Single Pulse Shock Tube Study of Allyl Radical Recombination - The

Article pubs.acs.org/JPCA

Single Pulse Shock Tube Study of Allyl Radical Recombination Aleksandr Fridlyand,† Patrick T. Lynch,‡ Robert S. Tranter,‡ and Kenneth Brezinsky*,† †

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, Illinois 60607, United States ‡ Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: The recombination and disproportionation of allyl radicals has been studied in a single pulse shock tube with gas chromatographic measurements at 1−10 bar, 650−1300 K, and 1.4−2 ms reaction times. 1,5-Hexadiene and allyl iodide were used as precursors. Simulation of the results using derived rate expressions from a complementary diaphragmless shock tube/laser schlieren densitometry study provided excellent agreement with precursor consumption and formation of all major stable intermediates. No significant pressure dependence was observed at the present conditions. It was found that under the conditions of these experiments, reactions of allyl radicals in the cooling wave had to be accounted for to accurately simulate the experimental results, and this unusual situation is discussed. In the allyl iodide experiments, higher amounts of allene, propene, and benzene were found at lower temperatures than expected. Possible mechanisms are discussed and suggest that iodine containing species are responsible for the low temperature formation of allene, propene, and benzene.

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complementary studies on allyl recombination. In the first part of this study, Lynch et al.13 examined the recombination of allyl radicals in the high temperature fall-off regime (10−120 Torr and 900−1700 K) using thermal decomposition of 1,5hexadiene and allyl iodide (C3H5I) as allyl precursors in a diaphragmless shock tube, with laser schlieren densitometry measurements (DFST/LS). The pressure dependent rate constants for allyl iodide dissociation R3 and the recombination of allyl to 1,5-hexadiene R1 were determined using revised thermochemistry for allyl, allyl iodide, and 1,5-hexadiene. An upper limit on the disproportionation channel R2 of allyl radical self-reaction leading to allene and propene (C3H6) was also estimated to be 5% of R1.

INTRODUCTION Understanding the combustion chemistry and formation of polycyclic aromatic hydrocarbons (PAH) as a precursor to soot formation, necessitates good understanding of the chemistry of resonantly stabilized radicals.1 Allyl (a-C3H5) radicals have been shown to play an especially important part in PAH formation by either undergoing a series of reactions to form propargyl (C3H3), which recombine into various C6H6 isomers, or by directly reacting with propargyl to yield various cyclic and aromatic species.2−7 In competition with this chemistry is the recombination of allyl radicals to 1,5-hexadiene (15HD) via reaction R1,6 as well as disproportionation leading to allene (aC3H4) and propene (C3H6) R2, as seen with other free radicals.8 Recombination and disproportionation may therefore be important in accurately predicting the formation of PAHs in flames. a‐C3H5 + a‐C3H5 = 15HD

(R1)

a‐C3H5 + a‐C3H5 = a‐C3H4 + C3H6

(R2)

C3H5I = a‐C3H5 + I

The present work extends the experimental study of allyl radical recombination to higher pressures (1−10 bar) using the single pulse shock tube technique. These experiments were conducted at very dilute initial conditions in argon, maintaining near isothermal conditions for the reaction period (1.4−2 ms). The hot gases are rapidly quenched by the arrival of the rarefaction wave and are sampled for analysis by gas chromatographic methods. The experimental results consist of measured mole fractions of major stable species formed from pyrolysis of 1,5-hexadiene and allyl iodide (ppm in argon) as functions of temperature and pressure. Simulation of the data assuming a closed, homogeneous, constant pressure reaction

The dissociation of allyl radicals to allene and hydrogen, the isomerization to propyne (p-C3H4), and the subsequent formation of propargyl radicals has been well studied experimentally and theoretically over a wide range of conditions.9−12 The recombination and disproportionation of allyl radicals at combustion relevant conditions, however, has received little attention. A single pulse shock tube has been used to study the recombination of allyl radicals and probe some of the secondary reactions at temperatures and pressures relevant to combustion. The work presented here is the second part of a pair of © 2013 American Chemical Society

(R3)

Received: March 8, 2013 Revised: May 10, 2013 Published: May 16, 2013 4762

dx.doi.org/10.1021/jp402391n | J. Phys. Chem. A 2013, 117, 4762−4776

The Journal of Physical Chemistry A

Article

with chemical thermometers; and any random error in these measurements manifests itself as scatter in the reaction temperatures reported in the present work. The reaction time was determined from the pressure trace of the end-wall transducer and was taken as the difference in time between the arrival of the incident shock wave and the time at which the pressure at the end-wall falls to 80% of the maximum.17 The uncertainty in the reported reaction times was estimated to be ±0.1 ms. After the reaction zone was quenched by the arrival of the rarefaction wave (determined from the end-wall pressure trace), samples of the gases were withdrawn through the port in the end-wall from the core of the gases to minimize contamination effects from the boundary layer and contact surface mixing. A schematic of the end section of the shock tube showing the gas sampling port is included in the Supporting Information. The samples were collected through an evacuated, stainless steel tube connected directly to gas injection ports attached to gas chromatography (GC) columns in an Agilent 7890A GC oven, fitted with two detectors, a flame ionization detector (FID), and a thermal conductivity detector (TCD). Dead volume effects were minimized by using compact fittings and flowing the initial portion of the sampled gas through the gas sample loops on the GC, before injecting a later portion; similar to the arrangement used with the HPST.18 Two external chemical thermometers15 were used to calibrate the experimental temperatures for reflected shock temperatures >1000 K. Cyclopropanecarbonitrile (CPCN) was used to calibrate the temperature in the range of 1000−1100 K, and 1,1,1-trifluoroethane (TFE) was used to calibrate the experimental temperature in the range of 1200−1300 K. The temperature between 1100 and 1200 K was interpolated. The average reaction temperature, as experienced by the chemical thermometers, is computed from eq 1.

environment allowed rate coefficients to be determined for allyl recombination and disproportionation, the primary focus of the work. The pyrolysis of 1,5-hexadiene was examined at three pressures in the 1−10 bar range and temperatures in the 850−1300 K range. Allyl iodide pyrolysis was studied at only one pressure (4.5 bar), but extended the range of experimental temperatures for the allyl recombination and disproportionation down to 650 K. Quite unexpectedly, a low temperature route to benzene formation was discovered in the allyl iodide experiments. Furthermore, the allyl radical exhibits unique behavior that requires additional considerations in the interpretation of the single pulse shock tube experiments, and the reactions in the cooling wave have to be taken into account. Typically, in single pulse shock tube experiments, reaction in the cooling wave is negligible and can be ignored.

