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The oxidation of ethylbenzene near 1060K
COMBUSTION A N D F L A M E 63:251-267 (1986)
251
The Oxidation of Ethyibenzene Near 1060K T. A. LITZINGER, K. BREZINSKY, and I. GLASSMAN Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544
Flow reactor data from the oxidation of ethylbenzene are analyzed to deduce the major reactions involved in removing the ethyl group. Three major routes are found: (i) direct cleavage of the sidechain followed by the oxidation of the benzyl radical, (ii) displacement of the ethyl by a radical species, and (iii) abstraction of a hydrogen from the ethyl group that leads to the formation of styrene from which the removal of the vinyl group occurs through displacement or oxidative attack. Because of the importance of styrene in the ethylbenzene mechanism, results from a styrene oxidation are also reported. The experimental results are used to derive quantitative information on the relative importance of the three major paths through linear regression of equations derived with a steady state analysis. Finally the reaction sequence beginning with abstraction shows a strong analogy to results for the oxidation of ethane, and suggests that some results for the alkanes can serve as guides in understanding the results for higher normal alkylated aromatics.
INTRODUCTION Past work on the oxidation of normal alkylated aromatics has pointed to the fact that after the sidechain is removed the oxidation of these fuels is essentially that of benzene [1, 2]. Therefore recent work has concentrated on the process of removal of the sidechain. In particular, a recent paper discusses the removal of the methyl from toluene plus some striking similarities between the oxidation of methane and toluene [3]. Many of the reactions of the benzyl radical that lead to the removal of the sidechain are analogous to reactions of the methyl radical under similar conditions. The present work investigates the oxidation of ethylbenzene and styrene which is found to be a key intermediate in the oxidation of ethylbenzene. Again the process of sidechain removal is emphasized, and three parallel paths will be presented to explain the experimental results. These three paths involve homolysis of a methyl group from the sidechain, radical displacement, Copyright © 1986 T. A. Litzinger et al. Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
and radical abstraction of a hydrogen from the ethyl group. The abstraction path is seen to produce large amounts of styrene and also shows similarities to reactions of ethane. This analogy can be used to formulate qualitative predictions for early reactions of higher normal alkylated aromatics. Such predictions may eventually lead to relatively simple models for the oxidation of normal alkylated aromatics.
EXPERIMENTAL A P P A R A T U S A N D RESULTS The Princeton turbulent flow reactor (Fig. 1) was used to obtain the chemical samples during the oxidation of the aromatic fuels. This reactor consists of three basic sections--a heat source, a mixing section for introduction of fuel and oxidizer, and the test section where samples are taken. In all of the experiments the major constituent of the gas stream is nitrogen, which comprises over 97% of the flow on a molar basis. The heat source is a plasma torch which raises a small amount of nitrogen to tempera-
252
T. A. LITZINGER ET AL.
PLASMA INLET S~C'R(~ - - 1
Fig. l. Schematic of the chemical kinetic flow reactor apparatus.
tures in excess of 10,000K; the remaining cold nitrogen is then mixed with the effluent from the torch to achieve the desired reaction temperature. A short distance downstream of the heat source oxygen enters the hot nitrogen and mixes with it prior to the addition of fuel. Fuel vapor produced by the evaporator system is injected into the oxygen/nitrogen stream through four ports at the throat of a converging-diverging nozzle. Injection at the throat produces excellent mixing and yields a homogeneous mixture of fuel and oxidizer at the inlet of the test section which consists of a quartz duct 10 cm in diameter and 1 m in length. The flow reactor which operates at 1 atm pressure has a temperature range of 900-1300K, and it is maintained at nearly adiabatic conditions by insulation and thermostatically controlled heaters around the test section and nozzle. With a 10 cm duct, flow velocities of 400-1200 cm/s can be achieved which are equivalent to a Reynolds number of approxi-
mately 4000-12,000. Thus the reactor always operates at Reynolds numbers in excess of the minimum for turbulent flow in a tube. The measured radial profiles of mean velocity are found to approximate plug flow even at the exit of the test section. Because of the plug flow profile and the good initial mixing, samples taken along the centerline of the test section are sufficient to characterize the chemistry. Gas samples are extracted at 15 uniformly spaced locations along the test section with a water-cooled probe, and at each sample point, the temperature is recorded by a thermocouple which is attached to the probe. The samples are stored in stainless steel loops of 12 cm 3 volume; these loops are contained inside a cylindrical chamber which is heated to prevent condensation of the sample species. After the sample storage chamber is connected to a gas chromatograph, the analysis proceeds automatically. At present the system yields concentrations of hydrocarbons from methane to species with 14
THE OXIDATION OF ETHYLBENZENE NEAR 1060K TABLE 1
Experimental Conditions Ethylbenzene Equivalence ratio
Styrene
0.64
0.95
1.3
0.56
Initial temp. (K) 1066 Temp. rise (K) 17
1056 10
1056 10
1056 10
Mole fraction Fuel Oxygen Nitrogen Total flow (moles/s)
1.1E-3 1.7E - 2 0.98
0.9E- 3 1.0E- 2 0.98
0.99
0.69
1.0E- 3 0.9E- 3 0,8E- 2 1.6E- 2 0.99 0.98 0.61
1.1
carbon atoms. On-line meters measure the concentration of oxygen, carbon monoxide, and carbon dioxide. When necessary the quantitative analysis is augmented with mass spectroscopy to identify unknown products. Emphasis must be placed on the fact that no radical species are detected because of the sampling and analysis techniques which are used. Further information of the flow reactor and associated systems is available in Refs. [41 and [5]. The experimental conditions for the oxidation of ethylbenzene and styrene are presented in Table 1. Both the ethylbenzene and styrene were of 99% purity from Aldrich Chemical; the inhibitor in the styrene was below detectability limits of the GC analysis because of the low fuel concentration in the experiments. The initial temperature of 1060K and the rather low fuel concentration of 1000 ppm were selected to allow " e a r l y " reactions, i.e., those involved in removing the sidechain, to be observed. In addition, the chosen conditions resulted in slow reaction such that the experiments were nearly isothermal. The initial fuel concentration was intentionally fixed for all experiments to eliminate it as a variable; therefore the stoichiometry was varied by changing the oxygen concentration. Finally the total molar flow rates were changed for each experiment in anticipation of
253
the fact that leaner conditions would result in faster reaction. Adjusting the total flow changes the residence time in the test section and allows additional control over the portion of the reaction which is observed. In summary, the fuel and oxygen concentrations, the initial temperature, and the total flow rate were selected so that the ethylbenzene was only partially oxidized in order that the reactions which remove the sidechain could be studied in detail. Figures 2a-2c illustrate the chemical species which are observed in the oxidation of ethylbenzene for the fuel lean experiment. The presence of products from the oxidation of the ethylbenzene in the first sample is due to the reaction that occurs in the diffuser located before the test section (Fig. 1). Note that the equivalence ratio, phi, is defined as the actual fuel/oxidizer ratio divided by the stoichiometric fuel/oxidizer ratio. The ordinate for these plots is mole fraction of species, and the abscissa is the flow time from injection of the fuel to the point where the sample is removed. Since the flow is nearly onedimensional and the position of the sample extraction is accurately known, the calculation of the residence time is readily performed. The fuel data in Fig. 2a indicate that the fuel is being consumed in reactions that have an apparent first-order character. Subsequent to the rapid fuel decay, a relatively large quantity of styrene is formed and consumed. The appearance of styrene as the ethylbenzene disappears strongly suggests that the fuel is being converted to styrene. The other major aromatic products include toluene and benzaldehyde, which peak later than styrene, and benzene and phenol, which rise throughout the reaction zone. In addition to these major aromatic products, methylphenylketone, benzofuran, and phenylacetylene are observed at ppm concentrations but are not plotted. The nonaromatic species from cyclopentadiene down to methane are characteristic of the oxidation of benzene [1, 2]. Finally, Fig. 2d shows that the total amount of carbon is conserved in the reaction zone, and thus no major intermediate products remain undetected. The results for the other two ethylbenzene experiments are shown in Figs. 3 and 4.
254
T. A. LITZINGER ET AL.
CSIHIO x 0.5
× z
Ixl O
O
0
10 20 30 40 50 60 70 80 90 100 TIME FROM INdECTION OF" FUEL ( r n s e c )
Fig. 2. Results for the lean oxidation of ethylbenzene (PHI = 0.64). a) ethylbenzene (CsHlo), styrene (CsHs), toluene (C7H8), benzaldehyde (C7H60), benzene (C6H6), and ethylene (C2H4).
2.5
z 1.5
_o
_w l -"'ta"°< -'l 1
/
o .5 C4H4
0
, ~
0
_
I
,
I
,
I
t
I
,
I
i
I
,
I
10 20 30 40 50 60 70 80 90 100 TIME FROM INJECTION OF FUEL ( r n s e c )
Fig. 2b. Phenol (C6H60), cyclopentadiene (C5H6) , vinylacetylene (C4H4), and acetylene (C2H2).
POSTULATED
REACTION ROUTES
Before the discussion of the reaction paths, the effect of the mixing of the fuel and oxidizer on the growth of radical species must be discussed to explain the importance of reactions between radicals and the fuel. Because the total mixing
time is short compared with the observed reaction time, the mixing will not dramatically affect the homogeneous chemistry in the test section. However, the mixing process reduces the induction period relative to shock tube experiments on the oxidation of alkylated aromatic fuels. For
THE O X I D A T I O N OF E T H Y L B E N Z E N E NEAR 1060K
255
.8,
W X Z .6
_o I.0.< t~
.4 W J 0
.2
/
/_C3"s C2H6
--
-
-
~
~.
-
= vC4.-HS; • 0
L ~ ]
I
10
20
0
,
I
,
30
I
,
40
I
,
50
I
i
60
I
,
70
l
t
80
I
90
,
I
100
T I M E FROM I N J E C T I O N O F F U E L ( r e s e t )
Fig. 2c. Methane (CH4). ethane (C2H0, butadiene ( C 4 H 6 ) (C3's).
,
and three carbon species
1.2 02 -q m ° 0
1.1 ~" m ;O >. --4 C ;:0 1 m
1..5
X
Z 0 t--
0 < n," t t. hi --I 0
CTOT
1
~
o
~ x .9
0
' 0
10
20
,:30
40
50
60
70
80
90
I 100
m I
.8
T I M E FROM I N J E C T I O N O F F U E L ( m a e c )
Fig. 2d. Carbon monoxide (CO), molecular oxygen (02), summation of carbon atoms in the products (CTOT), and temperature profile in the reaction zone.
e x a m p l e , under the conditions of the lean experiment, the induction time for ethylbenzene is approximately 60 ms based on a correlation o f the shock tube results [6]; h o w e v e r , in the flow reactor, most o f the fuel is c o n s u m e d in 80 ms. Evidently the mixing process produces conditions that result in a very rapid growth of highly reactive species, like hydrogen and o x y g e n
atoms or hydroxyl radicals, and therefore rapid fuel consumption. During the mixing process, many " p o c k e t s " of fuel and o x y g e n exist which have widely varying stoichiometries, and s o m e pockets will have quite short induction times based upon their stoichiometry. Therefore at the entrance to the test section, radical species are at sufficient levels such that they dominate reac-
256
T . A . LITZINGER ET AL.
