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The mechanism and kinetic analysis of C4H4 + C4H4 (but-1-ene-3-yne) reaction with features of H-transfer in combustion
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 3 2 4 9 e3 2 5 8
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The mechanism and kinetic analysis of C4H4 þ C4H4 (but-1-ene-3-yne) reaction with features of H-transfer in combustion Peng Liu, He Lin*, Zhenwu He, Yiran Zhang, Bin Guan, Zhen Huang Key Laboratory for Power Machinery and Engineering of Ministry of Education, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
article info
abstract
Article history:
Detailed mechanisms of C4H4 þ C4H4 reaction were investigated by accurate ab initio
Received 10 October 2015
density functional theory B3LYP/6-311 þ G(d,p) calculations, as well as CBS-QB3 calcula-
Received in revised form
tions. It was found that styrene, phenylacetylene and 5-membered ring structure can be
14 December 2015
formed in C4H4 þ C4H4 reaction through five parallel and competing pathways. Three
Accepted 14 December 2015
pathways are featured with H-transfer and dehydrogenation reactions, and the other two
Available online 13 January 2016
pathways are characterized with the forming and breaking of CeC bond. The H-transfer reactions play a vital role in stabilizing intermediates. It was found that the H-transfer
Keywords:
reaction is significantly dependent on the relative position of two C atoms, the degree of
Polycyclic aromatic hydrocarbons
saturation of the second C atom, and the electronic environment of molecule. Generally, an
But-1-ene-3-yne
ortho-position unsaturated C atom with relative small positive atomic charge is preferable
Density functional theory
in H-transfer reaction. The yield of products and reaction rate coefficients were evaluated
Kinetic mechanism
by using RiceRamspergerKasselMarcus theory with solving master equation at
H-transfer
different combustion temperatures and pressures (T ¼ 800e2500 K, P ¼ 0.1e10 atm). The kinetic results indicate that styrene is the dominant product and the formation of other two products can be ignored when temperature is lower than 1600 K. The formations of phenylacetylene and 5-membered ring structure are only facilitated at higher temperature, but dependent on the combustion pressure. Specifically, phenylacetylene is favored by higher pressure and higher temperature, while 5-membered ring structure is favored by lower pressure and higher temperature. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The polycyclic aromatic hydrocarbons (PAHs) generated from incomplete combustion of fossil fuel have negative influence on human health and atmospheric haze [1e5]. PAHs are highly
carcinogenic and mutagenic [6e11], and are also considered as the precursor of soot [7,12]. On the other hand, soot and carbon nano-material have been widely applied to the biomedical, catalytic, and automotive industries owing to their special physical characteristic, such as low density, high specific modulus, and antithermal shock [13e16]. In recent decades,
* Corresponding author. Tel./fax: þ86 21 34207774. E-mail address: [email protected] (H. Lin). http://dx.doi.org/10.1016/j.ijhydene.2015.12.038 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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great efforts have been made to understand PAHs evolution in combustion [14,17e23]. The PAHs formation and growth are dominantly controlled by the first aromatic ring (benzene, or its derivative) [24,25], which mainly formed by addition reactions of C4Hx species with acetylene (C2H2) [26e28], cyclization reactions of C6Hx and combination reaction of two propargyl radicals (C3H3) [29e31]. Once formed, the first aromatic ring can grow up to large PAHs based on the hydrogen abstraction C2H2 addition (HACA) mechanism. Several popular soot models such as ABF [32], USC [33], and KM2 [25] were developed based on the formation of the first aromatic ring and subsequent HACA mechanism. However these soot models need further improvement, and it has been shown that the concentrations of various PAHs are under-predicted [25]. Recently, the formation of first aromatic ring has drawn great attention in both theoretic and experimental studies [34e39]. Trogolo et al. [36] investigated the probability of the first ring formation by reaction of C3H3 with but-1-ene-3-yne (C4H4) using density functional theory (DFT). They found that 5-membered ring, 6-membered ring, 7-membered ring and open-chain products can be formed. The 5-membered ring product is the dominant product in a wide temperature and pressure range, and the yield is more than 80%. Hansen et al. [34] investigated the formation of benzene in a fuel-rich 1-hexene flame using flame-sampling molecular-beam timeof-flight mass spectrometry. They concluded that the Hassisted isomerization of fulvene contributes most (64%) to the benzene formation. However, the leading pathway in the formation of aromatic ring may vary with combustion condition, and the self-recombination reactions have been confirmed to be important. Stein et al. [38] proposed that the reaction of C3H3 þ C3H3 irreversibly generates benzene, and is the primary contributor for benzene formation in low pressure acetylene flame. Furthermore, the recombination reaction of two cyclopentadienyl radicals (cC5H5) has been verified to be the main channel for the formation of naphthalene [39]. However, the similar reaction of C4H4 þ C4H4 has not been investigated yet. In premixed ethyne [40], benzene [41], toluene [42], and gasoline [43] flames, C4H4 is abundant and the measured mole fraction is up to 2 103. In view of the importance of C3H3 þ C3H3 and cC5H5 þ cC5H5 reactions in forming aromatic ring, we would like to raise a topic that the reaction of C4H4 þ C4H4 may contribute to the formation of first aromatic ring. The objective of this study is to investigate the detailed mechanism of C4H4 þ C4H4 reaction in combustion. To this end, the potential energy surfaces (PES) of C4H4 þ C4H4 reaction are calculated using DFT method. Subsequently, the RiceRamsperger-Kassel-Marcus (RRKM) theory with solving master equation is employed to evaluate the reaction rate coefficients and the yield of products. It was found in this study that there is a large number of H-transfer reactions in C4H4 þ C4H4 reaction. Therefore, the characteristics of Htransfer reaction are discussed.
