Rearrangement of Tetrahydrotricyclopentadiene Using Acidic Ionic

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Rearrangement of Tetrahydrotricyclopentadiene Using Acidic Ionic Liquid: Synthesis of Diamondoid Fuel Lei Wang, Ji-Jun Zou,* Xiangwen Zhang, and Li Wang Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: As a work successive to AlCl3 catalytic isomerization of tetrahydrotricyclopentadiene (THTCPD) [Wang, L.; Zhang, X.; Zou, J.-J.; Han, H.; Li, Y; Wang, L. Energy Fuels 2009, 23 (5), 2383
2388], the reaction using chloroaluminate ionic liquid (IL) was investigated. It is found that IL catalysis gives totally different product distribution. The endo-cycloproyl fragments of THTCPD that cannot be isomerized by AlCl3 catalysis are transferred to exo conformation easily. Furthermore, the hydrocarbons are transferred to diamondoids, including methyl-1,2-tetramethyleneadamantane, methyl-diethyl-adamantane, and methyl-diamantane via skeletal rearrangement, with the first diamondoid as the primary and dominant product (selectivity > 80%). The IL shows much higher activity than superacid CF3SO3H, attributed to the synergetic effect of strong acidity and the novel solvent environment. Increasing the temperature, IL dosage, and AlCl3 in IL can promote the rearrangement rate with a slight decrease in the selectivity of methyl-1,2-tetramethyleneadamantane. The presence of trace water in IL forms superacid and induces considerable cracking byproducts, and solvent typically used in rearrangement decreases the reaction rate. The diamondoid-based product shows a low freezing point and high hydrogen content and, thus, is superior as a high-energy-density fuel. This work may open a door for facile synthesis of diamondoid fuel.

1. INTRODUCTION The rapid expansion of aeronautical and space technologies has been raising higher requirements for aerospace fuels, and a high volumetric energy content is specifically preferred for volumelimited aircrafts, such as missiles, rockets, and spacecrafts.1
5 High-energy-density fuels provide much more propulsion energy than conventional distillated fuel and, thus, significantly increase the payload and flight range. They generally possess a compact cyclic structure to afford a high density and high energy content, such as tricyclic JP-10 (with a density of 0.94 g/mL and an energy content of 39.6 MJ/L) and pentacyclic RJ-5 (with a density of 1.08 g/mL and an energy content of 44.9 MJ/L).1,5 Besides, the low-temperature performance of fuels, typically freezing point, is critical because they often encounter harsh environments, such as cold weather and high altitudes. JP-10 is the current standard missile fuel attributed to its low freezing point (
79 °C), and RJ5 is abolished because of its high freezing point (about 0 °C). Diamondoids are hydrocarbons that have carbon framework superimposable on the diamond lattice, in which the adamantane cage is the basic unit.6
8 Naturally occurring diamondoids are first discovered in some crude oil and natural gas condensates. Lower diamondoids, such as adamantane, diamantane, and triamantane, have been synthesized, but molecules containing more than four adamantane cages have not been prepared because of their complicated structure.9
11 Owing to their compact structure, diamondoids has been regarded as a new kind of highenergy-density fuel, and the alkyl-substituted derivatives are specifically attractive because they show a much lower freezing point than their parent molecules. It is reported that diamondoid fuels derived from some mobile reservoirs composed of alkyladamantanes, alkyl-diamatanes, and alkyl-triamantanes exhibit much higher density and volumetric energy content than JP-10.1 r 2011 American Chemical Society

However, the amount of naturally occurring diamondoid fuel is very limited, and studies on synthetic diamondoid fuel are very scarce. Previously, we studied the AlCl3 catalytic isomerization of tetrahydrotricyclopentadiene (THTCPD, C15H22) to synthesize high-energy-density fuel, in which the endo-norbornyl fragments of THTCPD are isomerized into exo configurations but the endocyclopropyl fragments are kept unchanged.12 It is expected that, in the presence of strong acid, the endo-cyclopropyl fragments could be isomerized and the low-temperature performance of fuel would be further improved. Cheng et al. used superacid CF3SO3H as the catalyst and obtained an unexpected C15H24 mixture.13 They proposed that the products are multiple-ring molecules formed via the addition
rearrangement reaction. Ionic liquid (IL) has been widely investigated as a novel and green solvent and/or catalyst for many reactions.14
16 Specifically, acidic IL is used to synthesize JP-10 and adamantane.17
19 Here, we present the chloroaluminate IL catalytic rearrangement of THTCPD. It is found that IL not only facilitates the isomerization of endo-cyclopropyl fragments but also induces notable skeletal rearrangement toward alkyl-diamondoids. This work may open a door for facile synthesis of diamondoid fuel.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. THTCPD (>99%) was synthesized according to previous work.20
22 Cation and anion precursors of IL, namely, triethylamine hydrochloride (Et3NHCl, >99%) and anhydrous AlCl3 (>99%), were purchased and used without purification. To Received: December 15, 2010 Revised: February 28, 2011 Published: March 22, 2011 1342

