ARTICLE pubs.acs.org/JPCA
Pyrolysis of n-Heptane: Experimental and Theoretical Study Tao Yuan,† Lidong Zhang,*,† Zhongyue Zhou,† Mingfeng Xie,† Lili Ye,† and Fei Qi*,†,‡ † ‡
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
bS Supporting Information ABSTRACT: An experimental study of n-heptane pyrolysis (2.0% n-heptane in argon) has been performed at low pressure (400 Pa) within the temperature range from 780 to 1780 K. The pyrolysis products were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Photoionization mass spectra and photoionization efficiency spectra were measured to identify pyrolysis products, especially radicals and isomers. Mole fraction profiles of pyrolysis products versus temperature were also measured, indicating that H2, CH4, C2H2, and C2-C6 alkenes are major pyrolysis products of n-heptane. Meanwhile, the thermal decomposition pathways of n-heptane have been investigated using theoretical calculation. The calculation results are in good agreement with the experimental measurement. On the basis of the experimental observation and theoretical calculation, the pyrolysis channels of unimolecular dissociation are proposed to understand the pyrolysis process of n-heptane.
1. INTRODUCTION As common components with high concentration in practical fuels, high attention is paid to unbranched long-chain alkanes (n g 4). Among them, n-heptane is mostly used as a primary reference fuel for octane rating in internal combustion engines1 and also an autoignition surrogate for diesel oil, since it has a cetane number of 56. Therefore, the pyrolysis and oxidation of n-heptane have been broadly investigated because of practical interests and for a better knowledge of the chemical mechanism of unbranched long-chain alkanes. In the past 2 decades, many studies on the pyrolysis,2-9 oxidation,1,4-6,10-13 and combustion14-26 of n-heptane have been performed. Some previous studies on pyrolysis should be mentioned here. By using GC methods, Kundru and his coworkers studied the pyrolysis of n-heptane in a tubular flow reactor at atmospheric pressure3 and high pressures.7 They identified the main products of n-heptane pyrolysis to be methane, ethylene, propylene, 1-butene, 1-pentene, and 1-hexene. In 1997, Held and co-workers developed a simplified hightemperature pyrolysis and oxidation model for n-heptane,4 based on their flow reactor experiment. In their work, they pointed out that n-heptane first decomposes to four different heptyl radicals, and heptyl suffers β-scission reactions to produce CH3, C2H5, and C2-C6 alkenes. Recently, methyl radical concentration during the oxidation and pyrolysis of n-heptane was measured by Davidson et al. using a shock tube.6 They concluded that the r 2011 American Chemical Society
overall decomposition rate of n-heptane was determined from the rate of formation of CH3. In 1995, Alexiou et al.2 carried out pyrolysis of toluene/n-heptane mixtures using a shock tube and studied the process of soot formation. On the basis of previous experimental results, Agafonov et al.5 provided a detailed kinetic model of soot formation for pyrolysis and oxidation of toluene and n-heptane in a shock tube. Garner et al.8 studied the pyrolysis of saturated and unsaturated C7 hydrocarbons using a highpressure shock tube. They compared the difference of saturated and unsaturated analogs. Furthermore, Zamostny et al.9 carried out the pyrolysis studies on a series of C1-C12 hydrocarbons including n-heptane in a micropyrolysis reactor and analyzed the products by online GC/FID/TCD. Their work exhibited the general trend of main products in relation to the feedstock structural characteristics, such as linearity, saturation, and number of substituent. However, most of these works were carried out at 1 atm or higher pressures. Meanwhile, there are many published kinetic mechanisms. Ranzi et al. presented an oxidation model of n-heptane.11 The pyrolysis submechanism of Ranzi’s model contains four kinds of reactions: (1) chain initiation to form alkyl radicals, (2) H-abstraction on n-heptane to form the four different alkyl radicals, (3) isomerization of alkyl Received: October 8, 2010 Revised: January 24, 2011 Published: February 15, 2011 1593
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Figure 1. Photoionization mass spectra of n-heptane pyrolysis taken at the temperature of 1480 K and different photon energies as labeled in the figure.