Calculation details All the transition state and local minima structures involved in C4H4 þ C4H4 reaction were fully optimized by DFT B3LYP
hybrid functional [44e46] with the 6-311 þ G(d,p) basis [46,47]. The same method was applied to calculate vibrational frequencies, which were scaled by a factor of 0.967 [48,49]. Zeropoint energies (ZPE) were calculated by the CBS-QB3 method [50]. The T1 diagnostics were performed at CCSD(T)/6311 þ G(d,p) level [51] with DFT/B3LYP/6-311 þ G(d,p) geometries to assess whether the single-determinant reference state CBS-QB3 method is suitable. A large T1 value suggests that the multireference electron correlation procedure is needed, and the threshold value of T1 is considered as 0.02 for the closedshell system [52]. The ZPE of the structures with multireference feature were recalculated at CASPT2(8,8)/B3LYP/6311 þ G(d,p) [53] level with the CASSCF(8,8)/B3LYP/6311 þ G(d,p) geometries [54]. The eight active orbitals are completely composed of the p orbitals. The intrinsic reaction coordinate (IRC) [55] calculations were carried out for every transition states to ensure that the transition states were connected with the reactant and product. The structures of all species were optimized at their ground state. All quantum chemical calculations were performed using Gaussian 09 suit of programs [56]. Based on the calculated quantum chemical parameters (including energy barriers, moments of inertia, and vibrational frequencies), the yield of products and rate coefficients of overall reaction (C4H4 þ C4H4 / product) were calculated using a combined RRKM/master equation method via MultiWell suite of codes (MultiWell-2013.1) [57,58]. Each of the three entrance channels were simulated separately, and the total yield were calculated by weight-averaging the rate coefficients of the three entrance reactions. For example, the branching ratio of k(T,P)entrance reaction 1/(k(T,P)entrance reaction 1 þ k(T,P)entrance reaction 2 þ k(T,P)entrance reaction 3), where k(T,P)entrance reaction 1 is the rate coefficients of entrance reaction 1, was used to scale the yield of products evolved from entrance reaction 1. In each RRKM/master equation simulation, the rate coefficients of product formation were the multiplier of high pressure limit rate coefficients of entrance reaction and the yield of product at steady state. Here, the rate coefficients of entrance reaction were calculated based on the rate coefficients of the unimolecular backward reaction and corresponding equilibrium constants. The rate coefficients of overall reaction were the sum of the rate coefficients of product formation in each channel (three channels in this study). The detail parameters used in MultiWell calculations were presented to facilitate the reproduction of calculation results. Specifically, the energy grain size in the first segment of the double array was set as 10 cm1, and the maximum energy was set as 100,000 cm1 in density and sum of states calculation. The exponential-down model with <△Edown> ¼ 260 cm1 was used to describe the collisional energy transfer [59]. Argon was selected as the bath gas collider in this study, and its LennardeJones parameter of s A and 114 K respectively [59]. The Lenand ε/kB is 3.47 nardeJones parameters of other structures were assumed to be equal to that of phenylacetylene with s ¼ 5.72 A and ε/kB ¼ 535.6 K [60]. The internal rotation modes with real frequencies less than 150 cm1 were distinguished by graphically visualizing the normal mode vibrations. All internal rotations were regarded as 1-D unsymmentrical hindered rotations [61], and the torsional potential energy and rotational constants
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 3 2 4 9 e3 2 5 8
were calculated through the relaxed scan with dihedral angle step of 10 at DFT/B3LYP/6-311 þ G(d,p) level. The symmetry number corrections were carried out according to the method proposed by Duncan [62,63]. Quantum tunneling corrections were also performed for all H-transfer reactions [64]. The translational and vibrational temperatures were considered to be same. The number of stochastic trials was varied from 1 104 to 1 106 to keep the statistical fluctuations within 3%.
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the T1 diagnostics for all structures involved in C4H4 þ C4H4 reaction were provided in Table S1 in supplementary material. The ZPE of the structures with multireference character were replaced with the values calculated by CASPT2(8,8)/B3LYP/6311 þ G(d,p) method. The comparisons of the CASPT2 and CSB-QB3 energies for structures with multireference character were presented in Table S2 in supplementary material.
Styrene pathway
Results and discussion Potential energy surface of the C4H4 þ C4H4 reaction The PES of C4H4 þ C4H4 reaction are composed of five parallel and competing pathways, which lead to the formation of styrene (10), phenylacetylene (14), and 5-membered ring structure (17, similar to pentalene), as shown in Fig. 1. Three pathways including C4H4 þ C4H4 / 2 / 7 / … / 10, C4H4 þ C4H4 / 2 / 11 / 14 þ H2, and C4H4 þ C4H4 / 2 / 12 / … / 14 þ H2 are characterized by numerous H-transfer reactions on the 6-membered ring. The other two pathways including C4H4 þ C4H4 / 3 / … / 10 and C4H4 þ C4H4 / 15 / … / 17 are featured by the forming and breaking of CeC bond. To make the reaction pathways clear, each product pathway was drawn respectively. In addition,
There are two pathways leading to the formation of styrene (10), C4H4 þ C4H4 / 2 / 7 / 8 / 9 / 10 and C4H4 þ C4H4 / 3 / 4 / 5 / 6 / 10, as shown in Fig. 2. In the first pathway, the entrance reaction step is given by the recombination of two C4H4 molecules (C4H4 þ C4H4 / 2), which leads to the formation of adduct 2 with 6-membered ring structure. This reaction needs to overcome the energy barrier of 27.0 kcal/mol. Subsequent, a H atom on C(6) atom transfers to adjacent C(7) atom with the energy barrier of 53.2 kcal/mol (2 / 7), which gives birth to an unsaturated C(8) atom. There are two H atoms on C(5) atom and one of them will transfer to the unsaturated C(8) atom, leading to the formation of structure 8 (7 / 8) with the energy barrier of 28.6 kcal/mol. In structure 8, the C(1) atom connects with two H atoms, but the meta-position C(3) atom is unsaturated. The more stable species 10 can be formed if one H atom on C(1) atom transfers to the meta-position unsaturated C(3) atom.
Fig. 1 e Potential energy surface of the C4H4 þ C4H4 reaction.
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Fig. 2 e Potential energy surface of the styrene pathway.
