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Primary steps in the oxidation of pyridine and pyrrole added to a lean methaneoxygenargon flame
C O M B U S T I O N A N D F L A M E 38:89-102 (1980)
89
Primary Steps in the Oxidation of Pyridine and Pyrrole Added to a Lean Methane-Oxygen-Argon Flame T H O M P S O N M. SLOANE, RICHARD J. BRUDZYNSKI, and JOHN W. RATCLIFFE Physical Chemistry Department, General Motors Research Laboratories, Warren. Michigan 48090
An understanding of the fuel-nitrogen oxidation mechanism is important for modelingcombustionof fuels containing fuel nitrogen and for developing nitric oxide emission control strategies. The work reported here identifiesthe primary reactions for pyridine and pyrrole in a lean methane-oxygen-argon flame. A major intermediatein the breakdown of pyridine in the flame, C3HsCN, was foundto be the same as the product of the reaction of O(3p) with pyridineobserved under single collision conditions. The analogous product of pyrrole oxidation in the flame, C2HsCN, was an isomer of the product observed from the reaction of O(3P) with pyrrole, C2HsNC. Other intermediatesfound in the early stages of the flame could be identified as reaction products of H and OH with pyridine and pyrrole. The importantring-opening step in the flame appears to be the decomposition of hydroxypyridineor hydroxypyrroleproduced by the reaction of O(3P) or OH with pyrrole or pyridine.
INTRODUCTION Nitrogen that is chemically bound in a fuel can be a major source o f nitric oxide when the fuel is burned [1, 2]. Liquid fuels made from coal will contain relatively large amounts o f fuel nitrogen [3], much of it in the form of pyridines and pyrroles [ 4 - 6 ] . If future liquid fuels are to be made from coal, an understanding of the fuelnitrogen oxidation mechanism is a fundamental necessity for modeling combustion o f these fuels and for developing nitric oxide emission-control strategies The object o f the work reported here is to identify the products of the initial steps in the oxidation of pyridine and pyrrole by a combination of two techniques: direct observation o f reaction products under single reactive collision conditions and molecular beam mass spectrometer sampling of a flame to which has been added a small amount of pyridine or pyrrole. The major primary reactants with fuel molecules in a flame are the radicals H, O, and OH. By analogy with other aromatic hydrocarbons [7-9] the oxygen atom reaction with pyridine or pyrrole is likely to lead to ring rupture in a single elemenCopyright © 1980 by The Combustion Institute Published by Elsevier North Holland, Inc.
tary step under single collision conditions. We have therefore chosen to study first the reaction o f O(3P) with pyridine and pyrrole under single collision conditions to determine whether the reactions proceed in the same way as the previously studied O(3P) + aromatic hydrocarbon reactions. We would also like to verify whether molecules found in a pyridine- or pyrrole-doped flame could originate from the reaction o f O atoms with pyridine or pyrrole. If the O atom reaction with pyridine and pyrrole proceeds in a manner similar to the O + benzene reaction, we believe that our knowledge o f the reactions o f OH [8, 9] and H [10] with aromatic hydrocarbons will help us to identify the reactions responsible for the appearance o f other nitrogen-containing molecules in the flame. One drawback to this procedure is that it is difficult with our present experimental arrangement to perform reaction-product analysis of oxygen atom reactions with aromatic hydrocarbons under typical combustion conditions. We can take a small step in the direction o f finding out how higher temperature affects the reaction mechanism for O(3p) + pyridine and pyrrole by looking for
0010-2180/80/040089+14501.75
90 changes in the reaction mechanism as the rotational and vibrational temperature of pyridine or pyrrole is increased to 570 K while holding the distribution of collision energies approximately constant. We do not presently have the capability to increase the collision energy at fixed reactant internal energy.
EXPERIMENTAL The apparatus employed here for reaction-product studies under single collision conditions has been described previously [7]. A free jet containing oxygen atoms was produced by flowing the products of a microwave discharge in 02 past a small orifice in the end of the quartz flow tube. The flow tube was contained in a vacuum chamber pumped by a 6-in. diffusion pump. The hydrocarbon source consisted of a heated Pyrex nozzle which was positioned at 90 ° with respect to the centerline of the oxygen atom jet. The centerlines of the two jets intersected 0.15 cm downstream from each source. The products of reactive collisions were detected by a differentially pumped quadrupole mass spectrometer positioned at 0 ° relative to the oxygen atom jet centerline. A number of previously described tests [7, 8] were used to insure that the products observed were due to reactions in the collision zone and not due to reactions on surfaces or in the ionizer. Ionization efficiency measurements were made of sufficiently intense product signals. These measurements yielded product ionization potentials which were helpful in distinguishing between isomers. The distributions of relative kinetic energies of collision are shown in Fig. 1 for 0 + pyridine collisions have pyridine source temperatures of 300 and 600 K, and in Fig. 2 for 0 + pyrrole. The calculations have been described previously [10]. To calculate these distributions we have assumed that the velocity distributions of both reactants were described by Maxwell-Boltzmann distributions and that the intensity of each source was proportional to the cosine of the angle from the jet centerline. A comparison of the distributions at the two temperatures shows the change in hydrocarbon source temperature leaves the distribution of
THOMPSON M. SLOANE ET AL. relative kinetic energies virtually unchanged. The vibrational and rotational energies of the hydrocarbons should increase by 10-30 kJ/mol, however. This allows us to observe whether the reaction mechanism changes due to addition of this amount of internal energy. With this apparatus we were not able to observe possible changes in the reaction mechanism with increasing translational energy at constant internal energy. We hope to add this capability in later experiments. The flat-flame burner experiments were performed with a molecular beam mass spectrometer system very similar to that used by others [11, 12]. The apparatus has been described in more detail in Ref. [13]. A mixture of CH 4 (9.2%), O z (21.5%) Ar (69.2%), and pyridine or pyrrole (0.15%) was fed through a single gas inlet line in the base of the burner. The equivalence ratio of this mixture was 0.9. The burner was mounted on a translation stage to allow sampling of the flame gas at varying distances downstream from the burner face. The flame was sampled through a 0.005-cm hole in the tip of a quartz cone having a 40 ° apex angle. The resulting molecular beam was differentially pumped and collimated before it entered the mass spectrometer chamber. A chopper wheel in front of the mass spectrometer was used for phase sensitive detection of flame components. Either a fast electrometer and lock-in amplifier combination or synchronous pulse counting were used to process the ion signals from the mass spectrometer. Mass interferences due to fragmentation in the ionizer were minimized by using low electron energies when appropriate. The mass search was concentrated at masses 50 and above because the aim of this work was a characterization of the initial steps of pyridine and pyrrole oxidation rather than a full investigation of pyridine and pyrrole flame chemistry. Such a complete study will be the subject of future work. RESULTS O(SP) + Pyridine Products were observed from this reaction under single collision conditions at m/e = 95 and 67. The
OXIDATION OF PYRIDINE AND PYRROLE
91
OO o •
10
0
0•
o%°
O(3P) * PYRIDINE
•0 • •
0 Oe
•
•
o
e•
300 K
o 600 K
• 0 °o 0 O O
E D
°%o •
@ o
<
Z L.L.J Z
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-g °°
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0.0
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I
I
I
2.0
4.0
6.0
8.0
10.0
12.0
RELATIVE KINETIC ENERGY (kJ/tool)
Fig. 1. Calculated distribution of relative energy of collision for 0(3p) + pyridine intersecting at 90°. Pyridine source temperatures are 300 and 600 K. product at mass 95 is the O a t o m - p y r i d i n e complex. This signal was small enough to require an O atom signal attenuation o f 16% to distinguish it from background noise, so it is possible that much o f the signal was due to complexes which had been collisionally stabilized. The signal increased to 1.25 times its 300-K value when the pyridine source was heated to 570 K, but was never strong enough to allow a measurement o f its ionization efficiency.
