Thermal Decomposition and Kinetics of a High-Energy-Density

Jan 5, 2016 - Key Laboratory for Green Chemical Technology of Ministry of ... of C5 species may form naphthalene and 2,3-dihydro-4-methyl-1H-indene...
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Thermal Decomposition and Kinetics of a High-Energy-Density Hydrocarbon Fuel: Tetrahydrotricyclopentadiene (THTCPD) Baoman Guo, Ya Wang, Li Wang, Xiangwen Zhang, and Guozhu Liu* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: Thermal decomposition of tetrahydrotricyclopentadiene (THTCPD, C15H22), a high-energy-density hydrocarbon fuel, was conducted in a batch reactor at 385−425 °C to investigate its kinetics and decomposition products. The reaction activation energy and pre-exponential coefficient were established as 248.5 kJ mol−1 and 1.5 × 1015 s−1, respectively. The detailed analysis of the decomposition products indicated that THTCPD was first cracked into ethylene, C5 (1,3-cyclopentadiene, cyclopentene, and cyclopentane), benzene, and C10 (JP-10 and its isomers) and then to form secondary products. The possible primary mechanism was that the cleavage of the C−C bond of THTCPD produced diradicals, which were further converted into monoradicals through intermolecular hydrogen abstraction, and then the monoradicals generated primary products through βscission, isomerization, and intermolecular hydrogen abstraction reactions. Possible secondary decomposition of primary products (C10 and C5 species) may form small molecules (C1−C4 species, methyl- and ethyl-cyclopentane, etc.), while some bimolecular reactions of C5 species may form naphthalene and 2,3-dihydro-4-methyl-1H-indene. This study may provide possible fundamental experimental information and kinetics for the potential application of THTCPD fuel. viscosity of THTCPD is 35.3 mPa s (at 20 °C) and drops to 19.1 mPa s after blending with 20% (in volume) JP-10 with the density above 1.0 g/cm3.17 Thermal pyrolysis is another effective method when the fuel was used as the coolant for the thermal management system.2,18−20 For instance, it was reported that the dynamic viscosities of RP-1 and RP-2 were reduced by 12.1−39.3 and 15.0−40.6% after different degrees of thermal stressing, repectively.21 In addition, the small molecular products generated from the pyrolysis process are beneficial for the ignition and combustion in the engine.22,23 Until now, few studies have been reported on the decomposition of THTCPD. Du et al. 24 synthesized THTCPD-X via the Diels−Alder addition of the DCPD and cyclopentadiene, and then tricyclopentadiene (TCPD) was hydrogenated. This THTCPD-X with a freezing point of 48− 49 °C could not be used directly as a HEDHF in the advanced aircrafts. Thermal pyrolysis of THTCPD-X was further investigated at 700−880 °C for 3−10 s at 0.1 MPa using a pyrolysis−gas chromatography/mass spectrometry. Major products were ethylene, propylene, cyclopentene, cyclopentadiene, benzene, and toluene. According to the experimental results and free radical mechanism, the primary mechanism consisting of nine pathways was speculated on the basis of the energy estimations from density functional calculations. Besides, they reported the conversion of THTCPD-X at different temperatures and also calculated the activation energy and pre-exponential coefficient of the reactions (Ea = 6.67 × 104 kJ mol−1, and A = 133.75 s−1). The activation energy was much higher than the C−C bond

1. INTRODUCTION With the development of new generated advanced aircrafts in recent years, the fuel should satisfy some technical requirements, such as high-density, high-volumetric-heat, and excellent low-temperature properties, long-term storability, etc. Particularly, for a volume-limited aircraft, the high-energy-density hydrocarbon fuels (HEDHFs) are the most desired, which would make the fuel tank occupy space as small as possible, and thus, the remaining space can be assigned to other components. On the other hand, the HEDHFs could extend the flight range almost 15% over that achievable with conventional fuels. Therefore, a new generation of hydrocarbon fuels with high energy density has attracted increasing focus during the past 2 decades.1−6 It is generally accepted that to synthesize liquid hydrocarbon fuels is an effective way to gain HEDHFs. Thus far, a series of HEDHFs, including cyclic dimers and trimers, diamondoid, and high-strained caged fuel, have been synthesized. Among them, tetrahydrotricyclopentadiene (THTCPD, C15H22) has received much attention because of its remarkable properties of density (1.03 g/cm3) and volumetric heat (43.3 MJ/L), approximately 9% higher than those of JP-10 (0.94 g/cm3 and 39.6 MJ/L).4−6 THTCPD could be synthesized from the Diels−Alder addition of dicyclopentadiene (DCPD) and cyclopentadiene, followed by hydrogenation and isomerization reactions.7−13 Despite its high-energy density, there is still a disadvantage restricting its application. For instance, the viscosity of THTCPD is ca. 35.3 mPa s at 20 °C, which is ca. 11 times higher than that of JP-10 (3.0582 mPa s at 20 °C).14 Obviously, the high viscosity will have a fatal effect on its atomization and, thus, the ignition and combustion.15,16 Noting this drawback, several methods were proposed to resolve this problem. A simple method is to blend it with other lower viscosity components.1 For example, the © 2016 American Chemical Society