Therefore, the direct H-transfer reaction between C(1) atom and meta-position unsaturated C(3) atom was investigated, but the corresponding transition state seems not to exist via the relaxed scan using DFT method. It was found that the H transfer from C(1) atom to the meta-position unsaturated C(3) atom needs two reaction steps, namely 8 / 9 and 9 / 10 in this study. In reaction step 8 / 9, H transfer reaction between C(1) and the ortho-position C(2) atoms happens with the energy barrier of 28.5 kcal/mol. Once formed, structure 9 will isomerize to the more stable species 10 with a low energy barrier (5.0 kcal/mol), via the other H transfer reaction between C(2) and the ortho-position C(3) atoms. Here, the facility of H transfer in reaction 9 / 10 was ascribed to the unsaturated C(3) atom. In the second pathway, the entrance reaction step is given by C4H4 þ C4H4 / 3, which involves the formation of C(3)-C(7) and C(3)-C(8) bonds with the energy barrier of 50.1 kcal/mol. The intramolecular rotation takes place in subsequent reaction step 3 / 4, which leads to the formation of structure 4 with 4-membered ring. The isomerization reaction (3 / 4) is featured by the breaking of C(3)-C(8) bond and the forming of C(4)-C(8) bond with an extremely low energy barrier (0.6 kcal/ mol). In the following reaction step 4 / 5, the C(1) atom combines with the C(7) atom with the energy barrier of 34.4 kcal/mol. The C(3)-C(7) bond in structure 5 is weak and will break when the H atom on C(1) atom transfers to the ortho-position C(7) atom (5 / 6). The product 10 forms after the H atom on C(7) atom in structure 6 transfers to the paraposition unsaturated C(3) atom (6 / 10). The energy barriers of the latter two reactions 5 / 6 and 6 / 10 are 53.7 and 31.3 kcal/mol respectively.
Phenylacetylene pathway The structure 2 can split other two pathways leading to the formation of phenylacetylene (14), namely C4H4 þ C4H4 / 2 / 11 / 14 þ H2 and C4H4 þ C4H4 / 2 / 12 / 13 / 14 þ H2, as shown in Fig. 3. In the reaction step 2 / 11, the H atom on C(1) atom in structure 2 transfers to the meta-position unsaturated C(3) atom with the energy barrier of 45.7 kcal/mol. Subsequent, two adjacent C atoms (C(5) and C(6)) in structure 11 simultaneously eliminate a H atom respectively in the reaction 11 / 14 þ H2. The energy barrier of the 2-H atoms dehydrogenation reaction is as high as 98.4 kcal/mol. In this study, the 1-H atom dehydrogenation from structure 11 is unlikely due to the high stability of structure 11. In the other pathway, the H atom on C(6) atom in structure 2 transfers to the para-position C(2) atom (2 / 12), followed by the other H-transfer reaction between C(2) and ortho-position C(3) atoms (12 / 13). Two H atoms located on C(1) and C(5) atoms are eliminated respectively in the reaction step 13 / 14 þ H2. The energy barrier of these reaction steps are 40.9 (2 / 12), 30.8 (12 / 13), and 95.8 kcal/mol (13 / 14 þ H2). Briefly speaking, the product 14 can be formed from structure 2 by H-transfer reactions and subsequent 2-H atoms dehydrogenation reaction.
5-Membered ring structure pathway The 5-membered ring structure (17) can be formed via the pathway C4H4 þ C4H4 / 15 / 16/17, as shown in Fig. 4. The entrance reaction in this pathway is the recombination of two C4H4 molecules involving the formation of C(4)-C(5) and C(3)C(6) bonds, which gives structure 15 with 4-membered ring
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Fig. 3 e Potential energy surface of the phenylacetylene pathway.
Fig. 4 e Potential energy surface of the 5-membered ring structure pathway.
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(33.6 kcal/mol). In the subsequent reaction 15 / 16, the C(1)C(8) bond forms and the existing C(3)-C(6) bond breaks with the energy of 38.1 kcal/mol, which results in the formation of 8-membered ring structure 16. It is notable that the C(3) and the para-position C(7) atoms are unsaturated, they will combine with each other to form the product 17 with energy barrier of 29.5 kcal/mol. In summary, it appears that the C4H4 þ C4H4 reaction system provides an interesting opportunity for the formation of the first aromatic ring in this study. The styrene (10), phenylacetylene (14) and 5-membered ring structure (17) are the stable products (102.8, 64.8, and 37.6 kcal/mol below the reagents, respectively), meaning that the whole reaction is exothermal and favored in combustion. In addition, styrene and phenylacetylene have been confirmed to play important role in PAHs evolution in flame [29,65]. However, their concentration are under-predicted by current popular soot models [25]. With the help of these new pathways obtained in C4H4 þ C4H4 reaction system, the accuracy of soot models may be better in terms of PAHs concentration.
The H-transfer reaction on 6-membered ring The H-transfer reaction plays important role in the evolution of PAHs in combustion, as it can determine the fate of the active site on PAHs surface, as well as the stability of PAHs molecule [66,67]. In this study, numerous H-transfer reactions on 6-membered ring are involved in C4H4 þ C4H4 reaction, and can be divided into five categories according to two characteristics of reactant. One is the relative position of two C atoms
(including ortho-position, meta-position and para-position), where the H-transfer reaction happens. The other is the degree of saturation of the second C atom that prepare to accept the H atom (unsaturated or saturated). Specifically, the H atom can transfer to the meta-position unsaturated C atom (2 / 11), to the para-position unsaturated C atom (6 / 10), to the ortho-position unsaturated C atom (9 / 10 and 12 / 13), to the para-position saturated C atom (2 / 12) and to the ortho-position saturated C atom (8 / 9), as shown in Fig. 5. The energy barriers of five categories H-transfer reactions are ranked in the following order: 9 / 10 (5.0 kcal/mol) < 8 / 9 (28.5 kcal/mol) < 12 / 13 (30.8 kcal/mol) < 6 / 10 (31.3 kcal/ mol) < 2 / 12 (40.9 kcal/mol) < 2 / 11 (45.7 kcal/mol). The rank indicates that the H atom is likely to transfer to the orthoposition C atoms instead of transferring to the para-position and meta-position C atoms, as the energy barriers of the latter two are higher than that of the former. As can be seen from Fig. 5, the degree of saturation of the second C atom has significant influence on the energy barriers of H-transfer reactions. The H atom transferring to the unsaturated C atoms is generally favored, and to the saturated C atoms is unsupported in terms of energy barrier. On the other hand, some H transfer reactions have similar reaction behaviors, but their energy barrier differs greatly. For example, the energy barrier of H transfer reactions 9 / 10 is lower than that of reaction 12 / 13 by 25.8 kcal/mol although both reactions are featured by the H atom transferring to the orthoposition unsaturated C atom. To explore the potential reasons for this difference, the Mulliken atomic charges of 9 and 12 molecules were investigated and shown in Fig. 6. It was
Fig. 5 e The H-transfer reactions involved in C4H4 þ C4H4 reaction, the values given above the arrow stand for the energy barrier of corresponding H-transfer reaction.
