Thermal Decomposition Kinetics and Mechanism of 1,1

Jun 24, 2014 - Thermal decomposition of 1,1′-bicyclohexyl, a potential surrogate component of high-density hydrocarbon fuels, was performed in a ...
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Thermal Decomposition Kinetics and Mechanism of 1,1′-Bicyclohexyl Lei Yue,† Xiaomei Qin,† Xi Wu,† Yongsheng Guo,*,† Li Xu,† Hujun Xie,‡ and Wenjun Fang*,† †

Department of Chemistry, Zhejiang University, Hangzhou 310027, China Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310035, China

‡

S Supporting Information *

ABSTRACT: Thermal decomposition of 1,1′-bicyclohexyl, a potential surrogate component of high-density hydrocarbon fuels, was performed in a batch-type reactor to investigate its thermal stability. A first-order kinetic equation is supposed to correlate the decomposition process, and the apparent rate constants, ranging from 0.0223 h−1 at 683 K to 0.1979 h−1 at 713 K, are determined. The Arrhenius parameters are determined with the pre-exponential factor A = 6.22 × 1020 h−1 and the activation energy Ea = 293 kJ·mol−1. Compared with four typical hydrocarbon compounds, the thermal stability trend is observed in the order of n-dodecane ≈ 1,3,5-triisopropylcyclohexane > bicyclohexyl > n-propylcyclohexane > decalin. Cyclohexane and cyclohexene are found to be the primary products due to the relatively low energy of the C−C bond connecting the two cyclohexyl rings. Bicyclohexyl decomposes into cyclohexane and cyclohexene equivalently at the beginning of the reaction. A probable mechanism on the basis of quantum calculation and GC-MS analyses for the decomposition of bicyclohexyl is proposed to explain the product distribution. It is shown that the formation of decomposition products is mainly obtained through hydrogen transfer, β-scission, isomerization, or dehydrogenation.

1. INTRODUCTION Endothermic hydrocarbon fuels are used as both propellants and coolants in hypersonic aircrafts.1−3 These kinds of hydrocarbon fuels circulate through the cooling passages located on the hot wall surface to absorb the waste heat before they are injected into the combustion chamber. The fuels decompose into small molecules through endothermic reactions during the cooling process, and it is known as regenerative cooling.4 However, the coke formation is accompanied by the decomposition of hydrocarbon fuels when the concentration of coke precursor reaches a certain extent.5 So, it is important to investigate the thermal stability of any candidates of hydrocarbon fuels. The pyrolysis of cyclic alkanes, as the main components of jet fuels, for example, JP-5, JP-7, JP-8C, and RP-1, has been widely studied in the literature. Billaud et al.6 carried out a study of the thermal decomposition of cyclohexane at 810 °C by using the technique of plug flow reactor. The decomposition of cyclohexane into ethylene, hydrogen, 1,3-butadiene, and cyclohexene was detected, and a mechanism was proposed to explain the experimental results. Kim et al.7 investigated the mechanism for coke formation during the thermal decomposition of methylcyclohexane. Alkyl substituted C5-ring hydrocarbons were identified as causing the formation of polycyclic aromatic hydrocarbons, which were precursors of cokes. Bruno and co-workers8,9 studied the thermal decomposition kinetics of n-propylcyclohexane and 1,3,5-triisopropylcyclohexane in detail. The first-order rate constant for the thermal decomposition of 1,3,5-triisopropylcyclohexane was observed to be 5 times larger than that of n-propylcyclohexane due to the different side chains and tertiary carbons. Yu et al.10,11 studied the thermal decomposition of n-butylcyclohexane and decalin. The cracking reactions of n-butylcyclohexane, dominated by side-chain cracking, were explained by freeradical mechanisms. The decomposition of decalin was © 2014 American Chemical Society