4
FCSHIO r-
2.5
x 0.5
I 2
~
C2H4
x x
g
z 0
1.5
C2H2 o
C7H6@
.5
C4H4_\ .
0
20
40
60
80
100
120
140
20
TIME FROM INJECTION OF FUEL ( m s e c )
4-0
60
BO
.
.
CSHS =
.
1 O0
120
140
TIME FROM INJECTION OF FUEL ( r e s e t )
Fig. 3a.
Fig. 3b.
1.2
.8 CH4
o
r x
Z 0
1.5
1.1
~ A A A . A A ' A A ~ A ~~ M P ~
x ,6
C
CO <
1
~,o;/ o
0 ~
C2H6 .2
-
.
.
.
.
.
.
.
.
.
x
9
.5
C3'
.8 20
40
60
80
1 O0
120
140
TIME FROM INJECTION OF FUEL ( r e B e c )
20
40
60
80
1 O0
120
140
TIME FROM INJECTION OF FUEL ( r n s e c )
Fig. 3d.
Fig. 3c.
Fig. 3. Results (a)-(d) for the near stoichiometric oxidation of ethylbenzene (PHI = 0.95); same species designation as Fig. 2.
tion with major stable hydrocarbons over initiation reactions like hydrogen abstraction by molecular oxygen. In this section, three reaction paths are postulated to explain the experimental results for ethylbenzene; the initial reactions of the three sequences are displacement and abstraction by radical species and homolysis. For each reaction route, a sequence of reactions is proposed which is consistent with the observed reaction prod-
ucts. Following this qualitative discussion of the routes, their relative contributions to the consumption of the ethylbenzene are estimated in the analysis section. A. H o m o l y s i s o f the Ethyl Group Homolysis of the ethyl group begins the reaction sequence shown in reactions 1-4 where represents the aromatic ring: q~C2Hs--*~bt~H2+ CH3,
(1)
THE OXIDATION OF ETHYLBENZENE NEAR 1060K
257
2.5! C8H10
x 0.5
,~,,3 x
x
C8H8 =
-
C2H
z
1.5
_o
,52
I-
b
C6H6
b-
u_
o
0
1
.5
I
20
40
60
80
1 O0
120
,
I
140
160
0 0
20
TIME FROM INJECTION OF F U E L ( r e s e t )
40
60
80
1 O0
120
140
160
TIME FROM INJECTION OF F U E L ( m a e c )
Fig. 4a.
Fig. 4b.
I 1.2
.8 0 1.5 x z 0
1.1
x
CH4
.6
A
A
L
•
A
±
±
±
:
±
±
"-
±
±
~.
±
z
5
4
CTOT
z=
~.4
1
m
× o
0
C H6
.2
co ,o
0
oo
0
TIME FROM INJECTION OF F U E L ( m s e c )
, , , , i ,_i
~
, j. 9 ~
'
~ L. .8
20
40
60
80
1 O0
120
140
160
TIME FROM INJECTION OF F U E L ( m s e c )
Fig. 4c.
Fig. 4d.
Fig. 4. Results (a)-(d) for the rich oxidation of ethylbenzene (PHI = 1.3); same species designation as Fig. 2.
~b(~H2+(O, HO2)-~q~CHO+(H, O H + H ) ,
(2)
@CHO + X--* ¢CO + XH,
(3)
~bCO--*6 + CO.
(4)
In addition to benzaldehyde formed in reaction 2, this sequence would yield the stable products, toluene, benzene, and phenol through additional reactions of the benzyl and phenyl radicals [1, 3]. Because of the large activation energy of the
homolysis [7-9], it is not expected to initiate the most important route at the temperature of this study. In fact a calculated high pressure rate constant for this reaction [9] would predict that this route accounts for no more than 15 % of the fuel consumption observed in the experiments. This route will become increasingly important as temperature rises and thus it is included for completeness.
258
T . A . LITZINGER ET AL.
B. Displacement of the Ethyl Group Another path which can occur in this system is the displacement of the ethyl group by a radical species. Hydrogen displacement of the methyl group of toluene is a well documented reaction [10, 11]; thus, reaction 5 would appear to be a feasible step for ethylbenzene:
q~C2H5+ H - ~ H + C2H5.
(5)
Clearly the hydrogen displacement could account for some of the benzene observed. Based on the large amounts of phenol detected, another possibility seems to be displacement by a hydroxyl radical, reaction 6:
(Figs. 2a, 3a, and 4a) tend to support the postulate of displacement reactions as a major reaction path. In fact the ethylene shows quite similar growth curves to both the benzene (and phenol) as indicated by the data figures. However, alternative routes to benzene and ethylene exist in the last path to be discussed, and therefore the presence of these species is not conclusive proof of the importance of the displacement reaction.
C. Radical Abstraction of a Hydrogen from the Sidechain
The role of such reactions for aromatics is a current topic of investigation [12, 13], and the experimental evidence indicates that only abstraction occurs for hydroxyl in the temperature range of this study. In the oxidation of the aromatic ring several routes to phenol are possible [1]; thus, the phenol found in the present experiments can be explained without the hydroxyl displacement reaction. The fate of the ethyl radical formed by the displacement reactions can shed some light on the mechanism. Under the conditions of this study the expected reactions of the ethyl radical are
The final sequence to be discussed begins with the abstraction of a hydrogen from the ethyl group of ethylbenzene by a radical species such as H, O, or OH. Two types of hydrogens exist on the sidechain, benzylic and primary, and the benzylic will be easier to abstract because of their weaker bonding [16]. At present, limited information is available on the selectivity of various radical species toward benzylic versus primary hydrogens; therefore, the actual extent to which the benzylic abstraction dominates cannot be estimated from rate data. The abstraction of a benzylic hydrogen will lead to the formation of styrene as indicated in reactions 10 and 11, where X represents O, H, or OH, the species that are expected to dominate abstraction:
C2H5 ~ C2I-I4+ H,
(7)
~C2H5 + X'-*~bCHCH3 + XH,
(10)
C2H5 + O2"-~C2H4 + HO2,
(8)
q~(~HCH3--~q~C2H3+ H.
(11)
C2H5 + RH-'~ C2H6 + I~.
(9)
As large amounts of styrene are observed in the products, this route is apparently quite significant. In the case of primary hydrogen abstraction (reaction 12), the
~bC2H5+ OH ~ ~bOH + C2H5.
(6)
In reaction 9, RH represents any hydrocarbon and I~ is the corresponding radical formed by abstraction of a hydrogen. At 1060K with a representative molecular oxygen concentration of 1 E - 7 mole/cm 3, the rates of reactions 7 and 8 are, respectively, I E5 and 2 E4 s-1 [14, 151. An estimate of the rate constant for formation of ethane via reaction 9 is 1 El3 e x p ( - 5 E 3 / R T ) cm3/mole s, where R is the ideal gas constant, 1.987 cal/mole K. With an approximate RH concentration of 1000 ppm the rate of reaction 9 is 1 E4 s -1. Thus reaction 7 is dominant, and over 90% of the ethyl radicals formed will become ethylene. The large amounts of ethylene
~bC2H5+ X--~0CH2CH2 + XH
(12)
fate of the radical formed is not so clear. The two possible reactions are q~CH2t~H2~ q~+ C2H4,
(13)
~bCH2CH2--*t~C2H3+ H.
(14)
Unfortunately, rate constants are not available for these two reactions to determine whether one of these reactions is dominant; thus, approximate rate constants must be considered. At
THE OXIDATION OF ETHYLBENZENE NEAR 1060K 1000K, these two paths have quite close heats of reaction of approximately 37 kcal/mole. An intrinsic activation energy of about 2 kcal/mole must be added to the endothermicity of reaction 14; no intrinsic activation energy is expected for reaction 13 because of the relatively high electron affinity of the phenyl radical [ 17, 18]. Thus the activation energy for reactions 13 and 14 have values of approximately 37 and 39 kcal/ mole, respectively. Both of the decomposition reactions produce a stable and a radical species and therefore should have tight transition states and similar preexponential factors [19]. Because the transition state for the loss of the vinyl group should have a greater change in entropy than the loss of a hydrogen, reaction 13 should have a somewhat larger preexponential by approximately a factor of four. Thus the reaction leading to phenyl and ethylene appears to be more important since it is expected to have a larger preexponential and a lower activation energy. However, these estimates are not of sufficient accuracy to rule out completely some contribution of reaction 14. In summary the abstraction of a benzylic hydrogen from the ethyl group will produce styrene, and the abstraction of a primary hydrogen will produce either a phenyl radical or styrene. In order to complete the picture of the removal of the sidechain the reactions of styrene must be considered. For styrene the reactions 15, 17, and 18 are possible: q~C2H3+ X ~ b C H I ~ H + XH,
(15)
~bCHt~H ~ ~bCCH + H,
(16)
~bC2H3+ H--*~bH + C2H3,
(17)
~bC2H3+ O---,oxygenated species.
(18)
The abstraction reaction [Eq. (15)] is suggested by the observation of phenylacetylene presumably resulting from reaction 16. In the ethylbenzene experiments only small amounts of phenylacetylene were observed indicating that reaction 15 is relatively slow compared with the competing steps reaction 17 and 18. Previous work [1, 2] on the oxidation of ethylbenzene suggested that reaction 17 accounted for the major consumption of the
259
styrene based upon the correlation of the benzene and ethylene profiles. However, some portion of both of these products is explained by the displacement reaction [Eq. (5)]. In addition, this earlier study proposed that the toluene and benzaldehyde that were observed could be explained completely by the homolysis route, i.e., reactions 1-4. In order to verify whether the homolysis route was indeed sufficient, the rate data of Ref. [9] was used to estimate the maximum contribution of this route. When the predicted rate of growth of products from the homolysis route was compared with the experimental results, the predictions were always too small. Also the agreement was the poorest for the experiments with the greatest oxygen concentration, which strongly suggests that oxidation reactions like reaction 18 are occurring and that they are producing benzyl radicals or benzaldehyde. In order to facilitate the determination of the important reactions of styrene, it was oxidized under conditions similar to the lean ethylbenzene experiment. The data from the oxidation of styrene are presented in Figs. 5a-5c for fuel lean conditions at a temperature of approximately 1060K. Due to difficulty with the polymerization of styrene in the evaporator system only one experiment was performed. These results suggest that displacement of the vinyl group is occurring because of the close correlation of the benzene and ethylene. Also, the presence of large amounts of benzaldehyde early in the reaction zone indicates that oxidative attack on styrene is significant because no other routes to benzaldehyde exist for styrene. The actual reactions that occur in the oxidation of styrene are uncertain; however, the work of Sloane and Brudzynski [20] on styrene reactions with atomic oxygen provides some insight to the actual path of reaction 18. In their study an adduct of mass 120 was observed and was identified as phenylacetaldehyde along with a fragment of mass 91 which was the benzyl radical. In addition, calculations of the frontier electron densities of styrene were presented and
260
T . A . LITZINGER ET AL.