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Fig. 6 e Mulliken atomic charge of structures 9 and 12.
found that the reaction of 9 / 10 with lower energy barrier is reasonable because the unsaturated C atom (acceptor) and redundant H atom (donor) mainly interact with each other via coulomb repulsive force in 9 and 12 molecules, and the repulsive force in 9 molecule is weaker than that in 12 molecule due to the unsaturated C atom in 9 molecule having smaller positive Mulliken atomic charge.
Kinetic analysis In this study, the reaction rate coefficients at various pressures (0.1, 1, and 10 atm) were calculated based on RRKM theory with solving the master equation in the temperature range of 800e2500 K, and presented in Table 1. To evaluate the rate calculation error, the theoretically assessed rate coefficients for phenyl þ acetylene reaction were compared with experimentally measured data [11]. The calculated rate coefficients show a very good agreement with experimental values, and the maximum deviation is within 3 times at 1000 K, as shown in Table 2. The comparison results indicate that the theoretical method used in this study is reliable. The calculation details of phenyl þ acetylene reaction are provided in Tables S5eS6 in the supplementary material. The yield of three products were calculated at a wide range of temperatures (800e2500 K) and pressures (0.1, 1, and 10 atm), which are relevant to combustion environment. As shown in Fig. 7(a)e(c), the yield trend of three products can be
divided to two stages according to temperature. In the first stage, the yields of three products are almost constant when the temperature is lower than 1600 K, and product 10 is the dominant product (with yield more than 95%). In the second stage where the temperature is higher than 1600 K, the competitive advantage of product 10 decreases with temperature, whereas that of products 14 and 17 increase with temperature. For example, the yield of product 17 predicted at 0.1 atm exceeds that of product 10 at 2000 K, and the yield of product 14 exceeds that of product 10 at 2400 K, as shown in Fig. 7(a). The yield distribution of products in C4H4 þ C4H4 reaction is significant sensitive to the pressure, as shown in Fig. 7(a)e(c). The temperature window for product 10 formation in the first stage is broaden at higher pressure. For example, the temperature that the yield of product 10 drops to 95% is 1600 K at 0.1 atm, 1800 K at 1 atm, and 1900 K at 10 atm. Furthermore, the importance of product 14 is highlighted at elevated pressure. For example, the yield of product 14 at 2500 K is only 6.5% at 0.1 atm, but increases to 36.4% and 42.1% at 1 atm and 10 atm respectively. This is reasonable because the energy barrier of the 2-H dehydrogenation reaction in the formation pathway of product 14 is relative high, and a higher temperature and pressure facilitate its formation. On the other hand, the formation of product 17 is unfavorable at elevated pressure. At 0.1 atm, the product 17 is the major product in C4H4 þ C4H4 reaction with the yield of 90% at 2000 K. As pressure increases, the yield of product 17 is more close to that of other two products.
Table 1 e Rate coefficients of overall reaction in the form of ATnexp(E/RT). Reaction C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4
þ þ þ þ þ þ þ þ þ
C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4
A (cm3mol1s1) / 10 / 14 / 17 / 10 / 14 / 17 / 10 / 14 / 17
9.51 1.65 3.76 1.27 2.52 7.68 9.36 1.68 1.59
1057 107 1018 1063 106 1016 1063 1011 1015
n 13.43 1.689 1.204 14.41 2.969 0.7244 14.48 2.031 0.2245
E (kcal/ Pressure mol) (atm) 61.64 64.92 69.30 70.86 93.24 68.16 73.5 108.64 67.28
0.1 0.1 0.1 1 1 1 10 10 10
Table 2 e Comparison of rate coefficients for the phenyl þ acetylene reaction. k (cm3mol1s1)
T (K)
Theoretical rates 1000 1100 1200 1300 1400 1500
5.99 1.20 2.10 3.31 4.80 6.54
1010 1011 1011 1011 1011 1011
Experimental rates [11] 2.13 3.02 4.05 5.18 6.41 7.68
1011 1011 1011 1011 1011 1011
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Fig. 7 e Product distribution of the C4H4 þ C4H4 reaction as function of temperature, (a) 0.1 atm, (b) 1 atm, (c) 10 atm.
Conclusion In this work, the reaction pathways for C4H4 þ C4H4 reaction have been investigated using DFT method, followed by the kinetics analysis via RRKM theory with solving master equation. The corresponding H-transfer reactions on the 6membered ring have been discussed in terms of energetics. The following conclusion can be remarked. It was found that five parallel and competing pathways exist in C4H4 þ C4H4 reaction, and lead to the formation of styrene (10), phenylacetylene (14) and 5-membered ring structure (17). The forming and breaking of CeC bond is the main theme in the formation pathways of products 10 and 17. The product 14 can be formed by H-transfer and subsequent dehydrogenation reactions. The H-transfer reaction helps some intermediates evolve to more stable isomers. It was found that the H-transfer reaction is significantly dependent on the relative position of two C atoms, the degree of saturation of the second C atom, and the electronic environment of molecule. The H-transfer reaction with the H atom transferring to the ortho-position C
atom is more likely to happen, and the reaction with the H atom transferring to the meta-position C atom is kinetically unfavorable. Furthermore, the energy barrier of H-transfer reaction may be lower if the second C atom is unsaturated and has smaller positive atomic charge. The kinetic results indicate that styrene is the dominant product with yield more than 95% at low temperature (T < 1600 K) and various pressures. At higher temperature, the yield of styrene decreases with temperature, while the yields of phenylacetylene and 5-membered ring increase with temperature. The yields of three products at high temperature are sensitive to the combustion pressure. Specifically, styrene and phenylacetylene are favored by higher pressure, while 5membered ring structure is favored by lower pressure.
Acknowledgment This work was supported by National Natural Science Foundation of China (91441129, 51210010) and the National Basic Research Program of China (973 Program) (2013CB228502).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.12.038.
