH formation path-
ways from CPD pyrolysis, including our previous CPD-indene
experimental results,26-28 are summarized in Figure 5.
Computational results suggest that the CPDyl radical recom-
bination route leading to naphthalene competes with the
intramolecular addition route of the CPDyl radical to CPD,
leading to indene at low and medium temperatures under
pyrolytic conditions where the contribution of routes involving
carbon fragments is expected to be low.15,31 With respect to the
naphthalene production from CPD, the calculated reaction rate
constants at the high-pressure limit indicate that the CPD-to-
naphthalene mechanism of Melius et al.24 is the major contribu-
tor to the naphthalene production at low combustion tempera-
tures (T < 1000 K) whereas an alternative C-C bond scission
route suggested by Wang et al.15 becomes important only at
higher temperatures.30,31
The observed high selectivity of indene, benzene, and
naphthalene and the observation of similar, though not identical,
intermediates as those proposed support the computational
results. Failure to observe the computationally predicted 4- and
7-methylindene and 9,10-dihydronaphthalene intermediates
might be due to rearrangement of these compounds to more
stable compounds that were observed; namely, 2- and 3-meth-
ylindenes and 1,2-dihydronaphthalene. The crossover of indene

12416 J. Phys. Chem. A, Vol. 114, No. 47, 2010
Kim et al.
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and naphthalene yields was observed around 775 °C, much
lower than that predicted by the computational studies.15,24,26-28
One possible explanation of this discrepancy is the suggestion
of Wang et al.15 that the inclusion of H abstraction reaction in
the calculation would greatly improve the formation rate of
naphthalene.
The loss of a methyl group in the CPD-to-indene pathway26-28
leads to methylcyclopentadiene formation, ring expansion to
form the observed cyclohexadienes, and then dehydrogenation
to form benzene.34,35 Similarly, naphthalene might also be
formed from indene via the observed methylindene and dihy-
dronaphthalene intermediates. Further PAH growth by addition
of cyclopentadienyl moieties was observed. Reaction of indene
and CPD produces fluorene, phenanthrene, and anthracene.26
Reaction of two indenes produces benzo[a]fluorene, benzo[b]-
fluorene, chrysene, benz[a]anthracene, and benzo[c]phenan-
threne.27 CPD-acenaphthylene reaction produces fluoranthene.16
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Conclusions
High selectivity for benzene, indene, and naphthalene prod-
ucts were observed in CPD pyrolysis over the temperature range
550-950 °C. A crossover of indene and naphthalene yields was
observed, as suggested in the computational studies. Although
not all of the intermediates expected on the basis of computa-
tional studies were detected, the experimental results support
the pyrolytic growth of PAH via CPDyl addition pathways at
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Acknowledgment. The support of the National Science
Foundation (collaborative research grant CTS-0210089) is
gratefully acknowledged.
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Supporting Information Available: A schematic diagram
of the experimental apparatus; axial temperature profiles of the
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