THE J O U R N A L OF
PHYSICAL CHEMISTRY Registered in U.S. Patent Office 0 Copyright, 1979, by the American Chemical Society
VOLUME 83, NUMBER 12
J U N E 14, 1979
A Direct Mass Spectrometric Study of the Mechanism of Toluene Pyrolysis at High Temperaturest Richard
D. Smith
Balielle Memorial Institute, Chemical Methods and Kinetlcs Section, Physical Sciences Department, Pacific Northwest Laboratoty, Rlchland, Washlngton 99352 (Recelved December 15, 1978) Publication costs asslsted by Batteile Memorial Institute
The detailed mechanism of the high-temperature pyrolysis of toluene has been studied over a large range of pressures and temperatures by using mass spectrometric techniques. The vapors leaving a high-temperature Knudsen cell equipped with a gas inlet line were examined by using modulated molecular beam mass spectrometric techniques. The pyrolysis products from both quartz and tungsten Knudsen cells were examined over a pressure range of to 1torr and temperatures up to 1800 “C. At the lower pressures, only products with molecular weights lower than C7H8were observed, including CH3,C2H2,C3H3,C4H2,C4H3,C4H4,C5H5,C6H6,and C7H7. At higher pressure, these products undergo bimolecular processes to form heavier compounds, up to at least C2,,H12. To help define the reaction mechanism, the pyrolyses of C6Ht3CH3and C6H&D3were also studied, as well as toluene in the presence of H2,C2H2,and C6Hs. These studies allowed an unambiguous determination of most of the major processes occurring during toluene pyrolysis.
Introduction The mechanism of toluene pyrolysis has attracted considerable attention for over a century.l Since the classic work of Szwarc2over 30 years ago, interest in the detailed reaction kinetics and mechanisms has produced a voluminous l i t e r a t ~ r e . ~In- ~many ~ studies, the rate constant of the initial unimolecular reaction C7H8 C7H7 + H (1) and the determination of the heat of formation of the C7H7 benzyl radical have been of primary interest.2-8 Other studies have been concerned with product identification and the detailed mechanism of the subsequent bimolecular and higher order processes following reaction l.9-22 In spite of the extensive studies of toluene pyrolysis, the details remain confused. There is even disagreement on the relative importance of reaction 1 compared to a unimolecular process involving cleaving of the C-C bond +
This paper is based on work performed under the United States Department of Energy, formerly Energy Research and Development Administration, Contract EY-76-C-06-1830. 0022-3654/79/2083-1553$01 .OO/O
between the phenyl and methyl g r o ~ p s . ~ ~ ~ @ ~ ~ J C&,CH3 C6H5 4- CH:, (2) A few of the secondary reactions postulated to be relevant to toluene pyrolysis have been studied separately; most notably the kinetics of the reactions of H atoms23-2sand methyl radicals26-28with toluene have been investigated in detail. The situation for the more complex processes yielding higher molecular weight products is less well defined and, for the most part, limited to speculation concerning the most easily rationalized products (e.g., bibenzyl and dimethyldiphenyls). The detailed reaction mechanism can best be examined by using techniques which directly probe the species present in the high-temperature reaction zone. The only previous “direct” studies of toluene pyrolysis were lowpressure flow studies which used conventional mass spectrometric detection. In the early study of Ingold and Lossing,lo the temperature is only crudely defined and conditions are such that many wall collisions may occur outside the high-temperature zone or on mass spectrometer ion source surfaces before analysis. Processes on cool +
0 1979 American Chemical Society
1554
The Journal of Physical Chemistty, Vol. 83, No. 12, 1979 QUAORUPOE M A S S SPECTROFIlETER
KNUDSEN CELL, ELEMENT, TUNGSTEN HEAT SHIELDS
Figure 1. Schematic illustration of the high-temperature Knudsen cell and modulated molecular beam mass spectrometer constructed for this study. For precise alignment, the entire furnace assembly may be moved about the plane perpendicular to the molecular beam.