considered to be dominated by cracking reactions under low pressure and by isomerization reactions under high pressure. Xing et al.12 studied the thermal cracking of JP-10 (exotetrahydrodicyclopentadiene) under pressure, and a mechanism was proposed to explain the product distribution. As mentioned above, a lot of researches have been reported about the decomposition of monocyclic, bicyclic, and polycyclic alkanes. However, few researches were reported about the decomposition of separated-bicyclic alkane. 1,1′-Bicyclohexyl (C12H22) is a high-density hydrocarbon compound, which can be found in kerosene-based fuel.13 It has two separated cyclohexyl rings with the total carbon number of 12. In consideration of the similar molecular structure to decalin and the same carbon number as n-dodecane, the comparison of physicochemical properties between bicyclohexyl and these two typical model fuels is listed in Table 1. The density, viscosity, and net heating value of bicyclohexyl are relatively similar to those of cis-decalin and are much higher than those of n-dodecane. It is obvious that bicyclohexyl will hopefully be an important surrogate component of the highdensity hydrocarbon fuels. In this work, the thermal decomposition of bicyclohexyl in a batch reactor was carried out as a function of reaction time at different temperatures. The rate constants of decomposition at various temperatures were determined, and the Arrhenius parameters were evaluated from the rate constants. A probable mechanism, along with theoretical computations for the decomposition of bicyclohexyl, is proposed to explain the product distribution. The experimental and calculational results are provided for comparing the thermal stability of bicyclohexyl Received: May 12, 2014 Revised: June 24, 2014 Published: June 24, 2014 4523

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Table 1. Physicochemical Properties of Bicyclohexyl, n-Dodecane, and cis/trans-Decalin density/g·cm−3 (293 K) viscosity/mPa·s (293 K) flash point/K freezing point/K net heating value/MJ·L−1 a

bicyclohexyl

n-dodecane

cis-decalin

trans-decalin

0.8862a 3.750d 347e 276.75g 37.70g

0.7486b 1.507b 347f 263.42g 32.86g

0.8968c 3.355c 332c 230g 38.05g

0.8698c 2.112c 327c 243g 36.84g

Ref 15. bRef 19. cRef 21. dRef 16. eRef 17. fRef 20. gRef 18.

Figure 1. Schematic diagrams of (a) experiment procedure and (b) reaction apparatus: (1) furnace; (2) reactor; (3) temperature measurement tube; (4) liquid phase tube; (5) reactor cover; (6) needle valve; (7) pressure gage; (8) thermocouple; (9) safety valve; (10) bolt. temperature (Tc) of bicyclohexyl is 731 K, and its critical pressure (pc) is 2.5 MPa.14 2.2. Procedure. Thermal decomposition of bicyclohexyl was performed in a batch reactor made of stainless steel. The experimental procedure and detailed structure of the reactor are illustrated in Figure 1. The experimental temperatures were ranged from 683 to 713 K in intervals of 10 K, and the reaction time was set from 0 to 5 h in intervals of 1 h. To eliminate the metal effects on the catalysis for decomposition of bicyclohexyl, the inside of the reactor was covered tightly with a quartz pool. The sample with a mass of around 25.0 g

with other hydrocarbon compounds and for developing a new advanced hydrocarbon fuel.

2. EXPERIMENTAL SECTION 2.1. Materials. 1,1′-Bicyclohexyl (CAS Registry No. 92-51-3, C12H22, mass fraction 99%,) was obtained from Aladdin Chemical Reagent Co., China. The reagent was checked with the purity of 99.5% by an Agilent 7890/5975C gas chromatography−mass spectrometer (GC-MS) and used without further purification. The critical 4524

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Table 2. Values of Conversion and Rate Constants for Thermal Decomposition of Bicyclohexyl under Different Conditions conversion (wt %) T (K)

0h

1h

2h

3h

4h

5h

k (h−1)

