Importance of Fundamental sp, sp2, and sp3 Hydrocarbon Radicals in

May 14, 2012 - mass number intervals of 24, 26, or 14 indicating the role of the particular corresponding radicals, ethynyl (C2H), ethenyl. (C2H3), or...
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Importance of Fundamental sp, sp2, and sp3 Hydrocarbon Radicals in the Growth of Polycyclic Aromatic Hydrocarbons Bikau Shukla*,† and Mitsuo Koshi‡ †

Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California 90089-1453, United States ‡ Department of Engineering Innovation, The University of Tokyo, 7-3-1, Bunkyo-ku, Hongo, Tokyo, 113-8656, Japan S Supporting Information *

ABSTRACT: The most basic chemistry of products formation in hydrocarbons pyrolysis has been explored via a comparative experimental study on the roles of fundamental sp, sp2, and sp3 hydrocarbon radicals/intermediates such as ethyne/ethynyl (C2H2/C2H), ethene/ethenyl (C2H4/C2H3), and methane/methyl (CH4/CH3) in products formations. By using an in situ timeof-flight mass spectrometry technique, gas-phase products of pyrolysis of acetylene (ethyne, C2H2), ethylene (ethene, C2H4), and acetone (propanone, CH3COCH3) were detected and found to include small aliphatic products to large polycyclic aromatic hydrocarbons (PAHs) of mass 324 amu. Observed products mass spectra showed a remarkable sequence of mass peaks at regular mass number intervals of 24, 26, or 14 indicating the role of the particular corresponding radicals, ethynyl (C2H), ethenyl (C2H3), or methyl (CH3), in products formation. The analysis of results revealed the following: (a) product formation in hydrocarbon pyrolysis is dominated by hydrogen abstraction and a vinyl (ethenyl, C2H3) radical addition (HAVA) mechanism, (b) contrary to the existing concept of termination of products mass growth at cyclopenta fused species like acenaphthylene, novel pathways forming large PAHs were found succeeding beyond such cyclopenta fused species by the further addition of C2Hx or CH3 radicals, (c) production of cyclopenta ring-fused PAHs (CP-PAHs) such as fluoranthene/corannulene appeared as a preferred route over benzenoid species like pyrene/coronene, (d) because of the high reactivity of the CH3 radical, it readily converts unbranched products into products with aliphatic chains (branched product), and (e) some interesting novel products such as dicarbon monoxide (C2O), tricarbon monoxide (C3O), and cyclic ketones were detected especially in acetone pyrolysis. These results finally suggest that existing kinetic models of product formation should be modified to include the reported novel species and their formation pathways. It is expected that outcomes of this study will be useful to understand the products formation from reactors to interstellar atmospheres as well as the growth mechanism of carbon nanomaterials.

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hydrocarbons (PAHs) and soot that causes severe health hazards.8−10 One of the main causes of the inability to control both of these processes is incomplete understanding of fundamentals of hydrocarbon pyrolysis. Moreover, to date, it is accepted that the formation of PAHs resulting in soot is one of the major hurdles that is limiting the efficiency of engines as well as the purity and controllability of carbon nanomaterials. Hence, understanding the formation pathways of PAHs is still a challenge in combustion research11,12 and carbon nanomaterial synthesis1−7 as well as a key to understanding the interstellar

ydrocarbon pyrolysis is one of the most important chemical processes from a scientific, technological, economic, environmental, and health point of view. Major technological and economical advancements strongly depend on hydrocarbons pyrolysis. For example, most of the advanced carbon materials, such as carbon nanotubes,1 nanohorns,2 nanofibers,3 graphene,4 nanodiamonds,5 fullerenes,6 and polymers,7 are synthesized on a mass scale from hydrocarbon pyrolysis via a chemical vapor deposition (CVD) technique. However, it is hard to grow these materials with higher purity and with selective properties. Similarly, most automobile engines still use fossil fuels that contain a multitude of hydrocarbons and because of lower efficiency of present engines a significant fraction of fuel is instantly pyrolyzed to produce environmental pollutants such as polycyclic aromatic © 2012 American Chemical Society