radicals, and (4) decomposition to form olefins and smaller radicals. Held et al. presented a semiempirical reaction mechanism for n-heptane oxidation and pyrolysis.4 Curran et al. presented a oxidation model of n-heptane, which has been validated by the oxidation of n-heptane in flow reactors, shock tubes, and rapid compression machines.1 There are few low-pressure data on n-heptane pyrolysis from previous work. And free radicals were not detected previously. Hence, detailed low-pressure pyrolysis data are necessary for kinetic model development. In this work, the pyrolysis of n-heptane has been investigated using a molecular-beam mass spectrometric technique coupled with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Benefiting from the tunability of synchrotron light, this method is able to detect the intermediates including radicals and distinguish isomers of pyrolysis process.27-30 Mole fraction profiles of pyrolysis products are deduced from near-threshold photoionization mass spectra taken at different temperatures. On the basis of the experimental data, ab initio calculations were performed to explain the experimental measurement and shed further light on the n-heptane pyrolysis pathways.
2. EXPERIMENTAL METHOD The experimental work was carried out at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. A detailed description on the pyrolysis study has been published previously.27,29,30 Two beamlines were used for this study.31,32 One was to utilize undulator radiation and the other bending magnet radiation from the 800 MeV electron storage ring. Undulator radiation was dispersed by a 1 m Seya-Namioka monochromator equipped with a laminar grating (1500 grooves/mm, Horiba Jobin Yvon) covering the photon energies from 7.80 to 24.00 eV. The monochromator was calibrated with the known ionization energies of the inert gases. The energy resolving power (E/ΔE) is about 1000. The average photon flux can reach a magnitude of 1013 photons/s. A gas filter was used to eliminate higher-order harmonic radiation with Ne or Ar filled in the gas cell. The beamline from the bending magnet provided photon energy range from 6.90 to 11.81 eV, which was described in detail previously.31 A LiF window was used to eliminate higher-order harmonic radiation for this beamline. A silicon photodiode (SXUV-100, International Radiation Detectors, 1594
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The Journal of Physical Chemistry A Inc.) was used to monitor the photon flux for normalizing ion signals. At the photon energy below ∼10.5 eV, where ionization energies (IEs) of most pyrolysis products are located, the beamline with the bending magnet can provide higher photon flux than the beamline with undulator radiation. Therefore, we use the beamline with the bending magnet to perform measurements at photon energies below 10.5 eV. The experimental setup consists of a pyrolysis chamber with a high-temperature furnace, a differentially pumped chamber with a molecular-beam sampling system, and a photoionization chamber with a reflection time-of-flight mass spectrometer (RTOF-MS). With the variation of photon energy, a series of mass spectra can be taken at the specified temperature for measurement of photoionization efficiency (PIE) spectrum. The integrated ion intensity for a specific mass is normalized by the photon flux and plotted as a function of the photon energy, which yields a PIE spectrum containing precise information of IEs of corresponding species. In this work, the PIE spectra were measured at the pyrolysis temperature of 1480 K. Considering the cooling effect of the molecular beam,19 the experimental error for determined IEs is within 0.05 eV. To keep near-threshold ionization and avoid fragmentation, we scanned the temperature at some selected photon energies. The mixture of n-heptane and Ar was fed into a 6 mm i.d. and 300 mm length alumina flow tube with 100 mm inside the furnace. n-Heptane was diluted with Ar, which was controlled by a MKS mass flow controller with the flow rate of 500 sccm, and the uncertainty of Ar is (1 sccm. n-Heptane was controlled by a liquid chromatography pump with the liquid flow rate of 0.066 mL/min (equivalent to 10.2 sccm in gas phase) at room temperature. The uncertainty of n-heptane is (0.001 mL/ min. After the pump, n-heptane was vaporized and mixed with Ar in a vaporizer kept at a temperature of 400 K. Thus, the total flow rate is 510.2 sccm, and the inlet mole fraction of n-heptane is calculated to be 2.00%. To reduce collisions of the pyrolysis species and detect the primary and secondary products containing radicals, the pressure of the pyrolysis chamber was kept at 400 Pa in this work, which was controlled by a MKS throttle valve. The temperature was measured by a Pt-30% Rh/Pt-6% Rh thermocouple inserted into the heating region inside the flow tube. The temperature uncertainty is within (50 K.