R2

683 693 703 713

0.74 0.80 1.02 1.16

2.81 5.47 7.59 16.22

5.28 9.00 19.26 35.06

6.63 14.31 29.02 44.37

9.67 20.38 37.25 47.24

11.03 25.68 42.17 52.04

0.0223 0.0577 0.1136 0.1979

0.99 0.98 0.99 0.99

was injected into the reactor. Before each run of reaction, N2 was purged into the reactor to replace the air. Then, the reactor was heated to the target temperature with the heating rate of 5 K/min. After reaction for a certain time at each given temperature, the reactor was removed from the furnace into a water-cooling device, which could be cooled to room temperature soon. The volume of the gaseous products was determined by the water displacement method. The liquid residuals were taken out and kept for the following analyses. 2.3. Analyses of Products. The gaseous products were analyzed by GC (GC 9790). The content of hydrogen was determined by the GC equipped with a stainless steel column and a thermal conductivity detector (TCD). The column temperature was kept constant at 363 K. The hydrocarbon gaseous products were determined by the GC equipped with a capillary column and a flame ionization detector (FID). The GC was programmed from 323 to 393 K at the rate of 5 K/min with an initial isothermal period of 3 min. The liquid residuals were determined by an Agilent 7890/5975C GC-MS. The GC was equipped with a capillary column, programmed from 323 to 533 K at the rate of 10 K/min with an initial isothermal period of 2 min. The transfer line temperature was 523 K, and the quadrupole Mass Spectrometer Detector (MSD) temperature was set at 423 K. The mass range was from 35 to 350 amu.

Figure 3. Arrhenius plot for rate constant (k) versus temperature (T) of thermal decomposition of bicyclohexyl.

3. RESULTS AND DISCUSSION 3.1. Conversion of Bicyclohexyl. Thermal decomposition of bicyclohexyl was performed at 683, 693, 703, and 713 K with

Figure 4. Comparison of Arrhenius plots for different kinds of hydrocarbon compounds.

3.2. Rate Constants and Activation Energy. A simplified assumption of the first-order reaction is used to describe the thermal decomposition of bicyclohexyl. The rate constant can be calculated by the following equation: 1 1 k = ln (2) t 1−x

Figure 2. First-order plots for thermal decomposition of bicyclohexyl at different temperatures.

where x is the conversion, t is the reaction time, and k is the apparent first-order rate constant. The plots of ln[1/(1 − x)] versus t for thermal decomposition of bicyclohexyl at temperatures ranging from 683 to 713 K are shown in Figure 2. It is worth noting that the plot of ln[1/(1 − x)] versus t at 713 K is not linear at the whole time range from 0 to 5 h. This result may be attributed to the increasing pressure in the reactor with prolonging the reaction time, which is a negative factor for decomposition reaction, and the formation of olefins, which are inhibitors of free radical reactions and lead to significant self-inhibition of the decomposition of paraffin.22

the reaction time from 0 to 5 h. The conversion of bicyclohexyl is calculated as conversion = (m0 − ml c)/m0

(1)

where m0 is the initial mass of bicyclohexyl injected into the reactor, ml is the mass of liquid residuals, and c is the mass fraction of bicyclohexyl in the liquid residuals. The values of conversion of bicyclohexyl under different reaction conditions are listed in Table 2. Clearly, the conversion increases with increasing reaction temperature or time. 4525

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Table 3. Carbon and Hydrogen Balance of the Samples after Decomposition under Different Conditions gaseous productsa

liquid residuals b

t (h)

C

H

MW

0 1 2 3 4 5

11.93 11.80 11.64 11.49 11.30 11.20

21.87 21.62 21.33 21.06 20.69 20.50

165.07 163.19 161.02 158.94 156.33 154.90

0 1 2 3 4 5

11.94 11.59 11.29 10.90 10.44 10.13

21.88 21.25 20.70 19.97 19.14 18.55

165.14 160.39 156.19 150.73 144.41 140.16

0 1 2 3 4 5

11.91 11.33 10.47 9.84 9.41 9.22

21.83 20.78 19.20 18.00 17.19 16.84

164.73 156.74 144.82 136.05 130.06 127.43

0 1 2 3 4 5

11.88 10.76 9.54 9.16 9.07 9.13

21.77 19.73 17.48 16.75 16.58 16.59

164.27 148.85 131.90 126.73 125.42 126.17

C

H

total MW

C

H

12.00 12.00 12.00 12.00 12.00 12.00

22.00 21.99 21.99 21.99 21.97 21.97

35.24 22.42

12.00 12.00 12.00 12.00 12.00 11.99

22.00 21.99 22.00 21.99 22.03 22.08 22.00 22.00 22.03 22.00 22.04 22.08 22.00 22.01 22.08 22.05 22.11 22.07

