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Mechanism of soot initiation in methane systems
Prog. gnerg3 Combust. Sci. 1989. Vol. 15, pp. 273-285 Printed in Great Britain. All rights reserved
0360--1285/89 $0.00 + .50 iF 1989 Pergamon Press plc
MECHANISM OF SOOT INITIATION IN METHANE SYSTEMS M. WEISSMAN*~ a n d S.W. BENSONt * McDonnell Douglas Research Laboratories, St. Louis, Missouri 63166, U.S.A. "?Donald P. and Katherine B. Loker Hydrocarbon Research Institute, University of Southern California. Los Angeles, CA 90089-1661, U.S.A. Received 22 June 1989
Abstract--Thermochemical and kinetic evaluations of the very rapid elementary radical reactions consuming the C2H: produced in a chlorine catalyzed polymerization of CH4 are presented. An earlier examination of the data and mechanism leading to C2H2 supports a methyl and chloromethyl recombination path to C2 hydrocarbons. The relative yield of CH3 and CH~CI depends on the excess of methane. In the CH4 system consumption of C2 species to ultimately form benzene is shown to proceed by a stepwise addition of CH 3 radicals to C, Hm species. When n is even the dominant species is an unsaturated polyolefin molecule. When n is odd the dominant species is a conjugated, unsaturated radical such as allyl, pentadienyl, benzyl, etc. Mono-olefins or saturated molecules are rapidly stripped to these species by radical catalyzed dehydrogenations. In the current system chloromethyl radicals are equivalent kinetically to methyl and play a dominant role. Their addition to unsaturated species produces chlorinated radicals that dechlorinate rapidly or recombine with chloromethyl to produce dichlorohydrocarbons that dehydrochlorinate very rapidly. A very important reaction in the sequence is the isomerization of propenyl and chloropropenyl radicals to allyl and chlorallyl by a 1-3 H (or Cl) atom shift. Its high pressure Arrhenius parameters at 1300K are estimated to be log [k(sec-l)] = 13.7 - 37/0 where 0 = 2.303 R T in kcal/mol. It appears likely that benzene conversion to soot also proceeds via a CH3/CH:CI radical, sequential addition mechanism. Stoichiometry considerations applied to the product yield distribution support the role of methyl and chloromethyl predicted by the proposed mechanism. Ionic pathways are shown to be insignificant in the formation of aromatics. CONTENTS
1. Introduction 2. Mechanism of the Early Stages 3. Polymerization of C: Species 3.1. Polymerization induced by C2 radicals 3.2. Polymerization induced by CH3 3.3. Polymerization pathways involving CH2C1 4. Stoichiometry Considerations Regarding the Relative Role of CH3 and CH2CI 5. Modeling the Reaction 6. Ionic Pathways 7. Conclusions Acknowledgements References
273 274 276 276 277 280 281 283 284 285 285 285
flames: with the added assumption that the butadienyl radical arises from the addition of C~ H~ to C2 H.~ (Fig. The initiation of soot in the pyrolysis and combus- 1). In 1984, we published some experimental results on tion of hydrocarbons has been investigated from the point of view of radical, heterogeneous and ionic the pyrolysis of CH3CI, pure and with selected mechanisms. In their now, well-known study, additives3 near 1300K. The derived reaction Howard, Bittner, and Cole ~ found that the rates of mechanism was similar in many ways to the sootaddition of the butadienyl radical to acetylenes agree forming reaction sequence proposed in the combuswithin an order of magnitude with the rates of tion of CH4 .: Thus C2 H2 arises in the same succession formation of aromatic compounds in butadiene of steps following the recombination of 2CH3 flames. This mechanism was extrapolated to methane radicals. However, H-atom abstraction is dominated by C1 rather than by H or OH which has the advantage of simplifying the reaction mechanism. Current mailing address: University of Southern CaliforThe description of the reaction mechanism in our nia. Department of Chemical Engineering, Los Angeles, CA previous work 3 stopped at C:H: production since the 90089-1211. U.S.A. 273 I. INTRODUCTION
JPECS 15:4-A
274
M . WEISSMAN a n d S . W , BENSON
CzH 6
Polyocetyiene
"
C H,~'--'I=" C H 3"-~'~,"~H~ M'-~-'~" C.,Z H,R I CH3 i i
~ ~
---i=. | -r
Call3M.--M--~CzH2.---~,-a H_/~/'C3H'~ ~ ~ C , H 2 qE
o
CO
CO
CO
CO
~
(-~ f,
CO
FIG. 1. M e c h a n i s t i c p a t h w a y s f o r t h e f o r m a t i o n o f m o n o c y c l i c a r o m a t i c species in a f u e l - r i c h n a t u r a l g a s flame?
known pathways of aromatization or polymerization (through addition of C2H3 or C4H~ or abstraction of H and formation of C: H) were not fast enough to account for the consumption of C2 H 2 in those particular experimental conditions. Despite the crude character o f the data, some
features of the reaction, regarding alternate homogeneeus pathways of conversion of acetylene to benzene and soot became apparent on further examination of the system. Some of these results, presented in this paper, may have a wider applicability.
2. M E C H A N I S M O F T H E EARLY STAGES
Initiation
(M)
CH3CI. f
Buffer systems
t
~
CH3 +
C[
ti)
C[ + CH3C[
"
HCt
+
CH2Ct
CH3 + HOt
"
CH4 + Ct
(1) (2)
(M) 2CH3
) C2He
CaH6 +
(t)
Ct
a,
HC[
+
C2H,,
+
C2Hs
+
C~H 3
(M)
C:Hs Ct
+
C2H3
Termination steps and c ons e c uti ve reactions CH3 +
C2H4 m
~
CH4 +
CH2Ct.
= (M)
m
I,
C2HsCi HCt
+
(t') C2H4
C2H4Ct 2
(t")
C2HaCI.2
)
HCt
+
C2H3Ct
C2HaCI
m
HC[
+
C2H a
The overall reaction is not a chain so that the first step, the dissociation t f CH3 CI determines its overall rate of disappearance. As soon as even 1% of HCI is formed in the system, the HCI/CI reactions are so fast that they act as a buffer system leading to near equilibrium concentrations of all the hydrocarbon radicals in the system. In particular, they bring about an effective equilibrium between CH3 and CHzCI radicals, and CH4 and CH3CI, molecules: CH3Ct
H + C2H~
CH2Ct
2CH~CL
+
HCt
(M)
CaHsCt
CH3
H
•
(a)
Thus, (CH:Ct) (CH3)
(CH3Ct) =
Ka
(CH4)
=
~
.
Note that K~ = 1(1"1(2 since step (a) = step 1 + step 2. At the high temperatures of our study K, > 1 so that CH2CI tends to be the dominant radical in the system. Since A/-~ = 4.3 kcal, K, is not very temperature dependent. At 1300K, Ka = 4.0 using ff'(CH2CI) = 57.7 eu.
275
Soot initiation in methane systems The C2 product distribution is determined by the radical ratio and the termination rate constants. A b o v e 1000K, the terminations tend to become pressure dependent. This leads to a rapidly decreasing importance of step t relative to t' and t". The latter two steps have less pressure sensitivity than t but, more significantly, have rapid decomposition
wit
reaction. We thus expect to see little or no C2 H6 in our system when we start with a 2:1 ratio of CH4 to CH3C1. The radical concentrations are given by steady state
~CH,~
=
/
kt
J
Conversion (percent) 20 40 60 Time (s) 0.035 0.058 0.096 Specie - l o g C, (mol/1)
75 0.1,,4
CH 4 CH~CI HC1 H~ C,H 4 C2H,
2.51 2.94 2.76 3.46 3.76 3.84 4.43
2.56 3.19 2.71 3.23 3.77 3.81 4.48
6.3 7.9 7.8 8.9 8.7
6.4 7.9 7.8 9.0 8.5
6.1
6.4
2.41 2.53 3.13 3.95 3.95 4.12 4.73
2.46 2.73 2.91 3.73 3.81 3.91 4.73
H
6.1 8.1 7.8 9.1 9.0
6.2 8.0 7.8 9.1 8.9
C4H5 CH, C1
5.6
5.9
C6H 6
C4 H6 CH~ CI C2H 3
~5.2
~ 10.4
*k-"~ + ~ k t J
~2~ .
13)
U n d e r the conditions obtaining in our system where k, is effectively much smaller than k~, and k,, (CH3) reduces to:
I':""l' [~.
~CH~) -- I . ~ . 1
-~-~
l
(4)
while, --~
TABLE la. Radical concentrations (C~) derived assuming partial equilibrium of the buffer reactions (RH + Cl --, HCI + R). RH concentrations were measured. (CH4)o:(CH3CI) o = 1.1:l.0, 1310 K, Po = 0.887 atm.
~c,,ct~ = ,~c.,~ [ l . k, k,. ]-~
[ki,c,,ct)]~
1+~¢,
(5)
kt
L(CH4) kv _1
The rate of loss of CH3CI is then given by: -d(CH3CD dt
(6)
= (I+~)kjtCH~Ct)
where a is very close to unity in most of our studies. This value is deduced from our observation of product species which showed that in the absence of added CH4, some o f the CH3CI is converted to CH4 while in the presence o f excess CH4 there seems to be little or no net change in CH4 concentration. It was shown in Ref. 3 how kj was derived from the experimental decay rate o f CH3 CI taking into account the overall stoichiometry o f the reaction. The value was close to the calculated fall-off rate constant of the unimotecular decomposition, k~ was corrected as well for fall-off behavior using the curves published by Warnatz 4 (Table 1). Using the CH~ steady state radical concentrations derived above and the reaction: ct + CH4 ,- "
HCt + CH3
TABLE lb. Relative radical equilibrium concentrations Radical
CH 3
C: H 3
C4 Hs
q~
Cl
Relative concentration at 20% conversion 75%
1 1
10-17 10-t 4
10-4
10-3.0
10-2.0
10-29
10-4
10-2.6
10-1.5
10-2.~
Radical
CH=CHCH3
CH,-CH=CH,
CHzC] .C3H4C1 C3H4C1 C5H~C1:(1.5) (PrCI) (AllC1)
Relative concentration at 20% conversion
l 0 -4t
10 - t l
10 -05
10 -41
10 -18
10 -3.2
H
C5H7C1:(4.5)
10 -zs
Radical or molecule
C_, I"L CI
C 6 H 9 CI 2
C 6 H 9 C1
C4 H7 C1
C~ H 8C1
Relative concentration at 20% conversion
I0 -°°
10-' o
10-1.5
10-o.4
10-2.0
Radical or molecule
C4 H~
C6 H9 C1
Relative concentration at 20°/0 conversion
10- 04
I 0 - ~~
M, WEISSMANand S.W. BENSON
276
the CI c o n c e n t r a t i o n has been derived for several degrees o f conversion. Next, f r o m similar buffer reactions, the c o n c e n t r a t i o n s o f the o t h e r radicals were derived (Table 1). These e q u i l i b r i u m c o n c e n t r a tions were used in the following estimations. In T a b l e 2 are given the t h e r m o c h e m i c a l p a r a m e t e r s a n d in T a b l e 3 the kinetic p a r a m e t e r s o f the e x a m i n e d species a n d reactions. W h e r e e x p e r i m e n t a l d a t a were unavailable r e a s o n a b l e values were derived by g r o u p additivity a n d the e s t i m a t i o n m e t h o d s developed in Ref. 7.