Received: October 3, 2015 Revised: December 21, 2015 Published: January 5, 2016 230

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Energy & Fuels dissociation energy.25 Clearly, they made a mistake in their calculation. To the best of our knowledge, there is little work on the THTCPD pyrolysis under high pressure, which is typical working conditions for the advanced active cooling technology (3.5−7 MPa).2 According to the recent work, the high pressure may have a remarkable effect on the mechanism and kinetics of decomposition of hydrocarbon fuel.26−29 Therefore, it is still necessary to do more work and, thus, to obtain more insights into thermal decomposition and kinetics of THTCPD under high pressure. The objective of this work is to investigate the high-pressure thermal decomposition of THTCPD in a stainless-steel batch reactor at different conditions (temperatures and residence times). The reaction kinetics under this experimental condition was established, and major decomposition products were identified and analyzed. The possible primary mechanisms and main secondary reactions of THTCPD were proposed. This study may provide possible fundamental experimental data for the potential application of THTCPD fuel.

2. EXPERIMENTAL SECTION 2.1. Materials. THTCPD used in this study was synthesized by Wang et al. in the following methods. First, it was prepared through the Diels−Alder addition of endo-DCPD and cyclopentadiene at 160 °C to form TCPD. Then, TCPD was hydrogenated with Pd−B/ γAl2O3 amorphous catalyst at 130 °C for 10 h under 1.5 MPa hydrogen pressure, followed by the isomerization in the presence of the AlCl3 catalyst.10,11,30 The synthetic product was a mixture with various isomers of THTCPD and few amounts of JP-10 and heavy components. Its properties are listed in Table 1.6,11 No further process was performed with it, and six isomers were detected. Figure 1 shows the chromatogram of THTCPD isomers and their structures.

Figure 1. (a) Chromatograph of THTCPD isomers and (b) their structures.

Table 1. Properties of THTCPD Used in This Study properties

values

chemical composition (wt %) THTCPD JP-10 >C15 density at 20 °C (g/cm3) volumetric heat value (MJ/L) pour point (°C) flash point (°C) viscosity at 20 °C (mPa s) critical pressure (MPa) critical temperature (°C)

97.15 1.88 0.97 1.0326 44.1 −60 116 35.3 3.03a 528.3a

a

Figure 2. Scheme of the batch reactor: (1) 15 mL reactor, (2) highpressure cap, (3) two-way spherical valve, (4) temperature indicator, (5) thermocouple, and (6) pressure gauge. residual liquid was collected and weighed. For each condition, at least two parallel experiments were performed to ensure the reproducibility, and the reproducibility errors were less than 5%. It is worth mentioning that the THTCPD temperature was lower than the set temperature of the furnace at the beginning of experiments. Usually, it takes 20−30 min to reach the set temperature. In this study, the time when THTCPD reaches the given temperature was regarded as the beginning time of reaction. 2.3. Analysis Method. The gaseous product was analyzed by Agilent 6820 gas chromatography (GC 6820) equipped with both a flame ionization detector (FID) and thermal conductivity detector (TCD). Hydrocarbon was analyzed using the FID and a K/Al2O3/S column (50 m × 0.53 mm × 0.5 μm), programmed from 40 to 100 °C at a rate of 4 °C/min with an isothermal period of 7 min after heating. Hydrogen was analyzed using the TCD and a HP-plot, 5A molesieve column (30 m × 0.53 mm × 25 μm), programmed at 70 °C for 3 min. The gaseous product was identified and quantitatively measured using the standard gas samples. The feed and liquid products were identified using Agilent 7890/ 5975 gas chromatography/mass spectrometry (GC/MS) with a HP-5

Calculated with the group contribution method.

