Pyrolysis of n-Butylbenzene at Various Pressures: Influence of Long

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Pyrolysis of n‑Butylbenzene at Various Pressures: Influence of Long Side-Chain Structure on Alkylbenzene Pyrolysis Yan Zhang,† Chuangchuang Cao,† Yuyang Li,*,‡,§ Wenhao Yuan,§ Xiaoyuan Yang,§ Jiuzhong Yang,*,† Fei Qi,‡,§ Tzu-Ping Huang,∥ and Yin-Yu Lee∥ †

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China ‡ Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration (CISSE), Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China § Key Laboratory for Power Machinery and Engineering of MOE, Shanghai Jiao Tong University (SJTU), Shanghai 200240, People’s Republic of China ∥ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan S Supporting Information *

ABSTRACT: This work investigates the pyrolysis of n-butylbenzene, which widely exists in transportation fuels and their surrogate mixtures. Both reactive and stable pyrolysis products were comprehensively detected with synchrotron vacuum ultraviolet photoionization mass spectrometry. Their mole fractions versus temperature were also evaluated at 30, 150, and 760 Torr. A kinetic model of n-butylbenzene pyrolysis was developed, and new data were used to validate the model. On the basis of the modeling analysis, the benzylic C−C bond dissociation that forms the benzyl radical and the propyl radical was found to be a key decomposition reaction of n-butylbenzene at all investigated pressures, whereas H abstraction provided increasing contributions with increasing pressure. Compared with small alkylbenzenes, such as toluene and ethylbenzene, n-butylbenzene demonstrates different pyrolysis characteristics and chemistry because of the existence of its long alkyl side chain. n-Butylbenzene has a higher pyrolysis reactivity and lower decomposition temperature regions, which inhibit the further decomposition of the benzyl radical and the formation of highly unsaturated C2−C4 products. As a result, conventional combination reactions between aromatic radicals and highly unsaturated C2−C4 species are only minor formation pathways for indene and naphthalene in nbutylbenzene pyrolysis, while fuel-specific pathways become crucial instead. Furthermore, combination reactions involving the benzyl radical and the phenyl radical are crucial for the formation of many PAHs, especially phenanthrene and fluorene. The results in this work reveal the strong influence of side-chain length on the pyrolysis chemistry of alkylbenzenes and indicate a further need for exploring the influences of other structural features.

1. INTRODUCTION Aromatic hydrocarbons constitute one of the major component families in practical transportation fuels.1,2 Aromatics are widely used as key components in gasoline,2 kerosene,3 and diesel oil4 surrogates. The combustion of aromatics is also recognized for the abundant production of polycyclic aromatic hydrocarbons (PAHs) and soot.5,6 These major air pollutants are extremely harmful to human health and environmental security.5,7 In Figure 1, n-butylbenzene is a typical alkylbenzene with a longer side-chain length than that of small alkylbenzenes such as toluene and ethylbenzene. As a result, n-butylbenzene has been

selected by many researchers as a representative aromatic component in jet fuels3,8 and diesel oils.4 In addition, nbutylbenzene is also recognized as a model fuel for large alkylbenzenes, such as n-pentylbenzene, n-hexylbenzene, etc.9 Many previous experimental studies have been carried out on n-butylbenzene combustion and were mainly focused on the oxidation chemistry of n-butylbenzene. In speciation work, Roubaud and co-workers11 measured low-temperature oxidation products of n-butylbenzene in a rapid compression machine (RCM) at pressures above 14 bar using gas chromatography (GC). Pousse and co-workers12 measured the stable species in a premixed methane/n-butylbenzene flame at 6.7 kPa using GC. Diévart and Dagaut13 measured the jetstirred reactor (JSR) oxidation products of n-butylbenzene at 10 atm using GC. Husson and co-workers14 also used GC to investigate the JSR oxidation of n-butylbenzene at temperatures from 550 to 1100 K, atmospheric pressure, and various equivalence ratios. Yuan and co-workers15 measured the Received: September 21, 2017 Revised: November 9, 2017 Published: November 21, 2017

Figure 1. Structure and bond dissociation energies (BDEs) of nbutylbenzene (unit: kJ/mol).10 © 2017 American Chemical Society

