Fuel 91 (2012) 164–169

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Isomerization of tetrahydrodicyclopentadiene using ionic liquid: Green alternativefor Jet Propellant-10 and adamantane

Lei Wang, Ji-Jun Zou ⇑, Xiangwen Zhang, Li WangKey Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China

a r t i c l e i n f o

Article history:Received 24 August 2010Received in revised form 1 November 2010Accepted 22 July 2011Available online 10 August 2011

Keywords:TetrahydrodicyclopentadieneJet Propellant-10AdamantaneIsomerizationIonic liquid

0016-2361/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.07.038

⇑ Corresponding author. Tel./fax: +86 22 27892340E-mail address: jj_zou@tju.edu.cn (J.-J. Zou).

a b s t r a c t

The isomerization of endo-tetrahydrodicyclopentadiene (endo-THDCPD) to exo-THDCPD (Jet Propellant-10) and adamantane was investigated using ionic liquid (IL) as acid catalyst to explore an alternativegreener than widely applied AlCl3-based operation. ILs composed of various 1-alkyl-3-methylimidazo-lium chlorides ([RMIM]Cl) and metal chlorides were checked, and [BMIM]Cl/AlCl3 was the best. Theeffects of acidity and dosage of IL, reaction temperature and time were studied for endo- to exo-isomer-ization of THDCPD. The reaction occurs quickly under mild conditions with both conversion and selectiv-ity beyond 98%. Purifying the reactant can suppress deactivation of IL; and the used IL can be recycledfour times by heating under vacuum after each run. By adjusting the reaction to severe conditions, i.e.higher IL dosage, higher temperature and longer time, exo-THDCPD is further isomerized to adamantane.But both the conversion and selectivity are much lower than those of endo- to exo- isomerization. Thehighest yield of adamantane is 50.9%, which can be further improved by some additives like 1-bromoad-amantane. Compared with other methods, IL catalysis has many advantages from energy and environ-mental perspectives.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Endo-tetrahydrodicyclopentadiene (endo-THDCPD), the hydro-genated product of endo-dicyclopentadiene that is C5 fraction ofstream cracking, has two very important isomers, namely exo-THDCPD and adamantane, see Scheme 1. Exo-THDCPD, with smallamount of additives, constitutes the standard high-energy-densityfuel Jet Propellant 10(JP-10) [1,2]. Its volumetric energy (39.6 MJ/L)is about 20% higher than conventional petroleum distillates, andthe low freezing point (�79 �C) and high flash point (54 �C) satisfythe requirements of both land and air applications. Actually, thisfuel has been used widely to expand the range of cruise missiles.Adamantane has a symmetric tricyclic structure resembling thatof diamond. Although thermally stable, the four bridgehead sitesof adamantane are chemically reactive and prone to take part inmany reactions like substitution, oxidation and alkylation. Theresulting derivatives have found many important applications inpharmaceuticals, polymers and lubricants [3,4].

Originally exo-THDCPD was synthesized through isomerizationof endo-THDCPD using Brönsted acid like sulfuric acid, now Lewisacid like AlCl3 is used for industrial batch operation [5–7]. Ada-mantane was initially isolated in small amount from petroleummixtures, then Schleyer reported a simple synthesis route via

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.

endo-THDCPD isomerization catalyzed by large amount of AlCl3

that is nowadays applied on industrial scale [8–10]. AlCl3-basedprocesses inevitably bring up many environmental concerns likecorrosion of equipment, troublesome product separation, no recy-clability of catalyst and thus catalyst disposal problem.

Superacid [11], heteropolyacid [12] and zeolite [13] have beeninvestigated for endo- to exo-isomerization of THDCPD. Similar toAlCl3, liquid superacid brings many environmental problems. Het-erogeneous reaction has advantages such as easy separation ofproduct and catalyst recyclability, but high temperature, highhydrogen pressure and large amount of catalyst are necessary toobtain acceptable yield. Moreover, deactivation of catalyst causedby serious coke formation is a big problem. Zeolites [14] andimmobilized AlCl3 [15] have also been used to synthesize adaman-tane but the yield is still very low.