3. POLYMERIZATION OF C2 SPECIES
3.1. Polymeri-ation Induced by C., Radicals As m e n t i o n e d earlier 7 the reactions that have been p r o p o s e d to lead to a r o m a t i c s from C~H., via vinyl radicals: /
x
c~,~ + C2H~
C,H,
(\ , ~ x , , ~ ' )
/'
C,Hs + C~H2
C~H,
~
)
~
'
(,)
(3)
TABLE 2. Thermochemical data for species considered A/-/7 in kcal/mol, S~' and Cp in cal/mo[
Compound C1 HC1 CH3C1 CH4 CH3 CH2CI C2H4 C2 H3 C.,H2 C2H propenyl-I allyl propylene butene-I buten-l-yl-3 butene- l-yl-4 butadiene
trans-pentene-2 1,3 pentadienyl-5 1,3 pentadiene penten-2-yl-I (trans) penten-2-yl-4 (trans) penten-2-yl-5 1,3 hexadiene-yl-5 1,3 hexadienyl-6 cyclohexenyl-3 1,3 hexadiene
trans-hexene-3-yl-6 trans-hexene-3 3 chloropropenyl- 1 (Pr-CI) 1 chloroallyl (All-C1) 1,4 dichlorobutene-I (C4H6CI 2 (1,4)) 3,4 dichlorobutene-I (C4 H6C12 (3,4)) 1,5 dichloropentene- 1-yl-3 (C5H7C12 (1,5)) 5 chloropentadiene- 1,3 (C 5H~ CI) 1 , 6 dichlorohexene-2-yl-4 (C6H9C12 (1,6)) 6 chlorohexadiene- 1,3 (C 6H 9CI) 5 chloropentene- l-yl-3 (C 5H eCI) 3 chloropropene-I 4,5 diehloropentene- 1-yl-3 (C 5HTCI2 (4,5))
A/~f 300 K
S~' 300 K
300 K
600 K
C~n( 73 900 K
28.9 22.0 19.6 17.9 35.1 29.1 12.5 66 54.2 127.0 61.1 41 4.9 0.1 33 48 26.5 - 7.9 53 18.2 27 24 38.0 44 60 30 13.4 34 - 12.7 56,3
39.5 44.7 56.0 44.5 46.4 57.7 52.4 56 48.0 49.6 64.9 62.1 63.8 73.5 71.5 75 68.2 83.7 74.6 76.5 83.0 82.0 85.7 87.9 89.7 73.2 86.3 94.4 90.5 74.4
5.2 7.0 9.8 8.5 8.8 9.29 10.3 0.9 10.6 8.9 14.9 15.0 15.5 20.6 20.0 20.2 18.8 25.8 23.3 24.1 25.0 25.2 25.4 27.6 28.5 24.7 29.2 30.5 30.9 17.60
5.4 7.1 14.6 12.5 11.5 12.6 16.9 15. I 14.0 10.7 23.3 24.1 25.8 35.3 33.8 33.5 29.7 44.0 39.7 41.1 42.7 42.5 42.4 47.0 46.5 47.2 50.6 51.8 53.5 25.90
5.3 7.4 18.0 16.1 13.5 14.7 21.2 18.0 15.8 11.9 29.0 29.7 32.7 44.5 42.1 41.6 37.4 55.8 47.7 50.0 53.4 53.4 52.9 58.4 57.7 59.9 61.8 64.7 67.6 31.02
5.2 7.8 20.3 18.7 15.1 16.1 24.2 19.9 17.2 12.7 33.4 33.8 37.1 51.1 47.5 47.4 42. I 64.3 53.6 56.7 61.1 61.1 60.4 66.4 65 68.0 70.2 74.0 77.8 34.36
5.2 8.1 21.7 20.6 16.2 17.2 26.2 21.3 18.3 13.3 35.8 36.0 40. I 54.7 48.3 50.3 45.4 68.7 56.8 60.3 65.2 65. I 64.3 70.4 69.5 7 l. 1 74.7 78.7 83.1 37.22
25,6
74.7
17.74
26.48
31.47
34.60
37.29
-
13.9
91.0
26.08
40.59
48.60
53.79
58.38
-
16,5
91.0
27.16
39.60
46.87
51.74
56.11
14,7
72.6
29.54
47.19
56.00
61.39
66.06
17.0
89.2
26.42
41.43
50.72
54.42
56.25
6,2
107.9
36.14
57.18
69.15
74.82
78.61
7,2
95.9
32.20
52.73
63.91
71.06
77.33
18.9
89.8
28.24
45.75
55.87
62.32
67.98
6.4
73.3
18.10
29.43
36.50
38.50
41.81
-
-
1200 K
1500 K
Soot initiation in methane systems TABLE3. Arrhenius rate parameters for reactions at 1300K. Estimations made with methods described in Ref. 7 N, REACTIONS
IogA~ M
-~S =
E
t
C2H3+CzH2 -- C4Hs
8.9
6.9
~H~÷HCt -- C,#6*CI
9.3
5.0
3
C4Hs+CzHz-- C,sH.,,
8.9
6.9
4
CH3 + C2Hz~ "~*
9.5
9.1
' 13.8
40.0
9.3
5,0
5
"~* ~ -- ~ + H
"~'. + HCI -- ~ '
7
~
8
"~.-
9
2 "~" -- C
t0
C+CI--
ti
"~" + C2H2 -- C "
+ Cl
+ CI -- @ + HCI ~
12 C "
~+HCl
~
15
CH~+ ~
14
CI+ (~-- H C I + ( -
-- (
15 cH 3 . ~ -
C
t5.7
27.9
12.4
45.0
t5.5
36.0
47
(~HzCl +AC3H 5 -
t0.0
0
10.7
0
13.3
40.0
43 C3HTCIz(4,51 -- (~5H7CI+Cl
44 ~ HzCI+ CsH?CI - - CHzCl- CH--2"~H- CHz-CH2CI t0,0
0
49
~HzCI+C4H 6 -- CsHoCI
8.7
4,0
o
50
C~HaCI -- C~H7 + CI
t5.0
55.4
9.0
4.0
to 3.o
9.2
2.4
22
~ -- ~.~ + H
14.0
45,0
23
(~ + C I - - ~ , ~ +HCI
tt.0
0
24 C H 3 + ~. -- ~,
10.7
0
25 ~'~ +CI -- ~ + HCr
II,0
0
26
~--~,~
f5.5
42.5
27
C -- ~[~
tL9
50.0
28
0
29
~ -- ~
11.O
O
13.5
29.0
9.5
t3.0
+ H
t5.4
26.0
c + @ - ~+Hc~ cH~. ~ - ~
11.o
1o3
o o
9.0
8.0
t3.7
30,0
5t
~H2CI+AC3H421 -- CI HeC- CHz-CH = CHCI
CH3+ ~".,~ C I -
53 ~
CI -- ~
0
t0.0
0
CI
+ CI
54 CI " v ' ~ " ~ CI -- CI ~
+CI
9,0
4,0
14.0
27.0
t4.0
27.0
Note: For some reactions that are not rate determining only approximate rate parameters were used
3.2. Polymerization Induced by CH~ The steady state concentration of CHa radicals is much larger than that of C2H3 in our system. In order for CH3 to account for the consumption of C2H2, we require a rapid addition of CH3 to C2 H2. In addition, the product of addition must have available further reaction pathways compatible with the experimental rate. The rate of addition of CH3 to C2H2 forming propenyl has been measured at low (300-500 K) temperatures: 5 Ca3 + C2H~
I0,0
CI
~
! 521 CHzCI + 'g~'NCI - - C [ - ~
( Pr -Cl)
56
C4HrCI
4~ C4H7CI - - Call6 + HCI
tto0
93 93
35 ~3H4C1-3 -- CHz- C'H - CHCI ( A l i - CI)
0
(C6H9C1~(1,5))
fo.4
(~ ( - "C( - C
34 ~.HZO+CzHz - C!CHz-CH : ~H
28.5
C6HgCI - - C6He+ HCI
48.0
32
13,7
45 C6HgC12(t.5) -- C6HsCI + CI
ts.5
~
28.5
46
( + H
~)
t4.O
(~,.~HtCI)
6.9
O
31 (~ -- ~
421CsH?CLz(I.5)-- HzC=CH-CH=CH=CH,zCI+ e l
0
48.0
50 CH3+ ~ - -
0
9.0
11.0
+ HCI
10.0
ft.0
t3,8
÷ X
~ H2C-"E"~--CH = CHCI- CH2CI (CsHTCLz (4, 5))
0
+ H
+ Cl -- ~
41
10.3
....~+ HCI
+ H
0
t,0
~
,9 ~ . ~ + 20 ~ + 2I ~ . , +
t0.0
(c~H~c12(1,51)
57.0
CI + LI "x-
(.
40 CH2CJ+ C4H5CI-- CIH~-CH2-CH--'~- CHCI
~t.0
t6 t8
REACTIONS
t5.7
t7
-- ~
TABLE 3. (continued)
kc oI/moI
2
6
277
-J~-.
.
(4)
(C4H6CIz(t , 4))
-- CHZ= CH-CHCI- CHzCI
37
(C,H6Zlz(l 2)) 58 C4HsCIzH,41 -- HCI - CaHsCl
f0.6
5t.0
C4H6CIz(!,2) -- HCI + C4HsCI
10.6
40,0
39
cannot account for the rate of consumption of C: H: in the described system because buffer reactions such as (2): Ct +
C,H,
C,)115 +
HCt
(-21
produce much too low concentrations of C, H5 and
C2 H3.
We have found, however, 6 that such additions may exhibit significant positive curvatures in Arrhenius plots due to their nontrivial heat capacity of activation] The rate constant at ] 300 K derived with the transition state model described 6 (] 07.8 M- ] sec- ~) predicts a rate of reaction compatible with the observed global flux of C from CH3CI to products. A factor of 0.5 due to fall-off behavior estimated by Dean m is within the accuracy limits of the estimates 00+°4). The propenyl radical can undergo back reaction ( - 4 ) , dehydrogenate to methylacetylene (5), hydrogenate to propene (6) followed by the dehydrochlorination of propene to allyl (7) or isomerize directly to ally] (8):
278
M. WEISSMAN and S.W. BENSON
(-4) s.
CH3
(5)•
+
~
+HOt ~,,r6~
k-4
C2H2
~
w.
1079 sec-I
+ H
ks = 1068$ec -1
+ Ct
k6 = 108'51/mol-sec; k6(HCt) = 105osec-I
(7) D ~ Ct
(8)
=
+ aCtk, ~ ks
/~:-.~
The fastest reaction is ( - 4 ) , leading to a nearequilibrium propenyl radical concentration of 10-1°'2M. Reactions (5) and (6) are minor; methylacetylene and propene yields were about 4% or less in preliminary runs in accord with the equilibria involved. In order for reaction (4) to be effective in consuming propenyl radicals, isomerization to allyl (8) would have to have a rate constant ks /> 107"6s~-1. It is generally believed that such isomerizations, involving the strain of creating a cyclic, 4-membered transition state:
lOl°.8;k,(Ct)=
1028sec -1
1077 Sec-1
The reverse reaction ( - 8) turns out to be the fastest yielding a concentration of allyl of 10-72M. The rate of recombination of 2 ~ (g) was found by Golden ~ to be equal to the rate of 2CH 3 which in this system species has a very low equilibrium constant (Kg) and, therefore, the product of recombination (9), the 1.5 hexadiene, is in too low concentration even when converted by C1 atoms to a stabilized radical (10). Its concentration is given by the equilibrium (9) since R _ 9 > RI0.
Although the addition of ~ to C: H2 seems to be an attractive pathway leading by isomerization (12) to a highly stabilized radical with the possibility of growing linearly by adding CH3, the thermo-dynamics are not favorable enough, R-ll > Ri2, '~.' Eq. (I1) = 10-~"SM, a n d ~ = 10-SM. This concentration is too small to make it effective in recombination reactions or further growth. The more favored, rapid cyclization (12') forms the stable cyclopentene-yl-4. This is followed by unimolecular loss of H and H formation of the stable cyclopentadiene. The concentration of the latter is limited though by the small concentration of pentadienyl radicals. H2C/~CH2 The recombination of ~ with CH3 is, however, competitive although the equilbrium concentration have prohibitively high activation energies. We have estimated the activation energy as being equal to the o f / ~ - is 10-72M while (CH3) is 10-62M because the strain of a cyclobutene ring (29.6 kcal/mol) + the larger termination rate constant compensates for this. intrinsic energy of H metathesis by vinyl from ethane This larger rate constant arises from a number of (7 kcai/mol). The frequency factor was estimated with factors. A factor of 2 arises from symmetry, another the methods described in Ref. 7 to be /> 1013"TSeC-l. factor of 2 from the fact that allyl has two active sites With these parameters the isomerization becomes while CH3 only one and a factor of about 2.5 arises competitive and, we can conclude that in general, 1,3 from the fact that CH3 + ~ will not be as far in the H-shift reactions from vinylic to allylic radicals, can fall-off as 2CH3. 1-Butene undergoes the back reaction ( - 13) faster be important at high temperatures. The following reactions of the allyl have been than the successive chlorine attack (14) which forms examined:
F i
[@ ...
1'
c--.]
(-8)
_
+
".
D
C2H2
I
+
~
CH3
(13)~ ~
(12')
_-'-
©
B
tv
+
H
@-"
÷ H +
RH .
Soot initiation in methane systems the stabilized l-butene-3-yl radical: (-13) ~o
~
(14)Ct
(
m
+
CH3
+
HC(
.
The equilibrium concentration, of butene-1 o f 10-65M, (calculated from g13 ) is sufficiently high to allow a competitive chlorination (14). We have neglected the formation of the unstabilized primary radical. The butene-l-yl-3 radical can reach relatively high concentrations as well, namely 10 -7.3 to 10 -7.8 (from reaction 14) but not as high as allyl. Its recombination with CH3 is probably not a major pathway of formation of 1-methylbutadiene (17). The dehydrogenation to butadiene (18) is faster even than the reverse reaction ( - 14) and therefore reaction (14) will not be equilibrium. Butadiene production is thus determined by step 14: (-14) +
HCt
+
CH 3
,~
15 -
~+
¢ C
~,
CH~
21
C
+ Ct
+
.