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EXPERIMENTAL SECTION The experiments were conducted in the new University of Illinois at Chicago (UIC) low pressure, single pulse shock tube (LPST). Its design is similar to the high pressure, single pulse shock tube (HPST) at UIC.14,15 The reflected shock pressures of 10 bar to subatmospheric extend the experimental pressure range accessible in the laboratory from the 15−1000 bar over which the HPST operates. Both of the single pulse shock tubes operate at similar reaction times of 1−3 ms and reaction temperatures of 650−2000 K. The design and operation of the LPST has been described in detail previously.16 Only a brief overview is presented here. The LPST consists of a 4420 mm long, stainless steel driven section extendable to 5360 mm, with an ID of 63.5 mm. The driver section is 1219 mm long, with an ID of 101.6 mm, and stainless steel construction. A 91 L dump tank is connected to the driven section at a 45° angle near the diaphragm section in order to suppress the transmitted portion of the reflected shocks. The shock tube is fired using the double-diaphragm technique. A buffer section separates the driven and driver section using two polyester film diaphragms (0.0127−0.1016 mm thickness) with the thinner diaphragm closest to the driven section. During experiments, the buffer section is filled to an intermediate pressure between the driven and driver pressures such that neither of the diaphragms burst. To fire the shock tube, the buffer section is evacuated until the pressure difference across the driver-side diaphragm is high enough to burst it, subsequently bursting the driven side diaphragm. The pressure near the end of the driven section is monitored using seven PCB 113A21 pressure transducers. Six of the transducers are mounted flush in the side-wall, spaced 70 mm between centers with the last transducer positioned 20 mm from the end-wall. One transducer is mounted coaxially with the shock tube, flush with the end-wall. The side-wall transducers measure the time of arrival of the incident shock in order to determine the velocity of the incident shock near the end-wall. The end-wall transducer is used to monitor the reaction pressure and determine the reaction time. All pressure traces were collected using two PCB 482C signal conditioners and two PCI-DAS4020/12 high-speed data acquisition cards. No significant shock attenuation was detected at the experimental conditions reported here. Differences of less than 1% between the extrapolated and averaged shock velocities were observed between the side-wall transducers with no consistent up or down trend. Incident shock arrival times are related to the experimental temperature through calibration

T = ( −E /R )/ln[ln(1 − x)/At ]

(1)

x = (F0 − Ff )/F0

(2)

E and A in eq 1 are the Arrhenius rate constant parameters for thermal decomposition of TFE19 or the total isomerization rate of CPCN.20 The parameters t is the reaction time, x is the extent of reaction calculated from eq 2, where F0 and Ff are the initial and postshock mole fractions of the chemical thermometer. The calibration produces a relationship between the incident shock velocity and the reaction temperature, which is subsequently used to determine the reaction temperature of the 1,5-hexadiene and allyl iodide experiments. The high pressure limit rate constant reported for the CPCN20 isomerization, which was found to be very close to the high pressure limit at ∼2.5 bar, was used for the temperature calibration of 10, 4.5, and 1 bar shocks. The high pressure limit rate constant reported for the TFE unimolecular decomposition19 was used for the temperature calibration of 10 and 4.5 bar shocks. For the 1 bar experiments, rate constant parameter reported at ∼2.5 bar19 for TFE was used, which is 25−35% lower than the high pressure limit in the 1200−1300 K range, with possible systematic error discussed below. Below 1000 K, no chemical thermometer was necessary. Very dilute mixtures in argon are very close to the ideal gas conditions, and the reaction temperature can be reliably determined using 1dimensional, shock tube analysis21 and measured incident shock velocity. The computed reaction temperatures showed good agreement with temperatures determined from CPCN 4763



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calibration between 1000 and 1050 K. The primary source, if any, of systematic error in the reported experimental temperatures are due to uncertainties in the rate constants of TFE and, to a smaller extent, CPCN. These result in systematic uncertainties of ±15 K up to 1300 K.16 Both of the precursors were purchased from Sigma-Aldrich. Allyl iodide was of 98% purity, 1,5-hexadiene was of 97% purity. Both purity levels were confirmed by GC-FID analysis of liquid samples. Test mixtures were prepared manometrically using calibrated MKS Baratron manometers by first degassing the liquid, using a freeze−thaw process under vacuum, and diffusing a small amount of the precursor into an evacuated, stainless steel tank. The tank was then filled with ultrahigh purity argon (99.999% purity), passed through an additional oxygen trap. The test mixtures were allowed to equilibrate overnight; their compositions were confirmed with GC analysis of gas samples from the tank. Normally the mixture tanks and transfer lines are maintained at 100 °C to prevent condensation, and the composition is monitored with periodic GC analysis. 1,5-Hexadiene was sufficiently stable that mixtures were prepared in the heated tank. However, allyl iodide was found to be thermally unstable and was consequently kept in a room temperature mixture tank. The shock tube was maintained at room temperature for all experiments. Species from the 1,5-hexadiene experiments were analyzed using an HP-PLOT Q column and a FID detector. Species from the allyl iodide experiments were analyzed using HPPLOT Q and HP-1MS columns simultaneously eluting to separate FIDs. The HP-1MS was only used for quantification of allyl iodide, which did not elute on the HP-PLOT Q. Product species were identified by a combination of mass spectrometry, using off-line sample analysis on a HP 5973 mass selective detector (MSD) and measuring the retention times for authentic standards. Species calibration on the GC was performed with standard, gaseous, calibration mixtures purchased from Sigma-Aldrich and Airgas with the exception of 1,5-hexadiene and allyl iodide. Calibration mixtures for 1,5hexadiene and allyl iodide were prepared using the calibrated MKS Baratron manometers. The relative error in species calibrations was estimated to be ±5−10%. For each individual experiment, the driven section was filled after first evacuating and flushing it with ultrahigh purity argon. The mixture was shocked to a desired pressure and temperature and was then quenched by the arrival of the rarefaction wave. Gas samples were then withdrawn and analyzed to determine the mole fraction (in ppm) of species formed during reaction. Any random error, either from velocity measurements or species analysis on the GC, is manifested in the experimental results as scatter in the data. It is minimized by firing multiple shocks at similar conditions.

of 1,5-hexadiene. The reaction times for these experiments were in the range of 1.4−2 ms and reaction temperatures were 850− 1300 K. The major species detected in addition to 1,5hexadiene were propene, allene, and propyne. At the high end of the temperature range, small amounts of methane (CH4), ethylene (C2H4), acetylene (C2H2), 1,3-butadiene (1,3-C4H6), 1-butene (1-C4H8), benzene (C6H6), and a few others in trace amounts were also detected. Example results for 10 bar experiments are shown in Figure 1; individual data points are

RESULTS AND DISCUSSION A total of 41 experiments with 1,5-hexadiene and 20 experiments with allyl iodide were performed over a temperature range of 650−1300 K and a number of different reaction pressures. The mole fractions of the species as a function of the temperature are reported here. Each experimental set spanning a wide range of temperatures also sweeps a very narrow range of experimental pressures and reaction times, which are given in the Supporting Information. 1,5-Hexadiene. Three sets of experiments, corresponding to nominal shock pressures of 10 (9−10.2), 4.5 (4.1−5.3), and 1 bar (0.8−1.0 bar), were conducted to investigate the pyrolysis

connected with lines to guide the eye. The species detected agree well with the expected product distribution of the allyl radical recombination/disproportionation and decomposition from the DFST/LS work.13 For most of the experiments reported here, the total carbon recovered was greater than 90%. A figure showing carbon recovered for each 1,5-hexadiene shock is included in the Supporting Information. The 10, 4.5, and 1 bar experiments were conducted with initial mole fractions of 104, 95, and 89 ppm, respectively. By normalizing the species profiles with the initial mole fraction of the precursor, the variation in the results due to the differences in the initial conditions is eliminated and the effect of pressure

Figure 1. (a) Experimental results for 1,5-hexadiene pyrolysis at 10 bar for primary products and the parent precursor. (b) Experimental results for minor products of 1,5-hexadiene pyrolysis at 10 bar. Individual data points are connected with lines to guide the eye.

