Coke Formation during Thermal Decomposition of Methylcyclohexane

Article pubs.acs.org/EF

Coke Formation during Thermal Decomposition of Methylcyclohexane by Alkyl Substituted C5 Ring Hydrocarbons under Supercritical Conditions Joongyeon Kim,† Sun Hee Park,† Chang Hun Lee,† Byung-Hee Chun,† Jeong Sik Han,‡ Byung Hun Jeong,‡ and Sung Hyun Kim*,† †

Department of Chemical and Biological Engineering, Korea University, 1 Anam-Dong, Sungbuk-Ku, Seoul 136-701, Korea Agency for Defense Development, Jochiwongil 462, Yuseong, Daejon, Korea

‡

ABSTRACT: In this study, a mechanism for coke formation of methylcyclohexane (MCH) was investigated by sampling products in a batch type reactor during thermal decomposition. Alkyl substituted C5 ring hydrocarbons, which have a 5-member ring structure with alkyl groups as side chains, were the major products of thermal decomposition of MCH before coke formation (below 465 °C). The composition of alkyl substituted C5 ring hydrocarbons gradually increased without any decrease during the reactions before coke formation. The composition of alkyl substituted C5 ring hydrocarbons decreased after considerable increase, when coke formation occurred as 0.084 g coke/ml MCH for 10 h at 465 °C. Coke was formed via an increase in polycyclic aromatic hydrocarbons (PAHs). Based on this result, it was hypothesized that the abundance of alkyl substituted C5 ring hydrocarbons caused the formation of PAHs and coke. To identify the role of alkyl substituted C5 ring hydrocarbons on coke formation, thermal decomposition of MCH with 1,2,3,4-tetrahydroquinoline (THQ) was performed. THQ was used as the hydrogen donor, which inhibited coke formation. When coke formation was completely inhibited with 5.0 wt % THQ at 465 °C, the composition of all alkyl substituted C5 ring hydrocarbons decreased compared to the experiment without THQ. In addition, PAHs were not completely formed throughout the reaction. These results demonstrated that an abundance of alkyl substituted C5 ring hydrocarbons caused the formation of PAHs, which were precursors for coke under supercritical conditions. Králiková et al. examined MCH pyrolysis in the gas phase.16 Experiments were performed in a tubular reactor at 1 bar and temperatures of 650−820 °C. The main purpose of their investigation was to examine coke formation. They reported that dehydrogenation of cycloalkane radicals was the dominant reaction for coke formation in gas phase. This result is different from the coke formation mechanism in supercritical phase of our present study. Zeppieri et al. also conducted a mechanistic study to examine thermal decomposition of MCH in the gas phase and at atmospheric pressure.17 Details of the mechanisms, including abstraction, C−C bond homolysis, and β-scission, were provided to explain the product distribution. However, the major products from the thermal decomposition of MCH in the gas phase were different from those of our study. Lai et al.18 examined thermal decomposition of MCH in the supercritical phase. They reported that ring contraction was the dominant reaction in the thermal decomposition of MCH. Ring contraction refers to the conversion of C6 ring hydrocarbons to C5 ring hydrocarbons. Although their works dealt with the thermal decomposition of MCH in the supercritical phase, the mechanism of coke formation from thermal decomposition of MCH was not considered. Inhibition of coke formation from hydrocarbons is important because coke causes fouling of lines in plants or aircraft.19−25 Thus, to suppress coke formation, it is necessary to determine the mechanism of coke formation.

1. INTRODUCTION It is important to investigate the thermal decomposition of methylcyclohexane (MCH) because MCH is subject to various hydrocarbon fuels including gasoline and aircraft fuels such as JP-9 and kerosene type fuels.1,12,13 Pyrolysis of other cyclic hydrocarbons, notably cyclohexane, has been the subject of numerous investigations. Previous research about the thermal decomposition of cyclic hydrocarbons focused on initial reaction, reaction rate, primary products, major products, and the effects of temperature and pressure on the pyrolysis of cyclic hydrocarbons.2−6 There are also several theoretical studies on modeling and kinetic mechanisms of cyclic hydrocarbons oxidation and pyrolysis.7−11 However, there have been only a limited number of previous studies on thermal decomposition of MCH.4−8 Some of the previous studies on thermal decomposition of MCH were carried out in the presence of steam. Bajus et al. conducted thermal decomposition of MCH in the presence of steam in a tubular reactor at atmospheric pressure.14 In their study, steam was used to maintain the constant conditions needed for thermal decompositions, and the role of steam as a reactant was neglected. Kinetic parameters and selectivity of specific products were obtained at temperatures of 700−800 °C. Pant et al. performed modeling for thermal decomposition of MCH.15 They also conducted experiments with MCH in the presence of steam in a flow reactor at 1 bar and temperatures between 680 and 800 °C. On the basis of their results, they developed a model for the thermal decomposition of MCH, which consisted of 25 reactions. © 2012 American Chemical Society

Received: May 5, 2012 Revised: July 7, 2012 Published: July 9, 2012 5121

dx.doi.org/10.1021/ef3007678 | Energy Fuels 2012, 26, 5121−5134

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2. EXPERIMENTAL SECTION

The aim of this study was to determine the mechanism of coke formation from MCH under supercritical phase because MCH contained in aircraft fuels could be thermally decomposed at high temperature and pressure during hypersonic flight. We investigated the mechanism of coke formation by focusing on variations in the compositions of alkyl substituted C5 ring hydrocarbons, which were the primary products of thermal decomposition. Herein, alkyl substituted C5 ring hydrocarbons such as alkylcyclopentane and alkylcyclopentene were defined as a 5-member ring hydrocarbon with alkyl groups as side chains. Also, to identify the role of alkyl substituted C5 ring hydrocarbons on coke formation, thermal decomposition of MCH in the presence of 1,2,3,4-tetrahydroquinoline (THQ) was performed. In this study, we investigated coke formation in the supercritical phase by conducting thermal decomposition of MCH in a batch type reactor. Unlike other studies, the 0.5 mL product in the reactor was sampled every 90 min during the reactions. This procedure is important to determine the composition of products at specific residence times and to identify the mechanisms of thermal decomposition.

