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Acid-Catalyzed Isomerization of Tetrahydrotricyclopentadiene: Synthesis of High-Energy-Density Liquid Fuel Lei Wang, Xiangwen Zhang, Ji-Jun Zou,* Hong Han, Yunhua Li, and Li Wang Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China ReceiVed December 29, 2008. ReVised Manuscript ReceiVed February 28, 2009
Here we used AlCl3 to transform tetrahydrotricyclopentadiene (THTCPD) into a high-energy-density liquid fuel. The reactant contains three inseparable isomers (I, II, and III). Quantum computation shows the possibility of endo fragments of THTCPD turning into exo counterparts. However, experiment indicates the isomerization only happens on norbornyl fragments but cyclopropyl fragments remain unchanged. Reactants I and II containing both norbornyl and cyclopropyl fragments are slowly isomerized, and the reactions are reversible, whereas III without a cyclopropyl fragment is converted quickly. The effects of the reaction conditions were studied. A 5% concentration of AlCl3 shows enough catalytic activity. A temperature higher than 15 °C is not preferred because it lowers the equilibrium conversion. A halohydrocarbon solvent such as 1,2-dichloroethane is necessary. Pseudo-first-order reversible kinetics was established for the isomerization of I and II. The heat of reaction is 22.94 and 17.69 kJ/mol, respectively. The resulting mixture shows good potential for advanced propulsion due to its high energy content and low freezing point.
1. Introduction Over the past few decades, there has always been a drive to increase the volumetric energy content of aviation fuels.1,2 Given a volume-fixed oil tank, high-energy-density fuels provide more propulsion energy than conventional refined fuels, thus increasing the payload, range, and speed of aircraft. Alternatively, when high-energy-density fuels are used, the fuel tank can be smaller without impairing the flight performance. Therefore, more space could be designed for ordinance, electronics, and other components. This is specifically useful for volume-limited aircraft such as missiles and rockets. For propulsion application, however, there are still some constraints that must be satisfied, such as low freezing point, low viscosity, suitable flash point, low toxicity, compatibility with vehicle materials, and longterm storage stability. Especially, low-temperature performance is a critical factor. To ensure the fuel transferring and piping systems function properly in cold environments, such as cold weather, high latitude area, and high space, the fuel should retain low viscosity at low temperature, and ice formation is unacceptable. JP-10 is one of the most successful high-energy-density fuels, which is presently used as the standard missile fuel by the U.S. Navy and Air Force.1-3 It has relatively high density and energy content (0.94 g/mL and 39.6 MJ/L, respectively) due to its strained polycyclic structure. Its low-temperature property is very outstanding (freezing point of -79 °C, viscosity of 18 mPa · s at -40 °C).3 JP-10 is composed of a pure component, * To whom correspondence should be addressed. E-mail: jj_zou@ tju.edu.cn. Telephone and fax: 86-22-27892340. (1) Chung, H. S.; Chen, C. S. H.; Kremer, R. A.; Boulton, J. R. Recent development in high-energy density liquid hydrocarbon fuels. Energy Fuels 1999, 13 (3), 641–649. (2) Edwards, T. Liquid fuels and propellants for aerospace propulsion. J. Propul. Power 2003, 19 (6), 1089–1107. (3) Bruno, T. J.; Huber, M. L.; Laesecke, A.; Lemmon, E. W.; Perkins, R. A. Thermophysical Properties of JP-10, NISTIR 6640; National Institute of Standards and Technology: Boulder, CO, 2006.