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Because the hydrocarbon mixture is acceptable for fuel-related application, no further work was performed to obtain individual isomers. Figure 1 shows the total ion chromatogram of these mixtures obtained by GC
MS analysis. The starting material is composed of three THTCPD isomers (I, II, and III; see Scheme 1). Previous theoretical computation has shown that the endo fragments of THTCPD can be converted to thermodynamically stable exo configurations. Because the isomerization is a carbocation process, the two endo fragments (endo-norbornyl and endo-cyclopropyl) of THTCPD should be converted stepwise via possible pathways of I f IV f V, I f II f V, and III f VI. In the AlCl3 catalytic reaction, only the I f IV, II f V, and III f VI routes, in which endo-norbornyl fragments are converted to exo configurations, are carried out but the endo-cyclopropyl fragments remain unchanged probably because the energy barrier is very high and the carbonium and/or intermediate is not stable enough in the solvent. When IL is used as a catalyst, isomer IV disappears and the amount of isomer V increases, indicating that the endo to exo isomerization of cyclopropyl fragments happens smoothly, through the I f IV f V and/or I f II f V route. In addition, a new isomer VII is formed, and its structure will be elucidated later. The most attractive result of the IL catalytic reaction is the presence of C15H24 and C15H26 in the product. Figure 2a shows typical mass spectra of C15H24. The parent peak (m/z 204) is the most intensive (base peak, 100% relative abundance), and the abundance of other ions does not exceed 40%, which is characteristic of the adamantane-containing structure.9 The M-69 peak (m/z 135, 15%) gives direct evidence for the existence of the adamantane unit. The M-56 peak (m/z 148, 37%) is the second most intensive, which means that the molecule tends to lose four carbon atoms together, and thus, there should be a sixmembered ring fused on the adamantane unit. In addition, there should be one more methyl group according to the molecular formula and the relatively intense M-15 peak (m/z 189, 12%). Therefore, C15H24 is methyl-1,2-tetramethyleneadamantane containing one six-membered ring fused on the 1,2 position of adamantane and one methyl substituent on adamantane. Similar to the case of THTCPD, there may be many stereoisomers for C15H24 and the methyl group may be positioned on different carbon atoms of the adamantane unit. As shown in Figure 1, at least five C15H24 isomers are formed.

produce IL, a defined amount of Et3NHCl and AlCl3 were mixed in a three-neck flask and stirred at 50 °C for 60 min under protection of dry N2. The molar composition of AlCl3 in IL was referred to as x = n(AlCl3)/(n(AlCl3) þ n(Et3NHCl)). 2.2. Rearrangement Reaction and Analysis. The reaction was carried out in a 50 mL three-neck flask equipped with a magnetic stirrer and reflux condenser. A total of 0.05 mol (10.1 g) of THTCPD was put into the flask placed in the oil bath, and then a defined amount of freshly prepared IL was added when the temperature reached the set value. To exclude the influence of moisture, the bibulous chloroaluminate IL was prepared freshly before each reaction and the reaction was conducted under the protection of dry N2. After the reaction, the stirring was stopped, the resulting mixture was separated into two phases, and then the upper hydrocarbon layer was recovered by decantation. Qualitative analysis of the product was conducted using Agilent 6890/5975 gas chromatography
mass spectrometry (GC
MS) equipped with a HP-5 capillary column (30 m  0.5 mm) and a Nicolet Magna-560 infrared spectrometer. Quantitative analysis was conducted using Agilent 7890 GC equipped with an AT-SE-54 capillary column (50 m  0.32 mm) and a flame ionization detector.