3. COMPUTATIONAL METHOD On the basis of the experimental observation, the channels of unimolecular dissociation and H attack were calculated using the Gaussian 03 program.33 The geometries of the reactants, transition states, and products were optimized using the hybrid density functional B3LYP method34,35 with the 6-311G(d,p) basis set.36-38 The vibrational frequencies were computed at the same level for characterizing the nature of structures and used for computing zero-point energy (ZPE) corrections. For more accurate evaluation of the energies, further single-point energy calculations were performed at the MP2/6-31G(d), MP2/6311þþG(3df,2p), and CCSD(T)/6-31G(d) levels with the optimized geometries.39-43 The approximate E[CCSD(T)/6311þþG(3df,2p)] energies employed are E[CCSD(T)/6311þþG(3df,2p)] = E[CCSD(T)/6-31G(d)] þ E[MP2/6311þþG(3df,2p)] - E[MP2/6-31G(d)] þ ZPE. The highprecision calculations were used to obtain the main reaction pathways in n-heptane pyrolysis.
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Figure 2. PIE spectrum of (a) m/z = 39, (b) m/z = 40, (c) m/z = 42, (d) m/z = 56, (e) m/z = 70, and (f) m/z = 84 from pyrolysis of n-heptane at 1480 K.
4. RESULTS AND DISCUSSION 4.1. Identification of Pyrolysis Species. All detected pyrolysis products of n-heptane can be observed with relatively high signal intensities at the temperature 1480 K. Therefore, we select this temperature to illustrate mass spectra and PIE spectra. Figure 1 shows six photoionization mass spectra taken at the temperature of 1480 K with the photon energies of 9.0, 9.5, 10.0, 10.5, 11.0, and 11.7 eV, respectively. The number and intensity of peaks change with photon energy. At 9.0 eV, only a couple of peaks are observed at m/z = 29 and 41. At 9.5 eV, the detected peaks are observed at m/z = 29, 41, 54, 68, and 71. At 10.0 eV, a number of pyrolysis intermediates are detected at m/z = 15, 29, 39, 40, 41, 42, 54, 56, and 58. The number of peaks increases as the photon energy increases. A number of peaks with m/z from 15 to 100 can be observed at the photon energy 11.7 eV, which belong to C1-C7 hydrocarbons. They may be pyrolysis intermediates or fragment ions from photoionization. Assignment of these species, which can be distinguished by different IEs, is performed with the aid of PIE spectra. Figure 2 illustrates the PIE spectra of six hydrocarbon species at m/z = 39, 40, 42, 56, 70, and 84 at 1480 K. For m/z = 39 (Figure 2a), the onset is 8.66 eV, which corresponds to the IE of propargyl radical (IE = 8.67 eV44). As shown in Figure 2b, two onsets are observed at 9.70 and 10.36 eV for m/z = 40, respectively, which correspond to IEs of allene (IE = 9.69 eV44) and propyne (IE = 10.36 eV44). The PIE spectra of allene and propyne are used to simulate the PIE spectrum of m/z = 40, indicating 45% allene and 55% propyne at 1480 K. For m/z = 42 (Figure 2c), the onset is 9.72 eV, corresponding to the ionization threshold of propylene (IE = 9.73 eV44). Two onsets are observed at 9.11 and 9.55 eV for m/z = 56 (Figure 2d), 1595