Unfortunately we have a measurement of butadiene in only one experiment; therefore, the estimations regarding the fate of butadiene have to be viewed with caution. Slightly higher values by a factor of 2 than the one used would make the fit really good. Reaction (21) is in equilibrium. The product, pentene-l-yl-3 is in too low a concentration to contribute via recombination with CH3; however, it competitively forms 1,3 pentadiene by step (22) in rather high c ~ncentration (10- ~4 M) sufficient for effective chlorine attack (23) to produce the highly stabilized, 1,3 pentadienyl-5 radical. The concentration of this radical is high, in comparison to CH3; their recombination rate should also be high producing 1,3 hexadiene in sufficiently high concentration (10 -6.6M) to permit effective chlorine attack (25) to form a stabilized 1,3 hcxadiene-5-yl radical. The concentration of the latter (10-84M) is not high enough to effectively recombine with CH3 but high enough to dehydrogenate to 1,3,5 hexatriene. This compound which is highly stabilized, could reach high equilibrium concentrations if its reactivity were not so
C[
16 C -
+
HCt
H.
The attractive pathway to benzene formation which starts with the addition of C2H 3 to butadiene (19) is not predominant here because of the tow concentrations of C, H3 and C4H6. The unimolecular reaction (18) is quite fast and allows large concentrations of butadiene to build up if reaction ( - 18) is the only sink for it. Butadiene concentration estimated from Kj~ is 10 -3.: M. The detected concentrations were however smaller by almost two orders of magnitude. This indicates the existence of additional very fast butadiene consumption routes. Production of C4H5 ( - 2 ) by chlorine attack is not significant because of the small (CI/HCI) ratio which maintains small C4H5 concentrations. The same is true for the addition of allyl (20). The addition of CH3 (21) is however a competitive route:
•+"
279
high. In a succession of rapid reactions (27-29) it forms benzene: "
C
+
"C
Ct
+ CH3
-"
~
+
H
+
HCt _
"
25'(CL)
"~
26°
© ©
+ H
C 27
At 1300K isomerization of 1,3 cyclohexadiene to the 1,4 compound can occur by 1,2 H shift (E ~ 62 kcal) competitively with CI atom attack. This estimate is based on the biradical mechanism for isomerization of cyclopropane derivatives and vinyl cyclopropane/ In that case rapid dehydrogenation produces benzene + H: in a unimolecular step:
M. WEISSMAN and S.W. BENSON
280 _
"
~
+ Ct
-HC~"
©
+
H2
÷
Ha.
The fate of the benzene has been analyzed using thermochcmical data. The formation and the reactions of phcnyl radical sccm to have no importance in the normal pyrolysis of CH3CI. It was important only in the experiments in which benzene was added initiallyto the reactants. A mechanism similar to that consuming C2H2, involving CH3 additions, dehydrogenations, Hmetathesis by CI, formation of stabilized radicals which recombine with CH3 when they arc in really high concentration or dehydrogenate, seems to be operational in the sequence benzene-naphthalene as well: +
30© -
Ct
31
CH3
©.
+
HCL
CH2Ct
+ C3H,Ct
D
CtCH~CH2~CH=CHCt
(36)
a
CtCH2~CHCL--CH~"CH:
(37)
rapidly produces isomers of dichlorobutene that dehydrochlorinate to 1-chlorobutadiene (38,39): C4H~CI2
-
HCt + C4HsCt .
CH3
(38.39)
The dehydrochlorination is fast enough even if it occurs thermally and not by chemical activation. The thermal step is slower though than the reverse of reactions (36) and (37). Very fast steps (38) and (39) are followed by further additions of chloromethyl radicals to produce isomers of dichloropentenyl radical: 4O
-
~
CHaCt
CH3
+
CsH~Ct2(I,5)
C4HsCt
41
43
CsHTCL2(4,5)
30 _' " ~
C[
CH3
C
t
+ ('t
(40)
32
33
~
=,
(Ell3) . ~
veral steps
We estimate that the addition-substitution reaction (30) of CH3 with benzene will have a rate constant of about 10Sl/mol-sec at 1300 K with an activation energy of about 13 kcai. Toluene once formed will be brought to equilibrium with benzyl radical by the HCI/C1 buffer. Benzyl will add CH3 to form ethyl benzene; that leads with C1 to styrene from which are formed next, sequentially, methyl styrene, phenyl allyl, phenyl butene, phenyl butene-yl, phenyl butadiene, and finally naphthalene. 3.3. Polymerization Pathways Involving CHeCI The addition of CH2CI to C2H 2 forms a chloropropenyl radical that can isomerize (35) to an allylic radical by a 1-3 H-atom (or Cl-atom) shift analogous to the propenyl radical: + C2H2
P
CH'---CH~CH2Ct
CH"-'CH~CHaC t
D
CHCt~CH~CH2
CH2CL
determined by the partial equilibrium (34) is high enough to ensure a rate of reaction (35) competitive with the overall production of carbon. The recombination of chloroallyl (AI1CI) and chloromethyl radicals (36,37):
(34) •
(35)
Since R-34 < R35, reaction (36) is rate determining (Table 3).* The concentration of chloroallyi (AIIC1) * In the 4--center transition state for the 1-3 CI atom shift, there is estimated to be 6 kcal less strain than in a 1-3 H atom shift. E~5 ~ 30kcal.
They dechlorinate at rates close to the reverse of their formation (42,43) and competitive with the overall flux of carbon atoms. These reactions are followed by further addition of CH2C1 (44) and formation of dichlorohexenyl radical: CH:Ct
+
CsH~Ct
~
C6H9C1.~(1,6)
(44)
which contains enough energy to dechlorinate through chemical activation. The thermal reaction though, is fast too, faster than the reverse of the formation reaction (44): C6HgCt2
=
C( + C6H9C((6) "
(45)
6-chlorohexadiene dehydrochlorinates competitively to hexatriene (46): C6HgCt
."
HCI.
+
C6H a
(46)
that forms benzene through hydrogen elimination at a very fast rate. The formation of allyl radicals through l-chloropropcne, C3H4Ct
+
HCI.
C~HsC(
~-
C~HsC(
"
C3Hs
+
CL
+ Ct
is too slow to be competitive with the overall reaction. In systems containing sufficient excess of methane, where CH 3 radicals have significant concentrations and form allyl radicals through reactions (4,8) described above, chloromethyl radicals can recombine with allyl:
Soot initiation in methane systems CH2Ct
+
C~Hs
m
CaHTCt-4
(47)
281
4. S T O I C H I O M E T R Y CONSIDERATIONS REGARDING RELATIVE ROLE OF CH 3 AND
C4H'/C[
o,
C,IH¢
÷
HCt.
(48)
Reaction (48) is expected to be faster than the comparable decomposition of EtCI. CH~CI *
C4H6
~
CsHsCt-5
(49)
The 5-chloro pentenyl radical has no rapid unimolecular path available to it except the reverse so that step (49) is in equilibrium. Much slower than ( - 4 9 ) is the unimolecular step (50) which is faster than competing bimolecular steps: CsHaCt-5
P
~ ' / " ~ C I
+
H .
(50)
5-Chloropentadiene 1-3 can react rapidly with CH3 or CH2CI to form hexadienes:
c.3 I
51,5i
THE
CHzCI
The mechanism described above predicts correlations between the CH4/CH3CI ratio and the change in final product yield distribution via the change in the CH3/CH2CI ratio with C H J C H 3 C I and the changing of the predominant role of these radicals in the recombination and polymerization reactions. We have found that, despite their crude character, the experimental data confirm the change in product distribution with the CH4/CH3C1 ratio predicted by the mechanism. Thus, from three hydrogen atoms available in CH3CI, one is tied up in the formation of HCI. If no CH4 is formed, both remaining hydrogen atoms form H2. I f ~ moles of CH4 are formed, (1 - 2ct) moles of hydrogen will be formed. If on the contrary, fl moles CH4 are consumed, more hydrogen than is available from CH3CI, (1 + 2fl) moles, is formed (Table 4).
~ c t
}
CH:Ct Y
53,54
+
Ct
ct
The 6-chloro hexadiene-l,3 formed in (54) will rapidly lose HCI in a 4-center process to form hexatriene 1,3,5 which then cyclizes as in steps (27), (28), and (29) to benzene. Thus, the chloromethyl radical forms competitive polymerization sequences which include its addition to double and triple bonds, dechlorination of the adduct or its recombination with chloromethyl followed by dehydrochlorination of the product or recombination and dechlorination of the monochlororadical formed. A critical step is the isomerization of the chloropropenyl radical into the allyl form which involves a 1-3 H-atom shift. This reaction and not the one that precedes it (the addition of CH:C1 to C, H2) is the critical bottleneck in the polymerization of C2H2 in this system. The step is fast enough to be compatible with the overall rate of reaction.
Since CH4 is formed from CH 3 and the maximum amount of CH3 is limited by stoichiometry and kinetics, the yield of CH4 can be used to indicate how much c n 3 was left to participate in the recombination reactions that form acetylene and in the polymerization reactions of acetylene and higher compounds. The precision with which we have measured CH4 is relatively poor and therefore we have followed the H2 concentration since it is correlated with the CH4 concentration (Table 4). From the table it can be seen that if CH4 is formed in the reaction then (H2)/ A(CH3CI) < 1 (where A(CH3CI) is the amount of CH3CI consumed at the given time of reaction). If CH4 is neither formed nor consumed. (H2)/ A(CH3CI) = 1 and if CH4 is consumed. (H:)/ A(CH3C1) > 1. If methane is consumed in an amount equal to the
TABLE4. Theoretical relative final product yields in the pyrolysis of methylchloride with and without added methane CH4
C(s) A C H 3CI
H: ACH~CI
HCI
ACH4
A C H 3C1
A C H 3C1
1 -
1
absent initially and formed in reaction
1-
present initially and not consumed in reaction
1
1
1
0
present initially in large excess and consumed in reaction
1 + fl
1 ~- 23 (>1)
1
-fl
2~
(<1)
M. WEISSMANand S.W. BENSON
282
methylchloride a m o u n t consumed, the formation of C , ' s and their polymers occurs only through CH3 radicals and (H.,)/A(CH3CI) = 3.
CL
CH3CL
)-
CH3 + Ct
÷ CH(
~
CH3 + HCL
2CH3
~
C2H6
C:H~ ~
2C(s) + 3H2
net: CH4 + CH3Ct -'--.~"
2C(s) + 3H: + HCt
Thus, if (H:)/A(CH3C1) > 3 a pyrolysis o f CH4 was induced. In Table 5 are given experimental data of (H2)/ A(CHaC1) and (CH4)/(CH3C1) per time interval at various reaction times. The data crudely confirm that as long as CH4/ CH3C1 < 2.5 4- 0.5, (A(H:)/A(CH3C1))o.I~ ~ 0.3. This latter is the ratio between the concentration changes in H 2 and CH3C1 in a time interval of 0.1 sec. F o r (2.5 + 0.5) < (CH4)/(CH3C1)) < (4.5 + 0.5), ( A ( H : ) / A ( C H 3 C I ) ) 0 ~ gradually increases to ! and for (CH4)/(CH3CI) > 4.5 _ 0.5, (A(H:)/A(CH3 C1))0~ ~ > 1. The latter situation, characteristic for CH4 rich systems where CH3 starts to play a predominant role is not encountered throughout a whole run.
Even in the experiment with the largest excess of methane used, ((CH4)o/(CH3CI)o = 2), CA(H,.)/ A(CH3CI))/01,~ > 1 only after 50% decomposition. Since in c o m p u t e r simulation of the reaction one cannot be limited to the description of a steady state but must model the reaction from its start and at all (CH4)o/(CH3C1)o ratios used in the experiments, a shorter or longer initial period is characterized by a predominance of CH,,C1 radicals. Then CH.,CI reactions must be included in the mechanism when modeling the reaction. ~m~, is limited by stoichiometry to 0.5 (when no H., is formed and all hydrogen from CH3C1 transforms into HCI and CH4). The reaction mechanism places an additional constraint on :Cmax: CH3Ct
I"
CH3 + CL
Ct + CH3Ct
---
HCt + CH:Ct
CH3 + CH3Ct
)
CH4 + CH~Ct
2CH:Ct
)
CzH2 + 2HCt
C2H:
)
2C(s) + H2
net: 3CH3Ct
)
2HCL + 2C(s) + H2 + CH4
Thus, ~m~, = (ACH4)/(ACH3CI) = 1/3. In order for ~ to have its m a x i m u m value, all CH3
TABLE 5. Experimental hydrogen yields and CH4./CH3CI ratios at various time intervals T(K)
~ (sec)
1310
CH 4
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0
0.28
1.00
2.27
3.57
8.0
7.7
0.33
0.27
0.28
0.27
0.26
0.26
0.19
0.25
0
0
1.13
1.24
4.96
7.6
10.36
0.35
0.47
0.56
2.5
0.8
1.74
CH3C1 H2 ACH3C1 AH,
t t0
0
se~
1260
CH 4
3.08
14.3
28.5
22.8
CH3C1 H,
0
22.9
27.0
0.67
0.70
3.25
3.75
ACH3C1 AH~,
1360
CFI4
0.23
0.40
0.71
1.44
2.13
3.22
4.90
6.75
0.24
0.43
0.51
0.24
0.70
1.0
CH3CI H2 ACH 3Cl All,. AACH~ C1/0.t s¢¢
4.0
0.90
Soot initiation in methane systems radicals would have to form C H 4 . IfCH3 is consumed to polymerize C 2 H 2 , the C H 4 concentration will be smaller and H2 will be increased. However, if CH2CI is polymerizing C: H2, all CH3 is free to form CH4 and the C:H ratio of 1:0.5 is conserved. The experimental data in which CH4 was not present in the beginning but is formed in the reaction are close to these proportions: =
(CH4)max
(C) max =
oexp = 0.74
0.23 (I-(,)
= 0.77
(H2)m,x = 0.52 (1-2(*) = 0.54. As ~ < 0.33, s o m e c n 3 must have reacted otherwise than by forming CH4. This agrees with the formation of small quantities of C:H4 where CH3 presumably had to be involved. It is also interesting to note that C2 H4 has smaller concentrations than C2 H: and starts to build up later than C2H2. This is expected since in the early parts of the reaction, (CH4)/(CH3 CI) is very small and the predominance of CH:CI o v e r C H 3 is large. Only later, when (CH3CI) diminishes considerably and CH4 is formed, the concentration of CH3 presumably increases and its role in the recombination reactions (CH3 + CH2C1 C2H5C1 ---, C2H4 HCl) becomes significant. The hydrogen profiles lag behind the C(s) profile, indicating that it is formed mainly as described by the mechanism in the polymerization reactions and that other hydrogen producing reactions that occur early in the mechanism, such as C:H~ --, H + C2H4 and C2H3 --. H + C2H2 do not play a significant role in
+
o+i\
283
the initial part of the reaction for all (CH+)o/(CH3C1)o ratios used in the experiments.