683 K

693 K

a

2.39 1.48 703 K

6.55 4.62

1.94 1.77 1.45 1.36 713 K

5.34 5.10 4.58 4.43

28.66 26.30 22.03 20.76

12.00 12.00 12.00 12.00 12.00 11.99

2.06 1.21 0.94 0.94 0.95

5.63 4.17 3.72 3.73 3.76

30.40 18.73 15.04 14.98 15.12

12.00 12.00 11.99 12.00 11.99 11.99

A blank section means no gaseous products detected. bMW = molecular weight.

Figure 5. Average molecular weights (MW) of (a) liquid residuals and (b) gaseous products as a function of reaction time (t) for the decomposition of bicyclohexyl at different temperatures.

where A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature. The Arrhenius plot of the first-order rate constants for thermal decomposition of bicyclohexyl is shown in Figure 3. The Arrhenius parameters determined from a linear regression of the data are A = 6.22 × 1020 h−1 and Ea = 293 kJ·mol−1. Thermal stability is important for the performance of any hydrocarbon fuels. The kinetic data for the thermal decomposition of bicyclohexyl and those of four hydrocarbon compounds from references8,9,11,23 are compared in Figure 4. It is observed that the rate constant for decomposition at a given temperature is in the order: k(n-dodecane) ≈ k(1,3,5triisopropylcyclohexane) > k(bicyclohexyl) > k(n-propylcyclohexane) > k(decalin). In other words, decalin is the most stable

When the decomposition conversion is relatively high, it may not be suitable to use the first-order reaction to describe the kinetics of thermal decomposition of bicyclohexyl. So, for the decomposition at 713 K, the experimental data with the reaction time from 0 to 3 h are employed to calculate the rate constant (k). The k values obtained from the plots of the kinetic data are listed in Table 2, along with the linearly dependent coefficient (R). It is shown that the first-order kinetic assumption is suitable for thermal decomposition of bicyclohexyl with the conversion less than 45%. According to the rate constants at different temperatures, the apparent activation energy can be determined by using the Arrhenius equation

ln k = ln A − Ea /RT

(3) 4526

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Supporting Information. To ensure the credibility of product distribution, the balances of C and H atoms before and after the thermal decomposition are compared in Table 3. It can be seen that all of the ratio of C to H for the samples after decomposition under different conditions are close to the original C/H ratio of bicyclohexyl (12:22). This means that the elemental balance is satisfactory. The changes of the average molecular weights (MW) of the liquid residuals and the gaseous products against the reaction time are plotted in Figure 5. The MW values are observed to decrease significantly with the increase of reaction time or temperature, which means that more and more small molecules are formed with increasing reaction time or temperature. So, the decomposition depth can be reflected quantitatively to a certain extent by the values of MW obtained from GC-MS analyses. At 713 K, the MW values of liquid residuals decrease continually from 164 to 127 as the reaction time increases from 0 to 3 h, and they exhibit no significant change around 126 from 3 to 5 h, which suggests the decomposition depth is not further intensified. This result can be attributed to the competition of the decomposition of bicyclohexyl and the condensation of cycloalkene. The MW values of gaseous products decrease with the increase of reaction time and temperature, which is mainly caused by the high hydrogen production under these conditions. At 713 K, the MW values of gaseous products are about 15 with no significant change from 3 to 5 h, which is consistent with the variation trend of liquid residuals. In the gaseous products, hydrogen, methane, ethane, ethylene, propane, propylene, butane, butene, and butadiene are detected. The relative contents of gaseous products are shown in Figure 6. With the increase of reaction time, the content of hydrogen increases, while the content of other products decreases. In contrast with our previous work,25 the contents of hydrogen, methane, ethane, and propane are always larger than those of others, such as ethylene and propylene. This is attributed to the considerable difference of the residence

Figure 6. Relative contents of gaseous products from thermal decomposition of bicyclohexyl.