3. RESULTS AND DISCUSSION 3.1. Identification of Rearrangement Products. Both the starting and resulting hydrocarbons are mixtures of many configurations that cannot be separated by vacuum distillation.

Figure 1. Total ion chromatogram of the product derived from AlCl3 and IL catalytic reactions (I
VII, C15H22 isomers; 4, C15H24; 0, C15H26).

Scheme 1. Isomerization/Rearrangement Pathway of THTCPD

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Figure 3. IR spectra of the product derived from AlCl3 and IL catalytic reactions.

Figure 4. Product distribution versus time in IL catalytic rearrangement of THTCPD (temperature, 80 °C; IL (x = 0.67)/THTCPD molar ratio, 1:1).

Figure 2. Mass spectra of (a) C15H24, (b) C15H26, and (c) C15H22 (VII) along with anticipated fragments.

The mass spectra of C15H26 are shown in Figure 2b. It also shows characteristics of alkyl-adamantane: the most intensive parent peak (m/z 206, 100%), M-71 peak (m/z 135, 32%), and M-15 peak (m/z 191, 16%). The difference is that the M-29 peak (m/z 177, 12%), hinting at the existence of an ethyl group, is comparatively strong, and there should be two ethyl groups according to the molecular formula. Therefore, C15H26 is methyl-diethyl-adamantane containing one adamantane unit plus two ethyl groups and one methyl group. Figure 2c shows that C15H22 (VII) is methyl-diamantane (MDAM) based on the two predominant peaks (parent peak at m/z 202, 25%, and M-15 at m/z 187).23 Infrared (IR) spectra shown in Figure 3 also support the formation of diamondoids in IL catalytic rearrangement. The spectrum of the AlCl3 catalytic product is similar to that of the

starting material, confirming that they have the same skeleton, despite different stereoconfigurations. However, the product of IL catalysis shows distinctly different characteristic bands, indicative of significantly skeletal changes. The bands around 1453 and 1360 cm
1 are the asymmetric and symmetric bending of the bridge methylene group, and the band around 1346 cm
1 is the symmetric bending of the bridgehead methine in the adamantane unit, respectively.24 The band around 1374 cm
1 is the symmetric bending of the methyl substituent. 3.2. Illustration of the Rearrangement Pathway. The rearrangement of polycyclic hydrocarbons is a complex thermodynamically controlled carbocation process, in which the most stable isomer with the lowest free energy should be the final product. Many researchers have found that alkanes containing three and more cycles tend to rearrange to an adamantanecontaining skeleton. The most typical case is that tricyclo[5.2.1.02,6]decane can be isomerized to adamantane in the presence of strong acid or superacid.25,26 Similarly, the rearrangement of tetracyclo[6.5.1.02,7.09,13]tetradecane and pentacyclo[8.2.1.14,7.02,9.03,8]tetradecane finally produces 1,2-tetramethyleneadamantane, which is the most stable and thermodynamically preferred molecule among C14H22 isomers, as confirmed by both 1344

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Table 1. Product Distribution of THTCPD Isomerization/Rearrangement Catalyzed by Different Catalysts product distribution (wt %) entry 1

a

catalyst AlCl3

C15H22 (I þ II þ III)

C15H22 (IV)

C15H22 (V)

C15H22 (VI) 14.5

C15H22 (VII)

C15H24

C15H26

byproduct

12.5

65.6

7.4

2b

CF3SO3H

8.2

48.2

11.9

15.8

1.8

13.8

0.3

0

3c

IL

2.0

16.9

14.5

4.3

53.1

2.4

6.8

4d

IL

3.2

18.6

15.0

3.9

53.4

2.4

3.5

Reaction conditions: catalyst dosage, 5 wt %; dichloroethane/THTCPD weight ratio, 1:1; temperature, 15 °C; time, 5 h. b Reaction conditions: catalyst dosage, 300 wt %; dichloromthane/THTCPD weight ratio, 11.8:1; temperature, 0 °C; time, 24 h. c Reaction conditions: catalyst dosage, 200 wt %; temperature, 80 °C; time, 3 h. d Reaction conditions: same as entry 3, unless the product in entry 1 is used as the starting material. a

