Electronic Supplementary Information for

Importance of Fundamental sp, sp2 and sp3

Radicals in the Growth of Polycyclic Aromatic

Bikau Shukla1* and Mitsuo Koshi2

1Department of Aerospace and Mechanical Engineering University of Southern California, Los Angeles, California 90089-1453, USA

2 Department of Engineering Innovation, The University of Tokyo, 7-3-1, Bunkyo-ku, Hongo, Tokyo, 113-8656, JAPAN

* Corresponding Author: Bikau Shukla Department of Aerospace and Mechanical Engineering, University of Southern California Los Angeles, California 90089-1453, USA Phone: 323-717-0016 Fax: 213-740-8071 Email: [email protected], [email protected]

1. Results

Temperature dependent mass spectra of gas phase products of pyrolysis of , and

are shown in figures S1-S3. Briefly, mass spectra of products of acetylene pyrolysis at

temperatures 1186-1476 K and constant pressure of 24.15 Torr with constant residence time of 0.69s are

shown in figure S1. Mass spectra of products of ethylene pyrolysis at temperatures of 1134-1485 K with

a constant pressure of 12.33 Torr and residence time 0.69s are shown in figure S2. 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 0.59s are shown in figure S3. The most probable species have been

assigned for the significant mass peaks.

-0.032 C4H2 C6H2

-0.027

C3H4 C8H2 1476 K -0.022

1418 K -0.017

-0.012

1347 K Normalized Intensity/a.u. Normalized

-0.007

1274 K C4H4 -0.002 1186 K

30 50 70 90 110 130 150 170 190 210 230 250 270 m/z

Figure S1. Mass spectra of products of acetylene pyrolysis at five different temperatures and constant

pressure of 24.15 torr with residence time 0.69s. 2

Assignment of species for the mass peaks of acetylene pyrolysis products (figure S1): A clear and regular sequence of mass peaks can be seen at regular 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 corresponding to abstraction (-

H) and acetylene addition (+C2H2). In the beginning at 1186 K, the major product’s peak at m/z = 52, most probably vinylacetylene, together with a weak peak at m/z = 78 () were observed. With increasing temperature up to 1347 K (m/z = 52) appeared and dominated over vinylacetylene to be 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.

C4H2

C4H4 C2H4

1485 K

C3H4 1406 K

1347 K

1277 K

C5H6 1206 K Normalized Intensity/a.u. Normalized C4H6 1134 K

m/z

Figure S2. Mass spectra of products of ethylene pyrolysis at six different temperatures and constant pressure of 12.33 torr with residence time 0.69s. 3

Assignment of species for the mass peaks of ethylene pyrolysis products (figure S2): 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 . Briefly, at low temperatures (<1277 K), 1,3 butadiene (m/z =54) was observed as the major product with many methyl radical influenced products like , toluene, methylnaphthalene at m/z = 42, 92, 142. At moderate temperatures (1277- 1400 K), the major product was changed to vinylacetylene (m/z = 52) and enhancement in methyl radical initiated products was seen. At high temperatures (>1400 K), once again change of major product from vinylacetylene (m/z = 52) to diacetylene (m/z = 50) together with the growth of polyacetylenes was observed.

Assignments of species for the mass peaks of products of acetone pyrolysis (figure S3): At low temperature (1140 K), the products are limited to C2 to C6 species while with increasing temperatures the number of species increased up to m/z = 252. Moreover, clear signals at regular intervals of mass number 14/12 like m/z 78, 92, 104,116, 128, 142, 154, and so on. (m/z = 42) was observed as major product up to 1236 K while at 1320 K benzene (m/z = 78) became the major product (Figure S3).

The most unique feature of this mass spectra is the detection of dicarbon monoxide (C2O, m/z = 40) produced by the abstraction of hydrogen from ketene (H2C=CO, m/z = 42), (C3O, m/z = 52), cyclic ketones and of course a small peak for acetyl radical at m/z = 43.