surfaces undoubtedly play a role in the observation of biphenyl, bibenzyl, and "carbon" by these workers at reported temperatures of 1150-1300 "C. A much more recent study of toluene pyrolysis has been reported by Spokes and Benson,22who studied the pyrolysis a t 1600 "C in an alumina vessel of open design. These experiments found only the stable species C6H6and C2Hzas hydrocarbon products, with the major product being CO (apparently due to surface-related reactions). It appears that both these previous studies were plagued by problems in the interpretation of the mass spectra due to the high electron energies of 50 and 70 eV, respectively. In the present work we have applied direct mass spectrometric modulated molecular beam sampling techniques to examine the products of toluene pyrolysis over a pressure range of approximately to 1torr and a t temperatures up to 1800 "C. The direct sampling of vapors from a high-temperature Knudsen cell allows examination of the radical and unstable molecular intermediates of toluene pyrolysis over a large range of temperatures and pressures. Thus, the present work has two goals: first, to show that directed modulated beam mass spectrometric methods are suitable for unraveling the complex chemistry inherent in hydrocarbon pyrolyses (in this regard, the apparatus, techniques, and interpretation are discussed in detail), and second, to examine the details of toluene pyrolysis a t high temperatures. Our approach has been to determine the dominant unimolecular and bimolecular reactions over a range of temperatures and pressures. To help define the mechanism of toluene pyrolysis, the present study has utilized appropriately labeled samples (C6H5CD3and C6H,'3CH3). Additional studies by use of C2H2,C6H6and D2 additives have also been performed. Thus, the present work has allowed, for the first time, an unambiguous determination of many of the major reaction pathways in toluene pyrolysis.
Experimental Section Instrumental Arrangement. Figure 1gives a schematic illustration of the high-temperature furnace-modulated molecular beam mass spectrometer constructed for this work. The high-temperature furnace is capable of heating a Knudsen cell to temperatures of 3000 "C. The Knudsen cells used in this work, fabricated from either tungsten or quartz, are placed on a three-rod tungsten pedestal in the high-temperature zone. Each Knudsen cell (2.5 cm long, 1.25 cm diameter) has a 4-mm diameter black-body hole for optical pyrometer measurements and a second hole (opposite the orifice) where the junction of a tungstend% rhenium vs. tungsten-26% rhenium thermocouple is placed. The gas inlet consists of a 0.20-cm i.d. tube, press
Richard D. Smith
fit into the cap of the tungsten Knudsen cell, and a Swagelok connection to the gas inlet line outside the high-temperature zone. Capillary tubing just outside the high-temperature region limits the time the gas spends in the heated region of the inlet line. Knudsen cells having orifices of 0.4 mm and 2 mm diameters were used in this work. The tungsten Knudsen cells were machined so that the wall near the orifice was as thin as possible. Clausing factors were greater than 0.95 in all cases. The Knudsen cells are heated radiatively by using a cylindrical tungsten mesh heating element (8 cm long, 3.8 cm diameter). The heating element is surrounded by seven tungsten heat shields and a water-cooled jacket. Placement of additional thermocouples at the top and bottom of the cell showed that any temperature gradient across the cell is less than 10 "C. Generally, at less than 2000 "C, the optical pyrometer (corrected) measurements agree with the thermocouple readings to within f10 "C, which is assumed to be the total uncertainty. At higher temperatures (>1500 "C) a TransData, Inc., Hall effect watt transducer also provides precise information from which the Knudsen cell temperature may be deduced after calibration with thermocouples. The temperature is regulated by using a Research, Inc., 640 U process controller utilizing the input from either the thermocouple or watt transducer. The temperature is controlled by using either set point operation or a Data-Trak programmer. Species leaving the Knudsen cell pass through a series of holes in the heating element, heat shields, and cooling jacket. The holes are sufficiently large to prevent reflected molecules from passing through the beam skimmer and being detected by the mass spectrometer. A pressure of to N/m2 (lo-' to torr), depending upon the pressure in the Knudsen cell, is maintained in the hightemperature furnace region by using a turbomolecular pump. The entire high-temperature furnace assembly may be moved in a plane perpendicular to the molecular beam, by using a set ot microslides, to allow precise positioning of the furnace relative to the molecular beam skimmer. The beam skimmer allows a well-collimated molecular beam to enter the analyzer section, where a pressure of less N/m2 (5 X torr) is maintained by than 7 X differential pumping. The molecular beam is modulated by using a rotating toothed wheel and subsequently passes through a mass spectrometer ion source. The modulated beam technique allows