namely, exo-tetrahydrodicyclopentadiene (exo-THDCPD). It is synthesized using endo-dicyclopentadiene (endo-DCPD), a natural dimer of cyclopentadiene (CPD), as the feedstock. The feedstock is first hydrogenated to give long-term storage stability.4,5 However, the resultant endo-THDCPD is a solid, so an acid-catalyzed isomerization is performed to convert it into the exo-isomer.6,7 It can be seen that the isomerization reaction is a critical step in synthesizing JP-10. To satisfy the propulsion requirements of next-generation advanced aircraft, researchers are seeking fuels with higher volumetric energy content. This can be achieved by fabricating compact hydrocarbons with a polycyclic structure and high C/H ratio. Unfortunately, it seems not easy to produce fuel with both a high energy content and an acceptable low-temperature performance. For example, use of RJ-5 (with a density of 1.08 g/mL and volumetric energy content of 44.9 MJ/L), a highenergy-density fuel, was finally ceased due to its high freezing point (>0 °C) and high cost.1 Recently, some newly developed methods have been applied to exploit high-energy-density materials. For example, the physicochemical properties and propulsion performance of some caged hydrocarbons have been evaluated using quantum computation and group contribution methods.8-10 Some new structures such as norbornadiene dimer have been synthesized using ionic liquid,11 and cyclopropanefused hydrocarbons have also been prepared via cyclopropanation reaction.12 (4) Zou, J.-J.; Zhang, X.; Kong, J.; Wang, L. Hydrogenation of dicyclopentadiene over amorphous nickel alloy catalyst SRNA-4. Fuel 2008, 87 (17), 3655–3659. (5) Liu, G.; Mi, Z.; Wang, L.; Zhang, X. Kinetics of dicyclopentadiene hydrogenation over Pd/Al2O3 catalyst. Ind. Eng. Chem. Res. 2005, 44 (11), 3846–3851. (6) Janoski, E. J.; Schneider, A.; Ware, R. E. Isomerization of tetrahydrotricyclopentadiene to a missile fuel additive. U.S. Patent 4,086,286, 1978. (7) Xing, E.; Zhang, X.; Wang, L.; Zou, J.; Mi, Z. Greener synthesis of JP-10: utilization of zeolites to replace AlCl3. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 2006, 51 (2), 535–537.
10.1021/ef801139h CCC: $40.75 2009 American Chemical Society Published on Web 04/17/2009
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Wang et al. Scheme 1. Synthetic Route of THTCPD
Considering the success of JP-10, it is expected that hydrocarbons based on CPD trimer (TCPD) may present a higher density and acceptable low-temperature properties.13,14 In addition, CPD is the main component of C5 streams in oil refineries. Its abundance and low cost make it ideal for developing high-density fuels. TCPD can be synthesized through the Diels-Alder addition between DCPD and CPD. We have studied the effects of the reaction conditions on this reaction15,16 and prepared Pd-B/γ-Al2O3 catalyst to hydrogenate TCPD into tetrahydrotricyclopentadiene (THTCPD).17,18 However, the resultant product is solid at room temperature. Therefore, an isomerization treatment is necessary to transform it into liquid fuel. In this work, we conducted fundamental research on the isomerization of THTCPD. The widely used Lewis acid catalyst AlCl3 was used.19 The possible isomerization pathways were first analyzed with quantum computation and subsequently confirmed by experiment. Then the effects of the reaction conditions including temperature, catalyst amount, and solvent were studied. Finally, pseudo-first-order reversible kinetics was established. The properties of the obtained product were also evaluated. 2. Experimental and Computational Methods 2.1. Materials Synthesis. THTCPD was prepared in our laboratory as briefly described in Scheme 1. First, TCPD was synthesized (8) Kokan, T. S.; Olds, J. R.; Seitzman, J. M.; Ludovice, P. J. Characterizing high-energy-density propellants for space propulsion applications. J. Thermophys. Heat Transfer 2008, 22 (4), 727–740. (9) Osmont, A.; Catoire, L.; Go¨kalp, I. Physicochemical properties and thermochemistry of propellanes. Energy Fuels 2008, 22 (4), 2241–2257. (10) Qiu, L. M.; Ye, D. Y.; Wei, W.; Chen, K. H.; Hou, J. X.; Zheng, J.; Gong, X. D.; Xiao, H. M. DFT studies toward the design and properties of high-energy density hydrocarbon fuel. J. Mol. Struct.: THEOCHEM 2008, 866 (1-3), 63–74. (11) Nguyen, M. D.; Nguyen, L. V.; Jeon, E. H.; Kim, J. H.; Cheong, M.; Kim, H. S.; Lee, J. S. Fe-containing ionic liquids as catalysts for the dimerization of bicycle[2.2.1]heap-2,5-diene. J. Catal. 