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DOI: 10.1021/acs.energyfuels.7b02855 Energy Fuels 2017, 31, 14270−14279

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Energy & Fuels intermediates of premixed n-butylbenzene flames at 4.0 kPa using synchrotron VUV photoionization mass spectrometry (SVUV-PIMS). In global combustion parameter measurements, Roubaud and co-workers11 recorded the ignition delay times (IDT) at 14 bar using RCM for n-butylbenzene. Ribaucour and co-workers16 recorded the IDTs of n-butylbenzene between 640 and 840 K using RCM. Nakamura and co-workers9 obtained the trends in IDT for n-butylbenzene over a wide temperature range using both shock tube and RCM at various equivalence ratios. Husson and co-workers14 also recorded the IDTs over wide conditions for n-butylbenzene. Comandini and co-workers17 and Mehl and co-workers18 recorded the laminar burning velocities at 1 atm for n-butylbenzene. On the basis of experimental achievements, many kinetic models were constructed for n-butylbenzene oxidation.9,12−16,18,19 Pyrolysis of hydrocarbons has a variety of practical applications in energy utilization and conversion, such as the utilization of fossil fuels,20 active cooling of scramjet engine,21−23 production of useful chemicals, 24,25 waste disposal,26 and material synthesis.25,27 On the other hand, pyrolysis of hydrocarbons not only provides specific validation on pyrolytic reactions in combustion kinetic models28,29 but also furthers the understanding of the crucial PAH chemistry in the soot formation mechanism that mainly occurs under pyrolytic conditions.30,31 As reviewed by Moldoveanu,25 investigations of the pyrolysis of aromatics is very limited compared with those of alkanes, especially for long side-chain alkylbenzenes. Taking n-butylbenzene as an example, only Yu and co-workers32 have studied its pyrolysis under near-critical and supercritical conditions to date. However, the GC method used in their work could only provide information on stable products, whereas no kinetic model was reported for nbutylbenzene pyrolysis. As a result, the pyrolysis chemistry of long side-chain alkylbenzenes is insufficiently understood.25 In this work, the pyrolysis of n-butylbenzene was investigated at 30, 150, and 760 Torr with SVUV-PIMS. The pyrolysis products, especially radicals, isomers, and PAHs, were identified, and their mole fractions were also evaluated. A detailed kinetic model of n-butylbenzene pyrolysis was developed and validated against new experimental data. Key pathways in n-butylbenzene decomposition and PAH formation were discussed, which revealed differences between the pyrolysis of n-butylbenzene and small alkylbenzenes.

Figure 2. (a) Schematic diagram of the flow reactor pyrolysis apparatus and (b) measured centerline temperature profiles.

Table 1. Experimental Conditions P/Torr

T/K

total flow rate/sccm

fuel %

Ar %

30 150 760

985−1226 938−1153 839−1060

1000 1000 1000

1 1 1

99 99 99

treated as a constant. n-Butylbenzene (99%) was provided by Shanghai Macklin Biochemical Co., Ltd. After vaporization, the n-butylbenzene/ argon mixture was supplied to the reactor at all three pressures. The pyrolysis species were sampled by a quartz nozzle and eventually ionized by the synchrotron light. The ions were detected by a reflectron time-of-flight mass spectrometer. For intermediate identification and mole fraction evaluation, the methods have been introduced in detail.35 The uncertainties were within ±25% for species with known photoionization cross sections (PICSs) and a factor of 2 for species with estimated PICSs, while the PICS data can be found in our online database.38

3. KINETIC MODELING The present kinetic model of n-butylbenzene (A1C4H9) pyrolysis was developed from our recent kinetic model for the n-butylbenzene flames15 with the pyrolytic reactions updated and the oxidation reactions removed. For unimolecular C−C bond dissociation of n-butylbenzene, the calculated rate coefficients from Husson and co-workers14 were adopted. For unimolecular C−H bond dissociation of n-butylbenzene, rate coefficients were taken from analogous reactions of ethylbenzene (A1C2H5).39 For H abstraction by H and CH3, rate coefficients were acquired from analogous reactions of npropylbenzene (A1C3H7).40 For H abstraction by C2H5, rate coefficients were taken from Husson and co-workers.14 Ipsosubstitution reactions on n-butylbenzene by H/CH3 to form benzene (A1)/toluene (A1CH3) and the 1-butyl (pC4H9) radical were incorporated, while the rate coefficients were referred to similar reactions of ethylbenzene. For decomposition reactions of phenylbutyl radicals, the recommended rate coefficients by Tsang41 were adopted. Reactions of PAH formation were taken from our previously proposed PAH