sample among these hydrocarbons, while n-dodecane or 1,3,5triisopropylcyclohexane is the least stable one. The thermal stability of bicyclohexyl or n-propylcyclohexane is intermediate. The results can be explained from the following aspects. Cycloalkanes are usually more stable than corresponding linear alkanes due to the expected high stability of cyclohexane rings.24 So, n-dodecane is less stable than the cyclic alkanes. The C−C bond connecting the two cyclohexyl rings in bicyclohexyl molecule is easily broken, while the C−C bond in the rings of decalin is relatively stable.21 Meanwhile, hydrocarbon compounds with tertiary carbons (from branching or ring substitution) are always less stable than related compounds that do not have tertiary carbons. As a result, bicyclohexyl and n-propylcyclohexane are less stable than decalin. As for the low stability of 1,3,5-triisopropylcyclohexane, it can be attributed to the three side chains and six tertiary carbons in one molecule.8 3.3. Product Distribution. The detected gaseous products and liquid residuals with the relative contents and masses after decomposition are listed in detail in Tables S1−S6 of the

Figure 7. Relative contents of liquid products from thermal decomposition of bicyclohexyl at (a) 683 K; (b) 693 K; (c) 703 K; (d) 713 K. 4527

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Figure 8. Contents of cyclohexane and cyclohexene in liquid residuals at (a) 683 K; (b) 693 K; (c) 703 K; (d) 713 K.

Figure 9. Possible radicals from the cleavage of C−C and C−H bonds of bicyclohexyl.

at the beginning of the reaction. At 693 K, the content of cyclohexene begins to be smaller than that of cyclohexane when the reaction time reaches 5 h. The same phenomena are observed when the reaction time reaches 3 h at 703 K and only 2 h at 713 K, respectively. This demonstrates that cyclohexane is more stable than cyclohexene under the reaction conditions in the present work. There is a similarity that cyclohexene would not be consumed obviously until its content reaches 11% in the liquid residuals. At low reaction temperature and for short reaction time, cyclohexene with a low concentration is relatively stable and would not be consumed. As the increase of reaction time or temperature, cyclohexene is gradually consumed to form aromatics or decompose into small molecules due to the secondary reactions, such as ring-opening reaction, dehydrogenation, and condensation.26,27 At 713 K, the content of cyclohexene exhibits clearly an extreme value around 2 h, which suggests that cyclohexene is a typical intermediate product during the decomposition process of bicyclohexyl. 3.4. Hypothetical Mechanism. The breaking of C−C bonds in bicyclohexyl molecules can produce biradicals, which are listed in Figure 9 and marked as radicals R1 to R4. For comparison, single radicals are produced by the loss of hydrogen atoms from bicyclohexyl molecules, which are also listed in Figure 9 and marked as radicals R5 to R8. The dissociation enthalpies from bicyclohexyl molecule to different free radicals calculated using Gaussian 09 software package28 are listed in Table 4. The lowest dissociation

time. Longer residence time leads to more consumption of olefins due to the secondary reactions. Because of quite complicated compositions of the liquid residuals, it is usually difficult and also unnecessary to give a detailed analysis for every product. On the basis of the GC-MS analysis results, the liquid products are mainly classified as cycloparaffin, cyclic olefin, aromatic, and linear alkane. Then, the relative contents of the four kinds of species and bicyclohexyl in liquid residuals as a function of reaction time are compared in Figure 7. It is shown that the contents of cycloparaffin and cyclic olefin are much larger than those of aromatic and linear alkanes. Some distinctive features are observed at different temperatures. At 683, 693, and 703 K, the content of each species increases with increasing reaction time. At 713 K, the content of cyclic olefin increases at the early stage of reaction, but it begins to decrease after 2 h. From Supporting Information, Tables S1−S4, it is shown that cyclohexane and cyclohexene are primary products for thermal decomposition of bicyclohexyl and that the amounts of them are always larger than those of other products. As compared in Figure 8, the content of cyclohexane for decomposition at 683 K is almost equal to that of cyclohexene. Besides cyclohexane and cyclohexene, few other products are observed in the product distribution (see Supporting Information, Table S1). It indicates that bicyclohexyl decomposes into cyclohexane and cyclohexene equivalently by breaking the C−C bond connecting the two cyclohexyl rings 4528