2008, 258 (1), 5– 13. (12) Chang, H. O.; Dai, I. P.; Joong, H. R.; Jeong, S. H. Syntheses and characterization of cyclopropane-fused hydrocarbons as new high energetic materials. Bull. Korean Chem. Soc. 2007, 28 (2), 322–324. (13) Hashizume, M.; Uchida, T.; Aida, F.; Suzuki, T.; Inomata, Y.; Matsumura, Y. Tetracyclodecenene compositions and process for producing the same. U.S. Patent 6,512,152, 2006. (14) Burdette, G. W.; Schneider, A. I. Exo-terahydrotricyclopentadiene, a high density liquid fuel. U.S. Patent 4,401,837, 1983. (15) Zhang, X.; Jiang, K.; Zou, J.; Wang, L.; Mi, Z. Continuous oligomerization of dicyclopentadiene at elevated pressure for synthesis of high-energy-density fuel. J. Chem. Ind. Eng. (China, Chin. Ed.) 2007, 58 (10), 2658–2663. (16) Zhang, X.; Jiang, Q.; Xiong, Z.; Zou, J.; Wang, L.; Mi, Z. DielsAlder addition of dicyclopentadiene with cyclopentadiene in polar solvents. Chem. Res. Chin. UniV. 2008, 24 (2), 175–179. (17) Zou, J.-J.; Xiong, Z.; Zhang, X.; Wang, L.; Mi, Z. Preparation of Pd-B/gamma-Al2O3 amorphous catalyst for the hydrogenation of tricyclopentadiene. J. Mol. Catal. A 2007, 271 (1-2), 209–215. (18) Zou, J.-J.; Xiong, Z.; Zhang, X.; Liu, G.; Wang, L.; Mi, Z. Kinetics of tricyclopentadiene hydrogenation over Pd-B/gamma-Al2O3 amorphous catalyst. Ind. Eng. Chem. Res. 2007, 46 (13), 4415–4420. (19) Busca, G. Acid catalysts in industrial hydrocarbon chemistry. Chem. ReV. 2007, 107 (11), 5366–5410.
via the Diels-Alder addition of DCPD and CPD at 160 °C. Then it was hydrogenated with Pd-B/γAl2O3 at 130 °C for 10 h according to a published procedure.17,18 Gas chromatographic analysis indicated the obtained THTCPD is a mixture of three isomers, namely, 77.7% isomer I, 8.5% isomer II, and 13.8% isomer III (please see below). It should be noted that these isomers could not be separated by high-vacuum distillation. Since a mixture fuel is acceptable for practical application, no further work was done to get individual isomers. 2.2. Isomerization Reaction. The typical isomerization procedure was as follows. A 15.0 g sample of endo-THTCPD dissolved in solvent was charged into a 50 mL three-neck flask equipped with a magnetic stirrer. The flask was placed in an oil batch to control the temperature. After the temperature reached the set value, a defined amount of anhydrous AlCl3 was quickly added under a flow of N2. That time was regarded as the beginning of the reaction. The reaction temperature ranged from -5 to +50 °C, the mass amount of AlCl3 relative to THTCPD varied from 1% to 8%, and the concentration of THTCPD was from 33% to 75%. The reactant samples were withdrawn using a microsyringe at fixed intervals for analysis. The samples were neutralized with NaOH solution (0.3 M) and then washed with water. The analysis was conducted using an HP4890 gas chromatograph equipped with an AT-SE-50 capillary column (50 m × 0.32 mm × 0.33 µm) and a flame-ionization detector. Nitrogen was used as the carrier gas. 2.3. Theoretical Computation. To determine the possible isomerization pathways, the geometry optimization and total energy computation of THTCPD isomers were performed using density functional theory with the M052X and B3LYP methods. The total energies calculated by the two methods were corrected with zeropoint energy using scaling factors of 0.964 and 0.9804, respectively. The Gaussian 03 software package was used in all calculations.20
3. Results and Discussion 3.1. Theoretical Analysis on Isomerization Pathways. DCPD is composed of the endo-isomer and a small amount of the exo-isomer. As shown in Scheme 1, each DCPD isomer has two CdC double bonds, one in the norbornyl (NB) ring and the other in the cyclopentyl (CP) ring. During the Diels-Alder addition, CPD may attack the NB or CP ring to form NB or CP adducts. Moreover, the newly formed CdC double bond may be orientated in the exo or endo position. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2004.