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Figure 2. (a) Comparison of normalized (by initial mole fraction of precursor) experimental and simulation results of 1,5-hexadiene decomposition. Model predictions for 15HD are the simulation results for 1,5-hexadiene + 0.5 × allyl predicted by simulation: (b) propene, (c) allene, and (d) propyne. Experimental results are for nominal experimental pressures of 1 bar, 89 ppm initial 15HD, blue triangles; 4.5 bar, 95 ppm initial 15HD, red circles; and 10 bar, 104 ppm initial 15HD, green diamonds. Simulation results for the nominal experimental pressures of 1 bar, blue solid lines; 4.5 bar, red dashed lines; and 10 bar, green dotted lines. Each shock was simulated individually, and data points were connected with a straight line for clarity and ease of comparison to experimental results.

on the recombination and decomposition of allyl radicals can be examined, as shown in Figure 2. Each shock was simulated individually and the data points connected with straight lines for clarity. The simulation results in Figure 2a for 1,5-hexadiene include half the allyl radicals predicted during simulation to account for the quenching process in the shock tube, discussed in detail later. Within experimental error, no significant pressure dependence is noticeable between the experimental results at each pressure in the temperature range of 850−1300 K. The principle stable products of 1,5-hexadiene pyrolysis are allene and propene, which were initially detected in significant quantities at approximately 1080 K. The primary product of allene isomerization is propyne, which was first detected in substantial amounts around 1140 K. The observed propyne mole fractions in Figure 2d suggest that isomerization of allene to propyne is pressure independent at the conditions of the current work. Allyl Iodide. Since no significant pressure dependence in allyl recombination was observed in the 1−10 bar range of the 15HD experiments, allyl iodide as a precursor for allyl radicals was studied only at a nominal experimental pressure of 4.5 bar

(4.0−5.75 bar), with an initial mole fraction of 87 ppm. The relatively weak C−I bond (43 vs 61 kcal/mol for C−C bond) led to formation of allyl radicals at a lower temperature than 1,5-hexadiene and consequently permitted the study of allyl radical recombination and disproportionation over a wider temperature range (650−1300 K). The major species detected were identical to those from the 1,5-hexadiene experiments, as shown in Figure 3. The carbon recovered for all allyl iodide shocks was greater than 90%. A figure showing the carbon recovered for each allyl iodide shock is included in the Supporting Information. Initial examination of the allyl iodide experimental results shows formation of allene and propene in significant quantities around 1000 K, which is 80 K lower than for the 1,5-hexadiene experiments, Figure 4b. In Figure 1, no decomposition in 15HD is apparent at 1000 K, which could explain the lack of propene and allene at this temperature. However, it will be shown in the modeling section that allyl radicals do form in significant quantities at as low as 1000 K in the pyrolysis of 1,5hexadiene but that they predominantly recombine back into 15HD at low reaction temperatures and in the cooling wave 4765



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positively identified using both the HP-PLOT Q and HP-1MS columns by analyzing shock samples on the MSD as well as by running a standard gaseous mix of aromatics on the GC. Two test mixtures, made from two separately purchased samples of allyl iodide, showed formation of similar quantities of benzene at the same shock temperatures. Benzene was the only cyclic species detected in significant quantities at these temperatures. However, in the gas chromatograms, some very minor peaks that could correspond to other cyclic species, including 1,3cyclohexadiene, were detected. However, the yields were too low for positive identification and quantification. Furthermore, the analytical methods used are similar to those in prior investigations of benzene formation from propargyl recombination7,22 where eight stable C6H6 isomers were observed including benzene. The lack of peaks corresponding to additional C6H6 species in the allyl iodide experiments indicates that formation of benzene at low temperatures is not due to propargyl recombination. Possible sources of benzene are discussed further in the modeling section. Modeling. A 47 reaction mechanism was used to simulate the experimental pyrolysis results for 1,5-hexadiene and allyl iodide. The mechanism, with the relevant rate constants and references, is shown in Table 1. The single pulse shock tube experiments were simulated using a closed, homogeneous, constant pressure reactor, as implemented in CHEMKIN 10112.23 Reaction pressure, reaction time, average reaction temperature (as determined from the calibration discussed in the experimental apparatus section), and initial mole fraction of the precursor in argon for each shock were provided as input. Given the dilute conditions, the simulations were essentially isothermal. The simulated mixture composition at the end of the reaction time was compared to experimental results at each shock temperature for the 1,5-hexadiene and allyl iodide experiments in Figures 2 and 5, respectively. Thermodynamic properties (NASA polynomials interpreted by CHEMKIN) were taken from the thermodynamic database compiled by Goos, Burcat, and Ruscic,24 except for allyl radicals, 1,5-hexadiene, and allyl iodide, which were taken as the revised values determined in the DFST/LS work.13 Reactions in the Cooling Wave. In a typical single pulse shock tube experiment, accumulation of radicals is very low. When the reaction zone is rapidly quenched by the arrival of the rarefaction wave, high activation energy reactions are shut down and the remaining radicals predominantly recombine to stable molecules. The net effect is a slight perturbation of the mole fractions of the species that were formed during reaction at high temperature. In most cases, this discrepancy contributes little error to predictions of stable intermediates17 due to the low accumulation of radicals involved. However, in the current work, a unique situation exists. Allyl radicals are very stable, and over most of the temperature range of the current work, they react predominantly by recombination to 1,5-hexadiene, and an equilibrium is established between allyl radicals and 1,5hexadiene. Simulations indicate that even at the lowest temperatures of the experiments on 1,5-hexadiene pyrolysis, where there is no apparent reaction as seen in Figure 1a, there is actually a relatively high accumulation of the allyl radicals in the shock heated gases. These radicals are obviously not observed in the samples withdrawn from the shock tube because they recombine in the cooling wave to 1,5-hexadiene. In Figure 6, comparison is shown of experimental 1,5-hexadiene mole fractions and those predicted by the simulation. Clearly, the simulations under-predict the experimental results. Also

Figure 3. (a) Experimental results for allyl iodide pyrolysis (87 ppm initial C3H5I mole fraction) at 4.5 bar for primary products and the parent precursor. (b) Experimental results for minor products of allyl iodide pyrolysis at 4.5 bar. Individual data points are connected with lines to guide the eye.

that quenches the high temperature reactions. This suggests that the disproportionation and decomposition of allyl radicals alone do not account for the formation of allene and propene at lower temperatures in pyrolysis of allyl iodide and that an additional mechanism is active. Surprisingly, benzene was also detected in much more significant quantities in the allyl iodide experiments than those with 1,5-hexadiene, Figure 4c. The peak benzene mole fraction from pyrolysis of allyl iodide is 4 ppm at ∼1075 K, Figure 3b, whereas in Figure 1b the benzene mole fraction steadily increases above 1100 K to 1 ppm at the highest temperature of the 1,5-hexadiene pyrolysis experiments. These observations, similar to the differences in allene and propene formation between the 1,5-hexadiene experiments and the allyl iodide ones, suggest that different mechanisms are operating to produce benzene. The onset of formation of propyne for both experiments at exactly the same temperature, ∼1140 K, Figure 4b, in both the 1,5-hexadiene and allyl iodide experiments suggests that propyne is produced by the same mechanism in both experiments. The formation of benzene in the low temperature allyl iodide experiments was completely unexpected, and additional steps were taken to confirm its presence. It was 4766



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Figure 4. Comparison of normalized experimental results at 4.5 bar with 15HD concentrations from experiments with 1,5-hexadiene (closed symbols) and allyl iodide (open symbols) as the allyl radical precursor. All results are normalized by half the number of potential allyl radicals from each precursor, respectively: (a) 1,5-hexadiene; (b) allene, red circles; propene, blue squares; and propyne, green triangles; (c) benzene.