Received: March 8, 2012 Accepted: April 20, 2012 Published: May 14, 2012 5007

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chemistry of carbonaceous dust formation13−16 in galaxies. Thus, understanding hydrocarbon pyrolysis with special focus on gas-phase products formation pathways up to PAHs is inevitable. Over a century, enormous studies have been conducted on pyrolysis of hydrocarbons and their mixtures by combustion, atmospheric, and materials scientists. Still, it is considered as an extremely complex process because of the involvement of very active free radicals and the intermediate chemistry of which is difficult to understand. It is well-known that in the pyrolysis of most aliphatic hydrocarbons, C2Hx and CHx hydrocarbon radicals/intermediates, such as ethyne/ethynyl (C2H2/C2H), ethene/ethenyl (C2H4/C2H3), and methane/methyl (CH4/ CH3), are produced as major active species that further initiate and accelerate the formation of a wide variety of products ranging from small to large species including PAHs. Thus, it is very important to understand the role of these fundamental species in products formation up to PAHs. Certainly, extensive pyrolysis studies have been performed on these fundamental hydrocarbons, but most of them are limited to the determination of initial decomposition pathways and their rate constants. In general the gas-phase products identification studies on pyrolysis of acetylene17−25 and ethylene26−34 are limited to the products up to benzene (C6H6) while studies on pyrolysis of acetone (capable of producing methyl radical more easily than others)35−40 are limited to initial decomposition channels,35−37 rate calculations,38,39 and dissociation dynamics.40 To our knowledge the detection of large products has not been reported and neither has there been a comparison of the roles of C1 and C2 fundamental species in PAHs formation. Thus, observation of large PAHs and their formation pathways from these fundamental hydrocarbons is still in demand. Regarding the products formation pathways up to PAHs from aliphatic hydrocarbons, among several reported mechanisms hydrogen abstraction and acetylene addition (HACA)41−43 is widely discussed even though it has been commented as a slow mechanism to compete with the speed of PAHs and soot growth processes.44,45 This is also supported by kinetic models constructed based on the HACA, which generally over-/underpredicts especially the aromatic species concentration compared to experimentally measured values.33 It should be noted that, in this mechanism, produced large PAHs to be benzenoid (having only benzene rings) or nonbenzenoid/CP-PAHs (having benzene rings fused with cyclopenta rings) strongly depends on the fate of a particular growth pathway initiating from naphthalene. If phenanthrene is preferably produced as a result of two-step HACA from naphthalene, the further resulting products will be a benzenoid species, pyrene. Similarly, if acenaphthylene is the preferred product of HACA from naphthalene, further resulting product will be fluoranthene. Over 2 decades since the HACA mechanism was reported,41−43 quite frequently it has been discussed such as D’Anna and co-workers46,47 that the former HACA route proceeding from naphthalene efficiently forms a benzenoid PAH, pyrene via phenanthrene, while later route terminates at the formation of cyclopenta ring-fused products like acenaphthylene/pyracyclene. To our knowledge, there is no experimental or theoretical basis for such expectations. Moreover, recently Richter and Howard11 and Marsh and Wornat48a have reported that in the presence of naphthalene and C2H/C2H2 acenaphthylene is preferably produced from 1-naphthyl + C2H2 compared to 2-

ethynylanphthalene production from 2-naphthyl + C2H2. On the other hand, 2-ethynylnaphthalene can either be changed to 2,3-tetracyclonaphthalene or further grow into phenanthrene. Thus, formation of phenanthrene from naphthalene via HACA cannot be a dominant path. In this scenario, even though acenaphthylene is always observed to be produced significantly, further addition of C2Hx is completely ignored until now. Very recent work of Wang and Violi48b has indicated that HACA should not stop at acenaphthylene/pyracyclene. Rather, further products mass growth from acenaphthylene should be the preferred route of products formation. On the basis of the presence of the cyclopenta ring in the curved graphene layer present in the core region of soot49 and also fullerenes, there is a strong possibility of fluoranthene formation rather than pyrene from naphthylene by the HACA. Thus, it is necessary to clarify this issue. Motivated by the above-mentioned limitations, the main objectives of this study were to explore the roles of C2Hx and CH 3 species in products formation and also to look experimentally at the possibility of further HACA from acenaphthylene. For this purpose gas-phase products of the pyrolysis of acetylene, ethylene, and acetone (as it produces methyl radical more easily than others) were detected using an in situ direct sampling vacuum ultraviolet (VUV) single-photon ionization (SPI) time-of-flight mass spectrometry (TOFMS) technique. Detected gas-phase products include ethylene (m/z = 28) to large PAH (m/z = 324).

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EXPERIMENTAL APPROACH Details of the experimental setup have been described elsewhere.50 Briefly, the apparatus consists of a quartz reaction tube wrapped with a tungsten heater placed into a source chamber aligned with the linear TOFMS. A UV laser pulse (355 nm) was loosely focused into the frequency-tripling cell filled with xenon (pressure = ∼7.45 Torr) to generate VUV photons at 118 nm (10.5 eV). The UV beam was separated and removed by a LiF crystal prism in order to avoid the (1 + 1) ionization by 355 and 118 nm photons. It is worth mentioning here that VUV photon was found only blind to ionize ethylene (C2H4, IP = 10.51 eV) beyond its ionization energy of 10.5 eV. The 40% acetylene, ethylene, or acetone and 60% He were supplied through the mass flow controllers into the reaction tube at temperatures of ∼1100 to ∼1500 K and a constant pressure and residence time. The gaseous product molecules were continuously sampled through a pinhole and were collimated by a 1.0 mm orifice skimmer mounted at 3.0 mm from the pinhole. The molecular beam was introduced into the ionization region of the TOFMS, and the molecular species were ionized by the 118 nm photons. The ionized species were accelerated by the electrodes, electronic lenses, and deflectors through the field-free drift tube to the MCP (multichannel plate). Mass spectra were recorded at different temperatures with constant pressure and residence time. Observed mass spectra were normalized against NO signal intensity recorded at each step before sample introduction to the reactor.