2.2. Thermal Decomposition of THTCPD. Thermal decomposition of pure and mixture hydrocarbons had been performed in batch reactors to investigate their reaction kinetics and machanisms.31−48 Herein, thermal decomposition of THTCPD was carried out in a 15 mL stainless-steel batch reactor (shown in Figure 2) treated by HNO3 solution and NaOH solution successively to minimize the wall catalytic effect. A total of 5 mL of THTCPD was added to the reactor, and then the reactor was purged with nitrogen 5 times to completely remove the air and finally was pressurized to 0.5 MPa of nitrogen. Then, the reactor was put into an electrically heated furnace with a pre-setting temperature. To measure the temperature of THTCPD, a thermocouple was directly inserted ca. 1 cm above the liquid. The pressure was observed from the pressure gauge. After thermal stressing for a given period, the reactor was immediately taken out and cooled by the water. The gas product was collected using the water replacement method to obtain the volume. After that, the 231

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Figure 3. Typical chromatograms of liquid residuals from THTCPD at different decomposition conversions: (a) THTCPD feed fuel, (b) x = 7.2%, (c) x = 18.6%, (d) x = 39.4%, (e) x = 54.5%, and (f) x = 68.6%. Panel ii represents the rectangular section in panel i. Products: (A) cyclopentene, (B) cyclopentane, (C) methlcyclopentane, (D) ethylcyclopentane, (E) cis-octahydropentalene, (F) 3a,4,5,6,7,7a-hexahydro-4,7-methanoindene, (G) JP-10, (H) cyclopentylcyclopentane, (I) endo-THDCP, (J) 2,3-dihydro-4-methyl-1H-indene, (K) 1,3-cyclopentadiene, (L) benzene, and (M) 3cyclopentylcyclopentene. MS column (30 m × 0.25 mm × 0.25 μm), programmed from 5 to 130 °C at a rate of 5 °C/min with an isothermal period of 2 min and from 130 to 270 °C at a rate of 10 °C/min with an isothermal period of 5 min after heating. They were quantitatively analyzed by an Agilent GC 7890 A-FID equipped with a PONA column (50 m × 200 μm × 0.5 μm). The column was programmed from 40 to 300 °C at a rate of 5 °C/min with an isothermal period of 2 min before and after heating. Figure 3 exhibits representative chromatograms of liquid residuals at different extents of thermal decomposition of THTCPD. It can be observed that THTCPD was situated in the retention time of 37−42 min, and the light and heavy components were situated in 2−34 and 44−56 min, respectively. Besides, the feed fuel contained ca. 1.9 wt % JP-10 and ca. 1 wt % heavy components. Conversion was defined as the mass decrease of the THTCPD part relative to the mass of the original THTCPD part and calculated by the following equation:26,37−40,46,49

In this study, the molar yield of i, Yi, was defined as the mole ratio of product i to the initial feed and selectivity of i, Si, was defined as the mole ratio of product i to the decomposed feed. They were used to describe the product distributions and calculated as follows: Yi =

m1iw1i M m 0 Mi

Si =

Yi x

where Mi and M are molar masses of the product i and THTCPD.

3. RESULTS AND DISCUSSION 3.1. Conversion and Kinetic Analysis. Thermal decomposition of THTCPD was conducted as 385, 400, 415, and 425 °C for different reaction times, as shown in Figure 4a. Conversion of THTCPD increases with increasing the reaction time and temperature. Some researchers have investigated the thermal decomposition of mixture fuels in batch reactors using first-order kinetic analysis. Bruno and co-workers37−40 studied the thermal decomposition of a series of mixture fuels, such as RP-1, RP-2, Jet-A, JP-7, JP-TS, and JP-900, in batch reactors. There were

t = 42 min ⎡ m1∑t1= 37 min w1i ⎤ 1 ⎥ × 100% x = ⎢1 − t0 = 42 min ⎥ ⎢ ∑ m w 0 t0 = 37 min 0i ⎦ ⎣

where m0 is the mass of feed fuel, m1 is the mass of liquid residual, and w0i and w1i are the corresponding mass fractions of component i in the THTCPD part. 232

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Figure 5. Arrhenius plot for thermal decomposition of THTCPD.