energy than the bond dissociation energy25 to reform propargyl radicals. Allyl Radical Recombination and Disproportionation. Reaction rate constants describing the primary dissociation of 1,5-hexadiene −R1 and allyl radical recombination R1, were computed for 1, 4, and 10 bar using the Gorin-type calculation and thermochemistry as described in the DFST/LS work.13 Prior experimental studies have shown that the disproportionation reaction R2 is minor compared to recombination, and the best fit to the current work was obtained with a branching ratio kR2/kR1 = 0.003. Not only was the loss of 1,5-hexadiene well simulated but the onset of formation of allene and propene during experiments, shown in Figure 2b,c, was captured as well. This branching ratio is consistent with the estimated upper limit from Lynch et al.13 of 0.05 at the higher temperatures and lower pressures of the DFST/LS part of this work. It is also consistent with Selby et al.26 (0.03) and James and Kambanis27 (0.008). The present simulation results reinforce the assertion that the disproportionation channel for allyl + allyl reaction is very small and may be even less significant than previously estimated.

shown in Figure 6 are the predicted allyl radical mole fractions. It was assumed that the allyl radicals would recombine into 1,5hexadiene during the quenching process, and prior to analysis, the simulation results with this assumption are shown in Figure 2. Throughout the simulation results shown here, similar corrections have been made. The reactions in the cooling wave for experiments with allyl iodide as the allyl radical precursor are not as clear-cut. In these experiments, not only are the allyl radicals present at large amounts but the I atoms also accumulate in large quantities. Thus, during cooling there will be some combination leading to a mixture of 1,5-hexadiene, allyl iodide, and I2. The importance of reactions in the cooling wave for allyl radicals is unusual. Recombination reactions of other resonantly stabilized radicals have been studied in single pulse shock tubes without reactions in the cooling wave being significant. However, in these cases, either a very strong bond is formed or isomerization of the adduct to more stable species is favored. An example of the first case is recombination of phenyl radicals to biphenyl where the central C−C bond dissociation energy is ∼119 kcal/mol8 and is sufficient to prevent dissociation back to phenyl radicals. Recombination of propargyl radicals7 is a good example of the latter case. Here, the activation energies for isomerization of the adducts are at least 20 kcal/mol lower in

15HD = a‐C3H5 + a‐C3H5

(−R1)

The simulation of the allyl iodide experiments, using the mechanism described in Table 1 is shown in Figure 5. The rate 4767



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Table 1. Reaction Mechanism Used to Simulation Allyl Iodide and 1,5-Hexadiene Pyrolysis between 950 and 1300 K (Modified Arrhenius Rate Constant Units Are cm3, mol, s, kcal) no.

reaction

−R1

15HD = a-C3H5 + a-C3H5

R2

2a-C3H5 = a-C3H4 + C3H6

R3 R4

C3H5I = a-C3H5 + I a-C3H5 = a-C3H4 + H

R5

C3H6 = a-C3H5 + H

R6

H + a-C3H4 = H + p-C3H4

R7

a-C3H4 = p-C3H4

R8 C3H5I = a-C3H4 + HI R9 I + C3H5I = I2 + a-C3H5 R10 a-C3H5 + HI = C3H6 + I R11 I + I + M = I2 + M Third Body Enhancement: I2/20 R12 C3H5I + H = a-C3H5 + HI R13 H + H + M = H2 + M Third Body Enhancement: H2/0/AR/0.63 R14 H + I2 = HI + I R15 I + H + M = HI + M R16 H + HI = H2 + I R17 HI + HI = H2 + I2 R18 2H + H2 = 2H2 R19 15HD + H = a-C3H4 + a-C3H5 + H2 R20 15HD + H = a-C3H5 + C3H6 R21 H + a-C3H4 = CH3 + C2H2

R22

H + a-C3H4 = CH3CCH2

R23

H + p-C3H4 = CH3CCH2

R24

H + p-C3H4 = CH3CHCH

R25

H + p-C3H4 = CH3 + C2H2

R26

CH3 + C2H2 = CH3CHCH

R27

c-C3H5 = a-C3H5

R28 R29

H + p-C3H4 = C3H3 + H2 H + a-C3H4 = C3H3 + H2

log(A)

n

Ea

pressurea

47.86 39.78 35.50 40.98 32.90 28.61 66.32 79.72 79.79 48.64 13.46 13.70 13.88 13.95 15.39 25.37 7.24 39.89 48.68 63.80 13.30 13.31 14.37

−9.7 −7.3 −6.0 −9.3 −6.8 −5.5 −15.5 −19.3 −19.1 −10.0 0.0 0.0 0.0 0.0 −0.3 −3.2 2.0 −7.8 −10.0 −15.5 0.0 0.0 0.0

72.68 69.39 67.62 12.47 9.18 7.41 66.10 95.34 97.50 80.30 80.00 80.00 80.00 80.00 6.44 13.17 4.52 78.45 88.69 66.10 6.80 4.89 −1.50

1 bar 4 bar 10 bar 1 bar 4 bar 10 bar 4 bar 1 bar 4 bar 10 bar 0.36 bar 2.3 bar 8 bar 14 bar 1 bar k1c 10 bar k1c 10 bar k2c 1 bar 10 bar

12.00 18.25

0.0 −1.0

0.00 0.00

est.13 39

14.63 21.30 13.68 13.41 16.95 12.00 13.00 20.10 16.23 4.09 8.52 53.28 51.90 40.45 35.41 53.46 51.98 39.84 34.83 51.14 50.59 39.76 40.64 12.54 14.24 44.08 43.78 35.93 32.48 35.71 34.00 4.55 3.82

0.0 −1.9 0.0 0.0 −0.6 0.0 0.0 −1.8 −0.6 2.7 1.1 −12.6 −11.8 −9.4 −7.6 −12.5 −11.7 −9.1 −7.3 −12.6 −11.9 −9.5 −9.6 0.4 0.0 −10.2 −9.7 −8.4 −7.0 −7.8 −6.9 2.8 3.1

0.43 0.00 0.66 43.80 0.00 0.00 0.00 15.00 14.75 6.34 8.89 16.73 18.29 7.85 7.15 16.85 18.33 7.46 6.72 15.43 16.92 8.77 9.40 5.46 7.13 18.73 20.56 12.36 12.36 26.32 26.81 4.82 5.52

38 38 38 38 39 est.13 est.13 12

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comment/ref. 13

0.3% of k−R1 p.w.