5. MODELINGTHEREACTION In Fig. 2 are represented the product yields predicted by the mechanism. The timescale of CH3CI decay and product build-up is shown and the profiles of the major products match closely the experimental profiles. The C.,H4 profile peaks somewhat lower (0.015 molar fraction) than the experimental profile (0.025 molar fraction). This can be adjusted varying the recombination rate constants within their accuracy limits. The acetylene profile peaks, as expected, earlier than the C2 H4 profile. The maximum is however higher (0.08 mole fraction) than the experimental peak (0.06 mole fraction). The decline of the profile (0.07) is also slower than the experimental profile (0.04). We have found that including steps of polymerization of C 6 H 6 t o naphthalene and of naphthalene to C(s) that involve CH3 and CH2CI radicals, the profile of C2H2 can approach the experimental profile since the CH2CI radicals are removed from the C2H2 formation reactions (Fig. 3). Since the stoichiometric eoefficients of C H 3 and CH:C1 in its reaction with naphthalene to soot is a global, empirical number, a more detailed modeling would be useful only if data on intermediate species concentrations were available. We feel, however, that it is reasonable to assume that C~ radicals are consumed in polymerization steps of naphthalene and higher aromatics in paths similar to those described above in the polymerization of C2-C~
o
0.50
O.08
o.oG
°'~5[k o.oo, ,----, 0.00 0.25 0.50
U 0.75
+:
::;:F "0.00
TIME (SEC)
"'.
:
0.25 0.50 TIME {SEC)
o:F/
0.75
0.025 (exp) iI" .......
0.020
0.015
5 ~o.o~o 0.005
0.00
'350.0 I
I
I
0.25 0.50 TIME (SEC)
0.75
0.50[
'~ 3.0, ::
0.25 0.50 TIME (SEC)
1 0,75
2 " 0 1 ~
,'
1.0
i
O0 I / 0.00
I 0.25
I 0.50
TIME (SEC)
J 0.75
'0 7.5
0.75[
w 5"0t/~
o5op
o.25
,3ooot-
5
t275.0~--/ t250.0 } 0.00
7---
0.00
'
i I 0.25 0.50 TIME (SEC)
0.75
o.2ot/ o.,st/
o. g-/ /
o.,q-/
0.05 / 0.00
I I 0.25 0.50 TIME (SEC)
0.75
0.01 0.00
I 0.25
0.50
TIME (SEe)
0.75
0.00 /
0,00
J 0.25 0,50 TIM[ (SEC]
FIG. 2. Computer simulation of the pyrolysis of methyl chloride without methane addition. Benzene is described to convert to carbon and hydrogen in one global step.
0"75
M. WEISS/dAN and S.W. BENSON
284
2 . 2 ~ . . , .+o6 . . . . . . . . . . .
%- . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
/
0.9
t.
o.,
O
0 ~o
2o.,p \
o 0 . ~
>o+
\
I
° .......
..-0 ..........
0
< o.61-
\
.'+, .,... ............. ..o.+b
\."
0.2;
.,
I
° •
•
................ -.+:.+ +:;'/
,,..
o.3 -
o - .... .' •
--I--~
..;. •
. . . . .
+
_
_
-
.
-
o., t-
.........
~
o.2F
........
~. . . .
0 O b~/'~'L4~ : ~ " ~1
~-m---~
0.0 0.0
0.t
0.2
0.3
0,4
TIME
,3[ ,.2
---v. - -
. . . . Ol " " " C H ~
.... O .... C
--~--
.... o . . . . H a
2*CzH 4
I
TIME
0.7
-~ . . . . . . . . . . . . . .
•
.o
~
]
0.3
0.4.
(SEC)
---v.- • 2* C z H a
. . . . c---. CH 4 --~-2*CzH4
2"C a H a
__~_
0.2
CH 3 CI
. , - - . o - - CH 3 Cl
• •
0.6
(SEC)
.bo....~....c-....o-....e- .........
uI'"
t.0
0.5
;
0.1
---o-C .. -o-+ - H 2
included and experimental product yield profiles can be used to adjust rate parameters within their accuracy limits.
b 6. I O N I C
PATHWAYS
0.9 0
0.8
o.z ~, 0.6
~ 0.5 ~ 0.4 o_ 0.3
.........:::8 .....~u ........
• .e" ; " o'~'
0.2 OJ 0.0'
0.0
~""~d~'["
t 0.1
l
i
'
'
0.2
0.3
I
0.4
TIME (SEC) ----o---- C H 3 C I
...-o...- CH4 l - - ~ - - -- 2 * C z
H4
.--v.-- 2 " C z H z • .-o-.-- C ---~
....
Hz
FIG. 3. Computer simulation of the pyrolysis of methyl chloride without and with methane additive. Empirical reactions of polymerization of benzene with CH: C1 and CH3 to soot, HC1, and H: are included in the mechanism. compounds and that the reaction mechanism described, with its reasonable estimates of thermochemistry and rate parameters provides a fast enough route to soot initiation in this CI/CH+ system. The inclusion of the formation and consumption of CH2CI, in the mechanism needs further experimental support. The formation seems to be fast enough and the consumption seems to rapidly reform CH2CI radicals so that the flux of carbon from CH3C1 to C(s) is not perturbed. Systems with large excess of methane in which little or no soot is formed should be in principle completely modelable since no empirical reactions must be
There has been much discussion on the role of ions in soot formation. We have demonstrated here that within uncertainties of the order of a factor of 2 radical pathways are quite capable of accounting for the formation of aromatic molecules at 1300 K in the absence of oxygen containing species. At the higher temperatures reached in flame systems 1600-2000 K, H and H2 can play the same role as is played in the present system by the C1/HCI buffer. At the higher temperature all relative rates will be faster, particularly endothermic steps and radical concentrations will also be higher. Hence there seems to be no difficulty in accounting for aromatic formation via radical pathways. Is it still possible that ions play a role? The halogen system is perhaps the most interesting from this point of view since CI atoms have the highest electron affinity (3.6eV) compared to all common species expected in flames and as a consequence one expects equilibrium ion concentrations to reach their highest levels. Let us consider as an example the equilibrium: A R +
CL
-
R+ +
Ct-
.
If R has an ionization potential comparable to that of the t-butyl radical (6.90 + 0.1 eV) then reaction A is endotbermic by 3.3 eV -- 76 kcal. The entropy change is about - 2 . 8 eu due mainly to spin changes so that at 1500K, AGA = 80kcal and KA (1500 K) -- 10- t m7. Assuming that we have electroneutrality and (R +) -- ( C I - ) then we see that the equilibrium ion concentrations are about 1 0 - 6 of the
Soot initiation in methane systems
285
mean of the product of (CI)(R). We have seen that the radical pathways have rate constants for the rate determining steps which are already within factors of 10- ~to 10 -2 of collision frequencies. Thus in order for ion pathways to contribute to aromatic formation they would have to have cross sections for reactions at 1500K which are from 104 to 106 larger than geometric cross sections. Langevin cross sections for ion-molecule reactions at 300K are geometric or smaller and decrease with increasing temperature. Hence equilibrium ion concentrations will not contribute measurably to aromatic formation. It has frequently been suggested that super-equilibrium concentrations of ions may be found in radical-radical reactions such as CH + O CHO + + e - . The difficulty with such proposals is that they invoke the participation of relatively rare species such as CH with very high heats of formation (142 kcal). Even if such reactions occurred at every collision their frequency would not be sufficient to produce ion concentrations in excess of the equilibrium concentration calculated here. One can see that in terms of the joint heats of formation, to produce ions, any radical-radical reaction must have sufficient energy to exceed the ionization potential of the neutral products of the same reaction. In consequence it is very unlikely that chemi-ionization processes can ever produce ion concentrations in flames sufficiently in excess of equilibrium to be of importance in the production of aromatics.
(5) Reasonable thermochemistry and kinetic parameters produce a computer simulation of the available data. When soot is formed, an empirical number of CH 3 and CH2C1 radicals was included in a globally described step of polymerization of C r H 6 to C(s). In a CH4 rich system when the yield of soot is small, the computer simulation should be entirely nonempirical. Can one generalize these conclusions to other systems? Probably not. Methane systems such as the one we have studied are unique in being dominated by the methyl radical. Systems starting with C~ hydrocarbons would probably be dominated by the allyl radical while benzene systems would show a similar dominance by phenyl radicals. A toluene system would be expected to have benzyl radicals as the dominant species. It is important to realize that above 1500 K differences in bond dissociation energies of as much as 7 kcal produce less than a factor of 10 difference in equilibrium populations of radical species. A-factors and concentration play a much more important role than they do at lower temperatures. An important application of this which is a special feature of the systems we examined is provided by the CI/HCI buffer. The very rapid reactions of both of these species with molecules and radicals respectively virtually guaranteed equilibrium between R and RH species. In higher temperature flame systems H/H2 reactions can play a similar role.
7. CONCLUSIONS
Acknowledgements--This work has been supported by
The analysis of the condensation process leading to the formation of polynuclear aromatics in methane systems reveals a number of unexpected results. (1) The process is dominated by the sequential addition of methyl to stabilized radicals or dienes ultimately to form C,H,, species. (2) For any given value of n, the number of carbon atoms in a radical or molecular species, growth is achieved by addition of CH3 radicals. When n is even the dominant species is an unsaturated molecule with significant conjugation or aromatic delocalization. When n is odd the dominant species is a stabilized radical. (3) Radicals with even numbers of carbon atoms are minor species as are molecules with odd numbers of carbon atoms. (4) Chloromethyl radicals are kinetically equivalent with methyl radicals forming competitive polymerization sequences. The fast additional routes available to chlorinated species (dehydrochlorination and dechlorination) that can proceed at these temperatures even through chemical activation, might explain the higher propensity to soot of chlorinated hydrocarbons when compared with related hydrocarb o n s lsomerization reactions proceeding via 1-3 Hshifts become important at high temperatures.
Grants from the National Science Foundation, and the U.S. Army Research Otfice, and by the Independent Research and Development program of McDonnell Douglas Corporation. REFERENCES
1. COLE, J. A., BITTNER,J. D. and HOWARD,J., Radical pathways in soot formation, Symposium on Combustion Chemistry, T. Sloane (Ed.) (1984). 2. TAQAN,M., Dissertation, MIT (1984). 3. WElSSMAN,M. and BENSON,S. W., Int. J. Chem. Kinet. 16, 307 (1984). 4. WARNATZ,J., Combustion Chemistry. W. C. Gardiner, Jr., (Ed.), Springer-Verlag (1984). 5. CRC, Handbook o f Biomolecular and Termolecular Reactions, Vols I and II, J. A, Kerr and S. J. Moss, (Eds), CRC Press, Boca Raton, Florida (1981). 6. WEISSMAN,M. and BENSON, S. W., Poster PS58, X X Syrup. (Int.) Combust., Ann Arbor, Michigan, 1984; J. Phys. Chem. (1988). 7. BENSON,S. W., Thermochemical Kinetics (2nd Edn) J. Wiley and Sons, New York (1976). 8. GOLDEN,D. M., J. Phys. Chem. 83, 108 (1979); Cohen. N., X I X Syrup. (Int.) Combust. 31 (1982). 9. BENSON,S. W. and O'NEAL, E., Kinetic Data on Unimolecular Gas Phase Reactions. Butterworth, London (1972). 10. DEAN,A., Int. Conf. on Chem. Kinet.. Washington. D.C., 1985 (in press). 11. ROSSl, M., KING. K.D. and GOLDEN, D. M., J Am. Chem. Soc. 101, 1223 (1979).