Thermal decomposition of MCH was performed in a batch type reactor made of stainless steel. Metal could be the catalysts for thermal decomposition of liquid hydrocarbons.26 To better understand the process of thermal decomposition of MCH, it is necessary to prevent the MCH from contacting the metal. Thus, the inside of the reactor was covered by quartz to eliminate any effects caused by the metal. For each test, 60 mL of MCH was placed in the reactor. Before each reaction, the inside of the reactor was purged with N2 gas for 10 min to prevent oxidation of MCH by oxygen in the air. After purging, the reactor was pressurized to initial pressure of 15 bar with N2. Then, an electrical heating jacket was used to heat the MCH to the target temperature with heating rate of 10 °C/min. When the temperature of MCH reached target temperatures for each test, the pressure of the reactor reached 50 bar. Therefore, thermal decomposition of MCH (Tc: 298.95 °C. Pc: 34.8 bar) was conducted under supercritical conditions. The 0.5 mL product in a reactor was sampled every 90 min during the reactions. The 0.5 mL sample was small enough not to affect the concentration and pressure inside the reactor. The sampled products were then cooled down to room temperature and separated into gaseous and liquid phases. Thermal decomposition products were identified by gas chromatography (GC). The composition of the gaseous products was analyzed using GC/TCD (thermal conductivity detector) and GC/FID (flame ionization detector). Hydrogen was analyzed using GC/TCD, which was equipped with a stainless steel column packed with a molecular sieve 5A. The temperature of the oven for GC/TCD was increased from 40 to 90 °C at a heating rate of 10 °C/min. Hydrocarbon gases from C1 to C4 were analyzed using GC/FID, which was equipped with a capillary column. The temperature of the oven for GC/FID was increased from 40 to 100 °C at a heating rate of 10 °C/min. The compositions of liquid products were analyzed by GC/MSD (Agilent 7890A Series GC Custom, 5975C inert MSD Standard Turbo EI) using a HP-5 ms column. The temperature of the oven was increased from 35 to 290 °C at a heating rate of 10 °C/min. For quantitative analysis of the samples, calibration tests were performed. Standard samples were provided by Chem Service Inc. A calibration curve, which provided the relationship between peak area of the chromatogram and weight percent, was obtained, and the molar concentration was calculated by the molecular weight and weight percent of each product. The liquid products, whose qualifications were above 80%, are listed in Tables 1−5, where the blanks were doubtful products because the qualifications were below 80%. Based on the retention time of known products, the doubtful products were classified by number of carbons.

Figure 1. Conversion of MCH during thermal decomposition at temperatures ranging from 405 to 465 °C.

Table 1. Thermal Decomposition Products of MCH during the Reaction at 405 °C reaction time (h) type

name

gas liquid

methane cyclohexane cyclopentane, 1,1-dimethylcyclopentane, 1,3-dimethyl-, transhexane, 3-methylcyclopentane, 1,2-dimethyl-, transcyclohexane, methylcyclopentane, ethylcyclohexene, 1-methylcarbon balanceb

a

MF

a

CH4 C6H12 C7H14 C7H14 C7H16 C7H14 C7H14 C7H14 C7H12

0.70

2.20

0.33

3.70

5.20

6.70

8.20

9.70

0.33

0.01 0.34 0.22

0.01 0.34 0.29

0.02 0.34 0.37 0.13

0.02 0.34 0.47 0.14

0.04 0.35 0.58 0.15

0.15 99.15 0.09 0.26 100.00

0.25 98.80 0.16 0.26 100.01

0.44 98.51 0.21 0.26 100.01

0.65 98.01 0.27 0.26 100.02

0.91 97.55 0.34 0.27 100.02

1.20 97.06 0.40 0.28 100.04

0.12 99.56

100.00

Molecular formula. b carbon balance (%) =

carbon amount after reaction in sample carbon amount before reaction in sample × 100 5122


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Table 2. Thermal Decomposition Products of MCH during the Reaction at 425 °C reaction time (h) type gas

liquid

name

MF

hydrogen methane ethane propane propylene cyclopentene, 3-methylcyclopentane, methylcyclopentene, 1-methylcyclopentene, 3-methylcyclohexane cyclopentane, 1,1-dimethylhexane, 3-methylcyclohexene cyclopentane, 1,3-dimethylcyclopentane, 1,3-dimethyl-, transcyclopentane, 1,2-dimethyl-,trans 2-hexene, 2-methylcyclohexane, methylcyclopentane, ethylcyclohexene, 4-methyltoluene cyclohexene, 1-methylcyclohexane, 1-methyl-3-propyl-

H2 CH4 C2H6 C3H8 C3H6 C6H10 C6H12 C6H10 C6H10 C6H12 C7H14 C7H16 C6H10 C7H14 C7H14 C7H14 C7H14 C7H14 C7H14 C7H12 C7H8 C7H12 C10H20