one to study unstable or reactive species in the beam because of the total absence of surface-related phenomena; the molecules studied do not collide with surfaces after leaving the Knudsen cell. Modulation of the beam allows the complete elimination of background species from the mass spectra.29 The modulated beam is analyzed by using one of two interchangeable Extranuclear quadrupole mass spectrometers. For highest sensitivity, the axially mounted mass spectrometer may be positioned within 1.5 cm of the chamber divider (or approximately 7 cm from the Knudsen cell). This mass spectrometer may also be moved back approximately 15 cm to allow phase angle spectrometry (Figure 1). In phase angle spectrometry, the phase shifts resulting from the mass-dependent flight times of molecular species between the chopper and ionizer (measured with a PAR 124A Lock-In amplifier equipped with the phase meter option) allow one to determine the molecular weight for species in the effusive beam (to &5%),after calibration with known species.29 (It should be noted that the highest Knudsen cell pressures employed in this work are somewhat above the efflusive flow region, defined as where the ratio of the mean free path over the orifice
High-Temperature Pyrolysis of Toluene
diameter is >>1.However, phase-angle spectrometry can still be practiced into the transition flow region with proper calibration.) Alternatively, the molecular beam may be analyzed by using a second mass spectrometer operated in the crossed-beam mode (Figure 1). This has the advantage of a greatly reduced noise level a t high temperatures (where scattered and excited neutral species may collide with the off-axis electron multiplier in the axial mode) a t the cost of a reduction in sensitivity, depending on operating parameters, on the order of 10. Analysis of Mass Spectrometric Data. The species flowing from Knudsen cells having 2 mm diameter orifices were examined for toluene pyrolysis a t temperatures up to approximately 1900 "C for the tungsten and 1400 "C for the quartz cells. The time required for temperature equilibration after a change in temperature ranged from -20 min a t low temperatures to less than 1 min a t 1800 "C. The toluene pressure in the Knudsen cells was varied over more than 3 orders of magnitude. Relative pressures were determined by using calibrated gas flows at room temperature and measuring the pressure in the first differentially pumped (furnace) chamber at various pumping speeds. The absolute pressures are known only approximately since they must be estimated on the basis of gas flow into the cell, the cell temperature, and the cell orifice size. These estimates of Knudsen cell pressure (corrected for temperature) range from approximately torr), well into the effusion range to N/m2 (W5to at the lowest pressures, to 1to 10 N/m2 to 10-1torr), into the transition flow range at the highest pressures. A different Knudsen cell with a 0.4 mm diameter orifice was used to obtain pressures approximately 20 times higher. The identification and determination of the relative concentrations of the various hydrocarbon species is a complex task requiring a combination of mass spectrometric techniques. The primary problem which had to be overcome in obtaining concentration profiles was in distinguishing parent ions of the various pyrolysis products from ions formed by fragmentation of larger species, particularly those fragment ions of toluene. Mass spectra of the products of toluene pyrolysis were recorded at electron energies of 10, 11, 12, 13, 14, 15, 17, and 25 eV, a t small pressure increments, and at 50 "C temperature increments, for both the tungsten and quartz cells. Since many of the products of toluene pyrolysis coincide with fragment ions of toluene, a methodology had to be developed to distinguish parent and fragment ions. The fragmentation pattern for toluene, in contrast to the assumption commonly employed in high-temperature mass spectrometry, was found to be highly temperature dependent. For example, at room temperature, the only detectable fragment ion a t 17 eV is C7H7+. However, at 1200 "C, C3H3+,C4H4+,C5H5+,and C7H6+fragment ions are also observed in abundances comparable to ions resulting from the corresponding neutral species. To resolve this problem, it is necessary to work a t sufficiently low electron energies to avoid contributions by fragmentation to a given m / e ratio, while maintaining an electron energy sufficiently high to obtain the desired sensitivity. The most useful tool in this regard is phase-angle spectrometry, which allowed an unambiguous determination of the molecular weight (to *5%) of the neutral precursor of a given ion (in the effusion flow range). For cases where the resolution of phase-angle spectrometry is insufficient to resolve parent and fragment ions (e.g., C4H3+from C4H3 or C4H4or C7H7+from C7H7or C7HB),it is necessary to rely on the behavior of the relative ion intensities as a function of electron energy. In each case, appearance
The Journal of Physical Chemistry, Vol. 83, No. 12, 1979
1555