13 10 RRKMb 30

12

11 0.3% of R3 p.w. 37 34 38

1 bar k1c 10 bar k1c 1 bar k2c 10 bar k2c 1 bar k1c 10 bar k1c 1 bar k2c 10 bar k2c 1 bar k1c 10 bar k1c 1 bar k2c 10 bar k2c 1 bar k1c 10 bar k1c 1 bar k2c 10 bar k2c 1 bar 10 bar 1 bar k1c 10 bar k1c 1 bar k2c 10 bar k2c 1 bar 10 bar

12

12

12

12 12

12 12 12



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Table 1. continued no.

reaction

Third Body Enhancement: H2/0/AR/0.63 R30 c-C3H4 = p-C3H4 R31

c-C3H4 = a-C3H4

R32

C3H3 + H = p-C3H4

R33

C3H3 + H = a-C3H4

R34

C3H3 + H = c-C3H4

R35

CH3 + C2H3(+M) = C3H6(+M)

Troe Centering Parameterse: 0.175/1341/60000/10140 R36 C3H6 + H = C2H4 + CH3 R37 C3H6 + H = a-C3H5 + H2 R38 C3H6 + H = CH3CCH2 + H2 R39 C3H6 + CH3 = a-C3H5 + CH4 R40 C3H6 + CH3 = CH3CCH2 + CH4 R41 a-C3H4 + CH3 = C3H3 + CH4 R42 C3H6 + H(+M) = n-C3H7(+M) Troe Centering Parameterse: 1/1000/1310/48097 R43 C3H6 + H(+M) = i-C3H7(+M)

pressurea

log(A)

n

Ea

37.09 37.22 26.40 35.70 29.90 24.03 29.50 23.94 21.03 18.51 13.10 58.63

−7.5 −7.2 −4.6 −6.9 −5.1 −3.1 −5.0 −3.2 −3.0 −2.0 0.0 −11.9

45.55 48.01 43.92 51.30 4.86 3.26 4.71 3.26 2.69 2.05 0.00 9.77

22.20 5.23 5.60 0.34 −0.08 12.11 13.12 38.80

−2.4 2.5 2.5 3.5 3.5 0.0 0.0 −6.7

11.20 2.49 9.79 5.68 11.70 7.70 3.26 7.00

k∞ k0

13.12 42.94

0.0 −7.5

1.56 4.72

k∞ k0

1 bar 10 bar 1 bar 10 bar 1 bar 10 bar 1 bar 10 bar 1 bar 10 bar k∞ k0

comment/ref. 11 11 11 11 11 9

9 40 40 40 40 9 9

9

Troe Centering Parameterse: 1/1000/645/6844 R44 H + CH3(+M) = CH4(+M)

39

Troe R45 R46

39 39

Troe R47 Troe

16.14 −0.5 0.54 k∞ 33.42 −4.8 2.44 k0 Centering Parameterse: 0.783/74/2941/6964 and Third Body Enhancement: H2/2/CH4/2/C2H6/3/AR/0.7 H + CH4 = CH3 + H2 8.82 1.6 10.84 2CH3(+M) = C2H6(+M) 16.83 −1.2 0.65 k∞ 41.53 −7.0 2.76 k0 Centering Parameterse: 0.619/73.2/1180/9999 and Third Body Enhancement: H2/2/CH4/2/C2H6/3/AR/0.7 C2H4(+M) = H2 + C2H2(+M) 12.90 0.4 86.77 k∞ 51.20 −9.3 97.80 k0 Centering Parameterse: 0.7345/180/1035/5417 and Third Body Enhancement: H2/2/CH4/2/C2H6/3/AR/0.7

39

a

For reactions with rate constants given at particular pressures, the rate constant at experimental pressures during simulation is computed by linear interpolation of log(k) as a function of log(P), as implemented by the PLOG function in CHEMKIN 10112. bRRKM calculated parameters using parameters provided by Fernandes et al.10 using ⟨ΔEdown⟩ = 430 cm−1. cTotal rate constant is the sum of k1(T) and k2(T) as implemented in CHEMKIN 10112 by declaring duplicate reactions. eTroe centering parameters for Fc(T) = (1 − a)exp(−T/T***) + a exp(−T/T*) + exp(−T*/T) provided as: a/T***/T*/T**.

Allyl Radical Secondary Chemistry. The short mechanism used to simulate DFST/LS results was used as a starting point for describing secondary chemistry in the LPST experiments and expanded to better simulate the single pulse shock tube results. Sensitivity analysis was used to identify key reactions and species. Results of a sensitivity analysis for 15HD at 1140 K and 4.5 bar is shown in Figure 7 for an experiment with 15HD as the reagent. Under these conditions, approximately 50% of 1,5-hexadiene is consumed in the 1.6 ms reaction time. In Figure 7, two plots are given showing both how the sensitivity coefficients change with time and the most sensitive reactions at the end of an experiment. Initially the mole fraction of 15HD is most sensitive to dissociation of the molecule and recombination of allyl radicals as the equilibrium between them is established. As the experimental time increases, reaction R4, dissociation of allyl radicals to allene and H, quickly becomes the most sensitive reaction. The next most important reactions in order (although far less significant) are disproportionation R2 and dissociation of propene to allyl

constant for reaction R3, describing primary unimolecular dissociation of allyl iodide, was computed at 4 bar using the same Gorin-type calculation presented by Lynch et al.13 The simulation results, with corrections for reactions in the quenching process, using the derived rate constants and thermochemistry from the DFST/LS work, agree well with the observed decomposition of allyl iodide and production of 1,5-hexadiene system over the range 650 to 1300 K. Thus, there is good agreement between the DFST/LS studies of Lynch et al.13 and the current work concerning the rate of dissociation of allyl iodide and 1,5-hexadiene and the recombination/disproportionation of allyl radicals. Because of the short reaction times in DFST/LS studies, the work of Lynch et al. shed little light on the secondary chemistry of the pyrolysis of 1,5-hexadiene and allyl iodide. The single pulse shock tube experiments, however, are influenced by further reactions, apart from those in the cooling wave, and these are discussed next. 4769



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Figure 6. Comparison of experimental (open symbols) and simulated results (closed symbols) for 4.5 bar experiments with 1,5-hexadiene as the allyl radical precursor: 1,5-hexadiene, red circles; allyl radicals, blue triangles. The large amount of allyl radicals formed is not seen experimentally because they recombine or react during the quenching process. This leads to the under prediction of 15HD seen here if the allyl radicals are not allowed to recombine at the end of the reaction period.

H + a‐C3H4 = a‐C3H5

At the upper end of the experimental temperature range, approximately 1150−1300 K, in the 1,5-hexadiene experiments, the model in Table 1 gives good agreement with the observed propene mole fractions and the onset of propyne formation. However, the present mechanism over predicts allene mole fractions at high temperatures and all three experimental pressures. The precise source of the over prediction is not clear. Figure 8 shows normalized sensitivity plots for the allene mole fraction at 1200 K at 4.5 bar and 1.6 ms reaction time, with 15HD as the reagent. Above 1150 K, reactions involving H atoms become more important. In the present mechanism, the most important reaction consuming allene at higher temperature is isomerization of allene to propyne via an H enhanced pathway R612 and also directly via reaction R7.11

Figure 5. Comparison of simulation results and experimental data at 4.5 bar from pyrolysis of allyl iodide (87 ppm initial C3H5I). Open symbols are experiments and closed symbols are model simulations. (a) C3H5I, blue triangle; 15HD, red circles; and allyl radical, green diamonds. (b) Allene, red circles; propene, blue triangles; and propyne, green diamonds.

and H, R5. The lack of sensitivity to any reactions other than R1 and −R1 at short reaction times is consistent with the DFST/LS studies of Lynch et al.13 a‐C3H5 = a‐C3H4 + H

(R4)

C3H6 = a‐C3H5 + H

(R5)

(−R4)

H + a‐C3H4 = p‐C3H4 + H

(R6)

a‐C3H4 = p‐C3H4

(R7)

The primary reactions influencing formation of allene at 1200 K, as shown in Figure 8, are dissociation of propene to allyl and H, R5, dissociation of allyl to allene + H, R4, and to a smaller extent, reactions of propene with H (R36−R38 in Table 1). The rate coefficients for reaction R4 have already been discussed, and any variation to try and improve the simulation of the higher temperature data degrades the low temperature fit. However, at the higher temperatures, reactions of allyl radicals and H-atoms do influence the predicted mole fractions of allene and propene. The reverse reaction of R5 (a-C3H5+ H) has two pathways, either formation of propene −R5 or allene and hydrogen −R5a.