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RESULTS

Mass spectra of gas-phase products of acetylene pyrolysis at temperatures of 1186−1476 K and constant pressure of 24.15 Torr with constant residence time of 0.69 s are shown in Supporting Information Figure S1, mass spectra of products of 5008

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Figure 1. Typical mass spectra of products of pyrolysis of (a) acetylene at 1403 K and pressure of 32.96 Torr showing the sequential addition of C2H/C2H2, (b) ethylene at 1278 K and pressure of 35 Torr showing the sequential addition of C2H3, and (c) acetone observed at 1320 K and pressure of 10.38 Torr with residence time of 0.59 s showing the sequential mass growth by CH3 additions (gray lines) and also by C2H3 radicals (black lines).

ethylene pyrolysis at temperatures of 1134−1485 K with a constant pressure of 12.33 Torr and residence time 0.69 s are shown in Supporting Information Figure S2, and mass spectra of products of acetone pyrolysis observed at temperatures of 1140−1320 K with a constant pressure of 10.38 Torr and residence time 0.59 s are shown in Supporting Information Figure S3. The most probable species have been assigned for the significant mass peaks. However, for the comparative study typical mass spectra of products of acetylene, ethylene, and acetone showing sequential mass peaks at regular mass number intervals are shown in Figure 1. Particularly, Figure 1a includes the sequences of species at regular interval of mass number 24 produced from acetylene pyrolysis at 1403 K and 32.96 Torr, Figure 1b shows sequences of species at the interval of mass

number 26 produced from ethylene pyrolysis at 1278 K and 35 Torr, and Figure 1c contains the sequences of products at interval of mass number 14 produced from acetone pyrolysis at 1320 K with a constant pressure of 10.38 Torr. The most probable species assigned for the products of acetylene, ethylene, and acetone pyrolysis observed in Supporting Information Figures S1−S3 and Figure 1 are listed in Table 1. Observed remarkable points in these figures (Supporting Information Figures S1−S3 and Figure 1) are the following: (1) The number of products increased toward higher masses with increasing temperature, and detected products include small aliphatic to the large PAHs up to m/z = 324 amu (Supporting Information Figures S1−S3). (2) Products of acetylene pyrolysis show a clear trend of mass peaks at equal mass 5009

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of ethylene pyrolysis, there is the presence of some methyl radical contributed products that include small aliphatic products to the large methyl-substituted PAHs such as propene (m/z = 42) produced from C2H3 + CH3, toluene (m/z = 92) product of C6H6 + CH3, and methylnaphthalene (m/z = 142) a product of naphthalene + CH3, whereas these methylsubstituted species were not observed in acetylene (ethyne,C2H2) pyrolysis products. This observation indicates that CH3 radical is also produced from vinyl radical most probably by reaction R1.

Table 1. Species Detected during Pyrolysis of Acetylene, Ethylene, and Acetone

C2H3 + C2H3 = CH3 + C3H3

(R1)