potentials (AP's) were measured and compared to literature values.30 In all cases where reliable AP's exist, values within *0.5 eV were determined for species identified in this work. Additionally, in most cases, an electron energy which gives only parent ions could be chosen. This is possible becguse all species in this work are expected to yield stable parent ions and fragment ions usually have AP's at least several electron volts higher. Although the results presented in the figures and tables represent data obtained a t 10-14 eV, an optimum single electron energy which eliminates nearly all fragmentation, but still allows the detection of all species (except H and H2),is 12 eV. The quadrupole mass spectrometer used in the present study has an effective mass range of 3-1300. Thus, while H and Hz can be detected a t higher electron energies, sensitivity is greatly reduced and, therefore, relative H and Hz concentrations could only be determined a t high pressures. The relative concentrations, obtained as described above, must then be corrected for ionization, transmission, and detection efficiency a t the appropriate temperature, pressure, and electron energy. Direct calibration was possible for CHI, C2H2,CzH4, C3H4,C4H2, C6H6,and C7HB For the remaining species, relative detection efficiencies were estimated by using standard gas mixtures (to determine transmission efficiencies) and estimated ionization efficiencies. Because of the necessary approximations, the absolute concentrations (after exclusion of H and H,) may be in error by as much as a factor of 2 to 3 in some cases. However, relative concentrations as a function of temperature are believed to be good to better than &lo%, since these data are derived directly from relative ion currents. Comparison of results for tungsten and quartz Knudsen cells with 2 mm diameter orifices shows only very minor differences in the observed concentrations. C6H6did seem to be produced in greater concentrations in the tungsten cell (by a factor of about 1.5); however, the minor differences appear not to suggest contributions due to heterogeneous catalysis. Preliminary measurements of the unimolecular rate constants in both cells show no differences outside of experimental scatter.31 Additionally, there is little evidence of darkening or surface deposits in the cells even after many hours of operation at high temperatures and pressures of 1 N/m2 (lo-, torr). Also, the formation of a surface deposit by pyrolysis of another compound (benzene or isopropyl iodide) did not affect the products or reaction rates for toluene.31 (The formation of a black "sooty" film was observed on and around the beam skimmer, suggesting the polymerization of reactive species on the cool surfaces.) Collectively, these observations suggest that the nature of the surface is not important in the high-temperature pyrolysis of toluene. Other workers have suggested that catalysis is rarely important under the conditions of high temperatures and low pressure.32 In addition to experiments with pure toluene, additional experiments were run by using D,, CzH2,and C6H6additives and isotopically labeled toluene (Stohler Isotope Chemicals, Waltham, MA). The C6H5CD3had a stated isotopic enrichment of 99.5% and was shown to be better than 98% C6H5CD3by mass spectrometric analysis. The C6H513CH3had a stated isotopic enrichment of 90%, confirmed by mass spectrometric analysis. Results and Discussion
The concentrations of the major products (excluding H2 and H), as a function of temperature at approximately torr total pressure, are given in Table I. Figures 2-4 and
The Journal of Physlcal Chemlstty, Vol. 83, No. 72, 1979
1558
Richard D. Smith
TABLE I: Concentrationa of Hydrocarbon Products in the Pyrolysis of Toluene temp, "C species 900 1000 1100 1200 1300 1400 CHI 0.31 11 1.5 24 16 0.02
0.09
-0.01
0.12 0.09 0.06 0.01 0.03 0.03 0.02 0.01 1.2 0.13
-0.01
0.18 0.08 0.06 0.13 0.03 14 86
0.44 2.0 0.14 0.09 1.3 0.19 0.21 0.68 0.20 0.20 0.33 0.10 3.5 0.31 0.01 0.24 0.64 4.4 1.2 1.0 37 33
0.15 0.90 0.17
30 69
40 56
0.07 0.10 0.08 0.01 0.02 0.01 0.03 0.20 0.03
2.4
4.9 37 1.7 5.2 5.4 0.8 0.1 6.3 0.7 1.2 0.7 0.2 1.0 0.1 0.6 0.8 1.3 11 0.6 0.3 2.5 1.1 0.03 0.6 0.15 0.4 0.08 0.4 0.06 0.2 0.17 0.07
16 0.78 0.20 5.0 0.78 0.25 4.1 0.4 0.8 0.8 0.3 2.8 0.3 0.2 0.7 1.1
10 1.7 1.0 16 8.4 0.45 0.33 0.5 0.07 0.2 0.09 0.18 0.6 0.09
1500
1600
1800
5.2 5.0 65 3.0 3.2 1.2 0.3
2.1 3.8 78 4.0 2.5 0.4 0.3
0.26 0.4 80 4.5 1.5 0.2 0.5
5.5 0.4 0.7 0.1 0.01 0.1
5.0 0.3 0.3
12 0.1
0.35 0.36 0.7 7.0 0.05 0.01 0.1
0.3 0.1 0.25 3.0
0.3
0.02
0.01 0.2
0.4
0.06 0.03 0.15
0.01
0.05 0.01 0.01
0.5
0.01
a Mole percent excluding H and H, (see text), for a pressure corresponding to approximately 500 arbitrary units in Figures 2-4 and 7 (Le., approximately 1 N/m2 or lo-, torr) for a tungsten Knudsen cell with a 2-mm orifice.
6-
5-
4-
3-
2-
/- -I
1-
1
10
100
1000
PRESSURE IARBITRARY UNITS1
Figure 2. Concentrations of C7H8,C,H7, C2H2,and CH, in a quartz Knudsen cell at 1280 OC, as a functlon of pressure on a scale pro-
1
IO
100
1000
PRESSURE IARBITRARY U N I T S )
portional to approximately 10-~-10-~to 10-'-10-* torr.
Figure 3. Concentrationsof C7H6,C7H5,C8He,C5H5,C4H2,C3H3,C2H4, and CH, in a quartz Knudsen cell at 1280 OC, as a functlon of pressure.