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Scheme 2. Possible Isomerization Pathways of THTCPD
Therefore, the reaction may produce as many as eight TCPD stereoisomers. According to literature work,21 we confirmed the TCPD obtained contains three isomers with negligible unidentified components. As a result, the starting material THTCPD used in this work is a mixture of three isomers. As shown in Scheme 2, I and II are the endo,exo,endo and endo,exo,exo isomers of NB adducts, whereas III is the endo,exo,endo isomer of CP adducts. During the isomerization reaction, the fragment in the endo orientation can be transformed into the exo orientation via Wagner-Meerwein rearrangement.22 As shown in Scheme 2, the endo-cyclopropyl fragment of isomer I may be converted into the exo orientation to form isomer II, and then the remaining endo-norbornyl fragment of isomer II may be transformed into the exo orientation. Thus, exo,exo,exo-isomer V is the ultimate product. Alternatively, the reaction may proceed via exo,exo,endo-isomer IV. For isomer III, which has a symmetry structure, endo,exo,exo (exo,exo,endo)-isomer VI may be the intermediate and exo,exo,exo-isomer VII should be the final product. Molecular computation provides a method of evaluating the stability of molecules and predicting the reaction pathway. The calculated total energies of seven THTCPD isomers are demonstrated in Figure 1. Although different computational methods and basis sets give different total energies, the tendency is the same. For the NB adducts, the energy decreases in the order I > IV > II > V; thus, the thermal preference follows the reverse order V > II > IV > I. For the CP adducts, the thermal preference is VII > VI > III. Therefore, the reactions prefer exo configurations, in agreement with the proposed pathways.
Figure 1. Relative energy of THTCPD isomers calculated using different methods. (The total energy of VII calculated using M052X/ 6-31g(d), M052X/6-311g(d,p), and B3LYP/6-31g(d), is -584.455305, -584.59943735, and -584.496874 au, respectively.)
3.2. Experimental Verification of Isomerization Routes. Some experimental runs were conducted to determine the reaction route of each isomer. At first, the reaction was conducted at -5 °C. It is found that the concentrations of I and II do not change during the reaction, suggesting that the temperature is too low for them to isomerize. However, the isomerization of isomer III takes place easily at this temperature, in which two products are formed. As show in Figure 2, the lumped composition of reactant and two products keeps a constant concentration, confirming no other components are involved in the reaction. The concentration-time curves show the reaction is a typical consecutive process with an intermediate. According to previous analysis, the intermediate is determined as isomer VI and the final product is VII. The reaction finishes in 120 min and is irreversible. When the reaction temperature was increased to 0 °C, the III f VI f VII transformation finished in a very short time (about 10 min). Figure 3 shows the concentrations of I and II gradually decrease, indicating they are also transformed to other configurations. In this case two additional components are formed, which are supposed to be isomers IV and V according to the aforementioned theoretical analysis. If the reaction proceeds via the I f II f V route, the concentration of isomer II should go up first and then decline, like the case of isomer VI. However, this phenomenon is not observed, so this route is ruled out, and the transformation of isomer I most likely takes place through the pathway I f IV f V. If so, intermediate IV should show a volcano-shaped concentration-time curve.
Figure 2. Composition-time profiles for the isomerization of III (temperature -5 °C, AlCl3 amount 3%, solvent 1,2-dichloroethane, THTCPD concentration 50%).
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Figure 4. Conversions of I, II, and III at different temperatures (AlCl3 amount 3%, solvent 1,2-dichloroethane, THTCPD concentration 50%, reaction time 30 min).
Figure 3. Composition-time profile for the isomerization of I and II (temperature 0 °C, AlCl3 amount 5%, solvent 1,2-dichloroethane, THTCPD concentration 50%).