0360--1285/89 $0.00 + .50 iF 1989 Pergamon Press plc
MECHANISM OF SOOT INITIATION IN METHANE SYSTEMS M. WEISSMAN*~ a n d S.W. BENSONt * McDonnell Douglas Research Laboratories, St. Louis, Missouri 63166, U.S.A. "?Donald P. and Katherine B. Loker Hydrocarbon Research Institute, University of Southern California. Los Angeles, CA 90089-1661, U.S.A. Received 22 June 1989
Abstract--Thermochemical and kinetic evaluations of the very rapid elementary radical reactions consuming the C2H: produced in a chlorine catalyzed polymerization of CH4 are presented. An earlier examination of the data and mechanism leading to C2H2 supports a methyl and chloromethyl recombination path to C2 hydrocarbons. The relative yield of CH3 and CH~CI depends on the excess of methane. In the CH4 system consumption of C2 species to ultimately form benzene is shown to proceed by a stepwise addition of CH 3 radicals to C, Hm species. When n is even the dominant species is an unsaturated polyolefin molecule. When n is odd the dominant species is a conjugated, unsaturated radical such as allyl, pentadienyl, benzyl, etc. Mono-olefins or saturated molecules are rapidly stripped to these species by radical catalyzed dehydrogenations. In the current system chloromethyl radicals are equivalent kinetically to methyl and play a dominant role. Their addition to unsaturated species produces chlorinated radicals that dechlorinate rapidly or recombine with chloromethyl to produce dichlorohydrocarbons that dehydrochlorinate very rapidly. A very important reaction in the sequence is the isomerization of propenyl and chloropropenyl radicals to allyl and chlorallyl by a 1-3 H (or Cl) atom shift. Its high pressure Arrhenius parameters at 1300K are estimated to be log [k(sec-l)] = 13.7 - 37/0 where 0 = 2.303 R T in kcal/mol. It appears likely that benzene conversion to soot also proceeds via a CH3/CH:CI radical, sequential addition mechanism. Stoichiometry considerations applied to the product yield distribution support the role of methyl and chloromethyl predicted by the proposed mechanism. Ionic pathways are shown to be insignificant in the formation of aromatics. CONTENTS
1. Introduction 2. Mechanism of the Early Stages 3. Polymerization of C: Species 3.1. Polymerization induced by C2 radicals 3.2. Polymerization induced by CH3 3.3. Polymerization pathways involving CH2C1 4. Stoichiometry Considerations Regarding the Relative Role of CH3 and CH2CI 5. Modeling the Reaction 6. Ionic Pathways 7. Conclusions Acknowledgements References
273 274 276 276 277 280 281 283 284 285 285 285
flames: with the added assumption that the butadienyl radical arises from the addition of C~ H~ to C2 H.~ (Fig. The initiation of soot in the pyrolysis and combus- 1). In 1984, we published some experimental results on tion of hydrocarbons has been investigated from the point of view of radical, heterogeneous and ionic the pyrolysis of CH3CI, pure and with selected mechanisms. In their now, well-known study, additives3 near 1300K. The derived reaction Howard, Bittner, and Cole ~ found that the rates of mechanism was similar in many ways to the sootaddition of the butadienyl radical to acetylenes agree forming reaction sequence proposed in the combuswithin an order of magnitude with the rates of tion of CH4 .: Thus C2 H2 arises in the same succession formation of aromatic compounds in butadiene of steps following the recombination of 2CH3 flames. This mechanism was extrapolated to methane radicals. However, H-atom abstraction is dominated by C1 rather than by H or OH which has the advantage of simplifying the reaction mechanism. Current mailing address: University of Southern CaliforThe description of the reaction mechanism in our nia. Department of Chemical Engineering, Los Angeles, CA previous work 3 stopped at C:H: production since the 90089-1211. U.S.A. 273 I. INTRODUCTION
JPECS 15:4-A
274
M . WEISSMAN a n d S . W , BENSON
CzH 6
Polyocetyiene
"
C H,~'--'I=" C H 3"-~'~,"~H~ M'-~-'~" C.,Z H,R I CH3 i i
~ ~
---i=. | -r
Call3M.--M--~CzH2.---~,-a H_/~/'C3H'~ ~ ~ C , H 2 qE
o
CO
CO
CO
CO
~
(-~ f,
CO
FIG. 1. M e c h a n i s t i c p a t h w a y s f o r t h e f o r m a t i o n o f m o n o c y c l i c a r o m a t i c species in a f u e l - r i c h n a t u r a l g a s flame?
known pathways of aromatization or polymerization (through addition of C2H3 or C4H~ or abstraction of H and formation of C: H) were not fast enough to account for the consumption of C2 H 2 in those particular experimental conditions. Despite the crude character o f the data, some
features of the reaction, regarding alternate homogeneeus pathways of conversion of acetylene to benzene and soot became apparent on further examination of the system. Some of these results, presented in this paper, may have a wider applicability.
2. M E C H A N I S M O F T H E EARLY STAGES
Initiation
(M)
CH3CI. f
Buffer systems
t
~
CH3 +
C[
ti)
C[ + CH3C[
"
HCt
+
CH2Ct
CH3 + HOt
"
CH4 + Ct
(1) (2)
(M) 2CH3
) C2He
CaH6 +
(t)
Ct
a,
HC[
+
C2H,,
+
C2Hs
+
C~H 3
(M)
C:Hs Ct
+
C2H3
Termination steps and c ons e c uti ve reactions CH3 +
C2H4 m
~
CH4 +
CH2Ct.
= (M)
m
I,
C2HsCi HCt
+
(t') C2H4
C2H4Ct 2
(t")
C2HaCI.2
)
HCt
+
C2H3Ct
C2HaCI
m
HC[
+
C2H a
The overall reaction is not a chain so that the first step, the dissociation t f CH3 CI determines its overall rate of disappearance. As soon as even 1% of HCI is formed in the system, the HCI/CI reactions are so fast that they act as a buffer system leading to near equilibrium concentrations of all the hydrocarbon radicals in the system. In particular, they bring about an effective equilibrium between CH3 and CHzCI radicals, and CH4 and CH3CI, molecules: CH3Ct
H + C2H~
CH2Ct
2CH~CL
+
HCt
(M)
CaHsCt
CH3
H
•
(a)
Thus, (CH:Ct) (CH3)
(CH3Ct) =
Ka
(CH4)
=
~
.
Note that K~ = 1(1"1(2 since step (a) = step 1 + step 2. At the high temperatures of our study K, > 1 so that CH2CI tends to be the dominant radical in the system. Since A/-~ = 4.3 kcal, K, is not very temperature dependent. At 1300K, Ka = 4.0 using ff'(CH2CI) = 57.7 eu.
275
Soot initiation in methane systems The C2 product distribution is determined by the radical ratio and the termination rate constants. A b o v e 1000K, the terminations tend to become pressure dependent. This leads to a rapidly decreasing importance of step t relative to t' and t". The latter two steps have less pressure sensitivity than t but, more significantly, have rapid decomposition
wit
reaction. We thus expect to see little or no C2 H6 in our system when we start with a 2:1 ratio of CH4 to CH3C1. The radical concentrations are given by steady state
~CH,~
=
/
kt
J
Conversion (percent) 20 40 60 Time (s) 0.035 0.058 0.096 Specie - l o g C, (mol/1)
75 0.1,,4
CH 4 CH~CI HC1 H~ C,H 4 C2H,
2.51 2.94 2.76 3.46 3.76 3.84 4.43
2.56 3.19 2.71 3.23 3.77 3.81 4.48
6.3 7.9 7.8 8.9 8.7
6.4 7.9 7.8 9.0 8.5
6.1
6.4
2.41 2.53 3.13 3.95 3.95 4.12 4.73
2.46 2.73 2.91 3.73 3.81 3.91 4.73
H
6.1 8.1 7.8 9.1 9.0
6.2 8.0 7.8 9.1 8.9
C4H5 CH, C1
5.6
5.9
C6H 6
C4 H6 CH~ CI C2H 3
~5.2
~ 10.4
*k-"~ + ~ k t J
~2~ .
13)
U n d e r the conditions obtaining in our system where k, is effectively much smaller than k~, and k,, (CH3) reduces to:
I':""l' [~.
~CH~) -- I . ~ . 1
-~-~
l
(4)
while, --~
TABLE la. Radical concentrations (C~) derived assuming partial equilibrium of the buffer reactions (RH + Cl --, HCI + R). RH concentrations were measured. (CH4)o:(CH3CI) o = 1.1:l.0, 1310 K, Po = 0.887 atm.
~c,,ct~ = ,~c.,~ [ l . k, k,. ]-~
[ki,c,,ct)]~
1+~¢,
(5)
kt
L(CH4) kv _1
The rate of loss of CH3CI is then given by: -d(CH3CD dt
(6)
= (I+~)kjtCH~Ct)
where a is very close to unity in most of our studies. This value is deduced from our observation of product species which showed that in the absence of added CH4, some o f the CH3CI is converted to CH4 while in the presence o f excess CH4 there seems to be little or no net change in CH4 concentration. It was shown in Ref. 3 how kj was derived from the experimental decay rate o f CH3 CI taking into account the overall stoichiometry o f the reaction. The value was close to the calculated fall-off rate constant of the unimotecular decomposition, k~ was corrected as well for fall-off behavior using the curves published by Warnatz 4 (Table 1). Using the CH~ steady state radical concentrations derived above and the reaction: ct + CH4 ,- "
HCt + CH3
TABLE lb. Relative radical equilibrium concentrations Radical
CH 3
C: H 3
C4 Hs
q~
Cl
Relative concentration at 20% conversion 75%
1 1
10-17 10-t 4
10-4
10-3.0
10-2.0
10-29
10-4
10-2.6
10-1.5
10-2.~
Radical
CH=CHCH3
CH,-CH=CH,
CHzC] .C3H4C1 C3H4C1 C5H~C1:(1.5) (PrCI) (AllC1)
Relative concentration at 20% conversion
l 0 -4t
10 - t l
10 -05
10 -41
10 -18
10 -3.2
H
C5H7C1:(4.5)
10 -zs
Radical or molecule
C_, I"L CI
C 6 H 9 CI 2
C 6 H 9 C1
C4 H7 C1
C~ H 8C1
Relative concentration at 20% conversion
I0 -°°
10-' o
10-1.5
10-o.4
10-2.0
Radical or molecule
C4 H~
C6 H9 C1
Relative concentration at 20°/0 conversion
10- 04
I 0 - ~~
M, WEISSMANand S.W. BENSON
276
the CI c o n c e n t r a t i o n has been derived for several degrees o f conversion. Next, f r o m similar buffer reactions, the c o n c e n t r a t i o n s o f the o t h e r radicals were derived (Table 1). These e q u i l i b r i u m c o n c e n t r a tions were used in the following estimations. In T a b l e 2 are given the t h e r m o c h e m i c a l p a r a m e t e r s a n d in T a b l e 3 the kinetic p a r a m e t e r s o f the e x a m i n e d species a n d reactions. W h e r e e x p e r i m e n t a l d a t a were unavailable r e a s o n a b l e values were derived by g r o u p additivity a n d the e s t i m a t i o n m e t h o d s developed in Ref. 7.