7 give the concentrations of several of the major pyrolysis products as a function of pressure at 1280 "C. In these figures, the arbitrary pressure scale ranges from, and is proportional to, approximately to torr at low pressure to approximately 10" to 10-1torr at high pressure. These results were obtained in a quartz Knudsen cell with a 2-mm orifice. At low pressures the residence time in this cell is calculated to be 3 X (M/T)1/2s, where M and T are the molecular weight and t e m p e r a t ~ r e . ~ ~ As already mentioned, the results for quartz and tungsten Knudsen cells were quite similar and, hence, no
distinction will be made in the following discussion. The large amount of experimental results cannot be presented; however, this is unnecessary since only minor variations in relative concentrations of the numerous higher molecular weight products are observed in the 1100-1400 "C temperature range. Higher molecular weight products are observed in only very small amounts outside of this temperature range (see Table I). In the following sections, we present and discuss the results for toluene pyrolysis near the low- and highpressure limits. In practice, these pressure ranges are
The Journal of Physical Chemistty, Vol. 83, No. 12, 1979
High-Temperature Pyrolysis of Toluene I. 2
I
1557
product decreases at higher pressures is consistent with the occurrence of subsequent unimolecular and bimolecular reactions. At temperatures as low as 1100 "C, it was apparent that part of the C7H7product of reaction 3 could undergo decomposition. Results at higher temperatures (12oO-1400 "C) suggest two pathways for this reaction C7H7 -+ C5H5 + C2H2 (6) C7H7 C4H4 + C3H3 (7) The fact that the C5H5,C4H4,C3H3,and CzH2 concentrations are pressure dependent and that at 1200 "C these products are not produced at a pressure low enough to disallow bimolecular processes indicate that C7H7 produced by reaction 1 does not undergo subsequent unimolecular reactions at temperatures less than approximately 1250 "C. In addition to C4H4,both C4H3and C4Hzare observed at 1200 "C, with the abundances of these products relative to C4H4 increasing at higher temperatures. The C5H5may also undergo further decomposition by Hz elimination, producing C5H3 in detectable amounts at temperatures above 1300 "C. At temperatures of 1300 "C and higher, the C7H7products of reaction 1may decompose via reactions 6 and 7 . The CH3,C2H2,and C3H3products of these reactions are quite stable and no decomposition products (e.g., CH2, CZH, C3H2, etc.) were observed at temperatures up to 1850 "C. An attempt was made to determine the branching ratio of C7H7between reactions 6 and 7 . This proved to be difficult since reactions 1and 3 produce C7H7with varying amounts of internal energy, and, hence, quite different branching ratios. At low pressures, the branching ratio hG/k7 was determined to be approximately 4 f 2 at 1250 "C. The ratio decreases at increased temperatures, suggesting a lower activation energy for reaction 6. Notably absent from the reaction products at low pressures are any products with molecular weights above that of toluene. The reaction of C7H7radicals with C7H8 does not yield higher molecular weight products at low pressures (-10" torr), and even at elevated pressures to 1 torr) the reaction of C7H7with C7H8or C7H7appears to be relatively slow. The results suggest that a steadystate situation involving C7H7and C7H8is attained at low pressures. This process almost certainly involves reaction 3 and its reverse reaction, since the reverse of reaction 1 is unlikely under the present circumstances. The results for C6H513CH3 help to define the reaction mechanism at low pressure. The results show that at pressures just high enough to allow a fraction of the H atoms to undergo bimolecular reactions, the carbon atoms in toluene have not scrambled. Thus, the CH3 and C6H6 products of reaction 8 are better than 90% (corrected for the degree of isotopic purity in toluene) labeled and unlabeled, respectively. +
01
' " " " ' I
1
'
'
A l I M l ~
"
LOO
10
"
'
~
~
'
I
1000
PRESSURE IARBITRRRY UNITS)
Flgure 4. Concentrations of CBH5,C5H3,C4H,, C,H,, C3H5,C3H4in a quartz Knudsen cell at 1280 "C,as a function of pressure.