.¢
~ . 5
4
x Z3
o_
I-O < w U-2 hi / O
c7H8o _,,..,.-.,-----7~.~-" V c8.6
_...~-~
c7.8
c8.10
O 0
I0
20 TIME
,.30 FROM
4-0
50
60
INJECTION
OF
70 FUEL
80
90
I00
(maec)
Fig. 5. Results for the lean oxidation of styrene (PHI = 0.56). (a) Styrene (C8H8), ethylbenzene (CsHtd, toluene (O7tt8), benzaldehyde (C7H60), benzene (C6H6) and ethylene (C2tt4).
2.5
'dbJ
2
x Z 0
1.5
I-0
< n," u_ b.l _I 0
I
,~
0
0
C6H60 .5
0
10
20 30 4-0 50 60 70 80 90 TIME FROM INJECTION OF" FUEL (rnsec)
100
Fig. 5b. Phenol (C6H60), cyclopentadiene (C5H6), and acetylene (C2H2).
indicate that the density is highest on the terminal carbon of the vinyl group. Thus, the highly electrophilic oxygen atom will preferentially attach to the terminal carbon. Even though this study was conducted at low temperature it can still serve to give some guidance to the
present higher temperature study particularly because the electron densities should not change dramatically with temperature. Hence the suggested path is
~C2H 3 + O--* ~bCH2CHO-'* O(~H2 + HCO.
(19)
THE OXIDATION OF ETHYLBENZENE NEAR 1060K
261
'd- .8 Ld × Z .6 0 )-tJ < /E h_ .40
CH4
.2
.
~
L I ~',~t"~
0 0
10
20
C
. 30
2
H
6
v 40
L 50
60
70
q 80
F K 90
100
TIME FROM INJECTION OF FUEL ( r n s e c ) Fig.
5c.
Methane
(cn4),
ethane
(C2H6) , butadiene
(C4H6) ,
and v i n y l a c e t y l e n e
(C4H4),
1.2
x Z 0 I-0 < l,
rq 1.1 ~" qJ rq ;0
O2
00 1 . 5
-_ *
=_
.
.
.
.
. TEMP
C 1
1 CTOT
v x
0
.9
• AI *.
0 0
10
m
t
20
.30
4-0
50
60
70
80
90
I
mI
.8
100
TIME FROM INJECTION OF FUEL ( r n s e c ) Fig. 5d. C a r b o n monoxide ( C O ) , molecular oxygen (02), summation of carbon atoms in
the products ( C T O T ) ,
and temperature profile in the reaction zone.
The difficulty in applying such a reaction sequence in the present case is that no phenylacetaldehyde was conclusively identified in the samples. Thus reaction 19 can be proposed as a plausible path for oxidation of styrene but it is not certain to be the only path. Alternative reactions include the addition of atomic oxygen to the carbon attached to the ring; such a reaction could produce methyl phenyl ketone
which is found in the products. Also, addition of oxygen to the ring may occur, and reactions involving the hydroxyl radical appear possible as well [20]. The information available from the ethylbenzene and styrene experiments and the literature suggest that the abstraction route leads to the formation o f styrene from the ethylbenzene. The vinyl group of the styrene is then removed by
262
T . A . LITZINGER ET AL.
10
--
164 Z O O
16 ~ bJ -J 0
PHI 0.64
Fuel
Rodlcal
0.95
v O
• •
1.3
16° -
1(~ 7 I 0
1 20
t
I
t __L~
I
, ~__,
40
60
80
1 O0
TIME FROM INJECTION
I 120
,
I 14-0
,
I 160
OF F U E L ( m s ° c )
Fig. 6. Ethylbenzene profiles (open symbols) and estimated radical concentrations (solid symbols) based upon the consumption rate of ethylbenzene for the experiments of Figs. 24,
displacement or it is oxidized to yield a benzyl radical. Finally this benzyl radical is oxidized as indicated in reactions 2-4, and the process of sidechain removal is then complete. A N A L Y S I S OF R E S U L T S In this section the implication of the apparent first order disappearance of the ethylbenzene is explored in the context of the proposed mechanism. This first order behavior, illustrated in Fig. 6, implies that the radical pool exists under quasi-steady conditions. Furthermore, the steady nature of the radical pool is exploited to produce estimates of the relative importance of the three proposed reaction paths. Physically, the steady state exists because the fuel and products consume radicals so quickly that the consumption and production of radicals approximately balance. The homolysis route has been shown to be a minor one, less than 15% of the fuel consumption; therefore, in the following analysis only radical reactions will be considered. The overall rate of fuel consumption can be written in terms
of an average rate constant for the various reactions and the sum of the concentration of the radical species, H, O, and OH, denoted by IX]. Because no significant route for reforming the fuel is anticipated, the resulting equation is d[F] dt
-
k[X][F].
(20)
When considered in the context of Eq. (20), the apparent first order behavior of the fuel is seen to require that the radical pool exist under quasisteady conditions for an isothermal experiment. An average value of the rate constant for radical reactions with the fuel was selected on the basis of available rates. An estimated rate constant for the displacement of the methyl from toluene is 1.5E12 at 1060K [11], and it should be indicative of the rate of ethyl displacement. In order to estimate the rate of abstraction of hydrogens from the ethyl group, rate data on the abstraction of primary hydrogens from alkanes were combined [14, 15] with the measured rate constant for abstraction of benzylic hydrogens from toluene by hydroxyl radicals [21]. Based
THE OXIDATION OF ETHYLBENZENE NEAR 1060K upon the chosen rate constants and estimated radical concentrations for the lean experiment, an average rate constant for hydrogen abstraction from the ethyl group is 3.3E12 cm3/mole s at 1060K. For the estimated radical concentrations of the rich experiment, the average rate constant for abstraction is 3.2E12. Given the uncertainty in the estimated rate constants for abstraction and displacement, the value of 3E12 cm3/mole s was used as representative of radical reactions with the fuel. From the experimental results, the value of k[X] throughout the reaction zone can be determined by k[X] =
1 d[F] [F]
,
(21)
dt
and then the average value of k is used to estimate the total concentration of H, O, and OH. The results of these calculations are presented in Fig. 6 and clearly indicate the quasisteady nature of the radicals. The contribution of the conversion of fuel to styrene to the total rate of fuel consumption can be estimated through extension of this steady state of radicals to the phenylethyl radicals formed by abstraction. To illustrate the method used, the styrene formed from the benzylic radical will be considered. If the benzylic radical is denoted by 1~, its rate equation is d[l~] -= kl0[X][F] - kll [1~], dt
(22)
where k~0 represents the average rate of hydrogen abstraction by radicals, X, and k~ is the rate of decomposition to give styrene. Because the fuel disappears in a first order manner its time behavior is [F] = [Fl0e -*[xlt.
(23)
Now given the fixed rate constants due to the isothermal conditions of the experiments, substitution of Eq. (23) into Eq. (22) allows direct integration to obtain [1~] = [ F ] ( k l o [ X ] ) / ( k l l - k[X]) + b e - * l l t ,
(24)
where b is a constant of integration and depends on the initial conditions of the problem. Based on the large amounts of styrene formed
263
in the experiments the value of kl0[X] should be of the same order as k[X] for fuel disappearance which ranges from 10 to 40 s - i . The value o f k l l is estimated to be 1El4 e x p ( - 4 5 E 3 / R T ) , which gives a value of 2E4 s- l at 1060K. Clearly, k~l ~> kl0[X], and therefore Eq. (24) reduces to [1~] = kl0[X_......~][F] + b e - kll t. k~l
(25)
For any reasonable initial condition, the second term in Eq. (25) is negligible on a time scale of a millisecond because kll is very large. Thus, [1~] can be further approximated to [l~l = k~0IX_~][F]. ktl
(26)
Inspection of Eq. (22) shows that this solution for [1~] corresponds to the steady state solution. The formation of styrene, S, via the decomposition of the benzylic radical leads to a rate equation for styrene of d[S]
= k~ [1~] - L [S],
(27)
dt
where L = kiT[H] + k18 [O]. Now substitution of the steady state solution for [1~], Eq. (26), produces diS] dt
= kt0[X][F] - L[S].