3. POLYMERIZATION OF C2 SPECIES
3.1. Polymeri-ation Induced by C., Radicals As m e n t i o n e d earlier 7 the reactions that have been p r o p o s e d to lead to a r o m a t i c s from C~H., via vinyl radicals: /
x
c~,~ + C2H~
C,H,
(\ , ~ x , , ~ ' )
/'
C,Hs + C~H2
C~H,
~
)
~
'
(,)
(3)
TABLE 2. Thermochemical data for species considered A/-/7 in kcal/mol, S~' and Cp in cal/mo[
Compound C1 HC1 CH3C1 CH4 CH3 CH2CI C2H4 C2 H3 C.,H2 C2H propenyl-I allyl propylene butene-I buten-l-yl-3 butene- l-yl-4 butadiene
trans-pentene-2 1,3 pentadienyl-5 1,3 pentadiene penten-2-yl-I (trans) penten-2-yl-4 (trans) penten-2-yl-5 1,3 hexadiene-yl-5 1,3 hexadienyl-6 cyclohexenyl-3 1,3 hexadiene
trans-hexene-3-yl-6 trans-hexene-3 3 chloropropenyl- 1 (Pr-CI) 1 chloroallyl (All-C1) 1,4 dichlorobutene-I (C4H6CI 2 (1,4)) 3,4 dichlorobutene-I (C4 H6C12 (3,4)) 1,5 dichloropentene- 1-yl-3 (C5H7C12 (1,5)) 5 chloropentadiene- 1,3 (C 5H~ CI) 1 , 6 dichlorohexene-2-yl-4 (C6H9C12 (1,6)) 6 chlorohexadiene- 1,3 (C 6H 9CI) 5 chloropentene- l-yl-3 (C 5H eCI) 3 chloropropene-I 4,5 diehloropentene- 1-yl-3 (C 5HTCI2 (4,5))
A/~f 300 K
S~' 300 K
300 K
600 K
C~n( 73 900 K
28.9 22.0 19.6 17.9 35.1 29.1 12.5 66 54.2 127.0 61.1 41 4.9 0.1 33 48 26.5 - 7.9 53 18.2 27 24 38.0 44 60 30 13.4 34 - 12.7 56,3
39.5 44.7 56.0 44.5 46.4 57.7 52.4 56 48.0 49.6 64.9 62.1 63.8 73.5 71.5 75 68.2 83.7 74.6 76.5 83.0 82.0 85.7 87.9 89.7 73.2 86.3 94.4 90.5 74.4
5.2 7.0 9.8 8.5 8.8 9.29 10.3 0.9 10.6 8.9 14.9 15.0 15.5 20.6 20.0 20.2 18.8 25.8 23.3 24.1 25.0 25.2 25.4 27.6 28.5 24.7 29.2 30.5 30.9 17.60
5.4 7.1 14.6 12.5 11.5 12.6 16.9 15. I 14.0 10.7 23.3 24.1 25.8 35.3 33.8 33.5 29.7 44.0 39.7 41.1 42.7 42.5 42.4 47.0 46.5 47.2 50.6 51.8 53.5 25.90
5.3 7.4 18.0 16.1 13.5 14.7 21.2 18.0 15.8 11.9 29.0 29.7 32.7 44.5 42.1 41.6 37.4 55.8 47.7 50.0 53.4 53.4 52.9 58.4 57.7 59.9 61.8 64.7 67.6 31.02
5.2 7.8 20.3 18.7 15.1 16.1 24.2 19.9 17.2 12.7 33.4 33.8 37.1 51.1 47.5 47.4 42. I 64.3 53.6 56.7 61.1 61.1 60.4 66.4 65 68.0 70.2 74.0 77.8 34.36
5.2 8.1 21.7 20.6 16.2 17.2 26.2 21.3 18.3 13.3 35.8 36.0 40. I 54.7 48.3 50.3 45.4 68.7 56.8 60.3 65.2 65. I 64.3 70.4 69.5 7 l. 1 74.7 78.7 83.1 37.22
25,6
74.7
17.74
26.48
31.47
34.60
37.29
-
13.9
91.0
26.08
40.59
48.60
53.79
58.38
-
16,5
91.0
27.16
39.60
46.87
51.74
56.11
14,7
72.6
29.54
47.19
56.00
61.39
66.06
17.0
89.2
26.42
41.43
50.72
54.42
56.25
6,2
107.9
36.14
57.18
69.15
74.82
78.61
7,2
95.9
32.20
52.73
63.91
71.06
77.33
18.9
89.8
28.24
45.75
55.87
62.32
67.98
6.4
73.3
18.10
29.43
36.50
38.50
41.81
-
-
1200 K
1500 K
Soot initiation in methane systems TABLE3. Arrhenius rate parameters for reactions at 1300K. Estimations made with methods described in Ref. 7 N, REACTIONS
IogA~ M
-~S =
E
t
C2H3+CzH2 -- C4Hs
8.9
6.9
~H~÷HCt -- C,#6*CI
9.3
5.0
3
C4Hs+CzHz-- C,sH.,,
8.9
6.9
4
CH3 + C2Hz~ "~*
9.5
9.1
' 13.8
40.0
9.3
5,0
5
"~* ~ -- ~ + H
"~'. + HCI -- ~ '
7
~
8
"~.-
9
2 "~" -- C
t0
C+CI--
ti
"~" + C2H2 -- C "
+ Cl
+ CI -- @ + HCI ~
12 C "
~+HCl
~
15
CH~+ ~
14
CI+ (~-- H C I + ( -
-- (
15 cH 3 . ~ -
C
t5.7
27.9
12.4
45.0
t5.5
36.0
47
(~HzCl +AC3H 5 -
t0.0
0
10.7
0
13.3
40.0
43 C3HTCIz(4,51 -- (~5H7CI+Cl
44 ~ HzCI+ CsH?CI - - CHzCl- CH--2"~H- CHz-CH2CI t0,0
0
49
~HzCI+C4H 6 -- CsHoCI
8.7
4,0
o
50
C~HaCI -- C~H7 + CI
t5.0
55.4
9.0
4.0
to 3.o
9.2
2.4
22
~ -- ~.~ + H
14.0
45,0
23
(~ + C I - - ~ , ~ +HCI
tt.0
0
24 C H 3 + ~. -- ~,
10.7
0
25 ~'~ +CI -- ~ + HCr
II,0
0
26
~--~,~
f5.5
42.5
27
C -- ~[~
tL9
50.0
28
0
29
~ -- ~
11.O
O
13.5
29.0
9.5
t3.0
+ H
t5.4
26.0
c + @ - ~+Hc~ cH~. ~ - ~
11.o
1o3
o o
9.0
8.0
t3.7
30,0
5t
~H2CI+AC3H421 -- CI HeC- CHz-CH = CHCI
CH3+ ~".,~ C I -
53 ~
CI -- ~
0
t0.0
0
CI
+ CI
54 CI " v ' ~ " ~ CI -- CI ~
+CI
9,0
4,0
14.0
27.0
t4.0
27.0
Note: For some reactions that are not rate determining only approximate rate parameters were used
3.2. Polymerization Induced by CH~ The steady state concentration of CHa radicals is much larger than that of C2H3 in our system. In order for CH3 to account for the consumption of C2H2, we require a rapid addition of CH3 to C2 H2. In addition, the product of addition must have available further reaction pathways compatible with the experimental rate. The rate of addition of CH3 to C2H2 forming propenyl has been measured at low (300-500 K) temperatures: 5 Ca3 + C2H~
I0,0
CI
~
! 521 CHzCI + 'g~'NCI - - C [ - ~
( Pr -Cl)
56
C4HrCI
4~ C4H7CI - - Call6 + HCI
tto0
93 93
35 ~3H4C1-3 -- CHz- C'H - CHCI ( A l i - CI)
0
(C6H9C1~(1,5))
fo.4
(~ ( - "C( - C
34 ~.HZO+CzHz - C!CHz-CH : ~H
28.5
C6HgCI - - C6He+ HCI
48.0
32
13,7
45 C6HgC12(t.5) -- C6HsCI + CI
ts.5
~
28.5
46
( + H
~)
t4.O
(~,.~HtCI)
6.9
O
31 (~ -- ~
421CsH?CLz(I.5)-- HzC=CH-CH=CH=CH,zCI+ e l
0
48.0
50 CH3+ ~ - -
0
9.0
11.0
+ HCI
10.0
ft.0
t3,8
÷ X
~ H2C-"E"~--CH = CHCI- CH2CI (CsHTCLz (4, 5))
0
+ H
+ Cl -- ~
41
10.3
....~+ HCI
+ H
0
t,0
~
,9 ~ . ~ + 20 ~ + 2I ~ . , +
t0.0
(c~H~c12(1,51)
57.0
CI + LI "x-
(.
40 CH2CJ+ C4H5CI-- CIH~-CH2-CH--'~- CHCI
~t.0
t6 t8
REACTIONS
t5.7
t7
-- ~
TABLE 3. (continued)
kc oI/moI
2
6
277
-J~-.
.
(4)
(C4H6CIz(t , 4))
-- CHZ= CH-CHCI- CHzCI
37
(C,H6Zlz(l 2)) 58 C4HsCIzH,41 -- HCI - CaHsCl
f0.6
5t.0
C4H6CIz(!,2) -- HCI + C4HsCI
10.6
40,0
39
cannot account for the rate of consumption of C: H: in the described system because buffer reactions such as (2): Ct +
C,H,
C,)115 +
HCt
(-21
produce much too low concentrations of C, H5 and
C2 H3.
We have found, however, 6 that such additions may exhibit significant positive curvatures in Arrhenius plots due to their nontrivial heat capacity of activation] The rate constant at ] 300 K derived with the transition state model described 6 (] 07.8 M- ] sec- ~) predicts a rate of reaction compatible with the observed global flux of C from CH3CI to products. A factor of 0.5 due to fall-off behavior estimated by Dean m is within the accuracy limits of the estimates 00+°4). The propenyl radical can undergo back reaction ( - 4 ) , dehydrogenate to methylacetylene (5), hydrogenate to propene (6) followed by the dehydrochlorination of propene to allyl (7) or isomerize directly to ally] (8):
278
M. WEISSMAN and S.W. BENSON
(-4) s.
CH3
(5)•
+
~
+HOt ~,,r6~
k-4
C2H2
~
w.
1079 sec-I
+ H
ks = 1068$ec -1
+ Ct
k6 = 108'51/mol-sec; k6(HCt) = 105osec-I
(7) D ~ Ct
(8)
=
+ aCtk, ~ ks
/~:-.~
The fastest reaction is ( - 4 ) , leading to a nearequilibrium propenyl radical concentration of 10-1°'2M. Reactions (5) and (6) are minor; methylacetylene and propene yields were about 4% or less in preliminary runs in accord with the equilibria involved. In order for reaction (4) to be effective in consuming propenyl radicals, isomerization to allyl (8) would have to have a rate constant ks /> 107"6s~-1. It is generally believed that such isomerizations, involving the strain of creating a cyclic, 4-membered transition state:
lOl°.8;k,(Ct)=
1028sec -1
1077 Sec-1
The reverse reaction ( - 8) turns out to be the fastest yielding a concentration of allyl of 10-72M. The rate of recombination of 2 ~ (g) was found by Golden ~ to be equal to the rate of 2CH 3 which in this system species has a very low equilibrium constant (Kg) and, therefore, the product of recombination (9), the 1.5 hexadiene, is in too low concentration even when converted by C1 atoms to a stabilized radical (10). Its concentration is given by the equilibrium (9) since R _ 9 > RI0.
Although the addition of ~ to C: H2 seems to be an attractive pathway leading by isomerization (12) to a highly stabilized radical with the possibility of growing linearly by adding CH3, the thermo-dynamics are not favorable enough, R-ll > Ri2, '~.' Eq. (I1) = 10-~"SM, a n d ~ = 10-SM. This concentration is too small to make it effective in recombination reactions or further growth. The more favored, rapid cyclization (12') forms the stable cyclopentene-yl-4. This is followed by unimolecular loss of H and H formation of the stable cyclopentadiene. The concentration of the latter is limited though by the small concentration of pentadienyl radicals. H2C/~CH2 The recombination of ~ with CH3 is, however, competitive although the equilbrium concentration have prohibitively high activation energies. We have estimated the activation energy as being equal to the o f / ~ - is 10-72M while (CH3) is 10-62M because the strain of a cyclobutene ring (29.6 kcal/mol) + the larger termination rate constant compensates for this. intrinsic energy of H metathesis by vinyl from ethane This larger rate constant arises from a number of (7 kcai/mol). The frequency factor was estimated with factors. A factor of 2 arises from symmetry, another the methods described in Ref. 7 to be /> 1013"TSeC-l. factor of 2 from the fact that allyl has two active sites With these parameters the isomerization becomes while CH3 only one and a factor of about 2.5 arises competitive and, we can conclude that in general, 1,3 from the fact that CH3 + ~ will not be as far in the H-shift reactions from vinylic to allylic radicals, can fall-off as 2CH3. 1-Butene undergoes the back reaction ( - 13) faster be important at high temperatures. The following reactions of the allyl have been than the successive chlorine attack (14) which forms examined:
F i
[@ ...
1'
c--.]
(-8)
_
+
".
D
C2H2
I
+
~
CH3
(13)~ ~
(12')
_-'-
©
B
tv
+
H
@-"
÷ H +
RH .
Soot initiation in methane systems the stabilized l-butene-3-yl radical: (-13) ~o
~
(14)Ct
(
m
+
CH3
+
HC(
.
The equilibrium concentration, of butene-1 o f 10-65M, (calculated from g13 ) is sufficiently high to allow a competitive chlorination (14). We have neglected the formation of the unstabilized primary radical. The butene-l-yl-3 radical can reach relatively high concentrations as well, namely 10 -7.3 to 10 -7.8 (from reaction 14) but not as high as allyl. Its recombination with CH3 is probably not a major pathway of formation of 1-methylbutadiene (17). The dehydrogenation to butadiene (18) is faster even than the reverse reaction ( - 14) and therefore reaction (14) will not be equilibrium. Butadiene production is thus determined by step 14: (-14) +
HCt
+
CH 3
,~
15 -
~+
¢ C
~,
CH~
21
C
+ Ct
+
.