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Table 1. List of Intermediates Measured in the Pyrolysis of n-Heptane IE (eV) m/z
formula
2
H2
15 16 26 27 28 29
CH3 CH4 C2H2 C2H3 C2H4 C2H5
species hydrogen methyl radical
1080
1730
16.53
12.13k
1.3 10
-2
1080
1530
10.50
5.6948
5.0 10
-3
1180
1780
16.53
44.3049
5.9 10
-3
1380
1780
16.53
55.6850
1.6 10
-4
1080
1680
10.50
13.3251
3.7 10
-2
1080
1630
11.70
8.9150
6.0 10
-4
1080
1430
10.50
14.93l
9.83
10.51
ethyl radical
3.7 10-2
15.43
8.59
ethylene
σ (Mb)g
15.43
11.40
vinyl radical
EX (eV)f
XMc
12.61
acetylene
TM (K)e
this workb
9.84
methane
TF (K)d
literaturea
8.32
12.60 11.40 8.57 10.49 8.31
-4
30 39
C2H6 C3H3
Ethane propargyl radical
11.52 8.67
11.52 8.66
3.4 10 2.1 10-3
1180 1280
1480 1780
13.00 10.50
40.3452 9.0051
40
C3H4
propyne
10.36
10.36
9.9 10-4
1280
1780
11.00
43.8453
9.70
8.0 10
-4
1280
1780
10.00
6.6453
1.7 10
-3
1080
1630
10.50
6.0954
5.8 10
-3
1080
1480
10.50
10.7353
3.5 10
-4
1380
1780
10.50
33.8350
5.2 10
-4
1180
1580
10.50
17.4850
-4h
i
1080
j
1480
10.50
12.6555
1180
1530
10.50
10.00l
i
1080
j
1380
10.50
10.00l
1380
1780
10.50
31.8150
1080
1380
10.50
10.00l
10.50
9.9156
40 41 42 52 54
C3H4 C3H5 C3H6 C4H4 C4H6
allene
9.69
allyl radical
8.13
propylene
9.73
1-buten-3-yne
9.58
1,3-butadiene
9.07
8.12 9.72 9.57 9.06
56
C4H8 C4H8
1-butene 2-butene
9.55 9.13
9.55 9.11
9.0 10
68
C5H8
1,3-pentadiene
8.60
8.61
2.7 10-4
70
C5H10
1-pentene
9.49
9.50
C5H10
2-pentene
9.04
9.03
78
C6H6
benzene
9.24
9.23
84 100
C6H12 C7H16
1-hexene
9.44
n-heptane
9.93
4.7 10
-4h
2.4 10-5
9.45 9.94
1.7 10
-4
2.0 10
-2
j
1080
Reference to NIST Chemistry Webbook. The uncertainty for IEs is (0.05 eV. The maximum mole fractions. TF refers to the initial temperature for formation of species. e TM refers to the temperature relating to the maximum mole fraction. f Energy of the mole fraction is calculated. g PI cross section is used. h The value is the total maximum mole fraction of species with the same mass. i The value of all species with the same mass. j The temperature is the initial decomposition temperature. k Measured value from cold Ar and H2 gas. l Estimated value. a
44 b
c
d
Figure 3. Stack plot of photoionization mass spectra of n-heptane pyrolysis taken at the photon energy of 10.5 eV with different temperatures labeled in the figure.
respectively, which correspond to 2-butene (IE = 9.13 eV44) and 1-butene (IE = 9.55 eV44). 1-Butene is found to be the dominant butene isomer from the PIE spectra. i-Butene, another butene
isomer with an IE (9.22 eV44) between those of 1- and 2-butene, may also exist. However its concentration is too low to reveal its onset on the PIE spectra, which is consistent with the fact that 1596
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Figure 6. The calculated direct bond dissociation energies of n-heptane at the CCSD(T) level (in kcal/mol). Figure 4. Mole fraction profiles of n-heptane, methyl radical, and major products (H2, CH4, C2H2, and C2H4). Symbols represent the mole fractions of corresponding species, and B-spline curves are used to connect the symbols.