2. EXPERIMENTAL METHOD The experiments were carried out at the National Synchrotron Radiation Laboratory (NSRL) and National Synchrotron Radiation Research Center (NSRRC). Detailed descriptions of the beamlines can be found in our previous work,33,34 and brief descriptions are provided in the Supporting Information. The flow reactor pyrolysis apparatus has been updated compared to our previous pyrolysis apparatus.31,35,36 Figure 2a shows the schematic diagram. Detailed descriptions of the pyrolysis apparatus are presented in the Supporting Information, while only the improvements are described here. To achieve a broader hightemperature region, the length of the heating wire was extended from 150 to 224 mm, and the density of the heating wire was doubled at the first-quarter and last-eighth lengths instead of the previously uniform density. These changes are the major updates from the previous apparatus. The centerline temperature profiles measured with the same method in our previous studies35,37 are illustrated in Figure 2b. In this work, experiments were carried out at 30, 150, and 760 Torr, and the experimental conditions are shown in Table 1. On the basis of our previous work,30,35 the differences of axial pressure between the inlet and outlet of the reactor were found to be quite small (less than 10%) at 30 Torr and higher pressures; thus, the pressure can be 14271


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Figure 3. Results for n-butylbenzene and the MAH species at three pressures. Symbols and lines denote measurements and simulations, respectively (as in Figures 4, 5, and 10).

Figure 4. Results for the C3−C7 species at three pressures.

formation mechanisms,31,39,42,43 while some key reactions for the formation of indene and naphthalene were updated with the pressure-dependent rate constants calculated by Mebel and co-workers.44,45 In addition, the present model also incorporated fuel-specific reactions for PAH formation. For species involved in the fuel submechanism, thermodynamic and transport data were mainly referred from the models of Nakamura9 and Pousse.12 The species and reaction numbers are 182 and 877, respectively. The reaction mechanism and the thermodynamic data are provided in the Supporting

Information. The simulation was performed with the measured temperature profiles as input parameters using Chemkin-PRO software.46

4. RESULTS AND DISCUSSION In this work, several radicals, isomers, monocyclic aromatic hydrocarbons (MAHs), and PAHs were identified. Rate of production (ROP) and sensitivity analyses were carried out at 30 and 760 Torr. On the basis of the modeling analyses, the nbutylbenzene consumption and PAH formation are discussed 14272


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Energy & Fuels to reveal the key pathways in both processes. Table S1 in the Supporting Information lists the species mentioned here. 4.1. Decomposition of n-Butylbenzene. Figure 3 shows the results for n-butylbenzene and the MAH species, including phenylbutenes (A1C4H7), n-propylbenzene, ethylbenzene, styrene (A1C2H3), phenylacetylene (A1C2H), toluene, the benzyl radical (A1CH2), and benzene, at three pressures. Figures 4 and 5 present the results for the smaller species. As

Figure 7. Sensitivity analysis of n-butylbenzene at 1180 K, 30 Torr and 1040 K, 760 Torr.

butylbenzene decomposes at approximately 300 K earlier than toluene and 80 K earlier than ethylbenzene under similar pyrolytic conditions. The ROP analysis in Figure 6 demonstrates that n-butylbenzene decomposes principally via unimolecular decomposition and radical attack. In the pyrolysis of hydrocarbons, the chain reaction systems were concluded to be initiated by the unimolecular decomposition reactions.19,47 For n-butylbenzene, its decomposition is dominantly initiated from the benzylic C−C bond dissociation reaction (R1) that produces the benzyl radical and the n-propyl (nC3H7) radical, because this bond is the weakest (320 kJ/mol10) in the nbutylbenzene molecule. R1 consumes 88% of n-butylbenzene at 30 Torr and 53% at 760 Torr. The sensitivity analysis of nbutylbenzene in Figure 7 also demonstrates that R1 is very important for the consumption of n-butylbenzene.