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The biradicals can abstract hydrogen atoms to be single radicals. The possible decomposition processes of all the single radicals are shown in Figure S1 of the Supporting Information. It is indicated that the products can be formed through hydrogen transfer, β-scission, isomerization, and dehydrogenation. The products are mainly consisted of alkyl substituted cyclohexane, cyclopentane, benzene, indene, and their derivates. On the basis of the quantum calculation results in Table 4, radical R1 is the most stable one. Radical R1 abstracts one hydrogen atom to be cyclohexane, or it loses one hydrogen atom to be cyclohexene. This is why the contents of cyclohexane and cyclohexene are always larger than those of other products. As shown in Supporting Information, Tables S1−S4, methylcyclopentane and spiro[5.6]dodecane are also major products. The potential energy profile for thermal decomposition pathways of bicylcohexyl to these four major products is shown in Figure 10. The rate constants for the elemental steps are calculated and listed in Table 5, along with the tunneling transmission coefficients as a function of temperature. Cyclohexene could decompose into butadiene and ethylene through β-scission or change to benzene through dehydrogenation. Radical R1 undergoes ring-opening reaction to decompose into small molecules. C5-ring product is obtained from radical R1 through isomerization reaction. The processes of other products obtained from radical R2 to radical R8 can be similarly described. All of the products can be verified in the GC-MS analyses.

Table 4. Dissociation Enthalpies and Free Energies from Bicyclohexyl Molecule to Several Free Radicals

enthalpy of reaction 1 indicates that it is most likely to produce radical R1 from bicyclohexyl molecule.

Figure 10. Potential energy profile for bicyclohexyl decomposition to form the major products. Free energy is in kcal/mol. 4529

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Table 5. Activation Free Energies and Rate Constants for the Elemental Steps with the Tunneling Transmission Coefficients T (K) path ai

path aii

path b

683 693 703 713 683 693 703 713 683 693 703 713

ΔG* (kcal·mol−1) 101.7

1.040 1.039 1.038 1.037 1.576 1.559 1.543 1.528 1.106 1.103 1.100 1.097

126.7

150.5

× × × × × × × × × × × ×

10 10−16 10−16 10−16 10−25 10−24 10−24 10−23 10−33 10−32 10−31 10−30

k(T) (L·mol−1·s−1) 4.079 1.220 3.536 9.950 6.128 2.368 8.807 3.158 1.033 5.181 2.483 1.139

× × × × × × × × × × × ×

10−17 10−16 10−16 10−16 10−25 10−24 10−24 10−23 10−32 10−32 10−31 10−30

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China under Grant Nos. 20973154, 21173191, 21273201, and J1210042.