(28)
Finally substitution for F in terms of d F / d t from Eq. (20) gives d[S]
d[F]
dt
-dt
( k~o[X])/( k [ X ] ) - L [S],
or
d[S] dt
d[F] f - L [S],
(29)
dt
where f is the rate of conversion of fuel to styrene normalized by the total rate of fuel consumption. In Eq. (29), f represents the fraction of the fuel consumption rate due to the production of styrene via the abstraction route. Analysis of the route to styrene through abstraction of a primary hydrogen yields an equation of the same form as Eq. (29). Therefore, Eq. (29) represents the general rate equa-
264 tion for styrene. From the experimental results, IS], d[S]/dt, and d[F]/dt are known and linear regression analysis [221 on Eq. (29) produces values for f and L. The results from the regression are listed in Table 2 along with the two standard deviation limits. These results show that the rate of production of styrene is nearly 50% of the fuel consumption rate regardless of the equivalence ratio. Also, the loss of styrene through radical reactions increases with increasing oxygen concentration which is the expected trend because the rate constants should be the same in all three experiments and the radical concentrations increase with increasing oxygen as shown in Fig. 6. A similar analysis can be performed for the amount of benzene formed of these experiments. However, because two routes to benzene exist, displacement and abstraction, the resulting value cannot be assigned to any one path. Therefore the only conclusion which can be drawn from the results for benzene formation is that the sum of the rates of displacement and primary hydrogen abstraction contribute approximately 30% of the fuel consumption. The value of 30% is derived simply by difference, i.e., 100% less the contributions by the styrene producing and the homolysis routes. DISCUSSION A. R e a c t i o n M e c h a n i s m Based on the analysis of the experimental results for ethylbenzene, three major paths have been proposed for removal of the ethyl group; the reaction sequences are summarized in Table 3. From the rate constant for the homolysis reaction and the analysis of the consumption of fuel to produce styrene, the relative importance of the three paths can be assessed. At 1060K the homolysis rate is 15% of the fuel consumption rate for the rich experiment and only about 5 % for the lean one; therefore it is a minor path. Clearly the significance of this route will increase with increasing temperature because the homolysis has an activation energy of approximately 70 kcal/mole [9]. The abstraction route (reaction 10-18) is the most significant of the
T . A . LITZINGER ET AL. TABLE 2 Results of Regression Analysis on Eq. (29)
Equivalence ratio
f
L (1/s)
0.64 0.95 1.3
0.54 _+ 0.04 0.49 _+ 0.04 0.49 + 0.04
26 +_ 2 9 _+ 1 7 _+ 1
TABLE 3 Proposed Reaction Routes
Homolysis Route ~C2H5 --" ¢CH2 + CH3 $(2H2 + (O, HO2) ~ $CHO + (H, OH + H) ~bCHO + X ~ q~CO + XH 0(20 -" ~ + CO
(1) (2) (3) (4)
Displacement Reaction ~C2H5 + H ~ q~H + C2H5
(5)
Abstraction Route ~C2H5 + X --* ~b(~HCHa + XH ~CHCHa ~ q~C2H3 + H q~C2H5 + X "-~ ~bCHz(~H2 + XH ~CH2CH 2 --~ (~ + C2H4 ~bCH2CH2 --b q~C2H3 + H t~C2H3 + X --* ~CHCH + XH ~CHt~H ~ tkCCH + H ¢~C2H3 + FI "-~ (~I-I + C2H 3 ~C2H3 + O "-~ OCHzCHO -* ¢~CH2 + HCO
(10) (11) (12) (13) (14) (15) (16) (17) (19)
three; this route accounts for at least 50% of the fuel consumption rate based on the regression analysis on the production of styrene. Because abstraction of a primary hydrogen leads to a phenyl radical or styrene (reactions 13 and 14), 50 % is a lower bound on the contribution of this route to the consumption of fuel. Since multiple paths exist to the phenyl radical and its associated products, benzene and phenol, the contri-
THE OXIDATION OF ETHYLBENZENE NEAR 1060K bution of primary abstraction cannot be determined unambiguously. Finally, the displacement route contributes 30% or less of the fuel disappearance rate; the uncertainty in this value lies in the fact that the abstraction route also contributes to the concentrations of benzene and phenol.
B. Steady State Analysis Although the experimental results clearly demonstrate the existence of a quasi-steady radical pool, questions remain as to the applicability of a steady state analysis to an oxidation experiment. Phenomenologically, the existence of a steady state concentration of an intermediate requires a balance between the sources and sinks of the species. In the present study the species of interest are the radicals, H, O, and OH, and the sources are decomposition of the phenylethyl radical and the H + 02 branching reaction. The sink for these radicals is primarily abstraction of hydrogen from the fuel. In a more quantitative sense, the steady state of a species requires that either the source or sink be much greater than the rate of change of the species [23]. If this condition is satisfied, both the source and sink will be much greater than the rate of change, and the balance is ensured. For the radical pool in the present experiments, the magnitude of the sink term is known from the fuel disappearance, i.e., d[F]/ dt = - k[X][F]. From the fuel rich experiment the minimum value of the k[X][F] is 1 . 5 E - 7 mole/cm3 s. The order of magnitude of the rate of change of the radicals is available from the results in Fig. 6; the maximum value of d[X]/dt is 2 E - 11 mole/cm 3 s. Clearly the sink term for the radicals is very much greater than the rate of change even if the average rate constant selected to calculate the radical concentration were changed by an order of magnitude. Therefore the present experiments easily meet the mathematical requirement for a steady state of the radical pool. Another indication that the results derived from the steady state analysis are reasonable is the rate constant for consumption of styrene which is obtained from the regression analysis.
265
First note that the rate of styrene consumption in Table 2 show the reasonable trend that increasing oxygen leads to increased rate of styrene consumption. Because the experiments were all at the same initial temperature and nearly isothermal, the increased rate of styrene consumption can only be explained by an increase in the radical concentration with increasing oxygen. Indeed the estimated radical concentrations in Fig. 6 show an increase with increasing oxygen concentration. When the disappearance rates for styrene are divided by the estimated radical concentrations, an approximate rate constant for radical attack on styrene is obtained. The values of this rate constant are 2.6E12, 2.4E12, and 2.2E12 cm3/mole s; these values agree quite well with the rate for ethylene reacting with oxygen atoms which is 5E12 cm3/mole s at 1060K [15]. The consistency and reasonable magnitude of the rate constant lend validity to the approach.
C. Analogy between Ethane and Ethylbenzene In the introduction, mention was made of the analogy that was found between certain reactions in a methane oxidation and the reactions found to be important in the removal of the sidechain of toluene [3]. Such analogies can also be seen in the present investigation when the proposed reactions of ethylbenzene and styrene are compared with those of ethane and ethylene. Under conditions similar to those of the present study, ethane is known to lose a hydrogen through radical abstraction and then the ethyl radical becomes ethylene through a unimolecular step or reaction with molecular oxygen [15, 24]. The conversion of ethylbenzene to styrene is the precise analog of this sequence of reactions. Once the ethylene has formed it may be attacked by an oxygen atom or it may undergo hydrogen abstraction to form a vinyl radical which will produce acetylene. Again the analogs of these reactions are found in the results of the present study, i.e., reactions 15, 16, and 18. Finally the homolysis of ethylbenzene is seen to be the analog of the homolysis of ethane; in the present system this reaction plays a more impor-
266
T . A . LITZINGER ET AL.
tant role because it has a lower activation energy than the corresponding reaction of ethane. The analogy between alkanes and the normal alkylated aromatics may allow use of the characteristics of alkanes to understand and to some extent to predict the behavior of higher normal alkylated aromatics. Of particular interest is the reaction sequence in normal alkanes in which radical abstraction of hydrogen is followed by the formation of an olefin [25]. For example, consider the propane reactions 30-33: C3H8 + X"-'~ n - C3H7 + XH,
(30)
C3H8+ X~i-
(31)
C3H7 + X H ,
n - C3H7-~ C2H4 + CH3,
(32)
i - C3H7-'r C3H6 + H.
(33)
Given the analogy, reactions 34-39 would be predicted for n-propylbenzene: q~C3H7 + X~0(~HC2H5 + XH,
(34)
0 C 3 H 7 + X---~q~C2nafH2 + XH,
(35)
q~C3H 7 + X~thCH2(~HCH3 + XH,
(36)
q~CHC2Hs---~thC2H3+ (~H3,
(37)
0C2H4CH 2--~t~fH2 + C2H4,
(38)
OCH2(~HCH3-~ ~C3H5 + H.
(39)
In the context of this analogy, a simplification may be possible for normal alkylated aromatics because the benzylic hydrogen bond energy is at least 10 kcal less than primary or secondary carbon-hydrogen bond energy; the lower benzylic C - H bond energy is due to the resonance of the benzyl radical which is formed by the abstraction. (In addition to lowering the bond strength, the resonance of the benzylic radical will affect the magnitude of the preexponential for the abstraction of benzylic hydrogens [26].) The lower energy of the benzylic bond means that the fraction of the benzylic hydrogens which are abstracted will be higher than the ratio of the number of benzylic hydrogens to the total number of hydrogens in the sidechain. The degree to which the abstraction is weighted toward the benzylic hydrogens will depend on the nature of the radicals in the system. However, conditions might allow the abstraction of
benzylic hydrogens to dominate the process, and this would mean a great simplification as then only reactions 34 and 37 would occur. For example, the dominance of styrene as the product of radical attack on the alkyl group has been observed in the pyrolysis of n-butylbenzene [27, 28]. If abstraction of benzylic hydrogen dominated in the present oxidation study, any normal alkylated aromatic would form mostly styrene, and the modeling of the abstraction route which is the most complex of the three routes would be greatly simplified. CONCLUSIONS From a flow reactor study of the oxidation of ethylbenzene at 1060K and atmospheric pressure, three reaction paths have been postulated for the removal of the ethyl group, and the contribution of each path to the rate of consumption of ethylbenzene has been estimated. The most significant route, which represents at least 50% of the fuel consumption rate, begins with the abstraction of a hydrogen from the ethyl group. Subsequently the phenylethyl radicals resulting from the abstraction decompose to form styrene or a phenyl radical. Further reactions of the styrene which complete the process of sidechain removal are displacement of the vinyl group by a hydrogen atom or oxidative attack on the vinyl. Displacement of the ethyl group from the ethylbenzene is found to contribute less than 30% of the fuel consumption rate. Finally, the rate of homolytic cleavage in the ethyl group to form a benzyl radical represents no more than 15 % of the fuel consumption rate at 1060K, the temperature of this investigation.
This work was supported by the Air Force Office of Scientific Research under Contract F49620-82-K-O011 and a general grant f o r combustion research from the Mobil Research and Development Corporation. REFERENCES 1.
Venkat, C., Brezinsky, K., and Glassman, I., Nine-
2.
The Combustion Institute, 1982, p. 143. Venkat, C., MSE Thesis, Chemical Engineering Department, Princeton University, 1981.
teenth (International) S y m p o s i u m on C o m b u s t i o n ,
THE OXIDATION 3.
OF ETHYLBENZENE
NEAR 1060K
Brezinsky, K., Litzinger, T. A., and Glassman, I., Int. J. Chem. Kin. 16:1053-1074 (1984). 4. Hautman, D. J., PhD Thesis, Mechanical and Aerospace Engineering Department, Princeton University, 1980. 5. Dryer, F. L., PhD Thesis, Mechanical and Aerospace Engineering Department, Princeton University, 1972. 6. Miyama, H., J. Phys. Chem. 75:1501-1504 (1971). 7. Swarc, M., J. Chem. Phys. 17:431-435 (1949). 8. Davis, H. G., Int. J. Chem. Kin. 15:469-474 (1983). 9. Robaugh, D. A., and Stein, S. E., Int. J. Chem. Kin. 13:445-462 (1981). 10. Amano, A., Horie, O., and Hanh, N. H., Chem. Lett. 10:917-920 (1972). 11. Benson, S. W., and Shaw, R., J. Chem. Phys. 47:4052-4055 (1967). 12. Lin, C.-Y., and Lin, M. C., Paper No. 86, 1984 Technical Meeting of the Eastern Section of the Combustion Institute. 13. Madrovich, S., and Felder, W., Paper No. 1, 1983 Technical Meeting of the Eastern Section of the Combustion Institute. 14. Warnatz, J., in Combustion Chemistry (W. C. Gardiner, Ed.), Springer-Verlag, New York, 1984, Chapter 5. 15. Westbrook, C. K., and Dryer, F. L., Prog. Energy Combust. Sci. 10:1-57 (1984). 16. McMillen, D. F., and Golden, D. M., Ann. Rev. Phys. Chem. 33:493-532 (1982).