Ionic liquid (IL) has been widely investigated as green catalystdue to its advantages such as negligible vapour pressure, nonflam-mability and tunable acidity [16–18]. Both the type and applicationof IL have been expanding rapidly in the last two decades. Specif-ically, chloroaluminate IL has been used in many reactions, likeisomerization [19–23], alkylation [24–26], Friedel–Crafts reaction[27], oligomerization [28,29], transesterification [30], and Diels–Alder addition [31]. In this paper, we present a fundamental studyon chloroaluminate IL catalytic isomerization of THDCPD with thepurpose to develop a green alternative for JP-10 and adamantanesynthesis.

Table 1Isomerization of THDCPD using different acidic ILs.

Anion Cation Cendo or Cexo (%) Sexo (%) SAD (%)

endo-THDCPD ? exo-THDCPDa

CuCl [BMIM]Cl 0FeCl3 [BMIM]Cl 5.2 100 0ZnCl2 [BMIM]Cl 8.7 100 0AlCl3 [BMIM]Cl 98.2 98.2 1.8AlCl3 [HMIM]Cl 97.6 98.5 1.5AlCl3 [OMIM]Cl 96.8 99.1 0.9

exo-THDCPD ? adamantaneb

AlCl3 [BMIM]Cl 100 35.8 64.9AlCl3 [HMIM]Cl 100 33.5 63.5AlCl3 [OMIM]Cl 100 30.4 62.8

a Reaction conditions: IL(x = 0.60) 10 mol%, temperature 50 �C, time 0.5 h.b Reaction conditions: IL(x = 0.67) 100 mol%, temperature 80 �C, time 4 h.

2500 2400 2300 2200 2100

[BMIM]Cl/AlCl3

[BMIM]Cl/ZnCl2

[BMIM]Cl/FeCl3

pure acetonitrile

Abs

orba

nce

(a.u

.)

Wavenumber (cm-1)

[BMIM]Cl/CuCl

Fig. 1. FT-IR spectra of [BMIM]Cl/MCln(x = 0.64) ILs.

2500 2400 2300 2200 2100

x(AlCl3)=0.71

x(AlCl3)=0.67

x(AlCl3)=0.64

x(AlCl3)=0.60

x(AlCl3)=0.55

Abs

orba

nce

(a.u

.)

Wavenumber (cm-1)

pure acetonitrile

x(AlCl3)=0.50

Fig. 2. FT-IR spectra of [BMIM]Cl/AlCl3 ILs with different mole fraction of AlCl3.

0.50 0.55 0.60 0.65 0.70

0

20

40

60

80

100

Con

vers

ion

or S

elec

tivity

(%

)

x(AlCl3) in IL

Cendo

Sexo

SAD

Fig. 3. Effect of mole fraction of AlCl3 in ILs on endo- to exo- isomerization ofTHDCPD. (reaction conditions: IL 10 mol%, temperature 50 �C, time 0.5 h).

40 50 60 70 80 90 1000

20

40

60

80

100

Con

vers

ion

or S

elec

tivity

(%

)

Temperature (°C)

Cendo

Sexo

SAD

Fig. 4. Effect of temperature on endo- to exo- isomerization of THDCPD. (reactionconditions: IL(x = 0.67) 3.3 mol%, time 0.5 h).

adamantaneendo-THDCPD exo-THDCPD

Scheme 1. Three isomers of THDCPD.

L. Wang et al. / Fuel 91 (2012) 164–169 165

2. Experimental

2.1. Preparation of IL

All reagents (>99.0% purity) were used directly without anypurification. Cation and anion of IL were composed of commercial1-n-alkyl-3-methylimidazolium chlorides ([RMIM]Cl,>99.0% pur-ity, water content < 0.03%) and anhydrous metal halides (MCln,> 99.0% purity), respectively. To prepare IL, defined amount of[RMIM]Cl and MCln were added into a three-neck flask and stirredat 50 �C for 60 min under protection of dried nitrogen. A symbol xwas used to denote the mole fraction of metal halide in IL, i.e.x = n(MCln)/[n(MCln)+n([RMIM]Cl)]. Acidity of IL was characterizedbased on FT-IR spectroscopy recorded on Nicolet 560 FTIR spec-trometer using acetonitrile as probe molecule, according toreported procedure [32,33].