Figure 5. Results for the C1−C2 species and argon (Ar) at three pressures.

seen in Figures 3−5, the present model can reproduce the fuel consumption and the formation of most decomposition products. The ROP and sensitivity analyses for fuel decomposition were performed at 1180 K, 30 Torr and 1040 K, 760 Torr, in which approximately 75% of the fuel was consumed and most of the decomposition products were abundantly produced. The reaction networks in n-butylbenzene decomposition at both pressures are exhibited in Figure 6. Furthermore, Figure 7 shows the sensitivity analysis of nbutylbenzene at 30 and 760 Torr. Compared to toluene31 and ethylbenzene39 pyrolysis, nbutylbenzene pyrolysis shows a higher reactivity; that is, n-

A1C4 H 9 = A1CH 2 + nC3H 7

(R1)

Among the two products of R1, the benzyl radical is resonantly stabilized and consequently has high concentrations among the radicals, whereas the n-propyl radical can easily decompose to ethylene (C2H4) and the CH3 radical by β-C−C scission reactions at both 30 and 760 Torr, which leads to the large production of the CH3 radical. As a result, the combination reaction between the benzyl radical and the CH3 radical (R2) produces ethylbenzene with high concentrations at both pressures. R2 is the dominant consumption

Figure 6. Reaction network in n-butylbenzene decomposition at 1180 K, 30 Torr and 1040 K, 760 Torr. The black and red colors denote reaction fluxes at 30 and 760 Torr, respectively. 14273


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butylbenzene since R2 can terminate radicals and compete with R4 and R5 in the consumption of the CH3 radical. In addition to H abstraction, the ipso-substitution of nbutylbenzene is another radical attack reaction class. The most significant ipso-substitution reaction is the one by H that forms benzene and the n-butyl radical (R6). Its contributions to the consumption of n-butylbenzene are approximately 1% and 5% at 30 and 760 Torr.

reaction of the benzyl radical and its contributions are 71% at 30 Torr and 47% at 760 Torr. The ROP analysis results are also consistent with the experimental observation that the concentration of ethylbenzene at 30 Torr is the highest (Figure 3d). In addition, the benzyl radical also reacts with H and C2H5 to produce toluene and n-propylbenzene. Similar to ethylbenzene pyrolysis,39 the unimolecular decomposition reactions of the benzyl radical producing fulvenallene (C7H6) and cyclopentadienyl radical (C5H5) contribute only negligibly to the benzyl consumption, which is different from toluene pyrolysis31 under similar conditions. Figure 4a,b displays the results for fulvenallene and the cyclopentadienyl radical at 30 Torr, whereas, at 150 and 760 Torr, their concentrations become too low to be detected. Compared with toluene, nbutylbenzene and ethylbenzene have longer alkyl side chains and a benzyl C−C bond and, consequently, have enhanced pyrolysis reactivities and relatively low decomposition temperature regions (see Figure 8). This is the main reason for the

(R3)

A1C4 H 9 + CH3 = A1C4 H8‐D + CH4

(R4)

A1C4 H 9 + CH3 = A1C4 H8‐A + CH4

(R5)

A1C4 H 9 + H = A1 + pC4 H 9

(R6)

The decomposition of phenylbutyl radicals mainly proceeds via β-C−C scission reactions (R7−R10). The β-C−H scission reactions that form phenylbutene isomers contribute less than 7% in total to the consumption of four types of phenylbutyl radicals, while 1-phenyl-2-butene (A1C4H7-2) is the most favored product. In addition, the 4-phenyl-1-butyl (A1C4H8-A) and 1-phenyl-1-butyl radicals can also undergo an isomerization reaction (R11) between each other. At 760 Torr, R11 consumes 87% of the 4-phenyl-1-butyl radical, while, at 30 Torr, R11 turns to the reverse direction and consumes approximately 17% of the 1-phenyl-1-butyl radical.

Figure 8. Simulated mole fraction profiles of toluene (A1CH3), ethylbenzene (A1C2H5), and n-butylbenzene (A1C4H9) at 30 Torr using the toluene model,31 ethylbenzene model,39 and present model. The temperature profiles measured in this work were used in the simulations of all three alkylbenzenes.