17.
267
Janousek, B. J., and Brauman, J. I., Gas Phase Ion Chemistry, Academic Press, New York, 1979, Vol. 2, p. 53. 18. Benson, S. W., Thermochemical Kinetics, 2nd Ed., Wiley, New York, 1976, p. 194. 19. Benson, S. W., ibid., p. 95. 20. Sloane, T. M., and Brudzynski, R. J., J. A. Ch. S. 101:1495-1499 (1979). 21. Tully, F. P., Ravishankara, A. R., Thompson, R. L., Nicovich, J. M., Shah, R. C., Reuter, N. M., and Wine, P. H., J. A. Ch. S. 85:2262-2269 (1981). 22. Volk, W., Applied Statistics f o r Engineers, 2nd Ed., McGraw-Hill, New York, 1969, p. 261. 23. Williams, F. A., Combustion Theory, Addison Wesley, Reading, MA, 1965, p. 369. 24. Cohen, R., PhD Thesis, Mechanical and Aerospace Engineering Department, Princeton University, 1977. 25. Dryer, F. L., and Glassman, I., Prog. Astronaut. Aeronaut. 62:255-306 (1979). 26. Rossi, M.,andGolden, D . M . , J . A . C . S . 101:12301235 (1979). 27. Albright, L. F., Carnes, B. L., and Corcoran, W. H., (Eds.), Pyrolysis: Theory and Industrial Applications, Academic Press, New York, 1983, p. 132. 28. Rebick, C., Frontiers o f Free Radical Chemistry (W. A. Pryor, Ed.), Academic Press, New York, 1983, p. 81.
Received 17 May 1985; revised 29 July 1985
251
The Oxidation of Ethyibenzene Near 1060K T. A. LITZINGER, K. BREZINSKY, and I. GLASSMAN Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544
Flow reactor data from the oxidation of ethylbenzene are analyzed to deduce the major reactions involved in removing the ethyl group. Three major routes are found: (i) direct cleavage of the sidechain followed by the oxidation of the benzyl radical, (ii) displacement of the ethyl by a radical species, and (iii) abstraction of a hydrogen from the ethyl group that leads to the formation of styrene from which the removal of the vinyl group occurs through displacement or oxidative attack. Because of the importance of styrene in the ethylbenzene mechanism, results from a styrene oxidation are also reported. The experimental results are used to derive quantitative information on the relative importance of the three major paths through linear regression of equations derived with a steady state analysis. Finally the reaction sequence beginning with abstraction shows a strong analogy to results for the oxidation of ethane, and suggests that some results for the alkanes can serve as guides in understanding the results for higher normal alkylated aromatics.
INTRODUCTION Past work on the oxidation of normal alkylated aromatics has pointed to the fact that after the sidechain is removed the oxidation of these fuels is essentially that of benzene [1, 2]. Therefore recent work has concentrated on the process of removal of the sidechain. In particular, a recent paper discusses the removal of the methyl from toluene plus some striking similarities between the oxidation of methane and toluene [3]. Many of the reactions of the benzyl radical that lead to the removal of the sidechain are analogous to reactions of the methyl radical under similar conditions. The present work investigates the oxidation of ethylbenzene and styrene which is found to be a key intermediate in the oxidation of ethylbenzene. Again the process of sidechain removal is emphasized, and three parallel paths will be presented to explain the experimental results. These three paths involve homolysis of a methyl group from the sidechain, radical displacement, Copyright © 1986 T. A. Litzinger et al. Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
and radical abstraction of a hydrogen from the ethyl group. The abstraction path is seen to produce large amounts of styrene and also shows similarities to reactions of ethane. This analogy can be used to formulate qualitative predictions for early reactions of higher normal alkylated aromatics. Such predictions may eventually lead to relatively simple models for the oxidation of normal alkylated aromatics.
EXPERIMENTAL A P P A R A T U S A N D RESULTS The Princeton turbulent flow reactor (Fig. 1) was used to obtain the chemical samples during the oxidation of the aromatic fuels. This reactor consists of three basic sections--a heat source, a mixing section for introduction of fuel and oxidizer, and the test section where samples are taken. In all of the experiments the major constituent of the gas stream is nitrogen, which comprises over 97% of the flow on a molar basis. The heat source is a plasma torch which raises a small amount of nitrogen to tempera-
252
T. A. LITZINGER ET AL.
PLASMA INLET S~C'R(~ - - 1
Fig. l. Schematic of the chemical kinetic flow reactor apparatus.
tures in excess of 10,000K; the remaining cold nitrogen is then mixed with the effluent from the torch to achieve the desired reaction temperature. A short distance downstream of the heat source oxygen enters the hot nitrogen and mixes with it prior to the addition of fuel. Fuel vapor produced by the evaporator system is injected into the oxygen/nitrogen stream through four ports at the throat of a converging-diverging nozzle. Injection at the throat produces excellent mixing and yields a homogeneous mixture of fuel and oxidizer at the inlet of the test section which consists of a quartz duct 10 cm in diameter and 1 m in length. The flow reactor which operates at 1 atm pressure has a temperature range of 900-1300K, and it is maintained at nearly adiabatic conditions by insulation and thermostatically controlled heaters around the test section and nozzle. With a 10 cm duct, flow velocities of 400-1200 cm/s can be achieved which are equivalent to a Reynolds number of approxi-
mately 4000-12,000. Thus the reactor always operates at Reynolds numbers in excess of the minimum for turbulent flow in a tube. The measured radial profiles of mean velocity are found to approximate plug flow even at the exit of the test section. Because of the plug flow profile and the good initial mixing, samples taken along the centerline of the test section are sufficient to characterize the chemistry. Gas samples are extracted at 15 uniformly spaced locations along the test section with a water-cooled probe, and at each sample point, the temperature is recorded by a thermocouple which is attached to the probe. The samples are stored in stainless steel loops of 12 cm 3 volume; these loops are contained inside a cylindrical chamber which is heated to prevent condensation of the sample species. After the sample storage chamber is connected to a gas chromatograph, the analysis proceeds automatically. At present the system yields concentrations of hydrocarbons from methane to species with 14
THE OXIDATION OF ETHYLBENZENE NEAR 1060K TABLE 1
Experimental Conditions Ethylbenzene Equivalence ratio
Styrene
0.64
0.95
1.3
0.56
Initial temp. (K) 1066 Temp. rise (K) 17
1056 10
1056 10
1056 10
Mole fraction Fuel Oxygen Nitrogen Total flow (moles/s)
1.1E-3 1.7E - 2 0.98
0.9E- 3 1.0E- 2 0.98
0.99
0.69
1.0E- 3 0.9E- 3 0,8E- 2 1.6E- 2 0.99 0.98 0.61
1.1
carbon atoms. On-line meters measure the concentration of oxygen, carbon monoxide, and carbon dioxide. When necessary the quantitative analysis is augmented with mass spectroscopy to identify unknown products. Emphasis must be placed on the fact that no radical species are detected because of the sampling and analysis techniques which are used. Further information of the flow reactor and associated systems is available in Refs. [41 and [5]. The experimental conditions for the oxidation of ethylbenzene and styrene are presented in Table 1. Both the ethylbenzene and styrene were of 99% purity from Aldrich Chemical; the inhibitor in the styrene was below detectability limits of the GC analysis because of the low fuel concentration in the experiments. The initial temperature of 1060K and the rather low fuel concentration of 1000 ppm were selected to allow " e a r l y " reactions, i.e., those involved in removing the sidechain, to be observed. In addition, the chosen conditions resulted in slow reaction such that the experiments were nearly isothermal. The initial fuel concentration was intentionally fixed for all experiments to eliminate it as a variable; therefore the stoichiometry was varied by changing the oxygen concentration. Finally the total molar flow rates were changed for each experiment in anticipation of
253
the fact that leaner conditions would result in faster reaction. Adjusting the total flow changes the residence time in the test section and allows additional control over the portion of the reaction which is observed. In summary, the fuel and oxygen concentrations, the initial temperature, and the total flow rate were selected so that the ethylbenzene was only partially oxidized in order that the reactions which remove the sidechain could be studied in detail. Figures 2a-2c illustrate the chemical species which are observed in the oxidation of ethylbenzene for the fuel lean experiment. The presence of products from the oxidation of the ethylbenzene in the first sample is due to the reaction that occurs in the diffuser located before the test section (Fig. 1). Note that the equivalence ratio, phi, is defined as the actual fuel/oxidizer ratio divided by the stoichiometric fuel/oxidizer ratio. The ordinate for these plots is mole fraction of species, and the abscissa is the flow time from injection of the fuel to the point where the sample is removed. Since the flow is nearly onedimensional and the position of the sample extraction is accurately known, the calculation of the residence time is readily performed. The fuel data in Fig. 2a indicate that the fuel is being consumed in reactions that have an apparent first-order character. Subsequent to the rapid fuel decay, a relatively large quantity of styrene is formed and consumed. The appearance of styrene as the ethylbenzene disappears strongly suggests that the fuel is being converted to styrene. The other major aromatic products include toluene and benzaldehyde, which peak later than styrene, and benzene and phenol, which rise throughout the reaction zone. In addition to these major aromatic products, methylphenylketone, benzofuran, and phenylacetylene are observed at ppm concentrations but are not plotted. The nonaromatic species from cyclopentadiene down to methane are characteristic of the oxidation of benzene [1, 2]. Finally, Fig. 2d shows that the total amount of carbon is conserved in the reaction zone, and thus no major intermediate products remain undetected. The results for the other two ethylbenzene experiments are shown in Figs. 3 and 4.
254
T. A. LITZINGER ET AL.
CSIHIO x 0.5
× z
Ixl O
O
0
10 20 30 40 50 60 70 80 90 100 TIME FROM INdECTION OF" FUEL ( r n s e c )
Fig. 2. Results for the lean oxidation of ethylbenzene (PHI = 0.64). a) ethylbenzene (CsHlo), styrene (CsHs), toluene (C7H8), benzaldehyde (C7H60), benzene (C6H6), and ethylene (C2H4).
2.5
z 1.5
_o
_w l -"'ta"°< -'l 1
/
o .5 C4H4
0
, ~
0
_
I
,
I
,
I
t
I
,
I
i
I
,
I
10 20 30 40 50 60 70 80 90 100 TIME FROM INJECTION OF FUEL ( r n s e c )
Fig. 2b. Phenol (C6H60), cyclopentadiene (C5H6) , vinylacetylene (C4H4), and acetylene (C2H2).