The mass 67 product signal was also weak with the pyridine source at 300 K, but this signal increased by a factor of 6 when the source was heated to 570 K. The measured ionization efficiency at this source temperature is shown in Fig. 3. The figure shows the ionization efficiency which has been corrected for the spread in energy of the ionizing electrons [14]. The ionization potential obtained is in the neighborhood o f 10.1 eV, which agrees moderately well with the previously
92
THOMPSON M. SLOANE ET AL.
0
o•
° oo° %0 •
10
o O0
O(3P) + PYRROLE
° ° °
•
°
°
300
o 600
° 0
o~Oo~o
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o
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4.0
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8.0
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120
RELATIVE KINETIC ENERGY ( k J / m o l )
Fig. 2. Calculated distribution of relative kinetic energy of collision for 0(3P) + pyrrole intersecting at 90°. Pyrrole source temperatures are 300 and 600 K.
obtained value of 10.39 eV for the unsaturated open-chain cyanide, CaHsCN [15]. This product corresponds to CO elimination from the 0 + pyr°dine complex, and is an analogous reaction path to the reaction of O(ap) with benzene to give CO and open-chain C5H6. This product is probably not pyrrole because the ionization potential of pyrrole is 8.2 eV [15]. To verify that it was indeed CO and not C2H4 being eliminated from
the 0 + pyridine complex, the experiment was repeated with pyridine-d 5. The product detected has a mass of 72 (C4DsN), the expected CO elimination product. If C2D 4 had been eliminated from the complex, a product at mass 68 (CaDNO) would have been observed. Other product paths searched for, but not found, were NO + C5H5, HCN + C 4 H 4 0 , and CsH4NO + H.
OXIDATION OF PYRIDINE AND PYRROLE
93
O(3p) +
MASS 67
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916.2 (9.5 eV)
964.4 (10.O)
1012.6 (10.5)
ELECTRON ENERGY (kJ) Fig. 3. Corrected ionization efficiency measurement for the mass 67 product observed from the reaction of 0(3P) with pyridine. The pyridine source temperature was 570 K.
o ( a p ) + Pyrrole The two products observed from this reaction were the O + pyrrole complex at m/e = 83 and the complex decomposition product, C2HsCN, at m/e = 55. The ionization efficiency measurement
for the complex is shown in Fig. 4. The ionization potential obtained is about 10.0 eV. To our knowledge, the ionization potential of this molecule has not been previously measured. The complex signal increased by 50% when the pyrrole source was heated from 370 to 570 K. The
94
THOMPSON M. SLOANE ET A L
O O(3p) +
MASS 83 I
H
O O t-
10
O
O
< v
>-
0
Z iii Z
O O O
0-D
0
0
0
0 0 o° I
868.0 (9.0 eV)
916.2 (9.5)
0 I1 964.4 (10.0)
I 1012.6 (10.5)
ELECTRON ENERGY (kJ) Fig. 4. Corrected ionization efficiency measurement for the mass 83 product from 0(3p) + pyrrole. The pyrrole source temperature was 300 K.
signal at m/e = 55 decreased to about half its 300-K value when the source was heated to 570 K. The ionization efficiency of the mass 55 product is shown in Fig. 5. The ionization potential obtained is about 10.8 eV at a pyrrole source temperature of 300 K. The two most likely possibilities
are C2HsNC (11.2 eV) [15] and C2I-I5CN (11.8 eV) [15]. Normally our measurements come within +0.2 eV of the accepted values o f ionization potentials for reaction products. We are confident in our electron energy calibration because, in separate experiments, we obtained ionization potentials
OXIDATION OF PYR1DINE AND PYRROLE
95
O(3p) +
MASS 55 I
H
O
O e-
10 ,.,C}
< >I-Ct) Z UJ I-. Z
O
0
O 0
0
00000000
0
I
,
1
964.4 (10.0 eV)
1012.6 (10.5)
1060.8 (11.0)
ELECTRON ENERGY (kJ) Fig. 5. Corrected ionization efficiency measurement for the mass 55 product from 0(3) + pyrrole. The pyrrole source temperature was 300 K. for argon, pyridine, and C2HsCN that were within 0.1 eV of the literature values. The stated uncertainty for the C2HsNC measurement is e0.3 eV [15], so our value is in reasonable agreement with that obtained for C2HsNC. Product paths searched for, but not found, were NO + C4H 5 and HCN + C3H40.
Pyridine-Doped CH4-O2-Ar Flame Figure 6 shdws the intensity of pyridine, methane, CO, and mass 67 as a function of distance from the burner surface, as well as the temperature. Since our object in this work was only to identify reaction products in the flame, we did not call-
96
THOMPSON M. SLOANE ET AL. --
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CH4-O2-Ar FLAME DOPED
--O
WITH PYRIDINE
CH 4
CO
--
M A S S 67
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15
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1000
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500
ON 1
2
3
4
5
6
7
8
D I S T A N C E F R O M BURNER FACE ( m m )
Fig. 6. Relative intensity profile of CH4, CO, CsH5N, mass 67, and temperature in the pyridine-doped CH4--O2-Ar flame.
brate our measurements for absolute mole fractions. The methane and pyridine profiles were arbitrarily normalized at 1.0 mm to determine how fast they disappear in the flame relative to each other. The CO profile was included to provide a reference point in the flame. The CO peak location did not depend on the addition of pyridine, although the CO concentration was greater in the region 2-4.5 mm downstream from the burner with pyridine added to the flame. Molecu-
lar nitrogen should make a negligible contribution to mass 28 because of the low quantity of pyridine and the lean air-fuel ratio. The mass 67 peak is located very near the position of maximum pyridine disappearance. Its ionization efficiency measured at its peak is shown in Fig. 7. The ionization potential obtained is about 10.3 eV, in reasonable agreement with the value of 10.1 eV obtained in the single collision reaction of O(3P) with pyridine. This is strong evidence
OXIDATION OF PYRIDINE AND PYRROLE
97
M A S S 67 IN PYRIDINE-DOPED
(3
CH4-O2-Ar FLAME
(3 (3 ,m E:
(3
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,
1012.6 (10.5 eV) ELECTRON ENERGY(kJ)
I
1060.8 (11.0)
Fig. 7. Corrected ionization efficiency measurement for the mass 67 component of the pyridine-doped flame. This measurement was made 0.4 cm downstream from the burner face where the gas temperature was 1700 K. that the O + pyridine reaction product at mass 67 is the same molecule as is found in the flame, C3HsCN. Other products in the initial stage of the flame were found at masses 96, 95, 83, and 68. The signal was too small in each case to obtain an ionization potential measurement. These measurements were obtained with an electron energy of 14.0 eV.