theoretical calculation and experiment.27
31 THTCPD molecules in the present work have a structure very similar to these two polycyclic tetradecanes. The only difference is that isomers I, II, IV, and V have one additional bridge methylene group connecting the 3,6 position of tetracyclo[6.5.1.02,7.09,13]tetradecane, while isomers III and VI possess a five-membered ring connecting the two norbornane fragments instead of a four-membered ring of pentacyclo[8.2.1.14,7.02,9.03,8]tetradecane. It naturally leads to the conclusion that THTCPD can also be rearranged to 1,2-tetramethyleneadamantane derivates. Figure 4 shows the evolution of isomers during the IL catalytic reaction. Original THTCPD (I þ II þ III) is converted to exo isomers (IV þ V þ VI) in 30 min, indicating that the conformational transformation occurs very quickly. However, the skeletal rearrangement is not so fast (see the slow increase of diamondoids in the product). C15H24 is the dominant product and accounts for more than 90% of the diamondoids, whereas C15H26 and MDAM are minor products. According to the above-mentioned analysis, THTCPD undergoes both conformational isomerization and skeletal rearrangement, as illustrated in Scheme 1. Only endo to exo isomerization of norbornyl fragments (I f IV, II f V, and III f VI) occurs in the AlCl3 catalytic reaction. In the IL catalytic reaction, however, the endo to exo isomerization of both norbornyl and cyclopropyl fragments (I f II f V, I f IV f V, and III f VI) proceeds readily. Moreover, considerable skeletal rearrangement happens with methyl-1,2-tetramethyleneadamantane as the primary and dominant product. Methyl-diethyl-adamantane is formed when the six-membered ring fused on the adamantane unit of methyl1,2-tetramethyleneadamantane is broken. It is not clear whether methyl-diamantane is formed directly from THTCPD or the former two alkyl-adamantanes. In addition, some byproducts, such as 2-pentene, methylcyclopentane, cyclohexane, methylcyclohexane, bicyclo[3.3.0]octane, 1-ethylcyclohexene, bicyclo[4.3.0]nonane, and tricyclo[5.2.1.02,6]decane, are formed through the dissociation of the polycyclic structure, which also provides extra hydrogen necessary for the formation of diamondoids via hydrogen abstraction and transfer. 3.3. Comparison to Other Catalytic Rearrangements. Table 1 shows the product distribution in different acid catalytic reactions. It should be noted that the reactions involving AlCl3 and CF3SO3H were conducted under optimal conditions reported in the literature.12,13 As aforementioned, no diamondoid is formed when AlCl3 is used as a catalyst. In the presence of superacid CF3SO3H and a large amount of solvent, some diamondoids are formed but the concentration is relatively low. It is worth noting that the reaction conditions are identical to Cheng’s work, in which the C15H24 molecules were regarded as multi-ring

Figure 5. Effect of AlCl3 molar composition on IL catalytic rearrangement of THTCPD (temperature, 80 °C; time, 3 h; IL/THTCPD molar ratio, 1:1).

hydrocarbons.13 However, the present work shows that the conclusion is not correct and the product should be diamondoids. In comparison to the CF3SO3H catalytic reaction, the IL catalytic rearrangement has many advantages: much higher activity in the skeletal rearrangement of THTCPD, less dosage of catalyst, no need of a solvent, no corrosive chemicals, and more environmentally benign. The outstanding performance of IL may be attributed to two points. First, the acidity of chloroaluminate anions in IL is strong enough to catalyze the reaction that is usually catalyzed by superacid. Second, IL itself works as a novel solvent to provide a strong polar and electrostatic environment that greatly stabilizes the carbonium and/or intermediate.32 3.4. Effects of Reaction Conditions. In this study, we focus on the preparation of diamondoids via skeletal rearrangement, so all of the THTCPD isomers, including I, II, III, IV, V, and VI, are regarded as reactants, whereas the dissociative small molecules are regarded as byproducts. C15H26 is chosen as the target product because it is the primary and dominant component of diamondoids. Also, entries 3 and 4 in Table 1 show that, regardless of the starting THTCPD, the reaction affords similar product distribution. The type of anions in IL is tunable depending upon its molar AlCl3 composition (denoted as x) as follows: ½Et3 NHCl þ AlCl3 f ½Et3 NHþ þ ½AlCl4 
½AlCl4 
þ AlCl3 f ½Al2 Cl7 
½Al2 Cl7 
þ AlCl3 f ½Al3 Cl10 
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Figure 6. Effect of the temperature on IL catalytic rearrangement of THTCPD [time, 3 h; IL (x = 0.67)/THTCPD molar ratio, 1:1].