0.73

2.23

0.36

3.73

5.23

6.73

8.23

9.73

0.07 0.10 0.01 0.01 0.01

0.10 0.10 0.01 0.01 0.01

0.10 0.11 0.01 0.01 0.01

0.11 0.13 0.01 0.01 0.01 0.05 0.08 0.35 0.24 0.51 1.67

0.02

0.05

0.36 0.34

0.37 0.59

0.10 0.39 0.87

0.16 0.40 1.09

0.24 0.44 1.35

0.25 0.15 0.15 0.44

0.52 0.31 0.30 0.90

0.80 0.49 0.46 1.38

1.03 0.64 0.61 1.78

1.26 0.81 0.77 2.21

97.82 0.24

95.85 0.50 0.12

93.75 0.77 0.19

91.46 0.88 0.26

89.71 1.26 0.34

0.13

0.27

0.42

0.55

0.70

99.08

0.11 0.09 0.09 99.61

0.13

99.47

carbon balance

99.96

99.90

99.78

99.82

1.55 1.04 0.98 2.73 0.12 86.96 1.56 0.43 0.14 0.88 0.01 0.12 0.11 0.11 99.90


liquid

name

MF

hydrogen methane ethane ethylene propane propylene butane cyclopentene 1-pentene cyclopentene, 3-methylcyclopentane, methyl1,3-cyclopentadiene, 1-methylcyclopentene, 1-methylcyclohexane benzene cyclopentene, 1,5-dimethylcyclopentane, 1,1-dimethylhexane, 3-methylcyclohexene cyclopentane, 1,3-dimethylcyclopentane, 1,3-dimethyl-, transcyclopentane, 1,2-dimethyl-, transheptane

H2 CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C5H8 C5H10 C6H10 C6H12 C6H8 C6H10 C6H12 C6H6 C7H12 C7H14 C7H16 C6H10 C7H14 C7H14 C7H14 C7H16

cyclopentene, 4,4-dimethyl2-hexene, 2-methyl2-heptene

C7H12 C7H14 C7H14

0.77 0.01

2.27 0.05

3.77

5.27

6.77

8.27

9.77

0.34 0.40 0.02 0.01 0.01 0.01

0.44 0.85 0.07 0.01 0.02 0.02

0.54 1.18 0.12 0.01 0.04 0.02

0.09

0.60 1.56 0.20 0.02 0.07 0.03 0.01 0.19 0.09 0.39 0.60 0.17 1.50

0.63 2.12 0.32 0.02 0.11 0.04 0.01 0.24 0.14 0.44 0.86 0.21 1.71

1.67 0.09 3.40

2.19 0.09 3.64

2.24 2.29 2.24 5.09 0.07 0.17 0.25 0.18 0.09

2.27 2.47 2.43 5.30 0.07 0.20 0.25 0.19 0.10

0.06 0.08

0.22 0.18

0.10

0.38

0.78

0.14 0.06 0.31 0.36 0.13 1.18

0.35

0.49

0.75

1.17

0.75

1.65

2.41

2.98

0.78 0.47 0.45 1.25

1.63 1.09 1.05 2.77

2.10 1.61 1.55 3.92

2.12 2.01 1.95 4.68

0.08

0.13 0.22 0.18 0.09

0.31

0.11

0.15 0.09 5123

0.18 0.09


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Table 3. continued reaction time (h) type

name

MF

3,5-dimethylcyclopentene cyclohexane, methylcyclopentane, ethylcyclohexene, 4-methylcyclohexane, methylenecyclopropane, 1-methyl-1-isopropenyl1-ethylcyclopentene toluene cyclohexene, 1-methylcyclohexane, 1,3-dimethyl-, ciscyclohexene, 3,5-dimethylcyclohexane, ethylethylbenzene benzene, 1,3-dimethylp-xylene 1-ethyl-4-methylcyclohexane cyclohexane, 1-ethyl-4-methyl-, cisbenzene, 1,3-dimethyl1-ethyl-4-methylcyclohexane cyclohexane, 1-ethyl-1-methylbenzene, 1-ethyl-3-methylm-menthane, (1S,3R)−(+)−-

C7H12 C7H14 C7H14 C7H12 C7H12 C7H12 C7H12 C7H8 C7H12 C8H16 C8H14 C8H16 C8H10 C8H10 C8H10 C9H18 C9H18 C8H10 C9H18 C9H18 C9H12 C10H20

cyclohexane, 1-methyl-3-propylbenzene, 1,2,4-trimethylcyclohexane, 1-methyl-3-propylcyclohexane, 1-methyl-4-(1-methylethyl)-, cisbenzene, 2-propenyl-

C10H20 C9H12 C10H20 C10H20 C9H10

benzene, 1-methyl-3-propyl-

C10H14

indan, 1-methyl-

C10H12

0.77 99.53

2.27

3.77

5.27

6.77

8.27

9.77

0.06 69.67 2.75 0.92 0.11 0.16

0.09 61.85 3.04 1.01 0.13 0.24

0.74 1.59 0.07

1.29 1.70 0.10 0.16 0.11 0.10 0.39 0.23 0.32 0.14 0.14 0.19 0.28 0.13 0.08 0.32 0.26 0.10 0.17 0.28 0.23 0.11 0.09 0.10 0.15 0.08 0.22 0.08 0.05 1.36 98.58

0.11 56.94 3.18 1.04 0.15 0.34 0.13 1.95 1.72 0.11 0.18 0.13 0.17 0.60 0.24 0.38 0.17 0.21 0.22 0.31 0.21 0.09 0.35 0.29 0.14 0.18 0.28 0.27 0.11 0.09 0.10 0.16 0.09 0.30 0.08 0.11 0.14 1.87 98.95