We believe these compounds are C s H s N O H , CsHsNO, C4H5NO, and C4H40 (or C4H6N), respectively. Pyrrole-Doped C H 4 - 0 z - A r Flame Figure 8 shows the profiles of pyrrole, CO, mass 55, and temperature in the pyrrole-doped flame.
98
THOMPSON M. SLOANE ET AL.
CH4-O2-Ar FLAME DOPED WITH PYRROLE
\o
--O--
CYCLO-C4 H5 N
--@---r-l--
CO MASS 55 TEMPERATURE
12000 u
f f
15
1500
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DISTANCE FROM BURNER FACE (mm) Fig. 8. Relative intensity profiles of pyrrole, CO, mass 55, and temperature in the pyrrole-doped flame.
The small peak in the mass 55 profile, although larger than our uncertainty limits based on ion counting statistics, was not reproducible. Repeated measurements of that region of the profile indicated that the noise level there is much higher than that based on counting statistics alone. In any event, that region of the profile is
not important for the purpose of the present work. A corrected ionization efficiency measurement of the mass 55 signal at its peak is shown in Fig. 9. The ionization potential obtained from this measurement is in excellent agreement with the accepted value for C2HsCN of 11.8 eV. As men-
OXIDATION OF PYRIDINE AND PYRROLE
99
0
MASS 55 IN PYRROLE-DOPED CH4-O2-Ar FLAME n
0 ¢-
O 10
0
< >t-
0
Z LU
I-Z
0 0 0 0--
0
0000
0
0
i
1109.1 (11.5 eV)
0
I
i
1157.3 1205.5 (12.0) (12.5) ELECTRON ENERGY (kJ) Fig. 9. Corrected ionization efficiencymeasurement for the mass 55 component of the pyrrole-doped flame measured at the peak in its profile.
tioned previously, we obtained a value of 11.811.9 eV for C2HsCN in a separate experiment. It would not be surprising for C2HsCN to be formed in the flame and C2HsNC in the reaction product analysis experiments because the isocyanide should isomerize rapidly to the cyanide at the temperature of peak mass 55 signal in the flame. The C2HsCN peak occurs near the position of maximum pyrrole disappearance. It could either
be a direct reaction product of pyrrole, or the result of the decomposition of C4H5NO which is formed early in the flame and which dissociates in a hotter region of the flame. Other products in the flame were observed at masses 83, 71, 69, and 54. We believe these molecules are C4HsNO , CaH5NO, C4HvN, and C3H4N, respectively. Since these last three products are not found in the pyridine-doped flame, this gives
100
THOMPSON M. SLOANE ET AL.
further evidence that the mass 67 product in that flame is unlikely to be pyrrole. DISCUSSION
From the data obtained, a reasonably consistent picture emerges of the initial stages of pyridine and pyrrole oxidation in a lean methane-oxygenargon flame. Based on our results for OOP) + pyridine and on previous knowledge of H, O, and OH reactions with unsaturated hydrocarbons, the following reactions are likely: M
OH + C s H s N ~ C 5 H s N O H *
~ CsH5NOH
(1)
-~ H + CsHsNO,
(2)
M
O(aP) + CsI-IsN-. CsI-I,sNO*~ C5H5NO
(3)
CO + H3C-CI-I=CHCN (or H2C=CHCH2CN), (4) M
H + C4HsN -> C4H6N* ~ C4H6 N,
(5)
M
0 + C4HsN ~ C4HsNO* --* C4HsNO.
(6)
M denotes any other flame component which can collisionally stabilize the vibrationally and rotationally excited molecule denoted with an asterisk. The more massive products of these reactions have all been observed in the flame. No other observable species were present at role > 50. Reaction (4) could yield either isomer, so the oxygen atom reaction would be expected to proceed as follows [161: o(ap) + HaC-CH=CHCN --" HaCCO + CH2CN,
(7) H3C-CH 2 + COCN,
(8) o(ap) + H2C=CHCHzCN ~ HCO + CH2CH2CN. (9)
No products were observed at mass 54 (CaH4N), however. Either the amounts of these products were too small to detect or another reaction mechanism giving products with masses less than 50 is important with these cyanides [17]. The reaction of OH with C4HsN could proceed in a number of ways [18], but no products were observed for these reactions either. It is clear that other experiments need to be done to find out how this unsaturated cyanide degrades in the flame. We were concerned about the possibility that the mass 67 flame species might not be a reaction product of 0(3p) + pyridine, but rather a fragment produced by electron bombardment of the 0 + pyridine complex in the ionizer. This possibility could not be completely excluded in our 0(3p) + benzene experiment or in our present 0 0 P ) + pyridine experiments. The CsH 6 product observed from 0 0 P ) + benzene might be a fragment, according to molecular beam results from another laboratory [19]. However, C5H6 has been observed in a benzene-oxygen flame to have an ionization potential of 8.5 eV, independent of position in the flame [201 and therefore independent of temperature. By comparison, in the same flame, the benzene fragment C6H 5 was identified by its variation in appearance potential over a range of about 1 eV depending on its position [20]. The ionization potential of a polyatomic molecule should not depend strongly on its temperature, but the appearance potential of a fragment ion might very well decrease with increasing temperature. We have measured the ionization potential of the mass 67 signal at three positions in the flame corresponding to a temperature range of 1400-1660 K, and the values obtained lie within the range of 9.95-10.3 eV. No systematic variation with temperature was noted. Since this range is within our experimental error, it is unlikely that our measured mass 67 in the flame is a fragment of the 0 0 P ) + pyridine complex or of any other parent compound. The reaction scheme which explains reasonably well our observations for the pyrrole-doped flame is the following: OH + cyclo-e4HsN -* C4HsNOH* ~ H + C4H5NO, (10)
101
OXIDATION OF PYRIDINE AND PYRROLE M
o(ne)+cyclo-C4~'~-~C4I~NO *-~ C4H~O
(11)
CO + C2HsCN, (12) H
H + cyclo-C4HsN ~ C4H6N ~ C4HTN,
(13)
OH + CaHsCN -~ C2H4CN + H20.