2.2. Isomerization reaction

Isomerization reaction was carried out in a 50 mL three-neckflask equipped with reflux condenser, magnetic stirrer and gasinlet for nitrogen flow to maintain inert atmosphere. 13.6 g(0.1 mol) self-produced endo-THDCPD (purity > 98.5%) was addedin reactor and heated to defined temperature in oil bath, thenthe pre-prepared IL was added under stirring. After the reactionis completed, the mixture was allowed to separate into two layers,

Table 2Effect of mole fraction of AlCl3 and dosage of ILs on endo- to exo- isomerization ofTHDCPD.

x(AlCl3) inIL

IL dosage relative to endo-THDCPD(mol%)

Cendo

(%)Sexo

(%)SAD

(%)

0.55 100 98.8 99.1 0.390.55 25 93.5 100 00.55 16.7 23.7 100 00.55 10 17.1 100 00.6 100 99.5 90.2 3.70.6 14.3 98.8 97.9 0.30.6 10 98.2 98.1 00.6 6.3 51.2 100 00.6 5 19.8 100 00.6 5b 45.5 100 00.67 100 100 90.2 5.60.67 14.3 99.1 97.0 1.10.67 10 98.8 98.3 0.40.67 5 98.5 98.4 00.67 3.3 98.3 98.7 00.67 2.8 23.4 98.9 00.67 2.8b 47.5 100 0

a Reaction conditions: temperature 50 �C, time 0.5 h.b Reactant is purified via distillation.

1 2 3 4 50

20

40

60

80

100

Con

vers

ion

or S

elec

tivity

(%

)

Reuse times

Cendo

(used IL is treated)

Sexo

-(used IL is treated)

Cendo

-(used IL is untreated)

Sexo

-(used IL is untreated)

Fig. 5. Reuse of IL in endo- to exo- isomerization of THDCPD. (reaction conditions:IL(x = 0.67) 3.3 mol%, temperature 50 �C, time 0.5 h).

0 2 4 6 8 10 12 14 16

0

10

20

30

40

50

60

70

80

Con

vers

ion

or S

elec

tivity

(%

)

Time (h)

Cexo

SAD

SDHN

SPoly

Fig. 6. Isomerization of exo-THDCPD to adamantane with prolonged reaction time.(reaction conditions: IL(x = 0.67) 100 mol%, temperature 80 �C).

166 L. Wang et al. / Fuel 91 (2012) 164–169

and the upper hydrocarbon layer was obtained by decantation. Incase excess adamantane precipitated in the bottom, hexane wasused to recover all products. In some cases, the remained IL layerwas heated at 100 �C under vacuum for 2 h for recycling. The com-position of hydrocarbons was analyzed using an Agilent 7890 gaschromatograph equipped with AT-SE-54 capillary column(50 m � 0.32 mm � 0.33 lm) and flame-ionization detector.

For exo- to endo-THDCPD isomerization, endo-THDCPD wasconverted to exo-THDCPD and small amount of adamantane, sothe conversion of endo-THDCPD (Cendo), and selectivity of exo-THDCPD (Sexo) and adamantane (SAD) were used to assess the activ-ity. For exo-THDCPD to adamantane isomerization, exo-THDCPDwas converted to adamantane along with some decalin and somepolymers. In this case, the conversion of exo-THDCPD (Cexo), andselectivity of adamantane (SAD), decalin (SDHN) and polymers (Spoly)were calculated.

3. Results and discussion

3.1. Determination of ILs

It should be noted that the reaction conditions to produceexo-THDCPD and adamantane are dramatically different. The

former occurs easily under mild conditions but the latter happensonly under severe conditions. Firstly, several 1-n-alkyl-3-methyl-imidazolium halides were checked, as shown in Table 1. Theconversion of endo-THDCPD and selectivity of exo-THDCPD donot change with the types of cation. As to reaction towardsadamantane, the selectivity of adamantane decreases very slightlyas the alkyl group of cation becomes longer. Overall, the effect ofcation on isomerization is negligible and it is the anion that deter-mines the activity.