characterized by the absence of higher molecular weight products in the former and their presence in the latter (except at the highest temperatures). Thus, the lowpressure range is not limited to the simple unimolecular reaction 1, resulting from energy transfer on gas-wall collisions, and includes reactions of the products of reaction 1. These reactions involve H-atom reactions with C7H7 and C7H8and subsequent thermal pyrolysis processes at higher temperatures. The distinction between unimolecular and bimolecular reactions is readily made by noting variations of concentration with pressure; indeed, the pressure could be lowered sufficiently to eliminate nearly all bimolecular reactions. Low Pressure. At the low-pressure limit, the pyrolysis of unlabeled toluene is dominated by a simple H-atom elimination process. C7H8 C7H7 + H (1)
-
This reaction was confirmed by the observation of H atoms at temperatures as low as 1000 "C. The unimolecular process involving cleavage of the methyl C-C bond C6H5CH3
+
C6H5
CH3
(2)
does occur but accounts for less than 0.5% of the total reaction over the temperature range 1000-1400 "C. The C6H5concentration increases with pressure indicating it is produced rapidly in a second-order reaction. Upon increasing the temperature for a constant gas flow (- 1014molecules/s) into the Knudsen cell, one observes increased contributions from both bimolecular reactions and pyrolysis of these products. The reaction of H atoms with toluene increases rapidly in importance at higher temperqtures due to both an increasing rate constant and H-atom concentration. Analysis of the pressure dependence indicates that H atoms react with toluene via at least two distinct channels:
+ C7H8 H + C7Ha H
-
C7H7 + Hz
(3)
C6H6 + CH3
(4)
+
Large increases in the concentrations of each of these products, with the exception of C7H7,are observed when the toluene pressure is increased near the low-pressure limit. While both of those reactions have been postulated previously, it should be noted that we have no convincing evidence for the corresponding reaction yielding methane and a phenyl radical. H + C7H8 C6H5 + CHI (5) +
The fact that the relative concentration of the C6H5
*
+
*
H + C6H5CH3 CsH6 CH3 (8) Previous 14C labeling (toluene-I-14C)experiments by Takeuchi et a.LZosupport this observation; however, in their experiments, they were unable to determine the precise reaction mechanism producing unlabeled methane at their lower temperatures (-800 "C). They report that C-atom scrambling is much more important at 1000 "C, where they find a nearly statistical distribution of 14C. These findings are consistent with the present results which indicate increased C-atom randomization at higher temperatures. The results for the decomposition of C7H7in reactions 6 and 7 are more ambiguous. Table I1 gives the fraction of 13Clabel for the CzHz,C3H3,C4H4,and C5H5products
Richard D. Smith
The Journal of Physical Chemistry, VoL 83, No. 12, 7979
1558
TABLE 11: Fraction of 13C Label Retained in the Products of C,H, Pyrolysis at 1280 'Cu species exptl calcd CH, CP, C3H3 C'lH, CJ-4 CJ-4
100 29 43 57 71
94* 4 37 i: 3 56t 5 40+ 4 651. 5 8+5
0
0
0
0
Obtained at low pressures and corrected for an isotopic enrichment of 9 0 % and naturally occurring * 3C.
0
0
0 L O h PRESSURE 0 HIGF P R E S S U R E
0
.. . b
0%
.
I 800 O
0
1200
1000
la00
TEMPERATURE loci
Figure 6. Relative rate of intramolecular H,D-atom exchange as a function of temperature at both low (0) and high pressures (0). The relative rate is derived from the [ m l e 9 6 ] / [ m / e951 ratio for CBH5CDB.
0 0 0 0 0
I-
A m 0.4' 800
. O n P
0
0
1 1000
lZ00
1400
TEMPERATURE l°Cl
Flgure 5. Relative rates of H and D loss from CeH& at low pressure (0). The ratio of [ m l e 9 3 ] / [ m l e941 vs. temperature for C&CD3 at both low pressures (on the order of torr) and high pressures (0) (on the order of I O - * to lo-' torr).