Using the experimental data of Du et al.,24 we re-estimated the kinetics parameters. The activation energy and preexponential coefficient of the reactions were established as 134.4 kJ mol−1 and 4.4 × 10−5 s−1. The re-estimated activation energy was also lower than that of this study (248.5 kJ mol−1). The possible reason for this difference may be caused by the error in their estimation of the residence time. It is interesting to note that THTCPD has a structure similar to that of JP-10. The difference is that THTCPD has one more five-membered ring than JP-10. In general, the decomposition rate of hydrocarbon fuel will increase with the molecular weight in homologous series.50,51 As expected, the decomposition rate of THTCPD is about 10 times larger than that of JP-10 (4.4 × 10−6 s−1 at 395 °C and 1.7 × 10−5 s−1 at 415 °C).47 The experimental activation energy of thermal decomposition of JP10 in the literature was 256.4 kJ mol−1 (in a batch reactor) and 261.1 kJ mol−1 (in a flow reactor).47,52 In comparison to the result of 248.5 kJ mol−1 in this study, it suggests that THTCPD is slightly less stable than JP-10. Except for the difference in the carbon number with a similar structure, it also can be explained that THTCPD has eight tertiary carbons, while JP-10 has six tertiary carbons.53−55 3.2. Gaseous Products. Not only THTCPD but also primary products would decompose into gaseous products with increasing conversion; therefore, the amount of gaseous components would increase, leading to the increase of the pressure in the reactor (Figure S1 of the Supporting Information). Hydrogen and C1−C5 components were detected in the gaseous product, with hydrogen and C1−C3 dominating. Distributions of the major components are shown in Figure 6. Obviously, ethylene exceeds other components at the start and then decreases with the reaction time as a general trend. It was probably formed via the β-scission reaction. Its selectivity decreases all along with conversion (Figure 9a), which may imply the decline trend of the β-scission reaction. On the other hand, it was assumed that ethylene was consumed by secondary reactions during the long reaction time.26 Hydrogen was probably formed via dehydrogenation, and its molar yield increases at first and then levels off. Its selectivity increases, then declines, and almost remains at that level (Figure S2e of the Supporting Information), suggesting that the dehydrogenation process does not increase any more. The molar yield and selectivity of methane, ethane, and propane increase all along (Figure 6 and Figure S2 of the Supporting Information), indicating that they are favorable for the formation of alkanes with extending time, which may be explained that bimolecular reactions, such as hydrogen abstraction become more favored with increasing pressure.18,25,56,57 The molar yield of propene was low.

Figure 4. (a) Conversion and (b) plots of ln(1/(1 − x)) as a function of time at different temperatures for thermal decomposition of THTCPD (black squares, 385 °C; red circles, 400 °C; blue upwardfacing triangles, 415 °C; and green downward-facing triangles, 425 °C).

various components in these fuels, and each component may decompose through various pathways and into various products. Besides, the primary products may continually yield second products through second reactions. Therefore, it is not practical to analyze the kinetics component by component. Thus, it is necessary to simplify the complex process, and the whole thermal decomposition of these fuels was treated as a simple first-order reaction. Xing et al.41 used the similar method to describe the thermal decomposition of the aviation fuel. Yu and Eser43 investigated the thermal decomposition of C10−C14 normal alkanes and their mixtures and found that the reactions could be exhibited well by apparent first-order kinetics. Herein, the first-order reaction was used to the kinetic analysis for the global thermal decomposition of the mixture THTCPD in batch reactors. The global kinetic model was as follows:37−40 A→B where A is the reactant and B is the product. In addition, if the experimental data in this study was exhibited well by the apparent first-order kinetics, it could justify the assumption.37,38 The rate constant was calculated using the first-order equation 1 1 k = ln t 1−x where t is the reaction time (s), x is the conversion, and k is the observed first-order rate constant (s−1). It should be pointed out that t is the time after reduction of preheat time. On the basis of the first-order rate constants at different temperatures, the activation energy can be derived from the Arrhenius equation. The rate constants at each temperature determined by plotting ln(1/(1 − x)) against t through the linear fitting approach are shown in Figure 4b. The first-order rate constants at 385, 400, 415, and 425 °C were established as 2.7 × 10−5, 7.6 × 10−5, 2.2 × 10−4, and 3.5 × 10−4 s−1, respectively. The activation energy and pre-exponential coefficient determined by plotting ln k as a function of 1/T (Figure 5) were 248.5 kJ mol−1 and 1.5 × 1015 s−1, respectively. 233