Fig. 1 depicts the FT-IR spectra of IL/CH3CN mixtures. Pureacetonitrile has two bands around 2292 cm�1 and 2253 cm�1 as-signed to CN stretching vibrations. The vibrations shift to higherfrequencies once acetonitrile is absorbed on Lewis acid species ofIL, and it is known that higher wavenumber corresponds to stron-ger acidity [32,33]. ILs containing CuCl, ZnCl2 or FeCl3 have weakLewis acidity, and IL with AlCl3 possesses the strongest acidity.For endo- to exo- isomerization of THDCPD, only chloroaluminateIL exhibits considerable activity, see Table 1. One can see that theisomerization activity is closely connected with IL’s acidity. Dueto the very high activity, [BMIM]Cl/AlCl3 is chosen for endo- toexo- isomerization of THDCPD. It is natural that only this chloroa-luminate IL is suitable for the adamantane-formation reactionbecause it requires stronger acidity.

3.2. Effect of reaction conditons on endo- to exo- isomerization ofTHDCPD

The types of acid species (anions) in IL are dependent on themole fraction of AlCl3 (denoted as x) in IL, as follows:

½BMIM�Clþ AlCl3 ! ½BMIM�þ þ ½AlCl4��

½AlCl4�� þ AlCl3 ! ½Al2Cl7��

½Al2Cl7�� þ AlCl3 ! ½Al3Cl10��

Since these species have different acid strength (the order is[Al3Cl10]� > Al2Cl7]� > AlCl4]�), x value obviously shows significantinfluence on isomerization, as shown in Figs. 2 and 3. Whenx 6 0.50, anion in IL is [AlCl4]� without any acidity, and the IRvibration bands of IL/CH3CN mixture is identical to those of pureacetonitrile. Accordingly the isomerisation reaction of endo-THDCPD does not happen. When 0.50 < x 6 0.55, some acidic[Al2Cl7]� anions appear and isomerization occurs, which indicatesthat the acidity of [Al2Cl7]� is strong enough to facilitate endo- toexo- isomerization of THDCPD. However, the conversion ofendo-THDCPD is relatively low because of insufficient concentration

endo-THDCPD endo-THDCPD exo-THDCPDexo-THDCPD+ +

Adamantane

Decalin + DecalinPolymers

PolymersAdamantane+

Scheme 2. Reaction pathway of THDCPD isomerization.

20 40 60 80 1000

10

20

30

40

50

60

70

Con

vers

ion

or S

elec

tivity

/%

IL dosage (mol%)

Cexo

SAD

SDHN

Fig. 7. Effect of IL dosage on exo-THDCPD to adamantane isomerization. (reactionconditions: IL(x = 0.67), temperature 80 �C, time 4 h).

0.55 0.60 0.65 0.70 0.75

0

20

40

60

80

100

Con

vers

ion

or s

elec

tivity

(%)

x AlCl3

in IL

Cexo

SAD

SDHN

Fig. 8. Effect of mole fraction of AlCl3 in IL on exo-THDCPD to adamantaneisomerization. (reaction conditions: IL(x = 0.67) 100 mol%, temperature 80 �C, time4 h).

L. Wang et al. / Fuel 91 (2012) 164–169 167

of acid species. The concentration of [Al2Cl7]� increases with theincrease of x value, correspondingly the reaction is acceleratedsignificantly. When x is increased to 0.6, the reaction proceeds veryquickly and the conversion of endo-THDCPD reaches 98.2% in30 min. One may note that there is no adamantane formed whenx < 0.64, and its selectivity is <1% even with x = 0.71 where some[Al3Cl10]� anions with very strong acidity are formed, suggestingthat the reaction towards adamantane is difficult to take place.