(14)
The above reactions do not explain the appearance of mass 71 (probably C2H5CNO) and mass 43 (either C2HaO or HCNO). Mass 71 could arise from the reaction o(ap) + C2HsCN ~ C2HsCNO,
(15)
although this is purely speculative. Reaction-product analysis for the reaction of 0(ap) with cyanides remains to be determined. Mass 43 would have to be explained on the basis of speculation also. It is clear that flame reaction mechanism studies are needed for the cyanides produced by oxygen atom attack on pyridine and pyrrole.
spectrometer is 35% less at a pyridine source temperature of 600 than 300 K. The observed 25% increase in the mass 95 signal is then a net 92% increase in the amount of 0 + pyridine complex that is formed which has a long enough lifetime to reach the detector. The net increase in the amount of the 0 + pyrrole complex formed with heating of the pyrrole source is about 130%. Since the complex should decompose more rapidly at the higher total energy, there is apparently a barrier to formation of the complex which can be overcome with vibrational or rotational energy of the heterocyclic hydrocarbon. This result is unusual in that there have been very few examples [10, 21-24] of an exothermic process which is enhanced by increasing the internal energy of one of the reactants, in one case, the presence of a small barrier was indicated by a positive activation energy for the reaction of H atoms with benzene [25-27]. Unfortunately, no kinetic information is presently available for the reaction of 0(ap)with pyridine or pyrrole. We have not yet performed the heated hydrocarbon source experiments with 0(ap) + aromatic hydrocarbons for a comparison with the results obtained here.
Reaction Dynamics of 0(ap) with Pyridine and Pyrrole Our results show that there is a strong analogy between the reaction of 0(3p) with these aromatic heterocyclic compounds and the reaction of 0(ap) with benzene [7]. The same spin-forbidden mechanism probably explains the 0(ap) + pyridine and pyrrole results. The increase in the amount of mass 67 signal with increasing pyridine source temperature cannot be interpreted as an increase in the cross section for formation of that product because its angular and energy distribution is unknown. However, the change in the 0 + pyridine complex signal can be interpreted if it is assumed that the velocity distributions of both beams are described by a Boltzmann velocity distribution and if the intensity distributions of both sources are cosine distributions. Since heating of the pyridine source changes the velocity distribution, the distribution of velocities in the center of mass is changed as well. The fraction of collisions whose center of mass velocity vector is pointed toward the mass
CONCLUSIONS We have found that the same products observed under single collision conditions from the reaction of 0(ap) with pyridine are also found in a pyridine-doped lean methane-oxygen-argon flame. The 0 + pyrro]e adduct and an isomer of the C2H5NC product were observed in the pyrroledoped flame. This shows that the important ringfragmenting step in the flame is likely to be the reaction of 0(ap) with pyridine and pyrrole and the decomposition of hydroxypridine or hydroxypyrrole produced by the reaction of pyridine and pyrrole with OH. The addition cross section of 0(ap) atoms to pyridine and pyrrole is enhanced by increased vibrational and/or rotational energy of the hydrocarbon. In the flame the decomposition products of this adduct are probably CO and an unsaturated (in the case of pyridine) or saturated (in the case of pyrrole) cyanide. How these cyanides react further in a flame will be the subject of future work.
102
REFERENCES 1. Pershing, D. W., Martin, G. B., and Berkau, E. E., AIChE Symp. Set. 71:19 (1975). 2. Pershing, D. W. and Wendt, J. O. L., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, p. 389. 3. Sternberg, H. W., Raymond, R., and Schweighardt, F. K.,Science 188:49 (1975). 4. Hayatsu, R., Scott, R. G., Moore, L. P., and Studier, M. H.,Nature 257:378 (1975). 5. Axworthy, A. E.,AIChESymp. Ser. 71:43 (1975). 6. Peterson, R. C., Lucht, R. P., and Laurendeau, N. M., Paper No. 36, 1978 Fall Technical Meeting, Eastern Section of the Combustion Institute, Miami Beach, Florida. 7. Sloane, T. M.,J. Chem. Phys. 67:2267 (1977). 8. Sloane, T. M., Chem. Phys. Lett. 54:269 (1978). 9. Kenley, R. A., Davenport, J. E., and Hendry, D. G., J. Phys. Chem. 82:1095 (1978). 10. Sloane, T. M., and Brudzynski, R. J.,J. Chem. Phys., (to be published). 11. Lazzara, C. P., Biordi, J. C., and Papp, J. F., Combust. Flame 21:371 (1973). 12. Peeters, J., and Mahnen, G., Thirteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1973, p. 133. 13. Sloane, T. M., and Ratcliffe, J. W., Mass Spectrometer Sampling of the Quench Zone of a Methane-OxygenArgon Flame, Research Publication GMR-2845, 1978 (unpublished). 14. Winters, R. E., Collins, J. H., and Courchene, W. L., J. Chem. Phys. 45:1931 (1966).
T H O M P S O N M. S L O A N E ET AL. 15. Franklin, J. L., Dillard, J. C., Rosenstock, H. M., Herron, J. T., Draxl, K., and Field, F. H., Ionization Potentials, Appearance Potentials, and Heats of Formation of Gaseous Positive Ions, NSRDS-NBS 26, U.S. Government Printing Office, Washington, D.C., 1969. 16. Kanofsky, J. R., Lucas, D., and Gutman, D., Fourteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1973, p. 285. 17. Bonanno, R. J., Timmons, R. B., Stief, L. J., and Klemm, R. B.,J. Chem. Phys. 66:92 (1972). 18. Slagle, I. R., Gilbert, J. R., Graham, R. E., and Gutman, D., Int. J. Chem. Kinet. Symp. Series, 317 (1975). 19. Buss, R., private communication. 20. Bittner, J., private communication. 21. Sloane, T. M., Tang, S.-Y., and Ross, J., J. Chem. Phys. 57:2745 (1972). 22. Cross, R. J., Lee, L., Saunders, M., and Auerbach, A., Paper #258, 178th ACS National Meeting, Washington, D.C., 1979. 23. Moy, J., Bar-Ziv, E., and Gordon, R. J., J. Chem. Phys. 66:5439 (1977) and references cited therein. 24. Spencer, J. E., and Glass, G. P.,Int. J. Chem. Kinet. 9:97 (1977);ibid., p. 111. 25. Hoyermann, K., Preuss, A. W., and Wagner, H. G., Ber. Bunsenges. Phys. Chem. 79:156 (1975). 26. Kim, P., Lee, J. H., Bonanno, R. J., and Timmons, R. B.,J. Chem. Phys. 59:4593 (1973). 27. Sauer, Jr., M. C., and Mani, I., J. Phys. Chem. 74: 59 (1970). Received 9 July 1979; revised 15 October 1979.