93.82 0.69 0.20

85.49 1.58 0.48

77.18 2.27 0.74 0.09 0.09

0.41

0.12 0.94

0.34 1.34

0.07

0.12

0.23 0.24 0.09

0.11 0.16

0.16 0.23

0.18 0.12

0.26 0.19

0.09 0.08

0.13 0.19 0.11

1H-indene, 2,3-dihydro-5-methyl2-methylindene naphthalene, 1,2,3,4-tetrahydronaphthalene C11−C13 carbon balance

0.06 0.07

C10H12 C10H10 C10H12 C10H8 0 99.95

0 99.30

0.13 99.06

0.42 98.70

0.72 98.51


liquid

name hydrogen methane ethane ethylene propane propylene butane cyclopentene 1-pentene pentane, 3-methylhexane 2-pentene, 4-methylcyclopentene, 3-methylcyclopentane, methyl1,3-cyclopentadiene, 1-methyl-

MF

0.80

H2 CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C5H8 C5H10 C6H14 C6H14 C6H12 C6H10 C6H12 C6H8

2.30 0.29 0.04 0.02 0.06 0.08 0.07

0.19 0.16

5124

3.80

5.30

6.80

8.30

9.80

2.99 1.44 0.25 0.03 0.12 0.10 0.02 0.18 0.12

3.10 2.39 0.64 0.04 0.21 0.10 0.04 0.24 0.25

3.58 3.59 1.33 0.04 0.78 0.11 0.14 0.25 0.34

0.04

0.05 0.06 0.43 1.65 0.21

0.06 0.06 0.40 2.17 0.19

3.74 4.36 1.49 0.04 0.79 0.12 0.19 0.23 0.40 0.02 0.06 0.06 0.36 2.55 0.19

4.44 5.69 1.50 0.04 0.86 0.15 0.21 0.19 0.37 0.03 0.05 0.05 0.28 2.26 0.16

0.37 0.77 0.17


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name

MF

cyclopentene, 1-methylcyclohexane benzene cyclopentene, 1,5-dimethylcyclopentane, 1,1-dimethylhexane, 3-methylcyclohexene cyclopentane, 1,3-dimethylcyclopentane, 1,3-dimethyl-, transcyclopentane, 1,2-dimethyl-, transheptane cyclopropane, trimethylmethylene2-hexene, 2-methyl2-heptene, (E)cyclohexane, methylcyclopentane, ethylcyclohexene, 4-methyl1,3,5-heptatriene, (E,E)1-ethylcyclopentene toluene cyclohexene, 1-methylcyclohexane, 1,3-dimethyl-, cis-

C6H10 C6H12 C6H6 C7H12 C7H14 C7H16 C6H10 C7H14 C7H14 C7H14 C7H16 C7H12 C7H14 C7H14 C7H14 C7H14 C7H12 C7H10 C7H12 C7H8 C7H12 C8H16

cyclopentane, 1-ethyl-3-methyl-, transcyclopentane, 1-ethyl-3-methylcyclohexane, 1,4-dimethyl1,3-dimethyl-1-cyclohexene cyclohexene, 3,5-dimethylcyclohexane, ethylethylbenzene pentalene, octahydro-, cisp-xylene 1-ethyl-4-methylcyclohexane cyclohexane, 1-ethyl-4-methyl-, trans-

C8H16 C8H16 C8H16 C8H14 C8H14 C8H16 C8H10 C8H14 C8H10 C9H18 C9H18

cyclohexane, 1-ethyl-4-methyl-, cis-

C9H18

benzene, (1-methylethyl)cyclohexane, propylbenzene, propylbenzene, 1-ethyl-3-methylbenzene, 1,2,4-trimethyl-

C9H12 C9H18 C9H12 C9H12 C9H12

benzene, 1-ethyl-2-methyl-

C9H12

0.80

2.30

3.80

5.30

6.80

8.30

9.80

0.64

1.36

1.54

1.45

1.35

1.07

0.67 0.07 1.87

1.93 0.50 2.99

3.28 0.96 3.29

4.02 1.01 3.04

4.53 0.83 2.69

4.69 0.66 2.09

1.77 1.31 1.27 3.10

1.75 2.07 2.05 4.39 0.08 0.19 0.27 0.08 58.36 2.63 0.91 0.12 0.12 1.87 1.49 0.11

1.23 2.25 2.24 4.33 0.07 0.25 0.27 0.08 43.73 2.66 0.85 0.14 0.16 4.16 1.30 0.31 0.13

0.87 2.06 2.07 3.66 0.07 0.25 0.25 0.07 33.66 2.33 0.68 0.12 0.16 6.10 1.01 0.28 0.13

0.17

0.19

0.16 0.13 0.18

0.16 0.21 0.42

0.63 0.36 0.16 0.21 0.21 0.27

1.30 0.39 0.18 0.40 0.22 0.28 0.13

0.14 0.23 0.67 0.12 1.92 0.38 0.18 0.58 0.21 0.25 0.16

0.12 0.10 0.13 0.53 0.12 0.11 0.09 0.32 0.31 0.27 0.16 0.17 0.12

0.16 0.10 0.21 0.87 0.17 0.12 0.15 0.29 0.30 0.40 0.15 0.16 0.12

0.13

0.23

0.08 0.49 0.07 0.07

0.09 0.66 0.09 0.08

0.61 1.82 1.85 2.91 0.05 0.23 0.25 0.07 23.28 1.93 0.50 0.09 0.15 8.25 0.73 0.27 0.13 0.08 0.04 0.19 0.13 0.12 0.26 1.01 0.14 2.66 0.36 0.18 0.79 0.15 0.19 0.19 0.05 0.22 0.11 0.26 1.27 0.22 0.11 0.24 0.22 0.25 0.52 0.10 0.13 0.10 0.06 0.27 0.08 0.08 0.79 0.11 0.09