Unfortunately we have a measurement of butadiene in only one experiment; therefore, the estimations regarding the fate of butadiene have to be viewed with caution. Slightly higher values by a factor of 2 than the one used would make the fit really good. Reaction (21) is in equilibrium. The product, pentene-l-yl-3 is in too low a concentration to contribute via recombination with CH3; however, it competitively forms 1,3 pentadiene by step (22) in rather high c ~ncentration (10- ~4 M) sufficient for effective chlorine attack (23) to produce the highly stabilized, 1,3 pentadienyl-5 radical. The concentration of this radical is high, in comparison to CH3; their recombination rate should also be high producing 1,3 hexadiene in sufficiently high concentration (10 -6.6M) to permit effective chlorine attack (25) to form a stabilized 1,3 hcxadiene-5-yl radical. The concentration of the latter (10-84M) is not high enough to effectively recombine with CH3 but high enough to dehydrogenate to 1,3,5 hexatriene. This compound which is highly stabilized, could reach high equilibrium concentrations if its reactivity were not so
C[
16 C -
+
HCt
H.
The attractive pathway to benzene formation which starts with the addition of C2H 3 to butadiene (19) is not predominant here because of the tow concentrations of C, H3 and C4H6. The unimolecular reaction (18) is quite fast and allows large concentrations of butadiene to build up if reaction ( - 18) is the only sink for it. Butadiene concentration estimated from Kj~ is 10 -3.: M. The detected concentrations were however smaller by almost two orders of magnitude. This indicates the existence of additional very fast butadiene consumption routes. Production of C4H5 ( - 2 ) by chlorine attack is not significant because of the small (CI/HCI) ratio which maintains small C4H5 concentrations. The same is true for the addition of allyl (20). The addition of CH3 (21) is however a competitive route:
•+"
279
high. In a succession of rapid reactions (27-29) it forms benzene: "
C
+
"C
Ct
+ CH3
-"
~
+
H
+
HCt _
"
25'(CL)
"~
26°
© ©
+ H
C 27
At 1300K isomerization of 1,3 cyclohexadiene to the 1,4 compound can occur by 1,2 H shift (E ~ 62 kcal) competitively with CI atom attack. This estimate is based on the biradical mechanism for isomerization of cyclopropane derivatives and vinyl cyclopropane/ In that case rapid dehydrogenation produces benzene + H: in a unimolecular step:
M. WEISSMAN and S.W. BENSON
280 _
"
~
+ Ct
-HC~"
©
+
H2
÷
Ha.
The fate of the benzene has been analyzed using thermochcmical data. The formation and the reactions of phcnyl radical sccm to have no importance in the normal pyrolysis of CH3CI. It was important only in the experiments in which benzene was added initiallyto the reactants. A mechanism similar to that consuming C2H2, involving CH3 additions, dehydrogenations, Hmetathesis by CI, formation of stabilized radicals which recombine with CH3 when they arc in really high concentration or dehydrogenate, seems to be operational in the sequence benzene-naphthalene as well: +
30© -
Ct
31
CH3
©.
+
HCL
CH2Ct
+ C3H,Ct
D
CtCH~CH2~CH=CHCt
(36)
a
CtCH2~CHCL--CH~"CH:
(37)
rapidly produces isomers of dichlorobutene that dehydrochlorinate to 1-chlorobutadiene (38,39): C4H~CI2
-
HCt + C4HsCt .
CH3
(38.39)
The dehydrochlorination is fast enough even if it occurs thermally and not by chemical activation. The thermal step is slower though than the reverse of reactions (36) and (37). Very fast steps (38) and (39) are followed by further additions of chloromethyl radicals to produce isomers of dichloropentenyl radical: 4O
-
~
CHaCt
CH3
+
CsH~Ct2(I,5)
C4HsCt
41
43
CsHTCL2(4,5)
30 _' " ~
C[
CH3
C
t
+ ('t
(40)
32
33
~
=,
(Ell3) . ~
veral steps
We estimate that the addition-substitution reaction (30) of CH3 with benzene will have a rate constant of about 10Sl/mol-sec at 1300 K with an activation energy of about 13 kcai. Toluene once formed will be brought to equilibrium with benzyl radical by the HCI/C1 buffer. Benzyl will add CH3 to form ethyl benzene; that leads with C1 to styrene from which are formed next, sequentially, methyl styrene, phenyl allyl, phenyl butene, phenyl butene-yl, phenyl butadiene, and finally naphthalene. 3.3. Polymerization Pathways Involving CHeCI The addition of CH2CI to C2H 2 forms a chloropropenyl radical that can isomerize (35) to an allylic radical by a 1-3 H-atom (or Cl-atom) shift analogous to the propenyl radical: + C2H2
P
CH'---CH~CH2Ct
CH"-'CH~CHaC t
D
CHCt~CH~CH2
CH2CL
determined by the partial equilibrium (34) is high enough to ensure a rate of reaction (35) competitive with the overall production of carbon. The recombination of chloroallyl (AI1CI) and chloromethyl radicals (36,37):
(34) •
(35)
Since R-34 < R35, reaction (36) is rate determining (Table 3).* The concentration of chloroallyi (AIIC1) * In the 4--center transition state for the 1-3 CI atom shift, there is estimated to be 6 kcal less strain than in a 1-3 H atom shift. E~5 ~ 30kcal.
They dechlorinate at rates close to the reverse of their formation (42,43) and competitive with the overall flux of carbon atoms. These reactions are followed by further addition of CH2C1 (44) and formation of dichlorohexenyl radical: CH:Ct
+
CsH~Ct
~
C6H9C1.~(1,6)
(44)
which contains enough energy to dechlorinate through chemical activation. The thermal reaction though, is fast too, faster than the reverse of the formation reaction (44): C6HgCt2
=
C( + C6H9C((6) "
(45)
6-chlorohexadiene dehydrochlorinates competitively to hexatriene (46): C6HgCt
."
HCI.
+
C6H a
(46)
that forms benzene through hydrogen elimination at a very fast rate. The formation of allyl radicals through l-chloropropcne, C3H4Ct
+
HCI.
C~HsC(
~-
C~HsC(
"
C3Hs
+
CL
+ Ct
is too slow to be competitive with the overall reaction. In systems containing sufficient excess of methane, where CH 3 radicals have significant concentrations and form allyl radicals through reactions (4,8) described above, chloromethyl radicals can recombine with allyl:
Soot initiation in methane systems CH2Ct
+
C~Hs
m
CaHTCt-4
(47)
281
4. S T O I C H I O M E T R Y CONSIDERATIONS REGARDING RELATIVE ROLE OF CH 3 AND
C4H'/C[
o,
C,IH¢
÷
HCt.
(48)
Reaction (48) is expected to be faster than the comparable decomposition of EtCI. CH~CI *
C4H6
~
CsHsCt-5
(49)
The 5-chloro pentenyl radical has no rapid unimolecular path available to it except the reverse so that step (49) is in equilibrium. Much slower than ( - 4 9 ) is the unimolecular step (50) which is faster than competing bimolecular steps: CsHaCt-5
P
~ ' / " ~ C I
+
H .
(50)
5-Chloropentadiene 1-3 can react rapidly with CH3 or CH2CI to form hexadienes:
c.3 I
51,5i
THE
CHzCI
The mechanism described above predicts correlations between the CH4/CH3CI ratio and the change in final product yield distribution via the change in the CH3/CH2CI ratio with C H J C H 3 C I and the changing of the predominant role of these radicals in the recombination and polymerization reactions. We have found that, despite their crude character, the experimental data confirm the change in product distribution with the CH4/CH3C1 ratio predicted by the mechanism. Thus, from three hydrogen atoms available in CH3CI, one is tied up in the formation of HCI. If no CH4 is formed, both remaining hydrogen atoms form H2. I f ~ moles of CH4 are formed, (1 - 2ct) moles of hydrogen will be formed. If on the contrary, fl moles CH4 are consumed, more hydrogen than is available from CH3CI, (1 + 2fl) moles, is formed (Table 4).
~ c t
}
CH:Ct Y
53,54
+
Ct
ct
The 6-chloro hexadiene-l,3 formed in (54) will rapidly lose HCI in a 4-center process to form hexatriene 1,3,5 which then cyclizes as in steps (27), (28), and (29) to benzene. Thus, the chloromethyl radical forms competitive polymerization sequences which include its addition to double and triple bonds, dechlorination of the adduct or its recombination with chloromethyl followed by dehydrochlorination of the product or recombination and dechlorination of the monochlororadical formed. A critical step is the isomerization of the chloropropenyl radical into the allyl form which involves a 1-3 H-atom shift. This reaction and not the one that precedes it (the addition of CH:C1 to C, H2) is the critical bottleneck in the polymerization of C2H2 in this system. The step is fast enough to be compatible with the overall rate of reaction.
Since CH4 is formed from CH 3 and the maximum amount of CH3 is limited by stoichiometry and kinetics, the yield of CH4 can be used to indicate how much c n 3 was left to participate in the recombination reactions that form acetylene and in the polymerization reactions of acetylene and higher compounds. The precision with which we have measured CH4 is relatively poor and therefore we have followed the H2 concentration since it is correlated with the CH4 concentration (Table 4). From the table it can be seen that if CH4 is formed in the reaction then (H2)/ A(CH3CI) < 1 (where A(CH3CI) is the amount of CH3CI consumed at the given time of reaction). If CH4 is neither formed nor consumed. (H2)/ A(CH3CI) = 1 and if CH4 is consumed. (H:)/ A(CH3C1) > 1. If methane is consumed in an amount equal to the
TABLE4. Theoretical relative final product yields in the pyrolysis of methylchloride with and without added methane CH4
C(s) A C H 3CI
H: ACH~CI
HCI
ACH4
A C H 3C1
A C H 3C1
1 -
1
absent initially and formed in reaction
1-
present initially and not consumed in reaction
1
1
1
0
present initially in large excess and consumed in reaction
1 + fl
1 ~- 23 (>1)
1
-fl
2~
(<1)
M. WEISSMANand S.W. BENSON
282
methylchloride a m o u n t consumed, the formation of C , ' s and their polymers occurs only through CH3 radicals and (H.,)/A(CH3CI) = 3.
CL
CH3CL
)-
CH3 + Ct
÷ CH(
~
CH3 + HCL
2CH3
~
C2H6
C:H~ ~
2C(s) + 3H2
net: CH4 + CH3Ct -'--.~"
2C(s) + 3H: + HCt
Thus, if (H:)/A(CH3C1) > 3 a pyrolysis o f CH4 was induced. In Table 5 are given experimental data of (H2)/ A(CHaC1) and (CH4)/(CH3C1) per time interval at various reaction times. The data crudely confirm that as long as CH4/ CH3C1 < 2.5 4- 0.5, (A(H:)/A(CH3C1))o.I~ ~ 0.3. This latter is the ratio between the concentration changes in H 2 and CH3C1 in a time interval of 0.1 sec. F o r (2.5 + 0.5) < (CH4)/(CH3C1)) < (4.5 + 0.5), ( A ( H : ) / A ( C H 3 C I ) ) 0 ~ gradually increases to ! and for (CH4)/(CH3CI) > 4.5 _ 0.5, (A(H:)/A(CH3 C1))0~ ~ > 1. The latter situation, characteristic for CH4 rich systems where CH3 starts to play a predominant role is not encountered throughout a whole run.
Even in the experiment with the largest excess of methane used, ((CH4)o/(CH3CI)o = 2), CA(H,.)/ A(CH3CI))/01,~ > 1 only after 50% decomposition. Since in c o m p u t e r simulation of the reaction one cannot be limited to the description of a steady state but must model the reaction from its start and at all (CH4)o/(CH3C1)o ratios used in the experiments, a shorter or longer initial period is characterized by a predominance of CH,,C1 radicals. Then CH.,CI reactions must be included in the mechanism when modeling the reaction. ~m~, is limited by stoichiometry to 0.5 (when no H., is formed and all hydrogen from CH3C1 transforms into HCI and CH4). The reaction mechanism places an additional constraint on :Cmax: CH3Ct
I"
CH3 + CL
Ct + CH3Ct
---
HCt + CH:Ct
CH3 + CH3Ct
)
CH4 + CH~Ct
2CH:Ct
)
CzH2 + 2HCt
C2H:
)
2C(s) + H2
net: 3CH3Ct
)
2HCL + 2C(s) + H2 + CH4
Thus, ~m~, = (ACH4)/(ACH3CI) = 1/3. In order for ~ to have its m a x i m u m value, all CH3
TABLE 5. Experimental hydrogen yields and CH4./CH3CI ratios at various time intervals T(K)
~ (sec)
1310
CH 4
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0
0.28
1.00
2.27
3.57
8.0
7.7
0.33
0.27
0.28
0.27
0.26
0.26
0.19
0.25
0
0
1.13
1.24
4.96
7.6
10.36
0.35
0.47
0.56
2.5
0.8
1.74
CH3C1 H2 ACH3C1 AH,
t t0
0
se~
1260
CH 4
3.08
14.3
28.5
22.8
CH3C1 H,
0
22.9
27.0
0.67
0.70
3.25
3.75
ACH3C1 AH~,
1360
CFI4
0.23
0.40
0.71
1.44
2.13
3.22
4.90
6.75
0.24
0.43
0.51
0.24
0.70
1.0
CH3CI H2 ACH 3Cl All,. AACH~ C1/0.t s¢¢
4.0
0.90
Soot initiation in methane systems radicals would have to form C H 4 . IfCH3 is consumed to polymerize C 2 H 2 , the C H 4 concentration will be smaller and H2 will be increased. However, if CH2CI is polymerizing C: H2, all CH3 is free to form CH4 and the C:H ratio of 1:0.5 is conserved. The experimental data in which CH4 was not present in the beginning but is formed in the reaction are close to these proportions: =
(CH4)max
(C) max =
oexp = 0.74
0.23 (I-(,)
= 0.77
(H2)m,x = 0.52 (1-2(*) = 0.54. As ~ < 0.33, s o m e c n 3 must have reacted otherwise than by forming CH4. This agrees with the formation of small quantities of C:H4 where CH3 presumably had to be involved. It is also interesting to note that C2 H4 has smaller concentrations than C2 H: and starts to build up later than C2H2. This is expected since in the early parts of the reaction, (CH4)/(CH3 CI) is very small and the predominance of CH:CI o v e r C H 3 is large. Only later, when (CH3CI) diminishes considerably and CH4 is formed, the concentration of CH3 presumably increases and its role in the recombination reactions (CH3 + CH2C1 C2H5C1 ---, C2H4 HCl) becomes significant. The hydrogen profiles lag behind the C(s) profile, indicating that it is formed mainly as described by the mechanism in the polymerization reactions and that other hydrogen producing reactions that occur early in the mechanism, such as C:H~ --, H + C2H4 and C2H3 --. H + C2H2 do not play a significant role in
+
o+i\
283
the initial part of the reaction for all (CH+)o/(CH3C1)o ratios used in the experiments.