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C4H4

C=C=O

2

:C=C=O H

C4H6 1320 K

:C=C=C=O 1236 K

3

CCOCH

Normalized Intensity/a.u. Normalized 3

CO H

1140 K

m/z

Figure S3. A typical mass spectra of products of acetone pyrolysis observed at temperatures 1140-1320

K and pressure of 10.38 torr with residence time of 0.59s.

2. Discussion

Since the growth mechanism of large products above benzene have been explained in the main text, only the initial decomposition phenomena together with the growth pathways of smaller products are discussed here.

Mechanism of initial decomposition and formation pathways of smaller products up to benzene: In the case of acetylene pyrolysis, observed mass spectra (figure S1) clearly indicate that below 1300 K the self-decomposition rate of acetylene was extremely slow while above that it became faster showing

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21 consistency with Duran et al. Similar to several studies, two different major products; C4H4 at low temperature (<1300 K) and C4H2 at high temperatures (>1300 K) were observed. Appearance of vinylacetylene (C4H4) as a major product at low temperature indicates that it is produced by the direct dimerization of acetylene (SR1) as reported by Kieffer et al.25

C2H2 + C2H2 → C4H4 SR1

Appearance of benzene together with vinylacetylene from the beginning of acetylene pyrolysis indicates that benzene is formed from an unsaturated aliphatic products of C2H2 and C4H4 by the ring cyclization via reaction (SR2).24 . C H + C H → . Cyclization 1, 5 shift 4 4 2 2 . or . . . SR2

At high temperature, observation of diacetylene (m/z = 50) as the major product in addition to vinylacetylene indicates that the most probable candidate for a weak peak at m/z = 76 should be benzyne produced from C4H2+ C2H2. Existence of benzyne backs up its contribution in the formation of benzene via a secondary route.

Contrary to acetylene pyrolysis, thermal decomposition of ethylene was initiated by the self- decomposition rather than dimerization. In the beginning at 1134 K, the major peak was observed at m/z

= 54 assigned to be 1,3-butadiene formed via reactions (SR3-6) indicating that thermal decomposition of

30,31 ethylene is initiated by the formation of vinyl radical (C2H3).

C2H4 + M → C2H3 + H + M SR3

C2H4 + H → C2H3 + H2 SR4

C2H4 + C2H3 → 1,3 - C4H6 + H SR5

C2H3 + C2H3 → 1,3 - C4H6 SR6

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With increasing temperature, at 1206 K observation of a peak at m/z = 52, most probably vinylacetylene can be produced from 1,3-butadiene by hydrogen elimination or abstraction (SR7-9) as reported by

Hidaka et al. 34 such as,

1,3 - C4H6 + H → 1,3 - C4H5 + H2 SR7

1,3 - C4H5 → C4H4 + H SR8

1,3-C4H6 → C4H4 + H2 SR9

Similarly observation of mass peaks at m/z = 42 and 40, most probably propene and , are produced by reactions (SR10, SR11).

C2H3 + CH3 → C3H6 SR10

C3H6 → C3H4 + H2 SR11

Like in the case of acetylene, benzene (m/z = 78) was observed from the beginning of pyrolysis together with 1,3 butadiene indicating its production by the Diels-Alder reaction51 of 1,3-butadiene addition to ethylene via the formation of cyclohexene (SR12, SR13).

1,3-C4H6 + C2H4 → C6H10 SR12

C6H10 → C6H6 + 2H2 SR13

Based on the presence of other radical/neutral species, the contribution of other possible routes (SR14-

SR18) forming benzene cannot be ignored,

C4H4 + C2H3 → C6H6 + H SR14

C4H5 + C2H3 → C6H7 + H SR15

C6H7 → C6H6 + H SR16

C3H3 + C3H3 → C6H6 SR17

C3H4 + C3H3 → C6H6 + H SR18

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The observation of a mass peak at m/z = 66 can be cyclopentadiene produced by the reactions SR19 and

SR20.