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which are more stable than the endo- and exo- and fivemembered ring.35,58 Isomers of JP-10 also exhibited different stability. It was found that conversion of endotetrahydrodicyclopentadiene (endo-THDCP) was a little higher than that of JP-10, while adamantane was consumed only at temperatures above 1000 K.58 Also, there existed isomerization of JP-10 and its isomers during thermal decomposition of JP-10. For instance, isomers (endo-THDCP and adamantine) in the feed fuel increased with conversion,33 which may be caused by the isomerization. Figure 3 shows that C5 and C10 components are the predominant liquid products, which increase rapidly with conversion. Besides, molar yields of methyl- and ethylcyclopentane and cis-octahydropentalene are relativity large. Figure 8 shows their distributions. Cyclopentane increases with the reaction time, which may be explained that bimolecular (especially hydrogen abstraction) reactions become more favored as pressure increases;18,57 therefore, cyclopentyl prefers to form cyclopentane rather than cyclopentene. The molar yield of cyclopentene passes through a maximum value at each temperature. Likewise, 1,3-cyclopentadiene shows a similar trend but decreases to zero at last, suggesting that it is an intermediate product. The reason for their decreasing trends was probably that secondary reactions consumed olefins with the long residence time.26 For instance, 1,3-cyclopentadiene may decompose into methane, ethylene, etc. or form naphthalene and 2,3-dihydro-4-methyl-1H-indene.59,60 Secondary reactions of cyclopentene may form 1,3-cyclopentadiene via dehydrogenation or form gaseous components via the ringopening pathway.61,62 Selectivity of 1,3-cyclopentadiene decreases all along, and that of cyclopentene presents the downtrend at the conversion of ca. 15% (in Figure 9b). The probable reason for the decline tendency seemed to be that the middle weight free radicals, such as C10 radicals, would tend to be converted to stable alkanes through bimolecular (particularly hydrogen abstraction) reactions before they split off small molecules, and this process would be intensified by pressure.25,26,50 For instance, with pressure increasing, the C10 radicals would tend to react in the pathway to form JP-10 or other C10 components rather than successive β-scission to form C5, such as 1,3-cyclopentadiene and cyclopentene. However, cyclopentene and cyclopenditene were the leading components in the study by Du et al., where THTCPD decomposed through β-scission all along to the end with short residence time and atmospheric pressure.24 It is in accordance with the result that the molar yield of major C10 components increases at first. Especially JP-10 increases significantly at first but then declines slightly (Figure 8e). The decline tendency was not observed at 385 °C, which may be due to the insufficient time at a low temperature, despite the relatively high conversion. Selectivity of JP-10 increases and then decreases at the conversion greater than 35% (Figure 9d). The reason for the decline is not completely clear. An explanation may be that JP-10 itself decomposed into small molecules. Likewise, both endo-THDCP and 3a,4,5,6,7,7ahexahydro-4,7-methanoindene increase first and then decrease probably as a result of further decomposition. Selectivity of endo-THDCP decreased at the conversion of ca. 25%. Cyclopentylcyclopentane and 2,3-dihydro-4-methyl-1H-indene increase all along with the reaction time, and the latter may be formed from 1,3-cyclopentadiene via self-recombination.63

Figure 6. Distribution of major gaseous products from thermal decomposition of THTCPD at different temperatures and times: (a) 385 °C, (b) 400 °C, (c) 415 °C, and (d) 425 °C (black squares, hydrogen; red circles, methane; blue upward-facing triangles, ethane; green downward-facing triangles, ethylene; magenta left-facing triangles, propane; and brown right-facing triangles, propene).

However, Du et al.24 reported that ethylene and propylene were leading gaseous products, which may be the result of the short residence time and low pressure. 3.3. Liquid Products. Isomers of THTCPD exhibit different concentration variations (Figure 7), reflecting their

Figure 7. Isomer molar concentrations relative to the feed fuel during thermal stressing of THTCPD at 415 °C (isomers: black squares, I; red circles, II; blue upward-facing triangles, III; green downward-facing triangles, IV; magenta left-facing triangles, V; and brown right-facing triangles, VI).