(as well as CH3 and C6H6)at 1280 "C. Also given is the fraction of 13Clabel expected if one completes carbon atom randomization prior to decomposition. These results indicate a significant fraction (perhaps as much as 10-20%) of the C7H7radicals which decompose do not completely randomize the carbon atoms. Alternatively, there may be no carbon atom randomization but only a limited specificity in the decomposition process. Regardless, the results show that the 13Clabel is more likely to reside in the C2H2and C3H3fragments than predicted on a statistical basis. Thus, the following processes are apparently favored: *CH2
I
H
Cleavage of the labeled benzyl radical via reaction 9 has been postulated previously;16however, the present work has been the first to demonstrate its occurrence. The results for pyrolysis of C6H5CD3are not as readily interpreted as those for carbon labeling. This is not unexpected since H atoms are expected to exchange more rapidly and with lower activation energies than C atoms. Figure 5 gives the ratio of D-atom loss to H-atom loss ([mle 93+]/[m/e 94'1 recorded at 11eV and corrected for I3C contributions and labeling error) from 800 to 1500 "C, a t the low-pressure limit (circles) and at approximately torr (squares). The results in Figure 5 show that the H and D atoms (assuming H- or D-atom loss from the methyl group) are well scrambled in toluene either prior to or during the decomposition. The ratio increases from about 0.43 a t 800 "C to about 0.56 at 1500 "C. By assuming purely random elimination of an H or D atom, one would predict a ratio of 0.6. Thus, the present results show either
that H and D atoms scramble fully prior to decomposition or that elimination of a ring hydrogen atom is just as likely as elimination of a methyl hydrogen atom. This latter possibility is less likely since the activation energy for elimination of a ring hydrogen is usually assumed to be higher than for elimination from the methyl group. The fact that the ratio of D-atom loss to H-atom loss is somewhat less than expected on the basis of purely random elimination (0.6) may be due to an isotope effect favoring H-atom loss. A t higher temperatures the ratio approaches 0.6, consistent with this explanation; however, the higher temperature data must be interpreted with caution since they may include small contributions due to intermolecular H, D exchange reactions, which apparently dominate at higher pressures (Figure 5 ) . This phenomenon is demonstrated in Figure 6, which gives the relative rate of intermolecular H, D exchange due to reactions 11 and 12 or equivalent processes. Figure 6 shows the lowC7H5D3 + C7H5D2 C7H4D4 C7HsD (11) C7H5D3 + CYH4D3
--
+ C7H4D4 + C7HbD2
(12)
pressure results above 1200 "C include contributions from these or analogous H,D-atom exchange reactions (such as H- and D-atom reactions with toluene). The rate of the H,D exchange at high pressure is much greater, as one expects from the above discussion. On the basis of these results, one might expect to find complete H,D-atom randomization in the products of reactions 4, 6, and 7 (e.g., CH3, C2H2,C3H3,C4H4,C5H5, and CGHG). The results do show considerable H,D scrambling; however, they also clearly show that several products are measurably enriched in H or D, most notably the products of reaction 4. Table I11 gives typical H,D distributions for various products obtained at 1250 "C for a tungsten Knudsen cell with a 2-mm orifice at a pressure torr. Results for C4X4(X = H or of approximately D) are not included since the contributions due to C4Xz and C4X3could not be resolved. The results show that CX3 is enriched in deuterium while CsXG is deuterium poor; this is consistent with the l3C labeling results and supports the mechanism suggested by reaction 8. The results suggest that reaction 13 is more likely than reaction 14 and are consistent with previous X + CGH5CD3 C & j X + CDa (13)
-
X
+ C G H ~ C D ~CeH5D + CDzX -+
(14)
observations of H-atom addition to the ring. These results
The Journal of Physical Chemistty, Vol. 83, No. 12, 1979
High-Temperature Pyrolysis of Toluene
TABLE 111: Hydrogen and Deuterium Atom Distribution for the Products of C,H,CD, Pvrolvsis at 1250 Ca species mle exptl
06
1559
I
calcdb
~
CH 3 CH2D CHD, CD3
15 16 17 18
0.07 i 0.12 i 0.21 i: 0.60 i
0.03 0.03 0.03 0.03
0.18 0.51 0.29 0.03
C,H, C,HD C A
26 27 28
0.34 t 0.02 0.53 i 0.02 0.13 t 0.02
0.36 0.54 0.11
C3H3 C3H2D
39 40 41 42
0.20 t 0.40 i 0.40 f 0.92 ?
0.1 0.1 0.1 0.02
0.18 0.54 0.27 0.02
65 66 67 68 69 70
0.08 f 0.34 i 0.48 * 0.10 f 0 0
0.03 0.03 0.03 0.02
0.03 0.29 0.51 0.18 0 0
78 79 80 81 82 83 84
0.34 ?: 0.47 0.15 i 0.04 f 0 0 0
0.05 0.05 0.05 0.05
0 0.11 0.64 0.36 0 0 0
C3HD2
C3D3 CSH, C5H'J C5H3D2 C5H2D3
C5HD4 C,DS C6H6 C6HSD
C6H4D2 C6H3D3 C6H2D4 C6HD3 C6D6
a At approximately torr in a tungsten Knudsen cell having a 2-mm orifice. Calculated on the basis of statistical formation from C,HsD3 for the C,X,, C3X, , and C,X, products and on the basis of statistical formation of C,H,D, and C,H6D, formed in a reaction of X with C,H, D, , by assuming an equal probability of X being H or D.