C3H3 + C2H3 → C5H6 SR19

C3H4 + C2H3 → C5H6 + H SR20

About the initiation of acetone decomposition, similarly to ethylene pyrolysis, acetone pyrolysis was initiated via self-decomposition. Based on the observation of a mass peak at m/z = 43 for which the most probable candidate is acetyl radical, and observation of a peak for the major product at m/z = 42 that should be ketene (CH2 = CO) produced from acetyl radical by hydrogen elimination, it is quite clear that acetone decomposition was initiated via reaction SR21 into acetyl (CH3CO) and methyl (CH3) radicals together with hydrogen abstraction reaction SR22, showing consistency with other studies.35-37

On the other hand our results also satisfy the further conversion of resulting acetyl/acetone radicals into ketene via reactions SR23-SR2436 as well as decomposition of a fraction of acetyl radical into CO through reaction (SR25).35,37

CH3-CO-CH3 → CH3CO + CH3 SR21

CH3-CO-CH3 + CH3/ H → CH3-CO-CH2 + CH4/ H2 SR22

CH3-CO-CH2 → CH2 = CO + CH3 SR23

CH3CO → CH2 = CO + H SR24

CH3-CO → CO + CH3 SR25

Thus, a mass peak at m/z = 28 that must be contributed dominantly towards by monoxide compared to ethylene, because ethylene can be only produced from methyl radical (produced from acetone) via ethane production and subsequent elimination of hydrogen, but a mass peak for ethane was not observed. This suggests that a small fraction of acetone was decomposed into methyl radials and

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. But at high temperature contribution of reaction SR25 can be considerable resulting in the contribution of ethylene in mass peak at m/z = 28.

A significant mass peak at m/z = 40 from the beginning of pyrolysis at 1140 K and its enhancement with subsequent reduction in ketene with increasing temperature and finally its domination over ketene clearly indicates that it must be produced from ketene by the elimination of hydrogen (SR26), thus it should be dicarbon monoxide (C2O)

CH2 = CO → :C = CO + H2 SR26

Another interesting point is the observation of some cyclic ketones. 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 at 1140

K indicate that the most probable candidates for those mass peaks should be the derivatives of acetone radical produced by hydrogen abstraction from acetone by the reaction (R43). These derivatives of acetone radical should be cyclic ketones such as cyclopropanone and 2-cyclopropene-1-one and tricarbon monoxide (C3O) produced by the sequential hydrogen elimination as shown in reaction

(SR27). But presence of a mass peak at m/z = 50 with increasing temperature indicates that 2- cyclopropene-1-one can be decomposed into acetylene and carbon monoxide. Acetylene so produced can produce vinylacetylene via dimerization, thus mass peak at m/z = 50 can be contributed to by both vinylacetylene and tricarbon monoxide (C3O). This is supported by the significant enhancement in the production of benzene (m/z = 28) as well as carbon monoxide (m/z = 28).

CH3 CH2 CH SR27 -H -H O -H O 2 O 2 C C C :C=C=C=O CH2 CH2 CH 52 56 54 - CO C H C H + 2 2 2 2 C4H4 52

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For the formation of benzene, appearance of C5 species at m/z = 66 and 68 and another peak at m/z = 80 suggest the formation of benzene by the reaction of cyclopentadiene and methyl radicals by the following reaction (SR28).

CH3 CH2. CH2 C +H /-H2 + CH3 +H /-H2 - H 1.

CH3 - H 2. + CH3 . SR28

On the other hand, presence of a diacetylene peak at high temperature with enhancement in benzene suggest that benzene is also produced by the reaction of C4 and C2 species like in the case of acetylene and ethylene (SR2, SR16-18).

Growth of Polyacetylenes- In both cases of acetylene and ethylene pyrolysis at high temperatures significant enhancement in peaks at m/z = 50 (diacetylene) and 74 (triacetylene) and appearance of a new weak peak at m/z = 98 (tetra-acetylene) support the polymerization of acetylene (SR29, SR30).

C4H2 + C2H2 → C6H2 + H2 SR29

C6H2 + C2H2 → C8H2 + H2 SR30

Particularly, in the case of ethylene pyrolysis observation of these species indicate that at high temperature conversion of ethylene/vinyl radical into acetylene is a dominant path way.

3. References:

As cited references are common in the main article and in the supporting information, for detail please see the reference section in the main text.

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