Nevertheless, neither of the newly formed compounds shows this characteristic. In fact, both compounds increase continuously during the reaction. This means that the expected consecutive isomerization of I does not occur. Therefore, the most possible reaction pathway is I f IV and II f V. As evidence, Figure 3 shows the consumed I and II are respectively compensated by a formed product. Although the amounts of I + IV and II + V slightly vary, they are still within the limit of the experimental error. Figure 3 also shows that I and II are not completely transformed, suggesting the I f IV and II f V reactions are reversible. The experimental results show the expected I f II and IV f V routes scarcely occur under the present reaction conditions. The reaction was conducted at 100 °C for a prolonged period, yet no endo to exo isomerization of the cyclopropyl fragment is observed. This suggests that the endo-cyclopropyl fragment of THTCPD is extremely difficult to convert into the exo configuration, although it is possible in thermodynamics. It has been pointed out that the carbonium ion formed on the cyclopropyl fragment may be a dead end, which may not function as the isomerization intermediate.23 It is also possible that the transformation of the cyclopropyl fragment needs an energetically unfavorable intermediate or transition state. Figure 4 further compares the reaction rate of the three isomerization reactions at different temperatures. The reaction of isomer III is much faster than the other two reactions. In 30 min, the conversion of III is 41% at -5 °C, and a 100% conversion is reached at 0 °C. For I and II, the conversion is (21) Nakagawa, K.; Iwase, S.; Ishii, Y. 13C-NMR-speltren von thermischen oligomeren des cyclopenadiens. Bull. Chem. Soc. Jpn. 1977, 50 (9), 2391–2395. (22) Schleyer, P. v. R.; Donaldson, M. M. The relative stability of bridged hydrocarbons. II. J. Am. Chem. Soc. 1960, 82 (17), 4645–4651. (23) McCaulay, D. A. Mechanism of acid-catalyzed isomerization of the hexanes. J. Am. Chem. Soc. 1959, 81 (24), 6437–6443.
Figure 5. Effect of temperature on the isomerization of THTCPD (AlCl3 amount 5%, solvent 1,2-dichloroethane, THTCPD concentration 50%).
less than 60% at 15 °C, clearly indicating a higher temperature and longer reaction period are necessary. The unique structure of III, which contains two norbornyl rings, makes it easy to isomerize, whereas the cyclopropyl ring in I and II decelerates the transformation of the adjacent norbornyl ring. It can be seen that the cyclopropyl structure in THTCPD molecules is the major factor hindering the isomerization reaction. 3.3. Effects of the Reaction Conditions. The effects of the reaction conditions including temperature, catalyst amount, and solvent were studied to establish a suitable operation. To do this, it seems not necessary to study the three isomers separately, so we regarded them as a whole. A preliminary run was first conducted at the highest temperature in the chosen range (50 °C) and the highest catalyst amount (8%), and no byproduct was detected. Therefore, the selectivity of the isomerization reaction is regarded as 100%. Figure 5 shows the effect of temperature on the reaction. The reaction rate is accelerated as temperature increases, and the time to reach equilibrium is shortened. It takes more than 3 h to reach equilibrium when the temperature is below 15 °C. The time is reduced to 90 min when the temperature is higher than 40 °C. However, the equilibrium conversion is also reduced. Especially, the equilibrium conversion is dramatically lowered at 50 °C. This is because the equilibrium constant of the exothermic reaction decreases with rising temperature. Figure 6 shows the effect of the catalyst amount on the reaction. The reaction rate is increased with an increase of the AlCl3 amount, but the equilibrium is not influenced. The equilibrium is reached within 160 min when the catalyst amount rises to 5%. This reaction time is acceptable for practical
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Figure 6. Effect of the AlCl3 amount on the isomerization of THTCPD (temperature 15 °C, solvent 1,2-dichloroethane, THTCPD concentration 50%).
Figure 8. Effect of the reactant concentration on the isomerization of THTCPD (temperature 15 °C, AlCl3 amount 5%).
Figure 7. Effect of solvent on the isomerization of THTCPD (temperature 15 °C, THTCPD concentration 50%): I, cyclohexane; II, ethyl acetate; III, chlorobenzene; IV, toluene; V, chloroform; VI, 1,2dichloroethane.