also suggest that H,D-atom randomization in toluene is not extensive prior to reaction at 1250 "C, whereas at 1350 "C the results indicate extensive randomization of H and D atoms. The results for C2H2,C3H3,and C5H5products indicate nearly complete randomization of H and D atoms at 1250 "C. The C3H3and C5H5products may show some evidence of D enrichment and H enrichment, respectively, but these ratios are, for the most part, within the estimated error limits (Table 111). Complete randomization is expected because C7H7shows complete H,D-atom randomization prior to decomposition (Figure 5 ) . Additionally, deuterium isotope effects are expected to be negligible in the case of carbon-carbon bond cleavage. These observations expose an interesting fact. Hydrogen atom elimination from toluene suggests complete randomization between ring and chain hydrogens. However, the results in Table I11 for reactions 13 and 14 show that toluene has not completely randomized H atoms prior to reaction. These apparently conflicting observations can only be consistent if the barrier to H,D exchange is smaller than that for H- or D-atom elimination from C7X8. Thus, if sufficient internal energy has accumulated in the toluene molecule for decomposition, sufficient energy will already be available for hydrogen atom exchange between the ring and the methyl group. While the results show that hydrogen atoms in the C7H7 radical are completely scrambled, the 13C labeling experiments indicate the carbon atoms are not completely scrambled. Thus, the conversion between the benzyl radical and a cycloheptatrienyl radical (assumed necessary for C-atom exchange) is sufficiently slow to prevent complete carbon atom randomization on the time scale of the present experiments.
I
1
,
, , I
10
I , # # '
1W
1 8
##I
1000
I
PRESSURE (ARBITRARY LNlTS'
Figure 7. Concentrations of CI3HB, C,?Hs, COH7,C8H8,C8H7,and C8Hg in a quartz Knudsen cell at 1280 O C , as a function of pressure.
High Pressure. A t pressures above approximately torr, the various species described in the last section react sufficiently fast with C7H8,C7H7,or other species so that their products may be detected, Table I gives the product distribution (excluding H and H,) at a pressure of approximately lo-, torr for temperatures up to 1800 "C. The dependence upon pressure of several of the higher molecular weight reaction products is shown in Figure 7 over the same pressure range as the lower molecular weight products in Figures 2-4. Thus, the initial set of radicals and highly reactive species may undergo reaction to form higher molecular weight compounds if the pressure is increased sufficiently. This behavior is illustrated in the intermediate pressure region of Figures 3,4, and 7, where a number of highly reactive species (C3H3,C4H3,C5H3, C5H5,C6H4,and C6H5) show maxima in their concentrations and apparently react (by radical-molecule or radical-radical processes) to form heavier compounds (C8-CI4). A further increase in pressure allows these products to undergo reactions to form even heavier compounds. This is shown by the product distribution in Table IV, obtained by using a Knudsen cell with a smaller (0.4 mm) orifice, to allow both increased pressure (10-l to 1 torr) and increased reaction time.32 The results also indicate a continuation of this trend at higher pressures (trace amounts of CZ4species were detected). Previous results,14which suggest that tar accounts for as much as 10% of the products of toluene pyrolysis at atmospheric pressure, support this belief. The complex chemistry which occurs during toluene pyrolysis at higher pressures (Z10-3 torr for the 2 mm orifice Knudsen cells) cannot be unraveled without more knowledge of the chemistry of the various hydrocarbon radicals. As noted above, the concentration profiles as a function of pressure provide some indication of the relative reactivities of the various radicals but provide no information on the radical precursors of specific higher molecular weight products. Additional information on the formation mechanism of the heavier species can be obtained by using C6H&3CH3labeled toluene; however, only a very limited amount of information can be gained by using C6H5CD3. Deuterium labeling experiments with toluene-d3 were of no value in discerning reaction mechanisms at higher pressures over the temperature range examined in this work. The results at higher pressures in Figures 5 and 6 show that both intermolecular and intramolecular H,D exchange are rapid, and one expects nearly complete H,D scrambling at higher temperatures. This is indeed observed, with all the higher molecular weight products
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The Journal of Physical Chemistry, Vol. 83, No. 12, 1979
TABLE IV: Concentrationa of Toluene Pyrolysis Products in a Small Orifice Knudsen Cellb species CH3 CH, C2HZ CZH, C3H3 CJ-4 C3Hs ',HZ
C4H3 C4H4 C,H, CSH, CSH, 'SH6
',HZ
C6H4 C6H5 C6H6
C7Hs C7H6
C,H, C;H, 'EH,
CSH, C8H8
concn, mol %
H7 C9H8 C10H6
CIOH, cl 1 H7
CI 1 H, 1 H9
CllHIO CIP, 'lZH9
C1,HlO
c,3H9 c,,H, BH1
1'
0 1
ClA 1'
qH1 0
CIA 1'
fraction containing 0, 1,or 2 3~ a t o m 8
1.5 0.5 0.2 2.0 0.2 0.1 0.1 0.1 1.0 0.2 0.3 1.4 0.3 0.1 0.1 0.8 0.2 0.3 0.1 0.03 0.08 0.04 0.07 0.05
c 9
1'
TABLE V: Percentage of Carbon-13 Label Associated with Higher Molecular Weight Productsa
concn, mol %
species
32