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The mechanism and kinetic analysis of C4H4 þ C4H4 (but-1-ene-3-yne) reaction with features of H-transfer in combustion Peng Liu, He Lin*, Zhenwu He, Yiran Zhang, Bin Guan, Zhen Huang Key Laboratory for Power Machinery and Engineering of Ministry of Education, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
article info
abstract
Article history:
Detailed mechanisms of C4H4 þ C4H4 reaction were investigated by accurate ab initio
Received 10 October 2015
density functional theory B3LYP/6-311 þ G(d,p) calculations, as well as CBS-QB3 calcula-
Received in revised form
tions. It was found that styrene, phenylacetylene and 5-membered ring structure can be
14 December 2015
formed in C4H4 þ C4H4 reaction through five parallel and competing pathways. Three
Accepted 14 December 2015
pathways are featured with H-transfer and dehydrogenation reactions, and the other two
Available online 13 January 2016
pathways are characterized with the forming and breaking of CeC bond. The H-transfer reactions play a vital role in stabilizing intermediates. It was found that the H-transfer
Keywords:
reaction is significantly dependent on the relative position of two C atoms, the degree of
Polycyclic aromatic hydrocarbons
saturation of the second C atom, and the electronic environment of molecule. Generally, an
But-1-ene-3-yne
ortho-position unsaturated C atom with relative small positive atomic charge is preferable
Density functional theory
in H-transfer reaction. The yield of products and reaction rate coefficients were evaluated
Kinetic mechanism
by using RiceRamspergerKasselMarcus theory with solving master equation at
H-transfer
different combustion temperatures and pressures (T ¼ 800e2500 K, P ¼ 0.1e10 atm). The kinetic results indicate that styrene is the dominant product and the formation of other two products can be ignored when temperature is lower than 1600 K. The formations of phenylacetylene and 5-membered ring structure are only facilitated at higher temperature, but dependent on the combustion pressure. Specifically, phenylacetylene is favored by higher pressure and higher temperature, while 5-membered ring structure is favored by lower pressure and higher temperature. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The polycyclic aromatic hydrocarbons (PAHs) generated from incomplete combustion of fossil fuel have negative influence on human health and atmospheric haze [1e5]. PAHs are highly
carcinogenic and mutagenic [6e11], and are also considered as the precursor of soot [7,12]. On the other hand, soot and carbon nano-material have been widely applied to the biomedical, catalytic, and automotive industries owing to their special physical characteristic, such as low density, high specific modulus, and antithermal shock [13e16]. In recent decades,
* Corresponding author. Tel./fax: þ86 21 34207774. E-mail address: [email protected] (H. Lin). http://dx.doi.org/10.1016/j.ijhydene.2015.12.038 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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great efforts have been made to understand PAHs evolution in combustion [14,17e23]. The PAHs formation and growth are dominantly controlled by the first aromatic ring (benzene, or its derivative) [24,25], which mainly formed by addition reactions of C4Hx species with acetylene (C2H2) [26e28], cyclization reactions of C6Hx and combination reaction of two propargyl radicals (C3H3) [29e31]. Once formed, the first aromatic ring can grow up to large PAHs based on the hydrogen abstraction C2H2 addition (HACA) mechanism. Several popular soot models such as ABF [32], USC [33], and KM2 [25] were developed based on the formation of the first aromatic ring and subsequent HACA mechanism. However these soot models need further improvement, and it has been shown that the concentrations of various PAHs are under-predicted [25]. Recently, the formation of first aromatic ring has drawn great attention in both theoretic and experimental studies [34e39]. Trogolo et al. [36] investigated the probability of the first ring formation by reaction of C3H3 with but-1-ene-3-yne (C4H4) using density functional theory (DFT). They found that 5-membered ring, 6-membered ring, 7-membered ring and open-chain products can be formed. The 5-membered ring product is the dominant product in a wide temperature and pressure range, and the yield is more than 80%. Hansen et al. [34] investigated the formation of benzene in a fuel-rich 1-hexene flame using flame-sampling molecular-beam timeof-flight mass spectrometry. They concluded that the Hassisted isomerization of fulvene contributes most (64%) to the benzene formation. However, the leading pathway in the formation of aromatic ring may vary with combustion condition, and the self-recombination reactions have been confirmed to be important. Stein et al. [38] proposed that the reaction of C3H3 þ C3H3 irreversibly generates benzene, and is the primary contributor for benzene formation in low pressure acetylene flame. Furthermore, the recombination reaction of two cyclopentadienyl radicals (cC5H5) has been verified to be the main channel for the formation of naphthalene [39]. However, the similar reaction of C4H4 þ C4H4 has not been investigated yet. In premixed ethyne [40], benzene [41], toluene [42], and gasoline [43] flames, C4H4 is abundant and the measured mole fraction is up to 2 103. In view of the importance of C3H3 þ C3H3 and cC5H5 þ cC5H5 reactions in forming aromatic ring, we would like to raise a topic that the reaction of C4H4 þ C4H4 may contribute to the formation of first aromatic ring. The objective of this study is to investigate the detailed mechanism of C4H4 þ C4H4 reaction in combustion. To this end, the potential energy surfaces (PES) of C4H4 þ C4H4 reaction are calculated using DFT method. Subsequently, the RiceRamsperger-Kassel-Marcus (RRKM) theory with solving master equation is employed to evaluate the reaction rate coefficients and the yield of products. It was found in this study that there is a large number of H-transfer reactions in C4H4 þ C4H4 reaction. Therefore, the characteristics of Htransfer reaction are discussed.