Fig. 4 shows that proper temperature is necessary for isomeri-zation reaction. There is a considerable increase in the conversionwhen temperature rises from 40 �C to 50 �C, and it does notincrease with temperature any more. On the contrary, the selectiv-ity of exo-THDCPD drops with reaction temperature due to the

formation of adamantane. For synthesizing exo-THTCPD, the opti-mal temperature is 50 �C. Table 2 shows that the catalyst dosageneeded for the reaction greatly depends on the mole fraction ofAlCl3, that is, higher the x value is, smaller the dosage is. It is worthnoting that both the conversion and selectivity exceed 98% with3.3 mol% IL(x = 0.67) used.

3.3. Deactivation and recycling of IL for endo- to exo- isomerization ofTHDCPD

From Table 2, one may note that, when dosage of IL is small, theconversion of endo-THDCPD is very low that cannot be promotedby expanding reaction time, suggesting that some acid speciesare deactivated or poisoned. Impurities in the reactant like oxidesand absorbed water may be detrimental for the catalyst. So it waspurified by vacuum distillation, and this pre-purification doessignificantly improve the reaction. The conversion is increasedfrom 19.8% to 45.5% when 5 mol% IL(x = 0.60) is used, and from23.4% to 47.5% when 2.8 mol% IL(x = 0.67) is used, see Table 2.

Using purified endo-THDCPD as reactant, the reuse of IL wasconducted in two ways. In first operation, the IL layer was directlyused as catalyst for another run. As shown in Fig. 5, the activityquickly decreases with the increase of recycling times, indicatinggradual deactivation of IL. The reaction produces some polymer-ized by-products, which is very considerable in adamantane-formation reaction (see below). Some of them may enter the ILlayer and be transferred into polymers in subsequent runs. Thesepolymers called as acid soluble oil in reference [20] will accumu-late and finally deactivate chloroaluminate species. To preventthe formation of polymers, the used IL was heated under vacuumafter each run to remove residues with relatively low boiling point.In this case, IL can be used for four runs without obvious activityloss, indicating the treatment retards deactivation. However, inthe fifth run, the activity becomes very low again, probablybecause the treatment cannot completely eliminate the formationand accumulation of polymers in IL. In addition, leaching of chlo-roaluminate into hydrocarbon layer may become obvious afterseveral runs, which lowers the concentration of active speciesand thus decreases the activity.

3.4. Isomerization of exo-THDCPD to adamantane

In endo- to exo- isomerization of THDCPD, a phenomenon worthnoting is that adamantane appears with trace amount under someconditions, i.e. high mole faction of AlCl3 in IL, high catalyst dosageand high reaction temperature. One may expect that adjusting thereaction conditions will produce considerable adamantane. Underthese conditions, isomerization of endo- to exo-THDCPD takes placealmost quantitatively in less than 10 min and then adamantaneappears, suggesting that adamantane is formed via isomerizationof exo-THDCPD. Fig. 6 shows that the conversion of exo-THDCPDgoes up with reaction time and reaches 56% in 15 h, whereas theselectivity of adamantane slowly decreases from 75.1% to 60.8%.It is clear that the reaction rate of adamantane-forming is much

Table 3Isomerization of THDCPD using various acid catalysts.

Reaction conditions Cendo (%) Sexo (%) SAD (%) Ref.

Catalyst dosage Temperature (�C) Time (h) Solvent/reactant ratio

110 wt% CF3SO3H rt 24 CCl4 (1/1) 100 99.2 [11]2.2 wt% AlCl3 68–121 3 98 99 1 [7]60 wt% AlCl3 0 1 CH2Cl2 (1/1) 98 98 [6]20 wt% FH-SSY 195 2 99 97.6 2.4 [13]10.8 wt% CF3SO3H 50 0.5 99.0 92.0 5.510.8 wt% AlCl3 50 0.5 99.0 91.0 5.310.8 wt% [BMIM]Cl/AlCl3(x = 0.67) 50 0.5 98.3 98.77.4 wt% [Et3NH]Cl/AlCl3(x = 0.67) 80 0.5 98.1 98.0270 wt% CF3SO3H + SbF5 (1/1) 0-rt 7 100 0.1 98 [11]AlCl3, 98 wt% 50 6 Dichloroethane (3/4) 100 62.1 [10]100 wt% HY 270 5 Cyclohexane (50/1) 2.5 MPa H2 100 69.5 15.2 [14]

168 L. Wang et al. / Fuel 91 (2012) 164–169

lower compared with that of endo- to exo- isomerization, and theformation of by-products is inevitable. Although the startingmaterial is endo-THDCPD, reaction in this stage is actually theexo-THDCPD ? adamantane one, so the activity is assessed usingthe conversion of exo-THDCPD and selectivity of adamantane.