The rate constants for R4 at different pressures in Table 1 were taken from the experimental and RRKM treatment by Fernandes et al.10 Miller et al.12 have calculated rate constant expressions for −R4 as part of a study of reactions on the C3H5 potential energy surface, and their theoretically derived values are in agreement with the measurements by Fernandes et al. However, the best simulation of the experimental 15HD, allene, and propene mole fractions was achieved using the exact rate constant expressions derived by Fernandes et al. and the revised thermochemistry from Lynch et al. for the allyl radical. The model presented in Table 1 includes the expressions of Fernandes et al.

H + a‐C3H5 = C3H6

H + a‐C3H5 = a‐C3H4 + H 2 4770

(−R5) (−R5a)



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Figure 7. Normalized sensitivity as a function of time for 1,5-hexadiene at 1140 K, 4.5 bar, and 1.6 ms reaction time for 15HD (95 ppm initial mole fraction) as the precursor. Inset bar plot shows ranked end-point normalized sensitivity at 1.6 ms.

Figure 8. Normalized sensitivity as a function of time for allene at 1200 K, 4.5 bar, and 1.6 ms reaction time for 15HD (95 ppm initial mole fraction) as the precursor. Inset bar plot shows ranked end-point normalized sensitivity at 1.6 ms.

Recent theoretical calculations for reaction −R5a by Jasper28 indicate that, at the high pressure limit, the formation of propene is dominant and k−R5/k−R5a ≈ 100 with k∞,−R5 from Harding et al.29 However, the pressure dependency of reaction −R5 is unknown, and thus, the pressure and temperature dependence of branching between the two pathways is not well-defined. Hidaka et al., however, have studied propene pyrolysis in a single pulse shock tube (1200−1800 K), using IR laser kinetic absorption spectroscopy.30 They obtained rate coefficients for propene dissociation R5 over the range 0.36−14 bar and estimated a rate constant of 1 × 1013 cm3/mol·s for the −R5a allene + H2 channel. Their rate constant is similar to the values used by Davis et al. (1.8 × 1013 cm3/mol·s)9 in their experimental and modeling study of propene pyrolysis and oxidation in an atmospheric pressure flow reactor (∼1200 K). Hidaka and Davis’s estimates for k−R5a are a factor of 8−10 greater than the calculated rate of Jasper at 1300 K and are considered here to be an upper limit. Using the forward rate

value of Hidaka et al. at 1 bar to compute the reverse rate constant for reaction −R5 gives k−R5/k−R5a ≈ 5.9 at 1300 K, in agreement with calculations by Jasper but with more significant contribution from the abstraction reaction. In the present work, using the pressure dependent rate constant for both product channels of the allyl + H reaction from either Hidaka et al. or Davis et al. led to simulation results as good as those presented in Figure 2c. The propene mole fractions are simulated accurately throughout, although there is a small under prediction for T > 1150 K. The allene mole fractions were predicted accurately up to the maximum in the experimental data and then consistently over predicted at higher temperatures suggesting some removal route is missing from the model. The best fit to the experimental 1,5-hexadiene pyrolysis experiments was obtained assuming that reaction R5, the addition of H-atoms to allyl radicals, was the sole reaction, i.e., the allene + H2 channel is negligible, and using rate coefficients from Hidaka et al. for kR5. The effect of varying k−R5 4771



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Figure 9. Normalized sensitivity as a function of time for propyne at 1200 K, 4.5 bar, and 1.6 ms reaction time for 15HD as the precursor. Inset bar plot shows ranked end-point normalized sensitivity at 1.6 ms.

of a detailed model to fully capture the extended secondary chemistry is beyond the scope of the current work where the primary purpose is to examine the initial recombination reaction and early stages of the secondary chemistry. The current model captures both the loss of 1,5-hexadiene and the formation of the major products at all but the highest temperatures using rate coefficients either derived from or consistent with the work of Lynch et al.13 Allyl Iodide. The dissociation of allyl iodide and the subsequent secondary reactions are potentially more complicated than that of 15HD. Scission of the weak C−I bond is undoubtedly the primary pyrolysis path generating an iodine atom in addition to allyl radicals, whereas from 15HD only allyl radicals are formed. Typically, I atoms are considered unreactive at the conditions found in shock tube experiments,7,32,33 and normally this is an excellent assumption. A second complication arises in that iodinated species can also dissociate by HI elimination, here, reaction R8. This possibility was examined in the DFST/LS portion13 of this work and found to be negligible at those reaction conditions. Consequently, the only reactions that should be of importance for simulating the LPST allyl iodide experiments are those considered in the lower temperature portion of the DFST/LS by Lynch et al.,13 i.e., unimolecular dissociation of allyl iodide and the allyl radical recombination/disproportionation chemistry. As expected over much of the temperature range of the current work, the loss of allyl iodide and formation of 15HD from the recombination of allyl radicals are well simulated using rate coefficients obtained in the DFST/LS part of this work. However, at the lower end of the temperature range and much longer observation times of the single pulse shock tube experiments presented here, not all of the experimental results could be simulated with the assumption that the iodine atoms would not participate in reaction and that the HI elimination channel R8 was entirely negligible. These observations are quite unusual and are discussed below.

was explored through brute force methods. Reducing the rate constant for R5 by a factor of 2 led to an under prediction of propene at higher temperatures, indicating that a large fraction of the propene formed is via association of allyl with H, −R5. Increasing the rate constant for R5 (factor of 2) increased the simulated yields of both allene, consistent with the sensitivity analysis results for allene in Figure 8 and propene. The final major product, which is only formed at high temperatures, is propyne. The normalized results for a sensitivity analysis of propyne formation at 1200 K, 4.5 bar, and 1.6 ms reaction time using 15HD as the precursor are shown in Figure 9. The primary reactions influencing formation of propyne are dissociation of allyl, R4 (discussed before), and isomerization of allene to propyne via R6 and R7. Kiefer et al. found isomerization of allene to propyne to be pressure dependent in high temperature and low pressure shocks.31 However, the pressure effect is expected to be small at present experimental conditions, and none was discernible from the experimental results in Figure 2c,d. The most sensitive reaction reducing the amount of propyne predicted is the dissociation of propene to allyl and H, R5, although at the temperatures of the current work the rate of this reaction will be very slow. Currently, a satisfactory solution to the over prediction of the allene mole fractions at high temperatures has not been found. Simple variation of rate coefficients within their uncertainties is not sufficient as an improvement in the predicted high temperature allene simulation invariably degrades other predictions. It was noted earlier that in addition to the main species observed experimentally there were indications of other species whose GC peaks were too small for positive identification and quantification. From their elution times, it is likely that they are species with more than 3 carbon atoms. Thus, it is likely that the over prediction of allene is due to insufficient detail in the model forming C4 and larger hydrocarbons. Tests with additional reactions leading to C4+ species (such as a-C3H5 + CH3 = 1-C4H8) were considered with varying degrees of success. These reactions were found to somewhat improve the predictions of the minor species at the highest temperatures of the present experiments, but none of them led to different conclusions regarding the allyl recombination and disproportionation reactions. Development

C3H5I = a‐C3H5 + HI

(R8)