0.45 1.41 1.44 2.14 0.04 0.18 0.24 0.06 17.95 1.49 0.39 0.08 0.12 9.03 0.55 0.24 0.11 0.07 0.04 0.17 0.11 0.05 0.23 1.26 0.15 3.14 0.31 0.16 0.92 0.12 0.15 0.18 0.04 0.23 0.09 0.27 1.57 0.24 0.10 0.28 0.18 0.21 0.61 0.09 0.11 0.08 0.06 0.30 0.09 0.07 0.84 0.12 0.12

0.34

0.10

99.54

0.25 0.10 80.19 1.74 0.64

0.32 1.15

0.11

0.09 0.13

0.06 0.07 0.23 0.08 0.14 0.10

benzene, 1,2,4-trimethylcyclohexane, 1-methyl-2-propyl-

C9H12 C10H20

m-menthane, (1S,3S)-(+)benzene, (2-methylpropyl)benzene, 1-methyl-4-(1-methylethyl)-

C10H20 C10H14 C10H14

cyclohexane, butylindane indene benzene, 1,4-diethyl-

C10H20 C9H10 C9H8 C10H14

0.07

0.32 0.27 0.13 0.16 0.14 0.09

0.07 0.29

5125


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name

MF

benzene, 1-methyl-3-propyl-

C10H14

benzene, 4-ethyl-1,2-dimethylbenzene, 1-methyl-2-propylbenzene, 2-ethyl-1,4-dimethyl-

C10H14 C10H14 C10H14

0.80

2.30

3.80

5.30

6.80

8.30

9.80

0.11

0.15 0.07 0.09

0.20 0.11 0.09 0.12

0.30 0.21 0.09 0.15 0.06 0.06 0.23 0.08 0.06 0.75 0.06

0.22 0.13 0.08 0.18 0.07 0.12 0.15 0.08 0.02 0.85 0.07 0.05 0.08 0.05 0.09 0.05 0.13 0.62 0.16 0.96 0.23 0.20 0.15 1.54 14.5 4.06 97.7

0.11

1-phenyl-1-butene

C10H12

indan, 1-methyl-

C10H12

benzene, 1-methyl-2-(1-methylethyl)-

C10H14

0.12 0.23

0.08

0.06 0.11

0.09 0.43

0.08 0.60

benzene, 1,2,4,5-tetramethyl-

C10H14

1H-indene, 2,3-dihydro-5-methyl-

C10H12

0.12

0.07 0.25

1H-indene, 2,3-dihydro-4-methyl2-methylindene naphthalene, 1,2,3,4-tetrahydro-

C10H12 C10H10 C10H12

0.22 0.08 0.10 0.06 0.15 2.34 0.10 97.90

0.40 0.11 0.14 0.08 0.40 5.46 0.10 97.94

naphthalene C11−C13 C14 carbon balance

C10H8 0.32 99.98

96.95

0.05

0.06 0.05 0.07

0.09 0.41 0.08 0.61 0.14 0.15 0.12 0.71 8.13 0.83 97.58

0.11 0.52 0.12 0.79 0.19 0.18 0.12 1.46 10.6 2.88 97.9

Table 5. Thermal Decomposition Products of MCH in the Presence of THQ 5.0 wt % during the Reaction at 465 °C reaction time (h) type gas

liquid

name

MF

hydrogen methane ethane ethylene propane propylene butane cyclopentene 1-pentene cyclopentane, methyl1,3-cyclopentadiene, 1-methyl-

H2 CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C5H8 C5H10 C6H12 C6H8

cyclopentene, cyclohexane benzene cyclopentane, cyclohexene cyclopentane, cyclopentane, cyclopentane,

C6H10 C6H12 C6H6 C7H14 C6H10 C7H14 C7H14 C7H14

3-methyl-

1,1-dimethyl1,3-dimethyl1,3-dimethyl-, trans1,2-dimethyl-, trans-

3,5-dimethylcyclopentene cyclohexane, methylcyclopentane, ethylcyclohexene, 4-methylcyclohexane, methylenecyclopentene, 1,5-dimethyl-

C7H12 C7H14 C7H14 C7H12 C7H12 C7H12

0.8

2.3

3.8

5.3

6.8

8.3

9.8

0.10 0.05 0.01

0.17 0.11 0.02 0.01 0.01 0.01

0.29 0.29 0.05 0.01 0.02 0.02

0.31 0.39 0.09 0.02 0.04 0.03 0.01

0.40 0.65 0.15 0.02 0.07 0.03 0.01 0.14

0.13

0.36

0.49 1.08 0.22 0.02 0.09 0.03 0.01 0.23 0.13 0.74 0.09 0.09 1.70

0.09

0.21

0.47

1.19

0.20 0.27 0.25 0.16 0.16 0.41

0.27 0.64 0.64 0.43 0.41 1.08

0.34 0.94 0.88 0.65 0.62 1.58

0.46 1.23 1.06 0.86 0.84 2.03

0.96 2.12 1.65 1.51 1.48 3.31 0.10 0.16

74.22 0.23

67.97 0.61 0.18

63.2 0.91 0.27

61.54 1.18 0.37

56.46 1.95 0.65

0.28

76.47

0.13 5126

1.73 2.57 1.66 1.84 1.81 3.74 0.16 0.21 0.09 47.48 2.28 0.81 0.12 0.24


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name

MF

toluene cyclohexene, 1-methylcyclohexane, 1,4-dimethyl-, transethylbenzene benzene, 1,3-dimethyl1-ethyl-4-methylcyclohexane