5. MODELINGTHEREACTION In Fig. 2 are represented the product yields predicted by the mechanism. The timescale of CH3CI decay and product build-up is shown and the profiles of the major products match closely the experimental profiles. The C.,H4 profile peaks somewhat lower (0.015 molar fraction) than the experimental profile (0.025 molar fraction). This can be adjusted varying the recombination rate constants within their accuracy limits. The acetylene profile peaks, as expected, earlier than the C2 H4 profile. The maximum is however higher (0.08 mole fraction) than the experimental peak (0.06 mole fraction). The decline of the profile (0.07) is also slower than the experimental profile (0.04). We have found that including steps of polymerization of C 6 H 6 t o naphthalene and of naphthalene to C(s) that involve CH3 and CH2CI radicals, the profile of C2H2 can approach the experimental profile since the CH2CI radicals are removed from the C2H2 formation reactions (Fig. 3). Since the stoichiometric eoefficients of C H 3 and CH:C1 in its reaction with naphthalene to soot is a global, empirical number, a more detailed modeling would be useful only if data on intermediate species concentrations were available. We feel, however, that it is reasonable to assume that C~ radicals are consumed in polymerization steps of naphthalene and higher aromatics in paths similar to those described above in the polymerization of C2-C~
o
0.50
O.08
o.oG
°'~5[k o.oo, ,----, 0.00 0.25 0.50
U 0.75
+:
::;:F "0.00
TIME (SEC)
"'.
:
0.25 0.50 TIME {SEC)
o:F/
0.75
0.025 (exp) iI" .......
0.020
0.015
5 ~o.o~o 0.005
0.00
'350.0 I
I
I
0.25 0.50 TIME (SEC)
0.75
0.50[
'~ 3.0, ::
0.25 0.50 TIME (SEC)
1 0,75
2 " 0 1 ~
,'
1.0
i
O0 I / 0.00
I 0.25
I 0.50
TIME (SEC)
J 0.75
'0 7.5
0.75[
w 5"0t/~
o5op
o.25
,3ooot-
5
t275.0~--/ t250.0 } 0.00
7---
0.00
'
i I 0.25 0.50 TIME (SEC)
0.75
o.2ot/ o.,st/
o. g-/ /
o.,q-/
0.05 / 0.00
I I 0.25 0.50 TIME (SEC)
0.75
0.01 0.00
I 0.25
0.50
TIME (SEe)
0.75
0.00 /
0,00
J 0.25 0,50 TIM[ (SEC]
FIG. 2. Computer simulation of the pyrolysis of methyl chloride without methane addition. Benzene is described to convert to carbon and hydrogen in one global step.
0"75
M. WEISS/dAN and S.W. BENSON
284
2 . 2 ~ . . , .+o6 . . . . . . . . . . .
%- . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
/
0.9
t.
o.,
O
0 ~o
2o.,p \
o 0 . ~
>o+
\
I
° .......
..-0 ..........
0
< o.61-
\
.'+, .,... ............. ..o.+b
\."
0.2;
.,
I
° •
•
................ -.+:.+ +:;'/
,,..
o.3 -
o - .... .' •
--I--~
..;. •
. . . . .
+
_
_
-
.
-
o., t-
.........
~
o.2F
........
~. . . .
0 O b~/'~'L4~ : ~ " ~1
~-m---~
0.0 0.0
0.t
0.2
0.3
0,4
TIME
,3[ ,.2
---v. - -
. . . . Ol " " " C H ~
.... O .... C
--~--
.... o . . . . H a
2*CzH 4
I
TIME
0.7
-~ . . . . . . . . . . . . . .
•
.o
~
]
0.3
0.4.
(SEC)
---v.- • 2* C z H a
. . . . c---. CH 4 --~-2*CzH4
2"C a H a
__~_
0.2
CH 3 CI
. , - - . o - - CH 3 Cl
• •
0.6
(SEC)
.bo....~....c-....o-....e- .........
uI'"
t.0
0.5
;
0.1
---o-C .. -o-+ - H 2
included and experimental product yield profiles can be used to adjust rate parameters within their accuracy limits.
b 6. I O N I C
PATHWAYS
0.9 0
0.8
o.z ~, 0.6
~ 0.5 ~ 0.4 o_ 0.3
.........:::8 .....~u ........
• .e" ; " o'~'
0.2 OJ 0.0'
0.0
~""~d~'["
t 0.1
l
i
'
'
0.2
0.3
I
0.4
TIME (SEC) ----o---- C H 3 C I
...-o...- CH4 l - - ~ - - -- 2 * C z
H4
.--v.-- 2 " C z H z • .-o-.-- C ---~
....
Hz
FIG. 3. Computer simulation of the pyrolysis of methyl chloride without and with methane additive. Empirical reactions of polymerization of benzene with CH: C1 and CH3 to soot, HC1, and H: are included in the mechanism. compounds and that the reaction mechanism described, with its reasonable estimates of thermochemistry and rate parameters provides a fast enough route to soot initiation in this CI/CH+ system. The inclusion of the formation and consumption of CH2CI, in the mechanism needs further experimental support. The formation seems to be fast enough and the consumption seems to rapidly reform CH2CI radicals so that the flux of carbon from CH3C1 to C(s) is not perturbed. Systems with large excess of methane in which little or no soot is formed should be in principle completely modelable since no empirical reactions must be
There has been much discussion on the role of ions in soot formation. We have demonstrated here that within uncertainties of the order of a factor of 2 radical pathways are quite capable of accounting for the formation of aromatic molecules at 1300 K in the absence of oxygen containing species. At the higher temperatures reached in flame systems 1600-2000 K, H and H2 can play the same role as is played in the present system by the C1/HCI buffer. At the higher temperature all relative rates will be faster, particularly endothermic steps and radical concentrations will also be higher. Hence there seems to be no difficulty in accounting for aromatic formation via radical pathways. Is it still possible that ions play a role? The halogen system is perhaps the most interesting from this point of view since CI atoms have the highest electron affinity (3.6eV) compared to all common species expected in flames and as a consequence one expects equilibrium ion concentrations to reach their highest levels. Let us consider as an example the equilibrium: A R +
CL
-
R+ +
Ct-
.
If R has an ionization potential comparable to that of the t-butyl radical (6.90 + 0.1 eV) then reaction A is endotbermic by 3.3 eV -- 76 kcal. The entropy change is about - 2 . 8 eu due mainly to spin changes so that at 1500K, AGA = 80kcal and KA (1500 K) -- 10- t m7. Assuming that we have electroneutrality and (R +) -- ( C I - ) then we see that the equilibrium ion concentrations are about 1 0 - 6 of the
Soot initiation in methane systems
285
mean of the product of (CI)(R). We have seen that the radical pathways have rate constants for the rate determining steps which are already within factors of 10- ~to 10 -2 of collision frequencies. Thus in order for ion pathways to contribute to aromatic formation they would have to have cross sections for reactions at 1500K which are from 104 to 106 larger than geometric cross sections. Langevin cross sections for ion-molecule reactions at 300K are geometric or smaller and decrease with increasing temperature. Hence equilibrium ion concentrations will not contribute measurably to aromatic formation. It has frequently been suggested that super-equilibrium concentrations of ions may be found in radical-radical reactions such as CH + O CHO + + e - . The difficulty with such proposals is that they invoke the participation of relatively rare species such as CH with very high heats of formation (142 kcal). Even if such reactions occurred at every collision their frequency would not be sufficient to produce ion concentrations in excess of the equilibrium concentration calculated here. One can see that in terms of the joint heats of formation, to produce ions, any radical-radical reaction must have sufficient energy to exceed the ionization potential of the neutral products of the same reaction. In consequence it is very unlikely that chemi-ionization processes can ever produce ion concentrations in flames sufficiently in excess of equilibrium to be of importance in the production of aromatics.
(5) Reasonable thermochemistry and kinetic parameters produce a computer simulation of the available data. When soot is formed, an empirical number of CH 3 and CH2C1 radicals was included in a globally described step of polymerization of C r H 6 to C(s). In a CH4 rich system when the yield of soot is small, the computer simulation should be entirely nonempirical. Can one generalize these conclusions to other systems? Probably not. Methane systems such as the one we have studied are unique in being dominated by the methyl radical. Systems starting with C~ hydrocarbons would probably be dominated by the allyl radical while benzene systems would show a similar dominance by phenyl radicals. A toluene system would be expected to have benzyl radicals as the dominant species. It is important to realize that above 1500 K differences in bond dissociation energies of as much as 7 kcal produce less than a factor of 10 difference in equilibrium populations of radical species. A-factors and concentration play a much more important role than they do at lower temperatures. An important application of this which is a special feature of the systems we examined is provided by the CI/HCI buffer. The very rapid reactions of both of these species with molecules and radicals respectively virtually guaranteed equilibrium between R and RH species. In higher temperature flame systems H/H2 reactions can play a similar role.
7. CONCLUSIONS
Acknowledgements--This work has been supported by
The analysis of the condensation process leading to the formation of polynuclear aromatics in methane systems reveals a number of unexpected results. (1) The process is dominated by the sequential addition of methyl to stabilized radicals or dienes ultimately to form C,H,, species. (2) For any given value of n, the number of carbon atoms in a radical or molecular species, growth is achieved by addition of CH3 radicals. When n is even the dominant species is an unsaturated molecule with significant conjugation or aromatic delocalization. When n is odd the dominant species is a stabilized radical. (3) Radicals with even numbers of carbon atoms are minor species as are molecules with odd numbers of carbon atoms. (4) Chloromethyl radicals are kinetically equivalent with methyl radicals forming competitive polymerization sequences. The fast additional routes available to chlorinated species (dehydrochlorination and dechlorination) that can proceed at these temperatures even through chemical activation, might explain the higher propensity to soot of chlorinated hydrocarbons when compared with related hydrocarb o n s lsomerization reactions proceeding via 1-3 Hshifts become important at high temperatures.
Grants from the National Science Foundation, and the U.S. Army Research Otfice, and by the Independent Research and Development program of McDonnell Douglas Corporation. REFERENCES
1. COLE, J. A., BITTNER,J. D. and HOWARD,J., Radical pathways in soot formation, Symposium on Combustion Chemistry, T. Sloane (Ed.) (1984). 2. TAQAN,M., Dissertation, MIT (1984). 3. WElSSMAN,M. and BENSON,S. W., Int. J. Chem. Kinet. 16, 307 (1984). 4. WARNATZ,J., Combustion Chemistry. W. C. Gardiner, Jr., (Ed.), Springer-Verlag (1984). 5. CRC, Handbook o f Biomolecular and Termolecular Reactions, Vols I and II, J. A, Kerr and S. J. Moss, (Eds), CRC Press, Boca Raton, Florida (1981). 6. WEISSMAN,M. and BENSON, S. W., Poster PS58, X X Syrup. (Int.) Combust., Ann Arbor, Michigan, 1984; J. Phys. Chem. (1988). 7. BENSON,S. W., Thermochemical Kinetics (2nd Edn) J. Wiley and Sons, New York (1976). 8. GOLDEN,D. M., J. Phys. Chem. 83, 108 (1979); Cohen. N., X I X Syrup. (Int.) Combust. 31 (1982). 9. BENSON,S. W. and O'NEAL, E., Kinetic Data on Unimolecular Gas Phase Reactions. Butterworth, London (1972). 10. DEAN,A., Int. Conf. on Chem. Kinet.. Washington. D.C., 1985 (in press). 11. ROSSl, M., KING. K.D. and GOLDEN, D. M., J Am. Chem. Soc. 101, 1223 (1979).