Figure 5. Mole fraction profiles of (a) C2 and C3 intermediates and (b) C4 to C6 intermediates.
i-butene cannot be produced from direct decomposition of nheptane. Two onsets are observed at 9.03 and 9.50 eV for m/z = 70 (Figure 2e), respectively, which correspond to IEs of 2-pentene (IE = 9.04 eV44) and 1-pentene (IE = 9.49 eV44). Here, the 2-butene and 2-pentene must be emphasized. Because the signals of 2-butene and 2-pentene are very low, their mole fractions are not calculated. On the other hand, cis- and trans-2-butene (also
for 2-pentene) can hardly be distinguished from the PIE spectrum due to their very close IEs. For m/z = 84 (Figure 2f), the onset is 9.45 eV, corresponding to an IE of 1-hexene (IE = 9.44 eV44). By the way, all the species of n-heptane pyrolysis detected in this experiment have been identified. Table 1 lists the detected species and provides their IEs from the literature44 and this work. 4.2. Mole Fraction Evaluation. The relative ion intensity of observed pyrolysis species versus the pyrolysis temperature can be obtained from the measurements of near-threshold photoionization mass spectra taken at different temperatures, as shown in Figure 3. Different photon energies, including 16.5, 13.0, 11.7, 11.0, 10.5, 10.0, 9.5, and 9.0 eV, were used to perform nearthreshold measurements of pyrolysis products for evaluation of their model fractions. For a detailed explication of the evaluation method, one can refer to our previous work.27 At 16.5 eV, argon was selected as a reference species to determine the mole fraction of n-heptane and H2; at 13.0 and 11.7 eV, acetylene and ethylene were selected as reference species, respectively; at 11.0, 10.5, and 10.0 eV, n-heptane was selected as the reference species; and at photon energies below the IE of n-heptane, propargyl radical was selected as the reference species. The mole fractions of the species with known photoionization cross sections have an uncertainty of (25%, and those with estimated photoionization cross sections have an uncertainty of a factor of 2 (-50% to ∼100%). Table 1 lists the initial temperature for formation of species (TF), the maximum mole fractions of pyrolysis species (XM), the temperature relating to the maximum mole fraction (TM), the photon energy (EX), and photoionization cross section (σ) used for each species in the mole fraction calculation. About 20 pyrolysis species have been observed in this work, whose mole fraction profiles are presented in Figures 4 and 5. The mole fraction profiles of n-heptane, methyl radical, and major products (H2, CH4, C2H2, and C2H4) are illustrated in Figure 4. The initial decomposition temperature of n-heptane is around 1080 K, and its concentration decreases gradually from the initial value of 2 10-2 to a full decomposition at about 1730 K. The concentrations of H2, CH4, and C2H2 increase continuously with increasing temperature, and their TFs are 1080, 1180, and 1380 K, respectively. The radical CH3 has a peak-shape mole fraction profile. The TF of CH3 is 1080 K and the maximum mole fraction is 1.34 10-2 at 1530 K. C2H4 decomposes to smaller and more stable molecules, such as C2H2 and H2 at high temperature. Thus, C2H4 also has a peak-shape mole fraction profile with the TM of 1630 K and XM of 3.70 10-2. 1597
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Figure 7. The calculated n-heptane unimolecular decomposition pathways for formation of alkenes at the CCSD(T) level (in kcal/mol).