(7) The most remarkable point in the mass spectra of acetone products (Supporting Information Figure S3 and Figure 1c) is the observation of dicarbon monoxide (C2O) at m/z = 40 and of tricarbon monoxide (C3O) at m/z = 52, whereas observation of the same peaks in ethylene/acetylene are contributed by propyne and vinylacetylene. (8) The strongest mass peak for C4H2 (m/z = 50) from acetylene and the weakest from acetone while the strongest mass peak for C4H4 (m/z = 52) observed from ethylene may be due to contribution of both CH2CCCH2 and HCCCHCH2. (9) In the case of acetone pyrolysis, peaks observed at m/z = 56 and 54 correspond to cyclic ketones formed by hydrogen abstract/elimination from acetone followed by cyclization and further hydrogen elimination. Assignment of Species for the Mass Peaks of Acetylene Pyrolysis Products (Supporting Information Figure S1 and Figure 1a). In the mass spectra of gas-phase products of acetylene pyrolysis, a clear and regular sequence of mass peaks can be seen at mass number interval of 24, such as at m/z = 78, 102, 126, m/z = 128, 152, 176, m/z = 202, 226, 250, and so on. This regular sequence corresponds to hydrogen abstraction (− H) and acetylene addition (+ C2H2). In the beginning at 1186 K, the major product peak at m/z = 52 is most probably vinylacetylene with a weak peak at m/z = 78 (benzene). With increasing temperature up to 1347 K diacetylene (m/z = 52) appeared and dominated over vinylacetylene as the major product. At higher temperatures (>1347 K) triacetylene (m/z = 74, C6H2) and tetra-acetylene (m/z = 98, C8H2) were observed, indicating that polymerization of acetylene is favored only at high temperatures. Assignment of Species for the Mass Peaks of Ethylene Pyrolysis Products (Supporting Information Figure S2 and Figure 1b). Similarly to acetylene pyrolysis, in the pyrolysis of ethylene a sequence of products were observed at a regular interval of mass number ∼26/27 such as m/z = 78, 104; 92, 116; 102, 128, 154; 152, 178, 204; 202, 228, and so on. These sequences of mass peaks correspond to hydrogen abstraction and C2H4/C2H3 addition. At low temperatures the observed products seemed to be largely contributed to by methyl radical. Briefly, at low temperatures (1400 K), once again a change of major product from vinylacetylene (m/z = 52) to diacetylene (m/z = 50) together with the growth of polyacetylenes was observed. Assignments of Mass Peaks of Products of Acetone Pyrolysis (Supporting Information Figure S3 and Figure 1c). At low temperature (1140 K), the products are limited to

number intervals of 24, corresponding to stepwise addition of ethynyl/ethyne (C2H/C2H2) (Figure 1a). (3) Products of ethylene pyrolysis show a clear trend of mass peaks at regular mass number intervals of 26, corresponding to stepwise addition of ethenyl/ethene (C2H3/C2H4) (Figure 1b). (4) Products of acetone pyrolysis show a clear trend of mass peaks at regular mass number intervals of 14/12, corresponding to stepwise addition of methyl/methane CH3/CH4 together with ethenyl (C2H3) addition products at mass number intervals of 26 (Figure 1c). (5) At high temperatures (>1300 K), in both cases of acetylene and ethylene pyrolysis, there is observation of polyacetylenes such as triacetylene (m/z = 74) and tetracetylene (m/z = 98). (6) Similar to acetone, in the case 5010

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C2−C6 species, whereas with increasing temperatures the number of species increased up to m/z = 252. Moreover, typical mass spectra of products (Figure 1c) show clear signals at regular interval of mass number 14/12 corresponding to Habstraction and CH3 addition like m/z = 78, 92, 104, 116, 128, 142, 154, and so on. Ketene (m/z = 42) was observed as major product up to 1236 K, whereas at 1320 K benzene (m/z = 78) became the major product (Supporting Information Figure S3). The most unique feature of these mass spectra is the detection of dicarbon monoxide (C2O, m/z = 40), tricarbon monoxide (C3O, m/z = 52), cyclic ketones, and of course a small peak for acetyl radical at m/z = 43.

the standard reference (0 kcal/mol) because other isomers (II− IV) were found more stable compared to isomer I. Moreover, among these isomers qualitatively isomer II is the most stable one and thus seems to be the preferred HACA product of acenaphthylene. This speculation is strongly backed up by the very recent theoretical study by Wang and Violi48 in which they have reported that the 1,2 double bond in acenaphthylene is much more reactive than the 3,4 and 4,5 aromatic bonds. On the basis of this, it is easy to understand the formation of fluoranthene, an isomer of pyrene, from naphthylene by reaction R2. Following the formation of fluoranthene the