Calculation details All the transition state and local minima structures involved in C4H4 þ C4H4 reaction were fully optimized by DFT B3LYP
hybrid functional [44e46] with the 6-311 þ G(d,p) basis [46,47]. The same method was applied to calculate vibrational frequencies, which were scaled by a factor of 0.967 [48,49]. Zeropoint energies (ZPE) were calculated by the CBS-QB3 method [50]. The T1 diagnostics were performed at CCSD(T)/6311 þ G(d,p) level [51] with DFT/B3LYP/6-311 þ G(d,p) geometries to assess whether the single-determinant reference state CBS-QB3 method is suitable. A large T1 value suggests that the multireference electron correlation procedure is needed, and the threshold value of T1 is considered as 0.02 for the closedshell system [52]. The ZPE of the structures with multireference feature were recalculated at CASPT2(8,8)/B3LYP/6311 þ G(d,p) [53] level with the CASSCF(8,8)/B3LYP/6311 þ G(d,p) geometries [54]. The eight active orbitals are completely composed of the p orbitals. The intrinsic reaction coordinate (IRC) [55] calculations were carried out for every transition states to ensure that the transition states were connected with the reactant and product. The structures of all species were optimized at their ground state. All quantum chemical calculations were performed using Gaussian 09 suit of programs [56]. Based on the calculated quantum chemical parameters (including energy barriers, moments of inertia, and vibrational frequencies), the yield of products and rate coefficients of overall reaction (C4H4 þ C4H4 / product) were calculated using a combined RRKM/master equation method via MultiWell suite of codes (MultiWell-2013.1) [57,58]. Each of the three entrance channels were simulated separately, and the total yield were calculated by weight-averaging the rate coefficients of the three entrance reactions. For example, the branching ratio of k(T,P)entrance reaction 1/(k(T,P)entrance reaction 1 þ k(T,P)entrance reaction 2 þ k(T,P)entrance reaction 3), where k(T,P)entrance reaction 1 is the rate coefficients of entrance reaction 1, was used to scale the yield of products evolved from entrance reaction 1. In each RRKM/master equation simulation, the rate coefficients of product formation were the multiplier of high pressure limit rate coefficients of entrance reaction and the yield of product at steady state. Here, the rate coefficients of entrance reaction were calculated based on the rate coefficients of the unimolecular backward reaction and corresponding equilibrium constants. The rate coefficients of overall reaction were the sum of the rate coefficients of product formation in each channel (three channels in this study). The detail parameters used in MultiWell calculations were presented to facilitate the reproduction of calculation results. Specifically, the energy grain size in the first segment of the double array was set as 10 cm1, and the maximum energy was set as 100,000 cm1 in density and sum of states calculation. The exponential-down model with <△Edown> ¼ 260 cm1 was used to describe the collisional energy transfer [59]. Argon was selected as the bath gas collider in this study, and its LennardeJones parameter of s A and 114 K respectively [59]. The Lenand ε/kB is 3.47 nardeJones parameters of other structures were assumed to be equal to that of phenylacetylene with s ¼ 5.72 A and ε/kB ¼ 535.6 K [60]. The internal rotation modes with real frequencies less than 150 cm1 were distinguished by graphically visualizing the normal mode vibrations. All internal rotations were regarded as 1-D unsymmentrical hindered rotations [61], and the torsional potential energy and rotational constants
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 3 2 4 9 e3 2 5 8
were calculated through the relaxed scan with dihedral angle step of 10 at DFT/B3LYP/6-311 þ G(d,p) level. The symmetry number corrections were carried out according to the method proposed by Duncan [62,63]. Quantum tunneling corrections were also performed for all H-transfer reactions [64]. The translational and vibrational temperatures were considered to be same. The number of stochastic trials was varied from 1 104 to 1 106 to keep the statistical fluctuations within 3%.
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the T1 diagnostics for all structures involved in C4H4 þ C4H4 reaction were provided in Table S1 in supplementary material. The ZPE of the structures with multireference character were replaced with the values calculated by CASPT2(8,8)/B3LYP/6311 þ G(d,p) method. The comparisons of the CASPT2 and CSB-QB3 energies for structures with multireference character were presented in Table S2 in supplementary material.
Styrene pathway
Results and discussion Potential energy surface of the C4H4 þ C4H4 reaction The PES of C4H4 þ C4H4 reaction are composed of five parallel and competing pathways, which lead to the formation of styrene (10), phenylacetylene (14), and 5-membered ring structure (17, similar to pentalene), as shown in Fig. 1. Three pathways including C4H4 þ C4H4 / 2 / 7 / … / 10, C4H4 þ C4H4 / 2 / 11 / 14 þ H2, and C4H4 þ C4H4 / 2 / 12 / … / 14 þ H2 are characterized by numerous H-transfer reactions on the 6-membered ring. The other two pathways including C4H4 þ C4H4 / 3 / … / 10 and C4H4 þ C4H4 / 15 / … / 17 are featured by the forming and breaking of CeC bond. To make the reaction pathways clear, each product pathway was drawn respectively. In addition,
There are two pathways leading to the formation of styrene (10), C4H4 þ C4H4 / 2 / 7 / 8 / 9 / 10 and C4H4 þ C4H4 / 3 / 4 / 5 / 6 / 10, as shown in Fig. 2. In the first pathway, the entrance reaction step is given by the recombination of two C4H4 molecules (C4H4 þ C4H4 / 2), which leads to the formation of adduct 2 with 6-membered ring structure. This reaction needs to overcome the energy barrier of 27.0 kcal/mol. Subsequent, a H atom on C(6) atom transfers to adjacent C(7) atom with the energy barrier of 53.2 kcal/mol (2 / 7), which gives birth to an unsaturated C(8) atom. There are two H atoms on C(5) atom and one of them will transfer to the unsaturated C(8) atom, leading to the formation of structure 8 (7 / 8) with the energy barrier of 28.6 kcal/mol. In structure 8, the C(1) atom connects with two H atoms, but the meta-position C(3) atom is unsaturated. The more stable species 10 can be formed if one H atom on C(1) atom transfers to the meta-position unsaturated C(3) atom.
Fig. 1 e Potential energy surface of the C4H4 þ C4H4 reaction.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 3 2 4 9 e3 2 5 8
Fig. 2 e Potential energy surface of the styrene pathway.