Figure 9. Arrhenius plots for the reversible isomerization of I and II (temperature 0 °C, AlCl3 amount 5%, solvent 1,2-dichloroethane, THTCPD concentration 50%).
application, so it seems not necessary to further increase the catalyst amount. The acid-catalyzed isomerization of alkanes proceeds via an ionic chain mechanism, in which the solvent may greatly influence the reaction. Figure 7 compares the effect of some solvents. With the absence of solvent, no isomerization happened even at 100 °C. The conversions are extremely low when nonhalide solvents such as cyclohexane with weak polarity and ethyl acetate with medium polarity are used, so the polarity of the solvent seems not a significant factor for the reaction. AlCl3 is hardly soluble in cyclohexane, ethyl acetate, and THTCPD itself. It has been reported that solid AlCl3 is not a strong Lewis acid;24 thus, a suitable solvent for AlCl3 is very necessary. Halohydrocarbons can greatly improve the reaction due to the ability to dissolve AlCl3. Specifically, 1,2-dichloroethane is most active among the solvents used because it can dissolve AlCl3 very well. Besides, this solvent may be helpful to form the carbonium ion transition state. Figure 8 shows the effect of the THTCPD concentration on the reaction. There is an optimal concentration for the highest equilibrium conversion. On one hand, the reaction requires sufficient solvent to dissolve the catalyst and stabilize carbonium ion. On the other hand, a low reactant concentration lowers the reaction rate. Therefore, a balance between the solvent amount and reactant concentration should be determined for the reaction. 3.4. Pseudo-First-Order Reaction Kinetics. When determining the apparent reaction kinetics, the III f VI f VII (24) Murthy, J. K.; Gross, U.; Ru¨diger, S.; Rao, V. V.; Kuman, V. V.; Wander, A.; Bailey, C. L.; Harrison, N. M.; Kemnita, E. Aluminum chloride as a solid is not a strong Lewis acid. J. Phys. Chem. B 2006, 110 (16), 8314–8319.
reaction is excluded because it proceeds too quickly to take enough samples. As mentioned above, I and II exhibit typical reversible characteristics, and the reactions are concentrationdependent. Therefore, a pseudo-first-order reversible reaction is proposed as follows: kf
AhB kr
(1)
where reactant A represents isomer I or II and product B represents isomer IV or V and kf and kr represent the forward and reverse reaction rate constants, respectively. The kinetic equation is expressed as d[B] ) kf[A] - kr[B] (2) dt where [A] and [B] represent the concentrations of A and B, respectively. The integration of eq 2 gives [B] )
kf[A]o {1 - exp[-(kf + kr)t} kf + kr
(3)
where [A]o represents the initial concentration of A. The equation was solved using the concentration-time data obtained in 1,2-dichloroethane solvent. Figure 9 shows the Arrhenius plots of the rate constants of I f IV and II f V at different temperatures. The activation energies for the forward and reverse reactions of I f IV are 18.27 and 41.21 kJ/mol, and 11.95 and 29.64 kJ/mol for those of II f V, respectively. The corresponding heat of reaction is 22.94 and 17.69 kJ/mol, respectively. 3.5. Properties of the Isomerized Product. The starting material is a solid with a melting point of 35 °C. After
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isomerization at 15 °C for 160 min, the resultant mixture contains 11.1% I, 1.4% II, 66.6% IV, 7.1% V, and 13.8% VII. It is a colorless and transparent liquid with a freezing point below -40 °C. The density of the mixture is 1.04 g/mL, 11.8% higher than that of JP-10, and the volumetric energy content is 44.1 MJ/L, 11.6% higher than that of JP-10. It can be seen that the synthesized fuel has a very high energy and good lowtemperature performance. 4. Conclusions The AlCl3-catalyzed isomerization of THTCPD has been conducted to convert it into a high-energy-density liquid fuel. Theoretical analysis shows the exo fragments of THTCPD molecules are preferred in thermodynamics, compared with the
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endo counterparts. However, experimental results indicate only endo-norbornly fragments can be converted into the exo counterparts. The reaction routes are I f IV, II f V, and III f VI f VII, and the former two are reversible. The suitable reaction conditions are as follows: AlCl3 amount 5%, temperature 15 °C, 1,2-dichloroethane solvent, THTCPD concentration 50%. An equilibrium conversion of 87% is reached in 160 min. The activation energies of the forward and reverse reactions are 18.27 and 41.21 kJ/mol for I f IV and 11.95 and 29.64 kJ/mol for II f V, respectively. The isomerized product shows a density of 1.04 g/mL, a volumetric energy content of 44.1 MJ/L, and a freezing point below -40 °C, which is very promising for advanced propulsion. EF801139H