POSTULATED
REACTION ROUTES
Before the discussion of the reaction paths, the effect of the mixing of the fuel and oxidizer on the growth of radical species must be discussed to explain the importance of reactions between radicals and the fuel. Because the total mixing
time is short compared with the observed reaction time, the mixing will not dramatically affect the homogeneous chemistry in the test section. However, the mixing process reduces the induction period relative to shock tube experiments on the oxidation of alkylated aromatic fuels. For
THE O X I D A T I O N OF E T H Y L B E N Z E N E NEAR 1060K
255
.8,
W X Z .6
_o I.0.< t~
.4 W J 0
.2
/
/_C3"s C2H6
--
-
-
~
~.
-
= vC4.-HS; • 0
L ~ ]
I
10
20
0
,
I
,
30
I
,
40
I
,
50
I
i
60
I
,
70
l
t
80
I
90
,
I
100
T I M E FROM I N J E C T I O N O F F U E L ( r e s e t )
Fig. 2c. Methane (CH4). ethane (C2H0, butadiene ( C 4 H 6 ) (C3's).
,
and three carbon species
1.2 02 -q m ° 0
1.1 ~" m ;O >. --4 C ;:0 1 m
1..5
X
Z 0 t--
0 < n," t t. hi --I 0
CTOT
1
~
o
~ x .9
0
' 0
10
20
,:30
40
50
60
70
80
90
I 100
m I
.8
T I M E FROM I N J E C T I O N O F F U E L ( m a e c )
Fig. 2d. Carbon monoxide (CO), molecular oxygen (02), summation of carbon atoms in the products (CTOT), and temperature profile in the reaction zone.
e x a m p l e , under the conditions of the lean experiment, the induction time for ethylbenzene is approximately 60 ms based on a correlation o f the shock tube results [6]; h o w e v e r , in the flow reactor, most o f the fuel is c o n s u m e d in 80 ms. Evidently the mixing process produces conditions that result in a very rapid growth of highly reactive species, like hydrogen and o x y g e n
atoms or hydroxyl radicals, and therefore rapid fuel consumption. During the mixing process, many " p o c k e t s " of fuel and o x y g e n exist which have widely varying stoichiometries, and s o m e pockets will have quite short induction times based upon their stoichiometry. Therefore at the entrance to the test section, radical species are at sufficient levels such that they dominate reac-
256
T . A . LITZINGER ET AL.
4
FCSHIO r-
2.5
x 0.5
I 2
~
C2H4
x x
g
z 0
1.5
C2H2 o
C7H6@
.5
C4H4_\ .
0
20
40
60
80
100
120
140
20
TIME FROM INJECTION OF FUEL ( m s e c )
4-0
60
BO
.
.
CSHS =
.
1 O0
120
140
TIME FROM INJECTION OF FUEL ( r e s e t )
Fig. 3a.
Fig. 3b.
1.2
.8 CH4
o
r x
Z 0
1.5
1.1
~ A A A . A A ' A A ~ A ~~ M P ~
x ,6
C
CO <
1
~,o;/ o
0 ~
C2H6 .2
-
.
.
.
.
.
.
.
.
.
x
9
.5
C3'
.8 20
40
60
80
1 O0
120
140
TIME FROM INJECTION OF FUEL ( r e B e c )
20
40
60
80
1 O0
120
140
TIME FROM INJECTION OF FUEL ( r n s e c )
Fig. 3d.
Fig. 3c.
Fig. 3. Results (a)-(d) for the near stoichiometric oxidation of ethylbenzene (PHI = 0.95); same species designation as Fig. 2.
tion with major stable hydrocarbons over initiation reactions like hydrogen abstraction by molecular oxygen. In this section, three reaction paths are postulated to explain the experimental results for ethylbenzene; the initial reactions of the three sequences are displacement and abstraction by radical species and homolysis. For each reaction route, a sequence of reactions is proposed which is consistent with the observed reaction prod-
ucts. Following this qualitative discussion of the routes, their relative contributions to the consumption of the ethylbenzene are estimated in the analysis section. A. H o m o l y s i s o f the Ethyl Group Homolysis of the ethyl group begins the reaction sequence shown in reactions 1-4 where represents the aromatic ring: q~C2Hs--*~bt~H2+ CH3,
(1)
THE OXIDATION OF ETHYLBENZENE NEAR 1060K
257
2.5! C8H10
x 0.5
,~,,3 x
x
C8H8 =
-
C2H
z
1.5
_o
,52
I-
b
C6H6
b-
u_
o
0
1
.5
I
20
40
60
80
1 O0
120
,
I
140
160
0 0
20
TIME FROM INJECTION OF F U E L ( r e s e t )
40
60
80
1 O0
120
140
160
TIME FROM INJECTION OF F U E L ( m a e c )
Fig. 4a.
Fig. 4b.
I 1.2
.8 0 1.5 x z 0
1.1
x
CH4
.6
A
A
L
•
A
±
±
±
:
±
±
"-
±
±
~.
±
z
5
4
CTOT
z=
~.4
1
m
× o
0
C H6
.2
co ,o
0
oo
0
TIME FROM INJECTION OF F U E L ( m s e c )
, , , , i ,_i
~
, j. 9 ~
'
~ L. .8
20
40
60
80
1 O0
120
140
160
TIME FROM INJECTION OF F U E L ( m s e c )
Fig. 4c.
Fig. 4d.
Fig. 4. Results (a)-(d) for the rich oxidation of ethylbenzene (PHI = 1.3); same species designation as Fig. 2.
~b(~H2+(O, HO2)-~q~CHO+(H, O H + H ) ,
(2)
@CHO + X--* ¢CO + XH,
(3)
~bCO--*6 + CO.
(4)
In addition to benzaldehyde formed in reaction 2, this sequence would yield the stable products, toluene, benzene, and phenol through additional reactions of the benzyl and phenyl radicals [1, 3]. Because of the large activation energy of the
homolysis [7-9], it is not expected to initiate the most important route at the temperature of this study. In fact a calculated high pressure rate constant for this reaction [9] would predict that this route accounts for no more than 15 % of the fuel consumption observed in the experiments. This route will become increasingly important as temperature rises and thus it is included for completeness.
258
T . A . LITZINGER ET AL.
B. Displacement of the Ethyl Group Another path which can occur in this system is the displacement of the ethyl group by a radical species. Hydrogen displacement of the methyl group of toluene is a well documented reaction [10, 11]; thus, reaction 5 would appear to be a feasible step for ethylbenzene:
q~C2H5+ H - ~ H + C2H5.
(5)
Clearly the hydrogen displacement could account for some of the benzene observed. Based on the large amounts of phenol detected, another possibility seems to be displacement by a hydroxyl radical, reaction 6:
(Figs. 2a, 3a, and 4a) tend to support the postulate of displacement reactions as a major reaction path. In fact the ethylene shows quite similar growth curves to both the benzene (and phenol) as indicated by the data figures. However, alternative routes to benzene and ethylene exist in the last path to be discussed, and therefore the presence of these species is not conclusive proof of the importance of the displacement reaction.
C. Radical Abstraction of a Hydrogen from the Sidechain
The role of such reactions for aromatics is a current topic of investigation [12, 13], and the experimental evidence indicates that only abstraction occurs for hydroxyl in the temperature range of this study. In the oxidation of the aromatic ring several routes to phenol are possible [1]; thus, the phenol found in the present experiments can be explained without the hydroxyl displacement reaction. The fate of the ethyl radical formed by the displacement reactions can shed some light on the mechanism. Under the conditions of this study the expected reactions of the ethyl radical are
The final sequence to be discussed begins with the abstraction of a hydrogen from the ethyl group of ethylbenzene by a radical species such as H, O, or OH. Two types of hydrogens exist on the sidechain, benzylic and primary, and the benzylic will be easier to abstract because of their weaker bonding [16]. At present, limited information is available on the selectivity of various radical species toward benzylic versus primary hydrogens; therefore, the actual extent to which the benzylic abstraction dominates cannot be estimated from rate data. The abstraction of a benzylic hydrogen will lead to the formation of styrene as indicated in reactions 10 and 11, where X represents O, H, or OH, the species that are expected to dominate abstraction:
C2H5 ~ C2I-I4+ H,
(7)
~C2H5 + X'-*~bCHCH3 + XH,
(10)
C2H5 + O2"-~C2H4 + HO2,
(8)
q~(~HCH3--~q~C2H3+ H.
(11)
C2H5 + RH-'~ C2H6 + I~.
(9)
As large amounts of styrene are observed in the products, this route is apparently quite significant. In the case of primary hydrogen abstraction (reaction 12), the
~bC2H5+ OH ~ ~bOH + C2H5.
(6)
In reaction 9, RH represents any hydrocarbon and I~ is the corresponding radical formed by abstraction of a hydrogen. At 1060K with a representative molecular oxygen concentration of 1 E - 7 mole/cm 3, the rates of reactions 7 and 8 are, respectively, I E5 and 2 E4 s-1 [14, 151. An estimate of the rate constant for formation of ethane via reaction 9 is 1 El3 e x p ( - 5 E 3 / R T ) cm3/mole s, where R is the ideal gas constant, 1.987 cal/mole K. With an approximate RH concentration of 1000 ppm the rate of reaction 9 is 1 E4 s -1. Thus reaction 7 is dominant, and over 90% of the ethyl radicals formed will become ethylene. The large amounts of ethylene
~bC2H5+ X--~0CH2CH2 + XH
(12)
fate of the radical formed is not so clear. The two possible reactions are q~CH2t~H2~ q~+ C2H4,
(13)
~bCH2CH2--*t~C2H3+ H.
(14)
Unfortunately, rate constants are not available for these two reactions to determine whether one of these reactions is dominant; thus, approximate rate constants must be considered. At
THE OXIDATION OF ETHYLBENZENE NEAR 1060K 1000K, these two paths have quite close heats of reaction of approximately 37 kcal/mole. An intrinsic activation energy of about 2 kcal/mole must be added to the endothermicity of reaction 14; no intrinsic activation energy is expected for reaction 13 because of the relatively high electron affinity of the phenyl radical [ 17, 18]. Thus the activation energy for reactions 13 and 14 have values of approximately 37 and 39 kcal/ mole, respectively. Both of the decomposition reactions produce a stable and a radical species and therefore should have tight transition states and similar preexponential factors [19]. Because the transition state for the loss of the vinyl group should have a greater change in entropy than the loss of a hydrogen, reaction 13 should have a somewhat larger preexponential by approximately a factor of four. Thus the reaction leading to phenyl and ethylene appears to be more important since it is expected to have a larger preexponential and a lower activation energy. However, these estimates are not of sufficient accuracy to rule out completely some contribution of reaction 14. In summary the abstraction of a benzylic hydrogen from the ethyl group will produce styrene, and the abstraction of a primary hydrogen will produce either a phenyl radical or styrene. In order to complete the picture of the removal of the sidechain the reactions of styrene must be considered. For styrene the reactions 15, 17, and 18 are possible: q~C2H3+ X ~ b C H I ~ H + XH,
(15)
~bCHt~H ~ ~bCCH + H,
(16)
~bC2H3+ H--*~bH + C2H3,
(17)
~bC2H3+ O---,oxygenated species.