C7H8 C7H12 C8H16 C8H10 C8H10 C9H18

o-xylene cyclohexane, 1-ethyl-4-methyl-, transcyclohexane, 1-ethyl-4-methyl-, cisbenzene, 1-ethyl-2-methyl-

C8H10 C9H18 C9H18 C9H12

cyclohexane, 1-methyl-3-propylbenzene, 1,3,5-trimethylcyclohexane, 1-ethyl-1-methyl-

C10H20 C9H12 C9H18

cyclohexane, 1-methyl-4-(1-methylethyl)-, transindane benzene, 1-propynylbenzonitrile, 4-methylo-toluidine indan, 1-methylm-ethylbenzonitrile

C10H20 C9H10 C9H8 C8H7N C7H9N C10H12 C9H9N

naphthalene, 1,2,3,4-tetrahydronaphthalene 1H-indene, 2,3-dihydro-1,1-dimethyl-

C10H12 C10H8 C11H14

quinoline naphthalene, 1,2,3,4-tetrahydro-6-methyl1H-indole, 1-methylindole naphthalene, 1-methylquinoline, 2-methylquinoline, 1,2,3,4-tetrahydro-

C9H7N C11H14 C9H9N C8H7N C11H10 C10H9N C9H11N

0.8

2.3

3.8 0.38

5.3

6.8

8.3

9.8

0.56

0.22 0.72

0.67 1.23

1.79 1.42 0.11 0.27 0.61 0.38 0.17 0.34 0.22 0.30 0.53 0.30 0.27 0.15 0.15 0.14 0.08 0.97 0.09 0.14 0.25 0.36 0.75 0.24 0.16 0.12 0.17 0.07 0.15 12.87 0.16 0.12 0.79 0.49 0.33 0.467 0.19 0.10 1.03 0.23 96.54

0.13 0.22 0.27

0.09

0.15

0.18

0.19 0.17 0.24 0.25 0.21 0.17

0.10 0.16

0.16

0.46

0.71

0.77

0.12

0.21

0.13 0.22

0.36

0.59

0.72

0.91 0.08 0.13 0.23 0.14 0.74 0.11

quinoline, 4-methyl1H-Indole, 6-methylquinoline, 2-ethylcarbon balance

0.13

0.14 9.78

2.62

5.42

0.10

0.17

0.27 0.18

22.91

20.13

18.33

0.10 0.55 0.21 0.15 14.04

0.35

0.71

C10H9N C9H9N C11H11N 100.2

99.6

98.9

98.3

0.14 10.30 0.12 0.60 0.21 0.26 7.524 0.16 0.76 0.18 97.68

0.15 12.59 0.12 0.75 0.22 0.29 2.155 0.19 0.83 0.20 97.06

temperatures up to 465 °C. MCH started to decompose at 405 °C.

Scheme 1. Methylcyclohexane (MCH, C7H14) and the Major Isomers Formed during the Thermal Decomposition of MCH

conversion of MCH (%) (initial concn of MCH) − (final concn of MCH) = × 100 (initial concn of MCH) (1)

The conversion of thermal decomposition after 10 h was 2.5, 12.6, 42.8, and 82.0% at 405, 425, 445, and 465 °C, respectively (Figure 1). The conversion of MCH increased and the products of thermal decomposition were diversified as the temperature increased. Variations in the composition of the thermal decomposition products of MCH during the reactions are shown in Tables 1−5. At 405 °C, the thermal decomposition products of MCH consisted of only C7 hydrocarbons, as shown in Figure 2a. Except for 1-methylcyclohexene and methane, all products were isomers

3. RESULTS AND DISCUSSION 3.1. Thermal Decomposition of MCH and Product Analysis. Thermal decomposition of MCH was performed at 5127


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Figure 2. Composition of products during thermal decomposition of MCH classified by number of carbons at (a) 405 °C, (b) 425 °C, (c) 445 °C, and (d) 465 °C.

of MCH (C7H14), which were alkyl substituted C5 ring hydrocarbons. As described, alkyl substituted C5 ring hydrocarbons have 5-member ring structures with alkyl groups as

side chains. The isomers of MCH were 1,1-dimethylcyclopentane(1,1-DMCP), 1,2-dimethylcyclopentane(1,2-DMCP), 1,3dimethylcyclopentane(1,3-DMCP), and ethylcyclopentane (ECP). 5128


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Scheme 2. Alkyl Substituted C5 Ring Hydrocarbons Produced from Thermal Decomposition of MCH

Figure 3. Classification of thermal decomposition products of MCH sampled at the end of the reaction (10 h) at temperatures ranging from 405 to 465 °C.

The molecular structures of the four isomers are shown in Scheme 1. There were no linear and branched hydrocarbon products at 405 °C. These results indicate that MCH do not readily undergo ring-opening reactions at 405 °C. At 425 °C, C6 hydrocarbons as well as C7 hydrocarbons were formed with small amount of C1, C2, C3, and C10 hydrocarbons, as shown in Figure 2b. However, the composition of C7 hydrocarbons was even higher than those of C6 hydrocarbons.

C7 hydrocarbons were mostly composed of alkyl substituted C5 ring hydrocarbons such as 1,1-DMCP, 1,2-DMCP, 1,3-DMCP,

Figure 4. Composition of thermal decomposition products of MCH at 445 °C: (a) alkyl substituted C5 ring hydrocarbons, (b) aromatic hydrocarbons. 5129


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Figure 5. Composition of thermal decomposition products of MCH at 465 °C: (a) alkyl substituted C5 ring hydrocarbons, (b) aromatic hydrocarbons.