In Figure 10, the normalized simulation results of 15HD and allyl iodide at 4.5 bar are compared, assuming I atoms are inert and HI elimination is negligible. The agreement appears 4772



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hypothesized that these reactions involve iodine containing species. Sensitivity analysis of allyl iodide at 850 K, 4.5 bar, and 1.6 ms reaction time is shown in Figure 11. At the longer time scales of the single pulse shock tube experiments, the recombination of allyl radicals to 15HD, R1, is the most influential. The next most important reactions, in order, are abstraction of the iodine atom from allyl iodide by another iodine atom, R9, HI elimination reaction, R8, and unimolecular dissociation of allyl iodide, R3. Incorporation of reactions R9 and R8 in the reaction mechanism greatly improved the simulations of allene as shown by the results in Figure 5b. C3H5I + I = a‐C3H5 + I 2

(R9)

Reaction R8 was considered negligible in the DFST/LS work by Lynch et al. on allyl iodide dissociation.13 However, the LPST results presented here clearly show some sensitivity to it. This difference arises primarily due to the different time scales and temperatures involved in the two sets of experiments and from the fact that laser schlieren experiments are essentially insensitive to a weak channel in reaction R8 due to the low enthalpy of reaction.13 The branching ratio between C−I scission and HI elimination from C3H5I was refined by treating it as a variable parameter. This resulted in kR8/kR3 = 0.003, which improved the prediction of the temperature at which allene was produced in allyl iodide experiments and is consistent with an upper limit of 0.01 suggested by Lynch et al.13 The final refinement to the mechanism was the addition of reaction R10, derived by Rossi and Golden in an investigation of the allyl + HI metathesis reaction in a very low pressure pyrolysis reactor.34 Inclusion of this significantly improved the prediction of propene formation. These changes are included in the mechanism presented in Table 1, and the simulation results in Figure 5 were produced with them. The allyl radical mole fraction increases throughout the simulation of allyl iodide pyrolysis, Figure 5, and cannot be assumed to simply recombine into 1,5-hexadiene, as was done for 1,5-hexadiene simulations, and to necessitate additional considerations.

Figure 10. Comparison of simulation results at 4.5 bar for allyl iodide (half-filled symbols) and 15HD (open symbols) pyrolysis experiments, assuming negligible HI elimination channel for allyl iodide. (a) 15HD, blue circles; propene, red triangles; (b) allene, blue circles; propyne, red triangles. All results are normalized by half the number of potential allyl radicals from each precursor, respectively. This shows that, assuming iodine is inert in LPST experiments, it does not reproduce the difference in yields of 15HD and formation of allene and propene at lower temperatures for allyl iodide experiments.

a‐C3H5 + HI = C3H6 + I

(R10)

Approximate simulations of the quenching processes in CHEMKIN, by defining a nonconstant pressure profile, indicated that the allyl radicals recombine about evenly into both allyl iodide and 1,5-hexadiene in the cooling wave. Assuming some of the excess allyl radicals are accounted for by the lack of low temperature benzene mechanism (discussed in the next section) and an even split in recombination between allyl iodide and 1,5-hexadiene, simulation results with this approximation of reactions in the cooling wave for allyl iodide and 15HD are shown in Figure 12. This simulation of allyl iodide and 15HD, accounting for quenching, is as good as shown in Figure 5a, with small disagreement at temperatures >950 K where the high temperature benzene mechanism is active. However, assuming part of the allyl radicals at lower temperature is accounted for by a low temperature benzene mechanism, the agreement between simulations and experiments is excellent. The only remaining uncertainty is associated with the lack of detail in the mechanism to account for formation of C4+ species at higher temperatures, which was discussed before. While the mechanism in Table 1 captures the most important features of the allyl iodide experimental results, it

extremely good with all the major species being well predicted. This comparison is quite encouraging as it suggests that the source of allyl radicals has no effect on the subsequent chemistry, at least for T > 1000 K. However, if one considers Figure 4, where the experimental results for 15HD and allyl iodide pyrolysis are compared, then it is clear that the actual yield of 15 HD was smaller in the experiments with allyl iodide as the allyl precursor than those with 1,5-hexadiene. Additionally, allene and propene appeared at a temperature 80 K lower in the C3H5I experiments than 15HD ones. These results suggest that there is a competing mechanism in the single pulse shock tube allyl iodide experiments, producing products similar to those of recombination/disproportionation of allyl radicals as well as benzene at lower temperatures than seen in the 15HD pyrolysis experiments. After considerable trial and error exploring alternate pathways and rates of reaction, it is 4773



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Figure 11. Normalized sensitivity as a function of time for allyl iodide at 850 K, 4.5 bar, and 1.6 ms reaction time for allyl iodide as the precursor. Inset bar plot shows ranked end-point normalized sensitivity at 1.6 ms.

temperatures, a second larger benzene peak is observed in the allyl iodide work that is replicated in shape and magnitude in the 15HD results. Thus, it is hypothesized that there is a distinct low temperature mechanism leading to benzene in the allyl iodide experiments. As shown before, accounting for formation of benzene at low temperature is necessary to account for the large number of allyl radicals accumulated for allyl iodide experiments, Figure 12. It is not presently clear exactly how benzene is formed at low temperatures. One potential route is through recombination of propargyl radicals. However, in the current work, propargyl is not formed until much higher temperatures compared to the maximum of the benzene peak in Figure 3b. Additionally, propargyl recombination has been extensively studied,7 and there are eight stable C6H6 isomers that would have been observed in the gas chromatograms if benzene was being formed from the propargyl radicals in the current work. Thus, there must be some other pathway to benzene. Kunichica et al. examined the pyrolysis of allyl iodide (as well as chloride and bromide) using a heated, narrow bore, silica reaction tube.35 In the reaction temperature range of 1070− 1270 K, reaction times of 0.8−2 ms, and atmospheric pressure, they detected propene as a major product and smaller quantities of allene, propyne, 1,5-hexadiene, and benzene. A direct comparison with the LPST results is difficult because the propene/allene ratio is much larger in Kunichica et al.’s experiments than the current work, and it is also not clear what effect surface reactions in their narrow bore reaction tube may have had, if any, on the chemistry. Kunichica et al. proposed a mechanism for benzene formation, which was initiated by allyl radicals or iodine atoms abstracting an H atom from C3H5I leaving a C3H4I radical. These then combine to form benzene, eliminating two HI molecules. Obviously this mechanism was not described in terms of elementary reaction steps, but it bears features that are compatible with the current work. It is also not clear if benzene is being produced solely behind the reflected shock wave or if it is being formed as radicals and atoms are mopped up during the quenching phase. Further, evidence of iodine mediated formation of benzene from hydrocarbons can be found in static cell experiments by Millineaux and Rayley36

Figure 12. Simulation (open symbols) of allyl iodide (blue triangles) and 1,5-hexadiene (red circles), assuming a low temperature mechanism for benzene accounts for part of the excess allyl seen experimentally, and the rest recombines evenly split into 15HD and allyl iodide.

is somewhat speculative and will remain so until a better grasp of reactions such as R9 and R10 is obtained and the reactions in the cooling wave are better understood. Efforts have been made to detect HI in both the current work and the experiments of Lynch et al.13 However, it has proved elusive but may simply be present in quantities below the detection limits of the analytical instruments. Benzene Formation. The mechanism presented in Table 1 does not contain reactions that account for benzene formation. Its presence in the pyrolysis of allyl iodide, albeit at low mole fractions compared to the other stable species, was unexpected at relatively low temperatures. The benzene profile in the allyl iodide experiments peaks at around 1050 K before decaying and then begins to rise again above 1250 K, Figure 3b. It is beyond the scope of the current work, but experiments have been performed with both 15HD and C3H5I at temperatures in excess of 1300 K in the LPST, and at these higher 4774



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who observed benzene when I2−n-hexane mixtures were heated to 770 K in a quartz cell. Development of a mechanism for low temperature benzene production from allyl iodide pyrolysis is beyond the scope of this work. The above studies are included as suggestions that an iodine mediated mechanism may be involved.