Transformation from endo- to exo-THDCPD is easy because itonly undergoes conformational isomerization. However, isomeri-zation from endo-THDCPD to adamantane is a complex, thermody-namics-controlling and much slow skeletal rearrangement becauseit involves hydride abstractions and multiple Wagner–Meerweinrearrangements. In this process the conversion from [exo-THDCPD]+ to [adamantane]+ is the rate-determining step. Alongwith the rearrangement, decalin is also formed through dispropor-tionation, and other by-products formed through polymerization.The reaction pathway is briefly illustrated in Scheme 2.

As seen in Fig. 7, the dosage of IL has to be very high to giveconsiderable conversion. The conversion of exo-THDCPD linearlyincreases with IL dosage, along with a slight decrease in theselectivity. Fig. 8 shows that, acidity is a crucial factor for ada-mantane synthesis and the conversion of exo-THDCPD increasesquickly with mole faction of AlCl3. The conversion of exo-THDCPD reaches 92.3% when x = 0.71. Unfortunately, strong acid-ity also induces considerable side reactions, as a result the selec-tivity of adamantane decreases from 77.6% to 55.2%, with theyield being 50.9%.

3.5. Comparison of IL catalytic isomerization with other methods

Isomerization of endo-THDCPD to exo-THDCPD and adamantaneusing various acid catalysts are listed in Table 3. All existingoperations offer high yield of exo-THDCPD. CF3SO3H-based opera-tion can be conducted at room temperature, but the dosage ofcatalyst and solvent are very high. As to AlCl3-based reaction, hightemperature and long reaction time, and/or large amount ofcatalyst and solvent, are necessary, whereas zeolite-catalyticreaction has to be conducted at significantly high temperature.IL-based route in the present work gives high yield (>97%) inshorter reaction time (0.5 h), and there is no solvent used, alsoliquid–liquid biphasic reaction makes product separation andcatalyst recycling easy. If IL is replaced by same amount(10.8 wt%) of AlCl3 or CF3SO3H, the yield of exo-THDCPD will belower (about 91%). Considering the 4-times recycling, the averagecatalyst consumption (2.7 wt%) is actually very low. The reactiontemperature is just a little higher than room temperature thatcan be easily controlled; also extra energy for heating or coolingis saved.

For exo-THDCPD production, industrial AlCl3-catalytic operationis in batch process because the catalyst cannot be properly

separated and recycled. The phase separation and recycling abilitymakes IL-based operation suitable for continuous flow reactionsystem. However, some technical problems have to be solved forscaling up. Fortunately, there are already some pilots or industrialprocess involving IL publicly announced, and new continuous reac-tors are developed specifically to facilitate the mixing of differentphases, separation of product and recycling of catalyst inside thereactor system [18,25,26,29], which can be applied in the presentcase. Imidazolium salt used in the present work is a little expen-sive, which makes the IL-based operation less attractive in eco-nomics. Since the activity of IL is totally decided by anions, it isexpected that cheaper reagent can be used to reduce cost. Asshown in Table 3, when trialkylammonium chloride (Et3NHCl)whose cost is comparable with AlCl3 is applied to replace[RMIM]Cl, the yield of exo-THDCPD is still very high (96.1%).

As to adamantane synthesis, both catalyst dosage and reactiontemperature are very high for all operations. The selectivity of zeo-lite-catalytic reaction needs huge improvement, and many caresmust be taken to deal with acid in superacid-catalytic reaction.AlCl3-based reaction is still the best choice by now. Along withthe mentioned shortcomings, another problem is that, the formedadamantane sediment is mixed with AlCl3 due to low solubility inunreacted exo-THDCPD, thus product separation and purification isvery troublesome. Although IL is not as active as AlCl3 and super-acid, it still has some advantages. Adamantane sediment in IL lay-ers can be easily recovered by filtration, and highly pure productcan be achieved via simple washing and drying. It is also found thatsome additives significantly improve the yield of adamantane. Forthe 4-h reaction with 100 mol% IL(x = 0.67) at 80 �C, addition of1 wt% 1-bromoadamantane increases the yield of adamantanefrom 17.8% to 34.1%.