Figure 5a shows mole fraction profiles of C2 and C3 intermediates, including C2H5, C3H3, C3H4 (allene and propyne), C3H5, and C3H6. As seen from the figure, TFs of these species are in the range 1080-1280 K. Among them, the TFs of C3H3 and C3H4 are obviously high, and their mole fractions increase with the increasing temperature. Furthermore, the mole fractions of allene and propyne are close to each other. That is consistent with their close stabilities. Both Yu et al.45 and Pomerantz et al.46 have performed calculations on allene and propyne with highly precise theoretical methods. The calculation results show that the enthalpies of allene and propyne are very close to each other. However, C2H5, C3H6, and C3H5 have peak-shape mole fraction profiles. The TM of C2H5 is about 1430 K, and the peak positions of C3H6 and C3H5 shift toward high temperature. The measured XMs for C2H5, C3H5, and C3H6 are 5.96 10-4, 1.70 10-3, and 5.76 10-3, respectively. Figure 5b shows mole fraction profiles of C4-C6 intermediates, including C4H4, C4H6, C4H8, C5H8, C5H10, C6H6, and C6H12. Among them, the TFs of alkenes are relatively low, and the TFs of C4H8, C5H10, and C6H12 are about 1080 K. The main reason is that alkenes are the primary decomposition products in n-heptane pyrolysis. With the increase of carbon atom number, the peak positions of C4H8, C5H10, and C6H12 shift toward low temperature and the XMs of these three species decrease, which are 8.95 10-4, 4.73 10-4, and 1.70 10-4, respectively. As degree of unsaturation increases, the TFs of hydrocarbons tend to higher temperature; e.g., the TFs of C4H6, C5H8, C4H4, and C6H6 are 1180, 1180, 1380, and 1380 K, respectively. The concentrations of C4H4 and C6H6 increase continuously with temperature increasing, and their XMs are 3.45 10-4 and 2.44 10-5 at 1780 K, respectively. From experimental measurement, H2, CH4, C2H2, and CnH2n (n = 2-6) are the main products in n-heptane pyrolysis, and the concentrations of alkenes obey the order of C2H4 > C3H6 > C4H8 > C5H10 > C6H12. The TFs of these alkenes are all at about 1080 K, and the TMs decrease with the increase of molecular weights. Chakraborty et al.7 indicated that the main pyrolysis products of n-heptane were methane, ethylene, ethane, propylene, 1-butene, 1-pentene, and 1-hexene, with their tubular
Table 2. Calculated Enthalpy Change (ΔrH, kcal/mol) for Some Reactions in n-Heptane Pyrolysis NIST44 this work: reactions
a
ΔrH (298 K)
ΔrH (0 K)a
ΔrH (0 K)
C7H16 f 1-C7H14 þ H2
29.8
28.1
26.5
C7H16 f 2-C7H14 þ H2 C7H16 f 3-C7H14 þ H2
27.2 27.4
25.3 25.6
24.2 24.5
C7H16 f 1-C6H12 þ CH4
17.0
15.7
15.7
C7H16 f 1-C5H10 þ C2H6
19.6
18.8
19.2
C7H16 f 1-C4H8 þ C3H8
20.0
19.4
19.6
C7H16 f C3H6 þ C4H10
19.7
19.1
19.2
C7H16 f C2H4 þ C5H12
22.3
21.5
21.5
The data were calculated on the basis of the ΔrH (298 K) in NIST.
reactor at pressures from 0.1 to 2.93 MPa. Most of their results are consistent with our experiment. 4.3. Decomposition Pathways of n-Heptane. The primary unimolecular decomposition and H-attack reaction pathways in the n-heptane pyrolysis are presented in Figures 6-9. The optimized geometries of the reactants and transition states are displayed in Figure 10. Detailed geometry information for all species involved in the thermal decomposition pathways is listed in the Supporting Information. Compared to combustion, the unimolecular decomposition reactions are more important in pyrolysis reactions.47 The main unimolecular decomposition pathways of n-heptane include direct bond-cleavage reactions, alkene formation reactions, and alkane þ singlet carbene reactions. Figure 6 shows the direct bond dissociation energies of n-heptane. The direct C-C bond cleavage reactions are the lowest energy pathways in all the unimolecular decomposition pathways, and bond dissociate energies (BDEs) for formation of C1 þ C6, C2 þ C5, and C3 þ C4 are 86.8, 86.7, and 87.8 kcal/mol, respectively. This is consistent with experimental observations that the TFs of CH3 and C2H5 are the lowest (about 1080 K) among all observed 1598
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Figure 8. The calculated n-heptane unimolecular decomposition pathways for formation of carbenes at the CCSD(T) level. The energies of singlet carbenes are shown in this figure (in kcal/mol).