89
Primary Steps in the Oxidation of Pyridine and Pyrrole Added to a Lean Methane-Oxygen-Argon Flame T H O M P S O N M. SLOANE, RICHARD J. BRUDZYNSKI, and JOHN W. RATCLIFFE Physical Chemistry Department, General Motors Research Laboratories, Warren. Michigan 48090
An understanding of the fuel-nitrogen oxidation mechanism is important for modelingcombustionof fuels containing fuel nitrogen and for developing nitric oxide emission control strategies. The work reported here identifiesthe primary reactions for pyridine and pyrrole in a lean methane-oxygen-argon flame. A major intermediatein the breakdown of pyridine in the flame, C3HsCN, was foundto be the same as the product of the reaction of O(3p) with pyridineobserved under single collision conditions. The analogous product of pyrrole oxidation in the flame, C2HsCN, was an isomer of the product observed from the reaction of O(3P) with pyrrole, C2HsNC. Other intermediatesfound in the early stages of the flame could be identified as reaction products of H and OH with pyridine and pyrrole. The importantring-opening step in the flame appears to be the decomposition of hydroxypyridineor hydroxypyrroleproduced by the reaction of O(3P) or OH with pyrrole or pyridine.
INTRODUCTION Nitrogen that is chemically bound in a fuel can be a major source o f nitric oxide when the fuel is burned [1, 2]. Liquid fuels made from coal will contain relatively large amounts o f fuel nitrogen [3], much of it in the form of pyridines and pyrroles [ 4 - 6 ] . If future liquid fuels are to be made from coal, an understanding of the fuelnitrogen oxidation mechanism is a fundamental necessity for modeling combustion o f these fuels and for developing nitric oxide emission-control strategies The object o f the work reported here is to identify the products of the initial steps in the oxidation of pyridine and pyrrole by a combination of two techniques: direct observation o f reaction products under single reactive collision conditions and molecular beam mass spectrometer sampling of a flame to which has been added a small amount of pyridine or pyrrole. The major primary reactants with fuel molecules in a flame are the radicals H, O, and OH. By analogy with other aromatic hydrocarbons [7-9] the oxygen atom reaction with pyridine or pyrrole is likely to lead to ring rupture in a single elemenCopyright © 1980 by The Combustion Institute Published by Elsevier North Holland, Inc.
tary step under single collision conditions. We have therefore chosen to study first the reaction o f O(3P) with pyridine and pyrrole under single collision conditions to determine whether the reactions proceed in the same way as the previously studied O(3P) + aromatic hydrocarbon reactions. We would also like to verify whether molecules found in a pyridine- or pyrrole-doped flame could originate from the reaction o f O atoms with pyridine or pyrrole. If the O atom reaction with pyridine and pyrrole proceeds in a manner similar to the O + benzene reaction, we believe that our knowledge o f the reactions o f OH [8, 9] and H [10] with aromatic hydrocarbons will help us to identify the reactions responsible for the appearance o f other nitrogen-containing molecules in the flame. One drawback to this procedure is that it is difficult with our present experimental arrangement to perform reaction-product analysis of oxygen atom reactions with aromatic hydrocarbons under typical combustion conditions. We can take a small step in the direction o f finding out how higher temperature affects the reaction mechanism for O(3p) + pyridine and pyrrole by looking for
0010-2180/80/040089+14501.75
90 changes in the reaction mechanism as the rotational and vibrational temperature of pyridine or pyrrole is increased to 570 K while holding the distribution of collision energies approximately constant. We do not presently have the capability to increase the collision energy at fixed reactant internal energy.
EXPERIMENTAL The apparatus employed here for reaction-product studies under single collision conditions has been described previously [7]. A free jet containing oxygen atoms was produced by flowing the products of a microwave discharge in 02 past a small orifice in the end of the quartz flow tube. The flow tube was contained in a vacuum chamber pumped by a 6-in. diffusion pump. The hydrocarbon source consisted of a heated Pyrex nozzle which was positioned at 90 ° with respect to the centerline of the oxygen atom jet. The centerlines of the two jets intersected 0.15 cm downstream from each source. The products of reactive collisions were detected by a differentially pumped quadrupole mass spectrometer positioned at 0 ° relative to the oxygen atom jet centerline. A number of previously described tests [7, 8] were used to insure that the products observed were due to reactions in the collision zone and not due to reactions on surfaces or in the ionizer. Ionization efficiency measurements were made of sufficiently intense product signals. These measurements yielded product ionization potentials which were helpful in distinguishing between isomers. The distributions of relative kinetic energies of collision are shown in Fig. 1 for 0 + pyridine collisions have pyridine source temperatures of 300 and 600 K, and in Fig. 2 for 0 + pyrrole. The calculations have been described previously [10]. To calculate these distributions we have assumed that the velocity distributions of both reactants were described by Maxwell-Boltzmann distributions and that the intensity of each source was proportional to the cosine of the angle from the jet centerline. A comparison of the distributions at the two temperatures shows the change in hydrocarbon source temperature leaves the distribution of
THOMPSON M. SLOANE ET AL. relative kinetic energies virtually unchanged. The vibrational and rotational energies of the hydrocarbons should increase by 10-30 kJ/mol, however. This allows us to observe whether the reaction mechanism changes due to addition of this amount of internal energy. With this apparatus we were not able to observe possible changes in the reaction mechanism with increasing translational energy at constant internal energy. We hope to add this capability in later experiments. The flat-flame burner experiments were performed with a molecular beam mass spectrometer system very similar to that used by others [11, 12]. The apparatus has been described in more detail in Ref. [13]. A mixture of CH 4 (9.2%), O z (21.5%) Ar (69.2%), and pyridine or pyrrole (0.15%) was fed through a single gas inlet line in the base of the burner. The equivalence ratio of this mixture was 0.9. The burner was mounted on a translation stage to allow sampling of the flame gas at varying distances downstream from the burner face. The flame was sampled through a 0.005-cm hole in the tip of a quartz cone having a 40 ° apex angle. The resulting molecular beam was differentially pumped and collimated before it entered the mass spectrometer chamber. A chopper wheel in front of the mass spectrometer was used for phase sensitive detection of flame components. Either a fast electrometer and lock-in amplifier combination or synchronous pulse counting were used to process the ion signals from the mass spectrometer. Mass interferences due to fragmentation in the ionizer were minimized by using low electron energies when appropriate. The mass search was concentrated at masses 50 and above because the aim of this work was a characterization of the initial steps of pyridine and pyrrole oxidation rather than a full investigation of pyridine and pyrrole flame chemistry. Such a complete study will be the subject of future work. RESULTS O(SP) + Pyridine Products were observed from this reaction under single collision conditions at m/e = 95 and 67. The
OXIDATION OF PYRIDINE AND PYRROLE
91
OO o •
10
0
0•
o%°
O(3P) * PYRIDINE
•0 • •
0 Oe
•
•
o
e•
300 K
o 600 K
• 0 °o 0 O O
E D
°%o •
@ o
<
Z L.L.J Z
00 o
o Ooo o
Dog
%0 000
-g °°
gig l l
Oooo
o
o~
o
/ '~o
.". ~% •,,o, O~o00 0-(~0
0.0
"
° • 0#
I
I
I
I
I
I
2.0
4.0
6.0
8.0
10.0
12.0
RELATIVE KINETIC ENERGY (kJ/tool)
Fig. 1. Calculated distribution of relative energy of collision for 0(3p) + pyridine intersecting at 90°. Pyridine source temperatures are 300 and 600 K. product at mass 95 is the O a t o m - p y r i d i n e complex. This signal was small enough to require an O atom signal attenuation o f 16% to distinguish it from background noise, so it is possible that much o f the signal was due to complexes which had been collisionally stabilized. The signal increased to 1.25 times its 300-K value when the pyridine source was heated to 570 K, but was never strong enough to allow a measurement o f its ionization efficiency.