A1C4 H8‐D = A1C2 H3 + C2H5

(R7)

A1C4 H8‐C = A1CH 2 CHCH 2 + CH3

(R8)

A1C4 H8‐B = A1CH 2 + C3H6

(R9)

A1CH 2 CH 2 + C2H4 = A1C4 H8‐A

(R10)

A1C4 H8‐A = A1C4 H8‐D

(R11)

Among the MAH products, ethylbenzene and styrene have the highest concentrations. As mentioned above, ethylbenzene formation is dominated by the combination of the benzyl radical and the CH3 radical. The formation of styrene in nbutylbenzene pyrolysis is more complex than that of ethylbenzene. In the pyrolysis of small alkylbenzenes such as toluene31 and ethylbenzene,39 styrene formation is dominated by stepwise H-loss from ethylbenzene. If styrene is still mainly formed from this pathway in n-butylbenzene pyrolysis, the formation of styrene would occur later than that of ethylbenzene. However, as observed from Figure 3, styrene is formed approximately 50 K earlier than ethylbenzene at 760 Torr, which implies additional main formation pathways of styrene. The ROP analysis indicates the β-C−C scission of the 1-phenyl-1-butyl radical (R7) contributes approximately 65% at 30 Torr and more than 70% at 760 Torr to the formation of styrene, which explains the early production of styrene. In addition, the β-C−H scission of the 2-phenylethyl (A1CH2CH2) radical can also produce styrene. The latter mainly comes from the β-C−C scission of the 4-phenyl-1-butyl radical. However, the dominant decomposition reaction of the 2-phenylethyl radical is β-C−C scission to produce the phenyl (A1−) radical and ethylene. As a result, the reaction sequence A1C4H8-A → A1CH2CH2 → A1C2H3 contributes only less than 10% to styrene formation. Figure 9 presents sensitivity analysis results for styrene, which also verifies the importance of the 1-phenyl-1-butyl radical to the formation of styrene. R4 has the maximum

phenomenon of fulvenallene and the cyclopentadienyl radical since the unimolecular decomposition reactions of the benzyl radical must overcome fairly high energy barriers.48,49 Instead of the decomposition reactions, the self-combination and combination with small species control the consumption of the benzyl radical. A1C2 H5 = A1CH 2 + CH3

A1C4 H 9 + H = A1C4 H8‐D + H 2

(R2)

H abstraction of n-butylbenzene by H and CH3 is another key reaction class, which mainly leads to four phenylbutyl radicals (e.g., R3−R5). These reactions contribute approximately 11% and 45% together to the decomposition of nbutylbenzene at 30 and 760 Torr, respectively. Among them, the 1-phenyl-1-butyl (A1C4H8-D) radical is the dominant product. According to the ROP analysis, the reactions to produce the 1-phenyl-1-butyl radical (e.g., R3 and R4) contribute approximately 5% and 18% of n-butylbenzene consumption at 30 and 760 Torr, respectively. The sensitivity analysis of n-butylbenzene in Figure 7 also indicates that H abstraction reactions are crucial for n-butylbenzene decomposition, especially for the 760 Torr case. Among the radicals, CH3 is most important for H abstraction due to its great production from the decomposition of the n-propyl radical. This also explains the positive sensitivity of R2 for n14274