Figure 9. The calculated H-abstraction by H attack and subsequent β-scissions pathways in n-heptane pyrolysis (in kcal/mol).
products. There are four different positions of H atoms in nheptane. The dissociation energies of four different C-H bonds are relatively higher than that of C-C bonds, which are 97.8 (C1/C7-H), 94.9 (C2/C6-H), 95.3 (C3/C5-H), and 95.2 (C4-H) kcal/mol. Figure 7 shows the unimolecular decomposition pathways for formation of alkenes. There are three H2 elimination pathways to form different heptenes (C7H14) with four-member-ring transition states TS1, TS2, and TS3, and their barriers are close (about 110 kcal/mol). The calculated energies for products are 26.5 (1heptene þ H2), 24.2 (2-heptene þ H2), and 24.5 (3-heptene þ H2) kcal/mol, respectively. These are consistent with the reference data (see Table 2).44 Other alkenes (from C2 to C6) can
also be formed via H transfer and C-C bond cleavage with fourmember-ring transition states. Their barriers are about 110 kcal/ mol (TS4-TS8). These pathways perform a similar behavior: the H transfers toward the meta position C atom in the transition states. The calculated energies of products for five reactions, of which form C2-C6 alkenes, are in agreement with the NIST data, with a difference smaller than 1 kcal/mol (see Table 2). Further C-C bond cleavage can result in the formation of alkane þ singlet carbene products with three-member-ring transition states (Figure 8). The calculated barriers at the CCSD(T) level are about 95 kcal/mol (TS9-TS14), which is about 15 kcal/mol lower than those of the reactions to form alkane þ alkene products. The energy difference between the lowest 1599
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Figure 10. Optimized geometries of the reactant and transition states at the B3LYP/6-311G(d,p) level.
singlet and triplet states (S-T gap; triplet states are more stable) of the carbene products is about 9.2 kcal/mol for CH2 (Figure 8), and the energies of other triplet state carbene were not shown in Figure 8. The interconversion between singlet and triplet states is limited; thus, the main products are the singlet carbenes. The energies of products in Figure 8 are related to singlet carbenes þ alkanes. However, the singlet carbene products (not including 1 CH2) are not stable and are ready to form alkene. Besides, the βscission following the H-abstraction reactions by H atom can also form the alkene products, which will be discussed in the next paragraph. Thus alkenes are the major products in n-heptane pyrolysis, which is in agreement with the experimental measurement. The radical attack reactions will not be neglected with the increase of the radical concentration. Among them the main reactions are the H attack reactions, displayed in Figure 9. The calculations show that the barrier of the H-abstraction of terminal groups by H attack (TS15) is higher, and the barrier is 12.1 kcal/ mol, which is 2.9 kcal/mol higher than that of other reactions (TS16-TS18). The subsequent β-scissions of the formed heptane radicals can produce alkene and alkyl radicals. The calculated barriers of the β-scissions (TS19-TS23) are 29.230.2 kcal/mol. The further scissions of the formed alkyl radicals can produce smaller products. The calculations show that the barriers of H attack reactions are obviously lower than that of unimolecular decomposition reactions. Thus, the H attack reactions will be more important when the concentration of H radical increases. By the calculation of the n-heptane decomposition pathways, the initial temperatures and concentrations of pyrolysis products
can be explained reasonably. Furthermore, a detailed kinetic model for low-pressure pyrolysis of n-heptane is in development, and the decomposition mechanism of n-heptane will be presented in the near future.
5. CONCLUSIONS An experimental study of n-heptane pyrolysis has been performed with tunable synchrotron VUV photoionization and molecular-beam mass spectrometry. Isomeric identification has been performed on the basis of the measurements of photoionization mass spectra and photoionization efficiency spectra, and mole fraction profiles are obtained by scanning the pyrolysis temperature at selected photon energies near the ionization threshold. The TFs of some small radicals, such as CH3 and C2H5, are relatively lower, and H2 and CnH2n (n = 2-6) are the major products in n-heptane pyrolysis. Furthermore, the XMs of alkenes decrease as the number of carbon atoms increases. On the basis of the experimental results, a theoretical study has been carried out at the CCSD(T) level, and the unimolecular decomposition, H-abstraction reactions by H radical, and the subsequent β-scissions reactions were computed. The calculations show that the direct C-C bond cleavage reactions have lower barrier in the unimolecular decomposition, and the alkenes and H2 are major pyrolysis products. That is consistent with the experimental measurement. The calculated results can assist in understanding the experimental observations. The study of pyrolysis channels of n-heptane enriches the high-temperature chemistry of n-heptane. 1600
dx.doi.org/10.1021/jp109640z |J. Phys. Chem. A 2011, 115, 1593–1601
The Journal of Physical Chemistry A
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed geometry information for all species involved in the thermal decomposition pathways. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (L.Z.),
[email protected] (F.Q.).