The mass 67 product signal was also weak with the pyridine source at 300 K, but this signal increased by a factor of 6 when the source was heated to 570 K. The measured ionization efficiency at this source temperature is shown in Fig. 3. The figure shows the ionization efficiency which has been corrected for the spread in energy of the ionizing electrons [14]. The ionization potential obtained is in the neighborhood o f 10.1 eV, which agrees moderately well with the previously
92
THOMPSON M. SLOANE ET AL.
0
o•
° oo° %0 •
10
o O0
O(3P) + PYRROLE
° ° °
•
°
°
300
o 600
° 0
o~Oo~o
•
C
O o
o
Oee° 00
o
• 0
C~O0
o
°°°0
00%
•
. 0
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0
0
• oOo
0 °0
••
0 ~)<~00
o o
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~o •
~ o0 o 0
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~ Oo
o
%-
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O.0
I
I
I
I
I
I
2.0
4.0
6.O
8.0
10.O
120
RELATIVE KINETIC ENERGY ( k J / m o l )
Fig. 2. Calculated distribution of relative kinetic energy of collision for 0(3P) + pyrrole intersecting at 90°. Pyrrole source temperatures are 300 and 600 K.
obtained value of 10.39 eV for the unsaturated open-chain cyanide, CaHsCN [15]. This product corresponds to CO elimination from the 0 + pyr°dine complex, and is an analogous reaction path to the reaction of O(ap) with benzene to give CO and open-chain C5H6. This product is probably not pyrrole because the ionization potential of pyrrole is 8.2 eV [15]. To verify that it was indeed CO and not C2H4 being eliminated from
the 0 + pyridine complex, the experiment was repeated with pyridine-d 5. The product detected has a mass of 72 (C4DsN), the expected CO elimination product. If C2D 4 had been eliminated from the complex, a product at mass 68 (CaDNO) would have been observed. Other product paths searched for, but not found, were NO + C5H5, HCN + C 4 H 4 0 , and CsH4NO + H.
OXIDATION OF PYRIDINE AND PYRROLE
93
O(3p) +
MASS 67
O
E:
© 10
< v
>I-(/') Z
©
U,J
I-Z
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© © 0
©
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916.2 (9.5 eV)
964.4 (10.O)
1012.6 (10.5)
ELECTRON ENERGY (kJ) Fig. 3. Corrected ionization efficiency measurement for the mass 67 product observed from the reaction of 0(3P) with pyridine. The pyridine source temperature was 570 K.
o ( a p ) + Pyrrole The two products observed from this reaction were the O + pyrrole complex at m/e = 83 and the complex decomposition product, C2HsCN, at m/e = 55. The ionization efficiency measurement
for the complex is shown in Fig. 4. The ionization potential obtained is about 10.0 eV. To our knowledge, the ionization potential of this molecule has not been previously measured. The complex signal increased by 50% when the pyrrole source was heated from 370 to 570 K. The
94
THOMPSON M. SLOANE ET A L
O O(3p) +
MASS 83 I
H
O O t-
10
O
O
< v
>-
0
Z iii Z
O O O
0-D
0
0
0
0 0 o° I
868.0 (9.0 eV)
916.2 (9.5)
0 I1 964.4 (10.0)
I 1012.6 (10.5)
ELECTRON ENERGY (kJ) Fig. 4. Corrected ionization efficiency measurement for the mass 83 product from 0(3p) + pyrrole. The pyrrole source temperature was 300 K.
signal at m/e = 55 decreased to about half its 300-K value when the source was heated to 570 K. The ionization efficiency of the mass 55 product is shown in Fig. 5. The ionization potential obtained is about 10.8 eV at a pyrrole source temperature of 300 K. The two most likely possibilities
are C2HsNC (11.2 eV) [15] and C2I-I5CN (11.8 eV) [15]. Normally our measurements come within +0.2 eV of the accepted values o f ionization potentials for reaction products. We are confident in our electron energy calibration because, in separate experiments, we obtained ionization potentials
OXIDATION OF PYR1DINE AND PYRROLE
95
O(3p) +
MASS 55 I
H
O
O e-
10 ,.,C}
< >I-Ct) Z UJ I-. Z
O
0
O 0
0
00000000
0
I
,
1
964.4 (10.0 eV)
1012.6 (10.5)
1060.8 (11.0)
ELECTRON ENERGY (kJ) Fig. 5. Corrected ionization efficiency measurement for the mass 55 product from 0(3) + pyrrole. The pyrrole source temperature was 300 K. for argon, pyridine, and C2HsCN that were within 0.1 eV of the literature values. The stated uncertainty for the C2HsNC measurement is e0.3 eV [15], so our value is in reasonable agreement with that obtained for C2HsNC. Product paths searched for, but not found, were NO + C4H 5 and HCN + C3H40.
Pyridine-Doped CH4-O2-Ar Flame Figure 6 shdws the intensity of pyridine, methane, CO, and mass 67 as a function of distance from the burner surface, as well as the temperature. Since our object in this work was only to identify reaction products in the flame, we did not call-
96
THOMPSON M. SLOANE ET AL. --
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CH4-O2-Ar FLAME DOPED
--O
WITH PYRIDINE
CH 4
CO
--
M A S S 67
~ " ---
15
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/O
10
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C5 H5 N TEMPERATURE
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W
1000
0
W
500
ON 1
2
3
4
5
6
7
8
D I S T A N C E F R O M BURNER FACE ( m m )
Fig. 6. Relative intensity profile of CH4, CO, CsH5N, mass 67, and temperature in the pyridine-doped CH4--O2-Ar flame.
brate our measurements for absolute mole fractions. The methane and pyridine profiles were arbitrarily normalized at 1.0 mm to determine how fast they disappear in the flame relative to each other. The CO profile was included to provide a reference point in the flame. The CO peak location did not depend on the addition of pyridine, although the CO concentration was greater in the region 2-4.5 mm downstream from the burner with pyridine added to the flame. Molecu-
lar nitrogen should make a negligible contribution to mass 28 because of the low quantity of pyridine and the lean air-fuel ratio. The mass 67 peak is located very near the position of maximum pyridine disappearance. Its ionization efficiency measured at its peak is shown in Fig. 7. The ionization potential obtained is about 10.3 eV, in reasonable agreement with the value of 10.1 eV obtained in the single collision reaction of O(3P) with pyridine. This is strong evidence
OXIDATION OF PYRIDINE AND PYRROLE
97
M A S S 67 IN PYRIDINE-DOPED
(3
CH4-O2-Ar FLAME
(3 (3 ,m E:
(3
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,
1012.6 (10.5 eV) ELECTRON ENERGY(kJ)
I
1060.8 (11.0)
Fig. 7. Corrected ionization efficiency measurement for the mass 67 component of the pyridine-doped flame. This measurement was made 0.4 cm downstream from the burner face where the gas temperature was 1700 K. that the O + pyridine reaction product at mass 67 is the same molecule as is found in the flame, C3HsCN. Other products in the initial stage of the flame were found at masses 96, 95, 83, and 68. The signal was too small in each case to obtain an ionization potential measurement. These measurements were obtained with an electron energy of 14.0 eV.