stability difference. The amounts of I, II, III, IV, V, and VI in the feed fuel (5 mL) were 6.3 × 10−4, 4.8 × 10−3, 1.3 × 10−2, 2.9 × 10−3, 5.3 × 10−4, and 2.7 × 10−3 mol, respectively. Among them, isomers III and VI decrease the most rapidly with a reduced amount of ca. 80% in 0.65 h. Both isomers IV and V decrease by ca. 40%, and isomer II decreases steadily by ca. 6% in 0.65 h. However, isomer I shows an increasing trend before 1.5 h, which may imply the probable isomerization between these isomers. It was likely that II−VI isomers were isomerized into I in this condition. However, it was calculated that the overall decomposition rate was much faster (ca. 20 times) than the isomerization rate. Therefore, the isomerization had a negligible effect on the decomposition reactions. It could be concluded that the stability at 415 °C was in this order: I > II > V > IV > III ≥ VI. Isomer I is the most stable, which may be due to the structure of adamantine and the six-membered ring, 234

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Figure 8. Major liquid product distribution from thermal decomposition of THTCPD (black squares, 385 °C; red circles, 400 °C; blue upwardfacing triangles, 415 °C; and green downward-facing triangles, 425 °C): (a) 1,3-cyclopentadiene, (b) cyclopentene, (c) cyclopentane, (d) 3a,4,5,6,7,7a-hexahydro-4,7-methanoindene, (e) JP-10, (f) endo-THTCPD, (g) cyclopentylcyclopentane, (h) 2,3-dihydro-4-methyl-1H-indene, (i) methlcyclopentane, (j) ethylcyclopentane, and (k) cis-octahydropentalene.

Panels i−k of Figure 8 show that methyl- and ethylcyclopentane and cis-octahydropentalene increase monotonously with time. They may be formed from the secondary decomposition of JP-10, as evidenced by the presence of them in the thermal decomposition of JP-10.64 It should be noted that cyclopentylcyclopentane was much more than its olefins (3-cyclopentyl- and 1-cyclopentylcyclopentene), and methyl- and ethyl-cyclopentane and cisoctahydropentalene were much more than those of related olefins (1-methyl- and 1-ethyl-cyclopentene and 1,2,3,3a,4,6ahexahydropentalene) (Figure S4 of the Supporting Information), which agrees well with the conclusion that a high pressure and long residence time are beneficial for the formation of alkanes.18,57 In this study, benzene and naphthalene are important aromatics to obtain some knowledge of mechanistic insights. Benzene is a primary product and increases with extending time in general. However, its selectivity decreases all along with conversion (Figure 9c), and the reason will be discussed later. As well-known, 1,3-cyclopentadiene contributes to naphthalene.58,61,63,65 Here, it was found that naphthalene increased when 1,3-cyclopentadiene decreased (Figures 8a and 10b). It is likely that 1,3-cyclopentadiene was turned into naphthalene. Also, secondary reaction of cyclopentene could form 1,3cyclopentadiene;61 therefore, naphthalene increased despite zero 1,3-cyclopentadiene.

Figure 9. Selectivity of ethylene, cyclopentene, 1,3-cyclopentadiene, benzene, JP-10, and endo-THDCP versus conversion for thermal decomposition of THTCPD (black squares, 385 °C; red circles, 400 °C; blue upward-facing triangles, 415 °C; and green downward-facing triangles, 425 °C). When conversion was less than 20%, the content of JP-10 in the feed fuel was excluded in the calculation of selectivity because it accounted for a large proportion.

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a low concentration (9.34 mol % JP-10 in N2). Vandewiele et al.58,61 demonstrated that the diradical reacted with JP-10 through hydrogen abstraction, which suggested that the bimolecular reactions became significant in the high mole fractions of JP-10. Xing et al.56 proposed the hypothetical mechanism for thermal decomposition of JP-10 under pressure of 0.1−3.9 MPa, where diradicals absorbed hydrogen atoms to generate single radicals. Three diradicals (R1, R2, and R3) were assumed to participate in the primary mechanism. It was reported that the bond dissociation energy (BDE) was helpful to the predictions of significant elementary steps.25 Herein, the BDE values of each C−C bond were calculated (Table S1 of the Supporting Information) to demonstrate their possibilities (R1, R2, and R3). The mechanism is in satisfactory agreement, with ethylene being the most among gaseous products in the initial stage. It is probable that ethylene and benzene were formed from R1 through several steps of β-scission, 1,3-cyclopentadiene from R1 or C10 radicals, such as R22 and R31, via some steps of βscission, and cyclopentene from path 2 or 3 through one or some steps of β-scission. As the pressure increases, the bimolecular reactions have an advantage over β-scission reactions. Accordingly, selectivity of ethylene, cyclopentene, and benzene decreased all along, while selectivity of cyclopentene first increased and then decreased (Figure 9). It seems that path 2 is the only source to generate cyclopentane in the primary reactions. JP-10 can be gained from R22 and R31 via intermolecular hydrogen abstraction, which probably would be intensified with pressure, and thus, its molar yield increases significantly first (Figure 8e). EndoTHDCP could be obtained from the decomposition of THTCPD or the isomerization of JP-10 in the feed in the initial stage. 3a,4,5,6,7,7a-Hexahydro-4,7-methanoindene could be generated from path 2 through isomerization and β-scission reactions. 3-Cyclopentylcyclopentene could be obtained from