’ ACKNOWLEDGMENT This research was supported by Chinese Academy of Sciences, Natural Science Foundation of China (10805047 and 50925623), and Ministry of Science and Technology of China (2007CB815204 and 2007DFA61310). ’ REFERENCES (1) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 1998, 114, 149. (2) Alexiou, A.; Williams, A. Fuel 1995, 74, 153. (3) Pant, K. K.; Kunzru, D. J. Anal. Appl. Pyrolysis 1996, 36, 103. (4) Held, T. J.; Marchese, A. J.; Dryer, F. L. Combust. Sci. Technol. 1997, 123, 107. (5) Agafonov, G. L.; Naydenova, I.; Vlasov, P. A.; Warnatz, J. Proc. Combust. Inst. 2007, 31, 575. (6) Davidson, D. F.; Oehlschlaeger, M. A.; Hanson, R. K. Proc. Combust. Inst. 2007, 31, 321. (7) Chakraborty, J. P.; Kunzru, D. J. Anal. Appl. Pyrolysis 2009, 86, 44. (8) Garner, S.; Sivaramakrishnan, R.; Brezinsky, K. Proc. Combust. Inst. 2009, 32, 461. (9) Zamostny, P.; Belohlav, Z.; Starkbaumova, L.; Patera, J. J. Anal. Appl. Pyrolysis 2010, 87, 207. (10) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Flame 1995, 101, 132. (11) Ranzi, E.; Gaffuri, P.; Faravelli, T.; Dagaut, P. Combust. Flame 1995, 103, 91. (12) Dagaut, P.; Togbe, C. Energy Fuels 2009, 23, 3527. (13) Dagaut, P.; Togbe, C. Fuel 2010, 89, 280. (14) Doute, C.; Delfau, J. L.; Akrich, R.; Vovelle, C. Combust. Sci. Technol. 1997, 124, 249. (15) Bakali, A. E.; Delfau, J. L.; Vovelle, C. Combust. Sci. Technol. 1998, 140, 69. (16) Doute, C.; Delfau, J. L.; Vovelle, C. Combust. Sci. Technol. 1999, 147, 61. (17) Ingemarsson, A. T.; Pedersen, J. R.; Olsson, J. O. J. Phys. Chem. A 1999, 103, 8222. (18) Inal, F.; Senkan, S. M. Combust. Flame 2002, 131, 16. (19) Kamphus, M.; Liu, N. N.; Atakan, B.; Qi, F.; McIlroy, A. Proc. Combust. Inst. 2002, 29, 2627. (20) Inal, F.; Senkan, S. A. Fuel 2005, 84, 495. (21) Zhang, H. Z. R.; Eddings, E. G.; Sarofim, A. F. Energy Fuels 2007, 21, 677. (22) Song, J. O.; Yao, C. D.; Liu, S. Y.; Xu, H. J. Energy Fuels 2008, 22, 3806. (23) D’Anna, A.; Ciajolo, A.; Alfe, M.; Apicella, B.; Tregrossi, A. Proc. Combust. Inst. 2009, 32, 803. (24) Song, J. O.; Yao, C. D.; Liu, S. Y.; Tian, Z. Y.; Wang, J. Fuel 2009, 88, 2297. (25) Yao, C. D.; Cheng, C. H.; Liu, S. Y.; Tian, Z. Y.; Wang, J. Fuel 2009, 88, 1752. (26) Yu, W.; Wei, L. X.; Ma, Z. H.; Huang, Z. H.; Yuan, T.; Tian, Z. Y.; Li, Y. Y. Chin. Sci. Bull. 2009, 54, 1477. (27) Zhang, T. C.; Wang, J.; Yuan, T.; Hong, X.; Zhang, L. D.; Qi, F. J. Phys. Chem. A 2008, 112, 10487.
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