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negative sensitivity coefficients due to the crucial role of the CH3 radical in the H abstraction of n-butylbenzene. The dominant consumption reactions of styrene are close to those in the previous studies39,50 and thus not repeated herein. Allylbenzene (A1CH2CHCH2) is also an important secondary decomposition product of n-butylbenzene, which is mainly produced from the β-C−C scission reaction of the 1-phenyl-2butyl (A1C4H8-C) radical at both pressures. Its further decomposition products are the indanyl (C9H9-1) radical and the phenylallyl (A1CHCHCH2-3) radical, which are both important precursors of indene (C9H8). The decomposition of allylbenzene can also produce benzene and the allyl radical (aC3H5) by H attack. Furthermore, the phenylbutene isomers that are produced from the β-C−H scission of phenylbutyl radicals mainly undergo stepwise H-loss reactions to produce phenylbutadiene (A1C4H5), whereas almost all phenylbutadiene is converted to the 1-methyleneindan-2-yl radical (C9H7CH2). 1-Phenyl-1-butene (A1C4H7-3) can also undergo the allylic C−C bond dissociation reaction to produce the phenylallyl radical and the CH3 radical. Figures 4 and 5 also present the results of C1−C4 products at three pressures. Compared with toluene31 and ethylbenzene39 pyrolysis, C2−C4 alkenes are produced at high concentrations in n-butylbenzene pyrolysis. The maximum measured mole fractions of ethylene are approximately 1 × 10−2 in this work (1% n-butylbenzene/99% Ar), and the maxima are approximately 1 × 10−3 and 2 × 10−3 in toluene31 (5% toluene/95% Ar) and ethylbenzene39 (1% ethylbenzene/99% Ar) pyrolysis, respectively. Furthermore, C3−C4 alkenes were not detected in toluene31 and ethylbenzene39 pyrolysis. The ROP analysis demonstrates that the formation of C2−C4 alkenes in this work is dominated by the decay of the alkyl side chain, which reveals the strong influence of the long alkyl side chain on the product pool of aromatic fuels. In contrast to the abundant production of C2−C4 alkenes, a smaller quantity of highly unsaturated C2−C4 products are

Figure 9. Sensitivity analysis of styrene at 1040 K, 760 Torr and 1180 K, 30 Torr.

positive sensitivity coefficients at both 30 and 760 Torr because it is the dominant formation reaction of the 1-phenyl-1-butyl radical. However, the decomposition reaction of the 4-phenyl1-butyl radical to form the 2-phenylethyl radical and ethylene (R10) has negative sensitivity coefficients for the formation of styrene, especially at 30 Torr. This phenomenon results from the influence of R11 on the 1-phenyl-1-butyl radical that is the dominant precursor of styrene. At 760 Torr, R11 can convert most of the 4-phenyl-1-butyl radical to 1-phenyl-1-butyl radical to promote the formation of styrene. Meanwhile, R10 competes with the consumption of the 4-phenyl-1-butyl radical and consequently has a negative sensitivity coefficient to the formation of styrene. At 30 Torr, the reverse reaction of R11 converts the 1-phenyl-1-butyl radical to the 4-phenyl-1-butyl radical, which enhances the inhibition effect of R10 to the formation of styrene. In addition, the reactions that consume the CH3 radical to form ethane and ethylbenzene also have

Figure 10. Results for major PAHs at three pressures. 14275


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Figure 11. Reaction network of PAH formation at 1060 K and 760 Torr. H-abs and Iso denote H atom abstraction and isomerization reaction, respectively.

models of Husson,14 Mehl,18 and this work are compared in Figure S2 in the Supporting Information. Figure 10a displays the results for indene, which has a fused benzene ring and cyclopentadiene ring structure. Indene is generally considered one of the most important bicyclic PAHs in combustion. Many possible formation pathways, such as C7+C2 and C6+C3, were proposed in previous studies51−54 and play important roles in indene formation in toluene31 and ethylbenzene39 pyrolysis. However, key formation pathways of indene in n-butylbenzene pyrolysis are different and are strongly influenced by the fuel structure and different distributions of the PAH precursors. Due to the lack of highly unsaturated C2 and C3 species, the conventional C7+C2 and C6+C3 pathways have minor contributions to indene formation in this work. Instead, the fuel-specific pathways through the indanyl radical and the phenylallyl radical, which are mentioned in last section, dominate the formation of indene. These pathways can be summarized as A1C4H9 → A1C4H8-C → A1CH2CHCH2 → C9H9-1 (or A1CHCHCH2-3) → C9H8. At 760 Torr, the H-loss reaction of the indanyl radical (R12) contributes over 70% to the formation of indene, and the cyclization of the phenylallyl radical (R13) contributes approximately 15%. However, at 30 Torr, the formation of indene is almost totally controlled by R12, and the contribution of R13 is negligible (see Figure S1).