Figure 10. Yields of (a) benzene and (b) naphthalene at different temperatures and times (black squares, 385 °C; red circles, 400 °C; blue upward-facing triangles, 415 °C; and green downward-facing triangles, 425 °C).

3.4. Possible Mechanism. According to the analysis of major gaseous and liquid products, it can be concluded that ethylene, C5 (1,3-cyclopentadiene, cyclopentene, and cyclopentane), C10 (3a,4,5,6,7,7a-hexahydro-4,7-methanoindene, JP10, 3-cyclopentylcyclopentene, and endo-THDCP), and benzene are primary products. In combination with the major product distribution, it is assumed that the primary reaction could proceed as the route shown in Figure 11. THTCPD, with a polycyclic structure, is liable to form a diradical. It was hypothesized that the diradical reacted with the THTCPD molecule to eliminate one radical, and thus, the diradical was converted to a monoradical. That is advisable in the batch reaction with increasing pressure, where the concentration of THTCPD is relatively high. Some researchers studied diradicals from JP-10. Herbinet et al.66 found that the diradical was isomerized through internal transfer of the H atom under atmospheric pressure, where mole fractions of JP10 were low (diluted to 0.7−4% in helium). Li et al.67 reported that the diradicals generated primary products with a double bond via the intramolecular hydrogen transfer process under a low pressure (667 Pa). Gao et al.68 predicted that the diradicals underwent an intramolecular disproportionaton process under

Figure 11. Possible primary mechanism for thermal decomposition of THTCPD (A, intermolecular hydrogen abstraction; B, β-scission; and C, isomerization). 236

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R22 through β-scission and intermolecular hydrogen abstraction reactions. The above mechanism discussed the formation of primary products. Secondary reactions mainly included that C 5 components probably decompose into small molecules, such as methane, ethane, ethylene, etc., through the ring-opening pathway or form naphthalene and 2,3-dihydro-4-methyl-1Hindene. Besides, JP-10 and its isomers could probably decompose into methyl- and ethyl-cyclopentane, cis-octahydropentalene, and gaseous components.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02326. Pressure variation (Figure S1), gaseous product selectivities (Figure S2), some major liquid product selectivities (Figure S3), liquid product identification (Figure S4), diradicals from THTCPD (Figure S5), and calculation of the bond dissociation energy of THTCPD (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-22-27892340. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION Thermal decomposition of THTCPD, a high-density hydrocarbon fuel, was performed in a stainless-steel batch reactor at 385−425 °C. The activation energy and pre-exponential coefficient were 248.5 kJ mol−1 and 1.5 × 1015 s−1, respectively. The detailed analysis of the decomposition products showed that THTCPD was probably first cracked into ethylene, C5 (1,3-cyclopentadiene, cyclopentene, and cyclopentane), benzene, and C10 (JP-10 and its isomers) and then probably to form small molecules (methane, ethane, and propane, methyland ethyl-cyclopentane, etc.), naphthalene, and 2,3-dihydro-4methyl-1H-indene. Methane and ethane were predominant in gaseous products. C5 and C10 components were leading liquid products. On the basis of the product distribution, a possible primary mechanism was that diradicals from the cleavage of the C−C bond of THTCPD were converted into monoradicals through intermolecular hydrogen abstraction to eliminate one radical and then the monoradicals generated primary products through β-scission, isomerization, and intermolecular hydrogen abstraction reactions. Further decomposition of C10 and C5 species into small molecules and some bimolecular reactions forming naphthalene and 2,3-dihydro-4-methyl-1H-indene may be the main secondary reactions.

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ACKNOWLEDGMENTS

The authors sincerely acknowledge financial support of the National Natural Science Foundation of China (21522605). Grateful thanks are also given to Dr. Enxian Yuan and Chan Wu for their help in the calculation of the BDE of THTCPD. 237

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