(4) Miller, J. A.; Melius, C. F. Kinetic and Thermodynamic Issues in the Formation of Aromatic Compounds in Flames of Aliphatic Fuels. Combust. Flame 1992, 91, 21−39. (5) McEnally, C. S.; Pfefferle, L. D.; Atakan, B.; Kohse-Höinghaus, K. Studies of Aromatic Hydrocarbon Formation Mechanisms in Flames: Progress Towards Closing the Fuel Gap. Prog. Energy Combust. Sci. 2006, 32, 247−294. (6) Matsugi, A.; Suma, K.; Miyoshi, A. Kinetics and Mechanisms of the Allyl + Allyl and Allyl + Propargyl Recombination Reactions. J. Phys. Chem. A 2011, 115, 7610−7624. (7) Tang, W.; Tranter, R. S.; Brezinsky, K. Isomeric Product Distributions from the Self-Reaction of Propargyl Radicals. J. Phys. Chem. A 2005, 109, 6056−6065. (8) Tranter, R. S.; Klippenstein, S. J.; Harding, L. B.; Giri, B. R.; Yang, X.; Kiefer, J. H. Experimental and Theoretical Investigation of the SelfReaction of Phenyl Radicals. J. Phys. Chem. A 2010, 114, 8240−8261. (9) Davis, S. G.; Law, C. K.; Wang, H. Propene Pyrolysis and Oxidation Kinetics in a Flow Reactor and Laminar Flames. Combust. Flame 1999, 119, 375−399. (10) Fernandes, R. X.; Giri, B. R.; Hippler, H.; Kachiani, C.; Striebel, F. Shock Wave Study on the Thermal Unimolecular Decomposition of Allyl Radicals. J. Phys. Chem. A 2005, 109, 1063−1070. (11) Miller, J. A.; Klippenstein, S. J. From the Multiple-Well Master Equation to Phenomenological Rate Coefficients: Reactions on a C3H4 Potential Energy Surface. J. Phys. Chem. A 2003, 107, 2680−2692. (12) Miller, J. A.; Senosiain, J. P.; Klippenstein, S. J.; Georgievskii, Y. Reactions over Multiple, Interconnected Potential Wells: Unimolecular and Bimolecular Reactions on a C3H5 Potential. J. Phys. Chem. A 2008, 112, 9429−9438. (13) Lynch, P. T.; Annesley, C. J.; Aul, C. K.; Yang, X.; Tranter, R. S. Recombination of Allyl Radicals in the High Temperature Fall-Off Regime. J. Phys. Chem. A 2013, DOI: 10.1021/jp402484v. (14) Tranter, R. S.; Fulle, D.; Brezinsky, K. Design of a High-Pressure Single Pulse Shock Tube for Chemical Kinetic Investigations. Rev. Sci. Instrum. 2001, 72, 3046−3054. (15) Tranter, R. S.; Sivaramakrishnan, R.; Srinivasan, N.; Brezinsky, K. Calibration of Reaction Temperatures in a Very High Pressure Shock Tube Using Chemical Thermometers. Int. J. Chem. Kinet. 2001, 33, 722−731. (16) Fridlyand, A.; Mandelbaum, A.; Brezinsky, K. Pyrolysis of nHeptane and Oxidation in Mixtures of Ethylene/Methane and isoOctane. J. Propul. Power 2013, 29, 732−743. (17) Tang, W.; Brezinsky, K. Chemical Kinetic Simulations Behind Reflected Shock Waves. Int. J. Chem. Kinet. 2006, 38, 75−97. (18) Comandini, A.; Malewicki, T.; Brezinsky, K. Online and Offline Experimental Techniques for Polycyclic Aromatic Hydrocarbons Recovery and Measurement. Rev. Sci. Instrum. 2012, 83, 034101. (19) Tsang, W.; Lifshitz, A. Kinetic Stability of 1,1,1-Trifluoroethane. Int. J. Chem. Kinet. 1998, 30, 621−628. (20) Lifshitz, A.; Shweky, I.; Kiefer, J. H.; Sidhu, S. S. Thermal Isomerization of Cyclopropanecarbonitrile. The Use of Two Chemical Thermometers in Single Pulse Shock Tube Experiments. In Proceedings of the 18th International Symposium on Shock Waves, Sendai, Japan, 1991. (21) Gaydon, A. G.; Hurle, I. R. The Shock Wave in an Ideal Gas. In The Shock Tube in High-Temperature Chemical Physics; Reinhold Pub. Corp.: New York, 1963; pp 9−27. (22) Tranter, R. S.; Tang, W.; Anderson, K. B.; Brezinsky, K. Shock Tube Study of Thermal Rearrangement of 1,5-Hexadiyne over Wide Temperature and Pressure Regime. J. Phys. Chem. A 2004, 108, 3406− 3415. (23) Chemkin 10112; Reaction Design: San Diego, CA, 2012. (24) Goos, E.; Burcat, A.; Ruscic, B. Extended Third Millenium Ideal Gas and Condensed Phase Thermochemical Database for Combustion with Updates from Active Thermochemical Tables. http://garfield. chem.elte.hu/Burcat/burcat.html (accessed 18 Sept 2012). (25) Miller, J. A.; Klippenstein, S. J. The Recombination of Propargyl Radicals and Other Reactions on a C6H6 Potential. J. Phys. Chem. A 2003, 107, 7783−7799.

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CONCLUSIONS The recombination and disproportionation of allyl radicals has been studied in a single pulse shock tube using 1,5-hexadiene and allyl iodide as precursors. This set of experiments in the 1− 10 bar range, temperatures of 650−1300 K, and reaction times of 1.4−2 ms is a complementary set of experiments for a diaphragmless shock tube, with laser schlieren densitometry study at low pressures, high temperatures, and short observation times. The rate constants for dissociation of allyl iodide and allyl recombination derived in the latter study showed excellent agreement with present experimental results. No pressure dependence in allyl recombination/disproportionation was observed at present experimental conditions. Simulation of these pyrolysis experiments indicated that a significant amount of allyl radicals accumulate and form equilibrium with the precursor. Unexpectedly, a low temperature pathway to benzene from allyl iodide was observed. Sensitivity analysis on an expanded mechanism highlighted uncertainties in allyl + H chemistry. Higher yields of allene and propene at lower temperatures in allyl iodide experiments lead to a hypothesized iodine driven reaction mechanism that competes with recombination/disproportionation at lower temperatures. Low temperature benzene formation pathways are speculated as motivation for future studies.

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ASSOCIATED CONTENT

S Supporting Information *

The pressure, temperature, reaction time, and the mole fractions (in ppm) of species detected for each shock; schematic of the end section of the low pressure shock tube; and figures depicting carbon recovered for all experiments. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(K.B.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Aspects of this work were performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, U.S. Department of Energy, under contract number DE-AC02-06CH11357.

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REFERENCES

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