produced in n-butylbenzene pyrolysis compared with the pyrolysis of small alkylbenzenes. For example, the maximum mole fractions of acetylene was less than 1 × 10−4 in this work, whereas the maxima were observed to be more than 5 × 10−3 and 5 × 10−4 in toluene31 and ethylbenzene39 pyrolysis, respectively. Furthermore, the propargyl radical was detected in the pyrolysis of toluene31 and ethylbenzene39 but undetected in this work. These phenomena result from the lower pyrolysis temperatures of n-butylbenzene than those of the small alkylbenzenes. These highly unsaturated C2−C4 products are generally produced from the unimolecular decomposition reactions of large radicals such as the benzyl radical and the fulvenallenyl (C7H5) radical in toluene31 and ethylbenzene39 pyrolysis. However, since the pyrolysis of n-butylbenzene proceeds at relatively low temperatures compared with small alkylbenzenes, the decomposition pathways of these large radicals are dramatically suppressed, which result in the weak production of these highly unsaturated C2−C4 products. 4.2. Formation of PAHs. Figure 10 presents the results for PAHs. The ROP analysis was performed at 1200 K, 30 Torr and 1060 K, 760 Torr, in which almost all PAHs reached relatively high concentrations. Furthermore, Figure 11 shows the main formation pathways of indene, naphthalene (A2), biphenyl (P2), fluorene (C13H10), diphenylmethane (C13H12), and phenanthrene (A3) based on the ROP analysis at 760 Torr, since the formation of PAHs is more intense at this pressure than at 30 Torr. The reaction network of indene and naphthalene formation at 30 Torr has several differences compared to those at 760 Torr (Figure S1 in the Supporting Information). Furthermore, simulations of several PAHs using

C9H 9‐1 = C9H8 + H

(R12)

A1CHCHCH 2 ‐3 = C9H8 + H

(R13)

As seen in Figure 10a, the concentration of indene is quite low at 30 Torr, which is different from the pyrolysis of small alkylbenzenes.31,39 According to the discussion above, it is 14276


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(C14H12), which explains the low concentration of stilbene at 30 Torr. The subsequent stepwise H-loss reaction sequence of stilbene is the dominant formation pathway of phenanthrene at both pressures. As a result, the deficient production of stilbene at 30 Torr inhibits the formation of phenanthrene at the same pressure. In addition to the self-combination, the benzyl radical can also undergo the combination reaction with the phenyl radical, which dominates diphenylmethane formation. The stepwise H-loss reaction sequence of diphenylmethane controls the formation of fluorene. Furthermore, the phenyl radical plays a significant role in biphenyl formation via the combination with benzene at both pressures, while its self-combination reaction also provides a minor contribution at 30 Torr.

obvious that the production of the 1-phenyl-2-butyl radical and allylbenzene plays a significant role in indene formation. The ROP analysis indicates that the formation of the 1-phenyl-2butyl radical consumes nearly 6% of n-butylbenzene at 760 Torr, while its formation only consumes approximately 1% of n-butylbenzene at 30 Torr. The weak production of the 1phenyl-2-butyl radical at 30 Torr results in the negligible formation of allylbenzene and consequently a much lower concentration of indene at 30 Torr. For the decomposition of indene, the main pathway at both pressures is the H abstraction of indene that produces the indenyl (C9H7) radical (R14 and R15). The combination reaction between the indenyl radical and the CH3 radical (R16) contributes almost all to the production of 1-methylindene (C9H7CH3), which is mainly decomposed to the C9H7CH2 radical. C 9H8 + H = C 9H 7 + H 2

(R14)

C9H8 + CH3 = C9H 7 + CH4

(R15)

C9H 7 + CH3 = C9H 7CH3

(R16)

C14 H14 = A1CH 2 + A1CH 2

(R17)

5. CONCLUSIONS Flow reactor pyrolysis of n-butylbenzene was investigated at 30, 150, and 760 Torr in this work. The pyrolysis products were detected and quantified using SVUV-PIMS. A pyrolysis model with 182 species and 877 reactions was developed and validated against these new data. On the basis of the modeling analyses, the benzylic C−C bond dissociation forming the benzyl radical and the n-propyl radical was revealed to be the dominant decomposition reaction at the investigated pressures. The H abstraction of n-butylbenzene exhibited increasing contributions with increasing pressure. Due to the existence of the long alkyl side-chain structure, the pyrolysis reactivity of nbutylbenzene was much higher than that of small alkylbenzenes such as toluene and ethylbenzene. Therefore, the decomposition temperature regions of n-butylbenzene became much lower under similar conditions. The lower temperature inhibited the further decomposition of the benzyl radical, especially under the 150 and 760 Torr conditions, and the formation of highly unsaturated C2−C4 products was also suppressed. As a result, conventional combination reactions between aromatic radicals and highly unsaturated C2−C4 species provided limited contributions to indene and naphthalene formation in n-butylbenzene pyrolysis. Instead, aromatics with unsaturated side chains, such as phenylbutenes and phenylbutadiene, became crucial for indene and naphthalene formation. Furthermore, the combination reactions involving the benzyl radical and the phenyl radical were crucial for the formation of PAHs such as bibenzyl, stilbene, phenanthrene, diphenylmethane, fluorene, and biphenyl. The findings of this work revealed the strong influence of side-chain length on the pyrolysis chemistry of alkylbenzenes. In the future, more effort is needed to understand the influences of other structural features regarding the pyrolysis chemistry of MAHs, such as branched, multisubstituted, unsaturated, and oxygenated side-chain structures.