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DISCUSSION Considering the main purpose of this study to explore the roles of fundamental C1 and C2 radicals in the growth of large products including PAHs and subsequently to check the possibility of further product formation from acenaphthylene on addition of these radicals, in this section the products growth pathways forming fluoranthene/corannulene from naphthalene via acenaphthylene by the sequential addition of C1 and C2 radicals will be discussed. However, considering the page limitation and also the involvement of well-known reaction pathways in the formation of smaller products up to benzene, initial decomposition channels together with the formation mechanism of smaller products have been explained in the Supporting Information. It should be noted that the reaction pathways discussed here are solely based on the observed sequences of mass peaks, that is m/z ratio. Growth Pathways of Large PAHs (Larger Than Naphthalene). In the case of acetylene pyrolysis, two sequences of mass peaks were observed at the regular interval of mass number 24, such as mass peaks at m/z = 128, 152, 176 and at m/z = 202, 226, 250 (Supporting Information Figure S1 and Figure 1a), and they are ascribed to be the PAHs produced by the addition of ethynyl (C2H) radical (EAM)15,16 or by the HACA.41−43 As the growth of all these species seemed to be started together at 1347 K, there should a close correlation in their growth process. The well-known species for m/z = 128 and 152 are naphthalene and acenaphthylene11,48a produced by two-step EAM/HACA. Similarly, to date the reported candidate for m/z = 202 is pyrene formed from phenanthrene (m/z = 178) by the HACA. But unfortunately, the presence of a mass peak at m/z = 202 and absence of a mass peak for phenanthrene (m/z = 178) in Supporting Information Figure S1 and Figure 1a clearly indicate that the dominant candidate for the mass peak at m/z = 202 should not be pyrene. On the other hand following the sequence, the presence of a mass peak just before m/z = 202 at m/z = 176 having 26 mass number difference suggests that the most probable candidate for m/z = 202 can be fluoranthene produced from acenaphthylene via a species (m/z = 176) by the addition of acetylene (ethyne, C2H2). Although until now it is accepted that mass growth by HACA stops at acenaphthylene formation,46,47 based on the observation of a mass peak at m/z = 176 just next to acenaphthylene (m/z = 152) and its correlation with m/z = 202 because of their mass difference by 24/26, it is clear that further growth from acenaphthylene via HACA is possible to form large PAHs.However, as there are four possible sites available with acenaphthylene for the attachment of C2H2/C2H there are four possible candidates I−IV (as shown above with their energy calculated by B3LYP/6-31G (d,p)) for m/z = 176. The energies of these isomers are calculated considering isomer I as

consecutive species for the mass peaks at m/z = 226, 250 should be most probably benzo[ghi]fluoranthene and corannulene produced by the HACA, as HACA is efficient for filling triple fusing (4 Cs bay structure) sites50 and can be shown by reaction R3.

However, in the case of ethylene pyrolysis products (Supporting Information Figure S2 and Figure 1b), besides some peaks for alkyl-PAHs produced from the reaction of PAH + CH3, one can see a very interesting trend of mass peaks at regular mass number interval of 26 corresponding to hydrogen abstraction followed by vinyl radical addition (+ C2H3/− H), called herein after as HAVA. In some cases the resulting product is associated with −2 mass number product that further continues the sequence of HAVA. It is important to note that with increasing temperature initially observed mass peaks diminished with simultaneous enhancement in −2 mass number peaks indicating that the former is changed to the latter by hydrogen elimination. The observed products mass number sequence can be shown as m /z = 128, 154 ( −2 = 152) 178, 204 ( −2 = 202) 228 ( −2 = 226) 252 ( −2 = 250) ... 5011

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Appearance of a mass peak at m/z = 154 associated with a −2 mass number peak at m/z = 152 (acenaphthylene) only in the case of ethylene pyrolysis indicates clearly that acenaphthylene is not produced here by the direct HACA route from naphthalene; rather it is produced from a species with mass 154 by hydrogen elimination. Thus, at first it is important to know the most probable candidate for m/z = 154. On the basis of the presence of sufficient C2H3/C2H4 in the reaction mixture, the most probable candidate for the mass peak at m/z = 154 that was the next sequential peak after naphthalene (m/z = 128) should be acenaphthene (produced from naphthalene + C 2 H 3 , by reaction R4) which can easily produce a

pyrolysis of C2Hx revealed that mass growth accelerated by C2 radical/neutral species dominantly results in the formation of especially externally and internally cyclopenta fused species (CP-PAHs) like acenaphthylene, fluoranthene, benzo[ghi]fluoranthene, and corannulene. Exactly the same information about the formation routes of PAHs was obtained from the analysis of acetone pyrolysis results. Products of acetone pyrolysis also include many alkylPAHs, especially methyl-PAHs such as methylnaphthalene (m/ z = 142), methylacenaphthylene (m/z = 166), methylfluoranthene (m/z = 216), and methylbenzo[ghi]fluoranthene (m/z = 240), and support the presence of sufficient CH3 radicals in the reaction mixture and hence the observation of mass peaks at regular sequence of mass number 14 (Supporting Information Figure S3, Figure 1c). Thus, the observed species must be produced by the active role of CH3 radicals. In the meantime, it is remarkable that ethenyl/ethynyl-PAHs were observed significantly in comparison to ethyl-PAHs (Figure 1c). It suggests that ethyl-PAHs produced by the consecutive addition of methyl radical to the methyl-PAHs are thermally less stable, which explains why they are easily converted to ethenyl/ethynyl-PAHs which may subsequently converted to more stable cyclic products through hydrogen eliminations. This conversion pathway of ethyl-PAHs to ethenyl/ethynylPAHs or CP-PAHs shown in reaction R6 must be taken into