(18)
The abstraction reaction [Eq. (15)] is suggested by the observation of phenylacetylene presumably resulting from reaction 16. In the ethylbenzene experiments only small amounts of phenylacetylene were observed indicating that reaction 15 is relatively slow compared with the competing steps reaction 17 and 18. Previous work [1, 2] on the oxidation of ethylbenzene suggested that reaction 17 accounted for the major consumption of the
259
styrene based upon the correlation of the benzene and ethylene profiles. However, some portion of both of these products is explained by the displacement reaction [Eq. (5)]. In addition, this earlier study proposed that the toluene and benzaldehyde that were observed could be explained completely by the homolysis route, i.e., reactions 1-4. In order to verify whether the homolysis route was indeed sufficient, the rate data of Ref. [9] was used to estimate the maximum contribution of this route. When the predicted rate of growth of products from the homolysis route was compared with the experimental results, the predictions were always too small. Also the agreement was the poorest for the experiments with the greatest oxygen concentration, which strongly suggests that oxidation reactions like reaction 18 are occurring and that they are producing benzyl radicals or benzaldehyde. In order to facilitate the determination of the important reactions of styrene, it was oxidized under conditions similar to the lean ethylbenzene experiment. The data from the oxidation of styrene are presented in Figs. 5a-5c for fuel lean conditions at a temperature of approximately 1060K. Due to difficulty with the polymerization of styrene in the evaporator system only one experiment was performed. These results suggest that displacement of the vinyl group is occurring because of the close correlation of the benzene and ethylene. Also, the presence of large amounts of benzaldehyde early in the reaction zone indicates that oxidative attack on styrene is significant because no other routes to benzaldehyde exist for styrene. The actual reactions that occur in the oxidation of styrene are uncertain; however, the work of Sloane and Brudzynski [20] on styrene reactions with atomic oxygen provides some insight to the actual path of reaction 18. In their study an adduct of mass 120 was observed and was identified as phenylacetaldehyde along with a fragment of mass 91 which was the benzyl radical. In addition, calculations of the frontier electron densities of styrene were presented and
260
T . A . LITZINGER ET AL.
.¢
~ . 5
4
x Z3
o_
I-O < w U-2 hi / O
c7H8o _,,..,.-.,-----7~.~-" V c8.6
_...~-~
c7.8
c8.10
O 0
I0
20 TIME
,.30 FROM
4-0
50
60
INJECTION
OF
70 FUEL
80
90
I00
(maec)
Fig. 5. Results for the lean oxidation of styrene (PHI = 0.56). (a) Styrene (C8H8), ethylbenzene (CsHtd, toluene (O7tt8), benzaldehyde (C7H60), benzene (C6H6) and ethylene (C2tt4).
2.5
'dbJ
2
x Z 0
1.5
I-0
< n," u_ b.l _I 0
I
,~
0
0
C6H60 .5
0
10
20 30 4-0 50 60 70 80 90 TIME FROM INJECTION OF" FUEL (rnsec)
100
Fig. 5b. Phenol (C6H60), cyclopentadiene (C5H6), and acetylene (C2H2).
indicate that the density is highest on the terminal carbon of the vinyl group. Thus, the highly electrophilic oxygen atom will preferentially attach to the terminal carbon. Even though this study was conducted at low temperature it can still serve to give some guidance to the
present higher temperature study particularly because the electron densities should not change dramatically with temperature. Hence the suggested path is
~C2H 3 + O--* ~bCH2CHO-'* O(~H2 + HCO.
(19)
THE OXIDATION OF ETHYLBENZENE NEAR 1060K
261
'd- .8 Ld × Z .6 0 )-tJ < /E h_ .40
CH4
.2
.
~
L I ~',~t"~
0 0
10
20
C
. 30
2
H
6
v 40
L 50
60
70
q 80
F K 90
100
TIME FROM INJECTION OF FUEL ( r n s e c ) Fig.
5c.
Methane
(cn4),
ethane
(C2H6) , butadiene
(C4H6) ,
and v i n y l a c e t y l e n e
(C4H4),
1.2
x Z 0 I-0 < l,
rq 1.1 ~" qJ rq ;0
O2
00 1 . 5
-_ *
=_
.
.
.
.
. TEMP
C 1
1 CTOT
v x
0
.9
• AI *.
0 0
10
m
t
20
.30
4-0
50
60
70
80
90
I
mI
.8
100
TIME FROM INJECTION OF FUEL ( r n s e c ) Fig. 5d. C a r b o n monoxide ( C O ) , molecular oxygen (02), summation of carbon atoms in
the products ( C T O T ) ,
and temperature profile in the reaction zone.
The difficulty in applying such a reaction sequence in the present case is that no phenylacetaldehyde was conclusively identified in the samples. Thus reaction 19 can be proposed as a plausible path for oxidation of styrene but it is not certain to be the only path. Alternative reactions include the addition of atomic oxygen to the carbon attached to the ring; such a reaction could produce methyl phenyl ketone
which is found in the products. Also, addition of oxygen to the ring may occur, and reactions involving the hydroxyl radical appear possible as well [20]. The information available from the ethylbenzene and styrene experiments and the literature suggest that the abstraction route leads to the formation o f styrene from the ethylbenzene. The vinyl group of the styrene is then removed by
262
T . A . LITZINGER ET AL.
10
--
164 Z O O
16 ~ bJ -J 0
PHI 0.64
Fuel
Rodlcal
0.95
v O
• •
1.3
16° -
1(~ 7 I 0
1 20
t
I
t __L~
I
, ~__,
40
60
80
1 O0
TIME FROM INJECTION
I 120
,
I 14-0
,
I 160
OF F U E L ( m s ° c )
Fig. 6. Ethylbenzene profiles (open symbols) and estimated radical concentrations (solid symbols) based upon the consumption rate of ethylbenzene for the experiments of Figs. 24,
displacement or it is oxidized to yield a benzyl radical. Finally this benzyl radical is oxidized as indicated in reactions 2-4, and the process of sidechain removal is then complete. A N A L Y S I S OF R E S U L T S In this section the implication of the apparent first order disappearance of the ethylbenzene is explored in the context of the proposed mechanism. This first order behavior, illustrated in Fig. 6, implies that the radical pool exists under quasi-steady conditions. Furthermore, the steady nature of the radical pool is exploited to produce estimates of the relative importance of the three proposed reaction paths. Physically, the steady state exists because the fuel and products consume radicals so quickly that the consumption and production of radicals approximately balance. The homolysis route has been shown to be a minor one, less than 15% of the fuel consumption; therefore, in the following analysis only radical reactions will be considered. The overall rate of fuel consumption can be written in terms
of an average rate constant for the various reactions and the sum of the concentration of the radical species, H, O, and OH, denoted by IX]. Because no significant route for reforming the fuel is anticipated, the resulting equation is d[F] dt
-
k[X][F].
(20)
When considered in the context of Eq. (20), the apparent first order behavior of the fuel is seen to require that the radical pool exist under quasisteady conditions for an isothermal experiment. An average value of the rate constant for radical reactions with the fuel was selected on the basis of available rates. An estimated rate constant for the displacement of the methyl from toluene is 1.5E12 at 1060K [11], and it should be indicative of the rate of ethyl displacement. In order to estimate the rate of abstraction of hydrogens from the ethyl group, rate data on the abstraction of primary hydrogens from alkanes were combined [14, 15] with the measured rate constant for abstraction of benzylic hydrogens from toluene by hydroxyl radicals [21]. Based
THE OXIDATION OF ETHYLBENZENE NEAR 1060K upon the chosen rate constants and estimated radical concentrations for the lean experiment, an average rate constant for hydrogen abstraction from the ethyl group is 3.3E12 cm3/mole s at 1060K. For the estimated radical concentrations of the rich experiment, the average rate constant for abstraction is 3.2E12. Given the uncertainty in the estimated rate constants for abstraction and displacement, the value of 3E12 cm3/mole s was used as representative of radical reactions with the fuel. From the experimental results, the value of k[X] throughout the reaction zone can be determined by k[X] =
1 d[F] [F]
,
(21)
dt
and then the average value of k is used to estimate the total concentration of H, O, and OH. The results of these calculations are presented in Fig. 6 and clearly indicate the quasisteady nature of the radicals. The contribution of the conversion of fuel to styrene to the total rate of fuel consumption can be estimated through extension of this steady state of radicals to the phenylethyl radicals formed by abstraction. To illustrate the method used, the styrene formed from the benzylic radical will be considered. If the benzylic radical is denoted by 1~, its rate equation is d[l~] -= kl0[X][F] - kll [1~], dt
(22)
where k~0 represents the average rate of hydrogen abstraction by radicals, X, and k~ is the rate of decomposition to give styrene. Because the fuel disappears in a first order manner its time behavior is [F] = [Fl0e -*[xlt.
(23)
Now given the fixed rate constants due to the isothermal conditions of the experiments, substitution of Eq. (23) into Eq. (22) allows direct integration to obtain [1~] = [ F ] ( k l o [ X ] ) / ( k l l - k[X]) + b e - * l l t ,
(24)
where b is a constant of integration and depends on the initial conditions of the problem. Based on the large amounts of styrene formed
263
in the experiments the value of kl0[X] should be of the same order as k[X] for fuel disappearance which ranges from 10 to 40 s - i . The value o f k l l is estimated to be 1El4 e x p ( - 4 5 E 3 / R T ) , which gives a value of 2E4 s- l at 1060K. Clearly, k~l ~> kl0[X], and therefore Eq. (24) reduces to [1~] = kl0[X_......~][F] + b e - kll t. k~l
(25)
For any reasonable initial condition, the second term in Eq. (25) is negligible on a time scale of a millisecond because kll is very large. Thus, [1~] can be further approximated to [l~l = k~0IX_~][F]. ktl
(26)
Inspection of Eq. (22) shows that this solution for [1~] corresponds to the steady state solution. The formation of styrene, S, via the decomposition of the benzylic radical leads to a rate equation for styrene of d[S]
= k~ [1~] - L [S],
(27)
dt
where L = kiT[H] + k18 [O]. Now substitution of the steady state solution for [1~], Eq. (26), produces diS] dt
= kt0[X][F] - L[S].