Figure 5 shows the effect of AlCl3 molar composition on the rearrangement. When x e 0.50, skeletal rearrangement does not happen because the [AlCl4]
anion in IL has no acidity. When 0.50 < x e 0.55, some acidic [Al2Cl7]
anions are formed and the rearrangement occurs, suggesting that the acidity of [Al2Cl7]
is strong enough to catalyze the rearrangement of THTCPD to diamondoid. The concentration of acidic [Al2Cl7]
linearly increases when x ranges from 0.55 to 0.67; as a result, the rearrangement rate goes up linearly. With x beyond 0.67, some [Al3Cl10]
anions with stronger acidity are formed but the improvement in the conversion of THTCPD is limited. The selectivity of C15H24 decreases with the increase of x, whereas a considerable byproduct appears, especially when x > 0.67. This indicates that polycyclic hydrocarbons are prone to cracking when the acidity is too strong. Figure 6 shows the effect of the temperature on the rearrangement. The reaction can take place at room temperature, and the conversion is 3-fold when the temperature ranges from 20 to 60 °C. This tendency continues at higher temperature, but the acceleration rate slows down. The selectivity of C15H24 does not change much, although a decreasing tendency is observed in the range studied. It is notable that there is an obvious decrease in the selectivity of MDAM with the increase of the temperature, hinting that a high temperature is not preferred for the formation of MDAM. Figure 7 shows the effect of IL dosage on the rearrangement. The reaction is promoted with the increase of IL dosage because the concentration of acidic anion is increased, accompanied with a slight decrease in the selectivity of C15H24. When the molar IL/THTCPD ratio exceeds 1.0, the conversion of THTCPD no longer increases. The effect of some additives and solvents on rearrangement is also studied, as shown in Table 2. The conversion of THTCPD is promoted when a trace amount of H2O (0.5 wt %) is introduced, but the formation of C15H24 is suppressed dramatically. The presence of trace water in chloroaluminate IL may generate some Brønsted acid, and the conjugation of Brønsted acid and Lewis acid produces superacid with the Hammett acidity of
18, noting that of CF3SO3H superacid is
14.1.33
35 It is very possible that superacid species result in cracking of C15H24. Generally, many rearrangements have to be conducted in the solvent to enhance the formation and stabilization of carbonium. However, the presence of solvent is deleterious to the IL catalytic reaction, because IL itself already provides a perfect solvent environment that will be disturbed by an additional solvent.

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Figure 7. Effect of IL dosage on IL catalytic rearrangement of THTCPD [temperature, 80 °C; time, 3 h; IL (x = 0.67)].

Table 2. Effect of the Additive/Solvent on IL Catalytic Rearrangement of THTCPD additive/

CTHTCPD

SC15H24

SMDAM

SC15H26

Sbyproduct

solvent

(%)

(%)

(%)

(%)

(%)

none

66.8

87.0

5.5

3.6

3.9

0.5 wt % H2O

85.5

76.5

6.1

3.8

13.6

50 wt %

61.2

86.1

5.1

4.7

4.1

63.6

85.7

5.3

5.2

3.8

58.8

86.6

5.1

4.8

3.6

chloroform 50 wt % dichloroethane 50 wt % methylbenzene

3.5. Properties of Diamondoid Fuel. After vacuum distillation to eliminate small-molecule byproducts, a mixture containing 70.1% C15H24, 5.5% MDAM, and 1.5% C15H26 along with remaining THTCPD is obtained. In comparison to AlCl3 catalytic conformational isomerization,12 IL catalytic skeletal rearrangement lowers the freezing point of fuel significantly (from
41 to about
70 °C), confirming that alkyl-substituted diamondoids can perform better at low temperatures. The hydrogen content is increased from 10.89 to 11.43%, which will favor the ignition and combustion of fuel and decrease the soot formation in the engine nozzle. Although the density slightly decreases from 1.04 to 1.01 g/ cm3, the diamondoid fuel is expected to show much better performance in propulsion application.