corresponding −2 mass species acenaphthylene by the hydrogen elimination. In the same way like reaction R2 and on the basis of energy of the isomers I−IV, the most preferred candidate for a mass peak at m/z = 178 should be 1ethenylacenaphthylene that can easily react with C2H3 radical to form dihydrofluoranthene (m/z = 204) which further changes to fluoranthene (m/z = 202) by hydrogen elimination. This whole reaction sequence can be shown by reaction R4. Considering the presence of a mass peak for m/z = 66, most probably cyclopentadiene, and indene (m/z = 116), a small contribution of phenanthrene (produced from C9H8 + C5H5) in the mass peak m/z = 178 and of pyrene (produced from phenanthrene via HACA) in the mass peak m/z = 202 cannot be ruled out. Following the same pattern of HAVA, assigning the candidates for mass peaks at m/z = 228, 254; 226, 250 and their formation routes are not difficult. Briefly, fluoranthene (m/z = 202) on addition of C2H3 forms dihydrobenzo[ghi]fluoranthene (m/z = 228) which via hydrogen elimination converted to benzo[ghi]fluoranthene (m/z = 226) that on further addition of C2H3 produces dihydrocorannule (m/z = 252), and this resulting product finally changes to corannulene (m/z = 250) by the subsequent hydrogen elimination by reaction R5. These observed mass sequences in the products of

account to understand the growth mechanism of PAHs accelerated by methyl radicals in the following explanations. Regarding the growth of large PAHs initiating from naphthalene under the presence of sufficient CH3 radicals produced from acetone, the observed sequence of mass peaks at m/z = 128, 142, and 154 can be easily assigned to naphthalene, 1-methylnaphthalene, and acenaphthene (produced by hydrogen elimination from species with mass 156, 1-ethylnaphthalene) produced by reaction R7. Similarly, another sequence

of mass peaks at m/z = 152, 166, and 178 can be assigned to acenaphthylene, 1-methylacenaphthylene, and 1-ethenylacenaphthylene (produced from 1-ethylacenaphthylene by H2 elimination), respectively. Following the enhancement in a −2 mass number peak at m/z = 176 with suppression in growth of 1-ethenylacenaphthylene (m/z = 178) with increasing temperature, it can be expected that the former should be 1ethynylacenaphthylene produced by hydrogen elimination from 1-ethenylacenaphthylene. Production of all of these species can 5012

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Finally, in all cases of acetylene, ethylene, and acetone pyrolysis, the produced large species are dominated by CPPAHs including corannulene (m/z = 250) over benzenoid PAHs like pyrene. This does not mean that formation pathways carried by C1 and C2 radicals never form benzenoid species like pyrene. Of course, after the formation of phenanthrene by biphenyl + C2 or indene + C5, further growth via addition of C2Hx radicals result in pyrene-like products, but in the present case since phenanthrene was not observed as a preferred product, production of benzenoid species is rather hard. This indicates that contrary to the commonly expected soot precursor coronene/pyrene (without any experimental evidence), the final soot precursor might be a CP-PAH that initiates the nucleation of soot. The observation of corannulene-like internally fused cyclopenta ring in the curved graphene layers in soot particles in the core region49 also backs up the above speculation that corannule can be an essential component of the soot. In addition, corannulene is a fundamental unit of C60 fullerene. Formation Pathways for Products from Benzene to Naphthalene. In the case of acetylene pyrolysis products (Supporting Information Figure S1 and Figure 1a), there are two candidates for the mass peak at m/z = 126, benzenediethynyl produced from phenylacetylene by the HACA or naphthyne produced by benzyne + diacetylene. Although it is hard to specify the isomer from TOFMS, significantly high concentrations of benzene, phenylacetylene, and acetylene over benzyne and the regular mass sequence at m/z = 78, 102, 126 favor the former route (reaction R11). Even

be represented by reaction R8. Considering the presence of smaller peaks for indene (m/z = 116) and C5 species that can

combine to form phenanthrene, a minor contribution of phenanthrene in mass peak at m/z = 178 cannot be ruled out. Certainly it cannot be a dominant contributor, otherwise its methyl derivative methyl-phenanthrene should be observed at m/z = 192. Hence, further growth of phenanthrene into pyrene cannot be expected for the mass peak at m/z = 202. Thus, the next sequence of mass peaks at m/z = 202, 216, and 228 are most probably fluoranthene, methylfluoranthene, and dihydrobenzo[ghi]fluornathene (produced by hydrogen elimination from species with mass 230, ethylfluornathene) produced by reaction R9. By the same way, the last sequence

observed at m/z = 226, 240, and 250 can be assigned to benzo[ghi]fluoranthene, methylbenzo[ghi]fluoranthene and corannulene (produced by two steps hydrogen elimination from species with mass 254, ethylbenzo[ghi]fluornathene) formed by reaction R10. Among the mass peaks that

in the case of benzenediethynyl, considering the site preference for the attachment of acetylene (ethyne,C2H2) to phenylacetylene, there are three possible candidates, o-, m-, and pbenzenediethynyl, among which o-benzenediethynyl is a less preferred product. This might be the reason that mass peak at 126 is dominant compared to naphthalene (128) because naphthalene is only produced from o-benzenediethynyl.16,17,46,47 The presence of a significant mass peak for benzenediethynyl compared to naphthalene also indicates that naphthalene must be produced by a non-HACA path from the reaction of benzene/phenyl and vinylacetylene/its radical by reaction R12. C6H6 /C6H5 + C4 H3/C4 H4 = C10H8 + H