4. Conclusions

The isomerization of tetrahydrodicyclopentadiene using acidic[RMIM]Cl/MCln IL has been investigated to develop a green alterna-tive for JP-10 and adamantane. Chloroaluminate IL is much moreactive than ILs with FeCl3, ZnCl2 or CuCl2, whereas the chain lengthof [RMIM]Cl has negligible effect on the reaction. The conversion ofendo-THDCPD and selectivity of exo-THDCPD are >98% with3.3 mol% IL(x = 0.67) used. By removing impurities in reactantand polymerized by-prodcuts formed during reaction, IL can beused for four runs without obvious activity loss. Under severeconditions with large catalyst dosage and high temperature,exo-THDCPD can be further isomerized to adamantane. The selec-tivity of adamantane is generally <70% due to the formation ofby-products like decalin and polymers. The highest yield ofadamantane reaches 50.9%, and utilization of some additives can

L. Wang et al. / Fuel 91 (2012) 164–169 169

further improve the yield. The IL-based reaction is more environ-mentally benign than the widely applied AlCl3-based operation.

Acknowledgments

This work is supported by Program for New Century ExcellentTalents in University, National Natural Science Foundation of China(20906069), and the Foundation for the Author of National Excel-lent Doctoral Dissertation of China (200955).

References

[1] Bruno TJ, Huber ML, Laesecke A, Lemmon EW, Perkins RA. Thermophysicalproperties of JP-10, NISTIR 6640. National Institute of Standards andTechnology: Boulder, CO; 2006.

[2] Zou J-J, Zhang X, Kong J, Wang L. Hydrogenation of dicyclopentadiene overamorphous nickel alloy catalyst SRNA-4. Fuel 2008;87:3655–9.

[3] Fort RC. Adamantane: The Chemistry of Diamond Molecules. NewYork: Dekker; 1976.

[4] Dahl JE, Liu SG, Carlson RMK. Isolation and structure of higher diamondoids,nanometer-sized diamond molecules. Science 2003;299:96–9.

[5] Cristol SJ, Seifert WK, Soloway SB. Bridged polycyclic compounds. X. thesynthesis of endo and exo-1, 2-dihydrodicyclopentadienes and relatedcompounds. J Am Chem Soc 1960;82:2351–6.

[6] Schneider A, Ware RE, Janoski EJ. Isomerization of endo-tetrahydrodicyclo-pentadiene to a missile fuel diluent. US Patent 4086284; 1978.

[7] Norton RV, Howe SC. Production of high energy fuel. US Patent 4270014; 1981.[8] Schleyer PvR. A simple preparation of adamantane. J Am Chem Soc

1957;79:3292.[9] Schleyer PvR, Donaldson MM. The relative stability of bridged hydrocarbons. II.

endo- and exo-trimethylenenorbornane. the formation of adamantane. J AmChem Soc 1960;82:4645–51.

[10] Zhang X, Qiu L, Tao R, Mi Z. Studies on the AlCl3 catalyst systems for thesynthesis of adamantane. Petrochem Techn 1999;28:546–8.

[11] Olah GA, Farooq O. Chemistry in superacids. 7. Superacid-catalyzedisomerization of endo- to exo-trimethylenenorbornane (tetrahydrodicyclo-pentadiene) and to adamantane. J Org Chem 1986;51:5410–3.

[12] Jeong BH, Han JS, Ko SW, Lee JH, Lee BJ. Deactivation and reuse of cesium-containing heteropolyacid for the isomerization of THDCPD. J Ind Eng Chem2007;2:310–3.

[13] Xing E, Zhang X, Wang L, Mi Z. Greener synthesis route for Jet Propellant-10:the utilization of zeolites to replace AlCl3. Green Chem 2007;9:589–93.