Figure 10c shows the results for naphthalene. Many pathways can produce naphthalene in n-butylbenzene pyrolysis. The ROP analysis shows that, at both 30 and 760 Torr, naphthalene is mainly produced from the decomposition of the C9H7CH2 radical directly or via methyleneindene (C9H6CH2). In addition, the H-loss of the 1,2-dihydronaphthalen-4-yl radical (H2A2-2) provides a minor contribution to naphthalene production. Compared with toluene31 and ethylbenzene39 pyrolysis, the reaction between the benzyl radical and the propargyl radical and the reaction between the 2-phenylvinyl (A1CHCH) radical and acetylene only provide minor contributions to naphthalene formation due to the insufficient production of these highly unsaturated C2 and C3 species. Similar to indene, the concentration of naphthalene at 30 Torr is also negligible. The reason is that the deficient formation of indene at 30 Torr inhibits the dominant source of the 1methyleneindan-2-yl radical, i.e., C9H8 → C9H7 → C9H7CH3 → C9H7CH2. Instead, the 1-methyleneindan-2-yl radical is almost totally produced from the phenylbutadiene decomposition pathway at 30 Torr (see Figure S1). However, the carbon flux of the latter pathway is much smaller than that of the former, which leads to the suppressed formation of naphthalene at 30 Torr. According to above discussion, aromatics with long unsaturated side chains such as phenylalkenes and phenyldialkenes are significant for indene and naphthalene formation in n-butylbenzene pyrolysis. The relatively low pyrolysis temperatures of n-butylbenzene compared with those of the small alkylbenzenes not only reduce the formation of highly unsaturated C2−C4 products and limit the conventional combination pathways for indene and naphthalene formation but also promote combination reactions between aromatic radicals. For example, the self-combination of the benzyl radical that forms bibenzyl (C14H14) (R17) contributes approximately 35% to the consumption of the benzyl radical at 1060 K and 760 Torr. Approximately 90% of bibenzyl undergoes H abstraction to produce the 1,2-diphenylethyl (C14H13) radical at both pressures. The dominant decomposition pathway of the 1,2-diphenylethyl radical is to produce styrene and the phenyl radical, which contribute approximately 95% and 75% to its consumption at 30 and 760 Torr, respectively. The remaining 1,2-diphenylethyl radicals mainly decompose to stilbene

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02855. Beamlines; pyrolysis apparatus; list of C5 and larger species; main formation pathways of indene and naphthalene; and comparison of simulated mole fraction profiles of indene, naphthalene, stilbene, and dibenzyl (PDF) 14277


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Reaction mechanism (TXT) Thermodynamic data (TXT)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-21-34204115 (Y.L.). *E-mail: [email protected]. Tel: +86-551-63602073 (J.Y.). ORCID

Yuyang Li: 0000-0002-0900-9234 Wenhao Yuan: 0000-0002-1102-9752 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (91641205, 51622605, U1432130, 51476155), Shanghai Science and Technology Committee (No. 17XD1402000), China Postdoctoral Science Foundation (2016M600312), and National Postdoctoral Program for Innovative Talents (BX201600100). The authors appreciate Ms. Meirong Zeng, Mr. Tianyu Li, Ms. Wei Li, and Mr. Jiabiao Zou for their technical assistance.

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Pyrolysis of n-Butylbenzene at Various Pressures: Influence of Long

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