Therefore, the direct H-transfer reaction between C(1) atom and meta-position unsaturated C(3) atom was investigated, but the corresponding transition state seems not to exist via the relaxed scan using DFT method. It was found that the H transfer from C(1) atom to the meta-position unsaturated C(3) atom needs two reaction steps, namely 8 / 9 and 9 / 10 in this study. In reaction step 8 / 9, H transfer reaction between C(1) and the ortho-position C(2) atoms happens with the energy barrier of 28.5 kcal/mol. Once formed, structure 9 will isomerize to the more stable species 10 with a low energy barrier (5.0 kcal/mol), via the other H transfer reaction between C(2) and the ortho-position C(3) atoms. Here, the facility of H transfer in reaction 9 / 10 was ascribed to the unsaturated C(3) atom. In the second pathway, the entrance reaction step is given by C4H4 þ C4H4 / 3, which involves the formation of C(3)-C(7) and C(3)-C(8) bonds with the energy barrier of 50.1 kcal/mol. The intramolecular rotation takes place in subsequent reaction step 3 / 4, which leads to the formation of structure 4 with 4-membered ring. The isomerization reaction (3 / 4) is featured by the breaking of C(3)-C(8) bond and the forming of C(4)-C(8) bond with an extremely low energy barrier (0.6 kcal/ mol). In the following reaction step 4 / 5, the C(1) atom combines with the C(7) atom with the energy barrier of 34.4 kcal/mol. The C(3)-C(7) bond in structure 5 is weak and will break when the H atom on C(1) atom transfers to the ortho-position C(7) atom (5 / 6). The product 10 forms after the H atom on C(7) atom in structure 6 transfers to the paraposition unsaturated C(3) atom (6 / 10). The energy barriers of the latter two reactions 5 / 6 and 6 / 10 are 53.7 and 31.3 kcal/mol respectively.
Phenylacetylene pathway The structure 2 can split other two pathways leading to the formation of phenylacetylene (14), namely C4H4 þ C4H4 / 2 / 11 / 14 þ H2 and C4H4 þ C4H4 / 2 / 12 / 13 / 14 þ H2, as shown in Fig. 3. In the reaction step 2 / 11, the H atom on C(1) atom in structure 2 transfers to the meta-position unsaturated C(3) atom with the energy barrier of 45.7 kcal/mol. Subsequent, two adjacent C atoms (C(5) and C(6)) in structure 11 simultaneously eliminate a H atom respectively in the reaction 11 / 14 þ H2. The energy barrier of the 2-H atoms dehydrogenation reaction is as high as 98.4 kcal/mol. In this study, the 1-H atom dehydrogenation from structure 11 is unlikely due to the high stability of structure 11. In the other pathway, the H atom on C(6) atom in structure 2 transfers to the para-position C(2) atom (2 / 12), followed by the other H-transfer reaction between C(2) and ortho-position C(3) atoms (12 / 13). Two H atoms located on C(1) and C(5) atoms are eliminated respectively in the reaction step 13 / 14 þ H2. The energy barrier of these reaction steps are 40.9 (2 / 12), 30.8 (12 / 13), and 95.8 kcal/mol (13 / 14 þ H2). Briefly speaking, the product 14 can be formed from structure 2 by H-transfer reactions and subsequent 2-H atoms dehydrogenation reaction.
5-Membered ring structure pathway The 5-membered ring structure (17) can be formed via the pathway C4H4 þ C4H4 / 15 / 16/17, as shown in Fig. 4. The entrance reaction in this pathway is the recombination of two C4H4 molecules involving the formation of C(4)-C(5) and C(3)C(6) bonds, which gives structure 15 with 4-membered ring
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 3 2 4 9 e3 2 5 8
Fig. 3 e Potential energy surface of the phenylacetylene pathway.
Fig. 4 e Potential energy surface of the 5-membered ring structure pathway.
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(33.6 kcal/mol). In the subsequent reaction 15 / 16, the C(1)C(8) bond forms and the existing C(3)-C(6) bond breaks with the energy of 38.1 kcal/mol, which results in the formation of 8-membered ring structure 16. It is notable that the C(3) and the para-position C(7) atoms are unsaturated, they will combine with each other to form the product 17 with energy barrier of 29.5 kcal/mol. In summary, it appears that the C4H4 þ C4H4 reaction system provides an interesting opportunity for the formation of the first aromatic ring in this study. The styrene (10), phenylacetylene (14) and 5-membered ring structure (17) are the stable products (102.8, 64.8, and 37.6 kcal/mol below the reagents, respectively), meaning that the whole reaction is exothermal and favored in combustion. In addition, styrene and phenylacetylene have been confirmed to play important role in PAHs evolution in flame [29,65]. However, their concentration are under-predicted by current popular soot models [25]. With the help of these new pathways obtained in C4H4 þ C4H4 reaction system, the accuracy of soot models may be better in terms of PAHs concentration.
The H-transfer reaction on 6-membered ring The H-transfer reaction plays important role in the evolution of PAHs in combustion, as it can determine the fate of the active site on PAHs surface, as well as the stability of PAHs molecule [66,67]. In this study, numerous H-transfer reactions on 6-membered ring are involved in C4H4 þ C4H4 reaction, and can be divided into five categories according to two characteristics of reactant. One is the relative position of two C atoms
(including ortho-position, meta-position and para-position), where the H-transfer reaction happens. The other is the degree of saturation of the second C atom that prepare to accept the H atom (unsaturated or saturated). Specifically, the H atom can transfer to the meta-position unsaturated C atom (2 / 11), to the para-position unsaturated C atom (6 / 10), to the ortho-position unsaturated C atom (9 / 10 and 12 / 13), to the para-position saturated C atom (2 / 12) and to the ortho-position saturated C atom (8 / 9), as shown in Fig. 5. The energy barriers of five categories H-transfer reactions are ranked in the following order: 9 / 10 (5.0 kcal/mol) < 8 / 9 (28.5 kcal/mol) < 12 / 13 (30.8 kcal/mol) < 6 / 10 (31.3 kcal/ mol) < 2 / 12 (40.9 kcal/mol) < 2 / 11 (45.7 kcal/mol). The rank indicates that the H atom is likely to transfer to the orthoposition C atoms instead of transferring to the para-position and meta-position C atoms, as the energy barriers of the latter two are higher than that of the former. As can be seen from Fig. 5, the degree of saturation of the second C atom has significant influence on the energy barriers of H-transfer reactions. The H atom transferring to the unsaturated C atoms is generally favored, and to the saturated C atoms is unsupported in terms of energy barrier. On the other hand, some H transfer reactions have similar reaction behaviors, but their energy barrier differs greatly. For example, the energy barrier of H transfer reactions 9 / 10 is lower than that of reaction 12 / 13 by 25.8 kcal/mol although both reactions are featured by the H atom transferring to the orthoposition unsaturated C atom. To explore the potential reasons for this difference, the Mulliken atomic charges of 9 and 12 molecules were investigated and shown in Fig. 6. It was
Fig. 5 e The H-transfer reactions involved in C4H4 þ C4H4 reaction, the values given above the arrow stand for the energy barrier of corresponding H-transfer reaction.