We believe these compounds are C s H s N O H , CsHsNO, C4H5NO, and C4H40 (or C4H6N), respectively. Pyrrole-Doped C H 4 - 0 z - A r Flame Figure 8 shows the profiles of pyrrole, CO, mass 55, and temperature in the pyrrole-doped flame.
98
THOMPSON M. SLOANE ET AL.
CH4-O2-Ar FLAME DOPED WITH PYRROLE
\o
--O--
CYCLO-C4 H5 N
--@---r-l--
CO MASS 55 TEMPERATURE
12000 u
f f
15
1500
cD A
v
10
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rr
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k-
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2
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- -
kl.I I--
j -"-'i 3
4
5
6
7
500
8
DISTANCE FROM BURNER FACE (mm) Fig. 8. Relative intensity profiles of pyrrole, CO, mass 55, and temperature in the pyrrole-doped flame.
The small peak in the mass 55 profile, although larger than our uncertainty limits based on ion counting statistics, was not reproducible. Repeated measurements of that region of the profile indicated that the noise level there is much higher than that based on counting statistics alone. In any event, that region of the profile is
not important for the purpose of the present work. A corrected ionization efficiency measurement of the mass 55 signal at its peak is shown in Fig. 9. The ionization potential obtained from this measurement is in excellent agreement with the accepted value for C2HsCN of 11.8 eV. As men-
OXIDATION OF PYRIDINE AND PYRROLE
99
0
MASS 55 IN PYRROLE-DOPED CH4-O2-Ar FLAME n
0 ¢-
O 10
0
< >t-
0
Z LU
I-Z
0 0 0 0--
0
0000
0
0
i
1109.1 (11.5 eV)
0
I
i
1157.3 1205.5 (12.0) (12.5) ELECTRON ENERGY (kJ) Fig. 9. Corrected ionization efficiencymeasurement for the mass 55 component of the pyrrole-doped flame measured at the peak in its profile.
tioned previously, we obtained a value of 11.811.9 eV for C2HsCN in a separate experiment. It would not be surprising for C2HsCN to be formed in the flame and C2HsNC in the reaction product analysis experiments because the isocyanide should isomerize rapidly to the cyanide at the temperature of peak mass 55 signal in the flame. The C2HsCN peak occurs near the position of maximum pyrrole disappearance. It could either
be a direct reaction product of pyrrole, or the result of the decomposition of C4H5NO which is formed early in the flame and which dissociates in a hotter region of the flame. Other products in the flame were observed at masses 83, 71, 69, and 54. We believe these molecules are C4HsNO , CaH5NO, C4HvN, and C3H4N, respectively. Since these last three products are not found in the pyridine-doped flame, this gives
100
THOMPSON M. SLOANE ET AL.
further evidence that the mass 67 product in that flame is unlikely to be pyrrole. DISCUSSION
From the data obtained, a reasonably consistent picture emerges of the initial stages of pyridine and pyrrole oxidation in a lean methane-oxygenargon flame. Based on our results for OOP) + pyridine and on previous knowledge of H, O, and OH reactions with unsaturated hydrocarbons, the following reactions are likely: M
OH + C s H s N ~ C 5 H s N O H *
~ CsH5NOH
(1)
-~ H + CsHsNO,
(2)
M
O(aP) + CsI-IsN-. CsI-I,sNO*~ C5H5NO
(3)
CO + H3C-CI-I=CHCN (or H2C=CHCH2CN), (4) M
H + C4HsN -> C4H6N* ~ C4H6 N,
(5)
M
0 + C4HsN ~ C4HsNO* --* C4HsNO.
(6)
M denotes any other flame component which can collisionally stabilize the vibrationally and rotationally excited molecule denoted with an asterisk. The more massive products of these reactions have all been observed in the flame. No other observable species were present at role > 50. Reaction (4) could yield either isomer, so the oxygen atom reaction would be expected to proceed as follows [161: o(ap) + HaC-CH=CHCN --" HaCCO + CH2CN,
(7) H3C-CH 2 + COCN,
(8) o(ap) + H2C=CHCHzCN ~ HCO + CH2CH2CN. (9)
No products were observed at mass 54 (CaH4N), however. Either the amounts of these products were too small to detect or another reaction mechanism giving products with masses less than 50 is important with these cyanides [17]. The reaction of OH with C4HsN could proceed in a number of ways [18], but no products were observed for these reactions either. It is clear that other experiments need to be done to find out how this unsaturated cyanide degrades in the flame. We were concerned about the possibility that the mass 67 flame species might not be a reaction product of 0(3p) + pyridine, but rather a fragment produced by electron bombardment of the 0 + pyridine complex in the ionizer. This possibility could not be completely excluded in our 0(3p) + benzene experiment or in our present 0 0 P ) + pyridine experiments. The CsH 6 product observed from 0 0 P ) + benzene might be a fragment, according to molecular beam results from another laboratory [19]. However, C5H6 has been observed in a benzene-oxygen flame to have an ionization potential of 8.5 eV, independent of position in the flame [201 and therefore independent of temperature. By comparison, in the same flame, the benzene fragment C6H 5 was identified by its variation in appearance potential over a range of about 1 eV depending on its position [20]. The ionization potential of a polyatomic molecule should not depend strongly on its temperature, but the appearance potential of a fragment ion might very well decrease with increasing temperature. We have measured the ionization potential of the mass 67 signal at three positions in the flame corresponding to a temperature range of 1400-1660 K, and the values obtained lie within the range of 9.95-10.3 eV. No systematic variation with temperature was noted. Since this range is within our experimental error, it is unlikely that our measured mass 67 in the flame is a fragment of the 0 0 P ) + pyridine complex or of any other parent compound. The reaction scheme which explains reasonably well our observations for the pyrrole-doped flame is the following: OH + cyclo-e4HsN -* C4HsNOH* ~ H + C4H5NO, (10)
101
OXIDATION OF PYRIDINE AND PYRROLE M
o(ne)+cyclo-C4~'~-~C4I~NO *-~ C4H~O
(11)
CO + C2HsCN, (12) H
H + cyclo-C4HsN ~ C4H6N ~ C4HTN,
(13)
OH + CaHsCN -~ C2H4CN + H20.