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REFERENCES

(1) Edwards, T. Liquid fuels and propellants for aerospace propulsion: 1903−2003. J. Propul. Power 2003, 19, 1089−1105. (2) Huang, H.; Spadaccini, L. J.; Sobel, D. R. Fuel-cooled thermal management for advanced aeroengines. J. Eng. Gas Turbines Power 2004, 126, 284−293. (3) Maurice, L. Q.; Lander, H.; Edwards, T.; Harrison, W. E. Advanced aviation fuels: a look ahead via a historical perspective. Fuel 2001, 80, 747−756. (4) Jiang, R. P.; Liu, G. Z.; Zhang, X. W. Thermal cracking of hydrocarbon aviation fuels in regenerative cooling microchannels. Energy Fuels 2013, 27, 2563−2577. (5) Venkataraman, R.; Eser, S. Characterization of solid deposits formed from short durations of jet fuel degradation: carbonaceous solids. Ind. Eng. Chem. Res. 2008, 47, 9337−9350. (6) Billaud, F.; Chaverot, P.; Berthelin, M.; Freund, E. Thermal decomposition of cyclohexane at approximately 810 °C. Ind. Eng. Chem. Res. 1988, 27, 759−764. (7) Kim, J.; Park, S. H.; Lee, C. H.; Chun, B. H.; Han, J. K.; Jeong, B. H.; Kim, S. H. Coke formation during thermal decomposition of methylcyclohexane by alkyl substituted C5 ring hydrocarbons under supercritical conditions. Energy Fuels 2012, 26, 5121−5134. (8) Gough, R. V.; Widegren, J. A.; Bruno, T. J. Thermal decomposition kinetics of 1,3,5-triisopropylcyclohexane. Ind. Eng. Chem. Res. 2013, 52, 8200−8205. (9) Widegren, J. A.; Bruno, T. J. Thermal decomposition kinetics of propylcyclohexane. Ind. Eng. Chem. Res. 2009, 48, 645−659. (10) Yu, J.; Eser, S. Thermal decomposition of jet fuel model compounds under near-critical and supercritical conditions. 1. nbutylbenzene and n-butylcyclohexane. Ind. Eng. Chem. Res. 1998, 37, 4591−4600. (11) Yu, J.; Eser, S. Thermal decomposition of jet fuel model compounds under near-critical and supercritical conditions. 2. decalin and tetralin. Ind. Eng. Chem. Res. 1998, 37, 4601−4608. (12) Xing, Y.; Fang, W. J.; Xie, W. J.; Guo, Y. S.; Lin, R. S. Thermal cracking of JP-10 under pressure. Ind. Eng. Chem. Res. 2008, 47, 10034−10040. (13) Song, C. S.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Pyrolytic degradation studies of a coal-derived and a petroleum-derived aviation jet fuel. Energy Fuels 1933, 7, 234−243. (14) Sebastian, H. M.; Yao, J.; Lin, H. M.; Chao, K. C. Gas-liquid equilibrium of the hydrogen/bicyclohexyl systems at elevated temperature and pressures. J. Chem. Eng. Data 1978, 23, 167−170.

ASSOCIATED CONTENT

S Supporting Information *

The calculation process of the balances of C and H number before and after decomposition. Computational details of the quantum calculations. Distributions and masses of liquid and gaseous products. Hypothetical mechanism for thermal decomposition of bicyclohexyl. This material is available free of charge via the Internet at http://pubs.acs.org.

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3.923 1.174 3.407 9.598 3.889 1.519 5.707 2.066 9.339 4.697 2.257 1.038

−17

Notes

4. CONCLUSION Thermal decomposition of bicyclohexyl was carried out in a batch reactor, and a hypothetical mechanism was proposed on the basis of quantum calculation to explain the product distribution. It is shown that bicyclohexyl decomposes into cyclohexane and cyclohexene equivalently at the beginning of the reaction. Cyclohexene is gradually consumed to form aromatics or decompose into small molecules due to the secondary reaction, which suggests cyclohexene is an intermediate product during the decomposition of bicyclohexyl. The decomposition products are mainly composed of alkyl substituted cyclohexane, cyclopentane, benzene, indene, and their derivates. The first-order rate constants for the thermal decomposition of bicyclohexyl are determined from 683 to 713 K and are compared with four model hydrocarbon fuels. The thermal stability is in the order of n-dodecane ≈ 1,3,5triisopropylcyclohexane > bicyclohexyl > n-propylcyclohexane > decalin. From these findings, a suitable adjustment of product distribution should be conducive to the performance of the cooling and combustion of a hydrocarbon fuel. On the basis of the experimental and calculation results, the secondary reactions could be controlled to a certain degree by regulating the reaction conditions to dominate the selectivity of the products. Because of its physicochemical properties and suitable thermal stability, bicyclohexyl will hopefully be an important surrogate component of high-density hydrocarbon fuel. However, it is noted that the relatively high viscosity can lead to its unsatisfactory properties of fluidity, which means bicyclohexyl might be blended with the low-viscosity fuel in a future practical application.

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kTST (L·mol−1·s−1)

κW

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-0571-88981416. Fax: +86-0571-88981416. (W.F.) *E-mail: [email protected]. (Y.G.) 4530

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dx.doi.org/10.1021/ef501077n | Energy Fuels 2014, 28, 4523−4531