The most abundant aromatic hydrocarbons at 445 °C were monocyclic aromatic hydrocarbons (MAHs) such as benzene and toluene. Generally, alkyl substituted C5 ring hydrocarbons easily convert to MAHs at higher temperatures.27 Although no decrease in the composition of alkyl substituted C5 ring hydrocarbons was observed at 445 °C (Figure 4a), it was hypothesized that those hydrocarbons started to convert to MAHs at 445 °C. The composition of aromatic hydrocarbons such as monocyclic and bicyclic hydrocarbons is displayed in Figure 4b. Under supercritical conditions, ring contraction from C6 rings to C5 rings is the dominant reaction.18,26,28 Ring contraction from C6 rings to C5 rings is related to the cage effect under the supercritical conditions. The cage effect refers to the phenomenon where the C6 rings become more compact structures such as C5 rings under the high pressure environment associated with the supercritical phase. On the other hand, the cage effect discourages C6 rings from opening to form large and linear structures.26,28 This effect explains why alkyl substituted C5 ring hydrocarbons were the major products for the thermal decomposition of MCH. The cage effect occurred at all temperatures used in this study. 3.2. Variations in Compositions of Alkyl Substituted C5 Ring Hydrocarbons and Mechanism for Coke Formation. Unlike other temperatures employed in this study, coke formation (0.084 g coke/ml MCH) was observed only at 465 °C after 10 h. Also, there were some distinctive features in

and ECP, which were the four major products from the thermal decomposition of MCH as at 405 °C. Also, C6 hydrocarbon products contained some alkyl substituted C5 ring hydrocarbons (1-methylcyclopentene, 3-methylcyclopentene, and methylcyclopentane). Based on these results, most of the products at both 405 and 425 °C were alkyl substituted C5 ring hydrocarbons. The composition of products at 445 °C is shown in Figure 2c. C7 and C6 hydrocarbons were the major products, and those hydrocarbons were also mostly composed of alkyl substituted C5 ring hydrocarbons. Products from thermal decomposition of MCH were classified as cycloalkane/cycloalkene including alkyl substituted C5 ring hydrocarbons, aromatic hydrocarbons, and linear/branched hydrocarbons for each temperature in Figure 3, where the composition of each product were measured at the end of each reaction (10 h). When the decomposition temperature was between 405 and 445 °C, cycloalkanes/cycloalkenes were the major products and they mostly consisted of alkyl substituted C5 ring hydrocarbons. Based on these results, alkyl substituted C5 ring hydrocarbons were determined to be the major products for thermal decomposition of MCH when the temperature was between 405 and 445 °C. All alkyl substituted C5 ring hydrocarbons such as 1,2-dimethylcyclopentane, 1,3-dimethlycyclopentane, and methlycyclopentane produced from thermal decomposition of MCH are displayed in Scheme 2. A distinctive feature of thermal decomposition of MCH at 445 °C was the sudden increase in the composition of aromatic hydrocarbons after 10 h compared to 405 and 425 °C (Figure 3). 5130


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Scheme 3. Representative Monocyclic, Bicyclic, and Polycyclic Aromatic Hydrocarbons Formed during the Thermal Decomposition of MCH

Figure 6. Composition of thermal decomposition products of MCH at 465 °C for 17.3 h.

the products distribution at 465 °C. Below 465 °C, the compositions of every product increased with reaction time (Figure 2a−c). However, at 465 °C, the compositions of C7 and C6 hydrocarbons, which mainly consisted of alkyl substituted C5 ring hydrocarbons, increased in the early stage of thermal decomposition due to the ring contraction reaction, but they started to decrease after 5.3 and 9.8 h, respectively (Figure 2d), and the composition of MAHs was significantly higher at 465 °C than 445 °C (Figures 4b and 5b). These results were attributed to an increase in the conversion of alkyl substituted C5 ring hydrocarbons to MAHs when coke formation occurred at 465 °C. As described previously, it is well-known that alkyl substituted C5 ring hydrocarbons easily convert to MAHs at higher temperatures. Therefore, it was hypothesized that the

abundance of C5 ring hydrocarbons at the middle stage of the reaction significantly affected the formation of MAHs. Another distinctive feature of the thermal decomposition of MCH at 465 °C was the increase in the compositions of C11−C13 and C14 hydrocarbons. Those hydrocarbons were mainly composed of bicyclic and polycyclic aromatic hydrocarbons (PAHs). The molecular structures of representative aromatic hydrocarbons are displayed in Scheme 3. PAHs are precursors for coke formation27,29 and are produced from MAHs.29,30 At 445 °C, PAHs were not produced at all due to the low composition of MAHs (6.3 mol % for 10 h, Figure 4b). In the same manner, at 465 °C, the increase in the composition of PAHs (4.4 mol % for 10 h, Figure 5b) could be explained by the high composition of MAHs (28.9 mol % for 10 h, Figure 5b). To confirm formation of 5131