(14)
The above reactions do not explain the appearance of mass 71 (probably C2H5CNO) and mass 43 (either C2HaO or HCNO). Mass 71 could arise from the reaction o(ap) + C2HsCN ~ C2HsCNO,
(15)
although this is purely speculative. Reaction-product analysis for the reaction of 0(ap) with cyanides remains to be determined. Mass 43 would have to be explained on the basis of speculation also. It is clear that flame reaction mechanism studies are needed for the cyanides produced by oxygen atom attack on pyridine and pyrrole.
spectrometer is 35% less at a pyridine source temperature of 600 than 300 K. The observed 25% increase in the mass 95 signal is then a net 92% increase in the amount of 0 + pyridine complex that is formed which has a long enough lifetime to reach the detector. The net increase in the amount of the 0 + pyrrole complex formed with heating of the pyrrole source is about 130%. Since the complex should decompose more rapidly at the higher total energy, there is apparently a barrier to formation of the complex which can be overcome with vibrational or rotational energy of the heterocyclic hydrocarbon. This result is unusual in that there have been very few examples [10, 21-24] of an exothermic process which is enhanced by increasing the internal energy of one of the reactants, in one case, the presence of a small barrier was indicated by a positive activation energy for the reaction of H atoms with benzene [25-27]. Unfortunately, no kinetic information is presently available for the reaction of 0(ap)with pyridine or pyrrole. We have not yet performed the heated hydrocarbon source experiments with 0(ap) + aromatic hydrocarbons for a comparison with the results obtained here.
Reaction Dynamics of 0(ap) with Pyridine and Pyrrole Our results show that there is a strong analogy between the reaction of 0(3p) with these aromatic heterocyclic compounds and the reaction of 0(ap) with benzene [7]. The same spin-forbidden mechanism probably explains the 0(ap) + pyridine and pyrrole results. The increase in the amount of mass 67 signal with increasing pyridine source temperature cannot be interpreted as an increase in the cross section for formation of that product because its angular and energy distribution is unknown. However, the change in the 0 + pyridine complex signal can be interpreted if it is assumed that the velocity distributions of both beams are described by a Boltzmann velocity distribution and if the intensity distributions of both sources are cosine distributions. Since heating of the pyridine source changes the velocity distribution, the distribution of velocities in the center of mass is changed as well. The fraction of collisions whose center of mass velocity vector is pointed toward the mass
CONCLUSIONS We have found that the same products observed under single collision conditions from the reaction of 0(ap) with pyridine are also found in a pyridine-doped lean methane-oxygen-argon flame. The 0 + pyrro]e adduct and an isomer of the C2H5NC product were observed in the pyrroledoped flame. This shows that the important ringfragmenting step in the flame is likely to be the reaction of 0(ap) with pyridine and pyrrole and the decomposition of hydroxypridine or hydroxypyrrole produced by the reaction of pyridine and pyrrole with OH. The addition cross section of 0(ap) atoms to pyridine and pyrrole is enhanced by increased vibrational and/or rotational energy of the hydrocarbon. In the flame the decomposition products of this adduct are probably CO and an unsaturated (in the case of pyridine) or saturated (in the case of pyrrole) cyanide. How these cyanides react further in a flame will be the subject of future work.
102
REFERENCES 1. Pershing, D. W., Martin, G. B., and Berkau, E. E., AIChE Symp. Set. 71:19 (1975). 2. Pershing, D. W. and Wendt, J. O. L., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, p. 389. 3. Sternberg, H. W., Raymond, R., and Schweighardt, F. K.,Science 188:49 (1975). 4. Hayatsu, R., Scott, R. G., Moore, L. P., and Studier, M. H.,Nature 257:378 (1975). 5. Axworthy, A. E.,AIChESymp. Ser. 71:43 (1975). 6. Peterson, R. C., Lucht, R. P., and Laurendeau, N. M., Paper No. 36, 1978 Fall Technical Meeting, Eastern Section of the Combustion Institute, Miami Beach, Florida. 7. Sloane, T. M.,J. Chem. Phys. 67:2267 (1977). 8. Sloane, T. M., Chem. Phys. Lett. 54:269 (1978). 9. Kenley, R. A., Davenport, J. E., and Hendry, D. G., J. Phys. Chem. 82:1095 (1978). 10. Sloane, T. M., and Brudzynski, R. J.,J. Chem. Phys., (to be published). 11. Lazzara, C. P., Biordi, J. C., and Papp, J. F., Combust. Flame 21:371 (1973). 12. Peeters, J., and Mahnen, G., Thirteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1973, p. 133. 13. Sloane, T. M., and Ratcliffe, J. W., Mass Spectrometer Sampling of the Quench Zone of a Methane-OxygenArgon Flame, Research Publication GMR-2845, 1978 (unpublished). 14. Winters, R. E., Collins, J. H., and Courchene, W. L., J. Chem. Phys. 45:1931 (1966).
T H O M P S O N M. S L O A N E ET AL. 15. Franklin, J. L., Dillard, J. C., Rosenstock, H. M., Herron, J. T., Draxl, K., and Field, F. H., Ionization Potentials, Appearance Potentials, and Heats of Formation of Gaseous Positive Ions, NSRDS-NBS 26, U.S. Government Printing Office, Washington, D.C., 1969. 16. Kanofsky, J. R., Lucas, D., and Gutman, D., Fourteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1973, p. 285. 17. Bonanno, R. J., Timmons, R. B., Stief, L. J., and Klemm, R. B.,J. Chem. Phys. 66:92 (1972). 18. Slagle, I. R., Gilbert, J. R., Graham, R. E., and Gutman, D., Int. J. Chem. Kinet. Symp. Series, 317 (1975). 19. Buss, R., private communication. 20. Bittner, J., private communication. 21. Sloane, T. M., Tang, S.-Y., and Ross, J., J. Chem. Phys. 57:2745 (1972). 22. Cross, R. J., Lee, L., Saunders, M., and Auerbach, A., Paper #258, 178th ACS National Meeting, Washington, D.C., 1979. 23. Moy, J., Bar-Ziv, E., and Gordon, R. J., J. Chem. Phys. 66:5439 (1977) and references cited therein. 24. Spencer, J. E., and Glass, G. P.,Int. J. Chem. Kinet. 9:97 (1977);ibid., p. 111. 25. Hoyermann, K., Preuss, A. W., and Wagner, H. G., Ber. Bunsenges. Phys. Chem. 79:156 (1975). 26. Kim, P., Lee, J. H., Bonanno, R. J., and Timmons, R. B.,J. Chem. Phys. 59:4593 (1973). 27. Sauer, Jr., M. C., and Mani, I., J. Phys. Chem. 74: 59 (1970). Received 9 July 1979; revised 15 October 1979.