4. CONCLUSION With the presence of chloroaluminate ionic liquid, THTCPD is easily converted to alkyl-diamondoids via skeletal rearrangement. The products include methyl-1,2-tetramethyleneadamantane, methyl-diethyl-adamantane, and methyl-diamantane, with methyl-1,2-tetramethyleneadamantane as the primary and dominant product (selectivity > 80%). IL shows much higher activity than superacid CF3SO3H, attributed to its acidic [Al2Cl7]
and polar solvent environment. Stronger acidity, higher temperature, or higher dosage of IL can accelerate the rearrangement but reduce the selectivity of methyl-1,2-tetramethyleneadamantane slightly. The presence of trace water induces a considerable decrease in the selectivity of diamondoids because of the cracking 1346

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’ AUTHOR INFORMATION Corresponding Author

*Telephone and Fax: 86-22-27892340. E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by the Program for New Century Excellent Talents in University, the National Natural Science Foundation of China (20906069), and the Foundation for the Author of National Excellent Doctoral Dissertation of China (200955). ’ REFERENCES (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, 641–649. (2) Edwards, T. Liquid fuels and propellants for aerospace propulsion. J. Propul. Power 2003, 19, 1089–1107. (3) Harbey, B. G.; Wright, M. E.; Quintana, R. L. High-density renewable fuels based on the selective dimerization of pinenes. Energy Fuels 2010, 24, 267–273. (4) Devener, B. V.; Perez, J. P. L.; Jankovich, J. J.; Anderson, S. L. Oxide-free, catalyst-coated, fuel soluble, air-stable boron nanopowders as combined combustion catalyst and high energy density fuel. Energy Fuels 2009, 23, 6111–6120. (5) Bruno, T. J.; Huber, M. L.; Laesecke, A.; Lemmon, E. W, Perkins, R. A. Thermophysical Properties of JP-10; National Institute of Standards and Technology (NIST): Boulder, CO, 2006; NISTIR 6640. (6) Dahl, J. E.; Liu, S. G.; Carlson, R. M. K. Isolation and structure of higher diamondoids, nanometer-sized diamond molecules. Science 2003, 299, 96–99. (7) Oomens, J.; Polfer, N.; Pirali, O.; Ueno, Y.; Maboudian, R.; May, P. W.; Filik, J.; Dahl, J. E.; Liu, S. G.; Carlson, R. M. K. Infrared spectroscopic investigation of higher diamondoids. J. Mol. Spectrosc. 2006, 238, 158–167. (8) Schleyer, P. v. R. A simple preparation of adamantane. J. Am. Chem. Soc. 1957, 79, 3292. (9) Fort, R. C.; Schleyer, P. v. R. Adamantane: Consequences of the diamondoid structure. Chem. Rev. 1964, 64, 277–300. (10) Schwertfeger, H.; Fokin, A. A.; Schreiner, P. R. Diamonds are a chemist’s best friend: Diamondoid chemistry beyond adamantane. Angew. Chem., Int. Ed. 2008, 47, 1022–1036. (11) Cage Hydrocarbons; Olah, G. A., Ed.; John Wiley and Sons: New York, 1990. (12) Wang, L.; Zhang, X.; Zou, J.-J.; Han, H.; Li, Y; Wang, L. Acidcatalyzed isomerization of tetrahydrotricyclopentadiene: Synthesis of high-energy-density liquid fuel. Energy Fuels 2009, 23, 2383–2388. (13) Cheng, S.; Liou, K.; Yen, D.; Liao, C. Preparation method of high density fuels by the addition-rearrangement of compound pentacyclo[7,5,1,02,8,03,7,010,14]pentadecane (C15H22). U.S. Patent 5,220,085, 1993. (14) Rogers, R. D.; Seddon, K. R. Ionic liquids—Solvents of the future. Science 2003, 302, 792–793. (15) Parvulescu, V. I.; Hardacre, C. Catalysis in ionic liquids. Chem. Rev. 2007, 107, 2615–2665. (16) Yoo, K; Namboodiri, V. V.; Varma, R. S.; Smirniotis, P. G. Ionic liquid-catalyzed alkylation of isobutane with 2-butene. J. Catal. 2004, 222, 511–519.

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dx.doi.org/10.1021/ef101702r |Energy Fuels 2011, 25, 1342–1347