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is shown in Figure 6. The composition of MAHs achieved maximum at 11.3 h (27.4 mol %) and started to decrease. Production rate of MAHs in Figure 6 started to decrease after the maximum composition of alkyl substituted C5 ring hydrocarbons. On the other hand, the composition of PAHs gradually increased without any decrease during the reactions. Since the composition of MAHs decreased after maximum, it is certain that alkyl substituted C5 ring hydrocarbons first converted to MAHs and then converted to PAHs. As a result, it was supposed that the abundance of alkyl substituted C5 ring hydrocarbons resulted in the formation of PAHs and coke at 465 °C. A schematic of coke formation from MCH is shown in Scheme 4. Unfortunately, it is difficult to describe schemes of the mechanism for coke formation from alkyl substituted C5 ring hydrocarbons, because a significant amount of PAHs (about 40 species) were formed at 465 °C, and most (20 species among 40 species) could not be identified by GC/MSD. However, types of aromatic hydrocarbons such as monocyclic, bicyclic, and polycyclic could be determined by retention time of GC/MSD analysis. The composition of aromatic hydrocarbons during the thermal decomposition of MCH is displayed in Figure 5b. MAHs were the most abundant aromatic hydrocarbons during the reaction (28.9 mol % for 10 h). MAHs were formed at the initial stage of the reaction (before 2.3 h) and the rate of formation significantly increased after 2.3 h. Bicyclic aromatic hydrocarbons were formed after 2.3 h, and their compositions gradually

Scheme 4. Schematic of Coke Formation from Thermal Decomposition of MCH under Supercritical Conditions

PAHs from MAHs, an additional thermal decomposition experiment of MCH was performed at 465 °C for 17.3 h and

Figure 7. Composition of thermal decomposition products of MCH in the presence of THQ 5.0 wt % at 465 °C: (a) alkyl substituted C5 ring hydrocarbons, (b) aromatic hydrocarbons. 5132


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of THQ, which stabilizes hydrocarbon radicals. When THQ was added to the thermal decomposition of MCH, the composition of alkyl substituted C5 ring hydrocarbons decreased. Also, the formation of PAHs and coke was inhibited. Based on these results, it is supposed that abundance of alkyl substituted C5 ring hydrocarbons lead coke formation through monocyclic/ polycyclic aromatic hydrocarbons. Coke formation could be avoidable by suppressing the formation of alkyl substituted C5 ring hydrocarbons even if the temperature is high enough for the production of coke.

increased to 12.8 mol % after 10 h. Polycyclic aromatic hydrocarbons were formed after 5.3 h and their compositions were lower than those of other aromatic hydrocarbons (4.4 mol % for 10 h). Formation of PAHs started later than that of monocyclic and bicyclic aromatic hydrocarbons because PAHs were produced from those aromatic hydrocarbons. 3.3. Inhibition of Coke by 1,2,3,4-Tetrahydroquinoline and Composition of Alkyl Substituted C5 Ring Hydrocarbons. As described previously, ring contraction from C6 rings to C5 rings was the dominant reaction during the thermal decomposition of MCH under the supercritical phase. During ring contraction, MCH is initially converted into intermediate radicals, then these radicals are converted to alkyl substituted C5 ring hydrocarbons.18 1,2,3,4-tetrahydroquinoline (THQ) stabilizes hydrocarbon radicals by donating a hydrogen.31,32 If monocyclic/bicyclic/polycyclic aromatic hydrocarbons and coke decreased during thermal decomposition of MCH due to a reduction in alkyl substituted C5 ring hydrocarbons by adding THQ, it can be assumed that alkyl substituted C5 ring hydrocarbons cause coke formation through monocyclic/ bicyclic/polycyclic aromatic hydrocarbons. To test the above hypothesis, thermal decomposition of MCH was carried out in the presence of THQ. Coke formation was completely inhibited during the thermal decomposition of MCH in the presence of 5.0 wt % of THQ at 465 °C. During thermal decomposition of MCH in the absence of THQ, the composition of alkyl substituted C5 ring hydrocarbons initially increased (0 mol % → 19.7 mol %, from 0 to 5.3 h) but started to decrease up to the end of the reaction (19.7 mol %→ 13.1 mol %, from 5.3 to 10 h) (Figure 5a). The compositions of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons gradually increased and were 28.9 mol %, 12.8 mol %, and 4.4 mol %, respectively, after 10 h (Figure 5b). During thermal decomposition of MCH in the presence of THQ, the compositions of alkyl substituted C5 ring hydrocarbons gradually increased to only 15.1 mol % after 10 h, and there was no decrease after the initial increase (Figure 7a). The composition of monocyclic and bicyclic hydrocarbons was very low (5.7 mol % and 0.2 mol %, respectively) after 10 h, and PAHs were not completely formed (Figure 7b). As a result, the reduction in the compositions of alkyl substituted C5 ring hydrocarbons decreased the composition of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons during the thermal decomposition of MCH in the presence of THQ. These combined results indicate that alkyl substituted C5 ring hydrocarbons cause coke formation through monocyclic, bicyclic, and polycyclic aromatic hydrocarbons.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-2-3290-3297. Fax: +82-2-926-6102. E-mail: kimsh@ korea.ac.kr. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by Korea University Grant and the Human Resources Development in the Korea Institute of Energy Technology Evaluation and Planning (20114010203050) grant funded by the Korea government Ministry of Knowledge Economy.

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REFERENCES

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4. CONCLUSION Thermal decomposition for coke formation of MCH was investigated by sampling products in a batch type reactor during the reaction. MCH started to decompose at 405 °C and coke formation occurred at 465 °C. Alkyl substituted C5 ring hydrocarbons, such as 1,1-DMCP, 1,2-DMCP, 1,3-DMCP, and ECP, were the major products before coke formation. This result can be explained by the cage effect, which is caused by the high pressure of the supercritical phase and makes MCH (C6 rings) more compact structure such as C5 rings. During thermal decomposition of MCH, an abundance of alkyl substituted C5 ring hydrocarbons resulted in the formation of PAHs, which were converted to coke. To examine the roles of alkyl substituted C5 ring hydrocarbons on coke formation, thermal decomposition of MCH was conducted in the presence 5133


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