[14] Navrátilová M, Sporka K. A Synthesis of adamantane on commerciallyavailable zeolitic catalysts. Appl Catal A 2000;203:127–32.

[15] Wu L, Ji M, He M, Cai T. Synthesis of adamantane catalyzed by an activeimmobilized aluminium chloride catalyst. Chin J Catal 2007;28:585–7.

[16] Parvulescu VI, Hardacre C. Catalysis in ionic liquids. Chem Rev2007;107:2615–65.

[17] Rogers RD, Seddon KR. Ionic liquids-solvents of the future. Science2003;302:792–3.

[18] Olivier-Bourbigou H, Magna L, Morvan D. Ionic liquids and catalysis: recentprogress from knowledge to applications. Appl Catal A 2010;373:1–56.

[19] Zhang R, Meng X, Liu Z, Meng J, Xu C. Isomerization of n-pentane catalyzed byacidic chloroaluminate ionic liquids. Ind Eng Chem Res 2008;47:8205–10.

[20] Berenblyum AS, Katsman EA, Karasev YZ. The nature of catalytic activity anddeactivation of chloroaluminate ionic liquid. Appl Catal A 2006;315:128–34.

[21] Huang M, Wu J, Shieu F, Lin J. Isomerization of exo-tetrahydrodicyclopentadi-ene to adamantane using an acidity-adjustable chloroaluminate ionic liquid.Catal. Commun. 2009;10:1747–51.

[22] Huang M, Chang J, Lin J, Lin K, Wu J. Method for producing exo-tetrahydrodicyclopentadiene using ionic liquid catalyst. US Patent 7488860;2009.

[23] Huang M, Wu J, Shieu F, Lin J. Isomerization of endo-tetrahydrodicyclo-pentadiene over clay-supported chloroaluminate ionic liquid catalysts. J MolCatal A 2010;315:69–75.

[24] Yoo K, Namboodiri VV, Varma RS, Smirniotis PG. Ionic liquid-catalyzedalkylation of isobutane with 2-butene. J Catal 2004;222:511–9.

[25] Qiao C, Zhang Y, Zhang J, Li C. Activity and stability investigation of[BMIM][AlCl4] ionic liquid as catalyst for alkylation of benzene with 1-dodecene. Appl Catal A 2004;276:61–6.

[26] Ladnak V, Hofmann N, Brausch N, Wasserscheid P. Continuous, ionic liquid-catalysed propylation of toluene in a liquid-liquid biphasic reaction modeusing a loop reactor concept. Adv Synth Catal 2007;349:719–26.

[27] Valkenberg MH, Castro C, Holderich WF. Friedel-crafts acylation of aromaticscatalysed by supported ionic liquids. Appl Catal A 2001;215:185–90.

[28] Thiele D, Souza RF. The role of aluminum species in biphasic butenedimerization catalyzed by nickel complexes. J Mol Catal A 2007;264:293–8.

[29] Wasserscheid P, Eichmann M. Selective dimerisation of 1-butene in biphasicmode using buffered chloroaluminate ionic liquid solvents-design andapplication of a continuous loop reactor. Catal Today 2001;66:309–16.

[30] Liang X, Gong G, Wu H, Yang J. Highly efficient procedure for the synthesis ofbiodiesel from soybean oil using chloroaluminate ionic liquid as catalyst. Fuel2009;88:613–6.

[31] Bae HW, Han JS, Jung S, Cheong M, Kim HS, Lee JS. Polymer-supportedchloroaluminate catalysts for the Diels–Alder reaction of cyclopentadiene withmethyl methacrylate. Appl Catal A 2007;331:34–8.

[32] Yang Y, Kou Y. Determination of the Lewis acidity of ionic liquids by means ofan IR spectroscopic probe. Chem Commun 2004;2:226–7.

[33] Wu Q, Dong B, Han M, Xin H, Zuo Y, Jin Y. Studies on acidity of chloroaluminateionic liquids and its catalytic performance for alkylation of benzene with long-chain alkenes. Spectrosc Spectr Anal 2007;27:460–4.