(R12)

On the other hand, in the case of ethylene, the most probable candidates for mass peaks at m/z = 78 and 104 are benzene (C6H6) and styrene (C8H8), a product of C6H6 + C2H3. A mass peak at m/z = 102 should be phenyacetylene produced from styrene by the hydrogen elimination which on further addition of C2H3 results in naphthalene (m/z = 128). Production of these species can be shown by reaction R13. Formation of styrene from unsaturated aliphatic species such as vinylacetylene and 1,3-butadiene52 cannot be neglected.

correspond to methyl addition products, especially for aromatic hydrocarbons, some peaks such as 78 and 104; 102, 128 and 154; 152, 178 and 204; 202 and 228 also satisfy the sequence of 26 for vinyl radical addition (Supporting Information Figure S3, Figure 1c). Thus, production of these species seems to be contributed by the HAVA. 5013

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C6H5CH 2 /C6H5CH3 + C3H4 /C3H3 = C10H8 + H 2 + H (R18)

Formation of Dicarbon Monoxide, Tricarbon Monoxide, and Cyclic Ketones from Acetone. Because of the similarity in the mechanism of initial decomposition and the formation pathways of smaller products with other studies, they are explained in detail in the Supporting Information. The formation routes of dicarbon monoxide (C2O), tricarbon monoxide (C3O), and some cyclic ketones, that were observed most probably for the first time in the products of acetone pyrolysis, are discussed here. From the beginning of the pyrolysis of acetone, a significant mass peak was observed at m/ z = 40 and was found to be enhanced with subsequent reduction in ketene (m/z = 42) with increasing temperature. Finally, it dominated over ketene (formation of ketene can be found in the Supporting Information). It clearly indicates that the candidate for m/z = 40 must be produced from ketene via H2 elimination (reaction R19); thus, it should be dicarbon monoxide (C2O).

Different from acetylene and ethylene, in the case of acetone pyrolysis products, the sequential mass peaks at m/z = 78, 92, and 106 with mass number interval of 14 can be designated to be benzene, toluene, and ethylbenzene. On the other hand, subsequent appearance and domination of the mass peaks at m/z = 102 and 104 following the disappearance of ethylbenzene with increasing temperature (Supporting Information Figure S3) clearly indicates that ethylbenzene is thermally unstable and is converted to styrene and phenylacetylene via hydrogen elimination by reaction R14.

CH 2 = CO → :CCO + H 2

Similarly, the presence of mass peaks at m/z = 56, 54, 52 associated with the mass peak of acetone (m/z = 58) from the beginning of pyrolysis indicate that the most probable candidates for them should be the derivatives of acetone radical produced by hydrogen abstraction from acetone. They can be cyclic ketones, such as cyclopropanone and 2cyclopropene-1-one, and tricarbonmonoxide (C3O) produced by the sequential hydrogen elimination as shown in reaction R20. However, with increasing temperature appearance of a

Similarly, observation of a mass peak at m/z = 118 (at 14 mass number interval from styrene) associated with another mass peak at m/z = 116 (indene) and with increasing temperature disappearance of the former with enhancement in the latter peak suggest that the former should be omethylstyrene (1-ethenyl-2-methylbenzene) that is converted to indene via ring cyclization as shown in reaction R15.

Considering the presence of indene and methyl radical in the reaction mixture, formation of naphthalene (m/z = 128) in the acetone case might be dominated by the reaction of indene and methyl radical (reaction R16) over its formation from reaction

mass peak for diacetylene (C4H2) at m/z = 50 suggests that 2cyclopropene-1-one might be decomposed into acetylene and carbon monoxide, and acetylene so produced can form vinylacetylene via self-recombination; thus, the mass peak at m/z = 52 can be contributed to by both vinylacetylene (C4H4) and tricarbonmonoxide (C3O). This is also supported by the significant enhancement in the production of benzene (m/z = 28) as well as carbon monoxide (m/z = 28). Implications of These Results. As is well-known, all aliphatic hydrocarbons preferably pyrolyze via β-sessions. Especially alkanes finally produce vinyl radials as the main building block, whereas if the starting hydrocarbon contains an odd number of carbon atoms some methyl radicals are also produced. Only at high temperatures vinyl (ethenyl, C2H3) radical is converted to acetylene (ethyne, C2H2); thus, on the basis of the above discussion, an overview of products formation pathways carried by C1 and C2 radicals is presented in Figure 2. It clearly suggests that in general products formation in the pyrolysis of hydrocarbons at moderate temperature is dominated by the HAVA mechanism. However, at high temperatures either the products produced from the HAVA are converted to HACA products by hydrogen elimination or vinyl radicals are initially changed to acetylene