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Fig. 6 e Mulliken atomic charge of structures 9 and 12.
found that the reaction of 9 / 10 with lower energy barrier is reasonable because the unsaturated C atom (acceptor) and redundant H atom (donor) mainly interact with each other via coulomb repulsive force in 9 and 12 molecules, and the repulsive force in 9 molecule is weaker than that in 12 molecule due to the unsaturated C atom in 9 molecule having smaller positive Mulliken atomic charge.
Kinetic analysis In this study, the reaction rate coefficients at various pressures (0.1, 1, and 10 atm) were calculated based on RRKM theory with solving the master equation in the temperature range of 800e2500 K, and presented in Table 1. To evaluate the rate calculation error, the theoretically assessed rate coefficients for phenyl þ acetylene reaction were compared with experimentally measured data [11]. The calculated rate coefficients show a very good agreement with experimental values, and the maximum deviation is within 3 times at 1000 K, as shown in Table 2. The comparison results indicate that the theoretical method used in this study is reliable. The calculation details of phenyl þ acetylene reaction are provided in Tables S5eS6 in the supplementary material. The yield of three products were calculated at a wide range of temperatures (800e2500 K) and pressures (0.1, 1, and 10 atm), which are relevant to combustion environment. As shown in Fig. 7(a)e(c), the yield trend of three products can be
divided to two stages according to temperature. In the first stage, the yields of three products are almost constant when the temperature is lower than 1600 K, and product 10 is the dominant product (with yield more than 95%). In the second stage where the temperature is higher than 1600 K, the competitive advantage of product 10 decreases with temperature, whereas that of products 14 and 17 increase with temperature. For example, the yield of product 17 predicted at 0.1 atm exceeds that of product 10 at 2000 K, and the yield of product 14 exceeds that of product 10 at 2400 K, as shown in Fig. 7(a). The yield distribution of products in C4H4 þ C4H4 reaction is significant sensitive to the pressure, as shown in Fig. 7(a)e(c). The temperature window for product 10 formation in the first stage is broaden at higher pressure. For example, the temperature that the yield of product 10 drops to 95% is 1600 K at 0.1 atm, 1800 K at 1 atm, and 1900 K at 10 atm. Furthermore, the importance of product 14 is highlighted at elevated pressure. For example, the yield of product 14 at 2500 K is only 6.5% at 0.1 atm, but increases to 36.4% and 42.1% at 1 atm and 10 atm respectively. This is reasonable because the energy barrier of the 2-H dehydrogenation reaction in the formation pathway of product 14 is relative high, and a higher temperature and pressure facilitate its formation. On the other hand, the formation of product 17 is unfavorable at elevated pressure. At 0.1 atm, the product 17 is the major product in C4H4 þ C4H4 reaction with the yield of 90% at 2000 K. As pressure increases, the yield of product 17 is more close to that of other two products.
Table 1 e Rate coefficients of overall reaction in the form of ATnexp(E/RT). Reaction C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4
þ þ þ þ þ þ þ þ þ
C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4 C4H4
A (cm3mol1s1) / 10 / 14 / 17 / 10 / 14 / 17 / 10 / 14 / 17
9.51 1.65 3.76 1.27 2.52 7.68 9.36 1.68 1.59
1057 107 1018 1063 106 1016 1063 1011 1015
n 13.43 1.689 1.204 14.41 2.969 0.7244 14.48 2.031 0.2245
E (kcal/ Pressure mol) (atm) 61.64 64.92 69.30 70.86 93.24 68.16 73.5 108.64 67.28
0.1 0.1 0.1 1 1 1 10 10 10
Table 2 e Comparison of rate coefficients for the phenyl þ acetylene reaction. k (cm3mol1s1)
T (K)
Theoretical rates 1000 1100 1200 1300 1400 1500
5.99 1.20 2.10 3.31 4.80 6.54
1010 1011 1011 1011 1011 1011
Experimental rates [11] 2.13 3.02 4.05 5.18 6.41 7.68
1011 1011 1011 1011 1011 1011
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Fig. 7 e Product distribution of the C4H4 þ C4H4 reaction as function of temperature, (a) 0.1 atm, (b) 1 atm, (c) 10 atm.
Conclusion In this work, the reaction pathways for C4H4 þ C4H4 reaction have been investigated using DFT method, followed by the kinetics analysis via RRKM theory with solving master equation. The corresponding H-transfer reactions on the 6membered ring have been discussed in terms of energetics. The following conclusion can be remarked. It was found that five parallel and competing pathways exist in C4H4 þ C4H4 reaction, and lead to the formation of styrene (10), phenylacetylene (14) and 5-membered ring structure (17). The forming and breaking of CeC bond is the main theme in the formation pathways of products 10 and 17. The product 14 can be formed by H-transfer and subsequent dehydrogenation reactions. The H-transfer reaction helps some intermediates evolve to more stable isomers. It was found that the H-transfer reaction is significantly dependent on the relative position of two C atoms, the degree of saturation of the second C atom, and the electronic environment of molecule. The H-transfer reaction with the H atom transferring to the ortho-position C
atom is more likely to happen, and the reaction with the H atom transferring to the meta-position C atom is kinetically unfavorable. Furthermore, the energy barrier of H-transfer reaction may be lower if the second C atom is unsaturated and has smaller positive atomic charge. The kinetic results indicate that styrene is the dominant product with yield more than 95% at low temperature (T < 1600 K) and various pressures. At higher temperature, the yield of styrene decreases with temperature, while the yields of phenylacetylene and 5-membered ring increase with temperature. The yields of three products at high temperature are sensitive to the combustion pressure. Specifically, styrene and phenylacetylene are favored by higher pressure, while 5membered ring structure is favored by lower pressure.
Acknowledgment This work was supported by National Natural Science Foundation of China (91441129, 51210010) and the National Basic Research Program of China (973 Program) (2013CB228502).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.12.038.
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