(28)
Finally substitution for F in terms of d F / d t from Eq. (20) gives d[S]
d[F]
dt
-dt
( k~o[X])/( k [ X ] ) - L [S],
or
d[S] dt
d[F] f - L [S],
(29)
dt
where f is the rate of conversion of fuel to styrene normalized by the total rate of fuel consumption. In Eq. (29), f represents the fraction of the fuel consumption rate due to the production of styrene via the abstraction route. Analysis of the route to styrene through abstraction of a primary hydrogen yields an equation of the same form as Eq. (29). Therefore, Eq. (29) represents the general rate equa-
264 tion for styrene. From the experimental results, IS], d[S]/dt, and d[F]/dt are known and linear regression analysis [221 on Eq. (29) produces values for f and L. The results from the regression are listed in Table 2 along with the two standard deviation limits. These results show that the rate of production of styrene is nearly 50% of the fuel consumption rate regardless of the equivalence ratio. Also, the loss of styrene through radical reactions increases with increasing oxygen concentration which is the expected trend because the rate constants should be the same in all three experiments and the radical concentrations increase with increasing oxygen as shown in Fig. 6. A similar analysis can be performed for the amount of benzene formed of these experiments. However, because two routes to benzene exist, displacement and abstraction, the resulting value cannot be assigned to any one path. Therefore the only conclusion which can be drawn from the results for benzene formation is that the sum of the rates of displacement and primary hydrogen abstraction contribute approximately 30% of the fuel consumption. The value of 30% is derived simply by difference, i.e., 100% less the contributions by the styrene producing and the homolysis routes. DISCUSSION A. R e a c t i o n M e c h a n i s m Based on the analysis of the experimental results for ethylbenzene, three major paths have been proposed for removal of the ethyl group; the reaction sequences are summarized in Table 3. From the rate constant for the homolysis reaction and the analysis of the consumption of fuel to produce styrene, the relative importance of the three paths can be assessed. At 1060K the homolysis rate is 15% of the fuel consumption rate for the rich experiment and only about 5 % for the lean one; therefore it is a minor path. Clearly the significance of this route will increase with increasing temperature because the homolysis has an activation energy of approximately 70 kcal/mole [9]. The abstraction route (reaction 10-18) is the most significant of the
T . A . LITZINGER ET AL. TABLE 2 Results of Regression Analysis on Eq. (29)
Equivalence ratio
f
L (1/s)
0.64 0.95 1.3
0.54 _+ 0.04 0.49 _+ 0.04 0.49 + 0.04
26 +_ 2 9 _+ 1 7 _+ 1
TABLE 3 Proposed Reaction Routes
Homolysis Route ~C2H5 --" ¢CH2 + CH3 $(2H2 + (O, HO2) ~ $CHO + (H, OH + H) ~bCHO + X ~ q~CO + XH 0(20 -" ~ + CO
(1) (2) (3) (4)
Displacement Reaction ~C2H5 + H ~ q~H + C2H5
(5)
Abstraction Route ~C2H5 + X --* ~b(~HCHa + XH ~CHCHa ~ q~C2H3 + H q~C2H5 + X "-~ ~bCHz(~H2 + XH ~CH2CH 2 --~ (~ + C2H4 ~bCH2CH2 --b q~C2H3 + H t~C2H3 + X --* ~CHCH + XH ~CHt~H ~ tkCCH + H ¢~C2H3 + FI "-~ (~I-I + C2H 3 ~C2H3 + O "-~ OCHzCHO -* ¢~CH2 + HCO
(10) (11) (12) (13) (14) (15) (16) (17) (19)
three; this route accounts for at least 50% of the fuel consumption rate based on the regression analysis on the production of styrene. Because abstraction of a primary hydrogen leads to a phenyl radical or styrene (reactions 13 and 14), 50 % is a lower bound on the contribution of this route to the consumption of fuel. Since multiple paths exist to the phenyl radical and its associated products, benzene and phenol, the contri-
THE OXIDATION OF ETHYLBENZENE NEAR 1060K bution of primary abstraction cannot be determined unambiguously. Finally, the displacement route contributes 30% or less of the fuel disappearance rate; the uncertainty in this value lies in the fact that the abstraction route also contributes to the concentrations of benzene and phenol.
B. Steady State Analysis Although the experimental results clearly demonstrate the existence of a quasi-steady radical pool, questions remain as to the applicability of a steady state analysis to an oxidation experiment. Phenomenologically, the existence of a steady state concentration of an intermediate requires a balance between the sources and sinks of the species. In the present study the species of interest are the radicals, H, O, and OH, and the sources are decomposition of the phenylethyl radical and the H + 02 branching reaction. The sink for these radicals is primarily abstraction of hydrogen from the fuel. In a more quantitative sense, the steady state of a species requires that either the source or sink be much greater than the rate of change of the species [23]. If this condition is satisfied, both the source and sink will be much greater than the rate of change, and the balance is ensured. For the radical pool in the present experiments, the magnitude of the sink term is known from the fuel disappearance, i.e., d[F]/ dt = - k[X][F]. From the fuel rich experiment the minimum value of the k[X][F] is 1 . 5 E - 7 mole/cm3 s. The order of magnitude of the rate of change of the radicals is available from the results in Fig. 6; the maximum value of d[X]/dt is 2 E - 11 mole/cm 3 s. Clearly the sink term for the radicals is very much greater than the rate of change even if the average rate constant selected to calculate the radical concentration were changed by an order of magnitude. Therefore the present experiments easily meet the mathematical requirement for a steady state of the radical pool. Another indication that the results derived from the steady state analysis are reasonable is the rate constant for consumption of styrene which is obtained from the regression analysis.
265
First note that the rate of styrene consumption in Table 2 show the reasonable trend that increasing oxygen leads to increased rate of styrene consumption. Because the experiments were all at the same initial temperature and nearly isothermal, the increased rate of styrene consumption can only be explained by an increase in the radical concentration with increasing oxygen. Indeed the estimated radical concentrations in Fig. 6 show an increase with increasing oxygen concentration. When the disappearance rates for styrene are divided by the estimated radical concentrations, an approximate rate constant for radical attack on styrene is obtained. The values of this rate constant are 2.6E12, 2.4E12, and 2.2E12 cm3/mole s; these values agree quite well with the rate for ethylene reacting with oxygen atoms which is 5E12 cm3/mole s at 1060K [15]. The consistency and reasonable magnitude of the rate constant lend validity to the approach.
C. Analogy between Ethane and Ethylbenzene In the introduction, mention was made of the analogy that was found between certain reactions in a methane oxidation and the reactions found to be important in the removal of the sidechain of toluene [3]. Such analogies can also be seen in the present investigation when the proposed reactions of ethylbenzene and styrene are compared with those of ethane and ethylene. Under conditions similar to those of the present study, ethane is known to lose a hydrogen through radical abstraction and then the ethyl radical becomes ethylene through a unimolecular step or reaction with molecular oxygen [15, 24]. The conversion of ethylbenzene to styrene is the precise analog of this sequence of reactions. Once the ethylene has formed it may be attacked by an oxygen atom or it may undergo hydrogen abstraction to form a vinyl radical which will produce acetylene. Again the analogs of these reactions are found in the results of the present study, i.e., reactions 15, 16, and 18. Finally the homolysis of ethylbenzene is seen to be the analog of the homolysis of ethane; in the present system this reaction plays a more impor-
266
T . A . LITZINGER ET AL.
tant role because it has a lower activation energy than the corresponding reaction of ethane. The analogy between alkanes and the normal alkylated aromatics may allow use of the characteristics of alkanes to understand and to some extent to predict the behavior of higher normal alkylated aromatics. Of particular interest is the reaction sequence in normal alkanes in which radical abstraction of hydrogen is followed by the formation of an olefin [25]. For example, consider the propane reactions 30-33: C3H8 + X"-'~ n - C3H7 + XH,
(30)
C3H8+ X~i-
(31)
C3H7 + X H ,
n - C3H7-~ C2H4 + CH3,
(32)
i - C3H7-'r C3H6 + H.
(33)
Given the analogy, reactions 34-39 would be predicted for n-propylbenzene: q~C3H7 + X~0(~HC2H5 + XH,
(34)
0 C 3 H 7 + X---~q~C2nafH2 + XH,
(35)
q~C3H 7 + X~thCH2(~HCH3 + XH,
(36)
q~CHC2Hs---~thC2H3+ (~H3,
(37)
0C2H4CH 2--~t~fH2 + C2H4,
(38)
OCH2(~HCH3-~ ~C3H5 + H.
(39)
In the context of this analogy, a simplification may be possible for normal alkylated aromatics because the benzylic hydrogen bond energy is at least 10 kcal less than primary or secondary carbon-hydrogen bond energy; the lower benzylic C - H bond energy is due to the resonance of the benzyl radical which is formed by the abstraction. (In addition to lowering the bond strength, the resonance of the benzylic radical will affect the magnitude of the preexponential for the abstraction of benzylic hydrogens [26].) The lower energy of the benzylic bond means that the fraction of the benzylic hydrogens which are abstracted will be higher than the ratio of the number of benzylic hydrogens to the total number of hydrogens in the sidechain. The degree to which the abstraction is weighted toward the benzylic hydrogens will depend on the nature of the radicals in the system. However, conditions might allow the abstraction of
benzylic hydrogens to dominate the process, and this would mean a great simplification as then only reactions 34 and 37 would occur. For example, the dominance of styrene as the product of radical attack on the alkyl group has been observed in the pyrolysis of n-butylbenzene [27, 28]. If abstraction of benzylic hydrogen dominated in the present oxidation study, any normal alkylated aromatic would form mostly styrene, and the modeling of the abstraction route which is the most complex of the three routes would be greatly simplified. CONCLUSIONS From a flow reactor study of the oxidation of ethylbenzene at 1060K and atmospheric pressure, three reaction paths have been postulated for the removal of the ethyl group, and the contribution of each path to the rate of consumption of ethylbenzene has been estimated. The most significant route, which represents at least 50% of the fuel consumption rate, begins with the abstraction of a hydrogen from the ethyl group. Subsequently the phenylethyl radicals resulting from the abstraction decompose to form styrene or a phenyl radical. Further reactions of the styrene which complete the process of sidechain removal are displacement of the vinyl group by a hydrogen atom or oxidative attack on the vinyl. Displacement of the ethyl group from the ethylbenzene is found to contribute less than 30% of the fuel consumption rate. Finally, the rate of homolytic cleavage in the ethyl group to form a benzyl radical represents no more than 15 % of the fuel consumption rate at 1060K, the temperature of this investigation.
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Received 17 May 1985; revised 29 July 1985