R12. Similarly, based on the presence of C5 species, formation of naphthalene by reaction R17 can also be expected. Formation of naphthalene from methylindene (reaction R16) is also reported by McEnally and Pfefferle53 and Lifshitz et al.54

2C5H5 = C10H8 + H 2

(R19)

(R17)

Considering the observation of significant mass peaks for propyne and toluene at m/z = 40 and 92, formation of naphthalene via reaction R18 also cannot be ignored completely. 5014

dx.doi.org/10.1021/ac3006236 | Anal. Chem. 2012, 84, 5007−5016

Analytical Chemistry

Article

mechanism in galaxies. Hence, the construction of kinetic models including the HAVA and modified HACA for better understanding the production of soot and atmospheric carbonaceous dust as well as carbon nanomaterials will be an interesting subject for future studies.

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CONCLUDING REMARKS In this study with the help of an in situ time-of-flight mass spectrometric analysis of gas-phase products of acetylene, ethylene, and acetone pyrolysis, the following have been deduced: (1) In aliphatic hydrocarbons pyrolysis, products formation strongly depends on the fundamental C1 and C2 radicals and dominantly followed by hydrogen abstraction with vinyl radical addition (HAVA), which means they involve some non-HACA pathways for the formation of products. (2) Large PAHs up to m/z = 324 were detected. (3) On the basis of the observed sequential mass peaks at the regular mass number interval of 24 in the case of acetylene and 26 and 14 in the case of ethylene and acetone, some new reaction pathways resulting in the formation of CP-PAHs were investigated. (4) Contrary to the existing concept of termination of HACA pathways at an acenaphthylene-like structure, the HACA-based formation pathways were also found proceeding beyond acenaphthylene to form large CP-PAHs via further addition of ethyne/ethynyl (C2H2/C2H) species. (5) Regarding the alkyl-substituted products, only methyl-substituted products were found to be stable, i.e., even ethyl-substituted products were found to be converted to ethenyl/ethynyl-substituted products at high temperatures. (6) Particularly, in the case of acetone pyrolysis, some new and interesting products like dicarbon monoxide (C2O), tricarbon monoxide (C3O), and derivatives of cyclic ketones were detected.

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Figure 2. Overview of products mass growth process by different radicals, namely, by vinyl radical (the bold arrows), ethynyl radical (medium arrows), and methyl radical (thin arrows).

ASSOCIATED CONTENT

S Supporting Information *

Temperature-dependent mass spectra of products of acetylene, ethylene, and acetone pyrolysis, explanation of their initial decomposition phenomena, and the formation pathways of smaller products up to benzene. References are common in the main text as well as in the supporting information, with the exception of ref 51, thus, please see main text for the references. This material is available free of charge via the Internet at http://pubs.acs.org.

which accelerates HACA pathways. Moreover, these results revealed important and novel pathways proceeding beyond cyclopenta fused PAH like acenaphthylene, resulting in the growth of CP-PAHs and by this way providing a new route in the existing HACA mechanism. Second, active roles of C1 and C2 species in products growth indicate that, especially in aliphatic pyrolysis/oxidation, they can readily add aliphatic branches on products to produce PAHs/soot with aliphatic chains, which have been recently observed in flames.55a,b Theoretical study has found that such PAHs with aliphatic chains can initiate soot nucleation rather faster.55c Third, some novel products like dicarbon monoxide (C2O), tricarbon monoxide (C3O), and cyclic ketones were observed in the case of acetone pyrolysis and large products up to PAHs (m/z = 324) were observed in all cases. Finally, novel reactions reported here for the formation of fluoranthene and other CP-PAHs from acenaphthylene are very important because they dominantly contribute in PAHs formation and until now these reactions have been overlooked. Thus, we strongly suggest that kinetic models should be updated including these new reactions, and hopefully modified models will be able to predict experimental results more closely, especially in fuel-rich conditions for which existing models are inadequate. Additionally, these results are useful for understanding the growth mechanism of fullerenes and other carbon nanomaterials as well as the carbonaceous dust formation

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AUTHOR INFORMATION

Corresponding Author

*Phone: 323-717-0016. Fax: 213-740-8071. E-mail: bikaushu@ usc.edu, [email protected]. Notes

The authors declare no competing financial interest.

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